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Thermal and Chemical Stability and Adhesion Strength of Pt Nanoparticle Arrays Supported on Silica Studied by Transmission Electron Microscopy and Atomic Force Microscopy Aaron S. Eppler,†,‡ Gu1 nther Rupprechter,§ Erik A. Anderson,‡ and Gabor A. Somorjai*,†,‡ Department of Chemistry, UniVersity of California, Berkeley, California 94720, Materials Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720, Chemical Physics Department, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany ReceiVed: February 18, 2000
The thermal, chemical, and mechanical stability of Pt nanoparticles supported on silica has been measured with transmission electron microscopy (TEM) and atomic force microscopy (AFM). The nanoparticle arrays were fabricated using electron beam lithography, which produced uniform particle sizes (20 ( 1 nm) and uniform interparticle distances (150 ( 1 nm). TEM studies provided information about the array periodicity, particle dimensions, and crystallinity of individual particles. Before heat treatments, individual Pt nanoparticles were found to be polycrystalline with crystalline domain sizes of 4-8 nm. After heating to 1000 K in high vacuum (10-7 Torr) and 1 atm H2, the crystalline domain sizes within individual particles grew larger, without noticeable deformation of the array. A similar enlargement of crystalline domains was seen in 1 atm O2 at a lower temperature of 700 K. Using contact mode AFM, the height, periodicity, and adhesion of the particles were determined. On a newly prepared sample, Pt particles were displaced from the silica support by the AFM tip with approximately 10 nN lateral force. The interfacial adhesion energy between the Pt and SiO2 was on the order of 1 mJ/m2, which is relatively weak bonding. After heating, the Pt particles could not be displaced by the AFM tip, suggesting that heat treatments had increased the bonding between the Pt and SiO2. The stability and uniformity of the nanoparticle arrays make them ideal model catalysts for reactions in either oxidizing or reducing conditions.
Introduction Using electron beam lithography, arrays of Pt nanoparticles have been fabricated on silicon oxide substrates. The Pt/SiO2 arrays are ideal model catalysts, since the particle size and periodicity can be easily controlled by changing the lithographic pattern.1,2 However, before subjecting these model catalysts to the high pressures and high temperatures used in most catalytic reactions, it is important to characterize the stability of the arrays. Previous work by Johansson et al. on similar model systems of Pt particles supported on CeO2 showed that the particles were either significantly recrystallized or disintegrated, depending on the size, in an oxygen atmosphere, but not in a noble gas or hydrogen atmosphere.3 In another study of a similar system of Pt crystals deposited from solution onto a carbon grid, Wang and co-workers found that surface melting occurred for crystalline particles with diameters between 5 and 20 nm at a temperature of about 900 K.4 In this paper, the structural changes occurring in Pt nanoparticle arrays supported on silica and the microcrystalline structure of individual nanoparticles has been investigated with transmission electron microscopy (TEM) both before and after heating in a vacuum (10-7 Torr), H2, and O2. In all environments, the arrays were stable, while the size of the crystallites within one particle increased as the temperature was raised. Possible mechanisms of crystallization, such as surface melting and intraparticle diffusion of Pt, are discussed. The adhesion †
University of California. Lawrence Berkeley National Laboratory. § Fritz-Haber-Institut der Max-Planck-Gesellschaft. ‡
between the Pt nanoparticles and the SiO2 support was investigated with atomic force microscopy (AFM), both before and after heating the arrays. Our results show that the nanoparticle arrays are stable under both oxidizing and reducing conditions, although heat treatments changed the crystallinity of the particles and increased the adhesion of the Pt particles to the SiO2 substrate. Experimental Section Samples. The nanoparticle arrays were fabricated using electron beam lithography. The first step in the EBL fabrication process was to spin-coat poly(methyl methacrylate) (MW ) 950 000) onto a Si(100) wafer with 5 nm thick SiO2 on the surface. Computer-designed patterns were then “written” into the polymer layer with a highly collimated electron beam (Leica column) generated by a field emission source. With a beam current of 600 pA and accelerating voltage of 100 kV, the beam diameter was approximately 8 nm. A dose of 2500 µC/cm2 (4 × 10-16 C/site) was used to expose the PMMA, resulting in a dwell time of about 0.6 µs at each particle site. Following dissolution of the exposed polymer, a 15 nm thick film of Pt was then deposited on the sample by electron beam evaporation. Finally, the remaining resist was removed (“lifted-off”) by dissolution and the metal particles of the prescribed pattern remained on the substrate. With this high degree of spatial resolution, 36 mm2 arrays with approximately 109 metal particles were produced. All samples used in this experiment and some of their important characteristics are listed in Table 1. Three different
10.1021/jp0006429 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/19/2000
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TABLE 1 sample
diameter (nm)
interparticle distance (nm)
experiment
A B C D
25 40 20 40
200 200 200 150
heated in a vacuum/TEM heated in H2/TEM heated in O2/TEM adhesion/AFM
samples were used for the TEM thermal stability studies: sample A (25 nm diameter particles) was heated to 1200 K in a vacuum, sample B (40 nm diameter particles) was heated to 1000 K in 760 Torr of H2, and sample C (20 nm diameter particles) was heated to 700 K in 760 Torr of O2. Sample D, with 40 nm diameter particles, was used for AFM adhesion studies. The pattern of the arrays was a square lattice with either 150 or 200 nm (center-to-center) pitch, the interparticle distance (centerto-center). The diameter of the particles and pitch of the sample were accurate to within (1 nm. The particle height on each sample was 15 nm. Transmission Electron Microscopy. Bright field (BF) and dark field (DF) images were acquired with a JEOL 200CX equipped with a heating stage for in situ temperature studies. The microscope was operated at 200 kV with typical beam current densities of 0.5-5 A/cm2. For high-resolution work, a JEOL ARM-1000 was used at 800 kV and a Philips CM300FEG/ UT with a field emission source was used at 300 kV. TEM disks with electron transparent regions were prepared by cutting and thinning a disk from the patterned wafer, which is a 0.5 mm thick Si wafer covered with native SiO2 (≈50 Å thick) and the Pt nanoparticles (Figure 1). First, the catalyst was mounted with a polymer adhesive to a glass slide, with the particles facing the slide. A 3 mm diameter disk was cut from the catalyst using an ultrasonic disk cutter (Gatan). The sample was then thinned mechanically using a polisher (Minimet) to 200 µm thickness and then to near transparency with a Dimpler (VCR). The Dimpler milled a convex hole on the backside of the sample, making the center of the sample the thinnest part. Finally, the sample was thinned by bombardment with Ar+ at 15 kV and a 15 degree angle, creating an electron transparent region in the center of the sample. Atomic Force Microscopy. A Park Scientific Instruments M5 was used for AFM measurements. Before taking measurements with the AFM, the arrays were cleaned with a stream of dry nitrogen. Both constant force and constant height modes were used to investigate the samples and each mode provided the same experimental results. The force constant of the AFM cantilever was not measured experimentally but reported by the manufacturer (Silicon-MDT) to have a typical value of 0.6 N/m and a range between 0.15 and 1.5 N/m. The AFM images provided information about the height and periodicity of the particles, but not the diameter of the particles, since the curvature of the AFM tip was convoluted in the image. Deconvolution techniques were not employed, since particle diameters were accurately measured with the TEM. Results and Discussion TEM Characterization of Untreated Arrays. Bright field and dark field images showed that all three samples characterized with the TEM (samples A-C) were polycrystalline with 3-6 nm crystalline domain sizes prior to heat treatments (Figures 2a,b, 3a,b, and 4a,b). There were no significant differences between the three samples, except for the particle diameters. The difference in diameters was not considered an important parameter leading to differences in stability because
Figure 1. Schematic depicting the preparation of TEM disks from Pt nanoparticle arrays.
all of the particles contained on the order of 100 000 atoms. With typical beam current densities of about 1 A/cm2, no changes in the arrays were observed with the TEM in the absence of heating. Therefore, it was unlikely that the electron beam in the TEM had a direct influence on the stability of the arrays, although Nepijko et al. found that higher current densities destabilized Pt particles grown on γ-Al2O3.5 TEM Characterization of Heat-Treated Arrays. Heating in High Vacuum (10-7 Torr). An array of 25 nm particles (sample A) was heated in a vacuum (10-7 Torr) from 300 to 1200 K over 6 h and continuously monitored in situ with TEM. The temperature was initially increased to 800 K within 2 h and then increased hourly in increments of 100 Κ, eventually reaching a final temperature of 1200 K. The particles never migrated across the support at any temperature (Figure 5a). However changes were observed in the crystalline structure within a single particle. Up to 1000 K, the particles essentially maintained their original polycrystalline morphology. After heating for 1 h at 1000 K, bright and dark field images showed that the crystalline domains in the particles had grown in size (Figure 2c,d). After heating for 1 h at 1200 K, some of the particles appeared to lose all of their polycrystalline nature (data not shown). Regularly oriented single crystals were not observed at any temperature and this was probably because the SiO2 support was amorphous and did not provide any preferential ordering at the interface. Individual particles did not migrate over the surface at any temperature, although some new features were observed after heating at 1200 K for 1 h. The chemical identity of these new features was not determined. Further evidence for crystallization of the particles can be seen in high-resolution TEM images taken before and after the heat treatments (Figure 6a,b). In the image taken before heat treatments (Figure 6a), lattice planes of Pt show that the crystallites within one particle do not have a specific orientation with respect to one another, demonstrating the polycrystalline nature. In Figure 6b, the polycrystalline nature of the particle can no longer be seen. More complete information about the internal structure of the particle and true crystallinity is difficult
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Figure 2. TEM images of sample A. Particle diameters were 25 ( 2 nm. (a) Bright field (BF) before any treatment and (b) the corresponding dark field (DF) image. (c) BF after heating in a vacuum at 1200 K for 1 h and (d) the corresponding DF image.
Figure 3. TEM images of sample B. Particle diameters were 40 ( 2 nm. (a) Bright field (BF) before any treatment and (b) the corresponding dark field (DF) image. (c) BF after heating in 1 atm H2 at 1000 K for 3 h and (d) the corresponding DF image.
to obtain from these images, due to the lack of information about the orientation of the grains.6
Heating in 760 Torr of H2. When heating in high-pressure gas, following changes in situ with TEM was not possible, so
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Figure 4. TEM images of sample C. Particle diameters were 20 ( 2 nm. (a) Bright field (BF) before any treatment and (b) the corresponding dark field (DF) image. (c) BF after heating in 1 atm O2 at 700 K for 4 h and (d) the corresponding DF image.
Figure 5. Low-magnification images showing that the array of Pt nanoparticles remained intact during heating in (a) vacuum at 1200 K, (b) 760 Torr of H2 at 1000 K, and (c) 760 Torr of O2 at 700 K.
the sample was first characterized with the TEM, then removed and heated in a cell with the appropriate gas, and then analyzed again with the TEM. After characterizing a newly prepared array (sample B) with TEM (Figure 3a,b), the sample was heated at 700 K in 760 Torr of H2 for 3 h in a glass cell. As with the vacuum sample, they array remained intact during heating (Figure 5b). Inspection of the particles after heating at 700 K for 3 h showed no changes in the array or particle structure (data not shown). However, the same TEM sample was then heated at 1000 K in 760 Torr of H2 for 3 h. As in a vacuum, under 760 Torr of H2 at 1000 K, the crystalline domains within one particle grew significantly larger, as seen in BF and DF images (Figure 3c,d). Heating in 760 Torr of O2. Sample C was characterized with TEM before heat treatments (Figure 4a,b) and then heated in
760 Torr of O2 at 700 K for 4 h. As with samples A and B, the array remained intact during heating (Figure 5c). However, in contrast to the particles treated in a vacuum (sample A) and H2 (sample B), after heating in O2 at 700 K, the size of crystallites within a particle grew larger (Figure 4c,d). Further experiments at higher temperatures were not carried out in O2. Discussion of TEM Data. Two mechanisms were considered for crystallite growth: (1) melting of the particles and (2) Pt diffusion within a particle. The melting of nanoparticles can be influenced by the radius of curvature of the particle, since this generates a pressure gradient across the interface between the particle and the surrounding medium. There are many reports that theoretically and experimentally describe the dependence of melting temperature on particle size.7-11 There is also a large discussion about the importance of surface “pre-melting” on
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Figure 6. High-resolution TEM images of sample A: (a) before heat treatments and (b) after heating in high vacuum at 1200 K for 1 h.
TABLE 2 symbol
physical meaning
value
Fs To ∆Hfus γSVa γSLa Fl
density of solid Pt melting temperature, bulk enthalpy of melting surface tension solid-vapor surface tension solid-liquid density of liquid
2.14 × 104 kg/m3 2069 K 1.005 × 105 J/kg 2.6-3.5 N/m2 0.334 J/m2 1.98 × 104 kg/m3
a
Pluis, calculated.11
the melting temperature of small particles and bulk metals.12-15 Most of these articles focus on particles much smaller than those described in this paper. Our Pt particles have between ca. 30 000 and 100 000 atoms, while most of these previous studies focus on particles having less than 1000 atoms, where size dependent properties become more significant. Nevertheless, the possibility of a decrease in melting temperature was considered and is discussed below. A thermodynamic equation describing the effect of the radius and the melting temperature and vapor pressure of particles with curved surfaces has been previous derived by Pawlow16 and further expanded on by Castro et al.9 to take into account the changes of surface tension with temperature. Since the modifications by Castro resulted in small (
