On-Site Characterization of Electrocrystallized PlatinumNanoparticles 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 modifiedcarbon film deposited on gold mesh grids. These Pt-modified surfaces were characterized by transmissionelectron microscopy and energy dispersive spectroscopy. Particles deposited on bare carbon surfaces exhibitedpolytetrahedral crystallographic morphology. Meanwhile, laterally dendritic growth of platinum formedby aggregation of primary particles (3-5 nm) were found on the sol-gel modified carbon surfaces. Thisis 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 themicrostructures of electrochemically produced particles, facilitating function and structure-correlatingstudies.

Introduction

Structured fine metals are the active catalysts in a widerange of applications due to their high specific surfacearea. Among these, supported metal catalysts play asignificant role in many chemical reactions.1 This is veryimportant with expensive noble metal catalysts such asplatinum, rhodium, gold, and others.2 The support playsa major role determining the mechanical and thermalstability of the particles while helping them in a highlydispersed state. Supports most widely used are carbon,silica, alumina, and zeolites. Therefore, utilization ofsupported metal catalysts are economically and practicallyfavorable.

Many efforts have been made to the preparation ofcolloidal metal particles with emphasis on controlled sizeand size distribution. Some results also showed the controlof particle shape and crystallographic morphology.3 How-ever, most of these colloidal metal particles have to becapped with ligands (e.g., polymers, thiols) as stabilizers,which are likely to be detrimental to their catalytic activity.Research on supported, unstabilized metal particles havebeen also extensively studied, but only few papers dealwith shape-controlled deposition.3b,c

Due to the unique role of platinum catalysts in chemicalprocesses and fuel cells,4 studies have been reported onthe synthesis of platinum particles supported on various

matrixes 5 including carbon and conducting polymers,nonconducting polymers, and inorganic polymers. Mostof these studies report spherical or granular platinumparticles on these supports. 5 Carbon-supported platinumis the most utilized catalysts in fuel cells and relatedtechnologies.6 Traditionally, dispersed platinum is pro-duced by impregnation methods (chemical reduction orthermal decomposition of platinum6a-e compounds). Elec-trochemical methods also provide an attractive way toproduce dispersed platinum electrodes.6f-i

Here, we report the electrochemical preparation ofplatinum particles on amorphous carbon film and sol-gelmodified carbon film deposited on gold mesh grids.Transmission electron microscopy (TEM) and energydispersive spectroscoy (EDS) revealed a substrate-de-pendent morphology of these electrocrystallized platinumnanoparticles. On the unmodified carbon film, facetedpolytetrahedral crystals were formed while on the sol-gel modified carbon film, two-dimensional (2D) nucleationand lateral growth of porous platinum flakes with highspecific surface area were observed.

Experimental Section

Gold mesh grids (300 mesh) for transmission electron mi-croscopy covered with an amorphous carbon film were used asthe working electrode (C/Au, Electron Microscopy Sciences). Thefunctionalized silane modified C/Au electrode was prepared by

(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. B1997, 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.

(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.

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a sol-gel procedure using 0.5% bis[(3-triethoxysilyl)propyl]tetrasulfide (SIS, Petrarch Systems) in an ethanol solution using0.1 M HCl as catalyst (SIS:H2O ) 1:8, molar ratio). After standingovernight, an aliquot (0.5 µL) of this solution was cast on theC/Au carbon film surface, and dried overnight before use. Thismodified electrode was denoted as SIS/C/Au.

Platinum particles were grown on the C/Au and SIS/C/Ausurfaces from a 0.1 M H2SO4 solution containing 1.0 mM PtCl4(Aldrich) by a pulsed potentiostatic method. The potential steppedfrom +600 to -200 mV vs Ag/AgCl with pulse width of 200 ms.All experiments were conducted at room temperature in aspecially designed electrochemical cell using a platinum wirecounter electrode and a Ag/AgCl reference electrode. Only thecarbon or SIS-modified carbon face of the Au grid electrodecontacted the solution.

The platinum particles grown on the carbon and SIS modifiedcarbon surfaces were observed by TEM (Philips TEM201) andanalyzed by EDS (JEOL JSM-5800 LV Scanning Microscope).

Results and DiscussionPulsed Potentiostatic Deposition. Figure 1 shows

typical current-time transients for a 200-ms pulse in the1.0 mM PtCl4, 0.1 M H2SO4 plating solution on the C/Au(a) and SIS/C/Au (b) electrodes. Similar to electrodepo-sition of Pt on basal plane-oriented graphite surfaces,6i

the current-time behavior on C/Au indicated an instan-taneous nucleation and diffusion-limited growth mech-anism for the deposition of platinum on the C/Au surface.On the SIS/C/Au surface, the initial current delay may bedue to a pseudocapacitive charge transport processes7

within the SIS xerogel thin film upon changing theelectrode potential. The current decayed with t-1/2 andapproached that given by the Cottrell equation, indicativeof a process controlled by planar diffusion. Our resultsare consistent with those for the electrodeposition of metalson polyaniline in its reduced state7 (i.e. at potentials wherethe film is electronically insulating), where the kineticsof nucleation were described adequately by an “instan-taneous nucleation”.

The current density on SIS/C/Au is lower than that onC/Au during all the deposition process, indicating a smalleramount of platinum involved during the electrodeposition

on SIS/C/Au surface, which is reasonable considering theinsulating properties and the blocking effect of the SISsol-gel thin film. EDS analysis results show that theplatinum content is higher on Pt/C/Au than that on Pt/SIS/C/Au, indicating larger amount of platinum depositedon C/Au. Formation of the initial nuclei is a crucial stepduring the electrodeposition process.8 Substrates withdifferent surface free energy influence the early stage ofgrowth of electrodeposited metal particles.5b,6a The dif-ference in surface free energy on C/Au and SIS/C/Ausurfaces results in different nucleation and growth kineticsfor platinum electrodeposition. Differences in kineticsresult in a different microstructural morphology for thedeposited platinum on these substrates.

Microstructures by TEM. Figure 2 shows the TEMimages and the histogram of platinum particles elec-trodeposited on C/Au surfaces. It is clear that polytetra-hedral crystals are formed on the amorphous carbon films.This is distinct from the previously reported spherical orgranular platinum particles5 on carbon materials (mostlypyrolytic graphite or HOPG) or conducting polymers (PPy,PAn, ...). The nucleation behavior and the growth mech-anism of a metal on an inert substrate are dependent onthe overpotential and the interaction between the metalcrystallites and the substrate.6a The surface free energyand microstructure of amorphous carbon deposited on Augrid differ from those of pyrolytic graphite surfaces; thelatter usually have a lower surface energy and a well-defined geometric structures.6b The heterogeneous char-acter of an amorphous carbon surface may promote therandom nucleation of platinum on active sites, and furthergrowth of platinum clusters on these active sites.

The crystallographic habit of gold particles grown oncarbon electrodes at different potentials has beenreported.5b,c The crystal characteristics were related tothepotential-dependentsurface reconstructionofAu (111),Au (110), and Au (100) surfaces. Platinum colloidal crystalswith different shapes have been produced in polymericsolutions.3d,e,f Oriented surfaces have been shown3e toincrease selectivity and reactivity in certain catalyticreactions. The faceted platinum particles on amorphouscarbon could be exploited for applications in electro-catalysis.

Figure 3 shows the TEM images and the histogram ofplatinum particles electrodeposited on SIS/C/Au surface.Compared with the unmodified C/Au, there are significantdifferences on the SIS/C/Au. The platinum particles onSIS/C/Au are much smaller and highly dispersed. Themean diameter of platinum particles is 17 nm on SIS/C/Au surface, while 138 nm on C/Au surface. The particlenumber density is about 1 order of magnitude higher onSIS/C/Au (1010 cm-2) than that on C/Au (109 cm-2). A closerexamination of the morphology of the particles revealedthat spongelike lateral 2D growth of dendritic platinumparticles are found on SIS/C/Au, in comparison to thecompact 3D platinum crystals grown on C/Au. Further-more, the 2D “cauliflower” platinum aggregation on SIS/C/Au consists of smaller primary particles with diameteraround 3-5 nm. There coexist primary particless“monomer”, “dimer”, “trimer”, “tetramer”sand 2D flake-like “polymer”. As is evident from the histogram, thereare two peaks on the size distribution, a main peak sittingaround 10-15 nm which consists of more than 60% of thetotal particles and a shoulder sitting around 35 nm whichconsists of about 15% of the total particles. This may reflectthat 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 the1.0 mM PtCl4, 0.1 M H2SO4 plating solution on the C/Au (a) andSIS/C/Au (b) electrodes. Potential was stepped from +600 to-200 mV vs Ag/AgCl.

760 Langmuir, Vol. 15, No. 3, 1999 Guo and Guadalupe

and/or thermokinetically favorable during the electrodepo-sition on the surface. Experiments to synthesize particleswith controlled size distribution are being pursued.

These nanometer-scale spongelike structures have alarger surface area that is an ideal prerequisite fortechnological applications as catalysts. Instead of the 2Ddendritic structure shown here, a 3D-related morphologywas found for zinc and palladium colloids entrapped inblock copolymer micelles.9 Primary particles (about 10nm) aggregated into stable cauliflowerlike superstruc-ture. These superstructures do not affect the optical and

electronic properties of the small primary particles, suchas band gap, exciton frequency, and resonator strength.9Here we provide a good opportunity to study suchproperties for supported 2D dendritic superstructure, aswell as their catalytic and electrocatalytic performance.Compared with colloidal particles stabilized, i.e., cappedwith ligands, where particle stabilization is generallyaccompanied by passivation, SIS/C/Au-supported Pt offersthe possibility of particle stabilization without passivationor with only partial passivation. We anticipate that thechemical interaction between sulfur domains and plati-num enables higher stability to the particles. Electro-chemically synthesized platinum particles on basal planeoriented graphite were reported as weakly physisorbed

(9) Antonietti, M.; Goltner, C. Angew. Chem., Int. Ed. Engl. 1997,36, 910

b

a

Figure 2. TEM images and for Pt/C/Au surfaces at low andhigh magnifications and the corresponding histogram.

Figure 3. TEM images for Pt/SIS/C/Au surfaces at low andhigh magnifications and the corresponding histogram.

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on the graphite surface so that conventional AFM andSTM imaging would wipe the particles from the surface.6i

Proposed Deposition Mechanism. As stated above,the heterogeneous character of amorphous carbon filmspromote a random nucleation and 3D crystallization of Ptparticles on the C/Au surface. Meanwhile, on the SIS/C/Au surface, sulfur domains might form during sol-gelmodification, which could template the nucleation and2D growth, probably assisted by some affinity betweenthe sulfur domain and the platinum. To further under-stand the 2D dendritic growth (aggregation) of electrode-posited platinum on SIS/C/Au surface, the electrochemicalbehavior of K3Fe(CN)6 was studied with the SIS sol-gelmodified electrode. Figure 4 shows the cyclic voltammo-grams of the modified electrode in a blank electrolytesolution (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 5µA) and the potential separation of the redox peaksincreased substantially (from 100 to 550 mV) on the SISsol-gel modified carbon surface, indicating a large block-ing effect of the insulating sol-gel thin film.10 However,the redox process on the modified surface indicated thatthe micropores in the SIS sol-gel thin film are accessiblefor the redox species to reach the carbon surface. Thevoltammetric behavior resembles that of a microelectrodearray.11 Deviation from the ideal sigmoidal ultramicro-electrode behavior may be due to the nonuniformity of thesize and spacing of the micropores on the SIS/C/Au surface.This feature may contribute to the formation of the 2D-dendriticplatinumaggregation.12 Oneparticleaggregationmight form around/above one (ultra)microelectrode. Thecoexistence of “monomer”, “dimer”, “trimer”, ..., and

“polymer” morphologies may reflect different geometricmicrostructures, surface free energies and microporositiesof the SIS sol-gel matrix. From the kinetics of ferricyanideon the bare carbon electrode and SIS xerogel film modifiedcarbon electrode, the fractional porosity or electroactivearea can be estimated from the equation:11a κapp ) κo(1 -θ). κo is the heterogenerous electron-transfer rate constantof ferricyanide on the bare carbon electrode, and κapp isthat on SIS xerogel film modified electrode; θ is the surfacecoverage of SIS xerogel film. Therefore, the uncoveredelectrode surface or fractional porosity (1 - θ) can beestimated to be 0.62%. It indicated that only a very smallportion of the overall electrode surface is available for theelectrodeposition of Pt particles. A simple scheme reflect-ing the above factors is proposed in Figure 5 for theplatinum deposition on C/Au and SIS/C/Au surfaces.

Conclusions

Electrodeposition of platinum particles on amorphouscarbon surface and sol-gel modified carbon surface ongold grids have been performed. TEM and EDS charac-terization provide direct information on the microstructureand morphology of the deposited platinum particles.Polytetrahedral platinum crystals were formed on theC/Au surface, which showed oriented crystallization.Meanwhile 2D dendritic cauliflower-like platinum ag-gregations consisted of smaller primary particles werefound on SIS/C/Au surface. This structure possesses ahigh surface area, a property that is advantageous for thepreparation of highly active noble metal catalysts at areduced cost.

Acknowledgment. This project is funded by DOE-EPSCoR (046138). Special thanks to Eng. Camilo Cangani,Research Centers in Minority Institutions (RCMI, PR-03051), and Janet Figueroa and Gabriel Cruz, MaterialsCharacterization Center (UPR-Rı́o Piedras) for their helpwith the TEM and EDS experiments, respectively.

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(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.

Figure 4. Cyclic voltammograms on SIS sol-gel modifiedelectrode in 0.4 M KCl solution without (a) and with 5 mMK3Fe(CN)6 (b). Scan rate: 20 mV/s.

Figure 5. Proposed nucleation and growth mechanism forelectrodeposition of platinum on C/Au and SIS/C/Au surfaces.

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