Influence of solution composition on sprayed ZnO nanorods properties and formation process: Thermoanalytical study of the precursors

ZnO is an important inorganic material for numerous applications. Different physical and chemical methods have been applied to deposit ZnO. Spray pyrolysis method being simple, rapid and low-cost is amongst the many options and has been chosen for this study. Fabrication of ZnO nanorods crystals by chemical spray pyrolysis was performed using an acidic solution of ZnCl2 1 or a basic ammonia-containing solution of ZnCl2 2. All layers were studied using X-ray diffractometry and Scanning Electron Microscopy. The formation of ZnO nanorods from 2 appeared at 450 °C, whereas spraying acidic solution 1 yielded ZnO nanorods like morphology at 550 °C. Thermal decomposition of precursors for ZnO layers prepared by de-watering of acidic aqueous solution of ZnCl2 with pH = 3 (1) and basic solution of ZnCl2 and NH4OH with pH = 10 (2) was monitored by simultaneous thermogravimetric and differential thermal analysis (TG/DTA) in air coupled online with evolved gas analyses by Fourier transformed infrared spectroscopy (FTIR). The precursor (1) is ZnCl2 *nH2O; the precursor (2) is a mixture of (NH4)3(ZnCl4)Cl, Zn(NH3)2Cl2, NH4Cl(NH3)3 and Zn(OH)2 phases. The thermal decomposition of (1) and (2) in the temperature range of 30–700 °C consists of two steps with total mass losses of 86.2% and 93.8%, respectively. The main evolved gases from (1) are H2O and HCl, whereas the main evolved gases from (2) are H2O, NOx and NH3. Degradation of (1) and (2) is completed by 670 and 620 °C, respectively. The final decomposition product of (1) and (2) at 700 °C is ZnO. This study shows that the use of basic solutions enables to decrease the temperature of ZnO formation and the deposition temperature of ZnO nanorods layers.