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Synthesis of ZnO Nanoparticles at Low Temperature by Hydrothermal Treatment Using the Sol-Gel Method

Research on nano-crystalline materials has been increased enormously in recent years. Because the several areas of application for this new type of materials in nanometer scale. These nano-scale properties depend on the size and appear when the size of the particles is approximately Bohr's diameter for the exciton [1]. Zinc oxide (ZnO) is an n-type semiconductor that shows a hexagonal wurtzite crystal structure, with a P63mc space group and parameters of a = b = 0.3250 nm and c = 0.5207 nm with quantum confinement effects. For the synthesis of zinc oxide nanoparticles, the sol-gel method showed several advantages over other techniques, due to the good stability of the solution, low temperature of hydrothermal treatment, besides not adding any type of surfactant or doping agent [2]. The nanoparticles of zinc oxide of this work were prepared by using zinc nitrate hexahydrate (Aldrich), citric acid (Aldrich) and sodium hydroxide (J. T. Baker). In the first step, three solutions were prepared. The first one was prepared by dissolving 0.04 mol of citric acid in 0.6 mol of distilled water. The second one was prepared by dissolving 0.02 mol of zinc precursor in 0.6 mol of distilled water and, the third one by dissolving 0.04 mol of sodium hydroxide in 1.2 mol of distilled water. For the second step, the zincsolution was added slowly (drop by drop) to citric acid solution. Then, Na-solution was added slowly (drop by drop) to zinc-citric acid solution and vigorous stirred for 30 min, too. The gel obtained was aged in a closed glass flask at 100 °C for 1 day. After, the hydrothermal treatment, the sample was washed with distilled water and centrifuged at 12000 rpm for 10 minutes. Then, the material was dried at 80 °C for 1 day. Finally, it was necessary to calcine the sample at 560 °C for 1 h to eliminate the residual organic.