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Photosensitized singlet oxygen and its applications

A surface science perspective on TiO2 photocatalysis

Nitrogen-doped titanium dioxide as visible-light-sensitive photocatalyst: designs, developments, and prospects.

An overview on limitations of TiO2-based particles for photocatalytic degradation of organic pollutants and the corresponding countermeasures

This feature article summarizes recent research on and development of semiconductor-based photocatalyst materials that are applicable to environmental remediation and/or chemical synthesis purposes. A wide variety of TiO2 particles and/or films have been studied during the past 30 years because they are the most stable and powerful photocatalysts leading to the degradation of various organic pollutants. The photocatalytic performance of other semiconductor materials such as ZnO, SnO2, WO3, Fe2O3 and CdS has also been intensively investigated. A general limitation in the efficiency of any photocatalytic process is the recombination of the photogenerated charge carriers, i.e., of electrons and holes, following bandgap illumination. Considerable efforts have been made to suppress this recombination and hence to enhance the charge carrier separation and the overall efficiency by means of coupling of different semiconductors with desirable matching of their electronic band structures, or incorporation of noble metal nanoclusters onto the surface of semiconductor photocatalyst particles. Modification of the physicochemical properties, such as particle size, surface area, porosity and/or crystallinity of the semiconductor materials, and optimization of the experimental conditions, such as pH, illumination conditions and/or catalyst loading, during photocatalytic reactions have also been carefully addressed to achieve high reaction rates or yields. To utilize solar energy more efficiently, i.e., to extend the optical absorption of the mostly UV-sensitive photocatalysts into the visible light range, numerous research groups have contributed to the development of novel visible light active photocatalysts. With the application of semiconductors with narrower bandgaps such as CdS, Fe2O3 and WO3 being straightforward choices, doping of wide bandgap semiconductors like TiO2 has been the most popular technique to enhance the catalysts' optical absorption abilities. Research on mixed-oxide-based semiconductor photocatalysts with deliberately modulated band structures has also attracted tremendous attention in the past decade, concentrating on, for example, the generation of H2 and/or O2 from H2O splitting, and the degradation of organic pollutants under visible light irradiation. Both theoretical calculations and experimental results have convincingly shown that the developed materials can serve as highly efficient photocatalysts that are both environmentally and economically significant.

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Photoelectrocatalytic materials for environmental applications

The commercially available TiO2-catalyst (Degussa P25) was modified with nanosized platinum and silver particles by the photoreduction method to obtain different noble metal loading (0.5 and 1 wt.%). The characterization of the synthesized catalysts was carried out by the BET method, XPS, TEM and the adsorption of the model pollutant. The degradation of oxalic acid has been studied in aqueous solution photocatalyzed by band-gap-irradiated TiO2, modified with nanosized platinum or silver particles. The photocatalytic activity of TiO2, modified with noble metal, is approximately double that of the semiconducting support. The adsorption properties of the catalysts, as well as the noble metal content on the surface of the support, influence the efficiency of the photocatalytic process. The reaction rate of photocatalytic degradation of the oxalic acid follows a zero kinetic order according to the Langmuir–Hinshelwood model. The increase of the quantum yield of the photodestruction reaction of the studied model pollutant is due to the formation of Schottky barriers on the metal–semiconductor interface, which serve as efficient electron traps, preventing the electron–hole recombination.

Photocatalytic properties of TiO2 modified with platinum and silver nanoparticles in the degradation of oxalic acid in aqueous solution

The commercially available TiO2-photocatalyst (Degussa P25) was modified with nanosized gold particles by the photoreduction method at four different values of the pH factor of the medium. The characterization of the synthesized catalysts was carried out by the BET method, X-ray photoelectron spectroscopy (XPS), TEM and the adsorption of the model pollutant. A remarkable influence of the pH on the particle size of Au was registered upon investigating the catalysts by XPS and TEM methods. The size of the gold nanoparticles on the TiO2 surface decreases with increase in the pH of the medium. The degradation of oxalic acid has been studied in aqueous solution photocatalyzed by band-gap-irradiated TiO2, modified with nanosized gold particles. The photocatalytic activity of TiO2, modified with gold, was found to increase with the decrease of the size of the gold nanoparticles on the surface of the photocatalytic material. The maximal value of the photocatalytic activity (twice higher than that of the semiconducting support) is registered in the case of gold photoreduction at pH 7. The adsorption properties of the catalysts, as well as the size of the noble metal nanoparticles on the surface of the support, influence the efficiency of the photocatalytic process. The reaction rate of photocatalytic degradation of the oxalic acid follows a zero kinetic order according to the Langmuir–Hinshelwood model. The increase of the quantum yield of the photodestruction reaction of the studied model pollutant is due to the formation of Schotky barriers on the metal-semiconductor interface, which serve as efficient electron traps, preventing the electron–hole recombination.

Influence of the size of gold nanoparticles deposited on TiO2 upon the photocatalytic destruction of oxalic acid

Water soluble zinc (II) 2,9,16,23-phthalocyanine tetrasulfonic acid [ZnPc(SO 3 H) 4 ], zinc (II) 2,9,16,23-phthalocyanine tetracarboxylic acid [ZnPc(COOH) 4 ] and sterically hindered bulky zinc(II) 2,9,16,23-tetrakis(8-quinolineoxy-5-sulfonicacid)-phthalocyanine [ZnPc(QnSO 3 H) 4 ] complexes have been studied as photocatalysts for oxidation of 2-mercaptoethanol and sodium thiosulfate. The ZnPc(QnSO 3 H) 4 complex exhibited the highest photocatalytic activity due to its low aggregation degree in aqueous medium. In the presence of tetrabutylammonium chloride, quinoline and ethanol additives the monomerization degree of the ZnPc(SO 3 H) 4 and ZnPc(COOH) 4 complexes is increased, their photocatalytic activity being close to that of ZnPc(QnSO 3 H) 4 . Because of high aggregation degree of ZnPc(SO 3 H) 4 and ZnPc(COOH) 4 in viscous medium, the quantum yields of singlet dioxygen generation and the photocatalytic oxidation rates of the sulfur-containing compounds are decreased. The photocatalytic activity of ZnPc(QnSO 3 H) 4 is slightly changed in viscous medium and in the presence of additives. The low aggregation degree of this complex allowed to establish enhanced and lowered rates of photocatalytic oxidation of 2-mercaptoethanol and sodium thiosulfate, respectively, on increasing medium viscosity. The effect of medium viscosity on ZnPc(QnSO 3 H) 4 photocatalyzed oxidation of the two substrates is discussed.

Effect of metal phthalocyanine complex aggregation on the catalytic and photocatalytic oxidation of sulfur containing compounds

The photooxidation of Na2S or Na2S2O3, catalyzed by Al2O3, TiO2 or WO3 superficially modified with metal-free phthalocyanine complex (Pc), cobalt phthalocyanine complex (CoPc) and copper phthalocyanine complex (CuPc), has been studied upon irradiation with visible light. The rates of the substrate photooxidation, catalyzed by the phthalocyanine complexes supported on TiO2 or WO3, are much higher than those registered with the same complexes, anchored on Al2O3. The high photocatalytic activity of the samples is explained by an electron transfer from the conduction band of the excited phthalocyanine semiconductor particles to the conduction band of the TiO2 or WO3 supports. The increase of the quantum yield of the photocatalytic redox process comes as a result of the additional formation of superoxide radicals on the TiO2 or WO3 conduction bands. A complete oxidation of Na2S or Na2S2O3 to sulfates has been registered in the presence of the studied photocatalysts.

Phthalocyanine modified TiO2 or WO3-catalysts for photooxidation of sulfide and thiosulfate ions upon irradiation with visible light

The photocatalytic oxidation of phenols by oxygen has been studied in alkaline aqueous medium upon irradiation with visible light. Metal Zn(II) and Al(III) mono- and polynuclear phthalocyanine complexes, soluble in water, have been used as photocatalysts. The studied polynuclear complexes represent two-dimensional polymers, formed of condensed phthalocyanine macrocycles. The number of the phthalocyanine units in them is 3 or 4. No visible batochromic shifting is observed in the Q-band electron transition ( λ =665–680 nm) of the polynuclear complexes with respect to their mononuclear analogues. This fact is an indication of a low π–π electron delocalization between the phthalocyanine units of the polynuclear complexes in the excited state. The photocatalytic activity per mole of the polynuclear complexes at pH=7 or 13 is much higher than that of the respective mononuclear phthalocyanine complexes. The high photocatalytic activity of the polymers might be explained by the proceeding of an intra-molecular triplet–triplet energy transfer between the phthalocyanine macrocycles in accordance with the mechanism of Dexter. The singlet oxygen, obtained by photon-induced energy transfer from the excited mono- and polynuclear photosensitizers, dominates the initial steps of photooxidation. The rate of photocatalytic oxidation of phenols depends on the pH of the medium and on the phthalocyanine complex aggregation degree. The products of the oxidation of phenols in alkaline medium are the same in the both cases of using mono- and polynuclear phthalocyanine complexes as catalysts. The formation of quinones is an intermediate step during the photocatalytic oxidation of phenols at pH=13 within the process of destruction of the substrate. Maleic and fumaric acids as well as carbon dioxide have been registered as final products of the photooxidation.

L. Bilyarska

Papers

Photocatalytic properties of TiO2 modified with platinum and silver nanoparticles in the degradation of oxalic acid in aqueous solution

Influence of the size of gold nanoparticles deposited on TiO2 upon the photocatalytic destruction of oxalic acid

Effect of metal phthalocyanine complex aggregation on the catalytic and photocatalytic oxidation of sulfur containing compounds

Phthalocyanine modified TiO2 or WO3-catalysts for photooxidation of sulfide and thiosulfate ions upon irradiation with visible light

Photooxidation of phenols in aqueous solution, catalyzed by mononuclear and polynuclear metal phthalocyanine complexes