TiO2 photocatalysis and related surface phenomena

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

The remarkable achievement by Fujishima and Honda (1972) in the photo-electrochemical water splitting results in the extensive use of TiO 2 nanomaterials for environmental purification and energy storage/conversion applications. Though there are many advantages for the TiO 2 compared to other semiconductor photocatalysts, its band gap of 3.2 eV restrains application to the UV-region of the electromagnetic spectrum ( λ  ≤ 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 TiO 2 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.

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Visible-light activation of TiO2 photocatalysts: Advances in theory and experiments

This paper reviews recent studies on the semiconductor photocatalysis based on surface-modified TiO2 of which application is mainly focused on environmental remediation. TiO2 photocatalysis that is based on the photoinduced interfacial charge transfer has been extensively studied over the past four decades. A great number of modification methods of semiconductor photocatalysts have been developed and investigated to accelerate the photoconversion, to enable the absorption of visible light, or to alter the reaction mechanism to control the products and intermediates. In this regard, various modification methods of TiO2 are classified according to the kind of surface modifiers (metal-loading, impurity doping, inorganic adsorbates, polymer coating, dye-sensitization, charge transfer complexation) and their effects on photocatalytic reaction mechanism and kinetics are discussed in detail. Modifying TiO2 in various ways not only changes the mechanism and kinetics under UV irradiation but also introduces visible light activity that is absent with pure TiO2. Each modification method influences the photocatalytic activity and mechanism in a way different from others and the observed modification effects are often different depending on the test substrates and conditions even for the same modification method. Better understanding of the modification effects on TiO2 photocatalysis is necessary to obtain reliable results, to assess the photoconversion efficiency more quantitatively, and to further improve the modification methods.

Surface modification of TiO2 photocatalyst for environmental applications

Photocatalytic water treatment: solar energy applications

Diffuse reflectance and transmission flash photolysis have been used to study the primary reaction intermediates in titanium dioxide (TiO{sub 2}) photosensitized reactions of potassium iodide, 2,4,5-trichlorophenol, methyl viologen dichloride, potassium tetrachloroplatinum(II), tris(1,10-phenanthroline)-iron(II) perchlorate, N,N,N{prime},N{prime}-tetramethyl-p-phenylenediamine, and thianthrene. In the cases where the product of OH radical mediated oxidation is different from that of direct electron transfer oxidation, only products of direct electron transfer oxidation are observed. The products of direct electron transfer and OH radical mediated oxidation of iodide and tetrachloroplatinum are identical. The transient decay kinetics in TiO{sub 2} powder systems appear to be first order with a Gaussian distribution of reaction rate constants reflecting the distribution of particle radii.

Titanium dioxide photosensitized reactions studied by diffuse reflectance flash photolysis in aqueous suspensions of TiO2 powder

Diffuse reflectance flash photolysis has been used to study the kinetics of (SCN){sub 2}{sup {sm bullet}{minus}} decomposition in aqueous suspensions of titanium dioxide (Degussa P-25). (SCN){sub 2}{sup {sm bullet}{minus}} is formed when SCN{sup {sm bullet}}, from oxidation of thiocyanate, SCN{sup {minus}}, by photoexcited TiO{sub 2}, reacts with excess SCN{sup {minus}}. The transient absorption spectra of (SCN){sub 2}{sup {sm bullet}{minus}} obtained on transparent colloidal TiO{sub 2} or on TiO{sub 2} powder suspended in water have maxima at 480 nm and similar absorption band shapes. A two-parameter kinetic model, developed by Albery to describe kinetics in several heterogeneous systems, is applied for the first time to diffuse reflectance measurements. The kinetics of (SCN){sub 2}{sup {sm bullet}} decomposition as a function of laser intensity, KSCN concentration, pH, and ionic strength ({mu}) have been investigated by using this model.

Titanium dioxide photooxidation of thiocyanate: (SCN)2.cntdot.- studied by diffuse reflectance flash photolysis

Pulse radiolytic studies of the reaction of pentahalophenols with OH radicals: formation of pentahalophenoxyl, dihydroxypentahalocyclohexadienyl, and semiquinone radicals

Pulse radiolysis has been used to optically and kinetically characterize the transient free radicals formed by the reaction of H{sup {sm bullet}}, N{sub 3}{sup {sm bullet}}, or OH{sup {sm bullet}} with 2,4,5-trichlorophenol (TCP-OH). The hydroxy-2,4,5-trichlorocyclohexadienyl radical (H-TCP-OH) formed by H{sup {sm bullet}} addition to TCP-OH (k = (1.1 {plus minus} 0.2) {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1}) has an absorption maximum at 360 nm with {epsilon}{sub 360} = (4700 {plus minus} 1000) M{sup {minus}1} cm{sup {minus}1}. The 2,4,5-trichlorophenoxyl radical (TCP-O{sup {sm bullet}}) formed by the reaction of N{sub 3}{sup {sm bullet}} with TCP-O- (k = (4.3 {plus minus} 0.8) {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1}) has an absorption maximum at 430 nm with {epsilon}{sub 430} = (3600 {plus minus} 600) M{sup {minus}1} cm{sup {minus}1}. The dihydroxy-2,4,5-trichlorocyclohexadienyl radical (HO-TCP-OH) formed by OH{sup {sm bullet}} radical addition to TCP-OH (k = (1.2 {plus minus} 0.1) {times} 10{sup 10} M{sup {minus}1} s{sup {minus}1}) has an absorption maximum at 320 nm with {epsilon}{sub 320} = (5300 {plus minus} 250) M{sup {minus}1} cm{sup {minus}1}. The principal isomer formed by the reaction of OH{sup {sm bullet}} with TCP-OH is the 1,6-dihydroxy-2,4,5-trichlorocyclohexadienyl radical, which has a pK{sub a} of 4.8 {plus minus} 1.0. The solutionmore » absorption from each radical decays via second-order kinetics on the millisecond time scale.« less

Pulse radiolysis of 2,4,5-trichlorophenol: Formation, kinetics, and properties of hydroxytrichlorocyclohexadienyl, trichlorophenoxyl, and dihydroxytrichlorocyclohexadienyl radicals

Both direct spectroscopic observation of intermediates formed on irradiated semiconductor surfaces and product analysis from inter- and intramolecular competition experiments with multifunctional organic substrates implicate direct interfacial electron transfer from an adsorbed organic donor as a primary process in photocatalytic oxidation reactions. Time-resolved diffuse reflectance spectroscopy is used to measure kinetic profiles for the decay of photogenerated transients on opaque, photocatalytically active metal oxide powders. The implications of such studies in the mechanistic characterization of photocatalytic detoxification are considered.

R. Barton Draper

Papers

Titanium dioxide photosensitized reactions studied by diffuse reflectance flash photolysis in aqueous suspensions of TiO2 powder

Titanium dioxide photooxidation of thiocyanate: (SCN)2.cntdot.- studied by diffuse reflectance flash photolysis

Pulse radiolytic studies of the reaction of pentahalophenols with OH radicals: formation of pentahalophenoxyl, dihydroxypentahalocyclohexadienyl, and semiquinone radicals

Pulse radiolysis of 2,4,5-trichlorophenol: Formation, kinetics, and properties of hydroxytrichlorocyclohexadienyl, trichlorophenoxyl, and dihydroxytrichlorocyclohexadienyl radicals

Control of Photocatalytic Oxidative Selectivity on Irradiated TiO2 Powders: A Diffuse Reflectance Kinetic Study