Optical and structural characterization of metal oxides and carbon nitride compounds for the development of organic/inorganic hybrid systems

Metal oxides (MOs) play a key role in many areas of chemistry, physics and materials science. They are characterized by a wide variety of high technological interest compounds (e.g. TiO2, Fe3O4, Fe2O3, Y2O3, ZnO). The still increasing interest for this class of materials is mainly due to the ability to take advantage of their diversity to find new important applications in several research field. Their applicability, strengthened by the low cost, safety, ease of synthesis, ranges from (photo)catalysis to microelectronics and in vivo biological studies. Most of the MOs-based devices present outstanding performance derived from their reduced dimensions. To date, the research on nanostructured MOs is highly active in order to deeper gain knowledge about surface states and their influence on chemical and physical properties, motivating an always more multidisciplinary approach. However, some characteristics of oxides can limit their use, e.g. high band gap with almost negligible absorption in the visible, lack of optical response (luminescence) or conduction band edge not sufficiently negative to promote proton reduction (limited photocatalytic activity). The introduction of doping elements in the MO matrix to reduce these limitations can have the drawbacks of the new defects generation at the surface and/or in the bulk. Other limitations can be overcome by creating hybrid materials, such as organic and inorganic systems. In such scenario, it is necessary a detailed study aimed to characterize the properties of organic material and the means of implementation of the hybrid system. In the first part of the present thesis, we examine the stability of some exemplar metal oxides systems. The aim of this work is to extend the comprehension of nanostructured MOs phase transformation and to analyse the conditions for which the transformation takes place. The second part presents a study on carbon nitride (CN) based molecular materials performed with spectroscopy and X-ray diffraction experiments. Our characterizations reveal that CN materials present high chemical and physical stability and good control of the optical properties. Finally, we propose a structural and optical characterization of some carbon-nitride/ metal oxides hybrids. Firstly, we focus on Tb3+ doped Y2O3 system, a good candidate for nanosized phosphors, where organic passivation of the surfaces by melamine (C3H6N6) has the dual role of replacing species for hydroxides quenchers and activators of Tb by energy transfer mechanism. Then, we conclude with TiO2/CN hybrids for photocatalysis applications, realized by chemical methods and atomic layer deposition, stressing the importance of functional and terminating groups of molecular systems.