D.R. Jennison

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.

Cu interactions with α-Al2O3(0001): effects of surface hydroxyl groups versus dehydroxylation by Ar-ion sputtering

XPS studies and first principles calculations compare Cu adsorption on heavily hydroxylated sapphire (0001) with a dehydroxylated surface produced by Ar{sup +} sputtering followed by annealing in O{sub 2}. Annealing a cleaned sapphire sample with an O{sub 2} partial pressure of {approximately}5 x 10{sup {minus}6} Torr removes most contaminants, but leaves a surface with {approximately}0.4ML carbon and {approximately}0.4ML OH. Subsequent light (6 min.) Ar ion sputtering at 1 KeV reduces the carbon to undetectable levels but does not dehydroxylate the surface. Further sputtering at higher Ar ion excitation energies (>2 KeV) partially dehydroxylates the surface, while 5 KeV Ar ion sputtering creates oxygen vacancies in the surface region. Further annealing in O{sub 2} repairs the oxygen vacancies in the top layers but those beneath the surface remain. Deposition of Cu on the hydroxylated surface at 300 K results in a maximum Cu(I) coverage of {approximately}0.35 ML, in agreement with theoretical predictions.

Computing accurate surface energies and the importance of electron self-energy in metal/metal-oxide adhesion

Abstract Starting with density functional theory (DFT) results, we correct surface energies using a jellium model as applied to surface electron densities obtained from the first principles calculations. We apply these results to the computed work of adhesion for Pd(1xa01xa01) to α-Al2O3(0xa00xa00xa01). Here polarization bonding dominates at the interface and is well described by DFT, but the surface energies of both the metal and the oxide have substantial errors due to electron self-exchange and self-correlation. We show that this correction is quite large for the generalized gradient approximation (GGA), thus explaining the difference between GGA results for metal/metal-oxide binding and those obtained by the local density approximation, where an accidental cancellation in errors produces better agreement with experiment. After the corrections, both methods produce results that are within the experimental error bars.

Effect of oxide vacancies on metal island nucleation

Abstract Point defects on oxide surfaces, presumably oxygen vacancies, are traditionally considered preferential nucleation centers for metal island formation. In a series of first-principles calculations for transition and noble metal nucleation on MgO(1xa00xa00), we show that the propensity for neutral anion vacancies to nucleate metal islands is strongly element dependent: To the right in a period, where d-shell filling is substantial, vacancies typically inhibit nucleation, whereas the opposite holds for far-left elements. This bears significant implications for the system-specific design of metal–oxide interface properties.

Structure of an ultrathin TiOx film, formed by the strong metal support interaction (SMSI), on Pt nanocrystals on TiO2(110)

Abstract We use first-principles density functional theory to study ultrathin TiOx films on Pt(1xa01xa01). The preferred interface with Pt has Ti with O as an overlayer. However, this ordering, preferred over Ti/O/Pt by 2.9 eV/Ti–O unit, produces >10% stress. This explains a complex structure, seen using STM, of TiOx bilayers encapsulating Pt(1xa01xa01) nanofacets: the energetics of stress relief is about ten times that of differences in the various possible O/Ti/Pt(1xa01xa01)-layer site occupations, thus favoring dislocation formation. A structure is found that is stable. It consists of a series of linear misfit dislocations at the relatively weak Ti/Pt interface that are 6/7 Ti/Pt rows wide. In addition, strong interactions at the O/Ti interface and O-layer strain also cause the Ti/Pt interface to abruptly change from hcp- to fcc-site Ti, producing linear “canyons” (Ti/Pt dislocation cores occur between these stripes). Furthermore, alternating hcp- and fcc-site triangles, each with ten O-atoms, are separated by bridging O in an abrupt O/Ti misfit dislocation, thus producing a zigzag pattern. The above dislocations release strain along both the x- and y-directions in the surface plane. However, we have been unable to find a stable structure if the zigzag ends consist of O, but stability is found if they consist of Ti. Finally, reverse bias STM images indicate the ends might indeed different than the line portions of the zigzag features.

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.

Ultrathin alumina film Al-sublattice structure, metal island nucleation at terrace point defects, and how hydroxylation affects wetting

In this paper, we include for discussion three topics of current interest in metal oxide surface science. Using first principles density functional theory (DFT) [1] calculations, we have investigated: (1) the atomic-scale structure of experimentally-relevant ultrathin alumina films, (2) the role of common point defects in metal island nucleation on oxide terraces, and (3) the growth and morphology of metals on oxide surfaces which have high concentrations of a common impurity.

STM-induced void formation at the Al2O3/Ni3Al(1 1 1) interface

Under ultrahigh vacuum conditions at 300 K, the applied electric field and/or resulting current from an STM tip creates nanoscale voids at the interface between an epitaxial, 7.0 A thick Al2O3 film and a Ni3Al(1 1 1) substrate. This phenomenon is independent of tip polarity. Constant current (1 nA) images obtained at +0.1 V bias and +2.0 V bias voltage (sample positive) reveal that voids are within the metal at the interface and, when small, are capped by the oxide film. Void size increases with time of exposure. The rate of void growth increases with applied bias/field and tunneling current, and increases significantly for field strengths >5 MV/cm, well below the dielectric breakdown threshold of 12±1 MV/cm. Slower rates of void growth are, however, observed at lower applied field strengths. Continued growth of voids, to ∼30 A deep and ∼500 A wide, leads to the eventual failure of the oxide overlayer. Density functional theory calculations suggest a reduction–oxidation mechanism: interfacial metal atoms are oxidized via transport into the oxide, while oxide surface Al cations are reduced to admetal species which rapidly diffuse away. This is found to be exothermic in model calculations, regardless of the details of the oxide film structure; thus, the barriers to void formation are kinetic rather than thermodynamic. We discuss our results in terms of mechanisms for the localized pitting corrosion of aluminum, as our results suggest nanovoid formation requires just electric field and current, which are ubiquitous in environmental conditions.