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Soft-Chemical Approach of Noble Metal Nanowires Templated from Mesoporous Silica (SBA-15) through Vapor Infiltration of a Reducing Agent Azusa Takai,† Yoji Doi,† Yusuke Yamauchi,*,†,‡,§ and Kazuyuki Kuroda*,† Department of Applied Chemistry and Majors in Applied Chemistry and Nanoscience and Nanoengineering, Faculty of Science & Engineering, Waseda UniVersity, Ohkubo 3-4-1, Shinjuku, Tokyo 169-8555, Japan, World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, and Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ReceiVed: October 28, 2009; ReVised Manuscript ReceiVed: March 3, 2010
Noble metal (Pt, Ag, and Au) nanowires are synthesized by using mesoporous silica (SBA-15) powders as templates through vapor infiltration of a reducing agent (dimethylamine borane, DMAB) under the same reduction conditions. Because SBA-15 has micropores connecting mesochannels, Pt nanowires are connected and periodically packed, reflecting the micropores and the mesochannel arrangements in the original mesoporous silica. On the other hand, other metal (Ag and Au) nanowires are mainly unconnected. Such a difference is attributed to the relatively faster deposition rate of Ag and Au than Pt. Therefore, Ag and Au tend to grow rapidly along the mesochannels, and the metal deposition in micropores is insufficient to occur. The length of the metal nanowires is controlled by the reduction time. As a typical case, the surface plasmon resonance spectra of the Au nanowires embedded in SBA-15 change depending on the length of the nanowires. The present reduction is a soft-chemical, straightforward, and general approach, which has advantages for the synthesis of metal nanowires in large amounts. 1. Introduction Currently, nanostructured metals, such as nanoparticles and nanowires, are attracting considerable attention due to their unique properties and a broad range of potential applications, such as optics, electronics, and magnetism.1 Nanostructured metals show high specific surface areas, which are quite effective for the enhancement of chemical reactions on the metal surface as nanocatalysts.2 In addition, surface plasmon resonance, a typical quantum size effect, is observed.3 Various rigid templates, such as porous anodic alumina membranes,4 zeolites,5 and mesoporous materials,6–18 have been mainly utilized for the preparation of nanostructured metals so far, although soft templates, including lyotropic liquid crystals (LLCs), have also been applied.19,20 In particular, mesoporous silica is advantageous as a template from the viewpoint of its chemical and mechanical stabilities. Because the pore size, pore geometry, and pore alignment can be precisely controlled by changing the synthetic conditions,21 various replicated metal structures can be rationally prepared by selecting desirable mesoporous silica. Pt nanoparticle arrays were synthesized from a cage-type mesoporous silica film (Pm-3m), and the size of embedded nanoparticles and their arrangements perfectly reflect the original mesostructures.7 Metal nanowire networks with periodic space are prepared from bicontinuous cubic mesoporous silica (Ia-3d).8 One-dimensional metal nanowires are synthesized from 2D-hexagonal mesoporous silica, and nanowires and nanoparticles can be selectively synthesized by optimizing the metal deposition method.9 When mesoporous silica with mi* To whom correspondence should be addressed. E-mail addresses:
[email protected] (Y.Y.);
[email protected] (K.K.). † Waseda University. ‡ WPI Center for MANA, NIMS. § PRESTO, JST.
cropores connecting mesochannels is used, periodically arranged nanowires are obtained, and the mechanical stabilities, electroconductivities, and specific surface areas become higher than those of nanowires aggregated without pillars supporting each other. Among several synthetic approaches of metal nanowires studied so far, one of the common methods is solution-based synthesis using diluted precursor solutions. This synthesis can control the length and crystallinity and be applied to various metals (e.g., Au and Ag) by adding organic molecules as capping agents.1–3 The solution-based synthesis is a simple and onestep process for preparing a large number of metal nanowires, but there are several problems to be encountered. First, guest species cannot efficiently reach all metal surfaces because of the presence of capping organic molecules on the surfaces. When aggregated nanowires are formed on electrodes after evaporation of solvents, it is quite difficult for molecules with specific size to be selectively adsorbed or reacted inside nanopores due to the nonuniform pores in the aggregates. Furthermore, the diameter of nanowires is not easily tuned by the solution-based synthesis. In contrast, the template-synthesis using mesoporous silica as rigid templates, as described above, can appropriately control the structures on a nanometer scale, which can overcome those problems. In addition, mesoporous silica is important not only as a template but also as a catalyst support of nanocasted metals. Fukuoka et al. reported that mesoporous silica (FSM-16)-supported Pt nanoparticles show an excellent activity in the preferential oxidation reaction of CO.12 Although replicated metals with various structures and compositions have been reported using mesoporous silica, the study of metal deposition methods inside mesopores is not advanced. In most methods reported previously, metal depositions are carried out under hard conditions. For example, the
10.1021/jp910288x 2010 American Chemical Society Published on Web 04/05/2010
Noble Metal Nanowires hydrogen and thermal reduction methods are simple without any pretreatment, but high temperature is unfavorable because of large energy consumption.13 The chemical vapor deposition and glow discharge plasma reduction methods require high pressure and inert gas replacement.14 UV reduction appears to be a better method, but an organic solvent vapor as a radical source is necessary, which is undesirable from an environmental viewpoint.15 In addition, the UV reduction conditions need to be carefully optimized depending on the metal species. Electrodeposition method can be applied to only mesoporous films adhered to conductive substrates, which is not suitable for mass production.10,16 Therefore, it is urgently necessary to develop a straightforward reduction method that has the potential and wide applicability to various metal species and is also environmentally friendly. The use of reducing agents dissolved in aqueous solutions is reported for the preparation of metal nanowires and mesoporous metals by soft-template or template-free methods.1,17 This method is close to ideal because it is widely applicable to various metal species and both special equipment and thermal treatment are unnecessary. However, this method is not suitable for replication synthesis using mesoporous silica as a hard template because metal ions confined in mesopores tend to flow to outer surfaces due to the dissolution of metal ions into solutions containing reducing agents. Here, we demonstrate a facile soft-chemical synthesis of nanostructured metals using a vapor infiltration process of a sublimed reducing agent. Mesoporous silica powders with metal species are placed with a reducing agent in a closed vessel under atmosphere. Because dimethylamine borane (DMAB) sublimes at near room temperature, energy consumption (like high temperature, high pressure, and UV) can be suppressed. In addition, a very small amount of DMAB is sufficient for metal reduction, leading to potentially inexpensive production. In previous reports, we reported this reduction method for the synthesis of mesoporous metals using LLCs consisting of selfassembled surfactants.20 However, the variety of the metal species has been quite limited to Pt and Pt-based alloy (PtRu and PtNi). Most kinds of the metals cannot reflect the order of LLC mesostructures because LLCs tend to be collapsed during metal deposition. In contrast, when mesoporous silica is used as a rigid template, replicas with a wide variety of metals can nicely reflect the original structure of mesoporous silica.6c,8a,13b,c The mesostructures of replicas are inverse when we suppose the structure of mesoporous metals is positive. The extension of the reduction method to general routes is quite effective, which can be demonstrated by the preparation of metal nanowires using mesoporous silica. In this study, mesoporous silica SBA-15 with a 2D-hexagonal symmetry was selected as a nanotemplate. A couple of noble metal nanowires (Pt, Ag, and Au) were synthesized by reflecting the original mesostructures of the nanotemplate under the same reduction conditions. Nanowires show anisotropic optical or magnetic properties based on their one-dimensional morphology and high aspect ratios.10 The plasmon resonances vary with the diameters, lengths, and aspect ratios.11 Furthermore, we successfully controlled the lengths by the reduction time and the absorption wavelengths of the plasmon resonance of nanowires. This reduction method has advantages for the synthesis of noble metal nanowires. 2. Experimental Procedure 2.1. Materials. Tetraethoxysilane ((C2H5O)4Si, TEOS, Kishida Chemical Co.) was used as a Si source. Pluronic P123 was purchased from Aldrich Chemical Co. Ethanol and hydrochloric acid were purchased from Junsei Chemical Co. and Kanto
J. Phys. Chem. C, Vol. 114, No. 17, 2010 7587 SCHEME 1: Schematic View of the Synthesis of Metal Nanowires from Mesoporous Silica (SBA-15) through a Vapor Infiltration Method
Chemical Co., respectively. N-trimethoxysilylpropyl-N,N,Ntrimethylammonium chloride (50% in methanol, TPTAC, Gelest) was used for the surface modification of SBA-15. Hydrogen hexachloroplatinate(IV) hexahydrate (H2PtCl6 · 6H2O), silver(I) nitrate (AgNO3), and hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 · 3H2O) were purchased from Kanto Kagaku Co. and used as metal sources. Dimethylamine borane ((CH3)2NH · BH3, DMAB, Kanto Kagaku Co.) was used as a reducing agent. Sodium hydroxide (NaOH, Wako Pure Chemical Industries, Ltd.) was used for the removal of silica. Dichloromethane (CH2Cl2, Wako Pure Chemical Industries, Ltd.) was used to remove Au species from the outer surface of SBA-15. 2.2. Preparation of Pt and Ag Nanowires. The procedure is shown in Scheme 1. SBA-15 powders were prepared according to the literature.22 SBA-15 (0.1 g) was immersed into 0.45 g of a 30 wt % H2PtCl6 · 6H2O aqueous solution and placed under a reduced pressure to incorporate metal species into SBA15 by capillary force. The color of the SBA-15 turned orange, suggesting that Pt species were incorporated into SBA-15. The Pt-incorporated SBA-15 was placed in a closed container (380 mL) with 0.1 g of DMAB in a dish at 40 °C for 4 d for the reduction of Pt species (Pt/SBA-15). To separate Pt nanowires, Pt/SBA-15 was dispersed in 3 M NaOH and stirred for 1 day to dissolve SBA-15, and Pt nanowires were finally obtained after washing with water and ethanol. For preparing Ag nanowires, 0.45 g of a 10 wt % aqueous solution of AgNO3 was used instead of H2PtCl6 · 6H2O, and the other procedure was same as that for the preparation of Pt nanowires. 2.3. Preparation of Au Nanowires. N-Trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TPTAC)-functionalized SBA-15 (TPTAC-SBA-15) was prepared and used as a template.13b,c TPTAC-SBA-15 (0.1 g) was immersed into 1.2 g of a 5 wt % ethanol solution of HAuCl4 · 3H2O, and the solution was stirred for 6 h. The product was isolated by filtration and dispersed in 100 mL of dichloromethane. Yellow powders were obtained after vacuum drying, indicating that Au species were incorporated into TPTAC-SBA-15. Both the reduction of Au species and the removal of TPTAC-SBA-15 were performed by the same procedures as those for the preparation of Pt and Ag nanowires. 2.4. Characterizations. Small-angle X-ray scattering (SAXS) patterns were measured using a Rigaku Nano viewer with Cu KR radiation (40 kV, 30 mA). Powder XRD patterns at higher angles were measured with a Rigaku RINT-Ultima III with Cu KR radiation (40 kV, 40 mA) at a scanning rate of 1.0°/min. TEM images and ED patterns were taken by a JEOL JEM2010 microscope using an accelerating voltage of 200 kV. Powder samples for the TEM observation were dispersed in ethanol by ultrasound and mounted on a carbon-coated microgrid (Okenshoji Co.). Energy-dispersive X-ray spectroscopic (EDS) analyses were obtained by TEM. Solid-state 29Si magicangle spinning (MAS) NMR measurements were performed on a JEOL JNMCMX-400 spectrometer at a resonance frequency of 79.42 MHz with a pulse width of 45° and a recycle delay of 100 s. The spectra were deconvoluted using a Gaussian function
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Figure 1. SAXS patterns of (a) SBA-15, (b) Pt/SBA-15 (Pt and SBA15 composite), and (c) Pt nanowires (after removal of SBA-15).
on Spinsight software, version 4.3.2. Chemical shifts for 29Si MAS NMR were referenced to poly(dimethylsiloxane) at -33.8 ppm. FT-IR spectra of the products in KBr pellets were obtained on a Perkin-Elmer Spectrum One spectrometer with a nominal resolution of 0.5 cm-1. N2 adsorption-desorption isotherms were measured by a Quantachrome Autosorb-1 apparatus. The pore size distribution curves were calculated from the adsorption branch by the NLDFT method. CHN analysis was performed using a Perkin-Elmer PE-2400II apparatus. Inductively coupled plasma emission spectroscopy (ICP) analysis was conducted with an IRIS Intrepid (Thermo Electron Corporation) to analyze the amount of Pt, Ag, and Au. The yields of Pt and Ag nanowires were calculated as the molar ratio of deposited metals/ dropped metal ions (Pt or Ag). In the case of Au nanowires, the amount of incorporated Au ion into SBA-15 was measured. Then, the total yield was calculated as the molar ratio of deposited Au/incorporated Au ion. In all cases, the amounts of deposited metals were measured after treating metal/SBA-15 composites with aqua regia. Diffuse-reflectance UV-vis-NIR was measured with a V-670 spectrometer (JASCO Co.). 3. Results and Discussion 3.1. Pt Nanowires. Figure 1 shows the SAXS patterns of SBA-15 before and after Pt deposition. The original SBA-15 before Pt deposition had three peaks assignable to (10), (11), and (20) in a highly ordered 2D-hexagonal symmetry with d10 spacing of 9.3 nm. After Pt deposition by the vapor infiltration method with DMAB, several intense peaks were observable; these peaks were located at the same positions as the original SBA-15. However, the intensity of the peaks was slightly lower than that of the original SBA-15. Due to a pore filling effect, the scattering contrast tended to be weaker. A similar phenomenon was noted in previous reports.13b,c,18 The TEM images of Pt/SBA-15 are displayed in Figure 2a and b. The ordered arrangements of silica mesochannels were well retained after the Pt deposition for all the observed regions. The Pt deposited inside the mesochannels showed a nanowire morphology of about 7 nm in diameter, and the size corresponded to the mesochannel diameter of the original SBA15. It has been previously shown that the top surfaces of SBA15 particles possess many open mesopores.23 Therefore, DMAB molecules can infiltrate into the inside of particles. Although the deposition of Pt near the entrance is considered to occur
Takai et al. preferably to that in the inner part of the SBA-15 particles, Pt was densely deposited not only at the mesopore entrance but also at the inner parts of the particles, meaning that DMAB vapor easily infiltrated the SBA-15 particles (Figure 2a). Another important point is that no Pt bulk deposition on the outer surfaces of SBA-15 particles was observed, indicating that all Pt species were impregnated in the mesochannels by capillary force under a reduced pressure condition (see the Experimental Procedure section). The length of the Pt nanowires was over 1 µm, and the length can be tuned by the reduction period. In a later section, the length control of Au nanowires is demonstrated by changing the reduction period (see section 3.3). After the complete dissolution of silica, one-dimensional Pt nanowire arrays were obtained, as confirmed by EDS analysis. Because SBA-15 possesses micropores in mesopore walls (which was confirmed by N2 adsorption-desorption isotherms (Figure S1)), the Pt nanowires were connected and periodically attached to one another. The arrangement of the Pt nanowires was characterized by SAXS (Figure 1c). Two peaks were observed, similar to the (10) and (20) planes of a 2D-hexagonal symmetry. The peak positions were similar to those of SBA15 and Pt/SBA-15, indicating successful replication from mesoporous silica. The reduction of the peak intensities is due to a heavy scattering effect of Pt. Another reason is the reduction of the single domain size of the mesoscale ordering in the Pt nanowires. Figure 2d-f shows the TEM images of the Pt nanowires after the removal of silica. Bundle nanowires were observed, indicating that nanowires were connected with each other through the metals deposited in micropores of SBA-15. The interval of the nanowires was in accordance with the results of SAXS analysis (Figure 1c). The arrangement was retained even after an ultrasonic treatment for 1 h, showing a good mechanical stability. The Pt nanowires consisted of connected nanoparticles of 3-5 nm in size and showed rough surfaces (Figure 2e). The diameter of Pt nanowires was around 7 nm, and the nanowires were composed of nanoparticles connected in a random orientation. The lattice fringes on each nanoparticle had d-spacings of 0.23 nm and about 70O of dihedral angle which correspond to the (111) planes of the Pt fcc structure (Figure 2f). One nanoparticle showed single crystallinity, and the size of the domains with single atomic crystallinity was about two nanoparticles. But, the crystallinity did not extend any further. The crystallinity over a wide area was characterized by a selectedarea electron diffraction (ED) pattern. Ringlike patterns composed of bright intense spots were observed and assigned to the Pt fcc structure in Figure 2f (inset), proving that the nanowires have polycrystallinity. From the wide-angle XRD patterns of the macroscopic crystallinity, a diffraction pattern assignable to the lattice spacing of the Pt fcc structure was observed (Figure 2c), consistent with the result from the ED patterns (Figure 2f, inset). The crystal domain was about 7 nm by the Scherrer formula, which is almost identical to one or two nanoparticle size in the Pt nanowires (Figures 2e). These results are consistent with the above TEM data. So far, Pt nanowires have been synthesized by several methods. Pt nanowires synthesized in the presence of organic molecules had smooth surfaces and high crystallinities because the molecules act as capping agents. Previous reports show that Pt nanowires synthesized in mesoporous silica under severe reduction conditions (e.g., under high temperature or UV irradiation) exhibit similar characteristics to Pt nanowires synthesized as above.13,15 In contrast, Pt nanowires synthesized in the present study had bumpy surfaces and low crystallinity due to the mild reduction
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Figure 2. (a) and (b) TEM images of Pt/SBA-15 (Pt and SBA-15 composite). (b) is a highly magnified image of (a). (c) Wide-angle θ-2θ XRD pattern of Pt/SBA-15. (d)-(f) TEM images of Pt nanowires (after removal of SBA-15). The inset of (f) is an ED pattern.
Figure 3. TEM images of (a) Ag/SBA-15 (Ag and SBA-15 composite); (b)-(d) Ag nanowires (after removal of SBA-15); (e) wide-angle θ-2θ XRD pattern of Ag/SBA-15.
condition by vapor infiltration with DMAB. The surface roughness of the nanowires is strongly related to the size of the single crystal in the nanowires. If the crystallite size is much larger than the width of the nanowire, the crystal growth absolutely proceeds only along the long axis of the nanowires due to high confinement of crystal growth by rigid silica walls. In this case, the smooth surface of the facets is exposed to the mesopore surface as reported by previous reports.13,15 On the other hand, the Pt nanowires synthesized in this study consist of nanocrystals in smaller size than the nanowire width, which means that Pt nanocrystals were connected along the mesochannels. The BET-specific surface area of Pt nanowires was around 40 m2 · g-1, as measured by N2 adsorption-desorption isotherms. This value is higher than that of around 30 m2 · g-1 calculated roughly for smooth, unconnected, and well-dispersed nanowires of 7 nm in diameter. For unconnected Pt nanoparticles of 3 nm in diameter, the specific surface area is calculated to be around 93 m2 · g-1, which is about three times higher than that of the nanowires. The bumpy morphology of Pt nanowires synthesized
in this study is based on the connected nanoparticles of 3-5 nm in size (Figure 2e). We can conclude that the bumpy morphology of the Pt nanowires contributes to the increase of the surface areas. In general, nanostructured Pt with higher surface areas shows higher catalytic activities.24 Therefore, the Pt nanowires reported here should have a potential for efficient catalytic activities. 3.2. Ag Nanowires. Ag nanowires were synthesized by the same procedure using aqueous solutions including Ag species instead of Pt species. The SAXS patterns of Ag/SBA-15 indicated three peaks that are consistent with the (10), (11), and (20) lattice planes of a 2D-hexagonal mesostructure of SBA15 (Figure S2). TEM images proved that the Ag nanowires deposited inside the mesochannels were unconnected (Figure 3a), which was in contrast to the findings for the above-described Pt system (Figure 2a and b). After the removal of silica, no bundle morphology was observed (Figure 3b and c). Indeed, no peaks of a low-angle SAXS pattern were observed (data not shown). We consider
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Figure 4. TEM images of (a) Au/TPTAC-SBA-15 (Au and TPTAC-SBA-15 composite); (b)-(d) Au nanowires (after removal of SBA-15). The inset of (d) is an ED pattern. (e) Wide-angle θ-2θ XRD pattern of Au/TPTAC-SBA-15.
such a difference from the case of Pt to be due to the metal growth characteristics. As is well-known, Ag is normally deposited in a manner similar to dendritic growth.25 In fact, the deposition rate of Ag is faster than that of Pt, and, as a result, Ag crystals grow rapidly along the mesochannels, and it is difficult to fill Ag in the micropores of the walls. Thus, the Ag deposition did not continuously extend to neighboring mesochannels through the micropores but preferably grew along the long axis of the main mesochannels. After the removal of silica, the Ag nanowires were about 7-8 nm in diameter (Figure 3b and c). The length of the nanowires was over 1 µm. The diameter was similar to that of Pt nanowires (Figure 2e). From the highly magnified TEM images, the single crystal domains in the smooth nanowires of Ag extended (at least 40 nm) in the long axis direction of the nanowires (Figure 3d). The d-spacing of the lattice fringes (about 0.23 nm) was assignable to the Ag fcc (111) planes. The Pt and the Ag nanowires differed in surface roughness. Ag nanowires had a smoother surface, although SBA-15 had corrugated mesopore surfaces.26 This difference is attributed to the single crystalline domain size of the nanowires, which indicates the presence of Ag nanowires with large-sized crystal domains. Because the crystal growth direction is defined by the mesochannels, the crystals grow only the long axis direction of the nanowires, and the nanowires with smooth surfaces were formed. The selectedarea ED patterns showed ringlike diffractions with intense spots (Figure 3d, inset). The ED patterns of the Ag nanowires were similar to those of Pt nanowires, but the brightness of the intense spots was higher. Wide-angle θ-2θ XRD patterns (Figure 3e) showed higher intensity and narrower fwhm (full-width at halfmaximum) values than those of Pt nanowires (Figure 2c). Nanometer-sized Ag shows the surface plasmon resonance; however, we did not measure the resonance of Ag nanowires because the surface of Ag is easily oxidized. Instead, we measured the plasmon resonance on more stable Au nanowires, which will be shown in the next section. 3.3. Au Nanowires. First, SBA-15 powders without any modification were used as templates for the synthesis of Au nanowires. After the Au deposition, a few nanowires were observed inside the mesochannels (Figure S3). Unlike the Pt and Ag nanowires, almost none of the Au nanowires reflected the original mesostructures. Therefore, it is suggested that the mesochannels were partially destroyed during Au deposition and/or the Au species were reduced on the external surfaces of
SBA-15 particles. After the removal of silica, both nanowires and large particles were observed (Figure S3). This is attributed to the high diffusion of the metal species. The mobility of metal species differs depending on the kind of metals. Au species should have higher mobility than Pt or Ag species.13b,c At the very early stage of the Au deposition, Au nuclei should be preferably deposited at the entrance of the mesopores because DMAB molecules easily access from near the mesopore entrance. When the diffusion of the metal species inside the mesochannels is higher, the crystal growth of metals proceeds rapidly from the deposited nuclei at the mesopore entrance, and bulk metal is deposited on the outer surface of the SBA-15 particles. For the preparation of Au nanowires, the mesopore surfaces of SBA-15 were modified with organic groups to suppress the rapid diffusion of Au species. TPTAC-functionalized SBA-15 (TPTAC-SBA-15) was used as a nanotemplate. Yang et al. reported on the preparation of Au nanowires using TPTACfunctionalized MCM-41, MCM-48, and SBA-15,13b,c although the reduction method was completely different. Negatively charged Au species (i.e., AuCl4-) were ion-exchanged with Clderived from TPTAC and strongly interacted with positively charged propyltrimethylammonium (-PTA+) groups. This interaction suppressed the rapid diffusion of Au species and led to the synthesis of Au nanowires inside the mesochannels. The SAXS patterns before and after the surface modification of SBA-15 showed three diffraction peaks assignable to the (10), (11), and (20) planes of a 2D-hexagonal symmetry (not shown). The intensity of all peaks was lower than that of the original SBA-15 due to the reduction of the scattering contrast by organic modification within the mesopores. Typical TEM image demonstrated a highly ordered 2D-hexagonal mesostructure (Figure S4). The modified SBA-15 was also characterized by the IR (Figure S5) and 29Si MAS NMR (Figure S6). The characterization results are described in the captions of these figures. The Au nanowires were successfully embedded inside the mesochannels of TPTAC-SBA-15 without any bulk Au deposition (Figure 4a). The Au nanowires of about 7-8 nm in diameter successfully reflected the mesostructures (Figure 4b), which was completely different from the result using nonmodified SBA15 as a template (Figure S3). Similar to Ag nanowires, the Au nanowires were deposited inside the mesochanels independently. As is known, Au is normally deposited in a manner similar to dendritic growth.25 The Au nanowires were not connected
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Figure 5. TEM images of Au/TPTAC-SBA-15 (Au and TPTAC-SBA-15 composite) for (a) 6 h, (b) 1 day, (c) 4 days, and (d) 7 days of reduction. All scale bars show 50 nm in length.
through micropores (Figure 4a). The crystallinity of the nanowires was characterized on the basis of highly magnified TEM images (Figure 4c and d) and wide-angle XRD (Figure 4e). A single crystal domain consisting of lattice fringes with d-spacing of ca. 0.23 nm extended to longer than 40 nm in one nanowire. The lattice direction was not retained by the attached neighboring nanowires, which resulted in assembled nanowires showing polycrystallinity on a macroscopic scale (Figure 4c and d). The selected-area ED pattern supports the above TEM results (Figure 4d). The crystallinity of the Au nanowires was higher than those of Pt and Ag nanowires (Figure 3e). Judging from the other two TEM images of nanowires (Figure S7), specific crystal growth direction in Au nanowires is not observed. Also, the selected-area ED patterns show ringlike patterns indicative of randomly oriented crystals in the nanowires (Figure 4d, inset). Similar phenomenon was observed for the Ag nanowires which were synthesized by using nonmodified mesoporous silica (Figure 3d and Figure S8). We roughly compared the crystallite sizes of Pt, Ag, and Au nanowires by wide-angle XRD patterns. The order of the crystallinities is Au > Ag . Pt by comparing the estimated fwhm of the fcc (111) diffraction peaks (Au, ∼ 0.3O; Ag;, ∼ 0.4O; Pt, ∼ 0.6O), which is consistent with the TEM images. In a previous study by Wang et al., the same noble metal nanowires in the mesochannels of SBA-15 were prepared by glow discharge plasma reduction from the same metal ions (H2PtCl6, AgNO3, and HAuCl4).6c They confirmed that the crystallinity depends on the metal species, which is similar to our results using the vapor infiltration of DMAB method as another reduction method. We suppose that the difference of crystallinities is due to the dendritic growth kinetics of metal ions25 inside the mesopores. When the dendritic growth is preferred, metal ions assemble to a deposited nucleus before the formation of nuclei. Crystals grow from one nucleus with retaining the crystallinity. As a result, single crystal domains grow and the crystallinity in nanowires becomes high. The difference of the mobility of metal ions in the mesopores should greatly affect the yields of metal nanowires. The higher the mobility is, the larger the amount of metal deposition per time becomes. In the case of Pt, the deposited weight of the Pt nanowires prepared using 1.0 g of SBA-15 was 18.0 mg · g-1 and the yield was calculated to be about 6% (ICP analysis), which is calculated from the amount of the initially introduced Pt species into SBA-15 and that of the finally deposited Pt nanowires. This yield is much lower than those of the Ag and Au nanowire systems, which is consistent with the mobility of Pt ions being the lowest of the three. The deposited weight of Ag nanowires was 114 mg · g-1, and the yield was calculated to be about 70%, which is about 11 times higher than the Pt system. The yield of the Au nanowires was calculated to be about 72% (109 mg · g-1), which is similar to that of Ag nanowires. This result means Au species in the mesochannels had high mobility even capped with TPTAC.
The uniform Au nanowires were successfully synthesized using TPTAC-SBA-15 as a template. The surface modification with TPTAC was effective for suppressing the rapid grain growth of Au in our vapor infiltration method. Au nanowires tended to coalesce after removal of TPTAC-SBA-15, observed by TEM (Figure S9). Au atoms near the surfaces of the nanowires have high mobility due to the high surface energy of Au. Nanometer-sized and unmodified Au usually coalesce4e because the surface energy of the larger-size Au is lower. In contrast, no coalescence of Au nanowires embedded in TPTACSBA-15 was observed. Therefore, mesoporous silica is quite effective for not only nanotemplates but also hosts for Au nanowires. After removal of silica, Au nanowires can be stable by surface modification with appropriate capping groups, such as thiol groups.11a The aspect ratios of the replicated nanowires were controlled by the applied reduction time. As a typical example, the aspect ratio of the Au nanowires was controlled by the reduction time. Anisotropic Au nanowires have potential applications, such as imaging and polarizing,27 because nanosized Au shows the surface plasmon resonance (SPR) in a visible region. When the reduction period was 6 h, Au nanoparticles of about 4 nm in diameter were mainly observed (Figure 5a). As the reduction time increased, the Au nanowires became longer (i.e., the aspect ratio was higher) (Figure 5b-d). The Au nanowires grew from the tips of the nanoparticles. These TEM images proved that the Au crystal growth proceeded along the mesochannels without connection to neighbors in mesochannels through the micropores. Therefore, the reduction period is critical for the nanowire length and aspect ratio. The optical properties of Au/TPTAC-SBA-15 were investigated. The SPR absorption peaks of Au nanowires varied with their aspect ratios. The color of Au/TPTAC-SBA-15 after 4 days of reduction (Au-4d/TPTAC-SBA-15) was different from Au/ TPTAC-SBA-15 after 1 day of reduction (Au-1d/TPTAC-SBA15). Au-1d/TPTAC-SBA-15 was dusky orange (Figure 6a, inset), while Au-4d/TPTAC-SBA-15 was dark purple (Figure 6b, inset). The SPR of Au-1d/TPTAC-SBA-15 (Figure 6a) had an obvious peak at around 500 nm, suggesting the morphology of all the deposited Au nanoparticles, which is consistent with the TEM image (Figure 5a). The Au-4d/TPTAC-SBA-15 (Figure 6b) also showed one peak at the same position as Au1d/TPTAC-SBA-15, which is derived from the transverse plasmon mode of the nanowires.3b The absorption intensity was relatively higher than that of Au-1d/TPTAC-SBA-15, which is attributed to the increase in the deposited amount of Au. Furthermore, very broad SPR absorption was observed in the entire region over 700 nm assignable to the longitudinal plasmon mode, suggesting that Au-4d/TPTAC-SBA-15 takes a nanowire morphology. Longitudinal plasmon absorptions were not observed for Au-1d/TPTAC-SBA-15. When the aspect ratio is strictly controlled, a sharp absorption band is observed. The broad absorption in the spectrum of the Au-4d/TPTAC-SBA-
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Takai et al. for the observation of the UV-vis-NIR spectra. The present study was supported by the Global COE Program “Practical Chemical Wisdom” from the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT). This work was also supported by a Grant-in-Aid for Scientific Research (No. 19850031) from the Japan Society for the Promotion of Science (JSPS). One of the authors (A.T.) is grateful for financial support provided through a Grant-in-Aid for a JSPS Fellow from MEXT. SupportingInformationAvailable: N2 adsorption-desorption isotherms, SAXS patterns, TEM images, IR spectra, and solidstate 29Si NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 6. UV-vis-NIR spectra and visual observations of (a) Au1d/TPTAC-SBA-15 and (b) Au-4d/TPTAC-SBA-15 (Au and TPTACSBA-15 composite).
15 indicates that the aspect ratios of the nanowires varied widely.12 The SPR spectra also showed that the nanowires grew from the nanoparticles with the increase in the reduction time. The diameter of the nanowires did not change during the growth, indicating that Au was deposited in the mesochannels. The nanowire growth observed by the SPR spectra corresponds to that from the TEM observations (Figure 5). A small absorption band at around 1400 nm is thought to be due to mesoporous silica.28 The morphologies, colors, and corresponding SPR absorption of the deposited Au in SBA-15 are controlled by the reduction time. 4. Conclusion We demonstrated the successful synthesis of noble metal (Pt, Ag, and Au) nanowires by using mesoporous silica through the vapor infiltration of a reducing agent. The replicated Pt nanowires are periodically arranged, reflecting the mesochannel ordering of the original mesoporous silica. The specific surface area of the Pt nanowires is around 40 m2 · g-1, which is attributed to the bumpy morphologies of the Pt nanowires. On the other hand, other metal (Ag and Au) nanowires are mainly isolated. Such a difference is attributed to the metal growth characteristics. The reduction method reported here is widely applicable to various kinds of metal species under a simple reduction condition. Massive amounts of metal nanowires can be synthesized under a mild reduction condition, which should be suitable for environmentally friendly processes. Moreover, the length of the metal nanowires differed with the reduction times; in contrast, the diameter of the nanowires reflected exactly that of the original mesochannels. The surface plasmon resonance spectra of the Au nanowires embedded in SBA-15 change, depending on the nanowire length. The vapor infiltration method should be applied to synthesize nanostructured metals from versatile templates and metal species. The synthesized metal nanowires and metal nanowire/nanotemplate composites hold a great promise as innovative metal nanomaterials for various applications, including optical and electronic nanodevices. Acknowledgment. The authors acknowledge Dr. N. Suzuki (MANA, NIMS) for the SAXS measurements, Mr. M. Fuziwara (Waseda University) for the TEM observations, and Dr. Y. Hagiwara, Mr. C. Urata, Mr. T. Imai, and Mr. T. Sakakibara (Waseda University) for 29Si MAS NMR and IR measurements. In addition, the authors acknowledge the JASCO Corporation
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