Role of Sulfur Incorporation in p-Type Nickel Oxide (p-NiO) on n-Type Silicon (n-Si) Photoelectrodes for Water Oxidation Reactions

Abstract

NiO is a promising electrocatalyst for photoelectrochemical (PEC) oxygen evolution reaction (OER); the phase transformation into Ni(OH)(2)/NiOOH is a key step for the OER, requiring additional overpotential. The morphological changes due to this phase transformation might expose the underlying Si surfaces in alkaline solutions and result in critical stability issues. Here, we report that the incorporation of sulfur (S) into NiO films can not only accelerate the initial charge transfer characteristics but also alleviate the film strain by its rapid dissolution during the phase transformation, indicative of the prolonged photoelectrochemical (PEC) operation. Atomic layer deposition (ALD) chemistry using bis(dimethylamino-2-methyl-2-butoxy)nickel and hydrogen peroxide was introduced to grow thin NiO films and was applied to PEC water splitting for the first time. The growth rates, structures, surface roughness, work functions, and optical properties were fully studied by X-ray reflectivity, X-ray diffraction, atomic force microscopy, and ultraviolet photoelectron spectroscopy. The PEC properties of n-Si/NiO were investigated with different thicknesses of ALD-NiO and by incorporation with elemental S. We found that introducing S results in a rapid, yet uniform, conversion of cubic NiO to layered Ni(OH)(2)/NiOOH structures, avoiding cracks and delamination during electrochemical conditioning processes. Our Ni(OH)(2)/NiOOH on Si photoelectrodes from S-NiO showed stable and optimal performance under strong alkaline conditions (pH 13) for approximately 120 h at photocurrents of approximately 20 mA.cm(-2).