EMSL: Igor Lyubinetsky's Publications

Chambers SA, T Ohsawa, CM Wang, I Lyubinetsky, and JE Jaffe. 2009. "Band Offsets at the Epitaxial Anatase TiO2/n-SrTiO3(001) Interface." Surface Science 603(5):771-780. Abstract We have used high-energy-resolution x-ray photoelectron spectroscopy to measure valence band offsets at the epitaxial anatase TiO2(002)/n-SrTiO3(001) heterojunction prepared by molecular beam epitaxy, Within experimental error, the valance band offset is zero for anatase thicknesses between 1 and 7 monolayers. The conduction band offset is also zero by virtue of the fact that both anatase and SrTiO3 exhibit the same bandgap value (~3.2 eV). In one set of experiments, the interface included a partial monolayer of fluorine remaining from the HF etch that was used to prepare the substrate. The F could not be removed without Ar ion sputtering and annealing, which in turn resulted in ~0.15 eV of band bending, indicating the presence of interfacial defects. The band offsets were measured to be approximately 0 eV as well when the F was removed. Density functional theory predicts the valence band offset for the clean interface to be 0.5 eV. Inclusion of interfacial F reduces the theoretical band offset to 0.2 eV, much closer to experiment, and suggesting that the interface dipoles created by F and sputter defects have a major effect on the band offset.

Du Y, NA Deskins, Z Zhang, Z Dohnalek, M Dupuis, and I Lyubinetsky. 2009. "Imaging Consecutive Steps of O2 Reaction with Hydroxylated TiO2(110): Identification of HO2 and Terminal OH Intermediates." Journal of Physical Chemistry C 113(2):666-671. doi: 10.1021/jp807030n Abstract The hydroperoxyl (HO2) species is believed to be a key intermediate in many heterogeneous photochemical processes, but generally metastable and thus hard to prove. We report here that for the first time, we directly imaged stable, adsorbed HO2 species during O2 reaction with a partially hydroxylated TiO2(110). We also found terminal hydroxyl groups, another critical but never directly observed intermediates. By imaging species and tracking site-specific reactions with high-resolution scanning tunneling microscopy, and determining the energies and configurations with density functional theory calculations, we provide molecular-level insight into the underlying reaction mechanisms. These results are expected to have far reaching implications for various catalytic systems involving the interconversion of O2 and H2O.

Du Y, NA Deskins, Z Zhang, Z Dohnalek, M Dupuis, and I Lyubinetsky. 2009. "Imaging Consecutive Steps of O2 Reaction with Hydroxylated TiO₂(110): Identification of HO₂ and Terminal OH Intermediates." Journal of Physical Chemistry C 113(2):666-671. Abstract We report results of the combined experimental and theoretical investigation of the molecular oxygen reaction with a partially hydroxylated TiO₂(110) surface. The consecutive steps of both primary and secondary site-specific reactions have been tracked with high-resolution scanning tunneling microscopy (STM). For the first time, we have directly imaged stable, adsorbed hydroperoxyl (HO₂) species, which is believed to be a key intermediate in many heterogeneous photochemical processes but generally metastable and “elusive” until now. We also found terminal hydroxyl groups, another critical but never directly observed intermediates. A conclusive evidence that O₂ reacts spontaneously with a single bridging OH group as an initial reaction step is provided. The experimental results are supported by density functional theory (DFT) calculations that have determined species energies and configurations. Reported observations provide a basis for a consistent description of the elementary reaction steps and offer molecular-level insight into the underlying reaction mechanisms. In a broader perspective, the results are expected to have far reaching implications for various catalytic systems involving the interconversion of O₂ and H₂O.

Du Y, NA Deskins, Z Zhang, Z Dohnalek, M Dupuis, and I Lyubinetsky. 2009. "Two Pathways for Water Interaction with Oxygen Adatoms on TiO2(110)." Physical Review Letters 102(9):Art. No. 096102. doi:10.1103/PhysRevLett.102.096102 Abstract Scanning tunneling microscopy and density functional theory studies show that oxygen adatoms (Oa), produced during O2 exposure of reduced TiO2(110) surfaces, alter the water dissociation/recombination chemistry through two distinctive pathways. Depending on whether H2O and Oa are on the same or adjacent Ti4+ rows, Oa facilitates H2O dissociation and proton transfer to form a terminal hydroxyl pair, positioned along- or across-Ti row, respectively. The latter process has not been reported previously, and it starts from “pseudo-dissociated” state of water. In both pathways, the subsequent reverse proton transfer results in H2O recombination and statistical oxygen atom scrambling, as manifested by an apparent along- or across-row motion of Oa’s.

Ohsawa T, I Lyubinetsky, Y Du, MA Henderson, V Shutthanandan, and SA Chambers. 2009. "Crystallographic Dependence of Visible-light Photoactivity in Epitaxial TiO2−xNx Anatase and Rutile." Physical Review. B, Condensed Matter and Materials Physics 79(8):Article number: 085401. doi:10.1103/PhysRevB.79.085401 Abstract Nitrogen-doped TiO2 materials have been shown to exhibit visible-light photoactivity, but the operative mechanism(s) are not well understood. Here we use structurally and compositionally well-defined epitaxial films of TiO2−xNx anatase (001) and rutile (110) (x~0.02) to show a qualitative difference between the visible-light activities for the two polymorphs. Holes generated by visible light at N sites in anatase (001) readily diffuse to the surface and oxidize adsorbed trimethyl acetate while the same in rutile (110) remain trapped in the bulk. In light of the low doping densities that can be achieved in phase-pure material, conventional wisdom suggests that holes should be trapped at N sites in both polymorphs. Although the detailed mechanism is not yet understood, these results suggest that the hole hopping probability is much higher along the [001] direction in N-doped anatase than along the [110] direction in N-doped rutile.

Ohsawa T, I Lyubinetsky, Y Du, MA Henderson, V Shutthanandan, and SA Chambers. 2009. "Crystallographic Dependence of Visible-Light Photochemistry in Epitaxial TiO2-xNx Anatase and Rutile." Physical Review. B, Condensed Matter and Materials Physics 79(8):Art. No. 085401. Abstract All films were grown by plasma assisted molecular beam epitaxy (PAMBE) in a custom chamber described elsewhere (1). Epitaxial films of TiO2-xNx(001) (x ≤ ~0.02) anatase were grown by PAMBE on undoped or Nb-doped (0.02 at. %) SrTiO3(001) (STO) and undoped LaAlO3(001) (LAO). Similarly, TiO2-xNx(001) (x ≤ ~0.02) rutile epifilms were grown on rutile TiO2(110). The growth and physical properties of N-doped anatase on LAO(001) and N-doped rutile on TiO2(110) have been described in detail elsewhere (2-4). In what follows, we describe the growth details for N-doped anatase on STO(001). The PAMBE chamber is connected to an x-ray photoelectron spectrometer (XPS) chamber and a photodesorption chamber. The former is equipped with a Gamma Data/Scienta SES 200 analyzer and a monochromatic AlK x-ray source. The latter includes a molecular dosing apparatus for TMAA, a Hg arc lamp, and a quadrupole mass spectrometer. The STO substrates were etched in buffered HF and annealed in flowing O2 at 1 atm. at 950oC for 8 hours. The etch dissolved SrO terraces and the oxygen anneal resulted in mass transport of the discontinuous TiO2 microterraces, resulting in an atomically flat, TiO2 terminated surface with a minimum step height of 4 Å (5). This treatment left some residual fluorine on the surface which could not be removed by annealing. The measured F 1s binding energy was ~684.0 eV, which is close to that exhibited by SrF2 – 684.6 eV (6). Based on this binding energy and the high degree of thermal stability, we conclude that F substitutes for O in the lattice. Under this assumption and using atomic photoemission cross sections (7), the F mole fraction within the anion sublattice is estimated to be ~0.05 within the probe depth of XPS at normal emission (~45 Å).

Petrik NG, Z Zhang, Y Du, Z Dohnalek, I Lyubinetsky, and GA Kimmel. 2009. "Chemical Reactivity of Reduced TiO2(110): The dominant role of surface defects in oxygen chemisorption." Journal of Physical Chemistry C 113(28):12407-12411. doi:10.1021/jp901989x Abstract O2 chemisorption on reduced, rutile TiO2(110) with various concentrations of oxygen vacancies (Ov) and bridging hydroxyls (OHb) is investigated with scanning tunneling microscopy, temperature programmed desorption and electron-stimulated desorption. On the annealed surface, 2 oxygen molecules can be chemisorbed per Ov. The same amount of O2 chemisorbs on surfaces where each Ov is converted to two OHb’s by exposure to water (i.e. 1 O2 per OHb). Surfaces with few or no Ov’s or OHb’s can be created by exposing the hydroxylated surface to O2 at room temperature, and the amount of O2 that chemisorbs on these surfaces at low temperatures is only ~20% of the amount on the annealed (reduced) surface. In contrast, the amount of chemisorbed O2 increases by more than a factor of two when the OHb concentration is enhanced – without changing the concentration of sub-surface Ti interstitials. The results indicate that the reactivity of TiO2(110) is primarily controlled by the amount of electron-donating surface species such as Ov’s and/or OHb’s, and not Ti3+ interstitials.

Zhang Z, Y Du, NG Petrik, GA Kimmel, I Lyubinetsky, and Z Dohnalek. 2009. "Water as a Catalyst: Imaging Reactions of O-2 with Partially and Fully Hydroxylated TiO2(110) Surfaces." Journal of Physical Chemistry C 113(5):1908-1916. Abstract The reactions of molecular oxygen with bridging hydroxyl groups, OHb, formed by H2O dissociation on bridging oxygen vacancies of TiO2 (110) are studied at low and high OHb coverages as a function of the O2 exposure, using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and electron simulated desorption (ESD) techniques. On partially hydroxylated surfaces, the sudden simultaneous disappearance of oxygen vacancies and oxygen adatoms formed by O2 dissociation is observed at high O2 exposures. On fully hydroxylated TiO2 surfaces, which enable us to compare results of STM, TPD and ESD studies, most of OHb’s are removed via reacting with O2. Hence, fully hydroxylated TiO2 surfaces can be converted to nearly stoichiometric surfaces, albeit with some amount of adsorbed molecular water. Formation of mobile H2O molecules and water-assisted diffusion of the reactants plays an important role in the kinetics of the processes on both partially and fully hydroxylated surfaces.

Du Y, Z Dohnalek, and I Lyubinetsky. 2008. "Transient Mobility of Oxygen Adatoms upon O2 Dissociation on Reduced TiO2 (110)." Journal of Physical Chemistry C 112(7):2649-2653. doi:10.1021/jp077677u Abstract Tracking the same region of the reduced TiO2 (110) surface by scanning tunneling microscopy before and after oxygen exposure at room temperature confirms that O2 molecules dissociate only at the bridging oxygen vacancies, with one O atom healing a vacancy and other O atom bonding at the neighboring Ti site as an adatom. Majority (~81%) of O adatoms are found separated from the original vacancy positions, by up to two lattice constants along [001] direction. Since at room temperature the thermal diffusion of O adatoms has been found to be rather small, with experimentally estimated activation energy of ~1.1 eV, we conclude that observed lateral distribution of the oxygen adatoms is attained through a nonthermal, transient mobility in the course of O2 dissociation. Unlike for other known cases of the dissociation of the diatomic molecules where both “hot” adatoms accommodate at the equivalent sites, in the studied system the oxygen atoms filling the vacancies are locked into the bridging oxygen rows and only the O adatoms are relatively free to move. The transient motion of the hyperthermal oxygen adatoms on the TiO2 (110) surface occurs exclusively along the Ti troughs.

Groves JF, Y Du, I Lyubinetsky, and DR Baer. 2008. "Focused ion beam directed self-assembly (Cu2O on SrTiO3 ): FIB pit and Cu2O nanodot evolution." Superlattices and Microstructures 44(4-5):677-685. doi:10.1016/j.spmi.2008.01.016 Abstract A gallium focused ion beam has been used to create discrete pits on the surface of a SrTiO3 (100) surface with the idea that these pits will serve as the nucleation sites for subsequent Cu2O quantum dot growth. Immediately after pit formation and following wet chemical etching and thermal annealing of the surface, the concentration of gallium within these pits has been analyzed using a high-resolution Auger system,. Using atomic force microscopy, the geometry of the pits has also been determined following etching and annealing. Growth of Cu2O quantum dots on the patterned surfaces has been performed. Growth of Cu2O quantum dots within the pits is the primary mode of dot formation. In several samples, dot growth within pits appears to occur by a two-step process with pits filling prior to initiation of a second, distinct phase of quantum dot growth above the plane of the original SrTiO3 surface.