No abstract

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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The UV photoproduction of a hydrophilic TiO 2 (110)(1×1) surface has been investigated in a pressurized ultrahigh vacuum apparatus under controlled conditions of hydrocarbon concentration in oxygen gas at 1 atm pressure. Water droplet contact angles have been measured continuously as the droplet is exposed to UV irradiation, yielding the first observations of a sudden wetting process during irradiation. Using hexane as a model hydrocarbon, it is found that when low partial pressures of hexane are present, the sudden onset of surface wetting occurs during UV irradiation after an induction period under photooxidation conditions. The induction period to reach the critical condition for sudden wetting increases when the partial pressure (and equilibrium surface coverage) of hexane is increased. These results indicate that the removal of adsorbed hydrocarbons by photooxidation is the critical factor leading to the UV-induced hydrophilicity phenomenon on TiO 2 . The phenomenon does not occur in the absence of O 2 gas. A concept concerned with kinetic screening of the TiO 2 -H 2 O interface from O 2 by water droplets is presented to explain the observation of sudden wetting in our experiments, compared to gradual wetting which is observed following UV irradiation in all other experiments reported in the literature. Complementary infrared spectroscopy measurements of the effect of UV irradiation in an O 2 atmosphere on adsorbed Ti-OH groups and on adsorbed H 2 O on the surface of a high-area TiO 2 powder show that no spectroscopic changes occur. This indicates that UV-induced changes in the -OH coverage or the nature of -OH bonding to TiO 2 , as suggested by others, cannot be used to explain the photoinduced hydrophilicity effect.

The hole-induced photodesorption of chemisorbed O2 from a TiO2(110) single crystal has been employed to monitor the kinetics of electron-hole pair (e-h) formation and hole trapping. Excitation is produced by 3.4 +/- 0.05 eV photons at 110 K. Two separate O2 desorption processes have been found which are characteristic of low photon fluxes and high photon fluxes. At a critical photon flux, Fhnu(crit), the slow O2 photodesorption process suddenly converts to a fast process, signaling the saturation of hole traps in the TiO2 crystal. Consequently, this allows photogenerated holes to more efficiently reach the surface, causing more rapid O2 photodesorption. The estimated bulk concentration of hole traps is approximately 2.5 x 10(18) cm(-3), involving a fraction of about 3 x 10(-5) of the atomic sites in the bulk. Both the slow and fast O2 photodesorption processes are described by a rate law that is proportional to Fhnu(1/2), indicating that the steady-state concentration of holes, [h], is governed by second-order e-h pair recombination kinetics. Effective use is made of a hole scavenger molecule, adsorbed methanol (CH3OH), to probe the role of added hole traps on the rate of the photodesorption of adsorbed O2 molecules and on the magnitude of Fhnu(crit).

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 Nions 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 Ndoping 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.

The adsorption and thermal desorption of CO 2 bound to both oxidized and reduced TiO 2 (110) surfaces has been studied using temperature-programmed desorption. For the stoichiometric and fully oxidized surface, a single thermal desorption feature (E d ) 48.5 kJ/mol) is measured and attributed to CO 2 bound to regular fivefold coordinated Ti 4+ atoms. For the fully reduced TiO 2 (110) surface, CO 2 binds not only to regular sites, but also to oxygen vacancy sites (E d ) 54.0 kJ/mol), created by thermal annealing. The variation in the characteristic CO 2 desorption kinetics was measured as a function of the surface reduction temperature, and the systematic production of increasing levels of surface defects is observed in the temperature range of 600-1100 K. This investigation was complimented by a comparison of the characteristic CO 2 desorption features from a TiO 2 (110) surface which was prepared by sputtering and direct annealing, without annealing in O 2 flux. It was found that after annealing to temperatures above 900 K, the CO 2 thermal desorption is very similar for surfaces prepared by the two methods, regardless of surface preparation.