Highly porous, ultra-low refractive index coatings produced throughrandom packing of silicated cellulose nanocrystals
article
The use of cellulose nanocrystals (CNCs) in optical materials has been extensively studied. Key in mostapplications reported to date is the chiral nematic ordering of CNCs. Here, we demonstrate that randompacking of silicated CNCs can also yield materials with interesting optical properties, i.e., highly porous,ultra-low refractive index coatings. Needle-shaped CNCs with an aspect ratio of 25 were extracted fromAvicel, and subsequently covered with a silica layer. In one single dip coating step, highly porous coatingsof CNC-silica core–shell particles were deposited on glass slides and silicon wafers. The lowest refractiveindex achieved was 1.03, which corresponds to a porosity of 94%; the thickness of these coatings rangedfrom 101 nm to 239 nm. The substrates, coated with a layer of CNC-silica core–shell particles, were heatedto 450◦C for two hours. Cellulose was removed through pyrolysis, which resulted in porous coatingsof sub-micron sized hollow silica rods. The porosity increase generated through pyrolysis of cellulose,however, was largely compensated by the decrease in packing porosity due to shrinkage of the coating.
TNO Identifier
528378
ISSN
09277757
Source
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 487, pp. 1-8.
Pages
1-8
Files
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