Alexander I. Kolesnikov

Role of Surface Structure on Li-Ion Energy Storage Capacity of Two-Dimensional Transition-Metal Carbides

A combination of density functional theory (DFT) calculations and experiments is used to shed light on the relation between surface structure and Li-ion storage capacities of the following functionalized two-dimensional (2D) transition-metal carbides or MXenes: Sc2C, Ti2C, Ti3C2, V2C, Cr2C, and Nb2C. The Li-ion storage capacities are found to strongly depend on the nature of the surface functional groups, with O groups exhibiting the highest theoretical Li-ion storage capacities. MXene surfaces can be initially covered with OH groups, removable by high-temperature treatment or by reactions in the first lithiation cycle. This was verified by annealing f-Nb2C and f-Ti3C2 at 673 and 773 K in vacuum for 40 h and in situ X-ray adsorption spectroscopy (XAS) and Li capacity measurements for the first lithiation/delithiation cycle of f-Ti3C2. The high-temperature removal of water and OH was confirmed using X-ray diffraction and inelastic neutron scattering. The voltage profile and X-ray adsorption near edge structure of f-Ti3C2 revealed surface reactions in the first lithiation cycle. Moreover, lithiated oxygen terminated MXenes surfaces are able to adsorb additional Li beyond a monolayer, providing a mechanism to substantially increase capacity, as observed mainly in delaminated MXenes and confirmed by DFT calculations and XAS. The calculated Li diffusion barriers are low, indicative of the measured high-rate performance. We predict the not yet synthesized Cr2C to possess high Li capacity due to the low activation energy of water formation at high temperature, while the not yet synthesized Sc2C is predicted to potentially display low Li capacity due to higher reaction barriers for OH removal.

Dynamics of water confined in single- and double-wall carbon nanotubes.

Using high-resolution quasielastic neutron scattering, we investigated the temperature dependence of single-particle dynamics of water confined in single- and double-wall carbon nanotubes with the inner diameters of 14+/-1 and 16+/-3 A, respectively. The temperature dependence of the alpha relaxation time for water in the 14 A nanotubes measured on cooling down from 260 to 190 K exhibits a crossover at 218 K from a Vogel-Fulcher-Tammann law behavior to an Arrhenius law behavior, indicating a fragile-to-strong dynamic transition in the confined water. This transition may be associated with a structural transition from a high-temperature, low-density (<1.02 gcm(3)) liquid to a low-temperature, high-density (>1.14 gcm(3)) liquid found in molecular dynamics simulation at about 200 K. However, no such dynamic transition in the investigated temperature range of 240-195 K was detected for water in the 16 A nanotubes. In the latter case, the dynamics of water simply follows a Vogel-Fulcher-Tammann law. This suggests that the fragile-to-strong crossover for water in the 16 A nanotubes may be shifted to a lower temperature.

Phonon density of states in MgB2

We report inelastic neutron scattering measurements of the phonon density of states in Mg 11B2, which has a superconducting transition at 39.2 K. The acoustic phonons extend in energy to 36 meV, and there are highly dispersive optic branches peaking at 54, 78, 89, and 97 meV. A simple Born-von Kàrmàn model reproduces the mode energies, and provides an estimate of the electron-phonon coupling of lambda approximately 0.9. Furthermore, the estimated boron and magnesium contributions to the isotope effect are in qualitative agreement with experiment. The data confirm that a conventional phonon mechanism, with moderately strong electron-phonon coupling, can explain the observed superconductivity.

Quasielastic and inelastic neutron scattering investigation of fragile-to-strong crossover in deeply supercooled water confined in nanoporous silica matrices

We investigated, using quasi-elastic and inelastic neutron scattering, the slow single-particle dynamics of water confined in laboratory synthesized nanoporous silica matrices, MCM-41-S, with pore diameters ranging from 10 to 18 A. Inside the pores of these matrices, the freezing process of water is strongly inhibited down to 160 K. We analysed the quasi-elastic part of the neutron scattering spectra with a relaxing-cage model and determined the temperature and pressure dependence of the Q-dependent translational relaxation time and its stretch exponent β for the time dependence of the self-intermediate scattering function. The calculated Q-independent average translational relaxation time shows a fragile-to-strong (FS) dynamic crossover for pressures lower than 1600 bar. Above this pressure, it is no longer possible to discern the characteristic feature of the FS crossover. Identification of this end point with the predicted second low-temperature critical point of water is discussed. A subsequent inelastic neutron scattering investigation of the librational band of water indicates that this FS dynamic crossover is associated with a structural change of the hydrogen-bond cage surrounding a typical water molecule from a denser liquid-like configuration to a less-dense ice-like open structure.

SEQUOIA: A Newly Operating Chopper Spectrometer at the SNS

A fine resolution chopper spectrometer (SEQUOIA) recently received first neutrons at the SNS. The commissioning phase of the instrument is underway. SEQUOIA is designed to utilize neutrons of an incident energy (Ei) between 10-2000 meV. A monochromatic beam is provided on a sample, 20 m from the decoupled ambient temperature H2O moderator, by filtering the white beam with a Fermi chopper located 18 m from the source. After interacting with the sample, neutrons are detected by an array of 3He linear position sensitive tubes located on a vertical cylinder with a radius of 5.5 m. This contribution presents current results from the commissioning experiments and compares SEQUOIAs actual and predicted performance. These commissioning experiments include characterization of the beam by monitors, determination of the chopper phase offsets, and runs with V and C4H2I2S. The predicted performance is provided by analytical calculations and Monte Carlo simulations.

Large phonon band gap in SrTiO3 and the vibrational signatures of ferroelectricity in ATiO3 perovskites: First-principles lattice dynamics and inelastic neutron scattering

We report on first-principles density functional perturbation theory calculations and inelastic neutron scattering measurements of the phonon density of states, dispersion relations, and electromechanical response of PbTiO3, BaTiO3, and SrTiO3. The phonon density of states of the quantum paraelectric SrTiO3 is found to be fundamentally distinct from that of ferroelectric PbTiO3 and BaTiO3 with a large, 70‐ 90 meV, phonon band gap. The phonon dispersion and electromechanical response of PbTiO3 reveal giant anisotropies. The interplay of covalent bonding and ferroelectricity strongly modulates the electromechanical response and gives rise to spectacular signatures in the phonon spectra. The computed charge densities have been used to study the bonding in these perovskites. Distinct bonding characteristics in the ferroelectric and paraelectric phases give rise to spectacular vibrational signatures. While a large phonon band gap in ATiO3 perovskites seems to be a characteristic of quantum paraelectrics, anisotropy of the phonon spectra correlates well with ferroelectric strength. These correlations between the phonon spectra and ferroelectricity can guide future efforts at custom designing still more effective piezoelectrics for applications. These results suggest that vibrational spectroscopy can help design novel materials.

Neutron spectroscopic investigation of dynamics of water ice

Quantitative studies of the properties of water require detailed investigation of the intramolecular and intermolecular interactions acting between the atoms and the molecules. Experimental information about the strength of the hydrogen bond interaction in water can be obtained directly by measuring its vibrational spectra. This is because a particular vibrational mode (or phonon) is determined by the interatomic force constants, which in turn are the double-differentials of the potential function at its minima. Therefore, measuring dynamical properties is one of the most powerful ways of investigating interatomic potentials in a given material. In this article we report series of inelastic neutron scattering studies for various forms of exotic ices, the aim of the investigation is to provide a better understanding of the nature of hydrogen bonding in water, which has considerable implications to large segment of scientific community for which the properties of water are important.

Inelastic Neutron Scattering Study of Confined Surface Water on Rutile Nanoparticles

The vibrational density of states (VDOS) for water confined on the surface of rutile-TiO(2) nanoparticles has been extracted from low temperature inelastic neutron scattering spectra. Two rutile-TiO(2) nanoparticle samples that differ in their respective levels of hydration, namely TiO(2) x 0.37 H(2)O (1) and TiO(2) x 0.22 H(2)O (2) have been studied. The temperature dependency of the heat capacities for the two samples has been quantified from the VDOS. The results from this study are compared with previously reported data for water confined on anatase-TiO(2) nanoparticles.

A comparison of four direct geometry time-of-flight spectrometers at the Spallation Neutron Source

The Spallation Neutron Source at Oak Ridge National Laboratory now hosts four direct geometry time-of-flight chopper spectrometers. These instruments cover a range of wave-vector and energy transfer space with varying degrees of neutron flux and resolution. The regions of reciprocal and energy space available to measure at these instruments are not exclusive and overlap significantly. We present a direct comparison of the capabilities of this instrumentation, conducted by data mining the instrument usage histories, and specific scanning regimes. In addition, one of the common science missions for these instruments is the study of magnetic excitations in condensed matter systems. We have measured the powder averaged spin wave spectra in one particular sample using each of these instruments, and use these data in our comparisons.

Structure and Stability of SnO2 Nanocrystals and Surface-Bound Water Species

The structure of SnO2 nanoparticles (avg. 5 nm) with a few layers of water on the surface has been elucidated by atomic pair distribution function (PDF) methods using in situ neutron total scattering data and molecular dynamics (MD) simulations. Analysis of PDF, neutron prompt gamma, and thermogravimetric data, coupled with MD-generated surface D2O/OD configurations demonstrates that the minimum concentration of OD groups required to prevent rapid growth of nanoparticles during thermal dehydration corresponds to ~0.7 monolayer coverage. Surface hydration layers not only stabilize the SnO2 nanoparticles but also induce particle-size-dependent structural modifications and are likely to promote interfacial reactions through hydrogen bonds between adjacent particles. Upon heating/dehydration under vacuum above 250 °C, nanoparticles start to grow with low activation energies, rapid increase of nanoparticle size, and a reduction in the a lattice dimension. This study underscores the value of neutron diffraction and prompt-gamma analysis, coupled with molecular modeling, in elucidating the influence of surface hydration on the structure and metastable persistence of oxide nanomaterials.