Experimental and First-Principles Study of an Optimized Cylindrical-Shaped ZnO/g-C₃N₄ Heterojunction for Enhanced Adsorption and Photocatalytic Degradation of Methylene Blue Dye

In the present study, we have synthesized a ZnO/g-C₃N₄ heterojunction by adjusting the concentration of zinc nitrate hexahydrate and explored the impact of this varying concentration on the material's physical properties via experimental methods and density functional theory (DFT) calculations. The band gap energies were assessed via UV-visible spectra, and band gap values of ZnO/g-C₃N₄ vary in the range of 2.55-2.70 eV, and the band gap of g-C₃N₄ (CN) has a value of 2.76 eV. Thus, heterojunctions exhibit a slightly narrower band gap compared to pure CN, and DFT calculations have also supported this observation. DFT study predicts the stability of the ZnO/CN heterojunction, further confirmed by experimental studies. The existence of the elements (Zn, O, C, and N), along with their valence states, was determined by X-ray photoelectron spectroscopy (XPS) analysis. The heterojunctions demonstrate significantly improved dye degradation percentage toward MB dye compared to the individual components. With the optimized concentration of ZnO, the ZnO/CN (ZCN4) heterojunction achieved a maximum dye degradation efficiency of 97%. The superior performance of the ZCN4 heterojunction has been ascribed to the optimized zinc precursor concentration, enhanced charge separation, reduced recombination rate of charge carriers, and narrowest band gap among the synthesized heterojunctions. The charge carrier dynamics were investigated through photoluminescence (PL) and electron paramagnetic resonance (EPR) analyses, revealing reduced recombination of charge carriers. Scavenger experiments identified superoxide anions as the dominant reactive species in the photocatalytic degradation process. The influence of pH on the photocatalytic performance was systematically examined, with point of zero charge (pH_PZC) measurements providing valuable insights into the surface charge interactions under varying pH conditions. The recyclability of the ZCN4 heterojunction was also evaluated over multiple cycles, demonstrating its stability and reusability. Furthermore, the Fourier transform infrared spectroscopy (FTIR) analysis of the recycled sample provided insights into the mineralization of the adsorbed dye molecules, confirming the degradation of organic pollutants. These findings underscore the potential of ZnO/CN heterojunctions as efficient photocatalysts for environmental remediation applications.