Templated Synthesis of Highly Ordered Mesostructured Nanowires

and stacked-donut structures, have the unique features of hierarchical organization, modulated ... as coaxially multilayered stacked-donuts, single- a...
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NANO LETTERS

Templated Synthesis of Highly Ordered Mesostructured Nanowires and Nanowire Arrays

2004 Vol. 4, No. 12 2337-2342

Yiying Wu,† Tsachi Livneh,† You Xiang Zhang,† Guosheng Cheng,† Jianfang Wang,† Jing Tang,‡ Martin Moskovits,† and Galen D. Stucky*,†,‡ Department of Chemistry and Biochemistry and Materials Department, UniVersity of California, Santa Barbara, California 93106 Received August 19, 2004; Revised Manuscript Received September 21, 2004

ABSTRACT A general methodology that utilizes confined mesoporous silica as template for preparing highly ordered mesostructured nanowires and nanowire arrays is developed. The prepared Ag, Ni, and Cu2O nanowires, with unprecedented mesostructures of coaxially multilayered helical, and stacked-donut structures, have the unique features of hierarchical organization, modulated surface morphology, high surface area, and chirality. Surface-enhanced Raman spectra from a silver mesostructured-nanowire bundle are presented.

Nanowires have attracted extensive interest in view of their interesting electronic and optic properties and for their potential applications as building blocks for nanoelectronic and nanophotonic devices.1 To control the transport of electric carriers and phonons through nanowires and thereby enhance their functionalities, great efforts have been made to create nanowires with controllable structural complexity, such as longitudinal heterostructured2,3 and superlattice nanowires,4-6 radial heterostructured nanowires7 and, recently, branched nanowires.8,9 These diverse nanowire morphologies can all be classified as solid structures, since for these cases, the nanowire structure is continuous in both longitudinal and radial directions. A less fully explored nanowire structural motif is the mesostructured/mesoporous nanowires/nanofibers.10,11 With their porosity, tunable mesopore size and orientation, and extremely high surface area, such nanowires/nanofibers hold considerable promise in applications such as sensing, catalysis, and nanofluidics. Previously reported fiber-like mesostructures are limited to silica fibers with micronscale12,13 and nanoscale10,11 diameters using surfactanttemplated sol-gel methods. Many opportunities, therefore, still exist for preparing other mesostructured nanowire materials. Li et al., for example, reported an interesting method to prepare porous metallic nanowires using confined nanosphere assemblies as template,14 which produced pore morphologies based on stacked spherical cavities with diameters >100 nm. Here we report a broadly applicable * Corresponding author. E-mail: [email protected]; Telephone: (01)-805-893-4872; Fax: (01)-805-893-4120. † Department of Chemistry and Biochemistry. ‡ Materials Department. 10.1021/nl048653r CCC: $27.50 Published on Web 10/14/2004

© 2004 American Chemical Society

methodology that utilizes confined mesoporous silica as a template for preparing highly ordered mesostructured nanowires with meso-ordering periodicity of ∼13 nm. Ag, Ni, and Cu2O nanowires with unprecedented mesostructures such as coaxially multilayered stacked-donuts, single- and doublehelices are obtained. This method, based on electrochemical deposition, can be readily applied to the preparation of nanowires of a broad range of materials: metals, semiconductors, oxides, and polymers. Specifically, we demonstrate the effectiveness of the Ag mesostructured nanowires synthesized using this technique as nanoengineered substrates for surface-enhanced Raman spectroscopy (SERS). Compared to those for solid nanowires, the SERS spectra from rhodamine 6G (R6G) molecules adsorbed on an aligned mesostructured nanowire bundle show significantly less sensitivity to the excitation light polarization, due to their unique modulated surface morphologies. The procedure for preparing mesostructured nanowires is schematically shown in Figure 1. The template used has a hierarchical porous structure composed of mesoporous silica confined inside the nanoscale channels of a porous anodized alumina (PAA) membrane. PAA membranes have been used as templates for the preparation of solid nanowires with controlled diameter and aspect ratio for many years.15-17 The additional fabrication of a mesostructured hierarchical template within the PAA nanochannels allows the architecture of nanowires to be controlled on multiple length scales: the alumina channel diameter and depth define the overall nanowire diameter and length, while the confined silica mesostructure defines the nanowire’s detailed fine texture.

Figure 1. Preparation procedure of the mesostructured nanowires using confined mesoporous silica as template.

The preparation of ordered mesoporous silica inside PAA channels (step 1 in Figure 1) is achieved by using sol-gel dip-coating method through the surfactant-templated evaporation-induced self-assembly process (see ref 18 for synthesis details). Nonionic triblock copolymers, such as poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EO20PO70-EO20; Pluronic P123), are used as the structuredirecting surfactant. The as-prepared silica/P123 composite mesostructure is subsequently calcined at 400-500 °C to form mesoporous silica. The void space in the mesoporous silica is then backfilled with other materials using electrochemical deposition (step 2 in Figure 1). This “casting” process creates an inverted replica of the original silica mesopores. The final step is to release the deposited materials from the surrounding oxide matrix using selective chemical etching (step 3 in Figure 1). Figure 2 shows the mesoporous silica structure formed inside PAA channels with average diameter of 60 nm. The side-view transmission electron microscope (TEM) image of a cross section of mesoporous-silica-loaded PAA membrane (Figure 2a) shows the composite structure to consist of rows of bright contrast regions with ordered mesoscale texture. In the 2-µm-thick PAA membranes used in this work, the mesoporous silica uniformly fills the entire alumina channel. Figure 2b shows a bundle of mesoporous silica nanofibers released from the alumina channels by selective chemical etching using phosphoric acid (which dissolves alumina but leaves silica intact). The ordered mesoporous internal structure is clearly identified along each nanofiber. In a separate study we showed that under the synthesis conditions employed here, the confined silica mesostructures formed inside the alumina nanochannels consist of coaxially multilayered coiled cylindrical mesopores.18 Figure 2c schematically shows the three triple-layer structures formed inside channels with diameters ranging from 55 to 73 nm: stackeddonuts (left), double helix (D-helix; middle), and single helix (S-helix; right). With further reduction in channel diameter, 2338

Figure 2. (a) Cross-section TEM image of the mesoporous-silicaloaded PAA membrane with average alumina channel diameter of 60 nm. The method to prepare the TEM cross section is to use a focused ion beam system (FEI DB235 duel-beam) to cut a 100 nm thick slice out of the silica-loaded PAA membrane, which was subsequently transferred to a TEM sample grid using a micromanipulator. Scale bar: 100 nm. (b) Released mesoporous silica nanofibers from the confining alumina matrix using selective chemical etching by 5 wt % phosphoric acid for 8 h at 65 °C. Scale bar: 100 nm. (c) Schematic mesopore morphologies of the confined silica mesostructures formed inside alumina channels with 55-73 nm diameters: stacked donuts (left), D-helix (middle), and S-helix.

the confined silica mesostructures evolve from coaxial triplelayer, to double-layer and to single-layer structures.18 The size of alumina nanochannels can, therefore, be used to rationally tune the confined silica mesostructures formed inside. Other synthesis parameters, such as the surfactant used, the hydrophilicity/hydrophobicity of the channel wall and the precursor compositions, also influence the mesopore size, orientation, and morphology of the resulting mesostructures.11,18-20 Electrochemical deposition was carried out at room temperature with AC peak-to-peak voltage of 80 V at 200 Hz. The electrolyte solutions used consisted of 0.05 M AgNO3 and 0.5 M H3BO3 for Ag deposition, 0.05 M NiSO4 and 0.5 M H3BO3 for Ni deposition, and 0.05 M Cu(NO3)2 for Cu2O deposition. Electrochemical deposition is a versatile technique for depositing a wide range of materials including metals, semiconductors, and conducting polymers. In addition Nano Lett., Vol. 4, No. 12, 2004

Figure 4. TEM images of released mesostructured nanowires composed of (a,b) Ag, (c) Ni, and (d) Cu2O. Insets in (b-d) are the SAED patterns indexed as (from inner rings): (b) Ag (111) (200) (220) and (113), (c) Ni (111) (200) (220) and (113), (d) Cu2O (110) (111) (200) (112) and (220). A vague NiO (111) ring appears on SAED pattern of (c). Scale bars: 100 nm.

Figure 3. SEM images of released Ag mesostructured nanowire arrays. (a) Large-area image. Scale bar: 1 µm. (b) High-magnification side-view image showing the ordered mesoscale fine textures on the nanowires. Underneath is vertically aligned nanowire array. Scale bar: 100 nm.

to the three materials reported here, other metallic and semiconducting mesostructured nanowires can be readily prepared using the same synthesis approach. The optimal deposition time depends on the depth of the alumina channels. For the ∼2 µm channels used here, a 3 min deposition time was used. The deposited mesostructured nanowires were released by immersing the samples in 0.1 M NaOH for 10 min at 65 °C to etch away the alumina. Figure 3 shows the scanning electron microscopy (SEM) images of the released Ag mesostructured nanowire bundles and arrays. The fine structure of the nanowires is clearly visible in the high-magnification SEM image (Figure 3b). TEM characterization provides a clearer view of mesostructures along the longitudinal axis of individual nanowires composed of Ag (Figure 4a,b), Ni (Figure 4c), and Cu2O (Figure 4d). The selected area electron diffraction (SAED) patterns (inset in Figure 4b-d) show that the as-prepared nanowires are crystalline. We calculated the average single-crystalline domain size using the Scherrer formula; the mean crystal size is similarly 8 nm for the Ag, Ni, and Cu2O nanowires. There are previous reports on electrochemical growth of single-crystalline metal nanowires such as Ag, Cu, and Au in track-etched polycarbonate and anodic alumina membranes by the careful control of overpotential.21 However, there are no reports that we are Nano Lett., Vol. 4, No. 12, 2004

aware of that have successfully obtained single-crystalline Ni nanowires by electrodeposition. The highly coiled nanochannels in the mesoporous silica template used in our study are also a factor in obtaining single-crystalline nanowires, since the nanowire needs to either continuously change its growth direction or to accommodate defects to release the stress. The control of crystal orientation correlation between the neighboring coaxial layers is another challenging issue that depends on the nucleation control and the availability of micropores connecting the adjacent layers. The single crystal growth in our highly coiled and interconnected nanochannels is an interesting topic that we will further investigate. The mesostructures of the nanowires are determined by the geometry of the silica mesopores, the mesoporemesopore interconnectivity, and the effectiveness of the electrochemical deposition. As mentioned above, diverse helical and stacked-donut mesoporous silica structures form inside the alumina channels, and the diameter of nanochannels determines the mesopore layers that can be accommodated.18 In practice, a distribution of channel diameter exists among the large quantity of channels in a PAA membrane (channel density ∼1010/cm2), which depends on the quality of the starting aluminum foil and the anodization processes. In addition, for the coaxially multilayered mesopore morphologies (e.g., the triple-layer architecture in Figure 2c), electrochemical deposition can effectively fill all the layers or just the outer layer. These factors result in the diverse nanowire mesostructures observed in detailed TEM examination of individual nanowires (Figure 5). Figure 5a shows an S-helix nanowire with an average pitch of 13 nm. The morphology is like a tight spring. Figure 5b shows a nanowire with stacked-donut structure. The average spacing between adjacent donuts is 13 nm. The image contrast implies that the two side regions of the nanowire 2339

Figure 5. TEM images showing different architectures of the prepared Ag mesostructured nanowires: (a) single-helix; (b) stacked-donuts; (c) core-shell with a single-helix shell and a solide nanowire core; (d) core-shell with S-helix core; (e) core-shell with double-helix shell and single-helix core; (f) coaxial threelayered structure (indicated by the arrow); (g) longitudinally heterostructured nanowire with a double-helix segment (indicated by the arrow) sandwiched between two single-helix segments; (h) end view of the closely packed mesostructured nanowire bundle showing the circular cross-sections of individual nanowires. Inset of (h) shows a 103 nm diameter cross-section with three concentric Ag layers and an inner hollow core. Scale bars: 40 nm in (a-g) and 700 nm in (h).

are hollow, while the middle region possesses interior structure. Coaxial core-shell multilayered nanowires are prevalent among the prepared nanowire mesostructures. Figure 5c shows a nanowire consisting of an S-helix shell and a solid nanowire core. Figures 5d and 5e show two other core-shell nanowires with S-helix cores. The nanowire in Figure 5e has a shell structure of double-helix (D-helix). Figure 5f shows a nanowire with coaxial three-layered structure. The outer two layers are stacked-donuts and the interior core is a solid nanowire running through the holes of the stacked donuts. In previous carbon backfilling studies of SBA-15 structures, it has been confirmed that there are micropores connecting the 2-D hexagonal mesopores.22,23 We believe that this is also the case in our confined mesoporous silica structures. The deposited Ag in the connecting micropores between the coaxial layers, therefore, provides anchoring points in the core-shell structures. We also observed the coexistence of different mesostructures on the same Ag nanowire. As an example, Figure 5g shows a longitudinally heterostructured nanowire with a D-helix segment (indicated by the arrow) sandwiched between S-helix segments. Figure 5h is an end-view TEM image of the aligned nanowire bundle showing the circular shaped cross sections of individual nanowires. On close examination, one finds that some nanowires are hollow while others have coaxial multilayered structures. An example is shown in the inset of Figure 5h. The 103-nm-diameter nanowire shown in cross section consists of three coaxial Ag outer layers and a hollow core. These mesostructured nanowires possess several distinct structural features compared with solid nanowires. (1) Higher 2340

surface area. Considering only the outer surface, a nanowire with stacked-donuts mesostructure has 3.14 (π) times greater surface area than that for a solid nanowire with the same diameter. The enhancement will be doubled if the inner surface for the hollow mesostructured nanowires is included. (2) Periodically modulated surface morphology. As shown in the TEM images (Figures 4 and 5), the outer surface of both the helical and the stacked-donut nanowires exhibits a modulated outline. The surface morphology influences the surface plasmon resonance for metallic nanowires, which, for example, might be useful in plasmonic applications such as surface-enhanced Raman spectroscopy. (3) Hierarchical organization. The mesostructured nanowires prepared in this work are composed of coaxial multilayers, in which the constituent individual layers are either a tight single helix, an intertwined double helix, or a regular stacking of nanorings. Such a hierarchical and regular organization provides a potential mechanism for controlling the electric carrier and phonon transport, as well as tuning the nanowire’s mechanical performance. (4) Chirality for helical nanowires. The S-helix and D-helix meostructures are chiral objects with pitch on the nanoscale (13 nm for S-helix and 27 nm for D-helix). These can lead to potentially interesting optical effects including very large circular dichroism. If helical wires with high enantio-purity can be prepared, they can form useful media for separating racemic mixtures or for chirally directing chemical reactions. The enantio-purity of the helical nanowires (left-handed vs right-handed) and their selective control are under investigation. The activity of the Ag mesostructured nanowires as SERSactive systems was demonstrated by measuring SERS spectra of R6G molecules adsorbed on them. The SERS enhancement is known to be highly dependent on the local geometry and morphology of the Ag substrate (for reviews, see refs 24-26). Both calculation and experiments have shown that the local electromagnetic field in the interstice between two Ag nanoparticles can reach very high values. The field enhancement is both wavelength- and polarizationdependent.27-29 For two interacting particles, only fields polarized along the interparticle axis are strongly enhanced.29 Bundles of aligned, strongly interacting Ag solid nanowires when used as SERS substrates also showed large electromagnetic field enhancement,30,31 with strong dependence on the polarization of the incident light. Significant Raman enhancement is achieved only when the excitation polarization is perpendicular to the nanowires’ longitudinal axes.31 For the Ag mesostructured nanowires obtained in this work, the periodically modulated surface morphology results in the existence of regular nanoscale crevices along the longitudinal axis on the various mesostructures observed including the stacked nanorings (donuts) and the S- and D-helices. Moreover, when released from the alumina matrix in which they were produced, the systems of aligned mesostructures became strongly interacting laterally. It is, therefore, interesting to determine (1) if these systems are good candidates for SERS enhancement, and (2) which of their complex geometrical details dominate the field enhancements and therefore affect the polarization dependence of the SERS. Nano Lett., Vol. 4, No. 12, 2004

Figure 6. (a) SERS spectra of R6G molecules adsorbed on a bundle of aligned Ag mesostructured nanowires with excitation polarized parallel (indicated as L) and perpendicular (indicated as P) to the nanowire longitudinal axis. The sample was prepared by dropping a drop of 10-5 M R6G methanol solution onto the Ag nanowire bundle and letting the solution dry naturally. The Ag nanowire bundle was on a TEM grid, which helped to check the structure and oriention of the nanowires under TEM. The lower right inset shows the TEM image of the nanowire bundle (scale bar: 1 µm) with the area inside the rectangle magnified in the upper left inset (scale bar: 60 nm). (b) Schematic model of an S-helix nanowire bundle illustrating the surmised location of the adsorbed R6G molecules (not to scale). In the schematic, the interpitch distance of the helix is exaggerated for clarity. (blue-centered stars): R6G molecule adsorbed in the crevices of the S-helix; (redcentered stars): R6G molecule adsorbed at interstitial locations between nanowires.

Figure 6a shows the SERS spectra measured from R6G molecules adsorbed on a bundle of parallel Ag mesostructured nanowires excited at 514.5 nm (cw Ar ion laser) polarized along (IL) and perpendicular (IP) to the axes of the Ag nanowires. The relative intensities observed with the two polarizations (IL/IP) was ∼2. The diameters of the mesostructured nanowires in the bundle are 55 ( 15 nm. The spectra show characteristic bands of R6G molecules that were significantly less sensitive to the polarization than was observed for the aligned solid nanowire bundle,31 which shows IP/IL ∼ 10. This difference can be qualitatively understood in terms of the structural difference between the two systems. For a bundle made up of parallel solid nanowires, the only SERS “hot” regions are the interstitial Nano Lett., Vol. 4, No. 12, 2004

spaces between the individual nanowires. Consequently, it will be light polarized perpendicular to the nanowires’ axes that is expected to excite the local surface plasmon resonance strongly. In contrast, for a bundle of mesostructured nanowires, two hot SERS regions may exist: the interstitial space between nanowires as in the case of the solid nanowires, and the nanoscale crevices between the structural elements (nanorings or helical strand) of the individual mesostructured nanowires, for which localized surface plasma are excited by light polarized parallel to the nanowire’s longitudinal axis. Therefore, the significantly reduced polarization sensitivity observed with these systems arises from the presence of two hot SERS regions with orthogonal polarization preferences. A schematic representation is shown in the Figure 6b for an S-helix nanowire bundle. However, the same arguments apply to all the mesostructures found in the template, for which the relative abundance of stacked-donuts to S-helices to D-helices was found to be approximately 0.35 to 0.58 to 0.07, respectively.18 The aligned and regular nanowire mesostructures possess the following structural elements that impact on the Raman enhancement. Each individual strand is composed either of closely stacked nanorings or of relatively tightly wound helices. (i) The interstices between the nanorings or between neighboring (and almost touching) pieces of the helix are high-field locations (although ones with a lower field enhancement than what would exist at the single interstice in a single pair of equivalently spaced nanorings). (ii) Each nanoring is an oblate object for which the field-enhancement is expected to be highest at spots along its largest circular perimeter where the curvature is largest.32 (If the object is a donut, this would also hold for the circular perimeter defining its hole.) (iii) This is offset, however, by the fact that (for a fully adsorbate-covered nanoring) the number of molecules occupying this high-field perimeter (and hence contributing to the observed Raman intensity) would be lower than those occupying the relatively less curved portion between nanorings. (iv) When the alumina matrix is removed, the individual nanowire strands of the mesostructure can approach each other very closely, potentially creating a very large number of closely spaced interstices in which the field enhancement would be high. Mechanisms (i) and (iii) lead to large enhancements when the light is polarized longitudinally, i.e., along the long axes of the mesostructured strands, while mechanisms (ii) and (iv) would contribute to high enhancements when the incident light is polarized across the strands. Our observed IL/IP ∼ 2 implies that the first set of mechanisms dominate somewhat. In summary, this paper reports a generic platform for synthesizing highly ordered mesostructured nanowires and arrays of various compositions and functionalities. Nanowires with unprecedented mesoscale textures of stacked-donuts, S-helices, and D-helices have been successfully prepared. Such nanowire architectures enhance the surface area and provide a possible strategy for reducing the effective nanowire diameter, controlling the transport of electric carriers and phonons through nanowires, and tuning their mechanical performance. 2341

Acknowledgment. This work was supported by the National Science Foundation under award number DMR 0120967 and award number DMR 02-33728, and partially supported by the MRSEC Program of the National Science Foundation under Award No. DMR 00-80034. References (1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (2) Hu, J. T.; Min, O. Y.; Yang, P. D.; Lieber, C. M. Nature 1999, 399, 48. (3) Zhang, Y.; Ichihashi, T.; Landree, E.; Nihey, F.; Iijima, S. Science 1999, 285, 1719. (4) Wu, Y. Y.; Fan, R.; Yang, P. D. Nano Lett. 2002, 2, 83. (5) Bjork, M. T.; Ohlsson, B. J.; Sass, T.; Persson, A. I.; Thelander, C.; Magnusson, M. H.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Nano Lett. 2002, 2, 87. (6) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (7) Lauhon, L. J.; Gudiksen, M. S.; Wang, C. L.; Lieber, C. M. Nature 2002, 420, 57. (8) Dick, K. A.; Deppert, K.; Larsson, M. W.; Martensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nat. Mater. 2004, 3, 380. (9) Wang, D.; Qian, F.; Yang, C.; Zhong, Z. H.; Lieber, C. M. Nano Lett. 2004, 4, 871. (10) Wang, J. F.; Zhang, J. P.; Asoo, B. Y.; Stucky, G. D. J. Am. Chem. Soc. 2003, 125, 13966. (11) Yang, Z. L.; Niu, Z. W.; Cao, X. Y.; Yang, Z. Z.; Lu, Y. F.; Hu, Z. B.; Han, C. C. Angew. Chem., Int. Ed. 2003, 42, 4201. (12) Huo, Q. S.; Zhao, D. Y.; Feng, J. L.; Weston, K.; Buratto, S. K.; Stucky, G. D.; Schacht, S.; Schuth, F. AdV. Mater. 1997, 9, 974. (13) Marlow, F.; McGehee, M. D.; Zhao, D. Y.; Chmelka, B. F.; Stucky, G. D. AdV. Mater. 1999, 11, 632.

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Nano Lett., Vol. 4, No. 12, 2004