Gradient Mo Engineering in [100]-Oriented CuWO₄ Films for Boosted Photoelectrochemical Water Splitting

Copper tungstate (CuWO₄) is a promising candidate for photoelectrochemical (PEC) water splitting, yet its practical performance is severely limited by inefficient charge transfer. Herein, a gradient Mo-doped CuWO₄ film (CuW_xMo_{1- x}O₄) with preferential [100] crystal orientation was constructed via a facile hydrothermal route. Owing to the distinct crystallization behaviors of isopolytungstate and isopolymolybdate species, a continuous W/Mo compositional gradient is established along the direction perpendicular to the conductive substrate, giving rise to a stepwise band profile across the film thickness. Such stepwise band engineering, synergistically coupled with crystallographic orientation control, effectively accelerates the separation and transport of photogenerated carriers. As a result, the CuW_xMo_{1- x}O₄ film delivers a high photocurrent density of 1.53 mA cm^-2, and presenting limited decay in 50 h. This work highlights gradient band engineering as an effective doping paradigm to overcome charge-transfer limitations in CuWO₄ and provides a general composition gradient strategy for high-performance photoelectrodes by integrating stepwise energy bands with tailored crystal orientation.