1：Experimental Design and Method Selection:

The study employed sol–gel dip-coating process for the deposition of CuO thin films, followed by thermal annealing to achieve pure CuO phase. A TiO2 protective layer was also deposited using the sol–gel method to enhance stability.

2：Sample Selection and Data Sources:

FTO substrates were used for the deposition of CuO and TiO2 layers. The films were characterized using SEM, XRD, and UV-visible absorption spectroscopy.

3：List of Experimental Equipment and Materials:

High-resolution scanning electron microscope (Hitachi SU70), Bruker D8 Discover diffractometer, Agilent Technologies Cary Series spectrophotometer, Solartron Ametek 2087 A Potentiostat/Galvanostat/impedancemeter.

4：Experimental Procedures and Operational Workflow:

The CuO films were synthesized by dip-coating from sol–gel precursor solutions, followed by thermal annealing. The TiO2 layer was deposited similarly and annealed at 500 °C. Photoelectrochemical performances were evaluated in a three-electrode configuration under front-side light illumination.

5：Data Analysis Methods:

The crystalline structure was analyzed using XRD, morphology by SEM, and optical properties by UV-visible absorption spectroscopy. Photoelectrochemical performances were evaluated through chronoamperometry and linear sweep voltammetry.