T. Tao, J. Cho
Keywords: TiO2, rutile, anatase, ZnO, hydrothermal, hierarchical organization, hybrid, nanorod
Summary:Ceramics coatings have received great attention for use as a functional surface, as well as a protective and barrier coating. We report a low-temperature hydrothermal processing of nanostructured ceramic coatings that enable their surface to be very effective in decomposing organic dyes and toxic species and also inactivating harmful bacteria. In particular, vertically aligned nanorod arrays have been designed, whose surfaces are tailored to have the maximum surface reactivity for the toxic agents or bacteria. Such nanostructured films are grown from in-situ precipitated nanoparticles when precursor solution gets above a critical saturation point during a hydrothermal processing. Depending on its pH, temperature, and solution chemistry, these primary nanoparticles are evolved into diverse shapes of secondary nanostructures (e.g., nanorods, nanoblades, nanoclusters) via hierarchical organization. The goal of this study is to tailor these nanostructures for enhanced photoresponses of ceramic surfaces that can also benefit antibacterial activities. Recent progresses made at fabricating nanostructured coatings of TiO2 (rutile, anatase) and ZnO are presented here. In particular, these coatings based upon TiO2 (anatase) nanoclusters and ZnO nanorods exhibited very high photocatalytic deactivation of methylene blue (MB) dyes. Strong bleaching of the MB dye is seen after 2 hours of UV irradiation, where the degradation rate depends on specific microstructures generated from those secondary nanostructures. Photocatalytic activity gives a good indication for the antimicrobial activity required for prevention of biofilm formation. The aforementioned TiO2 (anatase) and ZnO surfaces indeed displayed promising antibacterial behavior under UV light, but also under dark environment. For this testing, the surfaces of ZnO- and TiO2-coated coupons were contaminated with standardized inoculum of Gram-negative (Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus) bacteria. ZnO-coated samples showed a strong reduction of bacterial contamination even in darkness with almost 4-log reduction after 3 hours. Under UV light, the bacterial killing activity was much accelerated within 15 minutes. Antimicrobial test of ZnO coating was repeated with S. aureus (Gram positive) bacteria. After 2 hours on ZnO-coated surfaces, almost 4-log reduction was observed in darkness, as well as under UV within 10 minutes. TiO2 (anatase) coating also showed similar antibacterial activities under both dark and UV environment. Dark performance suggests that the shape and surface of the nanostructures play an important role in antibacterial activities as they may inhibit nutrient uptake when in contact with certain bacteria. Importantly, all of the above results were reproduced with the coatings made on various substrates including oxides, polycarbonates, Teflon, Ti, and stainless steels. It indicates the applicability of a current processing protocol into numerous applications due to its low enough processing temperatures that will not damage the substrates. Especially, solution processing has shown versatility to generate various nanostructures as well. Further structural developments are, therefore, possible by building a core-shell nanostructure made of rutile and anatase (or ZnO). Such hybrid structure becomes smarter so it can work under visible light, room light, UV light, or even dark environment, for water and chemical effluent purification, organic contamination removal and biomedical applications.