Photocatalytic Evolution of Hydrogen Peroxide: A Minireview

: Hydrogen peroxide (H 2 O 2 ) has demonstrated applicability in a wide range of applications, spanning from a bleaching agent in the pulp industry, environmental remediation, and fuel cell technology. Industrial scale synthesis, either by the anthraquinone method or catalytic oxidation of hydrogen gas, has serious drawbacks which are related with energy demanding and multi-step processes. An alternative green strategy involves the photocatalytic synthesis of H 2 O 2 . All that is needed is the renewable energy of the sun, a semiconducting species absorbing in the visible region, water, and oxygen. In this minireview, we describe the evolution of research milestones that have been achieved within the recent decades regarding the development of functional photocatalytic systems. In the early studies, back in the 1980’s, TiO 2 -based systems were mostly investigated. However, due to the large band gap of titania (3.2 eV), alternative semiconductors were studied which strongly absorb in the visible region. Thus, a variety of semiconductor families have been investigated, such as doped titania systems, other metal oxides, metal sulﬁdes, organic semiconductors, metal-organic frameworks, carbon nitride systems, etc. In parallel, the development of functional dopants onto the surface of the main semiconductor has lead to both the inhibition of electron-hole recombination and H 2 O 2 degradation. The current minireview collectively provides the studies of the higher H 2 O 2 production rates and offer some suggestions for the near future.


Introduction
Photocatalytic processes have been recognized as an alternative approach towards the achievement of a variety of chemical transformations [1]. The latter are related with a wide range of applications, including issues that are related to the environment (degradation of polluting species) [2] as well as energy conversion (synthesis of "green" fuels) [3]. The photocatalytic transformations take place through the redox reactions of either holes or electrons of photoexcited semiconducting species. Both oxidative and reductive paths result in the generation of either reactive species or neutral substances. In many cases, aqueous systems in an oxygen atmosphere are used in photocatalytic processes, so oxygenbased photo-adducts are expected to be formed. The main reactive oxygen species are hydroxyl radicals, singlet oxygen, superoxide anion radical, and hydrogen peroxide. In photocatalysis, the driving force towards high-yield half-reactions depends on the energy difference between either the conduction or valence band of the photocatalyst with the corresponding potentials of the half reactions [4].
Hydrogen peroxide (H 2 O 2 ) is considered as a green oxidant for either organic or inorganic structures. Regarding the synthetic strategies of hydrogen peroxide, an increased interest has been given by researchers, since the substance is considered both as a promising fuel as well as an environmentally friendly substance towards oxidative synthetic approaches [5]. Regarding the utilization as a potential fuel, this is supported by the fact that its transportation and storage are relatively safe processes due to the physical state of the substance (liquid) at ambient conditions. Since the side products in its reactions are not dangerous (oxygen and water), H 2 O 2 has been widely applied as a bleaching agent in the paper industry [6] or in wastewater treatment [7].
Until now, an energy-demanding multi-step industrial approach has been widely utilized towards the synthesis of H 2 O 2 , the so-called anthraquinone (AQ) process [8]. This approach involves a combination of energy-demanding chemical reactions, extraction steps by harmful organic media, and thus, the production of unwanted wastes. This multistep process cannot be considered as a green technology protocol. Thus, alternative green strategies need to be developed in order to synthesize hydrogen peroxide in large quantities, without harming the environment. In the recent decades or so, an alternative synthetic process involves the direct compounding of hydrogen and oxygen gases in the presence of a nanostructured catalyst. It is noted that the noble metals (or its alloys) that are used as catalytic systems increase the cost of the protocol drastically. Some of the nanostructured metals possess active crystallographic planes, which catalyze the decomposition of H 2 O 2 . This leads to lower production rates of the oxidant substance. Furthermore, the catalyst-based reactions of hydrogen and oxygen gases often result in an explosion, thus, it is imperative to purge the reactor with an inert gas (Ar, N 2 , etc.). All the aforementioned drawbacks have restricted the applicability of the process on the industrial scale. Alternative approaches by using renewable energy sources have been developed in the last 20 years or so. Specifically, hydrogen peroxide synthesis has been accomplished through photoexcitation of semiconductor-based systems in an oxygen-saturated atmosphere [9]. Due to the environmentally friendly character of the process, the photocatalytic synthesis of H 2 O 2 has attracted great interest and this is reflected by the huge production of related publications. Therefore, the timing is correct in order to demonstrate the current progress that has been achieved in the development of functional catalytic systems for the photosynthesis of H 2 O 2 .
In this review, we first describe the photocatalytic paths that were suggested to take place during the synthesis of H 2 O 2 in some seminal works. Subsequently, we discuss the various photocatalytic systems that have been used so far for the aforementioned synthetic transformation. In addition, some comparative data are illustrated within a comparative Table, in which the most efficient photocatalytic systems have been selected. The review aims to demonstrate the current status in the literature and even propose new avenues to the development of functional photocatalytic systems.

Redox Reactions during Photocatalytic Synthesis of H 2 O 2 -Early Considerations
Semiconductor-based photocatalytic reactions include three main processes [10]. Firstly, absorption of photon energy takes place by the semiconducting species. Great caution should be given to the energy band gap of the semiconductor and the amount of incident light in order to achieve a successful excitation. The first step results in the enrichment of the conduction band by electrons and the valence band by holes. The photoinduced electrons and holes separate from their bound state (exciton) and escape to the surface of the nanostructured semiconductor. The aforementioned carriers may react with a variety of species in the interfacial region between the semiconductor surface and the adjacent liquid environment. As a competitive process, the recombination of electrons and holes may take place, resulting in low efficiency photocatalytic reactions.
The basic principles of photocatalysis stand also for the synthesis of H 2 O 2 . In total, the aforementioned substance may be generated through either two-electron reduction of oxygen or two-hole oxidation of water. Starting from the reductive path and specifically through the oxygen radical anion species ( • O 2 − ), the following two reduction paths are valid: The former involves the so-called disproportionation reaction, whereas the latter involves the participation of conduction band electrons. Other than the reductive path, the hole-mediated oxidation of water may take place. This can be described in the following equation: The aforementioned oxidative reaction may be further divided in two process, one hole-mediated and the other which is considered as dimerization.
The seminal studies in the photoinduced synthesis of H 2 O 2 involved mainly the utilization of TiO 2 aqueous suspensions as the environment for hydrogen peroxide generation. As stated previously, such photoinduced synthetic processes may result in either reductive or oxidative paths. In the former case, the peroxide may be formed by a stepwise reduction of dissolved oxygen gas, in which two electrons are consumed in total [11]. The intermediate species that is formed is an oxygen radical anion. In parallel, a direct one-step reduction of oxygen takes place under a two-electron mechanism.
Early studies suggested that H 2 O 2 was solely formed by reductive paths of photoexcited TiO 2 through the oxygen radical anion species [12]. The authors observed that no H 2 O 2 was formed under inert atmosphere (nitrogen-purged solution), implying that H 2 O 2 was generated from the reduction site of the photocatalyst, its conduction band. Furthermore, the presence of Cu 2+ ions was found to act catalytically towards the enhanced generation of H 2 O 2 by the following reactions: Different conclusions were drawn by Diesen et al. [13] concerning the generation of H 2 O 2 in a de-aerated environment. The authors concluded that the peroxide was generated through the dimerization of hydroxyl radicals at such conditions. To further elucidate the origin of the H 2 O 2 , radical trapping experiments were conducted by using the radical scavenger tris(hydroxymethyl) aminomethane (Tris). The results showed that, in a deoxygenated system, no H 2 O 2 could be detected due to hydroxyl radical scavenging by the Tris reagent. This experiment actually supported the suggestion of the authors that H 2 O 2 formation took place by the dimerization of the hydroxyl radicals.
As stated previously, the oxidative character of the holes may result in redox transformations of water substance. These include either the oxidation of water to O 2 (four-hole mechanism), the oxidation of water to H 2 O 2 (two-hole mechanism) [14], or the oxidative transformation of water to hydroxyl radicals, which may be dimerized in order to form H 2 O 2 .

Titanium Dioxide (TiO 2 )
The photocatalyst which has been studied most in depth is titanium dioxide (titania, TiO 2 ). The aforementioned semiconductor demonstrates enhanced photostability, biocompatibility, yet its large band gap results in a negligible absorption to visible light wavelengths. The photocatalytic production of H 2 O 2 in aqueous TiO 2 suspensions has been studied in the seminal work of Hoffmann and co-workers, back in 1988 [15]. It was found that an appreciable amount of peroxide was detected in the presence of electron donors such as acetates. The latter were oxidized by the holes in the valence band of a semiconductor, interfering with the recombination inhibiting step. Beside H 2 O 2 , organic peroxides were formed, such as CH 3 OOH and CH 3 OOCH 3 . Kinetic analysis of the H 2 O 2 production rate revealed that after 5 min irradiation with 350 nm light source, a concentration of 0.9 µM was estimated. Analogous results were found in the work of Goto et al. [16]. The authors used isopropanol as a hole scavenger, so that the oxidative generation of H 2 O 2 was inhibited efficiently. The reduction products from molecular oxygen were found to be different, depending on the crystal lattice of titania photocatalyst. It was demonstrated that the main product was a superoxide ion when rutile particles are used, whereas hydrogen peroxide was mainly generated when anatase particles were used as the photocatalyst. Furthermore, it was found that the utilization of isopropanol as a hole scavenger rules out the scenario of the oxidative production of H 2 O 2 . Mechanistic investigation exhibited some interaction between isopropanol and superoxide ions towards the formation of acetone and hydrogen peroxide.
Hydrogen peroxide evolution, formed through a reductive strategy was attempted by the work of Selli and co-workers [17]. The hole scavengers that were used were formic acid and benzoic acid, respectively. The degradation rate of both hole scavengers was maximum at pH 4.2 and decreased either at lower or higher pH values. The observed trends were correlated with the electrostatic interactions between the substrate and the photocatalyst surface at different acidity ranges. In the case of formic acid photodegradation, no appreciable amount of hydrogen peroxide was formed. The authors attributed this result to the fact that the TiO 2 surface was not significantly protected by adsorbed formic acid molecules. This could result in the formation of Ti-peroxo complexes (Ti IV -OOH). Electron transfer processes, induced by photogenerated species, may lead to either the reduction or oxidation of the peroxo function.
The tendency of aromatic hole scavengers to contribute to more enhanced hydrogen peroxide yields when compared with aliphatic ones, was also shown by other works. The utilization of aromatic alcohols as efficient hole scavengers towards the generation of high yield hydrogen peroxide was demonstrated by Shiraishi et al. [18]. The irradiation of titania suspensions in the presence of a benzylic alcohol under an oxygen atmosphere resulted in the generation of H 2 O 2 at a concentration of 40 mM. The enhanced H 2 O 2 yield was suggested to be due to the efficient formation of side-on coordinated peroxo species on the semiconductor surface that were produced via the reaction of benzylic alcohols and O 2 in water. Such peroxo functions consist of three-membered rings of one Ti 4+ and two oxygen atoms. The utilization of benzaldehyde as a hole scavenger was not effective for H 2 O 2 formation, nor was the visible light irradiation of benzylic alcohol-containing suspensions.
By using visible light irradiation, heteroatom-doped titania was studied as photocatalysts for the selective synthesis of either oxygen radical anions or hydrogen peroxide [19]. The authors demonstrated that the sulfur-doped TiO 2 surpassed the N-doped TiO 2 in the ability to produce oxygen radical anions, while the N-doped TiO 2 surpassed the Sdoped TiO 2 in producing hydrogen peroxide. The different behavior in the photocatalytic processes was attributed to the multivalence character of the sulfur moieties, which may promote the decomposition of H 2 O 2 . Visible light irradiation for 10 min gave rise to an H 2 O 2 concentration of 0.055 µM by using the N-doped TiO 2 system. The corresponding value for the S-doped material was below 0.01 µM.
An efficient production of H 2 O 2 was demonstrated by Xiao and co-workers through a two-step hydrothermal process in autoclave conditions (Figure 1) [20]. In the first step, the authors treated commercial P25 TiO 2 nanoparticles in an alkaline aqueous medium under hydrothermal conditions. The TiO 2 nanotubes (TNT) were isolated bearing Na + as counterions. By an ion-exchange process, the sodium ions were replaced by protons in order to acquire the adduct HTNT. In a subsequent step, the latter adduct was mixed with citric acid (CA) and the whole mixture was hydrothermally treated in order to obtain a hybrid assembly of HTNT and carbon dots (CD). Irradiation under visible light resulted in an H 2 O 2 production rate of 4.8 mM/h. The presence of acidic protons on the TNT surface was crucial for the acceleration of the reaction between molecular oxygen and electrons towards the formation of H 2 O 2 . It is noted that irradiation with a source emitting at λ > 365 nm demonstrated decreased H 2 O 2 yields, when compared with the values that were acquired at the first two hours of irradiation. towards the formation of H2O2. It is noted that irradiation with a source emitting at λ > 365 nm demonstrated decreased H2O2 yields, when compared with the values that were acquired at the first two hours of irradiation. In an analogous study, the coating of both heteroatom-doped graphitic nanostruc tures (N,S-codoped graphene dots) and anionic perfluorosulfonic polymer (Nafion) onto nanostructured TiO2 surface resulted in the enhanced generation of H2O2 [21]. The hybrid demonstrated much higher photocatalytic activity under simulated sunlight irradiation than in the case of visible light. In the latter case, the dual-doped graphene dots acted as potential photosensitizer, contributing to the electronic enrichment of a TiO2 conduction band. Regarding the simulated sunlight irradiation, the optimized hybrid (5% Nafion SNGr@TiO2) demonstrated an H2O2 production of 745 μΜ within 2 h of irradiation. The authors suggested that the Nafion layer on the TiO2 surface accelerated the oxygen diffu sion from the liquid medium for electron scavenging. In parallel, the available protons on the catalyst surface increased in the presence of an anionic polymer coating. Nafion load ing that was higher than 5 wt% gave rise to lower H2O2 production rates. This could be explained due to excessive coverage of the semiconductor surface, making the physica adsorption of hole scavengers impossible.
In an alternative approach, TiO2@carbon composites were prepared by carbonization of aromatic compounds that were adsorbed onto the surface of inorganic oxide [22]. Spe cifically, pyrolytic decarboxylation of either benzoic acid or 1-naphthoic acid resulted in the synthesis of core-shell TiO2@carbon hybrids. The pyrolytic process was suggested to generate aryl radicals, which react either with similar species or aromatic carboxylic acids with the subsequent formation of a protecting carbon layer. The naphthoic acid-derived hybrid contained a larger population of oxygenated groups, with the latter being predom inantly epoxide rings. The sp 2 -hybridized domains near the epoxide groups were found In an analogous study, the coating of both heteroatom-doped graphitic nanostructures (N,S-codoped graphene dots) and anionic perfluorosulfonic polymer (Nafion) onto nanostructured TiO 2 surface resulted in the enhanced generation of H 2 O 2 [21]. The hybrid demonstrated much higher photocatalytic activity under simulated sunlight irradiation, than in the case of visible light. In the latter case, the dual-doped graphene dots acted as potential photosensitizer, contributing to the electronic enrichment of a TiO 2 conduction band. Regarding the simulated sunlight irradiation, the optimized hybrid (5% Nafion-SNGr@TiO 2 ) demonstrated an H 2 O 2 production of 745 µM within 2 h of irradiation. The authors suggested that the Nafion layer on the TiO 2 surface accelerated the oxygen diffusion from the liquid medium for electron scavenging. In parallel, the available protons on the catalyst surface increased in the presence of an anionic polymer coating. Nafion loading that was higher than 5 wt% gave rise to lower H 2 O 2 production rates. This could be explained due to excessive coverage of the semiconductor surface, making the physical adsorption of hole scavengers impossible.
In an alternative approach, TiO 2 @carbon composites were prepared by carbonization of aromatic compounds that were adsorbed onto the surface of inorganic oxide [22]. Specifically, pyrolytic decarboxylation of either benzoic acid or 1-naphthoic acid resulted in the synthesis of core-shell TiO 2 @carbon hybrids. The pyrolytic process was suggested to generate aryl radicals, which react either with similar species or aromatic carboxylic acids with the subsequent formation of a protecting carbon layer. The naphthoic acidderived hybrid contained a larger population of oxygenated groups, with the latter being predominantly epoxide rings. The sp 2 -hybridized domains near the epoxide groups were found to act as catalytic sites for the two-electron reduction of oxygen, thus, making the naphthoic-acid-derived hybrid a more efficient photocatalyst.
Covalent immobilization of polyoxometalates (PW 9 O 24 ) 9− onto TiO 2 was demonstrated by Wu et al. [23]. The hydrothermally synthesized TiO 2 -polyoxometalate photocatalyst demonstrated higher efficiency for H 2 O 2 generation than the corresponding one of hybrid, prepared by the physical mixing of the components. By using benzyl alcohol as a hole scavenger, the photocatalytic system generated about 38 µmol H 2 O 2 within two hours of irradiation. The attachment of polyoxometalate clusters in the hybrid was suggested to lead to enhanced light absorption and charge carrier transport. The material which has been recycled four times has shown similar activity regarding the H 2 O 2 yield.

Metal Nanoparticle-Decorated Titania
In most of the aforementioned studies, the H 2 O 2 yield was in the micromolar scale. The low yield was ascribed to the reaction of H 2 O 2 with surface Ti-OH groups, resulting in the formation of surface peroxide species. The latter were found to be reduced by the excited electrons of the semiconductor conduction band. A versatile approach by which the H 2 O 2 degradation may be inhibited is the displacement of hydroxyl moieties of the TiO 2 surface by other ions, such as F − [24]. The production of H 2 O 2 at a millimolar scale was achieved as a consequence of the suppression of the degradation reaction. The H 2 O 2 formation rate was fully consistent with the surface speciation, acquiring the maximum value when the whole population of hydroxyls was displaced by fluorides.
Besides the displacement of surface hydroxyl moieties, decoration of the TiO 2 surface by metal cocatalyst nanoparticles was studied in detail. The enhanced photoactivity stems from the interfacial electron transfer between TiO 2 and a metal cocatalyst. The latter process gives rise to efficient carrier separation, whereas the oxidation sites (TiO 2 surface) and reduction sites (metal nanoparticle surface) are spatially separated. Teranishi et al. [25] have demonstrated the drastic enhancement of H 2 O 2 generation by TiO 2 that is loaded with Au nanoparticles. By using ethanol as a hole scavenger, the hybrid photocatalyst was able to photogenerate H 2 O 2 on an 8 mM level under UV irradiation for 24 h. The most efficient activity appeared for the hybrids containing Au nanoparticles of diameter in the range between 7 and 8 nm. Although the Au nanoparticles act as an efficient catalytic site for two-electron reduction of O 2 , an additional side-reaction may occur in strongly adsorbed H 2 O 2 molecules. The latter may be reduced on the metal surface, giving hydroxyl radicals and anions, respectively. Thus, Au particles promote the formation and catalyze the decomposition of H 2 O 2 , simultaneously. Furthermore, the same group studied the effects of both temperature and pH on the yield of H 2 O 2 [26]. It was found that as the temperature and pH of the solution decreased, the H 2 O 2 generation was enhanced. At 5 • C and pH 2, the yield of H 2 O 2 reached the value of 17 mM after 23 h of irradiation. The quantum efficiency for H 2 O 2 formation was estimated to be 22% by assuming a two-electron mechanistic scheme. The authors suggested that H 2 O 2 was preferentially grown onto the Au surface, which strongly supports the avoidance of the decomposition process onto the TiO 2 surface. In order to inhibit the undesired side-reaction of H 2 O 2 decomposition through the formation of Ti-OOH species, Shiraishi and coworkers [27] have synthesized TiO 2 that is loaded with AuAg bimetallic alloy nanoparticles. The reason for the study of the alloy-based system was that the formed H 2 O 2 could be adsorbed onto the neat Au surface and decompose via a reductive path towards the generation of hydroxy radicals and anions, respectively. UV photoirradiation for 12 h of Au/TiO 2 afforded 1.5 mM H 2 O 2 , whereas the utilization of the alloy as a cocatalyst resulted in the highest photocatalytic activity, yielding 3.4 mM of H 2 O 2 . It was found that the double effect of the alloy particles involved both the carrier separation in the TiO 2 /alloy interface as well as the decreased H 2 O 2 adsorption onto the Au nanoparticles.
Analogous TiO 2 /Au nanostructures were grown onto silicon nanowires by a combination of molecular layer deposition, thermal annealing, and liquid phase reduction protocols [28]. The photocatalytic production of H 2 O 2 took place without the need of Solid-phase photocatalytic films of TiO 2 /Au onto glass substrate showed an enhanced yield of H 2 O 2 formation ( Figure 2) [29]. The TiO 2 film was deposited by spin coating, followed by calcination at 550 • C in order to acquire porosity. The Au nanoislands were deposited using a thermal or e-beam evaporation, with a size ranging between 2 and 20 nm. The enhanced photocatalytic activity was attributed to the co-existence of both small-and large-size Au nanoislands, which possess size-dependent work functions, thus minimizing the recombination of electron-hole pairs. After 20 min of irradiation, the H 2 O 2 yield reached a value of 1.5 mM. The performance enhancement was about 80 times higher than the one of neat TiO 2 .
nergies 2022, 15, x FOR PEER REVIEW 7 of 21 protocols [28]. The photocatalytic production of H2O2 took place without the need of sacrificial organic substances and reached a plateau at about 15 h of irradiation with a 365 nm light source. The maximum concentration of H2O2 was estimated at 38 μM. Solid-phase photocatalytic films of TiO2/Au onto glass substrate showed an enhanced yield of H2O2 formation ( Figure 2) [29]. The TiO2 film was deposited by spin coating, followed by calcination at 550 °C in order to acquire porosity. The Au nanoislands were deposited using a thermal or e-beam evaporation, with a size ranging between 2 and 20 nm. The enhanced photocatalytic activity was attributed to the co-existence of both small-and large-size Au nanoislands, which possess size-dependent work functions, thus minimizing the recombination of electron-hole pairs. After 20 min of irradiation, the H2O2 yield reached a value of 1.5 mM. The performance enhancement was about 80 times higher than the one of neat TiO2. An alternative passivation component for the inhibition of H2O2 degradation onto a TiO2 surface involves tin oxide (SnO2) [30]. The SnO2/TiO2 heterostructure, prepared by annealing at 500 °C, demonstrated the best performance. Decoration of the hybrid with 0.1 wt% Au resulted in an H2O2 yield of about 16 mM after 26 h of irradiation. The SnO2 passivation layer was responsible for the inhibition of H2O2 decomposition. An optimal SnO2 content was 4 wt%, while an excess oxide content was suggested to cause a shielding effect on the photo-absorption of the main semiconductor, TiO2. Similar three-component hybrids were prepared by a liquid-phase compounding protocol [31]. SnO2 nanorods were grown under autoclave conditions at 180 °C. Photoexcitation under visible light resulted in an H2O2 production rate of about 60 μM within 6 h.

Other Oxides
Besides using titania-based hybrids as the photocatalytic system for H2O2 formation the group of Hoffmann studied the activity of ZnO under ultraviolet irradiation [32]. In the presence of organic acids as the hole scavengers, aqueous ZnO suspensions were shown to generate H2O2 concentrations up to 2 mM. The order of efficiency of hole scavengers was as follows: formate > oxalate > acetate > citrate. The suggested mechanism for the oxidative decomposition of hole scavengers involved the generation of methyl radicals and carbon dioxide, in the case of acetate anion. Isotopic labeling investigation demonstrated that all the oxygen in the H2O2 adduct originated from molecular oxygen gas. In an analogous study, immobilized ZnO nanorod arrays were used as a photocatalyst for H2O2 generation [33]. The formation rate depended on the synthetic conditions of the ZnO An alternative passivation component for the inhibition of H 2 O 2 degradation onto a TiO 2 surface involves tin oxide (SnO 2 ) [30]. The SnO 2 /TiO 2 heterostructure, prepared by annealing at 500 • C, demonstrated the best performance. Decoration of the hybrid with 0.1 wt% Au resulted in an H 2 O 2 yield of about 16 mM after 26 h of irradiation. The SnO 2 passivation layer was responsible for the inhibition of H 2 O 2 decomposition. An optimal SnO 2 content was 4 wt%, while an excess oxide content was suggested to cause a shielding effect on the photo-absorption of the main semiconductor, TiO 2 . Similar three-component hybrids were prepared by a liquid-phase compounding protocol [31]. SnO 2 nanorods were grown under autoclave conditions at 180 • C. Photoexcitation under visible light resulted in an H 2 O 2 production rate of about 60 µM within 6 h.

Other Oxides
Besides using titania-based hybrids as the photocatalytic system for H 2 O 2 formation, the group of Hoffmann studied the activity of ZnO under ultraviolet irradiation [32]. In the presence of organic acids as the hole scavengers, aqueous ZnO suspensions were shown to generate H 2 O 2 concentrations up to 2 mM. The order of efficiency of hole scavengers was as follows: formate > oxalate > acetate > citrate. The suggested mechanism for the oxidative decomposition of hole scavengers involved the generation of methyl radicals and carbon dioxide, in the case of acetate anion. Isotopic labeling investigation demonstrated that all the oxygen in the H 2 O 2 adduct originated from molecular oxygen gas. In an analogous study, immobilized ZnO nanorod arrays were used as a photocatalyst for H 2 O 2 generation [33]. The formation rate depended on the synthetic conditions of the ZnO nanorods, namely the starting concentrations of the precursor substances. The maximum concentration of H 2 O 2 that was achieved was about 1.5 µM, after 6 h of UV irradiation.
By using visible wavelengths for excitation, Shiraishi and co-workers [34] have assessed the photocatalytic performance of BiVO 4 @Au hybrids for H 2 O 2 formation. After a 10 h irradiation, the H 2 O 2 yield reached a concentration of about 40 µM in pure water. Due to the favorable band alignment between the BiVO 4 conduction band and the oneand two-electron reduction potentials of oxygen species, the photoexcitation of the hybrids selectively promoted the generation of H 2 O 2 by the two-electron mechanism. Furthermore, a fraction of H 2 O 2 was also originated from water oxidation, which was supported by appropriate photocatalytic experiments by using AgNO 3 as an electron acceptor.
Tada and co-workers [35] have studied the utilization of photocatalysts consisting of BiVO 4 and Cu(II) complexes. Visible light irradiation in the presence of ethanol as a hole scavenger demonstrated that the presence of copper bis(acetonato)-type complexes was crucial towards the enhancement of photocatalytic performance. The positive contribution was attributed to the charge separation enhancement from BiVO 4 to the complex structure. The H 2 O 2 yield was up to 120 µM after 90 min of irradiation. No apparent decay of the photocatalytic activity was observed after five cycles. In the absence of a sacrificial agent, the corresponding production rate was 58 µM/h, yet the degradation of the copper complex took place simultaneously.
An alternative electron bridge component that was compounded with BiVO 4 nanostructures was reduced graphene oxide (rGO) (Figure 3) [36]. The latter nanostructure may induce efficient electron transfer due to its extended conjugate network of sp 2 -hybridized carbon atoms. The synthetic protocol was based on an electrostatic self-assembly approach, by using a precursor state of rGO, the graphene oxide.
nanorods, namely the starting concentrations of the precursor substances. The maximum concentration of H2O2 that was achieved was about 1.5 μM, after 6 h of UV irradiation.
By using visible wavelengths for excitation, Shiraishi and co-workers [34] have assessed the photocatalytic performance of BiVO4@Au hybrids for H2O2 formation. After a 10 h irradiation, the H2O2 yield reached a concentration of about 40 μΜ in pure water. Due to the favorable band alignment between the BiVO4 conduction band and the one-and two-electron reduction potentials of oxygen species, the photoexcitation of the hybrids selectively promoted the generation of H2O2 by the two-electron mechanism. Furthermore, a fraction of H2O2 was also originated from water oxidation, which was supported by appropriate photocatalytic experiments by using AgNO3 as an electron acceptor.
Tada and co-workers [35] have studied the utilization of photocatalysts consisting of BiVO4 and Cu(II) complexes. Visible light irradiation in the presence of ethanol as a hole scavenger demonstrated that the presence of copper bis(acetonato)-type complexes was crucial towards the enhancement of photocatalytic performance. The positive contribution was attributed to the charge separation enhancement from BiVO4 to the complex structure. The H2O2 yield was up to 120 μΜ after 90 min of irradiation. No apparent decay of the photocatalytic activity was observed after five cycles. In the absence of a sacrificial agent, the corresponding production rate was 58 μM/h, yet the degradation of the copper complex took place simultaneously.
An alternative electron bridge component that was compounded with BiVO4 nanostructures was reduced graphene oxide (rGO) (Figure 3) [36]. The latter nanostructure may induce efficient electron transfer due to its extended conjugate network of sp 2hybridized carbon atoms. The synthetic protocol was based on an electrostatic self-assembly approach, by using a precursor state of rGO, the graphene oxide. The maximum H2O2 yield reached a value of 300 μΜ after a 2h irradiation process The optimal production rate was achieved in a rGO loading of 1 wt%. Higher loading led to the lowering of the H2O2 yield. This was attributed either to a shielding effect to the photo-absorption or blocking of the semiconductor active sites. Another form of mixed metal oxide nanostructures as potentially active photocatalysts includes the nickel titanate (NiTiO3), which was synthesized by a hydrothermal strategy [37]. The maximum concentration of H2O2 reached 2.5 mM when nickel titanate was irradiated for 1 h in an oxygensaturated aqueous suspension. The data suggested that H2O2 was formed by either The maximum H 2 O 2 yield reached a value of 300 µM after a 2 h irradiation process. The optimal production rate was achieved in a rGO loading of 1 wt%. Higher loading led to the lowering of the H 2 O 2 yield. This was attributed either to a shielding effect to the photo-absorption or blocking of the semiconductor active sites. Another form of mixed metal oxide nanostructures as potentially active photocatalysts includes the nickel titanate (NiTiO 3 ), which was synthesized by a hydrothermal strategy [37]. The maximum concentration of H 2 O 2 reached 2.5 mM when nickel titanate was irradiated for 1 h in an oxygen-saturated aqueous suspension. The data suggested that H 2 O 2 was formed by either photogenerated electrons or holes. This could be explained by performing the following two experiments: (a) saturation of the medium with an inert gas gave rise to a decreased H 2 O 2 yield; (b) a further decrease was observed by using methanol as a hole scavenger in a nitrogen-saturated environment.
Hollow MoO 3 @SnS 2 nanotubes were synthesized by a two-step hydrothermal treatment at moderate temperatures [38]. Hydrogen peroxide was rapidly formed over the hybrid catalyst surface, reaching 100 µM within 100 min of irradiation. The corresponding activity of the hybrid that was prepared by physical mixing of the components was relatively lower (about 10 µM), implying their close contact in the hydrothermally prepared hybrid. Photocatalytic experiments were held in the presence of both hydroxy radical and oxygen radical anion scavengers, demonstrating a 90% decrease of the H 2 O 2 yield. The hybrid demonstrated enhanced photostability after four cycles of testing.

Metal Sulfides
Lee and co-workers [39] demonstrated the sustainable synthesis of H 2 O 2 in the absence of organic hole scavengers by a CdS-reduced graphene oxide (rGO) photocatalyst in sunlight with water and oxygen as resources. Under hydrothermal conditions, hybrids were prepared with a maximum rGO content at 30 wt%. The sample containing 20 wt% rGO showed optimum photocatalytic activity, which was five times higher than the one of neat CdS after 12 h irradiation.
By using a synchronous crystallization at room temperature, Zhang et al. [40] have synthesized Co 9 S 8 /Mn 3 O 4 hybrids. The adoption of Z-scheme irradiation gave rise to enhanced separation of the carriers (electrons and holes). An optimal photocatalytic system generated a maximum 1.6 mM of H 2 O 2 after 6 h irradiation, without using hole scavenger and purging pure oxygen gas. The Z-scheme contributed to the fact that the electrons in the conduction band of Co 9 S 8 were the reducing agents for oxygen transformation by a two-electron mechanism, whereas the holes in the valence band of Mn 3 O 4 generated a relatively high population of hydroxyl radicals, through which additional H 2 O 2 could be produced.
Kang and co-workers [42] have developed an SnS 2 /In 2 S 3 Type II heterostructure, coupled with carbon dots (CD). In this system, CD were suggested to act as an electron sink, regulate the kinetics of the carriers in the heterojunction, and then further increase the electron-hole separation efficiency of the SnS 2 /In 2 S 3 /CD hybrid. On the other hand, CD were considered as the photo-active sites for the oxygen reduction to H 2 O 2 . The hybrid heterostructure displayed superior activity with a H 2 O 2 yield of about 1112 µmol·h −1 ·g −1 .

Metal-Organic Frameworks (MOFs)
Metal-organic frameworks, consisting of metal cation species and organic linkers, have been shown to act as potential photocatalytic systems for the generation of hydrogen peroxide [43]. Yamashita and co-workers [44] synthesized an MOF that was composed of Ti 8 O 8 (OH) 4 clusters and 2-aminoterephthalic acid, MIL-125-NH 2 , which was applied for photocatalytic H 2 O 2 synthesis due to its visible light absorption. The photoactivity stems from the ligand-to-cluster charge transfer transition. The authors studied the oxygen reduction reaction by the radical anion of the Ti-based cluster in the presence of benzyl alcohol which acted as hole scavenger. Furthermore, it was concluded that the catalytic activity could be greatly enhanced by the deposition of nickel oxide nanoparticles onto the MOF structure. The metal oxide was suggested to accelerate the H 2 O 2 formation through disproportionation of the oxygen radical anion. After 8 h of irradiation, the H 2 O 2 yield was about 8 mM in the NiO/MOF hybrid.
In a similar study, Yu and co-workers [45] have fabricated MIL-125-NH 2 @ZnS heterostructures by a two-step solution-phase protocol. The optimized MIL-125-NH 2 @ZnS hybrid demonstrated a high photocatalytic activity for O 2 reduction with an H 2 O 2 production rate of about 120 mM/g/h. It was concluded that heterojunction coverage plays a crucial role in the regulation of the photocatalytic properties of hybrid nanostructures.
Analogous studies with the same MOF, modified with a hydrophobic aliphatic substance, were carried out in a biphasic system of benzyl alcohol/water (Figure 4) [46,47]. In the amino functionality of the MOF structure, a number of hydrophobic chains were attached through an amidation reaction [46]. After a 3 h irradiation, the H 2 O 2 yield was estimated to reach values up to 15 mM. The catalyst activity was found to be strongly dependent on the relative volume of water as well as the pH of the aqueous environment. In the case of using an organophosphonic acid (OPA) as a hydrophobic agent, the activity enhancement was attributed to the preservation of the access of the inner pores, due to the selective adsorption of the hydrophobic chain to the outermost surface of the MOF structure [47]. The oxidation adduct, benzaldehyde, was selectively diluted in the organic phase, whereas H 2 O 2 to the aqueous phase. After a 3 h irradiation, the H 2 O 2 yield was estimated to be about 1.3 mM.
Energies 2022, 15, x FOR PEER REVIEW 10 of 21 disproportionation of the oxygen radical anion. After 8 h of irradiation, the H2O2 yield was about 8 mM in the NiO/MOF hybrid. In a similar study, Yu and co-workers [45] have fabricated MIL-125-NH2@ZnS heterostructures by a two-step solution-phase protocol. The optimized MIL-125-NH2@ZnS hybrid demonstrated a high photocatalytic activity for O2 reduction with an H2O2 production rate of about 120 mM/g/h. It was concluded that heterojunction coverage plays a crucial role in the regulation of the photocatalytic properties of hybrid nanostructures.
Analogous studies with the same MOF, modified with a hydrophobic aliphatic substance, were carried out in a biphasic system of benzyl alcohol/water (Figure 4) [46,47]. In the amino functionality of the MOF structure, a number of hydrophobic chains were attached through an amidation reaction [46]. After a 3 h irradiation, the H2O2 yield was estimated to reach values up to 15 mM. The catalyst activity was found to be strongly dependent on the relative volume of water as well as the pH of the aqueous environment. In the case of using an organophosphonic acid (OPA) as a hydrophobic agent, the activity enhancement was attributed to the preservation of the access of the inner pores, due to the selective adsorption of the hydrophobic chain to the outermost surface of the MOF structure [47]. The oxidation adduct, benzaldehyde, was selectively diluted in the organic phase, whereas H2O2 to the aqueous phase. After a 3 h irradiation, the H2O2 yield was estimated to be about 1.3 mM. Furthermore, in situ synthesis of MIL-125 MOF was accomplished in the presence of 4,4′,4″,4‴-(pyrene-1,3,6,8-tetrayl) tetrabenzoic acid linker [48]. Due to the introduction of the ligand, the morphology of MOF crystallites contained more structural defects and possessed slightly larger BET surface areas and pore volumes. The most efficient photocatalyst accomplished a remarkable H2O2 production rate of 1654 μΜ/h under visible-light irradiation (λ > 400 nm) using triethanolamine as a hole scavenger.
Ti-doped zirconium (2-amino terephthalic acid) MOF structures, modified by an organophosphonic acid, demonstrated an even higher photocatalytic activity towards the generation of H2O2 in a biphasic system [49]. The hydrophobic OPA/Zr92.5Ti7.5-MOF exhibited a remarkable H2O2 production rate of 9.7 mM/h under the irradiation of visible light (λ > 420 nm), which was about 4.5 times higher than that of the parent zirconium-based MOF (Zr100-MOF). The enhanced activity was attributed to the effective Ti-doping, which Furthermore, in situ synthesis of MIL-125 MOF was accomplished in the presence of 4,4 ,4 ,4 -(pyrene-1,3,6,8-tetrayl) tetrabenzoic acid linker [48]. Due to the introduction of the ligand, the morphology of MOF crystallites contained more structural defects and possessed slightly larger BET surface areas and pore volumes. The most efficient photocatalyst accomplished a remarkable H 2 O 2 production rate of 1654 µM/h under visible-light irradiation (λ > 400 nm) using triethanolamine as a hole scavenger.
Ti-doped zirconium (2-amino terephthalic acid) MOF structures, modified by an organophosphonic acid, demonstrated an even higher photocatalytic activity towards the generation of H 2 O 2 in a biphasic system [49]. The hydrophobic OPA/Zr 92.5 Ti 7.5 -MOF exhibited a remarkable H 2 O 2 production rate of 9.7 mM/h under the irradiation of visible light (λ > 420 nm), which was about 4.5 times higher than that of the parent zirconium-based MOF (Zr 100 -MOF). The enhanced activity was attributed to the effective Ti-doping, which played the role of effectively promoting the charge transfer from photoexcited linkers of MOF, inhibiting the recombination of photogenerated electron-hole pairs.

Carbon-Based Semiconductors
Semiconducting low molecular weight substances, such as biscoumarin-containing acenes, have been used as potential photocatalysts for H 2 O 2 generation [50]. Visible light irradiation of thin films of such substances resulted in high efficiency oxygen reduction, with neat water being the hole scavenger. It was found that the presence of two carbonyl functionalities was the crucial catalytic site towards the reductive transformation of oxygen. The optimum H 2 O 2 yield that was acquired was 2.5 µg H 2 O 2 /mg catalyst/hour.
A melamine foam has been used as a precursor for the preparation of carbon-based supports by an annealing process [51]. During the thermal process, cobalt hydroxide was compounded with the foam structure. After the annealing, Co 3 O 4 nanoparticles were supported onto the carbon-based framework. The presence of oxygen vacancies resulted in efficient H 2 O 2 generation in the absence of a hole scavenger. The carbon support showed electron-acceptor character, thus catalyzing the oxygen reduction reaction. The optimal H 2 O 2 production rate was 3.78 mmol/h/g.
Shiraishi and co-workers [52] synthesized resorcinol-formaldehyde resins as potential metal-free photocatalysts. Such polymeric systems were comprised of benzenoid-quinoid conjugated domains, leading to a broad light absorption up to 700 nm. After 24 h of irradiation, the H 2 O 2 maximum amount that was generated was about 3.3 mM. The aforementioned systems were prepared by a base-catalyzed polycondensation process. The same group studied the synthesis of such networks under acidic hydrothermal conditions [53]. After 24 h of irradiation, the H 2 O 2 maximum amount that was generated was about 2.7 mM, when using HCl during the synthetic protocol. Similarly, by using oxalic acid for pH adjusting, such resorcinol-formaldehyde networks were doped with poly(3-hexylthiophene) through an autoclave reaction [54]. The highly dispersed conjugated polymer within the network of the crosslinked resin generated charge transfer complexes with the conduction band of the resin. This facilitated efficient transfer of the photogenerated conduction band electrons through the polythiophene backbone. After 6 h of irradiation, the H 2 O 2 concentration reached a maximum value of about 2.3 mM.
Analogous polycondensation adducts, such as 4-methoxybenzaldehyde/procyanidin networks, were tested as metal-free photocatalytic systems for H 2 O 2 generation [55]. The production rate of hydrogen peroxide reached a value of about 1385 µmol/h/g. The H 2 O 2 amount depended strongly on the chosen sacrificial agent as well as the acidity of the aqueous system. Moreover, the integration of carbon dots (CD) within the aforementioned 4-methoxybenzaldehyde/procyanidin network afforded an efficient photocatalytic system towards the H 2 O 2 generation in real seawater [56]. CD were shown to act as both the cocatalytic active site as well as the efficient electron acceptor/donor component, improving the catalytic efficiency in the hybrid assembly. The maximum yield of H 2 O 2 for the optimal hybrid catalyst was estimated to be 1776 µmol/g/h, in seawater.
Ring-shaped macrocyclic triazine-based systems, bearing either acetylene or diacetylene moieties, have been tested as potential H 2 O 2 photocatalysts [57]. The enhancement of photocatalytic activity was attributed to the presence of carbon-carbon triple bonds, which modulate the electronic properties of the frameworks as well as suppress electron-hole recombination.
Similarly, acetylene and diacetylene moieties have been merged with heptazine networks towards the synthesis of ring-shaped semiconductors ( Figure 5) [58]. The photocatalytic experiments were conducted in O 2 -saturated pure water under visible-light irradiation (λ > 420 nm). The maximum H 2 O 2 production rate was estimated to be about 69 µmol/h. Alternative ring-shaped systems, which construct covalent organic frameworks (COFs), include the ones that are synthesized by coupling reaction between 2,2 -bipyridine-5,5 -diamine and triformylphloroglucinol [59]. The optimized photosynthetic rate of H 2 O 2 reached a value of 1042 µM/h under one standard sunlight at 298 K.  Regarding the chemical architecture of conjugated polymers bearing acetylenic moieties, linear systems of the 3-[(4-ethynylphenyl)ethynyl] pyridine repeating unit have been studied as potential photocatalytic systems [60]. The catalyst was shown to withstand irradiation times up to about 10 h. At longer times, decomposition was found to take place. At 1.5 h irradiation time, the H2O2 production rate was about 3 mM.
Kang and co-workers [61] have used an organic semiconductor that was synthesized by a coupling reaction between 9,10-dibromoanthracene and trimethylsilylacetylene. The aforementioned system was studied for the photocatalytic H2O2 generation in polar organic media containing trace amounts of water. Under visible light, for an acetonitrile (MeCN) medium containing about 1.5 vol% water, the catalyst was found to generate H2O2 with a rate of 3923 μmol/g/h. Furthermore, in a commercially available MeCN with water content below 0.01%, 8.8 mM, H2O2 was generated after 12 h reaction, suggesting the water impurity in MeCN was completely removed.

Graphitic Carbon Nitride (g-C3N4) Systems
Among various photocatalysts, polymeric carbon nitride (C3N4) is a promising candidate for H2O2 production due to its (i) simple synthesis by a calcination process, (ii) structure consisting of earth-abundant carbon and nitrogen, (iii) effective bandgap size for visible light absorption, and (iv) suitable position of the conduction band for the reduction of oxygen in aqueous environment [62][63][64].
Visible light irradiation of a polymeric semiconductor, graphitic carbon nitride (g-C3N4) was found to selectively produce H2O2 in the presence of ethanol as a hole scavenger [65]. Electron spin resonance and Raman analysis revealed that the high H2O2 selectivity was attributed to the efficient formation of 1,4-endoperoxide species on the g-C3N4 lattice. This suppressed the reduction of O2 to superoxide radical (one-electron mechanism), selectively promoting the reduction of O2 to H2O2 (two-electron mechanism). It is noted that the valence band potential of g-C3N4 lies at 1.4 V (versus the NHE), rendering the water oxidation process not favorable thermodynamically. Thus, the modification of the band alignment of the semiconductor seems to be imperative. Regarding the chemical architecture of conjugated polymers bearing acetylenic moieties, linear systems of the 3-[(4-ethynylphenyl)ethynyl] pyridine repeating unit have been studied as potential photocatalytic systems [60]. The catalyst was shown to withstand irradiation times up to about 10 h. At longer times, decomposition was found to take place. At 1.5 h irradiation time, the H 2 O 2 production rate was about 3 mM.
Kang and co-workers [61] have used an organic semiconductor that was synthesized by a coupling reaction between 9,10-dibromoanthracene and trimethylsilylacetylene. The aforementioned system was studied for the photocatalytic H 2 O 2 generation in polar organic media containing trace amounts of water. Under visible light, for an acetonitrile (MeCN) medium containing about 1.5 vol% water, the catalyst was found to generate H 2 O 2 with a rate of 3923 µmol/g/h. Furthermore, in a commercially available MeCN with water content below 0.01%, 8.8 mM, H 2 O 2 was generated after 12 h reaction, suggesting the water impurity in MeCN was completely removed.

Graphitic Carbon Nitride (g-C 3 N 4 ) Systems
Among various photocatalysts, polymeric carbon nitride (C 3 N 4 ) is a promising candidate for H 2 O 2 production due to its (i) simple synthesis by a calcination process, (ii) structure consisting of earth-abundant carbon and nitrogen, (iii) effective bandgap size for visible light absorption, and (iv) suitable position of the conduction band for the reduction of oxygen in aqueous environment [62][63][64].
Visible light irradiation of a polymeric semiconductor, graphitic carbon nitride (g-C 3 N 4 ) was found to selectively produce H 2 O 2 in the presence of ethanol as a hole scavenger [65]. Electron spin resonance and Raman analysis revealed that the high H 2 O 2 selectivity was attributed to the efficient formation of 1,4-endoperoxide species on the g-C 3 N 4 lattice. This suppressed the reduction of O 2 to superoxide radical (one-electron mechanism), selectively promoting the reduction of O 2 to H 2 O 2 (two-electron mechanism). It is noted that the valence band potential of g-C 3 N 4 lies at 1.4 V (versus the NHE), rendering the water oxidation process not favorable thermodynamically. Thus, the modification of the band alignment of the semiconductor seems to be imperative. Further decoration of carbon nitride surface with electron-deficient aromatic diimide units of pyromellitic type has been shown to shift the potentials of either valence or conduction band of the main semiconductor [66]. After 48 h irradiation, the H 2 O 2 production rate was estimated to be 1.65 mM. The proposed mechanism involves two-photon excitation by which two electron/hole pairs were formed ( Figure 6). Water molecules are oxidized by the holes, whereas molecular oxygen is reduced by the electrons, thus forming a superoxo radical which subsequently transforms into 1,4-endoperoxide species. Protonation of the latter adduct produces hydrogen peroxide.
FOR PEER REVIEW 13 of 21 rate was estimated to be 1.65 mM. The proposed mechanism involves two-photon excitation by which two electron/hole pairs were formed ( Figure 6). Water molecules are oxidized by the holes, whereas molecular oxygen is reduced by the electrons, thus forming a superoxo radical which subsequently transforms into 1,4-endoperoxide species. Protonation of the latter adduct produces hydrogen peroxide. In a similar study, biphenyl diimide was used as a co-catalyst for g-C3N4 [67]. At 24 h irradiation, the maximum production rate was up to about 0.4 mM. In a subsequent study by the Shiraishi group, pyromellitic diimide-doped C3N4 was compounded with boron nitride (BN) and reduced graphene oxide (rGO) flakes [68]. The photoexcited electron was migrated from the g-C3N4 conduction band to rGO, leading to the two-electron reduction of O2 to H2O2, whereas the corresponding holes were transferred to BN, leading to the efficient oxidation of water. The integration of dual cocatalysts inhibited the electron-hole recombination process. After 24 h irradiation, the H2O2 production rate was estimated to be about 1.2 mM.
Heteroatom-doped C3N4 was synthesized through a solid-state thermal polymerization of either melamine or urea in the presence of an appropriate precursor. Kang and coworkers [69] have calcined a mixture of urea and phosphonitrilic chloride in order to acquire P-doped C3N4. This heteroatom-doped catalyst showed a very high photocatalytic activity for H2O2 generation, with a rate of 1968 μmol/g/h. Similarly, boron-doped C3N4 was synthesized through a solid-state thermal polymerization of melamine in the presence of KBH4 salt [70]. The resulting material demonstrated a leaf-vein-like morphology, In a similar study, biphenyl diimide was used as a co-catalyst for g-C 3 N 4 [67]. At 24 h irradiation, the maximum production rate was up to about 0.4 mM. In a subsequent study by the Shiraishi group, pyromellitic diimide-doped C 3 N 4 was compounded with boron nitride (BN) and reduced graphene oxide (rGO) flakes [68]. The photoexcited electron was migrated from the g-C 3 N 4 conduction band to rGO, leading to the two-electron reduction of O 2 to H 2 O 2 , whereas the corresponding holes were transferred to BN, leading to the efficient oxidation of water. The integration of dual cocatalysts inhibited the electron-hole recombination process. After 24 h irradiation, the H 2 O 2 production rate was estimated to be about 1.2 mM.
Heteroatom-doped C 3 N 4 was synthesized through a solid-state thermal polymerization of either melamine or urea in the presence of an appropriate precursor. Kang and co-workers [69] have calcined a mixture of urea and phosphonitrilic chloride in order to acquire P-doped C 3 N 4 . This heteroatom-doped catalyst showed a very high photocatalytic activity for H 2 O 2 generation, with a rate of 1968 µmol/g/h. Similarly, boron-doped C 3 N 4 was synthesized through a solid-state thermal polymerization of melamine in the presence of KBH 4 salt [70]. The resulting material demonstrated a leaf-vein-like morphology, which was found to promote the generation of defect sites. The optimal photocatalytic activity for H 2 O 2 synthesis reached 300 µM/h. Further derivatization of boron-doped C 3 N 4 by zinc polyphthalocyanine took place via copolymerization of 1,2,4,5-tetracyanobenzene and ZnCl 2 [71]. The resulting assembly demonstrated an enhanced H 2 O 2 production rate of 114 µmol/g/h by using a Z-scheme heterojunction. Ohno and co-workers [72] have developed a method towards the doping of C 3 N 4 lattice by single Sb atoms, by solid-state annealing of a melamine-NaSbF 6 mixture. The single Sb sites were found to act as an electron acceptor, thus acting as the O 2 photoreduction sites. Simultaneously, the accumulated holes at the N atoms of the C 3 N 4 domains adjacent to the Sb sites accelerated the H 2 O oxidation kinetics.
In an analogous approach, K/PO 4 -doped C 3 N 4 was synthesized through a solidstate thermal polymerization of melamine in the presence of K 2 HPO 4 salt [73,74]. Thus, the incorporation of both potassium and phosphate species was carried out onto the carbon nitride framework. These incorporated species modified the surface and charge transfer properties, thus enhancing the photoactivities for the generation of H 2 O 2 . The high photoactivity of doped C 3 N 4 towards the production of H 2 O 2 (1.7 mM after a 7 h irradiation) was attributed to (1) the enhanced light absorption at UV wavelengths, (2) the increased lifetime of the transient species, (3) the effective interfacial charge transfer to oxygen, and (4) the inhibited decomposition of in situ generated H 2 O 2 [73].
Similar hybrid photocatalysts were synthesized through the thermal processing of C 3 N 4 precursors with either potassium-based ionic substances [75,76], phosphate salts [77], or thermally annealed MOFs [78]. After visible light irradiation for a 10 h period, the H 2 O 2 concentration was estimated to be 5 mM, which was more than five times higher than the one that was achieved by calcined neat urea [77].
Alternatively, an ionothermal method was applied between C 3 N 4 and a mixture of alkali chlorides (Figure 7) [79]. It was suggested that either potassium or sodium cations were positioned at specific sites of the C 3 N 4 lattice, containing nitrogen/oxygen-based anionic species. The synergistic effect of doping and defect engineering resulted in the enhancement of photocatalytic performance with an H 2 O 2 production rate of 10.2 mmol/h/g, a value which was about 90 times higher than the one of neat C 3 N 4 .
nergies 2022, 15, x FOR PEER REVIEW 14 of 2 developed a method towards the doping of C3N4 lattice by single Sb atoms, by solid-state annealing of a melamine-NaSbF6 mixture. The single Sb sites were found to act as an elec tron acceptor, thus acting as the O2 photoreduction sites. Simultaneously, the accumulated holes at the N atoms of the C3N4 domains adjacent to the Sb sites accelerated the H2O oxidation kinetics.
In an analogous approach, K/PO4-doped C3N4 was synthesized through a solid-state thermal polymerization of melamine in the presence of K2HPO4 salt [73,74]. Thus, the in corporation of both potassium and phosphate species was carried out onto the carbon nitride framework. These incorporated species modified the surface and charge transfe properties, thus enhancing the photoactivities for the generation of H2O2. The high pho toactivity of doped C3N4 towards the production of H2O2 (1.7 mM after a 7 h irradiation was attributed to (1) the enhanced light absorption at UV wavelengths, (2) the increased lifetime of the transient species, (3) the effective interfacial charge transfer to oxygen, and (4) the inhibited decomposition of in situ generated H2O2 [73].
Similar hybrid photocatalysts were synthesized through the thermal processing o C3N4 precursors with either potassium-based ionic substances [75,76], phosphate salts [77], or thermally annealed MOFs [78]. After visible light irradiation for a 10 h period, the H2O2 concentration was estimated to be 5 mM, which was more than five times highe than the one that was achieved by calcined neat urea [77].
Alternatively, an ionothermal method was applied between C3N4 and a mixture o alkali chlorides (Figure 7) [79]. It was suggested that either potassium or sodium cation were positioned at specific sites of the C3N4 lattice, containing nitrogen/oxygen-based an ionic species. The synergistic effect of doping and defect engineering resulted in the en hancement of photocatalytic performance with an H2O2 production rate of 10.2 mmol/h/g a value which was about 90 times higher than the one of neat C3N4. In an analogous work, a Cu2(OH)PO4/g-C3N4 hybrid photocatalyst was synthesized by compounding within an autoclave reactor [80]. The aforementioned hybrid exhibited a full-spectrum-response from UV to the near IR region and the excitation mechanism involved a Z-scheme module. A Cu2(OH)PO4 ionic substance was suggested to form pho togenerated electrons, which were recombined with the holes of a g-C3N4 valence band Simultaneously, Cu2(OH)PO4 may adsorb O2 molecules, which is significantly importan in the photocatalytic process. The Cu2(OH)PO4/g-C3N4 heterojunction catalyst with a Cu2(OH)PO4 of 20 wt % showed an optimal H2O2 concentration of 7.2 mM, over 13 and In an analogous work, a Cu 2 (OH)PO 4 /g-C 3 N 4 hybrid photocatalyst was synthesized by compounding within an autoclave reactor [80]. The aforementioned hybrid exhibited a full-spectrum-response from UV to the near IR region and the excitation mechanism involved a Z-scheme module. A Cu 2 (OH)PO 4 ionic substance was suggested to form photogenerated electrons, which were recombined with the holes of a g-C 3 N 4 valence band. Simultaneously, Cu 2 (OH)PO 4 may adsorb O 2 molecules, which is significantly important in the photocatalytic process. The Cu 2 (OH)PO 4 /g-C 3 N 4 heterojunction catalyst with a Cu 2 (OH)PO 4 of 20 wt % showed an optimal H 2 O 2 concentration of 7.2 mM, over 13 and 31.3 times higher than the ones of neat g-C 3 N 4 and Cu 2 (OH)PO 4 , respectively. The photocatalytic generation of H 2 O 2 under a Z-scheme mechanism was assessed by using Bi 4 O 5 Br 2 /g-C3N4 hybrids [81]. The optimal H 2 O 2 production rate was 124 µM/h. The electrons and holes of the hybrid were mainly derived from the g-C 3 N 4 conduction band and the Bi 4 O 5 Br 2 valence band, respectively.
Metal phosphides have been applied as functional cocatalysts for C 3 N 4 -based systems. Specifically, cobalt phosphide (CoP) nanoparticles were embedded within the network of carbon nitride [82]. The optimal catalyst with 1.76 wt% CoP content demonstrated enhanced photocatalytic efficiency with an H 2 O 2 production rate of 140 µM after 2 h irradiation time, which was about 4.6 and 23.3 times higher than the ones of neat g-C 3 N 4 and CoP, respectively. In an analogous study, carbon nitride and WO 3 nanoparticles have been compounded through a solid-state calcination of dicyandiamide and ammonium paratungstate mixture [83]. The authors proposed oxygen-enriched carbon nitride models, which were proven to generate 1,4-endoperoxide species more efficiently, rather than superoxide radicals, through theoretical calculations and experimental investigation. Under an oxygen-saturated atmosphere and the presence of isopropanol as a hole scavenger, the highest yield that was achieved by the hybrid calcined at 500 • C reached 730 µmol after 5 h. Chen and co-workers [84] fabricated graphitic carbon nitride that was enriched with cyano groups, via solid-state thermal annealing of NaCl-dicyandiamide mixtures. The cyano groups were suggested to adjust the band structure of C 3 N 4 as well as act as oxygen adsorption sites. The resulting photocatalysts exhibited superior activity towards the H 2 O 2 generation. Irradiation in the visible region afforded a rate of 7 mM/h, whereas the corresponding rate under simulated sun conditions was 16 mM/h.
The covalent anchoring of aromatic substances onto the C 3 N 4 surface has been used in order to inhibit carrier recombination phenomena. Kim and co-workers [85] have attached anthraquinone molecules onto the carbon nitride surface. The suggested mechanism involves the hydrogenation reaction of anthraquinone moieties to AQH 2 and the subsequent generation of H 2 O 2 by oxygen reduction, mediated through the dehydrogenation reaction of AQH 2 intermediate substance. A H 2 O 2 production rate of 361 µmol/g/h at 380 nm excitation wavelength was achieved using the hybrid material in the presence of an organic electron donor (2-propanol).
Regarding the hybridization of carbon nitride with graphitic allotropes, Zhao and co-workers [86] have applied a protocol, leading to the covalent attachment of carbon nanotubes (CNTs) onto the carbon nitride surface, by an amidation reaction. The presence of the conductive co-catalyst was found to enhance the reducing ability of photoexcited semiconductor. The g-C 3 N 4 -CNTs hybrid exhibited remarkable catalytic performance for photocatalytic H 2 O 2 generation in the presence of formic acid as a hole scavenger (32.6 µmol/h).
In an interesting work, Kang and co-workers [87] have studied the synergistic effect of C 3 N 4 , a conductive needle coke and a green microalga (Chlorella vulgaris) towards the photocatalytic generation of H 2 O 2 . In this three-component system, the needle coke served as cocatalyst to enhance the separation of photoinduced electron-hole pairs of g-C 3 N 4 , which provides oxidation sites for partial H 2 O 2 evolution. The living C. vulgaris participated to the H 2 O 2 evolution through an oxygen reduction process. The optimal three-component photocatalyst exhibited a H 2 O 2 production rate of 0.98 µmol/h. By using the aforementioned principle of the dual cocatalyst, Li et al. [88] have compounded C 3 N 4 with organic small molecules and N,S-doped carbon dots. The organic substance was a carbazole derivative which acted as the active site of oxygen reduction reaction (ORR), and carbon dots were suggested to participate as the active site of water oxidation reaction (WOR) [89]. The three-component catalyst exhibited a remarkable H 2 O 2 production rate of 2203 µmol/h/g.

State-of-the-Art and Future Prospects
In Table 1, we selected the works which have exhibited the higher H 2 O 2 production rates. As clearly shown, the most efficient photocatalytic systems involve the ones that are related to either porous carbon nitride assemblies, metal organic frameworks, or carbonbased conjugated semiconducting nanostructures. However, the most important aspects seem to be tailored doping with heterostructures as well as the generation of functional defect sites. These may enhance the selectivity of the two-electron oxygen reduction reaction and suppress the electron-hole recombination process. Furthermore, the dopants act as oxygen adsorption sites, thus enhancing the photocatalytic activity. This puts into discussion the very important aspect of high surface area assemblies. Various protocols should be developed, based on sacrificial template approaches, in order to acquire doped semiconductors with available catalytic sites. The combination of chemical modification and porosity enhancement may pave the way towards the development of highly efficient photocatalytic systems in aqueous environments. Concerning the integration of such hybrids in industrial scale facilities, some promising results have been achieved, yet additional work has to be performed. The main parameter which needs to be addressed is the cost of the process, including the development of facilities utilizing the solar light as an excitation source. A significant advance in the field could be achieved by catalytic systems which are synthesized by one-pot processes. This would significantly decrease the cost of the whole process in the synthetic part. As shown in Table 1, C 3 N 4 -based nanostructures are considered as potentially efficient photocatalytic systems for H 2 O 2 evolution. This is greatly supported by the fact that such systems are derived by one-step annealing processes. Issues that are related with electron-hole recombination phenomena, could be resolved by developing functional Z-scheme binary systems acquiring spatial separation of the carriers. Our strong belief is that the industrialscale H 2 O 2 evolution and subsequent utilization is much closer to achievement, when compared with the state-of-the-art methods that were acquired a couple of years ago. The H 2 O 2 yield as well as the response of the catalysts to visible light have been widely enhanced in the recent years. Further research should be focused on the development of photocatalytic systems with high recyclability.