Crystal-Structure-Dependent Photocatalytic Redox Activity and Reaction Pathways over Ga₂O₃ Polymorphs

Differentiated crystal structures generally affect the surface physicochemical properties of catalysts, causing variety in catalytic activity between polymorphs. However, the underlying mechanism has not been completely revealed, especially the influence of surface physicochemical properties on photocatalytic redox activity and the reaction mechanism. In this work, we reveal the mechanism of surface redox properties on different crystal forms of gallium oxide from a molecular level. α-Ga₂O₃ and β-Ga₂O₃ exhibit a slight difference in catalytic oxidation of organic pollutants due to comprehensive influencing factors, including their valence band position, reactive oxygen species, and pore structure properties related to the adsorption-reaction-desorption process. But the catalytic reduction ability of CO₂ is obviously different due to the large differences of interaction between the surface of crystal structures and CO₂ molecules, which are critical to determine the catalytic performance and reaction pathways. The enhanced adsorption and activation of CO₂ on the α-Ga₂O₃ surface could promote the reduction reaction efficiency. Moreover, the large energy barrier of CH₂* formation on β-Ga₂O₃ makes the formation of methane (CH₄) relatively difficult compared to that on α-Ga₂O₃. The yield rate of CH₄ (1.8 μmol·g^-1·h^-1) on α-Ga₂O₃ is three times better than that on β-Ga₂O₃ (CH₄: 0.6 μmol·g^-1·h^-1). The current findings can offer novel insights into the understanding of crystal-structure-dependent photocatalytic performances and the design of new catalysts applied in energy conversion and environmental purification by crystal structure-tuning.