Modulation of photo-generated solvated electrons for ammonia synthesis via facet-dependent engineering of heterojunctions

Photocatalytic reduction of N-2 to NH3 is kinetically hindered because of the formation of high-energy reaction intermediates and intrinsically low efficiency of hot-carrier dynamics at typical reactant/photocatalyst interface. Direct injection of energized solvated electrons into a reactant medium has the potential to effectively overcome these barriers. Here, we demonstrate that when the two semiconducting materials, CdS and LDHs, are structurally arranged by forming a heterojunction, solvated electrons can be effectively generated, transported, and eventually promote the photocatalytic activity of ammonia synthesis in the form of solvated electrons in an aqueous solution. In particular, when 003 or 012 crystal face of LDHs and 002 facet of CdS form a heterojunction, the built-in electric field (BIEF) promotes the bulk-charge separation (BCS) of hot carriers from LDHs and CdS to the reaction media efficiently. As demonstrated in our theoretical calculations, solvated electrons form at the H2O/photocatalyst interface and drive the reduction of N-2 and the formation of-N2H in the rate-determining step. Using in-situ DRIFTS, we were able to capture the formation of imine (-NH=NH) as one of the reaction intermediates that sheds light on the atomistic mechanism of the photocatalyzed reaction. DFT calculation also indicates that CdS@LDHs-3 (003 crystal face of LDHs and 002 facet of CdS) has the minimum relative energy for the rate-determining step and is therefore the most favorable configuration for the ammonia synthesis reaction, as confirmed by the experimental results.

Automated summary

The structurally arranged CdS and LDHs form a heterojunction to effectively generate and transport solvated electrons, thereby promoting the photocatalytic activity of ammonia synthesis. This configuration facilitates bulk-charge separation and drives the reduction of N-2, leading to improved efficiency in the rate-determining step for ammonia synthesis.