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J. Phys. Chem. C 2008, 112, 4168-4171
Magnetic and Thermal Expansion Properties of Vertically Aligned Fe Nanotubes Fabricated by Electrochemical Method X. J. Xu,† S. F. Yu,*,† S. P. Lau,† L. Li,‡ and B. C. Zhao§ School of Electrical and Electronics Engineering, Nanyang Technological UniVersity, Block S2, Nanyang AVenue, Singapore 639798, Singapore, AdVanced Materials for Micro- and Nano-Systems, Singapore-MIT Alliance, National UniVersity of Singapore, 4 Engineering DriVe 3, Singapore 117576, Singapore, and School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, 1 Nanyang Walk, Block 5, Singapore 637616, Singapore ReceiVed: December 10, 2007; In Final Form: December 30, 2007
A simple electrochemical method, which involves the use of anodic alumina membranes (AAMs) as the template, was used to synthesize vertically aligned Fe nanotubes. It is shown that the thickness of the Au cathode electrode on the AAM and the use of a direct electrodeposition technique are the two key factors to realize Fe nanotubes. It is noted that the wall thickness of the Fe nanotubes can be preciously controlled by the deposition time and the nanotubes can change to nanowires as the deposition time prolongs. Magnetic and thermal expansion properties of both Fe nanotubes and nanowires are studied and compared. It is found that the Fe nanotubes have a large coercive force and are physically robust to the change in temperature as compared with Fe nanowires.
1. Introduction Template-based synthesis using anodic alumina membranes (AAMs) has attracted extensive attention for the fabrication of one-dimensional arrays of nanostructures. This is because the template-based synthesis can offer a convenient way to realize uniform aligned nanostructures inside the template matrices.1,2 In addition, a wide range of aligned nanostructures such as metals, semiconductors, and polymers can be grown via the AAMs.3,4 In particular, some groups have reported the nanofabrication of metallic nanotubes by using template-based synthesis.5-11 In fact, it is highly desired to obtain a precise control on the growth of metallic nanotubes, with which the physical properties of nanotubes can be carefully studied, for their potential applications in data storage devices, nanoscale fluidics, chemical and biological separations, sensors, as well as catalysts. Direct electrochemical deposition, with which the composition, morphology, and thickness of the deposited materials can be easily controlled, is a simple and flexible technique to fabricate nanostructures.12-15 However, the fabrication of largescale nanostructures by direct electrochemical deposition in an AAM template generally obtains wire-like nanostructures. In this paper, we report a simple electrochemical method for synthesizing large-scale metallic nanotubes using an electrochemical deposition method to grow vertically aligned metallic nanotubes in AAMs. As an example, Fe nanotubes and nanowires were fabricated by the electrochemical method. It can be shown that the Fe nanotubes exhibit larger anisotropic magnetic response than that of nanowires. Furthermore, the Fe * To whom correspondence should be addressed. E-mail:
[email protected]. † School of Electrical and Electronics Engineering, Nanyang Technological University. ‡ Advanced Materials for Micro- and Nano-Systems, National University of Singapore. § School of Physical and Mathematical Sciences, Nanyang Technological University.
nanotubes are physically robust (i.e., small thermal expansion coefficient) to temperature change when compared with nanowires. Therefore, the Fe nanotubes are a promising ferromagnetic material to realize durable data storage devices operating in harsh environment. 2. Experimental Details The AAM template used in this work was prepared by an anodization process.16,17 Al foil with purity of 99.999% was used as the starting material. After degreased with acetone and annealed at 450 °C for 4 h in vacuum of about 5 × 10-5 Torr, the Al foil was electrochemically polished at 23 V in a mixture of perchloric acid and absolute ethanol (with volume ratio of 1:9) for 3 min. The pretreated Al foil was then anodized in an electrolyte of 0.3 M H2C2O4 for 3 h to form an alumina layer. The alumina layer was removed by immersing the Al foil into a mixture of 6 wt % H3PO4 and 1.8 wt % H2CrO4 at 60 °C for 8 h. The Al foil was then anodized again (in the electrolyte of 0.3 M H2C2O4) for 10 h, and then the alumina barrier layer was removed in 5 wt % H3PO4 solution at 30 °C. As a result, the freestanding AAM template was obtained. The Fe nanotubes were grown inside the AAM template by a direct electrochemical method with a constant potential mode. This can be done by a common two-electrode plating cell with graphite as the counter electrode. The surface ratio between the cathode and anode electrodes is about 1:3. A mixture of 120 gL-1 FeSO4‚7H2O and 45 gL-1 H3BO3 in aqueous solution was selected as the electrolyte. The electrolyte was kept at 50 °C and a direct current (dc) bias of -1.35 V was applied during the deposition. To observe the thickness change of the nanotube wall, the deposition of the nanotubes was prolonged from 60 to 150 min. 3. Results and Discussion Figure 1 describes the growth procedures of the Fe nanotube array using the electrochemical deposition technique. Prior to
10.1021/jp711597r CCC: $40.75 © 2008 American Chemical Society Published on Web 02/23/2008
Properties of Vertically Aligned Fe Nanotubes
Figure 1. Schematic representation of the fabrication of the Fe nanotube array in the AAM.
electrodeposition, a thin layer of Au was sputtered onto one side of the AAM template. To avoid covering the all of the pores, the thickness of Au is controlled to be about 50 nm (i.e., slightly less than the diameter of the pores). If the thickness of Au is larger than 50 nm, then a hole-filling effect (i.e., Au residue on the edge of the pores will accumulate to fill up the pores) may occur. In addition, the Au layer serves as a cathode electrode. Therefore, the thickness of Au layer should not be
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4169 too thin otherwise the resistivity of the cathode electrode will be significantly increased. It must be noted that the Au layer also works as catalyst to grow an Fe nanotube inside the AAM template. Fe is preferentially deposited onto the Au accumulated at the edge of the pores and then grows along the axis direction of the nanochannels of the AAM to form nanotubes. The length of the nanotubes is usually dependent on the deposition time, and the longest length of the nanotubes is confined by the thickness of the AAM template. Furthermore, the outside diameter of the nanotubes is restricted by the diameter of the pores. Compared with other methods for the preparation of nanotubes,5,11,18 our method needs neither the immobilization process of the metallic nanoparticles in the AAM5 nor elevated temperatures.18 The assistance of a polymer in the electrolyte11 is also not required. Hence, it is believed that the method can achieve high-yield synthesis of metallic nanotubes by electrochemical deposition in the AAM. On the other hand, the wall thickness of the nanotubes obtained from our method can be controlled by the duration of electrodeposition. This implies that the growth of well-aligned nanotubes can be precisely controlled. Figure 2a shows the top-view of the field emission scanning electron microscopy (FESEM) image of the Fe nanotubes grown inside the AAM template. For the preparation of FESEM imaging, the AAM template was partially dissolved by a solution of 5 wt % NaOH at room temperature for 30 min. The inset in Figure 2a shows the FESEM image of the Fe nanotubes at higher
Figure 2. (a) Surface SEM image of Fe nanotubes, the inset is the image with larger magnification, (b) TEM image of a singe Fe nanotube, the upper and the bottom insets are the SAED of the nanotube and the XRD of the nanotubes, respectively, (c) EDS image of the Fe nanotubes, and (d) plot of the wall thickness dependence on the electrodeposition time.
4170 J. Phys. Chem. C, Vol. 112, No. 11, 2008 magnification. It is observed that most of the nanotubes have open ends which indicated that the Fe nanotubes had only grown on the pore walls. The outside diameter of the Fe nanotubes spreads uniformly over a narrow range, being about 55-60 nm, which corresponds well to the pore size (i.e., 55-60 nm in diameter) of the AAM template. Hence, the outside diameter of the nanotubes can be adjusted by choosing an AAM template with a different pore size. It can conclude that the deposition technique can achieve Fe nanotubes having a uniform wall thickness. Figure 2b shows the transmission electron microscopy (TEM) image of a Fe nanotube. For the preparation of TEM imaging, the sample was immersed in a 5 wt % NaOH solution to dissolve the AAM template and then washed by deionized water. Small drops of solution with Fe nanotubes were dropped on a Cu grid. It is noted that the Fe nanostructure is hollow and the nanotube is straight and uniform along its whole length. The upper inset in Figure 2b shows the selected-area electron diffraction (SAED) pattern of the Fe nanotube. The corresponding SAED pattern shows that the Fe nanotubes are polycrystalline. The bottom inset in Figure 2b is the X-ray diffraction (XRD) pattern of the as-grown Fe nanotube arrays embedded in AAM. There is only a (110) diffraction peak, which can be indexed to the bodycentered Fe structure, indicating that the Fe nanotubes have a preferential orientation along the [110] direction. The broadened peak originates from the presence of amorphous AAM. Figure 2c shows the energy dispersive spectrometer spectrum (EDS) of the Fe nanotubes. The dominant Fe peaks (i.e., largest count) of the spectrum come from the Fe nanotubes. The Cu and C peaks observed from the spectrum are due to the use of a carboncoated TEM Cu grid and the Al and O peaks are originated from the AAM template. Figure 2d plots the wall thickness dependence of the Fe nanotubes vs deposition time. It can be seen that the wall thickness increases as the deposition time increases. When the deposition time reaches 60 min, the wall thickness is found to be about 11 nm. Inset A in Figure 2d shows the corresponding SEM image. As the deposition time increases to 90 and 120 min, the wall thicknesses of the nanotubes reach about 18 and 22 nm, respectively. When the deposition time reaches about 150 min, the nanotubes become nanowires as shown in inset B in Figure 2d. Hence, it is verified that either nanotubes or nanowires can be fabricated by adjusting the corresponding deposition time. Magnetic properties of Fe nanowires have been studied by a number of groups.14,15 The typical sizes of the Fe nanowires range from 10 to 100 nm in diameter and 0.1 to 1 µm in length. But there is no report on the magnetic properties of Fe nanotube arrays. To understand the magnetism in the Fe nanotube arrays, a vibrating sample magnetometer (VSM) was used to measure the hysteresis loops of the sample at room temperature. The magnetic measurements were carried out using a VSM attached to the physical property measurement system (PPMS, Quantum Design). Parts a and b of Figure 3 plot the normalized magnetic moment, M/Ms, vs the applied external magnetic field, H, of the Fe nanotubes and nanowires, respectively. Both Fe nanotubes and nanowires have the same outside diameter and length of about 55 nm and 1 µm, respectively. The wall thickness of the Fe nanotube is about 11 nm. As shown in parts a and b of Figure 3, both the Fe nanotubes and nanowires exhibit anisotropic magnetic properties. For the Fe nanotubes with H applied perpendicular to the AAM surface (parallel to the nanotube axis), the saturation of M is observed for H equal or greater than 4 kOe which is similar to that of the
Xu et al.
Figure 3. Magnetic hysteresis loop for (a) Fe nanotube arrays and (b) Fe nanowire arrays. The inset represents the M-H data near H ) 0.
nanowires (see Figure 3b). On the other hand, if H is applied parallel to the AAM surface (perpendicular to the nanotube axis), the saturation of M is observed for H equal or greater than 8 kOe which is smaller than that of the nanowires (i.e., 9 kOe) as shown in Figure 3a. The ratio of remanent and saturated magnetization, Mr/Ms, and coercive force, Hc, for the nanotubes with H perpendicular (parallel) to AAM surface are found to be 0.53 and 1071 Oe (0.016 and 124 Oe), respectively. On the other hand, the values of Mr/Ms and Hc of the nanowires with H perpendicular (parallel) to AAM surface are found to be 0.35 and 980 Oe (0.005 and 32 Oe), respectively. The values of Mr/ Ms and Hc of the nanowires are less than that of the nanotubes (i.e., see the insets in Figure 3) indicating that it is more difficult for the nanotubes to reach saturation. This is because the magnetic moment is rigidly fixed by the surface defects so that the increase in surface area (i.e., increase of surface defects) increases the values of Mr/Ms and Hc in the nanotube structure. In fact, it can be shown that the values of Mr/Ms and Hc reduce with the increase of wall thickness of the Fe nanotubes. Figure 4 shows the percentage variation of thermal expansion of the as-grown nanotubes and nanowires embedded inside the AAM, 100% × [dT - dRT]/dRT, vs temperature, T, where dT and dRT are the interplanar spacing along the (110) axis of the Fe nanostructures at T and room temperature, respectively. The corresponding measurement method for the thermal expansion of the nanostructure can be found in ref 19. In brief, an in situ XRD was used to determine the thermal expansion of the nanostructures. The samples were placed inside a vacuum cell, in which the vacuum was kept at 5 × 10-5 Torr and the temperature was controlled within (1 °C, with a heating rate of 5 °C per minute. At each testing point, the temperature was held for 10 min before the XRD measurement. By measuring
Properties of Vertically Aligned Fe Nanotubes
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4171 coefficients of the annealed samples are in general larger than that of the as-grown nanostructures. As a result, the physical structure of the nanotubes embedded inside the AAM is in general more robust to the change of temperature than that of the nanowires. As the majority of the ferromagnetic materials expands or contracts with variation in temperature, their applications in harsh environment (i.e., space applications) as data storage devices may result in plastic deformation and fracture. The Fe nanotubes, which have a small thermal expansion coefficient value, may find application in data storage devices for extreme temperature conditions. Conclusions
Figure 4. Plot of the percentage variation of the axial thermal expansion of nanotubes and nanowires vs temperature.
the variation of diffraction angle, θ, vs T, the change of axial lattice parameters of the nanostructures can be deduced (i.e., dT,RT can be obtained from 2dT,RT sin(θ) ) λ, where λ ) 1.5406 Å). In addition, the influence of thermal annealing on the percentage variation of thermal expansion of both nanostructures was studied. This can be done by annealing the samples at 800 °C in vacuum (i.e., ∼5 × 10-5 Torr) for 2 h. It is observed that the as-grown Fe nanotubes exhibit nearly no increase in axial lattice parameter for the range of temperature (from room temperature to 800 °C). The axial lattice parameter of the annealed nanotubes is only slightly increased with temperature. On the other hand, the increase in axial lattice parameter is nearly 0.02% and 0.1% for the as-grown and annealed nanowires, respectively. In general, Fe nanowires have percentage variation of thermal expansion much larger than that of Fe nanotubes. As discussed in ref 19, the thermal expansion of the nanowires embedded inside AAM may be affected by two factors, (1) the contraction caused by the vacancies and (2) the collective expansion induced by surface tension of the nanowires, which are also limited by the confinement effect of the AAM. In fact, the influence of the confinement effect of the AAM on the nanotubes is much less than that on the nanowires (i.e., due to the hollow structure of the nanotubes) so that the axial expansion of the nanotubes is less than that of nanowires. Furthermore, as the vacancies (i.e., dislocation of atoms) inside the annealed samples are much less than that of the as-grown samples, there will be less vacancies to compensate for the expansion inside the annealed samples when compared with the as-grown ones. Therefore, the thermal expansion
In summary, a simple method is proposed to fabricate vertically aligned nanotubes inside the nanopores of AAMs. It is shown that the outside diameter and length of the nanotubes can be controlled by the pores sizes and thickness of the AAMs. In addition, the wall thickness of the nanotubes can be controlled by electrodeposition time. On the other hand, the magnetic and thermal expansion properties of both Fe nanotubes and nanowires are studied and compared. It is found that the formation of the nanotube structure increases the coercive force of the nanostructure. Furthermore, thermal expansion of the nanotube structure is negligible when compared with that of the nanowire structure. Hence, it is believed that the proposed deposition technology offers a new and convenient route to fabricate Fe nanotube arrays. The precise control on the growth of the Fe nanotube structure will allow the realization of durable data storage devices operating in harsh environment. Acknowledgment. This work was supported by MoE Grant ARC 2/06. References and Notes (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
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