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Nanostructured TiO2 samples with primary particle size in the 4−20 nm range were prepared by either hydrothermal (H) or thermal (T) treatment of an amorphous precursor, and their behavior under UV illumination at 77 K was studied by means of EPR spectroscopy. The samples of the H series present the smallest crystallite size and after irradiation in a vacuum show some Ti3+ centers. In contrast, under these conditions only weak signals associated with oxygenated radicals are observed for the T samples. However, when oxygen is preadsorbed, several oxygenated complexes (O-, O2 -, O2H•, and O3 -) are photogenerated in proportions that depend on the characteristics of the material. Subsurface O- species are exclusively detected in the case of the samples of the H series, whereas ozonide radicals and surface O- are stabilized on materials with larger crystalline domains. These oxygenated complexes are thermally unstable, and they disappeared after warming to room temperature in the case of the T samples, but they are transformed to O2 - on the surface of the hydrothermally treated TiO2. Since adsorbed water and different types of free hydroxyls are present on these materials, as revealed by FTIR, a number of surface reactions have to be considered in order to account for the formation and stability of such photogenerated species.

Heterogeneous photocatalysis is a process of great potential for pollutant abatement and waste treatment. In order to improve the overall performance of the photoprocess, heterogeneous photocatalysis is being combined with physical or chemical operations, which affect the chemical kinetics and/or the overall efficiency. This review addresses the various possibilities to couple heterogeneous photocatalysis with other technologies to photodegrade organic and inorganic pollutants dissolved in actual or synthetic aqueous effluents. These combinations increase the photoprocess efficiency by decreasing the reaction time in respect to the separated operations or they decrease the cost in respect of heterogeneous photocatalysis alone, generally in terms of light energy. Depending on the operation coupled with heterogeneous photocatalysis, two categories of combinations exist. When the coupling is with ultrasonic irradiation, photo-Fenton reaction, ozonation, or electrochemical treatment, the combination affects the photocatalytic mechanisms thus improving the efficiency of the photocatalytic process. When the coupling is with biological treatment, membrane reactor, membrane photoreactor, or physical adsorption, the combination does not affect the photocatalytic mechanisms but it improves the efficiency of the overall process. The choice of the coupling is related to the type of wastewater to be treated. A synergistic effect, giving rise to an improvement of the efficiency of the photocatalytic process, has been reported in the literature for many cases.