Electrochemical and surface analytical studies of self-assembled monolayers of three aromatic thiols on gold electrodes

Three thiols with three aromatic rings and different structure – terphenyl-4-methanethiol (TPMT), terphenyl-4-thiol (TPT), and anthracene-2-thiol (AT) – have been used to form self-assembled monolayers (SAM) on vapour-deposited and flame-annealed Au films on glass substrates. All three SAMs effectively block the anodic formation of Au oxide, indicating densely packed layers which prevent the access of water and hydrated ions through the organic layer to the metal surface. The film improves its inhibiting properties with duration of exposure to the thiol solutions, reaching completion after 1 hour [1]. The charge-transfer reaction of the Fe(CN)₆ ^3–/Fe(CN)₆ ^4– system is blocked for TPMT films with an insulation of the π-electron system from the Au surface by the methylene group. TPT and especially AT films show the current density of the redox reactions. It is proposed that the charge transfer occurs via the aromatic molecules of the SAMs to the Au surface. Electronic Publication     

Metal Nanoparticles on Polymer Surfaces: 5. Catalytic Activity of Colloidal Platinum Films Incorporated in Polystyrene Surface Layer

The fundamental possibility to enlarge Pt nanoparticles in monolayer ensembles formed on polystyrene surfaces by the adsorption from hydrosol in solution of isopropanol and K₂PtCl₄ is demonstrated for the first time. The enlargement of “seeding” nanoparticles is performed after their preliminary incorporation (partial embedding) into the polymer surface layer by the annealing of a system within the range between “surface” and “bulk” glass transition temperatures of polystyrene. It is shown that a colloidal film of metallic platinum with a thickness up to 200 nm is formed in the course of enlargement and it is mechanically fixed in the polymer surface layer. Such a system exhibits, over a long time, high catalytic activity in the model reaction of methyl viologen reduction with hydrogen.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 3, 2005, pp. 398–403.Original Russian Text Copyright © 2005 by Rudoy, Sukhov, Dement’eva, Abkhalimov, Vereshchagina, Kartseva, Ershov.     

An in situ STM study on the long-range surface restructuring of Au(1 1 1) in a non-chloroaluminumated ionic liquid

We report an in situ STM study of a potential-dependent long-range surface restructuring of Au(1 1 1) electrode in neat 1-butyl-3-methylimidazolium tetrafluoroborates (BMIBF₄) ionic liquid. Au(1 1 1) undergoes a significant long-range surface restructuring upon cathodic excursion to −1.0 V vs. Pt quasi-reference. The restructuring involves the formation of tiny pits, which then develops into a stable worm-like network with an average width of the network grids 2 nm. Electrochemical annealing occurs at the cathodic limit with the presence of a reduction product of cation BMI⁺. A smooth surface is recovered with the appearance of the typical (√3 × 22) reconstruction of Au(1 1 1). The surface restructuring is reestablished upon anodic excursion to −1.3 V after the adsorbed reduction product is oxidized. The long-range surface restructuring phenomenon is tentatively explained as a result of partial charge transfer to the weakly adsorbed BMI⁺, which reduces the metal–metal cohesive energy. In addition, the synergetic effect of the counter anion BF₄⁻ may also be involved. The results provide a knowledge of Au(1 1 1) electrode behavior in the neat ionic liquid and are beneficial to understanding in situ STM results involving surface morphological changes in such a media.     

密度泛函理论研究十二烷硫醇在Au(111)面上的吸附

采用第一性原理方法研究了十二烷硫醇(C₁₂H₂₅SH)分子在Au(111)面上未解离和解离吸附的结构、能量和吸附性质,在此基础上分析判断长链硫醇分子在Au(111)面吸附时S―H键的解离, 以及分子链长度对吸附结构和能量的影响. 计算了S原子在不同位置以不同方式吸附的系列构型, 结果表明在S―H键解离前和解离后,均存在两种可能的表面结构, 直立吸附构型和平铺吸附构型; 未解离的C₁₂H₂₅SH分子倾向于吸附在top位, 吸附能为0.35-0.38 eV; H原子解离后C₁₂H₂₅S基团倾向于吸附在bri-fcc位, 吸附能量为2.01-2.09 eV. 比较分析未解离吸附和解离吸附, 发现C₁₂H₂₅SH分子未解离吸附相较于解离吸附要稳定, 未解离吸附属于弱化学吸附.局域电子态密度和差分电荷密度分析进一步验证了S―H解离后S原子与表面之间成键的数目增加, 而且键合更强. 同时我们发现长链硫醇的吸附能量较短链硫醇的吸附能量略大, S原子与表面Au原子之间的距离略小.     

The effect of the cooling atmosphere in the preparation of flame-annealed Pt(111) electrodes on CO adlayer oxidation

The effect of the cooling atmosphere on the rate of CO adlayer oxidation on flame-annealed Pt(111) has been studied. Cooling of a flame-annealed Pt(111) electrode in air results in a higher amount of crystalline defects compared to Pt(111) cooled in a hydrogen–argon stream. Although the blank profiles in 0.5 M H₂SO₄ of Pt(111), cooled in air and under oxygen exclusion, are virtually identical, CO adlayer oxidation occurs at significantly lower overpotentials on the former electrode. Three voltammetric peaks are observed for subsaturated CO adlayer oxidation on Pt(111), cooled in Ar+H₂ mixture, while only two peaks develop in the case of a Pt(111) surface cooled in air. Random crystalline defects, introduced via cooling of a flame-annealed Pt(111) in air, enhance CO adlayer oxidation, and apparently also suppress the third high-potential peak observed on a quasi-perfect (111) surface. The high sensitivity of the saturated CO adlayer oxidation to the presence of crystalline defects on Pt(111) can hence be used as a straightforward, sensitive, though qualitative method to assess the degree of crystalline order of the electrode.     

Chemically and Electrochemically Induced Surface Reconstruction of Au Single Crystal Studied by the Nonlinear Optics

The rotational anisotropy of the optical second harmonic generation (SHG) from Au (Ⅲ) surface was studied as a measure of the surface reconstruction before and after the self-assembly (SA) of thiols on the gold facet. The SA reaction was undertaken on the √3×23 reconstructed Au(Ⅲ) surface which was achieved consistently throughout the reacition by the electrochemical potential control in an aqueous HCIO₄, solution. The polarization dependence of the anisotropic SHG pattern showed that the initial surface symmetry of the reconstructed √3×23 structure was neither settled nor deformed into the 1×1 structure by the monolayer deposition of SA-overcoat which was expected to show √3×√3 hexagonal structure.     

A novel route to obtain metal and oxide nanoparticles co-existing on a substrate

In this work we present a novel route to cover large surfaces with metal and oxide nanoparticles (NPs) by growing and annealing of metallic bilayers. We have used this method to fabricate ensembles of Au and α-Fe₂O₃ NPs on silica substrates from Fe/Au bilayers. The morphology of the hybrid nanostructures and the presence of defects and disorder can be tuned through the processing parameters as the initial film thickness and the annealing conditions. The proximity effects between both types of NPs alter their physical properties. In particular, we observe that the presence of α-Fe₂O₃ NPs modifies the surface plasmon resonance of Au NPs.     

Optimization of the electrophoretic deposition of YBa2Cu3O7−x superconducting large area coatings

Large area coatings (>10cm²) of the high temperature superconductor YBa₂Cu₃O_7–x (x = 0.1 – 0.2) (YBCO) have been prepared by scale up an electrophoretic deposition technique using silver sheets and Si-wafers coated with Ag or Au as substrates. Several parameters, like the kind of the solvent, the applied voltage, the distance between the electrodes, the initial concentration of the suspension and the temperature during the electrophoresis were investigated in order to attain high deposition rates, as well as uniform YBCO coatings with the proper stoichiometry. To obtain a strongly adherent and dense coating a subsequent appropriate sintering and annealing procedure has been developed. The coatings obtained were characterized for their stoichiometry and superconducting properties by X-ray diffraction (XRD), Raman spectroscopy and magnetic measurements. The homogeneity and thickness of the films and the average grain size of the deposited particles have been investigated by optical and scanning electron microscopy (SEM).     

Substrate effects in poly(ethylene glycol) self-assembled monolayers on granular and flame-annealed gold

Poly(ethylene glycol) (PEG) self-assembled monolayers (SAMs) are surface coatings that efficiently prevent nonspecific adhesion of biomolecules to surfaces. Here, we report on SAM formation of the PEG thiol CH3O(CH2CH2O)17NHCO(CH2)2SH (PEG(17)) on three types of Au films: thermally evaporated granular Au and two types of Au films from hydrogen flame annealing of granular Au, Au(111), and Au silicide. The different Au surfaces clearly affects the morphology and mechanical properties of the PEG(17) SAM, which is shown by AFM topographs and force distance curves. The two types of SAMs found on flame-annealed Au were denoted "soft" and "hard" due to their difference in stiffness and resistance to scratching by the AFM probe. With the aim of nanometer scale patterning of the PEG(17), the SAMs were exposed by low energy (1 kV) electron beam lithography (EBL). Two distinctly different types of behaviour were observed on the different types of SAM; the soft PEG(17) SAM was destroyed in a self-developing process while material deposition was dominant for the hard PEG(17) SAM.     

Decoupling surface reconstruction and perchlorate adsorption on Au(111)

On Au(111) electrodes, the investigation of ClO₄⁻ adsorption is hampered by a simultaneous surface reconstruction. We demonstrate that these two processes can be decoupled in cyclic voltammograms by a proper choice of the scan rate and of the initial potential. Our approach allowed the establishment of a relation between potentials of zero charge for the reconstructed and unreconstructed Au(111) surfaces.     

Structure and voltage tunable dielectric properties of sol–gel derived Bi1.5MgNb1.5O7 thin films

Bi_1.5MgNb_1.5O₇ (BMN) thin films were fabricated on Au/Ti/SiO₂/Si(100) substrates using a sol?Cgel spin coating process. Thermo decomposition of the BMN precursor gel was discussed. The structures, morphologies, dielectric properties and voltage tunable dielectric properties were investigated. The deposited films showed a cubic pyrochlore structure after annealing at 550?°C or higher temperatures. With the annealing temperature increased from 500 to 800?°C, the root-mean-square surface roughness of the films increased from 0.6 to 6.8?nm. Additional phase, MgNb₂O₆, emerged after annealing at 800?°C due to the volatilization of Bi element. The dielectric properties and tunability of the films were annealing temperature dependent. BMN thin films annealed at 750?°C had a high dielectric constant of 135 and low dielectric loss of 0.002 at 1?MHz. The high tunability of 31.3?% and figure of merit of 156.5 were obtained under an applied electric field of 1?MV/cm at room temperature.     

X-ray Photoelectron Spectroscopy of the Surface of Solid Electrolyte La0.88Sr0.12Ga0.82Mg0.18O3 – α

The composition and chemical state of elements on the surface of solid oxygen-conducting electrolyte La_0.88Sr_0.12Ga_0.82Mg_0.18O_{3 –} is studied by x-ray photoelectron spectroscopy before and after annealing it at 973 K in a CO₂ atmosphere. Products of interaction between doped lanthanum gallate and CO₂ are localized in a surface layer 8–10 nm thick. The annealing does not lead to the formation of chemical carbon compounds with metals on the electrolyte surface. Surface layers of the electrolyte both in the initial state and after the annealing are rich in strontium oxide and contain some lanthanum hydroxide.     

Low energy electron diffraction and cyclic voltammetry studies of flame-annealed platinum single crystals

Low energy electron diffraction (LEED) and cyclic voltammetry were used to examine the surface structure of flame-annealed platinum (100), (110), and (111) electrodes. Well-defined LEED patterns were obtained for these surfaces but only after heat treatment in vacuo at 400°C in order to remove surface contaminants. The cyclic voltammograms correspond to those expected for these low index platinum surfaces from prior studies.     

In situ scanning tunneling microscopy of molecular assemblies of cobalt(II)- and copper(II)-coordinated tetraphenyl porphine and phthalocyanine on Au(100)

Molecules of copper(II) and cobalt(II) 5,10,15,20-tetraphenyl-21H,23H-porphine (CuTPP and CoTPP) and cobalt(II) phthalocyanine (CoPc) are spontaneously adsorbed onto reconstructed Au(100) substrate from a benzene solution containing each individual complex. In situ scanning tunneling microscopy (STM) was used to examine the real-space arrangement and the internal molecular structure of each of the individual molecules in 0.1 M HClO4 under potential control. The adsorption of CuTPP and CoTPP produced the same highly ordered square array with an intermolecular spacing of 1.44 nm on a reconstructed Au(100) surface. These molecular superlattices and the underlying reconstructed Au(100) predominated between 0 and 0.9 V, but lifting of the reconstructed Au(100) surface and elimination of the ordered adlayers occurred at more positive potentials. Molecular resolution STM revealed propeller-shaped admolecule with its center imaged as a protrusion for Co(II) and a depression for Cu(II). In contrast, the spontaneous adsorption of CoPc molecules resulted in a rapid phase transition from the reconstructed Au(100) surface to the (1 x 1) phase, coupled with the production of locally ordered, square-shaped arrays with an intermolecular distance of 1.65 nm. This molecular adlayer and the Au(100)-(1 x 1) remained unchanged when the potential was modulated between 0 and 1.0 V. These results indicate that the subtle variation in the molecular structure of adsorbate influenced not only its spatial arrangement but also the structure of the underlying Au(100) substrate.     

A study of the growth and CO electrooxidation behaviour of PtRh alloys on Pt{1 0 0} single crystals

The formation of quasi-crystalline PtRh two dimensional films supported on Pt{1 0 0} is described. For the first time, the voltammetry of PtRh{1 0 0} single crystal alloys covering the whole range of alloy composition is reported. Synthesis follows a similar procedure to that described previously for the formation of PtPd alloys supported on Pt{h k l} but with some important provisos concerning the final annealing step. CO electrooxidation was used as a probe reaction and for certain PtRh{1 0 0} surface alloys, unusually high electrocatalytic activity was observed relative to monometallic Pt and Rh electrodes. The flame annealing of a “PtRhPt sandwich” precursor structure was found to be the best method of forming the PtRh alloy surfaces. For PtRh films annealed under nitrogen, significant phase separation was observed in agreement with previous surface science studies of PtRh adlayers on Pt{1 0 0} annealed in the absence of oxygen. In addition, an excess of Rh in the “sandwich structure” from which the alloy was formed tended to preclude good alloy formation. It is suggested that the protocols for thin film formation described may prove useful in many other important electrocatalytic systems.     

In situ and real-time surface differential diffraction (SDD) study of Cd underpotential deposition on Au(1 1 1)

In situ and real-time surface differential diffraction (SDD) has been used to study the underpotential deposition (UPD) of Cd on Au(1 1 1) in sulfuric acid media. Comparison of SDD results in sulfate electrolytes with and without the presence of Cd²⁺ ions reveals that the surface reconstruction associated with the sulfate adsorption and desorption dominates the structural effect. It is also found that the reconstructed gold surface is stable upon Cd UPD process. In the initial stages of UPD, Cd atoms bind to the surface in bridge sites. This is followed by an adlayer structure with Cd adsorption in threefold hollow sites before Au/Cd intermixing takes place.     

Effect of Glass Substrates on the Formation of Gold-Silver Colloids in Nanocomposite Thin Films

A sol-gel route to synthesize nanocomposite thin films containing phase separated metal colloids of gold (Au) and silver (Ag) was developed. Ag—Au colloids were prepared in silica films using dip coating technique. The annealing of the samples in air results in the formation of phase separated Ag and Au colloids in SiO₂ thin films, showing the surface plasmon peaks at 410 nm and 528 nm. For the synthesis of phase separated Ag and Au colloids on float glass substrates, formation of the silver colloids was found strongly dependent on the surface of the float glass. On the tin rich surface formation of both gold and silver colloids took place, whereas, on the tin poor surface the formation of only gold colloids was observed. The surface dependence of the formation of silver colloids was attributed to the presence of tin as Sn²⁺ state on the glass surface, which oxidizes into Sn⁴⁺ during heat treatment, reducing Ag⁺ into silver colloids.     

Structure Sensitivity in the Electrocatalytic Reduction of CO2 with Gold Catalysts

An understanding of the influence of structural surface features on electrocatalytic reactions is vital for the development of efficient nanostructured catalysts. Gold is the most active and selective known electrocatalyst for the reduction of CO₂ to CO in aqueous electrolytes. Numerous strategies have been proposed to improve its intrinsic activity. Nonetheless, the atomistic knowledge of the nature of the active sites remains elusive. We systematically investigated the structure sensitivity of Au single crystals for electrocatalytic CO₂ reduction. Reaction kinetics for the formation of CO are strongly dependent on the surface structure. Under‐coordinated sites, such as those present in Au(110) and at the steps of Au(211), show at least 20‐fold higher activity than more coordinated configurations (for example, Au(100)). By selectively poisoning under‐coordinated sites with Pb, we have confirmed that these are the active sites for CO₂ reduction.     

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.     

Thermally Driven Structural Order of Oligo(Ethylene Glycol)-Terminated Alkanethiol Monolayers on Au(111) Prepared by Vapor Deposition

To probe the effects of deposition temperature on the formation and structural order of self-assembled monolayers (SAMs) on Au(111) prepared by vapor deposition of 2-(2-methoxyethoxy)ethanethiol (CH₃O(CH₂)₂O(CH₂)₂SH, EG2) for 24 h, we examined the surface structure and electrochemical behavior of the resulting EG2 SAMs using scanning tunneling microscopy (STM) and cyclic voltammetry (CV). STM observations clearly revealed that EG2 SAMs vapor-deposited on Au(111) at 298 K were composed of a disordered phase on the entire Au surface, whereas those formed at 323 K showed improved structural order, showing a mixed phase of ordered and disordered phases. Moreover, at 348 K, uniform and highly ordered EG2 SAMs on Au(111) were formed with a (2 × 3√3) packing structure. CV measurements showed sharp reductive desorption (RD) peaks at −0.818, −0.861, and −0.880 V for EG2 SAM-modified Au electrodes formed at 298, 323, and 348 K, respectively. More negative potential shifts of RD peaks with increasing deposition temperature are attributed to an increase in van der Waals interactions between EG2 molecular backbones resulting from the improved structural quality of EG2 SAMs. Our results obtained herein provide new insights into the formation and thermally driven structural order of oligo(ethylene glycol)-terminated SAMs vapor-deposited on Au(111).