Synthesis of High-Purity Boron Nitride Nanocrystal at Low Temperatures

Synthesis of High-Purity Boron Nitride Nanocrystal at Low Temperatures Li

Hou,†

Faming

Gao,*,†

Guifang

Sun,†

Huiyang

Gou,†

and Min

Tian‡

Department of Chemical Engineering, Yanshan UniVersity, Qinhuangdao 066004, China, and Department of Chemistry, UniVersity of Ottawa, Ottawa, Canada

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 3 535-540

ReceiVed October 24, 2006; ReVised Manuscript ReceiVed NoVember 27, 2006

ABSTRACT: High-purity nanocrystalline boron nitride has been successfully synthesized by a simple synthesis method using amorphous B powder and NaN3 as the reactants and anhydrous CH3CN as the solvent at 380 °C. Results from XRD, FT-IR, EELS, and BET absorption measurements suggest that the synthesized product can be indexed as pure hexagonal BN with lattice constants of a ) 2.497 Å and c ) 6.678 Å, and the specific surface area of product is 52.19 m2/g. The TEM images show the multiplex belt spherelike, fiberlike, sheetlike, and tubelike morphologies of the products. Here, the multiplex belt spherelike and sheetlike structures are reported for the first time. When the temperature of reaction and the solvent are controlled, BN products with particular morphologies can be selectively produced. The optical properties of the product are observed in the PL spectra, which shows that the as-prepared BN emits strong visible luminescence at 580 nm (λex ) 325 nm). In recent years there has been increasing interest in materials with specific nanomorphologies because of the expectation of special properties.1-11 It is well accepted that there is a close relationship between the morphology and properties of inorganic materials, that is, morphologies determine the properties since the crystal shape dictates the interfacial atomic arrangement of the material. Hexagonal boron nitride (hBN) is an excellent material for its application in various engineering and refractory areas for its unique properties, for example, high resistance against chemical corrosion by acids, refractory properties in vacuum up to 2273 K, inactivity to most of molten metals, high electric resistivity on the order of 1013 V cm, and good thermal conductivity.12-14 All the above advantages have attracted many researchers to engage in the preparation of BN. Several studies have reported on boron nitride nanomaterials such as hBN nanotubes, nanocapsules, nanoparticles, and clusters.15-17 In this paper, we present two new nanostructures of hBN, multiplex belt spherelike and sheetlike, which are formed in the solvothermal process at 380°C using amouphous B powder and excess NaN3 as reagents and anhydrous CH3CN as solvent. Some fascinating features are described in the present work. (i) BN nanospheres are of uniform size and consist of rolled nanobelts in a highly close-packed assembly; (ii) sheetlike BN nanocrystallines with large surface area show a fine single-crystalline nature, and (iii) individual BN nanotubes with perfect tubiforms were also found in sample. Although the yield is low, the synthesis by this solvothermal method is reported for the first time. (iv) The photoluminescence measurements suggest that the as-prepared products possess interesting optical properties. Also the production of these nanostructures, which may possess interesting mechanical and electronic properties, bring great research value to materials science. All the manipulations were carried out in a dry glove box with flowing Ar. In the typical process, NaN3 (0.1 mol) and amorphous B powder (0.12 mol) were put into a stainless steel autoclave (50 mL capacity), and then the autoclave was filled with anhydrous CH3CN up to 60% of the total volume. The autoclave was sealed and maintained at 380 °C for 14 h in a * To whom correspondence [email protected]. † Yanshan University. ‡ University of Ottawa.

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furnace; then it was allowed to cool to room temperature naturally. The products were collected and washed with absolute ethanol and distilled water several times to remove the impurities. Then the final product was dried in vacuum at 80 °C for several hours. X-ray powder diffraction (XRD) pattern was carried out on a D/max-2500/PC X-ray diffractometer with Cu KR radiation (λ ) 0.15418 nm) to study the phases in the product. Scanningelectron micrographs (SEM) of the sample were taken with a SEM KYKY-2800 scanning-electron microscopy. A few powder samples were placed onto silver glue, which was adhered to the SEM stainless steel sample holder. The morphologies of sample were characterized by transmission electron microscopy (TEM) using a JEM-2010 transmission electron microscope with EDS and PEELS. Transmission electron diffraction (TED) was used to investigate the phase structure of the powder. Electron energy-loss spectroscopy (EELS) and Fourier-transform infrared absorption spectra (FT-IR) using an E55+FRA106 spectrometer were used to evaluate the results. The specific surface areas of samples were determined by the N2 adsorption method at 77 K using the NOVA-2000 instrument. The samples were degassed at 473 K for several hours prior to analysis. The specific surface areas were calculated from the adsorption branch according to the Brunauer-Emmett-Teller (BET) method. Room-temperature PL measurement of the dry powder were recorded with a FL3-11 fluorescence spectrophotometer using an excitation wavelength of 325 nm and a slit width of 2 nm. Figure 1 shows the XRD pattern of the sample. All eight peaks at d spacings of 3.3439, 2.1597, 2.0670, 1.8189, 1.6673, 1.5552, 1.2499, and 1.1703 can be indexed as a pure hexagonal BN (002), (100), (101), (102), (004), (103), (110), and (112) respectively. The cell parameters calculated by using the leastsquares method from these diffraction data are a ) 2.497 Å and c ) 6.678 Å, which are in a good agreement with the literature, and the deviation was lower than 1% compared with the reported values of a ) 2.51 Å and c ) 6.69 Å (JCPDS, 85-1068). No diffraction peaks for other phases or materials (such as cBN or B) are observed in XRD patterns, indicating a high purity and crystallinity of the final products. The infrared absorption spectrum of the as-prepared sample also showed the formation of hBN nanocrystals as shown in Figure 2, which displays three strong vibrations at 806.2, 1379,

10.1021/cg060747m CCC: $37.00 © 2007 American Chemical Society Published on Web 02/02/2007

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Figure 1. XRD pattern of the synthesized BN sample.

Figure 2. FT-IR spectra of the synthesized BN sample.

and 3427 cm-1. The vibrations at 1379 and 806.2 cm-1 are the fingerprints of sp2-bond BN. The former is caused by the stretching vibration of the B-N bond, and the latter can be attributed to the out-of-plane bending vibration of the B-N-B ring.18,19 hBN shows two transverse optical (TO) phonon modes at 767 (783) and 1367 (1510) cm-1 and two longitudinal optical (LO) phonon modes at 778 (828) and 1610 (1595) cm-1.20 These phonon modes shifted to 800 and 1372 cm-1 for multiwalled hBN tubes and 811 and 1377 cm-1 for polycrystalline hBN.21 Buckled hBN shows BN vibrations at 790 and 1395 cm-1.22 Turbostratic hBN shows phonon modes at 792 and 1384 cm-1. For the as-prepared BN, the vibrations are shifted to 800, 811 (LO-TO splitting), and 1379 cm-1, which indicates that the sample has a polycrystalline or tubular structure. The vibrations at 3427 cm-1 come from the moisture adsorbed on the surface of the hBN nanocrystals.23 An SEM photograph of the as-prepared sample is shown in Figure 3a. The white particles can be defined as BN, based on chemical analysis, under which the silver glue appears as a light gray. Spherelike products, which are in fact built from two or three small spheres, are clearly shown in the picture, and the large white particles in the image are usually caused by the agglomeration of several sheetlike and fiberlike particles. Meanwhile, the long individual BN nanofibers or nanotubes can also be found in the picture (marked by the yellow rectangles). From the data obtained from the XRD calculations and SEM micrographs, particle size histograms can be drawn as shown in Figure 3b. The particle sizes range from 0.4 to 1.4 µm, as determined from the images, which shows a relatively wide size distribution. The BET method based on the low-temperature

Hou et al.

absorption of nitrogen can be used to determine the specific surface area of the powder. In this work, commercial hBN powder (purity, 99%, 1 µm) was also studied as a reference. Figure 3c shows the BET plots of the amorphous boron used in the study, commercial hBN, and the product, The specific surface area of the as-prepared hBN is 52.19 m2/g, calculated with the multipoint BET equation, and is roughly a 9- and 2-fold increase over those of the amorphous boron (6.197 m2/g) and the commercial hBN (25.31 m2/g). The big specific surface area of the product implies that there is a textural looseness in the sample. The TEM images and selected area electron diffraction (SAED) patterns of as-prepared BN are shown in Figure 4. The proportion of the products with the sphere structure is about 20%. From Figure 4a and b, it can be found that BN nanospheres with an average diameter of 300 nm are in fact built from rolled nanobelts in a highly close-congregated assembly. The selective enlargement in Figure 4b shows that the nanobelts with a high length/width ratio are very thin, and they partially bend to look like a whisker structure. These nanobelts congregate with one end at the center of sphere and the other end perpendicular to the BN spherical surface, showing a radial structure. The nanoscale formation of beltlike building units can be realized by homogeneous precipitation, which can also be organized into the BN microspheres via an aggregation. A typical SAED pattern from several sheets of the BN sphere is shown as the inset of Figure 4a and exhibits two clear diffraction rings corresponding to the (100) snf (110) crystal planes of hexagonal BN. Fibers of boron nitride were also found under TEM observation, and the proportion of the fiberlike BN is as high as 30-40%. The dispersed fiberlike particles with an average size of 250 × 20 nm are shown in Figure 4d. All the BN fibers arrange randomly without any regular pattern. However, in Figure 4c, the sample exhibits a regular arrangement of fibers. It indicates that there is an abundant quantity of uniform and straight BN nanofibers with diameters varying from 10 to 20 nm and lengths up to several hundred nanometers which are aligned with one another to form a highly closed-packed assembly. The diffraction rings (in the inset of Figure 4c) from inner to outer, at d spacings of 3.33, 2.13, and 1.26 Å, match the hBN (002), (100), (110) planes, in good agreement with the XRD results. Apart from the spherelike and fiberlike particles, the majority of the as-obtained boron nitride crystals show a sheetlike morphology with a high two-dimensionality, as shown in Figure 4e and f. The average width and length of these BN sheets range are 1-2 and 3-5 µm, respectively. The highest length/width ratio is found to be 10, and the sheets display an average thickness of around 20 nm. The ultrathin sheets bend and appear transparent under the transmission electron microscope. The inserted SAED pattern (Figure 3e) which was taken from the thin part of a BN sheet is consistent with the single-crystalline nature. The two sets of brighter spots correspond to the (100) and (110) diffraction of hBN. When the sheetlike samples are compared with those with spherelike or fiberlike morphologies, the crystallinity of the former is obviously higher than the latter. A further investigation into the microstructure of the sheetlike BN was performed by HRTEM. From Figure 4f, which was taken from a section of sheet in Figure 4e marked with a circle, it can be seen clearly that the sheet is composed of well-crystallized graphitic boron nitride basal plane structures, and the average distance between the neighboring fringes is about 0.33 nm, which is consistent with the interplanar distance of 0.333 nm in bulk hBN.24 Interestingly, individual BN nanotubes were also prepared by this solvother-

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Figure 3. SEM photography and textural parameters of the samples: (a) representative SEM photography of hBN nanocrystals, (b) particle size distribution of hBN nanocrystals, and (c) BET plots of amorphous boron used in the study, commercial hBN, and the as-prepared hBN sample.

mal method at 380 °C. On the basis of the observation of TEM, the estimated yield of BN nanotubes is less than 5%. Figure 4g shows the tubular morphologies of the as-prepared BN products. It can be seen that the straight and uniform nanotube have outer diameters ranging from 80 to 90 nm and inner diameters ranging from 55 to 60 nm. The corresponding electron diffraction pattern from the tube wall can be indexed as a layered hexagonal boron nitride with the (002) basal planes (as shown with the shorter arrows) perpendicular to the tube axis. The corresponding HRTEM image (as shown in Figure 4h) obtained from the marked area of the nanotube in Figure 4g revealed that the nanotube’s wall is composed of about 35 shells and the spacing between the visible fringes in the tube wall is about 0.33 nm, which agrees with the (002) interplannar spacing of hexagonal BN. Although it is extremely difficult to make any quantitative statement about the degree of crystallinity, it is clear that the tubes have a good crystalline graphitelike layer structure. An important aspect of the reaction of NaN3 with amorphous B using CH3CN as the solvent is that the temperature can be as low as 380 °C; BN nanotubes are not obtained at such a low temperature by the other method of synthesis. Furthermore, there is no need for an additional catalyst in the present procedure.

The electron energy-loss spectroscopy corresponding to Figure 4a, c, e, and g exhibits hexagonal B-N bonding, as shown in Figure 4 i, j, k, and m, respectively. The typical ionization edges are found at 188 and 401 eV, which shows the characteristic K-shell ionization edges of B and N. Furthermore, the presence of a sharp π* peak and the shape of a σ* peak for both B and N signals are consistent with the sp2-bonded hBN. Elemental quantification of the EELS signals reveal that the B/N atomic ratios in panels i, j, k, and m are 0.99, 1.05, 1.08, and 0.97, respectively. Taking into consideration the experimental error of about 10% resulting mainly from background subtraction when the EELS spectra are analyzed, the B/N atomic ratio is about 1:1. To investigate the influence of solvent and reaction temperature on the formation of the nanocrystalline BN with different morphologies, a series of experiments were carried out (as shown in Table 1) with the procedure similar to that mentioned in the experimental section. BN powders, prepared without solvent at 550 °C, only consist of uniform needlelike hBN particles. The result is similar to that of Shi and co-workers.25 Heating at 450 °C could produce a large aggregate of hBN product, but the crystallinity was poor. When the temperature

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Figure 4. Series of TEM images showing the morphology of structures described in the text and corresponding EELS. The insets are electrondiffraction patterns of each structure.

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Table 1. Different Contents of hBN with Different Morphologies

group number 1 2 3

reactant B, NaN3, CH3CN B, NaN3, CH3CN B, NaN3, CH3CN

reaction temp 80%

380 °C

multiwhisker spherelike

20%

fiberlike sheetlike tubelike sheetlike

30-40% 35-45% 450 °C

7 8

0%

350 °C

4

6

morphology of hBN

approximate content of hBN with different morphologies

550 °C 450 °C 85% ∼90% 0%

is below 400 °C, no BN could be obtained. However, to date, BN nanocrystals were successfully synthesized at 380 °C using anhydrous CH3CN as solvent, and the morphologies of the samples obtained in CH3CN exhibit many differences. This result may be related to the corresponding change of pressure in the autoclave. NaN3 begins to decompose with increasing temperature, resulting in N2, and the solvent begins to vaporize (80 °C), which can result in high pressure in the autoclave. High pressure in the autoclave is beneficial to the formation of crystalline hBN. According to the crystal-growth mechanism in solution phase, nanocrystallites possess more surface energy and have greater solubility. Thus, some of the in situ formed BN nanopaticles grow into one-dimensional nanofibers, nanowhiskers, nanotubes, and two-dimensional nanosheets, which follows the thermodynamic law of reducing surface energy to greatest extent. In this experiment, the melting Na (melting point of 98 °C) which was obtained by the decomposition of NaN3 may play the role of catalyst for the formation of nanotube. However, the exact reason is not presently known. It was found that an optimum reaction condition for the nanocrystalline BN was at the temperature of 380 °C. Variation of the reaction time in the range of 10-14 h at 380 °C did not significantly affect the crystallinity. Only agglomerate fiberlike BN nanocrystals, as shown in Figure 3d, could be obtained at 350 °C. No BN could be obtained when the temperature was lower than 330 °C (thermal decomposition temperature of NaN3). When the reaction temperature was increased to 410 °C, the yield of hBN with well-crystalliized sheetlike structures reached as much as 80%; the fibers almost disappeared, and a very low yield of BNNTs was found. If the temperature is >450 °C, the product is contaminated by violent acetonitrile carbonization. It is well accepted that morphologies determine the properties since the crystal shape dictates the interfacial atomic arrangement of the material. The formation of different shapes including multiplex belt spherelike, fiberlike, sheetlike, and tubelike BN nanocrystals is expected to present special optical properties. The PL spectrum was used to investigate the optical properties of the synthesized hBN; the commercial hBN with a layered structure was also studied under the same experimental condi-

Figure 5. PL spectra of the commercial and synthesized hBN.

tions as a reference. Figure 5 presents the photoluminescence spectra of both the commercial and as-prepared hBN excited with a 325 nm laser source at room temperature. Two emission peaks are distinctly found in both samples, corresponding to 392 and 551 nm for the commercial hBN and 425 and 580 nm for the as-prepared hBN, respectively. The collected PL emission definitely indicates a red shift (around 30 nm) of the as-prepared hBN compared to that of the commercial hBN, which caused by the formation of rolled nanosheets, nanofibers, and nanotubes. It is of noteworthy that the commercial hBN showed much weaker intensity at 551 nm, while the intensity sharply increases at 580 nm for the as-prepared hBN. The TEM images show that the as-prepared BN has unusual crystal shapes, which depicts that the structure configuration may be responsible for the largely variable PL properties. That is to say, hBN has a layered structure (like sheets) with a layered interval of 3.33 Å. When these hBN sheets are rolled up to form some onedimensional structures, the electronic structure of the BN may undergo a change because of the high curvature. Recent theoretical calculation suggested that the large degree of curvature may cause the appearance of sp3 hybridization.26 The as-prepared hBN rolled nanosheets, nanotubes, and nanofibers may contain sp3-bonded structures and have a size confinement effect, and thus, they show a strong PL emission at 580 nm. The emission at 580 nm may be tentatively assigned to the defect-trapped states (vacancy-type defect). The quantum confinement effect of the BN nanofibers favors the formation of defect-trapped states and results in strong luminescence emission.27,28 The as-prepared BN composed of multiplex belt spherelike, fiberlike, sheetlike, and tubelike structures are fabricated at present, and they show strong visible PL bands at 580 nm, which suggests that the products may possess interesting optical properties and facilitate future nanoscale device application. In summary, nanocrystals BN with spherelike, fiberlike, sheetlike, and tubelike morphologies were successfully synthesized using B and NaN3 as the reactants and anhydrous CH3CN as solvent at a temperature of 380 °C. This is a successful example of the preparation of hBN with different morphologies via this solvothermal method. The synthesis of hBN with multiplex belt spherelike and sheetlikes structures is reported for the first time here. On the basis of TEM observation and TED analysis, the novel spherelike BN is formed by selfassembly of nanobelts, and the sheetlike BN shows perfect single-crystalline nature. This self-assembly process in a sealed system may be extended prepare other inorganic materials with

540 Crystal Growth & Design, Vol. 7, No. 3, 2007

more abundant nanostructures. Moreover, the sample emits a strong visible luminescence at 580 nm, which suggests that the products may possess interesting optical properties and facilitate future nanoscale device applications. Acknowledgment. The authors acknowledge the financial support from the National Natural Science Foundation of China (Nos. 50472050 and 50672080) and the Program for New Century Excellent Talents in University, and A Foundation for the Author of National Excellent Doctoral Dissertation of PR China (No. 200434). References (1) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 66, 409. (2) Yan, C.; Xue, D. J. Phys. Chem. B 2005, 109, 12358. (3) Gautam, U. K.; Vivekchand, S. C.; Govindaraj, A.; Kulkami, G. U.; Selvi, N. R.; Rao, C. N. R. J. Am. Chem. Soc. 2005, 127, 3658. (4) Mathiowitz, E.; Jacob, J. S.; Jon, Y. S.; Carino, G. P.; Chickering, D. E.; Chaturvedi, P.; Santos, C. A.; Vijayaraghavan, K.; Montgomery, S.; Bassett, M.; Morrell, C. Nature 1997, 386, 410. (5) Xiao, X. C.; Jeffrey, W.; Elam, S.; Trasobares, O. A.; Carlisle, J. A. AdV. Mater. 2005, 17, 1496. (6) Ma, R.; Bando, Y.; Zhu, H.; Sato, T.; Xu, C.; Wu, D. J. Am. Chem. Soc 2002, 124, 7672. (7) Chen, L. Y.; Gu, Y. L.; Li, Z. F.; Qian, Y. T.; Yang, Z. H.; Ma, J. H. J. Cryst. Growth 2005, 273, 646. (8) Luo, T.; Liu, J. W.; Chen, L. Y.; Zeng, S. Y.; Qian, Y. T. Carbon 2005, 43, 755. (9) Tang, K. B.; Qian, Y. T.; Zeng, J. H.; Yang, X. G. AdV. Mater. 2003, 15, 448.

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