Sensors and Actuators B 96 (2003) 717–722 ZnO sol–gel derived porous film for CO gas sensing Hyun-Wook Ryu a , Bo-Seok Park a , Sheikh A. Akbar b , Woo-Sun Lee a , Kwang-Jun Hong a , Youn-Jin Seo c , Dong-Charn Shin a , Jin-Seong Park a,∗ , Gwang-Pyo Choi b a Chosun University, 375 Susuk-Dong, Gwangju 501-759, South Korea b CISM, The Ohio State University, 2041 College Road, Columbus, OH 43210, USA c Daebul University, Yeongam-Gun, Jeollanam-Do 526-702, South Korea Received 28 March 2003; received in revised form 28 March 2003; accepted 28 July 2003 Abstract Fine powder of ZnO was synthesized by the sol–gel method, which was then calcined followed by spin coating on an alumina substrate and tested for CO gas sensing. With variation in the calcination temperature, the shape of gel powder changed into various morphologies: sheet, needle, and sphere. The grain growth rate increased above 700 ◦ C and the larger grains showed higher degree of crystallization. The organic element in gel powders evaporated below 300 ◦ C. The temperature dependence of the electrical conductance showed the sigmoidal shape, but the temperature range of the constant conductance narrowed with the decrease in the calcination temperature. The optimum sensing properties were observed for the specimen calcined at 500 ◦ C, and it degraded with the increase in calcination temperature because of the larger grain size that limits the surface area for gas–solid reaction. © 2003 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Thick film; CO gas sensor; Sol–gel 1. Introduction Lately, there has been an increasing interest in the ma- terial and electrical properties of inorganic compound that are insulators at low temperatures, but are good conductors at high temperatures [1]. Among them, SnO 2 , TiO 2 , ZnO, Fe 2 O 3 , and ZrO 2 have been used for applications such as gas sensors, varistors, and electrodes [2–9]. In particular, n-type semiconductors such as SnO 2 , TiO 2 , Fe 2 O 3 , and ZnO have extensively been used for detecting reducing gases such as CO, CH 4 and alcohol [7–9]. Zinc oxide (ZnO) is extensively used as a commercial varistor material with good performance characteristics be- cause of its sinterbility to high density at as low as 800 ◦ C. Although efforts are continuing for CO gas sensing using the hetero structure of SnO 2 and ZnO [7,10,11], experimen- tal results on pure ZnO for CO sensing is lacking and it may be related to rapid grain growth and densification. For gas sensors, it is necessary to have a porous microstructure with small particle size yielding large ratio of the surface area to the bulk [8]. In this study, sol–gel technique [2,12,13] was used to ob- tain homogeneous and fine particles of ZnO. We report on ∗ Corresponding author. Tel.: +82-62-230-7193; fax: +82-62-232-0302. E-mail address: jsepark@mail.chosun.ac.kr (J.-S. Park). the optimum processing condition for ZnO powder prepara- tion, phase and morphological characterization of the mate- rial and its electrical properties as a CO gas sensor. 2. Experimental Zinc acetate dihydrate ((CH 3 CO 2 ) 2 Zn·2H 2 O) was dis- solved in a solution of ethyl alcohol and acetylacetone (CH 3 COCH 2 COCH 3 ) at 60 ◦ C for 1 h. The gel powders, after drying at 140 ◦ C for 48 h, were calcined in the tem- perature range of 300–900 ◦ C for 4 h. Pt wires (0.25 mm diameter) were attached to Au pads on an alumina substrate that had printed Au electrode in a comb-tooth configuration. The porous ZnO sensor film was laid on top of the electrode using the spin coating technique followed by annealing at 450 ◦ C for 30 min before the elec- trical and sensing measurement. A mass flow controller (MFC) controlled the concentra- tion of CO gas. The two-probe dc measurement technique was used to measure the electrical and sensing properties. A multimeter (HP34401A) interfaced with a computer sys- tem recorded the electrical reading and displayed simulta- neously. X-ray diffraction (XRD: Rigaku D/Max-3c), scanning electron microscopy (FE-SEM: Hitachi S-4700), thermo 0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.07.010