The Influence of the Bulk Reduction State on the Surface Structure and Morphology of Rutile TiO 2 (110) Single Crystals (original) (raw)
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J Phys Chem B, 2000
We have investigated the relationship between different types and amounts of bulk defects and the surface morphology of TiO 2 (110) single crystals prepared by annealing in ultrahigh vacuum and in oxygen. Rutile TiO 2 (110) specimens were cut from the same crystal and were heated in a furnace to different temperatures which resulted in different states of reduction (colors of the crystals). After characterization of the bulk defects with electron paramagnetic resonance (EPR), the specimens were studied with scanning tunneling microscopy (STM), low-energy He + ion scattering (LEIS), and work function measurements. EPR reveals that darker rutile crystals exhibit higher concentrations of extended Ti 3+ related bulk defects such as crystallographic shear planes (CSP), with a decrease in substitutional and interstitial defects as compared to lighter crystals. Surface structures with (1 × 2) features are preferably formed upon UHV annealing on these darker crystals. LEIS measurements show that all of the crystals' (110) surfaces are reoxidized upon annealing in 18 O 2 (573 K, 1 × 10 -6 mbar, 10 min) and that the 18 O surface content is proportional to the bulk reduction state. UV-visible adsorption spectra and resistivity measurements also scale with the reduction states of crystals. Only the (1 × 1) structure is observed on the surface of slightly reduced crystals. Annealing in oxygen induces additional metastable structures, i.e., TiO 2 clusters on blue crystals and rosette networks on dark blue crystals.
Surface Science, 2006
The titanium dioxide rutile (0 1 1) (equivalent to (1 0 1)) surface reconstructs to a stable (2 · 1) structure upon sputtering and annealing in ultrahigh vacuum. A previously proposed model (T.Phys. Rev. Lett. 93 (2004) 036104/1) containing onefold coordinated oxygen atoms (titanyl groups, Ti@O) is supported by Scanning Tunneling Microscopy (STM) measurements. These Ti@O sites are imaged bright in empty-states STM. A few percent of these terminal oxygen atoms are missing at vacuum-annealed surfaces of bulk-reduced samples. These O vacancies are imaged as dark spots. Their number density depends on the reduction state of the bulk. Double vacancies are the most commonly observed defect configuration; single vacancies and vacancies involving several O atoms are present as well. Formation of oxygen vacancies can be suppressed by annealing a sputtered surface first in vacuum and then in oxygen; annealing a sputtered surface in oxygen results in surface restructuring and a (3 · 1) phase. Anti-phase domain boundaries in the (2 · 1) structure are active adsorption sites. Segregation of calcium impurities from the bulk results in an ordered overlayer that exhibits domains with a centered (2 · 1) periodicity in STM.
Effects of the crystal reduction state on the interaction of oxygen with rutile TiO2(110)
Catalysis Today, 2012
The interaction of O 2 with reduced rutile TiO 2 (1 1 0)-(1 × 1) has been studied by means of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD) and photoelectron spectroscopy (PES). It is found that the interaction of O 2 with TiO 2 (1 1 0) depends strongly on the reduction state of the TiO 2 (1 1 0) crystal. High-resolution STM studies revealed that the energy barrier for the non-vacancyassisted, 2nd O 2 dissociation channel decreases with increasing crystal reduction. Additionally, it is found in the STM studies that the Ti interstitial diffusion is slightly more facile in high-reduced TiO 2 (1 1 0) crystals compared to low-reduced ones. Accompanying TPD studies revealed that the line shape of the O 2-TPD peak occurring between ∼360 K and ∼450 K depends on the crystal reduction state. For high-reduced TiO 2 (1 1 0) crystals characterized by large terraces most O 2 molecules desorb at ∼386 K, whereas O 2 desorption is peaking at ∼410 K for low-and medium-reduced crystals. Furthermore, the O 2-TPD experiments revealed a highly non-linear behavior of the O 2 desorption peak integrals as function of the crystal reduction state. The presented results point to an ionosorption model where the adsorbates withdraw the excess charge (Ti 3+) from the near-surface region at temperatures < ∼360 K and where Ti interstitials react with oxygen species on the surface at temperatures ≥ ∼360 K.
Electrically-active defects in reduced and hydrogenated rutile TiO2
2020
We report on electrically-active defects located between 0.054 and 0.69 eV below the conduction band edge in rutile TiO2 single crystals subjected to reducing and hydrogenating heat treatments. Deep-level transient spectroscopy measurements recorded on TiO2 samples subjected to different heat treatments are compared. In samples annealed in H2 gas, three defect levels are commonly observed. One of these levels, E192, located 0.43 eV below the conduction band edge is tentatively assigned to a hydrogen-impurity complex. Two levels at 0.054 and 0.087 eV below the conduction band edge, which were present after all different heat treatments, are tentatively assigned as being related to O vacancies or Ti self-interstitials. Deep-level transient spectroscopy spectra of samples heat-treated in N2 display a larger number of defect levels and larger concentrations compared to samples heat-treated in H2 gas. N2 treatments are performed at considerably higher temperatures. Four energy levels loc...
Influence of surface and subsurface defects on the behavior of the rutile TiO 2 (110) surface
physica status solidi (a), 2006
We carried out a theoretical study of the rutile TiO 2 (110) surface using ab initio techniques. We investigate the effects of both surface and subsurface defects. These defects consist of a missing oxygen atom in the crystalline structure, the oxygen vacancy. We show that the defect formation energy of oxygen vacancies strongly depends on the position of the defect in the material. Our results imply that subsurface defects could play a role in the reactivity of the surface.
Extented defects in slightly reduced rutile TiO2
2015
The aim of this work is to study the influence of the dislocations network , introduced by plastic deformation on the formation of extended defects called crystallographic shear plans ( c.s.p.) . For this purpose, a rig allowing deformation under compression at high temperature and controlled reducing atmosphere, has been used. In order to preserve to ambiant temperature, the obtained samples microstructures at high temperature , a quenching system under secondary vacuum, was set up. Observation under transmission electronic microscope of Ti O2-x quenched samples, has confirmed the efficiency of our quenching system and that these structures can be analysed as solid solution of intrinsic punctual defects. Precipitates of Fe2Tin-202n-2 type in the shape of plates 22 ème Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 perpendicular to [1 0 0 ] directions and appearing at low temperature could be observed in these compounds. Observation under transmission electronic microscope o...
The Effect of Defect Disorder on the Electronic Structure of Rutile TiO2-x
Defect and Diffusion Forum, 2006
The purpose of this work is to study the effect of bulk point defects on the electronic structure of rutile TiO2. The paper is focused on the effect of oxygen nonstoichiometry in the form of oxygen vacancies, Ti interstitials and Ti vacancies and related defect disorder on the band gap width and on the local energy levels inside the band gap. Ab initio density functional theory is used to calculate the formation energies of such intrinsic defects and to detect the positions of these defect induced energy levels in order to visualize the tendency of forming local mid-gap bands. Apart from the formation energy of the Ti vacancies (where experimental data do not exist) our calculated results of the defect formation energies are in fair agreement with the experimental results and the defect energy levels consistently support the experimental observations. The calculated results indicate that the exact position of defect energy levels depends on the estimated band gap and also the charge...
Defects at the (1 1 0) surface of rutile TiO2 from ab initio calculations
International Journal of Hydrogen Energy, 2012
Cation vacancies DFT a b s t r a c t Defect segregation of oxygen vacancies, protons, titanium vacancies, and the mutual complex between the two latter (i.e. Ruetschi type defects) at the (1 1 0) surface of rutile TiO 2 has been studied through ab initio Density Functional Theory calculations (DFT).
Study of Clean TiO2(110) Surface by Scanning Tunneling Microscopy and Spectroscopy
Langmuir, 1996
The effect of annealing in UHV on the topographic and electronic structure of the TiO2(110) surface was studied by scanning tunneling microscopy and spectroscopy. Different (1×n) reconstructions with domain sizes of 5-10 nm were clearly identified in the crystallographic orientation [001]. Two other types of reconstruction with periodicities of 1.69 and 1.82 nm were detected perpendicular to the [11 h1] and [1 h13] crystallographic shear directions. The comparison of I/V and dI/dV spectra recorded on (1×1) and (1×2) terraces and on a disordered phase suggests a definite difference in the oxidation state of the surface Ti atoms of these arrangements.
The geometric structure of intrinsic defects at TiO2(110) surfaces: an STM study
Surface Science, 1995
We report on a scanning tunneling microscopy study of TiO2(ll0) surfaces. Different types of intrinsic defects are produced by a high-temperature treatment at T > 1070 K in UHV, i.e. (001) single-steps, (001) line defects, and oxygen vacancies occur as predominant types of defects. Atomically resolved STM images make it possible to determine their geometric structures. With additional input from voltage-dependent (STS) images, simple structural models are derived for electrostatically neutral (001) single-steps at TiO2(ll0) surfaces and for line defects. The latter may formally be described by titanium vacancies and oxygen vacancies along the [001] direction. * Corresponding author.