Deciphering the photocatalytic hydrogen generation process of Fresnoite Ba2TiGe2O8 by electronic structure and bond analyses

In addition to enhancing the activity of already-known photocatalysts, developing new ones is always desired in photocatalysis, giving more opportunities to approach practical applications. Most photocatalysts are composed of d0 (i.e. Sc3+, Ti4+, Zr4+) and/or d10 (i.e. Zn2+, Ga3+, In3+) metal cations, and a new target catalyst is Ba2TiGe2O8 containing both. Experimentally, it exhibits a UV-driven catalytic H2 generation rate of 0.5(1) μmol h−1 in methanol aqueous solution, which could be enhanced to 5.4(1) μmol h−1 by loading 1 wt% Pt as the cocatalyst. Most interestingly, theoretical calculations together with analyses on the covalent network could help us to decipher the photocatalytic process. The electrons in O 2p non-bonding orbitals are photo-excited to either Ti–O or Ge–O anti-bonding orbitals. The latter interconnect with each other to form an infinite two-dimensional network for electron migration to the catalyst surface, while the Ti–O anti-boding orbitals are rather localized because of the Ti4+ 3d orbitals; thus, those photo-excited electrons mostly recombine with holes. This study on Ba2TiGe2O8 containing both d0 and d10 metal cations gives an interesting comparison, suggesting that a d10 metal cation is probably more useful to construct a favorable conduction band minimum for the migration of photo-excited electrons.