Atomic layer deposition of SiO2 and TiO2 in alumina tubular membranes:: Pore reduction and effect of surface species on gas transport

The pore diameters in alumina tubular membranes were progressively reduced via SiO2 and TiO2 atomic layer deposition (ALD) using sequential surface reactions. The SiO2 ALD was accomplished using alternating exposures of SiCl4 and H2O. The TiO2 ALD was achieved using alternating exposures of TiCl4 and H2O. The reduction of the pore diameter was observed using in situ N-2 conductance measurements. The total conductance, C-t = Q/Delta P-t, was measured using a mass flow controller to define a constant gas throughput, Q, and two capacitance manometers to monitor the total pressure drop, Delta P-t. These N-2 conductance measurements revealed that the SiO2 and TiO2 ALD progressively reduced the pore diameter fr om an initial diameter of 50 Angstrom to molecular diameters. Using an aperture model for the conductance, the pore diameter was found to decrease at a rate of 1.3 +/- 0.1 Angstrom per SiCl4/H2O AB cycle during SiO2 deposition and 3.1 +/- 0.9 Angstrom per TiCl4/H2O AB cycle during TiO2 deposition. The N-2 conductance measurements were also very sensitive to the functional groups on the surface of the pores. The SiCl4 and TiCl4 exposures leave the pore surface terminated with SiCl* and TiCl* surface species, respectively. Subsequent exposure to H2O converts these surface species to SiOH* or TiOH* species. During SiO2 deposition, the pore diameters are 0.9 +/- 0.2 Angstrom larger after the H2O exposure replaces the SiCl* species nith the SiOH* species. During TiO2 deposition, the pore diameters are 0.7 +/- 0.2 Angstrom larger after the H2O exposure replaces the TiCl* species with the TiOH* species. These pore diameter differences are somewhat smaller than the differences predicted by steric interaction calculations of band lengths and van der Waals hard sphere radii for these surface functional groups. The N-2 conductance measurements illustrate that gas transport through microscopic pores is determined by pore diameters and the effect of the surface species.