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Fabrication of Copper Nanowire Encapsulated in the Pore Channels of SBA-15 by Metal Organic Chemical Vapor Deposition Ying Zhang,†,‡ Frank Leung-Yuk Lam,† Xijun Hu,*,† Zifeng Yan,‡ and Ping Sheng§ Department of Chemical Engineering, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, School of Chemistry & Chemical Engineering, China UniVersity of Petroleum, Dongying 257061, Shandong, China, and Department of Physics, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ReceiVed: May 16, 2007; In Final Form: June 6, 2007
Copper nanowires encapsulated in 6-nm pore channels of SBA-15 were synthesized by metal organic chemical vapor deposition at a low temperature in this study. It was found that chemically modifying the SBA pore surface by carbon deposition helps to synthesize better quality copper nanostructures in the pore channels of SBA-15. More importantly, using hydrogen as a reducing agent and carrier gas can produce significantly improved copper nanowires encapsulated in SBA-15. This enhancement is due to the chemical decomposition and reduction of organometallic precursor (copper(II) acetylacetonate) into smaller fragments that can more easily penetrate the interior of the SBA-15 porous structure. The dimensions of the encapsulated metal nanowire can be tailored by using different pore-sized substrates. This is the first time tuned copper nanowires encapsulated in the nanopore channels of well-defined porous structure have been synthesized under mild conditions.
1. Introduction One-dimensional (1D) nanostructures such as wires, rods, belts, and tubes have become the focus of intensive research areas owing to their unique applications in the fabrication of mesoscopic physics and nanoscale devices.1 Manufacturing nanodevices requires a metallic connection between each nanoassembly to build up a whole interconnected circuit system. Copper has been identified as a potential candidate because of its high conductivity. It promotes the electromigration that increases the device operating frequency and allows the use of high current densities.2-4 Synthesis of some metallic nanowires has been reported in the literature,4-7 but very few studies were mentioned on the synthesis of Cu nanowires encapsulated in porous structures. This may be due to the high temperature requirement for the vaporization of pure copper seed. Thus far, preparation of copper nanowires and nanorods can be achieved by template synthesis using chemical reduction of coppercontaining compounds by using colloidal assemblies as templates8-10,14 and electrochemical deposition.11-13 The main drawbacks of using template synthesis are the requirement of the energy-consuming reaction condition and low product yield. Liu and Bando have shown a one-step method that is the socalled vacuum vapor deposition for the preparation of copper nanowires and nanorods.5 Such a method, however, requires a high temperature for the vaporization of copper metal (above 1073 K). Other researchers have tried the metallic impregnation in the liquid phase to synthesize nanostructured products while maintaining dimensional monodispersion. Ziegler et al.,15 who * Corresponding author. Telephone: (852) 2358 7134. Fax: (852) 2358 0054. E-mail:
[email protected]. † Department of Chemical Engineering, Hong Kong University of Science and Technology. ‡ China University of Petroleum. § Department of Physics, Hong Kong University of Science and Technology.
Figure 1. Schematic diagram of the rotating tubular CVD reactor system.
suggested using supercritical carbon dioxide as a solvent medium, have succeeded in synthesizing copper nanowires and nanotubes in the mesoporous template with a pore size of about 5.5 nm. A regular pattern of the copper nanowire can be observed, however, and such a copper-filling process requires undergoing rigorous conditions (high pressure of 34.5 MPa and high temperature of 800 K) that are energetically unfavorable and difficult to apply in industry. Ye et al.16 synthesized copper nanorod in carbon nanotubes with pore diameter of about 2030 nm by supercritical fluid, which is even more complicated. The key idea of this work is to synthesize an ordered copper nanowire encapsulated in the pore channels of well-defined mesoporous SBA-15 by a low-temperature chemical vapor deposition (CVD) method. It is a simple technique for copper deposition because of its low deposition and decomposition temperatures and no requirement of a copper seed layer as mentioned in the electroplating technique.17-19 In such a deposition process, organometallic precursor is first deposited in the porous surface of the SBA-15 pore channels. To improve the quality of copper nanowire with the SBA-15 pore channels, hydrogen is introduced simultaneously during the precursor deposition, which acts as a reducing agent to chemically react with the organocupric precursor to form smaller organocupric fragments. These small fragments can more easily penetrate the
10.1021/jp073786x CCC: $37.00 © 2007 American Chemical Society Published on Web 08/09/2007
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Figure 2. TEM and XRD results of copper deposition in SBA-15 without hydrogen. (a) Copper deposition twice, (b) copper deposition three times, and (c) XRD image of (b).
Figure 3. TEM images of copper deposition in SBA-15 in the hydrogen atmosphere. (a) SBA-Cu-2, (b) SBA-Cu-3, and (c) SBA-Cu-4.
Figure 4. Proposed intermediate fragments during precursor decomposition in the presence of hydrogen.
SBA-15 pore channels and then be further decomposed and reduced to elemental copper by the flow of hydrogen gas, leading to the formation of a continuous copper nanostructure in the SBA-15 pore channel. 2. Experimental Section 2.1. Sample Preparation. A mesoporous material, SBA-15, was chosen as the substrate to encapsulate copper nanowires. The preparation procedure of SBA-15 can be found elsewhere.19,20 Practically, Pluronic 123, which was a triblock copolymer, was used as the template molecule. During preparation, Pluronic 123 was first dissolved in hydrochloric acid solution (1.6 mol/L) at 308 K under stirring. Tetraethylorthosilicate was slowly added to the solution at 308 K until the solution became homogeneous. The mixture was placed in an autoclave for 24 h at 373 K for aging. The washed cakelike solid was calcined at 823 K for 6-9 h with a heating rate of 1 K/min. The obtained white powder solid, blank SBA-15 with a pore diameter of 6 nm, can be used as the substrate for the copper deposition process. To investigate the effect of substrate surface properties on precursor deposition, the blank SBA-15 was pretreated with carbon deposition. A total of 0.25 g of SBA-15 was dried at
383 K for 12 h, and then the carbon coating was conducted in situ at 1073 K using a gas mixture of nitrogen (0.8 mL/s) and acetylene (0.2 mL/s). The SBA-15 coated with a thin layer of carbon was denoted as SBA-(C). For copper deposition, a rotated CVD reactor system was used, and a schematic diagram is shown in Figure 1. The CVD reactor is a stainless steel column with an inner diameter of 1 cm. Hydrogen, which acted as a reducing reactant and carrier gas, was supplied to the CVD reactor at a flow rate of 0.5 mL/s. The solid mixture of SBA(C) and the precursor, copper(II) acetylacetonate (Aldrich Chemical Co., 97%), with a mass ratio of 1:1 was housed inside the reactor vessel. The whole system was evacuated (2 kPa) using a vacuum pump and heated to 463 K for 30 min to sublime the solid organometallic precursor. Deposition was assumed to start when the system temperature reached 623 K. After a deposition process of 30 min, it was stopped by rapidly cooling the reactor to room temperature using compressed air flow. The deposition was conducted repeatedly for four times to improve the quality of copper nanowire encapsulated in SBA15 pore channels. The samples obtained from different deposition times were denoted as SBA-(C)-Cu-x where x was the cycle of deposition. For comparison, the copper nanowires prepared
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Figure 5. Mechanism of Cu deposition in pore channels of SBA-15 under hydrogen atmosphere.
Figure 6. TEM images of the deposited copper on the carbon-coated SBA-15. (a) SBA-(C)-Cu-2, (b) SBA-(C)-Cu-3, and (c) SBA-(C)-Cu-4.
was conducted by a unit of Quantachrome Autosorb-1 to determine the specific BET surface area, pore volume, and pore size of the obtained product. Morphology of the developed copper nanowire was examined by the TEM instrument, JEOL 2010F, using an accelerating voltage of 200 keV. The chemical composition of the product was determined through the XRF analysis by a JEOL X-ray fluorescence spectrometer. 3. Results and Discussion
Figure 7. XRD spectra of the deposited copper on the carbon-coated SBA-15.
using the blank SBA-15 substrate, which was not pretreated by carbon coating, were denoted as SBA-Cu-x. 2.2. Characterization. The obtained samples were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray fluoroscence (XRF) analyses. The XRD spectra were recorded by a PANalytical Powder X-ray diffractometer using Cu ΚR radiation (wavelength 1.54 Å) operating at 40 kV and 40 mA. Nitrogen adsorption at 77 K
In this study, utilization of hydrogen and pretreatment of carbon coating on SBA-15 are crucially important in the copper deposition process. The effect of hydrogen on the copper deposition is first discussed in this section. Figure 2a,b shows the TEM images of the copper-SBA-15 composite where copper deposition has been done two and three times, respectively, without hydrogen carrier gas. It is seen that the deposited copper (black dots) is agglomerated, leading to the formation of spherical copper nanoparticles that are mainly distributed on the exterior surface of SBA-15. The crystallinity of such copper nanoparticles was determined by an XRD spectrum, as illustrated in Figure 2c, which shows a distinctive pattern emerging as sharp narrow peaks at 2θ ) 43.297°, 50.466°, and 74.130° corresponding to the (111), (200), and (220), respectively. The presence of these three peaks confirms that the deposited copper is in elemental form. This observation implies
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Figure 8. TEM images and EDS spectra of pores filled with and without copper in SBA-(C)-Cu-4. (a) TEM image, (b) HRTEM image of Cu nanowire confined in channels of SBA-15, (c) EDS spectra of area A, and (d) EDS spectra of area B.
Figure 9. (a) TEM image of the Cu nanowire arrays after removing silica frame of SBA-15. (b) EDS spectra of the Cu nanowire arrays in area A.
that the crystal Cu(acac)2 cannot be broken down completely without reacted gas, which tends to form bigger aggregations of Cu(acac)2 and deposits on the exterior surface of the template rather than penetrate the pore channels of SBA-15. Therefore, the Cu(acac)2 can only be mainly decomposed and deposited on the external surface of SBA-15. Such a steric hindrance of organometallic precursor is found to be a critical problem for the metal deposition into the pore channels of substrate. To overcome this problem, pure hydrogen was introduced into the system during the deposition process. Hydrogen served as both reducing agent and carrier gas. Figure 3 illustrates the TEM images of the copper-SBA-15 composite prepared in the presence of hydrogen during the deposition process at a flow rate of 0.5 mL/s, two (SBA-Cu-2), three (SBA-Cu-3), and four (SBA-Cu-4) times. It is seen that numerous short black lines and small black dots that resemble copper nanowires and nanoparticles are found in the pore channels and surface of SBA-15. Comparing Figures 2 and 3, it can be clearly demonstrated that using hydrogen as a reducing
agent is extremely important to deposit copper nanowires into the interior pore channels of SBA-15. Hydrogen reduction of organometallic precursor is a common step for the metal coating process. However, very few researchers have addressed its application in the metal deposition in porous materials. Basically, the precursor, Cu(acac)2, can be chemically decomposed by hydrogen, leading to the formation of some smaller mass fragments that are CH3+, CH3CO+, and COCH2COCH+ ions21-24 and smaller copper organic complexes, as shown in Figure 4. Compared with the original large-sized precursor, the small-sized copper organic complexes diffuse much faster into the pore channels of SBA-15 and then further decompose inside the pores to deposit copper nanowires. To provide a general picture of copper deposition into the pore channels of SBA-15, a simple deposition mechanism of copper is postulated as displayed in Figure 5. During hydrogen reduction, the copper-containing precursor ligand is gradually decreased in size because of the fragment detachment. The
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Figure 10. Low-angle XRD spectra of blank SBA-15 and SBA-(C)Cu-4.
smaller copper organic complex can penetrate the pore channels of SBA-15 more easily, resulting in continuous copper deposition, agglomeration, and wire formation. Therefore, in the CVD process, hydrogen not only plays a role on the chemical reduction of cupric precursor but also enhances the mass transfer of the metallic precursor into the internal pore channels of SBA-15. To further increase the deposition amount of copper, the blank SBA-15 was pretreated with carbon coating to modify the surface from hydrophilic to hydrophobic nature. The hydrophobic surface of carbon-modified SBA-15 can enhance the adsorption of organometallic precursor because they can interact better. In this case, acetylene was used as the carbon source to deposit carbon on both interior and exterior surfaces of SBA15. Figure 6 displays the TEM images of the SBA-(C)-Cu samples. In general, it shows more continuous black solid lines within the SBA-15 pore channels compared with the case of non-carbon-coated SBA-Cu samples (Figure 3). Such black lines are believed to be copper nanowires because their growth direction is analogous to the pore configuration. There are also some black dots found on the exterior surface of SBA-15, which are copper nanoparticles (Figure 6a) formed during CVD. This further confirms that copper nanowires can only be obtained inside the pore channels of SBA-15. On the external surface, copper particles rather than wires are formed. These copper particles on the external surface of SBA-15 can be removed by post-treatment. By increasing the cycles of copper deposition from two to four times, as shown in ascending order in Figure 6a-c, more continuous copper nanowires are gradually formed and the copper density becomes higher. This means that it is possible to obtain patterned copper nanowires by repeated copper deposition. The crystallinity of both copper nanoparticles and nanowires is determined by XRD spectra that are shown in Figure 7. The characteristic peak of 43.297° corresponding to the (111) of crystalline copper plane orientation becomes intensified as the deposition cycles increase. Introducing carbon as a surfacemodifying substance on SBA-15 helps the copper deposition in porous media because the hydrophobic surface provides a high adsorption affinity of organometallic precursor. The XRD spectra in Figure 7 show the presence of elemental copper, but from it we cannot deduce whether the corresponding peak comes from the deposited copper nanowire. To confirm this, energy-dispersive spectrometer (EDS) analysis was conducted. Figure 8a shows the magnified image of SBA-(C)-Cu-4 with two marked areas: area A (copper nanowire) and area B
Zhang et al. (void space of the pore). The corresponding EDS spectra are shown in Figure 8c,d, displaying three peaks of resemblance of Cu ΚR, Κβ, and L shell electrons at 8.040, 8.900, and 0.920 keV, respectively. The presence of these three peaks indicates that copper atom is detected in those areas. However, the intensity measured in area A is much stronger than that in area B. This confirms that copper not only is deposited in the pores of SBA-15 but also forms a wire structure in such a confined space. To directly observe the atomic structure of Cu nanowires within the pore channels of SBA-15, a high-resolution TEM was performed, and the result is shown in Figure 8b for a selected area in Figure 8a. It can be seen that the copper nanowire consists of many small nanocrystals. Clear lattice fringes can be observed with a measured spacing between adjacent lattice planes of about 1.808 and 2.088 Å, which are attributed to the (200) and (111) family of planes of Cu, respectively. To study the quality (density and intactness) of copper nanowires inside the pore channels of SBA-15, Figure 9a displays the TEM image of the free-standing copper nanostructure after the silicate matrix of SBA-15 is removed using a 2.5 M NaOH solution.25 The obtained pure copper nanowires and their corresponding EDS analysis are shown in Figure 9. It can be seen that dense arrays of copper nanowires are present. The pure Cu wires seem to be larger than the pore size of the substrate. This could be the result of the adjacent nanowires collapsed to each other to form large rods when the supporting silica frame is removed. The EDS analysis of the copper nanowire array structure (Figure 9b) shows strong Cu signals, further indicating that these nanostructures are pure copper. The Si and O signals presumably come from the dissolved silica, and the Ni and C signals are from the carbon-coated nickel grid used for TEM analysis. Figure 10 shows the XRD patterns of blank SBA-15 and SBA-(C)-Cu-4. Strong (100), (110), and (200) diffraction peaks are observed, indicating the presence of long-range order in this sample. These peaks are at 0.9080°, 1.5609°, and 1.7977° 2θ corresponding to the d spacing of 97.29, 56.59, and 49.14 Å, respectively. After coating with carbon and the formation of copper nanowires in the pores of the substrate, the mesoporous framework can still be maintained (Figure 8). The framework slightly shrinks, and the (100), (110), and (200) peaks shift to 0.9521°, 1.6055°, and 1.8893° 2θ, corresponding to the d spacing of 92.79, 55.03, and 46.76 Å, respectively, because of the nanowire formation in the pores. The good maintenance of the similar structure as blank SBA-15 also indicates that the copper filling in the pore channels forms copper nanowires. 4. Conclusions Copper nanowires encapsulated in 6-nm pores of SBA-15 have been successfully synthesized via a simple route of lowtemperature metal organic chemical vapor deposition (MOCVD) technique. Copper was uniformly distributed inside the pores with a high density that was confirmed by EDS analysis. This approach requires only a mild working condition and is hence of relatively low cost. It was found that the surface modification of SBA-15 by carbon coating provides a better adsorption affinity of organometallic precursor, which increases the amount of copper deposited within the pores of SBA-15. The addition of hydrogen as the reducing agent and carrier gas helps the dissociation of organometallic precursor to form smaller sized cupriferous organic intermediates, which can more easily diffuse into the pore channels and be deposited there. Thus, copper
Fabrication of Copper Nanowire by MOCVD nanowires can be formed within the SBA-15 pore channel rather than only on the exterior surface of the substrate. It is believed that this method is a very promising technique to synthesize other metal nanostructures confined in a narrow space, which can have many potential optical and electronic applications. Acknowledgment. This project was supported by the Research Grants Council (RGC) of Hong Kong under the Central Allocation Vote scheme (Grant No. CA04/05.SC02). References and Notes (1) Wang, Z. L. AdV. Mater. 2000, 12, 1295. (2) Hwang, S. T.; Shim, I.; Lee, K. O.; Kim, K. S.; Kim, J. H.; Choi, G. J.; Cho, Y. S.; Choi, H. J. Mater. Res. 1996, 11, 1051. (3) Whitman, C.; Moslehi, M. N.; Paranjpe, A.; Velo, L.; Omstead, T. J. Vac. Sci. Technol., A 1999, 17, 1893. (4) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (5) Liu, W.; Bando, Y. AdV. Mater. 2003, 15, 303. (6) Wang, W. Z.; Wang, G. H.; Wang, X. S.; Zhan, Y. J.; Liu, Y. K.; Zheng, C. L. AdV. Mater. 2002, 14, 67. (7) Wang, W. Z.; Zhan, Y. J.; Wang, G. H. Chem. Commun. 2001, 727. (8) Tanori, J.; Pileni, M. P. AdV. Mater. 1995, 7, 862. (9) Tanori, J.; Pileni, M. P. Langmuir 1997, 13, 639. (10) Lisiecki, I.; Filankembo, A.; Sack-Kongehl, H.; Weiss, K.; Pileni, M.-P.; Urban, J. Phys. ReV. B 1999, 61, 4968.
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