Preparation and characterization of inverted opals

Resumen

Inverted opals based on polystyrene have been prepared in this thesis, by a method of selfassembly which consists of four steps: synthesis of the colloidal particles, ordering them into crystals, filling the interstices with CdS, TiO2 or SiO2 and removing the polymeric template. Three different types of polystyrene microparticles have been prepared by classic polymerization techniques. These polystyrene microparticles are polymeric and present an hydrodynamic diameter of about 1 µm (with a narrow distribution of sizes) and a negative surface charge density (due to surface sulphate groups). The difference between the three types of microparticles consists of the forces which stabilize the colloidal dispersion: electrostatic, steric or by coating (core-shell). The application of core-shell particles to colloidal crystallization improves the quality of the deposited structures. The ordering of the colloidal particles to form a colloidal crystal has been developed by three different techniques: electrophoresis, electrophoresis assisted by hydrodynamic flows and vertical deposition. Particles with a very high surface charge density (electrostatically stabilized dispersions) have been used in the first two cases, in order to improve the deposition forced by electric fields. The application of hydrodynamic flows to electrophoresis improves considerably the quality of the deposited structures. However, core-shell particles are more suitable in the case of vertical deposition because the hydrophilic coating reduces the problems of coalescence and coagulation, typical from large particles (>1 µm). The colloidal crystals have been characterized structurally (number of layers, size of domains and type of packing) by optical microscopy and S.E.M.. The optical properties of these crystals have been determined by optical microscopy, S.E.M., Fraunhofer diffraction and UV-Vis spectroscopy. The interstices of the colloidal crystal have been infiltrated with CdS by chemical electrodeposition. Moreover, colloidal crystals infiltrated with TiO2 or SiO2 have been prepared by codeposition. Monodisperse dispersions of TiO2 nanoparticles have been synthesized by sol-gel techniques, while the SiO2 are commercial particles. Depending on the working pH, the nanoparticles present negative or positive surface charge density. Moreover, the size of the nanoparticles (between 3.5 and 11 nm) is regulated by the time of deposition. In the case of TiO2, better structures have been obtained with positive nanoparticles (with opposite charge to the polystyrene microparticles). The infiltration of SiO2 nanoparticles is improved by using nanoparticles coated with a shell of polyvinylpyrrolidone. The removing of the polymeric template in order to obtain the inverted opal has been developed by either thermal procedures or chemical methods. The working parameters have been optimized in order to obtain high-quality structures with the both ones. However, thermal methods allows obtaining the inverted opal in a crystalline phase with higher refractive index, which would improve their optical properties. The structural and optical characterization of both the infiltrated and the inverted opals have been developed by the same techniques as the colloidal crystals.