Synthesis of nano-sized ZnO using drop wise method and its performance as moisture sensor

Nano-sized ZnO was synthesized through the hydroxide precipitation using drop wise method. Pellets of powder were made and they were heat-treated as 150, 300, 450 and 550 °C for 3 h and tested for its moisture-sensing ability. Each heat-treated pellet was exposed to humidity under control conditions and variations in resistance with the variations in humidity were measured. Sensitivity to moisture and activation energy of electrical transport were evaluated. Highest average sensitivity is 5.35 MΩ/%RH when the sample is heat-treated at temperature 550 °C. The activation energy reaches maximum (0.074 eV) after thermal annealing above 300 °C and from the value 0.018 eV of the sample calcined at 200 °C. Reversibilities of the resistance with relative humidity (%RH) for different annealing temperatures were also observed. These were excellent when the sensing element was heat-treated at 550 °C. SEM and XRD of the sensing material calcined at different temperatures were studied. Scanning electron micrographs show that synthesized zinc oxide is in the form of nanosheets. Minimum average thickness of ZnO nanosheets is found to be 35 nm for sensing element annealed at 550 °C.