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Synthesis and self-organization of γ-Fe2O3 nanoparticles by hydrothermal chemical vapor deposition
Materials Letters 59 (2005) 3375 – 3377 www.elsevier.com/locate/matlet
Synthesis and self-organization of g-Fe2O3 nanoparticles by hydrothermal chemical vapor deposition Ya Zhou, Zhengjun Zhang*, Yang Yue Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China Received 9 September 2004; accepted 25 April 2005 Available online 20 June 2005
Abstract We report here the synthesis of g-Fe2O3 nanoparticles in the presence of the water vapor, by thermally decomposing ferrocene at ¨1100 -C in a vacuum of 10 2 Torr. The g-Fe2O3 nanoparticles synthesized, interestingly, self-organized into straight lines of several ten micrometers long and several ten nanometers wide, on planar substrates. This could bring arrays of aligned, nanometer-wide lines of g-Fe2O3 nanoparticles in areas ¨ several hundred micrometers in diameter, thus providing an alternative idea to fabricate patterns of iron particles on the nanometer scale, i.e., by simply deoxidizing the g-Fe2O3 nanoparticles. The structure and the magnetic properties of the g-Fe2O3 particles were also investigated. D 2005 Published by Elsevier B.V.
Materials with reduced dimensions, for instance, nanotubes, nanowires, nanorods, and nanoparticles, etc., are of substantially different properties from their bulk state counterparts [1,2]; and have attracted great interest in the past decade. For example, the ferromagnetic g-Fe2O3 particles gain superparamagnetism when the size touches the nanometer-scaled realm [3,4]. Tremendous effort has thus been devoted, in the past years, to develop new approaches to synthesize materials of reduced dimensions in a controlled manner, understand their dimensionproperty relationship, and to explore the possibility of applying these materials into real devices. Ferromagnetic g-Fe2O3 is a renowned magnetic recording material [3], and can be applied in magnetic refrigeration [5], information storage, controllable drug delivery [6], etc. The synthesis of g-Fe2O3 nanoparticles is therefore of considerable interest [7 – 9]. It has commonly been a recognized iron in environments with water and can be easily oxidized into a-Fe2O3. This is the reason why scientists claimed the possible existence of water in Mars when a-Fe2O3 was observed. We reported here, however, g-
* Corresponding author. E-mail address: [email protected] (Z. Zhang). 0167-577X/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.matlet.2005.04.063
Fe2O3 nanoparticles could also be formed with the presence of water, i.e., by decomposing ferrocene at a relatively high temperature in a low vacuum. The g-Fe2O3 nanoparticles so-produced, as we observed, could self-organize into nanometer-scale lines on planar silica substrates; thus forming arrays of aligned lines within areas of several hundred micrometers in size. Once deoxidized, these patterns might serve as nanometer-scale templates for the growth of other nanomaterials such as carbon nanotubes. The approach we employed in this study was similar to that we used to produce circular patterns of a-Fe2O3 nanoparticles [10]. The difference for producing a-Fe2O3 and g-Fe2O3 is that we used a mixture of ferrocene and water only (in case of a-Fe2O3, xylene was also used), and the decomposition temperature was higher. Prior to the CVD operation, the silicon substrate (capped with a silica layer of ¨125 nm) was cleaned in sequence in acetone, alcohol, and deionized water baths supersonically. Then it was mounted inside a quartz tube furnace. The tube was pumped down to a vacuum on the order of 10 2 Torr, heated up to a temperature of ¨1100 -C. When the temperature was stabilized, a pre-vaporized mixture of deionized water and ferrocene was introduced into the tube for ¨2 min. After that, the tube was cooled to room temperature gradually. The sample was examined with X-ray diffraction (XRD),
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Y. Zhou et al. / Materials Letters 59 (2005) 3375 – 3377
Fig. 1. SEM images of the lines of g-Fe2O3 nanoparticles formed on the silica surface: (a) an overview at low magnifications; (b) parallel lines from an area of (a); (c) parallel lines at two neighboring domains; and (d) discontinuous lines showing the size of the particles.
micro Raman spectroscopy and scanning electron microscope (SEM), to observe the morphology and determine the structure of the deposits. Fig. 1(a) –(d) shows typical SEM micrographs of the gFe2O3 lines formed on the silica surface after deposition. The deposition was carried out with 15 ml water and 1 g
Fig. 2. (a) An XRD pattern and (b) a Raman spectrum of the g-Fe2O3 nanoparticles.
Fig. 3. Comparison of the SEM micrographs of the lines taken with (a) secondary electrons and (b) backscattered electrons.
Y. Zhou et al. / Materials Letters 59 (2005) 3375 – 3377
Fig. 4. The magnetic hysteresis loop of the g-Fe2O3 nanostructures.
ferrocene. One sees that the silica substrate surface was fully covered with lines of nanoparticles, and that within areas of several hundred micrometers in size, the lines are parallel to each other (see Fig. 1(a) –(b)). As seen from Fig. 1(c), within neighboring areas, the parallel lines trimly divide the substrate surface into distinctive domains by the way of their different orientations. The lines are normally several ten nanometers in width, and several ten to several hundred micrometers long. The lines are formed with nanoparticles. From the discontinuous lines shown by Fig. 1(d), we can clearly see the size of the nanoparticles. Another feature shown by Fig. 1(c) – (d) is that the lines were mainly deposited inside the straight cracks, which might be caused by the interaction between the water vapor and the silica surface at such high temperatures. The nanoparticles forming the lines, different from our previous observations at lower temperatures [10], are gFe2O3. This is interesting, as normally in environments with water, a-Fe2O3 is preferably formed. Fig. 2(a) and (b) shows an XRD pattern and a Raman spectrum of the lines. The XRD pattern was taken with Cu Ka radiation by fixing the incident beam at an angle of ¨3-. The Raman spectrum was taken with an Ar+ laser with a wavelength of 514 nm. From the XRD pattern one sees clearly the (211), (220), (310) and (311) diffraction peaks at 2h positions of ¨23.77-, 30.24-, 33.88- and 35.63-, respectively. These match quite well the documented XRD data for g-Fe2O3 [11]. The Raman spectrum demonstrates also the characteristic peaks in the range of 100¨1100 cm 1. The position and intensity of the peaks are in good agreement with those of g-Fe2O3 [12], which indicate strong peaks at ¨120 cm 1, 270 – 310 cm 1, 340 –380 cm 1, 630 cm 1, 660 cm 1, and 720 cm 1. These suggest that the materials produced by the present approach are g-Fe2O3. Energy dispersive X-ray (EDX) spectrometer analysis (not shown) indicated that these lines contain iron and oxygen. Fig. 3 shows SEM micrographs of the lines taken with secondary electrons (Fig. 3(a)) and backscattered electrons (Fig. 3(b)), respectively. One sees that the lines were formed with heavier elements than silicon. Another evidence for that the lines consisted of gFe2O3 is the magnetic property measurement. Fig. 4 shows a
3377
typical magnetic hysteresis loop of g-Fe2O3 lines, taken with a Lake Shore 7307 Vibrating Sample Magnetometer (VSM) at room temperature. The loop is typically ferromagnetic and gives a coercivity value of ¨160 Oe. These indicate that the lines produced by the present study consist of g-Fe2O3. The lines formation, as shown by Fig. 1(c) – (d), might be due to the interaction of the water vapor with the silica surface at high temperatures, through a mechanism similar to that of the formation of circular patterns of a-Fe2O3 nanoparticles observed previously [10]. Linear cracks formed on the silica surface due to the exposure of silica surface to the water vapor at high temperatures, and then iron nanoparticles formed by the decomposition of ferrocene were oxidized into g-Fe2O3 and deposited inside these cracks; thus forming the linear patterns. The oxidization of iron nanoparticles into g-Fe2O3 rather than a-Fe2O3 in the presence of the water vapor is intriguing and needs further investigation. We have also investigated the influence of temperature and the relative content of ferrocene over water on the formation of the lines of g-Fe2O3 nanoparticles. It was found that at temperatures below 1050 -C g-Fe2O3 nanoparticles could not be formed, while circular patterns of aFe2O3 nanoparticles were formed [10]. Higher contents of water lead to denser and discontinuous lines of g-Fe2O3 nanoparticles, while higher contents of ferrocene lead to the formation of a continuous film of g-Fe2O3. In summary, we have shown that even in the presence of water vapor, g-Fe2O3 nanoparticles could be formed by decomposing ferrocene, and that the nanoparticles could self-organize into arrays of aligned lines of nanometer scale, within an area of several hundred micrometers in size. This work might bring forward a new CVD method to produce gFe2O3 nanostructures, and a new idea to fabricate patterns of catalyst on the nanometer-scale, for growth nanomaterials such as carbon nanotubes. The present study was supported by the National Natural Science Foundation of China and in part by the Ministry of Education of China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
A.P. Alivisatos, et al., Adv. Mater. 10 (1998) 1297. S.H. Tolbert, A.P. Alivisatos, Annu. Rev. Phys. Chem. 46 (1995) 595. G. Bate, J. Magn. Magn. Mater. 100 (1991) 413. D. Vollath, D.V. Szabo, Nanostruct. Mater. 6 (1995) 941. R.D. McMichael, et al., J. Magn. Magn. Mater. 111 (1982) 29. S.P. Bhatnagar, R.E. Rosensweig, J. Magn. Magn. Mater. 149 (1995) 198. R. Massart, C. R. Acad. Sci. Paris, Ser. C 291 (1980) 1. H. Liu, T. Hihara, K. Sumiyama, K. Suzuki, Phys. Status Solidi 169 (1998) 153. D. Vollath, D.V. Szabo, R.D. Taylor, J.O. Willis, J. Mater. Res. 12 (1997) 2175. Z.J. Zhang, B.Q. Wei, P.M. Ajayan, Appl. Phys. Lett. 79 (2001) 4207. JCPDS card, No. 39-1346. Wang, et al., Lunar Planet. Sci. (1998) 1819.
Synthesis and self-organization of g-Fe2O3 nanoparticles by hydrothermal chemical vapor deposition Ya Zhou, Zhengjun Zhang*, Yang Yue Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China Received 9 September 2004; accepted 25 April 2005 Available online 20 June 2005
Abstract We report here the synthesis of g-Fe2O3 nanoparticles in the presence of the water vapor, by thermally decomposing ferrocene at ¨1100 -C in a vacuum of 10 2 Torr. The g-Fe2O3 nanoparticles synthesized, interestingly, self-organized into straight lines of several ten micrometers long and several ten nanometers wide, on planar substrates. This could bring arrays of aligned, nanometer-wide lines of g-Fe2O3 nanoparticles in areas ¨ several hundred micrometers in diameter, thus providing an alternative idea to fabricate patterns of iron particles on the nanometer scale, i.e., by simply deoxidizing the g-Fe2O3 nanoparticles. The structure and the magnetic properties of the g-Fe2O3 particles were also investigated. D 2005 Published by Elsevier B.V.
Materials with reduced dimensions, for instance, nanotubes, nanowires, nanorods, and nanoparticles, etc., are of substantially different properties from their bulk state counterparts [1,2]; and have attracted great interest in the past decade. For example, the ferromagnetic g-Fe2O3 particles gain superparamagnetism when the size touches the nanometer-scaled realm [3,4]. Tremendous effort has thus been devoted, in the past years, to develop new approaches to synthesize materials of reduced dimensions in a controlled manner, understand their dimensionproperty relationship, and to explore the possibility of applying these materials into real devices. Ferromagnetic g-Fe2O3 is a renowned magnetic recording material [3], and can be applied in magnetic refrigeration [5], information storage, controllable drug delivery [6], etc. The synthesis of g-Fe2O3 nanoparticles is therefore of considerable interest [7 – 9]. It has commonly been a recognized iron in environments with water and can be easily oxidized into a-Fe2O3. This is the reason why scientists claimed the possible existence of water in Mars when a-Fe2O3 was observed. We reported here, however, g-
* Corresponding author. E-mail address: [email protected] (Z. Zhang). 0167-577X/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.matlet.2005.04.063
Fe2O3 nanoparticles could also be formed with the presence of water, i.e., by decomposing ferrocene at a relatively high temperature in a low vacuum. The g-Fe2O3 nanoparticles so-produced, as we observed, could self-organize into nanometer-scale lines on planar silica substrates; thus forming arrays of aligned lines within areas of several hundred micrometers in size. Once deoxidized, these patterns might serve as nanometer-scale templates for the growth of other nanomaterials such as carbon nanotubes. The approach we employed in this study was similar to that we used to produce circular patterns of a-Fe2O3 nanoparticles [10]. The difference for producing a-Fe2O3 and g-Fe2O3 is that we used a mixture of ferrocene and water only (in case of a-Fe2O3, xylene was also used), and the decomposition temperature was higher. Prior to the CVD operation, the silicon substrate (capped with a silica layer of ¨125 nm) was cleaned in sequence in acetone, alcohol, and deionized water baths supersonically. Then it was mounted inside a quartz tube furnace. The tube was pumped down to a vacuum on the order of 10 2 Torr, heated up to a temperature of ¨1100 -C. When the temperature was stabilized, a pre-vaporized mixture of deionized water and ferrocene was introduced into the tube for ¨2 min. After that, the tube was cooled to room temperature gradually. The sample was examined with X-ray diffraction (XRD),
3376
Y. Zhou et al. / Materials Letters 59 (2005) 3375 – 3377
Fig. 1. SEM images of the lines of g-Fe2O3 nanoparticles formed on the silica surface: (a) an overview at low magnifications; (b) parallel lines from an area of (a); (c) parallel lines at two neighboring domains; and (d) discontinuous lines showing the size of the particles.
micro Raman spectroscopy and scanning electron microscope (SEM), to observe the morphology and determine the structure of the deposits. Fig. 1(a) –(d) shows typical SEM micrographs of the gFe2O3 lines formed on the silica surface after deposition. The deposition was carried out with 15 ml water and 1 g
Fig. 2. (a) An XRD pattern and (b) a Raman spectrum of the g-Fe2O3 nanoparticles.
Fig. 3. Comparison of the SEM micrographs of the lines taken with (a) secondary electrons and (b) backscattered electrons.
Y. Zhou et al. / Materials Letters 59 (2005) 3375 – 3377
Fig. 4. The magnetic hysteresis loop of the g-Fe2O3 nanostructures.
ferrocene. One sees that the silica substrate surface was fully covered with lines of nanoparticles, and that within areas of several hundred micrometers in size, the lines are parallel to each other (see Fig. 1(a) –(b)). As seen from Fig. 1(c), within neighboring areas, the parallel lines trimly divide the substrate surface into distinctive domains by the way of their different orientations. The lines are normally several ten nanometers in width, and several ten to several hundred micrometers long. The lines are formed with nanoparticles. From the discontinuous lines shown by Fig. 1(d), we can clearly see the size of the nanoparticles. Another feature shown by Fig. 1(c) – (d) is that the lines were mainly deposited inside the straight cracks, which might be caused by the interaction between the water vapor and the silica surface at such high temperatures. The nanoparticles forming the lines, different from our previous observations at lower temperatures [10], are gFe2O3. This is interesting, as normally in environments with water, a-Fe2O3 is preferably formed. Fig. 2(a) and (b) shows an XRD pattern and a Raman spectrum of the lines. The XRD pattern was taken with Cu Ka radiation by fixing the incident beam at an angle of ¨3-. The Raman spectrum was taken with an Ar+ laser with a wavelength of 514 nm. From the XRD pattern one sees clearly the (211), (220), (310) and (311) diffraction peaks at 2h positions of ¨23.77-, 30.24-, 33.88- and 35.63-, respectively. These match quite well the documented XRD data for g-Fe2O3 [11]. The Raman spectrum demonstrates also the characteristic peaks in the range of 100¨1100 cm 1. The position and intensity of the peaks are in good agreement with those of g-Fe2O3 [12], which indicate strong peaks at ¨120 cm 1, 270 – 310 cm 1, 340 –380 cm 1, 630 cm 1, 660 cm 1, and 720 cm 1. These suggest that the materials produced by the present approach are g-Fe2O3. Energy dispersive X-ray (EDX) spectrometer analysis (not shown) indicated that these lines contain iron and oxygen. Fig. 3 shows SEM micrographs of the lines taken with secondary electrons (Fig. 3(a)) and backscattered electrons (Fig. 3(b)), respectively. One sees that the lines were formed with heavier elements than silicon. Another evidence for that the lines consisted of gFe2O3 is the magnetic property measurement. Fig. 4 shows a
3377
typical magnetic hysteresis loop of g-Fe2O3 lines, taken with a Lake Shore 7307 Vibrating Sample Magnetometer (VSM) at room temperature. The loop is typically ferromagnetic and gives a coercivity value of ¨160 Oe. These indicate that the lines produced by the present study consist of g-Fe2O3. The lines formation, as shown by Fig. 1(c) – (d), might be due to the interaction of the water vapor with the silica surface at high temperatures, through a mechanism similar to that of the formation of circular patterns of a-Fe2O3 nanoparticles observed previously [10]. Linear cracks formed on the silica surface due to the exposure of silica surface to the water vapor at high temperatures, and then iron nanoparticles formed by the decomposition of ferrocene were oxidized into g-Fe2O3 and deposited inside these cracks; thus forming the linear patterns. The oxidization of iron nanoparticles into g-Fe2O3 rather than a-Fe2O3 in the presence of the water vapor is intriguing and needs further investigation. We have also investigated the influence of temperature and the relative content of ferrocene over water on the formation of the lines of g-Fe2O3 nanoparticles. It was found that at temperatures below 1050 -C g-Fe2O3 nanoparticles could not be formed, while circular patterns of aFe2O3 nanoparticles were formed [10]. Higher contents of water lead to denser and discontinuous lines of g-Fe2O3 nanoparticles, while higher contents of ferrocene lead to the formation of a continuous film of g-Fe2O3. In summary, we have shown that even in the presence of water vapor, g-Fe2O3 nanoparticles could be formed by decomposing ferrocene, and that the nanoparticles could self-organize into arrays of aligned lines of nanometer scale, within an area of several hundred micrometers in size. This work might bring forward a new CVD method to produce gFe2O3 nanostructures, and a new idea to fabricate patterns of catalyst on the nanometer-scale, for growth nanomaterials such as carbon nanotubes. The present study was supported by the National Natural Science Foundation of China and in part by the Ministry of Education of China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
A.P. Alivisatos, et al., Adv. Mater. 10 (1998) 1297. S.H. Tolbert, A.P. Alivisatos, Annu. Rev. Phys. Chem. 46 (1995) 595. G. Bate, J. Magn. Magn. Mater. 100 (1991) 413. D. Vollath, D.V. Szabo, Nanostruct. Mater. 6 (1995) 941. R.D. McMichael, et al., J. Magn. Magn. Mater. 111 (1982) 29. S.P. Bhatnagar, R.E. Rosensweig, J. Magn. Magn. Mater. 149 (1995) 198. R. Massart, C. R. Acad. Sci. Paris, Ser. C 291 (1980) 1. H. Liu, T. Hihara, K. Sumiyama, K. Suzuki, Phys. Status Solidi 169 (1998) 153. D. Vollath, D.V. Szabo, R.D. Taylor, J.O. Willis, J. Mater. Res. 12 (1997) 2175. Z.J. Zhang, B.Q. Wei, P.M. Ajayan, Appl. Phys. Lett. 79 (2001) 4207. JCPDS card, No. 39-1346. Wang, et al., Lunar Planet. Sci. (1998) 1819.