- Email: [email protected]
Boron-doped carbon microspheres
Materials Chemistry and Physics 114 (2009) 973–977
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Boron-doped carbon microspheres Kartick C. Mondal a,b , André M. Strydom c,∗ , Zikhona Tetana a,b , Sabelo D. Mhlanga a,b , Michael J. Witcomb b,d , Josef Havel e , Rudolph M. Erasmus b,f , Neil J. Coville a,b,∗∗ a
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, WITS 2050, South Africa DST/NRF Centre of Excellence in Strong Materials, WITS 2050, South Africa c Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa d Microscopy and Microanalysis Unit, University of the Witwatersrand, WITS 2050, Johannesburg, South Africa e Departments of Chemistry and Physical Electronics, Faculty of Science, Masaryk University, Kotláˇrská 2, 611 37 Brno, Czech Republic f School of Physics, University of the Witwatersrand, WITS 2050, South Africa b
a r t i c l e
i n f o
Article history: Received 14 May 2008 Received in revised form 24 October 2008 Accepted 3 November 2008 Keywords: CVD method Boron-doped carbon microsphere TEM Raman spectroscopy Electrical conductivity
a b s t r a c t A chemical vapor deposition (CVD) procedure has been used for the synthesis of boron-doped carbon microspheres (CSMs) using BF3 /MeOH as the boron source, and acetylene as the carbon source. The boron-doped carbon microsphere samples were characterized by transmission electron microscopy (TEM), Raman spectroscopy and laser ablation mass spectrometry analysis. The average diameter and the shell thickness of the carbon microspheres are strongly influenced by the boron content. The intensity of the D-band laser excitation line increased after the boron incorporation into the carbon microspheres. Electrical conductivity of the boron-doped carbon microspheres has been measured. The conductivity of the B-doped microsphere sample is lower than that of the undoped sample by about two orders of magnitude. This could be ascribed to a higher degree of charge localization which impedes the charge transport in this material compared to the undoped material. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Carbon is a useful building block that can create a wide range of structures. The successful growth of diamond films, the fullerene molecule C60 and its family members Cn , e.g. C70 and C80 , as well as carbon nanotubes (CNTs) have attracted much attention in recent years [1]. In particular, an enormous amount of research has to date focused on the synthesis and applications of CNTs. Another form of carbon, which is graphitic in nature, has also been described in the literature. This form is spherical in shape and although carbon spheres have been known for decades, studies on the synthesis of spherically shaped carbon has become topical in the last few years. These spherically shaped materials have been referred to as nanosized carbon spheres [2], carbon nanobeads, carbon spherules [3–5], nanoballs [6], carbon pearls [7] or simply as carbon microspheres (CMSs) [8]. In many instances CMSs have been synthesized accidentally as a by-product in the synthesis of CNTs by chemical vapor deposition (CVD) methods. Interest in products
∗ Corresponding author at: Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa. Fax: +27 11 4892339. ∗∗ Corresponding author at: Molecular Sciences Institute and School of Chemistry, University of the Witwatersrand, WITS 2050, South Africa. Fax: +27 11 7176749. E-mail addresses: [email protected] (A.M. Strydom), [email protected] (N.J. Coville). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.11.008
made from graphitic carbon relates to their high strength, high thermal resistance, light weight and conductivity. Further, carbon spheres have also been used in high strength composites, catalyst carriers, lubricants, electronic devices and as wear-resistant materials [9]. Recent studies have shown that carbon microspheres can be fabricated using CVD procedures involving the direct pyrolysis of hydrocarbons [9,10]. The main advantage of using the CVD process to make carbon microspheres relates to the facile synthesis processing that does not require a catalyst for their synthesis. Hence, unlike CNT and carbon nanofibre (CNF) synthesis the materials are very pure and thus require no post-reaction treatments to remove graphitic materials and catalysts. The doping of carbon materials is known to modify their conductivity properties. In particular, N doping of CNTs leads to n-type semiconductor materials [11] while B doping leads to p-type semiconducting CNTs [12]. To our knowledge no boron doping of CMSs has previously been reported. In this study we report on the synthesis of boron-doped carbon microspheres (B-CMSs) using acetylene as carbon source and BF3 in MeOH as boron source. No catalyst was used. We also report on the measurement of the physical properties of the B-CMSs as the B doping level is varied. Doped carbon microspheres can be considered as a promising new material for the fabrication of new generation electronic devices.
974
K.C. Mondal et al. / Materials Chemistry and Physics 114 (2009) 973–977
2. Experimental A horizontal tubular furnace was used to synthesize carbon microspheres by the pyrolysis of acetylene. The acetylene was introduced into the reactor at a flow rate of 100 ml min− . A pyrolysis temperature of 800 ◦ C was employed. This pyrolysis reaction generated a soot deposit on the inner wall of the quartz tube in the hot region of the reactor. After the system was cooled, the soot (2 g after 2 h reaction) was collected, weighed and analyzed. A blank run was recorded with acetylene and MeOH (no BF3 ) and the carbon microspheres produced were found to be the same size and gave the same yield as those produced from acetylene in the absence of MeOH. Boron addition was achieved by bubbling acetylene gas through a 1.3-M BF3 in MeOH solution (Sigma–Aldrich) and this procedure generated boron-doped carbon microspheres. Raman spectra were recorded using a Jobin-Yvon T64000 Raman spectrometer equipped with an Olympus BX40 microscope attachment and a liquid nitrogen cooled charge coupled device detector. All the samples were measured after excitation with three different laser wave lengths (488 nm, 514.5 nm and 647 nm). Power at the sample was kept low (∼1 mW) to avoid localized heating. Transmission electron microscopy (TEM) studies of the samples were carried out at 197 kV using a Philips CM200 TEM equipped with a LaB6 emitter, an Oxford ISIS EDX super ultrathin windowed detector and a Gatan Model 678 Imaging Filter (GIF). Electron energy loss spectroscopy (EELS) measurements were conducted on regions of spheres over holes in carbon lacey support films for periods up to 300 s. The detection limit for simulation of the spectra using Gatan EELS Advisor software was estimated to be of the order of 1 at % for our system. Determination of boron in carbon microspheres was undertaken using laserablation inductively coupled plasma optical emission spectrometry (LA-ICP-OES). The LA-ICP-MS system consists of a UP—213 Laser Ablation System (New Wave Research) and ICP-MS spectrometer Agilent 7500 series. The laser was operated at the 5th harmonic frequency (213 nm). Experimental calculations used were: repetition rate 20 Hz, laser spot diameter 55 m, laser irradiance 12.3 J cm−1 , raster 1 mm × 1 mm (3 raster count). The carbon microsphere powder or calibration mixtures were directly applied to double-sided tape on a glass slide and ablated into the ICP-MS spectrometer. For calibration, a mixture of activated carbon (ALDRICH, DarcoG60, 100 mesh powder) and boric acid (p.a. Lachema Brno) in the content range of B = 0–1.5% was used (sensitivity <0.05%). Electrical conductivity measurements were performed using the dc-resistivity facility of a Physical Property Measurement System (Quantum Design, San Diego). The sample materials were compacted into a specially manufactured cell using a standard procedure across the series of samples to facilitate comparison of both relative and absolute conductivity values. Since no binder was used, our measurements present data on the bare conductivity of the synthesized microsphere material. High purity gold wires were used for electrical contacts, in a four-probe contacting method incorporating current reversal in order to negate contact and thermal voltages.
3. Results and discussions Fig. 1 shows TEM images of the carbon products of the assynthesized material in the presence and absence of the boron source. The diameter distribution is presented graphically in Fig. 2. The figure shows that the microspheres synthesized in the absence of the boron source have a broad size distribution range (350–700 nm). The diameters of carbon microspheres obtained in the presence of the boron source are larger and more uniform in size (650–800 nm). The larger and more uniform size of the microspheres synthesized in the presence of boron may be due to a number of factors. For example, the boron can act as a carbon etching retardant, similar to the proposals made by Redlich et al. [14]. Better graphitization of the carbon as a result of boron doping might also lead to a higher resistance to sphere etching and hence to better size uniformity. Alternatively, boron substitution results in a decrease in the distance between adjacent carbon layers i.e. a contraction of the structure [15]. This can lead to an enhancement of the graphene edge growth resulting in an overall increase of the microsphere size after addition of boron to the feed. As shown in the TEM images (Fig. 1A, B, and insets), a light contrast layer encapsulates the darker interior of the spheres clearly revealing the core/shell geometry of the carbon microsphere [13]. The uniform thickness of the outer shell is about 25 nm for carbon microspheres synthesized in the absence of a boron source. However, the thickness of the outer shell (see inset to Fig. 1) decreased
Fig. 1. TEM images of the carbon spheres synthesized (A) in the absence of a boron source and (B) in the presence of a boron source. Insets: show TEM images of the selected portion of the carbon sphere.
significantly (thickness of the outer diameter ≈11 nm) when produced in the presence of a boron source. This indicates that the boron modifies not only the average diameter and its statistical variance but also the wall thickness of the spheres. EDX and EELS measurements undertaken at the nanometer level failed to detect any boron in the lattice of the carbon microspheres. However, LA-ICP-OES detected the presence of boron in those microspheres synthesized in the presence of the boron source. The amount of boron present was measured at 0.13 ± 0.03% (atom%). The prepared boron doped and undoped carbon microspheres were studied by Raman spectroscopy. Raman measurements were performed using 488 nm, 514.5 nm and 647 nm laser excitation wavelengths (Fig. 3). All the spectra have two major peaks located at approximately 1345 cm−1 and 1597 cm−1 respectively, indicating the graphitic structure of the carbon microspheres [16]. A weak band at around 1047 cm−1 was observed for both undoped and
K.C. Mondal et al. / Materials Chemistry and Physics 114 (2009) 973–977
Fig. 2. Diameter distribution of the microspheres synthesized in the presence and absence of boron source.
Fig. 3. Raman spectra of the boron-doped carbon spheres with different laser excitation wavelengths.
975
boron-doped carbon microspheres at 514.5 nm laser excitation. A similar peak was also observed in our earlier studies using a different synthesis procedure [6]. Another weak band appeared between 1221 cm−1 and 1234 cm−1 in all the laser excitation wavelengths. The origin of these two peaks is not currently understood. Comparison of the D- and G-band positions for the undoped and doped CMSs shows a small red shift for the doped CMSs relative to the undoped spheres for the 488 nm and 514.5 nm results. The 647 nm results will be commented on in a subsequent paragraph. Miao et al. [17] reported a red shift of the G-band position with an increase in CMS size for 488 nm excitation. The difference in the G-band positions for our 488 nm results and those of Miao et al. are comparable. It is difficult to attribute a certain difference to a particular sphere diameter due to the distribution of sphere diameters in the samples. Of importance is the correlation in the trend, i.e. a red shifted G-band position is associated with a larger average diameter sphere. These results are consistent with the TEM results (Fig. 1) which showed an increase in diameter of the carbon microspheres in the presence of a boron source. This result is consistent with boron atoms or clusters having been incorporated into the carbon microspheres. Comparison of the G-band positions obtained with the three different laser wavelengths for each of the undoped and doped spheres shows that within the precision of band positions for carbon, there is little evidence of dispersion of the G-band for the undoped sample, while the doped sample displays a small degree of dispersion. Since the G-band only exhibits dispersion in more disordered carbon [18], this points to the doped spheres having a more disordered structure than the undoped spheres. It is suggested that this disordered-induced dispersion of the G-band contributes to the data for the 647 nm excitation not following the trend for the sizeinduced red shift of the G-band position as discussed in the previous paragraph. The dispersion observed for the D-band is a function of the laser wavelength used for the excitation and is essentially independent of the type of graphitic material being studied [18b]. The other conspicuous observation is that the intensity of the D-band laser excitation line has increased after boron incorporation into the carbon microspheres (Table 1 and Fig. 3). Normally the ID /IG ratio increases on (i) increasing the amount of amorphous carbon in the material and/or (ii) decreasing the graphite crystal size. The D-band intensity is thus correlated with the number of defects present. The ID /IG ratio is also consistent with a decrease in the crystallization (or graphitization) of the spheres with boron incorporation. The increase of the G-band width of the carbon microspheres after boron incorporation (Table 1 and Fig. 3) indicates a decrease in graphitization after boron incorporation. Fig. 4 shows the combined results of electrical conductivity measurements on (A) undoped (B) B-doped carbon microsphere samples. The overall electrical conductivity, (T), is significantly decreased in the doped product as seen by a comparison of the room-temperature values for (B-doped) = 0.011 ( m)−1 and (undoped) = 2.51 ( m)−1 . There are several factors that have to be considered when comparing this B-induced suppression of the electrical conductivity. The most obvious is the perturbation on electronic states and, more specifically, on the prevalence of charge carrier species at those energy levels that are amenable to electrical current conduction. The reduction of the electrical conductivity upon doping with boron, which commonly acts as a hole-dopant, would suggest that holes are the minority carriers in the carbon microsphere matrix. This conjecture has not been tested by e.g. Hall coefficient measurements on these samples. The effect of holedoping on the mobility of charge carriers is a less obvious factor that might determine the conductivity in the present case, but one which is also of potential importance. It is furthermore useful to consider the influence on electrical conductivity of the observed
976
K.C. Mondal et al. / Materials Chemistry and Physics 114 (2009) 973–977
Table 1 Raman spectra data with different laser excitation wavelengths of the carbon sphere synthesized in the presence and in the absence of boron source. Laser wavelength (nm)
488 514 647
Undoped carbon sphere
Boron-doped carbon sphere
ID /IG
G-band width (cm−1 )
D-band position (cm−1 )
G-band position (cm−1 )
ID /IG
G-band width (cm−1 )
D-band Position (cm−1 )
G-band position (cm−1 )
0.62 0.56 0.47
42.2 44.5 58.7
1338.0 1341.3 1351.9
1597.0 1593.0 1595.5
0.61 0.86 0.69
49.2 56.9 56.0
1339.3 1345.4 1335.5
1601.6 1597.0 1592.6
change in average diameter of microspheres upon doping with B. Since phase-translational coherence is not expected to be of importance in the conduction of electrical current in the glassy matrix of microspheres, the presence of B dopant atoms that are bonded externally to individual microspheres is therefore not expected to be influential on the electrical conductivity. Atomic boron has a covalent diameter of 0.164 nm, which is extremely small compared to even the ≈500 nm diameter of an undoped microsphere, and from the TEM images of Fig. 1 there is, as expected, no indication of the inter-sphere distance being increased upon doping. The temperature variation of electrical conductivity for the undoped microsphere sample, see Fig. 4A, is well represented by the expression = 0 exp[−(T0 /T)n ], with n = 1/4, over the entire range of temperature (2–300 K) used in our experiments. This particular type of stretched exponential behavior is one of a few variations of the variable-range-hopping (VRH) model of electrical conductivity [19], and represents a phonon-assisted mechanism of charge transport, instead of the more usual conductor-like band conductivity. The n = 1/4 exponent of VRH conductivity is commonly associated with amorphous semiconductors and semiconducting glasses, and we therefore conclude that the network of carbon microspheres constitute an amorphous matrix for electrical conductivity. The conductivity of the B-doped microsphere sample on the other hand,
see Fig. 4B, is lower than that of the undoped sample by about two orders of magnitude. This could be ascribed to a higher degree of charge localization which is impeding the charge transport in this material compared to the undoped material. However, as our experimental investigations into the properties of this material are still exploratory, the importance of charge localization compared to other mechanisms such as dopant-induced charge carrier scattering awaits future studies. Both the doped and undoped microsphere samples show a variable-range hopping class of conductivity. This is a phononassisted process showing the involvement of electrons in charge transport. The introduction of boron severely reduces the conductivity. Since B can be expected to drain electrons from the lattice, this suggests that the conductivity in these samples may reasonably be expected to be of an electronic origin. If a simple decrease in charge carrier concentration was the major result of B-doping, then one would expect the conduction mechanism to change from VRH towards more simpler insulating or activation behavior, as opposed to the VRH process that seems robust against B-doping. The localization length is an indication of the degree of spatial confinement of the electronic wave function. Our analysis of the conductivity of the B-doped microsphere sample in terms of the VRH model, see Fig. 4B, leads to the conclusion that in this case the n = 1/4 glassy conductivity is found only at higher temperatures, i.e. above about 170 K. The conductivity towards lower temperatures is higher than what would have been predicted from an extrapolation (see dashed line, Fig. 4B) of the high temperature, n = 1/4 glassy conductivity. This change of behavior is conceivably due to extended electronic states dominating the conductivity at higher temperatures through hopping between localized states, whereas at low temperatures the conductivity is largely determined by the overlapping localized states of increased localization length. Further electrical transport measurements such as Hall coefficient measurements to determine the majority carrier species and its concentration, and magnetic field measurements are needed to further characterize the charge transport in carbon microsphere material. Measurements involving different dopant levels as well as studies involving dopant elements of different electron affinities are planned. 4. Conclusions
Fig. 4. Temperature dependence of the electric conductivity in (A) undoped and (B) B-doped carbon microsphere samples. The solid lines depict least-squares fits to the data as explained in the text.
We have accomplished our goal of synthesizing boron-doped carbon microspheres by applying a chemical vapor deposition method. The LA-ICP-OES analysis revealed the presence of boron in the carbon microspheres even though the EELS and EDX data do not reveal its presence (below detection limit). We have shown that the average diameters of the carbon microspheres and their shell thickness are strongly influenced by the boron content. The D- and G-band positions have been shifted towards the red for the boron-doped carbon microspheres when 488 nm and 514.5 nm laser excitation was used in the laser Raman analysis. Evidence from Raman band positions and the ID /IG ratio was presented that indicate a decrease in the graphitization of the CMSs with boron incorporation.
K.C. Mondal et al. / Materials Chemistry and Physics 114 (2009) 973–977
Our electrical conductivity measurements reveal behavior in both B-doped and in undoped microsphere samples that are generally associated with three-dimensional, semiconducting amorphous materials. The conductivity of the B-doped microsphere sample is lower than that of the undoped sample by about two orders of magnitude. This could be ascribed to a higher degree of charge localization which is impeding the charge transport in this material compared to the undoped material. These results warrant further studies to investigate the possibilities of tuning of the electrical and thermal transport properties in carbon microsphere samples by means of suitable doping. Acknowledgements We thank the University of the Witwatersrand, the University of Johannesburg and the NRF/DST Centre of Excellence in Strong Materials for the financial support. Grants from The Ministry of Education, Youth and Sports, project MSM0021622411 and the Academy of Sciences of the Czech Republic, project KAN101630651 are acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2008.11.008. References [1] [2] [3] [4]
Z.C. Kang, Z.L. Wang, J. Mol. Catal. A: Chem. 118 (1997) 215. Z.C. Kang, Z.L. Wang, J. Phys. Chem. 100 (1996) 5163. Q. Wang, H. Li, L. Chen, X. Huang, Carbon 39 (2001) 2211. V.G. Pol, M. Motiei, A. Gedanken, J. Calderon-Moreno, M. Yoshimura, Carbon 42 (2004) 111. [5] F. Li, Q. Qian, F. Yan, G. Yuan, Carbon 44 (2006) 128.
977
[6] X. Liu, B. Huang, N.J. Coville, Carbon 40 (2002) 2791. [7] (a) L.P. Biró, G.I. Márk, Z.E. Horváth, K. Kertész, J. Gyulai, J.B. Nagy, Ph. Lambin, Carbon 42 (2004) 2561; (b) A. Levesque, Vu Thien Binh, V. Semet, D. Guillot, R.Y. Fillit, M.D. Brookes, T.P. Nguyen, Thin Solid Films 464–465 (2004) 308. [8] A. Ma, X. Wang, T. Li, X. Liu, B. Xu, Mater. Sci. Eng. A 443 (2007) 54. [9] Y.Z. Jin, C. Gao, W.K. Hsu, Y. Zhu, A. Huczko, M. Bystrzejewski, M. Roe, C.Y. Lee, S. Acquah, H. Kroto, D.R.M. Walton, Carbon 43 (2005) 1944. [10] H. Qian, F. Han, B. Zhang, Y. Guo, J. Yue, B. Peng, Carbon 42 (2004) 761. [11] C.P. Ewels, M. Glerup, J. Nanosci. Technol. 5 (2005) 1345. [12] (a) R. Sen, B.C. Satishkumar, A. Govindaraj, K.R. Harikumar, G. Raina, J.-P. Zhang, A.K. Cheetham, C.N.R. Rao, Chem. Phys. Lett. 287 (1998) 671; (b) J. Yu, J. Ahn, S.F. Yoon, Q. Zhang, A. Rusli, B. Gan, K. Chew, M.B. Yu, X.D. Bai, E.G. Wang, Appl. Phys. Lett. 77 (2000) 1949; (c) J. Liu, S. Webster, D.L. Carrol, J. Phys. Chem. B 09 (2005) 15769; (d) M. Terrones, A.M. Benito, C. Manteca-Diego, W.K. Hsu, O.I. Osman, J.P. Hare, D.G. Reid, H. Terrones, A.K. Cheetham, et al., Chem. Phys. Lett. 257 (1996) 576; (e) K.C. Mondal, A.M. Strydom, R.M. Erasmus, J.M. Keartland, N.J. Coville, Mater. Chem. Phys. 111 (2008) 386–390. [13] K.C. Mondal, L.M. Cele, M.J. Witcomb, N.J. Coville, Catal. Commun. 9 (2008) 494. [14] Ph. Redlich, J. Loeffler, P.M. Ajayan, J. Bill, F. Aldinger, M. Ruhle, Chem. Phys. Lett. 260 (1996) 465–470. [15] M. Endo, C. Kim, T. Karaki, T. Tamaki, Y. Nishimura, M.J. Mattthews, S.D.M. Brown, M.S. Dresslhaus, Phys. Rev. B 58 (1998) 8991–8996. [16] (a) N. Chandrabhas, A.K. Sood, D. Sundararaman, S. Raju, Y.S. Raghunathan, G.V.N. Rao, V.S. Sastry, T.S. Radhakrisnan, Y. Hariharan, A. Bharath, C.S. Sundar, Pramana 42 (1994) 375; (b) R.I. Nemanich, G. Lucovsky, S.A. Solin, in: M. Balkanski (Ed.), Proceedings of the International Conference on Lattice Dynamics, Paris: Flammarion, 1978, p. 619. [17] J.-Y. Miao, D.W. Hwang, V. Kuppala, V. Narasimhulu, P.-I. Lin, Y.-T. Chen, S.-H. Lin, L.-P. Hwang, Carbon 42 (2004) 813. [18] (a) W.K. Hsu, S. Firth, Ph. Redlich, M. Terrones, H. Terrones, Y.Q. Zhu, N. Grobert, A. Schilder, R.J.H. Clark, H.W. Kroto, D.R.M. Walton, J. Mater. Chem. 10 (2000) 1425; (b) E.F. Antunes, A.O. Lobo, E.J. Corat, V.J. Trava-Airoldi, A.A. Martin, C. Verissimo, Carbon 44 (2006) 2202; (c) Q.-H. Yang, P.-X. Hou, M. Unno, S. Yamauchi, R. Saito, T. Kyotani, Nano Lett. 5 (2005) 2465. [19] (a) N.F. Mott, Philos. Mag. 19 (1969) 835; (b) N.F. Mott, M. Kaveh, Adv. Phys. 34 (1985) 329.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Boron-doped carbon microspheres Kartick C. Mondal a,b , André M. Strydom c,∗ , Zikhona Tetana a,b , Sabelo D. Mhlanga a,b , Michael J. Witcomb b,d , Josef Havel e , Rudolph M. Erasmus b,f , Neil J. Coville a,b,∗∗ a
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, WITS 2050, South Africa DST/NRF Centre of Excellence in Strong Materials, WITS 2050, South Africa c Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa d Microscopy and Microanalysis Unit, University of the Witwatersrand, WITS 2050, Johannesburg, South Africa e Departments of Chemistry and Physical Electronics, Faculty of Science, Masaryk University, Kotláˇrská 2, 611 37 Brno, Czech Republic f School of Physics, University of the Witwatersrand, WITS 2050, South Africa b
a r t i c l e
i n f o
Article history: Received 14 May 2008 Received in revised form 24 October 2008 Accepted 3 November 2008 Keywords: CVD method Boron-doped carbon microsphere TEM Raman spectroscopy Electrical conductivity
a b s t r a c t A chemical vapor deposition (CVD) procedure has been used for the synthesis of boron-doped carbon microspheres (CSMs) using BF3 /MeOH as the boron source, and acetylene as the carbon source. The boron-doped carbon microsphere samples were characterized by transmission electron microscopy (TEM), Raman spectroscopy and laser ablation mass spectrometry analysis. The average diameter and the shell thickness of the carbon microspheres are strongly influenced by the boron content. The intensity of the D-band laser excitation line increased after the boron incorporation into the carbon microspheres. Electrical conductivity of the boron-doped carbon microspheres has been measured. The conductivity of the B-doped microsphere sample is lower than that of the undoped sample by about two orders of magnitude. This could be ascribed to a higher degree of charge localization which impedes the charge transport in this material compared to the undoped material. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Carbon is a useful building block that can create a wide range of structures. The successful growth of diamond films, the fullerene molecule C60 and its family members Cn , e.g. C70 and C80 , as well as carbon nanotubes (CNTs) have attracted much attention in recent years [1]. In particular, an enormous amount of research has to date focused on the synthesis and applications of CNTs. Another form of carbon, which is graphitic in nature, has also been described in the literature. This form is spherical in shape and although carbon spheres have been known for decades, studies on the synthesis of spherically shaped carbon has become topical in the last few years. These spherically shaped materials have been referred to as nanosized carbon spheres [2], carbon nanobeads, carbon spherules [3–5], nanoballs [6], carbon pearls [7] or simply as carbon microspheres (CMSs) [8]. In many instances CMSs have been synthesized accidentally as a by-product in the synthesis of CNTs by chemical vapor deposition (CVD) methods. Interest in products
∗ Corresponding author at: Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa. Fax: +27 11 4892339. ∗∗ Corresponding author at: Molecular Sciences Institute and School of Chemistry, University of the Witwatersrand, WITS 2050, South Africa. Fax: +27 11 7176749. E-mail addresses: [email protected] (A.M. Strydom), [email protected] (N.J. Coville). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.11.008
made from graphitic carbon relates to their high strength, high thermal resistance, light weight and conductivity. Further, carbon spheres have also been used in high strength composites, catalyst carriers, lubricants, electronic devices and as wear-resistant materials [9]. Recent studies have shown that carbon microspheres can be fabricated using CVD procedures involving the direct pyrolysis of hydrocarbons [9,10]. The main advantage of using the CVD process to make carbon microspheres relates to the facile synthesis processing that does not require a catalyst for their synthesis. Hence, unlike CNT and carbon nanofibre (CNF) synthesis the materials are very pure and thus require no post-reaction treatments to remove graphitic materials and catalysts. The doping of carbon materials is known to modify their conductivity properties. In particular, N doping of CNTs leads to n-type semiconductor materials [11] while B doping leads to p-type semiconducting CNTs [12]. To our knowledge no boron doping of CMSs has previously been reported. In this study we report on the synthesis of boron-doped carbon microspheres (B-CMSs) using acetylene as carbon source and BF3 in MeOH as boron source. No catalyst was used. We also report on the measurement of the physical properties of the B-CMSs as the B doping level is varied. Doped carbon microspheres can be considered as a promising new material for the fabrication of new generation electronic devices.
974
K.C. Mondal et al. / Materials Chemistry and Physics 114 (2009) 973–977
2. Experimental A horizontal tubular furnace was used to synthesize carbon microspheres by the pyrolysis of acetylene. The acetylene was introduced into the reactor at a flow rate of 100 ml min− . A pyrolysis temperature of 800 ◦ C was employed. This pyrolysis reaction generated a soot deposit on the inner wall of the quartz tube in the hot region of the reactor. After the system was cooled, the soot (2 g after 2 h reaction) was collected, weighed and analyzed. A blank run was recorded with acetylene and MeOH (no BF3 ) and the carbon microspheres produced were found to be the same size and gave the same yield as those produced from acetylene in the absence of MeOH. Boron addition was achieved by bubbling acetylene gas through a 1.3-M BF3 in MeOH solution (Sigma–Aldrich) and this procedure generated boron-doped carbon microspheres. Raman spectra were recorded using a Jobin-Yvon T64000 Raman spectrometer equipped with an Olympus BX40 microscope attachment and a liquid nitrogen cooled charge coupled device detector. All the samples were measured after excitation with three different laser wave lengths (488 nm, 514.5 nm and 647 nm). Power at the sample was kept low (∼1 mW) to avoid localized heating. Transmission electron microscopy (TEM) studies of the samples were carried out at 197 kV using a Philips CM200 TEM equipped with a LaB6 emitter, an Oxford ISIS EDX super ultrathin windowed detector and a Gatan Model 678 Imaging Filter (GIF). Electron energy loss spectroscopy (EELS) measurements were conducted on regions of spheres over holes in carbon lacey support films for periods up to 300 s. The detection limit for simulation of the spectra using Gatan EELS Advisor software was estimated to be of the order of 1 at % for our system. Determination of boron in carbon microspheres was undertaken using laserablation inductively coupled plasma optical emission spectrometry (LA-ICP-OES). The LA-ICP-MS system consists of a UP—213 Laser Ablation System (New Wave Research) and ICP-MS spectrometer Agilent 7500 series. The laser was operated at the 5th harmonic frequency (213 nm). Experimental calculations used were: repetition rate 20 Hz, laser spot diameter 55 m, laser irradiance 12.3 J cm−1 , raster 1 mm × 1 mm (3 raster count). The carbon microsphere powder or calibration mixtures were directly applied to double-sided tape on a glass slide and ablated into the ICP-MS spectrometer. For calibration, a mixture of activated carbon (ALDRICH, DarcoG60, 100 mesh powder) and boric acid (p.a. Lachema Brno) in the content range of B = 0–1.5% was used (sensitivity <0.05%). Electrical conductivity measurements were performed using the dc-resistivity facility of a Physical Property Measurement System (Quantum Design, San Diego). The sample materials were compacted into a specially manufactured cell using a standard procedure across the series of samples to facilitate comparison of both relative and absolute conductivity values. Since no binder was used, our measurements present data on the bare conductivity of the synthesized microsphere material. High purity gold wires were used for electrical contacts, in a four-probe contacting method incorporating current reversal in order to negate contact and thermal voltages.
3. Results and discussions Fig. 1 shows TEM images of the carbon products of the assynthesized material in the presence and absence of the boron source. The diameter distribution is presented graphically in Fig. 2. The figure shows that the microspheres synthesized in the absence of the boron source have a broad size distribution range (350–700 nm). The diameters of carbon microspheres obtained in the presence of the boron source are larger and more uniform in size (650–800 nm). The larger and more uniform size of the microspheres synthesized in the presence of boron may be due to a number of factors. For example, the boron can act as a carbon etching retardant, similar to the proposals made by Redlich et al. [14]. Better graphitization of the carbon as a result of boron doping might also lead to a higher resistance to sphere etching and hence to better size uniformity. Alternatively, boron substitution results in a decrease in the distance between adjacent carbon layers i.e. a contraction of the structure [15]. This can lead to an enhancement of the graphene edge growth resulting in an overall increase of the microsphere size after addition of boron to the feed. As shown in the TEM images (Fig. 1A, B, and insets), a light contrast layer encapsulates the darker interior of the spheres clearly revealing the core/shell geometry of the carbon microsphere [13]. The uniform thickness of the outer shell is about 25 nm for carbon microspheres synthesized in the absence of a boron source. However, the thickness of the outer shell (see inset to Fig. 1) decreased
Fig. 1. TEM images of the carbon spheres synthesized (A) in the absence of a boron source and (B) in the presence of a boron source. Insets: show TEM images of the selected portion of the carbon sphere.
significantly (thickness of the outer diameter ≈11 nm) when produced in the presence of a boron source. This indicates that the boron modifies not only the average diameter and its statistical variance but also the wall thickness of the spheres. EDX and EELS measurements undertaken at the nanometer level failed to detect any boron in the lattice of the carbon microspheres. However, LA-ICP-OES detected the presence of boron in those microspheres synthesized in the presence of the boron source. The amount of boron present was measured at 0.13 ± 0.03% (atom%). The prepared boron doped and undoped carbon microspheres were studied by Raman spectroscopy. Raman measurements were performed using 488 nm, 514.5 nm and 647 nm laser excitation wavelengths (Fig. 3). All the spectra have two major peaks located at approximately 1345 cm−1 and 1597 cm−1 respectively, indicating the graphitic structure of the carbon microspheres [16]. A weak band at around 1047 cm−1 was observed for both undoped and
K.C. Mondal et al. / Materials Chemistry and Physics 114 (2009) 973–977
Fig. 2. Diameter distribution of the microspheres synthesized in the presence and absence of boron source.
Fig. 3. Raman spectra of the boron-doped carbon spheres with different laser excitation wavelengths.
975
boron-doped carbon microspheres at 514.5 nm laser excitation. A similar peak was also observed in our earlier studies using a different synthesis procedure [6]. Another weak band appeared between 1221 cm−1 and 1234 cm−1 in all the laser excitation wavelengths. The origin of these two peaks is not currently understood. Comparison of the D- and G-band positions for the undoped and doped CMSs shows a small red shift for the doped CMSs relative to the undoped spheres for the 488 nm and 514.5 nm results. The 647 nm results will be commented on in a subsequent paragraph. Miao et al. [17] reported a red shift of the G-band position with an increase in CMS size for 488 nm excitation. The difference in the G-band positions for our 488 nm results and those of Miao et al. are comparable. It is difficult to attribute a certain difference to a particular sphere diameter due to the distribution of sphere diameters in the samples. Of importance is the correlation in the trend, i.e. a red shifted G-band position is associated with a larger average diameter sphere. These results are consistent with the TEM results (Fig. 1) which showed an increase in diameter of the carbon microspheres in the presence of a boron source. This result is consistent with boron atoms or clusters having been incorporated into the carbon microspheres. Comparison of the G-band positions obtained with the three different laser wavelengths for each of the undoped and doped spheres shows that within the precision of band positions for carbon, there is little evidence of dispersion of the G-band for the undoped sample, while the doped sample displays a small degree of dispersion. Since the G-band only exhibits dispersion in more disordered carbon [18], this points to the doped spheres having a more disordered structure than the undoped spheres. It is suggested that this disordered-induced dispersion of the G-band contributes to the data for the 647 nm excitation not following the trend for the sizeinduced red shift of the G-band position as discussed in the previous paragraph. The dispersion observed for the D-band is a function of the laser wavelength used for the excitation and is essentially independent of the type of graphitic material being studied [18b]. The other conspicuous observation is that the intensity of the D-band laser excitation line has increased after boron incorporation into the carbon microspheres (Table 1 and Fig. 3). Normally the ID /IG ratio increases on (i) increasing the amount of amorphous carbon in the material and/or (ii) decreasing the graphite crystal size. The D-band intensity is thus correlated with the number of defects present. The ID /IG ratio is also consistent with a decrease in the crystallization (or graphitization) of the spheres with boron incorporation. The increase of the G-band width of the carbon microspheres after boron incorporation (Table 1 and Fig. 3) indicates a decrease in graphitization after boron incorporation. Fig. 4 shows the combined results of electrical conductivity measurements on (A) undoped (B) B-doped carbon microsphere samples. The overall electrical conductivity, (T), is significantly decreased in the doped product as seen by a comparison of the room-temperature values for (B-doped) = 0.011 ( m)−1 and (undoped) = 2.51 ( m)−1 . There are several factors that have to be considered when comparing this B-induced suppression of the electrical conductivity. The most obvious is the perturbation on electronic states and, more specifically, on the prevalence of charge carrier species at those energy levels that are amenable to electrical current conduction. The reduction of the electrical conductivity upon doping with boron, which commonly acts as a hole-dopant, would suggest that holes are the minority carriers in the carbon microsphere matrix. This conjecture has not been tested by e.g. Hall coefficient measurements on these samples. The effect of holedoping on the mobility of charge carriers is a less obvious factor that might determine the conductivity in the present case, but one which is also of potential importance. It is furthermore useful to consider the influence on electrical conductivity of the observed
976
K.C. Mondal et al. / Materials Chemistry and Physics 114 (2009) 973–977
Table 1 Raman spectra data with different laser excitation wavelengths of the carbon sphere synthesized in the presence and in the absence of boron source. Laser wavelength (nm)
488 514 647
Undoped carbon sphere
Boron-doped carbon sphere
ID /IG
G-band width (cm−1 )
D-band position (cm−1 )
G-band position (cm−1 )
ID /IG
G-band width (cm−1 )
D-band Position (cm−1 )
G-band position (cm−1 )
0.62 0.56 0.47
42.2 44.5 58.7
1338.0 1341.3 1351.9
1597.0 1593.0 1595.5
0.61 0.86 0.69
49.2 56.9 56.0
1339.3 1345.4 1335.5
1601.6 1597.0 1592.6
change in average diameter of microspheres upon doping with B. Since phase-translational coherence is not expected to be of importance in the conduction of electrical current in the glassy matrix of microspheres, the presence of B dopant atoms that are bonded externally to individual microspheres is therefore not expected to be influential on the electrical conductivity. Atomic boron has a covalent diameter of 0.164 nm, which is extremely small compared to even the ≈500 nm diameter of an undoped microsphere, and from the TEM images of Fig. 1 there is, as expected, no indication of the inter-sphere distance being increased upon doping. The temperature variation of electrical conductivity for the undoped microsphere sample, see Fig. 4A, is well represented by the expression = 0 exp[−(T0 /T)n ], with n = 1/4, over the entire range of temperature (2–300 K) used in our experiments. This particular type of stretched exponential behavior is one of a few variations of the variable-range-hopping (VRH) model of electrical conductivity [19], and represents a phonon-assisted mechanism of charge transport, instead of the more usual conductor-like band conductivity. The n = 1/4 exponent of VRH conductivity is commonly associated with amorphous semiconductors and semiconducting glasses, and we therefore conclude that the network of carbon microspheres constitute an amorphous matrix for electrical conductivity. The conductivity of the B-doped microsphere sample on the other hand,
see Fig. 4B, is lower than that of the undoped sample by about two orders of magnitude. This could be ascribed to a higher degree of charge localization which is impeding the charge transport in this material compared to the undoped material. However, as our experimental investigations into the properties of this material are still exploratory, the importance of charge localization compared to other mechanisms such as dopant-induced charge carrier scattering awaits future studies. Both the doped and undoped microsphere samples show a variable-range hopping class of conductivity. This is a phononassisted process showing the involvement of electrons in charge transport. The introduction of boron severely reduces the conductivity. Since B can be expected to drain electrons from the lattice, this suggests that the conductivity in these samples may reasonably be expected to be of an electronic origin. If a simple decrease in charge carrier concentration was the major result of B-doping, then one would expect the conduction mechanism to change from VRH towards more simpler insulating or activation behavior, as opposed to the VRH process that seems robust against B-doping. The localization length is an indication of the degree of spatial confinement of the electronic wave function. Our analysis of the conductivity of the B-doped microsphere sample in terms of the VRH model, see Fig. 4B, leads to the conclusion that in this case the n = 1/4 glassy conductivity is found only at higher temperatures, i.e. above about 170 K. The conductivity towards lower temperatures is higher than what would have been predicted from an extrapolation (see dashed line, Fig. 4B) of the high temperature, n = 1/4 glassy conductivity. This change of behavior is conceivably due to extended electronic states dominating the conductivity at higher temperatures through hopping between localized states, whereas at low temperatures the conductivity is largely determined by the overlapping localized states of increased localization length. Further electrical transport measurements such as Hall coefficient measurements to determine the majority carrier species and its concentration, and magnetic field measurements are needed to further characterize the charge transport in carbon microsphere material. Measurements involving different dopant levels as well as studies involving dopant elements of different electron affinities are planned. 4. Conclusions
Fig. 4. Temperature dependence of the electric conductivity in (A) undoped and (B) B-doped carbon microsphere samples. The solid lines depict least-squares fits to the data as explained in the text.
We have accomplished our goal of synthesizing boron-doped carbon microspheres by applying a chemical vapor deposition method. The LA-ICP-OES analysis revealed the presence of boron in the carbon microspheres even though the EELS and EDX data do not reveal its presence (below detection limit). We have shown that the average diameters of the carbon microspheres and their shell thickness are strongly influenced by the boron content. The D- and G-band positions have been shifted towards the red for the boron-doped carbon microspheres when 488 nm and 514.5 nm laser excitation was used in the laser Raman analysis. Evidence from Raman band positions and the ID /IG ratio was presented that indicate a decrease in the graphitization of the CMSs with boron incorporation.
K.C. Mondal et al. / Materials Chemistry and Physics 114 (2009) 973–977
Our electrical conductivity measurements reveal behavior in both B-doped and in undoped microsphere samples that are generally associated with three-dimensional, semiconducting amorphous materials. The conductivity of the B-doped microsphere sample is lower than that of the undoped sample by about two orders of magnitude. This could be ascribed to a higher degree of charge localization which is impeding the charge transport in this material compared to the undoped material. These results warrant further studies to investigate the possibilities of tuning of the electrical and thermal transport properties in carbon microsphere samples by means of suitable doping. Acknowledgements We thank the University of the Witwatersrand, the University of Johannesburg and the NRF/DST Centre of Excellence in Strong Materials for the financial support. Grants from The Ministry of Education, Youth and Sports, project MSM0021622411 and the Academy of Sciences of the Czech Republic, project KAN101630651 are acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2008.11.008. References [1] [2] [3] [4]
Z.C. Kang, Z.L. Wang, J. Mol. Catal. A: Chem. 118 (1997) 215. Z.C. Kang, Z.L. Wang, J. Phys. Chem. 100 (1996) 5163. Q. Wang, H. Li, L. Chen, X. Huang, Carbon 39 (2001) 2211. V.G. Pol, M. Motiei, A. Gedanken, J. Calderon-Moreno, M. Yoshimura, Carbon 42 (2004) 111. [5] F. Li, Q. Qian, F. Yan, G. Yuan, Carbon 44 (2006) 128.
977
[6] X. Liu, B. Huang, N.J. Coville, Carbon 40 (2002) 2791. [7] (a) L.P. Biró, G.I. Márk, Z.E. Horváth, K. Kertész, J. Gyulai, J.B. Nagy, Ph. Lambin, Carbon 42 (2004) 2561; (b) A. Levesque, Vu Thien Binh, V. Semet, D. Guillot, R.Y. Fillit, M.D. Brookes, T.P. Nguyen, Thin Solid Films 464–465 (2004) 308. [8] A. Ma, X. Wang, T. Li, X. Liu, B. Xu, Mater. Sci. Eng. A 443 (2007) 54. [9] Y.Z. Jin, C. Gao, W.K. Hsu, Y. Zhu, A. Huczko, M. Bystrzejewski, M. Roe, C.Y. Lee, S. Acquah, H. Kroto, D.R.M. Walton, Carbon 43 (2005) 1944. [10] H. Qian, F. Han, B. Zhang, Y. Guo, J. Yue, B. Peng, Carbon 42 (2004) 761. [11] C.P. Ewels, M. Glerup, J. Nanosci. Technol. 5 (2005) 1345. [12] (a) R. Sen, B.C. Satishkumar, A. Govindaraj, K.R. Harikumar, G. Raina, J.-P. Zhang, A.K. Cheetham, C.N.R. Rao, Chem. Phys. Lett. 287 (1998) 671; (b) J. Yu, J. Ahn, S.F. Yoon, Q. Zhang, A. Rusli, B. Gan, K. Chew, M.B. Yu, X.D. Bai, E.G. Wang, Appl. Phys. Lett. 77 (2000) 1949; (c) J. Liu, S. Webster, D.L. Carrol, J. Phys. Chem. B 09 (2005) 15769; (d) M. Terrones, A.M. Benito, C. Manteca-Diego, W.K. Hsu, O.I. Osman, J.P. Hare, D.G. Reid, H. Terrones, A.K. Cheetham, et al., Chem. Phys. Lett. 257 (1996) 576; (e) K.C. Mondal, A.M. Strydom, R.M. Erasmus, J.M. Keartland, N.J. Coville, Mater. Chem. Phys. 111 (2008) 386–390. [13] K.C. Mondal, L.M. Cele, M.J. Witcomb, N.J. Coville, Catal. Commun. 9 (2008) 494. [14] Ph. Redlich, J. Loeffler, P.M. Ajayan, J. Bill, F. Aldinger, M. Ruhle, Chem. Phys. Lett. 260 (1996) 465–470. [15] M. Endo, C. Kim, T. Karaki, T. Tamaki, Y. Nishimura, M.J. Mattthews, S.D.M. Brown, M.S. Dresslhaus, Phys. Rev. B 58 (1998) 8991–8996. [16] (a) N. Chandrabhas, A.K. Sood, D. Sundararaman, S. Raju, Y.S. Raghunathan, G.V.N. Rao, V.S. Sastry, T.S. Radhakrisnan, Y. Hariharan, A. Bharath, C.S. Sundar, Pramana 42 (1994) 375; (b) R.I. Nemanich, G. Lucovsky, S.A. Solin, in: M. Balkanski (Ed.), Proceedings of the International Conference on Lattice Dynamics, Paris: Flammarion, 1978, p. 619. [17] J.-Y. Miao, D.W. Hwang, V. Kuppala, V. Narasimhulu, P.-I. Lin, Y.-T. Chen, S.-H. Lin, L.-P. Hwang, Carbon 42 (2004) 813. [18] (a) W.K. Hsu, S. Firth, Ph. Redlich, M. Terrones, H. Terrones, Y.Q. Zhu, N. Grobert, A. Schilder, R.J.H. Clark, H.W. Kroto, D.R.M. Walton, J. Mater. Chem. 10 (2000) 1425; (b) E.F. Antunes, A.O. Lobo, E.J. Corat, V.J. Trava-Airoldi, A.A. Martin, C. Verissimo, Carbon 44 (2006) 2202; (c) Q.-H. Yang, P.-X. Hou, M. Unno, S. Yamauchi, R. Saito, T. Kyotani, Nano Lett. 5 (2005) 2465. [19] (a) N.F. Mott, Philos. Mag. 19 (1969) 835; (b) N.F. Mott, M. Kaveh, Adv. Phys. 34 (1985) 329.