Sputtered Nickel Oxide Thin Films on n-Si(100)/SiO2 Surfaces for Photo-Electrochemical Oxygen Evolution Reaction (OER): Impact of Deposition Temperature on OER Performance and on Composition before and after OER

Magnetron sputtered nickel oxide thin films deposited on the native oxide of crystalline n-Si(100) wafers are studied in dependence of the substrate deposition temperature (600 degrees C, 400 degrees C, 200 degrees C, and room temperature) using X-ray and synchrotron excited photoemission spectroscopy as well as cyclic-voltammetry under illumination. We show that the substrate temperature during nickel oxide sputtering governs the composition of the pristine NiO(x)film and the OER performance. Two dedicated nickel oxide species are found with Ni2+ corresponding to stoichiometric NiO while Ni3+ indicates an oxygen rich NiOx (x > 1) phase. With decreasing deposition temperature, the ratio of Ni3+/Ni2+ in the pristine NiO(x)film increases. Information depth dependent synchrotron related photoemission spectroscopy further suggests that oxygen rich NiOx is found on top of the surface and at the grain boundaries. The OER onset potential improves from 1.55 V to 1.1 V in correlation to an increasing Ni3+/Ni2+ ratio in the pristine NiO(x)film and an increasing emission from a nickel oxyhydroxide phase (h-NiOx) after photo-assisted cyclic-voltammetry in alkaline solution. Upon electrochemical treatment, a reconditioning process is observed with the formation of h-NiOx that consists of Ni(OH)(2) and NiOOH, while NiOx disappears. (c) 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited.