Es

Da

b

a

ARRAA

KGTSPL

1

taectdwctssboowritteb

0d

Sensors and Actuators B 153 (2011) 89–95

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

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

ffect of thickness of platinum catalyst clusters on response of SnO2 thin filmensor for LPG

ivya Haridasa, Arijit Chowdhurib, K. Sreenivasa, Vinay Guptaa,∗

Department of Physics and Astrophysics, University of Delhi, North Campus, Delhi 110007, IndiaAcharya Narendra Dev College, University of Delhi, Kalkaji, New Delhi 110019, India

r t i c l e i n f o

rticle history:eceived 19 May 2010eceived in revised form 5 October 2010ccepted 11 October 2010

a b s t r a c t

This paper reports the sensing response characteristics of rf-sputtered SnO2 thin films (90 nm thick)loaded with platinum catalyst cluster of varying thickness (2–20 nm) for LPG detection. The enhancedresponse (5 × 103) was obtained for 200 ppm LPG with the presence of 10 nm thin and uniformly dis-tributed Pt catalyst clusters on the surface of SnO2 thin film at a relatively low operating temperature

vailable online 15 October 2010

eywords:as sensorin oxideputtering

(220 ◦C). The high response for LPG is shown to be primarily due to the enhanced catalytic activityfor adsorbed oxygen on the surface of SnO2 thin film besides the spill over mechanism at elevatedtemperature.

© 2010 Elsevier B.V. All rights reserved.

latinumPG

. Introduction

Semiconducting oxide materials found potential application inhe field of gas sensors for detection of toxic and explosive gasest the domestic, industrial and environmental sites [1–6]. Sev-ral metal oxides including ZnO, TiO2, Fe2O3, WO3 and SnO2 areommonly investigated materials for such applications [2]. Amonghese SnO2 is the most widely used material in practical devicesue to its excellent inherent property of the exchange of oxygenith atmosphere [3–10]. Thin films are advantages over their bulk

ounterpart for the gas sensor with enhanced response characteris-ics due to increase in surface to volume ratio [2]. However the gasensors made of pure metal oxide exhibit low response and poorelectivity [3]. The performance of metal-oxide sensor is enhancedy incorporating suitable catalyst in the sensing layer. The effectf various catalytic additives (noble metals and transition metalxides) on the gas sensing properties of the metal oxide has beenidely studied [2,7]. The additives not only enhance the sensor

esponse but also improve the response speed along with reductionn the operating temperature [3,4]. The enhanced response charac-

eristics of a sensor due to incorporation of catalysts is attributedo several factors, including electronic and physico-chemical prop-rties of the sensing matrix, surface potential and inter-crystallitearriers, sizes of crystallites and so on [2–16]. Therefore the cata-

∗ Corresponding author. Tel.: +91 9811563101.E-mail address: drvin gupta@rediffmail.com (V. Gupta).

925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2010.10.013

lyst additives can lead to an improvement in the sensor responseby means of a selective promotion of the desired molecule reactionat the chosen sites [17]. The size of the additives is also consid-ered to be important [10], and the catalysts must be homogenouslydistributed in the sensing materials to improve the gas sensingproperties. In spite of extensive literature on catalyst additives inSnO2, very few efforts have been made towards the studies withthe emphasis on the amount of additive to be present in the sensingmaterial to achieve enhanced response [4].

LPG is a potential domestic fuel and is explosive in nature, there-fore the detection of its leakage is very important. Noble metalssuch as Pt and Pd have been widely used as catalysts to improvethe response characteristics of SnO2 gas sensor for detection ofLPG [3,5,6]. In general, the studies on the LPG sensors show eitherpoor sensing response (2–500) or very high operating temperature(>300 ◦C) along with a slow response speed (200–400 s). In our pre-vious work we have studied the effect of various catalysts includingnickel, nickel oxide, silver, silver oxide, platinum, platinum oxideand lead, on the sensing response characteristics of rf-sputteredSnO2 thin film for LPG detection [3]. A significant improvement inboth the sensing response and the response speed was observedusing a sensor structure consisting of uniformly distributed Pt cat-alyst clusters on the surface of SnO2 thin film [3]. Similar results

were obtained by various workers for SnO2 based LPG sensor [7,10].However, no efforts have been made towards the study on theamount of Pt metal catalyst to be required for enhanced sen-sor response characteristics, besides the fact that it can influencethe spill over process and modify the space charge region at the

9 nd Actuators B 153 (2011) 89–95

socirLtLri

2

sdwpvfiTmiariupsuato(fTf

hawsttivcswtr

3

frXaatTa(2r

thickness of film, and shows a decrease at higher thickness [21].It is well known that the sensing response of the sensor increaseswith increasing roughness of the film, because the number of theactive adsorption sites for oxygen or hydrocarbon molecules on thesensor surface is increased [22]. The presence of large roughness

0 D. Haridas et al. / Sensors a

urface of sensing SnO2 thin film. In the present work the quantityf platinum catalyst dispersed on the surface of SnO2 thin film in thelusters forms has been varied. It was of interest to understand thenfluence of the thickness of Pt clusters especially in the nanoscaleange on the response characteristics of SnO2 thin film sensor forPG. The sensing response of the SnO2 thin film loaded with 10 nmhin and uniformly distributed Pt islands for detection of 200 ppmPG is about ∼5 × 103 at 220 ◦C operating temperature with fastesponse (∼181 s) and recovery time (∼46 s) and is relatively bettern comparison to other LPG sensors that have been reported so far.

. Experimental

SnO2 thin films of 90 nm thickness were deposited by rf-puttering technique on the corning glass substrate. The optimizedeposition conditions for SnO2 thin film used in the presentork are reported elsewhere [3]. Interdigital electrodes (IDEs) oflatinum were patterned on the corning glass substrate using con-entional photolithography prior to the deposition of SnO2 thinlm, and were used to measure sensing response characteristics.he location of IDEs underneath the sensing SnO2 film allowed theeasurement of actual changes in the resistance that was occurring

n the bulk of the SnO2 layer. The as grown SnO2 thin films werennealed at 300 ◦C in air for two hours for achieving a stable sensoresistance. The sensor (SnO2–Pt) structure was made after deposit-ng platinum (Pt) metal catalyst on the surface of SnO2 thin filmsing a shadow mask having uniformly distributed closely packedores of diameter 600 �m. The dispersal of Pt catalyst clusters isuch that the sufficient surface of sensing SnO2 thin film remainsncovered and is available for the exchange of oxygen from thetmosphere, which is expected to play a crucial role in enhancinghe LPG response characteristics. The crystallographic orientationf the deposited films has been studied using X-ray diffractionXRD). The X-ray source used is Cu K� radiation (� = 1.5405 A). Theormation of SnO2 film has been confirmed by FTIR spectroscopy.he surface morphology of the sensor was studied using Atomicorce microscopy (AFM) in the contact mode.

The thickness of platinum clusters loaded on SnO2 surfaceas been varied in the range from 2 to 20 nm. Sensor responsend response speed of all prepared sensor structures (SnO2–Pt)ere measured over the temperature range 60–260 ◦C. The sen-

or response is defined as S = Ra/Rg, where Ra is the resistance ofhe sensor in air (i.e. in the absence of LPG), and Rg is the resis-ance of the sensor in the presence of the LPG. Initially, the sensors maintained at a fixed temperature having a definite resistancealue (Ra), subsequently LPG (200 ppm) is introduced into the testhamber through a fine calibrated leak. The resistance of the sen-or falls in magnitude and the final resistance (Rg) value is recordedhen it stabilizes within ±2% of its magnitude. The time taken for

he fall in the value of resistance from Ra to Rg is defined as theesponse time of the sensor.

. Results and discussion

The SnO2 thin film deposited on corning glass substrate wasound to be uniform, transparent and strongly adherent. No Braggeflection corresponding to any plane of SnO2 was observed in theRD pattern of as grown SnO2 thin film indicating the growth ofmorphous film. However film becomes polycrystalline after annnealing treatment at 300 ◦C in air for two hours. Fig. 1 showshe XRD pattern of SnO2–Pt sensor structure annealed at 300 ◦C.

he diffraction peaks observed at around 2� = 26.6◦, 33.9◦ and 51.8◦

re ascribed to the reflections corresponding to (1 1 0), (1 0 1) and2 1 1) planes of tetragonal SnO2 respectively. XRD peaks around� = 38.6◦ and 44.7◦ are associated with (1 1 1) and (2 0 0) planesespectively of FCC platinum respectively [18].

Fig. 1. X-ray diffractogram of SnO2–Pt clusters sensor structure.

SnO2 thin film has also been deposited under similar process-ing conditions on KBr pellet and annealed at 300 ◦C for 2 h for FTIRstudy. The IR spectrum of the annealed SnO2 film is shown in Fig. 2.The absorption peak around 606 cm−1 corresponds to the SnO2 lat-tice mode [19]. The absorption bands observed at around 3356,1379 and 1094 cm−1 in Fig. 2 can be assigned to the –OH groupsattached directly to the surface oxygen of the SnO2−x [20], andthe absorption band at 1633 cm−1 is attributed to H–O–H in planedeformation. The interaction of the O–OH− group can be seen at1461 cm−1. Fig. 3 shows the typical AFM images of the surface ofSnO2 thin film loaded with the Pt catalyst clusters as a function ofPt thickness. AFM images (Fig. 3) clearly depicts that the surfaceroughness of sensor increases with increase in thickness of Pt clus-ter up to 10 nm and the roughness decreases with further increasein the thickness (>10 nm) of Pt clusters. The observation is in agree-ment with literature where surface roughness increases for small

Fig. 2. FTIR spectra of the SnO2 thin film.

D. Haridas et al. / Sensors and Actuators B 153 (2011) 89–95 91

um isl

lP

3

ss

Fig. 3. AFM images of SnO2–Pt cluster sensors with varying thickness of platin

eads to enhanced sensing response for sensor having 10 nm thickt clusters in the present study.

.1. LPG sensor response

The sensing response characteristics of all prepared sensortructure having varying thicknesses of Pt clusters have beentudied and the variation of sensor response as a function of tem-

ands: (a) 2 nm, (b) 4 nm, (c) 6 nm, (d) 8 nm and (e) 10 nm, (f) 12 nm, (g) 20 nm.

perature for 200 ppm LPG is shown in Fig. 4. The sensor responsewas found to increase with increase in temperature and exhibitsa maximum value at a certain temperature (Topt), and thereafter

decreases (Fig. 4). The observed increase in the response could berelated either to an increase in the sensor resistance (Ra) in air, ora decrease in resistance (Rg) in the presence of the LPG, and for anenhanced sensor performance it is desired that both the changesoccur simultaneously. It may be noted from Fig. 4 that the response

92 D. Haridas et al. / Sensors and Act

Fsw

clbttsttn6ftrit

daH

Ft

ig. 4. Variation of response as a function of temperature for all prepared sensortructures (varying thickness of Pt catalyst). Inset shows the plot of sensor responseith thickness of Pt clusters at the operating temperature for 200 ppm LPG.

haracteristics depends strongly on the thickness of the pt clustersoaded on the surface of SnO2 thin film. The response was found toe relatively higher for the sensor structures having Pt clusters inhe thickness range 8–10 nm. Furthermore the operating tempera-ure of the sensor was also relatively lower (220 ◦C). Inset of Fig. 4hows the variation of sensor response obtained at their operatingemperature for 200 ppm LPG concentration as a function of thehickness of Pt catalyst cluster loaded on SnO2 surface. It may beoted from Fig. 4 that the response increases continuously from.0 × 100 to 4.9 × 103 with increasing thickness of Pt metal clustersrom 2 nm to 10 nm. However, response start decreasing with fur-her increase in thickness of the Pt cluster (>10 nm). The maximumesponse obtained at 10 nm thickness of Pt clusters indicates themportance of the presence of an optimum quantity of catalyst on

he surface of the sensing SnO2 layer.

The resistance (Ra) of all the sensors in air shows a continuousecrease with increasing temperature up to 130 ◦C (Fig. 5) and isttributed to the semiconducting behavior of sensing SnO2 layer.owever above 130 ◦C an increase in sensor resistance with tem-

ig. 5. Variation of resistance Ra in air for all prepared sensors as a function ofemperature.

uators B 153 (2011) 89–95

perature was observed in all samples indicating that some othermechanism is also playing role in determining the sensor resis-tance in air. The observed behavior of sensor resistance in air (Ra) isin agreement to the investigations on semiconducting metal-oxidesreferred to in literature for temperature dependent physisorptionand chemisorption process of oxygen from atmosphere on its sur-face [23]. The oxygen molecules are physisorbed on SnO2 filmsurface, that act as an electron acceptor and create an O2

− specieson its surface. Electrons on the SnO2 film surface are trapped bythe adsorbed oxygen species through a charge transfer mechanism.The negative charge creates the space charge region, and a poten-tial barrier is formed at the interface of the metal oxide with air[3,4] thereby results in a large sensor resistance (Ra) in air. Theobserved decrease in Ra (Fig. 5) at lower temperatures (<130 ◦C)corresponds to the decrease in the resistance of semiconductor(SnO2) material with increasing temperature, as more charge car-riers are able to move to the conduction band from the valenceband and become available for conduction. It is reported that athigher temperature (>130 ◦C), the oxygen activity on the surface ofmetal-oxide increased and O2

− transforms to O−, therefore the sen-sor resistance (Ra) shows an increase with increasing temperature(Fig. 5) in the range 130–200 ◦C.

It may be noted from Fig. 5 that the sensor resistance Ra increaseswith increase in thickness of the platinum catalyst islands from 2to 8 nm, and thereafter shows a decrease with further increase inthickness. The initial increase in sensor resistance with the pres-ence of Pt catalyst clusters of increasing thickness is attributedto the fact that the work function of platinum (6.35 eV) is morein comparison to that of SnO2 (4.18 eV). The loading of Pt onSnO2 film leads to the formation of space charge barrier at themetal–semiconductor interface due to transfer of electrons fromsemiconducting SnO2 layer towards the platinum metal catalyst.The reduction in concentration of electrons in the conduction bandof SnO2 due to the formation of Schottky barrier results in anincrease in the value of Ra at room temperature (Fig. 5). Further-more the enhanced activity of adsorbed oxygen on the uncoveredsurface of SnO2 thin film due to the presence of Pt metal catalyticclusters at relatively higher temperature (>130 ◦C) may also con-tribute towards the increase in Ra. The observed decrease in Ra athigher thickness of Pt cluster (>8 nm) may be due to pulling outof oxygen by the catalyst Pt from the underlying SnO2 thin film.The presence of oxygen vacancies in the SnO2 thin film resultsin an increase in electron concentration in the conduction bandand thereby decreases the sensor resistance (Ra) as observed in thepresent study for higher thickness (>8 nm) of Pt clusters (Fig. 5).

The variation of resistance (Rg) measured in the presence of200 ppm LPG for all prepared sensors having different thicknessof Pt clusters is shown in Fig. 6 as a function of temperature. Theresistance, Rg of all the sensor structure decreases to a minimumvalue at a particular temperature (Topt) and thereafter increasesslowly with further increase in temperature. The observed behav-ior clearly suggest that initially (below operating temperature) theprocess of chemisorption is irreversible, and the reaction becomereversible at higher temperature (>Topt). The value of Rg was alsofound to depend significantly on the quantity of Pt catalyst clusterson the surface of sensing SnO2 thin film (Fig. 6).

The observed enhanced response (Ra/Rg) of a sensor structurecould be related either to a large resistance in air (Ra), or a smallresistance (Rg) in the presence of LPG, and for an enhanced sensingperformance it is preferred that both of the changes occur accord-ingly in a novel sensor structure. The presence of platinum catalyst

clusters on the surface of SnO2 thin film is important to enhancethe response of the sensor to LPG. A plausible explanation for theenhanced sensitivity observed with 10 nm thick Pt clusters can beas follows. Since the diameter of the Pt clusters remains the samein all sensor structures (only the thickness is different), the sensing

D. Haridas et al. / Sensors and Actuators B 153 (2011) 89–95 93

Ft

aontarvatovoifvcst

pousiocu1matttc(w(ittt

s

Table 1Sensor response characteristics of SnO2/Pt clusters with varying thickness of Pt.

SnO2–Pt island sensor (Pt islandthickness (nm))

Sensorresponse

Responsetime (s)

Recoverytime (s)

2 6 2 44 10 2 106 119 2 108 555 12 34

ig. 6. Variation of sensor resistance (Rg) in the presence of LPG as a function ofemperature for different thickness of Pt islands.

ction of the sensor seems to be governed mainly by the amountf adsorbed oxygen present on the SnO2 surface. When the thick-ess of the platinum clusters on the surface of SnO2 film is lower,he activity of adsorbed oxygen is less due to presence of smallmount of catalysts, thereby giving relatively lower value of sensoresistance in air (Ra). The increase in content of Pt catalyst will acti-ate the oxygen adsorption of the uncovered surface of SnO2 film,nd therefore sensor response increases with increase in Pt clus-er thickness (up to 10 nm) due to increase in Ra. As the thicknessf Pt cluster increases beyond the optimum thickness (>10 nm) thealue of sensor resistance (Ra) should further increase, however, webserved a decrease in the value of Ra (Fig. 5). The observed decreasen Ra may be due to pulling out of oxygen by the Pt catalyst clustersrom the underlying SnO2 thin film. Therefore increase in oxygenacancies in the SnO2 thin film results in an increase in electrononcentration in the conduction band, and thereby decreases theensor resistance (Ra) as observed in the present study for higherhickness (>10 nm) of Pt clusters (Fig. 5).

The Pt catalysts are well known for activation of the spilloverrocess after dissociation of hydrocarbons (LPG) on the surfacef sensing SnO2 layer [10]. Therefore the interaction of LPG withncovered SnO2 layer leads to a substantial decrease in the sen-or resistance (Rg) due to desorption of adsorbed oxygen therebyncreasing the concentration of electron in the conduction bandf SnO2 after releasing the trapped electrons. Since, increase inontent of Pt catalyst will activate the oxygen adsorption of thencovered surface of SnO2 film and becomes maximum when0 nm thin Pt clusters is being loaded, the spilled dissociatedolecules of LPG get a chance to interact with ample amount of

dsorbed oxygen thereby releasing more number of trapped elec-rons. This accounts for the appreciable fall in the resistance (Rg) ofhe SnO2 thin film sensor loaded with 10 nm thin Pt clusters. Thus,he observed enhanced response (Ra/Rg) of the SnO2–Pt (10 nm)luster structure is related to an increase in the resistance in airRa) and also a very small resistance (Rg) in the presence of LPGhere both changes occurred simultaneously. At a higher thickness

>10 nm) the platinum clusters pulls out oxygen from the underly-ng SnO2 thin film, and less amount of platinum clusters is available

o dissociate and spill over the LPG molecules. Therefore the resis-ance of sensor having thick Pt clusters (>10 nm) does not expectedo fall to a much lower value and giving relatively lower response.

In summary, a high Ra value for 10 nm thin Pt cluster sensortructure indicates enhanced oxygen adsorption activity on the

10 4909 181 4612 123 143 6820 35 97 20

uncovered SnO2 surface. As a case in point McAleer et al. [24,25]reported pinning of Fermi-level of SnO2 with use of additiveswherein Schottky barrier on SnO2 surface influenced gas sensingbut only when additive size was limited in the range 1–10 nm athigher thickness values the affect was conspicuous by its absence.Pt clusters on the SnO2 surface spill over the dissociated impingingLPG molecules which are then able to interact with ample amountof adsorbed oxygen on uncovered SnO2 surface at 220 ◦C. Release ofthe trapped electrons increase the conductivity and hence explainan appreciable fall in the value of Rg.

It may be noted that four sensor structures of SnO2–Pt (10 nm)and SnO2 thin film were prepared under similar processing con-ditions in different batches in the present study and tested for200 ppm LPG. The sensing response and operating temperature ofthese sensor structures were found to be reproducible with in anaccuracy of ±5% and ±2% respectively. The observed results con-firm that the sensor structure having 10 nm thin Pt clusters exhibitsenhanced response. The same sensor was also tested for LPG detec-tion after a period of 10 weeks, and no significant degradation inthe sensing response characteristics were observed, indicating thehigher stability of the prepared sensor structure (SnO2–Pt clus-ters). The sensing response characteristics such as sensor responsetime and recovery time obtained for SnO2 thin film loaded with(catalyst) platinum clusters (2–20 nm thin) for 200 ppm of LPG arepresented in Table 1. It may be noted from Table 1 that the responsespeed as well as recovery time improves with decrease in the thick-ness of Pt catalyst islands; however the sensing response exhibits acompeting effect (Fig. 4). Although the sensor having an optimumthickness (10 nm) of Pt catalyst clusters exhibited the highest sens-ing response (∼5 × 103), the response time (181 s) and recoverytime (46 s) were slightly higher. The increase in response time wasexpected as the sensor loaded with Pt cluster of 10 nm thicknessshowed the roughest surface. It is well known from the literaturethat the more the rough is the surface; more would be the diffu-sion of gas. The trapping of gas molecules on the rough surfacewould enhance its response time, thereby confirming a significantincrease in response time for the sensor having 10 nm Pt cluster.Therefore sensor structure (SnO2–Pt) having 8 nm thin Pt catalystsseems to be more suitable for detection of 200 ppm LPG exhibitingmoderate response (5.5 × 102) with relatively fast response speed(12 s). The sensor response was found to increase by more than twoorders of magnitude when Pt metal clusters of 8–10 nm thicknesswere loaded on the surface of SnO2 thin film.

The study of sensor response as a function of gas concentra-tion is also important for practical applications. Hence the responsecharacteristics of prepared sensors (SnO2–Pt (10 nm) clusters) asa function of LPG concentration are carried out at the operatingtemperature (Fig. 7). It was observed that the response increasescontinuously with increase from 12 × 100 to 5 × 103 with increasein the concentration of LPG from 10 to 200 ppm, and thereafter

response gets saturate at a value of 5.8 × 103. The detection limitof prepared sensors in the present study is 10 ppm. Fig. 8 depictsthe selectivity of the SnO2–Pt (10 nm) cluster sensor structure. Thesensor has been tested for six different gases (LPG, CH4, NH3, CO2,

94 D. Haridas et al. / Sensors and Act

F

HtcoohtaeCntotibthbS

Fd

[

ig. 7. Variation of sensor response as a function of concentration of LPG (in ppm).

2, H2S) of 200 ppm concentration for a temperature range of 60 ◦Co 280 ◦C. It is observed that the SnO2 sensor having 10 nm thin Ptlusters showed the highest response (5 × 103) towards 200 ppmf LPG, and very low response (less than 3 orders) was observed forther gases, like sensing response for hydrogen is second best butaving a low response of about 50. Platinum as a catalyst preferen-ially dissociates each hydrocarbon (HC) entity in the alkane chains nascent hydrogen and carbon, both of which are then able tonhance reactivity with adsorbed oxygen on SnO2 surface formingO2 and H2O respectively [26]. It is important to note that simulta-eous removal of adsorbed oxygen for forming CO2 and H2O leadso enhanced sensing response of Pt–SnO2 to LPG compared to H2nly. Further the bond dissociation energy of H–H bond is greaterhan both C–H and C–C bonds thereby confirming that it is eas-er to dissociate a chain of hydrocarbons linked with C–H or C–Conds in comparison to H–H bonds in hydrogen gas. Hence, Pt clus-ers loaded onto SnO2 surface, give an enhanced response towardsydrocarbon detection in comparison to any other gas. Hence, LPG

eing a mixture of hydrocarbons is detected preferentially by thenO2–Pt sensor structure under study.

ig. 8. Variation of sensor response as a function of temperature for 200 ppm of sixifferent gases.

[

[

[

[

[

[

[

[

[

[

[

[

[

[

uators B 153 (2011) 89–95

4. Conclusions

The sensing response characteristics of rf-sputtered SnO2 thinfilm (90 nm) loaded with Pt catalyst clusters has been studied fordetection of 200 ppm LPG. Response of SnO2–Pt sensors to LPGdepends strongly on the quantity of platinum catalyst clustersavailable on its surface. The enhanced response (∼5 × 103) wasobtained with the presence of 10 nm thin Pt clusters on SnO2 sur-face for LPG detection at a relatively low operating temperature(220 ◦C) with a response speed of 180 s. The response of SnO2–Ptcluster sensor having 8 nm thin Pt shows a moderate response of5.5 × 102 but at a relatively higher response speed of 12 s. A criticalthickness of 8–10 nm for the platinum clusters allows the enhancedcatalytic activity for adsorbed oxygen besides the spill over mech-anism in the SnO2–Pt cluster sensors. The prepared sensor exhibitsenhanced stability (>10 weeks) and good selectivity.

References

[1] G.N. Chaudhari, A.M. Bende, A.B. Bodade, S.S. Patil, S.V. Manorama, Detection ofliquid petroleum gas using mixed nanosized tungsten oxide-based thick filmsemiconductor sensor, Talanta 69 (2006) 187–191.

[2] G. Sberveglieri, Recent developments in semiconducting thin-film gas sensors,Sens. Actuators B 23 (1995) 103–109.

[3] D. Haridas, K. Sreenivas, V. Gupta, Improved response characteristics of SnO2

thin film loaded with nanoscale catalysts for LPG detection, Sens. Actuators B133 (2008) 270–275.

[4] A. Chowdhuri, V. Gupta, K. Sreenivas, Fast response H2S gas sensing charac-teristics with ultra-thin CuO islands on sputtered SnO2, Sens. Actuators B 93(2003) 572–579.

[5] S. Chakraborty, A. Sen, H.S. Maiti, Selective detection of methane and butaneby temperature modulation in iron doped tin oxide sensors, Sens. Actuators B115 (2006) 610–613.

[6] F. Pourfayaz, A. Khodadadi, Y. Mortazavi, S.S. Mohajerzadeh, CeO2 doped SnO2

sensor selective to ethanol in presence of CO, LPG and CH4, Sens. Actuators B:Chem. 108 (2005) 172–176.

[7] A.S. Ryzhikova, A.N. Shatokhina, F.N. Putilin, M.N. Rumyantseva, A.M. Gaskov,M. Labeau, Hydrogen sensitivity of SnO2 thin films doped with Pt by laserablation, Sens. Actuators B 107 (2005) 387–391.

[8] M. Risti, M. Ivanda, S. Popovi, S. Musi, Dependence of nanocrystalline SnO2

particle size on synthesis route, J. Non-Cryst. Solids 303 (2002) 270–280.[9] G.-J. Li, X.-H. Zhang, S. Kawi, Relationships between sensitivity, catalytic activ-

ity, and surface areas of SnO2 gas sensors, Sens. Actuators B 60 (1999) 64–70.10] I. Matko, M. Gaidi, J.L. Hazemann, B. Chenevier, M. Labeau, Electrical properties

under polluting gas CO/of Pt- and Pd-doped polycrystalline SnO2 thin films:analysis of the metal aggregate size effect, Sens. Actuators B 59 (1999) 210–215.

11] S.-J. Hong, J.-I. Han, Low-temperature catalyst adding for tin–oxide nanostruc-ture gas sensors, IEEE Sens. J. 5 (2005) 12–19.

12] M. Nishibori, W. Shin, K. Tajima, L.F. Houlet, N. Izu, T. Itoh, I. Matsubara,Long-term stability of Pt/alumina catalyst combustors for micro-gas sensorapplication, J. Eur. Ceram. Soc. 28 (2008) 2183–2190.

13] C.-H. Hana, D.-W. Honga, I.-J. Kima, J. Gwak, S.-D. Hana, K.C. Singh, Synthesisof Pd or Pt/titanate nanotube and its application to catalytic type hydrogen gassensor, Sens. Actuators B 128 (2007) 320–325.

14] F. Haghighat, A. Khodadadi, Y. Mortazavi, Temperature-independent ceria- andPt-doped nano-size TiO2 oxygen lambda sensor using Pt/SiO2 catalytic filter,Sens. Actuators B 129 (2008) 47–52.

15] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Grain size effects on gas sensitivity ofporous SnO2-based elements, Sens. Actuators B 3 (1991) 147–155.

16] G. Zhang, M. Liu, Effect of particle size and dopant on properties of SnO2-basedgas sensors, Sens. Actuators B 69 (2000) 144–152.

17] A. Cabot, A. Vila, J.R. Morante, Analysis of the catalytic activity and electricalcharacteristics of different modified SnO2 layers for gas sensors, Sens. ActuatorsB 84 (2002) 12–20.

18] S. Fujihara, Y. Ogino, H. Imai, S.-H. Kim, Synthesis and structural characteriza-tion of thermally stable Pt/SnO2 nanocomposite particles, Nanotechnology 16(2005) 1396–1399.

19] F.F. Bentley, L.D. Smithson, A.L. Rozek, Infrared Spectra and CharacteristicFrequencies, Interscience Publishers, New York, London and Sydney, 1968(spectrum number: 1556).

20] D.E. Milligan, M.E. Jacox, Infrared spectroscopic evidence for the species HO2,J. Chem. Phys. 38 (1963) 2627–2632.

21] M.C. Salvadori, D.R. Martins, M. Cattani, DLC coating roughness as a function offilm thickness, Surf. Coat. Technol. 200 (2006) 5119–5122.

22] B.-K. Min, S.-D. Choi, SnO2 thin film gas sensor fabricated by ion beam deposi-tion, Sens. Actuators B 98 (2004) 239–246.

23] A. Gurlo, Interplay between O2 and SnO2: oxygen ionosorption and spectro-scopic evidence for adsorbed oxygen, Chem. Phys. Chem. 7 (2006) 2041–2052.

24] J.F. McAleer, P.T. Mosely, O.W. Norris, D.E. Williams, Tin oxide gas sensors, J.Chem. Soc., Faraday Trans. 83 (1987) 1323.

d Act

[

[

B

DtPMmao

AaD

D. Haridas et al. / Sensors an

25] J.F. McAleer, P.T. Mosely, O.W. Norris, D.E. Williams, B.C. Tofield, Tin dioxidegas sensors. Part 2. The role of surface additives, J. Chem. Soc., Faraday Trans.84 (1988) 441.

26] J. Girard, Principles of Environmental Chemistry, Jones and Bartlett PublishersInc., Canada, 2004, pp. 1–50.

iographies

ivya Haridas received the BSc and MSc degrees in physics in 2003 and 2005, respec-ively, from the University of Delhi, New Delhi where she is currently pursuinghD. Presently she is an assistant professor at the Department of Physics in Keshavahavidyalaya, University of Delhi. Her research work is in the area of condensedatter physics and is focused on gas sensors which involve the development of LPG

nd CNG sensors. Her other research interests are in the growth and characterizationf metal oxide films for their structural and optical properties.

rijit Chowdhuri was born in Delhi, India, in October 1974. He received the MScnd PhD degrees in physics in 1997 and 2004, respectively, from the University ofelhi. Presently he is an assistant professor in the Department of Physics, Acharya

uators B 153 (2011) 89–95 95

Narendra Dev College, University of Delhi, Delhi, India. His research interests are inthin film and nanoparticles based gas sensors, including investigating the variousgas sensing mechanisms.

K. Sreenivas received the BSc and MSc degrees in applied physics from Andhra Uni-versity, Visakapatnam, India, in 1977 and 1980, respectively, and the MPhil and PhDdegrees in physics from the University of Delhi, New Delhi, India, in 1982 and 1985,respectively. He is presently working as a professor in the Department of Physicsand Astrophysics at the University of Delhi. Also he is the director of UniversityScience and Instrumentation Center, Delhi University. His research interests are inpyroelectric infrared (IR) detectors, acoustoelectric interaction in thin-film-basedsurface acoustic wave devices, and materials for gas sensing applications.

Vinay Gupta received the BSc, MSc, and PhD degrees in physics in 1987, 1989,and 1995, respectively, from the University of Delhi, New Delhi, India. In 1995, he

worked as a senior lecturer in physics at Deen Dayal Upadhyaya College, Universityof Delhi. Presently, he is professor in the Department of Physics and Astrophysics,University of Delhi. His current research interests are in piezoelectric thin films andlayered structures, semiconductor and surface acoustic wave (SAW) sensors, ferro-electric films for electrooptic applications, and nanostructured oxide materials formultifunctional application.