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.

Sub-stoichiometric MoO_3-x nanostructures with plasmonic absorption via creating oxygen vacancies have attracted extensive attentions for many intriguing applications. However, the synthesis of one-dimensional (1D) plasmonic MoO_3-x nanostructures with widely tunable plasmonic absorption has remained a significant challenge because of their serious morphological destruction and phase change with increasing the concentration of oxygen vacancies. Here we demonstrate a surface-ligand protected reduction strategy for the synthesis of 1D MoO_3-x nanobelts with tunable plasmonic absorption in a wide wavelength range from 200 to 2,500 nm. Polyethylene glycol (PEG-400) is used as both the reductant to produce oxygen vacancies and the surface protected ligands to maintain 1D morphology during the formation process of MoO_3-x nanobelts, enabling the widely tunable plasmonic absorption. Owing to their broad plasmonic absorption and unique 1D nanostructure, we further demonstrate the application of 1D MoO_3-x nanobelts as photothermal film for interfacial solar evaporator. The surface-ligand protected reduction strategy provides a new avenue for the developing plasmonic semiconductor oxides with maintained particle morphology and thus enriching their wide applications.