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 photocataly tic 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, 1,2,4trichlorobenzene lies flat. The adsorption of 4-chlorophenol is accompanied by a decrease in the TÍO2 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 visible light photocatalytic activity of TiO₂ for 4-chlorophenol decomposition. It is also found that the molecule is stable against thermal decomposition at temperatures of up to ∼473 K, which is well above the temperature range used for photocatalytic decomposition studies.