Polypyrrole, α-Fe2O3 and their hybrid nanocomposite sensor: An impedance spectroscopy study
Graphical abstract
Introduction
In recent years, organic conducting polymers such as polyaniline, polythiophene, polypyrrole and their derivatives have shown a great potential in technological applications. Their good environmental stability, light weight, chemically tunable properties and ease of synthesis makes them useful in optoelectronic, electromechanical and electronic devices [1], [2], [3]. One of the conducting polymer, polypyrrole, is widely used in sensor application because of their room temperature operation, high sensitivity and selectivity towards various target gases [4], [5], [6]. Furthermore, the monomer pyrrole can be easily oxidized and water soluble [4], [5]. However some of the drawbacks with polypyrrole is their solubility, lack of specificity and limited mechanical strength [7], [8] Recently, inorganic metal-oxide semiconductors, such as SnO2, Fe2O3, WO3, ZnO, etc. have been extensively investigated for gas sensor applications because of their capability of detecting various toxic gases under different conditions [9]. However, the disadvantage with metal-oxide semiconductor based sensors is their higher operating temperature, which reduces sensor life; increases power consumption and limits the portability [10], [11]. Combining the properties of organic and inorganic materials will help in the generation of the new class of gas sensing materials with different and synergistic properties called as hybrid nanocomposites. Such materials can remove the drawbacks coming from the individual counterparts and also improves the gas sensing properties [12], [13]. There are several reports available to investigate such kind of hybrid nanocomposites for the application of gas sensor [14], [15], [16], [17].
Generally, detection of gases is carried out by measurement of dc resistance of the sensor films as well as dc measurements gives information on the sensor response. On the other hand, ac measurements offer a powerful tool to examine the nature of conduction processes [18], [19]. An impedance spectroscopy is an electrical technique that has been widely used for characterizing physical properties of electrochemical materials, conduction mechanism in solid state electrolytes, mechanisms of chemical reactions and measuring characteristics as well as nature of conduction processes in chemical sensors [18], [19], [20], [21]. Impedance spectroscopy technique has been widely used for distinguishing between different contributions to sensor response arising from different sources like bulk, grain boundaries, and electrode-sample interface regions. Impedance spectra have been plotted in the form of imaginary component of impedance as a function of real component with varying frequency as a parameter. Different contributions to overall resistivity of the sensors have been determined by fitting of the experimental curve to an equivalent circuit [19]. The impedance spectroscopy technique system is perturbed by a time dependent potential in the form of V = V0sin(ωt), while the output signal is given by I = I0sin(ωt + Ф), where Ф is the phase angle and ω = 2πf (f = frequency of A.C. Signal). The ratio of V/I is a complex number which determines the impedance (Z) at the corresponding frequency. The real part (Z′) and imaginary part (Z″) of the impedance and phase angle depends on the particular nature of the dominant conductive behavior, such as capacitive, inductive or resistive present in the system at a given frequency range [22].
In the present paper, impedance spectra of PPy, α-Fe2O3 and PPy/α-Fe2O3 nanocomposite thin films in presence of air and oxidizing NO2 gas was carried out and proposed sensing mechanism is explored. Thin films of PPy, α-Fe2O3 and PPy/α-Fe2O3 nanocomposites were deposited on glass substrates using spin coating technique and characterized using FTIR, FESEM, TEM and XPS techniques. The impedance spectroscopy measurements in this study were carried out from low frequencies to high frequencies (20 Hz–10 MHz).
Section snippets
Experimental and characterization techniques
Polypyrrole was prepared by chemical oxidative polymerization method at room temperature, α-Fe2O3 nanoparticles were prepared by sol–gel method and annealed at 700 °C and PPy/α-Fe2O3 hybrid nanocomposites were prepared by solid state synthesis methods at room temperature (38 °C). The detailed synthesis procedure of PPy, α-Fe2O3 nanoparticles and PPy/α-Fe2O3 hybrid nanocomposites has been described in our previous reports [23], [24], [25]. Films of PPy, α-Fe2O3 and PPy/α-Fe2O3 hybrid
FTIR analysis
The chemical structure of α-Fe2O3, PPy and PPy/α-Fe2O3 hybrid nanocomposites was analyzed using FTIR spectroscopy and displayed in Fig. 2. FTIR spectrum for α-Fe2O3 (Fig. 2(a)) with peaks at 679 cm−1 and 1023 cm−1 is attributed to FeO vibration modes [24].
The characteristic peaks observed at 1416 and 1567 cm−1 are due to OH bending vibrations. While, the broad peak at 3436 cm−1 is due to OH group [24]. FTIR spectrum of PPy (Fig. 2(b)) shows the peaks at 792 and 926 cm−1 are belongs to the CH
Conclusions
Impedance spectroscopy measurements before and after exposure of NO2 gas have been carried out on PPy, α-Fe2O3 and PPy/α-Fe2O3 nanocomposite films. FTIR and XPS studies demonstrate that the formation of PPy, α-Fe2O3 and PPy/α-Fe2O3 nanocomposites. FESEM studies showed that, the PPy, α-Fe2O3 and PPy/α-Fe2O3 films consists of granular morphology. Impedance spectroscopy studies demonstrate that upon interaction with NO2, adsorbed oxygen on the surface of PPy, α-Fe2O3 and PPy/α-Fe2O3 grains was
Acknowledgment
Authors (VBP) are grateful to DAE-BRNS, for financial support through the scheme No. 2010/37P/45/BRNS/1442.
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