Charge polarization at catalytic metal-support junctions : Part B : Theoretical modeling of Kelvin probe force microscopy

Existing models for the analysis of Kelvin probe microscopy experiments are extended and used to analyze the experimental electrical potential profiles for a Pt/TiO2 model nanoparticle. The derived model reproduces in detail the Kelvin probe image that reveals a characteristic ring-shaped negative charge zone at the surface around the particle: A planar negative charge zone at the surface of the support extends beyond the diameter of the Pt particle. It is compensated mostly by a planar layer of positive charges in the metal across the interface, and by a smaller number of positive charges at the metal-air interface. These latter charges determine the positive electrical potential of the metal particle, and they are likely responsible for the extent of the metal-support interaction in catalytic reactions.http://pubs.acs.org/journal/jpcafh2017-06-30hb2016Chemistr

Reaction kinetics of muonium with the halogen gases (F2, Cl2, and Br2)

Copyright @ 1989 American Institute of PhysicsBimolecular rate constants for the thermal chemical reactions of muonium (Mu) with the halogen gases—Mu+X2→MuX+X—are reported over the temperature ranges from 500 down to 100, 160, and 200 K for X2=F2,Cl2, and Br2, respectively. The Arrhenius plots for both the chlorine and fluorine reactions show positive activation energies Ea over the whole temperature ranges studied, but which decrease to near zero at low temperature, indicative of the dominant role played by quantum tunneling of the ultralight muonium atom. In the case of Mu+F2, the bimolecular rate constant k(T) is essentially independent of temperature below 150 K, likely the first observation of Wigner threshold tunneling in gas phase (H atom) kinetics. A similar trend is seen in the Mu+Cl2 reaction. The Br2 data exhibit an apparent negative activation energy [Ea=(−0.095±0.020) kcal mol−1], constant over the temperature range of ∼200–400 K, but which decreases at higher temperatures, indicative of a highly attractive potential energy surface. This result is consistent with the energy dependence in the reactive cross section found some years ago in the atomic beam data of Hepburn et al. [J. Chem. Phys. 69, 4311 (1978)]. In comparing the present Mu data with the corresponding H atom kinetic data, it is found that Mu invariably reacts considerably faster than H at all temperatures, but particularly so at low temperatures in the cases of F2 and Cl2. The current transition state calculations of Steckler, Garrett, and Truhlar [Hyperfine Interact. 32, 779 (986)] for Mu+X2 account reasonably well for the rate constants for F2 and Cl2 near room temperature, but their calculated value for Mu+Br2 is much too high. Moreover, these calculations seemingly fail to account for the trend in the Mu+F2 and Mu+Cl2 data toward pronounced quantum tunneling at low temperatures. It is noted that the Mu kinetics provide a crucial test of the accuracy of transition state treatments of tunneling on these early barrier HX2 potential energy surfaces.NSERC (Canada), Donors of the Petroleum Research Fund, administered by the American Chemical Society, for their partial support of this research and the Canada Council

Superparaelectric phase in the ensemble of non-interacting ferroelectric nanoparticles

For the first time we predict the conditions of superparaelectric phase appearance in the ensemble of non-interacting spherical ferroelectric nanoparticles. The superparaelectricity in nanoparticle was defined by analogy with superparamagnetism, obtained earlier in small nanoparticles made of paramagnetic material. Calculations of correlation radius, energetic barriers of polarization reorientation and polarization response to external electric field, were performed within Landau-Ginzburg phenomenological approach for perovskites Pb(Zr,Ti)O3, BiFeO3 and uniaxial ferroelectrics rochelle salt and triglycine sulfate.Comment: 28 pages, 7 figures, 3 Appendices, to be submitted to Phys. Rev.

Unusual enhancement of effective magnetic anisotropy with decreasing particle size in maghemite nanoparticles

In magnetic nanoparticles (NPs), the observed increase in the effective magnetic anisotropy Keff with the decrease in particle size D is often interpreted, sometimes unsuccessfully, using the equation Keff = Kb + (6KS/D), where Kb is the bulk-like anisotropy of the core spins and KS is the anisotropy of spins in the surface layer. Here, we test the validity of this relation in γ-Fe2O3 NPs for sizes D from 15 nm to 2.5 nm. The samples include oleic acid-coated NPs with D = 2.5, 3.4, 6.3, and 7.0 nm investigated here, with results on 14 other sizes taken from literature. Keff is determined from the analysis of the frequency dependence of the blocking temperature TB after considering the effects of interparticle interactions on TB. For the γ-Fe2O3 NPs with D \u3c 5 nm, an unusual enhancement of Keff with decreasing D, well above the magnitudes predicted by the above equation, is observed. Instead the variation of Keff vs. D is best described by an extension of the above equation by including Ksh term from spins in a shell of thickness d. Based on this core-shell-surface layer model, the data are fit to the equation Keff = Kb + (6KS/D) + Ksh{[1−(2d/D)]−3−1} with Kb = 1.9 × 105 ergs/cm3, KS = 0.035 ergs/cm2, and Ksh = 1.057 × 104 ergs/cm3 as the contribution of spins in the shell of thickness d = 1.1 nm. Significance of this result is discussed

Proton NMR for Measuring Quantum-Level Crossing in the Magnetic Molecular Ring Fe10

The proton nuclear spin-lattice relaxation rate 1/T1 has been measured as a function of temperature and magnetic field (up to 15 T) in the molecular magnetic ring Fe10. Striking enhancement of 1/T1 is observed around magnetic field values corresponding to a crossing between the ground state and the excited states of the molecule. We propose that this is due to a cross-relaxation effect between the nuclear Zeeman reservoir and the reservoir of the Zeeman levels of the molecule. This effect provides a powerful tool to investigate quantum dynamical phenomena at level crossing.Comment: Four pages, to appear in Phys.Rev.Let

A homebuilt ESE spectrometer on the basis of a high-power Q-band microwave bridge

We present a Q-band spectrometer which was built recently at the Institute of Physical Chemistry of the University of Stuttgart. It allows us to perform the field-sweep electron spin echo (ESE), pulsed electron-nuclear double resonance (ENDOR), relaxation and electron spin echo envelope modulation experiments both at room and low (down to 1.5 K) temperatures. The spectrometer consists of an electromagnet, digital field controller, pulsed microwave bridge, probehead, cryostat, radio frequency unit, pulse programmer and data acquisition electronics. The Q-band microwave bridge with 10.8 W output power is based on a two-stage IMPATT-diode pulse amplifier. The commercial Varian electromagnet system is controlled by a 24-bit home-built digital controller. The external devices are interfaced to the two PCs via GPIB and LAN. The spectrometer control software was developed in Visual C++. It consists of two programs running synchronously on the control PCs. The spectrometer is equipped with a cylindrical TE 011 cavity constructed both for ESE and for pulsed ENDOR. The cavity fits into a liquid He cryostat thus allowing low-temperature experiments. An 8-bit data acquisition digitizer is used to collect the echo signals, and the PBESR-PRO-400 digital word generator orchestrates the pulse experiments and sets pulse sequences of the microwave bridge. The spectrometer performance is demonstrated on nitrogen impurities in a polycrystalline synthetic diamond, on silver clusters supported on NaA zeolite and electron-irradiated tooth enamel. © 2008 Springer-Verlag

Size-Dependent Materials Properties Toward a Universal Equation

Due to the lack of experimental values concerning some material properties at the nanoscale, it is interesting to evaluate this theoretically. Through a “top–down” approach, a universal equation is developed here which is particularly helpful when experiments are difficult to lead on a specific material property. It only requires the knowledge of the surface area to volume ratio of the nanomaterial, its size as well as the statistic (Fermi–Dirac or Bose–Einstein) followed by the particles involved in the considered material property. Comparison between different existing theoretical models and the proposed equation is done

Annealing of gold nanostructures sputtered on polytetrafluoroethylene

Gold nanolayers sputtered on polytetrafluoroethylene (PTFE) surface and their changes induced by post-deposition annealing at 100°C to 300°C are studied. Changes in surface morphology and roughness are examined by atomic force microscopy, electrical sheet resistance by two point technique, zeta potential by electrokinetic analysis and chemical composition by X-ray photoelectron spectroscopy (XPS) in dependence on the gold layer thickness. Transition from discontinuous to continuous gold coverage takes place at the layer thicknesses 10 to 15 nm and this threshold remains practically unchanged after the annealing at the temperatures below 200°C. The annealing at 300°C, however, leads to significant rearrangement of the gold layer and the transition threshold increases to 70 nm. Significant carbon contamination and the presence of oxidized structures on gold-coated samples are observed in XPS spectra. Gold coating leads to a decrease in the sample surface roughness. Annealing at 300°C of pristine PTFE and gold-coated PTFE results in significant increase of the sample surface roughness

Molecular Dynamics of the Muonium-C60 Radical in Solid C60

The molecular dynamics and electronic structure of the μ+-C60 radical in crystalline C60 have been studied using muon spin rotation and relaxation. At room temperature μ+-C60 appears to be in a state of quasifree rotation. At the critical temperature TS=260 K the local electronic structure and molecular dynamics change discontinuously as expected for a first-order phase transition. The correlation times for reorientation are remarkably close to those determined by recent NMR experiments on C60, suggesting that the molecular dynamics of μ+-C60 are strongly coupled to those of its C60 neighbors