Surface layering at the mercury-electrolyte interface

In Situ and Operando X-ray Scattering Methods in Electrochemistry and Electrocatalysis

Electrochemical and electrocatalytic processes are of key importance for the transition to a sustainable energy supply as well as for a wide variety of other technologically relevant fields. Further development of these processes requires in-depth understanding of the atomic, nano, and micro scale structure of the materials and interfaces in electrochemical devices under reaction conditions. We here provide a comprehensive review of in situ and operando studies by X-ray scattering methods, which are powerful and highly versatile tools to provide such understanding. We discuss the application of X-ray scattering to a wide variety of electrochemical systems, ranging from metal and oxide single crystals to nanoparticles and even full devices. We show how structural data on bulk phases, electrode-electrolyte interfaces, and nanoscale morphology can be obtained and describe recent developments that provide highly local information and insight into the composition and electronic structure. These X-ray scattering studies yield insights into the structure in the double layer potential range as well as into the structural evolution during electrocatalytic processes and phase formation reactions, such as nucleation and growth during electrodeposition and dissolution, the formation of passive films, corrosion processes, and the electrochemical intercalation into battery materials.

Electrosynthesis of Quasi-Epitaxial Crystals on Liquid Metals

Electrosynthesis of single-crystalline metallic and intermetallic particles with a preferred orientation onto liquid metal electrodes has been performed. Liquid gallium electrodes immersed in aqueous alkaline electrolytes without any molecular additive or external solid seeding substrates were used to electroreduce separately Pb2+, Bi3+, Pd2+, and Mn2+. The crystallinity, composition, and orientation of the electrodeposition products were characterized by using scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, grazing incidence X-ray diffraction, and energy-dispersive X-ray spectroscopy. Electrodeposition of Pb and Bi results in the incipient formation of two-dimensional (2D) nuclei that subsequently direct the growth of Pb and Bi single crystals along the most close-packed [111] and [0001] directions, respectively. The absence of any intervening surface oxides and a low electroreduction flux are necessary to avoid polycrystalline dendrite formation. Under comparable conditions, the electrodeposition of Pd and Mn results in single-crystalline intermetallic particles at the interface. Each crystal exhibits a preferred orientation consistent with the unique atomic packing of the near-surface region of the liquid Ga. The presented study suggests a new concept in electrodeposition processes where the liquid metal structure imparts quasi-epitaxial growth in a system in which the electrode material specifically has no crystallinity or long-range order. This study is thus the first demonstration of highly oriented electrodeposition at a liquid/liquid interface under ambient conditions, highlighting the unique solvation environment of liquid metal interfaces for forming thin metallic and intermetallic films.

Role of chemisorbing species in growth at liquid metal-electrolyte interfaces revealed by in situ X-ray scattering

Liquid-liquid interfaces offer intriguing possibilities for nanomaterials growth. Here, fundamental interface-related mechanisms that control the growth behavior in these systems are studied for Pb halide formation at the interface between NaX + PbX₂ (X = F, Cl, Br) and liquid Hg electrodes using in situ X-ray scattering and complementary electrochemical and microscopy measurements. These studies reveal a decisive role of the halide species in nucleation and growth of these compounds. In Cl- and Br-containing solution, deposition starts by rapid formation of well-defined ultrathin (∼7 Å) precursor adlayers, which provide a structural template for the subsequent quasi-epitaxial growth of c-axis oriented Pb(OH)X bulk crystals. In contrast, growth in F-containing solution proceeds by slow formation of a more disordered deposit, resulting in random bulk crystal orientations on the Hg surface. These differences can be assigned to the interface chemistry, specifically halide chemisorption, which steers the formation of these highly textured deposits at the liquid-liquid interface.

Growth at liquid-liquid interfaces differ inherently from that on solids, making it attractive for nanomaterial formation. Here, the authors use X-ray scattering to derive a detailed microscopic picture of lead-halide growth on liquid mercury that reveals the key importance of anion adsorption.

In Situ X-Ray Techniques for Electrochemical Interfaces

This article reviews progress in the study of materials using X-ray-based techniques from an electrochemistry perspective. We focus on in situ/in operando surface X-ray scattering, X-ray absorption spectroscopy, and the combination of both methods. The background of these techniques together with key concepts is introduced. Key examples of in situ and in operando investigation of liquid-solid and liquid-liquid interfaces are presented. X-ray scattering and spectroscopy have helped to develop an understanding of the underlying atomic and molecular processes associated with electrocatalysis, electrodeposition, and battery materials. We highlight recent developments, including resonant surface diffraction and time-resolved studies.

Nucleation and Growth of PbBrF Crystals at the Liquid Mercury-Electrolyte Interface Studied by Operando X-ray Scattering

Detailed in operando studies of electrochemically induced PbBrF deposition at the liquid mercury/liquid electrolyte interface are presented. The nucleation and growth were monitored using time-resolved X-ray diffraction and reflectivity combined with electrochemical measurements, revealing a complex potential-dependent behavior. PbBrF deposition commences at potentials above -0.7 V with the rapid formation of an ultrathin adlayer of one unit cell thickness, on top of which (001)-oriented three-dimensional crystallites are formed. Two potential regimes are identified. At low overpotentials, slow growth of a low surface density film of large crystals is observed. At high overpotentials, crossover to a potential-independent morphology occurs, consisting of a compact PbBrF deposit with a saturation thickness of 25 nm, which forms within a few minutes. This potential behavior can be rationalized by the increasing supersaturation near the interface, caused by the potential-dependent Pb²⁺ deamalgamation, which changes from a slow reaction-controlled process to a fast transport-controlled process in this range of overpotentials. In addition, growth on the liquid substrate is found to involve complex micromechanical effects, such as crystal reorientation and film breakup during dissolution.

Salt induced reduction of lysozyme adsorption at charged interfaces

A study of lysozyme adsorption below a behenic acid membrane and at the solid-liquid interface between aqueous lysozyme solution and a silicon wafer in the presence of sodium chloride is presented. The salt concentration was varied between 1 mmol L(-1) and 1000 mmol L(-1). X-ray reflectivity data show a clear dependence of the protein adsorption on the salt concentration. Increasing salt concentrations result in a decreased protein adsorption at the interface until a complete suppression at high concentrations is reached. This effect can be attributed to a reduced attractive electrostatic interaction between the positively charged proteins and negatively charged surfaces by charge screening. The measurements at the solid-liquid interfaces show a transition from unoriented order of lysozyme in the adsorbed film to an oriented order with the short protein axis perpendicular to the solid-liquid interface with rising salt concentration.

A novel X-ray diffractometer for studies of liquid-liquid interfaces

The study of liquid-liquid interfaces with X-ray scattering methods requires special instrumental considerations. A dedicated liquid surface diffractometer employing a tilting double-crystal monochromator in Bragg geometry has been designed. This diffractometer allows reflectivity and grazing-incidence scattering measurements of an immobile mechanically completely decoupled liquid sample, providing high mechanical stability. The available energy range is from 6.4 to 29.4 keV, covering many important absorption edges. The instrument provides access in momentum space out to 2.54 Å(-1) in the surface normal and out to 14.8 Å(-1) in the in-plane direction at 29.4 keV. Owing to its modular design the diffractometer is also suitable for heavy apparatus such as vacuum chambers. The instrument performance is described and examples of X-ray reflectivity studies performed under in situ electrochemical control and on biochemical model systems are given.

In situ X-ray studies of adlayer-induced crystal nucleation at the liquid-liquid interface

Crystal nucleation and growth at a liquid-liquid interface is studied on the atomic scale by in situ Å-resolution X-ray scattering methods for the case of liquid Hg and an electrochemical dilute electrolyte containing Pb(2+), F(-), and Br(-) ions. In the regime negative of the Pb amalgamation potential Φ(rp) = -0.70 V, no change is observed from the surface-layered structure of pure Hg. Upon potential-induced release of Pb(2+) from the Hg bulk at Φ > Φ(rp), the formation of an intriguing interface structure is observed, comprising a well-defined 7.6-Å-thick adlayer, decorated with structurally related 3D crystallites. Both are identified by their diffraction peaks as PbFBr, preferentially aligned with their axis along the interface normal. X-ray reflectivity shows the adlayer to consist of a stack of five ionic layers, forming a single-unit-cell-thick crystalline PbFBr precursor film, which acts as a template for the subsequent quasiepitaxial 3D crystal growth. This growth behavior is assigned to the combined action of electrostatic and short-range chemical interactions.

Effect of headgroup on the physicochemical properties of phospholipid bilayers in electric fields: size matters

The effect of molecular structure on ensemble structure of phospholipid films has been investigated. Bilayers of dimyristoyl phosphatidylethanolamine (DMPE) were prepared on Au(111) electrodes using Langmuir-Blodgett and Langmuir-Schaeffer deposition. Capacitance and charge density measurements were used to investigate the adsorption behavior and barrier properties of the lipid bilayers. In situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was employed to investigate the organization of the molecules within the bilayer. DMPE bilayers exhibit lower capacitance than bilayers formed from the related lipid, dimyristoyl phosphatidylcholine (DMPC). The infrared data show that these results can be explained by structural differences between the bilayers formed from each molecule. DMPE organizes into bilayers with hydrocarbon chains tilted at a smaller angle to the surface normal, which results in a thicker film. The hydrocarbon chains contain few conformational defects. Spectra in the carbonyl and phosphate stretching mode regions indicate low solvent content of DMPE films. Both of these effects combine to produce films with lower capacitance and enhanced barrier properties. The results are explained in terms of the differences in structure between the constituent molecules.

Two-dimensional order in mercury-supported langmuir films of fatty diacids

The structure of mercury-supported Langmuir films of dicarboxylic acid molecules with 13 ≤ n ≤ 22 carbons is studied by X-ray methods and surface tensiometry. The molecules lie surface-parallel, forming mono-, bi-, or trilayers, depending on coverage. All films exhibit a full 2D order of the same single-molecule oblique unit cell. In particular, the distinct odd-even structure difference of 3D crystals of the same molecules is not observed. The unit cell's width and angle show a small systematic decrease with n, while the length increases commensurately with the molecular length. These results show the films to consist of closely packed, extended, polymer-like chains of diacid molecules, bound by their carboxyl end groups. Evidence is presented for the inclusion of a single mercury atom in the carboxyl-carboxyl bond. The possible conformation of this bond and implications of the parity-independent structure are discussed.

X-ray reflectivity at polarized liquid-Hg-aqueous-electrolyte interface: challenging macroscopic approaches for ion-specificity issues

We report Angstrom-resolved x-ray reflectivity analysis of externally polarized liquid-Hg surface in contact with molar LiCl, LiBr, and MgSO4 aqueous electrolytes. Interpretation of reflectivity curves demonstrates a dependence of Hg-surface layering on both applied potential and ion nature. It further highlights how interfacial polarization degree impacts electron density profiles at a molecular scale. These profiles indicate accumulation of anions and cations at the Hg surface. Upon decrease of the potential from the point of zero charge, anions are gradually expelled from the Hg surface. The study challenges traditional thermodynamic approaches for deriving countercharge composition at the Hg-electrolyte-solution interface from macroscopic Hg-surface tension data. It further dismisses the long-standing approximation that assimilates the Hg surface to a smooth, perfect chemically inert conductor with a uniformly smeared-out surface charge density.

Checkerboard self-patterning of an ionic liquid film on mercury

Å-resolution studies of room temperature ionic liquid (RTIL) interfaces are scarce, in spite of their long-recognized importance for the science and many applications of RTILs. We present an Å-resolution x-ray study of a Langmuir film of an RTIL on mercury. At low (high) coverage [90 (50) Å2/molecule] a mono-(bi)layer of surface-parallel molecules is found. The molecules self-assemble in a lateral ionic checkerboard pattern, unlike the uniform-charge, alternate-ion layers of this RTIL at its bulk-solid interface. A 2D-smectic order is found, with molecules packed in parallel stripes, forming long-range order normal to, but none along, the stripes.