Cellulosic materials are widely used in daily life for paper products and clothing as well as for emerging applications in sustainable packaging and inexpensive medical diagnostics. Cellulose has a high density of hydroxyl groups that create strong intra-and interfiber hydrogen bonding. These abundant hydroxyl groups also make cellulose superhydrophilic. Schemes for hydrophobization and spatially selective hydrophobization of cellulosic materials can expand the application space for cellulose. Cellulose is often hydrophobized through wet chemistry surface modification methods. This work reports a new modification method using a combination of atomic layer deposition (ALD) and atmospheric heating to alter the wettability of purely cellulosic chromatography paper. We find that once the cellulosic paper is coated with a single ALD cycle (1cy-ALD) of Al 2 O 3 , it can be made sticky superhydrophobic after a 150 °C ambient post-ALD heating step. An X-ray photoelectron spectroscopy investigation reveals that the ALD-modified cellulosic surface becomes more susceptible to adsorption of adventitious carbon upon heating than an untreated cellulosic surface. This conclusion is further supported by the ability to use alternating air plasma and heat treatments to reversibly transition between the hydrophilic and hydrophobic states. We attribute the apparent abruptness of this wetting transition to a Cassie−Wenzel-like phenomenon, which is also consistent with the sticky hydrophobic wetting behavior. Using scanning probe methods, we show that the surfaces have roughness at multiple length scales. Using a Cassie−Wenzel model, we show how a small change in the surface's Young's contact angleupon adsorption of adventitious carboncan lead to an abrupt increase in hydrophobicity for surfaces with such roughnesses. Finally, we demonstrate the ability to spatially pattern the wettability on these 1cy-ALD-treated cellulosic papers via selective heating. This ALD-treated hydrophobic paper also shows promise for microliter droplet manipulation and patterned lab-on-paper devices.

Cellulosic nanomaterials can improve the performance of various products and can be renewably sourced. In this study, nanocellulosic paper (nanopapers) is chemically and physically altered with simple gas-phase processing to achieve enhanced mechanical performance. Cellulosic nanofibril paper is exposed to single cycles of trimethylaluminum (TMA) and water to modify the surface and subsurface chemistry with small quantities of aluminum oxide. Precursor exposure times are found to significantly influence the amount of inorganic deposited within the cellulosic structure and its crystallinity. This result differs from the common assumption that exposing cellulose to TMA will lead to an “atomic layer deposition (ALD)” type of process in which self-limited surface saturation is quickly achieved. These results suggest that with extended exposure times, the TMA precursor finds new pathways to chemically or physically alter the cellulosic material. Through the x-ray photoelectron spectroscopy analysis, we find that cellulose undergoes a decomposition process during the TMA exposure and/or subsequent reaction with H2O, creating at least one additional pathway to inorganic uptake. Interestingly, uniaxial tensile strength measurements reveal that longer TMA exposure times significantly increase the nanopaper's elongation at break and ultimate tensile strength, with only a modest loss in Young's modulus. While similar inorganic loading can be achieved with multiple ALD cycles, mechanical toughness exhibits significantly less change than for the increased TMA exposure times. X-ray diffraction suggests that the TMA exposures are transforming crystalline portions of the nanocellulose into amorphous structures. These amorphous regions lead to crazing, which increases the strain to break and toughness of the nanopaper.