Probing the Kinetics of Crystallite Growth in Sol-Gel Derived Metal-Oxides Using Nanocalorimetry

In this work, we show that fast-scanning chip nanocalorimetry can be used to kinetically separate the heat flow contributions of crystallite growth and precursor decomposition that occur during the thermal decomposition of metal-oxide sol-gel precursors. We illustrate the technique using zinc acetate dihydrate, a common precursor used in the synthesis of zinc oxide films for electronics. Through an appropriately defined heating sequence consisting of precursor decomposition, followed by rapid quenching and subsequent zinc oxide crystallite growth, it is shown that the exothermic peaks corresponding to the growth of zinc oxide crystals can be kinetically separated from the endothermic peaks associated with precursor decomposition. The kinetic separation of these processes enables an analysis to be performed on the crystallite growth kinetics of zinc oxide within a zinc acetate matrix, as it occurs during the sol-gel process. Through a quantitative analysis, we estimate the activation energy of crystallite growth, confirming Johnson-Mehl-Avrami growth kinetics at low heating rates, and extract a time-temperature-transformation (TTT) diagram to visualize and quantify isocrystalline surfaces. Such fundamental knowledge regarding the evolution of crystallinity as a function of heating conditions is useful in the much broader sense of application development, serving as a guide to direct the precise thermokinetic engineering of crystallinity in sol-gel derived metal-oxide films. In particular, we emphasize its significance for electronic (ideally high crystallinity), optoelectronic and thermoelectric (ideally low crystallinity) applications.