High-Efficiency Broadband C₃N₄ Photocatalysts: Synergistic Effects from Upconversion and Plasmons

A plasmon and upconversion enhanced broadband photocatalyst based on Au nanoparticle (NP) and NaYF₄:Yb³⁺, Er³⁺, Tm³⁺ (NYF) microsphere loaded graphitic C₃N₄ (g-C₃N₄) nanosheets (Au-NYF/g-C₃N₄) was subtly designed and synthesized. The simple one-step synthesis of NYF in the presence of g-C₃N₄, which has not been reported in the literature either, leads to both high NYF yield and high coupling efficiency between NYF and g-C₃N₄. The Au-NYF/g-C₃N₄ structure exhibits high stability, wide photoresponse from the ultraviolet (UV), to visible and near-infrared regions, and prominently enhanced photocatalytic activities compared with the plain g-C₃N₄ sample in the degradation of methyl orange (MO). In particular, with the optimization of Au loading, the rate constant normalized with the catalysts mass of the best-performing catalyst 1 wt % Au-NYF/g-C₃N₄ (0.032 h^–1 mg^–1) far surpasses that of NYF/g-C₃N₄ and g-C₃N₄ (0.009 h^–1 mg^–1) by 3.6 times under λ > 420 nm light irradiation. The high performance of the Au-NYF/g-C₃N₄ nanocomposite under different light irradiations was ascribed to the distinctively promoted charge separation and suppressed recombination, and the efficient transfer of charge carriers and energy among these components. The promoted charge separation and transfer were further confirmed by photoelectrochemical measurements. The 1 wt % Au-NYF/g-C₃N₄ exhibits enhanced photocurrent density (∼6.36 μA cm^–2) by a factor of ∼5.5 with respect to that of NYF/g-C₃N₄ sample (∼1.15 μA cm^–2). Different mechanisms of the photodegradation under separate UV, visible, and NIR illuminations are unveiled and discussed in detail. Under simulated solar light illumination, the involved reactive species were identified by performing trapping experiments. This work highlights the great potential of developing highly efficient g-C₃N₄-based broadband photocatalysts for full solar spectrum utilization by integrating plasmonic nanostructures and upconverting materials