Nanostructured Transparent CuWO ₄ Films with Highly-Oriented Crystallinity Preferential to a (010) Plane as Efficient and Robust Photoanode for Visible-Light-Driven Water Oxidation

CuWO₄ is an n-type semiconductor with a narrow bandgap (Eg ≈ 2.2-2.4 eV), making it a promising visible-light-driven photoanode for water splitting, but it remains underexplored due to critical performance limitations. A nanostructured CuWO₄ film (denoted as T,A-CuWO₄) with high optical transparency and anisotropic crystal growth exhibiting preferential (010) and (020) planes, was synthesized from a homogeneous precursor solution containing CuCl₂ and WCl₆ in a mixed solvent of ethylene glycol (EG) and methyl imidazole (MeIm) as effective ligands via a facile spin-coating technique. The T,A-CuWO₄ electrode demonstrates a higher incident photon-to-current efficiency (IPCE) of 7.8% at 420 nm and 1.23 V vs RHE, compared to the CuWO₄ films prepared without either of EG or MeIm (denoted as CuWO₄ (w/o EG) (1.0%) or CuWO₄ (w/o Im) (3.5%), respectively), which is notably high among optically transparent CuWO₄ electrodes. The high performance of T,A-CuWO₄ film with a Faradaic efficiency (FEO2) of 98% and exceptional long-term stability over 60 h, stands out among state-of-the-art CuWO₄-based photoanodes for PEC water oxidation. The higher IPCE value for the T,A-CuWO₄ electrode is ascribed to the higher rate constant (kO2 = 25 s⁻¹) for water oxidation on the surface of the T,A-CuWO₄ electrode than those of the CuWO₄ (w/o EG) (4.6 s⁻¹) and CuWO₄ (w/o Im) (7.6 s⁻¹) electrodes. The higher kO2 is explained by the well-interconnected small particles of the T,A-CuWO₄ electrode, compared to irregularly shaped larger particles in CuWO₄ (w/o Im) and worm-like shaped large particles for CuWO₄ (w/o EG) film, as well as facilitated water oxidation at the active (010) facet (and (020) facet) resulting from its anisotropic crystallization.