Highly porous, ultra-low refractive index coatings produced through random packing of silicate cellulose nanocrystals

The use of cellulose nanocrystals (CNCs) in optical materials has been extensively studied. Key in most applications reported to date is the chiral nematic ordering of CNCs. Here, we demonstrate that random packing 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 from Avicel, and subsequently covered with a silica layer. In one single dip coating step, highly porous coatings of CNC-silica core–shell particles were deposited on glass slides and silicon wafers. The lowest refractive index achieved was 1.03, which corresponds to a porosity of 94%; the thickness of these coatings ranged from 101 nm to 239 nm. The substrates, coated with a layer of CNC-silica core–shell particles, were heated to 450 °C for two hours. Cellulose was removed through pyrolysis, which resulted in porous coatings of 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.