WO3 - Tungsten (VI) Trioxide

Tungsten trioxide has a number of polymorphs but investigations into photocatalytic properties focus upon the semiconducting monoclinic form which is stable at room temperature.


Figure 1: Crystal structure of monoclinic WO3

In the bulk, pure tungsten oxide has an indirect band gap of 2.6 eV. As a metal oxide it shows excellent chemical stability and resilience to photocorrosion effects. It has been used successfully as a photocatalyst for both water splitting and waste-water treatment reactions.

Electrochromism

Tungsten oxide exhibits the interesting property of electrochromism - a permanent, reversible colour change upon the application of electrical fields. When a cation such as H+ or Li+ is incorporated into the WO3 lattice, for example via electrochemical injection, a tungsten bronze, MxWO3, is formed. To maintain charge neutrality W5+ cations are created in the structure, in addition to the W6+ cations found in pure WO3. The W5+ colour centres are able to take part in optical absorption processes giving the compound a dark blue colour, in contrast to the pale yellow of pure WO3.

This effect is used in gas sensing applications and as the basis for electrchromic window technologies (smart windows).

Nanoparticles and Sol Gel Synthesis

Thin films of tungsten oxide oxide nanoparticles may be deposited using one of four sol-gel processing methods.

  • Anhydrous. Based upon the dissolution of WCl6 in ethanol the very low concentration of water molecules in films prepared by this method makes them particularly suitable for electrochromic applications.
  • Hydrogen Peroxide. Tungsten metal or oxide dissolved in hydrogen peroxide forms a peroxotungstic acid that may be dried in air to form tungsten oxide. Typically, films grown via this method exhibit low crystallinity, large grain sizes and a band gap of 3 eV rendering it unsuitable for photocatalyst processing.
  • Acidification. Aqueous Na2WO4 is acidified with HClaq or another strong acid to form tungstic acid. This method introduces non-volatile impurities that prevent the crystallisation of pure WO3 during subsequent heat treatments.
  • Ion Exchange. A proton exchange resin is used to prepare pure, concentrated aqueous tungstic acid from which may be polymerised through careful control over processing conditions and the use of organic additives. The absence of non-volatile impurities (such as Na+) allows high purity films to be prepared.