Light-responsive color switching of self-doped TiO2−x/WO3·0.33H2O hetero-nanoparticles for highly efficient rewritable paper

Smart materials that reversibly change color upon light illumination are widely explored for diverse appealing applications. However, light-responsive color switching materials are mainly limited to organic molecules. The synthesis of inorganic counterparts has remained a significant challenge because of their slow light response and poor reversibility. Here, we report a seeded growth strategy for the synthesis of TiO2−x/WO3·0.33H2O hetero-nanoparticles (HNPs) with networked wire-like structure of ∼ 10 nm in diameters that enable the highly reversible light-responsive color switching properties. For the TiO2−x/WO3·0.33H2O HNPs, Ti3+ species self-doped in TiO2−x nanoparticles (NPs) act as efficient sacrificial electron donors (SEDs) and Ti-O-W linkages formed between TiO2−x and WO3·0.33H2O NPs ensure the nanoscale interfacial contact, endowing the HNPs enhanced photoreductive activity and efficient interfacial charge transfer upon ultraviolet (UV) illumination to achieve highly efficient color switching. The TiO2−x/WO3·0.33H2O HNPs exhibits rapid light response (< 15 s) and long reversible color switching cycles (> 180 times). We further demonstrate the applications of TiO2−x/WO3·0.33H2O HNPs in ink-free, light-printable rewritable paper that can be written on freehand or printed on through a photomask using UV light. This work opens an avenue for designing inorganic light-responsive color switching nanomaterials and their smart applications.