Surface-decorated ZnO nanoparticles and ZnO nanocoating on electrospun polymeric nanofibers by atomic layer deposition for flexible photocatalytic nanofibrous membranes3

Water pollution is a growing environmental issue which severely threatens human health. Consequently, development of novel materials for water purification and waste treatment is an important research topic. Functional nanomaterials exhibiting photocatalytic properties, along with a very high surface area, have been widely investigated, since these nanostructures are quite effective in the degradation of organic contaminants under UV light and sunlight. For instance, metal oxides such as ZnO and TiO2 5 are very well known for their photocatalytic activity and therefore, these materials in the form of nanoparticles, nanorods and nanofibers have been studied intensely for water purification purposes. However, the brittle nature of these metal oxides often causes problems when they are used as a membrane. Therefore, designing photocatalytic membranes with flexible character is highly desired. For instance, flexible membranes composed of electrospun polymeric nanofibers functionalized with ZnO nanorods, formed by using a hydrothermal method have been shown to be effective for the photocatalytic degradation of organic molecules, so they have the potential for water purification and organic waste treatment. However, novel materials are always needed for the development of advanced filtering systems for water treatment. In this work, polymeric nanofibers surface-decorated with ZnO nanoparticles (NPs) and a ZnO nanocoating were fabricated using a two-step approach: electrospinning and atomic layer deposition (ALD) (Fig. 1 and Fig. S1, ESI3). Initially, polymeric (nylon 6,6) nanofibers with an average fiber diameter of y80 nm were produced via electrospinning technique. In the next step, ZnO was grown onto smooth surfaces of polymeric nanofibers via ALD by altering the deposition parameters. Altering the ALD parameters resulted in various ZnO morphologies on the polymeric nanofibers; surface-decorated ZnO NPs and highly dense ZnO NPs, and a continuous ZnO nanocoating with a uniform thickness (y27 nm). The resulting ZnO nanostructures onto nanofibers are illustrated schematically in Fig. 1. The scanning electron microscopy (SEM) images clearly elucidated that the ALD process did not destroy the fibrous structure of the polymer. In addition, when compared to the pristine polymeric nanofiber, nylon–ZnO nanofibers had rougher surfaces due to the deposition of ZnO NPs and the nanocoating (Fig. 2 and Fig. S2, ESI3). Overall, the sample decorated with highly dense ZnO NPs exhibited the highest surface roughness due to the presence of a greater number of individual ZnO NPs (Fig. 2c and 3b). The transmission electron microscopy (TEM) images clearly revealed the morphologies of ZnO deposited on polymeric nanofibers (Fig. 3 and Fig. S3, ESI3). For the sample shown in