Temperature‐controlled synthesis of bismuth tungstate with enhanced photochromic properties

Abstract This study explores the photochromic properties of bismuth tungstate (Bi 2 WO 6 ) synthesized via a solid‐state method at various temperatures (300°C–1000°C) to optimize light‐responsive behavior for potential applications. The Bi 2 WO 6 samples, prepared from tungsten oxide (WO 3 ) and bismuth oxide (Bi 2 O 3 ) through heat treatment, were analyzed using X‐ray diffraction (XRD), UV‐Vis Diffuse Reflectance Spectroscopy (UV‐DRS), and scanning electron microscopy (SEM) techniques. XRD results revealed the formation of the Bi 2 WO 6 phase at higher temperatures, while the Bi 14 W 2 O 27 phase, formed at 400°C–500°C, exhibited an oxygen‐deficient structure enabling significant color shifts. Band gap analysis showed that samples synthesized at 400°C and 500°C had a lower band gap (2.63 eV) compared to those prepared at higher temperatures, suggesting enhanced electronic properties due to oxygen vacancies. SEM images showed nanoscale crystal facets in the 400°C and 500°C samples, which facilitated effective sunlight excitation and stable color changes. Under sunlight exposure, these samples transitioned from yellow to greenish brown within 5 min, quickly reaching color saturation, while samples synthesized at higher temperatures showed minimal photochromic response. This work is the first to report on the photochromic properties of Bi 2 WO 6 synthesized through solid‐state methods, highlighting the role of oxygen vacancies, reduced band gap, and nanoscale morphology on enhancing photochromic performance.