Gas and humidity sensors based on iron oxide–polypyrrole nanocomposites

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Abstract

Nanocomposites of iron oxide and polypyrrole were prepared by simultaneous gelation and polymerization process. This resulted in the formation of mixed iron oxide phase for lower polypyrrole concentration, stabilizing to a single cubic iron oxide phase at higher polypyrrole concentration. The composites in the pellet form were used for humidity and gas sensing investigations. Their sensitivity to humidity was found to increase with increasing concentration of polypyrrole. Gas sensing was performed for CO2, N2 and CH4 gases at varying pressures. The sensors showed a linear relationship between sensitivity and pressures for all the gases studied. The sensors showed highest sensitivity to CO2 gas.

Introduction

The increased concern about environmental protection has led to continuous expansion in sensor development. Humidity sensors have attracted lot of attention in medical and industrial fields. The measurement and control of humidity is important in many areas including industry (paper, electronic), domestic environment (air conditioner), medicine (respiratory equipment), etc. Also, gas sensors having good sensitivity to gases such as methane, hydrogen, carbon dioxide, carbon monoxide, etc. are utmost in demand. Different criteria are used for measuring sensitivity to humidity and gases, like changes in mechanical, optical and electrical properties [1], [2]. Electrical detection is the most commonly used and is based on the change in resistance or capacitance of the sensor on exposure to water vapor and gases.

In recent years, inorganic semiconducting oxides like zinc oxide (ZnO), aluminium oxide (Al2O3), titanium oxide (TiO2), tin oxide (SnO2), iron oxide (Fe2O3), etc. have been studied extensively and have emerged as economical sensors for monitoring toxic gases and humidity [3], [4], [5], [6]. The sensitivity of these sensors to gas and humidity depends on the microstructure. This can be achieved by adopting special techniques of preparation or by doping impurities. It has been found that doping of SO42−, Ti, Sn, Zn, Si, etc. in α-Fe2O3 has been found to improve the sensing capabilities [7], [8], [9], [10]. The sol–gel process is an excellent method of producing highly porous and nanosized ceramics and oxides [11], [12] and these have been extensively investigated as sensor materials for gas and humidity sensing [13], [14], [15].

Several organic semiconducting materials, viz. conducting polymers such as polypyrrole, polyaniline, polythiophene, polyacetylene, etc. find a variety of applications as electronic and optoelectronic devices [16], [17]. Many conducting polymers have shown changes in resistivity on exposure to different gases and humidity [18], [19], [20]. The demand of conducting polymers in sensor application has been tremendous due to its ease of synthesis. These materials exhibit interesting electrical properties such as their ability to oxidize and reduce at specific electrochemical potential. Conducting polymers like polypyrrole and polyaniline have shown capability in sensing technology and are used as sensors for air borne volatile organic compounds referred to as electric nose, especially for detection of alcohols, NO2, etc. Polypyrrole has shown interesting ammonia, NH3, and NO2 gas sensing properties at room temperature [18], [19]. Though polypyrrole is highly sensitive to gases, yet it shows saturation effect at higher concentration of gases [18].

The inorganic sensors based on oxides have been found to be less sensitive to gases and humidity as compared to the conducting polymers, which show high sensing behavior due to their porous nature. However, the instability of some of the conducting polymers in air has limited their commercialization as sensors [16].

In the present paper, the inorganic–organic hybrid nanocomposite containing polypyrrole as the organic part and iron oxide as the inorganic part have been used for studying humidity and gas sensing properties. Such types of nanocomposite have shown to possess small grain size and high stability in air [21], [22]. To the best of our knowledge, this is the first ever attempt made to study these composites as humidity and gas sensors.

In order to determine the grain size and structural properties of the nanocomposites, several structural investigations using X-ray, IR, scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques have been carried out for varying polypyrrole concentration and the results are being presented here. The sensing properties of these nanocomposites were studied at different values of relative humidity (RH) and gas pressures for CO2, N2 and CH4.

Section snippets

Experimental details

The composites of iron oxide and polypyrrole were prepared by adding different amounts of pyrrole monomer to a mixture of ferric nitrate and methoxy ethanol in a certain specific ratio. This mixture was heated at 150 °C to evaporate the solvent, resulting in composite powders. These powders were then annealed at 300 °C. The powders were then compressed into pellets of diameter ∼1.2 cm and thickness ∼0.1 cm for further studies.

The crystal structure of the powders was examined by using a Rigaku

Results and discussions

Fig. 1a and b shows the X-ray diffraction pattern for the nanocomposites. The X-ray diffraction pattern for powder having lower concentration of polypyrrole (5%) has shown presence of both hexagonal and cubic phases of iron oxide (Fig. 1a). Addition of 15% polypyrrole results in a single cubic phase (Fig. 1b) with lattice constant a=8.54 Å. The FTIR transmission spectra of the powders using KBr pellets having different concentration of polypyrrole was recorded in the range 400–4000 cm−1 to

Conclusion

The nanocomposites of iron oxide and polypyrrole prepared by the sol–gel process were used to investigate the humidity and gas sensing properties. These nanocomposites showed better humidity sensing as compared to α-Fe2O3. The sensitivity was found to be high for the sensor with higher polypyrrole concentration. The possible mechanism for such an increase has also been suggested. A sharp increase in sensitivity was observed at higher values of RH, depending on the polypyrrole concentration.

Acknowledgements

The authors wish to thank Dr. Harkishan and Mr. Bheekam Singh, National Physical Laboratory, New Delhi for extending help in carrying out humidity studies. We would also like to thank Dr. N.C. Mehra, USIC, New Delhi for carrying out SEM and TEM studies. One of the authors (K.S.) wishes to thank CSIR, New Delhi, for providing fellowship and all other financial helps.

Komilla Suri received her MSc from Department of Physics, Delhi University and is currently working as senior research fellow in the same department. As a PhD student, her research interests include conducting polymers for various applications.

References (24)

  • C. Cantalini et al.

    Microstructure and humidity sensitive characteristics of α-Fe2O3 ceramic sensor

    J. Am Ceram. Soc.

    (1992)
  • H. Yagi et al.

    Humidity sensor using Al2O3, TiO2 and SnO2 prepared by sol–gel method

    J. Ceram. Soc. Jpn.

    (1992)
  • Cited by (0)

    Komilla Suri received her MSc from Department of Physics, Delhi University and is currently working as senior research fellow in the same department. As a PhD student, her research interests include conducting polymers for various applications.

    S. Annapoorni is a PhD in Physics from IIT, Chennai (1990). At present she is reader in Delhi University. Her research interests include hydrogen absorption in metals, magnetic properties of rare earth transition metals, metal oxides and conducting polymers. The recent interest is in magnetic and sensing properties of nanocomposites.

    A.K. Sarkar is a PhD in Analytical Chemistry from Burdwan University, India. He is working as senior scientist and head of chemistry section at the National Physical Laboratory, New Delhi. He has 35 years research experience in modern instrumental methods of analysis and has published over 90 research papers in different fields of chemistry. He holds five Patents including one each from USA and South Africa.

    R.P. Tandon is a PhD from the Department of Physics, Delhi University. He is a visiting scientist at MIT (USA), Queens University (Canada) and is currently a professor of physics in the Delhi University. Earlier, he was senior scientist at the National Physical Laboratory, New Delhi. His research interests include ceramics, polymers and glasses and has published over 100 research papers in these areas.

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