Mechanism of oxygen electroreduction on gold surfaces in basic media

The mechanism of the electroreduction of oxygen on Au surfaces in basic media is examined using surface-enhanced Raman scattering (SERS) measurements and density functional theory (DFT) calculations. The spectroscopy reveals superoxide species as a reduction intermediate throughout the oxygen electroreduction, while no peroxide is detected. The spectroscopy also shows the presence of superoxide after the addition of hydrogen peroxide. The calculations show no effect of OH addition to the Au(100) surface with regard to O-O length. These results suggest that the four-electron reduction of O(2) on Au(100) in base arises from a disproportionation mechanism which is enhanced on Au(100) relative to the other two low Miller index faces of Au.     

Dioxygen and hydrogen peroxide reduction with hemocyanin model complexes

Three copper polypyridyl complexes were examined as electrocatalysts for the oxygen reduction reaction (ORR): a Cu-N(3) complex, [Cu-[tris(6-methylpyridin-2-yl)methane]-(NCMe)]PF(6) (1); a related Cu(2)N(6) derivative, [Cu(2)-[1,2-bis(6-(bis(6-methylpyridin-2-yl)methyl)pyridin-2-yl)ethane]-(NCMe)(2)](PF(6))(2) (2); and the CuN(4) species [Cu-[tris(pyridin-2-ylmethyl)amine]](ClO(4))(2) [3](ClO(4))(2). Compared to other copper complexes, [3](ClO(4))(2) exhibits the highest reported ORR onset potential for a Cu complex of 0.53 V vs reversible hydrogen electrode at pH 1. The Cu(2)N(6) hemocyanin model is more active than the CuN(3), but both are less active than the CuN(4) complex. The results indicate that copper polypyridyl complexes are promising cathode catalysts for ORR.     

Nitrate adsorption and reduction on Cu(100) in acidic solution

Nitrate adsorption and reduction on Cu(100) in acidic solution is studied by electrochemical methods, in situ electrochemical scanning tunneling microscopy (EC-STM), surface enhanced Raman spectroscopy (SERS), and density functional theory (DFT) calculations. Electrochemical results show that reduction of nitrate starts at -0.3 V vs Ag/AgCl and reaches maximum value at -0.58 V. Over the entire potential region interrogated adlayers composed of nitrate, nitrite, or other intermediates are observed by using in situ STM. From the open-circuit potential (OCP) to -0.22 V vs Ag|AgCl, the nitrate ion is dominant and forms a (2 x 2) adlattice on the Cu(100) surface while nitrate forms a dominantly c(2 x 2) structure from -0.25 to -0.36 V. The interconversion between the nitrate and nitrite adlattices is observed. DFT calculations indicate that both nitrate and nitrite are twofold coordinated to the Cu(100) surface.     

Model Ge microstructures as anodes for Li-ion batteries

We examine the properties of microstructured Ge electrodes for Li-ion battery applications. Model-microfabricated single-crystalline Ge electrode structures are used to investigate the effects of Cu coating and partial discharging on cycle life. Results show that the Ge microstructures insert Li more isotropically than do comparable ones comprised of Si. A model Ge microbar electrode with a Cu coating is capable of 95 % coulombic efficiency after 40 cycles when the amount of charge is limited. The microstructured Ge electrode is found to exhibit poor performance at higher delithiation rates (above C/5) relative to microstructured Si electrodes. These results provide an understanding of the effects of electrochemical processes on model-microstructured Ge electrodes which may ultimately aid in the development of advanced anodes for Li-ion batteries.     

A reconsideration of acoustic invariance for place of articulation in diffuse stop consonants: evidence from a cross-language study

This study explored the claim that invariant acoustic properties corresponding to phonetic features generalize across languages. Experiment I examined whether the same invariant properties can characterize diffuse stop consonants in Malayalam, French, and English. Results showed that, contrary to theoretical predictions, we could not distinguish labials from dentals, nor could we classify dentals and alveolars together in terms of the same invariant properties. We developed an alternative metric based on the change in the distribution of spectral energy from the burst onset to the onset of voicing. This metric classified over 91% of the stops in Malayalam, French, and English. In experiment II, we investigated whether the invariant properties defined by the metric are used by English-speaking listeners in making phonetic decisions for place of articulation. Prototype CV syllables--[b d] in the context of [i e a o u]--were synthesized. The gross shape of the spectrum was manipulated first at the burst onset, then at the onset of voicing, such that the stimulus configuration had the spectral properties prescribed by our metric for labial and dental consonants, while the formant frequencies and transitions were appropriate to the contrasting place of articulation. Results of identification tests showed that listeners were able to perceive place of articulation as a function of the relative distribution of spectral energy specified by the metric.     

Picosecond laser for performance of efficient nonlinear spectroscopy from 10 to 21 microm

Laser tunability from 10 to 21 microm is obtained by use of an optical parametric oscillator based on a KTP crystal followed by a difference-frequency stage with a CdSe crystal. An all-solid-state picosecond Nd:YAG oscillator mode locked by a frequency-doubling nonlinear mirror is used for synchronous pumping.     

Potential dependent organization of water at the electrified metal-liquid interface

In situ infrared visible sum frequency generation spectroscopy (SFG) is used to examine the structure of water at the Ag-water interface in NaF and KF electrolyte solutions. Water is observed in environments associated with both the electrode surface and the diffuse double layer. Peaks are observed that are correlated with low-order water, water interacting with electrolyte ions, specifically adsorbed water to the electrode surface, and hydronium. Spectra obtained from a thiol-modified Ag surface enabled discrimination between surface-bound water and that in the double layer. The water organization is dependent on applied potential, with the observed intensities for specifically adsorbed and ion solvating water diminishing near the pzc.     

Oxygen electroreduction through a superoxide intermediate on bi-modified Au surfaces

The mechanism of the electroreduction of oxygen on bare and Bi-submonolayer-modified Au(111) surfaces is examined using surface enhanced Raman scattering (SERS) measurements along with detailed density functional theory (DFT) calculations. The spectroscopy reveals the presence of superoxide-level species at potentials where oxygen is reduced. These species are not present in solutions absent either oxygen or Bi at these potentials. The spectroscopy also reveals the presence of Bi-OH species which are associated with peroxide reduction. Detailed calculations show oxygen associates much more strongly with Bi in the (2 x 2) configuration on Au(111) relative to the bare Au surface. Additionally, the O-O bond is elongated following O2 association, which follows as a consequence of Bi-O bond formation and partial oxidation of the Bi adatom. These results show for the first time that the four-electron electroreduction of oxygen electroreduction occurs via a series pathway on the Bi-modified surface in acid solution.     

Peroxide electroreduction on bi-modified Au surfaces: vibrational spectroscopy and density functional calculations

The mechanism of the electroreduction of peroxide on Bi-submonolayer-modified Au(111) surfaces is examined using surface-enhanced Raman scattering (SERS) measurements along with detailed density functional theory (DFT) calculations. The spectroscopy shows the presence of Bi-OH and Bi-O species at potentials just positive of that where peroxide is reduced. These species are not present in solutions absent either peroxide or Bi. DFT calculations show that peroxide is unstable relative to Bi-OH when Bi is present in the (2 x 2) configuration on Au(111) known from previous work to be catalytically active. The spacing between Bi adatoms is such that peroxide association with two Bi cannot occur without O-O bond cleavage. The full Bi monolayer is catalytically inactive and exhibits none of the Bi-OH or Bi-O signals seen for the active surface. The calculations show that as the Bi coverage becomes greater and the Bi adatom spacing becomes smaller, peroxide can adsorb on Bi without O-O bond rupture. These results indicate an important role for M-OH species in peroxide electroreduction.     

AFM force measurements between SAM-modified tip and SAM-modified substrate in alkaline solution

The reversible desorption and adsorption of ethanethiol (ET) and hexadecane thiol (HDT) self-assembled monolayers (SAMs) on gold substrates are addressed with potential-dependent AFM force measurements where both tip and substrate potentials are controlled independently. For HDT-modified tip and substrate, the potential dependence of the force curve corresponds to the observed voltammetric features. The adhesion interaction between HDT-modified tip and substrate exhibits a large adhesion, whereas the adhesion is reduced to one-quarter of its original value after HDT on the substrate is removed. The presence of both attractive features on the approach curve and large adhesion on retraction after thiol desorption are ascribed to micelle formation from the desorbed, insoluble, thiols above the Au surface. For the ET-modified tip and substrate, the force curve evinces time-dependent recovery after the thiol adsorption peak which arises from the finite time of diffusion of the desorbed thiol back to the substrate. However, the force curves exhibit little potential dependence when the ET-desorbed tip is interacted with ET-modified substrate.