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J. Phys. Chem. C 2007, 111, 11872-11878
Characterization of Poly(3-octylthiophene)/Silver Nanocomposites Prepared by Solution Doping Eniko _ Pinte´ r,† Zoltan A. Fekete,† Otto´ Berkesi,† Pe´ ter Makra,‡ A Ä gnes Patzko´ ,§ and Csaba Visy*,† Department of Physical Chemistry, Department of Experimental Physics, and Department of Colloid Chemistry, UniVersity of Szeged, H-6701 Szeged, Hungary ReceiVed: April 4, 2007; In Final Form: May 16, 2007
Poly(3-octylthiophene)/silver nanocomposites (P3OT/Ag) were prepared by impregnating the polymer powder in silver perchlorate salt solutions. The total silver content, up to 6 wt %, was determined by inductively coupled plasma atomic absorption spectroscopy (ICP-AAS). The electric conductivities of the composites were measured and correlated with the silver content. Conductivity increased by more than 5 orders of magnitude with silver doping. The incorporated silver was speciated by X-ray diffraction (XRD). Silver was found in the form of both Ag and AgCl, predominantly in metallic nanocrystallites. The size distribution of the nanoparticles, determined from transmission electron microscopy (TEM), was found to be bimodal with two maxima around 3 and 17 nm, correlating with the two forms of silver in the composite. Increased conductivity was interpreted by results obtained by photoacoustic Fourier transform infrared spectroscopy (PAS-FTIR). The observed large Seebeck coefficient of P3OT is promising from the point of view of thermoelectric application.
Introduction Electronically conducting polymers are remarkable advanced materials, since under certain conditions they have a considerable electric resistance connected generally to semiconductor behavior, and they can be transformed into a metallic conducting state, as well.1-7 Composites of these polymers with inorganic matter, combining the different properties of components, provide new opportunities for tuning their behavior. During the past decade, a new branch of materials sciences nanostructured materialsshas developed, and the special character of nanosized solid materials has received great attention. Recently, the importance of metal nanoparticles has been summarized in a feature article,8 emphasizing their photophysical, photochemical, and photocatalytic aspects of the unique electronic and chemical properties, which have drawn the attention of chemists, physicists, biologists, and engineers because of the special opportunities opened by these materials in the development of a new generation of nanodevices. The importance of such composite materials lies in the fact that in these cases nanometals are dispersed in the matrix of the other components, which ensures their stabilization. Gold nanoparticles have been stabilized in alkane thiolates9 forming selfassembled monolayers of clusters containing ca. 4 nm gold cores. Tetraoctylammonium bromide capped gold nanoparticles exhibited surface binding properties,10 facilitating the formation of ordered arrays of gold nanoparticles. Phosphine-stabilized metal nanoparticles proved to be excellent precursors to specially functionalized nanoparticles11 from which blocks possessing well-defined metallic cores can be built. Various procedures for preparing nanoparticles are available from the literature,12-14 including new methods using templates.15,16 * Corresponding author. E-mail:
[email protected]. † Department of Physical Chemistry. ‡ Department of Experimental Physics. § Department of Colloid Chemistry.
A special group of composite materials is based on conjugated polymers. This family of composites may contain the conducting component either dispersed in inorganic matrix17 or, vice versa, the conductive organic matrix may incorporate the other phase. Conducting polymer films containing nanodispersed catalytic particles have been patented as a new type of composite material for technological applications,18 where the importance of the electrochemical conductance and the removal of higher molecular weight catalyst poisons have been outlined. Different conducting polymers can be easily synthesized by both chemical and electrochemical routes,19,20 and preparation of various polymers in both aqueous and nonaqueous solutions has been reported.21-24 They have been successfully applied as the conducting matrixes of composite materials incorporating, e.g., TiO2,25-30 V2O5,31 Fe2O3,32 Fe3O4,33 Fe,34 and Cu35 particles, but most works give account of embedding noble metals such as Pd, Au, and Pt.36-42 Conductivity improvement of polypyrrole (PPy) films by a vacuum deposited silver coating was reported.43 A previous work in our laboratory demonstrated a simple procedure for preparing polypyrrole/Ag nanocomposites.44 In these composites the chemical form of the incorporated silver depended on the doping anion used during the polymerization, and silver content, forming nanocrystals within the composite, stabilized the electric conductance of polypyrrole. In the current work, we report on the preparation and characterization of analogous silver nanocomposites with one of the poly(3-alkylthiophenes),45 the poly(3-octylthiophene) (P3OT)46-48 based silver nanocomposite. The characterization of polythiophene/silver composite revealed new properties compared to the polypyrrole based analogue, originating from the fundamental difference between the two polymers. The stable form of polypyrrole in air is the well-conducting state, while polythiophenes are more stable in their nonconducting form. However, as we show here for the first time, the conductivity of the latter proved to dramatically increase with the incorpora-
10.1021/jp0726440 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007
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Figure 1. Scheme of apparatus for measuring the thermoelectric coefficient.
tion of small amounts of silver (below 3 mass % total Ag content in the composite), leading to a conductivity more than 5 orders of magnitude higher. The conductivity increase becomes even more important when considering the measured extraordinary thermoelectric effect of P3OT, characterized by a Seebeck coefficient of 1283 µV/K (an outstandingly high value compared to inorganic thermoelectric materials). Experimental Procedures Chemicals (analytical grade) were purchased from Aldrich. P3OT powder was chemically synthesized through the oxidation of the monomer by FeCl3 in water-free chloroform solutions. The product obtained was filtered and washed thoroughly with ethanol to remove any residues from the oxidant. P3OT was then impregnated by stirring the polymeric material in nitromethane solutions of AgClO4 (c ) 5 × 10-4-2 × 10-3 mol/dm3). The decanted solid was dried at 80 °C. The dried powder was used in the X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopic (EDX) experiments. Some portions of the substance were dissolved for the elementary analysis, while others were compressed into tablets for the conductivity measurements by applying a pressure of p ) 740 MPa. The silver concentration was determined by the inductively coupled plasma atomic absorption spectroscopy (ICP-AAS) technique after dissolution in a sulfuric acid/H2O2 mixture (piranha solution). The silver incorporated into the polymer matrix was speciated by wide-angle X-ray diffraction (WAXRD) (Philips PW-1830 X-ray diffractometer). For irradiation the Cu KR line, λ ) 0.1542 nm, was applied (cathode at 40 kV and 30 mA), and Bragg scattering was recorded in the range of 2θ ) 2-40°. Transmission electron microscopy was performed using a Philips CM 10 microscope operating at an acceleration voltage of 100 kV. The size distribution of the particles incorporated into the P3OT was determined by applying this instrument supplied with a Megaview II camera and using the UTHSCSA Image Tool 2.00 computer program. Scanning electron microscopy was performed with a Hitachi 4700 FESEM apparatus, equipped with a Ro¨ntec QC2 X-FLASH detector for energy dispersive X-ray spectral analysis.
Figure 2. Electric conductivities (relative to neat P3OT of 0% Ag) of P3OT/Ag nanocomposites vs Ag concentrations (mass % determined by ICP-AAS). The curve is drawn to guide the eye.
Photoacoustic Fourier transform infrared (PAS-FTIR) spectra were recorded on a Bio-Rad Digilab Division FTS-65A/896 Fourier transform infrared spectrometer, using an MTEC-200 detector. For the measurement of the electric conductivity, the fourpoint-probe method in a square geometry arrangement has been used. A 130 Hz alternating current was applied through one electrode couple, and the voltage was measured between the other two contacted points. Impedance data were extracted from the Fourier spectra of the voltage response, with an especially developed data acquisition system interface on a PC.
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Figure 3. (a) Full WAXRD spectra obtained with P3OT/Ag nanocomposites at different silver concentrations (mass % determined by ICP-AAS). The bottom curve is that of the undoped polymer, and all others have been displaced for clarity. (b, c) Details of WAXRD spectra from (a) plotted vs calculated d-spacing. Center-of-gravity peak positions are marked for the highest silver concentration (top curve), as well as for the undoped polymer (bottom curve). (d) Details of WAXRD spectra from (a), plotted vs calculated d-spacing. Center-of-gravity peak positions are marked for the highest silver concentration (top curve), as well as for the undoped polymer (bottom curve). (e) Details of WAXRD spectra from (a), plotted vs calculated d-spacing. Silver and silver chloride peak positions are labeled.
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SCHEME 1: Possible Reactions of AgClO4 with P3OT
The thermoelectric coefficient of P3OT was determined from the voltage-temperature difference curves recorded in an apparatus drawn schematically in Figure 1.
Figure 5. (a) TEM picture of P3OT/Ag(6%) composite. The bar indicates a size of 100 nm. (b) Histogram of particle sizes from (a).
Results and Discussion Figure 2 shows the electric conductivities of the POT/Ag nanocomposites versus the analytical concentration of the noble metal component (in mass %, measured by the ICP-AAS technique). As P3OT is stable in its reduced, nonconducting form, the conductivity of the neat samples is small. Already at small doping levels the conductivity increases dramatically, with the effect leveling off above 3% Ag. Since the volume fraction of the metallic component is under 1% (i.e., very far from percolation), this change can only be explained by variation in the electronic structure of the polymer. The oxidized state of P3OT is stabilized by the inorganic component, which must be related to a redox reaction with the polymer, where the affinity of the thiophene sulfur toward metallic silver, already observed by other groups,49-52 is probably promoting the redox interaction of Ag+ with P3OT. The predominantly metallic chemical state of the noble metal in the composites is confirmed by WAXRD analysis. As seen in Figure 3a, the survey exhibits prominent scattering at the Bragg angle 2θ ) 38.1° that corresponds to Ag(111). As shown with magnified details of the spectra in Figure 3e, the intensity of this peak grows with the silver concentration. Also observed are peaks due to AgCl(200) and AgCl(111) at 2θ ) 32.2° and
Figure 6. SEM picture of P3OT/Ag(6%) composite, with secondary electron detection. (a) Magnification ) 1000; (b) magnification ) 100000.
Figure 4. Photoacoustic FTIR spectra of P3OT/Ag composites, with baselines subtracted; top to bottom: 3, 1, 0.5, 0.1, and 0 mass % Ag content (as measured by ICP-AAS). Curves have been displaced for clarity.
2θ ) 27.7°, respectively, but the contributions of these are only significant at the highest concentration studied here. The presence of the chloride ions seen here suggests that, during the oxidative polymerization of its monomer, a small fraction of P3OT is produced in the oxidized state binding counteranions from the FeCl3 solution. This form then can react with the silver salt solution to yield AgCl. These observations can be interpreted by either the redox interaction of silver perchlorate with the undoped P3OT, or the ion exchange with doped P3OT/Clsegments, according to Scheme 1. According to a more detailed analysis of the WAXRD spectra, one can observe that the Ag(111) peak width is nearly constant across the range of concentrations, indicating no variation of the particle sizes. A lower limit on the sizes of particles contributing to the signal can be estimated from the peak broadening with the Debye-Scherrer equation, which yields 17 nm in good accord with the TEM analysis (presented later). Other details of the spectra at smaller Bragg angles, presented in Figure 3b-d, provide insight into the structural changes of the polymer matrix itself upon doping with silver. The dominant first-order reflection at 2.16 nm (2θ ≈ 4°) (Figure 3b) is indicative of the lamellar interlayer spacing for
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Figure 7. SEM picture of P3OT/Ag(6%) composite, with backscattered electron detection. (a) Magnification ) 1500; (b) magnification ) 45000.
P3OT (for the fully regioregular form this was established at 2.08 nm).53,54 Upon the silver nanoparticle incorporation this spacing shifts to a higher value of ca. 2.3 nm. A similar increase is also seen in the second- and third-order reflections (2θ ≈ 8° and 12°, respectively) (Figure 3c), which have been ascribed to simple one-layer and partially interdigitated two-layer structures,55 respectively. Conversely, a small but definite decrease is observed for the smallest spacing (2θ ≈ 24°) (Figure 3d) for P3OT: 0.38 nm in the neat material53-55 shifted to ca. 0.36 nm in the composites. This peak is characteristic of the chain-to-chain repeat distance within the π-stacked polythiophene structure,50,51 and its change must result from stronger attraction between the backbones in the doped state as compared to the undoped one. This closer stacking, with increasing interchain interaction as the silver content is increased, is in perfect agreement with the conductivity increase in Figure 2. Also seen in Figure 3d is a broad amorphous halo centered around 0.45 nm “d-spacing” (2θ ≈ 20.5°) which has been attributed to side chain disorder; this diffuse feature does not change significantly upon the impregnation procedure. PAS-FTIR spectra of the composites, presented in Figure 4, had very high absorption covering the entire mid-IR range, likely caused by phonon coupling with the electrically conducting doped polymer matrix. This unstructured broad-band background swamped the signal (raw data not shown), precluding quantitative analysis. Nevertheless, as shown in Figure 4, qualitative vibrational spectra exhibiting peaks characteristic30,52,56-58 of both the undoped and doped P3OT, could be recovered after an adaptive background subtraction fitting. A picture of the nanocomposite taken by TEM is shown in Figure 5a. The inorganic component particles in the form of darker spots in the organic matrix are clearly seen. By applying the special UTHSCSA Image Tool 2.00 particle finder and size determination software, the data from Figure 5a have been analyzed for the size distribution. The resulting histogram, Figure 5b, exhibits the largest size around 20 nm, with the distribution characterized by two maxima at 3 and 19 nm. The latter agrees with the effective particle size from the WAXRD peak broadening mentioned earlier. Secondary electron detection SEM pictures are displayed in Figure 6. This technique gives rise to bright spots for particles with the heavy silver atoms, over the darker background of the polymer. Figure 7 presents two more SEM pictures, taken with backscattered electron detection. From the region depicted an EDX analysis was taken, summarized in Table 1. The high Ag content (compared to the 6 mass % determined by ICP-AAS) indicates that this measurement sampled over a locally silverenriched area of the composite. The lack of iron signal shows that the amount of either ferric or ferrous ions left over from the initial chemical polymerization step is negligible. On the other hand, the chlorine content is likely from Cl- anions (originating from the FeCl3 oxidant salt), which were bound as
TABLE 1: Energy Dispersive X-ray Analysis of the P3OT/ Ag(6%) Composite element carbon (aluminum contaminant) sulfur chlorine silver b
mass %a
atom %a
ratio to Sb
ratio to Agb
47.75 ( 3.37 80.84 ( 5.70 9.11 ( 0.82 14.08 ( 3.81 (0.33) 13.99 ( 0.79 8.87 ( 0.50 7.51 ( 0.52 4.31 ( 0.30 30.42 ( 7.95 5.74 ( 1.50
1.00 1.55 ( 0.41 0.49 ( 0.04 0.75 ( 0.20 0.65 ( 0.17 1.00
a Estimated errors are given by the EDX peak fitting software. Estimated errors are calculated from the propagation of errors.
doping anions to oxidized P3OT segments. Although this measurement does not give direct evidence of whether the chlorine here is present as AgCl (the presence of which as a minor component was proven by WAXRD, mentioned above at Figure 3e) or attached to the polymer as a counterion, a semiquantitative assessment can be made considering the atomic ratios shown in Table 1 along with the possible reactions depicted in Scheme 1. In the redox reaction, Scheme 1a, there is 1 equiv of chlorine in the neighborhood of the precipitating silver metal. In the ion exchange, Scheme 1b, there are 2 equiv of chlorine (i.e., one chloride and one perchlorate) in the neighborhood of the precipitating AgCl. Thus the low Cl:Ag atomic ratio (close to unity within the error limit) seen in Table 1 implies the prevalence of Scheme 1a and, therefore, the presence of metallic silver here. The atomic ratio relative to sulfur shows a remarkable depletion of carbon (compared to the C:S stoichiometry of 12:1 in P3OT). In conjunction with the local silver excess also observed, a tentative explanation for this is an oriented rearrangement of the polythiophene backbone with its sulfur sites moved near the silver particles. Comparing the current results with those from the earlier investigation on PPy/Ag,44 it is striking that the PPy made with FeCl3 oxidant formed exclusively AgCl upon impregnating with silver salt solution, whereas for P3OT it is a minor component here besides the metallic silver that is predominantly produced. This observed difference between the behavior of the two systems is in good agreement with the well-known difference in the redox properties of the two polymers: while the stable form of PPy is the oxidized state, polythiophenes are more stable in their neutral form. Therefore, more chloride dopant is available in the chemically polymerized PPy than in P3OT; on the other hand, the neutral P3OT has an affinity for producing silver via the reaction route in Scheme 1a. This redox change also provides the observed dramatic increase of the conductivity, which is very low for the nondoped P3OT. The simple silver doping process, leading to an exponential decrease in the resistance, promises to be useful in consideration of the thermoelectric behavior of polythiophenes.59,60 The P3OT tablet compressed from its powder exhibited an extremely large Seebeck coefficient. From the slope of the voltage-temperature difference curve in Figure 8 a value of 1283 µV/K is obtained,
Polythiophene-Silver Nanocomposites
Figure 8. Plot of thermoelectric behavior for P3OT.
which is more than 6 times larger than the one for Bi2Te3 crystal, considered the best inorganic thermoelectric material by now. The value is comparable to or even larger than those obtained for other polythiophenes.59,60 Since the thermoelectric coefficient to the second power is included in the “figure of merit”, expressing the efficacy of thermoelectric instruments, the P3OT/ Ag composite of increased conductivity61 may be applied in such devices for the exploitation of new energy sources. Conclusions This study demonstrated that the simple impregnation method with nonaqueous silver salt solution is applicable to produce metal nanoparticles in P3OT matrix. The silver is predominantly in the metallic form rather than in the form of AgCl salt. This observation is in marked contrast with that previously seen in the case of polypyrrole, where AgCl was exclusively formed when Cl- dopant was present (originating from the ferric chloride oxidant). The difference may be explained by the redox interaction of Ag+ with neural P3OT, combined with the affinity of silver for the sulfur sites of this polymer. The P3OT/Ag composites exhibit a dramatic increase of conductivity with silver contents below 1%. This large effect is assumed to be due to stabilization of a partially oxidized P3OT when in contact with the silver nanoparticles. The extremely large thermoelectric coefficient of P3OTs compared to such materials at presentsmay lead to applications in thermoelectric power generating devices. Acknowledgment. This work has been sponsored by the Hungarian National Scientific Research Fund (OTKA T042539) and by the Hungarian National Office of Research and Technology (NKTH) and the Agency for Research Fund Management and Research Exploitation (KPI) under Contract No. RET-07/ 2005. C.V. is an ECS member. References and Notes (1) Jonas, F.; Heywang, G. Electrochim. Acta 1994, 39, 1345. (2) Lenz, D. M.; Ferreira, C. A.; Delamar, M. Synth. Met. 2002, 126, 179. (3) MacDiarmid, A. G. ReV. Mod. Phys. 2001, 73, 701. (4) Heeger, A. J. ReV. Mod. Phys. 2001, 73, 681. (5) Alhalasah, W.; Holze, R. J. Solid State Electrochem. 2005, 9, 836. (6) Pickup, N. L.; Shapiro, J. S.; Wong, D. K. Y. Anal. Chim. Acta 1998, 364, 41. (7) Wallace, G. G.; Zhao, H.; Too, C. O.; Small, C. J. Synth. Met. 1997, 84, 323. (8) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729.
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