Síntesis de nanoestruturas metálicas de oro y plata a partir de precursores organometálicos: Propiedades plasmónicas y catalíticas stars

Abstract

This work is devoted to the synthesis, characterization and study of the plasmonic and catalytic properties of gold or silver metal nanoparticles and gold-silver bimetallic nanoparticles, obtained through an organometallic approach, i.e. without the need of an external reducing agent and using organometallic compounds under mild reaction conditions and short reaction times. This study has been carried out by the use of gold(I), silver(I) and bimetallic gold(I)-silver(I) organometallic precursor complexes bearing pentafluorophenyl ligands or similar organometallic gold(I) complexes bearing also imidazolium of N-heterocyclic carbene ligands and their decomposition in the presence of different stabilizing ligands. The use of this organometallic approach has allowed us to synthesize new metal nanostructures by exerting a strict control on the size, morphology and composition of the obtained nanoparticles. Thus, bimetallic gold-silver alloy nanoparticles, ultrathin bimetallic gold-silver alloy nanowires, bimetallic core-shell silver-gold nanoprisms and carbene-stabilized ultrasmall gold nanoparticles have been obtained. Finally, we have carried out the study of the plasmonic properties of the different bimetallic nanostructures obtained and the catalytic properties of mono- and bimetallic gold-silver nanoparticles in the reduction of 4-nitrophenol. Hence, this work is divided into five chapters: The first chapter is focused on the synthesis and characterization of spherical bimetallic gold-silver nanoparticles constituted by an alloy of both metals and stabilized with hexadecylamine. The use of different amounts of the stabilizing agent and solvents of different boiling points such as tetrahydrofuran, toluene or mesitylene has permitted to exert an excellent control on the size and metal composition of the nanoparticles, allowing us to carry out a deep study of the plasmonic properties of the obtained nanoparticles. This work is completed with a study of the nanoparticles formation by the use of the nuclear magnetic resonance technique, which has provided interesting information about the mechanism and the molecular species involved in the formation of the nanostructures. In the second chapter of this work the synthesis and characterization of unidimensional ultrafine bimetallic gold-silver nanostructures stabilized with oleic acid is shown. We have studied the influence of the reaction conditions on the shape of the obtained nanostructures. Thus, we have carried out the decomposition of the organometallic precursor [Au2Ag2(C6F5)4(Et2O)]n by employing different amounts of oleic acid in the presence or not of an organic solvent (tetrahidrofuran). We have also carried out a complete structural characterization of the nanostructures by using advanced transmission electron microscopy and EDX techniques. The work is completed with the study of the mechanism of formation of the nanostructures through UV-Vis absorption spectroscopy and nuclear magnetic resonance, which has allowed us to know mechanism and the molecular species involved in the formation of these bimetallic nanostructures. In the third chapter the synthesis of bimetallic core-shell silver-gold nanostructures is described. We have carried out the decomposition of the organometallic complex [Au(C6F5)(tht)] over silver seed nanoparticles, obtained by decomposition of [Ag(C6F5)]n in the presence of hexadecylamine. This strategy of successive reactions has permitted the growth of prismatic nanostructures over the spherical silver seed nanoparticles. The use of different gold:silver molar ratios has allowed us to exert a good control on the size and the shape of the synthesized nanostructures. We have carried out the characterization of the core-shell morphology through HAADF-STEM and EDX techniques. In the same way as in the former chapters we have studied the formation of the new nanostructures by the use of UV-Vis absorption spectroscopy, transmission electron microscopy and nuclear magnetic resonance, what has provided important information about the mechanism of formation and the molecular species involved in the nanostructure formation. The fourth chapter is focused on the synthesis and characterization of new organometallic complexes of the type [CxMIM][Au(C6F5)2], [CxMIM][Au(C6F5)Cl] y [Au(C6F5)(NHCx)] (MIM = 1-methyl-3-octylimidazolium, 1-methyl-3- dodecylimidazolium, 1-methyl-3-octadecylimidazolium where x = 8 or 18; NHC = 1- methyl-3-octylimidazolium-2-ilidene, 1-methyl-3-dodecylimidazolium-2-ilidene, 1- methyl-3-octadecylimidazolium-2-ilidene where x = 8, 12 or 18) and their use as precursors for the synthesis of ultrasmall (1-2 nm) gold nanoparticles. This type of gold complexes bear in the coordination sphere of the metal centre all the features needed for the formation and stabilization of the nanoparticles, since they display low melting points, due to the presence of long alkyl chains in the imidazolium or carbene ligands, what permits a fast change to the liquid phase at moderate temperatures and pentafluorophenyl ligands, which could give rise to a reductive elimination process under thermal conditions obtaining gold(0) atoms and decafluorobiphenyl as byproduct. We complete this work with the study of the reaction of formation of the nanoparticles and of their surface state through nuclear magnetic resonance and theoretical DFT calculations, which rely interesting information about the mechanism of formation and stabilization of the nanoparticles. Finally, the fifth chapter of this work deals with the synthesis and characterization of silver, gold and gold-silver nanoparticles stabilized with polyvinylpyrrolidone (PVP) polymer and their deposition over inorganic substrates such as 3-aminopropyl functionalized silica or silica nanoparticles. We have used these nanomaterials as catalysts in the reduction of 4-nitrophenol to 4-aminophenol, in the presence of an excess of NaBH4. The same reaction has been catalysed using ultrasmall gold nanoparticles, described in the previous chapter, also supported on functionalized silica. The study of the reduction of 4-nitrophenol catalysed by the nanoparticles has been monitored through 1H nuclear magnetic resonance, using the PVP-stabilized nanoparticles, and through UV-Vis absorption spectroscopy in the case of silicasupported nanoparticles, studying in the latter the pseudo-first order kinetics of 4- aminophenol formation.