Self-Limiting Ni-Co Electrodeposited GaAs for Solar Water Oxidation
Authors: Xu Yin , Zangari Giovanni 
Published: February 2020

Photoelectrochemical water splitting has demonstrated the conversion of solar energy into hydrogen and simultaneously overcome the intermittent property of the sunlight.1 n-type GaAs is among the ideal photoanodes for water oxidation, with an optimal bandgap (1.43 eV) to absorb the visible solar spectrum (<867 nm) while providing a larger photovoltage compared to silicon.2 One of the major drawbacks of GaAs is the instability in water-based solution under illumination, which leads to decomposition or formation of insulating oxide layer. Therefore, a protective layer with high transmittance and minimal Fermi level pinning effect on GaAs is desirable. Electrodeposition with a self-limiting growth feature allows the formation of nanosized islands or ultrathin layer of metal hydroxide or oxy-hydroxide on the substrate,3 providing a potential solution to fabricate high-performance, low-cost protective layer on GaAs. To the best of the authors’ knowledge, this deposition method has not been reported on GaAs substrate for the application of solar water oxidation.
In this work, galvanostatic electrodeposition without boric acid buffer at a well-controlled pH was applied to grow thin Ni-Co hydroxide layer/particles on GaAs substrate. XPS indicates that the surface is mainly composed of Ni/Co oxyhydroxides and the depth profiling measurements evidence that the electrodeposited Ni-Co layer thickness (10 nm) is independent of the deposition time. The results prove that the deposition follows a self-limiting process where the formation of (oxy) hydroxide layer on the surface inhibit the growth, attributed to the local pH gradient resulted from the proton reduction. Compared with Ni deposition, the Ni-Co deposition exhibit a slower growth rate of the oxyhydroxides, implying that the grain size of the layer is finer. Ni-Co was deposited onto the GaAs photoanode and photocurrent measurements were in a K3Fe(CN)6/K4Fe(CN)6 aqueous solution under 1.5G AM solar illumination, which exhibits high photocurrent density (8.9 mA cm-2 for 0.5s-deposited Ni-Co) at 1.23V vs. RHE, an early onset potential of ~0.5V vs. RHE and a slower photocorrosion compared with Ni deposited GaAs. The photocurrent decreases with the increasing deposition time due to a decrease of collected photons; Fermi level pinning was observed when a continuous and relatively thick layer was formed.

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Figure 1. (a) Potential vs. deposition time for Ni and Ni-Co on GaAs substrate. The potential change features marked in the rectangles indicate the formation of (oxy)hydroxide on the surface which inhibit further growth.3 (b) XPS depth profile by Ar-ion milling of Ni-Co deposited GaAs with different deposition time, which evidences the film thickness is independent of the deposition time. (c) Linear scanning voltammetries of Ni-Co deposited GaAs with a deposition time of 0.5s and 1.0s under 1.5G AM solar illumination.

Figure 1