Spontaneous formation of a tungsten trioxide sphere-in-shell superstructure by chemically induced self-transformation

A process of sequential self-transformation used to prepare in a single step high yields of crystalline tungsten trioxide hollow microspheres with amorphous internal cores that are detached from the enclosing crystalline shell was investigated. Electroc-diffraction analysis of shell fragments from individual hollow microspheres showed polycrystalline powder patterns. The apparent porous structure of the shell wall observed by scanning electron microscopy (SEM) was confirmed by Brunauer-Emmet-Teller (BET) analysis. The time dependence of the sphere-in-shell formation mechanism was elucidated by TEM and X-ray diffraction (XRD) analysis of structures isolated from the reaction mixture maintained at 80°C for various aging times. It was observed that the tungsten trioxide hollow microspheres showed a continuous absorption band below 470 nm.