Hierarchical-Oriented Attachment: From One-Dimensional Cu(OH)₂ Nanowires to Two-Dimensional CuO Nanoleaves

One-dimensional (1D) Cu(OH) 2 nanowires are converted into to two-dimensional (2D) CuO nanoleaves on a large scale, in which hierarchical-oriented attachment is involved. The whole transition process is captured and studied with the assistance of transmission electron microscopy (TEM) and high-resolution TEM images. Polycrystalline Cu(OH) 2 nanowires first evolved into single crystalline Cu(OH) 2 nanoleaves by an oriented attachment. In this process, Cu(OH) 2 nanoparticles (about 3 nm in diameter), which constructed the Cu(OH) 2 nanowires, rotated, oriented, and attached to produce single crystalline Cu(OH) 2 nanoleaves. Then, these single crystalline Cu(OH) 2 nanoleaves converted into single crystalline CuO nanoleaves via a reconstructive transformation, which consisted of the nucleation of CuO followed by a two-step oriented attachment of the CuO particles. In the formation of CuO nanoleaves, when the highest energy surfaces were eliminated by oriented attachment (from zero-dimensional nanoparticles to 1D nanoribbons), the subordinate high-energy surfaces became the highest one, and the oriented attachment took place sequentially on these subordinate surfaces (from 1D nanoribbons to 2D nanoleaves). The attached particles with angles caused by the unfinished rotation of nanoparticles in the formation of single crystalline nanoleaves, which was supposed to be a necessary process for oriented attachment, were observed under the experimental conditions. This revealed the potentiality for the formation of 2D or three-dimensional nanostructures by hierarchical-oriented attachment. These findings may be helpful for understanding the nanocrystal growth process and the oriented attachment mechanism in depth.