2538
J. Phys. Chem. C 2010, 114, 2538–2543
High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy Investigations of Bimetallic Nickel Bismuth Nanomaterials Created by Electron-Beam-Induced Fragmentation William D. Pyrz,† Sangmoon Park,‡ Douglas A. Blom,§ Douglas J. Buttrey,† and Tom Vogt*,| Department of Chemical Engineering, Center for Catalytic Science and Technology, UniVersity of Delaware, Newark, Delaware 19716, Department of Engineering in Energy and Applied Chemistry, Silla UniVersity, Busan 617-736, Republic of Korea, Electron Microscopy and NanoCenter, UniVersity of South Carolina, Columbia, South Carolina 29208, and NanoCenter and Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed: NoVember 11, 2009; ReVised Manuscript ReceiVed: January 5, 2010
Using high-angle annular dark-field scanning transmission electron microscopy, we investigated electronbeam-induced fragmentation (EBIF) processes in NiBi alloys. We are able to establish the presence of very small clusters of nanoparticles that eluded detection in earlier TEM work and, furthermore, confirm the existence of core-shell particles with a Bi core and a Ni-Bi shell. On the basis of these and earlier observations, we propose a general mechanism for the creation of core-shell particles using EBIF. Introduction The controllable and reproducible deposition of nanoparticles is of vital importance in existing and emerging technological applications, such as catalysis,1 single-electron devices,2–4 and optoelectronic devices.5,6 Bismuth-based nanomaterials have been the subject of several studies due to their unique properties at the nanoscale, such as highly anisotropic Fermi surfaces, low carrier densities, small effective masses, and long carrier mean free paths.7 Small Bi nanowires less than 50 nm in diameter exhibit semiconducting properties that differ from the typical bulk semimetallic behavior.8 The alloying of nickel with bismuth has been suggested for practical applications, such as the catalytic synthesis of large-diameter single-walled carbon nanotubes9 and viable alternatives to the lead-based solders currently employed in the electronics industry for printed circuit boards.10 BiNi-based intermetallic compounds are also remarkable due to the onset of superconductivity at low temperatures.11,12 A common challenge arising in the synthesis of nanoparticles is their tendency to agglomerate due to attractive van der Waals and other forces that minimize the total surface energy. Surfactants with lyophobic or lyophilic chemical groups and anionic, cationic, or zwitterionic charges are often employed to prevent agglomeration. Adsorption of surfactants onto nanoparticle surfaces will significantly alter the surface energies and, depending upon the application, may change the physical and chemical properties. When employing physical deposition methods based on highenergy electron beams, similar to those used in a transmission electron microscope (TEM), one can rapidly transfer both thermal energy and electrical charge to matter. Under electrical and thermal insulating conditions, material instabilities can then lead to the fragmentation of particles and result in the formation * To whom correspondence should be addressed. E-mail: tvogt@ mailbox.sc.edu. † University of Delaware. ‡ Silla University. § Electron Microscopy and NanoCenter, University of South Carolina. | NanoCenter and Department of Chemistry and Biochemistry, University of South Carolina.
of extended fields of nanoparticles. The observed particle size of the fragments and their composition vary as a function of the distance from the parent particle. Materials such as Au,13 Pb,14 Bi,15 NiBi nanowires,10 Fe-doped SnO2, and several metal azides16,17 have been reported to display electron-beam-induced fragmentation (EBIF). Prior work on NiBi nanowires found that prolonged electron irradiation led to the decomposition of the nanowires, and no further attempt to quantify or describe the phenomena was reported.10 Efforts to rationalize this fragmentation process are limited. Ru proposed that the fragmentation observed in Au particles was primarily the result of a combination of thermal and electrical effects induced by the electron beam, which was further enabled by the presence of excess vacancies and voids in the micrometer-sized parent particles.13 Lu et al. described the fragmentation to be caused by a rapid buildup of heat and pressure that induced both a phase transformation and a subsequent nanoparticle dispersion.16 In this and our previous work,18 micrometer-sized hydrothermally synthesized Bi-Ni particles are used as precursors for electron-beam-induced fragmentation (EBIF) events that produce micrometer-sized reproducible fields of nonoverlapping bimetallic nanoparticles of varying composition and size (Figure 1). We propose that the 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 disperses due to the rapid accumulation of charge, generating a field of crystalline Bi-rich nonoverlapping bimetallic nanoparticles. For an EBIF event to occur, a parent particle must be relatively large with a diameter of at least a few micrometers and have a dense consolidated morphology. It was also noted that the parent particle, regardless of size, must be isolated on the carbon support film and not in contact with a metal bar from the TEM grid support. Parent particles that were small or had a wire or meshlike morphology were not susceptible to fragmentation and were stable under the electron beam. It is believed that this selective fragmentation behavior is related to the ease with which a parent particle can dissipate the thermal energy and charge of the electron beam. For large, isolated parent particles with a consolidated morphology, it is
10.1021/jp9107443 2010 American Chemical Society Published on Web 01/27/2010
HAADF-STEM of Bimetallic NiBi Alloys Created by EBIF
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2539
Figure 1. Set of images extending in the radial direction from the parent particle, showing the particle size as a function of distance. The radial distance is on the order of 5 µm.
Figure 2. Phase diagram for the binary Ni-Bi system adapted from Feschotte et al.19
likely that there are limited points of contact with the conductive carbon substrate, precluding effective dissipation of heat and charge. Conversely, a wire or meshlike parent particle is likely to have multiple points of contact with the conductive carbon substrate and is thus better suited to dissipate both thermal energy and charge. The same dissipation principles are likely to apply for a smaller parent particle because it will have a higher proportion of its total surface area in contact with the conductive substrate. We proposed that, in the case of Bi-Ni particles, the rapid heating of matter by the electron beam induces incongruent melting of the parent particle so as to generate nanometer-sized pockets of liquid of the lower-melting component (Bi) that are finely distributed throughout the accompanying solid phase. We suggest that the formation of these liquid pockets provides a very high interfacial surface area that can accommodate significant charge accumulation. This high local charge density is suggested to be a driving mechanism for the subsequent fragmentation process. The likelihood of fragmentation appears to increase to a certain threshold near 80-85 mol % Bi. Above this level, the development of a contiguous liquid with a lower surface area seems to be favored, probably due to the larger proportion of the parent particle that melts during the initial heating under the electron beam; at some compositional threshold, we expect this to wet the underlying conductive carbon film, enhancing charge dissipation and preventing fragmentation. The peritectic nature of the liquidus curve displayed in Figure 2 for both of the compound line phases,19 NiBi and NiBi3, ensures incongruent (partial) melting and compositional variations that lead to the observed bimetallic product compositions as a function of distance from the parent particle. The apparent absence of overlapping nanoparticles strongly suggests that the droplets from which these nanoparticles form are charged by the incident electron beam. The observation of two distinct particle size distributions18 can be explained by analyzing the expected partial melting behavior of the intermediate Bi-Ni compounds as a function of temperature (Figure 2). On the basis of the phase diagram, the higher-melting BiNi phase is expected to partially melt at 919 K, forming a mixture that is approximately 25% liquid/
75% solid. The lower-melting Bi3Ni phase incongruently melts at 740 K, forming a mixture that is approximately 75% liquid/ 25% solid. The exact solid/liquid fractions would depend on the particle temperature. Given an EBIF event involving parent particles of similar size, we expect the higher Bi-content parent to provide larger nanoparticle fields with an increased upper bound for the nanoparticle size distribution relative to a particle with lower Bi content because the former is expected to contain a higher volume fraction of liquid. In the following, we use high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to investigate EBIF processes of NiBi particles and observe the presence of (1) small clusters of nanoparticles not detected in our earlier TEM work and (2) nanometer-sized core shell particles with a Bi core and BiNi shell. These observations allow us to propose a general mechanism for EBIF processes. Experimental Methods Hydrothermal synthesis of 3:1 Bi/Ni micrometer-sized particles was performed as reported previously.18 The resulting particles were prepared for EBIF and subsequent characterization by ultrasonic dispersion in acetone by deposition of 1-2 drops of the suspension onto a 200 or 300 mesh Cu TEM grid coated with an amorphous carbon film. A JEOL JEM2100F field emission gun transmission electron microscope (TEM) with a CEOS aberration corrector for electron probe generation was used in STEM mode for both the EBIF event and the characterization. Initial selection of particles for EBIF was done using the shadow image created in an STEM when the electron probe is fixed and the sample is far from the focal plane of the objective lens. The electrons in the probe are spread out over a large area whose size depends on the convergence angle of the probe-forming aperture and the difference between the focal plane of the microscope and the specimen. A large 200 µm diameter probe-forming aperture was used to provide both a larger field of view and more current in the probe for the EBIF event. Once a particle with the desired size and morphology was located, EBIF could be commenced by moving the specimen closer to the focal plane of the microscope. This is equivalent to increasing the convergence of the beam onto a small area of the specimen in TEM mode, although with a much smaller total beam current than is typical for TEM probe formation. Subsequent to EBIF, a much smaller aperture that was suitable for formation of an electron probe on the order of 0.1 nm was used to record high-angle annular dark-field (HAADF) images of the results of the EBIF. The image in Figure 3 is typical of a nanoparticle field following an EBIF event in the JEM2100F. As expected, the parent particle is surrounded by a field of nanoparticles. Similar to the nanoparticle fields reported previously,18 the nanoparticle size decreases as the radial distance from the parent particle increased. The overall size of this nanoparticle field is somewhat smaller than
2540
J. Phys. Chem. C, Vol. 114, No. 6, 2010
Figure 3. Typical nanoparticle field following an EBIF event in the aberration-corrected JEOL2100F TEM.
Pyrz et al. expected, but not surprising, because the parent particle in this case is significantly smaller than those analyzed in the our earlier work.18 Moderate-magnification HAADF-STEM images of larger nanoparticles in close proximity to the parent particle are shown in Figure 4. The image in Figure 4a reveals that the particles do not overlap and are well-dispersed on the carbon substrate. However, the HAADF-STEM image acquired with an aberration-corrected microscope reveals an important difference in the region between the nanoparticles that was not evident when using the noncorrected TEM.18 Careful examination of this region reveals several features with very low contrast that are enhanced in Figure 4b. In the contrast-adjusted image, there is clear evidence of a second nonoverlapping nanoparticle population with significantly smaller particle sizes that exists between the larger nanoparticles. High-magnification HAADF images of the large and small nanoparticles are shown in Figures 5 and 6. Starting with the larger nanoparticle population, the image in Figure 5a depicts a well-crystallized nanoparticle with well-defined facets. This observation is representative of all of the nanoparticles analyzed
Figure 4. Moderate-magnification aberration-corrected HAADF images showing nanoparticles in close proximity to the parent particle that coexist with a second nanoparticle population that is significantly smaller in size.
Figure 5. High-resolution aberration-corrected HAADF-STEM image of (a) a well-crystallized nanoparticle and (b) a nanoparticle containing a twin boundary.
HAADF-STEM of Bimetallic NiBi Alloys Created by EBIF
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2541
Figure 6. High-resolution aberration-corrected HAADF-STEM images showing two small nanoparticles with ordered interiors and edges not terminated by well-ordered crystalline faces.
Figure 7. Moderate-magnification aberration-corrected HAADF images showing a core-shell-type nanoparticle morphology that was discovered at the far edge of the nanoparticle field.
using conventional high-resolution studies.18 The image in Figure 5b displays a second nanoparticle that contains a clear domain wall. The high-resolution HAADF images in Figure 6 show significantly smaller nanoparticles near the edge of the nanoparticle field and located at a much larger radial distance away from the parent particle than the nanoparticles in Figure 5. Both nanoparticles in these images display clear lattice fringes but have rather poorly defined edges. Close examination of the edges reveals significant atomic roughness. Furthermore, there are numerous single atoms, dimers, trimers, and small loosely associated atomic clusters residing near the nanoparticle. The vacant space on the carbon substrate surrounding these nanoparticles is also covered by these small atomic clusters. Directly above the large nanoparticle in Figure 6a are several small atomic islands that are 1-2 nm in diameter. The dim contrast in these clusters suggested that they are very thin and may only be a single atom thick in the direction perpendicular to the beam. In addition to the well-ordered, partially (or incompletely) ordered nanoparticles and the loosely associated atomic clusters, there was yet another unique morphology that was observed at
the outermost edges of the nanoparticle field. Moderatemagnification HAADF images of these nanostructures are shown in Figure 7. Several of the nanoparticles in these images appear to have a core-shell structure in which a bright-contrast core is enclosed by a ring of material exhibiting diminished contrast. Higher-magnification images of these structures are shown in the panel of images displayed in Figure 9. In each of these images, an abrupt change in contrast that is only one-quarter to one-half a nanometer thick at the edge of the nanoparticles is observed. In addition to the change in contrast, the atomic-level coherency between the two regions appears to be different. Because the image contrast for the HAADF technique is proportional to the square of the atomic number Z, the shift in both the image contrast and the atomic level coherency appears to indicate a possible phase or composition change across the interface. Several possible phase combinations exist in the Bi-Ni system that could account for the observed contrast pattern. These include (1) a Bi core with a Bi3Ni shell, (2) a Bi core with a BiNi shell, (3) a Bi core with a Ni shell, (4) a Bi3Ni core with a BiNi shell, (5) a Bi3Ni core with a Ni shell, and (6)
2542
J. Phys. Chem. C, Vol. 114, No. 6, 2010
Pyrz et al.
Figure 8. Core-shell particle (6 nm, core diameter ∼ 5.5 nm, and shell ∼0.5 nm). The ratio of the mean of the Z contrast of the shell compared to the mean of the contrast of the core is 0.63.
Figure 9. High-resolution aberration-corrected HAADF images showing nanoparticles with a core-shell-like structure. These nanoparticles were located at the outer edges of the nanoparticle field.
a BiNi core with a Ni shell. It is likely that combinations 3, 5, and 6 can be ruled out because the EDS analyses in our previous work18 indicated that the Ni concentration of particles dropped as the radial distance from the parent particle increased. Analyzing the contrast of various particles, we find that the contrast variation we find experimentally of 0.63 is closest to the Z2 ratio (see Figure 8) of a Bi/BiNi core-shell particle (0.578). The next closest ratios would be Bi3Ni/BiNi (0.733)
and Bi/Bi3Ni (0.793), followed by unlikely combinations Bi3Ni/ Ni (0.157) and Bi/Ni (0.123). Therefore, the Z contrast we observe is most likely for a core-shell structure with a Bi core and a Bi-Ni alloy as a shell. This assignment is also consistent with the proposed fragmentation mechanism based on the Bi-Ni phase diagram. If the complete fragmentation event is not instantaneous and actually includes several sequential steps, then the formation
HAADF-STEM of Bimetallic NiBi Alloys Created by EBIF of these particles can be explains as follows: (1) For the highBi content alloys expected in the 3:1 Bi/Ni sample, the incongruent partial melting would lead to the formation of an initial liquid that is very rich in Bi. (2) This liquid would then disperse in a short period of time, forming nanoparticle fields. (3) Following the initial fragmentation event, the remaining material is continuously being irradiated by the electron beam and induces a partial melting/atomization, creating single atoms or small atomic clusters that are expelled and then gradually coat the nanoparticles that have already been dispersed. Because a large fraction of the Bi in the parent material has already been dispersed, the composition of this secondary coating material should be richer in Ni. (4) If enough material is dispersed through this secondary process, then the nanoparticles could be encapsulated by a thin film that may crystallize into an alloy phase (BiNi or Bi3Ni). This secondary coating process may be analogous to what can be observed using physical or chemical vapor deposition techniques. Evidence supporting this secondary coating mechanism is the direct HAADFSTEM observation of individual atoms, dimers, trimers, and small atomic clusters covering the carbon film in the areas surrounding the nanoparticles. The fact that this coating was only detected on the smallest nanoparticle could be related to their thickness. A thin coating could very easily go unnoticed or is incomplete on larger nanoparticles. An example displaying what appears to be an incomplete coating can be seen in Figure 5a,b, which is the same particles shown in Figure 5a,b. In both images, the superimposed arrows point to regions of the large nanoparticles that appear to be partially coated. In Figure 5b, this coating appears to be amorphous. The preliminary analyses of the nanoparticle fields using conventional STEM without aberration correction would not have been able to resolve these core-shell structures because the probe that was used had a diameter that was larger than the thickness of the shell. The high-resolution bright-field phase contrast imaging studies would also not have detected the presence of such a thin shell structure for the smallest nanoparticles because structural fluctuations were commonly observed due to the high-intensity electron beam. These fluctuations certainly would have blurred the contrast of the core-shell structure. Conclusions The observed EBIF behavior of Bi-Ni particles points to the potential use this deposition technique has to make unique nanoparticles using high-energy electron beams. Some general principles are necessary for EBIF to occur: First, one end member should have a much lower melting temperature than the other. Second, intermediate compounds in the phase diagram
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2543 need to melt incongruently. Third, the particle has limited contact with the substrate such that both the thermal energy and the electrical charge dissipation is inhibited. These conditions 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 results in Coulombic forces that drive or significantly facilitate the fragmentation process and ensures that the resulting nanodroplets do not have contact with one another. Crystallization upon deposition may be so rapid as to preclude ordered line compound formation, resulting in a random or weakly correlated atomic distribution in the nanoparticles. Following the initial EBIF event and dispersion of a high percentage of the lowmelting element, prolonged electron beam exposure appears to lead to single- or multiatom sputtering processes that deposit individual atoms and small atomic clusters across the adjacent nanoparticle fields and supports. At large distances from the parent particle, the sputtering processes may lead to the formation of core-shell structures with shells rich with the higher-melting component. We make a case that, for certain bimetallic nanoparticles, EBIF has the potential to become a size-, composition-, and morphology-selective physical deposition method. This might be able to be expanded to ternary or higher-order compositions. References and Notes (1) Stakheev, A. Y.; Kustov, L. M. Appl. Catal., A 1999, 188, 3. (2) 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. (3) Davidovic, D.; Tinkham, M. Appl. Phys. Lett. 1998, 73, 3959. (4) Devoret, M. H.; Esteve, D.; Urbina, C. Nature 1992, 360, 547. (5) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (6) Skomski, R. J. Phys.: Condens. Matter 2003, 15, R841. (7) Yang, B. J.; Li, C.; Hu, H. M.; Yang, X. G.; Li, Q. W.; Qian, Y. T. Eur. J. Inorg. Chem. 2003, 3699. (8) Heremans, J.; Thrush, C. M.; Lin, Y. M.; Cronin, S.; Zhang, Z.; Dresselhaus, M. S.; Mansfield, J. F. Phys. ReV. B 2000, 61, 2921. (9) Kiang, C. H. J. Phys. Chem. A 2000, 104, 2454. (10) Ould-Ely, T.; Thurston, J. H.; Kumar, A.; Respaud, M.; Guo, W. H.; Weidenthaler, C.; Whitmire, K. H. Chem. Mater. 2005, 17, 4750. (11) Fujimori, Y.; Kan, S.; Shinozaki, B.; Kawaguti, T. J. Phys. Soc. Jpn. 2000, 69, 3017. (12) Matthias, B. T. Phys. ReV. 1953, 92, 874. (13) Ru, Q. Appl. Phys. Lett. 1997, 71, 1792. (14) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Appl. Phys. Lett. 2002, 80, 309. (15) 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. (16) Lu, B.; Wang, C. S.; Zhang, Y. H. Appl. Phys. Lett. 1997, 70, 717. (17) Herley, P. J.; Jones, W. Z. Z. Phys. D 1993, 26, S159. (18) Pyrz, W. D.; Park, S.; Vogt, T.; Buttrey, D. J. J. Phys. Chem. C 2007, 111, 10824. (19) Feschotte, P.; Rosset, J. M. J. Less-Common Met. 1988, 143, 31.
JP9107443
