Sub-monolayer control of the growth of oxide films on mesoporous materials
The use of atomic layer deposition (ALD) as a way to deposit good-quality oxide films on the inner surfaces of mesoporous materials in a controlled fashion was explored for a number of systems, combining SBA-15, MCM-41 and FDU-12 materials and spherical mesoporous silica particles (SMSP) with Al2O3, TiO2, and SiO2 films. Advantage was taken of the well-defined nature of the pores in most of these materials to use N2 adsorption–desorption isotherms as a way to characterize the deposition process. It was seen that while the average size of the pores decreases monotonically with the number of ALD cycles used (because of the newly deposited layers), their size distribution remains as narrow as in the original samples, an observation that attests to the even coverage of the surfaces in a layer-by-layer fashion. This was confirmed further by transmission electron microscopy. Additional measurements of pore volumes and surface areas were used to further evaluate the effectiveness of the ALD processes. It was found that excessive exposure of the solids to the precursors during each ALD cycle lead to condensation at the bottom of the pores and to subsequent pore plugging, whereas insufficient doses prevent full coverage of the surfaces, leaving the original silica in the back of the pores exposed. These problems can be minimized by tuning the process parameters, but become more acute with small-pore samples such as MCM-41 or SMSP. With samples having large pores connected via small windows (FDU-12), film growth takes place inside the pores without affecting the size of the windows. Finally, the deposition of silicon oxide films is much slower than that of the other oxides, but can still be achieved using a modified experimental setup.