Titanium dioxide electron transfer

A mechanism which has been proposed for the operation of this type of photocell is illustrated in Figure 10.7, although it is not fully established in detail. In the proposed mechanism, it is suggested that absorption of light by the dye (or sensitiser, S) raises the dye to its first excited state S. In the excited state, S releases an electron into the conducting band of the titanium dioxide electrode, at the same time forming oxidised sensitiser, S +. At the counter-electrode, an electron is transferred to the... 

Minero, C., Mariella, G., Maurino, V., and Pelizzetti, E. (2000) Photocatalytic transformation of organic compounds in the presence of inorganic anions. 1. Hydroxyl-mediated and direct electron-transfer reactions of phenol on a titanium dioxide-fluoride system. Langmuir,... 

The attention devoted to supramolecular sensitizers containing multifold chromophoric and electroactive centers arises from the construction of molecular devices based on nanometric and well-defined molecular architectures [4]. The use of these species for sensitization of titanium dioxide has provided fundamental insights into interfacial electron-transfer processes. 

Intercalation of cations into a framework of titanium dioxide is a process of wide interest. This is due to the electrochromic properties associated with the process (a clear blue coloration results from the intercalation) and to the system s charge storage capabilities (facilitated by the reversibility of the process) and thus the potential application in rocking-chair batteries. We have studied alkali-metal intercalation and ion diffusion in the Ti02 anatase and spinel crystals by theoretical methods ranging from condensed-phase ab initio to semiempirical computations [65, 66]. Structure relaxation, electron-density distribution, electron transfer, diffusion paths and activation energies of the ion intercalation process were modeled. 

Therefore, it is reasonable to conclude that upon irradiation of titanium dioxide under oxygen, the electron transfer from water to the positive hole mainly results in hydroxylation of the aromatic... 

Titanium dioxide photocatalyzed oxidation of neat tetralin was previously reported to give its hydroperoxide (14). Reinvestigation showed that tetralol and tetralone are also formed in acetonitrile possibly through electron transfer from tetralin to the positive holes (27). 

Keywords electron transfer, heterogeneous catalysis, photochemistry, photoelectrochemistry, Titanium dioxide... 

This paper concentrates on a detailed description of the primary events occurring immediately after the absorption of a photon within a single titanium dioxide particle in an aqueous environment. This restriction was made, because 1) titanium dioxide seems to be the most active photocatalyst, and 2) the photocatalytic treatment of polluted water seems to be a promising application for an interfacial electron transfer serving the environment. 

In most experiments and applications with titanium dioxide photocatalysts, molecular oxygen is present to act as the primary electron acceptor. Usually the electrons trapped as Ti(III) are transferred to dioxygen adsorbed at the semiconductor surface yielding peroxyl radical anions (reaction (7.16)) [16]. 

Interfacial hole transfer dynamics from titanium dioxide (Degussa P 25) to SCN has been investigated by Colombo and Bowman using femtosecond time-resolved diffuse reflectance spectroscopy [6c]. A dramatic increase in the population of trapped electrons was observed within the first few picoseconds, demonstrating that interfacial charge transfer of an electron from the SCN" to a hole on the photoexcited titanium dioxide effectively competes with electron-hole recombination (reactions (7.12) - (7.15)) on an ultrafast time scale [6c]. 

The strongest evidence for direct hole oxidation as the principal step in the photooxidation step comes from a recent study performed by Draper and Fox that failed to detect any of the expected intermediate hydroxyl radical adducts following diffuse reflectance flash photolysis of several titanium dioxide/substrate combinations [33]. In each case where the product of hydroxyl radical-mediated oxidation was known to be different from that of direct electron transfer oxidation, the authors observed only the products of the direct electron-transfer oxidation. 

Still in the electron transfer field, a useful benzylation procedure is based on the heterogeneous sensitization by titanium dioxide. In this case, methylbenzenes, benzylsilanes and phenylacetic acids are used as donors and electron-withdrawing substituted alkenes have the double role of... 

Figure 7.17 Modes of Ti02 photosensitization (a) photosensitization with organic or inorganic chromophores chemisorbed onto titanium dioxide surface (b) formation of surface complexes exhibiting metal-to-band charge transfer transitions (MBCT) (c) bulk doping resulting in formation of acceptor or donor levels and (d) formation of composite semiconductors. A denotes the electron acceptor, D the electron donor...

Another way of carrying out electron-transfer mediated oxidation reactions is to use semiconductors as catalysts (Mozzanega et al., 1977). Titanium dioxide will, photocatalyse the oxidation of substituted toluenes to benz-aldehydes by electron transfer from toluene into the photogenerated hole. The electron in the conduction band will reduce oxygen giving the superoxide anion. Reaction of the superoxide anion with the hydrocarbon radical cation produces the aldehyde. A similar mechanism has been used to explain the observation that dealkylation of Rhodamine B (which contains N-ethyl groups) occurs when the dye is irradiated in the presence of cadmium sulphide (Watanabe et al., 1977). 

Typically, the rate of simple (outer-sphere) electron-transfer reactions, such as Fe(CN)e + e - Fe(CN)6 , is much slower at titanium dioxide than at metallic electrodes . This is consistent both with the flat shape of the voltammetric peaks in Fig. 2 and their shift to more negative potentials with increasing the sweep rate. The kinetics of the cathodic reactions at Ti02 appear to be markedly affected by the co-adsorption of some anions from the supporting electrolyte. The phosphate ions are not unique to cause such effects arsenate, fluoride and certainly other anions are expected to act in a similar way. 

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