Bromine Adsorption and Thermal Stability on Rh(111): A Combined XPS,LEED and DFT Study

This study addresses a fundamental question in surface science: the adsorption of halogens on metal surfaces. Using synchrotron radiation-based high-resolution X-ray photoelectron spectroscopy (XPS), temperature-programmed XPS, low-energy electron diffraction (LEED) and density functional theory (DFT) calculations, we investigated the adsorption and thermal stability of bromine on Rh(111) in detail. The adsorption of elemental bromine on Rh(111) at 170 K was followed in situ by XPS in the Br 3d region, revealing two individual, coverage-dependent species, which we assign to fcc hollow- and bridge-bound atomic bromine. In addition, we find a significant shift in binding energy upon increasing coverage due to adsorbate-adsorbate interactions. Subsequent heating shows a high thermal stability of bromine on Rh(111) up to above 1000 K, indicating strong covalent bonding. To complement the XPS data, LEED was used to study the long-range order of bromine on Rh(111): we observe a (√3×√3)R30° structure for low coverages (≤0.33 ML) and a star-shaped compression structure for higher coverages (0.33–0.43 ML). Combining LEED and DFT calculations, we were able to visualize bromine adsorption on Rh(111) in real space for varying coverages.     

Coverage‐Dependent Variation of Adsorption Configurations of Methionine on Ge(100)

The adsorption configurations of methionine molecules on the Ge(100) surface have been studied by using DFT calculations, core‐level photoemission spectroscopy (CLPES), and low‐energy electron diffraction (LEED) to scrutinize the adsorption structure as a function of coverage. At first, we obtained two important and stable structures. One is the most stable structure between these structures described as an “O H dissociated‐N dative‐S dative‐bonded structure” and the other is a less stable adsorption structure of these indicating an “O H dissociated‐S dative‐bonded structure” by using DFT calculations. We also performed CLPES to clarify our DFT calculation results. Through the spectral analysis of the S 2p, C 1s, N 1s, and O 1s core‐level spectra, we acquired the reasonable results that also revealed quite different bonding configurations depending on the methionine coverage. At low coverage (ca. 0.30 ML), a single type of sulfur and charged nitrogen peaks, which indicate an “O H dissociated‐N dative‐S dative‐bonded structure”, were observed. On the other hand, two types of sulfur peaks with thiol formation and two nitrogen peaks with neutralized and charged characteristics were monitored at a higher coverage (0.60 ML and above), which can be described as an “O H dissociated‐S dative‐bonded structure”. Hence, we can clearly demonstrate that our results obtained from CLPES spectra and DFT calculations are matched well with each other. Moreover, we additionally confirmed that the relative population of the two types of thiols and amines being included in methionine in between half monolayer induces a surface reorientation in the ordering from 2×1 to 1×1 employing LEED. This interesting variation of the methionine adsorbed on the Ge(100) surface by coverage dependence will be precisely discussed by using DFT calculations, CLPES, and LEED.     

The influence of carbon on the adsorption of CO on a Rh(100) single crystal

The influence of carbon on the adsorption of CO from a Rh(100) single crystal has been studied by a combination of experimental techniques: Temperature Programmed Desorption (TPD), Low Energy Electron Diffraction (LEED), and High Resolution Electron Energy Loss Spectroscopy (HREELS). These experimental techniques were combined with a computational approach using Density Functional Theory (DFT). Using this combination of techniques, we have shown that surface carbon greatly influences adsorbed CO and we have determined the exact magnitude of this interaction. Furthermore, we have demonstrated that carbon does not remain fully on the surface; at higher coverage it diffuses partially to subsurface positions. The presence of these subsurface species significantly influences the adsorbates on the surface.     

Interaction of SO3 with c-ZrO2(111) films on Pt(111)

Single-crystalline sulfated c-ZrO2(111) films of the cubic (c) type have been prepared by reactive deposition of Zr onto Pt(111) in an O2 atmosphere and subsequent exposition to a SO3 atmosphere. The morphology, atomic structure, and composition have been examined by scanning tunneling microscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy, and density functional theory (DFT) calculations. The clean c-ZrO2(111) films display a (2x2) surface structure. During SO3 exposure at room temperature, a clear (radical3xradical3)R30 degrees structure develops. At about 700 K, the SO3-induced (radical3xradical3)R30 degrees structure disappears and the bright (2x2) LEED pattern of the clean ZrO2 films reappears. The energies of plausible c-ZrO2(111)/SO3 structures have been examined by DFT. The (radical3xradical3)R30 degrees structure found in the experiments turned out to be the most stable one for temperatures below 700 K. At temperatures around 700 K, a disordered low coverage structure may exist, which can not be observed by conventional LEED. A comparison of cubic zirconia surfaces with the alternative tetragonal system yields similar results for the SO3 adsorption in the DFT calculations and shows that c-ZrO2 surfaces are good models for the industrial used tetragonal ZrO2 supports.     

Effect of Erosive Agents on Surface Characteristics of Nano-Fluorapatite Ceramic: An In-Vitro Study

Erosive beverages cause dissolution of natural teeth and intra-oral restorations, resulting in surface characteristic changes, particularly roughness and degradation. The purpose of this study was to evaluate the surface roughness and topography of a dental ceramic following immersion in locally available erosive solutions. A total of 160 disc specimens of a nano-fluorapatite type ceramic (12 mm diameter and 2 mm thickness) were fabricated and equally distributed into two groups (n = 80) and then evenly distributed among the following five testing groups (n = 16): lemon juice, citrate buffer solution, 4% acetic acid, soft cola drink, and distilled water which served as a control. The surface roughness (Ra) and topography were evaluated using a profilometer and scanning electron microscope at baseline, 24 h, 96 h, and 168 h respectively. Data were analyzed using ANOVA and Tukey’s multiple comparisons (p ≤ 0.05). Surface changes were observed upon exposure to all acidic beverages except distilled water. Amongst all immersion media, 4% acetic acid produced the most severe surface roughness across all time periods (i.e., baseline, 24 h, 96 h, and 168 h). A statistically significant difference in the surface roughness values between all immersion media and across all four time intervals was observed. Erosive agents had a negative effect on the surface roughness and topography of the tested ceramic. The surface roughness increased with increased storage time intervals.     

Molecular structures of two metal tetrakis(tetrahydroborates), Zr(BH4)(4) and U(BH4)(4): equilibrium conformations and barriers to internal rotation of the triply bridging BH4 groups

The molecular structures of Zr[(mu-H)(3)BH](4) and U[(mu-H)(3)BH](4) have been investigated by density functional theory (DFT) calculations and gas electron diffraction (GED). The triply bridged bonding mode of the tetrahydroborate groups in the former is confirmed, but both DFT calculations and GED structure refinements indicate that the BH(4) groups are rotated some 12 degrees away from the orientation in which the three bridging B-H bonds are staggered with respect to the opposing ZrB(3) fragment. As a result the symmetry of the equilibrium conformation is reduced from T(d) to T. Bond distances and valence angles are as follows (DFT/GED): Zr-B = 232.2/232.4(5) pm; Zr-H(b) = 214.8/214.4(6) pm; B-H(b) = 125.3/127.8(8) pm; B-H(t) = 119.4/118.8(17) pm; angle ZrBH(b) = 66.2/65.6(3) degrees; the smallest dihedral angle of type tau(BZrBH(b)) = 48/45(2) degrees. DFT calculations on Hf(BH(4))(4) indicate that the structure of this molecule is very similar to that of the Zr analogue. Matrix-isolation IR spectroscopy and DFT calculations on U(BH(4))(4) show that while the polymeric solid-state structure is characterized by terminal triply bridging and metal-metal bridging bidentate BH(4) groups, all BH(4) groups are triply bridging in the gaseous monomer. Calculations with one of the two nonbonding 5f electrons on U occupying an a(1) and the other distributed equally among the three t(2) orbitals indicate that the equilibrium conformation has T(d) symmetry, i.e. that the three B-H(b) bonds of each tetrahydroborate group are exactly staggered with respect to the opposing UB(3) fragment with tau(BUBH(b)) = 60 degrees. Calculations including spin-orbit interactions indicate that Jahn-Teller distortions from T(d) symmetry are either absent or very small. The best agreement between observed and calculated GED intensity data was obtained for a model of T(d) symmetry, but models of T symmetry with dihedral angles tau(BUBH(b)) > 42 degrees cannot be ruled out. Bond distances and valence angles are as follows (DFT/GED): U-B = 248.8/251.2(4) pm; U-H(b) = 227.7/231.5(6) pm; B-H(b) = 126.0/131.6(5) pm, B-H(t) = 119.5/117.8(11) pm; angle UBH(b) = 65.6/63.1(3) degrees. It is suggested that the different equilibrium conformations of the three molecules are determined primarily by repulsion between bridging H atoms in different tetrahydroborate groups.     

Bonding mechanism and atomic geometry of an ordered hydroxyl overlayer on Pt(111)

Exposing water to a (2 x 2)-O precovered Pt(111) surface at 100 K and subsequently annealing at 155 K led to the formation of a well-ordered (square root 3 x square root 3)R30 degrees overlayer. The structure of this overlayer is determined by DFT and full dynamical LEED calculations. There are two O containing groups per (square root 3 x square root 3)R30 degrees unit cell and both occupy near on-top positions with a Pt-O bond length of (2.11 +/- 0.04) A. DFT calculations determined the hydrogen positions of the OH species and clearly indicate hydrogen bonds between the neighboring adsorbed OH groups whose interaction is mainly of electrostatic nature. A theoretical comparison with H(2)O shows the hybridization of OH on Pt(111) to be sp(3).     

Prediction of Henry's law constant of benzene derivatives using quantum chemical continuum‐solvation models

Semiempirical (SM2, SM5.4A, MST‐AM1, COSMO‐AM1) and ab inito (HF/PCM‐vdW, MP2//PCM‐vdW, COSMO‐DFT) dielectric continuum‐solvation models as well as the surface‐tension model SM5.0R are analyzed with respect to predicting Henry's law constant at 25°C using a compound set of benzene and 39 benzene derivatives. Both hydrophilic and hydrophobic compounds are covered with a total variation in Henry's law constant of almost eight orders of magnitude corresponding to 44 kJ/mol, and the data set is selected such that there are cases where subtle changes in the molecular structure result in substantial changes of the free energy of solvation. The calculations with SM2, COSMO‐AM1, and COSMO‐DFT include solution‐phase geometry optimization, and the ab initio results refer to polarized basis sets of double‐zeta quality, with two gradient‐corrected functionals (BPW and BLYP) being used for the DFT‐based models. The results show considerable differences in performance between the different continuum‐solvation models, and among the methods yielding solvation free energies the systematic error ranges from −0.9 kJ/mol (SM5.0R) to 12.1 kJ/mol (MP2//PCM‐vdW). In particular, the nonelectrostatic solvation energy contributions of SM2, SM5.4A, MST‐AM1, and PCM‐vdW do not correlate with each other, and with PCM‐vdW omission of the nonelectrostatic component significantly improves the relative trend. The best statistics after scaling through linear regression are achieved with the electrostatic component of MP2//PCM‐vdW (r=0.94) and with COSMO‐DFT (r=0.93). The discussion includes detailed analyses of pecularities associated with certain functional groups, deviations from the expected relationship between dipole moment and solvation energy, and a simple approach to model dispersion interaction and cavitation energy by surface area terms that differentiate between individual atom types. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 17–34, 2000     

Structural Evolution of Water on ZnO(10 0): From Isolated Monomers via Anisotropic H‐Bonded 2D and 3D Structures to Isotropic Multilayers

The surface chemistry of water on zinc oxides is an important topic in catalysis and photocatalysis. Interaction of D₂O with anisotropic ZnO(10 0) surfaces was studied by IR reflection absorption spectroscopy using s‐ and p‐polarized light incident along different directions. Interpretation of the experimental data is aided using isotopologues and DFT calculations. The presence of numerous species is revealed: intact monomers, a mixed 2D D₂O/OD adlayer, an anisotropic bilayer, and H‐bonded 3D structures. The isolated water monomers are identified unambiguously at low temperatures. The thermally induced diffusion of water monomers occurs at elevated temperatures, forming dimers that undergo autocatalytic dissociation via proton transfer. Polarization‐ and azimuth‐resolved IR data provide information on the orientation and strength of H‐bonds within the 2D and 3D structures. Ab initio molecular dynamics simulations reveal strong anharmonic couplings within the H‐bond network.     

Epitaxial growth of single-crystalline Al(2)O(3) films on Cr(2)O(3)(0001)

Thin, crystallographically oriented single-crystalline Al2O3 films can be grown epitaxially on Cr2O3(0001) by codeposition of Al vapor and O2 at a substrate temperature of 825 K. The properties and growth of these films were monitored by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), low-energy ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). Two routes of preparation were investigated: (i) stepwise growth by alternating deposition of Al at room temperature and subsequent exposure to O2 at elevated temperatures; (ii) codeposition of Al and O2 at T > 800 K. The first route was consistently found to result in the growth of a complex interfacial oxide followed by the growth of polycrystalline Al2O3. The second mode of preparation provided homogeneous and ordered, probably (0001)-oriented, films of Al2O3 that maintained a LEED pattern up to a thickness around 10 A. The surface sensitive Cr MVV Auger transition at 34 eV was completely attenuated once the Al2O3 layer had reached a thickness of 6 A, pointing to film homogeneity at an early stage. This was confirmed by the absence of a significant Cr signal in LEIS spectra.     

Effects of surface pressure on the structure of the monolayer formed at the air/water interface by a non-ionic surfactant

The monolayer formed at an air/water interface by the synthetic non-ionic surfactant, 1,2-di-O-octadecyl-rac-glyceryl-3-(omega-methoxydodecakis (ethylene glycol)) (2C18E12) has been characterized using Langmuir trough measurements, Brewster angle microscopy (BAM), and neutron reflectometry. The BAM and reflectometry studies were performed at four different surface pressures (pi) in the range 15-40 mN/m. The BAM studies (which give information on the in-plane organisation of the surfactant layer) demonstrate that the 2C18E12 molecules are arranged on the water surface to form distinct, approximately circular, 5 microm diameter domains. As the surface pressure is increased these domains retain their size and shape but are made progressively more close-packed, such that the monolayer is made more or less complete at pi=40 mN/m. The neutron reflectometry measurements were made to determine the structure of the interfacial surfactant layer at pi=15, 28, 34 and 40 mN/m, providing information on the thickness of the 2C18E12 alkyl chains', head groups' and associated solvent distributions (measured along the surface normal), along with the separations between these distributions, and the effective interfacial area per molecule. Partial structure factor analyses of the reflectivity data show that the effective interfacial area occupied decreases from 217 A2 per 2C18E12 molecule at pi=15 mN/m down to 102 A2 at pi=40 mN/m. There are concomitant increases in the widths of the surfactant's alkyl chains' and head groups' distributions (modelled as Gaussians), with the former rising from 12 A (at pi=15 mN/m) up to 19 A (at pi=40 mN/m) and the latter rising from 13 A (at pi=15 mN/m) up to 24 A (at pi=40 mN/m). The compression of the monolayer is also shown to give rise to an increased surface roughness, some of which is due to the thermal roughness caused by capillary waves, but with a significant contribution also coming from the intrinsic/structural disorder in the monolayer. At all surface pressures studied, the alkyl chains and head groups of the 2C18E12 are found to exhibit a significant overlap, and this increases with increasing pi. Given the various trends noted on how the structure of the 2C18E12 monolayer changes as a function of pi, we extrapolate to consider the structure of the monolayer at pi>40 mN/m (making comparison with its single chain (CnEm) counterparts) and then relate these findings to the observations recorded on the structure and solute entrapment efficiency of 2C18E12 vesicles.     

Enantioselective organic syntheses using chiral transition metal complexes V. (2S,3S)-Bis(dibenzophospholyl)butane, a rigid (S,S)-CHIRAPHOS analog

The P–Ph cleavage of phenyldibenzophosphole (1) with lithium in THF gives lithium dibenzophospholide (2). Reaction of 2 with ethyleneglycol ditosylate produces the known chelate ligand 1,2-bis(dibenzophospholyl)ethane (3) in good yield. Similarly, 2 and (2R,3R)-butanediol ditosylate give the new chiral chelate ligand (2S,3S)-bis(dibenzophospholyl)butane (4). Ligand exchange of [CpRu(PPh₃)₂Cl] with 3 or 4 yields the halfsandwich complexes [CpRu(C₁₂H₈PC₂H₄PC₁₂H₈)Cl] (5) and [CpRu((S,S)-C₁₂H₈PCHMeCHMePC₁₂H₈)Cl] (6). Complex 6 was characterized crystallographically (monoclinic, space group P2₁ (no. 4), a=820.6(4), b=1501.0(3), c=1172.8(6) pm, β=108.87(2)°, V=1.367(1)×10⁹ pm³, Z=2). The most conspicuous feature of the structure of 6 is the perfect coplanarity of the two dibenzophosphole moieties imposed by their steric interaction with the Cp ligand. Complex 6 and the thiophene complex [CpRu((S,S)-C₁₂H₈PCHMeCHMePC₁₂H₈)(SC₄H₄)]BF₄ (7) derived therefrom are remarkably unreactive with regard to ligand substitutions. A possible explanation is the lack of intramolecular

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–C stabilization en route to the transition state of ligand substitution. The enantiomeric purity of 6 and 7 could nevertheless be demonstrated by conversion to diastereomerically pure [CpRu((S,S)-C₁₂H₈PCHMeCHMePC₁₂H₈)((S)-CNCHMePh)]BF₄ (8).     

Magnetism in polyoxometalates: anisotropic exchange interactions in the Co3II moiety of [Co3W(D2O)2(ZnW9O34)2](12-)--A magnetic and inelastic neutron scattering study

The ground-state properties of a Co3II moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low-temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na12[Co3W(D2O)2(ZnW9O34)2].40D2O (Co3). The ferromagnetic Co3O14 cluster core consists of three octahedrally oxo-coordinated CoII ions. According to the single-ion anisotropy and spin-orbit coupling of the octahedral CoII ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co3 spin cluster is anisotropic and is expressed as H = -2 sigma a = x,y,z (Ja12 S1a S2a + Ja23 S2a S3a), where Ja are the components of the exchange interactions between the CoII ions. To reproduce the INS data, different orientations of the two anisotropic J tensors must be considered, and the following conditions had to be introduced: Jx12 = Jy23, Jy12 = Jx23, Jz12 = Jz23. This result was correlated with the molecular symmetry of the complex. The following set of parameters was obtained: Jx12 = Jy23 = 1.37, Jy12 = Jx23 = 0.218, and Jz12 = Jz23 = 1.24 meV. This set also reproduces in a satisfactory manner the specific heat, susceptibility, and magnetization properties of Co3.     

Anisotropic surface energetics and wettability of macroscopic form I paracetamol crystals

Advancing (theta(A)) and receding (theta(R)) contact angles were measured with several probe liquids on the external facets (201), (001), (011), and (110) of macroscopic form I paracetamol crystals as well as the cleaved (internal) facet (010). For the external crystal facets, dispersive surface energies gamma(d) calculated from the contact angles were found to be similar (34 +/- 1 mJ/m(2)), while the polar components varied significantly. Cleaving the crystals exposed a more apolar (010) surface with very different surface properties, including gamma(d) = 45 +/- 1 mJ/m(2). The relative surface polarity (gamma(p)/gamma) of the facets in decreasing order was (001) > (011) > (201) > (110) > (010), which agreed with the fraction of exposed polar hydroxyl groups as determined from C and O 1s X-ray photoelectron spectroscopy (XPS) spectra, and could be correlated with the number of non-hydrogen-bonded hydroxyl groups per unit area present for each crystal facet, based on the known crystal structures. In conclusion, all facets of form I paracetamol crystals examined exhibited anisotropic wetting behavior and surface energetics that correlated to the presence of surface hydroxyl groups.     

Activation and isomerization of n-butane on sulfated zirconia model systems--an integrated study across the materials and pressure gaps

Butane activation has been studied using three types of sulfated zirconia materials, single crystalline epitaxial films, nanocrystalline films, and powders. A surface phase diagram of zirconia in interaction with SO(3) and water was established by DFT calculations, which was verified by LEED investigations on single-crystalline films and by IR spectroscopy on powders. At high sulfate surface densities a pyrosulfate species is the prevailing structure in the dehydrated state; if such species are absent, the materials are inactive. Theory and experiment show that the pyrosulfate can react with butane to give butene, H(2)O and SO(2), hence butane can be activated via oxidative dehydrogenation. This reaction occurred on all investigated materials; however, isomerization could only be proven for powders. Transient and equilibrium adsorption measurements in a wide pressure and temperature range (isobars measured via UPS on nanocrystalline films, microcalorimetry and temporal analysis of products measurements on powders) show weak and reversible interaction of butane with a majority of sites but reactive interaction with <5 micromol g(-1) sites. Consistently, the catalysts could be poisoned by adding sodium to the surface in a ratio S/Na = 35. Future research will have to clarify what distinguishes these few sites.     

Tuning of the emission efficiency and HOMO-LUMO band gap for ester-functionalized {Al(salophen)(H2O)2}+ blue luminophors

A series of [AlL(H(2)O)(2)(NO(3))] complexes, with L standing for an ester substituted salophen-type ligand, has been synthesized, and the luminescence properties have been investigated. These derivatives differ by the nature of the ester-R group introduced at the C5 position of their salicylidene rings (i.e., phenyl, 7a,a'; naphthyl, 7b,b'; pentafluorophenyl, 7c,c'; and p-nitrophenyl, 7d) and by the bis-imino bridge (i.e., 1,2- phenylene, 7a-d; and 1,2-naphthalene, 7a'-c'). All the complexes are characterized by luminescence in the blue range, the chemical diversity having no effect on the emission wavelength (480-485 nm). However, the emission efficiency was found to be strongly dependent on the Schiff-base ligand with quantum yields ranging from ? = 22% to 44%, the highest values being for the salophen derivatives with the electron-withdrawing ester-R groups (7a, 34%; 7a', 23%; 7b, 31%; 7b', 22%; 7c, 40%; 7c', 29%, and 7d, 44%). Both the electrochemical data and DFT calculations show that the HOMO-LUMO band gap is modified as a function of the ester R group (from 2.92 to 3.16 eV, based on the redox potentials). The crystal structures for the N,N'-bis(5-(phenoxycarbonyl)salicylidene)-1,2-phenylenediamine and the N,N'-bis(5-(p-nitrophenoxycarbonyl)salicylidene)-1,2-phenylenediamine aluminum complexes (7a and 7d) are reported.     

Calcium adsorption on MgO(100): energetics, structure, and role of defects

The adsorption of Ca on the MgO(100) surface at 300 K has been studied using microcalorimetry, in combination with LEED, AES, ISS, work function, sticking probability measurements, and density functional theory (DFT) calculations. The MgO(100) thin films (approximately 4 nm thick) were grown epitaxially on a 1 microm thick Mo(100) single-crystal. The sticking probability of Ca on MgO(100) at 300 K is unity. On the basis of AES and ISS measurements, it was determined that Ca grows mainly as 3D particles on the MgO(100) surface with a density of approximately 1 x 10(12) islands/cm2. Ca adsorbs initially at defect sites with a very high heat of adsorption (approximately 410 kJ/mol). DFT calculations attribute this high initial heat to Ca binding to kink sites (376 kJ/mol), step sites (205 kJ/mol), and lower concentrations of stronger binding sites. The heat of adsorption decreases rapidly with coverage, reaching a minimum of 162 kJ/mol at approximately 0.3 ML, where Ca is mainly adding to small 3D Ca clusters. Afterward, it increases to the value of bulk Ca heat of sublimation (178 kJ/mol) at approximately 1.2 ML, attributed to the increase in stability with increasing Ca particle size. A 1.0 eV decrease of the work function with Ca coverage from 0 to 0.3 ML indicates that Ca adsorbed at defects is cationic, in agreement with calculations showing that Ca donates electron density to the MgO. Light ion sputtering of the MgO(100) surface generates point defects, but these do not change the heat of adsorption versus coverage, implying that they do not nucleate Ca particles. Oxygen vacancies are a likely candidate; DFT calculations show that F and F+ center vacancies bind Ca more weakly than terrace sites. More extensive sputtering creates extended defects (such as steps and kinks) that adsorb Ca with heats of adsorption up to approximately 400 kJ/mol, similar to that at the intrinsic defect sites.     

Cosegregation-induced formation of surface compounds on (110) and (111) oriented surfaces of bcc alloys with 3d and 4d metals

Cosegregation phenomena were studied on the (110) and (111) surfaces of Fe-3.5%Mo-N single crystals by means of Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). On both surface orientations cosegregation of molybdenum and nitrogen was found to cause the formation of two-dimensional MoN surface compounds which are stabilized by strong chemical interactions between the two solutes. The maximum surface coverages of the segregants, which were established at temperatures around 500°C, correspond to less than a full monolayer of the MoN surface compounds. LEED investigations revealed a complex diffraction pattern of the MoN covered (110) alloy surface, while a (1 × 1) structure was observed on the (111) surface. However, no facetting of either surfaces occurred. This finding is in sharp contrast to previous results obtained for ferritic alloys with various 3d metals such as Fe-15%Cr-N and Fe-3%V-C,N. It is concluded that the maximum MoN surface coverage is too small to induce the facetting of the bcc(110) and bcc(111) alloy surfaces.     

Structure evolution of hydrogenated carbon films from amorphous carbon to fullerene‐like nanostructure

A–C:H (hydrogenated amorphous carbon) films were deposited by pulsed direct‐current (d.c.) plasma enhanced chemical vapor deposition on silicon substrates. This study investigated the structural and mechanical evolution of the as‐deposited films with fullerene‐like nanostructure. The results showed that pulsed d.c. negative bias (?500 ~ ?1000 V) signally influenced the growth rate, hardness, surface roughness, sp³ content, and friction behavior of the films. As the pulsed d.c. negative bias voltage increased, the sp³ content, surface roughness, hydrogen content and the friction coefficient of the films decreased; however, the growth rate and the hardness increased. The films deposited at ?1000 V with fullerene‐like microstructure display a nanohardness of about 19.7 GPa and the smallest friction coefficient (~0.06). The evolution on mechanical and structural properties of the films are explained by the a–C:H growth mechanism based on the interaction on plasma‐surface interface and the subsurface reactions in the film. Copyright © 2014 John Wiley & Sons, Ltd.     

Stability and structure in alpha- and beta-Keggin Heteropolytungstates, [X(n)(+)W12O40)](8-n)(-), X = p-block cation

beta-[SiW(12)O(40)](4)(-) (C(3)(v) symmetry) is sufficiently higher in energy than its alpha-isomer analogue that effectively complete conversion to alpha-[SiW(12)O(40)](4)(-) (T(d)) is observed. By contrast, beta- and alpha-[AlW(12)O(40)](5)(-) (beta- and alpha-1; C(3)(v) and T(d), respectively) are sufficiently close in energy that both isomers are readily seen in (27)Al NMR spectra of equilibrated (alpha-beta) mixtures. Recently published DFT calculations ascribe the stability of beta-1 to an electronic effect of the large, electron-donating [AlO(4)](5)(-) (T(d)) moiety encapsulated within the polarizable, fixed-diameter beta-W(12)O(36) (C(3)(v)) shell. Hence, no unique structural distortion of beta-1 is needed or invoked to explain its unprecedented stability. The results of these DFT calculations are confirmed by detailed comparison of the X-ray crystal structure of beta-1 (beta-Cs(4.5)K(0.5)[Al(III)W(12)O(40)].7.5H(2)O; orthorhombic, space group Pmc2(1), a = 16.0441(10) A, b = 13.2270(8) A, c = 20.5919(13) A, Z = 4 (T = 100(2) K)) with previously reported structures of alpha-1, alpha- and beta-[SiW(12)O(40)](4)(-), and beta(1)-[SiMoW(11)O(40)](4)(-).