Li-Qiong Wang

The adsorption of liquid and vapor water on TiO2(110) surfaces: the role of defects

Abstract The adsorption of liquid and vapor water on defective and nearly defect-free TiO 2 (110) surfaces has been studied using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). The study focuses on examining electronic defects as created in vacuum and after exposure to both liquid and vapor water. Defective surfaces were prepared by electron-beam exposure and Ar + bombardment. With exposure up to 10 4 L low vapor pressure ( −5 Torr) water to defective surfaces, little change on Ti3d defect intensity was observed. However, defect intensities were greatly reduced after exposing defective surfaces to ∼ 10 8 L higher vapor pressure (0.2–0.6 Torr) water for 5 min. More significantly, XPS and UPS spectra showed that electron-beam induced defects were completely removed upon liquid water exposure, while defects created by Ar + bombardment were only partially removed. Surface defects created by Ar + bombardment were removed more readily than sub-surface defects. Water adsorption on the surface has been quantified using the OH signal from the O 1s photopeak. For a nearly defect-free surface, water coverage was ∼ 0.02 ML at 10 4 L exposure to low vapor pressure water, ∼ 0.07 ML at 10 8 L exposure to higher vapor pressure water, and ∼ 0.125 ML with liquid water exposure, respectively.

Creation of variable concentrations of defects on TiO2(110) using low-density electron beams

Low density (~μAcm2) 0.48 and 1.0 keV electron beams have been used to create surface defects on a TiO2(110) surface. These electron-beam induced defects were examined primarily by X-ray photoelectron spectroscopy (XPS) with supporting ultraviolet photoemission spectroscopy (UPS). Glancing and normal emission XPS spectra of nearly defect-free surfaces revealed that Ti atoms on the surface were similar to the bulk Ti, while some surface oxygen atoms were different from the bulk oxygen. XPS of Ti 2p32 was used to quantify the defect concentration and to examine the defect electronic structure. Based on our calculation of defect concentrations and the comparison of our results with results and models from the literature, we conclude that oxygen vacancies induced by electron beams in the current study are mostly from the bridging oxygen sites, in agreement with the previous work. A range of defect concentrations with similar electronic structure, mainly composed of Ti3+, have been induced by low-density electron beams. Beam energy and exposure were the experimental variables. The rates of defect formation at low beam exposure were beam-energy dependent, with a faster growth rate at 0.48 keV than at 1.0 keV. These defects were similar to those by thermal annealing in vacuum, but a higher concentration of defects could be obtained with longer beam exposure. However, the e-beam induced defects were different from those produced by Ar+ ion bombardment since both this and previous studies have found defects produced by Ar+ ion bombardment to be complex, with a variety of different local environments where oxygen and titanium surface atoms coexist.

Comparative second harmonic generation and X-ray photoelectron spectroscopy studies of the UV creation and O2 healing of Ti3+ defects on (110) rutile TiO2 surfaces

SHG studies on polished and etched (110) rutile TiO2 single-crystal surfaces suggest that above band gap, low-energy (4.7 eV) ultraviolet photons create stable Ti3+ surface defects in UHV. Such defects can be healed by subsequently exposing the surface to O2 gas. These results are similar to recently reported measurements on polished and etched (001) rutile single-crystal surfaces. Observations on the (001) surfaces were interpreted as the photodesorption and re-adsorption of molecular oxygen bound loosely to Ti3+ defects on the surface as a Ti4+:O−2 complex. For the current (110) study, XPS was used to confirm the defect type and to quantify the density of Ti3+ defects created. Defects on the surfaces studied using XPS were generated using photons of even lower energy (3.4 eV), indicating that the oxygen species removed was very loosely bound. The same defect creation and healing processes were also observed on a nearly defect-free thermally annealed single-crystal surface using XPS. Whether bridging oxygen ions on the (110) surface or O−2 ions are the photo-labile species has yet to be determined. Transient photodesorption of O2 at high oxygen pressure also was observed using SHG. SHG has proven to be a sensitive probe of surface defects, consistent with XPS results, under conditions ranging from UHV to atmospheric.

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

Abstract 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.

Chemical sensors based on dielectric response of functionalized mesoporous silica films

Dielectric response of mesoporous silica films was monitored as a function of several gas-phase chemical species. The effects of humidity, ammonia, and methane on dielectric constant and dissipation factor of films subjected to different chemical treatments are described. Dielectric constant and dissipation factor of partially dehydroxylated films were found to be highly sensitive to both water vapor and ammonia in air. The capacitive devices based on mesoporous silica films show potential for use in chemical sensors.

ELECTRONIC STRUCTURE CALCULATIONS OF SMALL MOLECULE ADSORBATES ON (110) AND (100) TIO2

Electronic structure calculations were performed for water and methanol adsorbed onto model structures for ideal (110) and (100) rutile TiO2 surfaces. Both molecular and dissociated forms of water and methanol were examined for each surface, and the reaction coordinate for interconversion of the two forms. Our results indicate a strong thermodynamic favor for the dissociative form in each case; however, there is a potential energetic barrier to proton transfer to generate the dissociative form for the (110) surface. Based upon the structural differences between these surfaces, it is proposed that multiple hydrogen bonding interactions with the adsorbed species facilitate proton transfer.

Interactions of methanol with stoichiometric and defective TiO2(110) and (100) surfaces

Interactions of CH3OH with stoichiometric (nearly defect-free) and defective TiO2(110) and (100) surfaces have been studied using x-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy. The CH3OH saturation coverage was increased by increasing the number of defects created by electron-beam exposure or Ar+ ion bombardment. A small percentage of any defects produced were healed upon the saturation exposure (defect healing means a reduction in defect signal observed by the photoemission measurements). The structural influence on the adsorption and surface defect reactivity was found to be less significant for CH3OH than for H2O. The CH3OH coverages at a given exposure and defect reactivity were comparable for both the (100) and (110) surfaces.

Comparative SHG and XPS studies of interactions between defects and N2O on rutile TiO2(110) surfaces

Abstract In previous studies on TiO 2 (110) surfaces O 2 has been shown to heal Ti 3+ defects generated by above band-gap UV or low-energy electron-beam exposure. Electron affinity, rather than oxygen content may dictate whether a molecule will interact with a defect to result in defect healing. This paper examines interactions between TiO 2 (110) surfaces and N 2 O which has a substantial electron affinity. N 2 O is seen to heal defects at comparable exposures to those needed for O 2 healing. XPS indicates that N 2 O heals defects by dissociating such that no nitrogen remains on the surface. XPS could detect no difference between surfaces healed by N 2 O and those healed by O 2 . However, SHG indicates that surfaces healed by N 2 O differ slightly from surfaces similarly healed by O 2 , suggesting that O 2 may interact with defects on these surfaces to form Ti 4+ :O 2 − complexes.

Interactions of small molecules with TiO2(110) surfaces: The role of defects

In this article, the adsorption behavior and the surface redox reactivity among small molecules including N2O, O2, H2O and HCOOH on TiO2(110) surfaces are compared. New measurements of the interaction of HCOOH with defect‐free and defective TiO2(110) surfaces and initial results from N2O adsorption on defects are combined with earlier work involving O2 and H2O. X‐ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) were used to monitor the amount of species adsorbed on the surface and the presence of electronic defects. The defective surfaces were prepared by Ar+ bombardment and electron‐beam exposure. Both O2 and N2O ‘‘heal’’ defects (defect healing means an observation of the reducing of defect state intensity in both XPS and UPS spectra) even if they do not readily stick on the TiO2(110) surfaces at 300 K. H2O also heals defects, but at a much slower rate than either O2 or N2O. In contrast, HCOOH exposure produces little healing even though the rate of adsorption for HC...

Interaction of HCOOH with stoichiometric and reduced SrTiO3(100) surfaces

Interaction of formic acid with stoichiometric (TiO2-terminated) and reduced SrTiO3(100) surfaces has been investigated using temperature programmed desorption (TPD), and x-ray photoelectron spectroscopy (XPS). Formic acid was dissociated to form formate and a surface proton below 250 K on both stoichiometric and reduced SrTiO3(100) surfaces. Formate was decomposed primarily through dehydration to produce CO and H2O, instead of through dehydrogenation to produce CO2 and H2, on both surfaces. Formaldehyde produced from decomposition of formate was also observed on both surfaces. On stoichiometric surfaces, formaldehyde was produced through bimolecular coupling of two formates on low-coordination Ti cation sites. However, on the reduced surface, formaldehyde formation involves the reduction of surface formates through the oxidation of reduced Ti cations. XPS results show that surface defects on reduced SrTiO3(100) surfaces were reoxidized significantly upon exposure to 30 L HCOOH at 300 K, in contrast to de...