Final Report for AOARD Grant 15 IOA 101 " Photonic Nanoparticle - Doped Architectures for Enhancing Solar - to - Fuel Photocatalytic Conversion

Research funded by this award focused on the synthesis of unique plasmonic nanoparticles, preparation of core-shell particles to enhance the generation of hydrogen via the photocatalytic splitting of water, and mechanistic studies of the charge transfer of such core-shell particles. A particular emphasis centered on the synthesis and study of hollow gold-silver nanoshells (GSNSs) having tunable localized surface plasmon resonances (LSPRs). We prepared three sizes of GS-NSs with LSPR maxima centered at 500, 700, and 900 nm. Subsequently, we coated the GSNSs with a shell of silica (SiO2) having two different thicknesses, 17 nm and 42 nm. The generated GS-NSs (uncoated and SiO2-coated) were then coated with the photocatalyst, which in our case is the semiconductor zinc indium sulfide (ZnIn2S4; ZIS). After evaluating the photocatalytic water splitting activities, we found that GS-NS(700 nm)@SiO2(17 nm)@ZIS has the most enhancement factor (2.6) compared to that of ZIS without GS-NSs. The lifetime of photo-generated electrons in the conduction band of the photocatalyst was observed using timeresolved photoluminescence (TRPL). The increase of electron density can be retrieved by comparing the charge-carrier dynamics with and without the excitation of GS-NSs. To prepare a better coverage of the ZIS coating on the SiO2 surface, 3-mercaptopropyltrimethoxysilane (MPS) was used modify the SiO2 surface. The ZIS shell was then grown on the SiO2 surface to afford composite particles having a smooth morphology. We found that decreasing the concentration of photocatalyst precursor and increasing the content of SiO2 leads to a better coverage of the ZIS shell. We are also exploring the use of nanoshells coated with tin oxide (SnO2) rather than silica (SiO2) and coating the GS-NSs with zincand antimony-doped SnO2. Furthermore, we developed new routes for the preparation of silver, platinum, and palladium seed nanoparticles and demonstrated their use in the facile synthesis of Ag, Pt, Pd, and Pt/Ag nanoshells. Separate studies are exploring the use of photonic iron oxide (Fe3O4) nanoparticles as co-catalysts in photocatalytic water splitting. Our new synthetic procedures pave the way to generate complex composite structures having controlled nanoscale composition for optimizing solar hydrogen production. DISTRIBUTION A. Approved for public release: distribution unlimited. Professor T. Randall Lee Professor Tai-Chou Lee University of Houston National Central University