Photocatalysis is one of the techniques used for the eradication of organic pollutants from wastewater. In this study, Co–ZnO was tested as a photocatalyst for the degradation of methyl orange under irradiation of visible light. Co–ZnO loaded with 5%, 10%, and 15% Co was prepared by the precipitation method. The advanced techniques including X-ray diffraction, X-ray photoelectron spectroscopy, diffuse reflectance UV–visible spectroscopy, photoelectrochemical measurements, temperature-programmed desorption, photoluminescence, and fluorescence spectroscopy related to OH^• measurements were used for characterization of prepared Co–ZnO. Experiments showed that 10% Co–ZnO was a highly efficient catalyst for the photodegradation of methyl orange as compared to ZnO. The enhanced photocatalytic activity of Co–ZnO is attributed to the implantation of Co which inhibits the electron–hole recombination. A 100 mg/L solution of methyl orange dye was completely degraded within 130 min. The reaction kinetics has been described in terms of the Eley–Rideal mechanism.

The geometric, thermodynamic and electronic properties of Pd–graphene nanocomposites are comprehensively studied through quantum mechanical methods. Geometries of these clusters are optimized with the well-calibrated Minnesota functional M06-2X. The adsorption energies calculated at the M06-2X/LANL2DZ level show better agreement with those calculated from MP2/ANO-RCC-VDZP. Two different representative models for graphene, coronene and hexabenzocoronene, are used. The adsorption energies analysis reveals that the interaction energies increase with the size of the adsorbed cluster. However, for Pd_n/hexabenzocoronene, the interaction energies show a sudden drop at Pd₈/hexabenzocoronene. The difference in behavior between the interaction energies of Pd_n/hexabenzocoronene and Pd_n/coronene is attributed to the edge effect present in coronene. The electronic properties, including highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), Fermi level, molecular electrostatic potential (MEP), dipole moment, vertical ionization potential (VIP), vertical electron affinity (VEA), chemical hardness (η), softness (S) and chemical potential (μ) are studied. The VIP and VEA reveal that Pd_n/coronene clusters are stable in nature with the least reactivity. The HOMO–LUMO energy gaps are reduced with the increase in cluster size. The electronic properties show irregular trends, where the most favorable electronic properties are obtained for Pd₇/coronene and Pd₁₀/coronene.