Brownmillerite-Type Crystalline Ca2FeCoO5 Ultrasmall Particles with Single-Nanometer Dimensions as an Active Cocatalyst for Oxygen Photoevolution Reaction

Brownmillerite-type Ca₂FeCoO₅ (CFCO) is one of the most effective catalysts for oxygen evolution reaction (OER), comparable with noble metal oxides. In this study, crystalline CFCO ultrasmall particles with nanometric dimension are synthesized by a reverse micelle method on TiO₂ nanoparticles. The particle size decreases with decreasing molar ratio of water to surfactant. The precursors of CFCO must be calcined after loading on TiO₂ nanoparticles to achieve CFCO ultrasmall particles with several nanometers in size. Interaction between the precursors and TiO₂ is speculated to suppress aggregation of the precursors during calcination. The photocatalytic activity of TiO₂ for OER is improved by loading of CFCO ultrasmall particles with 5 nm, whereas the activity decreases by loading of CFCO with more than 15 nm. Photocatalytic activity of the most active CFCO/TiO₂ is comparable to that of RuO₂/TiO₂. Both the lower edge of conduction band and higher edge of valence band of CFCO are lower and higher than those of TiO₂, respectively, leading to transfer of excited holes and electrons transferred to CFCO and recombination. When the particle size of CFCO becomes several nanometric dimensions, the transferred holes rapidly reach the surface of CFCO and oxidize water molecules before recombination with electrons.