Photocatalytic Hydrogen Evolution via Water Splitting : A Short Review

Photocatalytic H2 generation via water splitting is increasingly gaining attention as a viable alternative for improving the performance of H2 production for solar energy conversion. Many methods were developed to enhance photocatalyst efficiency, primarily by modifying its morphology, crystallization, and electrical properties. Here, we summarize recent achievements in the synthesis and application of various photocatalysts. The rational design of novel photocatalysts was achieved using various strategies, and the applications of novel materials for H2 production are displayed herein. Meanwhile, the challenges and prospects for the future development of H2-producing photocatalysts are also summarized.


Introduction
The development of renewable green energy sources is a critical challenge for modern society.H 2 is environmentally friendly, renewable, and considered to be an ideal candidate for an economically and socially sustainable fuel [1][2][3][4][5][6], and was previously regarded as an alternative energy source.Interestingly, some researchers also found that H 2 -rich water has neuron effects owing to its antioxidant properties.Although the deep mechanism is not clear, more and more researchers made an effort to study the biological function of H 2 [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21].To date, almost all H 2 gas production processes in the industry are based on natural gas, coal, petroleum, or water electrolysis.These traditional preparation methods are limited due to the associated CO 2 emissions and high energy consumption.Hence, it is urgent to develop a low-cost method for efficient H 2 generation and, thus, support the emerging H 2 economy.
The sun provides an energy output of ~3 × 10 24 J per year, which is approximately 12,000 times higher than the current energy demand.Therefore, solar energy can act as a sustainable alternative energy source in the future.To date, the transformation of solar energy into H 2 via water splitting is deemed as a desirable H 2 preparation method to solve the energy crisis [22,23].
The proper use of H 2 requires insight into the physical properties of H 2 molecules.As we know, the lengths and strengths of hydrogen bonds are exquisitely sensitive to temperature and pressure.Meanwhile, the charges of H 2 molecules also vary with temperature [24] because the spin direction of the nucleus in the H 2 molecule changes depending on the temperature, and an energy difference occurs between H 2 molecules.The para-H 2 fraction changes with temperature, and it is necessary to understand the characteristics of H 2 molecules according to temperature [25].During the reaction, hydrogen can be used safely at room temperature; however, it is rather dangerous in high-temperature environments.
As we know, H 2 gas, often called dihydrogen or molecular H 2 , is a highly flammable gas with a wide range of concentrations between 4% and 75% by volume.Meanwhile, H 2 is the world's lightest gas.The density of H 2 is only 1/14 of that of air.At 0 • C, the density of H 2 is only 0.0899 g/L at standard atmospheric pressure, which is the smallest-molecular-weight substance; it is mainly used as a reducing agent.The enthalpy of combustion is about −286 kJ/mol, which can be displayed by the following equation: 2H 2 (g) + O 2 (g) → 2H 2 O(l) + 572 kJ (286 kJ/mol).Currently, H 2 is the main industrial raw material and the most important industrial gas.It has various applications in the petrochemical, electronic, and metallurgical industry, as well as in food processing, float glass, fine organic synthesis, aerospace, and other fields.At the same time, H 2 is also an ideal secondary energy source.Owing to the properties of H 2 , the aerospace industry uses liquid H 2 as fuel.Now, it is common to produce H 2 from water gas rather than using high-energy-consuming water.The produced H 2 is used in large quantities in the cracking reaction of the petrochemical industry and the production of ammonia.Unfortunately, all H 2 production methods are highly energy (thermal and electrical) demanding, which limits their application.Thus, it is crucial to find a new method of H 2 production.
Fujishima and Honda first reported photocatalytic water splitting using a TiO 2 electrode in 1972 [26].Research on solar H 2 production attracted researchers in various fields, such as (1) chemists for the design and synthesis of various catalysts to investigate structure-property relationships; (2) physicists to fabricate semiconductor photocatalysts with novel electronic structures, as predicted by theoretical calculation; and (3) material scientists to construct unique photocatalytic materials with novel structures and morphologies [27][28][29][30].When photocatalysts are illuminated at wavelengths which are suitable to their band gap energy, after the excitation, the charge carriers will either combine or transfer to the surface of the photocatalysts to participate in photocatalytic reactions.For the generation of efficient semiconductor photocatalysts, long-lived charge carriers and high stability are required [31][32][33].
Significant developments were made toward H 2 generation via water splitting over the last several decades by a number of talented researchers [34][35][36][37][38].
Herein, we attempt to sum up the advances achieved to date.Therefore, we briefly summarize the background related to various photocatalysts for H 2 generation and the achievements of high-efficiency photocatalysts.The main synthesis routes and modifications for adjusting the band structure to harvest light and enhance charge separation are also discussed.

Principle of H 2 Generation via Water Splitting
In the pioneering study by Fujishima and Honda [27], electrochemical cells were made up for the splitting of the water into H 2 and O 2 , as shown in Figure 1.While the TiO 2 electrode was under ultraviolet (UV) light irradiation, water oxidation (oxygen evolution) occurred on its surface, while the reduction reaction (H 2 evolution) occurred on the surface platinum black electrode.With this study in mind, semiconductor photocatalysts were later developed by Bard et al. in their design of a novel photocatalytic system.Figure 2a shows a display of hydrogen evolution by photocatalysts.The photocatalytic reaction occurring on the semiconductor photocatalysts can be divided into three parts: (1) obtaining photons with energy exceeding that of the photocatalyst's band gap, generating electron and hole pairs; (2) separating carriers by migration in the semiconductor photocatalyst; and (3) reaction between these carriers and H2O [39][40][41][42][43][44][45][46].In addition, electron-hole pairs will combine with each other simultaneously.As shown in Figure 2b, while photocatalysts are involved in hydrogen evolution, the lowest position of the conduction band (CB) should be lower than the reduction position of H2O/H2, while the position of the valence band (VB) should be higher than the potential of H2O/O2 [47-50].Various photocatalysts were reported to decompose water into H2 and O2 (Equation 1).As we know, the hydrogen evolution reaction can be separated into two parts: oxidation for the evolution of O2 (Equation 2) and water reduction to produce H2 (Equation 3) [51][52][53][54][55][56]:

Photocatalysts for Water Splitting
Many photocatalysts were created as photocatalysts for hydrogen evolution.Based on these species, they can be divided into three major parts: (1) graphene-based photocatalyst; (2) graphitic carbon nitride (g-C3N4)-based photocatalysts; and (3) heterojunction photocatalysts (semiconductorsemiconductor or semiconductor-(metal, element)).Figure 2a shows a display of hydrogen evolution by photocatalysts.The photocatalytic reaction occurring on the semiconductor photocatalysts can be divided into three parts: (1) obtaining photons with energy exceeding that of the photocatalyst's band gap, generating electron and hole pairs; (2) separating carriers by migration in the semiconductor photocatalyst; and (3) reaction between these carriers and H 2 O [39][40][41][42][43][44][45][46].In addition, electron-hole pairs will combine with each other simultaneously.As shown in Figure 2b, while photocatalysts are involved in hydrogen evolution, the lowest position of the conduction band (CB) should be lower than the reduction position of H 2 O/H 2 , while the position of the valence band (VB) should be higher than the potential of H 2 O/O 2 [47][48][49][50].Figure 2a shows a display of hydrogen evolution by photocatalysts.The photocatalytic reaction occurring on the semiconductor photocatalysts can be divided into three parts: (1) obtaining photons with energy exceeding that of the photocatalyst's band gap, generating electron and hole pairs; (2) separating carriers by migration in the semiconductor photocatalyst; and (3) reaction between these carriers and H2O [39][40][41][42][43][44][45][46].In addition, electron-hole pairs will combine with each other simultaneously.As shown in Figure 2b, while photocatalysts are involved in hydrogen evolution, the lowest position of the conduction band (CB) should be lower than the reduction position of H2O/H2, while the position of the valence band (VB) should be higher than the potential of H2O/O2 [47-50].Various photocatalysts were reported to decompose water into H2 and O2 (Equation 1).As we know, the hydrogen evolution reaction can be separated into two parts: oxidation for the evolution of O2 (Equation 2) and water reduction to produce H2 (Equation 3) [51][52][53][54][55][56]:

Graphene-Based Photocatalysts
Recently, graphene-based photocatalysts attracted significant attention for enhancing photocatalytic H 2 production performance.Graphene is used to enhance photocatalytic efficiency owing to its novel structure and electrochemical properties (Figure 3).

Graphene-Based Photocatalysts
Recently, graphene-based photocatalysts attracted significant attention for enhancing photocatalytic H2 production performance.Graphene is used to enhance photocatalytic efficiency owing to its novel structure and electrochemical properties (Figure 3).To date, many reports regarding the synthesis of graphene-based photocatalysts with improved photocatalytic efficiency were published.Graphene is a well-known two-dimensional (2D) material, which can improve surface area, and its 2D membrane-like structure imparts unique electrochemical properties [57][58][59][60].Generally speaking, photocatalysts prepared by simple physical mixing with graphene will involve only a bit of direct contact with the graphene sheets.This small amount of contact between the photocatalyst and graphene results in weak interactions and inhibits charge transfer rates.Hence, the synthesis of photocatalysts with more interactions is highly needed.
Previously, Kim et al. synthesized novel graphene oxide (GO)-TiO2 photocatalysts [58] in 2013, comprising a core-shell nanostructure with enhanced photocatalytic efficiency (Figure 4).The improved H2 production activity compared to that of TiO2 revealed that the utilization of the coreshell structure enhanced photocatalytic efficiency.This novel structural design offers threedimensional (3D) close contact between the materials and provides more active sites, which will enhance the charge separation rate and H2 production efficiency [61][62][63].To date, many reports regarding the synthesis of graphene-based photocatalysts with improved photocatalytic efficiency were published.Graphene is a well-known two-dimensional (2D) material, which can improve surface area, and its 2D membrane-like structure imparts unique electrochemical properties [57][58][59][60].Generally speaking, photocatalysts prepared by simple physical mixing with graphene will involve only a bit of direct contact with the graphene sheets.This small amount of contact between the photocatalyst and graphene results in weak interactions and inhibits charge transfer rates.Hence, the synthesis of photocatalysts with more interactions is highly needed.
Previously, Kim et al. synthesized novel graphene oxide (GO)-TiO 2 photocatalysts [58] in 2013, comprising a core-shell nanostructure with enhanced photocatalytic efficiency (Figure 4).The improved H 2 production activity compared to that of TiO 2 revealed that the utilization of the core-shell structure enhanced photocatalytic efficiency.This novel structural design offers three-dimensional (3D) close contact between the materials and provides more active sites, which will enhance the charge separation rate and H 2 production efficiency [61][62][63].
Previously, Kim et al. synthesized novel graphene oxide (GO)-TiO2 photocatalysts [58] in 2013, comprising a core-shell nanostructure with enhanced photocatalytic efficiency (Figure 4).The improved H2 production activity compared to that of TiO2 revealed that the utilization of the coreshell structure enhanced photocatalytic efficiency.This novel structural design offers threedimensional (3D) close contact between the materials and provides more active sites, which will enhance the charge separation rate and H2 production efficiency [61][62][63].Currently, many researchers are more interested in visible-light-driven photocatalysts, which are achieved using band-gap modification or taking graphene as a photosensitizer to broaden the visible-light adsorption range [64-66].Significant efforts were conducted for building visible-light response systems because of the UV-only response of TiO 2 , and its nontoxic properties [67].Recently, it was found that graphene regulating TiO 2 involves visible-light adsorption activity.The carbon-layered structure of graphene with enriched π electrons forms bonds with titanium atoms.As a result, this strong interaction will shift the band position and reduce the band gap [68][69][70].Lee et al. [71] also achieved a lower band gap using a graphene/TiO 2 photocatalyst.The improved photocatalytic efficiency of the graphene/TiO 2 composite owes to the band-gap regulation, which consequently promotes charge transfer rates through the graphene sheets.

g-C 3 N 4 -Based Photocatalysts
Currently, carbon-nitride-based photocatalysts receive significant attention for their photocatalytic H 2 generation owing to a unique electronic structure (Figure 5) [72][73][74][75][76][77].This section summarizes recent significant achievements in building C 3 N 4 -based photocatalysts for H 2 evolution.Methods including nanostructure regulation, band-gap modification, dye sensitization, and heterojunction fabrication are highlighted herein.Currently, many researchers are more interested in visible-light-driven photocatalysts, which are achieved using band-gap modification or taking graphene as a photosensitizer to broaden the visible-light adsorption range [64-66].Significant efforts were conducted for building visible-light response systems because of the UV-only response of TiO2, and its nontoxic properties [67].Recently, it was found that graphene regulating TiO2 involves visible-light adsorption activity.The carbonlayered structure of graphene with enriched π electrons forms bonds with titanium atoms.As a result, this strong interaction will shift the band position and reduce the band gap [68][69][70].Lee et al. [71] also achieved a lower band gap using a graphene/TiO2 photocatalyst.The improved photocatalytic efficiency of the graphene/TiO2 composite owes to the band-gap regulation, which consequently promotes charge transfer rates through the graphene sheets.

g-C3N4-Based Photocatalysts
Currently, carbon-nitride-based photocatalysts receive significant attention for their photocatalytic H2 generation owing to a unique electronic structure (Figure 5) [72][73][74][75][76][77].This section summarizes recent significant achievements in building C3N4-based photocatalysts for H2 evolution.Methods including nanostructure regulation, band-gap modification, dye sensitization, and heterojunction fabrication are highlighted herein.Recently, carbon nitride attracted significant attention following the pioneering research of Wang et al. in 2009 for photocatalytic hydrogen evolution [78,79].The assumed structure of C3N4 is a 2D framework with the tri-s-triazine linked by tertiary amines (Figure 6); it is thermally stable and chemically stable.Pioneering studies regarded g-C3N4 as a visible-light-driven phorocatalyst with a band gap of approximately 2.7 eV and an appropriate band position for water splitting [80-85].Hence, g-C3N4 is an ideal candidate for photocatalytic H2 evolution.H2 generation performance using g-C3N4 can be promoted with noble-metal particles such as Au or Pd, which obtain electrons in the CB to inhibit the charge recombination rate [86][87][88][89][90].Many researchers are developing metal-free photocatalysts for H2 evolution, and recent reports involved the introduction of non-noble-metal catalysts into g-C3N4 photocatalysts, displaying enhanced photocatalytic performance compared to noble-metal catalysts [91][92][93][94][95]. Hou et al. [86] synthesized MoS2/g-C3N4 composite photocatalysts (Figure 7) in 2018.MoS2/g-C3N4 increased the surface area and decreased the barrier when the electrons transported, thereby improving the charge transfer rate.The formation of band alignment enabled electron transfer from the CB (g-C3N4) to MoS2.Therefore, the MoS2/gC3N4 nanojunction significantly enhanced H2 evolution efficiency, achieving the highest H2 evolution rate and an optimum quantum efficiency of up to 2.1% (420 nm), which was higher than g-C3N4/Pt.Reproduced with permission from Reference [72]; copyright (2014), American Chemical Society.

Metal-Loading-Based Photocatalysts
Metal loading is also regarded as a useful method for photocatalytic enhancement.Song et al. [96] constructed Ag-rGO-TiO2 composite photocatalysts (Figure 8) in 2018.In order to analyze the photocatalytic mechanism of the architectural Ag-TiO2 and Ag-rGO-TiO2 composites, their structures with Ag nanocubes for light absorption and TiO2 nanosheets were well displayed.The difference between Ag-TiO2 and Ag-rGO-TiO2 is the interface between Ag nanocubes and TiO2 nanosheets, which enhances the electron transfer capability.For Ag-TiO2, the direct contact between the two materials results in the formation of Ag ( 100)/(001) TiO2 interface.Meanwhile, for Ag-rGO-TiO2, both Ag(100)/rGO and rGO/(001) TiO2 interfaces are formed by rGO.As mentioned above, the synergistic Reproduced with permission from Reference [72]; copyright (2014), American Chemical Society.

Metal-Loading-Based Photocatalysts
Metal loading is also regarded as a useful method for photocatalytic enhancement.Song et al. [96] constructed Ag-rGO-TiO 2 composite photocatalysts (Figure 8) in 2018.In order to analyze the photocatalytic mechanism of the architectural Ag-TiO 2 and Ag-rGO-TiO 2 composites, their structures with Ag nanocubes for light absorption and TiO 2 nanosheets were well displayed.The difference between Ag-TiO 2 and Ag-rGO-TiO 2 is the interface between Ag nanocubes and TiO 2 nanosheets, which enhances the electron transfer capability.For Ag-TiO 2 , the direct contact between the two materials results in the formation of Ag (100)/(001) TiO 2 interface.Meanwhile, for Ag-rGO-TiO 2 , both Ag(100)/rGO and rGO/(001) TiO 2 interfaces are formed by rGO.As mentioned above, the synergistic effect of Ag(100)/rGO and rGO/(001)TiO 2 interfaces, rather than the Ag(100)/(001) TiO 2 interface, offers quicker electron transfer.As shown in Figure 8, no Schottky barrier is formed between Ag and TiO 2 , and the hot electrons on the surface of TiO 2 flow back to Ag and then recombine with holes.Meanwhile, for the Ag-rGO-TiO 2 sample, no barrier is necessary to facilitate the electron transfer.The electrons generated on the surface of Ag nanocubes with smaller work function flow to rGO via a contact so as to equilibrate the electron Fermi distribution on the interface [97,98].Moreover, the rGO nanosheets can act as conductive channels, further transferring the electron to the rGO/TiO 2 interface.Owing to the light absorption of rGO, the transferred electrons within the rGO nanosheets can be further transferred to the CB of TiO 2 under light excitation.The proposed photocatalytic mechanism of Ag-rGO-TiO 2 is illustrated in Figure 8.

Z-Scheme Photocatalysts
An illustration of Z-scheme water splitting is shown in Figure 9.During an H2 evolution reaction, the reactions which happen on the surface of photocatalysts include the reduction of protons by CB electrons and the oxidation of an electron donor (D) by VB holes, yielding the corresponding electron acceptor (A), as follows: On the other hand, the forward reactions on an O2 evolution photocatalyst are as follows: where the electron acceptor generated by the paired H2 evolution photocatalyst is converted to D, and the water oxidation process occurs via the valence band holes.Thus, the water-splitting process can be achieved.

Z-Scheme Photocatalysts
An illustration of Z-scheme water splitting is shown in Figure 9.During an H 2 evolution reaction, the reactions which happen on the surface of photocatalysts include the reduction of protons by CB electrons and the oxidation of an electron donor (D) by VB holes, yielding the corresponding electron acceptor (A), as follows: On the other hand, the forward reactions on an O 2 evolution photocatalyst are as follows: where the electron acceptor generated by the paired H 2 evolution photocatalyst is converted to D, and the water oxidation process occurs via the valence band holes.Thus, the water-splitting process can be achieved.
On the other hand, the forward reactions on an O2 evolution photocatalyst are as follows: where the electron acceptor generated by the paired H2 evolution photocatalyst is converted to D, and the water oxidation process occurs via the valence band holes.Thus, the water-splitting process can be achieved.Amal et al. reported a Z-scheme photocatalytic water-splitting system using Ru/SrTiO 3 and partially reduced GO (PRGO)/BiVO 4 (Figure 10) in 2011 [100].As described in the report, the PRGO/BiVO 4 (O 2 photocatalyst) and Ru/SrTiO 3 :Rh (H 2 photocatalyst) were attached due to surface charge modification in acidic conditions, as depicted in Figure 10.Under irradiation, electrons are excited from the VB (BiVO 4 ) or an impurity level in Rh (Ru/SrTiO 3 :Rh) to the CB.We can indicate that the PRGO does not contribute to the electron and hole generation.In other words, the RGO in this work acts as an electron conductor.PRGO transfers the electrons from the CB of BiVO 4 to the Ru/SrTiO 3 :Rh.Meanwhile, the electrons in Ru/SrTiO 3 :Rh reduce the water to H 2 on the surface of the Ru co-catalyst, while the holes left in BiVO 4 oxidize the water to O 2 .Additionally, the PRGO provides a pathway for photogenerated electrons in the BiVO 4 photocatalyst.Each reaction can migrate as follows: reduction of water, transfer of electrons to PRGO, and transfer of holes to PRGO for oxidation.Because the majority of the photocatalyst surface is surrounded by water and only relatively small portions are in contact with PRGO [101], most electrons in Ru/SrTiO 3 :Rh and holes in BiVO 4 are used for water splitting.Amal et al. reported a Z-scheme photocatalytic water-splitting system using Ru/SrTiO3 and partially reduced GO (PRGO)/BiVO4 (Figure 10) in 2011 [100].As described in the report, the PRGO/BiVO4 (O2 photocatalyst) and Ru/SrTiO3:Rh (H2 photocatalyst) were attached due to surface charge modification in acidic conditions, as depicted in Figure 10.Under irradiation, electrons are excited from the VB (BiVO4) or an impurity level in Rh (Ru/SrTiO3:Rh) to the CB.We can indicate that the PRGO does not contribute to the electron and hole generation.In other words, the RGO in this work acts as an electron conductor.PRGO transfers the electrons from the CB of BiVO4 to the Ru/SrTiO3:Rh.Meanwhile, the electrons in Ru/SrTiO3:Rh reduce the water to H2 on the surface of the Ru co-catalyst, while the holes left in BiVO4 oxidize the water to O2.Additionally, the PRGO provides

Defect Engineering Photocatalyst
Among the various photocatalyst designs, the defect engineering strategy is regarded as an important way of modifying the photocatalysts.Defects are places where the atoms or molecules in the materials are disrupted, and they greatly influence photocatalytic performance.The defects in the lattice of photocatalysts not only act as an electron-hole recombination center, but also break the electronic structure and display a scattering center for electron and hole travel.Nevertheless, the positive effect of defects in photocatalytic performance enhancement were also recognized with the development of defect photocatalysts and the development of the photocatalytic field.
Chen et al. reported the synthesis of a bismuth subcarbonate (Bi 2 O 2 CO 3 , BOC) with controllable defect density (BOC-X) (Figure 11) in 2018.The BOC-X with defect density displayed a photocatalytic nitrogen fixation of 957 µmol•L -1 under irradiation within 4 h, which was 9.4 times higher than that of pristine BOC.This photocatalytic performance enhancement of BOC-X can be attributed to the surface defects.These defects contribute to the defect levels in the forbidden band, which improves the light harvest percentage.Meanwhile, surface defects can also inhibit the electron-hole recombination rate to promote the separation efficiency of charge carriers.Photocatalytic nitrogen fixation by BOC-X is displayed in Figure 11.If the light energy is higher than the band-gap energy, the electrons on the VB surface of BOC-X are transferred to the CB and react with N 2 to form NH 3 .Moreover, some of the VB electrons are transferred to the defect level and then react with N 2 .However, if the light energy is lower than the band-gap energy, the electrons of BOC-X are also excited from VB to the defect level and then participate in the reaction.Defects modulate the band gap of BOC-X and improve the light absorption range, thereby enhancing the carrier transport, and leading to photocatalytic enhancement.

Heterojunction Photocatalysts
During the H2 evolution reaction, the formed electron-hole charges are transferred to the surface of the photocatalyst for the next step of the reaction or recombine with each other [102][103][104][105][106]. To better reveal this point, we assumed it as a simple case [107]: the influence of gravity on a man jumping (Figures 12a,b).When a man (electron) jumps from the ground (VB) to the sky (CB), it can return to the floor immediately (recombination of the electron and hole) owing to gravity.In order to let the people rise off the floor (separation of the charge carrier pairs), an instrument (semiconductor B) can be used (Figures 12c,d).Subsequently, the previously mentioned people can drop to the instrument rather than the ground (inhibition of the electron and hole pair recombination).Although the inhibition of electron-hole recombination rate is an urgent issue, it can be achieved via suitable construction of materials.Many methods were conducted to achieve better electron-hole pair separation rate, such as element combining [108,109], metal doping [110,111], or the use of heterojunctions [112,113].Among these strategies, heterojunctions were proven to be the most desirable method for achieving efficient photocatalysis due to their improved separation ability of electron-hole pairs (Figure 12d).

Heterojunction Photocatalysts
During the H 2 evolution reaction, the formed electron-hole charges are transferred to the surface of the photocatalyst for the next step of the reaction or recombine with each other [102][103][104][105][106]. To better reveal this point, we assumed it as a simple case [107]: the influence of gravity on a man jumping (Figure 12a,b).When a man (electron) jumps from the ground (VB) to the sky (CB), it can return to the floor immediately (recombination of the electron and hole) owing to gravity.In order to let the people rise off the floor (separation of the charge carrier pairs), an instrument (semiconductor B) can be used (Figure 12c,d).Subsequently, the previously mentioned people can drop to the instrument rather than the ground (inhibition of the electron and hole pair recombination).Although the inhibition of electron-hole recombination rate is an urgent issue, it can be achieved via suitable construction of materials.Many methods were conducted to achieve better electron-hole pair separation rate, such as element combining [108,109], metal doping [110,111], or the use of heterojunctions [112,113].Among these strategies, heterojunctions were proven to be the most desirable method for achieving efficient photocatalysis due to their improved separation ability of electron-hole pairs (Figure 12d).
inhibition of electron-hole recombination rate is an urgent issue, it can be achieved via suitable construction of materials.Many methods were conducted to achieve better electron-hole pair separation rate, such as element combining [108,109], metal doping [110,111], or the use of heterojunctions [112,113].Among these strategies, heterojunctions were proven to be the most desirable method for achieving efficient photocatalysis due to their improved separation ability of electron-hole pairs (Figure 12d).A heterojunction is regarded as the connection between two kinds of photocatalysts with different band structures, which leads to a new band arrangement [114,115].Generally, three kinds of composite photocatalysts are developed (Figure 13).As shown in Figure 13a, the CB and VB of A are a bit over and under the band position of B, respectively [116].As a result, when the light irradiates, the generated electrons and holes are transferred to the CB and VB of B. Because the generated electrons and holes move to the same photocatalyst, the recombination rate of electron-hole pairs is not efficiently inhibited.The photocatalytic process happens on photocatalyst B with a mild potential requirement; thus, the photocatalytic ability of the photocatalyst using this heterojunction will be lower than others.As dispalyed in Figure 13b, the band positions of CB and VB are over that of photocatalyst B. Hence, during the photocatalytic reaction, the generated electron moves to photocatalyst B, while the holes are transferred to photocatalyst A, which leads to the formation of long lived electron-hole pairs [117][118][119].Parallel to Figure 13a, the photocatalytic performance of the type-II composite photocatalysts is inhibited by the redox process occurring on B. Meanwhile, as displayed in Figure 13c, the band structure of type-III composite photocatalysts is parallel to type II, apart from the interlaced gap changing into non-overlapping band gaps [120,121].Thus, the generated electron-hole pairs cannot be transferred between the two photocatalysts, resulting in them being inappropriate for long lived electron-hole pair separation.We can determine that the type-II heterojunctions are desirable for enhancing redox ability due to their optimum structure for long-lived electron-hole separation.In previous reports, great efforts were conducted to synthesize type-II composite photocatalysts, including g-C 3 N 4 /TiO 2 [122], WO 3 /BiVO 4 [123], WO 3 /g-C 3 N 4 [124], and BiPO 4 /g-C 3 N 4 [125].
resulting in them being inappropriate for long lived electron-hole pair separation.We can determine that the type-II heterojunctions are desirable for enhancing redox ability due to their optimum structure for long-lived electron-hole separation.In previous reports, great efforts were conducted to synthesize type-II composite photocatalysts, including g-C3N4/TiO2 [122], WO3/BiVO4 [123], WO3/g-C3N4 [124], and BiPO4/g-C3N4 [125].14b,c).We can see that the holes left on the n-type catalyst are transferred to the p-type catalyst, providing a negative specie.The electron-hole pair distribution keeps moving until a Fermi-level equilibrium is achieved [126][127][128].The generated active species move through the internal electric field of the composite photocatalysts, resulting in Yu et al. designed CdS/NiS composites photocatalysts using various heterojunctions in 2012, which greatly enhanced the hydrogen evolution performance.As shown in Figure 14a, around 20 nm of NiS particles were loaded onto the CdS uniformly, which supported a close connection between CdS and NiS.The formation of p-n heterojunctions facilitates charge transfer between the NiS and CdS, and inhibits charge-carrier recombination (Figure 14b,c).We can see that the holes left on the n-type catalyst are transferred to the p-type catalyst, providing a negative specie.The electron-hole pair distribution keeps moving until a Fermi-level equilibrium is achieved [126][127][128].The generated active species move through the internal electric field of the composite photocatalysts, resulting in long-lived electron-hole pair separation rates.Thus, the electron-hole recombination rate is efficiently inhibited owing to the synergistic effect between the two photocatalysts.The photocatalytic H 2 production rate over CdS/NiS composite photocatalysts with 5 wt.%NiS was found to be higher than that of the CdS and 1 wt.%Pt/CdS (Figure 14d).More NiS doping resulted in a reduction in photocatalytic efficiency due to NiS catalysts reducing the number of redox sites during the reaction.
efficiently inhibited owing to the synergistic effect between the two photocatalysts.The photocatalytic H2 production rate over CdS/NiS composite photocatalysts with 5 wt.%NiS was found to be higher than that of the CdS and 1 wt.%Pt/CdS (Figure 14d).More NiS doping resulted in a reduction in photocatalytic efficiency due to NiS catalysts reducing the number of redox sites during the reaction.

Summary and Perspectives
Over the last several decades, photocatalysis was shown to be a promising method for H2 production.Even though the principles controlling photocatalytic activity in the developed semiconductors were identified, several aspects remain unclear.Therefore, practical applications and the commercialization of photocatalytic H2 production require further research.Meanwhile, the charge transfer among photocatalysts due to the influence of structure and electrochemical properties is also not very clear, while the influence of various preparation methods on the catalytic performance is not well understood.The development of improved photocatalysts will benefit from advances in science.Improved building of novel co-catalysts will arise from using efficient catalysts.Many researches are underway investigating new synthesis methods for sample preparation and novel system construction.Herein, we concluded the most prominent achievements associated with H2 production via photocatalysis.We hope this report will assist further research efforts regarding the development of photocatalysts.

Conflicts of Interest:
The authors declare no conflict of interest.

Summary and Perspectives
Over the last several decades, photocatalysis was shown to be a promising method for H 2 production.Even though the principles controlling photocatalytic activity in the developed semiconductors were identified, several aspects remain unclear.Therefore, practical applications and the commercialization of photocatalytic H 2 production require further research.Meanwhile, the charge transfer among photocatalysts due to the influence of structure and electrochemical properties is also not very clear, while the influence of various preparation methods on the catalytic performance is not well understood.The development of improved photocatalysts will benefit from advances in science.Improved building of novel co-catalysts will arise from using efficient catalysts.Many researches are underway investigating new synthesis methods for sample preparation and novel system construction.Herein, we concluded the most prominent achievements associated with H 2 production via photocatalysis.We hope this report will assist further research efforts regarding the development of photocatalysts.

Figure 5 .
Figure 5. Proposed mechanism of graphitic carbon nitride (g-C 3 N 4 )-based photocatalysts.Reproduced with permission from Reference [72]; copyright (2014), American Chemical Society.Recently, carbon nitride attracted significant attention following the pioneering research of Wang et al. in 2009 for photocatalytic hydrogen evolution [78,79].The assumed structure of C 3 N 4 is a 2D framework with the tri-s-triazine linked by tertiary amines (Figure 6); it is thermally stable and chemically stable.Pioneering studies regarded g-C 3 N 4 as a visible-light-driven phorocatalyst with a

Figure 6 .
Figure 6.Schematic display of the structure of g-C 3 N 4 .Reproduced with permission from Reference [72]; copyright (2014), American Chemical Society.H 2 generation performance using g-C 3 N 4 can be promoted with noble-metal particles such as Au or Pd, which obtain electrons in the CB to inhibit the charge recombination rate[86][87][88][89][90].Many researchers are developing metal-free photocatalysts for H 2 evolution, and recent reports involved the introduction of non-noble-metal catalysts into g-C 3 N 4 photocatalysts, displaying enhanced photocatalytic performance compared to noble-metal catalysts[91][92][93][94][95].Hou et al.[86] synthesized MoS 2 /g-C 3 N 4 composite photocatalysts (Figure7) in 2018.MoS 2 /g-C 3 N 4 increased the surface area and decreased the barrier when the electrons transported, thereby improving the charge transfer rate.The formation of band alignment enabled electron transfer from the CB (g-C 3 N 4 ) to MoS 2 .Therefore, the MoS 2 /gC 3 N 4 nanojunction significantly enhanced H 2 evolution efficiency, achieving the highest H 2 evolution rate and an optimum quantum efficiency of up to 2.1% (420 nm), which was higher than g-C 3 N 4 /Pt.

Figure 9 .
Figure 9. Diagram of photocatalytic water splitting using a Z-Scheme system.Reproduced with permission from Reference [98]; Copyright (2010), American Chemical Society.

Figure 9 .
Figure 9. Diagram of photocatalytic water splitting using a Z-Scheme system.Reproduced with permission from Reference [98]; Copyright (2010), American Chemical Society.

Figure 12 .
Figure 12.Schematic display of (a) the influence of gravity on a person jumping, (b) electron-hole pair combination using a photocatalyst, (c) utilization of a stool to keep the person from returning to the ground, (d) electron-hole pairs separated in composite catalyst.Reproduced with permission from Reference [105]; copyright (2010), John Wiley & Sons, Inc.

Figure 12 .
Figure 12.Schematic display of (a) the influence of gravity on a person jumping, (b) electron-hole pair combination using a photocatalyst, (c) utilization of a stool to keep the person from returning to the ground, (d) electron-hole pairs separated in composite catalyst.Reproduced with permission from Reference [105]; copyright (2010), John Wiley & Sons, Inc.

Figure 14 .
Figure 14.(a) SEM image of CdS/NiS composite catalysts; (b,c) illustration of electron-hole pairs with CdS/NiS composite photocatalysts; (d) contrast of photocatalytic efficiency of CdS with different NiS content.Reproduced with permission from Reference [105]; copyright (2017), John Wiley & Sons, Inc.

Funding:
This research was supported by the Korea Evaluation Institute of Industrial Technology (KEIT) through the Carbon Cluster Construction project [10083586, Development of petroleum-based graphite fibers with ultra-high thermal conductivity] funded by the Ministry of Trade, Industry, & Energy (MOTIE, Korea), and the Commercialization Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science and ICT (MSIT) [2018_RND_002_0064, Development of 800 mA•h•g −1 pitch carbon coating materials].

Figure 14 .
Figure 14.(a) SEM image of CdS/NiS composite catalysts; (b,c) illustration of electron-hole pairs with CdS/NiS composite photocatalysts; (d) contrast of photocatalytic efficiency of CdS with different NiS content.Reproduced with permission from Reference [105]; copyright (2017), John Wiley & Sons, Inc.