Solid State Fluorination on the Minute Scale: Synthesis of WO_{3− x}F_x with Photocatalytic Activity

Solid state reactions are notoriously slow, because the rate-limiting step is diffusion of atoms or ions through reactant, intermediate, and product crystalline phases. This requires days or even weeks of high temperature treatment, consuming large amounts of energy. Metal oxides are particularly difficult to react, because they have high melting points. The study reports a high-speed solid state fluorination of WO₃ with Teflon to the oxyfluorides WO_3–xFx on a minute (<10 min) scale by spark plasma sintering, a technique that is used typically for a high-speed consolidation of powders. Automated electron diffraction analysis reveals an orthorhombic ReO₃-type structure of WO_3–xFx with F atom disorder as demonstrated by ¹⁹F magic angle spinning nuclear magnetic resonance spectroscopy. The potential of this new approach is demonstrated by the following results. i) Mixed- valent tungsten oxide fluorides WO_3–xFx with high F content (0 < x < 0.65) are obtained as metastable products in copious amounts within minutes. ii) The spark plasma sintering technique yields WO_3–xFx nanoparticles with high photocatalytic activity, whereas the corresponding bulk phases obtained by conventional solid-state (ampoule) reactions have no photocatalytic activity. iii) The catalytic activity is caused by the microstructure originating from the processing by spark plasma sintering.