Electrochimica Acta 46 (2001) 4205– 4211 www.elsevier.com/locate/electacta
Nanostructured palladium –polypyrrole composites electrosynthesised from organic solvents N. Cioffi a, L. Torsi a,*, I. Losito a, L. Sabbatini a, P.G. Zambonin a, T. Bleve-Zacheo b a
Dipartimento di Chimica, Uni6ersita degli Studi di Bari, 4, 6ia Orabona, I-70126 Bari, Italy b Istituto di Nematologia Agraria A. V., Consiglio Nazionale delle Ricerche, Bari, Italy Received 19 December 2000; received in revised form 25 January 2001
Abstract Palladium–polypyrrole nanostructured composite films can be easily synthesised using a two-step procedure comprising the electrochemical synthesis of palladium nanoparticles (Pd-NPs) that are subsequently potentiostatically deposited onto a polypyrrole thin film electrosynthesised from an acetonitrile solution. The composite thin films have good conductivity and their transmission electron micrographs show that the metallic inclusions have a mean diameter of about 5 nm with a homogeneous size distribution. X-ray photoelectron spectroscopy (XPS) analysis reveals the presence of two surface oxidation states for the as-synthesised Pd-NPs as well as for those deposited on the films. On the basis of experimental findings, a structural model for the Pd-NPs is proposed. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Electrosynthesis; Polypyrrole; Palladium nanoparticles; Nanostructured composites; Conducting polymers
1. Introduction In the recent years, metal nanoparticles (Me-NPs) have attracted a lot of interest because of their possible use in many technologically relevant applications. A computer search through the Chemical Abstracts [1] shows that more than 600 papers have been published, just during the year 2000, in international journals dealing with the synthesis, characterisation or application of metal nanoparticles/nanoclusters. Electrochemical procedures for the synthesis of Me-NPs, although less diffuse than chemical routes, have already proven to be a powerful tool to prepare spherical and cylindrical NPs in a wide range of metals [2–10]. These preparations are commonly based on the electrolysis of a * Corresponding author. Tel.: + 39-0805-442019; fax: + 390805-442026. E-mail address:
[email protected] (L. Torsi).
sacrificial metallic anode and/or of metal ions, carried out in the presence of surfactants. The Me-NPs produced often have a core-shell structure [11] in which the metallic core is surrounded by a layer composed of surfactant molecules and adsorbed ions that give morphological and chemical stability to the colloid. In the case of the Pd-NPs employed in the present study, the shell is thin enough not to prevent the electrochemical [10] or catalytic [4] activity of the metal. Very frequently, nano-sized metals are supported on an inert material in order to be employed as catalysts or as active elements in a device [12]. In fields such as Electrocatalysis and Sensing Devices, electrodes based on conducting polymers electrochemically modified with metallic inclusions are used frequently. However, while in the past years the resulting composite materials were composed of micro-sized metallic particles [13– 17], recently all-electrochemical syntheses of nanostructured metal– polymer composites have produced
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inclusions as small as 10 nm [18]. In a previous paper [10], we reported on all-electrochemical routes to prepare palladium – polypyrrole (Pd –PPy) nanocomposites from aqueous solutions. The procedure exploited the charge properties of the Pd-NPs and produced electroactive composite films in which the Pd-NCs maintained a small diameter (3 –5 nm) and narrow size dispersion. In the present work, we present an alternative electrochemical procedure that is based on the exclusive use of organic solvents, speeding up the whole preparation process and leading to composites having high conductivity values.
Briefly, a standard three-electrode cell comprising a Pd sacrificial anode and a Pd counter electrode, both having an area of about 6.25 cm2 and set at a distance of 3 mm was used. The reference electrode was Ag/AgNO3 (0.1 M) in acetonitrile and the electrolyte was tetraoctylammoniumbromide (TOAB) (0.1 M) in 1:3 CH3CN (MeCN)/tetrahydrofuran (THF). The anode potential was set at + 1.5 V versus reference and the current densities ranged between 3.0 and 4.5 mA/cm2. The synthesis, always lasting for 3 h, resulted in a colloidal dispersion of Pd nanoclusters in MeCN/THF, also containing a certain concentration of TOAB. The final amount of Pd in the colloid, evaluated from the total charge involved during the synthesis, was 6 –11 mg/ml.
2. Experimental
2.4. Cyclic 6oltammetric analysis
2.1. Materials and electrochemical apparatus
The electroactivity of the Pd-NPs was investigated by means of CV analysis. Pd-NPs were washed and dissolved in acetone as previously reported [10] and then were pre-concentrated on a graphite disk electrode (0.08 cm2 area) by drop-casting a 5 ml aliquot of the colloid on the electrode in a nitrogen environment. The CV responses of the bare graphite and of the elicited Pd-NPs-modified graphite working electrodes were measured in aqueous H2SO4 (0.5 M) using an Ag/AgCl, KClsat reference electrode and a Pt sheet as the counter electrode.
Pyrrole, purchased from Aldrich, was purified by vacuum distillation and stored under nitrogen at − 27 °C. All other chemicals were ACS reagent grade and used without any further purification. Platinum sheets (99.99%) and graphite rods (99.997%) were purchased from Goodfellow. Electrochemical experiments were performed using an EG&G, Princeton, Applied Research 263 potentiostat– galvanostat and conventional three-electrode cells. Pt sheets (area ca. 1 cm2) were used as counter electrodes and as substrates of the Pd –PPy samples employed for the spectroscopic characterisation. Before use, Pt electrodes were polished and pre-treated as reported elsewhere [19]. Disk working electrodes used for cyclic voltammetry (CV) were obtained by pressfitting a graphite rod into a Teflon tube. Ag/AgNO3 (0.1 M) in acetonitrile ( +350 mV vs. SCE) was used as the reference electrode for all the electrosyntheses.
2.2. Polypyrrole electrosynthesis Polypyrrole (PPy) thin films were electrochemically deposited in their oxidised form on Pt or ITO (indium tin oxide)-covered glass electrodes from a solution containing pyrrole (0.4 M) and N(C2H5)4BF4 (0.1 M) in acetonitrile. The deposition potential was fixed at +0.6 V versus reference. The solution was previously deoxygenated by bubbling nitrogen and a nitrogen environment was kept in the cell throughout the synthesis. The polymerisation charge density was in the 45 –65 mC/ cm2 range, corresponding [20] to a PPy average thickness of 100 –150 nm.
2.3. Nanoparticles electrosynthesis The Pd-NPs electrosynthesis was carried out following a procedure that has already been reported [2,10].
2.5. Pd–PPy composites electrosynthesis The Pd–PPy nanocomposites were grown either on Pt or ITO working electrodes using a two-step procedure. Firstly, PPy films were electrodeposited as described before, then they were immersed in a Pd-NPs solution (obtained by a ten-time dilution of the as-synthesised colloid with previously deoxygenated MeCN/ THF in a 1:3 ratio) and potentiostatically treated by square-wave cathodic pulses having the following voltage sequence: − 0.5 V, −1.0 V, − 0.5 V versus reference, each step lasting 4 ms. The charge densities of the pulsed deposition experiments ranged from 7 to 10 mC/cm2. After the synthesis, the composite films were extensively washed with THF and MeCN and subjected to ex situ characterisation.
2.6. Transmission electron microscopy (TEM) and four probe conducti6ity measurements The TEM analysis of the NPs was performed by putting a drop of the suspension on 400 mesh copper grids and allowing it to dry. Due to the poor adhesion of PPy onto ITO [21], the composite films grown on this substrate could be easily detached. TEM measurements were directly performed on self-standing Pd –PPy thin films sandwiched between
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two 200 mesh copper grids. All the samples were observed at 100 kV under a PHILIPS 400T TEM. Four probe conductivity measurements were performed on composite films detached from the ITO substrate and supported on insulating adhesive tapes.
2.7. X-ray photoelectron spectroscopy (XPS) analysis XPS measurements were performed using a Leybold LHS10 spectrometer equipped with unmonochromatised Al Ka and Mg Ka sources. Survey spectra were acquired in fixed retarding ratio mode (B=3) and high-resolution spectra (for C1s, O1s, Pd3d, Br3d, N1s, F1s regions) were recorded in fixed analyser transmission mode, with a pass energy of 50 eV. Calibration of the binding energy scale was performed by taking the alkyl component of the C1s photoelectron peak (binding energy =284.8 eV) as the internal reference. Data analysis was performed as reported in Ref. [22]. The full width at half maximum (FWHM) values used to fit the Pd3d region were derived from a previous analysis of a bulk Pd standard that was sputtered in order to remove the surface PdO layer before measuring the spectrum. XPS analysis of the as-synthesised Pd-NPs was performed on a 100 ml aliquot deposited by spin-coating (at 2000 rpm for 60 s) on a platinum sheet having an area of approximately 1 cm2.
Fig. 1. CV response of Pd-NPs (solid line) pre-concentrated at a graphite electrode and cycled in H2SO4 (0.5 M). The response of the bare graphite electrode cycled in the same electrolyte solution is reported as the dotted line. The scan rate is 100 mV/s.
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3. Results and discussion
3.1. Pd-NPs electroacti6ity In Fig. 1, the solid line is relevant to the CV response (first two cycles) of the Pd-NPs (particles mean diameter 3.9 9 0.9 nm) pre-concentrated at a graphite electrode and cycled in H2SO4 (0.5 M). The redox couple located at + 1.3 and + 0.4 V has already been attributed to oxidation/reduction processes involving the Pd-NPs [10]; it is most likely due to the oxidation of the Pd metallic core, located at + 1.3 V, which is followed by a PdO reduction at 0.4 V. The flat dotted line curve in the same figure is the CV response of the bare graphite electrode when cycled in the same electrolyte. It is evident that Pd-NPs are electroactive (though the electroactivity is different from that exhibited by bulk Pd [10]), thus proving that the stabilising shell is thin enough to not completely passivate the NPs.
3.2. Pd–PPy nanocomposites electrosynthesis After the separate synthesis of the PPy film and of the colloidal palladium, the latter was diluted ten times and used as the electrolyte solution for the potentiostatic deposition of the Pd-NPs on PPy. Extending a synthetic approach that has already given appreciable results on similar materials [10,23], the Pd-NPs deposition was accomplished by square-wave cathodic pulses that have already proven not to significantly modify the conductivity of PPy thin films [24]. In fact, by adjusting the duration of the pulses, it is possible to minimise the conductivity loss of the polymer, enhancing the NPs deposition. Compared to a previous study [10], that was based on the use of aqueous solutions which required a certain manipulation of the colloidal palladium, in the present case all the synthetic steps were carried out in organic solvents, exploiting the following advantages: (i) the synthesis has been considerably speeded up and simplified; (ii) since the PPy conductivity is much higher when it is electrodeposited from acetonitrile solutions of pyrrole and N(C2H5)4BF4 [25], the Pd –PPy composites obtained in the present study have good conductivity values. In Fig. 2, low and high magnification TEM images of a pulse-conditioned Pd –PPy film are reported. The flat pale-grey or darker globular zones are ascribed to the PPy thin film, which is almost transparent to the electron beam, while the dark small spherical dots are due to the metallic core of the nanoparticles. It is evident that a particularly high Pd-NPs loading is achieved and the resulting composite is truly nanostructured in that highly dispersed, not aggregated palladium clusters with average diameter of 5.0 9 1.1 nm are deposited.
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3.3. Pd–PPy nanocomposites electroacti6ity The electrochemical characterisation of the composites presented a number of experimental problems that prevented a direct comparison between the CV responses of the Pd-NPs and of the Pd – PPy films. Firstly, it was not possible to perform CVs on the composites in sulphuric acid, since in this electrolyte the composite film is almost completely removed during the first anodic sweep. Moreover, when cycled in organic solvents (for instance, N(C2H5)4BF4 (0.1 M) in acetonitrile), PPy shows an overoxidation peak that makes it difficult to distinguish unambiguously the electrochemistry of the Pd-NPs. In fact, the position of the PPy peak depends strongly upon the polymer thickness and spreads over 300 mV, in the 1200 –1500 mV range. However, it is important to note that when cycled in aqueous KCl (0.1 M), the Pd –PPy films obtained from organic solvents presented a behaviour similar to that already reported for the Pd –PPy films synthesised from aqueous solution [10].
3.4. Conducti6ity measurements
Fig. 2. Low and high magnification TEM images of a pulsedeposited Pd – PPy composite (average thickness 100 nm; PdNPs deposition charge density 7mC/cm2) prepared as described in the text. Table 1 Four probe conductivity data relevant to PPy and Pd–PPy nanocomposites Material
|/V−1 cm−1
PPy deposited from MeCN solution containing Py and N(C2H5)4BF4 [25] PPy cathodically pulsed in a MeCN/THF 1:3 solution containing 0.01M TOAB Pd–PPy nanocomposites deposited from organic solvents Pd–PPy nanocomposites deposited from aqueous solution, as reported in Ref. [10]
50 139 6 49 9 10 0.9 9 0.3
The material’s conductivity was measured by the four probe technique and the data are reported in Table 1. In order to quantitatively appreciate the effect of the cathodic pulses on the final conductivity of the composites, a blank experiment was carried out by treating a pristine PPy film in a Pd-NPs-free solution (composed of TOAB (0.01 M) in 1:3 MeCN/THF) with the pulses program for the same time interval employed for the Pd-NPs deposition. In the table, the conductivity values recorded for the pulsed PPy films and for the Pd – PPy composites are compared to the conductivity reported in literature for good quality, pristine PPy films (deposited from acetonitrile solutions containing pyrrole and N(C2H5)4BF4) and to that measured on Pd –PPy films prepared from aqueous solution as reported in Ref. [10]. The values indicate that the pulses partially reduce the PPy film, determining a limited conductivity loss (less than one order magnitude). On the other hand, a rise in the conductivity is measured for the Pd-NPs containing composite prepared from organic solvents. Concerning the electrical properties of Pd –PPy films deposited from aqueous solution, it is well known that PPy electrosynthesised from aqueous solutions has conductivity values lower than those of films grown in organic media [25]. The same trend can be observed by also comparing the results relevant to the Pd – PPy nanocomposites. The films grown under the experimental conditions of the present study hold, therefore, besides probable catalytic activity, also a good conductivity and, as such, they look promising as active layers in sensing devices.
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Fig. 3. Survey XP spectrum of a pulse-deposited Pd – PPy composite (average thickness 100 nm; Pd-NPs deposition charge density 7 mC/cm2).
3.5. XPS analysis A typical XPS survey spectrum relevant to the Pd – PPy nanocomposite is shown in Fig. 3. Besides the presence of carbon and nitrogen, i.e. the constituents of polypyrrole, signals due to palladium (Pd3d and Pd Auger) are also observed, thus confirming the presence of Pd-NPs (Pd atomic percent about 1%) on the polymer surface. Small signals due to fluorine (F1s and
Fig. 4. Detailed Pd3d XP spectra relevant to: (a) as-synthesised Pd-NPs spin-coated on a platinum sheet; (b) pulse-deposited Pd–PPy composite (average thickness 100 nm; Pd-NPs deposition charge density 7 mC/cm2).
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Auger) and bromine (Br3d) are also present. The former are likely due to BF− 4 anions, acting as dopant ions during PPy electrosynthesis; the latter is related to bromide, either as free anion, arising from the electrolyte, or as a ligand for Pd(II). The presence of an oxygen signal, although partially overlapped with the Pd3p3/2 signal and thus not quantifiable, is worth noting, since it suggests a partial surface oxidation of the material. The chemical state of palladium has been investigated by considering its high-resolution XP spectrum. In Fig. 4, the Pd3d signals relevant to as-synthesised Pd-NPs (a) and Pd–PPy composite (b) are compared. In both cases two components were adopted to fit the signal, each one consisting of two peaks having a 2:3 intensity ratio, typical of the spin-orbital splitting of the 3d level. The low binding energies doublet has been assigned to Pd(0), even if the peak positions are slightly higher than the value usually reported for bulk Pd(0) [26]; however, this finding is in agreement with the shift towards higher binding energies reported for metal clusters [27] when their size is decreased. The high binding energies, more intense, doublet has been assigned to Pd(II) as PdBr24 − [26]. However, the presence of oxygen in the survey spectrum of Pd –PPy suggests that a minor contribution due to PdO cannot be excluded in this case. This contribution could be responsible for the higher amount of Pd(II) found on the composite. Moreover, it is interesting to note that the tetrabromopalladate anions can be present both in the colloidal palladium and in the composites either as free ions or as species adsorbed on the Pd-NPs. As far as the Pd–PPy films are concerned, the presence of high amounts of free PdBr24 − anions is not expected, since the cathodic pulses adopted for the Pd-NPs deposition do not justify the inclusion of negatively charged doping ions in the material. In Fig. 5, the results relevant to an angle-resolved XPS analysis of a Pd –PPy film are reported. The rotation of the sample was used to characterise thinner and thinner layers of the material [28]. In the figure, dmax is the thickness of the surface layer sampled in the commonly employed experimental conditions (approximately 10 nm for polymeric films [28]) and d is the thickness of the layer that has been effectively sampled at the specified rotation angle. Consequently, lower d/dmax values are relevant to thinner surface layers. In Fig. 5a, the dependence of the atomic percentages of the most representative elements from the d/dmax ratio is reported. As it is evident from the figure, the carbon content strongly increases as d/dmax decreases, thus lowering all the other atomic percentages. High surface carbon percentages can be ascribed to oil contamination deriving from the vacuum system and also to the octyl chains of the ammonium salt bound to the PdNPs. In contrast to the other elements, the palladium
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pyrrolic nitrogen (NH) and the iminic nitrogen (NC). The last two components can be unambiguously ascribed to a polypyrrole film and can be used as markers for the PPy matrix. In the following discussion, the sum of their areas will be addressed as ‘pyrrolic nitrogen’. The ratio between Pd and the pyrrolic nitrogen increases as the sampled layer decreases, thus clearly indicating that the Pd-NPs are deposited on the PPy layer. However, it is not possible to exclude that a minor amount of Pd-NPs can enter the PPy pores, thus penetrating the film for distances deeper than the layers sampled in the present study.
3.6. Nanoparticles structure
Fig. 5. Most representative surface atomic percentages (a) and palladium/(pyrrolic nitrogen) ratio (b) dependence upon the thickness of the sampled layer, expressed as a ratio between the maximum sampled thickness, dmax, and the thickness of the effectively sampled layer d.
content is almost constant, indicating a surface segregation of the Pd-NPs. This last finding is also demonstrated by Fig. 5b. This figure has been obtained after a preliminary fitting of the N1s signal, that always gave three components: the positively charged nitrogen (N+ due to both the alkyl ammonium salts and the polarons of the conductive PPy film), the pure
Fig. 6. Structural model proposed for the Pd-NPs.
In a previous work [10], we found strong experimental evidences suggesting that the TOAB-stabilised Pd-NCs behave like bulky cations in aqueous solution, according to a structure already proposed by Reetz et al. [11], in which the tetraalkylammonium cations are the most abundant ionic species adsorbed on the particles’ surface and give them a net positive charge. This allowed us to deposit high amounts of Pd-NPs on PPy by simply applying cathodic potentials or pulses to PPy-modified electrodes. When organic solvents are used as the deposition medium, the apparent electrochemical behaviour of the NPs is not so definitive. In fact, Pd –PPy nanocomposites are obtained by applying cathodic pulses to PPy, but a certain amount of Pd-NPs inclusions can also be obtained using positive polarisations, even if in this case the NPs often undergo aggregation in the composite (TEM data not shown). It is worth noting that TEM micrographs recorded after a blank test, i.e. an experiment in which a PPy film was immersed in the Pd-NPs solution in absence of polarisation, washed and analysed, showed that only negligible amounts of clusters had adhered to the PPy film in these conditions. Other papers dealing with TOAB-stabilised chemically synthesised Pd-NPs [29,30] present a model, more complex than the one proposed by Reetz et al. [11], that seems adequate to describe the behaviour of our Pd-NPs during their deposition on PPy and it is also confirmed by the XPS results of the present study. The model presents a NP structure in which a Pd(0) core is surrounded by negative charges (the charge carriers are not specified) and by a shell composed of tetraoctylammonium ions, counterbalanced by more external bromide ions. Our XPS data indicate the presence of PdBr24 − on the surface of both as-synthesised and supported Pd-NPs; the tetrabromopalladate anions could be then the negatively charged species hypothesised in Refs. [29,30]. Consequently, a possible structure for the Pd-NPs could be the one described in Fig. 6.
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4. Conclusions A simple, all-electrochemical procedure has been used to synthesise nanostructured Pd – PPy composite thin films from organic solvents. The approach is based on the electrosynthesis of palladium nanoparticles that are subsequently deposited on the polymer matrix using cathodic pulses, which have proven to be successful in giving high loading of highly dispersed Pd-NPs in composite films having good conductivity values. XPS characterisation allowed us to address the surface composition of both the Pd-NPs and the Pd– PPy films. On the basis of literature information and our spectroscopic and electrochemical evidences, a model for the Pd-NPs structure in the colloidal solution employed in the present study is proposed. Work is in progress to exploit the Pd –PPy thin films as sensing layers in devices such as gas or chemical sensors.
Acknowledgements Ministero della Ricerca Scientifica e Tecnologica (MURST) is acknowledged for financial support. Professor C. Malitesta is gratefully acknowledged for helpful discussions, Mr A. Tambone is thanked for his help in acquiring XPS data. Professor A. Valentini is gratefully acknowledged for his skilled help in performing the four probe conductivity measurements. Part of this work has been carried out in the framework of the Center of Innovative Technology for Signal Detection and Processing, University of Bari.
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