Dépôt chimique en phase vapeur assisté par pulvérisation pulsée (PSECVD) pour la synthèse de matériaux fonctionnels avec des propriétés catalytiques et optiques en vue des utilisations dans les domaines de l’environnement et l’énergie

Abstract

This dissertation reports on the application of the PSE-CVD technique involving a one-step synthesis method of functional thin film materials such as metals, single and mixed oxides, which permits significant waste material reduction. The investigations of this thesis focus on the interplay between the properties of the synthesized films as well as the understanding of the deposition process and controlled growth of thin films. Cobalt metal thin films were deposited using ethanol solutions of Co(acac)2 and Co(acac)3, and the physico-chemical properties of the grown films were characterized in terms of structure, surface and morphology. Co(acac)2 enabled the growth of cobalt metal at lower temperatures than Co(acac)3. The difference in deposition temperature was attributed to the ability of ethanol to reduce Co(acac)2 better than Co(acac)3. For films deposited from Co(acac)3, the use of a nickel seed layer improved the growth rate, while a decrease in the deposition temperature when using a nickel seed layer was only observed with Co(acac)2 precursor. Furthermore, the growth behavior under the latter conditions was monitored with a unique UHV-compatible PSE-CVD reactor directly attached to an XPS system and ascribed to an enhancement of its catalytic reduction by ethanol. In the case of single-oxide deposition, the focus was on the synthesis of thin Cu2O films at low temperature, with particular attention given to an innovative approach to deposit thin films by water addition in the liquid feedstock. The water addition was found to have significant effect on lowering the band gap energy (Eg) as well as on the catalytic performance of the deposited films towards CO oxidation. The results exhibited that both the surface composition and optical properties are in good correlation with the catalytic activity. This finding is tentatively attributed to a reduced crystallite grain size and more surface oxygen species generated when water was added in the feedstock. For mixed oxides, a series of Co-Fe oxide thin films was prepared and comprehensively characterized. It was found that the increase of the Co content in an α-Fe2O3 matrix results in an increase of the band gap energy (Eg), permitting to improve the optical property as well as the catalytic performance towards CO oxidation. Specifically, it appears that the cobalt ferrites were very active for the total oxidation of some pollutants in the gaseous phase at low temperature; also, no CO was observed during the oxidation process. The promising performance of cobalt ferrite catalysts is attributed to a redox mechanism that involves the synergistic effects of the morphology, reducibility, and lattice oxygen mobility. Accordingly, the PSE-CVD technique is judged to be an appropriate method for synthesizing thin film functional materials and is expected to assist further synthesis of new efficient thin films. It could thus contribute to paving the way for catalytic and optical applications.