Surface structure and phase transition of K adsorption on Au(111): by ab initio atomistic thermodynamics

We studied the interactions between atomic potassium (K) and Au(111) at a range of coverage (i.e., Θ(K) = 0.11-0.5 monolayer (ML)) by ab initio atomic thermodynamics. For K on-surface adsorption, we found that K energetically favors the three-fold hollow sites (fcc or hcp), while the most significant surface rumpling was obtained at the atop sites. The incorporation of gold atoms in the adsorbate layer gradually becomes energetically favorable with increasing K coverage. We proposed a possible model with a stoichiometry of K(2)Au for the (2 × 2)-0.5 ML phase observed in lower energy electron diffraction (LEED): one K at atop site and the other K as well as one Au adatom at the second-nearest fcc/hcp and hcp/fcc, respectively. Clear theoretical evidences were given for the ionic interaction of K on Au surface. Additionally, phase transitions were predicted based on chemical potential equilibrium of K, largely in line with the earlier reported LEED observations: the clean surface → (√3 × √3)R30° → (2 × 2), and (2 × 2) → (√3 × √3)R30° reversely at an elevated temperature.     

金属-有机配合物分子在Au(111)表面的吸附行为

用STM对含氧桥的金属-有机配合物[Cu₂(μ-O)(dptap)4(NO₃)₂]分子在Au(111)表面的吸附行为进行了研究. STM结果表明, 该分子同时存在非解离吸附和解离吸附, 大部分分子在Au(111)面形成有规则的排列, 少量分子发生解离吸附, 并形成(√3×√3)R30°Cu原子吸附结构. 探讨了两种吸附现象共存的起因.     

Molecular orientations and interfacial structure of C60 on Pt(111)

Molecular orientations and assembled structures of C(60) molecules on Pt(111) have been characterized by low-temperature scanning tunneling microscopy for coverage between 0.1 ML and 1.5 ML. At room temperature, C(60) molecules preferentially decorate the steps and nucleate into single layer islands (SLIs) with hexagonal close-packed structures upon increasing coverage. C(60) islands comprise two differently oriented C(60)∕Pt(111)-(√13?×?√13) R13.9° phases, in which five types of molecular orientation of C(60) carbon cage configurations are clearly identified by the high-resolution scanning tunneling microscopy image. Further annealing treatment leads to more uniform molecular orientation without apparent aggregation of C(60) SLIs. As coverage increases above 1 ML, domains corresponding to (2√3?×?2√3) R30° superstructure appear. To explain the above transformation, an interfacial reconstruction model is proposed according to the detailed study of the molecular adsorption structures in different domains.     

Structure of CO2 adsorbed on the KCl(100) surface

The structure and dynamics of the adsorbate CO(2)/KCl(100) from a diluted phase to a saturated monolayer have been investigated with He atom scattering (HAS), low-energy electron diffraction (LEED), and polarization dependent infrared spectroscopy (PIRS). Two adsorbate phases with different CO(2) coverage have been found. The low-coverage phase is disordered at temperatures near 80 K and becomes at least partially ordered at lower temperatures, characterized by a (2√2×√2)R45° diffraction pattern. The saturated 2D phase has a high long-range order and exhibits (6√2×√2)R45° symmetry. Its isosteric heat of adsorption is 26 ± 4 kJ mol(-1). According to PIRS, the molecules are oriented nearly parallel to the surface, the average tilt angle in the saturated monolayer phase is 10° with respect to the surface plane. For both phases, structure models are proposed by means of potential calculations. For the saturated monolayer phase, a striped herringbone structure with 12 inequivalent molecules is deduced. The simulation of infrared spectra based on the proposed structures and the vibrational exciton approach gives reasonable agreement between experimental and simulated infrared spectra.     

Structural phases formed by NO(2)∕CO co-adsorption on Au{111} surfaces

Exposing a Au{111} surface to NO(2) and then to CO at temperatures around 120 K in ultra-high vacuum gives rise to molecular overlayers in which the two species are co-adsorbed, which we have investigated using low-temperature scanning tunnelling microscopy. Under NO(2)-rich conditions, a (√7 × √7)R19.1° phase with 3:1 NO(2):CO stoichiometry forms. Under CO-rich conditions, this phase co-exists with other phases having 2:1 and 1:1 NO(2):CO stoichiometries and different symmetries, and with bare Au surface. Structural models for these phases are discussed. Individual domains of the (√7 × √7)R19.1° phase are chiral, by virtue of the arrangement of their achiral components, an observation that may have more general implications.     

Relationship between the c(4×2) and the (√3×√3)R30° phases in alkanethiol self-assembled monolayers on Au(111)

Transformation between the two well-known phases of alkanethiol monolayers on Au(111), c(4×2) and (√3×√3)R30°, has been studied using scanning tunneling microscopy in ultra-high vacuum. Among the many versions of the c(4×2) phases observed, one particular structure where a lateral shift of adsorbate by as much as 0.17 nm within the unit cell is found. This lateral shift along the [112[combining macron]] direction corresponds to the movement of one adsorbed unit, towards its nearest neighbour from one hollow site to another (fcc to hcp, or hcp to fcc).     

Adsorption and decomposition of cyclohexanone (C6H10O) on Pt(111) and the (2 × 2) and (√3 × √3)r30°-Sn/Pt(111) surface alloys

Adsorption and decomposition of cyclohexanone (C(6)H(10)O) on Pt(111) and on two ordered Pt-Sn surface alloys, (2 × 2)-Sn/Pt(111) and (√3 × √3)R30°-Sn/Pt(111), formed by vapor deposition of Sn on the Pt(111) single crystal surface were studied with TPD, HREELS, AES, LEED, and DFT calculations with vibrational analyses. Saturation coverage of C(6)H(10)O was found to be 0.25 ML, independent of the Sn surface concentration. The Pt(111) surface was reactive toward cyclohexanone, with the adsorption in the monolayer being about 70% irreversible. C(6)H(10)O decomposed to yield CO, H(2)O, H(2), and CH(4). Some C-O bond breaking occurred, yielding H(2)O and leaving some carbon on the surface after TPD. HREELS data showed that cyclohexanone decomposition in the monolayer began by 200 K. Intermediates from cyclohexanone decomposition were also relatively unstable on Pt(111), since coadsorbed CO and H were formed below 250 K. Surface Sn allowed for some cyclohexanone to adsorb reversibly. C(6)H(10)O dissociated on the (2 × 2) surface to form CO and H(2)O at low coverages, and methane and H(2) in smaller amounts than on Pt(111). Adsorption of cyclohexanone on (√3 × √3)R30°-Sn/Pt(111) at 90 K was mostly reversible. DFT calculations suggest that C(6)H(10)O adsorbs on Pt(111) in two configurations: by bonding weakly through oxygen to an atop Pt site and more strongly through simultaneously oxygen and carbon of the carbonyl to a bridged Pt-Pt site. In contrast, on alloy surfaces, C(6)H(10)O bonds preferentially to Sn. The presence of Sn, furthermore, is predicted to make the formation of the strongly bound C(6)H(10)O species bonding through O and C, which is a likely decomposition precursor, thermodynamically unfavorable. Alloying with Sn, thus, is shown to moderate adsorptive and reactive activity of Pt(111).     

Adsorptive behavior of dimethylglyoxime on Au(111)

Dimethylglyoxime (DMG) adsorbed on Au(111) was investigated using electrochemical scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM experiments revealed three different structures of adsorbed DMG at open circuit potential (~0.07 V versus Ag/AgCl): (2√3×2√3)R30°-α, (2√3×4√3)R30°-β, and (2√3×4√3)R30°-γ. The coverage of adsorbed DMG obtained using XPS was 0.33. A combination of structural and quantitative information identified the adsorbed DMG as an anionic tetramer, held together by intermolecular hydrogen bonding and arrayed in three ordered patterns. Domains of adsorbed DMG underwent phase transitions between the observed structures, most likely due to the influence of the STM tip. However, a significant correlation between the observed structures and the imaging conditions was not found. The ordered layers existed only at open circuit potential as evidenced by their disappearance when the potential was shifted to 0.2 or -0.15 V. The ordered layers were also removed by immersion in a solution of Ni(2+), implying that the adsorbed DMG was converted to a soluble dimer complex with the Ni(2+) ion. This particular observation is discussed in terms of the rigidity of the organic network.     

Molecular orientation and lattice ordering of C60 molecules on the polar FeO/Pt(111) surface

C(60) molecules assemble into close packing layer under the domination of the intermolecular interaction when deposited onto Pt(111)-supported FeO layer kept at 400 K. From corresponding high resolution scanning tunneling microscopy (STM) image, a kind of C(60) molecular orientational ordering stabilized by the intermolecular interaction is revealed as C(60)/FeO(111)-(√133 × √133) R17.5° structure and determined from the commensurability between the C(60) nearest-neighbor distance and the lattice of the underlying oxygen layer. Moreover, due to the inhomogeneously distributed work function of the underlying FeO layer, the C(60) molecular electronic state is periodically modulated resulting in a bright-dim STM contrast. In addition, one coincidence lattice ordering is determined as 8 × 8 superstructure with respect to the C(60) primitive cell, which overlays a 3 × 3 moiré cell of the underlying FeO layer.     

Low-temperature adsorption of H2S on Ag(111)

H(2)S forms a rich variety of structures on Ag(111) at low temperature and submonolayer coverage. The molecules decorate step edges, exist as isolated entities on terraces, and aggregate into clusters and islands, under various conditions. One type of island exhibits a (√37×√37)R25.3° unit cell. Typically, molecules in the clusters and islands are separated by about 0.4 nm, the same as the S-S separation in crystalline H(2)S. Density functional theory indicates that hydrogen-bonded clusters contain two types of molecules. One is very similar to an isolated adsorbed H(2)S molecule, with both S-H bonds nearly parallel to the surface. The other has a S-H bond pointed toward the surface. The potential energy surface for adsorption and diffusion is very smooth.     

Preoxidation of CO on Os-modified Pt(111): a comparison with Ru-modified Pt(111)

The variation in CO adsorption structures during the preoxidation of CO on Os-modified Pt(111) (Pt(111)/Os) was investigated using cyclic voltammetry and electrochemical scanning tunneling microscopy. The spontaneous deposition of Os on Pt(111) resulted in randomly scattered islands with a coverage range of 0.13-0.54. During preoxidation on Pt(111)/Os, a phase transition from (2 × 2)-α to (√19 × √19) via the transient structures of (2 × 2)-β and (1 × 1) took place as on unmodified Pt(111). As the amount of Os increased, however, the transient structures of (2 × 2)-β and (1 × 1) appeared at lower potentials with higher populations. When the population of the transient structures was greater than 50%, an oxidative CO stripping process took place to the structure of (√19 × √19), completing the preoxidation. These observations strongly support the idea that the presence of Os increases the mobility of adsorbed CO by electronic modification of the Pt(111) surface (electronic effect). In addition, the results obtained with Pt(111)/Os were compared with those of Pt(111)/Ru.     

Electrochemical Stability of the Reconstructed Fe3O4(001) Surface

Establishing the atomic-scale structure of metal-oxide surfaces during electrochemical reactions is a key step to modeling this important class of electrocatalysts. Here, we demonstrate that the characteristic (√2×√2)R45° surface reconstruction formed on (001)-oriented magnetite single crystals is maintained after immersion in 0.1 M NaOH at 0.20 V vs. Ag/AgCl and we investigate its dependence on the electrode potential. We follow the evolution of the surface using in situ and operando surface X-ray diffraction from the onset of hydrogen evolution, to potentials deep in the oxygen evolution reaction (OER) regime. The reconstruction remains stable for hours between −0.20 and 0.60 V and, surprisingly, is still present at anodic current densities of up to 10 mA cm⁻² and strongly affects the OER kinetics. We attribute this to a stabilization of the Fe₃O₄ bulk by the reconstructed surface. At more negative potentials, a gradual and largely irreversible lifting of the reconstruction is observed due to the onset of oxide reduction.     

Ab initio studies of the electronic structure of L-cysteine adsorbed on Ag(111)

We have performed ab initio calculations for the adsorption of L-cysteine on Ag(111) using density functional theory. We have focused on two possible adsorbed species: the L-cysteine radical (?S-CH(2)-CH-NH(2)-COOH) adsorbed almost flat at a bridge site, slightly displaced toward an fcc location, and the zwitterionic radical Z-cysteine (?S-CH(2)-CH-NH(3)(+)-COO(-)) adsorbed at a bridge site, shifted to a hcp site forming a (4 × 4) unit cell (θ = 0.06) and a (√3 × √3) R 30° unit cell (θ = 0.33), respectively. Special attention has been paid to the electronic structure of the system. The adsorbate-silver bond formation has been exhaustively investigated by analyzing the density of states projected onto the different atoms of the molecule, and by charge density difference calculations. A complicated interplay between sp and d states of silver in the formation of bonds between the adsorbates and the surface has been found. The role of the carboxyl group in the interaction with the surface has been also analyzed.     

Electrodeposition of copper on a Pt(111) electrode in sulfuric acid containing poly(ethylene glycol) and chloride ions as probed by in situ STM

This study employed real-time in situ STM imaging to examine the adsorption of PEG molecules on Pt(111) modified by a monolayer of copper adatoms and the subsequent bulk Cu deposition in 1 M H(2)SO(4) + 1 mM CuSO(4)+ 1 mM KCl + 88 μM PEG. At the end of Cu underpotential deposition (~0.35 V vs Ag/AgCl), a highly ordered Pt(111)-(√3 × √7)-Cu + HSO(4)(-) structure was observed in 1 M H(2)SO(4) + 1 mM CuSO(4). This adlattice restructured upon the introduction of poly(ethylene glycol) (PEG, molecular weight 200) and chloride anions. At the onset potential for bulk Cu deposition (~0 V), a Pt(111)-(√3 × √3)R30°-Cu + Cl(-) structure was imaged with a tunneling current of 0.5 nA and a bias voltage of 100 mV. Lowering the tunneling current to 0.2 nA yielded a (4 × 4) structure, presumably because of adsorbed PEG200 molecules. The subsequent nucleation and deposition processes of Cu in solution containing PEG and Cl(-) were examined, revealing the nucleation of 2- to 3-nm-wide CuCl clusters on an atomically smooth Pt(111) surface at overpotentials of less than 50 mV. With larger overpotential (η > 150 mV), Cu deposition seemed to bypass the production of CuCl species, leading to layered Cu deposition, starting preferentially at step defects, followed by lateral growth to cover the entire Pt electrode surface. These processes were observed with both PEG200 and 4000, although the former tended to produce more CuCl nanoclusters. Raising [H(2)SO(4)] to 1 M substantiates the suppressing effect of PEG on Cu deposition. This STM study provided atomic- or molecular-level insight into the effect of PEG additives on the deposition of Cu.     

Adsorption of CO and O2 on W2C(0001)

CO, O(2), and H(2) adsorption on a clean W(2)C(0001)√13×√13 R ± 13.9° reconstructed surface at room temperature (RT) were investigated using high-resolution electron energy loss spectroscopy (HREELS). The W(2)C(0001) adsorbs CO molecularly and adsorbs O(2) dissociatively, but does not adsorb H(2) at RT. In the CO adsorption system, two C-O stretching (antisymmetric CCO stretching) modes were found at 242.3 meV (1954 cm(-1)) and at 253.0 meV (2041 cm(-1)). The low-frequency site is occupied at first with subsequent conversion to the high-frequency site with increasing coverage. Additionally, a small peak was apparent at 104.5 meV (843 cm(-1)), and a middle peak at 50-51 meV (400-410 cm(-1)), which are assignable to a symmetric stretching mode and a hindered translational mode, respectively, of a CCO (ketenylidene) species. These observations are consistent with the CO adsorption model on top of the surface carbon. For oxygen adsorption, two adsorption states were found at 65.2-68.1 meV (526-549 cm(-1)) and 73.6 meV (594 cm(-1)): typical frequencies to oxygen adsorption on metal surfaces. Results suggest that atomic oxygen adsorption occurred on a threefold hollow site of the second W layer.     

Complex orientational ordering of C60 molecules on Au(111)

The orientation and adsorption site for C(60) molecules on Au(111) has been studied using low temperature scanning tunneling microscopy. A complex orientational ordering has been observed for molecules inside the "in-phase" (R0°) domain. A 7-molecule cluster consisting a central molecule sitting atop of a gold atom and 6 tilted surrounding molecules is identified as the structural motif. The 2√3 × 2√3-R30° phase consists of molecules bonding to a gold atomic vacancies with a preferred azimuthal orientation. The quasi-periodic R14° phase is composed of groups of similarly oriented molecules with the groups organized into a 4√3 × 4√3-R30° like super-lattice unit cell.     

STM and voltammetric studies on the structure of a 4-pyridinethiolate monolayer chemisorbed on Au(100)-(1×1) surface

The structure for a 4-pyridinethiolate monolayer chemisorbed on the Au(100)-(1×1) single crystal surface was characterized by in situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV). In situ STM observation showed a well-ordered p(√2 R 45°×5 R 53.1°) structure (abbreviated as √2×5) for the surface modified with either 4-pyridinethiol (4-PySH) or bis(4-pyridyl)disulfide (4,4′-PySSPy) in a 0.05 M HClO₄ solution. On the Au(100)-(1×1) surface, 4-PySH molecules formed a dimer structure with the S–S bond length of about 2.4 nm. The observed dimer structure is similar to that previously reported on the Au(111) surface, and the orientation of the pyridine ring is mostly perpendicular to the surface normal. However, the adsorbed molecules were more densely packed (√3/√2 times) on the Au(100) surface than on the Au(111) surface. The surface excess was estimated to be 5.8 (±0.2)×10⁻¹⁰ mol cm⁻² based on the voltammetric charge for the reductive desorption. This value is in good agreement with that (5.7×10⁻¹⁰ mol cm⁻²) calculated from the parallelogrammic (√2×5) unit cell. The Au(100)-(1×1) surface modified with 4-PySH gave a well-defined electrochemical response of cytochrome c.     

First-principles study on the reconstruction induced by the adsorption of C60 on Pt(111)

The adsorption of C60 on a Pt(111) surface and the origins of the √13 × √13R13.9° or 2√3 × 2√3R30° reconstruction of the C60/Pt(111) system have been investigated by means of first-principles calculations. In agreement with the experimental observations, our calculations reveal that the C60 molecule binds covalently on the Pt(111) surface. The C60 molecule adsorbs on the Pt(111) surface with the center of a hexagonal ring located on top of a surface Pt atom. The surface Pt atom can be removed easily, forming a Pt vacancy upon the adsorption of C60 molecule. Our calculation results show that the strong covalent bonds between C60 and the Pt(111) surface and the formation of adatom-vacancy pairs in the C60/Pt(111) system may be the main driving forces promoting the substrate reconstructing pattern observed in experiments.     

The crystal structure of barium lead hexaaluminate phase II

A refinement was performed on the average crystal structure of barium lead hexaaluminate phase II ((Ba_0.8Pb_0.2)_2.34Al_21.0O_33.84) using single crystal X-ray diffraction data, giving a final R value of 0.030 with space group symmetry $\text{P}\text{6}\text{m2}$. The structure is essentially of a β-alumina type, but contains a lot of defects and interstitials. Inside the spinel block were found Ba(Pb) ions at 12-coordinated polyhedral sites, formed by complex defects, including triple Reidinger defects, in the same unit cell. Barium lead hexaaluminate phase II was found to consist of two kinds of unit cells with formulae “(BaPb)_3.0Al_20.0O_35.0” and “Ba_2.0Al_22.0O_34.0” in a 1 : 2 ratio; these three cells combine to form an a√3 × a√3 superstructure.     

Formaldehyde interaction with adsorbed oxygen on open-circuit platinum electrodes in aqueous sulfuric acid solutions

The open circuit potential transients and cathodic potentiodynamic pulses were measured upon formaldehyde (methylene glycol) interaction with pre-adsorbed oxygen (O_ads) on Pt/Pt and pc Pt electrodes in aqueous sulfuric acid solutions. The slowest interaction of CH₂(OH)₂ with O_ads was observed in the high coverage range of the electrode surface (θ_O ～ 0.2 0.8 to 1). The process rate in this range is determined by the direct reaction of O_ads with CH₂(OH)₂ molecules from the bulk solution. In the middle surface oxygen coverage range (θ_O 0.2 to 0.8), CH₂(OH)₂ interaction with O_ads takes place by the mechanism of “conjugated reactions”. The kinetic parameters of reactions for CH₂(OH)₂, HCOOH, and CH₃ OH were compared. The rate of CH₂(OH)₂-O_ads interaction on Pt electrodes in the high oxygen coverage range was found higher by an order of magnitude than that of HCOOH and by two orders of magnitude than in the case of CH₃OH.