Controlled Loading of Small Aun Clusters (n = 10–39) onto BaLa4Ti4O15 Photocatalysts: Toward an Understanding of Size Effect of Cocatalyst on Water-Splitting Photocatalytic Activity

We have recently succeeded in loading extremely small monodisperse gold clusters (1.2 ± 0.3 nm) as cocatalysts on a water-splitting BaLa 4 Ti 4 O 15 photocatalyst using a glutathione-protected Au 25 cluster (Au 25 (SG) 18 ) as a precursor; an improved photocatalytic activity of 2.6-fold was obtained when compared with that of BaLa 4 Ti 4 O 15 onto which large gold nanoparticles (10–30 nm) were loaded. In the current study, the controlled loading of a series of ultrasmall Au n clusters onto BaLa 4 Ti 4 O 15 using various Au n (SG) m clusters ( n = 10, 15, 18, 22, 25, 29, 33, 39) was examined. The results revealed that the use of a highly stable cluster as a precursor is essential for achieving control over the loading of the gold clusters. Additionally, the origin of the improved photocatalytic activity owing to the ultra miniaturization of the cocatalyst was reconsidered herein based on the photocatalytic activities of the obtained photocatalysts. The results strongly suggested that the activity per gold atom on the surface decreased significantly owing to the ultra miniaturization of the cocatalyst and that the origin of the improved activity is the increase in the number of surface gold atoms at a rate that overcomes the reduction effect in their activity.