Electrochromic WO3−x films with reduced lattice deformation stress and fast response time

Electrochromic properties of electrochemically deposited and etched (EDE) WO_3−x films have been investigated using voltammetry and nanogravimetry to elucidate the amount of residual stress associated with lattice polarization and deformation in WO_3−x nanoparticles. The cathodic WO_3−x deposition from pertungstic acid solution and unusual properties of the cathodic electroetching of the oxide in tetraethyl ammonium chloride solution are reported and elucidated on the basis of Electrochemical Quartz Crystal Nanogravimetry (EQCN) measurements. The stress enhanced resonant frequency shift was observed upon WO_3−x film coloration. However, the stress enhancement appeared to be much lower (up to 4-6 times) than that measured for films synthesized by other methods. The stress reduction in WO_3−x films under study has been attributed to the stress relaxing propensity of EDE film to suppress the compressive stress wave. A considerable isotopic effect has been observed in nanogravimetry of the H⁺ and D⁺ ion intercalation into WO_3−x films. We have found that the isotopic effect is primarily due to the true mass loading difference between hydrogen and deuterium ions, for the same concentration of color centers (2.65 × 10²¹ cm⁻³), since EQCN frequency shifts associated with stress in the film for H⁺ and D⁺ are very close to each other.