Efficient degradation of organic pollutants under solar irradiation using an n–p ZnO/NiO heterostructure†

In the present work, we have synthesized an n-ZnO/p-NiO (1 : 1 molar ratio) heterostructure via the hydrothermal method, effectively coupling n- and p-type semiconductors. Raman spectroscopy and X-ray diffraction (XRD) analyses confirmed the successful synthesis of ZnO/NiO composite nanopowders. Based on UV-vis absorbance/reflectance data, the direct optical bandgap is estimated to be around 3.02 and 3.31 eV, attributed respectively to the ZnO and NiO phases. Additionally, the photoluminescence spectrum shows three broad emission bands in the visible range around 410, 453, and 507 nm, attributed to defects such as metal interstitials, anti-oxygen sites, and oxygen vacancies, respectively. These defects will contribute to enhancing the electrical conductivity of the ZnO/NiO nanocomposite. The electrical conductivity σ_dc is approximately 10⁻⁸ S cm⁻¹ higher than that of pure NiO, confirming the improved conduction properties of the prepared nanocomposite. Additionally, the BET analysis revealed a significant specific surface area, which is favorable for catalytic applications. The photocatalytic activities of the ZnO/NiO heterojunction in the degradation of methylene blue (MB) and methyl orange (MO) dyes were investigated under natural solar irradiations. Photocatalytic degradation efficiencies were estimated through the evaluation of the decrease of the dye's characteristic absorbance bands (MB: 664 nm, MO: 464 nm). The nanocomposite shows good photocatalytic activity after 180 min, with a faster degradation rate of MB.