NANO LETTERS
Assembly and Interaction of Au/C Core−Shell Nanostructures: In Situ Observation in the Transmission Electron Microscope
2005 Vol. 5, No. 10 2092-2096
Eli Sutter,* Peter Sutter, and Yimei Zhu Center for Functional Nanomaterials, BrookhaVen National Laboratory, Upton, New York 11973 Received August 1, 2005; Revised Manuscript Received September 13, 2005
ABSTRACT Using transmission electron microscopy, we identify the temperature-dependent interaction pathway of carbon-supported Au nanoparticles. At low temperature (room temp. to 400 °C), Au nanoparticles predominantly interact by coalescence initiated by an atomic Au bridge. At high temperature (400−800 °C), the particles assemble into Au/C core−shell nanostructures. C-shells around individual nanoparticles passivate their surface and prevent coalescence. Ultimately coalescence does occur via rupture of the passivating shells, which invariably follows the assembly of C sheets enveloping and compressing multiple closely spaced Au/C nanoparticles.
The encapsulation of metals in carbon fullerene cages has attracted much interest recently due to the enormous technological promise and the expected novel and exotic physical, chemical, and electronic properties of encapsulated nanoparticles.1-5 Metals such as Co, Ni, Al, and Cu have been encapsulated in carbon fullerene cages via arc discharge processes.1-5 Despite the large interest in these nanoheterostructures, the encapsulation process remains poorly understood in part due to its complexity and the extreme formation conditions that it requires. Of particular importance are particle-particle interactions during thermal processing, such as Ostwald ripening or coalescence, which can profoundly alter the size distribution, and hence the size-dependent functionality, of nanoparticle ensembles. It has been demonstrated that carbon modification6,7,8 and metal particle encapsulation9 can result from intense electron irradiation in a transmission electron microscope (TEM). This finding suggests a route for studying the formation and the subsequent interaction and transformation of encapsulated metal nanoparticles under controlled conditions via real-time TEM observations. In this letter, we use real-time high-resolution TEM imaging to study the interaction of pure and carbon-shellencapsulated Au nanoparticles. Generally, interactions between supported metal nanoparticles are governed by a number of factors, including the random walk mobility of the particles, their surface structure and termination, and the * To whom correspondence should be addressed. Tel: 631-344-7179. Fax: 631-344-3093. E-mail:
[email protected]. 10.1021/nl051498b CCC: $30.25 Published on Web 09/27/2005
© 2005 American Chemical Society
generation and surface diffusion of adatoms on the nanoparticles and on the support. All of these factors depend on temperature and are likely affected by irradiation with energetic electrons. For the Au/C system, our TEM observations identify two global regimes of Au nanoparticle interactions that differ in the degree to which C is assembled into ordered graphene sheets. At low temperatures (room temperature to 400 °C), this structural transformation does not occur, and C plays a minor role. Hence, this regime provides conditions for identifying the pathway of interactions between pure Au nanoparticles. At elevated temperatures above 400 °C, Au efficiently catalyzes the assembly of curved graphene shells enclosing the nanoparticles. In this regime, changes in the interaction pathway of Au nanoparticles due to their encapsulation in a carbon shell can be considered. Our in situ experiments are carried out in a JEOL JEM 3000F field emission TEM equipped with a Gatan 652 hightemperature sample holder. For the metal nanoparticles, gold is chosen for its chemical inertness and stability in air. The specific system considered here is Au particles with ∼5 nm diameter dispersed on ultrathin amorphous carbon films supported by standard Cu grids. The experiments are carried out in the temperature range between room temperature and 800 °C at pressures below 2 × 10-5 Pa and at electron irradiation intensities between 2 and 50 A/cm2. Brief sample illumination at high intensity is used to accelerate interactions between the particles. The high-resolution TEM images are recorded with low electron intensity to prevent any uncon-
Figure 1. Sequence of transmission electron microscopy images characteristic of the interaction of Au nanoparticles in the temperature range between room temperature and 400 °C. See text for details.
trolled structural changes and further interaction of the nanoparticles. Representative TEM images characteristic of the interaction of Au nanoparticles in the temperature range between room temperature (RT) and 400 °C are shown in Figure 1. The image sequence, obtained at RT, follows the interaction of three nanoparticles over a time period of 8 min. Figure 1a shows the initial configuration. All three nanoparticles have approximately the same size (∼5.5-6 nm). The distance between particle 1 and its nearest neighbor, particle 2, is ∼4 nm. Particles 2 and 3 are more closely spaced, only ∼1 nm apart. We consistently find that nearest-neighbor separations of the order of 1 nm initiate a subtle coalescence process that ultimately leads to the formation of a single particle with combined volume. One such coalescence event is documented in Figure 1b-f. The process is triggered by the assembly of an atomic bridge between nanoparticles 2 and 3 (Figure 1b), which connects the two particles in a dumbbell configuration. The total length of the dumbbell in Figure 1b is 13.5 nm. The bridge expands laterally while its length initially remains fixed at the original separation of the two nanoparticles (Figure 1c). Subsequent further inNano Lett., Vol. 5, No. 10, 2005
creases in the width of the bridge are accompanied by a simultaneous decrease in overall length of the dumbbell (Figure 1d) until the width of the bridge becomes comparable to the now reduced diameter of the nanoparticles (4.5 nm). Finally, the dumbbell transforms into one larger particle that assumes a compact shape (Figure 1f). Our observations demonstrate that the coalescence of pure Au nanoparticles is initiated by the formation of a stable atomic bridge once the separation is reduced below a critical value. The gap width spanned by this bridge is about 1 nm, close to the length of stable atomic gold bridges observed in Au-Au break junctions.10,11 The position where the bridge forms appears to be determined by the relative orientation of the nanoparticles, which consist of small facets, ridges, and vertexes. If the closest distance between neighboring particles joins parallel facets, a bridge cannot be established, even if that distance is of the order of the critical distance of about 1 nm. Also, a vertex or ridge on one particle protruding toward a facet on its neighbor does not cause an Au bridge to form. Repeated observations show that bridging only occurs when closely spaced nanoparticles are oriented such that facet vertexes or ridges on the two particles align, 2093
a condition that is often established dynamically during the experiment via fluctuations in the particle shapes.12 Even at room temperature there clearly is significant diffusion of Au adatoms on the surface of the nanoparticles,13,14 which is manifest here via continuous shape changes of the particles. Surface sites such as terraces, steps, and facet vertexes with different coordination present different equipotential landscapes for diffusing Au atoms. As a result, adatom residence times, as well as the lifetimes of transient structures such as small metastable clusters at the particle surface, can differ substantially at these sites.14 Our observations suggest that clusters forming at low-coordinated sites, such as facet ridges and vertexes, can have sufficiently long lifetimes so that, ultimately, the gap between them can be bridged by continued attachment of adatoms, leading to the formation of a stable atomic bridge as observed in Figure 1 b. The fact that adjacent nanoparticles invariably evolve to join via an atomic bridge if they are spaced sufficiently close raises the question if the assembly of the bridge is aided by some form of direct interaction between metastable clusters on the neighboring particles. Under the conditions of our experiment in the electron microscope, electric fields may be most likely to mediate such interactions. Although the Au nanoparticles in this study are nominally held at the same electric potential through their contact via the conducting carbon support, we cannot exclude that small potential differences on the order of few volts could be established under 300 keV electron illumination.15 Scanning tunneling microscopy experiments, with tip-sample separation comparable to the distance between coalescing nanoparticles, have shown that electrical potential differences below 3 V can induce directed diffusion,16 the assembly of metastable surface structures,16 and field evaporation at Au surfaces.17 Although our data offer no conclusive evidence for such a scenario, we speculate that electric fields between adjacent Au particles could assist in the steering and controlled buildup of clusters on both sides, i.e., drive a directed assembly of a contact in the form of an atomic Au bridge. At elevated temperatures between 425 and 800 °C, the interaction between Au nanoparticles is modified drastically. The most striking observation is that even when the nanoparticles are closely spaced (∼1 nm) coalescence is not observed over long time intervals. Figure 2 shows a TEM image of an ensemble of closely spaced Au nanoparticles, held at 550 °C. This ensemble remained unchanged over 2 h. A key difference compared to lower sample temperatures is that the amorphous carbon support becomes modified at high temperature. In the vicinity of the Au nanoparticles, graphene fragments and layers form. Away from the nanoparticles, the carbon remains homogeneously amorphous, suggesting that the crystalline Au surface of our particles catalyzes and provides a template for the ordering and crystallization of C. Even disordered graphene fragments efficiently passivate the particles against coalescence. Under more intense electron irradiation (50 A/cm2) the graphene fragments close and form regular symmetric layers around the nanoparticles until complete shell closure is achieved.9,18 The inset of Figure 2 shows a Au nanoparticle 2094
Figure 2. Transmission electron microscopy image of a group of closely spaced Au nanoparticles at 550 °C. A high-resolution image of Au/C core-shell nanoparticle is shown in the inset.
completely encapsulated in a carbon shell. The C-shells typically consist of 2-5 closed graphene sheets with a separation of d ∼ 0.3-0.4 nm, in good agreement with the interlayer spacing in bulk graphite (0.34 nm). Thus temperatures higher than 425 °C and electron beam irradiation drive the assembly of Au/C core-shell nanoparticles. The graphene shells provide efficient passivation, and despite a close spacing of the nanoparticles comparable to that at which atomic bridges form at lower temperature, contact and coalescence of the particles are prevented even over long periods of time. The question thus arises if Au/C core-shell nanoparticles can indeed interact and coalesce. It has been shown that annealing of arc-discharge generated Co/C and Cu/C coreshell particles over extended time periods (10-12 h) at very high temperatures (1100 °C) leaves behind empty graphene shells, from which the metallic core has been removed.5 This observation suggests that there may indeed be pathways for interaction and coalescence of metal/C core-shell particles. Instead of high-temperature annealing, we use an intense electron beam at lower temperatures to stimulate such processes and less intense irradiation to view them with high spatial and temporal resolution. This method allows us to observe the modified interaction pathway of Au/C coreshell nanoparticles. A typical sequence of TEM images showing the interaction of the nanoparticles stimulated by electron beam irradiation at 475 °C is shown in Figure 3. Figure 3a is obtained prior to exposure of the nanoparticles to an intense electron beam. The remaining images in Figure 3 are recorded in 10 s intervals. During each interval the nanoparticles are exposed to a 50 A/cm2 electron beam for 1 s, followed by imaging at a reduced intensity of 2 A/cm2. Figure 3a shows a group of three particles that are ∼1 nm apart and are very similar, almost spherical in shape. In close proximity of the nanoNano Lett., Vol. 5, No. 10, 2005
Figure 3. Sequence of transmission electron microscopy images characteristic of the interaction of Au/C core-shell nanoparticles in the temperature range between 400 and 800 °C. See text for details.
particles, the amorphous carbon has already undergone structural modifications and consists of disordered graphene fragments. Upon intense electron irradiation, the graphene segments straighten and close around the nanoparticles (Figure 3b). The segments transform into shells that tightly wrap the individual particles and closely follow their shape. Importantly, some of the graphene sheets wrap around the entire ensemble (Figure 3c). Once the assembly of the enveloping sheets is initiated, two simultaneous processes Nano Lett., Vol. 5, No. 10, 2005
take place (Figure 3b-f): (i) the particles change shape and (ii) gradually form a more compact group. The shape change is most pronounced in particle 2: its projected shape develops from almost circular (Figure 3a) via a trapezoidal intermediate shape (Figure 3d) to a nearly triangular projection (Figure 3f). Simultaneously, particles 1 and 3 develop long straight facets on the sides adjacent to particle 2. The C-sheets separating the particles adjust by gradually reducing their curvature, and finally appear almost planar in these contact 2095
areas. Once such straight segments have developed, coalescence of the particles is suddenly initiated. In Figure 3g, particles 1 and 2 already have formed a rather large Au bridge between them while a bridge between particles 2 and 3 is just being established, enabled by a visible rupture in the graphene sheets at the position of the bridge. At a somewhat later stage (Figure 3h-j), the coalescence process continues with particles 1 and 2 forming a single entity evolving toward a compact shape, and particle 3 joining it via a laterally expanding bridge. At the conclusion of the coalescence (Figure 3k) one larger Au particle is formed and three empty carbon shells of different sizes remain in its vicinity. Finally, the large, single Au nanoparticle assumes its equilibrium shape, bounded by well-defined facets that we identify as the (100) and (111) major stable facets of Au (Figure 3l). At elevated temperatures between 425 and 800 °C, we find the interaction of C-supported Au nanoparticles modified by the assembly of an ordered C-shell consisting of several nested graphene layers. The shell plays a 2-fold role in the interaction of the particles: it passivates the surface of the Au particles and initially inhibits thermally driven coalescence via an atomic Au bridge as observed in the lowtemperature regime. Ultimately, however, the self-assembled graphene sheets stimulate coalescence by providing an unexpected mechanical driving force for the breakup of the core/shell structure. Intense electron irradiation stimulates considerable structural rearrangement, and drives the symmetrization and closure of graphene shells around the individual particles and, importantly, around entire ensembles of closely spaced particles. Once formed, shells enclosing multiple particles exert a very high pressure on the enclosed ensemble, driven by their tendency to achieve perfect spherical shape and the ideal bond length and angles of curved graphene layers. The resulting forces exerted on the particle ensemble drive dramatic shape changes, as observed in Figure 3a-f: Particles 1 and 3, enveloped by the shared graphene layer, compress particle 2 which adjusts by assuming a triangular projected shape. This promotes the formation of straight contact areas between the particles and in the sections of the graphene sheets that separate them. At this point, our image sequence shows the onset of coalescence between the particles, made possible by a rupture in the passivating shell providing a contact pathway. Stable, ordered graphene sheets invariably have a well-defined curvature, which matches that of our original 5 nm Au particles. Particles with large equilibrium facets (Figure 3l) never become embedded in ordered C-sheets, but rather assemble layers that curve outward, away from the particle. This suggests that only curved sheets are stable and that strain in highly deformed core-shell structures (Figure 3f) is the likely cause of the observed shell rupture. Finally, the C-shell
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rupture opens a pathway between adjacent particles separated by the shell thickness of ∼1 nm. This triggers a coalescence process that is equivalent to the one at room temperature: a Au bridge is established between the particles, and expands laterally, until a new, compact shape is achieved via continued mass transport. Encapsulation of metal nanoparticles with layered structures, such as carbon, boron nitride, or molybdenum disulfide shells, could be an attractive method for preventing coalescence and associated uncontrolled changes in the size distribution of particle ensembles with size-dependent functionality. Our observations show that thermally induced, Au catalyzed crystallization of C leads to small, disordered graphene segments loosely wrapped around the Au nanoparticles. Such disordered C-shells provide surprisingly efficient passivation against particle coalescence. Intuitively, one would expect a more perfect coating consisting of regular, completely closed graphene shells to provide an even better barrier against interaction. However, our observations demonstrate the contrary. The nonequilibrium conditions necessary to achieve shell closure invariably drive the formation of extended sheets that enclose entire ensembles of particles and generate an unexpected mechanical driving force for the rupture of the passivating shells, thus enabling rapid and uncontrolled coalescence. Acknowledgment. This work was performed under the auspices of the U.S. Department of Energy, under Contract No. DE-AC02-98CH1-886. References (1) McHenry, M. E.; Majetich, S. A.; Artman, J. O.; DeGraef, M.; Staley, S. W. Phys. ReV. B 1994, 49, 11358. (2) Banhart, F.; Hernandez, E.; Terrones, M. Phys. ReV. Lett. 2003, 90, 185502. (3) Sato, Y.; Yoshikawa, T.; Okuda, M.; Fujimoto, N.; Yamamuro, S.; Wakoh, K.; Sumiyama, K.; Suzuki, K.; Kasuya, A.; Nishina, Y. Chem. Phys. Lett. 1993, 212, 379. (4) Sun, X.-C.; Nava, N. Nano Lett. 2002, 2, 765. (5) Jiao, J.; Seraphin, S. J. Appl. Phys. 1998, 83, 2442. (6) Banhart, F.; Ajayan, P. M. Nature 1996, 382, 433. (7) Ugarte, D. Nature 1992, 359, 707. (8) Banhart, F. Rep. Prog. Phys. 1999, 62, 1181. (9) Xu, B. S.; Tanaka, S.-I. Acta Mater. 1998, 46, 5249. (10) Rodrigues, V.; Ugarte, D. Phys. ReV. B 2001, 63, 073405. (11) Coura, P. Z.; Legoas, S. G.; Moreira, A. S.; Sato, F.; Rodrigues, V.; Dantas, S. O.; Ugrate, D.; Galvao, D. Nano Lett. 2004, 4, 1187. (12) Iijima, S.; Ichihashi, T. Phys. ReV. Lett. 1986, 56, 616. (13) Bovin, J.-O.; Wallenberg, R.; Smith, D. Nature 1985, 317, 47. (14) Iijima, S.; Ichihashi, T. Jpn. J. Appl. Phys. 1985, 24, L125. (15) We do not observe directed diffusion jumps such as those for Au particles on insulating SiO2 support,12 i.e., substantial charging of the particles can be excluded. (16) Kim, J.; Uchida, H.; Yoshida, K.; Kim, H.; Nishimura, K.; Inoue, M. Jpn. J. Appl. Phys. 2003, 42, 3616. (17) Mamin, H. J.; Guethner, P. H.; Rugar, D. Phys. ReV. Lett. 1990, 65, 2418. (18) Ugarte, D. Carbon 1995, 33, 989.
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