Photocatalysis Involving a Visible Light-Induced Hole Injection in a Chromate(VI)–TiO 2 System (original) (raw)
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Visible-light activation of TiO2 photocatalysts: Advances in theory and
Though there are many advantages for the TiO2 compared to other semiconductor photocatalysts, its band gap of 3.2 eV restrains application to the UV-region of the electromagnetic spectrum ( ≤ 387.5 nm). As a result, development of visible-light active titanium dioxide is one of the key challenges in the field of semiconductor photocatalysis. In this review, advances in the strategies for the visible light activation, origin of visible-light activity, and electronic structure of various visible-light active TiO2 photocatalysts are discussed in detail. It has also been shown that if appropriate models are used, the theoretical insights can successfully be employed to develop novel catalysts to enhance the photocatalytic performance in the visible region. Recent developments in theory and experiments in visible-light induced water splitting, degradation of environmental pollutants, water and air purification and antibacterial applications are also reviewed. Various strategies to identify appropriate dopants for improved visible-light absorption and electron–hole separation to enhance the photocatalytic activity are discussed in detail, and a number of recommendations are also presented.
Environmental Science & Technology, 2004
A representative polyoxometalate, R-12-tungstophosphatic acid (PW 12 3-, POM), is loaded on the surface of TiO 2 particles used as a cocatalyst to gain further insights into the underlying reaction mechanism and to estimate the feasibility of using the new POM/TiO 2 cocatalyst in the photocatalytic degradation of 2,4-dichlorophenol (DCP) in aqueous media. Loading the PW 12 3species on the surface of TiO 2 enhances charge separation in the UVilluminated TiO 2 , thereby accelerating the hydroxylation of the initial DCP substrate but not the mineralization of DCP, which is somewhat suppressed in the presence of the polyoxometalate. An increase in the load of POM increases the concentration of aromatic intermediates, and more toxic intermediates, such as 2,6-dichlorodibenzo-p-dioxin, 2,4,6-trichlorophenol, are detected in the PW 12 3-/TiO 2 system. By contrast, cleavage of the whole conjugated structure of DCP predominates in TiO 2 only dispersions. Strong ESR signals for the superoxide radical anionic species, O 2 •-(HO 2 • radicals in acidic media; pH < 5), are detected in TiO 2 only dispersions; signals of O 2 •are much weaker in the TiO 2 / POM composite system under otherwise identical conditions. Experimental results infers that enhancement of charge separation in TiO 2 photocatalysis does not always result in improvement of the efficiency of mineralization of organic substrates, and the reaction between organic radical cations and the formed superoxide radical anions may be responsible for the mineralization of the chlorophenol.
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.
Titania is one of the most widely used benchmark standard photocatalysts in the field of environmental applications. However, the large band gap of titania and massive recombination of photogenerated charge carriers limit its overall photocatalytic efficiency. The former can be overcome by modifying the electronic band structure of titania including various strategies like coupling with a narrow band gap semiconductor, metal ion/nonmetal ion doping, codoping with two or more foreign ions, surface sensitization by organic dyes or metal complexes, and noble metal deposition. The latter can be corrected by changing the surface properties of titania by fluorination or sulfation or by the addition of suitable electron acceptors besides molecular oxygen in the reaction medium. This review encompasses several advancements made in these aspects, and also some of the new physical insights related to the charge transfer events like charge carrier generation, trapping, detrapping, and their transfer to surface are discussed for each strategy of the modified titania to support the conclusions derived. The synergistic effects in the mixed polymorphs of titania and also the theories proposed for their enhanced activity are reported. A recent venture on the synthesis and applications of anatase titania with a large percentage of reactive {001} facets and their band gap extension to the visible region via nonmetal ion doping which is a current hot topic is briefly outlined.
Photoanodic reactions occurring at nanostructured titanium dioxide films
Electrochimica Acta, 2006
The reasons of unusually large differences observed in photocurrent efficiencies for the oxidation of various organic and inorganic substrates at nanostructured TiO 2 photoelectrodes are discussed. The "redox cycling", where a product of the hole transfer acts, in turn, as scavenger for the photogenerated electrons, appears as a frequent cause of weak photocurrents. Such a recombination mechanism operates not only under open-circuit conditions but also in the presence of large anodic bias applied to the conducting support of the TiO 2 electrode. Experiments conducted in the presence of both, an efficient hole scavenger, formate ion, and an electron acceptor, methyl viologen dication, showed that, in a major part of the anodically polarized nanostructured TiO 2 film, the quasi-Fermi level of electrons remains actually close to the conduction band edge potential. Importantly, addition to the solution of an electron acceptor causes large drop of the photocurrent both under weak and intense UV illumination generated by an argon-ion laser. On the other hand, a large number of organic molecules undergoing essentially irreversible photooxidation (e.g., to form CO 2 and H 2 O) generate high photocurrents at the nanostructured TiO 2 electrodes. Such reactions, when occurring in the presence of large concentrations of the reactants, may involve direct hole transfer.
Titanium-Dioxide-Based Visible-Light-Sensitive Photocatalysis: Mechanistic Insight and Applications
Catalysts
Titanium dioxide (TiO2) is one of the most practical and prevalent photo-functional materials. Many researchers have endeavored to design several types of visible-light-responsive photocatalysts. In particular, TiO2-based photocatalysts operating under visible light should be urgently designed and developed, in order to take advantage of the unlimited solar light available. Herein, we review recent advances of TiO2-based visible-light-sensitive photocatalysts, classified by the origins of charge separation photo-induced in (1) bulk impurity (N-doping), (2) hetero-junction of metal (Au NPs), and (3) interfacial surface complexes (ISC) and their related photocatalysts. These photocatalysts have demonstrated useful applications, such as photocatalytic mineralization of toxic agents in the polluted atmosphere and water, photocatalytic organic synthesis, and artificial photosynthesis. We wish to provide comprehension and enlightenment of modification strategies and mechanistic insight, a...
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.