Reversible Photodoping of TiO₂ Nanoparticles for Photochromic Applications

Observations on the strong photochromic effect of crystalline TiO2 quantum dots (mean size ≈ 4 nm) are presented. The synthesized quantum dots consist of irregularly shaped anatase TiO2 nanoparticles (NPs) and are dispersed in butanol (8% by mass). Obtained NPs exhibit a dramatic photoresponse to UV light, enabling effective transmittance modulation in a broad wavelength range extending from the visible to near-infrared region, and even the thermal black body radiation regime beyond 10 μm. The exceptional photoresponse is attributed to hole-scavenging by butanol, TiO2 self-reduction, injection of electrons to the conduction band, and consequent localized surface plasmon resonances in NPs. The observed optical effect is reversible, and the initial high transmittance state can be restored simply by exposing the NPs to air. The applied NP synthesis route is economic and can be easily scaled for applications such as smart window technologies.