A Promising Three-Step Heat Treatment Process for Preparing CuO Films for Photocatalytic Hydrogen Evolution from Water

Copper (II) oxide (CuO) nanostructures were prepared on fluorine-doped tin oxide (FTO) using a three-step heat treatment process in a sol–gel dip-coating method. The precursor used for the dip-coating process was prepared using copper acetate, propan-2-ol, diethanolamine, and polyethylene glycol 400. Dip-coated films in layers of 2, 4, 6, 8, and 10 were prepared by drying each layer at 110 and 250 °C for 10 and 5 min, respectively, followed by calcination at 550 °C for 1 h. The films were applied toward photocatalytic hydrogen evolution from water. The X-ray diffraction (XRD) pattern of the films confirmed the tenorite phase of pure CuO. Raman spectroscopy revealed the 1A_g and 2B_g phonon modes of CuO, confirming the high purity of the films produced. The CuO films absorb significant photons in the visible spectrum due to their low optical band gap of 1.25–1.33 eV. The highest photocurrent of −2.0 mA/cm² at 0.45 V vs reversible hydrogen electrode (RHE) was recorded for CuO films consisting of six layers under 1 sun illumination. A more porous surface, low charge transfer resistance, and high double-layer capacitance at the CuO/electrolyte interface observed for the films consisting of six layers contributed to the high photocurrent density attained by the films. CuO films consisting of six layers prepared using the conventional two-step heat treatment process for comparative purposes yielded 65.0% less photocurrent at 0.45 V vs RHE compared to similar films fabricated via the three-step heating method. The photocurrent response of the CuO nanostructures prepared using the three-step heat treatment process is promising and can be employed for making CuO for photovoltaic and optoelectronic applications.