Superhydrophobic surfaces, such as lotus leaves , exhibit extraordinary water repellent and self-cleaning properties. Recently, superhydrophobic surfaces were combined with photocatalytic materials to produce multifunctional surfaces. Reports on the preparation of superhydrophobic photocatalytic surfaces are limited and the techniques are problematic. Either they were created under extreme experimental conditions or the catalyst particles were embedded in the polymer matrix with reduced surface contact area, leading to lower photocatalytic activity. Scaling these techniques to produce large areas would be difficult. In addition, some films were not robust and lost their superhydrophobicity upon light irradiation.
Here we present a novel method for preparing superhydrophobic surfaces composed of photocatalytic particles. No chemical modification of the particle surfaces was required to achieve superhydrophobicity. Superhydrophobic surfaces were fabricated by printing polydimethylsiloxane (PDMS) into arrays of cylindrical cones that measure 400 µm base diameter, 25 µm tip diameter and 1000 µm tall on 500 µm pitch. Catalyst particles of silicon phthalocyanine dispersed in a glass matrix, which we have shown to generate singlet oxygen (1O2) under 669nm light irradiation, were adhered onto the printed PDMS posts. Superhydrophobicity can be maintained, even with hydrophilic catalytic particles, due to this significant hierarchical structure. The surface has a water contact angle of 160° and maintains superhydrophobicity in contact with water under visible irradiation from a diode laser.
Another important feature of the high aspect ratio primary roughness is that it enables easy access to the plastron, i.e. the stable layer of air under the solid-liquid interface. Printing the PDMS posts on a porous membrane, and supporting the membrane over a plenum, provides a means to control the gas composition of the plastron and thus study catalysis at the solid-liquid-gas interface. The generation of 1O2 in the plastron region and the trapping of this short-lived reactive species after it travels across gas-liquid interface and is solvated in the supported solution were demonstrated. Because 1O2 generates no waste or byproducts, the fabricated superhydrophobic and photocatalytic composite surface can be used in water purification and disinfection, such as oxidizing toxic organic molecules and deactivating bacteria, as well as the synthesis important intermediates. Quantification of 1O2 as a function of plastron gas composition and flow rate was achieved using 9, 10-anthracene dipropionic acid as the trapping agent.
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