Gold

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Investigation of Structural and Electrical Properties of Polyaniline/Gold Nanocomposites Asma B. Afzal,†,‡ M. Javed Akhtar,*,† Muhammad Nadeem,† and M. M. Hassan‡ Physics DiVision, PINSTECH, P.O. Nilore, Islamabad 45650, Pakistan, and Department of Chemical and Material Engineering, PIEAS, Islamabad 45650, Pakistan ReceiVed: March 26, 2009; ReVised Manuscript ReceiVed: July 31, 2009

Gold nanoparticles are synthesized by the reduction of gold salt using extract of tea leaves in 1-methyl-2pyrrolidinone (NMP) solution. Transmission electron microscopy (TEM) confirmed the formation of gold nanoparticles having average size of ∼20 nm. These nanoparticles have been incorporated in NMP solution of polyaniline emeraldine base (PANIEB) to cast the nanocomposite films. Thermogravimetric analysis (TGA) revealed that the thermal stability of the nanocomposites has improved as compared to the pure PANI. TEM confirmed the presence of gold nanoparticles in polyaniline matrix. The impedance spectroscopic studies showed that gold nanoparticles have considerable effects on the electrical properties of PANI by reducing the charge trapping centers and increasing conducting channels, which causes substantial decrease in the real part of impedance. 1. Introduction The nanosized metallic particles have attracted attention of the materials community due to their unique properties.1,2 In recent years, the synthesis of organic/inorganic nanocomposites has become the subject of extensive studies. The nanocomposites containing organic polymers and inorganic particles in nanoscale regime provide a completely new class of materials with novel properties.3-5 Conducting polymers are interesting materials in modern technology because of their potential applications such as electromagnetic radiation shielding, antistatic coatings and sensors.6,7 Polyaniline (PANI) is unique among the family of conjugated polymers due to its good environmental stability, ease of preparation, inexpensiveness and reversible control of conductivity both by charge-transfer doping and protonation. The polyaniline emeraldine base (PANIEB) consists of equal number of reduced [-(C6H4)NH(C6H4)NH-] and oxidized [-(C6H4)Nd(C6H4)dN-] repeat units. It is the most stable form of PANI and its conductivity can be enhanced by incorporation of metal nanoparticles. It is insoluble in common organic solvents (such as chloroform, xylene, THF, etc.) and mostly soluble in 1-methyl-2-pyrrolidinone (NMP) and can be cast into a flexible film with residual NMP, about 10-18% by weight, as a plasticizer. During film casting, the slow controlled evaporation of the solvent freezes the rapidly changing diblock nature of the polymer leading to phase separation.8 The residual NMP in the resulting film causes the microphase separation into reduced repeat units and oxidized repeat units, which affects the bulk conductivity of the polymer.9 A number of studies have shown that the electrical and mechanical properties of the conducting polymers can be improved by the incorporation of metal nanoparticles. For instance, Breimer et al.10 observed that the electrical conductivity of polypyrrole has been enhanced by incorporating gold nanoparticles into photosynthesized polypyrrole films. Zhou et al.11 reported the synthesis of novel stable nanometer-sized metal (M ) Pd, Au, Pt) colloids protected by a π-conjugated polymer. * To whom correspondence should be addressed. E-mail: javeda@ pinstech.org.pk. Phone: +92-51-9290231. Fax: +92-51-9290275. † PINSTECH. ‡ PIEAS.

Cho et al.12 examined the electrical properties of contacts formed between conducting polymers and noble metal nanoparticles (platinum, gold, and silver) using current sensing atomic force microscopy. Sarma et al.13 have prepared PANI-gold nanocomposites by first reducing gold salt solution and then polymerizing aniline in the same medium. Ma et al.5 employed one-step synthesis of water-soluble gold nanoparticles/PANI composite for glucose sensing applications; whereas Pillalamarri et al.14 also used one-pot synthesis method in which composite materials consisting of polyaniline nanofibers decorated with noble-metal (Ag or Au) nanoparticles were synthesized with γ-radiolysis. The electrical conductivity of the composites increased with the loading of nanometals in the polymer. Wang et al.15 synthesized nanosized metallic particles via reduction of the metal salts by PANIEB in both NMP and aqueous media; as a result of these reactions, the polyaniline is converted to a higher oxidation sate. Recently, we have studied the structural and electrical properties of PANI/Ag nanocomposites and found that silver nanoparticles have profound effects in improving the thermal stability and electrical conductivity of PANI.16 In the present study, we report a new method for the synthesis of gold nanoparticles. These nanoparticles were suspended in NMP solution and their different concentrations were incorporated in PANI to cast NMP plasticized PANI-Au nanocomposite films. This method is cheap and clean with minimum risk of contamination to get the nanocomposite. Various techniques including Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) were used to characterize the PANI/gold nanocomposites for structural characterization and impedance measurements have been carried out for the determination of the electrical properties. 2. Experimental Section 2.1. Materials. Aniline monomer (Riedel-de-Hae¨n) was distilled under vacuum before use. Ammonium peroxydisulfate (APS, Riedel-de-Hae¨n), NMP (Panreac), HAuCl4 (Alfa Aeser), and HCl (Panreac) were used as-received without further purification. Deionized water was used throughout the experiment.

10.1021/jp902725d CCC: $40.75  2009 American Chemical Society Published on Web 09/11/2009

Properties of Polyaniline/Gold Nanocomposites 2.2. Synthesis. Gold nanoparticles can be synthesized by using various plant extracts;17 in the present study, extract of black tea leaves was used to reduce the gold salt to gold nanoparticles. Black tea leaves are rich in polyphenolic compounds such as theaflavins and thearubigins that belong to catechin group of flavinols. Many of them have well-defined antioxidant properties that are directly correlated to the total phenolics content of tea;18 recently, it has been suggested that quercetin may play an important role in the reduction of Au(III) to gold nanoparticles.19 We have used the extract of black tea leaves by soaking 2.0 g of tea leaves in 20 mL of NMP overnight. NMP extract of tea was then filtered using a common filter paper and stored at 4 °C until further use. An aqueous solution of HAuCl4 (50 mL, 0.5 mM) was refluxed for 5-10 min, and 1 mL of a warm (50-60 °C) NMP extract of tea was added to it quickly. Reflux was continued for another 40 min until the appearance of a deep-red solution of gold nanoparticles. The particle solution was filtered through 0.45 µm Millipore syringe filters to remove any precipitate. Gold nanoparticles were then pelletted twice using a benchtop centrifuge at 13 000 rpm for 30 min and resuspended in NMP to make a solution having 72 µg/mL of gold nanoparticles for further use. PANIEB was prepared by chemical oxidation of aniline with APS as oxidant in 1 M HCI solution.20 In a typical procedure, aniline (20 mL) was dissolved in 300 mL of 1 M HC1 and cooled to 5 °C in an ice bath. A precooled solution (200 mL) of 11.5 g APS in 1 M HC1 was added to the aniline solution dropwise over a period of ∼2 min under constant stirring. After ∼1.5 h, the precipitates were collected on a Buchner funnel and washed with 1 M HCI. The polyaniline hydrochloride so obtained was converted into PANIEB by treatment with 0.1 M NH4OH and drying under dynamic vacuum for 48 h at room temperature. The PANIEB powder obtained was stored for further study. PANIEB film was made by dissolving PANIEB powder in NMP. In a typical procedure, 1 g of finely ground PANIEB was stirred magnetically in 250 mL of NMP at room temperature for ∼8 h; an intense blue solution was formed, which was filtered through a Buchner funnel indicating the formation of PANIEB.21 In addition, we performed the UV-vis spectroscopy (not shown here), where a peak at about 620 nm was observed that confirmed that PANIEB was synthesized.22 We used three concentrations (0.24, 0.48, and 0.72 wt %) of gold nanoparticles (in NMP solution) and mixed with the NMP solution of PANIEB; mixture of both solutions was stirred in ultrasonic bath for ∼12 h. This mixture was poured in Petri dish and the solvent was evaporated at 120 °C to cast the PANI/gold nanocomposite films. Hereafter, these polymer films are referred as PANI-Au1 (0.24 wt % Au), PANI-Au2 (0.48 wt % Au), PANI-Au3 (0.72 wt % Au) and pure PANI. Pure PANI film had also been prepared using the above-mentioned procedure. 2.3. Characterization. FTIR spectra of PANI films were recorded using Nicolet 6700 FTIR with ATR in the 4000-400 cm-1 range for 50 scans. The TGA of the films was carried out on a Mettler Toledo in the temperature range of 50-850 °C under nitrogen atmosphere at heating rate of 10 °C/min. The gold nanoparticles and the nanocomposites were characterized by TEM, JEM-1010 (JEOL) operating at 120 kV. Specimens for TEM examination were prepared by slow evaporation of one drop of Au/NMP and PANI/Au solutions on a carbon-coated copper mesh grid. Impedance measurements of all PANI samples were performed using an Alpha-N Analyzer, Novocontrol (Germany) in the frequency range 0.1 e f e 106 Hz at room temperature.

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Figure 1. FTIR absorption spectra of (a) Pure PANI (b) PANI-Au1 (c) PANI-Au2 and (d) PANI-Au3; inset shows the shift of 1242 cm-1 band.

WINDETA software was used for data acquisition, which has been fully automated by interfacing the analyzer with a PC. Before impedance experiments the dispersive behavior of the leads was carefully checked to ensure the absence of any extraneous inductive or capacitative coupling in the experimental frequency range. The ac signal amplitude used for all these studies was 0.5 V. The ac resistivity measurements were performed on films having a diameter of 13 mm and a thickness of ∼2 µm. Contacts were made by silver paint on opposite sides of the films, which were cured at 70 °C for 3 h. In this study the results of the complex impedance are presented as Z ) Z′ + jZ′′ and permittivity as ε ) ε′ - jε′′, where Z′, ε′ and Z′′, ε′′ are the real and imaginary parts of impedance and permittivity, respectively. The relationship between ε and Z is given by ε )(Z-1/jωCc), where ω ) 2πf and Cc is the capacitance of the measuring cell.23 3. Results and Discussion In order to determine the possibility of interactions between gold nanoparticles and PANI matrix, FTIR spectroscopy of the pure PANI as well as nanocomposite films were performed (Figure 1). All of the characteristic absorption bands for the PANIEB16,24 were observed in pure PANI (Figure 1a). The bands at 3284 are assigned to N-H stretching. The presence of a strong CdO stretching band at 1671 cm-1 indicates that the PANIEB film contains residual NMP solvent. The bands at 1582 and 1487 cm-1 are attributed to CdC stretching mode of vibration for the quinoid and benzoid units, while the bands at 1274 and 1242 cm-1, shown in the inset of Figure 1, are due to C-N and CdN cm-1 stretching modes, respectively. The bands at 1169 and 827 cm-1 are the distinctive features of C-H inplane and C-H out-of-plane bending, respectively. These characteristic bands of PANIEB can also be seen in the infrared spectra of the nanocomposite films (Figure 1b-d) confirming the formation of PANI in all samples. However, we note a shift in some peak positions indicating that gold nanoparticles and the polymer have an interaction between them. The shift in peaks positions, associated with CdC (1582 cm-1) by ∼7 cm-1 and CdN (1242 cm-1) by ∼17 cm-1 (the shift of this band toward lower wavenumber is shown in the inset of Figure 1), is due to stretching of the quinoid ring. We do not observe any significant change in the peaks’ positions associated with the benzoid ring.

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Figure 2. Thermogravimetric analysis of (a) Pure PANI, (b) PANIAu1, (c) PANI-Au2, and (d) PANI-Au3.

From these results we can infer that gold nanoparticles may reside more close to the imine nitrogen of the PANI. These results support the conduction mechanism in PANI/gold nanocomposites proposed by Tseng et al.25 From Figure 1, it can be seen that intensities of the bands have been reduced due to the presence of Au nanoparticles. The reduced intensity of the bands can be attributed to the interactions between gold nanoparticles and PANI matrix. We have observed a similar behavior in PANI/Ag nanocomposites.16 The TGA measurements of the pure PANI and PANI-Au nanocomposites, having different concentration of Au, are shown in Figure 2. The degradation occurs in three steps. The evaporation of moisture is observed up to 100 °C; the second weight loss from 140 to 435 °C is due to the evaporation of NMP.26 For pure PANI, the major weight loss occurs after 435 °C indicating the structural decomposition of the polymer (Figure 2a). An improvement in the thermal stability of the nanocomposite can be seen, which increases with an increase in the nanofiller content. The onset of thermal degradation is shifted toward higher temperatures by about 40 °C for the composites, having highest concentration of gold nanoparticles, that is, PANI-Au3 (Figure 2d). These results are in agreement with the thermal decomposition data of the PANI/silver nanocomposite,16 where it was shown that the composite decomposes at higher temperature when PANI is loaded with silver nanoparticles. It is important to point out that at 850 °C; the weight loss of the nanocomposites is 5, 10, and 20% less in PANIAu1, PANI-Au2, and PANI-Au3, respectively, when compared with pure PANI. The improved thermal stability of the nanocomposites can be attributed to the reduced mobility of the polymer chains that in turn suppresses the free radical transfer via interchain reactions. As a result, the process of degradation will be slowed and decomposition will take place at higher temperature.27 Figure 3 shows transmission electron micrographs of gold nanoparticles in NMP and PANI/Au nanocomposite film. It can be seen that pure gold nanoparticles in NMP have narrow size distribution, having ∼20 nm average particle size, as estimated from TEM (Figure 3a). In the case of nanocomposites, we note that spherical shaped Au nanoparticles, having mean diameter of ∼20 nm, are distributed in the polymer matrix (Figure 3b). Although some agglomerates are formed, this may be due to the synthesis of nanocomposite films at 120 °C for 16 h. The

Afzal et al. number of these particles increases as the concentration of gold is increased in PANI/Au nanocomposite films. The impedance spectroscopy was employed at room temperature to explore the electrical properties of PANI films containing gold nanoparticles. Figure 4a shows the real part of the impedance of pure PANI and nanocomposite films when plotted as a function of frequency; we note that the effect of high frequency on Z′ for both pure PANI and PANI/Au nanocomposites is small. Below 5000 Hz, there is slight dispersion of Z′ but around 30 Hz a sharp change in the real part of the impedance can be observed. At low frequency, the real part of the impedance have the values of 3.0 × 108, 1.65 × 108, 3.71 × 107, and 2.57 × 107 Ω for pure PANI, PANI-Au1, PANIAu2, and PANI-Au3, respectively. It can be inferred that incorporation of 0.24, 0.48, and 0.72 wt % gold nanoparticles in PANI cause 2-, 8-, and 12-fold decrease in resistivity when compared with the pure one. The plots for the imaginary part of impedance versus frequency, as shown in Figure 4b, are resolved into two peaks, the strong peak at low frequency and a weak peak at high frequency for all PANI samples. Both of these peaks are similar to that reported earlier in the impedance studies of PANI/Ag nanocomposites,16 which may be due to microphase separation of the polymer chains.9,28,29 In the case of nanocomposites both peaks are suppressed but this decrease is more distinct in PANI-Au2 as compared to that of PANIAu1. From these outcomes we observe that incorporation of the gold nanoparticles results in the frequency shift of both peaks. In PANI-Au3 loading of gold nanoparticles shifts the strong peak in low frequency region and the weak peak in high frequency region from 2 to 5.2 and 1088 to 674 Hz, respectively. Thus in PANI/Au nanocomposite films both peaks are suppressed and a shift in the peaks position is evident; the inset of Figure 4 shows the weak peaks positions for PANI-Au2 and PANI-Au3 films. From these results, we can infer that incorporation of gold nanoparticles inside the PANI matrix may lead to a faster charge transfer than pure one. It may be suggested that in pure PANI the charge is transferred by two phases, that is, the phase of oxidized repeat units and the phase of reduced repeat units; whereas, in the case of nanocomposite films, gold nanoparticles provide additional conducting channels for charge transportation, which are responsible for relaxation process.30 Figure 5 shows the typical impedance (Z ) Z′ + jZ′′) plane plots between real (Z′) and imaginary (Z′′) parts of the impedance for pure PANI and PANI/Au nanocomposite samples. It contains two arcs, a small one at high frequency preceding a large one at low frequency. Previously, it has been reported that in NMP plastisized PANI films, the peak at lower frequency is due to the conductivity relaxation of the phase with oxidized repeat units and that at higher frequency to the relaxation of the phase with reduced repeat unit. It was also observed that the resistivity of the reduced repeat units is lower than that of the oxidized repeat units.9,29 On this basis it has been suggested that the small arc at high frequency with low value of resistivity is due to the conductivity relaxation of the reduced repeat units and the large arc at low frequency having high value of resistivity is because of the relaxation of the oxidized repeat units.16,28 The intersection values of the two arcs representing two phases give the resistance corresponding to the ac values. The reduced repeat units intersect the Z′ axis at the left-hand side and its extension on the right-hand side intersects the same axis at a point termed as Rr (resistance of the phase of reduced repeat units). The extension of the oxidized repeat units on the right-hand side intersects the real part of impedance axis at a point defined as Ro (resistance of the phase of oxidized repeat

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Figure 3. TEM micrographs of (a) gold nanoparticles in NMP and (b) PANI-Au2.

Figure 5. Impedance plane plots of pure PANI and PANI/Au nanocomposite films. Inset is enlarged portion of the impedance plane plots of first small arcs.

Figure 6. Equivalent circuit model used for fitting of pure PANI and PANI/Au nanocomposites.

Figure 4. (a) Real part Z′ and (b) imaginary part Z′′ vs log f for pure PANI and PANI/Au nanocomposite films; insets show relaxation of second peak at higher frequencies.

units). From Figure 5, we note that the impedance plots show two well-resolved arcs representing both types of repeat units. A comprehensive decrease in the size of impedance plane plots has been observed with the addition of even very low concentration of gold nanoparticles (PANI-Au1). This decrease in the impedance can be attributed to the charge transfer between PANI and gold nanoparticles. With the amplification of frequency the imine nitrogen of PANI may donate electrons to gold nanoparticles. Consequently, the gold nanoparticles become more negatively charged, whereas PANI chains become more positively charged. Therefore, the electrical conductivity of PANI/ Au nanocomposite significantly increases.25

Figure 6 shows the proposed equivalent circuit model of the resistance and constant phase element (CPE), which has been employed for the fitting of impedance plane plots of pure PANI and nanocomposites. The first small and second large semicircles, representing the phases of reduced repeat units and oxidized repeat units, have been modeled with equivalent circuit configuration (Rr, CPEr) and (Ro, CPEo), where subscripts “r” and “o” stand for the phases of reduced and oxidized repeat units, respectively. The values of the simulated electrical circuits derived from impedance plane plots of PANI samples having different concentrations of embedded gold nanoparticles are listed in Table 1. In the pure PANI sample, the phase of reduced repeat units has values Rr ) 2.15 × 107 Ω, CPEr ) 7.93 × 10-12 F, and of oxidized repeat units has Ro) 2.90 × 108 Ω, CPEo ) 4.25 × 10-10 F. When we compare present results with those previously reported PANI/Ag nanocomposites,16 we observe a similar trend in both cases. Table 1 shows that the addition of gold nanoparticles to PANI results in substantial rise in the CPE parameters. The resistance, Rr of the phase of reduced repeat units and Ro of the phase of oxidized repeat units decrease from 2.15 × 107 to 1.6 × 107 Ω and from 2.90 × 108 to 1.69 × 108 Ω, respectively, in PANI-Au1. However, when

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TABLE 1: Fitting Parameters Calculated from Equivalent Circuit Model pure PANI CPEr (F) nr Rr (Ω) CPEo (F) no Ro (Ω)

7.93 0.94 2.15 4.25 0.85 2.90

× 10

-12

× 107 × 10-10 × 108

PANI-Au1 -11

2.61 × 10 0.95 1.6 × 107 1.00 × 10-9 0.86 1.69 × 108

PANI-Au2 1.59 0.95 1.78 2.10 0.90 3.88

× 10

-10

× 106 × 10-9 × 107

PANI-Au3 1.60 0.96 1.52 1.60 0.91 2.75

× 10-10 × 106 × 10-9 × 107

concentration of Au nanoparticles is increased from PANI-Au2 to PANI-Au3, Rr decreases from 1.6 × 107 to 1.52 × 106 Ω and Ro decreases from 1.69 × 108 to 2.75 × 107 Ω, respectively. These results indicate that incorporation of gold nanoparticles in PANI causes a sharp increase in the capacitance and a comprehensive decrease in the resistance30 of both phases. The value of nr (CPEr of reduced part) increases from 0.94 to 0.96 and no (CPEo of oxidized part) from 0.85 to 0.91 when pure PANI is loaded with more Au nanoparticles (PANI-Au3). When n is close to 1, the CPE resembles a capacitor; in the case of pure PANI, both nr and no are less than 1; therefore CPE behaves like a nonideal capacitor.31 With the addition of gold nanoparticles, nr and no are approaching to 1; however, the phase of oxidized repeat units shows more heterogeneity as compared to that of reduced repeat units. The variation of real part of permittivity, ε′, and imaginary part of permittivity, ε′′, versus frequency for pure PANI and PANI/Au nanocomposites are shown in Figure 7. Two dielectric relaxations are observed in the spectrum of the permittivity, the low and high frequency relaxations. The low frequency relaxation is generally associated with the structural relaxation and a dynamic glass relaxation (R-relaxation).28 In the present study, the low frequency peak can be assigned to the dielectric relaxations of the phase of the oxidized repeat units. The high frequency relaxation (as shown in the inset of Figure 7b), also called as Maxwell-Wagner (M-W) relaxation,32 is due to the dielectric relaxations of the phase of the reduced repeat units. We note that the relaxations of the reduced repeat units become more distinct as the gold nanoparticles are incorporated in PANI when compared with pure PANI. Nanocomposites show higher value of permittivity at all frequencies of measurement as compared to the pure PANI. In the conducting polymers instead of permanent dipoles, there are strong charge (polaron and bipolaron) trapping centers33,34 the localized motion of which serves as an electric dipole under applied external electric field.28 This electric field causes the localized charge carriers to hop to neighboring sites resulting in the dielectric relaxations. Such charge hopping forms a continuous network allowing the charges to travel through the entire physical dimensions of the sample and causes electrical conduction.35 In the absence of strong charge trapping centers, the charge hopping could extend throughout the sample leading to a continuous current at low frequencies.36 When PANI is loaded with gold nanoparticles the charge trapping centers are reduced, thereby leading to a large number of charge participations in the relaxation process; as a result an increase in the conductivity of PANI is observed.

Figure 7. (a) Imaginary part ε′′ and (b) real part ε′ vs log f for pure PANI and PANI/Au nanocomposite films; inset shows enlarged portion at higher frequencies.

confirmed the formation of PANI in all samples. Comparison of the thermal properties of pure PANI and the nanocomposite films showed that the thermal stability is improved by about 40 °C. TEM results showed that nanoparticles are embedded in the PANI films. The electrical properties of the nanocomposites were determined by impedance spectroscopy. The conductivity relaxation analysis suggested microphase separation of NMP plasticized films into phases of reduced and oxidized repeat units. The conductivity of PANI films increases with increase in the concentration of embedded gold nanoparticles. Acknowledgment. We gratefully acknowledge the financial support of the Higher Education Commission of Pakistan through the Indigenous Scholarship Scheme for Ph. D. studies of Asma Binat Afzal in Science and Technology (Batch II). We are grateful to Dr. Irshad Hussain for the synthesis of gold nanoparticles.

4. Conclusions

References and Notes

We have synthesized gold nanoparticles by the reduction of HAuCl4 using extract of tea leaves in NMP; TEM revealed that average particle size is ∼20 nm. These nanoparticles were successfully incorporated in PANIEB films, using a physical process with minimum risk of chemical contamination. FTIR

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