Band Gap Narrowing of Zinc Orthogermanate by Dimensional and Defect Modification

Dimensional reduction and defect control are powerful techniques for enhancing the physical and chemical properties of raw materials. Herein, we report the new type of sheetlike or two-dimensional-like zinc orthogermanate (Zn₂GeO₄, denoted as S-ZGO) with a one-step hydrothermal reaction and its application to photocatalytic water splitting. S-ZGO is directly grown along the surface of the Zn foil, since the growth rate of a crystal facet can be modified through the restricted reaction of dissolved precursor (GeO₂) with a solid precursor (Zn foil) during the hydrothermal reaction. For further modification, the oxygen vacancies are introduced on the surface of S-ZGO using thermal hydrogen treatment and photodeposition-driven low amount of Pt/RuO₂ co-catalysts loading. Notably, this reduced dimension decreases the band gap to 4.09 eV for S-ZGO (from 4.5 eV for the bulk), and the hydrogenation of S-ZGO further decreases the band gap to 3.88 eV. The origin of band gap narrowing is demonstrated with the density functional theory showing increased density of states at the edge of the conduction band (CB) and valence band (VB), and a new defect level between the CB minimum and VB maximum. As a possible application, we demonstrate that S-ZGO/H₂ loaded with Pt/RuO₂ exhibits the H₂ rate of 167.0 μmol h^-1 g^-1 and the O₂ rate of 83.0 μmol h^-1 g^-1, a8 times those of the rodlike ZGO photocatalysts.