Cobalt phosphate-ZnO composite photocatalysts for oxygen evolution from photocatalytic water oxidation

Cobalt based oxygen evolution catalysts (Co-Pi) were loaded on the surface of ZnO by photochemical deposition in a neutral phosphate buffer solution containing Co ²⁺ ions. Structural, morphological, and optical properties of the samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-vis diffuse reflectance spectra techniques. The Co-Pi phase formed was amorphous and was deposited on the surface of ZnO uniformly as a layer of nanoparticles. The enhanced activity for oxygen evolution was directly observed from photocatalytic water oxidation over Co-Pi loaded ZnO. The oxygen produced in the first hour was more than 4 times of that obtained over ZnO alone. The results suggest that Co-Pi played the role of cocatalyst, which can trap photogenerated holes, leading to the enhancement of electron and hole separation efficiency. Further studies showed that the mixture of cobalt phosphate and ZnO exhibited similar enhancement in activity for oxygen evolution which could be due to the oxidation of nonactive cobalt(II) phosphate to active Co-Pi with higher oxidation states of cobalt upon light illumination during photocatalytic water oxidation process. In both systems, ZnO photocorrosion was observed based on inductively coupled plasma, XRD, and FESEM analyses.