Polyaniline-Pt and polypyrrole-Pt nanocomposites: Effect of supporting type and morphology on the nanoparticles size and distribution

Synthetic Metals 203 (2015) 22–30

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Polyaniline-Pt and polypyrrole-Pt nanocomposites: Effect of supporting type and morphology on the nanoparticles size and distribution Hugo G. Lemos, Sydney F. Santos, Everaldo C. Venancio * Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas (CECS), Universidade Federal do ABC (UFABC), Av. dos Estados 5001, Bairro Bangu, Santo André, SP, 09210-580, Brazil

A R T I C L E I N F O

A B S T R A C T

Article history: Received 17 November 2014 Received in revised form 20 January 2015 Accepted 5 February 2015 Available online xxx

The size and the distribution of platinum nanoparticles on the polyaniline-Pt (PANI-Pt) and polypyrrolePt (PPy-Pt) nanocomposites are extremely sensitive to the polymer matrix morphology. This is observed when the polymer matrix morphology was changed from nanofibers to nanotubes. TEM results showed platinum nanoparticles with size diameter as low as 2 nm in the case of polpyrrole nanofiber and polyaniline nanofiber matrices. In addition, FTIR and UV–vis results showed that polyaniline matrix with different morphologies, nanofibers and nanotubes, presented different chemical structures, which resulted in a different size and distribution of the platinum nanoparticles, as it was observed from the SEM, platinum X-ray mapping and TEM results. The DSC thermal analysis results showed that the presence of platinum nanoparticles in the polymer matrix resulted in a greater thermal stability when compared to the polymer matrix. It was the first time that the effect of PANI and PPy morphology on the platinum dispersion and distribution is demonstrated. ã 2015 Published by Elsevier B.V.

Keywords: Polyaniline Polypyrrole Morphology Platinum Nanocomposite

1. Introduction Polyaniline (PANI) and polypyrrole (PPy) are intrinsic conductive polymers with applicability in several areas such as molecular electronics, energy storage and conversion devices, flexible electronics (FET – field-effect transistors), systems for cell growth and differentiation, and supports for catalysts [1]. The use of conducting polymers as supports for catalysts has demonstrated advantages of these materials when compared to conventional supports such as high surface area which allows a significant reduction in the amount of catalyst used [2]. The nanostructured tridimensional arrangement of these polymers allows a good dispersion of low amount of active metal nanoparticles resulting in a significant increase in the available active area for catalytic reaction. The deposition of metal nanoparticles on conducting polymers can be carried out by using electrochemical [2] or chemicals methods [3–5]. The conducting polymers, PANI [3–5] as well as PPy [6], have different oxidation states and the fully reduced state of these polymers reacts spontaneously with platinum, ruthenium and palladium salts. This is because the standard potential of the

* Corresponding author. Tel.: +551149968200. E-mail address: [email protected] (E.C. Venancio). http://dx.doi.org/10.1016/j.synthmet.2015.02.006 0379-6779/ ã 2015 Published by Elsevier B.V.

reduced state of these functional polymers is negative (lower than
0.1 V vs. SCE). Thus, controlling the oxidation state and the deposition time, small amounts of catalytic species may be incorporated on a tridimensional nanostructured polymeric matrix. Several studies involving the synthesis of PANI and PPy conducting polymers based nanocomposites with metallic nanoparticles have been published [7–9]. According to the literature, PANI-Pt nanocomposites may be obtained using several different metallic precursors, reducing agents and methods [10–12]. PANI-Pt has been prepared using the metallic precursor H2PtCl6 itself as oxidizing agent during the polymerization of aniline monomers [13,14]. Moreover, PANI-Pt nanocomposite with good dispersion of the metallic nanoparticles was obtained by the dissolution of the polymeric matrix in a solution containing Pt nanoparticles synthesized by the reduction of the metallic ions in the presence of sodium citrate. [15] Hirao et al. [16–18] published a series of pioneering work describing the study of different hybrid systems of polyaniline (and polyaniline derivatives, such as poly(2-methoxy-oxyaniline-5sulfonic acid)) with transition metals (Cu(II), Fe(III), Pd(II) and V (III) and transition metals nanoparticles (Pd and iron oxide). It was pointed out that the polyaniline and polyaniline Cu(II) e Fe(III) complexes are considered to contribute as efficient systems for electron transfer, which was found to be the first known example for the catalyst redox reaction achieved by conducting polymers

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[16]. It was shown in detail the role of polyaniline as a redox mediator for catalysis in organic redox reactions, with better yield in the presence of molecular oxygen [17]. The role of molecular oxygen in the catalytic process is depicted and it is related to the reversible interconversion of different oxidation states in p-conjugated polymers such as polyanilines. It was also shown that there are different types of coordination between transition metals ions and polyanilines, where it was found that the complexation proceeds via the quinoid moieties. Polyanilines and different transition metals complexes interact in different ways, which was shown by the UV–vis absorption spectra results. Regarding the formation of polyaniline-transition metal nanocomposites, it was presented three methods to obtain polyaniline-Pd (nanoparticles): (i) direct reduction approach, in which metal ion and polyaniline are mixed and then reduced to form the metal nanoparticles; (ii) template method, where are involved two steps, the first is the polyaniline-metal ion complex formation, and the second is the reduction of the metal ion of the coordinated complex; this method is time-dependent, since the polyaniline-metal ion coordination process needs to reach an equilibrium state;

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additionally, the Pd metal ion ligand also interfere on the resulting conjugated system, which may result in a network or single strand systems; (iii) ligand exchange method, which is a reductant-less method, where two metal nanoparticle ligand are used, one is the ligand that contains the metal nanoparticle (e.g., starch, ligand A), and the other ligand is the polyaniline (ligand B). In this method, the metal nanoparticles reduction is carried out in the presence of ligand A, and then ligand exchange reaction from ligand A to ligand B, the polyaniline, resulting in formation of polyaniline-metal nanoparticles system. The aim of this method is to preserve the polyaniline ligand initial oxidation state, which might interfere on the catalytic properties of the system. The authors [18] also presented the key parameters to address the design and redox function of conjugated complexes with polyanilines or quinonediimines, which are expected to control their dynamic redox properties and high selectivity to electron transfer reactions. The complexation of poly(ortho-toluidine) (POT) with Pd(II) was carried out in an organic solvent. The quinonediimines moieties were available for complexation. The role of the p-conjugated system is to provide electronic communication between the redox

Fig. 1. SEM micrographies: polymers matrices of (A) polyaniline nanofibers – PANI-1, (C) polyaniline nanotubes – PANI-2 and (E) polypyrrole nanofibers – PPy. At the right side the nanocomposite of their respective matrices. The SEM images show the better dispersion of the Pt nanoparticles in the PANI-1-Pt (B) and PPy-Pt (F) nanocomposites. The PANI-2-Pt presented a large number of Pt agglomerates (D).

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sites, where polyanilines are promising materials to reach such system. p-conjugated ligands, such as polyanilines, present at least more than two coordination sites. Therefore, at least two systems with different structures can be designed. The resulting structure also depends on the number of coordination sites of the metal ligands. Furthermore, as already mentioned, electrochemical methods have been carried out to deposit platinum nanoparticles on the polymeric matrix [19]. The formation of platinum nanoparticles was obtained by reducing the K2PtCl4 salt in the presence of formic acid [20]. The use of reducing agents has also been reported by Nyczyk et al. [21] to obtain PANI-Pt nanocomposites in the presence of sodium borohydride (NaBH4) and having PtCl4 salt as metal precursor. This synthesis route resulted in well-dispersed platinum nanoparticles. However, it is important to mention that only few works on the synthesis of PPy-Pt nanocomposites have been reported in the literature [22–24]. Additionally, PANI morphology strongly depends on the synthesis method adopted, as reported previously, obtaining it in fibers, tubes, spheres, hollow spheres and flake-like shapes from nanometric to micrometric sizes [5]. As it was pointed out previously, “—there are as many different types of polyaniline as there are people who make it!” [5]. In the case of PPy, fiber and spheres are the major morphologies obtained depending on the synthesis method [25]. Notwithstanding the progresses on the synthesis of conducting polymers and their (nano) composites reported in the literature over the last years, from the best of our knowledge there is a lack of information concerning the effect of PANI and PPy morphology on the platinum nanoparticle average size and distribution in PANI-Pt and PPY-Pt nanocomposites. Therefore, in this research we investigate the effect of different synthesis routes in PANI-Pt and PPy-Pt based nanocomposites. Specifically, the morphology of the polymeric matrices was modified in order to evaluate its effect on the average size and dispersion of the platinum nanoparticles.

peroxydisulfate (APS, 0.10 mol L
1) in the presence of 1.0 mol L
1 HCl aqueous solution. The polymerization reaction was performed during 24 h under rigorous mechanical magnetic stirring. In order to obtain nanotubes of polyaniline (PANI-2) [5], it was used the same molar concentration above mentioned. However, the oxidative polymerization reaction was carried out in a weak acid medium (pH 2; HCl, 0.010 mol L
1). Nanofibers (PANI-1) and nanotubes (PANI-2) of polyaniline were non-doped using ammonium hydroxide (NH4OH, 0.20 mol L
1) aqueous solution by 24 h.The precipitate was collected on a Buchner funnel by using a filter paper (9.0 cm in diameter, Nalgon Ref. 3550) and then washed with high-purity water. Finally, polyaniline samples were dried under dynamic vacuum for 6 h at 60  C. 2.2.2. Nanofibers of polypyrrole matrix The synthesis of polypyrrole [25] was carried out by oxidizing pyrrole monomer (0.12 mol L
1) with APS (0.040 mol L
1) in 1.0 mol L
1 HCl aqueous solution. In order to obtain nanofiber morphology, CTAB was used (0.025 mol L
1). The precipitate was collected on a Buchner funnel by using a filter paper (9.0 cm in diameter, Nalgon Ref. 3550) and then

2. Experimental 2.1. Materials All the chemicals used were reagent grade. Aniline, pyrrole, ammonium peroxydisulfate ((NH4)2S8), APS), hydrochloric acid (HCl, 37%), ammonium hydroxide (NH4OH, 28%) and cetyltrimethylammonium bromide (CTAB) were purchased from Synth. Sodium tetrahydridoborate (NaBH4) and platinum (IV) chloride were purchased from Merck. N-methyl-2-pyrrolidone (NMP) (anhydrous, 99.5%) was purchased from Sigma–Aldrich. All reagents had highpurity having no necessity of further purification processes. Highpurity water used in this study for chemical synthesis is the water purified using Millipore Milli-Q deionizing system. 2.2. Synthesis of conducting-polymer matrices The synthesis of conducting-polymer matrices was carried out based on procedures reported by MacDiarmid and co-wokers [4,5,25]. The oxidative chemical polymerizations of aniline and pyrrole samples were carried out using aniline and pyrrole monomers, (NH4)2S2O8 and HCl at room temperature. Conducting polymers containing different morphology were obtained according to the procedures described in Sections 2.2.1 and 2.2.2. 2.2.1. Nanofibers and nanotubes of polyaniline matrices Nanofibers of polyaniline (PANI-1) [4] were obtained by the oxidation of aniline (0.10 mol L
1) with ammonium

Fig. 2. XRD patterns for (A) PANI 1 and PANI-1-Pt nanocomposite, (B) PANI 2 and PANI-2-Pt nanocomposite, and (C) PPy and PPy-Pt nanocomposite.

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washed with high-purity water until no bubble has been observed in the filtrate. Finally, it was dried under dynamic vacuum for 6 h at 60  C. 2.3. Synthesis of PANI-Pt and PPy-Pt nanocomposites For the synthesis of the conducting polymers-based nanocomposites, Pt nanoparticles were obtained by reducing Pt+4 ions from PtCl4 precursor in an aqueous solution according to the method described elsewhere [21]. It was used 0.2 mol of Pt per 1.0 mol of PANI mer and 1.0 mol of PPy mer. The polyaniline and polypyrrole molar amounts were calculated based on the tetramer units of polyaniline (C24H18N4) and 4 polypyrrole (C16H12N4), respectively. Amounts of 0.050 g of each sample obtained as described in Section 2.1 (PANI-1, PANI-2 and PPy-1) were introduced in three different beakers containing 5.0  10
3 mol L
1 of PtCl4 aqueous solution. The reactants were kept under rigorous mechanical magnetic stirring for 5 min. Then, NaBH4 powder was added into the mixtures using a 2.5 NaBH4/Pt molar ratio. After 2 h, the three samples (PANI-1-Pt, PANI-2-Pt and PPy-1-Pt) were filtered, washed and as previously described.

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2.4. Characterization of PANI-Pt and PPy-Pt nanocomposites 2.4.1. Morphological characterization Scanning electron microscopic (SEM) analysis was performed using a Jeol JSM 6701F – field emission electron microscope equipped with EDX microanalysis. High resolution transmission electron microscopy (HR-TEM) observations were carried out using a, 200 KV field-emission microscope (FEI, Tecnai G2F20). X-ray diffractometry (XRD) was carried out using a Bruker D8 Focus diffractometer (Cu-Ka radiation). 2.4.2. FTIR and UV–vis studies FTIR analysis was carried out using a Varian 660-IR spectrometer with an ATR accessory. The spectra were collected in the range of 400–4000 cm
1, with 2 cm
1 of resolution. UV–vis analysis was carried out using a Varian Cary 50 spectrometer using quartz cuvettes (1 cm path length). The solid samples were dissolved in 5 mL of N-methyl-2-pyrrolidone (NMP) (Aldrich) and then shaken for 30 s. Samples were then diluted with an additional NMP. NMP was used as the reference.

Fig. 3. TEM bright-field images of PANI-1-Pt nanocomposite.

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2.4.3. Thermal characterization TGA analysis was performed on a Q 600 thermogravimetric balance from TA Instruments. The nanocomposite samples were heated up under inert atmosphere (N2) from 25 to 900  C at constant rate of 10  C/min. 3. Results and discussion 3.1. SEM and X-ray diffraction analysis Fig. 1 shows the SEM images of different conducting polymers matrices (PANI-1, PANI-2 and PPy) and their nanocomposites (PANI-1-Pt, PANI-2-Pt and PPy-Pt), respectively. By the SEM analysis, it was observed that a better distribution of the platinum nanoparticles was obtained on the matrices composed by PANI-1 and PPy nanofibers. Otherwise, analyzing the PANI-2-Pt SEM micrograph, it was noted the presence of platinum agglomerates resulting in a poor dispersion. The nanocomposites were also characterized by XRD analysis. The diffraction patterns are shown in Fig. 2 for the all the investigated matrices and the nanocomposites. The PANI-1 diffraction pattern (Fig. 2A) is composed by two bands partially overlapped with the most intense one at 20 and a shoulder at around 29 . Conversely, the diffraction pattern of PANI-2 presents several peaks indicating a higher degree of crystallinity, as previously reported in the literature [26,27]. The different features of the XRD patterns of PANI-1 and PANI-2 may be due to a tubularlike structure of PANI-2. It is known that nanotube-like polyaniline nanostructures contains some oligomer and phenazine structures

which might present a higher degree of crystallinity as compared with nanofiber-like polyaniline structures [28]. In addition, later results published in the literature has described these structures present in some polyaniline nanostructure as adduct of aniline which might be formed in the earlier stages of the polymerization reaction of aniline [5,29], which might present higher degree of crystallinity as compared with conventional structures of polyaniline [27]. The presence of these structures [5,29] might not favor the formation of platinum nucleation sites. The XRD patterns of the nanocomposites show the diffraction peaks of the Pt (111) and (2 0 0) planes in addition to the peaks and bands of the respective matrices. 3.2. Transmission electron microscopy TEM bright-field images of the PANI-1-Pt, PANI-2-Pt and PPy-Pt samples are shown in Figs. 3–5, respectively. These results indicated differences on the microstructure of the investigated nanocomposites. Fig. 3A–D (PANI-1-Pt) indicates a relatively large dispersion of platinum nanoparticle, with particle sizes ranging from 2 up to 10 nm, with average diameter of 4.5 nm with standard deviation (SD) of 2.4 nm. From these images it is also possible to observe that most of these particles are agglomerated and these agglomerates (with tens of nanometers) are relatively welldispersed onto the polymeric matrix. In the PANI-2-Pt sample, the Pt nanoparticles have sizes slightly larger with average diameter of about 5.2 nm (SD of 1.5 nm), as observed in Fig. 5. The microstructure of this sample also comprises agglomerates of Pt nanoparticles, but in this case these agglomerates are much

Fig. 4. TEM bright-field for PANI-2-Pt nanocomposite.

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larger and poorly distributed when compared to the PANI-1-Pt sample. These results are in agreement with the idea of low interaction between the platinum nanoparticles and the nanotubelike polyaniline chains. The microstructure of the PPy-Pt sample is shown in Fig. 5. The mean diameter of the Pt particles is about 4.8 nm (SD of 1.8 nm), which is close to the value observed for the PANI-1-Pt sample. Taking into account that low particle size, sharp size distribution, and homogeneous distribution of particles on the polymeric matrix are desirable for practical purposes, the PPy-Pt shows the most interesting combination of microstructural features. The presence of a conducting polymer network apparently acts as a steric stabilizer agent for the platinum nanoparticles. The aniline and pyrrole polymerization processes were carried out using relatively low concentration of monomers and oxidizing agent, which resulted in the formation of nanofibers with smaller diameters. Thereby, the presence of these polymeric chains adsorbed on the platinum nanoparticles surface might act as a diffusion barrier for the growth of Pt+4 species, preventing the formation of larger platinum particles. 3.3. FTIR and UV–vis results The PANI matrices degree of protonation can be evaluated by the shift in the peak position related to the quinoid band and to the benzoid band (Fig. 6), which are located at 1591 cm
1 and 1501 cm
1, respectively. In addition, a third peak located at 1160 cm
1 is related to the C—C deformation plan. Fig. 6A shows the PANI-1-Pt FTIR spectrum where a shift in the peak positions located at 1583 cm
1 and 1482 cm
1 can be observed. In addition, it

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was observed a new peak at 1106 cm
1. In the literature, it is reported that the shifts of quinoid and benzoid bands to lower wavenumber, in addition to the emergence of a new band at 1106 cm
1 are attributed to the PANI protonation process [30]. The PANI-2-Pt spectrum (Fig. 6B) presents a shift in the peak position (1577 cm
1) related to the quinoid band. However, no changes were observed for the peaks associated with the benzoid band and the deformation plan. The low displacement of these peaks and the absence of a new peak at 1106 cm
1 might be associated with lower protonation degree of PANI resulted by the low amount and dispersion of the platinum nanoparticles [21]. These results are in good agreement with those obtained from the morphological studies (Figs. 1 and 2) indicating the lower capacity of the nanotube structures in acting as steric stabilizer agent. Fig. 6C presents the PPy-Pt spectrum indicating the peak associated with the stretching of C¼C bond at 1573 cm
1, whereas the stretching vibration band of the C—N bond appears at 1473 cm
1. The vibration of the pyrrole ring occurs at 1213 cm
1, whereas the deformation of the C—H and N—H bonds appears at 1051 cm
1. These small changes in the peak positions can be related to a possible interaction of platinum particles and the PPy polymer chains. The UV–vis spectra of the emeraldine base form of the polyaniline matrices and polyaniline-Pt nanocomposites are presented in Fig. 7. It is interesting to point out that there was a shift of the band attributed to a local charge transfer between a quinoid ring and the adjacent imine–phenyl–amine units observed in the matrices (630 nm for nanofibers and 600 nm for nanotubes) to lower wavelength values for both nanocomposites. It

Fig. 5. TEM bright-field for PPy-Pt nanocomposite.

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with the as we know polyaniline emeraldine base portion of the nanotube matrix structure. This might also be related to the TEM results, where it was observed the formation of agglomerates which are larger and poorly distributed when compared to the platinum nanoparticles observed in the PANI-1-Pt sample. As it can be seen, the spectra for polyaniline nanotubes are different from the as known “conventional” emeraldine base oxidation state for polyaniline. It was demonstrated in a previous work of MacDiarmid and co-wokers [5] that the synthesis depends on the synthesis conditions used (e.g., the initial pH of the polymerization medium). Depending on the experimental condition used to obtain polyaniline, a multitude of morphologies can be obtained, such as nanofibers, nanotubes, hollow microspheres, micro- and nano-rods, microflakes, for example [1]. The formation of such variety of structures is related to the formation of oligoanilines at the initial stage of the polymerization reaction, which might act as a template for the further growth of polyaniline as we know. In addition, as it was pointed out previously [17], the production of of polyaniline-transition metals nanoparticles depends on the method used to obtain these systems, where the interaction of polyaniline and transition metals ions can result in different types of morphology due to the self-assembling process that the system might undergoes. In this work it was used the so called direct reduction approach described by Amaya and Hirao [17] and we believe that initial matrices morphology, polyaniline nanofibers and polyaniline nanotubes, are not changed significantly.

Fig. 6. FTIR spectrums (A) PANI-1-Pt, (B) PANI-2-Pt and (C) PPy-Pt.

might indicate that the oxidation degree of the polyaniline matrices was increased after the NaBH4 assisted reduction of Pt (IV) ions. In Fig. 7B the shift of this absorption band is lower to the nanotube matrice, which might indicate that the Pt atoms interact

Fig. 7. UV–vis absorption spectra for (A) polyaniline emeraldine base oxidation state matrices, nanofibers (PANI-1) and nanotubes (PANI-2), and (B) polyaniline-Pt nanocomposites, with nanofibers (PANI-1-Pt) and nanotubes (PANI-2-Pt) matrices, respectively. Solvent, N-methyl-2-pyrrolidone (NMP).

Fig. 8. TGA results (A) PANI-1-Pt, (B) PANI-2-Pt and (C) PPy-Pt.

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3.4. Thermogravimetric analysis The thermal stability of the nanocomposites is shown in Fig. 8. The presence of platinum nanoparticles in the polymeric matrix of PANI-1-Pt nanocomposite resulted in a greater thermal stability when compared to the TGA results for PANI-1 matrix. From the TGA curves (Fig. 8A), it can be observed a weight loss between 25 and 120  C due to the removal of adsorbed water molecules. However, only pure PANI-1 sample showed a high loss of weight due to the degradation of the polymer matrix between 400  C and 600  C whereas the nanocomposite showed greater stability up to 600  C. The TGA curves of PANI-2 and PANI-2-Pt nanocomposite (Fig. 8B) are very close, indicating similar thermal stability, with appreciable weight loss after 181  C. The lack of improvement for the thermal stability showed by the PANI-2-Pt sample can be associated to the lower amount of platinum nanoparticle as compared with the PANI-1-Pt nanocomposite. In addition, it was observed some differences between the PANI-1 (Fig. 8A) and PANI2 (Fig. 8B) samples. The PANI-1 (nanofibers) showed a higher stability (Fig. 7A) up 400  C, which is described in the literature as a result of the cross-linking formation due to the polyaniline isomerization process [31,32]. These results showed that the difference in the chemical structure between PANI-1 (nanofibers) and PANI-2 (nanotubes) depletes the isomerization process which is observed in the PANI-1 structure. It is interesting to point out the work done by Wang et al. [32], where it was shown that HCl-doped polyaniline nanofibers could undergo mild degradation of the polymer backbone between 164  C and 200  C before the major degradation at 400  C. Thereby, the interaction among the platinum nanoparticles and the PANI-1-Pt cross-linked structure enhances the thermal stability of the PANI-1-Pt nanocomposite. These results corroborate those obtained from the morphologic and spectroscopic characterizations. Therefore, it can be inferred that different chemical structures between the investigated PANI matrices resulted in different interactions with Pt nanoparticles (a stronger interaction in the case of PANI-2 matrix). These results demonstrate the possibility of tuning the microstructure of PANI-Pt nanocomposite by controlling the matrix morphology. Fig. 8C presents the thermogravimetric curves for polypyrrole and PPy-Pt nanocomposite. No significant change in thermal stability is observed between these samples. The lack of changes in the stability probably could be associated with the relatively low content of platinum nanoparticle on the PPy-1-Pt nanocomposite. Further investigations are still necessary to shed some light on the different thermal stabilities observed for PANI-1-Pt and PPy-Pt despite the similarities concerning the microstructure of these composites. Additionally, some slight difference in thermal stability between the PPy matrix and the PPy-Pt nanocomposite observed above 720  C (Fig. 8C) might be due to a catalytic effect of platinum nanoparticles on the PPy degradation process (oxidation), which was not observed for PANI matrix nanocomposites (Fig. 8A and B). 4. Conclusions The results showed that a better distribution and dispersion of the Pt nanoparticles were obtained for polymer nanofiber matrices (PANI-1-Pt and PPy-1-Pt) than the polymer nanotube matrix (PANI-2-Pt). TEM results show a strong influence of the matrix morphology on the nanocomposite microstructure, mainly concerning the nanoparticle size, distribution and degree of agglomeration. Smallest polymers nanostructures (nanofibers) might provide large number of heterogeneous nucleation sites for nucleating Pt nanoparticles, avoiding their agglomeration. Therefore, better distribution of these nanoparticles throughout the matrix can be reached, also improving their size control. Moreover,

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FTIR spectra suggest an interaction between the Pt and the polymeric chains. The slighter shift of bands observed on PANI-2-Pt indicates that a difference of chemical structure of the PANI samples resulted in a smaller interaction between the Pt and PANI nanotubes contributing to a lower dispersion of the Pt nanoparticles.

Acknowledgements The authors gratefully acknowledge the financial support of FAPESP (process number 2011/16615-9) and CNPq (process number 481647/2011-2) and the Universidade Federal do ABC (UFABC). They also acknowledge use of the facilities at the CEM-UFABC and to Maria Alice Martins (Embrapa/CNPDIA) for the thermal analysis.

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