Conducting polypyrrole modified with ferrocene for applications in carbon monoxide sensors

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Abstract

Polypyrrole (PPy) was modified with ferrocene (Fc) by direct incorporation during polymerization so as to increase its sensitivity for carbon monoxide (CO) gas sensor applications. Synthesis was carried out by chemical oxidative polymerization in the presence of ferrocene using ferric chloride. The presence of these species in PPy was confirmed by different characterization techniques. The CO response measurements were carried out for the sensors fabricated in a surface cell mode with active PPy + Fc films by exposing to CO gas (300 ppm) at room temperature and it was found that the response as well as the recovery characteristics of these materials is very fast and dependent on the film composition. The response factor was found to be maximum for a 1.32 mol% Fc content in the polymer. These various results can be explained on the basis of interaction of CO molecules with Fe atoms of the ferrocene together with charge transfer interaction with the PPy chain.

Introduction

The importance of environmental gas monitoring is now recognized as an important area and much research has been focused on the development of suitable gas-sensitive materials for continuous monitoring and setting off alarms for hazardous chemical vapors present beyond specified levels [1], [2]. Conducting polymers are good candidates for the development of chemical or electrochemical sensors for many reasons: (i) ease of fabrication, (ii) low cost, (iii) easy to interface with electronic devices such as MEMS, and (iv) possibility of using the same polymer with different modifications so as to have selectivity to different gases or a signature in electronic noses. There are a few reports available in literature on the application of conducting polypyrrole and polyaniline for gas sensors [3], [4], [5], [6], [7] which are mainly associated with detection of organic solvent vapors, ammonia, chlorine and other similar hazardous gases. Recently, there has been considerable interest in exploiting organic substances such as porphyrins, phthalocyanines and doped conductive polymers [8], [9], [10] for these applications. On the other hand, the detection of carbon monoxide using such polymers is not extensively studied. Conventionally, carbon monoxide (CO) has been detected mostly by using a catalyst which is held at high temperature and subsequently monitoring the changes in the temperature due to the reaction with a help of thermistor/pellistor [11], [12], [13]. MOSFET type gas sensors with tin oxide substrates coated on silicon chips were reported [14]. Alternatively, solid polymer electrolytes with proton conducting membranes have also been employed for detecting CO [15], [16], [17] by measuring the voltage generated in a fuel cell mode. However, it would be advantageous to fabricate a dry sensor with simpler design such as a chemiresistor using a sensitive material having electronic conduction rather than ionic processes so as to obtain faster response. In such a case, the electronic conductors would have to be functionalized in order to increase their sensitivity to gases at room temperature. We have modified conducting polypyrrole with iron complexes such as ferrocene (Fc) so as to improve its sensitivity to CO and obtained excellent response characteristics, which are reported in this paper.

Section snippets

Synthesis of ferrocene modified polypyrrole

Ferrocene (Sigma Aldrich, high purity grade) was incorporated in polypyrrole (PPy) during the oxidative polymerization reaction of pyrrole using FeCl3 as an initiator and doping agent [18], [19]. Two reaction media were used viz. methanol as well as distilled water. In the latter case, as ferrocene was not so soluble in water, FeCl3 was added first and then a desired amount of ferrocene (Fc) was added, followed by monomer introduction. The molar ratio of pyrrole (Py) to FeCl3 was maintained 1:1

Results and discussion

The incorporation of ferrocene in PPy was confirmed by various characterization techniques. Fig. 2 shows the FT-IR spectra of the PPy synthesized in the presence of ferrocene by a chemical route. The peaks at 1108, 999 and 811 cm−1 arising from the out-of-plane vibration of cyclopentadiene and the 476 cm−1 peak due to the asymmetric ring metal stretching vibration, which is characteristic of ferrocene molecules, are clear evidence of the presence of ferrocene in the sample. The major polypyrrole

Conclusions

The above studies clearly show that polypyrrole can be suitably modified with ferrocene so as to increase its response to carbon monoxide. There is an optimum composition at which the maximum response is obtained even at low concentration (300 ppm) of the CO gas. Typical sensors made in a surface cell configuration can be used in dry state at room temperature and it has fast response characteristics since the sensing process involves only electronic charge transport and interactions with the

S. Radhakrishnan is a senior scientist at the Polymer Science and Engineering, National Chemical Laboratory, Pune with more than 26 years experience in the R&D activities. He has been project leader for a number of nationally important projects involving electro-active polymers for applications in sensors and actuators. His research includes development of polymer blends and composites, conducting polymers, nanocomposites and materials for electronic applications.

References (33)

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S. Radhakrishnan is a senior scientist at the Polymer Science and Engineering, National Chemical Laboratory, Pune with more than 26 years experience in the R&D activities. He has been project leader for a number of nationally important projects involving electro-active polymers for applications in sensors and actuators. His research includes development of polymer blends and composites, conducting polymers, nanocomposites and materials for electronic applications.

Santosh Paul graduated from Cochin University of Science and Technology in Polymer Technology 2002. He received MSc and MTech degrees in Polymer Science & Technology from Cochin University in 1999 and 2002, respectively. He is currently carrying out his PhD research in application of conducting polymers at Polymer Science and Engineering, National Chemical Laboratory, Pune.

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