Ga₄B₂O₉: An Efficient Borate Photocatalyst for Overall Water Splitting without Cocatalyst

Abstract

Borates are well-known candidates for optical materials, but their potentials in photocatalysis are rarely studied. Ga³⁺-containing oxides or sulfides are good candidates for photocatalysis applications because the unoccupied 4s orbitals of Ga usually contribute to the bottom of the conducting band. It is therefore anticipated that Ga₄B₂O₉ might be a promising photocatalyst because of its high Ga/B ratio and three-dimensional network. Various synthetic methods, including hydrothermal (HY), sol–gel (SG), and high-temperature solid-state reaction (HTSSR), were employed to prepare crystalline Ga₄B₂O₉. The so-obtained HY-Ga₄B₂O₉ are micrometer single crystals but do not show any UV-light activity unless modified by Pt loading. The problem is the fast recombination of photoexcitons. Interestingly, the samples obtained by SG and HTSSR methods both possess a fine micromorphology composed of well-crystalline nanometer strips. Therefore, the excited e^– and h⁺ can move to the surface easily. Both samples exhibit excellent intrinsic UV-light activities for pure water splitting without the assistance of any cocatalyst (47 and 118 μmol/h/g for H₂ evolution and 22 and 58 μmol/h/g for O₂ evolution, respectively), while there is no detectable activity for P25 (nanoparticles of TiO₂ with a specific surface area of 69 m²/g) under the same conditions