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Formation of heterostructures with p-type oxides such as NiO and CuO is one of the effective methods for improving the photocatalytic performance of ZnO. Such systems are often synthesized through template-based growth techniques that involve many steps. We have prepared ZnO-NiO composites through a facile, template-free, low-temperature sonochemical route. High-resolution transmission electron microscopy analysis indicates the formation of ZnO-NiO heterostructures. Photocatalytic activity of ZnO-NiO nanocomposites in the decomposition of methylene blue dye under solar irradiation is found to be much larger than that of both pure ZnO (1.26 × 10 −2 min −1 ) and NiO (0.31 × 10 −2 min −1 ) establishing synergistic effects. The rate constant increases with increase in the percentage of NiO in the composite and is 6.00 × 10 −2 min −1 for sample with the highest percentage of NiO. Rate constants for the second and third runs are estimated to be 4.4 × 10 −2 and 4.2 × 10 −2 min −1 which are promising. The main mechanism of enhancement of photocatalytic activity of the composites is identified as the more effective separation of the photogenerated free charge carries due to the internal electric field at the ZnO-NiO interface. Sharp decrease in the relative intensity of the band-band emission of ZnO at ~ 380 nm in the case of composite samples and analysis of the relative position of the conduction band and valence band edges of ZnO and NiO support the proposed mechanism.

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ZnCdO nanocrystalline powder is synthesized by the simple solution method at room temperature. Synthesized powder was characterized by powder x-ray diffraction (XRD), scanning electron microscope with x-ray spectrometry (EDX), thermo gravimetry–differential thermal analysis (TG–DTA) and other spectroscopic techniques. The XRD pattern exhibited a mixture of hexagonal ZnO and cubic cadmium oxide phases. The crystallite size of the prepared powder was found to be around 28 nm. Surface morphology of sample was determined by scanning electron microscopy and the distribution of Zn, Cd and oxygen species in the prepared sample was identified by chemical composition mapping through EDX. TG–DTA curves indicate the thermal stability of the prepared sample. The photoluminescence spectrum shows the bands in ultra-violet and blue emission regions. The Fourier transform infrared spectrum demonstrates the presence of metal oxide vibrations.

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