Tuning Exciton–Phonon Coupling by Isotope Engineering: Regulated Charge Carrier Evolution in Carbon Nitride Photocatalysts

ABSTRACT Polymeric carbon nitride (C 3 N 4 ) has emerged as a promising photocatalytic material, yet its photoconversion efficiency remains constrained by strongly bound excitons that impede free carrier generation. While structural or electronic modifications can lower the exciton binding energy, the detrimental role of exciton–phonon coupling in nonradiative decay is often overlooked. Here, we present an isotopic strategy by integrating hydrothermally synthesized deuterated carbon dots (d‐CD) into C 3 N 4 to construct d‐CD/C 3 N 4 composites, with negligible alteration to the bandgap. Temperature‐dependent photoluminescence spectroscopy reveals that deuteration not only reduces the exciton binding energy from 72.8 to 60.9 meV, but also weakens the exciton–phonon coupling strength from 977 to 794 meV. This suppression of exciton–phonon coupling mitigates nonradiative recombination, prolonging the charge carrier lifetime from 0.117 to 0.570 ms, as confirmed by transient photovoltage measurements. The stabilized long‐lived carriers further facilitate electron extraction, evidenced by enhanced electron transfer to methyl viologen as an effective electron mediator. As a result, d‐CD/C 3 N 4 exhibits a 1.7‐fold improvement in photocatalytic hydrogen evolution relative to its non‐deuterated counterpart. These findings underscore isotope engineering as an effective approach to regulate exciton–phonon interactions and charge carrier dynamics across the ps−ms timescale, offering a new dimension for tuning the optoelectronic behavior of C 3 N 4 ‐based photocatalytic systems.