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Sensors and Actuators B 161 (2012) 1114– 1118

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

j o ur nal homep a ge: www.elsev ier .com/ locate /snb

hort communication

ow temperature operating SnO2 thin film sensor loaded with WO3 micro-discsith enhanced response for NO2 gas

njali Sharmaa, Monika Tomarb, Vinay Guptaa,∗

Department of Physics and Astrophysics, University of Delhi, Delhi 110007, IndiaDepartment of Physics, Miranda House, University of Delhi, Delhi 110007, India

r t i c l e i n f o

rticle history:eceived 19 August 2011eceived in revised form 5 October 2011ccepted 7 October 2011vailable online 13 October 2011

ACS:1.15.Cd7.07.Df

a b s t r a c t

Highly sensitive and novel sensor structure comprising of SnO2 film and WO3 micro-discs has beendeveloped for trace level detection of NO2 gas at lower operating temperature. Loading of WO3 on SnO2

film surface in the form of uniformly distributed micro-discs (8 nm thin, 600 �m dia.) was found toenhance the sensing response (5.4 × 104) with fast response speed (67 s) at 100 ◦C. Spill over of NO2

gas molecules by WO3 micro-discs on the uncovered surface of SnO2 film and subsequently its reactionkinetics at Sn and W sites could be responsible for modulation of depletion width at n–n semiconductorheterojunction and may play an important role in enhanced response characteristics.

© 2011 Elsevier B.V. All rights reserved.

1.07.Bc

eywords:ensoremiconductor

nO2 thin filmsO2 gas

. Introduction

Out of the main key environmental pollutants, the presence ofigh level of nitrogen dioxide (NO2) is of greatest concern as it led toevere health hazardous including nerve system and asthma prob-em [1]. In air NO2 is transformed into gaseous nitric acid and toxicrganic nitrates, and contributes to the production of acid rain. NO2lays a major role in atmospheric reactions that produce ground-

evel ozone, a major component of smog [2]. Also NO2 is a precursoro nitrates, which contribute to increased respirable particle levelsn the atmosphere. Exhaust nitrogen oxides from combustion facil-ties such as automobiles and industrial sites are also hazardous touman health. Therefore, the development of sensors for efficientnd fast monitoring of NO2 gas is essentially important.

Few efforts have been made towards the development of NO2as sensors based on SAW devices, solid electrolytes, surfacelasmon resonance, semiconductor oxides etc. [3–6]. However,ensors based on semiconductor oxides are generally simple in

onstruction, low in cost and show high stability, even in corro-ive environments. Amongst various semiconductor oxides, SnO2nd WO3 are attractive for the detection of NO2 gas [7–11]. Higher

∗ Corresponding author.E-mail addresses: vgupta@physics.du.ac.in, drvin gupta@rediffmail.com

V. Gupta).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.10.014

response, fast response time along with low operating tempera-ture are the key issues for realization of an efficient NO2 gas sensorand it is difficult to design a sensing element exhibiting all theserequirements. In sensors where sensitivity is high, either operatingtemperature is high or response speed is poor, and where operat-ing temperature is low, the sensor response degrades with slowresponse. Meng et al. have studied the NO2 gas sensing character-istics using WO3 nanoparticles and found high sensor response of4700 for 1 ppm NO2 gas at 50 ◦C, but the response time is quitehigh (∼4 min) and the sensor did not recover its initial resistancevalue at all [12]. There are few reports on the detection of NO2 gasat room temperature using carbon nanotubes (CNTs) doped SnO2thin films as well, but the sensor response is found to be very low(sensor response is 1.7 towards 100 ppm of NO2 gas) [13]. A highsensor response is reported (∼33,300) at 500 ppm of NO2 gas, forWO3 doped SnO2 film however, the operating temperature (150 ◦C)is quite high [14]. In the present work, an effort has been made todesign a novel sensor structure by integrating the sensing SnO2 thinfilm with WO3 micro-discs for reliable detection of low concentra-tions of NO2 gas at much lower operating temperatures with fastresponse and recovery times.

2. Experimental

Tin oxide (SnO2) thin films of 430 nm thickness were depositedusing RF diode sputtering technique on the corning glass substrates.

A. Sharma et al. / Sensors and Actuators B 161 (2012) 1114– 1118 1115

O3-di

AA1tpopvwWmtbNpcSt3fimc

gtstoptwtitTrtiettoeoot1r

Fig. 1. Top view of the SnO2–W

metallic Tin (Sn) target (99.999% pure) was sputtered in 70%r + 30% O2 gas ambient at a sputtering pressure of 16 mTorr at50 W RF power. The Platinum (Pt) inter digital electrodes (IDEs) ofhickness ∼100 nm were patterned on the surface of corning glassrior to the deposition of SnO2 thin film. The fabrication detailsf Pt IDEs are explained in our previous work [15]. The depositionarameters of SnO2 thin films were optimized to achieve a loweralue of initial sensor resistance in air (Ra). Two sensor designsere prepared in the present study (i) SnO2 thin film loaded withO3 layer (8 nm thin), and (ii) SnO2 thin film loaded with WO3icro-discs (8 nm thin and 600 �m dia.). The dense WO3 ceramic

arget of 99.99% purity was ablated in 100% O2 at 10 mTorr pressurey Pulse Laser Deposition technique using the fourth harmonics ofd:YAG laser (� = 266 nm) with a repetition rate of 10 Hz at an out-ut power of 100 mJ/pulse. The loading of WO3 micro-discs werearried out using a shadow mask of 600 �m pore size. Thickness ofnO2 and WO3 films was measured using a surface profiler (Dek-ak 150A). The prepared sensor structures were annealed in air at00 ◦C for 3 h to stabilize the sensor resistance. The bare SnO2 thinlms and composite sensor structures having continuous layer oricro-discs of WO3 (Fig. 1) are named as SnO2-pure, SnO2–WO3-

ont. and SnO2–WO3-discs respectively.A specially designed gas sensor test rig (GSTR) consisting of a

lass test chamber was used to detect the NO2 gas. The humidity inhe test chamber was about 30 RH% while doing the sensing mea-urements. Volume of the test chamber was taken to be 11.0 l andarget NO2 gas was injected in the test chamber through a syringef 0.1 ml at the time of taking response for 10 ppm of NO2 gas. Airani gauge with a rotary pump was used to control the flow ofarget gas in the test chamber. Vacuum of the order of ∼10−3 Torras first created in the test chamber and subsequently a mixture of

he known concentration of target gas and clean synthetic air wasntroduced till the test chamber acquired the atmospheric pressureo ensure that the target gas was free from any other disturbing gas.he measurements were carried out in static mode. At the time ofecovery of the senor resistance, target gas was flushed out of theest chamber (by creating vacuum again) and the clean dry air wasntroduced into the chamber. The surface of porous SnO2 are gen-rally covered with chemisorbed and physisorbed water at roomemperature in normal laboratory condition (ca. 40–60 RH%) andhen shows ionic (H+) conductivity, and the effect of polarizationn sensor resistance cannot be ruled out. However, these effects arexpected to be low in the SnO2 thin film having low resistivity asbserved in the present work. It may be noted that the resistance

f SnO2 sensor prepared in present study was unstable at roomemperature and become stable only after a thermal treatment at10 ◦C in air for 1 h. Therefore, all prepared sensor structures wereefreshed at a temperature of 110 ◦C prior to measure the sensor

scs sensor structure on Pt IDEs.

response. The sensor was placed on a temperature controlled heat-ing block inside the glass test chamber and spring loaded platinisedcontacts were used to measure the sensor response as a function oftemperature (75–225 ◦C). At each temperature the sensor was firststabilized in air to obtain a stable resistance value. Target gas (NO2)of specific concentration was introduced into the test chamber andchanges in the sensor resistance were recorded after every secondusing a data acquisition system consisting of a digital multi-meter(model: Keithley 2700) interfaced with a computer.

NO2 is an oxidizing gas and the sensor response for the same isdefined as

S = Rg − Ra

Ra(1)

where, Ra and Rg are the resistances of the sensor element in thepresence of atmospheric air and target gas respectively. The timetaken by the sensor element to acquire the 90% of its maximumresistance value in the presence of target oxidizing gas was mea-sured to find out the senor response time. Once the maximumresistance value is attained, the target gas was flushed out of thetest chamber and sensor element was allowed to regain its initialresistance value (Ra) in atmospheric air. Time taken by the sensorto reacquire a value higher by 10% of its initial resistance value inthe presence of atmospheric air is measured to find out the sensorrecovery time.

3. Results and discussions

Crystalline structure and surface morphology of the pre-pared SnO2–WO3 composite sensing element was studied usingBragg–Brentano (�–2�) scan of a X-ray Diffractometer (Bruker D8Discover) using the Cu K�1 source (� = 0.154 nm) and Atomic forcemicroscopy (Veeco DICP2) respectively. The XRD pattern (Fig. 2) ofthe annealed SnO2–WO3 composite sensing element shows reflec-tions corresponding to (1 1 0), (1 0 1) and (2 1 1) planes of SnO2 andare in good agreement to the reported results on SnO2 thin films[16]. However, no XRD peak corresponding to WO3 is observedin any prepared sensor structures, indicating the growth of amor-phous WO3 (8 nm thin). The inset in Fig. 2 shows the AFM image ofthe surface morphology of WO3 in the composite sensing element(SnO2–WO3-discs). The AFM image show the elongated grains withchannels indicating a rough surface morphology as desired for a gassensor exhibiting excellent response characteristics.

The response characteristics of SnO2-pure, SnO2–WO3-cont.

and SnO2–WO3-discs sensing elements were carried out in thepresence of 10 ppm NO2 gas concentration as a function of tem-perature (75–250 ◦C). The response (Fig. 3) for all sensor structuresis found to be maximum at a temperature of 100 ◦C (operating

1116 A. Sharma et al. / Sensors and Actuators B 161 (2012) 1114– 1118

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0 20 0 40 0 600 80 0 100 0 120 0 14 00-1x104

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SnO2- Pure SnO2- WO3- cont . SnO2- WO3- dis cs

10 ppm NO2 gasTopt.= 10 0 oC

Fig. 4. Change in resistance with time when gas is inserted in and flushed out

ig. 2. X-ray diffractogram of the SnO2 thin films annealed in air at 300 C. Inset:FM image of the surface morphology of WO3 in the composite sensing element

SnO2–WO3-discs).

emperature). The sensor design comprising of SnO2 film loadedith WO3 micro-discs exhibits the maximum response ∼5.4 × 104

n comparison to other sensor structures. The sensor response ofnO2 film was found to increases from 1.4 × 104 to 5.4 × 104 withoading of WO3 micro-discs (Fig. 3), while a decrease in responseo 5.8 × 103 is observed with continuous over layer of WO3. Thebserved results clearly indicate that the availability of the surfacef both the WO3 and SnO2 is important for the interaction of targetas molecules for exhibiting enhanced response at lower temper-ture. It is important to point out that the response (1.4 × 104) ofare SnO2 film (SnO2-pure sensor) is much higher in comparisono the corresponding value reported in literature by other workersspecially at lower temperature (∼100 ◦C) and is attributed to theeposition of SnO2 thin film with porous and rough morphology17]. It is interesting to note that the room temperature resistancef sensor (Ra) increases after integrating WO3 with SnO2 thin filmsnd are 2.9 k�, 9.8 k� and 4.5 k� for SnO2-pure, SnO2–WO3-cont.

nd SnO2–WO3-discs sensor structure respectively. Integration ofO3 both in the form of a continuous over-layer or as micro-discs

esults in the formation of n–n semiconductor hetrojunction. The

60 80 10 0 120 14 0 160 180 20 0 220 24 0 260

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50000

60000

Res

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SnO2-pure SnO2-WO3-cont. SnO2-WO3-discs

ig. 3. Variation of response with temperature of sensor structures: SnO2-pure,nO2–WO3-cont. and SnO2–WO3-discs towards 10 ppm of NO2 gas.

from the chamber for SnO2-pure, SnO2–WO3-cont. and SnO2–WO3-discs sensorsstructures at operating temperature (Topt.) of 100 ◦C for 10 ppm of NO2 gas.

n–n junction formed at the outer surface of SnO2 is not expected somuch to affect the total resistance of sensor having IDEs located inthe innermost region of sensing layer if the SnO2 layer is dense andof large thickness. In the present study the SnO2 layer was highlyporous with a thickness of about 430 nm. The significant increaseobserved in sensor resistance from 2.9 k� to 9.8 k� with presenceof WO3 continuous layer, confirm the effect of n–n junction on thesensor resistance in the present sensor structures. Since, the workfunction of WO3 (4.41 eV) is slightly greater in comparison to thatof SnO2 (4.18 eV), a depletion region in SnO2 is formed near thejunction whereas an accumulation region is created in WO3 dueto the transfer of electrons from SnO2 to WO3 [18]. Therefore, thesensor resistance is found to be more for hetrojunction in com-parison to that of bare SnO2 sensor. Furthermore, presence of largeamount of WO3 on the surface of SnO2 for SnO2–WO3-cont., resultsin much higher value of Ra in comparison to corresponding valuefor SnO2–WO3-discs.

The change in sensor resistance obtained at 100 ◦C operatingtemperature for all the three sensor structures as a function oftime is shown in Fig. 4. Initially a 10 ppm NO2 gas is introducedinto the test chamber and subsequently gas is flushed out aftersome time. The value of resistance of SnO2-pure sensor was foundto increase slowly (from 2.72 k� to 38.63 M�) after exposure withNO2 gas, and is attributed to the oxidizing nature of target gas. How-ever, a rapid increase in the value of sensor resistance from 1.4 k�to 75 M� was observed for SnO2–WO3-discs sensor structure onNO2 gas interaction. Surprisingly a relatively small increase fromRa to Rg (4.3 k� to 25 M�) was noted for SnO2–WO3-cont. sen-sor structure (even smaller in comparison to SnO2-pure sensor),showing the importance of the availability of uncovered surfaceof SnO2 thin films besides WO3 catalysts for interaction with tar-get NO2 gas. The interaction of NO2 target gas with only WO3 layermay be responsible for the low response of SnO2–WO3-cont. sensorstructure.

The sensor response characteristic parameters (response,response time and recovery time) obtained for the SnO2-pure,SnO2–WO3-discs and SnO2–WO3-cont. sensing elements at 100 ◦Cfor 10 ppm NO2 gas are summarized in Table 1. It may be seenthat the presence of WO3 catalyst on the SnO2 surface either in

the form of continuous layer or discs reduces both the responseand recovery times to a greater extent (Table 1). Therefore, pres-ence of WO3 over SnO2 surface enhances the rate of the reactionof NO2 gas with sensing element. The enhanced sensor response

A. Sharma et al. / Sensors and Actuat

Table 1Sensing parameters obtained for three different sensor structures: SnO2-pure,SnO2–WO3-cont. and SnO2–WO3-discs at operating temperature of 100 ◦C for10 ppm of NO2 gas.

Structure type (with thickness) Response Responsetime

Recoverytime

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SnO2-pure 1.4 × 10 4.13 min 33.43 minSnO2–WO3 cont. 5.8 × 103 1.6 min 17.8 minSnO2–WO3-discs 5.4 × 104 67 s 17.05 min

5.4 × 104) with relatively fast response speed of 67 s was obtained,hen WO3 is loaded in the form of micro-discs on SnO2 film sur-

ace. The observed much higher values of sensor response for thenO2–WO3-discs composite sensing element with fast responsepeed at low operating temperatures of 100 ◦C towards 10 ppmO2 gas is encouraging and is much higher in comparison to thevailable reports in the literature.

Since, NO2 is an oxidizing gas, NO2 molecules trap the electronsrom the free Sn sites available on the surface of SnO2 thin filmorming NO2

−, and therefore the resistance of the SnO2 sensingayer increases from Ra to Rg [19]. When NO2 gas is removed, theO2

− species desorb as NO gas molecule from the SnO2 surface.he desorption of NO2

− leaves behind oxygen species (O−) whichs released slowly as oxygen molecule (O2) from the SnO2 film sur-ace thereby causing a slow recovery especially at low operatingemperature (Fig. 3).

It is seen that the dispersal of WO3 in the form of micro-iscs over the surface of SnO2 sensing layer, enhances the sensoresponse, besides reducing the response and recovery timesowards 10 ppm NO2 target gas. When the target NO2 gas moleculesnteract with the surface of SnO2–WO3-discs sensor, it attacks theree Sn sites available on the uncovered SnO2 surface as well as

sites on the surface of WO3 discs. As explained earlier, NO2olecules trap electrons from the Sn sites forming NO2

− speciesnd increasing the resistance of the underneath SnO2 sensing layer.owever, NO2 gas molecules after interaction with WO3 disc sur-

ace at lower temperatures (∼100 ◦C) are adsorbed at W sites inhe form of ONO− (nitrito type adsorbates) after extracting elec-rons and subsequently dissociate into nitrosyl type adsorbatesuch as NO+ and NO− along with O− [20,21]. At lower temperatures∼100 ◦C) the NO+ adsorbates are superior in number than NO−

ypes [21]. This lead to the transfer of electrons from WO3 oxideso species related to NO2 chemisorption which creates Schottkynergy barrier at the surface yielding a large resistance of the film.herefore, sensing mechanism of NO2 gas is mainly due to spilloverf NO+ adsorbates along with O− from the surface of WO3 micro-iscs on the uncovered surface of SnO2 film. Ghimbeu et al. havelso reported the formation of NO+ species on the surface of WO3fter interaction with NO2 gas molecules leading to high responset 150 ◦C [22]. NO+ interact with the available Sn sites on SnO2 sur-ace and extracts electrons from free Sn sites in the same manners NO2 gas molecules interact with Sn sites which increases theesistance of the sensor structure.

O+ + Sn2+ → NO(g) + Sn3+ (2)

(Sn3+ + O−) → 2Sn2+ + O2(g) (3)

The NO+ species extract electrons from the SnO2 surface therebyncreasing the depletion width in the SnO2 layer near the junctionnd resulting in further increase in the resistance of SnO2–WO3-iscs sensor structure. After trapping one electron from Sn site NO

s released to atmosphere and spilled over O− specie releases one

lectron to the Sn site and released to atmosphere. Thereby, reduc-ng the sensor resistance and thus sensor recovers back to the initialesistance. Therefore NO2 and NO+ species simultaneously reactith the uncovered surface of SnO2 thin film resulting in significant

ors B 161 (2012) 1114– 1118 1117

increase in the sensor resistance (SnO2–WO3-discs structure) incomparison to the corresponding increase in case of other twosensor structures (SnO2-pure and SnO2–WO3-cont. structure). Dur-ing desorption, NO2

− and NO+ species are released as NO and O2molecules from the uncovered surface of SnO2–WO3-discs sensorthus reducing the depletion width in the SnO2 layer near the junc-tion and sensor recovering back to its initial resistance value (Ra).

It is important to note that the sensor structure SnO2–WO3-cont.shows a much lower response as compared to that observed forpure-SnO2 and SnO2–WO3-discs structures indicating that the spillover mechanism is dominant over the Fermi energy level mecha-nism. This may be attributed to the fact that the NO2 moleculesinteract with the W sites only while the free Sn sites are not avail-able due to presence of continuous WO3 over layer on SnO2 surfacein SnO2–WO3-cont. sensor structure, thereby resulting in poorsensing response. The capturing of electrons by NO2 species fromthe surface of WO3 continuous layer results in a reduction in theaccumulation of charge in WO3 over layer near the junction. There-fore a relatively small increase in the resistance of SnO2–WO3-cont.sensor structure is observed in the presence of sensing gas. On con-trary, SnO2–WO3-discs sensor structure is showing the enhancedresponse in comparison to other sensors due to spill over of NO+

species from the WO3 micro-discs on the uncovered surface of SnO2layer. To the best of our knowledge, spill over mechanism for NO2gas sensing has not been reported and from our results we can inferthat the spill over mechanism is dominating over the Fermi energylevel mechanism.

It is important to note that response and recovery times of thesensor structures having WO3 on the SnO2 surface either in theform of micro-discs or continuous over layer are lower comparedto the corresponding values obtained for bare SnO2 film (SnO2-puresensor). The obtained results clearly indicates that the presence ofWO3 micro-discs enhances the rate of reaction between the targetNO2 gas molecules and the Sn sites available on uncovered surfaceof SnO2 thin films, resulting in a fast response and recovery speeds(Fig. 4). In the SnO2–WO3-discs sensor structure, the spill over ofdissociated NO+ ions by WO3 discs on the uncovered surface ofSnO2 film is responsible for the enhanced sensor response.

4. Conclusions

A novel sensor design structure having SnO2 thin film loadedwith 8 nm thin WO3 discs has been prepared yielding enhancedsensor response (5.4 × 104) at a much lower operating temperature(100 ◦C) with fast response and recovery times of 67 s and 17 minrespectively for 10 ppm NO2 gas. Presence of WO3 in the form ofmicro discs on the SnO2 film surface is identified to enhance therate of reaction between target gas and sensing layer resulting inmodulation of depletion width, thereby improving the responseand recovery times. Furthermore, the origin of enhanced sensorresponse for the same sensor structure is may be due to the spillovermechanism due to WO3 micro discs and simultaneous interactionof NO2 and NO+ species at Sn sites on the uncovered SnO2 surfacehaving porous and rough microstructure. The combination of boththe modulation of space charge region due to n–n junction (dueto interaction of NO2 with WO3) and changes in resistance due toextraction of electrons from SnO2 sensing layer (because of directinteraction of spilled over NO2 species with uncovered SnO2 layer)were responsible for enhanced response of SnO2–WO3-disc sensorstructure.

Acknowledgements

The authors are thankful to Department of Science and Technol-ogy (DST), National Program on Smart Sensors (ADA) and University

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f Delhi for the financial support for carrying out this research work.ne of the authors (AS) is grateful to the Council of Scientific and

ndustrial Research (CSIR) for the research scholarship.

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Biographies

Anjali Sharma was born in Delhi, India, in June 1985. She received her B.Sc. and M.Sc.degrees in Physics in the year 2006 and 2008 respectively, from University of Delhi.Presently she is a Junior Research Fellow pursuing her Ph.D. program from Universityof Delhi, Delhi. Her research interests are in gas sensor systems that include sensorcharacterization and development of metal oxide films for sensor coatings. She isalso working towards the fabrication of MEMS based Electronic-Nose for gas sensingapplications.

Monika Tomar was born in Delhi, India, in April 1976. She received her B.Sc., M.Sc.and Ph.D. degrees in Physics in 1996, 1998 and 2005 respectively, from the Universityof Delhi. Presently she is Assistant Professor at Miranda House, University of Delhi,India. Her research interests include piezoelectric thin films for Surface acousticswave devices and sensors, oxide thin films and nanostructures for gas sensing andbiosensing applications, photonic devices etc.

Vinay Gupta was born in Mujjaffar Nagar (U.P), India, in March, 1967. He received hisB.Sc., M.Sc., and Ph.D. degrees in physics 1987, 1989 and 1995, respectively from theUniversity of Delhi, India. Subsequently he joined DDU College, University of Delhi

as Assistant professor. Presently he is Professor in the Department of Physics andAstrophysics, University of Delhi, India. He is a senior member of IEEE. His currentresearch interests are in piezoelectric, ferroelectric and semiconducting thin films,gas/bio sensors, electro-optic applications, oxide nanostructures for multi functionalapplications etc.