TiO₂ metasurfaces: From visible planar photonics to photochemistry

TiO₂ metasurfaces have been intensively studied in the past few years. To date, the TiO₂ metadevices only used their high reflective index (n). The controllable light extinction coefficient (k) of TiO₂ has not been exploited yet. Here, we converted TiO₂ metasurfaces to black TiO₂ metasurfaces and explored their new opportunities in photochemistry. A complementary metal oxide semiconductor (CMOS)-compatible technique has been developed to reversibly and precisely control the absorption of TiO₂ metasurfaces without spoiling their internal nanostructures. Consequently, two types of black TiO₂ metasurfaces were realized for photochemical experiments. The metasurface with an ultrawide absorption band can substantially enhance the white light absorption and accelerate the solar-based photochemistry process by a factor of 18.7. The other metasurface with an absorption band of <20 nm only responded to the resonant wavelengths, making the photochemistry process capable of being monitored in real time. In addition, the reversible switch between normal and black states makes TiO₂ metasurfaces suitable for dynamic metadevices as well.