Highlights

Figure 1. "nonclassical growth mechanism” concept for the self-assembling mechanism.

We propose a growth mechanism that follows a “nonclassical crystallization” concept for the self-assembling mechanism of approximately 10-nmsized building units into peanut-like ZnO and/or microsphere-like hydrozincite particles.
Zinc oxide (ZnO) is a very interesting inorganic material because of its specific chemical, surface and microstructural characteristics. ZnO particles can be grown in many different nano- and micro-scale forms. The morphological diversity influences the particles’ properties, thus suitable particles can be used for various novel applications. On the other hand, the formation mechanism and the understanding of the crucial parameters concerning the control of the particles’ growth and morphology are still a great challenge for an increasing number of researchers.
The primary hindrance to such investigations has been the absence of appropriate techniques to probe the in-situ growth of nanocrystals. Furthermore, we were surprised to find no investigation where the combination of an insitu technique and an ex-situ technique was used to study the growth mechanism, starting with the first building units up to final micro-sized particles. Finally, from a fundamental chemical viewpoint, we still lack an understanding of ZnO particle growth, starting from molecular precursors to complexation reactions of the precipitation in many cases.
The above-mentioned challenges were a great motivation in our research. We have undertaken an in-depth study of several of these long-held notions using a combination of time-resolved experiments

involving the SAXS beamline at the synchrotron Elettra and ex-situ electron-microscopy (TEM and FE-SEM) techniques. The particles were prepared by the precipitation of zinc nitrate with urea. Depending on the reaction conditions, ZnO, hydrozincite, or a mixture of both phases was detected in our system. The condensation and complexation reactions led to the formation of nanoparticle building units up to a size of 10 nm. Afterwards, the nanoparticles immediately self-assembled into micro-sized particles. The molecular precursors and complexation reactions of the formation process were numerically predicted in the frame of the partial-charge model (PCM). We proposed a growth mechanism that follows a “nonclassical crystallization” concept for the self-assembling mechanism of approximately 10-nm sized building units into peanut-like ZnO and/or microsphere-like hydrozincite particles as presented in Figure 1. The influence of the reaction conditions on the particles’ formation kinetics and the phase composition were also defined.

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The growth mechanism of zinc oxide and hydrozincite: a study using electron microscopies and in situ SAXS;
M. Bitenc, P. Podbršček, P. Dubček, S. Bernstorff, G. Dražić, B. Orel and Z. Crnjak Orel;
Cryst.Eng.Comm. 14, 3080 (2012).
10.1039/C2CE06134A