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Technology for production of magnetic carbon nanopowders doped with iron and cobalt nanoclusters
Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Technology for production of magnetic carbon nanopowders doped with iron and cobalt nanoclusters T. Gеgechkori a, G. Маmniashvili a, E. Kutelia b, L. Rukhadze b, N.I. Maisuradze b, B.G. Eristavi b, D.I. Gventsadze b, А. Аkhalkatsi c, T. Gavasheli c, D. Daraselia c, D. Djaparidze c, А. Shengelaya c a
Ivane Javakhishvili Tbilisi State University Andronikashvili Institute of Physics, 6 Tamarashvili Street 0177 Tbilisi, Georgia Georgian Technical University, 69, Kostava Street Tbilisi 0175, Georgia c Ivane Javakhishvili Tbilisi State University, 3, Chavchavadze Avenue, Tbilisi 0128, Georgia b
art ic l e i nf o Article history: Received 4 December 2013 Received in revised form 13 March 2014 Accepted 6 May 2014 Keywords: Carbon nanopowder Magnetic clusters Core–shell Iron Cobalt Doped nanoparticles Scanning electron microscope Auger electron spectroscope Vibrating sample magnetometer NMR
a b s t r a c t Q3 Complex scanning electron microscope and Auger electron spectroscope structure and composition measurements, along with the vibrating sample magnetometer and NMR studies, were carried out on carbon nanoparticles doped with magnetic clusters, which were synthesized by a technology which combines the ethanol vapor pyrolysis method and the chemical vapor deposition process in a horizontal continuous reactor with certain temperature gradients and controlled partial oxygen pressure. The structure and composition data of the synthesized magnetic carbon nanopowders showed that the nanopowders consisted of randomly distributed carbon nanoparticle aggregations that are 200 nm in diameter doped with magnetic clusters. The magnetometry and NMR data are in agreement with the results of the structure analysis, pointing to the existence of a significant superparamagnetic contribution to the synthesized carbon nanoparticles doped with cobalt nanoclusters. & 2014 Published by Elsevier B.V.
1. Introduction The study of carbon nanoparticles (CNP) and nanotubes (CNT) promises to be an important field that could develop a new age of ultrastrong, light, conductive and versatile materials. Interest in these materials has been permanently growing in the last 20 years in connection with the development of nanotechnologies for controlling the size and shape of nanoparticles as well as creating ways for modification of their properties by doping with different atoms and clusters [1–3]. Particular interest is associated with the fabrication of core– shell structured nanocomposites on the CNP and CNT bases doped with ferromagnetic clusters for the synthesis of nanoscale functional materials which generate properties that are not available in any other materials [3]. In [4] the possibility of the synthesis of carbon nanopowders doped with magnetic clusters of iron oxide and cobalt atoms was studied using a novel technology which combines the ethanol
E-mail address: [email protected] (T. Gеgechkori).
vapor pyrolysis method and the chemical vapor deposition (CVD) process. In [5] the features used for the synthesis technique were briefly summarized, which are partially different from those described in the literature methods based on pyrolysis. The main features of our method are as follows: 1. the substrate (catalyst) is a massive polished plate (iron or cobalt), instead of metal powder; 2. the synthesis was carried out at a partial oxygen pressure of 10 25 Torr, for which an oxygen pump was included in a closedloop cycle. The carbon nanopowders synthesized on ground iron and cobalt plates consist of core–shell structured nanocomposite particles in the form of ferromagnetic cluster cores of the corresponding atoms coated with pyrocarbon shells. The carbon shells offer protection to the ferromagnetic impurities (cores) as well as introduce new properties to the hybrid nanostructures. Therefore, the possible field of application of magnetic carbon nanopowders is enormous. But it should be noted that the nanoscale magnetism of carbon nanoparticles doped with ferromagnetic clusters and magnetic atom-filled carbon nanotubes is poorly known. Aimed at better understanding of the structure–property relationship and the development of novel functional materials, the study of the nanoscale ferromagnetic materials has attracted
http://dx.doi.org/10.1016/j.jmmm.2014.05.007 0304-8853/& 2014 Published by Elsevier B.V.
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67 compressor (4) connected in series, an oxygen pump with a partial 68 oxygen pressure gage (5), an ethanol container (6) and a valve 69 (7) for releasing (or collecting) excess hydrogen. 70 The synthesis process was carried out in the mode of recircula71 tion of the main reagents, which are products of ethanol pyrolysis 72 in a closed-loop cycle at a monitored partial pressure of oxygen. The samples for analysis were taken from synthesized nanopow- Q5 73 74 ders obtained in a free poured state after they were shaken off 75 from the surfaces of the iron and cobalt catalyst plates. 76 The morphology and particle distribution of the synthesized 77 carbon nanopowders and the surfaces of the catalyst plates were 78 examined using a DSM-960 SEM (Opton, Germany). The chemical 2. Experimental procedure 79 composition and element distribution in the metal-doped carbon 80 nanopowders were examined by the Auger Electron Spectroscopy The synthesis of carbon nanopowder samples doped by iron 81 (AES) method using the LAS-2000 instrument (Riber, France). The and cobalt atoms was realized on surfaces of ground iron and 82 magnetometry measurements of the synthesized nanopowders cobalt plates placed in the temperature regions of 1200 1C, 700 1C 83 were carried out using a VSM (Cryogenic Limited, USA) making it and 500 1C in a horizontal continuous reactor of the experimental 84 possible to carry out the measurements over a temperature range setup [4]. 85 of 1.7–293 K and magnetic fields up to 5 T. NMR characterization The scheme of the proposed technology combining the pyr86 of the cobalt-doped carbon nanopowders was conducted using a olysis of ethanol vapors and the process of CVD is shown in Fig. 1. 87 magnetic video-pulse excitation technique [6]. This figure also depicts the temperature distribution in the 88 active zone of the reactor. During the experiment, when the 89 furnace is in the process of forming the magnetic powder, its 90 temperature is measured by a thermocouple located near the 91 center of the reaction pipe at a distance of 2 mm from the wall. 3. Results and discussion 92 The process setup is a closed-loop system which consists of a 93 horizontal pipe furnace (1) to provide the given temperature Representative SEM images of the prepared carbon nanopowders 94 distribution in the active section of the reactor (2). A platedoped with cobalt atoms synthesized on the surface of cobalt plates 95 shaped iron or cobalt catalyst (3) is placed in this section. The at temperatures of 1200 1C and 500 1C are shown in Fig. 2a and 2b, 96 circulation circuit of the reactants with the reactor consists of a respectively. The nanoparticles of the powder synthesized at 1200 1C 97 are individual uniform spheroids of 200–250 nm in diameter 98 (Fig. 2a), whereas the nanoparticles of the powders synthesized at 99 500 1C have smaller dimensions with a large diameter distribution 100 over the range of 70–150 nm (Fig. 2b), which in this case is the result 101 of the self-assembly processes of smaller (r50 nm) carbon nano102 particles doped with cobalt atoms (clusters). 103 In Fig. 3a, b and Fig. 3c, d SEM images of the doped carbon 104 nanopowders synthesized on the surface of iron substrate-catalyst in 105 the presence of oxygen at 1200 1C and 700 1C, respectively, are shown. 106 As is seen, in both cases carbon nanopowders doped by atoms 107 (clusters) of iron oxide consist of randomly distributed and 108 individually joined short-range ordered uniform nanoparticles of 109 spherical aggregations 150 nm in diameter, as result of the self110 assembly process of smaller ( 30⧸50 nm) nanoparticles. During 111 this process the morphology and size distribution of carbon 112 nanoparticles (doped with iron oxide clusters) are practically the 113 same as those of the nanoparticles synthesized at 1200 1C and 114 Fig. 1. Experimental setup used to produce the magnetic carbon nanopowder. 700 1C (comparing Fig. 3b and Fig. 3a). 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 Fig. 2. SEM images of magnetic carbon nanopowders doped with cobalt atoms synthesized at 1200 1C, nanoparticle diameter 200–250 nm (a) and 500 1C, nanoparticle 132 diameter 70–150 nm (b). considerable attention in recent years due to their potential applications in many fields such as sensors, nanolubricants, nano-biotechnology, and magneto-optical devices. In relation to the above-noted information, the aim of this work is the complex study of the morphology, size, chemical composition and magnetic properties of carbon nanopowders doped by iron and cobalt atoms using such modern analytic instruments as a scanning electron microscope (SEM), an Auger electron spectroscope (AES), a vibrating sample magnetometer (VSM) and NMR.
Please cite this article as: T. Gеgechkori, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.05.007i
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Fig. 3. SEM images of magnetic carbon nanopowders doped with iron clusters, synthesized at 1200 1C, nanoparticle diameter 100–200 nm (a and b) and 700 1C, nanoparticle diameter 100–150 nm (c and d).
The chemical composition and thickness of the carbon shell on ferromagnetic dopant impurities in nanoparticles synthesized at 1200 1C on the surface of iron and cobalt substrates were defined from differential Auger spectra recorded for the same part of sample surface according to the method used in [4]. A layer-by-layer analysis was conducted using a technique of spraying with argon ions accelerated up to 1 eV energy. The selected density of the ion stream allowed the removal of layers at the speed of approximately 10 Å/min. The layer-by-layer analysis was stopped at the appearance of significant-intensity triple L3VV transition peaks of iron and cobalt atoms in the Auger spectra, and an evaluation of carbon shells' thicknesses on the magnetic cluster cores was made. In Fig. 4a and b differential Auger spectra are shown in the energy range 100–900 eV recorded before the moment of appearance on them of the intense Auger transition peaks of iron and cobalt, respectively. The appearance of triple L3VV transition peaks of iron atoms with the simultaneous appearance of the KL2.3L2.3 peaks of the Auger transition of oxygen atoms for the carbon nanopowder synthesized on the iron substrate surface took place after a 20-min exposure of the sample surface to the argon ions (Fig. 4a). The appearance of the significant-intensity triple L3VV peaks of cobalt atoms in the Auger spectrum of carbon nanopowder synthesized on the cobalt substrate took place after a 100-min exposure to argon ions (Fig. 4b). The absence of any other peaks, besides the KL2.3L2.3 intensive peak of the Auger transition of carbon atoms, and the above-mentioned Auger peaks of iron, oxygen and cobalt atoms, points to the fact that the synthesized carbon nanopowders doped with iron and cobalt are of high purity, and their composition corresponds to the realized conditions of the synthesis. It also points to the fact that the nanoparticles in both synthesized powders have the structure of a core–shell type, which was formed as a result of
the interaction of pyrolysis products of ethanol with the surface of substrate – the catalyst according to the gas “corrosion” mechanism at high temperatures. To such a mechanism of formation of doped carbon nanoparticles points also the SEM observation of ground plate-substrate surface after the interaction with the products of ethanol pyrolysis at 1200 1C. So, in Fig. 5a and b different magnification SEM images are presented of the ground iron plate-substrate surface after shaking off the carbon nanopowder synthesized at 1200 1C. It is seen that, along with traces of grinding scratches, at relatively small magnification (Fig. 5a) the surface has a typical microstructure which is a characteristic of the gas corrosion interaction, and at large magnifications (Fig. 5b) the nanoparticles remained bonded by nanobridges with the substrate surface, which are commensurated with nanoparticles in the freely poured doped carbon powder. Along with the structural and chemical characteristics of the samples, great significance is attributed to the data on their magnetic properties, because they define the most interesting practical applications of magnetic nanostructures. The VSM magnetization measurements of the synthesized carbon nanopowders doped with iron and cobalt atoms were carried out in the zero-field-cooled (ZFC) and field-cooled (FC) modes with the curves taken over the range 3 K to 296 K for different values of cooling field HFC. The ZFC data were obtained first by cooling the sample from room temperature to the lowest temperature of 3 K in the zero applied field. Then a magnetic field was applied and the magnetization was measured with increasing temperature, whereas in the FC mode the sample was cooled from room temperature to the basal temperature in a magnetic field. The results of the VSM measurements of the ZFC and FC dependences and the magnetization hysteresis curves are shown in Fig. 6a and b and Fig. 7a and b for the magnetic carbon nanopowders doped with cobalt atoms and synthesized at 1200 1C and 500 1C, respectively.
Please cite this article as: T. Gеgechkori, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.05.007i
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Fig. 4. Auger spectra of the synthesized carbon powder doped with iron oxide clusters (a) and cobalt atoms (b) after the exposure to argon ions accelerated up to 1 keV for 20 min (a) and 100 min (b), respectively. Synthesis temperature is 1200 1C.
Fig. 5. SEM images of ground iron plate after shaking off the doped carbon nanopowder particles synthesized at 1200 1C from its surface.
The NMR experiments were carried out using an incoherent spin-echo spectrometer at liquid nitrogen temperature [6]. The experimental conditions were chosen to be optimal for the intensities of the echo signals to be studied. Fig. 8 shows the obtained NMR spectra of the cobalt-doped carbon nanopowder composite synthesized at 1200 1C (sample S1, o) and the coarse-grained cobalt powder taken at optimal RF pulse durations (sample S2, ♥) with a mean grain size D of 10 μm synthesized by an alloying method [7].
As is known [8] the NMR spectra of the coarse-grained Co at 77 K contain two intensive peaks at frequencies of 216 and 218.6 MHz, corresponding to nuclei arranged in the centers of the domain walls of FCC and the edges of the domain walls of HCP phases, and a peak at 225 MH. z corresponding to the nuclear located in the centers of domain walls of HCP phase. The shape of the NMR spectra points to the fact that in cobalt nanopowder (S1) prevails over the FCC phase, as well as in coarse-grained powder. For the characterization of microscopic magnetic properties of MNP we use the method of magnetic video-pulse excitation [6] where the influence of a magnetic videopulse field (pulsed magnetic field) on the nuclear spin echo in magnetic materials was studied. The investigation includes the determination of nuclear spin echo dependences in the sample on the amplitude Нd and duration τm of magnetic video-pulse (MVP) applied in the interval between RF pulses or between the second RF pulse and echo signal, as well as during the RF pulse action (Fig. 9). The decrease of echo signal intensity at the switching-on of MVP in intervals between RF pulses or between the RF pulse and echo signal is related to the breakdown of phase coherence in a system of processing nuclear moments because of a change in the local nuclear frequencies caused by hyperfine field anisotropy under the action of MVP displacing the domain walls. The switching-on of MVP symmetric with respect to the second RF pulse results in the fact that the first and the second RF pulses successfully excite nuclei changing their positions in the domain walls defining their resonance frequencies and the RF field gain factor (η) [7]. At sufficiently short RF pulses the resonance frequency change could be neglected but the change of η results in the decrease of echo intensity. In [6] the influence of the timing and frequency spectra of MVP on the TPE signals in magnets was studied. It was shown that the character of the spectrum is mainly defined by the hyperfine field anisotropy of the investigated nuclei and domain wall mobilities. By recording the echo signal intensity dependence on the amplitude Нd (I(Нd)) one could determine quantitatively the value of MVP amplitude causing the domain wall to shift by an amount equal to its effective thickness. This fact enables a simple and quantitative assessment of the coercive force of a sample for nuclei contributing to the NMR Hd spin echo signal intensity. In Fig. 10 the experimental data are presented showing the echo signal intensity dependence on Нd for cobalt-doped carbon
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Fig. 6. Temperature dependence of magnetization of magnetic carbon nanopowder doped with cobalt atoms (Co–CN) synthesized at 1200 1C in zero-field cooled (ZFC) (curve 1) and field cooled (FC) (curve 2) cases at magnetic field 15 Oe. (a); Hysteresis curve of Co–CN sample (b).
Fig. 7. Temperature dependence of the magnetization of magnetic carbon nanopowder doped with cobalt atoms (Co–CN) synthesized at 500 1C in zero-field cooled (ZFC) (curve 1) and field cooled (FC) (curve 2) cases at magnetic field of 15 Oe. (a); Hysteresis curve of Co–CN sample (b).
Fig. 9. The scheme of magnetic video-pulse action on the TPE signal. The RF marks the timing and duration of RF pulse and Hd shows the amplitude and duration of magnetic video-pulse. The case of symmetric MVP action. Asymmetric action corresponds to MVP application between RF pulses.
Fig. 8. NMR echo spectra for Co-doped carbon nanopowder composite (S1, ○) and coarse-grained cobalt powder (S2, ♥) taken at optimal RF pulse durations.
nanocomposite S1. For comparison, similar data for coarse-grained Co powder with a mean grain size D of 10 μm synthesized by the alloying method [7] are presented, too. In [9] the NMR spectrometer used by us was described and provided with the additional MVP unit capable of generating MVP with amplitudes up to 500 Oe and durations up to 10 μs. A comparison of echo intensity dependences on the amplitude of exciting MVP for MNP and coarse-grained Co powder (Fig. 10a and b) makes it possible to easily reveal two important
peculiarities: first, the degree of symmetric MVP influence on MNP echo signal is lower and, second, the degree of asymmetric MVP influence on MVP echo signal is comparatively close to the one for coarse-grained Co (S2) allowing for lower the domain walls' mobility in it. This lower suppressive capability in Co-doped carbon nanopowder composite as compared with Co coarse-grained powder could be related to the significant increase of coercive force in that part of the cobalt nanopowder which contributes to the NMR signal. As is known [10,11], the decrease of cobalt powder grain size from values of the order of 1 μm down to 150 nm is accompanied by the significant increase of coercive force of the sample related to the decreasing sizes and increasing role of surface effects. This fact is also reflected in the signal echo intensity dependence on the value of the outer magnetic field. In all samples the echo signal intensity practically does not change
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Fig. 10. Two-pulse echo intensity dependences on MVP amplitude Нd for symmetric (a) and asymmetric (b) influences for Co powders at optimal NMR frequencies and RF pulse parameters. Im is echo amplitude at Hd ¼0. Co-doped carbon nanopowder composite (S1, o) and coarse-grained cobalt powder (S2, ♥)
Fig. 11. NMR echo intensity dependence on the value of outer magnetic field. Co-doped carbon nanopowder composite (S1, o) and coarse-grained cobalt powder (S2, ♥).
up to magnetic field values of the order of the mean demagnetizing field due to the presence of the domain walls [12]. But in the case of the nanopowder composite the mobility of the domain walls is essentially decreased which should be reflected in the character of I(Нo) dependence on the value of the outer magnetic field Нo. Besides, due to increasing coercive force for suppressing of the echo signal in the nanopowder larger outer magnetic fields would be necessary. The results of echo signal intensity measurements on the value of the outer magnetic field confirm these considerations (Fig. 11). The VSM magnetization measurements of the synthesized carbon nanopowders doped with iron atoms were carried out in the zero-field-cooled (ZFC) and field-cooled (FC) modes with curves taken between 3 and 296 K for different values of cooling field HFC. The ZFC data were obtained first by cooling the sample from room temperature to the lowest temperature of 3 K in the zero applied field, and then a magnetic field was applied and magnetization measured with increasing temperature; in the FC mode the sample was cooled from room to the lowest temperature in a magnetic field.
In Fig. 12 the results of VSM measurements of ZFC and FC dependences are presented for the carbon magnetic nanopowder doped with iron atoms as synthesized by us and in Fig. 13a and b hysteresis curves are shown for this sample taken at temperatures of T ¼10 K (a) and 140 K (b). Similar ZFC and FC results are presented for the carbon coated iron nanopowder from Sun Innovations, Inc. (USA) with a mean particle size of 25 nm (b). Fig. 13a and b shows the temperature dependence of the magnetization behavior at high magnetic fields B ¼0.4 T and 0.8 T that the particles in both cases behave ferromagnetically with Curie temperatures higher than 300 K. The ZFC–FC magnetization curves at a small applied magnetic field of 0.01 T indicate that the particles are thermally stable without blocking (or superparametric behavior). This result is in agreement with the assessment of blocking temperature TB for 20 nm sized Fe nanoparticles which should be much higher than 350 K [13]. Further confirmation of this conclusion could be found from [13,14] where results of similar measurements are presented for carbon coated iron nanoparticles with mean sizes of 75 nm and 7.5 nm, respectively, synthesized by different technologies. The maximum at 120 K apparently corresponds to Verwey transition in magnetite [15] taking place in bulk magnetite particles which are present in our powder due to apparently some oxidation processes.
4. Conclusion The complex SEM and AES structure and composition content measurements, along with the VSM study, were carried out on carbon nanoparticles doped with magnetic clusters which were synthesized via a technology which combines the method of pyrolysis of ethanol vapors and the CVD process in a horizontal continuous reactor with certain temperature gradients and controlled partial oxygen pressure. The structure and composition content data of the synthesized magnetic carbon nanopowders showed that the nanopowders consisted of randomly distributed carbon nanoparticle aggregations of 200 nm in diameter doped with magnetic clusters. The magnetometry data correspond with the structure data pointing
Please cite this article as: T. Gеgechkori, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.05.007i
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Fig. 12. Temperature dependence of magnetization of magnetic carbon nanopowder doped with iron atoms (Fe–CN) synthesized at 1200 1C in zero-field cooled (ZFC) (curve 1) and field cooled (FC) (curve 2) cases at magnetic field 100 Oe ( 0.01 T), 0.4 T and 0.8 T (a).
Fig. 13. Hysteresis curves for magnetic carbon nanopowder doped with iron atoms taken at temperatures T ¼ 10 K (coercive force Hc ¼310 G) (a) and 140 K (b) (coercive force Hc ¼ 270 G) (b).
to the existence of a significant superparamagnetic contribution in the synthesized magnetic nanopowders. [7]
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Technology for production of magnetic carbon nanopowders doped with iron and cobalt nanoclusters T. Gеgechkori a, G. Маmniashvili a, E. Kutelia b, L. Rukhadze b, N.I. Maisuradze b, B.G. Eristavi b, D.I. Gventsadze b, А. Аkhalkatsi c, T. Gavasheli c, D. Daraselia c, D. Djaparidze c, А. Shengelaya c a
Ivane Javakhishvili Tbilisi State University Andronikashvili Institute of Physics, 6 Tamarashvili Street 0177 Tbilisi, Georgia Georgian Technical University, 69, Kostava Street Tbilisi 0175, Georgia c Ivane Javakhishvili Tbilisi State University, 3, Chavchavadze Avenue, Tbilisi 0128, Georgia b
art ic l e i nf o Article history: Received 4 December 2013 Received in revised form 13 March 2014 Accepted 6 May 2014 Keywords: Carbon nanopowder Magnetic clusters Core–shell Iron Cobalt Doped nanoparticles Scanning electron microscope Auger electron spectroscope Vibrating sample magnetometer NMR
a b s t r a c t Q3 Complex scanning electron microscope and Auger electron spectroscope structure and composition measurements, along with the vibrating sample magnetometer and NMR studies, were carried out on carbon nanoparticles doped with magnetic clusters, which were synthesized by a technology which combines the ethanol vapor pyrolysis method and the chemical vapor deposition process in a horizontal continuous reactor with certain temperature gradients and controlled partial oxygen pressure. The structure and composition data of the synthesized magnetic carbon nanopowders showed that the nanopowders consisted of randomly distributed carbon nanoparticle aggregations that are 200 nm in diameter doped with magnetic clusters. The magnetometry and NMR data are in agreement with the results of the structure analysis, pointing to the existence of a significant superparamagnetic contribution to the synthesized carbon nanoparticles doped with cobalt nanoclusters. & 2014 Published by Elsevier B.V.
1. Introduction The study of carbon nanoparticles (CNP) and nanotubes (CNT) promises to be an important field that could develop a new age of ultrastrong, light, conductive and versatile materials. Interest in these materials has been permanently growing in the last 20 years in connection with the development of nanotechnologies for controlling the size and shape of nanoparticles as well as creating ways for modification of their properties by doping with different atoms and clusters [1–3]. Particular interest is associated with the fabrication of core– shell structured nanocomposites on the CNP and CNT bases doped with ferromagnetic clusters for the synthesis of nanoscale functional materials which generate properties that are not available in any other materials [3]. In [4] the possibility of the synthesis of carbon nanopowders doped with magnetic clusters of iron oxide and cobalt atoms was studied using a novel technology which combines the ethanol
E-mail address: [email protected] (T. Gеgechkori).
vapor pyrolysis method and the chemical vapor deposition (CVD) process. In [5] the features used for the synthesis technique were briefly summarized, which are partially different from those described in the literature methods based on pyrolysis. The main features of our method are as follows: 1. the substrate (catalyst) is a massive polished plate (iron or cobalt), instead of metal powder; 2. the synthesis was carried out at a partial oxygen pressure of 10 25 Torr, for which an oxygen pump was included in a closedloop cycle. The carbon nanopowders synthesized on ground iron and cobalt plates consist of core–shell structured nanocomposite particles in the form of ferromagnetic cluster cores of the corresponding atoms coated with pyrocarbon shells. The carbon shells offer protection to the ferromagnetic impurities (cores) as well as introduce new properties to the hybrid nanostructures. Therefore, the possible field of application of magnetic carbon nanopowders is enormous. But it should be noted that the nanoscale magnetism of carbon nanoparticles doped with ferromagnetic clusters and magnetic atom-filled carbon nanotubes is poorly known. Aimed at better understanding of the structure–property relationship and the development of novel functional materials, the study of the nanoscale ferromagnetic materials has attracted
http://dx.doi.org/10.1016/j.jmmm.2014.05.007 0304-8853/& 2014 Published by Elsevier B.V.
Please cite this article as: T. Gеgechkori, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.05.007i
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67 compressor (4) connected in series, an oxygen pump with a partial 68 oxygen pressure gage (5), an ethanol container (6) and a valve 69 (7) for releasing (or collecting) excess hydrogen. 70 The synthesis process was carried out in the mode of recircula71 tion of the main reagents, which are products of ethanol pyrolysis 72 in a closed-loop cycle at a monitored partial pressure of oxygen. The samples for analysis were taken from synthesized nanopow- Q5 73 74 ders obtained in a free poured state after they were shaken off 75 from the surfaces of the iron and cobalt catalyst plates. 76 The morphology and particle distribution of the synthesized 77 carbon nanopowders and the surfaces of the catalyst plates were 78 examined using a DSM-960 SEM (Opton, Germany). The chemical 2. Experimental procedure 79 composition and element distribution in the metal-doped carbon 80 nanopowders were examined by the Auger Electron Spectroscopy The synthesis of carbon nanopowder samples doped by iron 81 (AES) method using the LAS-2000 instrument (Riber, France). The and cobalt atoms was realized on surfaces of ground iron and 82 magnetometry measurements of the synthesized nanopowders cobalt plates placed in the temperature regions of 1200 1C, 700 1C 83 were carried out using a VSM (Cryogenic Limited, USA) making it and 500 1C in a horizontal continuous reactor of the experimental 84 possible to carry out the measurements over a temperature range setup [4]. 85 of 1.7–293 K and magnetic fields up to 5 T. NMR characterization The scheme of the proposed technology combining the pyr86 of the cobalt-doped carbon nanopowders was conducted using a olysis of ethanol vapors and the process of CVD is shown in Fig. 1. 87 magnetic video-pulse excitation technique [6]. This figure also depicts the temperature distribution in the 88 active zone of the reactor. During the experiment, when the 89 furnace is in the process of forming the magnetic powder, its 90 temperature is measured by a thermocouple located near the 91 center of the reaction pipe at a distance of 2 mm from the wall. 3. Results and discussion 92 The process setup is a closed-loop system which consists of a 93 horizontal pipe furnace (1) to provide the given temperature Representative SEM images of the prepared carbon nanopowders 94 distribution in the active section of the reactor (2). A platedoped with cobalt atoms synthesized on the surface of cobalt plates 95 shaped iron or cobalt catalyst (3) is placed in this section. The at temperatures of 1200 1C and 500 1C are shown in Fig. 2a and 2b, 96 circulation circuit of the reactants with the reactor consists of a respectively. The nanoparticles of the powder synthesized at 1200 1C 97 are individual uniform spheroids of 200–250 nm in diameter 98 (Fig. 2a), whereas the nanoparticles of the powders synthesized at 99 500 1C have smaller dimensions with a large diameter distribution 100 over the range of 70–150 nm (Fig. 2b), which in this case is the result 101 of the self-assembly processes of smaller (r50 nm) carbon nano102 particles doped with cobalt atoms (clusters). 103 In Fig. 3a, b and Fig. 3c, d SEM images of the doped carbon 104 nanopowders synthesized on the surface of iron substrate-catalyst in 105 the presence of oxygen at 1200 1C and 700 1C, respectively, are shown. 106 As is seen, in both cases carbon nanopowders doped by atoms 107 (clusters) of iron oxide consist of randomly distributed and 108 individually joined short-range ordered uniform nanoparticles of 109 spherical aggregations 150 nm in diameter, as result of the self110 assembly process of smaller ( 30⧸50 nm) nanoparticles. During 111 this process the morphology and size distribution of carbon 112 nanoparticles (doped with iron oxide clusters) are practically the 113 same as those of the nanoparticles synthesized at 1200 1C and 114 Fig. 1. Experimental setup used to produce the magnetic carbon nanopowder. 700 1C (comparing Fig. 3b and Fig. 3a). 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 Fig. 2. SEM images of magnetic carbon nanopowders doped with cobalt atoms synthesized at 1200 1C, nanoparticle diameter 200–250 nm (a) and 500 1C, nanoparticle 132 diameter 70–150 nm (b). considerable attention in recent years due to their potential applications in many fields such as sensors, nanolubricants, nano-biotechnology, and magneto-optical devices. In relation to the above-noted information, the aim of this work is the complex study of the morphology, size, chemical composition and magnetic properties of carbon nanopowders doped by iron and cobalt atoms using such modern analytic instruments as a scanning electron microscope (SEM), an Auger electron spectroscope (AES), a vibrating sample magnetometer (VSM) and NMR.
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Fig. 3. SEM images of magnetic carbon nanopowders doped with iron clusters, synthesized at 1200 1C, nanoparticle diameter 100–200 nm (a and b) and 700 1C, nanoparticle diameter 100–150 nm (c and d).
The chemical composition and thickness of the carbon shell on ferromagnetic dopant impurities in nanoparticles synthesized at 1200 1C on the surface of iron and cobalt substrates were defined from differential Auger spectra recorded for the same part of sample surface according to the method used in [4]. A layer-by-layer analysis was conducted using a technique of spraying with argon ions accelerated up to 1 eV energy. The selected density of the ion stream allowed the removal of layers at the speed of approximately 10 Å/min. The layer-by-layer analysis was stopped at the appearance of significant-intensity triple L3VV transition peaks of iron and cobalt atoms in the Auger spectra, and an evaluation of carbon shells' thicknesses on the magnetic cluster cores was made. In Fig. 4a and b differential Auger spectra are shown in the energy range 100–900 eV recorded before the moment of appearance on them of the intense Auger transition peaks of iron and cobalt, respectively. The appearance of triple L3VV transition peaks of iron atoms with the simultaneous appearance of the KL2.3L2.3 peaks of the Auger transition of oxygen atoms for the carbon nanopowder synthesized on the iron substrate surface took place after a 20-min exposure of the sample surface to the argon ions (Fig. 4a). The appearance of the significant-intensity triple L3VV peaks of cobalt atoms in the Auger spectrum of carbon nanopowder synthesized on the cobalt substrate took place after a 100-min exposure to argon ions (Fig. 4b). The absence of any other peaks, besides the KL2.3L2.3 intensive peak of the Auger transition of carbon atoms, and the above-mentioned Auger peaks of iron, oxygen and cobalt atoms, points to the fact that the synthesized carbon nanopowders doped with iron and cobalt are of high purity, and their composition corresponds to the realized conditions of the synthesis. It also points to the fact that the nanoparticles in both synthesized powders have the structure of a core–shell type, which was formed as a result of
the interaction of pyrolysis products of ethanol with the surface of substrate – the catalyst according to the gas “corrosion” mechanism at high temperatures. To such a mechanism of formation of doped carbon nanoparticles points also the SEM observation of ground plate-substrate surface after the interaction with the products of ethanol pyrolysis at 1200 1C. So, in Fig. 5a and b different magnification SEM images are presented of the ground iron plate-substrate surface after shaking off the carbon nanopowder synthesized at 1200 1C. It is seen that, along with traces of grinding scratches, at relatively small magnification (Fig. 5a) the surface has a typical microstructure which is a characteristic of the gas corrosion interaction, and at large magnifications (Fig. 5b) the nanoparticles remained bonded by nanobridges with the substrate surface, which are commensurated with nanoparticles in the freely poured doped carbon powder. Along with the structural and chemical characteristics of the samples, great significance is attributed to the data on their magnetic properties, because they define the most interesting practical applications of magnetic nanostructures. The VSM magnetization measurements of the synthesized carbon nanopowders doped with iron and cobalt atoms were carried out in the zero-field-cooled (ZFC) and field-cooled (FC) modes with the curves taken over the range 3 K to 296 K for different values of cooling field HFC. The ZFC data were obtained first by cooling the sample from room temperature to the lowest temperature of 3 K in the zero applied field. Then a magnetic field was applied and the magnetization was measured with increasing temperature, whereas in the FC mode the sample was cooled from room temperature to the basal temperature in a magnetic field. The results of the VSM measurements of the ZFC and FC dependences and the magnetization hysteresis curves are shown in Fig. 6a and b and Fig. 7a and b for the magnetic carbon nanopowders doped with cobalt atoms and synthesized at 1200 1C and 500 1C, respectively.
Please cite this article as: T. Gеgechkori, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.05.007i
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Fig. 4. Auger spectra of the synthesized carbon powder doped with iron oxide clusters (a) and cobalt atoms (b) after the exposure to argon ions accelerated up to 1 keV for 20 min (a) and 100 min (b), respectively. Synthesis temperature is 1200 1C.
Fig. 5. SEM images of ground iron plate after shaking off the doped carbon nanopowder particles synthesized at 1200 1C from its surface.
The NMR experiments were carried out using an incoherent spin-echo spectrometer at liquid nitrogen temperature [6]. The experimental conditions were chosen to be optimal for the intensities of the echo signals to be studied. Fig. 8 shows the obtained NMR spectra of the cobalt-doped carbon nanopowder composite synthesized at 1200 1C (sample S1, o) and the coarse-grained cobalt powder taken at optimal RF pulse durations (sample S2, ♥) with a mean grain size D of 10 μm synthesized by an alloying method [7].
As is known [8] the NMR spectra of the coarse-grained Co at 77 K contain two intensive peaks at frequencies of 216 and 218.6 MHz, corresponding to nuclei arranged in the centers of the domain walls of FCC and the edges of the domain walls of HCP phases, and a peak at 225 MH. z corresponding to the nuclear located in the centers of domain walls of HCP phase. The shape of the NMR spectra points to the fact that in cobalt nanopowder (S1) prevails over the FCC phase, as well as in coarse-grained powder. For the characterization of microscopic magnetic properties of MNP we use the method of magnetic video-pulse excitation [6] where the influence of a magnetic videopulse field (pulsed magnetic field) on the nuclear spin echo in magnetic materials was studied. The investigation includes the determination of nuclear spin echo dependences in the sample on the amplitude Нd and duration τm of magnetic video-pulse (MVP) applied in the interval between RF pulses or between the second RF pulse and echo signal, as well as during the RF pulse action (Fig. 9). The decrease of echo signal intensity at the switching-on of MVP in intervals between RF pulses or between the RF pulse and echo signal is related to the breakdown of phase coherence in a system of processing nuclear moments because of a change in the local nuclear frequencies caused by hyperfine field anisotropy under the action of MVP displacing the domain walls. The switching-on of MVP symmetric with respect to the second RF pulse results in the fact that the first and the second RF pulses successfully excite nuclei changing their positions in the domain walls defining their resonance frequencies and the RF field gain factor (η) [7]. At sufficiently short RF pulses the resonance frequency change could be neglected but the change of η results in the decrease of echo intensity. In [6] the influence of the timing and frequency spectra of MVP on the TPE signals in magnets was studied. It was shown that the character of the spectrum is mainly defined by the hyperfine field anisotropy of the investigated nuclei and domain wall mobilities. By recording the echo signal intensity dependence on the amplitude Нd (I(Нd)) one could determine quantitatively the value of MVP amplitude causing the domain wall to shift by an amount equal to its effective thickness. This fact enables a simple and quantitative assessment of the coercive force of a sample for nuclei contributing to the NMR Hd spin echo signal intensity. In Fig. 10 the experimental data are presented showing the echo signal intensity dependence on Нd for cobalt-doped carbon
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Fig. 6. Temperature dependence of magnetization of magnetic carbon nanopowder doped with cobalt atoms (Co–CN) synthesized at 1200 1C in zero-field cooled (ZFC) (curve 1) and field cooled (FC) (curve 2) cases at magnetic field 15 Oe. (a); Hysteresis curve of Co–CN sample (b).
Fig. 7. Temperature dependence of the magnetization of magnetic carbon nanopowder doped with cobalt atoms (Co–CN) synthesized at 500 1C in zero-field cooled (ZFC) (curve 1) and field cooled (FC) (curve 2) cases at magnetic field of 15 Oe. (a); Hysteresis curve of Co–CN sample (b).
Fig. 9. The scheme of magnetic video-pulse action on the TPE signal. The RF marks the timing and duration of RF pulse and Hd shows the amplitude and duration of magnetic video-pulse. The case of symmetric MVP action. Asymmetric action corresponds to MVP application between RF pulses.
Fig. 8. NMR echo spectra for Co-doped carbon nanopowder composite (S1, ○) and coarse-grained cobalt powder (S2, ♥) taken at optimal RF pulse durations.
nanocomposite S1. For comparison, similar data for coarse-grained Co powder with a mean grain size D of 10 μm synthesized by the alloying method [7] are presented, too. In [9] the NMR spectrometer used by us was described and provided with the additional MVP unit capable of generating MVP with amplitudes up to 500 Oe and durations up to 10 μs. A comparison of echo intensity dependences on the amplitude of exciting MVP for MNP and coarse-grained Co powder (Fig. 10a and b) makes it possible to easily reveal two important
peculiarities: first, the degree of symmetric MVP influence on MNP echo signal is lower and, second, the degree of asymmetric MVP influence on MVP echo signal is comparatively close to the one for coarse-grained Co (S2) allowing for lower the domain walls' mobility in it. This lower suppressive capability in Co-doped carbon nanopowder composite as compared with Co coarse-grained powder could be related to the significant increase of coercive force in that part of the cobalt nanopowder which contributes to the NMR signal. As is known [10,11], the decrease of cobalt powder grain size from values of the order of 1 μm down to 150 nm is accompanied by the significant increase of coercive force of the sample related to the decreasing sizes and increasing role of surface effects. This fact is also reflected in the signal echo intensity dependence on the value of the outer magnetic field. In all samples the echo signal intensity practically does not change
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Fig. 10. Two-pulse echo intensity dependences on MVP amplitude Нd for symmetric (a) and asymmetric (b) influences for Co powders at optimal NMR frequencies and RF pulse parameters. Im is echo amplitude at Hd ¼0. Co-doped carbon nanopowder composite (S1, o) and coarse-grained cobalt powder (S2, ♥)
Fig. 11. NMR echo intensity dependence on the value of outer magnetic field. Co-doped carbon nanopowder composite (S1, o) and coarse-grained cobalt powder (S2, ♥).
up to magnetic field values of the order of the mean demagnetizing field due to the presence of the domain walls [12]. But in the case of the nanopowder composite the mobility of the domain walls is essentially decreased which should be reflected in the character of I(Нo) dependence on the value of the outer magnetic field Нo. Besides, due to increasing coercive force for suppressing of the echo signal in the nanopowder larger outer magnetic fields would be necessary. The results of echo signal intensity measurements on the value of the outer magnetic field confirm these considerations (Fig. 11). The VSM magnetization measurements of the synthesized carbon nanopowders doped with iron atoms were carried out in the zero-field-cooled (ZFC) and field-cooled (FC) modes with curves taken between 3 and 296 K for different values of cooling field HFC. The ZFC data were obtained first by cooling the sample from room temperature to the lowest temperature of 3 K in the zero applied field, and then a magnetic field was applied and magnetization measured with increasing temperature; in the FC mode the sample was cooled from room to the lowest temperature in a magnetic field.
In Fig. 12 the results of VSM measurements of ZFC and FC dependences are presented for the carbon magnetic nanopowder doped with iron atoms as synthesized by us and in Fig. 13a and b hysteresis curves are shown for this sample taken at temperatures of T ¼10 K (a) and 140 K (b). Similar ZFC and FC results are presented for the carbon coated iron nanopowder from Sun Innovations, Inc. (USA) with a mean particle size of 25 nm (b). Fig. 13a and b shows the temperature dependence of the magnetization behavior at high magnetic fields B ¼0.4 T and 0.8 T that the particles in both cases behave ferromagnetically with Curie temperatures higher than 300 K. The ZFC–FC magnetization curves at a small applied magnetic field of 0.01 T indicate that the particles are thermally stable without blocking (or superparametric behavior). This result is in agreement with the assessment of blocking temperature TB for 20 nm sized Fe nanoparticles which should be much higher than 350 K [13]. Further confirmation of this conclusion could be found from [13,14] where results of similar measurements are presented for carbon coated iron nanoparticles with mean sizes of 75 nm and 7.5 nm, respectively, synthesized by different technologies. The maximum at 120 K apparently corresponds to Verwey transition in magnetite [15] taking place in bulk magnetite particles which are present in our powder due to apparently some oxidation processes.
4. Conclusion The complex SEM and AES structure and composition content measurements, along with the VSM study, were carried out on carbon nanoparticles doped with magnetic clusters which were synthesized via a technology which combines the method of pyrolysis of ethanol vapors and the CVD process in a horizontal continuous reactor with certain temperature gradients and controlled partial oxygen pressure. The structure and composition content data of the synthesized magnetic carbon nanopowders showed that the nanopowders consisted of randomly distributed carbon nanoparticle aggregations of 200 nm in diameter doped with magnetic clusters. The magnetometry data correspond with the structure data pointing
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Fig. 12. Temperature dependence of magnetization of magnetic carbon nanopowder doped with iron atoms (Fe–CN) synthesized at 1200 1C in zero-field cooled (ZFC) (curve 1) and field cooled (FC) (curve 2) cases at magnetic field 100 Oe ( 0.01 T), 0.4 T and 0.8 T (a).
Fig. 13. Hysteresis curves for magnetic carbon nanopowder doped with iron atoms taken at temperatures T ¼ 10 K (coercive force Hc ¼310 G) (a) and 140 K (b) (coercive force Hc ¼ 270 G) (b).
to the existence of a significant superparamagnetic contribution in the synthesized magnetic nanopowders. [7]
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