Papers

AVS2004 Session SS1-WeA: Metal Oxides and Clusters II: TiO@sub 2@ and Photocatalysis

Wednesday, November 17, 2004 2:00 PM in Room 210B

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		SS1-WeA-1 First-Principles Study of Intrinsic Defect Formation Energies in Tio₂ J. He, S.B. Sinnott (University of Florida) First-principles calculations are used to study intrinsic defects in bulk rutile TiO₂ and on the TiO₂ (110) surface. The approach is density functional theory (DFT) using the generalized gradient approximation (GGA) combined with plane-wave ultrasoft pseudopotentials. In particular, the formation energies of isolated vacancies, interstitials, and substituents with different charges, representing localized or distributed charge states, are evaluated. The DFT calculation results are combined with thermodynamic data to calculate the defect formation energies. In particular, the influence of temperature and oxygen partial pressure on the oxygen chemical potential is taken into account. The calculations suggest that fully charged Ti interstitials are more stable than oxygen vacancies at most oxygen partial pressures. Contrary to expectations, our preliminary results also indicate that the formation energy behavior of O vacancies at the TiO₂(110) surface is similar to the formation energy behavior of O vacancies in bulk TiO₂. These results are helping us to understand defect formation and segregation in TiO₂.
2:20 PM		SS1-WeA-2 Comparison of Physisorption on MgO(100) and TiO₂(110) Surfaces Z. Dohnálek, J. Kim, B.D. Kay (Pacific Northwest National Laboratory) Characterization of oxide surfaces represents one of the current challenges in surface science. In this study we employ physisorption of weakly bound species such as N₂, O₂, CH₄, and Ar to determine the distribution of binding sites on the surfaces of MgO(100) and TiO₂(110). Cooling of the oxide surfaces to cryogenic temperatures (T < 30K) is critical for conduction such measurements. Both surfaces were extensively studied in the past and represent an ideal platform to correlate surface sites with binding energies of various adsorbates. On MgO(100) only a single temperature programmed desorption (TPD) monolayer feature is observed for all the adsorbates. This is a result of simplicity of unreconstructed MgO(100) surface with checkerboard like arrangement of Mg²⁺ and O^2- ions. TPD spectra observed on TiO₂(110) are in sharp contrast with those from MgO(100). In this case the surface structure composed of rows of Ti⁴⁺ ions and bridge-bonded oxygens leads to two distinct adsorption geometries. The coverage of the adsorbates in these two geometries is approximately the same and it in agreement with 1:1 ratio of Ti⁴⁺ and bridge-bonded O sites. Typical defects are also probed on both surfaces. On MgO(100) their coverages are determined to be on the order of 15%. Lower coordination of the defect sites results in increased adsorbate binding energies. The effect of oxygen vacancies on the physisorption on TiO₂(110) is currently being investigated. Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.
2:40 PM		SS1-WeA-3 The Oxygen Chemistry on Rutile Titanium Dioxide E.K. Vestergaard, E. Wahlström, R. Schaub, J. Matthiesen, F. Besenbacher (Interdisciplinary Nanoscience Center, Denmark) The detailed understanding of the oxygen chemistry on titanium dioxide is an important issue for chemical and photo-chemical processes on this material. In particular, the detailed route for oxygen vacancy filling through oxygen exposure is important to understand the change of surface reactivity upon re-stoichiometrization. We present STM investigations resolving the atomic-scale details of adsorption, diffusion and reaction of oxygen molecules on the TiO₂(110) surface. By following the dynamical processes in real time with STM movies, we find that both the diffusion and the reaction between oxygen molecules have activation energies of approximately 0.35 eV and exhibit extremely low attempt frequencies of ~10⁶ s^-1. These findings are interpreted in a model where charge transfer processes from the TiO₂ conduction band to the adsorbed oxygen molecules govern the dynamics: Surface oxygen vacancies pin the chemical potential at 0.35 eV from the conduction band, in good agreement with the observed activation energy for diffusion of oxygen molecules.¹ The same activation energy is also found for the dynamics of larger oxygen clusters containing three or more atoms. The interaction between such clusters and the bridging oxygen rows on the TiO₂(110) surface are found to be essential for the understanding of the ability of oxygen to heal the bridging oxygen vacancies. Finally, the presented STM results are discussed in the context of active support materials for Au catalysts. ¹E. Wahlström et al., Science 303, 511 (2004).
3:00 PM		SS1-WeA-4 Charge Transfer-Induced Water Splitting on the Rutile TiO₂(110) Surface R. Schaub, E. Wahlström, E.K. Vestergaard, J. Matthiesen, F. Besenbacher (Interdisciplinary Nanoscience Center, Denmark) In direct contact with water, metal oxides are promising candidates in the search for renewable energy sources through direct photo-splitting of water as suggested by the seminal experiments of Fujishima and Honda¹. TiO₂ is in particular one of the most utilized photo-chemically active systems for waste water treatment. For such photo-chemical processes, surface bound water as well as hydroxyls have been proposed to be the major species which are photo-activated to form hydroxyl radicals responsible for the photochemical activity. By means of scanning tunneling microscopy (STM) we have identified a number of different water-derived adsorbates on the TiO₂(110) surface, and the details of their formation were revealed from time-resolved STM movies. We find strong evidence that the diffusion of hydroxyls, as well as H₂O dissociation, is linked to the electronic properties of the substrate in the surface region, in a similar manner to our previous results reported for the interaction of O₂ molecules with TiO₂(110)². Hence, charge transfer from the conduction band of the substrate to adsorbed molecules or reactants is identified as a key factor to understand their physical properties. In other words, chemical reactions can be promoted, not only by a localized "active" site (the traditional approach), but also by the more delocalized conduction band electrons. ¹ A. Fujishima and K. Honda, Nature 238, 37 (1972). ² E. Wahlström et al., Science 303, 511 (2004).
3:20 PM		SS1-WeA-5 Photodecomposition of Acetone on TiO₂(110) M.A. Henderson (Pacific Northwest National Laboratory) Although acetone is commonly used to evaluate the performance of oxide photocatalysts, little is known about the mechanistic details of its photo-oxidation. This study provides insights into the photodecomposition of adsorbed acetone using the (110) face of rutile TiO₂ as a model photocatalyst. In the absence of UV light, acetone desorbs from the clean TiO₂(110) surface without decomposition, exhibiting strong coverage-dependence in its temperature programmed desorption (TPD) peak that shifts from 350 K to below 250 K as the monolayer is populated. Acetone molecules desorbing at 350 K constitute about 0.25 ML and exhibit H/D exchange with surface hydroxyl groups. On the other hand, coadsorbed water displaces about 0.75 ML of the acetone monolayer into physisorbed states, but does not influence the remaining 0.25 ML that constitutes the 350 K TPD peak. These strongly bound acetone molecules are not associated with oxygen vacancies. Virtually no photodecomposition is observed in the absence of gas phase O₂. Exposure to UV light in gas phase O₂ results in conversion of acetone to acetate via cleavage of a carbonyl-methyl bond. A similar reaction mechanism occurs in the dark during the coadsorption of acetone and molecular oxygen preadsorbed at oxygen vacancies, suggesting that acetone photodecomposition is facilitated through the excited electron channel (e.g., via reaction with an O₂^- species) and not through oxidation by valence band holes. Photodesorption measurements reveal that the methyl group is ejected from the surface at 200 K but is retained on the surface at 100 K presumably by conversion into formate based on the absence of likely C₁ or C₂ species in TPD. The acetone photodecomposition cross section increases with increasing acetone coverage, but decreases with coadsorbed water. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences.
3:40 PM		SS1-WeA-6 Water Adsorption on the Rutile TiO2(011)-(2x1) Surface A. Klust, T.J. Beck, U. Diebold (Tulane University); C. DiValentin (Universita' degli Studi di Milano-Bicocca, Italy); A. Selloni (Princeton University) Titanium oxide is a promising material for photocatalysis. Previous studies have shown that the photocatalytic activity of rutile TiO₂ depends dramatically on the surface orientation with the (011) surface being the most active.¹ The TiO₂(011) surface forms a stable (2x1) reconstruction. Based on theoretical modelling and scanning tunneling microscopy (STM) studies, we propose a structural model for this surface. The surface is terminated by one-fold coordinated O atoms (titanyl groups), a feature that distinguishes it from all other known TiO₂ surfaces. We suggest that the titanyl groups are responsible for the high photocatalytic activity of the TiO₂(011) surface. Here, we present STM and ultraviolet photoemission (UPS) studies of water adsorption on the TiO₂(011) surface showing that water adsorb in both molecular and dissociative form at 110 K. First-principles calculations are in agreement with these results and provide detailed information on the structure of the adsorbed molecules. In contrast to the well - known (110) surface², water dissociation is not mediated by surface defects demonstrating the high chemical activity of the TiO₂(011)-(2x1) surface. ¹ G.S. Rohrer in: The Chemical Physics of Surfaces (2001), ed. D.P. Woodruff. ² I.M. Brookes et al. Phys. Rev. Lett. 87 (2001) 266103; R. Schaub et al. Phys. Rev. Lett. 87 (2001) 266104.
4:00 PM		SS1-WeA-7 Photoexcitation of TiO₂ and the Chemistry of Electrons and Holes J.T. Yates, Jr. (University of Pittsburgh); O. Diwald (Technical University of Vienna, Austria); D. Panayotov, T.L. Thompson (University of Pittsburgh); S.D. Walck (PPG Industries); T. Berger, E. Knözinger, M. Sterrer (Technical University of Vienna, Austria) TiO₂ is a useful photocatalyst for the destruction of trace quantities of organic molecules in the environment. Photoexcitation of TiO₂ occurs as a result of electron-hole pair excitation by UV radiation with energy above the bandgap (3.0 eV). It has been found that N doping from NH₃ may be used to lower the photothreshold for TiO₂ by 0.6 eV, whereas N doping by ion implantation using N₂⁺ leads to an increase in the photothreshold energy. Excited electrons in TiO₂ may be detected by EPR or IR spectroscopy, and hole formation in the valence band region may be detected by EPR as O^- species. Excited electrons are observed to transfer to adsorbed O₂ to produce the O₂^- species (superoxide). An adsorbed organic molecule, containing both S and Cl moieties has been shown to accept excited electrons when it is bonded to the surface by the Cl moiety, but not when bound by the S moiety, indicating that charge transfer occurs preferentially into the polyfunctional molecule by means of the more electronegative attachment group.
4:40 PM		SS1-WeA-9 +ACQ+AHs+Dye Sensitization of the TiO+AEA-Sub 2+AEA- polymorph Anatase (101) Single Crystal Surface by a Series of Dicarboxylated Thiacyanine Dyes+AHs+AH0- N. Ruzycki, S. Ushiroda, Y. Lu (Colorado State University); M.T. Spitler (ChemMotif, Inc.); B.A. Parkinson (Colorado State University) +ACQ-Body +AHs-The dye sensitization of the TiO+AEA-Sub 2+AEA- polymorph anatase (101) is an important system of study because of its application to dye-sensitized solar cells such as the Gr+AEA-um A+AEA-tzel cell. The electrons per photon quantum yields for the dye sensitization of anatase are reported to be higher than those for the rutile polymorph of TiO+AEA-Sub 2+AEA-. The reason that anatase is a better substrate for photovoltaic purposes is yet unknown. The efficiency of solar cells is closely tied to the yield and rate of the electron transfer from the dye molecule to the surface, factors that will be influenced by the geometry of the dye binding and ordering at the surface. Studies that link UHV and electrochemical experiments are a first step towards understanding the mechanism of dye binding to the surface of this material. Dye sensitization of the single crystal anatase (101) surface was studied using a series of dicarboxylated thiacyanine dyes that bind to the surface through the carboxylate group. An ultraviolet (UV) light treatment of the anatase (101) surface, immediately prior to dye adsorption, improves both the reproducibility of dye coverage and the incident photon-to-current efficiencies (IPCE) for sensitization. The UV treatment does not pit or roughen the anatase surface and results in high IPCEs of up to 0.5 percent. The adsorption isotherms and adsorption and desorption kinetics of these dyes were studied. The photocurrent spectra showed features associated with surface-bound dye monomers, dimers and aggregates that could be followed as a function of the dye surface coverage. UHV studies on the single crystal anatase (101) surface were undertaken, including STM, for adsorption of a molecule (bis-isonicotinic acid) that had approximately the same backbone structure and carboxylate group as the dyes in order to elucidate structural models for the dye binding to the surface. +AEA.
5:00 PM		SS1-WeA-10 Enhanced Photocatalytic Activity in Fe₂O₃/Cr₂O₃ Epitaxial Heterojunctions J.R. Williams, S.A. Chambers, M.A. Henderson (Pacific Northwest National Laboratory) The heterogeneous photocatalytic activity may be enhanced by the creation of an oxide-oxide heterojunction where the band alignment is such that photoexcited electron-hole pairs tend to separate at the interface, resulting in enhanced charge carrier lifetimes. Here we show that epitaxial heterojunctions of Fe₂O₃ and Cr₂O₃ do indeed exhibit enhanced photocatalytic activity relative to either pure material. The dependence of the enhancement on the heterojunction thickness will be discussed, as well as the wavelength dependence. The heterojunctions were grown on α-Al₂O₃ substrates by Oxygen Plasma Assisted Molecular Beam Epitaxy (OPA-MBE). Characterization by XPS and all photocatalysis experiments are performed in situ after growth with no air exposure at any time. Photocatalytic activity is measured by the extent of decomposition of trimethylacetic acid (TMA) adsorbed on the surface using a quadrupole mass spectrometer, XPS, and a Hg arc lamp with various filters as the excitation source. Specifically we have observed a factor of ~ 2-3 increase in photocatalytic activity for this type of heterojunction surface as compared to the same surface of either pure material.