Grand-canonical approaches to understand structures and processes at electrochemical interfaces from an atomistic perspective

Electrochemical interfaces between an electrode and an electrolyte are often covered by ions from the solution. These adsorbed ions can strongly modify the properties of the interfaces. Furthermore, in electrocatalysis, the reacting species typically have to get into contact with the surface of the electrocatalyst, where the reaction then proceeds. Hence the understanding of the interaction of solvated species with electrode surfaces and the determination of the resulting adsorbate structures as a function of electrochemical control parameters such as electrode potential and electrolyte concentrations are crucial in electrochemistry. Here, grand-canonical theoretical approaches to derive adsorbate structures at electrochemical interfaces from an atomistic perspective will be reviewed. Special emphasis will be put on the distinction between the validity of the approach and the approximations that are typically made when using this approach.     

Differential electrochemical mass spectrometry

Differential electrochemical mass spectrometry (DEMS) can be used not only to identify products or intermediates of continuous faradaic reactions, but also to characterize submonolayer amounts of adsorbates on polycrystalline and single crystal electrode surfaces by means of their desorption, because of its high sensitivity. One possibility to achieve this is to oxidize a carbonaceous species to CO(2), which is quantitatively detected in the mass spectrometer. Many adsorbates can also be desorbed at certain potentials as such, or as the hydrogenated product, allowing a more direct characterization of the adsorbate. In some cases, a nonreactive desorption can be induced by displacement with a second adsorbate, yielding additional information. Interfacing an electrochemical cell to a mass spectrometer via a porous Teflon membrane can be achieved with a variety of cells. These will be described together with their specific advantages and characteristics.     

The influence of electrolyte concentration on the adsorption of octadecanol on Au(111)

The influence of electrolyte concentration on the potential dependent adsorption and desorption of octadecanol to/from a Au(111) electrode was investigated utilizing electrochemical and elastically scattered light techniques. The electrolyte concentration was found to influence the potential driven changes of the adsorbed layer (adsorption and desorption). The capacitive changes in the adsorbed layer were found to occur at more negative potentials with lower electrolyte concentration. The changes in the optical measurement, used to measure the characteristics of the desorbed species, or aggregates, were also found to be affected similarly. The magnitude of the overall change in the scattered light intensity was slightly dependent on electrolyte concentration. The re-adsorption of the aggregates was influenced by electrolyte concentration. The scattered light signal for an intermediate adsorbed state (adsorbed aggregate) was more prevalent for higher electrolyte concentration, suggesting that these intermediates were somewhat different compared to lower electrolyte concentrations. The lower electrolyte concentration displayed a larger potential region where this intermediate was stable, but the intensity of the scattered light was much lower. The electrolyte concentration most strongly influenced the potentials of adsorption and desorption, as well as the potential region of stability for the adsorbed intermediates. The sweep rate also has an influence on the scattering characteristics of the desorbed species, suggesting a possible method for measuring the kinetics of the adsorption–desorption process or for controlling the character of the desorbed species. These changes were explained in terms of a mechanism for the wetting or de-wetting of a surface. The influence of electrolyte concentration provides another opportunity for investigating the dynamics of this adsorption–desorption process.     

In-situ voltage tunneling spectroscopy at electrochemical interfaces

Nanophysics at electrochemical interfaces, probing the physical properties of nanostructures, requires laterally resolved in-situ spectroscopy, in particular voltage tunneling spectroscopy (VTS), which is at present not yet established. In-situ spectroscopy is required to achieve reliable and reproducible measurements of the intrinsic properties of nanostructures in an electrochemical environment, which are mainly determined in small nanostructures by surface atoms rather than bulk atoms. In contrast to tunneling spectroscopy in ultrahigh vacuum, tip and substrate double-layer capacitances as well as Faradaic currents play an important role in voltage tunneling spectroscopy at electrochemical interfaces. Deoxygenation of the electrolyte, fast measurements using appropriate instrumentation, and minimization of the unisolated tip apex and substrate surface areas exposed to the electrolyte are the key parameters to achieve reliable in-situ voltage tunneling spectroscopy data at electrochemical interfaces. The presented data show that bias voltage intervals of more than 1000 mV can be utilized for spectroscopic investigations in aqueous electrolytes, which allow the in-situ study of discrete electronic levels in nanostructures.     

Effects of lateral diffusion on the dynamics of desorption

The adsorbate dynamics during simultaneous action of desorption and lateral adsorbate diffusion is studied in a simple lattice-gas model by kinetic Monte Carlo simulations. It is found that the action of the coverage-conserving diffusion process during the course of the desorption has two distinct, competing effects: a general acceleration of the desorption process, and a coarsening of the adsorbate configuration through Ostwald ripening. The balance between these two effects is governed by the structure of the adsorbate layer at the beginning of the desorption process.     

The constant phase element reveals 2D phase transitions in adsorbate layers at the electrode/electrolyte interfaces

Using one of the most understood and well-characterized electrochemical systems, Pt(111) surface in contact with H₂SO₄, we provide evidences that specific adsorption, 2D phase transitions in the adsorbate layers and, in general, structural effects in the double layer are largely responsible for the so-called frequency dispersion of the double layer. The results also show promise that parameters of the constant phase element (which is used in impedance spectroscopy to account for the frequency dispersion) obtained as a function of the electrode potential can be reasonably used to detect 2D phase transitions at the electrode/electrolyte interfaces. This would provide a better insight into the interface, increasing the impact of measurements made by electrochemical impedance spectroscopy.     

表面等离子体共振技术在电化学反应过程研究中的应用

简述了表面等离子体共振(SPR)的基本原理,并综述了表面等离子体共振技术在电化学反应过程中的应用。SPR技术可以无需任何标记原位实时地检测分子间的相互作用,也可用于连续监测吸附/脱附和缔合/解离过程。表面等离子共振光谱(SPRS)与电化学技术结合可用来同时表征和处理电极/溶液的界面,在电化学掺杂/去掺杂过程、吸附/脱附反应的研究、痕量物质的检测、薄膜厚度、介电常数的测定等方面的应用已取得了很大的进展。     

Chemical interactions of pyridine and cyanide with silver

From a quantitative determination of pyridine and cyanide adsorbed on a silver electrode, by a radiochemical technique, we have shown that the two adsorbate - silver systems are different. After a dissolution - redeposition electrochemical cycle the quantity of pyridine adsorbed depends on the charge transfer. For low charge transfer (<50 mC cm^?2) the quantity increases from three to nine monolayers and depends on the nature of the supporting electrolyte, which suggests the formation of new bonds between pyridine, Ag and the anion of the supporting electrolyte. For high charge transfers the quantity of pyridine increases, the rate of increase depending on the supporting electrolyte (KI>KCl>KClO₄); in our opinion this is due to a trapping of pyridine in the salt formed between the support electrolyte anion and silver. The quantity of pyridine adsorbed at the silver electrode which can be as large as 100 equivalent monolayers can explain part of the enhancement of the Raman signal observed for this system.After a dissolution - redeposition electrochemical cycle the quantity of cyanide adsorbed remains constant, the cyanide - silver system is reversible and the Raman enhancement observed at the rest potential, is due only to Ag-CN interactions.     

Transport effects in the oxygen reduction reaction on nanostructured, planar glassy carbon supported Pt/GC model electrodes

The role of transport and re-adsorption processes on the oxygen reduction reaction (ORR), and in particular on its selectivity was studied using nanostructured model electrodes consisting of arrays of Pt nanostructures of well-defined size and separation on a planar glassy carbon (GC) substrate. The electrochemical measurements were performed under controlled transport conditions in a double-disk electrode thin-layer flow-cell configuration; the model electrodes were fabricated by colloidal lithography techniques, yielding Pt nanostructures of well defined and controlled size and density (diameter: 140 or 85 nm, height: 20 or 10 nm, separation: from 1-2 to more than 10 diameters). The nanostructured model electrodes were characterized by scanning electron microscopy and electrochemical probing of the active surface area (via the hydrogen adsorption charge). The electrocatalytic measurements revealed a pronounced variation of the hydrogen peroxide yield, which increases by up to two orders of magnitude with increasing separation and decreasing size of the Pt nanostructures. Similar, though less pronounced effects were observed upon varying the electrolyte flow and thus the mass transport characteristics. These effects are discussed in a reaction model which includes (i) direct reduction to H(2)O on the Pt surface and (ii) additional H(2)O(2) formation and desorption on both Pt and carbon surfaces and subsequent partial re-adsorption and further reduction of the H(2)O(2) molecules on the Pt surface.     

Electrochemical polymerization of aniline in phosphoric acid

The electrochemical synthesis of common conductive polymers such as polyaniline in phosphoric acid is a little different from that in other acidic media such as sulfuric acid. Electropolymerization in phosphoric acid is difficult, and this electrolyte medium is not applicable for this purpose. However, it is possible to overcome this problem by the addition of a small amount of sulfuric acid. In this case, the electropolymerization process can be successfully performed when the phosphate ion is doped. For instance, polyaniline films electrodeposited from an electrolyte solution of phosphoric acid have good stabilities and useful morphologies. Interestingly, phosphate doping results in the formation of nanostructures, whereas the polymer surface is macroscopically smooth. In an appropriate ratio, a mixed electrolyte of H₃PO₄ and H₂SO₄ can be used for the electropolymerization of aniline; thus, H₂SO₄ acts as a required agent for successful polymer growth, and H₃PO₄ acts as a doping agent. In this case, a small amount of sulfate is incorporated into the polymer matrix, which does not participate in the electrochemical insertion/extraction process. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3304–3311, 2006     

Molecular simulation evidence for solidlike adsorbate in complex carbonaceous micropore structures

Adsorption of a model nitrogen vapor on a range of complex nanoporous carbon structures is simulated by grand canonical Monte Carlo simulation for a single subcritical temperature above the bulk freezing point. Adsorption and desorption isotherms, heats of adsorption, and three-dimensional singlet distribution functions (SDFs) were generated. Inspection of the SDFs reveals significant levels of solidlike adsorbate at saturation even in the most complex of the microporous solids considered. This strongly suggests that solidlike adsorbate will also occur for simple subcritical vapors adsorbed on real noncrystalline solids such as microporous carbons at temperatures above the bulk freezing point, supporting indirect experimental observations. The presence of significant levels of solidlike adsorbate has implications for characterization of microporous solids where adsorbate density is used (e.g., determination of pore volume from loading). Detailed consideration of the SDF at different loadings for a model microporous solid indicates solidlike adsorbate forms at distributed points throughout the pore space at pressures dependent on the nature of the local porosity. The nature of the local porosity also dictates the freezing mechanism. A local freezing/ melting/refreezing process is also observed. Introduction of mesoporosity into the model causes hysteresis between the adsorption and desorption isotherms. Adsorption in the hysteresis loop occurs by a series of local condensation events. It appears as if the presence of adjacent microporosity and/or adsorbate within it affects the pressure at which these events occur. Reversal of the condensation during desorption occurs throughout the mesoporosity at a single pressure; this pressure is unaffected by the presence of adjacent microporosity or the adsorbate within it. It is also shown that the empirical concept of "pore size" is not consistent for describing adsorption in the complex solids considered here. A new concept is, therefore, proposed that seeks to account for the factors that affect local adsorption energy: local geometry, microtexture, surface atom density, and surface chemistry.     

Electrochemical plasmonic sensors

The enormous progress of nanotechnology during the last decade has made it possible to fabricate a great variety of nanostructures. On the nanoscale, metals exhibit special electrical and optical properties, which can be utilized for novel applications. In particular, plasmonic sensors including both the established technique of surface plasmon resonance and more recent nanoplasmonic sensors, have recently attracted much attention. However, some of the simplest and most successful sensors, such as the glucose biosensor, are based on electrical readout. In this review we describe the implementation of electrochemistry with plasmonic nanostructures for combined electrical and optical signal transduction. We highlight results from different types of metallic nanostructures such as nanoparticles, nanowires, nanoholes or simply films of nanoscale thickness. We briefly give an overview of their optical properties and discuss implementation of electrochemical methods. In particular, we review studies on how electrochemical potentials influence the plasmon resonances in different nanostructures, as this type of fundamental understanding is necessary for successful combination of the methods. Although several combined platforms exist, many are not yet in use as sensors partly because of the complicated effects from electrochemical potentials on plasmon resonances. Yet, there are clearly promising aspects of these sensor combinations and we conclude this review by discussing the advantages of synchronized electrical and optical readout, illustrating the versatility of these technologies.     

Measurement of the charge number per adsorbed molecule and packing densities of self-assembled long-chain monolayers of thiols

We have applied a recently developed method (Langmuir 2006, 22, 5509-5519) to determine charge numbers per adsorbed molecule and packing densities in self-assembled monolayers (SAMs) of octadecanethiol (C18SH), a representative long-chain thiol. Our method yields values of area per molecule that are physically reasonable, in contrast to the popular reductive desorption method, which gives molecular areas that are smaller than those determined by the van der Waals radii. In a nonadsorbing electrolyte, we were able to model the dependence of the charge number per adsorbed molecule on the electrode potential, taking into account that the desorption process is a substitution reaction between the solvent and the adsorbate. We have also shown that the charge number per adsorbed thiol is affected by the specific adsorption of the anion of the electrolyte. In the latter case, the thiol competes for adsorption sites at the surface not only with water but also with the anion of the electrolyte, and this competition has an effect on the measured charge number.     

Regulating the Interactions of Adsorbates on Surfaces by Scanning Tunneling Microscopy Manipulation

Scanning tunneling microscopy (STM) manipulation has received wide attention in the surface science community since the pioneering work of Eigler to construct surface nanostructures in an atom by atom fashion. Lots of scientists have been inspired and devoted to study the surface issues with the help of STM manipulations and great achievements have been obtained. In this Minireview, we mainly describe the recent progress in applying STM manipulations to regulate the inter‐adsorbate and adsorbate–substrate interactions on solid surfaces. It was shown that this technique could not only differentiate intermolecular interactions but also construct molecular nanostructures by regulating different kinds of inter‐adsorbate interactions or adsorbate–substrate interactions.     

Silicon surface nano-oxidation using scanning probe microscopy

This paper focuses on the nano-oxidation of a silicon surface using scanning probe microscopes in air ambient and in UHV. Special emphasis is put in air ambient on the preparation of the surfaces and on the oxidation mechanism. The characteristics of the patterned nanostructures are reviewed versus the parameters which govern the process (tip–surface voltage, tip speed, humidity) as well as the kinetics models of the oxidation process. The oxide patterns can act as robust masks for dry or wet etching. Fabrication of nanostructures is presented and allows to realize electronic nanodevices. In UHV, there is no direct nano-oxidation of the surface by the microscope tip. First the surface is hydrogenated, second a local hydrogen desorption is performed with the STM tip and finally the bare desorbed area is exposed to oxygen. The desorption process is analyzed versus tip–surface voltage and tunneling current. The oxidation of a desorbed area using molecular or atomic oxygen is actually difficult to achieve.     

Surface and electrochemical analysis for the understanding of TiO2 nanopores/nanotubes changes in post‐elaboration treatment

The electrochemical stability of TiO₂ nanoarchitecture fabricated in fluoride electrolyte presented in this paper is related to 2D and 3D geometries that present a shift from nanopores toward nanotubes. The fabrication conditions involve a 60 V applied voltage for 2 hours of anodizing in order to create the ordered structures, in a mixture of low‐water glycerol electrolyte and fluoride. With the use of different ultrasonication times, a variety of nanotubes/nanopores were observed. The surface interfacial aspects were investigated mainly by surface microscopy and hydrophilic/hydrophobic balance for the grown structures ultrasonicated at various periods of time. The electrochemical behavior of the nanotube‐structured surface was performed by potentiodynamic evaluation and electrochemical impedance spectroscopy in a simulated body fluid solution. As a most important result, all surface analysis and electrochemical data interpretation permitted the proposition of a model for elaboration of different nanostructures from nanopores to nanotubes. These different surface nanoarchitectures were obtained as a result of ultrasonication at various periods of time. Copyright © 2010 John Wiley & Sons, Ltd.     

Copper hydroxide nanoneedle and nanotube arrays fabricated by anodization of copper

Cu(OH)2 nanoneedle and nanotube arrays were electrochemically synthesized by anodization of a copper foil in an aqueous solution of KOH. The nanoneedles and nanotubes were constructed from nanosheets of Cu(OH)2. Controlling the electrochemical conditions can qualitatively modulate the lengths, amounts, and shapes of Cu(OH)2 nanostructures. The composition of as-prepared Cu(OH)2 nanostructures has been confirmed by X-ray diffraction and select-area electron diffraction. The influences of the KOH concentration of the aqueous electrolyte, the reaction temperature, and current density on the morphology of Cu(OH)2 nanostructures were investigated, and the formation mechanism of the nanostructures is discussed. Furthermore, Cu(OH)2 nanoneedles can be successfully transformed to CuO nanoneedles with little morphology change by heating. This work developed a simple, clean, and effective route for fabrication of large area Cu(OH)2 or CuO nanostructured films.     

Controlled growth of two-dimensional and one-dimensional ZnO nanostructures on indium tin oxide coated glass by direct electrodeposition

A simple electrochemical deposition technique is used to deposit ZnO nanostructures with diverse morphology directly on ITO-coated glass substrates at 70 degrees C. The concentration of the Zn(NO 3) 2.6H 2O electrolyte is important to controlling the dimensionality of the nanostructures, with formation of one-dimensional (1D) nanospikes and nanopillars (with 50-500 nm diameter) below 0.01 M and of two-dimensional (2D) nanowalls and nanodisks (with 50-100 nm wall/disk thickness) above 0.05 M. Glancing-incidence X-ray diffraction study shows their wurtzite structure and confirms the change in the preferred crystal plane orientation with the dimensionality of ZnO nanostructures. UV-vis spectroscopy reveals a higher transmittance from 2D nanostructures than from 1D nanostructures and their optical direct band gaps estimated to be 3.12-3.27 eV. Depth-profiling X-ray photoemission studies show the presence of Zn(OH) 2 outer layers on the ZnO nanostructures, with a higher Zn(OH) 2 moiety for 2D nanostructures relative to 1D nanostructures. Furthermore, a substantial quantity of Cl (provided by the KCl supporting electrolyte) is detected throughout the 2D nanostructures only. The photoemission data therefore affirm our proposed growth mechanism that involves capping of the preferred [0001] growth direction by Cl (-) ions under fast hydroxylation kinetics condition as observed at a higher Zn(NO 3) 2.6H 2O electrolyte concentration.     

Fundamental study on the thermal regeneration stages of exhausted activated carbons: kinetics

In this work the thermal regeneration of activated carbons saturated with p-nitrophenol has been analysed. By thermogravimetry, it was possible to elucidate the different events taking place during the thermal treatment, and relate them to the type of adsorption in the interfacial system. It was found that the mass loss during thermal treatment comprises a complex process in which different stages are involved, such as drying, desorption of physisorbed adsorbate, breaking up of surface functional groups, cracking of products from adsorbate–surface-specific interactions, etc. The analysis of the textural and surface chemistry characteristics of the pristine and regenerated adsorbent confirmed the thermal desorption mechanisms. Moreover, a kinetic study based on temperature-programmed desorption and Suzuki models was performed, using thermogravimetry data at different heating rates (5–20 K min^?1). From this analysis, the values of activation energy involved in each degradation step were estimated.     

铂催化氧还原反应过程中磷酸的影响及抑制磷酸吸附策略

与低温(<100oC)质子交换膜燃料电池相比,磷酸掺杂PBI膜燃料电池可工作于100–200 oC,工作温度的提高有利于提高电极反应动力学速率、增加Pt催化剂对CO等毒物的耐受性,以及简化电池水管理等.然而,磷酸在Pt催化剂表面吸附较强,这将造成Pt一定程度的毒化.基于“第三体效应”,即在Pt表面预吸附某些小分子,可在一定程度上抑制磷酸吸附,然而预吸附分子同时也将占据Pt表面部分活性位点,因而Pt的催化性能最终由两个因素决定：磷酸抑制程度和预吸附分子在Pt表面的覆盖度.
　　本文系统考察了Pt表面预吸附分子覆盖度和预吸附分子链长对其催化氧还原反应(ORR)活性的影响.首先,通过控制预吸附了胺类分子的Pt电极的电位,得到表面具有不同覆盖度的Pt电极,考察了0.1 mol/L H3PO4电解液中Pt电极对ORR的催化活性随预吸附分子覆盖度的变化规律;为分离磷酸吸附和修饰分子吸附本身对Pt催化活性的影响,对比了0.1 mol/L HClO4电解液中Pt电极对ORR的催化活性随预吸附分子覆盖度的变化规律.进一步对比研究了不同链长胺分子——正丁胺(BA)、正辛胺(OA)及十二胺(DA)等作为修饰分子对Pt/C催化剂电催化ORR活性的影响.结果表明,随修饰分子在Pt表面覆盖度提高,在0.1 mol/L HClO4溶液中,由于预吸附分子占据Pt部分活性位,修饰后光滑Pt电极表面的本征活性单调下降;而在0.1 mol/L H3PO4中,修饰后光滑Pt电极表面的ORR活性呈现先升高后降低的趋势,当预吸附分子覆盖度约为20%时,其ORR活性最高,为未修饰的光滑Pt电极表面的1.67倍.这表明预吸附分子有效抑制了磷酸的吸附,且当预吸附分子覆盖度约为20%时,预吸附分子对Pt表面的占据与其抑制磷酸吸附的作用达到最佳平衡点.然而,当修饰分子BA, OA和DA在Pt表面覆盖度分别为38.6%,26.1%和26.1%时, Pt/C在0.1 mol/L H3PO4中的ORR催化活性接近,分别为未经修饰Pt/C电催化剂的1.7,1.8和2.0倍,这表明预吸附分子链长对ORR催化活性影响较小,表面预吸附分子抑制磷酸吸附的策略对Pt/C催化剂也同样适用.同时, Pt/C电极经BA, OA和DA修饰后,其在0.1 mol/L HClO4中的比表面活性分别为未经修饰Pt/C电催化剂的1.0,1.1和1.3倍,与修饰后光滑Pt电极表面本征ORR活性变化规律不一致.然而,与Pt在HClO4电解质中的ORR活性相比, ORR的半波电位仍有大约123 mV的差距,今后还需继续从催化剂的角度,如调控Pt表面的吸附特性,或从创新电解质的角度,如有机磷酸电解质等出发解决磷酸毒化的问题.