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Sensors and Actuators B 140 (2009) 500–507

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

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

hin film polypyrrole/SWCNTs nanocomposites-based NH3 sensor operated atoom temperature

guyen Van Hieua,∗, Nguyen Quoc Dunga, Phuong Dinh Tamb, Tran Trungc, Nguyen Duc Chienb,d

International Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT), No.1 Dai Co Viet Road, Hanoi, Viet NamHanoi Advanced School Science and Technology(HAST), Hanoi, Viet NamFaculty of Environment and Chemistry, Hung Yen University of Technology and Education, Hung Yen, Viet NamInstitute of Engineering Physics (IEP), Hanoi University of Technology (HUT), Hanoi, Viet Nam

r t i c l e i n f o

rticle history:eceived 13 January 2009eceived in revised form 24 March 2009ccepted 23 April 2009

a b s t r a c t

A PPY/SWCNTs nanocomposite-based sensor with relatively high sensitivity and fast response–recoverywas developed for detection of NH3 gas at room temperature. The gas-sensitive composite thin filmwas prepared using chemical polymerization and spin-coating techniques, and characterized by Fourier

vailable online 7 May 2009

eywords:as sensorarbon nanotubes

transformed infrared spectra and field-emission scanning electron microscopy. The results reveal that theconjugated structure of the PPY layer was formed and the functionalized SWCNTs were well-embedded.The effects of film thickness, annealing temperature, and SWCNTs content on gas-sensing properties ofthe composite thin film were investigated to optimize the gas-sensing performance. The as-prepared thinfilm PPY/SWCNTs composite sensor with optimized process parameters had a response of 26–276% uponexposure to NH3 gas concentration from 10 to 800 ppm, and their response and recovery times were

ctive

olypyrroleanocomposite around 22 and 38 s, respe

. Introduction

Conducting polymers such as polyaniline, polypyrrole (PPY),olythiophene, and their composites have been widely investi-ated as effective materials for chemical sensors [1–10]. Amonghe conducting polymers, PPY and its composites have attractedonsiderable attention because they are easily synthesized, haveelatively good environmental stability, and surface charge char-cteristics that can be easily modified by changing the dopantpecies into the materials during the synthesis. It has been demon-trated that there are many approaches in the enhancement ofhe mechanical strength, chemical stability, and gas-sensing prop-rties by combining PPY with organic and inorganic materialso form composites [6]. A large number of reports have beenublished on chemical polymerization and electrochemical tech-iques to prepare polypyrrole/inorganic (TiO2, Fe2O3, SnO2, WO3,tc.) and organic (poly[vinyl alcohol], carbon nanofibers, carbonanotubes)/polypyrrole nanocomposite sensors to detect a wide

ariety of gases such as NO2 [7], humidity [11–13], H2S [14], volatilerganic compounds (VOCs) [15,16], CO2 [17,18], and NH3 [19,20].

The special geometry of carbon nanotubes (CNTs) and theirmazing feature of being all-surface reacting materials offer great

∗ Corresponding author. Tel.: +84 4 38680787; fax: +84 4 38692963.E-mail addresses: hieu@itims.edu.vn, hieunv-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.2009.04.061

ly.© 2009 Elsevier B.V. All rights reserved.

potential applications such as in gas sensor devices working atroom temperature. It has been reported that CNTs are very sen-sitive to the surrounding environment. The presence of O2, NH3,NO2 gases and many other molecules can either donate or acceptelectrons resulting in an alteration of the overall conductivity[22,23]. Such properties make CNTs ideal for nano-scale gas-sensingmaterials, and CNTs field effect transistors (FETs) and conductive-based devices have already been demonstrated as gas sensors[24,25]. However, CNTs still have certain limitations for gas sen-sor application such as their long recovery time, limited gasdetect, and weakness to humidity and other gases. Therefore, thecomposites of PPY and CNTs have received a great deal of atten-tion for gas-sensing application. PPY/SWCNTs and PPY/MWCNTscomposites-based sensors have been already developed for thedetection of ethanol and NH3, respectively [16,26]. They have showna higher sensitivity than both PPY- and CNTs-based sensors sepa-rately over a wide range of gas concentrations at room temperature.However, the sensing properties of nanocomposite thin film havenot yet been investigated in depth.

NH3 gas presents many hazards to both humans and the envi-ronment. Due to its highly toxic characteristics, even low levelconcentrations (ppm) pose a serious threat. NH3 sensors based on

conventional materials such as SnO2 [27,28], TiO2 [29,30], In2O3[31,32], WO3 [33,34] and ZnO [35,36] have been developed withgood sensitivity and selectivity and fast response–recovery thatcan be used in detecting lower level ammonia gas presence. How-ever, most of the NH3 gas sensors are fabricated using metal oxides

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gled without any particle-like impurities. The morphology of theas-synthesized PPY/SWCNTs composite (see Fig. 3c) shows thatthe SWCNTs are well-embedded within the matrix of the PPY. TheFT-IR spectra and FE-SEM characterizations confirm that the as-synthesized SWCNTs/PPY nanocomposite prepared in the present

N. Van Hieu et al. / Sensors an

hich are only effectively operated at temperature ranges between50 and 400 ◦C, resulting in high power consumption and com-lexities in integration. Thus, there is a great need to develop aew class of materials for gas sensors that would have good per-

ormance working at room temperature. In this paper, we presenthe sensing properties of the PPY/SWCNTs nanocomposite thin film,repared by simple and straightforward in situ chemical polymer-

zation of pyrrole mixed with SWCNTs, toward low concentrationf NH3 (10–800 ppm) at room temperature. The influence of filmhickness, SWCNTs content, and annealing temperature are stud-ed to optimize the gas-sensing properties of the composite thinlm.

. Experimental

.1. Material synthesis and characterizations

All the chemicals used were of analytical grade. The SWC-Ts were produced by chemical vapor deposition (CVD, Shenzhenanotech Port Co. Ltd., diameter <2 nm, length: 5–15 �m, purity:90%), and they exist as agglomerates. Therefore, the SWCNTs wereunctionalized to enhance their dispersion in the solvent. The func-ionalization was carried out using a typical procedure, as follows:00 mg SWCNTs were suspended in 35 ml concentrated nitric acid15 M) and refluxed for 12 h in a silicone oil bath maintained at20 ◦C to modify the SWCNTs surface, then rinsed with distilled2O until the pH of the solution was neutral, and finally dried at0 ◦C in a vacuum oven (∼10−2 Torr).

The formation of the nanocomposite of PPY and functionalizedWCNTs (f-SWCNTs) was carried out by in situ chemical poly-erization of pyrrole monomer with f-SWCNTs. 3.0 ml sodium

odecylbenzenesolfonate (DBSA) was dissolved in a solution of0 ml water, then 0.1 g f-SWCNTs was added and stirred by ultra-onic for 30 min to obtain a well-suspended SWCNTs. The SWCNTsuspension was then transferred into a flask and allowed to stand attemperature of 0–5 ◦C for 30 min. The pyrrole monomer (1 ml) wasfterwards slowly added drop-wise to the SWCNTs suspension withonstant magnetic stirring for 30 min at the temperature of 0–5 ◦C,hen 0.345 g (NH4)2S2O8 (APS) was added to start the polymeriza-ion process lasting 12 h. After the polymerization was finished, theomposite powder formed was filtered and rinsed with DI water,ethanol, acetone, and ethanol until the filtrate became colorless.

he as-prepared composite powder was then dried in vacuum atoom temperature.

Fourier transformed infrared (FT-IR, Niconet 6700) and field-mission scanning electron microscope (FE-SEM, 4800 Hitachi,apan) were used to characterize the as-synthesized materials.

.2. Sensor preparation and characterizations

A certain amount of the nanocomposite powder was suspendednto 2 ml CHCl3 (chloroform, containing 0.5 ml DBSA) by ultra-onic stirring for 30 min. The composite suspension with differentontents (0.005, 0.01, 0.03, 0.1 g/ml) was prepared for variationf the film thickness by spin-coating at 3000 rpm. A silicon sub-trate with interdigitated-electrode on the top was used for theas-sensor fabrication. The interdigitated-electrode was fabricatedsing a conventional photolithographic method with a finger widthf 100 �m and a gap size of 140 �m. The fingers of interdigitated-lectrode were fabricated by sputtering 10 nm Ti and 200 nm Pt on

layer of silicon dioxide (SiO2) with the thickness of about 100 nm

hermally grown on top of the silicon wafer. The coating layers wereried in vacuum and then immersed in methanol to remove DBSA.inally, the samples were annealed at different temperatures (25,00, 300, 400 ◦C) for 40 min.

ators B 140 (2009) 500–507 501

The gas sensors were tested with NH3 using the injectiontechnique. More details about this testing system can be foundelsewhere [37]. The electrical resistance response during testingwas monitored by the Precision Semiconductor Parameter Ana-lyzer (HP4156A). The sensor (S) response for a given measurementwas calculated as follows: S = Rg/Ra, where Rg and Ra are electricalresistances of the sensor in tested gas and in air, respectively.

3. Results and discussion

3.1. Material characterizations

Fig. 1 presents the FT-IR spectra of PPY and PPY/SWCNTs com-posite. It is clear that PPY and the composite show very similarspectra. The peaks at 2917 and 2854 cm−1 are associated with fivemembered ring C–H stretching [21]. The stretching and bendingmotion of N–H in PPY appear at 3361 and 1645 cm−1, respectively[21,38]. The peaks at 1249 and 1086 cm−1 are due to C–N stretchingand C–H deformation vibrations of PPY [21,26,40]. Additionally, thepeaks at 991 and 853 cm−1 related to the in-plane and out-of-planevibration modes of N–H and C–H [38,39]. It can be recognized thatthe PPY and SWCNT/PPY composites show nearly identical num-bers and positions of the main IR bands, and that the characteristicpeaks of MWNTs are hardly seen. The intensity of the transmissionlight to the SWCNTs is very low, and the corresponding reflectiveor scattering light has to transfer the matrix layers of the conduct-ing polymers. However, the matrix layer of the PPY has absorbedmuch of this light, suggesting that the SWCNTs are well-embeddedwithin the matrix of the PPY [41,42].

Fig. 2 shows the FE-SEM images of f-SWCNTs (Fig. 2a), PPY(Fig. 2b), and the f-SWCNTs/PPY composite (Fig. 2c). The typicalmorphology of PPY indicates that the particle size of PPY is lowerthan 100 nm with spherical morphology. The obtained morphologyof the f-SWCNTs shows that many nanotubes are loosely entan-

Fig. 1. FT-IR spectra of (a) PPY and (b) PPY/SWCNTs (1 wt.%) nanocomposite.

502 N. Van Hieu et al. / Sensors and Actuators B 140 (2009) 500–507

Fig. 2. FE-SEM image of (a) f-SWCNTs, (b) PPY, and (c)PPY/SWCNTs composite.

Fig. 3. Response curve of SWCNTs/PPY composite sensor to NH3 at room tempera-ture.

Table 1Response–recovery time from this work and the literatures.

Sensor type Response time (s) Recovery time (s) References

PPY/SWCNT This workPPY thin film 240 202 [45]

SWCNTs film 60 120 [46]Single wire of PPY 840 1,800 [48]Single wire of SWCNTs 6000 43,200 [49]

work are similar to the carbon nanotubes/PPY composites pre-pared by previous reports such as through chemical polymerization[41,42] vapor phase polymerization [26], and electrochemical poly-merization [43,44].

3.2. Gas-sensing properties

Fig. 3 shows a typical response curve of the thin film SWC-NTs/PPY composite gas sensors during gas-sensing at roomtemperature. The response curve indicates that the resistance sig-nal varies with time over the two of cyclic tests. Before each cyclictest, the sensor was exposed to the air and the measured resistanceof the sensor was equal to Ra. At the beginning of each cyclic test, adesired NH3 gas was injected into the chamber (4 l). The measuredresistance changed gradually. After a certain time, the resistancechanged very slowly, almost reaching a stable value, Rg, correspond-ing to the response of the sensor to NH3 gas. The glass chamberwas then removed from the sensor to expose the sensor to the airagain. The measured resistance was restored to its original value, Ra.The 90% response time for gas exposure (t90%(air-to-gas)) and that forrecovery (t90%(gas-to-air)) were calculated from the resistance–timedata shown in Fig. 3. The t90%(air-to-gas) value is around 22 s, whilethe t90%(gas-to-air) value is around 38 s. As can be seen, these valuesare lower than those of both the PPY- and the CNTs-based NH3 gassensors reported in the literature [45,46]. The response–recoverytime of the SWCNTs/PPY composite is even shorter than that of thesingle wire and tube gas sensor devices made from PPY nanowiresand SWCNTs [48,49]. The response–recovery time comparison dataare indicated in Table 1.

Fig. 3 shows that the resistance of the sensor increases when it isexposed to NH3 gas (electron-donor). This behavior is similar to thatof the PPY- and SWCNTs-based sensors [4,7,10,17,22,24,27,46–49],in which both PPY and SWCNTs behave as p-type semiconduc-tors. This suggests that the PPY/SWCNTs composite also behavesas a p-type semiconductor, and is consistent with the case of thePPY/MWCNTs composite sensor prepared by vapor phase polymer-ization [26]. This also implies that the adsorption of NH3 on thePPY/SWCNTs composite results in reducing the number of holes inPPY and SWCNTs because NH3 is an electron-donating gas. Similarto PPY- and SWCNTs-based sensors, the electron charge transfer isthe main mechanism in changing the resistance of the PPY/SWCNTson adsorption of NH3 gas. However, upon NH3 adsorption, electroncharge transfer is likely to only occur between NH3 and PPY becausethe SWCNTs are well-embedded within the PPY.

The fast response–recovery of the PPY/SWCNTs composite sen-sor could be explained as follows. It is well-known that theadsorption process of NH3 gas on CNTs is attributed to physisorptionand chemisorption, whereas chemisorption is due to site-defecton the sidewall of CNTs [50–52], and the defects of CNTs areunavoidable during the synthesis and purification processes. Thus,NH3 chemically adsorbed on the CNTs are hardly removed uponair exposure, resulting in slow response and recovery of CNTs-

based sensors. For PPY/SWCNTs-based sensors, the adsorptionprocess of NH3 gas can only be by physisorption due to the factthat the SWCNTs are well-embedded in the PPY, and the site-defects on the sidewall of SWCNTs are functionalized with the

d Actuators B 140 (2009) 500–507 503

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N. Van Hieu et al. / Sensors an

PY during the polymerization process. It has also been foundhat the PPY/SWCNTs-based sensor in the present work exhibitshorter response–recovery time than that of the PPY-based sensorseported in previous works [45,48]. This could be attributed to theact that our sensors are made of the thin-film type deposited bypin-coating of the PPY/SWCNTs composite suspension. Anothereason could be that the composite thin film forms permanentano-channels due to the hallow-core of SWCNTs, enabling themmonia molecules to be diffused in and out of the PPY/SWCNTsomposite faster than that of the pristine PPY. This also meanshat the PPY/SWCNTs composite is more porous than the PPY, asas been already claimed by previous reports [16,26]. The thinlm and the porous film could be the reasons explaining why thePY/SWCNTs composite-based sensor exhibits a higher responseompared to that of PPY-based sensors. Additionally, SWCNTs arenown to be strong adsorbers of NH3 gas molecules [53] which mayelp the PPY to interact more easily with them. Therefore, this coulde another contribution of SWCNTs in the response enhancementf the PPY/SWCNTs composite sensor.

.2.1. The effect of film thicknessIt is well known that the thickness of the sensitive layer has

great influence on the gas-sensing performance of thin filmensors, which have provided a much better platform for produc-ng high performance gas sensors. However, the influence of thelm thickness of PPY as well as PPY/carbon nanotubes compos-

te on their gas-sensing performance has been lacking much thusar. As such, in the present work, we try to explore the effectf the thickness of the composite-sensing layer on the sensitiv-ty, to find the optimal thickness for the composite gas sensor. Asndicated in the previous section, different PPY/SWCNTs compos-te suspensions in chloroform were prepared for the gas sensorabrication with variation in the film thickness. We have not yetuccessfully characterized the film thickness, but we believe thathe film thickness decreases with the decrease of the content ofhe composite suspension. Fig. 4a shows the sensor response ofll as-fabricated sensors up to 150 ppm NH3 at room tempera-ure. It can be seen that the sensor response increases with theecrease of PPY/SWCNTs suspension content. This can be attributedo the decrease of the film thickness. Fig. 4b indicates that the elec-rical resistance of the composite film increases with decreasedontent of the suspension, and is confirmed by the decrease ofhe film thickness. The mechanism of the film thickness on theesponse of the PPY/SWCNTs composite sensor has not been yetnderstood thus far. However, a plausible explanation could beuggested based on the sensing mechanism of metal oxide semi-onductors as previously reported [54–56]. The response of thePY/SWCNTs composite sensor depends on the reactivity and diffu-ion of NH3 gas molecules inside the composite-sensing thin film.herefore, when the thickness of the film decreases, the sensoresponse increases due to the decrease of the diffusion length of theas molecules [56]. At room temperature, the diffusion length hasignificant meaning for gas-sensor performance. Therefore, as indi-ated in Fig. 4b, the sensor response increases with decreasing filmhickness.

.2.2. The effect of SWCNTs contentIt has been realized that the content of carbon nanotubes

trongly affects the morphology and the electrical property of thePY/CNTs thin film [16,26,41,57]. Therefore, it is predicted thathe sensitivity of the composites sensor is influenced by varying

he SWCNTs content in the composites. Thus, various PPY/SWCNTsomposites-based sensors, in which the SWCNTs content (weighatio of SWCNT to PPY) was varied at 0, 0.5, 1.0, 3.0, and 5.0 wt.%,ere prepared and characterized. Fig. 5 shows the sensing charac-

eristics of all the as-fabricated sensors up to 150 ppm NH3 at room

Fig. 4. The NH3 gas-sensing characteristics of PPY/SWCNTs composite at differentfilm thickness: (a) transient responses of the sensor to 150 ppm NH3; (b) the sensorresponse as a function of the film resistance.

temperature. It is clearly seen that the response to the NH3 gasof the PPY/SWCNTs composite gas sensor increases at first as theSWCNTs content is increased up to 1 wt.% but it decreases whenthe SWCNTs content is further increased up to 5 wt.%. Apparently,the composite sensor with the SWCNTs content of 1 wt.% shows thehighest response to NH3 at room temperature, with the responseto 150 ppm NH3 being about 2.2. This value is relatively higherthan that of PPY/MWCNTs composite NH3 gas sensor prepared byvapor phase polymerization [16]. As mentioned above, SWCNTswere covered by PPY in the composite film, and the increase ofSWCNTs content can result in the increase of the surface area of thePPY/SWCNTs composite, providing more active sites for adsorptionof NH3 gas, and thus an increase in the sensitivity is expected. How-ever, it is well-known that the SWCNTs are composed of metallicand semiconducting CNTs, and the increase of the SWCNTs contentmakes the composite electrically shorted, thus increasing the per-colation effect by the metallic carbon nanotubes [58]; furthermore,the conductivity of the composite is then dominated by the metallicCNTs. This suggests that both these effects could be responsible forthe decrease of the composite sensor response.

3.2.3. The effect of annealing temperatureIt has been reported that the heat-treatment process of the

PPY/CNTs composite strongly affects the electrical and gas-sensing

504 N. Van Hieu et al. / Sensors and Actuators B 140 (2009) 500–507

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response decreased with the increase of humidity [3,8]. The rea-

ig. 5. The NH3 gas-sensing characteristics of PPY/SWCNTs composite at differentWCNTs contents: (a) transient responses of the sensor to 150 ppm NH3; (b) theensor response as a function of the SWCNTs content.

roperties of the composite-sensing film because the active sites ofhe PPY on the SWCNTs are quenched by the heat-treatment [16].

Therefore, annealing temperature needs to be optimized tobtain the best performance of a gas-sensing device. We fabricatedour PPY/SWCNTs composite sensors at the same time, and car-ied out the heat-treatment at temperatures of 200, 300 and 400 ◦Cor three sensors, and the last was maintained at room temper-ture. Fig. 6a shows the response of the four sensors to 150 ppmH3 gas at room temperature. From Fig. 6b, it can be seen that

he optimal annealing temperature is 300 ◦C. The sensor responsencreases with an increase of the annealing temperature from roomemperature (25 ◦C) up to 300 ◦C. This suggests that the composites still contaminates with the solvent after drying by the vacuum.he solvent is supposed to be evaporated with the increase in thennealing temperature, and the composite material becomes moreorous. However, with further increase of the annealing tempera-ure, the number of the active sites of PPY would decrease, resultingn the decreases of the composite sensor response.

.2.4. The effect of operating temperatureAlthough the aim of this work was to develop room tempera-

ure gas sensors for NH3 detection, we also tested the compositeensor to 150 ppm NH3 at different temperatures such as 25, 40,0 ◦C to examine the effect of operating temperature on the sensi-ivity to NH3 gas and to find optimized operating temperature. The

Fig. 6. The NH3 gas-sensing characteristics of PPY/SWCNTs composite annealing atdifferent temperatures: (a) transient responses of the sensors to 150 ppm NH3; (b)the sensor response as a function of the annealing temperature.

obtained responses of the composite sensor are shown in Fig. 7.It turns out that the sensor response significantly decreases withthe increase in the operating temperatures (see Fig. 7b). We havealso tested the composite sensor at temperature of 100 ◦C, andfound that the sensor does not respond to NH3 gas (not shown).Our sensor samples can be compared to the PPY-based sensors inthe detection of ethanol and CO2 gases that have been previouslyreported [5,8], indicating that not only the PPY materials but alsothe composites using PPY materials show the highest sensitivityat room temperature. This emphasizes the advantage of the sen-sors based on conducting polymer over the metal oxide sensors[1]. There have been several works studying the mechanism of theinteraction between NH3 and PPY materials [3,8,21,26,48]. How-ever, the effect of the ambient temperature on the interaction hasbeen not clear thus far. It has been demonstrated that the humidityas well as ambient temperatures strongly affects the sensing prop-erties of PPY-based sensors. The effect of humidity on the sensingproperties of PPY sensors has already been explained in previousworks. The PPY sensor was obviously shown to have better sensi-tivity under dry conditions than under humid conditions, and the

son is that the water vapor was adsorbed faster than NH3, and themore water vapor was adsorbed, the less NH3 was adsorbed onthe surface of the PPY sensor at room temperature. The effect of theambient temperature on the sensing properties can be explained as

N. Van Hieu et al. / Sensors and Actuators B 140 (2009) 500–507 505

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ig. 7. The NH3 gas-sensing characteristics of PPY/SWCNTs composite at differentperating temperature: (a) transient responses of the sensors to 150 ppm NH3; (b)he sensor response as a function of operating temperature.

ollows. As discussed above, the sensing process of the SWCNTs/PPYomposite involves two steps: (i) diffusion of gas molecules in theensing film, and (ii) the reaction between them. Temperature cannfluence both of the two steps. An increasing temperature will shifthe equilibrium to desorption because adsorption always prefersow temperatures. Additionally, both SWCNTs and PPY are sensing

aterials with adsorption/desorption as their preponderant step1,2,22–24]. Therefore, the sensitivity of the SWCNTs composite willecrease with increased operating temperature. Nonetheless, weelieve that the ambient temperature and humidity associativelyffect the sensitivity of the SWCNTs/PPY composite-based sensors.

.2.5. The influence of NH3 gas concentrationThe PPY/SWCNTs sensor performed at room temperature was

ubjected to successive injections of NH3 gas. A step-wise increasen electrical resistance obtained with increasing NH3 gas concen-ration from 0 to 800 ppm NH3 in the air for the PPY/SWCNTsomposite sensor at room temperature can be seen in Fig. 8a.he response (Rg/Ra) of the sensor was calculated from Fig. 8a,nd was plotted as a function of NH3 concentration as shown in

ig. 8b. It was found that the response increases from 26 to 276% byarying NH3 gas concentration from 10 to 800 ppm. It seems thatur PPY/SWCNTs composite-based sensors show a higher responseo NH3 at room temperature in comparison to that of PPY-, PPYanowires-, PPY/MWCNTs-based sensors as reported previously

Fig. 8. Step-wise increase in electrical resistance obtained with increasing NH3 gasconcentration from air to 800 ppm NH3 in air (a); the sensor response as a functionof the NH3 gas concentration (b).

[9,19,45,48]. It should be noted that a relatively good linear relationbetween the response and NH3 gas concentration is observed sepa-rately in low (10–140 ppm) and high (140–800 ppm) concentrationranges of NH3 gas. Furthermore, the response of the PPY/SWCNTs-based sensor saturates as much higher NH3 gas concentration(>600 ppm) than that of PPY-based sensors [45]. This observationis consistent with PPY/MWCNTs composite-based sensor as pre-viously reported [9], suggesting that the PPY/carbon nanotubescomposite-sensing materials provide more active sites than thatof PPY material.

4. Conclusion

The thin film PPY/SWCNTs composite-based sensor has beensuccessfully prepared using the simple and straightforward insitu chemical polymerization of pyrrole mixed with SWCNTs andspin-coating method. The composite sensors have shown excel-lent sensitivity to NH3 gas at room temperature and relativelyfast response–recovery. The response of the PPY/SWCNTs com-posite thin film gas sensor strongly depends on the preparationprocess of the sensitive film, including film thickness, SWCNTs con-

tent and annealing temperature. This result also implies that theseconditions need to be optimized for practical applications of thecomposites of PPY/carbon nanotubes as the gas sensors, in gen-eral. The observations of the film morphology revealed that theSWCNTs are embedded in the PPY materials during the polymer-

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zation process. According to this result, the sensing mechanism ofPY/SWCNTs to NH3 gas has been discussed.

cknowledgments

The work was supported by the National Key Research Pro-ram for Materials Technology (Project No. KC 02-05/06-10), theouncil of Scientific and Engineering Research of Ministry ofducation and Training funded projects (Code B2008-01-217 and2008-21-09), the National Foundation for Science and Technol-gy Development (NAFOSTED) of Vietnam (for Basic Researchroject: 2009–2012) and the Key Basic Research Program for Appli-ation orientation (2009–2012).

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he has been the Director of the IEP, HUT. His research interests include: characteri-zations and modeling of MOS devices, nanomaterials for chemical sensor, biosensor,optoelectronic materials and devices, and MEMS devices. He has been a leader ofmany national research projects related to microelectronic devices and functional

N. Van Hieu et al. / Sensors an

iographies

guyen Van Hieu received his MSc degree from the International Training Instituteor Material 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,e worked as a post-doctoral fellow, Korea University. His current research inter-sts include nanomaterials nanofabrications, characterizations and applications tolectronic devices, gas sensors and biosensors.

guyen Quoc Dung received the BS degree in Chemistry at Hanoi University of Edu-ation in 2004, and MSc degree in materials science from the International Trainingnstitute of Material Science (ITIMS), Hanoi University of Technology (HUT), in 2006.er research interest is the development of conducting polymer and conductingolymer/carbon nanotubes composites gas-sensing applications.

huong Dinh Tam received MSc and PhD degree from the International Trainingnstitute for Materials Science (ITIMS), Hanoi University of Technology (HUT) in,004, and 2009, respectively. Currently, he is research lecturer at Hanoi Advanced

choool Science and Technology (HAST). His current research interests include nano-aterials nanofabrications, characterizations and applications to electronic devices

nd biosensors.

ran Trung received MSc degree in 1994 and PhD degree in 1998 from Depart-ent of Electrochemistry, Hanoi University of Technology. During 2000 and 2001

ators B 140 (2009) 500–507 507

he worked as a post-doctoral fellow in Pusan National University, Korea. At presenthe has been working as Associate Professor at Faculty of Environment and Chem-istry, Hung-Yen University of Technology and Education. His research activitiesare related with the design, fabrication and characterization of organic–inorganichybrids and nanomaterials for application to electronic devices and batterysystems.

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 worked as associated professor at the Institute of EngineeringPhysics (IEP), Hanoi University of Technology (HUT). From 1989 to 1990 he workedas a visiting professor at the Grenoble University, France. From 1992 to 2006 he wasa vice director of the International Training Institute for Materials Science (ITIMS),HUT, where he established a Laboratory of Microelectronics and Sensors. Since 2003

nanomaterials. Dr. Nguyen Duc Chien is a member of Physics Society of Vietnam andVietnamese Materials Research Society.