Atomic Layer Deposition of SiO₂ and TiO₂ in Alumina Tubular Membranes:  Pore Reduction and Effect of Surface Species on Gas Transport

Abstract

The pore diameters in alumina tubular membranes were progressively reduced via SiO 2 and TiO 2 atomic layer deposition (ALD) using sequential surface reactions. The SiO 2 ALD was accomplished using alternating exposures of SiCl 4 and H 2 O. The TiO 2 ALD was achieved using alternating exposures of TiCl 4 and H 2 O. The reduction of the pore diameter was observed using in situ N 2 conductance measurements. The total conductance, C t = Q /Δ 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, Δ P t . These N 2 conductance measurements revealed that the SiO 2 and TiO 2 ALD progressively reduced the pore diameter from an initial diameter of 50 Å 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 Å per SiCl 4 /H 2 O AB cycle during SiO 2 deposition and 3.1 ± 0.9 Å per TiCl 4 /H 2 O AB cycle during TiO 2 deposition. The N 2 conductance measurements were also very sensitive to the functional groups on the surface of the pores. The SiCl 4 and TiCl 4 exposures leave the pore surface terminated with SiCl* and TiCl* surface species, respectively. Subsequent exposure to H 2 O converts these surface species to SiOH* or TiOH* species. During SiO 2 deposition, the pore diameters are 0.9 ± 0.2 Å larger after the H 2 O exposure replaces the SiCl* species with the SiOH* species. During TiO 2 deposition, the pore diameters are 0.7 ± 0.2 Å larger after the H 2 O 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 bond 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.