ISSN 10637761, Journal of Experimental and Theoretical Physics, 2009, Vol. 109, No. 2, pp. 254–261. © Pleiades Publishing, Inc., 2009. Original Russian Text © I.S. Lyubutin, K.V. Frolov, O.A. Anosova, V.S. Pokatilov, A.V. Okotrub, A.G. Kudashov, Yu.V. Shubin, L.G. Bulusheva, 2009, published in Zhurnal Éksperimental’noі i Teoreticheskoі Fiziki, 2009, Vol. 136, No. 2, pp. 302–310.
ORDER, DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Phase States and Magnetic Properties of Iron Nanoparticles in Carbon Nanotube Channels I. S. Lyubutina, *, K. V. Frolova, O. A. Anosovaa, V. S. Pokatilovb, A. V. Okotrubc, A. G. Kudashovc,d, Yu. V. Shubinc, and L. G. Bulushevac aInstitute
of Crystallography, Russian Academy of Sciences, Moscow, 119333 Russia Moscow State Institute of Radioengineering, Electronics, and Automation, Moscow, 119454 Russia cNikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia d Novosibirsk State Technical University, Novosibirsk, 630092 Russia *email:
[email protected] b
Received March 9, 2009
Abstract—The structure, phase composition, and magnetic properties of carbon nanotubes filled with iron nanoparticles and obtained by thermolysis of a mixture of ferrocene and C60 fullerene or ferrocene and orthoxylene at a temperature of 800°C are investigated. Electron microscopy, Xray diffraction, and Möss bauer spectroscopy data lead to the conclusion that carbon nanotubes are multilayer systems partially filled with iron nanoparticles and/or nanorods. Metallic inclusions in nanotube channels form αFe, γFe, and Fe3C phases. The concentration of each phase in the samples is determined. It is shown that 10–20nm iron clusters in nanotubes exhibit magnetic properties typical of bulk phases of iron. High elasticity of carbon nan otube walls facilitates stabilization of the hightemperature γFe phase; the relative concentration of this phase in a sample can be increased by lowering the concentration of ferrocene in the initial reaction mixture. PACS numbers: 76.80.+y, 61.46.Fg, 75.75.+a, 52.50.Bb DOI: 10.1134/S1063776109080093
Carbon nanotubes (CNTs) exhibit unique elec tronic, mechanical, and optical properties, which ren ders them promising systems for many applications [1, 2]. Considerable attention is paid at present to the development of technologies for obtaining CNTs filled with metals or gaseous substances [3]. One of the most interesting applications of these materials is a nano container for magnetically ordered phases [4, 5].
For a high metaltocarbon ratio in the synthesis zone, a fraction of the metal is spent on filling the inner cavity of CNTs, forming metallic nanorods pro tected by graphite envelopes from the action of ambi ent oxygen [14]. The coincidence of the easy magneti zation axis with the CNT axis leads to a larger value of the coercive force of metallic nanoparticles as com pared to bulk materials [15]. This makes it possible to use CNTs filled with a magnetic phase as magnetic force microscope probes for superdense magnetic recording, as well as in xerography.
Catalytic chemical vapor deposition (CCVD) is one simple and easily controllable technique for CNT synthesis [6]. The growth of CNTs in this case can be initiated from organometallic compounds like phtha locyanines [7, 8], carbonyls [9], and metallocenes [5, 10], which are catalyst and carbon sources simulta neously. The advantage of this method is that catalyst particles form in situ, which simplifies the technolog ical procedure of CCDV synthesis. Thermolysis of organometallic compounds results in the formation of arrays of oriented CNTs deposited on the walls of the quartz reactor or on substrates in the reaction cham ber. Samples of oriented CNTs exhibit clearly mani fested anisotropy in their properties and can be used for preparing gas sensors [11], field cathodes [12], supercapacitors [13], as well as in other applications.
Iron nanoparticles encapsulated in CNTs were obtained as a result of decomposition of ferrocene and xylene at T = 700°C [16]. According to transmission electron microscopy (TEM) data, it was proposed that the hightemperature γFe phase and lowtempera ture αFe phase dominate in CNTs, while iron carbide forms near the CNT tip [16]. Xray diffracton was used to establish the close phase composition of nanoparti cles in CNTs, which were synthesized by ferrocene thermolysis at T = 830°C at the surface of oxidized sil icon substrates with nanometerthick iron or cobalt layers deposited on them [17]. However, TEM analysis of individual particles revealed that the metal phase of iron (in particular, γFe) can be concentrated near the CNT tip. “Root” growth of CNT was proposed, in which a nanotube is strongly coupled with the surface of the substrate and its elongation occurs due to the
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addition of carbon atoms to the open upper end [17]. Mössbauer spectroscopy revealed that the phase com position of metallic inclusions may change upon a variation in the ferrocene dissociation temperature and that Fe3C phase forms at a lower temperature (880°C), while the product obtained at T = 950°C contains only the αFe and γFe phases [18]. More over, the ratio of the metallic phases in the sample depends on the material of the surface at which ori ented CNTs grow [18, 19]. The conclusion drawn in [18] concerning predominant formation of Fe3C at lower temperatures of the CCVD synthesis does not agree with the presence of only the αFe and γFe phases in the tubes grown at T = 850°C from a 2.5% solution of ferrocene in cyclohexane [20], nor with the formation of a noticeable amount of Fe3C in the prod uct of dissociation of ferrocene and C60 fullerene at T = 1050°C. Thus, a unified concept of the mechanism of for mation of metallic particles encapsulated in graphite shells of CNTs has not been worked out. To create and optimize the conditions of CCVD synthesis of CNTs, detailed analysis of the structure and phase composi tion of the materials obtained under different condi tions is required. The most effective diagnostic meth ods are highresolution TEM, Xray diffraction, and Mössbauer spectroscopy. In this study, we investigate the structure, phase composition, and magnetic properties of CNTs filled with iron, which were obtained by thermolysis of reac tion mixtures of various compositions. According to Xray diffraction and Mössbauer spectroscopy data, it was found that iron nanoparticles are present in nano tubes in the form of four crystal phases and they exhibit magnetic properties unusual for nanoparticles. A variation in the phase ratio upon a change in the fer rocene concentration in the reaction mixture has been detected. 2. SAMPLE SYNTHESIS AND EXPERIMENTAL TECHNIQUE Samples of CNTs filled with iron were obtained by the CCVD method on the setup described in detail in [22, 23]. The source of catalyst was (C5H5)2Fe fer rocene; CNTs grew from C60 fullerene (sample I) or C6H4(CH3)2 orthoxylene (sample II). Silicon sub strates 12 × 12 mm in size were placed into the synthe sis zone and annealed in air at T = 800°C for 30 min for the formation of a thin oxide layer on the surface. The reactor was then evacuated and filled with argon. During the synthesis of sample I, an alundum boat containing a mixture of ferrocene and C60 fullerene in equal amounts was introduced into the reactor with a manipulator and was placed directly under the sub strates. In the synthesis of sample II, ferrocene solu tion (2% by mass) in orthoxylene was injected into the synthesis zone with the help of controllable pressure on a piston in the outer chamber containing the liquid
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reaction mixture. The supply system was adjusted to an injection of 1 cm3 of the initial reaction mixture with a period of 15 min. CNT synthesis was carried out at T = 800°C in an argon flow (500 cm3/min) under atmospheric pressure for 1 h. The carbon material formed on silicon substrates and quartz tube walls in the form of a dense black film. The structure of the samples was characterized using scanning electron microscopy (SEM) on a LEO EVO 40 device and highresolution TEM on a JEM 2010 microscope. Xray diffraction measurements were performed on a DRON SEIFERT RM4 spec trometer (CuKα radiation, graphite monochromator). Diffraction patterns were recorded in the step mode in a range of angles 2θ from 20° to 70°. The Mössbauer absorption spectra from 57Fe nuclei of the sample were recorded at 80 and 295 K using a standard MS1100Em spectrometer operating in the constant acceleration mode. The source of γ quanta (57Co(Rd)) was at room temperature. The CNT film was mechanically separated from the substrate and ground in the form of a thin layer, which was pressed between two 30µmthick aluminum foils. Isomeric shifts were measured relative to the calibration αFe sample (18µm thick foil annealed in hydrogen) at room temperature. Computer processing of the spec tra was carried out using Univem MS software. 3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1. ElectronMicroscopic Studies Figure 1 shows the images of lateral splits of the samples obtained by SEM. It can be seen that CNT arrays have a transverse orientation relative to the sil icon substrate surface. The CNT length corresponds to the array thickness and is virtually the same for sample I (about 23 µm) and sample II (about 24 µm). According to SEMbased estimates, both samples are also characterized by close values of average diameters of CNTs. The structure of individual CNTs in sample I was analyzed using the TEM technique. The choice of this sample was dictated by a high concentration of fer rocene (the source of metal in the initial reaction mix ture). The image obtained with a smaller magnifica tion using TEM (Fig. 2a) shows that the sample con tains multilayer CNTs with the inner cavity partly filled with metal. Metallic inclusions are represented by nanoparticles and/or nanorods. The outer diameter of CNTs varies from 24 to 40 nm, while the diameter of the inner cavity confining the size of inclusions is 14–20 nm (Figs. 2b and 2c). The linear dimensions of metallic nanorods vary over a wide range and attain several hundred nanometers. In addition, a small number of rounded metallic nanoparticles are also present outside CNTs (Fig. 2d). The structure of dif fraction fringes in the image of a metallic inclusion
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Fig. 1. Microphotographs of the lateral surface of CNT arrays by scanning electron microscopy. CNTs were grown on silicon sub strates as a result of dissociation of (a) ferrocene–C60 fullerene and (b) ferrocene—orthoxylene mixtures.
near the tip of a CNT (Fig. 2c) corresponds to iron carbide Fe3C. The length of a monocrystalline Fe3C rod exceeds 45 nm. Fixation of the carbide state of iron at the end of a CNT can be explained by the nanotube growth mechanism in the course of CCVD, in which the formation of the tubular form of graphite takes place after saturation of a metallic particle with carbon [24]. 3.2. Xray Diffraction Xray diffraction profiles revealed the presence of several phases in synthesized samples (Fig. 3). The reflection at 2θ ≈ 26° corresponds to (002) reflections from graphite layers of a CNT; its dominant intensity in the spectrum of sample II indicates a higher con centration of carbon particles in the material as com pared to metallic particles (Fig. 3b). Conversely, the peak from the αFe phase at 2θ = 44.7° dominates in sample I. The high intensity and small width of this peak (see Fig. 3a) suggest that αFe particles exhibit preferred anisotropy along the (110) crystallographic axis; consequently, nanorods in a CNT are repre sented by the αFe phase. In addition to the αFe phase, the diffraction pattern for sample I shows a set of reflections in the 2θ interval from 37° to 60° from the Fe3C iron carbide lattice, as well as reflections
from oxide phases of iron corresponding to the spinel structure (magnetite Fe3O4 or/and maghemite γ Fe2O3). The peaks observed at small angles (2θ < 23°) correspond to C60 fullerene, which was not consumed for the CNT formation. As compared to sample I, the Xray diffraction profile for sample II contains high intensity peaks from Fe3C, which are comparable in height with the (110) reflection from αFe, while peaks from iron oxide are almost completely absent. The peak at 2θ = 50.7° in the diffraction pattern of sample II can be attributed to γFe. Thus, analysis of Xray diffraction data leads to the qualitative conclu sion that iron in sample I is mainly represented by the αFe phase, while phases αFe and Fe3C in sample II are in almost equal amounts. An exact quantitative estimate of the phase ratio in the samples was obtained from Mössbauer spectroscopy data. 3.3. Mössbauer Spectroscopy Data The Mössbauer spectra of samples I and II mea sured at room temperature are shown in Fig. 4. The decomposition of the spectra into components revealed the presence of three sextets and one singlet. The sextets correspond to magnetically ordered phases, while the singlet indicates the presence of a nonmagnetic form of iron. The spectra of both sam
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Fig. 2. TEM microimages of individual nanoparticles from the CNT array obtained by dissociation of the ferrocene–C60 fullerene mixture.
ples display identical sets of resonance lines and differ only in the intensities of each component (Figs. 4a and 4b). A decrease in temperature to T = 80 K does not change the spectral characteristics qualitatively (see Fig. 5). As expected, cooling leads to a slight increase in magnetic field Heff and in the value of isomeric shift (IS) due to the temperature shift (secondorder Dop pler effect). No new magnetic components appear. The values of hyperfine interaction parameters for var ious spectral components (namely, hyperfine mag netic field Heff at iron nuclei, isomeric chemical shift IS, quadrupole shift QS, relative area S of the compo nent) are given in Table 1. Comparison of parameters with the available data for possible crystal phases based on Fe–C and Fe–O revealed that the most intense magnetic component corresponds to the αFe metallic phase (see Fig. 4). The next component (on the intensity scale) corre sponds to the iron carbide phase (Fe3C cementite). The component with a low intensity and with the highest magnetic field at iron nuclei corresponds to γFe2O3 iron oxide (maghemite). It should be noted
that the hyperfine interaction parameters for γFe2O3 and αFe2O3 oxides are close; for this reason, it is dif ficult to determine the type of iron oxide in nanopar ticles unambiguously. However, the crystal structures of these oxides are different: γFe2O3 maghemite has a cubic structure of the spinel type, while αFe2O3 hematite has an rhombohedral structure of the corun dum type. Xray, diffraction profiles (see Fig. 3) indi cate the presence of the spinel structure, but do not reveal the corundum structure. Consequently, we can assume with confidence that the magnetic component with a low intensity in the Mössbauer spectrum belongs to the γFe2O3 phase. At the same time, the Mössbauer data indicate the absence of Fe3O4 magne tite in both samples, which was predicted by Xray dif fraction data. The nonmagnetic singlet (Figs. 4 and 5) corresponds to the γFe phase with a facecentered cubic (fcc) structure. The concentration of each phase in samples I and II can be estimated quantitatively from the areas under the resonance lines under the assumption that the probability of the Mössbauer effect is the same for the
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(a) 3
Intensity
1 44
(b)
4
3 3 3 33
3 33
3
3
3 3
3
1 3 2 3 3 3 3
335
33
20
30
40
50
60 2θ, deg
Fig. 3. Xray diffraction profiles of (a) sample I obtained by thermolysis of a ferrocene–C60 fullerene mixture and (b) sample II synthesized from a 2% solution of ferrocene in orthoxylene. Reflections denoted by 1, 2, 3, 4, and 5 cor respond to carbon in CNT, αFe, Fe3C, Fe2O3, and γFe, respectively.
iron phases established for these samples. Analysis shows that approximately half the amount of iron in synthesized CNTs is in the pure metallic state, while
Fe3C
Relative absorption
αFe
C60 T = 295 K
(a)
γFe
−12
−8
−4
0
4
8 12 Velocity, mm/s Fe3C
αFe
Relative absorption
γFe2O3
γFe2O3
Xylene T = 295 K
(b)
γFe
−12
−8
−4
0
4
8 12 Velocity, mm/s
Fig. 4. Mössbauer spectra from 57Fe nuclei measured at room temperature in (a) sample I obtained by thermolysis of ferrocene and C60 fullerene and (b) sample II synthe sized from a 2% solution of ferrocene in orthoxylene.
the remaining part corresponds to the mixture of γFe austenite iron, Fe3C cementite, and γFe2O3 iron oxide. The relative amounts of ironcontaining phases obtained from the spectra recorded to a higher degree of accuracy at room temperature are given in Table 2. Nanoparticles of the αFe, γFe2O3, and Fe3C phases are magnetically ordered both at nitrogen tem perature (80 K) and at room temperature (295 K). The γFe phase remains nonmagnetic upon cooling to 80 K. In accordance with the Mössbauer data, the αFe fcc phase dominates in the CNT channels. With allow ance for the TEM data (see Fig. 2), we can conclude that it is this phase that forms extended clusters and nanowires in CNTs. The iron carbide (Fe3C) phase is mainly concentrated at the tips of nanotubes (see Fig. 2c). This phase was observed by many authors [16, 19, 25, 26] and is apparently essential in the syn thesis of Fecontaining CNTs. It was shown in [16] that the Fe3C phase is very important in the growth mechanism of a carbon nanotube with iron and is mainly concentrated at the tips of nanotubes. We can assume that nanoparticles of γFe2O3 iron oxide are not in the channels, but outside CNTs. Indeed, TEM images in Fig. 2d indicate the presence of rounded metallic particles that are not coated with graphite envelopes. During storage of a sample in lab oratory conditions, the unprotected surface of the metal will probably be oxidized by atmospheric air. The presence of the γFe austenite phase at room and even nitrogen temperatures appears peculiar. Under normal conditions, this phase may exist in bulk iron at temperatures above 912°C and/or under a high pressure [27]. Under atmospheric pressure and at room temperature, the stable phase of iron is the bcc αFe structure (space group Im3m). At T = 912°C, it is transformed into the fcc γFe structure (space group Fm3m) [28]. The αFe–γFe (bcc–fcc) structural transition under a high pressure is also known [27]. Thus, our experiments show that the hightemper ature γFe phase can be formed in the CNT channels
Relative absorption
258
Fe3C
αFe
γFe2O3
C60 T = 80 K
γFe
−12
−8
−4
0
4
8 12 Velocity, mm/s
Fig. 5. Mössbauer spectrum from 57Fe nuclei measured at 80 K in sample I obtained by thermolysis of ferrocene and C60 fullerene.
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Table 1. Hyperfine interaction parameters obtained from analysis of the Mössbauer spectra from the 57Fe nuclei in carbon nanotube samples synthesized by dissociation of the ferrocene–C60 fullerene mixture (sample I) and ferrocene–orthoxy lene mixture (sample II) Sample Sample I (295 K)
Phases
IS, mm/s (±0.01)
QS, mm/s (±0.01)
Heff, kOe (±1.0)
S, % (±2.0)
αFe
0.00
0.00
331.0
57.0
Fe3C
0.20
0.02
209.3
22.2
Fe2O3
0.33
–0.05
496.2
9.2
–
–
11.6
γFe
–0.09
αFe
0.11
0.01
335.0
56.6
Fe3C
0.30
0.00
245.4
21.0
Fe2O3
0.42
–0.07
511.2
9.8
γFe
0.02
–
–
12.6
Sample II (295 K) αFe
0.00
0.01
330.5
51.6
Fe3C
0.19
0.03
208.5
28.1
0.23
0.04
498.7
4.9
–
15.4
Sample I (80 K)
Fe2O3 Sample II (80 K)
γFe
–0.09
αFe
0.10
0.03
334.5
52.7
Fe3C
0.31
–0.01
245.3
26.6
Fe2O3
0.59
–0.11
513.0
5.0
γFe
0.04
–
–
15.7
even when the CNT synthesis temperature is below the α–γ transition point [28], and this phase can be stable under normal conditions also. Probably, ele vated pressure is created during synthesis in CNT channels, which facilitates the formation of the γFe phase at lower temperatures and its stabilization under normal conditions. After the completion of synthesis, a transition from the αFe to γFe phase should take place as a result of cooling; however, this transition is hampered for nanoparticles in a CNT cavity since the unit cell volume in αFe is 9% larger than the volume of the γFe unit cell. Owing to the high elasticity of CNTs [29], part of the γFe phase is preserved at room temperature immediately at the contact with the inner wall of the carbon tube [30]. This assumption is con firmed by the large value of the γFe/αFe phase ratio for sample II (see Table 2); the concentration of fer rocene in the initial reaction mixture used for synthe sizing this sample was much smaller than in the syn thesis of sample I. The average size of metallic inclu sions, which is determined by the concentration of ferrocene vapor in the reaction chamber, is smaller in sample II as compared to sample I. For this reason, we can expect a higher relative concentration of the γFe component in small Fe particles in view of the fact that the γFe phase must be in contact with the CNT wall. As the temperature is lowered to 80 K, the Möss bauer component of γFe does not experience mag netic splitting (see Fig. 5), indicating that the γFe phase in CNT channels remains nonmagnetic. How
–
ever, the width of the singlet resonance line at T = 80 K becomes 1.5 times larger than at T = 295 K, which may indicate the onset of the transition to the antiferro magnetic state [31]. An exchangeinduced anisotropy may appear at the interface between the ferromagnetic αFe phase and the antiferromagnetic γFe phase [32], which may lead to anomalous hysteretic proper ties [25]. Another interesting and peculiar property of iron nanoparticles in CNT channels discovered from the results of Mössbauer experiment is worth noting. Nanoparticles of iron compounds with a size on the order of 10–20 nm usually exhibit superparamagnetic or paramagnetic properties at room temperatures [33, 34]. However, our experiments revealed that nanosize Table 2. Relative amount of ironcontaining phases in car bon nanotube samples grown on silicon substrates as a result of dissociation of the ferrocene–C60 fullerene mixture (sample I) and ferrocene–orthoxylene mixture (sample II). Data have been obtained from Mössbauer spectra recorded at room temperature Iron phase Sample
αFe
Fe3C
γFe2O3
γFe
I
0.57
0.22
0.09
0.12
II
0.52
0.28
0.05
0.15
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iron clusters in CNTs exhibit magnetic properties typ ical of bulk phases of iron. This means that blocking temperature Tb for thermal fluctuations of the mag netic moments of iron (or the Curie/Néel tempera ture) is much higher than 300 K, which can be due to a strong exchange interaction between iron atoms in the CNT channels. The strong exchange interaction along a nanorod may lead to a 1D magnetic ordering. It was shown in [35, 36] that effective magnetic anisot ropy in 1D exchangecoupled systems of ferromag netic nanoparticles is stronger than in their 2D and 3D analogs. It has been established experimentally that coercive force Hc of quasionedimensional iron nanofilaments may be an order of magnitude higher than the value of Hc for bulk materials, which is very important for practical applications of nanofilaments [37]. It should be noted in conclusion that in analyzing the properties of CNTs containing iron nanoparticles in internal channels, the multiphase nature of iron compounds in CNTs should be taken into account. The application of Mössbauer spectroscopy in the study of phase states and magnetic properties of such systems is very effective and provides information that can obviate the use of other methods. ACKNOWLEDGMENTS The authors are grateful to A.V. Ishchenko, who performed TEM testing of the samples. This study was financially supported by the Russian Foundation for Basic Research (project no. 0603 32802) and the programs “Development of Higher Educational Potential” (project no. 2.1.2/5257) and “Nanotechnologies and Nanomaterials” of the Pre sidium of the Russian Academy of Sciences (project no. 274.1.10). REFERENCES 1. Yu. V. Lozovik and A. M. Popov, Usp. Fiz. Nauk 167 (7), 751 (1997) [Phys.—Usp. 40 (7), 717 (1997)]. 2. A. V. Eletskiі, Usp. Fiz. Nauk 167 (9), 945 (1997) [Phys.—Usp. 40 (9), 899 (1997)]. 3. M. Terrones, Annu. Rev. Mater. Res. 33, 419 (2003). 4. F. Banhard, N. Grobert, M. Terrones, J.C. Charlier, and P. M. Ajayan, Int. J. Mod. Phys. B 15, 4037 (2001). 5. C. N. R. Rao, R. Sen, B. C. Satishkumar, and A. Govindaraj, Chem. Commun. 15, 1525 (1998). 6. E. G. Rakov, Ross. Nanotekhnol. 3 (9–10), 89 (2008) [Nanotechnol. Russ. 3 (9–10), 575 (2008)]. 7. S. Huang, L. Dai, and A. W. H. Mau, J. Phys. Chem. B 103, 4223 (1999). 8. X. Wang, W. Hu, Y. Liu, C. Long, Y. Xu, S. Zhou, D. Zhu, and L. Dai, Carbon 39, 1533 (2001). 9. S. Liu, X. Tang, Y. Mastai, I. Felner, and A. Gedanken, J. Mater. Chem. 10, 2502 (2000). 10. R. Sen, A. Govindaraj, and C. N. R. Rao, Chem. Phys. Lett. 267, 276 (1997).
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JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS
Translated by N. Wadhwa
Vol. 109
No. 2
2009
