Water splitting on semiconductor catalysts under Visible Light (ChemSusChem 2009, 2, 471) - Large-scale production of H2 (Steam Reforming Reaction, Review Chem. Rev. 107, 3952) - In one hour, more energy strikes the Earth from the Sun than is consumed in a year (14TW) Figure 1. Global power potential of renewable noncarbogenic resources in terawatts. [a] Assuming solar conversion systems with 10% efficiency covering 0.16% of the land of the Earth - H2O 1/2 O2 + H2 - Honda and Fujishima in 1972 - Maximum efficiency (5.9%) is still far from practical application (10%)

Ⅰ Basic principles (ⅰ) photo-electrochemical cell disadvantage - Overall water-splitting reaction, ΔG0=+237.2 kJ mol-1, 2.46eV per molecule - Two types of photocatalysts (ⅰ) photo-electrochemical cell (ⅱ) particulate photocatalytic system disadvantage - poor separation of charge carriers - difficulty in the separation of H2 and O2 Advantage - simpler - less expensive Figure 2. Schematic diagram of the basic principle of the overall water-splitting reaction on a solid semiconductor photocatalyst

Water splitting in acid media oxidation : H2O + 2h+ 2H+ + 1/2 O2 reduction : 2H+ + 2e- H2 overall : H2O H2+ 1/2 O2 Using redox reagents (sacrificial reagents) Figure 3. Half-reactions of water splitting for H2 or O2 evolution reaction in the presence of sacrificial reagents

Ⅱ Photocatalysts Requirements (1) Suitable solar visible-light absorption(Eg) (2) Separation of photoexcited e- from holes (3) Minimization of energy losses related to charge transport (4) Minimization of recombination (5) Chemical stability to corrosion and photo corrosion in water (6) Kinetically suitable electron transfer properties from photocatalyst surface to water interface To achieve water splitting (1) The bottom of conduction band must be a more negative potential than the proton reduction potential (H+/H2 = 0 V, NHE at pH=0) (2) The valence band edge must exceed the oxidation potential of water (+1.23V, NHE at pH=0) (3) Minimum Eg = 1.23eV, shorter than 1010 nm (70% of all solar photons) (4) Because of unavoidable energy losses, Eg should be larger than 1.23eV

- Because of energy loss, optimum Eg is 2.0 – 2.2 eV Figure 4. Band-gap energies and relative band positions of different semiconductors relative to the water oxidation/reduction potential (vs. NHE) - Because of energy loss, optimum Eg is 2.0 – 2.2 eV - CdS and GaP are unstable, because anions of these materials are more easily oxidized than water

Ⅲ Strategies to Develop Visible-Light Photocatalysts - Maximum apparent quantum yield (AQY = 2H/I X 100 (%)) is 5.9%, Rh2-yCryO3/(Ga1-xZnx)(N1-xZnx) - 90% of photogenerated e- and holes are recombined right after excitation - Finding new-single phase materials - Tuning band gap energy - Surface modification by deposition of co-catalysts to reduce EA for gas evolution - Sensitization - Nano-design to control the size, morphology, and defects

Ⅲ.1 Band gap engineering 1. Cation or anion doping - Sb, Ta, Cr doped TiO2 and SrTiO3 - ZnS doped Cu - Ni, C doped TiO2 - Impurity levels generated by cation doping is discrete, Bad for migration of holes Figure 5. Band structure of a cation-doped, wide-band-gap semiconductor photocatalyst with a visible-light response

Figure 6. Band structure of an anion-doped, wide-band-gap semiconductor with a visible-light response - N substitutes for O atom mixing of the p states of the doped anion with O 2p states shift the band edge upward - Fewer recombination centers than cation doping

2. Semiconductor alloys Figure 7. Band structure of photocatalysts prepared from solid solutions of wide- and narrow-band-gap photocatalysts GaN – ZnO, ZnS – CdS, ZnS – AgInS2, CdS – CdSe

Ⅲ.2 Surface co-catalysts - Novel metals or metal oxides (Pt, Rh, NiO, RuO2 etc) - Capture of electrons or holes reducing recombination - Transfer of electrons and holes to surface water molecules reducing EA

Ⅲ.3 Sensitization 1. Organic dye sensitization and antenna effects - Population the CB of WBG semiconductors with electrons under visible-light illumination using chromophore - Dye-sensitized solar cells using single-crystal semiconductor electrode shows poor light-harvesting efficiencies and low photo-current densities - Porous films of nano-meter sized TiO2 has 1000 times layer surface area - Stacking chlorophyll-containing thylakoid membrane of chloroplast - QY ≒ 1 - Forward injection rate of electrons are several orders of magnitude slower than back-electron transfer from semiconductor to oxidized sensitizers

- Light harvesting can be improved by incorporating several chromophoric molecular components - Porphyrins - RuⅡ/OsⅡ, bis-bipyridine

One dimensional antenna system

2. Inorganic sensitization : composite semiconductor - CdS – TiO2 composite - Interparticle electron transfer decreases carrier recombination - Successful criteria 1) CB level of NBG semiconductor should be more negative than that of WBG semiconductor 2) CB level of WBG semiconductor should be more negative than the water reduction potential 3) Electron injection should be fast and efficient Figure 8. Band structure of a composite photocatalyst with an enhanced visible-light response, prepared by a mixture of wide- and narrow-band-gap photocatalysts.

Ⅲ.4 Nano design - Transport of photexcited carrier is determined by the crystal size, the crystalline structure, the nature and number structural defects, and the surface properties of photocatalysts - Defects can act as trapping or recombination centers depending on the nature and location. At surface, reaction facilities. - Diffusion length of charge carriers must be longer than the particle size - In the bulk or grain boundaries recombination center - Eg of crystalline semiconductor is a function of particle size - The size of particle is less than that of Bohr radius electrons and holes in the quantum-sized semiconductor are confined in the potential well No delocalization Eg ↑ - At nanometer range, surface vacancies act as trapping center (3~15nm) - CdS, HgSe, PbSe, CdSe, ZnO and TiO2 (size effect was studied)

- Synthesis of nano-particles (sol-gel, micelles and inverse micelles, hydrothermal, CVD, sonochemical) - Aggregation problem can be solved by incorporation into the framework of microporous hosts (zeolite, activated carbons etc) - Formation of 3D networks Antenna mechanism Transforming the photon energy from the absorption site to the reaction site primary particles (PM) Strong electronic coupling between PMs enhances the activity Secondary particles

Ⅲ.5 Multiphoton water splitting - Semiconductor with small band gaps can be combined to drive water oxidation/reduction separately via multiphoton processes Figure 9. Diagram of a dual photocatalyst system employing a redox shuttle(A/R)

Ⅳ Efficient Formulations for Water Splitting under Visible light

1. Titanium oxide and titanates - TiO2 (3.1eV) - Improve visible light absorption - V+5, Cr3+, Fe3+, Co2+, and Ni 2+ doping no significant reactivity - Sb+5 and Cr3+ codoped TiO2 active for O2 evolution under λ > 440nm using AgNO3 as sacrificial agent Sb5+ suppressed the formation of Cr6+ and oxygen defects Figure 10. Dependence of the photocatalytic activity of TiO2:Sb(x%)/Cr(2.3%) on the ratio Sb/Cr (0.5 g, catalyst, 0.05 molL-1 aqueous silver nitrate solution, 320 mL, 300 W Xe lamp). Adapted from Ref. [47].

- Anion doping such as N, S, C p states of doped anion mixed with O 2p orbitals raised the valence band edge narrowing Eg - Fused TiO2 with SrO, BaO, Ln2O3 SrTiO3, La2Ti2O7, Sm2Ti2O7 (3.2eV) (3.8eV) - S anion doping - Sm2Ti2S2O5 visible absorption up to 650nm active with sacrificial electron donor or acceptor - TiSi2 absorbs wide range of the solar spectrum Rh, Cr3+– Ta5+, Cr3+–Sb5+ Figure 11. UV/vis diffuse reflectance spectra of Sm2Ti2O7 and Sm2Ti2S2O5. Adapted from Ref. [111].

2. Tantalates and niobates - Corner sharing MO6 (M=Ta, Nb) is highly active under UV (4.0-4.7eV) without co-catalysts - Due to the ease of migration and separation of electron-hole pair - MTaO3 (M=Li, Na, K) is active under UV - MTaO2N (M=Ca, Sr, Ba). TaON, Sr2Nb2O7-xNx (x=1.5-2.8) is active under visible light Figure 12. UV-vis diffuse reflectance spectra of Ta2O5, TaON, Ta2N3 and MTaO2N (M: Ca, Sr, Ba). Adapted from Ref. [119].

3. Other transition-metal oxides - BiVO4 scheelite structure - Ag3VO4 perovskite structure - WO3 oxidizes water at moderately high rates with Ag+ or Fe3+ - Pt-modified WO3 in a tandem system with Cr/Ta doped Pt-SrTiO3 under UV Active for O2 with AgNO3 acceptor under UV O2 with NaIO3 H2 with iodide 4. Metal nitrides and oxynitrides - Nitride and oxynitride of d10 metal cations (Ga3+, Ge4+) is active without sacrificial agent under UV - Solid solution of GaN and ZnO (Ga1-xZnx)(N1-xOx) >3eV >3eV 2.4-2.8eV

- Zn 3d and N 2p electrons repulsion narrowed Eg - Rh oxide, Cr oxide as co-catalyst H2 and O2 evolves under visible light without sacrificial agent QY=5.9% at 420-440nm first example of one-step photo-splitting of water under visible light Figure 14. Hydrogen and oxygen evolution rates in overall water splitting on (Ga1-xZnx)(N1-xOx) loaded with 1 wt% Rh and 1.5 wt% Cr under visiblelight illumination (0.3 g catalyst; 370 mL aqueous solution adjusted to pH 4.5 with H2SO4). Adapted from Ref. [122].

5. Metal sulphides - Small Eg : achieve under visible light - Unstable because S2- anions are more susceptible to oxidation than water - Photo corrosion can be suppressed by using a Na2S/Na2SO3 mixture as electron donors - Cds wurtzite structure , Eg = 2.4eV absorbs visible light of < 510nm - Pt/CdS, QY = 25% for H2 production - Combining CdS with TiO2, ZnO, or CdO Figure 15. Hydrogen evolution rate from an aqueous solution of Na2S and Na2SO3, under visible-light irradiation over CdS and CdS–CdO–ZnO catalysts (reactant solution 150 mL, 0.1M Na2S, 0.04M Na2SO3, 150 W Xe lamp). Adapted from Ref. [134].

- Cd1-xZnxS Figure 17. Hydrogen evolution rate from an aqueous solution of Na2S and Na2SO3, under visible-light irradiation over solid solutions of Cd1-xZnxS with different Zn concentrations (x=0.2, 0.25, 0.30, and 0.35; reactant solution) 150 mL, 0.1M Na2S, 0.04M Na2SO3, 150 W Xe lamp). Adapted from Ref. [137].

conduction band of ZnS Narrowing Eg achieve for H2 - ZnS (Eg=3.66eV) - Doping with Cu2+, Ni2+, Pb2+ causes the transition from Mn+ levels to the conduction band of ZnS Narrowing Eg achieve for H2 with SO32-/S2- electron donor - Combining ZnS with AgInS2 and CuInS2 (CuAgIn)xZn2(1-x)S2 Pt loaded on (AgIn)0.22Zn1.56S2 highest activity for H2 (QY=20% at 420nm) Figure 19. Diffuse reflectance spectra of (CuAg)xIn2xZn2(1-x)S2 solid-solution photocatalysts (x=0, 0.01, 0.025, 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, and 0.5) Adapted from Ref. [142].