Tailoring nanopore formation in atomic layer deposited ultrathin films

Selectivity is a critical attribute of catalysts used in manufacturing of essential and fine chemicals. An excellent way to induce selectivity in catalysts is by using ultrathin films with tailored nanoporosity. For instance, nanopores can be created in atomic layer deposition (ALD) ultrathin over-coatings on supported metal nanoparticles by subjecting the coatings to high temperature annealing. These nanopores expose the active surface of the underlying metal nanoparticles. The dimensions of these nanopores can be tuned to impart shape selectivity: only reactants or products with a specific size or shape can fit inside the pore. In this work, the authors explore the underlying mechanism driving nanopore formation in ALD films. Ultrathin films of ALD TiO₂ (∼2.5 nm thick) and ALD Al₂O₃ (∼4.9 nm thick) were deposited on nonporous γ-Al₂O₃ nanoparticles. The pore formation and evolution were monitored in situ during thermal annealing using small-angle x-ray scattering (SAXS), and the crystallinity was monitored by in situ x-ray diffraction. A correlation between the nanopore formation and amorphous to crystalline phase transitions in the ALD layers was observed. The authors hypothesize that the pores form through the relaxation of stress induced by densification of the ALD films during the phase transitions. The authors developed a mathematical model to evaluate this hypothesis and found remarkable agreement between the model and the SAXS measurements.