Current Mechanistic Understanding of Surface Reactions over Water-Splitting Photocatalysts  Peng Zhang, Tuo Wang, Jinlong Gong  Chem  Volume 4, Issue 2, Pages 223-245 (February 2018) DOI: 10.1016/j.chempr.2017.11.003 Copyright © 2017 Elsevier Inc. Terms and Conditions

Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 1 Model Illustration, Scanning Tunneling Microscopy (STM) Observations, and Calculation Results of H2O Adsorption Configurations on Anatase (101) (A) Ball-and-stick model of anatase TiO2 with an adsorbed H2O molecule. H2O oxygen and hydrogen atoms are plotted in yellow and white, respectively, whereas TiO2 atoms are blue (Ti) and red (O). The dotted lines indicate the hydrogen bonds. (B and C) Two consecutive STM images of a 0.11 monolayer of water on anatase (101) (173 × 63 Å2, +3.5 V, 0.45 nA) were taken at a sample temperature of 190 K. The green and black arrows indicate ordered water clusters along the [11¯1¯] and [111¯] directions, respectively. (D) Difference image of (B) and (C); the white arrows indicate the hopping direction of selected water molecules. (E) Optimized structures of a water monomer (1a), and of different clusters of two (2a–2c), three (3a–3c), and four (4a) water molecules. The adsorption energy for each configuration is given in millielectronvolts per water molecule in relation to the adsorbed monomer. A positive value of the adsorption energy indicates that the configuration is less stable than isolated monomers. H2O oxygen and hydrogen atoms are plotted in yellow and white, respectively. Reprinted with permission from He et al.21 Copyright 2009 Nature Publishing Group. Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 2 Band Energetics at the Semiconductor-Electrolyte Interface (A–C) Band energetics at the n-type semiconductor-electrolyte interface before equilibrium (A), after equilibrium (B), and in quasi-static equilibrium under illumination (C). (D–F) Band energetics at the p-type semiconductor-electrolyte interface before equilibrium (D), after equilibrium (E), and in quasi-static equilibrium under illumination (F). Reprinted with permission from Walter et al.4 Copyright 2010 American Chemical Society. Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 3 Strategies for Enhancing the Voc of α-Fe2O3 Photoanode (A) Energy band schematics (quasi-static equilibrium under solar illumination) of the α-Fe2O3/Si nanowire dual-absorber photoanode. (B) J-V plots under simulated solar illumination (AM 1.5, 100 mW cm−2) in 1.0 M NaOH aqueous electrolyte (scan rate 10 mV s−1) for the α-Fe2O3/Si nanowire photoanode (red) and α-Fe2O3 on planar FTO (black). (A and B) Reprinted with permission from Mayer et al.38 Copyright 2012 American Chemical Society. (C) Schematic of the dual-junction perovskite solar cell/α-Fe2O3 photoanode tandem cell. (D) Faradic efficiency measurement for the dual-junction perovskite solar cell/α-Fe2O3 photoanode tandem cell. Red and black symbols correspond to the O2 and H2 gases measured by gas chromatography and the black and red lines correspond to the integration of the photocurrent with respect to time. Inset shows the photocurrent measured during the gas evolution. (C and D) Reprinted with permission from Gurudayal et al.39 Copyright 2015 American Chemical Society. (E) Schematic illustration of the main electron-hole recombination pathways in α-Fe2O3 nanorod photoanodes for solar water splitting and the three-step approach to reduce bulk, interface, and surface recombination. (F) Summary of the effect of the different processes on the photocurrent onset potential (Phones) and photocurrent density (Jph, at 0.8 V versus a reversible hydrogen electrode) for α-Fe2O3 nanorods. Ti/fur indicates the furnace-annealed α-Fe2O3:Ti nanorods, Ti/fla indicates flame-annealed α-Fe2O3:Ti nanorods, the “-d” suffix indicates that the sample was grown on the dense-layer-coated substrate; the “-OA” suffix indicates that the sample was etched with oxalic acid; and the “-FeOOH” suffix indicates that the sample was integrated with FeOOH nanoneedle cocatalysts. (E and F) Reprinted with permission from Cho et al.42 Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 4 Influence of Surface States to the Band Energetics at the Semiconductor-Electrolyte Interface (A and B) Influence of surface states to the band energetics at the n-type semiconductor-electrolyte interface under dark (A) and illumination (B) conditions. (C and D) Influence of surface states to the band energetics at the p-type semiconductor-electrolyte interface under dark (C) and illumination (D) conditions. Reprinted with permission from Du et al.51 Copyright 2014 American Chemical Society. Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 5 PEC Characterizations of the Effect of Strategies for Passivating Surface States (A) J-V curves of α-Fe2O3 photoanodes before ALD treatment (black circles), after three ALD cycles of Al2O3 and annealed to 400°C (blue diamonds), and after subsequent cobalt treatment (red triangles) in the dark (broken lines) and under simulated solar illumination (AM 1.5G 100 mW cm−2, solid curves). (B) Representation of the different capacitances determined by EIS and plotted versus the applied potential. The space charge capacitance (blue triangles) and the Helmholtz capacitance (red, diamonds) are plotted before (full markers, plain line) and after (empty markers, broken line) three ALD cycles of Al2O3. The inset is the electronic equivalent circuit representing the photoanode-electrolyte system used for EIS data modeling. (A and B) Reproduced from Le Formal et al.55 with permission of the Royal Society of Chemistry. (C) J-V curves of pristine Ta3N5 photoanode and samples with ALD-grown TiO2 overlayers (named as TiO2(n)-Ta3N5, where n indicates the number of ALD cycles during the deposition of TiO2). The J-V tests were conducted under AM 1.5G irradiation with an intensity of 100 mW cm−2. Reproduced from Zhang et al.56 with permission of the Royal Society of Chemistry. (D) J-V curves of 20 nm α-Fe2O3 electrodes annealed at 500°C (dashed blue) and 800°C (solid green) under H2O oxidation conditions at pH 13.6 and 1 sun illumination. Reprinted with permission from Zandi et al.58 Copyright 2014 American Chemical Society. (E) Photoresponse to 830 nm near-infrared light of a α-Fe2O3 photoanode reflected by a small but non-negligible photocurrent, with or without rare-earth-based upconversion nanoparticles (REUCNPs). Inset: energy band diagram showing the proposed mechanism for the photo response. Band-edge positions before unpinning (without the applied positive potential) are shown in gray. (F) When the surface of α-Fe2O3 was covered by NiFeOx, the photoresponse to 830 nm near-infrared light was annihilated. Inset: proposed mechanism for the prohibited transition with energy changes smaller than the band gap. (E and F) Reprinted with permission from Du et al.51 Copyright 2014 American Chemical Society. Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 6 In Situ ART-IR Spectroscopy for the Detection of Intermedia on a α-Fe2O3 Photoanode for PEC Water Oxidation (A) Schematic illustration of the setup used for in situ ART-IR measurements of a α-Fe2O3 working electrode (WE), platinum counter electrode (CE), Ag/AgCl reference electrode (RE), and ZnSe ATR crystal. A thin layer of electrolyte was introduced between the α-Fe2O3 WE and the ATR crystal. (B) J-V curves of an α-Fe2O3 electrode in the setup measured in contact with D2O in the dark (blue) and under illumination (dark red). (C) IR spectra of α-Fe2O3 scanned at constant applied potentials, from 1.13 to 1.63 V versus RHE, under illumination. (D) The spectra were measured at an applied potential of 1.63 V versus RHE under illumination in contact with D216O, D218O or a 1:1 ratio of D216O/D218O. Reprinted with permission from Zandi et al.59 Copyright 2016 Nature Publishing Group. Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 7 Band Energetics at the Photoelectrode-Cocatalyst and Cocatalyst-Electrolyte Interfaces (A and B) Band energetics at the n-type photoelectrode-cocatalyst and cocatalyst-electrolyte interfaces under dark (A) and illumination (B) conditions. (C and D) Band energetics at the p-type photoelectrode-cocatalyst and cocatalyst-electrolyte interfaces under dark (C) and illumination (D) conditions. Reprinted with permission from Nellist et al.69 Copyright 2016 American Chemical Society. Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 8 Characterization of a Co3O4 Loaded BiVO4 Photoanode for Solar Water Oxidation The mass ratio of Co3O4/BiVO4 was set to 2.0, 4.0, and 8.0 wt % (denoted as 2-Co/BV, 4-Co/BV, and 8-Co/BV, respectively). (A) Transmission electron microscopy (TEM) image of 4-Co/BV. (B) High-resolution TEM (HRTEM) image of 4-Co/BV. (C and D) J-V curves of different photoanodes measured with AM 1.5G illumination (100 mW/cm−2) for water oxidation without 1 M Na2SO3 (C) and sulfite oxidation with 1 M Na2SO3 (D). (E and F) Separation efficiencies of charge carriers in the bulk (bulk) (E) and on the surface (ηsurface) (F) of different photoanodes. Reprinted with permission from Chang et al.70 Copyright 2015 American Chemical Society. Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 9 Enhancing the Injection of Charge Carriers through Integrating Interlayers between Semiconductor Photoelectrodes and Cocatalysts (A) Diagram of the energy band under thermodynamic equilibrium in the dark for the n-Si/TiOx/ITO/NiOOH photoanode and a cross-sectional HRTEM image. (B) J-V curves of n-Si/TiOx/ITO (black), n-Si/TiOx/ITO/NiOOH (blue), and p++-Si/TiOx/ITO/NiOOH (red) photoelectrodes in 1 M LiOH at a 20 mV s−1 scan rate. Solid line, under 100 mW cm−2 AM 1.5G irradiation; dashed line, in the dark. (C) Injection efficiency (Φinj) and separation efficiency (Φsep) of the n-Si/TiOx/ITO (black) and n-Si/TiOx/ITO/NiOOH (blue) photoanodes under 100 mW cm−2 AM 1.5G illumination. (A–C) Reprinted with permission from Yao et al.72 Copyright 2016 American Chemical Society. (D) HRTEM image of a Co3O4/Fh/Ta3N5 photoanode. (E) J-V curves of Ta3N5, Fh/Ta3N5, Co3O4/Ta3N5, and Co3O4/Fh/Ta3N5 photoanodes under AM 1.5G simulated sunlight (100 mW cm−2) in 1 M NaOH aqueous solution (pH 13.6). (F) Charge storage versus potential curves of Ta3N5, Fh/Ta3N5, Co3O4/Ta3N5, and Co3O4/Fh/Ta3N5 photoanodes. (D–F) Reprinted with permission from Liu et al.73 Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 10 Characterizations of the Synergetic Cocatalytic Effect of Co3O4 and CDot Cocatalysts on a Fe2O3 Photoanode (A) HRTEM image of a C/Co3O4-Fe2O3 photoanode. (B) J-V curves of Fe2O3, C-Fe2O3, Co3O4-Fe2O3, and C/Co3O4-Fe2O3 photoanodes. (C) Schematic illustration of the fast/slow reaction processes on Co3O4 cocatalyst and the two-step two-electron reaction pathway for photocatalytic water oxidation on the C/Co3O4-Fe2O3 photoanode. (D) Photoluminescence spectra of the scopoletin assay of the electrolytes after solar water-splitting reactions with different photoanodes. Reprinted with permission from Zhang et al.86 Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 11 Gas Pressure inside a Bubble, Pb, as a Function of r The red line represents the value of Pb for which chemical equilibrium state is satisfied. Point A is the maximum value of the gas pressure, B is the equilibrium value for very small radii of the bubbles, and C corresponds to the critical value, rC. Reprinted with permission from Hernández et al.93 Copyright 2015 American Chemical Society. Chem 2018 4, 223-245DOI: (10.1016/j.chempr.2017.11.003) Copyright © 2017 Elsevier Inc. Terms and Conditions