Development of Gold and Gold/semiconductor Nanostructures for Plasmonic and Photo/Electro-Catalytic Applications

Abstract

A simple, scalable one-pot synthesis route for ‘highly active’ Au nanoparticles (NPs) using oxalic acid (OA) as reducing agent was developed. At [Au3+]:[OA] of 1:3 under 50 C and microwave heating yielded highly active facet exposed rhombic and squashed dodecahedron NPs (R&SD). Kinetics and mechanism of formation were studied in detail. The catalytic activity of these NPs were tested for 4-nitrophenol hydrogenation. Also, metal enhanced fluorescence (MEF) and 1O2 generation (MESOG) of bioactive Rose Bengal (RB) dye by these NPs were studied. The hydrogenation rate was almost double, MEF increased 5 times and MESOG generation increased significantly for R&SD compared to other shaped NPs. Such high plasmonic activity owns to electromagnetic field enhancement (hot-spots) at sharp edges and corners of Au NPs. For stable, long term use of NPs, they were loaded on natural kaolinite clay. They showed high activity in catalytic hydrogenation of 4-nitrophenol, SERS signal enhancement of RB. Similarly, kaolinite-TiO2 (KT) composite were successfully prepared by OA catalyzed sol-gel method and co-doped with N, S (KTN,S). TiO2 NPs of 5-10 nm got deposited uniformly over clay. Also, presence of electrolyte (E) and Au NPs on KT (AKT) on photocatalytic activity were tested. The methylene blue degradation efficiency for various catalysts were of the following order, P25<KT<KT-E<AKT<AKT-E<AKTN,S-E. Stability and reusability study showed just 2-4 % drop in activity after 5 cycles. Photocatalytic esterification was also highly efficient by KTN,S, converting about 85% of oleic acid to methyl oleate. Further, a new process was developed to prepare CdS QD sensitized hollow TiO2 NPs (CdS/h-TiO2) called ‘core utilization method’. They were tested as photoanode for photoelectrochemical (PEC) ‘water-splitting’ and compared with some standard NPs. PEC activity was also tested in presence of Au and Au@SiO2 core-shell NPs and found to have positive impact on the photoanode. The charge transfer resistance was considerably less and electrons lifetime was high for these photoanodes compared to similar reported systems.