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carbon fibers prepared by hot-filament chemical vapor deposition
Applied Surface Science 257 (2011) 4963–4967
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Composited BCN/carbon fibers prepared by hot-filament chemical vapor deposition Jiannan Lü, Hongdong Li ∗ , Pinwen Zhu ∗ , Xianyi Lü, Yingai Li State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 19 October 2010 Received in revised form 28 December 2010 Accepted 1 January 2011 Available online 5 January 2011 Keywords: BCN Carbon fibers FTIR spectrum Composite
a b s t r a c t The combined BCN/carbon fibers with porous configuration have been successfully prepared by hotfilament chemical vapor deposition (HF-CVD). The composited materials consist of carbon fiber inside covered by the cylindrical BCN films. The differences in the surface morphology and the diameter of the composite fibers are related to the different reactant gases. It is demonstrated that the elements of B, C, and N are chemically bonded with atomic-level BCN hybrid in the composite fibers. The resistance of the composite fibers is about 300 which is 10 times higher than that of the isolated carbon fibers (27.5 ). When the applying voltage increases up to 8–15 V, the BCN films have been broken down and the resistance of composite fibers decreases to the typical value of the carbon fibers. The composite fibers with porous configuration have the strongly capacity to adsorb oxygen. The findings suggest that the combined BCN/carbon fibers are favorable for achieving high performance nano-optoelectronic and sensor devices. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the compound boron–carbon–nitride (BCN) has attracted considerable attention for potential applications especially in the optoelectronic fields because the band gap of BCN can be adjusted with the composition [1]. Furthermore, various structural features of BCN have been realized [2]. In general, the BCN products were deposited on silicon substrate by chemical vapor deposition (CVD) [3,4] and physical vapor deposition (PVD) [5]. For example, the BCN films synthesized on the diamond film were reported to improve the field emission of diamond [6]. The other important carbonaceous material, carbon fiber (CF), has been widely applied in many fields owing to their outstanding electrical, thermal, and mechanical properties. Considering the unique properties of BCN, it is desirable to fabricate the combine the BCN/carbon fibers having potential wide applications (e.g., applied as high performance electrochemical electrode and absorption waves materials). Furthermore, CFs used as the substrate for BCN is favorable to prepare long BCN fibers and to enhance the electrical properties of BCN fibers. In this paper, the BCN-coated CFs are prepared by hot-filament chemical vapor deposition (HF-CVD). The carbon fiber is proposed as either the template or the carbon source during the BCN growth. Analyzed by FTIR spectroscopy, the elements of B, C, and N are
∗ Corresponding author. Tel.: +86 431 85168095; fax: +86 431 85168095. E-mail addresses: [email protected] (H. Li), [email protected], [email protected] (P. Zhu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.004
chemically bonded with atomic-level B–C–N hybrid in the products. The combined BCN–CFs show a varied resistance applied under different voltages. 2. Experimental The synthesis of BCN/CF composite structure was carried out in a HF-CVD system [7]. N2 , CH4 and/or carbon fibers, and trimethylborate (B(OCH3 )3 ) were used as the precursor source of N, C, and B, respectively. The B(OCH3 )3 was loaded in the chamber during the growth process by bubbling the H2 gas through the liquid B(OCH3 )3 precursors (the ambient temperature was kept at 25 ◦ C). The carbon fibers were placed on a molybdenum substrate. The vertical distance between the filament and carbon fibers was ∼1 cm. The temperatures (measured by an infrared thermometer) of the filament and substrate were about 2000 ◦ C and 400 ◦ C, respectively. The deposition pressure was kept at 30 Torr and the lasting time was 2 h. The detailed growth parameters for different samples (samples a–d) are summarized in Table 1 (for simplification, the B-flow rate is expressed by the corresponding bubbling H2 flow rate). The morphology and chemical composition of the samples were characterized by scanning electron microscopy (SEM, JSM-6480LV) equipped with electron energy dispersive X-ray spectroscopy (EDX). The chemical bond structure information of BCN was analyzed by Fourier transform infrared spectroscopy (FTIR, NICOLET AVATAR 370 DTGS spectrometer) from 400 to 4000 cm−1 . The samples were mixed with KBr powder and pressed into a pill for the IR transition examination. The resistance of samples was obtained
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Table 1 Deposition parameters for samples (a)–(d), B flow rate = carrier H2 flow rate for bubbling B(OCH3 )3 . Sample
Reaction gases
Flow-rate ratio (in sccm)
a b c d
– H2 :B:N2 H2 :B:N2 :CH4 B:N2
– 390:5:5 390:5:5:3 5:5
from the current–voltage (I–V) curves measured using Keithley sourcemeter 2400 by the two-point probe method. Two copper wires were attached on the two ends of the CFs and BCN/CFs bundles using silver paste as electrodes. All the examinations were performed at room temperature. 3. Results and discussion 3.1. Analysis of FTIR The FTIR spectra of the samples a–d are shown in Fig. 1. For sample a of pure CFs (curve a), there is a broad peak of around 1630 cm−1 appearing in the region of 1550 cm−1 to 1750 cm−1 (see in the inset), which is related to the typical hexagonal net of graphite [8]. The curves (b)–(d) are the IR spectra of the products synthesized under different growth conditions. The two absorption peaks at around 820 cm−1 and 1375 cm−1 are attributed to the B–N–B bending mode and B–N stretching modes of BCN, respectively [9]. Note that the intensities of the characteristic peaks at 820 cm−1 (1375 cm−1 ) for sample b (sample d) is stronger than that for samples c and d (samples b and c), which means that the strength of the vibration modes are varied dependent on the intrinsic features of those samples synthesized under different growth conditions. The peak at 880 cm−1 found in samples b–d are assigned to the characteristic peak of B(OCH3 )3 , which might be absorbed on the products. The intensities of this characteristic peak for samples b and c are stronger than that for sample d, suggesting that the decomposition ratio of B(OCH3 )3 for sample d is higher than that for samples b and c
Fig. 1. IR transmission spectra of samples a–d. The inset shows the enlarged curve in the region of 1550–1750 cm−1 for sample a.
synthesized in the H2 -rich ambient. The adsorption peak at around 1050 cm−1 originates from c-BN phase, which generally appears in BCN films [10]. Similarly, the peak at around 1100 cm−1 is corresponding to the B–C vibrations from amorphous Bx C phase [11], and the peak at 1330 cm−1 is from the amorphous BN phase [12]. The adsorption peaks at 1425 cm−1 and 1500 cm−1 are resulted from C N [13]. The band centered at around 1625 cm−1 attributed to sp2 C–N vibration [5] appears in the spectra of samples b and d, while nearly absent for sample c. It is worth pointing out that the introducing H2 plays an important role in the formation of BCN structure
Fig. 2. SEM images of the samples a–d. The inset is the cross-section images of sample b.
J. Lü et al. / Applied Surface Science 257 (2011) 4963–4967
Fig. 3. SEM images and the corresponding EDX spectra of samples a, b and d.
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determined by FTIR spectroscopy (by comparing the spectra of samples b and c to that of sample d), due to the enhanced etching mechanism and/or other complete reaction processes. Based on the above FTIR results, the atomic-level BCN hybrid structures have been achieved, though the exact composition of the samples fabricated under different conditions need further detailed investigations. 3.2. SEM morphology of BCN/CF system The SEM images of those samples are shown in Fig. 2. The raw carbon fibers (Fig. 2a, for sample a), have smooth surface with nearly uniform diameter of around 10 m. For samples b (Fig. 2b) and c (Fig. 2c), the porous surface is presented and the diameters increase up to 15 m and 30 m, respectively. Based on above FTIR results, it is reasonable that the BCN film has successfully obtained and coated on the carbon fibers leading to the enlarged diameters of samples b and c. Furthermore, observed from the cross-section images of sample b (the inset of Fig. 2b) and sample c (not shown), the composited BCN/CF shows clearly the contrast between inner and outer parts, which is speculated that the inner is CF and the outer is BCN layer. However, the morphology of sample (d) shows chain-liked configuration consisting of beads with a diameter of ∼3 m. The differences in the surface morphology for samples b–d are related to the variation of the gas sources which is discussed as below. The growth processes for the samples were related to the variation of gas sources and the CF template. In the H2 /B ambient without CH4 , the CF was acted as the template (sample b), the carbon fiber is seen as the carbon resource to synthesize BCN on CF. When the CH4 gas flowed into the reaction chamber (sample c), the carbon resources to produce BCN were provided by both the CH4 gas and carbon fiber. Due to more carbonaceous source applied, the diameter of sample c (∼30 m) is therefore 2 times that of sample b (∼15 m). The morphology of sample d (Fig. 2d) synthesized without H2 introducing shows chain-liked configuration consisting of beads which is significantly different from samples b and c synthesized in H2 -rich ambient. Since H2 can strongly decompose CH4 and B(OCH3 )3 , and consequently the large amount of active carbon- and boron-related radicals is provided for the BCN growth. In this case, BCN was deposited on the carbon fiber, as proposed for samples b and c. However, the CH4 and B(OCH3 )3 cannot be effectively decomposed by lack of enough H2 source (for the formation of sample d, no H2 was introduced and the H only originates from the carrier gas of H2 for bubbling B(OCH3 )3 ). The large amount of carbon source was thus mainly from the CFs consumed in the growing process, resulting the smaller diameter of sample d with respect to that of carbon fibers (sample a) and samples b and c. Note that the samples b and c are porous, which may be related to the H2 -etching process in such H2 -rich reaction environment. However, the formation mechanism of bead-chain-liked configuration for sample d is still not very clear and further work is now undergoing. The EDX results for samples a, b and d are shown in Fig. 3 for identifying the chemical composition. It can be seen that there are a little contents of Si in all the samples which may be from the raw materials or the environment during the growth process. The image of sample c is similar to sample b (not shown). It can be seen that there are B, C and N elements appearing in the examined products, indicating the existence of BCN phase that is consistent with the IR spectral results. It is worthy noting that there is oxygen signal appearing in the spectra. And the oxygen content of samples b and d are significantly higher than that of sample a. The contamination of oxygen is due to the adsorbed oxygen when the samples exposed in air. Compared with the adsorption capacity of CFs [14], the porous configuration of sample b can effectively improve the
capacity of oxygen, and then adsorbs more oxygen than pure CFs exposed in air. For sample samples d, the fibers are thinner (∼3 m in diameter) than sample b, and consist of large number of small beads (∼5 m). This configuration can provide bigger surface proportion than sample b leading to the stronger capacity of adsorbing oxygen in the air. Furthermore, the dense bead-net is favorable to restrain the gas escape from the composite fibers. Therefore, the oxygen absorption for sample d estimated from EDX result is much higher than that in other samples. Although the EDX examinations were carried out in the SEM equipment having high vacuum, the CFs covered porous BCN have the strong capacity to adsorb oxygen. 3.3. Electricity characteristics of BCN/CF system Fig. 4 shows the I–V characteristics of samples a–d at the voltage region of −20 V to +20 V. Because of difficulty in determining the effective cross-section area and the exact length of the examined fibers for test, the amount and length of the fiber bundles were for the four samples. For comparison, in the following discussions, the resistivity for the samples was represented by the resistance, though it is strongly dependent on the features of the samples. The nearly symmetric I–V plot passing through zero point for all samples indicates that the contact between the silver and fibers is ohmic contact. As shown in the V–I curve, the resistance of raw carbon fibers estimated is about 27.5 . For sample b, the resistance is close to 300 in the region of 0–15 V, and when the bias was increased to 15–20 V, the resistance was decreased to ∼27.6 , which is nearly the same to the value of CFs. This suggests that the coated BCN layer on CFs was electronically broken down and consequently the conductance of the sample is mainly attributed to the CFs inside at high bias applied (i.e., 15 V). For samples c and d, the similar I–V characteristics are obtained with respect to that of sample b. The resistance is 242 (165 ) for sample c (sample d), while the break down voltage is ∼13 V (∼8 V). The differences in the resistance and break down voltage between the samples are attributed to the corresponding especial structures of the combined BCN/CFs. For sample c, the thin chain-liked structure mainly leads to the enhanced conductibility and low break-down voltage. Importantly, up to the threshold voltage as the BCN layer is broken, the resistance values for BCN/CFs composite materials (samples b–d) become to ∼27.5 , which is quite close to the resistance of CFs (sample a). This reasonably supports the state that the variation of the resistance examined can safely reflect the varying resistivity for the four samples, as mentioned above. The detailed work on the resistivity of BCN/CFs would be performed on the single fiber, which
Fig. 4. Characteristic I–V curve of the sample.
J. Lü et al. / Applied Surface Science 257 (2011) 4963–4967
would propose more useful physical information. Since the surface morphology, structure, and thickness of the BCN layer on CFs can be controlled as mentioned above, the electrical properties of the BCN/CFs system are desirably to be consciously adjusted to realize wide applications in the future. 4. Conclusions In conclusion, the combined BCN/CFs, which consist of carbon fiber inside covered by the cylindrical BCN films with 5–10 m in thickness, have been synthesized by the HF-CVD system. Based on the FTIR results, it is demonstrated that the BCN is chemically bonded atomic-level hybrid. The different reactant gases can result in the different morphologies and diameter of composite fibers. H2 in the reactant gases can decompose CH4 and B(OCH3 )3 . The BCN films with porous configuration and small size have the strong capacity of the oxygen adsorption. The resistance value of BCN films is ∼300 which is 10 times higher than that of CF (27.5 ). The breakdown voltage for BCN films is depend on the d the different configuration lead to the different resistance and breakdown volt in BCN films, which is favorable for the applications in the electrical devices and sensors. Acknowledgements This work was financially supported by the Program for New Century Excellent Talents in University (No. 06-0303) and the National Natural Science Foundation of China under Grant (No. 50772041).
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References [1] Y. Tateyama, T. Ogitsu, K. Kusakabe, S. Tsuneyuki, S. Itoh, Proposed synthesis path for heterodiamond BC2 N, Phys. Rev. B 55 (1997) R10161–R10164. [2] J. Yang, T. Qu, The synthesization and character of precursory BCN, Acta Phys. Chim. Sin. 21 (2005) 1373–1377. [3] W.L. Wang, X.D. Bai, E.G. Wang, Direct synthesis of B–C–N single-walled nanotubes by bias-assisted hot filament chemical vapor deposition, J. Am. Chem. Soc. 128 (2006) 6530–6531. [4] J. Yu, E.G. Wang, Turbostratic boron carbon nitride films produced by biasassisted hot filament chemical vapor deposition, J. Appl. Phys. 87 (2000) 4022–4025. [5] Amy Y. Liu, R.M. Wentzcovitch, M.L. Cohen, Atomic arrangement and electronic structure of BC2 N, Phys. Rev. B 39 (1989) 1760–1765. [6] M.V. Ugarov, V.P. Ageev, A.V. Karabutov, UV laser induced interfacial synthesis of CN-BCN layers on diamond films in borazine and ammonia, Appl. Surf. Sci. 139 (1999) 359–363. [7] H.D. Li, T. Zhang, L.A. Li, X.Y. Lv, B. Li, Z.S. Jin, G.T. Zou, Investigation on crystalline structure, boron distribution, and residual stresses in freestanding boron-doped CVD diamond films, J. Cryst. Growth 312 (2010) 1986–1991. [8] G.H. Chen, W.G. Weng, Preparation and characterization of graphite nanosheets from ultrasonic powdering technique, Carbon 42 (2004) 753–759. [9] T. Sugino, Y. Etou, T. Tai, Dielectric constant of boron carbon nitride films synthesized by plasma-assisted chemical-vapor deposition, Appl. Phys. Lett. 80 (2002) 649–651. [10] R. Gago, I. Jiménez, F. Agulló-Rueda, J.M. Albella, Zs. Czigány, L. Hultman, Transition from amorphous boron carbide to hexagonal boron carbon nitride thin films induced by nitrogen ion assistance, J. Appl. Phys. 92 (2002) 5177–5182. [11] K. Shirai, S. Emura, S. Gonda, K. Yukinobu, Infrared study of amorphous B1−x Cx films, J. Appl. Phys. 78 (1995) 3392–3400. [12] Y. Wada, Y.K. Yap, M. Yoshimura, The control of B–N and B–C bonds in BCN films synthesized using pulsed laser deposition, Diamond Relat. Mater. 9 (2000) 620–624. [13] T. Sugiyama, T. Tai, T. Sugino, Effect of annealing on dielectric constant of boron carbon nitride films synthesized by plasma-assisted chemical vapor deposition, Appl. Phys. Lett. 80 (2002) 4214–4216. [14] R.W. Fu, L. Liu, The adsorbent research of active carbon fibers, New Carbon Mater. 12 (1997) 39–45.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Composited BCN/carbon fibers prepared by hot-filament chemical vapor deposition Jiannan Lü, Hongdong Li ∗ , Pinwen Zhu ∗ , Xianyi Lü, Yingai Li State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 19 October 2010 Received in revised form 28 December 2010 Accepted 1 January 2011 Available online 5 January 2011 Keywords: BCN Carbon fibers FTIR spectrum Composite
a b s t r a c t The combined BCN/carbon fibers with porous configuration have been successfully prepared by hotfilament chemical vapor deposition (HF-CVD). The composited materials consist of carbon fiber inside covered by the cylindrical BCN films. The differences in the surface morphology and the diameter of the composite fibers are related to the different reactant gases. It is demonstrated that the elements of B, C, and N are chemically bonded with atomic-level BCN hybrid in the composite fibers. The resistance of the composite fibers is about 300 which is 10 times higher than that of the isolated carbon fibers (27.5 ). When the applying voltage increases up to 8–15 V, the BCN films have been broken down and the resistance of composite fibers decreases to the typical value of the carbon fibers. The composite fibers with porous configuration have the strongly capacity to adsorb oxygen. The findings suggest that the combined BCN/carbon fibers are favorable for achieving high performance nano-optoelectronic and sensor devices. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the compound boron–carbon–nitride (BCN) has attracted considerable attention for potential applications especially in the optoelectronic fields because the band gap of BCN can be adjusted with the composition [1]. Furthermore, various structural features of BCN have been realized [2]. In general, the BCN products were deposited on silicon substrate by chemical vapor deposition (CVD) [3,4] and physical vapor deposition (PVD) [5]. For example, the BCN films synthesized on the diamond film were reported to improve the field emission of diamond [6]. The other important carbonaceous material, carbon fiber (CF), has been widely applied in many fields owing to their outstanding electrical, thermal, and mechanical properties. Considering the unique properties of BCN, it is desirable to fabricate the combine the BCN/carbon fibers having potential wide applications (e.g., applied as high performance electrochemical electrode and absorption waves materials). Furthermore, CFs used as the substrate for BCN is favorable to prepare long BCN fibers and to enhance the electrical properties of BCN fibers. In this paper, the BCN-coated CFs are prepared by hot-filament chemical vapor deposition (HF-CVD). The carbon fiber is proposed as either the template or the carbon source during the BCN growth. Analyzed by FTIR spectroscopy, the elements of B, C, and N are
∗ Corresponding author. Tel.: +86 431 85168095; fax: +86 431 85168095. E-mail addresses: [email protected] (H. Li), [email protected], [email protected] (P. Zhu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.004
chemically bonded with atomic-level B–C–N hybrid in the products. The combined BCN–CFs show a varied resistance applied under different voltages. 2. Experimental The synthesis of BCN/CF composite structure was carried out in a HF-CVD system [7]. N2 , CH4 and/or carbon fibers, and trimethylborate (B(OCH3 )3 ) were used as the precursor source of N, C, and B, respectively. The B(OCH3 )3 was loaded in the chamber during the growth process by bubbling the H2 gas through the liquid B(OCH3 )3 precursors (the ambient temperature was kept at 25 ◦ C). The carbon fibers were placed on a molybdenum substrate. The vertical distance between the filament and carbon fibers was ∼1 cm. The temperatures (measured by an infrared thermometer) of the filament and substrate were about 2000 ◦ C and 400 ◦ C, respectively. The deposition pressure was kept at 30 Torr and the lasting time was 2 h. The detailed growth parameters for different samples (samples a–d) are summarized in Table 1 (for simplification, the B-flow rate is expressed by the corresponding bubbling H2 flow rate). The morphology and chemical composition of the samples were characterized by scanning electron microscopy (SEM, JSM-6480LV) equipped with electron energy dispersive X-ray spectroscopy (EDX). The chemical bond structure information of BCN was analyzed by Fourier transform infrared spectroscopy (FTIR, NICOLET AVATAR 370 DTGS spectrometer) from 400 to 4000 cm−1 . The samples were mixed with KBr powder and pressed into a pill for the IR transition examination. The resistance of samples was obtained
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Table 1 Deposition parameters for samples (a)–(d), B flow rate = carrier H2 flow rate for bubbling B(OCH3 )3 . Sample
Reaction gases
Flow-rate ratio (in sccm)
a b c d
– H2 :B:N2 H2 :B:N2 :CH4 B:N2
– 390:5:5 390:5:5:3 5:5
from the current–voltage (I–V) curves measured using Keithley sourcemeter 2400 by the two-point probe method. Two copper wires were attached on the two ends of the CFs and BCN/CFs bundles using silver paste as electrodes. All the examinations were performed at room temperature. 3. Results and discussion 3.1. Analysis of FTIR The FTIR spectra of the samples a–d are shown in Fig. 1. For sample a of pure CFs (curve a), there is a broad peak of around 1630 cm−1 appearing in the region of 1550 cm−1 to 1750 cm−1 (see in the inset), which is related to the typical hexagonal net of graphite [8]. The curves (b)–(d) are the IR spectra of the products synthesized under different growth conditions. The two absorption peaks at around 820 cm−1 and 1375 cm−1 are attributed to the B–N–B bending mode and B–N stretching modes of BCN, respectively [9]. Note that the intensities of the characteristic peaks at 820 cm−1 (1375 cm−1 ) for sample b (sample d) is stronger than that for samples c and d (samples b and c), which means that the strength of the vibration modes are varied dependent on the intrinsic features of those samples synthesized under different growth conditions. The peak at 880 cm−1 found in samples b–d are assigned to the characteristic peak of B(OCH3 )3 , which might be absorbed on the products. The intensities of this characteristic peak for samples b and c are stronger than that for sample d, suggesting that the decomposition ratio of B(OCH3 )3 for sample d is higher than that for samples b and c
Fig. 1. IR transmission spectra of samples a–d. The inset shows the enlarged curve in the region of 1550–1750 cm−1 for sample a.
synthesized in the H2 -rich ambient. The adsorption peak at around 1050 cm−1 originates from c-BN phase, which generally appears in BCN films [10]. Similarly, the peak at around 1100 cm−1 is corresponding to the B–C vibrations from amorphous Bx C phase [11], and the peak at 1330 cm−1 is from the amorphous BN phase [12]. The adsorption peaks at 1425 cm−1 and 1500 cm−1 are resulted from C N [13]. The band centered at around 1625 cm−1 attributed to sp2 C–N vibration [5] appears in the spectra of samples b and d, while nearly absent for sample c. It is worth pointing out that the introducing H2 plays an important role in the formation of BCN structure
Fig. 2. SEM images of the samples a–d. The inset is the cross-section images of sample b.
J. Lü et al. / Applied Surface Science 257 (2011) 4963–4967
Fig. 3. SEM images and the corresponding EDX spectra of samples a, b and d.
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determined by FTIR spectroscopy (by comparing the spectra of samples b and c to that of sample d), due to the enhanced etching mechanism and/or other complete reaction processes. Based on the above FTIR results, the atomic-level BCN hybrid structures have been achieved, though the exact composition of the samples fabricated under different conditions need further detailed investigations. 3.2. SEM morphology of BCN/CF system The SEM images of those samples are shown in Fig. 2. The raw carbon fibers (Fig. 2a, for sample a), have smooth surface with nearly uniform diameter of around 10 m. For samples b (Fig. 2b) and c (Fig. 2c), the porous surface is presented and the diameters increase up to 15 m and 30 m, respectively. Based on above FTIR results, it is reasonable that the BCN film has successfully obtained and coated on the carbon fibers leading to the enlarged diameters of samples b and c. Furthermore, observed from the cross-section images of sample b (the inset of Fig. 2b) and sample c (not shown), the composited BCN/CF shows clearly the contrast between inner and outer parts, which is speculated that the inner is CF and the outer is BCN layer. However, the morphology of sample (d) shows chain-liked configuration consisting of beads with a diameter of ∼3 m. The differences in the surface morphology for samples b–d are related to the variation of the gas sources which is discussed as below. The growth processes for the samples were related to the variation of gas sources and the CF template. In the H2 /B ambient without CH4 , the CF was acted as the template (sample b), the carbon fiber is seen as the carbon resource to synthesize BCN on CF. When the CH4 gas flowed into the reaction chamber (sample c), the carbon resources to produce BCN were provided by both the CH4 gas and carbon fiber. Due to more carbonaceous source applied, the diameter of sample c (∼30 m) is therefore 2 times that of sample b (∼15 m). The morphology of sample d (Fig. 2d) synthesized without H2 introducing shows chain-liked configuration consisting of beads which is significantly different from samples b and c synthesized in H2 -rich ambient. Since H2 can strongly decompose CH4 and B(OCH3 )3 , and consequently the large amount of active carbon- and boron-related radicals is provided for the BCN growth. In this case, BCN was deposited on the carbon fiber, as proposed for samples b and c. However, the CH4 and B(OCH3 )3 cannot be effectively decomposed by lack of enough H2 source (for the formation of sample d, no H2 was introduced and the H only originates from the carrier gas of H2 for bubbling B(OCH3 )3 ). The large amount of carbon source was thus mainly from the CFs consumed in the growing process, resulting the smaller diameter of sample d with respect to that of carbon fibers (sample a) and samples b and c. Note that the samples b and c are porous, which may be related to the H2 -etching process in such H2 -rich reaction environment. However, the formation mechanism of bead-chain-liked configuration for sample d is still not very clear and further work is now undergoing. The EDX results for samples a, b and d are shown in Fig. 3 for identifying the chemical composition. It can be seen that there are a little contents of Si in all the samples which may be from the raw materials or the environment during the growth process. The image of sample c is similar to sample b (not shown). It can be seen that there are B, C and N elements appearing in the examined products, indicating the existence of BCN phase that is consistent with the IR spectral results. It is worthy noting that there is oxygen signal appearing in the spectra. And the oxygen content of samples b and d are significantly higher than that of sample a. The contamination of oxygen is due to the adsorbed oxygen when the samples exposed in air. Compared with the adsorption capacity of CFs [14], the porous configuration of sample b can effectively improve the
capacity of oxygen, and then adsorbs more oxygen than pure CFs exposed in air. For sample samples d, the fibers are thinner (∼3 m in diameter) than sample b, and consist of large number of small beads (∼5 m). This configuration can provide bigger surface proportion than sample b leading to the stronger capacity of adsorbing oxygen in the air. Furthermore, the dense bead-net is favorable to restrain the gas escape from the composite fibers. Therefore, the oxygen absorption for sample d estimated from EDX result is much higher than that in other samples. Although the EDX examinations were carried out in the SEM equipment having high vacuum, the CFs covered porous BCN have the strong capacity to adsorb oxygen. 3.3. Electricity characteristics of BCN/CF system Fig. 4 shows the I–V characteristics of samples a–d at the voltage region of −20 V to +20 V. Because of difficulty in determining the effective cross-section area and the exact length of the examined fibers for test, the amount and length of the fiber bundles were for the four samples. For comparison, in the following discussions, the resistivity for the samples was represented by the resistance, though it is strongly dependent on the features of the samples. The nearly symmetric I–V plot passing through zero point for all samples indicates that the contact between the silver and fibers is ohmic contact. As shown in the V–I curve, the resistance of raw carbon fibers estimated is about 27.5 . For sample b, the resistance is close to 300 in the region of 0–15 V, and when the bias was increased to 15–20 V, the resistance was decreased to ∼27.6 , which is nearly the same to the value of CFs. This suggests that the coated BCN layer on CFs was electronically broken down and consequently the conductance of the sample is mainly attributed to the CFs inside at high bias applied (i.e., 15 V). For samples c and d, the similar I–V characteristics are obtained with respect to that of sample b. The resistance is 242 (165 ) for sample c (sample d), while the break down voltage is ∼13 V (∼8 V). The differences in the resistance and break down voltage between the samples are attributed to the corresponding especial structures of the combined BCN/CFs. For sample c, the thin chain-liked structure mainly leads to the enhanced conductibility and low break-down voltage. Importantly, up to the threshold voltage as the BCN layer is broken, the resistance values for BCN/CFs composite materials (samples b–d) become to ∼27.5 , which is quite close to the resistance of CFs (sample a). This reasonably supports the state that the variation of the resistance examined can safely reflect the varying resistivity for the four samples, as mentioned above. The detailed work on the resistivity of BCN/CFs would be performed on the single fiber, which
Fig. 4. Characteristic I–V curve of the sample.
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would propose more useful physical information. Since the surface morphology, structure, and thickness of the BCN layer on CFs can be controlled as mentioned above, the electrical properties of the BCN/CFs system are desirably to be consciously adjusted to realize wide applications in the future. 4. Conclusions In conclusion, the combined BCN/CFs, which consist of carbon fiber inside covered by the cylindrical BCN films with 5–10 m in thickness, have been synthesized by the HF-CVD system. Based on the FTIR results, it is demonstrated that the BCN is chemically bonded atomic-level hybrid. The different reactant gases can result in the different morphologies and diameter of composite fibers. H2 in the reactant gases can decompose CH4 and B(OCH3 )3 . The BCN films with porous configuration and small size have the strong capacity of the oxygen adsorption. The resistance value of BCN films is ∼300 which is 10 times higher than that of CF (27.5 ). The breakdown voltage for BCN films is depend on the d the different configuration lead to the different resistance and breakdown volt in BCN films, which is favorable for the applications in the electrical devices and sensors. Acknowledgements This work was financially supported by the Program for New Century Excellent Talents in University (No. 06-0303) and the National Natural Science Foundation of China under Grant (No. 50772041).
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