Wilhelm Hebenstreit

Oxygen-induced restructuring of the TiO2(110) surface: a comprehensive study

We report a comprehensive experimental and theoretical study of the eVect of oxidizing a TiO 2 (110) surface at moderate temperatures. The surfaces are investigated with scanning tunneling microscopy (STM ), low-energy He+ ion scattering (LEIS ) and static secondary ion mass spectroscopy (SSIMS ). Flat (1◊1)-terminated TiO 2 (110) surfaces are obtained by sputtering and annealing in UHV at 880 K. These surfaces are exposed to oxygen gas at elevated temperatures in the range 470‐830 K. Formation of irregular networks of pseudo-hexagonal rosettes (6.5 A ˚ ◊ 6A ˚ ) and small (11:0] oriented (1◊1) islands along with {001}-oriented strands is induced at temperatures from 470 to 660 K. After annealing above 830 K, only regular (1◊1) terraces and white strands are observed. The composition of these oxygen-induced phases is quantified using 18O 2 gas in combination with LEIS and SSIMS measurements. The dependence of the restructuring process on annealing time, annealing temperature, and sample history is systematically investigated. Exposure to H 2 18O and air in the same temperature regime fails to induce the restructuring. UHV annealing of restructured, oxygen-enriched TiO 2 (110) surface smooths the surfaces and converts the rosette networks into strands and finally into the regular (1◊1) terraces. This is reported in an accompanying paper [M. Li, W. Hebenstreit, U. Diebold, Phys. Rev. B (1999), submitted ]. The rosette model is supported by first-principles density functional calculations which show a stable structure results, accompanied by significant relaxations from bulk-truncated positions. A mechanism for the dynamic processes of the formation of rosettes and (1◊1) islands is presented and the importance of these results for the surface chemistry of TiO 2 (110) surfaces is discussed.

Atomic resolution by STM on ultra-thin films of alkali halides: experiment and local density calculations

Abstract Atomically resolved scanning tunneling microscopy (STM) of ultra-thin NaCl films on Al(111) and Al(100) demonstrates that only one atomic species of NaCl is imaged as a protrusion. By comparison of the constant current STM images with ab initio calculations of the local density of states by means of the full-potential linearized augmented plane wave method, the protrusions could be attributed to the anion Cl – . The calculations shows that a higher density of occupied states at the Cl-sites than for the Na-sites around the Fermi level causes the STM contrast between Cl and Na. With increasing number of NaCl layers the density of states in the band gap is reduced and the apparent height of additional NaCl layers decreases. The maximum film thickness allowing successful imaging by STM was found to be three layers.

Adsorption of sulfur on TiO2(110) studied with STM, LEED and XPS: temperature-dependent change of adsorption site combined with O–S exchange

We have studied the adsorption of elemental sulfur on TiO 2 (110) for exposure at room temperature and at 300°C. Depending on the substrate temperature we found diVerent adsorption sites with scanning tunneling microscopy (STM ). At room temperature sulfur adsorbs on the titanium rows with a high mobility along the [001] direction. At 300°C sulfur adsorbs at the position of the bridging oxygen rows and forms short chains along the [11:0] direction at low coverages. High coverages result in the formation of a (3◊1) superstructure. X-ray photoelectron spectroscopy ( XPS) shows a 2 eV shift of the sulfur 2p peak to lower binding energy concurrently with the change of adsorption site from the titanium to the oxygen rows. For the (3◊1) structure a distinct Ti3+ shoulder appears at the Ti 2p 3/2 peak. Based on these measurements we present a model where sulfur replaces every third bridging oxygen atom of the substrate and the other bridging oxygens are completely removed.

Oxygen-induced restructuring of rutile TiO2(110): formation mechanism, atomic models, and influence on surface chemistry

The rutile TiO2(110) (1×1) surface is considered the prototypical ‘well-defined’ system in the surface science of metal oxides. Its popularity results partly from two experimental advantages: (i) bulk-reduced single crystals do not exhibit charging, and (ii) stoichiometric surfaces, as judged by electron spectroscopies, can be prepared reproducibly by sputtering and annealing in oxygen. We present results that show that this commonly applied preparation procedure may result in a surface structure that is by far more complex than generally anticipated. Flat, (1×1)-terminated surfaces are obtained by sputtering and annealing in ultrahigh vacuum. When re-annealed in oxygen at moderate temperatures (470–660 K), irregular networks of partially connected, pseudohexagonal rosettes (6.5×6 Awide), one-unit cell wide strands, and small (≈tens of A) (1×1) islands appear. This new surface phase is formed through reaction of oxygen gas with interstitial Ti from the reduced bulk. Because it consists of an incomplete, kinetically limited (1×1) layer, this phenomenon has been termed ‘restructuring’. We report a combined experimental and theoretical study that systematically explores this restructuring process. The influence of several parameters (annealing time, temperature, pressure, sample history, gas) on the surface morphology is investigated using STM. The surface coverage of the added phase as well as the kinetics of the restructuring process are quantified by LEIS and SSIMS measurements in combination with annealing in 18O-enriched gas. Atomic models of the essential structural elements are presented and are shown to be stable with first-principles density functional calculations. The effect of oxygen-induced restructuring on surface chemistry and its importance for TiO2 and other bulk-reduced oxide materials is briefly discussed.

Chemical ordering and reconstruction of Pt25Co75(100): an LEED/STM study

The surface of a disordered Pt25Co75(100) alloy has been investigated using quantitative LEED, AES and UHV-STM at room temperature. Atomic-resolution images reveal that it reconstructs with close-packed rows shifted by half the interatomic distance, from hollow to bridge sites. The density of shifted rows increases with the surface Pt concentration, leading to (1 × 5), (1 × 6) and (1 × 7) patterns. Segregation and chemical ordering lead to the formation of c(2 × 2) domains between the shifted rows. Chemical resolution was achieved with STM: the apparent height of the Pt atoms in the STM topographs is about 0.1–0.4 A above that of Co, whereas LEED shows that Pt atoms are geometrically ∼0.04 A higher. The composition was determined down to the fourth layer. An oscillatory segregation profile is observed, with Pt-rich layers (〈C1〉 = 62.6% Pt, 〈C3〉 = 53.5%) and Pt-depleted layers (〈C2〉 = 6.9%, 〈C4〉 = 2.7%). Chemical ordering is present in the third layer and the four-layer surface slab stabilises with a structure and a composition quite similar to that of the L12 PtCo3 phase. As regards the composition and ordering of the top layer, there is a remarkable agreement between chemically resolved STM analysis and LEED analysis.

THE RELATIONSHIP BETWEEN BULK AND SURFACE PROPERTIES OF RUTILE TiO2(110)

We report scanning tunneling microscopy and complementary spectroscopic measurements on TiO2(110) surfaces. We show data on (i) a surface restructuring process that results from annealing in oxygen; (ii) Pt clusters, grown at room temperature and encapsulated upon high temperature annealing; and (iii) adsorption of sulfur. In each case, heavily reduced, dark crystals show a very different behavior than more stoichiometric, light blue ones.

Scanning tunneling microscopy of binary-alloy surfaces: is chemical contrast a consequence of alloying?

Abstract Recent STM studies achieved chemical resolution on PtRh and PtNi alloy surfaces. By a first-principles method employing the Tersoff–Hamann model, we have simulated STM scans on PtRh and PtNi(100) surfaces by calculating the apparent heights of individual surface atoms. The difference in apparent heights between Pt and Rh atoms is caused by changes in the density of states due to alloying. The simulations for the PtNi(100) surface, however, yield apparent heights of Pt and Ni atoms below atomic resolution, indicating that in the experiment, tip–sample interactions are responsible for chemical and atomic resolution.

Segregation and reconstructions of PtxNi1 − x(100)

Abstract It is known that on (100) surfaces of PtxNi1 − x single crystals Pt segregates. With increasing Pt concentration in the surface the transition from unreconstructed Ni(100) to the pseudo hexagonal Pt(100) reconstruction occurs via a shifted row reconstruction and several pseudo hexagonal (n × 1) superstructures (n = 7, 12 and 19) consisting of similar (7 × 1) and (5 × 1) subcells. This was revealed by atomically resolved scanning tunnelling microscopy (STM). From low energy ion scattering measurements it becomes clear that the formation of the pseudo hexagonal structure leads to strong amplification of Pt segregation. Chemically resolved STM on the atomic scale shows that Pt prefers the highly coordinated four-fold hollow sites in the pseudo hexagonal structures and Ni is pushed into nearly on-top or bridge sites. Therefore the strong tendency of Pt to increase its coordination is proposed as the driving force of the reconstructions. Corrugations and chemical ordering measured by STM within the pseudo hexagonal reconstructions are confirmed by simulations based on embedded atom method potentials.

Segregation of impurities on Cr(100) studied by AES and STM

Abstract With increasing annealing temperature, Auger electron spectroscopy (AES) of a Cr(100) single crystal shows segregation of C, N and O as the dominating segregating species, indicating competitive segregation of these elements. An STM study of N structures shows a c(2 × 2) superstructure at N coverages up to 1 2 . The local N coverage can be increased by insertion of N-rich domain boundaries up to 2 3 , where a c(3√2 × √2)R ± 45° structure forms, followed by a first-order phase transformation to a p(1 × 1) structure. The existence of patches of the N-rich p(1 × 1) structure at coverages below 2 3 is probably due to additional carbon impurities stabilizing this structure. The possibility of inverse corrugation on the pure Cr(100) surface is discussed.

Interaction of oxygen with PtRh(100) studied with STM

Abstract The adsorption of oxygen at 500°C on a Pt50Rh50(100) single crystal surface was studied using UHV-STM and Auger electron spectroscopy. Images were taken of the p(3 × 1) phase; of a mixed phase with p(2 × 2), c(2 × 2) and (3 × 3) units; and of rhodium oxide patches. Possible models for these structures involving surface reconstruction are presented. Exposure of the p (3 × 1) O PtRh (100) to H2 at room temperature led to the conversion to the p(1 × 1) substrate structure. The ordering and composition of this substrate structure after reduction is discussed.