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

Tungsten oxide (WO3) and nano-rod sodium tungstate (Na2W2O7) 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 WO3·2/3H2O, WO3·1/3H2O and WO3. 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 Na2W4O13 appeared in amorphous matrix, and grew to bundles of rod-shaped Na2W2O7 and Na2W4O13 phase at 500 °C. Raman scattering spectroscopy data confirmed the formation of Na2W4O13 and Na2W2O7 at 500 °C. The understanding of crystallization behavior and sodium ion diffusion into WO3 thin film on different substrates may be helpful to the application area of electrochromic windows or photocatalyst.