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Hydrothermal preparation and characterization of octadecahedron Fe3O4 film
Journal of Alloys and Compounds 484 (2009) 207–210
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Hydrothermal preparation and characterization of octadecahedron Fe3 O4 film Jie Chen, Kelong Huang ∗ , Suqin Liu College of Chemistry & Chemical Engineering, Central South University, Changsha, Hunan 410083, China
a r t i c l e
i n f o
Article history: Received 10 November 2008 Received in revised form 5 March 2009 Accepted 14 March 2009 Available online 25 March 2009 Keywords: Hydrothermal method Fe3 O4 film Octadecahedron
a b s t r a c t A ‘green’ hydrothermal process was used to prepare Fe3 O4 film on steel foil using FeSO4 ·7H2 O as the iron source and H2 O2 as the oxidant. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM) were employed to characterize the film. The results showed that the film was composed of fine particles with regularly octadecahedron morphology. The average diameter of the particles ranged from 300 nm to 1 m. Furthermore, the probable mechanism for the formation of Fe3 O4 film with octadecahedron morphology was suggested. © 2009 Published by Elsevier B.V.
1. Introduction Magnetic particles have been widely studied because of their fascinating properties and wide range of potential application in ferrofluids, information storage and medicine. Among magnetic particles, iron oxides (Fe2 O3 and Fe3 O4 ) have been extensively investigated [1–3]. The intrinsic properties of metal particles are mainly determined by its size, shape, composition, crystallinity and structure [4–6]. In principle, one can control any one of these parameters to fine-tune the properties of this nanoparticle. Metals (most of them are face-centered cubic, or fcc) including magnetite tend to nucleate and grow into twinned and multiply twinned particles with their surfaces bounds by the low-energy {1 1 1} and {1 0 0} facets [7]. The presence of {1 1 0} facets is not commonly observed due to the higher energy. Over the past decades, researchers have proposed several synthesis methods for preparing thin Fe3 O4 film such as chemical vapor deposition (CVD), molecular beam epitaxy, pulsed laser deposition, and sputtering [8–11]. Among them, a hydrothermal process, which was extensively used for preparing ceramic powders, is being adopted to the preparation of thin metallic films. However, the hydrothermal formation of Fe3 O4 film is still not well studied. Zhu et al. [12] have prepared Fe3 O4 thin film on Ni substrate using hydrazine hydrate as the mineralizer, but hydrazine hydrate is toxic to human being and can cause long-term harm to the environment. In this paper, a harmless and cheap hydrothermal method was developed for the preparation of Fe3 O4 thin film with regularly octadecahedron morphology on the surface of steel foil using ferrous sulfate heptahydrate as the iron source and hydro-
∗ Corresponding author. Tel.: +86 8830827; fax: +86 0731 8879850. E-mail address: [email protected] (K. Huang). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2009.03.092
gen peroxide as the oxidant. To the best of our knowledge, reports pertaining to a practical method for the formation of Fe3 O4 with octadecahedron morphology were rare. 2. Experimental 2.1. Coating procedure All reagents used in this experiment were of analytical grade without further purification. A typical experimental procedure was as follows: steel (1.5 cm × 1 cm) was polished with sand paper and rinsed with acetone. FeSO4 ·7H2 O (2.502 g) was dissolved in 60 mL deionized water, 10 mL of polyethylene glycol 20000 solution (50 g L−1 ) was added to the FeSO4 ·7H2 O solution and stirred, followed by adding and stirring of 10 mL diluted ammonia (2.5%). After that, 0.27 mL H2 O2 was added into the solution slowly. The mixture was stirred for 5 min to obtain a homogeneous solution. Then, the solution was transferred into a 100 mL autoclave. The cleaned steel foil was put into the autoclave, and stored in the sealed autoclave at 160 ◦ C for 8 h. When finished, the autoclave was naturally cooled down to room temperature. Then, the blackened steel foil was washed by deionized water for several times and dried at 80 ◦ C. 2.2. Coating characterizations X-ray powder diffraction (XRD) patterns were determined using Japan D/max2550VB+ 18 kw diffractometer with Cu K␣ radiation ( = 1.54178 Å) under the potential 40 kV and current 300 mA. The morphology of the as-prepared Fe3 O4 film was characterized using JSM-6360LV scanning electron microscope. The morphology and size of the Fe3 O4 particles remaining in solution were characterized using JE01-1230 transmission electron microscope. The chemical composition of the particles scraped from the film was analyzed by Thermo-NORAN energy dispersive spectrometer (EDS).
3. Results and discussions The XRD patterns of the steel and Fe3 O4 film coated steel are shown in Fig. 1a and b, respectively. All the peaks of Fig. 1a can be indexed as Fe-Cr-434-L stainless steel, which were consistent with
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Fig. 1. XRD patterns of the steel foil (a) and the foil coated with Fe3 O4 film (b).
the values from the standard card (JCPDS no. 34-0396). Magnetite is believed to be the crystalline phase for the synthesized film as identified by the new diffraction peaks at 18.2◦ , 30.0◦ , 35.3◦ , 37.0◦ , 42.8◦ , 53.3◦ , 56.8◦ and 62.4◦ . These peaks correspond to seven indexed planes (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0), respectively, of magnetite. All the peaks of Fig. 1b except for those of the steel (Fig. 1a) can be indexed as face-centered cubic Fe3 O4 with cell parameters a = 8.4 Å, which was in good agreement with the values from the standard card (JCPDS no. 19-0629). The strong and sharp peaks indicated that the Fe3 O4 film was well crystallized. It is worth noting that the ratio between the intensities of the (4 0 0) and (3 1 1) diffraction peaks was higher than the conventional value (0.223 versus 0.2), indicating that the as-produced product were abundant in {4 0 0} facets, and the {4 0 0} planes tended to be preferentially oriented (or textured) parallel to the surface of the supporting substrate. The ratio between the intensities of the (2 2 0) and (3 1 1) peaks was also slightly higher than the conventional value (0.31 versus 0.3) because of the relative abundance of {1 1 0} facets on the surfaces of the product. Fig. 2 shows the optical photographs of the steel before (right) and after (left) the growth of Fe3 O4 film. The shiny steel surface turned to dark black color after 8 h of treatment, suggesting that the formation of Fe3 O4 on the surface of steel, rather than other iron oxide phases, such as Fe2 O3 . A possible mechanism for the formation of magnetite film in this process was proposed, as illustrated in Fig. 3. Firstly, Fe3 O4
Fig. 2. The optical photographs of the steel substrate before (right) and after (left) growth.
nucleuses (nanoparticles) were increasingly formed on the surface of the substrates. The active sites of substrates were used as the nucleation sites of Fe3 O4 . Secondly, the discontinuous Fe3 O4 crystal nucleuses formed on the surface of foil can serve as nucleuses for further hydrothermal growth of the thin Fe3 O4 film. Finally, the hydrothermal synthesis of Fe3 O4 in the solution was carried on. This reaction can be expressed as follows: Fe2+ + 2NH3 ·H2 O → Fe(OH)2 ↓ + 2NH4 + 3Fe(OH)2 + H2 O2 → Fe3 O4 ↓ + 4H2 O Fe3 O4 particles formed in solution can also deposit onto the surface of foil. The morphology of the as-prepared Fe3 O4 film was further characterized by scanning electron microscope. Fig. 4(a) and (b) displays the typical images of this SEM film under different magnifications, respectively. As observed in these two images, the film was composed of fine particles with edge-affected cubic (octadecahedron) morphology. Their surfaces were smooth, all corners and edges of these cubic particles were slightly truncated. Every octadecahedron particulate is composed of 18 well-defined crystal faces, which can be mainly indexed to {1 0 0} and {1 1 0} facets. The sizes of particles ranged from 300 nm to 1 m. A three-dimensional model of these particles on the film was shown in the inset in Fig. 4(a), which was in consistent with the result with XRD measurement. The chemical composition of the particle scraped from the film was analyzed by EDS, and the EDS spectrum is shown in Fig. 4(d). The strong peaks for Fe and O were observed in the spectrum with the atomic ratio of 1:1.32, which matches well with that of magnetite. The crystal growth and crystal morphologies are governed by extrinsic and intrinsic factors, such as the crystal structure, the surface energy, and growth environment. As illustrated by Wang [13], the shape of an fcc nanocrystal was mainly determined by the ratio
Fig. 3. A possible mechanism for the formation of magnetite film on steel foil.
J. Chen et al. / Journal of Alloys and Compounds 484 (2009) 207–210
209
Fig. 4. Low- (a) and high- (b), (c) magnification SEM images of the Fe3 O4 film. EDS of the film (d).
between the growth rates along 1 0 0 and 1 1 1 directions. Octahedral and tetrahedral bounded by the most stable {1 1 1} planes would be formed, while perfect cubes bounded by the less stable {1 0 0} planes would be obtained. The {1 0 0} and {1 1 1} are low-energy surfaces and their formation is expected. The presence of {1 1 0} facets is a unique feature of the Fe3 O4 octadecahedron particles in this work and is not commonly observed because the {1 1 0} facet has higher energy than that of the {1 1 1} and {1 0 0} facets. Firstly, the crystal structure of Fe3 O4 may have an important role in the formation of Fe3 O4 cubic-like particles. The Fe3 O4 crystal possesses the inversed cubic spinel structure in which oxygen anions form a closed fcc packing and iron cations locate at the interstice of the oxygen tetrahedron and octahedron. This symmetry cubic structure would be favorable to the formation of cube-like shapes of particles [14]. Besides, the Fe3 O4 nanoparticles are formed in the presence of the polymer PEG 20000 in this work. As the capping agent, PEG 20000 tended to adsorb on the particular high-energy facets of the nanoparticles. As the energy of {1 1 0} facets was higher than those of {1 1 1} and {1 0 0} facets, the PEG molecules preferentially adsorb on the {1 1 0} facets and then reduce the energy and growth rates of {1 1 0} facets. The surface of the Fe3 O4 octadecahedron particles was encapsulated by a micelle, which not only confines the geometry of the Fe3 O4 cubic-like particles but also stabilizes the surface atoms of the {1 1 0} facets. Furthermore, as shown in Fig. 4(c), the growth of particles in this process may follow the principle of Ostwald ripening, which involves the growth of larger particles at the expense of the smaller one driven by the tendency of the solid phase in
the systems to adjust it to achieve a minimum total surface free energy. 4. Conclusion Fe3 O4 octadecahedron film was successfully prepared on the surface of steel by a simple and harmless hydrothermal process using FeSO4 ·7H2 O as the iron source and H2 O2 as the oxidant. The as-prepared Fe3 O4 film was composed of fine particles with regular octadecahedron morphology. The diameter of the truncated cubic particles ranged from 300 nm to 1 m. The presence of {1 1 0} facets is a unique feature of the Fe3 O4 octadecahedron particles in this work and is not commonly observed. Meanwhile, the growth mechanism of Fe3 O4 film on steel foil was suggested. Acknowledgement We wish to thank the Innovation Projects for Graduates of Center South University (No. 1343-74335000009) for its financial support of this project. References [1] W. Weiss, W. Ranke, Prog. Surf. Sci. 70 (2002) 1. [2] M.K. Krause, M. Ziese, R. Hohne, A. Pan, A. Galkin, E. Zeldov, J. Magn. Magn. Mater 1097 (2002) 242–245. [3] D. Beydoun, R. Amal, G.K.C. Low, S. McEvoy, J. Phys. Chem. B 104 (2000) 4387. [4] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [5] C.M. Lieber, Solid State Commun. 107 (1998) 607.
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J. Chen et al. / Journal of Alloys and Compounds 484 (2009) 207–210
[6] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [7] Y. Sun, Y. Xia, Science 298 (2002) 2176. [8] K. Shalini, G.N. Subbanna, S. Chandrasekaran, et al., Thin Solid Films 424 (2003) 56. [9] W. Eerenstein, L. Kalev, L. Niesen, et al., J. Magn. Magn. Mater. 73 (2003) 258– 259.
[10] M.K. Krause, P. Esquinazi, M. Lorenz, et al., J. Magn. Magn. Mater. 1097 (2002) 242–245. [11] S. Ohta, A. Terada, Thin Solid Films 143 (1986) 73. [12] H. Zhu, D. Yang, L. Zhu, Surf. Coat. Technol. 201 (2007) 5870. [13] Z.L. Wang, J. Phys. Chem. B 104 (2000) 1153–1175. [14] J. Feng, H.C. Zeng, Chem. Mater. 15 (2003) 2829.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Hydrothermal preparation and characterization of octadecahedron Fe3 O4 film Jie Chen, Kelong Huang ∗ , Suqin Liu College of Chemistry & Chemical Engineering, Central South University, Changsha, Hunan 410083, China
a r t i c l e
i n f o
Article history: Received 10 November 2008 Received in revised form 5 March 2009 Accepted 14 March 2009 Available online 25 March 2009 Keywords: Hydrothermal method Fe3 O4 film Octadecahedron
a b s t r a c t A ‘green’ hydrothermal process was used to prepare Fe3 O4 film on steel foil using FeSO4 ·7H2 O as the iron source and H2 O2 as the oxidant. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM) were employed to characterize the film. The results showed that the film was composed of fine particles with regularly octadecahedron morphology. The average diameter of the particles ranged from 300 nm to 1 m. Furthermore, the probable mechanism for the formation of Fe3 O4 film with octadecahedron morphology was suggested. © 2009 Published by Elsevier B.V.
1. Introduction Magnetic particles have been widely studied because of their fascinating properties and wide range of potential application in ferrofluids, information storage and medicine. Among magnetic particles, iron oxides (Fe2 O3 and Fe3 O4 ) have been extensively investigated [1–3]. The intrinsic properties of metal particles are mainly determined by its size, shape, composition, crystallinity and structure [4–6]. In principle, one can control any one of these parameters to fine-tune the properties of this nanoparticle. Metals (most of them are face-centered cubic, or fcc) including magnetite tend to nucleate and grow into twinned and multiply twinned particles with their surfaces bounds by the low-energy {1 1 1} and {1 0 0} facets [7]. The presence of {1 1 0} facets is not commonly observed due to the higher energy. Over the past decades, researchers have proposed several synthesis methods for preparing thin Fe3 O4 film such as chemical vapor deposition (CVD), molecular beam epitaxy, pulsed laser deposition, and sputtering [8–11]. Among them, a hydrothermal process, which was extensively used for preparing ceramic powders, is being adopted to the preparation of thin metallic films. However, the hydrothermal formation of Fe3 O4 film is still not well studied. Zhu et al. [12] have prepared Fe3 O4 thin film on Ni substrate using hydrazine hydrate as the mineralizer, but hydrazine hydrate is toxic to human being and can cause long-term harm to the environment. In this paper, a harmless and cheap hydrothermal method was developed for the preparation of Fe3 O4 thin film with regularly octadecahedron morphology on the surface of steel foil using ferrous sulfate heptahydrate as the iron source and hydro-
∗ Corresponding author. Tel.: +86 8830827; fax: +86 0731 8879850. E-mail address: [email protected] (K. Huang). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2009.03.092
gen peroxide as the oxidant. To the best of our knowledge, reports pertaining to a practical method for the formation of Fe3 O4 with octadecahedron morphology were rare. 2. Experimental 2.1. Coating procedure All reagents used in this experiment were of analytical grade without further purification. A typical experimental procedure was as follows: steel (1.5 cm × 1 cm) was polished with sand paper and rinsed with acetone. FeSO4 ·7H2 O (2.502 g) was dissolved in 60 mL deionized water, 10 mL of polyethylene glycol 20000 solution (50 g L−1 ) was added to the FeSO4 ·7H2 O solution and stirred, followed by adding and stirring of 10 mL diluted ammonia (2.5%). After that, 0.27 mL H2 O2 was added into the solution slowly. The mixture was stirred for 5 min to obtain a homogeneous solution. Then, the solution was transferred into a 100 mL autoclave. The cleaned steel foil was put into the autoclave, and stored in the sealed autoclave at 160 ◦ C for 8 h. When finished, the autoclave was naturally cooled down to room temperature. Then, the blackened steel foil was washed by deionized water for several times and dried at 80 ◦ C. 2.2. Coating characterizations X-ray powder diffraction (XRD) patterns were determined using Japan D/max2550VB+ 18 kw diffractometer with Cu K␣ radiation ( = 1.54178 Å) under the potential 40 kV and current 300 mA. The morphology of the as-prepared Fe3 O4 film was characterized using JSM-6360LV scanning electron microscope. The morphology and size of the Fe3 O4 particles remaining in solution were characterized using JE01-1230 transmission electron microscope. The chemical composition of the particles scraped from the film was analyzed by Thermo-NORAN energy dispersive spectrometer (EDS).
3. Results and discussions The XRD patterns of the steel and Fe3 O4 film coated steel are shown in Fig. 1a and b, respectively. All the peaks of Fig. 1a can be indexed as Fe-Cr-434-L stainless steel, which were consistent with
208
J. Chen et al. / Journal of Alloys and Compounds 484 (2009) 207–210
Fig. 1. XRD patterns of the steel foil (a) and the foil coated with Fe3 O4 film (b).
the values from the standard card (JCPDS no. 34-0396). Magnetite is believed to be the crystalline phase for the synthesized film as identified by the new diffraction peaks at 18.2◦ , 30.0◦ , 35.3◦ , 37.0◦ , 42.8◦ , 53.3◦ , 56.8◦ and 62.4◦ . These peaks correspond to seven indexed planes (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0), respectively, of magnetite. All the peaks of Fig. 1b except for those of the steel (Fig. 1a) can be indexed as face-centered cubic Fe3 O4 with cell parameters a = 8.4 Å, which was in good agreement with the values from the standard card (JCPDS no. 19-0629). The strong and sharp peaks indicated that the Fe3 O4 film was well crystallized. It is worth noting that the ratio between the intensities of the (4 0 0) and (3 1 1) diffraction peaks was higher than the conventional value (0.223 versus 0.2), indicating that the as-produced product were abundant in {4 0 0} facets, and the {4 0 0} planes tended to be preferentially oriented (or textured) parallel to the surface of the supporting substrate. The ratio between the intensities of the (2 2 0) and (3 1 1) peaks was also slightly higher than the conventional value (0.31 versus 0.3) because of the relative abundance of {1 1 0} facets on the surfaces of the product. Fig. 2 shows the optical photographs of the steel before (right) and after (left) the growth of Fe3 O4 film. The shiny steel surface turned to dark black color after 8 h of treatment, suggesting that the formation of Fe3 O4 on the surface of steel, rather than other iron oxide phases, such as Fe2 O3 . A possible mechanism for the formation of magnetite film in this process was proposed, as illustrated in Fig. 3. Firstly, Fe3 O4
Fig. 2. The optical photographs of the steel substrate before (right) and after (left) growth.
nucleuses (nanoparticles) were increasingly formed on the surface of the substrates. The active sites of substrates were used as the nucleation sites of Fe3 O4 . Secondly, the discontinuous Fe3 O4 crystal nucleuses formed on the surface of foil can serve as nucleuses for further hydrothermal growth of the thin Fe3 O4 film. Finally, the hydrothermal synthesis of Fe3 O4 in the solution was carried on. This reaction can be expressed as follows: Fe2+ + 2NH3 ·H2 O → Fe(OH)2 ↓ + 2NH4 + 3Fe(OH)2 + H2 O2 → Fe3 O4 ↓ + 4H2 O Fe3 O4 particles formed in solution can also deposit onto the surface of foil. The morphology of the as-prepared Fe3 O4 film was further characterized by scanning electron microscope. Fig. 4(a) and (b) displays the typical images of this SEM film under different magnifications, respectively. As observed in these two images, the film was composed of fine particles with edge-affected cubic (octadecahedron) morphology. Their surfaces were smooth, all corners and edges of these cubic particles were slightly truncated. Every octadecahedron particulate is composed of 18 well-defined crystal faces, which can be mainly indexed to {1 0 0} and {1 1 0} facets. The sizes of particles ranged from 300 nm to 1 m. A three-dimensional model of these particles on the film was shown in the inset in Fig. 4(a), which was in consistent with the result with XRD measurement. The chemical composition of the particle scraped from the film was analyzed by EDS, and the EDS spectrum is shown in Fig. 4(d). The strong peaks for Fe and O were observed in the spectrum with the atomic ratio of 1:1.32, which matches well with that of magnetite. The crystal growth and crystal morphologies are governed by extrinsic and intrinsic factors, such as the crystal structure, the surface energy, and growth environment. As illustrated by Wang [13], the shape of an fcc nanocrystal was mainly determined by the ratio
Fig. 3. A possible mechanism for the formation of magnetite film on steel foil.
J. Chen et al. / Journal of Alloys and Compounds 484 (2009) 207–210
209
Fig. 4. Low- (a) and high- (b), (c) magnification SEM images of the Fe3 O4 film. EDS of the film (d).
between the growth rates along 1 0 0 and 1 1 1 directions. Octahedral and tetrahedral bounded by the most stable {1 1 1} planes would be formed, while perfect cubes bounded by the less stable {1 0 0} planes would be obtained. The {1 0 0} and {1 1 1} are low-energy surfaces and their formation is expected. The presence of {1 1 0} facets is a unique feature of the Fe3 O4 octadecahedron particles in this work and is not commonly observed because the {1 1 0} facet has higher energy than that of the {1 1 1} and {1 0 0} facets. Firstly, the crystal structure of Fe3 O4 may have an important role in the formation of Fe3 O4 cubic-like particles. The Fe3 O4 crystal possesses the inversed cubic spinel structure in which oxygen anions form a closed fcc packing and iron cations locate at the interstice of the oxygen tetrahedron and octahedron. This symmetry cubic structure would be favorable to the formation of cube-like shapes of particles [14]. Besides, the Fe3 O4 nanoparticles are formed in the presence of the polymer PEG 20000 in this work. As the capping agent, PEG 20000 tended to adsorb on the particular high-energy facets of the nanoparticles. As the energy of {1 1 0} facets was higher than those of {1 1 1} and {1 0 0} facets, the PEG molecules preferentially adsorb on the {1 1 0} facets and then reduce the energy and growth rates of {1 1 0} facets. The surface of the Fe3 O4 octadecahedron particles was encapsulated by a micelle, which not only confines the geometry of the Fe3 O4 cubic-like particles but also stabilizes the surface atoms of the {1 1 0} facets. Furthermore, as shown in Fig. 4(c), the growth of particles in this process may follow the principle of Ostwald ripening, which involves the growth of larger particles at the expense of the smaller one driven by the tendency of the solid phase in
the systems to adjust it to achieve a minimum total surface free energy. 4. Conclusion Fe3 O4 octadecahedron film was successfully prepared on the surface of steel by a simple and harmless hydrothermal process using FeSO4 ·7H2 O as the iron source and H2 O2 as the oxidant. The as-prepared Fe3 O4 film was composed of fine particles with regular octadecahedron morphology. The diameter of the truncated cubic particles ranged from 300 nm to 1 m. The presence of {1 1 0} facets is a unique feature of the Fe3 O4 octadecahedron particles in this work and is not commonly observed. Meanwhile, the growth mechanism of Fe3 O4 film on steel foil was suggested. Acknowledgement We wish to thank the Innovation Projects for Graduates of Center South University (No. 1343-74335000009) for its financial support of this project. References [1] W. Weiss, W. Ranke, Prog. Surf. Sci. 70 (2002) 1. [2] M.K. Krause, M. Ziese, R. Hohne, A. Pan, A. Galkin, E. Zeldov, J. Magn. Magn. Mater 1097 (2002) 242–245. [3] D. Beydoun, R. Amal, G.K.C. Low, S. McEvoy, J. Phys. Chem. B 104 (2000) 4387. [4] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [5] C.M. Lieber, Solid State Commun. 107 (1998) 607.
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[6] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [7] Y. Sun, Y. Xia, Science 298 (2002) 2176. [8] K. Shalini, G.N. Subbanna, S. Chandrasekaran, et al., Thin Solid Films 424 (2003) 56. [9] W. Eerenstein, L. Kalev, L. Niesen, et al., J. Magn. Magn. Mater. 73 (2003) 258– 259.
[10] M.K. Krause, P. Esquinazi, M. Lorenz, et al., J. Magn. Magn. Mater. 1097 (2002) 242–245. [11] S. Ohta, A. Terada, Thin Solid Films 143 (1986) 73. [12] H. Zhu, D. Yang, L. Zhu, Surf. Coat. Technol. 201 (2007) 5870. [13] Z.L. Wang, J. Phys. Chem. B 104 (2000) 1153–1175. [14] J. Feng, H.C. Zeng, Chem. Mater. 15 (2003) 2829.