Low concentration CO gas sensing properties of hybrid ZnO architecture A. Tamvakos a, ⁎, D. Calestani b , D. Tamvakos a , D. Pullini a , M. Sgroi a , A. Pruna c,d, ⁎⁎ a Centro Ricerche Fiat, 50 Strada Torino, 10043 Orbassano (TO), Italy b IMEM-CNR, Parco Area delle Scienze 37/A, 43100 Parma, Italy c University Politehnica of Bucharest, 313 Splaiul Independentei, 060042 Bucharest, Romania d Gemmate Technologies, Via Reano, 10090 Buttigliera Alta (TO), Italy abstract article info Article history: Received 3 November 2015 Received in revised form 14 February 2016 Accepted 29 February 2016 Available online 02 March 2016 ZnO thin film and nanowire morphologies were employed to fabricate CO gas sensors by using radio frequency magnetron sputtering and chemical vapour deposition growth. The sensing properties were measured at CO gas concentration ranging from 10 to 50 ppm at varying operating temperature. The results showed a 5-folded sensitivity and half-reduced response time for the hybrid material based on both thin film and nanowires with respect to thin film only, thus indicating the high potential of morphology tailoring, namely, the surface-to- volume ratio and depletion layer, for the enhancement of ZnO sensing performance. © 2016 Elsevier B.V. All rights reserved. Keywords: ZnO CO Sensor Film Nanowire Depletion layer 1. Introduction ZnO is an n-type semiconducting metal oxide which exhibits re- markable properties such as wide band gap (3.37 eV), great variety of available morphologies (thin film, nanowires, tetrapods etc.), amenabil- ity to doping and high chemical stability, beside being highly abundant in nature, low cost and non-toxic [1–3]. Over the last years, it received increased interest for a wide range of applications including gas sensors, optoelectronics, solar cells, light emitting devices, and optical wave- guide devices [4–8]. Thanks to its properties, ZnO is the most widely applied metal oxide material for monitoring various gases, especially for detecting vapours of ethanol, hydrogen, methanol, trimethylamine, ammonia, acetaldehyde, carbon dioxide, xylene, monoethanolamine, etc [9–11]. Given the high applicability of ZnO-based gas sensors in various fields such as automotive industry, monitoring of airborne pollutants, improvement of residential safety or detection of natural gas leaks, much effort is being devoted at industrial level for the development of highly sensitive nano-sensors based on ZnO [12]. In this regard, the de- velopment of gas sensors for the detection of carbon monoxide (CO) is an issue of the day, since CO is one of the most toxic gases. Because of its lack of odour, it can form undetected products by incomplete com- bustion of fuel in industry and in residences which convert it in one of the main environmental contaminants. Numerous methods have been proposed for the synthesis of ZnO nanomaterials. Radio frequency (RF) magnetron sputtering, pulsed laser deposition, electrochemical deposition and chemical vapour depo- sition growth (CVD) are some of the most employed techniques due to various applicability of the obtained materials [1,3,4,12–14]. RF sputtering technique has been indicated as a viable method of preparing metal oxide films for CO sensing applications due to the ease in control over the preferred crystalline orientation, growth at relatively low tem- perature, good interfacial adhesion to the substrate, and the high pack- ing density of the grown film [15]. On the other hand, CVD growth is used to produce high-purity bulk materials and powders, as well as fab- ricating composite materials via infiltration techniques [16]. As a function of synthesis method and resulting morphology, various models were proposed in order to explain the ZnO sensing properties. For example, thin films have been reported to be influenced by the crys- tallite size, d, of the sensor material in conjunction with the space charge depth, L, where the grain size control is recognized as the most sensitive condition [17,18]. On the other hand, in the case of nanostructured ma- terials, other parameters such as surface-to-volume ratio and depletion layer width need to be considered in order to explain the sensing char- acteristics. Since the surface-to-volume ratio is strictly related to the density of the adsorbed oxygen ions, it affects at a large extent the sen- sor response. For example, Hongsith et al. reported a higher ethanol sensing response for both aligned high aspect-ratio nanowires and Microelectronic Engineering 160 (2016) 12–17 ⁎ Corresponding author. ⁎⁎ Correspondence to: A. Pruna, University Politehnica of Bucharest, 313 Splaiul Independentei, 060042 Bucharest, Romania. E-mail addresses: ttamvakos@gmail.com (A. Tamvakos), ai.pruna@gmail.com (A. Pruna). http://dx.doi.org/10.1016/j.mee.2016.02.070 0167-9317/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee