Mimicking Natural Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme Water Splitting Systems

Abstract

Visible light-driven water splitting using cheap and robust photocatalysts is one of the most exciting ways to produce clean and renewable energy for future generations. Cutting edge research within the field focuses on so-called "Z-scheme" systems, which are inspired by the photosystem II-photosystem I (PSII/PSI) coupling from natural photosynthesis. A Z-scheme system comprises two photocatalysts and generates two sets of charge carriers, splitting water into its constituent parts, hydrogen and oxygen, at separate locations. This is not only more efficient than using a single photocatalyst, but practically it could also be safer. Researchers within the field are constantly aiming to bring systems toward industrial level efficiencies by maximizing light absorption of the materials, engineering more stable redox couples, and also searching for new hydrogen and oxygen evolution cocatalysts. This review provides an in-depth survey of relevant Z-schemes from past to present, with particular focus on mechanistic breakthroughs, and highlights current state of the art systems which are at the forefront of the field.

Scheme 1. A Typical Model of a Single Photocatalyst System for Water Splitting

Scheme 2. A Schematic Diagram of a Double Excitation Process on P680 and P700 in Natural Photosynthesis

Scheme 3. A Double Excitation System Based on a HEP and an OEP with a Shuttle of Ox, oxidant, and Red, reductant, molecules

Two dotted curves indicate the back reactions which should be suppressed.

Figure 1

Band edge positions of typical semiconductors for visible driven water splitting.

Figure 2

Speculated reaction mechanism using IO₃^–/I^– redox mediator (pH = 7). Reprinted with permission from ref (37). Copyright 2002 Elsevier.

Figure 3

(I) Photocatalytic O₂ evolution from aqueous AgNO₃ solutions (0.05 mol/L, 320 mL) under visible light irradiation (λ > 420 nm) on (a) BiVO₄ (s–t) and (b) BiVO₄ (s–m). (II) The working diagram of Z-scheme water splitting based on BiVO₄. Reprinted with permission from refs (44) and (36). Copyright 2001 American Chemical Society and 2004 The Chemical Society of Japan.

Figure 4

(I) Schematic band structures of a metal oxide (NaTaO₃) and metal (oxy)nitride (BaTaO₂N). (II) The working diagram of Z-scheme water splitting based on TaON. Reproduced with permission from refs (5) and (48). Copyright 2007 American Chemical Society and 2008 The Chemical Society of Japan.

Figure 5

(I) (a) Diffuse reflectance spectra of H₂WO₄ and WO₃. (b) Rates of O₂ evolution over H₂WO₄ and WO₃ in Fe(ClO₄)₃ aqueous solution (5 mM, pH 2.3) under visible light irradiation (Xe lamp, cut off filter: L-42, Y-44, Y-46, Y-48 ,Y-50, O-52). (II) The working diagram of Z-scheme water splitting based on H₂WO₄. Reproduced with permission from ref (55). Copyright 2017 The Royal Society of Chemistry.

Figure 6

(I) Crystal structure of (a) Sillén–Aurivillius oxychloride Bi₄NbO₈Cl and (b) its related intergrowth compound Bi₆NbWO₁₄Cl proposed by Aurivillius. (II) Time course of Z-scheme water splitting coupled with Ru/SrTiO₃:Rh photocatalyst via Fe³⁺/Fe²⁺ redox mediator. (III) The working diagram of Z-scheme water splitting based on Bi₄NbO₈Cl. Reproduced with permission from ref (56). Copyright 2016 American Chemical Society.

Figure 7

Proposed band structure and visible light response of Rh-doped SrTiO₃ photocatalyst. Reproduced with permission from ref (16). Copyright 2004 American Chemical Society.

Figure 8

(I) UV–vis diffuse reflectace (DR) spectra of ATaO₂N (A = Ca, Sr, Ba), TaON, Ta₃N₅, and WO₃. (II) The working diagram of Z-scheme water splitting based on ATaO₂N. Reproduced with permission from refs (11) and (60). Copyright 2011 The Chemical Society of Japan and 2008 Elsevier.

Figure 9

(I) Molecular structures of dyes. (II) The working diagram of Z-scheme water splitting based on dye-adsorbed Pt/H₄Nb₆O₁₇ and IrO₂–Pt/WO₃. Reproduced with permission from ref (62). Copyright 2013 American Chemical Society.

Figure 10

Time courses of photocatalytic evolution of H₂ (closed) and O₂ (open) using a mixture of dye-adsorbed [(a) NKX-2311 and NKX-2587, (b) NKX-2677 and NKX-2697, and (c) MK-1 and MK-2] Pt/H₄Nb₆O₁₇ (50 mg) and IrO₂–Pt/WO₃ (100 mg) suspended in 5 mM KI aqueous solution (pH ∼ 4.5, without adjustment) under visible light. Arrows indicate evacuation of gas phase. Reproduced with permission from ref (62). Copyright 2013 American Chemical Society.

Figure 11

(I) (a) Crystal structure and optical properties of graphitic carbon nitride. Schematic diagram of a perfect graphitic carbon nitride sheet constructed from melem units. (b) Experimental XRD pattern of the polymeric carbon nitride, revealing a graphitic structure with an interplanar stacking distance of aromatic units of 0.326 nm. (c) Ultraviolet–visible diffuse reflectance spectrum of the polymeric carbon nitride. Inset: Photograph of the photocatalyst. (II) The working diagram of Z-scheme water splitting based on g-C₃N₄. Reproduced with permission from refs (63) and (35). Copyright 2009 Macmillan Publishers Limited and 2014 American Chemical Society.

Figure 12

Mechanism of water splitting in a Z-scheme photocatalyst system consisting of Ru/SrTiO₃:Rh and BiVO₄ under visible-light irradiation using RGO as a mediator. Reproduced with permission from ref (99). Copyright 2011 American Chemical Society.

Figure 13

(a) Illustration of the preparation of the SrTiO₃:La,Rh/Au/BiVO₄:Mo sheet by the particle transfer method, and (b) schematic of overall water splitting on the Ru-modified SrTiO₃:La,Rh/Au/BiVO₄:Mo sheet. Reproduced with permission from ref (98). Copyright 2016 Macmillan Publishers Limited.

Figure 14

(a) Photograph of the ink used for screen printing the photocatalyst sheet. (b) Photograph of a 10 cm × 10 cm SrTiO₃:La,Rh/Au nanoparticle/BiVO₄:Mo printed sheet. (c) Time course of the water splitting reaction using a Ru-modified SrTiO₃:La,Rh/Au colloid (40 wt %)/BiVO₄:Mo printed sheet under simulated sunlight at 288 and 5 KPa. The sample (6.25 cm²) was photodeposited with RuCl₃·3H₂O (0.17 μmol). Reproduced with permission from ref (98). Copyright 2016 Macmillan Publishers Limited.

Figure 15

Squares (□), circles (○), and triangles (△) stand for Ru-loaded SrTiO₃:La,Rh/C/BiVO₄:Mo, Cr₂O₃/Ru-loaded SrTiO₃:La,Rh/C/BiVO₄:Mo, and a-TiO₂/Cr₂O₃/Ru-loaded SrTiO₃:La,Rh/Au/BiVO₄:Mo, respectively. Closed and open symbols represent hydrogen and oxygen, respectively. Photodeposition from RuCl₃·3H₂O (0.2 μmol), K₂CrO₄ (0.2 μmol), Ti peroxide (1.3 μmol), and the overall water-splitting reaction were carried out under Xe lamp (300 W) illumination (λ > 420 nm) at 288 K. The area of the photocatalyst sheets was 7.5 cm². Reproduced with permission from ref (107). Copyright 2017 American Chemical Society.

Figure 16

Scheme of photocatalytic water splitting. Reproduced with permission from ref (96). Copyright 2011 Cambridge University Press.

Figure 17

Forward and backward reactions in Z-scheme water splitting (potentials at pH 7).

Figure 18

Absorption spectra of aqueous solutions containing I^–, IO₃^–, or I₃^– (precursors: KI, KIO₃, or I₂, without pH adjustment).

Figure 19

Time courses of the hydrogen evolution (squares) and the conversion of iodide ion into triiodide ions (circles) as the result of the photocatalytic reactions using the Pt-loaded TiO₂ (TIO-5) particles. The reaction was carried out in the cell containing the photocatalyst (200 mg) and 1.0 mol dm^–3 aqueous solution of potassium iodide (50 mL) at pH 2.4. Reproduced with permission from ref (114). Copyright 1996 The Chemical Society of Japan.

Figure 20

pH dependence of the initial rate of the photocatalytic reaction using Pt-loaded TiO₂ (Kanto Chemicals) particles as the photocatalyst. The reaction rates were determined from the amounts of triiodide ions generated during the photoirradiation for 1 h. The photocatalyst was heat-treated at 500 °C for 1 h. Reproduced with permission from ref (114). Copyright 1996 The Chemical Society of Japan.

Figure 21

Time course of photocatalytic evolution of H₂ using Pt(0.5 wt %)TiO₂ (anatase, Ishihara ST-01) photocatalyst suspended in 0.1 M of NaI aqueous solution under UV light irradiation (λ > 300 nm, 400 W Hg lamp). Reproduced with permission from ref (11). Copyright 2011 The Chemical Society of Japan.

Figure 22

Time course of photocatalytic O₂ evolution over TiO₂ photocatalysts suspended in aqueous solution (400 mL, pH 11 adjusted by NaOH) containing (a)1 mmol of NaIO₃ and (b) 1 mmol of NaIO₃ and 16 mmol of NaI. Reproduced with permission from ref (11). Copyright 2011 The Chemical Society of Japan.

Figure 23

Adsorption properties of iodate (IO₃^–) and iodide (I^–) anions on various TiO₂ powders. Reproduced with permission from ref (11). Copyright 2011 The Chemical Society of Japan.

Figure 24

Time course of photocatalytic O₂ evolution over Pt (0.5 wt %)/WO₃ and Pt (0.5 wt %)/BiVO₄ (inset) suspended in aqueous solution (250 mL, pH 6.5 without adjustment) containing (a) 0.25 mmol of NaIO₃ and (b) 0.25 mmol of NaIO₃ and 10 mmol of NaI. Reproduced with permission from ref (11). Copyright 2011 The Chemical Society of Japan.

Figure 25

(a) Dependence of rates of gas evolution over a mixture of Pt-TiO₂-A1 and bare TiO₂-R2 photocatalysts upon the pH value of NaI solution (NaI: 0.1 M). (b) Dependence of rates of gas evolution over a mixture of Pt-TiO₂-A1 and bare TiO₂-R2 photocatalysts upon the concentration of NaI aqueous solution (pH 11). Reproduced with permission from ref (11). Copyright 2011 The Chemical Society of Japan.

Figure 26

Time course of photocatalytic evolution of H₂ and O₂ using a mixture of Pt (0.3 wt %)/SrTiO₃ (Cr, Ta 4 mol % doped) and Pt (0.5 wt %)/WO₃ photocatalysts suspended in 5 mM of NaI aqueous solution (pH 6.5 without adjustment) under visible light irradiation (λ > 420 nm). Triangles indicate H₂ evolution using Pt/SrTiO₃:Cr/Ta alone. Reproduced with permission from ref (11). Copyright 2011 The Chemical Society of Japan.

Figure 27

Absorption spectra of aqueous solutions containing Fe cation (precursor, FeCl₃·6H₂O or FeCl₂·4H₂O, pH 2.1, adjusted by HCl).

Figure 28

Initial rate of oxygen production as a function of the Fe³⁺ (open circles) and WO₃ (closed circles). All solutions were in 0.5 × 10^–2 mol dm^–3 H₂SO₄ for the variation of [Fe³⁺], 7.5 mg cm^–3 of WO₃ was used, and for the variation in [WO₃], 10^–2 mol dm^–3 of Fe³⁺ was present. Reproduced with permission from ref (119). Copyright 1982 The Royal Society of Chemistry.

Figure 29

Effect of Fe²⁺ concentration on the initial rate of oxygen formation over WO₃ photocatalyst. Reproduced with permission from ref (119). Copyright 1982 The Royal Society of Chemistry.

Figure 30

Adsorption isotherms of Fe³⁺ and Fe²⁺ ions on TiO₂ powder at 25 °C: for Fe³⁺ ions (open circles) and for Fe²⁺ ions (open squares). When both Fe²⁺ and Fe³⁺ ions were added to the solution, their adsorptivity was affected each other: for Fe³⁺ ions (closed circles) and for Fe²⁺ ions (closed squares). The molar ratio of Fe³⁺ ions: Fe²⁺ ions are described in the figure. Measurements were performed using TiO₂ (JCR TIO-5) powder. Reproduced with permission and addition of the correction in the horizontal axis from ref (122). Copyright 1997 American Chemical Society.

Figure 31

Presumed reaction mechanism of the photocatalytic decomposition of water using the Fe³⁺/Fe²⁺ redox system. Reproduced with permission from ref (19). Copyright 1997 Elsevier.

Figure 32

Alternative evolution of H₂ and O₂ gases using a RuO₂–WO₃ catalyst and Fe³⁺/Fe²⁺ redox system. Run 1: mixture of FeSO₄ (1 mmol) and H₂SO₄ (10 mmol) and distilled water (350 mL) was irradiated through a quartz glass filter (λ > 200 nm). Run 2: RuO₂(1 wt %)–WO₃ catalyst (1 g) was added to the solution after run 1 and light was irradiated through a Pyrex glass filter (λ > 300 nm). Run 3: catalyst powder was filtered from the solution (or sedimented by stop stirring) after run 2, and then the solution was irradiated again through a quartz glass filter (λ > 200 nm). Run 4: same as run 2. Reproduced with permission from ref (19). Copyright 1997 Elsevier.

Figure 33

(a) Schematics of the photocatalytic reaction cell for splitting water and (b) energy diagram of splitting water by combined photocatalytic reactions. Reproduced with permission from ref (124). Copyright 1969 The Royal Society of Chemistry.

Figure 34

Photocatalytic overall water splitting using (Pt (0.5 wt %)/SrTiO₃:Rh (1%)–(BiVO₄) system under visible light irradiation in an aqueous solution of (a) FeCl₃ (2 mmol L^–1) and (b) FeCl₂ (2 mmol L^–1). Open marks, H₂; closed marks, O₂. Catalyst, 0.1 g for each component; reactant solution, 120 mL, pH 2.4; light source, 300 W Xe-arc lamp with a cutoff filter (λ > 420 nm); cell, top irradiation cell with a Pyrex window. Reproduced with permission from ref (126). Copyright 2007 The Chemical Society of Japan.

Figure 35

Suppression of backward reactions by Fe³⁺ ions. Reproduced with permission from ref (126). Copyright 2007 The Chemical Society of Japan.

Figure 36

Photocatalytic O₂ evolution on the BiVO₄ photocatalyst under visible light irradiation in 2 mmol L^–1 aqueous FeCl₃ solutions with (a) 0, (b) 1, (c) 2, and (d) 5 mmol L^–1 of FeCl₂. Catalyst, 0.1 g; reactant solution, 120 mL, pH 2.4; light source, 300 W Xe-arc lamp with a cutoff filter (λ > 420 nm); cell, top-irradiation cell with a Pyrex window. Reproduced with permission from ref (126). Copyright 2007 The Chemical Society of Japan.

Figure 37

Overall water splitting by a (Ru/SrTiO₃:Rh)–(BiVO₄)–([Co(bpy)₃]^3+/2+) system under visible light irradiation. Reaction conditions: catalyst, 0.1 g each; starting reactant solution, 120 mL of an aqueous [Co(bpy)₃]SO₄ solution (0.5 mmol L^–1, initial pH 3.8); light source, a 300 W Xe-arc lamp with a cutoff filter (λ > 420 nm); cell, top-irradiation type. SrTiO₃:Rh powder was prepared by calcination at 1373 K with Sr 3% excess. Reproduced with permission from ref (92). Copyright 2013 American Chemical Society.

Figure 38

Energy diagrams of the Z-scheme photocatalyst system, (Ru/SrTiO₃:Rh)–(BiVO₄)–([Co(bpy)₃]^3+/2+), and the ratio of [Co(bpy)₃]²⁺ ions to [Co(bpy)₃]³⁺ ions during overall water splitting under basic, neutral, and acidic conditions. Reproduced with permission from ref (92). Copyright 2013 American Chemical Society.

Figure 39

(a) Two-compartment-reactor for H₂ evolution separated from O₂ evolution. (b) H₂ evolution separated from O₂ evolution for overall water splitting by a (Ru/SrTiO₃:Rh)–(BiVO₄)–([Co(bpy)₃]^3+/2+) system. Catalyst, 0.1 g (Ru/SrTiO₃:Rh), 0.3 g (BiVO₄); starting reactant solution, 0.5 mmol L^–1 of an aqueous [Co(bpy)₃]SO₄ solution, 300 mL; initial pH adjusted to 3.8; light source, two 300 W Xe-arc lamps with cutoff filters (λ > 420 nm). SrTiO₃:Rh powder was prepared by calcination at 1373 K with Sr 3% excess. Open and close marks indicate H₂ and O₂, respectively. Reproduced with permission from ref (92). Copyright 2013 American Chemical Society.

Figure 40

Cyclic voltammograms of 1 mM of K₆[SiW₁₁O₃₉Mn^II(H₂O)]·6H₂O and K₈[SiW₁₁O₃₉]·13H₂O in an aqueous solution containing KH₂PO₄ (0.5 M) as a supporting electrolyte (scan rate, 50 mV s^–1; initial potential, 0 V (vs Ag/AgCl); initial scan direction, toward more positive potential; working electrode, glassy carbon; reference electrode, Ag/AgCl; counter electrode, Pt coil). Reproduced with permission from ref (93). Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 41

Time courses of H₂ evolution over Ru/SrTiO₃:Rh from a phosphate solution (0.5 M) containing [SiW₁₁O₃₉Mn^II(H₂O)]^6– (photocatalyst, 0.1 g; the amount of water, 50 mL). Reproduced with permission from ref (93). Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 42

Adsorption amounts of SiW₁₁Mn on (a) SrTiO₃:Rh and (b) WO₃ particles in 0.5 M phosphate solution (amount of photocatalyst, 15 mg; amount of Milli-Q water, 10 mL; stirring time, 15 h in dark; BET surface area, SrTiO₃:Rh 5.9 m² g^–1, WO₃ 4.6 m² g^–1). Reproduced with permission from ref (93). Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 43

Time courses of O₂ evolution over PtO_x/WO₃ from an aqueous solution containing [SiW₁₁O₃₉Mn^III(H₂O)]^5–. Dotted lines indicate the theoretical amount of O₂ calculated from added [SiW₁₁O₃₉Mn^III(H₂O)]^5– (photocatalyst, 0.1 g; the amount of water, 100 mL). Reproduced with permission from ref (93). Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 44

X-ray photoelectron spectra of I 3d and Pt 4f for Pt (3 wt %)–anatase TiO₂ powders; fresh sample (a), after stirring in an aqueous solution of NaI (1 mM) for 12 h in dark (b), after photoreaction in an aqueous solution of NaI (1 mM) for 20 h under UV light irradiation (c). X-ray photoelectron spectrum of I-3d for bare anatase TiO₂ powder after stirring in an aqueous solution of NaI (1 mM) for 12 h in the dark (d). Reproduced with permission from ref (133). Copyright 2003 Elsevier.

Figure 45

Water formation from H₂ and O₂ in gas phase reaction over (a) fresh and (b) I-adsorbed 3 wt % Pt–TiO₂ photocatalyst powders. A mixture of H₂ and O₂ (H₂:O₂ = 2:1) gases was introduced to a closed gas-circulating system with a Pyrex glass cell containing 10 mg of photocatalyst powder without water. Reproduced with permission from ref (133). Copyright 2003 Elsevier.

Figure 46

Consumptions of H₂ and O₂ due to back-reactions with 2 mmol L^–1 aqueous FeCl₃ solution on the (Pt (0.3 wt %)/SrTiO₃:Rh)–(BiVO₄) (triangles) and (Ru (0.7 wt %)/SrTiO₃:Rh)–(BiVO₄) (circles) systems in the dark. Catalyst, 50 mg each; reactant solution, 120 mL, pH 2.4, adjusted by H₂SO₄. Reproduced with permission from ref (85). Copyright 2008 Elsevier.

Figure 47

Photocatalytic overall water splitting on (a) the (Ru (0.7 wt %)/SrTiO₃:Rh)–(BiVO₄) system and (b) the (Pt (0.1 wt %)/SrTiO₃:Rh)–(BiVO₄) system. Catalyst, 50 mg each; reactant solution, 2 mmol L^–1 of aqueous FeCl₃ solution, 120 mL, pH 2.4; light source, 300-W Xe-arc lamp (λ > 420 nm); cell, top-irradiation cell with a Pyrex glass window. Reproduced with permission from ref (85). Copyright 2008 Elsevier.

Figure 48

Time courses of photocatalytic H₂ evolution under visible light (λ > 410 nm) on NKX-2677 dye-adsorbed Pt(in)/H₄Nb₆O₁₇, Pt(in–out)/H₄Nb₆O₁₇, and Pt/TiO₂ photocatalysts suspended in (a) 0.1 M aqueous KI solution (pH ∼ 6.5, without adjustment) and (b) 0.1 M aqueous triethanolamine solution (pH ∼ 7, adjusted with HCl). Reproduced with permission from ref (62). Copyright 2013 American Chemical Society.

Figure 49

Conceptual schemes for suppression of backward reaction using nanostructured layered semiconductors. Reproduced with permission from ref (62). Copyright 2013 American Chemical Society.

Figure 50

Rates of O₂ evolution over ex-Ca₂Nb₃O₁₀/K⁺ loaded with 0.3 wt % of various metal oxides (PtO, RuO₂, IrO₂, Rh₂O₃, and CoO_x) in 5 mM NaIO₃ aqueous solution under UV light irradiation (λ > 300 nm, Xe lamp) and overpotentials for IO₃^– reduction on each catalyst. Reproduced with permission from ref (136). Copyright 2015 The Royal Society of Chemistry.

Figure 51

Dependence of rates of O₂ evolution over Pt(0.5 wt %)–WO₃ photocatalysts upon the calcination temperature after impregnation of H₂PtCl₆. The reaction was carried out in aqueous solution (250 mL) containing NaIO₃ (2 mM). Reproduced with permission from ref (59). Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 52

Current–potential curves of TaON electrodes loaded with Ru species and an unmodified TaON electrode in an aqueous Na₂SO₄ solution (0.1 M) containing NaIO₃ (1.0 mM) under dark conditions. Reproduced with permission from ref (137). Copyright 2017 The Royal Society of Chemistry.

Figure 53

Initial rate of O₂ evolution on Ru-x and Ru-x-Co photocatalysts (50 mg) prepared at different temperatures in an aqueous solution (250 mL) containing NaIO₃ (1.0 mM) under visible light (λ > 400 nm). Reproduced with permission from ref (137). Copyright 2017 The Royal Society of Chemistry.

Figure 54

Summary of the speculated role and activities of the cocatalysts loaded onto TaON. Reproduced with permission from ref (137). Copyright 2017 The Royal Society of Chemistry.

Figure 55

Photocatalytic O₂ evolution over (a) WO₃ without Cs-treatment, (b) Cs-WO₃ (first run) prepared by the IMP method, and (c) Cs-WO₃ (second run) under visible light irradiation. The dashed line shows the upper limit of O₂ evolution expected from the amount of Fe³⁺ (1260 μmol) added to the solutions. The second run reaction was performed by exchanging the reactant solution of the first run with fresh Fe₂(SO₄)₃ aqueous solution. Reproduced with permission from ref (138). Copyright 2010 American Chemical Society.

Figure 56

Time courses of the photocatalytic O₂ evolution over PtO_x/H–Cs–WO₃ (solid line) and PtO_x/WO₃ (dotted line) suspended in the solutions containing 10 mM NaI and different concentrations of I₃^– (circles, 1 mM; squares, 0.5 mM; triangles, 0.2 mM). Catalyst, 0.4 g; reactant solution, 300 mL; light source, 300 W Xe lamp attached with L-42 cutoff filter. The dashed lines (30, 75, and 150 μmol) show the upper limits of O₂ evolution (four-electron oxidation reaction) expected from the amount of I₂ (I₃^–) (reduced by two electrons) added to the solutions. Reproduced with permission from ref (80). Copyright 2013 The Royal Society of Chemistry.

Figure 57

Time courses of photocatalytic evolution of H₂ and O₂ using a mixture of 0.2 g of Pt/SrTiO₃:Cr Ta as the H₂ evolution photocatalyst and 0.4 g of PtO_x/H–Cs–WO₃ (circle plots) or PtO_x/WO₃ (triangle plots) as the O₂ evolution photocatalyst suspended in 10 mM NaI aqueous solution (300 mL, pH 6.8). Light source: 300W Xe lamp attached with L42 cutoff filter. Reproduced with permission from ref (80). Copyright 2013 The Royal Society of Chemistry.

Figure 58

(a–c) SEM images of WO₃ samples prepared via hydrothermal reaction at different temperatures (reaction time, 4 h). (d) TEM image of HT-315. Reproduced with permission from ref (91). Copyright 2016 The Chemical Society of Japan.

Figure 59

Adsorption isotherm (at 20 °C) of Fe cation on the surface of various WO₃ samples in aqueous solution with different concentration of FeCl₃ or FeCl₂ (amount of WO₃, 0.1 g; amount of solvent, 10 mL; time for mixing in dark under inert gas atmosphere, 15 h). Reproduced with permission from ref (91). Copyright 2016 The Chemical Society of Japan.

Figure 60

(a) Influence of Fe³⁺ concentration on the rate of O₂ evolution under light irradiation. (b) Initial rate of O₂ evolution on various WO₃ photocatalysts suspended in aqueous solutions containing Fe³⁺ (16 mM) and Fe²⁺ (0, 1.6, 3.1, 7.9, and 12.4 mM) under light irradiation (pH 2.1 adjusted with HCl). Reproduced with permission from ref (91). Copyright 2016 The Chemical Society of Japan.