Polypyrrole

19878

J. Phys. Chem. C 2008, 112, 19878–19885

Synthesis and Characterization of Palladium Nanoparticle/Polypyrrole Composites Svetlana V. Vasilyeva,*,†,‡ Mikhail A. Vorotyntsev,‡ Igor Bezverkhyy,§ Eric Lesniewska,§ Olivier Heintz,§ and Remi Chassagnon§ Inorganic Chemistry Department, A. I. Hertzen State Pedagogical UniVersity, St. Petersburg, Russia, ICMUB-UMR 5260 CNRS, UniVersity of Burgundy, Dijon, France, and ICB-UMR 5209 CNRS, UniVersity of Burgundy, Dijon, France

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ReceiVed: June 19, 2008; ReVised Manuscript ReceiVed: October 22, 2008

In this work, a simple non-template one-step method for the synthesis of 2.0-2.5 nm palladium nanoparticles encapsulated into a polypyrrole shell via direct redox reaction between palladium(II) acetate and pyrrole in acetonitrile medium is described. Palladium nanoparticles are found to be able to self-organize into spherical Pd/PPy composites. The size of the Pd/PPy composite particles and Pd content in the composite depend strongly on the concentration of the palladium salt component. The combination of elemental CHNS and thermogravimetric (TGA) analysis was used to determine a high (∼40 wt %) content of palladium, which is in a good agreement with EDX data. The process of Pd/PPy composite formation was studied with the use of UV-vis spectroscopy and AFM spectroscopy. Properties of the obtained material were characterized by means of FTIR, XPS, XRD, SEM, and TEM techniques. 1. Introduction Nanostructured transition metal clusters and colloids are of great interest as catalysts of different organic reactions,1-9 electrocatalysts in fuel cells,10,11 and active components in nanoelectronic sensor and optical devices.12 Properties of nanocluster composites depend not only on the nature of the incorporated metal but also on the particle size, their shape, spatial distribution of metal nanoclusters and the nature of stabilizing agents, preventing them from aggregation. The development of the new metal-containing composite materials for described applications must meet several key requirements: formation and stabilization of very small metal particles, their high loading and uniform distribution inside the stabilizing matrix. Since specified parameters can be modified by a proper choice of the applied synthetic procedure,13 the use of suitable methods of synthesis providing an effective control of these parameters is of critical importance. Another crucial factor for determining the properties of such composite materials is the medium surrounding the metal particles. During the last two decades the use of conducting polymers as supporting matrices for incorporation of metal nanoparticles attracted considerable attention.14-16 Produced “metal/polymer” nanocomposite materials ensure a functional contribution of both “host” and “guest” components leading to an interference of their physical-chemical properties and their efficient performance in different applications.17,18 Recently, many studies were focused on applying polymers as matrices for dispersed metal nanoclusters, especially, palladium as a very important and one of the most versatile of transition metals in catalytic applications.19,20 * Corresponding author. Present address: Department of Chemistry, Center for Macromolecular Science and Engineering, Leigh Hall 300, University of Florida, Gainesville, FL 32611-7200. Telephone: 352-3922000. Fax: 352-392-9741. E-mail: [email protected]. † Inorganic Chemistry Department, A. I. Hertzen State Pedagogical University. ‡ ICMUB-UMR 5260 CNRS, University of Burgundy. § ICB-UMR 5209 CNRS, University of Burgundy.

Among other well known π-conjugated conducting polymers, polypyrrole (PPy) has been of particular interest because of its high environmental stability and controllable electronic conductivity.21 Diverse electrochemical and chemical methods of preparation of Pd/PPy composites have been developed. Examples include electrochemical incorporation of palladium nanoparticles into electroactive polypyrrole matrices during electrochemical synthesis of PPy films;22 electrodeposition of metal nanoparticles on preformed electrodes (surface of electrodes was coated with thin polymer films);23-26 absorption of metal ions by polymer film/particles from the solution of transition metal salts, with their subsequent chemical reduction.27,28 Chemical methods basically represent three-step procedures: (1) PPy powder formation via chemical oxidation of pyrrole monomer; (2) chemical reduction of resulting electroactive polymer; (3) chemical sorption of Pd2+ ions on the undoped polymer surface and their subsequent reduction, with yields of 1.5-5 wt% of Pd in the composite.29,30 Only a few examples of direct chemical reduction of palladium ions during pyrrole oxidation have been reported until now, namely the syntheses of a Pd/PPy composite in an inverse microemulsion medium31 and in hydrochloric acid32 and bimetallic Cu/Pd/PPy composite formation in water.33 One of the prospective applications of composites with palladium nanoparticles is related to an easy transformation of Pd atoms under the influence of organic ligands into the corresponding Pd(0) complexes which represent a remarkable catalyst in numerous organic reactions of C-C bond formation. One can refer to Tsuji Trost reaction (allylic substitution reaction), Heck reaction (coupling of aryl or vinyl halide with olefins), Stille coupling (stannate with organic halide), Suzuki reaction (boronic acid with aryl halide), Sonogashira reaction (aryl halide with alcyne), etc.1–9 Another domain of such materials is based on various oxidation reactions on the surface of metallic palladium, owing to activation of the molecular oxygen. Moreover, the incorporation of metal particles inside an electron-conducting polymer matrix allows one to vary in a controlled manner (via the imposed electrode potential) the

10.1021/jp805423t CCC: $40.75  2008 American Chemical Society Published on Web 11/18/2008

Palladium Nanoparticle/Polypyrrole Composites oxidation state of the metal atoms which opens the prospect to realize electrocatalytic processes including those which take place in fuel cells. A very high surface-to-volume ratio for a nanodispersed metal gives the possibility of an enormous diminution of the amount of this precious metal compared to massive samples of the same metal. Another key factor for the functioning of such materials as electrocatalysts is the state of their surface. Mostly, e.g., if the material is obtained via incorporation of (presynthesized and then solubilized) metal nanoparticles from their colloid solution or via a one-pot procedure inside a microemulsion, the incorporated particles conserve their protective coverage inside the composite material while this additional layer affects the kinetics of reactions which are to be catalyzed by the metal. In this context the search for new synthetic methods resulting in ”bare” metal particles without any protective layer and stabilized only by the polymer matrix is of importance for further catalytic applications. This study proposes such a way to Pd-polymer composite material. Herein, we present a simple one-step method of palladium/ polypyrrole (Pd/PPy) composite preparation in organic solvent medium. During this process, a direct redox reaction between pyrrole monomer and palladium(II) salt in acetonitrile solution occurs. Since such one-pot procedure promotes a close interaction between simultaneously generated polypyrrole and palladium metal particles, a high content of the metal, small size of metal particles and their uniform distribution in produced Pd/polymer composite material could be achieved. In view of a complicated structure of these composite materials they were characterized by means of elemental CHNS analysis, thermogravimetric analysis (TGA), UV-vis electron spectroscopy, Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) supplemented by energy dispersive X-ray spectroscopy (EDS) and selected area diffraction (SAD) devices. 2. Experimental Section 2.1. Reactants. Pyrrole (98%) was purchased from Aldrich Chemical Co. and distilled under vacuum before use. Palladium (II) acetate (99.9+%, Sigma-Aldrich), iron (III) chloride (99%, Sigma-Aldrich) and acetonitrile (HPLC grade, Carlo Erba) were used as received. 2.2. Procedure. Synthesis of Pd/PPy Composite Ensembles. In typical experiments, 10 mM palladium(II) acetate (Pd(OAc)2) and 50 mM pyrrole (Py) solutions in acetonitrile (AN) were mixed dropwise or abruptly (this choice did not influence the further evolution of the UV-vis spectra of the mixed solutions). Two different molar ratios of initial reactants in the mixed solution were chosen: (1) 1:100 molar feed ratio (palladium salt to pyrrole molar ratio); (2) 1:5 molar ratio, giving rise to the formation of composite materialssPd/PPy(100) and Pd/PPy(5) respectively (here and later in the text). Solutions were agitated vigorously in ultrasonic bath up to the colloid appearance and subsequent precipitation of the composite. During the procedure initial orange color of palladium salt solution was changing to brown, then dark brown-green followed by colloid appearance and subsequent sedimentation of a black-green powder. A colloid appearance was observed in about 50-60 minutes for 1:100 mixture and in 5-10 min for a 1:5 mixture of initial reactants. The precipitate was centrifuged, washed several times with acetonitrile until effluent became colorless and pH-neutral, and then dried under vacuum for 5 h prior to characterization.

J. Phys. Chem. C, Vol. 112, No. 50, 2008 19879 2.3. Instrumentation. To determine elemental composition of Pd/PPy samples both elemental and thermogravimetric analysis (TGA) were used. Elemental analysis (determination of C, H, N, S) was performed by means of Flash EA 1112 Thermo Electron analyzer. TGA was conducted on a thermogravimetric analyser SETARAM B85 with a heating ramp of 5°C/min and with argon or hydrogen flow of 100 mL/min. Process of the formation of Pd/PPy composite was carried out employing UV-visible spectroscopy and AFM microscopy techniques. UV-visible absorption spectra were measured using VARIAN UV-visible spectrophotometer Cary 50 scan in 10 mm quartz cuvettes (Hellma Benelux) in the course of the processes of Pd/PPy composite formation. For UV-vis measurements Pd/ PPy composites were synthesized in a Schlenk-type glass vessel in deaerated acetonitrile under Ar atmosphere. 0.75 mL of 10 mM Pd(OAc)2 solution in AN was added to 15 mL of 50 mM pyrrole solution (molar feed ratio 1:100). Spectra of 0.48 mM solution of Pd(OAc)2 (concentration of palladium salt corresponds to the initial one in the reaction mixture) was also studied. The reaction mixture was stirred in ultrasonic bath for 3 hours. UV-vis spectra of 2 mL portions of the mixture were recording during the reaction time. Atomic force microscopy (AFM) measurements were carried out on a Nanoscope IIIA Quadrex (Veeco Instruments, CA, US). AFM images were recorded in oscillating contact mode with Si cantilever (spring constant was about 1 N/m) under controlled atmosphere at 5% relative humidity. In order to study how the size of Pd/PPy composite particles changes during the reaction time, AFM samples were prepared by putting drops of Pd-PPy solution taken at different stages of composite synthesis: (1) after colloid appearance and (2) after precipitation of the material, onto an ITO covered glass surface. After each stage drops of solutions have been dried in oven at 80 °C for 10 min. Prior to use, ITO plates were prepared by cutting to a suitable size, and degreased by treatment with distilled ethanol in ultrasonic bath, followed by drying in an oven at 80 °C for 3 hours. Transmission electron microscopy (TEM) studies of composites were carried out on Jeol JEM-2100 TEM with LaB6 source operating at accelerating voltage of 200 kV. Ultrathin windowed energy-dispersive X-ray spectrometer (EDS) attached to the TEM microscope was used to determine the chemical composition of samples. For TEM experiments samples were prepared by placing a drop of the solution of the synthesized composites onto a 300-mesh formvar film (polyvinyl formal resin) coated copper grid, with a subsequent solvent evaporation in contact with ambient air at room temperature. Solutions were prepared by redispersing in acetonitrile of the previously rinsed and dried Pd/PPy powder. Scanning electron microscopy (SEM) micrographs were obtained with JEOL JSM-6400F instrument at 15 kV accelerating voltage. Chemical analysis of the samples was performed using Oxford Instruments EDX analyzer. INCA software was used for acquiring images and EDX data. For SEM experiments several drops of Pd/PPy suspension were taken after precipitation of the material from the synthesis solution, placed on the conducting side of an ITO covered glass plate and dried before SEM measurements in oven at 80 °C for 15 min. These samples were analyzed directly without any metallization treatment. FTIR spectra in the range from 4000 to 500 cm-1 have been recorded in the transmission mode using a Bruker Vector 22 FT-IR spectrometer operating with a resolution of 4 cm-1. For

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TABLE 1: Elemental Composition of Composite Materials Determined with CHN or CHN/TGA Techniques Pd/PPy molar feed ratio

%C

%H

%N

% Pda

1:5 (CHN) 1:5 (TGA (H2) + CHN) 1:100 (CHN) 1:100 (TGA (H2) + CHN)

29.88 3.44 33.09 2.04

2.76 0 2.81 0

8.56 0.33 10.69 0.21

43.3 96.23 36.5 97.75

a Calculated palladium content in composites after heating samples in a hydrogen atmosphere.

FTIR measurements Pd/PPy powder was dispersed in thin transparent KBr disk pellets. XPS analysis was performed on a SIA100 spectrometer (Cameca Riber apparatus) using nonmonochromatized Al KR X-ray source (1486.6 eV photons). X-ray powder diffractograms were recorded on a CPS 120 INEL diffractometer with monochromated CuKR radiation at 40 mA and 40 kV. 3. Results and Discussion 3.1. Thermogravimetric (TGA) and Elemental CHNS Analysis. To determine elemental composition of the synthesized Pd/PPy composite material the combination of thermogravimetric TGA and elemental CHN analysis techniques was used (Table 1). First, elemental analysis of the Pd/PPy samples with molar feed ratios 1:5 or 1:100 for C, H, and N atoms (weight percentage) has been performed (Table 1, lines marked as (CHN)). Then, the samples were heated with the rate of 5 °C min-1 up to 600 °C under H2 atmosphere (100 mL of hydrogen min-1) for several hours up to a constant mass in order to remove volatile organic products (acetic acid, acetonitrile, pyrrole and its short-chain oligomers). Color change of the powder sample from greenish black to shining metallic gray has been observed. Weight loss during the TGA heating procedure was about 54% and 61% for Pd/PPy(5) and Pd/ PPy(100) respectively. Then, elemental analysis measurements were performed once again to determine the residual amounts of carbon, hydrogen (practically absent) and nitrogen (Table 1, lines marked as (TGA (H2) + CHN)). Assuming, that after thermal treatment in reductive H2 atmosphere the sample does not contain oxygen, these data for C, H and N allowed the determination of the amount of Pd. Then, owing to the information on the masses of the sample before and after the treatment, we calculated the palladium content in Pd/PPy(5) and Pd/PPy(100) composite materials in their initial state. The TGA and CHNS data were obtained several times for different Pd/ PPy samples synthesized with the use of the same procedure described above. Good reproducibility of the results of the TGA and elemental analyses testifies that developed method of synthesis allows to produce Pd/PPy composites with controllable composition and a high content of palladium - 43.3 ( 1 wt % and 36.5 ( 1 wt % in Pd/PPy(5) and Pd/PPy(100) composites respectively. 3.2. Study of the Process of Pd/PPy Composite Formation. UV-Vis Absorption Spectroscopy. While pure pyrrole absorbs in the UV range only, the absorption of polypyrrole extends to the visible range, giving valuable information about the oxidation state of PPy. Besides, UV-visible spectra allow one to trace the time variation of the concentration of the starting palladium species, Pd(II) cations, during the reaction with pyrrole in the mixed solution. Electronic spectra of metal nanoclusters, such as Cu, Ag, and Au34-36 show strong absorption bands due to excitation of the

Figure 1. UV-vis spectra recorded during the process of Pd/PPy(100) composite formation from Pd(OAc)2 + Py mixed solution, molar feed ratio 1:100. Dashed line 1: spectrum of 0.48 mM solution of Pd(OAc)2 (concentration of the palladium salt corresponds to the initial one in the reaction mixture).

surface plasmons. Many transition metals, including palladium, do not give distinct surface plasmon peaks and are not easy to observe.37 Besides, the presence of reducing and stabilizing agents and coexistence of metal cations and nanoclusters in colloid solution give rise to an overlapping and broadening of the absorption bands and complicate data interpretation.38 A few minutes after the pyrrole monomer addition to the palladium salt solution, an absorption band with maximum at 390 nm related to Pd+2 ions (dashed line 1 in Figure 1a showing its spectrum without pyrrole) vanishes while a shoulder at about 350-370 nm appears, the absorbance of this band is gradually increasing as the reaction proceeds. In contrast to conventional assignments of the peak at 300-370 nm to the presence of Pd+2 ions in solution,39 a more accurate approach (combination of net analyte signal and principal component analysis methods) has demonstrated that the absorption band in this wavelength interval reveals the formation of Pd nanoparticles, which absorb light via valence-conduction band transitions and also scatter light because of their nanometric size.40,41 The identification of particular absorption bands of PPy with electron transitions is complicated by the coexistence of different type of moieties, width of absorption bands, their overlapping, poor reproducibility of the maximum positions depending on the nature of dopants and conditions of preparation.42 Investigation of spectral characteristics of the electrochemically prepared polypyrrole showed the presence of the broad bands near 700 nm and 900 nm in UV-vis absorption spectra of fully oxidized PPy and their rapid disappearance in the spectra of the reduced PPy.43,44 Appearance and gradual increase of the absorbance band at 450-570 nm, 675 and 735 nm in the spectra of Pd/ PPy solution, attributed to the sum of the polarons and bipolarons,43-46 indicates the presence of free charge carriers (mostly polarons) in the polypyrrole component of the composite material. Existence of these bands and absence of the band near 900-950 nm in the spectra of Pd/PPy composite (Figure 1a) confirms the conclusion that PPy, synthesized with the use of Pd(OAc)2 as an oxidant of an excessive amount of pyrrole, is in a partially oxidized state. These results are in agreement with our XPS data, indicating the presence of both neutral and charged types of nitrogen atoms in this composite material, whereas a single type of nitrogen was registered by XPS in neutral PPy. 45 AFM Studies. In order to explore the process of the composite formation at different stages, AFM studies have been ac-

Palladium Nanoparticle/Polypyrrole Composites

J. Phys. Chem. C, Vol. 112, No. 50, 2008 19881

Figure 2. 3D (5 µm × 5 µm) AFM images: (a) Pd/PPy(100) and (b) Pd/PPy(5) spheres formed at different stages of the composite formation. The smaller and bigger spherical particles on AFM images correspond to early (stage of the colloid formation) and final (stage of the precipitation of the composite) time intervals of the synthetic procedure, respectfully.

Figure 3. (a) SEM image of Pd/PPy(100). (b) TEM image of Pd/PPy(5) composites.

complished. Pd/PPy samples for AFM studies were prepared as follows. Two drops of a mixed Pd(OAc)2 + Py solution were taken at two time moments after its preparation corresponding to different stages of the composite formation. The first drop was taken from the solution when a colloid light-scattering became visible, then it was put onto an ITO-covered glass plate and dried in an oven for 10 min at 80°C. The second drop was taken after the precipitation of the composite material, placed onto ITO plate above the layer left after evaporation of the first drop and also dried under the same conditions. AFM image (5 µm × 5 µm scale) for two drops of Pd/PPy(100) sample in Figure 2a shows the presence of smaller (40-60 nm in diameter) and bigger (250-300 nm) Pd/PPy composite spherical particles originated from the stages of colloid appearance and of subsequent precipitation of the material respectively. This image characterizes the process of particle growth and indicates how their size is changing during the synthesis. In the case of Pd/ PPy(5) sample the size range of the composite spheres varies from 350-400 nm to 600-850 nm (Figure 2b). The sizes of these spherical particles of Pd/PPy(100) and Pd/PPy(5) composite materials at the final stages of the precipitation reaction are in good agreement with SEM and TEM results. Comparison of AFM images of Pd/PPy(100) (Figure 2a) and Pd/PPy(5) (Figure 2b) systems shows that the size of composite spheres becomes greater as the Pd(OAc)2 content in the initial reaction mixture increases.

3.3. Characterization of the Material. SEM and TEM Studies. SEM and TEM studies of the synthesized Pd/PPy composites corroborated the AFM results and provided additional information on the chemical composition of the material and the internal structure of Pd/PPy particles. The micrographs in Figure 3, parts a and b, illustrate general morphological features of the Pd/PPy particles. On SEM image (Figure 3a) spherical particles of Pd/PPy(100) composite with average particle diameters in the range from 250 to 300 nm are clearly seen. Figure 3b shows a typical TEM image of the Pd/PPy(5) precipitate, which consists of nearly perfect spheres with average diameters in the range of 550-800 nm. As shown in Figure 3, parts a and b, in both cases Pd/PPy spherical particles are aggregated into groups and linear chains. The sizes of Pd/ PPy(100) and Pd/PPy(5) spherical particles shown in SEM and TEM images (Figure 3, parts a and b) are close to those demonstrated with AFM technique (Figure 2, parts a and b). Analysis of EDX spectra taken from the surface area of 500 nm×500 nm of Pd/PPy samples during SEM experiments reveals the presence of 41.0 and 34.5 wt % of palladium in Pd/PPy(5) and Pd/PPy(100) respectively, that is in a good agreement with CHNS/TGA data described above. Thus, the change in the molar ratio of the components in the mixture of initial ingredients allows to vary palladium content in Pd/PPy ensembles: it increases with an increase of palladium(II) acetate content in the reaction medium.

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Figure 4. TEM images: (a) Pd/PPy(5) and (b) Pd/PPy(100) composites.

Figure 5. TEM images of Pd/PPy(100) spheres before (a) and after (b and c) beam condensing. Corresponding selected area electron diffraction (SAD) patterns are given in insets. (d) X-ray diffractogram of Pd/PPy(100) composite.

The internal structure of Pd/PPy spherical particles in Figures 2 and 3 may be seen from high-resolution TEM images of their external region (Figure 4). One can notice a heterogeneous character of the sphere composed of a more transparent host material (presumably, polymer), with incorporated palladium nanoparticles of a higher absorbance for TEM electrons (dark elements with diameters within 2.0-2.5 nm). Farther from the border of the sphere image the spatial density of dark elements increases while the size of nanoelements does not change visibly. Similar internal structures with nanoelements in the same range were observed for both materials, Pd/PPy(5) and Pd/PPy(100) (Figure 4, parts a and b). During the TEM experiments performed under condensed beam exposure, one can expect the destruction of the polymer material, with liberation of encapsulated nanoparticles and their aggregation, as it is frequently observed for metal nanoclusters.47,48 Indeed, after 5 s of the imposed electron beam, an “explosion” of the Pd/PPy(100) spheres has been observed. Parts a-c of Figure 5 present TEM images with included selected area diffraction (SAD) patterns of Pd/polymer(100) spheres before (Figure 5a) and after (Figure 5, parts b and c) beam focusing.

Comparison of the images of the same spherical composite particle in Figure 5, parts a-c, shows a progressive increase of its transparency related to the diminution of the dark spots inside the particle. In parallel, new faceted crystalline palladium clusters with the sizes in the range of 50 to 60 nm appear outside the initial composite particles. These new elements are originated obviously from the coalescence of palladium nanoparticles liberated from the initial Pd/PPy composite particle under the electron beam exposure. Insets of Figure 5, parts a and c, present the corresponding SAD data from areas being specified by circles in Figure 5, parts a and c. A broad halo ring pattern in Figure 5a (SAD a) demonstrates a small size of Pd nanoparticles. This result is in good agreement with XRD data. XRD diffraction pattern of Pd/ PPy powder sample (Figure 5d) shows a weak signal at 40.1°, which corresponds to the (111) reflection of Pd(0) crystals.49 On the contrary, after beam focusing the SAD b pattern (Figure 5c) reveals the presence of distinct spots superimposed on broad halos a sharp circle which is representative of isolated and larger nanocrystals of metallic palladium. The pattern of a newly formed 50 nm cluster (SAD c) reveals a clearly crystalline

Palladium Nanoparticle/Polypyrrole Composites

Figure 6. EDS spectra obtained from selected areas shown in Figure 5, parts a and c, and normalized to the intensity of the palladium signal: Areas: (a) initial composite particle, (b) the same particle after condensed beam exposure, and (c) newly formed particles.

structure corresponding to the face-centered cubic (fcc) packing mode of palladium atoms within the cluster metal core. The EDS spectra (Figure 6) obtained from selected areas a, b, c shown in Figure 5, parts a and c, are normalized to the intensity of the palladium signal. Spectra EDS a and EDS b demonstrate different concentrations of palladium and carbon inside the same composite particle before and after the condensed beam exposure. In the spectrum for the initial composite particle (EDS a) the ratio of the atomic contents of Pd and carbon is close to 1:10 which is in a reasonable agreement with this ratio, about 1:8, for this composite calculated from the data of the elemental analysis in Table 1. This ratio for the same spatial area inside the particle changes to 1:60 after the beam treatment (EDS b). The tendency is in conformity with a much smaller number of dark spots inside the composite particle in Figure 5c, compared to that in Figure 5a. An opposite tendency is seen in spectrum EDS c for a newly generated big palladium cluster: a significant increase of the Pd-to-C atomic ratio up to about 1:1.4 (the carbon signal is probably originated in this case mostly from the formvar film on the surface of the TEM substrate), since the cluster was formed by aggregation of Pd nanoparticles liberated from the polypyrrole shell under the electron beam exposure. This transformation of the composite material under the action of the condensed electron beam was observed only for one of the studied systems, Pd/PPy(100). The other system, Pd/PPy(5), was stable in these conditions. FTIR Spectroscopy. Polypyrrole component of Pd/PPy nanocomposite was characterized by FTIR spectroscopy. The FTIR spectrum of Pd/PPy(5) (Figure 7) shows the presence of a series of IR bands characteristic for polypyrrole. A series of weak bands at wavenumbers between 1360 and 1200 cm-1 is due to aromatic and aliphatic C-N stretch vibrations.50-52 Band at 1040 cm-1 has been assigned to a combination C-H in-plane ring bending and the deformation of the five-membered ring which contains the CdC-N deformation.51,52 The bands at 950 cm-1 and around 785 and 680 cm-1 are attributed to CdC in-plane bending of pyrrole ring and C-H out-of-plane bending in PPy, respectively. The region between 1550 and 1450 cm-1 includes the most intense bands attributed to the combination of N-H stretch, CdN stretch, and aromatic CdC stretch vibrations of the fivemembered pyrrole ring. (refs 53 and 54 and references therein)

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Figure 7. FTIR spectrum of Pd/PPy(5).

The ratio of integrated intensities of the 1560 and 1480 bands has often been used to determine the conjugation length in PPy.55-57 Intensity of the antisymmetric ring stretching mode at 1560 cm-1 decreases relative to the intensity of the symmetric mode at 1480 cm-1 with the extension of the conjugation length, i.e. ratio I1560/I1480 should become smaller with an extension of delocalization. However, in the case of Pd/PPy composite synthesized using Pd(CH3COO)2 as an oxidant, the bands near 1550 cm-1 are superimposed with the band attributed to antisymmetric stretch of CdO in carboxylate group of acetate anions.58 It results in a broadening of 1560 cm-1 combinational band and shadowing of the band at 1480 cm-1, making such analysis unquantifiable. An intense signal at 1409 cm-1 corresponding to the symmetric stretch of CdO in carboxylate group58 indicates the presence of acetate charge-compensating anions in the doped PPy component of Pd/PPy composite, this observation being in a good agreement with XPS data presented below. X-ray Photoelectron Spectroscopy. Core-LeWel C1s XPS Spectra. The C1s spectrum of Pd/PPy(5) composite is presented in Figure 8a. A standard line-shape analysis shows that this spectrum can be decomposed into a series of GaussianLorentzian (30%) peaks59 with the EB (binding energy) values equal to 284.5, 286.7, and 289 eV. The main peak component at 284.5 eV has been assigned to C-H, C-C, and C-N species.59,60 In addition to this peak, there are two minor peak components in the spectrum at 286.7 and 289 eV (parameters of these peaks can only be determined with an uncertainty, because of their low intensity), which may be attributed to oxidized carbon species (C-O and OdC-O respectively)61 characteristic to the presence of acetate charge-compensating anions in the composite (though a partial surface oxidation of the material cannot be excluded). Core-LeVel N1s XPS Spectra. N1s spectrum (Figure 8b) was decomposed into two components with the EB values equal to 399.5 and 401 eV assigned to neutral nitrogen atoms (-NH-) and positively charged (N+) species in PPy. 62 Lower bindingenergy component (∼ 397.5 eV) attributed to uncharged deprotonated imine (dN-) nitrogen atoms or e.g. imine defects responsible for interruption of effective conjugation of PPy, is absent in the core-level N1s XPS spectrum of Pd/PPy composite material. This fact is also in a good agreement with the assertion that conjugation of PPy is extended in Pd/PPy nanocomposite.

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Figure 8. XPS spectra of Pd/PPy(5) composite material and their simulation as a set of Gaussian peaks: (a) core-level C1s spectrum, (b) core-level N1s spectrum, (c) Pd 3d spectrum, and (d) Pd 3d spectrum of Pd/PPy “refreshed” sample (heated for 3 h under Ar, and then treated by Ar sputtering).

Ratio of the areas of the nitrogen peaks at 401 and 399.5 eV (N+/-NH-) is 0.31 for Pd/PPy(5) and 0.21 for Pd/PPy(100) (spectra not shown), which correlates well with data for nanostructured PPy and Pd/PPy systems. 56,62 An increase of doping level of PPy shell of the composite material with the growth of Pd (II) salt concentration in the mixture of initial reactants is quite logic. Pd 3d XPS Spectra. Besides carbon and nitrogen signals originated from PPy and acetate anions present in polymer, Pd 3d and Pd Auger signals are observed in a survey XPS spectra of the material. Palladium content calculated from XPS data is 6.2-6.9 atomic % corresponding to 34.8-37.1 weight %. This value is slightly lower than the one given by the CHN analysis in Table 1, compensated by a bit higher concentrations of C and N, 33.2-38.4 and 8.9-9.7 wt %, correspondingly, the ratio of these elements being always close to 4, as in pyrrole unit. Pd 3d XPS spectrum (Figure 8c) was decomposed into two doublet components. Low-binding-energy doublet at 334.5 eV (Pd 3d5/2) and 339.8 eV (Pd 3d3/2) is attributed to the Pd(0) species 62,63 that indicates the presence of palladium metal nanoparticles in Pd/PPy composite. Another doublet with peaks at 336.5 eV (Pd 3d5/2) and 341.8 eV (Pd 3d3/2) corresponds to higher energies than the previous one which implies an oxidized state of these atoms. However, these binding energies are still lower than those (338 and 343.5 eV) attributed in literature to Pd(II) salts (e.g. Pd 3d5/2 peak energy is 337.8 eV in PdCl2 and 338.4 eV in Pd(OAc)2) but close to those for PdO, 336.1 eV (Pd 3d5/2) and 341.8 eV (Pd 3d3/2).27 The nature of these peaks is uncertain. They may be assigned to palladium atoms in an intermediate oxidation state between Pd(0) and Pd(II),62 for instance, to surface atoms in palladium clusters coordinated to PPy through N atom or to oxygen of an acetate anion located in the vicinity of the cluster’s surface. Alternatively, since oxygen signal is present in the XPS survey spectra of Pd/PPy,

PdO formation related to a partial surface oxidation of the composite material cannot be excluded, either. To distinguish between the “bulk” and “surface” origins of the higher-energy doublet of Pd, another portion of the same Pd/PPy(5) sample has been heated at 100 °C under argon atmosphere and it has been sputtered with argon, to “refresh” the surface by removing an oxidized layer of the material before XPS measurements. As it is seen from the Pd 3d XPS spectrum of the “refreshed” sample (Figure 8d), only a low-binding-energy doublet at 334.5 eV (Pd 3d5/2) and 339.8 eV (Pd 3d3/2) attributed to the Pd(0) species is present. The observation that the doublet with peaks at 336.5 eV (Pd 3d5/2) and 341.8 eV (Pd 3d3/2) disappeared after this surface treatment confirms the above hypothesis on the localization of the oxidized Pd atoms within a thin surface layer of the material, e.g. due to their bonding to PPy or to acetate anion or inside a surface oxide. A smaller increase of the peak energies for the observed doublet, 336.5 and 341.8 eV, compared to those for Pd(II) species62 may be related to different chemical states of Pd in these systems, inside a bulk phase in the latter case while the former signal may originate from Pd atoms on the surface of its metallic clusters which are bonded both to other Pd atoms and to a non-metal, e.g., oxygen. A high intensity of this “surface” signal (compared to that for Pd(0)) in Figure 8c is in conformity with a very small size of Pd clusters, 2.0-2.5 nm, found from Figure 4: a simple estimate predicts that for a nanoparticle with about 10 atoms in diameter about a half of all atoms are located at its surface and may correspondingly form chemical bonds with surrounding atoms. 4. Conclusions A composite material in the form of Pd/PPy spherical particles (with diameters within a few hundred nanometers) has been

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