How does photocatalytic activity depend on adsorption, composition, and other key factors in mixed metal oxide nanocomposites?

Single and mixed phases of Co₃O₄, NiO, and CuO nanocomposites were synthesized and investigated as photocatalysts under visible light for degradation of Congo Red dye. Selected parameters were monitored regarding their impacts on photocatalysis, including band gap, crystallite size, specific surface area, zeta potential, composition, and adsorption parameters such as adsorption capacity, equilibrium constant, rate, and thermodynamic quantities. The dominant adsorption mode was chemisorption. Adsorption capacity varied inversely with zeta potential due to repulsion between dye molecule and nanocomposites surface. As the metal content was varied, photocatalytic activity (PCA) depended strongly on type/number of oxides, on charge carrier separation, and adsorption capacity. Specific surface area was less influential to PCA. PCA had little or no dependence on crystallite size, surface charge, adsorption kinetics, equilibrium constant, and thermodynamic parameters. Variation of the band gap did not affect visible-light PCA since all nanocomposites (except NiO) exhibited band gaps in the near infrared.