Effects of Zinc Nitrate and HMTA on the Formation Mechanisms of ZnO Nanowires on Au Seed Layers

The ability to form ZnO nanowire arrays with dedicated morphological properties is crucial for the development of efficient piezoelectric devices such as piezoelectric nanogenerators and sensors. However, their integration typically requires the use of metallic seed layers for their synthesis by chemical bath deposition, from which their morphological control is still very limited. In this context, the formation mechanisms of ZnO nanowires from Au seed layers are carefully investigated for different precursor (i.e., zinc nitrate and hexamethylenetetramine (HMTA)) concentrations in the range of 1–100 mM, where drastic variations of the morphological properties are observed. By coupling in situ pH measurements and thermodynamic computations, we perform an in-depth analysis of the thermodynamic properties of the chemical bath, where the predominant role of the NO3 – ions in the evolution of the pH of the chemical bath is revealed. An original approach is further developed to carefully determine the hydrolysis ratio of HMTA molecules, which is found to vary in the range of 20–45% with the precursor concentration, and to directly impact the supersaturation ratio of Zn­(II) species. From these results, we identify the presence of three different growth regimes depending on the precursor concentrations, each of them giving rise to ZnO nanowire arrays with specific morphological properties. These results highlight the critical importance of the thermodynamic properties of the chemical bath in the formation process of ZnO nanowires from Au seed layers and provide key elements of understanding to efficiently optimize their morphology for their integration into piezoelectric devices.