Tungsten(VI) oxide, also known as tungsten trioxide or tungstic anhydride, WO3, is a chemical compound containing oxygen and the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores are treated with alkalis to produce WO3. Further reaction with carbon or hydrogen gas reduces tungsten trioxide to the pure metal.
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||231.84 g/mol|
|Appearance||Canary yellow powder|
|Melting point||1,473 °C (2,683 °F; 1,746 K)|
|Boiling point||1,700 °C (3,090 °F; 1,970 K) approximation|
|Solubility||slightly soluble in HF|
|P121/n1, No. 14|
Trigonal planar (O2– )
|Occupational safety and health (OHS/OSH):|
|Safety data sheet (SDS)||External MSDS|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Tungsten(VI) oxide occurs naturally in the form of hydrates, which include minerals: tungstite WO3·H2O, meymacite WO3·2H2O and hydrotungstite (of the same composition as meymacite, however sometimes written as H2WO4). These minerals are rare to very rare secondary tungsten minerals.
In 1841, a chemist named Robert Oxland gave the first procedures for preparing tungsten trioxide and sodium tungstate. He was granted patents for his work soon after, and is considered to be the founder of systematic tungsten chemistry.
The crystal structure of tungsten trioxide is temperature dependent. It is tetragonal at temperatures above 740 °C, orthorhombic from 330 to 740 °C, monoclinic from 17 to 330 °C, triclinic from -50 to 17 °C, and monoclinic again at temperatures below -50 °C. The most common structure of WO3 is monoclinic with space group P21/n.
Tungsten trioxide is a strong oxidizing agent: it reacts with rare-earth elements, iron, copper, aluminium, manganese, zinc, chromium, molybdenum, carbon, hydrogen and silver, being reduced to pure tungsten metal. Reaction with gold and platinum reduces it to the dioxide.
Tungsten trioxide is used for many purposes in everyday life. It is frequently used in industry to manufacture tungstates for x-ray screen phosphors, for fireproofing fabrics and in gas sensors. Due to its rich yellow color, WO3 is also used as a pigment in ceramics and paints.
In recent years, tungsten trioxide has been employed in the production of electrochromic windows, or smart windows. These windows are electrically switchable glass that change light transmission properties with an applied voltage. This allows the user to tint their windows, changing the amount of heat or light passing through.
In 2013, highly photocatalytic active titania/tungsten (VI) oxide/noble metal (Au and Pt) composites toward oxalic acid were obtained by the means of selective noble metal photodeposition on the desired oxide's surface (either on TiO2 or on WO3). The composite showed a modest hydrogen production performance.
In 2016, shape controlled tungsten trioxide semiconductors were obtained by the means of hydrothermal synthesis. From these semiconductors composite systems were prepared with commercial TiO2. These composite systems showed a higher photocatalysis activity than the commercial TiO2 (Evonik Aeroxide P25) towards phenol and methyl orange degradation.
In 1999, Reich and Tsabba propose a possible nucleation of superconducting regions with Tc = 90 K on the surface of Na-doped WO3 crystals. It would be the only superconducting material containing no copper, with Tc higher than the boiling point of liquid nitrogen at normal pressure. Later, it was reported results of the search for the possible superconducting state in tungsten oxides WO3−x with various oxygen deficiency 0 < x < 1. In samples with one particular composition WO2.9, the signatures of superconductivity with the transition temperature Tc = 80 K was observed in the magnetization measurements.
Recently, some research groups have demonstrated that non-metal surface such as transition metal oxides (WO3, TiO2, Cu2O, MoO3, and ZnO etc.) could serve as a potential candidate for surface-enhanced Raman spectroscopy substrates and their performance could be comparable or even higher than those of commonly used noble-metal elements. There are two basic mechanisms for this application. One is that the Raman signal enhancement was tuned by charge transfer between the dye molecules and the substrate WO3 materials. The other is to use the electrical tuning of the defect density in the WO3 materials by the oxide leakage current control in order to modulate the enhancement factor of the SERS effect.