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Abstract:

Atomic layer deposition (ALD) is a film growth technique to cover solid surfaces with ultrathin films of oxides. ALD proceeds in vapor phase at reduced pressure and involves chemisorption of a precursor compound and reaction in a cyclic process. ALD is appreciated for its excellent conformality on flat surfaces as well as high aspect ratio structures. ALD seldom had been applied to materials with channels narrower than 50 nm. In this work the application of ALD in materials with still narrower pores was explored. The aim was to investigate the possibility to generate BrØnsted acid sites for adsorption and catalysis by deposition of alumina, and of TiO2 for photocatalytic purposes. Nanopores according to IUPAC are classified as mesopores with diameters from 2 to 50 nm and micropores being smaller than 2 nm. This study was divided in two parts dealing with the investigation of nanoporous powders and films. The powders comprised ordered mesoporous silica materials such as Zeotile-4 composed of a mosaic of zeolitic nanoslabs and presenting complex tridimensional mesoporosity, and COK-12 material with hexagonal arrays of tubular mesopores. Combinations of micro- and mesopores are present in hierarchical zeolite powders such as ultrastable Y zeolite. Silica thin films with very high porosity were prepared using an original procedure by linking zeolitic nanoslabs into open mesoporous networks adapting the concept for synthesis of Zeotile-4 powder. Calcined films characterized by ellipsometric porosimetry (EP) presented pore sizes in the range of 6-18 nm and unprecedented porosities of 70-90% depending on the temperature of assembly of the nanoslabs. Unlike Zeotile-4, the films were disordered according to SAXS and TEM. These mesoporous thin films were extremely flexible, and exhibited a reversible shrinkage of up to 30% of the original thickness upon capillary condensation of adsorbate vapor. ALD of alumina was achieved using alternating pulses of trimethyl aluminum and water vapor. Titania deposition was achieved using tetrakis(dimethylamino)titanium (TDMAT) and water. The introduction of amorphous TiO2 into the pores of Zeotile-4 was successful. Zeotile-4 particles were partially filled and a TiO2 penetration front was observed. The penetration depth of the TiO2 deposit to the interior of individual Zeotile-4 particles and the alteration of porosity by the introduced TiO2 was determined using TEM of particle cross sections and N2 physisorption. The TiO2 penetration depth into individual particles was dependent on mesopore architecture, pore width and shape and ALD pulse length. The use of ALD leads to patterned TiO2 deposition in the anisotropic Zeotile-4 support material. In COK-12 with mono dimensional pore system the ALD process worked less well and lead to pore clogging.Aluminum was successfully introduced via ALD on silicate-based ordered mesoporous materials, hierarchical materials and mesoporous zeolites. The incorporation of aluminum via ALD gave rise to aluminum in four-, five- and six fold coordination detected via 27Al MAS NMR. The ALD treatment of these materials was found to enhance the catalytic activity in decane hydrocracking by creation of additional BrØnsted acid sites. The hydrocracking activity of ultrastable Y zeolites was significantly enhanced. ALD is particularly attractive for materials in which the incorporation of aluminum through direct synthesis is difficult to achieve, such as e.g. Zeotile-4. ALD of TiO2 in mesoporous silica films was also successful. TiO2 was deposited throughout the film as revealed with Rutherford Backscattering Spectroscopy and EP. The residual porosity and mechanical flexibility of the film evolved with the number of applied ALD cycles. Upon calcination the deposited amorphous TiO2 layer was fragmented into anatase nanoparticles useful in photocatalysis.Micro-ring resonators were coated with nanoporous aluminosilicate films synthesized via sol-gel method and with a mesoporous silicate film on which alumina ALD was applied. The acid sites in these aluminosilicate coatings enabled adsorption and sensing of ammonia vapor. A reversible response to ammonia with selectivity relative to CO2 was obtained with these sensors. The ammonia detection limit was estimated at about 5 ppm. The work opens perspectives on development of nano-photonic sensors for real-time, non-invasive, low cost and light weight biomedical and industrial sensing applications.