Synthesis of Platinum Nanowires in Organic−Inorganic Mesoporous Silica Templates by Photoreduction:  Formation Mechanism and Isolation

Abstract

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.