Thermal Reduction of Pd Molecular Cluster Precursors at Highly

of Palladium and Palladium–Cobalt Nanoparticles on Vulcan XC-72R for the ... Nanostructural Formation of Pd-Co Bimetallic Complex on HOPG Surfac...
0 downloads 0 Views 382KB Size
Download PDF
Langmuir 2004, 20, 8329-8335

8329

Thermal Reduction of Pd Molecular Cluster Precursors at Highly Ordered Pyrolytic Graphite Surfaces Ramonita Dı´az-Ayala, Lisandra Arroyo, Raphael Raptis, and Carlos R. Cabrera* Chemistry Department, University of Puerto Rico, Rı´o Piedras Campus, San Juan, Puerto Rico 00931-3346 Received December 29, 2003. In Final Form: April 28, 2004 Highly ordered pyrolytic graphite (HOPG) surfaces were modified by the adsorption of Pd molecular precursors from solution. Two palladium-containing molecular precursors were studied, a mononuclear one and a trinuclear one, to compare their affinities and distributions at substrate surfaces. To obtain Pd nanoparticles, these neutral molecular precursors were reduced under a hydrogen atmosphere. Thermogravimetric analysis was carried out to establish the behavior of these precursors at various temperatures. Understanding the thermal stability of these compounds is very important to establish the appropriate conditions to form metallic Pd. The modified surface has been characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy; also, the reductive process was monitored by XPS. Remarkable differences were observed between the mononuclear and trinuclear compounds in terms of dispersion, particle size, and homogeneity. The preference of the trinuclear compound was to deposit at HOPG defects, in contrast to that of the mononuclear one, which was agglomeration on all surfaces. After the application of this technique, not only Pd nanoparticles but also Pd nanowires were obtained.

Introduction During the past decade, there has been growing interest in a new class of materials for catalysis with specific chemical and physical properties, different from conventional bulk materials or from atoms, i.e., nanoparticles. The controlled formation of noble metal nanoparticles is useful in many areas of catalysis and plays a central role in the emerging area of nanotechnology. Most studies emphasize the preparation and control of the metal nanoparticle size distribution, because these could change not only the activity but also the selectivity of catalysis. Metal nanoparticles can be prepared by physical and chemical methods. The use of molecular or ionic precursors1-5 is one of the most common chemical methods used to control size distribution and composition. Palladium is used extensively in catalytic combustion,6 hydrogen sensors,7,8 hydrogen storage,9 and methanol oxidation.10 It is very important to understand the stability, formation, and decomposition of Pd species at various temperatures and in various environments, including the substrate. In this work, inorganic compounds [Pd(3,5-Ph2pzH)2Cl2] and [Pd3(µ-Phpz)6] were used as Pd nanoparticle precur* To whom correspondence should be addressed. Phone: (787) 764-0000 ext 4807. Fax: (787) 756-8242. E-mail: ccabrera@ cnnet.clu.edu. (1) Hills, C. W.; Nasher, M. S.; Frenkel, A. I.; Shapley, J. R.; Nuzzo, R. G. Langmuir 1999, 15, 690. (2) Nashner, M. S.; Frenkel, A. I.; Adler, D. L.; Shapley, J. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1997, 119, 7760. (3) Nashner, M. S.; Frenkel, A. I.; Somerville, D.; Hills, C. W.; Shapley, J. R.; Nuzzo, R.G. J. Am. Chem. Soc. 1998, 120, 8093. (4) Pan, C.; Dassenoy, F.; Casanove, M. J.; Phillipot, K.; Amiens, C.; Lecante, P.; Mosset, A.; Chaudret, B. J. Phys. Chem. B 1999, 103, 10098. (5) Toshima, N.; Yonesawa, T. New J. Chem. 1998, 1179. (6) Lyubovsky, M.; Pfefferle, L.; Datye, A.; Bravo, J.; Nelson, T. J. Catal. 1999, 182, 275. (7) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. (8) Walter, E. C.; Penner, R. M. Anal. Chem. 2002, 74, 1546. (9) Zu¨ttel, A.; Nu¨tzenadel, Ch.; Schmid, G.; Chartouni, D.; Schlapbach, L. J. Alloys Compd. 1994, 293, 472. (10) Dickinson, A.; James, D.; Perkins, N.; Cassidy, T.; Bowker, M. J. Mol. Catal. A 1999, 146, 211.

sors. A highly oriented pyrolytic graphite (HOPG) surface was modified by spontaneous deposition of two different metal ion precursors, and chemical reduction was carried out to promote the formation of palladium metallic particles. Changes in chemical or physical properties were analyzed by thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The catalyst electronic properties are affected by their morphology and crystallographic properties, which are in turn affected by the chemical environments of the respective precursor compounds. Thus, differences in the pretreatment resulted in different characteristics. Differences in affinities, morphology, and particle size and dispersion were observed between the two molecular precursors (MPs). The use of these inorganic compounds provides a versatile method to produce metal nanoparticles. After the application of this method, not only Pd nanoparticles but also Pd nanowires were obtained. Experimental Section Thermogravimetric Analysis. Thermogravimetric analyses were performed using a Perkin-Elmer model TGA-7. Samples of 3.0-4.0 mg of the molecular precursors were encapsulated in an alumina sample pan. The samples were heated to a maximum temperature of 1000 °C at 5 °C/min in air. X-ray Photoelectron Analysis. Survey and multiplex XPS spectra were obtained using a PHI 5600ci spectrometer with a Mg KR polychromatic source (15 kV, 400 W). The binding energies were corrected using the carbon contamination peak at 284.5 eV as a reference. Atomic Force Microscopy. A Nanoscope IIIa-Multimode atomic force microscope from Digital Instruments, with a scanning probe microscope controller equipped with a He-Ne laser (638.2 nm) and a type E scanner, was used for the AFM experiments. A standard Si3N4 cantilever was used for contact mode imaging, and the scan rate was 1 Hz. Sample Preparation. Molecular precursors [Pd(3,5-Ph2pzH)2Cl2] and [Pd3(µ-Phpz)6] were prepared following a literature procedure.11 A freshly cleaved piece of HOPG (spi-2) was used as the substrate. The adsorption of the various Pd molecular cluster precursors at exfoliated HOPG was carried out at room temperature by immersing the latter for 12 h in a 1 mM

10.1021/la0364688 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/11/2004

8330

Langmuir, Vol. 20, No. 19, 2004

Dı´az-Ayala et al.

Figure 1. (a) Molecular structure of [Pd(3,5-Ph2pzH)2Cl2] powder and (b) its thermogravimetric analysis under air atmosphere.

Figure 2. (a) Molecular structure of [Pd3(µ-Phpz)6] powder and (b) its thermogravimetric analysis under air atmosphere. dichloromethane solution of the precursor. Afterward, all samples were placed under vacuum for 1 h and then characterized by the methods previously mentioned. The reduction process was performed in a closed tube furnace by raising the temperature to 600 °C at a rate of 5 °C/min under a H2 stream. This temperature was selected because the total degradation of the organic groups occurs below this temperature, as was previously observed in a TGA experiment. Then, the samples were cooled in a H2 atmosphere to 63 °C and then in pure N2 to room temperature before the system was opened. The H2 stream was previously dried over activated molecular sieves and deoxygenated over BASF Catalyst R 3-11.

Results and Discussion Thermogravimetric Analysis. Thermogravimetric analysis was performed for the as-prepared sample to establish its behavior with linear changes in temperature. One of the compounds discussed in this paper is Pd(3,5-Ph2pzH)2Cl2, a palladium mononuclear complex (MP(I)) (Figure 1a) which decomposes in six steps (Figure 1b). A loss of approximately 4.0% of the initial mass, considering that this occurs in the first transition, before 100 °C, can be attributed to an impurity present in the sample. There are three other consecutive losses of mass from 90 to 350 °C, which account for an overall loss of 59.8% in mass. These steps are associated with the thermal decomposition of the organic compounds. A fifth step of mass loss was observed between 350 and 490 °C, corresponding to 19.0%. The final loss at 800 °C was minor, corresponding to 2.6%. The measured residue was 15.0%, close to the theoretical residue for Pd, which was 17.2%. The loss of mass for the palladium trinuclear complex (MP(II)) (Figure 2b) took place in two steps, although a third step could be seen at 800 °C. As discussed below, the latter is associated with PdO decomposition, just as in the

case of the mononuclear compound, and not to decomposition or loss of the organic groups of the compound. Figure 2a shows the molecular structure of MP(II), [Pd3(µ- Phpz)6]. The thermograms, such as that shown in Figure 2b, exhibit a stable temperature range from 0 to 123.0 °C and another from 420 to 590 °C. At 850 °C, the metal exists in its metallic state. In the first two massloss steps, from 123.5 to 309.1 °C, and from 333.8 to 417.1 °C, respectively, the phenyl and pyrazole groups are expelled. MP(II) experiences a loss of more than 50% of its mass prior to reaching 450 °C. The mass becomes constant, and remains so up to about 590 °C. From 590 to 645 °C, a relatively small increase in mass suggests the formation of palladium oxide, PdO. Farrauto and collaborators reported the formation of PdO at less than 600 °C in air.12 After reaching 800 °C, only 21.6% of the sample remains. This value is similar to that calculated for the amount of palladium contained in the sample, 27.1%. The residue is thus proposed to be Pd0. It follows that, to obtain Pd particles, the temperature must reach 850 °C in air. The temperatures reported in the literature for the decomposition of PdO to Pd range widely, from 650 to 800 °C, depending on the support; e.g., PdO on Al2O3 is reduced to Pd0 at 800 °C under H2.12 The limiting temperatures of the various segments in the thermograms cannot be considered to be exactly reproducible. The thermogravimetric method is dynamic, and the system never reaches equilibrium. Hence, the temperatures of the distinctive features on the curve will (11) Baran, P.; Marrero, C. M.; Raptis, R. G.; Pe´rez, S. Chem. Commun. 2002, 1012. (12) Farrauto, R. J.; Lampert, J. K.; Hobson, M. C.; Waterman, E. M. Appl. Catal., B 1995, 6, 263.

Thermal Reduction of Pd Molecular Cluster Precursors

Figure 3. High-resolution XPS 3d features obtained from (a) Pd foil, (b) MP(I)/HOPG, (c) the sample in (b) after H2 reductive treatment; (d) MP(II)/HOPG, and (e) the sample in (d) after H2 reductive treatment.

be dependent, to some extent, on such factors as the rate of heating and the sample size.13 Comparison of both TGA results demonstrates that Pd reduction (decomposition) takes place for MP(I) in the same region as that for MP(II), although, for the mononuclear compound, it was preceded by more steps than it was for the trinuclear compound. XPS Analysis. XPS data were obtained for a blank sample (clean and exfoliated HOPG substrate immersed for 12 h in solvent, CH2Cl2), as well as for a series of samples modified by immersion of the HOPG in Pd molecular precursor solutions and reduced at high temperature under a hydrogen atmosphere. The XPS spectra of the HOPG substrate without Pd molecular cluster precursor (before and after reduction) do not exhibit the photoemission peaks that were attributed to Pd, N, and Cl, which were present on the MP(I)- and MP(II)-modified HOPG surfaces. In the XPS surveys for MP(I) and MP(II) obtained after the hydrogen reductive process, some characteristic peaks disappeared, since the Pd signals moved toward lower binding energies, and in some cases, the intensities decreased. Although we observed in the XPS survey spectra the disappearance of some peaks, when we analyzed the various regions one by one, we observed the presence of some residual N and/or Cl. The corresponding high-resolution XPS Pd 3d spectra are shown in Figure 3. The spectra shown in Figure 3a are characteristic of a clean metallic Pd surface, which we used as a Pd reference. The Pd 3d spectra for HOPG modified with MP(I) and MP(II), before and after reductive treatment (parts b-e, respectively, of Figure 3) exhibit peaks due to Pd2+ and Pd0. During the reductive treatment, all of the organic portions of the molecular precursor decompose, as indicated by the TGA analysis, which is consistent with the XPS spectra, in which the peaks for N 1s and Cl 2p disappear. The spectra exhibited photoemission peaks at binding energies corresponding to O and C, even after the reductive process. However, these are attributed to the HOPG substrate. Our blank XPS spectra for CH2Cl2/HOPG before and after reductive treatment also exhibited these peaks. An increase in O2 signal may suggest some O2 contamination. In the lowresolution XPS spectra, decreases in the intensity of the Pd signals were observed, as well as a tendency of the Pd 3p signal to disappear. This behavior cannot be correlated (13) Ewing, G. W. Instrumental Methods of Chemical Analysis, 5th ed.; McGraw-Hill: New York, 1985; Caption 23, pp 430-443.

Langmuir, Vol. 20, No. 19, 2004 8331

with the behavior of the O 1s photoelectron line, due to interference with the Pd 3p3/2 peak, which makes it difficult to check for the presence of palladium oxide. The process that occurs and causes this change is not well understood and is under study. To understand and characterize the influence of the precursor [MP(I) or MP(II)] on the nature of the Pd deposit, we have enlarged our studies by examining in detail the chemical shifts in the 3d level of palladium in molecular precursor and in palladium metal. High-energy resolution XPS Pd 3d spectra obtained from the reference Pd foil after slight Ar sputtering (4 kV, 50 s) were characterized by peaks with binding energies (BEs) of 335.5 eV for 3d5/2 and 340.8 eV for 3d3/2, both distinctive for Pd metal.14-18 The decomposition of MP(I) and MP(II) under H2 was carried out mainly to prepare metallic Pd on the HOPG support. This process was monitored by XPS. Figure 5b reveals a shift of 2.8 eV (from 338.3 to 335.5 eV) in the Pd 3d5/2 level compared to the reference. The BE value for Pd 3d5/2 is attributed to Pd(II). For the first molecular precursor [Pd(3,5-Ph2pzH)2Cl2], in which Pd is present in an oxidation state of +2, there are two Cl atoms in positions trans to the two organic ligands. The literature reports BE values from 337.5 to 338.6 eV for Pd(II),14,17,19-21 particularly for PdCl2.17,18 Kumar et al.17 reported a BE of 337.9 eV for PdCl2 3d5/ 2, and Yamashita21 reported a value of 338.6 eV for Pd 3d5/2 in [PdII(en)2Cl2]. This positive BE shift of 2.8 eV observed in our clusters can be explained by the electronegativity of the ligand. As the ligands attached to Pd are more electron-withdrawing, the Pd halogen bond becomes more highly polarized, depleting the electron density around the Pd. During the reductive treatment, most of the MP(I) in the near-surface region probed by XPS is converted to metallic Pd (Figure 3c). Comparison of this spectrum with the reference spectrum (Figure 3a) indicates a broadening of the Pd 3d features on the high-BE side due to the presence of some remaining Pd2+ or possible PdO formation. Regardless of what we observe in the XPS survey spectra, for example, the vanishing of some photoemission peaks, we still observe traces of elements such as N and Cl. Several different effects could explain a possible palladium oxide formation. First, there could be some residual O contamination in the inlet H2 or N2 gas; second, during the cooling process, oxygen could diffuse out of the HOPG. The oxygen atmosphere effect cannot be considered strongly, because some studies have found that metallic Pt and Pd formed by H2 reduction of the oxides were not oxidized when exposed to air at room temperature.22 However, Cobden et al. explained the decomposition of palladium hydride particles23 in the same manner. Thus, if we have hydrogen adsorbed on our palladium particles, we cannot rule out the possibility that, during the cooling (14) Hermans, S.; Wenkin, M.; Devillers, M. J. Mol. Catal. A 1998, 136, 59. (15) Poole, R. T.; Kemeny, P. C.; Liesegang, J.; Jenkin, J. G.; Leckey, R.C. G. J. Phys. F: Met. Phys. 1973, 3, L46. (16) Fleisch, T. H.; Zajac, G. W.; Schreiner, J. O.; Mains, G. J. Appl. Surf. Sci. 1986, 26, 488. (17) Kumar, G.; Blackburn, J. R.; Albridge, R. G.; Moddeman, W. E.; Jones, M. M. Inorg. Chem. 1972, 11, 296. (18) Tressaud, A.; Khairoun, S.; Touhara, H.; Watanabe, N. Z. Anorg. Allg. Chem. 1986, 540, 291. (19) Nosova, L. V.; Stenin, M. V.; Nogin, Yu. N.; Ryndin, Yu. A. Appl. Surf. Sci. 1992, 55, 43. (20) Choudary, B. M.; Kumar, K. R.; Jamil, Z.; Thyagarajan, G. J. Chem. Soc., Chem. Commun. 1985, 931. (21) Yamashita, M.; Murase, I.; Ito, T.; Wada, Y.; Mitani, T.; Ikemoto, I. Bull. Chem. Soc. Jpn. 1985, 58, 2336. (22) Fleish, T. H.; Mains, G. J. J. Phys. Chem. 1986, 90, 5317. (23) Cobden, P. D.; Nieuwenhuys, B. E.; Gorodetskii, V. V.; Parmon, V. N. Platinum Met. Rev. 1998, 42, 141.

8332

Langmuir, Vol. 20, No. 19, 2004

Dı´az-Ayala et al.

Figure 4. XPS spectra of the Pd 3d binding energy region including curve fits, for (a) MP(I)/HOPG and (b) MP(II)/HOPG, both after H2 reductive treatment.

Figure 5. AFM image (10 µm × 10 µm × 400 nm) of freshly cleaved HOPG that was immersed in CH2Cl2 for 24 h and later placed under vacuum for 1 h.

process, hydrogen desorbs and contaminants such as oxygen may chemisorb on the particles. From the curve fitting (Figure 4b), the Pd 3d BE was obtained. The spin-orbit splitting between 3d5/2 and 3d3/2 is 5.26 eV, which allows the fitting of the 3d5/2 component individually, a simple operation. The fits yield two peaks in the 346.0-334.5 eV range. The peaks at 335.4 and 336.8 eV are assigned to metallic Pd and to a residue of the cluster, respectively. The position of the peak at 335.4 eV agrees with those observed in other studies of carbonsupported Pd19 and with the BE that we report for our Pd foil reference, 335.5 eV (Figure 5a), while the skewness toward the high-BE side (336.9 eV)24 is normally attributed to the effect of chemisorbed oxygen. The Pd region for our second molecular cluster precursor, MP(II), presents in Figure 3d binding energies for the Pd doublet at 339.1 and 344.4 eV for the 3d5/2 and 3d3/2 bands, respectively. The peak shape for Pd 3d appears similar to those for metallic Pd and MP(I)/HOPG; i.e., only one Pd species is present. Considering the BE value, as well as the structure of our MP(II), in which no halogens are present but each pyrazole ligand contributes one negative charge, we attribute this binding energy to Pd(II). The spectrum shown in Figure 3e represents the reduction of MP(II), leading to a displacement of binding energies toward lower values. The 3d5/2 and 3d3/2 peaks for MP(II)/HOPG after the reduction process clearly indicate the existence of at least two forms of Pd (Figure 4b). One is the pure metal at 336.0 eV, which is 0.5 eV higher than that for Pd bulk (reference foil). Nosova reported a BE for highly dispersed catalyst at 0.6 eV higher than that for bulk Pd.19 The second BE, at 337.78 eV, can be ascribed to a phase oxide or Pd2+. Although the value 336.0 eV is 0.5 eV higher than (24) Kim, K. S.; Gosmann, A. F.; Winograd, N. Anal. Chem. 1974, 46, 197.

that of the reference, it can be attributed to metallic Pd considering the high dispersion and small particle size that result from the deposition from the trinuclear complex (see the AFM discussion below) as well as the poor interaction between the Pd nanoparticles and the substrate surface. Many references report present high BE values for Pd0, depending on the nature of the supporting substrate, the dispersion, and the particle size.8,25-27 Here, we can talk about the poor interaction between our HOPG substrate and the molecular precursor, considering the high BE value for Pd0. After modification, the HOPG could not be washed with solvent, because in previous analytical work, the metallic deposit was found to be removed by washing. Mason,28,29 who studied cluster-support interactions for clusters of group VIII, concluded that, for less interactive substrates such as carbon, the metal-support interactions are weak, and the XPS peaks for the cluster shift to higher BEs relative to that of the bulk metal. Our molecular cluster precursors appear to be no exception. Several authors use HOPG as a substrate for metal nanoparticles, because the metal interactions with the graphite basal plane are very weak.30,31 On the other hand, others report an increase in BE and line width in core-level spectra as the dispersion increased and the particle size decreased. Particle size effects may be interpreted in terms of screening effects.32 Atomic Force Microscopy. The HOPG surface was imaged by AFM first without MP compounds; this was considered as a blank. Furthermore, the modified HOPG surfaces were imaged before and after the reductive process to elucidate any changes in the surfaces caused by their adsorption. The AFM images in Figures 5 and 6 present the surfaces of HOPG in each step of the procedure. The freshly cleaved HOPG surface was immersed in dichloromethane for 12 h and then placed under vacuum for 1 h. It can be observed in parts a and c of Figure 6 that the surface changes after modification with MP(I) and MP(II), respectively. Many circular features were observed, and these persisted upon the H2 reduction process. Before the reduction process, it was very difficult to determine the particle size by AFM. Circular aggregates were observed on the surfaces, and the heterogeneity of the surfaces, as well as the HOPG planarity loss, was clear. After the H2 reductive treatment, as seen in Figure (25) Fritsch, A.; Le´gare´, P. Surf. Sci. 1985, 162, 742. (26) Fleisch, T. H.; Hicks, R. F.; Bell, A.T. J. Catal. 1984, 87, 398. (27) Takasu, Y.; Unwin, R.; Tesche, B.; Bradshaw, A. M. Surf. Sci. 1978, 77, 219. (28) Mason, M. G.; Gerenser, L. J.; Lee, S. T. Phys. Rev. Lett. 1977, 39, 288. (29) Mason, M. G. Phys. Rev. B 1983, 27, 748. (30) Lee, I.; Chan, K. Y.; Phillips, D. L. Appl. Surf. Sci. 1998, 136, 321. (31) Penner, R. M. J. Phys. Chem. 2002, 106, 3339. (32) Takasu, Y.; Unwin, R.; Tesche, B.; Bradshaw, A. M.; Grunze, M. Surf. Sci. 1978, 77, 219.

Thermal Reduction of Pd Molecular Cluster Precursors

Langmuir, Vol. 20, No. 19, 2004 8333

Figure 6. AFM images (10 µm × 10 µm × 400 nm) of (a) HOPG with MP(I) before the reduction process, (b) HOPG with MP(I) after reduction, (c) HOPG with MP(II) before reduction, and (d) HOPG with MP(II) after reduction (z ) 400 nm).

6b, the agglomeration of many particles can be observed over the whole surface, in contrast to the high dispersion and high selectivity toward defects sites of MP(II), presented in Figure 6d. This behavior differs from that reported by Simone and collaborators, which was a higher palladium dispersion with chloride-containing catalyst.33 It is important to clarify that in the present work a different substrate and different adsorption and reduction processes were used. Thermal decomposition, as well as other parameters such as distribution and growth rate,34 is strongly dependent on the support materials. In this work we used HOPG as a substrate, which presents a weak interaction with the cluster of group VIII and noble metals. Both MP(I) and MP(II) were observed on basal plane surfaces of HOPG before the reduction. However, remarkable differences were observed between MP(I) and MP(II) after the reduction. The identification of these Pd particles was shown by XPS analysis as well as by X-ray fluorescence/energy dispersive spectroscopy (EDS) measurements. Figure 7 shows the EDS spectrum of an HOPG surface modified with MP(I) and the SEM image of the area used for the X-ray analysis. Figure 7a presents an area in which it is possible to observe a large number of particles, including regions of bare HOPG surface. Figure 7b shows the spectrum of a specific particle; the weight percents presented by this analysis for Pd in MP(I), in the two regions of the sample, were 15.13% and 80.22%, respectively. Similar spectra were found for MP(II), in which the weight percent presented was 8.47%. It is known that nucleation begins at surface defect sites where bounded adatoms remain trapped and immobile. Nucleation is spatially random on terraces, and (33) Simone, D. O.; Kennelly, T.; Brungard, N. L.; Farrauto, R. J. Appl. Catal. 1991, 70, 87. (34) Jak, M. J. J.; Konstapel, C.; Kreuningen, A.; Chrost, J.; Verhoeven, J.; Frenken, J. W. M. Surf. Sci. 2001, 474, 28.

Figure 7. (a) X-ray fluorescence spectrum of the reduced form of MP(I) on HOPG. This was taken on the scanning electron microscope image (10000×, reproduced at 60% of original size) presented in (b).

nuclei are also aligned at step edges on the surface. It can be considered that the growth model of MP present on HOPG surfaces is the Volmer-Weber model, in which the growth of individual metal particles on the surface depends on the number and proximity of neighboring particles. Thus, mobile Pd nanoparticles can move on the HOPG surface and adhere to other neighboring Pd nanoparticles. A mobile particle can act as a new nucleus, and the growth of some particles by coalescence causes them to exhibit elliptical shapes (Figure 8a,b), as observed for MP(I), versus the spherical shape observed for MP(II) (Figure 9a,b). Schneider et al. report spherical shapes for Pd clusters at low Pd loadings, while, at higher Pd concentrations, more elliptical shapes are exhibited.35 On the other hand, nanoparticles formed from the reduction of MP(II) remain uncoalesced, and in some cases,

8334

Langmuir, Vol. 20, No. 19, 2004

Dı´az-Ayala et al.

Figure 8. Particle size analysis of Pd particles obtained after Pd(3,5-Ph2pzH)2Cl2 reduction under a hydrogen atmosphere: (a) deflection image (1 µm × 1 µm); (b) height image (5 µm × 5 µm); (c) particle size histogram.

Figure 9. Particle size analysis of Pd nanoparticles obtained after [Pd3(µ-Phpz)6] reduction under a hydrogen atmosphere: (a) deflection image (1 µm × 1 µm); (b) height image (5 µm × 5 µm); (c) particle size histogram.

the particles align themselves as seen in Figure 9a, suggesting that the surface is in the first and second phases of nanowire formation.31 A histogram36 presented in Figure 8c shows the size distribution in the range from 0 to 225 nm, in which the most abundant particles are 50 nm for MP(II). The HOPG surface modified with MP(I) (Figure 8) shows more particles on its surface with low dispersion and with a high-frequency particle size, 200 nm, as shown in the histogram in Figure 8c. Meanwhile, the HOPG surface modified with MP(II) has a similar appearance, but a high dispersion is observed, and the high-frequency particle size is 50 nm, smaller than that for MP(I). These results are in accord with the BE reported for the Pd species of the MP(II), as previously discussed. It can be observed that on the sample with MP(I), although a low dispersion is observed, the surface looks more homogeneous: all of the nanoparticles exhibit the same shape and similar sizes. They are larger than those for MP(II), since, in the latter, the surface is heterogeneous. This behavior can be explained considering the precursor of the particle in each sample. The observed behavior for MP(II), shown in the AFM images (Figure 6d), may reveal that, during the reduction process, organic groups were expelled from MP(II) in the same temperature range as for MP(I) but not with the same mass-loss ratio, as shown in Figures 1b and 2b. It is suggested that all of these observations may be due to a combination of factors, including the use of a larger precursor, with more steric hindrance and/or (35) Schneider, J.; Wambach, C.; Pennemann, B.; Wandelt, K. Langmuir 1999, 15, 5765.

a low tendency to nucleate on HOPG terraces. This is proposed to be the cause of the larger preference for MP(II) to nucleate selectively along steps, in contrast to that for MP(I), which is nucleation on the whole basal plane of HOPG, generating a broad particle size. The methodology presented here can be considered as an alternative for the preparation of noble metal nanowires narrower than 50 nm. Conclusions Thermogravimetric analysis showed physical and chemical changes that were experienced by the precursor compounds when heated. Changes in the structures of the compounds due to the decomposition processes were observed by TGA analysis. The AFM studies of the molecular precursor and its reduced state at HOPG surfaces present the formation of circular features that may be caused by the solvent. The circular morphology of the particles after the reduction process was comparable to the morphology of the molecular precursors, before reduction, on HOPG. A low dispersion of particles and larger particle sizes, for mononuclear (MP(I)) complex, were observed in the AFM images. On the other hand, the trinuclear (MP(II)) complex gave rise to highly dispersed nanoparticles and numerous stripes of adatoms along HOPG defects. A high affinity of the mononuclear complex toward HOPG surfaces can partially explain these types (36) The histogram presents the focal distance as a size measure due to the particles not being spherical. Focal distance ) (x1 - 2(minor axis/major axis))(major axis). http://www.1728.com/ellipse.htm.

Thermal Reduction of Pd Molecular Cluster Precursors

of behavior. XPS spectra obtained for the two MPs after reduction at 600 °C showed the disappearance and displacement of some elements, as suggested by TGA, as well as changes in the chemical environment. A displacement toward lower binding energies suggests a reduction of the compound, as shown by the high-resolution Pd XPS spectra. Furthermore, the formation of palladium nanowires is proposed for MP(II) on HOPG with edge defects. Our results indicate that nanowires may be removed from HOPG surfaces by rinsing with solvent.

Langmuir, Vol. 20, No. 19, 2004 8335

Acknowledgment. We acknowledge the financial support of NASA-URC (Grant Number NASA- NCC31034) and the ARL Collaborative Technology Alliance in Power and Energy, Cooperative Agreement No. DAAD1901-2-0010. We express special thanks to Dr. Estevao Fachini for the technical assistance in the surface analysis, and the Materials Characterization Center (MCC) of the University of Puerto Rico. LA0364688