Adsorption of 2,2′-bipyridine at an Au(111)|ionic liquid electrified interface

The adsorption of added 2,2′-bipyridine (2,2′-BP) from 1-ethyl-2,3-dimethyl imidazolium bis(trifluoromethanesulfonyl)imide (EMMImNTf₂) at an Au(111) electrode has been investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Addition of 2,2′-BP to the ionic liquid clearly modifies the interfacial region as a result of the competition between 2,2′-BP and EMMImNTf₂ to occupy the electrode surface. Within the region of ideal polarizability, the 2,2′-BP adlayer undergoes structural changes, shown by the presence of peaks in the CV curves. Between −0.2 V and + 0.9 V, the capacitance–potential curves obtained from EIS data present a capacity maximum depending strongly on the ac frequency, which is typical pseudo-capacitive behavior indicative of a reorganization of the interfacial layer. At more positive potentials a true capacity value close to 10 μF.cm^− 2 and invariant with the potential suggests that the 2,2′-BP molecules adopt a perpendicular orientation with the nitrogen atoms facing the electrode surface, similar to their adsorption on gold from aqueous solutions.     

The potentials of zero charge of Pd(1 1 1) and thin Pd overlayers on Au(1 1 1)

The potential of zero charge (pzc) of Pd(1 1 1) has been determined in dilute NaF solutions by measuring the Gouy–Chapman minimum of the double-layer capacity. For a massive Pd(1 1 1) single crystal electrode a pzc of −0.12 V vs. SCE has been found. The corresponding values for thin Pd(1 1 1) overlayers on Au(1 1 1) have also been determined. While the pzc of the first, pseudomorphic Pd layer on Au(1 1 1) is −0.09 V vs. SCE, the pzc of a five monolayers thick Pd film on Au(1 1 1) is practically identical to the pzc of the massive Pd(1 1 1) electrode. By comparing pzc's and work functions for Au(1 1 1) and Pd(1 1 1), the dipole contribution to the potential drop across the Pd(1 1 1)/water interface is estimated.     

Electrochemical behavior of CO2 reduction on palladium nanoparticles: Dependence of adsorbed CO on electrode potential

The electrochemical reduction of CO₂ is strongly influenced by both the applied potential and the surface adsorption status of the catalyst. In this work a gas diffusion electrode (GDE) coated with Pd nanoparticles/carbon black (Pd/XC72) was used to study the electrochemical reduction of CO₂. Cyclic voltammetric (CV) analysis of Pd/XC72 between 1.5 V and − 0.6 V (vs. RHE) shows the formation of intermediates and the blocking of hydrogen absorption on the Pd nanoparticles (NPs) under a CO₂ atmosphere. The relationships between the Faradaic efficiency/current density and the applied potential reveal that the onset potential of CO formation is around − 0.4 V. Moreover, the presence of adsorbed CO was confirmed through CV analysis of Pd/XC72 under CO₂ and CO/He atmospheres. This demonstrates that H atoms and CO intermediates co-adsorb on the surface of the Pd NPs at an applied potential of around − 0.4 V. When the applied potential is more negative than − 0.6 V, adsorption of CO intermediates on the surface of the Pd NPs becomes dominant.     

Electrolytic in situ STM investigation of h-BN-Nanomesh

Single sheet boron nitride layers on thin rhodium (1 1 1) films were formed upon thermal decomposition of borazine under ultra-high vacuum (UHV) conditions. They were transferred and investigated in an electrolytic environment. In 0.1 M HClO₄, the presence and stability of the so-called nanomesh super structure with a lattice constant of 3.2 nm is established with electrochemical impedance spectroscopy, cyclic voltammetry and subsequent imaging with in situ scanning tunneling microscopy (STM) under potential control. In the electrolyte, the BN nanomesh acts as a dielectric layer with an unusual behaviour of the impedance, where the capacitive component is larger than on a Rh(1 1 1) reference sample. It exhibits reversible hydrogen adsorption and desorption at a potential of −600 mV vs. a saturated mercury sulphate reference electrode (MSE). The unit cell of the nanomesh is imaged by STM and shows hexagonally arranged two-dimensional pores with a diameter of 2 nm. At a fixed potential, the nanomesh was stable for long time, but after repeated potential sweeps between +260 and −540 mV vs. MSE, STM indicates roughening, though a 12 × 12 superstructure was recovered after annealing in UHV.     

Spontaneous deposition of Sn on Au(1 1 1). An in situ STM study

The tin adlayer formed by spontaneous deposition on Au(1 1 1) was characterized by cyclic voltammetry and in situ scanning tunneling microscopy (STM) in sulphuric acid solution. Cyclic voltammetry measurements showed oxidation peaks in the potential range −0.60 ⩽ E/V vs SSE ⩽ 0, which can be ascribed to the dissolution of the Sn adsorbed layer. STM images of the Au(1 1 1)/Sn modified surface showed that tin nucleated both on step edges and on the flat terraces forming two-dimensional islands. The anodic polarization of this modified surface produced the gradual dissolution of the Sn adlayer which was evidenced by the formation of some holes and the reduction of the initial terraces to many small islands. STM images with atomic resolution obtained on these islands displayed an hexagonal expanded atomic structure. After the anodic stripping of this Sn adsorbed layer the images exhibited the typical Au(1 1 1) terraces with a (1 × 1) atomic structure. However, at more anodic potentials another dissolution process was observed producing noticeable changes on the surface morphology which could be ascribed to the dissolution of a Au–Sn surface alloy.     

Direct electrochemistry and electrocatalysis of hemoglobin immobilized in bimodal mesoporous silica and chitosan inorganic–organic hybrid film

Hemoglobin modified electrode was successfully fabricated to realize direct electrochemistry by immobilizing of Hemoglobin (Hb) in bimodal mesoporous silica (BMS) and chitosan (CS) inorganic–organic hybrid film. Here, BMS acted as a support to immobilize Hb due to its large pores and CS acted as a binder to increase film adherence and stabilizer to prevent the leakage of Hb. The resulting electrode (Hb/BMS/CS) gave a well-defined, reversible redox couple for HbFe(III)/Fe(II) with a formal potential of about −0.32 V (vs. Ag/AgCl) in pH 7.0 phosphate buffer solution. Hb/BMS/CS electrode showed a better electrocatalytial performance to H₂O₂ with wider linear detection range, lower detection limit, and higher sensitivity than that at electrode without BMS. The improved electrocatalytic performance for Hb/BMS/CS electrode was possibly contributed to BMS bimodal structure, whose large pores with 10–40 nm provide favorable conditions for protein immobilization and small pores with 2–3 nm avoid the mass-transfer limitations. In addition, UV–Vis and FTIR spectra indicated that Hb well maintained its native structure in the hybrid film.     

One-step fabrication of chitosan–hematite nanotubes composite film and its biosensing for hydrogen peroxide

This work reports on a novel chitosan–hematite nanotubes composite film on a gold foil by a simple one-step electrodeposition method. The hybrid chitosan–hematite nanotubes (Chi–HeNTs) film exhibits strong electrocatalytic reduction activity for H₂O₂. Interestingly, two electrocatalytic reduction peaks are observed at −0.24 and −0.56 V (vs SCE), respectively, one controlled by surface wave and the other controlled by diffusion process. The Chi–HeNTs/Au electrode shows a linear response to H₂O₂ concentration ranging from 1 × 10⁻⁶ to 1.6 × 10⁻⁵ mol L⁻¹ with a detection limit of 5 × 10⁻⁸ mol L⁻¹ and a sensitivity as high as 1859 μA μM⁻¹ cm⁻².     

Measurements of differential capacitance in room temperature ionic liquid at mercury,glassy carbon and gold electrode interfaces

Differential capacitances were measured in 1-propyl-3-methylimidazolium tetrafluoroborate (PMIBF₄) ionic liquid at three different electrode substrates (Hg, GC (glassy carbon) and Au) as a function of potential. Essentially different capacitance–potential curves were obtained at different electrodes. From the parabolic electrocapillary curve measured at dropping Hg electrode in PMIBF_4, the potential of zero charge (PZC) was found to be −0.31 V vs. Ag/AgCl (wire). However, the capacitance–potential curve at Hg electrode was found not to show any valley related to the PZC, whereas at GC and Au electrodes a minimum was observed at 0.29 and −0.51 V, respectively. The results are in disagreement with the recent theoretical study which implies that the capacitance–potential curve should be of bell shape with the maximum value of capacitance at PZC. The parabolic capacitance–potential curve similar to those obtained in inorganic molten salts was also observed for the first time at GC electrode. Probable causes of the difference in their capacitance–potential curves were also discussed.     

Surface poisoning during electrocatalytic monosaccharide oxidation reactions at gold electrodes in alkaline medium

In the present study, the surface poisoning of electrocatalytic monosaccharide oxidation reactions at gold electrodes were investigated. In the cyclic voltammetric studies, the electrocatalytic oxidation of aldohexose and aldopentose type monosaccharides, aminosugars, acetyl-glucosamine and glucronamide were observed at gold plate electrodes in alkaline medium. However, in controlled-potential electrolytic studies ranging −0.3 to −0.2 V in reaction solutions, current flows during electrolyses decreased quickly with time, except when glucosamine was used as a substrate.Results from surface enhanced infrared adsorption (SEIRA) spectroscopic measurements at an evaporated gold electrode for the electrocatalytic oxidation of glucose in 0.1 mol dm⁻³ NaOH at −0.3 V and Gaussian simulated spectra indicated that the gluconic acid as a 2-electron oxidation product and/or its analogs adsorbed onto the gold surface. Electrochemical quartz crystal microbalance (EQCM) measurement results, along with surface adsorption results from surface poisoning at the gold electrode during electrolytic reactions, suggested that gluconic acid and/or its analogs adsorbed vertically onto electrode surfaces in a full monolayer packing-like conformation. In the case of the electro oxidation of glucosamine in 0.1 mol dm⁻³ NaOH at −0.2 V, the obtained SEIRA spectra and EQCM results, clearly indicated that the glucosaminic acid as a 2-oxidation glucosamine product did not strongly bind onto the gold electrode surface.     

Anodic stripping voltammetric analysis of trace arsenic(III) enhanced by mild hydrogen-evolution at a bimetallic Au–Pt nanoparticle modified glassy carbon electrode

A glassy carbon electrode (GCE) modified with electrodeposited bimetallic Au–Pt nanoparticles (Au–PtNPs) was applied to sensitively detect As(III) by linear sweep anodic stripping voltammetry (LSASV). In 0.5 M aqueous H₂SO₄, atomic hydrogen and molecular hydrogen were easily electrogenerated at the Pt sites on Au–PtNPs/GCE, which can chemically reduce As(III) to As(0) and enhance the cathodic preconcentration of As(0) at both the Pt sites and the neighboring Au sites. Since the As(0)–Au affinity is weaker than the As(0)–Pt affinity, the preconcentrated As(0) can be rapidly oxidized on/near the surface Au sites of Au–PtNPs/GCE, yielding sharper and higher LSASV current peaks. Under optimum conditions (700 s preconcentration at − 0.1 V, 5 V s^− 1), the LSASV peak current for the As(0)–As(III) oxidation responded linearly to As(III) concentration from 0.005 to 3.0 μM with a limit of detection (LOD) of 3.7 nM (0.28 ppb) (S/N = 3), while that for the As(III)–As(V) oxidation was linear with As(III) concentration from 0.01 to 3.0 μM with a LOD of 6.0 nM (0.45 ppb) (S/N = 3). This method was applied for analysis of As(III) in real water samples.     

In situ femtosecond spectroelectrochemistry of Au(1 1 1) in an aqueous chloride solution

In this paper we report the first in situ femtosecond spectroelectrochemistry experiment employing a broadband probe, allowing the measurement of ultrafast transient visible spectra with a fixed pump wavelength. We investigated in situ femtosecond transient reflectivity of a Au(1 1 1) electrode in contact with an aqueous KCl solution. The pump wavelength was set at 780 nm and a supercontinuum probe was employed, yielding ultrafast spectral information on the electron thermalisation dynamics in the range 450–650 nm. Electrochemical control allowed to investigate the dynamic response to different Cl^? adsorption conditions.     

In situ scanning tunneling microscopy imaging of mercury film electrodeposited on Ir(1 1 1)

In situ scanning tunneling microscopy (STM) was used to examine multilayer Hg film electrodeposited on a well-ordered Ir(1 1 1) single crystal electrode in 0.1 M HClO₄ + 1 mM Hg(ClO₄)₂. Topography STM scans showed that the Ir(1 1 1) – supported Hg film electrode contained well-defined terraces separated by monatomic steps (Δz = 2.3 Å). The STM’s tip could be used to induce local dissolution of the Hg deposit under proper operating conditions and the depth of the etched pit informed directly the thickness of Hg deposit. Although in situ STM imaging with a tungsten tip could not result in atomic structure of bare Hg film in 0.1 M HClO₄, it discerned highly ordered iodine adlayers, represented by a (2 × 8√3)rect – I structure, on the Hg film in solution containing potassium iodide. These STM results suggested that the Hg substrate could have an ordered atomic structure.     

Controlled-potential electrosynthesis of glucosaminic acid from glucosamine at a gold electrode

The electrocatalytic oxidation of d-glucosamine (2-amino-2-deoxy-d-glucose) in alkaline and neutral solutions was examined using a carbon felt electrode modified with 2 nm core sized gold nanoparticles (Au_2 nm nanoparticles) and a gold plate electrode. The electrocatalytic voltammetric oxidation curves of d-glucosamine were obtained in both solutions. The voltammetric responses for the electrocatalytic oxidation at a Au_2 nm nanoparticle-modified electrode in both alkaline and neutral solutions were almost the same to those at a gold plate electrode. The oxidized product was identified to be d-glucosaminic acid (2-amino-2-deoxy- d-gluconic acid) generated by the 2-electron oxidation product of d-glucosamine by electrospray ionization time-of-flight mass spectra (ESI TOF-MS). The HPLC results also indicated that the oxidation product was d-glucosaminic acid.The controlled-potential electrolysis of d-glucosamine was performed at the Au_2 nm nanoparticle-modified carbon felt electrodes in both alkaline and neutral solutions. In the alkaline solution, at a potential of −0.2 V, d-glucosaminic acid was formed with a current efficiency of 100%. In the neutral solution, electrolysis at 0.4 V on d-glucosaminic acid was obtained with current efficiencies of 70%.     

A new composite electrode of Ni-Ferrite/TiOx/Si(111): Preparations and photoelectrochemical studies

A composite electrode of Ni-ferrite/TiO_x/Si(111) was synthesized by grafting Ni²⁺Fe²⁺Fe³⁺–LDH–TiCl₃ (LDH: Layered Double Hydroxides) on n-Si(111) surface and calcined under 1100 °C. Photoelectric research results indicated that the electrode had good photovoltaic effects in an electrolyte solution containing 7.6 M HI and 0.05 M I₂, while platinum plate was used as counter-electrode. The observed photo-voltages (U_pv) and photocurrent densities (j_pc) of the electrode were at ?0.75 V and 5.35 mA/cm², respectively. Compared with electrodes of oxidized n-Si(111) crystal and n-Si(111) wafer covered by Ni-ferrites, j_pc of the electrode Ni-ferrite/TiO_x/Si(111) was increased greatly.     

Determination of hydrogen sulfide in gasoline by Au nanoclusters modified glassy carbon electrode

A promising hydrogen sulfide (H₂S) sensor was prepared by electrodeposition of Au nanoclusters on glassy carbon electrode (GCE) and the surface structure was characterized by SEM and EDAX. These flower-like form Au nanoclusters, which were made up of highly dense clustering Au nanorods with an average diameter of 20 nm and length up to 80 nm, had an average size about 600 nm and uniformly distributed on the GCE surface. The electrocatalytic oxidation of H₂S in gasoline was performed on this modified electrode, which had a satisfactory liner response to H₂S in the range of 1–80 ppm and a detection limit of 0.45 ppm (s/n = 3). This sensor was sensitive, selective and stable.     

Development of MEMS-based direct methanol fuel cell with high power density using nanoimprint technology

Performance of MEMS-based DMFC is low, because graphite-based porous electrodes show poor compatibility with MEMS technology. Nanoimprint technology was adopted in this paper to prepare fine pattern on proton exchange membrane (PEM) in MEMS-based DMFC as a promising alternative to the graphite-based porous electrodes. Micro-convex with the diameter of about 600 nm and the height of 50–70 nm was prepared on Nafion^® 117 membrane by the nanoimprint at 130 °C using silicon mold. Thick Pt film (20 nm) was deposited as catalyst directly on the nanoimprinted Nafion^® 117 membrane. Then the Pt-coated PEM was sandwiched with micro-channeled silicon plates to form a micro-DMFC. With passively feeding of 1 M methanol solution and air at room temperature, the as-prepared cell had the open circuit voltage (OCV) of 0.74 V and the maximum power density of 0.20 mW/cm². The measured OCV was higher than those (0.1–0.3 V) of the state-of-the-art MEMS-based DMFC with planar electrode and pure Pt catalyst.     

Electrochemical synthesis of Au/polyaniline–poly(4-styrenesulfonate) hybrid nanoarray for sensitive biosensor design

Au/polyaniline (PANI)–poly(4-styrenesulfonate) (PSS) hybrid nanoarray is fabricated for biomolecular sensing in neutral aqueous solutions. Firstly, an array of one-dimensional Au nanorods (diameter = ca. 200 nm, length = ca. 3 μm) is formed by a template-electrodeposition method using a porous anodic alumina membrane, and then a thin PANI–PSS composite layer is electropolymerized on the surface of the Au nanorods. The resulting Au/PANI–PSS hybrid nanoarray exhibits a quasi-reversible redox electrochemical process at ca. +0.11 V and electrocatalytic oxidation of reduced β-nicotinamide adenine dinucleotide (NADH) is attained with a detection limit of 0.3 μM in a neutral solution.     

EC-STM observation on electrochemical response of fluidic phospholipid monolayer on Au(1 1 1) modified with 1-octanethiol

Electrochemical scanning tunneling microscopy (EC-STM) was applied to observe phospholipid layers over thiol-modified gold substrates as a model biological cell membrane. On a monolayer of 1-octanethiol on Au (1 1 1), a synthetic lipid, 1,2-dihexanoyl-sn-glycero-3-phosphocholine, was introduced in a neutral 0.05 M NH₄ClO₄ buffer solution. The lipid molecules formed a fluidic layer at 0.0 V vs. RHE of the substrate electrode potential. By cycling the electrode potential between +0.2 V and −0.2 V, the lipid layer reversibly changed over between the fluidic phase and a striped/grainy structure. This structural change might involve partial decomposition and oligomerization of phospholipids. This method will contribute for molecular biology by revealing the nanometer-scale structure of cell membrane.     

3-D nanoporous gold thin film for the simultaneous electrochemical determination of dopamine and ascorbic acid

In this study, we demonstrated a novel fabrication method of three dimensional nanoporous gold thin film (NPGF) onto gold (Au) substrate using electrochemical deposition method. Scanning electron microscope (SEM) investigation reveals the formation of highly-ordered pores, approximately 30 nm in diameter and 150 nm thick. The NPGF-modified electrode shows a linear range (0.1–40 μM) for dopamine detection in the presence of ascorbic acid. The electrochemical measurements of mixtures of dopamine, ascorbic acid, and uric acid in human serum sample for real sample applications was also investigated based on differential pulse voltammetry (DPV) technique. These high sensitivity and selectivity features of the proposed NPGF biosensor offer great promise for real sample biosensor application.     

AFM study of the adsorption of pyrrole and formation of the polypyrrole film on gold surface

Pyrrole (Py) adsorption and following electropolymerisation processes onto partially atomically flat Au (111) surfaces from aqueous solution of 0.1 M Py plus 0.1 M LiClO₄ have been investigated by non-contact atomic force microscopy. The adsorbed layers were examined, firstly in-situ, in solution of Py plus electrolyte and in pure electrolyte, and secondly ex-situ, in dried condition in air atmosphere. AFM images show clearly that in all cases the surface of the Au (111) electrode is covered with polymolecular adsorbed layer of Py. Electropolymerisation of the adsorbed Py layer causes some typical changes in the nanostructure of the layer: relatively larger nuclei disappear and honeycomb-like structures and ensembles of the middle size nuclei appear.