Voso4

Graphene oxide (GO) was synthesized via a modified Hummer’s method with more details described in Supplementary Information^{54 (link)}. To prepare NVP@rGO, 0.69 g of NH₄H₂PO₄, 0.318 g of Na₂CO₃ and 0.364 g of V₂O₅ were dispersed in deionized water, followed by adding 0.72 g of oxalic acid ( ≥ 99.0 %, Sigma-Aldrich) at 70 °C. The mixture was added with 7.3 mL GO aqueous dispersion (11 mg mL⁻¹) under vigorous stirring, and then freeze-dried to obtain the solid NVP@GO precursor. The precursor was grounded using an agate mortar, followed by sintering at 850 ^oC with a heating rate of 2 ^oC min⁻¹ in Ar to obtain the NVP@rGO powder. NVPF@rGO was prepared via a one-step hydrothermal method. Briefly, 0.536 g of NaF, 3.51 g of NaH₂PO₄·2H₂O and 1.763 g of VOSO₄·xH₂O (degree of hydration 3–5, Sigma-Aldrich) were dissolved in 30 mL deionized water, followed by mixing with 7.8 mL of GO aqueous dispersion (11 mg mL⁻¹) for 1 h to obtain a uniform dispersion. The mixture was immediately transferred into a 45 mL Teflon-lined stainless steel autoclave and kept at 120 ^oC for 10 h. The resulted precipitates were centrifuged at 4,000 rpm using deionized water for 5 times, and the obtained solid was dried at 80 °C for 10 h in a vacuum oven to obtain the NVPF@rGO powder. For bare NVPF, no GO was added with all the other procedures remained the same.

The BiVO₄ electrodes were prepared as previously reported procedure [19 (link)]. Solutions for electrodeposition were prepared by dissolving 10 mM Bi(NO₃)₃ (98 %, Alfa Aesar) in a solution of 35 mM VOSO₄ (97 %, Sigma Aldrich) at pH < 0.5 with HNO₃ (65 %, Acros Organics). Then 2 M CH₃COONa (≥99.0 %, Alfa Aesar) was added, raising the pH to ∼5.1, which was then adjusted to pH 4.7 with a few drops of HNO₃. Acetate serves to stabilize other insoluble Bi (III) ions at pH 4.7. This mildly acidic pH condition must be used because at pH lower than 2 where Bi(III) is soluble, no film can be formed while at pH higher than 5, V (IV) precipitates from solution. A three-electrode cell was used for electrodeposition, with an fluorine-doped tin oxide (FTO, 8 Ω/□, Hartford Glass Co.) coated glass substrate as working electrode, a Ag/AgCl (4 M KCl) as reference electrode and a platinum foil as counter electrode. A potentiostat (Sloartron SI 1287) was used for electrodeposition. Deposition of amorphous Bi–V–O films was carried out potentiostatically at 1.9 V vs Ag/AgCl (4 M KCl) for 5 min at 70 °C (ca. 2 mA cm⁻²). The as-deposited films were converted to crystalline BiVO₄ and amorphous V₂O₅ by annealing at 500 °C for 1 h in air, and pure BiVO₄ was achieved by dissolving the V₂O₅ in 1 M KOH under stirring for 20 min.