WO3 Nano-Ribbons: Their Phase Transformation from Tungstite (WO3-H2O) to Tungsten Oxide (WO3)

Ahmadi, Majid, Satyaprakash Sahoo, Reza Younesi, Anand, P.S. Guar, et al., 2014

Abstract

Tungsten oxide (WO3) nano-ribbons (NRs) were obtained by annealing tungstite (WO3·H2O) NRs. The latter was synthesized below room temperature using a simple, environmentally benign, and low cost aging treatment of precursors made by adding hydrochloric acid to diluted sodium tungstate solutions (Na2WO4·2H2O). WO3 generates significant interests and is being used in a growing variety of applications. It is therefore important to identify suitable methods of production and better understand its properties. The phase transformation was observed to be initiated between 200 and 300 °C, and the crystallographic structure of the NRs changed from orthorhombic WO3·H2O to monoclinic WO3. It was rigorously studied by annealing a series of samples ex situ in ambient air up to 800 °C and characterizing them afterward. A temperature-dependent Raman spectroscopy study was performed on tungstite NRs between minus 180 and 700 °C. Also, in situ heating experiments in the transmission electron microscope allowed for the direct observation of the phase transformation. Powder X-ray diffraction, electron diffraction, electron energy-loss spectroscopy, and X-ray photoelectron spectroscopy were employed to characterize precisely this transformation.

Impact Statement

The collection of tungsten oxide (WO3) nano-ribbons produced by annealing tungstite (WO3-H2O) nano-ribbons. The nano-ribbons were created with a simple, ecologicallyfriendly and cost-effective treatment consisting of hydrochloric acid and diluted sodium
tungstate solutions (Na2WO2-2H2O). The researchers observed that the transformation from Na2WO4-2H2O to WO3-H20 initiated between 200° to 300° C. The nano-ribbons’crystallographic structure then changed from WO3H20 to monoclinic WO3. Ex situ and in situ methods allowed the researchers to study the phase transformation in detail, while powder X-ray diffraction, electron diffraction, electron energy-loss spectroscopy and X-ray photoelectron spectroscopy enabled precise characterization.