High-Yield Synthesis of Boron Nitride Nanosheets ... - ACS Publications

Jul 30, 2009 - Key Laboratory of Liquid Structure and Heredity of Materials of Ministry of Education, College of Materials. Science and Engineering, S...
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High-Yield Synthesis of Boron Nitride Nanosheets with Strong Ultraviolet Cathodoluminescence Emission Rui Gao, Longwei Yin,* Chengxiang Wang, Yongxin Qi, Ning Lun, Luyuan Zhang, Yu-Xian Liu, Le Kang, and Xianfen Wang Key Laboratory of Liquid Structure and Heredity of Materials of Ministry of Education, College of Materials Science and Engineering, Shandong UniVersity, Jinan 250061, China ReceiVed: April 27, 2009; ReVised Manuscript ReceiVed: June 24, 2009

Bulk quantities of hexagonal boron nitride (h-BN) nanosheets have been synthesized via a simple templateand catalyst-free chemical vapor deposition process at 1100-1300 °C. Adjusting the synthesis and chemical reaction parameters, the thickness of the BN nanosheets can be tuned in a range of 25-50 nm. Fourier transform infrared spectra and electron energy loss spectra reveal the typical nature of sp2-hybridization for the BN nanosheets. It shows an onset oxidation temperature of 850 °C for BN nanosheets compared with only about 400 °C for that of carbon nanotubes under the same conditions. It reveals a strong and narrow cathodoluminescence emission in the ultraviolet range from the h-BN nanosheets, displaying strong ultraviolet lasing behavior. The fact that this luminescence response would be rather insensitive to size makes the BN nanosheets ideal candidates for lasing optical devices in the UV regime. The h-BN nanosheets are also better candidates for composite materials in high-temperature and hazardous environments. 1. Introduction Since the early 1990s, much interest on low dimensional nanomaterials has been revived with the advent of fullerenes1 and carbon nanotubes.2 Structurally identical to graphite in nature, hexagonal boron nitride (h-BN) is one of the most important III-V group materials with in-plane trigonal sp2 bonding, with B and N atoms entirely substituting for C atoms in a graphitic-like sheet with almost no change in atomic spacing. With comparison to graphite carbon, h-BN has excellent mechanical properties and thermal conductivity and is much more thermally and chemically stable than graphite, which makes BN a better candidate for composite materials in hazardous environments.3 BN nanotubes were first reported by Chopra et al. via an arc-discharge route.4,5 In the past decade, considerable efforts have been devoted to BN nanotubes through laser ablation,6 the carbo-,8 amino-,9 template-assisted process,10 and plasma-based7 and ball milling techniques.11 h-BN manifests a structural variety similar to that of graphite, including onedimensional (1D), two-dimensional (2D), and three-dimensional (3D) crystalline structures. In addition to BN nanotube structure, a variety of h-BN nanomaterials with well-defined morphology and size such as nanocapsules,12 nanowires,13 core/shell heterostructures,14,15 nanospheres,16,17 and hollow nanoribbons18 have been successfully synthesized and characterized. Because of their fundamental significance in investigating the dependence of physical properties on dimensionality, BN nanosheets are emerging materials with unusual properties that are promising for nano-optoelectronic applications and composites. Recently, the two-dimensional structure of boron nitride sheets and suspended membranes via a micromechanical cleavage technique19 and via a chemical solution-derived method starting from single-crystalline hexagonal boron nitride have been reported.20 Generally, current synthesis methods make it difficult for the manipulation of BN nanosheets, and the amount of * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: +86-531-88396970.

available BN nanosheet materials is always very limited for applications. To the best of our knowledge, the bulk synthesis of BN nanosheets has not been reported hitherto. Furthermore, the BN nanostructures via the CVD route inevitably contain metallic or metal oxide impurities, which is not easy to eliminate.21 In contrast to a metallic or semiconducting property of carbon nanotubes (CNT),22 the band gap structure of h-BN shows semiconducting properties with a nearly constant band gap (5.5 eV), which is mainly dependent on chemical composition.23 Optical and fluorescence properties of h-BN deserved particular interest during the past decade because the first observation of an intense far-UV exciton emission,24,25 making this material a candidate for use in new light-emitting devices. For the band gap energy property of BN, Lauret et al. directly measured the optical properties of single-walled BN nanotubes using a UV-vis spectrophotometer.26 Cathodoluminescence and photoluminescence examination reveal strong luminescence in the ultraviolet range from BN nanotubes.27-29 While Arenal et al. carried out electron energy loss spectroscopy (EELS) measurements on the optical gaps of single-walled and multiwalled BNNTs,30,31 the reported optical data on h-BN display a notable scatter from 3.0 to 6.6 eV, strongly dependent on the preparation method. The optical properties of h-BN nanostructures are still not well understood because of poor sample quality and low light emission efficiency. Here, we report a one-step facile route for the production of bulk quantities of thin BN nanosheets. Bulk quantities of BN nanosheets with uniform size distribution were prepared via chemical vapor deposition at 1100-1300 °C. The thickness of the BN nanosheets can be tuned by adjusting the synthesis and reaction parameters such as temperature. Significant advantages of the synthesis route include a template-free, low-temperature, facile procedure and high yield. The structure, chemical composition, and thermal stability of h-BN nanosheets are systematically studied. The optical property study of BN nanosheets via UV-vis adsorption and cathodoluminescence

10.1021/jp904246j CCC: $40.75  2009 American Chemical Society Published on Web 07/30/2009

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(CL) spectra reveals strong cathodoluminescence emission in the ultraviolet range, indicating that the present novel BN nanosheets are highly promising for application in optical devices. 2. Experimental Section In a typical procedure, B2O3 and melamine were first mechanically mixed, and the mixed powders as precursors were put into a graphite crucible. The graphite crucible containing the precursors was placed at the center of a high-purity graphite induction-heated cylinder. After evacuation of the quartz tube to about 2-3 Torr, high-purity Ar was introduced as a carrier gas at a flow rate of 30 standard cubic centimeters per minute (sccm). After that, a N2 gas was introduced as a reaction gas with a flow rate of 25 sccm. An induction furnace was used for heating the graphite susceptor to temperatures of 1000-1350 °C. After maintaining these experimental conditions for about 1.0 h, a large yield product with white color can be obtained downstream of the graphite induction-heated cylinder. The phase components of the synthesized products were measured using X-ray diffraction (XRD) patterns recorded in a Rigaku D/max-kA diffractometer with Cu KR radiation (60 kV, 40 mA). The morphologies and chemical composition of the products were characterized via SU-70 type thermal field emission scanning electron microscopy (FE-SEM) and attached X-ray energy dispersive spectrometry (EDS). The boron nitride specimens were dispersed in ethanol and deposited onto carbon film-coated copper grids. The microstructures and chemical composition of the BN products were studied using transmission electron microscopy and selected area electron diffraction. A Fourier transform infrared (FTIR) spectroscopic study was carried out with a Bruker Vector 22 FT-IR spectrometer at 1 cm-1 resolution with 1000 scans. The structure instability at high temperatures of the BN products was measured by a DSC41 differential scanning calorimeter. In order to investigate the optical absorbent properties of the BN samples, a thin film prepared from BN powder was first prepared, and then the UV-vis adsorption spectrum was obtained with a TU-1901 diode-array spectrophotometer. A cathodoluminescence spectrophotometer attached to the SU-70 FE-SEM was used to investigate optical properties of the synthesized BN products. Nitrogen adsorption-desorption isotherms were determined at 77 K using an adsorption porosimeter (Quadrasorb SI). The surface area measurements were performed according to the BET method.

Figure 1. FE-SEM images of boron nitride nanosheets prepared at 1200 °C.

Figure 2. (a) SEM images BN nanosheetes obtained at 1200 °C. (b,c) Corresponding B and N elelemental mapping, repectively. (d) Energydispersive X-ray (EDX) spectrum.

3. Results and Discussion After synthesis, a high yield of white products can be obtained. The morphology and microstructure of the synthesized products prepared at a synthesis temperature of 1200 °C are characterized in a SU-70 thermal field emission scanning electron microscopy (FE-SEM) as shown in Figure 1. The synthesized products generally display nanosheet-like morphologies. The prepared BN nanosheets have an average diameter of 800-1000 nm and a thickness of 35 nm. From highmagnification FE-SEM images (Figure 1b,c,d), it is clearly shown that BN nanosheets display a uniform size distribution, and the BN surface of BN nanosheets is very smooth. The chemical composition of the products can be characterized by corresponding elemental mapping and a X-ray energy dispersive (EDS) spectrometer attached to a SU-70 FE-SEM as shown in Figure 2. It is indicated from the EDS spectrum and corresponding elemental mapping that the products are composed of pure B and N (EDS spectrum in Figure 2d), and the B and

Figure 3. (a) Magnified SEM image of BN nanosheetes prepared at 1200 °C. (b,c) Corresponding B and N elemental mapping, respectively.

N elements are homogeneously distributed along the whole BN nanosheet structures as depicted in elemental mapping in panel b and c of Figure 2. Figure 3 depicts elemental mapping for several BN nanosheets with high magnification, clearly revealing the products are composed of homogeneously distributed B and N elements. X-ray diffraction was used to investigate the crystal structure and phase components of the products. As shown in Figure 4,

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Figure 4. Typical X-ray diffraction pattern of hexagonal BN nanosheets with lattice constants of a ) 0.2504 nm and c ) 0.6656 nm.

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Figure 6. (a,b,c) Typical transmission electron microscopy images of BN products synthesized at 1200 °C. (d) Selected area electron diffraction (SAED) pattern recorded along the [001] zone axis of a h-BN crystal.

Figure 5. Characteristic FTIR spectrum of the BN nanosheets.

Figure 7. Typical electron energy loss (EEL) spectrum from a single h-BN nanosheet, clearly showing the nature of sp2 hybridization for the h-BN nanosheet.

all of the diffraction peaks corresponding to the (002), (100), (101), (102), (004), (104), (110), and (112) planes of hexagonal phase of BN with lattice constants of a ) 0.2504 nm and c ) 0.6656 nm. (JCPDS card 34-0421) No impurities can be detected in this pattern, which indicates that pure BN nanosheets can be obtained under the current synthesis conditions. It is should be noted that the XRD pattern is almost the same as that of bulk crystalline h-BN reported by Tanaguchi et al.,25 different from the XRD results reported in refs 3 and 5. The prepared products were further examined using Fourier transformation infrared spectroscopy (FTIR). FTIR is effective to characteristically determine the structural nature of the hexagonal BN. Single-crystalline h-BN shows BN vibrations at 1365 cm-1,25 while turbostratic h-BN shows phonon modes at 792 and 1384 cm-1. It has been reported that the phonon modes shift to 800 and 1372 cm-1 for multiwalled, while h-BN tubes shift to 811 and 1377 cm-1 for polycrystalline h-BN.32 The characteristic FTIR spectrum of the BN nanosheet products shown in Figure 5 reveals the structural nature and phase composition of synthesized h-BN products. In the FTIR spectrum, there are two strong vibrations at 1374 and 816 cm-1. The peak around 1374 cm-1 results from the in-plane B-N transverse optical modes of the sp2-bonded h-BN, while the peak centered at 816 cm-1 could be attributed to the B-N-B outof-plane bending vibration.33 No obvious absorption peaks associated with the starting materials and impurities are observed.

Panels a-c of Figure 6 depict typical transmission electron microscopy (TEM) microstructures of the BN products synthesized at 1200 °C. The synthesized BN products have diameters that range from 800 to 1000 nm and a small thickness of about 35 nm. The BN nanosheets are very thin and are almost transparent under TEM observation. A selected area electron diffraction (SAED) pattern taken from a single nanosheet is depicted in Figure 6d. This pattern can be indexed as that recorded along the [001] zone axis of the h-BN crystal, which clearly shows the single-crystalline nature of hexagonal BN nanosheets. The chemical composition and structural nature of the synthesized BN products were further characterized via electron energy loss spectroscopy (EELS). Figure 7 shows a typical EELS spectrum from single h-BN nanosheet. Only B and N characteristic K-edges can be clearly observed in the EELS spectrum. The sharp π* and σ* peaks are typical of the sp2-hybridized BN structure. Two distinct edges at 188 and 401 eV are revealed, corresponding to the B-K and N-K edges, respectively. Quantification of EELS spectra gives a B/N atomic ratio of about 1.0.18,34 Surface area measurements were performed according to the BET method. It is revealed that the synthesized BN nanosheets have a surface area of 172 m2g-1, larger than the 168 m2g-1 for activated BN,35 50 m2 g-1 for porous BN,36 and 26.8 m2g-1 for spherical BN nanoparticles.37 Pore size distribution calculated from the BET method is shown in Figure 8. It is indicated that

Synthesis of Boron Nitride Nanosheets

Figure 8. Pore size distribution calculated from the BET method for h-BN nanosheets.

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Figure 10. (a,b,c) FE-SEM images of BN nanosheets prepared at 1300 °C. (d) FE-SEM image of BN nanosheets prepared at 1100 °C.

Figure 11. UV-vis optical absorption spectra of BN nanosheets. Figure 9. Thermal stability of h-BN nanosheets analyzed via thermogravimetric analysis in air.

the synthesized h-BN nanosheets have a relatively large pore size distribution. The pore radius for the BN nanosheets is about 2.4 and 5.3 nm. The thermal stability of hexagonal BN nanosheets was analyzed via thermogravimetric analysis in air as illustrated in Figure 9. It reveals an onset temperature for oxidation of BN nanosheets of 850 °C compared with only about 400 °C for that of carbon nanotubes under the same conditions.38 In spite of the large surface area for the BN nanosheets, the nanosheets display high thermal stability under high temperatures in air. With comparison to the complete oxidation temperature of 700 °C for carbon nanotubes, the BN nanosheets are stable even to 1000 °C, with only a slight oxidation on the surface. The results demonstrate a more pronounced resistance to oxidation for h-BN nanosheets. This may allow BN nanosheets to replace carbon-nanostructured materials for high-temperature applications. Careful study on the relation between the synthesis parameters and structures of the prepared BN products showed that the thickness of the BN nanosheets can be tuned by controlling synthesis or reaction temperatures. The effect of synthesis temperatures on the size of the synthesized BN products is shown in Figure 10. Compared with BN products synthesized at a relatively lower temperature of 1200 °C, the BN nanosheets obtained at a higher temperature of 1300 °C have a thickness

of about 20 nm (Figure 10a,b, and c), smaller than that prepared at 1200 °C (Figure 1). With synthesis temperature decreasing, the thickness of the BN nanosheet increases. The BN nanosheets synthesized at 1100 °C have an average thickness of 50 nm (Figure 10d), larger than that obtained at 1200 and 1300 °C. The optical absorption properties reflect the electronic state of the materials and are widely used to determine the band gap of semiconductors. The analysis of the available literature data shows that the previously reported values of BN band gap energy are widely dispersed in the range between 3.6 and 7.1 eV, depending on different BN samples with different structures.18,24,26,28,39,40 For the single-walled BN nanotubes, three absorption lines at 4.45, 5.5, and 6.15 eV in UV-vis spectra are observed. It is considered that the absorption lines at 6.15 eV are related to near band gap energy, while lines at 4.45 and 5.5 eV result from the quantification involved by the rolling up of the hexagonal boron nitride (h-BN) plates. Zhu et al. reported band gap energy of about 3.8 for BN whiskers with cubic- and hexagonal-mixed structures via optical UV absorbance examination.39 To investigate the optical properties of the as-prepared h-BN nanosheets, we obtained the UV-vis optical absorption spectrum of BN nanosheets (Figure 11). It can be seen that an absorption peak around 251 nm corresponds to a band gap energy (Eg) of 4.94 eV for h-BN. Another two peaks at 307 nm (4.04 eV) and 365 nm (3.40 eV) can also be observed in the UV-vis spectrum. The two low-energy lines can be considered

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Figure 12. (a) SEM images of boron nitride nanosheets prepared at 1200 °C. (b) Cathodoluminescence images form the corresponding BN nanosheets. (e) CL spectrum from the BN nanosheets.

as optical transitions between van Hove singularities in the onedimensional density of states of BN nanosheets.26 The measured bang gap energy for the prepared BN nanosheets is smaller than that of the h-BN single crystal24 and BN nanoribbons.18 It is believed that structural changes in morphology and size may result in changes in the electronic state and resultantly lead to changes in band gap energy.40 The optical properties of the BN nanosheets were further investigated using a cathodoluminescence (CL) spectrometer attached to a SU-70 FE-SEM. Panels a and b of Figure 12 depict SEM and CL luminescence images of BN nanosheets obtained at 1200 °C, respectively. The BN nanosheets display strong CL luminescence emission in the ultraviolet range. Examining the spectra in more detail, we can see that the dominant luminescence signal at higher energy displays two peaks at 313 and 327 nm, corresponding to energy of 3.96 and 3.79 ev, respectively. The near band edge CL luminescence emission is not observed in the present CL spectrum from the as-prepared BN nanosheets. For the CL examination on BN materials, there has previously been reported diversed CL emission results on BN samples by different groups. The band edge optical nature of intense 215 nm (5.77 eV) CL luminescence for a hexagonal BN single crystal is generally accepted and reported.24,25 While CL emissions centered at 3.25 and 3.80 eV and weak CL emissions at 5.33 eV are also reported by Cheng et al. from BN nanoribbons.18 For BN nanotubes, a strong cathodoluminescence peak located around 3.3 eV and a weak peak at 4.1 eV was reported by Zhi et al,27 while Czerw reported 2.68, 3.31, and 5.3 eV for BNNTs via cathodoluminescence measurement.41 In the present result, the strong ultraviolet CL emission from the BN nanosheets can be attributed to the deep-level emissions associated with defect-related centers (B or N vacancy-type defect-trapped states). Most importantly, the luminescence peaks at 313 and 327 nm are very narrow and sharp, and the h-BN nanosheets display strong ultraviolet lasing behavior. Similar CL luminescence results sisplay BN nanosheets with a smaller thickness of about 25 nm as shown in Figure 13. In the CL spectrum, we can see the CL strong ultraviolet luminescence centered at 318 and 332 nm, corresponding to energy of 3.90 and 3.74 eV, respectively, and the shape of the CL curve line is almost same. It is revealed that the luminescence response would be rather insensitive to size for the h-BN nanosheets.

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Figure 13. (a) SEM images of boron nitride nanosheets prepared at 1300 °C. (b) Cathodoluminescence images form the corresponding BN nanosheets. (e) CL spectrum from the BN nanosheets.

4. Conclusion Bulk quantities of hexagonal BN nanosheets with unique size distribution were synthesized via a high-temperature chemical vapor deposition (CVD) route. Significant advantages of the synthesis route include a template-free, low-temperature, facile procedure and high yield. The synthesized h-BN nanosheets have diameters of 800-1000 nm, and the thickness of h-BN nanosheets can be tuned from 25 to 50 nm via adjusting synthesis temperatures. It shows an onset oxidation temperature of 850 °C for BN nanosheets compared with only about 400 °C for that of carbon nanotubes under the same conditions. Optical properties were systematically investigated via UV-vis adsorption spectrometry and cathodoluminescence spectrometry. It reveals a strong cathodoluminescence emission in the ultraviolet range from the h-BN nanosheets, indicating that the present novel BN nanosheets are highly promising for application in optical devices. The BN nanosheets can find potential applications in lasing, light-emitting diodes, and medical diagnosis devices. Because of their high thermal stability, h-BN nanosheets are also better candidates for composite materials in hightemperature and hazardous environments. Acknowledgment. We acknowledge support from the National Nature Science Foundation of China (50872071), Nature Science Foundation of Shandong Province (Y2007F03 and Y2008F26), Foundation of Outstanding Young Scientists in Shandong Province (No. 2006BS04030), Tai Shan Scholar Foundation of Shandong Province, and Gong Guan Foundation of Shandong Province (2008GG10003019). Note Added after ASAP Publication. This article was released ASAP on July 30, 2009. A change was made to the caption of Figure 5, and the article was reposted on August 20, 2009. References and Notes (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162. (2) Iijima, S. Helical microtubules of graphitic carbon. Nature 1991, 354, 56. (3) Golberg, D.; Bando, Y.; Tang, C. C.; Zhi, C. Y. Boron nitride nanotubes. AdV. Mater. 2007, 19, 2413.

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