Synthesis and First-Principles Studies of Single-Crystalline β-BC2N with Oxygen-Bearing Defects Li Hou,† Faming Gao,*,† Huiyang Gou,† Zhibing Wang,† and Min Tian‡ Department of Chemical Engineering, Yanshan UniVersity, Qinghuangdao 066004, China, and Department of Chemistry, UniVersity of Ottawa, Ottawa, Canada
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 6 1972–1976
ReceiVed July 28, 2007; ReVised Manuscript ReceiVed December 13, 2007
ABSTRACT: Single-crystalline B-C-N nanorods were prepared at 500 °C and less than about 20 MPa. Results from X-ray powder diffraction (XRD) and selected-area electron diffraction (SAED) measurements suggest that the synthesized single-crystalline B-C-N is of tetragonal structure and its lattice constants are a ) 7.12(5) Å and c ) 3.57(3) Å. The nanorods all are very straight, and the ratios of length to diameter are between 5 and 30. High resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) observations confirm the well-defined nanorods grown preferentially perpendicular to the [220] direction develop tetragonal morphologies. The energy-loss spectroscopy (EELS) and energy-dispersive X-ray (EDX) elemental mapping confirm that the nanorod is composed of B, C, N, and a trace of oxygen, and its stoichiometry is determined to be close to BC2N. First-principles calculations confirm that the as-prepared sample is superlattice phase of β-BC2N phase with defects of the CN′ and the ON′. In addition, it is a semiconductor material with a direct energy band gap 1.21 eV. The calculated hardness is comparable to that of diamond. Introduction The high-pressure high-temperature synthesis of cubic boron-carbonitride (c-BCN) compound has been frequently reported.1–5 The measuring hardness for c-BCN, 76 GPa, is greater than that of cubic BN, comparable to that of diamond.4 Therefore, boron-carbonitride would be an excellent material for highspeed cutting and polishing of ferrous alloys.6,7 On the other hand, theorists have attempted to predict the structure and properties of various boron-carbonitrides using first-principles techniques.8–13 Results show that the total energy of β-BC2N is lowest among the various possible structures. In other words, the synthesis of the β-BC2N phase seems to be the easiest. Optimizing cell parameters, a ) b ) 3.577, c ) 3.608, of β-BC2N,11 strictly said, indicates that it is of tetragonal structure. However, to our knowledge, the synthesis of the β-BC2N has not been reported yet. As is well-known, the synthesis of the ternary B-C-N compounds are more difficult, even compared to that of diamond and cubic BN. In this work, we attempt the synthesis of the BC2N compound under relatively low temperature and low pressure. Tetragonal oxygen-bearing compound BxCyNz nanorods have been prepared. Since the yield of the nanorods is only 15%, first-principles calculations have to be employed to study its detailed structure and properties. Theoretical results suggest that the as-prepared sample is a superlattice phase of β-BC2N phase with the defects CN′ and ONβ, and its calculated hardness is even greater than that of the β-BC2N phase. Experimental Procedures In our experiments, CH3CN was used as both carbon source and solvent after distillation treatment at 82 °C to remove the impurities and moisture. The other reagents were analytically pure and used without further purification. The solvothermal reaction was carried out in a stainless steel autoclave (50 mL in total capacity) under autogenous pressure. The experimental procedure was as follows: at first, 2 mL of liquid BBr3 was dissolved into 15 mL of anhydrous CH3CN, then 4.10 g * To whom correspondence should be addressed. E-mail:
[email protected]. † Yanshan University. ‡ University of Ottawa.
of NaN3 powders was added into the solution. After being stirred for 10 min, the solution mixture was transferred into a stainless steel autoclave, and then the autoclave was filled with anhydrous CH3CN up to 60% of the total volume. After the air was expelled by argon gas, the autoclave was sealed. The whole manipulation was carried out in a Ar-flowing glovebox. Then the autoclave was maintained at 500 °C for 24 h with autogenous pressure inside the autoclave. On the basis of the volume concentration of CH3CN and the equation of state of gas, the maximum pressure in autoclave was calculated less than about 20 MPa. After the autoclave was cooled to room temperature naturally, the black powders were collected and washed with absolute ethanol and distilled water in sequence to remove the possible excessive NaN3 and the byproduct NaBr. Then the product was dried in vacuum at 80 °C for several hours. X-ray powder diffraction (XRD) pattern was carried out on a D/max2500/PC X-ray diffractometer with Cu KR radiation (λ ) 0.15418 nm) to study the phases present. The morphologies of samples were characterized by a transmission electron microscopy (TEM; JEM-2010) with energy-dispersive X-ray (EDX) and PEELS attachments and a scanning electron microscope (SEM; XL30 ESEM FEG). Selectedarea electron diffraction (SAED) was used to investigate the phase structure of the product. The electron energy-loss spectroscopy (EELS) and EDX elemental mapping were used to determine the chemical composition of the sample.
Results and Discussion Structure analysis was performed using XRD and SAED. Figure 1a shows a typical X-ray diffraction of our synthesized sample. The obvious broad feature around 24° (marked by a rectangle) indicates the presence of a large amount of amorphous phase in the prepared sample. The crystalline phase (marked by triangle) is an unknown phase and it has not yet been reported. In order to identify the crystal structure of the unknown phase, the SAED patterns of the unknown phase along two zone axes were also presented in Figure 1, which were taken from several single-crystalline nanorods. From Figure 1b,c, it can be concluded that this unknown phase belongs to a tetragonal system. Its lattice constants were calculated by using XRD and SAED data, and fitted by the method of least-squares, to be a ) 7.12(5) Å and c ) 3.57(3) Å, respectively. Their interplanar spacings and corresponding Miller indices measured by XRD and SAED are listed in Table 1. The calculated results basing
10.1021/cg700713g CCC: $40.75 2008 American Chemical Society Published on Web 05/20/2008
First-Principles Studies of Single-Crystalline β-BC2N
Crystal Growth & Design, Vol. 8, No. 6, 2008 1973
Figure 1. XRD pattern of the synthesized sample (a) and transmission electron microscopy electron beam diffraction patterns of the new B-C-N phase along [001] (b) and [110 ] (c) axes. Table 1. Interplanar Spacings and Miller Indices for the Tetragonal Boron Carbonitride Measured by XRD and SAED d (Å) (expt) No.
hkl
d (Å) (cal)
XRD
SAED
1 2 3 4 5 6 7
220 310 221 311 420 421 610
2.5332 2.2657 2.0734 1.9190 1.6023 1.4644 1.1780
2.5184 2.2134 2.0295 1.9050 1.6208 1.4749 1.1732
2.5337 2.2195 2.0170 1.6091
on the simulative structure are given, too. It can be seen that the data of SAED at d-spacings of 2.533, 2.219, 2.017, and 1.609 Å, match (220), (310), (221), and (420) planes, in good agreement with the XRD results. Taking into consideration the influence of the impurity in final product to this phase and the calculated error originates from the small difference between the simulative and actual crystal structures, the agreement is satisfactory when comparing the calculated and experimental results listed in Table 1, with a deviation of less than 3% for XRD and SAED measurements. By combining with XRD and SAED and EELS spectra, we can conclude that the single crystalline nanorods are tetragonal-structured B-C-N with oxygen-bearing defects. Besides, another phase (marked by a circle in Figure 1a) being similar to crystal C (see PDF 18311) also appeared in the synthesized sample. The size and morphology of the nanorods were examined by TEM and SEM. Figure 2 shows the representative images of the as-prepared nanorods. According to the observation of some low magnification TEM and SEM images, the proportion of the products with rodlike structure is about 15%. All the nanorods are very straight as shown in Figure 2a, the diameters
varied from 50 to 210 nm, and the ratios of length to diameter are between 5 and 30. The SEM image of the as-prepared nanorod presenting the detailed structure information of the rod is shown in Figure 2b, which exhibits prism morphology. It can be clearly seen that the nanorods consist of four prismatic planes and a hemisphere-like tip. The hemisphere-like tip is shown in Figure 2b, which is the most common morphology for nanorods, as already revealed.14 The nanorods with a pyramid-like tip were also found as shown in Figure 2a. The regular morphological features suggest the single crystalline nature of nanorods. Further evidence for the formation of singlecrystalline nanorods could be found in the high resolution transmission electon microscopy (HRTEM) image shown in Figure 2c. The HRTEM image was taken from one part of the nanorod and clearly shows the lattice fringes which are consistent throughout the crystal. The measured spacing between adjacent lattice planes is 0.25 nm, corresponding to the (220) plane of as-prepared tetragonal single crystal, which indicates that the preferred growth direction of nanorods is perpendicular to the [220] direction. Combining the results of SEM and HRTEM observation in our experiment and correlative discussions in ref 15, it can be concluded that the side surfaces of the observed tetragonal prisms belong to the {220} type planes. A representative EEL spectrum corresponding to the nanorod in Figure 3a is shown in the inset to estimate the stoichiometry of the nanorods. The typical ionization edges at 190, 284, and 403 eV, corresponding to the characteristic K-shell ionization edges of B, C, and N, respectively.16,17 The important result here is that the presence of a much weaker π-type peak for B and C from the nanorod compared to that of g-BC2N and the loss of sharp peak at 403, which is the feature for the N K-edge that can be attributed to an sp2-like crystalline system.18 All
1974 Crystal Growth & Design, Vol. 8, No. 6, 2008
Hou et al.
Figure 2. (a) The representative TEM images of as-prepared nanorods; (b) field emission scanning electron microscopy of a single nanorod; (c) HRTEM image taken from a rod.
Figure 3. (a) Bright-field TEM image of an individual nanorod; (inset) a representative EEL spectra taken from nanorods; (b) concentration profiles of the B, C, and N species along the yellow line across the rod in (a); (c) STEM image of an individual nanorod in bright field mode exhibiting elemental mapping. EDX boron mapping (red), EDX carbon mapping (green), and EDX nitrogen mapping (white) of the section of rod marked with the yellow rectangle on the TEM image.
First-Principles Studies of Single-Crystalline β-BC2N
Crystal Growth & Design, Vol. 8, No. 6, 2008 1975 ( 0.06C1.00 ( 0.00N0.36 ( 0.04O0.14 ( 0.02).
Considering the fact that a part of oxygen may involve contributions from the moisture adsorbed on the surface of the as-prepared sample, we can safely conclude that the oxygen content is less than 5%. Elemental quantification of the integrated EELS signal of several nanorods all showed the simultaneous presence of four elements, B, C, N, and O, quantitive analyses give a chemical formula B0.23-0.27C0.48-0.55N0.16-0.22O0.03-0.08 for all nanorods analyzed. In order to study the distribution of B, C, N, and O species in the rods, a series of EELS measurements were carried out. A compositional profile for the four elements was evaluated from a series of spectra measured along the yellow line marked in Figure 3a with a spatial resolution of about 10 nm (Figure 3b). The numbers (in nm) on the horizontal axis correspond to the distance along the marked line starting from its left-hand side. We observe the relative homogeneous coexistence of B, C, N and O due to profile correlations of the four elements at the same position of the rod. There is no distinct change in the shape of peaks among B, C, and N, and the profile demonstrates that the C peaks notably correlate with those of B and N. Thus we can conclude that the nanorods possess a consistent B-C-N structure with a trace amount of oxygen. Further evidence for the homogeneous distribution of B, C, and N species can be provided by the energy-dispersive X-ray elemental mapping. Figure 3c shows a scanning transmission electron microscopy (STEM) image of an individual nanorod, as well as elemental maps, representing B, C, and N, respectively. Within the rectangle section marked in Figure 3c, it appears that the B, C, and N species coexist along the rod regions. Occasional nanoparticles were found to be of pure carbon (see C mapping). The detailed analysis of the B, C, and N maps reveal that the concentration of three species seems to be different; the C concentration exceeded that of B, then the lowest is N. The result is consistent with that of EELS analysis. Although the EDX elemental mapping gave analysis on only a small part of a rod due to its detection limits, the obtained
Figure 4. All the possible topologically different B8C16(N6CO) (after structural relaxation). Boron, carbon, nitride, and oxygen atoms are depicted in pink, gray, blue, and red, respectively. The relative bigger atoms in each crystal structue represent the defects of the CN′ and the ON′.
these indicate that the tetragonal phase possesses complicated bonding structures that are different from that of both cubic and graphite-like B-C-N phases. Besides, a very small peak at 532 eV, the K-edge excitation of oxygen, was also found in EELS spectra.19 A series of EELS spectra from individual nanorods, and from different parts of the same nanorod (edge, center, tip, and root) gave the same result that the oxygen absorption features were always seen. Also, quantitive analyses reveal the contents of O ranging from 4 ( 0.5 at.% (e.g., B0.45 ( 0.05C1.00 ( 0.00N0.36 ( 0.04O0.07 ( 0.01) to 7 ( 0.9 at.% (e.g., B0.50
Table 2. Structures of B8C16(N6CO), Lattice Parameters a, b, c in Å and Total Energy E in eV superlattice of R-BC2N
superlattice of β-BC2N a b c R β γ E
1
2
3
4
5
6
7
8
9
10
7.1657 7.1657 3.6085 90.62 90.62 88.73 -5335.84
7.1576 7.1576 3.6113 89.98 90.02 88.84 -5335.51
3.5714 3.5714 14.4317 90.00 90.00 89.18 -5335.73
3.5662 3.5662 14.5816 90.00 90.00 89.29 -5334.27
3.5721 3.5721 14.5157 90.00 90.00 89.06 -5334.77
7.1619 7.2313 3.5723 89.45 90.78 90.54 -5335.67
7.1757 7.2291 3.5699 89.49 90.86 90.61 -5335.24
3.5730 3.6136 14.2792 90.00 90.60 90.00 -5335.71
3.5762 3.6318 14.3054 90.00 91.25 90.00 -5334.23
3.5887 3.6114 14.3135 90.53 90.94 89.41 -5335.02
Table 3. Hardness Calculation of B8C16(N6CO) with the Structure of S1 in Figure 4, where dµ is Bond Length, Neµ is the Electron Density Expressed in Number of Valence Electrons Per Cubic Angstroms, fiµ Is Ionicity of Bond, Hv calc and Hv exp Are Calculated and Experimental Vickers Hardness, Respectivelyb bond number
bond type
dµ (Å)
Neµ
fiµ
HVµ
Hv calc(GPa)
B8C16(N6CO) (S1 structure)
2 CC 1 CC 1C O 14 CC 1 CC 2 BO 16 BC 2 BC 12 CN 12 BN 4 BN 4 BC 4 CC 4 CN
1.432 1.458 1.495 1.513 1.565 1.525 1.578 1.649 1.557 1.556 1.562 1.573 1.515 1.564
1.122 0.904 0.922 0.771 0.562 0.868 0.602 0.371 0.687 0.684 0.680 0.498 0.930 0.679
0 0 0.446 0 0 0.450 0 0 0.236 0.234 0.227 0.000 0.000 0.228
154.0 127.5 71.3 104.5 77.8 64.8 79.7 51.7 68.0 68.1 66.9 70.7 118.1 66.5
80
β-BC2N
a
For comparison, the hardnesses of β-BC2N also are listed. b Experimental value from ref 4, for comparison.
78
Hv expt (GPa)
76b
1976 Crystal Growth & Design, Vol. 8, No. 6, 2008
mappings of B, C, and N indicate that they are homogeneously coexistent within all the rods. According to the chemical composition of as-prepared sample B0.23-0.27C0.48-0.55N0.16-0.22O0.03-0.08 and considering electronic charge equilibrium, its chemical formula may be suggested as B8C16(N6CO), that is, B0.25C0.53N0.19O0.03, which can be further considered as BC2N with defects of the CN′ and ON′. The theoretical studies8–11 indicate BC2N structures which was obtained from a fully geometry relaxation of the substitute fcc diamond has the two most stable structures, namely, R-BC2N and β-BC2N. R-BC2N crystal can be obtained from a primitive diamond system with eight atomic positions per unit cell by substituting four carbon atoms with two nitrogen at positions (0 1/2 1/2) and (1/2 0 1/2) and two borons at (1/4 1/4 1/4) and (3/4 1/4 3/4). β-BC2N is obtained by replacing the carbon atoms in positions (0 0 0), (1/2 1/2 0) with two borons and in (1/4 3/4 3/4) and (3/4 1/4 3/4) with nitrogens. In both structures each electron-rich nitrogen is bonded with two electron poor boron atoms and two carbons in order to reach the fourth co-ordination. In the same way the boron atoms are connected with two nitrogens and two carbons. The B-B and N-N bonds have been avoided in both structures, since these bonds likely require a higher energy to form. The optimization results of the lattice parameters and the ions relaxation indicate that R-BC2N consists of an orthorhombic structure where carbon, nitride, and boron atoms are tetrahedrally coordinate and β-BC2N crystallizes in the tetragonal structure with lattice constants a ) b ) 3.577 Å, c ) 3.608 Å.8,10,11 On the basis of the ab initio calculation techniques, we can study the properties of a novel B8C16(N6CO) structure obtained from a full geometry relaxation of the substituted R-BC2N and β-BC2N superlattice. B8C16(N6CO) phases can be constructed by replacing nitrogen atoms with carbon and oxygen atoms. In Figure 4, we show the possible topologically different structures (after structural relaxation) for B8C16(N6CO) in 2 × 2 × 1 and 1 × 1 × 4 supercells of the substituted R-BC2N and β-BC2N. Calculations of the groundstate geometries on the above compounds were carried out in the framework of density functional theory (DFT) with the Material Studio.20 Optimizations are performed with CASTEP code. The interactions between the ions and the electrons are described by using Ultrasoft Vanderbilt pseudopotential and the electron-electron interaction is treated within the local density approximation LDA by the Ceperley-Alder exchange correlation potential. The calculations were performed using an energy cutoff of 310 eV for the plane wave basis set. The optimization of the lattice parameters and the ions relaxation were performed iteratively until the minimum on the total energy was met. The calculated structures of the 10 B8C16(N6CO) shown in Figure 4 are listed in Table 2 together with the calculated total energy E and the optimized lattice constant. Among the 10 structures studied in Figure 4, the structure of S1 has the lowest energy. It is pseudotetragonal structure (space group P1) with lattice constant a ) b ) 7.166, c ) 3.608, R ) 90.6, β ) 90.6, γ ) 88.7, which are in good agreement with the experimental values of our as-prepared sample. So the theoretical calculations confirm that the as-prepared sample is β-BC2N phase with defects of the CN′ and the ON′. The calculated direct energy band gap 1.21 eV indicates that it is a semiconductor material. The calculated bulk modulus and shear modulus is 379 and 408 GPa, respectively. Also, we have calculated its Vickers microhardness using a semiempirical method,11,21 and listed in Table 3. From Table
Hou et al.
3, it can be seen that the hardness of the synthesized B0.25C0.53N0.19O0.03 is higher than that of β-BC2N, which ranks next to diamond. Conclusion In summary, single-crystalline B0.25C0.53N0.19O0.03 nanorods were obtained at relatively low temperature and pressure. On the basis of the XRD and SAED analysis, it is well crystallized as a tetragonal structure, and the lattice constants are a ) 7.12(5) Å and c ) 3.57(3) Å. Both EELS and EDX elemental mapping measurements prove that the B, C, and N species coexist within all the nanorods. On the basis of first-principles calculations, the as-synthesized B-C-N nanorods may be suggested as the 2 × 2 × 1 superlattice phase of β-BC2N with defects of the CN′ and the ON′. A semiempirical method is employed to estimate its hardness, and the calculated hardness is higher than that of cubic BN, comparable to that of diamond. The band structure calculation indicates that it is a semiconductor material. This opens a new way for low-temperature synthesis of singlecrystalline B-C-N. Acknowledgment. The authors acknowledge the financial support from the National Natural Science Foundation of China (No.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). L.H. acknowledges the financial support from the Science Foundation of Yanshan University for the Excellent Ph.D. Students.
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