Sensors and Actuators B 144 (2010) 450–456

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Sensors and Actuators B: Chemical

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Gas-sensing properties of tin oxide doped with metal oxides and carbonnanotubes: A competitive sensor for ethanol and liquid petroleum gas

Nguyen Van Hieu a,∗, Nguyen Anh Phuc Duc b, Tran Trung c,Mai Anh Tuan a, Nguyen Duc Chien b

a International Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT), No. 1 Dai Co Viet Road, Hanoi, Viet Namb Institute of Engineering Physics, Hanoi University of Technology, Hanoi, Viet Namc Faculty of Environment and Chemistry, Hung Yen University of Technology and Education, Hung Yen, Viet Nam

a r t i c l e i n f o

Article history:Available online 31 March 2009

Keywords:Gas sensorTin oxideCarbon nanotubes

a b s t r a c t

SnO2 doped with metal oxides such as PtO2, PdO, La2O3 CuO, and Fe2O3 and multi-walled carbon nan-otubes (MWCNTs) thin films were prepared by the sol–gel method. Thin film gas sensors were fabricatedby spin-coating the sol onto interdigitated microelectrodes. The microstructure and morphology of thematerials were characterized by XRD, FE-SEM, and TEM. The results reveal that their SnO2 particle sizeis lower than 10 nm, and the MWCNTs doping is well embedded in the SnO2 matrix. The response of allthe sensors was studied for different concentrations of ethanol and liquid petroleum gases (LPG) and atdifferent operating temperatures. Comparative results reveal that the (1 wt%) PtO2-doped SnO2 sensorexhibits higher sensitivity to ethanol gas and LPG than the sensors doped with the other dopants. Espe-

cially, the (1 wt%) PtO2-doped SnO2 sensor shows higher selectivity to ethanol gas over LPG, while the(0.1 wt%, 20 < d < 40 nm)-doped SnO2 shows higher selectivity to LPG over ethanol gas in the same testing

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. Introduction

The hybrid materials made of semiconductor metal oxidesSMO) such as TiO2, SnO2 and WO3 and carbon nanotubes (CNTs)ave been given much attention in recent year for their variouspplications such as photocatalysis, anode material for lithium-on batteries and gas sensor [1–14]. The nanoarchitectures formingybrid materials between SMO and CNTs have been conducted inifferent ways such as SMO/CNTs composite [1–5], SMO-coatedNTs [6–8], SMO-filled CNTs [9] and CNTs-doped SMO [10–14]. Thepecial geometries and properties of the hybrid materials facilitateheir great potential applications as high-performance gas sensors.revious works have demonstrated that the hybrid materials cane used to detect various gases such as NH3, NO2, H2, CO, LPG,nd ethanol [4–6,10–14]. These works also reported that the hybridaterials gas sensors have a better performance compared to the

ensors used SMO as well as CNTs as sensing materials. Interest-ngly, the composite SnO2/CNTs and the CNTs-doped SnO2 sensorsespond to NH3 and NO2 at room temperature, respectively [4,10].his would reduce considerably the power consumption of the

∗ Corresponding author. Tel.: +84 4 38680787; fax: +84 4 38692963.E-mail addresses: hieu@itims.edu.vn,

ieunv-itims@mail.hut.edu.vn (N. Van Hieu).

925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2009.03.043

© 2009 Elsevier B.V. All rights reserved.

sensing-device. The CNTs are hollow nanotube and p-type semi-conductor, therefore the enhancement of the sensing performanceof the sensors based on CNTs/SnO2 hybrid materials in comparisonwith the sensors based on the separated materials was attributed toadditional nanochannel for gas diffusion and p/n junctions formedby CNTs and SnO2. These mechanisms were previously representedin [4,5,10].

Ethanol gas sensors are extensively used for the control ofdrunken driving and monitoring of fermentation and other pro-cesses in chemical industries, while LPG sensors are frequently usedin the detection of the gas leakages to prevent accidental explo-sion. The development of ethanol gas and LPG sensors based onSnO2 thin film technology offers great advantages such as highsensitivity, fast response, and low cost. Therefore, much effort hasbeen devoted to improve its sensitivity and selectivity by intro-ducing various dopants such as PtO2, CdO, La2O3, PdO, SiO2, andRuO2 [16–31] or by mixing with other metal oxides such as Nb2O3,Fe2O3, and ZrO2 [23,32,33]. It was found that among additives, SnO2sensors doped with La2O3 and CdO showed good performance toethanol gas [17–19], while the Pd-, Pt- and RuO2-doped SnO2 sen-

sors showed good performance to LPG [20,24]. In this paper, forthe first time, we study and compare the performance of variousmetal oxides-doped SnO2 and MWCNTs-doped SnO2 sensors forthe detection of ethanol gas and LPG. In the later, the MWCNTs withdifferent diameters were used for the doping.

N. Van Hieu et al. / Sensors and Actu

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ig. 1. XRD pattern of typical SnO2 thin film heat-treated at 500 (a), 600 (b), and00 ◦C (c).

. Experimental

Stannic acid gel was synthesized by hydrolyzing 0.2 M solutionf tin chloride (SnCl4) with ammonia, using the following reactiont room temperature:

nCl4 + 4NH4OH → SnO2·nH2O + 4NH4Cl + (2 − n)H2O (1)

he resulting precipitate was washed thoroughly by repeating therocedures of suspending the gel into deionized water and collect-

ig. 2. TEM image of SnO2 nanoparticles (a); FE-SEM image of a SnO2 thin film heat-trean MWCNTs-doped SnO2 (d).

ators B 144 (2010) 450–456 451

ing it back by filtration to remove Cl−. The SnO2 gel was suspendedin an aqueous ammonia solution (pH 10.5) and followed by stir-ring for 2 h, with a calculated amount of SnO2 gel in order toachieve 5 equivalent wt% SnO2 sol–gel solution. The suspension wastransferred to a Teflon-lined stainless steel autoclave and treatedhydrothermally at 200 ◦C for 10 h. The 1 equivalent wt% CuO-,Fe2O3-, La2O3-, and PtO2-99 wt% SnO2 sols were prepared by mix-ing the required amount of dissolutions of Cu(NO3)2, Fe(NO3)3,La(NO3)3 and PtCl4 (0.1 M) to the pure SnO2 sol.

Functionalized MWCNTs with different diameters (d < 10 nm,d = 20–40 nm, and d = 60–100 nm) were used for the fabrication ofthe MWCNTs-doped SnO2 sensors with a calculated amount of theMWCNTs in order to achieve 0.1 equivalent wt% MWCNTs–99.9 wt%SnO2 sol. The MWCNTs were functionalized by using a typical pro-cedure described as follows: 200 mg MWCNTs were suspended in35 mL concentrated nitric acid (15 M) and refluxed for 12 h in asilicone oil bath maintained at 140 ◦C to modify the MWCNTs sur-face, they were then rinsed with distilled H2O until the pH of thesolution was neutral, and finally they were dried at 80 ◦C in vac-uum oven. The immersion-probe ultrasonic generator with a highpower up to 500 W (Model VC-505, Sonics, US) was used for disper-sion of MWCNTs in SnO2 sol. The morphology and the crystallinephase of the films were characterized by using a field emission scan-ning electron microscope (FE-SEM; 4800 Hitachi, Japan) and X-raydiffraction (XRD, Philips XPert Pro), respectively. The dispersion ofthe MWCNTs in the SnO2 sol was characterized by TEM using a JEM-

100cx instrument with an accelerating voltage of 80 kV. The detailson the gas sensor fabrication and characterization were describedin our previous works [4,14]. All sensors were tested with variousconcentrations of ethanol gas (100–1000 ppm) and LPG (0.1–1%or 1000–10,000 ppm), and with different operating temperatures

ted at 500 ◦C (b); TEM image of an MWCNTs-doped SnO2 sol (c); FE-SEM image of

452 N. Van Hieu et al. / Sensors and Actuators B 144 (2010) 450–456

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gas and LPG (not shown here). Therefore, these characterizationsare in order to choose right heat-treated temperature for metaloxides- and MWCNTs-doped SnO2 thin films. In addition, it is wellknown that MWCNTs materials can be burned out at heat-treatedtemperatures higher than 550 ◦C.

ig. 3. Response of PtO2, Fe2O3, La2O3, and CuO-doped SnO2 sensors to 2500 ppm250 ppm) and LPG (2500 ppm) of (1 wt%) a PtO2-doped SnO2 sensor (c); step wiseir to 1000 ppm ethanol gas in air for (1 wt%) PtO2-doped SnO2 sensor operating at

190, 220, 240, 260, 290, 320, 340, and 360 ◦C). It should be notedhat technological application requires the ethanol and LPG sensoro be able to detect at least 200 ppm alcohol (∼0.6 g/L in the humanlood) and 0.24% (2400 ppm) LPG (lowest explosive level). Hence,ur gas-concentration range to be tested is within levels.

. Results and discussion

Fig. 1a–c shows the XRD patterns of the SnO2 samples after theeat-treatment at temperatures of 500, 600, and 700 ◦C, respec-ively. The XRD characterization was also carried out with the

WCNTs-doped SnO2 samples (not shown), but we observed noifferences. This is attributed to the use of very low doping con-ent of MWCNT and the well-embedded MWCNTs in SnO2 matrix,hich have already been reported in the literature [1,3,4,7]. It can

e seen that the heat-treated samples are well crystallized with alliffraction peaks which can be well indexed to the tetragonal rutiletructure of SnO2. The broad and well-defined reflections werebserved at 2� = 26.51, 33.67 and 51.78 corresponding to (1 1 0),1 0 1) and (2 1 1) planes, respectively, in the XRD spectrum of thennealed SnO2 thin films, which are in good agreement with thereviously reported [20,21], confirming the formation of a polycrys-alline SnO2 thin film. The estimated value of the lattice constantsere found to be a = b = 4.734 Å and c = 3.185 Å (JCPDS 21-1250). The

alue of the crystallite size of the heat-treated SnO2 thin film wasstimated by fitting the width of (1 1 0) reflection using Scherrer

ormula d = K�/ˇcos �, where K is 0.94, � is the X-ray wavelength, ˇhe peak full width half maxima (FWHM) and, � is the diffractioneak position.

The roughly estimated values of crystallite size of the sam-les heat-treated at 500, 600, and 700 ◦C are found to be about

%) LPG in air (a) and 250 ppm ethanol gas in air (b); the response to ethanol gasase in electrical resistance obtained with a increase in ethanol concentration from; (e) the sensor response versus ethanol gas concentration.

5.8, 6.2, and 7.1 nm, respectively. This indicated that the crystallitesizes do not significantly vary for heat-treating temperature rang-ing from 500 to 700 ◦C. Actually, we have already investigated thegas-sensing properties of blank SnO2 films heat-treated at thesetemperatures, and we have obtained similar responses to ethanol

Fig. 4. Response as a function of operating temperature of the PtO2-doped SnO2

sensors with varying PtO2 doping content.

N. Van Hieu et al. / Sensors and Actuators B 144 (2010) 450–456 453

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ig. 5. Response of (0.1 wt%) MWCNTs (with d < 10 nm; 20 nm < d < 40 nm; 60 nm < das in air (b); response to ethanol gas (250 ppm) and LPG (2500 ppm) of (0.1 wt%, 10 nbtained with an increase in LPG concentration from air to 10000 ppm (1%) LPG inhe sensor response versus LPG concentration.

The particle size and morphology of the SnO2 thin film charac-erized further by TEM and FE-SEM are shown in Fig. 2. The TEMmage (Fig. 2a) shows that the particle size is quite homogenous inhe range of 4–8 nm. The FE-SEM image (Fig. 2b) shows the mor-hology of the SnO2 thin film treated at 500 ◦C. It is shown thathe particle size is smaller than 10 nm. The MWCNTs-doped SnO2ample was characterized by TEM and FE-SEM as shown in Fig. 2cnd d, respectively. Fig. 2c shows the TEM image of the MWCNTs-ispersed SnO2 sol in which the black materials absorbed ontohe wall of the functionalized MWCNTs were believed to be SnO2anoparticles from the SnO2 sol. This is because it was recognizedhat the MWCNTs-SnO2 bonding can be formed naturally throughome physicochemical interactions such as Van der Waals force, Honding and other bonds. For example, the OH group on SnO2 mayossibly react with the OH and COOH groups on the functional-

zed MWCNTs in removing the H2O contained in the wet material,nd thus the bonding C O Sn or O C O Sn might form throughhe dehydration reaction that happens among the groups on thewo materials. However, this was not strongly explained and morentensive studies are needed to confirm this. The absorbed SnO2n the MWCNTs would grow up and enclose the MWCNTs duringhe heat-treatment. This observation was consistent with previouseports [1–3]. Fig. 2d shows the FE-SEM image of the MWCNTs-oped SnO2 film after heat-treatment at 500 ◦C. It can be seen thathe MWCNTs are well encapsulated with a SnO2 matrix and is stillresent after the heat-treatment at 500 ◦C.

The sensing characteristics of (1 wt%) metal oxides (PtO2, Fe2O3,

uO, La2O3)-doped sensors to ethanol gas and LPG have indicated

n Fig. 3. The sensor responses as a function of operating tempera-ure to LPG and ethanol gas are respectively shown in Fig. 3a and b.t seems that the optimized operating temperatures of the sensoro ethanol gas and LPG are around 350 and 250 ◦C, respectively.

nm) -doped SnO2 sensors to 2500 ppm (0.25%) LPG in air (a) and 250 ppm ethanol< 20 nm) MWCNTs-doped SnO2 sensor (c); step wise decrease in electrical resistancer (0.1 wt%, 10 nm < d< 20 nm) MWCNTs-doped SnO2 sensor operating at 240 ◦C; (e)

It can be recognized that all the metal oxides-doped SnO2 sen-sors show an improvement in their response to ethanol gas, whileonly CuO, PtO2, and La2O3-doped sensors show an improvementin the sensor response to LPG compared with undoped SnO2 sen-sors. However, this also depends on the operating temperature tobe selected. These observations are consistent and have been rea-sonably explained in the literature [30,31]. Our experimental datashow that (1 wt%) the PtO2-doped sensor has a better sensitivity toethanol gas and LPG as compared to that of the sensors doped withthe other dopants. For instance, at the operating temperature of240 ◦C, the response to 250 ppm ethanol gas and 2500 ppm (0.25%)LPG is around 101.9 and 2.1, respectively. These values are compa-rable with the data reported in the literature [17–20,31]. We haveplotted the responses to ethanol gas (250 ppm) and LPG (2500 ppm)of the (1 wt%) PtO2-doped SnO2 sensor as depicted in Fig. 3c. It canbe seen that the sensor has relatively good selectivity to ethanol gasover LPG. Fig. 3d shows the electrical resistance variations obtainedwith several steps of different ethanol concentration from air to1000 ppm ethanol in air for the (1 wt%) PtO2-doped SnO2 sensor atan operating temperature of 240 ◦C. As can be seen, upon switchingon ethanol gas, the film reaches the saturated resistance Rg in 50 sand at the end of the injection cycle, when dry air is introduced, itselectrical resistance returns to the original value (Ra). This fact is aproof of the reversibility of the process. The stepwise decrease of theelectrical resistance of the film is very consistent with an increas-ing amount of ethanol oxidation. Greater ethanol oxidation causedthe introduction of more electrons into the SnO2 surface and the

film became less resistive. Fig. 3e depicts the correlation betweenthe ethanol gas concentration and the response of the (1 wt%) PtO2-doped SnO2 sensor. It seems that the correlation lines were not goodlinear for such broad ethanol concentration (100–1000 ppm). How-ever, for practical applications of this sensor, the linear fit can be

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ell made between the sensor response and ethanol concentrationor narrower ethanol concentration ranges (e.g. 100–500 ppm and00–1000 ppm). The linear dependent of the ethanol concentrationn the sensor response of our PtO2-doped SnO2 sensor for the nar-ow ethanol concentration range is consistent with that previouseported [17,18].

Indeed, we have studied further the sensing properties of thetO2-doped SnO2 sensors, in which the PtO2 doping content wasaried from 0.2 to 2 wt% in comparison with SnO2 weight. The sens-ng characteristics of these sensors were measured with 250 ppmthanol at different operating temperatures, and the results arehown in Fig. 4. It can be seen that the (1 wt%) PtO2-doped SnO2ensor shows a higher response. From this, it can be concluded thathe PtO2 doping content of 1 wt% is the optimal value.

The 90% response time for gas exposure (t90%(air-to-gas)) and thator recovery (t90%(gas-to-air)) were calculated from the resistance-ime data shown in Fig. 3d. The t90%(air-to-gas) values are around 23 s,hile the t90%(gas-to-air) value is around 46 s. These results are quite

omparable with those of SnO2-based sensors reported previously17–20].

Recently, hybrid CNTs/SnO2 sensors have been extensivelynvestigated. Therefore, for comparison with metal oxides-dopednO2 sensors, we have prepared and characterized MWCNTs-dopednO2 sensors in the same route as metal oxides-doped sensors inhis work. Fig. 5 shows the sensing characteristics of SnO2 sensorsoped with different kinds of MWCNTs. It can be recognized thathe responses to ethanol gas and LPG of all MWCNTs-doped SnO2ensors are improved at a low region of operating temperature.his observation is consistent with previously reported data forhe hybrid sensors of CNTs/SMO [5,7,11,13,14]. To explain this, wepeculate that it may result from the fact that the doping of MWC-Ts in the SnO2 matrix can introduce nanochannels and additionaletero-junction between SnO2 (n-type) and CNTs (p-type). Bothhese effects do not cause the response improvement of the hybrid

WCNTs-doped sensor at high operating temperatures because theanochannels formed by the MWCNTs may not play any role foras diffusion into the SnO2 matrix at a high operating temperature.therwise, we believe that SnO2 (n-type)/MWCNTs (p-type) canot functionalize well at a temperature higher than 350 ◦C due tohe transition from semiconductor behavior to metallic one of theNTs. More detail on this mechanism can be found further in recentorks by us and others [4,5,10,14]. Additionally, it also observed that

he effect of MWCNTs on the response of the MWCNTs-doped SnO2ensors is not significant in the detection of LPG and ethanol gas. Iteems that (d = 10–20 nm) MWCNTs-doped SnO2 sensors have bet-er performance to LPG and ethanol gas at an operating temperatureange of 280–350 ◦C.

The specific surface area (SSA) of MWCNTs with diameter of <10,0-40 nm and 60–100 nm were 242.2, 112.2, and 45.2 m2/g, respec-ively. In principle, the material with a higher SSA would have betteras response. However, we have observed that the doping contents so small that it could not affect the SSA of the MWCNTs-dopednO2 materials. Thus, the SSA factor cannot be a piece of evidencen the difference in the sensor response. The observed effect can bexplained by the fact that the MWCNTs embedded in SnO2 behaves nanochannels for the gas diffusion in the SnO2 bulk material.owever, a larger diameter of MWCNTS (e.g. d = 60–100 nm) can

esult in the decrease of sensor response because such larger diam-ter of MWCNTs could not be well embedded in the SnO2 matrix,nd they begin to connect together, resulting in a shorter resistanceath of the MWCNTs-doped SnO2 sensors.

From Fig. 5c, we can see that MWCNTs-doped SnO2 sensors areore selective to LPG than to ethanol gas at an operating tem-

erature range of 280–350 ◦C. This effect is completely differentith the metal oxides-doped SnO2 sensors (see Fig. 3c) that will

e discussed further in the next paragraph. Fig. 5d depicts the

Fig. 6. Response of an undoped SnO2 sensor to 250 ppm ethanol and 2500 ppm(0.25%) LPG in air operating at a temperature range from 190 to 360 ◦C.

electrical resistance variations obtained with several steps of dif-ferent LPG concentration from air to 1% LPG in air for the (0.1 wt%,20 < d < 40 nm) MWCNTs-doped SnO2 sensor operating at 320 ◦C.Similar to the PtO2-doped SnO2 sensors in the detection of ethanol,the MWCNTs-doped SnO2 sensors shows a good reversibility in thedetection of LPG and the stepwise decrease of electrical resistiv-ity of the MWCNTs-doped SnO2 film is very consistent with theincreasing amount of LPG oxidation. More LPG oxidation causedthe introduction of more electrons into the SnO2 surface and thefilm became less resistive. Fig. 5e depicts the variation of responsewith LPG concentration in air for the MWCNTs-doped SnO2 sensorsat an operating temperature of 320 ◦C. It can be observed that theresponse does not increase linearly for the concentration range of0.1–0.6% (1000–6000 ppm). It seems that the response tends to sat-urate for an LPG concentration higher than 0.5% (5000 ppm). Thiscan be attributed to the fact that there would be an insufficientnumber of oxygen anions available on the surface of the MWCNTs-doped SnO2 materials for reaction with LPG.

The 90% response time for gas exposure (t90%(air-to-gas)) and thatfor recovery (t90%(gas-to-air)) were calculated from the resistance-time data shown in Fig. 5d. The t90%(air-to-gas) value is around 21 s,while the t90%(gas-to-air) value is around 36 s. It can be seen that theresponse times of the Pt- and MWCNTs-doped SnO2 sensors aresimilar, while the recovery time of the MWCNTs-doped sensor isrelatively shorter than that of the PtO2-doped SnO2 sensors. Thiscould be attributed to the formation of the nanochannels in SnO2materials by doping CNTs that can enhance the diffusion in and outof the gas molecules.

To study the effect of MWCNTs doping on the sensing proper-ties to ethanol gas and LPG, we plotted the response of undopedSnO2 sensor to 250 ppm ethanol gas and 2500 ppm (0.25%) LPG asshown in Fig. 6. It is indicated that the response of undoped SnO2sensors to 250 ppm ethanol gas is higher than that to 2500 ppm(0.25%) LPG over an operating temperature range of 190–360 ◦C.Therefore, this points out that the higher response of the MWCNTs-doped SnO2 sensors to LPG than to ethanol can be attributed to theMWCNTs doping. This is an interesting finding that cannot yet beclearly explained as of now. The pure SnO2 sensor is more sensitiveto ethanol than LPG even though the ethanol gas concentration isabout 10 times lower than the LPG concentration. This has also beenexplored in previous works [15]. The sensing mechanism of the

ethanol and LPG has long been known and widely adopted in pre-vious reports [24–30]. However, to explain why the ethanol is moresensitive than LPG, even though the former has a lower concentra-tion than the later, is still unclear. It has long been known that there

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s a dehydrogenation step in the reaction between ethanol/LPG andnO2 surface at elevated temperatures, and they can be describeds [25,30]:

Ethanol dehydrogenation

2CH3CH2OH + O2− → 2CH3CHO + H2O + e (2)

LPG dehydrogenation

2CH3CH2CH3 + O2− → 2CH3CH2CHO + H2O + e (3)

2CH4CH2CH2CH3 + O2− → 2CH4CH2CH2CHO + H2O + e (4)

2CH3CHCH2 + O2− → 2CH3CHCO + H2O + e (5)

t should be noted that the main constituent of LPG is propane∼85% by liquid volume), butane (∼2.5% by liquid volume) andropene (∼5% by liquid volume). Among these constituents,ropane and butane are more stable than propene, which is annsymmetrical alkene containing a double bond. Thus, propene isore prone to hydrogenation than to the dehydrogenation, which

an be used to compare with the dehydrogenation of ethanol gas.or 2500 ppm LPG, the propene concentration can be estimated toe about 125 ppm. This is the reason why the response of the SnO2ensors to 2500 ppm LPG is lower than that of 250 ppm ethanol gas.

It has been reported that the MWCNTs-doped SnO2 sensorshow a better performance compared with undoped SnO2 sen-or. In our case, for ethanol detection, the response of the (0.1 wt,0 <d < 40 nm) MWCNTs-doped SnO2 sensors (∼12.3 for 250 ppmt 260 ◦C, see Fig. 5b) is about three times higher than that of thendoped SnO2 sensor (∼3.4 for 250 ppm at 260 ◦C, see Fig. 5b).he reason for this was previously explained in detail [4,5,10,14].he question to raise here is that why the MWCNTs-doped SnO2

s more sensitive to 2500 ppm LPG than to 250 ppm ethanol asepicted in Fig. 5c. Further intensive investigation should be done tonderstand this phenomenon more comprehensively. The plausiblexplanation for the observed effect can be based are as follows: (i)he MWCNTs are hollow nanotubes that gas absorption could occurn the inside and outside of the MWCNTs [34], (ii) the methane (CH4)

olecules (e.g. propane and butane) can be physically adsorbedn the outgassed nanotubes (i.e., nanotube after oxygen exposure)35], and (iii) the oxygen molecules are strongly adsorbed on theefective sites of MWCNTs (adsorption energy is about 0.32 eV) [36]hat can serve as a reactive gas for the oxidation reactions of LPG.hese reasons can enhance Reactions (3) and (4). Additionally, theonsumption of the adsorbed oxygen can affect the electrical prop-rties of the MWCNTs [36,37], and the electrical resistance of theWCNTs-doped SnO2 film can be consequently changed.

. Conclusion

We have systematically investigated and compared the perfor-ance of metal oxide- and MWCNTs-doped SnO2 thin film sensors

o LPG and ethanol gas. We have found that the SnO2 doped with thewt% PtO2 sensor shows the highest response to ethanol gas andPG compared with that of SnO2 doped with the other dopants.mong carbon nanotubes-doped SnO2 sensors, the sensor dopedith 0.1 wt% MWCNTs with a diameter ranging from 20 to 40 nm

xhibits the highest response to ethanol gas and LPG. An interestingnding is that the PtO2-doped SnO2 sensor shows good selectivity

o ethanol gas over LPG, while, the MWCNTs-doped SnO2 sensorhows good selectivity to LPG over ethanol gas, at the same testingonditions. The gas-sensing mechanism of the hybrid sensor haseen discussed. However, further study is needed to understandetter the selectivity of the hybrid sensor to ethanol gas and LPG.

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ators B 144 (2010) 450–456 455

Acknowledgments

The work was supported by the National Foundation for Sci-ence and Technology Development (NAFOSTED) of Vietnam (forBasic Research Project: 2009-2012), the National Key Research Pro-gram for Materials Technology (Project No. KC 02-05/06-10), theresearch projects of Vietnam Ministry of Education and Training(Code B2008-01-217 and B2008-21-09) and Key basic research pro-gram for application orientation (2009-2012).

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iographies

guyen Van Hieu received his MSc degree from the International Training Instituteor Materials Science (ITIMS), Hanoi University of Technology (HUT) in 1997 and PhDegree from the Department of Electrical Engineering, University of Twente, Nether-

ands in 2004. Since 2004, he has been a research lecturer at the ITIMS. In 2007, he

ators B 144 (2010) 450–456

worked as a post-doctoral fellow at Korea University. His current research inter-ests include nanomaterials nanofabrications, characterizations and applications toelectronic devices, gas sensors and biosensors.

Nguyen Anh Phuc Duc received the BS degree in Engineering Physics from Instituteof Engineering in Physics, Hanoi University of Technology, Vietnam in 2005, his MScdegree in Materials Science from the International Training Institute for MaterialsScience (ITIMS), Hanoi University of Technology (HUT) in 2007, and he is currentlyworking toward his PhD degree at Leuven University, Belgium. His current researchinterests include oxide semiconductors nanoparticle for gas-sensing applications.

Tran Trung received his MSc degree in 1994 and his PhD degree in 1998 from theDepartment of Electrochemistry, Hanoi University of Technology. In 2000 and 2001,he worked as a post-doctoral fellow in Pusan National University, Korea. At presenthe is working as an Associate Professor at the Faculty of Environment and Chemistry,Hung-Yen University of Technology and Education. His research activities are relatedwith the design, fabrication and characterization of organic–inorganic hybrids andnanomaterials for application to electronic devices and battery systems.

Mai Anh Tuan received his MSc degree from the International Training Institute forMaterial Science (ITIMS), Hanoi University of Technology (HUT) in 1999. In 2004,he completed his PhD program at Universite Claude Bernard Lyon 1, France. Since2000, he has been working as a lecturer at ITIMS, HUT. He is now biosensor group-leader at ITIMS. His current research interests include biosensors for bio-medicaland environmental application, functional materials and IC packaging technology(materials consideration).

Nguyen Duc Chien received the engineering degree in Electronic Engineering atLeningrad Electrotechnical University, Russian, in 1976, and the MSc and PhD inMicroelectronics at Grenoble Polytechnique University, France, in 1985 and 1988,respectively. He has been working as Professor at the Institute of Engineering Physics(IEP), Hanoi University of Technology (HUT). From 1989 to 1990 he worked as a vis-iting professor at the Grenoble University, France. From 1992 to 2006 he was a vicedirector of the International Training Institute for Materials Science (ITIMS), HUT,where he established the Laboratory of Microelectronics and Sensors. Since 2003 he

optoelectronic materials and devices, and MEMS devices. He has been the leader ofmany national research projects related to microelectronic devices and functionalnanomaterials. Dr Nguyen Duc Chien is also a member of Physics Society of Vietnamand the Vietnamese Materials Research Society.