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Au-nanoparticles composite as electrode coating suitable for electrocatalytic oxidation
Electrochimica Acta 56 (2011) 3575–3579
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Poly(3,4-ethylenedioxythiophene)/Au-nanoparticles composite as electrode coating suitable for electrocatalytic oxidation Fabio Terzi 1 , Barbara Zanfrognini, Chiara Zanardi 1 , Laura Pigani 1 , Renato Seeber ∗,1 Department of Chemistry, University of Modena and Reggio Emilia, via G. Campi 183, 41125 Modena, Italy
a r t i c l e
i n f o
Article history: Received 2 July 2010 Received in revised form 21 September 2010 Accepted 22 September 2010 Available online 29 September 2010 Keywords: Gold nanoparticles Poly(3,4-ethylendioxythiophene) Composite materials Electrocatalysis Glucose oxidation
a b s t r a c t Composite materials consisting of poly(3,4-ethylenedioxythiophene) including Au nanoparticles, encapsulated by citrate anions, have been firmly deposited on an electrode surface through a simple method, taking advantage of the interaction between Au metal and thiophene polymeric backbone. A series of similar electrode coatings, also including different amounts of nanoparticles inside, has been characterised in terms of thickness and surface morphology, through different microscopic techniques. The electrocatalytic properties have been studied with respect to the oxidation of glucose in alkaline media, which is prevented from occurring on the pure organic material. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Gold nanoparticles (AuNPs) are well acknowledged to possess remarkable electrocatalytic properties [1]. In recent years, they have been widely investigated as electrode modifiers capable to promote the electro-oxidation of different species, such as alcohols and carbohydrates. The deposition of AuNPs on electrode surfaces can be carried out according to different methods. Chemically synthesised AuNPs can be deposited on a properly functionalised electrode surface achieving best control on the NPs properties: in particular, the dimension and size distribution of the NPs can be modulated by varying the synthesis parameters, such as the nature and concentration of the reducing agent and the concentration of the metal salt precursor. In an electrocatalytic context, AuNPs encapsulated in citrate ions are often used, due to different properties such as the narrow dimensional distribution and the good long-term stability, in addition to good accessibility of the electroactive species to the metal core [2]. In order to choose the most appropriate functionalisation of the electrode surface capable to fix AuNPs, different aspects have to be taken into account: (i) the anchoring of AuNPs has to bear many electrochemical cycles over the potential window required by the electrocatalytic processes; (ii) after grafting, the NP metal cores
∗ Corresponding author. Tel.: +39 059 2055027; fax: +39 059 373543. E-mail address: [email protected] (R. Seeber). 1 ISE members. 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.09.071
have to be still available for the interaction with the electroactive species in solution; (iii) the charge percolation along the deposit has to be most effective; (iv) the number of AuNPs deposited should be possibly quite high. With this respect, the use of conducting polymers (CPs) meets with all these requirements. When polythiophene derivatives are chosen, the chemical affinity of the heteroatoms in the polymeric backbone to the NP metal core allows stable interactions between the two components. The resulting CP/NP composite materials combine the electrocatalytic properties of metal NPs and the antifouling properties of the organic component [3]. Among different CPs, our attention has been focused on poly(3,4-ethylenedioxythiophene) (PEDOT) as the polymeric matrix. PEDOT has emerged as a particularly interesting polythiophene derivative, thanks to similar characteristics as the particularly reduced band-gap, the easy occurrence of p-doping, the high stability over different charge and discharge cycles and the possibility of easily electrogeneration even in aqueous solution [4–8]. Interesting applications of PEDOT as an electrocatalyst have recently been reported [9–12]; however, some species, such as alcohols and carbohydrates, do not undergo oxidation on PEDOT. Different approaches are reported in the literature, suitable to deposit chemically synthesised AuNPs inside PEDOT [3,13–17]. In this work, we apply and refine a very simple approach [18] for the assembly of PEDOT/AuNP composite, in which a drop of solution of pre-synthesised AuNPs, encapsulated by citrate ions, is deposited on a PEDOT modified electrode. Different experimental conditions have been explored in order to obtain a coating combining at best good stability and enhanced electrocatalytic per-
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formances. This deposition method leads to stable grafting of a high number of AuNPs, exploiting chemisorption of Au on thiophene. The composites have been characterised by microscopic and electrochemical techniques. The electrocatalytic oxidation of glucose, observed when using this modified electrode, demonstrates the availability of metal core to profitable interaction with species present in solution.
2. Experimental All chemicals were from Aldrich, of pure or puriss. grade in the case of those employed for electrochemical tests, and used as received. All solutions were prepared using Millipore water, 18 M cm resistivity. AuNPs encapsulated by citrate ions have been synthesised according to Ref. [19] and stored in the refrigerator. The mean diameter of the NPs and the estimated standard deviation for the relevant quasi-normal distribution were determined through TEM images, resulting of 14 nm and 1 nm, respectively. The UV–Vis spectrum shows a maximum absorption band at 520 nm, well in accord with the mean diameter estimated [20]. The electrochemical syntheses and characterisations of the electrode coatings were performed with an Autolab PGSTAT12 (Ecochemie) potentiostat/galvanostat, under control of GPES dedicated software. All electrochemical tests were performed in a single compartment three-electrode cell, at room temperature, under Ar atmosphere. A 2 mm diameter glassy carbon (GC) disk electrode was used as the basis for the working electrode, a GC rod and an aqueous Ag/AgCl, 3 mol dm−3 KCl (Amel) constituted the auxiliary and reference electrodes, respectively. All the potential values given are referred to such a reference electrode. Before every set of measurements, the bare GC electrode was polished subsequently with 1, 0.3 and 0.05 m alumina powder, until a mirror-like surface was obtained. Then, the electrode was rinsed with ultrapure water in an ultrasonic bath for 30 min, renewing the bath every 10 min. This procedure has been chosen in order to minimize the contamination of the surface due to the residual polishing material. The electrochemical syntheses of PEDOT films were performed under galvanostatic conditions, 0.4 mA cm−2 current density, for 20, 40, 80 or 160 s, in 10 mmol dm−3 EDOT, 0.1 mol dm−3 LiClO4 , aqueous solution. Particular attention was paid to the choice of the value of the current density, suitably low to prevent polymer overoxidation: the potential was checked never to exceed +0.95 V. The stabilisation of the polymeric coating was performed in 0.1 mol dm−3 phosphate buffer solution (PBS, pH = 7.0): first, PEDOT was neutralised by polarising the electrode at −0.5 V for 60 s; then, 10 subsequent potential scans were performed in the potential range −0.5 to +0.5 V, at 0.05 V s−1 potential scan rate, in order to allow the system to achieve the steady-state electrochemical response. After a careful washing of the electrode coating with abundant water and the removal of any drops of water from the electrode surface and housing, one drop (7 l) of NP solution (ca. 0.89 mmol dm−3 Au) was deposited onto the polymer film and left there for different times, in order to promote the adsorption of the anionic NPs, thanks to the interaction with the positive charges present on partially oxidised PEDOT chains and the chemical affinity between thiophene sulphur atoms and Au metal. Due to the hydrophobic Teflon® housing of the GC electrode, the deposited drop assumes a hemispherical shape, wetting only the PEDOT coating and leaving the Teflon® housing dry. During the deposition of the AuNPs, the electrode was placed inside a glass vessel saturated with aqueous vapours, in order to prevent evaporation of the drops deposited onto the electrodes. The resulting modified electrode was rinsed with water in order to remove the excess of AuNPs. The total amount of Au deposited on PEDOT films was determined through Inductively Coupled Plasma (ICP) spectroscopy. ICP
analyses were carried out using a Perkin Elmer Optima 4200 DV instrument. The samples were obtained by dissolving the coatings in a solution containing 1 mmol dm−3 KCN and 0.1 mmol dm−3 KOH; after a dipping time of 72 h, 0.1 mmol dm−3 HCl was added. Morphology, thickness and roughness of the polymeric coating were investigated by means of Atomic Force Microscopy (AFM). Images were acquired directly on the electrode support, by a CP Park Autoprobe instrument in tapping mode. A NSG-11 tip from NT-MDT with a 11.5 N m−1 typical force constant was used. Scanning Electron Microscope (SEM) images were acquired directly on the electrode support using an ESEM Quanta-200 instrument (FEI Company) equipped with an Energy Dispersive Spectrometer (EDS – INCA system, Oxford Instruments). 3. Results and discussion In order to choose the most suitable polymer substrate to deposit AuNPs, preliminary tests have been performed on the pure organic material. In particular, the electropolymerisation of EDOT to deposit PEDOT onto the electrode substrate was conducted under galvanostatic condition, at different electrolysis times; the relevant film thicknesses and morphologies have been studied. Actually, despite the importance of these aspects when aiming at gaining most detailed picture of the system under study, investigations on PEDOT grown on GC substrates are very rare in the literature [9]. Thickness measurements were conducted on dried modified electrodes by AFM technique. Similar measurements were ascertained to be reliable, since AFM images evidenced that the polymer film was quantitatively removed by carefully scratching the surface with a plastic tip [21,22]. Exemplificative AFM images are reported in Fig. 1. Scratching tests were carried out on a bare GC electrode, demonstrating that the tip is soft enough not to damage the underlying substrate. On the other hand, scratching is suitable to form a nearly linear furrow on the polymeric film, possessing a flat bottom, ca. 30 m wide. The edges of the furrow are relatively sharp, as evidenced in Fig. 1C and D. Though small amounts of debris are necessarily present on the edges, they do not interfere with the thickness measure. In addition, delamination or deformation of the polymer close to the scratch is absent, suggesting the induction of a negligible stress of the film. The quantitative estimation of the thickness was performed over at least 40 different profile plots belonging to 4 different areas of the surface for each electrode. It is worth noticing that the bare GC substrates were checked to possess a smooth enough surface not to affect significantly the morphology of the polymeric coatings. The results obtained for the different electrodes are reported in Table 1. Hereafter, the notations GC/PEDOT Xs or GC/PEDOT Xs/AuNP Ymin are used for the different modified electrodes, X and Y being the galvanostatic polymerisation time and the AuNP contact time, respectively. Prolonging the polymerisation time leads to the increase of the polymer film thickness. Although literature reports that the thickness is directly proportional to the polymerisation time [23–25], we could actually observe a non-linear trend, allowing us to conclude Table 1 Thickness and roughness values for the different electrode surfaces. Electrode Bare GC GC/PEDOT GC/PEDOT GC/PEDOT GC/PEDOT GC/PEDOT GC/PEDOT
Thickness (nm)
20s 40s 80s 160s 20s/AuNP 30min 20s/AuNP 60min
31 66 126 215 32 33
± ± ± ± ± ±
4 5 11 16 3 4
Roughness (RMS/nm) 3 5 7 9 20 6 6
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Fig. 1. AFM images of the surface of a (A) GC/PEDOT 20s and (B) GC/PEDOT 160s modified electrodes. (C) 60 m × 60 m AFM image of a scratch done on the GC/PEDOT 160s modified electrode and (D) relevant profile plot.
that, in our case, the polymerisation efficiency is not constant at time passing. 5 m × 5 m images collected on the modified electrodes (Fig. 1A and B) show that the films are homogenous and pinholes are absent. It is well known that PEDOT thin films, grown by electrochemical techniques, possess a close-packed granular 3D structure. As evidenced in Fig. 1A and B, the higher the thickness, the higher the roughness. The increase of the surface roughness is consistent with the proposed models [26–28] since the application of the polymerisation potential for longer time leads to polymer grains of increasing dimensions [24,29]. Due to the tip-sample convolution effect it is difficult to precisely estimate the polymer grain dimensions. Hence, the grain sizes were evaluated by SEM technique: the thinnest films possess a grain size < 100 nm, while the thickest ones exhibit a grain dimension between 300 and 450 nm. The lateral dimension and height of the polymer grains increase at increasing the polymerisation time. In addition, SEM images show that the PEDOT film consists of a homogeneous coating on GC surface, any pinholes being absent. After the polymerisation and the electrochemical neutralisation and stabilisation of the PEDOT polymeric coating in PBS, a drop of AuNP solution was deposited onto the electrode surface; the deposition was conducted in a water-saturated environment, so that the inclusion of AuNPs occurs thanks to the actual effective interaction of Au with PEDOT chains, rather than by simple ‘precipitation’, as a consequence of progressive evaporation of water during drying. The anchoring of AuNPs on PEDOT films is due to the high affinity of sulphur atoms for gold substrates [3]. The stabilising species, namely citrate anions, do not constitute an obstacle to the adsorption process: they are a labile enough encapsulating agent to be easily substituted by molecules possessing higher adsorption energy, such as PEDOT residues [30,31]. The contact time of the drop of AuNP solution with PEDOT was fixed to 60 min, after checking that the electrochemical responses do not significantly change for longer deposition times. The composites were studied by recording voltammetric responses in alkaline medium, i.e. at pH = 12 (0.1 mol dm−3 LiClO4 in 17 mmol dm−3 KOH, aqueous solution). It is well know that elec-
troactive species, such as glucose, considered in the following, are inert with respect to oxidation on Au at low pH [32]. On the one hand, high pH values favour the electrocatalytic process [33,34] but, on the other hand, we could ascertain that also cause irreversible degradation of PEDOT. With this respect, preliminary tests allowed us to choose pH = 12 as a good compromise. In any cases, voltammetric responses recorded in support electrolyte media were characterised by the presence of an anodiccathodic peak system ascribable to oxidation and reduction of Au. This confirms the formation of composite materials in which the NPs are in contact with the solution, and therefore available for (electrocatalytic) oxidation of species in solution. The thickest polymeric coating, i.e. GC/PEDOT 160s/AuNP 60min, results in poorly reproducible electrochemical responses; in addition, cyclic voltammograms show that the coating is not stable under similar experimental conditions and voltammetric responses at acceptable degree of repeatability cannot be obtained. This is ascribed to the excessive thickness of the polymeric coating; SEM images evidence that the inclusion of NPs into the thickest PEDOT film leads to strong modification of the morphology: the swelling of the film is observed when AuNPs are added to PEDOT grown for 160 s (see Fig. 2B and the relevant inset). Lowering the electropolymerisation time, the modified electrodes result in more stable voltammetric responses. With this respect, the most promising coating resulted GC/PEDOT 20s/AuNP 60min, on which repeatable background responses are obtained after few voltammetric cycles. As it is evident in Fig. 2A, the film results homogeneous, significant swelling being absent. For this reason, 20 s electrolysis time was chosen. Different deposition times for the subsequent AuNPs were explored, namely 60, 30 and 5 min contact times, aiming at tuning the NP concentration on the polymeric film. The amount of NPs deposited on the different substrates was estimated from the relevant cyclic voltammograms registered in support electrolyte media (Fig. 3). The values relative to the height of the peak ascribable to Au reduction for different deposition times show that the amount of deposited Au increases up to 30 min, while a much less marked further increase is evidenced at longer deposition times.
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Fig. 3. Voltammetric responses of a GC/PEDOT 20s electrode modified with different amounts of AuNPs in 17 mmol dm−3 KOH + 0.1 mol dm−3 LiClO4; 0.050 V s−1 potential scan rate.
demonstrate that the method proposed allows the obtainment of a composite coating containing a large number of AuNPs, estimated to be of the order of 1013 NPs × mm3 . In agreement with what observed in AFM images (Table 1), SEM images show that GC/PEDOT 20s/AuNP 30min possesses similar morphology to GC/PEDOT 20s/AuNP 60min (Fig. 2C). As a final check of the actual availability of the AuNPs to induce electrocatalytic charge transfers, the behaviour of GC/PEDOT 20s/AuNP 30min modified electrodes was investigated with respect to the oxidation of glucose in alkaline solution. The voltammetric trace, reported in Fig. 5, demonstrates that AuNPs are active in the electrocatalytic oxidation of glucose: while no redox peak is observed at the bare GC electrode or at the pure PEDOT film, the PEDOT/AuNPs modified electrode presents the typical voltammetric response expected for the oxidation of primary alcohols and sugars on Au in alkaline medium [35]. Two main oxidation peaks, located at −0.28 V (A) and at +0.25 V (B), appear in the forward anodic potential scan. Peak A is ascribed to the oxidation of glucose, while Peak B is due to the further oxidation of gluconolactone, generated at the potentials of peak A [36–38]. The shoulder exhibited by each peak is due to the presence of different crystallographic planes in the AuNPs, leading to small differences in the oxidation potentials [39]. The shape of the glucose voltammogram in Fig. 5 is in agreement with the well known behaviour of Au electrodes in
Fig. 2. Secondary electrons SEM images of the surface of (A) GC/PEDOT 20s/Au 60min; (B) GC/PEDOT 160s/Au 60min modified electrodes; (C) GC/PEDOT 20s/Au 30min. In the inset of (B) SEM image of GC/PEDOT 160s is reported.
These results have been confirmed by EDS data, on the basis of the Au/S peak ratio (see Fig. 4). Spectra acquired using EDS in different regions of different PEDOT coatings showed that the polymeric films are compositionally homogenous, major segregations of AuNPs being absent. A more accurate estimation of the amount of Au in the PEDOT films, with 30 min contact time of PEDOT with the AuNP solution, has been carried out by performing ICP analyses. The mean quantity of Au embedded inside each PEDOT film resulted 0.11 g. Taking into account the size of the AuNPs and the Au density together with the thickness of the layer (Table 1), the mean distance between two adjacent NPs is estimated to be around 16 nm. These results
Fig. 4. EDS spectra of GC/PEDOT 20s/AuNP Ymin electrodes; Y = 5, 30, 60 min. Spectra have been shifted for sake of clarity.
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control of repeatability of the responses under well defined experimental conditions. This aspect is behind the scope of the present paper. 4. Conclusions
Fig. 5. Voltammetric responses for a GC/PEDOT 20s/Au 30min modified electrode in 17 mmol dm−3 KOH + 0.1 mol dm−3 LiClO4 in the absence (dashed line) and presence of 1.3 mmol dm−3 glucose (solid line, first six voltammetric scans); 0.050 V s−1 potential scan rate.
A very simple method to anchor stably an effective electrocatalytic component, such as an AuNPs system, to a versatile polymeric matrix possessing antifouling peculiarities, such as PEDOT, leads to a composite electrode system that exhibits the combined properties of the two components. The composite deposits obtained under different experimental conditions are characterised and the electrocatalytic properties of the NPs, actively supported by PEDOT in preventing poisoning adsorption, are successfully tested with respect to quite a critical oxidation process. The values of the current density result quite high when compared with those reported in the literature [36,40]. References
aqueous solutions: at positive potentials at which gold hydroxides form, the system is electrocatalytically active, while the activity drops dramatically once they are converted to oxides. This holds in both the forward and the backward scans, which accounts for the odd look of the cyclic voltammetric trace [35–38]; this behaviour also led some authors to exploit the voltammetric backward trace for drawing a calibration line on which to estimate the glucose concentration in solution [36,40]. It is noteworthy that subsequent voltammetric cycles in a 1.3 mmol dm−3 solution of glucose in KOH evidenced that, except for the very first scan, repeatable responses are obtained, and that the coating is stable for at least 30 scans. The maximum current density for 1.3 mmol dm−3 glucose electroxidation is ca. 270 A cm−2 . Further investigations have been performed by testing the efficiency of the electrode coating at varying the glucose concentration, as reported in Fig. 6. As expected, the peak current of both anodic peaks increases at increasing the concentration of glucose. Due to the strong overlap of peaks A and B, after suitable background subtraction, the total charge spent in the overall oxidation process corresponding to both peaks was taken as the measured quantity, to relate to variable glucose concentrations (see inset in Fig. 6). To a first approximation, it is evident that the resulting plot does not exhibit a clear linear trend; however, any conclusions about the possible analytical value of the system developed require much more experimental work, also including a careful
Fig. 6. Linear sweep voltammetric responses of a GC/PEDOT 20s/Au 30min modified electrode in 17 mmol dm−3 KOH + 0.1 mol dm−3 LiClO4 , at different glucose concentrations (0.1–5.0 mmol dm−3 ); 0.050 V s−1 potential scan rate; the plot of the corresponding charge spent vs. glucose concentration is reported in the inset.
[1] G.C. Bond, C. Louis, D.T. Thompson, in: G.J.H. Hutchings (Ed.), Catalysis by Gold, Catalytic Science Series, vol. 6, World Scientific Publishing, London, 2006. ˛ [2] H. Bonnemann, R. Richards, in: A. Wieckowski, E.R. Savinov (Eds.), Catalysis and Electrocatalysis at Nanoparticle Surfaces, Marcel Dekker, 2003. [3] C. Zanardi, F. Terzi, L. Pigani, R. Seeber, M. Lechkov, S. Prandzheva (Eds.), Encyclopedia of Polymer Composites: Properties, Performance, Applications, Nova Publishers, New York, 2009, pp. 1–74, and references therein. [4] M.C. Morvant, J.R. Reynolds, Synth. Met. 92 (1998) 57. [5] M. Lapkowski, A. Pro, Synth. Met. 110 (2000) 79. [6] M. Dietrich, J. Heinze, G. Heywang, F. Jonas, J. Electroanal. Chem. 369 (1994) 87. [7] H.J. Ahonen, J. Lukkari, J. Kankare, Macromolecules 33 (2000) 6787. [8] L. Pigani, A. Heras, A. Colina, R. Seeber, J. Lopez-Palacios, Electrochem. Commun. 6 (2004) 1192. [9] S.S. Kumar, J. Mathiyarasu, K.L.N. Phani, V. Yegnaraman, J. Solid State Electrochem. 10 (2006) 905. [10] V.S. Vasantha, S.-M. Chen, J. Electroanal. Chem. 592 (2006) 77. [11] S. Biallazor, A. Kupniewska, Electrochem. Commun. 2 (2000) 480. [12] P. Manisankar, S. Viswanathan, A.M. Pusphalatha, C. Rani, Anal. Chim. Acta 528 (2005) 157. [13] B.C. Sih, M.O. Wolf, Chem. Commun. (2005) 3375. [14] G. Zotti, B. Vercelli, A. Berlin, Acc. Chem. Res. 41 (2008) 1098. [15] F. Terzi, C. Zanardi, V. Martina, L. Pigani, R. Seeber, J. Electroanal. Chem. 619–620 (2008) 75. [16] C. Zanardi, F. Terzi, L. Pigani, A. Heras, A. Colina, J. Lopez-Palacios, R. Seeber, Electrochim. Acta 53 (2008) 3916. [17] C. Zanardi, F. Terzi, R. Seeber, Sens. Actuators B: Chem. 148 (2010) 277. [18] S.H. Cho, S.-M. Park, J. Phys. Chem. B 110 (2006) 25656. [19] J. Turkevich, P.C. Stevenson, J. Hillier, Disc. Faraday Soc. 11 (1951) 55. [20] F. Terzi, C. Zanardi, B. Zanfrognini, L. Pigani, R. Seeber, J. Lukkari, T. Aaritalo, J. Kankare, J. Phys. Chem. C 113 (2009) 4868. [21] R.A. McAloney, M. Sinyor, V. Dudnik, M.C. Goh, Langmuir 17 (2001) 6655. [22] R.F.M. Lobo, M.A. Pereira-da-Silva, M. Raposo, R.M. Faria, O.N. Oliveira Jr., Nanotechnology 10 (1999) 389. [23] J. Roncali, Chem. Rev. 92 (1992) 711. [24] L. Micaroni, D. Dini, F. Decker, M.A. De Paoli, J. Solid State Electrochem. 3 (1999) 352. [25] G.S. Popkirov, E. Barsoukov, R.N. Schindler, J. Electroanal. Chem. 425 (1997) 209. [26] M.F. Suárez, R.G. Compton, J. Electroanal. Chem. 462 (1999) 211. [27] B.J. Hwang, R. Santhanam, Y.L. Lin, Electrochim. Acta 46 (2001) 2843. [28] T. Hernández-Pérez, M. Morales, N. Batina, M. Salmón, J. Electrochem. Soc. 148 (2001) C369. [29] M. Innocenti, F. Loglio, L. Pigani, R. Seeber, F. Terzi, R. Udisti, Electrochim. Acta 50 (2005) 1497. [30] T. Matsuura, H. Sakaguchi, Y. Shimoyama, J. Mater. Sci.: Mater. Electron. 14 (2003) 353. [31] Z. Gao, K.S. Siow, H.A.O. Chan, Synth. Met. 75 (1995) 5. [32] L.D. Burke, T.G. Ryan, Electrochim. Acta 37 (1992) 1363. [33] M.T.M. Koper, S.C.S. Lai, E. Herrero, in: M.T.M. Koper (Ed.), Mechanisms of the Oxidation of Carbon Monoxide and Small Organic Molecules at Metal Electrodes in Fuel Cell Catalysis – A Surface Science Approach, Wiley, 2009 (Chapter 6). [34] M. Betowska-Brzezinska, T. Uczak, R. Holze, J. Appl. Electrochem. 27 (1997) 999, and references therein. [35] L.D. Burke, P.F. Nugent, Gold Bull. 31 (1998) 39. [36] J. Zhao, J. Yu, F. Wang, S. Hu, Microchim. Acta 156 (2007) 277. [37] M. Pasta, R. Ruffo, E. Falletta, C.M. Mari, C. Della Pina, Gold Bull. 43 (2010) 7. [38] Y. Bai, W. Yang, Y. Sun, C. Sun, Sens. Actuators B: Chem. 134 (2008) 471. [39] R.R. Adzic, M.W. Hsiao, E.B. Yeager, J. Electroanal. Chem. 260 (1989) 475. [40] F. Kurniawan, V. Tsakova, V.M. Mirsky, Electroanalysis 18 (2006) 1937.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Poly(3,4-ethylenedioxythiophene)/Au-nanoparticles composite as electrode coating suitable for electrocatalytic oxidation Fabio Terzi 1 , Barbara Zanfrognini, Chiara Zanardi 1 , Laura Pigani 1 , Renato Seeber ∗,1 Department of Chemistry, University of Modena and Reggio Emilia, via G. Campi 183, 41125 Modena, Italy
a r t i c l e
i n f o
Article history: Received 2 July 2010 Received in revised form 21 September 2010 Accepted 22 September 2010 Available online 29 September 2010 Keywords: Gold nanoparticles Poly(3,4-ethylendioxythiophene) Composite materials Electrocatalysis Glucose oxidation
a b s t r a c t Composite materials consisting of poly(3,4-ethylenedioxythiophene) including Au nanoparticles, encapsulated by citrate anions, have been firmly deposited on an electrode surface through a simple method, taking advantage of the interaction between Au metal and thiophene polymeric backbone. A series of similar electrode coatings, also including different amounts of nanoparticles inside, has been characterised in terms of thickness and surface morphology, through different microscopic techniques. The electrocatalytic properties have been studied with respect to the oxidation of glucose in alkaline media, which is prevented from occurring on the pure organic material. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Gold nanoparticles (AuNPs) are well acknowledged to possess remarkable electrocatalytic properties [1]. In recent years, they have been widely investigated as electrode modifiers capable to promote the electro-oxidation of different species, such as alcohols and carbohydrates. The deposition of AuNPs on electrode surfaces can be carried out according to different methods. Chemically synthesised AuNPs can be deposited on a properly functionalised electrode surface achieving best control on the NPs properties: in particular, the dimension and size distribution of the NPs can be modulated by varying the synthesis parameters, such as the nature and concentration of the reducing agent and the concentration of the metal salt precursor. In an electrocatalytic context, AuNPs encapsulated in citrate ions are often used, due to different properties such as the narrow dimensional distribution and the good long-term stability, in addition to good accessibility of the electroactive species to the metal core [2]. In order to choose the most appropriate functionalisation of the electrode surface capable to fix AuNPs, different aspects have to be taken into account: (i) the anchoring of AuNPs has to bear many electrochemical cycles over the potential window required by the electrocatalytic processes; (ii) after grafting, the NP metal cores
∗ Corresponding author. Tel.: +39 059 2055027; fax: +39 059 373543. E-mail address: [email protected] (R. Seeber). 1 ISE members. 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.09.071
have to be still available for the interaction with the electroactive species in solution; (iii) the charge percolation along the deposit has to be most effective; (iv) the number of AuNPs deposited should be possibly quite high. With this respect, the use of conducting polymers (CPs) meets with all these requirements. When polythiophene derivatives are chosen, the chemical affinity of the heteroatoms in the polymeric backbone to the NP metal core allows stable interactions between the two components. The resulting CP/NP composite materials combine the electrocatalytic properties of metal NPs and the antifouling properties of the organic component [3]. Among different CPs, our attention has been focused on poly(3,4-ethylenedioxythiophene) (PEDOT) as the polymeric matrix. PEDOT has emerged as a particularly interesting polythiophene derivative, thanks to similar characteristics as the particularly reduced band-gap, the easy occurrence of p-doping, the high stability over different charge and discharge cycles and the possibility of easily electrogeneration even in aqueous solution [4–8]. Interesting applications of PEDOT as an electrocatalyst have recently been reported [9–12]; however, some species, such as alcohols and carbohydrates, do not undergo oxidation on PEDOT. Different approaches are reported in the literature, suitable to deposit chemically synthesised AuNPs inside PEDOT [3,13–17]. In this work, we apply and refine a very simple approach [18] for the assembly of PEDOT/AuNP composite, in which a drop of solution of pre-synthesised AuNPs, encapsulated by citrate ions, is deposited on a PEDOT modified electrode. Different experimental conditions have been explored in order to obtain a coating combining at best good stability and enhanced electrocatalytic per-
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formances. This deposition method leads to stable grafting of a high number of AuNPs, exploiting chemisorption of Au on thiophene. The composites have been characterised by microscopic and electrochemical techniques. The electrocatalytic oxidation of glucose, observed when using this modified electrode, demonstrates the availability of metal core to profitable interaction with species present in solution.
2. Experimental All chemicals were from Aldrich, of pure or puriss. grade in the case of those employed for electrochemical tests, and used as received. All solutions were prepared using Millipore water, 18 M cm resistivity. AuNPs encapsulated by citrate ions have been synthesised according to Ref. [19] and stored in the refrigerator. The mean diameter of the NPs and the estimated standard deviation for the relevant quasi-normal distribution were determined through TEM images, resulting of 14 nm and 1 nm, respectively. The UV–Vis spectrum shows a maximum absorption band at 520 nm, well in accord with the mean diameter estimated [20]. The electrochemical syntheses and characterisations of the electrode coatings were performed with an Autolab PGSTAT12 (Ecochemie) potentiostat/galvanostat, under control of GPES dedicated software. All electrochemical tests were performed in a single compartment three-electrode cell, at room temperature, under Ar atmosphere. A 2 mm diameter glassy carbon (GC) disk electrode was used as the basis for the working electrode, a GC rod and an aqueous Ag/AgCl, 3 mol dm−3 KCl (Amel) constituted the auxiliary and reference electrodes, respectively. All the potential values given are referred to such a reference electrode. Before every set of measurements, the bare GC electrode was polished subsequently with 1, 0.3 and 0.05 m alumina powder, until a mirror-like surface was obtained. Then, the electrode was rinsed with ultrapure water in an ultrasonic bath for 30 min, renewing the bath every 10 min. This procedure has been chosen in order to minimize the contamination of the surface due to the residual polishing material. The electrochemical syntheses of PEDOT films were performed under galvanostatic conditions, 0.4 mA cm−2 current density, for 20, 40, 80 or 160 s, in 10 mmol dm−3 EDOT, 0.1 mol dm−3 LiClO4 , aqueous solution. Particular attention was paid to the choice of the value of the current density, suitably low to prevent polymer overoxidation: the potential was checked never to exceed +0.95 V. The stabilisation of the polymeric coating was performed in 0.1 mol dm−3 phosphate buffer solution (PBS, pH = 7.0): first, PEDOT was neutralised by polarising the electrode at −0.5 V for 60 s; then, 10 subsequent potential scans were performed in the potential range −0.5 to +0.5 V, at 0.05 V s−1 potential scan rate, in order to allow the system to achieve the steady-state electrochemical response. After a careful washing of the electrode coating with abundant water and the removal of any drops of water from the electrode surface and housing, one drop (7 l) of NP solution (ca. 0.89 mmol dm−3 Au) was deposited onto the polymer film and left there for different times, in order to promote the adsorption of the anionic NPs, thanks to the interaction with the positive charges present on partially oxidised PEDOT chains and the chemical affinity between thiophene sulphur atoms and Au metal. Due to the hydrophobic Teflon® housing of the GC electrode, the deposited drop assumes a hemispherical shape, wetting only the PEDOT coating and leaving the Teflon® housing dry. During the deposition of the AuNPs, the electrode was placed inside a glass vessel saturated with aqueous vapours, in order to prevent evaporation of the drops deposited onto the electrodes. The resulting modified electrode was rinsed with water in order to remove the excess of AuNPs. The total amount of Au deposited on PEDOT films was determined through Inductively Coupled Plasma (ICP) spectroscopy. ICP
analyses were carried out using a Perkin Elmer Optima 4200 DV instrument. The samples were obtained by dissolving the coatings in a solution containing 1 mmol dm−3 KCN and 0.1 mmol dm−3 KOH; after a dipping time of 72 h, 0.1 mmol dm−3 HCl was added. Morphology, thickness and roughness of the polymeric coating were investigated by means of Atomic Force Microscopy (AFM). Images were acquired directly on the electrode support, by a CP Park Autoprobe instrument in tapping mode. A NSG-11 tip from NT-MDT with a 11.5 N m−1 typical force constant was used. Scanning Electron Microscope (SEM) images were acquired directly on the electrode support using an ESEM Quanta-200 instrument (FEI Company) equipped with an Energy Dispersive Spectrometer (EDS – INCA system, Oxford Instruments). 3. Results and discussion In order to choose the most suitable polymer substrate to deposit AuNPs, preliminary tests have been performed on the pure organic material. In particular, the electropolymerisation of EDOT to deposit PEDOT onto the electrode substrate was conducted under galvanostatic condition, at different electrolysis times; the relevant film thicknesses and morphologies have been studied. Actually, despite the importance of these aspects when aiming at gaining most detailed picture of the system under study, investigations on PEDOT grown on GC substrates are very rare in the literature [9]. Thickness measurements were conducted on dried modified electrodes by AFM technique. Similar measurements were ascertained to be reliable, since AFM images evidenced that the polymer film was quantitatively removed by carefully scratching the surface with a plastic tip [21,22]. Exemplificative AFM images are reported in Fig. 1. Scratching tests were carried out on a bare GC electrode, demonstrating that the tip is soft enough not to damage the underlying substrate. On the other hand, scratching is suitable to form a nearly linear furrow on the polymeric film, possessing a flat bottom, ca. 30 m wide. The edges of the furrow are relatively sharp, as evidenced in Fig. 1C and D. Though small amounts of debris are necessarily present on the edges, they do not interfere with the thickness measure. In addition, delamination or deformation of the polymer close to the scratch is absent, suggesting the induction of a negligible stress of the film. The quantitative estimation of the thickness was performed over at least 40 different profile plots belonging to 4 different areas of the surface for each electrode. It is worth noticing that the bare GC substrates were checked to possess a smooth enough surface not to affect significantly the morphology of the polymeric coatings. The results obtained for the different electrodes are reported in Table 1. Hereafter, the notations GC/PEDOT Xs or GC/PEDOT Xs/AuNP Ymin are used for the different modified electrodes, X and Y being the galvanostatic polymerisation time and the AuNP contact time, respectively. Prolonging the polymerisation time leads to the increase of the polymer film thickness. Although literature reports that the thickness is directly proportional to the polymerisation time [23–25], we could actually observe a non-linear trend, allowing us to conclude Table 1 Thickness and roughness values for the different electrode surfaces. Electrode Bare GC GC/PEDOT GC/PEDOT GC/PEDOT GC/PEDOT GC/PEDOT GC/PEDOT
Thickness (nm)
20s 40s 80s 160s 20s/AuNP 30min 20s/AuNP 60min
31 66 126 215 32 33
± ± ± ± ± ±
4 5 11 16 3 4
Roughness (RMS/nm) 3 5 7 9 20 6 6
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Fig. 1. AFM images of the surface of a (A) GC/PEDOT 20s and (B) GC/PEDOT 160s modified electrodes. (C) 60 m × 60 m AFM image of a scratch done on the GC/PEDOT 160s modified electrode and (D) relevant profile plot.
that, in our case, the polymerisation efficiency is not constant at time passing. 5 m × 5 m images collected on the modified electrodes (Fig. 1A and B) show that the films are homogenous and pinholes are absent. It is well known that PEDOT thin films, grown by electrochemical techniques, possess a close-packed granular 3D structure. As evidenced in Fig. 1A and B, the higher the thickness, the higher the roughness. The increase of the surface roughness is consistent with the proposed models [26–28] since the application of the polymerisation potential for longer time leads to polymer grains of increasing dimensions [24,29]. Due to the tip-sample convolution effect it is difficult to precisely estimate the polymer grain dimensions. Hence, the grain sizes were evaluated by SEM technique: the thinnest films possess a grain size < 100 nm, while the thickest ones exhibit a grain dimension between 300 and 450 nm. The lateral dimension and height of the polymer grains increase at increasing the polymerisation time. In addition, SEM images show that the PEDOT film consists of a homogeneous coating on GC surface, any pinholes being absent. After the polymerisation and the electrochemical neutralisation and stabilisation of the PEDOT polymeric coating in PBS, a drop of AuNP solution was deposited onto the electrode surface; the deposition was conducted in a water-saturated environment, so that the inclusion of AuNPs occurs thanks to the actual effective interaction of Au with PEDOT chains, rather than by simple ‘precipitation’, as a consequence of progressive evaporation of water during drying. The anchoring of AuNPs on PEDOT films is due to the high affinity of sulphur atoms for gold substrates [3]. The stabilising species, namely citrate anions, do not constitute an obstacle to the adsorption process: they are a labile enough encapsulating agent to be easily substituted by molecules possessing higher adsorption energy, such as PEDOT residues [30,31]. The contact time of the drop of AuNP solution with PEDOT was fixed to 60 min, after checking that the electrochemical responses do not significantly change for longer deposition times. The composites were studied by recording voltammetric responses in alkaline medium, i.e. at pH = 12 (0.1 mol dm−3 LiClO4 in 17 mmol dm−3 KOH, aqueous solution). It is well know that elec-
troactive species, such as glucose, considered in the following, are inert with respect to oxidation on Au at low pH [32]. On the one hand, high pH values favour the electrocatalytic process [33,34] but, on the other hand, we could ascertain that also cause irreversible degradation of PEDOT. With this respect, preliminary tests allowed us to choose pH = 12 as a good compromise. In any cases, voltammetric responses recorded in support electrolyte media were characterised by the presence of an anodiccathodic peak system ascribable to oxidation and reduction of Au. This confirms the formation of composite materials in which the NPs are in contact with the solution, and therefore available for (electrocatalytic) oxidation of species in solution. The thickest polymeric coating, i.e. GC/PEDOT 160s/AuNP 60min, results in poorly reproducible electrochemical responses; in addition, cyclic voltammograms show that the coating is not stable under similar experimental conditions and voltammetric responses at acceptable degree of repeatability cannot be obtained. This is ascribed to the excessive thickness of the polymeric coating; SEM images evidence that the inclusion of NPs into the thickest PEDOT film leads to strong modification of the morphology: the swelling of the film is observed when AuNPs are added to PEDOT grown for 160 s (see Fig. 2B and the relevant inset). Lowering the electropolymerisation time, the modified electrodes result in more stable voltammetric responses. With this respect, the most promising coating resulted GC/PEDOT 20s/AuNP 60min, on which repeatable background responses are obtained after few voltammetric cycles. As it is evident in Fig. 2A, the film results homogeneous, significant swelling being absent. For this reason, 20 s electrolysis time was chosen. Different deposition times for the subsequent AuNPs were explored, namely 60, 30 and 5 min contact times, aiming at tuning the NP concentration on the polymeric film. The amount of NPs deposited on the different substrates was estimated from the relevant cyclic voltammograms registered in support electrolyte media (Fig. 3). The values relative to the height of the peak ascribable to Au reduction for different deposition times show that the amount of deposited Au increases up to 30 min, while a much less marked further increase is evidenced at longer deposition times.
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Fig. 3. Voltammetric responses of a GC/PEDOT 20s electrode modified with different amounts of AuNPs in 17 mmol dm−3 KOH + 0.1 mol dm−3 LiClO4; 0.050 V s−1 potential scan rate.
demonstrate that the method proposed allows the obtainment of a composite coating containing a large number of AuNPs, estimated to be of the order of 1013 NPs × mm3 . In agreement with what observed in AFM images (Table 1), SEM images show that GC/PEDOT 20s/AuNP 30min possesses similar morphology to GC/PEDOT 20s/AuNP 60min (Fig. 2C). As a final check of the actual availability of the AuNPs to induce electrocatalytic charge transfers, the behaviour of GC/PEDOT 20s/AuNP 30min modified electrodes was investigated with respect to the oxidation of glucose in alkaline solution. The voltammetric trace, reported in Fig. 5, demonstrates that AuNPs are active in the electrocatalytic oxidation of glucose: while no redox peak is observed at the bare GC electrode or at the pure PEDOT film, the PEDOT/AuNPs modified electrode presents the typical voltammetric response expected for the oxidation of primary alcohols and sugars on Au in alkaline medium [35]. Two main oxidation peaks, located at −0.28 V (A) and at +0.25 V (B), appear in the forward anodic potential scan. Peak A is ascribed to the oxidation of glucose, while Peak B is due to the further oxidation of gluconolactone, generated at the potentials of peak A [36–38]. The shoulder exhibited by each peak is due to the presence of different crystallographic planes in the AuNPs, leading to small differences in the oxidation potentials [39]. The shape of the glucose voltammogram in Fig. 5 is in agreement with the well known behaviour of Au electrodes in
Fig. 2. Secondary electrons SEM images of the surface of (A) GC/PEDOT 20s/Au 60min; (B) GC/PEDOT 160s/Au 60min modified electrodes; (C) GC/PEDOT 20s/Au 30min. In the inset of (B) SEM image of GC/PEDOT 160s is reported.
These results have been confirmed by EDS data, on the basis of the Au/S peak ratio (see Fig. 4). Spectra acquired using EDS in different regions of different PEDOT coatings showed that the polymeric films are compositionally homogenous, major segregations of AuNPs being absent. A more accurate estimation of the amount of Au in the PEDOT films, with 30 min contact time of PEDOT with the AuNP solution, has been carried out by performing ICP analyses. The mean quantity of Au embedded inside each PEDOT film resulted 0.11 g. Taking into account the size of the AuNPs and the Au density together with the thickness of the layer (Table 1), the mean distance between two adjacent NPs is estimated to be around 16 nm. These results
Fig. 4. EDS spectra of GC/PEDOT 20s/AuNP Ymin electrodes; Y = 5, 30, 60 min. Spectra have been shifted for sake of clarity.
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control of repeatability of the responses under well defined experimental conditions. This aspect is behind the scope of the present paper. 4. Conclusions
Fig. 5. Voltammetric responses for a GC/PEDOT 20s/Au 30min modified electrode in 17 mmol dm−3 KOH + 0.1 mol dm−3 LiClO4 in the absence (dashed line) and presence of 1.3 mmol dm−3 glucose (solid line, first six voltammetric scans); 0.050 V s−1 potential scan rate.
A very simple method to anchor stably an effective electrocatalytic component, such as an AuNPs system, to a versatile polymeric matrix possessing antifouling peculiarities, such as PEDOT, leads to a composite electrode system that exhibits the combined properties of the two components. The composite deposits obtained under different experimental conditions are characterised and the electrocatalytic properties of the NPs, actively supported by PEDOT in preventing poisoning adsorption, are successfully tested with respect to quite a critical oxidation process. The values of the current density result quite high when compared with those reported in the literature [36,40]. References
aqueous solutions: at positive potentials at which gold hydroxides form, the system is electrocatalytically active, while the activity drops dramatically once they are converted to oxides. This holds in both the forward and the backward scans, which accounts for the odd look of the cyclic voltammetric trace [35–38]; this behaviour also led some authors to exploit the voltammetric backward trace for drawing a calibration line on which to estimate the glucose concentration in solution [36,40]. It is noteworthy that subsequent voltammetric cycles in a 1.3 mmol dm−3 solution of glucose in KOH evidenced that, except for the very first scan, repeatable responses are obtained, and that the coating is stable for at least 30 scans. The maximum current density for 1.3 mmol dm−3 glucose electroxidation is ca. 270 A cm−2 . Further investigations have been performed by testing the efficiency of the electrode coating at varying the glucose concentration, as reported in Fig. 6. As expected, the peak current of both anodic peaks increases at increasing the concentration of glucose. Due to the strong overlap of peaks A and B, after suitable background subtraction, the total charge spent in the overall oxidation process corresponding to both peaks was taken as the measured quantity, to relate to variable glucose concentrations (see inset in Fig. 6). To a first approximation, it is evident that the resulting plot does not exhibit a clear linear trend; however, any conclusions about the possible analytical value of the system developed require much more experimental work, also including a careful
Fig. 6. Linear sweep voltammetric responses of a GC/PEDOT 20s/Au 30min modified electrode in 17 mmol dm−3 KOH + 0.1 mol dm−3 LiClO4 , at different glucose concentrations (0.1–5.0 mmol dm−3 ); 0.050 V s−1 potential scan rate; the plot of the corresponding charge spent vs. glucose concentration is reported in the inset.
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