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.