Unlocking Time-Dependent Visual Response Mode Through Regulating the Photo-Induced Local Structure Evolution Process of C@TiO2:Sm,N

Abstract

A new time-dependent dual-mode visual response system is developed based on C@TiO2:Sm,N/PVA film. In situ irradiation (ISI)–electron paramagnetic resonance and ISI–X-ray photoelectron spectroscopy combined with DFT calculations reveal that the visual response behavior is attributed to the dynamic evolution of a distinct local structure containing TiO₆, TiO₅N, and SmO₆ units connected via the bridging oxygen atom. Upon UV irradiation, the localization of the photogenerated electrons induces a rapid release of bridging oxygen atoms, creating oxygen vacancies, effectively hindering the energy transfer from TiO₂ to Sm³⁺ and resulting in a strong luminescence modulation that reaches the theoretical maximum (99.9%) within 1 s. With prolonged UV exposure (3–20 s), the bridging oxygen vacancies act as electron traps, and the photogenerated electrons accumulate around the Ti atoms near them. This causes the film to transition from white to dark blue, achieving a high photochromic contrast (51.4%). A foundation for advanced information encryption and anticounterfeiting is established based on the different time scales of luminescence modulation and photochromism. This study confirms the application potential of the film in “burn after reading” and dynamic information encryptions and advanced anticounterfeiting labels, and it offers comprehensive insights into the design of smart photoresponsive materials.