Electronegativity-correlated selectivity of methanol versus methane in CO2 photoreduction on doped SnO2/TiO2 S-scheme heterojunctions.

Photocatalytic CO2 reduction into high-value fuels represents an effective strategy for achieving carbon neutrality. However, selective generation of multielectron products (CH4 or CH3OH) remains challenging. Herein, based on first-principles calculations, a SnO2/TiO2 heterojunction featuring a coherent heterointerface and a built-in electric field (BIEF) from TiO2 to SnO2 is rationally constructed. This S-scheme heterojunction accelerates charge separation and enhances CO2 adsorption and activation on the TiO2 reduction phase by elevating the d-band center. Further investigations into C and F doping of the TiO2 component reveal a strong correlation between dopant electronegativity and product selectivity. Low-electronegativity C doping enriches surface electrons, stabilizing the *CH3 intermediate and thermodynamically favoring its deep hydrogenation toward CH4. Conversely, high-electronegativity F doping induces surface electron depletion, which thermodynamically suppresses *CH3 formation and thereby locks the product at CH3OH; the water-bridged hydrogen-bond chain F···HOH···HOCH3 may further facilitate CH3OH desorption as a secondary synergistic effect. Furthermore, kinetic analysis demonstrates that water-assisted proton transfer alleviates strain in the transition state and lowers the activation barrier, consequently enhancing the reaction reactivity. This study elucidates the regulatory role of dopant electronegativity in heterojunction photocatalysis, providing valuable theoretical guidance for the rational design of highly selective CO2 reduction photocatalysts.