Enhanced Dopant Solubility and Visible-Light Absorption in Cr–N Codoped TiO2 Nanoclusters

A major obstacle toward employing TiO2 as an efficient photoactive material is related to its large optical band gap, strongly limiting visible light absorption. Substitutional doping with both donors and acceptors (codoping) potentially leads to a significant band gap reduction, but the effectiveness of the codoping approach remains limited by the low solubility of dopants inside TiO2. Here we show that nanostructured Cr and N codoped TiO2 thin films can be obtained by supersonic cluster beam deposition (SCBD) with a high concentration of dopants and a strongly reduced band gap. Complementary spectroscopic investigations show that doping effectively occurs in substitutional lattice sites, inducing dopant levels in the gap that are remarkably delocalized. The high surface-to-volume ratio, typical of SCBD nanostructured films, likely facilitates the dopant incorporation. The present results indicate that SCBD films are highly promising photoactive nanophase materials.