Recent Advances of MXenes‐Based Optical Functional Materials

Transition metal carbides/nitrides/carbonitrides (MXenes) exhibit tremendous potential for optical applications due to their diverse elemental composition and adaptable structural properties. Based on introducing the preparation methods and optical properties of MXenes, this review focuses on the latest advances in MXenes‐based optical functional materials, and analyzes the performance enhancement mechanisms of MXenes‐based optical functional materials for photothermal conversion and photocatalysis. The key scientific and technical bottlenecks in the field of MXenes‐based optical functional materials are pointed out, and the future development trends and research directions of MXenes‐based optical functional materials are prospected.


Introduction
21] For instance, intraband and interband electronic transitions directly determine the optical absorption and photon radiation behavior of materials. [22,23]Therefore, MXenes also have unique optical properties and have shown great potential in the field of optical functional materials. [24]owever, there are few systematic reviews on the optical properties of MXenes and corresponding MXenes-based optical functional materials.
This review first introduces the preparation methods and optical properties of MXenes, and then focuses on the latest application progresses of MXenes-based optical functional materials, mainly including fields of photothermal conversion and photocatalysis (MXenes-based photothermal conversion materials can be specifically applied to desalination, wearable device, and photothermal therapy, and MXenes-based photocatalysis materials can be specifically applied to water treatment, hydrogen production, CO 2 reduction, etc.).Moreover, the performance enhancement mechanisms of MXenes-based optical functional materials for photothermal conversion and photocatalysis processes are analyzed in detail, the key scientific and technical bottlenecks in the field of MXenes-based optical functional materials are pointed out, and the future development trends and research directions of MXenes-based optical functional materials are prospected.

Preparation Methods of MXenes
In the MAX phase, the M-X bonds are mainly covalent bonds and ionic bonds, and the bond strength is high. [25]The M-A bonds and A-A bonds are mainly metal bonds, and the bond strength is relatively weak. [26]Therefore, A atoms layer has the highest reactivity and is easier to be etched. [27]Researchers have prepared MXenes mainly by selectively etching the A atomic layer in the MAX phase. [28,29]The main methods include hydrofluoric acid (HF) etching, fluoride salt etching, fluorine-free molten salt etching, etc. [30,31] 2.1.HF Etching HF etching is the earliest method used to prepare MXenes.In 2011, Gogotsi et al. [32] found that HF could selectively etch the Al atomic layer in Ti 3 AlC 2 to form the accordion-shaped Ti 3 C 2 T x with van der Waals forces between layers (Figure 1a).The formation of the accordion-like structure was mainly attributed to the exothermic nature of the reaction and the release of H 2 during the removal of Al atoms.Alhabeb et al. [33] found that the concentration and etching time of HF are important factors affecting the preparation of Ti 3 C 2 T x (Figure 1b-f ).Higher HF concentration could shorten the etching time and improve the preparation efficiency of Ti 3 C 2 T x .However, the higher the HF concentration, the more defects in the obtained Ti 3 C 2 T x nanosheets, such as the smaller size of the nanosheets or the existence of holes in the nanosheets, resulting in reduced mechanical properties and conductive properties.
The HF etching method possesses the advantages of ease of operation and low reaction temperature.The disadvantage is that the etched multilayered MXenes need to be further intercalated by organic reagents (e.g., dimethyl sulfoxide, tetrabutylammonium hydroxide) to weaken the interlayer van der Waals force to peel off to produce few-layered or single-layered MXenes dispersion, leading to higher process complexity. [34]Moreover, the strong etching ability of HF will lead to defects in MXenes nanosheets, in which free carbon may form carbon spheres, affecting the purity and performance of MXenes.More importantly, HF is highly corrosive and toxic with high operational risk and harsh environmental impact.Therefore, it is particularly important to find more effective, milder, and more harmless preparation methods to protect human health and the environment.

Fluoride Etching
The fluoride salt etching is a method that uses fluoride salt and acid as an etchant to generate HF in situ to prepare MXenes.In 2014, Ghidu et al. [35] first used HCl/LiF solution to etch Ti 3 AlC 2 at 40 °C to prepare MXenes (Figure 2).During the etching process, Li þ was inserted into the accordion-shaped Ti 3 C 2 T x layers to expand the interlayer spacing, and could be directly layered into single-layered/few-layered Ti 3 C 2 T x nanosheets through subsequent ultrasonication or shaking treatment.The Ti 3 C 2 T x conductive clay prepared by this method had high electrical conductivity (1,500 S cm À1 ) and strong plasticity, and could be bent into complex shapes.Besides LiF, other fluoride salts have also been used Figure 1.a) Schematic of the exfoliation process for Ti 3 AlC 2 .Reproduced with permission. [32]Copyright 2011, Wiley-VCH.b) Schematic of synthesis of Ti 3 C 2 T x , where DMSO and TMAOH are used to be short for dimethyl sulfoxide and tetramethylammonium hydroxide.SEM images of c) Ti 3 AlC 2 powder, d) 5F-Ti 3 C 2 T x powder, e) 10F-Ti 3 C 2 T x powder, and f ) 30F-Ti 3 C 2 T x powder.Reproduced with permission. [33]Copyright 2017, American Chemical Society.
as etchants.Liu et al. [36] combined HCl and various fluoride salts (LiF, NaF, KF, NH 4 F) into mixed solutions for etching Ti 3 AlC 2 .The interlamellar spacing and surface structures of MXenes could be tuned by changing the type of fluoride salt.Mixing fluoride salts with other acids could also help to prepare MXenes.For instance, Guo et al. [37] found that replacing HCl with H 2 SO 4 can improve the reaction efficiency, and there were -SO 4 functional groups on the surfaces of the prepared Ti 3 C 2 T x .
The advantage of the fluoride salt etching method is that the prepared MXenes have larger sheet size and fewer defects. [38]In addition, since cations (such as Li þ and NH 4þ ) and water molecules can be inserted between MXenes nanosheets at the same time, the interlayer spacing of the obtained MXenes is larger than that of the above-mentioned MXenes prepared by HF etching, which is beneficial to the subsequent preparation of few-layered MXenes nanosheets. [39,40]Therefore, the fluoride salt etching method is more suitable for preparing MXenes with high electrical conductivity and large sheet sizes. [41]However, this method also has some disadvantages.For example, although the use of fluoride salt avoids the direct use of HF, the presence of H þ and F À in the solution will still lead to the release of HF, which is still harmful to human health to a certain extent.

Fluorine-Free Molten Salt Etching
Transition metal halides can react with Al atomic layers of MAX phase in the molten state to prepare MXenes.Huang et al. [42] used ZnCl 2 /NaCl/KCl mixed molten salt system to etch Ti 3 AlC 2 under nitrogen atmosphere.During the etching process, Zn 2þ reacted with Al atoms in Ti 3 AlC 2 , and the weakly bonded Al atoms were transformed into Al 3þ .The reduced Zn atoms then occupied the position of the Al atomic layer to form Ti 3 ZnC 2 .Finally, the excess ZnCl 2 etched the Zn atoms in Ti 3 ZnC 2 and eventually generated Ti 3 C 2 T x .Studies have shown that the ratio of ZnCl 2 and Ti 3 AlC 2 has a significant impact on the qualities of the final products.Since the process is fluorine-free and e) Fracture surface of a thicker rolled film (30 μm).TEM images of f ) single-and g) double-layer flakes, respectively.Insets show sketches of these layers.Reproduced with permission. [35]Copyright 2014, Springer Nature.nonaqueous, the Ti 3 C 2 T x surface obtained by ZnCl 2 etching was full of -Cl functional groups instead of -O, -F, and -OH functional groups.The types and properties of the surface functional groups of MXenes can be changed by adjusting the type of molten salt.Li et al. [43] used CuCl 2 /CuBr 2 /CuI mixed molten salt system to etch Ti 3 AlC 2 to prepare Ti 3 C 2 T x (T = Cl, Br, I, Figure 3).By adjusting the type and mass ratio of CuCl 2 / CuBr 2 /CuI, the functional groups on the surface of the prepared MXenes could be adjusted and controlled.
The advantages of the fluorine-free molten salt etching method are that it is chemically safer and can replace or eliminate the surface functional groups of MXenes with molten inorganic salts, which provides ways for designing and preparing MXenes with specific surface functional groups. [44]The disadvantages are that the operation usually requires high-temperature treatment and an extremely excessive amount of salt, and it is not easy to extract MXenes from the product.However, the overall research of this method is in the early stage, and more in-depth research on the physicochemical properties of the prepared MXenes is necessary.

Other Methods
To achieve a more green, efficient, and safe preparation of MXenes, researchers have been continuously explored and developed various new synthesis methods and processes.Shi et al. [45] used I 2 to etch Ti 3 AlC 2 in the 100 °C anhydrous acetonitrile (CH 3 CN) to form the accordion-shaped Ti 3 C 2 I x , followed by using dilute HCl to remove AlI 3 generated during the etching process.The instability of the -I functional group on the surface allowed the conversion of -I into -OH and -O in dilute HCl solution, thereby obtaining fluorine-free Ti 3 C 2 T x .Jawaid et al. [46] etched Ti 3 AlC 2 in a halogen organic solvent, and the obtained Ti 3 C 2 T x had uniform -Cl, -Br, or -I functional groups on the surface.Since the etching process was carried out in an inert gas atmosphere and the final product was stored in the tetrahydrofuran solvent, no transformation of the halogen groups on the surface occurred.Talapin et al. [47] used metal Ti, titanium chloride (TiCl 3 or TiCl 4 ), and carbon or nitrogen sources (including graphite, CH 4 , and N 2 ) as precursors to prepare a variety of MXenes by chemical vapor deposition (CVD, Figure 4).This method avoided the generation of hazardous waste associated with the etching step, improved the preparation efficiency, and was expected to achieve large-scale preparation at an industrial scale.Hart et al. [48] prepared Mo 2 TiC 2 T x by etching Mo 2 TiAlC 2 in HF and delaminating by tetrabutylammonium hydroxide (TBAOH) intercalation, which reduced the concentration of -F termini and resulted in tetrabutylammonium (TBA þ ) and H 2 O embedded.TBA þ is a large organic ion that can significantly increase chip spacing and resistance.
Table 1 summarizes the preparation methods of MXenes and their advantages and disadvantages.

Light Absorption
MXenes have excellent light absorption capabilities across the entire solar spectrum.The bandgap determines the range of photon energy that a kind of materials can absorb. [49]When the photon energy is higher than the bandgap, the photon energy is large enough to overcome the bandgap and excite electrons to .Reproduced with permission. [43]Copyright 2021, American Chemical Society.
transit from the valence band to the conduction band, endowing the materials with the ability to absorb light of this energy. [50]Xenes usually have a relatively small bandgap and thus can absorb a wide range of photon energy. [51]In contrast, other materials may only absorb light energy in specific narrow wavelength ranges.Second, there is a charge transfer effect between metal ions and carbon ions in the layered structure of MXenes, which can promote light absorption and electron transmission, and enhance the light absorption ability of MXenes. [52]Moreover, the metal atoms in the metal-carbon layer of MXenes can excite the surface plasmon resonance (SPR) effect.SPR is an interaction between electrons and photons that enhances light absorption. [53,54]In addition, the energy level distribution of MXenes can provide enough excited states to enable light to be absorbed efficiently.Its energy band structure can adjust the electron transfer and energy transmission between the metal layer and the carbon layer, and enhance the light absorption. [55]urthermore, MXenes have multiple parallel metal-carbon layers, and the interlayer gap can produce multiple reflection and scattering effects, which increases the propagation path of light in MXenes, thereby absorbing more light energy.In addition, studies have found that the type of functional groups on the surface of MXenes can also affect its light absorption properties. [56]Compared with the -F and -OH functional groups, Reproduced with permission. [47]Copyright 2023, AAAS.
the -O functional group is more electronegative, and the energy bandgap generated by the MXenes with the surface functional group -O will also be larger (Table 2).Therefore, in the low optical frequency range (usually infrared and microwave bands), MXenes with surface functional groups -O may have lower optical absorption.Compared with MXenes with -O surface functional groups, the metal features of MXenes without surface functional groups and with -F and -OH functional groups are beneficial to further enhance their light absorption in the low optical frequency range.In the visible spectral range, the -O functional group can introduce new electronic states and energy levels, increasing the light absorption capacity in the visible spectral range.Therefore, materials with surface functional groupsexhibit stronger visible light absorption, while MXenes with surface functional groups -F and -OH show weaker visible light absorption.In the ultraviolet spectral range, the photon energy is higher and matches the energy bandgap, so the microstructure changes caused by all surface functional groups may promote the interaction between photons and electrons and increase the light absorption intensity.

Nonlinear Optical Properties
MXenes also have excellent nonlinear optical properties. [57]The nonlinear optical effect refers to the polarization of the medium induced by a strong oscillating electric field under strong laser irradiation. [58]The polarization not only oscillates with the frequency of the applied light field, but also produces high-order harmonic oscillation, and even has a direct current electric field component.60] It has been found that at a given wavelength, the effective nonlinear absorption coefficient of Ti 3 C 2 T x decreases with the pulse energy. [61]It is shown that at low pulse energies, the one-photon saturable absorption process dominates.As the pulse energy increases, the two-photon absorption process will also occur. [62]

Localized Surface Plasmon Resonance Properties
Moreover, MXenes exhibit longitudinal and transverse localized surface plasmon resonance (LSPR) properties in the visible and near-infrared (NIR) bands. [63]There are many free electrons on the surface of MXenes.When the frequency of the incident light matches the resonant frequency of the surface electrons of MXenes, the electric field of the incident light can resonate with the surface electrons of MXenes and excite the resonant vibration HF is highly corrosive and toxic, posing health and environmental risks.
Susceptible to defects.

Fluoride etching
MXenes have larger sheet sizes and fewer defects.
Presence of H þ and F À in the solution still releases HF to some extent, impacting health.Allows for tuning surface functional groups.

Fluorine-free molten salt etching
Chemically safer, can replace or eliminate surface functional groups with molten inorganic salts.
Requires high-temperature treatment and a large amount of salt, not easy to extract MXenes from the product.
Provides a way to design and prepare MXenes with specific surface functional groups.

Chemical vapor deposition
Large transverse sizes and fewer defects.High fabrication cost limits the massive commercialization.

Electrochemical etching
High yield (over 90%) and no -F termination.
Requires special electrochemical equipment, electrolyte solutions, and relatively complicated operation.
Table 2. Calculated bandgaps of functionalized MXenes based on conventional (Perdew-Burke-Ernzerhof, PBE) and hybrid (Heyd-Scuseria-Ernzerhof 06, HSE06) functionals. [22]enes mode of the surface electrons.The electric field generated by this resonant vibrational mode then interacts with the incident light to form a localized electromagnetic field enhancement region called the plasmon resonance region. [64]The LSPR effect results in a change in the interaction of MXenes with light in a specific wavelength or frequency range.For example, at the resonant frequency, MXenes can significantly enhance the ability to absorb light, and the absorbed light energy is converted into heat, which is dissipated into the surrounding medium through vibrations scattered by the lattice, thereby increasing the surrounding temperature.
The optical properties of MXenes are highly dependent on the electronic structure such as energy bandgap, direct/indirect bandgap, topological properties, etc., and these electronic structures are affected by many factors, such as chemical composition, crystal structure, size, surface chemical groups, applied electric field, stress/strain, doping, electronic localized state, etc. [65][66][67] Therefore, it is very difficult to accurately determine the energy level structure of specific MXenes materials and their related optical properties.

MXenes-Based Photothermal Conversion Materials
Photothermal conversion is a way to directly acquire and utilize solar energy, in which the incident light energy is absorbed by materials and converted into heat energy for further utilization. [68]The main mechanism of photothermal conversion is that after the material absorbs the energy irradiated by sunlight, the vibration of the internal lattice or the oscillation of the electrons will increase the temperature of the material.Solar energy is a kind of inexhaustible, inexhaustible, low-cost, and clean energy on the earth. [69]The development of solar energy utilization can alleviate the threat of energy crisis caused by the excessive use of traditional fossil fuels, and is conducive to the formation of a green and sustainable energy system. [70]3]

Desalination
With rapid population growth and accelerated industrialization, people are increasingly concerned about the shortage of freshwater resources.Solar desalination technology, which uses sunlight to remove dissolved salts from seawater, is one of the effective solutions. [74]In recent years, researchers have proposed an interface solar evaporation method, which absorbs solar energy through photothermal conversion materials and converts it into heat energy at the air/water interface and locally promotes the evaporation of seawater.The photothermal material located on top of the water surface acts as a solar absorber and a steam evaporator to efficiently generate water vapor to collect fresh water.MXenes not only have excellent light-to-heat conversion performance, but also obtain excellent hydrophilicity and layered structure to enable rapid and selective migration of water molecules, providing active sites for gas escape.Therefore, MXenesbased materials are ideal photothermal materials for desalination for interfacial solar evaporation.
Wang et al. [75] used polyvinylidene fluoride (PVDF) as the substrate, and stacked Ti 3 C 2 T x layers on the PVDF substrate by vacuum-assisted filtration to obtain Ti 3 C 2 T x /PVDF composite films.The microstructures formed between the two-dimensional materials could be used as water channels.The water below was continuously pumped to the evaporating surface by capillary action.Polystyrene (PS) foam was fixed under the PVDF substrate as a heat insulation layer, so that the evaporation device could float freely on the water surface, and accumulate heat conversion energy at the liquid-gas interface, effectively promoting the process of seawater evaporation and desalination (Figure 5a).With the nearly 100% internal light-to-heat conversion efficiency of Ti 3 C 2 T x (η = (P/A)/I, where η represents the photothermal conversion efficiency, P represents the optical power absorbed by the sample, A represents the area irradiated by light, I represents the intensity of light, the temperature of Ti 3 C 2 T x /PVDF composite films was as high as 75 °C under the irradiation of 1 sun light intensity, which was higher than that of pure PVDF substrate (30 °C).Through reasonable material selection and design, this evaporation device could achieve solar-water evaporation conversion efficiency up to 84%, that is, each square meter of photothermal film could produce about 1.4 kg of fresh water per hour.Zhang et al. [76] prepared the Janus Ti 3 C 2 T x aerogel with vertically arranged pore structure by directional freezedrying.The Janus Ti 3 C 2 T x aerogel had the properties of one side being hydrophobic and the other side being hydrophilic.Under light conditions, the best evaporation rate of Janus Ti 3 C 2 T x aerogel was 1.46 kg•m À2 h À1 , and the evaporation efficiency was 87%.The reason was that the hydrophilic bottom layer of Janus Ti 3 C 2 T x aerogel provided abundant channels for seawater to transport to the entire evaporation surface.The LSPR effect of Ti 3 C 2 T x enabled the upper hydrophobic layer to fully absorb sunlight and convert it into heat energy, promoting the rapid evaporation of water.Then, the salt ions such as Na þ , Ca 2þ , K þ, and Mg 2þ quickly precipitated and stayed in the bottom hydrophilic layer to achieve efficient desalination (Figure 5b).Zha et al. [77] prepared Ti 3 C 2 T x /CNF composite films by coating Ti 3 C 2 T x on the surface of cellulose nanofibers (CNF) by spraying process (Figure 5c-h).The Ti 3 C 2 T x /CNF composite films had strong light absorption ability and exhibited excellent light-to-heat conversion and water transport properties.The evaporation rate under 1 solar radiation was 1.44 kg m À2 h À1 , and the evaporation efficiency was 85.8%.

Wearable Device
83] Wang et al. [84] prepared multifunctional Ti 3 C 2 T x /PET textiles (M-textiles) by decorating Ti 3 C 2 T x on the surface of polyethylene terephthalate (PET) fabrics through the solution dip-coating process (Figure 6a-d).M-textiles not only maintained the inherent flexibility, comfort, lightweight, and permeability characteristics of textile substrates, but also exhibited excellent photothermal performance.Under the irradiation of 100 W NIR light (780 nm), the M-textiles exhibited a wide temperature range of 40-204 °C within 3-10 cm.At the same time, the wearable M-textiles showed excellent photothermal stability in the test of cycling and long-term irradiation of NIR light.Wei et al. [85] prepared Ti 3 C 2 T x /waterborne polyacrylate coatings by solution blending, and then painted them on the leather surface.The surface temperature of Ti 3 C 2 T x /waterborne polyacrylate-coated leather was 5.4 °C higher than that of polyacrylate-coated leather under solar radiation.The main reason was that the light-to-heat conversion performance of Ti 3 C 2 T x made it generate heat itself.At the same time, the infrared emissivity of Ti 3 C 2 T x was low, and it was not easy to radiate and lose heat.Fan et al. [86] used silver nanoparticles (AgNPs) modified Ti 3 C 2 T x as photothermal fillers to prepare AgNP@Ti 3 C 2 T x / poly urethane (PU) composites (Figure 6e).The results showed that when the amount of AgNP@Ti 3 C 2 T x was 0.16 wt%, the temperature of the AgNP@Ti 3 C 2 T x /PU composites could rise to 106 °C when the light intensity was 600 mW cm À2 for 1 min, and the cracks could be completely healed after 5 min of light irradiation.Self-healing transparent AgNP@Ti 3 C 2 T x /PU composites had great potential in the field of photothermal wearable devices.In our previous Figure 5. a) Schematic of a high-efficiency solar steam generation system.Reproduced with permission. [75]Copyright 2019, American Chemical Society.b) Schematic of a high-efficiency solar steam generation system and the salt resistance strategy.Reproduced with permission. [76]Copyright 2019, American Chemical Society.c) Schematic illustration of the design of concept for the Ti 3 C 2 T x /cellulose film steam generator.d) Digital image of Ti 3 C 2 T x /cellulose film (diameter 15 cm and thickness 0.2 mm).The inserted image of a small flower folded with the Ti 3 C 2 T x /cellulose film exhibits good flexibility.e) Schematic illustration of the steam generator comprised of photothermal film as well as floating and insulating layers.f ) Water mass changes of bulk water, rGO/cellulose, and Ti 3 C 2 T x /cellulose films under the solar illumination of 1 sun.g) Water evaporation rates and solar steam efficiency of bulk water, rGO/cellulose, and Ti 3 C 2 T x /cellulose films under the solar illumination of 1 sun.h) Water evaporation rates of Ti 3 C 2 T x /cellulose film under the solar illumination of different intensities.Reproduced with permission. [77]Copyright 2019, American Chemical Society.

Photothermal Therapy
Photothermal therapy is an attractive tumor treatment method, which mainly uses light absorbers to convert photon energy into thermal energy to achieve precise local heating of tumors to .Reproduced with permission. [84]Copyright 2020, The Royal Society of Chemistry.e) Schematic of the photothermal effects produced from the AgNP@Ti 3 C 2 T x hybrids in the AgNP@Ti 3 C 2 T x /PU composite coating when irradiated with light.Reproduced with permission. [86]Copyright 2019, American Chemical Society.
cause tumor cell necrosis or apoptosis, while preventing surrounding healthy tissues from being damaged and reducing systemic adverse reactions.The LSPR effect of MXenes makes it possible to achieve high-efficiency light absorption and energy conversion in a specific spectral range. [88]Moreover, MXenes have low cytotoxicity and high biocompatibility, and are widely used in the field of photothermal therapy. [89,90]in et al. [91] applied soybean phospholipid (SP) functionalized Ti 3 C 2 T x (Ti 3 C 2 T x -SP) to the photothermal therapy of tumors (Figure 7a-d).The results showed that Ti 3 C 2 T-SP showed no obvious toxicity both in vitro and in vivo.By intravenously injecting Ti 3 C 2 T x -SP into mice, tumor cells in mice were eliminated under NIR light irradiation.The introduction of SP enhanced the stability of Ti 3 C 2 T x nanosheets in the dispersion, improving their applicability in tumor diagnosis and therapy.Han et al. [92] prepared porous Nb 2 CT x composite films by coating the uniform mesoporous silica shell on the surface of Nb 2 CT x based on the gel-sol method (Figure 7e).The photothermal conversion rate of porous Nb 2 CT x composite films in the NIR-II biological window was 28.6%, which could be used to photothermal therapy.It not only broadened the application of MXenes such as Nb 2 CT x in tumor photothermal therapy, but also provided an effective strategy for the surface engineering of MXenes.Shi et al. [93] developed the biodegradable Nb 2 CT x -PVP nanosheets for photothermal therapy of tumors.This single-atom-thick Nb 2 CT x nanosheets with a lateral size of about 150 nm had excellent photothermal performance with a wide NIR spectrum and extremely high photothermal conversion efficiency.The low photothermal conversion efficiency reached 45.65%, and it had good cyclic photothermal stability, and could perform efficient in vivo photothermal ablation of mouse tumor xenografts in the NIR-I and NIR-II windows.The in vitro toxicity of Nb 2 CT x -PVP to cells showed that more cells were killed in the NIR-I and NIR-II biological windows with increasing laser energy.Both in vitro cell experiments and in vivo mouse experiments proved that Nb 2 CT x -PVP had no obvious toxicity and could effectively kill tumor cells within the NIR-I and NIR-II windows.This work broadened the application prospects of MXenes-based materials in cancer photothermal therapy by rationally designing the composition of MXenes and exploring the related physicochemical properties.
In conclusion, MXenes-based photothermal conversion materials offer promising opportunities for harnessing solar energy for various applications.Photothermal conversion enables the direct utilization of sunlight by absorbing and converting it into heat energy.MXenes, with their unique properties, extend the solar spectral response to the NIR region, making them effective in fields where temperature control is crucial, including desalination, wearable devices, and photothermal therapy.MXenesbased photothermal materials hold significant promise for advancing renewable energy utilization, enhancing wearable device capabilities, and improving the effectiveness of cancer treatment through photothermal therapy.These materials have the potential to contribute to a more sustainable and technologically advanced future.

MXenes-Based Photocatalysis Materials
Photocatalysis is considered as a green technology with important application prospects in the fields of energy and environment.Under light conditions, when the photon absorbed by  [91] Copyright 2016, American Chemical Society.e) The scheme of the synthetic procedure and stepwise surface PEGylation/targeting modification of CTAC@Nb 2 CT x -MSN.Reproduced with permission. [92]Copyright 2018, Ivyspring.
the semiconductor photocatalytic materials is larger than its bandgap, the electrons on the valence band are excited to the conduction band to form photogenerated electrons, and the corresponding photogenerated holes are formed on the valence band. [94]Utilizing the redox properties of photogenerated electron-hole pairs can realize photocatalytic degradation of pollutants and hydrogen production by splitting water.Studies have shown that the introduction of MXenes as a co-catalyst into the photocatalytic reaction system is beneficial to improve the photocatalytic performance.Its performance enhancement mechanism is mainly as follows.1) MXenes are conducive to the growth and uniform loading of photocatalytic materials on their surface due to their rich functional groups on the surface, unique two-dimensional layered structure, and adjustable layer spacing, making them have a larger specific surface area than singlecomponent materials, which can provide more surface active sites, which is conducive to the adsorption and transfer of substances on its surface. [95]2) MXenes can significantly promote the separation and migration of photogenerated carriers in photocatalytic materials due to their excellent metal conductivity. [96]) MXenes have the full-spectrum absorption effect, which can improve the utilization of sunlight, and its photothermal conversion effect is also conducive to the photocatalytic reaction. [97]herefore, MXenes, as cocatalysts, are widely used in photocatalysis fields, such as water treatment, hydrogen production, and CO 2 reduction.

Water Treatment
Environmental pollution has become a global issue of common concern, with particular emphasis on the serious threat posed to human survival by harmful pollutants such as heavy metals, antibiotics, and organic dyes. [98]As a low-cost photoredox technology, photocatalysis technology is one of the effective means to solve the problem of environmental pollution.MXenes-based photocatalytic composites have been widely used in the field of photocatalysis water treatment due to their high separation efficiency of photogenerated charge carriers and large specific surface area. [99]ing et al. [100] prepared CoO/TiO 2 /Ti 3 C 2 T x composites by hydrothermal method, which could be used for photocatalytic degradation of phenol (Figure 8a,b).After visible light irradiation for 15 min, the degradation efficiency of CoO/TiO 2 /Ti 3 C 2 T x composites to phenol was about 96.6%.The enhancement mechanism could be attributed to the construction of the doubleinterface charge transport channel with the suitable energy band structure, which facilitated the separation of photogenerated carriers and generated a sufficient potential difference for redox reactions.Cui et al. [101] prepared Nb 2 O 5 /Nb 2 CT x composites by a simple hydrothermal method and applied them to the photocatalytic degradation of rhodamine B (RhB) and tetracycline hydrochloride (TC-HCl).The results showed that the Nb 2 O 5 / Nb 2 CT x composites exhibited excellent photocatalytic degradation performance.After 120 and 180 min of visible light irradiation, the maximum degradation efficiencies for RhB and TC-HCl were 98.5% and 91.2%, respectively, and the performance enhancement was attributed to the fact that the formation of Schottky junction between Nb 2 O 5 and Nb 2 CT x facilitated the effective separation of photogenerated carriers.In addition, Cui et al. [102] also applied the Bi 2 WO 6 /Nb 2 CT x composites prepared by the hydrothermal method to photocatalytic degradation of RhB, methylene blue (MB), and TC-HCl, and demonstrated excellent photocatalytic degradation of pollutants (Figure 8c-h).After 90 min of visible light irradiation, the maximum degradation efficiencies for RhB, MB, and TC-HCl were 99.8%, 92.7%, and 83.1%, respectively.Zhuge et al. [103] prepared flower-like CaIn 2 S 4 microspheres/Ti 3 C 2 T x composites by hydrothermal method and applied them to photocatalytic degradation of TC-HCl.After visible light irradiation for 150 min, the degradation efficiency of flower-like CaIn 2 S 4 microspheres/Ti 3 C 2 T x composites for TC-HCl was 92.0%.The degradation efficiency of TC-HCl hardly changed after 4 times of reuse, indicating its excellent stability and reusability.The enhanced performance of the flower-like CaIn 2 S 4 microspheres/Ti 3 C 2 T x composites for photocatalytic degradation of pollutants was mainly attributed to the formation of the Schottky junction between CaIn 2 S 4 and Ti 3 C 2 T x , which enabled the migration of photogenerated electrons from CaIn 2 S 4 to Ti 3 C 2 T x , thereby promoting the effective separation of photogenerated carriers.

Hydrogen Production
Hydrogen energy is a green and highly efficient clean energy source, possessing advantages such as high calorific values, diverse sources, and its combustion byproduct being water. [94]he technology of photocatalysis water splitting for hydrogen production can convert solar energy into hydrogen energy, which is one of the effective ways to solve the energy crisis. [104]owever, due to the disadvantages of easy recombination of photogenerated carriers in photocatalytic materials, the efficiency of photocatalysis water splitting to produce hydrogen is low, only 1%, and practical application cannot be realized.Studies have found that the introduction of MXenes into the photocatalysis hydrogen production system can effectively inhibit the recombination of photogenerated carriers and provide effective reactive sites for the activation of hydrogen protons, which is one of the potential solutions to improve the activity of photocatalysis water splitting for hydrogen production. [105]heng et al. [106] prepared CdLa 2 S 4 /Ti 3 C 2 T x composites by in situ growing CdLa 2 S 4 nanoparticles on the surface of Ti 3 C 2 T x , and used them for photocatalysis hydrogen production.Under visible light irradiation, the photocatalysis hydrogen production rate of CdLa 2 S 4 /Ti 3 C 2 T x composites was as high as 11,182.4μmol g À1 h À1 , which was about 13.4 times that of CdLa 2 S 4 (834.5 μmol g À1 h À1 ), mainly due to the fact that the excellent electrical conductivity of Ti 3 C 2 T x facilitated the separation and migration of photogenerated carriers.In addition, the apparent quantum efficiency (AQE) of the composites at 420 nm could reach 15.6%, and it could still maintain a high photocatalysis hydrogen production activity after 6 times of reuse.Xiao et al. [107] assembled CdS nanorods prepared by hydrothermal method in situ on Ti 3 C 2 T x nanosheets to prepare CdS/Ti 3 C 2 T x composites, which showed excellent photocatalytic performance and could be applied to photocatalysis hydrogen production.The maximum photocatalysis hydrogen production rate of CdS/Ti 3 C 2 T x composites was as high as 2,407.0μmol g À1 h À1 , which was 6.7 times higher than that of pure CdS nanorods (359.2 μmol g À1 h À1 ), mainly due to the unique Schottky junction which was formed between Ti 3 C 2 T x and CdS, which facilitated the efficient separation of photogenerated carriers.Zhuang et al. [108] prepared TiO 2 /Ti 3 C 2 T x composites by electrostatic self-assembly and applied them to photocatalysis hydrogen production.The highest hydrogen production rate of TiO 2 /Ti 3 C 2 T x composites could reach 6.9 mmol g À1 h À1 , which was about 3.8 times that of pure TiO 2 nanofibers (1.8 mmol g À1 h À1 ).In addition, the photocatalysis hydrogen production yield of the composites was almost unchanged after five reuses, indicating its excellent stability and reusability.Huang et al. [109] prepared flower-like ZnIn 2 S 4 /Ti 3 C 2 T x composites by solvothermal method and applied them to photocatalysis hydrogen production (Figure 9).Under simulated sunlight irradiation, the photocatalysis hydrogen production rate of the ZnIn 2 S 4 /Ti 3 C 2 T x composites could reach up to 978.7 μmol g À1 h À1 , which was 2.7 times that of pure ZnIn 2 S 4 (362.4μmol g À1 h À1 ).Meanwhile, the AQE of the composites was as high as 24.2% at 420 nm, and its enhanced photocatalysis performance could be attributed to the highly exposed reactive sites of the composite, which were beneficial for the adsorption and conversion of hydrogen protons.In addition, the tight interfacial contact of the composites also facilitated the migration of photogenerated carriers.Reproduced with permission. [100]Copyright 2021, Elsevier.c) The absorption of RhB in the presence of Bi 2 WO 6 , BN-0.5, BN-2, BN-5, and BN-10 in the dark, respectively.d) Photodegradation of RhB in the presence of Bi 2 WO 6 , BN-0.5, BN-2, BN-5, and BN-10 under visible light irradiation, respectively, and e) the corresponding first-order kinetic plots of RhB degradation.f ) The absorption of MB in the presence of Bi 2 WO 6 , BN-0.5, BN-2, BN-5, and BN-10 in the dark, respectively.g) Photodegradation of MB in the presence of Bi 2 WO 6 , BN-0.5, BN-2, BN-5, and BN-10 under visible light irradiation, respectively, and h) the corresponding first-order kinetic plots of MB degradation.Reproduced with permission. [102]Copyright 2019, Elsevier.

CO 2 Reduction
CO 2 is a cheap and renewable carbon resource, but it is also the main gas that causes the greenhouse effect.Therefore, the use of photocatalysis technology to directly convert CO 2 into small molecular organic compounds such as HCOOH, CH 3 OH, CH 4 , and cyclic carbonates or high value-added compounds such as pharmaceutical intermediates can effectively alleviate the greenhouse effect and energy shortage problems. [110]owever, limited by the low photocatalytic activity and the poor activation and reduction of CO 2 molecules, the research on photocatalytic reduction of CO 2 is still a challenge.MXenes are highly effective in enhancing the photogenerated carrier separation and migration efficiency of photocatalysts, and are widely used in photocatalytic reduction of CO 2 . [111]ow et al. [112] prepared TiO 2 /Ti 3 C 2 T x composites by in situ growth of TiO 2 on the surface of Ti 3 C 2 T x by calcination, and applied it to the photocatalytic reduction of CO 2 .Under simulated sunlight irradiation, the rate of photocatalytic reduction of CO 2 to CH 4 by TiO 2 /Ti 3 C 2 T x composites was  [109] Copyright 2022, Elsevier.0.2 μmol g À1 h À1 , which was 3.7 times that of the commercial TiO 2 photocatalytic reduction of CO 2 (0.05 μmol g À1 h À1 ).The enhanced photocatalytic activity was due to the high conductivity of Ti 3 C 2 T x which promoted the migration of photogenerated electrons and effectively inhibited the recombination of photogenerated carriers.In addition, the unique rice husk-like structure of the composites endowed it with a large number of surface active sites, which was beneficial to the adsorption of CO 2 .Li et al. [113] combined ZnO nanoparticles and surface alkalized Ti 3 C 2 T x to construct ZnO/Ti 3 C 2 T x -OH composites by electrostatic self-assembly (Figure 10a-d), and the photocatalytic reduction rate of CO 2 to CO and CH 4 was 30.3 and 20.3 μmol g À1 h À1 , about 6.5 times and 35.0 times that of pure ZnO nanoparticles.It was mainly attributed to the low CO 2 adsorption energy of Ti 3 C 2 T x -OH, which was beneficial to the adsorption of CO 2 .In addition, Ti 3 C 2 T x -OH, as a photogenerated electron acceptor, facilitated the activation of CO 2 adsorbed on its surface to CO and CH 4 .Cao et al. [114] prepared Bi 2 WO 6 /Ti 3 C 2 T x composites (Figure 10e,f ) by in situ growing Bi 2 WO 6 on the surface of Ti 3 C 2 T. The total rate of photocatalytic reduction of CO 2 to CH 4 and CH 3 OH (2.22 μmol g À1 h À1 ) was higher than that of Bi 2 WO 6 (0.48 μmol g À1 h À1 ) increased by 4.6 times, mainly due to the unique two-dimensional layered structure of the Bi 2 WO 6 /Ti 3 C 2 T x composites effectively shortening the transport path of photogenerated carriers, so that photogenerated electrons could be quickly transferred from Bi 2 WO 6 migrates onto Ti 3 C 2 T x .In addition, the photothermal conversion effect of Ti 3 C 2 T x was also conducive to promoting the photocatalytic reaction.
MXenes have found applications as cocatalysts in various photocatalysis fields, including water treatment, hydrogen production, and CO 2 reduction.
Water treatment: MXenes-based photocatalytic composites are effective in the removal of pollutants from water due to their high charge carrier separation efficiency and large specific surface area.Various studies have demonstrated their effectiveness in degrading pollutants like phenol, rhodamine B, and tetracycline hydrochloride.
Hydrogen production: Photocatalytic water splitting for hydrogen production is a promising approach to renewable energy.MXenes have been incorporated into photocatalytic systems to enhance carrier separation, resulting in significantly increased hydrogen production rates.CO 2 reduction: MXenes have been used to improve the efficiency of photocatalytic CO 2 reduction, converting carbon dioxide into valuable compounds like CH 4 and CH 3 OH.Their conductivity, surface properties, and photothermal effects play crucial roles in enhancing CO 2 reduction rates.
In conclusion, MXenes-based photocatalysis materials hold great potential for addressing environmental challenges, advancing renewable energy production, and contributing to the reduction of greenhouse gas emissions.Copyright 2021, The Royal Society of Chemistry.e) N 2 adsorption-desorption isotherms, pore size distributions (inset of (e)).f ) CO 2 adsorption curves of Bi 2 WO 6 /Ti 3 C 2 T x composites.Reproduced with permission. [114]Copyright 2018, Wiley-VCH.

Conclusion and Outlooks
MXenes-based optical functional materials have been widely used in the fields of photothermal conversion and photocatalysis, and have achieved certain achievements and progress, but there are still some key scientific and technical bottleneck problems to be solved urgently: 1) Using LiF/HCl to etch the MAX phase is still the mainstream method for preparing MXenes.Researchers are suggested to develop fluorine-free, safe, and efficient methods for preparing MXenes, and then develop new antioxidation MXenes to expand their application in the field of optical functional materials for wider application; 2) Researchers are recommended to systematically study the factors affecting the optical properties of MXenes, master the laws of light absorption ability of MXenes under different surface states, clarify the changes in optical properties of MXenes with different shapes or doped with different atoms, and realize the better application of MXenes in the field of optical functions; 3) It is worth considering carrying out systematic research on the biocompatibility (such as genotoxicity, chronic toxicity, and carcinogenicity), biodegradability, pharmacokinetics, biodistribution, and immunogenicity of MXenes-based photothermal conversion materials, which is particularly important for broadening its application in photothermal therapy; and 4) Researchers are suggested to clarify the constitutive relationship between the type of MXenes surface functional groups, the number of MXenes layers, and its photocatalytic performance, and then improve the performance of MXenes-based photocatalysis materials by adjusting the surface functional groups of MXenes and the number of MXenes layers.It is of great significance to promote the wider application of MXenes and MXenes-based materials in the field of photocatalysis.

Figure 2 .
Figure 2. a) Schematic of Ti 3 C 2 T x clay synthesis and electrode preparation.b) X-ray diffraction (XRD) patterns of samples produced by etching in LiF þ HCl solution.c) Transmission electron microscope (TEM) image of several flakes.The inset shows the overall selected area electron diffraction pattern.d) Scanning electron microscope (SEM) image of a fracture surface of a 4 μm thick film.The flexibility of the film is demonstrated in the inset.e)Fracture surface of a thicker rolled film (30 μm).TEM images of f ) single-and g) double-layer flakes, respectively.Insets show sketches of these layers.Reproduced with permission.[35]Copyright 2014, Springer Nature.

Figure 3 .
Figure 3. Characterization of halogenated Ti 3 C 2 T x .a) XRD patterns showing the (002), (004), and (006) peaks of the Ti 3 C 2 T x .b) SEM image of Ti 3 C 2 Br 2 .c) High-angle annular dark-field (HAADF) image and corresponding energy dispersive spectrometer (EDS) map of Ti 3 C 2 Br 2 .d) High-resolution transmission electron microscope (HR-TEM) image showing an enlarged view of the marked area in (c).e,f ) High-resolution scanning transmission electron microscope (HR-STEM) image and corresponding EDS map showing the atomic positions of Ti 3 C 2 Br 2 .g,h) HAADF image and corresponding EDS map showing the element distributions of Ti 3 C 2 I 2 and Ti 3 C 2 (ClBrI).Reproduced with permission.[43]Copyright 2021, American Chemical Society.

Figure 4 .
Figure 4. a) Schematic diagram of the synthesis of Ti 2 CCl 2 .b) XRD pattern and Rietveld refinement of Ti 2 CCl 2 prepared by reacting Ti, graphite, and TiCl 4 at 950 °C.c) XRD patterns of dispersible delaminated and sonicated Ti 2 CCl 2 .(Inset) Colloidal solution of the delaminated Ti 2 CCl 2 .d) SEM image and e) EDS elemental mapping of a Ti 2 CCl 2 stack.f ) High-resolution HAADF image and g) electron energy loss spectroscopy atomic column mapping representing the layered structure of Ti 2 CCl 2 .Reproduced with permission.[47]Copyright 2023, AAAS.
3 O 4 /PAA) nanofiber films, and deposited Ti 3 C 2 T x nanosheets on the surface of Fe 3 O 4 /PAA nanofiber films by vacuum-assisted filtration, and then prepared Ti 3 C 2 T x -(Fe 3 O 4 /polyimide (PI)) composite films with Janus structure by thermal imidization.The surface temperature of the wearable Ti 3 C 2 T x -(Fe 3 O 4 /PI) composite films could be effectively controlled by adjusting the optical power density of simulated sunlight.With the optical power densities of 50 and 200 mW cm À2 , the surface temperature of the Janus Ti 3 C 2 T x -(Fe 3 O 4 /PI) composite films could reach up to 35 and 95 °C, respectively.

Figure 6 .
Figure 6.a) Schematic of fabrication of M-textiles.Temperature evolution with increasing time at different distances and angles of b) 90°, c) 60°, and d) 30°.Reproduced with permission.[84]Copyright 2020, The Royal Society of Chemistry.e) Schematic of the photothermal effects produced from the AgNP@Ti 3 C 2 T x hybrids in the AgNP@Ti 3 C 2 T x /PU composite coating when irradiated with light.Reproduced with permission.[86]Copyright 2019, American Chemical Society.

Figure 7 .
Figure 7. a) Two therapeutic approaches based on photothermal effect of Ti 3 C 2 nanosheets (intravenous injection of Ti 3 C 2 -SP nanosheets and intratumoral injection of phase-changeable PLGA/Ti 3 C 2 implant).b) The Ti 3 C 2 -SP nanosheets reach and accumulate at tumor tissues by enhanced permeability and retention (EPR) effect via the intravenous injection of Ti 3 C 2 -SP nanosheets.c) Photothermal ablation of cancer cell by Ti 3 C 2 -SP exposed to NIR laser.d) Histopathological examinations via the hematoxylin and eosin staining of major organs (heart, liver, spleen, lung, and kidney) of the control group and three treatment groups (at different Ti 3 C 2 -SP dosages of 5, 10, and 20 mg kg À1 ) for 30 days.All the scale bars are 100 μm.Reproduced with permission.[91]Copyright 2016, American Chemical Society.e) The scheme of the synthetic procedure and stepwise surface PEGylation/targeting modification of CTAC@Nb 2 CT x -MSN.Reproduced with permission.[92]Copyright 2018, Ivyspring.

Figure 9 .
Figure 9. a) Schematic illustration of the preparation process of the ZnIn 2 S 4 /Ti 3 C 2 composites b) FESEM and c) HRTEM images of the prepared ZT10 sample.d) HAADF-STEM image of the ZT10 sample and the EDS mapping images of its Zn, In, S, and Ti atoms.e) Proposed photocatalytic H 2 evolution mechanism over ZnIn 2 S 4 /Ti 3 C 2 composite photocatalysts.Reproduced with permission.[109]Copyright 2022, Elsevier.

Table 1 .
A summary of the preparation methods of MXenes and their advantages and disadvantages.
Requires further treatment with organic reagents to obtain fewlayered or single-layered MXenes.Fast synthesis.