The Effect of Nitrogen Ion Implantation on the Photoactivity of TiO2 Rutile Single Crystals

The effect of impurity doping on the photoactivity of TiO 2 rutile single crystals was subjected to a combined surface-science and bulk-analysis study. The incorporation of nitrogen ions, N -, into TiO 2 single crystals was achieved by sputtering with N 2 + /Ar + mixtures and subsequent annealing to 900 K under ultrahigh vacuum conditions. This procedure leads to a 90 Å thick structurally modified near-surface region, which, by the use of cross sectional transmission electron microscopy, can be described as rutile grains imbedded within a monocrystalline strained rutile matrix. The presence of N - ions distributed in the first 200 Å below the surface was revealed by X-ray photoelectron spectroscopy, in agreement with sputter depth profiles obtained by secondary ion mass spectroscopy. The concentration of N - doping is about 10 20 cm -3 in the first 200 Å of the near-surface region. The photodesorption of O 2 was employed to study the changes in the photochemical properties of nitrogen-implanted crystals. The action curves for O 2 photodesorption exhibit an unexpected blueshift compared to undoped crystals. The effect is attributed to the deposition of electronic charge in the lower levels of the conduction band (band-filling mechanism), causing allowed indirect photoexcitation processes to shift to energies higher than the band gap.