ZIF-8-on-MIL-68(Ga) Derived S-scheme Heterojunction for Photocatalytic Reduction CO2 to CH4 in Pure Water

Abstract ZnGa₂O₄ possess the potential for simultaneously reduction CO₂ and oxide water with photogenerated electrons and holes, however, pure ZnGa₂O₄ suffers from wide bandgap and sluggish migration kinetics of charge carriers. In this work, the MOF-on-MOF precursors of ZIF-8-on-MIL-68(Ga) were first synthesized and in situ converted into the ZnGa₂O₄/ZnO heterostructure. After loading of Au cocatalyst, the ternary Au/ZnGa₂O₄/ZnO photocatalyst could reduce CO₂ into CH₄ in pure water without any sacrificial reagent, reaching a CH₄ yield of 21.1 umol g⁻¹ h⁻¹ as well as high selectivity of 93.8%. The unique MOF-on-MOF derived hollow structure provides large specific surface area, abundant heterogeneous interfaces as well as increases the light harvesting ability. Moreover, experimental results and DFT calculation reveal that the S-scheme heterojunction deriving from ZIF-8-on-MIL-68(Ga) could efficiently promote charge separation and enable strong photoredox ability while the decoration of Au cocatalyst could further lower the Gibbs free energy barriers for the rate-determining step, thereby leading to CH₄ production with high activity and selectivity. This study verified the strategy of converting MOF-on-MOF composites into S-scheme heterostructures was feasible, which could be extended to other MOF-on-MOF precursors to exploit the synergistic effects between different photocatalysis.