Advancing the capabilities of plasmonic and semiconductor nanostructures for light-powered applications

Light-powered applications are attracting increasing attention in the rapidly developing field of nanomaterials, especially in photocatalysis and biomedical technologies. Such progress requires effective designs and fabrication methods for high-performance nanoparticles. The fabrication of nano-sized particles significantly increases the surface-to-volume ratio, which leads to cost reductions. Wet chemistry synthesis of nanoparticles offers precise control over the size and morphology of nanoparticles, as well as good scalability for bulk applications. Many light-active nanostructures suffer from shortcomings that arise from limited properties inherent to the nature of their structures as well as the inability to utilize the high-intensity region of the solar spectrum. Rational designs of new plasmonic nanostructures can lead to remarkably unique optical properties that offer emergent applications. Doping metal oxide semiconductors can tune the bandgap and recombination rate of the semiconducting nanoparticles, making them more effective photocatalysts. On the other hand, noble metal-semiconductor hybridizations can offer unique synergies that are beneficial for the intended applications. Chapter 1 of this dissertation provides a detailed review of an important class of plasmonic nanoparticles know as "metal nanostars". Chater 2 describes the synthesis and characterization of an entirely new metal nanostar; namely, semi-hollow gold-silver nanostars (hAuAgNSts). This unique bimetallic plasmonic nanostructures offers a new operational window in the ultraviolet and visible regions of the electromagnetic spectrum for metal nanostars that is complementary to the existing optical windows of conventional silver nanostars (AgNSts) and gold nanostars (AuNSts). The capability to tune the localized surface plasmon resonance (LSPR) peak of hAuAgNSts in the visible region enables their usage in many solar-powered applications, such as photocatalysis or photovoltaics, and makes better use of the high-energy flux of this region of the solar spectrum. Importantly, the fabricated bimetallic nanostars exhibit greater stability than traditional AuNSts.
Chapter 3 of this dissertation reports a quick and easy method to fabricate cuprous oxide-coated silver core-shell nanoparticles Ag@Cu2O. The method offers tunable shell thicknesses, leading to highly tunable extinction behavior of the core-shell nanoparticles in the center of the solar spectrum. This semiconductor-plasmonic combination significantly reduces the recombination rate of photogenerated electron-hole pairs in the composite core-shell nanostructures; moreover, the Ag@Cu2O hybrid particles showed significantly enhanced hydrogen generation rates in photocatalytic tests.
Chapter 4 of this dissertation describes facile procedures for the syntheses of uniform, monodisperse titanium dioxide (TiO2) and doped titanium dioxide nanoparticles. The bandgap and electron-hole recombination rates of the doped particles were successfully reduced by doping separately with niobium and tantalum. Dually-doped NTTO nanoparticles exhibited an even larger bandgap reduction compared to both singly doped NTO or TTO nanoparticle analogs. The photocatalytic hydrogen evolution rates of the doped TiO2 nanoparticles were also enhanced when compared to pristine TiO2 nanoparticles and reached the highest rate in the dually doped NTTO nanoparticles.