Sn-TiO₂/PTA Nanocomposite Films for High-Contrast Rewritable Media with Visible-Light-Driven Black Coloration

Photochromic materials are pivotal for rewritable smart media, yet conventional systems suffer from sluggish kinetics, UV dependency, and low optical contrast. Herein, we present a visible-light-responsive Sn-TiO2/phosphotungstic acid (PTA) nanocomposite film mediated by polyvinylpyrrolidone (PVP) that addresses these challenges through interfacial engineering and bandgap modulation. Sn-doped TiO2 nanoparticles, synthesized hydrothermally, are covalently linked to phosphotungstic acid (PTA) clusters via PVP-assisted dispersion, enabling efficient charge separation under 450 nm illumination. The Sn-TiO2/PTA/PVP nanocomposite film achieves ultrafast coloration within 10 s, attributed to the reduction of W6⁺ to W5⁺ in PTA. The colored state exhibits remarkable air stability (48 h) and rapid recovery (<30 min) via H2O2 vapor, sustaining >80 reversible cycles without degradation. With a narrowed bandgap (2.23 eV) and broadband intervalence charge transfer (IVCT) absorption (600–800 nm), the film demonstrates high-contrast black-state coloration and 2-day legibility as a rewritable medium. This work overcomes the limitations of organic dyes and UV-dependent systems, offering an inorganic, eco-friendly platform for smart displays, anti-counterfeiting labels, and energy-efficient photochromic technologies.