Hierarchically porous tungsten oxidenanotubes with crystalline walls made of the metastable orthorhombic polymorph

Abstract

A simple route towards highly crystalline and hierarchically porous tungsten oxide nanotubes is presented. Interestingly, due to the limited wall thickness, the metastable orthorhombic high-temperature polymorph could be stabilized for the first time at room temperature without doping. In brief, ammonium metatungstate (AMT) is grafted over core-crosslinked poly(butadiene)-block-poly(2-vinylpyridine) (PB-b-P2VP) nanorods at pH 3. Coulomb interactions between protonated PB-b-P2VP nanorods (pK_a 4.5) and metatungstate anions promote hybrid formation. Subsequent calcination of the PB-b-P2VP–AMT hybrid material at 490 °C results in a non-woven structure of hierarchically porous and crystalline, orthorhombic tungsten oxide nanotubes as confirmed by electron microscopy, X-ray diffraction, and nitrogen physisorption.