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Self-Assembly of Ionic Liquids-Stabilized Pt Nanoparticles into Two-Dimensional Patterned Nanostructures at the Air-Water Interface Hongjun Chen and Shaojun Dong* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, and Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun, Jilin, 130022, P. R. China ReceiVed July 27, 2007. In Final Form: September 19, 2007 In this paper, we demonstrate the self-assembly of ionic liquids (ILs)-stabilized Pt nanoparticles into two-dimensional (2D) patterned nanostructures at the air-water interface under ambient conditions. Here, ILs are not used as solvents but as mediators by virtue of their pronounced self-organization ability in synthesis of self-assembled, highly organized hybrid Pt nanostructures. It is also found that the morphologies of the 2D patterned nanostructures are directly connected with the quantities of ILs. Due to the special structures of ILs-stabilized Pt nanoparticles, 2D patterned Pt nanostructures can be formed through the π-π stack interactions and hydrogen bonds. The resulting 2D patterned Pt nanostructures exhibit good electrocatalytic activity toward oxygen reduction.
Introduction Recently, patterning of colloidal nanoparticles is of great interest for their potential applications in science and technology.1 There is a great demand for the development of new building blocks as well as new fabrication techniques to assemble, pattern, and integrate nanoparticles into functional and ordered nanostructures for potential photonics or electronic devices application.2 Due to its simplicity, versatility, and low cost, self-assembly is an alternative means of nanofabrication. Meanwhile, guided by intermolecular or interparticle interaction force field, selfassembly has been successfully used to construct wirelike,3 sheetlike,4 diamondlike,5 chainlike,6 spongelike,7 and ringlike2 nanoparticle assembly motifs, even superlattices.8 Such selfassembly can be achieved via a variety of techniques (solvent evaporation, surface modification of nanoparticles with bifunctional molecules, application of external force field)2,6,9 in solution or on solid surfaces. Driven by the reduction in interfacial energy, the self-assembly of colloidal nanoparticles always occurs at gas-liquid,10 liquid-liquid,11 gas-solid,8a or liquid-solid12 interfaces and forms two-dimensional (2D) nanostructures. * Corresponding author. Fax: +86-431-85689711. E-mail: dongsj@ ciac.jl.cn. (1) (a) Lin, X. M.; Parthasarathy, R.; Jaeger, H. M. Appl. Phys. Lett. 2001, 78, 1915. (b) Pileni, M. P.; Lalatonne, Y.; Ingert, D.; Lisiecki, I.; Courty, A. Faraday Discuss. 2004, 125, 251. (2) Sau, T. K.; Murphy, C. J. Langmuir 2005, 21, 2923. (3) (a) Liu, J.; Raveendran, P.; Qin, G.; Ikushima, Y. Chem. Commun. 2005, 2972. (b) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (4) Tang, Z.; Zhang, Z.; Wang, Y.; Glotzer, S. C.; Kotov, N. A. Science 2006, 314, 274. (5) Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A. Science 2006, 312, 420. (6) (a) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304, 1787. (b) Salant, A.; Amitay-Sadovsky, E.; Banin, U. J. Am. Chem. Soc. 2006, 128, 10006. (c) Caswell, K. K.; Wilson, J. N. U.; Bunz, H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914. (7) Zhou, Y.; Antonietti, M. J. Am. Chem. Soc. 2003, 125, 14960. (8) (a) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (b) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (c) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Science 2004, 303, 821. (9) (a) Petroski, J. M.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. A 2001, 105, 5542. (b) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B 1999, 103, 3818. (10) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978.
Among them, the self-assembly of metal nanoparticles, especially Pt nanoparticles, attracts much attention due to their novel physical and chemical properties for a new generation of catalysts, electronics, and photonics.13 Room-temperature ionic liquids (ILs), a new class of environmentally benign solvents, have developed to be a focal point of interest in both academia and industry, because of their unique chemical and physical properties, such as high chemical and thermal stability, negligible vapor pressure, high conductivity, wide electrochemical window, and the ability to dissolve a large variety of organic and inorganic compounds.14 ILs have been used in various fields, including organic synthesis, catalysis, and electrochemistry.15 Most recently, the advantages of ILs in inorganic synthetic processes have been gradually realized and received more and more attention. Some nanostructured materials, such as metallic nanoparticles,16 mesoporous materials,7,17 hollow TiO2 microspheres,18 and CoPt nanorods,19 have been prepared in ILs. In this paper, we describe the synthesis of ILs-stabilized Pt nanoparticles in aqueous solution and spontaneous self-assembly into 2D patterned nanostructures at the air-water interface under ambient conditions. Here, ILs are used as mediators in virtue of (11) (a) Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Science 2003, 299, 226. (b) Li, Y.; Huang, W.; Sun, S. Angew. Chem., Int. Ed. 2006, 45, 2537. (12) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (13) (a) Lee, H.; Habas, S. E.; Kweskin, S.; Butcher, D.; Somorjai, G. A.; Yang, P. Angew. Chem., Int. Ed. 2006, 45, 7824. (b) Fu, X.; Wang, Y.; Wu, N.; Gui, L.; Tang, Y. Langmuir 2002, 18, 4619. (c) Song, Y.; Steen, W. A.; Pena, D.; Jiang, Y.-B.; Medforth, C. J.; Huo, Q.; Pincus, J. L.; Qiu, Y.; Sasaki, D. Y.; Miller, J. E.; Shelnutt, J. A. Chem. Mater. 2006, 18, 2335. (14) (a) Welton, T. Chem. ReV. 1999, 99, 2071. (b) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667. (15) (a) Dzyuba, S. V.; Bartsch, R. A. Angew. Chem., Int. Ed. 2003, 42, 148. (b) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (c) Endres, F.; Bukowski, M.; Hempelmann, R.; Natter, H. Angew. Chem., Int. Ed. 2003, 42, 3428. (16) Dupont, J.; Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R. J. Am. Chem. Soc. 2002, 124, 4228. (17) (a) Zhou, Y.; Antonietti, M. AdV. Mater. 2003, 15, 1452. (b) Zhou, Y.; Schattka, J. H.; Antonietti, M. Nano Lett. 2004, 4, 477. (c) Antonietti, M.; Kuang, D. B.; Smarsly, B.; Zhou, Y. Angew. Chem., Int. Ed. 2004, 43, 4988. (18) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386. (19) Wang, Y.; Yang, H. J. Am. Chem. Soc. 2005, 127, 5316.
10.1021/la702279b CCC: $37.00 © 2007 American Chemical Society Published on Web 11/01/2007
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Figure 1. Typical SEM (A-E) and TEM (F) images and SAED pattern (inset, Figure 1F) of the 2D patterned Pt nanostructures synthesized by using 200 µL of [EMIM]‚BF4.
their pronounced self-organization ability in the synthesis of self-assembled, highly organized hybrid Pt nanostructures. Indeed, the pronounced self-organization of imidazolium ILs is described as a polymeric supramolecule17c,20 and has been used for the preparation of some highly organized SiO2 and TiO2 nanostructures.7,17 To the best of our knowledge, the use of ILs-stabilized metal nanoparticles self-assembled into patterned nanostructures is seldom reported. The resulting 2D patterned Pt nanostructures combine the convenient handling of a larger film with a considerable high surface area and narrow particle size distribution and are expected to have potential applications in catalysis. Experimental Section Chemicals. Nafion (perfluorinated ion-exchange resin, 5 wt % solution in a mixture of lower aliphatic alcohols and water) was purchased from Aldrich. K2PtCl4, H2SO4, and ascorbic acid were purchased from Beijing Chemical Co. 1-Ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]‚BF4) was synthesized, purified, and characterized as described in the literature.21 All reagents were used as received without further purification. The water used was purified through a Millipore system. Synthesis. In a typical synthesis of 2D patterned Pt nanostructures, 2.1 mg (5 µmol) of K2PtCl4 was mixed with 200 µL (1.24 mmol) or 50 µL (0.31 mmol) of [EMIM]‚BF4, followed by adding 2 mL of purified water under sonication until K2PtCl4 was completely dissolved. After that, 17.6 mg (0.1 mmol) of ascorbic acid was (20) (a) Gozzo, F. C.; Santos, L. S.; Augusti, R.; Consorti, C. S.; Dupont, J.; Eberlin, M. N. Chem. Eur. J. 2004, 10, 6187. (b) Gelesky, M. A.; Umpierre, A. P.; Machado, G.; Correia, R. R. B.; Magno, W. C.; Morais, J.; Ebeling, G.; Dupont, J. J. Am. Chem. Soc. 2005, 127, 4588. (21) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 2133.
added into the mixed solution and then left under air at room temperature until a shiny film was formed at the air-water interface (See Figure S1, left). Instrumentation. The samples for transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectra (XPS) characterization were prepared by transferring the as-prepared products on a carbon-coated copper grid, silicon slide, or glass slide, respectively. TEM measurements were made on a JEOL 2000 transmission electron microscope operated at an accelerating voltage of 200 kV. SEM image was taken on a XL30 ESEM FEG scanning electron microscopy at an accelerating voltage of 20 kV. The XRD pattern was collected on a D/Max 2500 V/PC X-ray diffractometer using Cu (40 kV, 200 mA) radiation. XPS was performed on an ESCLAB MKII using Mg as the exciting source. The charging calibration was performed by referring the C1s to the binding energy at 284.6 eV. The operating pressure in the analysis chamber was below 10-9 Torr with an analyzer pass energy of 50 eV. Cyclic voltammetry was performed with a CHI 600 electrochemical workstation (CH Instrument Co.) in a conventional threeelectrode electrochemical cell with the 2D patterned Pt-nanostructuremodified ITO as the working electrode, a large platinum foil as the auxiliary electrode, and a saturated calomel electrode (SCE) as the reference electrode.
Results and Discussion Figure 1 shows the representative SEM and TEM images of the resulting nanostructure produced by using 200 µL of [EMIM]‚ BF4. It can be seen that a relatively smooth film decorated with many holes are formed (Figure 1A), which is in order to restore the equilibrium film thickness and grow bigger due to evaporation-
Self-Assembly of ILs-Stabilized Pt Nanoparticles
driven instability according to the literature.2 The sizes and shapes of the holes are not uniform, changing from ∼1 to ∼15 µm in diameter, as shown in Figure 1B. More interestingly, some particle networks are embedded in the big holes, inducing the film to be more like a patterned nanostructure. The higher-magnification image clearly reveals that many cracks are distributed on this film (Figure 1C), indicating that the film is composed of many small nanoclusters and formed through a self-assembly process. Figure 1D gives a representative SEM image of a big hole embedded with a network of circular particles, from which one can clearly see that the network is composed of hundreds of individual particles and these particles are directly connected with each other, which should be in connection with the local inhomogeneities in their stabilizing agents.22 From the highermagnification image (Figure 1E), it can be clearly seen that these particles have irregular edges, and some cracks can still be discerned on these particle surfaces, demonstrating that these particles are only nanoparticle agglomerates. The TEM image shown in Figure 1F further verifies that the whole film actually is a big aggregate of nanoparticles and is composed of millions of individual nanoparticles with an average diameter of about 4 nm. It can also be observed that the individual nanoparticles have kept their integrity, namely, their original size and structure, inside the nanostructure. The inset in Figure 1F shows a selected area electron diffraction (SAED) pattern with four rings indexed to the {111}, {200}, {220} and {311} diffractions, respectively, which corresponds to a face-centered cubic (fcc) Pt metal. The lattice spacing between the {111} planes is 0.23 nm, which is in good agreement with that of bulk Pt metal.23 When [EMIM]‚BF4 was reduced to 50 µL, the morphology of the resulting nanostructure changed greatly, as indicated in Figure 2. A representative SEM image of a large area of 2D Pt network is shown in Figure 2A. The network is ramified and distributes relatively uniformly. From the higher-magnification image, it can be clearly seen that the network is an interconnected beltlike structure (Figure 2B) and the cracks can still be discerned on the network surface (Figure 2C). The TEM image (Figure 2D) also reveals that so many Pt nanoparticles are linked one by one to form 2D Pt network. Therefore, it can be inferred that the quantities of [EMIM]‚BF4 have a great influence on the morphology of the resulting Pt nanostructures; however, the building blocks in these Pt nanostructures are the same, i.e., small Pt nanoparticles. That is to say, the resulting 2D patterned Pt nanostructures are formed through self-assembly of [EMIM]‚ BF4-stabilized Pt nanoparticles. The crystalline nature of the resulting Pt nanostructures is characterized by XRD. Figure 3 shows the XRD patterns of the Pt nanostructures produced by using 200 and 50 µL of [EMIM]‚ BF4, respectively. For each pattern, four peaks are observed that can be assigned to the {111}, {200}, {220}, and {311} diffraction peaks of fcc Pt metal (JCPDS card, 88-2343), which coincides with the above SAED data. Using the Scherrer equation and assuming spherical particles, the mean diameter of the Pt nanoparticles estimated from the half-width of the {111} diffraction peak is about 4.2 nm, which is in good agreement with the TEM results. Figure 4 shows the typical XPS spectra of the resulting nanostructures. As shown in Figure 4A, the survey spectrum of this sample clearly reveals the presence of Pt4f, C1s, N1s, B1s, F1s, and O1s, with Si2p coming from the underlying substrate. As seen in the high-resolution spectrum of the Pt4f electron (22) Zhang, Z. L.; Glotzer, S. C. Nano Lett. 2004, 4, 1407. (23) Chen, J.; Herricks, T.; Geissler, M.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 10854.
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Figure 2. Typical SEM (A-C) and TEM (D) images of the 2D patterned Pt nanostructures synthesized by using 50 µL of [EMIM]‚BF4.
Figure 3. XRD patterns of the 2D patterned Pt nanostructures synthesized by using 200 and 50 µL of [EMIM]‚BF4, respectively.
region (Figure 4B, curve a), the binding energies of Pt4f7/2 and Pt4f5/2 electrons are 70.4 and 73.7 eV, respectively, which indicates that the Pt state in the resulting nanostructure is Pt(0). The slightly lower binding energy (∼0.6 eV) than that of bulk polycrystalline Pt is the result of the intimate interaction between the Pt nanoparticles and their surrounding stabilizing agent.24 The inset in Figure 4B shows a very strong F1s signal with a binding energy at 686.1 eV. Compared with the Pt nano-
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Figure 5. Typical SEM images of the Pt film synthesized without the addition of [EMIM]‚BF4. (Inset) A high-magnification SEM image.
Figure 4. XPS spectra of the 2D patterned Pt nanostructures synthesized by using 200 µL of [EMIM]‚BF4. (A) Survey analysis. (B) High-resolution spectra of the Pt4f (curve a) and F1s (inset) electron region. Curve b in part B corresponds to the high-resolution spectrum of the Pt4f electron region of the Pt film produced without the addition of [EMIM]‚BF4.
particles synthesized in imidazolium ionic liquids reported in literature,25 the 2D patterned Pt nanostructures have a slightly lower binding energy of Pt4f and highly intense F1s signal. It is reasonable to believe that the formation of Pt nanoparticles in thus-formed 2D patterned Pt nanostructures with a slightly lower binding energy is caused by the abundance of surrounding F atoms with higher electronegativity.25 As a control experiment, the Pt film can also be obtained through PtCl42- directly reduced by ascorbic acid without any ILs. The Pt film obtained is directly attached on the walls of the glass vessel (see Figure S1, right). When examined by SEM, it can be seen that the Pt film is very rough (Figure 5) and is composed of many Pt particles, which are spherical, having an agglomerated nanostructure about 5-200 nm in diameter (Figure 5, inset), which agrees well with the reported literature.26 The binding energies of Pt4f7/2 and Pt4f5/2 electrons are located at 70.97 and 74.3 eV (Figure 4B, curve b), respectively, in good agreement with that of bulk polycrystalline Pt. In contrast, the 2D patterned Pt nanostructures are formed at the air-water interface and have lower binding energies when adding [EMIM]‚ BF4. Obviously, thus-formed 2D patterned Pt nanostructures are mediated by [EMIM]‚BF4. This viewpoint is also supported by the fact that different quantities of [EMIM]‚BF4 can produce various morphologies of the 2D patterned Pt nanostructures. ILs have low interface tensions, resulting in high nucleation rates,17c and small Pt nanoparticles can be easily generated. According to published literature, we know that the process of self-assembly (24) Schueller, O. J. A.; Pocard, N. L.; Huston, M. E.; Spontak, R. J.; Neenan, T. X.; Callstrom, M. R. Chem. Mater. 1993, 5, 11. (25) Scheeren, C. W.; Machado, G.; Teixeira, S. R.; Morais, J.; Domingos, J. B.; Dupont, J. J. Phys. Chem. B 2006, 110, 13011. (26) Chang, G.; Oyama, M.; Hirao, K. J. Phys. Chem. B 2006, 110, 1860.
Figure 6. Cyclic voltammograms of 2D patterned Pt-nanostructuremodified ITO electrode in N2-saturated (curve a) and air-saturated (curve b) 0.1 M H2SO4 at a scan rate of 50 mV s-1.
not only arises from but also depends on the nature of the stabilizing agents.2-8 Therefore, in this work, self-assembly of Pt nanoparticles into 2D patterned Pt nanostructures should rest with the stabilizing agent, i.e., [EMIM]‚BF4. On the basis of the above XPS analysis, the slightly lower binding energy of the 2D patterned Pt nanostructures indicates that Pt nanoparticles are interacted with some electron donor molecules. Regarding the special structures of [EMIM]‚BF4, it is reasonable to deduce that Pt nanoparticles are directly coordinated with BF4- anions, thus forming a hydrophilic area inside and hydrophobic area outside of the Pt nanoparticle structure, which can cause Pt nanoparticles that easily float at the air-water interface. Meanwhile, driven by the reduction in interfacial energy, the self-assembly of [EMIM]‚BF4-stabilized Pt nanoparticles should occur at the airwater interface. Here, it should be noted that [EMIM]‚BF4 is a short-chain IL and cannot preferentially self-assemble into an ordered micelle structure or liquid crystalline phase by the rearrangement of the hydrophobic and hydrophilic molecular chains in solution, such as long-chain alkylammonium surfactants and amphiphilic block copolymers. Therefore, π-π stack interaction of the neighboring imidazolium rings should be present in the formation of the 2D ordered nanostructures.17b Meanwhile, it has also been reported that 1,3-dialkylimidazolium ILs can form 2D polymeric supramolecular structure through hydrogen bonds between the cations and anions.17c,20a,27 On the basis of the above two considerations and by virtue of the special [EMIM]‚ BF4-stabilized Pt nanoparticle structure, Pt nanoparticles can (27) Dupont, J. J. Braz. Chem. Soc. 2004, 13, 341.
Self-Assembly of ILs-Stabilized Pt Nanoparticles
self-assemble into 2D patterned Pt nanostructures through π-π stack interactions and hydrogen bonds. The exact mechanism for the formation of the 2D patterned Pt nanostructures in this system warrants further investigation. After transferring the 2D patterned Pt nanostructures onto the ITO surface, the modified electrode exhibits good electrocatalytic activity toward oxygen reduction, and the reduction peak occurs at about 0.3 V (curve b) in air-saturated 0.1 M H2SO4 at a scan rate of 50 mV s-1, as shown in Figure 6. Curve a corresponds to cyclic voltammograms of the 2D patterned Pt-nanostructuremodified ITO electrodes in N2-saturated 0.1 M H2SO4. The good electrocatalytic ability can be attributed to the high surface areato-volume of the 2D patterned Pt nanostructures that led to the good electrocatalytic activity toward O2 reduction.
Conclusion In summary, we report the self-assembly of [EMIM]‚BF4stabilized Pt nanoparticles into 2D patterned nanostructures at the air-water interface, in which ILs of [EMIM]‚BF4 play an
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important role and its quantities have a great influence on the morphologies of the resulting 2D patterned Pt nanostructures. Due to the presence of π-π stack interactions and hydrogen bonds among the [EMIM]‚BF4-stabilized Pt nanoparticles, 2D patterned Pt nanostructures can spontaneously form at the airwater interface at ambient conditions. At the air-water interface, the resulting 2D patterned Pt nanostructures can be easily transferred onto an ITO slide as a modified electrode and exhibit good electrocatalytic activity toward oxygen reduction. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20427003, 20575064, 20675076). Supporting Information Available: Photographs of the 2D patterned Pt nanostructures synthesized by using 200 µL of [EMIM]‚ BF4 and Pt film produced without the addition of [EMIM]‚BF4. This material is available free of charge via the Internet at http://pubs.acs.org. LA702279B
