Silica nanowires

Figure 3.5 (a) Transmission electron microscopy (TEM) image of a biphase coaxial silicon nanowire consisting of a crystalline core and an amorphous sheath (b) A biphase silicon carbide nanowire, where the amorphous material has grown predominantly on one side of the nanowire. Silica nanowires can result from the oxidation of silicon/silica nanowires. 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

Figure 3. Schematic representation of the selective synthesis of metal nanowires and nanoparticles by the Sintering Controlled Synthesis approach, (a) Mesoporous silica, (b) impregnation of mesoporous silica with metal ions, (c) addition of water/alcohol vapors and UV-irradiation, or wet H2-reduction, (d) formation of metal nanowires, (e) dry H2-reduction, (f) formation of metal nanoparticles.

Similarly, monometallic Rh, Pd, and Au and bimetallic Pt-Rh and Pt-Pd nanowires were prepared in FSM-16 or HMM-1 by the photoreduction method [30,33,34]. The bimetallic wires gave lattice fringes in the HRTEM images, and the EDX analysis indicated the homogeneous composition of the two metals. These results show that the wires are alloys of Pt-Rh and Pt-Pd. Mesoporous silica films were also used as a template for the synthesis of uniform metal particles and wires in the channels [35,36]. Recently, highly ordered Pt nanodot arrays were synthesized in a mesoporous silica thin film with cubic symmetry by the photoreduction method [37]. The... 

Sumetsky, M. Dulashko, Y. Hale, A., Fabrication and study of bent and coiled free silica nanowires Self coupling microloop optical interferometer, Opt. Express 2004,12, 3521 3531... 

We also include in this class of quasi-2D nanostructured materials Titania deposited inside ordered mesoporous silica (because an inner coating of mesoporous silica may be realized), or nano-dot type Titania particles well dispersed in the ordered porous matrix. We do not consider here solids which contain linear or zig-zag type TiOTiO-nanowires in a microcrystalline porous framework, such as ETS-4 and ETS-10, notwithstanding the interest of these materials also as photocatalysts,146-151 because these nanowires are located inside the host matrix, and not fully accessible from the gas reactants (the reactivity is essentially at pore mouth). 

Ni thin films were used as catalysts by Jin et al. and Yan etal. to produce amorphous SiNW at 1200°C. No external supplies of Si were needed, but hydrogen gas was used. It was unclear whether the nanowires were amorphous silicon nanowires covered with silica layers, or silica nanowires. 

It is possible that the as-made nanowires contain some crystalline Si core, and subsequent reaction with oxygen in the air results in the formation of silica nanowires. Since hydrogen gas was used in the reaction, pure silicon nanowires were probably made first, followed by oxidation in air. In the following discussion, for simplicity reason we will assume that amorphous SiNW were made during catal)Tic reactions. 

Figure 15.1 Ordered nanoscale structures of mesoporous materials such as FSM-16 and HMM-1 (Et-HMM-1) as the silica and organosilica templates for surface-mediated synthesis of metal/alloy nanowires and nanoparticles.

This chapter deals with the selective preparation, TEM/EXAFS/XPS characterization and catalysis of mono- and bimetallic nanowires and nanoparticles highly ordered in silica FSM-16, organosilica HMM-1 and mesoporous silica thin films. The mechanism of nanowire formation is discussed with the specific surface-mediated reactions of metal precursors in the restraint of nanoscale void space of mesoporous silica templates. The unique catalytic performances of nanowires and particles occluded in mesoporous cavities are also reviewed in terms of their shape and size dependency in catalysis as well as their unique electronic and magnetic properties for the device application. 

It has been previously reported [21, 22] that metal colloids are formed by radiochemical reactions in water/alcohol solutions, in which the reduction of metal salts takes place by solvated electrons and free radicals produced under UV or y-ray irradiation. Ichikawa et al. have applied this photoreduction method to the surface-mediated reaction of metallic ions and succeeded in synthesizing metal/aUoy nanowires in the constrained cavities of mesoporous supports such as FSM-16 and MCM-41 [18-20, 23-25]. The adsorbed water and alcohol work not only as solvents in the nanoscale silica void space but also as a source of reducing species for metallic ions to metals under UV-vis and y-ray [11, 18, 19] irradiation. The results indicate the dense formation of Pt nanowires inside the charmels of mesoporous supports, such as FSM-16, which act as the templates. In fact, no any Pt wire is observed on the external surface of FSM-16 or amorphous silica surface. Short wires, 10 nm long, are also observed as a minor species in the samples in the initial stage of UV and y-ray irradiation. 

Mechanism for Formation of p( Nanowires in Mesoporous Silica Templates... 

Figure 15.13 Pictorial representation of proposed mechanism for Pt nanowire formation in mesoporous silica templates such as FSM-16 and HMM-1 under UV-irradiation of H2PtCl6/ FSM-16 and H2PtCl6/HMM-l in the presence of methanol and water vapor.

Isolation and Characterization of Metal/Alloy Nanowires Free from the Silica Supports 615... 

Figure 15.18 Proposed structures of (a) Pt nanorod wires in silica FSM-16 and (b) Pt nanonecklace wires in organosilica such as HMM-1 (Et-HMM) and HMM-p (Ph-HMM), which are used as the mesoporous templates for nanowire formation.

To study the promotion mechanism of Pt wire/FSM-16 in the PROX reaction, the Pt nanowires were extracted by HF/EtOH treatment from FSM-16, and the wires were again deposited on the external surface of FSM-16 from the ethanol solution. We found that the resulting external Pt wire/FSM-16 catalyst gave low TOFs (>35) and lower CO selectivity (>30%) in the PROX reaction [32]. This implies that the encapsulation of Pt wires in the silica channels of FSM-16 is a key to promote the selective CO oxidation in the PROX reaction. Furthermore, from the structural characterization by XANES, XPS and IR in CO chemisorption... 

We have observed similar IR bands (1520, 1352 and 1295 cm ) on the Pt wire/ FSM-16 sample in an in situ IR study of the PROX reaction. From these results, we propose that the selective CO oxidation in the PROX on Pt wire/FSM-16 proceeds through the reaction of a carboxyl intermediate (COOH) on Pt nanowires (and particles) supported on FSM-16 with active OH groups (Figure 15.26). CO reacts with an active silica surface OH of FSM-16 to convert the HCOO intermediate on Pt wires and particles into CO2, thereby leading to selective CO oxidation. The subsequent H2/O2 chemisorption generates active surface OH groups near the Pt wires and particles on FSM-16. Smaller HCOO intermediates due to the smaller OH interaction on Pt particle/HMM-1 and Pt necklace wire/HMM-1 may reflect in their lower TOFs and lower CO selectivity in the PROX reaction (Figure 15.25a and b). 

We have reviewed the state of the art of the synthesis and applications of metal/ alloy nanowires and nanoparticles by surface-mediated fabrication using different mesoporous silica templates. New nanoscale materials are appearing one after another by the use of many kinds of mesoporous materials as templates, and we are facing a rapid advance of this research field. So far, many studies have focused... 

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