Design of a Single‐Atom In–N 3 –S site to Modulate Exciton Behavior in Carbon Nitride for Enhanced Photocatalytic Performance

Abstract Rational tailoring of the local coordination environment of single atoms has demonstrated a significant impact on the electronic state and catalytic performance, but the development of catalysts beyond noble/transition metals is profoundly significant and highly desired. Herein, the main‐group metal indium (In) single atom is immobilized on sulfur‐doped porous carbon nitride nanosheets (In@CNS) in the form of three nitrogen atoms coordinated with one sulfur atom (In–N 3 −S). Both theoretical calculations and advanced characterization investigations clearly elucidated that the single‐atomic In–N 3 –S structures on In@CNS are powerful in promoting the dissociation of excitons into more free carriers as well as the charge separation, synergistically elevating electron concentration by 2.19 times with respect to pristine CNS. Meanwhile, the loading of In single atoms on CNS is responsible for altering electronic structure and lowering the Gibbs free energy for hydrogen adsorption. Consequently, the optimized In@CNS‐5.0 exhibited remarkable photocatalytic performance, remarkable water‐splitting and tetracycline hydrochloride degradation. The H 2 production achieved to 10.11 mmol h −1 g −1 with a notable apparent quantum yield of 19.70% at 400 nm and remained at 10.40% at 420 nm. These findings open a new perspective for in‐depth comprehending the effect of the main‐group metal single‐atom coordination environment on promoting photocatalytic performance.