NANO LETTERS
Fabrication, Structure, and Magnetic Properties of Highly Ordered Prussian Blue Nanowire Arrays
2002 Vol. 2, No. 8 845-847
Pingheng Zhou,† Desheng Xue,*,† Haiqing Luo,‡ and Xingguo Chen‡ Key Laboratory for Magnetism and Magnetic Materials of MOE, and School of Chemistry and Chemical Engineering, Lanzhou UniVersity, Lanzhou 730000, P. R. China Received May 16, 2002; Revised Manuscript Received June 7, 2002
ABSTRACT Highly ordered Prussian blue nanowire arrays with diameters of about 50 nm and lengths up to 4µm have been fabricated for the first time by an electrodepositing technology with two-step anodizing anodic aluminum oxide films. X-ray diffraction and transmission electron microscopy measurement results show that the Prussian blue nanowires are dense, continuous, and highly crystalline with an fcc cubic structure of a ) 10.14 Å. The Mo1 ssbauer spectrum and infrared spectrum at room temperature indicate that the nanowires are ferric ferrocyanide: one kind of iron is Fe3+ with high spin and the other is Fe2+ with low spin. Magnetic property measurement results indicate that the Curie temperature of Prussian blue nanowire decreases as the average numbers of magnetic interaction neighbors is reduced.
Theoretical predictions suggest that one-dimensional Ising model magnetic systems show no magnetic ordering at nonzero temperature,1 and a number of experimental results of quasi-one-dimensional molecular-based magnets and magnetic metal nanowires have been reported.2 However, since these quasi-one-dimensional molecular-based magnets are still three-dimensional solids, it is a challenge to decrease their dimensions further. So it is very interesting to fabricate the nanowire of molecular-based magnet, which is an excellent way to understand the theoretical one-dimensional Ising model magnetic systems. Recently, Prussian blue analogues have played an important role in the field of molecular magnets, and many unusual properties have been found.3 Prussian blue is a mixed-valent cyanoferrate of stoichiometry Fe3+4[Fe2+(CN)6]3‚xH2O with a face-centered-cubic structure in which Fe3+ is high spin with S ) 5/2 and Fe2+ is low spin with S ) 0. It shows a long-range ferromagnetic ordering at Tc ) 5.6 K in which magnetic interactions occur between the Fe3+ ions through the 10-Å long Fe3+-NC-Fe2+-CN-Fe3+ linkages.4 Prior work by N. D. Vernon has shown that Prussian blue films can be deposited directly by electrochemical reaction in Fe3+ and [Fe(CN)6]4- acidic solution.5 In addition, porous alumina is a good choice for a template because the pore diameters are easily adjusted to sizes enabling quantum confinement. We therefore fabricated nanoscale Prussian blue wires in * Corresponding author. E-mail
[email protected] † Key Laboratory for Magnetism and Magnetic Materials of MOE. ‡ School of Chemistry and Chemical Engineering. 10.1021/nl0256154 CCC: $22.00 Published on Web 07/03/2002
© 2002 American Chemical Society
porous alumina template. In this letter, conditions that yield continuous and highly crystalline nanowires are described, and some characteristics and magnetic properties of the nanowires are discussed. The highly ordered porous alumina templates were fabricated by a two-step anodizing process.6 First, aluminum foil (99.999%) was anodized in 0.3 M oxalic solution at 40 VDC for 15 min at 10 °C, then the oxide film was dissolved in a mixed solution of 0.2 M H2CrO4 and 0.4 M H3PO4 at 60 °C. Second, the newly patterned aluminum substrate was anodized for 3 h again. Electrodeposition of the Prussian blue nanowires was performed using a standard double-electrode cell. The work electrode (porous alumina template) and counter electrode (graphite) were submerged in a fresh solution of 0.02 M FeCl3‚6H2O, 0.02 M K3Fe(CN)6, 0.6 M H3BO4, and 0.5 M KCl. The nanowires were deposited under 1 Hz 13 VAC. The nanowires were separated from the alumina template by dissolving the alumina layer with 1 M aqueous solution of H2SO4 for transmission electron microscopy investigation. The electrochemical reaction can be formulated as 4Fe3+ + 3e-1 + 3[Fe(CN)6]3- + xH2O f 4Fe3+ + 3[Fe(CN)6]4- + xH2O f Fe3+4[Fe2+(CN)6]3‚xH2O A scanning electron micrograph (JSM-5600) of a porous alumina template is shown in Figure 1a, from which it is found that the nanopores are uniform and highly ordered with diameters of about 50 nm. A bright-field image of dispersed
Figure 1. (a) SEM image of porous alumina template, (b) TEM image of dispersed Prussian blue nanowires. Figure 4. The Mo¨ssbauer spectra of Prussian blue nanowires which were obtained at RT.
Figure 2. TEM image and SAED image of a single Prussian blue nanowire taken in the sequence of (a) the first SAED image, (b) the following dark-field image, (c) the bright-field image, (d) the last SAED image.
Figure 3. X-ray diffraction patterns of Prussian blue bulk and alumina template filled with Prussian blue nanowires, using Cu KR radiation.
Prussian blue nanowires with diameters of about 50 nm and lengths up to 4 µm (JEOL 2000) is shown in Figure 1b. Analysis of a single wire, both by imaging and electron diffraction, is shown in Figure 2. Figure 2a is the first selected area electron diffraction pattern of a single Prussian blue nanowire in which there are six clear diffraction spots. Figure 2b is the following dark-image of the same nanowire. Both indicate that the nanowire is dense, continuous, and highly crystalline. When the electron-beam irradiation keeps going on, the wire is broken and comprises many small crystal particles as shown in the later bright-field image (Figure 2c). The last selected area electron diffraction pattern of the same nanowire (Figure 2d) shows three circle patterns, which evolve from the diffraction spots of Figure 4a as the nanowire breaks. The reason is that the temperature of nanowire under electron-beam irradiation for several minutes rises to over 100 °C, which causes the water molecular to evaporate and the structure of the nanowire to break. In addition, both the diffraction spots and the diffraction circles above indicate that the Prussian blue nanowire has a face-centered-cubic structure. In conclusion, the Prussian blue nanowire is dense, continuous and highly crystalline, which will be regarded 846
as a good quasi-one-dimensional nanomagnet for the following magnetic study. A powder X-ray diffraction patterns (Rigaku/Max-2400 Diffractometer, with Cu KR radiation) for Prussian blue nanowires, shown in Figure 3, compares well to Prussian blue bulk.7 All peaks can be indexed to the cubic space group Fm3m, except the broad low-angle bulge, which is due to the amorphous Al2O3 template. The lattice parameter is 10.14 Å, which is almost same as the 10.11 Å of the Prussian blue bulk. The Mo¨ssbauer spectrum of Prussian blue (a constant acceleration with a source of 57Co in rhodium, the spectra were fitted with Lorentz line shapes), shown in Figure 4, consists of a doublet absorption peak and a singlet absorption peak that are ascribable to the high spin Fe3+ and low spin Fe2+, respectively.8 The infrared spectrum measurement at room temperature in the region from 2000 to 2200 cm-1 showed one peak at 2158 cm-1, which is due to the stretching of CN in the Fe2+-CN-Fe3+ links. So the oxidation state of the iron ions in Prussian blue nanowire can be expressed as Fe2+-CN-Fe3+. The quadrupole splitting of nanowire shifts from 0.391 mm/s to 0.596 mm/s compared with the Prussian blue bulk. The reason is that the electric field gradient of nanowires increases as the dimension is reduced. The isomer shifts of Fe3+ and Fe2+ ions of Prussian blue nanowires are 0.643 and 0.134 mm/s, respectively, which are a litter larger than that of Prussian blue bulk 0.629 and 0.117 mm/s. The line widths of Fe3+ and Fe2+ ions of Prussian blue nanowires are 0.580 and 0.467 mm/s, respectively, which increase with respect to that of Prussian blue bulk 0.485 and 0.294 mm/s. It is concluded that the symmetry environment of iron ions in nanowires decreases compared with that of Prussian blue bulk. The magnetic properties of the nanowires were investigated with the magnetic property measurement system (MPMS). The field-cooled magnetization versus temperature of Prussian blue nanowires at H ) 10 G shows a break at Tc ) 4.1 K, which is lower with respect to that of Prussian blue bulk Tc ) 5.6 K as shown in Figure 5. The susceptibility measurements were carried out from T ) 1.9 to 10 K. The linear part of the χM-1 versus temperature curve was fitted to the Curie-Weiss law χM ) C/(T - θ). It is concluded that The Weiss constant θ is 3.76 K, which indicates a long ferromagnetic interaction between the Fe3+ ions through the Nano Lett., Vol. 2, No. 8, 2002
and infrared spectrum measurement results indicate that the oxidation state of iron ions in Prussian blue nanowires can be expressed as Fe2+-CN-Fe3+. Magnetic measurement results indicate that the Curie temperature of Prussian blue nanowire decreases as the average numbers of magnetic interaction neighbors is reduced.
Figure 5. Field-cooled magnetization versus temperature curves at H ) 10 G, thermal variation of the susceptibility of Prussian blue nanowire, χMT versus T (inset).
cyanide bridge and Fe2+ ions. For Prussian blue analogues, the Curie temperature Tc is expressed as9
Tc )
2xZijZji|Jij| xSi(Si + 1)Sj(Sj + 1) 3kB
where i ) j ) Fe3+, Si ) Sj ) 5/2, Zij is the number of nearest magnetic interaction neighbors and kB is the Boltzmann constant. As the lattice parameter of nanowires does not change, we suppose that the magnetic interaction constant Jij between the Fe3+ ions of Prussian blue nanowires is almost same as that of Prussian blue bulk. On the basis of this equation, it is concluded that the decrease of Curie temperature Tc in Prussian blue nanowire perhaps comes from the diminution of the average number of nearest magnetic interaction neighbors as Prussian blue becomes a quasi-onedimension nanomagnet from a three dimension solid. In summary, we have demonstrated for the first time the fabrication of highly ordered Prussian blue nanowire arrays by direct electrodeposition into an alumina template. X-ray diffraction and transmission electron microscope measurement results show that nanowires are continuous and highly crystalline with an fcc cubic structure. Mo¨ssbauer spectrum
Nano Lett., Vol. 2, No. 8, 2002
Acknowledgment. This work is supported by the TransCentury Training Program Foundation for the Talent of MOE, P. R. China and EYTP of China. Professor Chen would like to give thanks to the Visiting Scholar Foundation of the Key Lab for Magnetism and Magnetic Material of MOE. References (1) (a) Ising, E. Z. Phys. 1925, 31, 253. (b) Newell, G. F.; Montroll, E. W. ReV. Mod. Phys. 1953, 25, 159. Ruelle, D. Commun. Math. Phys. 1968, 9, 267. (2) (a) Nakatani, K.; et al. Inorg. Chem. 1994, 6, 257. (b) Caneschi, A.; Gatteschi, D.; Renard, J. P.; et al. J. Am. Chem. Soc. 1989, 111, 785. (c) Yee, G. T.; et al. AdV. Mater. 1991, 3, 309. (d) Bray, J. W.; Interrante, L. W.; Jacobs, I. S.; Bonner, J. C. In Extended Linear Chain Compounds; Miller J. S., Ed.; Plenum: New York, 1983; Vol. 3. (e) Sellmyer, D. J.; Zheng M.; Skomski, R. J. Phys.: Condens. Matter 2001, 13, R433. (3) (a) Ohkoshi, S.; Abe, Y.; Fujishima, A.; Hashimoto, K. Phys. ReV. Lett. 1999, 62, 1285. (b) Sato, O.; Lyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 271, 49. (c) Sato, O.; Lyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 272, 704. (d) Ferlay. S.; Mallah, T.; Quahe`s, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701. (4) (a) Herren, F.; Fischer, P.; Ludi, A.; Ha¨lg, W. Inorg. Chem. 1980, 19, 956. (b) Hoden, A. N.; Williams, H. J.; Walsh, D. E. Phys. ReV. 1956, 103, 572. (5) Vernon, D. N. J. Electrochem. Soc. 1978, 125, 886. (6) Masuda, H.; Fukada, K. Science 1995, 268, 1466. (7) 0.02 M aqueous solution of K4Fe(CN)6 was immediately added to a vigorously stirred 0.02 M aqueous solution of FeCl3. The resulting precipitate was filtered and washed with methanol and diethyl ether. The average particle size obtained by transmission electron microscope is about 1.5 µm. (8) (a) Duncan, F.; Wigley, P. W. R. J. Chem. Soc. 1963, 1120. (b) Kemp, M. J.; Beasley, M. L.; Collins, R. L.; Millingan, W. D. J. Am. Chem. Soc. 1968, 90, 3201. (9) Ohkoshi, S.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Phys. ReV. B 1997, 56, 11642.
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