Structure and Magnetic Properties of Boron-oxide and Boron-nitride Coated Iron Nanocapsules

Transcription

1 J. Mater. Sci. Technol., 2010, 26(11), Structure and Magnetic Properties of Boron-oxide and Boron-nitride Coated Iron Nanocapsules W.S. Zhang 1,2), J.G. Zheng 3), E. Brück 4), P.Z. Si 1), D.Y. Geng 1) and Z.D. Zhang 1) 1) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, China and International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang , China 2) College of Physics & Electronic Engineering, Taizhou University, Taizhou , China 3) Electron Probe Instrumentation Center (EPIC), Northwestern University, 2225N Campus Drive, 1156 MLSF, Evanston IL , USA 4) Van der Waals-Zeeman Instituut, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands [Manuscript received September 10, 2009, in revised form July 12, 2010] The boron-oxide coated iron nanocapsules have been prepared by arc-discharge in a mixture of diborane and nitrogen, and then the boron-nitride coated iron nanocapsules by a subsequent annealing under a nitrogen atmosphere at 1100 C. After the arc-discharge, the boron-oxide coated iron nanocapsules form, which show an amorphous surface layer of B 2 O 3 (and/or B) and a core of γ-fe, α-fe, FeB phases. After being annealed, part of the α-fe phase transforms to the γ-fe phase, and the FeB phase decomposes while the BN phase forms. The BN shell structure formed in the BN encapsulating iron nanocapsules is incomplete. Magnetic properties of the boron-oxide coated and the boron-nitride coated iron nanocapsules were compared and discussed in terms of the particles sizes, the phase components, and the surface structures. KEY WORDS: Arc discharge; Nanocapsules; Magnetic properties 1. Introduction Magnetic nanoparticles can be widely used in magnetic storage media, magnetic toner in xerography, catalysis, ferrofluids, clinical drug delivery, etc. Nanocapsules are the nanoparticles with shell/core structure [1,2]. The nanocapsules have attracted great interest, because of the protective coating of nanoparticles, which are usually pyrophoric and easily oxidized. It is a common fact that the protective coating of nanoparticles has a paramount importance for their application in the ambient environment. The nanocapsules with different shells, such as graphite [3 5], tungsten disulphide [6,7], boron oxide [8], silica oxide [9], polymer [10] and boron nitride [11,12], have been synthesized. The structure and magnetic properties of Corresponding author. Ph. D.; address: wszhangimr@hotmail.com; wszhang@tzc.edu.cn (W.S. Zhang). nanoscaled materials are different from those of the bulk materials. The studies on magnetic nanoparticles are helpful for understanding the behavior of electrons and spins in low dimensions. It has been a direct probe on the superparamagnetic phenomenon in Co nanoparticles [13]. For nanoparticles, even a small deviation in the particle size can lead to a large change in the blocking temperature. Fundamental magnetic parameters, such as the barrier height of the anisotropy energy, the distribution of the energy barrier and the flipping times, can be estimated from zero-field-cooled (ZFC) and field-cooled (FC) temperature dependence of magnetization of the magnetic nanoparticles [8,14 17]. In the present work, we try to prepare the boron-oxide encapsulating iron nanoparticles, by arcdischarge in a mixture of diborane and nitrogen, then the boron-nitride coated iron nanocapsules by a subsequent annealing under a nitrogen atmosphere at 1100 C. The structures of the as-prepared and as-

2 1052 W.S. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(11), Intensity / a.u. (b) (a) -Fe FeB -Fe B 2 O / deg. Fig. 1 XRD spectrum of (a) as-prepared Fe(B) and (b) annealed Fe(BN) nanocapsules annealed nanoparticles were investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM). After the arc-discharge, the boron-oxide coating iron nanocapsules form, and after being annealed under a nitrogen atmosphere, the boron-nitride (BN) shell forms in the BN encapsulating iron nanocapsules, but its structure is not very complete. Magnetic properties of the boronoxide coated and the boron-nitride coated nanocapsules were compared. 2. Experimental Procedures Fe(B) nanocapsules sheathed with boron-oxide were prepared by arc-discharge in a mixture of diborane (B 2 H 6 ) and nitrogen (N 2 ) atmosphere. The anode was bulk Fe of 99.9% purity, while a flexible tungsten needle of 2 mm in diameter served as the cathode. The chamber was evacuated to Pa before the introduction of the gases. A mixture of B 2 H 6 (10%) and N 2 (90%) was introduced into the chamber, till the gas pressure in the chamber reached MPa. The gas mixture serves as a reactant gas and a source of hydrogen plasma. A voltage in the range of 20 to 30 V was applied between the cathode and the anode for evaporation. The deposits on the water-cooled wall of the chamber were collected for different experiments. The nanoparticles collected from the wall were nitrogenated at 1100 C for 3 h in a conventional furnace by using silicon carbide rods as heaters. The fine particles were first laid down on an aluminium oxide crucible lying in the aluminium oxide tube of the furnace, isolated from air by flowing pure nitrogen. The samples for transmission electron microscopy (TEM) observation were prepared in two steps: First, the deposit was dispersed in ethanol in an ultrasound bath and then a drop of the suspension was transferred onto a carbon-coated TEM mesh grid and the ethanol was allowed to evaporate. The samples were examined by a JEOL 2000EX HRTEM operating at 200 kv. XRD spectra were recorded at room temperature in a Riguku D/max-2500pc diffractrometer with CuKα radiation and a graphite monochromator. Magnetic measurements were performed using a superconducting quantum interference device (SQUID) magnetometer (MPMS 5S, Quantum Design) in applied fields up to 5 T. To obtain the ZFC- FC temperature dependence of magnetization, the samples were first cooled at zero field from room temperature to 5 K, and then the magnetization was measured under a magnetic field of 0.1 T from 5 K to room temperature; thus the ZFC curve was obtained. After that the samples was cooled from room temperature to 5 K again with the same field; thus the FC curves were obtained by recording this cooling process. 3. Results and Discussion Diffraction peaks in the XRD spectrum of the asprepared Fe(B) nanocapsules (shown as Fig. 1(a)), can be indexed as γ-fe, α-fe, FeB and B 2 O 3 phases, which are the main constituents in the cores. γ-fe is a high temperature phase, which is in a metastable state in the Fe(B) nanocapsules. From the XRD pattern, it is seen that the crystallization of α-fe is incomplete. This is because during the condensation of the nanoparticles, the cooling rate is so high that there is no enough time for a complete crystallization of α-fe. Figure 1(b) shows the XRD spectrum of the nanocapsules after annealing under a nitrogen atmosphere at 1100 C for 3 h. It is mainly composed of peaks of B 2 O 3 and α-fe. The FeB phase is decomposed after the heat treatment. Certainly, the γ-fe phase in the Fe(B) nanocapsules (shown in Fig. 1(a)) is unstable and after the heat treatment, it transforms to the α-fe phase. In our previous work [8], the XRD and Mössbauer experiments confirmed that the boron-oxide-coated Fe nanocapsules prepared in a mixture of B 2 H 6 (10%) and He (90%) showed an amorphous surface layer and a core of α-fe (and Fe(B) solid solution), γ-fe, FeB and/or Fe 3 B phases. The typical morphology of the as-prepared Fe(B) nanocapsules, produced in a mixture of B 2 H 6 (10%) and N 2 (90%), is represented in an HRTEM micrograph (Fig. 2(a)). The typical size of the capsules is in the range of 20 to 200 nm. It is clearly shown that these nanocapsules are composed of a crystalline core and an amorphous shell. Meanwhile, it is found that some boron nanoparticles also exist. The thickness of the shell is approximately 2 10 nm. In our previous work, it was confirmed that the phases in the shell of the Fe(B) nanocapsules, prepared in a mixture of B 2 H 6 (10%) and He (90%), are mainly amorphous boron oxides [8]. We can find that not only amorphous boron oxides, but also some amorphous boron exist

3 W.S. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(11), Fig. 2 HRTEM images of (a) as-prepared Fe(B) nanocapsules, (b) annealed Fe(BN)) nanocapsules, (c) Fe 3O 4 phases, (d) the coexistence of FeB and Fe 3O 4 phases in the annealed Fe(BN)) nanocapsules and (e) the voids between the shell and the core of a large annealed nanocapsule in the shell of the present Fe(B) nanocapsules. This might be ascribed to the different effects of N 2 and He on the processes of the condensation and the formation of the nanoparticles. After being annealed under nitrogen at 1100 C for 3 h, an incomplete multilayered crystalline structure is formed around the Fe nanoparticles (Fig. 2(b)). The spacing of 0.34 nm between the layers is close to the inter-planar distance of nm in bulk h-bn. This means that part of the shells transform from the amorphous boron oxides (and/or amorphous boron) to the crystalline BN-type phase. However, some parts of the shells (see also Fig. 2(b)) still keep in the amorphous state, even after being annealed at 1100 C. Nevertheless, the occurrence of the transformation from the amorphous boron oxides (and/or amorphous boron) to the crystalline BN phase suggests that the latter is more stable under a nitrogen atmosphere at 1100 C. After being annealed under a nitrogen atmosphere at 1100 C, the boron-nitride (BN) shell forms in the BN encapsulating iron nanocapsules, although its structure is not very complete. Besides those phases that have been observed from XRD spectrum, Fe 3 O 4 phases are also found in HRTEM image (Fig. 2(c)). The characteristic interplanar spacing nm and nm and the angles between corresponding lattice planes can be measured directly from this two-dimensional lattice image. It matches the characteristics of the Fe 3 O 4 phase examined in the zone axis. The characteristic inter-planar spacing nm and nm correspond respectively to (0 22) and (1 3 1) lattice planes of the Fe 3 O 4 phase. Figure 2(d) shows the HRTEM image of a core containing FeB and Fe 3 O 4 phases characterized by the interplanar lattice spacing nm and nm, respectively. It is obviously seen from Fig. 2(e) that the voids exist between the shell and the core of the large nanocapsules with the size about 100 nm. The possible reasons for the formation of the voids are as follows: (1) The shape of the core of the large nanocapsules is not very spherical, during the condensation. The procedure for the formation of the core/shell structure is so fast that the voids might form. This might be because the shape of the core of the large nanocapsules cannot fit well with the shape of the shell immediately when the core/shell structure forms. (2) During the nitrogenation at high temperatures, the decomposition of the FeB phase, the transformation of γ-fe to α-fe, the transformation from the amorphous boron oxides (and/or amorphous boron) to the crystalline BN phase all may contribute to the re-arrangements of the atoms in the core and shell of the nanocapsules. Thus, the voids might form in the large nanocapsules. The result of magnetization, measured at fields up to 0.8 T at room temperature, is shown in Fig. 3. The saturation magnetization of the annealed

4 1054 W.S. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(11), M / (Am 2 /kg ) Field / T Fe(BN) Fe(B) Fig. 3 Field dependence of magnetization measured at room temperature for the as-prepared Fe(B) and annealed Fe(BN) nanocapsules M / (Am 2 /kg) T = 5 K Field / T Fe(B) Fe(BN) Fig. 4 Field dependence of magnetization measured at 5 K for the as-prepared Fe(B) and annealed Fe(BN) nanocapsules Fe(BN) nanocapsules (8.35 Am 2 /kg) is slightly higher than that of the as-prepared nanocapsules (6.37 Am 2 /kg). Compared with the bulk iron, the saturation magnetization of the present nanocapsules is very low. The reasons are as follows: In the present cases, non-magnetic boron nanoparticles exist. The oxidation of the nanoparticles leads to the formation of the oxides on their surfaces, lowering the magnetization. The amorphous boron-oxides or the boron or the boron-nitrides in the shells are non-magnetic. Besides, the saturation magnetization of the nanoparticles decreases when the size of the system decreases to be in nanoscale. These matters indeed reduce the saturation magnetization of the nanocapsules. A small angle neutron scattering study concluded that nanocrystalline Fe consists of ferromagnetic Fe cores and non- or weak-magnetic interfaces [18], indicating that the disorder structure near the interfaces provided less magnetic moment than that of the ferromagnetic core regions. The detrimental effect of interfaces on the saturation magnetization increases with decreasing the particle size, since the volume fraction of the interfaces/surfaces increases rapidly. Thus, the decrease in the saturation magnetization for the present iron nanocapsules is mainly due to the presence of the non-magnetic boron nanoparticles, the non-magnetic shells, and the non- or weakmagnetic interfaces, leading to a decrease in the effective magnetic moment. The other possibility is that some boron may have remained trapped inside the Fe nanoparticles. Furthermore, the occurrence of the superparamagnetism in part of the nanoparticles with a very small size below its critical size would decrease the magnetization of the particles. Compared with that of the Fe(C) nanocapsules prepared (105 Am 2 /kg) by arc-discharge in CH [5] 4, the saturation magnetization of the present nanocapsules is quite lower. This indicates that the graphite shell of the Fe(C) nanocapsules has better effects for preventing the oxidation of the surfaces of the nanocapsules [5]. The saturation magnetization of the present nanocapsules is even much lower than that of the Fe(B) nanocapsules (57 Am 2 /kg) prepared by arcdischarge in a mixture of B 2 H 6 (10%) and He (90%) [8]. This implies that the He gas in the mixture has a better action for preventing the oxidation during the evaporation, than the N 2 gas. Field dependence magnetizations of these two kinds of nanocapsules were also measured at 5 K and applied fields up to 5 T (Fig. 4). The magnetizations recorded at 5 T are and Am 2 /kg for the as-prepared Fe(B) and as-annealed Fe(BN) nanocapsules, respectively. We believe that this difference is due to the existence of γ-fe phase in the as-prepared nanocapsules. Face-centered γ-fe is normally a high temperature phase, which shows antiferromagnetic properties below its Néel temperature. Goneser et al. [19] found the Néel temperature of 304 stainless steels is 38 K, and γ-fe precipitates with two different sizes stabilized in the copper lattice are 55 and 67 K, respectively. The γ-fe phase in the asprepared nanocapsules is paramagnetic at room temperature. After annealing, the γ-fe phase transforms to the α-fe phase. During annealing the nanocapsules, the oxygen impurities in the nitrogen gas could react with iron in the nanoparticles, reducing the magnetization of the annealed nanocapsules. Furthermore, after being annealed at the nitrogen, the decomposition of the FeB phase and the formation of BN could also contribute to the change of the magnetic properties of the nanocapsules. These competing reasons together result in a higher magnetization at low temperatures, but a lower one at room temperature of the as-prepared nanocapsules. At 5 K, both the nanocapsules cannot be saturated even at

5 W.S. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(11), M / (Am 2 / kg) Fe(B) FC Fe(B) ZFC Fe(BN) FC Fe(BN) ZFC Field = 0.1 T T / K Fig. 5 Zero-field-cooled (ZFC) and field-cooled (FC) (in a field of 0.1 T) temperature dependence of magnetization of the as-prepared Fe(B) and annealed Fe(BN) nanocapsules 5 T. The lack of saturation in the magnetization suggests the presence of antiferromagnetic interactions in both nanocapsules. In our case, both γ-fe and Fe 3 O 4 behave as antiferromagnet at 5 K. By increasing the applied field, the ferromagnetic part tends to saturate, whereas the antiferromagnetic part increases linearly. Besides, this also reflects the surface spin canting in the iron nanoparticles. It was easy to form canted spins at the surface, due to broken exchange bonds, which made it difficult to be saturated [20]. The temperature-dependent magnetizations of both the nanocapsules were measured in the ZFC and FC processes under a relatively low field of 0.1 T [8]. The temperature dependence of the magnetization between room temperature and 5 K, recorded at 0.1 T, is slightly different for the two kinds of the nanocapsules. The magnetization below room temperature of the as-prepared nanocapsules is slightly higher than that of the as-annealed one. As plotted in Fig. 5, there is a large thermal irreversibility between ZFC- FC curves for both the nanocapsules. From 50 to 300 K for the as-prepared nanocapsules (or 350 K for the annealed nanocapsules), the ZFC magnetization increases nearly linearly. The continuous increase of the ZFC magnetization corresponds to the depinning of multidomain particles [21 23]. Below 5 to 25 K, there is a sudden decrease in the magnetization of both the nanocapsules, implying that there is an unknown phase transformation at about 25 K. Further studies need to be done to identify this phase. 4. Conclusion The boron-oxide coated iron nanocapsules have been prepared by means of arc-discharge in a mixture of diborane and nitrogen, and then the boron-nitride coated iron nanocapsules by a subsequent annealing under a nitrogen atmosphere at 1100 C. After the arcdischarge, the boron-oxide coated iron nanocapsules form, which shows an amorphous B 2 O 3 (and/or B) surface layer and a core of γ-fe, α-fe, FeB phases. After annealing, part of the γ-fe phase transforms to the α-fe phase; the FeB phase decomposes while the BN phase forms. The BN structure formed in the BN encapsulating iron nanocapsules is not very complete. The saturation magnetization of the present nanocapsules is lower than that of Fe(C) nanocapsules prepared in CH 4 and the Fe(B) nanocapsules prepared in a mixture of B 2 H 6 (10%) and He (90%). The continuous increase of the ZFC magnetization corresponds to the depinning of the multidomain particles. Magnetic properties of the boron-oxide coated and the boronnitride coated iron nanocapsules are also compared and discussed in terms of the particles sizes, the phase components, and the surface structures. Acknowledgements This work was financially supported by the National Basic Research Program (No. 2010CB934603) of China, Ministry of Science and Technology China, the National Natural Science Foundation of China under Grant No , the Natural Science Foundation of Zhejiang Province under Grant No. Y REFERENCES [1 ] Z.D. Zhang: Encyclopedia of Nanoscience and Nanotechnology, Vol. 6, ed H.S. Nalwa, American Scientific Publishers, 2004, [2 ] Z.D. Zhang: J. Mater. Sci. Technol., 2007, 23, 1. [3 ] Y. Saito, T. Yoshikawa, M. Okuda, M. Ohkohchi, Y. Ando, A. Kasuya and Y. Nishina: Chem. Phys. Lett., 1993, 209, 72. [4 ] Y. Saito, T. Yoshikawa, M. Okuda, N. Fujimoto, K. Sumiyama, K. Suzuki, A. Kasuya and Y. Nishina: J. Phys. Chem. Solids, 1993, 54, [5 ] X.L. Dong, Z.D. Zhang, Y.C. Chuang and S.R. Jin: Phys. Rev. B, 1999, 60, [6 ] L. Margulls, G. Salitra, R. Tenne and M. Tallanker: Nature, 1993, 365, 113. [7 ] R. Tenne, L. Margulis, M. Genut and G. Hodes: Nature, 1994, 360, 444. [8 ] 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, W. Liu and X.G. Zhao: Phys. Rev. B, 2001, 64, [9 ] M.H. Wu, Y.D. Zhang, S. Hui, T.D. Xiao, S.H. Ge, W.A. Hines, J.I. Budnick and M.J. Yacaman: J. Appl. Phys., 2002, 92, [10] Y.E. Hapiro, E.G. Pykhteeva and A.V. Levashov: J. Colloid. Interface. Sci., 1998, 206, 168. [11] M. Kuno, T. Oku and K. Suganuma: Scripta Mater., 2001, 44, [12] T. Oku, I. Narita and H. Tokoro: J. Phys. Chem. Solids, 2006, 67, [13] S.L. Woods, J.R. Kirtley, S.H. Sun and R.H. Koch: Phys. Rev. Lett., 2001, 87,

6 1056 W.S. Zhang et al.: J. Mater. Sci. Technol., 2010, 26(11), [14] C.P. Bean and J.D. Livingston: J. Appl. Phys., 1959, 30, 120S. [15] C. Petit and M.P. Pileni: Appl. Surf. Sci., 2000, , 519. [16] J. van Lierop and D.H. Ryan: Phys. Rev. Lett., 2000, 85, [17] T. Johnson, J. Mattsson, P. Nordblad and P. Svedlindh: J. Magn. Magn. Mater., 1997, 168, 269. [18] W. Wagner, A. Wiedenmann, W. Petry, A. Geibel and H. Gleiter: J. Mater. Res., 1991, 6, [19] U. Goneser, C.J. Meechan, A.H. Muir, and H. Wiedersich: J. Appl. Phys., 1963, 34, [20] R.H. Kodama, A.E. Berkowitz, E.J. McNiff, Jt. and S. Foner: Phys. Rev. Lett., 1996, 77, 394. [21] X.X. Zhang, J. Tejada, J.M. Hernandez and R.F. Ziolo: Nanostruct. Mater., 1997, 9, 301. [22] X.X. Zhang, J.M. Hernandez, J.Tejada, R. Sole and X. Ruiz: Phys. Rev. B, 1996, 53, [23] J. Tejada, R.F. Ziolo and X.X. Zhang: Chem. Mater., 1996, 8, 1784.