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Palladium Nanostructures and Nanoparticles from Molecular Precursors on Highly Ordered Pyrolytic Graphite Ramonita Dı´az-Ayala, Esteban R. Fachini, Raphael Raptis, and Carlos R. Cabrera* Department of Chemistry and Center for Nanoscale Materials, UniVersity of Puerto Rico, Rio Piedras Campus, San Juan, Puerto Rico 00931-3346 ReceiVed March 3, 2006. In Final Form: July 7, 2006 Nanostructures and nanoparticles of palladium assembled on highly ordered pyrolytic graphite (HOPG) by the adsorption of palladium molecular precursors (MPs), in dichloromethane solutions, have been prepared. Self-assemblies of palladium nanostructures on HOPG were characterized by scanning electron microscopy (SEM), Auger electron spectroscopy (AES), transmission electron microscopy (TEM), and atomic force microscopy (AFM) techniques. In this work, palladium rings had a wide variety of sizes in the nanometer range, and the ring/tube structures were preserved after a reductive process in which palladium metallic nanoparticles were formed. Noncircular structures were observed at HOPG defects and atomic step sites, as well. It is proposed that the observed ring formation of the palladium molecular precursors on HOPG substrates is related to the functional groups in the MPs, van der Waals interactions between particles and between particle-substrate, as well as the wetting properties of the solvent. In the present work, we illustrate several examples of the formation and characterization of palladium complex tubes and the resulting palladium rings, via the reduction process.
Introduction Approaches to produced nanostructures have become increasingly important. Many techniques are based on scanning probe microscopies such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) using tips to assemble metal nanostructures or create “molecule corrals”,1-4 which are used as templates to confine molecule ensembles. However, molecular self-assembly offers a good alternative to create nanomaterials and nanomachines that would lead to inexpensive and easily manufactured processes. Different ideas of how to build nanomaterials do not have limits; they include all possible morphologies, compositions, and structures. A good example is the formation of ring structures. The fascination of ringlike atomic and quantum structures dates back to 1865 to Kekule’s famous proposal of the structure of benzene. Particularly interesting are the magnetic properties of such nonsimple connected quantum systems, which are related to the possibility of trapping magnetic flux in their interior. Trapping of a single flux quantum in a small molecule such as benzene is impossible with the magnetic fields available in today’s laboratories.5,6 However, the availability of submicrometer solid-state ring structures has triggered a strong interest due to that the optical properties of metal ring-shaped materials significantly change from the properties of compact spheres as presented by Aizpurua et al.7 Other authors have reported on the trapping of a magnetic flux in the interior of an electrically conducting ring, which leads to so-called “persistent * Corresponding author. E-mail:
[email protected]. (1) Patrick, D. L.; Cee, V. J.; Beebe, T. P., Jr. Science 1994, 265, 231. (2) Stevens, F.; Kolodny, L. A.; Beebe, T. P., Jr. J. Phys. Chem. B 1998, 102, 10799. (3) Patrick, D. L.; Cee, V. J.; Morse, M. D.; Beebe, T. P., Jr. J. Phys. Chem. B 1999, 103, 8328. (4) McBride, J. D.; Tassell, B. V.; Jachmann, R. C.; Beebe, T. P., Jr. J. Phys. Chem. B 2001, 105, 3972. (5) Lorke, A.; Luyken, R. J.; Govorov, A. O.; Kotthaus, J. P.; Garcia, J. M.; Pettrof, P. M. Phys. ReV. Lett. 2000, 84, 2223. (6) Garcı´a, J. M.; Medeiros-Ribeiro, G.; Schmidt, K.; Ngo, T.; Feng, J. L.; Lorke, A.; Kotthaus, J. P.; Pettrof, P. M. Appl. Phys. Lett. 1997, 71, 2014. (7) Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; Ka¨ll, M.; Bryant, G. W.; Garcı´a de Abajo, F. J. Phys. ReV. Lett. 2003, 90, 057401.
currents”.5 Therefore, methods to synthesize mesoscopic rings are important in the field of electronics and devices. Several methods for the preparation of rings made of semiconductor, metal, polymer, and other materials have been reported. These methods include droplet epitaxy,8 colloidal lithography,7 the use of porous template9 and genetically driven assemblies,10 and other examples of self-assembly.6,11,12 However, it is known that when a suspension droplet dries on a solid substrate, we often observe stripe patterns of particles, which remain on the substrate on which it rested after evaporation of the solvent.13 Many articles demonstrated that wetting properties, capillary forces, particle-particle, and particle-substrate interactions play a role in the self-organization of nanocrystals deposited on a substrate. Thermodynamically, the simple process of drying a thin liquid film from a wetted substrate might be considered the reversible counterpart to wetting, which enjoys a broad and well-developed literature.14 On the other hand, there is a thermodynamic description of the dewetting process that can occur when a volatile film evaporates from a substrate to which it is bound by both van der Waals and polar forces, showing that initial stages of the pattern formation can be due to the diffusioncontrolled solidification problem in two dimensions.15 Finally, wetting properties and its counterpart dewetting, evaporation-driven instabilities, such as hole nucleation,12,16 can be used to create large mesoscopic rings. This may lead to new (8) (a) Mano, T.; Kuroda, T.; Sanguinetti, S.; Ochiai, T.; Tateno, T.; Kim, J.; Noda, T.; Kawabe, M.; Sakoda, N.; Kido, G.; Koguchi, N. Nano Lett. 2005, 5, 424. (b) Mano, T.; Watanabe, K.; Tsukamoto, S.; Fujioka, H.; Oshima, M.; Koguchi, N. J. Cryst. Growth 2000, 209, 504. (9) (a) Yi, D. K.; Kim, D.-Y. Nano Lett. 2003, 3, 207. (b) Hobbs, K. L.; Larson, P. R.; Lian, G. D.; Keay, J. C.; Johnson, M. B. Nano Lett. 2004, 4, 167. (c) Yan, F.; Goedel, W. A. Nano Lett. 2004, 4, 1193. (10) Nam, K. T.; Peelle, B. R.; Lee, S.-W.; Belcher, A. M. Nano Lett. 2004, 4, 23. (11) Warburton, R. J.; Scha¨flein, C.; Haft, D.; Bickel, F.; Lorke, A.; Karral, K.; Garcı´a, J. M.; Schoenfeld, W.; Petroff, P. M. Nature 2000, 405, 926. (12) Ohara, P. C.; Gelbart, W. M. Langmuir 1998, 14, 3418. (13) (a) Adachi, E.; Dimitrov, A. S.; Nagayama, K. Langmuir 1995, 11, 1057. (b) Jaschke, M.; Butt, H.-J. Langmuir 1995, 11, 1061. (c) Lipson, S. G. Phys. Scr. 1996, T67, 63. (d) Hu, H.; Larson, R. G. Langmuir 2005, 21, 3963. (14) Gennes, P. J. G. ReV. Mod. Phys. 1985, 57, 827. (15) Samid-Merzel, N.; Lipson, S. G.; Tannhauser, D. S. Phys. ReV. E 1998, 57, 2906.
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routes of ring mesostructure formation.17 One of the most recent and interesting results in this area was reported by Dugas et al.,18 who used droplet evaporation for the manufacturing of DNA chip. The design of components that organize themselves into desired patterns and functions is the key to applications of self-assembly. Molecular self-assembly involves noncovalent or weak covalent interactions (van der Waals, electrostatics, hydrophobic interactions, hydrogen and coordination bonds).19 In the self-assembly of larger component meso- or macroscopic objects, interactions can often be selected and tailored and can include interactions such as gravitational attraction, capillary, and entropic interactions between others. During the past decade, self-assembly has emerged as a promising approach to defining material features on length scales smaller than those conveniently accessible by lithography.20 In the present work, we illustrate several examples of the formation and characterization of palladium complex tubes and the resulting palladium rings via the reduction process to which the modified HOPG surfaces with the palladium precursor were exposed. AFM and SEM results showed that two different processes are occurring during the Pd nanoparticles and Pd rings formation at HOPG surfaces: one at HOPG basal planes and another at the step edges. The particle size and morphology were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The composition of the particles was determined by Auger electron spectroscopy (AES), in its scanning Auger microscopy (SAM) mode. The static and dynamics of a thin film on a substrate have been discussed extensively by Sharma,21 de Gennes,14 and Israelachvili,22 and experiments on similar systems have been reported by Reiter23 and Brochard-Wyart24 and co-workers. However, these works are limited to nonvolatile films, within which mass is conserved. In our case, the film is in contact with vapor and the transfer of mass between the liquid/gas interface has to be taken into account. Experimental Section Sample Preparation. Molecular precursors, Pd(3-PhpzH)2Cl2 (MP(II)), Pd(3,5-Ph2pzH)2Cl2 (MP(I)), Pd(3,5-Me2pzH)2Cl2 (MP(III)), Pd(4-Br-3,5-Me2pzH)2Cl2 (MP(IV)), [Pd2(µ-3,5-Ph2pz)2(3,5Ph2pzH)2]Cl2‚H2O (MP(V)), [Pd2(µ-3,5-Me2pz)2(3,5-Me2pzH)2Cl2 (MP(VI)), and Pd3(µ-3-Phpz)6 (MP(VII)), were prepared following literature procedure.25 A freshly cleaved piece of HOPG (spi-2) was used as the substrate. The adsorption of various Pd molecular cluster precursors at exfoliated HOPG was carried out at room temperature by immersing the substrate for 1 h in a 1 mM dichloromethane solution of the precursor. After removal of the HOPG substrate from the Pd MP dichloromethane solution, the surface was allowed to air-dry. Subsequently, samples were placed in a drybox for 1 h and (16) (a) Elbaum, M.; Lipson, S. G. Phys. ReV. Lett. 1994, 72, 3562. (b) Ohara, P. C.; Heath, J. R.; Gelbart, W. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 1078. (17) (a) Mertig, M.; Thiele, U.; Bradt, J.; Leibiger, G.; Pompe, W.; Wendrock, H. Surf. Interface Anal. 1997, 25, 514. (b) Tang, J.; Ge, G.; Brus, L. E. J. Phys. Chem. B 2002, 106, 5653. (c) Millard, M.; Motte, L.; Ngo, T.; Pileni, M. J. Phys. Chem. B 2000, 104, 11871. (18) Dugas, V.; Broutin, J.; Souteyrand, E. Langmuir 2005, 21, 9130. (19) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (20) Brinker, C. J. MRS Bull. 2004, 29, 631. (21) Sharma, A. Langmuir 1993, 9, 861. (22) Israelachvili, J. H. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1992. (23) (a) Reiter, G. Phys. ReV. Lett. 1992, 68, 75. (b) Reiter, G. Langmuir 1993, 9, 1344. (24) (a) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084. (b) Redon, C.; Brochard-Wyart, F.; Rondelez, F. Phys. ReV. Lett. 1991, 66, 715. (25) Baran, P.; Marrero, C. M.; Raptis, R. G.; Pe´rez, S. Chem. Commun. 2002, 1012.
Dı´az-Ayala et al. then characterized by the aforementioned methods. 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 chosen because the total degradation of the organic groups occurs below this temperature, as was previously observed in TGA (thermogravimetric analysis) experiments.26 Afterward, the samples were cooled in a H2 atmosphere to 36 °C and then maintained under pure N2 until reaching room temperature and before opening the system. The H2 stream was previously dried over activated molecular sieves and deoxygenated over BASF Catalyst R 3-11. Atomic Force Microscopy (AFM). The atomic force microscopy experiments were performed on a Nanoscope IIIa from Digital Instrument using a silicon tip in ambient atmosphere and temperature. AFM images were obtained in both tapping (TM) and contact modes (CM). Scan rates were varied from 1 to 2 Hz. The images presented in this work were obtained repeatedly in several spots of the sample. All images (512 × 512 pixels) shown in this paper are presented essentially unfiltered; in some cases, the “Flatten” filter of the NanoScope software was used. Auger Electron Spectroscopy (AES). An Auger electron spectrometer (PHI 660) was used for surface composition analysis. Pd ring studies were carried out in a scanning Auger microprobe (SAM) mode of the instrument. Transmission Electron Microscopy (TEM). To evaluate the morphology of the particles at HOPG surfaces, transmission electron microcopy (TEM) analysis was done. For these studies, the thin films were peeled off from the HOPG substrate using an optical microscope and ultrathin twister, and finally they were mounted on support film, carbon Type-A, 300 mesh copper grids. TEM was done at a JEOL 2010 at 200 kV.
Results and Discussion Pd Ring Self-Assembly. The adsorption experiments on HOPG were performed with seven Pd molecular precursors (MPs) in dichloromethane. Table 1 presents the chemical formula, structure, and other important parameters of the different Pd molecular precursors. These Pd molecular precursors vary in the functional groups that substitute the pyrazole ligands as well as in their nuclearity. They are mononuclear, binuclear, or trinuclear Pd complexes (i.e., one, two, or three Pd atoms, respectively). Microscopy studies reveal how these Pd particle molecular precursors promote the interesting structures with Pd such as ring formation and other different nanoparticle structures at HOPG defects. To understand these observations, images of the modified HOPG surfaces were done with the contact (CMAFM) and tapping (TMAFM) AFM modes as well as by scanning electron microscopy (SEM). The observed surface structures were not present in blank samples (CH2Cl2/HOPG). On the other hand, Figure 1d presents an AFM height image of the Pd ring selfassembly promoted by the adsorption of MP(I) at the HOPG surface. This image shows important characteristics such as the heterogeneity of the surface, while in some areas rings are only found. In other regions, it is possible to see agglomeration or a combination of both. Many of these circular features persisted even after the H2 reduction process. Contact mode and tapping mode AFM images were used to demonstrate that the ring structures are not artifact produced by the mode of analysis. These AFM images show various rings; they have a wide difference in sizes, ranging from 63 to 1328 nm in diameter and with heights between 21 and 209 nm, on a HOPG surface modified with a MP(I). Theses circular aggregates were observed on the surfaces after the adsorption of the Pd MPs, the heterogeneity of the surfaces, as well as the HOPG planarity loss were clear. Another general observation is that the (26) Dı´az-Ayala, R.; Arroyo, L.; Raptis, R.; Cabrera, C. R. Langmuir 2004, 20, 8329.
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Table 1. Chemical Formula and Molecular Structure of the Palladium Molecular Cluster Precursors (Cheng, C.-H.; Lain, J.-S.; Wu, Y.-J.; Wang, S.-L. Acta Crystallogr. 1990, C46, 208)
heterogeneous nucleation is straightforward on the edge of a step on HOPG, or at linear defects. Figure 1b shows the Auger spectrum of a self-assembled monolayer ring of MP(I) rings in which the elemental composition was determined. The Auger spectrum revealed the presence of Pd, Cl, C, and N with an electron kinetic energy of 324, 176, 265, and 389 eV, respectively. A SEM and Auger electron spectroscopy mapping (i.e., scanning
Auger microscopy) of some Pd rings is presented in Figure 1a and c. Auger electron spectroscopy surface mapping confirmed that the Pd MPs accumulation coincides with the annular structure on the surface. Other Pd MPs presented similar behavior; Figure 2 showed the HOPG surfaces modified with binuclear precursor, MP(V), and for the trinuclear MP(VII) molecular precursor. The
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Figure 1. (a) Scanning electron microscopy (SEM) micrographs of Pd ring formation on HOPG surfaces, (b) Auger electron spectroscopy (AES) spectra at the Pd rings (red) and substrate (blue), (c) scanning Auger microscopy (SAM) mapping of MP(I) on HOPG, electron beam accelerating voltage, 3 keV, and (d) 3D tapping mode atomic force microscopy (TMAFM) (5 µm × 5 µm × 50 nm) images.
Figure 2. Atomic force microscopy (AFM) images (top and 3D view) of MP(V) and MP(VII) on HOPG surface from solution in CH2Cl2: (a) top view (10 µm × 10m × 10 nm) of MP(V) on HOPG and (b) X-ray fluorescence spectrum of (a). Inset shows the corresponding AFM-3D image. (c and d) Top and 3D AFM images (5 µm × 5 µm × 200 nm) of MP(VII) on HOPG surface, respectively.
characteristic rings that vary in height, wall thickness, and inner diameter were observed, as well. It is interesting to keep in mind that MP(I), MP(V), and MP(VII) have phenyl-derivatized pyrazole ligands. Remarkably, precursor-assembling differences
have been observed between Pd MPs that have pyrazole groups substituted with methyl versus the Pd MPs that have phenyl groups, although in some spectroscopy analysis they presented the same behavior. X-ray fluorescence/energy dispersive spec-
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Figure 3. Representative AFM height images of samples of MP(III), MP(VI), and MP(IV) at HOPG surfaces: (a) (10 µm × 10 µm × 400 nm) height image of MP(III) at HOPG, (b) top view AFM images of HOPG surfaces modified with MP(VI), and (c and d) top image and characteristic X-ray fluorescence spectrum of surfaces modified with MP(IV), respectively. Inset shows the corresponding SEM image, SEM micrograph of 1 µm at a magnification of 20 000 at 15 kV. These images were taken before the reduction process.
troscopy (EDS) measurements reveal similar results for all complexes with the exception of MP(IV), which showed the presence of Br. Carbon, palladium, and chloride were the major components of the samples. In some samples, an additional peak at 0.525 keV also revealed the presence of oxygen from HOPG surfaces. Figure 2b shows a typical EDS spectrum of an HOPG surface modified with a Pd MPs and the SEM image of the area used for the X-ray analysis. The absence of nitrogen is explained because it is at low concentration in comparison with carbon whose peak overlaps with the N peak. The signal will be too small to be distinguished from the carbon background. As can be seen in Figure 3b, annular structures were also observed for MP(VI), a binuclear compound, but they are absent for MP(III) (Figure 3a) that presents just one Pd atom in its formula. These last two precursors have 3,5-dimethylpyrazole as a ligand. The mononuclear, MP(III), was the only Pd MP in which after various samples and various areas of analysis we never saw circular features, but small particles with mean diameter of 185 nm were observed. The surface roughness was 10.6 nm. The dinuclear, MP(VI), presents a noncontinuous annular assembly, as can be seen in Figure 3b. The figures show different particles of MP(VI) in a wide range of hundreds of nanometers to few micrometers, as well in Figure 3d the characteristic EDS of the HOPG surface modified with MP(IV) is shown. The other palladium complex with dimethyl pyrazolate ligands was MP(IV), which presented a peculiar shape at the HOPG surface. The bonfire-like structure shown by this complex, seen in Figure 3c, gives us the reminiscence of the flowerlike morphology exhibited by the long-side-chain polyglutamates selfassembled on a mica surface in a work reported by Sohn et al.27 However, both systems are chemically different. (27) Sohn, D.; Kitaev, V.; Kumacheva, E. Langmuir 1999, 15, 1698.
In summary, we could observe that these Pd MPs present the tendency to form rings with the exception of MP(III); the rings may have an interrupted rim or a continued and dense rim and be the last example of bonfire-like structure. The present panorama suggests that the spacing between pyrazolate ligands that is larger for phenyl than methyl substituents influences the shape of the ring. The structures of the rings are governed by a competition of Pd MPs-Pd MPs, Pd MPs-solvent, and solution-surface interactions. At the moment, we are not able to address the reason for the differences observed in the rim of the rings. However, it was noticed that these differences in the rim morphology might be correlated with the interdigitation of the side substituent in the palladium molecular precursors. Initially, we classified our Pd MPs in two main groups: the first was those that have 3,5diphenylpyrazole (e.g., MP(I), MP(V), and MP(VII)), and the other was those that have 3,5-dimethylpyrazole (e.g., MP(III), MP(IV) and MP(VI)) as ligands. Nevertheless, to facilitate the following explanation, we divided the last group with another difference, those that have Br in position 4 of the pyrazole group (e.g., MP(IV)) and those that do not (e.g., MP(III) and MP(VI)). In other words, we now have three groups. The first group that has 3,5-dimethylpyrazole as ligand and does not have Br will be those that arrange over the HOPG surface like independent particles (Figure 3a and b) or on occasion form ringlike structures whose rims are not continuous; instead, it is composed by individual particles. The other group of this first division is the MP(IV), which has a bromide in position 4 of pyrazole, between the two methyl substituents, and was the group that presented the bonfire-like structure at the HOPG surface, seen in Figure 3c. The former group, which presents a ringlike structure with a continuous and dense wall, as shown in Figures 1 and 2, is from
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Figure 5. TEM micrographs of MP(I) obtained by immersing the grid in the CH2Cl2 solution, then drying the solvent: (a) scale 100 nm.
Figure 4. Scheme illustrating the process by which the volatile solution that wets the HOPG surface thins to a thickness at which patches form (top) and promote the ring formation (middle). An AFM image example of the rings observed at one of the MPs on HOPG surfaces (bottom).
the MPs that have phenyl groups as substituents of the pyrazole. We attributed this difference in morphology to the fact that phenyl groups are planar aromatic rings that present an attractive nonbonded interaction between its rings and are referred to by the literature as π-π interactions or π-stacking. The steric constraints associated with this formation of the ordered stacking structures have a fundamental role in self-assembly processes. The presence of these planar aromatic rings in our last group raises the possibility that π-π interactions may play a significant role in the self-assembly processes that lead to the formation of a dense and continuous rim of the ring structure. This explanation is consistent with the fact that once these surfaces (HOPG modified with this group) are exposed to the thermal reductive process in which the organic components are decomposed, the rim of these ring structures lost its continuity and shows individual particles, as seen in Figures 6, 8, and 9. On the other hand, we have the group that has phenyl groups instead of methyl groups, a nonplanar substitute in which H’s are in a tetrahedral position that cause a steric hindrance. This effect can be attributed to a steric effect: two molecular fragments cannot occupy the same region in space. This situation was worst when a bromide group was added between the methyl groups. This interaction provides parallel orientation of the molecule with respect to each other as a result of independent particles or fiberlike structure as in the case of molecular precursor with Br. Upon the self-assembly rings of the Pd MPs, with phenyl substitutes, the driving forces
for molecular precursors ligands are mainly due to the interaction of the side groups such as phenyl. These aromatic groups of adjacent Pd MPs are interdigitated and crystallize upon solvent evaporation. Considering that all Pd MPs present similar behavior, we selected MP(I) as a good representation to analyze the behavior of the complex with special interest to those that have phenyl groups. These precursors have the pyrazole ligand substituted with phenyl groups. This group form rings with a dense wall as presented in Figure 1. In a random selection of 30 rings, in different modified samples, the ring wall thicknesses varied from 117 to 508 nm. The cross-section of those rings showed average heights between 21 and 209 nm, suggesting that these structures are short tubes or wells. This phenomenon may be due to several factors acting simultaneously such as (i) partial evaporation of the solvent that contains the Pd cluster on the HOPG surface, in which the particle-volume fraction increases, inducing an increase in the interaction between particles,28,29 (ii) capillarity and surface tension of the solvent with the substrate, and (iii) attractive interactions between particle-particle, which may be stronger than those between particle-substrate as predict by the Hamaker constant using Lifshitz theory.22,30 When HOPG is immersed in dichloromethane-containing Pd particle precursors, the solutions wets the surface. The particles are then randomly dispersed in the flat droplet and are submitted to the Brownian motion. During this process, evaporation takes place as consequence of the dichloromethane high vapor pressure (Pv ) 58 kPa at 25 °C). At this point, a group of factors promote the behavior that is observed. The surface of the liquid acts like a stretched film, and a sample of liquid adopts the shape of a sphere if the surface tension outweighs the effects of gravity.31 For years, there has been a great interest in the dynamics of spontaneous pattern formation in a rupturing liquid film on a rigid substrate. Although most work has been on nonvolatile32 (28) Motte, L.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1995, 99, 16425. (29) Hu, H.; Larson, R. G. Langmuir 2005, 21, 3963. (30) Motte, L.; Lacaze, E.; Maillard, M.; Pileni, M. P. Appl. Surf. Sci. 2000, 604, 162-163. (31) Mortimer, R. G. Physical Chemistry, 1st ed.; Benjamin/Cumming Publishing Co.: California, 1993. (32) Moon, J.; Garoff, S.; Wynblatt, P.; Suter, R. Langmuir 2004, 20, 402.
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Figure 6. (a) Scanning electron microscopy (SEM) of HOPG modified with MP(I) after the reduction process, (b) Auger electron spectroscopy (AES) spectrum result of point analysis at the ring (red), center of the ring (green), and substrate (blue), (c) scanning Auger microscopy (SAM) mapping, electron beam accelerating voltage, 3 keV, and (d) the TMAFM (5 µm × 5 µm × 100 nm) image.
Figure 7. Transmission electron microscopy (TEM) patterns of palladium particles of MP(I) at HOPG surfaces after the reduction process. Scale: (a) 100 nm, (b) 20 nm, (c) 10 nm, and (d) 20 nm.
systems, the study of the evolution of volatile films33-35 has introduced a new dimension, in which the type of behavior is controlled by vapor pressure. The variety of phenomena that present this intermolecular interaction in the three-phase contact line region, where a liquid-vapor interface intersects a solid substrate, help us to understand our system in which we cannot discard a partial evaporation process. Leizerson et al.36 reported the pattern generated during the evaporation of water film on a cleaved mica substrate; the pattern consisted of an array of water drops of widely differing sizes. Other authors reported several (33) Sharma, A. Langmuir 1998, 14, 4915. (34) Zheng, L.; Wang, Y.-X.; Plawsky, J. L.; Wayner, P. C., Jr. Langmuir 2002, 18, 5170. (35) Leizerson, I.; Lipson, S. G. Langmuir 2004, 20, 8423. (36) Leizerson, I.; Lipson, S. G.; Lyushnin, A. V. Nature 2003, 422, 395.
possible behaviors that occur when a uniform layer of a fluid evaporates from a substrate to which it is bound by both van der Waals and polar forces.37 Beebe et al. created etch-pit templates or “molecular corrals” on the basal plane of HOPG that promote the formation of ring structures. Mechanism of Ring Formation. The purpose of this work is to present the process of Pd ring/tube pattern formation on HOPG surface. The annular structures observed are the result of a complex process involving spreading and drying of Pd MPs solution, rupture, and dewetting of the film. Different morphologies that depend on the Pd MPs ligands and the solvent have been observed as aforementioned. Table 2 presents qualitative (37) Samid-Merzel, N.; Lipson, S. G.; Tannhauser, D. S. Phys. ReV. E 1998, 57, 2906.
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Figure 8. Different microscopy techniques: (a) AFM, (b) SEM, and (c and d) TEM showed the changes in the rim of the rings of MP(V) on HOPG surfaces after the reduction process, a characteristic behavior presented.
information about some observed behavior of one of the Pd MPs, MP(I), under different parameters such as concentration, solvent, or substrate to determine the formation or not of ring structures. Upon drying, a droplet of liquid typically leaves a ring of solute on the substrate on which is rested. This phenomenon, which is caused by an evaporation-driven flow, is not new. It is best illustrated by the ringlike mark associated with food, salt, and coffee stains encountered on solid surface in everyday life.38 This spontaneous ring pattern formation promoted the investigation under ingeniously controlled laboratory condition as a model for this basic phenomenon.39 A particular challenge in this field is to find experimental systems that are sufficiently simple and well understood to allow a quantitative theoretical analysis that enables a direct comparison between the patterns observed in experiments and those predicted by theory. Several key differences in the present situation that resulted in the spontaneous formation of many mesoscopic rings are discussed. First, we have a solvent that completely wets the solid substrate, as a uniform liquid film. A liquid will spread into a film on a supporting substrate if the attractive forces between the substrate and the liquid molecules are greater than the cohesive forces of the liquid itself. This is called wetting, and a film formed on a plane substrate under these conditions has a uniform thickness and is in stable equilibrium.40 Qualitatively, we have (38) (a) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. ReV. E 2000, 62, 756. (b) Hu, H.; Larson, R. G. Langmuir 2005, 21, 3972. (c) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827. (39) Adachi, E.; Dimitrov, A. S.; Nagayama, K. Langmuir 1995, 11, 1057. (40) Lipson, S. G. Phys. Scr. 1996, T67, 63.
found that the Pd MPs solution in CH2Cl2 completely wets the HOPG surface. This behavior is in agreement with the literature that reports that organic solvents strongly wet the thick graphite substrate.17b The Hamaker value for CH2Cl2/HOPG interacting across another medium air is 4.58 × 10-20. This value reveals that it wets the surface. A positive value of Hamaker constant means attractive interactions, and a negative value implies repulsive forces.22,24a,30 Second, the solvent then evaporates due to its volatile. As explained by Samid-Merzel37 et al., one of the first in reporting the theory for volatile fluid, several scenarios are possible when a uniform layer of fluid on a substrate evaporates. The first takes place when the fluid does not wet the substrate, and the second scenario occurs if the film wets the substrate. The third scenario, which represents the best description of our system, is when a layer of volatile solvent with nonzero thickness remains on the substrate at all times, but for some range of mean thickness a uniform layer is unstable and the film breaks up into a two-phase system: surface/liquid and surface/ vapor. The Pd rings result from the pinning of the contact lines of nucleating holes in the thin liquid film. The border between liquid film and solid substrate is referred to here as the contact line. We performed a qualitative study using the stop-and-look method in which we could observe how the thin liquid layer film of the solvent remains at the substrate surface once it was removed from the solution. The remaining film was in contact with the atmospheric air, so that the vapor becomes unsaturated with respect to the film over the substrate, and then the layer starts to evaporate. It becomes unstable and breaks up by the creation of rings (see Figure 4). These rings are manifestly in multiple
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Figure 9. Ring structure images of MP(VII) on HOPG after the reduction process: atomic force microscopy images (10 µm × 10 µm × 200 nm) (a) top and (b) 3D view, and scanning electron microscopy (SEM) images at (c) 5000× and (d) 35 000×. Table 2. Qualitative Information about Some Observed Nanostructures Shown for MP(I) Solutions with Different Variables: Concentration, Solvent, and Substrate variables concentrations: 1 mM from CH2Cl2 0.1 mM from CH2Cl2 solvents: CH2Cl2 THF CHCl3 substrates: HOPG Pt Au mica
ring formation
no ring formation
× × × × × × × ×
×
sizes, formed at the pinned contact line of the recently formed pseudo-drops containing nano- or micrometer-sized particles. In a system of volatile liquid/surface interfaces, the particles are drawn by the outward solvent fluxes to the edge of the pinned drop, producing a thick ring, because the flow toward the stationary rim is required to compensate for evaporation losses in maintaining an equilibrium shape of the liquid-vapor interface.18,29,38 The particles (nonvolatile components) accumulate at a rim, around the periphery of the droplet, during evaporation. This process occurring on a substrate is thought to evolve in two distinct modes of evaporation: an initial pinned phase, which corresponds to a constant contact area evaporation with a diminishing contact angle, and a second mode, unpinned phase, in which the contact area diminishes while the contact angle remains fixed.18,38a,41 Our observation of ringlike deposits contrary to a uniform deposit (expected when the contact angle
are constant and contact area shrinks) indicates that the former mode of evaporation will be an explanation in our system, so that the addition of nonvolatile components to the liquid that accumulate at the contact line alters the dynamic of the contact line. Rings have been reported by a group of researchers, Dugas18 et al. and Deegan38a et al. Nevertheless, these rings have a different length scale than that shown in the present work. Our solvent does not form drops on the surface, but rather wets the surface, spreading out to a uniform thin film. In this sense, we believe that ring (tubes) formation occurs due to dewetting of the spread volatile solvent. During evaporation, dewetting may, nonetheless, occur by nucleation and spreading of dry patches. After various analysis of our system, changing the Pd MPs, the solution concentration, the solvent, and the substrate, as presented in Table 2 and compared with the reported literature, we established a transition to a partially wetting situation occurring as part of the process of the evaporation and dewetting process. As we discussed above, the Pd MPs solution completely wets the graphite substrate (HOPG) as well as an amorphous carbon (TEM grid) as a thin film. It evaporates because the system is exposed to the air and the equilibrium vapor pressure P° of solvent exceeds the actual partial pressure (P) of solvent above the film. Because of this condition (P° . P), the film would need to be thin, by evaporation, to physically small dimensions for the liquid to be in equilibrium with its vapor. Equilibrium is reached only when all of the liquid has evaporated. Before reaching this point, the liquid evaporates steadily as a thin film (41) (a) Bourge`s-Monnier, C.; Shanahan, E. R. Langmuir 1995, 11, 2820. (b) Shanahan, M. E. R. Langmuir 1995, 11, 1041. (c) Erbil, H. Y.; McHale, G.; Newton, M. I. Langmuir 2002, 18, 2636.
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until it reaches a thickness below a critical value, at which points instability occurs, in which very thin circular patches nucleated and spread over the film. This patch nucleation also occurs in nonvolatile wetting fluids, where the critical value is the final thickness that is larger than the molecular size.14,24a,42 It is in this last step in which the transition occurs. In other words, a transition to the partially wetting situation occurs, in which a pattern of drops were produced. Similar types of pattern were observed in water during evaporation13c,15 as well as during rupture of nonwetting films.23a Once this pattern is formed, we understand that the droplet evaporation process, previously discussed, proceeded. This interpretation is supported by the experimental results such as the concentration of the particles inside the rings is smaller than the concentration of particles outside (Figure 1), as well as the theory. This is consistent with our inference that these rings are due to holes opening (i.e., pinning) in the liquid film and pushing particles out into the rim. For a more detailed elucidation of the formation mechanism and thermodynamic description, however, further studies are necessary. Figure 5 is a micrograph of a typical view of rings that can be observed on the HOPG surface with the difference that this time it is at another surface (TEM GRID). This picture demonstrates that: (1) the rings are structures regularly formed in the system in which similar conditions repeated again, (2) the rings are in the region of the same size observed by AFM, and (3) there are no distinguished particles at the rim. Spectroscopy analysis of this region corroborates the presence of Pd, Cl, and N. Decomposition of Palladium Molecular Precursors by Heating in a Reductive Atmosphere. The decomposition of several of the Pd MPs under H2 was performed mainly to prepare metallic Pd particles. This process was done with Pd MPs powder and with HOPG-modified surfaces. The reductive process was monitored by XPS and FT-IR techniques. Some of the XPS26 and FT-IR43 results were previously reported, in which a reduction of Pd MPs, to metallic Pd, was confirmed. In these reports, we mentioned the formation of rings when using MP(I), MP(VII), and MP(V), although it was not discussed in detail. Because of the subject of the current Article, we have focused on the formation of the rings. For example, we report how its morphology changes as the substituents in the ligands of the molecular precursor change and the fact that the ring structure was conserved after the reductive process. We are only presenting some important and relevant results of XPS. The reductive process does not destroy the self-assembled rings. Figure 6d shows the ringlike nanostructures after the reductive process. They are the building blocks of the rings on the HOPG surface. The particles varied from micro- to nanosize. The cross-section of those rings shows the presence of small aggregates corresponding to bi- or trilayers. Motte et al. attribute heights between 10 and 15 nm for small aggregates corresponding to bi- or trilayers.44 Figure 6 shows a micrograph of rings with sizes in the range of a few micrometers. They are not continuous, as the particles gather together to form the rings generated by two regions. One region is composed of bright particle spots to create the circumference and the other region in the center of the ring. The results of point analyses, with Auger electron spectroscopy, of these two areas are shown in Figure 6b. From these spectra, it can be seen that the bright particles in the rings (42) (a) Brochard-Wyart, F.; di Meglio, J.-M.; Que´re´, D.; de Gennes, P. G. Langmuir 1991, 7, 335. (b) Cazabat, A.-M. Contemp. Phys. 1987, 28, 347. (43) Diaz-Ayala, R.; Raptis, R. G.; Cabrera, C. R. ReV. AdV. Mater. Sci. 2005, 10, 375. (44) Motte, L.; Lacaze, E.; Maillard, M.; Pileni, M. P. Appl. Surf. Sci. 2000, 162, 604.
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consisted of Pd and the black region in the center of the ring consisted of C and some Pd particles. Scanning Auger microscopy mapping makes these differences more noticeable (see Figure 6c). The result of mapping is in agreement with the result of point analysis in Figure 6b. Figure 7 presents the TEM images of the formation of spider web-like structure surrounding the particles. The particles surrounded by this structure may help them to stay together even after the reduction process. This spider web-like structure will be a formation of amorphous C around the particles as a consequence of the reduction process to which samples were exposed. The reductive process is in the absence of oxygen and the promotion of CO2 is not favored while the graphitic carbon is favored. Although in many TEM images reported by the scientific community, the particles are inside a matrix, in the present case they are slightly different. This behavior was observed more surprisingly for other TEM images at high resolution for other samples treated in the same form, while using other Pd molecular precursor. In addition, XPS analysis for the carbon (C1s) binding energy region of the Pd MP powder, before and after the reduction process, presents a displacement from 286.8 to 284.1 eV. The latter value is characteristic of graphitic carbon. The elemental identification of the particles observed in TEM was done by X-ray microanalysis (EDS). X-ray fluorescence peaks corresponding to Pd and other peaks such as C and Cu, which are the materials of the TEM grid used, are present. The high Hamaker constant value for metal and metal oxides that represent stronger interactions between particles can explain even the process after reduction. Even the particle-particle interactions are greater than the particle-substrate. Several of the particle in the TEM images presented a noticeable change in darkness that was related to coalescence of the particles during the reduction process. We are not the first to report a similar behavior.45 Figure 8 presents the ring self-assembled structures of MP(V) at HOPG surfaces after they were exposed to the thermal reductive process. As observed in the last four figures, the reductive process does not destroy all of the self-assembled rings. The principal difference, once the systems are reduced, is the loss of the continuity in the rim of the rings, and now many particles are observed. This behavior is in accordance with the previous discussed explanation in relation to the morphology of the rings and the Pd MPs. In that point, we separated the Pd MPs into two principal groups: first, one that has methyl groups such as MP(III), MP(VI), and MP(IV), which in addition has Br, and the other that has phenyl substitute. The former does not present rings after the reductive process. In general, the particles are dispersed on HOPG surfaces with some preferences toward HOPG defects, producing a linear structure that may lead to a nanowire formation. In the latter, the reductive process generates rings with separate Pd particles. These rings are not continuous as were those observed prior to the reduction process (see Figures 6-10). In the XPS analysis (not presented in this Article), we could observe, after the hydrogen reductive process, that some characteristic peaks such as photoemission peaks corresponding to Cl and N disappeared, and at the same time a displacement of Pd binding energy peak toward lower values confirmed the reduction of palladium.26
Conclusions Palladium molecular precursors (MPs) present a good alternative for the self-assembly of Pd rings and tubes. The presence of ring structures is due to the thinning, by evaporation, of a (45) Tsai, M. H.; Chen, S. Y.; Shen, P. Nano Lett. 2004, 4, 1197.
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Figure 10. Ring structure images of MP(IV) on HOPG after the reduction process: atomic force microscopy images (AFM) (5µm × 5 µm × 100 nm) (a) top and (b) 3D view, and scanning electron microscopy (SEM) images at (c) 3500× and (d) 100 000×.
completely wetted HOPG surface with MPs dichloromethane solution. The rings are formed when the thin film is unstable and the dewetting process begins. In this dewetting process, a transition from complete to partial wetting occurs, promoting hole nucleations followed by ring and tube nanostructure formation. The π-π interactions between the planar aromatic rings promote the dense wall observed for some ring structures, while the steric hindrance or the higher spacing caused by nonplanar groups are responsible for the interrupted rim, individual particles, and bonfire structures. The smooth surface and defect sites of the HOPG help in the arrangement adopted by Pd MPs during the evaporation/dewetting process and after the reduction process, especially, the arrangement of the particles at HOPG defects. The interdigitation that creates the continuous rim of the rings was destroyed once the samples were exposed to the hydrogen thermal reductive process and all organics components were decomposed. This reductive process promotes the formation of
Pd nanoparticles, maintaining the annular arrangement. The formation of a shell of amorphous carbon during the reductive process, enveloping the particles, is suggested as one of the reasons for the particles to maintain its position as part of the rim in the ring structures. This methodology may be used to prepare nanotubes of Pd complex as well as rings of Pd metal for applications in catalysis, nanoelectronics, and biosensors. Acknowledgment. We gratefully acknowledge financial support from NASA-URC (grant number NASA-NCC-1034). We are much obliged to Professor J. M. Yacaman (University of Texas at Austin) for the opportunity to visit his laboratory and for the TEM analysis. The use of the Surface Microscopy and Spectroscopy Facility, of the Materials Characterization Center (MCC), at the University of Puerto Rico, Rio Piedras Campus, is gratefully acknowledged. LA0605900
