Solution Processing of Topochemically Converted Layered WO3 for Multifunctional Applications

Transition metal oxides (TMOs) attracts huge interest because of their tuneable optoelectronic properties, high carrier mobility, stability against harsh environments, etc. Among these, tungsten oxide (WO3) is one of the most interesting metal oxide, which has been studied over the past few decades in several applications like sensors, electrochromic devices, supercapacitors, optoelectronic devices, water splitting, etc. WO3 has been utilized in several nanomaterials form like nanoparticles, nanorods, nanoflowers, nanoplates, nanosheets, etc. to enhance the performance in several applications. WO3 nanosheets which are having 2D morphology got special attention and are utilized in various applications.

Usually, nanosheets of any material are synthesized from their bulk counterpart using different types of exfoliation mechanisms, either through a top-down approach or a bottom-up approach. One of the common top-down mechanisms is liquid-phase exfoliation (LPE). However, the problem of WO3 lies in the non-layered structure. To have it layered, the WO3 structure must contain water molecules (WO3. nH2O). That is the reason there is no report in the literature on anhydrous layered WO3. Therefore no report on LPE of this kind of WO3 to obtain nanosheets.

However, recently we crossed this barrier by synthesizing layered WO3 using topochemical conversion route from commercially available tungsten disulfide (WS2), and importantly, converted WO3 is shown to have anhydrous behavior. Recently we have reported this discovery in Chemistry-A European Journal published by Wiley (https://doi.org/10.1002/chem.202100751).

WS2 is heated at 500C for 3 h in ambient conditions in a furnace and as a result, WS2 gets converted to WO3 retaining its layered morphology. This process is commonly known as topochemical conversion. For the general reader, the definition of this process can be read as:

“Here, for topochemical synthesis, it can be broadly defined as adding, extracting or substituting elements to or from precursors in an environment of liquid or gas for synthesis of new materials which retain the structure or, at least, morphology of precursors.”

Xiao et al, Chem. Soc. Rev., 2018, 47, 8744–8765

Further, converted bulk layer WO3 is subjected to LPE to obtain WO3 nanosheets. Before that, the solvent selection is the most important step to achieve successful exfoliation. Because all solvents are not equally efficient to exfoliate and stabilize the material in the dispersion. The surface tension and Hansen solubility parameter (HSP) of solvent should be close to that of solute in order to have successful exfoliation. Therefore twenty-two solvents have been tried and among them, isopropyl alcohol-water (1:1) co-solvent system is found to be the best.

In this paper, detailed studies on LPE of converted WO3 have been carried out. Finally, obtained dispersions in co-solvent system are used to fabricate thin films which are utilized in supercapacitor and photoelectrochemical water oxidation. This processing technique shows its potential which can also be extended in other applications. The whole synthesis-LPE-thin film fabrication process can be summarized by the following schematic.

The terms shown in the schematic are taken from the original paper. Interested readers can go through the original publication through this link. If you wish to have a copy of this paper, please contact me through my email or “Share Your Views” option.

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