Photoinduced reactivity of titanium dioxide

This paper summarizes recent research dealing with development of titanium dioxide (TiO2) used for environmental applications. TiO2 plays the most important role owing to its excellent chemical and physical properties. However, the TiO2 band edge lies in the UV region that makes them inactive under visible irradiation. In this regard, considerable efforts have been made to increase the visible light activity of TiO2 via the modification of its electronic and optical properties. Doping TiO2 using either anions or cations is one of the typical approaches that has been largely applied. Coupling TiO2 with a narrow bad gap semiconductor (MxOy/TiO2 or MxSy/TiO2) represents another approach. This work aims to encompass the new progress of TiO2 for an efficient application in water and wastewater treatment under visible light, emphasizes the future trends of TiO2 in the environment, and suggests new research directions, including preparation aspects for the development of this promising material.

Modified TiO2 For Environmental Photocatalytic Applications: A Review

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 photocatalytic activity of materials for water splitting is limited by the recombination of photogenerated electron–hole pairs as well as the back-reaction of intermediate species. This review concentrates on the use of electric fields within catalyst particles to mitigate the effects of recombination and back-reaction and to increase photochemical reactivity. Internal electric fields in photocatalysts can arise from ferroelectric phenomena, p–n junctions, polar surface terminations, and polymorph junctions. The manipulation of internal fields through the creation of charged interfaces in hierarchically structured materials is a promising strategy for the design of improved photocatalysts.

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Photocatalysts with internal electric fields

Defects are critically important for metal oxides in chemical and physical applications. Compared with the often studied oxygen vacancies, engineering metal vacancies in n-type undoped metal oxides is still a great challenge, and the effect of metal vacancies on the physiochemical properties is seldom reported. Here, using anatase TiO2, the most important and widely studied semiconductor, we demonstrate that metal vacancies (VTi) can be introduced in undoped oxides easily, and the presence of VTi results in many novel physiochemical properties. Anatase Ti0.905O2 was synthesized using solvothermal treatment of tetrabutyl titanate in an ethanol–glycerol mixture and then thermal calcination. Experimental measurements and DFT calculations on cell lattice parameters show the unstoichiometry is caused by the presence of VTi rather than oxygen interstitials. The presence of VTi changes the charge density and valence band edge of TiO2, and an unreported strong EPR signal at g = 1.998 presents under room temperatu...

Titanium-Defected Undoped Anatase TiO2 with p-Type Conductivity, Room-Temperature Ferromagnetism, and Remarkable Photocatalytic Performance

The present study discusses the effect of iron doping in thin films deposited by rf sputtering. Iron doping induces a structural transformation from anatase to rutile and electrical measurements indicate that iron acts as an acceptor impurity. Thermoelectric power measurement shows a transition between n-type and p-type electrical conduction for an iron concentration around 0.13 at.%. The highest p-type conductivity at room temperature achieved by iron doping was .

https://iopscience.iop.org/article/10.1088/0022-3727/31/10/004/pdf

Structural and electrical properties of Fe-doped thin films

TiO2 thin films were prepared by direct current (dc) reactive sputtering, using various kinds of supports such as glass, silicon, alumina, and glass coated with indium–tin oxide Samples were characterized by X-ray diffraction (XRD), and atomic force microscopy (AFM) Different TiO2 − x stoichiometries, crystal structures and morphologies were obtained by changing the parameters of the reactive gas The photocatalytic properties of the samples were tested on the degradation of phenol The best efficiency in respect with phenol mineralization was obtained for samples prepared using an Ar–H2O mixture as the reactive gas Indium–tin oxide supports provide the most efficient thin films The catalytic efficiency per unit area of the sputtered films is at least one order of magnitude better than that of a 50 m2 g−1, reference TiO2 powder

Photocatalytic degradation of phenol by TiO2 thin films prepared by sputtering

This paper reports on an investigation on fcc TiO1+x thin films with 0<x<1. The films were deposited by r.f. reactive sputtering and characterized by X-ray diffraction, electron probe microanalysis, X-ray photoelectron spectroscopy, atomic force microscopy, scanning tunneling microscopy, and electrical measurements. The films crystallized in the fcc phase with a lattice parameter a=0.419 nm, exhibit a gold like color, an electrical resistivity of about 400 μΩ cm at room temperature, and remarkable nanohardness values of about 23 GPa. The results of these experiments are discussed and compared to the archetypal fcc TiN coatings.

Mechanical and electrical properties of fcc TiO1+x thin films prepared by r.f. reactive sputtering

High rate and process control of reactive sputtering by gas pulsing: the Ti O system

Cerium-doped TiO2 thin films have been prepared by reactive RF sputtering. At low Ce concentration, X-ray diffraction indicates that the films have the anatase structure. Ce concentrations higher than 1.2 at.% result in an amorphization of the film which remains stable up to 873 K. The TiO2 electrical properties have been stabilized and improved by cerium doping, resulting in a lower conductivity (10-9 Ω-1m-1), a higher electrical breakdown strength (2 ×107 V/m), and a high value of the permittivity (45±5). The implementation of amorphous TiO2:Ce thin films as insulator layers in ZnS:Mn alternating current thin film electroluminescent devices (ACTFELD) results in a significant drop in the threshold operating voltage and a notable increase in the device brightness compared with ACTFELD containing Y2O3 or BaTa2O6 insulator layers. Rapid thermal annealing further improves the performance of the electroluminescent device.

A.R Bally

Papers

Structural and electrical properties of Fe-doped thin films

Photocatalytic degradation of phenol by TiO2 thin films prepared by sputtering

Mechanical and electrical properties of fcc TiO1+x thin films prepared by r.f. reactive sputtering

High rate and process control of reactive sputtering by gas pulsing: the Ti O system

Ce-doped TiO 2 Insulators in Thin Film Electroluminescent Devices