Ball-Milled P-CuGaO2 Particles As a Semitransparent Scaffolding Material for Dye-Sensitized Molecular Photocathodes
Authors: Kumagai Hiromu , Yamazaki Yasuomi , Ishitani Osamu 
Published: February 2024

Dye-sensitized photocathodes using a metal complex as a photosensitizer are attractive candidates for catalytic material conversion reactions such as H2 evolution and CO2 reduction (Figure A). Band position, conductivity, and morphology of the p-type semiconductor scaffold electrode significantly affect the electron injection into the metal complex and the subsequent reaction process. Consequently, the properties of the semiconductor material can be one of the important factors the catalytic activity of the photocathodes.
We have previously reported p-type semiconductor CuGaO2 for the scaffold materials that exhibits a higher photocurrent and a positive onset potential compared to the conventionally used NiO, when combined with a Ru-tris-bipyridine-type photosensitizer[1]. Owing to the thermophysical property of CuGaO2, the typical solid-state synthesis of CuGaO2 particles needs high-temperature calcination with around 1373 K under an inert atmosphere, resulting in the large particle size of several microns and the limited conductivity and small surface area. Therefore, the photocathode activity is still insufficient. Other processes such as solvothermal synthesis have been reported, but it is still difficult to control the particle size, shape and chemical state.
In this study, we examined the ball milling to synthesize nano-sized fine CuGaO2 particles and to prepare a semitransparent scaffold electrode for dye-sensitized photoelectrochemical reaction. Planetary ball milling has been reported as the means to obtain fine semiconductor particles[2], while it is necessary to adjust the processing conditions and perform detailed analysis because there is a possibility of damaging the semiconductor and impairing its physical properties.
The XRD pattern, UV-visible absorption spectrum, XPS, and XAFS measurements of the ball-milled CuGaO2 particles revealed that there was no obvious deterioration due to the treatment.The SEM image of the ball-milled CuGaO2 electrode implied that the particles, which have been miniaturized about less than several hundred nm by the treatment, formed a dense layer, showing the semi-transparency of the electrode (Figure B). Detailed observation also proved that the ball-milled particles keep the crystal habit. The photocathode with ball-milled CuGaO2 modified with Ru-tris-bipyridine-type complex exhibited a reduction current by photoexcitation of the photosensitizer, and the current value exceeded those of the photocathodes using an untreated CuGaO2 electrode and a NiO electrode under the same conditions. The results suggest that the the miniaturization improved the characteristics of the photocathodes. The current study showed a way to mass-produce high-performance semiconductor scaffold electrodes by a simple ball-milling process.
[1] H. Kumagai, O. Ishitani, et al., Chem. Sci.
2017, 8, 4242.; C. Windle, H. Kumagai, R. Abe, O. Ishitani, V. Artero, et al., J. Am. Chem. Soc., 2019, 141, 9593. [2] K. Okuno, H. Kumagai, H. Kato, et al., Sustain.
Energy Fuels, 2022,6, 1698.

Figure 1