Conception des nouveaux nanocomposites à base de cristaux photoniques d’opale inverse en TiO2 à des fins photocatalytiques de l’UV vers le visible

Abstract

Heterogeneous photocatalysis based semiconductor appears actually as a technological alternative for environment remediation due to the economic use of catalysts and solar energy . The photocatalytic process depends mainly on the characteristics of the mole cule to be degraded, the light source and the semiconductor type (gap, texture) acting as a photocatalyst. Three dimensionally ordered macroporous TiO 2 with inverse opal structure, belongs to the more general class of photonic crystal (PC) has been widely studied as very efficient photocatalyst due to its unique properties in terms of ordered uniform architecture of interconnected macropores in three directions of space. The presence of macropores allows easy transfer of reactant molecules, high accessibili ty to active sites, large surface area and enhanced light absorption due to multiple scattering and slow photons effect. Despite these positive points, the large band gap energy of TiO 2 limit s its efficient use of solar energy because TiO 2 is active only under UV irradiation. Moreover, the high recombination of photogenerated electron - hole pairs result in its low photocatalytic efficiency. To solve these problems, many strategies have been developed to extend the range of absorption of the solar spectrum and to suppress recombination by increasing the charge carrier separation, through doping the TiO 2 with a second component which promotes photon - to - electron conversion efficiency. This thesis focuses on improving the photocatalytic propertie s of TiO 2 under UV and visible irradiation. In particular, we have oriented our work towards the coupling of TiO 2 with other oxides such as BiVO 4 and ZnO, which leads to the formation of nano - heterojunction. The formation of heterojunction system s is a key factor on photocatalysis to improve photogenerated electron - hole pairs separation and to exalt interfacial charge transfer between the coupled semiconductors under the internal field induced by the different electronic band structures of matching semiconductors . As a result, the lifetime of charge carriers is prolonged and thus the ir recombination can be inhibited. In particular, the combination of TiO 2 - PC with narrow band gap semiconductor, BiVO 4, that acts as sensitizer leads to the formation of BiVO 4 / TiO 2 - PC nanocomposite with intimate contact. As a result, the absorption of visible light increases with low recombination rate. Photocatalytic system based on nanostructural semiconductor characteriz ed by quantum size effect (QSE), in particular ZnO QDs@TiO 2 - CP, exhibits enhanced photocatalytic efficiency. As a consequence of QSE, the increase of surface /volume ratio as the particle size is reduced, which leads to enhancement of the access of electrons and holes to the surface and the ability of t he semiconductors to accumulate high densities of excess charges with higher energies and therefore more provide efficient oxidation and reduction processes. Moreover, the well - matched energy band alignment between ZnO QDs and TiO 2 promotes significantly t he separation in space of the electron - hole pairs. The design of ternary photocatalyst by coupling three semiconductors ZnO, BiVO 4 and TiO 2 - PC, is a good method to achieve more efficient charge separation. The formation of a cascade of electronic energy s tructures within the ZnO/BiVO 4 / TiO 2 - PC nanocomposite induces favorable charge separation between three coupled semiconductors through interfacial potential gradient in the ternary system. This charge separation prevents the electrons and holes from recombination. Thus, the electron and holes have more oppo rtunities to participate in reduction and oxidation reactions for the degradations of the organic dye (Rhodamine B) on their surface.

Date of Award	9 Dec 2016
Original language	French
Awarding Institution	University of Namur
Sponsors	Ministère d'enseignement supérieur et de recherche scientifique (Tunisie)
Supervisor	BAO LIAN SU (Supervisor), Ramzi Bourguiga (Co-Supervisor), Olivier Deparis (President), Alain KRIEF (Jury), Luca Fusaro (Jury), Yu Li (Jury) & Alexandru Vlad (Jury)