<b>NiO Nanowire Arrays Based on AAO Templates for Photoelectrochemical Water Splitting </b>

Herein, we report the effective preparation and characterization of well-ordered vertically aligned NiO nanowire (NW) arrays, showing potential as a photoactive material for photoelectrochemical (PEC) water splitting, which is an environmentally friendly approach to solar hydrogen energy. The synthesis was based on a precise multi-stage procedure that consisted of an optimized two-step anodization process to generate Anodic Aluminum Oxide (AAO) templates with tailor-made pore sizes (65–94 nm), deposition of Ni through nano-channels by electro-deposition, and subsequent thermal oxidation to generate crystalline NiO. Structural characterizations assured the success of the synthesis: scanning electron microscopy showed dense and uniform arrays of vertically aligned high-aspect-ratio NWs, X-ray diffraction confirmed a phase-pure face-centered cubic NiO structure free from impurities. UV-vis diffuse reflectance spectroscopy characterization revealed that as-prepared samples exhibited an in-tense ultraviolet absorption and band gaps of 3.77 to 4.18 eV. In particular, this bandgap was observed to depend strongly on the NW diameter. Performance, as measured through applied bias photon-to-current efficiency, was architecture dependent and increased systematically with increasing diameter of the nanowires from 65 nm to 94 nm. This trend is rationalized as the result of an optimum trade-off at higher diameters whereby larger diameters afford a greater surface area to promote catalysis, as well as more efficient light trapping and critically axial charge collection, minimizing bulk recombination. The good performance of these photo response properties is therefore attributed to the synergetic effect of a large available surface area, an ordered one-dimensional structure for efficient charge transfer, and a suitable band gap for UV light harvesting. Unlike previous studies, this work establishes a clear diameter-dependent structure–performance relationship in AAO-template NiO NW arrays, providing quantitative insight into the interplay between NW geometry, light harvesting, and axial charge transport. In summary, we provide a template-based, controllable, and scalable synthetic strategy for generating transition metal oxide nano-arrays, providing an explicit design principle for optimizing next-generation nanostructured photo- electrodes for effective solar-driven hydrogen generation.