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 0.92 ML (1 ML ≈ 5.2 × 10¹⁴ 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 10⁴L 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.