IEEE SENSORS JOURNAL, VOL. 10, NO. 1, JANUARY 2010 39 Enhancement of Ethanol Sensing Properties by Alloying With ZnO Tetrapods Theerapong Santhaveesuk, Duangmanee Wongratanaphisan, and Supab Choopun Abstract—The tetrapods were synthesized using simple thermal oxidation method from Zn and mixture. The tetrapods exhibited single crystalline hexagonal wurtzite structure with the prefer growth direction of along the legs. The sensors based on tetrapods were fabricated and in- vestigated the ethanol sensing properties. The FE-SEM, HRTEM, SAED, XRD, and RS results suggested that alloy was formed with a slightly decrease of c-axis lattice parameter. The decrease of sensor resistance under ethanol atmosphere was observed and suggested that the tetrapods possessed n-type property of semiconductor similar to ZnO. The tetrapod sensors exhibited higher sensitivity than that of pure ZnO tetrapod sensors for entire ethanol concentra- tion with optimum operating temperature of 300 . Thus, the enhancement of sensitivity due to alloying with the ZnO tetrapods was observed and maybe explained by an increase of adsorbed oxygen ions due to substitution of Ti atom into Zn atom. Also, the slope value of the plot between and suggested that adsorbed oxygen ion species at the surface of the tetrapods was which was same as pure ZnO tetrapods. Finally, these results have an important implication for a development of ethanol sensors based on metal oxide semicon- ductors for alcohol breath analyzer. Index Terms—Alloying, gas sensor, sensitivity, tetrapods. I. INTRODUCTION Z NO metal oxide semiconductor with a direct wide-band gap (3.4 eV) and high excitation binding energy (60 meV) at room temperature [1] is well known as promising functional material. It has been widely used in many field areas, such as a light emitting diode [2], [3], transparent conducting oxide ma- terial [4], thermoelectric material [5], field emission device [6], and so on. Recently, alloy materials of ZnO with several ele- ments have been investigated, such as Mg [7], Tb [2], Al [8], and Ce [9]. ZnO alloying with Ti and/or has also been re- ceived great interested due to wide range applications similar to ZnO. Chung et al. [10] have studied electrical and optical prop- erties of doped ZnO films prepared by radio-frequency Manuscript received November 15, 2008; revised January 28, 2009; accepted March 12, 2009. Current version published December 09, 2009. The work of T. Santhaveesuk was supported by Commission on Higher Education, Thailand, under the program Strategic Scholarships for Frontier Research Network for the Joint Ph.D. Program Thai Doctoral degree and Graduated School from Chiang Mai University. The work of S. Choopun was supported by Thailand Research Fund (TRF). The associate editor coordinating the review of this paper and rec- ommending it for publication was Prof. Cristina Davis. The authors are with the APRL, Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand and ThEP Center, CHE, Bangkok 10400, Thailand (e-mail: supab@ science.cmu.ac.th; s_theerapong@yahoo.co.th; dwongrat@chiangmai.ac.th). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2009.2036048 magnetron sputtering. It was found that -doped ZnO film showed lower resistivity and higher band gap energy which can be used for transparent conducting oxide application. Also, Park and Ko [5] have studied thermoelectric property of for thermoelectric generator application, and Xiong and Jiang [11] have investigated ferromagnetic property of Ti-doped ZnO for spintronic application. However, gas sensing property of alloy for gas sensor application has been rarely re- ported especially ethanol sensor. In this work, ethanol sensing properties of the tetrapods prepared by simple thermal oxidation were inves- tigated under ethanol concentration of 50–1000 ppm. The sensitivity of sensor was improved by the effect of al- loying. The enhancement of the ethanol sensing properties was promising for application as alcohol breath analyzer. II. EXPERIMENTAL tetrapods were synthesized by a simple thermal oxidation method. Zn and powders (Zn with 20 mol of ) were weighed, mixed, and grounded in agate mortar for 2 h. Then, mixed powder in the amount of 2 g was loaded into an alumina crucible and put in the center of furnace at tem- perature of 1000 under normal atmosphere for a few min- utes. The alumina crucible was taken out of the furnace, and the white wool products were observed. The synthesized products were characterized by field emission scanning electron micro- scope (FE-SEM), energy dispersive x-ray spectrometry (EDS), transmission electron microscope (TEM), X-ray diffractometer (XRD), and Raman spectrometry (RS) for morphology, chem- ical composition, phase formation, and crystal structure. The tetrapods were fabricated as ethanol sensors by mixing tetrapods with polyvinyl alcohol, pasted onto the alumina sub- strate with gold inter-digital electrode to form thick films, and then dried in air for a few hours. The thick films were annealed at 400 under normal atmosphere for 4 h, and then cooled naturally to room temperature. The ethanol sensing response of the sensors was measured by using a volt-amperometric technique at the operating temperature of 240 –360 and at ethanol concentration of 50, 100, 200, 500, and 1000 ppm. Since application as alcohol breath analyzer was our goal, the ethanol vapor at various concentrations was generated from ethanol solution using alcohol simulator (GUTH laboratory Inc., Harrisburg USA). The alcohol simulator functioned to simulate alcohol concentration at conditions similar to exhaled human breath. The schematic diagram of ethanol sensing characteristic measurement was showed as in [12]. Typically, ethanol sensitivity is defined as [13], [14], where is the electrical resistance of the sensor in air, and is its resistance in ethanol-air mixed gas. 1530-437X/$26.00 © 2009 IEEE Authorized licensed use limited to: Chiang Mai University. Downloaded on December 18, 2009 at 05:08 from IEEE Xplore. Restrictions apply.