Hexagonal hydrated tungsten oxide nanomaterials: Hydrothermal synthesis and electrochemical properties

• Hydrothermal preparation of h-WO3. 1/3H2O nanomaterials using CnH(2n+1)SO4Na surfactants (n = 10, 12 and 14).
• Nanofibers obtained when n = 10 and nanoneedles obtained when n = 12 and 14.
• Nanofibers show redox reversible behavior with intercalation/de-intercalation process of alkaline cation (Li+, Na+ and K+).
• Intercalation/de-intercalation process more reversible in propylene carbonate than in water and more difficult for larger cations (Na+ and K+) than for small one (Li+).

Nanocrystalline hexagonal hydrated tungsten oxide h-WO3·1/3H2O has been synthesized by hydrothermal process using sodium tungstate as inorganic precursor and three n-alkyl chain sodium sulfate surfactants (n = 10, 12 and 14) as structure-directing templates. X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) have been used to characterize the structure, the morphology and the composition of the material. The length of the alkyl chain of the surfactant molecules has a marked effect on the morphology and, particularly, on the particle size of the material. Nanofibers (about 50 nm in diameter) are obtained when sodium decyl sulfate is used as surfactant. Whereas, nanoneedles are obtained with sodium dodecyl (about 60 nm in diameter) or sodium tetradecyl sulfate (about 80 nm in diameter) as surfactant.Thin films of h-WO3·1/3H2O deposited on ITO substrates were electrochemically characterized by cyclic voltammetry; they show the same behavior whatever the surfactant. The voltammograms show reversible redox behavior with doping/dedoping process corresponding to reversible cation intercalation/de-intercalation into the crystal lattice of the nanofibers. This process is easier in propylene carbonate than in aqueous solvent and is easier for the small Li+ cation than larger ones, Na+ and K+. This is attributed to probable presence of two different tunnel cavities in the h-WO3·1/3H2O lattice.

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