Improvement of Photocatalytic Water Splitting Activity by Facet-Selective Loading of Ultrafine Rhodium–Chromium Mixed-Oxide Cocatalyst
Authors: Kawawaki Tokuhisa , Oguchi Sota , Negishi Yuichi 
Published: December 2024

<Background>
Water-splitting photocatalysts capable of producing hydrogen from light and water have attracted much attention toward the realization of a carbon neutrality. In such semiconductor photocatalysts, there are crystal planes where excited electrons and holes can easily reach.1) Therefore, further enhancement of water splitting activity can be expected if the appropriate co-catalyst can be selectively loaded onto the appropriate crystal face. Although the conventional method of supporting cocatalysts by photoelectrodeposition can selectively support cocatalysts on crystalline surfaces, it is difficult to control the particle size due to the principle of the preparation method. On the other hand, we have developed a method to support ultrafine particles (~1 nm in diameter) on photocatalysts (cluster deposition method) and have succeeded in improving their water splitting photocatalytic activity.2,3) However, in this method, the fine auxiliary catalysts were loaded non-selectively on the crystalline surface, and the miniaturization effect was not efficiently utilized. In this study, we attempted to further improve the water splitting activity by developing a "crystal-facet-selective ultrafine catalyst loading method". A rhodium (Rh) complex, which acts as a hydrogen production assistant catalyst, is selectively loaded onto a crystal face where excited electrons can easily reach, while maintaining the fine state of the complex.
<Experimental>
Eighteen-facet strontium titanate (STO) with exposed {100} (hydrogen-producing) and {110} (oxygen-producing) planes was used as a photocatalyst support, and glutathionate-protected Rh complex (Rh:SG) was used as a catalyst precursor. First, a chromium oxide film was formed on STO, and then Rh:SG was adsorbed. In this process, 1) protection of the {110} surface by alcohol and 2) enhancement of adsorption on the {100} surface by pH adjustment and photo irradiation were attempted. The target photocatalyst was then prepared by calcination and photoirradiation (this method). As a comparison, photocatalysts supported by the conventional photoelectrodeposition method and the cluster adsorption method were also evaluated.
<Results and Discussion>
Transmission electron microscopy (TEM) revealed that fine Rh particles (average particle size: 1.2 ± 0.2 nm) were observed on the {100} plane of the STO prepared by this method, whereas these particles were hardly observed on the {110} plane, indicating that Rh particles were successfully loaded in a fine and crystal facet-selective manner (Fig. 1). From Fourier transform infrared spectroscopy measurements, it was confirmed that the alcohol added during the preparation selectively adsorbed on the {110} plane on STO, which is thought to have suppressed the adsorption of Rh:SG on the {110} plane. It is also suggested that photo-reductive desorption of the ligand occurred by photo-irradiation during Rh:SG adsorption, which promoted Rh:SG loading on the {100} plane where the excited electrons reach. The photocatalysts prepared by this method showed 2.6- and 1.4-fold higher water splitting activity than those prepared by conventional photodeposition and cluster adsorption, respectively (Fig. 1). This increase in water splitting activity is assumed to be induced by 1) the increase in specific surface area due to the miniaturization of Rh cocatalyst and 2) the enhancement of charge separation due to the selective loading of cocatalyst on the crystalline surface.4)
1) K. Mu et al., Energy Environ. Sci., 2016, 9, 2463‒2469.
2) D. Yazaki, T. Kawawaki, Y. Negishi et al., Small, 2023, 19, 2208287.
3) D. Yazaki, T. Kawawaki, Y. Negishi et al., Energy Adv., 2023, 2, 1148‒1154.
4) D. Hirayama, T. Kawawaki, Y. Negishi et al., submitted.

Figure 1