SYNTHESIS AND OPTICAL PROPERTIES OF ULTRAFINE METAL NANOPARTICLES ON DIELECTRIC ANTENNA PARTICLES

Abstract

Effective light energy conversion into other forms of energy in metal and metal compound nanoparticles has been of great interest in past decades. Being illuminated by incident light, electrons in the nanoparticles can be excited to higher energy states followed by deposition of energy into other molecules around their surface and the lattices in the following relaxation process. Ultrafine nanoparticles are thus preferred in these processes due to their high specific surface areas. Moreover, the portion of excited electrons with higher energies is higher in smaller nanoparticles than in larger ones. However, the overall light power absorbed by nanoparticles is proportional to the square of particle size, which causes the ultrafine nanoparticles not to efficiently absorb the incident light, or to drive further chemical or physical processes.Light antennae materials are usually employed to enhance the light absorption of these ultrafine nanoparticles. Plasmonic nanoparticles, e.g., Ag, Au, Cu, and Al nanoparticles, enhance the light absorption of loaded nanoparticles mainly through strong electromagnetic fields generated near their surfaces and have been proven to be effective light antennae to benefit the light energy conversion of ultrafine nanoparticles. On the other hand, spherical dielectric particles, e.g., silicon dioxide nanospheres, represent a different type of light antennae with the advantages of low cost, simple synthesis, and negligible Ohmic loss when being illuminated. When the sizes of high geometric symmetry dielectric nanospheres are comparable with the wavelength of the incident light, Mie scattering can happen based on the difference in refractive index between the sphere and the surrounding medium, generating size-dependent scattering resonances at various wavelengths. At these wavelengths, strong electric fields can be created on the surface of dielectric spheres to enhance the light absorption of the nanoparticles loaded on the surface. Previous works have shown that silica nanospheres with a diameter of several hundreds of nanometers can effectively enhance the light absorption of ultrafine Pt nanoparticles and benefit photocatalytic reactions, e.g., selective oxidation of benzyl alcohol. Over the past few years, this concept has been broadened to other ultrafine nanoparticles to study their novel photo-to-chemical/physical properties. However, the availability and comprehensive understanding of the optical properties of this class of composite particles still need to be improved. These challenges limit the further development of these composite materials in new light energy conversion processes. This dissertation aims at studying this class of novel ultrafine nanoparticles/dielectric sphere composite particles synthesis and optical properties. In Chapter 2, a synthesis protocol of ultrafine ruthenium oxyhydroxide nanoparticles on the surface of silica nanospheres’ surfaces is introduced. Unlike the traditional synthesis of nanoparticles in solution followed by a loading process, the method developed in this chapter only requires the injection of aqueous ruthenium salt solution into a silica nanosphere dispersion. The obtained ultrafine nanoparticles with sizes of 2-3 nm are characterized to be ruthenium oxyhydroxide (RuOOH) nanoparticles. The silica nanospheres are crucial in stabilizing these ultrafine RuOOH nanoparticles and enhancing their light absorption. Due to the presence of ruthenium-oxygen bonds in the nanoparticles, the absorbed photons are converted to heat and transferred to the surrounding media with a photo-to-thermal conversion efficiency close to the unity. Experimental results have shown that heat can be effectively used in accelerating the reaction rate of selective oxidation of benzyl alcohol by molecular oxygen. Kinetics data also have shown that these ultrafine RuOOH nanoparticles are able to activate molecular oxygen adsorbed on their surfaces, which represents a novel property of these ultrafine RuOOH nanoparticles that is not observed in other traditional ruthenium catalysts. In Chapter 3, a more general synthesis method of ultrafine metal and metal oxyhydroxide nanoparticles on silica nanospheres is developed, inspired by the synthetic route in Chapter 2. Instead of functionalizing silica surfaces with silane agents with amino groups, the silica surfaces are selectively etched by an aqueous base to create a high density of surface hydroxyl groups. These hydroxyl groups can provide basic sites to stabilize metal ions in aqueous dispersion, which are nuclei for the further growth of larger metal oxyhydroxide nanoparticles. In this chapter, more than ten kinds of metal ions are loaded onto silica spheres, forming oxyhydroxide nanoparticles with average sizes below 5 nm. Some oxyhydroxide nanoparticles can be reduced by 5% H2/N2 to form metal nanoparticles with their ultrafine sizes maintained. The synthesis protocol is promising in preparation of bimetallic samples. The composition and optical absorption of all obtained composite particles are analyzed, demonstrating the practicability of utilizing the reported method to prepare high-quality light-absorbing composite particles. In Chapter 4, the optical absorption property of the composite particle is systematically studied. Using ultrafine Pt nanoparticles as the light absorbing material, the light absorptions of composite particles consisting of silica spheres with diameters from 100 to 1100 nm loaded with these Pt nanoparticles are studied. Through the combination of theoretical calculation based on Mie theory and the measured optical absorption spectra, the scattering resonance peaks are successfully located in each sample. It is also found that the photonic crystal effect and the general absorption of Pt nanoparticles can contribute to the light absorption spectra, especially at higher wavelengths. The relationship between the general absorption of Pt nanoparticles and the packing density of the powder is further studied. The successful deconvolution of several components in the absorption spectra can guide the further rational design of composite particles in optical-related applications. In Chapter 5, the composite particle system is further broadened to using high refractive index zinc sulfide nanospheres as a light antenna. The use of a higher refractive index light antenna is promising for obtaining higher light absorption enhancement in loaded ultrafine nanoparticles, even though the sample is dispersed in organic media with a high refractive index as well. After the successful loading of Pt nanoparticles to the surface of silica-coated zinc sulfide nanospheres, a protocol for analyzing their light absorption spectra in organic media is proposed. Size-dependent scattering resonance peaks are observed in bare zinc sulfide nanospheres and can be utilized to enhance the light absorption of Pt nanoparticles, even when the sample is sealed in high refractive index polymeric matrices. The composite particles are further employed in photothermal tests, the results prove that the better light absorption enhancement using zinc sulfide than silica nanospheres. The results introduced in this dissertation represent the first systematic and comprehensive study of ultrafine metal and metal oxyhydroxide nanoparticles loaded on the surface of dielectric light antenna particles. The conclusions open an avenue to further rational design of high-performance light-absorbing composite particles to be used in photo-to-thermal/chemical processes.