Adsorption Of Halide Ions Research Articles

Perspectives and Impact of in-Situ X-Ray Techniques for Electrodeposition

In-situ surface x-ray diffraction has enabled an atomic/molecular-level understanding of the electrochemical interface, including its potential and time dependence, to be developed. [1] Specifically, for electrodeposition the influence of additives and adsorbates onto growth and nucleation behaviour could be established. Halogens on metal surfaces are prototypic adsorbate systems but the adsorption of halide ions especially on copper is also of major importance for on-chip metallization in ULSI microchip production. Halide ions on Cu surfaces form an inhibiting adsorbate layer with polyethylene glycol (PEG). Even though the influence of the additives combination on the shape evolution of the Cu deposit was subject of numerous studies, their precise role during the elementary steps of the deposition with regards to altering the charge distribution and dipole moment at the interface is largely not understood. [1] While information about the atomic structure of the electrode surface in electrochemical in-situ cells has been widely investigated, insight into the charge distribution and the structure of the electrolyte at the interface is still lacking. Combining x-ray spectroscopy and x-ray diffraction to gain site-specific information about the charge distribution at buried interfaces is a promising tool. [2,3,4] Studies on the metal-halide interface and how the use of surface x-ray scattering techniques can help to characterise electrochemical interfaces in-situ in order to link, structure and stability and morphology changes will be presented. [1, 4] Advances in these directions offer possibilities in elucidating atomic scale models of the electrochemical interface and thus will help to establish structure-stability-reactivity relationships which will help to understand growth kinetics and nucleation behaviour.

The Puzzling Processes at Electrode | Ionic Liquid Interface

J.M. Lehn stated in his Nobel prize lecture in 1988 that supramolecular chemistry is the chemistry of the intermolecular bond, covering the structure and functions of entities formed by the association of two or more chemical species [1]. The adsorption of organic molecules is the net result of the interactions between the molecules and the two phases, the metal and the electrolyte, and the interactions between the latter two.To this day, the universal understanding that the adsorption of organic molecules results in the formation of an ordered monolayer is based on the phenomenon observed in mainly aqueous solutions. However, the conception might not be straightforwardly transferrable to the organic additive + ionic liquid | electrode interface. For example, the existence of an interfacial multilayer structure of pure IL ions in contrast with an aqueous electrolyte has been previously shown in both experimental and computational studies. Furthermore, it has been shown that in ILs, if the ions form rigid layers on the electrode, it is necessary to apply an overpotential for interfacial restructuring. Therefore, studying the adsorption of organic additives in ionic liquids (IL) should contribute to a better understanding of metal | IL interfaces.In the given presentation we overview the characteristics of solid-liquid interface characteristics of various additives from ionic liquid media at different electrodes.Cyclic voltammetry, electrochemical impedance spectroscopy and in situ scanning tunnelling microscopy measurements were conducted to characterize the electrochemical behavior of the self-assembled layers of 4,4’-bipyridine (4,4’-BP) and 2,2’-bipyridine (2,2’-BP) at the Sb(111) | x-BP+1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) interface [2-3]. The specific adsorption of iodide ions was studied at Bi(111), Cd(0001) and pyrolytic graphite electrodes [4-6]. All the experiments were carried out in a three-electrode electrochemical cell in a glovebox.The halide ions are surface active ions with specific adsorption behavior [4-6]. The strong specific adsorption of halide ions at single crystal electrodes, as well as other electrodes, is a complicated process due to the partial charge transfer between adsorbed ions and the solid electrode surface. The properties of the adsorption layer depend on the electrode potential and the concentration of the surface-active ions in an electrolyte. The theory that the adsorption of organic molecules results in SAM formation is mainly based on the results observed in aqueous solutions. However, these findings may not be as straightforwardly linkable to the organic additive + ionic liquid | electrode interface. The analysis of cyclic voltammetry and impedance results revealed that 2,2′-BP and 4,4′-BP indeed adsorb at the Sb(111) interface, forming a thin dielectric layer at the electrode surface, confirmed by in situ STM measurements, resulting in the differential capacitance values nearly two times lower compared to EMImBF4. Acknowledgments:This work was supported by the Estonian Research Council grant PSG249, and by the EU through the European Regional Development Fund under project TK141 (2014-2020.4.01.15-0011).

Analysis of impedance: The distribution of capacitance in halide ion treated supercapacitors

The micro- and mesoporous carbon electrode material has an enough pore width capacity to allow to adsorb the small-sized halide- and alkali ions, even to adsorb ionic liquid ions. • Ionic liquid mixtures containing halide or alkali ions are used to treat carbon electrodes. • The CV and EIS characteristics of carbon-based supercapacitors are studied. • The EIS modeling is applied to analyze the electrochemical characteristics. • The capacitive and pseudocapacitive characteristics of supercapacitors are analyzed. • Pseudocapacitance originates in the redox activity and specific adsorption of halide ions. A combination of two energy-stored modes, faradaic and electrostatic, has been applied to improve the energy density of supercapacitors with ionic liquid salt mixtures via impregnation of micro- and mesoporous carbon electrodes. An enhancement of capacitance was observed in the supercapacitors containing halide ion treated carbon electrodes, caused by the pseudocapacitive processes related to redox-active halide ions within and near the micro- and mesoporous carbon electrodes. To highlight the contribution of pseudocapacitance, an equivalent circuit model was applied to analyze the impedance spectra in detail. Significant pseudocapacitive behavior was only observed in the supercapacitors with halide ion treated carbon electrodes. A comparable result for both electrical double layer capacitance and pseudocapacitance values was obtained based on the direct current electrochemistry measurements as well as impedance fitting results. Meanwhile, according to the resistive behavior associated with the mass-transfer and faradaic processes, halide ion treated systems were more reactive compared with alkali ion treated systems. This article offers a brief and fundamental understanding with a focus on the contribution and distribution of both resistive and capacitive behavior analyzed by electrochemical impedance spectroscopy coupled with equivalent circuit modeling.

The effect of specific adsorption of halide ions on electrochemical CO2 reduction.

In the electrochemical CO2 reduction reaction (CO2RR), halide ions could impose a significant effect on multi-carbon (C2+) product production for Cu-based catalysts by a combined contribution from various mechanisms. However, the nature of specific adsorption of halide ions remains elusive due to the difficulty in decoupling different effects. This paper describes a facile method to actively immobilize the morphology of Cu-based catalysts during the CO2RR, which makes it possible to reveal the fundamental mechanism of specific adsorption of halide ions. A stable morphology is obtained by pre-reduction in aqueous KX (X = Cl, Br, I) electrolytes followed by conducting the CO2RR using non-buffered and non-specifically adsorbed K2SO4 as the supporting electrolyte, by which the change of local pH and cation concentration is also maintained during the CO2RR. In situ spectroscopy revealed that the specific adsorption of halide ions enhances the adsorption of *CO intermediates, which enables a high selectivity of 84.5% for C2+ products in 1.0 M KI.

Fermi Level Equilibration at the Metal-Molecule Interface in Plasmonic Systems.

We highlight a new metal-molecule charge transfer process by tuning the Fermi energy of plasmonic silver nanoparticles (AgNPs) in situ. The strong adsorption of halide ions upshifts the Fermi level of AgNPs by up to ∼0.3 eV in the order Cl- < Br- < I-, favoring the spontaneous charge transfer to aligned molecular acceptor orbitals until charge neutrality across the interface is achieved. By carefully quantifying, experimentally and theoretically, the Fermi level upshift, we show for the first time that this effect is comparable in energy to different plasmonic effects such as the plasmoelectric effect or hot-carriers production. Moreover, by monitoring in situ the adsorption dynamic of halide ions in different AgNP-molecule systems, we show for the first time that the catalytic role of halide ions in plasmonic nanostructures depends on the surface affinity of halide ions compared to that of the target molecule.

(Invited) In Situ Studies of the Atomic Structure/Charge Distribution at the Electrochemical Interface

The presence of specifically adsorbed anions can significally affect the electrochemical reactivity of a metal electrode which is of major interest for galvanic deposition, etching, corrosion and electrocatalysis. Halogens on metal surfaces are prototypic adsorbate systems but the adsorption of halide ions especially on copper is also of major importance for on-chip metallization in ULSI microchip production. Halide ions on Cu surfaces form an inhibiting adsorbate layer with polyethylene glycol (PEG). Even though the influence of the additives combination on the shape evolution of the Cu deposit was subject of numerous studies, their precise role during the elementary steps of the deposition with regards to altering the charge distribution and dipole moment at the interface is largely not understood.To obtain insights into the influence of the applied potential and the screening by the electrolyte to better understand electrochemical processes and charge transfer during adsorption and deposition is a major goal .[1]Thus combining x-ray spectroscopy and x-ray diffraction to gain site-specific information about the charge distribution at buried interfaces is a promising tool. [2,3]We will present studies on the metal-halide interface and how the use of surface x-ray scattering techniques can help to characterise electrochemical interfaces in-situ in order to link, structure and stability. [3] Advances in these directions offer possibilities in elucidating atomic scale models of the electrochemical interface and thus will help to establish structure-stability-reactivity relationships which will help to understand growth kinetics.

Key Role of Direct Adsorption on SERS Sensitivity: Synergistic Effect among Target, Aggregating Agent, and Surface with Au or Ag Colloid as Surface-Enhanced Raman Spectroscopy Substrate.

It is widely accepted that the sensitivity of surface-enhanced Raman spectroscopy (SERS) is mainly manipulated by the electromagnetic enhancement mechanism (EM). Herein, we determined that the direct adsorption of the target on the SERS active surface is vital as well, through the systematic investigation of the SERS behavior of three positively charged molecules on negatively charged gold (Au) or silver nanoparticles (Ag NPs). Facilitated by the synergistic effect among the molecule, the surface, and the specific adsorbed halide ions (Cl-, Br-, and I-), high SERS sensitivity for trace target was realized, which was mainly from the directly adsorbed molecules. Noteworthy, little contribution from the nondirectly adsorbed molecules was discernible, although the EM enhancement was at the same level for these two surface species dwelling within a distance significantly less than 1 nm from the surface. Further, the related strategy for trace detection sheds light on how to realize sensitive SERS detection of new targets.

Effect of alkali and halide ion doping on the energy storage characteristics of ionic liquid based supercapacitors

The electrochemical properties of supercapacitors based on carbide-derived carbon electrodes doped with up to 5 wt% mixture of halide or alkali ion salts in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EMImTFSI) have been studied to exploit their specific energy storage mechanism. To separately highlight the effect of redox active mixture doped electrodes, neat EMImTFSI is applied as the electrolyte for all studied supercapacitor systems. A noticeable increase of capacitance for carbon electrodes doped with halide ions has been observed, as measured by three different electrochemical analysis techniques. Compared with neat EMImTFSI based system, carbon electrodes doped with alkali ion salt mixtures do not show a noticeable improvement of energy storage characteristics. Interestingly, both 5 wt% alkali and halide ion salt mixture doped system exhibit a stable high capacitance up to 124 F/g at 3 V. It is shown that this enhancement of capacitance is due to reversible faradaic processes and the specific adsorption of halide ions within the micro- and mesoporous electrode material and does not limit the bulk electrolyte properties. It is seen that the alkali and halide ion doped systems display greater electrochemical stability compared with neat EMImTFSI based system within constant voltage floating tests.

Enhancing CO2 electroreduction on nanoporous silver electrode in the presence of halides

The effects of halide ions (F−, Cl−, Br−, and I−) on nanoporous Ag electrode for reduction of CO2 to CO were systematically assessed. The electrochemical measurements indicate that the catalytic performance of nanoporous Ag is closely related to the affinity of halide ions to Ag. The current density and Faradaic efficiency for CO production simultaneously increase with the increase of the radius of halide ions from F− to Cl− and then to Br−, and the disparity between catalytic performance will further widen with the negative shift of potential. While the nanoporous Ag in the presence of I− first exhibits the poorest electrocatalytic performance at low potential (<−0.69 V vs. RHE), then reverses the situation at high potential (>−0.89 V vs. RHE) in comparison to the other halide ions. Tafel plots indicate that the adsorption of halide ions on the surface of nanoporous Ag influences mechanistic pathways for CO2 reduction reaction. 8 h durability test demonstrates that the nanoporous Ag is catalytically stable in a mixed electrolyte containing bromide with only less than 5% deactivation. The density functional theory calculations enlighten that the weakened interaction between Ag atoms on the surface of electrocatalyst in the presence of halides can significantly enhance the binding energy of the intermediate COOH*, and thus effectively lowers the overpotential of CO2 reduction reaction.

Electric double layer on a renewable liquid (Cd[sbnd]Ga) electrode in dimethyl sulfoxide solutions

The electric double layer (EDL) structure on a renewable liquid (CdGa) (0.3at.% Cd) electrode in solutions of surface-inactive electrolyte is studied in a solvent with a high donor number, namely, dimethyl sulfoxide (DMSO). The following parameters: the potential of zero charge (PZC) unaffected by specific adsorption of ions, the value of “corrected electrochemical work function”, and the potential drop due to chemisorption of solvent are obtained on (CdGa)/DMSO interface and are compared with analogous results on Hg/DMSO, (TlGa)/DMSO, (InGa)/DMSO, Ga/DMSO, (CdGa)/acetonitrile (AN), (CdGa)/gamma-butyrolactone (GBL), and (CdGa)/dimethylformamide (DMF) interfaces. The chemisorption of DMSO molecules in the Helmholtz layer shifts the potential of the (CdGa) electrode in the negative direction, which suggests that chemisorbed DMSO dipoles are oriented with their negative (oxygen) ends to the metal surface. It is found that the solvent chemisorption potential drop on the (CdGa) electrode increases in the series AN<GBL<DMF<DMSO, i.e. with increasing donor number of the solvent. The energy of metal–DMSO chemisorption interaction increases in the series (TlGa)<(CdGa)≈(InGa)<Ga, i.e. with an increase in the electron work function. These results indicate the donor-acceptor nature of the metal-solvent chemisorption interaction in the Helmholtz layer, where the solvent molecules are donors of the electron pair relative to the metal. In spite of a decrease in the electron work function when passing from (InGa) to (CdGa), the energies of their chemisorption interaction with DMSO are close. This result can be explained by a closer approach of DMSO dipoles to the surface of the (CdGa) electrode as compared with the (InGa) electrode. It contributes to a more efficient overlapping of the donor-acceptor levels of the solvent and metal during their chemisorption interaction. Based on the PZC shifts observed in 0.1 М LiCl, LiBr and LiI solutions with respect to the PZC in the solution of surface inactive electrolyte, the adsorbability of halide ions on the (CdGa)/DMSO interface is studied. It is shown that on the (CdGa)/DMSO interface, the adsorption of halide ions increases in the sequence Cl−<Br−<I−, whereas at the Hg/DMSO and Bi/DMSO interfaces, an opposite row of surface activity for these ions is observed.

Nanocrystal Inks: Photoelectrochemical Printing of Cu2O Nanocrystals on Silicon with 2D Control on Polyhedral Shapes

AbstractHere, a printing technology is reported that enables the functionalization of photoconducting materials with an arbitrary user‐defined pattern of nanocrystals. Two sets of information can be encoded into the same unit area; control over the polyhedral shape of individual nanocrystals and high lateral resolution (micrometer scale) for large 2D patterns (millimeter scale) of Cu2O that were grown over unstructured amorphous silicon. The key to this new technology is a parallel modulation of both the electrode kinetics at the silicon/electrolyte interface and the light‐assisted control of adsorption of halide ions on the growing Cu2O particles. This is achieved using local pixelation of a visible light stimulus by adapting to the field of photoelectrochemistry spatial light modulation more often used in super resolution microscopy. Any user‐defined pattern (i.e., any arbitrary bitmap image file or a sequence of images) is converted within seconds into an array of nanoparticles. The process does not rely on the use of physical masks or chemical templating and it is parallel, meaning, for example, that it is possible to guide the exclusive growth of cubic nanoparticles in an area defined at will by the operator, while the neighboring section(s) are being covered exclusively by octahedrons.

Characterization of the Passive Film Formed on Iron in Phosphoric Acid Solution: On the Use of a Double Modulation Technique

The interpretation of the response of an electrochemical system is usually based on the assumption that the current flowing through the electrode is the sum of two independent processes: the faradic current corresponding to the electrochemical processes and the capacitive current corresponding to the response of the interfacial capacitance. However, the separation of faradaic and capacitive currents is questionable in many different cases such as experiments performed in low-concentrated supporting electrolyte or the specific adsorption of diluted reactants. It was also shown that a charge injection to an ideal polarizable electrode results in two relaxations of the double layer corresponding to the charge neutralization on the electrode in the higher frequency domain and the charge rearrangement in the diffuse layer in a lower frequency range[1-3]. Passive layer and thin oxide films formed at a metal surface usually provide a natural protection of the metal resulting in an additional capacitive response in series with the electrical double layer. In the case of the contribution of such superficial phase, 2D- or 3D-film growth, the dielectric-like response of the interface can no longer be neglected. When these phases are kinetically involved in charge transfers, the decoupling can become particularly arbitrary. The modulation of the interfacial capacitance transfer function (MICTF) which was devised by Antaño-Lopez et al. for studying the influence of the ac and dc current on the interface capacitance [4] provides a unique way to overcome this difficulty. The MICTF provides the capacitive response of the interface at a frequency Ω when it is simultaneously scanned at a second frequency ω with the impedance spectrum. That is performed by a proper signal processing of the system response to a two- frequency, ω and Ω, composite measuring signal. It was used to study the double layer relaxation and specific adsorption of halide ions on mercury electrode [5] or to gather information on electron dynamics in TiO2 dye-sensitized solar-cells [6]. In this work, we report on the response of the oxide film formed on iron in phosphoric acid solution using this double modulation technique. A specific attention will be paid on the capacitive response of the electrode as a function of the potential so as to be able to provide a detailed analysis of both the EIS and the capacity responses with the MICTF technique compared with other data interpretation such as the Mott-Schottky theory. [1] F.C. Anson, R.F. Martin, C. Yarnitzky, Creation of nonequilibrium diffuse double layers and studies of their relaxation, J. Phys. Chem., 73 (1969) 1835-1842. [2] C. Yarnitzky, F.C. Anson, Mechanism of charging and discharging ionic double layers at electrodes, J. Phys. Chem., 74 (1970) 3123-3130. [3] S.W. Feldberg, Theory of relaxation of the diffuse double layer following coulostatic charge injection, J. Phys. Chem., 74 (1970) 87-90. [4] R. Antaño-Lopez, M. Keddam, H. Takenouti, A new experimental approach to the time-constants of electrochemical impedance: frequency response of the double layer capacitance, Electrochim. Acta, 46 (2001) 3611-3617. [5] E.R. Larios-Duran, R. Antaño-Lopez, M. Keddam, Y. Meas, H. Takenouti, V. Vivier, Dynamics of double-layer by AC Modulation of the Interfacial Capacitance and Associated Transfer Functions, Electrochim. Acta, 55 (2010) 6292-6298. [6] H. Cachet, M. Keddam, H. Takenouti, R. Antaño-Lopez, T. Stergiopoulos, P. Falaras, Capacitance probe of the electron displacement in a dye sensitized solar cell by an intermodulation technique: a quantitative model, Electrochim. Acta, 49 (2004) 2541-2549.

Specific Adsorption of Halide Ions on Iron Surface: A Combined Electrochemical and Monte Carlo Simulation Investigation

Specific Adsorption of Halide Ions on Iron Surface: A Combined Electrochemical and Monte Carlo Simulation Investigation

Recovery of Active Surface Sites of Shape-Controlled Platinum Nanoparticles Contaminated with Halide Ions and Its Effect on Surface-Structure

The interaction of halide ions (I−, Br−, Cl−) with well-cleaned faceted platinum (nanocube, cuboctahedral) nanoparticles and platinum polycrystalline is investigated in 0.5 M H2SO4 electrolyte. Under electrochemical conditions, the Pt surface gets poisoned with halide ad-atoms and it causes the attenuation of both hydrogen adsorption/desorption in the lower potential region (0.06–0.4 V) and electroxidation of Pt nanoparticles in the higher potential region (0.6–1.2 V). Above certain concentration (5 × 10−6 M), the strongly adsorbing I− ions mask the Hupd features. On the other hand, Br− and Cl− ions alter the peak features in the Hupd region, those are characteristic of different Pt surface sites. On excursion to higher potentials (∼>1.2 V), concurrent halogen evolution, Pt oxidation, and oxygen evolution are observed; the increase in peak intensity in the Hupd region reflects the reconstruction of the Pt surface. To remove the adsorbed halide ions from the Pt surface, an in-situ potentiostatic method is employed, which involves holding the working electrode at ∼0.03 V in 0.1 M NaOH solution. The cleanliness and retention of surface-structure are confirmed from the voltammograms recorded in the test electrolyte and the recovery of oxygen reduction reaction (ORR) activity after cleaning the Br− ion-contaminated Pt surface supports this conjecture.

Modulating the phases of iron carbide nanoparticles: from a perspective of interfering with the carbon penetration of Fe@Fe3O4 by selectively adsorbed halide ions.

Iron carbide nanoparticles (ICNPs) are considered to have great potential in new energy conversion, nanomagnets and biomedical applications due to their intrinsically peculiar magnetic and catalytic properties. However, the synthetic routes were greatly limited in morphology and phase controlled synthesis. In this article, we present a versatile solution chemistry route towards colloidal ICNPs (Fe2C-hexagonal and monoclinic syngony, Fe5C2-monoclinic syngony and Fe3C-orthorhombic syngony) derived from body centered cubic Fe@Fe3O4 by introducing heteroatoms to restrain their phase transformation. We found that the phases of Fe2C NPs could be controlled by direct phase transformation in the drastic thermally driven procedure (defined as thermodynamical manner). Meanwhile, the selective adsorption of Cl ions weakened the bonding between Fe and C atoms, thus interfering with the penetration of C atoms to form lower carbon content Fe5C2 and Fe3C NPs. The kinetic mechanisms were evaluated using density functional theory (DFT) simulations focusing on the bonding energy between Fe-C and Fe-Cl atoms. All the obtained ICNPs exhibited typically soft ferromagnetic properties with the highest saturation magnetization value of 101.2 emu g-1 and the highest Curie temperature of 497.8 K.

The Activity of Halide Ions and the Effect on the Electrochemical Stability of Bi(111) Electrode in a Mixture of Ionic Liquids

Ionic liquids are versatile solvents and electrolytes for many electrochemical applications such as secondary batteries, capacitors, solar cells and electrochemical deposition of metal coatings[1–3]. In order to rationally design and further develop such systems, however, a detailed understanding of the processes that govern the interfacial processes and stability of ionic liquid interfaces is required. Although a large number of experimental and theoretical studies of ionic liquid interfaces exist, the research regarding the electrochemistry of ionic liquid mixtures has been limited. It is thus the focus of the presentation to provide information about the concentration dependent activity of halide ions in a mixture of ionic liquids and to explore the effect of ion activity on the nanoscale stability of the Bi(111) electrode surface. Potentiometry, cyclic voltammetry, electrochemical impedance spectroscopy and in situ scanning tunneling microscopy (STM) have been applied in the present study in order to explore the concentration dependent activity of halide ions (iodide, bromide, chloride) and to demonstrate how the activity of halide ions affects the electrochemical stability and 2D layer formation at a Bi(111) electrode surface in a mixture of ionic liquids. It is observed that, different from aqueous and organic electrolyte solutions, the ionic liquid media promote ion aggregate formation and complex cations exist in many different valence states, dependent upon the concentration of complex forming ions. The effects of the ion activity are evident from in situ STM measurements, as the surface of a Bi(111) electrode is stable in both ionic liquids that contain no halide additions as well as those with a saturated amount of halide ionic liquids, but is unstable in mixtures containing a small or medium amount of halide ionic liquid additions. Electrochemical measurements also suggest that different processes and mechanisms are required in order to describe the concentration dependence of the specific adsorption of halide ions at a Bi(111) electrode[4]. In situ STM measurements demonstrate that the formation of a tightly packed 2D layer of specifically adsorbed ions is only formed in concentrated mixtures of halide ionic liquids, which results in the dissolution and re-deposition of bismuth ions to preferentially occur are terrace edge sites, thus maintaining the monocrystalline structure of the Bi(111) electrode surface. Acknowledgements This work was supported by the Estonian Ministry of Education and Research (projects no. IUT 20-13, PUT55 and PUT1107), European Development Fund (project 3.2.1101.12-0019), and Estonian Centres of Excellence (projects no. TK117T and TK141).

Strong Correlations Between Structural Order and Passive State at Water-Copper Oxide Interfaces

Fundamental understanding of coupled electrochemical processes at metal-oxide/aqueous media interfaces, e.g., interfacial structure evolution, metal dissolution, and solvation dynamics at the atomic level are of immense interest to corrosion research, surface chemistry, and electrochemistry in general. Using reactive force field (ReaxFF) molecular dynamics (MD) simulations of CuOx passive thin film on Cu substrate, we investigated the impact of non-stoichiometry of passive oxide film on its breakdown, and aqueous corrosion (in sea water) of the underlying copper. Upon exposure to aqueous media (halide concentration similar to seawater), we find that halide ions adsorb on to the passive oxide layer, and cause dissolution of Cu atoms via formation of water-soluble Cu-halide complexes; eventually, this results in the breakdown of the passive oxide layer, and consequently leads to corrosion of underlying Cu. Our MD simulations indicate that oxygen rich CuOx films show enhanced resistance to breakdown via impeding halide ion adsorption as well as metal dissolution; for instance, increasing oxygen stoichiometry from 0.2 to 0.79 causes by halide adsorption to drop by half. Thus, O-rich Cu-oxide films provide better corrosion resistance than O-deficient ones. Interestingly, we find that high O-content in the oxide film promotes ordering of water molecules at the water/oxide interface leading to 2D “ice-like” layers. These ordered water layers inhibit the adsorption of halide ions via a phenomenon similar to brine rejection. These findings clearly indicate the oxygen stoichiometry, dissolution kinetics, and solvation dynamics at the water/oxide interface are strongly correlated to each other [Narayanan et al. Electrochimica Acta 179, 386 (2015)]. To validate the predictions of theory, and to probe the initiation of pits during aqueous corrosion of Cu, we employ coherent x-ray diffraction imaging. This imaging technique coupled with large scale molecular dynamics simulations identify localized strain fields that arise in the Cu nanoparticles; this in turn, provides atomic scale insights into pit formation during aqueous corrosion.

Effect of Anionic Electrolytes and Precursor Concentrations on the Electrodeposited Pt Structures

AbstractElectrodeposition of functional metal surfaces has received great attention because of their useful applications. Recently, interesting electrodeposition behavior of Pt at −0.8 V (vs. Ag/AgCl) was reported, where underpotential deposited H (Hupd) layers played a unique role in the electrodeposition. Here, we report the effect of anionic electrolytes and precursor concentrations on the electrochemical deposition behavior of Pt. Depending on these two experimental parameters, two distinct Pt structures, monolayer Pt films and Pt spheres, were electrodeposited at −0.8 V. In addition to the underpotential deposited H (Hupd) layers formed at −0.8 V, the adsorption of Cl− also plays a significant role in determining the electrodeposited Pt structures. When the PtCl42− concentration was low and the Cl− concentration was high enough for the adsorption of PtCl42− to be blocked by the Hupd and Cl− layers, monolayer Pt films were electrodeposited. Otherwise, further electrodeposition of Pt spheres over the monolayer Pt films occurred. The effect of other halide ion adsorption and the controlled growth of Pt spheres during the Pt electrodeposition were also investigated. The electrochemical deposition behavior of Pt demonstrated in this work provides insight into the fabrication of functional Pt surfaces.

Insights from the Adsorption of Halide Ions on Graphene Materials.

Graphene has recently found applications in a wide range of fields. Density functional calculations show that halide ions can be adsorbed on pristine graphene, but only F(-) has an appreciable binding energy (-97.0 kJ mol(-1) ). Graphene materials, which are mainly electron donors, can be made strong electron acceptors by edge functionalization with F atoms. The binding strengths of halide ions are greatly enhanced by edge functionalization and show direct proportionality with the degree of functionalization Θ and increased charge transfer. In contrast, the adsorption strengths of metal ions on pristine graphene are clearly superior to those of halide ions but decline substantially with increasing degree of edge functionalization, and for Θ=100 %, the binding energies of -95.7, -44.8, and -25.9 kJ mol(-1) that are calculated for Li(+) , Na(+) , and K(+) , respectively, are obviously inferior to that of F(-) (-186.3 kJ mol(-1) ). Thus, the electronic properties of graphene are fundamentally regulated by edge functionalization, and the preferential adsorption of certain metal ions or anions can be facilely realized by choice of an appropriate degree of functionalization. Adsorbed metal ions and anions behave differently on gradual addition of water molecules, and their binding strengths remain substantial when graphene materials are in the pristine and highly edge functionalized states, respectively.

Study on the Behavior of Halide Ions Contained in Ionic Liquids on Au(111) Electrode Surface

In recent times, ionic liquids (IL) have been studied as alternatives to water and organic solvents because of their unique features, in particular, tunability and designability. The electrochemical window, viscosity, conductivity, and hydrophobicity of ILs can be tuned by altering their molecular structures. One of the applications of ILs is as an electrolyte solution for electrochemical devices. In the last decade, many researchers have investigated the electrochemical characteristics of ILs and the behavior of IL/electrode interfaces. On the other hand, the effect of impurity contained in ILs on electrode surface structure has not been well uncovered. Typical impurities are halide ions, alkaline metal ions, and water. It has been well known that the halide ions are adsorbed on metal electrode surface and affect electrochemical processes. Especially, chemically adsorbed halide ions are called “specific adsorption anions”. To understand the effect and the behavior of halide ions exist at electrode/ILs interfaces is important from both scientific and industrial point of views. In this study, the behavior of halide ions on the Au(111) electrode surface in two ionic liquids (IL), 1-buthyl-1-methylpyrrolidiniun bis(trifluoromethylsulfonyl)amide ([BMP]TFSA) and 1-buthyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([BMIM]TFSA), was investigated by monitoring the electrode surface structure. Potential dependences of the X-ray diffraction intensity, which originate from the Au(111)-(1 × 1) structure and the surface normal structure, were simultaneously measured with cyclic voltammograms. We considered the dependence of ion concentration and ion species. The electrolytes were prepared using [BMP]Cl, [BMP]Br, [BMP]I, [BMIM]Br, [BMP]TFSA, and [BMIM]TFSA. The electrolytes were dehydrated at 80 °C under vacuum for 24 h before use. A “drop cell” was used as the electrochemical cell. A working electrode was a Au(111) single crystal disk (Φ ≈ 5 mm). Surface X-ray scattering (SXS) experiments were conducted at BL14B1, SPring-8 using a κ-type diffractometer. A 30 keV X-ray was used to obtain a sufficient attenuation length in the drop cell arrangement. In [BMP]TFSA, cyclic voltammograms (CV) showed both halide ion species and their concentration dependence. In the ion species dependence, current values increased in the order of Cl-, Br-, and I-. The cathodic current peak position also shifted to negative potential in the same order. Further, the kinetics of surface reconstruction became faster as the concentration increased. On the other hand, the potential dependence of the surface (1 × 1) structure showed no common behavior. In [BMIM]TFSA, small dependence was observed in both CVs and surface structure. The results revealed that halide ions are co-adsorbed with ionic liquid molecules on electrode surfaces and they increase the mobility of surface atoms. In contrast, the Au(111) surface does not reconstruct to the (p × √3) structure even in the IL containing 200 mM Br−, suggesting that the interaction between halide ions and surface Au atoms is weaker than that between IL molecules and surface Au atoms, that is, the surface property is mainly governed by adsorbed IL molecules. Furthermore, compared with results measured in the two ILs, it was revealed that the effect of halide ions on the Au(111) electrode surface structure depends on the strength of the interaction between IL molecules and surface Au atoms.