Formation of WO3 nanotube-based bundles directed by NaHSO4 and its application in water treatment

Self-assembly can offer a very powerful tool for the design of novel materials and many different templates have been found to direct the formation of microstructures. In this article, we report a simple method for the self-assembly of three-dimensional (3D) WO3 nanotube bundles. It is demonstrated that not only can NaHSO4 act as a reactant, but also, more importantly, it played multiple key roles in the self-assembly processes, while NH4HSO4 and KHSO4 have none of these functions at all. As suggested, at first, WO3 ordered and layered structures can be generated by the hydrothermal reaction of NaHSO4 with Na2WO4 at 180 degrees C, and then sodium ions (Na+) inserted into the layer cause a continual curl of the WO3 outer-slice by the repel force of static electricity between Na+ ions and H+ ions on the surface of the WO3 slice. Herein, Na+ ions can dramatically promote the formation of WO3 single crystal slices, which are precursors of the self-assembly, and SO42- ions can bridge the WO3 slices as well as the nanotubes. In addition, it is found that the WO3 nanotube bundles still keep their original aggregation after template removal, and the bundle can be disassembled gradually under a long treatment time of aqueous ultrasonication. Furthermore, the application in wastewater treatment of WO3 nanotube bundles has been investigated.