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

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.