Learn tungsten oxide properties and applications quickly from Funcmater

Tungsten is a transition metal element with abundant reserve resources in the world.  Tungsten-based materials are generally characterized by high inherent density (>7g·cm-3), abundant frame structure, low price and easy availability, so they have been used in the fields of electrochemistry, photocatalysis and gas sensing.  In addition, as the cathode material of lithium ion battery, tungsten-based material has high theoretical specific capacity and little environmental pollution, so tungsten-based material has been widely concerned in recent years.

Tungsten oxide structure and properties

WO3 is a transition metal oxide and an important N-type semiconductor oxide.  WO3 crystal is composed of WO6 regular octahedron, in which W atom is located in the center of the octahedron and O atom is located at six angles. According to the tilt Angle and rotation direction of the WO6 regular octahedron, many different crystal structures are formed, such as:  Cubic phase (ReO3 type), monoclinic phase Ⅰ type (epsilon - WO3), monoclinic phase Ⅱ type (gamma - WO3), three phase (delta - WO3), are in photograph (beta WO3) and four phase (alpha WO3).  

Among them, γ-WO3 is the most stable state at room temperature.  In addition, hexagonal tungsten oxide (H-WO3) is another stable phase of WO3.  It also consists of WO6 octahedron, but has six-membered and ternary rings that share equatorial oxygen in the AB plane.  WO3 has a good application in photocatalysis, gas sensors, supercapacitors, electrochromic devices, and the research on electrode materials for lithium ion batteries is also improving day by day.

In addition, the actual existence of tungsten oxide in nature, oxygen vacancy phenomenon is widespread, tungsten oxide atomic ratio can not be in accordance with the strict stoichiometric ratio, some have five valence and four valence, so the general formula of tungsten oxide is written as WO3-X (x=0 ~ 1).  The reduction of WO3 is usually accompanied by structural changes. Non-stoichiometric tungsten oxide mainly includes W20O58, W18O49 and W24O68.

Tungsten oxide application

WO3 can be used in supercapacitors, electrochromic, lithium ion batteries, gas sensors, photocatalysis, photocatalysis and other fields.  WO3 is getting more and more attention, and some new application areas are being developed.  

1. The application of electrochromism  

Electrochromism is to apply external voltage to the color layer, electrolyte ions in the electrolyte are inserted into it, this process is a coloring process;  When the reverse voltage is applied, the electrolyte ions embedded in the color-changing layer are deembedded into the electrolyte, which is a decolorization process.  WO3 based electrochromic devices are mainly used in intelligent Windows and electrochromic displays, with advantages of low energy consumption, high contrast and high stability.  

2. Applications in the field of photocatalysis  

The field of photocatalysis mainly includes photodecomposition of water to produce hydrogen and degradation of organic matter.  To achieve the use of sunlight to decompose water to produce a large amount of hydrogen, the main is to find excellent performance of the catalyst.  Tungsten oxide is a semiconductor with a small band gap. It shows excellent photocatalytic performance when visible light is less than 500nm, and its conversion efficiency is close to 100%.  In the field of hydrolytic hydrogen production, nano-WO3 needs to form complementary energy band with other materials or reduce its energy band by doping, so as to achieve effective hydrolytic hydrogen production of WO3.  On the other hand, when organic pollutants are degraded, the resulting electron holes react on the pollutants and degrade into pollution-free substances.  

3. The application of gas sensitivity  

It refers to the reaction between gas sensitive material WO3 and oxidizing gas to change the impedance and optical characteristics of WO3 material.  The temperature of the system is the key to the gas sensitive properties of WO3. The surface electrons of WO3 can be captured by oxidizing gases (O3, NO2 and CO2, etc.), and the conductivity is inversely proportional to the adsorbed molecules.  However, when reacting with reducing gases, the electrical conductivity of the GAS sensitive layer of WO3 increases.  

4. Application of lithium ion battery  

Although WO3 materials are not as widely used in lithium ion batteries as traditional transition metal oxides, such as Co3O4 and Fe3O4, more and more attention has been paid to WO3 anode materials for lithium ion batteries, and WO3 also shows good electrochemical performance.  This is mainly due to the fact that WO3, as a cathode material, has a theoretical specific capacity of 693 mA HG-1, which is much higher than graphite and has a lower cost.  Compared with other transition metal oxides, WO3 has many advantages as a cathode material in lithium ion batteries: tungsten is the highest price state, good stability and high safety performance;  WO3 is a kind of benign semiconductor with high conductivity and dielectric constant.  WO3 has high density (7.16 Gcm-3) and higher volume than capacity.  

Yang et al., using organic acids as raw materials, successfully prepared cylindrical and biconical layered H-WO3 systems by simple hydrothermal method.  The length is about 3μm, mainly composed of a single nanowire, the diameter of which is about 20nm.  The synthesized nanomaterial was used as the anode material of lithium ion battery, and the initial specific capacitance was up to 1283.3 Mahg-1. After 200 weeks of cycling, the capacitance remained to 704.1 Mahg-1.  The electrode material has excellent electrochemical properties, mainly related to the multistage structure of the material, which can promote the electrolyte to penetrate into the material and shorten the diffusion distance of Li+.  The interconnected nanowires have good mechanical stability.  

5. The application of supercapacitors  

RuO2 is the most ideal electrode material for supercapacitors, but RuO2 metal is expensive, toxic and other factors limit its wide application, so it is necessary to develop alternative new electrode materials.  At present, tungsten oxide has the advantages of high quality and low price, many oxidation valence states, suitable crystal structure, non-toxicity and excellent electrochemical stability.  Therefore, it has been widely studied in the field of supercapacitor.