Modification of Electrode Surfaces: Deposition of Thin Layers of

Dec 28, 2009 - In this paper, we present a novel, easy, and effective procedure for obtaining .... That examination revealed that PPY−Au formed tigh...
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Anal. Chem. 2010, 82, 469–472

Letters to Analytical Chemistry Modification of Electrode Surfaces: Deposition of Thin Layers of Polypyrrole-Au Nanoparticle Materials Using a Combination of Interphase Synthesis and Dip-in Method Marianna Gniadek, Sylwia Modzelewska, Mikolaj Donten, and Zbigniew Stojek* Department of Chemistry, University of Warsaw, ul. Pasteura 1, PL-02-093 Warsaw Formation of thin layers of the composite material by a method based on interphase polymerization induced by a transport-controlled redox reaction is described. The obtained films were of 0.2-1 µm thickness, consisted of polypyrrole and gold nanoparticles (up to 13.5 at. %), strongly adhered to the substrate surface, and were uniform. Different carbon materials and glass wool were employed as the substrates. The first step in the synthesis was deposition of an organic layer on the substrate. This was followed by dipping the substrate in an aqueous solution containing an oxidizer and appropriate washing and drying the composite film. Composite materials consisting of conducting polymer and metal nanoparticles may combine interesting properties of both components. Conductive polymeric coatings provide unique electrochemical and optical properties and improve the stability of the metal nanoparticles1 that are often employed as the catalyst. The conductivity of the obtained composite material is usually improved vs the pure polymeric material.2,3 Polypyrrole (PPY) is one of the most widely investigated conducting polymers. It exhibits excellent stability under environmental conditions, good conductivity, and biocompatibility. Because of its properties, PPY has found many applications in such fields as sensors,4-7 fuel cells,8 batteries,9,10 capacitors,11 and anticorrosive coatings.12 * To whom correspondence should be addressed. E-mail: stojek@ chem.uw.edu.pl. (1) Chen, A.; Kamata, K.; Nakagawa, M.; Iyoda, T.; Wang, H.; Li, X. J. Phys. Chem. B 2005, 109, 18283–18288. (2) Alqudami, A.; Annapoorni, S.; Sen, P.; Rawat, R. S. Synth. Met. 2007, 157, 53–59. (3) Chen, W.; Li, Ch. M.; Chen, P.; Sun, C. Q. Electrochim. Acta 2007, 52, 2845–2849. (4) Jin, G.; Norrish, J.; Too, Ch.; Wallace, G. Curr. Appl. Phys. 2004, 4, 366– 369. (5) Dubois, M.-P.; Gondran, Ch.; Renaudet, O.; Dumy, P.; Driguez, H.; Fort, S.; Cosnier, S. Chem. Commun. 2005, 4318–4320. (6) Al-Mashat, L.; Tran, H. D.; Wlodarski, W.; Kaner, R. B.; Kalantar-zadeh, K. Sens. Actuators, B 2008, 134, 826–831. (7) Mailley, P.; Cummings, E. A.; Mailley, C. S.; Eggins, B. R.; Mcadams, E.; Cosnier, S. Anal. Chem. 2003, 75, 5422–5428. (8) Liu, H.; Shi, Z.; Zhang, J.; Zhang, L.; Zhang, J. J. Mater. Chem. 2009, 19, 468–470. 10.1021/ac902426c  2010 American Chemical Society Published on Web 12/28/2009

There are reports in the literature on the electrochemical deposition of polypyrrole on the surface of such substrates as metals,13,14 glassy carbon,15,16 graphite,17 and even textiles.18,19 In recent years, a number of synthetic methods of polypyrrolegold composite have been proposed. The most common way is the electrochemical synthesis performed by electrodeposition of the conductive polymer from a solution with suspended particles of the metal.20 Another approach to obtain the polypyrrole-Au composite is volume chemical synthesis. It was used to produce gold decorated nanoparticles of polypyrrole.21 In this two-step synthesis, a gold salt was not used as the monomer oxidizer. Direct oxidation of pyrrole by AuCl4- for the composite formation was proposed by Rao and Trivedi.22 Recently, we described a simple method of nobel metal-polymer composite formation based on the liquid-liquid interphase synthesis.23,24 In this paper, we present a novel, easy, and effective procedure for obtaining a conducting polymer-metal composite layer of uniform thickness tightly covering any substrate, including those with a very developed surface. The method involves the use of a liquid-liquid two-phase boundary and can be useful in the modification of electrode surfaces. The layers of obtained composite materials were characterized by scanning electron micros(9) Vatsalarani, J.; Geetha, S.; Trivedi, D. C.; Warrier, P. C. J. Power Sources 2006, 158, 1484–1489. (10) Wang, J.; Too, C. O.; Zhou, D.; Wallace, G. G. J. Power Sources 2005, 140, 162–167. (11) Fan, L.-Z.; Maier, J. Electrochem. Commun. 2006, 8, 937–940. (12) Zarras, P.; Anderson, N.; Webber, C.; Irvin, D. J.; Irvin, J. A.; Guenthner, A.; Stenger-Smith, J. D. Radiat. Phys. Chem. 2003, 68, 387–394. (13) Jamadade, S. A.; Puri, V. Appl. Surf. Sci. 2009, 255, 8281–8285. (14) Chung, S. M.; Paik, W.; Yeo, I. H. Synth. Met. 1997, 84, 155–156. (15) Li, C. M.; Sun, C. Q.; Chen, W.; Pan, L. Surf. Coat. Technol. 2005, 198, 474–477. (16) Chen, W.; Lu, Z.; Li, Ch. M. Anal. Chem. 2008, 80, 8485–8492. (17) Ge, D.; Wang, J.; Wang, Z.; Wang, S. Synth. Met. 2002, 132, 93–95. (18) Child, A. D.; Kuhn, H. Synth. Met. 1997, 84, 141–142. (19) Wu, J.; Zhou, D.; Too, C. O.; Wallace, G. G. Synth. Met. 2005, 155, 698– 701. (20) Chen, W.; Li, Ch.-M.; Yu, L.; Lu, Z.; Zhou, Q. Electrochem. Commun. 2008, 10, 1340–1343. (21) Ding, J.; Wang, H.; Lin, T.; Lee, B. Synth. Met. 2008, 158, 585–589. (22) Rao, Ch. R. K.; Trivedi, D. C. Synth. Met. 2007, 157, 432–436. (23) Gniadek, M.; Bak, E.; Stojek, Z.; Donten, M. J. Solid State Electrochem. 2009, DOI: 10.1007/s10008-009-0939-6. (24) Gniadek, M.; Donten, M.; Stojek, Z. Electrochim. Acta 2009, in press.

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copy (SEM), X-ray energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). EXPERIMENTAL SECTION Chemicals. Chloroauric acid (HAuCl4), ascorbic acid, nitrobenzene (NB), and acetone were purchased from POCh S.A. Pyrrole and lithium perchlorate (LiClO4) were purchased from Fluka. All reagents were of p.a. purity. Pure water was obtained from a Mili-Q Plus/Milipore purification system (conductivity of water: 0.056 µS cm-1). Scanning Electron Microscopy and Elemental Analysis. The SEM images were taken with a LEO 435 VP microscope (Germany), controlled by producer’s software via a PC. The device was working at a low sample current to give the highest possible resolution and detailed structure imaging. Under such conditions, the effect of charging-up was diminished. The pictures were obtained with the 15 keV electron-beam energy to obtain the best quality images. Before the samples were exposed to vacuum, they were washed with acetone and dried. Elemental analysis measurements were carried out with a multichannel EDS device (Ro¨ntec, model M1, Germany). Optical Measurements. The optical observations during the polymerization process were carried out using a light microscope (Axioobserver, Zeiss, Germany). Synthesis Procedure. For the formation of PPy-Au composites, the substrate materials such as graphite rods and plates, carbon foam, and fiber glass were first washed with ethanol and water and then dried. Next, to cover the substrates with a thin layer of organic phase, they were immersed, for 2 min, in a 1 M solution of pyrrole (monomer) in nitrobenzene diluted with acetone in either a 1:2 or 1:10 ratio. Both horizontal and perpendicular immersions worked well. After that, the substrates were removed from the monomer solution, and the excess of acetone was evaporated in air to obtain a thin film of monomer solution in nitrobenzene on the substrate surface. Afterward, the NB-monomer covered objects were immersed in either a 0.02 or 0.1 M HAuCl4 (oxidant) aqueous solution for various times (from 15 min to 1 h). Then, the substrate samples were immersed in ethanol to remove nitrobenzene and in water to remove remains of the oxidant. Next, they were dried in air. The percentage of the metal in the composite can be lowered by mixing AuCl4- with another oxidant, e.g., S2O82-. However, it is worth noting that the rate of the pyrrole polymerization caused by AuCl4- is apparently higher. We expect that the formed gold nanocrystals stand behind this; it is known that the nanostructured gold catalyzes various oxidation processes.25 The final polypyrrole layer thickness was estimated by subtracting the masses of the graphite rods determined before and after immersion to the nitrobenzene solution. The PPy density was taken as 1.5 g/cm3.26 The immersion of graphite rods into 1 M NB solutions of pyrrol diluted either 1:2 or 1:10 with acetone resulted in 22 and 5 µm layers of solution, 1.45 × 10-4 and 3.3 × 10-5 g/cm2 of pyrrole in the layers, and 1 and 0.25 µm polymer layers, respectively. (25) Astruc, D. Nanoparticles and Catalysis; Viley-VCH: Weinheim, Germany, 2008; pp 1-48. (26) Lopez Cascales, J. J.; Otero, T. F. J. Phys. Chem. 2004, 120, 1951–1957.

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RESULTS AND DISCUSSION The presented procedure of obtaining the composite material consisting of conducting polymer and noble-metal particles is based on the phenomenon described in the recently published work.23,24 In those papers, the formation of the bulk material was described; here, we show how to produce thin films of the composite tightly covering the surface of various solid materials. The NB phase was diluted with acetone, since acetone is highly volatile and effectively lowers the viscosity of NB. Finally, it is possible to obtain an ultrathin film of the monomer NB solution on any surface state of the modified material after evaporation of acetone from the monomer-nitrobenzene-acetone mixture. A scheme of the procedure is given in Scheme 1 in the Supporting Information. The polymerization reaction took place at the organic liquid/water interface and started immediately after dipping the substrate covered by a NB-monomer layer in the aqueous phase with the oxidant. The AuCl4- ions cause the oxidation of the monomer and the polymer growth while they are reduced to metallic gold and form crystals which are incorporated into the polymer network (3 Py + 7/3 AuCl4- f [-Py-Py-Py-]+ + 7/3 Au + 6H+ + 28/3 Cl-).27 The average, maximal amount of gold determined by the EDX method was ca. 13.5% (hydrogen atoms are not seen by the EDX) which agrees well with the theoretical estimation based on the stoichiometry of the reaction. The amount of gold incorporated into the polymer can be diminished by the use of a mixture of two oxidants, one of which is not reduced to a solid deposit. In a series of syntheses with mixtures of AuCl4--persulfate ions, we could get the content of Au in the range of 1-13 at. %. A disadvantage of the use of the persulfate oxidant is a slowdown, by a factor of 3-10, depending on the gold content, of the polymerization reaction. The worked out method allows the formation of the composite material to be evenly distributed over the surface of disks and rods of the conducting and nonconducting substrates. The deposits with larger gold crystals observed under a light microscope appeared as dark films with visible golden spots which tightly covered the surface of all used substrates. Under a SEM, the gold objects within the polymer matrix were of similar shape and their size was in the range of nanometers to micrometers. The thickness and the polymerization rate of the polymer could be controlled by the experimental conditions (oxidant and monomer concentrations and reaction time). An increase in the concentration of the gold ion and reaction time caused an increase in the layer thickness and led to enhanced aggregation of the gold nanocrystals (nanoplates in flowerlike objects). The layers of the composite deposited on the graphite rods and inlaid discs were examined using the SEM and EDS techniques. That examination revealed that PPY-Au formed tightly adhered layers on the substrate surface (see Figure S2 in the Supporting Information). The electrochemical signals of the PPY-Au material deposited on a graphite rod electrode are illustrated by a cyclic voltammogram in Figure S3 in the Supporting Information. PPY-Au was a black solid composite with small gold spots which were evenly distributed in all samples when the concentration of the oxidant was small (0.02 M AuCl4-), polym(27) Selvan, S. T.; Spatz, J. P.; Klok, H.-A.; Moller, M. Adv. Mater. 1998, 10, 132–134.

Figure 2. Powder XR diffractograms of a PPy-Au composite on a graphite disk: 0.02 M HAuCl4 and 1 M monomer.

Figure 1. SEM images of (A) PPy-Au synthesized with 0.02 M AuCl4- and 1 M monomer for 15 min and (B) an enlarged typical gold granule with characteristic nanometer-thick metal plates.

erization time was short (15 min), and the organic-phase layer was relatively thick, since it was formed from a 1 M pyrrole solution in nitrobenzene diluted only 1:2 with acetone. This is illustrated in Figure 1A. The detailed complex shape of a typical gold object is presented in Figure 1B. These gold spots consisted of ultrathin (nanosized) gold flakes growing together from one point in all directions. After longer (1 h) polymerization times and for more concentrated oxidant, the gold structures (spots) were larger and formed patches of a very developed surface, see Figure S1 in the Supporting Information. Typical diffractograms of the samples of PPY-Au on graphite disks obtained with a 0.02 M AuCl4- solution and for two polymerization times (15 min and 1 h) are presented in Figure 2. With an increase in polymerization time, a departure from the regular behavior of the intensities of (111) and (200) signals is observed. The (111) signal should be three times higher than the (200) one; in the figure, they are nearly of the same intensity after a 1 h synthesis. The mentioned departure in the ratio of the signal heights means that in time the nanocrystals (nanoplates) grow along the selected (111) plane. That conclusion was confirmed by the SEM images which show that the larger gold granules consist of small nanometer-size gold flakes (see Figure 1B). The signals seen in Figure 2 are broader than those expected for the regular size polycrystalline material. This can be explained by a significant drop in the size of the gold crystals incorporated into the composite material deposited on the graphite disk surface. The equation proposed by Scherrer many years ago28 that combines the half width of the diffraction peak with the size of the crystal present in the investigated substance is apparently (28) Patterson, A. L. Phys. Rev. 1939, 56, 978–982.

Figure 3. SEM images of (A) carbon foam covered by PPy-Au composite and (B) glass fiber material covered with PPy-Au composite.

useful in the investigations of the crystalline objects.29,30 An estimation of the crystal size done by Bruker’s commercial software TOPAS 3 indicated that the gold crystals incorporated into PPY-Au composite, after a 1 h synthesis, appear as ca. 50 nm thick nanocrystals. For shorter polymerization times (15 min), the mean gold-nanocrystal thickness is smaller: 30 nm. The results of these calculations are consistent with the SEM data. The procedure described above was used to cover the substrates of more complex shapes. A typical micrograph of carbon foam covered with PPY-Au is shown in Figure 3A. Here, (29) Alexandrov, I. V.; Enikeev, N. A. Mater. Sci. Eng., A 2000, 286, 110–114. (30) Donten, M.; Stojek, Z.; Cesiulis, H. J. Electrochem. Soc. 2003, 150 (2), C95– C98.

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the dilution of the pyrrole in nitrobenzene solution with acetone was particularly useful, since it made the liquid less viscous and allowed the easy penetration of the carbon foam. After a few of minutes of evaporation of acetone, a thin film of monomer solution covered the entire surface of the carbon foam. Finally, a solid film of composite material with evenly distributed gold crystals was obtained. The composite material covered the entire surface of the carbon foam. Like in the previous cases, the obtained PPY-Au material contained larger gold granules, which consisted of small nonosized leaflike gold crystals. A good coverage of the foam with PPY-Au composite was always obtained even with a short, 15 min polymerization time. We have also covered a nonconducting glassy fiber material. This type of material has a very developed surface. The developed procedure was successfully used to cover this material with the PPY-Au material. Already, after a short polymerization time (15 min), each fiber was covered with the composite material; see Figure 3B. In contrast to other substrates, the polymer-metal composite grew between fibers as thin plates. This made the surface of the modified material even more developed. After a 1 h polymerization time, the PPY-Au composite formed the structures tightly covering the glass-fiber material.

nanoparticles was possible by a method based on the intherphase synthesis. The obtained layers adhered well to substrates and covered well the even surfaces and all elements of strongly developed surfaces like those of carbon foam. On the surface and inside the fibrous substrates, the composite material consisted of nanometer thick films spread between the adjacent fibers. The ratio of polymer to Au could be controlled in the range of 1-13.5 at. % by the use of the appropriate oxidative mixture of the persulfate and gold anions.

CONCLUSIONS It appeared that the formation of thin and micrometer-thick layers of the composite material consisting of polypyrrole and Au

Received for review December 18, 2009.

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ACKNOWLEDGMENT Support for this work by the Polish Ministry of Science and Higher Education Grants N N204 244534 and N N204 131937 and by Mazovia PhD fellowship 756/101/09 is gratefully acknowledged.

SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

AC902426C

October

26,

2009.

Accepted