Photomediated Synthesis Of Silver Nanoparticles And Reaction Mechanism, Kinetic Studies

Silver nanoparticles are of great interest in the past decade due to their uniquephysical and chemical properties. Such particles have been used in the developmentof many important applications in the fields of optical, catalysis, surface enhancedRaman scattering (SERS), and biological diagnostics. Controlling the shape and sizeof silver nanoparticles is of considerable essential in manipulating their LSPR(localized surface plasmon resonances) properties and applications. Therefore, it isnot surprised that there has been a hot topic in the development of synthetic methodsfor preparing these nanostructures and investigating their size-and shape-dependentproperties. To date, the majority of the research has been focused on the preparationof isotropic silver particles with controlled size, and shape such as prism, disk, rod,icosahedra, decahedra, octahedra, wire, cube, and bipyramid. These nanoparticleshave been widely used in the field of biosensors and medical therapy.Photomediated and thermal synthesis are two main approaches that have beenemployed in the synthesis of silver nanoparticles. Thermal synthesis usually requiresstrong reducing agents and frequently high temperatures. It is a milestone thatMirkin and co-workers first proved that silver spherical nanoparticles can betransformed into triangular prisms by photochemical method in2001. A series ofefforts have been attempted to control the shape and size of silver nanoparticles byadjusting the wavelength, pH, and capping agents in photomediated synthesis. As weall know, temperature plays an important role in thermal synthesis of nanoparticles. Xia et al. reported the morphology of nanoparticles (and thus their optical properties)could be readily controlled by changing the reaction temperature. It has beendemonstrated that an increase in reduction rate due to an increase of the reactiontemperature could promote growth towards thermodynamically favorable structure.However, there are few reports that take into account the temperature effect during aphotomediated synthesis procedure.Considering above research background, silver nanoparticles are preparared andfinely controlled by photomediated approaches. The main contents and results of thisthesis are summarized as following:1. Temperature effects on photochemical synthesis of silver nanoparticles areinvestigated. The morphology of final products is strongly dependent on the reactiontemperature: nanodecahedra at a low temperature of20C; nanoprisms at a highertemperature of40C; mixture shapes at30C. An interesting transformation processis observed at a lower temperature of20C: silver nanoprisms are grown at first, andthen transformed into nanodecahedra completely. The reaction mechanisms arediscussed.2. To further investigate the influence of the irradiating wavelength, we designedand built a system consisting of a temperature controlled water-filled tank andwell-fitted light-emitting diode (LED) arrays. The irradiation wavelength is thencontrolled with a narrow-band emission (±5nm) in addition to control of the reactiontemperature in the photomediated synthesis. Remarkably, we find that bothnanoprisms and nanodecahedra can be controllably synthesized by adjusting thetemperature under shorter wavelength irradiation; but that nanoprisms are the soleproduct independent of reaction temperature with a longer irradiation wavelength of590nm. This is an unexpected result in the context of prior work and suggests thatthe mechanism for growth may be richer than previously anticipated. Based on theobservations, we propose a growth mechanism consisting of a primary step and asecondary step. The results provide the first indication that there is a primary stepthat is affected by the wavelength, presumably corresponding to the excitationenergy required for the transformation from seeds to nanodecahedra or nanoprisms, and a secondary step affected by the temperature, presumably leading to acompetition between kinetics and thermodynamics in the process. Kinetics istherefore important in determining the products.3. Localized surface plasmon resonances were modulated by adjusting themorphology and size of silver nanodecahedra and nanoprisms. Surface-enhancedRaman scattering (SERS) activities of the silver nanostructures were investigatedusing R6G and CV as probe molecules at various laser excitation wavelengths (514nm,633nm, and785nm). The results showed that the enhancement effect of SERSis modulated by the coupling effect between the localized surface plasmon resonanceof nanoparticles and the laser excitation wavelength. This work will be of greatsignificance in understanding the SERS enhancement mechanism and in thefabrication of nanoparticle films for biosensing.