Growth Kinetics and Morphological Evolution of ZnO Precipitated from Solution

This work characterizes the nucleation and growth kinetics of zinc oxide (ZnO) precipitated from aqueous hexamethylenetetramine (HMTA) zinc nitrate (Zn-(NO3)(2)) solutions observed by in situ and ex situ transmission electron microscopy. Quantitative comparisons between in situ beam-induced precipitation, in situ thermally activated precipitation, ex situ thermally activated precipitation, and ex situ electrochemistry provide insights into the rate limiting mechanism and the chemistry governing the reactions. All experiments indicate that isotropic ZnO precipitates directly from solution. These particles begin to aggregate and grow anisotropically shortly after nucleation. The conversion to anisotropic growth does not rely on coalescence despite the fact that the two are often observed to occur in concert. The results indicate that the reaction pathway for in situ beam-induced growth more closely mimics ex situ electrochemistry than ex situ chemistry. In situ and ex situ thermally activated growth processes proceed in a similar manner, although particle transport and aggregation are limited by the in situ geometry.