Chemical compound

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.[1] Tungsten ores are treated with alkalis to produce WO3. Further reaction with carbon or hydrogen gas reduces tungsten trioxide to the pure metal.[citation needed]

2 WO3 + 3 C → 2 W + 3 CO2 (high temperature)
WO3 + 3 H2 → W + 3 H2O (550 - 850 °C)

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.

History

In 1841, a chemist named Robert Oxland gave the first procedures for preparing tungsten trioxide and sodium tungstate.[2] He was granted patents for his work soon after, and is considered to be the founder of systematic tungsten chemistry.[2]

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.[1]

CaWO4 + 2 HCl → CaCl2 + H2WO4
H2WO4H2O + WO3

Another common way to synthesize WO3 is by calcination of ammonium paratungstate (APT) under oxidizing conditions:[2]

(NH4)10[H2W12O42] · 4 H2O → 12 WO3 + 10 NH3 + 10 H2O

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.[3] The most common structure of WO3 is monoclinic with space group P21/n.[2]

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.[citation needed]

WO3 + 2 Fe → W + Fe2O3
2WO3 + Pt → 2 WO2 + PtO2

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[4] and in gas sensors.[5] Due to its rich yellow color, WO3 is also used as a pigment in ceramics and paints.[1]

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.[6][7] 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.[8]

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.[9]

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.[10][11]

In 1999, Reich and Tsabba propose a possible nucleation of superconducting regions with Tc = 90 K on the surface of Na-doped WO3 crystals.[12] 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.[13]

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.[14][15] 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.[16] 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.[17]

References

  1. ^ a b c Patnaik, Pradyot (2003). Handbook of Inorganic Chemical Compounds. McGraw-Hill. ISBN 978-0-07-049439-8. Retrieved 2009-06-06.
  2. ^ a b c d Lassner, Erik and Wolf-Dieter Schubert (1999). Tungsten: Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds. New York: Kluwer Academic. ISBN 978-0-306-45053-2.
  3. ^ H. A. Wriedt: The O-W (oxygen-tungsten) system. In: Bulletin of Alloy Phase Diagrams. 10, 1989, S. 368, doi:10.1007/BF02877593.
  4. ^ "Tungsten trioxide." The Merck Index Vol 14, 2006.
  5. ^ David E Williams et al, "Modelling the response of a tungsten oxide semiconductor as a gas sensor for the measurement of ozone", Meas. Sci. Technol. 13 923, doi:10.1088/0957-0233/13/6/314
  6. ^ Lee, W. J.; Fang, Y. K.; Ho, Jyh-Jier; Hsieh, W. T.; Ting, S. F.; Huang, Daoyang; Ho, Fang C. (2000). "Effects of surface porosity on tungsten trioxide(WO3) films' electrochromic performance". Journal of Electronic Materials. 29 (2): 183–187. Bibcode:2000JEMat..29..183L. doi:10.1007/s11664-000-0139-8. S2CID 98302697.
  7. ^ K.J. Patel et al., All-Solid-Thin Film Electrochromic Devices Consisting of Layers ITO / NiO / ZrO2 / WO3 / ITO, J. Nano-Electron. Phys. 5 No 2, 02023 (2013)
  8. ^ Development of a high-performance photocatalyst that is surface-treated with cesium Archived 2010-05-20 at the Wayback Machine
  9. ^ Karácsonyi, É.; Baia, L.; Dombi, A.; Danciu, V.; Mogyorósi, K.; Pop, L.C.; Kovács, G.; Coşoveanu, V.; Vulpoi, A.; Simon, S.; Pap, Zs. (2013). "The photocatalytic activity of TiO2/WO3/noble metal (Au or Pt) nanoarchitectures obtained by selective photodeposition". Catalysis Today. 208: 19–27. doi:10.1016/j.cattod.2012.09.038.
  10. ^ Székely, I., et al. Synthesis of shape-tailored WO3 micro-/nanocrystals and the photocatalytic activity of WO3/TiO2 composites (2016) Materials, 9 (4).
  11. ^ Baia, L., et al. Preparation of TiO2/WO3 composite photocatalysts by the adjustment of the semiconductors' surface charge (2016) Materials Science in Semiconductor Processing, 42, pp. 66-71
  12. ^ Reich, S.; Tsabba, Y. (1999). "Possible nucleation of a 2D superconducting phase on WO single crystals surface doped with Na". The European Physical Journal B. 9: 1–4. doi:10.1007/s100510050735. S2CID 121476634.
  13. ^ https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A27208/datastream/PDF2/Shengelaya-2020-Signatures_of_filamentary_superconductivity_up-%28accepted_version%29.pdf[bare URL PDF]
  14. ^ G. Ou (2018). "Tuning Defects in Oxides at Room Temperature by Lithium Reduction". Nature Communications. 9 (1302): 1302. Bibcode:2018NatCo...9.1302O. doi:10.1038/s41467-018-03765-0. PMC 5882908. PMID 29615620.
  15. ^ S. Hurst (2011). "Utilizing Chemical Raman Enhancement: A Route for Metal Oxide Support Based Biodetection". The Journal of Physical Chemistry C. 115 (3): 620–630. doi:10.1021/jp1096162.
  16. ^ W. Liu (2018). "Improved Surface-Enhanced Raman Spectroscopy Sensitivity on Metallic Tungsten Oxide by the Synergistic Effect of Surface Plasmon Resonance Coupling and Charge Transfer". The Journal of Physical Chemistry Letters. 9 (14): 4096–4100. doi:10.1021/acs.jpclett.8b01624. PMID 29979872.
  17. ^ C. Zhou (2019). "Electrical tuning of the SERS enhancement by precise defect density control" (PDF). ACS Applied Materials & Interfaces. 11 (37): 34091–34099. doi:10.1021/acsami.9b10856. PMID 31433618. S2CID 201278374.

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