Aqueous Coassembly of Small TiO₂ Nanorods with Cellulose Nanocrystals into Transparent, Uniaxially Aligned Inorganic Thin Films for Waveplates

Abstract

Mesomorphic ceramic films comprising uniaxially oriented short nanorods with a modest aspect ratio are desired for a wide range of applications, including waveplates requiring optical transparency and optoelectronics benefiting from enhanced surface area. Fabrication of such films remains challenging but can, in principle, be facilitated by coassembly with relatively large companion nanorods. This idea is successfully demonstrated for 30 nm long TiO2 nanorods with an aspect ratio of 5 assisted by 200 nm long cellulose nanocrystals (CNCs) with an average aspect ratio of 20 in an aqueous suspension. Blade-coating is exploited as a cost-effective, scalable method for shear alignment to fabricate centimeter-scale, transparent thin films. The resulting dried, composite films contain over 50 wt % TiO2. Upon calcination, 260 nm thick mesomorphic ceramic coatings emerge with an optical birefringence at 0.09 and a transparency over 90% from 420 to 1690 nm. This simple method to align small nanorods through coassembly with CNCs could be generalized to fabricate a variety of transparent composite and inorganic thin film optical retarders.