Constructing a ZnO/CuCo₂O₄ p-n heterojunction photocatalyst for efficiently hexavalent chromium–phenol detoxification and nitrogen fixation

The design of low-cost, recyclable, high-efficiency, and visible light-based photocatalytic systems is being considered a potential new “green” approach to address challenges such as N₂ fixation and the removal of pollutants from wastewater. For this purpose, a ZnO/CuCo₂O₄ p-n heterojunction was synthesized by a simple method, and its photocatalytic activity was evaluated for N₂ fixation and hexavalent chromium–phenol detoxification. The ZnO/CuCo₂O₄ nanocomposite with oxygen vacancy showed better performance in ammonia production (3460 μmol L⁻¹ g⁻¹, 43.3, and 16.1-fold), chromium photo-reduction (505.8 × 10⁻⁴ min⁻¹, 25.5 and 5.57-fold), and phenol photo-oxidation (345 × 10⁻⁴ min⁻¹, 42.5 and 6.57-fold) compared to ZnO and CuCo₂O₄. XRD, XPS, and EPR analyses provide sufficient evidence for oxygen vacancy in the nanocomposite that enhances chemical adsorption and activation of N₂ molecules. FESEM, TEM, and HRTEM images clearly represent that the CuCo₂O₄ nanoparticles are superimposed on ZnO spindle nanoparticles and also reveal the heterojunction boundary between the two components. Photoelectrochemical analyses showed that the recombination of electron-hole pairs in the nanocomposite was minimized and that charge carriers could promote photocatalytic activity. Mott-Schottky measurements were used to study the band structure and by combining the results of the scavenging test the possible mechanism was described in detail. Recycling the ZnO/CuCo₂O₄ sample after photocatalytic reactions showed that the prepared nanocomposite is a reactive and stable photocatalyst after visible-light irradiation. This nanocomposite is hoped to develop new ideas for designing multifunctional photocatalytic systems with high photocatalytic activity.