Investigation of dielectric after-effects in poly- and single crystalline BaTiO3

Dielectric relaxation occurring in single- and polycrystalline BaTiO₃, following different polarization treatments of the specimens has been investigated in the temperature range 10 K<T<450 K. The observed relaxation phenomena are discussed in terms of polarization induced growth of extra-domains which, via corresponding enhancement of the total domain wall area, may account for the time dependence of the dielectric constant. On the basis of this relaxation model the activation parameters of the different processes are determined and discussed in terms of variations of the wall mobility within the different crystallographic phases occurring in BaTiO₃.     

Binding states of CO on single crystal planes of Pt

Flash desorption mass spectrometry and Auger electron spectroscopy are used to compare the binding states, desorption and adsorption kinetics, and adsorbate densities on the (111), (100), (110), (211), and (210) crystal planes of clean Pt. Desorption obeys first order kinetics for all states with activation energies of the most tightly bound states varying from 36 kcal mole^?1 on (211) and (210) to 26 kcal mole^?1 on (110) and (111). The sticking coefficient is nearly unity on (110) and (210) and is 0.24 on (100). Multiple binding state (or breaks in the desorption activation energy versus coverage) are observed on all planes. The saturation CO density at 300 K is highest on the (100), (210), and (211) planes and lowest on (110). Properties of (210) and (211) cannot be explained simply in terms of sites on the other planes, and adsorption indicates that none of the planes facet. Previous models of CO on (111) and (110) are compared with present results, and structures are suggested for the other planes.     

Electrochemical activity of Sn-modified Pt single crystal electrodes for ethanol oxidation

Low-index plane Pt single crystal electrodes modified by submonolayer deposition of Sn have been tested for ethanol oxidation in acidic media using cyclic voltammetry and chronoamperometry. The enhancement factor for ethanol oxidation depends on both substrate crystallography and Sn coverage. The optimum coverage was found to be around 0.2, 0.25 and 0.52 for (1 0 0), (1 1 1) and (1 1 0), respectively. The enhancement factor was found to decrease in the order: Sn/Pt (1 1 0) > Sn/Pt (1 0 0) > Sn/Pt (1 1 1). On the other hand, the current density obtained at 0.4 V after 15 min of electrolysis was found to decrease in the order: Sn/Pt (1 1 0) > Sn/Pt (1 1 1) > Sn/Pt (1 0 0).     

Structural dependence of intermediate species for the hydrogen evolution reaction on single crystal electrodes of Pt

Adsorbed hydrogen and water were measured during the hydrogen evolution reaction (HER) on the low and high index planes of Pt in 0.5 M H₂SO₄ using infrared reflection absorption spectroscopy. Hydrogen is adsorbed at the atop site (atop H) on Pt(110) during the HER, whereas adsorbed hydrogen at the asymmetric bridge site (bridge H) is found on Pt(100). The band intensity of the adsorbed hydrogen depends on temperature, indicating that the bands are due to the intermediate species for the HER. The band of the atop H appears on stepped surfaces with (110) step, whereas the asymmetric bridge H is observed on Pt(211) = 3(111)–(100) and Pt(311) = 2(111)–(100) that have (100) step. The absence of the atop H on Pt(100), Pt(211), and Pt(311) can be attributed to the relative stability of the bridge site.     

Diffusion and binding of CO on Pt nanowires

The diffusion and binding of CO molecules on self organized Pt nanowires on Ge(0 0 1), have been studied by room temperature scanning tunneling microscopy. CO molecules are found to bind at the bridge position of the Pt dimers. The CO molecules are mobile at room temperature and perform an “unrestricted” one-dimensional random walk in defect-free regions of the Pt nanowires.     

Non-equilibrium surface phase transitions during the catalytic oxidation of CO on Pt(100)

Isothermal low-pressure oscillations of the rate of catalytic CO oxidation on a Pt(100) surface could be established under appropriate conditions and were monitored through the accompanying periodic variation of the work function. Parallel observations by the Video-LEED technique demonstrated that these oscillations are associated with periodic transformations of the (long-range) surface structure from the reconstructed hex to the 1 × 1 phase and back, which is caused by varying surface concentrations of the reacting particles.     

Kinetic instabilities during the NO(x) reduction with hydrogen on Pt crystals studied with field emission on the nanoscale

This paper reviews field emission studies of kinetic instabilities occurring during the catalytic reduction of nitric oxide (NO) and nitrogen dioxide (NO(2)) by hydrogen on three-dimensional platinum crystals. Emphasis is placed on revealing that both field ion microscopy (FIM) and field electron microscopy (FEM) can image such instabilities under truly in situ reaction conditions with a lateral resolution on the nanoscale. In particular, oscillatory behavior with rapid ignition from a state of low to a state of high catalytic activity is demonstrated for both NO and NO(2) reduction. Results of a local chemical probing during FIM studies of the NO+H(2) reaction are also shown and provide clear evidence for the oscillatory behavior of water (detected as H(2)O(+) and H(3)O(+)) formation and for diffusion supply of NO into surface regions emptied during the stage of high catalytic activity. The rapid ignition ("surface explosion") of the catalytic cycle is discussed on the basis of an autocatalytic mechanism of the NO decomposition. On the (001) plane of the Pt crystal small island formation is seen to occur during the low-activity state of the catalytic cycle. Islands have a size equivalent to approximately 3 nm, move independently from each other, and do not merge when colliding. A tentative model is discussed associating islands with patches of hydroxyl groups. Very regular oscillatory behavior is demonstrated for the NO(2) reduction using FEM. Advantages as well as shortcomings of the FEM/FIM experimental approach are discussed and an outlook on future studies using local chemical probing will be given wherever appropriate. (c) 2002 American Institute of Physics.     

Structure and reactivity of surface oxides on Pt(110) during catalytic CO oxidation

We present the first structure determination by surface x-ray diffraction during the restructuring of a model catalyst under reaction conditions, i.e., at high pressure and high temperature, and correlate the restructuring with a change in catalytic activity. We have analyzed the Pt(110) surface during CO oxidation at pressures up to 0.5 bar and temperatures up to 625 K. Depending on the pressure ratio, we find three well-defined structures: namely, (i) the bulk-terminated Pt(110) surface, (ii) a thin, commensurate oxide, and (iii) a thin, incommensurate oxide. The commensurate oxide only appears under reaction conditions, i.e., when both and CO are present and at sufficiently high temperatures. Density functional theory calculations indicate that the commensurate oxide is stabilized by carbonate ions (CO3(2-)). Both oxides have a substantially higher catalytic activity than the bulk-terminated Pt surface.     

Laser-induced instabilities of a continuous thermal trace on a surface

Results are presented that corroborate the important role played by laser-induced vapor-phase oxidation in the onset of an instability of the irradiation trace and formation of self-organized structures.     

Pattern formation during the oxidation of CO on Pt{100}: a mesoscopic model

Constantly changing irregular patterns of carbon monoxide (CO) and oxygen are seen during CO oxidation on platinum crystals in the [100] orientation. Ours is the first reaction-diffusion model to reproduce this pattern formation on physically feasible length and time scales, faithfully incorporating the available experimental data. Numerical simulations show patterns made up of CO and oxygen fronts moving at similar speeds to those seen in experiments.     

Template preparation of Pt nanowire array electrode on Ti/Si substrate for methanol electro-oxidation

Platinum (Pt) nanowire array electrode is obtained by dc electrodeposition of Pt into the pores of anodic aluminum oxide (AAO) template on Ti/Si substrate. Transmission electron microscope (TEM) examination shows all the nanowires have uniform diameter of about 30 nm. The brush shapes Pt nanowire array electrode can be seen clearly by field emission scanning electron microscope (FESEM). Pt nanowire array electrode gives the X-ray diffraction (XRD) pattern of face-centered cubic (fcc) crystal structure. The electro-oxidation of methanol on this electrode is investigated at room temperature by cyclic voltammetry. The results demonstrated that the Pt nanowire array electrode will have good potential applications in portable power sources.     

Orientation of chemisorbed species from orthogonal-plane arups: Tilted CO on Pt{110} and upright CO on Pt{111}.

Angle-resolved photoemission spectra obtained with electron collection in the plane orthogonal to the incidence plane, using a He II photon source, are used to provide an accurate measure of the orientation of chemisorbed diatomic molecules. As previously established, CO in the Pt{111}c(4 × 2) structure is found to be chemisorbed in an upright configuration, whereas in the Pt{110}(2 × 1) p1g1 structure the CO molecular axis is tilted 26 ± 2° away from the surface normal, in a direction between [211] and [433].     

Monte Carlo simulation of temperature programmed reaction and surface explosion during CO oxidation on a Pt catalyst

A Monte Carlo model of CO oxidation on a Pt(111) surface that includes finite rates of adsorption-desorption and reaction and the effect of the catalyst temperature is presented. The results show that, as expected from the reaction-adsorption probabilities, the surface coverage changes from being almost completely covered by CO at low temperature (60°C), to being completely covered by oxygen at high temperature (160°C). Furthermore, it was found that an unstable state occurs when cooling down the oxygen covered surface from 160°C to 60°C. It is shown that if a site for CO adsorption is created under this metastable state, a surface explosion that propagates spatially occurs. Thus the MC simulations provide a method to describe a catalytic reaction on surfaces with strongly non-linear spatio-temporal dynamics.     

Chemisorption of CO on the unreconstructed Pt(100) surface

The chemisorption of CO on the clean, unreconstructed Pt(100)-1 × 1 surface was investigated by LEED and XPS. Three LEED patterns, c(2 × 2), (√2 × 3√2) R45° and c(4 × 2), were observed with increasing CO exposure and structure models corresponding to these LEED patterns were proposed. The absolute coverage of CO was determined by combining the O(1s) XPS data with coverage information derived from LEED. The maximum CO coverage thus obtained was θ = 0.75 and the initial sticking coefficient was determined to be s₀ = 0.6. This coverage calibration can also be utilized for other oxygen containing molecules by comparing the corresponding O(1s)it peak intensities.     

Interactions between NO and CO on Pt(111)

Temperature programmed desorption (TPD) of coadsorbed NO and CO on Pt(111) shows that no reaction occurs (less than 2%) up to the desorption temperature of NO. At 100 K, adsorption is competitive, but neither gas displaces the other from the surface. Coadsorbed CO causes the NO desorption temperature to be lowered by as much as 100 K, but NO does not affect the CO desorption temperature. TPD spectra for NO depend on which gas is adsorbed first, indicating that equilibrium between species is not established on the surface during desorption. Electron energy loss spectra show that the vibrational spectrum of each gas is only weakly affected by the other. When NO is adsorbed first, CO does not affect the ratio of bridged and terminal NO but lowers the frequencies of the bridged NO by approximately 50 cm^?1 and lowers the intensities of vibrational peaks of both species by a factor of about four. When CO is adsorbed first, the ratio of terminal to bridged NO increases for given coverage of NO, and the frequency of the bridged NO remains at the pure NO value. These results are explained in terms of CO island formation, repulsive interactions between NO and CO, and low adsorbate mobilities.     

Compatibility of field emitter studies of oscillating surface reactions with single crystal measurements: catalytic CO oxidation on Pt

Kinetic oscillations in catalytic CO oxidation on Pt have been studied on large (millimeter size) single crystal planes of Pt as well as on a Pt field emitter tip that exposes different crystal facets of nanometer size. In order to examine the compatibility of results from the two types of experiments, the regions of different dynamical behavior (bifurcation diagram) have been mapped out in p_CO,T-parameter space using a field electron microscope (FEM) and a field ion microscope (FIM). The comparison with the results of single crystal measurements shows that in the case of applied electrostatic fields less than 5 V nm⁻¹ (FEM), the field-induced effects are negligible, but they are significant for fields exceeding 12 V nm⁻¹ (FIM). The field-induced shift of the bifurcation diagram toward lower p_CO values, observed with FIM, is explained in terms of a field-modified interaction of CO and O₂ with Pt studied here with field ion appearance energy spectroscopy. With coadsorbed lithium (submonolayer coverage), the existence range for rate oscillations is shifted toward higher p_CO values. This shift is attributed to a redistribution of the electron density at the surface induced by alkali metal co-adsorption.     

Systematic experimental study of instabilities in the sealed CO2 laser

A systematic pressure-current experimental study of the wave instability regions in glow-discharge of different CO₂ laser mixtures has been accomplished. The forward ionization wave F^– has been found to be the main instability. At low currents (< 30 mA) and higher pressures (> 550 Pa) the laser discharge becomes stable. The highest output power at a given mixture ratio was achieved in these quiet regions. The normalized electric field dependences of wavenumber, phase and group velocities of the F^– wave are presented.     

CO adsorption and dissociation on iron oxide supported Pt particles

We studied CO adsorption on Pt particles deposited on well-ordered Fe₃O₄(1 1 1) thin films grown on Pt(1 1 1) by temperature programmed desorption (TPD). A highly stepped Pt(1 1 1) surface produced by ion sputtering and annealing at 600 K was studied for comparison. Structural characterization was performed by scanning tunneling microscopy and Auger electron spectroscopy. The TPD spectra revealed that in addition to the desorption peaks at ∼400 and 480 K, assigned to CO adsorbed on Pt(1 1 1) facets and low-coordination sites respectively, the Pt nanoparticles annealed at 600 K exhibit a desorption state at ∼270 K. This state is assigned to initial stages of strong metal support interaction resulting in partial Fe-Pt intermixing. On both Pt/Fe₃O₄(1 1 1) and stepped Pt(1 1 1) surfaces CO is found to dissociate at 500 K. The results suggest that CO dissociation and carbon accumulation occur on the low-coordinated Pt sites.