Abstract
Single crystal ZnO nanotube arrays were synthesized at low temperature in an aqueous solution containing zinc nitrate and hexamethylenetetramine. It was found that the pH value of the reaction solution played an important role in mediating the growth of ZnO nanostructures. A change in the growth temperature might change the pH value of the solution and bring about the structure conversion of ZnO from nanorods to nanotubes. It was proposed that the ZnO nanorods were initially formed while the reaction solution was at a relatively high temperature (~90 °C) and therefore enriched with colloidal Zn(OH)2, which allowed a fast growth of ZnO nanocrystals along the [001] orientation to form nanorods. A decrease in the reaction temperature yielded a supersaturated solution, resulting in an increase in the concentration of OH− ions as well as the pH value of the solution. Colloidal Zn(OH)2 in the supersaturated solution trended to precipitate. However, because of a slow diffusion process in view of the low temperature and low concentration of the colloidal Zn(OH)2, the growth of the (001) plane of ZnO nanorods was limited and only occurred at the edge of the nanorods, eventually leading to the formation of a nanotube shape. In addition, it was demonstrated that the pH might impact the surface energy difference between the polar and non-polar faces of the ZnO crystal. Such a surface energy difference became small at high pH and hereby the prioritized growth of ZnO crystal along the [001] orientation was suppressed, facilitating the formation of nanotubes. This paper demonstrates a new strategy for the fabrication of ZnO nanotubes on a large scale and presents a more comprehensive understanding of the growth of tube-shaped ZnO in aqueous solution at low temperature.
Highlights
ZnO is a type of semiconductor with a wide band gap (3.37 eV) and a large exciton binding energy (~60 meV at room temperature)
It is well known that for either solution growth or vapor deposition, the morphology of ZnO nanorods in terms of size and size distribution could be significantly affected by the uniformity and crystal size of the seeds, which act as initial sites for the crystal nucleation [29,30,31,32]
It is clear that the reaction process for attachment of an atom to the interface was limited by the low temperature, so that the growth rate was slow. This is a reason that the length of ZnO nanorods in Figure 6 was smaller than that of the nanorods grown at 90 °C. All these results suggest that (1) the surface energy difference of ZnO crystal between polar and non-polar face strongly depends on the pH of the reaction solution, and (2) the growth rate greatly depends on the temperature
Summary
ZnO is a type of semiconductor with a wide band gap (3.37 eV) and a large exciton binding energy (~60 meV at room temperature). To understand the mechanism of the formation of tube-shaped ZnO, we investigated the dependence of the pH value of the solution on the reaction time as well as on the temperature of the solution.
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