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 deg. C and crystal growth occurred at 500 deg. C. However, quite different crystallization behavior appeared on SG due to sodium ion diffusion. At 400 deg. C, rectangular shape crystals identified as Na2W4O13 appeared in amorphous matrix, and grew to bundles of rod-shaped Na2W2O7 and Na2W4O13 phase at 500 deg. C. Raman scattering spectroscopy data confirmed the formation of Na2W4O13 and Na2W2O7 at 500 deg. 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.