Room Temperature Polymorphism in WO₃ Produced by Resistive Heating of W Wires

Polymorphous WO₃ micro- and nanostructures have been synthesized by the controlled Joule heating of tungsten wires under ambient conditions in a few seconds. The growth on the wire surface is assisted by the electromigration process and it is further enhanced by the application of an external electric field through a pair of biased parallel copper plates. In this case, a high amount of WO₃ material is also deposited on the copper electrodes, consisting of a few cm2 area. The temperature measurements of the W wire agrees with the values calculated by a finite element model, which has allowed us to establish the threshold density current to trigger the WO₃ growth. The structural characterization of the produced microstructures accounts for the γ-WO₃ (monoclinic I), which is the common stable phase at room temperature, along with low temperature phases, known as δ-WO₃ (triclinic) on structures formed on the wire surface and ϵ-WO₃ (monoclinic II) on material deposited on external electrodes. These phases allow for a high oxygen vacancies concentration, which is interesting in photocatalysis and sensing applications. The results could help to design experiments to produce oxide nanomaterials from other metal wires by this resistive heating method with scaling-up potential.