Synergistic redox reactions toward co‐production of H2O2 and value‐added chemicals: Dual‐functional photocatalysis to achieving sustainability

Integrating H2O2 evolution with oxidative organic synthesis in a semiconductor‐driven photoredox reaction is highly attractive since H2O2 and high‐value chemicals can be concurrently produced using solar light as the only energy input. The dual‐functional photocatalytic approach, free from sacrificial agents, enables simultaneous production of H2O2 and high‐value organic chemicals. This strategy promises a green and sustainable organic synthesis with minimal greenhouse gas emissions. In this review, we first elucidate the fundamental principles of cooperative photoredox integration of H2O2 synthesis and selective organic oxidation with simultaneous utilization of photoexcited electrons and holes over semiconductor‐based photocatalysts. Afterwards, a thorough review on the recent advancements of cooperative photoredox synthesis of H2O2 and value‐added chemicals is presented. Notably, in‐depth discussions and insights into the techniques for unravelling the photoredox reaction mechanisms are elucidated. Finally, critical challenges and prospects in this thriving field are comprehensively discussed. It is envisioned that this review will serve as a pivotal guidance on the rational design of such dual‐functional photocatalytic system, thereby further stimulating the development of economical and environmentally benign H2O2 and high‐value chemicals production.


INTRODUCTION
In light of the energy and climate crises faced by modern society, tremendous efforts have seen invested onto developing sustainable and stable energy conversion system, of which among the technologies developed, photocatalysis has received multifarious interdisciplinary interests as an efficacious and facile strategy to produce green fuels, chemicals, and to battle the troublesome environmental pollution. 1,2Hydrogen peroxide, or H 2 O 2 , is a versatile substance that finds extensive applications in bleaching, organic syntheses, and disinfection, of which the pulp and paper industry accounts for the major usage of it. 3H 2 O 2 is endowed with intriguing features such as water solubility, high energy density (2.1 MJ kg −1 , 60% aqueous phase), and ease of storage.These characteristics therefore prompted its exploration in energy-related field as both oxidant and reductant in an innovative H 2 O 2 fuel cell.
Although the output voltage (1.09 V) is lower than a H 2 /O 2 fuel cell (1.23 V), considering the energy density is comparable to compressed hydrogen (3.5 MJ kg −1 ), and on top of the other merits of aqueous H 2 O 2 , this is but a minor disadvantage. 4Up to date, the anthraquinone oxidation (AO) process is the predominant H 2 O 2 production technology, and the process is constituted of four major steps, which necessitates exquisite energy input, noble-metal catalyst, and carbon-intense hydrogen that is derived from fossil fuels such as coal and natural gas.Even worse, the AO process generates toxic by-products that pose severe environmental issues. 5In addition to AO process, direct synthesis approach has been reported as alternative to the AO process, wherein this method requires simply H 2 , O 2 , and Pd-based catalysts. 6,7Yet this method succumbs to two major drawbacks, namely, the explosiveness of H 2 -O 2 mixture over a wide range of concentrations and realizing good selectivity for H 2 O 2 over the side product, water.Hence, there is a strong desire to establish a sustainable, costeffective, and inherently safe route for H 2 O 2 synthesis.In this regard, photocatalytic H 2 O 2 production emerges as an auspicious candidate to replace the conventional AO process.Photocatalytic H 2 O 2 synthesis can be realized through either two-electron water oxidation reaction (2e − WOR) or two-electron oxygen reduction reaction (2e − ORR). 8,9On the flip side, the electrocatalytic approach has also been demonstrated for H 2 O 2 production.The key advantage of photocatalysis over electrocatalysis is that the former allows the direct utilization of natural energy source (solar light), as in contrast to the latter that requires electricity from artificial manner.However, if the applied electricity originates from sustainable resources such as solar light, wind, tidal wave, etc., it will be highly appealing in the energy utilization standpoint.Moreover, there is a prominent connection between the photochemical and electrochemical routes in which the 2e − ORR or 2e − WOR is involved to realize H 2 O 2 production.This observation can deduce that the integration of 2e − ORR or 2e − WOR electrocatalysts into photocatalytic systems as cocatalysts will be auspicious to elevate the efficacy of H 2 O 2 production. 10Hitherto, in consideration of both thermodynamic and kinetic barriers, most research opted for 2e − ORR with employment of sacrificial agent for achieving practical H 2 O 2 yield.However, the utilization of sacrificial hole scavengers usually ends up producing undesirable waste chemicals, and such strategy has zero-to-little practicability due to considerations on sustainability, cost, and originality of the employed reagents. 11In light of these challenges, the concept "cooperative photoredox reaction coupling" has emerged in recent years, aiming to achieve effective photoredox chemical production in a dual-functional photocatalytic system, that is, both photogenerated electrons and holes are exploited to produce value-added chemicals.This concept has garnered significant attention in recent years, as it offers exciting prospects for green and sustainable photoredox chemical production.For instance, Wei et al. conjoined CO 2 reduction reaction with 2e − WOR to concurrently produce CO and H 2 O 2 . 12Both H 2 O 2 and CO evolution rates were impressive (17 000 and 7300 μmol g −1 h −1 , respectively), well suggesting the synergy of the novel concept.In another example, 2e − ORR was coupled with alcohol oxidation reaction for synchronous generation of furoic acid and H 2 O 2 . 13To date, although some reviews regarding the photocatalytic H 2 O 2 production process have been published, 8,[14][15][16] there is lack of a comprehensive review that focuses on the cooperative photoredox reaction coupling of H 2 O 2 and chemical synthesis, which is aimed to be addressed by this review.It is critically timewise to assess and present an up-to-date review on the recent progress of the state-of-the-art development in photocatalysis toward concurrent production of H 2 O 2 and value-added chemical.
In this review, a brief introduction on the general principles of photocatalysis is first presented, the fundamentals of photocatalytic H 2 O 2 production via 2e − ORR and 2e − WOR are then thoroughly discussed.Subsequently, we highlight the common or advanced techniques employed for reaction pathway and product analysis.The recent 5-year advances in cooperative photoredox synthesis of H 2 O 2 and valorized chemical, as catalyzed by various types of photocatalysts, which span across graphitic C 3 N 4 (g-C 3 N 4 ), metal sulfide, organic polymers, metal oxides, etc., are systematically outlined.Finally, a personal viewpoint on the current challenges and prospects of this specific photocatalytic application are highlighted, with the anticipation of stimulating new ideas and expanding the exploitation of cooperative photoredox reaction coupling.

FUNDAMENTALS OF PHOTOREDOX H O AND VALUABLE CHEMICAL SYNTHESIS
Over the past decades, extensive efforts have been invested into studying the principles of photocatalysis, whereby currently it is accepted that all photocatalytic reactions would follow three fundamental steps: (1) excitation of electrons from occupying valence band (VB) to the conduction band (CB) by photons absorption, of which the incident photons carry energy that could suffice the bandgap (E g ) between VB and CB, while generating positively charged holes in VB; (2) the subsequent separation of electron-hole pairs and their independent migration to the photocatalyst surface; and (3) the consumption of photogenerated electrons and holes by adsorbed chemical species, whence initiates the associating photoredox reactions.It is worthwhile to mention that the photoredox reactions are not photocatalyst surface bounded but can occur on the surface of corresponding oxidation or reduction cocatalyst. 179][20] Conspicuously, the photore- dox surface reactions are significantly suppressed given the low amount of available surface charges, leading to poor photocatalytic yield and efficiency.It can be deduced that the electronic properties and surface reactions are dependent on one another and synergistically correlated.

Dual-functional photoredox reactions beyond overall water splitting: concurrent H 2 O 2 and valuable chemical synthesis
According to the statistics from Web of Science, it is observed that over the past decade, there is an exponential growth of interest revolving H 2 O 2 in conjunction with the photocatalytic aspect, as indicated by the number of citations and publications (Figure 1).Up to date, there are numerous types of photocatalysts that have been reported to drive the photocatalytic synthesis of H 2 O 2 , which includes the families of g-C 3 N 4 , metal sulfide, organic polymer, metal oxide, etc. (Table 1).Each of these materials offers exclusive and intriguing features that have proven beneficial toward augmenting and improving their photocatalytic properties, thereby performing the photocatalytic reactions with outstanding efficacy.
2][73][74][75][76][77][78] The key idea of cooperative photoredox synthesis is to achieve the production of valuable chemicals at both the oxidation and reduction active sites of a rationally designed dual-functional photocatalytic system, contrasting with conventional photocatalysis that usually concede the oxidation power of photogenerated  2).Dual-functional photocatalytic system herein refers to the system with well utilization of the photoinduced electronhole pairs for producing value-added chemicals at both oxidation and reduction sites.Typically, this type of system is conceived by thoughtfully selecting the suitable oxidation substrates that act as proton sources. 79,80ollowing this idea, Xue et al. synthesized a phosphorusdoped g-C 3 N 4 co-loaded with cobalt phosphides, and the as-prepared photocatalyst was utilized for photocatalytic pure water splitting in an anaerobic reaction environment. 29The gas chromatography analysis and iodometric test evidenced the presence of H 2 and H 2 O 2 in stoichiometric ratio (1:1), implying that the water oxidation half-reaction was following a two-electron pathway.Apart from the one-step (Equation 1) or two-step (Equations 2 and 3) 2e − WOR, it has been demonstrated by numerous research that the 2e − ORR also produces H 2 O 2 . 8,71,72,77,81In this pathway, O 2 follows either the direct one-step protoncoupled electron transfer (Equation 4) or the sequential two-step single-electron reduction route (Equations 5-7) for H 2 O 2 evolution.A schematic diagram (Figure 3) is provided to visualize the reactions in a photocatalytic system.Conjunctionally, several past studies are encouraged for reference to inquire about the oxidation potential of common organic substrates. 82,83 OOH + e − + H + → H 2 O 2  0 = 1.05V (7)   From the thermodynamic perspective, 2e − WOR is less favorable compared to 4e − WOR, given its more positive redox potential.However, this pathway has been demonstrated to be kinetically advantageous over the latter, 84 attributed to the involvement of lesser photoexcited holes being consumed by the water molecules.Through this reaction, the nuisance of product separation is readily eliminated owing to the distinct phases of liquid H 2 O 2 and gaseous H 2 .In addition, recently several studies have demonstrated the coupling of 2e − WOR with 2e − ORR spearheaded for sole production of H 2 O 2 in a two-channel manner. 23,46,67Such strategy suppresses the HER activity to favor 2e − ORR and 2e − WOR at the reduction and oxidation active sites, respectively, and consequently exhibit extremely high H 2 O 2 evolution rate ranging from 1900 to 3500 μmol g −1 h −1 .This ingenious pathway coupling also exploits the redox capability of photogenerated charge carriers while mitigating the use of sacrificial agent.This points out an alternative and attractive design philosophy for synthesizing extraordinarily performing photocatalytic systems to achieve high H 2 O 2 production rate.Furthermore, very recent studies have exemplified the cooperative coupling of 2e − ORR with selective organic oxidation or 2e − WOR with CO 2 photoreduction towards generating H 2 O 2 and valuable chemicals such as dihydroisoquinoline derivatives (DHIQs), 42 furoic acid, 66 carbon monoxide, 12 etc., and they have exhibited unprecedentedly high yields for the redox products.Nonetheless, the massive potential of photoredox cooperative coupling of H 2 O 2 and chemical synthesis has remained to be explored, and in concurrent manner, the underlying reaction mechanisms should be thoroughly examined to gain insightful information for further improving the photocatalytic systems.In regard to this, computational and instrumental analyses play pivotal roles and must be synergistically interpreted, which will be discussed in the following section.

Reaction pathway identification
To ascertain the reaction pathway and mechanisms followed by a photocatalytic system that performed the photoredox production of H 2 O 2 and value-added chemicals, critical computational and instrumental analyses must be synergistically employed.For instance, preliminary theoretical studies of the surface electronic structure of the as-synthesized photocatalyst can be conducted via first-principle density functional theory (DFT) computations.In this context, DFT calculation has been well utilized for predicting the evolution propensity of reaction intermediates such as hydroxyl radical (⋅OH) and superoxide radicals (⋅O 2 − or ⋅OOH) that will emerge in 2e − WOR and 2e − ORR pathways, respectively. 45,48Owing to the accurately determined Gibbs free energies of formation of the intermediates, the forecasting of undergone reaction pathway by a designed photocatalyst is thus realizable.The DFT computational results can be jointly interpreted alongside in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to unveil the reaction mechanisms of a given photoreaction and is especially useful for 2e − ORR coupled with selective organic oxidation. 42,66The time-dependent emergence and disappearance of characteristic peaks of the functional groups observed in the in situ DRIFTS spectra act as solid supports to propose suitable photocatalytic reaction sub-steps.For instance, He et al. conducted both DFT computations and in situ DRIFTS analysis to study the photoreaction steps in the photoredox H 2 O 2 and furoic acid synthesis. 66he intensity of characteristic peaks of functional groups such as C 3 =C 4 of furan ring, C 5 =O of furfural (1699 cm −1 ) and furoic acid (1684 cm −1 ) gradually enhanced or diminished over time, thereby indicating evolution of furfuryl alcohol-furfural-furoic acid (Figure 4A).Additionally, the presence of hydroxyl radical is considered as strong evidence to prove the occurrence of 2e − WOR in a photocatalytic system, whereby the elec-tron paramagnetic resonance (EPR) spectroscopy can be employed to capture the free radical with 5,5-dimethyl-1pyrroline N-oxide (DMPO) acting as trapping agent. 11,23In detail, the EPR spectroscopy analysis is performed under light irradiation, with a blank reference at dark condition.A photocatalytic system that performs 2e − WOR under photoirradiation will have the photogenerated hydroxyl radical captured and formed DMPO-⋅OH adduct and exhibits characteristic peaks that can be observed in the EPR spectrum (Figure 4B).Similarly, the same techniques can be applied to verify the 2e -ORR pathway, 23,86 with the characteristic peaks of DMPO-O 2 − and DMPO-OOH in the EPR spectra (Figure 4C).The characteristic quartet of peaks is good evidence to deduce that the subjected photocatalyst follows a 2e − WOR or 2e − ORR pathway.However, to further support the conclusion, rotating disk electrode (RDE) and/or rotating ring-disk electrode (RRDE) analyses are recommended, whereby kinetic characteristics of the electrochemical reaction, such as average electron transfer number (n e-) can be determined.The linear sweep voltammograms (LSVs) obtained at the different rotating speeds via RDE over the applied potential range can be transformed into a Koutecky-Levich plot to obtain n e- (Figure 4D). 48,51Therefore, in the collaborative manner with the EPR spectra, the reaction pathway undertaken by the photocatalyst can be firmly validated.RRDE can serve the same purpose while providing the researchers with the conveniences in terms of omitting the mathematical transformations, that is, the average electron transfer number is obtainable through a simple equation (Equation 8), and for such usage, the current (or current density) versus time (It) curves are always accompanied (Figure 4E). 24,29,69The parameters required in the RRDE equation are rotating disk current (I d ), ring current (I r ), and collection efficiency (N; experimentally determined).
6,87 Regarding WOR, RRDE can offer details on the mechanisms and electrochemical stages of the WOR by monitoring the reduction (or oxidation) current responses.In brief, the RDE acts as the working electrode, and its potential is typically scanned from 0.8 to 2.0 V (vs.Ag/AgCl). Notably,he electrolyte employed is a phosphate buffer solution (pH 7) purged by N 2 /Ar gas before the electrochemical measurement, thus the possibility for 2e − ORR to occur is expelled.As demonstrated by the researchers, the potential range is effective to ensure the occurrence of water oxidation, and because the disk electrode is rotating during the measurement, the evolved product such as O 2 (if 4e − WOR) or H 2 O 2 (if 2e − WOR) will be swept to the ring electrode.On the flipside, a constant potential is applied on the ring electrode (usually Pt based) for the detection of products.In particular, the reduction potential of −0.23 V (or −0.3 V) and oxidation potential of 0.6 V (applied potentials are against Ag/AgCl electrode) are used to detect O 2 and H 2 O 2 , respectively.Here, there are two possible scenarios: (1) the photocatalyst carries out 4e -WOR and produces O 2 , where reduction current responses are observable on the ring electrode (Figure 3F); and (2) 2e -WOR is executed by the photocatalyst and H 2 O 2 is evolved, resulting in oxida-tion current responses on the ring electrode (Figure 3G). 47herefore, the exact WOR performed by the photocatalyst can be precisely vindicated.In the ORR perspective, H 2 O 2 selectivity can act as an inference to estimate the likelihood of 2e -ORR, which will prevail, as compared to 4e - ORR, which is determinable via Equation ( 9), and H 2 O 2 is determined in the same manner similar to that of the WOR scenario (Figure 4H).For this application, the electrolyte employed must be O 2 purged to achieve oxygen-rich condition prior to the RRDE measurement.
As a summary, a table is provided to comprehend the mentioned analytical methods, wherein the corresponding purpose(s) and inference(s) are included with respect to the context of this review (Figure 5).

Product quantification methods
Assessment of the photocatalytic performances, especially the production rate is of utmost importance in research study.It is common to report the production rate in either "μmol g −1 h −1 " or "μM g −1 h −1 ."The evolved organic products are usually quantifiable through high-performance liquid chromatography or gas chromatograph-mass spectrometry, which is extremely reliable if the equipment calibration is properly performed.The quantification techniques reported for H 2 O 2 include iodometry, titanium sulfate colorimetry, ammonium metavanadate (NH 4 VO 3 ) colorimetry, cerium sulfate titration, and potassium permanganate (KMnO 4 ) redox tiration. 45,53,56,72,88For instance, iodometry utilizes the redox reaction between iodide (I − ) ions and hydrogen peroxide, wherein the iodide ions are oxidized by hydrogen peroxide into triiodide (I 3 − ) ions under acidic condition.Owing to the strong absorption of ultraviolet-visible (UV-vis) light of triiodide ions at ∼351 nm, and a stoichiometric ratio of 1:1 between the formed triiodide ions and consumed hydrogen peroxide molecules, an inference of H 2 O 2 concentration can be done by cross-referencing to a calibration curve.Notably, the first three techniques use UV-vis spectrophotometry and establishment of calibration curve.To accurately quantify the amount of H 2 O 2 evolved during photoirradiation, the applied method of choice must be highly attentive with regards to the nature of the reaction medium, that is, the presence and concentration of organic compounds, the possible interferences contributed by organic compounds, and the detection limit.Iodometry is a facile method for quantifying photoevolved H 2 O 2 , requiring only two working solutions based on potassium iodide (KI) and potassium hydrogen phthalate (C 8 H 5 KO 4 ), along with minimal and simple apparatus.However, based on our own observation, iodometry is ineffective toward constructing a linear calibration curve for a high concentration aqueous benzyl alcohol reaction system, which is caused by the immiscibility of benzyl alcohol in water.The insoluble benzyl alcohol droplets disrupt the incident light during UV-vis spectrophotometry and result in strong noise around the maximum absorption peak, thus preventing correct reading of absorbance.An approach to resolve the issue is replacing water with acetonitrile or using water-acetonitrile mixture that can dissolve the benzyl alcohol, yet if high concentration of benzyl alcohol (≥10 vol%) is employed, there is a high probability of not acquiring a proper maximum absorbance peak.Moreover, a critical research conducted by Wei et al. suggested that certain aliphatic and aromatic compounds present in the reaction supernatant can obtrude H 2 O 2 measurement fulfilled by KMnO 4 titration. 89For instance, the KMnO 4 titration method tends to overestimate the concentration of H 2 O 2 due to the presence of ethanol and acetaldehyde, and the situation becomes even severe with respect to aromatic compounds as substantial errors are observed.Thus, it is advised to use KMnO 4 titration only when the anticipated H 2 O 2 concentration exceeds 1000 μM, of which influences of aliphatic compounds are mitigatable to certain extent (worse case: EtOH-20% error), but this is inapplicable for the case of aromatic organics.Alternately, the NH 4 VO 3 colorimetry demonstrated unprecedented high robustness and anti-inference properties, wherein it exhibited nominal error in all scenarios except in the low-medium-high concentration acetic acid and high concentration p-benzoquinone case.Although NH 4 VO 3 colorimetry has a relatively high detection limit (65.8 μM) compared to KMnO 4 titration (0.3 μM), 90 considering that cooperative photoredox synthesis of H 2 O 2 and value-added chemicals would generally yield medium to high concentration (>500 μM) of H 2 O 2 .As this is ascribed to the fact that most semiconductors (e.g., CdS, TiO 2 , g-C 3 N 4 ) have VB maximum matching the redox potential of common organic substrates, 83 this quantification method is versatile and reliable in quantifying H 2 O 2 in the presence of organic compounds.Other methods that are not featured here have been described by Freese et al. elsewhere. 91onetheless, the reported methods, along with their applicability and limitations are collectively summarized in Figure 6, which aims to serve as guidance for selecting the proper H 2 O 2 quantification method.
Summing up, a flowchart is presented to outline the procedures in performing the cooperative photoredox production of H 2 O 2 and selectively oxidized organic (Figure 7).In the end, we hope that this section can play a substantial role in better serving the community in advancing this photocatalytic application and to uncover the vast

g-C 3 N 4 -based photocatalyst
Carbon nitride (CN) has a collection of various allotropes, including g-C 3 N 4 , α-C 3 N 4 , β-C 3 N 4 , cubic-C 3 N 4 , and pseudocubic-C 3 N 4 .3][94] The 2D sheets of g-C 3 N 4 are composed of periodically ordered tri-s-triazine rings formed by sp 2 hybridized C and N atoms, wherein the unique aromatic C-N heterocycles bestow the g-C 3 N 4 exceptional chemical and thermal stability, making it a highly appealing photocatalyst. 95,96The pioneering work of applying g-C 3 N 4 as a photocatalytic material is first reported by Wang et al. in 2009, in which the material is employed for driving sacrificial overall water splitting (OWS). 97Ever since, the research efforts invested to uncover and exploit the potential of g-C 3 N 4 as visiblelight-excitable photocatalyst has vehemently increased.Normally, g-C 3 N 4 can be synthesized via simple thermal polymerization of carbon-and nitrogen-rich precursors, for example, melamine, thiourea, urea, dicyandiamide.
It is recognized that the bandgap of g-C 3 N 4 is ∼2.7 eV, with CB and VB positioned at approximately −0.8 and 1.9 eV (vs.normal hydrogen electrode at pH 0), respectively. 16,22,98,99The characteristics endow g-C 3 N 4 to be visible-light responsive and the satisfactory band positions can provoke the 2e − WOR pathway, as well as a diversified range of photocatalytic reactions such as CO 2 reduction, [100][101][102] pollutant degradation, [103][104][105] and N 2 fixation. 36,106However, as elucidated by the extensive researches, bulk g-C 3 N 4 itself suffers from detrimental issues such as inefficient charge separation, rapid charge recombination, low accessible surface area, and marginal visible-light harvesting, 39,41,107,108 which severely limit its practicability in performing 2e − WOR or photoredox reactions for H 2 O 2 production and selective chemical synthesis.As such, it is common to implement modifications, for example, heteroatom doping, 24,82 heterojunction construction, 76,77,109,110 or cocatalyst loading, 25,75 for mitigating the aforementioned impediments of g-C 3 N 4 to achieve prominent photocatalytic performances.Nevertheless, herein the recent progress of g-C  8C, the Gibbs free energy for surface atomic hydrogen adsorption (∆G H* ) on CoNiP was drastically closer to the thermoneutral condition (|∆G H* | = 0) than the other metal phosphides, implying its more superior performance in photocatalytic pure water splitting, and this coincided with the experimental results.In another work by the same authors, homogenous P-P/P-P bridges were constructed by incorporation of red phosphorus on g-C 3 N 4 , wherein Co x P was employed as cocatalyst (PCN-y-CP-z) (Figure 8D). 29he interesting part was the greater evolution rates for both H 2 O 2 and H 2 (274.4 and 305.2 μmol g −1 h −1 , respectively) in contrast to the previous CoNiP-PCN.Although UV-vis light irradiation was utilized in the photocatalytic reaction, the influence of light wavelength was relatively insignificant as demonstrated by the visible-light control experiments.The improved performances exemplified the more superior charge migration capability endowed by homogenous chemical bridging (P-P/P-P) compared to heterogenous bridging (P + -P δ− -Co/Ni δ+ ), and the in situ photodeposition of Co x P cocatalyst (Figure 8E).Additionally, if CoNiP cocatalyst is anchored instead of Co x P, an even enhanced photoactivity can be anticipated as rationalized from the lower ∆G H* .To this end, it is accentuated that the construction of homogenous or  29 heterogenous chemical bridging between the components in a photocatalytic system, by means of synergistical doping and cocatalyst engineering, provides an effective charge transfer channel, whence facilitates the migration of charge from the bulk of photocatalyst to cocatalyst, hence results in more effective photocatalytic surface reaction.

3.1.2
Single-atom cocatalyst/functional group decoration 2][113][114][115] For instance, when the MSA cocatalyst is anchored on g-C 3 N 4 , the absence of metallic bond leads to formation of diverse chemical interactions between the semiconductor surface and MSA cocatalyst, for example, metal-ligand coordination, which enables the unique metal-to-ligand charge transfer process. 11,40,116,118nspired by this idea, Li et al. prepared PCNNi, which was single-atom Ni loaded g-C 3 N 4 by a simple calcination method. 27Under visible-light irradiation, H 2 O 2 and H 2 production rates reached 26.6 and 24.0 μmol g −1 h −1 , respectively.Notably, the atomic dispersion of Ni cocatalyst on g-C 3 N 4 surface was vigorously evidenced by high-angle annular dark field scanning transmission electron microscopy and the Fourier transformed amplitude of extended X-ray absorption fine structure of Ni K-edge spectra, as illustrated in Figure 9A,B, respectively.The latter also revealed that the Ni atoms were coordinated with the N atoms in the g-C 3 N 4 framework.The DFT computations discovered that the transitional state of 2e − WOR (0.5554 eV) was significantly lower than the 4e − WOR (0.8330 eV) pathway, implying that HO-OH intermediates had higher formation propensity compared to O-O species (Figure 9C).Such strategy has also found its applicability in CO 2 photoreduction, 119 N 2 photofixation, 120 and pollutant photodegradation, 121 of which the synthesized photocatalysts exhibited spectacular photocatalytic performances compared to their pristine state.
Owing to its planar molecular structure, introducing functional group onto the surface of g-C 3 N 4 can straightforwardly increase the specific surface area without  22 destroying the structural integrity of g-C 3 N 4 , adjust its energy band structure and improve light absorptivity.Simultaneously, the functional group can act as charge sink to promote the migration of photogenerated holes and electrons.Through functional group modification, precise control of the delocalization of π-electrons in the g-C 3 N 4 planes can be realized, leading to the mitigation of charge carrier recombination. 122At the juncture, covalent chemical functionalization is one of the broadly applied techniques for introducing functional groups through the reaction between the functional group vectors and -C=Nor -NH 2 groups of the g-C 3 N 4 network.Nonetheless, such idea has been applied to explore the versatility of g-C 3 N 4 , as seen in several recent studies like dualfunctional ligands grafted g-C 3 N 4 , and B-N unit decorated g-C 3 N 4 . 123,124Herein, a study by Che et al. demonstrated fabrication of g-C 3 N 3.5 (O 0.5 H 0.5 ) via two-step calcination, followed by acid hydroxylation (Figure 9D). 22Without any further modifications, excellent H 2 O 2 and H 2 evolution rates of 909.8 and 947.7 μmol g −1 h −1 were demonstrated under visible-light irradiation, with an excellent apparent quantum efficiency (AQE) of 10.6% recorded at the 420 nm wavelength.This was a 14-fold enhancement in comparison to pristine g-C 3 N 4 .The outstanding photocatalytic performance was ascribed to the synergistical effect of increased exposure of melem oligomers by secondary calcination and hydroxylation of the surface dominant primary amine (-NH 2 ) group into -OH group.
The resultant Brunauer-Emmett-Teller (BET) surface area (Figure 9E) and light absorption (Figure 9F) were significantly enhanced compared to pristine g-C 3 N 4 .Interestingly, the -NH 2 group was identified to be a 4e − ORR (reversed 4e -WOR) active site, which resonated with the study by Fattahimoghaddam et al.By introducing -OH group, not only HER activity was favored (Figure 9G), but it also provided toxicity-resisting effect toward reactive oxygen species (ROS) and photoexcited holes for the photocatalyst. 125Nevertheless, the works highlighted the merits of applying surface functionalization onto g-C 3 N 4 to augment the surface electronic structure for concurrent superior active sites construction and suppression of competitive side reactions.aerobic condition (Figure 10B).The control experiments also indicated an inhibitory effect of trimesic acid-derived CDs (CDs2) toward the ORR activity in contrast to those of phloroglucinol based, as exemplified by ORR LSV curves.Additionally, ORR LSV curves corroborated the active sites for 2e − ORR and 2e − WOR were assignable to CDs1 and NCN, respectively (Figure 10C).Likewise, the two pathways were strongly supported by EPR radical trapping and RRDE tests.Following this idea, hexaketocyclohexane octahydrate-derived CDs (CRCDs) was devised by He et al., and subsequently anchored onto g-C 3 N 4 via simple calcination (Figure 10D). 127he highest yield of H 2 O 2 from the optimum composite (CN 3 -CRCDs) reached a whopping 3023 μmol g −1 h −1 (Figure 10E).The binary photocatalyst was verified by in situ transient photovoltage tests for possessing WOR-active CDs and ORR-active g-C 3 N 4 .It was worthwhile noting that the g-C 3 N 4 was infused with oxygen reduction capability through VB position shifting induced by a secondary solvothermal step.Nevertheless, the amazing photocatalytic performance of the CH 3 -CRCDs composite can be attributed to the increased absorptivity at wavelength less than 625 nm, appropriate band alignment of CN 3 -CRCDs, and efficient photoexcited charge separation and migration.Aside from CDs, zeolite imidazole framework (ZIF) can be composited with g-C 3 N 4 to enable the two-channel production of H 2 O 2 , as demonstrated by Zhao et al. with their ZIF-8/C 3 N 4 (Figure 10F). 23The photocatalyst presented a superb H 2 O 2 generation rate of 2641 μmol g −1 h −1 (Figure 10G), of which the covalent bonding between the ZIF-8 and g-C 3 N 4 provoked by the carbonization process gave rise to the promoted visible-light absorption capacity.
Nonetheless, the highly porous structure of ZIF-8/C 3 N 4 (Figure 10H) was particularly important to reinforce the molecular diffusion of O 2 and H 2 O 2 for providing sufficient reactant and evacuating the surface-active sites in accelerated manner.In conclusion, these works provided a new entry point to meet high H 2 O 2 productivity, where the photocatalyst suppresses HER activity in favor of 2e -ORR.Within such a system, the communal promotion between oxygen reduction and water oxidation cooperatively results in highly efficient H 2 O 2 photoproduction.

Metal sulfide-based photocatalyst
To date, among the available metal-based photocatalyst, metal sulfide has been receiving ever-growing interest due to its tunable bandgap, modulable chemical composition, earth-abundant sources, etc. Hitherto, the multifarious studies conducted on metal sulfide are mostly centered on transitional metals, such as Cd, Zn, Cu, In, and Sn, [128][129][130][131][132][133] of which possess the d 10 electronic configuration.The atomic orbitals of metal cation and sulfide anion are factually hybridized into intimately spaced molecular orbitals, which ultimately leads to the formation of CB and VB, respectively.In particular, the CB edge of transitional metal sulfide (TMS) is comprised of the d orbital of metal, along with its s and p orbitals, wherein oppositely, the VB edge is constituted of the 3p orbital of sulfur atom. 134Owing to the presence of d orbital, TMSs are usually endowed with negative CB, implying they have potent reduction capability as similar to metal oxides.However, the relatively higher position of S 3p orbital compared to O 2p orbital grants TMS with comparably weaker oxidation power, hence rendering a typical TMS to have strong reduction and mild oxidation power, along with narrower bandgap.The lineage of reported TMS photocatalysts includes singlemetal sulfides (e.g., CdS, MoS 2 ) and dual-metal sulfides (e.g., Zn 3 ln 2 S 6 , Zn x Cd 1-x S). 37,131,[133][134][135] Hitherto, in the perspective of cooperative coupling of H 2 O 2 with H 2 or selective chemical synthesis, CdS has caught the eyes of researchers due to its visible-light responsiveness (E g ≈ 2.4 eV), negative CB edge position, and efficacious photogeneration of electron-hole pairs. 36However, pristine CdS itself is severely obstructed by self-corrosion induced by its photoexcited holes, inherent toxicity, and rapid charge recombination, wherein the photogenerated charge lifetime is less than 2 ns. 106Aside from single-metal sulfides, dual-metal sulfides also hold great potential due to the versatility for engineering the bandgap by simple tuning of elemental composition.Yet they still suffer similar issues similar to that of CdS.Therefore, it is imperative to implement similar modification strategies as seen in the g-C 3 N 4 -based photocatalytic system to create an ideal TMS-based photocatalyst toward realizing bolstered photocatalytic H 2 O 2 production with concurrent H 2 or selective chemical synthesis.

Zn x Cd 1-x S with cocatalyst engineering
Astonished by its composition tunability, Zn x Cd 1-x S decorated with redox cocatalyst was prepared by Xu et al. via a two-step method (Figure 11A,B). 39The optimal composite, Co@NC/Zn 0.5 Cd 0.5 S/Co-Pi was able to cleave water into H 2 (613.8μmol g −1 h −1 ) and H 2 O 2 (592.7 μmol g −1 h −1 ) with near-stoichiometric evolution rate, which were 13.2 and 14.4 times higher compared to pristine Zn 0.5 Cd 0.5 S, respectively.In particular, the cocatalysts Co@NC and Co-Pi acted as electron-and hole-extracting sites, respectively, and they synergistically inhibited the recombination of charge carriers and elevated the charge mobility by extreme degree.This synergy was evidenced by the steady-state photoluminescence (PL) spectra, wherein the PL intensity of Co@NC/Zn 0.5 Cd 0.5 S/Co-Pi was quenched drastically compared to Zn 0.5 Cd 0.5 S (Figure 11C).The composite also exhibited the smallest arc radius in the Nyquist plots obtained from electron impedance spectroscopy, implying its alleviated charge transfer resistance (Figure 11D).Further investigation with the time-resolved PL spectroscopy elucidated that the photogenerated electron lifetime experienced a near fourfold increment from 1.04 to 3.83 ns after the loading of redox cocatalyst.The similar strategy was adopted by Song et al. for synthesizing nanospherical Zn 0.14 Cd 0.86 S wrapped by phosphide protection layer and anchored with Ni 3 Pi 2 cocatalyst (Figure 11E). 41The photocatalyst, denoted as ZCS/PO/Ni 3 Pi 2 performed 2e − WOR with maximum H 2 O 2 and H 2 generation rate of 1464 and 1375 μmol g −1 h −1 , respectively.Unveiling the nature, the Ni 3 Pi 2 cocatalyst underwent in situ conversion and evolved into reductive Ni 2 P and oxidative NiPi cocatalysts, substantiated the feasible occurrence of 2e − WOR (Figure 11F).The inspiring feat was the consecutive two-step in situ photosynthesis method for PO protection layer deposition and Ni 3 Pi 2 decoration, respectively (Figure 11G), which was ingeniously paired with OWS without undesired oxidation product generated while yielding impressive H 2 production rate (9260 and 9530 μmol g −1 h −1 ).This strategy fully exploited the usability of the photocatalyst preparation process.Nonetheless, the works emphasize the vital role of redox cocatalyst on TMS-based photocatalysts for optimizing the effectiveness of photoexcited charge separation and migration that are conducive for bolstering the surface photoredox reaction.Conversely, there has been reported study stressing the limited amelioration of photocatalytic performance induced by loading Reproduced with permission: Copyright © 2023, Elsevier B.V. 41 of single reduction or oxidation cocatalyst, 20 hence it is strongly suggested to divert attention toward the fabrication of redox cocatalysts, which is crucial for attaining a dual-functional photocatalyst endowed with extraordinary activity, stability, and reaction selectivity.

CdS with heterojunction engineering
Aside from cocatalyst loading, heterojunction construction is also an auspicious strategy for exploiting the merits of the individual semiconductors in a typical heterostructure. 110Up to date, the discovered heterojunctions are categorizable into type II, Z-scheme, S-scheme, p-n junction, etc., and among them, Z-scheme and Sscheme heterojunctions stand out due to their capabilities in preserving the more superior redox active sites that ultimately lead to the robust utilization of solar energy. 136n a related work, Wang et al. synthesized a Z-scheme heterostructure composed of zero-dimensional W 18 O 49 quantum dots and one-dimensional CdS nanorods, denoted as W 18 O 49 /CdS. 38The optimal composite, WCS300 was able to produce H 2 O 2 and H 2 at rate of 32.01 and 87.71 μmol g −1 h −1 , respectively.However, there is either lack of ground evidence or clear elucidation to support that the composite is following a Z-scheme charge transfer mechanism, though the authors did rationalize the pivotal importance of morphology in improving the overall photocatalytic performance.In this regard, a Z-scheme CdS hollow cubes@Znln 2 S 4 nanosheets (NSs) heterojunction was fabricated by Zhang et al.
(Figure 12A). 37The Z-scheme composite was capable of achieving near-stoichiometric evolution of H  interface, an internal electric field was constructed that facilitated electrons flow from Znln 2 S 4 to CdS, representing the Z-scheme mechanism (Figure 12B).Pioneeringly, Kelvin probe force microscopy-based spatially resolved surface photovoltage mapping technique was utilized to visually determine the Z-scheme charge migration pathway.As demonstrated in Figure 12C, under light irradiation, CdS@ZnIn 2 S 4 experienced a dramatically high surface potential change compared to that of pristine CdS (Figure 12D), wherein the signal was exclusively assigned to ZnIn 2 S 4 .This phenomenon conspicuously manifested the accumulation of photogenerated holes in ZnIn 2 S 4 , which was concordant with the Z-scheme mechanism.
In addition, Fang et al. prepared a BiOBr/CdS heterostructure via a solvothermal method (Figure 12E). 43otably, the heterojunction was applied to perform twochannel H 2 O 2 photoevolution with rate of 346.4 μmol g −1 h −1 , which is principally identical to that previously mentioned in the g-C 3 N 4 section.The DFT computational and experimental results corroborated that the composite was of S-scheme nature, as illustrated in Figure 12F.However, information regarding the surface free-energy level of intermediate species was either not provided or evaluated, which was crucial to unveil the underlying principles of the photocatalyst activity.To conclude, heterostructure construction provides the beneficial features, for example, exploitation of superior and spatially separated redox active sites, boosted charge separation efficiency, etc., which in the context of concurrent H 2 O 2 and H 2 or selective chemical synthesis, the photocatalyst poisoning caused by H 2 O 2 can be reduced or even mitigated, giving rise to long-term stability, and enhanced photocatalytic performances.

Other strategies
Other strategies reported for modifying TMS-based photocatalysts toward application on simultaneous H 2 O 2 and H 2 or selective chemical synthesis include homojunction construction and defect engineering. 40,42For instance, Cd 0.5 Zn 0.5 S nanotwin consists of distinct wurtzite and zinc blende crystal phase segments by itself is a structured homojunction that can inherently suppress selfphotooxidation (Figure 13A).Therein, Liu et al. synthesized a near-surface P-doped Cd 0.5 Zn 0.5 S nanotwin co-modified with red phosphorus and Co 2 P (CZS-P-Co 2 P) (Figure 13B). 40Importantly, homogenous phosphorus bridge was established between the P-doped surface and red phosphorus as well as Co 2 P (Figure 13C), which facilitated electron transport by acting as effective transfer channel, thereby yielding H 2 O 2 and H 2 in 801.3 and ∼550 μmol g −1 h −1 , respectively.This homogenous chemical bridging is similar to the g-C 3 N 4 -based PCN-y-CP-z, 29 and importantly, the performed DFT calculations on different transitional metal phosphide (TMP) cocatalysts (Co 2 P, Ni 2 P, and FeP) also predicted a similar trend in ∆G H* : Co 2 P was closer to thermoneutral conditions than other TMPs.Surprisingly, the photocatalytic performance of Ptloaded CZS fell behind all the studied TMP-modified CZS (Figure 13D).As for defective TMS-based photocatalyst, Luo et al. developed monodispersed sea urchin-like Zn 3 ln 2 S 6 with lattice sulfur defects for stoichiometric H 2 O 2 and DHIQs syntheses (Figure 13E).To the best of our knowledge, this pioneering work reported on coupling H 2 O 2 production with selective chemical synthesis.In this photocatalytic system, DHIQs were produced from selective oxidation of tetrahydroisoquinoline derivatives (THIQs), while H 2 O 2 was resulted from 2e − ORR.The photocatalytic activities were recorded at outstandingly high levels, with H 2 O 2 and DHIQs being evolved at 66 400 and 62 100 μmol g −1 h −1 , respectively, under visible-light irradiation, on top of that with exceptional THIQs conversion (90.0%) and selectivity toward DHIQs (92.1%).Such excellent performances were attributed to the introduction of lattice sulfur defects that not only accelerated transport and transfer of photoexcited charges, but also enriched In atoms with charges that led to facilitated adsorption and excitation of molecular oxygen.DFT computations confirmed that the charge-rich In sites possessed the lowest free energy for O 2 adsorption and activation, and also that of H 2 O 2 desorption (Figure 13F).Critically, time-dependent in situ Fourier transform infrared spectroscopy was able to detect characteristic bands of DHIQs (1500 cm −1 ), superoxide intermediate (⋅OOH) (1268 cm −1 ), and H 2 O 2 (1386 and 832 cm −1 ) during the photocatalytic reactions (Figure 13G), whence verified that DHIQs and H 2 O 2 were successfully synthesized.This work has therefore paved the way for coupling H 2 O 2 photoproduction with reactions beyond HER to achieve the photoredox production of valuable chemicals.

Organic polymer-based photocatalyst
Beyond the horizon of g-C 3 N 4 , other organic-based photocatalysts, for example, covalent organic frameworks (COFs), covalent triazine frameworks (CTFs), covalent heptazine frameworks (CHFs), donor-acceptor polymer (D-A), etc., have captured the enthusiasm of researchers in recent years.As discussed, g-C 3 N 4 by itself is insufficient to provide the adequate driving force for photocatalytic reactions to occur in satisfactory manner, hence augmentation strategies are incumbent to modify g-C 3 N 4 toward realizing enhanced photocatalytic performances.On the contrary, the other organic photocatalysts offer several intrinsic merits over g-C 3 N 4 , for instance, they are synthesizable from a wide range of organic compounds and synthesis methodologies, resulting in the formation of unique and vast chemical linkages with specialized functionalities. 118n addition to that, the physical and photoelectrical properties such as morphology, crystallinity, optical bandgap, band positions (lowest unoccupied molecular orbital and highest occupied molecular orbital), light absorptivity, etc., can be facilely modulated by rational design of reaction conditions and prudent selection of the organic precursors.The high polymerization level of the material also invokes insolubility and stability against water and common organic solvents, which allows easy separation through filtration and be reused continuously. 137Herein, the recent progress of organic-based photocatalysts applied for synchronic photocatalytic H 2 O 2 and H 2 or chemical synthesis are reported in sequential manner, starting with CTFs and CHFs, followed by the D-A polymer and eventually COFs.

Covalent triazine/heptazine frameworks
In a pioneering work, Chen et al. prepared CTFs bearing acetylene or diacetylene moieties, CTF-EDDBN and CTF-BDDBN, respectively, via trimerization reactions and subsequent exfoliations. 45In contrast to CTF-BPDCN that had no acetylene or diacetylene linkers and only produced H 2 O 2 via 2e − ORR, the former CTFs were able to generate H 2 O 2 by the two-channel mode (Figure 14A), with the maximum production rate recorded at 97.2 μmol g −1 h −1 by CTF-BDDBN NSs, which to the best of our knowledge, is the first to be reported for CTF-based photocatalyst.The introduction of acetylene and diacetylene moieties was verified to have contributed significantly to the formation of stable-delocalized charge in CTF-EDDBN and CTF-BDDBN, and improved the coplanar conformation that favored charge separation.Importantly, in situ characterizations and isotope labeling tests using H 2 18 O validated that the oxygen reduction and water oxidation followed by CTF-EDDBN and CTF-BDDBN were of two-electron mode (Figure 14B).
DFT computations showcased that the energy barrier associated with the formation of adsorbed OH was significantly lowered at the acetylene and diacetylene active sites compared to triazine (Figure 14C-E).This strongly evidenced the propensity of H 2 O 2 formation via the 2e − WOR on the acetylene or diacetylene-bearing CTFs.Taken this inspiration, Wu et al. fabricated (thio)urea-functionalized CTFs (Bpu-CTF and Bpt-CTF) (Figure 14F), 46 wherein the optimal CTF grafted with thiourea (Bpt-CTF) achieved remarkable photoproduction of H 2 O 2 at rate of 3268.1 μmol g −1 h −1 and AQE of 8.6% (400 nm).The astonishing photocatalytic performance was attributed to the improved polarization endowed by thiourea group that drastically elevated the charge separation and electron transfer.The time-dependent DFT calculations conspicuously indicated that the electron and hole accumulations were promoted at N 2p orbital and C=S groups, respectively, after the polar moiety was introduced (Figure 14G,H).Interestingly, the photocatalytic system was found to follow the cascaded 4e − WOR-2e − ORR pathway, 69 as evidenced by critical analyses, such as in situ DRIFTS, EPR spin trapping tests, and Ar-atmosphere control experiments.As a continuation of the research, Cheng et al. rationally designed CHFs carrying spatially separated redox centers, of which tri-s-triazine and acetylene (or diacetylene) moieties acted as reduction and oxidation centers, respectively (Figure 14I). 47The photocatalysts were applied for twochannel H 2 O 2 production, with the best photoevolution performance achieved by CHF-DPDA, the diacetylenebearer at a rate of 1725 μmol g −1 h −1 and an extremely high solar-to-chemical conversion (SCC) efficiency of 0.78%.By incorporating the spatially separated redox centers, charge separation was facilitated in CHF-DPDA owing to decreased exciton binding energy, wherein the photoexcited electrons and holes were accumulated at the tri-striazine and acetylene/diacetylene moieties, respectively (Figure 14J).In addition to that, the average lifetime of photogenerated charge carrier was extended to an impressive degree of 17.20 ns for CHF-DPDA, again demonstrating the advantages of decorating spatially separated redox centers onto a photocatalyst.Overall, these works highlighted the significance of embedding spatially separated redox functional groups to boost the photocatalytic performance of CTFs and CHFs toward achieving photoredox production of H 2 O 2 .

Donor-acceptor polymer
9][140][141] For typical D-A polymer, the acceptor is by nature electron deficient, and acts as electron trapping site where photoexcited electrons are generated and migrated from the donor.Conspicuously, the donor accumulates with photogenerated holes and acts as the oxidation center.Endowed by the conjugation and electronic pull-push effects, D-A polymers possess the traits of prominent light responsiveness and accelerated photoexcitons splitting, which are beneficial toward high photocatalytic activity. 142Fascinated by their intrinsic merits, Yang et al. synthesized a porous nanorod-like D-A polymer (NMT400) via a synergistic combination of supramolecule construction and calcination treatment, as demonstrated in Figure 15A,B. 48he tuning of ratio between benzene (D) and triazine (A) and simultaneous introduction of cyano-group linkages into the polymer structure were realized by the strategy (Figure 15C), which drastically changed the electronic structure, and resulted in the maximized two-channel photoevolution of H 2 O 2 at a rate of 270.9 μmol g −1 h −1 .The porous nanorod-like morphology was also crucial toward the photoactivity as it provided more accessible active surface areas and decreased the migration distance of excitons.The role of cyano-groups was further exemplified by DFT computation wherein they were capable of realizing the strong chemisorption of O 2 and enhancing the migration of electron to adsorbed O 2 (Figure 15D).This was in sharp contrast with the insignificant adsorption energies (>−0.15eV) exhibited by other moieties such as benzene, triazine, and amide groups.Nonetheless, this work demonstrated the synergy of morphology control and functionalization to attain ameliorated photocatalytic performances.In this regards, RF-DHAQ-2  15E). 50The resorcinol-formaldehyde (RF) framework was incorporated with 1,4-dihydroxyanthraquinone (DHAQ) to decrease the D/A ratio via the extended Stober method, and resulted in an excellent H 2 O 2 yield and SCC efficiency of 1820 μmol g −1 h −1 and 1.2%, respectively.In particular, the DHAQ can be reversibly transformed into 1,4-dihydroxyanthrahydroquinone, which promoted the formation of intermediate species due to the quenced energy levels of transition state compared to that of pristine RF (Figure 15F).This consequently diminished the energy obstruction in the 2e − ORR pathway, whence enhanced the overall photocatalytic production of H 2 O 2 .In short, these works conclusively stressed the pivotal role of morphology regulation and structural functionalization in the synthesis of D-A polymer for attaining excel photocatalysts.

3.3.3
Covalent organic framework COF is another hotspot in the realm of organic photocatalyst, where in recent years, it is deployed in substantial and diverse photocatalytic applications such as CO 2 reduction, [143][144][145][146] organic transformation, [147][148][149][150] pollu-tant abatement, 151 etc. Derivable from a significant number of precursor species and interlinked by rigid covalent bonding, the light-harvesting capability, charge transport behaviors, and redox reaction sites are tunable with high precision via rational selection of suitable monomer blocks and their reticulation within COFs. 152The reversible condensation reaction schemes such as Schiff base reaction, spiro-borane condensation, etc., are routinely used for COFs fabrication, 153 and the morphological analysis indicates that COFs are endowed with orderly porous crystalline network structure, which favored the suppression of charge recombination.Owing to these attributes, Kou et al. prepared bipyridine-based COF (COF-TfpBpy) for dualchannel photocatalytic H 2 O 2 production (Figure 16A), with the optimized photosynthesis rate reached 694.6 μmol g −1 h −1 . 51The bipyridine monomer was discovered to critically affect the overall photocatalytic performances by changing the electron transfer mode in the two-electron processes, as in contrast to non-bipyridine-based COF-TfpDaaq (Figure 16B).The DFT calculations explicitly verified that the adsorption energy for two H 2 O molecules onto the nitrogen atom of bypyridine moiety was lower than that of single H 2 O molecule, which corroborated that one-step 2e − WOR was followed (Figure 16C (i) and (ii)).
Further EPR spectroscopy and radical scavenger tests also provided strong evidences that both 2e − WOR and 2e − ORR followed the one-step process toward H 2 O 2 production upon the incorporation of bipyridine monomer.On another work, the strategy of cooperative construction of spatially separated redox centers and morphology control was adopted by Chang et al. for solvothermal synthesis of crystalline spherical TTF-BT-COF (Figure 16D,E). 52n this photocatalyst, tetrathiafulvalene (TTF) and benzothiazole (BT) moieties acted as effective hole and electron trapping sites, respectively, and performed the bichannel production of H 2 O 2 at rate and AQE of 2760 μmol g −1 h −1 and 11.19% (420 nm), respectively.The differential charge density analysis determined that charge accumulation and depletion zones were presented at the BT and TTF regions, respectively, ascribed to the more-electronegative S atom of BT compared to that of TTF (Figure 16F).This phenomenon depicted the distinct redox centers for facilitating charge separation and the feasible occurrence of 2e -ORR and WOR, of which supported by the quenched PL spectra and Nyquist plots of TTF-BT-COF.Withal, the synergistic effect of implementing multiple modifications, for example, morphology tuning and incorporation of spatial redox sites, has again demonstrated to vehemently improve the performances of photocatalytic systems.

Other photocatalysts
Metal oxide-based photocatalysts share similar reduction capabilities like that of metal sulfide but owing to its inherently lower position of O 2p orbital as aforementioned, it also possesses powerful oxidation sites.In addition, the syntheses of metal oxides are facile and adaptable to multifarious types of modifications, along with inherent lower-toxicity, biocompatibility, and good stability compared to metal sulfide. 5These characteristics therefore render this class of photocatalytic materials with high potential for H 2 O 2 photoevolution via 2e − ORR and 2e − WOR.For instance, Zhu et al. synthesized a titanium silicalite-1 (TS-1)-supported CoO nanodots (CoO-TS-1) (Figure 17A,B). 64The TS-1 substrate was crucial for adsorbing H 2 O 2 in virtue of enhancing the stability of CoO and facilitated the monodispersing of CoO nanodots.This strategy consequently led to an optimized photocatalytic activity for synchronous evolution of H 2 and H 2 O 2 at rates of 1460 and 1390 μmol g −1 h −1 , respectively, and extraordinarily extended stability beyond 168 h.Furthermore, as evidenced by o-tolidine UV-vis spectroscopy results, no H 2 O 2 was detected in the 24-h reaction supernatant, rather the ROS was in situ adsorbed by TS-1 (Figure 17C), which   17D).In direct contrast to conventional powder-suspension photocatalytic system, the innovative combination of floatable polystyrene spheres with the hydrophobic heterojunction gave rise to a gas-solid-liquid triphasic system that can expedite the photocatalyst separation and recycle process, along with improved O 2 diffusion (Figure 17E).17G). 69The anchoring of CDs accelerated the electron transfer from the CB of CoP to the surfaces of CDs, while suppressed the recombination of electronhole pairs as indicated by uphilled transient photocurrent responses compared to pristine CoP (Figure 17H).This enhancement therefore led to optimized production of H 2 and H 2 O 2 at 239 and 466 μmol g −1 h −1 , respectively, under saturated air condition.RRDE measurements of the electron transfer numbers in CoP and CDs were 3.6 and 1.7, respectively, which corroborated the cascaded pathway.
In continuation of the research, Ni 2 P replaced CoP in the composite photocatalyst, and this brought about a significantly ameliorated H 2 O 2 evolution rate (1080.4μmol g −1 h −1 ), but not for the case of H 2 .This can be attributed to the higher BET surface area of Ni 2 P/CDs composite and larger |∆G H* | of Ni 2 P that has been reported in multiple studies, respectively. 11,40Similarly, the competitiveness between HER and oxygen evolution reaction activities was observable as well; however, the reaction atmosphere was adjustable to tune the reaction selectivity (Figure 17I).

CONCLUSION AND PROSPECTS
In summary, this review revolved around the green concept of cooperative photoredox production of H 2 O 2 and value-added chemicals, along with the fundamental mechanisms of the solar-driven process.In contrast to the conventional energy-intensive anthraquinone process and sacrificial-agent involved photosystem, cooperative photoredox reaction coupling has emerged to be a favorable approach in establishing green-sustainable and costperformance efficient process.Nonetheless, the cooperative photoredox reaction coupling system accentuated on H 2 O 2 production has shown remarkable progress, and from the viewpoint of real-world applications, such system holds great potential in establishing a decentralized in situ, on-demand production of H 2 O 2 with down-scalability and avoiding the cost of storage and transportation.This is in direct contrary to the centralized anthraquinone process that is not down-scalable and requires multiple unit operations to producing H 2 O 2 .On top of that, since additional value-added organics are produced, the overall economic value of the process is enhanced.However, such a dual-functional photoredox system is still in its infancy, there remain simultaneously extensive potentials and longstanding challenges in this flourishing area requiring substantial and longterm efforts to further advance this photocatalytic field (Figure 18).
First, the absence of standardized measurement of performance, which enables an unambiguous point-to-point comparison of various photocatalysts has yet to be developed, posing a pivotal obstruction to foster the field.Till now, most reports favor two common measures that are (1) related to catalyst mass (μmol g −1 h −1 or μM g −1 h −1 ), and (2) correlated to light illumination power (AQE).A study has pointed out that the photocatalytic performance is not to be normalized with regard to catalyst mass due to the uncertain direct proportionality between the formation rate and mass of catalyst employed. 1,16This is due to that most papers tend not to report on the irradiation strength, and such variation hinders the direct comparison of reaction rates of photocatalysts reported, albeit such term is still reported in this review.In fact, AQE serves as a more appropriate indication of photocatalytic efficiency that is independent of illumination conditions but still associated with the experimental conditions and measurements.Yet a universal standard spectrum for AQE evaluation is absent and will result in certain differences between different researches.Alternatively, it is recommended that in future research, the SCC efficiency is to be included (Equation 10).SCC efficiency is acquired by applying the AM 1.5G filter and stood out as a useful indication to quantify the efficacy of energy conversion from solar energy to chemical energy.Since cooperative photoredox system exhibits potent possibility to achieve industrial practicability, it is necessary to achieve a SCC efficiency of more than 1%. 154,155Withal, to cogitate the economic feasibility of such an organic-involved process, a value-oriented metric of solar-to-value rate (SVR) can be reported with regard to the selected organic (Equation 11).SCC efficiency for H 2 O 2 is expressed in Equation (10).
SVR can also be expanded to include the cost of product collection and separation, catalyst preparation, and scaling factors, which necessitates comprehensive technoeconomic analysis and life-cycle assessment (LCA). 154][158] Additionally, integration of multiple driving forces, such as photonic, thermal, electric, and/or sonic energy, 140,141,159 onto a photocatalytic system represents a plausible strategy for boasting the overall efficiency of the cooperative photo redox reaction.For instance, the combination of piezoelectric effect and photocatalysis has been demonstrated to be an appealing strategy toward enhancing the photocatalytic performance of semiconductors.The amelioration arising from the piezoelectric effect is due to the external stress-induced internal electric field that facilitates charge separation, thereby leading to higher charge availability for the photocatalytic reactions.The external stress can be sourced from various origins, for example, ultrasonication, fluid-shear stress, bending stress, etc. 145,146 With the aims of further boosting the catalytical prowess, the commonly adopted strategies such as cocatalyst loading, defect engineering, and heterojunction construction, which have been utilized to enhance the charge migration behavior, can be synergistically integrated along with piezoelectric effect.Such strategy has seen in numerous research including N 2 fixation, pollutant degradation, and H 2 O 2 production, of which they have exhibited elevated photocatalytic performances compared to their pristine state of being excited by photoirradiation only. 149,150,160,161otably, a system in which 2e -WOR is coupled with CO 2 photoreduction is expected to improve tremendously, given the additional forces introduced to break the rigid thermodynamic barriers of the two reactions.At the juncture, this type of coupling reaction remains scant in the photocatalysis field, but as a direct mimicking of the natural photosynthesis process (so dubbed artificial photosynthesis), it is the ultimate goal that is destined to be struck.In the quest to search for the advanced photocatalyst for efficient 2e − WOR performance, metal oxides with concurrent suitable O* and OH* binding energies (Δ O ≳ 3.5eV and Δ OH ≲ 2.4eV) are suggested to be sought after for selective H 2 O 2 generation. 162From the perspective of electrocatalytic 2e − WOR, among the studied metal oxides, WO 3 , BiVO 4 , MnO 2 , and SnO 2 possess weak O adsorption energies, thus promote 2e − WOR to produce H 2 O 2 . 163otably, IrO 2 , PtO 2 , and RhO 2 feature strong O adsorption energies; therefore, they tend to favor the formation of O 2 (i.e., facilitate the 4e − WOR pathway).It has been mentioned before that electrocatalysts can be incorporated into a photocatalytic system as cocatalysts to ameliorate the efficacy of H 2 O 2 production.Here, it is recommended that the photocatalytic research community refers to the works of those excel in the electrocatalytic realm to gain insights for the rational design of advanced photocatalytic systems capable of driving the 2e − WOR.
Second, extensive efforts are required in uncovering the mechanisms of cooperative photoredox synthesis of H 2 O 2 and selectively oxidized organics.The investigations on the mechanisms of organic oxidation remain in the preparatory phase ascribed to the complexity of reaction pathway and shortcomings of conventional characterization techniques.The C-H bond activation and cleavage have generally been regarded as the initiation step of selective organic synthesis, 164 yet research regarding the thermodynamic constraints of CB minimum and VB maximum for the process is scarce.The information is of great scientific interest to steer the design philosophy of photocatalyst, thus in situ/operando characterization techniques are employable to this end.For instance, in situ DRIFTS, X-ray absorption spectroscopy, and EPR spectroscopy can be conjointly utilized to monitor the real-time evolution of reaction intermediates and products during photoirradiation.Going deeper, theoretical DFT calculations are necessary to be performed and interpreted alongside the experimental results, so to decipher the reaction pathway at the atomic/sub-atomic/molecular level.Therefore, through the concerted efforts of experimental and theoretical analyses, the shroud on the mechanisms of cooperative photoredox synthesis of H 2 O 2 and selectively oxidized organic can be unraveled, which will benefit the fine tuning of photocatalyst via rational decisions based on the uncovered mechanisms.Moreover, following the widespread and ever-bursting growth of artificial intelligence (AI) in recent years, a formidable tool has descended into the photocatalysis research field.The incorporation of AI enables an automated analysis of various photocatalytic materials, and by providing sufficient high-quality data sets, AI can rapidly identify the patterns exhibited by the photocatalyst.Therefore, the structure-property relationships can be unveiled, which is comparable to DFT calculations. 165,166Significantly, the utilization of AI can leverage high-throughput experiments and calculations, thus allowing sparse or expensive materials to be allocated onto the most promising candidates.Consequently, fast-track discovery and optimization of photocatalysts are realized through the concurrent high-throughput computational and experimental screenings.
Third, in order to achieve industrial feasibility, persistent operation of photocatalyst under low solar flux particularly beyond sunset is crucial to maximize the production of H 2 O 2 and value-added organics.To tackle the issue, persistent and memory-based photocatalysts are suggested for future exploration owing to their capability to operate under low solar flux beyond sunset, thereby satisfying the industrial requirement of day-tonight operability. 167For elucidation, Loh et al. has inaugurated persistent photocatalysis by utilizing defect-laden charge storage materials with multivalent states. 168Their study demonstrated an enhanced photoconductivity that sustained photocatalytic effect even without light supply, thereby enabling photoproduction during the night.Moreover, considering photocatalytic materials are mostly nanoscale, it is crucial to develop techniques that can facilitate large-scale production of photocatalysts without sacrificing its physicochemical properties and intrinsic photoactivity.This predicament is exceptionally important to race toward a practical pilot or industrial scale photocatalytic farm, albeit to the best of our knowledge, till now the synthesis of photocatalyst exerted in lab scale only.On the flipside, 3D printing (addictive manufacturing) represents a promising solution to address the particulate nature of photocatalysts and its burdensome post-reaction separation process. 169,170In this context, 3D printing technology provides a novel approach to construct the catalyst-supporting substrates with complex geometrical configurations, of which realizing the customization of the substrate structure.This strategy can ameliorate the exposed surface area, light-harvesting capability, mass and heat transfer phenomena of the photocatalyst, [171][172][173][174][175] thereby improving the photocatalytic performances and resolves the issues troubling the post-reaction separation process without significantly compromising the photocatalytic activity.In addition, spotlight should be given to the development of photoreactor for different operation mode as well.As factors such as light source, flow rate, reaction temperature, etc., are influential toward the photoactivity, they have to be assessed systematically during the design of photocatalytic reactor. 16The ideal reactor should be engineered in the way that maximized light penetration and absorption are realized, while the masstransport and heat-transfer phenomena are facilitated.In this regard, the scientific community can look into continuous flow photoreactor, which composed of consecutive but segmented quartz tubes. 176Such reactor not only allow the even distribution of light irradiation but also enhanced the mass-and heat-transfer phenomena.Herein, an enhanced mass-transport is beneficial to provide sufficient amount of reactant to the redox active sites of catalyst, and simultaneously allows the "good-timed" desorption of product molecules.In such way, the photocatalytic process can infinitely reach the ideal absorption-desorption dynamic equilibria.On the flipside, excellent heat-transfer is prominent to prevent the formation of local hotspot in the photoreactor, as in to prohibit the degradation of catalyst, reactant, product, or reaction intermediate.Additionally, the associated product overoxidation or side reactions are mitigatable due to continuous feeding of reactants and removal of products, thereby leading to outstanding selectivity of desired product.To this end, as a complementary fragment toward industrialization, extensive efforts should be devoted to integrating the major protocols (e.g., catalyst synthesis, characterizations, performance benchmarking, computational screening) into a fully automated system.The significance of automation is that all procedures are performed in-line with minimal human interventions, thus increasing experimental reproducibility and establishing standardization across different manifestos. 177There has been reported studies that utilized robotic formulation platforms for synthesis of collection of conjugated co-polymer photocatalysts, 178 and the use of robotic arm that traverses between different workbenches to perform catalyst preparation, photocatalytic reaction, and gas chromatography analysis. 179Nevertheless, this certainly calls for the multidisciplinary cooperation between material scientists, theoretical chemists, AI, robotic engineers, etc.
Looking ahead, despite the existing challenges, the cooperative photoredox synthesis of H 2 O 2 and selectively oxidized organics demonstrates the vast potential in con-current generation of valuable chemicals with effective utilization of solar power.There remain plenty of space to be explored in this field, especially the realization of photosystem performing the combination of highly efficient CO 2 photoreduction, which is considered the holy grail of photocatalysis, with 2e -WOR, which requires only water.Such coupling strategy can "kill two birds with one stone" whereas aforementioned it not only mimics the natural photosynthesis process (herein dubbed artificial photosynthesis), but also aligns with the global aspiration toward accomplishing the United Nations Sustainable Development Goals.Nonetheless, artificial photosynthesis by integrating CO 2 reduction with 2e − WOR offers copious of organic synthesis potential given the selective evolution of CO 2 reduction product can be fine-tuned via rational design of photocatalyst.It holds exceptional values for human society in terms of addressing the anthropogenic CO 2 emissions, energy crisis, and potentially replacing majority of the energy-intensive and environmentally hostile industrial processes today.

F I G U R E 1
Number of citations and publications recorded over the past 10 years with the keywords "photocataly*" and "H 2 O 2 ."Source: Web of Science, accessed on October 20, 2023.

F I G U R E 3
Schematic illustration of general photoexcitation process with redox potentials for reactive oxidation species (ROS) evolution.All redox potentials provided are at pH 0.

F I G U R E 5
Analytical methods for reaction pathway analysis in the application of cooperative photoredox production of H 2 O 2 and value-added chemicals.

F I G U R E 6
Selection of H 2 O 2 quantification method based on the presence of organics in the photocatalytic system.Abbreviations: ICM, iodometry colorimetric method; KRT, KMnO 4 redox titration; NCM, NH 4 VO 3 colorimetric method; SOF, selective organic formation.potential of cooperative photoredox reaction coupling system.

F
I G U R E 8 (A) Transmission electron microscopy (TEM) image of CoNiP-PCN composite (mazarine circles indicate CoNiP nanoclusters).(B) P 2p X-ray photoelectron spectroscopy (XPS) spectra of PCN and CoNiP-PCN.(C) Density functional theory (DFT) calculations of atomic hydrogen adsorption free energy for the cocatalysts Ni 2 P, Co 2 P, and CoNiP.Reproduced with permission: Copyright © 2019.Elsevier B.V. 11 (D) TEM image of PCN-y-CP-z.(E) Schematic illustration of homogenous P-P/P-P bridge on PCN-y-CP-z and the migration route of charge to participate in 2e + water oxidation reaction (WOR).Reproduced with permission: Copyright © 2022, Elsevier B.V.

F
I G U R E 9 (A) High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of PCNNi (red circles depict single-atom Ni).(B) Fourier transform amplitude of extended X-ray absorption fine structure (EXAFS) of Ni K-edge spectra.(C) Energy levels computation of 2e − water oxidation reaction (WOR) and 4e − WOR pathways for PCNNi.Reproduced with permission: Copyright © 2021, The Royal Society of Chemistry. 27(D) Synthesis strategy for g-C 3 N 3.5 (O 0.5 H 0.5 ) photocatalyst.(E) Volume of adsorbed gas measured by Brunauer-Emmett-Teller (BET) and (F) ultraviolet-visible (UV-vis) absorption spectra of prepared photocatalysts.(G) Cycle stability of g-C 3 N 3.5 (O 0.5 H 0.5 ) (red) and g-C 3 N 3.5 (N 0.5 H 1 ) (black).Reproduced with permission: Copyright © 2019, The Royal Society of Chemistry.

F
I G U R E 1 2 (A) Field emission scanning electron microscopy image of CdS@ZnIn 2 S 4 hollow cubes.(B) Band energy structure diagram of CdS@ZnIn 2 S 4 hollow-cube before contact, after contact, and migration pathway of photogenerated electrons upon light irradiation.Surface photovoltage mapping (SPVM) mapping image of (C) CdS@ZnIn 2 S 4 hollow cube and (D) CdS excited with 450 nm light.Reproduced with permission: Copyright © 2021, Elsevier B.V. 37 (E) Scanning electron microscopy (SEM) image of BiOBr/CdS.(F) Band energy structure diagram of CdS@ZnIn 2 S 4 hollow-cube before contact and after contact, and the migration pathway of photogenerated electrons upon light irradiation.Reproduced with permission: Copyright © 2023, Elsevier B.V.43 High-resolution transmission electron microscopy (HRTEM) image of Cd 0.5 Zn 0.5 S with illustration showing the boundaries of nanotwin.(B) HRTEM image of CZS-P-Co 2 P. (C) Illustration of constructed homogenous P bridges in CZS-P-Co 2 P. (D) Photocatalytic H 2 evolution rates in pure water splitting of prepared photocatalysts.Reproduced with permission: Copyright © 2022, Elsevier B.V. 40 (E) Scanning electron microscopy (SEM) image of Zn 3 ln 2 S 6 .(F) Free energy diagram for O 2 conversion to H 2 O 2 at different adsorption sites of Zn 3 ln 2 S 6 .(G) Time-dependent in situ Fourier transform infrared (FTIR) spectra of Zn 3 ln 2 S 6 in 25 mM tetrahydroisoquinoline derivatives (THIQ)/CH 3 CN solution.Reproduced with permission: Copyright © 2023 John Wiley & Sons, Inc.42

F I G U R E 1 4
(A) Schematic illustration of the two-electron reaction pathways toward H 2 O 2 production using different covalent triazine frameworks (CTFs).(B)The relative intensities of 16 O 2 and 18 O 2 before and after light irradiation as measured by gas chromatograph-mass spectrometry (GC-MS).Density functional theory (DFT)-computed free energy diagrams of (C) triazine ring, (D) acetylene moiety, and (E) diacetylene moiety.Reproduced with permission: Copyright © 2020, John Wiley & Sons, Inc. 45 (F) Synthesis scheme of Bpu-CTF and Bpt-CTF from different urea precursors.(G) Electron and (H) hole distribution of Bpt-CTF in selected excited state, as determined by time-dependent DFT calculations.Reproduced with permission: Copyright © 2022, John Wiley & Sons, Inc. 46 (I) Schematic illustration of H 2 O 2 photoevolution via the two-electron pathways on the spatially separated redox centers of covalent heptazine frameworks (CHFs).(J) DFT-calculated electron distributions in CHFs under photoirradiation (dashed circle is acetylene/diacetylene moiety).Reproduced with permission: Copyright © 2022, John Wiley & Sons, Inc.47

F I G U R E 1 7
(A) Transmission electron microscopy (TEM) and (B) high-resolution TEM (HRTEM) images of CoO-TS-1 (inset shows the d-spacing of CoO).(C) o-Tolidine ultraviolet-visible (UV-vis) absorption spectra of 24-h reaction supernatant and precipitate for CoO-TS-1 (inset shows the o-tolidine-induced color-change of reaction components).Reproduced with permission: Copyright © 2019, The Royal Society of Chemistry. 64(D) TEM image of TBO40.(E) Schematic illustrations of O 2 flow supply for the conventional and triphasic systems for O 2 reduction and furfuryl alcohol oxidation.(F) In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of TBO40 over the reaction time course with 10 min sampling interval.Reproduced with permission: Copyright © 2022, John Wiley & Sons, Inc. 66 (G) Scanning electron microscopy (SEM) image of CoP/carbon dots (CDs).(H) Transient photocurrent responses of CoP/CDs and pristine CoP.Reproduced with permission: Copyright © 2020, Elsevier B.V.

F I G U R E 1 8
Prospects in the realm of cooperative photoredox synthesis of H 2 O 2 and value-added chemicals aimed toward realizing the United Nations Sustainable Development Goals (UNSDGs).

Synthesis method Reaction medium (atmosphere) Light source Product evolution rate (µmol g −1 h −1 ) AQE (%) (wavelength) Stability (h) Reaction pathway Ref.
Progress summary of photocatalytic system for H 2 O 2 photoproduction coupled with HER or selective chemical synthesis.Development roadmap of (up) sacrificial photocatalytic systems for H 2 O 2 4,10,72,73,77-79 ; and (down) dual-functional photocatalytic systems for photoredox synthesis of H 2 O 2 and value-added chemicals. 25,37,42,45,66holes.The pioneering work by Xue et al. demonstrated a dual-functional photoredox system for concurrent H 2 O 2 and H 2 production, 11 since then multifarious types of materials have been reported to drive the photoredox synthesis of H 2 O 2 coupled with value-added chemicals (Figure TA B L E 1 F I G U R E 2 3 N 4 -based photocatalysts synthesized for simultaneously producing H 2 O 2 and H 2 or value-added chemicals is representatively reported. a rate of 239.3 μmol g −1 h −1 under visible-light irradiation.The significantly bolstered photocatalytic performance was attributed to the substitution of C by doped P atoms in the tri-s-triazine rings and the constructed heterogenous P + -P δ− -Co/Ni δ+ chemical bridge between the cocatalyst Co x Ni y P and PCN, as elucidated by the P 2p Xray photoelectron spectroscopy (XPS) spectra (Figure8B), respectively.The reaction pathway was rigorously verified being the 2e − WOR, as done by RRDE measurement and MnO 2 -induced decomposition of H 2 O 2 .As shown in Figure 2 O 2 and H 2 at rate of 604.8 and 540.3 μmol g −1 h −1 , respectively, in pure water splitting, accompanied by an AQE of 1.63% (400 nm).DFT investigations indicated the ZnIn 2 S 4NSs in the composite possessed an appropriately high surface free-energy (>3.5 eV) for adsorbing O atom, which favored the selectivity toward H 2 O 2 production via 2e − WOR.It also predicted that upon the formation of heterojunction 69(I) H 2 and H 2 O 2 evolution rates under N 2 , air, and O 2 atmospheres.
O 2 and H 2 through the 4e − WOR-2e − ORR cascaded pathway.For instance, Liu et al. reported a CoP/CDs composite with the CDs acted as electron sink (Figure of H 2 O 2 and the accompanying redox product.For instance, element doping, cocatalyst loading, and defect engineering facilitate the separation and migration of charge carriers, while creating spatially separated redox centers that facilitate the surface photoredox reactions.The decoration of single-atom cocatalyst or functional group aids in the construction of superior active sites and suppression of competitive side reactions.Alternatively, adopting the dual-channel strategy, which practices the communal enhancement of two-electron ORR and WOR, proves to be a potent approach for achieving the exclusive production of H 2 O 2 .Heterojunction engineering offers a means to effectively harness the strengths of two distinct semiconductors, including the superior VB and CB, while improving the long-term stability and charge transfer efficiency. and the applied modification strategies.The ameliorated photocatalytic properties (e.g., charge separation-transfer efficacy, optical absorption, macro/micro-chemical environment, improved spatial separation of redox centers, etc.) arising from the modifications are thoroughly reviewed, of which they are essential in enhancing the yield and selectivity