methodology for atomic layer deposition (ALD) synthesis.50-52 The surface sol-gel technique generally consists of two-half reactions: (1) nonaqueous condensation of metal-alkoxide precursor molecules with surface hydroxyl groups and (2) aqueous hydrolysis of the adsorbed metal-alkoxide species to regenerate surface hydroxyls. Iteration of the above sequential condensation and hydrolysis reactions allows the layer-by-layer coating of a selected metal oxide on a hydroxyl-terminated surface. Scheme 5.1 is a schematic diagram of the basic synthesis protocol for ultrathin TiO2 on SBA-15. The SBA-15 materials have hexagonally packed channels with pore sizes of ~7.4 nm. The large mesopores allow a facile transport of metal-alkoxide reactants inside the mesopores not only for monolayer but also for multilayer functionalization on the internal walls of SBA-15.
Figure 5.5 shows the variation of the pore size distribution as a function of cycles of surface-modification-based N2 adsorption isotherms. The pore size decreases with the modification cycle number. The reduction of the mesopore size for each cycle should be about twice the single-layer thickness. Accordingly, the effective single-layer thickness is about 6 to 7 A based on the above BET measurements. This value is close to those estimated from the frequency changes of a quartz crystal balance for ultrathin films prepared by the surface sol-gel process on 2-D substrates.47-49
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