Hexagonal hydrated tungsten oxide nanomaterials: Hydrothermal synthesis and electrochemical properties

Highlights

• Hydrothermal preparation of h-WO₃. 1/3H₂O nanomaterials using C_nH_(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⁺).

Abstract

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.

Introduction

Over the past few years, there has been a great deal of interest in the synthesis and the application of low-dimensional nanostructured materials such as MoO₂ [1], VO₂ [2], V₃O₇·H₂O [3] and ZnO [4]. One-dimensional (1D) tungsten oxide and tungsten oxide hydrates (WO₃·nH₂O) have received particular attention because of their potential uses in catalysis and as chemical- and photo-sensors for smart windows, energy storage and photocatalysis [5], [6], [7], [8], [9], [10]. Orthorhombic WO₃·1/3H₂O has been investigated for a number of applications due to its open structure [11]. In this structure when a plane perpendicular to the c-axis is considered, there are two types of octahedra. There is a first [WO₆] octahedral type where four oxygen atoms are shared with an adjacent octahedron in the equatorial ab plane and two other oxygen atoms with octahedra in the upper and lower ab planes. The second [WO₅(OH₂)] octahedral type contains one coordinated water molecule bonded along the c-axis opposite the WO bond [12], [13]. In the WO₃ hexagonal structure, the octahedra form six-membered rings in the equatorial plane (0 0 1) and the stacking of such planes along the c-axis leads to the formation of large tunnels [14], [15]. The tunnel structure gives these materials the ability to be intercalation hosts for monovalent ions (such as H⁺, Li⁺, Na⁺, etc.) by electrochemical oxidation or reduction [16], [17].

Generally these tungsten oxide nanomaterials are prepared using various techniques such as: thermal evaporation [18], flame spray technique [19], modified plasma arc gas condensation [20], liquid phase deposition [21], the reverse microemulsion-mediated method [22] and the hydrothermal method [17], [23], [24], [25], [26], [27]. Among these techniques, the hydrothermal approach stands out as a very promising route featuring low reaction temperature, and may be applicable to large scale processes.

Hexagonal structures of hydrated and non-hydrated tungsten oxide are generally metastable [15]; therefore, they must be prepared at relatively low temperatures. Recently Song et al. [28] reported on the electrochemical properties of one-dimensional h-WO₃ and h-WO₃·1/3H₂O nanostructures prepared by hydrothermal synthesis using different salts in order to understand the structures and morphologies effect on injection of lithium cation into the structure of tungsten oxide nanomaterials. The authors discussed the difference in electrochemical behaviors in terms of difference of reaction surface area between the two types of nanomaterials. In this paper we deal with the hydrothermal synthesis and structural characterization of h-WO₃·1/3H₂O nanocrystals with different morphologies using surfactants having different alkyl chain length. Using the h-WO₃·1/3H₂O nanofiber layers on indium-tin-oxide (ITO)-coated glass as electrode, the electrochemical properties were investigated by cyclic voltammetry (CV) in the presence of Li⁺, Na⁺ and K⁺ respectively, in aqueous electrolytic solution and in propylene carbonate electrolytic one.