Unraveling the photoelectrochemical behavior of Ni-modified ZnO and TiO2 thin films fabricated by RF magnetron sputtering

Abstract

Zinc oxide (ZnO) and titanium oxide (TiO2) thin films are fabricated by radio frequency magnetron sputtering, which allows fine control of the properties and compositions of semiconductor materials with practical application in solar-light-driven water splitting. Here, nickel is introduced as an effort to engineer the band structures and to enhance the photoelectrochemical performance of the TiO2 and ZnO photoanodes. An increase in the Ni concentration changes the preferred orientation of ZnO crystals and inhibits an anatase-to-rutile phase transformation in TiO2. Pristine ZnO and TiO2 thin films have columnar structures with average widths of 200 nm and 50 nm, respectively, and an increase in the Ni concentration reduces the width of the columnar structures. The results from X-ray photoelectron spectroscopy analysis reveal that Ni2+/Ni3+ ions are successfully introduced into the ZnO and TiO2 lattices, and oxygen vacancies are formed. The effect of Ni is also studied by Mott-Schottky analysis, Gärtner theory, and open circuit potential decays, revealing important changes in the optoelectronic features of the TiO2 and ZnO photoanodes. Enhancement in the photon absorption is integral for the higher activity in Ni-modified TiO2, whilst an efficient collection of charge carriers is rather determining in Ni-modified ZnO. In addition, the interaction of water molecules with the surfaces of pristine and Ni-modified ZnO and TiO2 thin films is explored using molecular modeling. Tailoring the optoelectronic properties through a suitable fabrication protocol can lead to efficient and cost-effective light-harvesting materials.