TiO2(110) single crystals, doped with nitrogen via an NH3 treatment at 870 K, have been found to exhibit
photoactivity at photon energies down to 2.4 eV, which is 0.6 eV below the band-gap energy for rutile TiO2.
The active dopant state of the interstitial nitrogen that is responsible for this effect exhibits an N (1s) binding
energy of 399.6 eV and is due to a form of nitrogen that is probably bound to hydrogen, which differs from
the substitutional nitride state with an N (1s) binding energy of 396.7 eV. Optical absorption measurements
also show enhanced absorption down to 2.4 eV for the NH3-treated TiO2(110). A co-doping effect between
nitrogen and hydrogen is postulated to be responsible for the enhanced photoactivity of nitrogen-doped TiO2
materials in the range of visible light.
A detailed discussion of the photochemistry of TiO 2 surfaces is presented, covering important work from the literature as well as more recent studies. The production and characterization of surface defects is discussed, and studies of the adsorption of molecular oxygen on these defects is presented. In addition, both chemical and physical methods for detection and measurement of defect sites on TiO 2 are reviewed. The role of nitrogen doping on shifting the photothreshold energy of TiO 2 is discussed and the active chemical state of the nitrogen is described, based on XPS N(1s) binding energies. Observations of charge transfer between excited TiO 2 and adsorbates are presented, and it is shown that the electronegativity of the attachment atom which forms the surface bond is important in governing charge transfer.
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