IEEE SENSORS 2006, EXCO, Daegu, Korea / October 22-25, 2006

Sensing characteristics of nano-network structure of polypyrrole for volatileorganic compounds (VOCs) gases

Cheol-Beom Lim, Joon-Boo Yu*, Do-Yeon Kim*, Hyung-Gi Byun***, Duk-Dong Lee**and Jeung-Soo Huh+

Department of Sensor and Display Engineering, *Department of Materials Science and Metallurgy, **School of ElectronicEngineering, Kyungpook National University, Sanyuk-Dong 1370, Daegu, Korea,

School of Electronic Information and Communication Engineering, Kangwon National University, Samcheok, Korea

Abstract- Nanowires of conducting polypyrrole (PPy) weresynthesized by chemical polymerization methode. The 0.18 molof pyrrole (Py) as monomer, 0.18 mol of dodecyl-benzenesulfonic acid (DBSA) as dopant and 0.26 mol of anhydrousiron( III) tri chloride (FeCl3) as oxidant were mixed and stirredin 200 ml of distilled water at 0 'C. The reaction was carriedout for three different times (6 h, 24 h, 40 h) and reaction ratewas 200 rpm. The morphology of PPy was evaluated by fieldemission scanning electron microscopy (FE-SEM). It wasfound that the morphology of nanowire PPy was affected byreaction time and aggregation rate. When the polymerizationtime was getting longer, nanowire structure was appearedclearly. The sensitivities of nanowire PPy were investigatedwith volatile organic compounds (VOCs).

I. INTRODUCTION

Volatile organic compounds (VOCs) are definedas hydrocarbon compounds with a boiling point below 2000Cthat are used as solvents in glue, and cleaning agents inchemical processing. Its speedy evaporation and toxic orcarcinogenic nature can make a deep concern to humanbeings, e.g. acetone, benzene, hexane, and toluene [1]. VOCsare also recognized as the main cause of sick housesyndrome, which is a product of poor indoor air quality. Thiscan cause or aggravate disease such as allergies, asthma,cancer, and emphysema [2]. Accordingly, in order to monitorVOCs, high performance sensor systems are required thatcan identify and measure gas and/or vapor species at roomtemperature. Gas chromatography (GC), mass spectrography(MS) and ultraviolet (UV) spectroscophy are helpful inquantifying odor gases, however, they are time-consuming,expensive, and seldom performed in real-time in the field.Therefore, these conventional methods must give a way to aspeedier procedure using an electronic nose composed of gassensors [3]-[5]. Conducting polymers have been developedand reported for use in odor detection and identification foryears [6]. Several advantages exist over other technologiessuch as few poisoning effects, room temperature operation,low power consumption, rapid response absorption /

desorption and long sensor lifetime [7]. It is well known thatthe properties of materials depend not only on their chemicalstructure, but also on their morphologies. For example,nanoscale materials possess unique properties on account oftheir finite small size and have wide range of applications tovariety of areas. Thus, the synthesis of nanoscale materialsattracted great interests in past 10 years. Among theconducting polymers, PPy is one of the most investigateddue to its high electrical conductivity and its relatively goodenvironmental stability and low toxicity. PPy nanowire hasbeen synthesized by electrochemical polymerization withscanning microneedle electrode or using microporousmembrane as a template [8]-[10]. Comparing with theelectrochemical method, the method of in-situ dopingchemical oxidation polymerization is simpler and cheaper inproducing large quantities of nanostructure PPy, because itovercomes the limitation of the area of electrode. In thispaper, we investigated the synthesis of nanowire structure ofPPy by in-situ doping chemical oxidation polymerizationmethod. And we fabricated VOCs sensor by this nanowirestructure ofPPy.

II. EXPERIMENT PROCEDURE

A. MaterialsThe monomer pyrrole (Aldrich) was distilled. Dodecyl-

benzene sulfonic acid (DBSA, Kanto Chemical Co. Inc),anhydrous iron (III) tri-chloride (FeCl3), and other reagentswere used as received.

B. Nanowire PPy synthesis by Chemical synthesis methodPyrrole monomer and DBSA were dissolved in lOOml of

deionized water by stirring vigorously. The molar ratio ofpyrrole/DBSA was kept to be 1:1. FeCl3 as an oxidizingagent was dissolved in deionized water and was added to theabove solution. The reaction was carried out for three

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IEEE SENSORS 2006, EXCO, Daegu, Korea / October 22-25, 2006

TABLE I. THREE DIFFERENT SAMPLE OF SYNTHESIS PPY

Sample 1Sample 2Sample 3

Py (mol)0.180.180.18

IFDBSA (mol)

0.180.180.18

11FeCl3 (mol)

0.260.26

Time (h)62440

different time (6 h, 24 h, 40 h) and reaction rate was 200 rpmand terminated by pouring methanol into the solution. Table1 shows three different samples of synthesis PPy. Theresultant powder was washed sequentially with methanol,distilled water and acetone, followed by filtering and dryingin a vacuum oven at 25 °C for 12 h.

C. SensorfabricationPPy VOCs gas sensor was fabricated by dipping method.

The dipping solution was prepared with 0.1 g of PPy powder,0.2 g of DBSA and 5 g of chloroform. After dipping,prepared sensor was dried in heating oven at 70 'C for 1 hunder nitrogen atmosphere. After drying, PPy sensor was

washed with methanol to remove DBSA. Then the PPysensor was heated by heating oven at 70 'C for 3 h undernitrogen atmosphere. Fig. 1 is a schematic diagram of theinterdigitated electrode structure used in this work. Itconsists of Pt/Pd alloy electrodes patterned onto the surfaceof an alumina substrate; the overlap electrode length is 6 mmand the electrode gap is 0.2 mm

D. Measuring system

The target vapor concentrations were precisely controlledby a flow system with mass flow controller (MFC),temperature controller, and measuring chamber, as shown inFig. 2. The concentration of VOC vapors was calculatedaccording to equation (1).

P x L

760 - LP *xL

+ L + L760 L

Ppy Solution

allf>>v

x 10 6

Figure 1. Schematic of PPy sensor

(1)

N,

CuIig bith

Figure 2. Schematic diagram of measuring system

where L and L' are bubbler gas flow rate and dilute gas flowrate (in sccm), respectively, and P* is the vapor pressure oftarget gas (in mmHg), and calculated according to equation(2).

lgp*=A B p*T+C

(2)

where A, B, and C are constants for a certain type of gas, Tis temperature. The operating temperature was set at 25 °Cusing a water cooling bath. Nitrogen was used as a carriergas. To measure the resistance change of sensors, 1.2V was

provided to the series circuit combining sensors and fixedresistances. The effective measure sensitivity (S) is definedas equation (3).

S KRR PX100% (3)

Ra is the initial resistance of the sensor and Rg theresistance of the sensor when it was exposed to the target gas.

III. RESULTS AND DISCUSSION

A. MorphologySEM measurement, as shown in Fig. 3, characterized the

morphology of nanowire structure of PPy prepared by thechemical oxidation polymerization. Different surfacemorphologies are obtained by different chemical reaction

Ppy layer time. Fig. 3 (a) shows the PPy image which has reaction timefor 6 h. Fig. 3 (b) shows the PPy image which is reaction

Pt (Al) time for 24 h and Fig. 3 (c) is 40 h. As shown in Fig. 3chemical reaction time was greatly affected to surface

El morphology of PPy. The nanowire structure was not

A1203(PCB) appeared in Fig. 3 (a). But, the nanowire- network structurewas appeared in Fig. 3 (b) and (c). When the reaction timewas getting longer nanowire structure appeared clearly. WithPPy which reaction time was 40 h, nanowire width was lessthan 100 nm. In case the PPy which reaction time was 24 h,nanowire width was from 100 nm to 200 nm.

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The thinner width and clearer nanowire structure of PPyhas a larger surface area. It means that VOCs gas can havemore adsorption to thinner and clearer nanowire structure ofPPy.

--1.21-I

t -0.9

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FeCIl 6H

.

300 ppm 600 ppm 1000 ppm

Concentration (ppm)

-1.5 L Benzene gas

FeCIl 6H

300 ppm 600 ppm 1000 ppm

Concentration (ppm)

Figure 4. Variation of sensitivity with VOCs gas concentration ofnanowire PPy sensors.

B. SensitivityThe sensitivity of nanowire structure of PPy sensor upon

exposure to four kinds ofVOCs gases (methanol gas, ethanolgas, toluene gas, benzene gas) in the range of 300 ppm to1000 ppm was shown in Fig. 4. The sensitivity of 40 hchemical reaction time of PPy sensor was much higher thanother two sensors to four kinds of VOCs gases. The 40 hchemical reaction time of PPy sensor has a largest surfacearea, because it has thinner width and clearer nanowirestructure. So, with 40 h reaction time PPy sensor with 40 hreaction time has more area for VOCs gases adsorption. Allof three kinds of PPy sensor were indicated that linearsensitivity increase during the VOCs gas concentrationincrease. Nanowire PPy sensors have better sensing propertyto alcohol type gases such as methanol and ethanol compare

with benzene type gases such as toluene and benzene,because, molecule size of alcohol type gases is smaller thanbenzene type gases. While alcohol type gases have a -OHradical, benzene type gases have not a -OH radical. The -

OH radical reacts and changes the resistance actively tonanowire PPy sensor

C. ReproducibilityFig. 5 shows the response behavior of PPy sensor with

40 h reaction time under VOCs (methanol, ethanol, toluene,benzene gas 1000 ppm) gases. The reproducibility of thesensor was tested for 5 cycles. The injection time was 5 minand recovery time was 5 min. As shown in Fig. 5, thesensitivity base-line was very stable under methanol,

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0 500 1000 1500 2000 2500 3000 3500

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Time (s)

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._

-0.5-0

-1 .0

500 1000 1500 2000 2500 3000 3500 4000

Time (s)

-In

OutBenzene gas 1000 ppm

0 500 1000 1500 2000 2500 3000 3500 4000

Time (s)

Figure 5. Reproducibility of nanowire PPy sensor to VOCs gases.

ethanol and toluene gases. But, the sensitivity base-line wasunstable under benzene gas.

IV. CONCLUSION

We have demonstrated synthesis of nanowire structure ofPPy by in-situ doping chemical oxidation polymerizationmethod. We investigated application to VOCs gas sensor

using nanowire structure of PPy. Surface morphologies ofPPy can be controlled by different reaction times. Nanowirestructure was appeared clearly in 40 h reaction time of PPy.The width of nanowire was less than 100 nm. So, surfacearea was largest in PPy with 40 h reaction time. Thesensitivity of PPy sensor with 40 h reaction time was higherthan other two sensors under VOCs gas (methanol gas,ethanol gas, toluene gas, benzene gas). The reproducibility of40 h reaction time of PPy sensor was stable under methanol,ethanol and toluene gases.

ACKNOWLEDGMENT

This work was supported in part by MIC and IITAthrough IT Leading R&D Support Project, by Korea Instituteof Environmental Science and Technology through Eco-Technopia 21 project. The authors thank to government forfinancial supports.

REFERENCES

[1] J. H Seinfeld, "Atmospheric chemistry and physics if air pollution,"John Wiley & sons, New York, 1995, pp. 36-42.

[2] N. Irving Sax and R. J. lewis, "Hawley's condensed chemicaldictionary," 11th ed, VNR, New York, 1987, pp. 177-178, 704-969

[3] T. Godish, "Indoor sir pollution control," Lewis Publishers, Michigan,USA, 1991, pp. 42-48.

[4] H.G. byun, K.C. persaud, S.M. khaffaf, P.J. Hobbs and T.H.Misselbrook, "Application of unsupervised clustering method to theassessment of malodour in agriculture using an array of conductingpolymer odour sensors," Computer and Electronics in Agriculture,vol. 17, pp. 233-247, 1997.

[5] J.W. Gardner, P.N. Bartlett, "A brief history of electronic noses,"Sensors and Actuators B, vol. 18, pp. 211-220, Mar. 1994.

[6] M.C. horrillo, J. Getino, L. Ares, I. Sayago and F.J. Gutierrez,"Measurements of VOCs with a semiconductor electron nose," J.Electrochem. Soc, vol. 145, pp. 2468-2489, 1998.

[7] K.C. persaud, S.M. khaffaf, J.S. Payune, A.M. Pisanelli, D.H. Leeand H.G. Byun, "Sensor array techniques for mimicking mammalianolfactory system," Sensors and Actuators B, vol. 36, pp. 267-273, Oct.1996.

[8] Yunze Long, Zhaojia Chen, Jean Luc Duvail, Zhiming Zhang,Meixiang Wan, " Electrical and magnetic properties of polyanilineFe3O4 nanostructures," Physica B, vol. 370, pp. 121-130, Sep. 2005.

[9] Yun Tian, Jixiao Wang, Zhi Wang, Shichang Wang,"Electroreduction of nitrite at an electrode modified with polypyrrolenanowires," Synthetic Metals, vol. 143, pp. 309-313, Dec. 2004.

[10] Kyung T. Kim, Sung M. Cho, "A simple method for formation ofmetal nanowires onflexible polymer film," Materials Letters, vol. 60,pp. 352-355, Aug. 2006

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