<p>Nanostructured systems showing reversible colour switching are envisaged to play a significant role in photo-switches, photo-optical sensors, smart windows, displays, optical storage memories. Most of the materials exhibiting reversible colour switching are organic compounds. However, their UV-light activation, low thermal and chemical stability, as well as harmful synthesis methods, are of limit for their extensive use. In this research, we have created an inorganic switchable photochromic

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... aterial exploiting: (i) TiO₂ ability of creating an exciton upon excitation, (ii) copper as the chromophore, and (iii) graphene's extraordinarily high electron mobility. Our material showed itself to be able to work under visible-light, its photochromic property being three times faster than conventional titania based photochromic materials, reaching a stable change in colouration after only 30 mins of visible-light irradiation (<i>versus</i> > 120 min in conventional Cu-TiO₂). With the addition of just 1 wt% graphene, the material exhibited a staggeringly stable photochromic switching over repeated cycles. These results relate to the best previously reported values for any form of TiO₂-based photochromic material. This is therefore an excellent candidate for smart self-cleaning windows, and other chromic devices and applications.</p>