Low-temperature atomic layer deposition of SiO 2 /Al 2 O 3 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 + H2 O process in order to prepare thin multilayer nanolaminates on self-standing CNF films. One to six stacks of SiO2 /Al2 O3 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 /Al2 O 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’.