NANO LETTERS
Template Synthesis of Nanoparticle Arrays of Gold and Platinum in Mesoporous Silica Films
2002 Vol. 2, No. 7 793-795
Atsushi Fukuoka,*,† Hidenobu Araki,† Yuzuru Sakamoto,† Noriaki Sugimoto,‡ Hiroshi Tsukada,‡ Yoko Kumai,‡ Yusuke Akimoto,‡ and Masaru Ichikawa*,† Catalysis Research Center and DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0811, Japan, and Toyota Central R&D Labs, Inc., Nagakute, Aichi 480-1192, Japan Received May 15, 2002
ABSTRACT Mesoporous silica films work as templates to form uniform nanoparticles of Au and Pt. The composite materials form an array of metal nanoparticles (diameter ca. 2.5 nm) in the one-dimensional mesopores (pore diameter 2.7 nm), showing an ordered structure in the TEM observation. The nanoparticles are isolated from the silica film by dissolving with a diluted HF solution, and in the presence of 1-dodecanethiol unsupported Au nanoparticles form a superlattice structure.
Metal and semiconductor nanocomposites are key precursors to higher-ordered structures in a so-called bottom-up approach in nanotechnology.1 Mesoporous materials such as FSM-16,2 MCM-41,3 and SBA-154 have large pores (2-10 nm) and high surface area (ca. 1000 m2 g-1), thus providing great opportunities in separation, catalysis, sensors, and synthesis of new nanostructured materials. Recently we have shown that powders of mesoporous silicas such as FSM-16 and hybrid HMM-15 can be used as templates to synthesize various metal nanowires and nanoparticles by photo- and H2-reduction.6 Powders of MCM-41 and SBA-15 have also been used as templates to prepare metal nanowires.7,8 On the other hand, recently developed mesoporous silica films9-12 would find attractive application in making nano-ordered devices in which the mesopores are filled with metal nanoparticles and nanowires. However, the synthesis of nanostructured metals in the mesoporous thin films has not been well explored.13 Here we report the template synthesis of nanoparticle arrays of Au and Pt in the mesoporous silica films. The mesoporous silica film was prepared by a sol-gel process under acidic conditions according to the published procedure.9 Tetramethoxysilane (TMOS) was used as a silicon source, and trimethylstearylammonium chloride (C18TAC) as a surfactant. The molar ratio of TMOS/C18TAC/ HCl/H2O was 1.0:0.98:0.003:8.9. The silica/surfactant com* Corresponding authors. (A. Fukuoka) E-mail:
[email protected]. (M. Ichikawa) E-mail:
[email protected]. Phone: +81 11 706 3696. FAX: +81 11 706 4957. † Hokkaido University. ‡ Toyota Central R&D Labs, Inc. 10.1021/nl0256107 CCC: $22.00 Published on Web 06/01/2002
© 2002 American Chemical Society
posite solution was dip-coated on a silicon wafer at a rate of 20 mm min-1 and dried at 373 K for 1 h. The samples were calcined in air at 673 or 773 K for 4 h to result in the formation of transparent films with a thickness of ca. 500 nm, in which residual N or C was not detected by elemental analysis. We found that the mesoporous structure after the calcination at 773 K was slightly damaged by the interaction with acidic aqueous solutions of HAuCl4‚4H2O or H2PtCl6‚ 6H2O in the following impregnation step. In contrast, the silica film calcined at 673 K was acid tolerant. Therefore, the silica film calcined at 673 K was used in the following experiments. Figure 1a shows the X-ray diffraction (XRD) patterns of the film sample (film/Si). One intense peak and one weak peak are observed at 2θ ) 2.1 and 4.3°, which are indexed to (100) and (200) reflections of a 2D hexagonal structure. The diffraction patterns indicate that the channels run parallel to the Si surface.10,12 The pore diameter is 2.7 nm by the BJH (Barrett-Joyer-Halenda) method. To incorporate metal precursors in the mesoporous channels, the film/Si samples (8 × 8 mm, 4 pieces) were immersed in an aqueous solution of HAuCl4‚4H2O or H2PtCl6‚6H2O (10 mg/2 mL). The mixture was irradiated with ultrasonic wave for 30 s under reduced pressure, and the samples were left in the solution for 24 h. Then the samples were rinsed with deionized water and dried under vacuum for 24 h. The metal precursors were reduced by H2 at 673 K for 2 h or by UV-visible irradiation;6c,d both of the methods gave similar results. In the XRD patterns of the Au/film/Si composite (Figure 1b), (100) and (200) peaks were observed at 2θ ) 2.7 and 5.4°, indicating the retention of the channel
Figure 1. XRD patterns of (a) mesoporous silica film on silicon wafer after calcination and (b) after incorporation and reduction of Au.
Figure 3. (a) TEM image of a cross section of Pt/film/Si. Scale bar: 50 nm. (b) TEM image of Pt/film.
Figure 2. TEM image of Au nanoparticles in mesoporous silica film. The inset is a selected area electron diffraction pattern.
structure upon metal loading. The peak positions are slightly shifted to high 2θ angles from those of film/Si, which may result from partial shrinkage of the mesoporous film by further condensation of the silicates in the incorporation process.8,9,11 The Au/film composite was peeled from the Si wafer in ethanol, and the suspension was deposited on a carbon microgrid for transmission electron microscope (TEM) observation. Figure 2 shows a TEM image of the Au/film composite, in which Au nanoparticles with a mean diameter of 2.5 nm are densely formed in the mesopores. It is interesting to note that the composite material forms an array of uniform Au nanoparticles in the mesopores, giving an ordered structure in the TEM observation. In the selected area electron diffraction pattern (Figure 2 inset), diffraction rings/spots are observed that are indexed to (111), (200), (220), and (311) reflections of fcc Au. This indicates the formation of crystalline Au metal. In the diffuse-reflectance UV-visible spectroscopy, the Au/film/Si composite sample gave a weak peak at ca. 580 nm due to the plasmon resonance of the Au nanoparticles. 794
Nanoparticle arrays of Pt metal were similarly formed in the mesoporous film by the photoreduction of H2PtCl6/film/ Si in the presence of vapors of water and methanol. Figure 3a shows a TEM image of a cross section of Pt/film/Si. Pt nanoparticles (diameter 2.5 nm) are packed in close vicinity to each other in the one-dimensional mesopores, and in some part the arrays of Pt particles show an ordered structure (Figure 2b). Pt nanoparticles were also synthesized by H2reduction of Cp′Pt(CH3)3 (Cp′ ) C5H4CH3) impregnated in the mesoporous film (see Supporting Information). The uniform Au and Pt nanoparticles were obtained for the mesoporous films in this study, while metal nanowires were formed for the powder samples of mesoporous silicas under similar preparation conditions.6-8 As mentioned above, compounds containing N or C were not detected for the mesoporous silica films after the calcination at 673 or 773 K, and metal nanoparticles were also formed in the mesoporous film calcined at 773 K. However, there remains a possibility that very small amounts of residual compounds such as amines and carbons were deposited on the internal channels to block the aggregation of the metal particles. Another explanation is the involvement of a 3D hexagonal structure in the major 2D hexagonal network of the mesoporous thin films.14 It was reported that CdS nanoparticles were formed in 3D hexagonal mesoporous silica films.15 Unsupported metal particles were obtained by dissolving the silica frameworks with a solution of HF/H2O/EtOH (HF 2-5 wt %). Triphenylphosphine (PPh3) or alkanethiols were added as a stabilizing ligand. The unsupported Au nanoparticles by addition of a PPh3/HF solution form aggregates of the Au particles (see Supporting Information). In contrast, the separated Au nanoparticles form ordered structures using Nano Lett., Vol. 2, No. 7, 2002
unsupported Au nanoparticles form the superlattice structure with alkanethiols. Studies on formation mechanism and applications of the nanoparticle/mesoporous film composites are currently in progress. Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan (No. 13650836). Supporting Information Available: TEM images of Pt nanoparticle/film composite prepared from Cp′Pt(CH3)3, unsupported Au nanoparticles with PPh3, C6H13SH, HS(CH2)10SH, and C18H37SH, and a proposed structure of unsupported Au nanoparticles with C12H25SH (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org. References
Figure 4. (a) TEM image of unsupported Au nanoparticles isolated from the mesoporous silica film with C12H25SH/HF. (b) Enlarged image of (a). (c) HRTEM image of one Au nanoparticle. (d) Unsupported Au nanowires/particles isolated with C12H25SH/HF.
alkanethiol/HF solutions. As shown in Figure 4a and b, 1-dodecanethiol (C12H25SH) gives a large area of a 2D monolayer of self-assembled Au particles with a hexagonal packing structure. The mean particle size is 2.5 nm and the distance between the centers of the Au particles (dAu-Au) is 4.5 nm. Thus the overlap of alkyl chains in 1-dodecanethiols on adjacent Au particles is estimated to be ca. 1.0 nm in this superlattice structure.16-18 With 1-octadecanethiol (C18H37SH) dAu-Au is also 4.5 nm, but 1-hexanethiol (C6H13SH) and 1,10-decanedithiol (HS(CH2)10SH) give a shorter dAu-Au of 3.5 nm. This suggests that the distance between the Au particles is controllable by the structure of thiols. Figure 4c shows a high-resolution TEM (HRTEM) image of one Au nanoparticle of 3.0 nm isolated from the film with C12H25SH/HF, where two crystal planes are clearly observed. The observed d spacing is 0.23 nm for both of the planes, and the dihedral angle is 70°, thus showing that the planes are equivalent to {111}. This result clearly indicates that the Au nanoparticle is a single crystal of Au metal covered with 1-dodecanethiol. Figure 4d is another TEM image of separation of the Au nanocomposite with C12H25SH/HF. Au nanowires (diameter 3 nm, length 20-30 nm) are observed among the Au nanoparticles. In conclusion, we have demonstrated the formation of uniform nanoparticles of Au and Pt of 2.5 nm using mesoporous silica film as the template. The nanoparticles are closely packed in the mesopores, forming an ordered array structure in the mesoporous network. The nanoparticles are isolated from the silica film by treating with HF. The
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