An anisotropically crystallized and nitrogen-doped CuWO4 photoanode for efficient and robust visible-light-driven water oxidation†

In this work, a nitrogen-doped CuWO₄ (N-CuWO₄) photoanode for photoelectrochemical (PEC) water oxidation was synthesized by a mixed metal-imidazole casting (MiMIC) method with 1-butylimidazole (BIm). The N-CuWO₄ electrode achieved PEC water oxidation at a wavelength of 540 nm that is longer than that of the film (neat-CuWO₄) prepared without BIm by 40 nm due to the N-doping, which was conscientiously characterized by spectroscopic and theoretical investigations. The N-CuWO₄ surface was compactly covered with worm-like particles of 100–500 nm diameter, which is responsible for the rigid adherence of the N-CuWO₄ film onto the substrate. The N-CuWO₄ film showed anisotropic crystallization of triclinic CuWO₄ with predominant growth of the (010) and (100) planes, in contrast to the isotropic crystallization observed for the neat-CuWO₄ film. The N-CuWO₄ electrode demonstrated superior performance for PEC water oxidation with an incident photon-to-electron conversion efficiency (IPCE) of 5.6% contributed by a charge separation efficiency (η_sep) of 12.3% and a catalytic efficiency (η_cat) of 51.9% (at 420 nm and 1.23 V), a faradaic efficiency (FE_O2) of 97% for O₂ evolution, and considerable stability for 40 h, which are advantageously comparable to those of the state-of-the-art CuWO₄-based photoanodes. The water oxidation rate constant (k_O2 = 1.6 × 10² s⁻¹) at the surface for the N-CuWO₄ electrode was higher than that (6.8 s⁻¹) for the neat-CuWO₄ electrode by 2 orders of magnitude, which is responsible for the high IPCE and η_cat of the N-CuWO₄ electrode. The higher k_O2 value for the N-CuWO₄ electrode is ascribed to the higher active site on the (100) facet for water oxidation at the surface.