Ultraviolet light-induced electron-hole pair excitations in anatase TiO(2) powders were studied by a combination of electron paramagnetic resonance and infrared spectroscopy measurements. During continuous UV irradiation in the mW.cm(-2) range, photogenerated electrons are either trapped at localized sites, giving paramagnetic Ti(3+) centers, or remain in the conduction band as EPR silent species which may be observed by their IR absorption. Using low temperatures (90 K) to reduce the rate of the electron-hole recombination processes, trapped electrons and conduction band electrons exhibit lifetimes of hours. The EPR-detected holes produced by photoexcitation are O(-) species, produced from lattice O(2-) ions. It is found that under high vacuum conditions, the major fraction of photoexcited electrons remains in the conduction band. At 298 K, all stable hole and electron states are lost from TiO(2). Defect sites produced by oxygen removal during annealing of anatase TiO(2) are found to produce a Ti(3+) EPR spectrum identical to that of trapped electrons, which originate from photoexcitation of oxidized TiO(2). Efficient electron scavenging by adsorbed O(2) at 140 K is found to produce two long-lived O(2)(-) surface species associated with different cation surface sites. Reduced TiO(2), produced by annealing in vacuum, has been shown to be less efficient in hole trapping than oxidized TiO(2).

The variety of functionalities and porous structures inherent to metal-organic frameworks (MOFs) together with the facile tunability of their properties makes these materials suitable for a wide range of existing and emerging applications. Many of these applications are based on processes involving interaction of MOFs with guest molecules. To optimize a certain process or successfully design a new one, a thorough knowledge is required about the physicochemical characteristics of materials and the mechanisms of their interaction with guest molecules. To obtain such important information, various complementary analytical techniques are applied, among which vibrational spectroscopy (IR and Raman) plays an important role and is indispensable in many cases. In this review, we critically examine the reported applications of IR and Raman spectroscopies as powerful tools for initial characterization of MOF materials and for studying processes of their interaction with various gases. Both the advantages and the limitations of the technique are considered, and the cases where IR or Raman spectroscopy is preferable are highlighted. Peculiarities of MOFs interaction with specific gases and some inconsistent band assignments are also emphasized. Summarizing the broad analytical possibilities of the IR and Raman spectroscopies, we conclude that it can be applied in combinations with other techniques to explicitly establish the structure, properties, and reactivity of MOFs.

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.

Transmission FTIR spectroscopy is used to explore the electronic structure of excited TiO 2 nanoparticles. Broad infrared spectral features in UVphotoexcited, n-doped, and thermally reduced titania are found to be well-described by two theoretical models, which independently account for the creation of free conduction band electrons and trapped localized electrons that occupy states within the band gap. The infrared spectra indicate that the trapped electrons reside at shallow donor levels that exist 0.12−0.3 eV below the conduction band minimum. IR excitation of the trapped electrons is evidenced by a broad feature in the spectra, which exhibits a maximum that corresponds to the energy of the donor level. These features are well described by a hydrogenic-effective mass model. In addition, free conduction band electrons have a dramatic effect on the infrared spectra by exhibiting a broad featureless absorbance that increases exponentially across the entire mid-IR range. This absorbance is the result of intraconduction band transitions, for which free electron coupling to acoustic phonons is required to conserve momentum. Both localized (within the band gap) and delocalized (within the conduction band) electrons are found to exist in TiO 2 when excess electrons (are created by different means: UV photoexcitation in the presence of a hole scavenger (methanol), irradiation with atomic hydrogen, and thermal removal of lattice oxygen.