Determining the Ionic and Electronic Contribution in Conductivity of

ARTICLE pubs.acs.org/JPCC

Determining the Ionic and Electronic Contribution in Conductivity of Polypyrrole/Au Nanocomposites Lakshinandan Goswami, Neelotpal Sen Sarma, and Devasish Chowdhury* Physical Sciences Division, Polymer Unit, Institute of Advanced Study in Science and Technology, Paschim Boragaon, Garchuk, Guwahati 781 035, Assam, India ABSTRACT: In this work we determine the ionic and electronic contribution in the conductivity of polypyrrole (PPy) and polypyrrole/gold (PPy/Au) nanocomposites having different amounts of Au NPs. Polypyrrole (PPy) was prepared using ammonium persulfate (APS) as an oxidizing agent. Polypyrrole/Au nanocomposite was prepared by adding gold (Au) nanoparticles (NPs) prepared separately to the polypyrrole reaction mixture. The gold nanoparticles were prepared by reducing HAuCl4 with cysteine. PPy and PPy/Au was characterized by UV
vis, Fourier Transform Infrared spectroscopy (FTIR), transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and thermogravimetric analysis (TGA). The ac conductivity and transport number was also measured of PPy and PPy/Au nanocomposites. Impedance measurement shows that incorporation of [PPy]:[Au] 1:0.0063, 1:0.019, and 1:0.0317 M Au NPs results in a 1.5-, 300-, and 8000-fold decrease, respectively, in the impedance value when compared with pure PPy. The ionic transport number shows that ionic conduction in the PPy/Au nanocomposites increases as the content of Au NPs is increased. The ionic conductivity increased from 50% for pure PPy to 90% in the case of PPy/Au nanocomposite. The impedance measurement of base (NaOH) treated ba-PPy and ba-PPy/Au nanocomposites shows that incorporation of Au NPs in the PPy does not help in increasing the ac conductivity of the composite as shown by PPy and PPy/Au nanocomposite in acidic condition. This decrease in ac conductivity can be attributed to deprotonation of the PPy chain in aqueous basic medium (pKa ≈ 9
10). The ionic transport number shows that unlike PPy and PPy/Au nanocomposites in acidic condition, base-treated ba-PPy and ba-PPy/Au nanocomposites do not have any ionic contribution in the conductivity of the nanocomposites.

’ INTRODUCTION Since the discovery of conducting polymers, which is also called ‘‘synthetic metals’’ as a result of its conduction properties, they have found diverse applications in electronic,1 optoelectronic2 and electromechanical devices,3 antistatic and anticorrosion coatings,4 sensors,5,6 batteries and supercapacitors, lightemitting diodes (LEDs),7 solar cells,8 electrochromic devices,9 and transparent electrode materials.10 Among the conducting polymer, polypyrrole is one of the most widely and extensively studied conducting polymers. The reasons for this intense focus on polypyrrole certainly lie in the fact that the monomer (pyrrole) is easily oxidized and commercially available. The polymer polypyrrole possesses several advantages including environmental stability and good redox properties as well as the ability to give high electrical conductivities. Nanocomposites are a subset of composites that take advantage of unique materials properties on the small scale. In general, polymer nanocomposites11,12 are materials in which nanoscopic inorganic particles, typically 10
100 nm in at least one dimension, are dispersed in an organic polymer matrix in order to dramatically improve the performance properties of the polymer such as increased conductivity, increased modulus, strength, and stiffness, improved barrier properties, improved solvent and heat resistance, decreased flammability, etc. Many such nanocomposites of r 2011 American Chemical Society

conducting polymer in general and polypyrrole in particular have been synthesized and improved properties studied. For example, core
shell carbon black/polypyrrole nanocomposites have shown a high discharge capacity and find use in electrochemical energy storage applications.13 Multilayered polypyrrole-coated carbon nanotubes have shown improved functional stability and electrical properties in comparison with bare PPy.14 Zhu et al. prepared conductive polypyrrole/tungsten oxide metacomposites with negative permittivity.15 Moreover, metalfilled polymer composites have the potential to address the needs of emerging dielectric technologies. There are also numerous references in the literature pertaining to increased conductivity in conducting polymer nanocomposites.16
18 These studies are limited to studying their electrical properties conductivity, impedance,19
21 resistivity, dielectric behavior,22 etc. In this paper we study and determine the electronic and ionic contribution in the conductivity of PPy/Au nanocomposites. In ionic conduction “ions” carry the current. In electronic conductors “electrons” carry the current. PPy was prepared by oxidation of pyrrole with ammonium Received: August 5, 2011 Revised: August 26, 2011 Published: August 26, 2011 19668

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peroxodisulfate in acidic medium, and Au NPs, prepared separately, were added to synthesize polypyrrole/Au nanocomposite. Au NPs were prepared using cysteine as a reducing as well as a stabilizing agent. Impedance measurement of PPy/Au nanocomposites show that there is increased conductivity of the PPy/Au nanocomposites with increased concentration of Au NPs. Ionic transport number measurements show that the ionic conduction in PPy/Au nanocomposites increases with increased concentration of Au NPs. The PPy and PPy/Au nanocomposites were treated with NaOH (base) and are referred to in the text as baPPy and ba-PPy/Au nanocomposites, respectively. While in the case of PPy and PPy/Au nanocomposites the conductivity is due to the ionic as well as the electronic contribution, the basetreated ba-PPy and ba-PPy/Au nanocomposites do not have any ionic contribution, as evident from the transport number measurement.

’ EXPERIMENTAL SECTION Materials. Pyrrole (98% v/v) and hydrogen tetrachloroaurate(III) chloride (HAuCl4) (99.99%) were purchased from Sigma Aldrich. Ammonium persulfate (APS, >98%) and L-cysteine monohydrate (>99%) were purchased from Merck. All chemicals were used as received without any further treatment. Synthesis of Polypyrrole and Gold/Polypyrrole Nanocomposite. A stock solution of pyrrole in HCl is made for preparing different batches of polypyrrole and polypyrrole/gold nanocomposite by dissolving 1 mL of pyrrole in 25 mL of 1 M HCl. To prepare polypyrrole, 5 mL from the pyrrole in HCl stock solution was added to 2 mL (1 M) of ammonium persulfate (APS) and stirred in an ice bath (0
2 °C) for 6 h. The dark green-colored precipitate was washed with HCl and Mili Q water several times and dried under vacuum. Polypyrrole/gold nanocomposite was prepared with different concentrations of [PPy]:[Au]. The different molar concentrations used were 1:0.0063 M, 1:0.019 M, and 1:0.0317 M labeled as PPy
Au A, PPy
Au B, and PPy
Au C respectively. The polypyrrole
gold nanocomposite was prepared by the same synthetic protocol as preparing polypyrrole described above; only gold nanoparticles prepared separately were added to the pyrrole
APS solution after 30 min and the solution stirred in an ice bath (0
2 °C) for another 6 h. Like before, the dark greencolored precipitate was washed with HCl and Mili Q water several times and dried under vacuum. The gold nanoparticles were prepared by reducing 0.65 mM HAuCl4 with 5 mM cysteine to obtain a pink color characteristic of Au NPs. In the case of base-treated PPy, referred to in the text as ba-PPy, 1 M NaOH was added dropwise until the pH of the solution was ∼9
10 with continuous stirring for another 1 h. The solution turns blue with addition of NaOH. Characterization. Transmission electron microscopy was carried out in a JEOL JEM 2100 with an operating voltage at 200 kV. UV
vis spectra were taken using a Shimadzu (UV1601PC) spectrophotometer. FT-IR spectroscopic measurements of PPy and PPy/Au nanocomposite were recorded in a Bruker FT-IR spectrometer. Samples for FT-IR measurements were prepared in the form of pellets by mixing 20 mg of IR spectroscopic-grade potassium bromide with 2 mg of dried samples. The spectra were recorded in transmission mode over 256 scans. Small-angle X-ray scattering (SAXS) was also done on a Bruker D8 Advance powder diffractometer using the capillary setup and Cu Kα (λ = 1.54 Å) as the incident radiation. Fitting of

Figure 1. (A) UV
vis spectrum of Au NPs prepared using cysteine as a reducing agent (red curve). UV
vis spectrum obtained using Mie’s plot, a computer-generated program considering Mie’s theory of gold nanoparticles in water. Also shown in the inset the photograph of Au NPs prepared. (B) Corresponding transmission electron microscope (TEM) image of the Au NPs. (C) Plot of the size distribution of Au NPs obtained from the TEM image.

the scattering profile was carried out using Nanofit software to determine the particle size distribution of the PPy
Au nanocomposite. Thermogravimetric analysis (TGA) was carried out in a Perkin-Elmer TGA 4000. Alternating Current Impedance and Transport Number Measurements. All ac impedance measurements were performed at room temperature under ambient conditions using an impedance analyzer (Hioki 3532-50). Typically, for ac impedance studies a pellet of size ≈ 13 mm diameter is made using a KBr press (Technosearch, Mumbai) and put between two stainless steel plates. Impedance measurement was carried out the frequency range from 42 Hz to 1 MHz. Tranport number measurement was performed using a homemade cell setup, and details are discussed in the Results and Discussion section.

’ RESULTS AND DISCUSSION Polypyrrole (PPy) was prepared using APS as an oxidizing agent. Polypyrrole/Au nanocomposite was prepared by adding gold (Au) nanoparticles (NPs) prepared separately to the polypyrrole reaction mixture. The gold nanoparticles were prepared by reducing HAuCl4 with cysteine. The UV
vis spectrum of the freshly prepared Au NPs using cysteine was taken and is shown in Figure 1. The spectrum gave a well-formed surface plasmon resonance band centered at 540 nm. Also shown in the inset is a photograph of Au NPs prepared. The absorbance and size distribution predicted from the Mie plot, a computergenerated program, was determined to be ∼40 nm. The expected absorbance spectrum obtained from Mie’s plot is also shown in Figure 1. A representative transmission electron microscope image of the Au NPs and the corresponding size distribution is also shown in Figure 1B. The size distributions measured from TEM images clearly show that the majority of the NPs have sizes 19669

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Figure 2. (A) UV
vis spectra of PPy reaction mixture after 5 min and after addition of NaOH. (Inset) Photograph of the corresponding reaction solutions. (B) Representative transmission electron microscope (TEM) images of pure PPy and PPy/Au nanocomposites.

Figure 3. (A) Representative plot of the log of the scattering intensity and the scattering vector q of the PPy/Au nanocomposite along with the best fit. (B) Size distribution plot derived of the PPy/Au nanocomposite.

(diameter) in the range of 30
40 nm. This is in complete agreement of the size obtained by UV
vis (Mie’s plot). Figure 2 shows the UV
vis spectrum of the PPy reaction mixture after 5 min (black line). Also shown in the inset is the photograph of the PPy reaction mixture after 5 min of reaction time, showing the green color of the PPy reaction mixture. The UV
vis spectrum of the PPy reaction mixture after addition of NaOH is also shown in Figure 2 (red line). The photograph of the reaction mixture shows that the color of the reaction mixture turns blue after addition of base (NaOH). Representative transmission electron microscope images of pure PPy and PPy/Au nanocomposite are shown in Figure 2B and 2C, respectively. The TEM image of PPy/Au nanocomposite clearly shows the Au NPs dispersed in the polymer matrix. Small angle X-ray scattering (SAXS) was performed on the PPy/Au nanocomposite. Figure 3 shows the log of the scattering intensity as a function of the magnitude of the scattering vector q, where q = 4π sin(θ)/λ. Here 2θ is the angle between the incident X-ray beam and the detector measuring the scattered intensity, and λ is the wavelength of the X-rays. The corresponding best fit using Bruker’s NanoFit software using a model “hard sphere” was done, and the size distribution of the polymer nanocomposite was determined to be 447 Å or 44.7 nm and is shown in Figure 3B.

The TGA measurements of the pure PPy and PPy/Au nanocomposites, having a different concentration of Au, are shown in Figure 4 done in the temperature range from 35 to 900 °C. The thermogram clearly shows the degradation of pure PPy and PPy/Au nanocomposite takes place in three major steps: the weight loss up to 100 °C due to evaporation of moisture, at 210 °C, and the major weight loss at 400 °C. Complete degradation of the polymer occurs at ∼750 °C for pure PPy as well for PPy/Au nanocomposite except PPy/Au C, where the content of Au NPs is high. Complete degradation of PPy/Au C nanocomposites takes place at a much higher temperature of ∼860 °C. Fourier transform infrared spectra (FTIR) of pure polypyrrole prepared in acidic medium and PPy/Au nanocomposites prepared using different amounts of Au NPs, i.e., PPy/Au A, PPy/Au B, and PPy/Au C, are shown in Figure 5. There is not much change in the peak position observed for pure PPy and PPy/Au nanocomposites. Peaks at 1530 and 1578 cm
1 can be assigned to typical polypyrrole ring vibration. Two peaks at 1402 and 1314 cm
1 correspond to C
N stretch and dC
H in-plane vibration. The peak near 1123 cm
1 arises due to breathing vibration of the pyrrole ring, and the peaks near the positions at 1062, 977, and 892 cm
1 can be assigned to the dC
H in-plane vibration, C
C out-of-phase deformation vibration, and dC
H out-of-plane vibration, indicating polymerization of the pyrrole. 19670

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Figure 4. Thermogravimetric analysis of pure PPy and PPy/Au nanocomposites having a different concentration of Au NPs.

Alternating Current Impedance Studies. Alternating current impedance measurement was carried out at room temperature to study the electrical properties of PPy and PPy/Au nanocomposites. Figure 6A shows the log of the real part of the impedance (Z0 ) of pure PPy and different PPy/Au nanocomposites having different ratios of PPy:Au as a function of frequency. It is evident from the plot that the effect of the high frequency on Z0 for both pure PPy and PPy/Au nanocomposites is small. While there is an observed sharp drop in Z0 at 1000 Hz for pure PPy, low PPy:Au, i.e., PPy/Au A shows a drop in Z0 at 5000 Hz. PPy/Au B and PPy/Au C did not show much decrease in Z0 values. At 42 Hz, the real part of impedance has values of 8.02  106, 6.62  106, 2.73  104, and 1.03  103 Ω for pure PPy, PPy/Au A, PPy/Au B, and PPy/Au C, respectively. The Z0 values show that incorporation of [PPy:Au] in the ratio of 1:0.0063, 1:0.019, and 1:0.0317 M Au NPs results in a 1.5-, 300-, and 8000-fold decrease, respectively, in the impedance value when compared with pure PPy. Similarly, Figure 6B shows the log of the imaginary part of the impedance (Z00 ) of pure PPy and PPy/Au nanocomposite as a function of log frequency. The plot reveals that there is observed a steep drop of log Z00 only for pure PPy at higher frequency, while none of the PPy/Au nanocomposite compositions showed such a steep drop in log Z00 values only at higher frequency. Figure 7A shows the Nyquist plot where
Zim or Z00 is plotted against Zreal or Z0 . There is a comprehensive decrease in the size of the impedance plane plots thst has been observed with addition of even a very low concentration of gold nanoparticles (PPy/Au A). This decrease in the impedance can be attributed to the charge transfer between PPy and gold nanoparticles. With the amplification of frequency the nitrogen of PPy may donate electrons to gold nanoparticles. Consequently, the gold nanoparticles become more negatively charged, whereas PPy chains become more positively charged. Therefore, the electrical conductivity of PPy/Au nanocomposite significantly increases. Mechanistic details are discussed in a later part of the paper. The variation of the real part of the permittivity, ε, and the imaginary part of permittivity, ε00 , versus frequency for pure PPy and different PPy/Au nanocomposites is shown in Figure 8. It is evident from the plot that both the real part of the permittivity, ε0 , and the imaginary part of the permittivity, ε00 , decrease with increased frequency. While for pure PPy and PPy/Au A ε0 remains more or less the same above 3000 Hz, in the case of PPy/Au B and PPy/Au C it is at 50 and 300 kHz, respectively, that ε0 becomes constant. Impedance measurement was also done on base-treated PPy (ba-PPy) and ba-PPy/Au nanocomposites. A Nyquist plot of baPPy and ba-PPy/Au is given in Figure 9A. There is an observed

Figure 5. Fourier transform infrared (FTIR) spectra of pure PPy and PPy/Au nanocomposites having a different concentration of Au NPs.

slight increase in the size of the impedance plane plots with addition of Au NPs. This shows that ac conductivity of the basetreated PPy did not change with addition of Au NPs like PPy/Au nanocomposite prepared in acidic condition. The plot of log Zreal vs log frequency and log Zim vs log frequency of ba-PPy and baPPy/Au nanocomposites is given in Figure 9B and 9C, respectively. Both plots are similar, and there is not much difference. The variation of the real part of the permittivity, ε0 , and the imaginary part of the permittivity, ε00 , versus the frequency for baPPy and ba-PPy/Au nanocomposites is shown in Figure 9D and 9E. The plots are similar for ba-PPy and ba-PPy/Au NPs. It is also to be noted that permittivity values are lower as compared with PPy and PPy/Au nanocomposites prepared in acidic medium. Ionic Transport Study. The total ionic transport number was determined by the standard Wagner polarization technique23,24 of pure PPy and PPy/Au nanocomposites and base-treated ba-PPy and ba-PPy/Au nanocomposites. When a voltage, V, is applied to the cell (which is 1 V in the present experiment), ionic migration will occur until the steady state is achieved. At the steady state, the cell is polarized and any residual current flows because of electron migration across the sample and interfaces. This is because the ionic currents through an ion-blocking electrode fall rapidly with time if the sample is primarily ionic. We used a homemade cell to measure the transport properties. The cell used to measure the ionic transport of pure PPy and different PPy/Au composites is shown in Figure 10A. The plot of polarized current versus time is shown in Figure 10. The initial total current decreases with time due to depletion of the ionic species in the sample and becomes constant in the fully depleted situation. Ionic transport number can then be calculated using the relation Tion = iT
ie/iT, where iT is the total current and ie is the current due to the electron. The values of Tion were determined to be 0.50, 0.62, 0.79, and 0.90 for pure PPy, PPy/Au A, PPy/Au B, and PPy/Au C, respectively. The value implies that pure PPy has 50% ionic conduction, PPy/Au A has 62% ionic conduction, PPy/Au B has 79% ionic conduction, and PPy/Au C has the highest 90% ionic conduction. Hence, the ionic conduction in the PPy/Au nanocomposites increases as the content of Au NPs is increased. On the other hand, there is no change in the polarizing current with time when PPy is treated with base (ba-PPy), as evident from the 19671

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Figure 6. (A) log of the real part Z0 and (B) imaginary part Z00 vs the log frequency (f) for pure PPy and PPy/Au nanocomposites.

Figure 7. (A) Impedance plane plot of pure PPy and different compositions of PPy/Au nanocomposite. (B) Enlarged portion of the impedance plane for small curves especially of PPy/Au nanocomposites.

Figure 8. (A) Real part ε0 vs log frequency for pure PPy and different PPy/Au nanocomposite. (B) Same as A with magnified y axis. (C) Imaginary part ε00 vs log frequency for pure PPy and different PPy/Au nanocomposites. (D) Same as C with magnified y axis.

polarized current vs time plot of ba-PPy in Figure 10C. The polarized current vs time plot of ba-PPy/Au nanocomposites also shows that the polarizing current does not change with time. This indicates that there is no ionic part contributing toward its

conductivity in both ba-PPy and ba-PPy/Au nanocomposites (ba-PPy/Au). The conductivity mechanism of inorganic semiconductors, e.g., CdS, CdSe, etc., and conducting polymers, e.g., polyaniline, 19672

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Figure 9. (A) Impedance plane plot of ba-PPy and ba-PPy/Au nanocomposites. (B) log of the real part Z0 and (C) the imaginary part Z00 vs the log frequency for ba-PPy and ba-PPy/Au nanocomposite. (D) ε0 vs log f for ba-PPy and ba-PPy/Au nanocomposite. (D) ε00 vs log f for ba-PPy and ba-PPy/Au nanocomposites.

polypyrrole, etc., is different and may be a direct result of interplay of the electronic properties and structural crystallinity of the material, while in inorganic semiconductors doping is done which will result in creation of charge carriers (electrons or holes) by injection or ejection of electrons in the valence or conduction bands, respectively. On the other hand, doping of conducting polymers (adding metal NPs) seems to take a different path; it creates accessible energy levels in the middle of the band gap between the highest occupied HOMO and the lowest unoccupied energy levels LUMO while maintaining valence and conduction bands which remain full and empty, respectively. As in the case of crystalline inorganic semiconductors, the charge carriers move in a continuum carrier path, but in the conducting polymer that may not be the case as a result of intrachain transport and charge recombination along the chains. We propose the following mechanism. The increase in the ac conductivity of PPy (prepared in acidic medium) with introduction of Au nanoparticles can be attributed to charge transfer between polypyrrole and gold nanoparticles. When a small voltage is applied, the lone pair of the electron on the nitrogen of the PPy may gain enough energy and get through the interface between the PPy and the gold nanoparticles and move onto the gold nanoparticles (Figure 11). Consequently, the gold nanoparticles become more negatively charged, whereas the

PPy will become more positively charged. The conductivity of the PPy
gold nanoparticle composite will increase dramatically after the charge transfer. There is the possibility of redox processes taking place in the polymer giving rise to a charge transfer resistance and a capacitance contribution to the predicted impedance behavior. A further consideration is the fact that ions and electrons are introduced into or expelled from the polymer film during a redox process in order to achieve electroneutrality. Within the conducting polymer, the impedance behavior can be derived from three elements: the resistance to ionic charge transport (Ri), the resistance to electronic charge transport (Re), and the capacitance associated with charge separation (C). The effect of the different types of charge carriers on ac impedance response, however, is more far reaching due to the possibility of coupling between them in addition to the simple combination of the three elements, Ri, Re, and C. The counterion on PPy, which may be Cl
generated by Au NPs with HCl, stabilizes the charge on the polymer and will also have a contribution toward ionic conductivity. To substantiate the role of the counterion in ionic conductivity, an experiment was done in which Au NPs was prepared with acid-free stabilizer using H2O2 instead of Cys and polymerization was carried out in H2SO4 medium. In this case the counterion will be SO42
instead of Cl
. 19673

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Figure 10. (A) Schematic representation of the cell circuit used in measurement of the transport number. (B) Polarized current vs time plot of pure PPy and different PPy/Au nanocomposites. (C) Polarized current vs time plot of pure ba-PPy and ba-PPy/Au nanocomposites.

Figure 11. Schematic representation depicting the charge transfer between polypyrrole and gold nanoparticles in PPy/Au nanocomposites.

PPy/Au nanocomposite was thus prepared using the same ratio [PPy]:[Au] as PPy/Au C. Impedance measurement showed that at 42 Hz the real part of the impedance has a value of 3.6  104 Ω in comparison to 1.03  103 Ω found in PPy/Au C. The one order higher impedance value can be attributed to the lower ionic mobility of the bulky counterion (SO42
) resulting in lower ionic conductivity. However, the transport number measurement shows that the ionic part remains the same, i.e., 90%, as in PPy/Au C. Treatment of PPy with a base (ba-PPy) results in a decrease in ac conductivity25,26 as a result of deprotonation of the PPy chain in aqueous basic medium (pKa ≈ 9). There is change in its electronic structure, probably formation of quinoid structure. Introduction of Au NPs also does not help in increasing the ac conductivity of the base-treated PPy/Au nanocomposites (ba-PPy/Au).

’ CONCLUSIONS In summary, we prepared different PPy/Au nanocomposites taking different ratios of [PPy]:[Au] NPs. Impedance measurement shows that incorporation of 1:0.0063, 1:0.019, and 1:0.0317 M [PPy]:[Au] NPs results in a 1.5-, 300-, and 8000-fold decrease in the

impedance value when compared with pure PPy. The ionic transport number shows that ionic conduction in the PPy/Au nanocompopsites increases as the content of Au NPs is increased. The ionic conductivity increased from 50% for pure PPy to 90% in the case of PPy/Au nanocomposite. Actually the impedance behavior depends on three factors: the resistance to ionic charge transport (Ri), the resistance to electronic charge transport (Re), and the capacitance associated with charge separation (C). Moreover, the counterion on PPy which stabilizes the charge on the polymer will also have a contribution toward ionic conductivity. Impedance measurement of base (NaOH) treated ba-PPy and ba-PPy/Au nanocomposites shows that incorporation of Au NPs in the PPy does not help in increasing the ac conductivity of the composite as shown by PPy and PPy/Au nanocomposite in acidic condition. This decrease in ac conductivity can be attributed to deprotonation of the PPy chain in aqueous basic medium (pKa ≈ 9
10). The ionic transport number shows that unlike PPy and PPy/Au nanocomposites in acidic condition, base-treated ba-PPy and ba-PPy/Au nanocomposites does not have any ionic contribution in the conductivity of the nanocomposites.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +91 361 2912073. Fax: +91 361 2279909. E-mail: [email protected] .

’ ACKNOWLEDGMENT L.G. thanks IASST for a fellowship. The authors thank Mr. Subhojit Das and the Central Instrumentation Facility, IIT Guwahati, for TEM measurements. We also acknowledge funding from DST, Government of India, through a project under the SERC Fast Track Scheme (SR/FTP/CS-45/2007) and CSIR, New Delhi (Project No. 01(2488)/11/EMR-II). 19674

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