Effect of surface roughness on diffusion-limited charge transfer

A theory is developed for diffusion-limited charge transfer on a non-fractally rough electrode. The perturbation expressions are obtained for concentration, current density and measured diffusion-limited current for arbitrary one- and two-dimensional surface profiles. The random surface model is employed for a rough electrode\electrolyte interface. In this model the gross geometrical property of an electrochemically active rough surface - the surface structure factor-is related to the average electrode current, current density and concentration. Under short and long time regimes, various morphological features of the rough electrodes, i.e. excess area (related to roughness slope), curvature, correlation length, etc. are related to the (average) current transients. A two-point Pade approximant is used to develop an all time average current expression in terms of partial morphological features of the rough surface. The inverse problem of predicting the surface structure factor from the observed transients is also described. Finally, the effect of surface roughness is studied for specific surface statistics, namely a Gaussian correlation function. It is shown how the surface roughness enhances the overall diffusion-limited charge transfer current.

Surface barrier effects in non-resonant microwave absorption by superconducting thin films of YBa2Cu3O7??

We present an unusual temperature dependence of hysteresis in the Lion resonant microwave absorption (NRMA) signals from superconducting thin films of YBa2Cu3O7-delta. We observe that the hysteresis increases with increase in temperature till T-c which we interpret as evidence for the presence of Bean-Livingston surface barriers (BLSB) in the single crystalline films.

Temperature-programmed desorption and surface reaction of thiophene over co-mo/.gamma.-Al2O3 hydrodesulfurization catalysts

The temperature-programmed desorption (TPD) and temperature-programmed surface reaction (TPSR) of thiophene over a series of Co-Mo/gamma-Al2O3, hydrodesulfurization (HDS) catalysts with varying Co to Mo ratios have been studied with the objective of understanding the promotional role of Co in the HDS reaction. As part of the study, the desorptions (TPD) and hydrogenations (TPSR) of butane, butene, and butadiene over these catalysts have also been investigated. The TPD of the hydrocarbons over catalysts containing no Co showed a single desorption profile while incorporation of Co created an additional site, with higher heats of desorption, without significantly affecting desorption from the original site. The TPSR measurements showed that the two sites had separate and independent activity for the hydrogenation of the C-4 hydrocarbons. The TPD of thiophene over catalysts with varying Co to Mo ratios showed a single desorption profile with identical heats of desorption, implying that Co does not affect or influence the adsorption sites for thiophene. The TPSR of the HDS of thiophene, however, showed that, although the products of the HDS reaction-butane, butene, and H2S-are the same irrespective of the Co content, the temperature profiles and the activation barriers for the formation of these species show considerable change with the Co/Co+Mo ratio. The results are discussed in light of the existing models for the promotional role of Co in the HDS reaction.

Methanol-to-gasoline (MTG) conversion over ZSM-5. A temperature programmed surface reaction study

The conversion of methanol to gasoline over zeolite ZSM-5 has been studied by temperature programmed surface reaction (TPSR). The technique is able to monitor the two steps in the process: the dehydration of methanol to dimethyl ether and the subsequent conversion of dimethyl ether to hydrocarbons. The activation barriers associated with each step were evaluated from the TPSR profiles and are 25.7 and 46.5 kcal/mol respectively. The methanol desorption profile shows considerable change with the amount of methanol molecules adsorbed per Bronsted site of the zeolite. The energy associated with the desorption process, (CH3OHH+-ZSM5 --> (CH3OHH+-ZSM5 + CH3OH, shows a spectrum of values depending on n.

Crystal Structure Engineering by Fine-Tuning the Surface Energy: The Case of CdE (E = S/Se) Nanocrystals

We prove that CdS nanocrystals can be thermodynamically stabilized in both wurtzite and zinc-blende crystallographic phases at will, just by the proper choice of the capping ligand. As a striking demonstration of this, the largest CdS nanocrystals (similar to 15 nm diameter) ever formed with the zinc-blende structure have been synthesized at a high reaction temperature of 310 degrees C, in contrast to previous reports suggesting the formation of zinc-blende CdS only in the small size limit (< 4.5 nm) or at a lower reaction temperature (<= 240 degrees C). Theoretical analysis establishes that the binding energy of trioctylphosphine molecules on the (001) surface of zinc-blende CdS is significantly larger than that for any of the wurtzite planes. Consequently, trioctylphosphine as a capping agent stabilizes the zinc-blende phase via influencing the surface energy that plays an important role in the overall energetics of a nanocrystal. Besides achieving giant zinc-blende CdS nanocrystals, this new understanding allows us to prepare CdSe and CdSe/CdS core/shell nanocrystals in the zinc-blende structure.

Nature of the Oxygen Species at Ni(110) and Ni(100) Surfaces Revealed by Exposure to Oxygen and Oxygen-Ammonia Mixtures: Evidence for the Surface Reactivity of O- Type Species

Adsorption of dioxygen at clean Ni(110) and Ni(100) surfaces gives rise to two prominent features in the O(1s) spectra at 530 and 531 eV due to O2- and O- type species, respectively. Interaction of ammonia with a Ni(100)-O surface where theta(oxygen) < 0.1 ML favors the dissociation of NH3 giving NHn, (n = 1, 2) and N(a) species. This is accompanied by a decrease in the intensity of the 531 eV feature. On the other hand. a Ni(100)-O surface where the oxygen species are mainly of the O2- type is unreactive, Coadsorption studies of NH3-O-2 mixtures show that at Ni(110) surfaces the uptake of both oxygen and ammonia increase with the proportion of oxygen in the NH3-O-2 mixture. The surface concentrations of the O- species and the NHn species also increase with the increase in the O-2/NH3 ratio while the slope of the plot of sigma(N) versus sigma(O-) is around unity. The results demonstrate the high surface reactivity of the O- species and its role in the dissociation of ammonia. Based on these observations, the possibility of the formation of a surface complex between ammonia and oxygen (specifically O-) is suggested. Results from vibrational spectroscopic studies of the coadsorption of NH3-O-2 mixtures are consistent with those from core-level spectroscopic studies.

Nonresonant microwave absorption studies of surface passivation of superconducting YBa2Cu3O7−δ thin films

The surfaces of laser ablated thin films of YBa2Cu3O7?? have been passivated with about 100 Å thick textured layer of Ca0.95Sr0.025Ba0.025Zr0.98Ta0.01Ti0.01O3. It is shown that this low loss dielectric material preserves the quality of the surface and also prolongs the aging process. The films (both passivated and as?deposited) have been studied for degradation on exposure to atmosphere and also on dipping directly in water. The technique of nonresonant microwave absorption is used to study the effects and extent of degradation in these films. © 1995 American Institute of Physics.

Inverse surface melting in confined clusters: Ar-13 in zeolite L

Monte Carlo and molecular dynamics simulations on an Ar-13 cluster in zeolite L have been carried out at a series of temperatures to understand the rigid-nonrigid transition corresponding to the solid-liquid transition exhibited by the free Ar-13 cluster. The icosahedral geometry of the free cluster is no longer preferred when the cluster is confined in the zeolite. The root-mean-squared pair distance fluctuation, delta, exhibits a sharp, well-defined rigid-nonrigid transition at 17 K as compared to 27 K for the free cluster. Multiple peaks in the distribution of short-time averages of the guest-host interaction energy indicate coexistence of two phases.; It is shown that this transition is associated with the inner atoms becoming mobile at 17 K even while the outer layer atoms, which are in close proximity to the zeolitic wall, continue to be comparatively immobile. This may be contrasted with the melting of large free clusters of 40 or more atoms which exhibit surface melting. Guest-host interactions seem to play a predominant role in determining the properties of confined clusters. We demonstrate that the volume of the cluster increases rather sharply at 17 and 27 K respectively for the confined and the free cluster. Power spectra suggest that the motion of the inner atoms is generally parallel to the atoms which form the cage wall.

Role of hydrophobic effect and surface charge in surfactant-DNA association

Ethidium bromide is one of the best known DNA intercalator. Upon intercalation inside DNA, the fluorescence due to ethidium bromide gets enhanced by many orders of magnitude. In this paper, we employed ethidium bromide as a probe for studying surfactant-DNA complexation using fluorescence spectroscopy and agarose gel electrophoresis. Surfactants of different charge types and chain lengths were used and the results were compared with that of the related small organic cations or salts under comparable conditions. The cationic surfactants induced destabilization of the ethidium bromide-DNA complex at concentrations in orders of magnitude lower than that of the small organic cations or salts. In contrast however, the anionic surfactants failed to promote any such destabilization of probe-DNA complex. DNA loses its ethidium bromide stainability in the presence of high concentration of cationic surfactant aggregates as revealed from agarose gel electrophoresis experiments. Inclusion of surfactants and other additives into the DNA generally enhanced the DNA double-strand to single strand transition melting temperatures by a few degrees, in a concentration-dependent manner and at high surfactant concentration melting profiles got broadened.

Thiophene adsorption studies on clean and hydrogen-preadsorbed MoS2(100) surface

It is known from temperature-programmed desorption studies that the binding energy of thiophene over Mo/gamma-Al2O3 and Co-Mo/gamma-Al2O3, hydrodesulfurization catalysts, is lower in the presence of hydrogen. The adsorption of thiophene on clean and hydrogen-adsorbed MoS2 was modelled using extended Huckel tight binding band structure calculations. In the eta(1) adsorption configuration the calculations show a lower binding energy for adsorption on the hydrogen-preadsorbed surface similar to that observed experimentally. The lowering is due to an increased occupancy of the Mo density of states in the presence of hydrogen.

Guest-Assisted Self-assembly of Organostannoxane Nanotubules on a Mica Surface

The reaction of n-BuSn(O)OH](n), and 9-hydroxy-9-fluorenecarboxylic acid in the presence of p-X-C6H4-OH (X = F, Br) afforded hydroxyl-rich hexameric organostannoxane prismanes. The crystal structures of these prismanes reveal guest-assisted supramolecular structures. Self-assembly of these compounds on a mica surface affords organooxotin nanotubules.

Hydrogen-bond dynamics near a micellar surface: Origin of the universal slow relaxation at complex aqueous interfaces

The dynamics of hydrogen bonds among water molecules themselves and with the polar head groups (PHG) at a micellar surface have been investigated by long molecular dynamics simulations. The lifetime of the hydrogen bond between a PHG and a water molecule is found to be much longer than that between any two water molecules, and is likely to be a general feature of hydrophilic surfaces of organized assemblies. Analyses of individual water trajectories suggest that water molecules can remain bound to the micellar surface for more than 100 ps. The activation energy for such a transition from the bound to a free state for the water molecules is estimated to be about 3.5 kcal/mol.

Renewable surface electrodes based on dopamine functionalized exfoliated graphite: NADH oxidation and ethanol biosensing

Exfoliated graphite (EG) was modified by covalently attaching dopamine (DA) (3,4-dihydroxyphenethylamine) through amide linkages, using -COOH groups introduced on the EG surface. The modified material was characterized by FT-IR spectroscopy, Xray photoelectron spectroscopy and electrochemical techniques. Composites of DA modified EG dispersed in organically modified silicates were prepared by a sol-get process. Electrodes were fabricated by casting the composites in glass tubes. The sol-gel based electrodes were found to be active for the electrocatalytic oxidation of NADH and biosensing of ethanol in presence of NAD(+) and alcohol dehydrogenase enzyme. The modified composite electrodes were found to be stable for several months. The surface of the electrode could be renewed just by mechanically polishing the electrode using emery sheets. The modified EG was also pressed and restacked in the form of a pellet and the use of this material as a binderless bulk-modified electrode was also demonstrated. The performance of sol-gel derived composite EG electrodes with binderless bulk-modified EG electrodes was compared. (C) 2002 Elsevier Science B.V. All rights reserved.

Slow orientational dynamics of water molecules at a micellar surface

The dynamics of water molecules near an aqueous micellar interface is studied in an atomistic molecular dynamics simulation of cesium pentadecafluorooctanoate (CsPFO) in water. The dipolar orientational time correlation function (tcf) and the translational diffusion of the water molecules are investigated. Results show that both the reorientational and the translational motion of water molecules near the micelle are restricted. In particular, the orientational tcf exhibits a very slow component in the long time which is slower than its bulk value by 2 orders of magnitude. This slow decay seems to be related to the slow decay often observed in experiments. The origin of the slow decay is analyzed.