Sensors and Actuators B 136 (2009) 230–234 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Two mechanisms of conduction in polycrystalline SnO 2  C. Malagù a,∗ , M.C. Carotta a , A. Giberti a , V. Guidi a , G. Martinelli a , M.A. Ponce b , M.S. Castro b , C.M. Aldao b a Department of Physics University of Ferrara, Via Saragat 1/C, I-44100 Ferrara, Italy b Institute of Materials Science and Technology (INTEMA), Universidad Nacional de Mar del Plata-CONICET, Juan B. Justo 4302, B7608FDQ Mar del Plata, Argentina article info Article history: Received 28 July 2008 Received in revised form 29 September 2008 Accepted 21 October 2008 Available online 30 October 2008 Keywords: Metal-oxide semiconductors Tunneling Impedance spectroscopy abstract Impedance spectroscopy was carried out on SnO 2 thick films exposed to different atmospheres. Oxy- gen desorption and adsorption during heating and cooling processes, respectively, onto the grain surface affects the resistance and electrical capacitance of the sensors. Double parabolic Schottky-type potential barriers at grain boundaries were assumed to analyze the tunneling and thermionic contributions to con- ductivity. In addition, the influence of the temperature on the oxygen diffusion into the grains annihilating oxygen vacancies was studied. It was deduced that in-diffusion substantially affects the conductivity. © 2008 Elsevier B.V. All rights reserved. 1. Introduction In the last two decades, gas sensors based on SnO 2 thick films became the dominant solid state devices for the gas detection in domestic, commercial, and industrial alarms [1,2]. It is generally accepted that, when oxygen chemisorbs, electrons transfer from the bulk to the surface of the grain modifying the barriers at the grain boundaries [3]. In particular, after oxygen adsorption, the barrier height and the depletion width become larger and, as a con- sequence, the sample resistance increases. Different factors (such as type of defects, morphology, and additives) contribute to the electrical response of the gas sensor [4–6]. The film conductivity is broadly used to characterize a sensor. Less attention has been paid to the electrical capacitance. Neverthe- less, adsorption of gaseous species on the grain boundaries induces changes in the sensor apparent capacitance [7]. Indeed, for non- overlapped Schottky barriers (when the grain diameter is greater than the depletion layer R ≫ ), the capacitance (C gb ) is related to the electron concentration in the bulk, N d , and the potential barrier height, V s , as [7] C gb ∝ (N d /V s ) 1/2 . (1)  Paper presented at the International Meeting of Chemical Sensors 2008 (IMCS-12), July 13–16, 2008, Columbus, OH, USA. ∗ Corresponding author. E-mail address: malagu@fe.infn.it (C. Malagù). Therefore, a decrease of the capacitance can be related to a higher barrier and/or to the reduction of the donor concentration due to the annihilation of oxygen vacancies. In this work, the resistance and electrical capacitance response of an undoped tin oxide thick film gas sensor is analyzed as a func- tion of temperature in an oxygen atmosphere. In order to explain these results, an electrical equivalent circuit that includes four different contributions to the overall impedance: grain boundary, bulk, electrode contact and the deep bulk traps presence was con- sidered (see Fig. 2). Thermionic and tunneling contributions to elec- trical conduction have been considered to estimate the V s and N d . 2. Experimental Analytical grades of SnCl 2 ·2H 2 O (Baker) were employed as a tin source. Tin dioxide was precipitated at pH 6.25 through dropwise addition of ammonium hydroxide (NH 4 OH) to an acid (HNO 3 ) solu- tion of tin (II) chloride. A series of washings with deionised water were carried out until negative reaction of the filtrate with a con- centrated silver nitrate (AgNO 3 ) solution. The obtained slurry, along with the required amounts of the dissolved additives, was added to a solution of citric acid in ethylene glycol in a 1:4 molar ratio. The mixtures were left at room temperature with constant stirring during 24 h aging with the purpose of promoting the formation of the metallic citrates. Later, a concentrated solution of NH 4 OH was slowly added until a crystalline solution was attained which was afterwards evaporated and heated to promote the polymerization. The solid was heated to 350 ◦ C for 12 h in the beaker to burn off the 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.10.015