Functionalisation of nanostructured silica for the immobilisation of plasmonic particles

Due to global warming, solutions are needed to reduce waste and energy consumption. One solution could be photocatalysis, which uses the solar spectrum. Thanks to this infinite source, a hope for zero-emission becomes possible. Plasmonic particles, namely nanoparticles that interact with light, can absorb this energy and give it back via thermal energy or electron transfer thanks to their localised surface plasmon resonance (LSPR). Those materials are currently in development and can be used in many applications. By supporting them on a solid, it is possible to enhance their photocatalytic effect. Moreover, it allows to combine several catalysts on the same support. Thanks to this advantage, cascade and tandem reactions can be developed, in order to synthesise complex molecules starting from commercial reagents. The first aim of this study was to produce the plasmonic particles. Gold nanorods (Au NRs) were chosen for their broader absorption range in the UV-vis-NIR spectrum compared to gold nanospheres. The synthesis involved the preparation of small gold particles as seeds, followed by anisotropic growth of the seeds to elongate them into nanorods. The aspect ratio (AR) of the nanorods was varied by adjusting the amount of silver nitrate (AgNO3) in the growth solution, allowing the tuning of the absorption spectrum. By introducing gold and palladium precursors into the growth solution, alloyed Au-Pd nanorods (Au-Pd NRs) were synthesised. Additionally, Au NRs used as anchor points can be coated by Au-Pd alloy, resulting in Au-Pd@Au NRs. Other plasmonic particles with spherical morphology were also synthesized for comparison, by reduction of a palladium precursor (Pd NPs) or a mixture of gold/palladium precursors (Au-Pd NPs) by NaBH4. The second part of this study was aiming at immobilising those plasmonic particles on silica (SiO2) nanospheres functionalised with different chemical entities (amino or thiol groups) in order to compare their immobilisation efficiency. UV-vis and SEM/TEM analysis revealed that the SiO2 NPs with thiol moieties (SiO2-SH) was the best candidate. Finally, the performance of the different supported photocatalysts was assessed by means of the Suzuki-Miyaura cross-coupling reaction. After some optimisations of the photocatalytic set-up, a comparison of the different plasmonic particles (Au NRs, Pd NPs and alloy Au-Pd NRs) supported on SiO2-SH was made to assess their activity with or without light exposure. Pd NPs displayed a better yield, but the Au-Pd NRs demonstrated a better photocatalytic effect.