Formation and diffusion of oxygen-vacancy pairs on TiO2(110)-(1x1)

We present the measurements for the diffusion of bridging oxygen vacancy (OV) crossover Ti rows via OV pairs (OVPs). Using a high-resolution scanning tunneling microscope (STM), we show that the OVs can be moved along the bridging oxygen rows driven by the STM tip at voltages higher than 3.0 V on TiO(2)(110)-(1x1) surface. It is found that the combination of OVPs leads to the formation of OVPs, which can diffuse crossover Ti rows under the mediation of OVs in adjacent bridging oxygen rows. The deduced diffusion activation energy for the diffusive OVPs from experiments is in agreement with first-principles calculations. The reaction activation energy of the OVPs with O(2) is lower than that of the OVs by 82 meV.     

Study of adsorption and decomposition of H2O on Ge(100)

The adsorption and decomposition of water on Ge(100) have been investigated using real-time scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. The STM results revealed two distinct adsorption features of H2O on Ge(100) corresponding to molecular adsorption and H-OH dissociative adsorption. In the molecular adsorption geometry, H2O molecules are bound to the surface via Ge-O dative bonds between the O atom of H2O and the electrophilic down atom of the Ge dimer. In the dissociative adsorption geometry, the H2O molecule dissociates into H and OH, which bind covalently to a Ge-Ge dimer on Ge(100) in an H-Ge-Ge-OH configuration. The DFT calculations showed that the dissociative adsorption geometry is more stable than the molecular adsorption geometry. This finding is consistent with the STM results, which showed that the dissociative product becomes dominant as the H2O coverage is increased. The simulated STM images agreed very well with the experimental images. In the real-time STM experiments, we also observed a structural transformation of the H2O molecule from the molecular adsorption to the dissociative adsorption geometry.     

The chemical nature of surface point defects on MoO3(010): adsorption of hydrogen and methyl.

We report density functional theory calculations using the Adaptive Coordinate Real-space Electronic Structure (ACRES) method of the terminal oxygen vacancy on the (010) surface of MoO3, within a (2 x 2) ordered array of vacancies on the surface. Analysis of the electronic structure of this surface shows that there are unoccupied dangling d(xz) and d(z)2 orbitals perpendicular to the surface that are created by the removal of terminal oxygen. The Mo-oxygen bonds surrounding the vacancy contract; however, the overall morphology of the surface is not drastically distorted. The vacancies alter the chemical character of the surface, as shown by studies of hydrogen and methyl binding. On both the "perfect" and vacancy surfaces, hydrogen was most strongly adsorbed over the terminal oxygen and most weakly bound over the symmetric bridging oxygen. Hydrogen is bound over the Mo atom, with a slightly smaller binding energy than hydrogen over the asymmetric bridging oxygen. The most favorable binding site for methyl on the vacancy surface is over the Mo atom exposed by removal of a terminal oxygen, whereas methyl bound to terminal oxygen is most stable on the perfect surface. There is no local minimum for adsorption over the symmetric bridging oxygen; instead, a methyl placed over this site moves toward the terminal oxygen vacancy. Analysis of the bonding shows that methyl is bound more strongly than hydrogen over the Mo atom because the C 2p orbital has better overlap with the Mo d(z)2 orbital than the hydrogen 1s. In addition, the steric repulsion observed for methyl over the perfect MoO3(010) surface is more easily relieved with the presence of the terminal oxygen vacancy.     

The Role of Holes in Borophenes: An Ab Initio Study of Their Structure and Stability with and without Metal Templates

An electron‐counting strategy starting from magnesium boride was used to show the inevitability of hexagonal holes in 2D borophene. The number (hole density, HD) and distribution of the hexagonal holes determine the binding energy per boron atom in monolayer borophenes. The relationship between binding energy and HD changes dramatically when the borophene is placed on a Ag(111) surface. The distribution of holes in borophenes on Ag(111) surfaces depends on the temperature. DFT calculations show that aside from the previously reported S1 and S2 borophene phases, other polymorphs may also be competitive. Plots of the electron density distribution of the boron sheets suggest that the observed STM image of an S2 phase corresponds to a sheet with a HD of 2/15 instead of a sheet with a HD of 1/5. The hole density and the hole distribution echo the distribution of vacancies and extra occupancies in complex β‐rhombohedral boron.     

Selective formation of cumulative double bonds (C[double bond]C[double bond]N) in the attachment of multifunctional molecules on Si(111)-7 x 7

The cumulative double bond (C[double bond]C[double bond]N), an important intermediate in synthetic organic chemistry, was successfully prepared via the selective attachment of acrylonitrile to Si(111)-7 x 7. The covalent binding of acrylonitrile on Si(111)-7 x 7 was studied using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM) and DFT calculations. The observation of the characteristic vibrational modes and electronic structures of the C[double bond]C[double bond]N group in the surface species demonstrates the [4 + 2]-like cycloaddition occurring between the terminal C and N atoms of acrylonitrile and the neighboring adatom-rest atom pair, consistent with the prediction of DFT calculations. STM studies further show the preferential binding of acrylonitrile on the center adatom sites of faulted halves of Si(111)-7 x 7 unit cells.     

The growth of silver on an ordered alumina surface

The growth of Ag on an ordered Al2O3 surface was studied by low energy ion scattering spectroscopy (LEIS), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). Three-dimensional (3D) growth of Ag clusters was observed with STM and LEIS, with the cluster size increasing with Ag coverage. The XPS core level binding energies and the Auger parameters indicate a weak interaction between the Ag clusters and the Al2O3 support. Final state effects are determined to be the primary contribution to the Ag core level binding energy shift. Nonzero order kinetics was observed for Ag desorption in TPD with the Ag sublimation energy decreasing with decreasing cluster size.     

Specific ethene surface activation on silver oxide covered Ag[111] from the interplay of STM experiment and theory

High-resolution scanning tunneling microscopy (STM) images at 5 K, simultaneously resolving the molecular adsorbate and the honeycomb structure of the well-defined Ag[111]-p(4 x 4)+Ag(1.83)O substrate, assign the adsorption site for ethene on the silver oxide surface. Ethene molecules are exclusively adsorbed above a particular subset of Ag(delta)(+) sites in the hexagonal rings of the oxide. Extensive density functional theory (DFT) slab calculations confirm that this is the most stable site, with an adsorption energy of 0.4 eV (39 kJ mol(-1)). Adsorption is accompanied by a large deformation of the hexagonal oxide ring and a significant increase in the C-C bond length. STM image simulations provide qualitative agreement with the experimental images, and the molecular orientation is discussed with the help of simple molecular orbital arguments.     

苯乙烯在Ag(111)和Ag(110)表面环氧化反应结构敏感性的理论研究

采用密度泛函理论(DFT)对苯乙烯在Ag(110)表面和Ag(111)表面的环氧化反应进行了计算研究. 经计算, 在Ag(110)表面预吸附氧原子更易吸附在3 重穴位(3h), 吸附能为-3.59 eV; 在Ag(111)表面预吸附氧原子的最稳定吸附位是fcc 位, 吸附能为-3.69 eV. 苯乙烯的环氧化反应过程首先经过一个金属中间体, 然后再进一步反应变为产物, 其中经过直链中间体较支链中间体更加有利. Ag(110)面的反应活化能一般大于Ag(111)面的, 并且微观动力学模拟结果表明, Ag(111)表面生成环氧苯乙烷的选择性要明显高于Ag(110)表面(0.38 与 0.003), 原因是Ag(111)面环氧化反应活化能小于苯乙醛及燃烧中间体的活化能, 而在Ag(110)上正相反.     

Gas phase synthesis and reactivity of Agn+ and Ag(n-1)H+ cluster cations

Multi-stage mass spectrometry (MSn) on [(M + Ag - H)x + Ag]+ precursor ions (where M = an amino acid such as glycine or N,N-dimethylglycine) results in the formation of stable silver (Ag3+, Ag5+ and Ag7+) and silver hydride (Ag2H+, Ag4H+ and Ag6H+) cluster cations in the gas phase. Deuterium labelling studies reveal that the source of the hydride can be either from the alpha carbon or from one of the heteroatoms. When M = glycine, the silver cyanide clusters Ag4CN+ and Ag5(H,C,N)+ are also observed. Collision induced dissociation (CID) and DFT calculations were carried out on each of these clusters to shed some light on their possible structures. CID of the Agn+ and Ag(n-1)H+ clusters generally results in the formation of the same Ag(n-2)+ product ions via the loss of Ag2 and AgH respectively. DFT calculations also reveal that the Agn+ and Ag(n-1)H+ clusters have similar structural features and that the Ag(n-1)H+ clusters are only slightly less stable than their all silver counterparts. In addition, Agn+ and Ag(n-1)H+ clusters react with 2-propanol and 2-butylamine via similar pathways, with multiple ligand addition occurring and a coupled deamination-dehydration reaction occurring upon condensation of a third (for Ag2H+) or a fourth (for all other silver clusters) 2-butylamine molecule onto the clusters. Taken together, these results suggest that the Agn+ and Ag(n-1)H+ clusters are structurally related via the replacement of a silver atom with a hydrogen atom. This replacement does not dramatically alter the cluster stability or its unimolecular or bimolecular chemistry with the 2-propanol and 2-butylamine reagents.     

Electronic structures of one-dimensional metal-molecule hybrid chains studied using scanning tunneling microscopy and density functional theory

The electronic structures of self-assembled hybrid chains comprising Ag atoms and organic molecules were studied using scanning tunneling microscopy (STM) and spectroscopy (STS) in parallel with density functional theory (DFT). Hybrid chains were prepared by catalytic breaking of Br-C bonds in 4,4″-dibromo-p-terphenyl molecules, followed by spontaneous formation of Ag-C bonds on Ag(111). An atomic model was proposed for the observed hybrid chain structures. Four electronic states were resolved using STS measurements, and strong energy dependence was observed in STM images. These results were explained using first-principles calculations based on DFT.     

Anticorrelation between Surface and Subsurface Point Defects and the Impact on the Redox Chemistry of TiO2(110)

By using a combination of scanning tunneling microscopy (STM), density functional theory (DFT), and secondary‐ion mass spectroscopy (SIMS), we explored the interplay and relative impact of surface versus subsurface defects on the surface chemistry of rutile TiO₂. STM results show that surface O vacancies (V_O) are virtually absent in the vicinity of positively charged subsurface point defects. This observation is consistent with DFT calculations of the impact of subsurface defect proximity on V_O formation energy. To monitor the influence of such lateral anticorrelation on surface redox chemistry, a test reaction of the dissociative adsorption of O₂ was employed and was observed to be suppressed around them. DFT results attribute this to a perceived absence of intrinsic (Ti), and likely extrinsic interstitials in the nearest subsurface layer beneath inhibited areas. We also postulate that the entire nearest subsurface region could be devoid of any charged point defects, whereas prevalent surface defects (V_O) are largely responsible for mediation of the redox chemistry at the reduced TiO₂(110).     

Adsorption energies of molecular oxygen on Au clusters

The adsorption properties of O(2) molecules on anionic, cationic, and neutral Au(n) clusters (n=1-6) are studied using the density functional theory (DFT) with the generalized gradient approximation (GGA), and with the hybrid functional. The results show that the GGA calculations with the PW91 functional systemically overestimate the adsorption energy by 0.2-0.4 eV than the DFT ones with the hybrid functional, resulting in the failure of GGA with the PW91 functional for predicting the adsorption behavior of molecular oxygen on Au clusters. Our DFT calculations with the hybrid functional give the same adsorption behavior of molecular oxygen on Au cluster anions and cations as the experimental measurements. For the neutral Au clusters, the hybrid DFT predicts that only Au(3) and Au(5) clusters can adsorb one O(2) molecule.     

Studies of chemisorbed tetracene on si(111)-7x7

The chemisorption of tetracene on the Si(111)-7x7 surface was studied using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. On the basis of the STM results and dimension analysis, two types of binding configurations were proposed. One of the configurations involves the di-sigma reaction between two C atoms of an inner ring with an adatom-rest atom pair on the substrate to give rise to an unsymmetrical butterfly structure. Tetracene in another configuration possesses four C-Si bonds that are formed via di-sigma reactions between the C atoms at the terminal rings with two center adatom-rest atom pairs within one-half of the surface unit cell. Besides, two other binding modes were proposed based on the dimension compatibility between the tetracene C and the substrate Si dangling bonds even though their identifications through the STM images are nonexclusive. Structural modeling and adsorption energies calculations were carried out using the DFT method. Factors affecting the relative thermodynamic stabilities based on the calculation results and the relative populations of tetracene in the different binding configurations as observed experimentally were discussed.     

CO2 adsorption on TiO2(110) rutile: insight from dispersion-corrected density functional theory calculations and scanning tunneling microscopy experiments

Adsorption of CO(2) on the rutile(110) surface was investigated using dispersion-corrected density functional theory and scanning tunneling microscopy (STM). On the oxidized surface the CO(2) molecules are found to bind most strongly at the five-fold coordinated Ti sites adopting tilted or flat configurations. The presence of bridging oxygen defects introduces two new adsorption structures, the most stable of which involves CO(2) molecules bound in tilted configurations at the defect sites. Inclusion of dispersion corrections in the density functional theory calculations leads to large increases in the calculated adsorption energies bringing these quantities into good agreement with experimental data. The STM measurements confirm two of the calculated adsorption configurations.     

The initial growth behavior of perylene on Cu(100)

Using scanning tunneling microscopy (STM) together with density functional theory (DFT) the growth behavior of perylene on the Cu(100) substrate has been investigated. As revealed by STM images, perylene molecules prefer to adopt lying configuration with their molecular plane parallel to the substrate, and two symmetrically equivalent ordered domains were observed. DFT calculations show that perylene molecule prefers to adsorb on the top site of substrate Cu atoms with its long molecular axis aligning along the [011] or [01-1] azimuth of the substrate which is the most stable adsorption geometry according to its highest binding energy. Consequently, two adsorption structures of c(8×4) and c(8×6), each containing two perylene molecules per unit cell, are proposed based on our STM images. The growth mechanism for ordered perylene domains on Cu(100) can be attributed to the balance between weak adsorbate-adsorbate interaction and comparable adsorbate-substrate interaction.     

Structural requirements and reaction pathways in dimethyl ether combustion catalyzed by supported Pt clusters

The identity and reversibility of the elementary steps required for catalytic combustion of dimethyl ether (DME) on Pt clusters were determined by combining isotopic and kinetic analyses with density functional theory estimates of reaction energies and activation barriers to probe the lowest energy paths. Reaction rates are limited by C-H bond activation in DME molecules adsorbed on surfaces of Pt clusters containing chemisorbed oxygen atoms at near-saturation coverages. Reaction energies and activation barriers for C-H bond activation in DME to form methoxymethyl and hydroxyl surface intermediates show that this step is more favorable than the activation of C-O bonds to form two methoxides, consistent with measured rates and kinetic isotope effects. This kinetic preference is driven by the greater stability of the CH3OCH2* and OH* intermediates relative to chemisorbed methoxides. Experimental activation barriers on Pt clusters agree with density functional theory (DFT)-derived barriers on oxygen-covered Pt(111). Measured DME turnover rates increased with increasing DME pressure, but decreased as the O2 pressure increased, because vacancies (*) on Pt surfaces nearly saturated with chemisorbed oxygen are required for DME chemisorption. DFT calculations show that although these surface vacancies are required, higher oxygen coverages lead to lower C-H activation barriers, because the basicity of oxygen adatoms increases with coverage and they become more effective in hydrogen abstraction from DME. Water inhibits reaction rates via quasi-equilibrated adsorption on vacancy sites, consistent with DFT results indicating that water binds more strongly than DME on vacancies. These conclusions are consistent with the measured kinetic response of combustion rates to DME, O2, and H2O, with H/D kinetic isotope effects, and with the absence of isotopic scrambling in reactants containing isotopic mixtures of 18O2-16O2 or 12CH3O12CH3-13CH3O13CH3. Turnover rates increased with Pt cluster size, because small clusters, with more coordinatively unsaturated surface atoms, bind oxygen atoms more strongly than larger clusters and exhibit lower steady-state vacancy concentrations and a consequently smaller number of adsorbed DME intermediates involved in kinetically relevant steps. These effects of cluster size and metal-oxygen bond energies on reactivity are ubiquitous in oxidation reactions requiring vacancies on surfaces nearly saturated with intermediates derived from O2.     

Two-dimensional polymer formation on surfaces: insight into the roles of precursor mobility and reactivity

We report on a combined scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) study on the surface-assisted assembly of the hexaiodo-substituted macrocycle cyclohexa-m-phenylene (CHP) toward covalently bonded polyphenylene networks on Cu(111), Au(111), and Ag(111) surfaces. STM and XPS indicate room temperature dehalogenation of CHP on either surface, leading to surface-stabilized CHP radicals (CHPRs) and coadsorbed iodine. Subsequent covalent intermolecular bond formation between CHPRs is thermally activated and is found to proceed at different temperatures on the three coinage metals. The resulting polyphenylene networks differ significantly in morphology on the three substrates: On Cu, the networks are dominated by "open" branched structures, on the Au surface a mixture of branched and small domains of compact network clusters are observed, and highly ordered and dense polyphenylene networks form on the Ag surface. Ab initio DFT calculations allow one to elucidate the diffusion and coupling mechanisms of CHPRs on the Cu(111) and Ag(111) surfaces. On Cu, the energy barrier for diffusion is significantly higher than the one for covalent intermolecular bond formation, whereas on Ag the reverse relation holds. By using a Monte Carlo simulation, we show that different balances between diffusion and intermolecular coupling determine the observed branched and compact polyphenylene networks on the Cu and Ag surface, respectively, demonstrating that the choice of the substrate plays a crucial role in the formation of two-dimensional polymers.     

Experimental and theoretical studies of surface nitrate species on Ag/Al2O3 using DRIFTS and DFT

Surface nitrate (NO3(-)) species on the Ag/Al2O3 play an important role in the selective catalytic reduction (SCR) of NOx. In this study, the formation and configuration of surface nitrate NO3(-)(ads) species on Ag/Al2O3 and Al2O3 in the oxidation of NO have been studied using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations. Different nitrates species (bridging, bidentate and monodentate) were observed by in situ DRIFTS and validated by DFT calculations results. Attention was especially focused on the proposal of two different bidentate nitrates species (a normal bidentate and an isolated bidentate). In addition, the thermal stability of different surface nitrate species was discussed based on the adsorption energies calculations, DRIFTS, and temperature-programmed desorption (TPD) results. It was suggested that the decomposition and desorption of the surface nitrate species could be controlled by kinetics.     

Cooperative effects in the activation of molecular oxygen by anionic silver clusters

A novel size dependence in the adsorption reaction of multiple O2 molecules onto anionic silver clusters Agn- (n = 1-5) is revealed by gas-phase reaction studies in an rf-ion trap. Ab initio theoretical modeling based on DFT method provides insight into the reaction mechanism and finds cooperative electronic and structural effects to be responsible for the size selective reactivity of Agn- clusters toward one or more O2. In particular, Agn- clusters with odd n have paired electrons and therefore bind one O2 only weakly, but they are simultaneously activated to adsorb a strongly bound second oxygen molecule. For the clusters Ag3O4- and Ag5O4-, this cooperative effect results in a superoxo-like, doubly bound O2 subunit with potentially high activity in catalytic silver cluster oxidation processes.     

Carboxylic acid chemistry at the Ge(100)-2 x 1 interface: bidentate bridging structure formation on a semiconductor surface

The reactions of acetic acid, acetic-d3 acid-d, and formic acid with the Ge(100)-2 x 1 surface have been investigated using multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The infrared and photoelectron data provide experimental evidence for an O-H dissociation product at 310 K. DFT calculations indicate that the O-H dissociation pathway is significantly favored, both kinetically and thermodynamically, over other potential reaction pathways. All of the carboxylic acids studied exhibit unexpected vibrational modes between 1400 and 1525 cm(-1), which are attributed to the presence of a bidentate bridging structure where both oxygen atoms interact directly with the surface.