Study of activation energy and humidity sensing application of nanostructured Cu-doped ZnO thin films
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In this paper we have reported analysis on activation energy and humidity sensing studies of Cu-doped ZnO thin films. Thin Films of undoped and Cu-doped ZnO nanomaterials were prepared. Undoped and Cu-doped ZnO thin films annealed at 600 °C showed the best results with sensitivity of 20.63 MX/%RH and 39.14 MX/%RH respectively in the 15–95% RH range. Low value of activation energy indicated that this sensing element had low operating temperature and could be used at room temperature. Other parameters like response time, recovery time, hysteresis, and aging effects were also studied. The crystallite size for the sensing element of pure ZnO annealed at 600 °C calculated from Scherrer’s formula is in the 24–38 nm range. For the sensing element of 7% Cu doped ZnO the range of crystallite size is 25–41 nm. The average grain size as measured from SEM micrograph for 7% Cu doped ZnO and pure ZnO sensing elements were 36 and 42 nm, respectively.
I. INTRODUCTION
There are various types of sensors that are used in day-to-day life for specific purposes. Humidity sensor is one of such devices. ZnO is a very versatile compound semiconductor with a band gap of about 3.3 eV. Zinc Oxide is a promising semiconductor for humidity as well as gas sensing applications. Scientists have been studying ZnO thin film and nanostructures for the detection of toxic gases, combustion gases, pollutants, and organic vapors. Humidity or gas systems are also used in various scientific research laboratories worldwide. Researchers are developing cutting edge humidity sensors that show superb sensitivity, low hysteresis, and other amazing properties. Humidity or gas sensors that are developed using ceramic material have attracted much attention due to their chemical and physical stability. By doping ZnO with different elements its electrical properties can be enhanced. Currently, scientists are focusing more and more on impedance or resistive type humidity sensors due to low cost and better performance. Resistive humidity sensors are also developed using polymers. Investigations were also carried out by Jeseentharani et al. for analyzing humidity sensing properties of the composites prepared by mixing 1:1 mole ratio of CuO–ZnO, CuO–NiO, and NiO–ZnO compound. The samples were sintered at 800 °C for 5 h and then subjected to resistance measurements as function of Contributing Editor: Xiaobo Chen a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.322
relative humidity (RH) in the range of 5–98% RH. It was noticed that CuO–NiO compound possessed the best humidity sensitivity. The response and recovery times of the CuO–NiO composites were 80 and 650 s, respectively.1 Kutty et al. have reported the varistor properties of polycrystalline ZnO:Cu.2 Chiou et al. and Bellini et al. extensively studied Cu as an important additive in the ZnO based varistors.3,4 The role of Copper in the Zinc oxide-based sensing materials is different from other dopants such as Mn, Co, Br, and Pt. Jayanti et al. reported that ZnO nanocrystals
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