Atomic N Modified Rutile TiO2(110) Surface Layer with Significant Visible Light Photoactivity

It has been recently emphasized that the surface of a photocatalyst plays crucial roles on its photocatalytic performance. By nitridizing the top layer of the rutile TiO2(110) surface using reactive atomic N flux, we report direct experimental evidence of significantly enhanced photocatalytic activity under both ultraviolet (UV) and visible light irradiations. The visible light activity of a nitridized surface is found to be comparable to the UV light activity of the pristine surface. On the basis of X-ray photoemission spectroscopy (XPS) measurements and density-functional theory (DFT) calculations, top surface N doping efficiently narrows the local band gap, ∼2.0 eV, which accounts for the visible light activity. Under visible light excitation, nearly all free charges contribute to the photocatalytic reactions. The improvement of photocatalytic activity is attributed to the N 2p add-on shoulder at the valence band maximum (VBM) as well as the strong exchange-splitting at the surface that settles the N 2pz states inside the conduction band of the TiO2 matrix, which avails efficient charge transfer.