Effect of Hydrogen Peroxide and Oxalic Acid on Electrochromic Nanostructured Tungsten Oxide Thin Films

Electrochromic tungsten oxide, WO3, nanomaterials have been prepared using sol-gel method from sodium tungstate, Na2WO4, and ion exchange resin. Thin films were prepared using spinner coating technique from the prepared nanomaterials. The effect of number of layers, and additive type; hydrogen peroxide and/or oxalic acid, were investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-VIS spectrophotometry, Fourier transfer infrared (FTIR), Thermal gravimetric analysis (TGA) and transmission electron microscopy (TEM) were used to characterize different properties of the prepared nanomaterials. Electrochemical measurements were used to evaluate the electrochromic properties. The thickness of the prepared WO3 thin films was ranged between 0.33 μm to 9.82 μm, depending on the number of applied layers. The crystal structure of pure WO3 without additive as well as with 4 % oxalic acid exhibited hexagonal structure. Monoclinic structure formed for samples contain 1% hydrogen peroxide and a mixture of 1% hydrogen peroxide and 4% oxalic acid. Hydrogen peroxide and oxalic acid additives led to an open porous structure and increased the diffusion coefficient from 5.3x10-14 to 3.1x10-9 cm2 s-1. High coloration efficiency of 64.2 cm2 C-1 was obtained for the open porous structure of the two-layer film prepared with W+ ion concentration of 0.1 M, and 1% hydrogen peroxide and 4% oxalic acid.