ZnO nanoparticles with different size were prepared by calcining the precursor, zinc carbonate hydroxide, at different temperature, and Pd/ZnO or Ag/ZnO composite nanoparticles with the noble metal content of w = 0.5% and 0.75% were obtained by photoreduction method using the smallest ZnO nanoparticles as photocatalysts. The as-prepared samples were also characterized by the techniques such as XRD, TEM, XPS, SPS and EFISPS, and their photocatalytic activity was evaluated by the photocatalytic degradation of gas phase n-heptane. The effects of the crystallite size and the deposit of noble Pd or Ag on the surface photovoltage responses and photocatalytic activity of ZnO nanaparticles were investigated, and the relations between the surface photovoltage spectrum and photocatalytic activity were discussed. It could be concluded that the photocatalytic activity of nanoparticles could be preliminarily evaluated by the measurement of their SPS. The results indicate that the intensity of SPS signal of ZnO nanoparticles decreases with decreasing size, while the photoactalytic activity increases; the intensity of SPS signal of ZnO nanoparticles decreases remarkably after depositing the appropriate noble metal on their surface, while the photocatalytic activity increases. In addition, the deactivity mechanism f the photocatalyst of ZnO nanoparticles was also analysed.

Key words: ZINC OXIDE, NANOPHASE MATERIALS, PHOTOCATALYSIS, OXIDATION, HEPTANE, CATALYST DEACTIVATION