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Synthesis of hexagonal boron carbonitride phase by solvothermal method
Diamond & Related Materials 13 (2004) 1757 – 1760 www.elsevier.com/locate/diamond
Synthesis of hexagonal boron carbonitride phase by solvothermal method Fu Lin Huang, Chuan Bao Cao *, Xu Xiang, Rui Tao Lv, He Sun Zhu Research Center of Materials Science, Beijing Institute of Technology, No. 5, Zhongguancun South Street, Beijing 100081, People’s Republic of China Received 28 June 2003; received in revised form 25 February 2004; accepted 7 March 2004 Available online 23 April 2004
Abstract A solvothermal reaction of CH3CNBCl3 and lithium nitride (Li3N) using benzene as the solvent has been successfully applied to prepare boron carbonitride at 300 jC and less than about 7 MPa. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) are used to confirm their chemical composition and atomic-level hybrid. X-ray diffraction (XRD) and transmission electron diffraction (TED) analysis indicate that the powders have hexagonal network structures. D 2004 Elsevier B.V. All rights reserved. Keywords: BCN; Solvothermal synthesis; High pressure crystal growth; Metastable phases
1. Introduction The ternary B – C –N compounds have attracted considerable attention in recent years for the potentially interesting properties of this system. Cubic BCN compounds (cBCN) are expected to combine the properties of diamond and cubic boron nitride (c-BN), whereas hexagonal structures (h-BCN) are expected to behave as semiconductors with various band gap energies depending on the composition and structure [1,2]. Because of these attractive features, the BCN materials have been investigated both theoretically [3– 5] and experimentally [6– 8]. In general, phases separation prevents actual bond hybridization between the pure carbon and boron nitride phases [9,10]. So far, most of the reported BCN materials have been the mixtures of the carbon and BN phases. Besides BN and carbons bonds, the formations of boron carbide and carbon nitride bonds are required to create hybridized BCN materials [1,2]. Because the phase separation is enhanced at high synthesis temperature, new approaches should be found to synthesize hybridized BCN materials at relatively low temperatures. There has been much effort to synthesize the new BCN phase by various techniques such as: nitriding of solidphase precursors at relatively high temperature [11]; gas phase reactions using CVD techniques [12 – 14]; solid * Corresponding author. Tel.: +86-10-68913792; fax: +86-1068915023. E-mail address: [email protected] (C.B. Cao). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.03.001
phases pyrolysis [15,16]. However, few reports are concerning solvothermal synthesis of boron carbonitride phase. Recently, the solvothermal technique has attracted much attention and has been used to synthesis some important semiconductors [17 – 21]. Here we report on the synthesis of hexagonal boron carbonitride through a solvothermal method. To the best of our knowledge, this is first time that the solvothermal method is successfully used to prepare BCN phase.
2. Experimental procedures In a typical synthesis, appropriate amounts of CH3CNBCl3 and Li3N were put into a stainless steel autoclave of 50 ml capacity, and then the autoclave was filled with liquid benzene up to 40% of the total volume. Care was taken to prevent water ingress during operation. The autoclave was maintained at 300 jC for 12 h and then allowed to cool to room temperature. During the reaction, the pressure was measured by the pressure gauge and the highest pressure recorded was less than 7 MPa. The final products of grey powder were collected and washed with ethanol, dilute chlorhydric acid and distilled water to eliminate the unreacted CH3CNBCl3, Li3N and by-products. Then the product was dried in vacuum at 100 jC for several hours. The composition and band structures of the samples were studied by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). X-ray diffraction (XRD) and transmission electron diffrac-
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tion (TED) were used to investigate the phase structure of the powder. X-ray powder diffraction pattern was obtained on a Rigaku D/max-J400 X-ray diffractometer with Ni-filtered Cu Ka radiation. Transmission electron diffraction (TED) was observed with a Philips Tecnai20 transmission electron microscopy and the accelerating voltage was 120 kV. XPS measurement was carried out on a Perkin-Elmer PHI5300 ESCA X-ray photoelectron spectrometer for the surface elemental analysis. FTIR spectrum was obtained on a Perkin-Elmer L-710 infrared spectrometer. Fig. 2. The TED pattern of boron carbonitride powder.
3. Results and discussion Structure analysis was performed using XRD and TED. Fig. 1 shows a typical XRD pattern. Combined with analysis of XPS and FTIR, the broad diffraction lines observed should attributed to h-BCN (00.2) {2h = 24.5j} and (10.0) {2h = 43j} planes suggest that the crystallite sizes are very small, and there are some defects in the layered structure. This means that the structure of the powder had basically hexagonal network, where the hexagonal BCN network layers are irregularly stacked with each other in the direction of the c axis. To further investigate the structure of the powder, we also observed its electron diffraction patterns by TEM. The products under TEM have piece-like structures and the electron diffraction pattern was taken from the edge of BCN pieces. The TED pattern is shown in Fig. 2, electron diffraction of the sample showed six-symmetrical spots, which indicates that the sample microscopically has a hexagonal structure. The dspacing calculated from the (10.0) diffraction in Fig. 2 is 0.212 nm, which is value similar to those of h-BN (d10.0 = 0.217 nm) and graphite (d10.0 = 0.213 nm). Therefore, the sample is composed of very small single crystals. Compared to the highly disordered materials, the content of single crystal is very low. We believed that a very small quantity of single crystals was embedded in the disordered network structures. In Fig. 2, we also observed halo ring
Fig. 1. The observed XRD spectrum of boron carbonitride powder.
pattern, which indicates the sample contains polycrystalline. From the innermost ring outward, the calculated interplanar spacing are 0.332, 0.212 and 0.118 nm. These d-spacings can be indexed to the (00.2), (10.0) and (11.2) of a hexagonal BCN phase. Compared with ASTM data of hBN and carbon, the d-values of the sample are slightly close to that of the graphite. From the d-spacings, we can ˚ and calculate out the lattice parameters of a = 2.52 A ˚ c = 6.58 A. In order to confirm the synthesis is hexagonal boron carbonitride and not h-BN or carbon, the XPS and FTIR analysis were carried out. The survey scan XPS spectrum indicate the presence of only B, C, N and a small amount of O, which may be due to surface adsorption, in the powder; Li and Cl were not detected. Fig. 3 shows the survey scan XPS spectrum and Fig. 4 shows deconvoluted B1s, C1s and N1s spectra. In Fig. 4(a), the deconvolution of B1s spectrum gives two peaks centered at 189.5 and 190.9 eV, respectively. It was reported that the B1s spectra have peaks at 188.4 and 189.4 for B4C and BC3.4, respectively, and 190.8 eV for hBN [22]. Thus, the resolved two peaks can be attributed to B – C and B – N bonding, implying that the B atoms in the powder bond with both C and N atoms. The chemical shift toward lower energy suggests a contribution of B – C bonding, since the electronegativity of C atoms is lower than that
Fig. 3. The survey scan XPS spectrum of the boron carbonitride powder.
F.L. Huang et al. / Diamond & Related Materials 13 (2004) 1757–1760
Fig. 4. The XPS spectra of the boron carbonitride powder from solvothermal synthesis: (a) B1s, (b) C1s, (c) N1s.
of N atoms. The full width at half-maximum (FWHM) for the B1s spectrum was 2.9 eV, while that for h-BN was 0.92 eV [12]. This wide peak is evidence that there are at least two types of bonding surrounding the B atoms. In Fig. 4(b), the deconvolution of the C1s spectrum gives two peaks centered at 284.3 and 285.7 eV, Considering that the binding energy of C1s involved in ethylamine is 285.6 eV, this allows us to assign the peak at 285.7 eV to C – N bonding. Since C1s peaks in B4C and BC3.4 center at 283.0 and 284.3 eV, respectively [12], and the peak in graphite almost appears at the same position (284.4 eV), the deconvolution peak at 284.3 eV can be regarded as a combined contribution of both C – B and C –C bonding. The FWHM for C1s is 2.7 eV; wider than that for graphite which is 0.35 eV [12]. From the C1s spectrum, we cannot observe the existence of the sp2 CMN bonding, since its peak of bonding energy is centered at 286.6 eV [23]. It is simply considered that the main bonding configuration surrounding C atoms of the powder is a C atom bonding with three C atoms and there are also C atoms involved in B – C and C – N bonding. In Fig. 4(c), the results of the deconvolution indicate that there are two types of N chemical states in the powder, and the peaks of their binding energies are centered at 398.4 and 399.4 eV, respectively. The former can be assigned to the N – B bonding and the latter to the N – C bonding [12]. The
1759
FWHM for the N1s spectrum is 2.8 eV, while that for h-BN is 0.88 eV [13]. Since the N1s peak in sp2 NMC is centered at 400.0 eV[24], the results of N1s spectrum show the absence of sp2 NMC. These results indicate that the N atoms bond both with B and N atoms and are consistent with those of B1s and C1s. XPS results described above suggest that all boron, carbon and nitrogen atoms are mixed atomically and are not separated in the form of graphite and h-BN. The atomic fractions in the powder were estimated from the integral areas of the XPS peaks calibrated by the corresponding sensitivity factors. For the sample shown in Fig. 3, it was found that the atomic percentages of B, C and N are 13.8, 58.3 and 19.6 at.%, respectively. To further investigate the nature of chemical bonding of the prepared powder, FTIR absorption spectra were measured. Fig. 5 shows the FTIR absorption spectrum. The strong absorption peak with a shoulder at 1563 cm 1 shows the existence of graphitelike sp2 domains, which is similar to that reported by Kaufman et al. [25] and Zhao et al. [26]. In the graphite single crystal, this band is IR forbidden, but become IR active when the symmetry of the graphite rings has been broken due to the incorporation of nitrogen atoms [25,26]. The peaks at 1095 and 805 cm 1 are attributed to the B – C stretching mode and the B –N – C out-of-plane bending vibration, respectively [27]. The peak at 3417 cm 1 corresponds to the vibrational modes of NH2 [24]. This is because there are a lot of H atoms in solvent benzene and source materials CH3CNBCl3. H atoms easily incorporate in the final product. Since the electronegativity of N is higher than that of C, it is probably that the NH bond is preferentially formed compared to CH bonds. The present FTIR data strongly support the results obtained by XPS, confirming that the powder is atomic-level hybrid composed of B, C and N atoms. The XPS and FTIR results also suggest that the powder is consistent with an extended inorganic boron carbonitride solid. We have made some further characterization of our products. From the analysis of XPS spectrum (the technique is believed to be accurate to within 5% f 10% for quantitative analysis), we obtain that our product predominately
Fig. 5. The FTIR spectrum of the boron carbonitride powder.
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F.L. Huang et al. / Diamond & Related Materials 13 (2004) 1757–1760
Table 1 The chemical composition of the product by elemental analysis
Acknowledgements
Elements
C
N
B
H
O
Weight percent
36.63
20.78
14.43
4.18
7.96
consists of B, C and N elements with a small amount of O (B: 13.8%, C: 58.3%, N: 19.6%, O: 8.3%), which is from the surface absorption. We also studied the chemical composition by usual combustion elemental analysis for C, N and H (Elementar Vario EL, Germany), by high-temperature thermal decomposition for O (Elementar Vario EL), by alkali (KOH) melting at 720 jC followed by Inductively coupled plasma-Atomic Emission Spectrometry (ICP-AES, PS-950) for boron. Table 1 shows the chemical composition of the product by elemental analysis. It is noted that the total content of B, C, N, O and H is only 83.98% due to the fact that the highest temperature of the combustion chamber is 950 jC and the sample with inorganic network structure was just partially combusted with some products uncombusted. And we think that the little amount of hydrogen may arise from the water absorbed on the surface of the sample from the atmosphere during the process of analysis. From the analysis of FTIR, it is noticed that there are no CHx vibrations patterns which located at 2900 cm 1. From the above data, we can say that the XPS analysis results are believable. From the above investigations and analysis, it can be concluded that the hexagonal boron carbonitride phase has been successfully prepared by the solvothermal process. The results demonstrate that the solvothermal process in a solvent under supercritical conditions, or near such a pressure – temperature domain, allows the synthesis of metastable phases such as BCxN. Compared with classic solid state chemistry methods, metastable phases can be prepared at much milder conditions by the method, which may provide a powerful route for preparing such kinds of materials.
4. Conclusions In summary, we first verified that extended inorganic boron carbonitride powders have been prepared by solvothermal method. The diffraction results indicate that the powder has hexagonal structures and contain very small single crystals and polycrystalline. The novel synthetic route may provide a new means of preparing boron carbonitride, which holds some promise for synthesizing metastable bulk boron carbonitride crystals by optimizing the reaction conditions and the selection of appropriate precursors and solvent.
Financial support from the National Natural Science Foundation of China is gratefully acknowledged.
References [1] A.Y. Liu, R.M. Wentzcovitch, M.L. Cohen, Phys. Rev., B 39 (1989) 1760. [2] W.R.L. Lambrecht, B. Segall, Phys. Rev., B 47 (1993) 9289. [3] Y. Tateyama, T. Ogitsu, K. Kusakabe, S. Tsuneyuki, Phys. Rev., B 55 (1997) 10161. [4] M. Wibbelt, H. Kohl, P. Kohler-Redlich, Phys. Rev., B 59 (1999) 11739. [5] H. Sun, S-H. Jhi, D. Roundy, M.L. Cohen, S.G. Louie, Phys. Rev., B 64 (2001) 094108. [6] M. Kawaguchi, Adv. Mater. 9 (1997) 615. [7] J.P. Nicolich, F. Hofer, G. Brey, R. Riedel, J. Am. Ceram. Soc. 84 (2001) 279. [8] Y.K. Yap, M. Yoshimura, Y. Mori, T. Sasaki, Appl. Phys. Lett. 80 (2002) 2559. [9] T. Sasaki, M. Akaishi, S. Yamaoka, Y. Fujiki, T. Oikawa, Chem. Mater. 5 (1993) 695. [10] S. Ulrich, H. Ehrhardt, T. Theel, J. Schwan, S. Westermeyr, M. Scheib, P. Becker, H. Oechsner, G. Dollinger, A. Bergmaier, Diam. Relat. Mater. 7 (1998) 839. [11] T.Ya. Kosolapova, G.N. Makarenko, T.I. Serebryakova, E.V. Purilutskii, O.T. Khorpyakov, O.I. Chernysheva, Porosk. Metall. 1 (1971) 27. [12] M.O. Watannbe, S. Itoh, K. Mizushima, T. Sasaki, Appl. Phys. Lett. 68 (1996) 2962. [13] J. Yu, E.G. Wang, G. Xu, Chem. Phys. Lett. 292 (1998) 531. [14] R. Gago, I. Jimenez, J.M. Albella, L.J. Terminello, Appl. Phys. Lett. 78 (2001) 3430. [15] L. Maya, L.A. Harris, J. Am. Ceram. Soc. 73 (1990) 1912. [16] R. Riedel, J. Bill, G. Passing, Adv. Mater. 3 (1991) 551. [17] Q. Fu, J.T. Jiu, H. Wang, C.B. Cao, H.S. Zhu, Chem. Phys. Lett. 301 (1999) 87. [18] Y. Xie, Y.T. Qian, W.Z. Wang, S.Y. Zhang, Y.H. Zhang, Science 272 (1996) 1926. [19] Y.D. Li, Y.T. Qian, H.W. Liao, Y. Ding, L. Yang, C.Y. Xu, F.Q. Li, G.E. Zhou, Science 281 (1998) 246. [20] X.Z. Zhao, R. Roy, K.A. Cherian, A. Badzlan, Nature 385 (1997) 513. [21] J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287 (2000) 1471. [22] M.O. Watanabe, T. Sasaki, S. Itoh, K. Mizushima, Thin Solid Films 334 (1996) 281. [23] Q. Lv, C.B. Cao, C. Li, J.T. Zhang, H.S. Zhu, X. Kong, X.F. Duan, J. Mater. Chem. 13 (2003) 1241. [24] D. Marton, K.J. Boyd, A.H. Al-Bayati, S.S. Todorov, J.W. Rabalais, Phys. Rev. Lett. 73 (1994) 118. [25] J.H. Kaufman, S. Metin, D.D. Saperstein, Phys. Rev., B 39 (1989) 13053. [26] X.-A. Zhao, C.W. Ong, Y.C. Tsang, Y.W. Wong, P.W. Chan, C.L. Choy, Appl. Phys. Lett. 66 (1995) 2652. [27] B. Yao, L. Liu, W.H. Su, J. Appl. Phys. 86 (1999) 2464.
Synthesis of hexagonal boron carbonitride phase by solvothermal method Fu Lin Huang, Chuan Bao Cao *, Xu Xiang, Rui Tao Lv, He Sun Zhu Research Center of Materials Science, Beijing Institute of Technology, No. 5, Zhongguancun South Street, Beijing 100081, People’s Republic of China Received 28 June 2003; received in revised form 25 February 2004; accepted 7 March 2004 Available online 23 April 2004
Abstract A solvothermal reaction of CH3CNBCl3 and lithium nitride (Li3N) using benzene as the solvent has been successfully applied to prepare boron carbonitride at 300 jC and less than about 7 MPa. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) are used to confirm their chemical composition and atomic-level hybrid. X-ray diffraction (XRD) and transmission electron diffraction (TED) analysis indicate that the powders have hexagonal network structures. D 2004 Elsevier B.V. All rights reserved. Keywords: BCN; Solvothermal synthesis; High pressure crystal growth; Metastable phases
1. Introduction The ternary B – C –N compounds have attracted considerable attention in recent years for the potentially interesting properties of this system. Cubic BCN compounds (cBCN) are expected to combine the properties of diamond and cubic boron nitride (c-BN), whereas hexagonal structures (h-BCN) are expected to behave as semiconductors with various band gap energies depending on the composition and structure [1,2]. Because of these attractive features, the BCN materials have been investigated both theoretically [3– 5] and experimentally [6– 8]. In general, phases separation prevents actual bond hybridization between the pure carbon and boron nitride phases [9,10]. So far, most of the reported BCN materials have been the mixtures of the carbon and BN phases. Besides BN and carbons bonds, the formations of boron carbide and carbon nitride bonds are required to create hybridized BCN materials [1,2]. Because the phase separation is enhanced at high synthesis temperature, new approaches should be found to synthesize hybridized BCN materials at relatively low temperatures. There has been much effort to synthesize the new BCN phase by various techniques such as: nitriding of solidphase precursors at relatively high temperature [11]; gas phase reactions using CVD techniques [12 – 14]; solid * Corresponding author. Tel.: +86-10-68913792; fax: +86-1068915023. E-mail address: [email protected] (C.B. Cao). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.03.001
phases pyrolysis [15,16]. However, few reports are concerning solvothermal synthesis of boron carbonitride phase. Recently, the solvothermal technique has attracted much attention and has been used to synthesis some important semiconductors [17 – 21]. Here we report on the synthesis of hexagonal boron carbonitride through a solvothermal method. To the best of our knowledge, this is first time that the solvothermal method is successfully used to prepare BCN phase.
2. Experimental procedures In a typical synthesis, appropriate amounts of CH3CNBCl3 and Li3N were put into a stainless steel autoclave of 50 ml capacity, and then the autoclave was filled with liquid benzene up to 40% of the total volume. Care was taken to prevent water ingress during operation. The autoclave was maintained at 300 jC for 12 h and then allowed to cool to room temperature. During the reaction, the pressure was measured by the pressure gauge and the highest pressure recorded was less than 7 MPa. The final products of grey powder were collected and washed with ethanol, dilute chlorhydric acid and distilled water to eliminate the unreacted CH3CNBCl3, Li3N and by-products. Then the product was dried in vacuum at 100 jC for several hours. The composition and band structures of the samples were studied by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). X-ray diffraction (XRD) and transmission electron diffrac-
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F.L. Huang et al. / Diamond & Related Materials 13 (2004) 1757–1760
tion (TED) were used to investigate the phase structure of the powder. X-ray powder diffraction pattern was obtained on a Rigaku D/max-J400 X-ray diffractometer with Ni-filtered Cu Ka radiation. Transmission electron diffraction (TED) was observed with a Philips Tecnai20 transmission electron microscopy and the accelerating voltage was 120 kV. XPS measurement was carried out on a Perkin-Elmer PHI5300 ESCA X-ray photoelectron spectrometer for the surface elemental analysis. FTIR spectrum was obtained on a Perkin-Elmer L-710 infrared spectrometer. Fig. 2. The TED pattern of boron carbonitride powder.
3. Results and discussion Structure analysis was performed using XRD and TED. Fig. 1 shows a typical XRD pattern. Combined with analysis of XPS and FTIR, the broad diffraction lines observed should attributed to h-BCN (00.2) {2h = 24.5j} and (10.0) {2h = 43j} planes suggest that the crystallite sizes are very small, and there are some defects in the layered structure. This means that the structure of the powder had basically hexagonal network, where the hexagonal BCN network layers are irregularly stacked with each other in the direction of the c axis. To further investigate the structure of the powder, we also observed its electron diffraction patterns by TEM. The products under TEM have piece-like structures and the electron diffraction pattern was taken from the edge of BCN pieces. The TED pattern is shown in Fig. 2, electron diffraction of the sample showed six-symmetrical spots, which indicates that the sample microscopically has a hexagonal structure. The dspacing calculated from the (10.0) diffraction in Fig. 2 is 0.212 nm, which is value similar to those of h-BN (d10.0 = 0.217 nm) and graphite (d10.0 = 0.213 nm). Therefore, the sample is composed of very small single crystals. Compared to the highly disordered materials, the content of single crystal is very low. We believed that a very small quantity of single crystals was embedded in the disordered network structures. In Fig. 2, we also observed halo ring
Fig. 1. The observed XRD spectrum of boron carbonitride powder.
pattern, which indicates the sample contains polycrystalline. From the innermost ring outward, the calculated interplanar spacing are 0.332, 0.212 and 0.118 nm. These d-spacings can be indexed to the (00.2), (10.0) and (11.2) of a hexagonal BCN phase. Compared with ASTM data of hBN and carbon, the d-values of the sample are slightly close to that of the graphite. From the d-spacings, we can ˚ and calculate out the lattice parameters of a = 2.52 A ˚ c = 6.58 A. In order to confirm the synthesis is hexagonal boron carbonitride and not h-BN or carbon, the XPS and FTIR analysis were carried out. The survey scan XPS spectrum indicate the presence of only B, C, N and a small amount of O, which may be due to surface adsorption, in the powder; Li and Cl were not detected. Fig. 3 shows the survey scan XPS spectrum and Fig. 4 shows deconvoluted B1s, C1s and N1s spectra. In Fig. 4(a), the deconvolution of B1s spectrum gives two peaks centered at 189.5 and 190.9 eV, respectively. It was reported that the B1s spectra have peaks at 188.4 and 189.4 for B4C and BC3.4, respectively, and 190.8 eV for hBN [22]. Thus, the resolved two peaks can be attributed to B – C and B – N bonding, implying that the B atoms in the powder bond with both C and N atoms. The chemical shift toward lower energy suggests a contribution of B – C bonding, since the electronegativity of C atoms is lower than that
Fig. 3. The survey scan XPS spectrum of the boron carbonitride powder.
F.L. Huang et al. / Diamond & Related Materials 13 (2004) 1757–1760
Fig. 4. The XPS spectra of the boron carbonitride powder from solvothermal synthesis: (a) B1s, (b) C1s, (c) N1s.
of N atoms. The full width at half-maximum (FWHM) for the B1s spectrum was 2.9 eV, while that for h-BN was 0.92 eV [12]. This wide peak is evidence that there are at least two types of bonding surrounding the B atoms. In Fig. 4(b), the deconvolution of the C1s spectrum gives two peaks centered at 284.3 and 285.7 eV, Considering that the binding energy of C1s involved in ethylamine is 285.6 eV, this allows us to assign the peak at 285.7 eV to C – N bonding. Since C1s peaks in B4C and BC3.4 center at 283.0 and 284.3 eV, respectively [12], and the peak in graphite almost appears at the same position (284.4 eV), the deconvolution peak at 284.3 eV can be regarded as a combined contribution of both C – B and C –C bonding. The FWHM for C1s is 2.7 eV; wider than that for graphite which is 0.35 eV [12]. From the C1s spectrum, we cannot observe the existence of the sp2 CMN bonding, since its peak of bonding energy is centered at 286.6 eV [23]. It is simply considered that the main bonding configuration surrounding C atoms of the powder is a C atom bonding with three C atoms and there are also C atoms involved in B – C and C – N bonding. In Fig. 4(c), the results of the deconvolution indicate that there are two types of N chemical states in the powder, and the peaks of their binding energies are centered at 398.4 and 399.4 eV, respectively. The former can be assigned to the N – B bonding and the latter to the N – C bonding [12]. The
1759
FWHM for the N1s spectrum is 2.8 eV, while that for h-BN is 0.88 eV [13]. Since the N1s peak in sp2 NMC is centered at 400.0 eV[24], the results of N1s spectrum show the absence of sp2 NMC. These results indicate that the N atoms bond both with B and N atoms and are consistent with those of B1s and C1s. XPS results described above suggest that all boron, carbon and nitrogen atoms are mixed atomically and are not separated in the form of graphite and h-BN. The atomic fractions in the powder were estimated from the integral areas of the XPS peaks calibrated by the corresponding sensitivity factors. For the sample shown in Fig. 3, it was found that the atomic percentages of B, C and N are 13.8, 58.3 and 19.6 at.%, respectively. To further investigate the nature of chemical bonding of the prepared powder, FTIR absorption spectra were measured. Fig. 5 shows the FTIR absorption spectrum. The strong absorption peak with a shoulder at 1563 cm 1 shows the existence of graphitelike sp2 domains, which is similar to that reported by Kaufman et al. [25] and Zhao et al. [26]. In the graphite single crystal, this band is IR forbidden, but become IR active when the symmetry of the graphite rings has been broken due to the incorporation of nitrogen atoms [25,26]. The peaks at 1095 and 805 cm 1 are attributed to the B – C stretching mode and the B –N – C out-of-plane bending vibration, respectively [27]. The peak at 3417 cm 1 corresponds to the vibrational modes of NH2 [24]. This is because there are a lot of H atoms in solvent benzene and source materials CH3CNBCl3. H atoms easily incorporate in the final product. Since the electronegativity of N is higher than that of C, it is probably that the NH bond is preferentially formed compared to CH bonds. The present FTIR data strongly support the results obtained by XPS, confirming that the powder is atomic-level hybrid composed of B, C and N atoms. The XPS and FTIR results also suggest that the powder is consistent with an extended inorganic boron carbonitride solid. We have made some further characterization of our products. From the analysis of XPS spectrum (the technique is believed to be accurate to within 5% f 10% for quantitative analysis), we obtain that our product predominately
Fig. 5. The FTIR spectrum of the boron carbonitride powder.
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Table 1 The chemical composition of the product by elemental analysis
Acknowledgements
Elements
C
N
B
H
O
Weight percent
36.63
20.78
14.43
4.18
7.96
consists of B, C and N elements with a small amount of O (B: 13.8%, C: 58.3%, N: 19.6%, O: 8.3%), which is from the surface absorption. We also studied the chemical composition by usual combustion elemental analysis for C, N and H (Elementar Vario EL, Germany), by high-temperature thermal decomposition for O (Elementar Vario EL), by alkali (KOH) melting at 720 jC followed by Inductively coupled plasma-Atomic Emission Spectrometry (ICP-AES, PS-950) for boron. Table 1 shows the chemical composition of the product by elemental analysis. It is noted that the total content of B, C, N, O and H is only 83.98% due to the fact that the highest temperature of the combustion chamber is 950 jC and the sample with inorganic network structure was just partially combusted with some products uncombusted. And we think that the little amount of hydrogen may arise from the water absorbed on the surface of the sample from the atmosphere during the process of analysis. From the analysis of FTIR, it is noticed that there are no CHx vibrations patterns which located at 2900 cm 1. From the above data, we can say that the XPS analysis results are believable. From the above investigations and analysis, it can be concluded that the hexagonal boron carbonitride phase has been successfully prepared by the solvothermal process. The results demonstrate that the solvothermal process in a solvent under supercritical conditions, or near such a pressure – temperature domain, allows the synthesis of metastable phases such as BCxN. Compared with classic solid state chemistry methods, metastable phases can be prepared at much milder conditions by the method, which may provide a powerful route for preparing such kinds of materials.
4. Conclusions In summary, we first verified that extended inorganic boron carbonitride powders have been prepared by solvothermal method. The diffraction results indicate that the powder has hexagonal structures and contain very small single crystals and polycrystalline. The novel synthetic route may provide a new means of preparing boron carbonitride, which holds some promise for synthesizing metastable bulk boron carbonitride crystals by optimizing the reaction conditions and the selection of appropriate precursors and solvent.
Financial support from the National Natural Science Foundation of China is gratefully acknowledged.
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