ARTICLE pubs.acs.org/crystal
Origin of Coproduced Boron Nitride and Carbon Helical Conical Fibers Laure Bourgeois,*,†,‡ Timothy Williams,†,§ Masanori Mitome,|| Richard Derrien,†,‡,^ Naoyuki Kawamoto,|| Dmitri Golberg,|| and Yoshio Bando|| Monash Centre for Electron Microscopy, ‡Department of Materials Engineering, and §School of Chemistry, Monash University, Victoria 3800, Australia International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba 305-0044, Japan
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bS Supporting Information ABSTRACT: The nucleation of curved graphitic structures such as carbon and boron nitride nanotubes in the absence of catalyst particles is still poorly understood. The nucleation and growth mechanisms of graphitic cones have been even more elusive. We investigate the formation of helical conical fibers of boron nitride (BN) and carbon (C) produced in a catalyst-free environment from the solid-state annealing of an amorphous BCN compound. Helical conical fibers consist of a single graphene sheet wrapped conically around an axis. We show that the cones grow radially in clusters, with each cluster arising from a common seed. The radial morphology originates from spherulite-like helicoidal polyhedral particles of BN that serve as templates for the BN and C cones. Evidence is presented that the original seed of the BN polyhedral particles is a ∼5 nm fullerene-like shell. We propose an idealized model for the seed structures that consists of multiply twinned hexagonal (h-)BN crystals containing a screw dislocation. This model provides additional insight into the view that conical graphitic structures originate from topological defects in graphene.
’ INTRODUCTION In the large family of carbon (C) and boron nitride (BN) curved graphitic structures that include fullerenes and nanotubes, conical structures occupy a special place: they are topologically unique, constituting the simplest departure from the hexagonal graphene structure.1 In the ideal case, their curvature is determined at a single point, the conical apex. For a seamless graphitic cone, this defect must be a C or BN ring such as a pentagon or a square.2 Truly seamless graphitic cones of C or BN have been observed as terminations of nanotubes2,3 and as stand-alone fibers.1,47 Theoretical calculations have promised advantageous electronic properties for both carbon (C) and boron nitride (BN) cones (refs 8 and 9, respectively), but the rarity of such structures, at least compared with nanotubes, has hampered any realization of their potential. There exists another type of conical structure, which is not seamless but consists of a single conical graphene sheet wound helically about the conical axis. While the carbon variant was discovered several decades ago10,11 and has remained a topic of interest,12,13 it is much more recently that helical cones of BN have been synthesized.1417 Faceted analogues of BN cones18 have also been reported, but overall their experimental observations remain rare. BN helical nanocones were observed to display remarkable “springlike” mechanical behavior;19 to date their properties remain largely unexplored. Similarly to other curved graphitic structures such as BN nanotubes, graphitic cones usually require the presence of nonBN or non-C particles to form. Their formation mechanisms have been proposed to involve templating, such as on SiC twin r 2011 American Chemical Society
crystals10,11 or diffusion-assisted growth via a metallic catalyst particle.7,18 BN helical conical nanofibers17,19 and coproduced BN and C microcones1416 are exceptions, both forming in the absence of catalysts by solid-state annealing of a BCN precursor material. While BN helical nanocones were shown to form on pre-existing BN particles,17 the formation of coproduced BN and C microcones has only been addressed superficially.16 It is the purpose of the present work to clarify the mechanisms associated with the nucleation and growth of these unusual structures. Coproduced BN and C microcones are micrometer-scale fibers consisting of a helically wound conical layer of graphene (C) or white graphene (BN)15,16 (Figure 1). Figure 1a shows one such fiber of pure BN composition, as imaged under bright-field transmission electron microscopy (BF-TEM); the wedge-shaped top and bottom surfaces of the fiber, indicative of a cone, are clearly visible. A high-resolution (HR) TEM image of the conical fiber’s tip (Figure 1b) reveals not only the continuous and straight nature of the BN layers (inset) all the way (within 1 nm) to the cone axis, but also darker, superperiodic fringes (arrows). These fringes correspond to the periodic alignment of the graphene layer along its [100] direction and, therefore, reflect the helical nature of the cones.16 The helical cone topology is illustrated schematically in Figure 1c. It is worth noting that the wrapping of the single graphene sheet can extend to a particle half Received: March 31, 2011 Revised: May 2, 2011 Published: May 24, 2011 3141
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The SEM observations shown here were all conducted in secondary electron image mode at 15 kV using a JEOL JSM-7001F instrument. TEM observations in both bright-field (BF) and dark-field (DF) modes were carried out on a JEOL JEM-2100F instrument operated at 200 kV. Chemical analysis involved the identification of core-loss edges in an electron energy loss spectrum collected using a Gatan Enfina electron energy loss spectrometer (EELS) or of K-edges in energy dispersive X-ray spectra acquired using a JEOL Si(Li) 50 mm2 detector. Spatially resolved chemical maps were generated from EELS spectrum images or EDX maps recorded in scanning (S)TEM mode with a 2 Å probe. Corresponding STEM images were obtained in annular dark-field (ADF) mode. A focused ion beam (FIB) milled specimen was prepared by embedding the region of interest in tungsten (W) and slicing with a gallium beam, in a Hitachi 2000S FIB microscope.
Figure 1. (a) Typical helical microcone as viewed under bright-field TEM. The top and bottom conical surfaces are clearly visible, defining a straight cone axis (red dashed line); two smaller cones are also present. (b) HRTEM image of the microcone tip, showing the excellent graphitic layer ordering; the black arrows and the inset show pseudoperiodic dark contrast associated with the helicity of the cone. (c) Schematic diagram of a helical microcone, consisting of a single graphene layer of conical shape winding around the cone axis.
a micrometer in width and several micrometers in length, or some 104 windings of the helix. The following describes a structural and chemical investigation by TEM and scanning electron microscopy (SEM) of the evolution of the BCN material from its original amorphous state to the final helical cone product. We find that the precursor structures to the microcones are helically wound BN shells consisting of turbostratic graphite domains of polyhedral shape. The results suggest an idealized model in which the original seed is a multiply twinned crystal of hexagonal BN containing a screw dislocation.
’ EXPERIMENTAL SECTION All samples were obtained from an amorphous BCN material annealed for 24 h at different temperatures and for different heating rates. The starting amorphous BCN material, with uniform BC4N composition,14 was prepared by heating at 1400 C a fused mixture of urea, boric acid, and saccharose, as reported in more detail in ref 20. This material will be referred to as 1400. To obtain the highest yield of pure BN and C cones, the highest solid-state annealing temperature possible (2200 C) and fastest heating ramp (30 min from room temperature) was applied to the amorphous BC4N compound under a flow of nitrogen gas.14,16 This sample was re-examined in the present study following careful electron microscopy specimen preparation to preserve the environment surrounding the conical objects, as described shortly. Investigating the development of the microcones required examination of samples annealed at lower temperatures (1800 and 2000 C), but most importantly, at considerably lower heating rates (0.5 C/ min, or a total heating ramp of ∼16 h).14 The samples and heat treatments will be hereafter referred to by the annealing temperature and the heating rate, R for rapid (∼70 C/min) and S for slow (0.5 C/min). Samples for TEM were prepared by gently crushing the powder material in an agate mortar, dispersing the fine powder in ultrapure ethanol via sonication for 1 min, and placing one drop of the dispersion onto the holey carbon film of a 3 mm TEM Cu grid. For SEM examination the same procedure as for TEM was applied, except that the sonication step was skipped in order to retain intact chunks of the original powder; the Cu grid was attached to a brass stub via a small piece of double-sided carbon tape.
’ RESULTS Figure 2a shows a representative aggregate of conical particles observed by SEM in the 2200R sample. In previous work16 we found that in this material 97% of the microcones were either pure BN or pure C, and that each composition corresponded to particles of distinct morphologies. Thus, long and thin columnar fibers were always of pure C, and large and squat particles were always of pure BN. The two morphological types are evident in Figure 2a and are labeled according to their deduced composition. In this work, we used this knowledge to identify the overall composition of individual conical particles. Although all cones appear isolated in the aggregate imaged by SEM in Figure 2a, more careful sample preparation revealed that they in fact occur radially in clusters of two to five fibers attached together (Figure 2bd). The cluster shown in Figure 2d was of a less common type, consisting of two BN cones and a C cone. The BCN chemical maps obtained for the green rectangular region marked in the inset confirmed the chemical composition deduced from particle shape; they also show the sharp boundary between the BN and C cones. HRTEM images (Supporting Information Figure 1) demonstrated that the C cone and its neighboring BN cone were structurally joined via highly distorted graphitic sheets. Clusters containing two or more C cones attached together were never found in this sample. This may be due to the inherent fragility of aggregates of needle-shaped fibers. Another possible explanation will be offered later. Given the large distance (>200 nm) separating neighboring cones in a given cluster (Figure 2bd), it appears unlikely that the foot of the cones in the 2200R sample constitutes the original seed of that cluster. Extensive searches by SEM of conical fibers still attached to unreacted amorphous material failed to uncover any such seeds. Therefore, we examined materials produced under conditions resulting in slower growth kinetics. A sample (denoted 2000S) slowly heated to 2000 C was examined. Many conical fibers are visible (see arrows on SEM image, Figure 3a). An example of two carbon microcones connected at their base by a faceted particle (arrows) is presented in Figure 3b. Contrary to the 2200R sample, angular-shaped particles (arrows, Figures 3b and c) were always observed at the base of conical fibers. Note the sharp transition from particle to fiber. Some of the fibers had well-formed conical caps, as in the 2200R sample (Figure 3b), while others exhibited a more rounded tip (Figure 3c). Another common particle type consisted of an irregular region capped with faceted conical surfaces (see arrows, Figure 3d). Note, in particular, the five facets of the top cap. The facets are 3142
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Figure 2. (a) SEM image showing an aggregate typical of the 2200R sample: it contains a large fraction of BN cones (see particles labeled BN) and C cones (labeled C) easily recognizable by their shape and size.16 (b and c) SEM images showing clusters of cones attached at their base. (d) BF-TEM image of a similar cone cluster to part c. Chemical mapping of the region framed in green in the inset ADF-STEM image reveals the sharp interface between the BN and C cones.
Figure 3. SEM images illustrating the main features of the 2000S sample: well-defined carbon cones shown at (a) low magnification and (b) higher magnification, (c) carbon fibers and (d) clusters of faceted conical particles (arrows). In parts b and c, the cones and fibers can be seen to emanate from angular-shaped particles (arrows).
highlighted by bright contrast at their edges. These features and the bright blotches also visible on the facets are probably caused by strain. Chemical analysis in the TEM showed that the fibers were always of carbon, sometimes with BN on their outer surface, while the faceted particles were always of pure BN (Supporting Information Figure 2a and b). Material heated slowly to 1800 C was found to be dominated by roughly equi-axed particles with distinct facets (Figure 4a and b)
Figure 4. SEM images illustrating the main features of the 1800S sample: clusters of polyhedral particles, as imaged at (a) low magnification and (b) higher magnification, and (c) short, connected conical particles. (d) The particle shown in part c was also imaged by TEM, which revealed diffraction contrast due to strain. In part b, the numbers 1 to 5 point to polyhedral vertices.
and the bright localized contrast mentioned earlier. The particle tended to exhibit triangular and rhombic facets as well as vertices formed from the meeting of three (arrows 3 and 4), five (arrow 2), and six facets (arrow 1) (Figure 4b). These vertices may be considered as the apex of faceted conical surfaces, similarly to the particle shown in Figure 3d. Smooth conical surfaces were also observed, though rarely (Figure 4c and d). The TEM view 3143
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Figure 5. (a) BF-TEM image of two faceted particles in the 1800S sample. The arrows point to tips as seen in projection; the particle on the right-handside has opened up into three conelike shells. (b) BCN composite EELS map of the left particle shown in part c, with EELS spectra for the two regions framed; the particle is pure BN (1), while the amorphous matrix around it has a composition close to BC4N (2). (c) Selected-area diffraction pattern of the particle on the left-hand-side of part a, and three dark-field TEM images using the three circled 00.2 reflections; the particle contains triangular domains ∼150 nm in size. (d) ADF-STEM image of a region just below frame 1 in part b, revealing superperiodic bright fringes every three (00.2) planes (see arrows and inset for enlarged view); the BCN EELS chemical map shows that the area is essentially pure BN.
(Figure 4d) of the short cones imaged under SEM (Figure 4c) reveals the presence of strain (see dark stripes). No fibers of the type found in the 2000S sample were observed. The crystal structure of the polyhedral particles of the 1800S sample was investigated by TEM (Figure 5). The distinct polyhedral shape and emerging conelike caps noted earlier are evident under BF-TEM (Figure 5a). EELS analysis showed all such particles to be of pure BN, whereas the surrounding material had a composition close to the BC4N of the precursor material (Figure 5b). Figure 5c demonstrates its polycrystalline nature and the presence of large crystals. Dark-field TEM images formed from each of the three (00.2)-type reflections circled in the SAED pattern reveal that these large crystallites are triangular in shape and meet at the center of the particle, at least in projection; this suggests a common origin at the
