title: Crossover from localized to diffusive water dynamics in carbon nanohorns: A comprehensive quasielastic neutron-scattering analysis.
abstract: Incoherent neutron scattering by water confined in carbon nanohorns was measured with the backscattering spectrometer SPHERES and analyzed in exemplary breadth and depth. Quasielastic spectra admit δ-plus-Kohlrausch fits over a wide q and T range. From the q and T dependence of fitted amplitudes and relaxation times, however, it becomes clear that the fits do not represent a uniform physical process, but that there is a crossover from localized motion at low T to diffusive α relaxation at high T. The crossover temperature of about 210 to 230 K increases with decreasing wave number, which is incompatible with a thermodynamic strong-fragile transition. Extrapolated diffusion coefficients D(T) indicate that water motion is at room temperature about 2.5 times slower than in the bulk; in the supercooled state this factor becomes smaller. At even higher temperatures, where the α spectrum is essentially flat, a few percentages of the total scattering go into a Lorentzian with a width of about 1.6μeV, probably due to functional groups on the surface of the nanohorns.
authors.affiliation: LLB, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France.
authors.forename: Marie-Claire
authors.initials: MC
authors.lastname: Bellissent-Funel
authors.affiliation: Center for Energy and Environmental Science, Shinshu University, 1-17-1 Wakasato, Nagano, Japan.
authors.forename: Katsumi
authors.initials: K
authors.lastname: Kaneko
authors.affiliation: Graduate School of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan.
authors.forename: Tomonori
authors.initials: T
authors.lastname: Ohba
authors.affiliation: Forschungszentrum Jülich GmbH, JCNS at MLZ, Lichtenbergstraße 1, 85747 Garching, Germany.
authors.forename: Marie-Sousai
authors.initials: MS
authors.lastname: Appavou
authors.affiliation: Forschungszentrum Jülich GmbH, JCNS at MLZ, Lichtenbergstraße 1, 85747 Garching, Germany.
authors.forename: Antti J
authors.initials: AJ
authors.lastname: Soininen
authors.affiliation: Forschungszentrum Jülich GmbH, JCNS at MLZ, Lichtenbergstraße 1, 85747 Garching, Germany.
authors.forename: Joachim
authors.initials: J
authors.lastname: Wuttke