Room temperature NO2 gas sensor based on PPy/α-Fe2O3 hybrid nanocomposites
Introduction
In order to prevent human from being harmed by toxic gas and to protect the environment, the detection of toxic gas has become increasingly important. The sensing materials, which are inorganic or organic, have been studied widely [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. But both of them have particular advantage and drawbacks. Though inorganic semiconductors, such as SnO2, Fe2O3, and ZnO, either undoped or doped have been studied extensively [1], [2], [3], [4], [5], [6] and have emerged as economical sensors for monitoring toxic gases, the disadvantages of semiconductor oxide sensors are their high operating temperature (350–450 °C) [15], [16], which increases power consumption, reduces sensor life, and limits the portability. Several organic semiconductors, such as polypyrrole, polyaniline, polythiophere, polyacetylene, etc. also have gas sensitivity at normal temperature [7], [8], [9], [10], [11], [12], [13], [14]. Polypyrrole is the most studied among these, but the self-limitations reduce the application largely. Because PPy is sensitive to electrochemical and chemical degradation and the conductivity exhibits a long-term irreversible decay due to the irreversible attack of oxygen present in the ambient, and the long response time due to the highly ordered structure [17].
To prepare the optimized properties materials, organic–inorganic sensing hybrids have been developed [18], [19], considering which can complement the characteristics of pure inorganic and organic materials. Nanocomposite thin films of polymer–inorganic nanoparticles are reported to have porous morphology caused by the solid-state polymerization [20], [21]. The porous morphology has added advantage for the gas sensing application due to readily penetration of gas molecules into the polymer matrix. The relating studies for gas sensors were studied. Benjamin et al. [22] reported the polypyrrole/SnO2 sensor had sensitivity to organic vapors at room temperature. Suri et al. [23] studied the pressure and humidity sensitivity of polypyrrole/iron oxide. These types of hybrid materials have been shown to possess small grain size and high stability in air [24], [25]. Nardis et al. [26] reported that cobalt porphyrin/tin dioxide had superior selectivity to methanol vapor than to CO. Hosono et al. [27] reported the polypyrrole/MoO3 hybrid thin film and their volatile organic compound gas sensing properties. But the research of the room temperature NO2 gas sensing properties of spin coated PPy/α-Fe2O3 hybrid nanocomposites by using solid state synthesis route has not been reported.
In the present communication, organic–inorganic hybrid nanocomposite containing polypyrrole as an organic part and iron oxide as an inorganic part was successfully prepared by solid state synthesis route. Hybrid nanocomposite thin films were prepared by spin coating technique on a glass substrate and have been used for studying gas sensing properties. The hybrid nanocomposite films were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The hybrid nanocomposite films were exposed to NO2 gas at room temperature and the electrical response was noted. For a comparison, thin films of iron oxide, and PPy were also prepared separately, and evaluated along with the PPy/α-Fe2O3 hybrid nanocomposite films for sensing NO2 gas at room temperature. We report our findings in this paper and discuss a plausible mechanism for the formation and electronic behavior of such hybrid nanocomposites.
Section snippets
Synthesis of polypyrrole
Polypyrrole (PPy) was synthesized by chemical oxidative polymerization technique using monomer pyrrole. Analytical grade ammonium per-sulfate APS, was used as oxidizing agent. The chemical polymerization was carried out in a beaker by mixing 0.1 M aqueous solution of pyrrole and 0.1 M of APS in 1:1 ratio by volume. The polymerization was carried out for a period of 3 h. After termination of the polymerization process, the precipitate obtained was filtered. The product was washed successively by
X-ray diffraction analysis
Fig. 2 shows the X-ray diffraction (XRD) patterns of polypyrrole, nano α-Fe2O3 and PPy/α-Fe2O3 (50%) hybrid nanocomposite. Fig. 2(a) shows the X-ray diffraction pattern of PPy which has a broad peak at 2θ=20–30° indicating the amorphous behavior of the polymer [30]. The broad peak results from the scattering of X-rays from PPy chain. Fig. 2(b) shows the X-ray diffraction pattern of nano α-Fe2O3. The X-ray diffraction peak of nano α-Fe2O3 at 2θ=24.16°, 33.11°, 35.62°, 40.86°, 49.46°, 54.11°,
Conclusions
In order to develop the new gas sensors, we put emphasis on organic–inorganic materials. PPy/α-Fe2O3 (organic–inorganic) hybrid nanocomposites prepared by solid state synthesis method and were used to investigate gas sensing properties. For comparison, the gas sensitivities of PPy and α-Fe2O3 were also studied. The presence of α-Fe2O3 nanoparticles in PPy matrix has been confirmed by X-ray diffraction, Fourier transform infra red spectroscopy and field emission scanning electron microscopy.
Acknowledgment
Authors (VBP) are grateful to DAE-BRNS, for financial support through the scheme no. 2010/37P/45/BRNS/1442.
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