Sensors and Actuators A 183 (2012) 34–42 Contents lists available at SciVerse ScienceDirect Sensors and Actuators A: Physical j ourna l h o me pa ge: www.elsevier.com/locate/sna Inﬂuence of humidity on impedance of SbSI gel A. Starczewska a,∗ , M. Nowak a , P. Szperlich a , B. Toro ´ n a , K. Mistewicz a , D. Stró ˙ z b , J. Szala c a Solid State Physics Section, Institute of Physics, Silesian University of Technology, Krasi´ nskiego 8, 40-019 Katowice, Poland b Institute of Material Science, University of Silesia, Bankowa 12, 40-007 Katowice, Poland c Department of Materials Science, Silesian University of Technology, Krasi´ nskiego 8, 40-019 Katowice, Poland a r t i c l e i n f o Article history: Received 28 October 2011 Received in revised form 4 May 2012 Accepted 13 June 2012 Available online 23 June 2012 Keywords: Humidity sensors Antimony sulfoiodide (SbSI) Nanowires Impedance spectroscopy a b s t r a c t This paper presents the effect of water vapor on the electrical response of antimony sulfoiodide (SbSI) nanowires obtained sonochemically to explore its application as a humidity sensor. For the ﬁrst time this material has been studied using impedance spectroscopy. The measurements have been made in nitrogen for various humidities and temperatures. The real part of the total complex impedance is found to decrease by three orders of magnitude with the increase of humidity from 10% to 85%. Inﬂuence of temperature and humidity on relaxation time of SbSI is reported. The least square ﬁtting of the Nyquist characteristics of the investigated gel allowed one to distinguish between different equivalent electric models of the SbSI gel. The changes of the parameters of the model with increasing temperature and humidity are presented. The polarization of water molecules is shown to be a major contributor to the capacitance–temperature characteristics of SbSI gel. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Up to now, there has been a great interest in the develop- ment of humidity sensors for air-conditioning systems, industrial equipments, etc. Many types of materials, such as electrolytes, ceramics, and polymers, have been proposed for humidity sensors by exploiting the variations in their electrical parameters [1]. Usu- ally humidity sensors are divided into two categories: the resistive and capacitive types. The former type of sensors is based on the variation of the real part of the impedance with relative humidity (RH), i.e., when real part (Z ′ ) is greater than the negative of imagi- nary part (-Z ′′ ) of sample impedance. On the other hand, when Z ′ is smaller than -Z ′′ the capacitive part of impedance dominates and the material is suitable as a capacitive-type sensor. In the case of intermediate values of the -Z ′′ /Z ′ ratio, a new-type of impedance metric sensor has been proposed for the detection of water vapour. In this case, the change in the complex-impedance of device is measured as a sensing signal (see e.g., [2]). The antimony sulfoiodide (SbSI) being a semiconductor ferro- electric has an unusually large number of interesting properties [3]. Among them there are the pyroeletric, pyro-optic, piezoelec- tric, electromechanical, electro-optic and other nonlinear optical effects. Recently, the SbSI gel was synthesized under ultrasonic irradiation [4]. During drying of this gel, the relatively big (of a few cm 3 volume), rigid but brittle samples of a so-called ∗ Corresponding author. Tel.: +48 32 603 41 67. E-mail address: Anna.Starczewska@polsl.pl (A. Starczewska). xerogel were obtained. This material is made up of large quan- tity nanowires single crystalline in nature [4]. The surface area of the sonochemically prepared SbSI was estimated as about 75 m 2 /g [4]. The high surface-to-volume ratios associated with these SbSI nanostructures, possessing semiconducting as well as ferroelectric properties, make their electrical properties extremely sensitive to species adsorbed on the surface. The investigations of infrared (IR) absorbance of the sonochemically fabricated SbSI gel have shown a strong adsorption of ethanol, ethoxide species, ethylene and water on the SbSI nanowires [5]. SbSI gel belongs to a complex sys- tem, hence, one can expect that its resistance–temperature and resistance–humidity properties are affected by semiconducting of nanowires, surface adsorption, grain-boundary potential bar- rier and ferroelectric–paraelectric phase transition. Unfortunately, these properties of the SbSI gel remain little known [6]. Since water is accompanied by its own dipole moment, the different amounts of the H 2 O molecules on/in a specimen result in the change of capac- itance. Study of the inﬂuence of humidity on properties of SbSI is important, partly because water vapour may cause an interference problem in sensing various gasses while this material is attractive for sensors [3,7]. Therefore, the aim of the present paper is to use impedance spectroscopy to investigate the effect of temperature and humidity on electrical properties of SbSI gel. 2. Experiment The SbSI was prepared sonochemically from the constituents (the elements antimony, sulfur and iodine), weighed in the stoi- chiometric ratio as described elsewhere [4]. When the preparation 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.06.009