Synthesis, Characterization & Extinction Property Of Silica Nanowires And One-dimensional Silica/Silver Nanoshells

Nanoshells consisting of a silica core and a noble metal shell have great potential applications for biomedicine, chemosensing and photonic crystals, et al., due to their tunable electric and optic properties based on the sizes and relative dimensions of the core radius and shell thickness. Currently the researches about silica/noble metal nanoshells are mainly focused on the spheric nanoshells. As to one-dimensional silica / noble metal nanoshells, however, only few reports can be found. In this dissertation, the silica/silver one-dimensional nanoshells are synthesized by means of direct and pretreatment electroless silver deposition using the silica nanowires derived from chrysotile as substrates, and the extinction properties of the silica/silver one-dimensional nanoshells in the range of UV-Vis-NIR are theoretically studied by the Discrete Dipole Approximation (DDA) theory as well. Main conclusions and innovations are obtained as follows:1. Preparation, characterization and forming mechanism of silica nanowiresSilica nanowires with high purity, great aspect ratio and even morphology have been prepared through physicochemical dispersing and acid leaching of natural mineral chrysotile, which provides novel thought and method for the synthesis of silica nanowires and deep-processing of chrysotile mineral.By physicochemical dispersing and acid leaching, silica nanowires have been prepared from the chrysotile with special nanotubular structure. The high purity silica nanowires are well-dispersed and well-proportioned, with lengths of several microns, diameters of 25-55nm, and chemical composition of SiO_1.8·0.6H₂O. Although the silica is amorphous, its structure is regular to some extent. The structural unit of the silica nanowires is the [SiO₄] tetrahedron and six-member silicon-oxygen ring with the tetrahedral positioned alternately up and down in the six-member ring. Through the study of characterization results, the forming mechanism of silica nanowires derived from chrysotile has been determined.2. Solution chemistry calculation of electroless silver deposition on the surface of silica nanowiresThe introduction of solution chemistry to the liquid phase preparation of nanomaterials provides theoretical foundation and direction for the experimental study, and supports the experiment design and result analysis with helpful information and assistance.According to the calculation principle, the concentration logarithmic diagrams, ionic partition diagrams and potential-pH equilibrium diagrams of Sn-H₂O, Ag-H₂O, NH₃-H₂O, Ag-NH₃-H₂O and HCHO-H₂O systems have been plotted in detail. From the analytical results of solution chemistry diagrams, basic experimental conditions of electroless silver deposition on the surface of silica nanowires have been determined, which validly supports the subsequent experiment study and result analysis.3. Preparation and characterization of one-dimensional silica / silver nanoshellsBy the methods of direct electroless silver deposition and pretreatment electroless silver deposition, one-dimensional silica/silver nanoshells have been synthesized, which introduces novel route for the fabrication of one-dimensional silica / noble metal nanoshells, and supports the synthesis of other one-dimensional core-shell nanomaterials with useful reference and consultation.Using silica nanowires as substrate and basing on the calculation of solution chemistry, silver layers have been coated on the surface of silica nanowires, which results in one-dimensional silica/silver nanoshells. The suitable reaction pH value and temperature are 12 and 30℃, respectively. Advance adsorption of Sn²⁺ on the surface of silica nanowires is helpful for the forming of tiny and dense silver nuclei, as a result, denser and evener silver coating layer can be formed through the method of pretreatment electroless silver deposition. The thickness of silver shell coated on the silica nanowires would be influenced by initial concentration of Ag(NH₃)₂⁺ complex. The thickness of silver shell can be enhanced from dozens of nanometers to more one hundred nanometers as the increase of Ag(NH₃)₂⁺ concentration from 0.05 to 0.25 mol·L^-1.4. Simulation of extinction properties of one-dimensional silica/silver nanoshellsSpecial optical property of one-dimensional silica/silver nanoshells have been theoretically investigated in details for the first time, which has important significance for the study on the properties and applications of this kind of novel nanomaterials.By means of discrete dipole approximation (DDA) theory, the extinction property in the range of UV-Vis-NIR of one-dimensional silica/silver nanoshells has been investigated for the first time. The extinction property of one-dimensional silica/silver nanoshells shows strong relation with those of spheric silica/silver nanoshells and silver nanocylinders. Extinction spectrum of one-dimensional silica/silver nanoshell consists of transverse mode and longitudinal mode resonance bands, which is mainly influenced by the structural parameters such as size of particles, ratio of silica diameter to silver shell thickness and ratio of length to diameter. When the thickness of silver shells is more than the electronic mean free path of silver, the extinction property of one-dimensional silica/silver nanoshells will be identical to that of the one-dimensional silver nanoparticles with the same size.