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.     

Electrochemical and STM studies of 1-thio-β-D-glucose self-assembled on a Au(111) electrode surface

In this study, a Au(111) electrode is functionalized with a monolayer of 1-thio-β-D-glucose (β-Tg), producing a hydrophilic surface. A monolayer of β-Tg was formed on a Au(111) surface by either (1) potential-assisted deposition with the thiol in a supporting electrolyte or (2) passive incubation of a gold substrate in a thiol-containing solution. For each method, the properties of the β-Tg monolayer were investigated using cyclic voltammetry (CV), differential capacitance (DC), and chronocoulometry. In addition, electrochemical scanning tunneling microscopy (EC-STM) was used to obtain images of the self-assembled monolayer with molecular resolution. Potential-assisted assembly of β-Tg onto a Au(111) electrode surface was found to be complicated by oxidation of β-Tg molecules. The EC-STM images revealed formation of a passive layer containing honeycomb-like domains characteristic of a formation of S(8) rings, indicating the S-C bond may have been cleaved. In contrast, passive self-assembly of thioglucose from a methanol solution was found to produce a stable, disordered monolayer of β-Tg. Since the passive assembly method was not complicated by the presence of a faradaic process, it is the method of choice for modifying the gold surface with a hydrophilic monolayer.     

The concerted and stepwise chemisorption mechanisms of isothiazole and thiazole on Si(100)?2?��?1 surface

The surface reaction pathways of isothiazole and thiazole on Si(100)?2?×?1 surface were theoretically investigated using multireference wavefunctions. In the case of isothiazole, the Si?CN dative adduct turned out to be the major surface product. In contrast, a direct reaction competition between a concerted [4?+?2]_CC cycloaddition and Si?CN dative adduct was found in the adsorption of thiazole. Therefore, it is concluded that the particular geometric arrangements of heteroatoms exhibit distinctly different initial surface reaction mechanisms.     

Structure and properties of Ni(110) surface coadsorbing chlorine and oxygen

The coadsorption of chlorine with oxygen on Ni(110) surface has been investigated by XPS, UPS, AES and work function measurements. The chlorine preadsorption drastically inhibits the further uptake of oxygen. On the contrary, precovered oxygen has hardly any influence on the additional adsorption of chlorine due to the incorporation of precovered oxygen into the subsurface driven by the chlorine coadsorption. ARXPS measurements provide the evidence for this coadsorption model. The thermal desorption of chlorine and oxygen from the coadsorption surface is very similar to that of both individual adsorbates under the same heating conditions, but the desorption temperature of both the adsorbates apparently decreases on the coadsorption surface. The coadsorption and thermodesorption mechanisms are also discussed in detail.     

Experimental studies of the NaCs 5(3)Π0 and 1(a)3Σ+ states

We report high resolution measurements of 372 NaCs 5(3)Π(0)(v, J) ro-vibrational level energies in the range 0 ≤ v ≤ 22. The data have been used to construct NaCs 5(3)Π(0) potential energy curves using the Rydberg-Klein-Rees and inverted perturbation approximation methods. Bound-free 5(3)Π(0)(v, J) → 1(a)(3)Σ(+) emission has also been measured, and is used to determine the repulsive wall of the 1(a)(3)Σ(+) state and the 5(3)Π(0) → 1(a)(3)Σ(+) relative transition dipole moment function. Hyperfine structure in the 5(3)Π(0) state has not been observed in this experiment. This null result is explained using a simple vector coupling model.     

Selected electrochemical properties of Pd–Au alloys: hydrogen absorption and surface oxidation

Hydrogen absorption into and surface oxidation of Pd–Au alloys in acidic solutions were studied by cyclic voltammetry (CV) and chronoamperometry (CA) coupled with the electrochemical quartz crystal microbalance (EQCM). The influence of alloy bulk and surface composition on the process of oxidation of absorbed hydrogen was examined. The stresses induced by hydrogen insertion in Pd–Au alloys were compared with the case of pure Pd. The potential corresponding to the formation of a monolayer of surface oxide was determined for Pd–Au alloys of different surface states. Electrochemical dissolution of Pd–Au alloys was investigated.     

Mechanistic studies of Fc-PNA(⋅DNA) surface dynamics based on the kinetics of electron-transfer processes

N‐Terminally ferrocenylated and C‐terminally gold‐surface‐grafted peptide nucleic acid (PNA) strands were exploited as unique tools for the electrochemical investigation of the strand dynamics of short PNA(?DNA) duplexes. On the basis of the quantitative analysis of the kinetics and the diffusional characteristics of the electron‐transfer process, a nanoscopic view of the Fc‐PNA(?DNA) surface dynamics was obtained. Loosely packed, surface‐confined Fc‐PNA single strands were found to render the charge‐transfer process of the tethered Fc moiety diffusion‐limited, whereas surfaces modified with Fc‐PNA?DNA duplexes exhibited a charge‐transfer process with characteristics between the two extremes of diffusion and surface limitation. The interplay between the inherent strand elasticity and effects exerted by the electric field are supposed to dictate the probability of a sufficient approach of the Fc head group to the electrode surface, as reflected in the measured values of the electron‐transfer rate constant, k⁰. An in‐depth understanding of the dynamics of surface‐bound PNA and PNA?DNA strands is of utmost importance for the development of DNA biosensors using (Fc‐)PNA recognition layers.     

UHV–EC and EC–STM studies of molecular chemisorption at well-defined surfaces: hydroquinone and benzoquinone on Pd(hkl)

The chemisorption, from aqueous solutions, of hydroquinone and benzoquinone onto well-defined Pd(111) and Pd(100) electrode surfaces has been studied by tandem electrochemistry, high-resolution electron energy loss spectroscopy and in situ scanning tunneling microscopy. The results indicate that hydroquinone is oxidatively chemisorbed to form a quinonoid species that is oriented parallel to the surface but with a slight tilt in which one of the para-oxygens is topographically just higher than the other. The same surface species is obtained when adsorption is from aqueous benzoquinone.     

Review of the molar heats of chemisorption and chemabsorption by crystalline oxides and the surface “homogeneity versus heterogeneity” problem

To obtain a solution of the surface “homogeneity versus heterogeneity” problem, the results of microcalorimetric measurements of the dependences of the molar heats of chemisorption and chemabsorption of different gases on the amounts of chemisorbed or chemabsorbed gases in more than 20 gas/metal-oxide systems, in which the molar chemisorption heats are coverage-independent over rather wide ranges of the surface coverages, are presented. In order to approach the states of the metal oxide samples to those in real catalytic processes catalyzed by these oxides, the coverage dependences of the heats of chemisorption of gases at the samples were measured for a number of gas/metal-oxide systems against the chemisorbed amounts of not only the gas under study but also of another gas chemisorbed previously. The calorimetric data-set is supplemented with data obtained by other methods capable of helping to solve the surface “homogeneity versus heterogeneity” problem. These data are discussed together with the data on chemisorption in more than 40 gas/metal systems for which homogeneity of the surfaces was stated in our previous review. The entire set of the measurements was published for several decades by about 40 different composite authors. The chemisorption and chemabsorption mechanisms are discussed. It is concluded that thermally stabilized powder metal and metal oxide surfaces are homogeneous relative to the chemical ability of their atoms in chemisorption and catalytic processes in line with Langmuir’s opinion and the band theory of solids.     

Computer aided simulation of hydrogen–oxygen (air) fuel cell with regard to the degradation mechanism of platinum catalyst on the cathode

A mathematical model of a hydrogen–oxygen (air) fuel cell that takes into account the phenomena of degradation of the cathodic platinum catalyst is presented. For potential cycling from 0.6 to 1.1 V with a scan rate of 0.1 V/s, depending on the platinum loadings, the following factors are found to prevail in the mechanism of electroactive surface degradation: the coalescence of platinum nanoparticles at large loadings and the platinum dissolution/redeposition and diffusion to the membrane at medium and low loadings. Based on mathematical simulation, the data on the discharge curves are calculated. The observed decrease in the discharge characteristics is attributed to the degradation of the catalyst active surface and the increased transport losses during accelerated stress testing.     

Equilibrium and kinetic studies on the adsorption of VB_(12) onto CMK-3

The adsorption of VB12 onto CMK-3 was studied as a function of temperature and initial VB12 concentration. The highest VB12 adsorption capacity was determined as 353.4 mg/g at 40℃. Adsorption data were well described by the Langmuir model, although they could be modelled by the Freundlich equation. The pseudo-first-order and pseudo-second-order kinetic models were applied to test the experimental data. The pseudo-second-oider kinetic model provided the best correlation of the experimental data compared to the pseudo-nrsi-order model.     

Quantum-Chemical Simulation of the Adsorption of OH– Ions on Au(111)

Russian Journal of Electrochemistry - Based on the cluster model of the metal surface, the OH– ion interaction with the gold surface is studied by the density functional method. The...     

Are dispersive forces relevant for CO adsorption on the MgO(0 0 1) surface?

Periodic ab initio B3-LYP calculations on the MgO(0 0 1)/CO system underestimate the CO binding energy value with respect to experiment. The flaw is in the B3-LYP functional, unable to compute dispersive interactions. Here we show how to evaluate this contribution by adopting a two-layer ONIOM scheme in which the B3-LYP crystal energy is improved by the MP2 energy computed on the Mg₉O₉/CO cluster. The final complete basis set extrapolated (MP2/∞:B3-LYP/VTZ) CO/Mg(0 0 1) binding energy of 13 kJ/mol is in good agreement with experiment, with about 7 kJ/mol deriving from dispersive interactions.     

Atomic structure and formation mechanism of identically sized Au clusters grown on Si(111)-(7×7) surface

Identically sized Au clusters are grown on the Si(111)-(7×7) surface by room temperature deposition of Au atoms and subsequent annealing at low-temperature. The topographical images investigated by in situ scanning tunneling microscopy show a bias-dependent feature. The current-voltage properties measured by scanning tunneling spectroscopy indicate some semiconducting characteristics of the Au adsorbed surface, which is attributable to the saturation of Si dangling bonds. These experimental results, combined with the simulated scanning tunneling microscopy images and the first-principles adsorption energy calculations, show that the Au cluster is most likely to have a Au(6)Si(3) structure. In the Au(6)Si(3) cluster, three adsorbed Au atoms replace the three Si center adatoms, forming a hollow triangle, while the replaced Si atoms and other three Au atoms connect into a hexagon locating within the triangle. The formation mechanism of this atomic configuration is intimately associated with the complicated chemical valences of Au and the specific annealing conditions.     

Adenine–Au and adenine–uracil–Au. Non-conventional hydrogen bonds of the anions and donator–acceptor properties of the neutrals

This is a study of adenine–Au and adenine–uracil–Au (neutral, anionic and cationic), applying the B3LYP density-functional approach. In these systems, the interaction is directly related to the charge; so that as the metal atomic charge increases, the bond strength also increases. Neutral molecules are weakly bonded, the interaction in the case of cations is mainly electrostatic and in the case of the anions, the extra electron is localized on the metal atom and consequently, non-conventional hydrogen bonds are formed. In the case of adenine–Au (anion), the H dissociation energy is similar to the electron dissociation energy, and therefore both reactions may be possible. Moreover, the Au anionic atom modifies the hydrogen bonds of the uracil–adenine base pair. This may be significant in the study of point mutations that may occur in the Watson–Crick dimmer of nucleic basis. The electron-donator properties of these compounds are analyzed with the aid of the donator–acceptor map (DAM), previously described. Adenine–Au, uracil–Au and adenine–uracil–Au are more effective electron donors, but poorer electron acceptors than adenine, uracil and adenine–uracil. If the electron acceptor properties of carotenoids such as β-carotene and astaxanthin are compared, there are indications that astaxanthin may act as an oxidant instead of an antioxidant with the uracil–adenine base pair. The oxidation of nucleic acid bases by carotenoids may have important consequences, as oxidative damage of DNA and RNA appears to be linked to cancer. This is something that demands further studies and for this reason, work concerning the reactivity of carotenoids with DNA-nitrogen bases is in progress.     

Combined adsorption of ammonia and oxygen on the surface of solid solutions of CdxHg1 ? xTe

The adsorption of ammonia, oxygen, and the NH₃ + O₂ mixture on solid solutions Cd _x Hg_{1 − x} Te is studied. The low adsorbability of the mixture, as compared to that of the individual components, is established. Conclusions are drawn as to the nature of its active centers. Schemes for the combined adsorption of ammonia and oxygen are proposed. The temperature areas of tentatively highest catalytic activity of CdTe and CdHgTe in the reaction of ammonia oxidation are established. The formation of gaseous nitrogen in the final products is demonstrated. The degree of ammonia’s transformation into nitrogen is 97–99%. It increased with a rise in temperature and did not depend on the initial ammonia concentration.     

Theoretical studies on the spectroscopic properties and the substituent effects of pyridyl triazole Os(Ⅱ) complexes

To explore the spectroscopic properties of pyridyl triazole Os(Ⅱ) complexes and how the substituent effects affect the spectroscopic properties of [Os(ptz)2L2] (L=PH3; ptzH=(2-pyridyl)-1,2,4-triazole) (1), [Os(bptz)2L2] (bptzH=3-tert-butyl-5-(2-pyridyl)-1,2,4-triazole) (2), [Os(fptz)2L2] (fptzH=3- (trifluoreomethyl)- 5-(2-pyridyl)-1,2,4-triazole) (3), and [Os(fbtz)2L2] (fbtzH=3-(trifluoreomethyl)-5-(4-tert-butyl- 2-pyridyl)-1,2, 4-triazole) (4), the density functional theory (DFT) method at the B3LYP level ...