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

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 attained PEC water oxidation at the longer wavelength of 540 nm by 40 nm compared to the film (neat-CuWO₄) prepared without BIm 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 in diameter, which is responsible for the N-CuWO₄ film adhering rigidly on the substrate. The N-CuWO₄ film showed anisotropic crystallization of triclinic CuWO₄ with predominant growth to [010] and [100] directions, in contrast to isotropic crystallization for the neat-CuWO₄ film. The N-CuWO₄ electrode demonstrated the superior performance for PEC water oxidation with incident photon-to-electron conversion efficiency (IPCE) of 5.6% contributed from charge separation efficiency (η_sep) of 12.3% and catalytic efficiency (η_cat) of 51.9% (at 420 nm and 1.23 V), Faraday efficiency (FE_O2) of 97% for O₂ evolution, and significant stability for 40 h, which compares advantageously with those for 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 for the N-CuWO₄ electrode. The higher k_O2 for the N-CuWO₄ electrode is ascribable to the higher active site on the (100) facet for water oxidation at the surface.