Sensing of carbon monoxide gas in reducing environments $

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1 Sensors and Actuators B 84 (2002) Sensing of carbon monoxide gas in reducing environments $ Prabir K. Dutta a,*, R.R. Rao a, S.L. Swartz b, C.T. Holt b a Center for Industrial Sensors and Measurements, The Ohio State University, 120 W, 18th Avenue, Columbus, OH 43210, USA b Nex Tech. Materials Ltd., 720 I Lakeview Plaza, Worthington, OH 43085, USA Received 4 October 2001; received in revised form 5 January 2002; accepted 17 January 2002 Abstract The change in resistance of thin-films of CuCl upon exposure to carbon monoxide is developed as the basis for sensing CO gas in reducing environments. Several methods of preparation of thin films of CuCl films were examined. Films prepared by solvent evaporation under ambient conditions showed good sensitivity for CO in the presence of hydrogen gas. The optimum sensing temperature was found to be 50 8C. We propose a hole conduction mechanism for CO sensing by CuCl films. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Metal halide sensors; Fuel cells; CO adsorption 1. Introduction Carbon monoxide (CO) sensors find extensive use as environmental monitors designed to alert occupants to dangerous levels of poisonous CO gas. These sensors can be calorimetric, based on changes of the electronic spectrum of a molecule upon adsorption of CO, as well as sensors based on photoelectric and ionization effects. [1]. Another class of CO sensors uses semiconducting metal oxides, and function at higher temperatures [2]. Most extensive studies have been done on SnO 2 with operational temperatures of C. The basis of sensing in SnO 2 relies on oxygen chemisorption on the SnO 2 grains, leading to the formation of a charge depleted region on the surface that impedes electron conduction across grains. Combustible gases, such as CO react with the adsorbed oxygen thereby increasing conductivity, and forms the basis of sensor response. We have exploited a similar mechanism in the semiconducting oxide TiO 2 to sense CO at temperatures up to 600 8C [3,4]. It is clear that in these metal oxide sensors, the presence of O 2 in the sensing environment is essential. In this paper, we report a strategy for sensing CO in reducing environments. The change in resistance of copper(i) chloride (CuCl) films upon exposure to CO is the basis for sensing. Since Cu(I) is stable under reducing conditions, the possibility exists to make measurements in a H 2 stream. An immediate application of such a sensor would be in $ Presented at Eurosensors XIII. * Corresponding author. address: dutta.1@osu.edu (P.K. Dutta). sensing CO in H 2 -rich streams that feed the anode in fuel cells. The CO in the fuel cell arises from reforming of hydrocarbons and its presence diminishes the electrocatalytic activity for H 2 oxidation. CO coverage on the Pt surface has been found to be the crucial factor for poisoning of the catalyst [5]. Levels of CO that need to be monitored are 0.2 1% in gases exiting the shift reactor and ppm in gas streams that need to be cleaned up by air oxidation. Our choice of CuCl as the active sensing material was based on reports that CuCl is a good adsorbent for CO [6]. Small CuCl crystallites impregnated on high surface area silica gel and alumina adsorb CO effectively. The adsorption of CO proceeds via formation of Cu(CO)Cl [7]. Sorption processes for CO removal have been designed with CuCl adsorbents and found to operate over wide temperature and CO concentration ranges [8]. Reversible adsorption of CO on CuCl polycrystalline films have been noted down to temperatures of 100 K. The morphology and CO adsorptive properties of CuCl polycrystalline films have been explained using SEM and FTIR techniques [9]. Electrical conductivity measurements of CuCl show that it is predominantly an ionic conductor [10 12]. Knauth and co-workers, in a series of papers have studied the conduction properties of CuBr under ambient conditions and also demonstrated that sputtered films of CuBr can be used for NH 3 sensing via resistance measurements [13 17]. They have interpreted the current voltage characteristics of the CuBr films with p-type semiconductivity of the films and a reduced copper deficiency on copper substrates. In this study, we examine various methods of preparation of CuCl and depositions on interdigitated gold electrodes, /02/$ see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S (02)

2 190 P.K. Dutta et al. / Sensors and Actuators B 84 (2002) followed by measurement of resistance changes upon exposure to CO in a H 2 environment. We report that for a certain set of preparation conditions of CuCl, reversible resistance changes upon exposure to CO in H 2 streams is observed. 2. Experimental Copper chloride (99.9%, Aldrich) was used as precursor in the preparation of CO sensors. Samples were prepared by Fig. 1. Change in resistance of CuCl films prepared by various methods upon exposure to 1000 ppm CO at 50 8C. (a) Film made by painting an ink containing CuCl, terpienol and TEOS, measurement made in N 2 ; (b) film made by solvent evaporation under anaerobic conditions, measurement in N 2 :H 2 (1:1); (c) film made by solvent evaporation in ambient air, measurement in N 2 :H 2 (1:1); (d) film made by solvent evaporation in vacuum oven, measurement in N 2 :H 2 (1:1).

3 P.K. Dutta et al. / Sensors and Actuators B 84 (2002) depositing films onto alumina substrates with a gold interdigitated electrode pattern. Several methods of film preparation were investigated. First method was a painting method using an ink prepared by mixing CuCl, Terpinol (few drops) and 10% tetraethylorthosilicate (TEOS). These samples were then heated at 400 8C for 3 h in N 2 and treated with hydrogen at 150 8C for 12 h. All the other methods were based on solvent evaporation. 50 mg of CuCl was dissolved in 5 ml of acetonitrile and nitrogen gas was bubbled through the solution. An alumina substrate was placed on a hot plate and heated to 90 8C. The CuCl solution was added drop wise to the hot substrate, acetonitrile evaporated quickly thereby depositing the CuCl film on the substrate. Another method involved solvent evaporation in a vacuum oven. An alumina substrate was placed in a small beaker and submerged in CuCl solution. The beaker was placed in a vacuum oven and acetonitrile was evaporated thereby leaving the thin film on the substrate. The sample was vacuum dried until all solvent was removed and treated with hydrogen at 150 8C for 2 h. Films prepared by all these three methods were characterized by XRD, electron microscopy and electrical measurements. 3. Results Resistance changes of the samples prepared by different methods upon exposure to CO in an inert or reducing stream were investigated. Below we describe the methods along with the sensing data. (a) Ink method: The heat treatment at 150 8C with H 2 was carried out for annealing the CuCl films after deposition. Resistance changes of the film upon CO exposure was monitored at temperatures between 25 and 400 8C. Resistance changes were only observed over a narrow range of temperatures, with the optimum being about 50 8C. Fig. 1a shows that the background resistance is of the order of 35 MO and shows an increase of a few MO upon exposure to 1000 ppm CO/N 2. (b) Solvent deposition: Three methods were tried. In all cases, the powder diffraction was typical of the nantokite structure of CuCl. (1) Anaerobic deposition: Extreme caution was taken to keep freshly crystallized CuCl from any oxidation during the deposition step from solution. The deposited film of CuCl on the substrate was white in color. Fig. 1b shows that there was no change in resistance upon exposure to 1000 ppm CO/N 2 at 50 8C. (2) Aerobic deposition (Method A): In this method, CuCl was dissolved in acetonitrile in an inert environment and then deposited on the alumina substrate under ambient conditions. Scanning electron micrographs (SEM) showed a porous structure of spherical aggregates of films with particle sizes of mm. The resistance change upon 1000 ppm Fig. 2. Comparison of sensitivities of a solvent evaporated CuCl film to CO (1000 ppm) in N 2 :H 2 (1:1) at various temperatures (R ¼ resistance with CO, R 0 ¼ resistance in background gas). CO/N 2 :H 2 (1:1) exposure at 50 8C is shown in Fig. 1c and the film shows an increased conductivity in the presence of CO. (3) Aerobic deposition (Method B): This method is similar to Method A except that the substrate was immersed in a CuCl/acetonitrile solution and the solvent evaporated under vacuum. SEM shows individual CuCl crystallites of reasonably uniform size of mm. Fig. 1d shows that the direction of the resistance change upon exposure to 1000 ppm CO/N 2 :H 2 (1:1) at 50 8C is similar to Method A, except that the response appears to be larger. The background resistance of these CuCl sensors was not influenced by H 2. In all cases, the optimum sensing temperature was found to be 50 8C, with no perceptible change in resistance over 100 8C. Fig. 2 shows the change in relative resistance upon exposure to 1000 ppm CO for a sensor prepared by Method A at various temperatures. Similar loss of sensitivity with temperature was noted with all films. The effect of humidity was also examined. Upon exposure to a humidified CO stream, the sensor response disappeared and could only be recovered if the gas stream was dried. 4. Discussion This discussion focuses on the comparison of the CuCl sensor with previously reported halide based sensors and concludes by proposing a mechanism to explain the data shown above. In the present study, we observe both an increase and decrease of conductivity upon CO exposure depending on the film preparation method. For films prepared by mixing CuCl and SiO 2, an increase in resistance is observed, whereas for all films made by solvent evaporation, a resistance decrease is observed upon CO exposure. There are two metal halides that have been previously reported for sensing studies, AgCl [18,19] and CuBr [13 17,20]. The species that have been examined are NH 3 and

4 192 P.K. Dutta et al. / Sensors and Actuators B 84 (2002) (CN) 2, and both these molecules form coordination complexes with the metal ion. In the majority of cases, it has been observed that exposure to the gas leads to an increase in resistance. Free standing films of AgCl and AgCl/Al 2 O 3 show increase in resistance upon exposure to NH 3 and (CN) 2 [19]. Sputtered films of CuBr also show increase in resistance upon NH 3 exposure [20]. The response time of these materials is typically much faster than the recovery once the gas stream is turned off. Several mechanisms have been considered for explaining the resistance increase. Adsorption of NH 3 on AgCl was proposed to lead to increase in interfacial resistances. Lauque et al. considered the interfacial resistance mechanism to explain the increase in resistance upon interaction of CuBr with NH 3 [17]. In addition, they also considered that increase of electron concentration in the space charge region upon Cu þ migration to the surface leads to an increase in electron concentration and thereby, a decrease in the majority carriers, which are holes. These effects are possibly also arising in the CuCl/SiO 2 films. Hydroxyl groups on the surface of TiO 2 in CuBr-TiO 2 composites can act as sites for adsorption of Cu(I) [21]. In the present case, the hydroxyl groups of SiO 2 may entrap Cu(I) ions and the interfacial resistance mechanism can explain the observations. The only other reported case where a decrease in resistance upon NH 3 exposure was observed is for annealed films of AgCl [19]. The increase in conductivity for the annealed films was based on the fact that Ag þ conductivity controls the response [18,19]. Upon introductionof NH 3, there is increased accumulation of Ag þ at the gas-solid interface due to complexation with NH 3, leading to increase in Ag þ vacancies in the bulk, resulting in increased conduction. Traces of NH 3 on AgCl prepared from Ag(NH 3 ) 2 Cl lead to considerably higher conductivities as compared to conventionally prepared AgCl, in agreement with the ionic conduction mechanism. Studies done in this paper used a Au/CuCl/Au electrode system. Current voltage curves show non-linear response for cells of the type Au/CuBr/Au and was explained as arising from blocking of the ionic current [17]. For Cu/CuX/ graphite cells, current was proposed to be exclusively carried by holes [22]. So, the conduction mechanism in the present study should primarily be through holes. There are two possible mechanisms to generate holes in copper halides. It has been noted that at low temperatures the conductivity of pure CuX varies due to traces of oxygen or halogens [22]. The oxygen can be incorporated into anion vacancies, creating holes. 1 2 O 2ðgÞþV Cl! O 0 Cl þ 2 h Oxygen doping to create holes has been noted in Hall effect measurements in an ambient environment [23]. Oxygen of the level of 10 ppm, is primarily associated with the surface because bulk diffusion is very slow at room temperature. Sulfur dissolved in CuCl also provides p-type conductivity because of substitution on anionic halogen sites [10]. However, considering that film preparation in this study involved a reduction treatment with H 2 at 150 8C, adsorbed O 2 doping is not playing an important role. The other mechanism to generate holes would be from the non-stoichiometry (Cu 1 x Cl) in the CuCl film. Defect chemistry suggests that copper vacancies are the majority defects in CuBr and these can ionize to give holes. Electronic hole conduction was found to be the dominant conduction mechanism for CuX (X ¼ Cl, Br) films made by evaporation and was explained as arising from Cu(II) formed during the evaporation process [24]. We propose the following tentative mechanism for the increased conductivity with CO exposure to solvent deposited films. The procedure of film formation via solvent evaporation under ambient conditions leads to the presence of Cu(II) on the surface of the CuCl crystallites. Hole migration between CuCl grains is impeded by the Cu(II) layer. Upon introduction of CO, the coordination complex Cu(CO)Cl is formed on the surface, resulting in loss of the barrier due to Cu(II). The response times are fast (seconds) even at moderate temperatures of 50 8C, since a surface controlled mechanism is determining the response. The observation that there was no response to CO if strict anaerobic conditions were followed during film preparation is indicative of the important role that surface Cu 2þ is playing in the sensing mechanism. Current studies are focused on better understanding of the mechanism. The interaction of CO with Cu(I) can be viewed as acid base rather than redox chemistry [18]. This would also be consistent with the temperature dependence, which shows that beyond 50 8C, there is a decrease in sensitivity due to the disruption of the Cu CO bonding. For annealed AgCl films, the change in conductivity upon NH 3 exposure diminished with temperature and explained by decreased complex formation [19]. Similar coordination chemistry arguments can also be made to explain the insensitivity to CO in the presence of water. Water will complex the Cu(II) ions on the CuCl surface, impeding the binding of CO to Cu(I) sites. 5. Conclusions A CO sensor for reducing environment applications is fabricated using thin films of CuCl. The CO sensor shows good sensitivity towards CO even in the presence of hydrogen. Sensitivity can be improved by preparing CuCl thin films by ambient solvent evaporation methods. Formation of Cu 2þ on the surface by oxidation during thin film formation seems to be a crucial step to get a good CO response. We also propose a hole conduction mechanism for CO sensing on CuCl films. Acknowledgements We acknowledge funding from NSF grant number EEC (PKD) and DOE grant number DE-FG02-99ER86099 (SS).

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