Growth Kinetics of ZnO Nanorods: Capping-Dependent Mechanism and Other Interesting Features

Abstract

Although the growth of nanocrystals has been investigated by several workers, investigations of the growth of 1-D nanostructures have been limited. We have investigated the growth kinetics of both uncapped and poly(vinyl pyrollidone) (PVP)-capped ZnO nanorods carefully by a combined use of transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) which provide direct information on size and shape and compensate for the deficiency of each other. Values of average length and diameter of the ZnO nanorods obtained by TEM and SAXS are comparable. In the presence of the capping agent, the length of the nanorods grows faster while the diameter becomes narrower. The length distribution shows periodic changes in the width in the case of the uncapped nanorods, a feature absent in the case of the capped nanorods. In the absence of the capping agent, we observe the presence of small nanocrystals next to the nanorods after a lapse of time. The occurrence of small nanocrystals as well as the periodic focusing and defocusing of the width of the length distribution tend support to the diffusion-limited growth model for the growth of uncapped ZnO nanorods. Accordingly, the time dependence of the length of uncapped nanorods follows the L-3 law as required for diffusion-limited Ostwald ripening, while the PVP-capped nanorods show a time dependence which is best described by a combination of diffusion and surface reaction with a L-3 + L-2 type behavior. Collapse of all distribution curves obtained at different times of the reaction into a single universal Gaussian in the case of the PVP-capped nanorods also shows that the growth mechanism is more complex than Ostwald ripening.