Electric conductance of metal nanowires at mechanically controllable break junctions under electrochemical potential control

We have developed the mechanically controllable break junction setup with an electrochemical cell (EC-MCBJ) to measure the electric conductance of metal nanowires under electrochemical potential control. The electric conductance of Au nanowires was investigated in 0.1 M Na₂SO₄ solution using EC-MCBJ. The conductance of the Au nanowires was quantized in units of G₀ (=2e²/h), showing clear features in the conductance histogram. The atomic contact with a specific conductance value was kept for >5 s, indicating the relatively high stability of the present EC-MCBJ system.     

Mechanical fabrication of metal nano-contacts showing conductance quantization under electrochemical potential control

We have mechanically fabricated Ni and Cu nano-constrictions in solution to study their quantized conductance behavior under electrochemical potential control. Conductance quantization was observed at both metals in solution at room temperature for the first time. The conductance of Cu nano-constriction was quantized in units of G₀(=2e²/h). A sharp 1G₀ peak was observed in the conductance histogram. For Ni, a rather broad peak at 1–1.5G₀ was observed in the histogram. The conductance quantization behavior was discussed by comparing previously documented results of nano-constrictions fabricated in air or ultra-high vacuum conditions, with those fabricated in solution.     

Fractional conductance quantization in metallic nanoconstrictions under electrochemical potential control

We study the electrical conductance of gold nanoconstrictions by controlling the electrochemical potential. At positive potentials, the conductance is quantized near integer multiples of G0(2e(2)/h) as shown by well-defined peaks in the conductance histogram. Below a certain potential, however, additional peaks near 0.5G(0) and 1. 5G(0) appear in the histogram. The fractional conductance steps are as stable and well defined as the integer steps. The experimental data are discussed in terms of electrochemical-potential-induced defect scattering and Fermi energy shift, but a complete theory of the phenomenon is yet to be developed.     

Sulfidation of a Cu submonolayer at the Au(1 1 1)/electrolyte interface - An in situ STM study

We describe the electrochemical preparation of an ultrathin copper sulfide film on Au(1 1 1) and its structural characterization by in situ STM. The first step, underpotential deposition of a Cu submonolayer from CuSO₄/H₂SO₄ solution, is followed by two electrolyte exchanges for (i) Cu-free (blank) H₂SO₄ solution and (ii) NaOH/Na₂S solution. The well-known (√3 × √3)R30° structure of the upd Cu layer is stable in the blank electrolyte for at least 2 h. After exposure to bisulfide, the Cu layer contracts and forms two-dimensional islands of two distinct ordered surface phases, i.e. a rectangular and, at higher potentials, a hexagonal phase, with Cu-free Au(1 1 1) regions between them, the latter exhibiting the characteristic (√3 × √3)R30°-S adlayer structure. Potential changes lead to a complex phase behaviour including HS⁻ ? S_x oxidation/reduction and, at strongly anodic potentials, dissolution of the Cu adlayer.     

Heterogeneous adsorption and catalytic oxidation of benzene, toluene and xylene over spent and chemically regenerated platinum catalyst supported on activated carbon

The heterogeneous adsorption and catalytic oxidation of benzene, toluene and o-xylene (BTX) over the spent platinum catalyst supported on activated carbon (Pt/AC) as well as the chemically treated spent catalysts were studied to understand their catalytic and adsorption activities. Sulfuric aqueous acid solution (0.1N, H₂SO₄) was used to regenerate the spent Pt/AC catalyst. The physico-chemical properties of the catalysts in the spent and chemically treated states were analyzed by using nitrogen adsorption-desorption isotherm and elemental analysis (EDX). The gravimetric adsorption and the light-off curve analysis were employed to study the BTX adsorption and oxidation on the spent catalyst and its modified Pt/AC catalysts. The experimental results indicate that the spent Pt/AC catalyst treated with the H₂SO₄ aqueous solution has a higher toluene adsorption and conversion ability than that of the spent Pt/AC catalyst. A further studies of H₂SO₄ treated Pt/AC catalyst on their catalytic and heterogeneous adsorption behaviours for BTX revealed that the activity of the H₂SO₄ treated Pt/AC catalyst follows the sequence of benzene > toluene > o-xylene. The adsorption equilibrium isotherms of BTX on the H₂SO₄ treated Pt/AC were measured at different temperatures ranging from 120 to 180 °C. To correlate the equilibrium data and evaluate their adsorption affinity for BTX, the two sites localized Langmuir (L2m) isotherm model was employed. The heterogeneous surface feature of the H₂SO₄ treated Pt/AC was described in detail with the information obtained from the results of isosteric enthalpy of adsorption and adsorption energy distributions. Furthermore, the activity of H₂SO₄ treated Pt/AC about BTX was found to be directly related to the Henry's constant, isosteric enthalpy of adsorption and adsorption energy distribution functions.     

Characterization of oxide layers formed on electrochemically treated Ti by using soft X-ray absorption measurements

The oxide layers of electrolytic oxidized titanium (Ti) were characterized using Ti L_2,3 and O K edge X-ray absorption. The spectra show that the structure of the oxide layers that are formed during a 1 min treatment are dependent on the concentration of the electrolyte (H₂SO₄ or Na₂SO₄) with which the Ti surface was treated, and also on the magnitude of the potential that was applied during the anodic oxidation process (100 V or 150 V). It is found that a potential of 150 V and an electrolyte concentration of 0.5 M or 1.0 M produces a layer of TiO₂ having rutile crystal structure.     

Electrochemical fabrication and characterization of nanocontacts and nm-sized gaps

Copper nanocontacts and molecular-sized nanogaps were prepared and characterized at electrified solid/liquid interfaces employing lithographic and electrochemical techniques. A dedicated four-electrode potentiostat was developed for controlling the electrochemical fabrication process and for monitoring the electrical characteristics of the nanostructures created. The formation and breaking of Cu nanocontacts exhibits conductance quantization characteristics. The statistical analysis of conductance histograms revealed a preferential stability of nanocontacts with integer values of G₀, with a clear preference for 1 G₀, 2 G₀ and 3 G₀. The growth of molecular-sized gaps shows quantized tunneling current, which is attributed to the discrete nature of Cu atoms, water molecules, and specifically adsorbed ions. PACS 73.23Ad; 73.63.Rt; 82.45.Yz; 85.35.-p     

Site-blocking effects of preadsorbed H on Pt(1 1 1) probed by 1,3-butadiene adsorption and reaction

The influence of hydrogen coadsorption on hydrocarbon chemistry on transition metal surfaces is a key aspect to an improved understanding of catalytic selective hydrogenation. We have investigated the effects of H preadsorption on adsorption and reaction of 1,3-butadiene (H₂CCHCHCH₂, C₄H₆) on Pt(1 1 1) surfaces by using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Preadsorbed hydrogen adatoms decrease the amount of 1,3-butadiene chemisorbed on the surface and chemisorption is completely blocked by the hydrogen monolayer (saturation) coverage (θ_H = 0.92 ML). No hydrogenation products of reactions between coadsorbed H adatoms and 1,3-butadiene were observed to desorb in TPD experiments over the range of θ_H investigated (θ_H = 0.6-0.9 ML). This is in strong contrast to the copious evolution of ethane (CH₃CH₃, C₂H₆) from coadsorbed hydrogen and ethylene (CH₂CH₂, C₂H₄) on Pt(1 1 1). Hydrogen adatoms effectively (in a 1:1 stoichiometry) remove sites from interaction with chemisorbed 1,3-butadiene, but do not affect adjacent sites. The adsorption energy of coadsorbed 1,3-butadiene is not affected by the presence of hydrogen on Pt(1 1 1). The chemisorbed 1,3-butadiene on hydrogen preadsorbed Pt(1 1 1) completely dehydrogenates to H₂ and surface carbon upon heating without any molecular desorption detected, which is identical to that observed on clean Pt(1 1 1). In addition to revealing aspects of site blocking that should have broad implications for hydrogen coadsorption with hydrocarbon molecules on transition metal surfaces in general, these results also provide additional basic information on the surface science of selective catalytic hydrogenation of butadiene in butadiene-butene mixtures.     

Influence of an electrostatic potential at the metal/electrolyte interface on the electron binding energy of adsorbates as probed by X-ray photoelectron spectroscopy

We present a study of the influence of an electrostatic potential across the Au(1 1 1)/4,4′-dithiodipyridine (PySSPy)-SAM/0.01 M NaOH interface on the electron binding energies of the N 1s and S 2p orbitals of the adsorbate, as probed by X-ray photoelectron spectroscopy (XPS). The observed systematic binding energy shift for N(1s) with electrode potential of −0.9 eV/V has been assigned to a so-called electrochemical shift, i.e., an apparent shift of the binding energy due to the potential drop in the electric double layer. No systematical shift was observed for the S(2p) core level of the thiol with electrode potential. The results are contrasted by measurements in 0.1 M H₂SO₄, where no electrochemical shift was found for the protonated nitrogen.     

Berberine as a natural source inhibitor for mild steel in 1 M H2SO4

Berberine was abstracted from coptis chinensis and its inhibition efficiency on corrosion of mild steel in 1 M H₂SO₄ was investigated through weight loss experiment, electrochemical techniques and scanning electronic microscope (SEM) with energy disperse spectrometer (EDS). The weight loss results showed that berberine is an excellent corrosion inhibitor for mild steel immersed in 1 M H₂SO₄. Potentiodynamic curves suggested that berberine suppressed both cathodic and anodic processes for its concentrations higher than 1.0 × 10⁻⁴ M and mainly cathodic reaction was suppressed for lower concentrations. The Nyquist diagrams of impedance for mild steel in 1 M H₂SO₄ containing berberine with different concentrations showed one capacitive loop, and the polarization resistance increased with the inhibitor concentration rising. A good fit to Flory-Huggins isotherm was obtained between surface coverage degree and inhibitor concentration. The surface morphology and EDS analysis for mild steel specimens in sulfuric acid in the absence and presence of the inhibitor also proved the results obtained by the weight loss and electrochemical experiments. The correlation of inhibition effect and molecular structure of berberine was then discussed by quantum chemistry study.     

Bias-dependence of the conductance of Au nanocontacts at 4 K

The bias-dependence of the quantized conductance in Au nanocontacts has been measured at liquid He temperature. A well-defined 1G₀ peak (G₀=2e²/h is the conductance quantum unit) appears in the conductance histogram. With increasing the bias, the 1G₀ peak decreases in height, while its position remains unshifted. This behavior of the 1G₀ peak is just the same as that observed at room temperature [H. Yasuda, A. Sakai, Phys. Rev. B 56 (1997) 1069]. The critical bias V_bc at which the 1G₀ peak disappears is ∼2.2 V, which is not much different from V_bc at 300 and 77 K. This weak temperature dependence of V_bc is compatible with high-current contact instability due to electromigration of contact atoms.     

Structures and magnetic properties for electrodeposited Co ultrathin films on copper

The formation of Co films on polycrystalline copper in diluted sulphuric acid was investigated by employing cyclic voltammetry (CV), atomic force microscopy, and in-situ magneto-optic Kerr effect (MOKE) techniques. By comparing CV measurements in the pure supporting electrolyte (11 mM K₂SO₄/1 mM H₂SO₄) and the cobalt sulphate solution (10 mM K₂SO₄/1 mM H₂SO₄/1 mM CoSO₄), peaks from voltammetric cycling for copper dissolution, readsorption of dissolved copper ions, cobalt bulk dissolution and oxidation of hydrogen could be resolved. As the electroplating time increases, the size of the Co clusters increases and the deposition of Co corresponds to island growth. The first hysteresis loop occurs at a Co thickness of 0.33 nm in the longitudinal configuration. For films thinner than 7 nm, the Kerr intensity increases linearly because the Curie temperature of the film is well above 300 K.     

Mechanisms of radical removal by SO2

It is well established from experiments in premixed, laminar flames, jet-stirred reactors, flow reactors, and batch reactors that SO₂ acts to catalyze hydrogen atom removal at stoichiometric and reducing conditions. However, the commonly accepted mechanism for radical removal, SO₂ + H(+M) ? HOSO(+M), HOSO + H/OH ? SO₂ + H₂/H₂O, has been challenged by recent theoretical and experimental results. Based on ab initio calculations for key reactions, we update the kinetic model for this chemistry and re-examine the mechanism of fuel/SO₂ interactions. We find that the interaction of SO₂ with the radical pool is more complex than previously assumed, involving HOSO and SO, as well as, at high temperatures also HSO, SH, and S. The revised mechanism with a high rate constant for H + SO₂ recombination and with SO + H₂O, rather than SO₂ + H₂, as major products of the HOSO + H reaction is in agreement with a range of experimental results from batch and flow reactors, as well as laminar flames.     

Structural, vibrational and dielectric properties of new potassium hydrogen sulfate arsenate: K4(SO4)(HSO4)2(H3AsO4)

A new compound, K₄(SO₄)(HSO₄)₂(H₃AsO₄) was synthesized from water solution of KHSO₄/K₃H(SO₄)₂/H₃AsO₄. This compound crystallizes in the triclinic system with space group P1¯ and cell parameters: a=8.9076(2) Å, b=10.1258(2) Å, c=10.6785(3) Å; α=72.5250(14)°, β=66.3990(13)°, γ=65.5159(13)°, V=792.74(3) Å³, Z=2 and ρ_cal=2.466 g cm⁻³. The refinement of 3760 observed reflections (I>2σ(I)) leads to R₁=0.0394 and wR₂=0.0755. The structure is characterized by SO₄²⁻, HSO₄⁻ and H₃AsO₄ tetrahedra connected by hydrogen bridge to form two types of dimer (H(16)S(3)O₄⁻?S(1)O₄²⁻ and H(12)S(2)O₄⁻?H₃AsO₄). These dimers are interconnected along the [1¯ 1 0] direction by the hydrogen bonds O(3)-H(3)?O(6). They are also linked by the hydrogen bridge assured by the hydrogen atoms H(2), H(3) and H(4) of the H₃AsO₄ group to build the chain S(1)O₄?H₃AsO₄ which are parallel to the “a” direction. The potassium cations are coordinated by eight oxygen atoms with K-O distance ranging from 2.678(2) to 3.354(2) Å.Crystals of K₄(SO₄)(HSO₄)₂(H₃AsO₄) undergo one endothermic peak at 436 K. This transition detected by differential scanning calorimetry (DSC) is also analyzed by dielectric and conductivity measurements using the impedance spectroscopy techniques. The obtained results show that this transition is protonic by nature.     

Study of the hydrogen diffusion and segregation into Fe-C-Mo martensitic HSLA steel using electrochemical permeation test

Diffusion and trapping mechanisms are studied in order to improve Hydrogen embrittlement (HE) resistance of high yield strength steels. Investigations were carried on Fe-C-Mo model steel with a quenched and tempered martensitic microstructure. Hydrogen diffusion was studied by using the electrochemical permeation technique. The influence of the charging current densities in 1 M H₂SO₄ at ambient temperature shows a relation between the apparent diffusion coefficient D_app and the apparent subsurface concentration of hydrogen C_0app. Two domains can be separated and are mainly associated with a competition between two distinct processes: hydrogen trapping and hydrogen diffusion. These results are correlated to the quantities of reversible and irreversible traps into the membrane. Moreover, the experimental results revealed that the apparent diffusion coefficient increases and the total amount of trapped hydrogen decreases with temperature. The activation energy of the diffusion process (0.26 eV) and the trapping process (0.58 eV) are supposed to be, respectively, affiliated with lattice diffusion and with trapping on incidental dislocations and/or on martensitic lath interfaces due to misorientations (geometric necessary dislocations).     

Diffusion-limited electrodeposition of ultrathin Au films on Pt(1 1 1)

The electrodeposition of Au on Pt(1 1 1) from electrolytes containing μM concentrations of was studied by in situ scanning tunneling microscopy. Under these conditions the Au flux is limited by diffusion in the electrolyte over a wide potential range, which allows to assess the effect of the electrochemical environment on the growth kinetics. Similar to gas phase metal deposition Au film growth proceeds via nucleation and lateral growth of Au monolayer islands, with the saturation island density strongly depending on the deposition potential and on the anion species in the electrolyte. For deposition in H₂SO₄ solution the saturation island density continuously increases with increasing potential between −0.2 and 0.5 V (SCE), whereas in Cl-containing H₂SO₄ it first decreases and then increases again. Following nucleation and growth theories this behavior can be attributed to potential-induced changes of the Au surface mobility, caused by changes in the density and structure of coadsorbed sulfate/bisulfate and chloride adlayers. Under conditions of high Au surface mobility multilayer growth proceeds via a typical Stranski-Krastanov growth mode, with layer-by-layer growth of a pseudomorphic Au film up to 2 ML and 3D growth of structurally relaxed islands at higher coverage, indicating thermodynamic control under these conditions.     

New quasi-one-dimensional tetracyanidoplatinate,Cs4[Pt(CN)4](CF3SO3)2: Synthesis,structure, and physical characterization

The synthesis and some physical properties of a new quasi-one-dimensional tetracyanidoplatinate, Cs₄[Pt(CN)₄](CF₃SO₃)₂ (CsCP(OTf)) are reported and described in comparison to the well-known K₂[Pt(CN)₄]Br_0.30·3.2H₂O (KCP). Single-crystal X-ray diffraction reveals Pt–Pt spacings to be greater than those of KCP by 5% longitudinal and 38% transverse, but much shorter than comparable spacings in other non-partially oxidized platinates. Anomalies are observed between temperatures 100 K and 200 K: (1) Longitudinal DC conductivity is two orders of magnitude higher and is non-monotonic with temperature, showing a minimum at around 170 K. (2) Nuclear magnetic resonance (NMR) longitudinal relaxation time T₁ is at least three orders of magnitude higher than that of KCP, and is also non-monotonic with temperature, showing a sharp peak at around 120 K. Since X-ray diffraction reveals no structural transition at 120 K, these suggest a possible lattice freezing or stiffening at around 120 K.     

Luminescence investigations on sulfate apatite Na6(SO4)2FCl:RE (RE=Dy, Ce or Eu) phosphors

A new phosphor in the Cl-F system doped with Dy, Ce and Eu has been reported. Characterization of this phosphor using XRD, PL and TL techniques is described. Polycrystalline Na₆(SO₄)₂FCl:Dy; Na₆(SO₄)₂FCl:Ce and Na₆(SO₄)₂FCl:Eu phosphors prepared by a solid state diffusion method have been studied for their X-ray diffraction, photoluminescence (PL) and thermoluminescence (TL)characteristics. The PL excitation and emission spectra of phosphors were obtained. Dy³⁺ emission in the host at 475 and 570 nm is observed due to ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transition, respectively, whereas the PL emission spectra of Na₆(SO₄)₂FCl:Ce phosphor shows the Ce³⁺ emission at 322 nm due to 5d→4f transition of Ce³⁺ ion. In Na₆(SO₄)₂FCl:Eu lattice, Eu²⁺ as well as Eu³⁺ emissions are observed. The emission of europium ion in this compound exhibits the blue as well as red emission. The TL glow curves of the same compounds have the simple structure with a prominent peak at 150, 175 and 200 °C. TL response, fading, reusability and trapping parameters of the phosphors are also studied. The TL glow curves of γ-irradiated Na₆(SO₄)₂FCl sample show one glow peak indicating that only one set of traps is being activated within the particular temperature range each with its own value of activation energy (E) and frequency factor (s). The trapping parameters associated with the prominent glow peak are calculated using Chen’s half width method. The release of hole/electron from defect centers at the characteristic trap site initiates the luminescence process in these materials. The intensity of the TL glow peaks increases with increase of the added γ-ray dose to the samples.     

Effect of Gd2O3 on the hydrogen evolution property of nickel–cobalt coatings electrodeposited on titanium substrate

The effect of rare earth compound, Gd₂O₃, on the microstructure and the hydrogen evolution property of Ni–Co alloy electrode was studied. The morphology and microstructure were characterized by SEM and XRD, respectively, and the electrocatalytic efficiency was evaluated on the basis of electrochemical data obtained from steady-state polarization curves, Tafel curves and electrochemical impedance spectroscopy measurements carried out in 0.50 M Na₂SO₄+0.10 M H₂SO₄ solution. It was found that the embedded Gd₂O₃ particles largely enhanced the electrocatalytic activity of Ni–Co alloy electrode to hydrogen evolution reaction.     

Application of Black Pearl carbon-supported WO3 nanostructures as hybrid carriers for electrocatalytic RuSex nanoparticles

RuSe_x electrocatalytic nanoparticles were deposited onto hybrid carriers composed of Black Pearl carbon-supported tungsten oxide; and the resulting system's electrochemical activity was investigated during oxygen reduction reaction. The tungsten oxide-utilizing and RuSe_x nanoparticle-containing materials were characterized using transmission electron microscopy, X-ray diffraction and electrochemical diagnostic techniques such as cyclic voltammetry and rotating ring-disk voltammetry. Application of Black Pearl carbon carriers modified with ultra-thin films of WO₃ as matrices (supports) for RuSe_x catalytic centers results during electroreduction of oxygen in 0.5 mol dm⁻³ H₂SO₄ (under rotating disk voltammetric conditions) in the potential shift of ca. 70 mV towards more positive values relative to the behavior of the analogous WO₃-free system. Also the percent formation (at ring in the rotating ring-disk voltammetry) of the undesirable hydrogen peroxide has been decreased approximately twice by utilizing WO₃-modified carbon carriers. The results are consistent with the bifunctional mechanism in which oxygen reduction is initiated at RuSe_x centers and the hydrogen peroxide intermediate is reductively decomposed at reactive WO₃-modified Black Pearl supports. The electrocatalytic activity of the system utilizing WO₃-modified Black Pearl supports has been basically unchanged upon addition of acetic acid, formic acid or methyl formate to the sulfuric acid supporting electrolyte.