Janus‐Type Hybrid Paper Membranes

Functional 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.