Publication date: 15th December 2014
Surface anodic corrosion of n-type GaN photo-anode is a serious problem for the photoelectrochemical water splitting. Recently, high stability and high efficiency were reported by NiO loading [1], however, the role of NiO have not been clarified. In this report, we investigated the effects of the electrolytes for n-type GaN with and without NiO loading.
The working electrode was n-type GaN (C.C. 2.0×1017 cm-3) grown on (0001) sapphire substrates by MOVPE. The NiO were deposited on the GaN surface and annealed. The counter electrode was Pt and the light intensity was controlled as 100 mW/cm2 by Xe-lamp. The electrolytes were 0.5 mol/L H2SO4 (pH 0.5), 1.0 mol/L KOH (pH 13.9), and 1.0 mol/L NaOH (pH 13.6).
The photocurrent changes of GaN with reaction time were investigated using 3 types of electrolytes. The n-type GaN photo-anodes were treated with and without NiO loading. The photocurrent density and stability were improved with the loading of NiO in NaOH as reported [1]. The time dependence of photocurrent was different between KOH and NaOH. The photocurrent of NiO loaded GaN was rapid decreasing from the start to 60min in KOH, however, the photocurrent of NiO loaded GaN was stable during 180min in NaOH. The surfaces were mirror-like for the NiO loaded GaN using basic solutions after 180min. In contrast, the photocurrents in H2SO4 showed rapid decrease and were similar for the n-type GaN with and without NiO loading. The surfaces changed frosted glass-like for the case of basic solutions without NiO and of H2SO4 electrolyte. The NiO on the GaN surface was completly desolved using H2SO4. These results suggest that the corrosion and stability of NiO loaded GaN are related to the electrolyte. The NaOH electrolyte is the best condition to keep the initial effect of NiO co-catalyst on n-type GaN in our experiments.
[1] T. Hayashi et al., Jpn. J. Appl. Phys. 51, 112601 (2012).
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