A surface X-ray diffraction study of TiO2(1 1 0)(3 × 1)-S

Surface X-ray diffraction has been used to investigate the structure of TiO₂(1 1 0)(3 × 1)-S. In concert with existing STM and photoemission data it is shown that on formation of a (3 × 1)-S overlayer, sulphur adsorbs in a position bridging 6-fold titanium atoms, and all bridging oxygens are lost. Sulphur adsorption gives rise to significant restructuring of the substrate, detected as deep as the fourth layer of the selvedge. The replacement of a bridging oxygen atom with sulphur gives rise to a significant motion of 6-fold co-ordinated titanium atoms away from the adsorbate, along with a concomitant rumpling of the second substrate layer.     

The 2 × 1 reconstruction of the rutile TiO2(0 1 1) surface: A combined density functional theory, X-ray diffraction, and scanning tunneling microscopy study

An extensive search for possible structural models of the (2 × 1)-reconstructed rutile TiO₂(0 1 1) surface was carried out by means of density functional theory (DFT) calculations. A number of models were identified that have much lower surface energies than the previously-proposed ‘titanyl’ and ‘microfaceting’ models. These new structures were tested with surface X-ray diffraction (SXRD) and voltage-dependent STM measurements. The model that is (by far) energetically most stable shows also the best agreement with SXRD data. Calculated STM images agree with the experimental ones for appropriate tunneling conditions. In contrast to previously-proposed models, this structure is not of missing-row type; because of its similarity to the fully optimized brookite TiO₂(0 0 1) surface, we call it the ‘brookite (0 0 1)-like’ model. The new surface structure exhibits two different types of undercoordinated oxygen and titanium atoms, and is, in its stoichiometric form, predicted to be rather inert towards the adsorption of probe molecules.     

Oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

The oxidation of the W(1 0 0) surface at elevated temperatures has been studied using room temperature STM and LEED. High exposure of the clean surface to O₂ at 1500 K followed by flash-annealing to 2300 K in UHV results in the formation of a novel p(3 × 1) reconstruction, which is imaged by STM as a missing-row structure on the surface. Upon further annealing in UHV, this surface develops a floreted LEED pattern characteristic of twinned microdomains of monoclinic WO_x, while maintaining the p(3 × 1) missing-row structure. Atomically resolved STM images of this surface show a complex domain structure with single and double W〈0 1 0〉 rows coexisting on the surface in different domains.     

SrTiO3(0 0 1) reconstructions: the (2 × 2) to c(4 × 4) transition

Scanning tunneling microscopy (STM) is used to investigate the (0 0 1) surface structure of Nb doped SrTiO₃ single crystals annealed in ultra high vacuum (UHV). Atomically resolved images of the (2 × 2) reconstructed surface are obtained after annealing a chemically etched sample. With further annealing dotted row domains appear, which coexist with the (2 × 2) reconstruction. The expansion of these domains with further annealing gives rise to the formation of a TiO₂ enriched c(4 × 4) reconstruction.     

Adsorption structure of acetic anhydride on a TiO2(1 1 0) surface observed by scanning tunneling microscopy

The carboxylic acids are stably adsorbed on TiO₂(1 1 0) surfaces at room temperature. To demonstrate the neutralization mechanism proposed by Ashima et al. [H. Ashima, W.-J. Chun, K. Asakura, Surf. Sci. 601 (2007) 1822.] that explains the stable adsorption of carboxylic acids, we studied the full-coverage adsorption structure of acetic anhydride on a TiO₂(1 1 0) surface by STM (scanning tunneling microscopy). We directly observed three postulated species on the TiO₂(1 1 0) surface; normal acetates (termed acetate A) forming a (2 × 1) ordered structure, a minor acetate species (termed acetate B) which was present between the bridging oxygen and the 5-fold Ti, and the oxygen vacancies. We determined the ratio of these three species. This ratio was in good agreement with the postulated conversion reaction of acetate B to A.     

Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)

We have studied hydrogen adsorption on the Ge(1 1 1) c(2 × 8) surface using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). We find that atomic hydrogen preferentially adsorbs on rest atom sites. The neighbouring adatoms appear higher in STM images, which clearly indicates a charge transfer from the rest atom states to the adatom states. The surface states near the Fermi-level have been followed by ARPES as function of H exposure. Initially, there is strong emission from the rest atom states but no emission at the Fermi-level which confirms the semiconducting character of the c(2 × 8) surface. With increasing H exposure a structure develops in the close vicinity of the Fermi-level. The energy position clearly indicates a metallic character of the H-adsorbed surface. Since the only change in the STM images is the increased brightness of the adatoms neighbouring a H-terminated rest atom, we identify the emission at the Fermi-level with these adatom states.     

Initial stages of the autocatalytic oxidation of the InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen

The initial stages of oxidation of the In-rich InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen (O₂) were studied using scanning tunneling microscopy (STM) and density functional theory (DFT). It was shown that the O₂ dissociatively chemisorbs along the rows in the [1 1 0] direction on the InAs surface either by displacing the row-edge As atoms or by inserting between In atoms on the rows. The dissociative chemisorption is consistent with being autocatalytic: there is a high tendency to form oxygen chemisorption sites which grow in length along the rows in the [1 1 0] direction at preexisting oxygen chemisorption sites. The most common site size is about 21-24 Å in length at ∼25% ML coverage, representing 2-3 unit cell lengths in the [1 1 0] direction (the length of ∼5-6 In atoms on the row). The autocatalysis was confirmed by modeling the site distribution as non-Poisson. The autocatalysis and the low sticking probability (∼10⁻⁴) of O₂ on the InAs(0 0 1)-(4 × 2)/c(8 × 2) are consistent with activated dissociative chemisorption. The results show that is it critical to protect the InAs surface from oxygen during subsequent atomic layer deposition (ALD) or molecular beam epitaxy (MBE) oxide growth since oxygen will displace As atoms.     

The adsorption of and reaction of NO2 on Ag(1 1 1)-p(4 × 4)-O and formation of surface nitrate

The adsorption and reaction of nitrogen dioxide on the Ag(1 1 1)-p(4 × 4)-O surface has been investigated with RAIRS, TPRS and STM. At 300 K NO₂ initially reacts with the oxygen overlayer to form nitrate in p(3 × 3) and p(4 × 4) structures, which convert to a new p(3 × 3) at saturation coverage. Surface pitting during nitrate adsorption is suggestive of the incorporation of silver atoms into the NO₃ structure. With heating NO₃ decomposes into NO₂ and O at 396 K and 497 K, and oxygen desorbs at 578 K.     

Quantitative determination of the local structure of H2O on TiO2(1 1 0) using scanned-energy mode photoelectron diffraction

O 1s scanned-energy mode photoelectron diffraction has been used to determine the local structure of molecular water on TiO₂(1 1 0). The adsorption site is found to be atop five-fold coordinated surface Ti atoms, confirming the results of published total energy calculations and STM imaging. The Ti-O_w bondlength is found to be 2.21 ± 0.02 Å, much longer than Ti-O bondlengths in bulk TiO₂ and for the formate (HCOO-) species adsorbed on this surface. This is consistent with relatively weak bonding, and in general agreement with total energy calculations, although all of the published calculations yield bondlengths somewhat longer than the experimental value. Structural optimisation based on the photoelectron diffraction data also provides some information on the associated substrate relaxation. In particular, the bondlength of the five-fold coordinated surface Ti atom to the O atom directly below shows the same contraction (relative to the bulk) as is found for the clean surface, reinforcing the picture of rather weak bonding of the water to this same Ti surface atom.     

Quasi-one-dimensional structures on the Si(1 1 1) surface induced by Ba adsorption

Ba-induced quasi-one-dimensional reconstructions of the Si(1 1 1) surface have been investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While the 3 × ‘2’ surface shows double-periodicity along the stripes in STM images consistent with half-order streaks observed in LEED patterns, no sign of the double-periodicity along the chain direction was detected for the 5 × 1 surface. The 5× stripes in STM images show internal structures with multiple rows. The two rows comprising the boundaries of a 5× stripe in the filled-state STM image are found to have 3a × √3/2 spacing across the stripe. The observation of the successive 3× and 2× spacings between the boundary rows supports a structural model proposed for the Ba-induced 5 × 1 Si reconstruction composed of honeycomb chains and Seiwatz chains. The highest coverage 2 × 8 surface does not reveal a quasi-1D row structure in STM images.     

Room-temperature-adsorption behavior of acetic anhydride on a TiO2(1 1 0) surface

We have studied the adsorption structure of acetic anhydride on a TiO₂(1 1 0) surface using XPS (X-ray photoelectron spectroscopy), LEED (low energy electron diffraction) and HREELS (high resolution electron energy loss spectroscopy) to determine the origins of the unique adsorption properties of carboxylic acids on a TiO₂(1 1 0) surface. The C 1s XPS data indicated that the saturation carbon amount of adsorbed acetic anhydride was 12 ± 3% larger than that of the adsorbed acetic acid. LEED showed p(2 × 1) weak spots for the acetic anhydride adsorbed surface. The HREELS spectra revealed the dissociative adsorption of acetic anhydride. Based on these findings, we concluded that the neutralization of the bridging oxygen atoms associated with the dissociative adsorption is necessary for the stable adsorption of carboxylates on the 5-fold Ti sites.     

Structure and properties of TiO<Subscript>2</Subscript> surfaces: a brief review

Titanium oxides are used in a wide variety of technological applications where surface properties play a role. TiO₂ surfaces, especially the (110) face of rutile, have become prototypical model systems in the surface science of metal oxides. Reduced TiO₂ single crystals are easy to work with experimentally, and their surfaces have been characterized with virtually all surface-science techniques. Recently, TiO₂ 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 TiO₂(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 TiO₂(110). Some of these studies have been instrumental in furthering the understanding of the ‘strong metal support interaction’ between group-VIII metals and TiO₂, as well as low-temperature oxidation reactions on TiO₂-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 TiO₂ anatase phase can also be made accessible to surface investigations. Received: 4 March 2002 / Accepted: 20 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-504/862-8279, E-mail: diebold@tulane.edu     

STM study of the initial stages of C60 adsorption on the Pt(1 1 0)-(1 × 2) surface

We have studied the initial stages of adsorption of C₆₀ on the Pt (1 1 0)-(1 × 2) surface by means of STM. At room temperature, fullerene molecules adsorb in the troughs between two adjacent Pt rows of the missing row reconstruction. Mobility over the terraces is negligible, denoting strong bonding with the surface, also testified by a well-defined orientation of fullerene monomers with respect to the substrate. Upon annealing at 750 K, molecular migration towards kinks and step edges occurs, where small islands nucleation begins. A commensurate registry with the substrate is maintained by small (5-10 molecules) C₆₀ aggregates, leading to expanded nearest-neighbour distances with respect to those found in hexagonal close packed fullerene ad-islands grown on other metallic substrates.     

水在光诱导制备TiO2(110)表面BBO氧空位过程中的抑制作用 (cited: 1)

利用程序升温脱附谱研究了用266 nm激光在表面制备不同氧空位浓度的方法．实验表明利用266 nm激光能够很容易产生表面氧空位．同样90 s光照,在光子数密度大于6.7×10¹⁶ photons/cm²s的时候,表面氧空位浓度和和光子数密度成线性关系．在光照过程中,没有观测到氧气和钛原子的脱附,表明利用266 nm激光在TiO₂(110)-(1×1)制备氧空位是一种有效而且温和的方式．而在表面提前吸附水利用激光产生氧空位的速率比干净的表面慢了两个数量级．进一步研究表面,预先吸附了水后,在光照过程中水更容易脱附,导致桥氧原子的脱附被抑制,降低了氧空位产生的速率     

金红石型TiO2(110)表面性质及STM形貌模拟

采用密度泛函理论从头计算了金红石型TiO₂(110)表面的相关性质,切片模型含有9层原子,采用化学整比表面结构,晶胞真空层厚度为1.5nm,原子价电子采用超软赝势表达.差分电子密度分布图发现原子附近区域电子密度分布以球对称为主,电子定域形成离子键的趋势较强,但在Ti和O原子之间存在较弱的共价键.模拟了金红石型TiO₂(110)表面结构的扫描隧道显微镜(scanning tunneling microscope,简称STM)图像,利用Tersoff-Hamann的成像理论,在+2V的正向偏压下,采用一系列变化的数值作为STM探针离表面桥式氧的距离,分析了相关态密度的变化,发现(110)表面的STM形貌凸起部分来自于5—Ti原子,而不是2—O原子(桥式氧),在TiO₂(110)表面结构成像中,电子效应起主导作用,证实了STM实验观察到的亮行是Ti原子的结果. 关键词： 功能材料 密态泛函理论 表面结构 STM像     

A study of the interaction between perylene and the TiO2(1 1 0)-(1 × 1) surface-based on XPS, UPS and NEXAFS measurements

The interaction between a semi-large aromatic hydrocarbon compound (perylene) and the TiO₂(1 1 0)-(1 × 1) surface under ultra high vacuum conditions has been probed by X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) methods. UPS measurements of the adsorbate system have been compared with an experimental UPS spectrum of perylene in the gas phase and a calculated spectrum obtained by means of density functional theory (DFT) methods. NEXAFS results of perylene molecules adsorbed on TiO₂(1 1 0)-(1 × 1) were compared with data from an α-phase perylene single crystal. A novel analysis of the valence data has been employed to show that no strong chemical interaction takes place between perylene and the TiO₂(1 1 0)-(1 × 1) surface. Furthermore, angle-dependent NEXAFS measurements and the growth curve results suggest that the perylene molecules are oriented flat down onto the TiO₂ substrate due to weak van der Waals interactions.     

Soft X-ray appearance potential spectroscopy (SXAPS) study of TiO2(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces

A soft X-ray appearance potential spectroscopy (SXAPS) apparatus with high sensitivity was built to measure non-derivative spectra. SXAPS spectra (non-derivative) of Ti 2p and O 1s for TiO₂(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces have been measured using low incident currents (about 10 μA/cm²) and a photon counting mode. Density of empty states on Ti and O sites are deduced by self-deconvoluting the spectra. The self-deconvoluted SXAPS spectra are qualitatively similar to those measured by X-ray absorption spectroscopy (XAS). The Ti 2p_3/2 spectrum shows two strong peaks which correspond to t_2g and e_g states. For the O 1s spectrum two strong peaks near the threshold are also found which can be ascribed to O 2pπ and O 2pσ states. These results suggest that the spectra almost obey the dipole selection rule, so-called the “approximate dipole selection rule”. The SXAPS spectra of Ti 2p and O 1s for the (1 1 0) and (0 0 1) surfaces resemble qualitatively, which is consistent with the XAS results. The spectra measured on the (1 1 0)-1 × 2 surface at an incident angle of 45° off normal to the surface and on the (1 1 0) surface sputtered by Ar ions indicate that SXAPS is very sensitive to the surface electronic states.     

Band bending at the Ni/Si(100)-2×1 submonolayer interface

Band bending at the Ni/Si(100)-2×1 interface has been monitored by using Si 2p core level photoemission spectra. Two nickel-induced Si 2p components appear in the initial interaction between Ni and Si(100)-2×1, which is confined at the top surface and the first subsurface layers. At Ni coverage less than 0.0375 ML, Ni atoms prefer the adamantane interstitial sites on the first subsurface, but switch to the pedestal sites on Si dimer rows at higher Ni coverage. The change in the preferred occupation sites of Ni atoms on the Si(100)-2×1 surface strongly affects the amount of band bending shift. The shift towards higher binding energy, when Ni atoms occupy the adamantane interstitial sites, is attributed to metal-induced-gap states. While Ni atoms occupy the pedestal sites, the band bending shift is reduced which is attributed to the passivation of surface states.     

Scanning tunnelling microscopy and spectroscopy of the reduced TiO2(1 0 0) surface

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to study the topographic and electronic structure of a reduced TiO₂(1 0 0) surface. The STM results showed that the TiO₂(1 0 0) surface is capable to form (1 × 7) reconstruction which can transform to (1 × 3) reconstruction due to reoxidation of the surface. The CITS results showed that the (1 × 7) reconstruction is much more metallic in compared to the (1 × 3) reconstruction showing pronounced surface states at energy 1.3 eV and 0.8 eV below the Fermi level and at energy 1.0-1.2 eV above the Fermi level.