Electrified Surfaces Research Articles

Transient charge-driven 3D conformal printing via pulsed-plasma impingement

Seamless integration of microstructures and circuits on three-dimensional (3D) complex surfaces is of significance and is catalyzing the emergence of many innovative 3D curvy electronic devices. However, patterning fine features on arbitrary 3D targets remains challenging. Here, we propose a facile charge-driven electrohydrodynamic 3D microprinting technique that allows micron- and even submicron-scale patterning of functional inks on a couple of 3D-shaped dielectrics via an atmospheric-pressure cold plasma jet. Relying on the transient charging of exposed sites arising from the weakly ionized gas jet, the specified charge is programmably deposited onto the surface as a virtual electrode with spatial and time spans of ~mm in diameter and ~μs in duration to generate a localized electric field accordantly. Therefore, inks with a wide range of viscosities can be directly drawn out from micro-orifices and deposited on both two-dimensional (2D) planar and 3D curved surfaces with a curvature radius down to ~1 mm and even on the inner wall of narrow cavities via localized electrostatic attraction, exhibiting a printing resolution of ~450 nm. In addition, several conformal electronic devices were successfully printed on 3D dielectric objects. Self-aligned 3D microprinting, with stacking layers up to 1400, is also achieved due to the electrified surfaces. This microplasma-induced printing technique exhibits great advantages such as ultrahigh resolution, excellent compatibility of inks and substrates, antigravity droplet dispersion, and omnidirectional printing on 3D freeform surfaces. It could provide a promising solution for intimately fabricating electronic devices on arbitrary 3D surfaces.

Electrified Interactions of Polyzwitterions with Charged Surfaces: Role of Dipole Orientation and Surface Potentials.

The zwitterionic groups possess strong dipole moments, leading to inter- or intrachain interactions among zwitterionic polymers. This study aims to demonstrate the interaction of polyzwitterions poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), and poly(carboxybetaine methacrylate) (PCBMA) with electrified surfaces, despite their electrically neutral nature. We studied the adsorption of polyzwitterions and their monomers on electrified surfaces by using an electrochemical quartz crystal microbalance with dissipation (EQCM-D). The interaction between zwitterionic molecules and charged surfaces is explored by adjusting the surface potentials. Interestingly, the adsorption of polyzwitterions can be influenced by external potential, primarily due to the formation of polyzwitterions restricting the mobility of zwitterionic groups, affecting the adsorption behavior of polyzwitterions based on the surface potential. The impact is determined by the arrangement of positive and negative ions within the zwitterionic groups, which are the dipole orientation. Additionally, surface potentials determine the adsorption rate, amount, and chain conformation of the adsorbed thin polyzwitterion layers. The effect of ionic strength was investigated by introducing electrolytes into the aqueous solutions to assess the range of influenced surface potentials.

Enhanced Heat Flow between Charged Nanoparticles and an Aqueous Electrolyte.

Heat transfer through the interface between a metallic nanoparticle and an electrolyte solution has great importance in a number of applications, ranging from nanoparticle-based cancer treatments to nanofluids and solar energy conversion devices. However, the impact of the surface charge and dissolved ions on heat transfer has been scarcely explored so far. In this study, we compute the interface thermal conductance between hydrophilic and hydrophobic charged gold nanoparticles immersed in an electrolyte using equilibrium molecular dynamics simulations. Compared with an uncharged nanoparticle, we report a 3-fold increase of the Kapitza conductance for a nanoparticle surface charge of +320 mC/m2. This enhancement is shown to be approximately independent of the surface wettability, charge spatial distribution, and salt concentration. This allows us to express the Kapitza conductance enhancement in terms of the surface charge density on a master curve. Finally, we interpret the increase of the Kapitza conductance as a combined result of the shift of the water density distribution toward the charged nanoparticle and an accumulation of the counterions around the nanoparticle surface which increase the Coulombic interaction between the liquid and the charged nanoparticle. These considerations help us to apprehend the role of ions in heat transfer close to electrified surfaces.

Theory of electrolyte solutions in a slit charged pore: Effects of structural interactions and specific adsorption of ions.

In this paper, we present a continuation of our research on modeling electrolyte solutions within charged pores. We make use of the model developed by Blossey et al. [Phys. Rev. E 95, 060602 (2017)], which takes into account the structural interactions between ions through a bilinear form over the gradients of local ionic concentrations in the grand thermodynamic potential, as well as their steric interactions through the lattice gas model. The structural interactions may describe the effects of the molecular structure of ions at a phenomenological level. For example, these effects include steric effects due to non-spherical shapes of ions, their conformational lability, and solvent effects. In addition, we explore their specific interactions with the pore walls by incorporating external attractive potentials. Our primary focus is on observing the behavior of ionic concentration profiles and the disjoining pressure as the pore width changes. By starting with the local mechanical equilibrium condition, we derive a general expression for the disjoining pressure. Our findings indicate that considering the structural interactions of ions leads to a pronounced minimum on the disjoining pressure profiles at small pore widths. We attribute this minimum to the formation of electric double layers on the electrified surfaces of the pore. In addition, our results demonstrate that the inclusion of the attractive interactions of ions with the pore walls enhances this minimum and shifts it to smaller pore thicknesses. Our theoretical discoveries may be useful for those involved in supercapacitor electrochemical engineering, particularly when working with porous electrodes that have been infused with concentrated electrolyte solutions.

Optical response of electro-tuneable 3D superstructures of plasmonic nanoparticles self-assembling on transparent columnar electrodes.

Electrically tuneable, guided self-assembly of plasmonic nanoparticles (NPs) at polarized, patterned solid-liquid interfaces could enable numerous platforms for designing nanoplasmonic optical devices with new tuneable functionalities. Here, we propose a unique design of voltage-controlled guided 3D self-assembly of plasmonic NPs on transparent electrodes, patterned as columnar structures-arrays of vertical nanorods. NP assembly on the electrified surfaces of those columnar structures allows formation of a 3D superstructure of NPs, comprising stacking up of NPs in the voids between the columns, forming multiple NP-layers. A comprehensive theoretical model, based on quasi-static effective medium theory and multilayer Fresnel reflection scheme, is developed and verified against full-wave simulations for obtaining optical responses-reflectance, transmittance, and absorbance-from such systems of 3D self-assembled NPs. With a specific example of small gold nanospheres self-assembling on polarized zinc oxide columns, we show that the reflectance spectrum can be controlled by the number of stacked NP-layers. Numerical simulations show that peak reflectance can be enhanced up to ∼1.7 times, along with spectral broadening by a factor of ∼2-allowing wide-range tuning of optical reflectivity. Smaller NPs with superior mobility would be preferable over large NPs for realizing such devices for novel photonic and sensing applications.

Self-assembled nanostructures in ionic liquids facilitate charge storage at electrified interfaces

Driven by the potential applications of ionic liquids (ILs) in many emerging electrochemical technologies, recent research efforts have been directed at understanding the complex ion ordering in these systems, to uncover novel energy storage mechanisms at IL-electrode interfaces. Here, we discover that surface-active ILs (SAILs), which contain amphiphilic structures inducing self-assembly, exhibit enhanced charge storage performance at electrified surfaces. Unlike conventional non-amphiphilic ILs, for which ion distribution is dominated by Coulombic interactions, SAILs exhibit significant and competing van der Waals interactions owing to the non-polar surfactant tails, leading to unusual interfacial ion distributions. We reveal that, at an intermediate degree of electrode polarization, SAILs display optimum performance, because the low-charge-density alkyl tails are effectively excluded from the electrode surfaces, whereas the formation of non-polar domains along the surface suppresses undesired overscreening effects. This work represents a crucial step towards understanding the unique interfacial behaviour and electrochemical properties of amphiphilic liquid systems showing long-range ordering, and offers insights into the design principles for high-energy-density electrolytes based on spontaneous self-assembly behaviour.

Solvate Ionic Liquids at Electrified Interfaces.

Solvate ionic liquids (SILs) are a promising electrolyte for Li-ion batteries; thus, their behavior at electrified interfaces is crucial for the operation of these batteries. We report molecular dynamics simulation results for a prototypical SIL of lithium triglyme bis(trifluoromethanesulfonyl)imide ([Li(G3)][TFSI]) sandwiched between electrified surfaces. At negatively charged as well as neutral electrodes, the electrolyte largely maintains the characteristics of SILs in terms of the interfacial Li+ ions' coordination by a similar number of oxygen atoms on G3 ligands as the bulk Li+ ions. The persistence of the complex ions is attributed to the 1:1 Li-G3 ratio in bulk SILs and the fact that G3 molecules readily adapt to the interfacial environment by aligning themselves with the surface to ensure good solvation of the interfacial Li+ ions. Nevertheless, the interfacial Li+ ions also display changes of solvation from that in bulk SIL by deviating from the molecular plane formed by the oxygen atoms on G3 ligands as electrodes become more negatively charged. Using density functional theory along with natural bond orbital calculations, we examine the effects of such structural distortion on the properties of the complex cation. Both the frontier orbital energies of the complex cation and the donor-acceptor interactions between Li+ ions and G3 ligands are found to be dependent on the deviation of Li+ ions from the molecular plane of the G3 ligands, which suggests that the electrochemical reduction of Li+ ions should be facilitated by the structural distortion. These results bear important implications for the nanostructures and properties of SILs near electrified interfaces during actual operations of Li-ion batteries and serve to provide guidance toward the rational design of new SIL electrolytes.

Synergistic Effects of Ions and Surface Potentials on Antifouling Poly(3,4-ethylenedioxythiophene): Comparison of Oligo(Ethylene Glycol) and Phosphorylcholine.

For electrified surfaces, ions and applied potentials play major roles in controlling the surface properties. Antifouling materials such as poly(ethylene glycol) and zwitterionic polymers that resist nonspecific protein binding and cell adhesion play a key role in various biomedical applications. In this study, we investigated and compared the antifouling properties of conducting polymers grafted with oligo(ethylene glycol) groups and phosphorylcholine (PC) groups in the presence of different anions and applied potentials. Considerable effort has been made to illustrate the different effects of manipulating the antifouling properties of these two surfaces. We prepared polymer films by applying electropolymerization to two functionalized (3,4-ethylenedioxythiophene) polymers containing triethylene glycol and PC groups, respectively. A quartz crystal microbalance with dissipation (QCM-D) was employed to characterize the negatively charged bovine serum albumin and positively charged lysozyme adsorption as a function of ionic concentration in the presence of various Hofmeister anions. The frequency changes corresponded to the protein or ion adsorption/desorption behavior on the surface. The anions adsorbed on polymer films to effectively enhance the hydration layer of the polymer surface and reduce nonspecific protein binding. We further integrated a potentiostat with the QCM-D to control the protein adsorption/desorption behaviors by applying potentials, and we conducted an electrochemical QCM-D study. Most importantly, with the synergistic effect of ions and surface potential, a nearly fresh polymer surface was regenerated. This study describes principles to maintain and regenerate the antifouling properties of electrified surfaces, which are critical for implanted bioelectronics applications.

Electronic structure calculations on electrolyte–electrode interfaces: Successes and limitations

Summary Density functional theory calculations on electrolyte–electrode interfaces are briefly reviewed in this article. The calculation methods are roughly categorized into two types: calculations on electrically neutral surfaces and electrified surfaces. The former methods have been successfully applied to reproduce observed trends in catalytic activities and to design new materials. The methods, however, cannot evaluate any electrochemical properties originated from surface electrifications. By this reason, several theoretical methods on electrified interfaces have been proposed. The methods can provide properties originated from charge transfers, such as electrosorption valency values, symmetry factors and potential-dependent spectroscopies, within self-consistent manners. Many limitations, however, still exist, and further studies are necessary particularly on effects of dynamical atomic motions on free energies.

Molecular dynamics study of room temperature ionic liquids with water at mica surface

Water in room temperature ionic liquids (RTILs) could impose significant effects on their interfacial properties at a charged surface. Although the interfaces between RTILs and mica surfaces exhibit rich microstructure, the influence of water content on such interfaces is little understood, in particular, considering the fact that RTILs are always associated with water due to their hygroscopicity. In this work, we studied how different types of RTILs and different amounts of water molecules affect the RTIL-mica interfaces, especially the water distribution at mica surfaces, using molecular dynamics (MD) simulation. MD results showed that (1) there is more water and a thicker water layer adsorbed on the mica surface as the water content increases, and correspondingly the average location of K+ ions is farther from mica surface; (2) more water accumulated at the interface with the hydrophobic [Emim][TFSI] than in case of the hydrophilic [Emim][BF4] due to the respective RTIL hydrophobicity and ion size. A similar trend was also observed in the hydrogen bonds formed between water molecules. Moreover, the 2D number density map of adsorbed water revealed that the high-density areas of water seem to be related to K+ ions and silicon/aluminum atoms on mica surface. These results are of great importance to understand the effects of hydrophobicity/hydrophicility of RTIL and water on the interfacial microstructure at electrified surfaces.

Fluctuating hydrodynamics for ionic liquids

We present a mean-field fluctuating hydrodynamics (FHD) method for studying the structural and transport properties of ionic liquids in bulk and near electrified surfaces. The free energy of the system consists of two competing terms: (1) a Landau–Lifshitz functional that models the spontaneous separation of the ionic groups, and (2) the standard mean-field electrostatic interaction between the ions in the liquid. The numerical approach used to solve the resulting FHD-Poisson equations is very efficient and models thermal fluctuations with remarkable accuracy. Such density fluctuations are sufficiently strong to excite the experimentally observed spontaneous formation of liquid nano-domains. Statistical analysis of our simulations provides quantitative information about the properties of ionic liquids, such as the mixing quality, stability, and the size of the nano-domains. Our model, thus, can be adequately parameterized by directly comparing our prediction with experimental measurements and all-atom simulations. Conclusively, this work can serve as a practical mathematical tool for testing various theories and designing more efficient mixtures of ionic liquids.

Emerging scanning probe approaches to the measurement of ionic reactivity at energy storage materials.

Many modern energy storage technologies operate via the nominally reversible shuttling of alkali ions between an anode and a cathode capable of hosting them. The degradation process that occurs with normal usage is not yet fully understood, but emerging progress in analytical tools may help address this knowledge gap. By interrogating ionic fluxes over electrified surfaces, scanning probe methods may identify features that impact the local cyclability of a material and subsequently help inform rational electrode design for future generations of batteries. Methods developed for identifying ion fluxes for batteries show great promise for broader applications, including biological interfaces, corrosion, and catalysis.

Non-Equilibrium Ionic Liquid-Electrode Interface at Elevated Temperature and Its Influence on Co2+ Reduction Process

Electrodeposition in Ionic Liquid (IL) media is still at its infancy stage. Recently much attention is devoted to exploration of the IL-electrode interface for better understanding of the electrochemical processes at the electrified surfaces. Herein, we provide a new perspective of the temperature-driven IL-substrate interface structuring. We observed an increase in Co2+ reduction kinetics induced by applying a high temperature gradient. In contrast, keeping the cell at a high constant temperature (100°C) leads to slower Co2+ reduction kinetics, it results in a shift of the reduction onset and evolution of the cyclic voltammogram shape with time. We could ascribe these phenomena to the disturbance of the electrochemical double layer (induced by heating) and its further slow reorganization at constant temperature. We showed that such non-equilibrium behavior can be beneficial for the synthesis of metallic thin films from ILs. Electrochemical quartz crystal microgravimetry, impedance spectroscopy, cyclic voltammetry and chronoamperometry were employed within this work.

Nonfaradaic nanoporous electrochemistry for conductometry at high electrolyte concentration.

Nanoporous electrified surfaces create a unique nonfaradaic electrochemical behavior that is sensitively influenced by pore size, morphology, ionic strength, and electric field modulation. Here, we report the contributions of ion concentration and applied ac frequency to the electrode impedance through an electrical double layer overlap and ion transport along the nanopores. Nanoporous Pt with uniform pore size and geometry (L2-ePt) responded more sensitively to conductivity changes in aqueous solutions than Pt black with poor uniformity despite similar real surface areas and enabled the previously difficult quantitative conductometry measurements at high electrolyte concentrations. The nanopores of L2-ePt were more effective in reducing the electrode impedance and exhibited superior linear responses to not only flat Pt but also Pt black, leading to successful conductometric detection in ion chromatography without ion suppressors and at high ionic strengths.

Influence of temperature on the capacitance of ionic liquid electrolytes on charged surfaces

In this work using molecular dynamics simulations we examine the temperature dependence of the differential capacitance of room temperature ionic liquid electrolytes near electrified surfaces. For electrodes with atomically flat surfaces our simulations show very weak temperature dependence of the differential capacitance (DC) with a slight decrease of DC with increasing temperature. For atomically corrugated surfaces where the ion dimensions are comparable to the size of the surface corrugation patterns, the influence of temperature on DC is much more pronounced. At low temperatures the DC dependence on electrode potential shows large variations with well-defined maxima and minima. However, with increasing temperature these features are significantly flattened. Also for these corrugated surfaces an abnormal positive slope of DC vs. temperature is observed in the narrow range of relatively low voltages. Analysis of changes in the electric double layer structure as a function of temperature allowed us to propose a new mechanism explaining the observed trends in capacitance as a function of temperature and surface topography. The obtained simulation results are discussed in light of available experimental data and help to discriminate between contradictory experimentally observed trends in DC temperature dependence reported for ionic liquid based electrolytes in the literature.

Reactivity of Long Chain α , ω-Dibromoalkanes at Palladized Solid Cathodes: Homocoupling vs Oligomerization

Copper–palladium (Cu–Pd) and silver–palladium (Ag–Pd) systems are used as cathode materials for reducing long chain -dibromoalkanes with . Such interfaces globally permit the cleavage of the two C–Br bonds in a single two-electron process. A spectacular potential shift is obtained in comparison with glassy carbon electrodes at which the total reduction corresponds to a four-electron process. Such potential shifts are typically of the order of 1 V. In particular, cathodic reactions are easily achieved at Ag–Pd interfaces. Under these conditions, supporting evidences are given for the formation (according to simultaneous or sequenced processes) of a transient -biradical with different lengths of the alkyl chain . Macroelectrolysis data show that palladized interface reactions give rise to heterogeneous couplings and, markedly, the formation of homodimers. Reductions of -dibromoalkanes in the presence of copper–palladium, palladium, and silver–palladium fine powders efficiently induce the deposit of Cu, Pd, and Ag nanoparticles at chemically inactive electrified surfaces (glassy carbon and graphite).

Reactivity of α,ω-dibromoalkanes at copper and silver cathodes: (I) Behaviour of 1,3-dibromopropane

Evidence is presented that copper, silver, and their alloys with palladium, may display an exceptional reactivity in the contact of α,ω-alkanes [Br–(CH 2) n –Br]. Thus, the use of coppered surfaces as cathode materials shows a spectacular potential shift ( ΔE ≈ 1 V) for the main reduction step when compared to glassy carbon taken as a standard interface. Such cathodic reactions are particularly easy and fast with 1,3-dibromopropane ( DB3). Thus, it is expected that a preliminarily transient [(CH 2) 3Cu (II),2Br −] monomer or polymer, is formed: the indirect reduction by use of 3D electrodes from copper powder in suspension may produce at electrified surfaces (made of glassy carbon, platinum, or nickel) fine deposits of copper nano-particles. Oppositely, silver (essentially by means of Ag–Pd alloy employed as cathode material) reacts with each C–Br bond and the observed cathodic behaviour is uniform with all α,ω-alkanes. A special attention is drawn on the nature of modified surfaces and the enhancement of catalysis kinetics brought by the use of copperized and silverized electrodes.

Multidimensional electrochemical imaging in materials science

In the past 20 years the characterization of electroactive surfaces and electrode reactions by scanning probe techniques has advanced significantly, benefiting from instrumental and methodological developments in the field. Electrochemical and electrical analysis instruments are attractive tools for identifying regions of different electrochemical properties and chemical reactivity and contribute to the advancement of molecular electronics. Besides their function as a surface analytical device, they have proved to be unique tools for local synthesis of polymers, metal depots, clusters, etc. This review will focus primarily on progress made by use of scanning electrochemical microscopy (SECM), conductive AFM (C-AFM), electrochemical scanning tunneling microscopy (EC-STM), and surface potential measurements, for example Kelvin probe force microscopy (KFM), for multidimensional imaging of potential-dependent processes on metals and electrified surfaces modified with polymers and self assembled monolayers.

Supramolecular Hydrophobic−Hydrophilic Nanopatterns at Electrified Interfaces

We have developed hydrophobic-hydrophilic nanopatterns at electrified surfaces via the self-assembly of amphiphilic molecules. For this purpose, we selected 5-hexadecyloxy isophthalic acid: this neutral amphiphile forms hydrogen-bonded rows that are commensurate with the Au(111) surface. The alkyl chains are interdigitated. The molecular organization of these nanopatterns depends strongly on the substrate potential, which reveals the hierarchical nature of the assembly. The new hydrophobic-hydrophilic nanopatterns are of special interest as templates for the formation of nanostructures of higher complexity.

De-electrization of plastic products by increasing surface conductivity

De-electrization of products manufactured from plastics is achieved by increasing conductivity — mainly of the surface. The electric charge is effectively drained from the electrified surfaces by coating the latter with conductive paints and lacquers or by manufacturing products directly from conductive plastics.