Charge separation characteristics of a high-activity, mixed-phase titania photocatalyst (Degussa P25) are probed by EPR spectroscopy. While previous proposals consider rutile as a passive electron sink hindering recombination in anatase, this research details the critical and active role of rutile in TiO2 formulations. The inactivity of pure-phase rutile is due in part to rapid rates of recombination. Yet, in mixed-phase TiO2, charges produced on rutile by visible light are stabilized through electron transfer to lower energy anatase lattice trapping sites. These results suggest that within mixed-phase titania (P25) there is a morphology of nanoclusters containing atypically small rutile crystallites interwoven with anatase crystallites. The transition points between these two phases allow for rapid electron transfer from rutile to anatase. Thus, rutile acts as an antenna to extend the photoactivity into visible wavelengths and the structural arrangement of the similarly sized TiO2 crystallites creates catalytic “hot spots” at the rutile−anatase interface.

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The adsorptive behavior of TiO2 under various illumination conditions was investigated. In the dark, Degussa P25, which contains both the anatase and rutile phases of TiO2, adsorbed significantly greater amounts of 2,4,5-trichlorophenol (TCP) than either pure anatase or pure rutile. Results in the literature and in our laboratory that appeared to indicate "photoenhanced adsorption" under ambient fluorescent lighting on P25 and anatase are in fact due to the photoreaction of TCP. On pure-phase anatase, all reactions are due to trace ultra-band-gap energy light present in the fluorescent lighting. On P25, however, reaction also occurs at sub-band-gap energies. TCP forms a charge-transfer complex with P25 that is activated by light wavelengths as long as 520 nm. The trichlorophenoxyl radicals resulting from chargetransfer couple with each other to form a suite of polyaromatic chlorinated products with detected masses as high as 1200 D. These products are not formed under UV irradiation and in fact are destroyed by conventional UV photocatalysis. No reaction occurs on pure-phase anatase or rutile as a consequence of irradiation with sub-band-gap light. Carbon mass balance was closed for all catalysts under all lighting conditions. Our results show that the wavelength of light is an important factor in determining products on P25. This knowledge can be used to avoid charge-transfer complex activation when the resulting products are undesirable. Alternatively, charge-transfer complexes on P25 may be exploitable for polymer syntheses. The differences between surface reactions on P25 and those on pure-phase TiO 2 may be explained in terms of the morphology of Degussa P25, wherein anatase-rutile interfaces give rise to active sites.