Defects at the rutile TiO2

The total energy of periodic TiO2 slabs using a self-consistent ab initio method calculated show that the 110 surface shows the lowest surface energy, and the 001 surface the highest.001 direction (the crystallographic c-axis). A Ti interstitial located in these channels is in an octahedral configuration, similar to the regular Ti sites. Consequently, the diffusing species in oxidation reactions of reduced TiaOb surfaces (where a>b/2 but probably less than b) produced by sputtering and/or Ti deposition is the Ti atom and not the not the O vacancy, as has been shown in a series of elegant experiments with isotopically labeled 18O and 46 Ti ...The surface science of TiO2

The diffusion mechanism for the various types of defects is quite different; oxygen migrates via a site exchange (vacancy diffusion) mechanism, while excess Ti diffuses through the crystal as interstitial atoms. The interstitial diffusion happens especially fast through the open channels along the 001 direction (the crystallographic c axis. A Ti interstitial located in these channels is in an octahedral configuration, similar to the regular Ti sites. Consequently, the diffusing species in oxidation reacions of reduced Tia Ob surfaces (where a> b/2 but probably less than b) produced by sputtering and/or Ti deposition is the Ti atom and not the O vacancy, as has been shown in a series of elegant experiments with isotopically labeled 18O and 46Ti.

The rutile 110 surface is the most stable crystal face and simple guidelines can be used to essentially predict the structure and stability of TiO2 (110) - (1x1). Because these concepts are very useful for the other crystal faces of TiO2 as well as other oxide materials, they ar exemplified for this surface. Although the TiO2 (110) surface is very stable, it nevertheless reconstrucs and restructures at high temperatures under both oxidizing and reducing conditions.

Surface science of TiO2

The bulk structure of reduced TiO2-x crystals is quite complex with a various types of defects such as doubly charged oxygen vacancies, TiO3+ and TiO4+ interstitials, and planar defects such as CSPs. The defect structure varies with oxygen deficiency which depends on temperature, gas pressure, impurities, etc. despite years of research the question of which type of defect is dominant in which region of oxygen deficiency is still subject to debate. It was shown that the dominant type are Ti interstitials in the region from TiO 1.9999 (from 3.7x10^18 to 1.3x19^19 missing O atoms per cubic centimetre). CS planes precipitate on cooling crystals across the TiO2-x (0 ≤ x≤ 0.0035) phase boundary. They show a very strong dependence on the cooling history and are absent in quenched specimen.
E Yagi physics Review B 54 (1996)
TiO2 electric properties in dependence on the bulk defect concentration has been investigated as well!




Structure, defects, and impurities at the rutile TiO2(0 1 1)-(2 × 1) surface: A scanning tunneling microscopy study


The titanium dioxide rutile (011) (equivalent to (101)) surface reconstructs to a stable (2 × 1) structure upon sputtering and annealing in ultrahigh vacuum. A previously proposed model (T.J. Beck, A. Klust, M. Batzill, U. Diebold, C. Di Valentin, A. Selloni, Phys. Rev. Lett. 93 (2004) 036104/1) containing onefold coordinated oxygen atoms (titanyl groups, Tidouble bond; length as m-dashO) is supported by Scanning Tunneling Microscopy (STM) measurements. These Tidouble bond; length as m-dashO 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.


Surface Science, Volume 600, Issue 19, 1 October 2006, Pages 4407-4417

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The surface science of titanium dioxide

…a better understanding and improvement of catalytic reactions is one main driving force for surface investigations on TiO2. because most heterogeneous catalysts consist of small metal clusters on an oxide support, many growth studies of metals on TiO2 were performed. These metal/TiO2 systems often serve as a model for other metal/oxide surfaces. Traditionally, TiO2 is a component in mixed vanadia/titania catalysts used for selective oxidation reactions…TiO2 is not suitable as a structural support material, but small additions of titania can modify metal-based catalysts in a profound way. The so called strong metal support interaction (SMSI) is, at least in part, due to encapsulation of the metal particles by a reduced TiO2 overlayer (see review by Haller and Resasco). Recently, this phenomenon was revisited using surface science techniques. The discovery that finely dispersed Au particles supported on TiO2 and other reducible metal oxides oxidize CO at low temperature has spurred some excitement in the surface science community.

The photoelectric and photochemical properties of TiO2 are another focus of active research. The initial work on the photolysis of water on TiO2 electrodes without an external bias, and the thought that surface defect states may play a role in the decomposition of water into H2 and O2, has stimulated much of the early work on TiO2. Unfortunately, TiO2 has alow quantum yield for the photochemical conversion of solar energy. The use of colloidal suspensions with the addition of dye molecules has been shown to improve efficiency of solar cells, and has moved TiO2-based photoelectrochemical converters into the realm of economic competitiveness.
By far, the most actively pursued applied research on titania is its use for photo assisted degradation of organic molecules. TiO2 is a semiconductor and the electron-hole pair that is created upon irradiation with sunlight may separate and the resulting charge carriers might migrate to the surface where they react with adsorbed water and oxygen to produce radical species. These attack any adsorbed organic molecule and can, ultimately, lead to complete decomposition into CO2 and H2O. The applications of this process range from purification of wastewaters; disinfection based on the bactericidal properties of TiO2, for example in operating rooms in hospitals; use of self-cleaning coatings on car windshields, to protective coatings of marble, eg. for preservation of ancient Greek statues against environmental damage. It was even shown that subcutaneous injection of a TiO2 slurry in rats, and subsequent near-UV illumination, could slow or halt the development of tumour cells.


Diebold U, The surface science of titanium dioxide, Surface Science Reports, Volume 48, Issues 5-8, January 2003, Pages 53-229
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Role of surface oxygen vacancies in photoluminescence of tin dioxide nanobelts

The role of surface oxygen vacancies in the optical properties of tin dioxide nanobelts is investigated in this paper. Using a first-principles approach, based on the density functional theory combined to a very accurate exchange correlation functional, we characterize SnO2 (1 0 1), that is the nanobelt largest surface. We show that the presence of surface oxygen vacancies leads to the appearance of (i) occupied states located at about 1 eV above the valence band and (ii) unoccupied states lying in resonance with the conduction band. Photoluminescence characterization performed on samples of SnO2 nanobelts at low temperature shows that the basic spectral features of luminescence are in excellent agreement with theoretical predictions.

Microelectronics Journal, In Press, Corrected Proof, Available online 27 September 2008





Stored energy, vacancies and thermal stability of ultra-fine grained copper

The stored energy and thermal stability of oxygen-free high conductivity copper processed by equal channel angular pressing up to 16 passes at room temperature was studied by differential scanning calorimetry. Stored energy increased with strain up to four passes, after which it saturated at 0.95 ± 0.05 J/g. This saturation value is 20% higher than from conventional cold rolling. The microstructure of the copper after eight passes was characterized by an average subgrain size of about 0.21 μm and high-angle boundary fraction of about 35%. The contributions to the stored energy from defects were calculated and compared, suggesting that the stored energy mainly originates from boundaries and vacancies. The restoration activation energy after eight passes was between 77 and 80 kJ/mol. The higher stored energy and lower activation energy compared to cold-rolled copper is attributed to excess vacancies.

Materials Science and Engineering: A, Volume 492, Issues 1-2, 25 September 2008, Pages 74-79




Electrochemical and photoelectrical properties of titania nanotube arrays annealed in different gases

Titania nanotube arrays fabricated by anodic oxidation of titanium foil were calcined in dry nitrogen, air, and argon at various temperatures for varied period of time. Changes in morphology and crystallinity of the nanotube arrays were studied by means of SEM and XRD. The influences of annealing conditions on the electrochemical and conductivity were investigated by electrochemical impedance spectroscopy (EIS), and the results showed that the electrical conductivities of TiO2 nanotube arrays calcined in nitrogen for 3 h were improved greatly as compared to the as-grown titania nanotube arrays or annealed in air or argon. Well defined oxidation and reduction peaks were observed during the cyclic voltammetric scan at 0.1 V/s in 10 mM K3[Fe(CN)6] solution. Photocurrent response in TiO2 nanotube arrays calcined in nitrogen was significantly enhanced. Reduction of tetravalent titanium cations and the formation of oxygen vacancies were ascribed to explain the improved electrochemical and photoelectrical properties of titania nanotube arrays.

Sensors and Actuators B: Chemical, Volume 134, Issue 2, 25 September 2008, Pages 367-372






An investigation of the thermal stability, crystal structure and catalytic properties of bulk and alumina-supported transition metal nitrides

The relationship between crystal structures and catalytic activities was investigated. The results indicated that metal nitrides with higher vacancy concentration exhibited higher activities for NO decomposition. There was a stronger interaction between the metal nitride phases and γ-Al2O3 support. It was suggested that Co4N/γ-Al2O3 exhibited thermal stability significantly higher than that of bulk counterpart, owing to the strong interaction between the Co4N phase and γ-Al2O3 support. We applied the XRD technique to examine the structural changes of Co4N/γ-Al2O3 catalysts during the reactions. The results indicated that the rapidly loss in catalytic activity was due to the bulk oxidation of Co4N/γ-Al2O3. In the NO–H2 reaction, the oxygen generated during NO dissociation was partly reduced by H2 and partly incorporated into the nitride lattice. By the addition of H2 in feed gas at 600 °C, one can retain the active Co4N/γ-Al2O3 phase by minimizing the presence of surface oxygen.

Journal of Alloys and Compounds, Volume 464, Issues 1-2, 22 September 2008, Pages 488-496



NOVEL OXIDE MBE




This work concentrates on the growth of novel epitaxial oxides in ultra-thin film form. The primary goal of this work is to learn how to use defects -- both point defects and extended defects such as surface and interfaces -- to modify the electronic structure of highly correlated oxides. To this end we study the physical structure using RHEED, LEED, x-ray diffraction, and STM as well as the electronic strucutre via high-resolution ARPES.

http://www.physics.ubc.ca/~quantmat/OxideMBE.html






Fe nanocrystal growth on SrTiO3„001…

We have investigated the structure and morphology of self-assembled iron nanocrystals supported on a SrTiO3(001)-c (4x2) substrate using scanning tunneling microscopy. Nanocrystals with a truncated pyramid shape were imaged, which result from the epitaxial growth of bcc Fe on SrTiO3(001).

Magnetic materials composed of densely packed nanocrystals are of interest because they exhibit different magnetic properties compared to the bulk solids. For example,
such materials can show different magnetization directions, have enhanced magnetic moments, or display lower Curie temperatures. Organized arrays of self-assembled magnetic nanocrystals also have possible applications in the area of high-density information storage media. Iron in the bcc structure is of particular interest because of its high magnetic moment and remnant magnetization. However, if iron is
grown in ultrathin film form it often adopts the fcc structure, where a small variation of the lattice constant or lattice distortion can result in drastic changes of magnetic phases including a low-moment ferromagnetic phase, an antiferromagnetic phase, a ferrimagnetic phase, and highmoment ferromagnetic phases. A description of the properties of nanocrystalline Fe therefore requires knowledge of the structure, size, shape, and distribution of the nanocrystals.
In this letter we report on the epitaxial growth of Fe nanocrystals on a SrTiO3 (001)substrate. Fe self-assembles into truncated pyramid nanocrystal domains. A precise analysis of the pyramidal cluster shape shows that Fe is bcc packed. The equilibrium nanocrystal shape is used to determine the adhesion energy of bcc Fe on SrTiO3. Interest in the SrTiO3 surface has emerged from its electronic properties and its use as a substrate for supported nanocrystal growth. The SrTiO3 (001) surface presents a multitude of different reconstructions8, depending on sample
preparation, which can be used for the growth of regular nanocrystals over macroscopic length scales. SrTiO3 crystallizes into the cubic perovskite structure with a 3.905 Å lattice parameter. In its pure form it has a 3.2 eV band gap that
would make it unsuitable for imaging in the scanning tunneling microscope (STM). To overcome this problem we use crystals doped with 0.5% weight Nb.

The STM images show that iron forms truncated pyramidnanocrystals on SrTiO3 (001). This crystal shape can only evolve from cubic packing. Cubic-packed Fe thin films
can exist in the fcc and bcc structures. The equilibrium truncated pyramid shape for the fcc structure has a _001_ top facet and four _111_ side facets, these being the lowest energy facets. On the other hand, in the bcc structure the lowest energy facets are the _001_ and the _011_ facets. This means that the equilibrium truncated pyramid shape for the bcc structure has a [001] top facet and four (011) side facets.
A truncated pyramid-shaped fcc Fe nanocrystal will have (111) side facets that have an angle of 54.7° with respect to the substrate and an angle of 70.6° between each other. The unit cell dimension for fcc Fe is afcc ~3.6 Å, and as the interplanar periodicity along the (001) direction is half the unit cell dimension, we would expect fcc nanocrystals to be quantized in heights of ~1.8 Å. However, if the Fe nanocrystals have bcc packing, their shapes are subtly different. The (011) side facets have a 45° angle with respect to the substrate and a 90° angle between each other. The unit cell dimension for bcc Fe is abcc=2.87 Å and the height quantization
is therefore ~1.44 Å. Our data show that the measured side facets angle is 90.8°±1.2°
and that the islands are quantized into heights of 1.4 Å multiples. We therefore conclude that the truncated pyramid nanocrystals reported on here have a bcc structure.

Fabien Silly and Martin R. Castella, Department of Materials, University of Oxford




Shape changes of catalytic RH nanoparticles during oxidation, reduction

Heterogeneous catalysts often consist of metal nanoparticles absorbed on oxide supports, and the size and shape of these nanoparticles are likely to be affected by conditions in the reactor such as temperature and oxidation state. However, such changes are not readily observed experimentally because many methods require vacuum conditions. A research team was able to examine the changes to rhodium nanoparticles on a MgO surface using high-resolution in situ x-ray diffraction, as well as transmission election microscopy. At elevated temperatures (570 K), these pyramid-shaped nanoparticles became flatter upon exposure to oxygen, which causes the formation of a surface oxide. The nanoparticles returned to their original shape after exposure to CO, which causes reduction of the surface.

[Shape Changes of Supported Rh Nanoparticles During Oxidation and Reduction Cycles, Science 19 September 2008:
Vol. 321. no. 5896, pp. 1654 - 1658 DOI: 10.1126/science.1160845]