Sensors and Actuators B 118 (2006) 208–214
Nanocomposites SnO2/Fe2O3: Sensor and catalytic properties M. Rumyantseva a,∗ , V. Kovalenko a , A. Gaskov a , E. Makshina a , V. Yuschenko a , I. Ivanova a , A. Ponzoni b , G. Faglia b , E. Comini b b
a Chemistry Department, Moscow State University, 1-3 Leninskie Gory, 119992 Moscow, Russia Sensor Lab. CNR-INFM – Dipartimento di Chimica e Fisica, Universit`a di Brescia, Via Valotti 9, 25133 Brescia, Italy
Available online 30 May 2006
Abstract Nanocomposites SnO2 /Fe2 O3 have been obtained in whole concentration range (0–100 mol% Fe2 O3 ) using wet chemical synthesis. The gas sensor properties of nanocomposites towards CO (40–150 ppm), ethanol (10–200 ppm), H2 S (2–10 ppm) and NO2 (50 ppb–10 ppm) have been studied by conductance measurements in temperature range 150–450 ◦ C. Balancing the SnO2 /Fe2 O3 molar ratio sensors performances can be tailored to obtain materials suitable for different applications. High responses towards ethanol together with low humidity effects have been observed for samples with a high Fe2 O3 content (x > 70 mol%). Acidic and redox properties of the samples were characterized by NH3 -TPD and H2 -TPR, respectively. Catalytic properties of nanocomposites were studied in ethanol oxidation using continuous-flow catalytic system. It is demonstrated that increase of Fe2 O3 content reduces the amount of surface acid sites and enhances the oxidizing capability of nanocomposites in ethanol oxidation. © 2006 Elsevier B.V. All rights reserved. Keywords: Semiconductor gas sensor; Tin dioxide; Iron oxide; Catalytic properties
1. Introduction
2. Experimental
Complex oxide nanocrystalline systems (nanocomposites) based on semiconductor oxide (SnO2 ) and catalyst (Fe2 O3 , MoO3 and V2 O5 ) are of great interest for creation of selective semiconductor gas sensors. Second component can form a solid solution in the bulk [1–5] or a monolayer on the surface of major phase crystallites [6–9], or an own phase. Depending on the molar ratio of components, each system differs in nanostructure, redox properties and acidity/basicity of the surface. These parameters determine sensor and catalytic properties of nanocrystalline oxide systems. In present work, we studied the sensor properties of SnO2 /Fe2 O3 nanocomposites with different component distribution toward CO, H2 S, NO2 , and ethanol. The effect of molar SnO2 /Fe2 O3 ratio on sensor properties of nanocomposites was investigated by various physical and physico-chemical methods. The materials prepared were also tested as catalysts in ethanol oxidation.
SnO2 -based samples were prepared by conventional hydrolysis of SnCl4 followed by impregnation of dried resulting gel by Fe(NO3 )3 [1]. Fe2 O3 -based samples were elaborated by hydroxides co-precipitation from solution containing SnCl4 and Fe(NO3 )3 with subsequent thermal annealing. The hydrolysis was realized using an aqueous solution of hydrazine monohydrate, N2 H4 ·H2 O [2,7]. Throughout this paper, compositions will be given as (Fe2 O3 )x (SnO2 )1−x with the ratio [Fe]/([Fe] + [Sn]) equal to 2x/(x + 1). The composition and nanostructure of the powders have been investigated by EDX spectroscopy, XRD, TEM, Raman spectroscopy, BET measurements [1,7] and M¨ossbauer spectroscopy [2]. It was demonstrated that depending on the molar ratio of components and annealing temperature nanocomposites SnO2 /Fe2 O3 exhibit SnO2 -based single phase system, or threephase system composed by SnO2 -, Fe3 O4 - and Fe2 O3 -based solid solutions, or two-phase system of SnO2 - and Fe2 O3 -based solid solutions, or Fe2 O3 -based single phase system [1]. Samples presented in this study differ in the distribution of minor component between the bulk and surface of the grains of major phase. Sample characteristics are summarized in Table 1.
∗
Corresponding author. E-mail address:
[email protected] (M. Rumyantseva).
0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.04.024
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Table 1 Sample characteristics x in (Fe2 O3 )x (SnO2 )1−x (EDX)
Tanneal (◦ C)
Phases (XRD)
Crystallite size (nm) (XRD, TEM)
1 0.975
300 500
␣-Fe2 O3 ␣-Fe2 O3
16 26
75 45
0.72
300
␣-Fe2 O3
16
125
0.04
300
SnO2
4
120
0.015
300
SnO2
4
150
0
300
SnO2
4
120
Sensitive materials have been deposited on alumina substrates provided by interdigitated Pt contacts on the front side and a Pt meander as heater on the backside. Measurements have been carried out by flow through technique in a temperature-stabilized sealed chamber (volume of 1 l) at 20 ◦ C under controlled humidity, working with a constant flux of 0.3 slm. Gas mixtures were generated by certified dry air bottles with diluted target gases concentrations and a humidity system. DC volt-amperometric measurements (U = 1 V) have been carried out to monitor samples’ electrical conductivity (G) during exposure to CO, ethanol, H2 S and NO2 . Responses have been calculated as G/G for reducing gases (CO, ethanol, H2 S) and as R/R for oxidising gas (NO2 ). The catalytic activity of SnO2 /Fe2 O3 nanocomposites in ethanol oxidation was studied at atmospheric pressure in a continuous-flow quartz microreactor. The powdered materials were pelletized under 50 MPa, crushed into 0.5–1.0 mm particles and placed into the reactor between two layers of quartz particles. Prior to catalytic experiment the samples were pretreated at 400 ◦ C in a flow of dry air for 6 h. The reactor was heated with a temperature controlled tubular furnace. Ethanol was delivered by passing the air flow of 60 ml/min through a saturator with ethanol at 0 ◦ C. The weight hourly space velocity (WHSV) was 1 g/(g h). The reaction temperature was varied from 200 to 400 ◦ C. The identification of reaction products was carried out by chromato-mass spectroscopy, using Hewlett–Packard 5890 gas chromatograph with a HP-FFIP column and MS detector HP 5971A. The O-containing reaction products were analyzed on a computer-interfaced M-3700 gas chromatograph equiped by a 6-m Carbowax column; light hydrocarbons, CO and CO2 were analyzed on a CHROM-5 chromatograph with 4-m Porapack-Q column. Temperature-programmed reduction with H2 (H2 -TPR) and adsorption of NH3 for temperature-programmed desorption experiments (NH3 -TPD) were performed using H2 /N2 (5%) and NH3 :N2 = 1:1 gas mixtures, respectively. Before the exper-
Surface area (BET)
Mutual distribution of components
Pure component [1] Sn4+ substitutes Fe3+ in metal positions in Fe2 O3 crystal structure [3] Sn4+ is distributed between bulk and surface of Fe2 O3 grains with formation of SnO2 -like small islands strongly bonded to the surface Fe3+ cations. [1,2] Fe3+ substitutes Sn4+ in metal positions in SnO2 crystal structure with the growth of Fe3+ concentration (segregation) in near surface region [1] Fe3+ substitutes Sn4+ in metal positions in SnO2 crystal structure [2] Pure component [1]
Tests performed Sensor properties
Catalytic properties
+ +
+
+
+
+
+
+ +
+
iments, the samples were pretreated for 2 h at 400 ◦ C in a flow of helium. NH3 -TPD was carried out in helium flow after purging the sample at 50 ◦ C during 60 min to decrease the amount of physisorbed ammonia. The temperature was increased with a rate of 8 ◦ C/min up to 800 ◦ C. 3. Results and discussion 3.1. Sensor properties Responses towards CO as function of working temperature are plotted in Fig. 1. Curves relative to samples with high Fe2 O3 content (x = 0.975 and x = 0.72) have similar shapes. Response intensities are maximized at the working temperature of 250 ◦ C, with intensity increasing with decreasing the iron oxide fraction. Further lowering this fraction down to x = 0.15 enhance such a behavior and increase the working temperature at which response intensity is maximized.
Fig. 1. Responses towards CO (150 ppm) as a function of the working temperature, RH = 30%.
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Fig. 4. Responses towards NO2 (2 ppm) as a function of the working temperature, RH = 30%. Fig. 2. Responses towards H2 S (10 ppm) as a function of the working temperature, RH = 30%.
Fig. 2 shows the sample’s responses towards 10 ppm H2 S versus operating temperature. (Fe2 O3 )0.975 (SnO2 )0.025 and (Fe2 O3 )0.72 (SnO2 )0.28 samples exhibit a similar behavior, with a response versus temperature curve peaked around 150 ◦ C. Such a peak disappeared in the case of the (Fe2 O3 )0.015 (SnO2 )0.985 nanocomposite. Calibration curves presented in Fig. 3 highlight that the sample (Fe2 O3 )0.72 (SnO2 )0.28 has the maximum response in low concentration range but nanocomposite (Fe2 O3 )0.015 (SnO2 )0.985 exhibits more pronounced response dependence versus H2 S concentration without saturation. Sensor’s response towards NO2 was checked within the temperature range 50–300 ◦ C. The shape of response versus temperature curve is similar for each sensor (Fig. 4). Sensor’s signals rapidly increase with the working temperature increasing from 50 to 100–150 ◦ C. Within this range of temperature, response increases from zero (50 ◦ C) to its maximum value. At higher temperatures, response decreases with a lower slope. Nanocomposite (Fe2 O3 )0.72 (SnO2 )0.28 exhibits best performances around 100 ◦ C, while samples (Fe2 O3 )0.975 (SnO2 )0.025 and (Fe2 O3 )0.015 (SnO2 )0.985 have best performances at 150 ◦ C. Sensor signal increases decreasing the sample Fe2 O3 content.
Calibration curves towards NO2 are plotted in Fig. 5. Such curves refer to sensor’s best working temperature. Each sensor (especially (Fe2 O3 )0.975 (SnO2 )0.025 ) starts to exhibit saturation effects around 2–5 ppm. For concentrations higher than 200 ppb, best performances have been obtained with nanocomposite (Fe2 O3 )0.72 (SnO2 )0.28 , but in low concentration range (50–200 ppb) its response becomes lower than observed for sensors with high Fe2 O3 content. Regards ethanol, best performances are reached at 250 ◦ C. High responses have been observed. As an example, a value of G/G = 24 has been obtained for a concentration of 10 ppm. For concentration levels within the 10–100 ppm range, sample (Fe2 O3 )0.015 (SnO2 )0.985 has performances similar to the ones of other sensors (Fig. 6). For higher concentrations, Fe2 O3 -rich sensors undergo saturation effects that have not been observed for (Fe2 O3 )0.015 (SnO2 )0.985 (see calibration curves in Fig. 7). Humidity effects have been investigated. Results highlight (Fig. 7) that responses of Fe2 O3 -rich sensors are slightly influenced by humidity, while for (Fe2 O3 )0.015 (SnO2 )0.985 response increases with decreasing the relative humidity (RH) value. This fact may be explained if one supposes that dissociative adsorption of water on SnO2 surface occurs easier than on Fe2 O3 one [10,11]. At higher humidity this process results in formation of additive OH-groups on SnO2 surface (Brønsted acid sites) and,
Fig. 3. Calibration curves towards H2 S, RH = 30%.
Fig. 5. Calibration curves towards NO2 , RH = 30%.
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Fig. 6. Responses towards ethanol measured at 250 ◦ C, with RH = 30%.
consequently, in decrease of sensor signal towards ethanol (see Section 3.2). Regarding response time it has been observed that sensors respond (and recover) within few minutes toward each tested gas. In summary, gas sensing tests highlights that the x = 0.015 sample exhibits highest response between the tested set of samples. This is consistent with observation that lowering the Fe2 O3 content increases surface area and reduces crystallite size (see Table 1), but the observed differences in sensing performances appear too high to be fully explained by such microstructural features. Further analysis has been carried out involving also pure compounds (x = 0 and 1 in Table 1). Measurements have been restricted to ethanol that exhibit a wide range of oxidation products suitable for a detailed analysis of catalytic process enhanced by different SnO2 :Fe2 O3 molar ratios. Temperature effect on ethanol sensing is reported in Fig. 8. A similar shape of this curves have been observed in ref. [12]. As it was for other gases, (Fe2 O3 )0.015 (SnO2 )0.985 demonstrated maximal sensor signal. Nanocomposites (Fe2 O3 )0.72 (SnO2 )0.28 , (Fe2 O3 )0.04 (SnO2 )0.96 and undoped SnO2 showed the same sensor response, but for the last maximal signal is shifted versus higher temperature range. Pure iron oxide has much lower sensitivity than other samples.
Fig. 7. Calibration curves towards ethanol at 250 ◦ C, RH = 30%, solid line; RH = 50%, dash line.
211
Fig. 8. Sensor response to ethanol vs. measurement temperature.
3.2. Acidic and redox properties The purpose of this study was to analyze the influence of SnO2 /Fe2 O3 molar ratio and specific surface area of nanocomposites on their sensor properties with respect to ethanol. The investigation of nature of surface adsorption sites may give complimentary information useful to explain the sensor properties of nanocomposites. Four samples were selected for these studies: pure oxides SnO2 and ␣-Fe2 O3 , and two nanocomposites (Fe2 O3 )0.04 (SnO2 )0.96 and (Fe2 O3 )0.72 (SnO2 )0.28 with a high sensitivity to ethanol. All selected samples, except pure Fe2 O3 , had similar surface area. Since SnO2 /Fe2 O3 nanocomposites were annealed at low temperature (300 ◦ C) they should contain surface hydroxyl groups [13], which could be considered as Brønsted acid centers. NH3 -TPD method allows obtaining information on the amount of acid sites, as well as on their acidic strength. NH3 -TPD profiles for the samples studied are shown in Fig. 9. Pure SnO2 shows a strong peak at 136 ◦ C that can be assigned to the physically adsorbed NH3 . This peak disappears when the temperature of pretreatment is increased up to 100 ◦ C. The peaks at 290 and 477 ◦ C can be assigned to desorption of ammonia chemisorbed on Lewis acid sites, which are coordinatively unsaturated Sn4+ cations with coordination number 4 and 5, respectively [14]. For pure iron oxide, the peak at 250 ◦ C can be associated with NH3 desorption from surface Fe3+ cations, which also play role of medium Lewis acid
Fig. 9. NH3 -TPD profiles of SnO2 /Fe2 O3 nanocomposites.
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Table 2 Surface acid sites of SnO2 /Fe2 O3 nanocomposites x in (Fe2 O3 )x (SnO2 )1−x (EDX)
A (mol/g)
A/SBET (mol/m2 )
1 0.72 0.04 0
33 93 130 350
0.44 0.74 1.1 2.9
± ± ± ±
0.09 0.08 0.1 0.3
centers [15]. The appearance of peak at 350 ◦ C in NH3 -TPD curve of (Fe2 O3 )0.72 (SnO2 )0.28 nanocomposite may be due to ammonia desorption from Sn4+ strongly bonded to the surface Fe3+ cations [2]. The amount of adsorbed NH3 decreases with the increase of Fe2 O3 content in the samples. The addition of iron oxide results in the diminishing of the physical adsorption of ammonia and the decrease of desorption energy, evidenced by the shift of TPD peaks to lower temperatures. Table 2 shows the amount of acid sites (A) determined from the NH3 TPD curves, supposing that each NH3 molecule desorbes from one site. To get information about redox properties of the nanocomposites, H2 -TPR experiments were performed (Fig. 10). The reduction of catalysts starts at about 200 ◦ C. Pure iron oxide shows two peaks at 363 and 534 ◦ C assigned to reduction of Fe2 O3 into Fe3 O4 and Fe, respectively [16]. According to ref. [17,18] tin oxide has only one reduction peak at 700–760 ◦ C, which can be ascribed to the reduction of SnO2 into metallic Sn. Nanocomposite (Fe2 O3 )0.72 (SnO2 )0.28 exhibits a strong peak at 568 ◦ C, that corresponds to the simultaneous reduction of iron and tin oxides into metals. A weak reduction peak at 338 ◦ C can be attributed to incomplete reduction of Fe2 O3 into Fe3 O4 . This peak is absent on the H2 -TPR spectrum of (Fe2 O3 )0.04 (SnO2 )0.96 . On the other hand, the broad peak at 300–400 ◦ C on the H2 -TPR profiles of (Fe2 O3 )0.04 (SnO2 )0.96 (Fe2 O3 )0.72 (SnO2 )0.28 indicates the reduction of SnO2 to Sn2+ , that may be possible for amorphous tin oxide [18]. Therefore,
Fig. 10. H2 -TPR profiles of SnO2 /Fe2 O3 nanocomposites.
SnO2 /Fe2 O3 nanocomposites have higher oxidizing capability than pure oxides, which is line with earlier observations for SnFe binary oxide catalysts [17]. 3.3. Catalytic properties The study of ethanol oxidation over pure oxides and two nanocomposites (Fe2 O3 )0.04 (SnO2 )0.96 and (Fe2 O3 )0.72 (SnO2 )0.28 has demonstrated that the major reaction product is acetaldehyde. Ethyl acetate, ethylene, ethyl ether and acetic acid are also observed in significant amounts (Fig. 11). Besides that, traces of acetone and some heavy products are also detected. The conversion of ethanol into ethyl acetate and acetic acid decreases with the increase of Fe2 O3 content in the catalysts. On the other hand, conversion of ethanol into acetaldehyde and products of total oxidation (CO2 , H2 O) increases with Fe2 O3 content. Pure Fe2 O3 has lower activity than (Fe2 O3 )0.72 (SnO2 )0.28 , that may be caused by lower surface area of pure iron oxide. The total conversion of ethanol at 200 ◦ C is rather low and increases sharply with temperature, reaching 100% at 300 ◦ C over the samples containing iron oxide whereas over pure tin oxide it achieves 100% at appreciably higher temperature (Fig. 12). Therefore, nanocomposites are better catalysts for oxidation than pure oxides, which is in a good agreement with H2 -TPR results. Testing of the catalysts for longer times show significant decay of catalytic activity with time on stream (Fig. 13). This suggests that the catalyst are deactivating due to the active sites blocking by carbonaceous deposits. The presence of methane in the products proves this assumption.
Fig. 11. Ethanol conversion into various products over pure oxides and SnO2 /Fe2 O3 nanocomposites: (a) at 250 ◦ C and (b) at 300 ◦ C.
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undoped SnO2 and this fact cannot be explained only by specific surface area change. The increase of Fe2 O3 content also leads up to the growth of oxidizing capability of SnO2 /Fe2 O3 nanocomposites in reaction with ethanol vapor. In sensor properties, this effect results in the diminution of temperature of maximum sensor signal. Moreover, the temperature of maximal sensor sensitivity correlates with a temperature of overall ethanol conversion in catalysis experiment. Analysis of whole data set indicates that addition of second component to tin or iron oxide improves both sensor and catalytic properties by changing the type and number of adsorption centers and activity of chemisorbed oxygen. Fig. 12. Temperature dependence of ethanol conversion over different SnO2 /Fe2 O3 nanocomposites.
The present experimental data can be interpreted in terms of a model taking into account the possible mechanisms of alcohol oxidation on oxide surfaces [19,20]. On the surface of oxide nanocomposites, ethanol converts via two reaction routes: dehydrogenation into acetaldehyde (which occurs predominantly in the presence of basic oxides) and dehydration into ethylene (which requires acid surface groups). Both intermediates can further oxidize to CO2 and H2 O:
The increase of Fe2 O3 content in nanocomposites results in the decrease of number of acid sites. This leads to the decrease of the contribution of dehydration into overall process evidenced by the argumentation of acetaldehyde in the products mixture. From the viewpoint of the sensor response, the dehydrogenation process is more attractive because the final reaction product, CH3 CHO, gives rise to a stronger response of semiconductor gas sensors than does the alkene, CH2 CH2 , resulting from the dehydration process [19]. Moreover, dehydration does not increase the electron concentration and, hence, has no effect on the sensor signal. This conclusion is in agreement with the data presented in Fig. 8: all SnO2 /Fe2 O3 nanocomposites demonstrate higher sensor signal towards ethanol as compared with
Fig. 13. Total conversion of ethanol vs. time on stream over different SnO2 /Fe2 O3 nanocomposites at 300 ◦ C.
4. Conclusions Nanocomposites of SnO2 /Fe2 O3 have been synthesized by wet chemical method and tested as gas sensors. Results highlight that balancing the SnO2 /Fe2 O3 molar ratio sensors performances can be tailored to obtain materials suitable for different applications. For example, x = 0.015 sample revealed the most sensitive towards each tested gas (CO, H2 S, NO2 , ethanol). Differently, as far as humidity effects are concerned, its interference is reduced by increasing the Fe2 O3 content to x = 0.7 or higher values. The sensitivity exhibited by the x = 0.015 sample can be partly ascribed to its high specific surface area (150 m2 /g) and reduced grain size (4 nm). The increase of Fe2 O3 content results in the decrease of number of acid sites on oxide surface and in the enhancement of the oxidizing capability of SnO2 /Fe2 O3 nanocomposites. High responses towards ethanol together with low humidity effects have been observed for samples with a high Fe2 O3 content (x > 0.7). Acknowledgement This work has been partly founded by the “Italy and Russia program of bilateral scientific cooperation 2003–2004”. References [1] M.N. Rumyantseva, V.V. Kovalenko, A.M. Gaskov, T. Pagnier, D. Machon, J. Arbiol, J.R. Morante, Nanocomposites SnO2 /Fe2 O3 : wet chemical synthesis and nanostructure characterization, Sens. Actuators B, Chem. 109 (2005) 64–74. [2] V.V. Kovalenko, M.N. Rumyantseva, P.B. Fabritchnyi, A.M. Gaskov, The unusual distribution of the constituants in the (Fe2 O3 )0.8 (SnO2 )0.2 nanocomposite evidenced by 57 Fe and 119 Sn M¨ossbauer spectroscopy, Mendeleev Commun. 14 (2004) 140–141. [3] W. Zhu, O.K. Tan, J.Z. Jiang, A new model and gas sensitivity of nonequilibrium xSnO2 -(1 − x)Fe2 O3 nanopowders prepared by mechanical alloying, J. Mater. Sci. Mater. Electron. 9 (1998) 275–278. [4] J.Z. Jiang, R. Lin, K. Nielsen, S. Mørup, K. Dam-Johansen, R. Clasen, Gassensitive properties and structure of nanostructured (␣-Fe2 O3 )x -(SnO2 )1−x materials prepared by mechanical alloying, J. Phys. D: Appl. Phys. 30 (1997) 1459–1467. [5] P. Bonzi, L.E. Depero, F. Parmigiani, C. Perego, G. Sberveglieri, G. Quattroni, Formation and structure of tin-iron oxide thin film CO sensors, J. Mater. Res. 9 (1994) 1250–1256. [6] Y.-C. Xie, Y.-Q. Tang, Spontaneous monolayer dispersion of oxides and salts onto surfaces of supports: applications to heterogeneous catalysis, Adv. Catal. 37 (1990) 1–43.
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Biographies M. Rumyantseva graduated in chemistry at the Moscow State University in 1992. She received her PhD in 1996 from Moscow State University and from National Polytechnic Institute of Grenoble. From 1997 until now, she has been employed in the Chemistry Department of Moscow State University, where she has been involved in the research on synthesis and characterization of semiconductor materials for gas sensors.
V. Kovalenko graduated in chemistry at the Moscow State University in 2003 and is currently a PhD student in this University. His work is focused on synthesis and characterization of new materials for gas sensors as well as on in situ study of sensing mechanism using M¨ossbauer spectroscopy. A. Gaskov leads the Laboratory of Diagnostics of Inorganic Materials of Chemistry Department of Moscow State University since 1995. He is doctor of science in inorganic chemistry (1988) and professor of chemistry (1993). He has more 30 years experience of research in semiconductor material science, including crystal and thin films growth, heterostructure synthesis, surface analysis, development of semiconductor materials for IR detectors and gas sensors. E. Makshina graduated in chemistry at the Moscow State University in 2004 and is currently a PhD student in this University. Her work deals with the synthesis and characterization of the binary mixed oxides supported on mesoporous molecular sieves as well as with testing the materials in complete and selective oxidation of alcohols. V. Yuschenko graduated in chemistry at the Moscow State University in 1966. She received her PhD in 1969 from Moscow State University. From 1970 until now, she has been employed in the Chemistry Department of Moscow State University. Her work focused on characterization of acid–base and redox properties of materials using temperature-programmed methods. I. Ivanova leads the Laboratory of Kinetics and Catalysis of Chemistry Department of Moscow State University since 2000. She is doctor of science in kinetics and Catalysis (1996). Her research interests lie in the field of heterogeneous catalysis, surface chemistry and application of in situ spectroscopic techniques to the investigation of the mechanism of heterogeneous catalysis. A. Ponzoni received the degree in physics from the University of Parma, Italy, in 2000. He is currently pursuing the PhD degree in Materials for Engineering student at the University of Brescia. His major research activity regards synthesis and characterization of metal oxide for gas sensing applications. G. Faglia received an MSc degree from the Polytechnic of Milan in 1991 with a thesis on gas sensors. Since then he has been studying to obtain the PhD in electronics by the University of Brescia. In 1992, he has been appointed as a researcher by the Thin Film Lab at the University of Brescia. He is involved in the study of the interactions between gases and the tin oxide surface and in sensor electrical characterization. In 1996, he has received the PhD degree by discussing a thesis on semiconductor gas sensors. E. Comini received graduate degree in physics from the University of Pisa in 1996. She is presently working on chemical sensors with particular reference to deposition of thin films by PVD technique and electrical characterization of MOs thin films. She also received PhD degree in material science from University of Brescia in 2000. She is now working at the University of Brescia.
