RT Dissertation/Thesis
T1 Nobel metal-TiO2 nanocomposites : synthesis, characterization and catalytic activity
A1 Nascimento, Ana Cláudia Lobão do
K1 2210.01 Catálisis
K1 2210.04 Química de Coloides
AB The work presented in this thesis is focused on the synthesis, characterization and catalytic activity of gold-TiO2 composites. We wanted to take advantage of the experience of the Colloid Chemistry Group, whose activity is strongly focused on the synthesis, characterization and evaluation of the formation mechanism of metal nanocrystals (mainly gold and silver) with size and shape control, which allows a fine-tuning of the optical response of these colloids in the UV-vis-NIR spectral range. Moreover, their experience also allowed the possibility to design core-shell nanoparticles of metal nucleus (e.g. silver, gold) surrounds by a layer of a material such as silica or titania. These core-shell systems allow control the properties of the colloid by means of careful modification of the dimensions of the core-shell geometry and of the nature of both the core and the shell. For instance, coating of gold nanoparticles with a semiconductor material like titanium dioxide make them interesting materials to be used for energy conversion of solar to electrical energy inside an electrochemical cell, for the photocatalytic degradation of organic pollutants and for chemical sensors, among others. In this context, we wanted, at first, coated gold nanoparticles with a uniform shell of titanium dioxide, Au@TiO2. However, nanoparticles coated with titanium dioxide are generally difficult to synthesize, because common titania precursors are highly reactive, and thus control over their hydrolysis and condensation is not straightforward. Different strategies were followed to achieve a homogeneous titania shell with controlled thickness. Only, an approach based on the combination of the layer-by-layer self-assembly technique and the hydrolysis and condensation of titanium (IV) butoxide allowed obtain core-shell gold-titania nanocomposites, as describe in Chapter 2. Characterization of these Au@TiO2 core-shell nanocomposites was carried out by UV-vis spectroscopy and electron microscopy techniques.In a second stage, composites of titanium dioxide doped by gold nanoparticles, TiO2@Au,were synthesized in a two-step protocol. The strategy is based on the preparation of spindle-shaped TiO2 nanoparticles and after the Au deposition following different strategies, as explored in Chapter 3. Is generally known, that nanostructured catalysts with porosity are one effective way to improve the surface area and the elementary processes in (photo)catalysis, namely they offer a shorter path for the reagents reached the surfaces actives. Therefore, we decided to synthesize nanoporous anatase titania nanoparticles with a high surface area. Also, it is well known that the catalytic behavior of TiO2@Au (photo)catalysts is strongly influenced by the size of the gold nanoparticles and by their reciprocal interaction. Therefore, TiO2 nanoparticles homogeneously doped with gold NPs with about 2 and 4 nm in diameter were obtained only by the method of deposition-precipitation with urea followed by reduction with sodium borohydride. The structural, physico-chemical and morphological properties of TiO2 and Au-doped TiO2 were examined by transmission electron microscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV-visible absorption spectroscopy, X- ray diffraction, dynamic light scattering, selected area electronic diffraction, and Brunauer-Emmett-Teller surface area studies.In Chapter 4, knowing that citrate stabilized gold nanoparticles are very efficiency catalyst for the reduction of ferricyanide ion to ferrocyanide by sodium borohydride, we studied the catalytic activity of TiO2@Au composites in the this reduction reaccion, in order to understand the importance of TiO2 support on kinetic results and provide a clear and relevant characterization of this catalytic system. Therefore, the catalytic activity of TiO2@Au catalyst was compared with the catalytic activity of anatase titania mesocrystals and citrate-stabilized Au nanoparticles. Furthermore, it is well know that the catalytic activity of TiO2@Au composites is affected by the gold nanoparticles size and the gold loading. So, TiO2@Au samples with different Au loading (Au particle size remained unchanged) and with gold nanoparticles of about 2 and 4 nm in diameter were used to study effect of the Au loading and the gold NP size effect, respectively on the reaction rate of reduction of ferricyanide by borohydride ions. Moreover, the colloidal stability, proven by reusing the TiO2@Au catalyst without loss of catalytic activity, during the reaction allows us to propose a mechanism of the catalyzed reaction after the kinetic analysis of the results.In Chapter 5, knowing that heterogeneous photocatalysis based on TiO2 has been the focal point of numerous investigations in recent years because of the chemical stability of this material, its lack of toxicity, and its potential utility for total destruction of organic compounds in polluted air and wastewater. Furthermore, TiO2 samples synthesized exhibits stronger absorption in the UV-visible range with a red shift in the band gap transitions. So, the photocatalytic activity for degradation of rhodamine B was investigated in water under visible light using the prepared TiO2/TiO2@Au samples. In addition to this, the electrons transfer from anatase titanium dioxide nanoparticles to the gold ions (Au1+/Au3+) in the TiO2@Au composites, prepared by deposition-precipitation with urea and before NaBH4 reduction, under sunlight irradiation was checked.In the last chapter, a general conclusions of this thesis are presented. Finally, a synopsis, in Spanish, of this dissertation is included at the end of the thesis as required by the regulation of University of Vigo.
YR 2016
FD 2016-01-22
LK http://hdl.handle.net/11093/730
UL http://hdl.handle.net/11093/730
LA eng
DS Investigo
RD 18-ene-2025