Sensing behaviors of polypyrrole sensor under humidity condition

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Abstract

The influence of temperature (0, 25, and 40 °C) and relative humidity (5, 10, and 20%) of polypyrrole (PPy) sensor were studied. The PPy sensors were prepared by chemical oxidative method. Initial resistance of the PPy sensor was increased with increase of humidity and decrease of temperature, respectively. Each sensitivity to methanol, water vapor, and mixture gas (methanol and water vapor) was measured, and then compared. Methanol and water vapor were recognized to different type gases in PPy sensors, and adsorption of methanol and water vapor was competed on the surface of sensors. The sensitivity was increased under low humidity and low temperature. The fastest response and recovery time was shown at 25 °C. The reproducibility of PPy sensor was excellent at various humidity conditions. The best operating temperature was investigated at 25 °C.

Introduction

The environmental importance is well understood and much research has focused on the development of suitable gas sensitive materials. Among many different types of gas sensors [1], [2], [3], metal oxide [4] and conducting polymer type sensors are most popular. The conducting polymer sensors have great advantages of high sensitivity toward volatile organic compounds (VOCs) gases, a lower detectable limit in the range of a few tens of parts per millions, and the potential to operate at near room temperature [5]. The conducting polymers like polythiophene, polypyrrole (PPy) [6], [7] and polyaniline show variations of their conductivity and initial resistance upon interaction with humidity [8], [9] and methanol gas.

In this paper, we fabricated a PPy sensor by chemical polymerization method [10], [11], [12], [13], and investigated sensing behavior to methanol gas under various humidity conditions [14], [15] and temperatures using flow system.

Section snippets

Fabrication of sensor

The PPy polymer was synthesized by chemical polymerization at 0 °C under atmospheric condition. The pyrrole (Aldrich) of 0.23 mol as monomer and dodecylbenzenesulfonic acid (DBSA, Kanto Chemical) of 0.12 mol as dopant were mixed and stirred. Ammonium persulfate (APS, Kanto Chemical) of 0.06 mol as oxidant was added to this solution for the purpose of initiation of polymerization. This solution was maintained for 6 h at 0 °C and then methanol was poured into this solution to terminate the

Results

In this study, the sensing properties were investigated by controlling range of temperature and relative humidity (RH). Initial resistance of the PPy sensors was shown in Fig. 2. It was increased according to increase of humidity and decrease of temperature.

It could be explained that the number of charge carrier hole in PPy layer were reduced by absorption water vapor from humid atmosphere.

To explain the aspect of methanol and water vapor absorption, measurements were performed in two different

Conclusions

Initial resistance was increased with the increasing humidity level and decreasing temperature. The sum of sensitivity to methanol and water vapor was larger than mixture gases. As the relative humidity increases from 5 to 20%, the sensitivity of methanol vapor was decreased. The reproducibility of PPy sensor was excellent during 10 on-off cycles by switching between N2 and 1000 ppm methanol gas, indicating that our sensor could be used cyclically at various humilities. The sensitivity of

Acknowledgments

Authors wish to appreciate the financial support of National Research Lab. from Ministry of Science and Technology, and KNURT Fund, 2003, Korea.

Jae-Hyun Cho received BS degree from the Department of Material Science and Metallurgy of Kyungpook National University, Daegu, Korea, in 2004. At present, he is working in MS degree course in the Department of Material Science and Metallurgy of Kyungpook National University. He is studying on VOCs monitoring system using conducting polymer sensor.

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Jae-Hyun Cho received BS degree from the Department of Material Science and Metallurgy of Kyungpook National University, Daegu, Korea, in 2004. At present, he is working in MS degree course in the Department of Material Science and Metallurgy of Kyungpook National University. He is studying on VOCs monitoring system using conducting polymer sensor.

Joon-Boo Yu received MS degree in Electronic Engineering in 1996 from Kwandong University, Korea and he is Ph.D candidate of the Department of Material Science and Metallurgy of Kyungpook National University. He is studying on VOCs monitoring system using conducting polymer sensor.

Jung-Suk Kim received her BS degree in Industrial Chemistry from Keimyung University in 2003. At present, she is working in MS degree course in Nano-Science Technology at Kyungpook National University. Her current research interest is conducting polymer gas sensor.

Sung-Ok Sohn received her BS, MS and Ph.D from Kyungpook National University, Taegu, Korea. Her research areas are polymer materials and semi-conducting polymer sensors. She is working as post-doctoral fellow.

Duk-Dong Lee received Ph.D degree from Yon-Sei University, Seoul, Korea, in 1984. He is currently a professor at the School of Electrical Engineering and Computer Science, Kyungpook National University, Daegu, Korea. He has performed research on semiconductor gas sensors since 1978, and also performed a research in the field of humidity and optical sensors.

Jeung-Soo Huh received Ph.D from the Department of Materials Science and Engineering (Electronic Materials) from the Massachusetts Institute of Technology, USA, in 1993. He is directing National Research Lab (Environmental Gas sensor) MOST, Korea. His research areas are chemical sensor, biosensor and e-nose system.

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