Ultralow-Density, Transparent, Superamphiphobic Boehmite Nanofiber Aerogels and Their Alumina Derivatives

L ow-density materials have been attracting interest from researchers.Since the invention of aerogels in 1931, 1 many researchers have produced aerogel materials with various chemical compositions. 2,3The most common one is silica aerogels, which show unique properties such as low thermal conductivity, high surface area, visible-light transparency, and low refractive index. 4,5One of the most important techniques for designing aerogels is the precise controls over pore properties including morphology and porosity, which leads to improved performance in, for example, thermal insulation and catalytic ability.In recent years, the number of reports on ultralow-density (<5 mg cm -3 ) monolithic materials has been growing. [6][7]][8][9][10][11][12][13] They are expected to expand the frontiers of science and technology, such as innovation for designing nanostructured materials and extended applications to electrodes and molecular collectors.Moreover, provided such materials show transparency, they extend the potential applications as photocatalysts, 14 optical sensors, 15 and detectors for elementary particle physics, in which special optical properties such as ultralow-reflective index are required.Since low-density porous materials reported so far consist of a cell structure with structural inhomogeneity in the range of visiblelight wavelengths or longer, solid skeletons seriously contribute to visible-light scattering and transparency is lost. 16To achieve transparent porous materials, the average and local sizes of the scattering objects (solid skeletons) must be controlled within several tens of nanometers.Among ultralow-density materials, there is only one reported to be transparent (light transmittance through a 10 mm-thick sample is, for example, above 70% at λ = 550 nm), as far as we know. 6However, these materials require techniques more laborious than that for other silica aerogels.In this paper, we report ultralow-density aerogels that have high visible-light transparency, obtained via a simple process.To achieve such materials, we used aqueous dispersion of boehmite nanofibers (BNF sol, consisting of boehmite and amorphous aluminum hydroxide, Figure 1), 17 which is commercially available.Aggregation of the boehmite nanofibers is induced by a pH swing from acidic to weakly basic; due to the decrease of electrostatic repulsion with rising pH, gelation is observed above pH ∼ 6. 18 Indeed, the BNF sol turns into gel immediately after adding a basic aqueous solution such as ammonia (Movie S1 in the Supporting Information); however, a too rapid increase in the local pH causes inhomogeneous aggregation, resulting in less transparent aerogels.To avoid this