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J. Phys. Chem. C 2007, 111, 10824-10828
Electron Beam-Induced Fragmentation and Dispersion of Bi-Ni Nanoparticles William D. Pyrz,† Sangmoon Park,‡ Tom Vogt,‡ and Douglas J. Buttrey*,† Center for Catalytic Science and Technology, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716, and NanoCenter and Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed: February 19, 2007; In Final Form: May 15, 2007
We report the use of electron beam-induced fragmentation (EBIF) to produce arrays of nonoverlapping bimetallic Bi-Ni nanoparticles. Factors influencing this fragmentation process include the precursor particle composition, morphology, melting temperature, type of melting (peritectic vs eutectic), and the ability to dissipate thermal energy and charge. Based on this study, a plausible mechanistic explanation for the observed trends is discussed based on partial melting and charge accumulation fueled by the high-energy electron beam. EBIF finds potential application in the controlled production of nanoparticle arrays that allow the efficient exploration of size- and composition-dependent physical and chemical properties.
Introduction High-energy electron beams, such as those used in a transmission electron microscope (TEM), can rapidly transfer both thermal energy and electrical charge to a specimen. Under conditions that sufficiently limit the dissipation of thermal energy and charge, particle fragmentation can occur, resulting in fields of nanoparticles. Several systems such as Au,1 Pb,2 Bi,3 and NiBi nanowires,4 as well as an oxide system of Fedoped SnO2 and several metal azides,5,6 have been reported to display electron beam induced fragmentation (EBIF). Despite being reported for multiple systems, attempts to rationalize this fragmentation are limited. Ru suggested that fragmentation in Au particles was caused by thermal and electrical effects induced by the electron beam in combination with excess vacancies and voids in micron-sized particles.1 Lu et al. described this phenomenon in oxide clusters to be a rapid buildup of heat and pressure that induced both a phase transformation and nanoparticle dispersion.5 Herley et al. described the formation of nanoparticles through a process that started with the melting of azide compounds, radiolytic decomposition evolving N2, and finally atomization to produce nanoparticles.6 Controlled deposition is of paramount importance in nanoparticleapplicationsincludingcatalysis,7 single-electrondevices,8-10 and optoelectronic devices.11,12 In large part, the desired sizedependent properties are a consequence of large surface-tovolume ratios. One major difficulty in the synthesis of nanoparticles is the tendency for agglomeration due to the high surface energy. In this work, we use micrometer-scale hydrothermally synthesized Bi-Ni parent particles in which we induce fragmentation to produce fields of non-overlapping bimetallic nanoparticles of varying composition and size. We argue that these parent particles, under conditions that limit the dissipation of thermal energy and electrical charge, melt incongruently to produce a Ni-rich solid and a Bi-rich liquid. The latter then fragments due to the rapid accumulation of charge and results in a field of crystalline Bi-rich non-overlapping bimetallic * Corresponding author. E-mail:
[email protected]. Phone: (302) 8312034. Fax: (302) 831-2085. † University of Delaware. ‡ University of South Carolina.
nanoparticles. The peritectic nature of the liquidus curve for both of the line phases NiBi and NiBi3 (Figure 1) ensures incongruent (partial) melting and compositional variation leading to the bimetallic product compositions observed.13 The absence of overlapping nanoparticles strongly suggests that the droplets from which these nanoparticles form accumulate surface charge from the incident beam. Analyses of the resulting nanoparticle fields showed that the distance between the “parent” particle and the nanoparticles correlates with both composition and size. Bismuth nanomaterials have generated significant attention due to their unique properties which include a highly anisotropic Fermi surface, low carrier densities, small effective masses, and long carrier mean free paths.14 At a diameter of less than 50 nm, bismuth nanowires begin to exhibit semiconducting properties that differ from the typical bulk semimetallic behavior.15 The addition of nickel to bismuth opens the door for practical applications that include catalytic synthesis of large-diameter single-wall carbon nanotubes16 and viable alternatives to the lead-based solders currently employed in the electronics industry for printed circuit boards.4 These intermetallic compounds have also drawn interest due to onset of superconductivity at low temperatures.17,18 The use of EBIF enables the production of an array of non-overlapping Ni-Bi nanoparticles varying in both size and composition that might allow one to screen the unique size- or composition-dependent physical and chemical properties in parallel.6 EBIF can be induced in ternary and higher-order compositions containing peritectics, particularly those containing low-melting elements, and thus provide a novel method to produce large non-overlapping nanoparticle arrays. Experimental Methods Hydrothermal reduction reactions with the following ratios of Bi(NO3)3‚5H2O (Alfa, 99.999%) and Ni(NH4)2(SO4)2‚6H2O (Alfa) were used to synthesize Bi-Ni intermetallics: 10 molar % excess in Ni, 10 molar % excess in Bi, Bi:Ni ratios of 1:1, 2:1, 3:1, 4:1, and pure Bi. Ni(NH4)2(SO4)2‚6H2O was dissolved into 10 mL distilled water and appropriate ratios of Bi(NO3)3‚ 5H2O were added to a 23 mL Teflon-lined Parr reactor. The solution was stirred for 15 min followed by the slow addition of 3 mL N2H4‚H2O (Alfa 99+). The solution was sealed in an
10.1021/jp071414i CCC: $37.00 © 2007 American Chemical Society Published on Web 06/29/2007
Fragmentation and Dispersion of Bi-Ni Nanoparticles
Figure 1. Phase Diagram for the Ni-Bi binary system adapted from Feschotte et al.13
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Figure 3. XRD patterns for the as-synthesized particles from the hydrothermal method at 150 °C. The synthesized samples are the 10 molar % excess Ni, 10 molar % excess Bi, and Bi:Ni ratios of 1:1, 2:1, 3:1, and 4:1. Reference patterns for phase identification are the BiNi(Mono), BiNi(Hex), Bi3Ni(Ortho), Ni, and Bi. The as-synthesized products are a mixture of the BiNi, Bi, and Bi3Ni phases.
Figure 2. (a) HAADF micrograph of the as-synthesized product prior to EBIF. (b) SEM image showing the product morphology which includes a wire-like mesh of small particles and (c) large aggregates of small particles.
autoclave and kept at 150 °C for 18 h. Following the reaction, the precipitate was washed with distilled water and ethanol, and then centrifuged. Phase identification was done using a Rigaku MiniFlex diffractometer with Cu KR radiation over a 2θ range from 20°60°. TEM analyses were performed on a JEOL 2010F equipped with a Schottky field emission gun operated at 200 keV with an ultrahigh-resolution pole piece providing a point resolution of 1.9 Å. Energy dispersive X-ray spectroscopy (EDS) provided elemental analysis and mapping using an EDAX Phoenix X-ray spectrometer with a resolution of 134 eV over a 40 keV range. EDS analysis was performed in STEM mode using a 0.5 to 0.7 nm diameter nanoprobe. All samples were prepared by first sonicating the nanoparticle solutions for 5 min, and then applying 1-2 drops of solution to a 200- or 300-mesh Cu grid coated with an ultrathin carbon film. Results and Discussion The as-synthesized Bi-Ni compounds and mixtures appeared black. Figure 2a shows a representative high-angle annular dark field (HAADF) image of the product. In our investigations we found particle sizes ranging from 50 nm to several microns in diameter. SEM images displayed in Figure 2b,c show that the Ni-Bi product consists of clusters of particles that form large irregularly shaped aggregates or wire-like meshes, where most particles exhibit features of both morphologies. There were no significant trends observed in the sample morphology as the precursor ratio was varied from Ni-rich to Bi-rich. The structures of the Bi-Ni compounds were identified using X-ray powder diffraction (XRD) as shown in Figure 3. Reference patterns for Ni, Bi, BiNi, and Bi3Ni were used for phase
Figure 4. (a) HAADF image displaying the non-overlapping nanoparticle field generated by the EBIF. (b) HAADF micrograph showing the post electron irradiation image of the particle in Figure (1a) displaying the drastic changes to the parent material. (c) Lowmagnification and (d) high-magnification SEM images after an EBIF event showing both the parent particle and nanoparticle size and morphology.
determination. Distinctive peaks appearing in the range 27° < 2θ < 34° allow the preliminary identification of Bi, BiNi, and Bi3Ni phases. For the 10% molar excess of Ni, the stoichiometric BiNi mixture, and the 10% molar excess of Bi samples, the dominant phase was the BiNi intermetallic with a trace of Ni impurity. For the Bi:Ni ratios 2:1, 3:1, and the 4:1, mixtures of Bi, BiNi, and Bi3Ni were found. Under convergent beam conditions, the Bi-Ni agglomerates fragment and produce radial fields of nanoparticles that extend uniformly for several hundred microns across the carbon film. The time scale of this phenomenon appears to be on the order of seconds. Figure 4 shows typical results observed after EBIF. Figure 4b shows the parent particle from Figure 2a, demonstrating the drastic changes that occur during EBIF. For the remainder of the discussion, the agglomerates present before EBIF will be referred to as the parent particle and references to nanoparticles will refer to the particles that are formed due to EBIF. Intriguingly, there are no overlapping nanoparticles in the micrographs suggesting the presence of repulsive forces due to surface charging induced by electron beam irradiation. Several of the fragmentation fields were subsequently imaged in the SEM, as shown in Figure 4c,d, and these images show that the
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Figure 5. The diagrams on the left display the possible melting paths for the compounds in the NiBi phase diagram. HAADF images in panels a and b show the formation of a porous pumice-like solid after EBIF. HAADF images show in panel c show typical images of a large nanoparticle field and, in panel d, a small nanoparticle field.
Figure 6. Particle size distributions as a function of radial distance resulting from EBIF showing two distinct size distributions (a) 1-50 nm in diameter and (b) 1-500 nm in diameter.
nanoparticles have either a hexagonal or a circular periphery and the parent material observed after EBIF reveals a porous network of particles. Following EBIF, three trends were seen that appear to be correlated with the composition of the parent particle. First, as the Bi:Ni precursor ratio increased, both the size of the nanoparticle field and the upper bound of the nanoparticle diameter within a given field increased as shown in the images in Figure 5c,d. Second, the measured particle size fell into one of two distinct size distributions (Figure 6). For each EBIF event studied, the size distribution was determined by taking sequential images in a series that extended in a radial direction from the parent particle. Within each image, the average particle size was determined and the distance marked as that from the edge of the parent particle to the center of the image field. Third, there appears to be a higher probability of EBIF initiation as the Bi: Ni precursor ratio increases to an optimum value of 3:1. Above
this optimum, rather than EBIF, the parent particle melts and evolves from an irregular morphology to spherical morphology. These trends from EBIF-favored to contiguous melt-favored transformation can be rationalized by taking into account the bulk Bi-Ni phase diagram. In the case of EBIF, rapid heating to produce partial melting leads to formation of nanoscale pockets of liquid finely distributed throughout the concomitant solid phase. We believe that the formation of these liquid pockets provides a very high interfacial surface area which can accommodate significant charge build up. This accumulation of charge might provide the driving mechanism for the fragmentation process. Evidence to support this is seen in the SEM images in Figure 5a,b, where the parent particle shows a porous morphology after fragmentation. The likelihood of fragmentation appears to increase with increasing liquid fraction to a certain threshold near 80-85% Bi. Above this level, the development of a contiguous liquid with a lower surface area seems to be favored, probably due to wetting of the underlying conductive carbon film that enhances charge dissipation. The observation of two particle size distributions can be explained by analyzing the composition dependence of the partial melting of the intermediate Bi-Ni compounds (Figure 5). The higher melting BiNi phase is expected to partially melt at 919 K, and form a mixture of 25% liquid/75% solid. The lower melting Bi3Ni phase incongruently melts at 740 K and forms a mixture that is 75% liquid/25% solid. Given an EBIF event involving parent particles of similar size, we expect higher Bi-content precursor ratios to give larger nanoparticle fields and a larger upper bound on nanoparticle size relative to a particle with lower Bi content, since the former is expected to contain a higher volume fraction of liquid. The composition of the nanoparticles was determined using EDS in STEM mode. Using the nanoprobe, individual nanoparticles from each EBIF event were examined and Figure 7a shows the nanoparticle composition as a function of radial distance from the parent particle. Interestingly, the nanoparticles always appeared to be single-phase solid solutions with compositions ranging from 50 to 100 atomic percent Bi, rather than ordered intermetallic phases, perhaps due to rapid cooling upon contact with the carbon film. As the particle size decreased, which also corresponded to a distance further away from the parent material, the Bi content approached 100%. As expected, nanoparticles generated from parent material with a 1:1 Bi:Ni
Fragmentation and Dispersion of Bi-Ni Nanoparticles
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Figure 8. (a) High-resolution image of a hexagonal shaped Bi particle showing the atomic structure looking down the [100] zone axis. (b) Corresponding FFT pattern for the image in panel a showing the simulated diffraction pattern. Figure 7. (a) Plot of atomic Bi % as a function of radial distance for individual nanoparticles using energy dispersive X-ray spectroscopy in STEM mode. (b) Elemental mapping showing the HAADF image, (c) Ni K-edge map, and (d) Bi M-edge map providing further evidence for bimetallic compositions in the large particles. The arrow indicates a region of pure Ni particles.
ratio showed an enhancement of the Ni content compared to more Bi-rich precursors. For all starting compositions, analysis of the residual parent particle after EBIF showed that it was very rich in Ni, with atomic compositions ranging between 70% and 98%. The observation of nanoparticles that differ in composition from their parent particle is not surprising due to the incongruent (peritectic) melting of the intermetallic NiBi and NiBi3 line phases. The incongruent melting initially leads to a Bi-rich liquid that is the first liquid dispersed from the parent particle. As the Bi-rich liquid is dispersed from the parent particle, subsequent melting will lead to liquid compositions with increased Ni mole fraction. X-ray mapping was used to provide further evidence to show that the nanoparticles were bimetallic. EDS elemental maps shown in Figure 7b-d show the HAADF image, the Ni K edge, and the Bi M edge respectively. Comparing the elemental maps to the original HAADF image reveals that the nanoparticles were in fact bimetallic and the uniformity in the images seems to indicate an even distribution of the elements within the nanoparticles, which further supports a single-phase solid solution. Close comparison of the Ni K-edge map to the Bi M-edge map also shows the appearance of pure Ni particles at the edge of the parent agglomerate. The structures of the nanoparticles with diameters ranging between 5 and 50 nm were determined using HREM imaging and Fast Fourier Transform (FFT) analysis, with an example shown in Figure 8. The images revealed that the particles were all single phase and no interfaces were observed to provide evidence of mixed-phase content. Figure 9 shows the measured lattice spacings as a function of particle size compared to the lattice spacings of elemental Bi (a ) 4.5462 Å, c ) 11.8626 Å).19 Measured d-spacings were consistent with the rhombohedral structure of pure Bi, and no distinguishable distortions were observed due to the solid solution with Ni, despite the smaller radius of Ni. The confidence in the d-spacing is limited to about (3%. Assuming an ideal solid solution, and using Vegard’s rule to estimate the difference in the lattice due to Ni substitution into rhombohedral Bi,20 we would not expect to be able to confidently resolve a change in lattice constants at the Ni concentrations measured in the 5-50 nm particles. Additionally, there was no observed correlation of lattice spacings with nanoparticle size. This result was in agreement with the size-
Figure 9. Plot showing the measured interplanar spacings as a function of particle size measured through the use of FFT analysis on HREM images. The horizontal lines represent the d-spacings (hkl listed on right) for the rhombohedral structure of Bi (JCPDS (a ) 4.5462, c ) 11.8626).19
dependent structural studies of nm-sized Bi clusters of Wurl et al.,21 but inconsistent with the studies showing a structural phase transition from rhombohedral to cubic or pseudo-cubic symmetry as reported by Oshima et al. for Bi crystallites larger than 8.4 nm in diameter.22 The lack of a structural transition may be due to the inclusion of Ni within the nanoparticles. Continuous structural fluctuations were observed for nanoparticle sizes below 5 nm, similar to those reported for other systems such as Au nanoparticles by Ajayan and Marks,23 and in Sn-Bi alloy nanoparticles by Lee and Mori.24 Our attempts to record the different structures for these very small nanoparticles were unsuccessful due time-resolution limits on image capture. We hope to capture such images under cryogenic conditions in the near future. Conclusions We believe that the observed EBIF behavior may be fairly common, although the Bi-Ni system is probably a special case in which this phenomenon is easily induced. The reasons for this seem to involve four contributing factors: (1) one of the end members has a much lower melting temperature than the other, (2) intermediate compounds in the phase diagram are incongruently melting, (3) the residual solid phase has a fibrous or porous morphology such that the surface area is very large, and (4) the particle is placed on a substrate in a manner that inhibits thermal energy and electrical charge dissipation. These combined factors lead to the formation of a highly dispersed liquid phase, rich in the low-melting component, which can accumulate significant charge while being rapidly heated in the electron beam. It seems that this dense accumulation of charge leads to Coulombic forces that drive the fragmentation process, and ensures that the resulting nanodroplets do not make contact with one another. Crystallization upon deposition may be so
10828 J. Phys. Chem. C, Vol. 111, No. 29, 2007 rapid as to preclude ordered line compound formation, leaving a random or weakly correlated atomic distribution in the nanoparticles. EBIF could become a viable method for the controlled deposition of bimetallic nanoparticles composed of at least one low-melting material in a phase diagram exhibiting incongruent melting. Further efforts are underway to test the generality of this concept. Systems of nonoverlapping bimetallic (or multimetallic) nanoparticles of varying size and composition, if deposited on a substrate, should lend themselves to analysis using various scanning probe techniques, thus allowing combinatorial searches for unique sizes and compositions that provide optimal properties for specific applications. Acknowledgment. W.P. and D.B. thank R. F. Lobo for helpful discussions and C. Ni with the Keck Microscopy facility for TEM access and assistance. T.V. and S.P. acknowledge support from the State of South Carolina and internal funds from the Vice President’s of Research & Health Science’s Office at the University of South Carolina. W.P. and D.B. acknowledge support from the U.S. Department of Energy, Grant # DE-FG0203ER15468. References and Notes (1) Ru, Q. App. Phys. Lett. 1997, 71, 1792-1794. (2) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. App. Phys. Lett. 2002, 80, 309-311. (3) Li, Y. D.; Wang, J. W.; Deng, Z. X.; Wu, Y. Y.; Sun, X. M.; Yu, D. P.; Yang, P. D. J. Am. Chem. Soc. 2001, 123, 9904-9905.
Pyrz et al. (4) Ould-Ely, T.; Thurston, J. H.; Kumar, A.; Respaud, M.; Guo, W. H.; Weidenthaler, C.; Whitmire, K. H. Chem. Mater. 2005, 17, 47504754. (5) Lu, B.; Wang, C.; Zhang, Y. App. Phys. Lett. 1997, 70, 717-719. (6) Herley, P. J.; Jones, W. Z. Phys. D 1993, 26, S159-S161. (7) Stakheev, A. Y.; Kustov, L. M. App. Catal., A 1999, 188, 3-35. (8) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323-1325. (9) Davidovic, D.; Tinkham, M. App. Phys. Lett. 1998, 73, 39593961. (10) Devoret, M. H.; Esteve, D.; Urbina, C. Nature 1992, 360, 547553. (11) Skomski, R. J. Phys.: Condens. Matter 2003, 15, R841-R896. (12) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Ann. ReV. Mater. Sci. 2000, 30, 545-610. (13) Feschotte, P.; Rosset, J. M. J. Less-Common Met. 1988, 143, 3137. (14) Yang, B. J.; Li, C.; Hu, H. M.; Yang, X. G.; Li, Q. W.; Qian, Y. T. Eur. J. Inorg. Chem. 2003, 3699-3702. (15) Heremans, J.; Thrush, C. M.; Lin, Y. M.; Cronin, S.; Zhang, Z.; Dresselhaus, M. S.; Mansfield, J. F. Phys. ReV. B 2000, 61, 2921-2930. (16) Kiang, C. H. J. Phys. Chem. A 2000, 104, 2454-2456. (17) Fujimori, Y.; Kan, S.; Shinozaki, B.; Kawaguti, T. J. Phys. Soc. Jpn. 2000, 69, 3017-3026. (18) Matthias, B. T. Phys. ReV. 1953, 92, 874-876. (19) JCPDS-ICDD Card No. 85-1329 (Bi metal). PCPDFWIN, version 2.02; May 1999. (20) Cullity, B. D; Stock, S. R. Elements of X-ray Diffraction; Prentice Hall: Englewood Cliffs, NJ, 2000. (21) Wurl, A.; Hyslop, M.; Brown, S. A.; Hall, B. D.; Monot, R. Eur. Phys. J. D 2001, 16, 205-208. (22) Oshima, Y.; Takayanagi, K.; Hirayama, H. Z. Phys. D 1997, 40, 534-538. (23) Ajayan, P. M.; Marks, L. D. Phys. ReV. Lett. 1989, 63, 279-282. (24) Lee, J. G.; Mori, H. J. Electron Microsc. 2003, 52, 57-62.
