Coupling colloidal chemistry with coordination chemistry: Design of hybrid nanomaterials by the assembly of plasmonic nanoparticles and functional coordination complexes

Resumo

Nanotechnology involves the design, characterization, production and application of structures, devices and systems by the control of the shape and size at the nanometer scale involving different fields. In the last decade, nanotechnology development has boosted the interest in hybrid nanomaterials. These materials are a complimenting combination of two (or more) nanoparticles (NPs) with enhanced performance characteristics that offer exciting opportunities. It allows the possibility of integrating materials with different physical and chemical properties to widen the range of practical applications. In this context, Au NPs have recently attracted a lot of attention due to the great opportunities that Au offers at the nanoscale. In fact, their facile synthesis and functionalization can be exploited for constructing hybrid nanoparticles showing multi-functionality. In this manner, different Au hybrid nanostructures have been developed exhibiting diverse sizes, shapes and compositions displaying novel physicochemical properties, opening the door to potential new applications. On the other hand, Coordination Polymers (CPs) possess besides interesting electronic properties, potential advantages over conventional inorganic nanomaterials such as structural and chemical versatility, high specific area and biodegradability, among others. Therefore, the integration of both Au and CPs in a single heterostructure has emerged as an appealing topic. However, suitable chemical design appears as one of the key factors to improve their applicability. The work described in this thesis is motivated by the purpose of designing and studying novel hybrid nanostructures formed by combining Au NPs with different CPs: i) Prussian Blue and its Analogues (PB and PBA), ii) Spin-Crossover compounds (SCO) and iii) Metal-Organic Frameworks (MOF). Taking into account the numerous possible heterostructures, it will be discussed why these tailored hybrid NPs are the most appropriate for magneto-optical, electrochemical and electrical applications. In chapter 1, it is described the optical properties and the synthesis of Au NPs as well as the main research efforts that have been made to combine CPs incorporating Au functionalities within the overall hybrid nanomaterials. The main results of this thesis are divided into three parts depending on the potential applications: magneto-optics, electrochemistry and electrical conductivity. Chapter 2 deals with the preparation of hybrid systems formed by metallic NPs decorated by electrostatic attraction onto PBA NPs of different sizes and nature. In this approach, the capping agent of the plasmonic NP is modified, thus, allowing to select the plasmonic NP (isotropic or anisotropic) and, therefore, to tune the plasmon band position in a broad range of the visible spectrum. The heterostructure keeps its plasmonic and magnetic properties becoming a suitable hybrid material for magneto-optical applications. In chapter 3, different heterostructures composed of Au and PBA (of NiFe and CoFe) are synthesized and evaluated as electrocatalysts for the oxygen evolution reaction. The core@shell heterostructures are found to be the most appropriate to exploit the Au properties (conductivity and electronegativity). In this way, through a suitable chemical design it can be greatly enhanced the electrochemical activity and stability of the electroactive PBA. In chapter 4, a straightforward protocol is carried out to overgrow a thin SCO over different plasmonic NPs. Moreover, this synthetic route was extended to MOF. It is observed that thanks to the metallic core and the naked surface of the ultrathin SCO/MOF shell, these core@shell NPs are more conductive than the pristine SCO NPs when contacted to electrodes. In future work, further development will be done by taking advantage of the plasmon properties of the plasmonic core to get a light-induced spin transition (SCO) and to promote the adsorption/desorption of guest molecules (MOF) to obtain advanced sensing devices. This Ph.D. thesis is expected to represent a significant advancement in the development of novel heterostructures as a result of the incorporation of Au NPs to CPs.