Hybrid Core−Shell Nanocomposites Based on Silicon Carbide

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Hybrid Core-Shell Nanocomposites Based on Silicon Carbide Nanoparticles Functionalized by Conducting Polyaniline: Electron Paramagnetic Resonance Investigations A. Kassiba,*,† W. Bednarski,†,⊥ A. Pud,‡ N. Errien,† M. Makowska-Janusik,§ L. Laskowski,§ M. Tabellout,† S. Kodjikian,∇ K. Fatyeyeva,‡ N. Ogurtsov,‡ and Y. Noskov‡ Laboratoire de Physique de l’Etat Condense´ UMR CNRS 6087, Faculte´ des Sciences et Techniques, UniVersite´ du Maine, 72085 Le Mans Cedex 9, France, Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Science, 1, Murmanska, 02094 KieV, Ukraine, Institute of Physics, UniVersity of Czestochowa, Al. Armii Krajowej 13/15, 42 200 Czestochowa, Poland, and Laboratoire des Oxydes et Fluorures UMR CNRS 6010, Faculte´ des Sciences et Techniques, UniVersite´ du Maine, 72085 Le Mans Cedex 9, France ReceiVed: February 4, 2007; In Final Form: May 25, 2007

Electron paramagnetic resonance (EPR) investigations were carried out on hybrid core-shell nanocomposites based on silicon carbide nanoparticles (SiC) and polyaniline (PANI) doped with camphor sulfonic acid (CSA). Charge carrier concentrations and thermal activations in low and high conducting polymers arranged as thin layers (2-5 nm) on the nanoparticle surfaces were investigated in a wide temperature range [4-430 K]. The EPR results and analyses are supported by complementary investigations of the macroscopic electrical conductivity and vibrational properties probed by Raman spectrometry. Temperature-dependent EPR measurements indicate paramagnetic susceptibilities with Curie-Weiss-like features and thermally activated spins. In contrast to the annealing stability of the nanocomposites, kinetic phenomena were witnessed on the EPR spectra of highly doped PANI. These phenomena correlate with the occurrence of transverse bipolarons which lead to irreversible structural changes in the polymer backbone. Polaron and bipolaron contributions to the electronic transport mechanism and the stability of the materials after annealing were found to be caused by doping rates of both the bare PANI and the nanocomposites.

1. Introduction The physical properties of functional hybrid nanocomposites were intensively investigated during the past decade. Among the reported functionalities, those in optics,1 electronics,2 photovoltaics,3 or optoelectronics4 are very promising. Some of these architectures are potentially suitable for technological applications such as solar cells, optical modulators or switches, and photoluminescent components in selective light windows. The active vectors of these functionalities consist of inorganic semiconducting nanocrystals, and in this context silicon carbide is a promising material due to its unique ability to adopt different crystalline polytypes which monitor the band gap as well as the electronic and optical properties.5-7 Moreover, in hybrid materials, the interface effects between organic and inorganic nanocrystals play a key role in their physical properties. Thus, an effective control of the surface states of the nanocrystals presents an important challenge to fine-tune their physical responses and, furthermore, their efficiency. One relevant approach is the surface modification of silicon carbide nanoparticles (SiC) nanoparticles, achieved by grafting a thin layer of semiconducting polymer with a nanometric thickness. The resulting nanocomposites are expected to benefit from the intrinsic properties of both components. On the one hand, SiC nanoparticles are characterized by versatile properties, such as * Corresponding author. E-mail: [email protected]. † Laboratoire de Physique de l’Etat Condense ´ , Universite´ du Maine. ‡ National Academy of Science, Ukraine. § University of Czestochowa. ∇ Laboratoire des Oxydes et Fluorures, Universite ´ du Maine. ⊥ Permanent address: Institute of Molecular Physics, Polish Academy of Sciences, Mariana Smoluchowskiego 17, 60-179 Pozna_, Poland.

dielectric behavior marked by interfacial polarizations,8-10 as well as vibrational and luminescence properties, which indicate the main role of surface states.6,7 On the other hand, with suitable acid doping, the polyaniline (PANI) conductivity can be modulated from a semiconducting to a metallic state.11 The easy synthesis, the key electronic and optical properties, as well as the relatively good thermal stability12,13 allow PANI to be a model material for core-shell nanocomposites with promising electroluminescent properties. Indeed, SiC nanoparticles functionalized at the surface by PANI lead to a large organicinorganic interface area, ensuring charge transports and recombinations leading to electroluminescent phenomena. Thus, the aim of this paper is to create hybrid nanocomposites based on SiC nanoparticles and PANI with different acid doping concentrations (camphor sulfonic acid is used, abbreviated as CSA) and to investigate the mechanism of charge transports in the nanocomposites with respect to that involved in the bare PANI. The charge carriers in these materials consist of polarons, with spin (1/2) and spinless bipolarons. Electron paramagnetic resonance spectroscopy (EPR) is a suitable tool to evaluate polaron concentrations and to probe their dynamic and thermal activations. The polaron concentration of different modified PANI composites was examined in a variable temperature range (4-430 K). In contrast, bipolarons are spinless without any EPR activity and only a comparison between the thermal evolution of the EPR active centers and the conductivity of the materials can shed light on the bipolarons contribution to the charge transport process. The dynamic of the polaron/bipolaron conversion was also probed from the time-dependent EPR signal measurements. The thermal stability was investigated and compared between the bare PANI and the nanocomposites after

10.1021/jp070966y CCC: $37.00 © 2007 American Chemical Society Published on Web 07/19/2007

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their annealing at suitable temperatures where charge carrier losses and structural modifications occur. Thus, the considered approaches are thought to contribute to a better understanding of the charge transport mechanism in hybrid nanocomposites and their stability prior to undergoing tests of their electroluminescent responses which will be considered in a separate contribution. The paper is organized as follows: the first part describes the experimental procedures including the synthesis and morphology of the nanocomposites by transmission electron microscopy. It also includes the basis of the experimental conditions for EPR, Raman, and dielectric spectroscopies. The second part describes the detailed results and analyses of the EPR investigations carried out in a wide range of temperatures and addresses their correlations with the structural and macroscopic electrical properties. The conclusion points out the main achievement of this work: mainly, the original procedure of nanocomposites synthesis, the specificity of the charge transport mechanisms, and the thermal stability of the different hybrid materials considered. 2. Experimental Details 2.1. Sample Synthesis. The investigated samples were prepared at 22 °C by aniline chemical polymerization in acid media (CSA) with the presence of SiC nanoparticles in the ratio 1.23 wt % with respect to the solvent. Slow magnetic stirring for 16-24 h achieves the PANI polymerization on the nanoparticle surfaces. In order to calculate the aniline quantity required to create the PANI shell on all SiC nanoparticles, it was presumed that the synthesized PANI constitutes a homogeneous surface layer on each SiC nanoparticle and did not form a separate phase. This presumption was based on the well-known PANI ability to precipitate on all surfaces presenting an aniline polymerization mixture.14 The parameters for these quantity evaluations are related to the specified thicknesses of PANI layers in the range of 2-5 nm, the specific gravity of PANI (1.2 g m-3), and the total polymerization of PANI. From these calculations, the necessary quantities and concentrations of aniline were determined with corresponding quantities of oxidant ammonium persulfate (APS) and CSA acid. The typical molar ratios of aniline:APS ) 1:1.25 were used, and in all syntheses the CSA concentration was 1 M. The obtained SiC-PANI-CSA nanocomposite dispersions were dialyzed against distilled water through a cellophane membrane to remove residual products (monomers, trimers, etc.) and impurities. This procedure was followed by water evaporation from the purified dispersions and by vacuum drying of the SiC-PANI nanocomposites to separate the final products for further characterizations. The color of the obtained powders was indicative of the doping level. It should be mentioned here that, after the dialysis procedure as well as after the washing procedure with a high quantity of water, PANI is in a partially dedoped state. In order to compensate the dopant losses after the dialysis procedure, an additional quantity of the dopant (equal to 10% of the theoretical content of dopant in completely doped PANI) was added to the PANI-CSA-SiC nanocomposites (or dialyzed individual PANI-CSA) dispersion before its drying. An exhaustive report of the synthesis process is reported elsewhere.15 For the sake of simplicity, hereafter we will refer to the PANICSA-SiC nanocomposites (or bare PANI), which were prepared without compensation of partial dedoping, as “low doped”. The samples with partially dedoping compensation will be referred to as “highly doped”. However, the exact doping level is determined by EPR measurements which evaluate the concentration of polarons in those media.

Figure 1. High magnification TEM images evidenced the location of PANI (amorphous aspects) around well-crystallized SiC nanoparticles (a), and a numerical simulation of a SiC nanoparticle surrounded by a PANI-CSA shell (b).

2.2. Transmission Electron Microscopy. Transmission electron microscopy (TEM) was used to reveal the polymer arrangement on the SiC nanoparticle surfaces. The TEM observations were carried out by using a JEOL 2010 microscope and an acceleration voltage of about 200 kV. The procedure consists of using a small quantity of the composite powder mixed with absolute ethanol and then submitting the mixture to an ultrasound stirring for 10 min in order to facilitate the dispersion of the grains. A droplet of the suspension was then deposited on a copper grid covered by an amorphous perforated carbon membrane. This substrate topology supports the evacuation of charges and limits the possible degradation of the compounds. Representative images of the composites are depicted in Figure 1 which shows a homogeneous covering of the nanoparticle surfaces by PANI at a thickness of about 1-2 nm. 2.3. EPR Measurements. EPR experiments were performed by using a Bruker CW X-band spectrometer operating at 9.39.9 GHz. Samples were placed into quartz tubes, and their EPR signals were recorded in the temperature range of 4-430 K with temperature stabilization ((0.5 K) by means of Oxford Instrument and Bruker GmbH cryostats. A magnetic field modulation of about 0.5 G and a microwave power on the order of 0.2 mW were typically used for all experiments to avoid line saturation. Intensities of the EPR spectra were calculated from the line parameters after deconvolution of EPR signal in terms of the derivative of normalized Lorentzian lines. 2.4. Raman Experiments. A confocal micro-Raman spectrometer with an Ar+ laser excitation at 514 nm was used at room temperature. For all the samples, the spectra were recorded in the same conditions and using the same set of parameters. Particular care was paid to the laser power which was limited down to 1 mW in order to prevent the overheating of the samples by the beam focused on an area of about 2 µm. 2.5. Dielectric and Electrical Measurements. Dielectric relaxation spectroscopy measurements were carried out over a wide frequency range (0.1 Hz to 3 MHz) at different temper-

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Figure 2. EPR spectra of PANI, SiC, and nanocomposites recorded at room temperature using the same experimental parameters (field modulation, microwave power). The amplification factor is indicated on the curves. Low intensity satellite lines are visible only for low doped polymers and SiC nanoparticles. The ratio A/B, which serves to identify the eventual dysonian EPR line shape, was evaluated for the marked line and found equal to 0.97 (0.01), i.e., less than unity, and then evaluated excluding the dysonian features of the EPR spectra.

atures between 153 and 473 K by using a Novocontrol broadband dielectric spectrometer. In the given frequency range, a Solartron SI1260 combined with a broadband dielectric converter (BDC) allowed impedance and dielectric measurements. The dielectric device was coupled to a Quatro temperature controller. Isothermal measurements were carried out on powder samples adjusted in a dielectric cell specially designed for powders. The temperature was measured in the immediate neighborhood of the sample with an accuracy greater than (0.1 K. For these measurements, the samples were made into cylindrical pellet forms with a thickness of about 2 mm and a diameter of about 5 mm. 3. Results and Discussion 3.1. EPR Spectra Features. Figure 2 summarizes the EPR spectra recorded at room temperature (RT) for different doping rates of PANI and nanocomposites. The spectra exhibit an asymmetrical shape with more or less resolved satellites depending on the doping level. Thus, for highly doped PANI and composites, the observed asymmetry may result either from the skin effect leading to dysonian EPR line features or from unresolved hyperfine or superhyperfine structures. In the first case, the height ratio A/B of the positive part to the negative one (Figure 2) should be greater than 1. However, no such requirement was found for the present EPR spectra. For this reason and whatever the doping rate, a deconvolution of the EPR spectra was performed by using derivatives of individual Lorentzian lines whose origins are justified from the involved paramagnetic centers and relevant interactions mainly with the neighboring nuclei as outlined below. 3.2. Origin of the Paramagnetic Centers. In the bare PANI, two main paramagnetic centers are expected from the acid

Kassiba et al. doping. The first is related to unpaired spins (polarons) involved in disordered regions of the polymer. The disorder is traduced by the heterogeneous doping of the polymer and also by the amorphous arrangement of the polymer chains acting as a barrier for the charge carriers’ mobility. These localized electronic centers interact with the surrounding nuclei which carry nuclear spins, and the resulting superhyperfine structure contributes to the EPR absorption line by more or less resolved satellites as observed in the low doped PANI. This is confirmed by the EPR spectra recorded at different frequencies, i.e., there are the same magnetic field separations between satellites at K, X, and S bands. So, with regard to these unresolved superhyperfine structures and the structural disorder around these paramagnetic species, their typical EPR signal exhibits relatively large line width. The second paramagnetic species is induced by the acid doping which creates active electronic centers as polarons in ordered regions of the polymer. This organization supposes filamentary arrangements of the chains as well as a crystallized area with metallic-like character. The polarons represent the charge carriers along the polymer chains, and their temperaturedependent dynamic contributes to the relaxation phenomena exhibited by EPR line width behavior. The EPR line from delocalized unpaired spins is characterized by a narrow shape due to motional averaging effects. When the nanocomposites doping level is relatively low, the EPR signal from SiC nanoparticles can be seen and occurs from dangling bonds involved at the outermost particle surfaces and from the carbon vacancies located in the particle cores. A detailed analysis of these line features from SiC nanoparticles was already reported.10,16 The EPR signal is composed of a central line which shows the main intensity flanked by weak satellites originating from hyperfine coupling with the 29Si nuclei (Figure 2a). By increasing the doping rate of the nanocomposites, no such details are resolved, and as shown in Figure 2, the EPR spectra are marked mainly by two contributing lines also observed for the relatively high doping rate of the bare PANI. The above arguments justify the deconvolution procedure carried out on the EPR spectra in all the investigated samples and are outlined below. 3.3. Deconvolution Procedure and Results. Figure 3 shows examples of experimental spectra deconvolution for low and highly doped PANI-CSA-SiC samples. For low doped PANI and composites, even if four individual lines are used to adjust the experimental spectra, the two satellite lines have a weak intensity in comparison with the central part of the spectrum. The line widths ∆Hpp and intensities obtained for satellite lines after the deconvolution procedure are then burdened with a large uncertainty due to their small intensities. They also have a small influence on the total spectral intensity over the whole temperature range studied. Furthermore, the significant contribution to the EPR is mainly from the central part accounted for by two derivatives of the Lorentzian line. For highly doped samples (PANI or nanocomposites), two lines account for all the spectra features throughout the whole temperature range. In carrying out the deconvolution and by using a reference sample, the double integration of the EPR spectra determines the overall spin concentrations (polarons) for the investigated samples at room temperature (Table 1). It is worth noting that, irrespective of the paramagnetic centers in silicon carbide nanoparticles, the unpaired spins from PANI dominate the overall spin density of the nanocomposites. As a matter of fact, the transport mechanism in the nanocomposites is monitored mainly by the PANI layers and the organicinorganic interfaces.

Hybrid Core-Shell Nanocomposites Based on SiC, PANI

Figure 3. Deconvolution of the EPR spectra by using derivatives of individual Lorentzian lines for (a) low doped and (b) highly doped PANI-CSA-SiC nanocomposite at room temperature.

TABLE 1: Absolute Unpaired Spin Concentrations in SiC Nanoparticles, Doped PANI, and Doped Nanocompositesa sample

concn [1017/g]

SiC PANI-CSA (low doped) SiC-PANI-CSA (low doped) PANI-CSA (highly doped) SiC-PANI-CSA (highly doped)

1.1 7.6 17 115 58

a Residual paramagnetic centers exist in SiC nanoparticles with low concentration compared to those involved in doped PANI or in doped nanocomposites.

3.4. Thermal Behavior of EPR Spectra. 3.4.a. EPR Line Widths. Excluding the weak satellites for a low doping rate, the spectra of all samples are marked mainly by two absorption lines with different characteristics, i.e., a narrow line from polarons in ordered regions and broad line from polarons localized in disordered regions.17 The line width dependencies versus the temperature for different doped samples are shown in Figure 4. In the low doped composite, only slight changes were noticed whatever the temperatures. In the highly doped composite, increasing the temperature from 4 to 100 K induced a narrowing of the line widths (Figure 4) due to the thermal activation of the polarons whatever their location, i.e., in ordered and disordered environments. Above 100 K, a small decrease is noticed on the narrow line while the broad line exhibits an increased line width with the maximum at 200 K.

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Figure 4. Temperature dependence of the peak-to-peak line widths ∆Hpp for (a) low doped and (b) highly doped PANI-CSA-SiC. A weak variation of ∆Hpp characterizes the EPR lines in the low doped sample (a) while large variations and singularities occur for the highly doped nanocomposite.

This behavior can be interpreted as due to interactions between the delocalized spins in the ordered regions and the localized ones in the disordered regions.17 However, alternative phenomena can also be invoked to explain the increase of the line width with respect to the temperature for the polarons in disordered regions. The line width increase might be a consequence of the hopping process of charge carriers giving rise to macroscopic conductivity. The process is characterized by a temperaturedependent transition rate probability between localized states as follows:

( )

Γ ∝ exp -

∆ kBT

with ∆ being the energy barrier between the localized states, kB the Boltzmann constant, and T the temperature. The corresponding EPR line width is inversely proportional to the characteristic hopping process time which is thermally activated. Its contribution to the line width would exhibit a thermal evolution as in Arrhenius’ law ∆H ∝ e-(∆/kBT) in analogy with a former EPR report.18 Above 200 K, the decrease of the broad line width (polarons in disordered regions) is primarily due to carrier diffusion and motional narrowing.19 3.4.b. Paramagnetic Spin Susceptibilities. The EPR line intensity is proportional to the paramagnetic spins susceptibility as well as to the unpaired spin (polarons) concentration. Its thermal variation is summarized in Figure 5 for the nanocomposites. While the low doped materials give rise to similar

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Figure 6. Temperature dependencies of the total EPR spectral intensity normalized by the intensity at 300 K: symbols, experimental points; lines, calculations according to eq 1 (see the text). A good agreement between experiments and the model (eq 1) is realized for all samples except for the highly doped PANI where a large discrepancy is found. The insert shows the expanded temperature range (4-430 K) for highly doped PANI and nanocomposites.

Figure 5. Typical EPR line intensity evolutions for PANI-CSA-SiC blend with (a) low and (b) high CSA doping levels. A similar line intensity evolution is noticed for localized and delocalized polarons in the low doped composites. Delocalized polarons dominate the contribution to the EPR line intensity above 100 K.

behavior of polarons in both ordered and disordered regions of the polymer shells, the variations are quite different in the highly doped sample. However, to exclude any misleading analysis based only on the main contributing lines from the deconvolution procedure, the forthcoming discussion deals with the overall intensity obtained by double integration of the EPR signals. Whatever the doping rate, the thermal evolutions of the spin susceptibilities were analyzed according to the following equation:20

χ(T) ) µBn(F) +

NCg2µB2 4kB(T - θ)

+

exp(-J/kBT) NTAg2µB2 (1) kBT 1 + 3 exp(-J/kBT) where µB is the Bohr magneton, n(F) is the density of states at the Fermi level F, kB is the Boltzmann constant, NC and NTA represent the concentrations of the Curie and thermally activated spin, respectively, g is the spectroscopic factor, and J is the interaction energy between neighboring polarons. The first term in eq 1 describes the temperature-independent Pauli susceptibility eventually involved in the polyaniline blends and originates from three-dimensional metallic regions whose size increases with the protonation level.21,22 The second term

is the Curie-Weiss-like contribution involved for uncorrelated localized centers or weakly interacting ones. The last term refers to thermally activated spins by a process such as the conversion between the spineless bipolarons and the polaronic paramagnetic centers.17,23 Figure 6 summarizes the temperature dependencies of double integrated EPR spectra intensities normalized by the value at 300 K. By using eq 1, the best adjustments of experimental curves are drawn as continuous lines, as in the case of low doped bare PANI, bare nanoparticles, and bare nanocomposites. For highly doped samples, the adjustments are shown by dotted lines, and the insert presents extended temperature range measurements, i.e., 4-430 K. Except for the highly doped bare PANI, a good agreement between experimental data and eq 1 is obtained for all the investigated samples. To summarize, silicon carbide susceptibility is described only by localized Curie-like spins. For the low doped samples, good adjustments were obtained by using the second and third term of eq 1, while in the highly doped nanocomposite, a small contribution of Pauli susceptibility is required. In all cases, the exchange interaction energy J between spins is almost the same within the uncertainty range. The values are about 855 (12) K and 801 (45) K for low doped bare PANI and nanocomposite, respectively, while 832 (35) K is obtained for highly protonated nanocomposites. This result indicates that the paramagnetic species are involved mainly in the doped polymer as bulk in the bare PANI or as shells in the nanocomposites. 3.5. Annealing Effect and Time-Dependent Spin Conversion. While the features of spin susceptibility in the nanocomposites are well accounted for by using eq 1, the highly doped PANI exhibits quite a singular behavior, and no relevant fits were achieved on the entire temperature range. In order to explain this discrepancy, time-dependent experiments were carried out to clarify the annealing effect on the stability of the samples as well as the possibility of an occurrence of some kinetic behavior. In this aim, annealing of the samples was performed in the same conditions at 430 K for 30 min. The EPR spectra of the low protonated samples (bare PANI, nanocomposites) and SiC nanoparticles did not show any timedependent behavior at high temperature (430 K for 30 min) nor after a fast cooling to room temperature (RT). Thus, no spin losses and no particular kinetic effect were revealed within the

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Figure 8. Raman spectra (excitation line: λ0 ) 514.5 nm) of the highly doped PANI-CSA and PANI-CSA-SiC recorded at room temperature before and after annealing. Note the appearance of the small band around 570 cm-1 in the Raman spectrum of highly doped PANI after annealing; the band correlates with the occurrence of phenazine-like structures (transverse bipolarons).

Figure 7. Time-dependent behavior of the EPR line width for the narrow line (delocalized polarons)sopen symbolssand integral intensitys filled symbolssrecorded (a) during annealing and temperature stabilization at 430 K and (b) after a rapid cooling down to 292 K for highly doped PANI-CSA. Continuous lines present an exponential law with characteristic times reported in the text.

range of experimental uncertainty. Similar features were also noticed in the highly doped nanocomposites, i.e., the changes on the EPR spectra were insignificant. The more drastic changes were observed after annealing the highly doped bare PANI. A line width broadening and a decreasing intensity at the fixed temperature 430 K were seen versus time (Figure 7a). During the 30 min annealing process, the EPR spectrum features were marked by a 2-fold decrease of the EPR signal intensity and a line broadening from the initial value of 0.95 to 1.4 G. An opposite relaxation behavior occurs for the line width and intensity after cooling the sample from 430 K down to RT. A crude analysis was undertaken to account for these kinetic processes by an exponential law with characteristic relaxation times. At 430 K, 10.8 (0.9) and 18.5 (2.2) min are characteristic times for the intensity and line width changes, respectively. At RT, the time-dependent process is longer than at 430 K but can also be described by an exponential law with the characteristic times 170 (6) and 236 (10) min for the intensity and line width, respectively. It is worth noting that, when the sample was annealed and rapidly cooled down to RT, the features of the EPR signal did not reflect the initial results. More precisely, after annealing, the final values of the EPR line intensity and line width at RT (tf∞) achieved 72% and 139% (1.75 G), respectively, in comparison with the initial values. Large losses of polaronic spins are then caused by the annealing treatments. The different behaviors observed in highly doped PANI and the low protonated polymer or in the nanocomposite samples can be justified from the polaron and bipolaron conversion process. On the one hand, the reversible process for the

thermally activated polarons (third term in eq 1) is important for the low protonated samples and highly doped nanocomposite in the intermediate and high temperature ranges. However, this process dominates the spin susceptibility behavior in highly doped PANI but only for an intermediate temperature range where a significant increase of paramagnetic susceptibility with the temperature is induced. Annealing studies clearly suggest that in the case of highly doped PANI, an important and partially reversible process is characterized by a long relaxation time to achieve the conversion equilibrium state between bipolaronic and polaronic states whatever the temperature range. These EPR experiments indicate drastic structural and electronic changes induced by annealing the highly doped PANI sample where the transport process is markedly different than those involved in the nanocomposites. Complementary measurements are then performed on the highly doped bare PANI and nanocomposites by Raman spectrometry and dielectric relaxation spectroscopy (DRS) and will be outlined briefly below. Raman measurements probe the structural features after annealing while the DRS technique states the correlation between the charge carriers (polarons, bipolarons) and the macroscopic conductivity of the samples. The Raman spectra, presented in Figure 8, were recorded by using an Ar+ laser excitation line at the wavelength λ0 ) 514.5 nm with the power of about 1 mW. The main changes on the Raman spectra of annealed samples consisted of an increase of the characteristic band centered at 574 cm-1, well marked for the PANI sample. According to references 24 and 25, this band is assigned to the formation of phenazine like structures produced by cross-linking among PANI chains. Moreover, as it was argued in reference 26, heterogeneous structures with cross-linked PANI are characterized by a lower conductivity value in comparison with homogeneous non-cross-linked ones. Thus, Raman experiments support the irreversible structural changes in the highly doped PANI through the creation of phenazine-like structures followed by an irreversible conversion between polarons and bipolarons. From the DRS measurements, the electrical conductivity (σ′) involved in the highly doped PANI and nanocomposites is

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Figure 9. Temperature dependence of dc-conductivity and paramagnetic spin susceptibility in the highly doped PANI. The marked temperature range corresponds to crossover regimes from conductivity dominated by polarons (carrying spins) to bipolarons (spinless quasiparticles).

deduced from the imaginary dielectric permittivity(′′) according to the relation σ′ ) ω0′′. Figures 9and 10) summarize the conductivity results compared to the unpaired spin (polarons) concentration obtained from EPR results. All the reported curves (conductivity and spin susceptibility) show the occurrence of the crossovers in their evolutions around the critical temperatures 300 and 375K. Between these temperatures, a similar evolution of conductivity is found for the PANI and nanocomposites. This contrasts with the behavior of the unpaired spin concentrations which show a higher slope for the nanocomposites than for PANI. These results indicate that polaron-bipolaron conversion occurs in the PANI in agreement with the Raman experiments (phenazine-like structures). In the nanocomposites, it seems that the conductivity is characterized mainly by thermally activated polarons. It is worth noting that, by a conjugation of EPR, Raman, and DRS measurements, relevant information on the structure and on the transport properties can be addressed. This is initially related to the occurrence of phenazine-like structures in the highly doped PANI as an irreversible process which created a strong effect of the annealing on the structural properties of the polymer. On the other hand, it was shown that, in the nanocomposites, the conductivity is similar to that of the PANI but with an improved structural stability due to the lesser

Kassiba et al.

Figure 10. Temperature dependence of dc-conductivity and paramagnetic spin susceptibility in the highly doped PANI-SiC composite. The marked temperature range corresponds to crossover regimes in the thermal activation of polarons responsible for the electrical conductivity. No particular spin losses (absence of bipolarons) are seen with regard to the highly doped PANI (Figure 9).

effects induced by annealing, i.e., no particular kinetic process and no spin losses. 4. Conclusions Consistent EPR investigations were carried out on original systems based on inorganic nanoparticles whose surfaces are functionalized by conducting PANI. Depending on the topology of the polymer location such as bulk-like arrangement in the bare PANI, or a surface-like arrangement in nanocomposites, the charge carrier dynamic, probed by EPR, exhibits very different behavior. Furthermore, EPR results show that the nanoparticles overcame the interchain bipolaron formation and the polaron conversion process. This is supported by the Raman spectra where no phenazine-like signatures were observed in the nanocomposites. Such structures, expected to appear with an increase of the polymer chains mobility, are no longer involved, or in anecdotic ratio, in nanocomposites due to the ability of the nanoparticles to isolate the protonated PANI chains. This fact is fully confirmed by the correct adjustment of the experimental susceptibility data according to the theory (eq 1) for the low protonated PANI and for PANI-SiC (long average distance between polaronic states) as well as in the highly doped PANI-SiC. On the other hand, in highly protonated PANI

Hybrid Core-Shell Nanocomposites Based on SiC, PANI without silicon carbide nanoparticles, no such agreement between the experimental data and the theory is realized because of the occurrence of irreversible structural changes through the phenazine-like structure formation with annealing. Finally, it is worth noting that the performed experiments indicate that the main role of the SiC nanoparticles on the nanocomposite performance includes improved thermal and structural stability associated with similar electrical responses. Acknowledgment. W.B. is grateful to the regional Pays de la Loire Council for financial support through a postdoctoral fellowship. Dr. J. Boucle´ (Optoelectronics Group, Cavendish Laboratory, Cambridge University) and Drs. M. Mayne and N. Herlin-Boime (LFP-CEA, Saclay, France) are acknowledged for the synthesis of the SiC nanoparticles. We thank Prof. A. Bulou (LPEC, Le Mans) for recording the Raman spectra and for his fruitful discussions. A.K. dedicates this article to the TARDIEU Service team of CHM, Le Mans, France. References and Notes (1) Yuwono, A. H.; Xue, J.; Wang, J.; Elim, H. I.; Ji, W.; Li Y.; White, T. J. J. Mater. Chem. 2003, 13, 1474. (2) Choudhury, K. R.; Winiarz, J. G.; Samoc, M.; Prasad, P. N. Appl. Phys. Lett. 2003, 82, 3, 406. (3) Erwin, M. M.; Kadavanich, A. V.; McBride, J.; Kippeny, T.; Pennycook, S.; Rosenthal, S. J. Eur. Phys. J. D 2001, 16, 1. (4) Boucle´, J.; Kassiba, A.; Makowska-Janusik, M.; Herlin-Boime, N.; Reynaud, C.; Desert, A.; Bulou, A.; Emery, J.; Sanetra, J.; Pud, A. A.; Kodjikian, S. Phys. ReV. B 2006, 74, 205417. (5) Charpentier, S.; Kassiba, A.; Bulou, A.; MOnthioux, M.; Cauchetier, M. Eur. Phys. J: Appl. Phys. 1999, 8, 111. (6) Kassiba, A.; Makowska-Janusik, M.; Boucle´, J.; Bardeau, J.-F.; Bulou, A.; Herlin-Boime, N. Phys. ReV .B. 2002, 66 (15), 155311. (7) Makowska-Janusik, M.; Kassiba, A.; Boucle´, J.; Bardeau, J.-F.; Kodjikian, S.; De´sert, A. J. Phys.: Condens. Mater. 2005, 17, 5101. (8) Kassiba, A.; Tabellout, M.; Charpentier, S.; Herlin-Boime, N.; Emery, J. R. Solid. State Commun. 2000, 115, 389. (9) Tabellout, M.; Kassiba, A.; Tkaczyk, S.; Laskowski, L.; Swiatek, J. J. Phys.: Condens. Mater. 2006, 18, 1143.

J. Phys. Chem. C, Vol. 111, No. 31, 2007 11551 (10) Kassiba, A. In Nanostructured Silicon based powders and composites; Editors A.P. Legrand, A.P., Ch. Se´ne´maud, Ch., Eds.; Francis & Taylor (GB): 2002. (11) Pud, A. A.; Tabellout, M.; Kassiba, A.; Korzhenko, A.; Rogalsky, S. P.; Shapoval, G. S.; Houze, F.; Schneegans, O.; Emery, J. R. J. Mater. Sci. 2001, 36 (14), 3355. (12) Reghu, M.; Cao, Y.; Moses, D.; Heeger, A. J. Phys. ReV. B 1993, 47, 1758. (13) MacDiarmid, A. G.; Epstein, A. J. Electronic Properties of Conjugated Polymers; Kuzmany, H., Mehring, M., Roth, S., Eds.; Springer: Berlin, 1992. Premvardhan, L.; Peteanu, L. A.; Wang, P.-C.; MacDiarmid, A. G. Synth. Met. 2001, 116, 157. Shimano, J. Y.; MacDiarmid, A. G. Synth. Met. 2001, 123, 251. (14) Stejskal, J.; Quadrat, O.; Sapurina, I.; Zemek, J.; Drelinkiewicz, A.; Hasik, M.; Kivka, I.; Proke, J. Eur. Polym. J. 2002, 38, 631. (15) Pud, A.; Noskov, Yu.; Kassiba, A.; Fatyeyeva, K.; Ogurtsov, N.; Makowska-Janusik, M.; Bednarski, W.; Tabellout, M.; Shapoval, G. J. Phys. Chem. 2007 (in press). (16) Charpentier, S.; Kassiba, A.; Emery, J.; Cauchetier, M. J. Phys.: Condens. Mater. 1999, 11, 4887. (17) Kahol, P. K. Solid. State Commun. 2001, 117, 37. Kahol, P. K.; Ho, C. J.; Chen, Y. Y.; Wang, C. R.; Neeleshwar, S.; Tsai, C. B.; Wessling, B. Synth. Met. 2005, 153, 169. (18) Rudolf, T.; Bo¨hlmann, W.; Po¨ppl, A. J. Magn. Res. 2002, 155, 45. (19) Liu, H. K.; Shih, C. C.; Wang, G. P.; Wu, T. R.; Wu, K. H.; Chang, T. C. Synth. Met. 2005, 151, 256. Kahol, P. K.; Pinto, N. J. Synth. Met. 2004, 140, 269. (20) Iida, M.; Asaji, T.; Inoue, M. B. Synth. Met. 1993, 55, 607 and references therein. (21) Kahol, P. K.; Guan, H.; McCormick, B. J. Phys. ReV. B. 1991, 44, 10393. Pinto, N. J.; Kahol, P. K.; McCormick, B. J.; Dalal, N. S.; Wan, H. Phys. ReV. B. 1994, 49, 13983. (22) Wang, Z. H.; Scherr, E. M.; MacDiarmid, A. G.; Epstein, A. J. Phys. ReV. B. 1992, 45, 4190. (23) Kahol, P. K.; Raghunathan, A.; McCormick, B. J. Synth. Met. 2004, 140, 216. (24) Boutaleb, N.; Benyoucef, A.; Salavagione, H. J.; Belbachir, M.; Morallo´n, E. Eur. Polym. J. 2006, 42, 733. (25) Soares, B. G.; Amorim, G. S.; Souza, F. G., Jr.; Oliveira, M. G.; Pereira da Silva, J. E. Synth. Met. 2006, 156, 91. (26) Pereira da Silva, J. E.; Temperini, M. L. A.; Co´rdoba de Tprresi, I. J. Braz. Chem. Soc. 2005, 16, 322.