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, similar to 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 2p(z) states inside the conduction band of the TiO2 matrix, which avails efficient charge transfer.