Surface Electrochemical Potential Research Articles

Bioactive conductive polymeric nanocomposite coating for titanium implants

Hydroxyapatite (HAp) coatings on titanium (Ti) substrates have attracted much attention owing to the combination of good mechanical properties of Ti and superior biocompactibility. In this study, an electrochemical deposition of hydroxyapatite was carried out by incorporating collagen peptide and polypyrrole. The developed coatings were subjected for in vitro mineralization for bioactivity in 1.5 Kukubo’s simulated body fluid and the performance was compared with that of the HAp coating developed by electrochemical deposition process. XRD, FTIR, SEM-EDS and AFM were used to investigate the crystallinity, chemical composition, microstructural morphology and surface roughness of the nanocomposite coatings. Electrochemical characterization and surface potential change of the coatings were carried out by EIS, CV studies. Enhanced biogrowth and adhesion was facilitated on HAp/Collagen/Polypyrrole nanocomposite coatings on Ti substrate. In summary, a simple electrochemical treatment can be used to fabricate highly adhesive bioactive coatings on implants that can interact effectively in body environment.

Electrokinetic, electrochemical, and electrostatic surface potentials of the pristine water liquid-vapor interface.

Although conceptually simple, the air-water interface displays rich behavior and is subject to intense experimental and theoretical investigations. Different definitions of the electrostatic surface potential as well as different calculation methods, each relevant for distinct experimental scenarios, lead to widely varying potential magnitudes and sometimes even different signs. Based on quantum-chemical density-functional-theory molecular dynamics (DFT-MD) simulations, different surface potentials are evaluated and compared to force-field (FF) MD simulations. As well explained in the literature, the laterally averaged electrostatic surface potential, accessible to electron holography, is dominated by the trace of the water molecular quadrupole moment, and using DFT-MD amounts to +4.35V inside the water phase, very different from results obtained with FF water models which yield negative values of the order of -0.4 to -0.6V. Thus, when predicting potentials within water molecules, as relevant for photoelectron spectroscopy and non-linear interface-specific spectroscopy, DFT simulations should be used. The electrochemical surface potential, relevant for ion transfer reactions and ion surface adsorption, is much smaller, less than 200 mV in magnitude, and depends specifically on the ion radius. Charge transfer between interfacial water molecules leads to a sizable surface potential as well. However, when probing electrokinetics by explicitly applying a lateral electric field in DFT-MD simulations, the electrokinetic ζ-potential turns out to be negligible, in agreement with predictions using continuous hydrodynamic models. Thus, interfacial polarization charges from intermolecular charge transfer do not lead to significant electrokinetic mobility at the pristine vapor-liquid water interface, even assuming these transfer charges are mobile in an external electric field.

(Invited) Local and Macroscopic Probes of Semiconductor/Electrocatalyst Photochemical Interfaces

Charge-carrier-selective interfaces between electrocatalyst particles and semiconductor light absorbers are critical for solar photochemistry but controlling their properties is challenging. In this talk I will show that the nanoelectrode tip of an atomic-force-microscope cantilever can sense the surface electrochemical potential of thin-film and nanoscale electrocatalysts coating semiconductor photoelectrodes during operation. This technique allowed us to unambiguously show that metal (oxy)hydroxide layers act as both hole collectors and oxygen-evolution catalysts on metal-oxide photoanodes such as Fe2O3 and BiVO4. We also discovered the critical role that heterogeneous interfacial barrier heights, and a related nanoscale pinch-off effect, play in building carrier-selective interfaces in semiconductor photoelectrodes for generating fuel from sunlight. As specific example, thin films and nanoparticle of Pt hydrogen-evolution catalysts on p-InP, a high-performance photocathode material, along with macroscopic and nanoscopic electrical and chemical analysis, are used to show how hydrogen alloying, the pinch-off effect for nanoscale contacts, and the formation of a native surface oxides all play different roles in creating charge-carrier-selective junctions. These measurements are compared and contrasted to a new approach to “wirelessly” measure interfacial for different contact materials by analyzing shifts in element-specific shifts in x-ray photoelectron emission energies. The sum of these new insights can be broadly applied to photocathodes, photoanodes, and overall water-splitting systems to control charge-carrier selectivity and improve performance.

(Invited) Nanoscale Electrocatalyst/Semiconductor Interfaces As Charge-Carrier-Selective Contacts in Photocatalytic and Photoelectrochemical Systems

Charge-carrier-selective interfaces between electrocatalyst particles and semiconductor light absorbers are critical for solar photochemistry but controlling their properties is challenging. In this talk I will show that the nanoelectrode tip of an atomic-force-microscope cantilever can sense the surface electrochemical potential of thin-film and nanoscale electrocatalysts coating semiconductor photoelectrodes during operation. This technique allowed us to unambiguously show that metal (oxy)hydroxide layers act as both hole collectors and oxygen-evolution catalysts on metal-oxide photoanodes such as Fe2O3 and BiVO4. We also discovered the critical role that heterogeneous interfacial barrier heights, and a related nanoscale pinch-off effect, play in building carrier-selective interfaces in semiconductor photoelectrodes for generating fuel from sunlight. As specific example, thin films and nanoparticle of Pt hydrogen-evolution catalysts on p-InP, a high-performance photocathode material, along with macroscopic and nanoscopic electrical and chemical analysis, are used to show how hydrogen alloying, the pinch-off effect for nanoscale contacts, and the formation of a native surface oxides all play different roles in creating charge-carrier-selective junctions. The sum of these new insights can be broadly applied to photocathodes, photoanodes, and overall water-splitting systems to control charge-carrier selectivity and improve performance.

Electrochemical Bacterial Enrichment from Natural Seawater and Its Implications in Biocorrosion of Stainless-Steel Electrodes.

Microbial electrochemical technologies have revealed the opportunity of electrochemical enrichment for specific bacterial groups that are able to catalyze reactions of interest. However, there are unsolved challenges towards their application under aggressive environmental conditions, such as in the sea. This study demonstrates the impact of surface electrochemical potential on community composition and its corrosivity. Electrochemical bacterial enrichment was successfully carried out in natural seawater without nutrient amendments. Experiments were carried out for ten days of exposure in a closed-flow system over 316L stainless steel electrodes under three different poised potentials (−150 mV, +100 mV, and +310 mV vs. Ag/AgCl). Weight loss and atomic force microscopy showed a significant difference in corrosion when +310 mV (vs. Ag/AgCl) was applied in comparison to that produced under the other tested potentials (and an unpoised control). Bacterial community analysis conducted using 16S rRNA gene profiles showed that poised potentials are more positive as +310 mV (vs. Ag/AgCl) resulted in strong enrichment for Rhodobacteraceae and Sulfitobacter. Hence, even though significant enrichment of the known electrochemically active bacteria from the Rhodobacteraceae family was accomplished, the resultant bacterial community could accelerate pitting corrosion in 316 L stainless steel, thereby compromising the durability of the electrodes and the microbial electrochemical technologies.

Elucidating the role of electrochemical polarization on the selectivity of the CO2 hydrogenation reaction over Ru

The hydrogenation of CO2 into high-value fuels is a potentially effective approach to reduce anthropogenic dependence on fossil fuels and effects of climate change. In this study, we evaluated the hydrogenation of CO2 into CO and CH4 under the electrochemical promotion of catalysis (EPOC) effect through experimental and computational studies using Ru nanoparticles. Ru nanoparticles (1–2 nm) supported on yttria-stabilized zirconia (YSZ) solid electrolyte were evaluated at 250 °C at atmospheric pressure. Under positive polarization, the methanation reaction was promoted and the competitive reverse water gas shift (RWGS) reaction was impeded. On the other hand, negative polarization resulted in suppressing permanently the methanation reaction and minimally affecting the RWGS reaction. To qualitatively rationalize the tuning of selectivity via EPOC, Density Functional Theory (DFT) computations were used to model the EPOC effect induced on the Ru (0001) surface. DFT computations uncovered that electric field effects together with a change in surface electrochemical potential between intermediates are responsible for the contrasting influence of EPOC on the CH4 and CO formation over Ru catalysts.

Re-examining the tetraphenyl-arsonium/tetraphenyl-borate (TATB) hypothesis for single-ion solvation free energies

Attempts to establish an absolute single-ion hydration free energy scale have followed multiple strategies. Two central themes consist of (1) employing bulk pair thermodynamic data and an underlying interfacial-potential-free model to partition the hydration free energy into individual contributions [Marcus, Latimer, and tetraphenyl-arsonium/tetraphenyl-borate (TATB) methods] or (2) utilizing bulk thermodynamic and cluster data to estimate the free energy to insert a proton into water, including in principle an interfacial potential contribution [the cluster pair approximation (CPA)]. While the results for the hydration free energy of the proton agree remarkably well between the three approaches in the first category, the value differs from the CPA result by roughly +10 kcal/mol, implying a value for the effective electrochemical surface potential of water of -0.4 V. This paper provides a computational re-analysis of the TATB method for single-ion free energies using quasichemical theory. A previous study indicated a significant discrepancy between the free energies of hydration for the TA cation and the TB anion. We show that the main contribution to this large computed difference is an electrostatic artifact arising from modeling interactions in periodic boundaries. No attempt is made here to develop more accurate models for the local ion/solvent interactions that may lead to further small free energy differences between the TA and TB ions, but the results clarify the primary importance of interfacial potential effects for analysis of the various free energy scales. Results are also presented, related to the TATB assumption in the organic solvents dimethyl sulfoxide and 1,2-dichloroethane.

Nanoelectrode Atomic Force Microscopy Probes Enable the in-Operando Measurement of Surface Electrochemical Potentials during Oxygen Evolution Catalysis

Heterogeneous electrochemical processes, including (photo)electrochemical water splitting to evolve hydrogen via semiconductors and/or electrocatalysts, are driven by the accumulation of charge carriers and thus the interfacial electrochemical potential gradients that promote charge transfer. Conventional electrochemical techniques measure/control potentials at the conductive substrate, but are unable to isolate processes and electrochemical potentials at the surface during operation.1 In this talk, we will show that a nanoelectrode tip of an atomic force microscope cantilever can effectively sense the surface electrochemical potential of electrocatalysts in operando.2 First, we will discuss fundamental aspects and capabilities of the probes used.3 We then will show how the technique allows for measurement of the surface potential and thickness-dependent electronic properties of cobalt (oxy)hydroxide phosphate (CoPi). We show that when CoPi is deposited on illuminated photoanodes like hematite (a-Fe2O3), it acts as both a hole collector and an oxygen evolution catalyst. We highlight the versatility of the technique by comparing surface potentials of CoPi-decorated planar and mesoporous hematite, as well as bismuth vanadate photoelectrodes.

Disentangling interfacial energetics

Understanding interfacial charge transfer in water-splitting photoelectrodes is complicated by the delicate interplay between catalyst and light absorber. Now, an approach based on atomic force microscopy is exploited to measure the surface electrochemical potential of nanostructured catalyst-coated electrodes in operando.

Potential-sensing electrochemical atomic force microscopy for in operando analysis of water-splitting catalysts and interfaces

Heterogeneous electrochemical phenomena, such as (photo)electrochemical water splitting to generate hydrogen using semiconductors and/or electrocatalysts, are driven by the accumulated charge carriers and thus the interfacial electrochemical potential gradients that promote charge transfer. However, measurements of the “surface” electrochemical potential during operation are not generally possible using conventional electrochemical techniques, which measure/control the potential of a conducting electrode substrate. Here we show that the nanoscale conducting tip of an atomic force microscope cantilever can sense the surface electrochemical potential of electrocatalysts in operando. To demonstrate utility, we measure the potential-dependent and thickness-dependent electronic properties of cobalt (oxy)hydroxide phosphate (CoPi). We then show that CoPi, when deposited on illuminated haematite (α-Fe2O3) photoelectrodes, acts as both a hole collector and an oxygen evolution catalyst. We demonstrate the versatility of the technique by comparing surface potentials of CoPi-decorated planar and mesoporous haematite and discuss viability for broader application in the study of electrochemical phenomena. Photoelectrochemical water splitting to produce H2 is dependent on the electrochemical potentials between photoelectrodes and the electrolyte, which are difficult to measure. Here the authors use an electrochemical atomic force microscope technique to sense the potential of catalyst-coated electrode surfaces in operando.

Electrochemical Control of Peptide Self-Organization on Atomically Flat Solid Surfaces: A Case Study with Graphite.

The nanoscale self-organization of biomolecules, such as proteins and peptides, on solid surfaces under controlled conditions is an important issue in establishing functional bio/solid soft interfaces for bioassays, biosensors, and biofuel cells. Electrostatic interaction between proteins and surfaces is one of the most essential parameters in the adsorption and self-assembly of proteins on solid surfaces. Although the adsorption of proteins has been studied with respect to the electrochemical surface potential, the self-assembly of proteins or peptides forming well-organized nanostructures templated by lattice structure of the solid surfaces has not been studied in the relation to the surface potential. In this work, we utilize graphite-binding peptides (GrBPs) selected by the phage display method to investigate the relationship between the electrochemical potential of the highly ordered pyrolytic graphite (HOPG) and peptide self-organization forming long-range-ordered structures. Under modulated electrical bias, graphite-binding peptides form various ordered structures, such as well-ordered nanowires, dendritic structures, wavy wires, amorphous (disordered) structures, and islands. A systematic investigation of the correlation between peptide sequence and self-organizational characteristics reveals that the presence of the bias-sensitive amino acid modules in the peptide sequence has a significant effect on not only surface coverage but also on the morphological features of self-assembled structures. Our results show a new method to control peptide self-assembly by means of applied electrochemical bias as well as peptide design-rules for the construction of functional soft bio/solid interfaces that could be integrated in a wide range of practical implementations.

The Surface Potential of the Water-Vapor Interface from Classical Simulations.

The electrochemical surface potential across the water-vapor interface provides a measure of the orientation of water molecules at the interface. However, the large discrepancies between surface potentials calculated from ab initio (AI) and classical molecular dynamics (MD) simulations indicate that what is being calculated may be relevant to different test probes. Although a method for extracting the electrochemical surface potential from AIMD simulations has been given, methods for MD simulations have not been clarified. Here, two methods for extracting the surface potential relevant to electrochemical measurements from MD simulations are presented. This potential is shown to be almost entirely due to the dipole contribution. In addition, the molecular origin of the dipole contribution is explored by using different potential energy functions for water. The results here show that the dipole contribution arises mainly from distortions in the hydration shell of the full hydrogen bonded waters on the liquid side of the interface, which is determined by the charge distribution of the water model. Disturbingly, the potential varies by 0.4 eV depending on the model. Although there is still no consensus on what that charge distribution should be, recent results indicate that it contains both a large quadrupole and negative charge out of the molecular plane, i.e., three-dimensional (3D) charge. Water models with 3D charge give the least distortion of the hydration shell and the best agreement with experimental surface potentials, although there is still uncertainty in the experimental values.

The thermodynamics of proton hydration and the electrochemical surface potential of water.

The free energy change for transferring a single ion across the water liquid/vapor interface includes an electrochemical surface potential contribution. Since this potential is not directly accessible to thermodynamic measurement, several extra-thermodynamic approaches have been employed to infer its sign and magnitude, with a resulting wide spread of values. Here, we examine further the thermodynamics of proton hydration and the electrochemical surface potential of water along three directions: (1) a basic relation of interfacial electrostatics and experimental results on ion distributions near a water/organic interface are employed to infer a solvent contribution to the electrochemical surface potential, (2) a re-analysis is performed of the existing bulk and cluster ion hydration data, and (3) extensive computational modeling is conducted to examine the size dependence of hydration enthalpy differences for the NaF ion pair between the small cluster and the converged bulk limits. The computational studies include classical polarizable models and high-level quantum chemical methods. The new theoretical analysis of existing experimental data and the combined classical/quantum modeling lead to results consistent with our previously derived proton hydration quantities.

Phosphonic Acid Adsorbates Tune the Surface Potential of TiO2 in Gas and Liquid Environments.

Controlled attachment of molecules to the surface of a material can alter the band structure energies with respect to the surrounding environment via a combination of intrinsic and bonding-induced dipoles. Here, we demonstrate that the surface potential of an application-relevant material, anatase TiO2, can be tuned over a broad energy range of ∼1 eV using a family of dipolar phosphonic acid-based adsorbates. Using TiO2 as an example, we show with photoelectron spectroscopy that these adsorbates are stable in a liquid environment (propylene carbonate). More interestingly, the tunability is substantially retained and follows trends in the computed bound dipole. The electrochemical surface potential is shown to vary over 600 meV, the highest range in electrolytes to the best of our knowledge. Using density functional theory calculations, we rationalize the measured trends and show that the effective dipole upon molecular adsorption and not the intrinsic dipole of the isolated molecules correlates with observed changes in surface potential. Control of the effective dipole, through judicious choice of robust surface species, can allow in situ tuning of energy levels and functionality at active surfaces for energy conversion and storage, biosensing, and molecular electronics.

Quasichemical analysis of the cluster-pair approximation for the thermodynamics of proton hydration.

A theoretical analysis of the cluster-pair approximation (CPA) is presented based on the quasichemical theory of solutions. The sought single-ion hydration free energy of the proton includes an interfacial potential contribution by definition. It is shown, however, that the CPA involves an extra-thermodynamic assumption that does not guarantee uniform convergence to a bulk free energy value with increasing cluster size. A numerical test of the CPA is performed using the classical polarizable AMOEBA force field and supporting quantum chemical calculations. The enthalpy and free energy differences are computed for the kosmotropic Na(+)/F(-) ion pair in water clusters of size n = 5, 25, 105. Additional calculations are performed for the chaotropic Rb(+)/I(-) ion pair. A small shift in the proton hydration free energy and a larger shift in the hydration enthalpy, relative to the CPA values, are predicted based on the n = 105 simulations. The shifts arise from a combination of sequential hydration and interfacial potential effects. The AMOEBA and quantum chemical results suggest an electrochemical surface potential of water in the range -0.4 to -0.5 V. The physical content of single-ion free energies and implications for ion-water force field development are also discussed.

The Intersection of Interfacial Forces and Electrochemical Reactions

We review recent developments in experimental techniques that simultaneously combine measurements of the interaction forces or energies between two extended surfaces immersed in electrolyte solutions-primarily aqueous-with simultaneous monitoring of their (electro)chemical reactions and controlling the electrochemical surface potential of at least one of the surfaces. Combination of these complementary techniques allows for simultaneous real time monitoring of angstrom level changes in surface thickness and roughness, surface-surface interaction energies, and charge and mass transferred via electrochemical reactions, dissolution, and adsorption, and/or charging of electric double layers. These techniques employ the surface forces apparatus (SFA) combined with various "electrochemical attachments" for in situ measurements of various physical and (electro)chemical properties (e.g., cyclic voltammetry), optical imaging, and electric potentials and currents generated naturally during an interaction, as well as when electric fields (potential differences) are applied between the surfaces and/or solution-in some cases allowing for the chemical reaction equation to be unambiguously determined. We discuss how the physical interactions between two different surfaces when brought close to each other (<10 nm) can affect their chemistry, and suggest further extensions of these techniques to biological systems and simultaneous in situ spectroscopic measurements for chemical analysis.

Surface Coverage and SEI Induced Electrochemical Surface Stress Changes during Li Deposition in a Model System for Li-Ion Battery Anodes

We report electrochemical surface stress and potential dependent matrix assisted laser desorption ionization (MALDI)-time of flight (TOF) mass spectrometry (MS) results combined with detailed density functional theory (DFT) analysis of Li deposition on a Au model system for Li ion battery anodes. Deposition of Li on Au surfaces at potentials >0.2 V vs. Li/Li+ occurs through the formation of a Li-Au surface alloy, a result that is predicted by DFT calculations. As the Au surface potential becomes more cathodic, compressive stress develops on the surface, a result again predicted from calculation. The compressive stress is completely removed by cycling the potential back to 2.0 V vs. Li/Li+ through delithiation of the surface alloy. Lithiation of the Au electrode during Li bulk alloy formation at potentials <0.2 V vs. Li/Li+ results in compressive stress, as expected. However, in this case residual tensile stress is observed following delithiation, the magnitude of which increases with increasing lithiation/delithiation cycles. Potential dependent MALDI-TOF MS analysis shows that solid electrolyte interphase (SEI) oligomers are formed during delithiation following Li bulk alloy formation and that these oligomers are the likely origin of the observed residual tensile stress. This residual tensile stress is not present when the carbonate solvent is replaced with an ionic liquid. These results show that surface stress is determined by Li-host atomistic interactions as well as the nature of the SEI.

Electrochemical Surface Potential Due to Classical Point Charge Models Drives Anion Adsorption to the Air-Water Interface.

We demonstrate that the driving forces for ion adsorption to the air-water interface for point charge models result from both cavitation and a term that is of the form of a negative electrochemical surface potential. We carefully characterize the role of the free energy due to the electrochemical surface potential computed from simple empirical models and its role in ionic adsorption within the context of dielectric continuum theory. Our research suggests that the electrochemical surface potential due to point charge models provides anions with a significant driving force for adsoprtion to the air-water interface. This is contrary to the results of ab initio simulations that indicate that the average electrostatic surface potential should favor the desorption of anions at the air-water interface. The results have profound implications for the studies of ionic distributions in the vicinity of hydrophobic surfaces and proteins.

Altered Membrane Structure and Surface Potential in Homozygous Hemoglobin C Erythrocytes

BackgroundHemoglobin C differs from normal hemoglobin A by a glutamate-to-lysine substitution at position 6 of beta globin and is oxidatively unstable. Compared to homozygous AA erythrocytes, homozygous CC erythrocytes contain higher levels of membrane-associated hemichromes and more extensively clustered band 3 proteins. These findings suggest that CC erythrocytes have a different membrane matrix than AA erythrocytes.Methodology and FindingsWe found that AA and CC erythrocytes differ in their membrane lipid composition, and that a subset of CC erythrocytes expresses increased levels of externalized phosphatidylserine. Detergent membrane analyses for raft marker proteins indicated that CC erythrocyte membranes are more resistant to detergent solubilization. These data suggest that membrane raft organization is modified in CC erythrocytes. In addition, the average zeta potential (a measure of surface electrochemical potential) of CC erythrocytes was ≈2 mV lower than that of AA erythrocytes, indicating that substantial rearrangements occur in the membrane matrix of CC erythrocytes. We were able to recapitulate this low zeta potential phenotype in AA erythrocytes by treating them with NaNO2 to oxidize hemoglobin A molecules and increase levels of membrane-associated hemichromes.ConclusionOur data support the possibility that increased hemichrome deposition and altered lipid composition induce molecular rearrangements in CC erythrocyte membranes, resulting in a unique membrane structure.

Eccentric Phenomena at Liquid Mercury Electrode/Solution Interfaces: Upward, Downward, and Circular Motions

The present paper describes a visualization of unidirectional and circular motions triggered by an electrochemical redox reaction at a charged, bent, and streamed liquid electrode/liquid solution interface. The novel circular motion that induces a conversion of electrochemical energy into mechanical energy could be visualized for the first time at a hanging mercury drop electrode (HMDE)/dimethyl sulfoxide (DMSO) solution interface via the electrochromic reaction of 2,1,3-benzothiadiazole (BTD) by using a CCD-color video camera. The observed motions are self-insisting in nature and are tunable into upward, downward, clockwise, and anticlockwise ones by an appropriate choice of the experimental conditions. This circular motion is visualized for the first time as the cause of the well-known cyclic voltammetric anodic current oscillation at the HMDE. Several small perturbations, for example, surface tension, surface motion, bulk motion, diffusional mass transport, and surface electrochemical potential are considered to be endlessly amplified by their coupling in a cyclic chain, resulting in such macroscopic motions at the electrode/solution interface. All of the phenomena can be explained on the basis of the modern theory proposed by Aogaki et al. for the polarographic streaming maxima of the first kind.