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

In this work, a nitrogen-doped CuWO<INF>4</INF> (N-CuWO<INF>4</INF>) photoanode for photoelectrochemical (PEC) water oxidation was synthesized by a mixed metal-imidazole casting (MiMIC) method with 1-butylimidazole (BIm). The N-CuWO<INF>4</INF> electrode achieved PEC water oxidation at a wavelength of 540 nm that is longer than that of the film (neat-CuWO<INF>4</INF>) prepared without BIm by 40 nm due to the N-doping, which was conscientiously characterized by spectroscopic and theoretical investigations. The N-CuWO<INF>4</INF> surface was compactly covered with worm-like particles of 100-500 nm diameter, which is responsible for the rigid adherence of the N-CuWO<INF>4</INF> film onto the substrate. The N-CuWO<INF>4</INF> film showed anisotropic crystallization of triclinic CuWO<INF>4</INF> with predominant growth of the (010) and (100) planes, in contrast to the isotropic crystallization observed for the neat-CuWO<INF>4</INF> film. The N-CuWO<INF>4</INF> 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 (eta<INF>sep</INF>) of 12.3% and a catalytic efficiency (eta<INF>cat</INF>) of 51.9% (at 420 nm and 1.23 V), a faradaic efficiency (FE<INF>O<INF>2</INF></INF>) of 97% for O<INF>2</INF> evolution, and considerable stability for 40 h, which are advantageously comparable to those of the state-of-the-art CuWO<INF>4</INF>-based photoanodes. The water oxidation rate constant (k<INF>O<INF>2</INF></INF> = 1.6 x 102 s-1) at the surface for the N-CuWO<INF>4</INF> electrode was higher than that (6.8 s-1) for the neat-CuWO<INF>4</INF> electrode by 2 orders of magnitude, which is responsible for the high IPCE and eta<INF>cat</INF> of the N-CuWO<INF>4</INF> electrode. The higher k<INF>O<INF>2</INF></INF> value for the N-CuWO<INF>4</INF> electrode is ascribed to the higher active site on the (100) facet for water oxidation at the surface.