Solar energy converted hydrogen is becoming much more attractive as an energy storage material recently. We are focusing on the hydrogen generation by using photoelectrochemical (PEC) water splitting as a solar energy conversion. GaN was believed to be a suitable material for evaluate this method because GaN has good chemical stability and high crystal quality. However, it is difficult to maintain stable photocurrent using the n-type GaN photoanode due to anodic corrosion in PEC reactions although GaN has good chemical stability. In order to overcome this corrosion problem, NiO-loading was proposed, however, the mechanisms of this NiO-loading on n-type GaN to improve the corrosion was obscured. Thus, we discuss the mechanisms and propose the model in this report.
The experiments were performed with using n-type GaN (C.C. 2.0×10¹⁷ cm^-3) as a working electrode grown by MOVPE. The NiO was deposited as NiO island shape by Ni(OH)₂ dispersed solution. The counter electrode was Pt and the light was Xe-lamp. The photocurrent density as functions of time was measured for 180 min.
Firstly, we investigated the dependence of electrolyte. The NiO worked only in basic solutions, and NiO dissolved in acidic solutions. Secondly, we indicated the differences of the properties of PEC between the NiO islands and NiO layer on n-type GaN. The NiO layer showed rarely reaction due to the high resistivity. In contrast, the NiO islands showed excellent catalytic properties. Thirdly, the effect of NiO loaded ratio for the photocurrent density was evaluated. When the NiO loaded ratio was smaller, the effect of suppression of anodic corrosion became weaker. Finally, the color of the loaded NiO was observed. The color of NiO changed from transparent to black after the PEC reaction. It is proposed that the NiO on GaN changes from Ni²⁺ to Ni³⁺ during the PEC reaction. This phenomenon is probably photochromism due to the hole transport from n-type GaN to NiO. Considering all these results, we propose the band diagram between GaN and NiO. The important point of this diagram is that the valence band of semiconductor photoanode should be lower than that of loaded catalytic material. In addition, OH^- concentration has an important role for PEC reaction probably because of the carrier transfer between the catalytic material and water. When the band diagram and the electrolyte condition are similar to this, the other catalytic material also expects to show similar effect.