Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.118
Publication date: 18th July 2019
In the photosynthesis-II (PS-II), light absorber and water oxidation reaction center are separated. According to this PS-II system, we investigated the separated system of GaN nanowires (NWs) photo-absorber and NiO water oxidation electrocatalyst. The NWs of photo-absorber for our experiments were semi-polar GaN compared to those of GaN single crystal.
To characterize the electrochemical properties of semi-polar GaN NWs, a working electrode was prepared from MOVPE grown NW arrays using a vapor-liquid-solid mechanism. The NiO was deposited by Ni(OH)2 dispersed solution. The counter electrode was Pt and the light was a Xe-lamp with an intensity of 500 mW/cm2. The electrolyte was 1.0 M NaOH (pH 13.4). The photocurrent densities were measured for 180 min without bias.
We measured the GaN NW arrays using cyclic voltammetry with light illumination to clarify the photoelectrochemical properties. Voc, turn-on voltage, and cathodic current of GaN NWs had completely different results compared to those of GaN single crystal. Especially, cathodic current of GaN NWs was significantly larger than those of GaN single crystal. The resulting which was electrochemical properties likely indicated that the surface absorption species are different between the GaN NWs and single crystal due to the surface orientations.
The photocurrent density of GaN NWs without NiO can be observed under the illumination. However, the photocurrent density was observed to decrease rapidly once the reaction started. The surface changed from having a frosted glass appearance before to a mirror-like appearance after the reaction. It means that the GaN NWs dissolved into the electrolyte during the PEC reaction. GaN NWs with NiO showed similar result for that without NiO. The NiO was loaded on GaN NWs thus, it is not good condition to work as catalyst of GaN NWs. GaN NWs with and without NiO were completely dissolved after the 180 min reactions from SEM image. From these results, we see that NiO didn’t work to prevent the anodic corrosion like with GaN single crystals from these results.
As shown here, the semi-polar GaN NWs sample shows completely different photoelectrochemical and electrochemical properties compared to GaN single crystal because of their surface crystal orientation.
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