Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

Photoinduced reactivity of titanium dioxide

The interest in heterogeneous photocatalysis is intense and increasing, as shown by the number of publications on this theme which regularly appear in this journal, and the fact that over 2000 papers have been published on this topic since 1981. This article is an overview of the field of semiconductor photocatalysis : a brief examination of its roots, achievements and possible future. The semiconductor titanium dioxide (TiO 2 ) features predominantly in past and present work on semiconductor photocatalysis; as a result, in the most of the examples selected in this overview to illustrate various points the semiconductor is TiO 2 .

An overview of semiconductor photocatalysis

4.3. Reaction Mechanism 2373 4.4. Asymmetric Synthesis 2374 4.5. Outlook 2374 5. Alternating Polymerization of Oxiranes and CO2 2374 5.1. Reaction Outlines 2374 5.2. Catalyst 2376 5.3. Asymmetric Polymerization 2377 5.4. Immobilized Catalysts 2377 6. Synthesis of Urea and Urethane Derivatives 2378 7. Synthesis of Carboxylic Acid 2379 8. Synthesis of Esters and Lactones 2380 9. Synthesis of Isocyanates 2382 10. Hydrogenation and Hydroformylation, and Alcohol Homologation 2382

Transformation of carbon dioxide.

The absolute energy positions of conduction and valence band edges were compiled for about 50 each semiconducting metal oxide and metal sulfide minerals. The relationships between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a catalyst or photocatalyst in aqueous redox reactions are reviewed. The compilation of band edge energies is based on experimental flatband potential data and complementary empirical calculations from electronegativities of constituent elements. Whereas most metal oxide semiconductors have valence band edges 1 to 3 eV below the H2O oxidation potential (relative to absolute vacuum scale), energies for conduction band edges are close to, or lower than, the H2O reduction potential. These oxide minerals are strong photo-oxidation catalysts in aqueous solutions, but are limited in their reducing power. Non-transition metal sulfides generally have higher conduction and valence band edge energies than metal oxides; therefore, valence band holes in non-transition metal sulfides are less oxidizing, but conduction band electrons are exceedingly reducing. Most transition-metal sulfides, however, are characterized by small band gaps (<1 eV) and band edges situated within or close to the H2O stability potentials. Hence, both the oxidizing power of the valence band holes and the reducing power of the conduction band electrons are lower than those of non-transition metal sulfides.

The absolute energy positions of conduction and valence bands of selected semiconducting minerals

Several commercial and homemade TiO{sub 2} samples were used as photocatalysts to prove that the physicochemical features as determined by the origin and preparation methods affect the photocatalytic behavior, in addition to the semiconducting properties. The photodegradation of aqueous phenol solution in TiO{sub 2} dispersion, carried out in a batch reactor, was used as a test reaction. The results show a great variability of photocatalytic behavior, and in particular it has been observed that the rutile phase is active or inactive according to the preparation conditions.

Influence of the preparation methods of titanium dioxide on the photocatalytic degradation of phenol in aqueous dispersion

The influence of deposited silver upon the photocatalytic activity of titania in the rutile and anatase allotropic forms has been studied in three different reactions. The common feature found for these three reactions was an electron transfer from illuminated TiO 2 to silver particles. In the first reaction (platinum photodeposition). it has been shown that the initial presence of metallic silver orientates the localization of subsequent Pt photodeposits with, in particular, a 100% selectivity to Pt deposition on top of silver particles or agglomerates previously deposited on anatase. In the second reaction (2-propanol oxidation), Ag deposit was found beneficial for the activity of rutile and detrimental for that of anatase. For rutile which is less active, silver helps for the electron-hole pair dissociation. By contrast, for anatase, the negatively charged Ag particles preferentially attract photoholes and become recombination centers, thus decreasing the photocatalytic activity. For the third reaction (dehydrogenation of 2-propanol), Ag/TiO 2 catalysts were found very poorly active, as expected for a group I-B metal. However, in the additional presence of platinum deposits, Pt/Ag/TiO 2 catalysts are as active as their Pt/TiO 2 homologues, thus confirming that Pt is deposited on top of silver with good electronic contacts between both metals. The three photocatalytic reactions have common electronic processes based on the photoelectron generation on titania and the subsequent electron transfer to the metal (s).

Influence of metallic silver and of platinum-silver bimetallic deposits on the photocatalytic activity of titania (anatase and rutile) in organic and aqueous media

The results of a study of photocatalytic degradation of phenol using aqueous oxygenated TiO2 (anatase) suspensions in a batch Pyrex photoreactor are reported The influence on the photodegradation rate of various parameters as pH, phenol and TiO2 content, oxygen partial pressure, anions present in the dispersions was investigated A complete oxidation of phenol was observed Intermediate compounds, catechol and quinone, were detected It was observed that the photodegradation also proceeded with sunlight radiation A mechanistic and kinetic model, which accounts for the results obtained, is given Likely reasons for inactivity of the rutile modification for this reaction are also given

Photocatalytic degradation of phenol in aqueous titanium dioxide dispersions

The production of ammonia from dinitrogen and water vapor at mild conditions of temperature and pressure on Fe{sup 3+}-doped titanium dioxide powders under UV radiation has been studied in a continuous photoreactor working in the gas-solid regime. A net activity decline was observed after a few hours of irradiation; this decline did not depend on the reactor temperature, the powder composition, or the amounts of produced ammonia. The highest activity is found when no excess iron is segregated at the surface; overall turnover numbers for dinitrogen reduction as high as six electrons per iron atom can then be reached before powder deactivation, showing the catalytic character of the participation of Fe in this process. An IR investigation of the active and spent specimens revealed that the irradiation determines the almost complete disappearance of the OH groups from the powder surface. Furthermore, an ESR study of all the powders showed that bulk Fe{sup 3+} ions are better electron traps than Ti{sup 4+} ions so that, when a UV photon generates a hole-electron pair, the electron can be stabilized on the iron ions.

Dinitrogen photoreduction to ammonia over titanium dioxide powders doped with ferric ions

The photoreduction of dinitrogen to ammonia in a gas-solid regime and the photodegradation of phenol in an aqueous liquid-solid regime were investigated by using a series of chromium-ion-containing titania specimens. For the same photoreactions, two iron-ions-containing titania specimens were also studied for comparison. For the photoreduction reaction it was found that the presence of the dopants is essential for the occurrence for the photoreactivity, the iron-ions-containing specimens being more effective than those containing chromium ions. For the photooxidation reaction the presence of the dopants instead is not essential; the photoreactivity of titania is not affected or, in some cases, detrimentally affected. The two contrasting behaviors are explained by taking into account the differences in the gas-solid and liquid-solid interfaces.

Antonino Sclafani

Papers

Influence of the preparation methods of titanium dioxide on the photocatalytic degradation of phenol in aqueous dispersion

Influence of metallic silver and of platinum-silver bimetallic deposits on the photocatalytic activity of titania (anatase and rutile) in organic and aqueous media

Photocatalytic degradation of phenol in aqueous titanium dioxide dispersions

Dinitrogen photoreduction to ammonia over titanium dioxide powders doped with ferric ions

Activity of chromium-ion-doped titania for the dinitrogen photoreduction to ammonia and for the phenol photodegradation