(Invited) Improvement of Photocatalytic Water Splitting Activity By Loading of Ultrafine Rhodium–Chromium Mixed-Oxide Cocatalyst
Authors: Negishi Yuichi 
Published: December 2024

For the improvement of the activity of water-splitting photocatalysts (WSP), it is effective to load metal/metal oxide nanoparticles (NPs), called cocatalysts. However, it is difficult to precisely control their size with the conventional methods. In this study, we synthesized ultrafine metal nanoclusters (NCs) with rhodium (Rh) and platinum (Pt) as promising metal species for hydrogen (H2) evolution reactions (HER), and used them as cocatalysts to enhance the activity of WSPs. The results showed that BaLa4Ti4O15 (BLTO; UV-light-driven photocatalyst) loaded with Rh NC and SrTiO3:Rh (visible-light-driven photocatalyst) loaded with Pt NC exhibited 1.5 and 1.4 times higher water splitting activity than that of prepared by conventional methods, respectively. From transmission electron microscope (TEM) images, it was confirmed that these metal NCs as cocatalysts were loaded finer and more monodisperse than the cocatalyst prepared by conventional methods. These results indicate that metal NCs can contribute to the activation of various photocatalysts by miniaturizing the cocatalysts.
<Introduction>
To achieve carbon neutrality, WSP, which can produce H2 from water and sunlight, have attracted much attention in recent years. In general, loading metal NPs, called cocatalysts, is effective in increasing the activity of WSP. Since the cocatalyst works as active sites for the reaction, it is necessary to enhance the functionality of the cocatalyst to create highly active WSP. However, it is difficult to precisely control the particle size and the electronic state of the conventional (photodeposition (PD) and impregnation (IMP)) methods 1,2,3). On the other hand, aggregates of metal atoms, called metal NCs, exhibit specific properties different from those of the bulk metal and can be synthesized while controlling their particle size and ligands. In this study, to improve their activity, we synthesized metal NCs using Rh and Pt (Rh NC and Pt NC), which are promising metal species for HER and loaded them on UV- and visible-light-driven photocatalysts, respectively.
< Results and discussion>
<Comparison of activity of BaLa4Ti4O15 (Rh NC vs. IMP method)>
We measured the water-splitting activity of Rh NC and IMP method, and found that Rh NC showed 1.5 times higher activity than IMP method (Fig. 1). Therefore, the particle size of the Rh cocatalyst in each method was confirmed from TEM images. As a result, it was confirmed that Rh NC (1.3 ± 0.3 nm) loaded finer and more monodisperse than the IMP method (3.0 ± 2.3 nm).
<Comparison of activity of SrTiO3:Rh (Pt NC vs. PD method)>
We measured the H2 evolution activity of the Pt NC and PD method, and found that Pt NC was 1.4 times higher activity than the PD method. Therefore, the particle size of the Pt cocatalyst in each method was confirmed from TEM images. As a result, it was confirmed that this method (1.7 ± 0.3 nm) was loaded finer and more monodisperse than the PD method (2.9 ± 0.6 nm).
<Factors of enhanced activity>
This improvement of the activity can be attributed to the increase in the number of active sites due to the increase in specific surface area resulting from the finer cocatalysts. Indeed, in Rh2–xCrxO3/BLTO and Pt NC/SrTiO3:Rh, the active sites with Rh and Pt NCs were found to be 2.1 and 1.7 times higher, respectively, than those with the conventional method. This factor suggest that the activity is increased when metal NCs are used as cocatalysts.
<Conclusions>
Metal NCs can load cocatalysts finer and more monodisperse than conventional (PD and IMP) methods. This miniaturization of the cocatalyst is considered to have increased the specific surface area of the cocatalyst and increased its active site, thereby increasing its activity. In addition, metal NCs were found to be useful for both UV- and visible-light-driven photocatalysts, indicating their high versatility.

Figure 1