John F. Evans

EC catalysis of ascorbic acid oxidation using plasma polymerized vinylferrocene film electrodes

Abstract The use of radio frequency plasma discharges of neat vinylferrocene to chemically modify pyrolytic graphite electrode surfaces is discussed. The plasma process yields a stable coating of electroactive polymer, the thickness of which may be varied by controlling the duration of the plasma discharge reaction. X-ray photoelectron spectroscopy has been utilized to characterize the modified electrode surfaces, and the results indicate that the surface composition varies as a function of placement of the substrate in the plasma reactor. The electrochemical behavior of the plasma polymerized vinylferrocene films has been investigated by cyclic voltammetry. The voltammograms show marked differences in charge transport kinetics in aqueous vs. nonaqueous (acetonitrile) media. The plasma-deposited films are shown to be electrocatalytically active in the oxidation of ascorbic acid in aqueous acidic solutions. The implications of charge transport, counter ion transport, and substrate (ascorbic acid) transport kinetics with respect to the efficiency of the electrocatalytic reaction are also discussed.

Kinetic and mechanistic study of the chemical vapor deposition of titanium dioxide thin films using tetrakis-(isopropoxo)-titanium(IV)

The mechanism of TiO2thin film deposition on single crystal TiO2 by chemical vapor deposition(CVD) using tetrakis(isopropoxo)titanium(IV) (TTIP) has been investigated. The structure of the rutile(100) surface has been shown to form a 1×3 reconstruction using reflection high‐energy electron diffraction. Temperature programmed reaction spectroscopy and molecular beam scattering have been employed to probe the kinetics of the reaction of TTIP and to identify reaction products. The deposition mechanism involves two parallel pathways to form TiO2: at lower temperatures (500–650 K) propene and isopropanol are formed as products; at higher temperatures (≳650 K) propene and water are formed as products in a second pathway that becomes dominant. An activation energy of 57±8 kJ/mol has been measured for the first pathway. The results for the single crystalsurface are compared to earlier work on polycrystalline substrates.

The physical and chemical characterization of electrochemically reformed silver surfaces

Abstract The effects of electrochemically oxidizing and reducing polycrystalline silver foils in aqueous chloride solutions are studied from a physical and chemical standpoint. Scanning electron microscopy indicates that the resulting surface is rough, consisting of nodular formations. Auger electron spectroscopic, secondary ion mass spectrometric and electrochemical results provide clear indication that a highly purified layer of metallic silver of increased surface area and activity is produced by the electrochemical reformation process. No evidence is found for the retention of intense Raman scatterers such as pyridine at these surfaces under ultrahigh vacuum conditions. The implications of the chemical and physical state of the reformed silver foils are discussed in the context of theories which have been proposed to explain the anomalously intense Raman spectra of scatterers adsorbed from solution onto these reformed silver surfaces. The origin of the 240 cm−1 Raman line commonly observed on electrochemically reformed silver is discussed and assigned to the Ag−Cl− stretching mode.

MOVPE of AlN and GaN by using novel precursors

Abstract The technique of low pressure MOVPE has been used to grow thin films of AlN and GaN on silicon and (0001) sapphire substrates using the single-source precursors diethylaluminum azide [Et2AlN3]3 (DEAA) and diethygallium azide [Et2GaN3]3 (DEGA). In-situ growth rate measurements have been performed for AlN growth on Si substrates. For comparison with conventional MOVPE growth, epitaxial films of AlxGa1−xN have also been deposited from triethylgallium (TEG), triethylaluminum (TEAl) and NH3 under similar conditions. The properties of the grown films are discussed in terms of precursor selection and growth conditions.

Chemical and mechanical properties of redox polymer-modified electrodes: Part I. A mechanical/electrochemical model

Abstract A model is developed to describe the coupling of the mechanical and electrochemical thermodynamics of redox polymer film conversion for systems in which there is a net change in the charge per redox site associated with this redox conversion. The electrostatically forced intrusion of compensating charge, in the form of ions of the supporting electrolyte, into the polymer film is predicted to result in non-Nernstian behavior because of the finite void volume within the film and the finite molar volume of the charge-compensating ion. The physical consequences of this phenomenon are the forced swelling of the polymer film, a process which in the presence of crosslinks (either covalent or ionic) requires energy input in excess of that needed to carry out the simple electron transfer to/from the redox sites. This energy is consumed in doing work against the elastic framework of the crosslinked polymer matrix, and when couched in terms of an elastic deformation, may be considered reversible in the thermodynamic sense. The model developed here treats only the simplest of such mechanical coupling in that reversible, isotropic stress-strain is proposed to account for the non-Nernstian thermodynamics of these redox polymer films.

Diamine/Silane-Modified controlled pore glass: The covalent attachment reaction from aqueous solution and the mechanism of reaction of bound diamine with copper(II)

Abstract The reaction of N -2-aminoethyl-3-aminopropyltrimethyoxysilane (en-silane) with the surface of controlled pore glass (CPG) in aqueous solution was examined. The reaction appears to involve the rapid (ca. l min) formation of a covalently bound monolayer of en-silane at room temperature. Multilayers of bound polymeric en-silane can be obtained by reaction at elevated temperature. The latter reaction is slower, presumably because of the more stringent steric requirements of polymer formation in chain growth away from the CPG surface. For en-silane coverages ranging from 1.7 to 8.5 μ mol m -2 , the mechanism for the complexation of copper(II) by ethylenediamine (en) bound to a CPG surface by reaction of en-silane with CPG was also investigated by thermometric and potentiometric titrimetry. A two-step mechanism is proposed. The data indicate that the first step of the surface reaction involves formation of a Cu(en) 4 complex as opposed to the Cu(en) complex formed in the first step of the solution reaction. The second step results in a complex similar to the Cu(en) 2 ; species formed in solution.

Chemical and mechanical properties of redox polymer-modified electrodes: Part III. Redox thermodynamics on linear polyvinylferrocene electrodes

The redox thermodynamics of thin films (< 200 nm) of linear polyvinylferrocene (LPVF) deposited by spincoating on polished electrode substrates have been studied in tetra-n-butylammonium perchlorate + acetonitrile solutions. Three distinct regions of thermodynamic behavior were revealed through analysis of Nernst plots (E vs. log(cFc+/cFc0)) constructed from controlled potential coulometric data. For films which were 0–3% oxidized, super-Nernstian slopes with values dependent upon electrolyte concentration were attributed to Donnan equilibrium contributions to the electrode potential. This form of LPVF thus appears to function as a simple anion exchanger. Further oxidation (up to ca. 50%) is accompanied by a transition to a second thermodynamic region characterized by sub-Nernstian behavior which is interpreted in terms of an increasing association of ferrocenium/perchlorate ion pairs into dimers or higher order aggregates. Oxidation above 50% results in an abrupt return to super-Nernstian behavior, for which a mechanical/electrochemical model is proposed. In this third region, the LPVF film is envisioned to be a homogeneous network structure characterized by a high concentration of ionic crosslinks and a capability for sustaining electrochemically induced stress arising from the forced accommodation of neutralizing counterions. The transitions from one thermodynamic region to another are consistent with increasing film ion content as oxidation proceeds. The complexity in thermodynamic redox behavior of LPVF results from the substantial potential-dependent changes in film ion concentration (0 to 6 M) along with a relatively high degree of structural freedom due to the lack of covalent crosslinks.

Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography IV. Partitioning vs. adsorption mechanism on various types of polymeric bonded phases

Abstract The partition and adsorption mechanisms of retention in reversed-phase liquid chromatography have been examined based on a comparison of the free energy of transfer of methylene groups from aqueous-organic mixtures to bulk hexadecane with those to a variety of polymeric bonded phases. The stationary phases studied include: conventional silica-based polymeric phases of various alkyl chain lengths, a so-called “horizontally polymerized” octadecyl phase on silica and a series of polybutadiene-coated zirconia phases. The data indicate that for methylene groups a partition-like mechanism is dominant on all phases. On the polybutadiene-coated zirconia and “horizontally polymerized” octadecyl phases the partition mechanism holds at all mobile phase compositions. In contrast on conventional polymeric silica phases the retention mechanism seems to become more adsorption-like at methanol compositions greater than about 70% (v/v).

Chemical and mechanical properties of redox polymer-modified electrodes: Part II. Redox thermodynamics of plasma-polymerized vinylferrocene electrodes

The thermodynamics of thin films ( 400 nm) shows that significant physical damage can occur during electrolysis. These and related observations suggest that the super-Nersntian behavior may be related to stress induced by the forced incorporation of the anion of the supporting electrolyte, as is required to maintain electroneutrality within the films. A mechanical/electrochemical model is employed to explain the origin of the excess free energy required to convert ferrocene sites to ferrocenium sites within the PPVF films.

Room-temperature water adsorption on the Si(100) surface examined by UPS, XPS, and static SIMS

Abstract The adsorption of water on the reconstructed Si(100)-(2 x 1) surface at room temperature is studied using an integrated multi-technique approach. UPS, XPS, and static SIMS surface examination in a variable temperature mode of experimentation indicates dissociative adsorption of water to hydroxyl (OH) and hydride (H) groups saturating the single dangling bonds on adjacent dimer-pair Si atoms of the reconstructed surface. No evidence of molecular water is found . Upon heating the surface the hydroxyl decomposes to bridge-bonded oxygen (Si-O-Si) and additional silicon hydride groups. This is complete at ≈400°C. Above 400°C the surface loses hydrogen, presumably as H 2 , and above 600°C the substoichiometric silicon oxide sublimes as SiO to return to the clean, reconstructed surface. The chemical state of oxygen on the surface is monitored as a function of temperature through the use of a Δ parameter (Δ = binding energy O(1s) - binding energy Si(2p)). These results are consistent with experimental evidence and surface chemistry first proposed by others. Assignment of the observed DOS features is made by comparison of the experimental DOS spectra to calculations for the system and for system analogs (SiOH radical, oxygen-saturated Si surface, hydrogen-saturated Si surface), and to another hydroxyl system (NaOH). Four major features of the saturated hydroxyl/hydride surface are found : 5 eV Si-H bond , Sisp 3 + H1s character; 6.4 eV O 2p lone pair , π symmetry nonbonding orbital; 7.6 eV O-H bond , O 2p + H1s character, σ symmetry; 11.9 eV Si-OH bond , sp 3 + O 2p character (binding energies relative to the Fermi level).