Article pubs.acs.org/JPCC
Bias-Induced Enhancement of Conductivity in Polypyrrole 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 S Supporting Information *
ABSTRACT: In this study we investigate the effect of application of a dc bias on chemically synthesized polypyrrole (PPy) and a PPy/Au nanocomposite. While PPy shows a more than 2 order permanent decrease in impedance (increase in conductivity), the PPy/Au nanocomposite on the other hand shows a temporary decrease in impedance, and in most cases the impedance increases with time. The permanent change in impedance observed in PPy is as an outcome of further polymerization of PPy resulting in increased molecular mass where shorter chains polymerize into longer chains on application of a bias, as indicated and confirmed by gel permeation chromatography. A probable mechanism of this observation is also discussed in this paper.
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INTRODUCTION Conducting polymers have gained considerable attention since their discovery three decades ago. The primary unique feature of a conducting polymer is its ability to obtain a wide range of conductivities depending on the doping level from those typical of insulators (98%) and L-cysteine monohydrate (>99%) were purchased from Merck. All the chemicals are used as received without any further treatment. Synthesis of Polypyrrole and the Gold/Polypyrrole Nanocomposite. Synthesis of polypyrrole and the polypyrrole/gold nanocomposite was carried out with a method already reported by this laboratory.12 In short, a stock solution of pyrrole in HCl was made for preparing different batches of polypyrrole and the polypyrrole/gold nanocomposite by dissolving 1 mL of pyrrole in 25 mL of 1 M HCl. To prepare polypyrrole, 5 mL of the pyrrole in HCl stock solution was added to 2 mL (1 M) of APS and the resulting solution stirred in an ice bath (0−2 °C) for 6 h. The dark-green-colored precipitate obtained was washed with HCl and Milli-Q water several times and dried under vacuum. The polypyrrole/gold nanocomposite was prepared with [PPy] (M):[Au] (M) = 1:0.0317. The polypyrrole/gold nanocomposite was prepared by the same synthetic protocol used to prepare polypyrrole described above; only gold nanoparticles prepared separately were added to the pyrrole− APS solution after 30 min, and the solution was stirred in an ice bath (0−2 °C) for another 6 h. As done previously, the darkgreen-colored precipitate was washed with HCl and Milli-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 pink color characteristics of Au NPs. Characterization. Transmission electron microscopy (TEM) was carried out with a JEOL JEM 2100 with an operating voltage of 200 kV. UV−vis spectra were taken using a Shimadzu (UV1601PC) spectrophotometer. Fourier transform infrared (FT-IR) spectroscopic measurements of PPy and the PPy/Au nanocomposite were recorded with 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. The X-ray diffraction (XRD) pattern was collected on a Bruker D8 Advance diffractometer using Cu radiation operating at 40 kV and 40 mA. Bias-Induced ac Impedance Measurements. All the ac impedance measurements were performed at room temperature under ambient conditions using an impedance analyzer (Hioki 3532-50). Typically for ac impedance measurement a pellet of ∼13 mm diameter with an average thickness of 0.14 mm was made using a KBr press (Technosearch, Mumbai) and put between two stainless steel plates. The impedance measurement was carried out typically in the frequency range of 42 Hz to 1 MHz. A dc bias was applied to the sample in the same sample setup used for measuring ac impedance. The sample setup with the pellet of sample between two stainless steel plates was connected to a homemade circuit discussed in detail in the Results and Discussion (see Figure 3 for details) for application of the dc bias. The bias provided was in the range from 0.5 to 2.0 V.
Figure 1. (A) Schematic representation depicting the PPy/Au nanocomposite. (B, C) Representative TEM images of PPy and the PPy/Au nanocomposite, respectively. (D) X-ray diffraction plot of the PPy/Au nanocomposite.
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RESULTS AND DISCUSSION
PPy was prepared using APS as the oxidizing agent to polymerize pyrrole (Merck) to polypyrrole in an ice bath
Figure 2. (A) Circuit diagram used to apply a bias to the sample. The sample is made in the form of a pellet and pressed against two stainless steel anvils. (B) log Z vs time of the PPy after application of a bias of 1 V and after the pellet was ground to make a powder and then a pellet was made again. The impedance measurement was done at 42 Hz. 6447
dx.doi.org/10.1021/jp300758d | J. Phys. Chem. C 2012, 116, 6446−6452
The Journal of Physical Chemistry C
Article
Figure 4. log Z vs time of the PPy/Au after application of different biases from 0.5 to 2.0 V taken (A) after 1 h and (B) after 1 day. The impedance measurement was at 42 Hz.
Figure 3. (A) log Z vs time of the PPy after application of different biases from 0.5 to 2.0 V with increments of 0.5 V and (B) log Z vs time of the same PPy measured after 1 day. The impedance measurement was at 42 Hz. (C) Corresponding frequency-dependent plot, log Z vs log frequency, after application of different amounts of bias. The frequency range measured was from 42 Hz to 1 MHz.
(0−2 °C). The PPy/Au nanocomposite was prepared by adding Au nanoparticles (NPs) prepared separately to the polypyrrole reaction mixture. Figure 1A shows a schematic diagram depicting the formation of the PPy/Au nanocomposite. The gold nanoparticles were prepared by reducing HAuCl4 with cysteine. The UV−vis spectrum of the Au NPs using cysteine has a wellformed surface plasmon resonance band centered at 539 nm (given in the Supporting Information, Figure S1). The absorbance and size distribution were predicted from MiePlot, a computer-generated program, and the size was determined to be ∼40 nm. Parts B and C of Figure 1 show the representative TEM images of PPy and the PPy/Au nanocomposite. The image of the PPy/Au nanocomposite (Figure 1C) clearly shows the distribution of Au NPs on the polymer. The size distributions of the Au NPs were calculated from the TEM
Figure 5. log Z vs time of the (A) PPy and (B) PPy/Au nanocomposite after application of biases of 0.1 and 0.3 V taken after 1 h and after 1 day. The impedance measurement was at 42 Hz. 6448
dx.doi.org/10.1021/jp300758d | J. Phys. Chem. C 2012, 116, 6446−6452
The Journal of Physical Chemistry C
Article
Figure 8. Stacked FT-IR spectra of PPy and the PPy/Au nanocomposite before and after application of 2 V.
Scheme 1
Figure 6. Gel permeation chromatogram profile of PPy after application of biases of (A) 0 V and (B) 2.0 V.
Scheme 2
Figure 7. Gel permeation chromatogram profile of PPy/Au nanocomposites after application of biases of (A) 0 V and (B) 2.0 V.
measurement of pure PPy with time at 42 Hz. Application of a 1 V bias to the sample resulted in a more than 2 order decrease of impedance (increase in conductivity). When a voltage V is applied to the sample, there is a possibility of ionic migration which will occur until a steady state is achieved. Through ionic transport number measurement, we have already shown that there is a polarizing current flowing in PPy and the PPy/Au nanocomposite12 and there is an ionic contribution in both PPy and the PPy/Au nanocomposite, with the ionic character increasing with increasing concentration of Au NPs. At the steady state, the cell is polarized and any residual current flows because of electron migration across the sample and interfaces.
image, and it is clearly shown that most of the NPs have sizes (diameters) in the range of 25−35 nm (Figure S2, Supporting Information), although some smaller particles are also visible in the image. The presence of Au NPs in the PPy/Au nanocomposite was also confirmed by recording the XRD pattern. The XRD plot shows patterns corresponding to the (111), (200), (220), and (311) diffraction peaks of Au. The effect of bias on the impedance of PPy and PPy/Au was studied. Figure 2 A shows a circuit diagram used to apply a dc bias to the sample. A potentiometer was used to provide a variable voltage to the sample. Figure 2B shows the impedance 6449
dx.doi.org/10.1021/jp300758d | J. Phys. Chem. C 2012, 116, 6446−6452
The Journal of Physical Chemistry C
Article
Scheme 3
change in the impedance value after application of 0.1 and 0.3 V and the order of the impedance value remains the same. The impedance of PPy was also measured after 1 day and is shown in Figure 5A. In the case with the PPy/Au nanocomposite (Figure 5B) application of 0.1 and 0.3 V results in a decrease in the impedance by 0.5 order, but the change is not permanent, and in fact, when measured again after 1 day, the impedance shows an increase (even more than that of PPy/Au before application of the voltage). As mentioned before, the pellet was preserved in a vacuum desiccator and used the next day. To know the fate of the polymer after application of a bias, GPC was done on the samples in THF using a Waters GPC system at room temperature (flow rate of 1 mL/min) comprising a refractive index detector (Waters 2414) and pump (Waters 515, HPLC pump). A 0.025% solution of polymer was prepared in THF. Calibration of the column was done with narrow standard polystyrene. The gel permeation chromatogram profile (Figure 6A) shows that the molecular mass of as-prepared PPy was determined to be 648 kDa. After application of a bias of 2 V, the GPC chromatogram shows (Figure 6B) that new peaks emerged corresponding to molecular masses of 895 and 2008 MDa. The GPC data give an indication of further polymerization of shorter PPy chains forming longer chains after introduction of a bias, and as a result, increased molecular mass was obtained. The GPC chromatogram profile of the PPy/Au nanocomposite is shown in Figure 7A. The chromatogram shows a peak corresponding to a molecular mass of 4466 MDa. On application of 2 V of dc bias there is a peak at lower eluting time corresponding to 26 kDa. The XRD pattern of the PPy/ Au nanocomposite after application of 2 V of dc bias also shows that the Au(111) peak disappeared, indicating a change in the PPy/Au nanocomposite (data provided in the Supporting Information, Figure S3). Thermogravimetric analysis (TGA) of the PPy/Au nanocomposite and PPy/Au nanocomposite after application of a bias (2 V) is provided in Figure S4, Supporting Information. The thermogram shows that the thermal stability decreases after application of a bias and the PPy/Au nanocomposite degrades at lower temperature. All this evidence hints at degradation of PPy/Au on application of a dc bias. Such degradation is not observed in the case of PPy. FT-IR studies were also carried out on PPy and the PPy/Au nanocomposite before and after application of a bias. The FTIR spectra are stacked in Figure 8. It is quite evident from the FT-IR spectra that characteristic peaks of polypyrrole, namely, the peak at 1570 due to polypyrrole ring vibration, the peaks at 1400 corresponding to C−N stretching, the peak near 1185 cm−1 due to breathing vibration of the pyrrole ring, and the peaks near 1092, 970, and 872 cm−1 assigned to the CH in-plane vibration, C−C out-of-phase deformation vibration,
This is because the ionic currents through an ion-blocking electrode fall rapidly with time if the sample is primarily ionic. Therefore, to ascertain that the change in impedance observed after application of a bias is not a result of ionic migration, the sample that was in the form of a “pellet” was again ground. Then a fresh pellet was made and the impedance measured again. Grinding will ensure that the pellet is neutralized and mixed homogenously to ensure the ions are no longer polarized. Impedance (Z) measurement after grinding showed that the impedance change is permanent and there is no substantial change in impedance after the sample is ground (Figure 2B). To know the role of voltage in the change in impedance, a different set of experiments was done in which different voltage biases were applied to PPy. Figure 3A shows the change in impedance observed plotted on a logarithmic scale vs time. The plot shows that there was a 2 order change in impedance after application of a bias of 0.5 V. Subsequent additional voltage applied of 1, 1.5, and 2.0 V did not further change the impedance drastically; the impedance decreased very marginally, but the order remained the same. It has to be mentioned here that the subsequent additional voltage was applied on the same sample. The change in impedance observed is permanent and remains the same even when measured after 1 day. Figure 3B shows the log Z vs time plot measured after 1 day. The pellet was preserved in vacuum desiccator, and the same pellet was for measurement the next day. The frequency-dependent change in impedance on application of different bias voltages on PPy was also carried out. Figure 3C shows the log Z vs log frequency plot at various voltages applied. It was observed that the change in impedance on application of a bias voltage is pronounced up to 5000 Hz, after which there is a steady decrease, and at 1 MHz the change is only less than 0.5 order. A similar study was done on the PPy/Au nanocomposite. Figure 4A shows the log Z vs time plot at different bias voltages. The plot clearly shows that the change in impedance observed after application of a bias is not as pronounced as in the case with PPy, although there is a 0.5 order change in the impedance value at 2.0 V. However, unlike with PPy, the impedance change is not permanent and the impedance value in most of the cases increases as evident from Figure 4B, which depicts the log Z vs time plot at different bias voltages taken of the same samples after 1 day. To learn about the changes in impedance in PPy and the PPy/Au nanocomposite on application of a dc bias of
