Interactions of HCOOH with stoichiometric and defective TiO2(110) surfaces

Interactions of HCOOH with stoichiometric (nearly defect-free) and defective TiO2(110) surfaces have been studied experimentally using X-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and theoretically using electronic structure calculations. The HCOOH saturation coverages were 0.58 ML. 0.77 ML, and 0.97 ML (1 ML approximate to 5.2 x 10(14) cm(-2)) for nearly defect-free surfaces, for electron-beam exposed surfaces, and for Ar+ ion bombarded surfaces, respectively, The excess formic acid adsorption quantitatively corresponds to the number of newly exposed sites created by electron-beam exposure. Electronic structure calculations show a strong adsorptive interaction for formate on cation sites on both stoichiometric and defective TiO2 surfaces, consistent with the experimental observations. In spite of adsorption at defect sites, little or no defect healing (defect healing means a reduction in defect signal observed by the photoemission measurements) was observed for either electron-beam exposed or Ar+ bombarded surfaces by HCOOH exposure up to 10(4) L at room temperature. However some healing: will occur if extra energy provided by electrons is introduced to breakdown formate species. In contrast to water adsorption, electronic structure calculations on defective TiO2 have found that formate is located in an asymmetric position with respect to the Ti3+ sites with a potential additional interaction with the Ti4+ site. (C) 1997 Elsevier Science B.V.