- Email: [email protected]
Preparation of carbon-coated magnetic iron nanoparticles from composite rods made from coal and iron powders
Fuel Processing Technology 86 (2004) 267 – 274 www.elsevier.com/locate/fuproc
Preparation of carbon-coated magnetic iron nanoparticles from composite rods made from coal and iron powders Jieshan Qiua,b,*, Yongfeng Lia, Yunpeng Wanga, Yuliang Ana, Zongbin Zhaoa, Ying Zhoua,b, Wen Lic a
Carbon Research Laboratory, School of Chemical Engineering, Center for Nano Materials and Science, State Key Lab of Fine Chemicals, Dalian University of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, China b State Key Laboratory for Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China c State Key Laboratory of Coal Conversion, CAS, P.O. Box 165, Taiyuan, Shanxi 030001, China Accepted 30 March 2004
Abstract We report the preparation of carbon-coated magnetic iron nanoparticles with diameters mainly around 40–55 nm from iron-filled coal-derived hollow carbon rods by arc discharge. These monodispersed carbon-coated iron nanoparticles were found in the soot-like deposits formed around the cathode. The transmission electron microscopy and X-ray diffraction studies show that these nanoparticles are perfectly coated with graphitic carbon shells with a thickness of 20–30 nm. A simple model is proposed to explain the growth of the carbon-coated metal nanoparticles in terms of the properties of coal-derived carbon and the arcing conditions. D 2004 Elsevier B.V. All rights reserved. Keywords: Carbon onions; Coal; Arc discharge; Encapsulated nanomaterials
* Corresponding author. Carbon Research Laboratory, School of Chemical Engineering, Center for Nano Materials and Science, State Key Lab of Fine Chemicals, Dalian University of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, China. Fax: +86 411 8363 3080. E-mail address: [email protected] (J. Qiu). 0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2004.03.006
268
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
1. Introduction In the past decade, the preparation of nanosized magnetic metal particles encapsulated by graphitic carbon shells has been actively pursued. This interest has been chiefly driven by the application potential of these carbon-coated magnetic metal nanomaterials in many fields as diverse as magnetic recording media, magnetic toner in xerography, contrast agent in magnetic resonance imaging and catalyst supports [1,2]. It is well known that metal particles in nanometer size tend to be easily oxidized under oxidizing medium. In the case of carboncoated nanoparticles, the outside carbon coating or shells can protect the nanosized metal particles against oxidization and degradation. Up to now, great efforts have been devoted to the iron-group metal and carbon system in which the majority of the encapsulated materials are found to be in metallic phase, such as bcc-Fe, fcc-Co and fcc-Ni [3–8]. To date, several approaches have been adopted in preparing carbon-coated nanosized metal particles [3,9–13], of which the arc discharge technique is commonly used because of its simplicity and low operating cost. For the conventional arc discharge technique, the preparation of carbon-coated nanomaterials is normally conducted in an inert gas atmosphere such as helium with a metal-filled graphite electrode as anode, resulting in various carbon-coated nanosized metal materials with polyhedral morphologies, of which the diameter or size varies in a wide range from several tens to several hundred nanometers [3]. In practice, those carbon-coated nanomaterials with narrow and uniform size distribution as well as perfect ball-like morphology would be more desirable. Our previous work proved that coal, a cheap and abundant carbon source in nature, is a good starting material for production of fullerenes, carbon nanotubes, nanocapsules, carbon micro-balls and carbon-coated nickel nanocrystals [14–21], which were achieved under different conditions and with or without catalysts. Here we report the preparation of carbon-coated iron nanoparticles with perfect ball-like shape and relative uniform size from composite rods made from a mixture of coal and iron powders.
2. Experimental A typical anthracite from the western part of China was used in this study. The proximate and ultimate analyses of the coal samples are shown in Table 1. The coal sample without any pretreatment was crushed and sieved to 150 Am, and fully dried at 383 K for 12 h before use. The coal powder was mixed with coal tar binder in a coal-to-binder ratio of 80:20% by weight, and subsequently pressed at about 10–20 MPa to form hollow coal rods which were put into an electric furnace and carbonized under flowing N2 to make hollow carbon rods. The furnace was first ramped at 3 K/min to 773 K for 1 h, then, was Table 1 Analysis data of coal sample Proximate analysis (%)
Ultimate analysis (%, daf)
M ad
Ad
V daf
C
H
N
S
O
2.78
3.92
15.18
87.86
3.66
0.73
0.36
7.39
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
269
further ramped at 10 K/min to 1173 K for 4 h before cooling down to room temperature. Finally, hollow carbon rods (O.D. 10 mm, I.D. 5 mm, length 100–120 mm) were obtained. The hollow carbon rods were filled with a mixture of iron and carbon powder, and both ends of the filled rod were sealed with graphite plugs. The carbon powder used here was obtained by re-crushing some of the coal-derived hollow carbon rods as described above. The ratio of iron powder to carbon powder in the mixture was 50:50 by weight. Both the iron and carbon powders had a size smaller than 150 Am. The iron-filled hollow carbon rods were used as the anode in the arc discharge experiments while the cathode was made by a graphite electrode with a diameter of 16 mm and length of 30 mm. The arc discharge experiments were conducted with a direct current of 50–70 A and a voltage of 30–50 V in He at 0.065 MPa. The distance between two electrodes was maintained at about 1–2 mm by manually feeding the anode. After arc discharge, soot-like materials deposited on the inner wall of the arc chamber and surface of cathode were collected and examined by transmission electron microscopy (TEM, JEM-2000EX operated at 100 kV) and X-ray diffractions (XRD, Rigaku D/max 2400, Cu Ka). The samples for TEM observation was dispersed in ethanol with ultrasonic treatment for several minutes, and several drops of the suspension were placed on a carbon-coated copper grid for TEM examination.
3. Results and discussion The TEM was used to scrutinize the soot-like deposits both on the arc reactor inner walls and around the cathode, which were collected after the arc discharge. The TEM study reveals that carbon-coated iron nanoparticles are present in quantity in the deposits around the cathode. Occasionally, similar nanoparticles were also found in the soot-like deposits on the inner chamber walls, but in most cases they were embedded in amorphous carbon globules, which is not uncommon in the case of the arc discharge of graphite-metal composite electrodes [3,6]. It should be noted that in our case with iron and coal-based carbon composite rods as anode, it is in the deposits around the cathode that carbon-coated iron nanoparticles with uniform diameters and nearly perfect sphere-shape were found. The typical TEM images of the carbon-coated iron nanoparticles are shown in Fig. 1. Fig. 1a is a low magnification image, showing that carbon-coated nanoparticles with a nearly perfect ball-like shape are abundant in the deposits around the cathode. It is interesting to note that particles with polyhedral morphologies are rarely observed. The detailed TEM study reveals that the nanoparticles are coated by carbon shells with a thickness of ca. 20–30 nm. The dark area in the core of these particles is iron metal, as shown in Fig. 1, which is confirmed by XRD study. All the nanoparticles seem to be monodispersed, and each of them can be clearly identified. Fig. 1b shows two nanoparticles with similar diameter. It is expected that this kind of discrete state would be of benefit to application of these carbon-coated nanoparticles. Fig. 2 shows the particle size distribution of the encapsulated iron nanoparticles in spherical graphitic shells, which is obtained from the TEM measurements. The diameter distribution of the encapsulated iron nanoparticles is shown in Fig. 2a, and the size distribution of the whole particles including the carbon shells is shown in Fig. 2b. It can be clearly seen that the diameters of carbon-coated iron nanoparticles from coal-based carbon
270
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
Fig. 1. Typical TEM images of carbon-coated iron nanoparticles prepared from iron-filled coal-derived hollow carbon rods. (a) A low magnification image showing many particles with nearly perfect spherical shape; (b) an image of two nanoparticles with similar diameter.
composite rods are in a range of 25–60 nm, and nearly 80% of the particles has a size of ca. 40–55 nm, which are quite narrow and uniform compared with carbon-coated metal nanoparticles prepared from graphite composite electrodes that have a wide size distribution in a range of 20–200 nm [3,6,8]. It has been found that the nanosized iron particles obtained in our experiments are uniformly coated with carbon shells with a
Fig. 2. Histogram of particle size distribution of the carbon-coated iron nanoparticles. (a) The diameter distribution of the carbon-coated iron nanoparticles in the core; (b) the outer diameter distribution of the carboncoated nanoparticles.
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
271
thickness of ca. 20–30 nm. This kind of nanoparticles may be of potential in application fields where the nanomaterials are required to be uniform in size. The typical XRD patterns of the carbon-coated iron nanoparticles prepared by arc discharge of iron and coal-based carbon composites are shown in Fig. 3, from which a sharp peak at 26.28 and a shoulder peak at 25.98 can be clearly seen, corresponding to the (002) plane of hexagonal graphite structure. The value of d 002 for these two peaks is 0.3401 and 0.3427 nm, respectively. This implies that the carbon shells outside the nanoparticles are graphitic layers. In addition to these two graphite peaks, another three strong peaks at 43.28, 44.58 and 50.68 could also be seen in Fig. 3, which can be assigned to the superposition of the g-Fe phase with fcc structure and a-Fe phase with bodycentered cubic (bcc) structure. It should be noted that almost no iron carbide phases are found in these carbon-coated iron nanoparticles. This means that most of the encapsulated iron crystallites or nanoparticles in the core of the carbon shells are in ordinary metallic phase. Similar metal nanoparticles with polyhedral morphology were prepared by Saito [3] and his colleagues, and in their case, graphite-metal composite electrodes were used. It should also be noted that in the XRD pattern shown in Fig. 3, there exist some weaker and broader diffraction peaks that cannot be indexed to both carbon and iron. These peaks might be due to the minerals contained in the coal-derived carbon. In other words, the mineral matter in coal might play a role in the formation of carbon-coated iron nanoparticles, which definitely cannot be ruled out though the detailed mechanism involved is not known at the moment. More work is needed to clarify whether and how the minerals influence the synthesis of carbon-coated iron nanoparticles. In addition to the XRD results, we have found by a very simple experiment that these carbon-coated iron nanoparticles in metallic phase have maintained their magnetic properties. We simply put the carbon-encapsulated iron nanoparticles into water or organic solvents such as acetone and hexane, and applied a magnetic force outside the container, the carbon-coated iron nanoparticles moved along the direction of the magnetic force applied. This means that for the iron nanoparticles encapsulated inside the carbon cages, most of the core crystallites are still in ordinary metallic phase, which is consistent with the above XRD
Fig. 3. X-ray diffraction pattern of carbon-coated iron nanoparticles prepared from iron-filled coal-derived hollow carbon rods.
272
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
results. This spectacular magnetic property of the carbon-encapsulated iron particles would make them an ideal candidate as catalyst supports in magnetically stabilized bed [22,23], where the graphitic carbon layers may work as support of various catalysts while the magnetic core makes the catalysts-supported particles move freely and be dispersed uniformly in the liquid phase catalysis bed when a magnetic force is applied. Therefore, the precious catalysts supported on the carbon-coated magnetic metal nanoparticles would be easily recovered under the effect of magnetic force after the reaction is finished. The TEM and XRD results clearly demonstrate that the nanosized magnetic iron particles coated with uniform graphite shells can be produced from carbon composite rods derived from coal and iron powders. These coal-based carbon encapsulated nanoparticles differ substantially from those graphite-based carbon encapsulated nanoparticles in several aspects. First, all the coal-based nanoparticles have a nearly perfect spherical shape, while the graphite-based nanoparticles normally have polyhedral morphologies. Second, the coal-based nanoparticles have a narrow diameter distribution with about 80% in the range of 40–55 nm, which is much smaller than the graphite-based nanoparticles reported in the literature. Third, almost all of the coal-based nanoparticles exist in discrete or monodispersed state, while the graphite-based nanoparticles tend to be embedded in other carbon impurities. The formation mechanism of these coal-based and carbon-coated iron nanoparticles with perfect ball-like shape is not known because the available information is very limited at the present time. Nevertheless, it must be related to the chemical structure of coalderived carbon. It is known that the chemical structure of coal-derived carbon is very complex and consists of graphite crystallites containing a few domains of 1–10 nm in which the basic units are small graphite crystallite (termed as SGCs thereafter) made of a few graphitic layers. The SGCs are jointed together by relative weak cross-links, and the cross-links would break first under plasma arcing conditions to release some of the SGCs as free particles. Some of the released SGCs might be directly incorporated into the carbon-coated iron nanoparticles without further decomposition as in the case of graphite process in which the graphite needs to be first decomposed into C1 and C2 species [24]. Another possibility that cannot be ruled out is that some SGCs might go through further decomposition to release aromatic species such as polycyclic aromatic hydrocarbons (PAHs) as well as C1 or C2 species. These smaller carbon-containing species are very active, and undoubtedly, would actively take part in the formation reaction of carboncoated iron nanoparticles as the basic building blocks with the presence of iron species. For a better understanding of how the carbon-coated iron nanoparticles are formed from coal-based carbon, a simple model is put forward to illustrate the growth process of the particles, in which the roles played by SGCs, PHAs, C1 and C2 species and iron metal are all taken into account. Fig. 4 schematically shows the growth process of carbon-coated iron nanoparticles from coal-based carbon, which can be divided into three stages. In the first stage as shown in Fig. 4a, a mixture consisting of various carbon species and iron particles that may be in liquid phase or quasi-liquid phase is formed. In the second stage, graphitic layers start to appear on the outside surface of the particles both due to the catalytic effect of Fe metal and the high temperature in the arc zone. These newly formed graphitic carbon shells stack up to wrap or surround the core crystallites, as shown in Fig. 4b. Under the constant bombardment of electrons and ions in arc plasma, the graphite
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
273
Fig. 4. Schematic illustration of the growth progress of carbon-coated iron nanoparticles. (a) Fe-C alloy particles in liquid phase, in which large carbon fragments including PAHs and SGCs are taken in; (b) particles with polyhedral morphology are formed and the graphitic carbon layers in parallel gradually start to grow from outside of the surface because of the high temperature in the arc and the catalytic effect of iron species, leading to the formation of carbon shells outside the iron nanoparticles, (c) well-developed carbon-coated iron nanoparticles are formed.
layers formed on the surface would realign, and at the same time, large amounts of big carbon fragments such as SGCs and PAHs would be continuously taken in and take part in the formation of carbon shells. In the last stage as shown in Fig. 4c, the carbon-coated iron nanoparticles with a perfect ball-like shape are formed. It is expected that these nanoparticles might possess interesting and attractive properties. In this regard, much work is needed in future.
4. Conclusions In summary, carbon-coated iron nanoparticles with perfect spherical shape and narrow size distribution have been prepared from iron-filled coal-derived carbon rods by arc discharge. The diameters of these encapsulated nanoparticles are in the range of 25–60 nm with the peak centered at ca. 45 nm. The growth mechanism of these particles is discussed in terms of the chemical structure of coal-based carbon and the arcing conditions. A simple model is proposed to schematically show the formation process of the nanoparticles, in which large carbon fragments released from coal-based carbon rods in the arc discharge process may play an important part.
Acknowledgements This work is partly supported by the National Natural Science Foundation of China (Nos. 20376011, 29976006), the Natural Science Foundation of Liaoning Province (Nos. 9810300701, 2001101003), the Education Ministry of China and National Basic Research Program of China (No. 2003CB615806).
References [1] S.A. Majetich, J.O. Artman, M.E. Mchenry, N.T. Nuhfer, Phys. Rev., B 48 (1993) 16845. [2] S. Seraphin, D. Zhou, J. Jiao, J. Appl. Phys. 80 (1996) 2097. [3] Y. Saito, Carbon 33 (1995) 979.
274 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274 R.S. Ruoff, D.C. Lorents, B. Chan, R. Malhotra, S. Subramoney, Science 259 (1993) 346. B. Bokhonov, M. Korchagin, J. Alloys Compd. 333 (2002) 308. R. Seshadri, R. Sen, G.N. Subbama, K.R. Kannan, C.N. Rao, Chem. Phys. Lett. 231 (1994) 308. M. Tomita, Y. Saito, T. Hayashi, Jpn. J. Appl. Phys. 32 (1993) 280. Y. Saito, T. Yoshikawa, M. Okuda, M. Ohlkohchi, Y. Aodo, A. Kasuya, Y. Nishina, Chem. Phys. Lett. 209 (1993) 72. J.R. Heath, S.C. O’Brien, Q. Zhang, Y. Liu, R.F. Curl, H.W. Kroto, F.K. Tittel, R.E. Smalley, JACS 109 (1985) 359. D. Ugarte, Chem. Phys. Lett. 209 (1993) 99. S.C. Tsang, J.S. Qiu, P.J.F. Harris, Q. Fu, N. Zhang, Chem. Phys. Lett. 322 (2000) 553. Z.D. Zhang, J.G. Zheng, I. Skorvanek, G.H. Wen, J. Kovac, F.W. Wang, J.L. Yu, Z.J. Li, X.L. Dong, S.R. Jin, W. Liu, X.X. Zhang, J. Phys., Condens. Matter 13 (2001) 1921. Z.D. Zhang, J.L. Yu, J.G. Zheng, I. Skorvanek, J. Kovac, X.L. Dong, Z.J. Li, S.R. Jin, H.C. Yang, Z.J. Guo, W. Liu, X.G. Zhao, Phys. Rev., B 64 (2001) 024404. J.S. Qiu, Y. Zhou, L.N. Wang, S.C. Tsang, Carbon 36 (1998) 465. J.S. Qiu, Y. Zhou, Z.G. Yang, L.N. Wang, F. Zhang, S.C. Tsang, P.J.F. Harris, Mol. Mater. 13 (2000) 377. J.S. Qiu, Y. Zhou, Z.G. Yang, D.K. Wang, S.C. Guo, S.C. Tsang, P.J.F. Harris, Fuel 79 (2000) 1303. J.S. Qiu, F. Zhang, Y. Zhou, H.M. Han, D.S. Hu, S.C. Tsang, P.J.F. Harris, Fuel 81 (2002) 1509. Y.F. Li, J.S. Qiu, Z.B. Zhao, T.H. Wang, Y.P. Wang, W. Li, Chem. Phys. Lett. 366 (2002) 544. J.S. Qiu, Y.F. Li, Y.P. Wang, C.H. Liang, T.H. Wang, D.H. Wang, Carbon 41 (2003) 767. J.S. Qiu, Y.F. Li, Y.P. Wang, T.H. Wang, Z.B. Zhao, Y. Zhou, F. Li, H.M. Cheng, Carbon 41 (2003) 2170. J.S. Qiu, Y.F. Li, Y.P. Wang, T.H. Wang, Z.B. Zhao, Y. Zhou, Y.G. Wang, Fuel 83 (2004) 615. R.E. Rosensweig, Science 204 (1979) 57. S. Brandani, G. Astaria, Chem. Eng. Sci. 51 (1996) 4631. P.J.F. Harris, Carbon Nanotubes and Related Structures, Cambridge Univ. Press, Cambridge, UK, 1999.
Preparation of carbon-coated magnetic iron nanoparticles from composite rods made from coal and iron powders Jieshan Qiua,b,*, Yongfeng Lia, Yunpeng Wanga, Yuliang Ana, Zongbin Zhaoa, Ying Zhoua,b, Wen Lic a
Carbon Research Laboratory, School of Chemical Engineering, Center for Nano Materials and Science, State Key Lab of Fine Chemicals, Dalian University of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, China b State Key Laboratory for Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China c State Key Laboratory of Coal Conversion, CAS, P.O. Box 165, Taiyuan, Shanxi 030001, China Accepted 30 March 2004
Abstract We report the preparation of carbon-coated magnetic iron nanoparticles with diameters mainly around 40–55 nm from iron-filled coal-derived hollow carbon rods by arc discharge. These monodispersed carbon-coated iron nanoparticles were found in the soot-like deposits formed around the cathode. The transmission electron microscopy and X-ray diffraction studies show that these nanoparticles are perfectly coated with graphitic carbon shells with a thickness of 20–30 nm. A simple model is proposed to explain the growth of the carbon-coated metal nanoparticles in terms of the properties of coal-derived carbon and the arcing conditions. D 2004 Elsevier B.V. All rights reserved. Keywords: Carbon onions; Coal; Arc discharge; Encapsulated nanomaterials
* Corresponding author. Carbon Research Laboratory, School of Chemical Engineering, Center for Nano Materials and Science, State Key Lab of Fine Chemicals, Dalian University of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, China. Fax: +86 411 8363 3080. E-mail address: [email protected] (J. Qiu). 0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2004.03.006
268
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
1. Introduction In the past decade, the preparation of nanosized magnetic metal particles encapsulated by graphitic carbon shells has been actively pursued. This interest has been chiefly driven by the application potential of these carbon-coated magnetic metal nanomaterials in many fields as diverse as magnetic recording media, magnetic toner in xerography, contrast agent in magnetic resonance imaging and catalyst supports [1,2]. It is well known that metal particles in nanometer size tend to be easily oxidized under oxidizing medium. In the case of carboncoated nanoparticles, the outside carbon coating or shells can protect the nanosized metal particles against oxidization and degradation. Up to now, great efforts have been devoted to the iron-group metal and carbon system in which the majority of the encapsulated materials are found to be in metallic phase, such as bcc-Fe, fcc-Co and fcc-Ni [3–8]. To date, several approaches have been adopted in preparing carbon-coated nanosized metal particles [3,9–13], of which the arc discharge technique is commonly used because of its simplicity and low operating cost. For the conventional arc discharge technique, the preparation of carbon-coated nanomaterials is normally conducted in an inert gas atmosphere such as helium with a metal-filled graphite electrode as anode, resulting in various carbon-coated nanosized metal materials with polyhedral morphologies, of which the diameter or size varies in a wide range from several tens to several hundred nanometers [3]. In practice, those carbon-coated nanomaterials with narrow and uniform size distribution as well as perfect ball-like morphology would be more desirable. Our previous work proved that coal, a cheap and abundant carbon source in nature, is a good starting material for production of fullerenes, carbon nanotubes, nanocapsules, carbon micro-balls and carbon-coated nickel nanocrystals [14–21], which were achieved under different conditions and with or without catalysts. Here we report the preparation of carbon-coated iron nanoparticles with perfect ball-like shape and relative uniform size from composite rods made from a mixture of coal and iron powders.
2. Experimental A typical anthracite from the western part of China was used in this study. The proximate and ultimate analyses of the coal samples are shown in Table 1. The coal sample without any pretreatment was crushed and sieved to 150 Am, and fully dried at 383 K for 12 h before use. The coal powder was mixed with coal tar binder in a coal-to-binder ratio of 80:20% by weight, and subsequently pressed at about 10–20 MPa to form hollow coal rods which were put into an electric furnace and carbonized under flowing N2 to make hollow carbon rods. The furnace was first ramped at 3 K/min to 773 K for 1 h, then, was Table 1 Analysis data of coal sample Proximate analysis (%)
Ultimate analysis (%, daf)
M ad
Ad
V daf
C
H
N
S
O
2.78
3.92
15.18
87.86
3.66
0.73
0.36
7.39
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
269
further ramped at 10 K/min to 1173 K for 4 h before cooling down to room temperature. Finally, hollow carbon rods (O.D. 10 mm, I.D. 5 mm, length 100–120 mm) were obtained. The hollow carbon rods were filled with a mixture of iron and carbon powder, and both ends of the filled rod were sealed with graphite plugs. The carbon powder used here was obtained by re-crushing some of the coal-derived hollow carbon rods as described above. The ratio of iron powder to carbon powder in the mixture was 50:50 by weight. Both the iron and carbon powders had a size smaller than 150 Am. The iron-filled hollow carbon rods were used as the anode in the arc discharge experiments while the cathode was made by a graphite electrode with a diameter of 16 mm and length of 30 mm. The arc discharge experiments were conducted with a direct current of 50–70 A and a voltage of 30–50 V in He at 0.065 MPa. The distance between two electrodes was maintained at about 1–2 mm by manually feeding the anode. After arc discharge, soot-like materials deposited on the inner wall of the arc chamber and surface of cathode were collected and examined by transmission electron microscopy (TEM, JEM-2000EX operated at 100 kV) and X-ray diffractions (XRD, Rigaku D/max 2400, Cu Ka). The samples for TEM observation was dispersed in ethanol with ultrasonic treatment for several minutes, and several drops of the suspension were placed on a carbon-coated copper grid for TEM examination.
3. Results and discussion The TEM was used to scrutinize the soot-like deposits both on the arc reactor inner walls and around the cathode, which were collected after the arc discharge. The TEM study reveals that carbon-coated iron nanoparticles are present in quantity in the deposits around the cathode. Occasionally, similar nanoparticles were also found in the soot-like deposits on the inner chamber walls, but in most cases they were embedded in amorphous carbon globules, which is not uncommon in the case of the arc discharge of graphite-metal composite electrodes [3,6]. It should be noted that in our case with iron and coal-based carbon composite rods as anode, it is in the deposits around the cathode that carbon-coated iron nanoparticles with uniform diameters and nearly perfect sphere-shape were found. The typical TEM images of the carbon-coated iron nanoparticles are shown in Fig. 1. Fig. 1a is a low magnification image, showing that carbon-coated nanoparticles with a nearly perfect ball-like shape are abundant in the deposits around the cathode. It is interesting to note that particles with polyhedral morphologies are rarely observed. The detailed TEM study reveals that the nanoparticles are coated by carbon shells with a thickness of ca. 20–30 nm. The dark area in the core of these particles is iron metal, as shown in Fig. 1, which is confirmed by XRD study. All the nanoparticles seem to be monodispersed, and each of them can be clearly identified. Fig. 1b shows two nanoparticles with similar diameter. It is expected that this kind of discrete state would be of benefit to application of these carbon-coated nanoparticles. Fig. 2 shows the particle size distribution of the encapsulated iron nanoparticles in spherical graphitic shells, which is obtained from the TEM measurements. The diameter distribution of the encapsulated iron nanoparticles is shown in Fig. 2a, and the size distribution of the whole particles including the carbon shells is shown in Fig. 2b. It can be clearly seen that the diameters of carbon-coated iron nanoparticles from coal-based carbon
270
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
Fig. 1. Typical TEM images of carbon-coated iron nanoparticles prepared from iron-filled coal-derived hollow carbon rods. (a) A low magnification image showing many particles with nearly perfect spherical shape; (b) an image of two nanoparticles with similar diameter.
composite rods are in a range of 25–60 nm, and nearly 80% of the particles has a size of ca. 40–55 nm, which are quite narrow and uniform compared with carbon-coated metal nanoparticles prepared from graphite composite electrodes that have a wide size distribution in a range of 20–200 nm [3,6,8]. It has been found that the nanosized iron particles obtained in our experiments are uniformly coated with carbon shells with a
Fig. 2. Histogram of particle size distribution of the carbon-coated iron nanoparticles. (a) The diameter distribution of the carbon-coated iron nanoparticles in the core; (b) the outer diameter distribution of the carboncoated nanoparticles.
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
271
thickness of ca. 20–30 nm. This kind of nanoparticles may be of potential in application fields where the nanomaterials are required to be uniform in size. The typical XRD patterns of the carbon-coated iron nanoparticles prepared by arc discharge of iron and coal-based carbon composites are shown in Fig. 3, from which a sharp peak at 26.28 and a shoulder peak at 25.98 can be clearly seen, corresponding to the (002) plane of hexagonal graphite structure. The value of d 002 for these two peaks is 0.3401 and 0.3427 nm, respectively. This implies that the carbon shells outside the nanoparticles are graphitic layers. In addition to these two graphite peaks, another three strong peaks at 43.28, 44.58 and 50.68 could also be seen in Fig. 3, which can be assigned to the superposition of the g-Fe phase with fcc structure and a-Fe phase with bodycentered cubic (bcc) structure. It should be noted that almost no iron carbide phases are found in these carbon-coated iron nanoparticles. This means that most of the encapsulated iron crystallites or nanoparticles in the core of the carbon shells are in ordinary metallic phase. Similar metal nanoparticles with polyhedral morphology were prepared by Saito [3] and his colleagues, and in their case, graphite-metal composite electrodes were used. It should also be noted that in the XRD pattern shown in Fig. 3, there exist some weaker and broader diffraction peaks that cannot be indexed to both carbon and iron. These peaks might be due to the minerals contained in the coal-derived carbon. In other words, the mineral matter in coal might play a role in the formation of carbon-coated iron nanoparticles, which definitely cannot be ruled out though the detailed mechanism involved is not known at the moment. More work is needed to clarify whether and how the minerals influence the synthesis of carbon-coated iron nanoparticles. In addition to the XRD results, we have found by a very simple experiment that these carbon-coated iron nanoparticles in metallic phase have maintained their magnetic properties. We simply put the carbon-encapsulated iron nanoparticles into water or organic solvents such as acetone and hexane, and applied a magnetic force outside the container, the carbon-coated iron nanoparticles moved along the direction of the magnetic force applied. This means that for the iron nanoparticles encapsulated inside the carbon cages, most of the core crystallites are still in ordinary metallic phase, which is consistent with the above XRD
Fig. 3. X-ray diffraction pattern of carbon-coated iron nanoparticles prepared from iron-filled coal-derived hollow carbon rods.
272
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
results. This spectacular magnetic property of the carbon-encapsulated iron particles would make them an ideal candidate as catalyst supports in magnetically stabilized bed [22,23], where the graphitic carbon layers may work as support of various catalysts while the magnetic core makes the catalysts-supported particles move freely and be dispersed uniformly in the liquid phase catalysis bed when a magnetic force is applied. Therefore, the precious catalysts supported on the carbon-coated magnetic metal nanoparticles would be easily recovered under the effect of magnetic force after the reaction is finished. The TEM and XRD results clearly demonstrate that the nanosized magnetic iron particles coated with uniform graphite shells can be produced from carbon composite rods derived from coal and iron powders. These coal-based carbon encapsulated nanoparticles differ substantially from those graphite-based carbon encapsulated nanoparticles in several aspects. First, all the coal-based nanoparticles have a nearly perfect spherical shape, while the graphite-based nanoparticles normally have polyhedral morphologies. Second, the coal-based nanoparticles have a narrow diameter distribution with about 80% in the range of 40–55 nm, which is much smaller than the graphite-based nanoparticles reported in the literature. Third, almost all of the coal-based nanoparticles exist in discrete or monodispersed state, while the graphite-based nanoparticles tend to be embedded in other carbon impurities. The formation mechanism of these coal-based and carbon-coated iron nanoparticles with perfect ball-like shape is not known because the available information is very limited at the present time. Nevertheless, it must be related to the chemical structure of coalderived carbon. It is known that the chemical structure of coal-derived carbon is very complex and consists of graphite crystallites containing a few domains of 1–10 nm in which the basic units are small graphite crystallite (termed as SGCs thereafter) made of a few graphitic layers. The SGCs are jointed together by relative weak cross-links, and the cross-links would break first under plasma arcing conditions to release some of the SGCs as free particles. Some of the released SGCs might be directly incorporated into the carbon-coated iron nanoparticles without further decomposition as in the case of graphite process in which the graphite needs to be first decomposed into C1 and C2 species [24]. Another possibility that cannot be ruled out is that some SGCs might go through further decomposition to release aromatic species such as polycyclic aromatic hydrocarbons (PAHs) as well as C1 or C2 species. These smaller carbon-containing species are very active, and undoubtedly, would actively take part in the formation reaction of carboncoated iron nanoparticles as the basic building blocks with the presence of iron species. For a better understanding of how the carbon-coated iron nanoparticles are formed from coal-based carbon, a simple model is put forward to illustrate the growth process of the particles, in which the roles played by SGCs, PHAs, C1 and C2 species and iron metal are all taken into account. Fig. 4 schematically shows the growth process of carbon-coated iron nanoparticles from coal-based carbon, which can be divided into three stages. In the first stage as shown in Fig. 4a, a mixture consisting of various carbon species and iron particles that may be in liquid phase or quasi-liquid phase is formed. In the second stage, graphitic layers start to appear on the outside surface of the particles both due to the catalytic effect of Fe metal and the high temperature in the arc zone. These newly formed graphitic carbon shells stack up to wrap or surround the core crystallites, as shown in Fig. 4b. Under the constant bombardment of electrons and ions in arc plasma, the graphite
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274
273
Fig. 4. Schematic illustration of the growth progress of carbon-coated iron nanoparticles. (a) Fe-C alloy particles in liquid phase, in which large carbon fragments including PAHs and SGCs are taken in; (b) particles with polyhedral morphology are formed and the graphitic carbon layers in parallel gradually start to grow from outside of the surface because of the high temperature in the arc and the catalytic effect of iron species, leading to the formation of carbon shells outside the iron nanoparticles, (c) well-developed carbon-coated iron nanoparticles are formed.
layers formed on the surface would realign, and at the same time, large amounts of big carbon fragments such as SGCs and PAHs would be continuously taken in and take part in the formation of carbon shells. In the last stage as shown in Fig. 4c, the carbon-coated iron nanoparticles with a perfect ball-like shape are formed. It is expected that these nanoparticles might possess interesting and attractive properties. In this regard, much work is needed in future.
4. Conclusions In summary, carbon-coated iron nanoparticles with perfect spherical shape and narrow size distribution have been prepared from iron-filled coal-derived carbon rods by arc discharge. The diameters of these encapsulated nanoparticles are in the range of 25–60 nm with the peak centered at ca. 45 nm. The growth mechanism of these particles is discussed in terms of the chemical structure of coal-based carbon and the arcing conditions. A simple model is proposed to schematically show the formation process of the nanoparticles, in which large carbon fragments released from coal-based carbon rods in the arc discharge process may play an important part.
Acknowledgements This work is partly supported by the National Natural Science Foundation of China (Nos. 20376011, 29976006), the Natural Science Foundation of Liaoning Province (Nos. 9810300701, 2001101003), the Education Ministry of China and National Basic Research Program of China (No. 2003CB615806).
References [1] S.A. Majetich, J.O. Artman, M.E. Mchenry, N.T. Nuhfer, Phys. Rev., B 48 (1993) 16845. [2] S. Seraphin, D. Zhou, J. Jiao, J. Appl. Phys. 80 (1996) 2097. [3] Y. Saito, Carbon 33 (1995) 979.
274 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
J. Qiu et al. / Fuel Processing Technology 86 (2004) 267–274 R.S. Ruoff, D.C. Lorents, B. Chan, R. Malhotra, S. Subramoney, Science 259 (1993) 346. B. Bokhonov, M. Korchagin, J. Alloys Compd. 333 (2002) 308. R. Seshadri, R. Sen, G.N. Subbama, K.R. Kannan, C.N. Rao, Chem. Phys. Lett. 231 (1994) 308. M. Tomita, Y. Saito, T. Hayashi, Jpn. J. Appl. Phys. 32 (1993) 280. Y. Saito, T. Yoshikawa, M. Okuda, M. Ohlkohchi, Y. Aodo, A. Kasuya, Y. Nishina, Chem. Phys. Lett. 209 (1993) 72. J.R. Heath, S.C. O’Brien, Q. Zhang, Y. Liu, R.F. Curl, H.W. Kroto, F.K. Tittel, R.E. Smalley, JACS 109 (1985) 359. D. Ugarte, Chem. Phys. Lett. 209 (1993) 99. S.C. Tsang, J.S. Qiu, P.J.F. Harris, Q. Fu, N. Zhang, Chem. Phys. Lett. 322 (2000) 553. Z.D. Zhang, J.G. Zheng, I. Skorvanek, G.H. Wen, J. Kovac, F.W. Wang, J.L. Yu, Z.J. Li, X.L. Dong, S.R. Jin, W. Liu, X.X. Zhang, J. Phys., Condens. Matter 13 (2001) 1921. Z.D. Zhang, J.L. Yu, J.G. Zheng, I. Skorvanek, J. Kovac, X.L. Dong, Z.J. Li, S.R. Jin, H.C. Yang, Z.J. Guo, W. Liu, X.G. Zhao, Phys. Rev., B 64 (2001) 024404. J.S. Qiu, Y. Zhou, L.N. Wang, S.C. Tsang, Carbon 36 (1998) 465. J.S. Qiu, Y. Zhou, Z.G. Yang, L.N. Wang, F. Zhang, S.C. Tsang, P.J.F. Harris, Mol. Mater. 13 (2000) 377. J.S. Qiu, Y. Zhou, Z.G. Yang, D.K. Wang, S.C. Guo, S.C. Tsang, P.J.F. Harris, Fuel 79 (2000) 1303. J.S. Qiu, F. Zhang, Y. Zhou, H.M. Han, D.S. Hu, S.C. Tsang, P.J.F. Harris, Fuel 81 (2002) 1509. Y.F. Li, J.S. Qiu, Z.B. Zhao, T.H. Wang, Y.P. Wang, W. Li, Chem. Phys. Lett. 366 (2002) 544. J.S. Qiu, Y.F. Li, Y.P. Wang, C.H. Liang, T.H. Wang, D.H. Wang, Carbon 41 (2003) 767. J.S. Qiu, Y.F. Li, Y.P. Wang, T.H. Wang, Z.B. Zhao, Y. Zhou, F. Li, H.M. Cheng, Carbon 41 (2003) 2170. J.S. Qiu, Y.F. Li, Y.P. Wang, T.H. Wang, Z.B. Zhao, Y. Zhou, Y.G. Wang, Fuel 83 (2004) 615. R.E. Rosensweig, Science 204 (1979) 57. S. Brandani, G. Astaria, Chem. Eng. Sci. 51 (1996) 4631. P.J.F. Harris, Carbon Nanotubes and Related Structures, Cambridge Univ. Press, Cambridge, UK, 1999.