Carbon Nanotubes as Nanoreactors for Fabrication of Single

NANO LETTERS

Carbon Nanotubes as Nanoreactors for Fabrication of Single-Crystalline Mg3N2 Nanowires

2006 Vol. 6, No. 6 1136-1140

Junqing Hu,*,† Yoshio Bando,†,‡ Jinhua Zhan,‡ Chunyi Zhi,‡ and Dmitri Golberg‡ International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, and AdVanced Materials and Nanomaterials Laboratories, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan Received February 2, 2006; Revised Manuscript Received March 27, 2006

ABSTRACT Due to fast decomposition of Mg3N2 in the presence of water in the atmosphere (Mg3N2 + 6H2O f 3Mg(OH)2 + 2NH3), the synthesis of single-crystalline Mg3N2 nanowires has been a challenge. Here, we demonstrate that carbon nanotubes may serve as nanoreactors for a simple thermal reaction process resulting in the first fabrication of high-quality, large-yield, single-crystalline Mg3N2 nanowires. The Mg3N2 nanowires are homogeneously sheathed over their entire lengths with very thin graphitic carbon tubular layers, which effectively prevent their decomposition (even when the samples are put into water or exposed to atmosphere for several months). We have systematically analyzed for the first time the Mg3N2 nanomaterial by means of transmission electron microscopy (TEM), high-resolution TEM, and electron diffraction. Successful fabrication of carbon sheath protected Mg3N2 nanowires may promote further experimental studies on their crystal structures and properties.

Driven by the prospective applications in mesoscopic physics and nanoscale technology, many material systems including group IV,1,2 III-V,3,4 and II-VI semiconductors,5,6 elemental metals and alloys,7,8 oxides,9,10 sulfides,11 carbides,12 and hydroxides13 have successfully been prepared in the form of one-dimensional (1D) nanostructuressnanowires, nanorods, and nanotubes using various methods. Magnesium nitride, Mg3N2, has an anti-bixbyite structure with the bodycentered cubic (bcc) unit cell (space group: Ia-3) known also for some other alkaline rare-earth metal (M3N2, M ) Be, Mg, Ca) nitrides.14,15 Mg3N2 is widely used as a catalyst and a nitriding agent during the preparation of various nitrides.16,17 Mg3N2 is a direct energy gap semiconductor (∼2.8 eV) and thus may serve as a potential high-temperature semiconducting material and/or component of a semiconductor heterostructure useful in nanoelectronics.18 Due to fast decomposition of Mg3N2 in the presence of water in the atmosphere (Mg3N2 + 6H2O f 3Mg(OH)2 + 2NH3),19 the synthesis of single-crystalline Mg3N2 nanowires has not yet been accomplished and remains a challenge. Carbon nanotubes (CNTs) can serve as nanoreactors for introducing reactive species and reactants, followed by desired reactions producing nanowires (that cannot be synthesized under normal conditions) in the confined con* Corresponding author. E-mail: [email protected]. † International Center for Young Scientists (ICYS). ‡ Advanced Materials and Nanomaterials Laboratories. 10.1021/nl060245v CCC: $33.50 Published on Web 04/29/2006

© 2006 American Chemical Society

figurations. If a thermally and chemically stable graphitic carbon is used as a protective sheath (or coating) on a given Mg3N2 nanowire, it would effectively prevent its decomposition in the presence of water in the atmosphere. Upon this motivation, in this Letter, we report on CNT utilization as nanoreactors for a simple thermal reaction process resulting in the first fabrication of high-quality, large-yield, singlecrystalline Mg3N2 nanowires. The Mg3N2 nanowires are homogeneously sheathed over their entire length with very thin graphitic carbon tubular layers, which effectively prevent the decomposition of as-formed Mg3N2 nanowires (even if the samples are put in water or exposed to air for several months). The Mg3N2 nanowires were prepared in a vertical highfrequency induction furnace. The furnace consists of a fusedquartz tube and an induction-heated cylinder made of highpurity graphite coated with a carbon fiber thermoinsulating layer (the coated carbon fiber layer is also used as substrate for collecting the Mg3N2 nanowires grown) and has two inlets on its top and base, respectively, and one outlet on its base, as illustrated in Figure S1 in the Supporting Information. A graphite crucible, containing 0.8 g of MgB2, 0.4 g of Ga2O3, and 0.3 g of activated carbon powders, was placed at the center cylinder zone. After evacuation of the quartz tube to ∼ 2 × 10-1 Torr, two pure N2 flows were introduced through the inlets at a flow rate of 0.5 L/min (top) and 2.0 L/min

Figure 1. (a) XRD pattern and (b) SEM image of the Mg3N2 nanowires.

(base), respectively, creating the ambient pressure in the furnace. The crucible was rapidly heated to and maintained at 1200 °C for 1.0 h, and then the power generator was switched off allowing the crucible to cool to room temperature naturally. The processing temperature was measured by an optical pyrometer with an estimated accuracy of (10 °C. After synthesis, thermoinsulating carbon fibers near the furnace top were densely covered with yellowish Mg3N2 nanowires. Figure 1a shows a X-ray diffraction (XRD; RINT 2200) pattern of an as-synthesized product. The pattern reveals sharp peaks which match well those peculiar to a bcc form of Mg3N2 (JCPDS (35-0778), a ) 9.9657 Å). A weak reflection at ∼3.36 Å indicates the presence of crystalline graphitic carbon (JCPDS (26-1076)); a broad peak at 2θ ) ∼20-30° may be attributed to some amorphous materials within the product. Scanning electron microscopy (SEM; JSM-6700F, at 10 kV) image, Figure 1b, reveals that the product is composed of numerous radiolaria-like ensembles composed of wirelike nanostructures with lengths of several tens of micrometers (also see Supporting Information, Figure S2). Transmission electron microscopy (TEM; JEM-3000F, at 300 kV) images, Figure 2a, show that the wires have a uniform diameter along their lengths, and most of them have a diameter of ∼80-120 nm, whereas just a few have a larger diameter of ∼150-200 nm. The clear contrast variations observed on the wires’ hemispherical end parts, as shown in an inset (right middle), suggest a different tip-end phase composition compared to the wire bodies. In fact, each nanowire is not a “monolithic” wire, but a composite wire, Nano Lett., Vol. 6, No. 6, 2006

Figure 2. (a) TEM images showing that Mg3N2 nanowires have a uniform diameter along the length. The inset depicts the wires’ tipend structure. (b, c) EDS and EELS spectra revealing that the wires are made of Mg, N (with a stoichiometry close to Mg3N2), and C.

which is sheathed over its entire length with a thin graphitic layer, or in other words, a wire is embedded in a CNT (shown later). An X-ray energy dispersive spectrum (EDS), Figure 2b, confirms that the wires are composed of Mg, N (a stoichiometry close to Mg3N2), and C, whereas the short rods/ droplets at the tip-ends (inside carbon sheath) are of metallic Ga (shown later). Electron energy loss spectra (EELS), Figure 2c, further indicate that the wires are made of Mg (Mg-K: 1305 eV), N (N-K: 401.6 eV), and C (C-K: 283.8 eV). Both EDS and EEL spectra confirm the nanowire high purity with respect to O contamination. Interestingly, most of the as-grown Mg3N2 nanowires display significant tapering at their tip-end segments, as shown in Figure 3a. In some cases, the tapered end can be a short metallic Ga rod, as shown in Figure 3b. Notably, careful TEM examination shows that uniform and thin carbon sheath on a given Mg3N2 nanowire extends along the wire toward both of its ends, i.e., the sheathed CNT is sealed at both ends, as shown in Figure 3a-c. This entirely prevents the Mg3N2 nanowire’s decomposition in the presence of water in the atmosphere. An EDS spectrum, Figure 3d, further shows that the short rods/droplets at the tip ends are made of metallic Ga. In fact, due to a thermal effect of 1137

Figure 3. (a-c) TEM images showing a uniform and thin carbon sheath on a Mg3N2 nanowire extending along the entire wire length and sealed at both ends. (d) EDS spectrum confirming that a short rod/droplet at the structure tip end is made of metallic Ga.

electron beam irradiation and a low melting point of Ga (bulk Ga: mp of 29.8 °C, but it has been shown that Ga remains liquid up to -80 °C when encapsulated in CNTs20), under the present TEM imaging conditions (a very low basic pressure of ∼1 × 10-5 Pa, a condenser aperture no. 2 of 70 µm in diameter), the Ga short rods or droplets within CNTs are in a liquid state, as suggested by an electron diffraction (ED) pattern (inset in Figure 3d). Since the first structure determination of Mg3N2 by Partin (in 1997),14 there has been no further research performed on its micro- or nanostructures. In the present work, the detailed Mg3N2 microstructure analysis was systematically performed for the first time by means of TEM, highresolution TEM (HRTEM), and ED. A high-magnification TEM image, Figure 4a, depicts a Mg3N2 nanowire sheathed with a very thin (∼6 nm) and uniform graphitic carbon layer. The inset (lower left) shows that the carbon layer indeed contains the ordered graphitic (002) sheets, with an interplanar spacing of 0.34 nm, as also suggested by the ED (shown later) patterns. Figure 4b is a HRTEM image of a wire, in which the lattice fringes of the (110) and (-110) planes with the d spacing of 0.70 nm, characteristic of bcc Mg3N2, can be clearly seen; the [001] crystallographic direction is parallel to the long axis direction of the wire, i.e., the growth direction. The corresponding ED pattern (upper-right inset) can be indexed as the [100] zone axis pattern of a Mg3N2 single crystal. Shown in Figure 4c is a HRTEM image of a wire, in which the measured d spacings of 0.70 nm correspond to the (10-1) and (-110) lattice fringes of the Mg3N2 crystal, respectively. The growth direction of the wire is determined to be parallel to the [011] crystallographic orientation. The ED pattern (upper-right inset) can be indexed as the [111] zone axis pattern of the Mg3N2 single crystal. Figure 4d is a HRTEM image of a wire. The measured d spacing of 0.70 and 0.29 nm are in agreement with those of the Mg3N2 (-110) and (-2-22) planes, respectively. The ED pattern (upper-right inset) can be assigned to the [112] zone axis pattern of the Mg3N2 crystal; in the pattern, the pairs of arcs peculiar to the (002) and (004) reflections are visible along the direction perpendicular to the CNT axis, suggesting that the tube wall 1138

Figure 4. (a) High-magnification TEM image showing a Mg3N2 nanowire homogeneously coated with a very thin graphitic layer. (b-e) HRTEM images and ED patterns revealing that the Mg3N2 nanowires are structurally uniform single crystals which are grown along one of four possible directions, i.e., [001], [011], [111], and [112] crystallographic orientations of a Mg3N2 crystal.

consists of the cylindrically stacked graphitic (002) planes. Preliminary studies reveal that this structurally uniform wire grows along the [111] direction. Shown in Figure 4e is a HRTEM image of a wire, in which the measured d spacings of 0.41 and 0.49 nm correspond to the (12-1) and (002) lattice fringes of the Mg3N2 crystal, respectively. The growth direction of the wire is determined to be parallel to the [112] crystallographic orientation. The ED pattern (upper-right inset) is coincident with the [012] zone axis pattern of the Mg3N2 crystal. Extensive HRTEM and ED examinations indicate the structural uniformity of these single-crystalline Mg3N2 nanowires that is well maintained along the entire lengths. The nanowires grow along one of four possible orientations, i.e., the [001], [011], [111], and [112] crystallographic directions of the Mg3N2 single crystal. Nano Lett., Vol. 6, No. 6, 2006

Figure 5. HRTEM image taken from the interface domains between a Mg3N2 nanowire and a C-coating. The image reveals that the Mg3N2 nanowire surface is structurally and compositionally uniform and no intermediate layers which may be attributed to MgO or Mg(OH)2 are present on the surface.

Shown in Figure 5 is a HRTEM image taken from a C-coating surface and the interface domains between a Mg3N2 nanowire and a C-coating (also see Supporting Information, Figure S3). As seen from the image, the C-coating has a uniform thickness along the length; the Mg3N2 nanowire surfaces are structurally and compositionally homogeneous and there are no intermediate layers which may be attributed to the covering MgO or Mg(OH)2 phases. It further confirms that a C-coating on a nanowire can effectively prevent the material decomposition in the presence of water in the atmosphere. The formation of single-crystalline Mg3N2 nanowires sheathed with CNTs proceeds via a simple thermal reaction process using MgB2, Ga2O3, activated carbon powders, and N2 as source materials. The reaction of Ga2O3 and activated carbon at a processing temperature (∼1200 °C) in a N2 flow takes place as follows: Ga2O3 (solid) + 2C (solid) f Ga2O (vapor) + 2CO (vapor), and Ga2O (vapor) + 3CO (vapor) f 2Ga (liquid) + C (solid) + 2CO2 (vapor).21,22 These reactions will produce a Ga vapor and carbon clusters. The Ga vapor condenses at a low temperature (∼700-800 °C) to form Ga droplets, and then the Ga droplets induce the anisotropic growth of CNTs; the simultaneous generation of the Ga vapor and carbon clusters promotes the Ga fillings of CNTs. The growth process of CNTs is schematically illustrated in Figure 6 (from step I to step IV). It is suggested that the CNT growth has occurred via a Ga-catalyzed vaporliquid-solid process. Meanwhile, a MgB2 powder is decomposed into a hot Mg vapor (while as-formed B still remains in the crucible). The Mg vapor then reacts with N2 and forms Mg3N2 nanosized clusters. These clusters are also transported by N2 and enter the growing CNTs where they deposit as Mg3N2 nucleus on the Ga-filling surface or on the inner tube wall (step V, Figure 6). During the ongoing reaction, more Mg3N2 clusters are generated, penetrate inside the tube, and form the Mg3N2 nucleus on the Ga-filling surface, this results in the Mg3N2 nanowire continuously growing (step VI). Finally, the growing CNT undergoes a self-sealing process (step VII), forming an entirely sealed Nano Lett., Vol. 6, No. 6, 2006

Figure 6. Schematic illustration of the growth process of a Mg3N2 nanowire coated with CNT: (I) Ga vaporizing, (II) Ga vapor condensing to form a Ga droplet, (III) CNT nucleating, (IV) CNT continuously growing and in situ Ga filling within it, (V and VI) a Mg3N2 nanowire continuously growing inside CNT, (VII) CNT sealing.

tube end. During the growth process of a Mg3N2 nanowire, CNT acts as a reactor or a template that retards the lateral growth of the Mg3N2 nucleus, thus confining the 1D nanowire growth inside the tube. The process is somewhat similar to the synthesis of carbide nanorods through a CNT confined reaction,3,12 except that in the present case CNTs additionally serve as a coating on the Mg3N2 nanowires. In our experiment, the influences of the source materials and processing reaction temperatures on the formation of CNTs and Mg3N2 nanowires were investigated. If the MgB2 powders were not included in the source materials while other reaction conditions were kept unchanged, the final product mainly consisted of CNTs (Supporting Information, Figure S4). The synthesis temperature mainly determines the volume of Ga fillings in CNTs: the higher the reaction temperature, the larger the volume of Ga fillings. The lower processing temperature (∼1200 °C) led to a very small quantity of Ga fillings in CNTs, and thus as-formed CNT has a spacious hollow cavity effectively used for the Mg3N2 nanowire growth. The higher processing temperature (e.g., ∼1400 °C) led to a considerable amount of Ga fillings (forming a Ga nanowire) within CNTs, and thus as-formed CNTs have a smaller hollow cavity used for the nanowire growth. Normally, Ga nanowires and Mg3N2 nanowires occupy ∼40% and ∼60% of the entire cavity of a given CNT, respectively. The Ga and Mg3N2 nanowire interfaces resulted in the formation of Ga-Mg3N2 nanowire heterojunctions inside CNTs (Supporting Information, Figure S5). In conclusion, a simple thermal reaction process using CNTs as nanoreactors has been developed for the first fabrication of high-quality, large-yield, single-crystalline Mg3N2 nanowires. The Mg3N2 nanowires are homogeneously sheathed over the entire lengths with very thin graphitic carbon tubular layers, which effectively prevent the decomposition of as-formed Mg3N2 nanowires in the presence of water in the atmosphere. Fabrication of a carbon layer stably preserving Mg3N2 nanowires may promote further experi1139

mental studies on their properties and crystal structures (the first microstructure studies using TEM technique on this material herein). It is believed that the developed simple method might be useful for the formation of protective carbon coatings on the III-V and II-VI semiconductor nanowires in order to prevent their surface oxidation and/or hydrogenation. Acknowledgment. This work was performed through the Special Coordination Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sport, Science, and Technology of the Japanese Government. Supporting Information Available: A diagram of the vertical induction furnace used, SEM images of wirelike nanostructures, HRTEM images of the interface domains, SEM images of carbon nanotube ensembles, and TEM image of nanowire heterojunctions. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208-211. (2) Wang, D. W.; Dai, H. J. Angew. Chem., Int. Ed. 2002, 41, 47834786. (3) Han, W. Q.; Fan, S. S.; Li, Q. Q.; Hu, Y. D. Science 1997, 277, 1287-1289. (4) Goldberger, J.; He, R.; Zhang, Y. F.; Lee, S.-K.; Yan, H. Q.; Choi, H.-J.; Yang, P. D. Nature 2003, 422, 599-602.

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NL060245V

Nano Lett., Vol. 6, No. 6, 2006