A Floatable Piezo-Photocatalytic Platform Based on Semi-Embedded ZnO Nanowire Array for High-Performance Water Decontamination

In the past few decades, water pollution has been becoming a serious global issue owing to the rapid development of the industry and widespread use of chemical and biological synthetic materials. Therefore, research on the aspect of pollutant removal for a natural water system is highly urgent and essential. Among numerous strategies, photocatalysis is one promising technology for disinfection and detoxification and has been demonstrated to be effective against various types of organic pollutants originating from industrial and agricultural wastes. This process mainly involves two reactions: oxidation and reduction. With the assistance of solar energy, photocatalysis is economical and applicable to versatile environmental conditions so that it has attracted considerable interest in recent years. For heterogeneous photocatalysis, the process is commonly realized by semiconductor photocatalysts, which can be categorized into two kinds in terms of their configurations: (1) slurry photocatalysts [ 1 , 2 , 3 ] and (2) immobilized photocatalysts. Among them, immobilized photocatalysts involve immobilized films/nanocomposites [ 4 , 5 , 6 ] and immobilized vertically grown nanowires (NWs) [ 7 , 8 , 9 ]. Slurry photocatalysts, including particles and 1D nanostructures, are the most commonly studied configurations, which are considered to possess high degradability owing to their large surface area-to-volume ratios and high homogeneity in aqueous solutions. With proper modification, excellent photocatalytic activities of these photocatalysts can be achieved, which are comparable to that of Degussa P25 [ 10 , 11 , 12 , 13 , 14 ]. However, some challenges such as easy agglomeration, complicated recovery from liquids, and increased absorption along the penetration depth of UV light hindered these photocatalysts from practical use. As an alternative route, immobilized film-like photocatalysts are preferable as practical options. This type of configuration possesses high recyclability, whereas their limited active surface area and mass transfer restraint become additional drawbacks. Recently, a photoreactor, especially microchannel-based photoreactor, has been suggested as one effective way to improve the performance of immobilized photocatalysts by increasing the surface-to-volume ratio and mass transfer and reducing the molecular diffusion distance. Such photoreactors serving as tools for the degradation of organic pollutants have demonstrated excellent reproducibility, stability, and degradability [ 15 , 16 , 17 , 18 ]. Moreover, a modification toward this design, such as employing immobilized NW photocatalysts, further improves its photodegradation rate owing to the larger active surface area in comparison with that of a film [ 19 , 20 ]. However, most of these reactors require external power to drive the polluted solution to flow across the reaction chamber, and a specific platform is usually built to bring it closer to the light source, which inevitably increase the cost for scalable water cleanliness. In this regard, a configuration possessing immobilized NW-like photocatalysts both autonomously floating and freely moving atop the water is desired to improve the utilization of sunlight and overcome the mass transfer issue.