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Sensors and Actuators B 137 (2009) 637–643

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

omparative study of nanocrystalline SnO2 materials for gas sensor application:hermal stability and catalytic activity

.G. Pavelkoa,∗, A.A. Vasilieva, E. Llobeta, X. Vilanovaa, N. Barrabésb, F. Medinab, V.G. Sevastyanovc

University Rovira i Virgili, Department of Electronic, Electrical and Automatic Control Engineering (DEEEA), 43007 Tarragona, SpainUniversity Rovira i Virgili, Chemical Engineering Department (DEQ), Tarragona, SpainN.S. Kurnakov Institute of General and Inorganic Chemistry RAS, Moscow, Russian Federation

r t i c l e i n f o

rticle history:eceived 3 October 2008eceived in revised form6 November 2008ccepted 10 December 2008vailable online 24 December 2008

a b s t r a c t

In this paper we discuss thermal stability of tin dioxide nanoparticles and long-term stability of theSnO2-based gas sensors. Relying on laser-spark element analysis and XRD analysis we compare crystallitegrowth kinetics of synthesized and commercial (Sigma–Aldrich) SnO2 with different impurity level. Wefound that commercial SnO2 nanopowder has lower thermal stability in comparison with synthesizedone, resulting in higher crystallite growth rate and poly-dispersed particle size after annealing. A specialattention is drawn to discussion of catalytic activity of the nanocrystalline materials. Using IR spectroscopy

eywords:as sensornO2

ong-term stabilityhermal stabilitympurity level

we studied chemical composition of the annealed materials, which allowed us to reveal in the synthesizedSnO2 intensive formation of Sn–OH bonding during annealing. We performed temperature programmedreduction (TPR) analysis to compare the catalytic activity of the materials with and without deposited Pdon SnO2. Long-term stability of the sensors made on the basis of synthesized and commercial SnO2 wasmeasured as a sensor signal deviation during 590 h of operation in 0.2, 0.6 and 1.0 vol. % of propane in air(50% RH at 20 ◦C).

rystallite growth

. Introduction

During last decades, the development of thick film materials forin dioxide based gas sensors enabled the fabrication of sensitivend relatively selective gas sensors. However, long-term stabilityf the sensors remains an important problem, which restricts thepplication of such devices in industrial and residential gas andre alarm systems, electronic noses, etc. [1,2]. The attempts toolve this problem by chemical modification of the sensing materialhowed that this approach gives hard to predict gas-sensing proper-ies, because each nanoparticle of SnO2 contains only few atoms ofopant. As a result, the number of admixture atoms is random andan be very far from the one calculated using average concentrationalues.

In this research, we apply a completely different approachhat consists of synthesizing a very pure tin dioxide materialith extremely low level of sodium, chlorine, and sulfur contam-

nation. Using different techniques (laser-spark element analysis,hermal X-ray diffraction, temperature programmed reduction andR spectroscopy) we study both home-made and commercial SnO2anopowders to estimate the influence of the impurities on crys-

∗ Corresponding author. Tel.: +34 977 558764; fax: +34 977 559605.E-mail address: rpavelko@yandex.ru (R.G. Pavelko).

925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2008.12.025

© 2008 Elsevier B.V. All rights reserved.

tallite growth rate during isothermal annealing at 700 ◦C, as wellas the catalytic activity and surface composition of SnO2 materials.These results are compared with the ones on long-term stability ofthe sensors fabricated using the same SnO2 materials. The role ofthe impurities in the long-term stability of SnO2-based sensors isdiscussed.

2. Experimental

2.1. Synthesis and characterization of the nanomaterials

As a precursor for the synthesis of tin dioxide nanopowder, weused tin (IV) acetate (Sigma–Aldrich) solution in acetic acid. Tinoxide was synthesized by the precipitation from this solution bydrop-by-drop addition of an aqueous NH3 solution. After the precip-itation, the sample was centrifuged and washed several times withdeionized water. Then the powder was dried carefully at 150 ◦C andannealed at 300 ◦C for 12 h. As a reference material SnO2 nanopow-der fabricated by Sigma–Aldrich (p/n 549657) was used.

The impurity content in both materials was measured by

laser-spark mass-spectrometry (EMAL-2). Transmission electronmicroscopy (TEM) was used to establish the morphology and toevaluate the particle size distributions (Jeol JEM 1011, operating at100 kV). FTIR spectra were recorded using JASCO 680 Plus spec-trometer (absorption mode, scan times—32, resolution—2 cm−1) at

6 d Actuators B 137 (2009) 637–643

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Table 1Concentration of main impurities in SnO2 materials.

SnO2 material Impurity content (wt. %)

Na Cl S

by several reasons. The usual process of formation of metal oxidesensing layers in semiconductor and thermocatalytic gas sensorsrequires annealing at 700–800 ◦C [5]. This could be used not onlyto remove carbon traces of organic vehicle, but for catalyst activa-tion and to stabilize phases as well. The latter sometimes requires

38 R.G. Pavelko et al. / Sensors an

oom temperature. The samples were mixed with KBr powder andressed into self-supporting disks.

The crystallite growth kinetics was studied by means of Siemens5000 diffractometer (Bragg-Brentano parafocusing geometry, ver-

ical � − � goniometer, Ni-filtered CuK� radiation) equipped withn Anton-Paar HTK10 heating platform (TXRD). The data wereecorded during isothermal annealing at 700 ◦C after 1, 6, 18 and0 h. Crystallite size was determined using the Fundamental Param-ters Approach convolution algorithm implemented in the programOPAS 3.0 [3]. It was assumed for all samples that only crystalliteize affects the XRD peak broadening and no effects of microstrainsere observed in the XRD line width. For the calculation of the crys-

allite size, we used three diffraction peaks corresponding to 110,01, 121 planes of SnO2 cassiterite structure. The calculations wereerformed according to the Scherrer’s law from the net integralidth of the peaks.

Temperature programmed reduction (TPR) study of synthesizednd commercial SnO2 was performed using a ThermoFinniganPD/R/O 1100 instrument equipped with a thermal conductivityetector (TCD). Before the TPR experiments, the samples (100 mgf each material) were dried for 24 h in helium flow (20 ml/min) at20 ◦C. After that, the reduction process in the reducing gas mix-ure flow (5% H2 in argon, flow rate was equal to 20 ml/min) wastarted at room temperature and finished at 800 ◦C at a heating ratef 10 ◦C/min. Hydrogen consumption was measured using thermalonductivity detector. The TPR study was carried out for pure oxidesnd for the oxides with deposited catalyst (1 wt. % of Pd).

N2 physisorption adsorption–desorption isotherms at 77 K waseasured using Micromeritics ASAP 2010 surface analyzer. Before

nalysis, all the samples were degassed in vacuum at 393 K for 6 h.

.2. Sensor fabrication

Both synthesized and commercial SnO2 materials were impreg-ated with water solution of palladium complex Pd(NH3)4(NO3)2.hen, the powders were dried at 110 ◦C, calcined at 300 ◦C for 10 hnd annealed at 500 ◦C in order to achieve the Pd complex decom-osition. The preset catalyst to carrier ratio was equivalent for allensing materials: wPd = 1 wt. %. Then the powders were mixed withhe solution of ethyl cellulose in terpineol to form printable ink. Thenk was deposited on sensor microheater platform, dried for 15 mint 150 ◦C and annealed at 700 ◦C for 15 min to reach the stabilizationf the material.

The sensor platforms were made of screen-printed refractorylass, platinum heaters and electrodes, assembled in TO-8 package.he details can be found elsewhere [4].

.3. Sensor characterization

The sensor responses were measured in 0.2, 0.6 and 1.0 vol. %f propane in synthetic air at 50% RH. The gas concentrations andas humidification were prepared with gas mixing system Envi-onics 4000 at 20 ◦C. Before the measurements all sensors weretabilized at operation temperature 450 ◦C during 100 h. The firsteasurements, taken for zero point, were carried out after the sen-

or stabilization, then after 330, 460 and 590 h of operation. Theensors were exposed to the target gas during 10 min, which wasollowed by 10 min exposition to pure carrier gas (synthetic air, 50%H). The sensor signal was defined as a ratio Rair/Rgas, where Rgas

s the sensing layer resistance in target gas atmosphere and Rairs the resistance in pure carrier gas. Long term stability was esti-

ated as a total deviation of sensor signal after certain periodsf time and was defined as �((Sn − Sn−1)/Sn−1) × 100%, where Sn

s sensor signal after 330, 460 or 590 h and Sn−1 is the previous sen-or (e.g. (S330 − S0)/S0, where S0 and S330 are sensor signals after thetabilization and 330 h of operation respectively).

Synthesized 0.010 0.009 0.001Commercial 0.228 0.328 0.011

3. Results and discussion

3.1. Material properties

The impurity level of synthesized and commercial SnO2 wasestimated by means of laser spark mass-spectrometry for morethan 20 widespread elements. It was found that commercial mate-rial is highly contaminated with Na and Cl ions (Table 1). High levelof contaminations is related, probably, with the use of tin chlo-ride and sodium compounds as precursor and precipitating reagent,respectively, in the preparation process of tin dioxide nanopowderused by Sigma–Aldrich. The material did not become notably purereven after six-fold washing in hot deionized water. Therefore, wecan conclude that byproducts of the SnO2 synthesis are partiallyincorporated in the crystalline lattice or tight pores of the materialand could not be removed completely using conventional chemicalmethods.

Synthesized and commercial SnO2 have BET surface area of148 m2/g and 17 m2/g, respectively, which corresponds to parti-cle sizes of 3 nm for synthesized and 26 nm for commercial SnO2(calculated assuming spherical particle shape).

TEM investigation of the oxides shows that synthesized SnO2consists of uniform particles with mean diameter 3–5 nm (Fig. 1).Particles of commercial SnO2 are polydisperse: the least diameteris ∼10 nm, while the largest is ∼70 nm (Fig. 2a). These results are ingood agreement with BET area of the materials.

Both materials have cassiterite type crystalline structure. Themean crystallite size of SnO2 nanopowders before heat treatment,calculated using TOPAS software, was equal to 1.3 and 30 nm forsynthesized and commercial SnO2, respectively. The temperaturerange for TXRD study of crystallite growth kinetics was chosen

Fig. 1. TEM image of synthesized SnO2 (Jeol, JEM 1011).

R.G. Pavelko et al. / Sensors and Actu

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ig. 2. TEM images of commercial SnO2 before (a) and after (b) annealing at 700 ◦Cor 1 h.

everal hours of annealing. In addition, the experimental error inrystallite size calculation for commercial material was equal to.6% at 700 ◦C, which increases significantly at lower temperaturesue to fewer changes in crystallite size.

Fig. 3 shows the results of the simulation of the crystallinerowth during annealing at 700 ◦C. The simulation was performedsing a commonly assumed parabolic law describing isothermalrystallite growth:

N − DN = Kt, (1)

here D0 is the initial crystallite size, D the current crystallite size,the rate constant, N the grain growth exponent factor and t the

nnealing time.The difference between both materials becomes more evident,

f we compare their integral crystallite growth rate. The integralate, calculated as �D/t (where �D the change in crystallite sizeor 30 h of annealing and t the annealing time in hours) was found

qual to 0.026 and 0.14 nm/h for synthesized and commercial SnO2,espectively. The main contribution to high integral growth rate ofommercial SnO2 was made during the first 2 h of annealing whenhe rate was by a factor of 7 higher than for the synthesized SnO2.

ig. 3. Crystallite size of synthesized and commercial SnO2 as a function of annealingime at 700 ◦C.

ators B 137 (2009) 637–643 639

The difference in crystallite growth rates could be explainedby several factors. First, the materials have different morphology,which complicates the comparison of the results. According to[6] for example, porosity of nanomaterials plays significant rolein crystallite growth kinetics. The nitrogen physisorption revealedthat commercial SnO2 has lower porosity in comparison with syn-thesized material by a factor of ∼2.5 (pore volume equals 0.16and 0.06 cm3/g for synthesized and commercial SnO2 respectively),which is believed should decrease thermal stability. However, themost drastic difference between two materials is related to theirchemical composition. The Na+ and Cl− content in commercial SnO2is more than by 20 times higher in comparison with synthesizedone. And it is well known that admixtures (especially with high dif-fusion coefficient like Na+ and Cl−) in a quantities as low as 10−6 to10−4 at. % could notably favor grain growth by increasing the defectdensity of the crystalline lattice [7].

After 30 h of isothermal annealing, TEM particle size for synthe-sized SnO2 equals ∼7 nm and particles remain uniform in shape,while for commercial SnO2 the particles are in the range 20–150 nm.The change in the particle size affects the electrical properties of thematerial and this will have negative impact on long-term stabilityof the sensor [8].

In addition, we found that after 1 h of isothermal annealing, anew phase appears on the surface of commercial SnO2 particles(Fig. 2b). This new phase is a result of the thermal migration ofimpurities (Na and Cl ions), which presence on the surface can leadto inhibition of catalytic processes [9].

FTIR spectroscopy of nanomaterials can provide useful informa-tion about chemical modification of the solid. The spectra wererecorded on JASCO 680 Plus spectrometer at room temperaturefor SnO2 samples, which undergone isothermal annealing at 700 ◦Cduring 1, 6, 18 and 30 h. The spectra of commercial and synthesizedpowders are shown in Fig. 4.

While the spectrum of commercial SnO2 is quite simple withonly one intense band between 400 and 800 cm−1, the spectrumof synthesized SnO2 shows several intense bands: a broad onebetween 400 and 850 cm−1, a peak around 1620 cm−1, and anotherbroad band at 2000–3650 cm−1. The first band is assigned to dif-ferent stretching vibration of Sn–O bond and during isothermalannealing. The maximum of the band shifts to the higher wavenumbers that is from 590 to 630 cm−1 after 30 h of annealing at700 ◦C. The right shoulder of the peak also becomes less intense.The similar phenomenon is observed for commercial material. Aweak shoulder near 510 cm−1 looses intensity with annealing time,while the peak at ∼650 cm−1 becomes more intense.

The shoulder in the region 520–560 cm−1 corresponds to ter-minal vibration of Sn–O bond, whereas vibrations at higher wavenumbers can be ascribed to bridged ones [10]. Thus, a decreasein adsorption band at around 500 cm−1 could be a result of ter-minal group lessening. This allows us to conclude that eitherchemical modification of the surface or growth processes occurduring isothermal annealing. The following discussion will showthat the chemical modification of the surface is more signifi-cant.

The bands in the range of 2000–3650 cm−1 can be assignedto hydroxyl stretching vibrations �OH(Sn–OH) of both, terminaland bridged groups, and to stretching vibration in hydrogen bondOH···O. During the annealing, the band becomes more intense andseveral shoulders appear on the right and on the left side of theband maximum, located at 3416 cm−1. The latter, together witha small left shoulder around 3230 cm−1, should be ascribed to

stretching vibrations of bridged Sn–OH bond. The right shoulderaround 3545 cm−1 is related, probably, to stretching vibrations ofterminal Sn–OH group [10]. The maximum of the band does notshift during annealing, which indicates that new types of hydroxylbonding do not take place on the surface. Commercial SnO2 has a

640 R.G. Pavelko et al. / Sensors and Actuators B 137 (2009) 637–643

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ig. 4. FTIR spectra in the range 400–4000 cm−1 of synthesized (a) and commercialb) SnO2 after drying and after isothermal annealing at 700 ◦C for 1, 6, 18 and 30 h.

eak in the same range 3250–3650 cm−1 with much lower inten-ity, which is an evidence of lower concentration of hydroxyl groupsn the surface. The broad nature of the band indicates that OHroups are bonded by several modes, including Sn–OH (termi-al groups), Sn–(OH)–Sn (bridged groups) and hydrogen bondingn–O···HO–Sn.

The band around 1620 cm−1 corresponds to the bending modef ligand-bonded molecular water (Fig. 5a). It is known thatonomeric, dimeric and polymeric water molecules have their

eformation modes at 1600, 1620 and 1633 cm−1, respectively [11].he evolution of the peak shape with annealing time indicates that,fter approximately 6 h, there are mostly dimers and polymers ofater molecules on the SnO2 surface (1616 and 1635 cm−1). The

ame situation is observed with commercial material. The weakeak at 1620 cm−1 is a superposition of two peaks at 1616 and635 cm−1. However, the quantity of adsorbed water increases withnnealing time only in the case of synthesized SnO2, whereas com-ercial material loses molecular water during annealing (decrease

n peak intensity).In a range of 900–1250 cm−1, a broad intense band appears after

h of annealing (Fig. 5b). According to numerous investigations, thisand corresponds to the deformation mode ıOH(Sn–OH) of termi-

al OH groups [10]. It is remarkable that the spectrum of the SnO2owder dried after wet synthesis does not have any correspondingibration mode. The intensity of the band increases with annealingime. The band maximum at 1040–1045 cm−1 does not shift duringeat treatment.

Fig. 5. FTIR spectra in the range 1300–1900 cm−1 (a) and 850–1300 cm−1 (b) of syn-thesized SnO2 after drying and after isothermal annealing at 700 ◦C for 1, 6, 18 and30 h.

Summarising, the FTIR spectra manifest that surfaces of synthe-sized and commercial SnO2 are very different from the chemicalpoint of view. The change in spectrum shape of synthesized SnO2 ismostly due to the intensive formation of Sn–OH groups and accu-mulation of molecular water during annealing. Taking into accountthat desorption of chemisorbed water in the range of 280–400 ◦Cresults in formation of catalytically active oxygen species (e.g. O2

−,O− and O2−) the synthesized SnO2 probably will demonstrate in thesame temperature range high catalytic activity in oxidation reactionand as a result higher sensitivity to reducing gases [12].

To investigate the oxidation activity of synthesized and com-mercial materials, we performed TPR study of SnO2 with andwithout Pd (Fig. 6). TPR profile of the synthesized SnO2 displaysone broad intense peak at 360 ◦C, followed by the onset of bulkreduction (that is the reduction of SnO2 particle bulk) starting from550 ◦C. Commercial SnO2 shows a different profile with small peakaround 200 ◦C and bulk reduction starting from 450 ◦C. The lowtemperature reduction peak can be attributed to the reduction ofsurface and subsurface of SnO2, while the bulk reduction of SnO2is observed at higher temperature (>500 ◦C). From the results onecan easily see that synthesized SnO2 demonstrates a higher surface

reduction compared with the commercial one. This indicates thepresence of high amount of active oxidation species, which prob-ably form due to thermal desorption of chemisorbed water. Thepresence of the latter in the synthesized SnO2 was proved by FTIRstudies.

R.G. Pavelko et al. / Sensors and Actuators B 137 (2009) 637–643 641

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ig. 6. TPR profiles of pure (dash line) and with deposited Pd catalyst SnO2 (boldine).

Whereas the deposition of 1% of Pd does not affect significantlyhe reduction profile of the commercial SnO2 (a slight increase inhe reduction of surface species is observed), the profile of syn-hesized Pd-SnO2 solid shows a clear shift to lower temperatures.his reduction peak is shifted from 360 ◦C to 200 ◦C. However,he reduction peak for bulk SnO2 species remained constant. The2 oxidation already starts at 100 ◦C and goes on up to 400 ◦C,emonstrating significant decrease in activation energy of oxi-ation processes. This fact is only observed for the synthesizedamples.

Thus, the TPR study has proved our assumption about high cat-lytic activity of synthesized SnO2 and it becomes apparent thatigh content of chemisorbed water could be an evidence for highxidation ability of material at elevated temperatures.

.2. Sensor characterization

Long-term stability measurements were performed for 590 h.ll sensors underwent phase stabilization at operating tempera-

ure 450 ◦C during 100 h. Then sensors were exposed to differentropane concentrations: 0.2, 0.6 and 1.0 vol. % in humid air (50%H at RT). The first measurement was carried out just after stabi-

ization, which is considered as zero point for long-term stabilityxperiment, then after 330, 460 and 590 h. We used sensing mate-ials on the basis of synthesized and commercial SnO2 with 1 wt. %f Pd.

As it can be seen from Fig. 7, the responses of the sensors differreatly with sensing materials. Sensors on the basis of synthesizednO2 have the highest resistance in humid air, which suggests loweroncentration of free charge carries in the bulk material. The lat-er could be a result of high surface population with the electroncceptors like oxygen species: O2

− and O− which is a result ofhemisorbed water desorption. On the other hand, the resistancender target gas is of the same order of magnitude as the resistancef the sensors on the basis of commercial SnO2

The use of synthesized SnO2 results in the highest signalsdefined as Rair/Rgas) to all propane concentrations. For example,he signal to 0.4% of propane in synthetic air at 50% RH after sensortabilization is equal to 42 for synthesized SnO2 and 4.5 for com-

ercial SnO2. The drastic difference between both materials can be

xplained by higher catalytic activity and high specific surface ofhe synthesized material.

A distinctive feature of commercial SnO2 is that the signals to allropane concentrations increase with operation time. The crystal-

Fig. 7. Sensor layer resistance of synthesized (a) and commercial (b) SnO2 with 1 wt.% of Pd to 0.4% of propane in synthetic air at 50% RH.

lite growth cannot lead to this phenomenon. We explain this relyingon TEM results, which demonstrated the formation of a new phaseon the SnO2 surface (Fig. 2b). After 100 h of phase stabilization at450 ◦C, chlorine impurities, known as catalyst inhibitor, cover thesurface causing poor sensitivity to target gas [9]. This inhibitor, inits turn, takes part in the oxidation processes and vacates the sur-face with oxidation byproducts. The elimination of the inhibitor (Cl)from the surface leads to the signal increment.

Unlike the previous, the sensing material on the basis of synthe-sized SnO2 demonstrates a slight decrease in signal value duringthe operation. We suppose that signal decrease is a result of crys-tallite thermal growth and surface diminution, which takes place,without any doubts, in both sensing materials. The difference is thatimpurities, apart from causing higher crystallite growth rate, leadto the ambiguity in the surface chemistry by interacting with targetgas and its oxidation products.

Fig. 8 shows results of long-term stability measurements. Theplot reflects total signal drift defined as �(Sn − Sn−1/Sn−1) × 100%(where Sn−1 is previous and Sn is current signal after a cer-tain operation time) against operating time for different propaneconcentrations. The highest signal drift is obtained for sensing

materials on the basis of commercial SnO2. The highest contribu-tion to the signal deviation for both types of sensors was observedafter 330 h of sensor operation. The total signal drift after 590 hvaries (depending on the target gas concentration) from 0.7% to1.4% for synthesized SnO2 and from 16% to 25% for commercial

642 R.G. Pavelko et al. / Sensors and Actu

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ig. 8. Total signal drift depending on propane concentration against operation timef the sensors.

nO2 in the concentration range under investigation. The hightability of the synthesized material can be explained only by ther-al stability of nanoparticles which is a result of low impurity

ontent.

. Conclusion

XRD crystallite size and TEM particle size analysis indicate theemarkable thermal stability of synthesized SnO2 with low impu-ity level. On the other hand commercial SnO2 with Na and Clmpurities has poor thermal stability resulting in higher crystalliterowth rate and poly-dispersed particle size after annealing. It washown that during isothermal annealing, impurities tend to formndividual phase on the particle surface of commercial SnO2.

FTIR results manifest significant difference in chemical compo-ition of the synthesized and commercial materials which mostlyue to the intensive formation of Sn–OH groups and accumulationf molecular water after annealing. This fact can be explained byigh density of surface defect due to low particle size of synthesizednO2.

TPR experiments manifest that the synthesized material hasigh catalytic activity even without deposited catalyst. This indi-ates that desorption of chemisorbed water in the range of80–400 ◦C results in formation of numerous oxygen species (e.g.2

−, O− and O2−), which are known as catalytically active ones inxidation processes.

As a result of high oxidation activity and high specific surfacerea sensors made on the basis of synthesized SnO2 have the highestignals to different propane concentrations. The low impurity levelf the synthesized materials results in the lowest signal drift.

We found that crystallite growth causes signal decrease by 1%uring 590 h of operation for this material. On the other hand,igh impurity concentration in commercial material leads to signal

ncrease by 25% after 590 h. This indicates that, apart from crys-allite growth (resulting in signal decrease), impurities take partn surface oxidation processes causing inhibition of the propanexidation and remarkable signal drift.

cknowledgments

This work has been funded in part by CICyT under Granto. TEC2006-03671. R. Pavelko gratefully acknowledges a Ph.D.

ators B 137 (2009) 637–643

scholarship from URV. Also authors would like to thank Dr. F.Gispert—Guirado from the Laboratory of X-Ray Diffraction (URV,Spain) for his contribution to XRD experiments and spectraprocessing.

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[4] A.A. Vasiliev, Physical and chemical principles of the design of gas sensorsbased on metal oxides and structures metal/solid electrolyte/semiconductor.Dissertation, Doctor of Science Degree, Moscow, 2004.

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Biographies

Roman G. Pavelko graduated from People’s Friendship University of Russia(Moscow) in 2003, obtained his PhD in Chemistry in 2007 at the Institute of Generaland Inorganic Chemistry, (Russian Academy of Science, Moscow). At present he isPhD student in University Rovira i Virgili (Tarragona, Spain) in the Electronic Engi-neering Department. His research interests concern synthesis of dispersed materials,material science, experimental and theoretical study of surface processes related tometal oxide gas sensors.

Alexey A. Vasiliev graduated from Moscow Institute of Physics and Technology in1980, obtained his PhD in 1986 for the “Study of the kinetics of low-temperaturereactions of atomic fluorine by ESR method”. Gained his Dr. of Science degree (habil-itation) in solid state microelectronics in 2004 for the investigation of “Physical andchemical principles of design of gas sensors based on metal oxide semiconductorsand MIS structures with solid electrolyte layer”. Recently he is working in Sensorgroup of the University Rovira i Virgili (Tarragona, Spain) and at Russian researchcenter Kurchatov Institute (visiting position). Research interests are related withthe study of the kinetics and mechanisms of heterogeneous processes related withchemical sensing, kinetics and mechanisms of electrochemical processes in liquidand solid electrolytes.

Eduard Llobet graduated in telecommunication engineering from the UniversitatPolitècnica de Catalunya (UPC), (Barcelona, Spain) in 1991, and received his PhD in1997 from the same university. He is currently an associate professor in the Elec-tronic Engineering Department at the Universitat Rovira i Virgili (Tarragona, Spain).His main areas of interest are in the design of semiconductor and carbon nanotubebased gas sensors and in the application of intelligent systems to complex odoranalysis.

Xavier Vilanova graduated in telecommunication engineering from the Universi-tat Politècnica de Catalunya (UPC), (Barcelona, Spain) in 1991, received his PhD in1998 from the same university. He is currently an associate professor in the Elec-tronic Engineering Department at the Universitat Rovira i Virgili (Tarragona, Spain).

His research activities are related to semiconductor gas sensors development andcharacterization, as well as, gas sensors systems design.

Noelia Barrabés received MS from Chemical Engineering Department in Rovira iVirgili University in 2006. She is currently finishing the PhD degree in chemicalengineering, working in catalysis, at the Rovira i Virgili University.

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FUr

Vc

R.G. Pavelko et al. / Sensors an

rancesc Medina is a professor in the Department of Chemical Engineering at theniversity Rovira I Virgili of Tarragona. His research interest is in the areas of mate-

ials science, catalysis, reactor design and chemical engineering.

ladimir G. Sevastyanov is graduated from Mendeleyev University of Chemi-al Technology of Russia in 1964. He is a professor, head of the Department of

ators B 137 (2009) 637–643 643

Physical Chemistry of Sensor Materials in the N.S. Kurnakov Institute of Gen-eral and Inorganic Chemistry (Moscow, Russia). His research activity is related tosynthesis and investigation of volatile coordination compounds of wide range of ele-ments (including actinides), ultra pure chemical compounds, dispersed oxides andcarbides.