Two-step hydrothermal synthesis of (NH4)xWO3 hollow spherical and hierarchical structures

Abstract

Hollow spherical and hierarchical structures of ammonium tungsten bronze ((NH₄)_xWO₃) were prepared via a two-step hydrothermal method. It was found that the (NH₄)_xWO₃ hollow spherical structure without adding citric acid, while the (NH₄)_xWO₃ hierarchical structure formed when adding citric acid at the second hydrothermal step. Both structures consist of nanorods and nanoparticles with connected frames. The possible formation mechanism of these two (NH₄)_xWO₃ structures was proposed and discussed. The current approach is simple, and can also be applied for the synthesis of other tungsten bronze materials with novel structures.