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

TiO2 photocatalysis and related surface phenomena

Jenny Schneider,*,† Masaya Matsuoka,‡ Masato Takeuchi,‡ Jinlong Zhang, Yu Horiuchi,‡ Masakazu Anpo,‡ and Detlef W. Bahnemann*,† †Institut fur Technische Chemie, Leibniz Universitaẗ Hannover, Callinstrasse 3, D-30167 Hannover, Germany ‡Faculty of Engineering, Osaka Prefecture University, 1 Gakuen-cho, Sakai Osaka 599-8531, Japan Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China

Understanding TiO2 photocatalysis: mechanisms and materials.

The photocatalytic degradation of azo dyes containing different functionalities has been reviewed using TiO2 as photocatalyst in aqueous solution under solar and UV irradiation. The mechanism of the photodegradation depends on the radiation used. Charge injection mechanism takes place under visible radiation whereas charge separation occurred under UV light radiation. The process is monitored by following either the decolorization rate and the formation of its end-products. Kinetic analyses indicate that the photodegradation rates of azo dyes can usually be approximated as pseudo-first-order kinetics for both degradation mechanisms, according to the Langmuir–Hinshelwood model. The degradation of dyes depend on several parameters such as pH, catalyst concentration, substrate concentration and the presence of electron acceptors such as hydrogen peroxide and ammonium persulphate besides molecular oxygen. The presence of other substances such as inorganic ions, humic acids and solvents commonly found in textile effluents is also discussed. The photocatalyzed degradation of pesticides does not occur instantaneously to form carbon dioxide, but through the formation of long-lived intermediate species. Thus, the study focuses also on the determination of the nature of the principal organic intermediates and the evolution of the mineralization as well as on the degradation pathways followed during the process. Major identified intermediates are hydroxylated derivatives, aromatic amines, naphthoquinone, phenolic compounds and several organic acids. By-products evaluation and toxicity measurements are the key-actions in order to assess the overall process.

TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations A review

Fluorescent carbon nanoparticles or carbon quantum dots (CQDs) are a new class of carbon nanomaterials that have emerged recently and have garnered much interest as potential competitors to conventional semiconductor quantum dots. In addition to their comparable optical properties, CQDs have the desired advantages of low toxicity, environmental friendliness low cost and simple synthetic routes. Moreover, surface passivation and functionalization of CQDs allow for the control of their physicochemical properties. Since their discovery, CQDs have found many applications in the fields of chemical sensing, biosensing, bioimaging, nanomedicine, photocatalysis and electrocatalysis. This article reviews the progress in the research and development of CQDs with an emphasis on their synthesis, functionalization and technical applications along with some discussion on challenges and perspectives in this exciting and promising field.

Carbon quantum dots and their applications

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 O2 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.

Semiconductor-mediated photodegradation of pollutants under visible-light irradiation

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.

Efficient Degradation of Toxic Organic Pollutants with Ni2O3/TiO2-xBx under Visible Irradiation

Visible-Light-Induced Aerobic Oxidation of Alcohols in a Coupled Photocatalytic System of Dye-Sensitized TiO2 and TEMPO

Imine derivatives are important building blocks for the synthesis of fine chemicals and pharmaceuticals. Generally, their synthesis involves condensation of an amine and a carbonyl compound. To circumvent the problem caused by the excessively active nature of ketones or aldehydes, an alternative strategy for the direct oxidation of amines has attracted much interest. Dioxygen, or more preferably dioxygen in the air, embodies the quintessential oxidant for chemical synthesis. Unfortunately, the reactivity of dioxygen is difficult to control; it typically reacts under harsh conditions with poor selectivity. Recently, much attention has been paid to organic synthesis photocatalyzed by TiO2. [3] In H2O, the selectivity of TiO2 photocatalysis is especially poor, so that, it is efficient in the degradation of organic pollutants. Even when the reactions are performed in inert organic solvent to prevent generation of OH radicals, the selectivity is usually very low owing to the unselective autooxidation of the photogenerated radicals. However, it has been shown that high selectivity can be realized in the TiO2 photocatalytic system when both generation of the OH radical and unselective autooxidation are avoided. By using O-labeling experiments, our research group has discovered that an oxygen-atom transfer dominates the photocatalytic transformation of alcohols into the corresponding carbonyl compounds on TiO2 in benzotrifluoride. This transformation has a different mechanism than that of noble-metal/transition-metal complex catalysis. All the experimental results support a mechanism as shown in Equation (1). Such a mechanism avoids the unselective autooxidation processes, which commonly occur in TiO2 photocatalysis in the presence of dioxygen, and hence should lead to high selectivity in the photocatalytic oxidation of alcohols. This mechanistic insight can direct us to envision new reactions. Herein, we report our findings on a series of benzylic amines that were selectively transformed into the corresponding imines using 1 atm of air as the oxidant by TiO2 photocatalysis in acetonitrile. The formation of imines involves a two-step process: a selective oxygenation step to generate the aldehydes and a subsequent condensation step to afford the imines. The high selectivity for the formation of imines is attributed to both the highly selective formation of aldehydes from amines [Eq. (2)] via a similar mechanism to

Selective Formation of Imines by Aerobic Photocatalytic Oxidation of Amines on TiO2

Several TiO2-based photocatalytic systems that have considerable visible light response have been developed, such as the photodegradation of organic pollutants on sensitized TiO2 by visible light, construction of visible-light-active novel TiO2 photocatalysts by matrix or surface modification. In this paper, we review briefly our recent progress in the TiO2 photocatalytic degradation of organic pollutants by visible light, some related work by other groups is also involved.

Chuncheng Chen

Papers

Semiconductor-mediated photodegradation of pollutants under visible-light irradiation

Efficient Degradation of Toxic Organic Pollutants with Ni2O3/TiO2-xBx under Visible Irradiation

Visible-Light-Induced Aerobic Oxidation of Alcohols in a Coupled Photocatalytic System of Dye-Sensitized TiO2 and TEMPO

Selective Formation of Imines by Aerobic Photocatalytic Oxidation of Amines on TiO2

Photocatalytic Degradation of Organic Pollutants Under Visible Light Irradiation