Photoelectrochemical characteristics of ferric tungstate

Active ferric tungstate was prepared by fusing an equimolar mixture of tungsten oxide and ferric oxide at 1100 °C and annealing at 800 °C for 20 h. Analysis of the electrode material by X-ray diffraction showed that its composition was Fe₂WO₆. When this material was illuminated by visible light in 0.1 M NaOH solution, an anodic photocurrent at a positive potential of 0.5 V (SCE) was obtained. Therefore, this material is considered as an n-type semiconductor. The d.c. conductivity of this material at 25 °C was 4 × 10⁻⁶ Ω⁻¹ cm⁻¹. In the dark, unexpectedly high anodic currents were observed at positive potentials of 0.8 V (SCE) in 0.1 M NaOH. These currents are attributed to the existence of a high density of electron-hole recombination centers within the band-gap of ferric tungstate. When dimethyl viologen (DMV) was used as an electroactive compound in the electrolyte, the anodic photocurrents increased significantly. The oxidation of DMV is thus expected to compete with the electron-hole recombination process. Furthermore, the process of electron-hole recombination was also predicted from the shape of the photocurrent transients under interrupted illumination. These transients exhibited first-order relaxation effects in the region of the onset time of the photocurrents. The band-gap energy of Fe₂WO₆ was found to be about 1.5 eV and its flat-band potential in 0.1 M NaOH was about −0.3 V (SCE). The photoelectrochemical properties of ferric tungstate are explained according to the formalism of the band model of the semiconductor/electrolyte interface. Received: 16 July 1997 / Accepted: 26 September 1997     

Strong and weak approximations of k-spacings processes

Summary We obtain strong approximation results for the empirical process and the quantile process based on non-overlapping k-spacings using a characterization of uniform spacings in terms of independent exponential random variables. Moreover the weak convergence of these processes in /q-metrics is discussed. Applications to spacing statistics are included.Part of this research was done while the author was a visiting scientist at the Department of Mathematics and Statistics, Carleton University, supported by Canadian NSERC operating grant of M. Csörg. This research was also supported by a research grant of the Research Center of the College of Science, King Saud University     

Base-catalyzed condensation of cyclopentadiene derivatives. Synthesis of fulvalene analogues: strong proaromatic electron acceptors

A series of proaromatic electron acceptors derived from fulvenes were synthesized from tetrachlorocyclopentadiene and previously unknown 1,4-dicyano- and 1,4-dialkoxycarbonyl-2,3-dimethoxy cyclopentadienes. Two reversible one-electron reductions steps observed for fulvalenes coalesce into one two-electron reduction step upon increasing the length of the conjugating bridge.     

Photocatalytic degradation of polyhydroxybutyrate films using titanium dioxide nanoparticles as a photocatalyst

Photocatalytic degradation of polyhydroxybutyrate (PHB) polymeric films (30 μm thickness) containing different concentrations of titanium dioxide (TiO₂) nanoparticles under ultraviolet (UV) irradiation (λ_max = 313 nm) has been studied. The activity of TiO₂ (0.001-0.005%) as a photocatalyst was determined by monitoring various functional group indices, weight loss in polymeric films and photodegradation rate constant (k _d) with irradiation time. Photodegradation was found to be highly dependent on the TiO₂ nanoparticles concentration and the UV irradiation time. The rate of PHB sample photodegradation was highest when the concentration of TiO₂ was 0.005% (by weight) and lowest when its concentration was 0.001%.

     

Cation-specific interactions with carboxylate in amino acid and acetate aqueous solutions: X-ray absorption and ab initio calculations

Relative interaction strengths between cations (X = Li (+), Na (+), K (+), NH 4 (+)) and anionic carboxylate groups of acetate and glycine in aqueous solution are determined. These model systems mimic ion pairing of biologically relevant cations with negatively charged groups at protein surfaces. With oxygen 1s X-ray absorption spectroscopy, we can distinguish between spectral contributions from H 2O and carboxylate, which allows us to probe the electronic structure changes of the atomic site of the carboxylate group being closest to the countercation. From the intensity variations of the COO (-) aq O 1s X-ray absorption peak, which quantitatively correlate with the change in the local partial density of states from the carboxylic site, interactions are found to decrease in the sequence Na (+) > Li (+) > K (+) > NH 4 (+). This ordering, as well as the observed bidental nature of the -COO (-) aq and X (+) aq interaction, is supported by combined ab initio and molecular dynamics calculations.     

Production and properties of a biosurfactant obtained from a member of the Bacillus subtilis group (PTCC 1696)

The production and properties of a biosurfactant, synthesized by a member of the Bacillus subtilis group (PTCC 1696) which was isolated from an Iranian oil field, have been investigated. The biosurfactant, which was produced as a primary metabolite associated with the growth of PTCC 1696, was able to reduce the surface and interfacial tension of media to 26.7 and 0.1 mN/m, respectively. Crude biosurfactant and acid precipitated biosurfactant have critical micelle concentrations of 10 and 100 mug/ml, respectively. The stability of the biosurfactant at different salinities, pH and temperature and also its emulsifying activity have been investigated. It is an effective surfactant at very low concentrations over a wide range of temperatures, pHs and salt concentrations and also has the ability to emulsify oil, which is essential for enhanced oil recovery.     

Resolution enhancement in solid-state NMR of oriented membrane proteins by anisotropic differential linebroadening

We demonstrate that a significant improvement in the spectral resolution may be achieved in solid-state NMR experiments of proteins in inhomogeneously disordered oriented lipid bilayers. Using 1H homonuclear decoupling instead of standard 1H heteronuclear decoupling, the 15N line widths may be reduced by up to seven times for such samples. For large oriented membrane proteins, such resolution enhancements may be crucial for assignment and structural interpretation.     

Excited-state symmetry and reorientation dynamics of perylenes in liquid solutions: time-resolved fluorescence depolarization studies using one- and two-photon excitation

The excited-state symmetry and molecular reorientation of perylene, 1,7-diazaperylene, and 2,5,8,11-tetra- tert-butylperylene have been studied by different fluorescence depolarization experiments. The first excited electronic singlet state was reached through one-photon excitation (OPE) and two-photon excitation (TPE). A 400 and 800 nm femtosecond laser pulse was used for this purpose, and data were collected by means of the time-correlated single-photon counting technique. It is found that the rotational correlation times for each perylene derivative are very similar in the OPE and TPE depolarization experiments. For the determination of the two-photon absorption tensor, a recently described theoretical model has been applied (Ryderfors et al. J. Phys. Chem. A 2007, 111, 11531). It was found that the two-photon process can be described by a 2 x 2 absorption tensor for which the components are solvent dependent and exhibit mixed vibronic character. In the dipole approximation this is compatible with a parity-forbidden two-photon absorption into the first excited singlet state.