Phase Engineering of WO₃ Nanoparticles Towards Efficient Photochromic Nanocomposites

Tungsten oxide (WO₃) is an attractive phase-sensitive chromogenic material whose photoinduced optical response produces reversible coloration of interest light responsive optical devices. Here, WO₃ nanoparticles (NPs) are synthesized via a simple surfactant-free sol-gel precipitation route by systematically varying acid content and reaction temperature then characterized to establish a correlation among synthesis conditions, crystalline phase morphology and photochromic response. Morphological, structural and optical characterizations show that by gradually heating the precursors at 90 °C and varying the acid content, predominantly sheet-like orthorhombic and hydrated-orthorhombic WO₃ NPs are yielded, characterized by an intense yellow color, optical band-gap values of approximately 2.4 eV, and negligible photochromism. In contrast, in a highly acidic environment. the temperature tuning promotes the formation of rod-like WO₃ NPs with whitish color and higher band gap. The crystalline structures change from hexagonal to hexagonal hydrogen bronze phase, decreasing the reaction temperature. Under UV irradiation, these samples exhibit markedly higher photochromic efficiency than the orthorhombic phase, up to 18-fold higher, due to more accessible proton intercalation pathways offered by the hexagonal tunnel structure. The incorporation into poly(vinylpyrrolidone) allows the production of nanocomposite films that preserve and enhance the photochromic response. Overall, the reaction temperature is identified as the key synthetic parameter controlling WO₃ crystalline phase that consequently affects photochromic behaviors, while polymer integration provides a practical route toward processable, flexible, and device-compatible photochromic coatings.