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1386-9477/$ - se

doi:10.1016/j.ph

�Correspondals Science (ITI

Viet Road, Han

E-mail addr

(N.V. Hieu).

Physica E 40 (2008) 2950–2958

www.elsevier.com/locate/physe

Inclusion of SWCNTs in Nb/Pt co-doped TiO2 thin-film sensor forethanol vapor detection

Nguyen Van Hieua,b,�, Nguyen Van Duya, Pham Thanh Huya,b, Nguyen Duc Chienb,c

aInternational Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT), No.1 Dai Co Viet Road, Hanoi, Viet NambHanoi Advanced School of Science and Technology (HAST), Hanoi University of Technology (HUT), Viet Nam

cInstitute of Engineering Physics (IEP), Hanoi University of Technology (HUT), Viet Nam

Received 30 July 2007; received in revised form 20 February 2008; accepted 21 February 2008

Available online 23 May 2008

Abstract

Nb-Pt co-doped TiO2 and the hybrid SWCNTs/Nb-Pt co-doped TiO2 thin films have been prepared by the sol–gel spin-coating process

for gas-sensor fabrication. Field emission scanning electron microscope (FE-SEM, TEM and X-ray diffraction (XRD) characterizations

indicated that the SWCNTs inclusion did not affect the morphology of the TiO2 thin film and the particle size. Additionally, the

SWCNTs were well embedded in the TiO2 matrix. The gas-sensing properties of Nb–Pt co-doped TiO2 thin films with and without

SWCNTs inclusion were investigated. The hybrid sensors with the inclusion of different SWCNTs contents are examined to elucidate the

effect of SWCNTs content on the gas-sensing properties. Experimental results revealed that the responses to ethanol of Nb–Pt co-doped

TiO2 sensors with SWNCTs inclusion increase by factors of 2–5 depending on the operating temperature and the ethanol concentration,

compared to that of the sensor without SWCNTs inclusion. Moreover, all hybrid sensors can operate with high sensitivity and stability at

a relatively low operating temperature (o335 1C). The responses of the hybrid sensors are greatly affected by SWCNTs content

inclusion. The optimized SWCNTs content of 0.01% by weight was obtained for our experiment. The improved gas-sensing performance

should be attributed to the additional formation of the p/n junction between SWCNTs (p-type) and TiO2 (n-type).

r 2008 Elsevier B.V. All rights reserved.

PACS: 61.48.De; 07.07.df; 81.07.De

Keywords: Titanium oxide; Carbon nanotubes; Gas sensor

1. Introduction

Semiconductor metal oxide (SMO) gas sensors areactually one of the most investigated groups of gas sensors.They have attracted great attention by many users andscientists interested in the field under atmospheric condi-tions due to their advantages such as high sensitivity topollutant gases, large number of detectable gases, fastresponse and recovery times, low cost, easy implementa-tion, and small size [1–4].

e front matter r 2008 Elsevier B.V. All rights reserved.

yse.2008.02.018

ing author at: International Training Institute for Materi-

MS), Hanoi University of Technology (HUT), No.1 Dai co

oi, Viet Nam. Tel.: +84 4 8680787; fax: +84 4 8692963.

esses: hieu@itims.edu.vn, hieunv-itims@mail.hut.edu.vn

The application fields of TiO2 material range fromcatalytic and electrochemical processes through opticalcoatings to gas-sensing devices [5–11]. TiO2-based sensorshave been increasingly paid attention due to its betterchemical stability at high operation temperatures and inharsh environment besides its sensing feasibilities com-pared to that of SnO2-based sensors [12,13]. Apparently,SnO2 was one of the first considered and still is the mostfrequently used material for gas-sensing applications [14].More recently, the detection of gas molecules of organiccompounds (alcohol, methanol, n-propanol, acetone,benzene) [15–18], oxidants (NO2, CO, O2) [19–22], andreductants (H2, NH3) [23–25] have been reported fornanosized TiO2. Like SnO2-based sensor, a great concern isthe effect of additives doping on the electronic structureand the gas-sensing properties of TiO2 material. So far, the

ARTICLE IN PRESSN.V. Hieu et al. / Physica E 40 (2008) 2950–2958 2951

doping effect has been studied as well as the benefits fromvarious additives such as Pd, Pt, Nb, La, Cu, W, Cr, andSn in improving sensitivity, selectivity, stability, and inreducing the operating temperature [12,26,31–34]. It hasbeen observed that TiO2 doped with Nb and Pt sensorshave a good performance in detecting ethanol vapor[27–31]. Our previous works have also shown that theNb–Pt co-doped TiO2 sensor has a good sensitivity toethanol vapor [29,30]. Unfortunately, the operating tem-perature of the TiO2-based sensors are still rather too high,normally in the range of 350–400 1C. This would result inhigh power consumption and difficulty of packaging.Accordingly, it would be inconvenient to develop portablehand-held alcohol tester for drunk-drive control usingTiO2-based sensors. Therefore, the decrease of the operat-ing temperature for the TiO2 sensor is very important forthe alcohol tester.

Carbon nanotubes (CNTs) have been the most activelystudied materials in recent years due to their uniqueelectrical, mechanical, and chemical properties, and muchattention has been paid to their application in various fieldsof nanotechnology [35]. It has been reported that the CNTsare very sensitive to the surrounding environment. Thepresence of O2, NH3, NO2 gases, and many othermolecules can either donate or accept electrons, resultingin an alteration of the overall conductivity [36–38]. Suchproperties make CNTs ideal for nanoscale gas-sensingmaterials, and CNTs field effect transistor (FETs) andconductive-based devices have already been demonstratedas gas sensors [39–43].

Recently, the combination of SMO with CNTs has beenexplored and many interesting findings have been obtained,in which various kinds of nanoarchitectures between SMOand CNTs have been made such as CNTs-doped SnO2,CNTs–SnO2 or TiO3 or WO3 composite, and CNTs coatedwith SnO2 [44–50]. This has motivated us to explore theinfluence of SWCNTs inclusion on sensing performance ofthe TiO2-based sensor to ethanol vapor. In this work, weinvestigate the influence of SWCNTs inclusion withvarying SWCNTs content on the sensing properties ofthe Nb–Pt co-doped TiO2 sensor.

2. Experimental

2.1. Materials synthesis and characterizations

One percent Nb, 0.5% Pt co-doped TiO2 sol wasprepared by the sol–gel method that was previouslyreported [29,30]. The precursors used to fabricate thesolutions are tetra propylortho titanate Ti(OC3H7)4 (99%),hydrogen hexachlo-platinate H2PtCl6 � xH2O (99.9%), nio-bium ethoxide Nb(OC2H5)5 (99%), and isopropanolC3H7OH (99.5%). All chemicals were obtained fromMerck with analytical grade. To synthesize the hybridSWCNT/Nb–Pt co-doped TiO2 material, the SWCNTswith the external diameter lower than 2 nm purchased fromShenzhen Nanotech Port Ltd. Co. (Shenzhen, China) [51]

were dispersed in the 1% Nb, 0.5% Pt co-doped TiO2 solsolution using an immersion-probe ultrasonic with a powerof 100W for 10min. The SWCNTs content was varied inthe range of 0.001–0.1wt% by weight (compared to TiO2).The film was deposited by spin coating on silica substrateat the speed of 4000 rpm for 20 s and the film thickness ofaround 320 nm was obtained by the Alpha Step Profiler.Hybrid sensors with different SWCNTs contents weresigned as S0, S1, S2, S3, S4, and S5, where S0, S1, S2, S3, S4,and S5 were 0, 0.001, 0.005, 0.01, 0.05, and 0.1wt% ofSWCNTs inclusion on Nb–Pt co-doped TiO2 sample,respectively. As-deposited films were dried for 30min at60 1C and then they were annealed at 500 1C for 30min.The morphology and the crystalline phase of the films werecharacterized using a field emission scanning electronmicroscope (FE-SEM; 4800 Hitachi, Japan). The disper-sion of SWCNTs in the TiO2 sol was characterized byTEM using a JEM-100cx instrument with an acceleratingvoltage of 80 kV. It should be noted that the solution wascarefully ultrasonicated before the deposition on a Cu/Rhgrid covered with formvar, and the grid was driedovernight before heat treatment and characterization. Inaddition, the microstructure of the sintered film wascharacterized by X-ray diffraction (XRD), using a Bruker-AXS D5005.

2.2. Gas-sensor fabrication and measurement of

gas-sensing properties

The fabrication of the gas sensor was carried out in thefollowing manner: (i) the interdigitated electrode wasfabricated using a conventional photolithographic methodwith a finger width of 100 mm and a gap size of 70 mm. Thefingers of the interdigitated electrode were fabricated bysputtering 10 nm Ti and 200 nm Pt on a layer of silicondioxide (SiO2) with a thickness of about 100 nm thermallygrown on top of a silicon wafer; (ii) the sensing layers weredeposited on top of the electrode and subsequentlysubjected to heat treatment.The gas-sensing measurements were carried out as

follows. The sensor was first placed on a hot plate andelectrically connected by tungsten needles, and then allwere loaded in a glass chamber (see Fig. 1). The desiredethanol gas concentrations, obtained by mixing ethanol gaswith air using a computerized mass flow control system(AALBORG model GFC17S-VALD2-A0200), were in-jected into the chamber subsequently. The injection of acertain amount of the mixed gas was accurately controlledby a computer. After a duration of time, the chamber waspurged with air and the experiment was repeated foranother cycle. The electrical-resistance response duringtesting was monitored by a precision semiconductorparameter analyzer HP4156A, which can be used to detecta very low electrical current (around 10�12A). This allowsus to measure the high resistance of the TiO2 films. Theresistance responses of the sensor in air ambient and uponexposure the ethanol pulses were monitored. The sensor

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Fig. 1. Apparatus for gas-sensor testing.

N.V. Hieu et al. / Physica E 40 (2008) 2950–29582952

response (Si ¼ 0, 1, 2, 3, 4) was defined as the ratio of thesensor resistance in air (Ra) and in ethanol gas (Rg).

Fig. 2. FE-SEM images: (a) the 0% SWCNTs inclusion sample and

(b) the 0.01wt% SWCNTs inclusion sample.

CNT

TiO2

Fig. 3. TEM image of SWCNTs coated with TiO2 after thermal treatment

(a) and (b) a bundle of SWCNTs.

3. Results and discussion

3.1. Microstructure characterizations

Fig. 2a and b presents the surface morphologies ofNb–Pt co-doped without (S0) and with (S5) SWCNTsinclusion samples, respectively. It can be seen that themorphology of the films is not clearly different betweenthe samples. Fig. 3a shows the TEM images of theTiO2/SWCNTs material after heat treatment. It also showsthat the SWCNTs still present and are embedded by a TiO2

material. The SWCNTs with a diameter lower than 2 nmwere used for the hybrid material preparation. Since, theSWCNT observed in Fig. 3a should be a bundle with adiameter of around 10 nm so that they could not dispersecompletely during the material preparation process. It wasvery difficult to find a single SWCNT embedded in theTiO2 matrix. As indicated in Fig. 3 (TEM image providedby the producer), there are also bundles with diametersof around 10 nm, which agrees with our observation.SWCNTs–TiO2 bonding can be formed naturally throughsome physicochemical interactions such as Van der Waalsforce, H bonding and other bonding. The interactionbetween –OH groups in the course of hydrolysis ofTi(OC3H7)4 and –COOH and –OH groups on SWCNTsformed by the purification process. This reveals that theTiO2 crystalline would increase and enclose the SWCNTsduring heat treatment. More details to explain theattachment of TiO2 nanoparticles on carbon nanotubescan be found elsewhere [52–55].

XRD pattern of the TiO2 thin film annealed at 500 1C isshown in Fig. 4 and it confirms that the film crystallized to

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Fig. 5. Response to ethanol for the sensor S3 operating at 380 1C.

N.V. Hieu et al. / Physica E 40 (2008) 2950–2958 2953

the anatase structure after heat treatment. The crystallitesize roughly estimated by the Scherrer equation is about10 nm. XRD was carried out with the highest SWCNTscontent samples (S5) but there was no SWCNTs peaksobserved in the XRD pattern due to the relatively lowcontent of SWCNTs in the materials.

3.2. Ethanol-sensing properties

To study the effect of SWCNTs inclusion on gas-sensingproperties, we measured the responses of all as-indicatedsensors to ethanol gas at different concentrations rangingfrom 125 to 1000 ppm and at different operating tempera-tures ranging from 290 to 400 1C.

Fig. 5 shows a typical response curve of the hybrid(0.01%) SWCNTs/Nb–Pt co-doped sensor (S3) at anoperating temperature of 380 1C. The response curve showsthat the measured resistance varies with time over a seriesof cyclic tests. At the beginning of a cyclic test, the sensorwas exposed to air, and then it was exposed to a certainconcentration of ethanol. Another cyclic test was con-ducted with a different concentration of ethanol gas. It wasfound that the response and recovery times are less than 5 sand the sensor response is repeated with the same ethanolconcentration after several cyclic tests.

Fig. 6a–d shows the response of all as-indicated sensorsas a function of operating temperature to different ethanolconcentrations of 125, 250, 500, and 1000 ppm, respec-tively. The operating temperature has an obvious influenceon the response of the Nb–Pt co-doped with and withoutSWCNTs inclusion. The highest response to ethanol gas ofthe Nb–Pt co-doped TiO2 sensor is obtained at 380 1C,whereas the responses of only selected hybrid SWCNTs/Nb–Pt co-doped TiO2 sensors (S3 and S4) are higher forethanol concentrations lower than 1000 ppm. Especially, it

seems that there is a slight shift of the optimal operatingtemperature of the hybrid sensor towards the lowertemperature region. It can also be seen that the responseof the Nb–Pt co-doped sensor becomes significantlyenhanced by including SWCNTs for ethanol concentra-tions lower than 250 ppm. Additionally, all the hybridSWCNT/Nb–Pt TiO2 co-doped sensors have higherresponses to ethanol gas at the low operating temperatureregion (o335 1C) compared to that of the Nb–Pt co-dopedsensor (S0). These are very important issues for practicalapplication, because one of the main applications for anethanol sensor is the screening of intoxicated drivers.Therefore, the ethanol sensor should be able to detect[C2H5OH] �200 ppm, which corresponds to �0.5 g ofC2H5OH per liter of blood and should also be operated atlow operating temperature to reduce the power consump-tion for a hand-held portable breath alcohol tester. Forhigher operating temperature (4335 1C), only selectedNb–Pt co-doped TiO2 including of SWCNTs sensors havehigher response, and the enhancement of the sensorresponse to ethanol gas also becomes significant forethanol concentrations of lower than 250 ppm.To study more details of the effect of SWCNTs inclusion

content on gas-sensing properties, we plotted the responsesof all the sensors to different ethanol concentrations(125, 250, 500 and 100 ppm) as a function of SWCNTscontent for two moderate operating temperatures of 305and 380 1C as shown in Fig. 7a and b, respectively. It canbe seen that the response to ethanol of the hybridSWCNTs/Nb–Pt co-doped TiO2 sensors is increased atfirst as the SWCNTs content increases up to 0.01% or0.05%, but it reduces when SWCNTs are further increasedto 0.1%, for an operating temperature of 305 1C. At anoperating temperature of 380 1C, the responses of thehybrid sensor are relatively lower or equal comparedto that of the sensor without the inclusion of SWCNTs.

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Sensor S0Sensor S1Sensor S2Sensor S3Sensor S4Sensor S5

400380360340320300 400380360340320300

Fig. 6. Sensitivity of S0, S1, S2, S3, S4, and S5 versus operating temperature to (a) 125 ppm ethanol and (b) 1000 ppm ethanol.

0

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pons

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Ethanol gasOperating Temp. 305°C

0.10.050.010.0051E-3

Fig. 7. Sensitivity as a function of SWCNTs doping concentration at

operating temperatures of 305 1C and 380 1C to (a) 125 ppm ethanol and

(b) 1000ppm ethanol.

N.V. Hieu et al. / Physica E 40 (2008) 2950–29582954

From Fig. 7, it seems that the SWCNTs inclusion contentin the range of 0.005–0.01% would be optimal.

For quantitatively showing response improvement of thehybrid SWCNTs/Nb–Pt co-doped TiO2 sensors comparedto that of the Nb–Pt co-doped TiO2 sensor, we plotted

normalized response, S1,2,3,4/S0 against the operatingtemperature in Fig. 8a–d. It can be seen that the responseto ethanol of the hybrid SWCNTs/Nb–Pt co-doped TiO2

sensor increases by the factor of two to five depending onthe SWCNTs content, operating temperature, and ethanolgas concentration. This factor decreases rapidly, withincreasing operating temperature when measuring at highethanol concentrations (500 and 1000 ppm). Therefore, thebest performance sensor should be selected in consideringthe operating temperature and ethanol concentration.Fig. 9a and b shows the relationship between the

response and ethanol gas concentration for the sensorsoperating at 308 and 380 1C, respectively. It can be seenthat the response of all the sensors to ethanol gas increasesrather steeply with increasing gas concentration, withoutshowing a saturation tendency up to 1000 ppm, and thecurves exhibit quasi-linearity. Moreover, the response to200 ppm ethanol has the value of 4–15, which can meetpractical application such as medical diagnostics andbreath alcohol tester. One can see that the response toethanol of the hybrid sensors increases more steeply withincreasing ethanol gas concentration compared to that ofthe sensor without SWCNTs inclusion, for the operatingtemperature of 308 1C. For the operating temperature of380 1C, the inverse effect was obtained (see Fig. 9b).One of the disadvantages of a TiO2-based sensor is that

the resistance of the sensor is relatively too high. This is

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300 320 340 360 380 400 300 320 340 360 380 400

Fig. 8. Normalized sensitivity S1, 2, 3, 4, 5/S0 against operating temperature at ethanol concentrations of (a) 125 ppm, (b) 250 ppm, (c) 500 ppm, and

(d) 1000ppm.

N.V. Hieu et al. / Physica E 40 (2008) 2950–2958 2955

inconvenient for practical application, because the elec-trical measurement circuit of the gas detection apparatus ismore complicated. As shown in Fig. 10, the resistance ofthe sensor decreases with increas in the operatingtemperature. This is attributed to the semiconductingbehavior of the TiO2 material. One can see that resistanceincreases with increasing the SWCNTs inclusion content.This issue should be considered once the hybrid sensorbetween CNTs and metal semiconductor oxide is used forpractical application.

3.3. Discussion and gas-sensing mechanism

The improved response of a Nb–Pt co-doped TiO2

sensor by SWCNTs inclusion at the low operatingtemperature region cannot be clearly explained yet.However, we speculate that it may result from the fact

that the inclusion of SWCNTs in the TiO2 matrix canintroduce nanochannels and additional hetero-junctionsbetween TiO2 (n-type) and CNTs (p-type). Both theseeffects do not cause the response improvement of thehybrid SWCNTs/Nb–Pt co-doped sensor at high operatingtemperatures. The nanochannels formed by SWCNTs maynot play any role in gas diffusion into the TiO2 matrix athigh operating temperature. Otherwise, we believe thatTiO2 (n-type)/SWCNTs (p-type) cannot function well attemperatures higher than 335 1C due to the transition froma semiconductor behavior to a metallic one of the CNTs.Furthermore, the inclusion of SWCNTs in a TiO2 matrixcauses a connection together and results in short resis-tances between the TiO2 nanoparticles, reducing thenumber of barriers between nanoparticles in a Nb–Ptco-doped TiO2 sensor, which plays an important role inthe surface sensing mechanism of the thin-film gas sensor.

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1000800600400200

a

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Fig. 9. The sensor response of S0, S1, S2, S3, S4, and S5 versus ethanol

concentration at operating temperature of (a) 305 1C and (b) 380 1C.

300

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320 340 360 380

Fig. 10. Resistance versus temperature for sensing layers of sensors S0and S3.

N.V. Hieu et al. / Physica E 40 (2008) 2950–29582956

This explains why the increase of SWCNTs dopingconcentration results in the decrease of the sensitivity ofthe TiO2 thin-film sensor.

As described in Section 3.1, the morphologies of thesample with and without SWCNTs inclusion are porous.The porous structures may promote the effect of ethanoladsorption on the interface, leading to an improvement inthe sensor response. However, there is no obviousdifference between them in their morphology. Conse-quently, they are not likely to contribute to a greatimprovement in the response at the low operatingtemperature region.The improvement of the TiO2 gas-sensor performance

and the reduction of the optimal operating temperature bySWCNTs inclusion have not been well understood to dateand not much literature has been reported on the relativework recently. Generally, the gas-sensing mechanism of ahybrid CNTs/SMO sensor has not yet been well demon-strated. However, in this study, we speculate the followingreasons to explain our experimental observations.It should be noted that the SWCNT is a perfect hollow

nanotube with a diameter lower than 2 nm. Thesenanotubes embedded in the TiO2 film will provide an easydiffusion for chemical gas accessing through the bulkmaterial. After the thermal treatment, these tiny SWCNTswere left in the bulk material derived to form thepermanent gas nanochannels as indicated in Fig. 3b. Theuse of SWCNTs can bring some advantages such asintroducing an identical open gas nano-channel throughthe bulk material, achieving a great surface-to-volumeratio, and providing good gas-adsorption sites due to insideand outside of SWCNTs [41]. Recently, Wei et al. [44] haveproposed a model relative to the p/n junction formedbetween SnO2 and SWCNTs, which was similar to the p/njunction of the Si semiconductor as reported by severalauthors [45,46,56,57]. They have demonstrated that theexistence of the hetero-junction can be used to detect gasesat low or room temperature. They have prepared a sensorbased on the structure of p–n Si semiconductor/SnO2. Thesemiconductor p–n/SnO2 gas sensor has been demon-strated to work at room temperature. They have proposedthat the change in barrier height or the change inconductivity of the sensitive layer of SnO2 may modulatethe depletion layer at the n/p-junction of the Si substrate.This change of the depletion layer in the n/p-junction,induced by the sensitive SnO2 layer, may cause animprovement in the performance of the gas sensor at lowoperating temperature. This model can explain and under-stand our experimental results. In the light of this model,TiO2 is well known as an n-type semiconductor. Thismeans that if ethanol molecules (reducing gas) adsorbedonto the surface of a TiO2 sensor, they increase the numberof conducting electrons due to the fact that oxygen ion onthe surface reacts with ethanol molecules and releaseselectrons, resulting in the decrease of the resistance of theTiO2 sensor.It has been reported that SWCNTs act like a p-type

semiconductor when they are used as gas-sensing materials[39–43]. As depicted in Fig. 3b, the SWCNTs wereembedded in TiO2 nanoparticles after thermal treatment,

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forming a good contact between SWCNTs and TiO2

nanoparticles. Hence, besides the barriers between TiO2

nanoparticles, there are additional p/n hetero-junctions,which are formed by Pt/(n-TiO2)/(p-SWCNT)/(n-TiO2)/Pt.This junction is strongly affected by the gas adsorption atlow or room temperature like the p/n junction as describedin Refs. [45,46,56,57]. The gas adsorption on a TiO2 surfaceinduces the change in barrier height or the change inconductivity of the sensing layer of TiO2 and maymodulate the depletion layer of the p/n hetero-junction ofSWCNTs and TiO2 nanoparticles. Therefore, the improve-ment of the gas-sensor performance and the shift of theoperating temperature toward the lower temperatureregion can be attributed to the amplification effects of thejunction combined with the gas reaction.

If the hybrid SWCNTs/Nb–Pt doped TiO2 sensorsoperate under temperatures ranging from 300 to 350 1C,there is a transition of behavior from the semiconductorto the metallic one of the CNTs and suppresses thesemiconducting tubes in the bundles embedded in the TiO2

matrix. More details about the transition behavior ofcarbon nanotubes at high temperature were previouslyreported [58–60]. It, therefore, results in the vanishing ofthe hetero-junctions of SWCNTs/TiO2 nanoparticles. Thismay explain why the hybrid sensors have no improvementin the sensitivity at high operating temperature region.

4. Conclusion

The hybrid SWCNTs/Nb–Pt co-doped TiO2 sensorshave been successfully fabricated for ethanol-sensingapplication. The SWCNTs inclusion are well embeddedby TiO2 nanoparticles that we cannot detect by XRD andalso by FE-SEM surface verification. The porosity andcrystallite size are only slightly affected by the SWCNTsdopant. The SWCNTs inclusion has exhibited an improve-ment of the Nb–Pt co-doped TiO2-based sensor perfor-mance to ethanol at an operating temperature lower than335 1C. Experimental results indicate that the response toethanol of a Nb–Pt co-doped TiO2 thin-film sensorincreases by a factor of 2–5 with the SWCNTs inclusioncontent up to 0.01% by weight. Moreover, the SWCNTs/Nb–Ptdoped TiO2 thin-film sensor can operate with goodresponse and stability at a relatively low-temperature rangeof 290–320 1C. The 0.01% SWCNTs-doped sensor is a bestchoice for sensing ethanol vapor, which seems to assurethat the hybrid sensor is suitable to be used as a portablebreath alcohol tester. Our results therefore suggest thatSWCNTs inclusion is an effective route to reduce theoperating temperature of the TiO2 thin-film sensor.

Acknowledgments

This work was financially supported by VLIR-HUTproject, Code AP05/Prj3/Nr03 and HAST project no. 01.The authors also acknowledge Grant no. 405006 (2006)from the Basic Research Program of the Ministry of

Science and Technology (MOST) and for the financialsupport from Third Italian-Vietnamese Executive Pro-gramme of Co-operation in S&T for 2006–2008 under theproject title, ‘‘Synthesis and Processing of Nanomaterialsfor Sensing, Optoelectronics and Photonic Applications’’.

References

[1] J. Puigcorbe, A. Cirera, J. Cerda, J. Folch, A. Cornet, J.R. Morante,

Sensors Actuators B 84 (2002) 60.

[2] N. Barsan, M. Schweizer-Berberich, W. Gopel, J. Anal. Chem. 365

(1999) 287.

[3] G. Korotcenkov, Sensors Actuators 34 (2005) 209.

[4] G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Crit. Rev. Solid

State Mater. Sci. 29 (2004) 111.

[5] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, Renew.

Sustainable Energy Rev. 11 (2007) 401.

[6] G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, C.A. Grimes,

Sol. Energy Mater. Sol. Cells 90 (2006) 2011.

[7] J.R. Durrant, S.A. Haque, E. Palomares, Coord. Chem. Rev. 248

(2004) 1247.

[8] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol.

C: Photochem. Rev. 1 (2000) 1.

[9] R.N. Pandey, K.S. Chandra Babu, O.N. Srivastava, Prog. Surf. Sci.

52 (1996) 125.

[10] M. Ferroni, V. Guidi, G. Martinelli, G. Faglia, P. Nelli,

G. Sberveglieri, Nanostruct. Mater. 7 (1996) 709.

[11] K. Zakrzewska, Vacuum 74 (2004) 335.

[12] A.M. Ruiz, G. Saka, A. Cornet, K. Shimanoe, J. Morante,

N. Yamazoe, Sensors Actuators B 103 (2004) 312.

[13] G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Crit. Rev. Solid

State Mater. Sci. 29 (2004) 111.

[14] M. Batzill, U. Diebol, Prog. Surf. Sci. 79 (2005) 47.

[15] A.M. Taurino, S. Capone, A. Boschetti, T. Toccoli, R. Verucchi,

A. Pallaoro, P. Siciliano, S. Iannotta, Sensors Actuators B 100 (2004)

177.

[16] S.H. Si, Y.S. Fung, D.R. Zhu, Sensors Actuators B 108 (2005) 165.

[17] L. Sun, L. Huo, H. Zhao, S. Gao, J. Zhao, Sensors Actuators B 114

(2006) 387.

[18] A. Teleki, S.E. Pratsinis, K. Kalyanasundaram, P.I. Gouma, Sensors

Actuators B 119 (2006) 683.

[19] L. Francioso, D.S. Presicce, A.M. Taurino, R. Rella, P. Siciliano,

A. Ficarella, Sensors Actuators B 95 (2003) 66.

[20] L. Francioso, D.S. Presicce, M. Epifani, P. Siciliano, A. Ficarella,

Sensors Actuators B 107 (2005) 563.

[21] A.M. Ruiz, A. Cornet, J.R. Morante, Sensors Actuators B 100 (2004)

256.

[22] Y.-K. Jun, H.-S. Kim, J.-H. Lee, S.-H. Hong, Sensors Actuators B

120 (2006) 69.

[23] G.K. Mor, M.A. Carvalho, O.K. Varghese, M.V. Pishko,

C.A. Grimes, J. Mater. Res. 19 (2004) 628.

[24] H.-S. Kim, W.-T. Moon, Y.-K. Jun, S.-H. Hong, Sensors Actuators

B 120 (2006) 63.

[25] C.M. Carney, S. Yoo, S.A. Akbar, Sensors Actuators B 108 (2005)

29.

[26] L. Francioso, D.S. Presicce, P. Siciliano, A. Ficarella, Sensors

Actuators B 123 (2007) 516.

[27] G. Sberveglieri, E. Comini, G. Faglia, M.Z. Atashbar, W. Wlodarski,

Sensors Actuators B 66 (2000) 139.

[28] A.M. Ruiz, A. Cornet, J.R. Morante, Sensors Actuators B 111–112

(2005) 7.

[29] D.T.T. Le, D.D. Vuong, N.V. Hieu, N.D. Chien, in: Proceedings of

the Eighth German–Vietnamese Seminar on Physics and Engineering,

2005, p. 122.

[30] D.T.T. Le, N.V. Hieu, N.D. Chien, in: Proceedings of the ninth Asia

Pacific Physics Conference (9th APPC), 2004, p. 391.

ARTICLE IN PRESSN.V. Hieu et al. / Physica E 40 (2008) 2950–29582958

[31] E. Comini, G. Faglia, G. Sberveglieri, Y.X. Li, W. Wlodarski,

M.K. Ghantasala, Sensors Actuators B 64 (2000) 169.

[32] K. Zakrzeka, M. Radecka, M. Rekas, Thin Solid Films 310 (1997) 161.

[33] R.K. Sharma, M.C. Bhatnagar, G.L. Sharma, Sensors Actuators B

45 (1997) 209.

[34] R.K. Sharma, M.C. Bhatnagar, G.L. Sharma, Sensors Actuators B

46 (1998) 194.

[35] E.T. Thostensona, Z. Renb, T.-W. Chou, Composites Sci. Technol.

61 (2001) 1899.

[36] H. Ulbricht, G. Moos, T. Hertel, Surf. Sci. 532–535 (2003) 852.

[37] S. Santucci, S. Picozzi, F. Di Gregorio, L. Lozzi, J. Chem. Phys. 119

(2003) 10904.

[38] A. Modi, N. Koratkar, E. Lass, B. Wei, Nature 424 (2003) 171.

[39] T. Someya, J. Small, P. Kim, C. Nuckolls, J.T. Yardley, Nano Lett. 3

(2003) 877.

[40] J. Li, Y. Lu, Q. Ye, M. Cinke, J. Han, M. Meyyappan, Nano Lett. 3

(2003) 929.

[41] A. Fujiwara, K. Ishii, H. Suematsu, H. Kataura, Y. Maniwa, S.

Suzuki, Y. Achiba, Chem. Phys. Lett. 336 (2001) 205.

[42] R. Ionescu, E.H. Espinosa, E. Sotter, E. Llobet, X. Vilanova,

X. Correig, A. Felten, C. Bittencourt, G. Van Lier, J.C. Charlier,

J.J. Pireaux, Sensors Actuators B 113 (2006) 36.

[43] A. Zahab, L. Spina, P. Poncharal, Phys. Rev. B 15 (2000) 10000.

[44] B-.Y. Wei, M.-C. Hsub, P.-G. Su, H.-M. Lin, R.-J. Wu, H.-J. Lai,

Sensors Actuators B 101 (2004) 81.

[45] W. Zhang, E.A. de Vasconcelos, H. Uchida, T. Katsube,

T. Nakatsubo, Y. Nishioka, Sensors Actuators B 65 (2000) 154.

[46] W. Zhang, H. Uchida, T. Katsube, T. Nakatsubo, Y. Nishioka,

Sensors Actuators B 49 (1998) 58.

[47] C. Bittencourt, A. Felten, E.H. Espinosa, R. Ionescu, E. Llobet,

X. Correig, J.-J. Pireaux, Sensors Actuators B 115 (2006) 33.

[48] Y.-L. Liu, H.-F. Yang, Y. Yang, Z.-M. Liu, G.-L. Shen, R.-Q. Yu,

Thin Solid Films 497 (2006) 355.

[49] R.A. Guirado-Lopez, M. Sanchez, M.E. Rincon, J. Phys. Chem.

C 111 (2007) 57.

[50] A. Wisitsoraat, A. Tuantranont, C. Thanachaynont, V. Patthana-

settakul, P. Singjiai, J. Electroceram. 17 (2006) 45.

[51] Shenzhen NanoTech Port., Co., Ltd., China,/http://www.nanotubes.

com.cnS.

[52] Q. Huang, L. Gao, J. Mater. Chem. 13 (2003) 1527.

[53] J. Sun, M. Iwasa, L. Gao, Q. Zhang, Carbon 42 (2004) 885.

[54] A. Gomathi, S.R.C. Vivekchand, A. Govindaraj, C.N.R. Rao, Adv.

Mater. 17 (2005) 2757.

[55] A. Jitianu, T. Cacciaguerra, R. Benoit, S. Delpeux, F. Beguin,

S. Bonnamy, Carbon 42 (2004) 1147.

[56] A. Kunimoto, N. Abe, H. Uchida, T. Katsube, Sensors Actuators B

65 (2000) 122.

[57] N. Savage, B. Chwieroth, A. Ginwalla, B.R. Patton, S.A. Akbar,

P.K. Dutta, Sensors Actuators B 79 (2001) 17.

[58] H.Q. Nguyen, J.S. Huh, Sensors Actuators B 12 (2006) 426.

[59] L. Valentini, I. Armentano, J.M. Kenny, C. Cantalini, L. Lozzi,

S. Santucci, Appl. Phys. Lett. 82 (2003) 961.

[60] L. Valentini, L. Lozzi, C. Cantalini, I. Armentano, J.M. Kenny,

L. Ottaviano, S. Santucci, Thin Solid Film 436 (2003) 95.