Porphyrin self-assembly at electrochemical interfaces: role of potential modulated surface mobility

The self-assembly of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP) on Au(111) electrodes was investigated. The adlayer structure was found to depend on the electrode potential. At positive potentials (>0.5V(SCE)), a disordered layer of TPyP is formed on the Au(111) electrode. STM images showed that the disordered molecules are immobile. At negative potentials (-0.2V(SCE)), however, the molecules are highly mobile and can no longer be imaged by STM, though they remain on the surface. At intermediate potentials (-0.2 to +0.2V(SCE)), the TPyP formed a highly ordered adlayer. Once the ordered adlayer is formed, it persists even after the potential is stepped to higher values (0.5-0.8 V(SCE)). These results can be explained by the role of potential modulated adsorbate-substrate interaction and surface mobility. This suggests the intriguing prospect of using electrode potential to tune surface interactions and to drive surface processes, e.g., molecular self-assembly, in electrochemical systems.     

In situ SERS and SHINERS study of electrochemical hydrogenation of p-ethynylaniline in nonaqueous solvents

Surface-enhanced Raman spectroscopy (SERS) studies of electrode/solution interfaces are important for understanding electrochemical processes. However, revealing the nature of reactions at well-defined single crystal electrode surfaces, which are SERS-inactive, remains challenging. In this work, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used for the first time to study electrochemical adsorption and hydrogenation reactions at single crystal surfaces in nonaqueous solvents. A roughened Au surface was also studied for comparison. The experimental results show that the hydrogenation of adsorbed p-ethynylaniline (PEAN) on roughened Au electrode surfaces occurred at very negative potentials in methanol because of the catalytic effect of surface plasmon resonance (SPR). However, because “hot electrons” were blocked by the silica shell of Au@SiO₂ nanoparticles and aprotic acetonitrile was an ineffective hydrogen source, surface reactions at Au(111) were inhibited in the systems studied. Density functional theory (DFT) calculations revealed that the PEAN triple bond opened, allowing adsorption in a flat configuration on the Au(111) surface via two carbon atoms. This work provides an advanced understanding of electrochemical interfacial processes at single crystal surfaces in nonaqueous systems.     

基于超微电极的原位电化学拉曼光谱

原位电化学拉曼光谱是一种重要的光谱电化学技术.基于超微电极的原位电化学拉曼光谱将拉曼光谱反映的结构信息与电极表面的电化学过程从实验上严格对应和关联,为深刻理解电化学反应机理提供依据.本文综述了采用超微电极作为工作电极的原位电化学拉曼光谱的研究方法和应用进展,总结了应用超微电极作为工作电极开展电化学拉曼光谱实验的方法和具有表面增强拉曼活性的超微电极制备方法,展示了如何利用在超微电极表面获得的拉曼光谱与界面电化学过程的严格关联研究单个锌颗粒电化学氧化过程、吡啶分子在Au电极表面的电化学吸附过程,以及如何利用该技术能以高的信噪比和灵敏度同时测量光电流与分子反应这一特性研究对巯基苯胺选择性光氧化反应.采用超微电极作为工作电极的原位电化学拉曼光谱技术极大拓展了拉曼光谱技术的研究范围,有望成为探索(光)电化学反应的有力工具.     

Ordered molecular assemblies of substituted bis(phthalocyaninato) rare earth complexes on Au(111): in situ scanning tunneling microscopy and electrochemical studies

Substituted bis(phthalocyaninato) rare earth complexes ML2 (M = Y and Ce; L = [Pc(OC8H17)8]2, where Pc = phthalocyaninato) were adsorbed onto single crystalline Au(111) electrodes from benzene saturated with either YL2 or CeL2 complex at room temperature. In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were used to examine the structures and the redox reactions of these admolecules on Au(111) electrodes in 0.1 mol dm(-3) HClO4. The CVs obtained with YL2- and CeL2-coated Au(111) electrodes respectively contained two and three pairs of redox peaks between 0 and 1.0 V (versus reversible hydrogen electrode). STM molecular resolution revealed that YL2 and CeL2 admolecules were imaged as spherical protrusions separated by 2.3 nm, which suggests that they were oriented with their molecular planes parallel to the unreconstructed Au(111)-(1 x 1). Both molecules when adsorbing from approximately micromolar benzene dosing solutions produced mainly ordered arrays characterized as (8 x 5 radical3)rect (theta = 0.0125). The redox reactions occurring between 0.2 and 1.0 V caused no change in the adlayer, but they were desorbed or oxidized at the negative and positive potential limits. The processes of adsorption and desorption at the negative potentials were reversible to the modulation of potential. Electrochemical impedance spectroscopy (EIS) and CV measurements showed that YL2 and CeL2 adlayers could block the adsorption of perchlorate anions and mediating electron transfer at the Au(111) electrode, leading to the enhancement of charge transfer for the ferro/ferricyanide redox couple.     

Sb在Au电极上不可逆吸附的电化学过程

用电化学循环伏安法和电化学石英晶体微天平(EQCM)技术研究了Sb在Au电极上不可逆吸附的电化学过程. 研究结果表明, 在-0.25 V到0.18 V(vs SCE)范围内, Sb可在Au电极上稳定吸附, 并且在0.15 V附近出现特征氧化还原峰. 根据EQCM实验数据, 在电位0.18 V时, Sb在Au电极上的氧化产物是Sb2O3; 同时Sb的吸附阻止了电解液中阴离子和水在Au电极上的吸附. 当电极电位超过0.20 V时, Sb2O3会被进一步氧化成Sb5+化合物, 同时逐渐从Au电极表面脱附.     

Transient hydroxyl formation from water on oxygen-covered Au(111)

We present evidence for the formation of transient hydroxyls from the reaction of water with atomic oxygen on Au(111) and investigate the effect of adsorbed oxygen on the hydrogen bonding of water. Water is evolved in peaks at 175 and 195 K in temperature programed reaction experiments following adsorption of water on oxygen-covered Au(111). The peak at 175 K is ascribed to sublimation of multilayers of water, whereas the peak at 195 K is associated with oxygen-stabilized water or a water-hydroxyl surface complex. Infrared reflection absorption spectra are consistent with the presence of molecular water over the entire range of coverages studied, indicating that isolated stable hydroxyls are not formed. Isotopic exchange of adsorbed (16)O with H(2)(18)O following adsorption and subsequent temperature programed reaction, however, indicates that transient OH species are formed. The extent of oxygen exchange was considerable--up to 70%. The degree of oxygen exchange depends on the initial coverage of oxygen, the surface temperature when preparing oxygen adatoms, and the H(2)(18)O coverage. The hydroxyls are short-lived, forming and disproportionating multiple times before water desorption during temperature programed reaction. It was also found that chemisorbed oxygen is critical in the formation of hydroxyls and stabilizing water, whereas gold oxide does not contribute to these effects. These results identify transient hydroxyls as species that could play a critical role in oxidative chemical reactions on gold, especially in ambient water vapor. The crystallinity of adsorbed water also depended on the degree of surface ordering and chemical modification based on scanning tunneling microscopy and infrared spectra. These results demonstrate that oxidation of interfaces has a major impact on their interaction with water.     

Organized Monolayers of Biological Macromolecules on Au(111) Surfaces

Single-crystal electrochemistry and scanning tunneling microscopy directly in aqueous electrolyte solution (in situ STM) are established in physical electrochemistry but new in studies of adsorption and interfacial electrochemistry of biological macromolecules. These high-resolution techniques have now been applied comprehensively to proteins and other biomolecules in recent studies, discussed in this report. Focus is on three systems. The first one is a pair of amino acids, cysteine and cystine. These are strongly adsorbed via thiolate and disulfide, respectively, with identical reductive desorption and in situ STM patterns. Long-range lateral order can be imaged to molecular resolution. The amino acids are also reference molecules for the blue single-copper protein Pseudomonas aeruginosa azurin. This protein assembles in two well-defined orientations. One applies on bare Au(111) to which the protein is linked via its surface disulfide group. This orients the copper center away from the electrode surface. The other mode is by hydrophobic interactions with variable-length alkanethiols self-assembled on Au(111). In this mode the copper center is directed towards the surface. Adsorption and long-range electron tunneling in both modes have been characterized in detail using different electrochemical and spectroscopic techniques, as well as STM. Other data show that penta-(A–T) oligonucleotide adsorbed via a covalently bound thiol linker also displays reductive desorption and in situ STM to molecular resolution. The three systems thus appear to open new perspectives for broader use of high-resolution electrochemical techniques in comprehensive investigations of large biological molecules.     

碱性介质中L-赖氨酸在纳米金膜电极上的吸附和氧化过程

采用原位红外反射光谱(in situ FTIRS)和循环伏安法(CV)研究了碱性介质中L-赖氨酸在纳米金膜电极(nm-Au/GC)上的解离吸附和氧化过程. 研究结果表明, 在碱性溶液中以阴离子形式存在的赖氨酸[-OOC—CH—NH2—(CH2)4—NH2]在低电位区间(-0.95~-0.80 V, vs. SCE)发生部分解离, 生成AuCN-物种(约2110 cm-1), 同时赖氨酸阴离子的羧基侧还可通过两个氧原子与金电极表面相互作用. 随着电位的升高, 吸附态CN-氧化产生NCO-, OCN-和AuCN, 其对应的红外吸收峰分别位于2254, 2168和2226 cm-1附近.     

In situ STM study of Au(111)/Os bimetallic surfaces: spontaneous deposition and electrochemical dissolution

We provide an electrochemical and structural characterization by in situ STM of Au(111)/Os electrodes prepared by spontaneous deposition of Os on Au(111). Surfaces with Os coverage values up to the saturation coverage were examined, from 10%. Using comparisons to previous work on Au(111)/Ru, Pt(111)/Ru, and Pt(111)/Os, we find that we may now generalize that Os deposits spontaneously faster than Ru and has a greater tendency to form 3-D structures. Additionally, the Au(111) substrate shows preferential step and near-step decoration in both cases, although it is less pronounced for Os than Ru. We also investigated the incremental dissolution of the Os from Au(111), to better understand electrochemical dissolution processes in general and to better control the Os deposit structure. The application of controlled electrochemical treatments (cyclic voltammetry up to increasingly positive values) significantly increased the dispersion of the Os deposit by generating smaller, more widely spaced islands. Upon voltammetry up to 0.75 V, the Au(111)/Os surface showed evidence of alloying and the formation of 3-D structures suggestive of strong Os-Os (oxidized) species interactions. The CO stripping results show the Au(111)/Os is not particularly effective for this reaction, but such results help to complete the overall picture of NM-NM catalytic combinations. Although the Au(111)/Os system itself is not catalytically active, the electrochemical manipulation of the deposit structure demonstrated here may be applied to other noble metal/noble metal (NM/NM) catalytic substrates to find optimal deposit morphologies.     

Electrochemical Behavior of Irreversibly Adsorbed Sb on Au Electrode

The electrochemical processes of irreversibly adsorbed antimony (Sb_ad) on Au electrode were investigated by cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM). CV data showed that Sb_ad on Au electrode yielded oxidation and reduction features at about 0.15 V (vs saturated calomel electrode, SCE). EQCM data indicated that Sb_ad species were stable on Au electrode in the potential region from −0.25 to 0.18 V (vs SCE); the adsorption of Sb inhibited the adsorption of water and anion on Au electrode at low electrode potentials. Sb₂O₃ species was suggested to form on the Au electrode at 0.18 V. At a potential higher than 0.20 V the Sb₂O₃ species could be further oxidized to Sb(V) oxidation state and then desorbed from Au electrode.     

In-situ monitoring of redox processes of viologen at Au(hkl) single-crystal electrodes using electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy

In-situ Raman/SERS studies of molecular adsorption/reaction behaviors at well-defined electrochemical interfaces are important for understanding the fundamentals of electrochemical processes. However, it is still a great challenge to perform such studies on model single-crystal surfaces as the smooth surface cannot support surface plasmon resonance (SPR). In this work, shell-isolated nanoparticle-enhanced Raman spectroscopy was combined with an electrochemical method (EC-SHINERS) to study the adsorption and redox transformation of a resonant molecule viologen HS-8V8H at Au(hkl) single-crystal electrodes. Changes in the molecular structure with potential were identified on different single-crystal surfaces, which explained the transformation process of viologen from V^2 + state to V⁺ and then V⁰. Facet-dependent SERS enhancement was also observed, which results from the different imaginary part of the dielectric function on Au(111), Au(100) and Au(110), and is supported by the FEM simulations. Furthermore, a nonlinear resonant Raman process has been directly observed in our experiments, which is consistent with the simulation results. These findings increase our understanding of the electrochemical behavior of molecules in model systems.     

碱性介质中甘氨酸在纳米金膜电极上的吸附和氧化

运用原位红外反射光谱（in situ FTIRS）和电化学石英晶体微天平（EQCM）在分子水平上研究了碱性介质中甘氨酸在纳米金膜电极上的解离吸附和氧化过程.结果表明,甘氨酸在很低的电位下（－0.8 V, vs SCE）就可发生解离吸附.其解离产物氰基(CN－)与电极表面存在较强的化学吸附作用，形成AuCN－物种(红外吸收谱峰位于2100 cm－1附近).吸附在纳米金膜表面的CN－给出红外吸收显著增强、红外谱峰方向倒反和半峰宽增加的异常红外效应特征.吸附态CN－在低电位抑制H2O和OH－的吸附，当电位高于0.2 V可氧化产生OCN－；进一步升高电位到0.3 V则形成.溶液相物种OCN－和对应的红外吸收峰分别为2169 cm－1和2145 cm－1.实验结果指出，金以的形式溶解是导致电极表面质量显著减少的主要原因.     

应用镍超微电极的电化学表面增强拉曼光谱技术研究

镍（Ni）电极在电化学中应用广泛。原位表征Ni电极表面的吸附物种有益于帮助理解电极反应历程、指导发展高效电催化剂。应用超微电极作为工作电极的电化学表面增强拉曼光谱技术结合了超微电极表面的传质特性和分子水平的高灵敏度表征,是研究Ni电化学的有力手段。本文所述的研究工作通过在金（Au）超微电极表面电吸附具有SERS活性的Au纳米粒子并恒电流沉积金属Ni薄层,制备并表征了具有SERS活性的Ni超微电极。在氢氧化钠溶液中的循环伏安实验和以4-甲基苯硫酚分子作为探针分子的SERS实验结果表明,沉积速率和沉积电量是影响超微电极表面Ni的覆盖度和SERS活性的关键因素。在吸附了直径为55 nm Au纳米粒子的、直径为10 μm Au的超微电极表面,以100 μA·cm^-2电流密度电沉积厚度约为5个原子层Ni的条件下,可获得Ni覆盖完好的、具有最强SERS活性的Ni超微电极。     

Angstrom-Scale Electrochemistry at Electrodes with Dimensions Commensurable and Smaller than Individual Reacting Species

In nature and technologies, many chemical reactions occur at interfaces with dimensions approaching that of a single reacting species in nano- and angstrom-scale. Mechanisms governing reactions at this ultimately small spatial regime remain poorly explored because of challenges to controllably fabricate required devices and assess their performance in experiment. Here we report how efficiency of electrochemical reactions evolves for electrodes that range from just one atom in thickness to sizes comparable with and exceeding hydration diameters of reactant species. The electrodes are made by encapsulating graphene and its multilayers within insulating crystals so that only graphene edges remain exposed and partake in reactions. We find that limiting current densities characterizing electrochemical reactions exhibit a pronounced size effect if reactant's hydration diameter becomes commensurable with electrodes’ thickness. An unexpected blockade effect is further revealed from electrodes smaller than reactants, where incoming reactants are blocked by those adsorbed temporarily at the atomically narrow interfaces. The demonstrated angstrom-scale electrochemistry offers a venue for studies of interfacial behaviors at the true molecular scale.     

Platinum nanofilm formation by EC-ALE via redox replacement of UPD copper: studies using in-situ scanning tunneling microscopy

The growth of Pt nanofilms on well-defined Au(111) electrode surfaces, using electrochemical atomic layer epitaxy (EC-ALE), is described here. EC-ALE is a deposition method based on surface-limited reactions. This report describes the first use of surface-limited redox replacement reactions (SLR(3)) in an EC-ALE cycle to form atomically ordered metal nanofilms. The SLR(3) consisted of the underpotential deposition (UPD) of a copper atomic layer, subsequently replaced by Pt at open circuit, in a Pt cation solution. This SLR(3) was then used a cycle, repeated to grow thicker Pt films. Deposits were studied using a combination of electrochemistry (EC), in-situ scanning tunneling microscopy (STM) using an electrochemical flow cell, and ultrahigh vacuum (UHV) surface studies combined with electrochemistry (UHV-EC). A single redox replacement of upd Cu from a PtCl(4)(2-) solution yielded an incomplete monolayer, though no preferential deposition was observed at step edges. Use of an iodine adlayer, as a surfactant, facilitated the growth of uniformed films. In-situ STM images revealed ordered Au(111)-(square root 3 x square root 3)R30 degrees-iodine structure, with areas partially distorted by Pt nanoislands. After the second application, an ordered Moiré pattern was observed with a spacing consistent with the lattice mismatch between a Pt monolayer and the Au(111) substrate. After application of three or more cycles, a new adlattice, a (3 x 3)-iodine structure, was observed, previously observed for I atoms adsorbed on Pt(111). In addition, five atom adsorbed Pt-I complexes randomly decorated the surface and showed some mobility. These pinwheels, planar PtI(4) complexes, and the ordered (3 x 3)-iodine layer all appeared stable during rinsing with blank solution, free of I(-) and the Pt complex (PtCl(4)(2-)).     

Enhanced stability of short- and long-chain diselenide self-assembled monolayers on gold probed by electrochemistry, spectroscopy, and microscopy

Well-ordered, compact, self-assembled monolayers (SAMs) of hexyl and dodecyl diselenides have been formed on oriented (111) gold surfaces. Monolayer formation has been effected by adsorption from neat diselenides as well as millimolar solutions of diselenides in alcohol. The monolayer formation is confirmed using electrochemical quartz crystal microbalance studies. The stability and permeability of the monolayers at various temperatures have been probed using reflection absorption infrared spectroscopy (RAIRS) and electrochemistry. The RAIRS studies in the dry state show the formation of highly ordered, compact structures when adsorbed from neat compounds compared to the monolayers adsorbed in the presence of alcohol. The monolayers adsorbed from neat diselenide are quite stable as a function of temperature irrespective of the chain length. The electrochemical studies based on the blocking behavior of the monolayers toward electron transfer between a diffusing species and the electrode surface reflect the stability and the compactness of the structure. The results point out that the presence of solvent molecules during the SAM formation hinders the organization of the monolayer structure, especially in the case of short-chain diselenide monolayers.     

Electrochemistry of self-assembled monolayers of the blue copper protein Pseudomonas aeruginosa azurin on Au(111)

We report the self-assembly and electrochemical behaviour of the blue copper protein Pseudomonas aeruginosa azurin on Au(111) electrodes in aqueous acetate buffer (pH=4.6). The formation of monolayers of this protein is substantiated by electrochemical measurements. Capacitance results indicate qualitatively that the protein is strongly adsorbed at sub-μM concentrations in a broad potential range (about 700 mV). This is further supported by the attenuation of a characteristic cyclic voltammetric peak of Au(111) in acetate solution with increasing azurin concentration. Reductive desorption is clearly disclosed in NaOH solution (pH=13), strongly suggesting that azurin is adsorbed via its disulphide group. An anodic peak and a cathodic peak associated with the copper centre of azurin are finally observed in the differential pulse voltammograms. These peaks are, interestingly, indicative of long-range electrochemical electron transfer such as paralleled by intramolecular electron transfer between the disulphide anion radical and the copper atom in homogeneous solution, and anticipated by theoretical frames. Together with reported in situ scanning tunnelling microscopy (STM) results they constitute the first case for electrochemistry of self-assembled monolayers of azurin, even redox proteins. This integrated investigation provides a new approach to both structure and function of adsorbed redox metalloproteins at the molecular level.     

Silver residues as a possible key to a remarkable oxidative catalytic activity of nanoporous gold

Recently, several forms of unsupported gold were shown to display a remarkable activity to catalyze oxidation reactions. Experimental evidence points to the crucial role of residual silver present in very small concentrations in these novel catalysts. We focus on the catalytic properties of nanoporous gold (np-Au) foams probed via CO and oxygen adsorption/co-adsorption. Experimental results are analyzed using theoretical models represented by the flat Au(111) and the kinked Au(321) slabs with Ag impurities. We show that Ag atoms incorporated into gold surfaces can facilitate the adsorption and dissociation of molecular oxygen on them. CO adsorbed on top of 6-fold coordinated Au atoms can in turn be stabilized by co-adsorbed atomic oxygen by up to 0.2 eV with respect to the clean unsubstituted gold surface. Our experiments suggest a linking of that most strongly bound CO adsorption state to the catalytic activity of np-Au. Thus, our results shed light on the role of silver admixtures in the striking catalytic activity of unsupported gold nanostructures.     

Conformational adaptation and selective adatom capturing of tetrapyridyl-porphyrin molecules on a copper (111) surface

We present a combined low-temperature scanning tunneling microscopy and near-edge X-ray adsorption fine structure study on the interaction of tetrapyridyl-porphyrin (TPyP) molecules with a Cu(111) surface. A novel approach using data from complementary experimental techniques and charge density calculations allows us to determine the adsorption geometry of TPyP on Cu(111). The molecules are centered on "bridge" sites of the substrate lattice and exhibit a strong deformation involving a saddle-shaped macrocycle distortion as well as considerable rotation and tilting of the meso-substituents. We propose a bonding mechanism based on the pyridyl-surface interaction, which mediates the molecular deformation upon adsorption. Accordingly, a functionalization by pyridyl groups opens up pathways to control the anchoring of large organic molecules on metal surfaces and tune their conformational state. Furthermore, we demonstrate that the affinity of the terminal groups for metal centers permits the selective capture of individual iron atoms at low temperature.     

Electrochemical dimerization of 2-(2'-thienyl)pyridine adsorbed on Au(111) observed by in situ fluorescence

The study of heterodentate molecules adsorbed on metal electrodes provides an opportunity to expand the functionality of modified surfaces while offering insights into the surface and intramolecular electronic interactions of organic adsorbates. The adsorption of 2-(2'-thienyl)pyridine, a molecule containing both pyridine and thiophene moieties, on a Au(111) electrode is reported. Adsorption was characterized by electrochemistry in neutral and basic aqueous electrolyte and was compared to that of pyridine. The aqueous electrochemistry of thiophene on Au(111) was also characterized for comparison purposes. At negative potentials, in the presence of 2-(2'-thienyl)pyridine, a diffuse, pi-bonded monolayer was formed, and a phase transition to a close-packed N- and/or S-bonded configuration was observed near -0.4 V in a 1 mM solution of adsorbate, similar to that seen in pyridine on Au(111). The thiophene-like oxidative dimerization of the molecule was confirmed at positive potentials using in situ fluorescence microscopy by comparison with the spectrum of the chemically synthesized dimer.