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Vol. 122, Issue 11, November 2010, pp.9-19



Morphological and Relative Humidity Sensing Properties of Pure ZnO Nanomaterial


1N. K. Pandey, 2Karunesh Tiwari

Sensors and Materials Research Laboratory, Department of Physics

University of Lucknow, Lucknow, 226007, U. P. India

Tel.: 1 +91 9415003998, 2 +091 9451037737

E-mail: 1 nkp1371965@rediffmail.com, tiwari.lu@rediffmail.com



Received: 30 March 2010   /Accepted: 18 November 2010   /Published: 30 November 2010


Abstract: In this paper we report the resistive type humidity sensing properties of pure ZnO nanomaterial prepared by solid-state reaction method. Pellets of pure ZnO nanocrystalline powder have been made with 10 weight % of glass powder at pressure of 260 MPa by hydraulic press machine for 3 hours. These pellets have been sintered at temperatures 200 C - 500 C in an electric muffle furnace for 3 hours at heating rate of 5C/min. After sintering, these pellets have been exposed to humidity in a specially designed humidity chamber at room temperature. It has been observed that as relative humidity increases, resistance of the pellets decreases for entire range of humidity i.e. 10 % to 90 %. The sensing element of ZnO shows best results with sensitivity of 11.13 MΩ/%RH for the annealing temperature of 400 C. This sensing element manifests lower hysteresis, less effect of aging and high reproducibility for annealing temperature 400 C. SEM micrographs show that the sensing elements manifest porous structure with a network of pores that are expected to provide sites for humidity adsorption. The average grain size calculated from SEM micrograph is 236 nm. XRD pattern shows peaks of hexagonal zincite. As calculated from Scherers formula, the average crystalline size for this sensing element is 59.4 nm. For this sensing element, the values of activation energy from the  Arrhenius plot is 0.041 eV for temperature range 200 C - 400 C and 0.393 eV for temperature range 400 C - 500 C. The adsorption of water molecules on the surface takes place via a dissociative chemisorption process leading to release of electrons. ZnO has electron vacancy. Hence, because of this reaction, the electrons are accumulated at the ZnO surface and consequently, the resistance of the sensing element decreases with increase in relative humidity.


Keywords: Zinc oxide, Electrical properties, Humidity sensor, Annealing


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