Advanced smart windows and inkless paper using transparent films of natural polymer enhanced with phosphotungstic acid and doped with zno nanoparticles

Phosphotungstic acid-based photochromic composites with rapid ultraviolet responsiveness represent a significant advancement in the development of smart functional materials. In this work, transparent hybrid films were engineered using ethyl cellulose, polyvinylpyrrolidone, and zinc oxide nanoparticles 45 nm, incorporated with phosphotungstic acid 180 nm, through a controlled solvent-evaporation method. The microstructural features and photochromic behavior of the resulting EC-PVP/ZnO/PWA films were systematically examined using transmission electron microscopy, atomic force microscopy, UV–Vis spectroscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy. Contact angle measurements revealed a reduced hydrophobicity, with a water contact angle of 60.1°, attributed to the presence of hydrophilic PEG segments. Under UV irradiation, the films exhibited a rapid and reversible color transition within 1–9 min, driven by photogenerated electrons from ZnO that promote PWA photoreduction through favorable energy-band alignment. The films subsequently returned to their original state within approximately 40 min. Reversible photochromism was preserved in oxygen-containing environments, during which the films adopted a light-blue appearance under visible-light exposure. XPS analysis confirmed a proton-coupled electron-transfer mechanism, indicated by an increase in the W 5 + /W ratio from 0.22 to 0.28 following UV irradiation. This study provides the first demonstration of PWA-integrated EC-PVP/ZnO hybrid films, offering promising potential for ink-free recording media, optoelectronic devices, and smart window technologies.