Janus‐Type Hybrid Paper Membranes

Abstract

AbstractFunctional paper‐based materials and devices have been increasingly attractive to scientists in the recent past. In particular, the possibility to functionalize the surface of paper fibers with tailor‐made coatings has broadened a possible scope of emerging application considerably. This work introduces novel functional paper membranes with adjustable gradient and Janus‐type wettability based on gradient and Janus‐type silica coating distribution along the paper cross‐section. Correlation of CLSM (distribution), thermogravimetric analysis (silica amount), and Kr‐BET (surface area; BET: Brunauer–Emmett–Teller) reveals an extremely low coating thickness, in the range of just a few nanometers, being sufficient to fully inverse paper wettability from hydrophilic to very hydrophobic excluding water. This asymmetric wettability, originating from an asymmetric silica distribution along the paper cross‐section, is established by synchronizing silane hydrolysis and condensation reaction rates with silane transport rates in paper within a simple and scalable one‐step drying process after having immersed a paper sheet into a tetraethoxysilane‐containing precursor solution. As silica by itself, like paper, is a hydrophilic material, the observed hydrophobicity is related to a reduction in cellulose fiber nanoscale porosity controlling water imbibition. While being relevant in manifold applications, these ultrathin, Janus‐type hybrid paper membranes are demonstrated to show directed gating and selective oil–water separation.