Humidity sensor characteristics based on ZnO nanostructure grown by sol-gel method

Abstract

This work deals with ZnO thin films, electrical and structural properties characterisation and relative humidity sensing test. Films were deposited on glass substrate by sol-gel method. The sols were prepared using zinc acetate mixed with 2-methoxyethanol and monoethanolamine (MEA) as solvent and stabiliser, respectively. Obtained films were post annealed at 500°C in ambient atmosphere for 2 h. The films' crystalline structure was investigated by means of X-ray diffraction (XRD) analysis. XRD patterns of ZnO thin films exhibit three strong peaks (101), (002) and (100) planes assigned to ZnO wurzite structure. ZnO films morphology was investigated by scanning electron microscope (SEM). The SEM images reveal that the synthesised ZnO films are composed of irregular fibre-like stripes and wrinkle network. Resistivity and mobility of films were characterised by means of four probe resistivity and Hall effect measurement. The thin films humidity sensing properties have been analysed through electrical resistance measurements at various temperatures and humidity levels. It was observed that the sensor resistance decreases from 22 MΩ to 0.40 MΩ accordingly with increasing relative humidity from 15% to 95% at 25°C. Sensor sensitivity has been explained on the basis of variation of electrical conductivity of water molecules adsorption on film surface owing to wrinkle network morphology. At relative humidity of 95%, we recorded a response time of 7 s at 100°C which reveals that temperature has strong influence on the response and recovery times. Obtained results suggest that the prepared ZnO film has interesting potential in humidity sensing.