Optical modulation and thickness-dependent structural evolution in CuO thin films for PEC-water splitting

Copper oxide (CuO) thin films were deposited by RF magnetron sputtering from a high-purity copper target in an argon–oxygen plasma with a fixed oxygen flow of 20 sccm, and the effect of film thickness (105–410 nm) on structural, optical, and photoelectrochemical properties was systematically evaluated. X-ray diffraction and field-emission scanning electron microscopy demonstrate that increasing thickness promotes progressive crystallite growth and microstructural densification, with the average grain size increasing by 28% from 13 ± 0.5 nm to 17± 0.5 nm, accompanied by reduced lattice disorder. These structural improvements directly modulate the optical response, leading to an 18% narrowing of the optical band gap from 1.98 to 1.62 eV and a 5% increase in refractive index from 2.7 ± 0.01 to 2.83 ± 0.03, reflecting enhanced optical density and stronger light–matter interaction. Photoelectrochemical measurements further reveal a clear thickness dependence, where the 410 nm CuO film delivers a maximum photocurrent density of approximately −2.8 mA cm -2 at −1.0 V versus RHE, indicating more efficient charge generation and transport. The results demonstrate that thickness modulation during room-temperature RF sputtering is a key factor linking microstructural evolution with optical and photoelectrochemical performance, enabling rational optimization of CuO thin films for PEC water-splitting applications. • CuO films deposited with controlled thicknesses 105–410 nm via RF sputtering. • Polycrystalline monoclinic CuO confirmed across all thicknesses. • Crystallite size increased from 13 ±0.5 nm to 17 ±0.5 nm with increasing thickness. • Optical band gap decreased from 1.98 eV to 1.62 eV; refractive index rose from 2.7 to 2.83. • PEC photocurrent improved with thickness; 410 nm film reached −2.88 mA/cm 2 at −1 V.