Controlled Growth of Mesostructured Crystalline Iron Oxide Nanowires

°C. The second-order template synthesis method for preparing the ordered array .... Synthesis of iron-filled carbon nanotubes with a great excess...
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J. Phys. Chem. B 2005, 109, 2546-2551

Controlled Growth of Mesostructured Crystalline Iron Oxide Nanowires and Fe-Filled Carbon Nanotube Arrays Templated by Mesoporous Silica SBA-16 Film Keying Shi, Yujuan Chi, Haitao Yu, Baifu Xin, and Honggang Fu* Laboratory of Physical Chemistry, School of Chemistry and Materials Science, Heilongjiang UniVersity, Harbin 150080, China ReceiVed: August 14, 2004; In Final Form: NoVember 29, 2004

The three-dimensional (3D) accessible pore structures (Im3hm space groups) of continuous mesoporous silica SBA-16 thin films have been prepared by a dip-coating technique in nonaqueous media under acidic conditions on indium-tin oxide glass (ITO). The films are oriented with the (111) crystal plane perpendicular to the surface of the film. On one hand, deposition of iron metal into the mesopores of SBA-16 films was achieved by using an electrochemical method. The Fe2O3 nanowire arrays were synthesized. The crystalline structures of porous Fe2O3 nanowires and nanorods were studied via TEM, SEM, and XRD. On the other hand, a small amount of Fe was deposited into the pores of the SBA-16 thin film as a catalyst, and carbon nanotube arrays formed inside the pores of SBA-16 film were fabricated by catalytic decomposition of acetylene at 700 °C. The second-order template synthesis method for preparing the ordered array of carbon nanotubes filled with Fe has been used. The carbon nanotubes are very uniform in diameter and length and are aligned vertically with respect to the SBA-16 film.

1. Introduction The templated growth method, which involves confined growth of metallic materials to a template (e.g., a pore) followed by removal of the template, provides a flexible and affordable synthetic route to a large variety of metal nanowires.1-3 Examples of such templates include hard templates4-7 (e.g., porous alumina films, track-etched polycarbonate films, and mesoporous silica) and soft templates.8 The new materials, such as carbon,9 noble metals,10,11 transition metal oxides,12,13 and sulfides,2 were achieved by using the conventional chemical methods. Since the discovery of carbon nanotubes by Iijima in 1991,14 these one-dimensional and hollow nanomaterials are attracting increasing scientific and technological interest for their excellent properties.15-17 Aligned CNT films produced on mesoporous silica substrate were reported 8 years ago by Li et al.18 They prepared mesoporous silica in iron nitrate aqueous solution so that iron oxide nanoparticles formed simultaneously with the mesoporous silica and were reduced to metal iron nanoparticles later on.19,20 The mesopore openings were not regularly arranged on the substrate, and, therefore, although the CNTs were wellaligned, the locations of the CNTs were also not regular. Our method is more advanced: (i) This fabrication method is capable of producing very uniform in diameter and length carbon nanotubes and controllable properties reproducibly. (ii) The CNTs were regular, parallel to each other, and hollow with open ends. The diameters of CNTs and the space between the CNTs could be controlled by changing the inner diameters of the pores and the thickness of the pore-wall of the mesoporous silica films. (iii) The growth of the helicity of CNTs may be controlled in comparison with ref 21. The individual Fe-filled carbon nanotubes (formed by our method) in a periodic superstructure created the unique possibility of studying novel mesoscopic collective excitations and cooperative phenomena due to electromagnetic coupling of tubes in the array.22,23 * Corresponding author. E-mail: [email protected].

In the present work, we demonstrate for the first time an electrochemical synthesis of iron-metal and iron-oxide nanorod arrays templated by mesoporous SBA-16 films. The secondelectrochemical deposition method was used for preparing a highly ordered array of carbon nanotubes filled with metal Fe. Iron (oxide) is a good choice because it is a very good adsorbate for nitric oxide24,25 and may have potential applications in (environmental) catalysis and medical science, and also it is in favor of generating CNTs. 2. Experimental Section Chemicals. Triblock PEO106-PPO70-PEO106 copolymer surfactant (F-127) is commercially available from BASF, Pluronic F-127 (Mav ) 12 600). Synthesis. The mesoporous silica films were deposited by dip-coating on indium-tin oxide glass (ITO).26 The pore size of the thin film can be varied from 80 to 200 Å. The Fe depositions were performed with a potentiostat at room temperature. The counter electrode was a platinum plate, and the thin film of SBA-16 on ITO substrate was the cathode. The Fe depositions were carried out at -1.4 and -2.0 V, and a solution of 0.03 mol/L (up to 0.1 mol/L) FeSO4 and 0.7 mol/L H3BO3 was used as the electrolyte. The used current density varied from 0.01 to 0.02 A/cm2. The deposition time was 10 min (for CNTs) or 1 h (for nanowires). Fe-deposited SBA-16 film was moved out from the cell followed by the treatment with a 1-4% HF aqueous solution, drying in air, and calcination. A small amount of Fe as catalyst was deposited into the bottom of the pore of SBA-16 film. The thin film deposited Fe was placed in a horizontal quartz reactor, and the temperature of the reactor was increased to 700 °C under a flowing gas composed of 10% H2 and 90% Ar. After that, a source gas composed of 50% C2H2, 2% H2, and 48% Ar was passed through the reactor. Aligned carbon nanotubes were then generated inside each pore by the catalytic decomposition of acetylene at 700

10.1021/jp0463316 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/28/2005

Controlled Growth of Iron Oxide Nanowires

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Figure 1. XRD patterns and pore size distribution curve of SBA-16: (a-e) theoretically calculated XRD pattern of SBA-16; (f) XRD pattern of calcined S (Im3hm) mesoporous silica SBA-16 film; (g) pore size distribution curve of SBA-16 (a, the unit cell parameter; R, the radius of spherical cages; Rc, the radius opening).

°C for 10 or 30 min. The metal Fe was then filled into the aligned carbon nanotubes by a second electrodeposition. Analyses. X-ray powder diffraction measurements of samples were carried out on a Rigaku D/Max-2400X X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å). N2 (BET) adsportion-desorption isotherms were measured at 77 K on a Micromeritics Instrument ASAP200. The samples were outgassed at 473 K for 2 h before adsorption measurement. SEM was carried out on a TECNAI XL 30-FEG scanning electron microscope with an operating voltage varying from 5 to 20 kV. The optimum resolution of the SEM was 1.5 nm. TEM (TECNAI 200) was employed for the study of the structure of SBA-16 mesoporous silica. For the high-resolution electron microscopy (HREM) observation, a JEOL-2010 electron microscope was employed. 3. Results and Discussion (a) SBA-16 Film on ITO. Continuous thin films of mesoporous silica SBA-16 with a three-dimensional accessible pore system have been prepared on indium-tin oxide (ITO) glass by a dip-coating technique using F-127 surfactant.26 The experimental results of N2 adsportion-desorption on the powder specimen indicated that a typical SBA-16 film has a BET surface area of 534.45 m2/g with a mean pore size of 160 Å and a total pore volume of 0.376 cm3/g. The produced mesoporous silica in thin film was examined by an X-ray diffractometer (Figure 1). Five well-resolved Bragg peaks, corresponding to the d spacings of 137.92, 97.397, 68.858, 56.225, and 51.129 Å (Figure 1f), were observed from the calcined film and can be indexed onto the SBA-16 unit cell as (110), (200), (220), (222), and (321), respectively, with the unit cell parameter a ) 194.794 Å. That the 1/dhkl versus m ) (h2 + k2 + l2)1/2 plot27 is a straight line indicates the good fit of the data to the Im3hm space group; the cavities are connected through mesoporous openings. Figure 1a-e shows the theoretically calculated XRD patterns of mesoporous silica SBA-16. It can be seen that the size of the spherical cages and opening are dependent on the ratio of the intensity of I110/I200. When the unit cell parameter is constant, the size of the radius opening affects the ratio of the intensity of I110/I200; for a ) 192.554 Å,

Figure 2. Images of the calcined mesoporous SBA-16 film: (a) SEM image of the SBA-16 film on the ITO substrate; (b) TEM image of the SBA-16 thin film showing the grain boundary, indicated by the arrow, between two domains on the [100] and [111] directions.

the ratio of I110/I200 of the SBA-16 film is about 4.450, R ) 0.49a ) 94.570 Å (a, the unit cell parameter; R, the radius of spherical cage), Rc ) 0.115a ) 29.256 Å (Rc, the radius opening connecting the cavities). Figure 1f is the XRD pattern of the

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Figure 3. (a) TEM image perpendicular to the direction of the channels-deposited Fe in the pores of the SBA-16 thin film (the inset at the tip left is a TEM image parallel to the direction of the channels of the SBA-16 thin film-deposited Fe); (b) TEM image of the iron oxide nanowire arrays; (c) HRTEM image of the Fe2O3 single-crystal nanowire (applied potential, 1.7 V; solution concentration, 0.05 mol/L FeSO4; electrode separation e 0.4 cm).

calcined SBA-16 film; the ratio of I110/I200 is approximated to 1, and the pattern is similar to Figure 1e, so the values of R and Rc are as follows: R ) 0.49a ) 94.570 Å, Rc ) 0.32a ) 61.617 Å. The size of the opening is about 120 Å, and the size of the spherical cage is about 189 Å. Figure 1g is the size distribution curve of the SBA-16 film. Horvath-Kawazoe analysis of the adsorption-desorption isotherm (BET) data for mesoporous silica SBA-16 gives pore sizes of 120-180 Å. This is consistent with the results of XRD. Figure 2a is the high magnification SEM image of the top plane view of the SBA-16 film on ITO substrate. It is clearly seen that the openings of the mesopores are almost uniform in size, which can be directly measured from the SEM image, and regularly distributed on the film surface with mostly hexagonal patterns corresponding to the (111) plane of SBA-16. The domain structure and ordered pore opening is about 100 Å in diameter. The structure of SBA-16, space group Im3hm, can be regarded as an amorphous silica framework with spherical cages at bodycentered sites connected by some mesoporous channels. By looking at the SEM image along the arrow directions in Figure 2a (the inset at the tip left is a side view of “a” area in Figure 2a), it shows a boundary between two oriented domains in a large-area mesoporous silica thin film.

The big black dot indicated by the white arrow is a spherical cage at body-centered sites. The diameter of the spherical cage is about 150 Å. The black cylinders indicated by the black arrows in Figure 2a are the openings of channel-packed hexagonally nanopores of the top surface of SBA-16. The high magnification SEM image (Figure 2a) shows that the morphology of the channel is similar to beads on an abacus and they are connected three-dimensionally. Figure 2b shows the grain boundary, indicated by the arrow, between two domains on the [100] and [111] directions. Although the whole film is not single crystal in Figure 2b, the film surface shows a very high degree of pore ordering. (b) Potentiostatic Electrochemical Template Synthesis of Nanowires (Fe) in Mesoporous Silica Thin Film. (1) Deposition of One-Dimensional Nanowires. The potentiostatic electrochemical template synthesis of nanowires (Fe) is performed in mesoporous silica thin film with nominal pore diameters dN from 80 to 200 Å. In the process of electrochemical deposition, nucleation is a very important process. On one hand, the competition between growth and nucleation determines the granularity of the deposit. The higher is the nucleation rate during deposition, the finer are the crystal grains of the deposit. On the other hand, the formations of the growing crystals are usually determined by the general appearance and structure of

Controlled Growth of Iron Oxide Nanowires

Figure 4. SEM image of iron oxide nanowires growth and formation in three-dimensional electrocrystallization (applied potential, 1.4-1.6 V for 1 h; solution concentration, 0.05 mol/L FeSO4; electrode separation e 0.4 cm).

the substrate (mesoporous silica film). With a higher growth rate of the crystal grains normal to the substrate surface, for instance, a fibrous structure of the deposit is obtained. Figure 3a presents the TEM top view and side view of Fe deposits, which was scraped off from the substrate. The visual angle of the image is perpendicular to the direction of the channels deposited Fe, but it is parallel in the inset at the tip left. The analysis of EELS indicates that besides the Si element, Fe and O elements also exist in the surface of the Fe-filled SBA-16 film. Figure 3b is the TEM image of iron oxide nanowire arrays, in which the Fe2O3 nanowires are parallel to each other and have uniform diameter. Figures 3c and d shows the images of iron oxide wires. HREM investigations demonstrate the structure of the nanowire to be single crystal. The two spacings of the image, as shown in Figure 3c, are 2.51 and 4.53 Å, which correspond to the [110] and [003] crystal plane of γ-Fe2O3, respectively. (2) Deposition of Three-Dimensional Nanowires. Metal deposition is a crystal growth process; the nucleation and growth rate depend on the different potentials27-30 and foreign substrate (S).31-34 The structure analysis indicates that the used SBA-16 film has three-dimensional accessible pore structures with the diameters of the spherical cavities and mesoporous openings within 160 and 100 Å, respectively, which is in agreement with the reported work in ref 35. The mesoporous silicas with 3D pore networks are used as a template of deposition Fe; their replicas are 3D patterned nanorods (see Figure 4). The high magnification SEM image (Figure 4) shows that the morphology of the three-dimensionally connecting iron oxide nanorods is like beads on an abacus and also demonstrates that the deposition is a crystal growth process of 3D transition metal Fe. The diameter of the smallest part of the nanorods (Figure 4) is about 150 Å, and that of the “beads” is in a range from 200 to 500 Å. The shape of the 3D nanorods is similar to the original shape of the SBA-16 channels (see Figure 2a), and the enlargement of the nanorod diameters is believed to be due to the oxidation of Fe to iron oxide during the calcinations. The difference of the morphologies of iron oxide nanowires is because of the structure of mesoporous silica film and applied potential. There is a grain boundary on the surface of SBA-16 films as shown in Figure 2b. Hence, even if the electrochemical

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Figure 5. Process model of deposition Fe, generation CNTs, and CNTs filled with Fe. (white cylinders) The [111] direction mesopores of the SBA-16 film channel. (b) Fe deposited in pores of the SBA-16 film as catalyst for CNTs. (orange cylinders) CNTs filled with Fe by secondelectrochemical deposition.

deposition would proceed at an exactly constant rate and a homogeneous growth over the whole 10 mm × 15 mm growing area (the effective area of the thin film of SBA-16 on ITO substrate as the cathode), the instant pore filling (rate) would still vary in different pores. Otherwise, the applied potential also affects the growth rate and morphology of the nanowire. If a lower applied potential is controlled during the deposition of Fe process, the growth rate of the crystal grains would be changed more slowly, and the 3D structure of the deposit would be obtained (like Figure 4). At the same time, there are some influential factors, such as the dissolution of the film, drying, calcination, etc. For example, during drying and calcination, the mechanical forces tear on the nanowires, and the nanowires were further oxidized into iron oxide leading to various different morphologies. (c) Template Synthesis of Carbon Nanotubes. A small amount of Fe deposited into the bottom of the pore of the SBA-16 film as catalyst grew carbon nanotubes. Figure 5 shows the process model of deposition Fe, generation CNTs, and CNTs filled with Fe. The counter electrode was a platinum plate, and the thin film of SBA-16 on ITO substrate was the cathode. The Fe depositions were carried out at -1.4 and -2.0 V and in a solution of 0.05 mol/L for 10 min. The CNTs arrays grew from the bottom pores of the SBA-16 film and out of the SBA-16 film with a very uniform diameter and hollow with open ends. The length of CNTs was controlled by CVD time. Figure 5 shows the SEM images of CVD carbon obtained for a 10 min deposition at 700 °C using ethylene as precursors. The carbon nanotubes formed in the pores of the SBA-16 film with the diameter of the large pore between 80 and 200 Å. The SEM images in Figure 6a and b show the results of micrographs of carbon nanotube arrays embedded in the ordered channel of mesoporous sillca SBA-16 film. The samples were peeled off from the ITO; thus, the CNTs and mesoporous sillca SBA-16 film were also peeled off in the same time without any etching followed by SEM investigation (as shown in 6b). The SEM images in Figure 6a and b were obtained without dissolution of the template film. It clearly shows that the carbon nanotubes grew inside the 3D cubic mesoporous silica films due to Fe deposited into the bottom of their nanopores. Hence, all of the CNTs are wrapped and

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Figure 6. SEM images of the aligned carbon nanotubes. (a, b) Side view of aligned CNTs formed inside the mesoporous silicate SBA-16 film [(a) and (b): the SBA-16 film without being treated by HF aqueous solution]. (c, d) Side view of CNTs on the surface of mesoporous silica thin film (applied potential, 1.7 V; solution concentration, 0.05 mol/L FeSO4; electrode separation e 0.4 cm). The time of Fe deposition was 10 min. Carbon nanotube arrays formed inside the pores of the SBA16 film were fabricated by catalytic decomposition of acetylene at 700 °C for 10 min.

embedded by the mesoporous silica. The array has excellent uniformity in size and position. (d) Array of Fe-Filled Carbon Nanotubes. A low magnification morphology image of the isolated carbon nanotubes is shown in Figure 7a. The inset at the tip left in Figure 7a is an enlarged cross-sectional view of a CNT end. Figure 7b shows the TEM images of the CNTs-deposited Fe. As shown in Figure 7, the CNTs have the open ends, similar to that of a previous report.36 The selected-area electron diffraction pattern of the Fe-filled carbon nanotubes indicates that the Fe nanowire is a single crystal, but carbon nanotube is polycrystalline. Because the carbon nanotubes are of uniform length and are open ended, a “second-order template synthesis method” was

Shi et al.

Figure 7. TEM images of carbon nanotubes: (a) a single carbon nanotube (the inset at the tip left is an enlarged view of the tube end); (b) the CNT deposited Fe (the inset is a SAED pattern from a single CNT filled with Fe).

used for preparing a highly ordered array of carbon nanotubes completely filled with metal. Figure 8 shows the TEM image of Fe-filled carbon nanotube arrays. It indicates that all of the CNTs are completely filled with Fe. The following features can be noted from these images. First, ethylene precursor can be decomposed in the pores and deposited on the inner pore wall to form carbon nanotubes under the experimental conditions. Second, the carbon nanotubes have the same outer diameter as that of the pores of the template film. Furthermore, these carbon nanotubes are a highly aligned ensemble, which is advantageous over the random orientation of the carbon nanotubes prepared by either arc discharge37 or other methods.38 Third, these carbon nanotubes have a highly ordered, parallel to each other, uniform diameter and are hollow with open ends (see Figure 6). Their

Controlled Growth of Iron Oxide Nanowires

J. Phys. Chem. B, Vol. 109, No. 7, 2005 2551 Foundation of Heilongjiang Province (Nos. 20171016, 20271019, 20301006). Electron microscopy was carried out in the Beijing Laboratory of Electron Microscopy, Institute of Physics. We are also thankful for comments from Prof. L.-M. Peng and Dr. W. Z. Zhou. References and Notes

Figure 8. TEM images of Fe-filled carbon nanotube arrays.

diameters vary with the inner diameters of pores of SBA-16 thin film. These extremely uniform arrays could be used in a variety of applications including high-density data storage, field emission displays, and infrared imaging detectors, etc. Looking further, our method allows individual carbon nanotube devices to be periodically assembled into ultradense nanoelectronic networks whose collective behavior could then be used to perform computational functions. 4. Summary The formation of low-dimensional metal oxide systems, the dependence on the inhomogeneities of substrate surface, the thickness of mesoporous silica film, the applied potential, and the electrolyte concentration39 are of great importance for electrocatalysis as well as the electrochemical fabrication of electronic nanodevices. Iron oxide nanowires grow and form in one-dimensional electrocrystallization at 1.7 V and a electrolyte concentration of 0.1 mol/L. With iron oxide in threedimensional electrocrystallization at 1.4-1.5 V and an electrolyte concentration of 0.1 mol/L, the morphology of the nanorods is like beads on an abacus and they are connected threedimensionally. The shape of the 3D nanorods is similar to the original shape of the SBA-16 channels. We have proposed an easy method for the growth of wellaligned patterned CNTs arrays and synthesized the ordered CNT arrays over large areas by decomposition of acetylene on Fe in 3D cubic ordered mesoporous silica films. The method of CVD carbon deposition in the bare SBA-16 film is controlled easily by the nanotube size, density, and arrays spacing by varying experimental conditions (such as changing the coating solution concentration or the coating time), and the pore size of cubic mesoporous silica SBA-16 film can be adjusted by adding organic swelling agents TMB. Thus, the desired length and size of CNT arrays can be obtained by catalytic decomposition of acetylene. Acknowledgment. This work was supported by Key Projects of the National Natural Science Foundation of China and Heilongjiang province (No. 20431013 ZJG0404), the National Natural Science Foundation of China, and the Postdoctoral

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