Voltage-dependent surface IR spectra of water at gold electrodes from machine-learned abinitio dipoles.

The electrochemical reduction of sulfur dioxide (SO2) has been studied in ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpy][NTf2]) at gold and platinum electrodes by cyclic voltammetry and in-situ electrochemical quartz crystal microbalance. A mechanism for the electrode reaction was proposed that the main process in the reduction was one electron-transfer process and the product was stable SO2 -• radical anion. While two anodic peaks were observed on the oxidation wave at both working electrodes. The anodic peak which appeared at a less negative potential was assigned to the oxidation of free SO2 -• radicals. However, the peak appearing at more positive potential was assigned to the oxidation of SO2 -• which was solvated into IL electrolyte on the electrode surface. Comparing gold and platinum electrodes, the gold electrode gave a better sensitivity due to the interaction between sulfur and gold surface. Based on the above proposed SO2 electrochemical reduction mechanism, real time SO2 detection by amperometric method was also investigated. The results demonstrated that the detection limit and sensitivity for SO2 was 39.4 ppm and 99 µA/[%SO2], respectively, in [Bmpy][NTf2] at gold electrode. A stable response was able to achieve over 30 days. Because of the attractive physical properties of ILs (e.g., less volatile and high thermal stability), the utilization of IL provides a novel electrochemical approach to develop a robust SO2 sensor.

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Relationship of two electroactive forms of horse heart cytochrome c at gold and glassy carbon electrodes in water and methanol

We present an ab initio method for calculation of the electronic structure and electronic transport of nanoscale systems coupled to electrodes with applied voltage bias. The method is based on the local density approximation of density functional theory and implemented in the framework of the tight-binding linear muffin-tin orbital approach in its atomic sphere approximation. A fully atomistic description of the electrodes and the nanosystem is used, and the self-consistent charge and electrostatic potential for the system under applied bias is calculated using the nonequilibrium Green's function (NEGF) approach. General expressions for the lesser Green's function and transmission coefficient obtained within NEGF theory are rewritten using auxiliary Green's functions that are defined by the inverse of the short-ranged structural constants. This reformulation of the theory with auxiliary Green's functions allows the use of very effective and well-developed tight-binding techniques. The method is applied to three systems: a single benzene di-thiol molecule coupled to (111) gold electrodes, a single gold atom coupled to (100) gold electrodes, and a single platinum atom coupled to (100) platinum electrodes.

Some dithiophosphonate derivatives were synthesized and the electrochemical reduction mechanism was investigated by cyclic voltammetry (CV), square wave voltammetry (SWV) and chronoamperometry (CA) in 0.1 M tetrabutylammoniumtetrafluoroborate (TBATFB) in acetonitrile at platinum (Pt) and gold (Au) electrodes. Dithiophosphonates showed a cyclic voltammetric reduction peak at about −1.1 V at Pt and −1.3 V at Au electrode (vs. Ag/Ag+) in this media. It was also shown that dithiophosphonates can be determined quantitatively in acetonitrile using a calibration graph. The number of electrons transferred were calculated as 2 using ferrocene as a reference compound at the UME electrode. Mechanism of dithiophosphonates was also examined on Pt and Au electrodes and electrochemical reduction of dithiophosphonates seems to follow an EC mechanism with an irreversible electron transfer step. The reaction product in the bulk electrolysis experiment was isolated and identified using proton-coupled P-31 NMR, 13C-NMR and IR spectroscopy. The adsorption tests for dithiophosphonates were revealed that no strong or weak adsorption phenomena exist on both Pt and Au electrodes. Simulation curves were acquired by DigiSim 3.03 version to investigate the reduction mechanism and to estimate the kinetic parameters for electrochemical and chemical steps.

The electrochemical oxygen reduction reaction is vital for applications such as fuel cells, metal air batteries and for oxygen gas sensing. Oxygen undergoes a 1-electron reduction process in dry ionic liquids (ILs) to form the electrogenerated superoxide ion that is solvated and stabilized by IL cations. In this work, the oxygen/superoxide (O2/O2 •-) redox couple has been used to understand the effect of mixing ILs with different cations in the context of developing designer electrolytes for oxygen sensing, by employing cyclic voltammetry at both gold and platinum electrodes. Different cations with a range of sizes, geometries and aromatic/aliphatic character were studied with a common bis(trifluoromethylsulfonyl)imide ([NTf2]-) anion. Diethylmethylsulfonium ([S2,2,1]+), N-butyl-N-methylpyrrolidinum ([C4mpyrr]+) and tetradecyltrihexylphosphonium ([P14,6,6,6]+) cations were mixed with a common 1-butyl-3-methylimidazolium ([C4mim]+) cation at mole fractions (x) of [C4mim]+ of 0, 0.2, 0.4, 0.6, 0.8, and 1. Both the redox kinetics and thermodynamics were found to be highly dependent on the cation structure and the electrode material used. Large deviations from "ideal" mixtures were observed for mixtures of [C4mim][NTf2] with [C4mpyrr][NTf2] on gold electrodes, suggesting a much higher amount of [C4mim]+ ions near the electrode surface despite the large excess of [C4mpyrr]+ in the bulk. The electrical double layer structure was probed for a mixture of [C4mim]0.2[C4mpyrr]0.8[NTf2] using atomic force microscopy measurements on Au, revealing that the first layer was more like [C4mim][NTf2] than [C4mpyrr][NTf2]. Unusually fast kinetics for O2/O2 •- in mixtures of [C4mim]+ with [P14,6,6,6]+ were also observed in the electrochemistry results, which warrants further follow-up studies to elucidate this promising behavior. Overall, it is important to understand the effect on the kinetic and thermodynamic properties of electrochemical reactions when mixing solvents, to aid in the creation of designer electrolytes with favorable properties for their intended application.

We present density functional theory-nonequilibrium Green’s function method for electron transport of dipyridazine and dipyridimine molecular junctions with gold, copper and nickel electrodes. Our investigation reveals that the junctions formed with gold and copper electrodes bridging dipyridazine molecule through thiol anchoring group enhance current as compared to the junctions in which the molecule and electrode were coupled directly. Further, nickel electrode displays weak decrease of current with increase of voltage at about 1.2 V. The result is fully rationalized by means of the distribution of molecular orbitals as well as shift in molecular energy levels and HOMO-LUMO gap with applied bias voltage. Our findings are compared with theoretical and experimental results available for other molecular junctions. Present results predict potential avenues for changing the transport behavior by not only changing the electrodes, but also the position of nitrogen atom and type of anchoring-atom that connect molecule and electrodes, thus extending applications of dipyridazine and dipyridimine molecule in future integrated circuits.

Electrochemical reduction of oxygen on gold and boron-doped diamond electrodes in ambient temperature, molten acetamide–urea–ammonium nitrate eutectic melt

Electrochemical CO2 reduction reaction (CO2RR) can harness renewable energy to convert CO2 into valuable chemicals and fuels. The nanostructured catalysts used in CO2RR are incorporated onto the gas diffusion electrodes using ionomer binders such as Nafion and Piperion, which form a thin layer that partially or fully covers the catalyst nanoparticles. Recent reports indicate that these ionomers can also influence the activity and selectivity of CO2RR. A high local pH has been proposed to facilitate CO2RR selectivity to multi-carbon products while inhibiting parasitic the hydrogen evolution reaction. While some reports suggest that anionic ionomers promote local pH,1,2 others hypothesize that cationic ionomers increase the local pH.3,4 However, no studies have directly measured local pH changes at the catalyst-ionomer interface with different ionomers.In this work, we employed in situ surface-enhanced Raman spectroscopy to directly measure local pH at the surface of a roughened gold electrode using a pH-sensitive probe molecule. We used the hydrogen evolution reaction as a model reaction because, like CO2RR, it also involves the consumption of proton or production of hydroxide ions, leading to a local pH rise upon application of current. The gold electrode was coated with an anionic ionomer (Nafion), a cationic ionomer (Piperion), and a neutral polymer. The rate of local pH increase for these polymer-coated electrodes was compared with that of a bare gold electrode across electrolytes of varying bulk pH and under different applied current densities. Our results reveal the influence of ionomers of different charge polarities on local pH under electrochemical conditions. The insights gained from this work will guide the selection of ionomers for tuning the local pH during electrochemical reactions.

Electronic transport property of 4,4′-bipyridine molecular junction

The effect on molecular transport due to chemical modification of the metal-molecule interface is investigated, using as an example the prototypical molecular device formed by attaching a p-disubstituted benzene molecule onto two gold electrodes through chemically different end groups. Using a first-principles based self-consistent matrix Green's function method, we find that depending on the end group, transport through the molecule can be mediated by either near-resonant-tunneling or off-resonant-tunneling and the conductance of the molecule varies over more than two orders of magnitude. Despite the symmetric device structure of all the molecules studied, the applied bias voltage can be dropped either equally between the two metal-molecule contacts or mostly across the source (electron-injecting) contact depending on the potential landscape across the molecular junction at equilibrium.

A nanocrystalline diamond (NCD) thin film is deposited on a gold electrode and oxygen terminated by r.f. oxygen plasma. An atomic force microscope (AFM) is used to induce electrostatically charged micrometer‐sized areas on the diamond film by applying bias voltages in the range between –30 V and +30 V on the AFM tip during scanning in contact mode. Trapped charge was detected by Kelvin force microscopy showing a contact potential difference of 150 mV for both polarities. Decrease of surface potential by 20 mV in the positively charged area and by 90 mV in the negatively charged area is observed after 15 h under ambient conditions. Possible charge trapping mechanisms are discussed in terms of electret‐like or semiconductor‐like behavior of NCD thin films. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied the single-molecule conductance of a family of curcuminoid molecules (CCMs) using the mechanically controlled break junction (MCBJ) technique. The CCMs under study contain methylthio (MeS-) as anchoring groups: MeS-CCM (1), the free-ligand organic molecule, and two coordination compounds, MeS-CCM-BF2 (2) and MeS-CCM-Cu (3), where ligand 1 coordinates to a boron center (BF2 group) and to a CuII moiety, respectively. We found that the three molecules present stable molecular junctions allowing detailed statistical analysis of their electronic properties. Compound 3 shows a slight increase in the conductance with respect to free ligand 1, whereas incorporation of BF2 (compound 2) promotes the presence of two conductance states in the measurements. Additional experiments with control molecules point out that this bistability is related to the combination of MeS- anchoring groups and the BF2 moiety within the structure of the molecules. Theoretical calculations show that this can be explained by the presence of two conformers once compound 2 is anchored between the gold electrodes. An energy minimum is found for a flat structure but there is a dramatic change in the magnitude and orientation of dipole moment (favouring a non-flat conformer in the presence of an external electric field) due to a conformational change of one of the terminal MeS- groups. The results thus point to an intricate interplay between the applied bias voltage and the molecule dipole moment which could be the basis for designing new molecules aiming at controlling their conformation in devices.

The enantioselectivity imparted to a gold electrode by modifying its surface with a self-assembled monolayer (SAM) of cysteine (Cys) was investigated for the electrochemical redox reaction of 3,4-dihydroxyphenylalanine (DOPA). A cyclic voltammetric study of the redox reaction revealed that the enantioselectivity was determined by the surface coverage of the gold electrode with Cys molecules. The electrode modified with approximately 1.8 x 10(14) Cys molecules cm(-2) exhibited enantioselectivity in the voltammogram for the oxidation and reduction of DOPA, while the voltammograms obtained by the electrodes with either more or less surface coverages did not exhibit significant enantioselectivity. It is suggested that the accessibility of DOPA to that area of the gold surface which is not blocked by Cys molecules at an optimum surface coverage, is required for the enantioselective redox reaction of DOPA to proceed.

The adsorpotion of 1, 6-hexanedithiol (HDT) at gold electrode was widely used to enzyme immobization. The effect of anions on adsorption of HDT at gold electrode was investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The results show that stability of HDT at gold electrode in Na2SO4 solution was worse than that in LiClO4 solution. LiClO4 is a good salt for electrolyte at HDT gold electrode application in the Tris Buffer system (20 mM Tris-HCl pH=7.1, 5 mM MgCl2).