Formation of nano-rod sodium tungstate film by sodium ion diffusion

Tungsten oxide (WO 3) and nano-rod sodium tungstate (Na 2W 2O 7) thin film was prepared by dip coating on slide glass (SG) and tin-doped indium oxide (ITO) coated glass in peroxotungstic acid (PTA) solution. Thermal analysis results showed that PTA is decomposed to tungsten oxide via multiple steps of WO 3·2/3H 2O, WO 3·1/3H 2O and WO 3. Monoclinic phase appeared on ITO glass at 400 °C and crystal growth occurred at 500 °C. However, quite different crystallization behavior appeared on SG due to sodium ion diffusion. At 400 °C, rectangular shape crystals identified as Na 2W 4O 13 appeared in amorphous matrix, and grew to bundles of rod-shaped Na 2W 2O 7 and Na 2W 4O 13 phase at 500 °C. Raman scattering spectroscopy data confirmed the formation of Na 2W 4O 13 and Na 2W 2O 7 at 500 °C. The understanding of crystallization behavior and sodium ion diffusion into WO 3 thin film on different substrates may be helpful to the application area of electrochromic windows or photocatalyst.