Titanium dioxide surface charge

The direct charge transfer to dichloroacetate proposed in reaction (7.21) requires that the scavenging molecules are adsorbed on the Ti02 surface prior to the adsorption of the photon. Otherwise, this reaction could not compete with the normal hole-trapping reactions (7.9) and (7.10). So the adsorption of the model compound DCA on the titanium dioxide surface prior to the bandgap excitation appears to be a prerequisite for an efficient hole scavenging. 

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...

Surface polarity can also be independently evaluated by physical means. deMayo and coworkers have assigned surface polarity of silica gel particles by observing shifts in the absorption spectra of absorbed spiropyrans which are sensitive to solvent polarity . Darwent and coworkers have shown that kinetic salt effects follow surface charge on colloidal titanium dioxide and, with zeta potential measurements, that surface area and charge could be separately evaluated... 

If so, one may expect products to result from chemical bond formation between the cation-radical-anion-radical pair, which are both paramagnetic and of opposite charge. In the latter route, there is a precedent for the formation of dioxetane intermediates of stable olefin cation radicals [51], as in the characterization by Nelsen and coworkers of a dioxetane cation radical from adamantylidene cation radical [52]. If a dioxetane is formed, either in neutral form or as a cation radical, the Ti02 surface can function in an additional role, that is, as a Lewis acid catalyst, to induce decomposition of the dioxetane. Since no chemiluminescence could be observed in these reactions, apparently Lewis acid catalysis provides a nonradiative route for cleavage of this high-energy intermediate. That Ti02 can indeed function in this way can be demonstrated by independent synthesis of the dioxetane derived from 1,1-diphenylethylene, which does indeed decompose to benzophenone when it is stirred in the dark on titanium dioxide. 

Physical techniques for evaluating surface polarity led deMayo and coworkers to assign relative rates of reaction on silica gel particles from shifts in the absorption spectra of absorbed spiropyrans [76, 77]. Similarly, Darwent and coworkers demonstrated that kinetic salt effects correlate with surface charge and with zeta potential measurements on colloidal titanium dioxide [80]. 

Ortyl TT, Peck GE. Surface charge of titanium dioxide and its effect on dye adsorption and aqueous suspension stability. Drug Dev Ind Pharm 1991 17 2245-2268. 

Materials 4-Chlorophenol, 4-chlororesorcinol, hydroquinone, and hy-droxyhydroquinone were purchased from Aldrich Chemical Co., Milwaukee, WI 4-chlorocatechol was purchased from TCI American, Portland, OR. Acetonitrile, hexane, methylene chloride, methanol, and pyridine were obtained from the Fisher Scientific Co., Fulton, CA. The photocatalyst was Degussa P-25 titanium dioxide [mainly anatase form surface area 50 m2/g pHzpc (pH at zero point of charge) 6.3 contains some impurities such as Al203 (<0.3%), Si02 (<0.2%), Fe203 (<0.01%), and HCl (<0.3%)]. 

Chadwick, M.D. et al.. Surface charge properties of colloidal titanium dioxide in ethylene glycol and water. Colloids Surf. A, 203, 229, 2002. 

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