Effect of citric acid on formation of oxides of Cu and Zn in modified sol-gel process: A comparative study

We report here the influence of citric acid concentration on the formation of sol-gel products in each of Cu and Zn systems by using respective metal nitrate as precursor and citric acid as gelling agent. The synthesized sol-gel products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray analysis (EDX), Fourier transform infra-red (FT-IR) spectroscopy and UV-Visible diffused reflectance spectroscopy (UV-Vis DRS). The influence of citric acid concentration on the formation of metal/metal oxide in each case was primarily investigated by varying the molar ratio of each metal nitrate (N) and citric (C) acid (N:C = 1:1, 1:2, 1:4, 1:6 and 1:8). It was observed that at low N:C molar ratios (1:1) and (1:2), the Cu system had only CuO and at high N:C molar ratio, lower oxidation state of copper (Cu2O and Cu) has resulted. Distinctly, irrespective of the N:C molar ratio, the sol-gel product of Zn system was only single phase of ZnO. The SEM observations confirmed that the grains of these two metal systems were spherical in nature. In each metal system, at high N:C molar ratio, small grain size has resulted. At high N:C ratio, lower oxidation state of metal ion is resulted where the metal system is susceptible for reduction. The susceptibility of metal ions to undergo reduction controlled the formation of end products in the sol-gel process.