Light-responsive color switching of self-doped TiO_2-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 TiO_2-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 TiO_2-x/WO₃·0.33H₂O HNPs, Ti³⁺ species self-doped in TiO_2-x nanoparticles (NPs) act as efficient sacrificial electron donors (SEDs) and Ti-O-W linkages formed between TiO_2-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 TiO_2-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 TiO_2-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.