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 °C, 400 °C, 200 °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 Ni 2+ corresponding to stoichiometric NiO while Ni 3+ indicates an oxygen rich NiO x (x > 1) phase. With decreasing deposition temperature, the ratio of Ni 3+ /Ni 2+ in the pristine NiO x film increases. Information depth dependent synchrotron related photoemission spectroscopy further suggests that oxygen rich NiO x 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 Ni 3+ /Ni 2+ ratio in the pristine NiO x film and an increasing emission from a nickel oxyhydroxide phase (h-NiO x ) after photo-assisted cyclic-voltammetry in alkaline solution. Upon electrochemical treatment, a reconditioning process is observed with the formation of h-NiO x that consists of Ni(OH) 2 and NiOOH, while NiO x disappears.