Hot Electron Back-Injection from Plasmonic Au Nanoparticles Boosts Photocatalytic CO ₂ Reduction over Boron-Doped Spherical g-C ₃ N ₄

Converting CO2 into high-value fuels through the synergistic interplay of donor-acceptor (DA) structure and photothermal effects presents a promising strategy for enhancing the carbon cycle and mitigating greenhouse gas emissions. In this work, a carbon nitride (g-C3N4) based photocatalyst, designated Au/BMNS-x, was engineered to integrate both a DA structure and Localized Surface Plasmon Resonance (LSPR) by simultaneously incorporating boron doping and Au nanoparticles (NPs) into g-C3N4. The plasmonic Au NPs generate a pronounced photothermal effect under irradiation, significantly elevating the local reaction temperature during CO2 photoreduction. Real-time infrared thermography demonstrated that Au/BMNS-2 reached a stabilized surface temperature of 148.1℃, which is 1.17 times higher than that of BMNS and 2.06 times greater than pristine g-C3N4. Under optimized conditions, Au/BMNS-2 exhibited a CO production rate 5.99 times higher than that of pristine g-C3N4, along with excellent structural stability and reusability over multiple cycles. In situ X-ray photoelectron spectroscopy (XPS) and femtosecond transient absorption spectroscopy (fs-TAS) provide direct evidence of hot electron back-injection from plasmonic Au NPs into BMNS, enriching electron density around the catalytic active sites. Crucially, the DA structure, synergistically coupled with the LSPR effect, enables highly efficient separation and ultrafast transfer of photogenerated charge carriers, thereby significantly enhancing overall photocatalytic performance. The reaction mechanism was further elucidated through in situ Fourier transform infrared spectroscopy (FT-IR) spectroscopy and density functional theory (DFT) simulation. This study offers a rational design strategy for multifunctional photocatalysts that harness both plasmonic and photothermal effects, opening new avenues for high-efficiency solar-driven CO2 conversion technologies.