Chun Haow Kung1 Beniamin Zahiri1 Pradeep Sow2 Walter Mérida1

1, University of British Columbia, Vancouver, British Columbia, Canada
2, BITS Pilani K.K. Birla Goa Campus, Goa, , India

The development of a smart surface with reversible wettability between water-attracting (hydrophilic) and water-repelling (hydrophobic) state in response to external stimuli is presented. Stimuli-responsive surfaces are of great interest due to their versatility of applications, including microfluidics, water harvesting, wastewater treatment and oil-water separation. Here, we present an electrochemical approach for fast and reversible wettability control on core-shell Cu-CuOx dendritic structure by manipulating the oxidation state of the CuOx shell phase. The wetting switching from superhydrophobic (contact angle > 150°) to superhydrophilic (contact angle < 10°) regime could be accomplished within a few seconds to a few minutes by applying a low voltage (< 1.5 V). The modulation of the magnitude and duration of the applied voltage controls precisely the rate and extent of the transition. The initial superhydrophobicity is fully regained by air drying at room temperature for 1 hour or mild heat drying at 100°C for 30 min. Microstructural analysis based on the scanning transmission electron microscopic high-angle annular dark-field imaging (STEM-HAADF), energy-dispersive X-ray spectroscopy (EDS) mapping and X-ray photoelectron spectroscopy (XPS) revealed the presence of CuOx surface film shielding the Cu core. Electrochemical analysis showed that the in-situ wetting transition is based on the Faradaic phase transformation of the surface bound CuOx groups. During electroreduction of the outermost oxide layer, the surface transitioned from low adhesive rolling state (lotus effect), to high adhesive pinning state (petal effect), and eventually to superwetting state with superior water-absorbing ability (fish scale wetting). Gravity-driven separation of oil-water mixtures demonstrated the functionality of the in-situ wettability switching of the Cu-CuOx core-shell nanostructures. We showed that the as-deposited Cu mesh exhibiting superhydrophobicity and superoleophilicity is effective for heavy oil-water separation. On the other hand, application of a small reduction voltage (< 1.5 V) remediated light-oil contaminated water. The voltage application drives the electrochemical reduction of the CuOx shell phase, converting the Cu mesh into superhydrophilic/underwater superoleophobic state. Depending on the specific treatment needs, the mesh can be switched between oil-removal and water-removal mode for water purification contaminated with organic solvents of different densities. The separation efficiencies for a series of oil-water mixtures are above 98% for 30 separation cycles, illustrating good recyclability of the mesh for long-term operation at industrial scale. The findings open a new avenue for exploration of various metal oxide materials for redox reaction-mediated wetting and adhesion tuning in areas such as selective droplet transportation, heat transfer manipulation, and controllable drug delivery.