Self-cleaning, broadband and quasi-omnidirectional antireflective structures based on mesocrystalline rutile TiO₂ nanorod arrays

We report excellent broadband and quasi-omnidirectional antireflective (AR) structures based on highly stable, self-cleaning, mesocrystalline rutile TiO₂ nanorod arrays, which were grown by a facile hydrothermal synthesis directly on Ti foils. Typically, each hierarchical nanorod is a single-crystal-like rutile TiO₂ mesocrystal comprising many [001]-oriented nanotips about 10-30 nm in diameter grown on the top of a [001]-oriented stem nanorod about 100-400 nm in diameter. These novel hierarchical mesostructures exhibit efficient suppression of reflection towards wavelengths ranging from visible to near infrared (NIR) region, with reflection <0.5% in the visible region and <2% in the NIR region, at a wide range of incident angles ranging from nearly normal to 45°. These excellent antireflection properties could be attributed to an optimized graded refractive index profile resulting from the hierarchical tips-on-rod structure of the mesocrystalline nanorods. The chemical and thermal stability of rutile TiO ₂ endows the prepared mesocrystalline TiO₂ AR structures with high stability. Moreover, the AR structures exhibit useful self-cleaning properties either under UV irradiation or through fluorosilane-modification.