Photocatalytic Hydrogen Production: Role of Sacriﬁcial Reagents on the Activity of Oxide, Carbon, and Sulﬁde Catalysts

: Photocatalytic water splitting is a sustainable technology for the production of clean fuel in terms of hydrogen (H 2 ). In the present study, hydrogen (H 2 ) production efﬁciency of three promising photocatalysts (titania (TiO 2 -P25), graphitic carbon nitride ( g -C 3 N 4 ), and cadmium sulﬁde (CdS)) was evaluated in detail using various sacriﬁcial agents. The effect of most commonly used sacriﬁcial agents in the recent years, such as methanol, ethanol, isopropanol, ethylene glycol, glycerol, lactic acid, glucose, sodium sulﬁde, sodium sulﬁte, sodium sulﬁde/sodium sulﬁte mixture, and triethanolamine, were evaluated on TiO 2 -P25, g -C 3 N 4 , and CdS. H 2 production experiments were carried out under simulated solar light irradiation in an immersion type photo-reactor. All the experiments were performed without any noble metal co-catalyst. Moreover, photolysis experiments were executed to study the H 2 generation in the absence of a catalyst. The results were discussed speciﬁcally in terms of chemical reactions, pH of the reaction medium, hydroxyl groups, alpha hydrogen, and carbon chain length of sacriﬁcial agents. The results revealed that glucose and glycerol are the most suitable sacriﬁcial agents for an oxide photocatalyst. Triethanolamine is the ideal sacriﬁcial agent for carbon and sulﬁde photocatalyst. A remarkable amount of H 2 was produced from the photolysis of sodium sulﬁde and sodium sulﬁde/sodium sulﬁte mixture without any photocatalyst. The ﬁndings of this study would be highly beneﬁcial for the selection of sacriﬁcial agents for a particular photocatalyst.


Introduction
Photocatalytic hydrogen (H 2 ) production via water splitting is a sustainable and renewable energy production technology with negligible impact on the environment [1] (Figure 1).H 2 is one of the most promising and clean energy sources for the future, with water as the only combustion product.After the invention of photo-electrochemical water splitting in 1972 [2] by Fujishima and Honda, nearly 9000 research articles have been published, outlining the use of various photocatalysts.In particular, most of the research works have been carried out using powder photocatalysts (except photo-electrochemical studies).The reported materials in the recent years are categorized as oxide  , carbon [3,81,, and sulfide [3,14,17,35,58,59,113,114,119,128,133,154,164,169,177,181,195,203,208,210,215,220,227,230,235, photocatalysts.Titanium oxide-P25 (TiO 2 -P25), graphitic carbon nitride (g-C 3 N 4 ), and cadmium sulfide (CdS) are the most extensively studied photocatalysts for water splitting.Many review articles have also been published [1,116,163,167,225,237,238,245, discussing the various features of the photocatalytic water splitting, such as fundamental concepts, theoretical principles, nature (morphology, surface characteristics, and optical properties) of the photocatalyst, role of co-catalyst/sacrificial reagents, mechanism, kinetics, etc.Nevertheless, there is still not many comprehensive studies to identify an appropriate sacrificial reagent with respect to the nature of a photocatalyst.
Catalysts 2018, 8, x FOR PEER REVIEW 2 of 35 In particular, most of the research works have been carried out using powder photocatalysts (except photo-electrochemical studies).The reported materials in the recent years are categorized as oxide , carbon [3,81,, and sulfide [3,14,17,35,58,59,113,114,119,128,133,154,164,169,177,181,195,203,208,210,215,220,227,230,235, photocatalysts.Titanium oxide-P25 (TiO2-P25), graphitic carbon nitride (g-C3N4), and cadmium sulfide (CdS) are the most extensively studied photocatalysts for water splitting.Many review articles have also been published [1,116,163,167,225,237,238,245, discussing the various features of the photocatalytic water splitting, such as fundamental concepts, theoretical principles, nature (morphology, surface characteristics, and optical properties) of the photocatalyst, role of cocatalyst/sacrificial reagents, mechanism, kinetics, etc.Nevertheless, there is still not many comprehensive studies to identify an appropriate sacrificial reagent with respect to the nature of a photocatalyst.[1].Reproduced with permission from Ref. [1].Copyright 2019, Elsevier.
Sacrificial agents or electron donors/hole scavengers play a prominent role in photocatalytic H2 production because the water splitting is energetically an uphill reaction (ΔH0 = 286 kJ mol −1 ).It is realized that methanol, triethanolamine, and sodium sulfide/sodium sulfite are the most commonly used sacrificial reagents for oxide, carbon, and sulfide photocatalysts, respectively.In most of the cases, fresh water (e.g., deionized water or double distilled water) has been used to evaluate the H2 production efficiency in a micro photo-reactor (volume in the range of 30 to 70 mL) with a strong light irradiation source (nearly ≤ 300 W).However, the vitality and utilization of this technology have not been comprehensively studied in a real environment.Moreover, the commercialization of this technology is still restrained by its poor efficiency and the use of expensive noble metals (like Pt, Au, Pd, Rh) as co-catalysts.Most of the published results do not have much consistency in terms of efficiency.For example, different efficiency values have been reported for pure TiO2 using methanol as a scavenger (Table 1).This discrepancy is ascribed to the following reasons: photo-reactor design, inert gas (Ar or N2) purging flow rate, light irradiation source, gas sampling method, gas chromatography (GC) analysis conditions, calculations, etc. Sacrificial agents or electron donors/hole scavengers play a prominent role in photocatalytic H 2 production because the water splitting is energetically an uphill reaction (∆H 0 = 286 kJ mol −1 ).It is realized that methanol, triethanolamine, and sodium sulfide/sodium sulfite are the most commonly used sacrificial reagents for oxide, carbon, and sulfide photocatalysts, respectively.In most of the cases, fresh water (e.g., deionized water or double distilled water) has been used to evaluate the H 2 production efficiency in a micro photo-reactor (volume in the range of 30 to 70 mL) with a strong light irradiation source (nearly ≤ 300 W).However, the vitality and utilization of this technology have not been comprehensively studied in a real environment.Moreover, the commercialization of this technology is still restrained by its poor efficiency and the use of expensive noble metals (like Pt, Au, Pd, Rh) as co-catalysts.Most of the published results do not have much consistency in terms of efficiency.For example, different efficiency values have been reported for pure TiO 2 using methanol as a scavenger (Table 1).This discrepancy is ascribed to the following reasons: photo-reactor design, inert gas (Ar or N 2 ) purging flow rate, light irradiation source, gas sampling method, gas chromatography (GC) analysis conditions, calculations, etc.
Oxidation : 2SO HS − (ads Path A:     The previous articles reported H 2 production efficiencies with various combinations of photocatalysts and sacrificial reagents.This study provides detailed information on the selection of sacrificial reagents and photocatalysts for H 2 production.The efficiencies of TiO 2 -P25, g-C 3 N 4 , and CdS were evaluated using methanol (MeOH), ethanol (EtOH), isopropanol (IPA), ethylene glycol (EG), glycerol (GLY), lactic acid (LA), glucose (GLU), sodium sulfide (Na 2 S), sodium sulfite (Na 2 SO 3 ), sodium sulfide/sodium sulfite mixture (Na 2 S/Na 2 SO 3 ), and triethanolamine (TEOA) as sacrificial reagents (organic and inorganic).The efficiency of a photocatalyst was described in terms of pH of medium and nature of the sacrificial agent (carbon chain length, alpha hydrogen, hydroxyl groups, binding interactions, etc).Besides, control experiments were executed to investigate the H 2 production with only sacrificial reagents under solar light irradiation in the absence of photocatalyst.

TiO 2 P25
Figure 2 shows the H 2 production efficiency of TiO 2 P25 using various sacrificial agents.H 2 production efficiencies of TiO 2 /EG, TiO 2 /GLY, TiO 2 /Na 2 S/Na 2 SO 3 , TiO 2 /GLU, TiO 2 /Na 2 S were found to be 190.2 µmol, 130.8 µmol, 126 µmol, 120 µmol, and 120 µmol, respectively.H 2 production efficiency of TiO 2 /MeOH system reduced to 81.6 µmol for the same period.The use of TEOA, EtOH, IPA, and Na 2 SO 3 as sacrificial reagents resulted in poor H 2 production, yielding 61.8 µmol, 49.8 µmol, 46.2 µmol, and 40.8 µmol, respectively.TiO 2 /LA mixture displayed the lowest yield of H 2 production (only 27.6 µmol).TiO 2 /EG mixture showed the maximum H 2 production (190.2µmol) efficiency as compared to all other combinations.This is ascribed to the faster charge transfer reaction in the TiO 2 /EG system compared to the photo-generated electron-hole recombination process [431,432].The length of the carbon chain, the number of hydroxyl groups, and dehydrogenation/decarbonylation characteristics of sacrificial agents are the primary features in controlling the H 2 production efficiency.Moreover, the following properties of sacrificial agents could also strongly influence the efficiency: polarity and electron donating ability, adsorption capability on the photocatalyst surface, the formation of by-products, and the selectivity for reaction with photo-generated holes (e.g., decarboxylation process) [10,94,[431][432][433][434][435][436].Carbon monoxide (CO) is one of the main intermediates for the alcohols with a short carbon chain.Hence, the adsorption of CO on the active sites of TiO 2 via chemisorption restricts further adsorption of alcohol on the photocatalyst surface [437].The removal of CO as CO 2 is the rate-determining step in H 2 production.It depends on the adsorption efficiency and the number of alpha hydrogens of the sacrificial agent [437].During the water-splitting process, the hydroxyl radical ( • OH) abstracts alpha hydrogen from the alcohol to create • RCH 2 -OH radical, which gets further oxidized into an aldehyde, carboxylic acid, and CO 2 [437].Bahruji et al. [437] suggested that alkyl groups connected to the alcohol (e.g., C x H y OH) could yield the respective alkanes (e.g., C x−1 ) during the water-splitting process.The alkane production rate was decreased with the increase of OH groups in alcohol [438].In the case of polyols, the hydrogen atoms from the alpha carbon could be easily extracted and evolved in the form of H 2 [438].The alpha carbon atoms could be oxidized into CO 2 .The C atoms without OH groups (other than alpha C atoms) would be evolved in the form of alkanes [438].Time-resolved transient absorption spectroscopy results revealed that carbohydrates and polyols (C2-C6) could rapidly react with ~50-60% holes (h + ) within 6 ns as compared to other alcohols [439,440].The OH groups could act as an anchor for the chemisorption of alcohols on the photocatalyst surface [438].The coordination efficiency of alcohols with the Ti sites relies on the number of OH groups and the carbon chain length.This type of linkage could be beneficial for the utilization of holes to improve the H 2 production and suppress the charge carrier recombination [438].The first principle calculations showed that the formation of gap levels in TiO 2 via the adsorption polyols could accelerate the hole trapping process [441].Though EG showed maximum efficiency for TiO 2 -P25, glycerol and glucose are the most appropriate sacrificial agents for any kind of oxide photocatalyst.This owes to their (glucose and glycol) most abundance, less toxicity, low cost, and they can readily undergo dehydrogenation as compared to other alcohols [40,63,435].
beneficial for the utilization of holes to improve the H2 production and suppress the charge carrier recombination [438].The first principle calculations showed that the formation of gap levels in TiO2 via the adsorption polyols could accelerate the hole trapping process [441].Though EG showed maximum efficiency for TiO2-P25, glycerol and glucose are the most appropriate sacrificial agents for any kind of oxide photocatalyst.This owes to their (glucose and glycol) most abundance, less toxicity, low cost, and they can readily undergo dehydrogenation as compared to other alcohols [40,63,435].

g-C3N4
H2 production efficiency of g-C3N4 with various sacrificial agents is shown in Figure 3.In this case, only the use of TEOA, Na2S, Na2SO3, and Na2S/Na2SO3 resulted in H2 production.H2 production efficiency of g-C3N4/Na2S (139.8 µmol) system was higher than that of g-C3N4/Na2S/Na2SO3 (127.2 µmol) and g-C3N4/Na2SO3 (5.4 µmol).g-C3N4/TEOA mixture showed the best efficiency (247.2 µmol) when compared to all other sacrificial agents.This can be ascribed to the fact that photo-corrosion and degradation of π conjugated structure [304] of amine rich g-C3N4 is secured by the effective binding of TEOA on the catalyst surface [112].TEOA excellently consumes the photo-generated holes, improves the dispersion of photocatalyst, and acts as a binding ligand to improve the interaction of g-C3N4 with water molecules [204,442].The results shown in Figure 3 also suggest that alcohols and glucose are not strongly adsorbed on the g-C3N4 surface for water-splitting reaction.This is attributed to the absence of hydrophilicity and surface characteristics (e.g., active sites, poor electrical conductivity, water oxidation ability) of g-C3N4 to facilitate a strong interfacial electron/hole transfer process on the catalyst surface.The poor crystallinity and basal planar structure of g-C3N4

g-C 3 N 4
H 2 production efficiency of g-C 3 N 4 with various sacrificial agents is shown in Figure 3.In this case, only the use of TEOA, Na 2 S, Na 2 SO 3 , and Na 2 S/Na 2 SO 3 resulted in H 2 production.H 2 production efficiency of g-C 3 N 4 /Na 2 S (139.8 µmol) system was higher than that of g-C 3 N 4 /Na 2 S/Na 2 SO 3 (127.2µmol) and g-C 3 N 4 /Na 2 SO 3 (5.4µmol).g-C 3 N 4 /TEOA mixture showed the best efficiency (247.2 µmol) when compared to all other sacrificial agents.This can be ascribed to the fact that photo-corrosion and degradation of π conjugated structure [304] of amine rich g-C 3 N 4 is secured by the effective binding of TEOA on the catalyst surface [112].TEOA excellently consumes the photo-generated holes, improves the dispersion of photocatalyst, and acts as a binding ligand to improve the interaction of g-C 3 N 4 with water molecules [204,442].The results shown in Figure 3 also suggest that alcohols and glucose are not strongly adsorbed on the g-C 3 N 4 surface for water-splitting reaction.This is attributed to the absence of hydrophilicity and surface characteristics (e.g., active sites, poor electrical conductivity, water oxidation ability) of g-C 3 N 4 to facilitate a strong interfacial electron/hole transfer process on the catalyst surface.The poor crystallinity and basal planar structure of g-C 3 N 4 endorse the electron-hole recombination [443].Moreover, high activation energy and overpotential are required for H 2 production on the g-C 3 N 4 surface [182,211].This could be rectified by the loading of noble metals or co-catalysts over g-C 3 N 4 or fabricating Z-scheme photocatalysts.In most of the studies, it was reported that g-C 3 N 4 acts as an outstanding template and there was no H 2 production on g-C 3 N 4 without any noble metal co-catalyst [444,445].The results also demonstrated that the light absorption capability, chemical stability, and suitable band edge positions of narrow band-gap g-C 3 N 4 are not the only decisive factors to enhance the H 2 production efficiency.
are required for H2 production on the g-C3N4 surface [182,211].This could be rectified by the loading of noble metals or co-catalysts over g-C3N4 or fabricating Z-scheme photocatalysts.In most of the studies, it was reported that g-C3N4 acts as an outstanding template and there was no H2 production on g-C3N4 without any noble metal co-catalyst [444,445].The results also demonstrated that the light absorption capability, chemical stability, and suitable band edge positions of narrow band-gap g-C3N4 are not the only decisive factors to enhance the H2 production efficiency.

CdS
Photocatalytic H2 production efficiency of CdS using various sacrificial agents is shown in Figure 4.The use of TEOA, Na2S, Na2SO3, Na2S/Na2SO3, and LA as sacrificial reagents resulted in H2 formation.CdS/TEOA system showed the maximum efficiency of 283.2 µmol of H2 as compared to all other sacrificial agents.The efficiency of CdS/Na2S, CdS/Na2SO3, CdS/LA systems was found to be 181.2µmol, 154.8 µmol, and 84 µmol, respectively.The mixture of CdS/Na2S/Na2SO3 showed the lowest H2 production of 54 µmol after 6 h.Bare CdS is not stable under prolonged light irradiation because the sulfide ions on its surface are rapidly oxidized into sulfur through the reaction with photo-generated holes (photo-corrosion -CdS + 2h + → Cd 2+ + S) [308,446,447].The sulfide oxidation of CdS can occur before the oxidation of water by holes [308,447].Hence, the H2 production efficiency of CdS highly relies on the effective binding of sacrificial agents on its surface.The results showed that amine and sulfide/sulfite might be strongly bound to the CdS surface and it could effectively consume the holes as compared to alcohol and sugars.It is obviously noted that H2 is produced in high alkaline (amine, sulfide, and sulfite) and acidic (LA) pH mixtures when compared to neutral pH (alcohols and sugar).LA is converted into pyruvic acid and CO2 during the water-splitting reaction; this may slightly influence the pH and polarity of the reaction mixture.The sulfide ions from Na2S stabilizes CdS surface to terminate the surface defects originated from photo-corrosion.The electronhole recombination process is strongly restrained by the sulfide ions at alkaline pH.When CdS is suspended in a water medium, thiol (Cd-SH) and hydroxyl (Cd-OH) groups are developed on its

CdS
Photocatalytic H 2 production efficiency of CdS using various sacrificial agents is shown in Figure 4.The use of TEOA, Na 2 S, Na 2 SO 3 , Na 2 S/Na 2 SO 3 , and LA as sacrificial reagents resulted in H 2 formation.CdS/TEOA system showed the maximum efficiency of 283.2 µmol of H 2 as compared to all other sacrificial agents.The efficiency of CdS/Na 2 S, CdS/Na 2 SO 3 , CdS/LA systems was found to be 181.2µmol, 154.8 µmol, and 84 µmol, respectively.The mixture of CdS/Na 2 S/Na 2 SO 3 showed the lowest H 2 production of 54 µmol after 6 h.Bare CdS is not stable under prolonged light irradiation because the sulfide ions on its surface are rapidly oxidized into sulfur through the reaction with photo-generated holes (photo-corrosion -CdS + 2h + → Cd 2+ + S) [308,446,447].The sulfide oxidation of CdS can occur before the oxidation of water by holes [308,447].Hence, the H 2 production efficiency of CdS highly relies on the effective binding of sacrificial agents on its surface.The results showed that amine and sulfide/sulfite might be strongly bound to the CdS surface and it could effectively consume the holes as compared to alcohol and sugars.It is obviously noted that H 2 is produced in high alkaline (amine, sulfide, and sulfite) and acidic (LA) pH mixtures when compared to neutral pH (alcohols and sugar).LA is converted into pyruvic acid and CO 2 during the water-splitting reaction; this may slightly influence the pH and polarity of the reaction mixture.The sulfide ions from Na 2 S stabilizes CdS surface to terminate the surface defects originated from photo-corrosion.The electron-hole recombination process is strongly restrained by the sulfide ions at alkaline pH.When CdS is suspended in a water medium, thiol (Cd-SH) and hydroxyl (Cd-OH) groups are developed on its surface, which are highly pH dependent [310].In the case of Na 2 S, the pH of the medium is alkaline, sulfide (S 2 − ) and hydrogen sulfide (HS − ) are formed when Na 2 S is dissolved in water [310].
During light irradiation, S 2 − and HS -are quickly oxidized into sulfate (SO 4 2− ) and polysulfide (S 4 2− , S 5 2− ) ions, respectively [310].The oxidation of sulfide by the photo-generated holes is much preferential as compared to the photo-corrosion of CdS [112].The precipitation of yellow colored polysulfide ions diminishes the photocatalytic efficiency via acting as an optical filter and competing with the H 2 generation reaction.This could be restricted by the addition of Na 2 SO 3 to generate more HS -and S 2 O 3 2− ions to enhance the photocatalytic activity [292].However, the results shown in Figure 4 suggest that H 2 production efficiency of Na 2 S/CdS or CdS/Na 2 SO 3 are higher than that of CdS/Na 2 S/Na 2 SO 3 .The reasons could be predicted by the photolysis experiments of sacrificial agents.The pH of TEOA/water mixture would be around 12, which could enhance the H 2 production efficiency via strong interfacial bonding on the CdS surface and its reaction with photo-generated holes [204].
surface, which are highly pH dependent [310].In the case of Na2S, the pH of the medium is alkaline, sulfide (S2 − ) and hydrogen sulfide (HS − ) are formed when Na2S is dissolved in water [310].During light irradiation, S2 − and HS -are quickly oxidized into sulfate (SO4 2− ) and polysulfide (S4 2− , S5 2− ) ions, respectively [310].The oxidation of sulfide by the photo-generated holes is much preferential as compared to the photo-corrosion of CdS [112].The precipitation of yellow colored polysulfide ions diminishes the photocatalytic efficiency via acting as an optical filter and competing with the H2 generation reaction.This could be restricted by the addition of Na2SO3 to generate more HS -and S2O3 2− ions to enhance the photocatalytic activity [292].However, the results shown in Figure 4 suggest that H2 production efficiency of Na2S/CdS or CdS/Na2SO3 are higher than that of CdS/Na2S/Na2SO3.The reasons could be predicted by the photolysis experiments of sacrificial agents.The pH of TEOA/water mixture would be around 12, which could enhance the H2 production efficiency via strong interfacial bonding on the CdS surface and its reaction with photo-generated holes [204].

Photolysis
Photolysis experiments were carried out for all sacrificial agents in water for 6 h of light irradiation without the additions of photocatalysts.Control experiments were also carried out in the absence of sacrificial agents to evaluate the efficiency of the photocatalyst.There was no H2 production in the absence of any sacrificial agents for TiO2-p25, g-C3N4, and CdS.The results of photolysis experiments with sacrificial reagents under solar light in the absence of photocatalysts are shown in Figure 5. Interestingly, a remarkable amount of H2 was evolved from Na2S/water (159 µmol), Na2SO3/water (51 µmol), and Na2S/Na2SO3/water (134.4 µmol) systems without photocatalyst.It was observed that the H2 production efficiency was increased with respect to the concentration of sulfide or sulfite.When compared to results obtained in the presence of photocatalysts, it could be observed that the photocatalysts, such as TiO2-p25 and g-C3N4, surprisingly reduced the actual H2 production efficiency of sulfide system.There was not a significant increment in the efficiency of

Photolysis
Photolysis experiments were carried out for all sacrificial agents in water for 6 h of light irradiation without the additions of photocatalysts.Control experiments were also carried out in the absence of sacrificial agents to evaluate the efficiency of the photocatalyst.There was no H 2 production in the absence of any sacrificial agents for TiO 2 -p25, g-C 3 N 4 , and CdS.The results of photolysis experiments with sacrificial reagents under solar light in the absence of photocatalysts are shown in Figure 5. Interestingly, a remarkable amount of H 2 was evolved from Na 2 S/water (159 µmol), Na 2 SO 3 /water (51 µmol), and Na 2 S/Na 2 SO 3 /water (134.4 µmol) systems without photocatalyst.It was observed that the H 2 production efficiency was increased with respect to the concentration of sulfide or sulfite.When compared to results obtained in the presence of photocatalysts, it could be observed that the photocatalysts, such as TiO 2 -p25 and g-C 3 N 4 , surprisingly reduced the actual H 2 production efficiency of sulfide system.There was not a significant increment in the efficiency of CdS/Na 2 S as compared to the photolysis of Na 2 S.However, the efficiency of CdS/Na 2 SO 3 was higher than that of Na 2 SO 3 photolysis.It is also noted that a high concentration of sulfide/sulfite mixture (in the range of 0.2 M to 1 M) was used in most of the studies for H 2 production [246,264,273,300,[448][449][450][451].In such cases, the photolysis of sulfide or sulfite solutions were not evaluated.Hence, the H 2 production should have been mainly originated via photolysis of sulfide/sulfite mixture rather than the photocatalytic effect.The photochemical reactions involved in H 2 generation from sulfide and sulfite solutions are described in detail from Equations ( 42)-( 62) [428][429][430].CdS/Na2S as compared to the photolysis of Na2S.However, the efficiency of CdS/Na2SO3 was higher than that of Na2SO3 photolysis.It is also noted that a high concentration of sulfide/sulfite mixture (in the range of 0.2 M to 1 M) was used in most of the studies for H2 production [246,264,273,300,[448][449][450][451].In such cases, the photolysis of sulfide or sulfite solutions were not evaluated.Hence, the H2 production should have been mainly originated via photolysis of sulfide/sulfite mixture rather than the photocatalytic effect.The photochemical reactions involved in H2 generation from sulfide and sulfite solutions are described in detail from Equations ( 42)-( 62) [428][429][430].Li et al. [430] investigated the photochemical generation of H2 from sulfide and sulfite mixture solutions.They found that the pH of sulfide/sulfite mixture (~13.14) was slightly decreased (~12.94) after the completion of photolysis experiments.The addition of a small amount of sulfite into the sulfide solution could not amplify the H2 production.Nevertheless, the elemental sulfur or polysulfide originated from HS -is efficiently consumed by SO3 2− to improve the photonic efficiency.It is strongly recommended to study the effect of photolysis when the sulfide/sulfite mixture is used as the sacrificial agent to evaluate the H2 production efficiency of the photocatalyst.The elemental sulfur could be filtered out by passing hydrogen sulfide (H2S) gas in the aqueous solution under nitrogen atmosphere [428].The resulting filtrate could be reused for photolytic H2 production [428].
Wang et al. [112] suggested that Na2S/Na2SO3, MeOH, and TEOA were the most appropriate sacrificial agents for sulfide, oxide, and carbon-based photocatalysts.However, the experiments were not performed to study the effect of photolysis, especially the sulfide or sulfite solutions.A high concentration of sacrificial agents was used to evaluate the photocatalytic activity.The photocatalytic experiments were not described in detail.Moreover, the effect of most earth abundant glucose and glycerol were not investigated on the H2 production efficiency.Sulfur dioxide (SO2) emission from the flue gas can be absorbed as Na2SO3 solution using dilute sodium hydroxide.The photolysis of such sodium sulfite solution is an eco-friendly way to produce H2 gas [429].Only a few studies were focused on using wastewater for H2 generation.Souza and Silva [310] studied the feasibility of using tannery sludge wastewater for photocatalytic H2 generation using CdS.The photolysis of sulfide-rich wastewater or industrial effluent is the foremost choice to produce green energy in a sustainable way via photocatalysis.Li et al. [430] investigated the photochemical generation of H 2 from sulfide and sulfite mixture solutions.They found that the pH of sulfide/sulfite mixture (~13.14) was slightly decreased (~12.94) after the completion of photolysis experiments.The addition of a small amount of sulfite into the sulfide solution could not amplify the H 2 production.Nevertheless, the elemental sulfur or polysulfide originated from HS -is efficiently consumed by SO 3 2− to improve the photonic efficiency.It is strongly recommended to study the effect of photolysis when the sulfide/sulfite mixture is used as the sacrificial agent to evaluate the H 2 production efficiency of the photocatalyst.The elemental sulfur could be filtered out by passing hydrogen sulfide (H 2 S) gas in the aqueous solution under nitrogen atmosphere [428].The resulting filtrate could be reused for photolytic H 2 production [428].Wang et al. [112] suggested that Na 2 S/Na 2 SO 3 , MeOH, and TEOA were the most appropriate sacrificial agents for sulfide, oxide, and carbon-based photocatalysts.However, the experiments were not performed to study the effect of photolysis, especially the sulfide or sulfite solutions.A high concentration of sacrificial agents was used to evaluate the photocatalytic activity.The photocatalytic experiments were not described in detail.Moreover, the effect of most earth abundant glucose and glycerol were not investigated on the H 2 production efficiency.Sulfur dioxide (SO 2 ) emission from the flue gas can be absorbed as Na 2 SO 3 solution using dilute sodium hydroxide.The photolysis of such sodium sulfite solution is an eco-friendly way to produce H 2 gas [429].Only a few studies were focused on using wastewater for H 2 generation.Souza and Silva [310] studied the feasibility of using tannery sludge wastewater for photocatalytic H 2 generation using CdS.The photolysis of sulfide-rich wastewater or industrial effluent is the foremost choice to produce green energy in a sustainable way via photocatalysis.

TOC Analysis
TOC analysis was carried out for solutions after the photocatalytic experiments, to assess the degradation of organic sacrificial agents in the solutions at the end of 6 h experiment.Table 2 summarizes the TOC results for the solutions of TiO 2 -p25.The results suggest the effective utilization of the organic sacrificial agents by TiO 2 -P25 for H 2 production.TOC was reduced almost half for most of the sacrificial agents after 6 h except for glucose.This is ascribed to the formation of more organic intermediates when glucose is utilized as the sacrificial agent.

Experimental
All the chemicals used were of analytical grade and used as received without further purification.TiO 2 P25 was purchased from Sigma Aldrich (Darmstadt, Germany), g-C 3 N 4 was synthesized by the calcination of urea at 550 • C for 2 h [452].CdS was synthesized via the hydrothermal method [453].Photocatalytic experiments were carried out using an immersion type reactor (Lelesil innovative systems, Thane, Maharashtra, India) as shown in Figure 6.All the reactions were carried out without any noble metal co-catalyst.The reactor is a tightly closed setup with a total volume of 1000 mL.Reactions were carried out using 500 mL of double distilled water with 0.5 g/L of photocatalyst and desired amount of sacrificial reagent (based on the literature).The empty headspace was kept constant at 500 mL for all reactions.The sacrificial agent concentration was fixed at 10% (alcohol, amine, acid) and 0.1 M (glucose, sodium sulfide, sodium sulfite, sodium sulfide/sodium sulfite mixture).The mixture was stirred under nitrogen purging for 1 h after which, the purging was stopped, and the reactor was closed immediately.The mixture was irradiated using a 300 W Xenon arc lamp without any UV cutoff filter (simulated solar light source).H 2 sampling was carried out for every 1 h using a 250 µL sample lock gas tight syringe.At the end of the photoreaction time, when organic sacrificial reagents were used, the mixture was filtered using a 0.45 µm micro-filter, and the filtrate was analyzed using a total organic carbon (TOC; Shimadzu, Japan) analyzer to measure the loss of reagents by mineralization.H 2 was analyzed using Agilent gas chromatography (USA) with thermal conductivity detector (TCD), manual injection, carrier gas N 2 , molecular sieve 5 A • column with 2-m length, front inlet temperature 140 • C, and detector temperature 150 • C.

Summary and Outlook
Different types of photocatalysts have been successfully investigated for H2 production using various organic and inorganic sacrificial agents.The surface of an oxide photocatalyst would be more suitable for polyols and sugars for adsorption as compared to amines and sulfides.Amines are the most appropriate of sacrificial agents for carbon and sulfide photocatalysts.CdS/TEOA (283.2 µmol), g-C3N4/TEOA (247.2 µmol), and TiO2/EG (190.2 µmol) are the three best systems with maximum H2 production in this study.H2 could also be generated via the direct photolysis of sodium sulfide solution in the absence of any catalyst.TiO2-p25 and g-C3N4 suppress the self-H2 generation efficiency of Na2S photolysis.More technological developments are required for the practical application of water-splitting in a scalable and economically feasible way.Stable, affordable, and active co-catalysts should be developed in the future to replace the expensive noble metals to achieve a significant amount of H2 production.In most of the studies, precious fresh water with a high concentration of sacrificial agents was used in a small reactor to generate H2.Hence, future studies should be focused on a pilot scale using industrial wastewater and seawater rather than using fresh water.

Summary and Outlook
Different types of photocatalysts have been successfully investigated for H 2 production using various organic and inorganic sacrificial agents.The surface of an oxide photocatalyst would be more suitable for polyols and sugars for adsorption as compared to amines and sulfides.Amines are the most appropriate of sacrificial agents for carbon and sulfide photocatalysts.CdS/TEOA (283.2 µmol), g-C 3 N 4 /TEOA (247.2 µmol), and TiO 2 /EG (190.2 µmol) are the three best systems with maximum H 2 production in this study.H 2 could also be generated via the direct photolysis of sodium sulfide solution in the absence of any catalyst.TiO 2 -p25 and g-C 3 N 4 suppress the self-H 2 generation efficiency of Na 2 S photolysis.More technological developments are required for the practical application of water-splitting in a scalable and economically feasible way.Stable, affordable, and active co-catalysts should be developed in the future to replace the expensive noble metals to achieve a significant amount of H 2 production.In most of the studies, precious fresh water with a high concentration of sacrificial agents was used in a small reactor to generate H 2 .Hence, future studies should be focused on a pilot scale using industrial wastewater and seawater rather than using fresh water.

12 + 2 USV
OH − + SO 2− 3 + S 0 (ads) → [HS 2− LETTER A WITH STROKE 023B Ȼ \textstrokecapitalc LATIN CAPITAL LETTER C WITH STROKE 023C ȼ \textstrokec \textslashc LATIN SMALL LETTER C WITH STROKE 023D Ƚ \textbarcapitall LATIN CAPITAL LETTER L WITH BAR 023E Ⱦ \textstrokecapitalt LATIN CAPITAL LETTER T WITH DIAGONAL STROK 023F ȿ \textrts LATIN SMALL LETTER S WITH SWASH TAIL 0240 ɀ \textrtz LATIN SMALL LETTER Z WITH SWASH TAIL represents species, which can undergo intramolecular charge transfer.

Figure 3 .
Figure 3. Photocatalytic H 2 production efficiency of g-C 3 N 4 using various sacrificial agents.

Figure 4 .
Figure 4. Photocatalytic H2 production efficiency of CdS using various sacrificial agents.

Figure 4 .
Figure 4. Photocatalytic H 2 production efficiency of CdS using various sacrificial agents.

Figure 6 .
Figure 6.(a) Schematic and (b) photograph of the reactor used for photocatalytic H2 production.

Figure 6 .
Figure 6.(a) Schematic and (b) photograph of the reactor used for photocatalytic H 2 production.

Sodium sulfide and sodium sulfite mixture (Na 2 S and Na 2 SO 3
[430]]:Two different reaction pathways are involved when sodium sulfide and sodium sulfite mixture is used as a sacrificial agent.HS − (aq) → HS − (ads)

Table 2 .
TOC of solutions after the completion of the photocatalytic reaction.