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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. [ABSTRACT FROM AUTHOR]