The semiconductor and the surface-adsorbed antenna molecule (dyes or other color species) can constitute efficient photochemical systems for environmental remediation. The major advantage of these systems is that they are able to achieve the degradation of organic pollutants by using visible light from the sun as energy and O(2) in the air as the oxidant under ambient conditions. In this tutorial review, the unique mechanistic characteristics, the constitution of photodegradation systems and their performance are described. The involved radical reactions during the degradation are also discussed.

To promote efficient use of solar energy, many studies have focused on the modification of TiO2 to extend its spectral response to visible region. Here we report a combined modification of TiO2 by two components: the nonmetal element boron and the metal oxide Ni2O3. The photocatalyst presents high photocatalytic activity in the visible region, which can efficiently degrade and mineralize toxic organic pollutants such as trichlorophenol (TCP), 2,4-dichlorophenol (2,4-DCP), and sodium benzoate. The dechlorination and mineralization results indicate the photocatalytic pathway via visible light excitation. The study demonstrates that the modification of TiO2 both to extend its spectral response to the visible region and to improve its catalytic efficiency can be achieved by doping with boron, a nonmetal, and Ni2O3, a metal oxide.

Surface-fluorinated TiO2 (F-TiO2) particles were prepared via the HF etching method. The surface characteristics of fluorinated TiO2, the adsorption modes of dyes, and the reaction pathways for the photocatalytic degradation of dye pollutants under visible light irradiation were investigated. It was found that, in the treatment of TiO2 by HF etching, F(-) not only displaces surface HO(-) but also substitutes some surface lattice oxygen. Using zwitterionic Rhodamine B (RhB) dye as a model, the change of the adsorption mode of RhB on F-TiO2 relative to that on pure TiO2 was validated by adsorption isotherms, X-ray photoelectron spectroscopy (XPS), and IR techniques for the first time. RhB preferentially anchors on pure TiO2 through the carboxylic (-COOH) group, while its adsorption group is switched to the cationic moiety (-NEt 2 group) on F-TiO2. Both the photocatalytic degradation kinetics and mechanisms were drastically changed after surface fluorination. Dyes with positively charged nitrogen-alkyl groups such as methylene blue (MB), malachite green (MG), Rhodamine 6G (Rh6G), and RhB all underwent a rapid N-dealkylation process on F-TiO2, while on pure TiO2 direct cleavage of dye chromophore ring structures predominated. The relationship between surface fluorination and the degradation rate/pathway of dyes under visible irradiation was also discussed in terms of the effect of fluorination on the surface adsorption of dyes and on the energy band structure of TiO2.

An uninvited guest: In the photocatalytic oxidation of alcohols to their corresponding aldehydes or ketones using TiO2 in organic solvents, such as benzotrifluoride (BTF), an oxygen atom transfer from the dioxygen to the α‐carbon atom of the alcohol dominates the reaction process.

Like a differential gear: Upon irradiation with visible light, excited dye molecules inject electrons into TiO2 and form dye radicals, which can oxidize TEMPO to TEMPO+, which in turn oxidizes alcohols in the presence of O2 selectively to corresponding aldehydes. One reason for the selectivity is that the irradiation is downshifted to visible light.

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The effect of metal ions (Cu 2+ , Fe 3+ , Zn 2+ , Al 3+ , and Cd 2+ ) on the photodegradation of several dyes: sulforhodamine B (SRB), alizarin red (AR), and malachite green (MG) has been investigated in aqueous TiO 2 dispersions under visible irradiation (λ > 420 nm). Trace quantities of transition metal ions such as Cu 2+ and Fe 3+ having suitable redox potentials alter the electron-transfer pathway involving the dye, O 2 and TiO 2 particles, and markedly depress the photodegradation of all three dyes under visible irradiation. Other metal ions, such as Zn 2+ , Cd 2+ , and Al 3+ , have only a slight influence on the photoreaction by altering the adsorption of dyes. Photogeneration of H 2 O 2 and reactive radicals, and the changes in fluorescence emission of SRB in TiO 2 aqueous dispersions were examined to elucidate the role of the metal ions. Addition of Cu 2+ or Fe 3+ decreases the reduction of O 2 by the conduction electrons, subsequently blocks the formation of reactive oxygen species (O -• , • OH), and depresses the degradation of dyes under visible irradiation. We deduce that the reduction of O 2 is essential for the photodegradation of dyes under visible irradiation.

Phosphate modified TiO2 photocatalysts were prepared by phosphoric acid treatment before or after TiO2 crystallization. Substrates with different structures were chosen to explore the photocatalytic activity of as-modified TiO2 under UV irradiation. It was found that the effect of phosphate modification is definitely attributed to the surface-bound phosphate anion, and the modification by phosphate can affect both the rates and pathways of photocatalytic reactions, which are of great dependence on the structures and properties of substrates. The degradation of substrates (such as 4-chloropehenol, phenol, and rhodamine B) with weak adsorption on the pure TiO2 was markedly accelerated by phosphate modification, while substrates (such as dichloroacetic acid, alizarin red, and catechol) with strong adsorption exhibited a much lower degradation rate in the phosphate modified system. A much higher amount of hydroxyl radical was produced in phosphate modified system. All of the experimental results imply that phosphate modification largely accelerates the hydroxyl radical attack, but hinders the direct hole oxidation pathway. A common operating mechanism for the phosphate modification, which can be applicable to other inert anions, is also discussed from the viewpoint of an anion-induced negative electrostatic field in the surface layer of TiO2 and the hydrogen bond between modification anion and H2O molecule.