Switching charge transfer direction in heterojunctions: A competition of two driving forces

The charge transfer pathway and corresponding heterojunction type rely on the combined effects of the material's inherent properties and the interface structure. Herein, we report a crystal facet-dependent switching of charge transfer direction in TiO₂/Ni₃S₄ composites via the competition of two driving forces (work function difference and interfacial bonding). Experimental and theoretical calculations demonstrate that when the dominant contact facet of Ni₃S₄ transitions from (022) to (113), the density of interfacial Ti-S bonds significantly increases. These Ti-S bonds, polarized by the electronegativity difference between S (2.58) and Ti (1.54), generate a driving force competing with the work function driving force. This competition can determine the charge transfer direction and the heterojunction type, leading to distinct active species generation: TiO₂/Ni₃S₄-3 (S-scheme) produce ^•O₂^-, ^•OH, and ^•SO₄^-, while TiO₂/Ni₃S₄-7 (type-II) generates only ^•OH and ^•SO₄^-, and ultimately resulting in different degradation pathways with ofloxacin as the model pollutant. This work highlights the important role of interfacial bonding in determining charge transfer direction, and introduces a "driving force competition" concept for regulating heterojunction types.