Preparation and characterization of quantum size zinc oxide: a detailed spectroscopic study

We report the synthesis of transparent colloidal suspensions of small zinc oxide particles in water, 2-propano1, and acetonitrile. Quantum (Q)-size effects are observed during particle growth and qualitatively interpreted by using a simple molecular orbital (MO) picture. The particles at the final stage of growth are approximately spherical in shape and consist of 2000-3000 ZnO molecules. They exhibit many of the photophysical properties of bulk zinc oxide. However, pronounced shifts in the absorption spectrum during the illumination of anoxic suspensions of ZnO reveal a distinctively different behavior of these small particles. Fluorescence spectra of the ZnO sols suggest that adsorbed electron relays are necessary to shuttle electrons from the conduction band into lower lying traps. Two fluorescence maxima are observed at the final growth stage of the ZnO particles. The bandgap fluorescence at 365 nm has an extremely short lifetime (τ < 100 ps), while the visible luminescence at 520 nm exhibits a slower biexponential decay (i.e., τ = 14 and 190 ns). The latter fluorescence is attributed to photogenerated electrons tunneling to preexisting, trapped holes. The low overall fluorescence quantum yield of Ф = 0.03 measured in these zinc oxide suspensions is indicative of radiationless transitions accompanying the emissions. A pronounced pH dependence of the Stern-Volmer constants obtained with various ionic substances, that effectively quench the 520-nm emission, is explained by specific adsorption to the charged particle surface. The zero point of charge (pH_(zpc)) of the aqueous colloidal suspension was determined to be 9.3 ± 0.2 by several independent methods