1. Energy and environmental issues at a global level are important topics. It is indispensable to construct clean energy systems in order to solve the issues. Hydrogen will play an important role in the system because it is an ultimate clean energy and it can be used in fuel cells. Moreover, hydrogen is used in chemical industries. For example, a large amount of hydrogen is consumed in industrial ammonia synthesis. At present, hydrogen is mainly produced from fossil fuels such as natural gas by steam reforming. Fossil fuels: coal, natural gass, petroleum In this process, fossil fuels are consumed and CO 2 is emitted. Hydrogen has to be produced from water using natural energies such as sunlight if one thinks of energy and environmental issues. (i) Electrolysis of water using a solar cell, a hydroelectric power generation, etc. (ii) Reforming of biomass. (iii) Photocatalytic or photoelectrochemical water splitting (artificial photosynthesis) A new pathway for solar energy utilization = artificial photosynthesis Photocatalytic conversion of water and CO 2 to liquid fuel Therefore, achievement of solar hydrogen production from water has been urged. There are several ways for solar hydrogen production. Conventional Hydrogen Generation CH 4 + H 2 O - CO + 3H 2 CO + H 2 O - CO 2 + H 2

2. Hydrogen production by water splitting Photocatalyst CB VB hvhv e-e- Reduction Oxidation h+h+ A A-A- D D+D+

3. Photocatalytic Water Splitting 2H + + 2e - → H 2 Photocatalytic Water Reduction H 2 O + 2h + → ½O 2 + 2H + Photocatalytic Water Oxidation H 2 O + hν → H 2 + ½O 2 (E = -1.23 eV) Photocatalytic Overall Water Splitting C.B. V.B. H+H+ H2H2 H2OH2O O2O2 charge recombination Low Quantum Yield ! Low Photo-Activity ! hv or heat h+h+ h+h+ e-e- e-e- hν≥ E g

4. O2O2 H2H2 O2O2 H2H2 hυhυ No need for H 2 /O 2 separation High efficiency Low stability Fabrication cost Low cost Low efficiency at present e-e- H2H2 H2OH2O Pt O2O2 H2OH2O S. C. hυhυ Two methods photoelectrochemicalphotocatalytic

5. E/eV Fe 2 O 3, WO 3 or TiO 2 Water Pt C. B. V. B. E 0 (H 2 O/O 2 ) E 0 (H 2 /H + ) Pt-Cathode Electrolyten-type Anode Bias 1.23eV H2H2 O2O2 2 e - + 2 H +  H 2 H 2 O  ½ O 2 + 2H + + 2e - E g > 1.23 eV Band positions Photocatalytic Water Splitting

7. Solar Light Spectrum UV Visible IR 3.26eV 1.59eV 2.07eV 600 nm 1.23eV 1008 nm 46% 38%17% 3.5 % 0.5 % 0% Two Requirements for Band Structures 1.Band gap: 1.23 eV < E g < 3.26 eV 2.Band positions

8. 0.0 +1.0 +2.0 +3.0 +4.0 E NHE (eV) (V) -4.5 -3.5 -5.5 -6.5 -7.5 -8.5 H + /H 2 H 2 O/O 2 (S 2- /S) (@pH=1) GaAs (n,p) 1.4eV GaP (n,p) 2.3eV CdSe(n) 1.7eV CdS(n) 2.4eV SnO 2 (n) 3.8eV TiO 2 (n) 3.2eV ZnO(n) 3.2eV SiC (n,p) 3.0eV WO 3 (n) 2.8eV Fe 2 O 3 (n) 2.2eV In 2 O 3 (n) 2.7eV Band Gaps and Band Positions Photobleach

9. 200 – 800 nm

10. 200 – 1000 nm

11. Anatase, Rutile, Brookite

12. 3. Experimental method for water splitting 3.1.The points that should be paid attention to evaluate photocatalytic water splitting are shown in Fig. 10.7 (i)Stoichiometry of H 2 and O 2 evolution. In water splitting, both H 2 and O 2 should form with a stoichiometric amount, 2:1, in the absence of a sacrificial reagent. Often H 2 evolution is observed with a lack of O 2 evolution. In this case, the amount of H 2 evolution is usually small compared with an amount of a photocatalyst. It is not clear if such a reaction is photocatalytic water splitting and it is important to clarify that it is not a sacrificial reaction. (ii) Time. Amounts of H 2 and O 2 evolved should increase with irradiation time. To check not only the value of activity or a gas evolution rate but also the time course is important. Repeated experiment is also informative. (iii) Turnover number (TON ). Amounts of H 2 and O 2 should overwhelm an amount of a photocatalyst. If the amounts are much less than the amount of photocatalyst we do not know if the reaction proceeds photocatalytically because the reaction might be due to some stoichiometric reactions. TON is usually defined by the number of reacted molecules to that of an active site (eq 6). 3. Experimental method for water splitting 3.1.The points that should be paid attention to evaluate photocatalytic water splitting are shown in Fig. 10.7 (i)Stoichiometry of H 2 and O 2 evolution. In water splitting, both H 2 and O 2 should form with a stoichiometric amount, 2:1, in the absence of a sacrificial reagent. Often H 2 evolution is observed with a lack of O 2 evolution. In this case, the amount of H 2 evolution is usually small compared with an amount of a photocatalyst. It is not clear if such a reaction is photocatalytic water splitting and it is important to clarify that it is not a sacrificial reaction. (ii) Time. Amounts of H 2 and O 2 evolved should increase with irradiation time. To check not only the value of activity or a gas evolution rate but also the time course is important. Repeated experiment is also informative. (iii) Turnover number (TON ). Amounts of H 2 and O 2 should overwhelm an amount of a photocatalyst. If the amounts are much less than the amount of photocatalyst we do not know if the reaction proceeds photocatalytically because the reaction might be due to some stoichiometric reactions. TON is usually defined by the number of reacted molecules to that of an active site (eq 6).

13. However, it is often difficult to determine the number of active sites for photocatalysts. Therefore, the number of reacted electrons to the number of atoms in a photocatalyst (eq 7) or on the surface of a photocatalyst (eq 8) is employed as the TON. The number of reacted electrons is calculated from the amount of evolved H 2. The TONs (7) and (8) are smaller than the real TON (6) because the number of atoms is more than that of active sites. Normalization of photocatalytic activity by weight of used photocatalyst (for example, mmol h -1 g -1 ) is not acceptable because the photocatalytic activity is not usually proportional to the weight of photocatalyst if an amount of photocatalyst is enough for a certain experimental condition. The amount of photocatalyst should be optimized for an each experimental setup. In this case, photocatalytic activity usually depends on the number of photons absorbed by a photocatalyst unless the light intensity is too strong. (iv) Quantum yield. The rate of gas evolution is usually indicated with a unit, for example mmol h -1. Because the photocatalytic activity depends on the experimental conditions such as a light source and a type of a reaction cell, the activities cannot be compared with each other if the reaction conditions are different from each other. Therefore, determination of a quantum yield is important. The number of incident photons can be measured using a thermopile or Si photodiode. However, it is hard to determine the real amount of photons absorbed by a photocatalyst in a dispersed system because of scattering. So, the obtained quantum yield is an apparent quantum yield (9). The apparent quantum yield is estimated to be smaller than the real quantum yield because the number of absorbed photons is usually smaller than that of incident light. It should be noteworthy that the quantum yield is different from the solar energy conversion efficiency that is usually used for evaluation of solar cells.

14. The number of photocatalysts that can give good solar energy conversion efficiency is limited at the present stage because of insufficient activities for the measurement. However, the solar energy conversion efficiency should finally be used to evaluate the photocatalytic water splitting if solar hydrogen production is considered. (v) Photoresponse. When a photocatalyst is irradiated with light of energy larger than the band gap, water splitting should proceed. An action spectrum is indispensable to see the photoresponse, especially for a photocatalyst with visible light response (band-path and interference filters are usually employed to obtain monochromatic light for the action spectrum measurement). Even if a material absorbs visible light it does not always show a photocatalytic activity by the excitation of the visible light absorption band. Cut-off filters are sometimes used to see the photoresponse. In this case the onset of the photoresponse can be measured. Water splitting by mechanocatalysis proceeds on some metal oxides under stirring and dark condition. 8,39 Some control experiments such as no photocatalysts or non-irradiation have to be carried out to confirm the photocatalytic reaction and neglect the possibility of the mechanocatalytic water splitting. There are many other points that researchers have to pay attention. The details of experiments for general photocatalysis are described in the literature by Ohtani. 40

18. Overall Water Splitting under UV Light TiO 2 (1972) 1% 23% Sr 2 Nb 2 O 7 (1999) 50% Ba-Sr 2 Nb 2 O 7 (2003) 50% La/NaTaO 2 (2003) 35% Ba-La 2 Ti 2 O 7 (2002) Kudo 30% K 2 La 2 Ti 3 O 10 (1997) Domen SrTiO 3 (1982) 1% 10% K 4 Nb 6 O 17 (1986) Domen Lee LeeLee Quantum Yield = 2× No. of H 2 molecules (or 4 × No. of O 2 molecules) No. of absorbed photons

23. Valence band controlled photocatalysts

24. DL = doping level Band Gap Engineering

25. Cation Doping : metal-doped La 2 Ti 2 O 7 Visible light absorption induced by interband formation in the bandgap of the original semiconductor. Only limited amounts of dopant could be introduced w/o disturbing the original structure. Substituted sites could become the sites for e - -h + recombination. New Single Phase Materials

26. Sr 2 Nb 2 O 7 NH 3 /1273K E g 's of (oxy)nitrides, (oxy)carbide and (oxy)sulfide are less than that of oxides due to negative shift of valence band. Many oxynitrides derived from nitridation of oxides are nonstoichiometric and contain oxygen defects. NH 3 /973K Nb 4d New Single Phase Materials Anion Doping: Sr 2 Nb 2 O 7-x N x

28. Undoped, single phase, photocatalyst PbBi 2 Nb 2 O 9 A layered perovskite of aurivillius phase (A) (B) A : TiO 2-x N x B : PbBi 2 Nb 2 O 9 New Single Phase Materials

31. Systems

32. Domen et. al., Nature, 440 (2006) 295. Solid solution Action spectrum

33. 1. New Single Phase Materials 2. Photosensitizer 3. Modification of UV Photocatalysts - Cation Doping - Anion Doping (Nitrides, Carbides, Sulfides) 4. Composite Photocatalysts 5. Solid Solution 1. New Single Phase Materials 2. Photosensitizer (PS) 4. Composite Photocatalysts Solid Solution CB VB Energy (Wide BG) (Narrow BG) (Visible light) Control of band gap and position Search for the Visible Light Photocatalysts 3. Modification (nm) A

36. Layered compound Composite Photocatalysts Bulk Oxide (n-type) Nanoparticles (p-type) Photocatalytic p-n nanodiodes Nano-Bulk Composite p-type n-type

37. PbBi 2 Nb 1.9 W 0.1 O 9 synthesized by the solid state reaction CaFe 2 O 4 synthesized by the sol-gel method. : PbBi 2 Nb 1.9 W 0.1 O 9 : CaFe 2 O 4 Photocatalytic p-n Nanodidoes Hydrothermal treatment 150 o C, 7 d

38. Photocatalytic p-n Nanodiodes p-type CaFe 2 O 4 n-type PbBi 2 Nb 1.9 W 0.1 O 9 UV-VisPhotocurrent CH 3 CHO decomposition

39. Catalyst loaded with 0.1 wt% Pt, 0.3 g; 450W W-Arc lamp (Oriel) with UV cut-off filter (λ ≥ 420 nm) Catalysts Band gapH 2 EvolutionO 2 Evolution E g (eV)λ ab (nm)μmol/gcathQ.Y. (%)μmol/gcathQ.Y. (%) CaFe 2 O 4 / PbBi 2 Nb 1.9 W 0.1 O 9 2.7545034.84.1667538 PbBi 2 Nb 2 O 9 2.884317.60.9552029 CaFe 2 O 4 1.99623 trace 0 0 TiO 2-x N y 2.77451 trace 022114 Water splitting with sacrificial agents (  420 nm) Aqueous AgNO 3 solution (200 mL, 0.05 M) Methanol 30 mL H 2 O 170 mL Angew. Chem. Int. Ed. 44 (2005) 4585

40. Photocatalytic p-n Nanodiodes with an Ohmic Junction W(CO) 6 PbBi 2 Nb 1.9 Ti 0.1 O 9 W/PbBi 2 Nb 1.9 Ti 0.1 O 9 WO 3 /W/PbBi 2 Nb 1.9 Ti 0.1 O 9 O2O2 Flow 2OH - O 2 + 2H + WO 3 W metal PbBi 2 Nb 1.9 Ti 0.1 O 9 (A) Appl. Phys. Lett., 89, 64103 (2006) WO 3 -5.24 eV -7.94 eV CdS -3.98 -6.38 TiO 2 -4.21 -7.41 WO 3 -5.24 -7.94

41. Catalyst loaded with 0.1wt% Pt, 0.3 g; light source, 450W W-Arc lamp (Oriel) with UV cut-off filter(λ ≥ 420nm). Catalysts Band GapH 2 EvolutionO 2 Evolution E g (eV)λ ab (nm)μmol/gcath 3 Q.Y.(%) μmol/gcath 3 Q.Y.(%) WO 3 /W/ PbBi 2 Nb 1.9 Ti 0.1 O 9 2.8643349.336.0674141 CaFe 2 O 4 / PbBi 2 Nb 1.9 W 0.1 O 9 2.7545034.84.1667538 PbBi 2 Nb 2 O 9 2.884317.60.9552029 CaFe 2 O 4 1.99623trace0 0 TiO 2-x N y 2.77451trace022114 Photocatalytic H 2 and O 2 evolution from water under visible light (  420 nm) Aqueous AgNO 3 solution (200 mL, 0.05 M) Methanol 30 mL H 2 O 170 mL

43. Synthesis of Coupled Semiconductor by Filling 1D TiO2 Nanotubes with CdS Subarna Banerjee, Susanta K. Mohapatra, Prajna P. Das and Mano Misra** Chemical and Materials Engineering, University of Nevada, Reno, Nevada 89557 Chem. Mater., 2008, 20 (21), pp 6784–6791 Schematic showing the thermodynamically favorable conduction bands for CdS and TiO 2.

44. CdS decomposes readily. CdS(s) + 2h + → Cd 2+ (aq) + S(s) Na 2 S + H 2 O → NaOH + Na + HS - CdS-TiO 2 NT + hv → e- + h + HS - + h + → H + + S - S + SO 3 2- → S 2 O 3 2- SO 3 2- + S 2 2- → S 2 O 3 2- + S 2- S 2 O 3 2- + 3H 2 O + 4h + → 2SO 3 2- + 6H + 2OH - + SO 3 2- → SO 4 2- + H 2 O 2H + + 2e- → H 2 So overall reaction is S 2- + SO 3 2- + 5H 2 O → 5H 2 + 2SO 4 2-

45. GroupsJournal PhotocatalystPhotocatalytic Reaction H 2 evolution (μmol/h) Kudo JACS (2004) (AgIn) x Zn 2(1-x) S 2 0.3g (3wt% Pt) K 2 SO 3 and Na 2 S solution 424 (λ ＞ 420 nm) Kudo Angew. Chem. Int. Ed. (2005) ZnS-CuInS 2 -AgInS 2 0.3g (0.765wt%), 0.25M K 2 SO 3 + 0.35M Na 2 S 2300 (λ ＞ 420 nm) Domen JPC. B (2004) Ln 2 Ti 2 S 2 O 5 1wt% Pt 0.2g, 0.01M Na 2 S + 0.01M Na 2 SO 3 24 (λ ＞ 420 nm) Domen Chem. Commun (2003) ZnIn 2 S 4 2wt% Pt 0.2g, 0.43M Na 2 S + 0.5M Na 2 SO 3 (200ml) 57 (λ ＞ 420 nm) Kudo Chem. Commun (2002) AgInZn 7 S 9 3wt% Pt 0.3g, 0.25M K 2 SO 3 + 0.35M Na 2 S 66.7 (λ ＞ 420 nm) YoshidaChem. Phys. Lett. (2003) LaMnO 3 /CdS (0.5:1 molar ratio) 0.1g (No Pt), 0.1 M Na 2 S + 0.5M Na 2 SO 3 + 1.0M NaOH (pH = 14) 40 (λ ＞ 420 nm) YoshidaChem. Phys. Lett (2004) CdS/ETS-4 Composite 0.1g (No Pt), 0.1M Na 2 S + 0.5M Na 2 SO 3 + 1.0M NaOH 16 (λ ＞ 420 nm) Yoshida JPC. B (2002) Intercalated CdS 0.3% Pt 0.1g 20 mL (0.1M Na 2 S) under xenon lamp(300W) 67 (λ ＞ 420 nm) Postech- CdS (bulk)-TiO 2 1wt% Pt 0.1g + 0.1 M Na 2 S-0.02M Na 2 SO 3 625 (λ ＞ 420 nm) CdS-AgGaS 2 1wt% Pt 0.1g + 0.1 M Na 2 S-0.02M Na 2 SO 3 472 (λ ＞ 420 nm) Sulfide Photocatalysts: Literature Survey

46. Fig. The amount of H 2 evolution with reaction time. Fig. Schematic of generalized scheme for H 2 S decomposition using photocatalyst. Process of Simultaneous Hydrogen Production and Decomposition of H 2 S dissolved in Alkaline Water

47. p-n nanocomposite photocatalysts EFEF n-type metal Ohmic contact e-e- e-e- 2H + H2H2 h+h+ 2OH - O 2 +2H + CB VB Schottkyp-n p/ohmic/n

48. Characteristics2003 Status2005 Target2010 Target2015 Target Solar-to-hydrogen Efficiency (%) 7.07.59.014.0 Durability (h)100100010,00020,000 Cost ($/kg-H 2 )-360225 DOE Technical Targets for Hydrogen from PEC Cell

49. Photocatalytic H 2 production