© Copyright 2001 American Chemical Society
OCTOBER 30, 2001 VOLUME 17, NUMBER 22
Letters Simple Preparation Method of Multilayer Polymer Films Containing Pd Nanoparticles Jianyun Liu, Long Cheng, Yonghai Song, Baifeng Liu, and Shaojun Dong* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, People’s Republic of China Received May 29, 2001. In Final Form: September 12, 2001 A simple route to the fabrication of multilayer films containing Pd nanoparticles is described. Following layer-by-layer assembly of PdCl42- and polycation, QPVP-Os (a quaternized poly(4-vinylpyridine) complexed with [Os(bpy)2Cl]2+/+), on 4-aminobenzoic acid-modified glassy carbon electrodes, the three-dimensional Pd nanoparticle multilayer films are directly formed on electrode surfaces via electrochemical reduction of PdCl42- sandwiched between polymers. The growth of PdCl42- is easy on electrode surfaces by electrostatic interaction, and the assembly processes are monitored by cyclic voltammetry and UV-vis spectroscopy. The depth profile analyses by X-ray photoelectron spectroscopy verify the constant composition of the Pd nanoparticle multilayer films. Atomic force microscopy proves that the as-prepared Pd nanoparticles are uniformly distributed with an average particle diameter of 3-7 nm. The resulting Pd nanoparticle multilayermodified electrode possesses high catalytic activity for the reduction of dissolved oxygen and oxidation of hydrazine compounds in aqueous solution.
Introduction Construction of nanoparticles is at the focus of materials research because of their unique electronic, catalytic, and optical properties.1 Noble metal nanoparticles are of fundamental interest and technological importance because of their applications as catalysts.2 In the most interesting and practically promising field, such as sensors and photo- or bio-electrochemical devices, the nanoparticles are utilized in the form of thin films deposited on suitable substrates. The layer-by-layer (LBL) assembly method initially developed for pairs of oppositely charged polyelectrolytes3 has been recently applied to the prepa* Corresponding author tel, +86-431-526-2101; fax, +86-431568-9711; e-mail,
[email protected]. (1) (a) Antonietti, M.; Go¨ltner, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 910-928. (b) Schmid, G.; Chi, L. F. Adv. Mater. 1998, 10, 515-526. (c) Weller, H. Angew. Chem., Int. Ed. 1998, 37, 1658-1659. (d) Alivisatos, A. P. Science 1996, 271, 933-937. (2) (a) Lewis, L. N. Chem. Rev. 1993, 93, 2693-2730. (b) Cho, G. L.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346-349. (c) Pei, J.; Li, X.-Y. J. Electroanal. Chem. 1998, 441, 245-258.
ration of the three-dimensional superstructure array of nanoparticles.4 It allows for the deposition of homogeneous, robust films with accurately controlled layer thickness and interlayer separation. However, most of the nanoparticles must be presynthesized through reduction of a (3) (a) Decher, G. Science 1997, 277, 1232-1237. (b) Lvov, Y.; Decher, G.; Haas, H.; Mo¨hwald, H.; Kalacher, A. Physica B 1994, 198, 89-91. (c) Kellogg, G. J.; Mayer, A. M.; Stockton, W. B.; Ferreira, M.; Rubner, M. F.; Satija, S. K. Langmuir 1996, 12, 5109-5113. (4) (a) Pastoriza-Santos, I.; Koktysh, D. S.; Mamedov, A. A.; Giersig, M.; Kotov, N. A.; Liz-Marza´n, L. M. Langmuir 2000, 16, 2731-2735. (b) Mamedov, A. A.; Ostrander, J.; Aliev, F.; Kotov, N. A. Langmuir 2000, 16, 3941-3949. (c) Aliev, F.; Correa-Duarte, M.; Mamedov, A. A.; Ostrander, J. W.; Giersig, M.; Liz-Marza´n, L. M.; Kotov, N. A. Adv. Mater. 1999, 11, 1006-1010. (d) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065-13069. (e) Schmitt, J.; Decher, G.; Dressich, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61-65. (f) Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640-7641. (g) Musick, M. D.; Keating, C. D.; Lyon, L. A.; Botsko, S. L.; Pena, D. J.; Holliway, W. D.; McEvoy, T. M.; Richardson, J. N.; Naton, M. J. Chem. Mater. 2000, 12, 2869-2881. (h) Blonder, R.; Sheeney, L.; Willner, I. Chem. Commun. 1998, 1393-1394. (i) Cassabneau, T.; Fendler, J. H. J. Phys. Chem. B 1999, 103, 1789-1793.
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metal salt by reducing agents in solution. Moreover, in most cases, the substrates used are the functionalized glass or conductive glass supports. In this paper, a direct synthetic method of threedimensional nanoparticle multilayers by a LBL technique on a glassy carbon electrode (GCE) surface is described. First, on the 4-aminobenzoic acid (4-ABA)-functionalized GCE,5 polycation QPVP-Os (a quaternized poly(4-vinylpyridine) complexed with [Os(bpy)2Cl]2+/+) and PdCl42are alternately assembled by electrostatic interaction. Second, PdCl42- sandwiched between the QPVP-Os layers is electrochemically reduced to yield zero valent Pd particles. Wrighton et al.6 ever reported the functionalization of photocathode surfaces by reduction of PtCl62incorporated into [PQ2+‚2Br-] polymer on the electrode surface. Here, we try to develop a step-by-step way to adsorb PdCl42- and QPVP-Os in order to obtain threedimensional Pd nanoparticle multilayer films. Thus, the nanoparticles can be uniformly distributed in the polymer; the amount of nanoparticles on carbon electrode surfaces can be controlled conveniently. Scheme 1 represents the synthetic route of the ideal Pd nanoparticle multilayer films.
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Figure 1. (A) Cyclic voltammograms of (PdCl42-/QPVP-Os)n/ 4-ABA/GCE in 0.1 M KCl solution with n ) 1, 3, 5, 7, 9, 11, and 13 (from inside to outside), respectively. Scan rate: 100 mV s-1. The inset shows the relationship of the Os-centered redox current vs the number of bilayers (PdCl42-/QPVP-Os). (B) UVvis absorption spectra of multilayer films (PdCl42-/QPVP-Os)n on PDDA-coated quartz substrates with n ) 2, 4, 6, 8, and 10 (from lower to upper curves), respectively. The inset shows the relationship of absorbance at 300 nm vs the number of bilayers.
Scheme 1. Schematic Illustration of the Formation of Pd Particle Multilayer Films
Figure 2. XPS spectra of (PdCl42-/QPVP-Os)6 multilayer film in the Pd (3d) (A) and Cl (2p) (B) level regions before (curve a) and after (curve b) electrochemical reduction.
The freshly polished GCE surface was first modified with 4-ABA so that the carbon surface was functionalized with a COOH group. Then, a polycation QPVP-Os monolayer was assembled on the negatively charged surface by electrochemically scanning in the QPVP-Os solution.5 PdCl42- was subsequently deposited on the QPVP-Os monolayer by electrostatic attraction. These PdCl42-/QPVP-Os bilayers were further assembled LBL on the electrode surface by alternate deposition. Because the polymer QPVP-Os is electroactive and conductive, electrochemical measurements can be performed to monitor the increase of the multilayer film. Figure 1A shows the cyclic voltammograms of the (PdCl42-/QPVP-Os)n/ 4-ABA/GCE with n ) 1, 3, 5, 7, 9, 11, and 13. The redox current of the Os-centered polymer is regularly enhanced with the number of bilayers, and the good linear relationship of the redox peak current of the multilayer film electrode to the number of bilayers indicates the uniform growth of the film (Figure 1A inset). The PdCl42-/QPVPOs multilayer film can also be built up on a glass substrate by dipping a poly(diallyldimethylammonium chloride) (PDDA)-coated glass substrate into a PdCl42- and QPVP(5) (a) Liu, J. Y.; Cheng, L.; Liu, B. F.; Dong, S. J. Langmuir 2000, 16, 7471-7476. (b) Cheng, L.; Liu, J. Y.; Dong, S. J. Anal. Chim. Acta 2000, 417, 133-142. (6) (a) Bruce, J. A.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 74-82. (b) Dominey, R. N.; Lewis, N. S.; Bruce, J. A.; Bookbinder, D. C.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 467-482. (c) Watanabe, S.; Regen, S. L. J. Am. Chem. Soc. 1994, 116, 8855-8856.
Os solution alternately. Figure 1B shows the UV-vis absorption spectra of 2, 4, 6, 8, and 10 bilayers of PdCl42-/ QPVP-Os film on the PDDA-coated quartz slide, respectively. A characteristic absorption band of QPVP-Os at 300 nm appeared. The linear increase in absorbance with the number of layers suggests a regular deposition of the film (Figure 1B inset). The ill-defined band at 230 nm arises from PdCl42- absorption, which is shifted to the longer wave as compared with that of the PdCl42- solution (217 nm) due to the interaction between PdCl42- and polymer.7 After it was deposited, PdCl42- sandwiched between two QPVP-Os layers of the multilayer film was reduced by electrolysis under constant potential at -0.8 V in N2saturated 0.1 M KCl solution. X-ray photoelectron spectrum (XPS) measurements were made to confirm the complete reduction of PdCl42- and the formation of Pd(0) on multilayer-modified GCE. As depicted in Figure 2A, the Pd (3d5/2) and Pd (3d3/2) peaks are present at 337.2 and 342.4 eV, respectively, prior to reduction. After electrochemical reduction, both of the peaks shift to 335.2 and 340.6 eV, respectively, which is consistent with the change in oxidation state from +2 to 0.8 Importantly, the peak strength of chlorine element weakens greatly after reduction (Figure 2B), while the peak strength of other elements such as N and Os has negligible changes. These indicate that after reduction, most of the chlorine is released from (7) Zhao, M. Q.; Crooks, R. M. Angew. Chem., Int. Ed. 1999, 38, 364366.
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Figure 3. Depth profile analysis of a multilayer film (Pdnano/ QPVP-Os)9 on a 4-ABA/GC plate. The data were obtained by XPS measurements in conjunction with Ar ion sputtering for different times.
the metal-ligand, and the rest balances the positive charge of QPVP-Os in the multilayer film. The depth profile analysis by XPS is further examined to reveal the expected elemental composition of the multilayer film as a function of depth in the multilayer film. A typical set of depth profile analysis were given in Figure 3. The XPS peaks integration areas of the elements N, Os, and Pd were used to roughly estimate the amount present in the multilayer. The Pd(0) signal was observed at about constant peak area until the multilayer film was completely sputtered away. The nearly identical change trends in the peak area of Os and N elements indicate a uniform distribution of QPVP-Os polymer in the multilayer films. These data show that Pd is partially entrapped in polymeric QPVP-Os. Therefore, the boundary between the QPVP-Os and the Pd(0) layer could be less distinctive. It is necessary to point out that in many instances the layered nature of the films is obscured by interpenetration of the subsequent layers and high interlayer roughness and by embedding of particles on the polymer chain.6,9 It has been reported that LBLassembled films made from only two components can often be represented as a homogeneous mixture of, for instance, nanoparticles and polyelectrolyte.10 Atomic force microscopy (AFM) imaging provides more detailed information involving the formation of Pd nanoparticles after electrochemical reduction of PdCl42- multilayer films and the surface morphology and homogeneity of the films. Figure 4 presents the typical tapping-mode AFM images of the multilayer films on (PdCl42-/QPVPOs)3/4-ABA/GCE. Before it is reduced (Figure 4A), the modified film exhibits a poor surface morphology because of the relatively rough surface of the glassy carbon (mean roughness 2.1 nm). After the film is electrochemically reduced (Figure 4B), Pd nanoparticles can be seen clearly. They are homogeneously distributed on the electrode surface, forming a densely packed film, where each particle is in contact with adjacent ones, which is consistent with that of XPS depth profile analyses (vide supra). Figure 4C shows the typical line scan image. It should be noted that tip convolution leads to a distorted, enlarged view of the true particle size in the x-y plane. Here, the average diameter of the nanoparticles was estimated to be 3-7 nm from the feature heights of numerous line scans, although the particle size is not very precise because of (8) (a) Kumar, G.; Blackburn, J. R.; Albridge, R. G.; Moddeman, W. E.; Jones, M. M. Inorg. Chem. 1972, 11, 296-300. (b) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1979. (9) Kost, K. M.; Bartak, D. E.; Kazee, B.; Kuwana, T. Anal. Chem. 1988, 60, 2379-2384. (10) Mamedov, A. A.; Kotov, N. A. Langmuir 2000, 16, 5530-5533.
Figure 4. Tapping-mode AFM image of a multilayer film (PdCl42-/QPVP-Os)3 on a 4-ABA/GC plate before (A) and after (B) electrochemical reduction. (C) Line scan on part of image B.
the roughness of the glassy carbon substrates. Clearly, Pd nanoparticles are relatively uniform with a narrow size distribution. PdCl42- could be easily adsorbed into
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the polymer,6 and polymer is known for its ability to stabilize particles by preventing agglomeration.11 It is reasonable that Pd nanoparticles can be uniformly distributed in the polymer. Importantly, the Pdnano/ polymer multilayer films result in smoothing of the surface with the mean roughness of 1.8 nm. It is because the resulting Pd nanoparticles fill in some vacancies and defects of the film. The amount and size of Pd particles can be adjusted by changing the layer number of the multilayer film. When the film thickness is increased, the nanoparticles become more closely packed and some agglomerated blocks begin to form (see Supporting Information), which is in agreement with other multilayer film systems.4g In particular, this sort of multilayer film can be considered a very homogeneous composite material, which does not have sharp interfaces between the polymer and the inorganic component. It is very valuable for catalysts and some device fabrications. It allows one to obtain uniform charge transport through the whole film. The use of nanoparticles superstructures for the creation of electrochemical devices is an extremely promising prospect.12 A series of electrochemical sensors were built by Au nanoparticles.13 Thus-prepared noble metal Pd nanoparticle multilayer films exhibit excellent catalytic activity for O2 reduction, which is an important electrode reaction in fuel cells.14 Figure 5 shows cyclic voltammograms of (Pdnano/QPVP-Os)6/4-ABA/GCE in the presence (solid line) and absence (dashed line) of O2. Clearly, there is a substantial catalytic effect on O2 reduction at the Pd particles-modified electrode. The catalytic reduction of O2 occurs at about 0.18 V, which is near the Os-centered reduction potential of QPVP-Os in the film (0.24 V). However, the multilayer film electrode prior to electrolysis only exhibits the redox reaction of QPVP-Os, and there is no catalytic activity for O2 reduction in this potential range (dotted line in Figure 5). Therefore, Pd particles play a main role in the catalytic reduction of O2. Here, QPVP-Os acts as a mediator of the catalytic reaction because of its good redox activity and conductivity. (11) (a) Kelaidopoulou, A.; Abelidon, E.; Kokkinidis, G. J. Appl. Electrochem. 1999, 29, 1255-1261. (b) Laborde, H.; Le´ger, J.-M.; Lamy, C. J. Appl. Electrochem. 1994, 24, 219-226. (c) Napporn, W. T.; Le´ger, J.-M.; Lamy, C. J. Electroanal. Chem. 1996, 408, 141-147. (d) Bradley, J. S.; Millar, J. M.; Hill, E. W. J. Am. Chem. Soc. 1991, 113, 4016-4017. (e) Hepel, M. J. Electrochem. Soc. 1998, 145, 124-133. (12) Shipway, A. N.; Lahav, M.; Willner, I. Adv. Mater. 2000, 12, 993-998. (13) (a) Lahav, M.; Shipway, A. N.; Willner, I.; Nielsen, M. B.; Stoddart, J. F. J. Electroanal. Chem. 2000, 482, 217-221. (b) Shipway, A. N.; Lahav, M.; Blonder, R.; Willner, I. Chem. Mater. 1999, 11, 13-15. (c) Lahav, M.; Shipway, A. N.; Willner, I. J. Chem. Soc., Perkin Trans. 2 1999, 1925-1931. (d) Lahav, M.; Gabai, R.; Shipway, A. N.; Willner, I. Chem. Commun. 1999, 1937-1938. (14) Baldauf, M.; Kollo, D. M. J. Phys. Chem. 1996, 100, 1137511381.
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Figure 5. Cyclic voltammograms of (Pdnano/QPVP-Os)6/4-ABA/ GCE in N2-saturated (dashed line) and O2-saturated (solid line) 1 M H2SO4 solution and of (PdCl42-/QPVP-Os)6/4-ABA/GCE before electrolysis in O2-saturated 1 M H2SO4 (dotted line).
Importantly, with increasing the number of nanoparticlecontaining layers, the catalytic current enhances. This conclusively demonstrates that the amount of nanoparticles increases with the number of layers, and the inner layer Pd nanoparticle surface is accessible to reactants in the solution. The Pd nanoparticle multilayers also possess high catalytic activity for the oxidation of hydrazine compounds (see Supporting Information). In summary, this work describes a simple method for fabricating Pd nanoparticle multilayer films based on LBL assembly of PdCl42- and polycation on 4-ABA-modified GCE followed by electrochemical reduction of the multilayer films. The resulting Pd nanoparticle multilayer films are very homogeneous without a clear interface, which is very useful for catalysis and device applications. This is a general approach to the preparation of metal nanoparticle multilayers with three-dimensional structure. Work is in progress on the fabrication of other metal nanoparticle multilayer films and their application as heterogeneous catalysts in fuel cells. Brief Summary 2-/QPVP-Os multilayer films were assembled on
PdCl4 4-ABA-modified GCE by electrostatic interaction. Threedimensional Pd nanoparticle multilayers were directly formed on electrodes by electrochemical reduction of PdCl42- sandwiched in a polymer layer. The resulting Pd nanoparticle multilayers do not have clear interfaces, which is favorable for charge transfer through the film. The Pd nanoparticles can effectively catalyze the reduction of O2 and the oxidation of hydrazine compounds. Acknowledgment. This work has been supported by the National Natural Science Foundation of China (No. 29835120). LA010784+
