Nickel oxide (NiO) is a wide band gap semiconductor material that is used as an electrochromic layer or an ion storage layer in electrochromic devices. In this work, the effect of annealing temperature on sol-gel NiO films was investigated. Fourier transform infrared spectroscopy (FTIR) showed that the formation of NiO via decomposition of the precursor nickel acetate occurred at about 300 °C. Meanwhile, an increase in roughness was observed by Atomic force microscope (AFM), and precipitation of a large number of crystallites was observed at 500 °C. X-ray Diffraction (XRD) showed that the NiO film obtained at such a temperature showed a degree of crystallinity. The film crystallinity and crystallite size also increased with increasing annealing temperature. An ultraviolet spectrophotometer was used to investigate the optical band gap of the colored NiO films, and it was found that the band gap increased from 3.65 eV to 3.74 eV with the increase in annealing temperature. An electrochromic test further showed that optical modulation density and coloring efficiency decreased with the increase in crystallite size. The electrochromic reaction of the nickel oxide film is more likely to occur at the crystal interface and is closely related to the change of the optical band gap. An NiO film with smaller crystallite size is more conducive to ion implantation and the films treated at 300 °C exhibit optimum electrochromic behavior.

Tungsten trioxide (WO₃) is a wide band gap semiconductor material that is used as an important electrochromic layer in electrochromic devices. In this work, the effects of the annealing temperature on the optical band gap of sol-gel WO₃ films were investigated. X-ray Diffraction (XRD) showed that WO₃ films were amorphous after being annealed at 100 °C, 200 °C and 300 °C, respectively, but became crystallized at 400 °C and 500 °C. An atomic force microscope (AFM) showed that the crystalline WO₃ films were rougher than the amorphous WO₃ films (annealed at 200 °C and 300 °C). An ultraviolet spectrophotometer showed that the optical band gap of the WO₃ films decreased from 3.62 eV to 3.30 eV with the increase in the annealing temperature. When the Li⁺ was injected into WO₃ film in the electrochromic reaction, the optical band gap of the WO₃ films decreased. The correlation between the optical band gap and the electrical properties of the WO₃ films was found in the electrochromic test by analyzing the change in the response time and the current density. The decrease in the optical band gap demonstrates that the conductivity increases with the corresponding increase in the annealing temperature.

Tungsten trioxide (WO₃) is a wide band gap semiconductor material, which is commonly not only used, but also investigated as a significant electrochromic layer in electrochromic devices. WO₃ films have been prepared by inorganic and sol-gel free ammonium tungstate ((NH₄)₂WO₄), with the modification of glycerol using the spin coating technique. The surface tension, the contact angle and the dynamic viscosity of the precursor solutions demonstrated that the sample solution with a 25% volume fraction of glycerol was optimal, which was equipped to facilitate the growth of WO₃ films. The thermal gravimetric and differential scanning calorimetry (TG-DSC) analysis represented that the optimal sample solution transformed into the WO₃ range from 220 °C to 300 °C, and the transformation of the phase structure of WO₃ was taken above 300 °C. Fourier transform infrared spectroscopy (FT-IR) spectra analysis indicated that the composition within the film was WO₃ above the 300 °C annealing temperature, and the component content of WO₃ was increased with the increase in the annealing temperature. The X-ray diffraction (XRD) pattern revealed that WO₃ films were available for the formation of the cubic and monoclinic crystal structure at 400 °C, and were preferential for growing monoclinic WO₃ when annealed at 500 °C. Atomic force microscope (AFM) images showed that WO₃ films prepared using ammonium tungstate with modification of the glycerol possessed less rough surface roughness in comparison with the sol-gel-prepared films. An ultraviolet spectrophotometer (UV) demonstrated that the sample solution which had been annealed at 400 °C obtained a high electrochromic modulation ability roughly 40% at 700 nm wavelength, as well as the optical band gap (E_g) of the WO₃ films ranged from 3.48 eV to 3.37 eV with the annealing temperature increasing.

Polyethylene oxide (PEO)-modified silicon dioxide (SiO₂)-doped crystalline tungsten trioxide (WO₃) films for use as electrochromic layers were prepared on indium tin oxide (ITO) glass by the sol–gel spin coating technique. The effects of the PEO template and SiO₂ on the electrochromic transmittance modulation ability of crystalline WO₃ films were investigated. Fourier transform infrared spectroscopy (FT-IR) spectra analysis indicated that PEO was decomposed after annealing at 500 °C for 3 h. X-ray diffraction (XRD) pattern analysis showed that both SiO₂ and PEO helped reduce the crystalline grain size of the WO₃ films. Atomic force microscope (AFM) images showed that the combined action of SiO₂ and PEO was helpful for achieving high surface roughness and a macroporous structure. An electrochromic test indicated that PEO-modified SiO₂-doped crystalline WO₃ films intercalated more charges (0.0165 C/cm²) than pure WO₃ crystalline films (0.0095 C/cm²). The above effects resulted in a good transmittance modulation ability (63.2% at 628 nm) of PEO-modified SiO₂-doped crystalline WO₃ films, which was higher than that of pure WO₃ crystalline films (9.4% at 628 nm).