Fig. 1. (Color online) Schematic of water splitting in photoanode-based PEC cell.

Fig. 2. (Color online) Ultimate photocurrent density and corresponding maximum STH of some semiconductors.

Fig. 3. (Color online) Comparison of electron–hole recombination in planar and 1D Fe₂O₃ nanorods arrays electrode.

Fig. 4. (Color online) (a–c) SEM and (d) TEM images of obtained Fe₂O₃ by a facile rapid dehydration strategy (RD-Fe₂O₃). (e) Current density–voltage curves of obtained Fe₂O₃ by a conventional temperature-rising route (C-Fe₂O₃) and RD-Fe₂O₃ collected at 10 mV/s in 1.0 M KOH aqueous electrolyte under AM 1.5G illumination and in the dark. The solid and dashed lines represent the data collected under back (solid lines) and front (dashed lines) illuminations, respectively. Reproduced from Ref. [36].

Fig. 5. (Color online) (a) Mott–Schottky (M–S) plots measured in 1 M NaOH solution. Conditions: 1 kHz frequency. (b) J–V curves of Sn-doped (green), Sn, Zr-doped α-Fe₂O₃(blue), and Sn, Zr-doped α-Fe₂O₃-NiOOH (red) in the dark and under illumination. Reproduced from ref 18. (c) M–S plots of undoped, Si-doped, Ti-doped, and co-doped α-Fe₂O₃ films at the frequency of 1000 Hz. (d) J–V characteristics of the undoped, Si-doped, Ti-doped, and codoped films in 1 M NaOH under 500 W full arc xenon lamp, the green line indicates dark currents of all the films. The inset is IPCE performance for the four α-Fe₂O₃ films mentioned above at 0.6 V vs. Ag/AgCl in 1 M NaOH. Reproduced from Ref. [56].

Fig. 6. (Color online) Comparison of the band structure of an n-type semiconductor photoanode in the presence and the absence of passivation layers inside a PEC cell. (a) High charge recombination at surface defects and inefficient water oxidation by the photogenerated holes. (b) Use of an OER catalyst layer, which promotes facile hole transfer across the interface to the electrolyte for improving water oxidation. (c) Use of thin non-catalytic passivation layers which suppress surface recombination and improve water oxidation.

Fig. 7. (Color online) (a) J–V curves of a bare hematite electrode (red solid line) and the same electrode after depositing 1 (orange dotted line), 2 (yellow short dashed line), 15 (green dashed double dotted line), 45 (teal long dashed line), and 90 (blue dashed single dotted line) mC/cm² CoPi catalyst at 5 mV/s. (b) Proposed mechanism of the catalytic process. (c) Anodic and cathodic transient currents at an applied bias of 1.05 V vs. RHE. Reproduced from Ref. [63].

Fig. 8. (Color online) (a) Scheme of charge transfer from Fe₂O₃ to H₂O through Ni(OH)₂ and/or IrO₂. (b) Chronoamperometry measurement of Ti-Fe₂O₃, Ti-Fe₂O₃/Ni(OH)₂, and Ti-Fe₂O₃/Ni(OH)₂/IrO₂ under a stepped potential. Reproduced from Ref. [20].

Fig. 9. (Color online) (a) High-resolution TEM images of Fe₂O₃/FeB photoanode. (b) Charge separation and (c) injection efficiency at 1.23 V vs. RHE for the Fe₂O₃ and Fe₂O₃/FeB photoanodes. Reproduced from Ref. [80]. (d, e) AFM images and (f, g) the corresponding SPVM images of (d, f) Fe₂O₃ and (e, g) Fe₂O₃|V_O|Fe_xS, respectively. Scale bars in (a–d), 200 nm. (h) Histograms of the SPV distributions on the Fe₂O₃and Fe₂O₃|V_O|Fe_xS electrodes. (i) Time evolution of SPV generation and decay with light on and off on Fe₂O₃ and Fe₂O₃|V_O|Fe_xS. All the SPV signals are obtained under 450 nm laser illumination at a light intensity of 4 mW/cm². (j) Photocurrent-potential (J–V) curves of the Fe₂O₃ and Fe₂O₃|V_O|Fe_xS photoanodes in a 1 M NaOH (pH ~ 13.6) aqueous electrolyte under AM 1.5G illumination and in the dark. Reproduced from Ref. [81].

Fig. 10. (Color online) Schematic illustration of (a) the type-II heterostructure, (b) p–n heterostructure, and (c) Z-scheme system without electron-mediators band alignments, and the correspondingly possible separation and transfer process of photoinduced electron–hole pairs of semiconductor photocatalysts. Reproduced from Refs. [92, 93].

Fig. 11. (Color online) Schematic for the energy band structure of the Fe₂O₃-NA/RGO/BiV_1–xMo_xO₄ heterojunction and the proposed mechanism of PEC water splitting. Reproduced from Ref. [105].

Fig. 12. (Color online) (a) TiSi₂ nanonet-based hematite nanostructure is essentially a core/shell arrangement where the core is the nanonet for effective charge collection and the shell is hematite for photocatalytic functionalities. The electronic band structure is shown in the enlarged cross-sectional view. (b) Low- and (c) high-magnification transmission electron microscopy (TEM) images showing the conformal coverage and crystallinity of hematite. (d) J–V curves of an Fe₂O₃/TiSi₂ heteronanostructure and a planar hematite film. (e) IPCE comparison of Fe₂O₃ with and without TiSi₂ nanonets. Reproduced from Ref. [116].