Preparation and Characterization of Silane-Stabilized, Highly Uniform

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Preparation and Characterization of Silane-Stabilized, Highly Uniform, Nanobimetallic Pt-Pd Particles in Solid and Liquid Matrixes Lawrence D’Souza and S. Sampath* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India Received April 3, 2000. In Final Form: August 1, 2000 Highly uniform, stable nanobimetallic dispersions are prepared in a single step in the form of sols, gels, and monoliths, using organically modified silicates as the matrix and the stabilizer. The Pt-Pd bimetallic dispersions are characterized by UV-vis, TEM, SEM, and XRD measurements. The evolution of silicate was followed by IR spectroscopy. XPS and CO adsorption studies reveal that the structure of the particles consists of a palladium core and a platinum shell. Electrocatalysis of ascorbic acid oxidation has been demonstrated using thin films of silicate containing the nanobimetal particles on a glassy carbon electrode.

Introduction Nanobimetallic particles, comprising two different metal elements, are of considerable interest from both scientific and technological points of view.1-12 The catalytic properties and the electronic structure of the nanomaterials can be tailored by changing the composition and structure.13-17 For example, unique surfaces are obtained whenever a second metal is added to a conventional metal catalyst. This may create a new property that may not be achieved by the monometallic components. Most of the reported literature on the stabilization of metallic particles has been limited to aqueous solutions in the presence of surfactants or water-soluble polymers as stabilizers. There have been recent attempts to use other matrixes as well.18-27 For example, long chain * To whom correspondence should be addressed. E mail: [email protected]. (1) Sinfelt, J. H. Bimetallic catalystssDiscoveries, concepts and applications; John Wiley: New York, 1983. Ponec, V.; Bond, G. C. Stud. Surf. Sci. Catal. 1995, 95, 175. Sinfelt, J. H. Acc. Chem. Res. 1987, 20, 134. (2) Rodriguez, J. A.; Goodman, D. W. J. Phys. Chem. 1991, 95, 4196. (3) Nieuwenhuys, B. E. In The chemical physics of solid surfaces and heterogeneous catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1993; Vol. 6, p 185. (4) Rodriguez, J. A. Surf. Sci. Rep. 1996, 24, 223. (5) Campbell, C. T. Annu. Rev. Phys. Chem. 1990, 41, 775. (6) Wuttig, M.; Feldmann, B.; Flores, T. Surf. Sci. 1995, 331-333, 659. (7) Hubbard, A. T. Heterog. Chem. Rev. 1994, 1, 3. (8) Attard, G. A.; Price, R. Surf. Sci. 1995, 335, 63. (9) Trasatti, S. Surf. Sci. 1995, 335, 1. (10) Sheu, B.; Chaturvedi, S.; Strongin, D. R. J. Phys. Chem. 1994, 98, 10258. (11) Henglein, A. Chem. Rev. 1989, 89, 1861. (12) Magruder, R. H.; Osborn, D. H., Jr.; Zuhr, R. A. J. Non-Cryst. Solids 1994, 176, 298. (13) Toshima, N.; Wang, Y. Langmuir 1994, 10, 4574. (14) Kolb, O.; Quaiser, S. A.; Winter, M.; Reetz, M. T. Chem. Mater. 1996, 8, 1889. (15) Toshima, N.; Wang, Y. Adv. Mater. 1994, 6, 245. (16) Schmid, G.; West, H.; Halm, J. O.; Bovin, J. O.; Grenthe, C. Chem. Eur. J. 1996, 2, 1099. (17) Schmid, G.; Lehnert, A.; Halm, J. O.; Bovin, J. O.; Grenthe, C. Angew. Chem., Int. Ed. Engl. 1991, 30, 874. (18) Meguro, K.; Nakamura, Y.; Hayashi, Y.; Torizuka, M.; Esumi, K. Bull. Chem. Soc. Jpn. 1988, 61, 347. (19) Ng Cheong Chan, Y.; Schroak, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 24. (20) Schmid, G. Chem. Rev. 1992, 92, 1709. (21) Torigoe, K.; Esumi, K. Langmuir 1992, 8, 59. (22) Schmid, G.; Lehnert, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 780.

surfactants21 such as quarternary ammonium compounds have been used to stabilize metallic particles in organic as well as inorganic solvents. A recent method proposed by Lev and co-workers24 involves the stabilization of nanosized metallic particles using sol-gel-derived aminosilicate matrixes. An inverse micelle was used by Martino and co-workers26 for the stabilization of gold nanoparticles. The disadvantage in this method is that the precursor for the gel was found to destabilize the micelles, resulting in a large particle size of the target metal. Mulvaney and co-workers28 used a two-step process involving the synthesis of metal particles and subsequently coating them with an appropriate aminosilane. Other methods include the use of porous alumina and silica as supports, and reduction of the metal ions was effected by the use of hydrogen at high temperatures.29,30 This was found to yield a wide particle size distribution. The method developed by Schubert and co-workers31 involves the attachment of metal particles to the silane precursors and subsequent polymerization. As for the bimetallic systems, Cu-Ru,32,33 Pd-Ni,34 CuNi,35 Pd-Ag,36 and Co-Pd,37 combinations stabilized in (23) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 801. (24) Bharathi, S.; Fishelson, N.; Lev, O. Langmuir 1999, 15 (6), 1929. (25) Mulvaney, P.; Underwood, S. Langmuir 1994, 10, 3427. (26) Martino, A.; Yamanaka, S. A.; Kawola, J. S.; Loy, D. A. Chem. Mater. 1997, 9, 423. (27) Esumi, K.; Shiratori, M.; Ishizuka, H.; Tano, T.; Torigoe, K.; Meguro, K. Langmuir 1991, 7, 457. Miner, R. S., Jr.; Namba, S.; Turkevich, J. Proceedings of the seventh Int. Congress on Catalysis; Seiyama, Tanabe, Eds.; Kodansha: Tokyo, 1981; p 160. (28) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (29) Ueno, A.; Suzuki, H.; Kotera, Y. J. Chem. Soc., Faraday Trans. 1983, 79, 127. (30) Lopez, T.; Bosch, P.; Moran, M.; Gomez, R. J. Phys. Chem. 1993, 97, 1671. (31) Breitscheidel, B.; Zieder, J.; Schubert, U. Chem. Mater. 1991, 3, 559. (32) Schubert, U.; Breitscheidel, B.; Buhler, B.; Egger, C.; Urbaniak, W. In Better Ceramics Through Chemistry; Symp. Proc., San Francisco, April 27-May 1, 1992; Hampden-Smith, M. J., Klemperer, W. G., Brinker, C. J., Eds.; MRS Proc., 1992; Vol. 271. (33) Schubert, U. New J. Chem. 1994, 18, 1049. (34) Morke, W.; Lamber, R.; Schubert, U.; Breitscheidel, B. Chem. Mater. 1994, 6, 1639. (35) Kaiser, A.; Gorsmann, C.; Schubert, U. J. Sol-Gel Sci. Technol. 1997, 2, 795. (36) Heinrichs, B.; Delhez, P.; Schoebrechts, J.-P.; Pirard, J.-P. J. Catal. 1997, 172, 322.

10.1021/la000496y CCC: $19.00 © 2000 American Chemical Society Published on Web 10/04/2000

Silane-Stabilized Nanobimetallic Pt-Pd Particles

silicate matrixes have been reported. The literature on the Pt-Pd bimetallic system27,38-40 reveals that the preparation conditions employed vary from polymer protection of the bimetallic colloid in aqueous media to laser vaporization of bulk alloys.41 Toshima and coworkers39 reported the preparation of polymer-protected Pt-Pd alloy colloids by simultaneously reducing the metal ions in water-ethanol solutions and studied their structure with the EXAFS technique. Pt-Pd alloy colloids in water in oil microemulsions were prepared by BoutonnetKizling and co-workers42 and further used for studying their catalytic behavior for isomerization reactions. Clusters of Pt-Pd on amorphous carbon were reported by Rousset and co-workers.41 It is desirable to have the bimetallic nanodispersions stabilized and incorporated in a solid matrix. This avoids coagulation and precipitation of the particles as happens in a solution phase. Second, the bimetallic catalysts can be recovered easily after the reaction, and hence elaborate separation procedures are avoided. In this direction, we are reporting the preparation and characterization of very highly uniformly distributed and stabilized nanobimetallic dispersions in silicate matrixes in the form of sols, gels, and thin films. We have extended the method of Lev and co-workers24 to stabilize bimetallic systems. To our knowledge, preparation and stabilization of bimetallic particles of Pt-Pd in silicate matrixes have not been reported. The advantages include the preparation of bimetallic particles in a single-step procedure, a very uniform distribution, small particle size, and the versatility of making the matrix in the form of sols, gels, films, and monoliths. We have exploited the method of sol-gel processing of materials and the inherent advantages of the organically modified silicates (Ormosils) to synthesize and stabilize nanobimetallic particles. The sol-gel-derived silicate used in the present studies acts as a solid matrix and an inherent stabilizer as well. We have been able to synthesize a variety of bimetallic combinations of nanoparticles involving Pt, Pd, Ag, and Au. The present paper details the preparation and characterization of Pt-Pd bimetallic particles. This combination is known to show a very high catalytic activity for selective hydrogenation reactions.43 The sol-gel-stabilized particles are characterized by various techniques, and the films containing these particles are used for the electrocatalytic oxidation of ascorbic acid. Experimental Section Chemicals. N′-[3-(Trimethoxysilyl)propyl]diethylene triamine (TPDT) and aminopropyltrimethoxysilane (APS) were obtained from Aldrich, St. Louis, MO. Hexachloroplatinate, palladium chloride, sodium borohydride, methanol, and all other chemicals used were of analytical grade. Double-distilled water was used. Apparatus. UV-visible spectra were recorded using a Hitachi 3000 spectrophotometer. The samples were in the form of sols or thin or thick films on glass or quartz substrates. TEM was carried out using a JEOL 3010 model operating at 300 kV. (37) Guczi, L.; Schay, A.; Stefler, G.; Mizukami, F. J. Mol. Catal. 1999, 141 (1-3), 177. (38) Harada, M.; Asakura, K.; Ueki, Y.; Toshima, N. J. Phys. Chem. 1992, 96, 9730. (39) Toshima, N.; Harada, M.; Yonezawa, T.; Kushihashi, K.; Asakuri, K. J. Phys. Chem. 1991, 95, 7448. (40) Wang, Y.; Toshima, N. J. Phys. Chem. B 1997, 101, 5301. (41) Rousset, J. L.; Renouprez, A. J.; Cadrot, A. M. Phys. Rev. B 1998, 58 (4), 2150. (42) Touroude, R.; Girard, P.; Maire, G.; Kizling, J.; BoutonnetKizling, M.; Stenius, P. Colloids Surf. 1992, 67, 9. (43) Toshima, N.; Kushihashi, K.; Yonezawa, T.; Hirai, H. Chem. Lett. 1989, 1769.

Langmuir, Vol. 16, No. 22, 2000 8511 Samples were prepared by evaporating a drop of the sol on a copper grid. Polymerization proceeded during evaporation of the solvent, and hence, the pictures reflect the distribution of metallic particles in the solid silicate samples. FT-IR spectra were recorded using a Bruker Equinox 55 IR model. SEM images of the solid silicate samples containing metallic particles were taken using a JEOL JSM-840A scanning microscope. XRD analyses were performed using a Shimadzu XD-D1 model operated at 30 kV and 30 mA. CO adsorption measurements were carried out by purging the sols containing the desired metal particles followed by equilibration for about 30 min. A few drops of the sol were placed on a KBr pellet and allowed to dry before the spectra was taken. X-ray photoelectron spectra were recorded on an ESCA-3 Mark II spectrometer (VG Scientific, U.K.) using Al KR radiation (1486.6 eV). Binding energies were calculated with respect to C(1s) at 285 eV. The analyses were performed on pellets of 8 mm diameter at a vacuum level of 10-9 Torr. Etching of the samples was carried out using Ar+ ion sputtering at 6 kV. Electrochemical measurements were carried out using an electrochemical system (CH 660A, CH Instruments, Austin, TX). Preparation of Bimetallic Dispersions. A typical preparation procedure for a bimetallic colloid containing a 100:1 molar ratio of the precursor and the metal salts is as follows: 130 µL of TPDT was added to 3.8 mL of methanol followed by 50 µL of H2O and 50 µL of 0.1 M HCl. This mixture was shaken for a few minutes. Hydrolysis of the silane precursor was initiated during this period. Five hundred microliters each of 0.01 M H2PtCl6 and 0.01 M PdCl2 were subsequently added to the silica sol. The mixture was found to be very homogeneous. After a few minutes, sodium borohydride (0.005 g) was added with vigorous stirring, and the reduction was followed by measuring the UV-vis absorbance of the sol. Various compositions of the Pt and Pd containing molar ratios of 1:1, 0.75:0.25, 0.5:0.5, and 0.25:0.75 of the salts were prepared using the same protocol as described above. Control experiments were carried out with individual metal colloids and also the physical mixture of previously formed individual nanometal colloids. In all the above preparations, no phase separation or segregation was observed. The color of the bimetallic sols ranged from less intense brown to intense brown depending on the Pt/Pd content. Individual colloids were light and dark brown for Pt and Pd, respectively. Preparation of Thin Films, Gels, and Thick Films. Thin and thick films of 0.1-10 µm could be cast on glass slides by the dip coating process. Gels and monoliths of a desired shape were obtained by allowing the solvent to evaporate. The dried material was found to shrink considerably, but slow evaporation of the solvent led to crack-free monoliths.

Results and Discussion An aminosilicate matrix is used to stabilize bimetallic platinum and palladium particles. The NH2 groups associated with the silane precursors are capable of stabilizing nanometallic particles. The general scheme of the sol-gel process and the reduction of the metal ions may be represented as

1. UV-Vis Spectroscopy. The absorbance spectra of the sols containing different compositions of the bimetallics are given in Figure 1. The measurements are carried out after ensuring complete reduction of metal ions based on

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Figure 1. Absorbance spectra of the TPDT-stabilized Pt-Pd sol: (A) Pt; (C) Pd; (B and D) Pt-Pd bimetallic colloids of molar ratios 0.75:0.25 and 0.25:0.75 of the Pt and Pd salts, respectively. Inset I: Absorbance spectra of 5 mM H2PtCl6 (G) and 2 mM PdCl2 (H) in methanol. Inset II: Absorbance spectra of the TPDT-stabilized bimetallic colloid (E) and the physical mixture of the same composition (F). The molar ratio of the silane to metal salts is 100:0.5:0.5.

the kinetics of reduction of individual components. Complete reduction of Pt(IV) and Pd(II) ions under the conditions employed in the present studies takes place in about 25 and 10 min, respectively. The absorbance behavior of the metallic colloids is found to be very similar to that with either of the silanes as stabilizer. Figure 1 represents the UV-visible spectra of platinum (A) and of palladium (C) colloids. The absence of peaks at 460 and 378 nm characteristic of unreduced Pt(IV) (1G), and at 440 and 325 nm characteristic of unreduced Pd(II) (1H), indicates complete reduction. The spectra of the individual metallic particles show broad absorption over the entire range, and this conforms to the computed spectra as given by Creighton and Eadon.44 The absorption behavior of the Pt-Pd bimetallic colloids (Figure 1B, D, and E) containing different compositions (0.75:0.25, 0.25: 0.75, and 0.5:0.5 molar ratios of the salts) is found to be different from the absorbance of individual components. These are similar to the reported spectra by Esumi and co-workers in a cyclohexane-chloroform medium and also by Turkevich and co-workers in an aqueous medium,27 suggesting that bimetallic colloidal particles are formed in the silicate matrix as well. To confirm the bimetallic nature of the particles, a physical mixture is prepared from the already prepared individual components and its absorbance is compared to that of a bimetallic Pt-Pd colloid of the same composition. It is found that the absorbance spectrum of the bimetallic colloid (0.5:0.5 molar ratio of H2PtCl6 and PdCl2) is different from that of the physical mixture of the same composition (Figure 1E and F). The change in the absorbance spectra of the bimetallic colloid from those of individual components can be primarily attributed to the change in dielectric function with mixing different metal atoms.45 The formation of bimetallic dispersions depends on the kinetics of reduction of individual components, and this in turn depends on the stability constants of the metalamine complexes. The stability constants of Pt-amine complexes are much higher than those of the corresponding Pd-amine complexes. The reported log β of the Pd(en) complex is 26.946 while that of the Pt(en) complex is 36.5.47 (44) Alan Creighton, J.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87 (24), 3881. (45) Torigoe, K.; Nakajima, Y.; Esumi, K. J. Phys. Chem. 1993, 97, 8304. (46) Meller, D. P.; Maley, L. Nature 1948, 161, 436. (47) Smith, R. M.; Martell, A. E. Critical Stability Constants, Vol. 2, Amines; Plenum Press: New York, 1975.

D’Souza and Sampath

The log β of the dien complex of Pd is reported to be 8.5,46 and that of the corresponding platinum analogue may be expected to be higher. On the basis of these values, it may be concluded that the ease with which palladium ions can be reduced will result in the stabilization of palladium particles by the amino groups of the silicate followed by that of platinum metal particles. This will result in a structure having a palladium core and a platinum shell. Indeed, this is observed on the basis of XPS studies and CO adsorption measurements, as will be explained later. The bimetallic dispersions prepared using tetraethoxysilane or methyltrimethoxysilane or trimethylamine as the stabilizer are not stable, and the sols immediately destabilize and a precipitate is formed. This brings out the dual role of aminosilanes as stabilizers of nanobimetallic particles. The presence of either one of the two functional groups, SiOH or NH2, is found to be insufficient to stabilize the nanoparticles. It is possible that the amino groups stabilize the nanobimetallic particles while the SiOSi and SiOH form a network surrounding the metallic particles. Additionally, the use of long alkyl chaincontaining matrixes is expected to stabilize the nanoparticles by keeping them far apart and thereby preventing coagulation. Mulvany and co-workers48 have reported the formation of a silica shell around gold nanoparticles prepared using silanes. Lev and co-workers24 postulated that silanes form a network around the nanometallic particles of gold, silver, and palladium. These studies indicate that the silicate polymer forms a shell around the nanometallic particles. The stability of the mono- as well as bimetallic colloidal dispersions is found to be very good. The particle size does not change with time. It is observed that the absorbance spectra of the sol immediately after preparation and after several months of storage are indistinguishable. This is true for both TPDT- and APS-stabilized metallic particles. No precipitation is observed even after prolonged storage. The sol-containing nanometallic particles can be gelled and subsequently dried to yield monoliths. Desired shapes in the form of thin and thick films of micron sizes (up to several tens of microns) and also very thick monoliths (of millimeter thickness) can be prepared. Films can be prepared by either a spin or dip coating technique. Typical absorbance spectra of a silicate film (0.858 mm thickness) containing nanobimetallic particles of composition 1:1 molar ratio of the salts are nearly the same as that of the sol (figure not shown). Addition of tetraethoxysilane to the sol decreases the cross-linking time but does not change the absorbance spectra. The spectra remained constant over several months, showing the excellent stability of the particles incorporated in the solid silicate matrix. 2. TEM Studies. The surface plasmon absorption is known to be affected by the particle size, the refractive index, and the viscosity of the medium, and hence, it is not possible to determine the particle size on the basis of the optical characteristics alone.25 A typical TEM image of a 100:5 molar ratio of APS and the metal salts (the molar ratio of the precursors of Pt-Pd is 1:1) shows (Figure 2) that the particle size distribution is very uniform and the shape is nearly spherical and very uniform. The average diameter of the particles ranges between 1 and 3 nm with the maximum close to 2 nm. This is shown in the histogram of particle size distribution (inset, Figure 2). The standard deviation calculated on the basis of Figure 2 works out to be 0.9 nm (N-2 ) 5). The TEM images of the dispersions obtained from both the TPDT and APS (48) Liz-Marzan, L. M.; Giersig, M.; Mulvany, P. Langmuir 1996, 12, 4329.


Langmuir, Vol. 16, No. 22, 2000 8513

Figure 3. FT-IR spectra of CO adsorbed on the TPDT-stabilized metallic particles: Pt (A); Pd (B); Pt-Pd bimetal of molar ratio 0.5:0.5 (C), 0.66:0.33 (D), and 0.25:0.75 (E).

Figure 2. TEM picture of the APS-stabilized Pt-Pd bimetallic colloid. The molar ratio of silane to metal salts is 100:5:5. The bar indicates 20 nm. The standard deviation is 0.9 nm (N-2 ) 5). The inset shows the particle size distribution.

precursors are found to be very similar. However, the average particle size is marginally larger in the APS matrix than in the TPDT matrix. Typically, the maximum in the histogram is found to be ∼2 nm for APS and ∼1.5 nm for TPDT stabilizer. The homogeneity and the distribution of the particles are found to be better in the TPDT matrix than in the APS matrix. However, the particle size distribution remains the same for different compositions of the Pt-Pd bimetallic colloids (0.75:0.25, 0.25:0.75, and 0.5:0.5 molar ratios of the salts) provided the same stabilizer is used. The uniform distribution of these particles in the silicate matrixes leads to a good catalyst, as exemplified in the use of these materials for the electrocatalysis of ascorbic acid oxidation. 3. Structure of Nanobimetallic Particles. a. CO Adsorption Studies. One of the powerful methods, by which the structure of bimetallic particles is followed, is based on CO adsorption on different sites. The resulting IR spectrum would enable us to decipher the core-shell structure of the bimetallic particles. The application of this method for the Pt-Pd system is well characterized in the literature.40 The IR spectra of the CO adsorbed on nanometallic colloids stabilized in a TPDT matrix are shown in Figure 3. A strong absorption band at 1965 cm-1 and a relatively less intense peak at 2030 cm-1 are observed when palladium monometallic particles are used (Figure 3B). The former is assigned to the CO adsorbed on the metal surface at the bridging site (bridging adsorption band), and the latter one corresponds to the CO adsorbed at a terminal site40 (linear adsorption band). It is reported that the relative intensities of these two peaks vary depending on the particle size.49 It is also shown (49) Bradley, J. S. In Clusters and colloids; Schmid, G., Ed.; Weinheim: Germany, 1994; p 510.

that amorphous Pd clusters of 1 nm size prepared by metal vapor deposition give a very strong linear absorption band.49 The polymer [poly(N-vinyl-2-pyrrolidone)] protected metal clusters are shown to exhibit identical absorbance spectra, and the features do not change in the particle size range 1-6 nm. The IR of the CO adsorbed on Pt nanometallic particles in a TPDT matrix shows an intense band at 2036 cm-1 (Figure 3A). This band is assigned to CO adsorbed in a linear type on the Pt surface. The spectra observed with bimetallic clusters of varying compositions are shown in Figure 3C, D, and E. A strong absorption band at 2037 cm-1 is observed for the CO adsorbed on the bimetallic colloid prepared using a 0.5: 0.5 molar ratio of the salts of Pt:Pd (Figure 3C). This reveals that the shell of the bimetallic cluster is made of platinum and the core is made of palladium metal. Figure 3D corresponds to a molar ratio of 0.65:0.35 of the Pt and Pd salts, respectively. The amount of Pt is higher than that of Pd, and hence, the formation of a bimetal with a similar structure of a Pd core and a Pt shell is favored. Indeed, this is observed in the IR spectra where only the CO adsorbed on platinum metal is shown. These results are in conformity with the conclusions based on absorbance measurements where the palladium(II) ions are found to get reduced faster than the platinum(IV) ions. The relative kinetics probably leads to the stabilization of palladium metal particles by the amino group of the silicate, and subsequently the platinum particles cover the palladium core. The bimetallic colloid prepared using a 0.25:0.75 molar ratio of the salts of Pt:Pd results in bands at 2026 and 1961 cm-1. This may be due to both metallic components, since the amount of Pt is insufficient to completely cover the Pd surface. The IR spectra of CO adsorbed on APS-modified metallic colloids are the same as those of TPDT-modified Pt-Pd bimetallic clusters. The Pt-Pd bimetallic clusters stabilized by poly(Nvinyl-2-pyrrolidone) have been shown to have similar structures to that of the present material, depending on the preparation conditions.40 It is interesting to compare the core/shell structure of the bimetallic clusters prepared by simultaneous reduction of Pd(II) and Pt(IV) ions in a

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Figure 4. XPS of a TPDT-stabilized Pt-Pd bimetallic colloid prepared from Pt and Pd metal salts of molar ratio 0.65:0.35.

Figure 5. FT-IR spectra of a TPDT-stabilized (A) and APSstabilized (C) Pt-Pd bimetallic colloid of composition 0.5:0.5. B and D correspond to the neat TPDT and APS precursors.

water-ethanol mixture with the present nanoclusters stabilized using silicates. The EXAFS and IR-CO probe studies of the bimetallic clusters in ethanol-water mixtures led to a structure having a platinum core and a palladium shell.39 The observed IR of the adsorbed CO was different for different compositions, the 1:1 molar ratio of the metal composition (as used in the present studies) showed bands at 2050 and 1906 cm-1, and the authors attributed this observation to be due to a varied surface composition of the clusters. In the silicatestabilized Pt-Pd bimetallic clusters, however, the shell consists totally of platinum particles for the 1:1 molar ratio. Hence, it can be concluded that the preparation procedure plays a large role in determining the structures of bimetallic clusters. b. XPS Studies. A typical XPS spectrum of the bimetallic sample, prepared from a 0.65:0.35 molar ratio of the Pt and Pd salts, after 35 min of etching, is given in Figure 4. As-prepared Pt-Pd bimetallic clusters do not show the presence of either platinum or palladium. This is due to the silicate network surrounding the metallic particles as explained above. Similar observations are reported for polymer-protected metal colloids.50 Once the silicate coating is removed by etching the sample, presence of metallic particles is observed. Peaks corresponding to platinum 4f7/2 and 4f5/2 levels are observed at 72.5 and 75.7 eV binding energies, respectively, after 10 min of etching. Small humps at 71.1 and 74.2 eV are also observed. These two sets of peaks are assigned to Pt2+ and Pt0 states, respectively. However, no palladium peak is observed in the first 30 min of etching. Palladium peaks due to 3d5/2 and 3d3/2 levels along with platinum 4f levels appear after 35 min of etching. Palladium peaks corresponding to the 3d levels are found to occur at 335 and 340 eV, and the data suggest that palladium is in the metallic form. Hence, it is confirmed from XPS measurements that platinum is the shell and Pd is the core of the bimetallic particles stabilized in silicate matrixes. Further, Pt is present in the oxidized form and the oxidation state of +2

Table 1. Assignment of FT-IR Peaksa

(50) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 22, 1179.

TPDT TPDT-BIM

APS

APS-BIM

assignmentsb

3280 2938 2838 1583 1467 1191

3359 2935 2842 1571 1479 1191

3369 2931 2871 1660 1384

ν(1° N-H) sym ν(CH2) asym ν(CH3) sym of SiOCH3 δ(NH2) δ(CH2) F(Si-O-CH3) ν(Si-O-Si) ν(C-O) sym ν(Si-OH) ν(CH2)

1056 935 821

3391 2934 2884 1665 1438 1129 1035 780

1058 923 813

1128 1035 781

a

References: Langmuir 1997, 13 (15), 3921-3926. Langmuir 1999, 15 (6), 1929-1937. Pouchert, C. J. The Aldrich Library of FT-IR spectra; Aldrich Chemical Co., Inc.: Milwaukee, WI, 1985. Silverstien, R. M.; Bassler, C.; Morrill, T. C. Spectroscopic identification of organic compounds, 3rd ed.; John Wiley & Sons, Inc.: New York, 1974. b ν ) stretching; δ ) deformation; F ) rocking.

suggests that PtO is formed on the surface while Pd is present in the metallic form as the core. The detailed XPS study of various bimetallic systems stabilized in silicate matrixes will be reported elsewhere. 4. FT-IR Studies. FT-IR studies are carried out to follow the evolution of silicate during the stabilization of metallic particles. Figure 5 shows the FT-IR spectra of TPDT- and APS-stabilized nanobimetallic particles. The spectra of neat silanes are also given for comparison purposes. All the spectra are taken for the gelled and dried silicate materials. Hence, these data represent the state of the completely polymerized silicate network containing metallic clusters. The peak assignments given in Table 1 agree well with the available data in the literature for silicate evolution.24 The prominent features of the spectra are the following: The unhydrolzed TPDT shows sharp bands at 2838 and 1191 cm-1 that are characteristic of methoxy groups. The bands corresponding to methoxy groups occur at 2842 and 1191 cm-1 for APS. In the crosslinked, metal-loaded silicate, these bands completely disappear, showing that the hydrolysis is complete. The bands appearing at 1129 and 1128 cm-1 for TPDT and APS matrixes confirm the presence of siloxane groups (SiOSi) in the polymerized material. It is reported by


Figure 6. FT-IR spectra of solid, TPDT-stabilized Pt-Pd bimetallic particles. Regions corresponding to N-H stretching and bending modes are shown. A and A′ correspond to pure silicate; B and B′, C and C′, and D and D′ correspond to 1, 2, and 4 mole % Pt and Pd with respect to silicate.

Buining and co-workers51 that aminosilane coating of mercaptosilane-modified gold clusters exhibits a sharp peak at 1115 cm-1 and this is attributed to the formation of a siloxane network. The SiO stretching that results from the uncondensed silanol groups is nearly absent in the TPDT matrix while it is observed to be very weak at 928 cm-1 for the APS matrix. These uncondensed silanol groups are quantified by pKa and impedance measurements and will be reported elsewhere. The band appearing at around 1600 cm-1 corresponds to the primary amine [δ(NH2)] of the silane. The position of this band shifts to higher wavenumbers as the silane is cross-linked and polymerized. The shift is from 1583 to 1665 cm-1 for neat TPDT (liquid) to polymerized TPDT (solid) and from 1571 to 1660 cm-1 for neat APS to polymerized APS. Further NH stretching bands around 3500 cm-1 are broadened in the cross-linked material. Similar broadening along with reduction in intensity is reported for the amine-capped gold colloids.52 The complexation of amine groups of silicates with metal particles, leading to their stabilization, has already been reported in the literature.24,53 IR spectral studies are carried out to decipher the interaction of metal particles with the amine groups of TPDT. Figure 6 shows the NH stretching and bending regions of TPDT-stabilized bimetallic particles with different metal loadings with respect to silicate. Completely cross-linked solid samples are used for this study. A systematic shift of the absorption peak from 3396 to 3348 cm-1 and from 1610 to 1587 cm-1 is observed with an increase in the loading of bimetal from 1 to 4 mol %. This constant shift toward a lower wavenumber region, reduced intensity, and peak broadening with an increase in metal loading is attributed to the binding of the NH2 group to metal particles. Intramolecular hydrogen bonding with the hydrogen of the silanol groups would also contribute to the above observations. 5. XRD and SEM Studies. Powder X-ray analysis is carried out to ascertain the nature of metallic particles. The XRD spectra of Pt, Pd, and Pt-Pd (1:1 molar ratio) (51) Buining, P. A.; Humbel, B. M.; Phillose, A. P.; Verkleji, A. J. Langmuir 1997, 13, 3921. (52) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (53) Puddephatt, R. J. The Chemistry of Gold; Elsevier: Amsterdam, 1978.

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bimetallic clusters result in very broad peaks (not shown). The peaks become sharp and the intensity increases when the samples are heated to ∼300 °C for a few hours. The diffractammograms are carried out using dried, powdered gels. The XRD patterns are observed to be similar for both TPDT- and APS-stabilized metallic particles. The observed reflections at ∼38° and ∼45° (2θ) correspond to (111) and (200) surfaces. The 2θ values observed for the bimetallic cluster are different from those of the monometallic components. The (200) surface that shows sharper peaks than the (111) surface occurs at an intermediate value of 2θ ) 45.46 for the bimetal compared to the individual components of 45.58 for platinum and 44.84 for palladium. It is difficult to compare the (111) reflection, since the peaks are broad. It is reported54 that the poly(N-vinyl-2-pyrrolidone)-protected bimetallic Pt/Pd clusters show very broad peaks depending on the composition. The particle size calculated on the basis of the peak width at half-maximum corresponds to 3.2 nm for Pt, 2.9 nm for Pd, and 3.5 nm for the Pt/Pd bimetal. These values are slightly larger than the particle size deduced from TEM micrographs. The XRD pattern is found to be very similar for various bimetal compositions of Pt and Pd. The SEM picture of the bimetallic clusters is given in Figure 7. The corresponding elemental mapping is also shown. This clearly reveals that the metal particles are very uniformly present all over the silicate matrix. The composition of the Pt/Pd salts used is a 1:1 molar ratio. The morphology and the metal distribution remain the same for different compositions of the bimetal and also for different stabilizers used in the present study. 6. Electrocatalysis of Ascorbic Acid Oxidation. The amperometric determination of ascorbic acid (vitamin C) is of much current interest, especially in bioelectrochemistry. It is a primary oxidant in human blood plasma and is present in mammalian brain along with several neurotransmitter amines such as dopamine. It is used as a preservative in food industries and is found in high concentration in some fruits. Many of the fundamental studies on ascorbic acid oxidation have been carried out on metal electrodes.55-57 Preliminary investigations have been carried out to detect ascorbic acid in the solution phase using a film of silicate containing Pt/Pd bimetallic clusters. The films are very sturdy when methyltrimethoxysilane (MTMOS) is copolymerized with TPDT during film formation. Typical preparation of the film involves mixing of 2 mL of methanol, 65 µL of TPDT, 17.5 µL of MTMOS, 125 µL of H2O and 125 µL of 0.1 M HCl. After about 5 min of stirring, 125 µL each of 0.01 M H2PtCl6 and 0.01 M PdCl2 are added followed by the addition of 0.005 g of NaBH4. This sol is later diluted with 4 mL of methanol. Complete reduction is ensured before the formation of the film. Glassy carbon is used as the substrate on which 2 µL of the sol containing Pt/Pd (molar ratio of TPDT/metal salts is 100:0.5:0.5) is spread, and the solvent is allowed to evaporate. Once the solvent evaporates, a stable silicate film containing the metallic particles is formed. The film is heated to 125 °C for a few hours for improved cross-linking. Figure 8 shows the voltammograms of a bimetal-modified glassy carbon electrode (GCE) in phosphate buffer of pH 7.2. The concentration of ascorbic acid used is 4 mM. It is seen that (54) Yonezawa, T.; Toshima, N. J. Chem. Soc., Faraday Trans. 1995, 91 (22), 4111. (55) Rueda, M.; Aldaz, A.; Sanchez-Burgos, F. Electrochim. Acta 1978, 23, 419. (56) Manzanares, M. I.; Solis, V.; Rita, H. de Rossi. J. Electroanal. Chem. 1996, 407, 141. (57) Popa, E.; Notsu, H.; Miwa, T.; Tryk, D. A.; Fujishima, A. Electrochem. Solid State Lett. 1999, 2 (1), 49.

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Figure 8. Cyclic voltammograms of ascorbic acid oxidation at a bimetallic colloid modified GCE at different scan rates of 10 (A), 25 (B), 50 (C), and 75 (D) mV/s, respectively. The concentration of ascorbic acid used is 4 mM. Inset (left): Voltammogram on a bare GCE at a scan rate of 10 mV/s. Inset (right): Oxidation current corrected for the base value versus the square root of the scan rate for the metal-modified GCE.

Figure 7. SEM picture of a TPDT matrix containing a 0.5:0.5 molar ratio of Pt-Pd. The elemental mapping is also shown: Pd (A) and Pt (B).

the oxidation currents start to increase at about -0.1 V versus SCE while the bare glassy carbon electrode (shown as an inset in the same figure) shows an onset of 0.1 V. This shows the electrocatalytic behavior of the bimetalmodified electrodes. The inset on the right side of Figure 8 shows a relationship between the oxidation peak currents and the square root of the scan rate for the modified electrode surface. This is found to be linear, passing through the origin, showing that the process is diffusionlimited through the film on the GCE. Figure 9 shows the steady-state response of the modified GCE at -0.1 V. Each addition is ∼2 mM ascorbic acid. The dilution effect is taken into consideration while the calibration graph is plotted. The response time is rather fast, of the order of

Figure 9. Steady-state response of the bimetallic colloidmodified GCE for ascorbic acid oxidation. Each addition corresponds to approximately 2 mM. Inset: Calibration curve at -0.1 V versus SCE.

a few tens of seconds, and the calibration curve (inset) shows that the linearity is observed up to 12 mM. Conclusions It has been shown that bimetallic particles of Pt and Pd can be prepared and stabilized in a single step using silane


precursors. The sol-gel process used here does not involve a separate stabilizer as in the case of solution-phase preparations of bimetallic particles. Additionally, the size distribution is very uniform and narrow. The size distribution is retained in both the liquid and solid phases. No aggregation or segregation of the particles is observed even after prolonged storage of several months. The silane monomers can play a dual role of stabilizing the nanoparticles as well as forming a solid matrix that would help in the use of these particles for various applications. The metal particles have been shown to be electrocata-

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lytically active toward ascorbic acid oxidation. These stabilized bimetallic particles are being used for other electrocatalysis reactions. The possibility of using various combinations of bimetallic surfaces for surface-enhanced Raman scattering studies is also being explored. Acknowledgment. The authors wish to thank Mr. P. Bera for the XPS studies. Financial support of the DST, New Delhi, India, is gratefully acknowledged. LA000496Y

Preparation and Characterization of Silane-Stabilized, Highly Uniform

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