Enhancement of bifunctional photocatalytic activity of boron-doped g-C₃N₄/SnO₂ heterojunction driven by plasmonic Ag quantum dots

Enhancement of photocatalytic charge transfer with reduced electron–hole (e⁻ + h⁺) recombination toward the targeted chemical reaction is a challenging task. In this study, we utilized boron (B) dopant as impurity levels, and the plasmonic Ag quantum dots (QDs), and g-C₃N₄/SnO₂ heterojunction nanocomposite under visible light (λ = 385–740 nm) as a nanocomposite photocatalytic system for energy and environmental applications. The introduction of B dopant with optimal level (BCN_1.0) of 1 wt% resulted in the fast migration of photoexcited electrons from valence band to the conduction band of g-C₃N₄ and reduced the migration distance of the electrons from valence band to the conduction band and improved surface redox reactions. The heterojunction formed with SnO₂ nanorods and nanoparticles enhanced the electron–hole transfer rate across the interface of the heterojunction (BCNS₁₅) with 15 wt% and improved the photocatalytic activity during redox reactions. The introduction of plasmonic Ag QDs (1 wt%) as a co-catalyst improved the H₂ generation efficiency (BCNS–Ag_1.0). Ultraviolet photoelectron spectroscopy results confirmed that the charge transfer process was initiated via the Z-scheme mechanism. Photoluminescence spectra identified the reduction of electron–hole recombination that was responsible for improved catalytic efficiency in the optimized heterojunction nanocomposite. The synergetic effect of B dopant, the visible light sensitization effect and the co-catalytic effect of the Ag QDs, the formation of a heterojunction of the B-g-C₃N₄/SnO₂/Ag (BCNS–Ag_1.0) nanocomposite resulted in 2.5 times higher methyl orange dye degradation efficiency which is 65.6% compared to pristine g-C₃N₄ (CN) 25.9%. Further, the photocatalysts were tested for photocatalytic H₂ generation, the optimized BCNS–Ag_1.0 nanocomposite heterojunction showed 3.2 times higher H₂ generation activity (10.2 mmol/h/g_cat) than pristine CN(3.1 mmol/h/g_cat) through Z-scheme mechanism. Finally, the BCNS–Ag_1.0 nanocomposite demonstrated excellent stability toward methyl orange degradation as well as H₂ generation for at least 4 cycles. Therefore, the BCNS–Ag_1.0 nanocomposite is a promising photocatalytic system for scale-up synthesis.