Unveiling Bulk Carrier Transport in LiTaGeO5 and NaTaGeO5 Photocatalysts through Integrated Crystal and Electronic Structure Analyses

The rational design of semiconductor photocatalysts with efficient bulk charge separation remains a major challenge for solar energy conversion. Herein, titanite-type LiTaGeO5 (LTGO) and NaTaGeO5 (NTGO) are investigated as model systems to elucidate how crystal structures and electronic structures govern carrier transport behavior in oxide semiconductors. Structural analyses reveal that both compounds contain quasi-one-dimensional [TaO6] octahedral chains separated by [GeO4] tetrahedra and alkali-metal cations. The substitution of local structural adjustments induces distinct local structural rearrangements within the framework while preserving the overall chain-like connectivity. Electronic structure and chemical bonding analyses demonstrate that the valence band maximum (VBM) is dominated by nonbonding O 2p orbitals, whereas the conduction band minimum (CBM) is contributed by Ta 5d–O 2p and Ge 4s–O 2s antibonding orbitals, revealing that the frontier electronic states involve multiple structural units rather than solely the [TaO6] chains. Nevertheless, both compounds exhibit preferential carrier transport along the [TaO6] chain direction due to the anisotropic connectivity of frontier-orbital-contributing structural units. Under UV irradiation, Pd-loaded LTGO and NTGO achieve hydrogen evolution rates of 36.7(4) and 59.6(3) μmol/h, respectively, with NTGO exhibiting superior photocatalytic performance. This work demonstrates that the spatial connectivity of frontier-orbital-contributing structural units, together with local structural modulation and chemical bonding characteristics, is crucial for understanding bulk carrier transport and photocatalytic behavior in oxide semiconductors.