Annealing of nanometer-sized zinc oxide particles

The processes occurring during annealing of particulate ZnO thin films and powders are examined in detail by size-determining methods (TEM and powder X-ray diffraction) and by thermal analysis. Water and carbon dioxide desorb from the surface at temperatures of about 100 degreesC. At higher temperatures (between 300 and 500 degreesC), two processes are observed: a change in the chemical composition of the surface by thermal degradation of adsorbed molecules and an increase of the geometric particle size. A structural model for nanometer-sized ZnO nanocrystals is proposed according to which the core of the particles consists mainly of pure ZnO surrounded by a shell containing Li+ and ZnO. Acetate ions adsorbed on this shell form the outermost layer. To account for the low energy shift in the absorption spectra of the larger particles, an interaction of the adsorbed acetate ions with the electrons inside the ZnO is suggested which contributes to the observed size-quantization of the tiny semiconductors.