[4-66]nh3 sensitive chem ire sis tor sensors using plasma function ali zed multiwall carbon...

7/31/2019 [4-66]NH3 Sensitive Chem Ire Sis Tor Sensors Using Plasma Function Ali Zed Multiwall Carbon Nanotubes-Conducti

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NH3 Sensitive Chemiresistor Sensors UsingPlasma Functionalized Multiwall Carbon

Nanotubes/Conducting Polymer Composites

Tai-Jin Kim, Si-Dong Kim*, Nam-Ki MinDepartment of Control and Instrumentation Engineering

Korea UniversityJochiwon, Korea

*Auto Electronic [email protected]

James Jungho Pak, Cheol-Jin Lee,Soo-Won KimSchool of Electrical Engineering

Korea UniversitySeoul, Korea

AbstractWe present a micromachined NH3 chemiresistor

sensor based on O2 plasma functionalized multiwall carbon

nanotube (MWCNT)/ conducting polymer (CP) composites,

and discuss its gas sensing mechanism. The FTIR spectra ofplasma-treated MWCNTs indicate that oxygenated groups are

created on the surface of CNTs after the plasma treatment.

The plasma-functionalized MWCNT/PANI sensor exhibits a

linear response of 3.34% per ppm NH3 for concentrations

ranging from 10 to 100 ppm, and a sensitivity of about 7 times

higher than that from a corresponding pristine MWCNT

sensor. This dramatic change in resistance is attributed to the

fact that the binding of the functional group with the defective

CNT is stronger than that with the perfect one.

I. INTRODUCTION

Ammonia (NH3) occurs naturally and is produced by

human activity. It is an important source of nitrogen which isneeded by plants and animals. No health effects have beenfound in humans exposed to typical environmentalconcentrations of ammonia. However, exposure to highlevels of ammonia can cause irritation and serious burns onthe skin and in the mouth, throat, lungs, and eyes. At veryhigh levels, ammonia can even cause death. Ammonia is alsoflammable at concentrations of approximately 15% to 28%by volume in air. The United States Occupational Safety andHealth Administration has set an acceptable 8h exposurelimit at 25 ppm and a short-term (15 min) exposure level at35 ppm by volume [1]. Thus many efforts have been madetowards the development of gas sensors for the propercontrol and detection of ammonia in the range of 0.1-100

ppm levels. There are many principles for measuringammonia[2]: metal-oxide gas sensors, catalytic ammoniadetectors, and conducting polymer sensors. Although metaloxide sensors are highly sensitive, they require highoperating temperature (>200 C) in order to maintain highsensitivity levels [2,3]. Conducting polymer-based sensorshave been observed to offer short response time [4,5];

however these devices suffer from limited sensitivity [2,5].Recently, it was reported [6,7] that carbon nanotubes couldbe potentially used in a wide variety of chemical, gas, andbiosensors. Carbon nanotube-based sensors offer significantadvantages over traditional gas sensors such assemiconducting metal oxide- and polymer-based gas sensors.They can be operated at room temperature, offer greaterconductivities, and are smaller than conventional gassensors. Most CNT-based chemiresistors uses the mixture ofCNTs with traditional bulk materials, typically polymer. Forthese nanocomposite devices, the matrix materials detects theanalyte of interest, and the CNTs embedded in the matrix actas electrical conduction pathways from the matrix to themetal electrodes. NH3 gas sensors with various devicearchitectures and matrix materials have been fabricated usingCNT itself [6,8,9] and CNT/polymer nanocompositematerials[10,11].

In this paper, we demonstrate a micromachined NH3chemiresistor sensor based on plasma-functionalizedmultiwall carbon nanotubes (p-MWCNT)/conductingpolymer(CP) to enhance the sensitivity and impart theselectivity to sensors. Plasma-treatment of carbon nanotubeshas been performed to facilitate the CNT-PC bond in thenanocomposite materials and to promote conduction betweenthe CNTs and the CP.

II. SENSORFABRICATION

Fig. 1 shows a NH3 sensor based on plasmafunctionalized MWCNT/conducting polymer(CP) composite.

MWCNTs were synthesized by the catalytic decompositionof C2H4 over Fe/Mo/Al2O3 catalyst at 923 K in an Ar/H2atmosphere. The procedure is like that described in theprevious paper [12]. A mixture of Fe(NO3)39H2O and Mowas dissolved in ethanol, and this solution was added toaluminum isopropoxide dissolved in ethanol. The catalystwas calcined at 923 K in O2 for 2h, followed by a

1-4244-2581-5/08/$20.00 2008 IEEE 208 IEEE SENSORS 2008 Conference

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mechanical grinding for several hours. For the production ofMWCNTs, C2H4, Ar, and H2 were introduced into the quartztube at flow rates of 300, 500, and 500 sccm, respectively.The MWCNTs were purified by the treatment with 3 N nitricacid. The MWCNTs (1g) and 3 N nitric acid (400 mL) wereadded into a glass flask, and the MWCNTs/ acid mixture wasthen subject to sonication for 30 min, followed by heating at

333 K for 12 h with continuously stirring. After the acidtreatment, the product was filtered on a membrane filter(PTFE 0.5m), washed to neutral pH, and dried at 393 K for12 h.

The microhotplate consists of the following stackedlayers: a low-stress Si3N4 layer, Pt/Ta layers for microheaterand thermometer(RTD), Si3N4 /SiO2 layers to insulate theheater from the electrodes, and Pt electrodes. The fabricationprocess starts with the LPCVD deposition of low-stresssilicon nitride layers (8000 ) to be used as a supportingmembrane for the devices. For microheater and RTD2000/200 thick Pt/Ta layers were deposited on the devicecenter by lift-off with optical lithography using negative

photoresist. The patterned heater and RTD were coveredwith 0.5-mm-thick CVD Si3N4 /SiO2 layers to isolate a 200nm thick Cr electrodes from the heater. The dispersedMWCNT solution was accurately drop deposited across Crelectrodes by a micromanipulator. The sample was heated upto 100 using microheater to enhance the adhesion of CNTfilm. Then, the oxygen plasma treatment was carried out at apower of 30 W. The temperature of the microheaters wascontrolled by external current. Polyaniline powder wasdissolved in methylpyrrolidone, in a polymer:solvent weightratio of 3:100, and sonicated for 5 hours. A 20 ml of 3wt%PANI solution in a 1 M HCl was stirred for 5 h and dried for24 h at room temperature. PANI films were prepared bydirect deposition from PANI solution onto the electrode set,

followed by air drying. A backside silicon etching with KOHat 80 was used to form a 1.5mm x 1.5mm Si3N4 membrane.

For comparative purposes, two NH3 sensors were alsoformed using different sensing materials; a pristine CNT andMWCNT/Nafion composites.

Fig. 1 (a) Micrograph of a microhotplate, and (b) schematic representationof the structure of micromachined MWCNT/CP NH3 sensors.

III. RESULTS AND DISCUSSION

The FTIR absorption spectra of plasma-treatedMWCNTs are showed in Fig.2. They show adsorption peakscorresponding to stretching vibrations of C=O and C-O. Thisindicates that oxygenated groups were created on the surfaceof CNTs after the plasma treatment. O2 plasma treatment isexpected to create defects such as vacancies in the sidewallsof CNTs, as well as at the open ends. If a vacancy isfunctionalized with O2, dissociation is observed, resulting inone C=O and C-O-C functional group [13,14]. Thus,treatment with oxygen plasma gives rise to carbonyl groupsrather than hydroxyl groups, although OH groups havehigher interaction energy with a perfect CNT wall ascompared to other types of oxygen-containing groups [13].

Fig. 2 FTIR absorption spectra of plasma-treated MWCNTs for differentplasma treatment time.

Fig. 3 shows the XPS survey spectra after the oxygenplasma treatment. These spectra reveal the presence ofcarbon and oxygen on the MWCNT samples. Theoxygenated surface groups are suggested to be the anchoring

sites for functional groups or gas molecules [14].

Fig. 3 XPS spectra of plasma-treated MWCNTs for different plasmatreatment time.

The concentration of oxygen evaluated by FTIR and XPSwas found to increase with increasing plasma treatment timeup to 60s and then gradually decrease for treatment timebeyond 60s. When a strong plasma treatment, 30W/90s or100W/30s, was used, the CNT surface was etchedchemically. This caused CNT to be thinner or bent, resulting

Cr electrode

Si3N4membrane Si

Pt heater

RTD MWCNT/PANI composite

Pt heater

Electrodes

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in the decrease of oxygen concentration on the sample, asshown Fig. 2 and 3.

Fig. 4 shows the response curves of plasma-functionalized MWCNT/PANI sensor for four consecutiveexposures to 100 ppm of NH3. After the first exposure toNH3, replacement of the analyte with N2 decreases the

resistance to a level that is maintained in subsequentrecoveries from exposure to NH3.

0 1000 2000 3000 4000 5000 6000

1.000

1.005

1.010

1.015

1.020

1.025

R/R0

Time (s)

N2

NH3

Fig. 4 Repetitive response of the plasma-functionalized MWCNT/PANI gassensor to NH3 gas.

This dramatic change in resistance is attributed to thefact that the binding of the functional group with thedefective carbon nanotube is stronger than that with theperfect one. For nanocomposite chemiresistor devices, theMWCNTs are primarily used to increase the conductivity ofthe materials. Generally, the matrix material detects theanalyte of interest, and the MWCNTs embedded in thematrix act as conduction pathways from the matrix to themetal electrodes. During doping, neutral PANI moleculesgains protons, forming N+-H chemical bonds (protonation).

So positively charged local centers placed at nitrogen areformed, as shown in Fig.5.

Fig. 5 NH3 gas detection mechanism of the plasma- functionalizedMWCNT-PANI gas sensor.

These doped states can be controlled by acid/basereactions. When PANI is exposed in ammonia gas, anammonia molecule takes up protons from PANA and thenthe protons on NH groups are transferred to NH3molecules, thus forming energetically more favorableammonium ions (NH4

+), while PANI itself turns into its base

form [4]. This is the PANI dedoping by deprotonation(Fig.6). This process is reversible, and when ammoniaatmosphere is removed, the ammonium ion can bedecomposed to ammonia gas and proton. The deprotonationreaction causes the electrical resistance of the PANI layer toincrease [15-17].

The sensitivity of the plasma-functionalized MWCNT-PANI sensor was also compared to those of pristineMWCNT and MWCNT/nafion sensors. As shown in Figs. 6and 7, the response of pristine MWCNT-based sensors arenon-linear, while the plasma-functionalized MWCNT/PANIsensor exhibits a linear response of 3.34% per ppm NH3 forconcentrations ranging from 10 ppm to 100 ppm, and asensitivity of about 7 times higher than that from acorresponding pristine MWCNT sensor. This high sensitivitymay be due to a presence of oxygen at the surface of O 2plasma-functionalized MWCNTs, providing the anchoringsites for functional groups.

A plasma-functionalized MWCNT/nafion sensor showsbetter sensitivity and better accuracy than a pristineMWCNT sensor, but with a much lower sensitivity and ahigher low limit (detection limit) around 20 ppm, comparedto plasma-treated MWCNT/PANI devices.

0 20 40 60 80 100

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

R : R-R0

R0

: Initial resistance

Pristine MWCNT

P-MWCNT

Concentration (ppm)

R/R

0

Fig. 6 Variation ofR/R0 of theplasma-functionalized MWCNT exposedto different NH3 concentrations.

0 20 40 60 80 100

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

MWCNT/PANI

P-MWCNT/PANI

Concentration (ppm)

R

/R0

Fig. 7 Variation ofR/R0 of the plasma-functionalized MWCNT/PANIgas sensorat 0-100ppm NH3 concentration.

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20 40 60 80 100

0.008

0.010

0.012

0.014

0.016

0.018

0.020

0.022

0.024MWCNT/Nafion

P-MWCNT/Nafion

Concentration (ppm)

R/R

0

Fig. 8 Variation ofR/R0 of the plasma-functionalized MWCNT/Nafiongas sensorat 0-100ppm NH3 concentration.

Fig. 9 depicts the variation of R/R0 with plasmatreatment, indicating linear increase in sensitivity withtreatment time up to 60s, and then decrease for treatmenttime beyond 60s, which may be due to the decreasednumber of defects on the MWCNT surface available forinteraction with PANI and NH3 molecules, as evident fromFigs.2 and 3.

0 20 40 60 80 1000.000

0.003

0.006

0.009

0.012

0.015

R/R

0

Plasma treatment time (s)

Plasma treatment power : 30W

Fig. 9 Variation ofR/R0 of the plasma-functionalized MWCNT/PANIgas sensorat 100ppm NH3 concentration.

IV. CONCLUSION

In summary, we have demonstrated a new O2 plasmafunctionalized multiwall carbon nanotube/ conductingpolymer composite chemiresistor sensor. The sensitivity of

MWCNTs/PANI composite sensor to NH3 gas was affectedsignificantly by oxygen plasma treatment. The presence ofoxygen on the surface of plasma functionalized MWCNTwas found to increase the sensitivity to NH3. The plasma-functionalized MWCNT/PANI sensor exhibited a linearresponse of 3.34% per ppm NH3 for concentrations rangingfrom 0 to 100 ppm. These results suggest thatnanocomposites of MWCNTs and conducting polymersoffer great promise for the practical realization of highsensitivity NH3 gas sensors.

ACKNOWLEDGMENT

This work was supported by grant No. K20601000002-07E0100-00210 from Korea Foundation for InternationalCooperation of Science Technology.

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