Solid State Fluorination on the Minute Scale: Synthesis of WO_3−xF_x with Photocatalytic Activity

Abstract

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 WO3–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 WO3–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 WO3–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 WO3–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.

A High‐Performance Asymmetric Supercapacitor Based on Tungsten Oxide Nanoplates and Highly Reduced Graphene Oxide Electrodes

“…It is an n ‐type semiconductor with variable oxidation states, high energy and packing density, and large pseudocapacitance [33] . It has been used not only in secondary batteries, [34] photocatalysis, [35] gas sensing, [36] or electrochemical [37] and solar [38] energy devices, but also as an electrode material for flexible and portable supercapacitors [39] . Much consideration has been devoted to its pseudocapacitor performance [40] .…”

A Generalized Method for High‐Speed Fluorination of Metal Oxides by Spark Plasma Sintering Yields Ta3O7F and TaO2F with High Photocatalytic Activity for Oxygen Evolution from Water

“…While HER photocatalysts containing precious metals (Pd, Pt) or earth abundant 3d transition metal oxides (e.g., TiO 2 , [ 6,7 ] Fe 2 O 3 , [ 8,9 ] CoO, [ 10 ] spinels [ 18 ] or nitrides, [ 14,17 ] and oxynitrides of the group 5 metals [ 19,20 ] have been in the scientific focus since many years, oxyfluorides of the 4d or 5d transition metals [ 21–28 ] received less attention, although they show good stability against photo‐ or chemical corrosion, are nontoxic, and have suitable bandgaps. [ 29,30 ] It has been proposed that the proton acidity in the oxide/hydroxide semiconductors Nb 3 O 7 (OH) [ 31 ] can promote the adsorption of water, CO 2 , or organic molecules.…”

“…The use of oxyfluorides like Nb 3 O 7 F [ 21–28 ] or TaO 2 F [ 22,24,26,28,33 ] for photocatalysis is hampered by synthetic and processing difficulties. [ 34 ] Their chemical synthesis requires fluorine gas, reactive fluorides, or hydrogen fluoride as toxic and corrosive sources of fluorine that make special equipment and techniques for handling indispensable.…”

“…The maximum fluorine content for TaO 2 F is comparable to that obtained in conventional reactions at 650 °C in quartz ampoules starting from Ta 2 O 5 and PTFE (ratio 1.5:1 for TaO 2 F and 1:3 for Ta 3 O 7 F). [24,25,27,28] However, fluorination in SPS reactions are much faster (9 min compared to 12 h for TaO 2 F and 36 h for Ta 3 O 7 F). The yields by SPS were 2 g after ≈9 min (compared to 0.2 g of TaO 2 F after 12 h and 0.8 g of Ta 3 O 7 F after ≈36 h by conventional high temperature chemistry).…”

Festkörperchemie

“…Mit einer Spark-Plasma-Sinteranlage synthetisierte die Gruppe um Tremel innerhalb weniger Minuten aus WO 3 und Teflon das Oxidofluorid WO 3-x F x (0 < x < 0,65). 13) Die dabei entstehenden Nanoteilchen sind photokatalytisch aktiv bei der Zersetzung organischer Moleküle.…”