Green synthesis and DFT insight into carbon-doped ZnO nanoparticles derived from rambutan peel for enhanced photocatalytic performance

In this study, carbon-doped ZnO nanoparticles (C–ZnO) were synthesized via a dual-functional green route using rambutan (Nephelium lappaceum L.) peel extract as both a natural reducing/stabilizing agent and an in situ carbon source, eliminating the need for external dopants. This biomass-derived strategy enables waste valorization, cost reduction, and controlled carbon incorporation for enhanced photocatalytic activity. Systematic optimization of calcination temperature (400–800 °C) and time (4–8 h) revealed that C–ZnO calcined at 600 °C for 6 h exhibited optimal structural and electronic properties, including moderate carbon content (8.18 wt%) and a narrowed band gap of 3.08 eV. XRD and Raman analyses confirmed the formation of wurtzite ZnO with substitutional/interstitial carbon incorporation and embedded graphitic carbon domains (ID/IG < 1), while FTIR validated Zn–O and carbon-related functional groups. Compared with pristine ZnO photocatalysts reported in the literature, the optimized C–ZnO sample exhibited markedly enhanced photocatalytic performance, achieving up to 99.75% methylene blue degradation under UV irradiation (180 min, pH 11, 10 mg L⁻¹ MB, 50 mg catalyst). Radical scavenging experiments identified •OH and h⁺ as the dominant reactive species, and the catalyst retained over 73% of its activity after five successive cycles. Density functional theory (DFT) calculations revealed that carbon doping introduces C 2p–O 2p hybridized states, narrows the band gap, and promotes charge redistribution within the ZnO lattice, thereby improving charge separation and photocatalytic efficiency. These combined experimental and theoretical results demonstrate that biomass-derived carbon doping is an effective and sustainable strategy for tuning the electronic structure of ZnO toward high-performance photocatalytic environmental remediation.