Mesoporous Oxyhalide Aggregates Exhibiting Improved Photocatalytic Activity for Visible-Light H2 Evolution and CO2 Reduction

Oxyhalides are promising visible-light photocatalysts for water splitting and CO₂ conversion; however, those exhibiting high activity for these reactions have rarely been reported. Here, we show that using water-soluble Ti complexes as precursors in the microwave-assisted hydrothermal synthesis of the oxyhalide photocatalyst Pb₂Ti₂O_5.4F_1.2 (PTOF) resulted in the production of nanoparticulate PTOF. The primary particle size of the synthesized PTOF ranged from several tens of nanometers to several hundreds of nanometers. Using Ti-citric acid or Ti-tartaric acid complexes as precursors, the PTOF was formed as mesoporous aggregates, compared with a bulky analogue (0.5–1 μm) prepared using a TiCl₄ precursor. The PTOF prepared from Ti-citric acid complex had a particle size of 50–100 nm and showed a one-order-of-magnitude greater activity for H₂ evolution from an aqueous ethylenediaminetetraacetic acid solution with the aid of a Rh cocatalyst. An apparent quantum yield (AQY) of 15.4 ± 1.0% at 420 nm, which is the highest among the reported oxyhalide photocatalysts, was achieved under optimal conditions. Although excess particle size reduction of PTOF lowered the H₂ evolution activity, the PTOF with the smallest possible primary particle size of 15–30 nm, prepared from Ti-tartaric acid complex, showed the highest activity toward the selective reduction of CO₂ into formate in a nonaqueous environment when combined with a binuclear Ru­(II) complex. The CO₂ reduction AQY was 10.4 ± 1.8% at 420 nm, a record-high value among metal-complex/semiconductor binary hybrid photocatalysts. This study highlights the importance of morphological control of oxyhalides for realizing their full potential as photocatalysts for artificial photosynthesis.