Pt nanowires are synthesized in organic−inorganic mesoporous silica HMM-1 (channel diameter 3 nm) by photoreduction, and the formation mechanism and isolation of the wires have been studied. UV-irradiation of HMM-1 impregnated with H2PtCl6 for 48 h in the presence of adsorbed water and methanol leads to the formation of Pt nanowires (diameter 3 nm, length up to 5 μm) in the one-dimensional channels. On the other hand, H2-reduction of H2PtCl6/HMM-1 at 443 K for 2 h results in the formation of Pt nanoparticles (diameter 3 nm) and short wires in the channels. The formation mechanism of the wires has been studied by transmission electron microscopy (TEM), X-ray absorption fine structure (XAFS), and powder X-ray diffraction (XRD), and it is suggested that Pt ions migrate in the water/methanol phase and are catalytically reduced on Pt particles to grow to wires. In the H2-reduction, however, the reduction of Pt ions is faster than the migration, thus resulting in the formation of nanoparticles. The Pt wires are isolated by removing HMM-1 with HF treatment (yield 66%). The unsupported Pt wires are stabilized by ligands such as [N(C18H37)(CH3)3]Cl and P(C6H5)3. Scanning tunneling microscope (STM) observation of the isolated Pt wires on highly oriented pyrolytic graphite (HOPG) confirms that the wires separated from HMM-1 have a necklace-like structure, while the wires from pure siliceous FSM-16 have a rodlike structure.
Mesoporous silica films were prepared on glass plates by a dip coating method, and the film/plate samples were used as a support for Au, Pt and Pd nanoparticles. By H 2 -or photo-reduction of metal precursors impregnated on the films, metal nanoparticles are formed with an ordered array structure in the mesopores. The nanoparticles are isolated by dissolving the silica film with a dilute HF solution, and the particles are stabilized with ligands such as 1-dodecanethiol and triphenylphosphine. The diameter of the separated nanoparticles is 2.5 nm with narrow distributions, showing that uniform particles are formed in the mesopores. Selective formation of metal nanoparticles rather than nanowires suggests that a 3D-hexagonal structure is involved in the 2D-hexagonal mesoporous silica films. To study the formation of nanoparticles in a 3D-hexagonal structure, HMM-2 powder was used as a support, and in fact Au and Pt nanoparticles are formed in the HMM-2 powders. The XRD study suggests that the Au nanoparticles in the film/plate have an anisotropic orientation in the mesoporous channels. The Au/film/plate sample gives a weak plasmon peak at 570-580 nm using diffuse-reflectance UV-visible spectroscopy, which is shifted to high wavelength by 50-60 nm compared with larger Au particles.
Nanowires of platinum and gold with high aspect ratios are synthesized in one-dimensional channels of ordered mesoporous organosilica HMM. Water-saturated hydrogen reduction (wet H 2 -reduction) is significantly effective for large-scale synthesis of long nanowires, while dry H 2 -reduction gives monodispersed metal nanoparticles. The nanowires are characterized by TEM, XRD, UV-vis, STM, and XAFS. The Pt and Au wires are 3 nm in diameter and several hundred nanometers to micrometers in length. The metal wires extracted from HMM with HF have a nanonecklace structure, while the wires from siliceous FSM-16 have a nanorod structure. The results indicate that the hydrophobicity or hydrophilicity of the internal pore gives a significant influence on the morphology of nanowires. The nanonecklace structure is also affected by the organic groups in HMM. The mechanism for formation of Pt nanowires in wet H 2 -reduction was studied. In the impregnation process, the [PtCl 6 ] 2ion is physically adsorbed on HMM, and the Pt(IV) ion is reduced to Pt(0) via Pt(II). Tiny Pt nanoparticles are formed in the mesoporous channels, and the water vapor in the H 2 flow enhances the migration of Pt ions, leading to the growth of long nanowires.
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