Surface and Interface Engineering for Photoelectrochemical Water Oxidation  Yongbo Kuang, Taro Yamada, Kazunari Domen  Joule  Volume 1, Issue 2, Pages 290-305 (October 2017) DOI: 10.1016/j.joule.2017.08.004 Copyright © 2017 Elsevier Inc. Terms and Conditions

Joule 2017 1, 290-305DOI: (10.1016/j.joule.2017.08.004) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 1 Photoelectrochemical Water-Splitting Cell (A) A non-biased PEC cell comprising a water oxidation photoanode and a hydrogen evolution photocathode; Ef,n and Ef,p represent the quasi-Fermi levels of electrons and holes under illumination, respectively. ηH and ηO represent the overpotential for hydrogen and oxygen evolution reaction, respectively. (B) Tandem and parallel configurations of photoanode and photocathode. Joule 2017 1, 290-305DOI: (10.1016/j.joule.2017.08.004) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 2 Photoelectrochemical Performance Improvement of Photoanodes via Surface Modification (A) Change of photocurrent density (Jph) and onset potential (Von). (B) Stability enhancement. Joule 2017 1, 290-305DOI: (10.1016/j.joule.2017.08.004) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 3 Possible Effects of Surface and Interface Engineering for Photoelectrochemical Water Oxidation Joule 2017 1, 290-305DOI: (10.1016/j.joule.2017.08.004) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 4 Photodriven Water Oxidation on Protected Semiconductors (A) Cross-sectional schematic of a photoanode stabilized against corrosion in a 1.0 M KOH(aq) by an electronically defective layer of TiO2 deposited by ALD. Instead of corroding the anode, the photogenerated holes are conducted through the TiO2 to patterned Ni electrocatalysts, where the holes are used to oxidize water to O2. (B) Photoelectrochemical behavior of TiO2-coated Si photoanodes in 1.0 M KOH(aq). The formal potential for oxidation of water to O2 (g) is labeled at 0.19 V versus saturated calomel electrode (SCE). (C) Elemental contrast image of the Si/TiO2/Ni interface by scanning transmission electron microscopy. The Ni film in this study was deposited to be continuous without patterning. (D) Chronoamperometry of an n-p+-Si photoanode coated with 44 nm of TiO2 and Ni islands for over 100 hr at 0.93 V versus SCE in 1.0 M KOH(aq). The photocurrent density versus time (red curve) was overlaid with the illumination intensity versus time (black curve). Reproduced from Hu et al.35 Copyright 2014, American Association for the Advancement of Science. Joule 2017 1, 290-305DOI: (10.1016/j.joule.2017.08.004) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 5 The Evolution of Ta3N5 Surface Energetics Stage I: fresh Ta3N5 free of H2O. Stage II: Ta3N5 with partial H2O adsorption due to exposure to ambient air. Stage III: Ta3N5 immersed in H2O; as a result of the strong surface adsorption and the solvent effect, the band-edge position shifts positively by 0.6 V. Stage IV: Ta3N5 with surface oxides; when Ta3N5 oxidizes H2O under illumination, its surface N atoms are displaced by O atoms, leading to a further change of the Fermi level by 0.2 V toward the positive direction. The horizontal lines correspond to the surface Fermi-level position of Ta3N5 in stages I–IV. Reproduced from He et al.48 Copyright 2016, Elsevier. Joule 2017 1, 290-305DOI: (10.1016/j.joule.2017.08.004) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 6 NiFeOx-Decorated Hematite Photoanode with Radically Improved Onset Potential (A) Steady-state J-V behaviors of various hematite photoelectrodes for oxygen evolution. The current densities of Si photocathode placed behind the hematite photoanode are shown to illustrate the meeting points. (B) Band diagram of unmodified hematite (gray) and NiFeOx-decorated hematite after regrowth treatments (red) under flat-band, quasi-equilibrium conditions. The Fermi-level shift (denoted as 1) is a direct result of the regrowth treatment. The hole quasi-equilibrium potential shift (denoted as 2) is due to the application of NiFeOx. Reproduced from Jang et al.39 Copyright 2015, Nature Publishing Group. Joule 2017 1, 290-305DOI: (10.1016/j.joule.2017.08.004) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 7 Schematic Diagram of the Particle Transfer Method Reproduced from Minegishi et al.102 Copyright 2013, Royal Society of Chemistry. Joule 2017 1, 290-305DOI: (10.1016/j.joule.2017.08.004) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 8 Self-Generation and In Situ Regeneration of Ni- and Fe-Based Oxygen Evolution Catalyst on Particulate BiVO4 Photoanodes (A) Schematic illustration of Ni-, Fe-based oxygen evolution catalyst (NiFe-OEC) self-generation and in situ regeneration over BiVO4 electrodes. (B) Photocurrent change of a bare Mo:BiVO4/Ni/Sn electrode under irradiation at 0.6 V versus RHE. The rapid increase in photocurrent indicates the self-generation of OECs. The insets are the scanning electron microscopy image of the electrode taken after catalyst modification (scale bar, 100 nm), and the current-potential curve of the activated electrode. (C) Schematic comparison of the site-specific catalyst regeneration and non-site-specific catalyst deposition process. When the concentration of Ni2+ is below 1 μM, the re-deposition of Ni-based catalyst occurs selectively at sites that suffer from the loss of catalyst because these sites are more active for Ni2+ oxidation, while the sites saturated with NiFe-OEC are more active for water oxidation. At higher concentrations of Ni2+, Ni2+ oxidation becomes non-site-specific as its reactivity increases, leading to catalyst overloading. Reproduced from Kuang et al.41 Copyright 2016, Nature Publishing Group. Joule 2017 1, 290-305DOI: (10.1016/j.joule.2017.08.004) Copyright © 2017 Elsevier Inc. Terms and Conditions

Figure 9 Dual-Working-Electrode Photoelectrochemistry and Semiconductor Catalyst Adaptive Junction (A) Device schematic showing independent electrical connections to the semiconductor and electrocatalyst. (B) Band-bending diagram and wiring schematic for DWE PEC analysis. The TiO2 potential Vsem and the catalyst potential Vcat are controlled relative to water oxidation potential by the primary working electrode (WE1) and the secondary working electrode (WE2), respectively, of a bipotentiostat (using a single Hg/HgO reference electrode). Ec and Ev are the conduction and valence bands of the semiconductor, respectively. Ef,n and Ef,p are the quasi-Fermi levels for electrons and holes, respectively. (C and D) The measured steady-state open-circuit semiconductor potential Vsem and calculated Voc across the semiconductor catalyst junction as a function of the catalyst potential for IrOx-coated TiO2 (C) and Ni(OH)2-coated TiO2 (D). The inset band diagrams illustrate how the different electrostatic potential drops at the catalyst solution interface (depicted by the spatial dependence of the vacuum electron energy Evac) for ion-permeable and ion-impermeable catalysts affects Voc. Reproduced from Lin et al.108 Copyright 2014, Nature Publishing Group. Joule 2017 1, 290-305DOI: (10.1016/j.joule.2017.08.004) Copyright © 2017 Elsevier Inc. Terms and Conditions