Chemical Synthesis of ZnO Nanocrystals

A new chemical process to synthesize pure ZnO nanocrystal colloidal for bioimaging applications is reported. Zinc acetate dihydrate was dissolved and refluxed in a methanol solution at a low temperature (68degC) and ambient environment. Biocompatible surface capping agents were introduced to the synthesis process to control the particle nucleation and growth and, therefore, the particle size and its surface chemistry. Five types of capping agents were investigated for their effectiveness in limiting the particle growth. Three capping agents-3-amino- propyl trimethoxysilane (Am), tetraethyl orthosilicate (TEOS), and mercaptosuccinic acid (Ms)-were found effective in capping the ZnO nanoparticles and limiting the growth of the particles, while the other two-3-mercaptopropyl trimethoxysilane (Mp) and polyvinylpyrrolidone (Pv)-caused agglomeration or forming large clusters in the solutions. Particles synthesized were in the size range of 10-30 nm after capping, and grew to 60 nm and 100 nm in 3 weeks and 6 weeks respectively during storage at ambient conditions. Refluxing time was found to affect only the first precipitation time. Washing by ethanol and slow drying were found critical in converting Zn(OH)₂ into ZnO. XRD analyses revealed forming of single phase ZnO Wurzite (P63mc) structure. TEM analysis determined the single crystal size of 6 nm. Photo- luminescence (PL) spectra showed high intensity in UV emission and low intensity in the visible emission, which imply a good surface morphology of the ZnO nanoparticles with few surface defects. Optical absorption spectra indicated absorption at the wavelength of 380 nm from the uncapped ZnO, corresponding to the bandgap of bulk ZnO. The capped ZnO absorbed at a shorter wavelength (350 nm) indicating a much smaller particle size. Capping effectiveness of each agent is discussed through possible capping mechanisms and chemical reactions.