Tungsten 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. Tungsten oxide occurs naturally in the form of hydrates, which include minerals: tungstite WO3·H2O, meymacite WO3·2H2O and hydrotungstite. These minerals are rare to very rare secondary tungsten minerals.
History
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.
Preparation
Tungsten trioxide can be prepared in several different ways. CaWO4, or scheelite, is allowed to react with HCl to produce tungstic acid, which decomposes to WO3 and water at high temperatures. Another common way to synthesize WO3 is by calcination of ammonium paratungstate under oxidizing conditions:
Structure and properties
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.
Uses
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. 2010- AIST reports a quantum yield of 19% in photocatalytic water splitting with a caesium-enhanced tungsten oxide photocatalyst. In 2013, highly photocatalytic active titania/tungsten oxide/noble metal composites toward oxalic acid were obtained by the means of selective noble metal photodeposition on the desired oxide's surface. 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 photocatalysisactivity than the commercial TiO2 towards phenol and methyl orange degradation. Recently, some research groups have demonstrated that non-metal surface such as transition metal oxides could serve as a potential candidate for SERS enhancement and their performance could be comparable or even higher than those of 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.
