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Solvothermal reduction synthesis and magnetic properties of polymer protected iron and nickel nanocrystals
Journal of Alloys and Compounds 365 (2004) 112–116
Solvothermal reduction synthesis and magnetic properties of polymer protected iron and nickel nanocrystals Yang-Long Hou1 , Song Gao∗ College of Chemistry and Molecular Engineering, State Key Laboratory of Rare Earths Materials Chemistry and Applications, Peking University, Beijing 100871, PR China Received 6 December 2002; received in revised form 7 April 2003; accepted 18 June 2003
Abstract Nanoscale Fe and Ni particles were prepared by a mild solvothermal reduction route in the presence of polymers. The metal nanocrystals were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). TEM and XRD results show that Fe and Ni nanocrystals have an average size ca. 65 and 15 nm, respectively. The polymers on the surface of nanocrystals prevent iron and nickel from being oxidized in air. Magnetic measurements indicate that Fe nanocrystals are soft ferromagnets, while Ni nanocrystals show a superparamagnetic behaviour. © 2003 Elsevier B.V. All rights reserved. Keywords: Metal nanoparticles; Solvothermal synthesis; Magnetism
1. Introduction
2. Experimental
Owing to the interesting magnetic properties and application potential in data records and storage, more attentions have been attracted on nanoscale magnetic transition metals-based materials, including Ni, Co and Fe [1,2]. For example, the order array of FePt nanoparticles is more promising in magnetic recording due to their large uniaxial magnetocrysalline anisotropy [Ku ≈ 7 × 106 J/m3 ] [3]. Organized blocks of small metal particles have been used to build single-electronic devices [4,5]. A number of physical and chemical routs, such as the thermal decomposition of organometallic precursor, the chemical reduction method and the arc-charge spilling route, were applied to prepare transition metal nanocrystals [6–10]. Recently, hydrothermal approach was also used for the preparation of nanometals such as Co, Ni and Ru [11,12]. Here polymer-protected solvothermal reduction route, a new strategy, was developed to prepare air-stable iron and nickel nanocrystals without observable oxidation.
In a typical experiment, 1 mmol Fe(acac)3 or Ni(acac)2 , 15 mmol poly-(N-vinyl-pyrrolidone) (PVP, Mr = 40 000), 2 ml 85% N2 H5 OH and 85 ml ethylenediamine were mixed in a 100 ml Teflon-lined stainless steel autoclave, then the pH value of solution was adjusted to ca. 12 by the addition of droplets of 1 M NaOH aqueous solution. The autoclave was allowed to heat up to 120–140 ◦ C and maintained at this temperature for 8–12 h. After the autoclave naturally cooled to room temperature, the precipitates were collected and washed with 1:1 methanol–CHCl3 and distilled water in sequence to remove the possible excess PVP, N2 H5 OH and other by-products. The products were dried in vacuum at 60 ◦ C for 12 h. Infrared spectroscopy on KBr pellets was recorded on a NICOLET Magna-IR 750 spectrophotometer. X-ray powder diffraction (XRD) was performed on a Rigaku Dmax /2000 diffractometer under Cu K␣ radiation. The particle size and its distribution were determined by transmission electron microscopy (TEM) in a JEOL 200CX (200 kV). The contents of iron or nickel in the products were determined on an inductively coupled plasma-atomic emission spectrometer (ICP). X-ray photoelectron spectroscopy (XPS) data were collected on a ESCA-lab5. Magnetic studies were carried
∗
Corresponding author. Fax: +86-10-6275-1708. E-mail address: [email protected] (S. Gao). 1 Present address: Department of Chemistry, The University of Tokyo, Tokyo, Japan. 0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00651-0
Y.-L. Hou, S. Gao / Journal of Alloys and Compounds 365 (2004) 112–116
out by using a Quantum Design SQUID and an OXFORD Maglab 2000 system.
3. Results and discussions 3.1. X-ray diffraction and TEM observation The reaction in ethylenediamine might be proposed as follows: M(acac)n + N2 H5 OH + OH− → M + N2 + H2 O
(1)
In addition to the protection of PVP, the nitrogen gas produced in the system supplied an inert ambience to keep nanocrystals from being oxidized. Cubic iron and nickel metals, as identified by X-ray power diffraction pattern of the products (Fig. 1), were formed after the reaction process. TEM photograph (Fig. 2a) indicates that the average size of iron nanoparticles is about ca. 65 nm, closing to the value calculated by Scherrer equation (61 nm) [13], and the distribution is relatively narrow. Due to the equilibrium of
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self-repulsion and attracting, self-assembly and agglomeration were occurred when magnetic Fe nanometals were dispersed on a carbon-coated copper grid. In the case of Ni nanocrystals, the size of nanoparticles is ca. 15 nm, close to the value of 16 nm calculated from Scherrer equation. As displayed in Fig. 2c, a certain extent of aggregations were observed, which might resulted from magnetic interaction and polymer adherence between smaller particles. It is well known that nanometer sized iron particles are easy to be oxidized in air. However, from XRD pattern (Fig. 1a), except for pure cubic iron, no other iron oxide (Fe2 O3 or Fe3 O4 ) phase was detected, which might result from the protection of polymer on the surface of nanoparticles. IR spectroscopy of the polymer-coated Fe particles is shown in Fig. 3a, suggesting the existence of organic components. Compared with IR spectroscopy of neat PVP (Fig. 3b), the vibration peak of carbonyl (1659 cm−1 ) in PVP has a ca. 60 cm−1 shift (1582 cm−1 ) in iron nanocrystals, which might be attributed to the interaction between polymers and the surface of iron. On the other hand, a possible thin oxide layer on the surface of iron particles might also protect metals against further oxidation. Because of nodection of iron oxides by XRD, only a very small fraction and/or poor crystalline exists, if any. The IR pattern of nickel particles is similar to that of iron particles. Figs. 4 and 5 show the XPS spectra of iron and nickel nanoparticles, respectively. The XPS data revealed that the surface of samples has no evident characteristic of metals before bombardment, as shown in Figs. 4b and 5b. This result is due to the presence of polymer on the surface of samples. After the bombardment of Ar ion with the voltage of 2 kV for 5 min, the spectra are agreement with that of the bulk metal materials [14]. It also suggests that the samples are coated with polymers, which protect nanoparticles from further oxidation. According to the results of ICP, the metal contents in iron and nickel nanocrystals are 95% and 59%, respectively. 3.2. Magnetic properties
Fig. 1. X-ray diffraction patterns of as-prepared iron (a) and nickel (b) particles.
The temperature dependence of magnetization for iron particles was measured in an applied magnetic field of 100 Oe in the range of 5 to 300 K using zero-field-cooling (ZFC) and field-cooling (FC) procedures. As shown in Fig. 6, the none cross of ZFCM and FCM curves in the temperature range of measurement suggests that the blocking temperature of the sample should be above 300 K, which might be caused by the large size of iron particles. Field dependence of magnetization was shown in Fig. 7. The magnetization values obtained at 5 and 300 K under 60 kOe field are 183 and 179 emu/g, respectively, slightly lower than that of bulk iron (Ms , 222 emu/g) and 40 nm-sized iron particles [8], suggesting magnetism was slightly reduced by PVP that coated on the surface of iron particles. As shown in the inset of Fig. 7, the coercivity (HC ) of iron nanoparticles is almost negligible even at 5 K, which is a very typical behavior for a
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Fig. 2. TEM image of iron (a) and nickel particles (c) together with the corresponding size distribution histograms (panels b and d, respectively).
soft magnet. Apparently, iron nanoparticles with the diameter of 65 nm show a bulk-like iron magnetic characteristic. The ZFCM and FCM of Ni sample are displayed in Fig. 8. With the increasing of temperature, ZFCM comes to a maximum at around room temperature, while FCM decreases monotonously, but the two curves have not reached a cross
point even at 330 K. Such a behavior is the characteristic of superparamagnetism. Assuming the blocking temperature (TB ) is 350 K, the effective magnetic anisotropy constant (K) of each particle can be estimated from the blocking temperature using the expression K = 25kb TB /V, where V is the volume of the particle and kb is the Boltzman con-
Fig. 3. Infrared spectroscopy of iron nanoparticles (a) and PVP (b).
Y.-L. Hou, S. Gao / Journal of Alloys and Compounds 365 (2004) 112–116
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Fig. 4. XPS spectra of Fe nanoparticles, (a) is the surface after bombardment by Ar+ with the voltage of 2 kV for 5 min and (b) is the surface of the original sample. Fig. 7. M–H curves and hysteresis loop of iron nanoparticles measured at 5 K and 300 K.
Fig. 5. XPS spectra of Ni nanoparticles, (a) is the surface after bombardment by Ar+ with the voltage of 2 kV for 5 min and (b) is the surface of the original sample.
Fig. 6. ZFC and FC magnetization curves of iron nanoparticles measured at 100 Oe field.
stant. For a 15 nm-sized Ni particle, the calculated value of K is 6.8 × 105 erg/cm3 , which is comparable to the magnetocrystalline anisotropy of bulk nickel, 2.3 × 105 erg/cm3 . The increasing of magnetocrystalline anisotropy is attributed to surface effect of nanoscale nickel particles. Similar behaviour was also observed in nanosclae Fe2 O3 system [16]. The hysteresis loop of Ni nanopartilces is shown in the inset of Fig. 8, the coercivity is 107 Oe at 5 K and 90 Oe at 293 K, respectively, comparable to the reported values for Ni nanoparticles [15]. This magnetization almost reaches the saturation of 55 emu/gNi at 10 kOe and 5 K, very close to the value of bulk nickel (57.3 emu/g). It suggests that the coat of PVP on nickel nanoparticles does not reduce surface magnetism, which coincides with the previous results [17].
Fig. 8. ZFC and FC magnetization curves and hysteresis loops of nickel nanoparticles at 5 K and 293 K.
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4. Conclusions We report the formation of cubic iron and nickel nanoparticles through a solvothermal reduction route, this method is facile and inexpensive. The passivation of metal surface by PVP gives stable nanometal particles in air. Moreover, the PVP has only slightly reduced the surface magnetism of nanoscale iron and nickel particles. Iron nanoparticles behave as a bulk-like soft ferromagnet below the measured temperature, whereas nickel nanocrystals have common features of superparamagnets. Acknowledgements We acknowledge the financial support from the National Natural Science Fund for Distinguished Young Scholars (20125104), State Key Project for Fundamental Research (G1998061305). References [1] D.L. Feldheim, C.A. Foss Jr. (Eds.), Metal Nanoparticles: Synthesis, Characterization and Applications, Marcel Dekker, New York, 2002.
[2] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [3] S. Sun, C.B. Murray, W. Dieter, L. Folks, A. Moser, Science 287 (2000) 1989. [4] A.P. Alivisatos, Science 272 (1996) 1323. [5] C.B. Murray, C.R. Kagan, M.G. Bawendi, Science 270 (1995) 1335. [6] S. Sun, C.B. Murray, J. Appl. Phys. 85 (1999) 4325. [7] C. Petit, A. Taleb, M. Pileni, J. Phys. Chem. B103 (1999) 1805; M. Green, P. O’Brien, Chem. Commun. (2001) 1912. [8] X.G. Li, S. Takahashi, K. Watanabe, Y. Kikuchi, M. Koishi, Nano Lett. 1 (2001) 475; N. Cordente, M. Respaud, F. Senocq, M.-J. Casanove, C. Amiens, B. Chaudret, Nano Lett. 1 (2001) 565. [9] S.J. Park, S. Kim, S. Lee, Z.G. Khim, K. Char, T. Hyeon, J. Am. Chem. Soc. 122 (2000) 8581. [10] Y.P. Sun, H.W. Rollins, R. Guduru, Chem. Mater. 11 (1999) 7. [11] Y.D. Li, L.Q. Li, H.W. Liao, H.R. Wang, J. Mater. Chem. 9 (1999) 2675. [12] S. Gao, J. Zhang, Y.F. Zhu, C.M. Che, New J. Chem. 24 (2000) 739. [13] H.P. Klig, L.E. Alexander, X-ray Diffraction Procedures, Wiley, New York, 1959. [14] J.F. Moudler, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1992. [15] J.H. Hwang, V.P. Dravid, M.H. Teng, J.J. Host, B.R. Elliott, D.L. Johnson, J. Mater. Res. 12 (1997) 1076. [16] E. del Barco, J. Asenjo, X.X. Zhang, R. Pieczynski, A. Julià, J. Tejada, R.F. Ziolo, Chem. Mater. 13 (2001) 1487. [17] T. Ould Ely, C. Amiens, B. Chaudret, E. Snoeck, M. Verelst, M. Respaud, J.-M. Broto, Chem. Mater. 11 (1999) 526.
Solvothermal reduction synthesis and magnetic properties of polymer protected iron and nickel nanocrystals Yang-Long Hou1 , Song Gao∗ College of Chemistry and Molecular Engineering, State Key Laboratory of Rare Earths Materials Chemistry and Applications, Peking University, Beijing 100871, PR China Received 6 December 2002; received in revised form 7 April 2003; accepted 18 June 2003
Abstract Nanoscale Fe and Ni particles were prepared by a mild solvothermal reduction route in the presence of polymers. The metal nanocrystals were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). TEM and XRD results show that Fe and Ni nanocrystals have an average size ca. 65 and 15 nm, respectively. The polymers on the surface of nanocrystals prevent iron and nickel from being oxidized in air. Magnetic measurements indicate that Fe nanocrystals are soft ferromagnets, while Ni nanocrystals show a superparamagnetic behaviour. © 2003 Elsevier B.V. All rights reserved. Keywords: Metal nanoparticles; Solvothermal synthesis; Magnetism
1. Introduction
2. Experimental
Owing to the interesting magnetic properties and application potential in data records and storage, more attentions have been attracted on nanoscale magnetic transition metals-based materials, including Ni, Co and Fe [1,2]. For example, the order array of FePt nanoparticles is more promising in magnetic recording due to their large uniaxial magnetocrysalline anisotropy [Ku ≈ 7 × 106 J/m3 ] [3]. Organized blocks of small metal particles have been used to build single-electronic devices [4,5]. A number of physical and chemical routs, such as the thermal decomposition of organometallic precursor, the chemical reduction method and the arc-charge spilling route, were applied to prepare transition metal nanocrystals [6–10]. Recently, hydrothermal approach was also used for the preparation of nanometals such as Co, Ni and Ru [11,12]. Here polymer-protected solvothermal reduction route, a new strategy, was developed to prepare air-stable iron and nickel nanocrystals without observable oxidation.
In a typical experiment, 1 mmol Fe(acac)3 or Ni(acac)2 , 15 mmol poly-(N-vinyl-pyrrolidone) (PVP, Mr = 40 000), 2 ml 85% N2 H5 OH and 85 ml ethylenediamine were mixed in a 100 ml Teflon-lined stainless steel autoclave, then the pH value of solution was adjusted to ca. 12 by the addition of droplets of 1 M NaOH aqueous solution. The autoclave was allowed to heat up to 120–140 ◦ C and maintained at this temperature for 8–12 h. After the autoclave naturally cooled to room temperature, the precipitates were collected and washed with 1:1 methanol–CHCl3 and distilled water in sequence to remove the possible excess PVP, N2 H5 OH and other by-products. The products were dried in vacuum at 60 ◦ C for 12 h. Infrared spectroscopy on KBr pellets was recorded on a NICOLET Magna-IR 750 spectrophotometer. X-ray powder diffraction (XRD) was performed on a Rigaku Dmax /2000 diffractometer under Cu K␣ radiation. The particle size and its distribution were determined by transmission electron microscopy (TEM) in a JEOL 200CX (200 kV). The contents of iron or nickel in the products were determined on an inductively coupled plasma-atomic emission spectrometer (ICP). X-ray photoelectron spectroscopy (XPS) data were collected on a ESCA-lab5. Magnetic studies were carried
∗
Corresponding author. Fax: +86-10-6275-1708. E-mail address: [email protected] (S. Gao). 1 Present address: Department of Chemistry, The University of Tokyo, Tokyo, Japan. 0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00651-0
Y.-L. Hou, S. Gao / Journal of Alloys and Compounds 365 (2004) 112–116
out by using a Quantum Design SQUID and an OXFORD Maglab 2000 system.
3. Results and discussions 3.1. X-ray diffraction and TEM observation The reaction in ethylenediamine might be proposed as follows: M(acac)n + N2 H5 OH + OH− → M + N2 + H2 O
(1)
In addition to the protection of PVP, the nitrogen gas produced in the system supplied an inert ambience to keep nanocrystals from being oxidized. Cubic iron and nickel metals, as identified by X-ray power diffraction pattern of the products (Fig. 1), were formed after the reaction process. TEM photograph (Fig. 2a) indicates that the average size of iron nanoparticles is about ca. 65 nm, closing to the value calculated by Scherrer equation (61 nm) [13], and the distribution is relatively narrow. Due to the equilibrium of
113
self-repulsion and attracting, self-assembly and agglomeration were occurred when magnetic Fe nanometals were dispersed on a carbon-coated copper grid. In the case of Ni nanocrystals, the size of nanoparticles is ca. 15 nm, close to the value of 16 nm calculated from Scherrer equation. As displayed in Fig. 2c, a certain extent of aggregations were observed, which might resulted from magnetic interaction and polymer adherence between smaller particles. It is well known that nanometer sized iron particles are easy to be oxidized in air. However, from XRD pattern (Fig. 1a), except for pure cubic iron, no other iron oxide (Fe2 O3 or Fe3 O4 ) phase was detected, which might result from the protection of polymer on the surface of nanoparticles. IR spectroscopy of the polymer-coated Fe particles is shown in Fig. 3a, suggesting the existence of organic components. Compared with IR spectroscopy of neat PVP (Fig. 3b), the vibration peak of carbonyl (1659 cm−1 ) in PVP has a ca. 60 cm−1 shift (1582 cm−1 ) in iron nanocrystals, which might be attributed to the interaction between polymers and the surface of iron. On the other hand, a possible thin oxide layer on the surface of iron particles might also protect metals against further oxidation. Because of nodection of iron oxides by XRD, only a very small fraction and/or poor crystalline exists, if any. The IR pattern of nickel particles is similar to that of iron particles. Figs. 4 and 5 show the XPS spectra of iron and nickel nanoparticles, respectively. The XPS data revealed that the surface of samples has no evident characteristic of metals before bombardment, as shown in Figs. 4b and 5b. This result is due to the presence of polymer on the surface of samples. After the bombardment of Ar ion with the voltage of 2 kV for 5 min, the spectra are agreement with that of the bulk metal materials [14]. It also suggests that the samples are coated with polymers, which protect nanoparticles from further oxidation. According to the results of ICP, the metal contents in iron and nickel nanocrystals are 95% and 59%, respectively. 3.2. Magnetic properties
Fig. 1. X-ray diffraction patterns of as-prepared iron (a) and nickel (b) particles.
The temperature dependence of magnetization for iron particles was measured in an applied magnetic field of 100 Oe in the range of 5 to 300 K using zero-field-cooling (ZFC) and field-cooling (FC) procedures. As shown in Fig. 6, the none cross of ZFCM and FCM curves in the temperature range of measurement suggests that the blocking temperature of the sample should be above 300 K, which might be caused by the large size of iron particles. Field dependence of magnetization was shown in Fig. 7. The magnetization values obtained at 5 and 300 K under 60 kOe field are 183 and 179 emu/g, respectively, slightly lower than that of bulk iron (Ms , 222 emu/g) and 40 nm-sized iron particles [8], suggesting magnetism was slightly reduced by PVP that coated on the surface of iron particles. As shown in the inset of Fig. 7, the coercivity (HC ) of iron nanoparticles is almost negligible even at 5 K, which is a very typical behavior for a
114
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Fig. 2. TEM image of iron (a) and nickel particles (c) together with the corresponding size distribution histograms (panels b and d, respectively).
soft magnet. Apparently, iron nanoparticles with the diameter of 65 nm show a bulk-like iron magnetic characteristic. The ZFCM and FCM of Ni sample are displayed in Fig. 8. With the increasing of temperature, ZFCM comes to a maximum at around room temperature, while FCM decreases monotonously, but the two curves have not reached a cross
point even at 330 K. Such a behavior is the characteristic of superparamagnetism. Assuming the blocking temperature (TB ) is 350 K, the effective magnetic anisotropy constant (K) of each particle can be estimated from the blocking temperature using the expression K = 25kb TB /V, where V is the volume of the particle and kb is the Boltzman con-
Fig. 3. Infrared spectroscopy of iron nanoparticles (a) and PVP (b).
Y.-L. Hou, S. Gao / Journal of Alloys and Compounds 365 (2004) 112–116
115
Fig. 4. XPS spectra of Fe nanoparticles, (a) is the surface after bombardment by Ar+ with the voltage of 2 kV for 5 min and (b) is the surface of the original sample. Fig. 7. M–H curves and hysteresis loop of iron nanoparticles measured at 5 K and 300 K.
Fig. 5. XPS spectra of Ni nanoparticles, (a) is the surface after bombardment by Ar+ with the voltage of 2 kV for 5 min and (b) is the surface of the original sample.
Fig. 6. ZFC and FC magnetization curves of iron nanoparticles measured at 100 Oe field.
stant. For a 15 nm-sized Ni particle, the calculated value of K is 6.8 × 105 erg/cm3 , which is comparable to the magnetocrystalline anisotropy of bulk nickel, 2.3 × 105 erg/cm3 . The increasing of magnetocrystalline anisotropy is attributed to surface effect of nanoscale nickel particles. Similar behaviour was also observed in nanosclae Fe2 O3 system [16]. The hysteresis loop of Ni nanopartilces is shown in the inset of Fig. 8, the coercivity is 107 Oe at 5 K and 90 Oe at 293 K, respectively, comparable to the reported values for Ni nanoparticles [15]. This magnetization almost reaches the saturation of 55 emu/gNi at 10 kOe and 5 K, very close to the value of bulk nickel (57.3 emu/g). It suggests that the coat of PVP on nickel nanoparticles does not reduce surface magnetism, which coincides with the previous results [17].
Fig. 8. ZFC and FC magnetization curves and hysteresis loops of nickel nanoparticles at 5 K and 293 K.
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4. Conclusions We report the formation of cubic iron and nickel nanoparticles through a solvothermal reduction route, this method is facile and inexpensive. The passivation of metal surface by PVP gives stable nanometal particles in air. Moreover, the PVP has only slightly reduced the surface magnetism of nanoscale iron and nickel particles. Iron nanoparticles behave as a bulk-like soft ferromagnet below the measured temperature, whereas nickel nanocrystals have common features of superparamagnets. Acknowledgements We acknowledge the financial support from the National Natural Science Fund for Distinguished Young Scholars (20125104), State Key Project for Fundamental Research (G1998061305). References [1] D.L. Feldheim, C.A. Foss Jr. (Eds.), Metal Nanoparticles: Synthesis, Characterization and Applications, Marcel Dekker, New York, 2002.
[2] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [3] S. Sun, C.B. Murray, W. Dieter, L. Folks, A. Moser, Science 287 (2000) 1989. [4] A.P. Alivisatos, Science 272 (1996) 1323. [5] C.B. Murray, C.R. Kagan, M.G. Bawendi, Science 270 (1995) 1335. [6] S. Sun, C.B. Murray, J. Appl. Phys. 85 (1999) 4325. [7] C. Petit, A. Taleb, M. Pileni, J. Phys. Chem. B103 (1999) 1805; M. Green, P. O’Brien, Chem. Commun. (2001) 1912. [8] X.G. Li, S. Takahashi, K. Watanabe, Y. Kikuchi, M. Koishi, Nano Lett. 1 (2001) 475; N. Cordente, M. Respaud, F. Senocq, M.-J. Casanove, C. Amiens, B. Chaudret, Nano Lett. 1 (2001) 565. [9] S.J. Park, S. Kim, S. Lee, Z.G. Khim, K. Char, T. Hyeon, J. Am. Chem. Soc. 122 (2000) 8581. [10] Y.P. Sun, H.W. Rollins, R. Guduru, Chem. Mater. 11 (1999) 7. [11] Y.D. Li, L.Q. Li, H.W. Liao, H.R. Wang, J. Mater. Chem. 9 (1999) 2675. [12] S. Gao, J. Zhang, Y.F. Zhu, C.M. Che, New J. Chem. 24 (2000) 739. [13] H.P. Klig, L.E. Alexander, X-ray Diffraction Procedures, Wiley, New York, 1959. [14] J.F. Moudler, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1992. [15] J.H. Hwang, V.P. Dravid, M.H. Teng, J.J. Host, B.R. Elliott, D.L. Johnson, J. Mater. Res. 12 (1997) 1076. [16] E. del Barco, J. Asenjo, X.X. Zhang, R. Pieczynski, A. Julià, J. Tejada, R.F. Ziolo, Chem. Mater. 13 (2001) 1487. [17] T. Ould Ely, C. Amiens, B. Chaudret, E. Snoeck, M. Verelst, M. Respaud, J.-M. Broto, Chem. Mater. 11 (1999) 526.