Interactions of 4-Chlorophenol with TiO2 Polycrystalline Surfaces: A Study of Environmental Interfaces by NEXAFS, XPS, and UPS

Despite a significant body of literature on the photocatalytic decomposition of 4-chlorophenol by TiO2 at liquid/solid and gas/solid interfaces, a fundamental understanding of the interaction of 4-chlorophenol with TiO2 is lacking. We present the first study of this interaction under well-defined UHV conditions by means of NEXAFS, time-dependent XPS, and UPS. XPS data show that the molecule adsorbs with the carbon framework intact and no scission of the C−Cl bond. The NEXAFS results indicate a coverage-dependent tilted geometry for the adsorbed molecule that is attached to the surface via a phenolate link. In contrast, because of the absence of an OH functionality, 1,2,4-trichlorobenzene lies flat. The adsorption of 4-chlorophenol is accompanied by a decrease in the TiO2 work function pointing to adsorbate → substrate charge transfer. Interestingly, the UPS data suggest that 4-chlorophenol adsorption leads to the photosensitization of TiO2, thus providing a basis for understanding recent results on the vi...