Fig. 1. (a) Current-voltage curves of the same 5MBVO film successively subjected to electrochemical treatment, air oxidation (day1, day2, day3, day7, and day14), and a repeated electrochemical treatment (day15) in the dark (dotted line) and under AM 1.5G 100 mW cm-2 simulated sunlight (solid lines). (b) Photocurrent densities of the same 5MBVO film successively subjected to electrochemical treatment, air oxidation (day1, day2, day3, day7, and day14), and a repeated electrochemical treatment (day15) at 1.23 VRHE.

Fig. 2. XPS spectra of Bi 4f (a), V 2p (b), O 1s (c), and Mo 3d (d) for the 5MBVO and 5MBVO-CV films. OL, OOH, and OH2O in (c) represent the lattice O2-, surface hydroxyl-like species, and adsorbed H2O molecules, respectively. All spectra have been calibrated by the C 1s peak (284.6 eV).

Fig. 3. (a) XRD patterns of the as-prepared MoOx film and MoOx-CV film. (b) Raman spectra of the MoOx and MoOx-CV films. (c) XPS spectra of Mo 3d for the MoOx and MoOx-CV films exposed to air at different stages (day1, day3, day7, day14, and day35) after the electrochemical treatment. All spectra have been calibrated by the C 1s peak (284.6 eV). (d) Mo5+/(Mo5+ + Mo6+) ratios of the MoOx-CV film exposed to air at different stages (day1, day3, day7, day14, and day35) after the electrochemical treatment, which were calculated according to the peak areas of Mo5+ and Mo6+ in (c).

Fig. 4. ηsep values (a), ηinj values (b), and TRPL spectra (c) of the 5MBVO and 5MBVO-CV films. The TRPL spectra were collected by exciting the electrodes at 360 nm and probing at 500 nm, and the TRPL curves were fit using a double-exponential function. (d) OCP values under AM 1.5 G irradiation (100 mW cm-2). (e) EIS curves (inset: equivalent circuit model) at 1 VRHE under illumination. (f) IPCE values at 1.23 VRHE of the 5MBVO and 5MBVO-CV films. The measurements in (d-f) were conducted in a 0.1 mol L-1 potassium phosphate (KPi) aqueous solution (pH = 7).

Fig. 5. (a) CV curves of bare FTO, the MoOx film, and the MoOx-CV film measured in a K2SO4 electrolyte containing the reversible redox couple ferricyanide/ferrocyanide, [FeIII(CN)6]3-/[FeII(CN)6]4-. (b) Schematic of electrochemical treatment inducing conductivity in the MoOx film.

Fig. 6. TEM images of 5MBVO (a) and 5MBVO-CV (b) particles. HRTEM images of 5MBVO (c) and 5MBVO-CV (d) particles corresponding to the framed areas in TEM images.

Fig. 7. Schematics of the Mo-doped BiVO4/MoOx composite electrode. Detailed energy diagrams for Mo-doped BiVO4, MoOx, and the electrolyte before (a) and after (b) the electrochemical treatment.

Fig. 8. Current-voltage curves of Mo-doped BiVO4 photoelectrodes with various Mo-doping concentrations before (a) and after (b) the electrochemical treatment in the dark (dotted lines) and under AM 1.5G 100 mW cm-2 simulated sunlight (solid lines). (c) Photocurrent densities of Mo-doped BiVO4 photoelectrodes with various Mo-doping concentrations at 1.23 VRHE before and after the electrochemical treatment. Current-voltage curves of the 5MBVO-CV (d), 10MBVO-CV (e), and 20MBVO-CV (f) films (10MBVO and 20MBVO films after the electrochemical treatment are designated as 10MBVO-CV and 20MBVO-CV, respectively) after different numbers of electrochemical treatment cycles (5, 25, 50, 75, and 100 cycles). (g) Current-voltage curves of the 10MBVO-CV film after electrochemical treatment at different threshold reduction potentials (-0.9, -1.0, -1.1, and -1.2 V vs. Ag/AgCl). (h) Photocurrent density of the 10MBVO-CV film at 1.23 VRHE as a function of threshold reduction potential. Photographs of the 10MBVO-CV film undergoing electrochemical treatment at -0.9 (left) and -1.2 (right) V vs. Ag/AgCl threshold reduction potentials are shown in the inset. (i) Schematic showing that electrochemical treatment cannot activate Mo-doped BiVO4 electrodes with high contents/thicknesses of MoOx segregations.