Polypyrrole-Fe 2 O 3 Nanocomposites with High Dielectric Constant: In Situ Chemical Polymerisation

Abstract

Novel nanocomposites of polypyrrole (PPy) dispersed with iron oxide (Fe₂O₃) particles have been synthesised by in situ chemical oxidative polymerisation of pyrrole in the presence of ammonium persulfate (APS) as an oxidising agent. The concentration of Fe₂O₃ was varied between 10-50wt% of PPy. The simultaneous polymerisation of pyrrole and oxide addition led to the complete synthesis of nanocomposites. A maximum dielectric constant of ∼28500 was observed at 20wt% of Fe₂O₃. The nanocomposites were characterised by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD analysis confirmed the structure and crystallinity of the nanocomposites, and a strong interaction between PPy and Fe₂O₃ particles was observed by FTIR technique. SEM and TEM images showed that Fe₂O₃ particles had been coated with PPy by establishing a network during the polymerisation process. The values of dielectric constant were obtained from capacitance measurements. The value of dielectric constant for nanocomposites with 20wt% of Fe₂O₃ was observed to be almost 12 times that of the pure PPy matrix. The high value of dielectric constant indicated a high packing density of Fe₂O₃ particles in PPy matrix. These nanocomposites have potential applications in electronic or biomedical devices.

1. Introduction

Research into conjugated polymers demonstrated that all polymers need not be insulators ¹ . These conjugated polymers have great scientific and technological significance due to their novel electrical, optical, electronic and optoelectronic characteristics^2,3. In the neutral state these materials demonstrate insulating or semiconducting characteristics^4–6 and find applications in different fields like, solar cells^7,8, sensors ⁹ , organic light emitting diodes^10,11 and optoelectronic devices ¹² .

Doping with metal oxides is one of the several ways to optimise the properties of these materials by carefully controlling the dopant into the polymer matrix ¹³ , resulting in new unique properties that cannot be achieved by single material^14,15. The inorganic materials due to their high surface to volume ratio are expected to alter the properties of organic polymer matrix rapidly. Above all, the main idea to synthesise these organic-inorganic networks is to obtain the unique composite materials demonstrating optimized properties between those of the organic and inorganic materials.

Among various polymers, polypyrrole (PPy) has reasonable thermal and environmental stability, significantly high electrical conductivity and simple route of synthesis^16–18. PPy has potential applications in electronic and electrochromic devices such as solar cells, super capacitors, energy storage^19–21, batteries, sensors^22–24, microwave absorption ²⁵ , antistatic coatings ²⁶ , charge storage ²⁷ , gas separation membranes ²⁸ and capacitors ²⁹ . Its optical, electrical and mechanical properties can be improved by carefully doping of metal oxides in the PPy matrix^30–32.

However there is not much research done on the dielectric properties of PPy-Fe₂O₃ nanocomposites. Herein we report the synthesis of the organic-inorganic composites by chemical oxidative polymerisation with different iron oxide contents in the polymer. The effects of concentration of filler metal oxide have been studied by exploring their structural, morphological and dielectric characteristics in order to assess the applications of such nanocomposites for electronic and related fields.

2. Experimental

2.1 Materials

All the chemicals were purchased from reputable companies. Pyrrole (analytical grade, 99%) was purchased from Sigma-Aldrich, stored at 5°C and vacuum distilled prior to use. APS and Fe₂O₃ were supplied by Merck. All other supplementary chemicals and solvents such as acetone, hydrochloric acid (HCl), chloroform and methanol were obtained from Fluka and used as received. Ultrapure deionised water (Seralpur delta) was used during all synthesis procedures.

2.2 Synthesis of PPy and PPy-Fe₂O₃ Nanocomposites

The chemical oxidative polymerisation method was carried out for the synthesis of PPy and PPy-Fe₂O₃ nanocomposites as described earlier ³³ . Vacuum distilled pyrrole (10 g) and ammonium persulfate (APS) (12.25 g) were dissolved separately in deionised water for synthesis of PPy-Fe₂O₃ nanocomposites. The pyrrole mixture was acidified gradually by dropwise addition of HCl (5 mL). At the next stage, 10wt% Fe₂O₃ (1 g) ultrasonically dispersed in deionised water was added to the pyrrole solution and the mixture was well stirred for 3 h. Afterwards, the solution of APS was added dropwise by dropping funnel into the mixture containing pyrrole, HCl and Fe₂O₃ under vigorous stirring. Same synthesis procedure was adopted for other concentrations of Fe₂O₃ i.e., 20, 30, 40 and 50 wt%. The prepared nanocomposites were designated as P-1, P-2, P-3, P-4 and P-5 respectively The suspension of obtained composites left overnight in fume hood. After ensuring the complete polymerisation, the suspensions were successively filtered and washed with deionised water. The precipitate was then dried under vacuum at 70°C for 24 h. To achieve complete homogeneity of the constituents, the dried precipitates were well crushed and ground for 1hour by an A-grade mortar and pestle, cleaned with acetone and deionised water. Pellets of the ground powder were prepared using a hydraulic press with a pressure of 30 kN applied for 2 min before further characterisation.

The powder X-ray diffraction patterns of samples were obtained by using an automated diffractometer, Bruker-AXS D8, using Cu Kα radiation. The operating voltage and current of the machine were maintained at 40 kV and 30 mA respectively. The samples were mounted on standard holders and diffraction spectra were recorded over the range of 10–60 degrees (2-theta) with a counting time of 3 s and step size 0.10 degree. Molecular structure was analysed through FTIR spectra recorded by Perkin Elmer FTIR spectrometer in the range from 500 to 3500 cmr ¹ . The surface morphology of nanocomposites was observed by an EVO50 ZEISS scanning electron microscope and a Philips CM 12 transmission electron microscope respectively. For the measurement of dielectric properties a Wayne Kerr LCR meter Model 4275 was used in the frequency range from 20 Hz to 20 MHz. The obtained data were transformed into dielectric constants by applying the relation:

$$C=\frac{{\epsilon }_r{\epsilon }_{o}A}{d}$$

(1)

where ε_r is relative permittivity, C is capacitance, ε_o is permittivity of free space, d and A are thickness and cross sectional area of the pellets respectively. The thickness and diameter of the samples were measured by a digital micrometer.

3. Results and Discussion

3.1 X-ray Diffraction (XRD)

XRD patterns reveal that pure PPy is amorphous ³⁴ and PPy-Fe₂O₃ nanocomposites are polycrystalline in nature due to the existence of crystalline material i.e., Fe₂O₃ whose diffraction pattern is shown in Figure 1 . The diffraction patterns of pure PPy along with its nanocomposites are depicted in Figure 2 . An increase in the intensity of diffraction peaks is observed with increasing concentration of Fe₂O₃ in the nanocomposites. For pure PPy, the existence of a broader peak at 2θ between 20–28° has already been reported, and corresponds to the characteristic peak of pure PPy ³⁵ . The appearance of this broad peak in all nanocomposites confirms the existence of PPy, and its intensity decreases with an increase in Fe₂O₃ content. Figures 2(a-f) show diffraction peaks corresponding to the (102), (104), (110), (113), (024) and (116) planes of Fe₂O₃ at 24.51, 33.28, 35.669, 40.880, 49.495 and 54.14 degrees 20 respectively. The observed diffraction peaks are well matched with the JCPDS data Card No. 13–534.

Figure 1

XRD pattern of Fe₂O₃

Figure 2

XRD patterns of (a) PPy (b) P-1 (c) P-2 (d) P-3 (e) P-4 (f) P-5

The variation in crystallite size of the nanocomposites as a function of Fe₂O₃ concentration was estimated by means of the Scherrer relation:

$$d=\frac{K\lambda }{\beta \cos \theta }$$

(2)

Here d is crystallite size for individual peak, K is the unit cell geometry dependent constant, whose value is typically between 0.85 to 0.99, λ is the wavelength of incident X-ray, β is the line broadening at full width at half maximum (FWHM) of the individual peaks and θ is the Bragg angle. For crystallite size measurement, the strongest peak corresponding to the (104) plane was selected. The average value of crystallites observed to be in the range from 18 to 36 nm. A slight shift in diffraction angle of the (104) diffraction peak is observed for all nanocomposites with respect to the Fe₂O₃ pattern as reported earlier ³⁶ . The structural parameters for nanocomposites are listed in Table 1 .

Table 1

Structural parameters for Fe₂O₃ and PPy-Fe₂O₃ nanocomposites

Sample	2θ (Degrees)	d-Spacing (Å)	β (Radians)	Cos θ (Degrees)	Thickness (nm)
Fe2O3	33.177	2.699	0.0804	0.958	18
P-1	33.400	2.682	0.0402	0.958	36
P-2	33.400	2.682	0.0425	0.958	34
P-3	33.540	2.671	0.0425	0.958	34
P-4	33.800	2.651	0.0452	0.958	32
P-5	33.440	2.679	0.0482	0.957	30

3.2 Fourier Transform Infrared Spectroscopy (FTIR)

Figure 3 depicts the FTIR spectra correlating the chemical structure of PPy and PPy-Fe₂O₃ nanocomposites. For pure PPy, FTIR spectrum is in close agreement with one reported earlier ³⁷ . The spectral bands observed at 780, 1090 and 1160 cmn ¹ are due to C–H out-of-plane ring deformation, C–H/ N–H in-plane deformation modes and C-H interplane bending respectively ³⁸ . The spectral bands at 1428 and 1550 cm⁻¹ are associated with CH₃ absorption and interacting C–C vibrations respectively ³⁹ . The spectral bands located at 1490 and 1728 cm⁻¹ exhibit C=C vibrations of the quinoid rings^40,41. It is noticeable that peak positions of all PPy-Fe₂O₃ nanocomposites are displaced towards higher wavenumbers than those observed in pure PPy. This displacement is likely to result from the process of active electronic interaction between PPy and Fe₂O₃ particles ⁴² . The inclusion of Fe₂O₃ causes the creation of hydrogen bonds between the NH protons and oxygen atoms on the Fe₂O₃. As a result N–H bond and the stretching intensity become weaker^43,44.

Figure 3

FTIR spectra of (a) PPy (b) P-1 (c) P-2 (d) P-3 (e) P-4 (f) P-5

3.3 Scanning Electron Microscopy (SEM)

Figure 4 (a) shows SEM images of PPy representing the formation of a porous spongy irregular agglomerated structure. Figures 4(b) to 4(f) depict the SEM images of PPy-Fe₂O₃ nanocomposites, showing an enhancement in the attachment of Fe₂O₃ particles to the polymer matrix, with an increase in Fe₂O₃ concentration. It is clearly visible from the micrographs that Fe₂O₃ particles (white colour) were embedded almost completely in the PPy matrix, confirming the formation of nanocomposites. Furthermore, the presence of Fe₂O₃ particles covered by PPy chain agglomerates and the resulting microstructure shows the presence of modified aggregated porous regions which would facilitate good electrical conductivity and dielectric response.

Figure 4

(a) SEM image of PPy (b) SEM image of P-1 (c) SEM image of P-2 (d) SEM image of P-3 (e) SEM image of P-4 (f) SEM image of P-5

3.4 Transmission Electron Microscopy (TEM)

For detailed surface characterisation of PPy-Fe₂O₃ nanocomposites, transmission electron microscopy of selected samples was carried out. The morphology shown from Figures 5(a) to 5(c) reveals the presence of two distinct regions, i.e. a black core of iron oxide particles well wrapped by outer grey shells of PPy with an average diameter ranging from 30 to 36 nm. This observation is evidence for successful polymerisation of polymer-metal oxide nanocomposites. The increment in Fe₂O₃ concentration seems helpful for it to occupy the porous sites of the polymer, which could result in improvement of various structural, electrical and magnetic properties.

Figure 5

(a) TEM image of P-1 (b) TEM image of P-3 (c) TEM image of P-5

3.5 Dielectric Constant Measurements

The dielectric constant (ε’) of a material refers to its capacity to store energy in the presence of an electric field, while the dielectric loss (ε”) expresses a concomitant loss or dissipation of energy^45,46. Under the influence of an applied electric field, a conductive material always experiences the induction of two types of currents: (i) displacement current and (ii) conduction current ⁴⁷ . The former type of current arises from localised bound charges, responsible for electronic, ionic, orientational and space charge polarisation (ε’) within the material. The latter type is typically induced due to mobile charges and is responsible for dielectric losses (ε”) within the material ⁴⁸ .

In composite materials, heterogeneity of organic and inorganic material would result in the domination of space charge polarisation, whereas in conjugated polymer system, polarons and/or bipolarons are mobile and free to hop between different sites along the polymer chain. The space charge polarisation arises from the restricted mobility of bound carrier dipoles, which leads to a form of orientational polarisation ⁴⁹ , responsible for a decreasing value of ε’ with an increase in frequency. Figure 6 shows the graph of ε’ vs. frequency for pure PPy and PPy-Fe₂O₃ nanocomposites. A sharp decrease in the value of ε’ is observed initially from 20 Hz to 1 kHz that would be well explained on the basis of space charge effects ⁵⁰ and interfacial polarisation ⁵¹ . During that process the charge carriers appearing from the Maxwell-Wagner-Sillars polarisation effect, are stored at the interfaces of the constituents. These interfaces could be either inner dielectric boundaries or external sample-electrode contacts ⁵² .

Figure 6

Variation in ε’ as function of frequency for (a) PPy (b) P-1 (c) P-2 (d) P-3 (e) P-4 (f) P-5

After 1 kHz, the sharp decrease in ε’ slowed down and became linear presented no further reasonable change in higher frequency regions. The decrease in ε’ at higher frequency region is due to the dielectric relaxation response of the material ⁵³ . The dielectric relaxation usually occurs due to delayed molecular polarisation within the external applied field ⁵⁴ . P-2 nanocomposite exhibits the highest values of ε’ among all the samples, which might be due to the strong interaction between PPy and Fe₂O₃ particles ⁵⁵ . The obtained ε’ values of PPy are 2408 and 371 at 20 Hz and 1 kHz respectively, whereas for the case of P-2 nanocomposite higher values of dielectric constant are obtained i.e., 28500 and 4169 at 20 Hz and 1 kHz respectively. For the higher frequency regions ε’ remained persistent, because induced moments could no longer synchronise themselves with the applied field in that frequency range. The dependence of ε’ on concentration of Fe₂O₃ is shown in Figure 6 (inset), presenting the highest value of ε’ for P-2 nanocomposite.

3.6 Dielectric Loss Measurements

Dielectric loss (ε”) response to frequency in PPy and PPy-Fe₂O₃ nanocomposites is shown in Figure 7 . The observed energy loss is due to the existence of moving dipoles between polymer and the metal oxide. These moving dipoles are a result of the strong interactions between the constituents. Since the existence of moving dipoles is less in nanocomposites as compared to PPy, that is the reason for increased value of dielectric loss in nanocomposites as compared to PPy and is confirmed experimentally. Dielectric loss varies with the particle size of the nanocomposites and is usually high for large particles ⁵⁶ . The inset of Figure 7 displays the highest value of ε” for P-2 nanocomposite.

Figure 7

Variation in ε” as function of frequency for (a) PPy (b) P-1 (c) P-2 (d) P-3 (e) P-4 (f) P-5

The values of ε” for PPy are 6032 and 361 at 20 Hz and 1 kHz respectively, whereas in the case of P-2 nanocomposite these values are 278472 and 26315 at 20 Hz and 1 kHz respectively. The observed values of ε’ and ε” for all the samples are listed in Table 2 .

Table 2

Fe₂O₃ loading effect on dielectric properties of PPy-Fe₂O₃ nanocomposites

Sample	Dielectric Constant at:		Dielectric Loss at:
	20 Hz	1 KHz	20 Hz	1 KHz
PPy	2408	371	6032	361
P-1	6814	570	41593	2030
P-2	28500	4169	278472	26315
P-3	25048	2865	193104	20789
P-4	13632	1340	130349	8359
P-5	12848	1183	95633	5923

The ratio of ε” to ε’ is a measure of energy dissipation of a material, denoted as “tan δ” provides the basic information about an efficient energy absorbing material. The dependence of tan δ on frequency for PPy and PPy-Fe₂O₃ nanocomposites is presented in Figure 8 . A decreasing trend in the value of tan δ is observed with gradual increase in frequency. The highest value of tan δ of each sample was found to be different depending on the loading of Fe₂O₃ content and it was found to reach a maximum at 20wt% Fe₂O₃ concentration. Those nanocomposites demonstrating high dielectric constant values in low frequency regions are suitable candidates for use in charge storing devices, electro-magnetic interference (EMI) shielding and decoupling capacitor applications ⁵⁷ .

Figure 8

Variation in tan δ for (a) PPy (b) P-1 (c) P-2 (d) P-3 (e) P-4 (f) P-5

4. Conclusion

PPy-Fe₂O₃ nanocomposites have been prepared by incorporation of Fe₂O₃ particles into a PPy matrix. A uniform dispersion of the Fe₂O₃ particles and strong interaction between the PPy and the Fe₂O₃ particles have been observed by XRD and FTIR respectively. An increasing trend of the dielectric constant up to 20wt% and then a gradual decrease up to 50wt% of Fe₂O₃, has been observed, with a maximum almost 12 times that of pure PPy. The surface morphology clearly reveals the presence of evenly-dispersed Fe₂O₃ particles in the PPy matrix, in accordance with their proportion and the successful formation of nanocomposites. The synthesised nanocomposites could be helpful in applications such as fabricating charge storing devices, electromagnetic interference (EMI) shielding and decoupling capacitor applications.

Conflicts of Interest

The authors declare that they have no conflict of interest.