## 2 Materials and Methods
### Instrumentation  
An InfraLUM FT-08 infrared spectrometer (Lumex, St. Petersburg, Russia) was used to record vibrational spectra of the complex solution. The spectra were recorded in the range of 350-2000 cm\({}^{-1}\). For analysis of the powders, their suspensions in rujol null were prepared and then placed as films between KBr lenses.  
A UV-Vis spectrophotometer SF-56 (OKB Spectr, St. Petersburg, Russia) was used to record spectra in the UV, visible, and near-IR ranges using a quartz cuvette (1 mm thick). The precursor solution was not additionally diluted during the study of its hydrothermal treatment behavior and was returned to the autoclave after recording spectra for further thermal treatment.  
Thermal behavior of the prepared mixture of the [WO\({}_{2}\)(\(C_{5}\)H\({}_{7}\)O\({}_{2}\))\({}_{2}\)] complex and tungsten oxide in the range of 25-600 \({}^{\circ}\)C was studied using the synchronous thermal analyzer SDT Q600 (TA Instruments, New Castle, DE, USA) in air flow (250 mL/min), heating speed 10\({}^{\circ}\)/min.  
X-ray diffraction analysis (XRD) of the WO\({}_{3}\) film grown on glass was performed using a D8-Advance diffractometer (Bruker, Billerica, MA, USA; CuK\(\alpha\) = 1.5418 A, Ni filter, E = 40 keV, I = 40 mA, range 20 5-80\({}^{\circ}\), signal accumulation time 0.3 s/point, resolution 0.02\({}^{\circ}\)).  
Raman spectroscopy of the WO\({}_{3}\)/glass film was performed in the range of 200-1200 cm\({}^{-1}\) on a R532 spectrometer (EnSpectr, Chernogolovka, Russia). The scan accumulation time was 500 ms, the number of scans was 150, and the laser wavelength was 532 nm.  
The microstructure of the films was studied using scanning electron microscopy (SEM, NVision 40 dual-beam workstation, Carl Zeiss, Inc., Jena, Germany). In the course of SEM measurements, we also used the Oxford Instruments X MAX 80 microprobe analyzer, with the help of which we clarified the chemical composition of the formed oxide films.  
The grown WO\({}_{3}\) films were also studied using atomic force microscopy (AFM) on the NT-MDT Solver PRO-M (NT-MDT, Zelenograd, Russia). Both surface morphology and local electrophysical properties of the material were characterized. For the latter purpose, Kelvin Probe Force Microscopy (KPFM) technique was used. Scanning was performed in the semicontact mode using ETALON HA_HR probes (ScanSens, Bremen, Germany) with a conductive W\({}_{2}\)C coating (probe tip curvature radius \(\leq 35\) nm). Within the framework of the KPFM, we also measured the value of the electron work function \(\varphi_{\rm oxide}\) for the oxide films.  
To study the electrochromic properties of the WO\({}_{3}\) film on a glass/ITO substrate, a specialized two-electrode cell was assembled. The glass/ITO substrate was used as the auxiliary electrode. A polymer electrolyte composed of a mixture of polyethylene glycol, propylene carbonate, and lithium perchlorate was placed between the WO\({}_{3}\) film and the auxiliary electrode (the ratio of polyethylene glycol monomers to the amount of lithium perchlorate was 8:1). After the cell was sealed, it was placed in a chamber of a UV-Vis-spectrophotometer SF-56 to measure its optical properties. The difference in potentials was supplied by potentiostat-galvanostat P-45X with an attachment for measurement of electrochemical impedance FRA-24M (Electrochemical Instruments, Chernogolovka, Russia). The same device was used to record cyclic voltammetry curves (speed of voltage change 50 mV/s).