Synthesis of nanocrystalline NiO/ZnO heterostructured composite powders by sol-gel auto combustion method and their characterizations

Nanocrystalline NiO/ZnO heterostructured composite powders were prepared by the sol-gel auto combustion method, based on nickel and zinc nitrate precursors and using diethanolamine (DEA) as novel fuel. The composition of different NiO and ZnO ratios, ranging from 100/0, 95/5, 90/10, 80/20, 60/40, 50/50, 40/60, 20/80, 10/90, 5/95 to 0/100, were studied. The structural, chemical bonding, morphological, optical, and fluorescence properties including the local atomic structure of each calcined sample were systematically investigated by means of X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), UV-visible diffuse reflectance spectroscopy (UV-DRS), photoluminescence (PL) spectroscopy, and synchrotron X-ray absorption spectroscopy (XAS), respectively. For the ZnO concentration below 20%, both XRD and Raman spectroscopy results revealed only the NiO phase. This conformed to the observation of Zn K-edge and Ni K-edge X-ray absorption near edge structure (XANES). The Zn ions found in the samples of low ZnO concentration exhibited six-fold coordination with oxygen atoms rather than the four-fold coordination found in the wurtzite (WZ) structure of ZnO. In contrast, the Ni ions which are found in the samples of low NiO concentration (<= 10%) are coordinated both tetrahedrally and octahedrally by four or six oxygen atoms, respectively, rather than the six-fold coordination which is usually observed for Ni ions in the rock salt (RS) form of NiO. All analytical results obtained from experimental XANES spectra were verified by the theoretical calculation of absorption spectra using the FEFF9.7 code. The UV-DRS results showed that there was an increase in the reflectance efficiency for both infrared and visible light conditions as the content of ZnO increases; meanwhile, the values for the energy gap (E-g) of all composite samples were higher than that of pure NiO and ZnO. In addition, the PL spectra revealed major blue emission bands observed at 490 nm when the excitation wavelength was 300 nm. As the ZnO phase developed, a variety of violet emission bands occurred within the range of 400 nm-450 nm, which was obviously related to the change in E-g. The intrinsic defects occurred in the NiO/ZnO composite powders were probably responsible for this phenomenon. (C) 2017 Elsevier B.V. All rights reserved.