Investigation of growth mechanisms for metal nanomaterials and metal nanoparticles -encapsulated TiO2 nanoparticles

• Main Authors: Kuo, Chien Lin, 郭建麟
• Other Authors: 黃國柱
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/86711357984691038126

博士 === 國立清華大學 === 化學系 === 100 === Coarsening process in nanoscale systems have been the subject of intensive research efforts. Among the number of processes which explains the coarsening phenomena, Ostwald ripening theory is most commonly adopted to rationalize the growth of large metal nanoparticles which are formed at the expense of small-sized nanoparticles of higher chemical potential energies. This theory did not describe whether the morphology of a metal nanoparticle plays a critical role in affecting the shape evolution of nanoparticles in the Ostwald ripening processes. In this thesis, we investigate the shape evolution among Ag nanoparticles of different morphologies in solutions, and measure chemical potential energies (or the electrochemical oxidation potentials) of Ag NPs of different morphologies, including, nanocubes, nanospheres, triangular plates, and decahedral nanoparticles with similar volume We show that the chemical potential energies of Ag NPs are strongly dependent on their morphologies, crystalline facets, surface to volume ratio (S/V ratio) and structural defects which were examined by selected area electron diffraction and X-ray diffraction. Chemical etching of Ag NPs by Fe (NO3)3 results in the preferential removal of atoms at edges/corners, and causes negative shift in the oxidation potentials. This clearly reveals the crystalline-facet dependent electrochemical behavior. To further understand the coarsening process of Ag nanowires in polyol process, we carried out the experiment, to examine the role of nitrate ion in the growth of silver nanowires. Ag nanowires (NWs) were formed via uniaxial growth of multiple twinned decahedral particles (MTPs) along the {111} facets. Herein, we show that the above MTP uniaxial growth mechanism for growth of nanorods (NRs) and short nanowires (NWs) is different from that for the growth of long Ag NWs. We provide experimental evidences to show that polycrystalline long Ag NWs (up to ∼100 μm) could be formed in high yield (∼90%) by a completely different growth mechanism via self-assembly of Ag NPs/NRs. Solution phase in-situ X-ray diffraction (XRD) measurements show that a strained face-centered tetragonal (fct) phase was gradually formed during the formation and growth of long Ag NWs, in addition to the normal face-centered cubic (fcc) phase. The strained fct phase disappears after partial etching by HAuCl4 and Fe (NO3)3. The working conditions for the MTP uniaxial growth mechanism and the nitrate-promoted self-assembly growth mechanism was compared and discussed. The as-synthesized Ag NWs were also utilized to fabricate conductive thin films (adhesive) for electrical applications. Metal-semiconductor nanocomposites belong to a particular class of popular heterostructures, for the reason that, when the metal is in contact with the semiconductor, the overall photo-abilities becomes very promising. One of the most straightforward examples of a nanoscale template that acts to guide for the growth of a plasmonic nanostructure is a metal nanoparticle. More recently, shape-controlled syntheses of metal nanostructures that can similarly be modified to act as supports for the deposition of a second element have been demonstrated, introducing the concept of shape as an additional variable that can impact the properties of the compound structure. Normally, different morphologies were bounded by different facets. To better understand the electron transfer in metal-semiconductor system with different facet via fabricated of Ag@TiO2 core-shell nanoparticles using different morphologies of Ag nanocrystals, such as, plate (which bounds by (111) facet), cube (which bounds by (100) facet, decahedron and rod by slight modification of titanium tetrabutoxide has been reported. Optical properties of these nanocrystals exhibited a slight bathochromic shift of surface plasmon resonance (SPR) of metallic nanocrystals after coating with amorphous titania (a-TiO2). As the Titania shell thickness increases, red shift in the SPR becomes dramatic. Further, temperature plays a critical role in the crystalline formation of titania shell (anatase, represented as c-TiO2). The photodegradation studies over Rhodamine B (RhB) under visible light illumination using Ag@c-TiO2 core-shell nanoparticles were compared with different morphologies of metal nanocrystal as core. The rate of photodegradation rate of RhB on Ag nanopalte@c-TiO2 is larger than that on Ag nanocube@c-TiO2. And the rate of photodegradation rate of RhB on Ag@c-TiO2 is 26 times higher than bare TiO2 (P25). Overall, we propose a photodegradation mechanism of RhB over Ag@TiO2 core-shell nanoparticles using ESR technique and near-infrared photoluminescence spectra. Besides, the metal nanoparticle in the titania ((TiO2) shell to improve electron transport, one dimensional titania nanostructures also reduce the resistance. Titania nanofiber/ hollow interior possess excellent electron mobility than nanoparticles. Electrospinning process is an easy way to fabricate ultra-long and well-aligned TiO2 nanofibers. Hence in the electrical applications, the mismatch of nanostructures would create a lot of defects in the nanomaterial. These defects would trap a lot of electrons while illumination. In the present study, we demonstrated the template-directed coating of TiO2 on SiO2 electrospun fiber with Ti metal and I2 via low temperature chemical vapor deposition. The thickness of titania shell was manipulated by the flow rate of the carrier gas. These nanostructures were characterized by XRD, EDS mapping and XPS. We also examined the photocurrent with hollow titania nanofibers under visible light illumination