The Colorful World of Tungsten Oxide: Classification and Characteristics Revealed

October 15, 2024

In the vast universe of chemistry, tungsten oxide (WO3-x) has become a striking existence with its unique colors and diverse properties. As a compound formed by the combination of tungsten and oxygen, tungsten oxide not only has rich colors, but also shows a wide range of application potential and unique physical and chemical properties. Today, let us walk into the colorful world of tungsten oxide and uncover the mystery of its classification and characteristics.

1. Classification of tungsten oxide

According to the different oxygen contents, tungsten oxide can be subdivided into many types, mainly including yellow tungsten oxide, orange tungsten oxide, blue tungsten oxide, purple tungsten oxide, white tungsten oxide and brown tungsten oxide.

(1) Tungsten trioxide

Tungsten trioxide (WO3), also known as yellow tungsten, is the most stable form of tungsten oxide. Its appearance is yellow to yellow-green powder, with a melting point as high as 1470-1475°C, a boiling point exceeding 1750°C, a density of approximately 7.16g/cm³, and is insoluble in water. Yellow tungsten sublimates significantly at 850°C and hardly reacts with inorganic acids except hydrofluoric acid. It can be slowly dissolved in ammonia or concentrated alkali solution; it reacts with chlorine to form oxychloride, but cannot react with bromine and iodine.

Yellow tungsten is widely used in the fields of photocatalysis, catalysis, gas sensors and battery materials due to its unique properties. For example, yellow tungsten can be used as the main component of photocatalysts for inorganic and organic synthesis, photolysis of water for hydrogen production and oxygen evolution, degradation of organic pollutants, etc. At the same time, yellow tungsten is also used to produce tungsten powder and tungsten carbide powder, which are then used to manufacture cemented carbide products, pure tungsten products, tungsten bars, tungsten wires, tungsten electrodes, etc.

(2) Orange tungsten oxide

Orange tungsten oxide, as a unique member of the tungsten chemical family, is significantly different from traditional yellow or yellow-green tungsten oxide (both follow the chemical formula of WO3). The display of this special color is likely due to subtle defects in its crystal microstructure, trace amounts of impurity doping, or the production of lower valence tungsten ions due to the partial reduction process. These factors work together to give it Unique orange hue.

Similar to other tungsten oxides, orange tungsten oxide also has high chemical activity and certain reducing properties. In addition, it may also exhibit specific electrical, optical or catalytic properties, which make it potentially useful in energy conversion, environmental protection, sensing devices and other fields.

(3) Blue tungsten oxide

Blue tungsten oxide (blue tungsten) is a general term for a type of mixed tungsten oxide in industry, including WO2.90, W20O58 and (NH4)х·WO3 (ammonium tungsten bronze). Among them, WO2.90 is the most common type of blue tungsten oxide, with a tungsten content of 80.00% to 80.80%, a density of 2.4 to 2.8 g/cm³, and a particle size of 12 to 19 μm. The color of blue tungsten oxide is affected by light, and is light blue under light and dark blue in the dark. It is mainly produced by processes such as thermal decomposition or wet reduction, and has excellent thermochemical stability, catalytic properties, adsorption, gas sensitivity, etc. It can be used to make metal tungsten powder, cemented carbide, gas-sensitive component materials, energy storage electrode materials, etc.

(4) Purple tungsten oxide

Purple tungsten oxide (W18O49 or WO2.72, purple tungsten) is a purple finely divided crystalline powder with a color ranging from dark purple to pure color. Its particle size is closely related to the raw material of ammonium paratungstate. Purple tungsten oxide has a unique crystal structure, such as loose particles composed of needle-shaped or rod-shaped grains. It not only has high chemical activity and high reducibility, but also exhibits good electrochromic properties and can undergo reversible color changes under the action of an electric field. This property makes it have broad application prospects in the field of electrochromic devices, such as automotive rearview mirrors, high-end architectural glass, etc. Purple tungsten can also be used to manufacture ultrafine tungsten powder and nano tungsten powder. Its unique crystal structure is conducive to the preparation of ultrafine tungsten powder by hydrogen reduction.

(5) White tungsten oxide

White tungsten oxide is a white amorphous powder with high thermal and chemical stability. It can remain unchanged for a long time in the air. It is a semiconductor material with certain photoelectric properties. White tungsten oxide is widely used in catalysis, electronics and ceramics. It can be used as a catalyst in various catalytic reactions. In electronic materials, it can be used to prepare metal oxide semiconductor materials and high-temperature thermoelectric materials.

(6) Tungsten dioxide

Tungsten dioxide (brown tungsten, WO2) is a brown powder of tungsten oxide with a rutile structure (monoclinic system). It is insoluble in water, but may be soluble in certain strong acid or alkaline solutions. It has a high specific gravity and good conductivity, reducibility and oxidizability. It is mainly used to make tungsten powder and tungsten trioxide. However, WO2 is easily oxidized to WO3 in the air, and under certain conditions, it will also react with other substances to produce tungsten oxides of different valence states.

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