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Chem. Mater. 2006, 18, 2468-2471
Facile Synthesis of Morphology-Controlled Platinum Nanocrystals Xinhua Zhong,*,†,‡ Yaoyu Feng,§ Ingo Lieberwirth,‡ and Wolfgang Knoll*,‡ Department of Chemistry, East China UniVersity of Science & Technology, Shanghai 200237, People’s Republic of China, Max-Planck Institute for Polymer Research, 55128 Mainz, Germany, and School of Life Science, Tongji UniVersity, Shanghai 200092, People’s Republic of China ReceiVed February 24, 2006 ReVised Manuscript ReceiVed April 11, 2006
Morphology-controlled synthesis of nanostructures is a great challenge in materials chemistry because the morphology (including dimensionality and shape) of most nanostructures can effectively tune their intrinsic chemical and physical properties.1 Nanostructured platinum is an example of considerable interest for many industrial applications because of its extraordinary properties.2 For example, it serves as a catalyst in the production of hydrogen from methane, in the reduction of pollutant gases exhausted from automobiles, and particularly in the direct methanol fuel cell (DMFC).3 It has been established that the catalytic reactivity of platinum nanostructures highly depends on the morphology of the nanoparticles,4 and therefore the design and synthesis of wellcontrolled shapes and sizes of platinum nanoparticles could be critical for their application, especially in the field of catalysis. For these reasons, much effort has been devoted in the past decade to the fabrication of platinum nanostructures with monodisperse sizes and well-defined morphologies. In the general synthetic methods of platinum nanostructures, Pt(IV) or Pt(II) precursors are reduced in the solution phase by reducing agents such as alcohol,5 sodium borohydride,6 or hydrogen7 in the presence of organic molecules or polymers as capping agents or morphology-directing re* To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +49 6131 379100. † East China University of Science & Technology. ‡ Max-Planck Institute for Polymer Research. § Tongji University.
(1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (2) Wieckowski, A.; Savinova, E. R.; Vayenas, C. G. Catalysis and Electrocatalysis at Nanoparticle Surfaces; Marcel Dekker: New York, 2003. (3) (a) Bell, A. T. Science 2003, 299, 1688. (b) Schulz, J.; Roucoux, A.; Patin, H. Chem. ReV. 2002, 102, 3757. (c) Pino, L.; Recupero, V.; Beninati, S.; Shukla, A. K.; Hegde, M. S.; Bera, P. Appl. Catal. A 2002, 225, 63. (d) Williams, K. R.; Burstein, G. T. Catal. Today 1997, 38, 401. (4) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (5) (a) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B 1999, 103, 3818. (b) Teng, X.; Liang, X.; Maksimuk, S.; Yang, H. Small 2006, 2, 249. (c) Chen, C.-W.; Akashi, M. Langmuir 1997, 13, 6465. (6) (a) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (b) Zhao, S.; Chen, S.; Wang, S.; Lie, D.; Ma, H. Langmuir 2002, 18, 3315. (c) Yang, J.; Lee, J.; Deivaraj, T.; Too, H. Langmuir 2003, 19, 10361. (d) Chen, S.; Kimra, K. J. Phys. Chem. B 2001, 105, 5397.
agents. The resultant Pt nanoparticles synthesized via this route are usually spherical or have undefined facets. Recently, Xia and others have also used inorganic species in place of organic surfactants as shape-directing reagents in a solutionphase chemical reducing process of the production of platinum nanoparticles, and a series of new morphologies were obtained.8 An alternative approach is using soft templates9 (such as micelles) or hard templates10 (such as mesoporous silica) to direct the formation of platinum nanocrystals with anisotropic morphologies. Although template-based syntheses can supply a predetermined morphology for the obtained nanoparticles, they are restricted by a number of limiting factors that include the tedious work involved in the preparation and removal of the templates or the limited range of morphological variation. The thermal decomposition of metal precursors in highboiling-point solvents such as amines or alcohols, which act as reactants as well as control agents for particles growth and architectural control, has been proven to be an effective route for the preparation of high-quality monodisperse nanocrystals including metal oxides11 and metal chalcogenides,12 but this method has not been extensively used in the preparation of metal nanocrystals. This approach is successful in tuning the morphology of the obtained nanocrystals because the selected surfactant can effectively kinetically control the growth rates of various facets of a seed, and both the reaction temperature and the chemical nature of the reaction media can tune the decomposition rate of the precursor. Another advantage of this approach is the ease of scale-up. Here, we extend this approach to the preparation of platinum nanostructures. Complex morphologies of porous flowerlike three-dimensional (3D) dendritic- and polypodlike, as well as multi-branched (tripod and tetrapod), platinum (7) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (b) Ahmadi, T. S.; Wang, Z. L.; Henglein, A.; El-Sayed, M. A. Chem. Mater. 1996, 8, 1161. (8) (a) Chen, J.; Herricks, T.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 2589. (b) Herricks, T.; Chen, J.; Xia, Y. Nano Lett. 2004, 4, 2367. (c) Teng, X.; Yang, H. Nano Lett. 2005, 5, 885. (d) Chen, J.; Herricks, T.; Geissler, M.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 10854. (9) (a) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838. (b) Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, C. J.; van Swol, F.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635. (c) Kijima, T.; Yoshimura, T.; Uota, M.; Ikeda, T.; Fujikawa, D.; Mouri, S.; Uoyama, S. Angew. Chem., Int. Ed. 2004, 43, 228. (10) (a) Melosh, N. A.; Boukai, A.; Diana, F.; Gerardot, B.; Badolato, A.; Petroff, P. M.; Heath, J. R. Science 2003, 300, 112. (b) Wakayama, H.; Setoyama, N.; Fukushima, Y. AdV. Mater. 2003, 15, 742. (c) Shirari, M.; Igeta, K.; Arai, M. Chem. Commun. 2000, 623. (d) Liang, H.-P.; Zhang, H.-M.; Hu, J.-S.; Guo, Y.-G.; Wan, L.-J.; Bai, C.-L. Angew. Chem., Int. Ed. 2004, 43, 1540. (e) Han, Y.; Kim, J.; Stucky, G. D. Chem. Mater. 2000, 12, 2068. (f) Shin, H. J.; Ryoo, R.; Liu, Z.; Terasaki, O. J. Am. Chem. Soc. 2001, 123, 1246. (11) For example: (a) Pinna, N.; Garnweitner, G.; Antonietti, M.; Niederberger, M. J. Am. Chem. Soc. 2005, 127, 5608. (b) Zhang, Z.; Zhong, X.; Liu, S.; Li, D.; Han, M. Angew. Chem., Int. Ed. 2005, 44, 3466. (c) Zhong, X.; Xie, R.; Sun, L.; Liberwirth, I.; Knoll, W. J. Phys. Chem. B 2006, 110, 2. (d) Zhong, X.; Knoll, W. Chem. Commun. 2005, 1158. (e) Yin, M.; Gu, Y.; Kuskovsky, I. L.; Andelman, T.; Zhu, Y.; Neumark, G. F.; O’Brien, S. J. Am. Chem. Soc. 2004, 126, 6206. (12) For example: (a) Jun, Y.-W.; Lee, S.-M.; Kang, N.-J.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 5150. (b) Jun, Y.-W.; Jung, Y.-Y.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 615.
10.1021/cm060463p CCC: $33.50 © 2006 American Chemical Society Published on Web 04/21/2006
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Figure 1. Representative EDX spectrum of Pt nanostructures. The additional copper and carbon signals are contributions from the carbon coated TEM copper grid.
nanostructures are produced by direct thermolytic reduction of trans-Pt(NH3)2Cl2 in different high-boiling-point solvents (oleylamine (OAm), hexadecylamine (HDA), or oleyl alcohol (OAl)), which also act as a reducing and capping agent. The obtained porous 3D dendritic and polypod-like Pt nanostructures may have additional advantages over the conventional spherical nanoparticles. On one hand, the porous dendritic or polypod-like structure can decrease the Pt consumption and thus save material and reduce cost; on the other hand, the 3D interconnected structure could achieve higher catalytic performance, because network structure could supply more absorption sites for reactant molecules. In a typical reaction for the synthesis, 30.0 mg (0.1 mmol) of trans-Pt(NH3)2Cl2 (99.95%, from Acros) and 5.0 mL of a solvent of choice, OAm (97%, from Acros), HDA (98%, from Aldrich), or OAl (90%, from Aldrich), were loaded in a 50-mL three-necked round-bottom flask, and the mixture was degassed at 90 °C under a vacuum of ∼1 mbar for 20 min under magnetic stirring. After the hour, the reaction vessel was filled with argon, and its temperature was raised to 150 °C and maintained until a clear light yellow solution was obtained. Then the temperature was increased to 250 °C with a heating rate of ∼15 °C/min and kept at this temperature for 10 min under argon flow. When the temperature was up to a certain value (250 °C in OAm, 210 °C in HDA, and 220 °C in OAl), the originally clear light yellow solution turned to yellowish-brown and soon to black (the process from yellowish-brown to black takes less than 30 s), indicating the decomposition of the Pt precursor and the formation of Pt nanoparticles. Subsequently, the reaction mixture was cooled to ∼60 °C, and 10 mL of toluene was added. The nanostructures (black precipitate) were isolated, purified by centrifugation, and washed with chloroform several times. The resulting organic ligand-coated Pt particles are re-dispersible in nonpolar solvents such as chloroform, toluene, or hexane and used for the following measurement without any size selection. It was found that OAm gives 3D dendritic nanostructures, HDA yields 3D polypod-like nanostructures, and OAl produces multibranched (tripod and tetrapod) Pt nanoparticles. The chemical composition of the obtained Pt nanostructures was determined using energy-dispersive X-ray (EDX) analysis. In the EDX spectrum (Figure 1), except for the copper and carbon signals from the transmission electron microscopy (TEM) grid, only peaks of Pt are observed. This indicates that the obtained nanostructures are composed of platinum exclusively. The structure and morphology of the
Figure 2. p-XRD spectra of platinum (a) nanodendrons, (b) nanopolypods, and (c) multibranches. The stated particle sizes were calculated from the Scherrer equation. The line p-XRD pattern (bottom) corresponds to bulk face-centered cubic platinum.
obtained Pt nanostructures were characterized using TEM (FEI Tecnai F20 at an acceleration voltage of 200 kV) and powder X-ray diffraction (p-XRD, Philips PW 1820). TEM samples were prepared by depositing a drop of dilute toluene dispersion of nanocrystals on a carbon film coated copper grid. p-XRD samples were prepared by depositing nanocrystal powder on a piece of Si(100) wafer. The p-XRD pattern of each platinum nanocrystal shape (Figure 2) reveals that the obtained Pt nanocrystals possess cubic structure with high crystallinity. All the diffraction peaks match well with Bragg reflections of the standard and phase pure face-centered cubic (fcc) structure of Pt (space group, Fm3m), with the measured lattice constant of this cubic phase being a ) 3.92 Å. As expected, the width of the diffraction peaks is considerably broadened and decreases with increasing particle size. By using the Scherrer formula, we can calculate the mean sizes of the nanocrystals from the peak width at half-maximum. Particle sizes obtained from the width of the (111) reflections are depicted in Figure 2. TEM images of the sample prepared in OAm are shown in Figure 3. Uniform 3D flowerlike dendritic structures were revealed by low-magnification TEM, as shwon in Figure 3A. The average diameter of the 3D dendrons is 39 ( 4 nm. The high-magnification TEM image in the inset of Figure 3A demonstrates the porous structure of the spherical 3D dendrons, which is built up by tens of elongated primary nanoparticles with average dimensions of ∼6 × 9 nm. It appears that these small primary nanoparticles are interconnected to one another to form larger secondary 3D dendritic architectures with recognizable boundaries or voids between the component subunits. A more detailed examination of the flowerlike dendrons by high-resolution TEM (HRTEM, Figure 3B) shows a lower contrast between the crystallites, which reveals that nanopores separate many primary particles. It has been previously reported that larger crystalline domain (mesocrystals) can be formed by the small primary nanoparticles via orientation alignment and recrystallization.13 The fact that the formation of mesocrystals via the arrangement of individual small primary nanoparticles is in accordance with the observation that the average size for the (13) (a) Co¨lfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576 and references therein. (b) Tan, Z.; Kotov, N. A. AdV. Mater. 2005, 17, 951 and references therein.
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Figure 3. TEM images and SAED pattern of platinum nanodendrons prepared in OAm. (A) Overview TEM image; the inset shows a TEM image of a dendron with high magnification. (B) HRTEM image. (C) SAED pattern of a single Pt nanodendron.
Figure 4. TEM images of platinum nanostructures. (A) Overview TEM image of nanopolypods prepared in HDA. The inset is the SAED of nanopolypods. (B) HRTEM image of nanopolypods. The inset is the SAED of a single nanopolypod. (C) Overview TEM image of multi-branched nanostructure prepared in OAl.
primary nanoparticle measured from TEM images is smaller than that determined from the p-XRD pattern using the Debye-Scherrer formula. In addition, the lattice fringes of parts of the neighboring individual is slightly deviating from the perfect alignment, which indicates slight misorientations between the primary nanoparticles. This observation in the HRTEM images is in agreement with the results from the selected area electron diffraction (SAED) pattern of a single dendron (shown in Figure 3C). In the measured SAED pattern, mainly two kinds of crystalline orientations are observed, which are indexed to be 111, 220, and 311 reflections of fcc Pt, but the diffraction spots in both orietations are slightly elongated. The elongated diffraction spots in the SAED pattern suggest the presence of multiple nanodomains with a small misorientation deviating from perfect single crystalline behavior. The average orientation distribution is estimated to be ∼9°, measured from the width of the arcs in the diffraction pattern. Few examples have been reported for the preparation of 3D porous Pt nanostructures.5b,8b It should be noted that, during the preparation of this manuscript, a very similar porous Pt nanostructure has been fabricated very recently in a complicated capping system.5b The reported porous Pt nanostructures in the literature could not survive in the heat annealing process because of Ostwald ripening (in less than 20 min most of the particles lost their porosity). In contrast, in our case the obtained Pt dendritic structures do survive for several hours in the process of heat annealing at the high reaction temperature without any change of their original morphology. The observed absence of the Ostwald ripening process is not unusual in the annealing process of colloidal metal nanocrystal systems, because most metal nanocrystals usually cannot dissolve in the reaction solvent system.14 Without the existence of monomers in the reaction media, Ostwald ripening thus becomes impossible. The long-term
fixation of the particle morphology as observed in our case is very convenient for the production of nanostructures with the desired morphology. Figure 4A,B shows the TEM images of Pt samples prepared in HDA, which yielded polypod-like structures. The nanopolypods exhibit a roughly spherical morpholoy with diameters averaging about 105 nm. Each nanopolypod was derived from the assembly of dozens of nanorods that have an average diameter of ∼11 nm and are ∼40 nm long. One end of the building unit nanorods is connected together with the other end sticking out from the central sphere at different directions. The diameter for each nanorod is not completely even, with a continuous decrease toward the end, which makes it look more like a finger. The SAED pattern for a collection of polypods, given as the inset in Figure 4A, indicates that the nanopolypods have a fcc structure, in agreement with the p-XRD results. HRTEM images of the polypods show well-defined two-dimensional lattice planes for the primary nanorods (Figure 4B). The interplane spacing shown in Figure 4B is about 0.229 nm, which should be assigned to the (111) plane of the fcc system of Pt. This result indicates that the growth of the building nanorods is along [110] direction. The HRTEM images clearly show different orientations for the building units in a single polypod, which illustrates that the polypods are polycrystalline. This is further confirmed by the electron diffraction pattern on a single isolated multipod (shown in the inset of Figure 4B). Figure 4C shows a typical overview TEM image of the platinum multi-branched nanocrystals obtained with OAl as (14) (a) Sun, S.; Muray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (b) Shevchenko, E. V.; Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 11480. (c) Talapin, D. V.; Haubold, S.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. J. Phys. Chem. B 2001, 105, 2260.
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the reaction medium. Most particles in the sample exhibited a well-defined, multi-branched morphology with ∼50% of three arms (tripod), ∼40% of four arms (tetrapod), and the rest ∼10% of dots or some irregular shape. The tripods or tetrapods are nearly uniform in size and morphology. The arms in a multipod are rice-shaped and nearly equal in length and diameter. The average lengh of arms of multipods is ∼11 nm, and the average diameter of the arms is ∼8 nm. Tripod and tetrapod shapes have recently attracted much attention owing to their potential use as building blocks in the fabrication of complex, multiple-terminal devices through self-assembly.15 A number of noble metals such as Pt,8a-c Au,16 and Rh17 have recently been demonstrated to form branched nanocrystals. Although the growth mechanism is yet to be completely understood, the use of some capping agents is imperative. The crystal surface reactivity was identified as the main driving force for the growth of anisotropic nanostructures. The nature of the solvent selected (including the capping and reducing capabilities) seems to play a crucial role in directing the morphology of the final structure in the studied system. The selected solvent would act as a ligand to form stable complexes with Pt2+. When the temperature is continuously raised and then surpassed a critical temperature, the Pt(II) complex is reduced by solvent molecules to zerovalent Pt0. As a result of the different reducing capabilities of the solvents used, the critical reduction temperature differs and thus results in a different temporal evolution of the monomer (Pt0) concentration. In addition, the reaction temperature also plays a key role in the determination of the crystal morphology. The temperature can affect the formation rate of the monomer Pt0 and the corresponding rates for nucleation and crystal growth. It has been estab(15) (a) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L.-W.; Alivisatos, A. P. Nature 2004, 430, 190. (b) Wang, D.; Lieber, C. M. Nat. Mater. 2003, 2, 355. (16) (a) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Nano Lett. 2004, 4, 327. (b) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. J. Am. Chem. Soc. 2003, 125, 16186. (17) (a) Zettsu, N.; McLellan, J. M.; Wiley: B.; Yin, Y.; Li, Z.-Y.; Xia, Y. Angew. Chem., Int. Ed. 2006, 45, 1. (b) Hoefelmeyer, J. D.; Niesz, K.; Somorjai, G. A.; Tilley, D. Nano Lett. 2005, 5, 435.
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lished that the monomer concentration can effectively determine the crystal morphology.8,18 Furthermore, according to the fast autocatalytic mechanism,19 when the Pt seeds reach a certain size (∼ 500 atoms), the Pt nanoparticles apparently become autocatalytic for the platinum reduction reaction. With the initiation of the reduction process, the whole process can proceed very quickly (our experimental results reveal that the whole process lasts less than 1 min). This property induces more complicated reduction kinetics, making it more difficult to illustrate the evolution of the morphology. The detailed mechanism for the morphology control is still under further investigation. In conclusion, we reported a straightforward solution-phase chemical approach to prepare complex 3D porous flowerlike dendritic- and polypod-like, as well as multibranched, Pt nanostructures by direct thermolytic reduction of transPt(NH3)2Cl2 in different high-boiling-point solvents (OAm, HDA, or OAl), which also act as a reducing and capping agent. The chemical composition, structure, and morphology of the obtained nanostructures were characterized using EDX analysis, p-XRD, and TEM. The obtained 3D Pt nanostructures may have additional advantages over the conventional spherical nanoparticles. We are working on the generalization of the current facile synthetic method to prepare many other noble metal nanocrystals. Research is in progress to explore the morphology control mechanism, the unusual properties, and the potential application as catalyst of these obtained nanostructures with complicated 3D morphologies. Acknowledgment. We thank the National Natural Science Foundation of China (No. 20501005), the Program for New Century Excellent Talents in University of China (NCET-050382), and the Deutsche Forschungsgemeinschaft (DFG) under the SFB 625 for financial support. X.Z. is grateful for the fellowship provided by the Alexander von Humboldt Foundation. CM060463P (18) (a) Kumar, S.; Nann, T. Small 2006, 3, 316. (b) Peng, X. AdV. Mater. 2003, 15, 459. (19) Henglein, A.; Ershov, B. G.; Malow, M. J. Phys. Chem. 1995, 99, 14129.
