Sensors and Actuators B 115 (2006) 247–251 Nano-crystalline Cu-doped ZnO thin film gas sensor for CO H. Gong a,∗ , J.Q. Hu a , J.H. Wang a , C.H. Ong a , F.R. Zhu b a Department of Materials Science and Engineering, National University of Singapore, Lower Kent Ridge, Singapore 119260, Singapore b Institute of Materials Research and Engineering, 3 Research Link, Kent Ridge, Singapore Received 8 February 2005; received in revised form 7 September 2005; accepted 13 September 2005 Available online 2 November 2005 Abstract A Cu-doped ZnO (CZO) film was prepared on a glass substrate by co-sputtering using ZnO and Cu targets. The CZO film possessed a columnar structure consisting of small crystals with an average grain size of around 5 nm. The CO-sensing properties of the CZO film were tested at operating temperatures of 150, 300, 350 and 400 ◦ C. It was found that CZO-based sensor exhibited the highest sensitivity to CO at 350 ◦ C. An obvious change in resistance of the CZO film was also observed when the sensor was exposed to 6 ppm CO at 150 ◦ C. The sensing mechanism of the nano-structured CZO sensor is discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Nano-crystalline Cu-doped ZnO; Gas sensor; CO 1. Introduction Several semiconducting oxides such as SnO 2 , ZnO, In 2 O 3 and indium tin oxide (ITO) are employed as gas sensors, by utilizing the changes of the electrical conductivity of these mate- rials upon exposing to target gases [1–5]. The utilization of ZnO in gas sensor applications has a long history. Systems composed mainly of ZnO were studied as chemoresistive materials to detect gases like H 2 [6], NH 3 [7], CH 4 [8],O 2 [9], seafood smell (TMA or trimethylamine) [10], ethanol [11] and CO [12]. It has been suggested that thin film ZnO gas sensors exhibit higher sensi- tivities compared to other forms of ZnO sensors [13]. Carbon monoxide (CO) is one of the most dangerous gases in air pollution and human life. CO is produced by incomplete combustion of fuels and commonly found in the emission of automobile exhausts, the burning of domestic fuels, etc. It is highly toxic and extremely dangerous because it is colorless and odorless. CO sensors are, therefore, required in various situations including the detection of smoldering fires. Nano- structured zinc oxide with diverse morphology of nano-wires, nano-rods and nano-belts has been extensively studied due to its unique physical properties such as wide band gap and large excitonic binding energy and electric conducting properties for ∗ Corresponding author. E-mail address: msegongh@nus.edu.sg (H. Gong). applications in short-wavelength optoelectronic devices, solar cells, and sensors [13,14]. The recent demonstration of gas sensors based on nano-ZnO has further stimulated substantial efforts to explore ZnO nano-structures for high gas sensitivity. However, a nano-ZnO sensor prepared by an arc plasma method did not show an expected high sensitivity even when exposed to CO at a concentration as high as 5000 ppm [14]. Nakamura et al. [15] and Choi and Choi [12] reported a sensitivity towards a few hundreds ppm CO by utilizing CuO–ZnO hetero-contacts, but the grains were not nano-sized. Among the various techniques, direct current and radio fre- quency (RF) magnetron techniques are widely used for prepar- ing ZnO and other metal oxides. The sputtering techniques enable the fabrication of uniform ZnO films on various substrates for device applications. In this work, we use co-sputtering of Cu and ZnO to produce a Cu-doped ZnO (CZO) film and assess its potential for a highly sensitive CO gas sensor. 2. Experimental The CZO thin film was deposited on a glass substrate with a dimension of 10 mm × 20 mm × 1.5 mm. A ZnO tar- get (99.999% pure) and a Cu target (99.99% pure) were used for fabricating CZO thin films. The electric powers used were 5.0 W cm -2 for ZnO and 1.5 W cm -2 for Cu targets. Argon was employed as the operating gas, and a constant partial pressure 0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.09.008