NaF Surface Research Articles

Towards improved structural stability and electrochemical properties of a Li-rich material by a strategy of double gradient surface modification

The electrochemical properties of layered Li-rich oxide Li1.2Ni0.13Co0.13Mn0.54O2 (LLO) can be enhanced by a facile NaF surface modification. We have clarified by XPS, HR-TEM and HAADF-STEM analyses that the mechanism is the formations of a gradient coating layer Na1-xLixF and a gradient LLO surface Li1.2-xNaxNi0.13Co0.13Mn0.54O2 via a Li+/Na+ exchange reaction. The exchange reaction occurs at the Li-slabs of LLO down to a depth of less than 15 nm. After such a double gradient surface modification, the LLO displays markedly improved cycle life and rate capability as well as suppressed voltage decay. It exhibits a capacity retention of 85% beyond 1000 cycles at 10 C and a voltage decay of only 0.0021 V per cycle at 0.5 C, revealing great application prospects in energy storage materials.

Insights into Working Mechanism of Alkali Metal Fluorides as Dopants of ZnO Films in Inverted Polymer Solar Cells

Alkali metal fluorides (AMFs) have been utilized as dopants of ZnO films in PTB7:PC71BM based inverted polymer solar cells (i-PSCs). As a result, power conversion efficiency (PCE) and device stability were obviously improved in i-PSCs with ZnO:AMF. In particular for the i-PSCs with ZnO:NaF (0.2 mol %), PCE of 8.64% was obtained in the fresh devices and the PCE still remained at 7.56% after i-PSCs were retained in the glovebox for 80 days, which were much better than the only ZnO devices. Results of photocurrent density–effective voltage characteristics, electron mobility, and capacitance–voltage characteristics demonstrated that doping ZnO with AMFs can effectively raise the charge carrier extraction, electron mobility, charge carrier density, and built-in potential in the i-PSCs. X-ray photoemission spectroscopy (XPS) measurements indicated that oxygen defects on the ZnO surface were reduced through doping with AMFs. Atomic force microscopy images showed that adding 0.2 mol % AMFs into ZnO did not lead to the lattice distortion of ZnO films. Scanning electronic microscopy–energy dispersive spectrum mapping and XPS depth profiling suggested that there were more Na+ species at the top of the ZnO:NaF surface than K+ or Cs+ at the top of the ZnO:KF or ZnO:CsF surface. Therefore, Na+ have more chances to meet the defects on the ZnO surface than K+ and Cs+ so that the i-PSCs with ZnO:NaF shows the best PCE. These results reveal that NaF is an effective, competitive, and prospective dopant of ZnO in i-PSCs.

Surface chemistry and physical properties of Nafion/polypyrrole/Pt composite membrane prepared by chemical in situ polymerization for DMFC

We modified Nafion by means of chemical in situ polymerization of pyrrole monomers with platinum (Pt) precursors for an application into an electrolyte of direct methanol fuel cells (DMFCs). SEM and EPMA exhibited the presences of polypyrrole and Pt at the surface region of Nafion, after diffusing and polymerizing pyrrole monomers with Pt precursors. XPS and FT-IR spectra were used to characterize the surface of Naf–Ppy–Pt composite membranes, demonstrating that pyrrolinum groups of polypyrrole were interacted with sulfonic groups or Pt precursors (PtCl 6 − or PtCl 4 −). After in situ polymerization of pyrrole monomers, the morphological reorganization of sulfonic groups in Naf–Ppy–Pt composite membranes occurred via electrostatic interaction. Thermal stability, proton conductivity, methanol permeability, and cell performance of composite membranes were analyzed by TGA, AC impedance, refractometer, and potentiostat. Naf–Ppy–Pt composite membranes had higher thermal stabilities of sulfonic groups and side chains than Nafion and Naf–Ppy as a result of the interaction between Nafion–SO 3 −⋯polypyrrole–NH 2 + and the presence of thermally stable Pt. The cell performance of Naf–Ppy–Pt 0 0 2 was enhanced significantly compared to that of Nafion under the specific condition, due to more reduction of methanol crossover than that of proton conductivity. Therefore, this synthetic method offers a facile way to improve physical properties of polymer electrolyte for the fabrication of advanced composite membranes.

Effects of polysiloxane coating of NaF on the release profile of fluoride ion from Bis-GMA/TEGDMA resin containing NaF.

The aim of this study was to regulate fluoride release from restorative resin containing NaF using N-(beta-aminoethyl)-gamma-aminopropylmethyldimethoxysilane (AMMS) and evaluate factors that regulate fluoride release from the resin. ESCA analysis, FT-IR measurements along with SEM observations demonstrated that a polysiloxane layer was formed on the surface of NaF treated with AMMS. Bis-GMA/TEGDMA resin containing NaF powder treated with AMMS released lower concentrations of fluoride for longer periods when compared with that containing untreated NaF. However, AMMS treatment of NaF was less effective for the regulation of fluoride released from the resin than gamma-methacryloxypropyltrimethoxysilane (gamma-MPTS) treatment, despite its higher hydrophobic polysiloxane layer formation. These findings may have been caused by the higher density of polysiloxane prepared with gamma-MPTS than that prepared with AMMS. The present findings suggested, therefore, that alkoxysilane should be chosen based not only on hydrophobicity but also the density of polysiloxane to effectively regulate fluoride release from the restorative resin containing NaF.

Recent Progress in Microtribology. Atomic-scale Tribology Studied by Frictional Force Microscope.

A two-dimensional frictional force microscope study has been made on an atomic scale tribology between a sharp tip of a single asperity of Si3N4 and an atomically flat surface of NaF(100), where the frictional force becomes two-dimensional vector. By increasing the normal load from 1.4 nN to 2.2 nN, the periodicity of the two-dimensional stick-slip motion of the tip is remarkably changed from lattice periodicity to atomistic periodicity. This load dependence of the periodicity qualitatively agrees with that of the tip-surface interaction due to surface relaxation by increasing the load, which is theoretically predicted using computer simulation. Within the load range where the atomistic periodicity appears, the load dependence of the amplitude of the friction shows anomalous behavior. Further, by increasing the load more than ∼3.6 nN it is suggested that the tip and/or NaF surface break down, because the two-dimensional stick-slip motion disappears.

Atomic force microscopic observation of step movements on NaCl(001) and NaF(001) with the help of adsorbed water

Cleaved (001) faces of NaCl, NaF and LiF single crystals have been observed with atomic force microscopy in air. Spontaneous movements of monatomic steps were observed, depending upon relative humidity, with the first two crystals. The step speed on NaCl showed a marked increase at about 60%, while only a gradual increase was observed with NaF. The step movements on NaCl were not disturbed very much by screw dislocations, but were substantially retarded by surface voids. The presence of adsorbed water in two-dimensionally and three-dimensionally condensed phases is a reasonable explanation for these effects. By fixing the AFM tip on flat terraces, crystalline blocks were formed on the NaF surface. They grew spontaneously, collecting salt from flat parts. The salt is pumped from the substrate to the top of the blocks. A mechanism to explain this is proposed here.

Load dependence of the periodicity in friction force images on the NaF(100) surface

Abstract We investigated load dependence of the frictional force between a NaF(100) surface and a Si3N4 asperity on an atomic scale using a two-dimensional frictional force microscope. As a result, we found that the periodicity in the frictional image changes with the load. Below the normal load of 1·9nN, the usual lattice periodicity of the NaF surface is observed. On the other hand, at the normal load between 1·9 and 2·8 nN, an anomalous periodicity of half the lattice periodicity is observed for the first time. At the normal load between 3·1 and 3·3 nN, the usual lattice periodicity is again observed. This load dependence of the periodicity consistent with the prediction of the computer simulation by Tang, Joachim, Devillers and Girard. Finally, above the normal load of 3·6 nN, the periodicity disappeared.

Two-dimensionally discrete friction on the NaF(100) surface with the lattice periodicity

Using the two-dimensional frictional force microscope, we observed two-dimensional discrete friction on the NaF(100) surface with its lattice periodicity, which is explained by the two-dimensional stick-slip model quantitatively. By the raster scan, such a discrete friction composes the lattice periodicity image of the NaF surface. This means that one of the AFM contrast mechanisms of the lattice periodicity of the ionic crystal is the discrete friction. On the other hand, with a normal load of 14 nN the discrete friction disappeared, so the number of contact atoms is estimated as a few atoms. Thus, even under a near single-atom friction regime the atomic scale friction is explained by the two-dimensional stick-slip model quantitatively.

Thermal and intrinsic stresses in single-crystal nickel films

The thermal and intrinsic stresses in single-crystal nickel films, in the thickness range 500-3000 AA, prepared by epitaxial growth onto the (100) cleaved surfaces of NaF, LiF, NaCl and MgO have been obtained. Calculated results from ferromagnetic resonance experiments show that the magnitudes and directions of the film stresses depend on the nature of the substrate conditions and on the sequential ordering of the deposited layers. The experimental results revealed that the thermal stresses, which are elastic and isotropic in character, are larger than the intrinsic stresses and can be of either compressive or tensile type. This type of stress disappears after the film is removed from the substrate. The present results also show that there is no single model which sufficiently accounts for the magnitudes of the intrinsic stresses in the strained films.

Evidence for threshold effects in positron diffraction from NaF and LiF.

We have measured the energy dependence of the intensity of a positron beam specularly reflected from the (100) surfaces of NaF and LiF. A 1-eV-wide peak located at an energy that decreases with increasing angle of incidence \ensuremath{\theta} is qualitatively identified as a beam threshold effect. Given the known small positron affinity for the solids, the narrowness of the peak is consistent with its being due to a true surface resonance. However, the dispersion appears to depart from the expected energy versus \ensuremath{\theta} trajectory parallel to the kinematic threshold. Additionally, a search in 50-meV steps did not reveal any fine structure in the peaks, possibly due to our 0.3-eV effective instrumental resolution. Further experiments to map the dispersion with greater precision, to measure the binding energies of the resonances, and to search for possible fine structure are proposed.

Low-energy positron-diffraction study of NaF and LiF.

Using a magnetically transported beam of slow positrons, we have measured the elastic backscattering probability versus energy for positrons normally incident on air-cleaved and vacuum-cleaved (100) surfaces of NaF and LiF. We observe enormous first-order Bragg peaks and several higher-order peaks, the positions of which imply an inner potential of -0.5\ifmmode\pm\else\textpm\fi{}0.2 and -0.1\ifmmode\pm\else\textpm\fi{}0.2 eV for NaF and LiF, respectively. The atomic plane spacings are equal to the bulk values within 1/2%. There are only slight differences between the data obtained using the freshly air-cleaved and the vacuum-cleaved surfaces. Surface disorder may be responsible for the presence of some weak Bragg reflections corresponding to directions that would otherwise be inaccessible to the incident positron beam. The temperature dependence indicates that the disorder would not have a thermal origin.

Adsorption isotherms of simple gases on sodium fluoride: Significance of two-dimensional critical pressure

On the NaF surface, two-dimensional condensation of Ar, CH 4, O 2 and N 2 occurred, although the adsorption isotherms of Kr did not show any indication of this phenomenon. The position of the step due to the two-dimensional condensation in the isotherm depended largely on the kind of adsorbed gas: the relative pressure at which the two-dimensional condensation occurs increased in the order N 2 < CH 4 < O 2 < Ar. The thermodynamic analysis of these experimental data proved that the ratio of the critical pressure to the vapor pressure of the bulk liquid or solid increased as the binding energy of gas to surface became small compared with the heat of bulk condensation. It was concluded that the Kr-NaF system was one of the special adsorption systems where the critical pressure was higher than the vapor pressure of the bulk liquid or solid at the critical temperature.

Two-dimensional condensation of water and alcohols on NaF

On the surface of NaF the adsorption isotherms of H 2O, D 2O, CH 3OH, C 3H 3OH and 1-C 3H 7OH as well as the infrared spectra of H 3O, D 2O, dilute HDO, CH 3OH and CH 3OD were measured. The adsorption temperatures of H 3O (253–308 K) were within the phase transition region where two phases of low and high density coexist, while those of CH 3OH, C 2H 5OH and 1-C 3H 3OH were yet within a super-critical region. The entropy of the 2D condensed H 2O on NaF was found to be 14.0 cal K −1 mol −1, which suggests that the condensed phase of water on NaF is liquid-like. The OD stretching band of dilute HDO in the 2D condensed water gives a maximum adsorption at ca. 2530 cm −1 with a half width of ca. 150 cm −1, being in good agreement with that in liquid water. Comparison of the integrated absorbance of the D 2O bending mode with that of the OD stretching mode suggests that the cluster size of the 2D condensed water on NaF decreases with increasing temperature. The 2D critical temperature and the occupied areas of these adsorbates enable us to conclude that the compatibility of the molecular size with the surface lattice is not important in the occurrence of the 2D condensation of the hydrogen-bonding molecules on NaF and that adsorbed molecules are randomly oriented on the surface to the extent similar to that in 3D liquid state.

Dispersion relations of surface phonons in LiF(001) and NaF(001)

Recent atom-scattering experimental data have stimulated a new investigation on the surface phonons of some alkali halides. In the framework of Green's-function method applied to breathing-shell model dynamics we have calculated the dispersion curves and a few $K$-selected densities of states of surface phonons for the (001) surface of NaF and LiF at room temperature. Particular attention is paid to the change in ion polarizabilities occurring at the surface and its effect on the Rayleigh wave frequencies at the zone boundary.

Selective adsorption of 3He and 4He on clean surfaces of NaF and LiF

Atomic beam scattering was used to study the gas-surface interaction between helium and ionic crystals. In particular, the binding energies of the selectively adsorbed states were measures for two isotopes of helium on the (001) surfaces of NaF and LiF cleaved in vacuum. Consistent results were obtained by avoiding incident conditions where band structure effects invalidate the free particle approximation. The energy levels can be fit to a 9-3 model potential.

Bound state vibrational spectrum of the 3–9 atom-surface interaction

The potential V(z) = C 9 z 9 − C 3 z 3 is a reasonable parametrization of the atom-surface interaction. We evaluate the discrete spectrum E n of bound states for this potential with arbitrary coefficients C 9 and C 3. The resulting form in the WKB approximation is E n = −D [1 − (n + l 2 ) L ] 6 , where L depends on the mass and D is the well depth. We find that the exact solution of the Schrödinger equation can be written in the same form, with n shifted slightly by an amount δ n , which we calculate. The results are applied to the case of He near a NaF surface, in which the calculated eigenvalue spectrum agrees well with experimental values.

The inelastic scattering of He and Ne from a (001) surface of sodium fluoride

The inelastic scattering of He and Ne atoms from a cleaved (001) surface of NaF has been studied at different surface temperatures. The study was similar to that for the He–LiF system, in that the intensity of the inelastic streaks in the vicinity of diffraction peaks was explored as a function of detector location and crystal rotation. For diffraction peaks not too close to the normal the phonon emission and absorption effects separated into regions on either side of the elastic peak, the latter events being on the side closest to the normal. At 140°K the phonon absorption events were less probable than phonon emission events, i.e., atoms tended to lose energy while at higher surface temperature the probability of energy gain increased. The positions of the streaks moved predictably in space by varying the incident momentum of the beam, a parameter which also influenced the intensity of the streaks. Comparisons have been made to the He–LiF system using the elastic constants of the two materials.

The scattering of helium atoms from adsorbed layers of molecules on a (001) surface of NaF

The adsorption behavior of CCl 4, EtOH and H 2O molecules on a cleaved (001) surface of NaF has been studied. Ordered layers were observed in the cases of EtOH and H 2O as was evidenced by many fractional order peaks in the scattering pattern of He atoms. The scattering from a layer of H 2O molecules was studied in greater detail where the effect of varying several parameters was qualitatively observed. The parameters studied were; degree of adsorption, temperature of deposition and annealing behavior. Further the temperatures of disruption of the layers were followed by observing the fall in height of a fractional order peak or the increase in height of a substrate diffraction peak. Finally, some direct evidence for the diffusion of physisorbed molecules has been obtained.

Scattering of He and Ne atoms from a (001) surface of sodium fluoride

The scattering of He and Ne atoms from a cleaved (001) surface of NaF has been studied at different surface temperatures and incident beam energies. The geometric peak shape expected for the beam detector arrangement was compared to the experimentally observed peaks under different conditions of surface temperature and incident energy. The (00) peak height was followed as a function of surface temperature for two values of the incident momentum, in order to extract some information about the role of the attractive part of the interaction potential during a collision. Finally, the peak heights of several diffraction peaks were determined as a function of the incident energy. Several minima were observed in the scattering pattern, which can be associated with bound state resonances and which have been the subject of considerable interest recently. In this case, several maxima were found which can also be associated with bound state resonances. The (00) peak, observed in the scattering of Ne atoms, also showed maxima and minima when the height was monitored, as a function of the incident energy.

The Inelastic Scattering of He from a (001) Surface of Sodium Fluoride

The inelastic scattering of He atoms from a cleaved (001) surface of NaF has been studied at different surface temperatures. The study was similar to that for the He–LiF system in that the intensity of the inelastic streaks in the vicinity of diffraction peaks was explored as a function of detector location and crystal rotation. At 140 K the phonon absorption events were less probable than phonon emission events, i.e. atoms tended to lose energy while at higher surface temperatures the probability of energy gain increased. The position of the streaks moved predictably in space by varying the incident momentum of the beam, a parameter which also influenced the intensity of the streaks.