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At least 703 records · Page 39Linked to original sources
PubMed2004

Large-amplitude Fourier transformed high-harmonic alternating current cyclic voltammetry: kinetic discrimination of interfering Faradaic processes at glassy carbon and at boron-doped diamond electrodes.

Significant advantages of Fourier transformed large-amplitude ac higher (second to eighth) harmonics relative to responses obtained with conventional small-amplitude ac or dc cyclic voltammetric methods have been demonstrated with respect to (i) the suppression of capacitive background currents, (ii) the separation of the reversible reduction of [Ru(NH(3))(6)](3+) from the overlapping irreversible oxygen reduction process under conditions where aerobic oxygen remains present in the electrochemical cell, and (iii) the kinetic resolution of the reversible [Ru(NH(3))(6)](3+/2+) process in mixtures of [Fe(CN)(6)](3-) and [Ru(NH(3))(6)](3+) at appropriately treated boron-doped diamond electrodes, even when highly unfavorable [Fe(CN)(6)](3-) to [Ru(NH(3))(6)](3+) concentration ratios are employed. Theoretical support for the basis of kinetic discrimination in large-amplitude higher harmonic ac cyclic voltammetry is provided.

Journal Article↗
PubMed2000

Laser activation voltammetry: selective removal of reduced forms of methyl viologen deposited on glassy carbon and boron-doped diamond electrodes

The effect of high-intensity laser pulses on the reduction of methyl viologen at glassy carbon electrodes in aqueous solution is investigated using laser activation voltammetry (LAV) under both channel flow and no-flow conditions and compared with the effect of conventional variable-temperature voltammetry. The reduction proceeds in two consecutive one-electron steps, and the neutral two-electron-reduction product of methyl viologen is shown by voltammetry and in situ optical microscopy to form two types of deposits, amorphous and crystalline, on the electrode surface. Laser activation voltammetry using a 10 Hz pulsed Nd-YAG 532 nm laser is shown to remove the deposits from the electrode surface at different laser intensities: the amorphous material is more easily ablated than the crystalline deposit. By conventional variable-temperature voltammetry, it is shown that the two stripping peaks disappear as the temperature is increased. However, with conventional heating, the opposite ease of removal is detected compared to the case of laser activation voltammetry: the stripping response associated with the crystalline material disappears at lower temperatures compared to that for the amorphous material. In the presence of high-intensity laser pulses (>0.17 W cm(-2)), glassy carbon surfaces are damaged and the voltammetric characteristics become poor. It is shown that, by the employment of a thin-film boron-doped diamond electrode grown using a chemical vapor deposition procedure on a tungsten substrate, much higher laser intensities can be applied and well-defined LAV signals can be obtained without deactivation of the electrode.

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PubMed1999

In situ monitoring of an escherichia coli fermentation using a diamond composition ATR probe and mid-infrared spectroscopy

A diamond composition ATR probe was used in situ to obtain IR spectra on replicate Escherichia coli fermentations involving a complex medium. The probe showed excellent stability over a 6-month operating period and was unaffected by either agitation or aeration. The formation of an unknown was observed from IR spectra obtained during the sterilization; subsequent experiments proved this to be a reaction product between yeast extract and the phosphates used as buffer salts. Partial-least-squares-based calibration/prediction models were developed for both glucose and acetate using in-process samples. The resulting models had prediction errors of +/-0.26 and +/-0.75 g/L for glucose and acetic acid, respectively, errors which were statistically equivalent to the estimated experimental errors in the reference measurements. Relative concentration profiles for the unknown formed during sterilization could be generated either by tracking peak height at an independent wavelength or by self-modeling curve resolution of the spectral region overlapping that of glucose. These profiles indicated that this compound was metabolized simultaneously with glucose; upon depletion of the glucose, when the microorganism switched to consumption of acetic acid, utilization continued but at a lower rate. The data presented provide an extensive characterization of the performance characteristics of this in situ analysis and clearly demonstrate its utility not just in the quantitative measurement of multiple known species but in the qualitative evaluation of unknown species.

Journal Article↗
PubMed2005

Synthesis and characterization of a porous magnetic diamond framework, Co3(HCOO)6, and its N2 sorption characteristic.

[Co3(HCOO)6](CH3OH)(H2O) (1), the isostructural analogue of the porous magnet of coordination framework [Mn3(HCOO)6](CH3OH)(H2O), and its desolvated form [Co3(HCOO)6] (2) were prepared and characterized by X-ray and neutron diffraction methods, IR, thermal analyses, and BET, and their magnetic properties were measured. The parent compound, 1, crystallizes in the monoclinic system, space group P21/c, a = 11.254(2) A, b = 9.832(1) A, c = 18.108(3) A, beta = 127.222(2) degrees , V = 1595.5(4) A3, Z = 4, R1 = 0.0329 at 180 K. It possesses a unit cell volume that is 9% smaller than [Mn3(HCOO)6](CH3OH)(H2O) due to the smaller radius of Co2+ ion. Compared with the parent compound 1, the desolvated compound 2 has slightly larger lattice with cell parameters of a = 11.2858(4) A, b = 9.8690(4) A, c = 18.1797(6) A, beta = 127.193(2) degrees , V = 1613.0(1) A3, R1 = 0.0356 at 180 K. The cell parameters of 2, obtained from neutron powder data at 2 K, are a = 11.309(2) A, b = 9.869(1) A, c = 18.201(3) A, beta = 127.244(8) degrees , V = 1617.3(5) A3. The pore volume reduces from 33% to 30% by replacing Mn by Co. The material exhibits a diamond framework based on Co-centered CoCo4 tetrahedral nodes, in which all metal ions have octahedral coordination geometry and all HCOO groups link the metal ions in syn-syn/anti modes. It displays thermal stability up to 270 degrees C. The compound easily loses guest molecules without loss of crystallinity, and it partly reabsorbs water from the atmosphere. Significant N2 sorption was observed for the desolvated framework suggesting that the material possesses permanent porosity. The magnetic properties show a tendency to a 3D long-range magnetic ordering, probably antiferromagnetic with a spin canting arrangement below 2 K.

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PubMedDate not supplied

Infrared and Raman spectroscopic investigations of the Nb(V) fluoro and oxofluoro complexes in the LiF-NaF-KF eutectic melt with development of a diamond IR cell.

A vacuum-tight cell for infrared spectroscopic investigations of extremely corrosive melts, e.g., molten fluorides, has been constructed and tested up to 750 degrees C. The cell has a gold-lined sample chamber and a diamond window transparent for the infrared light. It can be furnished with a gold piston that enables the recording of short-path-length FTIR spectra of liquid samples. Solutions of Nb(V) in LiF-NaF-KF eutectic (FLINAK) with and without oxide additions have been investigated by FTIR and Raman spectroscopy. The presence of NbF7(2-), NbOF5(2-), and NbO2F4(3-) complexes was established in the molten state at 600 degrees C. After solidification NbF7(2-) was still the only Nb(V) all-fluoro complex present. Three oxofluoro complexes, NbOF6(3-), NbOF5(2-), and NbO2F4(3-), have been identified in the solid state. Typical frequency regions for the different complexes are established. Finally, it was shown that K2NbF7 can be used as an indicator to determine the oxide content of the sample melts.

Journal Article↗
PubMed2000

Ferromagnetism in an extended three-dimensional, diamond-like copper(II) network: a new copper(II)/1-hydroxybenzotriazolato complex exhibiting soft-magnet properties and two transitions at 6.4 and 4.4 K.

The use of the substituted benzotriazole ligand btaOH (1-hydroxybenzotriazole) in copper(II) chemistry has yielded a structurally and magnetically very interesting complex. The [Cu2(O2CMe)4(H2O)2]/btaOH.H2O/aqueous NH3 (1:4:4, 1:3:3, 1:2:2) reaction system in MeOH gives dark brown-green [Cu(btaO)2(MeOH)]n (4) in approximately 80% yield. 4 crystallizes in the tetragonal space group P4(3)2(1)2 with (at 25 degrees C) a = 9.915(1) A, b = 9.915(1) A, c = 14.715(2) A, and Z = 4. The structure consists of a 3D, diamond-like copper(II) lattice. The CuII atom has a square pyramidal geometry with four btaO- ligands at the basal plane. The btaO- ion functions as a bidentate bridging ligand, with N(3) and the deprotonated oxygen being the ligating atoms. Dc and ac magnetic susceptibility measurements, together with low-field (10 G) and high-field (up to 5000 G) magnetization data, are consistent with ferromagnetic interactions on the scale of the crystal lattice with two critical temperatures: 6.4 and 4.4 K. The former critical temperature could correspond to a transition from a paramagnetic to a ferromagnetic state; the latter one, to a transition from a ferromagnetically ordered state to its 3D ordering. The magnetic data, along with the field dependence of the magnetization and the EPR data, are also in line with a soft magnet. Moreover, the EPR studies performed on 4 reveal unique features reported for the first time in the field of molecular magnetism.

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PubMed2003

Spectroscopic study of [Fe2O2(5-Et3-TPA)2]3+: nature of the Fe2O2 diamond core and its possible relevance to high-valent binuclear non-heme enzyme intermediates.

The spectroscopic properties and electronic structure of an Fe(2)(III,IV) bis-mu-oxo complex, [Fe(2)O(2)(5-Et(3)-TPA)(2)](ClO(4))(3) where 5-Et(3)-TPA = tris(5-ethyl-2-pyridylmethyl)amine, are explored to determine the molecular origins of the unique electronic and geometric features of the Fe(2)O(2) diamond core. Low-temperature magnetic circular dichroism (MCD) allows the two features in the broad absorption envelope (4000-30000 cm(-)(1)) to be resolved into 13 transitions. Their C/D ratios and transition polarizations from variable temperature-variable field MCD saturation behavior indicate that these divide into three types of electronic transitions; t(2) --> t(2) involving excitations between metal-based orbitals with pi Fe-O overlap (4000-10000 cm(-)(1)), t(2)/t(2) --> e involving excitations to metal-based orbitals with sigma Fe-O overlap (12500-17000 cm(-)(1)) and LMCT (17000-30000 cm(-)(1)) and allows transition assignments and calibration of density functional calculations. Resonance Raman profiles show the C(2)(h)() geometric distortion of the Fe(2)O(2) core results in different stretching force constants for adjacent Fe-O bonds (k(str)(Fe-O(long)) = 1.66 and k(str)(Fe-O(short)) = 2.72 mdyn/A) and a small ( approximately 20%) difference in bond strength between adjacent Fe-O bonds. The three singly occupied pi-metal-based orbitals form strong superexchange pathways which lead to the valence delocalization and the S = (3)/(2) ground state. These orbitals are key to the observed reactivity of this complex as they overlap with the substrate C-H bonding orbital in the best trajectory for hydrogen atom abstraction. The electronic structure implications of these results for the high-valent enzyme intermediates X and Q are discussed.

Circular Dichroism↗
PubMed2004

Amido-bridged Cu2N2 diamond cores that minimize structural reorganization and facilitate reversible redox behavior between a Cu1Cu1 and a class III delocalized Cu1.5Cu1.5 species.

A novel Cu(2)N(2) diamond core structure supported by an [SNS](-) ligand (1) ([SNS](-) = bis(2-tert-butylsulfanylphenyl)amido) has been prepared. This dicopper system exhibits a fully reversible one-electron redox process between a reduced Cu(1)Cu(1) complex, [[SNS][Cu]](2) (2), and a class III delocalized Cu(1.5)Cu(1.5) state, [[[SNS][Cu]](2)][B(3,5-(CF(3))(2)C(6)H(3))(4)] (3). Structural snapshots of both redox forms have been obtained to reveal remarkably little overall structural reorganization. The Cu...Cu bond distance nonetheless undergoes an appreciable compression (approximately 0.13 A) upon oxidation, providing a Cu...Cu distance of 2.4724(4) A in the mixed-valence state that is virtually identical to the Cu...Cu distance observed in the reduced form of the Cu(A) site of thiolate-bridged cytochrome c oxidase. Despite the low structural reorganization evident between 2 and 3, the [SNS](-) ligand is quite flexible. For example, square-planar geometries can prevail for divalent copper ions supported by [SNS](-) as evident from the crystal structure of [SNS]CuCl (4). Physical characterization for the mixed valence complex 3 includes electrochemical, magnetic (SQUID), EPR, and optical data. The complex has also been examined by density functional methods. An attempt was made to measure the rate of electron self-exchange k(s) between the Cu(1)Cu(1) and the Cu(1.5)Cu(1.5) complexes 2 and 3 by NMR line-broadening analysis in dichloromethane solution. While the system is certainly in the fast-exchange regime, the exchange process is too fast to be accurately measured by this technique. The value for k(s) can be bracketed with a conservative lower boundary of > or =10(7) M(-1) s(-1), a value that appears to be larger than other low molecular weight copper model complexes for which similar data is available. The unusually large magnitude of k(s) likely reflects the minimal structural reorganization that accompanies Cu(1)Cu(1) <--> Cu(1.5)Cu(1.5) interchange.

Journal Article↗
PubMed2005

Fluorinated nanodiamond as a wet chemistry precursor for diamond coatings covalently bonded to glass surface.

The chemical reaction between fluoro-nanodiamond (F-ND) powder, solubilized in o-dichlorobenzene, and a glass surface, functionalized with the silane coupling agent, 3-aminopropyltriethoxysilane (APTES), was found to proceed under heating at 130 degrees C for 24-40 h and to result in covalent bonding of F-ND particles to a glass substrate, forming a 10-40 nm thick nanocrystalline film. The observed process presents a novel and cost-effective approach to fabrication of diamond coatings on glass by using wet chemistry instead of CVD and can be extended to other materials.

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PubMed2005

A highly emissive Cu2N2 diamond core complex supported by a [PNP]- ligand.

A Cu2N2 diamond core structure, {(PNP)CuI}2 (2), supported by a [PNP]- ligand (1) ([PNP]- = bis(2-(diisobutylphosphino)phenyl)amide) has been prepared. 2 is highly emissive at ambient temperature in both the solid and solution states and is characterized by a relatively long-lived excited state (tau > 10 mus) and an unusually high quantum yield (phi > 0.65). These observations are consistent with a low degree of structural reorganization between the ground state of 2 and its excited state *2, and also with a high degree of steric protection of the two copper centers of 2 afforded by the bulky [PNP]- ligand. An estimate for the excited-state reduction potential of *2 (ca. -3.2 V vs Fc+/Fc), and the availability of two well-separated and reversible ground-state redox processes, suggests that bimetallic copper systems of these types may be interesting candidates to consider for photochemically driving multielectron redox transformations.

Journal Article↗
PubMed2005

Diamond-hexagonal semiconductor nanocones with controllable apex angle.

We report on the synthesis of nanostructured and crystalline tapered Si and Ge polyhedra via metal-catalyzed chemical vapor deposition. These Si and Ge nanocones (SiNCs, GeNCs) possess tips with near-atomic sharpness, micron-scaled bases, hexagonal cross-sections, and controllable apex angles. High-resolution transmission electron microscopy, selected-area electron diffraction and Raman scattering spectroscopy and analysis indicate that the SiNCs are of the diamond-hexagonal Si(IV) phase.

Gold↗
PubMed2006

Impact of grain-dependent boron uptake on the electrochemical and electrical properties of polycrystalline boron doped diamond electrodes.

A combination of high-resolution electrical and electrochemical imaging techniques, in conjunction with cathodoluminescence (CL), is used to investigate the electrochemical behavior of oxygen-terminated highly doped polycrystalline boron doped diamond (BDD). The BDD has a dopant density approximately 5 x 10(20) atoms cm(-3), grain size ca. 5-40 microm, and thickness 500 microm. CL imaging demonstrates that boron uptake is nonuniform across the surface of BDD, and conducting atomic force microscopy (C-AFM) highlights how this impacts on the local conductivity. While C-AFM shows no evidence for enhanced grain boundary conductivity, two characteristic conductivity domains are found with resistances of ca. 100 kOmega and ca. 50 MOmega. With the use of scanning electrochemical microscopy (SECM), local heterogeneities are also observed in the electroactivity of the BDD surface, consistent with the two different types of conducting regions. Local currents of the magnitude expected for metal-like behavior are observed in some regions, suggesting degenerative doping of the grains (supported by CL studies). In other regions, slower electron transfer is apparent. However, even for the reduction of Ru(NH(3))(6)(3+), which occurs at potentials far negative of the flat-band potential for oxygen-terminated BDD, all areas of the surface show some electroactivity. This study highlights that the spatially heterogeneous conductivity and corresponding electroactivity of BDD are readily resolved using a combination of C-AFM, SECM, and CL.

Journal Article↗
PubMed2006

Application of a quantum cascade laser for time-resolved, in situ probing of CH4/H2 and C2H2/H2 gas mixtures during microwave plasma enhanced chemical vapor deposition of diamond.

First illustrations of the utility of pulsed quantum cascade lasers for in situ probing of the chemistry prevailing in microwave plasma activated hydrocarbon/Ar/H2 gas mixtures used for diamond thin film growth are reported. CH4 and C2H2 molecules, and their interconversion, have been monitored by line-of-sight single pass absorption methods, as a function of process conditions (e.g., choice of input hydrocarbon (CH4 or C2H2), hydrocarbon mole fraction, total gas pressure, and applied microwave power). The observed trends can be rationalized, qualitatively, within the framework of the previously reported modeling of the gas-phase chemistry prevailing in hot filament activated hydrocarbon/H2 gas mixtures (Ashfold et al. Phys. Chem. Chem. Phys. 2001, 3, 3471). Column densities of vibrationally excited C2H2(v5=1) molecules at low input carbon fractions are shown to be far higher than expected on the basis of local thermodynamic equilibrium. The presence of vibrationally excited C2H2 molecules (C2H2(double dagger)) can be attributed to the exothermicity of the C2H3 + H <==> C2H2 + H2 elementary reaction within the overall multistep CH4 --> C2H2 conversion. Diagnostic methods that sample just C2H2(v=0) molecules thus run the risk of underestimating total C2H2 column densities in hydrocarbon/H2 mixtures operated under conditions where the production rate of C2H2(double dagger) molecules exceeds their vibrational relaxation (and thermal equilibration) rates.

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PubMed2006

Ab initio studies on nanoscale friction between fluorinated diamond surfaces: effect of model size and level of theory.

Interactions between two fluorinated diamond surfaces placed in contact with each other were investigated with quantum chemical Hartree-Fock and Møller-Plesset perturbation theory and basis sets def-SV(P), def-TZVP, and 6-31G. Two models, C(6)H(6)F(3)-C(24)H(24)F(12) and C(13)H(16)F(6)-C(22)H(24)F(10), were used to examine how model size and level of theory affect the atomic-scale friction, especially the coefficient of friction. Also of interest was a comparison of the interaction energies of the two models with different stacking configurations. The averages of the calculated friction coefficients fell within the range of values 0.28-0.43.

Journal Article↗
PubMed2006

Density functional theory investigation of product distribution following reaction of acrylonitrile on diamond (001)-2 x 1 surface.

The reaction of acrylonitrile with the C(001)-2 x 1 surface has been investigated by employing density functional cluster model calculations. The calculations revealed eight possible reaction pathways for acrylonitrile with the surface dimer. Full geometry optimized structures were obtained for all adducts, including intra- and interdimer reaction products. These results were analyzed in terms of both the total energy values and the detailed optimized geometries. We find that the reaction of acrylonitrile with the diamond (001) surface occurs primarily through its nonpolar C=C group and the intradimer [2+2](cc) product is the dominant product. All these results are in good agreement with the experimental work by Schwartz. It is noteworthy that the isomerization process plays an important role in the chemisorption process. Both intradimer [4+2] product and interdimer [2+2](cc) product can isomerize to the intradimer [2+2](cc) product. The present study shows that the isomerization between intradimer [4+2] product and intradimer [2+2](cc) product is slightly favorable over the direct path to formation of the intradimer [2+2](cc) product.

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PubMed2004

Surface functionalization of ultrananocrystalline diamond films by electrochemical reduction of aryldiazonium salts.

The surface functionalization of ultrananocrystalline diamond (UNCD) thin films via the electrochemical reduction of aryl diazonium cations is described. The one-electron-transfer reaction leads to the formation of solution-based aryl radicals, which in turn react with the UNCD surface forming stable covalent C-C bonds. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), ac impedance spectroscopy, and contact angle measurements have been employed to characterize the organic overlayer and estimate the surface coverage. The grafting of 3,5-dichlorophenyl groups renders the UNCD surface hydrophobic, whereas the attachment of 4-aminophenyl groups makes the surface relatively hydrophilic. The surface coverage, estimated from the electrochemical and XPS measurements, is as high as 70% of a compact monolayer. The aminophenyl terminated surface was obtained by electrochemical reduction of the tethered nitrophenyl groups. This two-step approach yields a UNCD surface with functional moieties available for the potential covalent coupling of a wide variety of biomolecules (e.g., DNA and proteins).

Journal Article↗
PubMed2004

Analysis of corrosion processes at the surface of diamond-like carbon protected zinc selenide waveguides.

A detailed surface analytical study on the corrosion behavior of unprotected and diamond-like carbon (DLC)-coated mid-infrared (MIR) waveguides used in remote sensing applications at strongly oxidizing conditions is presented. High-quality DLC films, with a thickness of 100 nm serving as MIR-transparent corrosion barrier, have been produced at the surface of zinc selenide (ZnSe) attenuated total reflection waveguides via pulsed laser deposition techniques. IR microscopy and atomic force microscopy are applied to investigate the chemical inertness of DLC-based membranes against aqueous solutions of hydrogen peroxide. These stability studies show that uncoated ZnSe waveguides are subject to severe chemical surface modifications, while DLC-protected waveguides maintain their optical properties and chemical integrity. In situ studies on the corrosion behavior by a recently developed approach combining scanning electrochemical microscopy (SECM) with Au/Hg amalgam ultramicroelectrodes in a scanning stripping voltammetry experiment provides additional insight into the mechanisms of the corrosion process. It is demonstrated that the combination of surface analytical techniques and, in particular, the innovative application of SECM with amalgam electrodes provides superior information on corrosion processes at the surface of optical waveguides. This detailed study confirms the efficiency of protective DLC coatings deposited onto IR-transparent optical waveguides, rendering this novel concept ideal for sensing applications in harsh environments.

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