CHARACTERIZATION OF ZINC OXIDEGOLD NANOCOMPOSITES SYNTHESIZED BY PHOTODEPOSITION TECHNIQUE FOR NANODEVICE APPLICATIONS

Abstract

In this work, gold nanoparticles (Au NPs) were assembled onto zinc oxide (ZnO) nanostructures through photoinduced assembly mechanism. The properties of ZnO were necessary to predict this mechanism. Firstly, ZnO nanostructured thin films were synthesized by thermal oxidation of zinc (Zn) thin films at different oxidation temperatures of 400, 450, 500, and 550°C. Then, Au NPs were formed onto those ZnO films by photoreduction of HAuCl4 aqueous solution under UVA light irradiation. The characteristics of ZnO nanostructures and zinc oxidegold (ZnOAu) nanocomposites were characterized by FESEM, XRD, XPS, AFM adhesion mapping, and UV-vis spectroscopy. Morphologies of ZnO nanostructures were comprised of short rods branched into their ZnO nanocluster base for all cases. XRD results showed that ZnO nanostructures were highly oriented to the preferred orientation of (002) and the highly texture coefficient of (002) plane. This suggested single crystallinity and that the majority of nanostructures prefer to grow along the caxis. XPS spectra peak of 531.6 eV can be indirectly indicated by the oxygen vacancy in ZnO. In addition, AFM adhesion mapping was used to assess the adhesion property of ZnO. It was found that the ZnO oxidizing at 450°C had the maximum intensity of the preferred (002) orientation and texture coefficient of (002) plane, the highest reduction of oxygen vacancy, and the maximum adhesion intensity, which resulted in the strongest surface polarity. Morphologies of ZnOAu nanocomposites were comprised of several Au NPs growth to encapsulate the end of rodlike ZnO nanostructures which had the strongest adhesion and the lowest surface energy on its site. It was clearly observed that the maximum amount of Au NPs assembly was obtained for ZnO films oxidized at 450 °C. Therefore, the photoinduced assembly mechanism can be simply explained in terms of surface polarity and surface energy. Finally, ZnOAu nanocomposites were applied as H2 sensors. Therefore, the H2 gas sensing properties of ZnOAu nanocomposites were investigated and compared with ZnO nanostructures at operating temperatures in the range of 150450C. It was clearly observed that H2 sensor based on ZnOAu nanocomposites exhibited the optimum operating temperatures at 200C, lower than the 350400C of the sensors based on ZnO nanostructures. This indicated the effect of Au NPs assembly that reduced the optimum operating temperature. In addition, H2 sensor based on ZnOAu nanocomposites had a higher sensor response than those sensors based on ZnO nanostructures. The enhancement of sensor response can be explained in terms of the reaction rate constant (kOxy) by considering the catalytic effect of Au. Moreover, ZnOAu nanocomposites H2 sensor operating at 150C had potential application in fuel cell units.