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Development of a Novel Deposition Method for Synthesis of Porous Oxide Thin Films Y. Gao and S. Elder Supported by PNNL Laboratory Directed Research and Development. Since mesoporous molecular sieves, such as hexagonally-ordered MCM-41, were discovered by Mobil Corporation scientists in 1992 (Kresge et al. 1992), the synthesis of mesoporous silica using assemblies of surfactant molecules to template the condensation of inorganic species has attracted considerable interest because of promising potential applications in catalysis and separation technologies. Typically, these materials are utilized in powder form. However, use of these materials in thin films will significantly improve their performance in some of the applications. The objective of this project is to develop a template-assisted vapor-phase technique for synthesis of thin porous films with controlled microstructure, composition, and film thickness. Unlike the solution routes, which are under thermodynamic equilibrium conditions, vapor-phase deposition is a non-equilibrium processing method. We have demonstrated the feasibility of the proposed template-assisted vapor-phase technique for synthesis of porous thin films. We have obtained key growth parameters for the processing. The porous thin films deposited on planar substrates using surfactants such as CTAC show hexagonally ordered pore structure of silica, similar to those obtained using the hydrothermal methods. For example, x-ray diffraction patterns of the porous thin films reveal only (h00) peaks [e.g., (100), (200)] of a hexagonal ordered structure. The ordered pore structure was also confirmed by cross-sectional transmission electron microscopy. The results indicate that the behavior of the surfactants under vacuum could be similar to that in the solution. Porous thin films with different film thickness, pore size, and pore ordering have been produced using the template-assisted vapor-phase technique. Mesoporous thin films were also deposited inside a macroporous structure. Two kinds of macropore size were used: 300 mm and <0.5 mm in diameter. For the macroporous structure with the large pore size, a thin layer (~1 mm) of mesoporous silica was typically deposited. The pore channels can be made in either parallel to the surface of the macropores or random orientations. The coated macroporous structure is envisioned to have great opportunities in catalysis applications. For example, our preliminary study showed more than 90% conversion in diesel combustion with only 20% of conventional catalyst loading. For the macroporous structure with the small pore size, mesoporous silica completely filled the macropores to form a composite (Figure 2.1). Such composite structures completely filled with mesoporous materials can be used as ceramic membranes. Compared to conventional ceramic membranes, these ceramic membranes can have higher permeability via small thickness and one-dimensional pore channels, and provide better species selectivity though the uniform and controlled pore size. These new ceramic membranes may have great potential in separation technologies. Figure 2.1. Plane-view TEM image of a mesoporous-silica filled macropore showing that one-dimensional mesopores are close-packed along the macropore surfaces and aligned parallel to the macropore surfaces regardless of the shape of the macropores. The insets recorded under a near-focus condition provide a clear view of the mesopore (dark) arrangement. Reference Kresge, C. T., M. E. Leonowicz, W. J., Roth, J. C. Vartuli, and J. S. Beck, Nature 359, 710 (1992). William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: June 12, 2001 Security & Privacy PNNL-13147