Diffusion by chemical bond hopping of single O2 and H on Si(111)-7×7 surfaces

Using a variable temperature STM to trace in detail the path of single particle movement, it is possible to derive diffusion parameters of individual atoms and molecules on solid surfaces as well as to probe the mechanisms. Below ˜370 °C, O₂ molecules adsorb on Si(111)-7×7 surfaces at the top site of Si-adatoms as bright image spots. An O₂ molecule can hop between two adatom sites within the half unit cell it adsorbs via two rest-atom sites. Above this temperature, it can either hop out of the half cell, or can go through other reaction pathways. In contrast, for H atoms, the adsorption sites are rest-atom sites. An H atom darkens the rest-atom in filled state image, but the surrounding adatoms will appear brighter because of a reverse charge transfer. Above ˜280 °C, it can hop to a neighbor rest atom site within the half cell via an adatom site. The adatom in the short lived intermediate state appears darker because of the saturation of its dangling bond. Above ˜340 °C, it can hop out of the half cell via two adatom sites. Thus diffusion of H and O₂ on this surface is achieved by hopping of chemical bonds via intermediate states. We have also derived site and pathway-specific activation energies and frequency factors and the potential energy curves for the hopping of O₂ and H on Si(111)-7×7 surfaces.     

Adsorption and migration growth of the Ce,O, and F adatoms on the CeO2 (111) surface: A density functional theory study

In order to investigate the microscopic behavior of the crystal surface growth of the fluorinated cerium dioxide polishing powder, the adsorption and migration of the Ce, O, and F atoms on the CeO₂ (111) surface were studied by using density functional theory with Hubbard correction +U. The adsorption energies of three single atoms at five high-symmetry sites and the migration activation energies along the migration pathway on the CeO₂ (111) surface were calculated. Results show that the most stable adsorption sites of the Ce, O, and F atoms were the O_h, Ce_bri, and Ce_t sites, respectively. The Ce atom migrated from the O_h to the O_t site. The O atom migrated from the Ce_bri to the O_bri site. The F atom migrated from the Ce_t to the O_h site. The migration activation energies of the Ce, O, and F atoms along the migration pathways were 1.526, 0.597, and 0.263 eV, respectively. The F adatom does not change the spatial configuration of the Ce and the O atoms. When the O vacancy occurs on the CeO₂ (111) surface, the F adatom can make up for the O vacancy defect.     

水在HfO2(111)和(110)表面的吸附与解离

运用广义梯度近似密度泛函理论方法(GGA-PW91)结合周期平板模型, 研究水分子在二氧化铪(111)和(110)表面不同吸附位置在不同覆盖度下的吸附行为. 通过比较不同吸附位的吸附能和几何构型参数发现:(111)和(110)表面铪原子(top 位)是活性吸附位. 水分子与表面的吸附能值随覆盖度的变化影响较小. 在(111)和(110)表面, 水分子都倾向以氧端与表面铪原子相互作用. 同时也计算了羟基、氧和氢在表面的吸附, Mulliken 电荷布居, 态密度及部分频率. 结果表明, 在两种表面羟基以氧端与表面铪相互作用, 氧原子与表面铪和氧原子同时成键, 而氢原子直接与表面氧原子相互作用形成羟基. 通过过渡态搜索, 水分子在(111)和(110)表面发生解离, 反应能垒分别为9.7和17.3 kJ·mol^-1, 且放热为59.9和47.6 kJ·mol^-1.     

Transport Processes at α‐Quartz–Water Interfaces: Insights from First‐Principles Molecular Dynamics Simulations

Car–Parrinello molecular dynamics (CP–MD) simulations are performed at high temperature and pressure to investigate chemical interactions and transport processes at the α‐quartz–water interface. The model system initially consists of a periodically repeated quartz slab with O‐terminated and Si‐terminated (1000) surfaces sandwiching a film of liquid water. At a temperature of 1000 K and a pressure of 0.3 GPa, dissociation of H₂O molecules into H⁺ and OH^? is observed at the Si‐terminated surface. The OH^? fragments immediately bind chemically to the Si‐terminated surface while Grotthus‐type proton diffusion through the water film leads to protonation of the O‐terminated surface. Eventually, both surfaces are fully hydroxylated and no further chemical reactions are observed. Due to the confinement between the two hydroxylated quartz surfaces, water diffusion is reduced by about one third in comparison to bulk water. Diffusion properties of dissolved SiO₂ present as Si(OH)₄ in the water film are also studied. We do not observe strong interactions between the hydroxylated quartz surfaces and the Si(OH)₄ molecule as would have been indicated by a substantial lowering of the Si(OH)₄ diffusion coefficient along the surface. No spontaneous dissolution of quartz is observed. To study the mechanism of dissolution, constrained CP–MD simulations are done. The associated free energy profile is calculated by thermodynamic integration along the reaction coordinate. Dissolution is a stepwise process in which two Si? O bonds are successively broken. Each bond breaking between a silicon atom at the surface and an oxygen atom belonging to the quartz lattice is accompanied by the formation of a new Si? O bond between the silicon atom and a water molecule. The latter loses a proton in the process which eventually leads to protonation of the oxygen atom in the cleaved quartz Si? O bond. The final solute species is Si(OH)₄.     

Structure Sensitivity in the Decomposition of Ethylene Carbonate on Si Anodes

The interaction of ethylene carbonate (EC) with Si surfaces is studied by density functional theory. The results show a strong structure sensitivity in the adsorption of EC on Si surfaces. While the adsorbed EC molecule readily decomposes on the Li/Si(111) surface, it does not dissociate on the Li/Si(100) and Li/Si(110) surfaces. On Si(111), the O atom at the top of EC is detached from the EC molecule and binds to the Li adatom, forming Li?O molecules. The mechanism of EC decomposition is the transfer of 2.4 electrons from the surface to the EC molecule, as well as the formation of a covalent bond between the Li adatom and the EC molecule. This result shows that in lithium‐ion batteries with Si anodes, dissociation of the solvent and formation of a solid electrolyte interphase layer start as soon as the Li atoms cover the anode surface.     

Adsorption and Decomposition Mechanism of 1,1‐Diamino‐2,2‐dinitroethylene on Al(111) Surface by Periodic DFT Calculations

The adsorption of 1,1‐diamino‐2,2‐dinitroethylene (FOX‐7) molecule on the Al(111) surface was investigated by the generalized gradient approximation (GGA) of density functional theory (DFT). The calculations employ a supercell (4×4×2) slab model and three‐dimensional periodic boundary conditions. The strong attractive forces between oxygen and aluminum atoms induce the N? O bond breaking of the FOX‐7. Subsequently, the dissociated oxygen atoms and radical fragment of FOX‐7 oxidize the Al surface. The largest adsorption energy is ?940.5 kJ/mol. Most of charge transfer is 3.31e from the Al surface to the fragment of FOX‐7 molecule. We also investigated the adsorption and decomposition mechanism of FOX‐7 molecule on the Al(111) surface. The activation energy for the dissociation steps of P2 con?guration is as large as 428.8 kJ/mol, while activation energies of other con?gurations are much smaller, in range of 2.4 to 147.7 kJ/mol.     

Adsorption of formic acid on Si(111)7 × 7 at room temperature: a valence band photoemission and Si2p photodesorption study

Formic acid is the simplest of the carboxylic acids and a model adsorption system for several surfaces. In spite of the simple structure, formic acid reactivity and photoreactivity may be quite complex. In this paper, a study is presented on the deuterated formic acid adsorption on Si(111)7 × 7 at room temperature. The study is performed both by valence band photoemission and by photon‐stimulated desorption as a function of time and of photon energy in the 90–120 eV range. A primarily adsorption on rest atoms is found. This is verified by monitoring rest atoms and adatom intensity as a function of formic acid exposure. Further checks were made to control that surface adatoms were still free to react after the adsorption of formic acid. The photon stimulated desorption produces 5 single positively charged fragments: D⁺, O⁺, OD⁺ CO⁺ and CDO⁺. Possible fragmentation mechanisms are discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Water Adsorption and Dissociation on Ceria‐Supported Single‐Atom Catalysts: A First‐Principles DFT+U Investigation

Single‐atom catalysts have attracted wide attention owing to their extremely high atom efficiency and activities. In this paper, we applied density functional theory with the inclusion of the on‐site Coulomb interaction (DFT+U) to investigate water adsorption and dissociation on clean CeO₂(111) surfaces and single transition metal atoms (STMAs) adsorbed on the CeO₂(111) surface. It is found that the most stable water configuration is molecular adsorption on the clean CeO₂(111) surface and dissociative adsorption on STMA/CeO₂(111) surfaces, respectively. In addition, our results indicate that the more the electrons that transfer from STMA to the ceria substrate, the stronger the binding energies between the STMA and ceria surfaces. A linear relationship is identified between the water dissociation barriers and the d band centers of STMA, known as the generalized Brønsted–Evans–Polanyi principle. By combining the oxygen spillovers, single‐atom dispersion stabilities, and water dissociation barriers, Zn, Cr, and V are identified as potential candidates for the future design of ceria‐supported single‐atom catalysts for reactions in which the dissociation of water plays an important role, such as the water–gas shift reaction.     

Zur Struktur eines ungewöhnlichen Tetramers des Lithiumphenolats: [C6H5OLi · C4H8O]4 · C6H5OH

The Structure of an unusual Tetramere of Lithium Phenoxide: [C₆H₅OLi · C₄H₈O]₄ · C₆H₅OH Single crystals of lithium phenoxide have been obtained from THF. In the structure (P 2₁/n, Z = 4, a = 11.69 Å, b = 21.15 Å, c = 18.55 Å, β = 91.11°) four lithium atoms and four phenoxide oxygen atoms are cubically arranged. Further, each lithium atom coordinates the oxygen atom of a tetrahydrofuran molecule. The ideal cubeform structure is disturbed by one phenol molecule which is coordinated in addition to four phenoxide and four THF molecules. Hence, one edge of the cube (Li4? O4) is substituted by the coordination of the phenol oxygen atom O5 with Li4 and hydrogen bonding between O4 and the hydroxy group of phenol. Van der Waals forces are the only interaction between these complexes.     

Dissociation mechanism of 2-propanol on a Si(111)-(7x7) surface studied by scanning tunneling microscopy

Adsorption of 2-propanol, (CH3)2CHOH, on a Si(111)-7x7 surface was studied by scanning tunneling microscopy. (CH3)2CHOH adsorbs equally on the faulted and unfaulted half unit cells by forming Si-OCH(CH3)2 and Si-H on an adatom and rest atom pair. Si-OCH(CH3)2 is consecutively increased in each half unit cell, and the adsorption is saturated when every half unit cell has three Si-OCH(CH3)2, which corresponds to 0.5 of the adatom coverage. The sticking probability for the dissociation of (CH3)2CHOH is independent of the adatom coverage from 0 to 0.4, but it depends on coverage at higher than 0.4. By counting the darkened adatoms, Si-OCH(CH3)2 on the center adatom (m) and that on the corner adatom (n), it was found the m/n ratio is ca. 4 for the first dissociation of (CH3)2CHOH in virgin half unit cell, but it becomes ca. 1.9 and 1.8 when two and three Si-OCH(CH3)2 are contained in a half unit cell. This result reveals that the dissociation probability of (CH3)2CHOH at the adatom-rest atom pair site is influenced by the nearest Si-OCH(CH3)2 in the half unit cell.     

β‐Y2Si2O7, a new thortveitite‐type compound,determined at 100 and 280 K

A new form of Y₂Si₂O₇ (diyttrium heptaoxodisilicate) has been synthesized which is isotypic with thortveitite, Sc₂Si₂O₇, and crystallizes in the centrosymmetric space group C2/m, both at 100 and 280 K. The Y³⁺ cation occupies a distorted octahedral site, with Y—O bond lengths in the range 2.239 (2)–2.309 (2) Å. The SiO₄ tetrahedron is remarkably regular, with Si—O bond lengths in the range 1.619 (2)–1.630 (2) Å. The bridging O atom of the Si₂O₇ pyrosilicate group shows a large anisotropic displacement perpendicular to the Si—O bond. Changes in lattice and structural parameters upon cooling are small with, however, a distinct decrease of the anisotropic displacement of the briding O atom. Structure solution and refinement in the non‐centrosymmetric space group C2 are possible but do not yield a significantly different structure model. The Si—O—Si bond angle of the isolated Si₂O₇ groups is 179.2 (1)° at 280 K in C2 and 180° per symmetry in C2/m. The C2/m structure model is favoured.     

Chemisorption of thiol‐functionalized metallocene molecules on Si(111)‐ Ag√3×√3 surface: A density functional theory study

Herein, chemical adsorption properties of the thiol‐functionalized metallocene molecules [M(C₅H₄SH)₂] on Si(111)‐Ag√3×√3 surface were investigated using density functional theory calculation. For this purpose, thiol‐modified ferrocene [Fe(C₅H₄SH)₂], osmocene [Os(C₅H₄SH)₂], and ruthenocene [Ru(C₅H₄SH)₂] molecules were attached on the surface via two different binding models. The more favorable chemical binding energies of [Fe(C₅H₄SH)₂], [Os(C₅H₄SH)₂], and [Ru(C₅H₄SH)₂] molecules were calculated as ?3.42, ?2.15, and ?2.00 eV, respectively. The results showed that the adsorption energies of metallocene molecules change independently by increasing the radius of metal ions where on going down the group of the periodic table. The calculated adsorption energies showed that [Fe(C₅H₄SH)₂] molecule was more stable on the Si(111)‐Ag√3×√3 surface. By calculating the electronic band structure for metallocene/Si(111)‐Ag√3×√3 surfaces, we identified a flat dispersion band in a part of the surface Brillouin zone. © 2015 Wiley Periodicals, Inc.     

Adsorption kinetics and patterning of a Si(111)-7 x 7 surface by dissociation of methanol

CH(3)OH undergoes dissociation on a Si(111)-7 x 7 surface via a two dimensionally free precursor. The sticking probability attained by the STM (scanning tunneling microscopy) was entirely coverage independent, where the observed image represented the final state of the adsorption. CH(3)OH dissociates equally on the faulted and unfaulted halves at room temperature. However, the dissociation at the center adatom-rest atom site is four times preferential to that at the corner adatom-rest atom site in each half unit cell. Such site selectivity, center/corner, changes with the occupation of adatoms in corresponding half unit cell, that is, center/corner=4 for the half unit cell with one reacted adatom, but 2.6 and 1.8 for the half unit cells with two and three reacted adatoms, respectively. Such site selectivity is well rationalized by the dissociation depending on the local conformation of the site instead of the local density of states (LDOS). The site selectivity of center/corner is well reproduced by considering the occurrence probability of the whole dissociation pattern. As the STM image represents the final state of the adsorption, if the final step of adsorption involves dissociation of molecule or precursor, the STM image reflects the dissociation probability depending on the local structure. On the other hand, if no dissociation of molecule or precursor is involved at the final step, the adsorption probability might depend on the LDOS. The adsorption of H(2)S, H(2)O, and NH(3) is also discussed from this general viewpoint of adsorption. The concept of a two dimensionally free precursor will be important to understand the kinetics of heterogeneous catalysis.     

Single Si atom supported on defective boron nitride nanosheet as a promising metal‐free catalyst for N2O reduction by CO or SO2 molecule: A computational study

The low‐temperature reduction of N₂O plays a significant role for solving the growing environmental and health issues caused by emission of this greenhouse gas. The aim of this study is to investigate the possible reaction pathways for the reduction of N₂O by CO or SO₂ molecule over Si‐doped boron nitride nanosheet (Si‐BNNS). According to our results, a B or N‐vacancy defect in BN sheet could be able to greatly stabilize the single Si adatom. The relatively large diffusion barrier for the Si atom over the defective BN sheet also indicates Si‐BNNS is stable enough to be utilized in catalytic reduction of N₂O. The large charge‐transfer from the surface to N₂O leads to the spontaneous dissociation of this molecule into N₂ molecule and an activated oxygen atom (O_ads). The O_ads moiety is then eliminated by CO or SO₂ molecule. The calculated activation energies and reaction energies reveal that the Si atom located on top of the B‐vacancy site has a large catalytic activity toward the reduction of N₂O by CO or SO₂.     

7‐Phenyl‐1‐oxa‐4‐thiaspiro[4.5]decan‐7‐ol stereoisomers

The stereoisomers of 7‐phenyl‐1‐oxa‐4‐thia­spiro­[4.5]­decan‐7‐ol, C₁₄H₁₈O₂S, have the same stereochemistry at the C atom bearing an OH group, i.e. axial OH and equatorial phenyl groups. However, the acetal S and O atoms are axial and equatorial, respectively, in one isomer and reversed in the second. Furthermore, the crystals of one isomer are composed of hydrogen‐bonded mol­ecules involving the hydroxyl H atom and the O atom of the five‐membered heterocyclic ring, with an O?O distance of 2.962 (3) Å, forming a polymeric chain along the b axis. The asymmetric unit of the other isomer is composed of two mol­ecules, wherein hydroxyl H atoms and the O atoms of the five‐membered heterocyclic rings display intramolecular O—H?O hydrogen bonds with O?O separations of 2.820 (2) and 2.834 (2) Å.     

Formation of Au nanocluster ordered array on Si(111)‐7 × 7 surface

We report a successful approach in fabricating well shape two‐dimensional Au nanocluster hexagonal array on an Si(111)‐7 × 7 surface. The size of Au hexagons is about 2.5 × 2.5 nm². The easiness of preparing large scale Au hexagonal array makes this approach usable to fabricate a wide range of other metal nanocluster arrays. Further deposition of small amount of Au nanoclusters on the Au hexagonal array surface will prefer to occupy the holes of the hexagonal array at room temperature. Finally, a reasonable structure of formatting the hexagonal Au nanocluster array on the Si(111)‐7 × 7 surface was proposed. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of substitution on the strength and nature of CH···N hydrogen bond in XCCH···NH3 complexes

The effect of substitution on the strength and nature of CH···N hydrogen bond in XCCH···NH₃ (X = F, Cl, Br, OH, H, Me) and NCH···NH₃ complexes were investigated by quantum chemical calculations. Ab initio calculations were performed using MP2 method with a wide range of basis sets. With tacking into account the BSSE and ZPVE, the values of BEs decrease. Replacement of the nonparticipatory hydrogen atom of HCCH by the electronegative atoms (F, Cl, and Br), lead to the BEs increases. The BE corresponding to the replacement of the nonparticipatory hydrogen atom of HCCH by the OH and CH₃ groups decreases. A far greater enhancement of the interaction energy arises from replacement of HCCH by the more acidic HCN. The natural bond orbital analysis and the Bader's quantum theory of atoms in molecules were also used to elucidate the interaction characteristics of these complexes. The electrostatic nature of H‐bond interactions is predicted from QTAIM analysis. In addition, the relationship between the isotropic and anisotropic chemical shifts of the bridging hydrogen and binding energy of complexes as well as electron density at N···H BCPs were investigated. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Secondary oxidation product on Si(111)-(7 x 7) characterized by isotope-labeled vibrational spectroscopy

The reaction of O(2) with Si(111)-(7 x 7) has been studied by electron energy-loss spectroscopy at 82 K. In addition to the losses due to Si-O-Si configurations, we observed two Si-O stretch modes depending on the coverage. A 146-meV peak appears at the initial reaction stage and was ascribed to a metastable product with one oxygen atom bonding on top of Si adatom and the other inserted into the backbond. The initial product is further oxidized to produce the second Si-O stretch peak at 150 meV. The secondary product was partially substituted with isotopes and analyzed with a simple model of coupled oscillators. The vibrational spectra reflect dynamical couplings between the isotopes, which is consistent with those predicted from the tetrahedral SiO(4) structure with one on top and three inserted oxygen atoms.     

A DFT study of H2O dissociation on metal‐precovered Fe (100) surface

The H₂O adsorption and dissociation on the Fe (100) surface with different precovered metals are studied by density functional theory. On both kinds of metal‐precovered surface, H₂O molecules prefer adsorb on hollow sites than bridge and top sites. The impurity energy difference is proportional to the adsorption energy, but the adsorbates are not sensitive to the adsorption orientation and height relative to the surface. The Hirshfeld charge analysis shows that water molecules act as an electron donor while the surface Fe atoms act as an electron acceptor. The rotation and dissociation of H₂O molecule occur on the Co‐ and Mn‐precovered surfaces. Some H₂O molecules are dissociated into OH and H groups. The energy barriers are about 0.5 to 1.0 eV, whose are consistence with the experimental data. H₂O molecules can be dissociated more easily at the top site on Co‐precovered surface 1 than that at bridge site on Mn‐precovered surface 2 because of the lower reaction barrier. The dispersion correction effects on the energies and adsorption configurations on Co‐precovered surface 1 were calculated by OBS + PW91. The dispersion contributions can improve a bit of the bond energy of adsorbates and weaken the hydrogen bond effect between adsorption molecules a little.     

Theoretical investigation of structural stability and lattice vibrations of U6Fe16Si7 and its interstitial carbide U6Fe16Si7C

A series of lattice inversion pair potentials are used to evaluate the phase stability and site preference for uranium intermetallics U₆Fe₁₆Si₇ and its interstitial carbide U₆Fe₁₆Si₇C. The calculated preferentially occupation site of the Si atom is found to be the 4a site. Interstitial C atom can only be located on the 4b site. Calculated lattice constants are found to agree with a report in the literature. It is noted that the total and partial phonon densities of states are first evaluated for the U₆Fe₁₆Si₇ and U₆Fe₁₆Si₇C compounds. The analysis for the inverted potentials explains qualitatively the contributions of different atoms to the vibrational modes.