Metal sulfides are highly active photocatalysts for water reduction to form H2 under visible light irradiation, whereas they are unfavorable for water oxidation to form O2 because of severe self-photooxidation (i.e., photocorrosion). Construction of a Z-scheme system is a useful strategy to split water into H2 and O2 using such photocorrosive metal sulfides because the photogenerated holes in metal sulfides are efficiently transported away. Here, we demonstrate powdered Z-schematic water splitting under visible light and simulated sunlight irradiation by combining metal sulfides as an H2-evolving photocatalyst, reduced graphene oxide (RGO) as an electron mediator, and a visible-light-driven BiVO4 as an O2-evolving photocatalyst. This Z-schematic photocatalyst composite is also active in CO2 reduction using water as the sole electron donor under visible light.

We report herein remarkable improvement of activity and stability of an Au25-loaded BaLa4Ti4O15 water-splitting photocatalyst. We first examined the influence of refining the gold cocatalyst on the individual reactions over the BaLa4Ti4O15 photocatalyst in this water-splitting system. The results revealed that refining the gold cocatalyst accelerates not only the hydrogen generation reaction, but also oxygen photoreduction reaction, which suppresses the H2 generation via photoreduction of protons. This finding suggests that photocatalytic activity will be enhanced if the O2 photoreduction reaction can be selectively suppressed by covering Au25 with a Cr2O3 shell which is impermeable to O2 but permeable to H+. Then, we developed new method for the formation of the Cr2O3 shell onto Au25. Our method utilizes the strong metal–support interaction between them. Water-splitting photoactivity of Au25–BaLa4Ti4O15 was improved by 19 times under an optimized coverage of the Cr2O3 shell. The Cr2O3 shell also elongated the lifetime of the photocatalysts by preventing the agglomeration of Au25 cocatalysts.

Glutathione-protected Au25 clusters were used to load monodisperse gold nanoclusters (1.2 ± 0.3 nm) onto BaLa4Ti4O15 to create photocatalysts. The photocatalytic activity of the resulting material for water splitting was determined to be 2.6 times higher than that of catalysts loaded with larger gold nanoparticles (10-30 nm) via conventional photodeposition.

Doped NaTaO (NaTaO :A, where A=Mg, Ca, Sr, Ba, or La) has arisen as a highly active photocatalyst for CO reduction to simultaneously form CO, H , and O using water as the electron donor when used with an Ag cocatalyst, under UV irradiation, and with 1 atm (0.1 MPa) of CO . The ratio of the number of reacted electrons/holes was almost unity, indicating that water was consumed as the electron donor. A liquid-phase reduction method for loading of the Ag cocatalyst was superior to photodeposition and impregnation methods. The Ag cocatalyst-loaded NaTaO :Ba was the most active photocatalyst in water with no required additives. The addition of bases, such as hydrogencarbonate, was effective to enhance the CO formation for Mg-, Ca-, Sr-, Ba-, and La-doped NaTaO photocatalysts with an Ag cocatalyst. Ca- and Sr-doped NaTaO photocatalysts showed especially high activity along with the Ba-doped photocatalyst in the aqueous NaHCO solution. The selectivity for the CO formation [CO/(CO+H )] on Ca-, Sr-, and Ba-doped NaTaO photocatalysts with Ag cocatalyst reached around 90 %.

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.