Low-temperature atomic layer deposition of SiO₂/Al₂O₃ multilayer structures constructed on self-standing films of cellulose nanofibrils

In this paper, we have optimized a low-temperature atomic layer deposition (ALD) of SiO₂ using AP-LTO® 330 and ozone (O₃) as precursors, and demonstrated its suitability to surface-modify temperature-sensitive bio-based films of cellulose nanofibrils (CNFs). The lowest temperature for the thermal ALD process was 80°C when the silicon precursor residence time was increased by the stop-flow mode. The SiO₂ film deposition rate was dependent on the temperature varying within 1.5-2.2 Å cycle^-1 in the temperature range of 80-350°C, respectively. The low-temperature SiO₂ process that resulted was combined with the conventional trimethyl aluminium + H₂O process in order to prepare thin multilayer nanolaminates on self-standing CNF films. One to six stacks of SiO₂/Al₂O₃ were deposited on the CNF films, with individual layer thicknesses of 3.7 nm and 2.6 nm, respectively, combined with a 5 nm protective SiO₂ layer as the top layer. The performance of the multilayer hybrid nanolaminate structures was evaluated with respect to the oxygen and water vapour transmission rates. Six stacks of SiO₂/Al₂O with a total thickness of approximately 35 nm efficiently prevented oxygen and water molecules from interacting with the CNF film. The oxygen transmission rates analysed at 80% RH decreased from the value for plain CNF film of 130 ml m^-2 d^-1 to 0.15 ml m^-2 d^-1, whereas the water transmission rates lowered from 630 ± 50 g m^-2 d^-1 down to 90 ± 40 g m^-2 d^-1.This article is part of a discussion meeting issue 'New horizons for cellulose nanotechnology'.