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Sensors and Actuators B 117 (2006) 426–430

Behavior of single-walled carbon nanotube-based gassensors at various temperatures of treatment and operation

Hong-Quang Nguyen, Jeung-Soo Huh ∗Department of Materials Science and Metallurgy, Kyungpook National University, 1370 Sankyuk-dong-Bukgu,

Daegu 702-701, Republic of Korea

Received 7 June 2005; accepted 22 November 2005Available online 6 January 2006

bstract

The behavior of single-walled carbon nanotube (SWNT)-based gas sensors at various conditions of operation was reported. The sensors wereabricated from SWNT powder by the screen-printing method, followed by an annealing pretreatment in open-air for 2 h at various temperatures tonhance the sensor characteristics. It was found that, the annealing at a suitable temperature not only removed the residual solvent and amorphousmpurities from the SWNTs, but also opened the nanotube-caps, resulting in enhancement the sensor sensitivity. Subsequently, the sensor annealedt 200 ◦C was employed for detection ammonia (NH3) in 500 sccm nitrogen (N2) flowing. After 10 min exposing to 5 ppm NH3 at room temperature,

he resistance of the sensor increased up to 8% in comparison with its initial value. The sensitivity increased as the NH3 concentration increasedut was diminished when NH3 concentration reached to 40 ppm. The sensor recovery was amended either by increasing the carrier gas flux or byeating in desorption time. By choosing a suitable regime of operation, in which the carrier gas flux, the heating duration and temperature wereppropriately controlled, the characteristics of the SWNT-based sensor such as recovery and reproducibility were significantly improved.

2005 Elsevier B.V. All rights reserved.

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eywords: Ammonia; Annealing; Carrier gas flux; Gas sensor; SWNT

. Introduction

Single-walled carbon nanotube (SWNT), a novel materialiscovered by Iljima and Ichihashi [1] in 1993, has attracted areat attention due to its extraordinary properties. In gas sens-ng aspect, SWNTs have tubular structure with large surface-o-volume ratio [2], which provides plenty sites for gaseous

olecules to adsorb. The one-dimensional quantum wire natureakes their electronic properties very sensitive to gas pres-

nce [2,3]. These factors enable the SWNTs to be an idealandidate for gas sensing materials. So far, a variety of gasesuch as NO2, NH3, CH4, O2, organic and inorganic vapors,tc., have been successfully detected by SWNT-based sensors3–11]. The adsorption of gaseous molecules either donates or

ithdraws electrons from the SWNT, leading to changes in theWNT electrical properties [4]. The advantageous properties of

he SWNT-based gas sensors such as high sensitivity and fast

∗ Corresponding author. Tel.: +82 53 950 5562; fax: +82 53 950 6335.E-mail addresses: quang nh2002@yahoo.com (H.-Q. Nguyen),

shuh@knu.ac.kr (J.-S. Huh).

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925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2005.11.056

esponse have been assured. The drawback of these sensors ishe slow and incomplete recovery that has been conceded. Kongt al. [3] reported that after the NO2 was replaced by pure argon,he conductance of the SWNT samples was observed to slowlyecover, and the typical recovery time was ∼12 h; Li et al. [5]tated that the recovery time was very long, on the order of0 h. To date, there have been a variety of attempts to overcomehis limitation. Kong et al. used heating at 200 ◦C in air to get aecovery of ∼1 h. Li et al. [5,6] used ultraviolet light to knock thedsorbed molecules out of the SWNT sites, thereby the recoveryime was accelerated to about 10 min [4]. Tsai et al. [7] reportedhat an effective desorption of oxygen and hydrocarbon from theurface of the SWNTs can be achieved by evacuating to highacuum at 500 K for several hours. All these methods improvedartly the recovery of the SWNT-based sensors, but are still notatisfied.

In order to improve the recovery of the SWNT-based sensors,t is necessary to understand the mechanism of gas adsorption

n the SWNT sites, thereby a suitable method for gas desorptionan be suggested. In this work, the behavior of the single-walledarbon nanotube-based gas sensors at various conditions ofperation was demonstrated. The considerable elements in fab-

and Actuators B 117 (2006) 426–430 427

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Table 1Temperature vs. supply voltage

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H.-Q. Nguyen, J.-S. Huh / Sensors

ication as well as operation of the SWNT-based sensors wereiscussed. From the experimental result, an appropriate regimef the sensor operation was suggested.

. Experimental

The SWNT powder used in our experiments was purchasedrom Iljin Nanotech Co., Korea. The typical diameter of the tubesnd bundles are 1–1.2 and 20 nm, respectively. The length of theubes is 2–20 �m and that of the bundles is 20 �m. The SEMnd TEM images of the SWNT powder are depicted in Fig. 1.

An amount of 30 mg SWNT powder was dispersed in 780 mg-terpineol (Aldrich, USA) by ultrasonic vibration for 2 h toake a sticky solution. This solution was printed on alumina

Al2O3) substrates with interdigital electrodes by a screen-rinting method to fabricate the SWNT-based sensors. Theseensors were divided into four groups: groups 1, 2 and 3 werennealed in open-air for 2 h at 100, 200 and 400 ◦C, respectively,hereas the rest was not annealed at all.

The sensors were inserted on a panel then introduced in a

ealed chamber of 1.6 l volume. Nitrogen (N2) and NH3 weresed as carrier and test gas, respectively. Prior to NH3 detection,he sensors were soaked in N2 flowing until the sensor resis-

ig. 1. SEM (a) and TEM (b) of the SWNT bundles. The SWNT exist in bundlesr ropes with diameter about 10–20 nm. The images show the residual impuritiesnd defects on the SWNT bundles.

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ances reached stationary values. The concentration of NH3 wasontrolled by altering the mass flow controllers. The heating tohe sensors was supplied by a dc power connected to the panel.he heating temperature was measured by a pyrometer and con-

rolled by changing the supply voltage. Table 1 presents theorrespondence between heating temperature and supply volt-ge. The resistance change of the sensors due to temperatureariation or NH3 adsorption was monitored, analyzed and storedy a computer with a DAQ (data acquisition board) and Lab-IEW software.

. Results and discussion

Fig. 2 represents the response of the sensors that werennealed at various temperatures to 10 ppm NH3 in 500 sccm2 flux at room temperature. It was found that the sensor sen-

itivity was strongly affected by annealing pretreatment. Theensors that was not annealed or annealed at 100 ◦C exhibited

lower sensitivity in comparison with the sensor annealed at

00 ◦C. The sensitivity, however, was reduced when annealingemperature was up to 400 ◦C. Obviously, the annealing temper-ture is essential in the SWNT-based sensor pretreatment.

ig. 2. Response of the SWNT-based sensors annealed at various temperatureso NH3 exposure. The sensor sensitivity was defined as S = (Rt − R0)/R0 × 100%,here Rt and R0 are the sensor resistances before and after exposing to NH3,

espectively.

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The effect of annealing on the SWNT bundles was con-idered to explain the behavior of the sensor sensitivity.n our experiments, the sensor sensitivity is defined as= (Rt − R0)/R0 × 100%, where Rt and R0 are resistance of the

ensor before and after NH3 exposure, respectively. The sensitiv-ty therefore depends on not only the resistance change (Rt − R0)ue to gas adsorption, but also the initial resistance (R0) of theensor. In the cases of the SWNTs annealed at a low tempera-ure (<200 ◦C) or not annealed, namely as-produced SWNTs,he tubes are generally closed at both ends [8]. For closed-nd SWNT bundles, there are three possible sites where gasolecules can be adsorbed: the grooves between adjacent tubes,

he interstitial channels between three or more tubes and the con-ex external surfaces [9,10]. In contrast, the SWNTs annealedt a high-enough temperature, namely heating treatment (HT-WNTs), are open-end tubes and there are four possible sitesor gas adsorption: the aforementioned three sites plus the innerhannel for each tube [9–12]. The annealing also causes annlargement in diameter of the tubes [13] making possible foras adsorption inside the tubes. Upon NH3 exposure, the gasolecules are adsorbed first inside the tubes, next in the inter-

titial channels and the grooves, finally on the convex externalurface of bundles [9–12]. At same experimental condition, themount of gas adsorbed on the HT-SWNT bundles is muchore than that on the as-produced ones, leading to a larger

hange in the sensor resistance. Therefore, the sensitivity of theT-SWNT-based sensors is generally higher than that of the

s-produced ones.If the annealing temperature reaches a certain threshold, how-

ver, a transition from semiconducting to metallic nanotube withigh resistivity will take place [14–17]. As a result, the initialesistance (R0) of the sensor will increase, leading to a decreasen the sensitivity, even though the amount of gas adsorbed on theWNT bundles may be increased. In fact, the sensor annealedt 400 ◦C exhibited a lower sensitivity than that of the sen-or annealed at 200 ◦C and this reality could be explained ashe above mentioned. Obviously, the annealing pretreatment isndispensable in manufacturing the SWNT-based sensors, buthich temperature to anneal is equally considerable. In our case,

he sensor annealed at 200 ◦C in open-air for 2 h exhibited bestensitivity in NH3 detection, and was employed from then on.

Fig. 3 shows the response of the optimized SWNT-basedensor to NH3 exposure with concentrations from 5 to 60 ppmt room temperature. The carrier gas flux was maintained at00 sccm throughout the experiment. After exposing to 5 ppmH3 for 10 min, the sensor resistance increased about 8%. It

an be inferred that the SWNT-based sensor could detect NH3ith a concentration level as low as sub-ppm at room tempera-

ure. The sensor sensitivity increased with an increase of NH3oncentration. The increase in sensitivity, however, was dimin-shed when the NH3 concentration reached to 40 ppm, indicatinghat a stationary state was approached. If the NH3 concentrationxceeded 40 ppm, the sensitivity regularly increased again. The

ame result was obtained and reported in a recent research [17].

The recovery of the sensor, however, was quite slow andncomplete. It might be due the residual NH3 molecules that hadeen adsorbed on the SWNT bundles and not entirely removed

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ig. 3. Response of the optimized SWNT-based sensor to NH3 exposure in

2 flowing (500 sccm) at room temperature. At concentration of 40 ppm, theensitivity is lower than expected, indicating a saturation state established.

y fluxing the carrier gas. The existence of residual moleculesn the SWNT evidences that the bonding between the SWNTnd NH3 molecules is quite strong. Since the adsorption of NH3as molecules on a defect-free SWNT is physisorption [10,18],e believe that the adsorption between NH3 molecules and theWNTs in this case consist of physisorption and chemisorption.he chemisorption might be due to the defects and impuritieso-existing with SWNTs in the bundles. These defects and impu-ities could produce dangling bonds in the tube edges, ends andalls. While the SWNTs exposed to NH3 gas, the danglingonds could be saturated by NH3 molecules, resulting in form-ng chemisorptions between the SWNTs and NH3 molecules19].

The strong bonding between NH3 molecules and the SWNTsequires a long time to degas and it causes the slow recoveryf the SWNT-based sensor. In the subsequent experiments, thearrier gas flux was intensified in desorption time to remove thedsorbed NH3 molecules from the SWNTs. The stronger carrieras flux was conducted, the better recovery the sensor exhibited.he improvement in recovery achieved by this method, however,as not satisfied, even though the maximum value of the massow controller was set as depicted in Fig. 4.

Another way to remove the adsorbed molecules is a heat-ng treatment in open-air [3]. The effect of heating on theWNT resistivity was investigated before using for gas des-rption. The SWNT-based sensor was heated from room tem-erature to 250 ◦C then cooling back to room temperature. Theeating–cooling cycle was repeated with temperatures of 350nd 400 ◦C in next two cycles. The response of the sensor resis-ance to temperature variation was graphed in Fig. 5. The curves

1B1A2 and A2B2A3 (Thermals 1 and 2) indicate that the sensoresistance decreased as temperature increased, and increased inooling process to room temperature. The response of the sensor

esistance to temperature variation was the behavior of a semi-onducting material. This response, however, only took placet a moderate temperature (<350 ◦C in our case). When heatingemperature exceeded to 350 ◦C (line B2B3 in curve A3B2B3C3,

H.-Q. Nguyen, J.-S. Huh / Sensors and Actuators B 117 (2006) 426–430 429

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Fig. 6. Response of the sensor to NH3 exposure with heating effect. The heatingat 100 ◦C was applied both in adsorption and desorption time.

ig. 4. The sensor recovery was improved by increasing the carrier gas flux.he improvement, however, was not satisfied.

ig. 5), the resistance increased in conjunction with a temper-ture increase. This is the behavior of a metallic material toemperature variation. The transition from semiconducting to

etallic behavior of the SWNT at high temperature is consis-ent with the previous studies [14–17], which supposed that theeating treatment might cause a structural change in the SWNT,eading to a change in its chirality. It is well known that the elec-rical properties of a SWNT is characterized by its chiral vector= na1 + ma2, where a1, a2 are the graphite primitive lattice vec-

ors, n and m are the integers. If (n − m)/3 or (2n + m)/3 is an inte-er, the tube is metallic, otherwise it is semiconducting [20,21].he heating temperature could cause a change in the chiralityf the SWNT, leading to a change in (n,m) values, i.e. changerom semiconducting to metallic property. During the coolingrocess, the structural change, as well as the transition, occurs

n the opposite tendency from semiconducting to metallic type.

Certainly, the heating at a moderate temperature could reducehe sensor resistance, therefore degrade the sensor sensitivity

ig. 5. Response of the SWNT resistance to temperature variation. Theeating–cooling process was carried out with nitrogen flowing at 500 sccm.

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ig. 7. Characteristics of the SWNT-based sensor operated at the optimizedondition.

n NH3 detection as indicated in Fig. 6. In order to avoid thismpact, the heating was not applied in sensing duration but inesorption time, for only 5 min at 70 ◦C. In addition, the carrieras flux was maximized to 1000 sccm for degassing. Fig. 7 showshe characteristics of the sensor at this condition. The figureerforms a fast response, high sensitivity, good recovery andxcellent reproducibility of the sensor to 20 ppm NH3 exposure.epending on each sensor and NH3 concentration, the heating

emperature and heating time may be appropriately adjusted.

. Conclusion

In this paper, the effect of various elements on the fabrica-ion and operation of the SWNT-based sensors was discussed.nnealing the SWNT sensors in open-air at a suitable dura-

ion and temperature not only removed the residual solvent andmorphous impurities from the SWNT raw, but also opened theWNT end-caps, resulting in improvement the sensor sensitiv-

ty. The sensor operating at room temperature exhibited a high

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ensitivity. At this temperature, it easily detected 5 ppm NH3 in00 sccm N2 with response time of 10 min.

When the sensor was heated to a high enough temperature, aransition from semiconducting to metallic might occurred dueo changes in the SWNT chirality and diameter. The sensor sen-itivity also was affected significantly by heating and it exhibitedest ability of NH3 detection at room temperature.

By increasing the carrier gas flux, combined with heating at0 ◦C for 5 min in desorption time, the sensor recovery was dra-atically improved. At the optimized condition of operation, the

ensor performed a fast response, high sensitivity, good recoverynd excellent reproducibility.

cknowledgement

The authors would like to thank the National Research Lab ofhe Kyungpook National University for their financial support.

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iographies

ong-Quang Nguyen received his BS degree at Vinh University in 1992, hisS degree at the Institute of Physics (National Center for Natural Science

nd Technology, Vietnam) in 2000. He is currently studying for his PhDegree at the Department of Materials Science and Metallurgy, Kyungpookational University, South Korea. His research project is carbon nanotubes

s gas sensing materials.

eung-Soo Huh received his BS and MS degrees in Materials Science andngineering from the Seoul National University in 1983 and 1985, respec-

ively. He received PhD degree from the Department of Materials Sciencend Engineering, Massachusetts Institute of Technology (USA) in 1994. He

s currently a professor at the Department of Materials Science and Metal-urgy, Kyungpook National University. He also is the director of Nationalesearch Lab of Environmental Gas Monitoring, Kyungpook National Uni-ersity, South Korea. His current research interests are metal oxide and nanoensor & electronic nose system.