Sensors and Actuators B 160 (2011) 1043–1049 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Alcohol-sensing characteristics of spray deposited ZnO nano-particle thin films C.S. Prajapati, P.P. Sahay ∗ Department of Physics, Motilal Nehru National Institute of Technology, Allahabad 211 004, India a r t i c l e i n f o Article history: Received 9 June 2011 Received in revised form 7 September 2011 Accepted 8 September 2011 Available online 14 September 2011 Keywords: ZnO nano-particle thin films Spray pyrolysis Alcohol sensing Sensor response a b s t r a c t ZnO nano-particle thin films were deposited on cleaned glass substrates by spray pyrolysis technique using the precursor solution of zinc acetate dihydrate [Zn(CH 3 COO) 2 ·2H 2 O]. Structural analyses and sur- face morphology of the resulting films were carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD analyses confirm that the films are polycrystalline zinc oxide, possessing hexag- onal wurtzite structure with crystallite size ∼25 nm. The SEM micrograph of the film shows a good uniformity and a dense surface having spherical-shaped grains. Alcohol sensing characteristics of the deposited films have been investigated for various concentrations of methanol, ethanol and propan- 2-ol in air at different operating temperatures. At 150 ppm concentration, the film shows maximum response (85.2%) to propan-2-ol at the operating temperature of 250 ◦ C; whereas at the same concentra- tion 150 ppm, the maximum responses to methanol and ethanol at 300 ◦ C are observed to be 75.8% and 52.4%, respectively. Also, the film exhibits selective high response to propan-2-ol, followed by ethanol and methanol, respectively at each operating temperature up to 275 ◦ C. This selectivity is more pronounced in the region of lower operating temperatures and concentrations. A possible reaction mechanism of alcohol sensing has been proposed. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The presence of volatile organic compounds (VOCs) in the environment has become a serious concern due to tightened envi- ronmental regulations on VOCs all over the world. Their speedy evaporation and toxic or carcinogenic nature can make the pres- ence of high concentrations of VOCs in air which is harmful to human beings [1]. The VOCs are also recognized as the main cause of sick house syndrome, which is a result of poor indoor air qual- ity [2]. Under the category of VOCs, alcohols (methanol, ethanol and propan-2-ol) are widely used in many applications. Methanol is a very useful organic solvent and widely used in manufacturing of dyes, colors, perfumes, formaldehyde, etc. However, it is highly toxic and often fatal to human beings. Ethanol is the most com- mon alcohol which is encountered by the people all over the world in day to day life. Detection of ethanol is necessary for the control of drunken driving and the monitoring of fermentation and other processes in chemical industries. Propan-2-ol, though not nearly as toxic as methanol, is oxidized by the liver into acetone by alco- hol dehydrogenase [3]. Symptoms of isopropyl alcohol poisoning include flushing, headache, dizziness, nausea, vomiting, etc. Thus, there is a need to monitor the presence of the alcoholic VOCs in air for environmental control and safety. ∗ Corresponding author. Tel.: +91 532 2271260; fax: +91 532 2545341. E-mail address: dr ppsahay@rediffmail.com (P.P. Sahay). Among the available metal oxide semiconductors for gas sensing applications, zinc oxide (ZnO) is a potential candidate for toxic and combustible gas sensors, due to which this material has drawn con- siderable interest to researchers for the development of an alcohol sensor having higher sensitivity along with optimization of selec- tivity [4–11]. The crystallite size and morphology of the ZnO thin films have great influences on the performance of the gas sensors based on these films [12,13]. In recent years, ZnO nano-particle thin films have received growing attention due to their interesting elec- trical and optical properties arising out of large surface-to-volume ratio, quantum confinement effect, etc. It has been established that ZnO nano-particle thin films can be prepared by chemical route techniques like spray pyrolysis [12], sol–gel [14], dip coat- ing [15], SILAR [16], etc. In the present investigation, we have used spray pyrolysis technique to prepare ZnO nano-particle thin films because in this technique, the deposition rate and the thickness of the films can be easily controlled over a wide range by changing the spray parameters. In the literature, a lot of studies on the sensor response to ethanol have been reported [17–21], but comparatively a little attention has been paid to the investigation of methanol and propane-2- ol sensors [22–24]. Ethanol sensors based on ZnO and Au-doped ZnO nanowires have been studied by Hongsith et al. [17]. They have shown that the response of Au-doped nanowires to ethanol is more as compared to that exhibited by the undoped nanowires. Hsueh et al. [18] have reported that the ethanol gas response at 300 ◦ C exhibited by the laterally grown ZnO nanowires is 61% for 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.09.023