Vertically aligned nanocrystalline Cu–ZnO thin films for photoelectrochemical splitting of water

Cu-incorporated nanocrystalline ZnO thin films were deposited on glass substrate by sol–gel. To a solution of zinc acetate 2-hydrate in dimethyl formamide, calculated quantities of copper acetate were added. The clear solution, obtained after 2 h of continuous stirring, was coated on ITO plates. Pre-annealing at 250 °C was followed by sintering at 400, 500, and 600 °C. XRD analysis revealed dominant evolution of hexagonal ZnO with a possible simultaneous growth of meta-stable cubic ZnO. AFM and SEM analysis indicated preferential growth of nanocrystallites along c-axis. Optical characterization led to two prominent absorption thresholds in the UV region; one matching with the band gap of bulk ZnO and the second at slightly higher energy, suggesting quantum confinement effect in nanocrystallites. Cu incorporation influenced the two band gap energies differently. Photoelectrochemical splitting of water using 1% at. Cu–ZnO film sintered at 600 °C resulted in 141% gain in photocurrent at zero bias.