Effects of Hexamethylenetetramine on the Nucleation and Radial Growth of ZnO Nanowires by Chemical Bath Deposition

The growth of ZnO nanowires by chemical bath deposition (CBD) is of great potential for their integration into nanoscale devices. However, the effects of the chemical precursors in solution are still under debate, such as the role of hexamethylenetetramine (HMTA). In order to tackle this issue, these effects are thoroughly disentangled from the effects of the structural morphology of the ZnO seed layer and investigated through a large number of nonequimolar CBDs over a broad range of chemical precursor concentrations and ratios. The analysis is further supported by thermodynamic simulations yielding theoretical solubility plots and speciation diagrams of Zn(II) species. It is shown that the ZnO deposited volume and, to some extent, the length of ZnO nanowires are directly related to the supersaturation in solution, which strongly depends on the chemical precursor concentration and pH. A slight excess of HMTA with respect to zinc nitrate is required to reach the largest axial growth rate of ZnO nanowires. In addition to act as a source of HO– ions, HMTA is found to act as a pH buffer over a broad range of chemical precursor concentrations and ratios, except for its largest excess. Additionally, it is unambiguously revealed that HMTA strongly reduces the radial growth of ZnO nanowires, by inhibiting the development of their nonpolar m-plane sidewalls. Importantly, HMTA also affects significantly the density of ZnO nanowires and hence their nucleation process, which is attributed to its significant interaction with the ZnO seed layer. The present findings give a deeper insight into the multiple roles of HMTA, which are an important step toward the ultimate control of the structural uniformity of ZnO nanowire arrays.