UV and Visible Light-Driven Production of Hydroxyl Radicals by Reduced Forms of N, F, and P Codoped Titanium Dioxide (original) (raw)
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Mechanism of the OH Radical Generation in Photocatalysis with TiO2 of Different Crystalline Types
Journal of Physical Chemistry C, 2014
The difference in the OH radical generation through photocatalytic reaction with distinct crystalline types of TiO 2 in aqueous suspension was explored by means of a fluorescence probe method. By employing two kinds of probe molecules with different adsorptivities, that is, coumarin and coumarin-3-carboxylic acid, we could detect OH radicals located near the TiO 2 surface and those in the bulk solution individually. The amount of the OH radicals near the TiO 2 surface was much larger than that in the solution, which diffused from the TiO 2 surface to the bulk solution. Rutile TiO 2 produced a much smaller amount of OH radicals as compared to anatase and anatase-contained TiO 2. On the addition of H 2 O 2 , the OH radical generation for pure anatase TiO 2 decreased but increased for rutile and rutile-contained TiO 2. This difference could be explained as follows: at the surface of anatase TiO 2 , trapped holes become adsorbed OH radicals, while at the rutile surface, Ti-peroxo (Ti-OO-Ti) produced by the combination of two trapped holes which correspond to the adsorbed H 2 O 2 , could work as a catalyst for the OH radical generation from water.
Abstract In this study the efficiency of different bare, doped and composite photocatalysts were compared, under UV and visible light irradiation in order to show a detailed picture of the relative performance of the best photocatalysts developed in our laboratories and the mostly investigated reference titanias. The syntheses of our photocatalysts were optimized in order to achieve maximum photocatalytic activity under UV and visible light irradiation. Non doped commercial (Aeroxide P25, Aldrich anatase) and synthesized titanias (produced by sol-gel and flame hydrolysis techniques) and nitrogen, iron, iodine doped and silver or gold deposited titanium dioxides were investigated with two model pollutants (phenol and oxalic acid) under identical experimental conditions. The material properties of these selected photocatalysts were thoroughly characterized by X-ray diffraction, diffuse reflectance spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray fluorescence spectroscopy and BET methods. The highest degradation rate of phenol was determined for the flame made titania sample with relatively low specific surface area (20 m2/g) when UV irradiation was applied. In contrast with that, our nitrogen doped photocatalyst with high specific surface area (139 m2/g) was the best for phenol degradation under visible light irradiation. Although the most efficient oxalic acid mineralization occurred with noble metal photodeposited samples under UV irradiation, this type of modification was detrimental when VIS irradiation is applied. The decomposition rate of oxalic acid was maximum under VIS irradiation using the iron and nitrogen doped photocatalysts. For both substrates and irradiation conditions our best photocatalysts were found to be significantly more active than Aeroxide P25 TiO2. Intermediate studies revealed that phenol degradation resulted in dihydroxy benzene intermediates, such as pyrocatechol and hydroquinone both under UV and visible light irradiation with our TiO2-N photocatalyst. The results of this comparative study could promote the determination of the optimal synthesis conditions of titanium dioxide based photocatalysts for a given organic pollutant in water.
A review on the visible light active titanium dioxide photocatalysts for environmental applications
Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO2) semiconductor materials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for a wide range of environmental and energy applications. One of the most significant scientific and commercial advances to date has been the development of visible light active (VLA) TiO2 photocatalytic materials. In this review, a background on TiO2 structure, properties and electronic properties in photocatalysis is presented. The development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed. Emphasis is given to the origin of visible light absorption and the reactive oxygen species generated, deduced by physicochemical and photoelectrochemical methods. Various applications of VLA TiO2, in terms of environmental remediation and in particular water treatment, disinfection and air purification, are illustrated. Comprehensive studies on the photocatalytic degradation of contaminants of emerging concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, cyanotoxins and volatile organic compounds, with VLA TiO2 are discussed and compared to conventional UV-activated TiO2 nanomaterials. Recent advances in bacterial disinfection using VLA TiO2 are also reviewed. Issues concerning test protocols for real visible light activity and photocatalytic efficiencies with different light sources have been highlighted.
Synthesis of Oxygen Deficient TiO2 for Improved Photocatalytic Efficiency in Solar Radiation
2021
The photocatalytic activities of TiO2 have been limited mainly to absorbing in the ultraviolet spectrum which accounts for only 5% of solar radiation. High energy band gap and electron recombination in TiO2 nanoparticles are responsible for its limitations as a photocatalyst. An oxygen deficient surface can be artificially created on the titanium oxide by zero valent nano iron through the donation of its excess electrons. A visible light active TiO2 nanoparticle was synthesized in the current investigation through simple chemical reduction using sodium boro-hydride. The physical and textural properties of the synthesized oxygen deficient TiO2 photocatalyst was measured using scanning/ transmission electron microscopy while FTIR, XRD and nitrogen sorption methods (BET) were employed for its further characterizations. Photochemical decoloration of orange II sodium dye solution in the presence of the synthesized TiO2 was measured using an UV spectrophotometer. The resulting oxygen defi...
Influence of water temperature on the photocatalytic activity of titanium dioxide
Reaction Kinetics, Mechanisms and Catalysis, 2012
In this study, it was shown that a possible explanation of increasing photocatalytic activity with temperature may be the fact that with increasing water temperature, the amount of hydroxyl radicals in water also increases, because the ionic product of water increases with an increase in temperature. For measurements of the amount of hydroxyl radicals, the fluorescence technique was used. Terephthalic acid was used as a hydroxyl radical scavenger. After inducing of TiO 2 , positive holes in the valance band may react with OH ions and produce • OH radicals, a strong oxidizing agent.
Materials Science in Semiconductor Processing, 2015
Research into the development of solar and visible light active photocatalysts has been significantly increased in recent years due to its wide range of applications in treating contaminants of emerging concern (CECs), endocrine disrupting compounds (EDCs), bacteria and cyanotoxins. Solar photocatalysis is found to be highly effective in treating a wide range of CECs from sources such as pharmaceuticals, steroids, antibiotics, phthalates, disinfectants, pesticides, fragrances (musk), preservatives and additives. Similarly, a number of EDCs including polycyclic aromatic hydrocarbons (PAHs), alkylphenols (APs), bisphenol A (BPA), organotins (OTs), volatile organic compounds (VOCs), natural and synthetic estrogenic and androgenic chemicals, pesticides, and heavy metals can be removed from contaminated water by using solar photocatalysis. Photocatalysis was also found effective in treating a wide range of bacteria such as Staphylococcus aureus, Bacillus subtilis, Escherichia coli, Salmonella typhi and Micrococcus lylae. The current review also compares the effectiveness of various visible light activated TiO2 photocatalysts for treating these pollutants. Doping or co-doping of TiO2 using nitrogen, nitrogensilver, sulphur, carbon, copper and also incorporation of graphene nano-sheets are discussed. The use of immobilised TiO2 for improving the photocatalytic activity is also presented. Decorating titania photocatalyst with graphene oxide (GO) is of particular interest due to GO's ability to increase the photocatalytic activity of TiO2. The use GO to increase the photocatalytic activity of TiO2 against microcystin-LR (MC-LR) under UV-A and solar irradiation is discussed. The enhanced photocatalytic activity of GO-TiO2 compared to the control material is attributed to the effective inhibition of the electron-hole recombination by controlling the interfacial charge transfer process. It is concluded that there is a critical need for further improvement of the efficiency of these materials if they are to be considered for bulk industrial use.
J Mod Green Energy , 2024
The term "photocatalysis" has recently gained high popularity, and various products using photocatalytic functions have been commercialized. Of all the materials that may be used as photocatalysts, titanium dioxide (TiO 2) is virtually the only one that is now and most likely will remain appropriate for industrial application. Water and air purification systems, sterilization, hydrogen evolution, self-cleaning surfaces, and photoelectrochemical conversion are just a few of the products and applications in the environmental and energy domains that make extensive use of TiO 2 photocatalysis. This is due to the fact that TiO 2 has the lowest cost, most stability, and most effective photoactivity. Furthermore, history attests to its safety for both people and the environment because it has been used as a white pigment since antiquity. This review discusses some important aspects and issues concerning different synthesis methods and their influence on the structure and properties of TiO 2 , as well as the concept of photocatalysis based on it as a promising biocompatible functional material that has been widely used in recent years. The advantages of TiO 2 applications in various fields of science and technology are discussed, including environmental protection, photocatalysis including self-cleaning surfaces, water and air purification systems, hydrogen liberation, photovoltaic energy, cancer diagnosis and therapy, coatings and dental products, etc. Information on the structure and properties of TiO 2 phases is presented, as well as modern methods of synthesizing functional materials based on it. A detailed review of the basic principles of TiO 2 photocatalysis is then given, with a brief introduction to the modern concept of TiO 2 photocatalysis. Recent advances in the fundamental understanding of TiO 2 photocatalysis at the atomic-molecular level are highlighted, and advances in TiO 2 photocatalysis from the perspective of design and engineering of new materials are discussed. The challenges and prospects of TiO 2 photocatalysis are briefly discussed.
Evaluating the Mechanism of Visible Light Activity for N,F-TiO2 Using Photoelectrochemistry
The improvement of the solar efficiency of photocatalytic materials is important for solar driven environmental remediation and solar energy harvesting applications. Photoelectrochemical characterization of nitrogen and fluorine codoped titanium dioxide (N,FTiO2) was used to probe the mechanism of visible light activity. The spectral photocurrent response under visible irradiation did not correlate with the optical absorption spectrum of the N,F-TiO2; however, open-circuit photopotential measurements provided better correlation to the optical absorption spectra. These observations suggest that electrons excited to the conduction band from the N-induced midgap state are rapidly trapped by defect levels below the conduction band. Reactive oxygen species (ROS) can be produced via the reduction of molecular oxygen by conduction band electrons leading to the oxidative degradation of organic pollutants, and singlet oxygen may play a role. If there is no loss in the band gap activity, as compared to undoped titania, then any additional visible light activity may give an overall improvement in the solar efficiency. The photocurrent response should not be used as a direct measure of photocatalytic activity for doped titania as the oxygen reduction pathway is vitally important for the generation of ROS, whereas hole transfer from dopant midgap states may not be so critical.
Journal of Catalysis, 2003
This review deals with the preparation of highly reactive titanium oxide photocatalysts and the clarification of the active sites as well as the detection of the reaction intermediates at the molecular level. Furthermore, we discuss the advancement of photofunctional systems and processes that can utilize visible and/or solar light. The photocatalytic reactivity of semiconducting TiO 2 powder was found to be dramatically enhanced by the loading of small amounts of Pt, which work to enhance the charge separation of the electrons and holes generated by light irradiation. Highly dispersed titanium oxide species prepared within zeolite frameworks or silica matrices showed unique photocatalytic performance much higher than that of conventional semiconducting TiO 2 photocatalysts. The potential for the effective utilization and conversion of solar energy makes research into the modification of the electronic properties of TiO 2 photocatalysts by such methods as advanced metal ion implantation to produce photocatalysts which are able to absorb and operate efficiently even under visible light irradiation one of the most important fields in photocatalysis research. This modification process can be applied not only to semiconducting TiO 2 photocatalysts but also to TiO 2 thin film photocatalysts, as well as titanium oxide photocatalysts highly dispersed within zeolite frameworks. Significantly, a new alternative method for directly preparing such visible-light-responsive TiO 2 thin film photocatalysts has been successfully developed by applying a RF magnetron sputtering deposition method.