Nb-Doped Colloidal TiO2 Nanocrystals with Tunable Infrared Absorption

We report a new colloidal synthesis of niobium-doped TiO 2 anatase nanocrystals (NCs) that allows for the preparation of ∼10 nm NCs with control over the amount of Nb doping up to ∼14%. The incorporation of niobium ions leads to the appearance of a tunable, broad absorption peak that ranges from the visible range to the mid-infrared. This optical behavior is attributed to the substitution of Nb 5+ on Ti 4+ sites generating free carriers inside the conduction band of the TiO 2 NCs as supported by optical and electron paramagnetic resonance spectroscopic investigations. At the same time, the incorporation of progressively more niobium ions drives an evolution of the shape of the NCs from tetragonal platelets to “peanutlike” rods.