Effect of isothermal holding time on hydrogen-induced structural transitions of WO3

Abstract

Tungsten trioxide (WO₃) has the ability to transform oxygen-deficient structures (WO_3−x; 0 ≦ x ≦ 1) at high temperatures under hydrogen. Because the band gap of WO_3−x depends on the amount of W⁵⁺ species resulting from oxygen vacancies, this material is expected to have unique applications. Herein, to elucidate the WO₃ reduction mechanism, we investigated the crystallographic changes of monoclinic WO₃ powder samples using X-ray and neutron diffraction measurements under different reduction conditions, namely, under hydrogen at 500 or 800 °C for isothermal holding times of 30 min or 22 h. During heating, the yellow color of WO₃ changed to various other colors, suggesting that WO₃ underwent different reactions with hydrogen depending on the temperature and isothermal holding time. The X-ray powder diffraction results indicated that the hydrogen-treated WO₃ crystals formed various oxygen-deficient structures, including stoichiometric WO_3−x, non-stoichiometric WO_3−x, and W metal. However, the formation of a single WO_3−x phase was extremely difficult. For the blue WO₃ sample obtained at short isothermal holding times, the total scattering analysis suggested that the oxygen vacancies in WO₃ gradually formed at local positions. Furthermore, the neutron powder diffraction measurements revealed that the reduction of WO₃ under hydrogen occurred on the surface. These results obtained by diffraction measurements enhance the knowledge in the chemical and physical properties of WO_3−x.