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Available online at www.sciencedirect.com

Sensors and Actuators B 129 (2008) 741–749

Alcohol sensing properties of sol–gel prepared Sr(II)-addedcobalt aluminate spinel composites

J. Judith Vijaya a,∗, L. John Kennedy b, G. Sekaran b, A. Meenakshisundaram c,R. Thinesh Kumar a, P. Amalthi a, K.S. Nagaraja a

a Department of Chemistry, Loyola Institute of Frontier Energy (LIFE), Loyola College, Chennai 600034, Indiab Department of Environmental Technology, Central Leather Research Institute (CLRI), Chennai 600020, India

c Catalysis Laboratory, R&D Centre, Chennai Petroleum Corporation Limited, Chennai 600068, India

Received 19 June 2007; received in revised form 12 September 2007; accepted 13 September 2007Available online 16 September 2007

bstract

Strontium(II)-added CoAl2O4 composites prepared by sol–gel technique were investigated for the sensing properties to alcohols like methanol,thanol, isopropanol and butanol. X-ray diffraction, scanning electron microscopy and nitrogen adsorption/desorption isotherms at 77 K weremployed to identify the structural phases, surface morphology and BET surface area of the composites, respectively. The composites wererepared with the molar ratios of Co:Sr as 1.0:0.0, 0.0:1.0, 0.8:0.2, 0.6:0.4, 0.4:0.6, and 0.2:0.8, keeping the aluminum molar ratio as constantnd were labeled as CoSA1, CoSA2, CoSA3, CoSA4, CoSA5 and SA, respectively. The samples sintered at 900 ◦C for 5 h, were subjected toc resistance measurements in the temperature range from 30 to 250 ◦C to study the alcohol vapor sensing characteristics. The results revealedhat the sensitivity factor (Sf) increased with temperature up to the optimum temperature at which maximum sensitivity is obtained and thereafterecreased. The sensitivity factor was found to increase with increase in concentration from 100 to 5000 ppm at the respective operating temperatures.mong the composites prepared CoSA5 showed the maximum sensitivity factor towards alcohols. The sensitivity factor and response and recovery

haracteristics showed that the order of sensing alcohols by the composites was butanol > isopropanol > ethanol > methanol, which could bexplained on the basis of oxidation of alcohols. The stability of the most sensitive composite CoSA5 was also discussed along with the mechanismf sensing. The response and recovery characteristics of CoSA1, CoSA5 and SA to alcohols were also reported.

2007 Elsevier B.V. All rights reserved.

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eywords: Spinels; Composites; Ceramics; Sol–gel technique; Alcohol sensors

. Introduction

Solid-state sensors are among the most versatile of all sen-ors, as they detect a wide variety of gases and can be used inany different applications. Different response characteristics

re achieved by varying the semiconductor materials, processingechniques and sensor operating temperature. Among the uniquettributes of the solid-state sensor are the abilities of the sensorso detect low ppm levels of gases as well as high combustible lev-

ls [1]. Earlier reports [2,3] have shown the potential of severalovel semiconducting materials as viable gas-sensing materials.ost of these materials are based on mixed metal oxides that

∗ Corresponding author. Tel.: +91 44 28178200; fax: +91 44 28175566.E-mail address: jjvijayaloyola@yahoo.co.in (J. Judith Vijaya).

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925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2007.09.031

hange in electrical response when the stoichiometric propor-ions of the metal centres are altered and they exhibit extrinsicemiconducting behavior with either p- or n-type conduction4]. In most cases the resistance change is observed when oxida-ive or reductive nature of the gas is applied to these materialsepending on the type of semiconductor used [5,6]. The sol–gelechnique is considered as the most promising technique to pre-are porous ceramic semiconducting sensors [7] as it allows forigh purity ceramics with homogeneous distribution of compo-ents on the atomic scale. Among the available materials, metalxides having the spinel-type structure are often used as modelystems owing to their structural features, which facilitate tai-

oring of relevant properties, and many physical and chemicalroperties of these materials are a function of cation distribution8]. Though magnesium aluminate spinels was used as humidityensors [9,10], there is no literature found regarding the metal

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42 J. Judith Vijaya et al. / Sensors a

luminate spinel and doped metal aluminate spinels used forhe purpose of sensing alcohols. In our previous work, we stud-ed the humidity sensing characteristics of cobalt aluminates andr(II)-added cobalt aluminates, and in the present study the alco-ol sensing properties of the same are examined. In our study wenvestigated the interaction characteristics of the four kinds oflcohols, i.e. methanol, ethanol, isopropanol and butanol, aimingo find how the number of methyl groups affects the sensitivity.he main objective of the present work was to study only theffect of addition of strontium on cobalt aluminate in the sensingf alcohols rather than the isomorphic substitution. The compos-tes prepared by sol–gel technology were characterized by X-rayiffraction, scanning electron microscopy and nitrogen adsorp-ion/desorption isotherm at 77 K. The dependence of electricalesponse of these pellets to alcohols was studied. The resultshow that they can be proposed as materials for alcohol sensingevices.

. Experimental procedure

Cobalt aluminate, Sr(II)-added cobalt aluminate and stron-ium aluminate spinel composites were prepared by a sol–geloute [7] with different molar ratios of strontium 0, 0.2, 0.4,.6, 0.8 and 1.0 added to CoAl2O4 and were labeled as CoSA1,oSA2, CoSA3, CoSA4, CoSA5 and SA, respectively. Thus,oSA1 represents pure cobalt aluminate and SA represents pure

trontium aluminate in the present study. The composites in theorm of thick pellets were then sintered at 900 ◦C for 5 h inn ambient air atmosphere. The samples were cooled down tooom temperature at the natural cooling rate of the furnace. Theample codes with corresponding molar ratio composition arehown in Table 1. The structural studies were carried out usingPhilips X’pert diffractometer for 2θ values ranging from 10

o 80◦ using CuK� radiation at λ = 0.154 nm. The surface mor-hology of the sintered porous compacts was determined by aeo–Jeol scanning electron microscope at the desired magnifi-ation. The surface area and pore size distribution were derivedrom the nitrogen adsorption–desorption isotherms using liquiditrogen at 77 K. The nitrogen adsorption–desorption isothermsf the composites were measured using an automatic adsorp-

ion instrument (Quantachrome Corp. Nova-1000 gas sorptionnalyzer). Prior to measurement, composites were degassed at50 ◦C for overnight. The surface area of the composites wasalculated using BET equation, which is the most widely used

able 1urface area parameters of CoSA1, SA and CoSA5 composites

. no. Parameters CoSA1 SA CoSA5

SBET (m2/g) 20.42 77.86 144.6Smic (m2/g) 0.000 33.96 52.64Smeso (m2/g) 20.42 43.9 91.96Total pore volume (cm3/g) 0.113 0.1041 0.131Micropore volume (cm3/g) 0.000 0.0201 0.0290Mesopore volume (cm3/g) 0.113 0.084 0.102Average pore diameter (nm) 22.15 5.347 3.640

BET: BET surface area, Smic: micropore surface area, Smeso: mesopore surfacerea.

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ctuators B 129 (2008) 741–749

odel for determining the specific surface area (m2/g). Theore size distribution was determined using the BJH (Barett,oyner and Halenda) method. In addition, the t-plot method8] was applied to calculate the micropore volume and exter-al surface area. The total pore volume was estimated as theiquid volume of the adsorbate adsorbed at a relative pressuref 0.99. All surface area measurements were calculated fromhe nitrogen adsorption isotherms by assuming the area of theitrogen molecule to be 0.162 nm2. The temperature depen-ent resistance experiments were carried out to determine thectivation energies of the samples using the linearised formf the expression, I = Ioexp−Ea/kT , where I was the current,a the activation energy, k the Boltzmann constant and T the

emperature. For this purpose, the samples were kept inside aylindrical furnace, which was connected to a microprocessorontrolled temperature programmer. The activation energy of theomposites was determined from the temperature dependenceonductance experiments in the temperature range 120–300 ◦Cnder ambient conditions.

The sensitivity tests were carried out in a testing chamberhat measures the surface resistance of the samples. Alcoholsmethanol, ethanol, isopropanol and butanol) were injected bymicro syringe into the test chamber and the sensing character-

stics of the sensor were observed by measuring the electricalesistance change of the sensor when the latter was exposedo alcohols. The volume of the test chamber was 0.003 m3. Asml of liquid in 1 m3 volume corresponds to a concentrationf 1 ppm, a typical injection of 0.3 ml of alcohol in 0.003 m3

orresponds to the gas concentration of 100 ppm [9]. Underhe exposure of reducing gas such as alcohol, its resistanceecreases. Its sensitivity factor Sf is defined as:

f = Rgas − Rair

Rair

ig. 1. Schematic diagram of the experimental set-up used to measure the alco-ol vapor responses.

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J. Judith Vijaya et al. / Sensors a

f alcohols. The schematic representation of the experimentaletup is shown in Fig. 1.

. Results and discussion

.1. N2 adsorption/desorption isotherms

Fig. 2 represents the nitrogen adsorption/desorptionsotherms at 77 K of the cobalt aluminate (CoSA1), Sr(II)-addedobalt aluminate, the mixed metal oxide composite that pos-essed maximum sensitivity (CoSA5), and strontium aluminateSA). The large increase in the nitrogen adsorption at the higherelative pressures indicates that the CoSA1 sample have only

mesoporous character with a relatively larger pore size inhe mesopore range of 2–50 nm. The nitrogen adsorption forA considerably increased at a low relative pressure, indicat-

ng the formation of micropores in addition to the presencef mesopores. This effect was found to be more pronouncedn the Sr-added cobalt aluminate (CoSA5), suggesting that theddition of strontium can introduce microporosity that couldead to an increase in BET surface area than the cases ofoSA1 and SA composites. The amount of nitrogen adsorbed

ncreased to 85.06 cm3/g for CoSA5, while it was only 73.10 and3.28 cm3/g for pure cobalt aluminate (CoSA1) and strontiumluminate (SA), respectively. The composite CoSA1 possessedvery low surface area, 20.42 m2/g, while the composite SA

ossessed 77.86 m2/g. The BET surface area increased con-iderably to 144.6 m2/g after the addition of strontium in theobalt aluminate composite. The addition of strontium retardshe growth of the bulk cobalt aluminate phase, leading to anncrease in the surface area, enhancement in porosity and intro-uction of micropores. The increase in surface area is due todecrease in grain size due to the addition of strontium in

he cobalt aluminate composites as a result of non-isomorphicubstitution. The pores could be generated within the matricesf three components of cobalt, strontium and aluminum oxide,nd consequently pores of different dimensions are produced.

he micropore surface area was obtained by subtracting theesopore surface area from the corresponding BET surface

rea. Micropore surface area was not observed in the CoSA1omposite, whereas SA possessed the micropore surface area

ig. 2. Nitrogen adsorption/desorption isotherms of (a) CoSA1, (b) CoSA6, andc) CoSA5 at 77 K.

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ctuators B 129 (2008) 741–749 743

f 33.96 m2/g, and CoSA5 possessed a comparatively a highicropore surface area of 52.64 m2/g. The results suggest that

he intergranular pores in CoSA1 were purely in the mesoporicange and for SA the formation of intergranular pores fell ino the micropore and mesoporic ranges. However, the Sr(II)-dded cobalt aluminate CoSA5 led to a significant increase inicropore and mesopore ranges in comparison with the other

omposites.The average pore diameters of CoSA1, CoSA5 and SA com-

osites were 22.15, 3.64 and 5.34 nm respectively, and thealues were obtained from the pore size distribution studies7]. The decrease in the average pore diameter for CoSA5omposite can be attributed to the formation of a consider-ble amount of micropores due to non-isomorphic substitutionf strontium in the cobalt aluminate matrices. The reductionn the average pore diameter of SA composite is attributed tohe reduction in the grain size of strontium aluminates. The

esopore volume was found to decrease with the formation oficropores in the composite CoSA5. The values of total pore

olume, micro and mesopore volume and average pore diame-er are given in Table 1. Therefore, the formation of microporesy the strontium addition in the CoSA5 matrices along withesopores would lead to enhanced adsorption with a higher

ensitivity factor than the pure cobalt aluminate (CoSA1) com-osites.

.2. X-ray diffraction studies

The X-ray diffraction patterns of cobalt aluminate (CoSA1),r(II)-added cobalt aluminate (CoSA2, CoSA3, CoSA4 andoSA5) and strontium aluminate (SA) samples are shown inig. 3. The XRD spectra of Fig. 3a showed peaks correspond-

ng to a cobalt aluminate spinel-like phase (JCPDS: 10-0458).ll Sr(II)-added composites showed the peaks corresponding toSrO.4Al2O3 (JCPDS: 09-38), SrO.2Al2O3 (JCPDS: 25-1208)nd SrO phase (JCPDS: 01-0886), which implied that the mate-ial was composed of mixed phases. The addition of strontiumetards the growth of the bulk cobalt aluminate phase on theurface and forms new phases 5SrO.4Al2O3 and SrO.2Al2O3.his multiphasing was observed more notably in CoSA5 andA composite samples.

.3. Scanning electron microscopy (SEM)

Fig. 4(a–c) depicts the intergranular porous structure of theomposite materials qualitatively. In the micrograph of cobaltluminate (CoSA1) and strontium aluminate (SA) the size ofhe grains is larger than that of the Sr(II)-added cobalt alumi-ate (CoSA5). In CoSA5, the particles were found to be sphericaln shape, but not in CoSA1 and SA composites. This suggestshat the addition of a larger amount of strontium in the cobalt

luminate matrix reduces the grain size with the formation ofntergranular pores that leads to micro and mesopores. Further-

ore, it is observed for CoSA5 that the grains connect each othernd/or aggregate to some extent with well-developed porosityhat is very important to sensing.

744 J. Judith Vijaya et al. / Sensors and Actuators B 129 (2008) 741–749

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ig. 3. X-ray diffraction spectra of (a) CoSA1, (b) CoSA2, (c) CoSA3, (d)oSA4, (e) CoSA5, and (f) SA.

.4. Gas sensing measurements

Prior to gas sensing measurements, the dc electrical resistancef the samples in the form of pellets was determined by a two-robe method instead of the van der Pauw four-probe method,s the present work was to measure the changes in surface con-uctivity as a function of applied field and current. Conductingilver paste was employed to ensure the Ohmic contacts. Theamples were electrically connected to a dc power supply and aeithley 485 picoammeter in series. Given the high resistivity of

he materials under investigation, the potential inaccuracy dueo contact resistance is assumed negligible. The sensitivity testsere carried out in a testing chamber designed for this purpose

Fig. 1) by injecting alcohols (methanol, ethanol, isopropanolnd butanol) using a micro syringe. The changes in resistance ofure and Sr(II)-added cobalt aluminate composites in the formf thick pellets when exposed to 1000 ppm of alcohols in theemperature range of 30–300 ◦C (room temperature is 30 ◦C) ishown in Fig. 5(a–d). It is observed that all the curves go up andeach their maxima, then decline again with increasing temper-ture and the addition of Sr(II) is beneficial in increasing the

ensitivity of the pure cobalt aluminate composites to alcohols.t is evident that the alcohol sensitivities vary with an increasen molar ratios of Sr(II) added. It is also observed that Sr(II)-dded metal aluminate composites have higher sensitivity than

msSe

Fig. 4. SEM images of (a) CoSA1, (b) CoSA5, and (c) SA composites.

ure metal aluminate composites. The temperature at which theaximum sensitivity occurs is fixed as the optimum operating

emperature to sense alcohols for the composites.The sensitivity factor to methanol of pure cobalt aluminate,

oSA1 was found to be 140 ± 1 at an optimum temperaturef 150 ◦C and for pure strontium aluminate SA it was 75 ± 2t 175 ◦C. But, the composite CoSA5 with higher strontiumontent showed a larger sensitivity factor of 249 ± 3 at an opti-um temperature of 150 ◦C. Thus, by the addition of Sr(II) the

ensitivity factor of the composites to methanol was increased.imilarly the sensitivity factors of CoSA1, CoSA5 and SA tothanol at 150 ◦C, 150 ◦C and 200 ◦C (optimum temperature)

J. Judith Vijaya et al. / Sensors and A

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titastcaluminate matrix is found to have higher sensitivity factors toalcohols.

It was also observed that the changes in resistance increasedwith an increase in alcohol concentration from 100 to 5000 ppm

ig. 5. Sensitivity factors of Sr(II) added CoAl2O4 composites to 1000 ppm ofa) methanol, (b) ethanol, (c) isopropanol, and (d) butanol at different tempera-ures.

ere 160 ± 2, 300 ± 5 and 95 ± 3, respectively. When the com-osites were exposed to isopropanol vapor the sensitivity factorsbtained for CoSA1, CoSA5 and SA at the respective operat-ng temperatures of 175 ◦C, 150 ◦C and 200 ◦C were 180 ± 2,50 ± 4 and 120 ± 4, respectively. The sensitivity factors ofoSA1, CoSA5 and SA to butanol were found to be 209 ± 2,80 ± 4 and 141 ± 3 at 200◦, 175◦ and 225 ◦C, respectively.

The optimum operating temperature to alcohols (methanol,thanol, isopropanol and butanol) vary with the composites. Its shown that at the optimum temperature most of the adsorbedxygen species would have reacted with alcohol vapors. Thisdsorbed oxygen creates a space charge region near the surfacef the composite by extracting electrons from the material. Alco-ol, being reducing in nature, removes adsorbed oxygen speciesrom the surface and re-injects the electrons back to the mate-ial, thereby decreasing the resistance. The maximum sensitivityactor at the respective optimum working temperatures indicates

hat the equilibrium density of chemisorbed oxygen ions is max-mal at this temperature. It is observed that the sensitivity of theoSA5 composite towards alcohol is the highest among all theomposites, which is consistent with the low activation energy of

F(o

ctuators B 129 (2008) 741–749 745

hat composite. The increase in sensitivity factor with an increasen operating temperature can also be attributed to the fact thathe thermal energy obtained was high enough to overcome thectivation energy barrier of the reaction, while the reduction inensitivity above the respective optimum temperatures was dueo the difficulty in exothermic vapor adsorption [10]. Hence theomposite CoSA5 with higher strontium content in the cobalt

ig. 6. Sensitivity factors of Sr(II)-added CoAl2O4 composites to (a) methanol,b) ethanol, (c) isopropanol, and (d) butanol of different concentrations at theirptimum temperatures.

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46 J. Judith Vijaya et al. / Sensors a

t their respective optimum operating temperatures, as shown inig. 6(a–c). The sensitivity factor increases up to 1000 ppm withhigher rate of increase in sensitivity and thereafter slows down

eaching near equilibrium. The Sr(II)-added composites showedigher sensitivity factors to alcohols than pure metal aluminatesCoSA1 and SA). The sensitivity factor at low concentrationas nearly a linear relationship with concentration, as there maye a sufficient number of pores for alcohol vapor adsorption.y increasing the concentration the rate of adsorption decreasesue to less assess of alcohol vapors into the filled pores, leadingo a less increase in sensitivity factor values [10].

From the above studies the increasing order of the sen-itivity for all the composites to alcohol vapors is in therder butanol > isopropanol > ethanol > methanol, which can bexplained on the basis of ease of oxidation. It has been reportedhat alcohols with increased CH2 groups are easily decom-osed and oxidized [11,12]. As a result butanol is most easilydsorbed into the sensor, followed by isopropanol, ethanol andethanol, respectively. Thus, the addition of Sr(II) into the metal

luminate matrix introduces micropores that causes the reduc-ion of average pore diameter, that comes closer to the molecularize of alcohols and leads to an enhanced adsorption, followedy oxidation.

The results from our studies showed that the compositeoSA5 that possessed both micro and mesopores showed larger

ensitivity factors towards alcohols. In this case the adsorptionrocess would have proceeded through a sequence of diffusionteps from the bulk phase into the mesopores and then to theicropores. Hence mesopores present in the composites behave

s an entrance for the movement of alcohol molecules from theulk phase to the inner micropores without any hindrance forhe adsorption, followed by oxidation using the adsorbed oxy-en. Thus, the presence of mesopores is important as it enablesn easy assess to the micropores not only for accelerating theiffusion with micropores but also increasing the equilibriumoverage of the micropore surface. At the same time, if theomposite would have possessed micropores alone then the porelockage might have occurred due to aggregation of adsorbateolecules at the pore entrance due to the smaller cross sectional

rea. And if the composite had only mesopores then desorp-ion of alcohols would be more favored than the adsorption dueo quite bigger pore size. Thus, by the addition of strontium

ore micropores are introduced in addition to the increase inhe amount of mesopores.

In addition the difference in sensitivity to alcohols amongoSA1, CoSA5 and SA composites could be compared with

heir activation energies. The activation energy of the compos-tes CoSA1, CoSA2, CoSA3, CoSA4, CoSA5 and SA obtainedere 0.319, 0.300, 0.291, 0.284, 0.278 and 0.110 eV, respec-

ively. From the results it is observed that by the addition ofr(II) on the metal aluminate matrix, the activation energy iseduced, thereby increasing the sensitivity to alcohols. In casef pure cobalt aluminate, when the metal oxides contacts are

ormed, a barrier will be formed at the interface that is fully char-cterized by the work function (the minimum energy required toake out electron from the surface) of the semiconductors insidehe energy gap. In equilibrium, if the work function is large, a

batt

ctuators B 129 (2008) 741–749

epletion layer will be developed in the semiconductor and givesise to a larger barrier height. This manifests a larger electricalesistance of the sensor material. But by the addition of Sr(II),he decrease in junction barrier height becomes more significantnd thereby increases the sensitivity to alcohols.

Ivanoskaya et al. [13] reported that the materials with multi-le phases are extremely sensitive to alcohols than the materialsith a single phase. The presence of multiple phases within the

omposite favors the effective course of multi-step process ofeductive–oxidative interactions. There are two ways of alcohololecule conversion possible at the oxide surface, dehydrogena-

ion and dehydration.

CH2 OH ⇔ RCHO(ads) + 12 H2 (dehydrogenation) (1)

CH2 OH ⇔ R CH = CH2 + H2O (dehydration) (2)

Besides, further oxidation of the formed products is possi-le at the oxide surface and it should result in sensor responserowth. Thus, the process most often proceeds through thexidizing dehydrogenation mechanism. Klissurski [14] alsoeported that the alcohol detection is predominantly realizeds dehydration process. The formation of aldehyde by dehy-rogenation is known to have higher molecular sensitivity of aemiconductor gas sensor than alkene [15,16].

The alcohol sensing mechanism is based on the changes in theonductance of the composites, and the oxygen adsorbed on theurface of the materials influences the conductance of the oxide-ased sensor. From TEP (thermoelectric power measurements)esults CoSA1 and SA were found to be p-type semiconductors.nd the mixed metal composite CoSA5 showed n-type semicon-uctivity. The sensing effect of CoSA1 and SA that are p-typeemiconductors can be explained on the basis of surface oxygenacancies. In p-type semiconductors, adsorbed oxygen accepthe electrons from valence band and hence increasing the holeoncentration.

2(g) ↔ O(ads) + O(ads) (3)

(ads) ↔ O−(ads) + h+ (4)

OH(g) + O−(ads) ↔ R O(ads) + OH(ads) + e− (5)

O(ads) + OH(ads) ↔ RCHO(ads) + H2O(g) (6)

OH(ads) ↔ H2O(g) + O−(ads) + h+ (7)

CHO(ads) ↔ RCHO(g) + vacantsites (8)

The adsorption of molecular oxygen takes place on the sur-ace of the composite through surface oxygen vacancies andissociative chemisorption of R OH with surface oxygen form-ng a surface alkoxy intermediate and a strongly chemisorbedurface hydroxyl species, releasing the trapped electrons to neu-ralize the bulk positive holes, hence giving electrical response.hen the cleavage of a C H bond in the alkoxyl intermediate

y nucleophilic attack from OH(ads), yielding RCHO and H2O,s shown by Eq. (6). The surface hydroxyl groups are assumedo be neutral species with respect to the lattice, and desorp-ion of weakly bound RCHO from the surface, provides new

J. Judith Vijaya et al. / Sensors and A

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cTorecovery characteristics. Better response would be expected forlarger concentration of the test gas adsorbed, because the reac-tion between the adsorbed reducing gas and oxygen speciesbecomes more favorable. The response and recovery times were

ig. 7. Resistance stability of CoSA5 to methanol, ethanol and isopropanol.

acant sites, as shown by Eq. (8), for the approaching alcohololecules.The sensing mechanism of CoSA5 with n-type semiconduc-

ivity is shown below. The atmospheric oxygen molecules aredsorbed on the surface of the composite in the form of O− and2− thereby decreasing the electronic conduction. The adsorp-

ion of O− is more reactive and thus makes the material moreensitive to the presence of a reducing gas like alcohol. At rel-tively low temperature, the surface preferentially adsorbs O2−nd the sensitivity of the material is consequently very small.s the temperature increases the dominant process becomes

he adsorption of O− and hence the sensitivity of the materialncreases.

2(gas) ↔ O2(ads) (9)

2(ads) + e− ↔ O2−

(ads) (10)

2−

(ads) + e− ↔ 2O−(ads) (11)

−(ads) + e− ↔ O2−

(ads) (12)

OH(g) + O−(ads) ↔ RCHO(ads) + H2O + e− (13)

CHO(ads) ↔ RCHO(g) + vacantsites (14)

Alcohols are easily oxidized to aldehyde and when the alco-ol vapor comes into contact with the surface oxygen ions ofhe oxide surface, it reacts with either O− or O2

− ions and getsxidized to aldehyde and water and liberates electrons whichctually causes the conductivity to increase, and desorption ofeakly bound RCHO from the surface provides a new vacant

ites for the surface approaching alcohol molecules.Therefore, the composite CoSA5 with smaller grain size,

ower activation energy, more micro and mesopores, larger sur-ace area that results in greater oxygen adsorption and higherensitivity than the other composites.

.5. Stability of MSA5 composites towards alcohols

To study the stability of CoSA5 composite with higherensitivity we continuously measured the resistance towards000 ppm of alcohols at their respective operating temperaturesor 100 h. It was found that the resistance was stable except a

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ctuators B 129 (2008) 741–749 747

mall variation of about 1 × 106 � within the studied duration.he stability of the composite CoSA5 towards methanol, ethanolnd isopropanol is shown in Fig. 7. Also after a month the com-osite sensitivity to alcohols was nearly the same with very lowariations in sensitivity factor (Sf ± 5) indicating good stabilitynd durability of the sensor prepared.

.6. Response and recovery characteristics

It is now well known that the sensing mechanism of semi-onductor gas sensor materials is a surface controlled process.he adsorption of test gases, which depends on both the typef gas and the sensor material, might affect both response and

ig. 8. Response and recovery plots of CoSA5 composite to (a) methanol at50 ◦C, (b) ethanol at 150 ◦C, (c) isopropanol at 150 ◦C, and (d) butanol at75 ◦C.

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tudied for CoSA1 (pure metal aluminate), CoSA5 (higher sen-itivity) and SA (pure strontium aluminate) composites at theirorresponding operating temperatures for three cycles to ensureepeatability. It is observed that the response and recovery timesre faster for CoSA5 than the pure CoSA1 and SA composites.his can be explained due to the presence of higher Sr(II) con-

ent on the metal aluminate matrix and the high sensitivity of theomposites. Thus, by the addition of Sr(II) to pure metal alumi-ates, the response and recovery times for sensing of alcoholsre improved considerably. The CoSA5 composites with n-typeemiconducting nature have quicker response and recovery thanhe other composites. When the sensor materials are maintainedt equilibrium at the operating temperature, adsorbed oxygenpecies in the form of O−, O2

− and O2−, etc. localize mobilelectrons, creating a depletion layer at the surface of the individ-al particles and inter-granular regions. Therefore the reactionf reducing test gases like alcohol vapors with the charged oxy-en species destroys the electron localization process and isbserved as a change in conductivity, thus giving rise to quickeresponse and recovery [17]. The response and recovery plot ofhe maximum sensitivity sample CoSA5 to methanol, ethanolnd isopropanol is shown in Fig. 8(a–d). Comparing the valuesf response and recovery time it can be seen that the responseo butanol is much longer than those to isopropanol, ethanolnd methanol. This can be explained by the steric and electrononating effect of the alcohol molecules [18]. Thus, by the addi-ion of Sr(II) the response and recovery characteristics of theomposites are improved.

. Conclusion

A study on the alcohol sensing characteristics of Sr(II)-addedoAl2O4 composites prepared by sol–gel technique has beenarried out. The results suggest that the increase in strontiumontent can increase the porosity, resulting in larger sensitivityactor values. The XRD patterns suggested the formation of aingle phase for cobalt aluminate composite while multiphasingas observed in the Sr(II)-added cobalt aluminate composites.

t was observed that the sensitivity of the composites to alco-ols was higher at the optimum temperature which varies withifferent composites. The sensing properties of the compositesere studied for different concentrations at the respective oper-

ting temperatures and found to be increasing with an increase inlcohol concentration. The increase in surface area and porosity,educed grain size with intergranular pores leading to both micrond mesopores, multiphasing nature, and decrease in activationnergy by Sr(II) addition makes the composite CoSA5 to be aetter material to detect alcohols.

cknowledgements

We thank Most Rev. Dr. A.M. Chinnappa, S.D.B, D.D., Ph.D.,rchbishop, Madras-Mylapore for extending constant encour-

gement and support throughout the study. We also thank theoyola College management and members of the department ofnvironmental technology, CLRI for extending support through-ut the study.

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ctuators B 129 (2008) 741–749

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iographies

. Judith Vijaya obtained her MSc in chemistry from Loyola CollegeAutonomous), Chennai, Tamil Nadu, India in 2000. Since 2001 she is workings a professor in the Department of Chemistry, Loyola College. Her main fieldsf interests are solid-state sensors, coordination chemistry and developing nanoaterials. Currently she is working on the development of mixed oxide ceramic

umidity and volatile organic compound sensors by sol–gel technique for indus-rial applications and the catalytic oxidation of volatile organic compounds using

etal oxide composites.

. John Kennedy obtained his MSc and MPhil degrees in physics in 1997

nd 1998 from Loyola College (Autonomous), Chennai, India. He received hishD degree in the interdisciplinary area of physics – environmental technol-gy in 2005 from the University of Madras. He is currently working as a youngcientist in the “Carbon materials research group” at Department of Environmen-al Technology, Central Leather Research Institute, Adyar, Chennai, India. His

nd A

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Gitgoaihohei

APyC

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Kat1984–1986. Since 1986, he has been with the Department of Chemistry, Loyola

J. Judith Vijaya et al. / Sensors a

ain fields of research are on the development of mesoporous carbons, carbonanotubes, energy storage using super capacitors, porous ceramic composites,emiconducting porous materials and heterogeneous catalysis.

. Sekaran obtained his MSc and PhD degrees from University of Madrasn 1978 and 1990, respectively. He is currently a senior scientist, headinghe Department of Environmental Technology, and ‘Carbon materials researchroup’, Central Leather Research Institute, Adyar, Chennai, India. He is an hon-rary Professor of Anna University in the faculty of leather technology. Hisreas of interests include heterogeneous catalysis applied to oxidation of organ-cs and inorganics in wastewater and in gaseous stream using carbon basedeavy metal doped catalysts. Anaerobic treatment of wastewater, developmentf electron rich carbon based catalyst, oxidation of refractory organics usingeavy metal doped mesoporous catalyst, development of photocatalysts, gen-ration of electrical energy through biocatalysts etc. are the other areas of hisnterest.

. Meenakshisundaram obtained his MSc from Loyola College in 1978 andhD from Indian Institute of Technology, Madras in 1983 in the area of catal-sis. He is currently working as Senior Manager (R&D) in Chennai Petroleumorporation Limited and is working in the area of catalysis related to petroleum

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ctuators B 129 (2008) 741–749 749

efining processes. His main interests are in the area of characterization andvaluation of refining catalysts and development of new catalysts and processesor fuel quality improvements. He is also a member of the Executive committeef catalysis society of India.

. Thinesh Kumar obtained his MSc from Loyola college in 2007 and his areasf interest include the catalytic oxidation of volatile organic compounds usingetal oxide composites.

. Amalthi obtained her MSc from Loyola college in 2007 and currently doinger MPhil in Loyola college. Her areas of interest include the catalytic oxidationf volatile organic compounds using metal oxide composites.

.S. Nagaraja obtained his MSc and PhD degrees from University of Madrasnd Indian Institute of Technology (IIT), Madras in 1978 and 1982, respec-ively. He was a visiting professor to De La Salle University, Philippines, during

ollege, Chennai, and is a member of the research team at Loyola Institutef Frontier Energy (LIFE). His interests include solid-state sensors, photo-oltaic/photo galvanic devices, bioinorganic and co-ordination chemistry andrecursor development for chemical vapor deposition (CVD) applications.