J. Phys. Chem. C 2008, 112, 1831-1836
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Platinum Thin Film Consisting of Well-Aligned Nanowires and Its Optical Behavior Takashi Suzuki,† Hirokatsu Miyata,*,‡ Takashi Noma,‡ and Kazuyuki Kuroda*,†,§,| Department of Applied Chemistry, Waseda UniVersity, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan, Canon Research Center, Canon Inc. 3-30-2 Shimomaruko, Ohta-ku, Tokyo 146-8501, Japan, Kagami Memorial Laboratory for Materials Science and Technology, Waseda UniVersity, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan, and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi-shi, Saitama 332-0012, Japan ReceiVed: September 11, 2007; In Final Form: NoVember 11, 2007
A nanostructured film composed of interconnecting aligned platinum nanowires is prepared by electrodeposition using a hard template of mesoporous silica film with uniaxially aligned mesochannels formed on a conductive substrate. Platinum is introduced into the mesopores through micropores, connecting adjacent mesopores, without influencing the mesoporous structure. The alignment of the nanowires is retained to some extent after the removal of the silica template, leading to the formation of a film with aligned platinum nanowires, although the structural regularity is largely degraded. Irrespective of the presence of the silica template, the obtained films show distinct macroscopic optical anisotropy in the visible region, reflecting the anisotropic localized surface plasmon resonance arising from individual nanowire, despite its interconnected structure. The aligned structure enables the observation of such an axis-dependent optical property in a macroscopic scale. This versatile replication method can be extended to the preparation of aligned nanowire assemblies with various compositions.
Introduction Nanowires have attracted growing interests because of their unique physical properties caused by their highly anisotropic morphology.1-3 Several methods, such as the vapor-liquidsolid process, step decoration, and replication of tubular templates, have been proposed for the preparation of nanowires.4-6 In particular, a method based on replication of a preformed nanoscale template is advantageous because of its versatility in materials, including various metals, semiconductors, and oxides, as well as the process facility.7-17 Various nanomaterials, e.g., mesoporous materials, porous anodic aluminum oxide (AAO), and tubular reverse micelles, can be used as nanotemplates.7-17 Among these nanotemplates, mesoporous silica films with tubular mesopores,18-27 prepared by self-assembly of surfactants, are especially useful because they work as a stable hard template, which allows the formation of a well-ordered nanowire assembly film on a substrate.9-13 However, the in-plane direction of each nanowire in the resultant assembly films reported so far is random without exception because the direction of the mesopores in the template mesoporous silica film is not controlled. Consequently, such films have macroscopically isotropic properties regardless of the anisotropy of individual nanowires, because the properties arising from long and short axes of nanowires are evenly included. To reflect the individual anisotropy on macroscopic properties, macroscopic alignment control of nanowires is indispen* To whom correspondence should be addressed. (K.K.) Phone and fax: +81-3-5286-3199. † Department of Applied Chemistry, Waseda University. ‡ Canon Research Center, Canon Inc. § Kagami Memorial Laboratory for Materials Science and Technology, Waseda University. | CREST, Japan Science and Technology Agency.
sable. The alignment can be achieved by controlling the alignment of mesopores in a mesoporous silica film template. Our group has succeeded in the precise control of in-plane arrangement of mesopores in mesoporous silica films prepared by using substrates with anisotropic surfaces,23-34 and the 2Dhexagonal mesoporous structure with uniaxially aligned mesopores is brought about by the interfacial interactions.23-30 The importance of the use of a mesoporous silica film with aligned mesopores is that it causes the alignment of nanowires in the plane of a substrate, in contrast to the vertical alignment that is achieved by using AAO as a template.14-17 The in-plane alignment of nanowires leads to macroscopically anisotropic properties in the plane of the film, which enables us to use the axis-dependent properties simply by rotating the substrate. This is more straightforward for practical applications of the aligned nanowires compared to those with the vertical alignment.35 Also, the detailed characterization of the nanowire assemblies with controlled in-plane alignment greatly contributes to a deeper understanding of the optical properties of nanowires, e.g., polarized light emission,36 giant birefringence,37 and anisotropic localized surface plasmon resonance (LSPR),38,39 which are significant in the “nanowire photonics” field. Here we report successful preparation of a platinum (Pt) nanowire assembly film, in which the alignment of each nanowire is totally controlled, using a mesoporous silica film with a uniaxially aligned mesoporous structure. Onto a conductive substrate as a working electrode Pt was electrodeposited through micropores connecting adjacent mesopores. The obtained films with the aligned Pt nanowires show a distinct macroscopic optical anisotropy, reflecting the anisotropic LSPR in each nanowire, despite the interconnected structure. The transmittance and the reflectance of visible light are largely dependent on the polarization with respect to the alignment
10.1021/jp077276e CCC: $40.75 © 2008 American Chemical Society Published on Web 01/23/2008
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SCHEME 1: Formation Process of Film Consisting of Well-Aligned Pt Nanowires
Figure 1. (A) θ-2θ scanning XRD profiles. The rubbing direction of the substrate was fixed perpendicularly to the projection of the incident X-ray. (B) In-plane φ scanning profiles. The samples are (a) the mesoporous silica film, (b) the Pt nanowires/silica nanocomposite film, and (c) the thin film consisting of well-aligned Pt nanowires.
direction of the nanowires. Such a film with nanostructure-based anisotropic properties will find many applications in optics. Experimental Methods Scheme 1 shows the schematic formation process of a film consisting of well-aligned Pt nanowires. A mesoporous silica film with uniaxially aligned mesopores was prepared by the evaporation-induced self-assembly (EISA) process followed by a thermal treatment to remove templates.23 Non-ionic surfactant, Brij56 (C16(EO)10), was used as the template for the formation of both mesopores and micropores that connect adjoining mesopores.40,41 Either an indium-tin oxide (ITO) or a tin oxide substrate coated with a rubbing-treated polyimide film was used as a substrate. The use of ITO and tin oxide substrates ensures high electronic conductivity as well as strong adhesion of the mesoporous silica film onto the substrate after calcination. The structural anisotropy in the polyimide film, brought about by the rubbing treatment, controls the alignment of mesopores through interfacial hydrophobic interactions between the polymer chains.26-30 The polyimide film was removed by a thermal treatment without causing peeling of the film from the substrate. A mesoporous silica film with aligned tubular mesopores formed on a conductive substrate was soaked in a solution of H2PtCl6 for electrodeposition of Pt. Using the underlying conductive substrate as a working electrode, Pt was electrodeposited, leading to the formation of a Pt-silica nanocomposite film. The silica wall was removed with an alkaline solution, leaving the uniaxially aligned Pt nanowires. Preparation of Substrates with a Rubbing-Treated Polyimide Coating. Substrates were coated with a polyimide precursor (polyamic acid) by spin coating and baked at 200 °C for 1 h in an air atmosphere for the formation of a polyimide film. The polyimide film on the substrates underwent a rubbing treatment using a nylon-covered cylindrical roller by following the details of the rubbing treatment described previously.26 Preparation of Mesoporous Silica Films. Brij56 and tetraethoxysilane (TEOS) were sequentially dissolved in ethanol (EtOH), and then 0.1 M hydrochloric acid and water were added to this solution. The final composition of the precursor solution was Brij56 0.080/TEOS 1.0/EtOH 22/H2O 5.0/HCl 0.0040. After 2 h of stirring, the substrate coated with rubbing-treated polyimide film was dip-coated with this precursor solution and subsequently dried for 1 day in a room temperature. The
surfactant and the polyimide film were removed by calcination at 400 °C for 1 h in air. Electrodeposition of Platinum. Electrodeposition was conducted in a 2 wt % H2PtCl6 solution using the conductive substrate as a working electrode, on which a mesoporous silica film formed, an Ag/AgCl electrode as the reference electrode, and a Pt plate as a counter electrode for 1 h by galvanostatic electroplating (1 mA‚cm-2) (Pt nanowire/silica nanocomposite film). The removal of the silica wall was performed with a 1 M sodium hydroxide solution at 50 °C (thin film consisting of well-aligned Pt nanowires). Characterization. The structure of the films was elucidated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution scanning electron microscopy (HRSEM). The θ-2θ scanning XRD was performed with a MAC Science M03XHP22 diffractometer using Mn-filtered Fe-KR radiation under the operating conditions of 40 kV and 20 mA. In-plane XRD and the 2θ scanning XRD were performed with a four-axis goniometer RIGAKU ATX-G diffractometer using Cu-KR radiation under the operating conditions of 50 kV and 300 mA, and a soller slit with a vertical divergence of 0.48° was used to obtain a parallel beam. The incident angle of X-ray in the in-plane geometry was set to 0.2°. The two-dimensional (2D) XRD patterns were recorded under a reflection geometry using synchrotron radiation at the Photon Factory on beamline 4A. The incident angle of X-rays was 0.1°. TEM images were recorded on a JEOL JEM-2010 instrument at an accelerating voltage of 200 kV. HR-SEM images were recorded on a Hitachi S-5500 at an accelerating voltage of 3 kV. The mesoporous silica film and Pt nanowire/silica nanocomposite film were also observed with an optical microscope (Olympus BH2). The absorption spectra were recorded on a Shimadzu UV-2500 PC spectrophotometer. Results and Discussion Mesoporous Silica Film with Uniaxially Aligned Mesochannels on a Conductive Substrate. The mesoporous silica films are successfully prepared on the conductive substrate without forming cracks. The profile of the θ-2θ scanning X-ray diffraction (XRD) recorded with the sample geometry in which the rubbing direction of the substrate is perpendicular to the projection of the incident X-rays (trace a in Figure 1A) is consistent with the two-dimensional (2D) hexagonal mesoporous structure with a structural period of d01 ) 3.7 nm. The peaks indicated as (•) can be assigned (1h1) and (1h2), which do not contribute to the XRD under the ideal θ-2θ scanning geometry.
Thin Film Consisting of Well-Aligned Pt Nanowires
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Figure 2. Optical microscopic image of the Pt nanowires/silica nanocomposite film, taken under a reflection mode. Pt was incorporated only in the left-half region of the micrograph.
However, the finite divergence angle of the X-ray beam, which is comparable to the small diffraction angle for this material, results in the appearance of the (1h1) and (1h2) peaks in the diffraction profile when the incident X-rays are directed parallel to the alignment direction of the mesochannels.42 The XRD pattern changes by rotating the substrate, and the peaks indicated as (•) in the trace a disappear (data not shown). The observed anisotropy in the θ-2θ XRD pattern indicates the structural anisotropy of the film, which is consistent with the uniaxially aligned mesoporous structure shown in our previous papers.23,26,27 The alignment of the mesochannels is characterized by using grazing incident in-plane XRD.23-27 Sharp in-plane diffraction peaks are observed only when the projection of the incident X-rays is perpendicular to the rubbing direction (see Supporting Information Figure S1). The peak positions in the in-plane XRD pattern are inconsistent with the θ-2θ scanning peak positions if an ideal 2D-hexagonal structure is supposed. This shows that the structure of the film deviates from the ideal 2D-hexagonal structure because of the selective structural shrinkage along the direction of film thickness. This anisotropic shrinkage is caused by the progress of condensation of silica walls especially in the thermal treatment.23-27 Because the mesoporous silica film is anchored to the substrate, the shrinkage does not take place in the plane of the film. The deviation from the ideal 2Dhexagonal structure of this mesoporous silica film is estimated to be ∼39% from the d01/d2h1 ratio (1.73 for the ideal 2Dhexagonal structure), which is estimated from the θ-2θ (d01 ) 3.7 nm, trace a in Figure 1A) and in-plane φ-2θχ scanning XRD (d2h1 ) 3.5 nm, trace a in Figure S1A) patterns.23-27 The distribution of the in-plane alignment of the mesopores is quantitatively estimated by measuring the in-plane rocking curve (φ-scanning profile), as shown in trace a in Figure 1B. The fullwidth at half-maximum (fwhm) of the peaks was estimated to be ∼3°. This shows that the tubular mesochannels are aligned perpendicularly to the rubbing direction with a very narrow alignment distribution. This shows the successful formation of a mesoporous silica film with uniaxially aligned mesochannels on a conductive substrate. Platinum Nanowires/Silica Nanocomposite Film (after the Electrodeposition). After the electrodeposition of Pt, the film turned uniformly gray in color. The optical micrograph of the film, taken under a reflection mode, is shown in Figure 2. To show the difference of the color clearly, the electrodeposition was partially performed, and Pt was incorporated only in the left-half region of the micrograph. The film morphology is intact
Figure 3. 2D-XRD patterns of (a) the Pt nanowires/silica nanocomposite film and (b) the thin film consisting of well-aligned Pt nanowires. The patterns were recorded with the geometry where the incident X-rays were perpendicular to the rubbing direction.
after the electrodeposition, suggesting the uniform incorporation of Pt. Both the positions of the θ-2θ scanning XRD peaks (Figure 1A, trace b) and the fwhm of the in-plane φ-scanning XRD peaks (Figure 1B, trace b) were not changed, which shows that Pt is incorporated in the pores without degrading the regular porous structure at all. The detailed mesostructure of the film after the Pt deposition is confirmed using two-dimensional (2D) XRD patterns (Figure 3). The 2D-XRD pattern measured for the Pt nanowire/silica nanocomposite film is shown in Figure 3A. The pattern was recorded with the geometry where the incident X-rays were perpendicular to the rubbing direction (parallel to the nanowires). The clear diffraction spots were observed, showing the retention of the highly ordered mesostructure after the Pt deposition. The wide angle 2θ scanning XRD pattern (Figure 4) of the film after the electrodeposition, with fixed θ ) 0.2° to avoid the influences of substrates, shows the two diffraction peaks assigned to the (111) and (200) diffractions of the face-centered cubic structure of Pt. The existence of these peaks in the XRD profile under the 2θ scanning geometry proves that the Pt nanowire assembly is polycrystalline. The size of the crystallites is estimated to be ∼3 nm by the Scherrer formula. Figure 5A shows the TEM image of the film after the electrodeposition. The diameter of the nanowires in this image is consistent with that estimated by XRD. The observed lattice fringes are parallel over several adjoining Pt nanowires, showing that the nanowires are electrodeposited through the micropores that connect the adjacent mesopores.43,44 This results in the formation of singlecrystalline Pt domains consisting of several nanowires.
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Figure 4. 2θ scanning XRD pattern of the Pt nanowires/silica nanocomposite film. Figure 6. HR-SEM top-view image of the thin film consisting of aligned Pt nanowires.
Figure 5. TEM images of (A) the Pt nanowires/silica nanocomposite film and (B) single Pt nanowire. The arrows in the TEM image (A) show that the lattice fringes of Pt are parallel over several adjoining Pt nanowires. The single Pt nanowire was obtained by sonic dispersion after the removal of the silica template.
Platinum Thin Film Consisting of Well-Aligned Nanowires (after Removal of the Silica Template). The film morphology is retained even after the removal of the silica template. The large shift of the XRD peak to higher angles as well as the broadening (Figure 1A, trace c) shows that the structural regularity is retained to some extent although the structure is considerably shrunk and degraded. The retention of the nanostructure of the film after the removal of the silica template is also confirmed by 2D-XRD as shown in Figure 3B. The diffraction spots assigned to (01), (10), and (1h1) can still be observed. The disappearance of the higher diffraction spots is the proof of structural degradation. The degradation is likely caused by the surface tension during the drying process after the removal of silica. Two diffraction peaks in the in-plane φ-scanning profile (trace c in Figure 1B) for the sample after the removal of silica show that the alignment of the nanowires is retained to some extent. However, the broadening of the diffraction peak in the φ-scanning profile shows that the in-plane alignment of the nanowires is also influenced. The retention of the alignment of nanowires was also confirmed by direct observation. The top-view image of the film after the removal of silica, recorded with a highresolution scanning electron microscope (HR-SEM), clearly shows the formation of high-density nanowires with uniaxial alignment (Figure 6). This is the first successful formation of the film consisting of well-aligned nanowires. As described above, the retention of the mesostructure as well as that of the film morphology strongly suggests that the electrodeposition proceeds through the micropores connecting adjacent mesopores. If the mesopores were completely isolated from each other, the
electrodeposition would not proceed because of the blockage of electron passes by insulating silica walls. The removal of the silica wall allows the observation of the isolated nanowire by TEM. Figure 5B shows the TEM image of an isolated nanowire. The cylindrical morphology with a uniform diameter of ∼3 nm is confirmed by this image. Optical Property of Well-Aligned Platinum Nanowires. For this well-aligned Pt nanowire assembly film, anisotropic optical properties, which reflect the anisotropic structure, are expected because the LSPR frequency depends on the size of crystallites.38,39,45 Generally, the plasmon resonance of Pt nanostructure (
