Synthesis of plasmonic core/shell nanostructures for broadband light absorption

Abstract

Core/shell nanostructures have attracted considerable interest as a step forward in development of single particles for various applications due to their size-dependent physical and chemical properties. In addition, composition of the location of the inner core and outer shell has been diversified, tailoring can be made in a way that complements or reinforces the disadvantages of any material. These unique and customizable properties have made core/shell nanoparticles an emerging nanomaterial that is extremely important for a wide range of applications such as electronic optical devices, energy storage materials, and catalytic applications. Especially in the field of optical materials, core/shell nanostructure is attractive. To induce plasmon resonance across a broad-range of wavelength, the size, composition of core and shell can be adjusted. In addition, the blending of core/shell nanoparticles can also show a broadband absorption. This dissertation begins with a review of core/shell nanoparticles and their possible applications. Chapter 2 will discuss brief review of core/shell nanoparticles, materials for the inner core and outer shell, and their possible applications such as photocatalyst and plasmonic nanofluid in direct solar thermal collectors. The research is divided into two parts (chapter 3 and 4). In the second part of the dissertation, we designed core/shell nanostructures of SiO2/TiO2, which were decorated with gold (Au) nanoparticles (NPs), to activate the visible light-driven photocatalytic reaction that is not feasible with only the SiO2@TiO2 core/shell nanostructures because there is no absorption in the visible wavelengths. The core/shell nanostructures were simply synthesized by hydrothermal method. In the core/shell structure, TiO2 covered the SiO2 core surface, which acts as a template. The attachment of additional Au NPs inside or outside the core/shell led to absorbance in visible wavelengths of light from the Sun, from which most radiation comes with a spectrum peak at yellow wavelength, owing to the localized surface plasmon resonance peak at approximately 550 nm. The SiO2@Au@TiO2@Au CSN (0.01 g) decomposed 1% of methyl orange (MO) solution in 15 mL deionized water in 1 h under the visible light. The visible light-driven photocatalytic reaction was ascribed to the localized surface plasmon resonance around the Au NPs that were deposited inner or outer the mesoporous TiO2 shell. In the third part of this dissertation, we proposed surface modified metal@SiO2 nanoparticles to improve the dispersion stability and absorption coefficient. Au@SiO2 and Ag@SiO2 nanoparticles were prepared by a low temperature two-step solution process and it showed no sedimentation for more than a month. Even though they were under the high temperature about 150 oC, they maintained their dispersion stability. In order to analyze the photo-thermal conversion of the proposed nanofluids, the absorption and scattering coefficients of plasmonic nanofluid should be known a priori. In this work, we proposed the measurement method which can distinguish the absorption and scattering coefficients, and the optical properties of the plasmonic nanofluids were precisely quantified. Consequently, Au@SiO2 nanofluid showed extremely low scattering albedo (0.011) in comparision to 0.3 of Ag@SiO2 nanofluid.