The majority of the nanowire synthesis methods utilize catalyst particles to guide the nanowire geometry. In contrast, catalyst-free methods are attractive for facile fabrication of pure nanowires without the need for catalyst preparation. Nonetheless, how nanowire growth is guided without a catalyst is still widely disputed and unclear. Here, we show that the nanowire growth during metal oxidation is limited by a nucleation of a new layer. On the basis of in situ transmission electron microscope investigations we found that the growth occurs layer by layer at the lowest specific surface energy planes. Atomic layers nucleate at the edge of twin boundary ridges and form a long-range ordering along the twin boundary. We anticipate our study to be a starting point to employ defects for nanowire growth control and consequently shaping the geometry of nanowires in a similar manner as in the catalyst-assisted growth method.
Keywords: atomic layer; in situ TEM; metal oxide; nanowires; noncatalytic growth; nucleation.