Nanoporous Pt with High Surface Area by Reaction-Limited

Jan 27, 2009 - Sakai , G., Yoshimura , T., Isohata , S., Uota , M., Kawasaki , H., Kuwahara , T., Fujikawa , D., and Kijima , T. Adv. Mater. 2007 19 2...
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Langmuir 2009, 25, 3115-3121

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Nanoporous Pt with High Surface Area by Reaction-Limited Aggregation of Nanoparticles B. Viswanath,† S. Patra,‡ N. Munichandraiah,‡ and N. Ravishankar*,† Materials Research Centre, and Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India ReceiVed September 7, 2008. ReVised Manuscript ReceiVed December 21, 2008 Nanoporous structures with high active surface areas are critical for a variety of applications. Here, we present a general templateless strategy to produce such porous structures by controlled aggregation of nanostructured subunits and apply the principles for synthesizing nanoporous Pt for electrocatalytic oxidation of methanol. The nature of the aggregate produced is controlled by tuning the electrostatic interaction between surfactant-free nanoparticles in the solution phase. When the repulsive force between the particles is very large, the particles are stabilized in the solution while instantaneous aggregation leading to fractal-like structures results when the repulsive force is very low. Controlling the repulsive interaction to an optimum, intermediate value results in the formation of compact structures with very large surface areas. In the case of Pt, nanoporous clusters with an extremely high specific surface area (39 m2/g) and high activity for methanol oxidation have been produced. Preliminary investigations indicate that the method is general and can be easily extended to produce nanoporous structures of many inorganic materials.

Introduction Porous structures with large surface areas are technologically important as supports for catalysts, as membranes, in scaffolding applications, and as surface stress-based sensors.1-5 In particular, noble metals with high surface areas are routinely employed as catalysts in fuel cells, as substrates for surface-enhanced Raman spectroscopy, and for sensor/actuator applications.2,3,5-10 Direct methanol fuel cells have attracted considerable attention as power sources for portable applications. The catalysts used in these fuel cells are typically based on bimetallics with Pt as the main constituent.11 One of the main issues relating to the catalyst involves devising means to reduce the weight of the expensive catalyst, and thus, nanoparticle morphologies with high active surface areas are desirable. Porous Pt has been successfully employed commercially as a catalyst for such applications. The most common methods for producing structures with large areas involve the use of soft or hard templates.1,8,9,12-15 A number of templateless techniques have also been developed to produce * To whom correspondence should be addressed. Telephone: 91-80-2293 3255. Fax: 91-80-2360 7316. E-mail: [email protected]. † Materials Research Centre. ‡ Inorganic and Physical Chemistry. (1) Attard, G. S.; Leclerc, S. A. A.; Maniguet, S.; Russell, A. E.; Nandhakumar, I.; Bartlett, P. N. Chem. Mater. 2001, 13, 1444. (2) 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. (3) Teng, X. W.; Liang, X. Y.; Rahman, S.; Yang, H. AdV. Mater. 2005, 17, 2237. (4) Viswanath, B.; Ravishankar, N. Nanotechnology 2007, 18, 475604. (5) Weissmuller, J.; Viswanath, R. N.; Kramer, D.; Zimmer, P.; Wurschum, R.; Gleiter, H. Science 2003, 300, 312. (6) Jiang, J.; Hall, T. D.; Tsagalas, L.; Hill, D. A.; Miller, A. E. J. Power Sources 2006, 162, 977. (7) Musthafa, O. T. M.; Sampath, S. Chem. Commun. 2007, 1, 67. (8) Rioux, R. M.; Song, H.; Hoefelmeyer, J. D.; Yang, P.; Somorjai, G. A. J. Phys. Chem. B 2005, 109, 2192. (9) Xu, C. W.; Wang, H.; Shen, P. K.; Jiang, S. P. AdV. Mater. 2007, 19, 4256. (10) Shuyan, G.; Hongjie, Z.; Xiaomei, W.; Jianhui, Y.; Liang, Z.; Chunyun, P.; Dehui, S.; Li, M. Nanotechnology 2005, 16, 2530. (11) Koczkur, K.; Yi, Q.; Chen, A. AdV. Mater. 2007, 19, 2648. (12) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838. (13) Mahima, S.; Kannan, R.; Komath, I.; Aslam, M.; Pillai, V. K. Chem. Mater. 2008, 20, 601. (14) Sakai, G.; Yoshimura, T.; Isohata, S.; Uota, M.; Kawasaki, H.; Kuwahara, T.; Fujikawa, D.; Kijima, T. AdV. Mater. 2007, 19, 237.

porous structures.4,11,16-19 Here, we report a templateless and surfactant-free method for synthesizing porous clusters of Pt with an extremely high specific surface area of 39 m2/g. We demonstrate that controlled aggregation of ultrafine nanoparticles of the order of 2-3 nm in the solution phase leads to the formation of porous structures with high surface areas and a high activity for methanol oxidation. The principle is general and can be applied to produce porous structures of metals/bimetals and other inorganics. Nanoparticles are generally stabilized in solutions electrostatically and/or sterically using surfactants or capping agents. In the absence of surfactants, the particles tend to agglomerate with time to form extended structures that settle down in the solution. The irreversible aggregation of colloidal particles has been studied in great detail, and the formation of nearly monodisperse aggregates has been shown for several systems.20-29 Based on the limiting assumptions of diffusion control and reaction control for aggregation, two different types of aggregate morphologies have been predicted. Diffusion control, in which particles adhere once they encounter each other, leads to the formation of open structures with lower fractal dimension, while reaction control, in which particles undergo multiple collisions before getting (15) Yamauchi, Y.; Takai, A.; Komatsu, M.; Sawada, M.; Ohsuna, T.; Kuroda, K. Chem. Mater. 2008, 20, 1004. (16) Sun, S. H.; Yang, D. Q.; Villers, D.; Zhang, G. X.; Sacher, E.; Dodelet, J. P. AdV. Mater. 2008, 20, 571. (17) Toberer, E. S.; Joshi, A.; Seshadri, R. Chem. Mater. 2005, 17, 2142. (18) Seshadri, R.; Meldrum, F. C. AdV. Mater. 2000, 12, 1149. (19) Peng, X.; Koczkur, K.; Nigro, S.; Chen, A. Chem. Commun. 2004, 2872. (20) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (21) Bogush, G. H.; Zukoski, C. F. J. Colloid Interface Sci. 1991, 142, 19. (22) Groenwold, J.; Kegel, W. K. J. Phys. Chem. B 2001, 105, 11702. (23) Lin, M. Y.; Lindsay, H. M.; Weitz, D. A.; Ball, R. C.; Klein, R.; Meakin, P. Nature 1989, 339, 360. (24) Matijevic, E. Chem. Mater. 1993, 5, 412. (25) Park, J.; Privman, V.; Matijevic, E. J. Phys. Chem. B 2001, 105, 11630. (26) Privman, V.; Goia, D. V.; Park, J.; Matijevic, E. J. Colloid Interface Sci. 1999, 213, 36. (27) Stradner, A.; Sedgwick, H.; Cardinaux, F.; Poon, W. C. K.; Egelhaaf, S. U.; Schurtenberger, P. Nature 2004, 432, 492. (28) Yakuti, k, I. M.; Shevchenko, G. P.; Rakhmanov, S. K. Colloids Surf., A 2004, 242, 175. (29) Yang, L. M.; Wang, Y. J.; Sun, Y. W.; Luo, G. S.; Dai, Y. Y. J. Colloid Interface Sci. 2006, 299, 823.

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attached, leads to the formation of more compact structures with higher fractal dimensions. The aggregation can be controlled by manipulating the solution conditions with low zeta potential conditions favoring the formation of diffusion-limited aggregates and a higher zeta potential leading to the formation of nearly monodisperse and compact reaction-limited aggregates with very large surface areas. Here, we report the formation of reactionlimited aggregates of Pt and demonstrate that these indeed have a high catalytic activity for methanol oxidation and in other catalytic applications.

Experimental Procedures For the synthesis of Pt clusters, we adopt a procedure where no capping agent is used. The synthesis protocol involves hydrothermal heating of an aqueous solution of chloroplatinic acid at 200 °C for 5 h. Owing to the difficulty in obtaining mechanistic understanding during hydrothermal synthesis, we investigate the formation mechanism of such porous structures by using a standard reducing agent such as NaBH4 and by controlling the aggregation of the nanoparticles by tuning the zeta potential of the solution. Synthesis. For the synthesis of nanoporous Pt, 1 mM aqueous solution of H2PtCl6 was prepared by dissolving 20 mg of H2PtCl6 in 40 mL of water (corresponding pH was found to be 3). This aqueous solution of H2PtCl6 was heated in an autoclave at 200 °C for 5 h without any addition of buffer, reducing agent, or capping agent. The products formed during the synthesis are sensitive to the duration of heating, as the pH of the solution fluctuates as reaction proceeds. For the synthesis of the aggregates at room temperature, NaBH4 has been used in the presence of inorganic buffer solutions (pH ) 7) without the use of capping agents. For the reduction by NaBH4, the order of addition of reagents and reducing agents was found to be critical. In the first step, 10 mM sodium borohydride solution was prepared in 35 mL of buffer solution. In the second step, 5 mL of aqueous solution of chloroplatinic acid (8 mM) was added into the buffer (pH ) 7) containing the NaBH4 solution under continuous stirring. Characterization. The reaction products were characterized using a scanning electron microscope (SEM) and transmission electron microscope (TEM). SEM was carried out using a FEI Sirion FESEM operated at 5-20 kV. TEM studies were carried out in a JEOL 200CX TEM operated at 160 kV and a Tecnai F30 field-emission TEM operated at 300 kV. X-ray diffraction (XRD) was carried out using a Philips MDL PW 1050/7 X-ray powder diffractometer. Brunauer-Emmett-Teller (BET) surface area measurement was carried out using Quantachrome Autosorb automated gas sorption system. Zeta potential measurements were carried out using a zeta potential analyzer. The pH was varied between 1 and 11 using diluted HCl and liquor ammonia. Cyclic voltammetry and chronoamperometric studies were carried out using a potentiostat/galvanostat EG&G PARC model Versastat II or Solartron model 1286. Electrode Preparation. For electrochemical oxidation of methanol, electrodes were prepared on amorphous carbon paper (Toray). A mixture of Pt particles (50 wt %) and acetylene black (45 wt %) was subjected to grinding in a mortar, and a few drops of Nafion suspension (Aldrich) were added to get a slurry. It was coated on carbon paper. Coating and drying steps were repeated to get the required loading level (2 mg cm-2) of the active material. Finally, the catalyst loaded carbon paper was dried at 80 °C for 12 h. For electrical connection, a copper wire was attached to the carbon paper with the help of a conducting silver point. A Sartorius balance model CP225D-OCE with 0.01 mg sensitivity was used for weighing the electrodes. Electrochemical Measurements. A conventional cell with a three electrode configuration was used for all experiments. A glass cell of about 70 mL capacity with suitable ground-glass joints to introduce a working electrode (Cpaper), Pt foil auxiliary electrodes, and a saturated calomel electrode (SCE) as reference was used for electrochemical studies. All solutions were prepared using doubly distilled water. Potential values are reported against SCE.

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Results and Discussion Microstructural Studies of Nanoporous Pt Aggregates. Hydrothermal synthesis in the absence of a capping agent yields phase-pure Pt confirmed by X-ray diffraction as shown in Figure 1A. Figure 1B is a secondary-electron image of the hydrothermally synthesized Pt showing nearly uniform spherical particles. The size distribution of the particles is shown in Figure 1C as a histogram, obtained from SEM and TEM images of over 200 Pt particles. The average particle size is 66 nm with a standard deviation of (13 nm. It is interesting to note that a narrow size distribution of particles is obtained although there is no surfactant/ capping agent used for synthesis. The low magnification TEM bright-field image (Figure 2A) shows the uniformity of the particle size, while the higher magnification bright-field image (Figure 2B) reveals that each particle is not a single crystal but comprises particles of much finer dimensions. Solid platinum nanoparticles of this dimension (∼70 nm) are expected to appear much darker in the bright-field images owing to the mass-thickness contrast expected from such particles. The faint contrast clearly suggests that there is significant porosity associated with these particles and suggests that these form by the aggregation of smaller particles. This is clearly seen in the dark-field image shown in Figure 2C where the individual nanoparticles appear bright. The high-resolution image (Figure 2D) also clearly reveals that the individual nanoparticles comprising the aggregates are of the order of 1-3 nm in diameter. In addition, it was possible to kinetically stabilize the individual nanoparticles for short times and transfer them to a grid for TEM observation. This clearly indicates that the larger particles of the order of 70 nm are formed by the aggregation of subunits of the order of 1-3 nm. The XRD peaks shown in Figure 1A are much sharper than that expected from very fine nanoparticles, which indicate that the coherent diffracting domains are much larger. The average particle size calculated from the XRD pattern taking the full width at half-maxima (fwhm) is 26 nm, which is much larger than the 2-3 nm particles and smaller than the 70 nm porous clusters. High resolution images from various clusters indicate that there is strong texture/orientation between adjacent grains with fringes extending across several particles (Supporting Information Figure 1). We believe that this is due to some recrystallization/grain rotation process that happens under the synthesis conditions that tends to form boundaries with low/ small misorientations in order to minimize strain energy. There is a possibility for oriented attachment to take place as we have shown recently for the case of Au nanowires.30 A recent study on the fusion of PbSe nanoparticles using in situ TEM indicates a sequence of thermally induced rotations that take place to reduce the interfacial free energy of the system.31 We believe that such a rearrangement leading to the formation of large coherent domains with low interfacial strains and energy is operative in this case also. Further studies are required to understand this aspect better. Formation Mechanism of Nanoporous Pt Aggregates. It is interesting to note that the aggregates that are formed have a very narrow size distribution. The formation of monodisperse colloids could arise by diffusional growth of nuclei that have formed in a nucleation burst32,33 or by secondary growth processes involving the aggregation of such nuclei. The TEM images clearly indicate that the structures seen here have formed as a result of aggregation. The formation of monodisperse aggregates has been (30) Halder, A.; Ravishankar, N. AdV. Mater. 2007, 19, 1854. (31) van Huis, M. A.; Kunneman, L. T.; Overgaag, K.; Xu, Q.; Pandraud, G.; Zandbergen, H. W.; Vanmaekelbergh, D. Nano Lett. 2008, 8, 3959. (32) La Mer, V. K. Ind. Eng. Chem. 1952, 44, 1270. (33) La Mer, V. K.; Dinegar, R. H. J. Am. Chem. Soc. 1950, 72, 4847.

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Figure 2. Bright-field images of aggregates of Pt synthesized under hydrothermal conditions at 423 K in the absence of any external reducing and capping agents at (A) low magnification and (B) high magnification showing their porous nature arising due to the aggregation of primary nanoparticles. (C) Dark-field image showing the size of the individual crystallites in the aggregate and (D) HRTEM images of the Pt clusters synthesized under hydrothermal conditions. The individual Pt nanoparticles comprising the aggregate are resolved in the lattice image.

Figure 1. (A) XRD of the hydrothermally synthesized porous Pt clusters, (B) SEM image of Pt clusters showing the uniform size, and (C) histogram of Pt cluster obtained over 200 Pt aggregates from SEM and TEM images.

observed in several systems.20-22,24-28,34,35 A kinetic model was described that successfully explains several features of the aggregation process in a semiquantitative way indicating that kinetics plays a major role in the size selection.24,25 It is also conceivable that the size selection arises as a result of two competing forces in the solution phase. The combination of shortrange attraction and long-range repulsion has been shown to result in the formation of equilibrium clusters during protein aggregation.27 Reduction in the surface energy drives the aggregation process, while the increase in electrostatic energy due to the double layer overlap provides the opposing force. The functional forms of each of these forces can be represented analytically using simplified assumptions and could lead to the existence of an equilibrium size for aggregation. Of course, kinetics also has an important role to play in the aggregation process. Here, we show the synthesis of stable, porous Pt clusters in the absence of any capping agents in aqueous medium by fine-tuning the zeta potential of the nanoparticles. The first step involves establishing a large driving force for nucleation such that a burst of nucleation takes place in the solution. The stability of the colloid against aggregation can then be tuned by varying the solution conditions. In the simplest form of the DLVO theory, the attractive potential is represented by the van der Waals force between the particles in the medium that is represented in terms of the Hamaker constant.36 The repulsive force between the particles arises due to the double layer overlap and can be estimated semiquantitatively by measuring the zeta potential of the system. While the attractive potential remains nearly constant, the repulsive part of the potential can be tuned over a large range by controlling the zeta potential through the pH, ionic strength, and temperature. The net interaction energy (34) Matijevic, E. Colloid J. 2007, 69, 29. (35) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (36) Leckband, D.; Israelachvili, J. Q. ReV. Biophys. 2001, 34, 105.

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Figure 4. Zeta potential measurement of Pt nanoparticles synthesized using NaBH4 at room temperature.

Figure 3. Interaction energy plotted as a function of separation distance for the interaction of two spherical Pt nanoparticles of 1.5 nm radius in aqueous medium. Double layer thickness, k ) 1.04 × 108 m-1; Hamaker constant, A ) 4 × 10-19 J. (A) Double layer repulsion and van der Waals attraction exist between two nanoparticles, and the net interaction energy at the pH of 6.7 is shown separately. (B) Net interaction energy at different pH values with the corresponding barrier is shown.

between two nanoparticles is the summation of the attractive and repulsive components and can be represented as follows.36

V ) [-Cvdw⁄r6] + [q1q2 e-k(r-σ) ⁄ 4πεε0r(1 + kσ)] where CvdW is a constant that is related to the Hamaker constant and r is the separation distance between two particles. σ is the radius of the particle, and k is the inverse double layer thickness. Using the above expression, the interaction energy between the two Pt nanoparticles at different pH values has been calculated. For simplification, it is assumed that the ionic strength of the solution remains constant and the inverse double layer thickness is taken to be 1 nm which is a typical value at low concentrations. In Figure 3, the interaction energy between two Pt nanoparticles of 1.5 nm radius as a function of separation distance is plotted. The attractive, repulsive, and net interaction energies are shown in Figure 3A. The surface charge density is calculated from the values of experimentally measured zeta potential at different pH values. Figure 3B shows the net interaction energies with corresponding barriers at different pH values (zeta potential). At very low pH (low zeta potential), the surface charge density is lower and the repulsive component is weaker, and hence, there is no barrier between two nanoparticles and the net interaction energy is attractive. At higher pH (higher zeta potential), the

Figure 5. Schematic of the colloidal stability and the aggregation process as a function of decreasing zeta potential is shown. (A) Stable, (B) reaction-limited aggregation (RLA), and (C) diffusion-limited aggregation (DLA). Corresponding TEM bright-field images synthesized at room temperature with the use of standard reducing agent NaBH4 at (D) pH ) 11 (stable), (E) pH ) 7 (RLA), and (F) pH ) 4 (DLA).

repulsive component increases and exhibits a barrier as is evident from Figure 3B. The described interaction shown in Figure 3 is only valid between two Pt nanoparticles. With each and every further addition of nanoparticles, the radius as well as the charge will undergo continuous change, and hence, calculation of the net interaction for the formation of cluster is complicated without the help of simulation. However, the overall trend of the interaction at different pH values will be same as the one give here. The above calculations indicate that it is possible to control the aggregation process by changing the pH and/or ionic strength of the solution. We present experimental realization of this fact using NaBH4 as the reducing agent for the synthesis of the Pt aggregates at room temperature. The variation of zeta potential with pH under this condition is shown in Figure 4. Figure 5A-C illustrates schematically different aggregation regimes depending on the interaction energy between the particles. Under low zeta potential conditions close to the isoelectric point, the electrostatic repulsive force between particles is negligible and thus it is likely that particles will adhere once they encounter each other in the solution. This leads to the formation of an open structure and falls under the diffusion-limited regime for the aggregation of

Nanoporous Pt with High Surface Area

particles. An increase in the zeta potential leads to an increase in the repulsive force between the particles and to the reactioncontrolled aggregation regime which results in the formation of compact, porous clusters as shown in Figure 5C. Pt nanoparticles can be kinetically stabilized for several hours under very basic conditions (pH ∼ 11) where the zeta potential of the solution is about -40 mV. From the variation in the zeta potential with pH, it is seen that the zeta potential varies from abut -40 mV at pH ∼ 11 to about -2 mV at pH ∼ 1. In order to achieve uniform Pt aggregates which are compact, the zeta potential should be in the medium range (in this case between -15 and -20 mV). Based on the mechanism discussed above, we have successfully synthesized Pt clusters at the pH of 7 where the zeta potential is around -20 mV. Figure 5D-F shows bright-field images of fine Pt nanoparticles synthesized at pH ) 11, 7, and 4, respectively. At pH ) 11, nanoparticles are kinetically stabilized for several hours in the solution. However, there could be aggregation during drying on the TEM grid. This results in the formation of aggregates in some regions while most of the particles are separated. While the Pt nanoparticles synthesized under high pH conditions are kinetically stable, they are unstable under acidic conditions and aggregate instantaneously to form diffusionlimited aggregates. The diffusion-limited aggregation (DLA) mechanism operates at lower zeta potential close to the isoelectric point of the medium. At pH ) 4, the surface charge is negligible, the attractive van der Waals force overcomes the electrostatic repulsive force, and hence the sticking probability of the nanoparticles undergoing collisions is almost one. As a result, two particles adhere permanently once they collide with each other, leading to the formation of less compact fractal structures. When the pH of the solution is close to 7, there exists a significant barrier for aggregation and aggregation leads to the formation of compact clusters. It is under these conditions that the specific surface area and hence the catalytic activity of the aggregate are at maximum. We exploit this reaction-limited aggregation regime to produce clusters with large surface areas that have very high catalytic activities. The cluster morphology also has important implications for the thermal stability of the catalyst. The fact that no surfactant has been used also ensures that the surface of the catalyst remains pristine and exhibits high activity. In principle, it is possible to get the uniform clusters of any material or even bimetallic clusters by manipulating the zeta potential, surface area, ionic strength of the solution, and kinetics of the primary particle formation. High Surface Area and Catalytic Activity of Nanoporous Pt Aggregates. The aggregation of ultrafine primary particles results in a very porous structure with a high surface area. Surface area and pore size distribution were measured by usingthemultiplepointBETmethodusingtheadsorption-desorption isotherms of nitrogen at 77 K. The lower part of the adsorption isotherm is used for measurement of specific surface area, whereas the desorption branch of the isotherm is used for pore size analysis. BET surface area measurements shown in Figure 6A reveal that the specific surface area of the nanoporous aggregates is 39 m2/g, which is significantly higher than the reported values for porous Pt produced without a template (less than 25 m2/g) and commercially available Pt black powder. On the other hand, the specific surface area of the DLA structure of Pt measured from the multipoint BET adsorption isotherm is only 8 m2/g. Barrett-Joyner-Halenda (BJH) pore size distribution analysis (Figure 6B) shows that the average pore diameter is about 3.1 nm. There have been previous reports on porous

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Figure 6. (A) Typical BET adsorption/desorption isotherm of porous Pt clusters used for surface area measurement. (B) BJH pore size analysis curve obtained for the porous Pt clusters synthesized under hydrothermal conditions.

metal aggregates using different methods.37-47 However, the surface area obtained here is significantly higher and the absence of surfactant ensures that the active surfaces are available for catalysis. The compact nanoporous structure exhibits much higher catalytic activity compared to the fractal-like aggregates. The catalytic reduction of 4-nitrophenol into 4-aminophenol, carried out using nanoporous aggregates (RLA), is found to be superior (37) Vidoni, O.; Philippot, K.; Amiens, C.; Chaudret, B.; Balmes, O.; Malm, J.-O.; Bovin, J.-O.; Senocq, F.; Casanove, M.-J. Angew. Chem., Int. Ed. 1999, 38, 3736. (38) Pelzer, K.; Vidoni, O.; Philippot, K.; Chaudret, B.; Collie`re, V. AdV. Funct. Mater. 2003, 13, 118. (39) Ramirez, E.; Jansat, S.; Philippot, K.; Lecante, P.; Gomez, M.; MasdeuBulto, A. M.; Chaudret, B. J. Organomet. Chem. 2004, 689, 4601. (40) Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, C. J.; vanSwol, F.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635. (41) Ewers, T. D.; Sra, A. K.; Norris, B. C.; Cable, R. E.; Cheng, C. H.; Shantz, D. F.; Schaak, R. E. Chem. Mater. 2005, 17, 514. (42) Shi, L.; Berkland, C. AdV. Mater. 2006, 18, 2315. (43) Teng, X.; Liang, X.; Maksimuk, S.; Yang, H. Small 2006, 2, 249. (44) Ullah, M. H.; Chung, W.-S.; Kim, I.; Ha, C.-S. Small 2006, 2, 870. (45) Yujiang, S.; Ying-Bing, J.; Haorong, W.; Pena, D. A.; Yan, Q.; Miller, J. E.; Shelnutt, J. A. Nanotechnology 2006, 17, 1300. (46) Zhong, X.; Feng, Y.; Lieberwirth, I.; Knoll, W. Chem. Mater. 2006, 18, 2468. (47) Hatakeyama, Y.; Umetsu, M.; Ohara, S.; Kawadai, F.; Takami, S.; Naka, T.; Adschiri, T. AdV. Mater. 2008, 20, 1122.

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to the diffusion-limited aggregates (DLA) due to the high specific surface area available for the reduction (Supporting Information Figure 2). Electrochemical oxidation of methanol has gained importance in energy conversion in the form of methanol-air fuel cells48 and also in methanol economy.49 Pt is an essential catalyst for electro-oxidation of methanol. The reaction proceeds by adsorption of methanol molecules on the catalyst surface, which is followed by stepwise oxidation leading to total conversion of methanol to CO2.50 As adsorption of methanol and the reaction intermediates is essential, nanoparticles of Pt with a large surface area and high porosity were considered to be useful for this reaction. The electrocatalytic oxidation of methanol using the Pt aggregates (RLA) as the catalyst was investigated using cyclic voltammetry and chronoamperometry. Figure 7A shows the cyclic voltammogram in 0.5 M H2SO4 containing 1 M CH3OH recorded at a sweep rate of 10 mV s-1. The typical behavior of Pt for methanol oxidation was observed with the forward oxidation peak occurring at 0.82 V and the backward oxidation peak occurring at 0.67 V. In Figure 7A, the forward peak current density (If) is about 67 mA cm-2, which is considered to be high for a nano-Pt particle loading level of 2 mg cm-2.51 The ratio of forward peak current (If) to the backward peak current (Ib), which is a measure of CO tolerance,7 is found to be 1.3. As the commercial catalysts for methanol oxidation have an If/Ib ratio close to unity,52 the value obtained in the present work is attributed to the nanoporous morphology of the Pt catalyst. Long-term oxidation of methanol was conducted at 0.83 V, and the variation of current with time was recorded. The electrolyte (0.5 M H2SO4 + 1.0 M CH3OH) was stirred with a Teflon-coated magnetic paddle during the experiment. A steady current of 1 mA cm-2 over 10 000 s of experiment indicates (Figure 7B) a durable catalytic activity of Pt particles for electro-oxidation of methanol. Electrochemical studies carried out using DLA platinum under identical conditions show lower voltammetric peak current density in comparison with RLA platinum. The peak current density of the DLA platinum electrode (Supporting Information Figure 3A) is 6 mA cm-2, which is several times lower than the peak current density (67 mA cm-2) obtained for the RLA platinum electrode. The chronoamperometric current of DLA platinum electrode tends to decrease with time (Supporting Information Figure 3B) against the constant current of RLA platinum. This result indicates that, in addition to the greater catalytic activity, the long-term stability of RLA platinum is superior to that of DLA platinum for methanol oxidation. The morphology of the RLA catalyst after 20 000 cycles is shown in the bright-field TEM image in Figure 7C. Surprisingly, there is no significant change in its morphology observed in the TEM even after 20 000 s. In addition, the thermal stability of the Pt clusters assessed by heating the catalyst (in dry condition) at 125 °C for a period of 50 h also shows no difference in morphology with respect to the as-synthesized one (Figure 8). Under similar conditions, the catalyst comprising isolated nanoparticles would undergo significant coarsening, leading to a loss in the catalytic activity. For isolated nanoparticles on a support, there is significant loss in the active/effective surface area due to contact with the capping agent and the support. However, this is not the case for the capping free porous aggregates (48) Surampudi, S.; Narayanan, S. R.; Vamos, E.; Frank, H.; Halpert, G.; LaConti, A.; Kosek, J.; Prakash, G. K. S.; Olah, G. A. J. Power Sources 1994, 47, 377. (49) Olah, G. A.; Goeppert, A.; Prakash, G. K. S. Beyond Oil and Gas: The Methanol Economy; Wiley VCH: Weinheim, 2006; p 1. (50) Peng, X.; Koczkur, K.; Chen, A. Nanotechnology 2007, 18, 305605. (51) Le´ger, J. M. J. Appl. Electrochem. 2001, 31, 767. (52) Raghuveer, V.; Manthiram, A. J. Electrochem. Soc. 2005, 152, A1504.

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Figure 7. Electrocatalysis behavior of Pt aggregate for methanol oxidation and its stability. (A) Cyclic voltammograms of Pt aggregate in 1.0 M CH3OH + 0.5 M H2SO4 with a sweep rate of 10 mV s-1. (B) Current-time curve obtained from chronoamperometric studies carried out at 0.83 V. Pt loading is 2 mg cm-2. (C) TEM bright-field image of Pt aggregate after 20 000 s confirming the morphological stability of the Pt clusters.

of the type shown here. Based on the current densities, the electrochemical and thermal stabilities, and the available surface area for catalysis, it is concluded that the aggregate morphology

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CaF2, Pd, and Au-Pt bimetallics (Supporting Information Figures 4 and 5).

Conclusion In summary, we have investigated the interaction of capping free nanoparticles in solution in order to control their aggregation. Nanoporous Pt aggregates with high surface area are formed by controlling the electrostatic repulsion between the nanoparticles. The reaction-limited aggregates of porous Pt are found to be potential candidates for methanol oxidation and 4-nitrophenol catalytic reduction. The morphological stability of the nanoporous Pt is confirmed by electron microscopy after subjecting it to chronoamperometric and heat treatment studies. It is important to note that the strategy presented here to produce uniform porous structures is free from capping agents/templates and can easily be extended to any other systems by controlling the parameters such as pH, ionic strength, and dielectric constant of the medium. The generality of the method is tested for a variety of inorganic materials including Au-Pt bimetallics and may thus provide a general strategy to produce high surface area inorganics for different applications.

Figure 8. Bright-field TEM image of porous Pt clusters heated at 125 °C for 55 h confirming their morphological stability.

provides significant advantages over the conventional nanoparticle catalysts. In order to obtain a higher current at a lower voltage, the conductivity of the support needs to be increased and proper contact between the catalyst and the support has to be ensured. Lowering the voltage is possible by the formation of bimetallic clusters of Pt and Ru using a similar strategy involving controlled aggregation which is under investigation. We have also successfully extended this strategy for achieving porous aggregates for a variety of materials such as hydroxyapatite, fluorapatite,

Acknowledgment. N.R. acknowledges funding from the Nanoscience Initiative of the Department of Science and Technology, Government of India. The Tecnai F30 TEM is a part of the National Electron Microscopy Facility at the Indian Institute of Science. Thanks are due to Mr. Mariappan, NAL for his help with the surface area measurements. Supporting Information Available: HRTEM image, UV-vis spectra, electrocatalysis behavior of a Pt aggregate, TEM and EDX analysis of porous bimetallic clusters of Au-Pt, and generality of the method to achieve uniform clusters for a variety of materials.. This material is available free of charge via the Internet at http://pubs.acs.org. LA802938D