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

The effect of impurity doping on the photoactivity of TiO2 rutile single crystals was subjected to a combined surface-science and bulk-analysis study. The incorporation of nitrogen ions, N-, into TiO2 single crystals was achieved by sputtering with N2 +/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 1020 cm-3 in the first 200 Å of the near-surface region. The photodesorption of O2 was employed to study the changes in the photochemical properties of nitrogen-implanted crystals. The action curves for O2 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.