Graphitic carbon nitride supported silver nanoparticles (AgNPs/g-C3N4): synthesis and photocatalytic behavior in the degradation of 2,4-dichlorophenoxyacetic acid

Graphitic carbon nitride supported silver nanoparticles (AgNPs/g-C₃N₄) with 1%, 3%, and 5% AgNPs were successfully synthesized by an "ex situ" method with ultrasound of a mixture of AgNP solution and g-C₃N₄. The AgNP solution was prepared by chemical reduction with trisodium citrate, and g-C₃N₄ was synthesized from the urea precursor. The supported nanoparticles were characterized by X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption–desorption (BET), Fourier transformation infrared (FTIR) and Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), photoluminescence spectroscopy (PL), electron paramagnetic resonance (EPR) and electrochemical impedance spectroscopy (EIS) Nyquist plots. The visible light-driven photocurrent measurement was performed by three on–off cycles of intermittent irradiation. The analyses show that AgNPs were evenly dispersed on g-C₃N₄, and have sizes ranging from 40 to 50 nm. The optical properties of the AgNPs/g-C₃N₄ material were significantly enhanced due to the plasmonic effect of AgNPs. The photocatalytic activity of catalysts was evaluated by 2,4-D degradation under visible light irradiation (λ > 420 nm). In the reaction conditions: pH 2.2; C_o (2,4-D) 40 ppm; a m/v ratio of 0.5 g L⁻¹, AgNPs/g-C₃N₄ materials exhibit superior photocatalytic activity compared to the pristine g-C₃N₄. The studies on the influence of free radicals and photogenerated holes, h⁺, show that ˙OH, O₂˙⁻, and h⁺ play decisive roles in the photocatalytic activity of AgNPs/g-C₃N₄. The TOC result indicates the minimal toxicity of the by-products formed during the 2,4-D degradation. In addition, the AgNPs/g-C₃N₄ catalytic activity under direct sunlight irradiation was similar to that under artificial UV irradiation. Based on these results, a possible mechanism is proposed to explain the enhanced photocatalytic activity and stability of AgNPs/g-C₃N₄. Theoretical calculations on the interaction between 2,4-D and g-C₃N₄, Ag/g-C₃N₄ was also performed. The calculated results show that the adsorption of 2,4-D on Ag-modified g-C₃N₄ is significantly more effective compared to pristine g-C₃N₄.