In Situ Monitoring of Atomic Layer Controlled Pore Reduction in Alumina Tubular Membranes Using Sequential Surface Reactions

The pore diameter in alumina tubular membranes with an initial diameter of 50 Å was systematically reduced using the atomic layer controlled deposition of Al2O3. The Al2O3 was deposited using sequential exposures of Al(CH3)3 (trimethylaluminum, TMA) and H2O in an ABAB... binary reaction sequence. The pore diameter reduction was monitored using in situ N2 and Ar conductance measurements. The conductance, C = Q/ΔP, was measured using a mass flow controller to define a constant gas throughput, Q, and a pair of capacitance manometers to monitor the transmembrane pressure drop, ΔP. Conductance measurements were periodically obtained at 298 K as a function of AB binary reaction cycles. These conductance measurements were consistent with a pore diameter reduction from 50 Å to ∼5−10 Å at a rate of ∼2.5 Å for each AB cycle. Conductance measurements were also performed during the Al2O3 deposition at 500 K after each half-reaction in the binary reaction sequence. The TMA half-reaction leaves the pore surface covered with AlCH3* surface species; the H2O half-reaction converts these surface species to AlOH*. Following the TMA exposures, the conductance was significantly lower than the conductance after the subsequent H2O exposure. The corresponding difference in pore diameter of ΔD ≈ 2 Å was close to the difference predicted by the van der Waals hard sphere diameters of 3.5−4.0 Å for a CH3 group and 2.9 Å for an OH group. These in situ conductance measurements demonstrate that the pore diameters in mesoporous membranes can be reduced to molecular dimensions with atomic layer control using sequential surface reactions. Pore diameters can be tailored for specific applications by varying the number of AB cycles and changing the nature of the terminating surface functional groups.