Tuning the electronic properties of a clean TiO2(1 1 0) surface via repeated sputtering and annealing: A KPFM and LC-AFM study (original) (raw)
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Applied Physics A Structure and properties of TiO 2 surfaces: a brief review
Titanium oxides are used in a wide variety of technological applications where surface properties play a role. TiO 2 surfaces, especially the (110) face of rutile, have become prototypical model systems in the surface science of metal oxides. Reduced TiO 2 single crystals are easy to work with experimentally, and their surfaces have been characterized with virtually all surface-science techniques. Recently, TiO 2 has also been used to refine computational ab initio approaches and to calculate properties of adsorption systems. Scanning tunneling microscopy (STM) studies have shown that the surface structure of TiO 2 (110) is more complex than originally anticipated. The reduction state of the sample, i.e. the number and type of bulk defects, as well as the surface treatment (annealing in vacuum vs. annealing in oxygen), can give rise to different structures, such as two different (1 × 2) reconstructions, a 'rosette' overlayer, and crystallographic shear planes. Single point defects can be identified with STM and influence the surface chemistry in a variety of ways; the adsorption of water is discussed as one example. The growth of a large number of different metal overlayers has been studied on TiO 2 (110). Some of these studies have been instrumental in furthering the understanding of the 'strong metal support interaction' between group-VIII metals and TiO 2 , as well as low-temperature oxidation reactions on TiO 2 -supported nanoscopic gold clusters. The growth morphology, interfacial oxidation/reduction reaction, thermal stability, and geometric structure of ultra-thin metal overlayers follow general trends where the most critical parameter is the reactivity of the overlayer metal towards oxygen. It has been shown recently that the technologically more relevant TiO 2 anatase phase can also be made accessible to surface investigations.
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.
Scientific Reports, 2019
We investigate the thermal reduction of TiO2 in ultra-high vacuum. Contrary to what is usually assumed, we observe that the maximal surface reduction occurs not during the heating, but during the cooling of the sample back to room temperature. We describe the self-reduction, which occurs as a result of differences in the energies of defect formation in the bulk and surface regions. The findings presented are based on X-ray photoelectron spectroscopy carried out in-operando during the heating and cooling steps. The presented conclusions, concerning the course of redox processes, are especially important when considering oxides for resistive switching and neuromorphic applications and also when describing the mechanisms related to the basics of operation of solid oxide fuel cells.
Photoelectrochemical spectroscopy studies of titanium dioxide surfaces: theory and experiment
Surface Science, 1991
We describe experimental and theoretical studies of titanium dioxide. Phot~lectr~he~~l data are presented for surfaces of known structure. The oxides were grown anodically on titanium metal. We report calculations of the electronic densities of states and optical conductivities of r-utile TiO, in finite-size samples disordered by the addition of oxygen vacancies for comparison with experiments. We conclude that in less disordered surfaces the electronic structure exhibited by the photospectra is very close to that of bulk rutile TiO,. In contrast, very thin oxides showed evidence of an electronic structure dominated by localized states near the band edge. More disordered oxides gave photospectra in which transitions associated with direct band gap transitions in bulk TiOZ seem to have been suppressed. Taking the structural information on the oxides into account, we argue that this suppression is probably a matrix element effect. 0039-6028/91/$03.50 0 1991 -Elsevier Science Publishers B.V. All rights reserved Xenon bmp Multiprogrammer
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.
Electrochimica Acta, 2014
Anodic oxidation of titanium in acid electrolytes allows to obtain a thin, compact oxide layer with thickness, structure, color, and electrical properties that vary with process parameters imposed, among which cell voltage has a key effect. Although oxidation kinetics have been investigated in several research works, a broader vision of oxide properties-including thickness and structure-still has to be achieved, especially in the case of very thin oxide films, few tens of nanometers thick. This is vital for engineered applications of nanostructured TiO 2 films, as in the field of memristive devices, where a precise control of oxide thickness, composition and structure is required to tune its electrical response. In this work, oxide films were produced on titanium with thickness ranging from few nanometers to 200 nm. Oxide thickness was estimated by coulometry and spectrophotometry. These techniques were then combined with C-AFM, which provided a deeper understanding of oxide thickness and uniformity of the metal surface and probed the presence of crystalline nano-domains within the amorphous oxide phase affecting the overall film electrical and optical properties. (M.V. Diamanti).
XRD and XPS Analysis of TiO2 Thin Films Annealed in Different Environments
材料科学与工程 中英文b版, 2014
Undoped and Nb-doped TiO 2 thin films have been fabricated on glass substrate by RF magnetron sputtering. The morphologic, structural and surface composition of these films before and after annealing in different environments were investigated by atomic force microscopy (AFM) imaging, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD data reveal that the crystallinity is improved when the films are Nb-doped and annealed in H 2 environment. The TiO 2 thin films annealed in H 2 environment exhibit only the anatase phase. The XPS analysis of TiO 2 with Nb indicates the maximum shift in binding energy of the Ti 2p peak. A mechanism for the incorporation of Nb in the TiO 2 lattice has been proposed.
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.
Photoelectron diffraction investigation of the structure of the clean TiO2(110)(1×1) surface
Physical Review B, 2007
The surface relaxations of the rutile TiO 2 (110)(1x1) clean surface have been determined by O 1s and Ti 2p 3/2 scanned-energy mode photoelectron diffraction. The results are in excellent agreement with recent low energy electron diffraction (LEED) and medium energy ion scattering (MEIS) results, but in conflict with the results of some earlier investigations including one by surface X-ray diffraction. In particular, the bridging O atoms at the surface are found to relax outwards, rather than inwards, relative to the underlying bulk. Combined with the recent LEED and MEIS results a consistent picture of the structure of this surface is provided. While the results of the most recent theoretical total-energy calculations are qualitatively consistent with this experimental consensus, significant quantitative differences remain.
Challenges in the Surface Analytical Characterisation of Anodic TiO 2 Films – a Review
Zeitschrift für Physikalische Chemie, 2005
Surface analytical findings of thick anodic passive layers on titanium are discussed and analysed in order to give a critical overview about possible artefacts for the system Ti/TiO 2 . In line with previous studies, the pretreatment of the surface can have an influence on the morphology of the passive film. On polycrystalline surfaces, the oxide film shows a different composition and morphology on differently oriented grains, but although the crystallographic orientation of the substrate influences the thickness and the morphology of the passive layer the defect density does not seem to depend so strongly on the substrate condition. Ion sputtering strongly alters the TiO 2 surface morphology and composition which can be directly seen in SEM images of the surface and indirectly from the change in XPS spectra throughout a sputter depth profile. The material removal is not homogeneous and sputtering leads to reduction of the surface oxide. In AES experiments the passive layer on titanium undergoes changes induced by the primary electron beam that is used for the analysis. All these effects make the Ti/Ti-oxide a challenging system particularly when employing surface analytical methods for its investigation. Recent developments like the growth of nanotube structures on titanium give the system a renewed importance.