Rapid Color‐Switching of MnO₂ Hollow‐Nanosphere Films in Dynamic Water Vapor for Reversible Optical Encryption

Abstract Drop‐casting manganese oxide (MnO₂) hollow nanospheres synthesized via a simple surface‐initiated redox route produces thin films exhibiting angle‐independent structural colors. The colors can rapidly change in response to high‐humidity dynamic water vapor (relative humidity > 90%) with excellent reversibility. When the film is triggered by dynamic water vapor with a relative humidity of ≈100%, the color changes with an optimal wavelength redshift of ≈60 nm at ≈600 ms while there is no shift under static water vapor. The unique selective response originates from the nanoscale porosity formed in the shells by randomly stacked MnO₂ nanosheets, which enhances the capillary condensation of dynamic water vapor and promotes the change of their effective refractive index for rapid color switching. The repeated color‐switching tests over 100 times confirm the durability and reversibility of the MnO₂ film. The potential of these films for applications in anti‐counterfeiting and information encryption is further demonstrated by reversible encoding and decoding initiated exclusively by exposure to human breath.