Multistage growth of monocrystalline ZnO nanowires and twin-nanorods: oriented attachment and role of the spontaneous polarization force
Our understanding of crystal growth mechanisms has changed deeply in the past few decades. Particularly, the oriented attachment of intermediate nanoparticles has been accepted to be a crucial crystal growth mechanism that is distinct from the traditional one involving nucleation and Ostwald ripening. However the details of the oriented attachment process are not readily observed experimentally, and little is known about the driving force and the dynamics involved in oriented attachment. In this respect, ZnO is an ideal material because it possesses strong spontaneous polarization which may easily drive oriented attachment during its crystal growth. We study experimentally and theoretically the complete crystal growth process (from primitive amorphous nanoclusters to ultimate single nanocrystals) of one-dimensional ZnO nanocrystals growing in water/ethanol at high temperatures. The results reveal that both axial (along the direction of the polarization axis) and lateral oriented attachment of the intermediates occurs during the growth process of the one-dimensional ZnO nanocrystals. Calculation based on the force and interaction model reveals that the axial oriented attachment driven by the spontaneous polarization force dominates the crystal growth of ZnO nanocrystals, and the van der Waals force also plays a role in driving oriented attachment. The study shows that oriented attachment of intermediate nanoparticle ensembles induces formation of the symmetric twin-nanorods. These results improve our understanding of the growth mechanism of nanocrystals in a liquid medium.