Application of the [WO2(C5H7O2)2] Complex in Hydrothermal Synthesis of WO3 Film and Study of Its Electrochromic Properties

The goal of this work was the synthesis study of the [WO2(C5H7O2)2] complex and its application as a precursor for the growth of WO3 films in hydrothermal conditions, as well as evaluating the microstructural features and electrochromic properties of the formed materials. Dioxotungsten acetylacetonate was synthesized in an aqueous medium and purified. It was found that during hydrothermal treatment of the alcohol solution of the complex, acetylacetonate ligands undergo partial destructive substitution by alkoxyl groups, intensifying at temperatures above 140 °C. Considering these data and using a [WO2(C5H7O2)2] solution, WO3 films were grown on glass and glass/ITO substrates. The resulting films had different microstructures according to scanning electron microscopy (SEM) and atomic force microscopy (AFM): the former consisted of submicron spheres (~500 nm), distinct nanoparticles (60–160 nm), and submicron- and micron-sized ridges, while the latter consisted of 1D structures (length 350 ± 25 nm, width 110 ± 25 nm). Using Kelvin probe force microscopy (KPFM), the electron work function was determined for the film on glass/ITO substrate (4.77 eV). It was found that the electrochemical coloration process of the obtained WO3 film can proceed in two stages, and the optical contrast is about 17.5% (at the wavelengths of 600–1100 nm). The results obtained show the prospects of applying the proposed approach to obtaining WO3 electrochromic films with a hierarchical microstructure with the hydrothermal method using the [WO2(C5H7O2)2] complex as a precursor.