Effect of Nitrogen-Containing Regulators on Graphene Quantum Dot/ZIF-8 Composites for Photocatalytic CO₂ Reduction

Graphene quantum dots (GQDs) with distinct optical responses were prepared from pyrene using urea, melamine, and 2,4-pyridinedicarboxylic acid as nitrogen-containing regulators and were subsequently combined with ZIF-8 for visible-light-driven CO₂ reduction. FT-IR, Raman, XRD, optical spectroscopy, representative high-magnification transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS) showed regulator-dependent structural, compositional, and optical differences. XPS detected surface nitrogen in all three GQD samples, with the highest N content in y-GQDs, while the relative N 1s component distributions differed across the series. At a nominal 4 wt% GQD addition, r-GQDs/ZIF-8 gave the highest observed mean CO and CH₄ formation rates of 23.51 ± 0.48 and 4.08 ± 0.15 μmol·g^-1·h^-1, respectively, corresponding to approximately 2.9- and 4.5-fold increases over pristine ZIF-8. This sample also showed the lowest fitted charge-transfer resistance, the highest mean photocurrent density, and the fastest qualitative time-resolved photoluminescence decay among the compared composites. Across three independent five-cycle tests, 83.78 ± 0.54% of the initial combined CO and CH₄ rate was retained. These results establish correlations among regulator identity, surface composition, optical relaxation, photoelectrochemical response, and catalytic activity, but do not determine a unique charge-transfer pathway or exclude contributions from surface basicity, CO₂ adsorption, and nominal-loading differences.