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On-Site Characterization of Electrocrystallized Platinum Nanoparticles on Carbon and Sol-Gel Thin Film Modified Carbon Surfaces Yizhu Guo and Ana R. Guadalupe* Department of Chemistry, P.O. Box 23346, University of Puerto Rico, San Juan, Puerto Rico 00931-3346 Received June 22, 1998. In Final Form: October 16, 1998 Platinum nanoparticles were electrocrystallized on amorphous carbon film and on sol-gel modified carbon film deposited on gold mesh grids. These Pt-modified surfaces were characterized by transmission electron microscopy and energy dispersive spectroscopy. Particles deposited on bare carbon surfaces exhibited polytetrahedral crystallographic morphology. Meanwhile, laterally dendritic growth of platinum formed by aggregation of primary particles (3-5 nm) were found on the sol-gel modified carbon surfaces. This is a new kind of morphology for electrodeposited platinum with a high specific and accessible surface area, holding great promise for Pt-catalyzed reactions. The characterization gives direct information on the microstructures of electrochemically produced particles, facilitating function and structure-correlating studies.
Introduction Structured fine metals are the active catalysts in a wide range of applications due to their high specific surface area. Among these, supported metal catalysts play a significant role in many chemical reactions.1 This is very important with expensive noble metal catalysts such as platinum, rhodium, gold, and others.2 The support plays a major role determining the mechanical and thermal stability of the particles while helping them in a highly dispersed state. Supports most widely used are carbon, silica, alumina, and zeolites. Therefore, utilization of supported metal catalysts are economically and practically favorable. Many efforts have been made to the preparation of colloidal metal particles with emphasis on controlled size and size distribution. Some results also showed the control of particle shape and crystallographic morphology.3 However, most of these colloidal metal particles have to be capped with ligands (e.g., polymers, thiols) as stabilizers, which are likely to be detrimental to their catalytic activity. Research on supported, unstabilized metal particles have been also extensively studied, but only few papers deal with shape-controlled deposition.3b,c Due to the unique role of platinum catalysts in chemical processes and fuel cells,4 studies have been reported on the synthesis of platinum particles supported on various (1) (a) Heterogeneous Catalysis, Principles and Applications, 2nd ed.; Bond, G. C., Ed.; Clarenden Press: Oxford, England, 1987. (b) Qi, Z.; Pickup, P. G. Chem. Commun. 1998, 15. (2) (a) Jarvi, T. D.; Sriramulu, S.; Stuve, E. M. J. Phys. Chem. B 1997, 101, 3649. (b) Seregina, M. V.; Bronstein, L. M.; Platonova, O. A.; Chernyshov, D. M.; Valetsky, P. M. Chem. Mater. 1997, 9, 923. (c) Kao, W.; Kuwana, T. J. Am. Chem. Soc. 1984, 106, 473. (3) (a) Tanori, J.; Pileni, M. P. Langmuir 1997, 13, 639. (b) Lu, D.; Okawa, Y.; Suzuki, K.; Tanaka, K. Surf. Sci. 1995, 325, L397. (c) Lu, D.; Okawa, Y.; Ichihara, M.; Aramate, A.; Tanaka, K.; J. Electroanal. Chem. 1996, 406, 101. (d) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (e) Ahmadi, T. S.; Wang, Z. L.; Henglein, A.; El-Sayed, M. A. Chem. Mater. 1996, 8, 1161. (f) Rodriguez, A.; Amiens, C.; Chaudret, B.; Casanove, M.; Lecante, P.; Bradley, J. S. Chem. Mater. 1996, 8, 1978. (4) (a) Ralph, T. R.; Hards, G. A. Chem. Ind. 1998, 4 May, 334. (b) Ralph, T. R.; Hards, G. A. Chem. Ind. 1998, 4 May, 337. (c) Ralph, T. P. Platinum Met. Rev. 1997, 41, 102. (d) Wilson, M. S, Gottesteld, S. J. Electrochem. Soc. 1992, 139, L28.
matrixes 5 including carbon and conducting polymers, nonconducting polymers, and inorganic polymers. Most of these studies report spherical or granular platinum particles on these supports. 5 Carbon-supported platinum is the most utilized catalysts in fuel cells and related technologies.6 Traditionally, dispersed platinum is produced by impregnation methods (chemical reduction or thermal decomposition of platinum6a-e compounds). Electrochemical methods also provide an attractive way to produce dispersed platinum electrodes.6f-i Here, we report the electrochemical preparation of platinum particles on amorphous carbon film and sol-gel modified carbon film deposited on gold mesh grids. Transmission electron microscopy (TEM) and energy dispersive spectroscoy (EDS) revealed a substrate-dependent morphology of these electrocrystallized platinum nanoparticles. On the unmodified carbon film, faceted polytetrahedral crystals were formed while on the solgel modified carbon film, two-dimensional (2D) nucleation and lateral growth of porous platinum flakes with high specific surface area were observed. Experimental Section Gold mesh grids (300 mesh) for transmission electron microscopy covered with an amorphous carbon film were used as the working electrode (C/Au, Electron Microscopy Sciences). The functionalized silane modified C/Au electrode was prepared by (5) (a) Itaya, K.; Matsushima, Y.; Uchida, I. Chem. Lett. 1986, 571. (b) Wang, Y.; Liu, H.; Jiang, Y. J. Chem. Soc., Chem. Commun. 1989, 1878. (c) Lopez, T.; Moran, M.; Navarrete, J.; Herrera, L.; Gomez, R.; J. Non-Cryst. Solids 1992, 147 & 148, 753. (d) Bartak, D. E.; Kazee, B.; Shimazu, K.; Kuwana, T. Anal. Chem. 1986, 58, 2756. (e) Kao, W.; Kuwana, T. J. Am. Chem. Soc. 1984, 106, 473. (f) Seregina, M. V.; Bronstein, L. M.; Platonova, O. A.; Chernyshow, D. M.; Valetsky, P. M. Hartmann, J.; Wenz, E.; Antonietti, M. Chem. Mater. 1997, 9, 923. (6) (a) Kinoshita, K.; Stonehart, P. Mod. Aspects Electrochem. 1997, 12, 183. (b) Kinoshita, K. Proc. Electrochem. Soc. 1979, 79-2, 144. (c) Kinoshita, K.; Lundquist, J.; Stonehart, P. J. Catal. 1973, 31, 325. (d) Kinoshita, K.; Stonehart, P. Electrochim. Acta 1975, 20, 101. (e) Kinoshita, K. J. Electrochem. Soc. 1990, 137, 845. (f) Takasu, Y.; Ohashi, N.; Zhang, X. G.; Murakami, Y.; Minagawa, H.; Sato, S.; Yahikozawa, K. Electrochim. Acta 1996, 41, 2595. (g) Jiang, L. C.; Derek, P. J. Electroanal. Chem. 1983, 149, 237. (h) Bipdra, P.; Yeager, E. Proc. Symp. Electrocryst. 1981, 6, 233. (i) Zoval, J. V.; Lee, J.; Gorer, S.; Penner, R. M. J. Phys. Chem. B 1998, 102, 1166.
10.1021/la980729+ CCC: $18.00 © 1999 American Chemical Society Published on Web 01/07/1999
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Pulsed Potentiostatic Deposition. Figure 1 shows typical current-time transients for a 200-ms pulse in the 1.0 mM PtCl4, 0.1 M H2SO4 plating solution on the C/Au (a) and SIS/C/Au (b) electrodes. Similar to electrodeposition of Pt on basal plane-oriented graphite surfaces,6i the current-time behavior on C/Au indicated an instantaneous nucleation and diffusion-limited growth mechanism for the deposition of platinum on the C/Au surface. On the SIS/C/Au surface, the initial current delay may be due to a pseudocapacitive charge transport processes7 within the SIS xerogel thin film upon changing the electrode potential. The current decayed with t-1/2 and approached that given by the Cottrell equation, indicative of a process controlled by planar diffusion. Our results are consistent with those for the electrodeposition of metals on polyaniline in its reduced state7 (i.e. at potentials where the film is electronically insulating), where the kinetics of nucleation were described adequately by an “instantaneous nucleation”. The current density on SIS/C/Au is lower than that on C/Au during all the deposition process, indicating a smaller amount of platinum involved during the electrodeposition
on SIS/C/Au surface, which is reasonable considering the insulating properties and the blocking effect of the SIS sol-gel thin film. EDS analysis results show that the platinum content is higher on Pt/C/Au than that on Pt/ SIS/C/Au, indicating larger amount of platinum deposited on C/Au. Formation of the initial nuclei is a crucial step during the electrodeposition process.8 Substrates with different surface free energy influence the early stage of growth of electrodeposited metal particles.5b,6a The difference in surface free energy on C/Au and SIS/C/Au surfaces results in different nucleation and growth kinetics for platinum electrodeposition. Differences in kinetics result in a different microstructural morphology for the deposited platinum on these substrates. Microstructures by TEM. Figure 2 shows the TEM images and the histogram of platinum particles electrodeposited on C/Au surfaces. It is clear that polytetrahedral crystals are formed on the amorphous carbon films. This is distinct from the previously reported spherical or granular platinum particles5 on carbon materials (mostly pyrolytic graphite or HOPG) or conducting polymers (PPy, PAn, ...). The nucleation behavior and the growth mechanism of a metal on an inert substrate are dependent on the overpotential and the interaction between the metal crystallites and the substrate.6a The surface free energy and microstructure of amorphous carbon deposited on Au grid differ from those of pyrolytic graphite surfaces; the latter usually have a lower surface energy and a welldefined geometric structures.6b The heterogeneous character of an amorphous carbon surface may promote the random nucleation of platinum on active sites, and further growth of platinum clusters on these active sites. The crystallographic habit of gold particles grown on carbon electrodes at different potentials has been reported.5b,c The crystal characteristics were related to the potential-dependent surface reconstruction of Au (111), Au (110), and Au (100) surfaces. Platinum colloidal crystals with different shapes have been produced in polymeric solutions.3d,e,f Oriented surfaces have been shown3e to increase selectivity and reactivity in certain catalytic reactions. The faceted platinum particles on amorphous carbon could be exploited for applications in electrocatalysis. Figure 3 shows the TEM images and the histogram of platinum particles electrodeposited on SIS/C/Au surface. Compared with the unmodified C/Au, there are significant differences on the SIS/C/Au. The platinum particles on SIS/C/Au are much smaller and highly dispersed. The mean diameter of platinum particles is 17 nm on SIS/C/ Au surface, while 138 nm on C/Au surface. The particle number density is about 1 order of magnitude higher on SIS/C/Au (1010 cm-2) than that on C/Au (109 cm-2). A closer examination of the morphology of the particles revealed that spongelike lateral 2D growth of dendritic platinum particles are found on SIS/C/Au, in comparison to the compact 3D platinum crystals grown on C/Au. Furthermore, the 2D “cauliflower” platinum aggregation on SIS/ C/Au consists of smaller primary particles with diameter around 3-5 nm. There coexist primary particless “monomer”, “dimer”, “trimer”, “tetramer”sand 2D flakelike “polymer”. As is evident from the histogram, there are two peaks on the size distribution, a main peak sitting around 10-15 nm which consists of more than 60% of the total particles and a shoulder sitting around 35 nm which consists of about 15% of the total particles. This may reflect that some of the 2D “dendrimers” are more kinetically
(7) Leone, A.; Marino, W.; Scharifker, B. R. J. Electrochem. Soc. 1992, 139, 438.
(8) Palomar-Pardare, M.; Ramirez, M. T. Gonzalez, I.; Serruya, A.; Scharitfker, B. R. J. Electrochem. Soc. 1996, 143, 155.
Figure 1. Current-time transients for a 200 ms pulse in the 1.0 mM PtCl4, 0.1 M H2SO4 plating solution on the C/Au (a) and SIS/C/Au (b) electrodes. Potential was stepped from +600 to -200 mV vs Ag/AgCl. a sol-gel procedure using 0.5% bis[(3-triethoxysilyl)propyl] tetrasulfide (SIS, Petrarch Systems) in an ethanol solution using 0.1 M HCl as catalyst (SIS:H2O ) 1:8, molar ratio). After standing overnight, an aliquot (0.5 µL) of this solution was cast on the C/Au carbon film surface, and dried overnight before use. This modified electrode was denoted as SIS/C/Au. Platinum particles were grown on the C/Au and SIS/C/Au surfaces from a 0.1 M H2SO4 solution containing 1.0 mM PtCl4 (Aldrich) by a pulsed potentiostatic method. The potential stepped from +600 to -200 mV vs Ag/AgCl with pulse width of 200 ms. All experiments were conducted at room temperature in a specially designed electrochemical cell using a platinum wire counter electrode and a Ag/AgCl reference electrode. Only the carbon or SIS-modified carbon face of the Au grid electrode contacted the solution. The platinum particles grown on the carbon and SIS modified carbon surfaces were observed by TEM (Philips TEM201) and analyzed by EDS (JEOL JSM-5800 LV Scanning Microscope).
Results and Discussion
Electrocrystallized Platinum Nanoparticles
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a
b Figure 2. TEM images and for Pt/C/Au surfaces at low and high magnifications and the corresponding histogram.
Figure 3. TEM images for Pt/SIS/C/Au surfaces at low and high magnifications and the corresponding histogram.
and/or thermokinetically favorable during the electrodeposition on the surface. Experiments to synthesize particles with controlled size distribution are being pursued. These nanometer-scale spongelike structures have a larger surface area that is an ideal prerequisite for technological applications as catalysts. Instead of the 2D dendritic structure shown here, a 3D-related morphology was found for zinc and palladium colloids entrapped in block copolymer micelles.9 Primary particles (about 10 nm) aggregated into stable cauliflowerlike superstructure. These superstructures do not affect the optical and
electronic properties of the small primary particles, such as band gap, exciton frequency, and resonator strength.9 Here we provide a good opportunity to study such properties for supported 2D dendritic superstructure, as well as their catalytic and electrocatalytic performance. Compared with colloidal particles stabilized, i.e., capped with ligands, where particle stabilization is generally accompanied by passivation, SIS/C/Au-supported Pt offers the possibility of particle stabilization without passivation or with only partial passivation. We anticipate that the chemical interaction between sulfur domains and platinum enables higher stability to the particles. Electrochemically synthesized platinum particles on basal plane oriented graphite were reported as weakly physisorbed
(9) Antonietti, M.; Goltner, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 910
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Figure 4. Cyclic voltammograms on SIS sol-gel modified electrode in 0.4 M KCl solution without (a) and with 5 mM K3Fe(CN)6 (b). Scan rate: 20 mV/s.
on the graphite surface so that conventional AFM and STM imaging would wipe the particles from the surface.6i Proposed Deposition Mechanism. As stated above, the heterogeneous character of amorphous carbon films promote a random nucleation and 3D crystallization of Pt particles on the C/Au surface. Meanwhile, on the SIS/C/ Au surface, sulfur domains might form during sol-gel modification, which could template the nucleation and 2D growth, probably assisted by some affinity between the sulfur domain and the platinum. To further understand the 2D dendritic growth (aggregation) of electrodeposited platinum on SIS/C/Au surface, the electrochemical behavior of K3Fe(CN)6 was studied with the SIS sol-gel modified electrode. Figure 4 shows the cyclic voltammograms of the modified electrode in a blank electrolyte solution (a) and a solution containing 5 mM K3Fe(CN)6 (b). Compared with the unmodified electrode (not shown), the peak current decreased substantially (from 32 to 5 µA) and the potential separation of the redox peaks increased substantially (from 100 to 550 mV) on the SIS sol-gel modified carbon surface, indicating a large blocking effect of the insulating sol-gel thin film.10 However, the redox process on the modified surface indicated that the micropores in the SIS sol-gel thin film are accessible for the redox species to reach the carbon surface. The voltammetric behavior resembles that of a microelectrode array.11 Deviation from the ideal sigmoidal ultramicroelectrode behavior may be due to the nonuniformity of the size and spacing of the micropores on the SIS/C/Au surface. This feature may contribute to the formation of the 2Ddendritic platinum aggregation.12 One particle aggregation might form around/above one (ultra)microelectrode. The coexistence of “monomer”, “dimer”, “trimer”, ..., and (10) Finklea, H. O. Electroanal. Chem. 1996, 19, 109. (11) (a) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663. (b) Gao, Z.; Siow, K. S. Electrochim. Acta 1997, 42, 315. (12) (a) Garacia-Pastoriza, E.; Mostany, J.; Scharifker, B. R. J. Electroanal. Chem. 1998, 441, 13. (b) Scharifker, B. R.; Mostany, J.; Serruya, A. Electrochim. Acta 1992, 37, 2503.
Guo and Guadalupe
Figure 5. Proposed nucleation and growth mechanism for electrodeposition of platinum on C/Au and SIS/C/Au surfaces.
“polymer” morphologies may reflect different geometric microstructures, surface free energies and microporosities of the SIS sol-gel matrix. From the kinetics of ferricyanide on the bare carbon electrode and SIS xerogel film modified carbon electrode, the fractional porosity or electroactive area can be estimated from the equation:11a κapp ) κo(1 θ). κo is the heterogenerous electron-transfer rate constant of ferricyanide on the bare carbon electrode, and κapp is that on SIS xerogel film modified electrode; θ is the surface coverage of SIS xerogel film. Therefore, the uncovered electrode surface or fractional porosity (1 - θ) can be estimated to be 0.62%. It indicated that only a very small portion of the overall electrode surface is available for the electrodeposition of Pt particles. A simple scheme reflecting the above factors is proposed in Figure 5 for the platinum deposition on C/Au and SIS/C/Au surfaces. Conclusions Electrodeposition of platinum particles on amorphous carbon surface and sol-gel modified carbon surface on gold grids have been performed. TEM and EDS characterization provide direct information on the microstructure and morphology of the deposited platinum particles. Polytetrahedral platinum crystals were formed on the C/Au surface, which showed oriented crystallization. Meanwhile 2D dendritic cauliflower-like platinum aggregations consisted of smaller primary particles were found on SIS/C/Au surface. This structure possesses a high surface area, a property that is advantageous for the preparation of highly active noble metal catalysts at a reduced cost. Acknowledgment. This project is funded by DOEEPSCoR (046138). Special thanks to Eng. Camilo Cangani, Research Centers in Minority Institutions (RCMI, PR03051), and Janet Figueroa and Gabriel Cruz, Materials Characterization Center (UPR-Rı´o Piedras) for their help with the TEM and EDS experiments, respectively. LA980729+
