Hydroxyl Dipoles Research Articles

Oxygen vacancy-induced hydroxyl dipole reorientation in hydroxyapatite for enhanced piezocatalytic activity

A new method to boost the piezocatalytic activity of hydroxyapatite (HAP) is proposed that oxygen vacancies were introduced to align the orientation of hydroxyl dipoles in parallel. A series of hydroxyapatites with different contents of oxygen vacancy were synthesized by hydrothermal method and high-temperature calcination. Among them, hydroxyapatite with a moderate oxygen-vacancy concentration (OVHAP-2 h) exhibited an excellent piezocatalytic removal efficiency of 98.43% for bisphenol A (BPA) within 12 min, and its degradation kinetic rate constant was almost 4 times that of pristine HAP. This could be ascribed to the introduction of oxygen vacancies which induce the transformation in the orientation of hydroxyl dipoles from disorderly to parallel. Finally, the possible piezocatalytic mechanism of OVHAP is proposed based on the free radical trapping experiments. This work provides a novel strategy for improving the piezocatalytic performance of HAP, and expands its application in environmental remediation by harvesting mechanical energy.

Oxygen Vacancy-Induced Hydroxyl Dipole Reorientation in Hydroxyapatite for Enhanced Piezocatalytic Activity

Oxygen Vacancy-Induced Hydroxyl Dipole Reorientation in Hydroxyapatite for Enhanced Piezocatalytic Activity

Sensing Properties of NiO Loaded SnO2 Nanoparticles—Specific Selectivity to H2S

NiO-loaded SnO2 powders were prepared involving two chemical procedures. The mesoporous SnO2 support was synthesized by a hydrothermal route using Brij 35 non-ionic surfactant as a template. The nickel loadings of 1 and 10 wt.%. NiO were deposited by the wet impregnation method. The H2S sensing properties of xNiO-(1-x)SnO2 (x = 0, 1, 10%) thick layers deposited onto commercial substrates have been investigated with respect to different potential interfering gases (NO2, CO, CO2, CH4, NH3 and SO2) over a wide range of operating temperatures and relative humidity specific for in-field conditions. Following the correlation of the sensing results with the morphological ones, 1wt.% NiO/SnO2 was selected for simultaneous electrical resistance and work function investigations. The purpose was to depict the sensing mechanism by splitting between specific changes over the electron affinity induced by the surface coverage with hydroxyl dipoles and over the band bending induced by the variable surface charge under H2S exposure. Thus, it was found that different gas-interaction partners are dependent upon the amount of H2S, mirrored through the threshold value of 5 ppm H2S, which from an applicative point of view, represents the lower limit of health effects, an eight-hour TWA.

Poly(vinyl alcohol) as a gas accumulation layer for an organic field-effect transistor ammonia sensor

Ammonia (NH3) gas sensors based on organic field-effect transistor (OFET) using poly(vinyl alcohol) (PVA) as a gas accumulation layer were fabricated. The results showed the percentage responses of saturation current (Ion) was 50.3% under 0.5ppm NH3, which is over one order of magnitude higher than that of pure PMMA dielectric. Also, it showed that there was a remarkable shift in the field-effect mobility after exposed to NH3 gas. Moreover, the percentage responses of Ion would increase to 66.9% and 30.2% under 0.5ppm and 0.2ppm NH3 after applied a gate voltage pulse, respectively. The sensing properties of these OFET gas sensors can realize absorption and desorption with opposite gate bias, exhibiting non-volatile states of Ion and VT under the programming-erasing bias as used for memory elements. By analyzing the morphologies of dielectric and organic semiconductor, and the electrical characteristics of OFET sensors, the interaction between NH3 and PVA will be strengthened under the influence of the applied electrical field on the hydroxyl dipoles. The bias-induced re-orientation of the hydroxyl dipoles can modulate the influence of NH3 on the trapping (absorption) and detrapping (desorption) processes of charge carriers at the pentacene/PVA interface.

Strong molecular weight effects of gate-insulating memory polymers in low-voltage organic nonvolatile memory transistors with outstanding retention characteristics

Organic nonvolatile memory transistors, featuring low-voltage operation (⩽5 V) and high retention characteristics (>10 000 cycles), are demonstrated by introducing high molecular weight poly(vinyl alcohol) (PVA) as a gate insulating layer. PVA polymers with four different molecular weights (9.5–166 kDa) are examined for organic memory devices with poly(3-hexylthiophene) channel layers. All devices show excellent p-type transistor behavior and strong hysteresis in the transfer curves, but the lower molecular weight PVA delivers the higher hole mobility and the wider memory window. This has been attributed to the higher ratio of hydroxyl group dipoles that align in the out-of-plane direction of the PVA layers, as supported by impedance spectroscopy (dielectric constants), polarized Fourier transform-infrared spectroscopy and synchrotron radiation grazing incidence X-ray diffraction measurements. However, outstanding retention characteristics (<4% current variation after 10 000 cycles) have been achieved with the higher molecular weight PVA (166 kDa) rather than the lower molecular weight PVA (9.5 kDa). Researchers have created an organic transistor that retains memory states for over 10,000 cycles with minimal, sub-5-volt power requirements. Flexible memory devices made from semiconducting plastics are significantly easier to fabricate than silicon-based materials. However, the high voltages usually needed to switch the on/off state of organic transistors are detrimental to device longevity. An international team led by Youngkyoo Kim from Kyungpook National University in Korea used insulating poly(vinyl alcohol) (PVA) films with high molecular weights to reduce the voltage demands of plastic circuits. The researchers inserted PVA films between a transparent electrode and a semiconducting polymer, and then fabricated the stack into a source/drain/gate transistor setup. The most stable data retention characteristics were achieved with the heaviest PVA films — a relationship the researchers attribute to hydroxyl dipole orientations at interfacial boundaries. Low-voltage (<5 V) organic memory transistors with outstanding retention stability (>10 000 cycles), which were achieved by employing both the P3HT channel layer and the PVA gate-insulating memory layer, were strongly influenced by the PVA molecular weights due to the different orientation of hydroxyl dipoles.

Structure refinement of sub-cubic-mm volume sample at high pressures by pulsed neutron powder diffraction: application to brucite in an opposed anvil cell

Neutron powder diffraction measurements of 0.9 mm3 of mixture of deuterated brucite and pressure medium were conducted at pressures to 2.8 GPa, using an opposed anvil cell and a medium-resolution diffractometer at Japan Proton Accelerator Research Complex pulsed neutron source. Spurious-free diffraction patterns were successfully obtained and refined to provide all structural parameters including Debye–Waller factors. Tilting of hydroxyl dipoles of brucite toward one of the three nearest-neighbor oxygen anions was confirmed to be substantial at pressure as low as 1.5 GPa. By this application, technical feasibility to analyze such a small sample has been newly established, which would be useful to extend the applications of neutron diffraction at high pressures.

ChemInform Abstract: Electrostatic Effects on (Di)terpene Synthase Product Outcome

AbstractReview: 60 refs.

Electrostatic effects on (di)terpene synthase product outcome

Terpene synthases catalyze complex reactions, often forming multiple chiral centers in cyclized olefin products from acyclic allylic diphosphate precursors, yet have been suggested to largely control their reactions via steric effects, serving as templates. However, recent results highlight electrostatic effects also exerted by these enzymes. Perhaps not surprisingly, the pyrophosphate co-product released in the initiating and rate-limiting chemical step provides an obvious counter-ion that may steer carbocation migration towards itself. This is emphasized by the striking effects of a recently uncovered single residue switch for diterpene synthase product outcome, whereby substitution of hydroxyl residues for particular aliphatic residues has been shown to be sufficient to "short-circuit" complex cyclization and/or rearrangement reactions, with the converse change further found to be sufficient to increase reaction complexity. The mechanistic hypothesis for the observed effects is hydroxyl dipole stabilization of the specific carbocation formed by initial cyclization, enabling deprotonation of this early intermediate, whereas the lack of such stabilization (i.e. in the presence of an aliphatic side chain) leads to carbocation migration towards the pyrophosphate co-product, resulting in a more complex reaction. This is further consistent with the greater synergy exhibited between pyrophosphate and aza-analogs of late, relative to early, stage carbocation intermediates, and crystallographic analysis of the monoterpene cyclase bornyl diphosphate synthase wherein mechanistically non-relevant counter-ion pairing between aza-analogs of early stage carbocation intermediates and pyrophosphate is observed. Thus, (di)terpene synthases seem to mediate specific reaction outcomes, at least in part, by providing electrostatic effects to counteract those exerted by the pyrophosphate co-product.

Microstructure evolution in aqueous solutions of cetyl trimethylammonium bromide (CTAB) and phenol derivatives

Surfactant aggregate microstructure changes induced by the addition of a series of phenolic organic dopants (phenol, cresol, 4-ethyl phenol and 4- n- and i-butyl phenol) to a micellar solution of cetyl trimethyl ammonium bromide (CTAB) have been studied using cryogenic transmission electron microscopy (cryo-TEM). As the concentration of each of the dopants is increased, there is a systematic reduction in the curvature of the observed microstructures. For phenol and cresol the microstructure changes from globular micelles to rod-like micelles and then to long worm-like micelles. For 4-ethyl phenol and 4-butyl phenol, the microstructure transforms from globular micelles to worm-like micelles to unilamellar vesicles and then bilamellar vesicles. The concentrations at which these morphological transitions take place reduce as the number of methyl substitutions on the phenolic ring is increased. These microstructure transitions are attributed to changes in the packing parameter of the surfactant complex as dopants penetrate to different degrees in the surfactant palisades layer, resulting from a balance of interaction between the surfactant tails and the dopant aromatic chain and the hydrogen bonding of the hydroxyl group with surrounding water. The alignment of the hydroxyl dipoles reduces the electrostatic field around the CTAB head groups, decreases interlayer repulsive interactions and allows membrane fluctuations to stabilize the bilamellar vesicles. These highly tunable microstructures can be exploited for templated materials synthesis, and have implications for the micellar-enhanced ultrafiltration process.

Incorporating protein conformational flexibility into the calculation of pH-dependent protein properties

A method for combining calculations of residue pKa's with changes in the position of polar hydrogens has been developed. The Boltzmann distributions of proton positions in hydroxyls and neutral titratable residues are found in the same Monte Carlo sampling procedure that determines the amino acid ionization states at each pH. Electrostatic, Lennard-Jones potentials, and torsion angle energies are considered at each proton position. Many acidic and basic residues are found to have significant electrostatic interactions with either a water- or hydroxyl-containing side chain. Protonation state changes are coupled to reorientation of the neighboring hydroxyl dipoles, resulting in smaller free energy differences between neutral and ionized residues than when the protein is held rigid. Multiconformation pH titration gives better agreement with the experimental pKa's for triclinic hen egg lysozyme than conventional rigid protein calculations. The hydroxyl motion significantly increases the protein dielectric response, making it sensitive to the composition of the local protein structure. More than one conformer per residue is often found at a given pH, providing information about the distribution of low-energy lysozyme structures.

In-source analysis of radiation curing of unsaturated polyester resin by charging current technique

The dielectric and conductive properties of unsaturated polyester (UP) resins undergo significant changes in the course of crosslinking reaction. In-source radiation curing of UP resins was studied by combining dielectric, thermal and charging current analyses. Dielectric permittivity (by stepwise changing of frequency) and charging current were measured in the course of cobalt-60 γ-irradiation at constant temperatures. The exothermic reaction could be monitored by differential thermal analysis (DTA), while the observed change of dielectric permittivity can show how the dielectric relaxation time distribution shifted through the experimental window represented by reaction temperature and by frequency range of dielectric spectrometer. The decrease in dielectric permittivity and in charging current may be mainly associated with the partial immobilization of permanent carbonyl and hydroxyl dipoles attached to the oligoester chain. The measurements of charging properties of UP resins during the curing seems very promising due to the ease with which the relaxation parameters and essential information on the molecular interactions and mobility can be derived. The electrical properties of UP resins are mainly determined by inter- and intramolecular hydrogen bonds. The shape of the charging current spectra during the curing of UP resins showed a heterogeneous nature of vitrification process. The results show that two relaxations occur: the first one is attributed to the increase in viscosity of liquid phase as oligoester chains are crosslinked prior to the gelation, and the second one to the further crosslinking in the gel phase, resulting in the vitrification of the system. The characteristic times to gelation and vitrification of UP resins in the course of radiation isothermal curing are decreasing functions of reaction temperature.

Neutron Diffraction Study of a One-Layer Monoclinic Chlorite

AbstractA monoclinic IIb-2 clinochlore from the Achmatov mine, Ural Mountains, U.S.S.R., was investigated by neutron diffraction. The formula, based on electron microprobe and wet chemical analyses, is (Mg4.54Al0.97Fe2+0.28 Fe3+0.18Mn0.01)(Si2.85Al1.15)O10(OH)8. A refinement based on 512 unique reflections converged in space group C2/m to a final R =.066. Cation disorder was found in the two octahedral positions of the 2:1 layer, whereas partial Mg and Al ordering occurs in the interlayer sheet. The two hydroxyl dipoles are roughly perpendicular to the interlayer sheet, forming weak to medium hydrogen bonds with O…O distances of 2.859 and 2.881 Å. The OH-dipole of the 2:1 layer is perpendicular to the (001) plane.

Test of short range effects in surface enhanced Raman spectroscopy: An elastic tunnelling study of pyridine — metal interfaces

The elastic tunnel current versus bias over the range ± 1 V in junctions of the type A1-I(X)-M where M = Pb, Ag, Cu or Au and X is 2- 3- or 4-pyridinecarboxaldehyde has been used to determine parameters which describe the barrier in each. Little variation attributable to the different dopants is observed but there are substantial effects attributable to the top metal M in both clean and doped junctions. It is suggested that electron densities of states are responsible. The potential step between dopant and metal M decreases in the sequence Au:Pb:Ag. Evidence is collated from a number of sources to suggest that hydroxyl dipole layers influence the metal-organic interface potential step both in aqueous-solution-doped tunnel junctions and in SERS systems.

Anomalous temperature dependence of paraelectric relaxation at very low temperatures

Hydroxyl doped KCl crystals were subjected to two dimensional stress in a (001)-plane so that the paraelectric hydroxyl dipoles can be described by a double-well system with a small tunneling splitting. In the temperature range from 0.025 to 0.3 K this system exhibits a dielectric relaxation rate that increases with decreasing temperature.

Paraelectric centers with low zero-field splitting

The OH - substituted in KCl for a Cl - has six [001] equilibrium orientations. If the crystal is subjected to two-dimensional compression in the (001)-plane the two orientations [001] and [00 1 ] have the lowest energy. Therefore at low temperatures the hydroxyl dipole behaves like a particle in a double well. Measurements of the dielectric constant along [001] down to 0.025 K indicate that this system has a tunneling splitting Δ' < 0.025K k B and rather unusual relaxation properties.

Isotope effects in paraelectric relaxation

Through measurements of the complex dielectric susceptibility in the temperature range between 0.3°K and a few °K we have studied the influence of the isotopic substitutions H→D and16O→18O on the relaxation time of the hydroxyl dipole in weakly doped KBr and RbCl. The H→D effect is present in both host lattices. The oxygen effect, however, is only for RbCl outside experimental error, indicating that in this host lattice the reorientation of the hydroxyl ion involves a considerable displacement of its center of mass.