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TUNGSTEN OXIDE NANOPARTICLES (WO3)

Nanoparticles research had progressed rapidly in the recent years mainly due to the unique properties of basic elements that are brought about by altering their atomic and molecular properties. By virtue of these properties, nanoparticles have found many applications in the field of biomedicine, cosmetics, electronics, coatings and plastics, etc. This article deals with the properties and applications of tungsten oxide nanoparticles.

Tungsten oxide is the chemical compound with the formula WO3. It is bronze-colored solid crystal in a monoclinic cell. The rutile-like structure features distorted octahedral WO6 centers with alternate short W–W bonds (248 pm). Each tungsten center has the d2 configuration which gives the material a high electrical conductivity.

 
Tungsten Oxide Nanoparticles

Tungsten 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 recent years, transition metal oxide structures have garnered considerable attention due to their unique properties. Among the numerous transition metal oxides, tungsten oxides have been of special interest because of their distinctive characteristics that have led to a number of applications and promise further developments. Such applications include gas and humidity sensors, optical devices, electrochromatic windows, catalysts and many more.

Numerous forms of tungsten oxide have been synthesized, with the stoichiometric formulas for all of forms being WOx, where 0 < x ? 3. Bulk tungsten oxides as well as tungsten oxide films are generally WO3, with WO2 forms also possible under some conditions. With the great interest in nanoscience and nanostructured materials over the past 15 years, researchers have developed nanostructured forms of tungsten oxide as well. These have ranged from WO3 nanocrystals on the order of only a few nanometers to one-dimensional nanorods of non-stoichiometric composition ranging from WO2.5 to WO2.9 (referred to as nanowires, nanoneedles, or nanowhiskers by some researchers).

PROPERTIES OF TUNGSTEN OXIDE

Tungsten metal can be easily oxidized in air or oxygen to form oxides. When occurring at temperature up to 327?C, this reaction forms WO3, with the thickness of the oxide layer dependent on both temperature and humidity. From 327?C to 400?C, a protective oxide layer of oxide is formed.

Tungsten oxide (WO3) nanopowder or nanoparticles are available in the form of nanofluids or faceted high surface area oxide particles exhibiting magnetism. Other forms in which these particles are available are dispersed, transparent, high purity and coated forms. Tungsten belongs to Block D, Period 6 while oxygen belongs to Block P, Period 2 of the periodic table.

Physical and Technical Properties

Properties
Chemical Symbol  WO3
Molar Mass 231.84 g/mol
Melting point 1473 °C
Boiling point 1700 °C
Density 7.16 kg/cm3
Electronic config. Tungsten [Xe] 4f14 5d4 6s2 Oxygen [He] 2s2 2p4

 

APPLICATIONS OF TUNGSTEN OXIDE

Tungsten oxide 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 electro chromic 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.

Electro chromic Devices

WO3 -based electro chromic (EC) devices which are normally seen in smart windows and EC displays, have been widely studied over the past few decades. These devices exhibit a good memory effect with low power consumption, high contrast and long-term stability. There are a few types of configurations for EC devices.

Photo catalytic Applications

A main goal in the area of photo catalysis is to find suitable materials for efficient solar hydrogen production and organic pollutant degradation. In 1969, Fujishima and Honda reported the first photo electrolysis of water using single crystal rutile-structured TiO2 under UV irradiation. Since then, TiO2 and other semiconductor materials including WO3 were intensively explored for their photo catalytic abilities.

Optical Recording Devices

Driven by the need for high-density and reversible information storage, optical recording is an advanced technology, which has seen application in everyday life. The initial exploration of WOx for this field used a combination of photo chromic and electro chromic effects to record digital images.

 

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