ZnO Nanorods Orientation Control with Seedless Hydrothermal Growth and Finding Suitable Morphology with Simulation for Tactile Sensors

Piezoelectric tactile sensors (PTSs) has been receiving a great deal of research interest due to increasing progress in the field of haptic/tactile devices. PTSs are the most important part in realizing electronic skin to mimetic the human skin, and with the help of ZnO nanorods (NRs) it is getting considerable attention due to its low-cost seedless hydrothermal growth process, and biological compatibility. Hydrothermal growth is a low-temperature (< 100 ℃) chemical-based growth of piezoelectric ZnO nanorods and can provide various orientations of ZnO nanorods by controlling the growth parameters. Till now most of the research was based on the vertical aligned c-oriented ZnO nanorods due to the expected better performance of PTSs as the d33 piezoelectric coefficient (c-orientation) has a much higher value. Our seedless growth process shows three main growth morphology of ZnO nanorods which are completely tilted in different directions, vertical and tilted mixed in most of the growths and completely vertical with substrate optimization. In order to achieve vertical alignment of ZnO NRs, we have focused on substrate pre-treatment, varying precursor concentration, and growth temperature and time. It is observed that the vacuum annealed substrates at 250–300 ℃ temperature for 20–25 min were in good agreement to assist vertical growth at a certain growth temperature and time. Furthermore, the three morphologies are simulated to obtain a quantitative understanding of the piezoelectric output. Simulation with the finite element method using COMSOL Multiphysics^® software shows that the tilted array geometry of NRs shows enhanced piezoelectric voltage over the vertical array of NRs which is in contrast with general view in this research.