Design of novel compositionally controlled hybrid and ternary nanostructures

Abstract

The size/shape dependent and unique physical and chemical properties presented by nanostructured materials have attracted great attention in several fields such as energy harvesting, optoelectronics and biomedicine, among others. Even though binary semiconductors have been some of the most studied systems until now, ternary and quaternary semiconductors have started to stand out due to the wide variety of compositions and, as a result, of properties they offer. The importance of hybrid nanomaterials is growing as well: the association of more than one material in the same nanostructure usually allows the preservation or even, the enhancement, of the different properties of the preliminary materials and combines them with the new ones originated from the interaction between the two domains.

This thesis is focused on the design of novel compositionally controlled hybrid and ternary nanostructures based on low toxic materials.

Firstly, a simple procedure at room temperature is reported for the synthesis of hybrid and ternary nanostructures of Ag-Au-Se and Ag-Au-S. The method consists in the reaction between pre-synthesised Ag2Se/Ag2S nanoparticles (NPs) and a Au(III) precursor. The reaction time, the concentration of gold solution, the surfactant nature and the Ag:Au ratio are the four key parameters that allow the control of the final product.

Regarding the Ag-Au-Se system, Au-Ag2Se hybrid nanoparticles (HNPs), Au-Ag3AuSe2 HNPs and Ag3AuSe2 NPs were successfully synthesised. In addition, Au-Ag3AuSe2 HNPs were tested as thermoelectric material, obtaining an improved response in comparison with the binary material (Ag2Se). The potential of Ag3AuSe2 NPs as Computed Tomography contrast agents was also tested, obtaining promising results in this field.

Concerning to the analogous system with sulphur, the higher miscibility of Au and S offers a more complex ternary diagram, with two ternary materials with different stoichiometries: Ag3AuS2 and AgAuS. A gradual transformation of Ag2S to Au2S was achievable by the proposed method, with the possibility of isolating Au-Ag2S HNPs, Au-Ag3AuS2 HNPs, Au-AgAuS HNPs, Au-Au2S HNPs and hollow Au2S NPs.

Secondly, another ternary system was studied: Ag-Cu-S. Even though this system also presents two different ternary materials (Ag3CuS2 and AgCuS), the direct hot injection method proposed here only allows the formation of the AgCuS stoichiometry. Two different mechanisms are reported, depending on the precursor of copper used in the synthesis. The

material was thermoelectrically characterized as well, but without showing a proper performance.

Thirdly, four novel nanostructures based on Cu-Pt-Se are described. They were synthesised by a reaction at high temperature between pre-synthesised Cu2-xSe NPs and a Pt(II) precursor. The nanomaterials were thoroughly structurally and morphologically characterized to study the impact of the Pt:Cu ratio in the final product. The larger the amount of platinum in the structure, the more efficient diffusion of the element occurs through the Cu-Se lattice, with the consequent and slow spell of selenium until its totality.

Finally, hybrophilic hybrid inorganic-organic nanocomposites formed by inorganic NPs (Au, Ag, Ag3AuSe2 i Au@Fe3O4) and a highly fluorescent low molecular weight Au(I) metallogelator are presented. Their coupling is mainly based on aurophilic/metallophilic interactions between atoms in the surface of the NPs and Au(I) atoms from the complex. Additionally, the Ag and Au nanocomposites were characterized by Raman Spectroscopy. It is well known that when a molecule is strongly coupled to a plasmonic nanoparticle, the intensity of the Raman peaks of the molecule are intensified. This phenomenon is known as Surface-Enhanced Raman Spectroscopy (SERS) and could be observed in both materials.

In summary, in this thesis five hybrid and ternary nanostructured systems, based on low toxic materials, have been synthesised, characterized and studied, following the aim of investigate alternative materials, which, in a future, could be applied in energy conversion and biomedicine fields.

En els últims anys, els materials ternaris i híbrids han començat a sorgir gràcies al gran ventall de composicions i, per tant, de propietats que ofereixen i que els donen la possibilitat d’aplicar-se en diversos camps, com ara l’emmagatzematge d’energia, l’optoelectrònica o la biomedicina. Aquesta tesis està centrada en el disseny de noves nanoestructures ternàries i híbrides basades en materials amb una toxicitat baixa.

En primer lloc, s’ha descrit un procediment simple a temperatura ambient per la síntesi de nanoestructures ternàries i híbrides d’Ag-Au-Se i d’Ag-Au-S que consisteix en la reacció entre nanopartícules d’Ag2Se i Ag2S sintetitzades prèviament i un precursor d’Au(III). El temps de reacció, la concentració del precursor d’or, la naturalesa del tensioactiu i la relació Ag:Au són els quatre paràmetres clau que permeten el control del producte final. Addicionalment, dos compostos del sistema Ag-Au-Se van ser caracteritzats termoelèctricament i com a agents de contrast en tomografia computada.

En segon lloc, s’ha estudiat un altre sistema ternari, format per Ag-Cu-S. El mètode d’injecció en calent proposat en aquesta tesi permet la formació del material amb estequiometria AgCuS. El material va ser caracteritzat termoelèctricament, tot i que no mostra resultats satisfactoris degut a la seva baixa conductivitat elèctrica.

En tercer lloc, es presenten quatre nanoestructures noves basades en Cu, Pt i Se, sintetitzades mitjançant una reacció a alta temperatura entre NPs de Cu2-xSe sintetitzades prèviament i un precursor de Pt(II). L’impacte de la relació Pt:Cu utilitzada en la síntesi en el producte final va ser estudiada. A mesura que la quantitat de platí augmenta en l’estructura, aquest es va introduint més eficientment en la xarxa cristal·lina del semiconductor de coure i seleni, expulsant gradual i lentament el seleni fins a la totalitat, augmentant així el caràcter metàl·lic de les nanoestructures finals.

Finalment, es descriuen uns compostos híbrids hidrofílics, formats a partir de NPs inorgàniques (Au, Ag, Ag3AuSe2 i Au@Fe3O4) i un complex d’Au(I) de baix pes molecular i altament fluorescent. El seu acoblament està basat, essencialment, en interaccions aurofíliques/metal·lofíques entre els àtoms de la superfície de la nanopartícula i els àtoms d’Au(I) del complex.