Fabrication of Metal Nanostructures in Mesoporous Silicas
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Fabrication of Metal Nanostructures in Mesoporous Silicas Kuei Jung Chao and Chia Min Yang Department of Chemistry, National Tsinghua University Hsinchu, 300, Taiwan ABSTRACT Mesoporous silicas were used as hosts for fabrication of nanostructured metals. Due to the confinement effect, densely packed metal nanowire bundles and nano-networks or highly dispersed metal nanoparticles were prepared inside the channels of mesoporous silicas by metal incorporation/reduction processes. The structure of nanostructured metals was analyzed by TEM and PXRD, and the composition was investigated by ICP-AES, EDX and EELS. INTRODUCTION Metal nanostructures have attracted growing interests recently, because of their unique physical and chemical properties of low dimensionality [1-3]. They could be synthesized within the confined space of a host material. For example, one-dimensional metal nanowire arrays have been synthesized in anodic alumina membrane with diameter of 13-100 nm [1-2,4-5]. To prepare metal nanostructures with even smaller size, ordered mesoporous silicas (e.g. MCM-41, MCM-48, SBA-15) with uniform pore diameters (1.5-30 nm) and tunable pore structures are considered to be promising hosts [6-8]. One advantage of mesoporous silica over other templates is the feasibility of modification of the pore walls to have desired properties [9-11]. In this report, we show that various metal nanostructures can be synthesized in the mesoporous silica hosts by following the process shown in figure 1. Preparations of various metal nanostructures including densely packed metal nanowire bundles and nano-networks or highly dispersed metal nanoparticles are demonstrated. Their structures as well as compositions were analyzed.
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Figure 1.
Schematic representation of fabrication of metal nanostructures in MCM-41.
EXPERIMENTAL Siliceous MCM-41 with one-dimensional channels and hexagonal symmetry as well as MCM-48 with bicontinuous channels and cubic symmetry were synthesized according to some known procedures [12-13]. The organic templates were removed by calcination at 813 K for 6 hours in airflow. Ionic metal precursors were incorporated on the intrachannel surface of calcined and modified mesoporous silica, and were then reduced to form metal/ silica composites in hydrogen flow at 373-573 K. The pore diameter and the pore volume of calcined MCM-41 and MCM-48 were determined by Barrett-Joyner-Halenda (BJH) method from nitrogen sorption isotherm. The powder X-ray diffraction (PXRD) was performed to determine the structure and the periodicity of the pores in host silicas. The pore wall thickness can be deduced from the pore diameter in combination with PXRD analysis. The metal content in the reduced metal/silica composite was mainly determined by inductively coupled plasma (ICP) analysis. The peak widths of PXRD of nanostructured metals were used to estimate the average crystalline domain size of the metal in the composite by a spherical model and Scherrer’s equation. The data was compared with the images from transmission electr
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