Strong, Rapid and Reversible Photochromic Response of Nb doped TiO₂ Nanocrystal Colloids in Hole Scavenging Media

Understanding photochromicity is essential for developing new means of modulating the optical properties and optical response of materials. Here, we report on the synthesis and exciting new photochromic behavior of Nb⁵⁺ doped TiO₂ nanoparticle colloids (NCs). We find that in hole scavenging media, Nb⁵⁺ doping significantly improves the photochromic response time of TiO₂ nanoparticles. In the infrared regime, Nb-doped TiO₂ NCs exhibit an order of magnitude faster photoresponse kinetics than the pristine TiO₂. Enhanced photochromic response is observed in the visible light regime as well. The transmittance of Nb-doped TiO₂ NCs drops to 10% in less than 2 minutes when irradiated by UV light in 500 nm range. The photochromic reaction is fully reversible. The physical origin of the high reaction rate is the high Nb⁵⁺ concentration. As a donor dopant, Nb⁵⁺ builds up a significant positive charge in the material, which leads to highly efficient electron accumulation during the UV irradiation and results in a rapid photoresponse. EPR experiments identify a new defect type from Nb⁵⁺ doping, which alters the physical mechanisms available for transmittance modulation. Our new NCs are economic to synthesize and highly suitable for switchable photochromic applications, e.g. smart windows for modulating visible light and infrared transmittance in built environments.