Tungsten is the advantage mineral in storage of our country, however, with serious inferiors development in utilization until nowadays. As an important intermediate product in the industrial chain of tungsten, tungsten oxide is now only recognized as the precursor of metal tungsten preparation, but it definitely deserves more.

As a matter of fact, tungsten oxide is an important multifunctional material with a wide spectrum of potential optoelectronic applications, such as photo/electro-chromism, photocatalysis and trace chemical detection, due to its structural flexibility and cation intercalated states. 

Recently, Prof. ZHAO Zhigang's group from Suzhou Institute of Nano-tech and Nano-bionics (SINANO) of the Chinese Academy of Sciences, starting from the complexity of both phase structure and stoichiometry among tungsten oxide materials, they briefly reviewed the attendant optoelectronic properties distinguished from other semiconductors.

Then, based on the original work of ZHAO’s group, recent advances on optoelectronic applications of tungsten oxide materials are described, not only covering traditional chromogenic applications, but also with a focus on the latest visible-light-driven photocatalysis, surface-enhanced Raman scattering (SERS), and photothermal applications.

Meanwhile, material designs by means of structure and stoichiometry control are involved, as a strategy to obtain improved performance with tungsten oxide in its traditional applications, and then an outlook on the bright future and possible research trends of this versatile material are given at the end of the paper. 

Previous work of ZHAO’s group has mostly focused on photocatalysis, aiming at the improved efficiency of photon-generated charge carrier separation in tungsten oxide materials.

The structure of WO₃ nanotubes decorated with Pt nanoparticles were designed for excellent activity in the photocatalytic degradation of gasous aldehyde.

Other new materials, such as WO₃ octahedron and the “three-in-one” agent H₂W_1.5O_5.5·H₂O for water treatment, were fabricated with acidified surface for an enhanced affinity towards heavy metal ions and dye molecules in waste water. 

Photochromic is another featured merit of tungsten oxide materials, which shares similar mechanism with a typical photocatalysis process. Given an effective trapping of charge carriers, the color of a tungsten oxide film can be turned into blue from transparent, which is due to the photochromic phenomenon.

However, slow coloration speed, poor reversibility and UV-response only are three common problems encountered by tungsten oxide when submitted to practical photochromic applications.

To solve these problems, ZHAO’s group has achieved extended photoresponse covering near-infrared region (700 nm) together with good reversibility based on flowerlike tungsten oxide-based organic-inorganic hybrid, and also gained tailored onset wavelength and significantly shortened photochromic response with a remote photochromic system based on CdS QDs-loaded WO₃ film. 

As a specialize research direction, ZHAO’s group has made several important progresses to propel the development of smart chromophore technique, including the innovation of host materials, electrolytes and device structures in the field of electrochromism.

Quantum-sized tungsten oxide materials were firstly obtained through a simple wet-chemical method, the excellent electrochromic performance of which was a result of the efficient matter/charge transport in quasi-zero dimensional nanomaterials.

For the first time, trivalent Al³⁺ ions were adopted as intercalated cations in an electrochromic process to overcome the deficiency of commonly used monovalent species (H⁺、Li⁺、Na⁺), with greatly improved response speed, coloration efficiency and cyclic stability.

In ZHAO’s group, multi-function and combination among emerging techniques has always been heavily emphasized in the device design, several new-concept electrochromic devices can be found in recently publish papers, such as flexible film with gradient color-changing, smart supercapacitors, and fast self-charging chromic cell. 

Now, ZHAO’s group devotes their attention to exploit pretty new applications for traditional tungsten oxide materials. For example, with sea-urchin-like substoichiometric W₁₈O₄₉ nanoparticles as SERS substrate, the detection limit concentration of R6G can be as low as 10^-7 M with the maximum enhancement factor of 3.4×10⁵, in the rank of the highest sensitivity among semiconducting materials, even comparable to noble metals without ‘hot spots’.

These findings reveal that with a suitable modulation of the oxygen vacancy density in semiconductor oxides, the SERS activity of which can be dramatically promoted, which break through the limitation of noble metal substrates in common SERS applications and provide important clues in the future design strategy for highly-efficient semiconducting SERS substrates. 

The results has been published in Advanced Materials entitled with “Tungsten Oxide Materials for Optoelectronic Applications”.