Solar‐Triggered Engineered 2D‐Materials for Environmental Remediation: Status and Future Insights

Modern‐day society requires advanced technologies based on renewable and sustainable energy resources to face the challenges regarding environmental remediation. Solar‐inspired photocatalytic applications for water purification, hydrogen and oxygen evolution, carbon dioxide reduction, nitrogen fixation, and removal of bacterial species seem to be unique solutions based on green and efficient technologies. Considering the unique electronic features and larger surface area, 2D photocatalysts have been broadly explored for the above‐mentioned applications in the past few years. However, their photocatalytic potential has not been optimized yet to the adequate level of practical and commercial applications. Among many strategies available, surface and interface engineering and the hybridization of different materials have revealed pronounced potential to boost the photocatalytic potential of 2D materials. This feature review recapitulates recent advancements in engineered materials that are 2D for various photocatalysis applications for environmental remediation. Various surface and interface engineering technologies are briefly discussed, like anion–cation vacancies, pits, distortions, associated vacancies, etc., along with rules and parameters. In addition, several hybridization approaches, like 0D/2D, 1D/2D, 2D/2D, and 3D/2D hybridization, etc., are also deeply investigated. Lastly, the application of these engineered 2D materials for various photocatalytic applications, challenges, and future perspectives is extensively explored.


Introduction
Modern society critically demands the availability of chemical supplies and sustainable energy sources that are extremely important for daily life's convenience, stability, and transportation. At the global level, energy production based on fossil fuels currently constitutes 85% of the total energy requirements. Rapid progress and development of novel technologies have encouraged the research community to develop environmentally friendly and sustainable methods for energy production. Global warming has drastically increased in the past few years, resulting from the ever-growing pollution content in environment due to excessive use of fossil fuels. [1][2][3][4] Due to increased population growth, industrialization and modernization have been enhanced rapidly worldwide, which is believed to escalate the energy demands by 2050. [5][6][7][8] Moreover, fossil fuels like natural gases, coal, and petroleum are rapidly consumed since the world relies on these fuels to meet energy Modern-day society requires advanced technologies based on renewable and sustainable energy resources to face the challenges regarding environmental remediation. Solar-inspired photocatalytic applications for water purification, hydrogen and oxygen evolution, carbon dioxide reduction, nitrogen fixation, and removal of bacterial species seem to be unique solutions based on green and efficient technologies. Considering the unique electronic features and larger surface area, 2D photocatalysts have been broadly explored for the above-mentioned applications in the past few years. However, their photocatalytic potential has not been optimized yet to the adequate level of practical and commercial applications. Among many strategies available, surface and interface engineering and the hybridization of different materials have revealed pronounced potential to boost the photocatalytic potential of 2D materials. This feature review recapitulates recent advancements in engineered materials that are 2D for various photocatalysis applications for environmental remediation. Various surface and interface engineering technologies are briefly discussed, like anion-cation vacancies, pits, distortions, associated vacancies, etc., along with rules and parameters. In addition, several hybridization approaches, like 0D/2D, 1D/2D, 2D/2D, and 3D/2D hybridization, etc., are also deeply investigated. Lastly, the application of these engineered 2D materials for various photocatalytic applications, challenges, and future perspectives is extensively explored.
distinct morphological features after the discovery of graphene unlocked new prospects in water treatment. [36] Gradual increase in bandgap energies and shifting of absorption region can be observed from graphene to molybdenum disulfide (MoS 2 ). Several innovative water treatment systems with outstanding separation, adsorptive and catalytic performances have been developed by controlling the structure, thickness, and size of these 2D materials. Such systems mainly include graphene membranes, [37] oil adsorbents, [38] ultrafast photocatalysts, [39] and various smart/self-healing structures. [40] Although many materials have been extensively studied and significant developments have been made in the field of photocatalysis. However, this process has low efficiency and deprived stability, which fall well short of the requirements for practical application. Three critical steps are involved during a typical photocatalytic process: 1) light-harvesting for exciton pairs generation, 2) separating and transferring the photogenic charges from bulk to surface, and 3) initiation of redox reactions through charges at surface or the corresponding active sites. [41] 2D materials can considerably enhance charge separation, light absorption, and interfacial redox reactions. [42] Unlike their analogous bulk forms, 2D materials offer overwhelming advantages due to their unique atomic properties. The generation of light-induced charge carriers is effectively boosted accredited to ultrathin nature and large surface area of 2D materials. Furthermore, the possibility of recombination is effectively reduced due to a decrease in diffusion length for charge carriers in ultrathin structures. In addition, the redox reactions are boosted due to the availability of numerous active sites in 2D materials enriched with coordination-unsaturated atoms at their surface. [43] However, the performance of these 2D materials has not yet reached the level of practical application demands. Researchers have adopted various surface and interfacial engineering strategies to enhance their photocatalytic potential further.
Producing defects in semiconductors is an effective method of tuning and improving the characteristics of 2D materials. [44] Apart from the most frequently studied defects in photocatalytic materials, i.e., oxygen vacancies, other defects like metal vacancies have also been explored recently. However, they are relatively difficult to handle. Using 2D materials in photocatalysis has made it possible for researchers to intentionally produce various types of defects with a reasonable degree of ease. Because of their thin structures, even a tiny amount of generated defects can produce the considerable potential to tailor a photocatalyst's bandgap, spin nature, carrier concentrations, etc. Moreover, it is much easier to produce defects in 2D materials to generate different phenomena than their bulk counterparts. Additionally, creating defects on the surface of bulky materials results in defects inside the bulk, which is highly detrimental to photocatalysis since these defects might function as recombination centers within the structure. [45] The researchers have emphasized the importance of defects in light-driven catalysis to design better and more robust photocatalytic sys-to encourage further exploring of defective 2D materials for various photocatalytic applications.
This feature review summarizes exciting developments in defective 2D materials and their potential application in photocatalytic environmental remediation. Some of the most significant and recent accomplishments in surface and interface engineering of 2D materials, for instance, vacancy associates, anion-cation vacancies, disorders, pits, and distortions, are presented. Furthermore, various approaches for the controlled formation of these defects, such as ball milling, vacuum activation, and chemical reduction, are presented. In addition, different hybridization engineering strategies and essential parameters for surface and interfacial engineering have also been stated. Lastly, the crucial effects of defects (both surface and interfacial) on the performance of these photocatalysts in pollutant removal, CO 2 reduction, water oxidation, hydrogen evolution, organic synthesis, and nitrogen fixation have been summarized.

Supremacy of 2D Materials
2D materials with single or few atom thicknesses (>5 nm) have high surface atom ratio, large surface area, and intrinsic quantum confined electrons that exhibit extraordinary optical, mechanical, and electronic properties and have great potential for the research of transistors, catalysis, optoelectronic, conversion, and the energy storage devices. [48][49][50][51][52][53] These materials present unique physiochemical behavior like electronic anisotropy, high surface activity, planar conductivity, and tunable energy structure. [54][55][56][57][58] The decrease in thickness of bulk substances to the atomic level, atomic structures will go through the apparent distinctions, including length and angle of bonds, coordination number, and formation of surface defects and disordering of surface atoms. As a result, 2D materials exhibit bulk properties and new features. [51,[59][60][61][62] Semiconductor photocatalytic materials have gained interest as they give promising solutions for environmental pollution and energy storage. These materials split the water into hydrogen and oxygen, eliminating pollutants and reducing CO 2 by solar light as an external driving force. [15,[63][64][65][66] Light absorption, migration, separation of charges, and surface redox reactions are the major steps for photocatalysis. After irradiation exposure, the photocatalysts absorb the light and produce electron and hole pairs in the conduction and valance bond. These photogenerated electron-hole pairs diffuse to the surface of a material and then migrate to the active sites before the surface reactions. The recombination of charges happens during this; surface atomic, crystal structure, particle size, and crystallinity affect the separation efficiency. At the end, target molecules adsorb on the material's surface and undergo charge injection and desorption to make final products. [59,[67][68][69][70] Now, many semiconductors exist for photocatalysis with tunable electronic and crystal structures. Although remarkable achievements are done to optimize the photocatalysis process, many photocatalysts show relatively low performance, which depends on the rational design of such materials. 2D materials have awakened a new aspect of this field because of the appropriate band structure. The 2D configuration can harvest more ultraviolet-visible radia-tions and have a large specific surface area. However, the absorption of photons is very limited in bulk materials due to the reflection and transmission of grain boundaries. Additionally, as atomic thickness decreases the migration distance, charge carriers quickly move to the surface area in 2D materials. It reduces the recombination possibility and enhances photocatalysis. Lastly, unique 2D structures with a high surface-atom ratio render many active sites for accelerating the reaction processes. Also, atomic escape energies become relatively small due to the decrease in thickness. Surface defects play a role in target molecule adsorption, building strong interplay, super activation process, and charge transfer. These features help photocatalysts display various features, and several scientific reports have been done. [71][72][73][74] It is urgent and desirable to present an inclusive review on this field to encourage further developments in this niche.

Computational Screening of 2D Materials for Photocatalysis
Thermodynamic Stability: Thermodynamic stability is an essential property of materials, and during the search for an effective photocatalytic material, this property should be checked first. In 2D materials, when two layers are brought towards each other, the energy of dispersion interaction continuously decreases, which shows that 2D materials are metastable and have no actual thermodynamic ground state. Therefore, the lack of 3D competing phases makes the dispersion energy stabilize bulk layered material instead of individual layers. However, 2D materials can make themselves kinetically stable, i.e., in the presence of different absorb rates, solvents or substrates like GaN, 2D silica AIN (aluminum nitride), materials like MoS 2 , [75] free-standing graphene [76] or BN can be stabilized. [77,78] The difference in the free energy of bulk materials and 2D materials having the same composition gives the value of thermal stability, given below (Equation 1).
2D materials must be stably suspended nanosheets in an aqueous solution. In Figure 1, blue bars illustrate the energy of formation for some experimentally synthesized, and red bars present the theoretically predicted 2D materials. Here, N 3D and N 2D are the numbers of atoms in corresponding unit cells, and E 3D and E 2D are the energies of bulk and single-layered materials. With weak van-der Waals (vdW) layered bulk species, 2D materials have low formation energies and therefore are extracted as freestanding single-layer flakes. [79,80] Several examples include graphene, [81] MoS 2 , [82] GaSe, [83] SnS 2 , [84] BN, [85] MoSe 2 , [85] WSe 2 , [85,86] SnSe, [84] and NbTe 2 . [85] On the other hand, 2D materials having formation energy of more than 200 meV/atom, like 2D group III-V elements, [87] 2D oxides, [88] and silicone, [80] are not primarily synthesized without a stable substrate. [89] Moreover, it is unlikely to achieve suspended flakes (freestanding) of 2D materials with a higher energy of formation. [90,91] It is depicted in Figure 1 that only some materials, such as ZnO, [92] AIN, [78] and silicone, [93] with higher formation energy, have been prepared on the surface of the substrate and not extracted in freestanding state.
Optical Absorption: The ability of a particular material to capture sunlight plays a vital role in splitting water molecules. Capturing the visible part of the light spectrum is preferable because it contains about 40% of the energy compared to the ultraviolent part, which only captures 5%. Optical absorption is obtained from the imaginary part for the dielectric function expression using GW or DFT approximations. Standard DFT wave functions and GW calculations are used to provide input of quasiparticle energies to BSE. [106,107] 2D materials show better efficiency for photocatalytic water splitting because of their significant absorption. BSE describes the optical spectrum and excitonic energies of 2D materials, such as the spectrum of MoS 2 , [102,108] and SnS 2 depicts the domination of excitonic states. [109] The imaginary part for permittivity of SnS 2 was determined from BSE and random phase approximation. This phase could not describe the electron and hole pairs; BSE spectrum shows three peaks below 3.2 eV (low energy range). The first peak (at 2.75 eV) is ascribed to SnS 2 optical bandgap, which is in agreement with the experimental value of 2.55 eV determined by UV spectroscopy and second peak corresponds to a different exciton and the third one (at 3.16 eV) is due to direct quasiparticle bandgap determined by GW method. The exciton binding energy differs between the third and the first peak for SnS 2 . The BSE value (0.41 eV) is close to bulk SnS 2 (0.4 eV), which is comparable to the exciton binding energy WS 2 (0.6 eV) [102] and MoS 2 (0.96 eV). [103]

Classification of 2D Photocatalysts
To date, a significant amount of 2D nanosheets have been synthesized by various chemical and/or physical methods, which are mainly divided into two types, layered and non-layer structural materials. Concerning layered materials, the in-plane layer is formed by connecting the in-plane atoms by strong chemical bonding. However, the weak vdW interaction is essential in stacking these monolayers to form bulk crystals. [110,111] The Figure 1. Formation energies of some experimentally synthesized (blue bars) and theoretically predicted but experimentally unsynthesized 2D materials (red bars). The materials in the yellow-shaded region, with ΔE f > 0.2 eV per atom, have only been extracted as single-layer or few-layer nanosheets on suitable substrates and are yet to be obtained in a free-standing or suspended form. Reproduced with permission. [94] Copyright 2015, American Chemical Society.

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representative layered material is graphite crystal, stacked by many graphene layers via weak vdW force. In addition, nitrides (such as g-C 3 N 4 , h-BN, GaN, and Ca 2 N), black phosphorus (BP), Xenes, transition metal dichalcogenides (TMDs), transition metal oxides (MOs) are also layered materials, as shown in Figure 2. [110] In recent years, material and engineering technology advances have enabled tremendous progress in catalysis, sustainable energy generation, and photocatalytic degradation. Novel spectroscopy and nanofabrication techniques provided the properties of primary materials and emerged their functionalities through configuration and structure engineering. It enables significant progress in multicomponent industrial catalysts, [112] superior chemical processing, electrocatalysis, [113] and photocatalysis. [114] Today's environmental growth and technology will require a diverse collection of materials, some of which are rare and unequally distributed on the planet, indicating economic feasibility and a possible challenge to their supply. Ironically, sustainability and the dangers involved with materials are often overlooked in academic research. Nevertheless, a fundamental point of view is the material's durability for the ultimate goal of green energy synthesis. Indeed, given the near-limitless supply of renewable energy (wind and solar energy), the materials used to convert it to actual energy (electrical) are obtained infrequently. Remarkably, the precious metals group, gallium, rare earth elements, aluminum cobalt, and a wide range of other elements [115] are crucial components of industrial catalysts that are widely used. If reliance on those materials can be reduced by substitution in future, they can be recycled more efficiently, avoiding monetary disruptions and accumulating reserves competitiveness. [116] The following properties should be present in these materials: Strengthening the catalyst's durability via material design (confinement and post modifications), minimizing the loading of noble metals while maintaining superior activity by maximizing available active surface area, replacing primary materials for more affordable and readily available alternatives (base metals and carbocatalysis), improvements in the synthesis and elimination of catalysts (green chemistry based catalyst recycle) and toxicolo gical and environmental impact assessment of catalyst products.
Another critical characteristic is the catalyst's nanostructure, which is used to analyze and compare various catalysts to determine their activity propensity. In general, the electronic structure and interfacial degree (smaller-sized materials have more atoms at the support tip) of photocatalysts are highly affected by their size (<10 nm). As a result, controlling the size and structure of photocatalytic semiconductor materials can result in unexpected and significant changes in their properties. [117] As a suitable candidate for photocatalysis, the most extensively studied 2D-layered materials-based photocatalysts include MOs, bismuth-based materials, metal hydroxides, metal composite oxides, metal chalcogenides, and metal-free photocatalytic materials. Therefore, techniques for the large-scale synthesis of 2D materials will be required for industrialization. Moreover, since specific applications of these materials are strongly dependent on characteristics such as their morphology and quality, mass-production techniques should also be developed to accommodate such requirements (Figure 3). [118] Fabricating 2D materials with controllable edge morphology, several layers, and a degree of crystallinity are critical for their use in highactivity catalytic applications. As a result, this section provides a concise overview of the fundamental properties of these 2D materials used in photocatalysis and brief descriptions of the synthesis strategies for 2D materials. [119]

Carbon-Based Photocatalysts
Typical carbon nanostructures are 0D (fullerenes and carbon quantum dots abbreviated as QDs), 1D (nanotubes of carbon), and 2D (graphene and its derivatives). The 2D materials can be thought of as derivatives of graphene sheets that are one atom thick. Since we are only interested in 2D materials, other structures (3D, 1D, and 0D) are beyond the reach of this feasibility study. The crystalline structures of graphene were revealed after the advent of X-ray diffraction methods in the 1930s. Following  [110] Copyright 2020, MDPI.
www.advmatinterfaces.de that, in 1947, the theoretical analysis proposed that isolated layers of graphene would exhibit distinct electrical features. Currently, these forecasts are still confirmed to be accurate. Mechanical exfoliation of graphite results in the formation of single graphene layers. Initially, this was shown by the micromechanical cleavage of graphite by an adhesive tape, which is well known as the "scotch tape method" recorded by Novoselov and Geim in 2004 that later earned them the Noble Prize in Physics in 2010. [120] Chemical vapor deposition techniques have been designed and are currently used to mass-produce graphene with minimal defect count. [121] Chemical exfoliation of bulk graphite can yield graphene layers. These methods are economical since they start with low-cost raw materials that easily mix with different chemical treatments to produce a variety of graphene and derivative materials, including graphene oxide (GO) and reduced graphene oxide (RGO). [122] Their surface chemistry and structural properties make them an ideal platform for stabilizing photocatalytic processes. For example, Ton et al. fabricated visible-light active TiO 2 /graphene nanocomposites with a significantly narrowed energy gap that demonstrated superior photocatalytic potential against various dyes. [123] Chen and co-workers produced visible light-active TiO 2 /GO composites having an energy gap of less than 2.43 eV for methyl orange degradation. [124] The improved photocatalytic activity of TiO 2 /graphene photocatalyst has been inferred to be due to close coupling between TiO 2 and graphene or GO that enables charge transfer across the interface and prevents excitons from recombining. Also, GO-formed p-or n-type junction in the prepared composite serves as a sensitizer, improving the light-driven activity of nanocomposites in the visible range.
Graphitic carbon nitride (g-C 3 N 4 ) is another carbon-based 2D material with remarkable physical, chemical, and electronic properties that are being widely used as a metal-free photocatalyst to degrade several dyes [125] such as Rhodamine B (RhB), [126] methyl orange, [127] methylene blue (MB), [128] and so on. [129] However, bare g-C 3 N 4 has poor dislocation conductivity, a small actual surface region, and a large recombination rate, which contribute to low photocatalytic efficiency. [130,131] Furthermore, g-C 3 N 4 in bulk has a layered 2D structure and an acceptable bandgap (≈2.7 eV) to absorb visible light effectively. The g-C 3 N 4 is acquired from delamination of its bulk form, typically formed by polycondensation or bulk reaction of N 2 -rich precursors. Ailan et al. investigated a straightforward top-down approach, adopting the oxidation etching approach to form g-C 3 N 4 nanosheets from bulk g-C 3 N 4 in air at elevated temperatures. [132] The g-C 3 N 4 nanosheets obtained with specific surface area of 306 m 2 g −1 were approximately 2 nm thin, more significant than the thickness of the bulk process. The quantum confinement effect enhances the ability of excitons to pass in the plane direction and increases their lifespan. Consequently, the photocatalytic efficiency of prepared nanosheets for H 2 processing was significantly improved.
Liu et al. demonstrated that N-deficient carbon nitride could be formed post-treatment via molten salt. In this study, bulk carbon nitride was ground with KCl and LiCl in an agate mortar. [133] Next, it was heated in air at 550 °C and naturally cooled to ambient temperature to synthesize N-deficient carbon nitride. [133] Besides, N-deficient carbon nitride can achieve synthesis based on the thermal treatment with a suitable gas etching agent for assisting in the selective removal of set atoms. [134][135][136] As opposed to direct clear thermal treating process, the gas etching approaches defect type in a controlled manner. Specifically, Li et al. synthesized carbon-abundant carbon nitride nanosheet with nitrogen vacancies via thermally treating carbon nitride in Figure 3. 2D films and heterostructures require high crystal quality and homogeneous thickness for applications for various applications. Reproduced with permission. [118] Copyright 2022, Nature Publishing Group. www.advmatinterfaces.de an N 2 atmosphere (Figure 4a). [136] Che et al. initially reported ultrathin carbon nitride synthesized based on the hydrogen bond intercalating influence exerted by NO 3 − . [137] Notably, the nitrogen vacancies strength exhibited by the ultrathin carbon nitride can receive the regulation from NO 3 − concentration at layer of insertion. Figure 4b presents the probable forming process regarding nitrogen defects. To investigate the ultrafast deflagration performance to the formation of defect-modified g-C 3 N 4 , flame images were captured by high-speed camera, as shown in Figure 4c, the whole process only lasts for 5 s. [138] Shen et al. [139] performed synergy of dopants and defects in graphitic carbon nitride with exceptionally modulated band structures for efficient photocatalytic oxygen evolution. In this study, boron dopants and nitrogen defects were simultaneously introduced into g-C 3 N 4 via simply calcining the mixture of g-C 3 N 4 and sodium borohydride (NaBH 4 ) in a nitrogen atmosphere at different temperatures (see Figure 4d). The obtained borondoped and N-deficient g-C 3 N 4 was denoted as BH x (x = 300, 350, 400, 450, and 500), where x represents the calcination temperature. As depicted in Figure 4d, during the calcination process, active hydrogen and boron released from NaBH 4 would react with the carbon and nitrogen atoms in the framework of g-C 3 N 4 and produce ammonia and alkanes gases, then BH x was finally obtained, in which amino (NH 2 ) was decomposed and cyano (NC) was introduced by breaking CNC bonds, along with the doping of boron atoms at carbon sites.
In another work Wang et al. [140] prepared a series of g-C 3 N 4 microtubes with tunable N-vacancy concentrations and porous wall structures were synthesized by an in situ soft-chemical method (Figure 4e). The novel synthesis involved calcining The process for the preparation of carbon-rich g-C 3 N 4 nanosheets through successively thermally treating carbon nitride in different atmosphere. Reproduced with permission. [136] Copyright 2017, John Wiley & Sons, Ltd. b) The stripping process and defect generation mechanism of ultrathin carbon nitride nanosheets, Reproduced with permission. [137] Copyright 2019, Elsevier B.V. c) Preparation process of defect-modified carbon nitride samples: calcination for the mixture of dicyandiamide (DICY) and NaN 3 . Reproduced with permission. [138] Copyright 2019, Elsevier B.V. d) Schematic illustration of the preparation process of BH x (boron-doped and nitrogen-deficient g-C 3 N 4 , top left) and the proposed structural changes in the heptazine units of carbon nitride as induced by NaBH 4 thermal treatment (bottom left). Reproduced with permission. [139] Copyright 2019, John Wiley & Sons, Ltd. e) Schematic representation of the morphological evolution of porous carbon nitride microtubules modified by nitrogen vacancies. Reproduced with permission. [140] Copyright 2019, American Chemical Society. f) Schematic illustration for the synthetic process of nanocage-like carbon nitride. Reproduced with permission. [141] Copyright 2019, The Royal Society of Chemistry.

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N-deficient rod like precursors, which were synthesized by the self-conversion of monomeric melamine for different hydrothermal treatment times. The morphological evolution of the porous tubular architecture is discussed in detail, aided by timeresolved hydrothermal experiments. The prepared porous g-C 3 N 4 microtubes exhibited clearly improved photocatalytic activity for nitric oxides degradation compared to bulk g-C 3 N 4 . The effects of N-vacancies on O 2 and nitric oxide adsorption activation, electron capture, and electronic structure and the effect of the tubular structure on oriented electron transfer were systematically investigated through experimental and computational studies, which led to the proposal of a mechanism for the enhanced nitric oxide removal activity. [140] Template-free synthesis of nanocage-like g-C 3 N 4 with high surface area and nitrogen defects for enhanced photocatalytic H 2 activity as shown in Figure 4f. [141] Recently, new exfoliation techniques (liquid phase) to synthesize g-C 3 N 4 nanostructures from their bulk counterparts have also been designed. For instance, Yang's group synthesized freestanding g-C 3 N 4 nanostructured materials by exfoliating g-C 3 N 4 powder in isopropanol; these nanostructures demonstrated high visible light-derived photocatalytic effectiveness for H 2 production. The H 2 evolution performance of exfoliated nanostructures was significantly greater than that of their nonexfoliated counterparts by a factor of >17 and significantly more significant relative to previous g-C 3 N 4 nanostructures by a factor of >8. [142] Besides its unique energy band structure, g-C 3 N 4 is responsible for photocatalysis applications like H 2 generation, CO 2 reduction, water purification, and decontamination.
Recently, g-C 3 N 4 -based heterogeneous catalysts are also being investigated as a tool for improving photocatalytic properties. [143,144] Hou et al. synthesized g-C 3 N 4 /N 2 -doped graphene/ MoS 2 ternary heterojunctions, demonstrating excellent photocatalytic efficiency under visible light to remove Cr(VI) in water. [145] It may be due to augmented light absorption, charge separation at the interface, or efficient charge transfer. Han et al. successfully inserted Co 3 O 4 into g-C 3 N 4 to entrap the photo genic holes in the product, resulting in efficient methylene orange degradation. [146] Some studies have demonstrated the high photocatalytic behavior of silver or gold nanoparticles decorated g-C 3 N 4 for the decomposition of methylene orange. It was due to the surface plasma resonance phenomenon and synergistic influence of the catalyst's electron sink action with gold or silver nanoparticles. [147,148] Numerous pollutants have been degraded using g-C 3 N 4 /carbon composites. Typically, g-C 3 N 4 /GO is used to degrade RhB and 2,4-dichlorophenol, [149] g-C 3 N 4 /CNT is used to degrade MB, [150] and g-C 3 N 4 /graphene is used to degrade RhB. [151] It is primarily due to the following factors: first, carbon materials may act as charge transfer and acceptor medium, significantly increasing the exciton separation ability. Second, carbon products can act as cocatalysts, increasing the available photocatalytic active sites. Thirdly, black carbon is more active at absorbing light. However, the loading amount must be carefully monitored since an abundance of black carbon may negatively impact light shielding. In recent years, g-C 3 N 4 photocatalysts with Z-scheme heterojunctions have attracted much because of their unique charge transfer procedures to degrade organic contaminants that can significantly improve photocatalytic performance. [152] The benefits of Z-scheme photocatalysts include cost savings, increased charge separation ability, elimination of light shielding, and optimization of redox potential. [153] Lu et al. identified a g-C 3 N 4 / Ag/MoS 2 ternary composite that degraded more efficiently than either MoS 2 or g-C 3 N 4 alone. [154]

Metal Composites
Metal composite oxides exhibited photocatalytic advantages, mainly formed with ultrathin thickness. [155] HNbWO 6 nanosheets have been extracted via the dispersion of HNbWO 6 •1•5H 2 O in an aqueous medium of tri ethanolamine, steady with acid/base effect and exfoliation supported by ion intercalation procedure. Atomic force microscopy (AFM) findings indicated that the prepared samples' thickness was between 1.8 and 2.0 nm, which is consistent with the single-layer reputation. Synthesized nanosheets exhibited a high capacity for photocatalytic H 2 production (158.9 µmol h −1 ). Additionally, metal composite oxides were synthesized through ion exchange reactions using ultrathin precursors. SnNb 2 O 6 nanosheets, for example, are synthesized using K 4 Nb 6 O 17 nanosheets and SnCl 2 as precursors. [156] It was retained in SnNb 2 O 6 with a 3 nm thickness through K 4 Nb 6 O 17 ultrathin thickness, as verified by AFM analysis. In contrast to bulk SnNb 2 O 6 , its nanosheets had a higher bandgap and a lower CB (conduction band) potential, indicating a favorable reduction capacity for photocatalytic H 2 evolution. Additionally, the charge transfer efficacy of SnNb 2 O 6 nanosheets was improved due to their ultrathin thickness. Additional analysis revealed an exceptional H 2 evolution performance over SnNb 2 O 6 nanosheets under visible light, which was approximately 14 times greater than the bulk structures.

2D Metal Oxides
Numerous MOs have large bandgaps, which provide attractive energy levels for redox reactions but frequently suffer from poor electron conductivity and reduced photocatalytic frequencies. Due to difficulty of accelerating the transfer of photogenic electrons in pure MOs, reducing the migration direction will be a more efficient way to boost photocatalysis. To do this, we can shrink the third dimension while extending the scale of the remaining two dimensions, resulting in a thin assembly with a large surface fraction. This 2D structure minimizes the distance between bulk and surface active sites for electron migration and maintains a high specific surface area. Certainly, fabricating MOs based on 2D materials is a cost-effective method for optimizing surface area and charge transfer, thus achieving a proficient photocatalytic efficiency. [42] Because the bulk of MO lacks layered architectures, only a few 2D MOs were first recognized for photocatalysis. Moreover, with new synthetic approaches and procedures, 2D nanosheets like TiO 2 , Fe 2 O 3 , Cu 2 O, ZnO, WO 3 , SnO, In 2 O 3 , CeO 2 , and HNb 3 O 8 were developed and used as photocatalyst. [157,158] Because of the simple nonlayered structure, several 2D MOs are challenging to shape using the straightforward ultrasonic exfoliation technique. As a result, several other techniques were used to monitor the shape of such materials. For instance, a lamellar organic/inorganic hybrid policy has been suggested to fabricate TiO 2 nanosheets. [157] The solvothermal method was used to produce lamellar TiO 2 octylamine hybrid precursors www.advmatinterfaces.de using Ti isopropoxide (Ti source), octylamine (capping reagent), and 2-phenyl ethanol (solvent) (Figure 5a-c). [42,159]  layers with acids, and stabilized layers were attained using a surfactant (tetrabutylammonium hydroxide). Attributed to the quantum confinement effect, the exfoliated WO 3 nanosheets had a higher bandgap than bulk WO 3 . Besides an exfoliation process, wet chemical techniques were used to achieve the direct preparation production of MOs nanosheets. Several MOs have been developed using a selfassembly technique involving ethylene glycol (co-surfactant) and polyethylene oxidepolypropylene oxide-polyethylene. [42] These MOs include TiO 2 , Fe 3 O 4 , Co 3 O 4 , ZnO, MnO 2 , and WO 3 . Furthermore, perovskite oxides have recently experienced a renaissance because of their improved efficiency in photocatalysis and solar cell applications. Numerous stratiform perovskites can be easily exfoliated to produce their layered perovskite nanosheets.
Various MOs, such as SnO 2 , WO 3 , TiO 2 , Fe 2 O 3 , and ZnO, have been broadly examined as photocatalysts over the last four decades. [160,161] TiO 2 was the most studied because of its high stability, adequate electronic structure, biocompatibility, and superior light absorption properties. The 2D nanostructures of Figure 5. Flakes of ultrathin TiO 2 : a) Synthesis scheme, Reproduced with permission. [157] Copyright 2016, Elsevier B.V. b) AFM image, c) AFM images of height profiles in (b) Reproduced with permission. [42] Copyright 2018 John Wiley & Sons, Ltd. d) Exfoliation of titanate crystals of the lepidocrocite type into TiO 2 nanosheets is depicted schematically. Reproduced with permission. [162] Copyright 2014, American Chemical Society.

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TiO 2 nanosheets obtained by exfoliating layered titanate have attracted interest for their potential use as photocatalysts. [162] These nanosheets exhibit semiconductor disposition just like their bulk counterparts and contain anatase and rutile phases but with somewhat higher bandgap due to size quantization. Ti 0.91 O 20.36 nanosheets, for example, demonstrated an energy gap value of 3.8 eV, which was more significant when compared to anatase titania (3.2 eV). [163] Top-down multistep access was well known during the exfoliation and intercalation of MOs to form nanosheets. [164] For instance, layered titanates were initially formed in the case of TiO 2 nanosheets through a traditional solid-state reaction between an alkali metal carbonates solution and TiO 2 at high temperature ( Figure 5d). Numerous layered MOs, including titanoniobate (Ti 5 NbO 14 , Ti 2 NbO 7 , and TiNbO 5 ), perovskite oxides (KLnNb 2 O 7 and RbLnTa 2 O 7 ), HCa 2-x Sr x Nb 3 , HNb 3 O 8 , and WO 3 were fabricated via similar solid-state reactions and wet-chemical exfoliation approaches. [155,165,166] Zhou and co-workers recently developed freestanding single-layer Bi 2 WO 6 nanosheets from cetyltrimethylammonium bromide (CTAB) using a wet chemical method. Several active sites were generated on the surface of singlelayer nanosheets since Bi atoms were not saturated on single layer, which directly produced h + s upon light irradiation. Quick exciton separation at a highly photoactive surface revealed that single-layer Bi 2 WO 6 has outstanding photocatalytic efficiency for photodegradation of RhB. [167] Titanoniobate nanosheets have improved photocatalytic efficiency in eliminating organic pollutants. [168] Using a simple wet chemical technique, Tae and co-workers recently investigated the development of multiple titanate nanosheets having diamond-like shapes with a typical lateral size of 30 nm. [169]

Metal Chalcogenides
Transition metal chalcogenides (TMCs) are materials with the chemical symbol M n X m , (M stands for a transition metal while X stands for a chalcogenide). These materials have been identified with group IV (Zr, Hf, and Ti), group VI (W and Mo), and group X (Pt and Pd), including Nb, Re, and Ta. [170] Layered TMCs constitute a structure of 2D layers lying on top of one another and are often presented stoichiometrically as MX 2 (Figure 6c). In a typical arrangement, each layer with a thickness of three atoms comprises a central metal atom; in contrast, the chalcogenide atoms are tightly bound above and below. Furthermore, these metal/chalcogen atoms are covalently bonded with a strong interaction while mild London dispersion forces keep the layers together. Consequently, the weakly bonded layers can easily be separated (exfoliated) in TMCs that initially contributed to their applications as high-efficiency lubricants.
TMCs have recently gained popularity as a graphene substitute due to their special physicochemical features that other 2D materials lack or cannot achieve. For instance, complex engineering of energy gaps is needed to allow graphene to be used as a transistor. Additionally, TMCs exhibit a broad range of electronic properties, including insulators, semiconductors, metals, and semimetals, with a high range of charge mobilities and direct/indirect bandgap based on their configuration. As a result, TMCs exhibit a host of highly favorable properties relating to charge transfer, magnetism, ion, small molecule intercalation, catalytic and optical properties. TMCs nanosheets perform a variety of roles in photoelectrocatalytic and photocatalysis applications. Mainly, they serve as photosensitizers by enhancing their ability to harvest photons in the visible zone of the electromagnetic spectrum as charge carriers and separators due to their adequate bandgap. Due to their unique electronic structure, metal chalcogenides generally exhibited a relatively large light absorption field, indicating they are potential candidates for various photocatalytic applications.
Several 2D TMCs have been developed recently, including SnS 2 , SnS, MoS 2 , CdS, SnSe, ZnIn 2 S 4 , In 2 S 3, and ZnSe. [171] Due to the critical layer structure of these 2D TMCs, synthetic techniques usually focus on exfoliation. For instance, bulk SnS 2 in formamide refluxing is required to break the weak vdWs interactions among the 2D layers to obtain hexagonal SnS 2 singlelayers. [172] Due to their favorable electronic properties, various TMCs nanosheets, such as MoSe 2 , MoS 2 , WSe 2 , WS 2 , TiS 2 , and SnS 2, have recently emerged as potential 2D materials for photocatalysis applications. [173][174][175][176] An illustration of 2D MoS 2 with 2H and 1T phases is given in Figure 6 possesses numerous active sites and a large surface area, allows for extensive interaction and effective reactions. As a result, MoS 2 -based heterostructures and composites have been demonstrated as promising materials, opening up new avenues for photocatalysis applications. The direct removal of organic pollutants and elimination of heavy metals have been thoroughly studied using photocatalytic activity based on MoS 2 materials. [177,178] Zhou et al. used primitive hydrothermal methods to fabricate a TiO 2 -MoS 2 monolayer hybrid photocatalytic activity with a 3D-layered structure. [179] The 3D-layered structure comprises a TiO 2 nanobelt core and a MoS 2 nanosheet shell (referred to as TiO 2 @MoS 2 ). It has a greater potential for adsorption and superior photocatalytic ability to degrade organic dyes. The alignment of energy levels between MoS 2 and TiO 2 is ideal for efficiently prohibiting  [183] Copyright 2015, The Royal Society of Chemistry. b) Crystal structures of MXenes in different layer arrangements by a chemical formula of M n+1 X n , where n is 1, 2, or 3 (M 2 X, M 3 X 2 , or M 4 X 3 ), "M" represents the early transition metals (Ti, V, Cr, and Nb, etc.), and "X" is carbon and/ or nitrogen. Reproduced with permission. [184] Copyright 2019, American Chemical Society.
www.advmatinterfaces.de photogenic exciton recombination and better charge transfer. Cheon et al. synthesized disk-shaped ZrS 2 nanosheets with lateral dimensions of 20 to 60 nm through a reaction between CS 2 and ZrCl 4 in oleylamine. [180] Soon, this process was generalized to include different transition metal selenide and sulfide nanosheets. In another study, MoS 2 nanosheets were synthesized using a solvothermal approach using the precursors' thiourea and (NH 4 ) 6 Mo 7 O 24 •4H 2 O. [181] Wang et al. showed that the ultraviolet light irradiation of Bi 2 O 2 CO 3 /MoS 2 composites significantly photodegraded RhB. This influence was ascribed to the synergy between MoS 2 and Bi 2 O 2 CO 3 . [182] Zhang et al. showed that vertically aligned MoS 2 layers exhibit high photocatalytic activity and can effectively degrade organic compounds in contaminated water. [3] One more TMCs material, WS 2 , had already shown a rare and valuable photocatalyst that operated across the entire solar spectrum. Sang et al. announced the discovery of WS 2 nanosheets with enhanced photocatalytic performance for UV to NIR (near-infrared) regions to degrade organic pollutants. [175] The methylene orange solution was nearly fully degraded in 100 min when exposed to UV light, approximately 90% of methylene orange was degraded in 300 min when exposed to visible irradiation, and approximately 80% degradation of methylene orange was recorded in 300 min when exposed to NIR light. Additionally, a solution of RhB dye was degraded using a WS 2 photocatalyst under NIR light. After 5 h of exposure, 60% degradation of RhB was evaluated, showing that WS 2 is an active photocatalyst in the NIR region. Several top-down approaches to formulating few-layers or single-layer metal chalcogenide nanosheets have been identified, including liquid-phase ultrasonic exfoliation, mechanical exfoliation, and lithium intercalation exfoliation. [170] Additionally, bottomup approaches like chemical processing and chemical vapor deposition offered potentially effective solutions for fabricating metal chalcogenide nanosheets, such as exfoliation techniques.

2D MXenes
Recent years have seen a boom in studies involving 2D MXenes in photocatalysis and the discovery of novel MXenes materials. While MXenes are incompetent for photocatalysis, they have potential to play a significant role in cocatalysts owing to the metallic conductivity, layered architecture, abundance of hydrophilic active sites, and stable functionalized capacity. [185] Yury's group first defined MXene as a novel 2D family member in 2011. It comprises early transition metal nitrides and carbides, while M in the formula denotes a transition metal, including Nb, Ta, or Ti, and X denotes carbon and N 2 . In a typical synthesis process, elective etching of group IIIA/IVA elemental layer (A layer) from the bulk M n+1 AX n is carried out to obtain MXenes. [186] This novel synthetic method provides MXene with abundant fluorine and hydroxyl functional groups on the soil, which is advantageous for further surface modification. In 2011, Naguib and co-workers treated Ti 3 AlC 2 for 2 h at room temperature in hydrofluoric acid; as a result, the exfoliated 2D Ti 3 C 2 layers were initially obtained through the selective etching of Al atoms from a ternary precursor of carbide Ti 3 AlC 2 . Because of exceptional electronic properties, large specific surface area, and hydrophilicity, this 2D material has been used in a wide variety of applications since then. Over the last decade, 20 different MXene materials have been synthesized, namely Ti 3 C 2 , TiNbC, Ti 2 C, V 2 C, Nb 4 C 3 , and Mo 2 C. [187][188][189][190] MXenes have also drawn the interest of several researchers for photocatalytic CO 2 reduction, H 2 evolution, removal of contaminants, and N 2 fixation. MXenes are used as a platform for one or more catalyst materials to increase photocatalysis. A Schottky barrier is formed among the semiconductor catalyst and MXene, which prevents photoinduced charge carriers from recombining. Additionally, the high basic surface area of MXenes produces an abundance of photocatalytic active sites. [191,192] Additionally, MXenes exhibit remarkable properties such as high electrical conductivity, chemical stability, environmental friendliness, hydrophilicity, and many surface functional sites that allow them to be used as extremely powerful adsorbents for a wide range of molecules and ions. Due to their increased electroconductivity, MXenes can significantly minimize hole and electron recombination. It is advantageous for the purification of toxic pollutants. When two cationic dyes (acid blue 80 and MB) were combined with Ti 3 C 2 T x under UV irradiation for 5 h, degradation up to 62% and 81% was achieved, respectively. [192] It is hypothesized that the development of TiO 2 and/or TiH 4 O 4 on surface of Ti 3 C 2 T x , when exposed to ultraviolet light, enhances photocatalytic performance. [193,194] Wang et al. [195] designed an aqueous droplet light heating system along with a thorough mathematical procedure, which combined leads to a precise determination of internal light-to-heat conversion efficiency of various nanomaterials (Figure 7a). To precisely evaluate the light-to-heat conversion efficiency of Ti 3 C 2 MXene, droplet-based light absorption and heat measurement system was carefully established based on literature with certain modification and the system setup is schematically presented in Figure 7b. The internal light-to-heat conversion efficiency of MXene, more specifically Ti 3 C 2 , was measured to be 100%, indicating a perfect energy conversion. Furthermore, a self-floating MXene thin membrane was prepared by simple vacuum filtration. In the presence of a rationally chosen heat barrier, it produced a light-to-water-evaporation efficiency of 84% under one sun irradiation, which is among the state of art energy efficiency for similar photothermal evaporation system. The outstanding internal light-to-heat conversion efficiency and great light-to-water evaporation efficiency reported in this work suggest that MXene is an auspicious light-to-heat conversion material and thus deserves more research attention toward practical applications. The laser light is partially adsorbed by MXene existing in the optical path of the laser beam inside the droplet (Figure 7c), which is a circular column with length of 2.6 mm and diameter of 0.85 mm. [195] Recently Que et al. [196] fabricated a hydrophobic surfaceenabled salt-blocking d-Ti 3 C 2 MXene membrane, which contains a salt-blocking d-Ti 3 C 2 nanosheet layer modified by trimethoxy(1H,1H,2H,2H-perfluorodecyl)silane (PFDTMS) for sunlight harvesting and a piece of commercial filter membrane for water supply, was fabricated for efficient and long-term stable solar desalination (Figure 7d). The whole solar steam generation device consists of three components: a hydrophobic d-Ti 3 C 2 membrane on a filter membrane as a solar absorber, vapor evaporator, and salt blocker, a piece of commercial polystyrene foam as a thermal insulator and floater, and nonwoven fabric as the water path which pumps water by the capillary www.advmatinterfaces.de effect ( Figure 7e). The schematic illustration of the evaporation process for the hydrophilic and hydrophobic d-Ti 3 C 2 membranes is shown in Figure 7f,g, respectively. Seawater infiltrates the membrane through the capillary effect for the hydrophilic membrane, and the salt continues crystallizing on the membrane surface with water evaporation. The hydrophobic membrane with 10 mg d-Ti 3 C 2 loading mass delivers a water evaporation rate of 1.31 kg m −2 h −1 , which is 3.12 times that of pristine seawater (0.42 kg m −2 h −1 ). Besides, the natural evaporation rate of the device in the dark shows a relatively low value of 0.19 kg m −2 h −1 , which is attributed to the hydrophobic feature of the membrane. Therefore, the corresponding solar steam conversion efficiency is up to 71% under only one sun, as shown in Figure 7h. The solar steam generation performance can be maintained well for at least 10 cycles, with each cycle sustaining for over 1 h. The evaporation rate can be maintained without decay for over 200 h, as shown in Figure 7i. [196] Another research group used a simple hydrothermal oxidation process to fabricate the (001) TiO 2 /Ti 3 C 2 combination. In this method, 2D Ti 3 C 2 T x MXene was used to incorporate TiO 2 , while Ti atoms on Ti 3 C 2 acted as nucleation sites. Thus, the atomic level interfacial heterojunction within TiO 2 and Ti 3 C 2 layers facilitated the reduction of defect-induced excitonic recombination.
The most abundant reactive species across the light-induced oxidation of MO are radical hydroxyl ions (OH). Additionally, the photodegradation of MB has been investigated using TiO 2carbon composites synthesized via ball grinding of 2D Ti 2 CT x . [197] MXene groups have been identified as promising materials to fabricate hybrid carbon/transition oxide. By significantly reducing the recombination of photogenic excitons, carbon nanocomposites will significantly enhance TiO 2 's photo catalytic potential. [198,199] To increase the photocatalytic performance of metal sulfides, a ternary Ti 3 C 2 -OH/ln 2 S 3 /CdS photocatalytic device was formulated through a straightforward hydrothermal synthesis process. [200] The fabricated samples had a spherical shape with a high specific surface area of 4-TIC (Ti 3 C 2 -OH/ln 2 S 3 / CdS with 4% Ti 3 C 2 -OH), which provided extra active sites and facilitated the adsorption of dye molecules. Additionally, the strong relation between the three materials and superior conductivity of Ti 3 C 2 -OH maintained the composites' vigorous degradation performance. Lu et al. defined the disintegration of MXene mixtures to produce TiO 2 -Ti 3 C 2 -CuO composites. [201] Due to their superior electrical features, hydrophilic surface terminations, and large specific surface area, MXenes are widely applied in photocatalytic water splitting. [202] However, since the water redox potentials of the majority of MXenes exceed their Figure 7. a) Schematic illustration of light-to-heat conversion by MXene Ti 3 C 2 . b) Experimental setup for droplet-based light-to-heat conversion experiment. c) Schematic of droplet with laser irradiation. Reproduced with permission. [195] Copyright 2017, American Chemical Society. Schematic illustration of d) the fabrication process of the hydrophobic d-Ti 3 C 2 membrane and e) hydrophobic d-Ti 3 C 2 membrane-based solar desalination device. Schematic illustration of the solar desalination process of f) hydrophilic and g) hydrophobic membranes. h) Solar thermal conversion efficiency, i) long-term realtime seawater weight loss through the hydrophobic d-Ti 3 C 2 membrane evaporation with 10 mg loading under one sun. Reproduced with permission. [196] Copyright 2018, The Royal Society of Chemistry.
www.advmatinterfaces.de band edges, they cannot be explicitly utilized as photocatalysts for water splitting to create H 2 . Zr 2 CO 2 and Hf 2 CO 2 are exceptions since they possess ideal band edge positions and outstanding optical absorption in UV-vis region, enabling them to be utilized for photocatalytic water splitting to create H 2 . Thus, for photocatalytic H 2 generation, MXenes primarily play cocatalysts' role, assisting photocatalysts in increasing photocatalytic performance. In N 2 fixation, Liu et al. developed and produced the first Ti 3 C 2 T x -derived TiO 2 @C/g-C 3 N 4 composites, demonstrating exceptional photocatalysts N 2 reduction performance (250.6 mmol g −1 h −1 ). The heterostructure of type II formed by TiO 2 and g-C 3 N 4 aided in the effective separation and migration of photogenic excitons. Furthermore, Ti 3+ formed during thermal treatment of Ti 3 C 2 T x could serve as photoinduced electron captors and as activation/adsorption sites for N 2 . [203] Effective harvesting of NIR light is yet a significant problem in photocatalysis; however, Ti 3 C 2 T x /TiO 2 catalyst was prepared recently by heating Ti 3 C 2 T x in a muffle furnace which demonstrated impressive photocatalytic behavior when exposed to both full-spectrum and monochromatic 740 nm light. The increased efficiency may be attributed to the NIR absorption through the plasmonic Ti 3 C 2 T x process and activation of N 2 via oxygen vacancies (V O ) formed on TiO 2 . [204]

Metal/Layered Double Hydroxides
Due to their superior physical and chemical properties, metal hydroxides or layered double hydroxides (LDHs)-based materials provide an innovative route for the design, optimization, and mechanistic study of photocatalyzed contaminant degradations. These properties provide a unique layered structure, bandgap engineering, large surface area, better anion exchange strength, and wide-range light absorption. Besides this, their chemical, architectural, and electronic properties can easily be tailored to a particular reaction and working condition. [37] Most research on LDHs-based photocatalysts has been devoted to controlling their morphological properties and preparing novel nanostructured complexes through methods (elemental doping, loading of functional groups). [205,206] Lately, a significant increase has been noted in the use of LDHs for applications such as photocatalytic oxidative removal of organic compounds, CO 2 elimination, water-gas shift reaction, and other significant chemical transformations. [207][208][209] LDHs are anionic clays characterized by their layered structures and a particular formula of [M 2+1−x M 3+x (OH) 2 ] x +(A n− x/n )·yH 2 O, M 3+ represents a trivalent metal ion including Fe 3 , Cr 3+ , or Al 3+ while M 2+ stands for a divalent metal ion like Zn 2+ , Mg 2+ , or Cu 2+ ; each metal cation on the terrace is connected to one or more OH groups, forming the backbone of LDHs. [210] A n− is a charge-balancing anion that is stable only under primary conditions and can take on the chemical identities of organic, inorganic, or complex heteropoly anions. Due to the simplicity of the cation guideline, the desired bandgap was developed in metal hydroxides by incorporating specific photoactive metal cations. Thus, the 2D metal hydroxide structure demonstrated significant potential for photocatalysis. For instance, 2D ZnAl-LDHs were synthesized through a reverse micelle approach to be applied as photocatalysts in CO 2 conversion. [211] Due to the ultrathin thickness of the ZnAl-LDH nanosheets, Vo was developed, resulting in the generation of Zn vacancy complexes. The Zn vacancy complexes can act as electrostatic traps for CO 2 photoreduction. As a result, ZnAl-LDH nanosheets demonstrated ominously enhanced photocatalytic potential for CO 2 reduction compared to bulk ZnAl-LDH. Tian et al. prepared oriented CuCr-LDHs films (16.5 m thick) on a Cu substrate employing electrophoretic deposition. They discovered that these CuCr-LDHs films effectively catalyze the visible light photo catalytic decomposition of 2,4,6-trichlorophenol, RhB, and Congo red. [212] The extraordinary degradation efficiency was due to the high crystallinity, the inclusion of meso-and micro porous nano structures, and the large surface area. Numerous new ultrathin metal hydroxides or LDHs, including ZnTi, and NiTi-LDH, have demonstrated exceptional efficiency in various photo catalytic applications. [213] Recently, LDHs research has expanded beyond the basic dicationic MgAl-LDHs to tricationic or tetracationic multiphased species, including the MgFeTi-LDHs, MgZnAlFe-LDHs, and post-calcination multiphase MOs-LDHs, most of which have shown encouraging photocatalytic properties. [209,214,215] Additionally, by loading, intercalation, doping, and chemical modification, LDHs may shape novel layered or nanocomposite photo catalysts with other materials. Ju et al. calcined ZnAlTi-LDO (ZnAlTi layered double oxide) to 500 °C to obtain ZnAlTi-LDO nano particles, which were then strategically designed to form nanocomplexes with C 60 and AgCl for visible-light photodegradation of bisphenol A at room temperature. [216] The LDO's photocatalytic efficiency was found to be highly susceptible to the pH of the reaction media. They discovered that a mildly acidic environment (pH = 5) promotes photoexcitation of dissolved oxygen by e − and the formation of free radicals such as •O 2and •OH; whereas, a strong pH under simple conditions results in a cathodic displacement of the valence band (VB), reducing photocatalytic efficiency.

Bismuth-Based Materials
Bi-based semiconductor photocatalysts have generated considerable interest owing to their unusual optical, electrical, and structural properties, as well as their exceptional photocatalytic redox behavior and stability. As compared to TiO 2 electronic band configuration, the VB of Bi-based materials is composed of hybridized orbitals of well-dispersed Bi (6s) and O (2p)Bi 6s orbital will both accelerate and narrow the mobilities of photogenic charge carriers. [217] In Bi  Zhang's group developed BiOCl monocrystalline nanosheets with 001 and 010 facets using a mild hydrothermal process involving pH adjustment. [218] BiOCl nanosheets (001) exhibited greater photo-oxidation behavior for methylene orange degradation when illuminated with UV irradiation. At www.advmatinterfaces.de the same time, BiOCl with 010 facets exhibited superior degradation efficiency when illuminated with visible irradiation leading to indirect photosensitization of dye. The Bi 2 WO 6 nanosheets with a single layer are synthesized using the surfactant cetyltrimethoxysilane. CTAB assisted the hydrothermal technique and the single-layer Bi 2 WO 6 was decorated with Br ions provided by CTAB and formed Coulomb repulsions, delaying the Bi 2 WO 6 stacking. [167] Additionally, long chains of CTA + ions with hydrophobic nature on the surface of Bi 2 WO 6 acted as an additional surface repulsion, preventing the growth of crystals along the c-axis. Numerous coordinative unsaturated Bi atoms were revealed and function as active sites at Bi (Figure 8a-d). As a result, Bi 2 WO 6 exhibited greatly improved photocatalytic performance for ozone removal in visible light. Wang's group fabricated BiOCl microflowers consisting of porous nanosheets with approximately 100% revealing [001] facets, demonstrating exceptional photoactivity to degrade various dyes, such as RhB, MB, and methylene orange. [219] At the terminal of [001] facets, there is a high concentration of O atoms, which favors the adsorption of cationic dye and collection of photogenic e − injected from an excited dye. It was established that dye photosensitization of BiOCl is a feasible method for increasing its absorption of visible light. As a result, it is suitable for photocatalytic degradation when www.advmatinterfaces.de exposed to visible irradiation. Additionally, the porous BiOCl (001) is hoped to be used in dye-sensitized solar cells.
Fornasiero and co-workers have investigated a Bi 2 O 3 /Bi 2 O 4-x composite as a possible photocatalyst (Figure 8e-g). As a result, they used UV-vis light in Bi 2 O 3 to induce surface changes, resulting in the formation of a Bi 2 O 3 /Bi 2 O 4-x nanocomposite arrangement. Thus, such surface modifications significantly improve the photocatalytic efficiency of MB. [ BiOCl nanosheets were 0.42 and 0.24 mM h −1 g −1 , respectively, with 3% NiO x filling. The findings suggest that 010-BiOCl nanosheets have a significantly higher charge carrier separation performance and photoabsorption capability due to the facet dependency of homogeneous carbon doping. [221] Composite photocatalyst schemes incorporate a wide range of heterostructures for improving overall photocatalytic performance. This technique enables the tuning of band structures, the promotion of carrier transition, the enhancement of operation and selectivity, and the manipulation of other factors necessary for achieving outstanding photocatalytic efficiency.
Additionally, hybrid systems provide suitable templates for in-depth studies of the critical interfacial effects of catalysis. 2D heterostructures have attracted substantial attention among the numerous composite photocatalysts because of many advantageous characteristics. [222,223] The contact area is expanded by using the wide specific area of 2D heterostructures, enabling intimate interaction and effective transfer of charges at the interface. Owing to this behavior, 2D hybrid systems have enormous potential for achieving superior photocatalytic potentials. [185,224] The heterojunction interfacial effect will significantly increase the lifetime of photogenic excitons in catalysts by effective separation that directly/indirectly leads to redox reactions such as photocatalytic organic degradation and H 2 generation. Numerous attempts were made to engineer 2D components or reinforce the interfacial force to shape capable 2D photocatalysts. [225] The 2D/2D heterojunctions have many catalytic advantages, including increased active sites due to larger specific surface area/interface regions and ultrathin structure. Charges quickly move due to the 2D components' low specific resistance and shorter traveling path in one dimension. Transparency as a result of the ultrathin structure aids in light harvesting.
Graphene, in particular, is an excellent medium to fabricate 2D composite photocatalysts. Besides its high electron conductivity, graphene's ample surface functional groups enable it to integrate easily with 2D semiconductors. The compact interaction enables the swift transfer of excitons from 2D semiconductors to graphene, facilitating the photocatalytic phase. [226,227] She et al. created an effective 2D α-Fe 2 O 3 /g-C 3 N 4 photocatalytic water-splitting catalyst. Due to their strong interface, electrons in CB of α-Fe 2 O 3 can swiftly move to VB of g-C 3 N 4 and recombine to photoinduced holes in VB of g-C 3 N 4 . Consequently, electron excitation in g-C 3 N 4 was most effective, and photoinduced electron recombination was firmly suppressed. [228] Zhu et al. synthesized another 2D/2D composite photocatalyst by utilizing g-C 3 N 4 . In contrast to the 2D α-Fe 2 O 3 /g-C 3 N 4 structure, Type-I heterojunction was fabricated by a hybrid structure. [229] The photoexcited e − in g-C 3 N 4 traveled to CB of BP when exposed to visible light, which combined with the e − produced in BP. The e − was captured collectively in the interfacial PN coordinate bond between g-C 3 N 4 and BP, extending the lifetime of the electrons. Correspondingly, the Janus bilayer junction was fabricated using 1T MoS 2 and single-layer BiOCl, demonstrating enhanced visible-light hydrogen evolution behavior. [230] The key to this Vo-oriented assembly lies in the metallic characteristic of MoS 2 monolayer and the asymmetric structure of 1L-BOC composed of only (Cl 2 ) layers and oxygen-deficient (Bi 12 O 17 ) layers. The enormous difference in electrostatic potential between the (Cl 2 ) and (Bi 12 O 17 ) sides produced an internal electric field vertically aligned from the latter to the former in this one-of-a-kind configuration. Internal electric fields eased the movement of photoinduced charge carriers to the (Bi 12 O 17 ) and (Cl 2 ) faces, respectively. As a result of the transfer of electrons to monolayers of MoS 2 via BiS bonds, light-induced H 2 evolution was achieved.

Engineering Protocols
Defects were shown to be important across photocatalytic processes. The material's intrinsic properties, including carrier concentration, coordination number, electrical conductivity, electronic structure, and microstructure, could be altered by engineering various defects. Apart from doping, defect engineering has always had a significant impact on 2D materials, as demonstrated in the case of photocatalysis. Because of the atomic-scale thin nature of 2D materials, even a very minute content of defects can exert huge effects on basic properties. Due to the lower escape energy of atoms in 2D materials, a wide range of various types of defects can be generated in contrast to the bulk materials. Thus, surface defects such as anion and cation vacancies, pits, associated vacancies, and distortions can be easily created to optimize the electronic properties of 2D materials. Herein, various surface defects in 2D photocatalysts and their beneficial properties are summarized.

Anion Vacancies
Due to the low formation energy of V O , they have been considered more prevalent and extensively studied defects in transition-MOs. [231] Due to the atomic thickness of the V O , the electronic structure and physiochemical properties of 2D materials are being effectively customized, affecting photocatalytic efficiency. [232] Along with altering the carrier concentration and electronic structure, engineered V O can aid in molecule activation, including O 2 , N 2 , and CO 2 , and enhanced photocatalytic efficiency. Zhang and co-workers discovered that the V O in BiOBr contains clustered electrons for back-donation, which can cause changes in the adsorbed N 2 molecule, lengthening the bond between N-atoms from 1.078 to 1.133 for free molecular nitrogen. [233] The N 2 molecule can also be effectively reduced to NH 3 using electrons transmitted through the www.advmatinterfaces.de interface from its excited BiOBr. Comparable to N 2 activation, it has been shown that V O in ZnAl-LDH is conducive to CO 2 activation. [234] An increase in the density of V O was observed upon gradual decrement in the thickness of prepared samples (210 to 2.7 nm) due to the emergence of various unsaturated coordinate Zn ions adjacent to the Vo.
Kong's group adopted the plasma engraving method to produce defects of V O and Ti 3+ on TiO 2 nanosheet's surface. [235] The electronic configuration of TiO 2 nanosheets undergoes considerable variation due to the manufactured defects, with bandgap decreasing (3.13-2.88 eV), along with upshifting of CB and VB edges forming a defective state in the forbidden gap. Due to the formation of this defective state, the H 2 generation activity reached dramatically twofold compared to pure TiO 2 . Besides facilitating the forming of intermediate bands in the bandgap, the V O in WO 3 atomic layers promotes the adsorption and activation of CO 2 into radical COOH• species. [236] The critical function of V O in WO 3 layers enables the creation of more CO and O 2 in the infrared region.
Lei and co-workers achieved totally controlled formation of Vo-rich and Vo-deficient In 2 O 3 nanosheets by rapidly heating In(OH) 3 nanosheets in the presence of oxygen or air. [237] They highlight the first synthesis of ultrathin cubic-In(OH) 3 sheets through a mesoscopic-assembly strategy and hence realize the fabrication of 5 atom thick In 2 O 3 porous sheets with V O via a fast-heating strategy. The number of oleate ions and their peculiar arrangement play a crucial role in forming In(OH) 3 2D sheets (Figure 9a). Initially, three oleate ions interact with one In 3+ ion to form an In-oleate complex via electrostatic interaction. The homogeneously dispersed oleate ions on the surface of In 3+ ions lead the complex to take on a hexagonal mesostructure, in which all the In 3+ ions are uniformly www.advmatinterfaces.de separated by one oleate ion. Meanwhile, the corresponding small-angle XRD pattern in Figure 9b demonstrates the presence of a hexagonal mesophase with a = 27.7 Å, which relatively consists with the length of one oleate ion. Additionally, a strong V O signal at g = 2.004 was detected in ESR (electron spin resonance spectroscopy) spectra, indicating that the Vorich In 2 O 3 contains the largest V O . The Vo innovation significantly altered the electronic structure of In 2 O 3 nanosheets with high Vo material. As shown by DRS (Diffuse reflectance spectroscopy) and XPS examination, Vo-doped In 2 O 3 sample exhibited a narrower energy gap, and an upshift was observed in VB tip. DFT calculations demonstrated that large density of states (DOS) was produced at valence band maxima. A new concentration of defects revealed that Vo-rich In 2 Figure 9e. Evidently, as displayed by the calculated DOS in Figure 9 h,i, the perfect 5 atom thick In 2 O 3 slab shows an increased DOS at the conduction band edge compared with the bulk counterpart, which indicates that more carriers can be effectively transferred to the conduction band minimum of the atomically thin perfect In 2 O 3 sheets. In addition, the calculated results in Figure 9g,h reveal that the presence of V O endows the In 2 O 3 2D structure with increased DOS at valence band maxima, which ensures a higher carrier concentration and hence increases the electric field in the space charge regions, thus achieving enhanced carrier separation.

Cation Vacancies
Cation vacancies impart a similar moderating impact on the electronic, physical and chemical features of metallic compounds, just like anion vacancies, leading to their diverse electronic and orbital distributions. [238] Compared to anion vacancies, metal cation vacancies are more difficult to engineer and control due to their high forming energy, making it much more difficult to decide their work. [231] Scientists have created many photocatalytic materials with cation vacancies and are investigating their effect on efficiency of the photocatalysts. [239][240][241] The 2D atomic layers, both with and without constrained cationic vacancies, can become excellent models for elucidating the structure-activity relationship in considerable detail. Vanadium vacancies (V v ) have been generated in BiVO 4 unit cell nanosheets (1.28 nm) in a variety of amounts ( Figure 10). To detect the types and quantities of defects in the synthetic o-BiVO 4 atomic layers, positron annihilation spectrometry (PAS) was performed and the results were shown in Figure 10c-e. As revealed by the positron lifetime spectra in Figure 10c, the orthorhombic-BiVO 4 (o-BiVO 4 ) atomic layers exhibited three-lifetime components, with the two longer life components (τ 2 , τ 3 ) could be ascribed to the large voids and the interface present in the samples. Jiao et al. synthesized dense ZnIn 2 S 4 layers (unit cell) with extensive or limited Zn vacancies using a simple hydrothermal method and temperature variance. [242] The TEM images revealed a sheet-like morphology, while the AFM parameters indicated that the layers of ZnIn 2 S 4 formed were unit-cell thick along the direction of c-axis.
The Zn vacancies-poor and rich characteristics of the investigated products were evaluated using zeta-potentials, EPR (Electron paramagnetic resonance spectroscopy), and PAS, demonstrating that ZnIn 2 S 4 layers with distinct Zn vacancy concentrations were successfully prepared, providing two ideal models for examining the relation among Zn vacancies and photocatalysts actions. [242] Due to the high Zn vacancies, ZnIn 2 S 4 layers exhibited significantly improved excitons separation performance, confirmed by PL (photoluminescence) analysis, surface photovoltage, and ultrafast transient absorption. Additionally, the plentiful Zn vacancies boosted the light-harvesting from 440 to NIR region, along with superior CO 2 adsorption and hydrophilicity. The distribution of charges in space near the CB's edge has been measured using DFT. The Zn vacancy is observed to increase the charge density of neighboring sulfur atoms, implying that electrons are being excited to CB more quickly. [242] Song et al. discovered that Ti vacancies in single-layer H 1.07 Ti 1.73 O 4 •H 2 O nanosheets would cause the development of numerous radical O species that interact with water molecules through H 2 bonds to form surface coordination. Consequently, a 10.5-fold increase in photocatalytic efficiency for H 2 evolution can be achieved compared to its layered equivalent. [243,244]

Associated Vacancies
Along with monatomic vacancies, associated vacancies have the potential to significantly alter the physical and chemical properties of semiconductors via multiatomic vacancy coupling. A published study revealed that high-energy facet treatment facilitates surface defects forming like associated vacancies. Atoms lost from the surface not only add mono vacancies but thereby associated vacancies as well. Due to the multi-atomic coupling of vacancies, these defects will intensely engineer the electronic properties and result in extraordinary electronic output. For example, by morphologically manipulating Bi 2 WO 6 nanosheets to form peony-like aggregations and nano-bipyramids, the exposed (100) and (113) facets can also be obtained. [246] As verified by XPS, PAS, and theoretical measurements, the "Bi-O" associated vacancies are present in the exposed high-energy (100) facets of Bi 2 WO 6 nano-bipyramids. The formed "Bi-O" associated vacancies lead to bandgap narrowing and increasing the exciton separation efficiency that, in turn, enhances the photocatalytic potential to degrade dyes. Conversely, due to Bi 2 WO 6 complex structures, along with crystal facet, grain boundary, morphology, capping agents, and a direct relationship with vacancy associates and photocatalytic behavior can be difficult to establish. [246] www.advmatinterfaces.de The 2D materials have been an excellent model for investigating simple structure-activity correlations since it allows for the careful introduction of defects on its surface while being consistent with other configurations. For example, dimension engineering was used to produce triple vacancies of V Bi in BiOCl nanosheets. [247] The PAS was used to verify the corresponding triple vacancy V Bi (Figure 11). When the outer Bi atoms surface is revealed in the BiOCl lattice, they can easily break out and form a vacancy. Since the thickness had been reduced to the atomic level, oxygen atoms associated with Bi atoms present inside an internal layer can escape more easily. Thus, controlled defects in BiOCl nanoplates were isolated, while in BiOCl nanosheets, their corresponding vacancy V Bi ′′′V O •• V Bi ′′′ was modified. Different defect forms undoubtedly impact the electronic structure, which ensures improved RhB adsorption due to the additional -ve charge. Due to the change in defect forms, ultrathin structures exhibited upshifting of CB and VB potentials that supported the mobility of charge carriers, allowing enhanced exciton separation. As a result, BiOCl nanosheets demonstrated significant solar photocatalytic behavior for pollutant elimination.

Distortions and Pits
Apart from atom-different vacancy-related defects, many other types of lattice defects, such as lattice dislocations, distortion, and disorders, can also have significant regulation effects on www.advmatinterfaces.de materials' electron configuration and physicochemical properties. [248] As compounds are limited to atomic thickness, a significant amount of breaking of interatomic bonds occurs, resulting in relatively active and wish-bonding surface atoms. As a result, 2D crystals would have high specific surface energy, making the structure extremely unstable. In order to achieve a much more stable thermodynamical state, crystals tend to gain minimum surface energy values in general. Surface distortions can effectively decrease the surface energy of 2D materials and boost their structural stability. Several lattice parameters, such as interatomic distance, bond angle, coordination number, and bond length, are affected by the generation of surface distortions. [249] Consequently, the electronic structure of the crystals is undoubtedly disturbed due to such changes, which tailor the photocatalytic activity. For instance, X-ray absorption fine structure spectroscopy (XAFS) results validated a notable change in the local arrangement of atoms in single-layer ZnSe in contrast to bulk structure. [250] The significant peaks observed at 2.11 and 3.63 Å for bulk ZnSe are ascribed to the closest Zn-Se, and Zn-Zn coordinates, respectively (Figure 12). However, a shift in Zn-Se peak to 2.17 Å with reduced intensity and a significant decrease in Zn-Zn peak intensity were observed for single-layer ZnSe.
In addition, SeSe bond lengths were extended from 4.012 Å to 4.11 Å for bulk and single-layer structures, and the bond angle between Zn-Se-Zn surface atoms was also compressed by 7.0° as well, in single-layer ZnSe in contrast to the bulk structure. These results suggested a significant disturbance in the local atomic arrangements arising from the surface distortions. Apart from that, the structural stability of single-layer ZnSe structures was greatly improved due to reduced surface energy from the distorted surface structure. Additionally, the charge migration ability in single-layer structures is improved significantly, attributable to higher DOS near CB edge, arising from surface distortions. Benefiting from such outstanding properties, a very stable and efficient water photo-oxidation performance was realized for single-layer ZnSe. Similar findings have also been reported for surface distortions in other 2D NiTi-LDH, SnS, and SnS 2 systems. [84,213,251] Zhao et al. reported that in contrast to Ti 4+ in bulk NiTi-LDH, very thin nanosheets (2 nm) of NiTi-LDH can acquire Ti 3+ with lower coordination numbers, which was verified from X-ray absorption near-edge structure (XANES) spectra as well. [213] The electronic configuration of thin NiTi-LDH nanosheets was effectively tailored due to immense structural distortions. Hence, a superior photocatalytic response can be achieved for O 2 evolution under visible irradiation. As a result, the photocatalytic oxygen evolution behavior under visible light can be improved.

Strategies for Tuning Defect Creation
Surface defects can efficiently modify materials' local atomic and electronic structure, electrical conductivities, and optical properties, further affecting physical and chemical features and light-driven catalytic efficiency. Hence, it is necessary to describe efficient methods for regulating the formation of defects and the intrinsic mechanism for defect formation.
Chemical Reduction Treatments: Chemical reduction is a powerful technique for engineering surface defects into semiconductors. [252,253] It can be used with reducing reagents such as NaBH 4 , CaH 2 , and N 2 H 4 or with reducing solvents such as ethylene glycol and glycerol. By chemical reduction with NaBH 4 , Bi et al. synthesized defective K 4 Nb 6 O 17 nanosheets (Figure 13a). The high reducibility of NaBH 4 reacts with the oxygen atoms in the lattice K 4 Nb 6 O 17 nanosheets during the reaction, leaving V O on the surface. [254] This V O may be used for bandgap narrowing via lowering CB edge, thereby increasing light absorption. Simultaneously, this will create a barrier for surface electrons, promoting charge separation and increasing H 2 evolution behavior. As MO is prepared using reducing solvents, the oxygen atoms are often lost, leaving V O in the lattice. For instance, during the BiOCl nanosheet preparation at 160 °C, highly reducing ethylene glycol can interact with O 2− at (001) facet much more quickly, forming V O . [255] Not only do the shaped V O expand the absorption edge of light spectrum, but they also allow the efficient capturing of photogenic electrons and molecular O 2 to form radical superoxide species.
Chemical therapies are often used to modify the surface of objects. Chemical additives can be used to enhance doping efficiency while causing less harm to the components. Chemical therapies are also used to control optoelectronic interface trap states. The sulfur vacancies in MoS 2 have been filled using thiol chemistry, where chemical reactions dominate the curing process. [256] The monolayer MoS 2 samples studied here are obtained by mechanical exfoliation from bulk crystals. As demonstrated in earlier works, a high density of sulfur vacancies exists in as-exfoliated MoS 2 . [257,258] These defects, which can act as catalytic sites for hydrodesulfurization reactions, are chemically reactive. Therefore, it is possible to repair the sulfur vacancies by thiol chemistry. For two reasons, they choose a specific molecule trimethoxysilane (MPS, Figure 13b, inset). i) The SC bond in MPS is weaker than other thiol molecules like dodecanethiol due to the acidic nature of CH 3 O groups, leading to a low energy barrier for the reaction. ii) The trimethoxysilane groups in MPS react with the SiO 2 substrate to form a selfassembled monolayer.
The reaction kinetics of sulfur vacancies and MPS by DFT is shown in Figure 13b. The MPS and generated sulfur vacancies undergo a two-step reaction with energy barriers of 0.22 and 0.51 eV, respectively. Therefore, the sulfur vacancies in Figure 13c,d are believed to be intrinsic rather than induced by electron irradiation. High-temperature annealing can also Figure 12. XAFS measurements and calculated DOS from Synchrotron radiation. a) Zn and Se K-edge extended XAFS oscillation function kχ(k) and b) Fourier transforms for ZnSe single layers, ZnSe-pa (pa: n-propylamine) single-layers, and bulk ZnSe, respectively; the red, blue, and black lines correspond to ZnSe single layers, ZnSe-pa single layers, and bulk ZnSe, respectively, c) The black, blue, and olive lines in the estimated DOS represent the total, Se sp, and Zn sp state, respectively; the calculated bandgaps for ZnSe single layers and bulk ZnSe were 1.25 and 1.07 eV, respectively. d) ZnSe single-layer structural model viewed in the (110) plane. Reproduced with permission. [250] Copyright 2012, Nature Publishing Group.
www.advmatinterfaces.de be used to solve the low energy barrier. The concentration of sulfur vacancies is decreased fourfold after MPS therapy. As a result, the monolayer MoS 2 field-effect transistor exhibited superior mobilities (>80 cm 2 V −1 s −1 ) even at room temperature. It is significantly greater than the sample size as packed. Another approach for defect healing is the sulfur vacancy selfhealing technique by utilizing poly(4-styrenesulfonate) (PSS) therapy. [259] The healing mechanism of PSS (acting as a catalyst) can be explained as the hydrogenation of PSS guiding sulfur adatom clusters onto the as-grown MoS 2 to fill the vacancies. Thus, electronic density of healed MoS 2 is reduced by a factor of 643, resulting in the fabrication of a lateral homojunction with a complete rectifying response. The efficiency of homojunction was significantly improved due to the elimination of lattice defect-induced local fields. Surprisingly, the three types of devices exhibit very different behavior (Figure 13e-h).
Ball Milling: Ball-milling may be used to break the surface of materials and introduce defects. As graphite is ball-milled, the particle size of the graphite decreases, and more edges/ defects are revealed, which would be useful for maximizing the catalytic activity. [260] Zhu et al. discovered that during the ball-milling process, various defects in BiPO 4 , such as V Bi and V O , were created. [261] However, since the generated vacancies/ defects were in bulk form, it prevented the isolation of photogenerated charges, and the photocatalytic activity was decreased. In 2D crystals, surface defects would predominate due to the extremely thin (atomic-scale) structure having numerous exposed atoms at the surface. Even so, this technique aims to introduce a variety of surface defects into 2D materials, thus  [254] Copyright 2014, John Wiley & Sons, Ltd. b) There are two energy barriers; the first one (0.51 eV) is due to the SH bond breaking, and the second one (0.22 eV) is due to SC bond breaking. The sulfur vacancies in the initial state are illustrated by dashed. The inset shows the chemical structure of MPS. c) As-exfoliated and d) top-side treated monolayer MoS 2 sample, showing the significant reduction of sulfur vacancy by MPS treatment. Red arrows highlight the sulfur vacancies. The overlaid blue and yellow symbols mark the position of Mo and S atoms, respectively. Scale bar, 1 nm. e) Typical σ-V g characteristics for as-exfoliated (black), top-side-treated (blue), and DS-treated (red) monolayer MoS 2 at T = 300 K. f) µ-T characteristics for the three devices at n = 7.1 × 10 12 cm −2 . Solid lines are the best theoretical fittings. The dashed red line shows T −0.72 scaling. Arrhenius plot of σ (symbols) and theoretical fittings (lines) for the g) as-exfoliated, h) top-side-treated MoS 2 . Reproduced with permission. [256] Copyright 2014, Nature Publishing Group.

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increasing photocatalytic efficiency. While the defect concentration can be adjusted by adjusting the ball milling time and strength, the type of defects is difficult to monitor.
Vacuum Activation: Additionally, vacuum activation is widely used for moderating defects into 2D materials. Xing's group presented an easy and inexpensive vacuum-activated low-temperature process for TiO 2 modification. [262] Without altering the crystal structure or crystallinity of TiO 2 , Ti 3+ and Vo can be added. As TiO 2 is heated to a sufficient temperature in a vacuum, the O atoms on the surface lack any external pressure restraint and appear to discharge from the surface. As a consequence of the increasing temperature and lengthening of time, Vo and Ti 3+ developed. By stretching the light-harvesting range and creating defective states to catch photogenic charge carriers, the vacuum-activated process will enhance photocatalytic operation for contaminant elimination and H 2 production. Since this technique falls into very mild treatment methods, generated defects can eventually vanish during the photocatalytic phase. However, these defects can be regenerated again by repeating the procedure and can be applied to several MOs, including ZnO, WO 3 , and MoO 3 . In addition, the V O concentrations can be effectively controlled by controlling the temperature and treatment time during the treatment. Moreover, this method had difficulty realizing other types of vacancies like S, B, or metal vacancies.

Hybridization Engineering
2D materials have an extremely high specific surface area, which enhances the importance of the surface state relative to the bulk inside. Charge carriers produced by photons are dispersed at the surface to participate in the oxidation/reduction reactions. Thus, the hybridization of surfaces to boost the effective ingestion of excitons is enviable in the absence of a 2D structure. In this section, consistent with surface hybridization, various 2D hybridization techniques with robust case studies are added, including QDs/2D materials, single atoms/2D materials, molecular/2D materials, and layered 2D/2D hybridization.

Single Atoms/2D Materials Hybridization
In order to boost photocatalytic performance, it is possible that nanoparticles could be reduced to single atoms. However, the fraction of monoatomic with unsaturated coordination bonds is maximized, enabling a high surface effect. [263] Zhang et al. pioneering work on monoatomic-based catalysis attracted attention in the photocatalysis domain. The monoatomic-dependent photocatalyst was focused on the dispersion or coordination of secluded monoatoms on the surface of the support material. Monoatomic-based strategies may enhance photocatalytic behavior and provide another method for adjusting selectivity. Additionally, active single atoms, chemical bonding between single atoms, and 2D materials-based supports have developed into a robust and straightforward charge transfer process. Thus, building a single atom/2D materials hybridization is highly desirable to achieve a superior photocatalytic response (Figure 14a). [264] Via calcination, protonation, and coupled exfo-liation, single Rh atoms have been scattered on uniform TiO 2 nanosheets (Figure 14b). [265] The water dissociation at the surface, as in states 2 or 4 (Figure 14c), is not the final state (generation of H 2 ) but only a reaction intermediate state. At least one or more transition states may influence the overall rate in the full reaction pathway to the final state. Hence, an Rh-doped titania nanosheet was prepared. The photocatalytic activity for hydrogen production from this nanosheet was 10 times that for an undoped nanosheet. The presence of single Rh atoms substituting for Ti 4+ in the nanosheet lattice was confirmed by TEM observation. It indicates that single Rh atoms can act as reaction centers for the photocatalytic reaction. Thus, crystal sites containing a single transition metal atom in the photocatalyst can act as cocatalysts. First-principles modeling methods support the experimental observations. These results will be useful in better understanding the role of the cocatalyst and the mechanism and will provide new insight for the design of advanced photocatalysts for water splitting. On the other hand, simulations reveal a metastable dissociatively adsorbed state for both the doped and the undoped system. The atomic configuration of the dissociated states is similar for both cases (Figure 14c,d), with an OH fragment adsorbed atop a (now seven-fold coordinated) Ti atom and the H atom adsorbed atop a surface bridging oxygen. Despite the similar geometries, the total energy cost for dissociation is 0.85 eV in the undoped system but only 0.48 eV when dissociated near the Rh atom. The different energy cost of dissociation, 0.85 eV versus 0.48 eV, suggests that more dissociated water molecules can be found when the nanosheets are doped. Based on the Boltzmann factor based on this difference, approximately six times more dissociated water molecules on the Rh-doped systems can be expected than the undoped ones. Additionally, HAADF-STEM techniques are employed to confirm the defects of 2D photocatalyst. In the HAADF-STEM image, distinct brightest spots indicated Rh atoms, while moderate brightness spots indicated Ti atoms (Figure 14e,f). In addition, the Extended X-ray absorption fine structure (EXAFS) analysis revealed that Rh atoms in prepared samples exhibited a chemical environment similar to that of Rh 2 O 3 , which exhibited the bonding with O atoms and undergoing oxidation. Guo and co-workers investigated using single Pt atoms as cocatalysts to enhance the hydrogen generation behavior of C 3 N 4 nanosheets under irradiation. [266] To form Pt single atoms/C 3 N 4 , a basic liquid phase reaction with H 2 PtCl 6 and C 3 N 4 was used in conjunction with low-temperature annealing. HAADF-STEM technique was used to determine the dispersion and structure of Pt. Specific, transparent spots have been observed to be uniformly scattered on graphitic-C 3 N 4 sheets, with 99.4% of Pt having a size greater than 0.2 nm, indicating that Pt exists entirely as monoatomic (Figure 14e-j). As the doping concentration of Pt exceeded 0.38%, the dispersion of Pt atoms became denser, and numerous nanoclusters were formed. The local atomic configuration of the Pt/C 3 N 4 has been investigated using extended EXAFS spectroscopy. The coordination number was evaluated to be approximately 5 for the Pt atoms, which confirmed the decoration of monoatomic on the top of g-C 3 N 4 surface having a bandgap value of 2.03 eV. The photocatalytic H 2 evolution behavior was significantly enhanced regarding the fabrication of a single Pt atom/C 3 N 4 structure. Pt/C 3 N 4 (0.16 wt% Pt loading) reached a production rate of nearly 318 µmol h −1 , about www.advmatinterfaces.de 50 times that of pure C 3 N 4 . Concurrently, prepared structures demonstrated remarkable stability during the photocatalytic H 2 generation after several cycles. The desirable quality of ultrafast transient absorption spectroscopy, surface trap states of C 3 N 4 was largely altered by the secluded single Pt atom, which increases the exciton life period and increases the possibility of e − engaging in H + reduction.
The observation that secluded metal atoms possess high surface energy was shown, as well as the possibility that these atoms cooperate closely with the supports surface. The hybridization energy scheme can become a local minimum by interacting with affected metal atoms with available defects on the support surface. Hence, these atoms could be secured and maintained in their stable state. Surface defects are more likely Figure 14. a) Merging single-atom-dispersed silver and carbon nitride to a joint electronic system via copolymerization with silver tricyanomethanide. Reproduced with permission. [264] Copyright 2016, American Chemical Society. b) Illustration of photocatalytic reaction centers in 2D titanium oxide crystals. c) Charge density and dissociation energetics for undoped and Rh-doped titania nanosheet. d) Top view images for charge density of nanosheet during water dissociation process for undoped and Rh-doped nanosheets. e) HAADF-STEM (200 Reproduced with permission. [265] Copyright 2015, American Chemical Society. g) Pt-CN HAADF-STEM picture. The size distribution of the bright spots is seen in the inset. h) Pt L 3 -edge EXAFS oscillations of Pt-CN, K 2 PtCl 6 , and Pt foil Fourier transform. i) Pt-CN schematic models. j) H 2 generating activity of g-C 3 N 4 and Pt-CN photocatalysts. Reproduced with permission. [267] Copyright 2016, John Wiley & Sons, Ltd.

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to form in 2D materials due to the extremely high specific surface area and minute atomic flee radiation. Thus, a monoatomic-anchored surface defect rich 2D structure can be constructed to enhance photocatalytic behavior. [42]

Quantum Dots/2D Materials Hybridization
Nanoparticles have dangling bonds between their coordinated unsaturated surface atoms. In order to reduce the size of nanoparticles any further, a higher proportion of atoms at the surface must be achieved in comparison to total atoms, and their average binding energy should be more significant. Thus, if the size of nanoparticles can be used to monitor the QDs and adjust the 2D materials, it is possible to produce strong interfacial coupling between them. Additionally, due to the small size of QDs, they may exhibit a strong dispersion on 2D materials, enhancing the photocatalytic properties. To improve Ag's operational effectiveness, Ag QDs with a diameter greater than 5 nm have been developed. [268] Upon irradiation with visible light, photocatalytic performance to degrade tetracycline hydrochloride (TCH), ciprofloxacin (CIP), and RhB was greatly enhanced following hybridization with BiOBr nanosheets. The study revealed that tuned Ag QDs could effectively activate molecular O 2 with the help of excited electrons, which decreases upon exposure to irradiation. Ag QDs will act as adsorption, charge separation, and reaction centers simultaneously, resulting in increased photocatalytic behavior. In order to reduce the service life of noble metals, metal-free or non-noble metal QDs are used instead. For example, N-doped carbon QDs (N-CQDs) of 3 nm diameter have been fabricated via a hydrothermal route and then adjusted on the atomically thin surface of BiOI nanosheets. [269] AFM images show that the average thickness of BiOI is about 0.9 nm, suggesting the single-layer configuration. After the introduction of N-CQDs, the N-CQDs/BiOI materials display a greatly prolonged lifetime of photogenerated charge carriers, as proved by time-resolved transient photoluminescence decay and instantaneous photocurrent. The atomically thin structure of BiOI ensures the strikingly fast bulk charge diffusion to the surface, and the modified N-CQDs with conjugated π structure can effectively promote the surface charge separation, resulting in a longer carrier lifetime. As a result, the concentration of active species and photocatalytic performance of the N-CQDs/BiOI photocatalyst were expressively increased.
Likewise, many other systems involving QD/2D materials hybridization that can be used to improve photocatalytic performance, including NiS 2 , Zn-Ag-In-S, and CdSe QDs, have been explored. [270] Kang and co-workers have utilized carbon nanodots (CDots) as a chemical catalyst to boost photocatalysts H 2 O splitting through C 3 N 4 significantly (Figure 15a-d). [271] They systematically confirmed that water-splitting photocatalysis by CDots-C 3 N 4 indeed proceeds via the stepwise 2e − /2e − two-step process, in which H 2 O oxidation to H 2 O 2 is the first and ratelimiting step, followed by the second and fast step of H 2 O 2 disproportionation to O 2 , which CDots chemically catalyze. Additionally, CQDs have also been involved in the disintegration of H 2 O 2 and O 2 evolution. As a result, exceptional photocatalytic H 2 O splitting efficacy can be achieved, with a solar to H 2 conversion performance of 2.0% and healthy stability of 200 days. Apart from the above results, several other systems regarding QD/2D material hybridization to promote photocatalytic performance, such as CdSe QDs, Zn-Ag-In-S QDs, NiS 2 QDs, and so on. [270,272,273] Recently, Ning et al. [272] developed a kind of cadmium-free Zn-Ag-In-S (ZAIS) colloidal QDs that shows remarkably photocatalytic efficiency in the visible (Figure 15h). More importantly, a nanocomposite based on the combination of 0D ZAIS colloidal QDs and 2D MoS 2 nanosheet is developed. This can leverage the strong light harvesting capability of colloidal QDs and catalytic performance of MoS 2 simultaneously. As a result, an excellent external quantum efficiency of 40.8% at 400 nm is achieved for colloidal QD-based hydrogen generation catalyst. This work presents a new platform for the development of high-efficiency photocatalyst based on 0D-2D nanocomposite. The morphology of the as-produced ZAIS-2 (higher Ag amount compared to pristine sample) QDs-MoS 2 nanocomposites using TEM (Figure 15e-g) [272] These above results undoubtedly demonstrated the superiority of QDs modification, and the QDs/2D configuration may be an effective alternative structure to achieve highefficiency photocatalytic behavior.

Molecular/2D Materials Hybridization
Because a single isolated atom is being used to engineer electronic structure, single molecular structures may also tune the electronic features by serving as a cocatalyst to improve photocatalytic action. Xia and co-workers synthesized H 2 O soluble molecular cocatalyst trifluoroacetic acid (TFA) to improve the H 2 evolution potential of K 4 Nb 6 O 17 . [269] Utilizing the reversible redox couple TFA/TFA including the highly active intermolecular radical responses, the TFA molecule acted as a solid hole supplier, efficiently transporting the photogenerated h + and resulting in increased charge separation potency. The TFA improvement consistently increased H 2 generation, and the highest rate reached 6344 µmol g −1 h −1 for the sample with 25:6 molar ratio. The maximal generation rate was ≈32 times greater as compared to bare K 4 Nb 6 O 17 , definitely indicating the effect of molecular cocatalyst.
Furthermore, molecular cocatalyst soluble in water can provide a good amount of available area to the photocatalyst by diffusing in the solution. Using C 3 N 4 subnanopores, molecular TiO 2 has been included in C 3 N 4 catalyst via an easy polycondensation of TiO 2 ion precursors and DICY. [274] TEM study obtained and verified the morphology of prepared TiO 2 -C 3 N 4 nanosheets having a thickness of approximately 3 nm. TiO 2 originated from elemental mapping and HAADF-STEM to be reliably dispersed with an isolated format on C 3 N 4 frame. As demonstrated in Figure 16a, controllable subnanopore engineering in 2D g-C 3 N 4 using molecular titanium-oxide incorporation (TiO-g-C 3 N 4 ) was achieved by a simple "bottom-up" polycondensation of precursors which contain DICY and titanium www.advmatinterfaces.de oxide ions. In detail, TiCl 4 and DICY were first dissolved in a cooled ammonium chloride solution, forming a colorless and transparent aqueous solution. During the dissolving process, TiCl 4 was rapidly hydrolyzed to TiO 2+ . [275] The corresponding TEM image in Figure 16b also verifies the clean ultrathin nanosheet with a sheet-like morphology as that of 2D g-C 3 N 4 . The AFM image reveals that the thickness of the TiO-CN2 nanosheet (one of the TiO-g-C 3 N 4 samples with higher molecular titanium-oxide incorporation) is about 3-3.3 nm. Furthermore, EDX mapping analyses were performed to identify the distribution of molecular titanium-oxide in the matrix of 2D g-C 3 N 4 . [275] The HAADF-STEM image of TiO-CN2 nanosheet also confirms that titanium-oxide was homogeneously dispersed in the framework of 2D g-C 3 N 4, as shown in Figure 16c. Moreover, at the edge of the TiO-2D g-C 3 N 4 nanosheet where the yellow circles are noted, it can be identified that the element Ti exists as an isolated atom with atomic size, suggesting that an isolated titanium-oxide species was coordinated in the subnanopores of 2D g-C 3 N 4 . All of the above characterization results demonstrate that 2D TiO-g-C 3 N 4 was successfully obtained by subnanopore engineering using homogeneous titanium-oxide incorporation in the framework of 2D g-C 3 N 4 . The XRD results clearly illustrate that molecular titanium-oxide incorporation in 2D g-C 3 N 4 does not destroy the primary building structure of the CN framework. [275] The solid-state 13 C NMR spectra also confirmed this result in Figure 16f. The two prominent peaks in the solid-state 13 C NMR spectra of 2D g-C 3 N 4 and TiO-CN2 are similar. The first peak at = 164.7 ppm is assigned to the C atoms in CN 2 (NH X ) and the second peak at = 164.7 ppm is attributed to the C atoms in CN3 (other sample with more higher TiO content). Also, the structure of 2D TiO-g-C 3 N 4 was further Figure 16. a) The synthesis of TiO-g-C 3 N 4 from "bottom-up" polycondensation of the intended precursors is depicted in this diagram, b) TEM, c) HAADF-STEM, d) AFM image of TiO-CN2 nanosheet, e) 2D g-C 3 N 4 and TiO-CN2 nanosheets XRD patterns, f) The solid-state 13 C NMR spectra of 2D g-C 3 N 4 and TiO-CN2, g) IR spectra of 2D g-C 3 N 4 and TiO-CN2. Reproduced with permission. [275] Copyright 2016, The Royal Society of Chemistry.
www.advmatinterfaces.de characterized using FTIR spectra (Figure 16g). [275] Upon TiO 2 molecule incorporation, prepared photocatalysts showed bandgap narrowing compared to pure C 3 N 4 with reduced CB position. It results from further Ti-O electron presence in the lattice and increases π-e − delocalization in the conjugated structure. Furthermore, the electronic structure of hybrid catalysts can also facilitate the isolation of charge carriers. Thus, TiO 2 -C 3 N 4 improved the light-induced degradation performance and generated more •OH radicals for pollutant removal. [275] While 2D structure allows fast transfer of charge carriers in bulk, deficiency of charge separation sites for surface charging would also ruin overall photocatalytic operation. A hybridization technique is necessary to facilitate charge separation at surface and, more importantly, the transfer of holes. Moreover, using water-soluble molecular materials as a homogeneous cocatalyst can significantly improve photocatalytic efficiency. So, perhaps evolved molecular cocatalyst strategies can potentially separate photogenerated carriers and thus enhance photocatalytic efficiency.

2D/2D Stackings Hybridization
Building 2D-2D stacks to boost the photocatalytic potential is a widely applied process. Lattice mismatch has been significantly reduced due to the comparable layered structures of 2D materials. Zhang and co-workers studied single-layer Bi 12 O 17 Cl 2 with surface Vo through an exfoliation technique based on Li intercalation. [230] The key to this V O -oriented assembly lies in the metallic characteristic of 1L (monolayer)-MoS 2 and the asymmetric structure of 1L-Bi 12  More interestingly, they found that all the MoS 2 sheets were anchored on the same surface in Bi 12 O 17 Cl 2 , which was further evidenced by their side-view TEM image (Figure 17m). These observations demonstrate the occurrence of an oriented assembly. As 1L-Bi 12 O 17 Cl 2 has an asymmetric structure consisting of (Cl 2 ) and V O of (Bi 12   www.advmatinterfaces.de provided direct, atomic-resolution shreds of evidence that this oriented assembly resulted in 2D Janus bilayer junctions of (Cl 2 )-(Bi 12 O 17 )-(MoS 2 ). [230] Since the concentration of charges surrounding (Bi 12 O 17 ) 2+ layer was larger in contrast to (Cl 2 ) 2− layer, photogenic excitons strived to (Bi 12 O 17 ) 2+ and (Cl 2 ) 2− end faces when exposed to light. The photogenic electrons traveled between the single layers of MoS 2 via BiS bonds, which allowed improved separation of excitons (ultralong carrier life of 3446 ns), as confirmed by transient absorption analysis. Using atomic size thickness, effective direct interface charging isolation, and ample active sites in MoS 2 , the fabricated hybrid bilayers showed superior visible light-derived H 2 evolution efficiency. Furthermore, using a holedonating agent such as ascorbic acid can significantly improve the H 2 production rate up to 33 mmol h −1 g −1 . Other than that, many 2D stacking studies, including MoS 2 /C 3 N 4 , C 3 N 4 /Bi 4 O 5 I 2 , Fe 2 O 3 /C 3 N 4 , MoS 2 /TiO 2 , NiO/Ca 2 Nb 3 O 10 , SnS 2 /C 3 N 4 , MoS 2 / CdS, ZnIn 2 S 4 /MoSe 2 and ZnCr-LDH/layered titanate have been reported. [276][277][278][279] These hybridized 2D/2D structures have many overwhelming advantages. For example, they offer a significant amount of available area at the 2D-2D interface and effectively decrease the barriers for electron migration with cocatalysts help. Consequently, they can immensely boost the interfacial charge transfer in the photocatalysts by allowing quantum tunneling phenomenon. In addition, these hybrid materials can significantly improve the light-harvesting property of the photocatalyst by easing the light-blocking phenomenon in cocatalysts as well. However, there is a dire need to alter the 2D components to strengthen the interfacial force between the layers to fabricate 2D/2D hybridized photocatalysts with superior activities.

3D/2D Hybridization
Instead of the beneficial effect of the 3D/2D heterostructure system, this binary system still greatly suffered from the inefficient photo charge carrier migration between the neighboring photocatalyst, consequently promoting the photo charge carrier recombination. [280][281][282] Thus, ensuring the smooth photo charge carrier migration between the neighboring photocatalyst in the binary g-C 3 N 4 /BiVO 4 heterostructure system is paramount to warrant better photocatalytic activity. Recently, the addition of carbonaceous materials, particularly RGO has triggered widespread interest in photocatalytic application owing to its beneficial effect as an electron shuttle for photo charge carrier migration across the heterostructure interface, thus alleviating the existing problem within the binary heterostructure system. [283] Equally important is the fact that green hydrogen production remains a boundless challenge hitherto and limited study has been focused on utilizing lake water as a source of hydrogen. Until now, most photocatalytic or photoelectrocatalytic hydrogen production was only employed as a chemicalbased electrolyte solution with sacrificial reagents in which their environmentally friendly approach can be questioned.
Additionally, despite many photocatalyst studies that have made epigrammatic progress, there is still limited analysis provided by the current literature for the bifunctional application of the as-developed photocatalysts for hydrogen generation and photodegradation studies. To date, the discharge of antibiotics to the aquatic environment stemming from the pharmaceutical industries has jeopardized the aquatic environment. Hence, it is of utmost significance that needs to be addressed and solved before it becomes irrevocable. [280] Samsudin and Sufian [280] fabricated 2D/3D g-C 3 N 4 /BiVO 4 photocatalyst decorated with RGO for boosted photoelectrocatalytic hydrogen production from natural lake water and photocatalytic degradation of antibiotics. The morphological structure of the 2D/3D g-C 3 N 4 /BiVO 4 decorated with RGO at different amounts of RGO loading was examined using FESEM analysis as shown in Figure 18. All of the composite heterostructure samples revealed the typical flower-type structure in a micro-size range, corresponding to 3D BiVO 4

Surface and Interface Engineering
In current history, photocatalytic activity has focused on issues related to structural engineering by clarifying photocatalytic processes and developing strategies for detailed synthesis and comprehensive characterizations. This structure creation's goal is relatively simple: boost each charge kinetics stage. Structural engineering mainly involves energy band, surface, and interface engineering to optimize the effects of selected primary measures. [284] The aim of energy band engineering (EBE) is to optimize the photocatalysts' photon-harvesting efficiency, thus promoting exciton pair production during the first step. The primary objective of interface engineering is to prevent negative e − -h + recombination to increase the number of e − s and h + s reaching the surface for redox reactions. Certainly, accumulating enough e − s and h + s on the surface might not guarantee the high current effectiveness of a light-driven catalytic mechanism. Surface engineering has proved to be a flexible way to improve adsorption properties and reaction species activation capabilities. The surface e − s or h + s are more easily involved in a redox reaction. [285] The EBE has made great efforts over the past decade, including developing narrow and wide bandgap semiconductors doping. [286] As we can determine from the research, this issue received only a few review articles, although the EBE was extensively summarized. [287] However, research focusing on surface and interface engineering showed the significance of their activities in optimizing photoconversion efficiency. Surface engineering, vacancy engineering, element doping, surface heterojunction formation and facet-dependent site control have proven effective in tuning photocatalyst properties. [288][289][290][291] The surface atoms, however, only comprise a tiny fraction of the bulk photocatalysts, typically producing an inconspicuous impact to improve their photoconversion efficiency. 2D structures, with thin thickness, massive lateral scale, and plentiful exposed surface atoms with controlled facets, establish perfect platforms for atomic surface engineering. First, the surface atoms in 2D materials can almost reflect the entire material's overall physical and chemical composition and include all details about the atomic structure. Second, 2D surface atoms are vulnerable to escape from the crystal lattice, thereby promoting vacancy engineering. Thirdly, surface atoms of 2D materials typically have low coordination numbers that can be considered alteration sites, www.advmatinterfaces.de facilitating surface modification and hybrid structure building. Meanwhile, the interface between two elements is the site for transition and differentiation in hybrid systems. The reliability of transferring charges through the interface is crucial to avoiding the recombination of charges. For example, the electric field generated inside the interfaces may drive the charge carriers to be spatially divided into various components. This spatial isolation limits bare material load recombination. [292] Because the photogenic excitons must pass through the interface in a typical catalytic process, several parameters such as interface compositions, zones, faults, electronic binding, facets, and band bending seem to have a major impact on charging and separation efficiency. [293] This section described a series of important surface and interface engineering parameters that compensate for or influence load kinetics and overall photocatalytic efficiency. These variables can help researchers develop more effective and stable 2D photocatalytic remediation systems. At the start of sections, specific surface and interface engineering rules will be explained. Then relevant parameters that define or affect charging kinetics performance will be elaborated. Along with the parameter-efficiency relationship, the architecture rules will direct us to maneuver the charging kinetics by selecting or adjusting parameters. It is the fundamental challenge of surface and interface engineering.

Design Rules and Parameters for Surface Engineering
Designing Rules: Photocatalysts' morphological architecture has some general guidelines. Initially, the surface of the engineered part must be on that a catalytic reaction takes place. Numerous exposed components in hybrid systems may not react. For example, solid semiconductors in Z-scheme photocatalysts merely contribute to transferring and recombining the charges. [294] Thus, the catalytic efficiency cannot be changed by changing these components' surface parameters (SPs).
Secondly, different reactions can affect different SPs. Therefore, rationally tuning suitable SPs is critical for behavior improvement, which should have been focused on a thorough understanding of surface kinetics. For example, tuning catalyst surface pore sizes may influence large molecules' Figure 18. FESEM images of the 2D/3D g-C 3 N 4 /BiVO 4 decorated with RGO at different amount of RGO loading along with the elemental mapping of the 1.2 wt% RGO@g-C 3 N 4 /BiVO 4 sample. Reproduced with permission. [280] Copyright 2020, Elsevier B.V. www.advmatinterfaces.de Figure 19. Schematic representation of essential parameters in surface engineering for photocatalysis: a) To boost photocatalytic CO 2 reduction, a Pt www.advmatinterfaces.de transport. While adjusting the pore size can be an effective technique for enhancing the photocatalytic abilities to remove pollutants, this technique is unsuitable for H 2 O splitting applications since the size of water molecules is far less than the dye molecules. [295] Thirdly, modifications to particular SPs will modify band composition and light-harvesting semiconductors absorption. For instance, it was argued that surface vacancies and compositions play an essential part in extending the solar absorption spectrum of semiconductors. In contrast, structures having textured surfaces have better light-reflecting properties that can help with better absorption of photons. [247,296] In such conditions, the role of SPs in improving photoconversion efficiency becomes much more difficult. In comparison, cocatalyst surface engineering is pretty simple, as light absorption does not change.
Fourthly, consider relations among various SPs; for instance, surface pore adjustment eventually contributes to surface area variation. Eventually, correlations between SPs and interface structures should be recognized, as interface structures significantly affect the catalytic efficiency surface. Under the above rules, surface design should be designed by optimizing a few essential factors: surface compositions, vacancies, pores, facets, areas, surface conditions, phases, and band bending.
Parameters: According to the rules above, surface design can be performed by selecting or optimizing important parameters, including surface compositions, phases, facets, areas, pores, vacancies, surface state, and band bending. Three surface engineering methods were used to increase the catalytic activity and selectivity of photocatalysis: 1) modify these SPs to provide a surface that is much more active and promote superior adsorption and activation capabilities for specific species; 2) Control the SPs to encourage an abundant number of charge carriers to enter catalyst' surface reaction sites, which dramatically increase the diffusion rate by reducing the diffusion distance for charge carriers; and 3) enhancing the redox reactions occurring at the catalyst's surface. After that, we will demonstrate a series of SPs, which can influence the photocatalytic process across the following three techniques.
Parameters-Surface Compositions: Surface composition is essential for catalytic reactions as it critically affects catalyst surface adsorption and activation activity. However, the surface composition is often associated with semiconductor light absorption, making it a sophisticated element in efficient tuning. Thus, a better method for tuning catalytic efficiency is adjusting cocatalyst surface compositions. Using nanosheets of Pt-doped TiO 2 (Pt as cocatalysts) for photocatalytic reduction of CO 2 in water, H 2 will become the primary product given that the surface of Pt cocatalysts plays an important role in H 2 O activation. [297] Furthermore, the selectivity for the reduction of CO 2 was improved by changing the surface composition of Pt cocatalyst through the slight coating of Cu 2 O (Figure 19a). Pt core transferred TiO 2 photogenerated e − s to Cu 2 O shell in this process, and Cu 2 O shell will serve as an active site for CH 4 and CO generation. Apart from improved activation of reactants, core-shell photocatalysts with cocatalysts have also been utilized for back reactions suppression during photocatalysis. For example, a significant back reaction between photocatalytic H 2 evolution reaction with the surrounding O 2 to produce H 2 O was observed for Rh as a cocatalyst. In order to overcome this issue, core-shell structures of Rh-Cr 2 O 3 cocatalysts were fabricated, which offered different active sites for H 2 -generation through Cr 2 O 3 surface. In contrast, Rh facilitated the transfer of photogenic electrons toward Cr 2 O 3 . [298] The Cr 2 O 3 surface effectively prohibited the production of water through back reaction. Additionally, varying surface compositions can play several crucial roles in surface engineering. One typical case is the photocatalytic splitting of H 2 O over LaMg 1/3 Ta 2/3 O 2 N along with RhCrO y cocatalyst to reduce H + . [299] Authors reported that the reduction of O 2 occurred on LaMg 1/3 Ta 2/3 O 2 N surface in opposition with oxidation of H 2 O while the accrued photogenic h + s oxidized nitrogen species to N 2 . In order to avoid O 2 reduction, LaMg 1/3 Ta 2/3 O 2 N was coated with amorphous TiOXH to be served as a selective pervasion layer (Figure 19b). After the selective infusion of O 2 and H 2 generated at the composite and coating interface, into the atmosphere, the reverse flow of O 2 into the coated layer was prohibited due to higher O 2 pressure in the layer, hence rendering the oxygen reduction. In some other cases, Ta 3 N 5 nanosheets synthesized via the thermal oxidation of Ta foils exhibited very poor performance in the photoelectrochemical (PEC) splitting of water due to intense recombination of photogenic excitons in surface passivation layer (Figure 19c). [300] By exfoliating the surface passivation layer thermally or mechanically, Ta 3 N 5 photocurrent was greatly improved.
Parameters-Surface Facets: Due to atomic arrangement variations, surface facets also play a crucial role in tuning the adsorption and activation capabilities. In light-harvesting semiconductors and cocatalysts, variation in exposed facets can contribute to distinct photocatalytic behavior and selectivity. [301,302] In a typical case, they have investigated photocatalytic CO 2 cocatalyst is selectively coated with Cu 2 O, Reproduced with permission. [297] Copyright 2013, John Wiley & Sons, Ltd. (b) To suppress the oxygen reduction reaction (ORR) reaction, a TiOXH-coated LaMg 1/3 Ta 2/3 O 2 N catalyst was used as a selective permeation layer. Reproduced with permission. [299] Copyright 2015, John Wiley & Sons, Ltd. c) after exfoliation of the surface passivation layer, a process for improving photocurrent. Reproduced with permission. [300] Copyright 2013, John Wiley & Sons, Ltd. (d) C 3 N 4 nanosheets operate as cocatalysts in a photocatalytic CO 2 reduction reaction with Pd nanocubes and nanotetrahedrons, Reproduced with permission. [301] Copyright 2014, The Royal Society of Chemistry. e) Internal electric fields produce the difference in charge diffusion distance between BOC-001 and BOC-010, Reproduced with permission. [218] Copyright 2012, American Chemical Society. f) Between the (010) and (110) facets of a BiVO 4 crystal, there is spatial charge separation. Reproduced with permission. [315] Copyright 2013, Springer Nature, g) TiO 2 with metallic and semiconducting MoS 2 nanosheets as cocatalysts: charge transfer characteristics. Reproduced with permission. [225] Copyright 2015, Springer Nature. h) 2D nanostructures have advantages of large surface area and short charge-diffusion distance to surface. Reproduced with permission. [304] Copyright 2015, American Scientific Publishers, i) CO 2 reduction via photocatalysis using porous Ga 2 O 3 as a photocatalyst. Reproduced with permission. [306] Copyright 2012, The Royal Society of Chemistry. j) In surface and bulk defects, the behavior of photogenerated electrons and holes is studied, Reproduced with permission. [309] Copyright 2013, The Royal Society of Chemistry. k) Surface band bending in both upwards and downwards semiconductors. Reproduced with permission. [312] Copyright 2015, The Royal Society of Chemistry. www.advmatinterfaces.de reduction in the presence of H 2 O using C 3 N 4 nanosheets and Pd nanocrystals as light-harvesting semiconductors and reduction cocatalysts, respectively (Figure 19d). [301] In a typical scenario, photocatalytic CO 2 reduction was examined using Pd reducing cocatalyst in C 3 N 4 nanosheets. Additionally, H 2 O was also reduced to H 2 during the process over Pd (100) nanocubes, whereas the reduction of CO 2 in water was mainly carried over Pd (111) nanotetrahedrons. The theoretical calculations also proposed lower potentials for CO 2 activation along with superior adsorption capabilities for Pd (111) and higher potentials for H 2 O adsorption on Pd (100). Apart from the adsorption and activation of reacting species, surface facets affect photoconversion efficiency with other impacts. For instance, semiconductor facets perpendicular to internal electric field orientation will be even more active in photocatalysis relative to facets parallel to direction. For example, BiOCl-001 nanosheets displayed better photocatalytic activity than BiOCl-010 nanosheets. [218] Since the separation and transport of photogenic charge carriers are greatly facilitated by the internal electric field induced in the crystal, a relatively short diffusion length is offered by the small [001]-facet in BOC-001 ( Figure 19e). Another crucial role played by the surface facets is the spatial exciton separation in semiconductors which can accumulate photogenic charge carriers on various facets to offer varying selectivity and catalytic activity. A characteristic instance is the (110) and (010) facets of BiVO 4 crystal. Studies found that the (110) and (010) facets can be decorated selectively with oxidation and reduction, respectively, owing to the spatial separation of charges. The spatial effect produces a marginal difference in the energy levels of CB and VB between (110) and (010) facets, resulting in separate deposition of e − and h + on (010) and (110) facets (Figure 19f).
Parameters-Surface Phase: The surface phase is another critical surface design parameter. A semiconductors bandgap is considered to be closely associated with its surface phase. [303] In addition, phase of cocatalysts is also critical to their performance as active sites and carrier transporters. The literature showed TiO 2 's superior photocatalytic H 2 output capacity where semiconducting (2H) and metallic (1T) nanosheets of MoS 2 have been employed as cocatalysts. [225] TiO 2 -incorporated MoS 2 nanosheets showed slightly better output rates of photocatalytic H 2 relative to TiO 2 -decorated 2H MoS 2 nanosheets. MoS 2 cocatalyst surface phases impact photoconversion efficiency from two distinct angles. Initially, active H 2 evolution sites have been found only on the edges of 2H MoS 2 sample, whereas the 1T MoS 2 samples offered active sites on the basal planes as well. Second, the charge mobilities in 1T sample were greater as compared to the 2H nanosheets. Higher and shorter diffusion rates distance mean that more photogenerated e − s in TiO 2 reach 1T MoS 2 reaction sites and involve in the reactions (Figure 19g).
Parameters-Surface Area: A larger surface area is known to allow higher photocatalytic activity because atoms only present at the surface of the photocatalyst take part in the redox reactions. Raising the surface-to-volume ratio allows additional active sites for the adsorption of surrounding species to react with each other. For example, newly invented 2D photocatalysts proved more efficient than their 3D counterparts (Figure 19h), partly attributed to an increased sur-face area. [304,305] In this scenario, g-C 3 N 4 nanosheets had a considerably greater surface area (384 m 2 g −1 ), resulting in a ninefold improvement in H 2 generation performance compared to their parent g-C 3 N 4 . [142] A major advantage of 2D photocatalysts has been the minimized distance from bulk to a surface (Figure 19h). [304] The minimal nanosheet thickness shortens path to reaction sites, decreasing likelihood of recombination loss across charge transfer.
Parameters-Surface Pores: Surface pore structures improve the surface-to-volume ratio, leading to better reactant capturing and adsorption. For example, in photocatalytic activity, CO 2 reduction with H 2 O, and porous Ga 2 O 3 showed a 400% higher conversion rate than commercial Ga 2 O 3 nanoparticles. [306] The superior reduction potential was ascribed to the better potential (300%) for CO 2 adsorption offered by greater surface area (200%) of porous Ga 2 O 3 (Figure 19i). In another study, Cu 3 (BTC) 2 MOF (Metal-organic framework) along with TiO 2 were utilized to fabricate core-shell nanostructures for light-derived reduction of CO 2 . Since CO 2 could penetrate through porous TiO 2 quite easily and get trapped within the pores of MOF, the selectivity and photoactivity of the prepared core-shell photocatalyst were highly enhanced. The trapped CO 2 in the MOF received photogenic electrons from TiO 2 and was reduced to CH 4 effectively. [307] Parameters-Surface Vacancies: For surface catalytic reactions, adsorption and activation processes frequently occur in coordinately unsaturated, thermodynamically unstable locations. Site bonds are prone to absorb reactants and charge carriers, and therefore, surface vacancy forms and numbers may influence catalytic activity and selectivity. Kong et al. and Yan's group showed that TiO 2 's photocatalytic activity is significantly improved after an increment in the concentration ratio of surface defects to bulk defects. [308,309] Normally, photogenerated e − s or h + s retained by large defects cannot continue to proceed toward surface reactions and serve as new recombination centers for the photogenic charge carriers. Conversely, photogenerated e − or h + trapped by surface defects could participate in the redox reactions with the adsorbed species to facilitate catalytic reactions (Figure 19j).
Parameters-Surface Band Bending: In semiconductors, tailoring various SPs can change the redox potentials of excitons since the surface variations cause band bending in semiconductors surface owing to disturbed bonding networks within the crystal structure. [310] For example, a downward and upward surface band bending was observed in p-type and n-type GaN semiconductors, respectively. This is attributable to the generation of defects and the insertion of dopants. [311] The downward bending facilitated the photogenic e − s to move below for reduction reactions, while h + were driven up for oxidation reactions due to the upward bending of surface bands (Figure 19k). [312] Consequently, lower O 2 and higher H 2 evolution were observed for the p-type GaN nanowires in light-driven H 2 O splitting, in contrast to n-type GaN. [311] In the structure of a typical semiconductor, band bending (downward/upward) would generate extra barriers for either redox reactions (oxidation or reduction) since they proceed simultaneously on a single surface, limiting the overall performance. However, suppose the active sites for both reactions can be separated spatially by utilizing various facets or com-www.advmatinterfaces.de ponents. In that case, the surface band bending can effectively boost photocatalytic performance. Semiconductors with polar surfaces, for instance, can offer facets with opposing charges on two sides by distinctively terminating the surface bonds. Hence improving the redox reactions activity separately. [313] On the other hand, various dopants have been utilized to regulate the Fermi level to tailor the surface band bending in non-polar semiconductors. For example, the downward band bending in p-type GaN has been effectively controlled by tailoring doping levels of Mg-dopant to boost the water oxidation potential. [311] The reaction conditions, like the nature of electrolyte and the interfacial contact with catalyst's surface, can also induce surface band bending in semiconductors. [313] In addition, particles' size plays a crucial role in tailoring the surface band bending as well; reducing the size of particles up to two times the width of surface space charge region results in incomplete relaxing of surface band bending to the bulk level, which is highly undesirable for surface reactions. From this behavior, it can also be inferred that all SPs are interconnected and depict interplay effects. [285,314]

Design Rules and Parameters for Interface Engineering
Rules: Likewise, to surface design, many principles must be explained for photocatalyst interface design. First, the interface between two elements seems to be where carriers migrate. Occasionally, hybrid structure interfaces do not use e − s or h + s transportation. Tuning interface parameters do not change catalytic efficiency. Magnetic semiconducting nanoparticls core-shell configurations collapse into this process, in which the magnetic core also contributes to magnetic separation after using a photocatalyst. [316] Other than this, the roles of interfaces in their designed systems should be fully understood by dynamic charge kinetics models before interface engineering. For instance, interfacial defects are mostly e − -h + recombination centers, thereby preventing interface charging. Consequently, removing interfacial defects would mostly boost photoconversion efficiency. Thus the interfacial defects are necessary to modulate the exciton recombination in solid-state semiconductor-semiconductor Z-scheme architectures. [317] Two-part job functions should be evaluated when an interface is designed to transfer e − s or h + s. [318] Generally, when two components interact, the role of the component to take e − s (or have h + s) must be higher compared to the other components beyond the interface (i.e., one to have e − s). Moreover, transition directions will rely on particular kinetic charging models on photoexcitation. The job functions must also satisfy the specifications of the respective models. After this, consider the relationship between various interface parameters. Interfacial compositions and facets, for example, significantly affect interfacial defect formation. In particular, close compositions and minor lattice mismatches for two components will minimize the likelihood of interface defects.
Parameters: Two mutually exclusive interface engineering approaches have been used for charge transport in PC. One will create an interface that provides a driving force for excitons' symmetric movement to separate them for surface reactions. The second approach would optimize the charge transfer efficiency by adjusting different interfacial parameters to mitigate the losses generated from excitonic recombination. If design rules are defined, parameters along with interface compositions, regions, defects, facets, electronic binding, and band bending should be customized smoothly.
Unlike the advantageous effects of surface defects, interface defects chiefly behave as e-h recombination sites and affect the transfer of photogenic charge carriers through the interfaces. Interfacial defects may be reduced by generating a singlecrystal interface between two components. These interfacial defects in Cu 2 O-Pd nanosheets are crucial to charge separation efficiency where a Schottky junction is formed. The interfacial defects in the widely employed Pd-decorated Cu 2 O structure prevent the interfacial charge transition between Cu 2 O and Pd. [293] Pd-Cu 2 O core-shell architectures have been established with superior interfaces for single-crystal structures to mitigate them. Conversely, delivering the h + s trapped in Pd cores across the Cu 2 O shells was challenging for surface reactions. A new Cu 2 OPd-graphene stack configuration was created to prevent this unfavorable condition by contacting Pd with Cu 2 O and GR. The single-crystal Cu 2 O-Pd interface avoids the defects in this structure, and the Pd-graphene interface provides transfer channels to the h + s for extraction, leading to increased output of photocatalytic H 2 (Figure 20d).
Another critical parameter in interface engineering is interfacial engineering. A wide interfacial region could theoretically have enough channels for effective charge transfer. In contrast to other structures, 2D layered stack systems had the highest interfacial region and the shortest distance between interface and surface for charge transfer. [319] Interfacial electron transfer values of 1.15108, 3.47108, and 1.06109 s −1 were estimated in a study for 0D-2D, 1D-2D, and 2D-2D TiO 2 -graphene nanosheets, respectively, emphasizing critical position of interfacial region in charge transfer. [320] Transfer of charges through the interfaces is often based on components' facets in interaction with one another. For example, TiO 2 (100)-graphene interfaces exhibited a superior charge transfer rate in contrast to TiO 2 (101)-graphene and TiO 2 (001)-graphene interfaces, allowing them to produce more photocatalytic H 2 . [321] The discrepancy was most likely due to formation of TiC bonds between (100) facets of TiO 2 and graphene, whereas the (001) (101) facets have been bound to graphene by TiOC bonds. Similarly, Pd nanocubes have been decorated on various facets of BiOCl nanoplates, exhibiting a broad range of interfacial hole-trapping capabilities through a Schottky junction. [322] The photocatalytic O 2 evolution efficiency of BiOCl (110)-Pd was found to be significantly higher than that of BiOCl (001)-Pd, owing to the Schottky barrier being more easily formed in BiOCl (110)-Pd. BiOCl (110)-Pd (100) had a slightly thinner interfacial boundary layer than BiOCl (001)-Pd (100). As a result, charge recombination was inhibited more efficiently in BiOCl (110)-Pd (Figure 20e). Similar to surface engineering, interfacial composition is the first critical parameter in interface engineering. The structure is related to constituents' light absorption and surface reaction capabilities. Consequently, interface structure tuning is often achieved by building new interfaces by adding novel components in the composite structures. The primary function of the added component is to generate a new interface www.advmatinterfaces.de for promoting charge conversion since it was never implicated in light-harvesting or redox reactions. A typical example is the interface engineering of Ta 3 N 5 semiconductor and CoO x cocatalyst for water oxidation. [323] Because of the difficulties of intimate interaction between the hydrophilic and hydrophobic surfaces of CoO x and Ta 3 N 5 , respectively, the surface of Ta 3 N 5 was coated with magnesia nanolayer to turn hydrophobic into hydrophilic surface. Apart from enhancing the interfacial interaction of CoO x and Ta 3 N 5 , it also has a passivation effect, lowering Ta 3 N 5 defect density. Greatly enhanced interfacial charge transfer and increased water oxidation performance (Figure 20a). Conductive interlayers can be added to improve transferring interfacial charge. For context, Fe 2 O 3 /rGO/BiV 1-x Mo x O 4 core-shell nanorods surpassed Fe 2 O 3 / BiV 1-x Mo x O 4 in PEC water splitting. [324] Performance improvement can be attributed to the electron-conducting properties of RGO nanosheet, which facilitate charge transfer between Fe 2 O 3 and BiV 1-x Mo x O 4 (Figure 20b). The electron density contour maps below the K 2 La 2 Ti 3 O 10 CB are given in Figure 20f. Apparently, the Ni3d+Ti3d hybrid orbitals spread from the interface region to the Ni bulk at Ni(111)-K 2 La 2 Ti 3 O 10 (101) interface, while the electron density was only localized within K 2 La 2 Ti 3 O 10 for Ni(111)-K 2 La 2 Ti 3 O 10 (002) interface. Hence, the transfer of photogenic e − s to Ni cocatalyst is much easier at the (111)Ni-(101)K 2 La 2 Ti 3 O 10 interface. Also, other interfacial parameters like defects and interfacial compositions played a crucial role in the electronic coupling at the interface. [285] As far as the charging kinetics is concerned, interfacial band bending could occur to achieve an equilibrium of e − s Fermi distributions between the two components, which is necessary for effective diffusion of charge carriers through the interface. Like surface band bending, the e − s can move downward with more +ve CB while h + s can move upward with more -ve VB along with the interfacial band bending (Figure 20g). [285,312] Moreover, the direction of charge carrier's migration is highly dependent on the band bending orientation, which is based on the work functions of coupled materials. Conversely, the charge transfer efficiency is highly affected by the degree of band bending, and speaking thermodynamically, larger band bending results in superior charge transfer.

Theoretical Insight
Based on computational studies investigating the bandgap and band edge criterion, scores of 2D materials have been predicted to be capable of spontaneous water splitting. [94] Figure 21 illustrates the band edge alignment for several families of 2D materials. Figure 21a shows the prediction by Zhuang et al. that the single-layer group-III monochalcogenides, GaS, GaSe, GaTe, InS, InSe, and InTe are suitable candidates for spontaneous photocatalytic water splitting. [326] Another class of 2D materials that have received much attention for its potential in photocatalysis is the transition metal dichalcogenides. [327][328][329][330] Figure 21b illustrates that the band edge positions of the single-layer TMDs like CrS 2 , MoS 2 , WS 2 , PtS 2 , and PtSe 2 make them suitable for photocatalytic splitting of water. [327,328] Further, studies on the electronic structure of vacancies and edges of MoS 2 show that these defects can provide catalytically active sites. [326,331,332] Liu et al. predicted that single-layer metal-phosphorustrichalcogenides, MPX 3 (M = Zn, Mg, Ag 0.5 Sc 0.5 , Ag 0.5 In 0.5, and X = S, Se) exhibit the intrinsic electronic properties suitable for spontaneous photocatalytic water splitting, see Figure 21c. [333] Single layer α-MNX (M = Zr, Hf; X = Cl, Br, and I) and β-MNX (M = Zr, Hf; X = Cl, Br) have been shown to be yet another class of 2D materials suitable for photocatalytic water splitting. [326] Furthermore, single-layer bismuth oxyhalides including BiOCl, BiOBr, and BiOI are suggested to exhibit photocatalytic activity for water splitting. [334] DFT-based calculations are a powerful tool for materials design. [335][336][337][338][339][340][341] As known, the traditional local density approximation and generalized gradient approximation functionals usually underestimate the bandgaps of semiconductors. [341,342] In contrast, the HSE06 hybrid functional usually predicts more accurate results of bandgaps concerning experimental results. [97,343] However, HSE06 functional uses the one-particle approximation in band energies' calculations, and there are still systematic errors compared with experimental data. [344] Hence, many investigations have been done to explore the accuracy of the bandgaps and band edge positions of semiconductors calculated by HSE06 hybrid functional. [345][346][347] It is found that HSE06 hybrid functional shows sizable errors in the ionization potential and electron affinity but is much better in MPS and is weaker than other thiol at predicting relative band positions due to error cancellation. [348] Most theoretical results of electronic structures discussed here are calculated by HSE06 hybrid functional. The optical absorption spectra are simulated by converting the complex dielectric function to the absorption coefficient α abs (ω) according to the relation (Equation 3): [341,349] 2 1 abs 1 where ε 1 (ω) and ε 2 (ω) are the real and imaginary parts of frequency-dependent complex dielectric function ε(ω), respectively, due to the tensor nature of the dielectric function, ε 1 (ω) and ε 2 (ω) are averaged over three polarization vectors (along x-, y-, and z-directions). In addition, the light adsorption calculated with enhance O 2 water oxidation by increasing the interfacial contact between Ta 3 N 5 and CoO x ., Reproduced with permission. [323] Copyright 2015, John Wiley & Sons, Ltd. b) To enhance charge transfer between Fe 2 O 3 and BiV 1-x Mo x O 4 , graphene was used as a conductive interlayer. Reproduced with permission. [324] Copyright 2012, American Chemical Society. c) The advantages of 2D layered stack structures in both large interfacial area and short chargediffusion distance from interface to surface. Reproduced with permission. [312] 10 . Reproduced with permission. [325] Copyright 2007, Chemical Society of Japan. g) In a semiconductor-based junction, interfacial band bending occurs, Reproduced with permission. [312] Copyright 2015, The Royal Society of Chemistry.

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HSE06 method can only serve as a guide in searching photocatalysts under one-particle approximation. An accurate light absorption spectrum will be obtained by taking excitonic effects into account, for example, the GW plus BSE approach. [341,350,351]

Environmental Remediation
As a result of the above, 2D materials demonstrated enormous photocatalytic benefits due to their microstructure, bandgap, electronic configuration, and surface composition. Additionally, as previously discussed, engineered defects can be used to modulate light absorption, electrical conductivity, electronic structure, carrier concentration, and interfacial catalysis mechanism, indicating an immense potential for enhancing photocatalytic action for various applications. The advanced photocatalytic activity in water oxidation, H 2 processing, CO 2 reduction, N 2 fixation, organic synthesis, H 2 O 2 production, and contaminants removal over defective 2D materials have been explored in this section (also see Table 1).

H 2 O Oxidation
Photocatalytic water splitting was already observed as a potentially game-changing strategy for producing safe and renewable hydrogen, with the largest energy density, has been identified as a possible energy carrier for storing energy from the sun in chemical bond energy between two H atoms. Due to the slow dynamics of the four-hole, half-reaction mechanism in water oxidation, it was critical for optimum splitting performance. As a result, increasing the efficiencies for light-derived H 2 O oxidation application by reasonable photocatalyst structure design is highly desirable. To design vis-light active semiconductors for H 2 O splitting, the bandgap and band positions must be optimized, charge separation must be adequate, charge movement must be easy, and the semiconductor must be durable in aqueous solutions. 2D architecture with a high defect density may represent an ideal structure for increasing O 2 generation activity.
Additionally, an appealing design strategy for meeting these criteria is to combine defective 2D materials (i.e., graphene, MoS 2 , and g-C 3 N 4 ) with suitable semiconductors. The photocatalytic separation of water into its components is observed as the Holy Grail of chemistry because it requires merely renewable energy sources, photocatalysts as a medium, and H 2 O as a reaction source. Although a significant move forward has been made, the efficacy of water splitting is still limited in the majority of photocatalytic methods. Generally, H 2 O oxidation is a slow and inefficient process in photocatalytic H 2 O splitting schemes due to the complex four h + s redox method. As a result, it is critical to propose a photocatalyst with a robust solar H 2 O oxidation system. Given this, Di et al. engineered rational pit defects in 2D BiOCl nanosheets by partially digging pits on previously prepared BiOCl nanosheets with ethylene glycol. [393] TEM and STEM images clearly showed the engineered pit defects on the (001) exposed facet (Figure 22).
According to DFT calculations, photogenerated e − s will move to gravitate toward the (001) facet in BiOCl, while h + s will migrate in the (110) direction. Because O 2 production is an absolute hole participating reaction, this will appear on the (110) facet, even though the h + s long migratory path to the (110) facet and the h + s distance towards the facet will eventually include significant e-h recombination. Furthermore, the introduced pit defects reduced the migration distance of h + s, thus increasing its h + s utilization. According to the DFT measurement, the engineered pit defects often marginally improve DOS at CB and VB edges, thus raising carrier concentration and facilitating electron excitation. Additionally, the abundance of unsatisfied chemical bonds accompanying defects created a favorable chemical atmosphere for reaction Figure 21. Band-edge positions of 2D materials compared to redox potentials of water: a) group-III monochalcogenides. Reproduced with permission. [79] Copyright 2013, American Chemical Society. b) TMDs. Reproduced with permission. [328] Copyright 2013, American Chemical Society, and c) metal phosphide trichalcogenides relative to the vacuum level. Reproduced with permission. [333] Copyright 2014, AIP Publishing LLC. Adv. Mater. Interfaces 2023, 10, 2202172 www.advmatinterfaces.de molecules to chemisorb and fostered photocatalytic water oxidation reactions. So, the pit-rich BiOCl nanosheet could generate O 2 at a rate of 56.85 mol g −1 h −1 , between 3 to 8 times faster than the BiOCl nanosheet and bulk BiOCl. In another study, ultrasonic exfoliation of lamellar hybrid intermediate (Zn 2 Se 2 )(propylamine) resulted in the formation of 4 atomic thin freestanding single-layers ZnSe. [250] Although the size of ZnSe had been atomically reduced, the local atomic structure had undergone remarkable improvements. Simultaneously, the SeSe bond lengths increased from 4.012 to 4.11. These findings established surface distortion in the single-layer structure, which decreased surface energy and exemplary stability of fabricated structures' single-layers. Additionally, surface deformation can increase DOS at the CB tip, ensuring an even higher charge carrier transfer rate. ZnSe single-layer exhibit high light-harvesting, improved exciton separation, and lower resistance to charge carriers due to their single-layer configuration with surface defects. As a result, single-layers ZnSe nanosheets demonstrated a 195-fold increase in photoconversion efficiency for H 2 O oxidation following Xe lamp irradiation relative to bulk ZnSe.
Correspondingly, defects engineered in other 2D photocatalysts, including Vo confined in In 2 O 3 , pits formed in WO 3 nanosheet, or surface distortions formed in ZnSe, SnS 2 , and SnS nanosheet, will show superior photocatalytic water oxidation behaviors. [237,251,394] Liu et al. developed a variety of pore structures in WO 3 nanosheets using a rapid heating technique on previously exfoliated WO 3 •2H 2 O nanosheets. [395] Given that the photogenerated h + s migration direction was along 001 facets in X-direction in W-O-W chains, the photogenerated h + s almost certainly experienced many charge carrier recombination, severely impairing damaging photoconversion efficiency. The pores formed effectively shorten the diffusion path of h + s and promote H 2 O oxidation to form O 2 at the WO 3 surface. Besides that, an abundance of dangling bonds along the pore environment provided favorable conditions for facile chemisorption of molecular reactions, which increased O 2 evolution kinetics. Photocatalytic H 2 O oxidation efficiency was increased by 18 times when pore-rich WO 3 . Moreover, nanosheets were compared to bulk WO 3 . It demonstrates an essential technique for increasing conversion efficiency with a 2D structure for photocatalytic H 2 O oxidation.

H 2 Evolution
Although photocatalytic solar energy conversion to hydrogen fuels is an ideal approach for future energy sustainably, the relatively low energy conversion efficiency still greatly limits its potential practical applications. The photocatalytic hydrogen production efficiency can be significantly improved by virtue of 2D configuration coupled with abundant surface defects. [46,396] Peng et al. [397] recently performed vacancy-induced 2H@1T MoS 2 phase incorporation on ZnIn 2 S 4 to boost photocatalytic hydrogen evolution. They discover the synergistic regulations of both structural and electronic benefits by introducing sulfur vacancies in a 1T-MoS 2 nanosheet host to prompt the transformation of the surrounding 1T-MoS 2 local lattice into a 2H phase, leading to dramatically enhanced photocatalytic hydrogen evolution activity. Multiple in situ spectroscopic and microscopic characterizations combined with theoretical calculations demonstrated that in plane sulfur vacancies as active sites could activate the proton. At the same time, the 2H@1T-MoS 2 phase incorporation can effectively regulate the electronic structure and further improve the conductivity. Therefore, the optimized ZnIn 2 S 4 @MoS 2 photocatalyst achieves a high photocatalytic hydrogen evolution activity of 23233 µmol g −1 with an apparent quantum yield of ≈5.09%. [46] The PEC measurements were performed to investigate the promoted interfacial charge separation. As presented in Figure 23a, the smaller arc radius of electrochemical impedance spectroscopy (EIS) Nyquist plot of ZnIn 2 S 4 @MoS 2 (4.8 at%) electrode than that of bare ZnIn 2 S 4 showed a much lower interfacial charge transfer resistance. Through fitting the EIS by equivalent circuit, charge transfer resistance (R ct ) of ZnIn 2 S 4 @MoS 2 electrode (18.9 kΩ) was much smaller than that of pristine ZnIn 2 S 4 (62.6 kΩ), suggesting an improved charge transfer efficiency in the presence of 2H@1T-MoS 2 phase-incorporation. [398,399] In addition, ZnIn 2 S 4 @MoS 2 (4.8 at%) electrode also showed an enhanced transient photocurrent response, compared with pristine ZnIn 2 S 4 counterparts under the identical conditions, which further confirmed the favored interfacial charge transfer (inset, Figure 23a). To further investigate the impact of 2H@1T-MoS 2 phase incorporation on charge separation and transfer property, Mott-Schottky measurement was employed (Figure 23b). The flat-band potential of ZnIn 2 S 4 @MoS 2 was measured to be -0.80 V versus normal hydrogen electrode (NHE), which indicated the MoS 2 loading had little influence on the conduction band edge of n-type ZnIn 2 S 4 . [400,401] The result also implied that the intrinsic sulfur vacancy defects of ZnIn 2 S 4 were homogeneously distributed in the surface region of ZnIn 2 S 4 .
Zhang et al. [402] developed a smart strategy to position MoS 2 QDs at the sulfur vacancies on a Zn facet in monolayered ZnIn 2 S 4 (Vs-M-ZnIn 2 S 4 ) to craft a 2D atomic-level heterostructure (MoS 2 QDs@Vs-M-ZnIn 2 S 4 ) as shown in Figure 24a. The electronic structure calculations indicated that the positive charge density of the Zn atom around the sulfur vacancy was more intensive than other Zn atoms. The sulfur vacancy confined in monolayered ZnIn 2 S 4 established an important link between the electronic manipulation and activities of ZnIn 2 S 4 . The sulfur vacancy acted as electron traps, prevented vertical transmission of electrons, and enriched electrons onto the Zn facet. The sulfur vacancy-induced atomic-level heterostructure sewed up vacancy structures of Vs-M-ZnIn 2 S 4 , resulting in a highly efficient interface with low edge contact resistance. Photogenerated electrons could quickly migrate to MoS 2 QDs through the intimate ZnS bond interfaces. As a result, MoS 2 QDs@Vs-M-ZnIn 2 S 4 showed a high photocatalytic hydrogenation activity of 6.884 mmol g −1 h −1 , 11 times higher than 0.623 mmol g −1 h −1 for bulk ZnIn 2 S 4 , and the apparent quantum efficiency reached as high as 63.87% (Figure 24b,c). [402] The holey carbon nitride nanosheets with carbon vacancies have been controlled and prepared by NH 3 treating of bulk counterpart at 510 °C (Figure 24d-g). [403] The formed defect-rich ultrathin structure can effectively tune the electronic structure with reduced bandgap and upshifted CB and VB positions. At the same time, a higher donor density and remarkably increased charge separation efficiency can be acquired in defective carbon nitride nanosheets. Compared with bulk carbon nitride, the defective carbon nitride nanosheets can display roughly 20 times improved photocatalytic hydrogen production activity under visible light (λ > 420 nm) irradiation, reaching 82.9 µmol h −1 , with no decrease in the hydrogen production rate during a 9 h measurement. In another case, by the cooperative utilization of ultrathin structure and surface V O , the photocatalytic hydrogen production rate of defective K 4 Nb 6 O 17 nanosheets can be significantly improved to 1661 µmol g −1 h −1 , about 7 and 21 times higher than defect-free K 4 Nb 6 O 17 ultrathin nanosheets and defect-free bulk K 4 Nb 6 O 17 , respectively. [254] This hydrogen production rate is the optimal report for defective 2D materials under visible light. The sulfur vacancies can also observe a similar function in a ZnIn 2 S 4 monolayer to build MoS 2 quantum dot/ZnIn 2 S 4 monolayer heterostructure with outstanding photocatalytic hydrogen production performance. [402] Wang and co-workers investigated the photocatalytic H 2 evolution of few-layered MoS 2 to C 3 N 4 nanosheets, [404] and C 3 N 4 was absorbed in (NH 4 ) 2 MoS 4 during the formation phase. At 350 °C, sulfidation was carried out using an H 2 O solution, followed by sulfidation using H 2 S gas. C 3 N 4 and MoS 2 have similar layer formation that reduces lattice mismatch and aid in the planar MoS 2 slab growth when the surface is C 3 N 4 . In response, an inorganic-organic 2D-2D stacking was established using G-like thin layer heterojunctions. As a result of the few-layered MoS 2 nanosheets, numerous H 2 evolution sites have been established. Additionally, due to the tunneling effect in thin interfacial layers of MoS 2 , these dispersed layers on the C 3 N 4 nanosheets could be more effective than multilayer MoS 2 . Based on the charge distinction between the help and the number of H 2 evolving sites affected by few-layer MoS 2 nanosheets, it was determined that the developed MoS 2 /C 3 N 4 2D junctions used to have superior H 2 generation activity than a pristine sample. Creating a direct Z-scheme heterostructure between g-C 3 N 4 and other semiconductors is a cost-effective way to maximize photocatalytic efficiency. A clear explanation is that the direct Z-Scheme photocatalyst 2D/2D Fe 2 O 3 /g-C 3 N 4 generates H 2 at a rate 13 times that of g-C 3 N 4 . [405] Because of the difference in work functions between g-C 3 N 4 (4.18 eV) and Fe 2 O 3 (4.34 eV), electrons will switch from g-C 3 N 4 to Fe 2 O 3 at the intimate 2D/2D interface. Thus, at the Fe 2 O 3 /g-C 3 N 4 interface, a built-in electric field is created, which becomes advantageous for photoinduced charge carrier transfer and separation.

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Additionally, a direct Z-scheme system that relies on the band structures of Fe 2 O 3 and g-C 3 N 4 has been established. After all, e − s formed in the CB of Fe 2 O 3 will pass through the intimate 2D/2D interface to the VB of g-C 3 N 4 and recombine with the h + s through d-p conjugation, hence impeding inhibiting photogenerated charge carrier recombination. As an effect, photoinduced e − s and h + s accumulate in g-C 3 N 4 in CB and Fe 2 O 3 in VB, respectively. This direct Z-scheme method will not only this direct Z-scheme method increase exciton separation performance but also generates a major driving force for the lightdriven splitting of water, thus increasing the ability of g-C 3 N 4 .
Lei et al. [406] developed a proof-of-concept strategy to enable visible-light photocatalytic activity of wide bandgap Ca 2 Nb 3 O 10 monolayer nanosheet by incorporating RGO nanosheet as a photosensitizer. The Ca 2 Nb 3 O 10 monolayer/RGO 2D-2D nanohybrids exhibit vastly elevated performance in photocatalytic H 2 evolution with a H 2 production rate of 820.76 µmol h −1 g −1 and TCH degradation reactions under the visible light irradiation. The combined experimental and theoretical results demonstrate that the electrons generated from the photoexcited RGO transfer to the Ca 2 Nb 3 O 10 monolayer and then participate in photocatalytic reactions. The constructed RGO sensitized monolayer perovskite photocatalyst nanohybrids are demonstrated as an efficient photosensitizer for enhancing visible light harvesting of wide bandgap semiconductor in solar-energy conversion, and elaborated in details of the charge Figure 23. a) EIS Nyquist plots and the transient photocurrent responses of pristine ZnIn 2 S 4 and ZnIn 2 S 4 @MoS 2 (4.8 at%) (inset, a), b) Mott-Schottky plots of ZnIn 2 S 4 and ZnIn 2 S 4 @MoS 2 (4.8 at%) in the dark, c) in situ DRIFT (diffuse reflectance infrared Fourier transform spectroscopy) spectra of water on the ZnIn 2 S 4 @MoS 2 (4.8 at%) surface with an increasing water amount (water-saturated flow under He for 2 h) and d) with irradiation under 300 Xe lamp for 2 h, and e) proposed mechanism for photocatalytic H 2 production in the ZnIn 2 S 4 @MoS 2 system under visible-light irradiation. Reproduced with permission. [397] Copyright 2021, Elsevier B.V. www.advmatinterfaces.de Figure 24. a) Formation mechanism of MoS 2 QDs@Vs-M-ZnIn 2 S 4 . Nitrogen adsorption-desorption isotherms of b) bulk ZnIn 2 S 4 and c) Vs-M-ZnIn 2 S 4 (insets are the corresponding pore size distribution curves). Reproduced with permission. [402] Copyright 2018, American Chemical Society. d) TEM image of defective carbon nitride nanosheets, e) EPR spectra, f) bandgap structures, and g) photocatalytic H 2 evolution performance, Reproduced with permission. [403] Copyright 2015, John Wiley & Sons, Ltd.

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transfer process of this type of nanohybrid photocatalyst (Figure 25).
In another study, Kwon and co-workers [407] reported a cathode based on WS 2 /p-Si structure for light-driven H 2 evolution; with an increase in thickness, fabricated WS 2 film exhibits a change in color from yellow to brown, and the absorbance of films increases as well. However, no shift in position of absorption peak was observed for the prepared thin films. The highest current density (8.375 mA cm −2 at 0 V) was achieved for 23 nm WS 2 /p-Si sample with the incident photon to current conversion efficiency of 72%. This study validated that the compositing of conventional semiconductors with TMCs such as MoS 2 and WS 2 could effectively attain superior water-splitting performance under light.
Despite the benefit of the 2D/2D heterojunction, charge carrier recombination occurs in the interlaminar space region owing to the weak vdW force in the layers. As a result, it is desirable to prompt an inherent driving force in the 2D semiconductor plane to facilitate photogenerated charge carriers' delocalization near photoexcited locations. The in plane heterostructure configuration generates a strong electric field within the system, guiding photogenic excitons to appropriate redoxactive sites. In-plane heterojunctions between 2H and 1T′ MoS 2 have been fabricated in the MoS 2 monolayer and employed for photocatalytic H 2 production. [408] Via thermal annealing of the MoS 2 nanosheet, the ratio of different MoS 2 phases can be regulated in a systematic manner. The in-plane 1T′ MoS 2 /2H MoS 2 annealed (60 °C) heterostructure showed maximum activity under visible light with an H 2 generation rate of 1500 µmol h −1 g −1 , and the MoS 2 /Al 2 O 3 -60 °C heterostructure revealed high stability in 0.1 m lactic acid aqueous solution. Due to graphene's outstanding carrier mobility, ultrahigh optical transmittance, and high electrical conductivity is touted as a versatile photosensitizer for enhancing photocatalytic reactions when exposed to visible light.

CO 2 Reduction
Due to the continued burning of fossil fuels and the exponential rise in CO 2 levels in the environment over the past decades, the oil crisis and climate change have sparked widespread concern. As a result, natural oil exploitation misuse is a matter of priority. Using an effective semiconductor photocatalyst, oxidation of CO 2 to chemical fuels like CO, CH 4 , CH 3 OH, CH 2 O 2 , and CH 2 O is a practicable way to mitigate the greenhouse effect and address the energy crisis. [409,410] However, converting CO 2 to other products is highly complex and challenging since CO www.advmatinterfaces.de bond in CO 2 has a high dissociation energy of 750 kJ mol −1 and requires the presence of several electrons. The formation of a CO 2 -intermediate through single electron transmission to allow the formation of CO 2 has been identified as rate-limiting step in proton-related reduction process. For initialization, a theoretical potential of -1.9 V versus NHE is needed, and a superior over potential is desired for actually exploited potentials. Since Inoue et al. in 1979 worked on the photoconversion of CO 2 to usable fuels, numerous photocatalysts for CO 2 reduction have been published. [411] Moreover, since CO 2 has strong thermodynamic stability (G = −394.4 kJ mol −1 ), releasing CO 2 to reactive carbon intermediates on the surface of photocatalysts is a major concern. [412] Thus, in addition to light absorption and the transfer and separation of photoinduced e − s and h + s, CO 2 adsorption and activation are critical for photocatalytic CO 2 reduction. Recent research demonstrates that defective 2D photocatalysts have a tremendous capacity for exhibiting exceptional CO 2 photoreduction activity. For instance, by reducing the thickness of ZnAl-LDH nanosheet, V O defects were added, creating Zn vacancy complexes. [234] The formed Zn vacancy complexes will act as traps for CO 2 and H 2 O molecules, promoting charge separation and enhancing CO 2 photoreduction activity to produce CO. Along with anion and cation vacancies, photocatalytic CO 2 reduction may be beneficial. A lamellar hybrid intermediate approach has been used to form Bi 2 WO 6 layers with a single unit cell thickness. [413] To produce oleate ions, sodium oleate formed an electrostatic bond with Bi 3+. Then, lamellar Bi oleate complexes are formed by self-assembling oleate ions in a tail-to-tail/head-tohead bilayer sequence to form a mesostructure. As Na 2 WO 4 was injected and hydrothermally refined, Bi 2 WO 6 was selfexfoliated and formed into a single-unit cell sheet. As synthesized, combined with a 300 W Xe lamp, a single-unit Bi 2 WO 6 layer is used as a photocatalyst for CO 2 photoreduction. The Bi 2 WO 6 powder was suspended in H 2 O along with an incredibly pure CO 2 gas that constantly bubbled in the solution. In the synthesis of CH 3 OH, an average rate of 75 µmol g −1 h −1 was observed over single unit cell Bi 2 WO 6 layers at over a 5 h period, nearly three and a half times faster than the values obtained with Bi 2 WO 6 nanocrystals and bulk Bi 2 WO 6 , respectively. Engineered Zn vacancies in one-unit-cell ZnIn 2 S 4 increased charge separation efficiency by allowing higher charge density and transport. [242] The defect-mediated successful charge separation results in a CO formation rate of 33.2 µmol g −1 h −1 for the Zn vacancy-rich ZnIn 2 S 4 nanosheet, approximately 3.6 times that of Zn vacancy-deficient ZnIn 2 S 4 nanosheet. This other research [414] demonstrated the facile one-step in situ hydrothermal syntheses of a 2D/2D g-C 3 N 4 / NiAl-LDH hybrid heterojunction. The negatively charged g-C 3 N 4 nanosheet can act as nucleation sites for the in situ growth of NiAl-LDH nanosheets, forming an intimate interface between the g-C 3 N 4 and NiAl-LDH nanosheet. The 2D/2D g-C 3 N 4 /NiAl-LDH exhibits a significantly higher CO evolution rate (8.2 µmol h −1 g −1 ) than pure g-C 3 N 4 (1.56 µmol h −1 g −1 ), NiAl-LDH (0.92 µmol h −1 g −1 ), and a physical mixture of g-C 3 N 4 and NiAl-LDH (2.84 µmol h −1 g −1 ). Moreover, the selectivity for CO is nearly 82% for the 2D/2D g-C 3 N 4 /NiAl-LDH (10%) photocatalyst. In addition, the 2D/2D g-C 3 N 4 /NiAl-LDH improved not only CO generation but also the evolution of H 2 and O 2 .
The CO 2 adsorption capability of Bi 2 WO 6 /RGO/g-C 3 N 4 was significantly greater than that of g-C 3 N 4 and Bi 2 WO 6 , owing to the CO 2 molecules' delocalized π-conjugated binding and the broad-conjugated structure of RGO, which established the special π-π conjugation interaction. The increased CO 2 adsorption capacity of the Bi 2 WO 6 /RGO/g-C 3 N 4 could benefit photocatalytic CO 2 reduction. In addition, due to the electric field associated with the 2D/2D interfaces formed in the Bi 2 WO 6 /RGO/g-C 3 N 4 heterojunctions, the 2D/2D interface creation accelerates the migration of charge carriers. Moreover, since the Bi 2 WO 6 , RGO, and g-C 3 N 4 have an intimate interface, photo induced e − s in Bi 2 WO 6 (CB) will rapidly merge with h + s in g-C 3 N 4 (VB) through RGO redox mediator, resulting in the e − s aggregation in g-C 3 N 4 (CB) and h + s in Bi 2 WO 6 (VB). Furthermore, after that, accumulated e − s in g-C 3 N 4 (CB) can be transferred to RGO, owing to RGO's excellent electron conductivity and storage power, resulting in an increased electron density on RGO surface. Thus, CO 2 molecules can be reduced to CH 4 and CO by accumulated e − s on the RGO surface, while the water molecules are being oxidized by h + s on Bi 2 WO 6 (VB) to form O 2 and protons. Formation of 2D/2D/2D Bi 2 WO 6 /RGO/g-C 3 N 4 hybrid Z-scheme heterojunction enhances photocatalytic CO 2 reduction efficiency and H 2 and O 2 generation with up to 92% selectivity for CO/CH 4 . Hou et al. used PO 4 and V O to synergistically enhance the CO 2 photoreduction activity of Bi 2 WO 6 atomic layers. [232] Functionalized Bi 2 WO 6 atomic layers exhibit a CH 3 OH formation average of 157 µmol g −1 h −1 , which is over two and a half times that of Bi 2 WO 6 atomic layers and bulk Bi 2 WO 6 , respectively. After 60 h of reaction time, there is no discernible loss of CO 2 reduction activity over the functionalized Bi 2 WO 6 atomic layers, implying the possibility of realistic solar fuel production.
Besides charge carrier separation, improving the catalyst's CO 2 adsorption capacity is critical to enhancing the photocatalytic CO 2 reduction efficiency. Using Ti 3 C 2 MXene's excellent electrical conductivity and excess of exposed metal sites, Ti 3 C 2 / Bi 2 WO 6 nanosheet was synthesized by 2D Bi 2 WO 6 on the surface of 2D Ti 3 C 2 . [415] The 2D/2D Ti 3 C 2 /Bi 2 WO 6 matrix established a strong interface between Ti 3 C 2 and Bi 2 WO 6 nanosheet, and the O or OH group on the Ti 3 C 2 surface aids Ti 3 C 2 in capturing photoinduced e − s from Bi 2 WO 6 . When exposed to light, it gets excited from Bi 2 WO 6 's VB and then moves to its CB. Since Bi 2 WO 6 's CB potential exceeds Ti 3 C 2 's Fermi stage, photoinduced e − s will then be passed from Bi 2 WO 6 to Ti 3 C 2 via the 2D/2D interface. Additionally, increased specific surface area and Ti 3 C 2 /Bi 2 WO 6 nanosheet pores promoted CO 2 adsorption. CO 2 molecules adsorbed on Ti 3 C 2 surface will react to CH 4 and CH 3 OH with photoinduced e − s. Additionally, the special photothermal conversion property of Ti 3 C 2 will provide energy to activate the catalyst, enhancing photocatalytic CO 2 performance. O 2 can also be formed as an H 2 O by-product during the photocatalytic CO 2 reaction (Figure 26a). However, since this study did not analyze H 2 processing, it is unclear if all photogenerated e − s are included in the CO 2 reduction process. Ye et al. used a basic mechanical mixing technique to combine surface alkalinized Ti 3 C 2 MXene as cocatalysts with commercially available P25, resulting in a significant increase in photocatalytic CO 2 RR. [416] www.advmatinterfaces.de Figure 26. a) For CO 2 to *CH 4 and **H 2 O conversion, the lowest amount of energy paths (PBE/DFT-D3 computations) were explored, catalyzed by Mo 3 C 2 . C, Mo, O, and H atoms are represented by grey, lilac, red, and white spheres, respectively. Reproduced with permission. [417] Copyright 2017 American Chemical Society. b) CO and CH 4 evolution rates photo catalytically over P25, 5Pt/P25, 5TC/P25, and 5TCOH/P25, Reproduced with permission. [418] Copyright 2016, American Chemical Society. c-f) P25 for CH 4 generation and photocatalytic CO 2 RR of c) TiO 2 /Ti 3 C 2 (TT-x) samples and images of TT550 obtained with d,e) FESEM, and f) TT650. Reproduced with permission. [233] Copyright 2015, American Chemical Society. g,h) Photoinduced e-migration technique at g) Ti 3 C 2 / Bi 2 WO 6 heterointerface and h) photocatalytic activity of Ti 3 C 2 /Bi 2 WO 6 at various Ti 3 C 2 /Bi 2 WO 6 mass ratios (0%, 0.5%, 1-5%). Reproduced with permission. [415] Copyright 2018, John Wiley & Sons, Ltd.

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After surface alkalinization, 5 wt% Ti 3 C 2 (OH) 2 -doped P25 (5TC OH/P25) shows a large rise in CH 4 release compared to unmodified 5TC/P25 (Figure 26b). The DFT study showed that CO 2 adsorption energy on TCF (F termination) exceeded CO 2 adsorption energy on TC-OH (OH termination). As a response, CO 2 molecules have been readily adsorbed on the TC-OH wall, creating activated CO 3 2− . Facilitating charging isolation, excellent electrical conductance, sufficient CO 2 adsorption, and activation sites on alkalinized MXene contributed significantly to photocatalytic progress. These findings demonstrated MXene's crucial position as an effective metal-free cocatalyst for synthetic photosynthesis.
In a similar in situ study reported by Xu et al., [419] conductive Ti 3 C 2 was decorated with TiO 2 nanoparticles via thermal annealing for fabricating TiO 2 /Ti 3 C 2 composites to generate CH 4 from CO 2 RR (Figure 26c). At high temperatures, oxidation of Ti 3 C 2 occurred, removing F functional groups and attaching O functional groups. The rice crust morphology of TT550 and TT650 was observed to be completely different from Ti 3 C 2 (Figure 26d-f). The e − s are efficiently transferred to TiO 2 due to Ti 3 C 2 conductivity, whereas the rice crust morphology offers abundant active sites to promote photocatalytic performance. However, straightforward evaluation of photocatalytic behavior for both TiO 2 /Ti 3 C 2 T x systems was difficult because of the morphology difference, various surface modification by MXene, different synthetic routes, and different phases of TiO 2 . Recently, the same group has fabricated a hybrid ultrathin Ti 3 C 2 /Bi 2 WO 6 2D/2D heterojunction (Figure 26g). [420] The separation and transport of photogenic charges are significantly improved due to intensive physical effects and electronic coupling. 2 wt% Ti 3 C 2 -modified Bi 2 WO 6 nanosheets record the highest CH 4 release rate than other stoichiometry (Figure 26h). Moreover, large interfacial contact surfaces of intimate 2D/2D heterojunction offer larger contact areas and shorted diffusion lengths at the interfaces, which generate superior charge mobilities in contrast to 1D/2D and 0D/2D heterojunctions. Intensive research on 2D/2D heterointerfaces has engendered new potential in photocatalyst designs based on layered heterojunctions.

N 2 Fixation
Ammonia (NH 3 ) is not only an essential chemical to provide a variety of important chemicals (fertilizers) but also an important energy carrier and fuel. [421,422] Currently, NH 3 is primarily synthesized in industry through the well-known Haber-Bosch process that needs high pressure and temperature. [423] The photocatalytic NH 3 synthesis from N 2 , H 2 O and daylight was fascinating since this reaction is being performed at room temperature and pressure. The theory of N 2 fixation through photosynthesis is close to that of CO 2 reduction. Under solar light, photogenerated e − s and h + s become excited, and afterward, h + s oxidize the H 2 O to form O 2 and protons, while the e − s reduces the N 2 and protons to NH 3 . [423][424][425][426] However, N 2 fixation seems to be more difficult than CO 2 reduction due to the higher dissociation energy of the NN triple bond (up to 962 kJ mol −1 ) and the poor binding force between molecular N 2 and the catalyst surfaces. [427] Usually, N 2 catalyst conversion is extremely harsh due to N 2 's poor affinity for solid-state catalysts and high-energy intermediates presence. The defect-rich surface, which contains abundant electron donors and active catalytic sites, can encourage photocatalytic N 2 fixation. [428] Latest studies show that 2D photocatalysts, including V O -BiOBr, Bi 5 O 7 I, and MoS 2 , were promising materials for effective photocatalytic N 2 fixation. [418,429,430] However, these 2D photocatalysts demonstrated low activity, indicating that further improvement in photoconversion efficiency is needed. Consequently, various faulty 2D photocatalysts and 2D heterojunction photocatalysts were formed for photocatalytic N 2 fixation during the last few decades.
Previous research has shown that with their excess localized e − s, V O can efficiently capture and activate inert N 2 molecules as electron trap centers. The NN bond length of N 2 can be prolonged to 1.133 Å via an end-on configuration on the O vacancies in (001) facet exposed BiOBr. [430] This bond length is between the triple bond length (1.078 Å) of free molecular nitrogen and the double bond length (1.201 Å) of diazene, suggesting the effective activation of N 2 . Subsequently, e − s from the excited BiOBr's CB could be smoothly injected into N 2 's π-antibonding orbitals, leading to photoreduction to ammonia. MoS 2 nanosheets with sulfur vacancies were developed and subsequently used for N 2 fixation when stimulated by V O -motivated N 2 activation. [430] MoS 2 nanosheets formed NH 3 at a rate of nearly 325 µmol g −1 after 10 h of simulated solar light irradiation. While commercially available bulk MoS 2 is incapable of photocatalytic NH 3 synthesis under identical test conditions, this demonstrates the exclusive MoS 2 nanosheet advantage for N 2 reduction. According to Mott-Schottky spectra, CB locations of MoS 2 nanosheets and bulk MoS 2 have been predicted are -0.35 and -0.24 V, respectively, since they were located below the N 2 thermodynamic reduction potentials by one or two e − s transition. As a result, it has been assumed that N 2 reduction using MoS 2 nanosheets involved multielectron coupled proton transfer. Owing to the n-type semiconductor essence of MoS 2 nanosheets, numerous free e − s exist, and these free e − s can combine with photogenerated excitons to form charge excitons (trions), which have been primarily found around Mo sites. The generated trions contain many e − s in a single-bound state, which is advantageous for multi-emigration reactions. If sulfur vacancies constrain N 2 , this was bound by trions by three Mo atoms following irradiation. A trion-supported sixelectron reduction method is obtained as N 2 is allowed by donating e-s from its bonding orbitals and taking e-s into its antibonding orbitals. Zhang et al. discovered that V O in CuCr-LDH nanosheet could cause sheet distortions, dramatically increasing N 2 chemisorption and favoring charge transfer from LDH to N 2 (Figure 27). [234] As an outcome, CuCr-LDH nanosheet exhibits outstanding photoreduction of N 2 to NH 3. Under UV-vis/sunlight illumination, optimized NH 3 concentration will reach 184.8 or 142.9 µmol L −1 in water at 25 °C, respectively. It was the most active material for NH 3 synthesis identified to date in pure water. After five consecutive cycles, no noticeable decrease in activity can be detected, indicating exceptional photocatalytic stability.
In addition, creating a 2D/2D heterojunction is also a good route to optimizing 2D photocatalyst photoconversion efficiency for N 2 conversion. For example, a 2D p-n heterojunction composed of AgCl/-Bi 2 O 3 nanosheet with a thickness of www.advmatinterfaces.de ≈2.7 nm has been synthesized and demonstrated good photocatalytic N 2 fixation efficiency. -Bi 2 O 3 was a p-type semiconductor in the AgCl/-Bi 2 O 3 heterojunction, whereas AgCl was an n-type semiconductor. Under sunlight, photogenerated e − s and h + s can be excited across AgCl and Bi 2 O 3 , and recombination of e − s and h + s can be hindered by creating an internal electric field within the space charge region. N 2 has chemically adsorbed on the active sites considering the vast unique AgCl/Bi 2 O 3 nanosheet surface areas and V O presence. Moreover, V O could insert photo induced e − s directly into chemically adsorbed N 2 molecules, weakening NN bond and activating N 2 . The active N 2 thus reacted with the H + present in the water to form NH 3 , which then get dissolved in it to form NH 4+ . [432] The 2D/2D g-C 3 N 4 /RGO hybrid heterojunction catalyst produces 42.4 times more NH 4+ than g-C 3 N 4 attributed to electrostatic reaction between g-C 3 N 4 and RGO. [433]

H 2 O 2 Production
H 2 O 2 (frequently used bleach and disinfectant) is a potential liquid fuel H 2 substitute that exhibits improved applications in rocket engines, fuel cells, and other fields because of its high energy density (3.0 MJL −1 ). As H 2 O 2 is more convenient for storing and transportation than compressed H 2 , it possesses great potential for future energy applications. Now, H 2 O 2 is produced mainly by anthraquinone technique and has some technical limitations due to toxic byproducts and high energy consumption. So, it is essential to adopt effective and clean methods for its production. Its photocatalytic production using H 2 O and O 2 has gained significant attention due to its clean, efficient, and safe process. [434] Photocatalytic consists of four electron reaction processes; H 2 O 2 reduction or H 2 O oxidation process should be enhanced to improve selectivity. Like, Wei et al. improved the selectivity of H 2 O 2 over g-C 3 N 4 by oxygen doping through the calcination of DICY and ammonium para tungstate. [435] AFM depicts that the nanosheet of g-C 3 N 4 is 2.1 nm thick, as presented in Figure 28a,b. UV-vis study demon strated the redshift on the absorption spectra compared to the pristine g-C 3 N 4 . XPS, FTIR, 13 C-NMR, and FTIR confirmed the existence of COC and OH groups in sample, which helped to optimize the O 2 absorption. The quantum efficiency of enhanced catalyst is 28.5%, 3.5 time higher than pristine g-C 3 N 4 , as shown in Figure 28c. The results for these catalysts showed good stability and no chance of deactivation over 20 h. DFT calculations and rotating disk electrode technique confirm oxygen doping which helps to generate 1,4-endoperoxide intermediate. This is very important for enhancing the selectivity and efficiency of two-electron ORR for producing H 2 O 2 shown in Figure 28d. Moreover, H 2 O 2 can be produced by modifying defects for g-C 3 N 4 . [436] Xie et al. established a g-C 3 N 4 for maximizing solar energy conversion efficiency with two synergistic N defects. One is responsible for oxygen activation, and the other is important for separation and excitation of photogenerated charges. [436] The mixture of KOH and g-C 3 N 4 is heated to obtain the NH x , catalyst, and N 2C vacancies. This enhanced sample demonstrated 152.6 µmol h −1 efficiency of H 2 O 2 production, 15 times higher than pristine g-C 3 N 4 .
Further, DFT results demonstrated that the decreased bandgap for g-C 3 N 4 and promoted carrier separation is due to the newly generated NH x vacancy. The N vacancy is considered the real active site for oxygen reduction and activation. Single metal atomic catalysts having uniformly distributed reaction  [437] Sb has +3 oxidation state and 4d 10 5s 2 electronic configuration, which form reactive centers with electron-hole pairs.
The O bond and the prevention of O 2 bond from overreaction for generating H 2 O are presented. This report has stated a complete mechanism for producing H 2 O 2 compared to the constructed catalyst shown in Figure 29. Photoelectrons and holes gathered on adjacent N and Sb atoms achieve effective carrier separation. Consequently, O 2 molecules get adsorbed at the Sb site and react with photoexcited electrons to form µ-peroxide under a two-electron reduction. Meanwhile, gathered holes at N atoms of the melem units near Sb sites accelerate water kinetics (oxidation). Without any sacrificial agent, H 2 O 2 photocatalytic production rate is about 12.4 mg L −1 in 2 h under visible light, 248 times more than the pristine g-C 3 N 4 . The determined solar chemical conversion rate and quantum efficiencies are 0.61% and 17.6%, respectively, at 420 nm. Using catalysts with a single atom to regulate the path of the reaction will find a better design for developing improved catalysts for more comprehensive applications.

Antimicrobial
The formation of reactive oxygen species (ROS) during PC will result in mineral, genetic, and protein leakage, resulting in cell death and a decrease in microbial load. Semiconductor photocatalysts based on MOs, for instance, TiO 2 , ZnO, MgO, and WO 3 , as well as nonmetal oxides, for example, g-C 3 N 4 , multiwall carbon nanotube, and GO, have been formed and used to inactivate microbial species. [438] Photocatalysts with a high surface area have more reactive sites on the surface, enhanced charge transfer, and separation efficiencies, all of which contribute to increasing photocatalytic activity. These properties make 2D photocatalysts an appealing prospect for effective microbial species removal from water. Huang et al. demonstrated that the presence of mesoporous g-C 3 N 4 , Escherichia coli K-12 can be successfully eliminated, achieving 100% inactivation efficiency after 4 h of visible region radiation. Moreover, they stated that mesoporous g-C 3 N 4 has a surface area 20 times that of bulk g-C 3 N 4 and that photogenerated h + s on the g-C 3 N 4 surface will aid in bacterial inactivation. [439] Recently, Kang et al. demonstrated the manufacture of visible light active porous g-C 3 N 4 nanosheet using two distinct methods: alternative heating and cooling and bacterial-inspired exfoliation. Porous g-C 3 N 4 nanosheet demonstrated superior water disinfection behavior when disinfecting E. coli due to its broad surface region, small bandgap , and improved transportation ability. [440] Rtimi et al. published a ground-breaking finding for bacterial inactivation using Cu/TiO 2 sputtered films. Also, in the dark, the films showed outstanding bactericidal properties. Cu ions have increasingly enhanced the inactivation process, even under low visible light irradiation conditions. The researchers monitor copper leaching and discover that the levels are only ppb, considered noncytotoxic by human standards. [441] Transition and noble metals are frequently used as dopants to increase the performance of photocatalytic disinfection in 2D materials. A transition metal could introduce an additional energy level into semiconductor material, facilitating electron-hole pair formation and broadening absorption toward the visible spectrum. Electrons migrating from one of these levels to CB require significantly less photon energy than e − s in Figure 28. a,b) AFM images of g-C 3 N 4 and O-doped g-C 3 N 4 . c) Diffuse reflectance absorption spectra (left axis) and apparent quantum yield (apparent quantum yield, right axis) of samples. d) Koutecký-Levich plots the ORR data measured by a rotating disk electrode. Reproduced with permission. [424] Copyright 2019, John Wiley & Sons, Ltd.
www.advmatinterfaces.de original semiconductors. Many transition metal-doped g-C 3 N 4 photocatalytic methods were investigated. However, none of them discusses its photocatalytic microbe's disinfection. The noble metals incorporation into g-C 3 N 4 photocatalysts may improve photocatalytic efficiency by generating charge carriers and extending the photocatalyst's spectral absorption into the visible light. Additionally, the metal species' surface plasmon resonance (SPR) effect can result in charge carriers' generation in g-C 3 N 4 . [442,443] In addition, noble metals can act as an electron sink, facilitating the separation of photogenerated charge carriers and thereby increasing the photoconversion efficiency of g-C 3 N 4 . [444] Specifically, photogenerated e − s is transferred from the g-C 3 N 4 (CB) to metal nanoparticles deposited on the g-C 3 N 4 surface, whereas the photogenerated hole remains on g-C 3 N 4 . This results in a successful separation of photogenerated charge carriers and consequent improvement of the efficiency of photocatalytic disinfection by generating ROS. For instance, Xu et al. demonstrated efficacy against Staphylococcusaureus using Ag-doped g-C 3 N 4 , which was prepared in two distinct ways (hydrothermal treatment and photo-assisted reduction). After 3 h of exposure, g-C 3 N 4 and Ag/g-C 3 N 4 composites inactivated almost 29.6 and 99.4% of bacterial cells, respectively. [445] Moreover, h + and O 2 − species are significant for bacterial inactivation by photocatalysis by Ag/g-C 3 N 4 (Ag/PCNO). Additionally, the high efficiency of Ag/PCNO was attributed to the SPR effect of Ag nanoparticles and the synergistic effect of PCNO molecules. Besides Ag, the combination of both nanoparticles of Au and g-C 3 N 4 may provide exceptional peroxidase activity toward the breakdown of H 2 O 2 to OH radicals and can effectively eradicate G+ and G− bacteria. In addition, it is effective at degrading existing DR biofilms and inhibiting the formation of new biofilms in vitro. Furthermore, g-C 3 N 4 is highly toxic to cancer cells. [446] In another study, Cao's group coated polydiallyldimethylammonium chloride (PDDA), which is a positively charged polyelectrolyte on cysteine (Cys)-modified MoS 2 nanosheets decorated with Ag + for effective bactericidal (in vitro) and rapid wound healing (in vivo) applications. [447] Authors highlighted that rather than being in the nanoparticles form; the decorated Ag was essentially in an ionic state (Figure 30a) that can significantly decrease the Ag wastage and toxicity to living creatures. The prepared Ag + modified nanosheets offered superior Figure 29. Mechanism of photocatalytic H 2 O 2 production. The white, gray, blue, red, and magenta spheres refer to hydrogen, carbon, nitrogen, oxygen, and Sb atoms, respectively. After shining visible light, the photogenerated electrons are localized at the Sb sites (with a blue glow). In contrast, the photogenerated holes are localized at the N atoms at the melem units (with a red glow). Subsequently, the dissolved O 2 molecules are adsorbed (orange arrows) onto the Sb sites and then reduced (blue arrows) via a 2e − transfer pathway by forming an electron µ-peroxide as the intermediate. Simultaneously, water molecules are oxidized (pink arrows) to generate O 2 by the highly concentrated holes on the melem units. Reproduced with permission. [437] Copyright 2021, Springer Nature Limited.
www.advmatinterfaces.de bactericidal performance in contrast to AgNO 3 solution or Ag nanoparticles (Figure 30b,c). Zhang and co-workers loaded TH, an antibiotic, on chitosan-modified MoS 2 nanosheets, as demon strated in Figure 30d. [448] The hybridization of 2D materials with other semiconductor materials is an appealing strategy for increasing the photocatalytic efficacy of these materials. The primary benefits involve broadening absorption into the visible region, separating excitons effectively by shifting e − s from higher to lower CB and h + s from higher to lower VB, and inhibiting photo corrosion of semiconductor materials. By hydrothermal calcination, micron-sized TiO 2 spheres have been enfolded with a g-C 3 N 4 hybrid structure (g-C 3 N 4 /TiO 2 ), and the (g-C 3 N 4 )/TiO 2 hybrid material was able to properly inactivate E. coli within 180 min of exposure to visible light. Likewise, hydrothermal temperature changes can affect photocatalytic inactivation efficiency. [449] A vertically aligned Z-scheme heterojunction was designed hydrothermally, joining g-C 3 N 4 and TiO 2 (anatase) with 001 facets. In  [447] Copyright 2017, American Chemical Society. d) Chitosan-modified MoS 2 nanosheets-TCH scheme for anti-biofilm applications. Reproduced with permission. [448] Copyright 2017, IOP Publishing. www.advmatinterfaces.de comparison to fabricated bare g-C 3 N 4 and TiO 2 , coupled band structures in the composite lead to better photocatalytic bactericidal potential. [450] Visible light enabled importantly, bi-based semiconductor materials gained significant interest in photocatalysis due to their extraordinary crystalline structures and optical absorption properties. Under visible light exposure, the photocatalytic disinfection activity of Bi 2 MoO 6 /g-C 3 N 4 nanosheet composites is enhanced. Moreover, photocatalytic disinfection activity of the Bi 2 MoO 6 /g-C 3 N 4 nanosheet has been enhanced at a Bi 2 MoO 6 loading of 20%. [451] A more comprehensive effort was made to develop a new composite of AgBr-Ag-Bi 2 WO 6 and to investigate the composites' bactericidal properties when exposed to visible light. In comparison to other binary composites, the resulting composite demonstrated efficient inactivation. The TEM photos confirmed the bacterial cell destruction, which was confirmed again by releasing potassium ions. [452] Xia et al. synthesized a monoclinic dibismuth tetraoxide (mg-C 3 N 4 /Bi 2 O 4 ) heterojunction using a simple hydrothermal method. For 1.5 h of visible light penetration, the optimum ratio of g-C 3 N 4 /Bi 2 O 4 in the composite (1:0.5) was capable of inactivating 6 log10 CFU mL −1 of E. coli, E. coli K-12 was much more effective relative to g-C 3 N 4 (1.5 log) and m-Bi 2 O 4 (4 log). [453] In particular, disinfection efficiency has not improved significantly with m-Bi 2 O 4 material, even though excessive m-Bi 2 O 4 can provide a recombination center for excitons, thus decreasing the ability to separate electron-hole pairs.

Removal of Pollutants
Any of the conventional waste treatment methods would trigger severe ecological problems. The oxidation of toxic organic compounds was a hot subject in medicine. Recent years were seen an increase in the study and novel technologies development, including photocatalytic water photodegradation of organic compounds dependent on effective solar energy conversion. [454] Semiconductor photocatalysis is considered one of the most powerful solutions to approach the severe environmental waste and energy crisis. [455,456] The use of photocatalysis tends to become a more attractive than conventional chemical oxidation approaches to decompose poisonous substances into harmless materials. Despite the benefits of large specific surface area, high adsorption potential of organic contaminants, and good light harvesting potential, 2D photocatalysts exhibit substantial application potential in pollutant removal. Moreover, photocatalytic behavior of 2D photocatalysts also desires is increased owing to rapid recombination of photogenerated e − s and h + s. The defective establishing and hybridized 2D photocatalyst is an efficient way to inhibit charge carrier recombination and to facilitate organic contaminants degradation. [42] Photocatalytic wastewater treatment using 2D materials is a well-known and proven technique. [457] Long carbon chains act as a capping reagent in ionic liquids, regulating crystal growth along c-axis. Sequentially, reaction pH was changed to 11, supplying OH − to replace Br and completing the dehalogenation process in the fabrication route of Bi 4  nanosheets demonstrated improved pollutant removal efficiency. Two bulk g-C 3 N 4 thermal treatments form porous g-C 3 N 4 nanosheets with surface carbon defects. [458] The ultrathin structure and surface defects will accelerate carrier separation between the bulk and the surface in a bidirectional fashion. Consequently, when exposed to visible light, the defective porous ultrathin g-C 3 N 4 demonstrated a 25.7-fold improvement in photocatalytic activity for RhB degradation elimination. Zhou et al. used primitive hydrothermal techniques to fabricate a TiO 2 -MoS 2 monolayer hybrid photocatalyst with a 3D-layered structure. [179] The 3D-layered structure comprises a TiO 2 nanobelt core and a MoS 2 nanosheet shell (referred to as TiO 2 @MoS 2 ). It seems to have a greater potential for adsorption and a higher photocatalytic ability for degrading organic dyes such as RhB. The energy levels between MoS 2 and TiO 2 are supposed to be ideal for charge transfer and recombination inhibition of photon-induced e − s and h + s. Zhang and co-workers discovered that V Bi -O′′′ would probably enhance the DOS at CB minimum, thus improving photon reaction and photoabsorption. [459] Simultaneously, V Bi -O′′′ defects in monolayer BiO 2-x can aid in more electron-hole pair separation. Consequently, defective monolayer BiO 2-x exhibits enhanced photocatalytic behavior when exposed to UV, visible, and NIR light for RhB and phenol removal. Mashtalir et al. degraded MB (a cationic dye) and acid blue 80 (AB80) (an anionic dye) using Ti 3 C 2 T x (Figure 31a,b). [460] UV irradiation has been used to accelerate the decay of MB and AB80. In dark, MB concentration decreases due to MB's negatively charged adsorption on Ti 3 C 2 T x surfaces. Following UV irradiation, a significant decrease in MB and AB80 concentrations of 81 and 62% have been observed in the suspended Ti 3 C 2 T x presence. This was observed that over a long period of time, Ti 3 C 2 T x oxidation to TiO 2 in the dissolved O 2 existence was evident, which merits extensive research in this area. Similar to G-TiO 2 nanocomposite, it is hypothesized that Ti 3 C 2 T x -assisted TiO 2 could be a trigger, promoting more development in this direction. Peng's group [224] adopted a hydrothermal route to partially oxidize Ti 3 C 2 to fabricate a composite comprising of TiO 2 with exposed {001}-facet and Ti 3 C 2 (Figure 31c,d).
Coupling a 2D photocatalyst with a cocatalyst is a safe way to promote organic contaminant oxidation. Due to its wide surface area, high adsorption potential, superior electron conductivity, and high thermal stability, RGO was coupled with WO 3 by in situ growth of WO 3 rectangular sheets on RGO. [461] The 2D/2D RGO-WO 3 process utilizes photogenerated e − s to convert O 2 to •O 2 -, and the photocatalyst and water combine to form the •OH radicals. Despite the strong oxidizing properties, the •O 2 and •OH radicals are used to degrade MB and RhB dyes. Compared to the hydroxyl, epoxy, and carboxylic functional groups, www.advmatinterfaces.de the RGO may interact directly with heterocyclic dye molecules. Additionally, interaction between the aromatic rings of the 2D/2D RGO-WO 3 and heterocyclic dye molecules promotes the formation of hydroxyl radicals, which enhances the dye molecules' degradation efficiency. The photodegradation rate of MB and RhB reaches 32 and 85%, respectively. The increased degradation rate of RhB is due to its higher positive potential at the COOH group than MB, which strengthens the bond between •O 2 − /•OH and the RhB. The above results indicate that a 2D cocatalyst with high electron conductivity and appropriate functional groups can likely improve the photodegradation efficiency of organic pollutants. Wang et al. demonstrated the intense photodegradation of RhB by Bi 2 O 2 CO 3 /MoS 2 composites through ultraviolet light irradiation. [182] This was due to the synergy effect between MoS 2 and Bi 2 O 2 CO 3 cocatalyst. The 2D/2D heterostructures (AgIO 3 /g-C 3 N 4 ) have been successfully developed for photocatalytic waste H 2 O treatment after vis light exposure by Li et al. [462] The g-C 3 N 4 nanosheet used as polymeric organic semiconductors has demonstrated superior visible light reaction. The photoconversion efficiency of AgIO 3 /g-C 3 N 4 heterostructures for organic dye degradation was extensively greater than single AgIO 3 /g-C 3 N 4 nanosheets. Significantly, the degradation reaction rate constant of RhB was ≈22.86 times that of the synthesized AgIO 3 /g-C 3 N 4 nanosheet sample in addition to heterostructures consisting of AgIO 3 and bulk g-C 3 N 4 (AgIO 3 /g-C 3 N 4 -B). This demonstrates 2D materials' role in the fabrication of heterojunction photocatalysts. The Z-scheme method is considered a perfect tactic for photocatalysis over the type II heterojunction due to the dual advantages of high redox capability and effective charge separation.
A basic reflux procedure has been used to fabricate a 2D/2D BiOBr/g-C 3 N 4 Z-scheme heterojunction that has been used to photocatalyzed the RhB and bisphenol A degradation. [463] The photocatalytic degradation observed and confirmed the presence of h + , •O 2 , and •OH as oxidative species, with •OH being the most critical species, while h + and •O 2 − had a minor contribution to the photocatalytic degradation reaction. Resulting in a significant gap in the BiOBr/g-C 3 N 4 configuration, excited e − s can pass from the g-C 3 N 4 (CB) to BiOBr (VB). At the same time, h + s can transfer from BiOBr (VB) to g-C 3 N 4 (CB) through 2D/2D interface, referred to as type II heterojunction. Even so, when the BiOBr/g-C 3 N 4 system flows through a type II heterojunction, no •O 2 − or •OH is generated due to a lack of appropriate reduction and oxidation potentials of the BiOBr and g-C 3 N 4 , respectively. A Z-scheme photocatalytic system for the BiOBr/g-C 3 N 4 system has been developed. Thus, photoinduced e − s may pass from BiOBr (CB) to g-C 3 N 4 (VB) and interact with h + s. The e − s in g-C 3   in Ti 3 C 2 T x . Reproduced with permission. [460] Copyright 2014, The Royal Society of Chemistry. Preparation of nanohybrids (001)TiO 2 /Ti 3 C 2 : c) charge transfer process, d) bandgap transition. Reproduced with permission. [224] Copyright 2016, American Chemical Society.
www.advmatinterfaces.de (0.1 wt%) significantly improves photocatalytic activity compared to pristine ZnO nanosheets. As the molecular weight of MoS 2 has been improved from 0.01 to 0.1 wt%, the photoconversion efficiency of ZnO/MoS 2 has been further increased and was decreased after the molecular weight of MoS 2 was increased to 1 wt%. A high concentration of MoS 2 blocks the sunlight that is being used to force photodegradation of MB, lowering the photoconversion efficiency. As a result of the interfacial effect between ZnO and MoS 2 , photogenerated e − s will move from the ZnO-CB to the MoS 2 -CB, significantly improving carrier separation and, thus, catalytic activity. Increased transport and separation of photogenerated h + s and e − s induced by the interfacial effect can be formed by introducing MoS 2 components into MoS 2 /ZnO heterostructures. After being transferred to the catalyst surface, the photogenerated charges react with O 2 and H 2 O, generating numerous reactive radicals (OH and superoxide anion radicals) that degrade dye molecules. In contrast to the heterojunction effect, P-loading induced defects in ZnO nanosheets promote photocatalytic activity by introducing an energy level difference between bandgaps.

Organic Synthesis
Photocatalytic organic synthesis was proposed as a viable method for reviving chemical reactions sustainably. [465][466][467] The prospect of lowering energy costs associated with chemically formed, solar light-driven chemical transformations with the assistance of semiconductors is enormous. Semiconductors may use solar light to generate excitons or heat carriers, which can be used to simulate chemical reactions at the catalyst's surface. Moreover, owing to the quick recombination of photoinduced charge carriers and the high oxidizing potential of the h + s, photoconversion efficiency and selectivity remain insufficient for large-scale implementation of the photocatalytic organic synthesis process. At large scale, the photocatalytic conversion's functionality and selectivity remain challenging. Weak interaction between O 2 molecules and photocatalyst surfaces, especially defect-free surfaces, is a significant factor in the ineffectiveness of photocatalytic organic formations. In addition, there is the issue of low selectivity, which can be produced from photo-generated h + s. Produced h + s have a high oxidizing potential and apply to nonselective over oxidation. [468] It was recently shown that defective 2D materials could selectively synthesize organic compounds under mild conditions. By engineering V O into WO 3 nanosheet, O 2 molecules are effectively activated into superoxide radicals, thus initiating organic aerobic coupling of amines to corresponding imines. [469] HAADF-STEM showed the surface atomic structure of WO 3 that demonstrated continuous and orderly lattice fringes in defect-deficient WO 3 nanosheet but minor lattice disorder and dislocations in defect-rich WO 3 nanosheet. This finding unquestionably established the presence of different defects in two samples. According to XAFS analysis, total coordination number of W atoms in defect-deficient WO 3 is 5.4, which is slightly less than expected value of 6, suggesting a local shortage of O atoms in defect-deficient WO 3 nanosheet. The slightly improved EPR signal strength at 2.002 g in defect-rich WO 3 also showed that V O were designed into the defect-rich WO 3 nanosheet. Based on first-principles calculations, it was determined that chemisorbed O 2 to a coordinatively unsaturated W site switches to a side-on mode upon WO 3 electron charging. The deficient WO 3 then donates 0.72 e − s to the adsorbed O 2 , raising the length of the OO bond from 1.22 to 1.47, favoring the creation of O 2 •− species. The technique effectively converted sunlight into the aerobic coupling of amines to their corresponding imines, achieving a six times increase in kinetic rate over defect-deficient WO 3 . Xiong et al. [469] activated the oxygen molecules to initiate the effective degradation of organic pollutants by generating V O into WO 3 ultrathin nanosheets. In order to fabricate defect-rich WO 3 nanosheets, the primary product (WO 3 ·H 2 O) was calcined at 673 K in an N 2 environment, while atmospheric conditions were adopted during calcination to generate defect-deficient nanosheets. HAADF-STEM results revealed a much smoother and flat surface of defect-rich nanosheets compared to defect-deficient WO 3 nanosheets. Moreover, controlled and continuous lattice fringes were simultaneously observed through atomic-scale HAADF-STEM images (Figure 32).
Xie and co-workers created V O in BiOBr nanoplates to enhance excitons dissociation and molecular O 2 activation. [470] According to DFT calculations, excitons become unstable as they approach V O . Furthermore, femtosecond time-resolved transient absorption spectroscopy showed that the BiOBr sample with Vo (BiOBr-OV) had a slightly longer photoinduced electron recovery lifetime than the BiOBr sample, allowing charge carrier migration for photocatalytic reactions. Remarkably, the superoxide radical produced on BiOBr-OV was highly efficient and selective in aerobic oxidative coupling of amines to imines. In contrast, the imine transformation efficiency was significantly lower in BiOBr. Li and co-workers investigated the BiOCl nanosheet's colloidal structure. [471] Single crystalline BiOCl colloidal (BiOCl C-UT) nanosheets with a thickness of almost 3.7 nm are produced via BiCl 3 hydrolysis in octadecylamine solution and in situ H 2 O formation via reaction in oleylamine and oleate solution. BiOCl nanosheets are also developed hydrothermally and are referred to as BiOCl H-UT nanosheets. The wettability of as-synthesized BiOCl nanosheets was determined using surface H 2 O touch angle measurements. BiOCl C-UT nanosheets exhibited an H 2 O contact angle of 116.3°, indicating that they are hydrophobic. Organic ligands seem to have capped the surface of BiOCl C-UT nanosheets during colloidal development. BiOCl H-UT nanosheets exhibited an H 2 O contact angle of 0°, indicating their super hydrophilic origin. Significant differences between BiOCl C-UT nanosheets and BiOCl H-UT nanosheets can significantly impact photocatalytic organic conversion.
Additionally, there is a high concentration of V O on BiOCl C-UT nanosheets, resulting in heavy absorption in the visible light spectrum. To take advantage of their hydrophobicity and enhanced light-harvesting ability, BiOCl C-UT nanosheets demon strated significantly enhanced photocatalytic activity for converting N-t-butylbenzylamine to N-t-butylbenzylimine. The 78% conversion ratio was observed with BiOCl C-UT nanosheets, while BiOCl H-UT nanosheets converted at a rate of approximately 15% after 1 h of Xe lamp irradiation. Moreover, the BiOCl C-UT nanosheet sample was extensively used to convert secondary amines to their respective imines, improving www.advmatinterfaces.de conversion selectivity and performance. The 2D photocatalysts were demonstrated to be an excellent alternative for photocatalytic organic formation. This approach can extend perceptions of organic conversion methods and pave the way for creating additional excellent organic transformation systems.

Conclusion and Perspective
Engineered 2D materials and their hybridizations are excellent materials for elementary photocatalytic processing and have many possible commercial uses. This systematic research Figure 32. a-f) Study of morphology, g) W L3 edge EXAFS spectra are Fourier transformed concerning commercial WO 3 , h) at room temperature, ESR spectra, i) V O positions in WO 3 lattice are depicted in this scheme, j) after irradiation with >400 nm at 298 K, cyclic analysis for defect rich WO 3 in catalytic aerobic coupling of benzylamine, k) diagram depicts entire light-driven catalytic reaction pathway. Reproduced with permission. [469] Copyright 2016, American Chemical Society.

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focused on significant progress in applying 2D materials for photocatalytic solar conversion. This study summarizes the classification and controlled fabrication process of defective 2D photocatalysts. Following that, methods for modifying the electronic structure of 2D materials and their photocatalytic properties are discussed, including surface and interface engineering and doping. In addition, additional hybridizations of 2D features, including single atoms/2D materials, QDs/2D materials, molecular/2D materials, and 2D-2D stacking compounds, are provided which will further improve photocatalytic properties. Finally, several photocatalytic applications, including H 2 evolution, H 2 O oxidation, CO 2 reduction, N 2 fixation, organic synthesis, antimicrobial activity, and contaminants degradation, were analyzed with a focus on observations into structureperformance relationships. These applications included H 2 O oxidation, H 2 evolution, CO 2 reduction, N 2 fixation, H 2 O 2 production, antimicrobial activity, and organic synthesis. Different surface defect forms, such as anion-cation vacancies, multivacancies, disorders, vacancy associates, pits, and distortions, have been used to adjust the microstructure, atom coordination number, electronic structure, carrier concentration, or electrical conductivity of 2D photocatalysts and thereby improve photocatalytic efficiency. Numerous effective methods for controlling defect formation have been described, including vacuum activation, chemical reduction, ball milling, and ultraviolet irradiation.
Additionally, the critical functions of surface defects in improving photocatalytic activity are suggested, including lowering the molecular activation energy, acting as an active site for direct reaction, increasing light absorption and carrier concentration, and acting as charge separation centers to facilitate surface charge separation. Numerous studies outlined in this section revealed that defective 2D materials with special electronic structures are advantageous for improved photocatalytic efficiency in processing O 2 , H 2 , CO 2 , and N 2 reduction to selective organic synthesis ammonium and pollutant elimination. Furthermore, owing to their special benefit, 2D heterojunctions such as Type I, Type II, Z-scheme, and Schottky heterojunction exhibit high performance in photocatalysis. Separating charge carriers from exposed active sites is critical for optimizing the photocatalytic efficiency of 2D photocatalysts. Constructing 2D/2D heterojunctions is a viable method for promoting charge carrier separation and increasing the percentage of exposed active sites, significantly improving the photocatalytic activity of 2D photocatalysts.
Despite rapid development in 2D materials for photocatalysis, this field faces many obstacles. Apart from the recent advances outlined here, research in this field is still in its infancy; issues and challenges in the design, synthesis, and application of defective 2D photocatalysts remain. Though various top-down and bottom-up approaches were used to synthesize 2D materials beyond graphene, large-scale preparation of 2D materials remains difficult. The mass development of 2D materials with specified surface defects would be critical for photocatalytic applications. To investigate diverse and abundant synthetic strategies for defect-rich 2D materials with atomic-scale thickness on a large scale, more diverse and abundant synthetic strategies should be investigated. Second, in contrast to well-developed 2D materials like hydroxides, MOs, and sulfides for photocatalysis. Some novel 2D materials along with layer oxyhalides (e.g., Bi 4 VO 8 Cl and FeOBr), multi-metal chalcogenides (e.g., C 3 N and C 2 N) and thiophosphates (e.g., CoPS 3 ) should be considered. 2D materials with an intrinsic non-vdW layer structure provide, in particular, tremendous photocatalytic potential, as ample surface atoms between bonds contribute to the formation of an excellent chemical mechanism conducive to molecular reaction chemisorption and catalytic dynamics. Thirdly, many 2D materials, especially those with a defect-rich architecture, would be unstable physicochemically. During the storage and photoreaction processes, isolated nanosheet can endure irreversible aggregation and structural disintegration, resulting in the loss of advanced structural characteristics. Moreover, along with surface V O , certain faults would be filled by ambient H 2 O or O 2 during longterm photocatalytic action, negating the microenvironment's distinct benefits.
Thus, methods for stabilizing these defective 2D photocatalysts must be studied. Fourth, since photogenerated e − s and h + s recombine quickly, sacrificial reagents have been used in the majority of photocatalytic reactions, including H 2 , O 2 evolution, and CO 2 reduction, to achieve high-efficiency half-reactions. It has been shown that using sacrificial reagents enhances the photocatalytic action of photocatalysts. Moreover, as the sacrificial reagents are consumed, the photocatalytic activity reduces, which undermines the application of 2D photocatalysts. Although forming 2D/2D heterojunctions is a viable method of enhancing charge carrier separation, more efficient 2D/2D heterojunctions are mostly needed for useful applications. Fifth, owing to rapid recombination of photogenerated electron-hole pairs, in most photocatalytic reactions, sacrificial reagents could be deployed to accomplish high-efficiency half-reactions, including H 2 evolution, O 2 evolution, and CO 2 reduction. It was shown that the use of sacrificial reagents enhances the photocatalytic action of photocatalysts. Even then, as the sacrificial reagents are absorbed, the photocatalytic activity reduces, which undermines the practical application of 2D photocatalysts. Sixth, a structure-activity relationship study in defective 2D photocatalysts is insufficient; additional research is required to establish an authentic relationship between surface defects, atomic thickness, and photocatalytic activity. Via sophisticated characterization techniques, in situ observation is needed to determine active sites and mechanisms involved during the photocatalytic phase. Ultimately, most tests involving defective 2D materials for photocatalysis remain in the manual trial-anderror stage. Numerous 2D materials with various components and defects can show a variety of photocatalytic properties that make them play an important role in a variety of photocatalytic applications. Bi 2 WO 6 nanosheet, for instance, is an outstanding photocatalyst for CO 2 reduction but is ineffective at producing H 2 . A mixture of ab initio DFT studies and systematic study of photocatalytic production involving different component or defect types can be used to effectively help in the detection of novel defect 2D photocatalysts from unexpected elemental combinations and defect shapes.
To summarize, tremendous progress has been made in the last few years in design of 2D material-based heterostructures, surface alteration, morphology regulation, and element doping. Therefore, the critical challenge in photocatalysis is www.advmatinterfaces.de improving applications such as CO 2 reduction, organic pollutant elimination, and solar fuel processing. Most work remains to be performed in designing and improving better photocatalysts. It can precisely tune the composition, shape, reaction sites on the surface, and bandgap of 2D material photocatalysts to allow interfacial reactions, light trapping, and charge carrier isolation and conversion. In certain cases, hybrid 2D materials resulted in decreased crystallinity and increased cracks, which is detrimental to photocatalytic behavior and charge transfer. As a result, these are critical stages in creating more efficient photocatalysts, such as the investigation of novel doping techniques and the precise regulation of the structure, surface state, and dopant distribution. The properties of photocatalysts are critical for optimizing photocatalytic performance and enabling a future paradigm shift in design principles. Over the last decade, working on 2D materials has advanced at a breakneck rate. Photocatalysts based on 2D materials can be critical in solving the environmental and energy problems associated with photochemical conversion aided by sunlight.
Furthermore, the majority of photocatalysis research is still in the manual trial-and-error stage, with many of the reaction mechanisms unclear. Certain underdeveloped, highly efficient 2D photocatalysts can be ignored due to the limited preparation processes. With the advent of DFT computing and machine learning, increased emphasis must be placed on developing more stable and efficient 2D photocatalysts and heterojunctions in two dimensions. Likewise, environmental considerations about the solar-powered 2D device are important for commercial applications. However, no research is being conducted on this subject at the moment. Biocompatibility evidence for 2D components used in biomedical applications may be used to approximate their environmental impact. While stable 2D binary compounds such as MoS 2 and MXenes are nontoxic, unstable MXenes such as tellurene are harmful. Further investigation of the 2D materials' long-term environmental impact is recommended.
We see the future challenges for the computational screening of materials for photocatalytic applications, mostly in the description of solvation and kinetics. The active development of implicit solvation models and their implementation into widely used software packages is expected to lead to an efficient description of the effect of the electrolyte on electronic properties and stability, including corrosion. A challenge for solvation models and computational corrosion studies is overcoming the complexity of the interaction and chemical reactions between solvated species and 2D materials. Regarding kinetics, computational methods that describe nonadiabatic dynamics and can assess the rate of electron transfer reactions and exciton dynamics are still being developed and not yet routinely applied or available in community codes. The enormous progress in the fields of 2D materials and computational methods suggests that many of those challenges will be tackled in the near future.