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Du Y.,Environmental Molecular science Laboratory | Deskins N.A.,Fundamental and Computational science Directorate | Deskins N.A.,Worcester Polytechnic Institute | Zhang Z.,Fundamental and Computational science Directorate | And 4 more authors.
Journal of Physical Chemistry C

A combination of scanning tunneling microscopy and density functional theory has been used to investigate the interactions between water molecules and terminal hydroxyls (OHt's) adsorbed on the TiO2(110) surface at 300 K. We show that OHt's have a significant effect on the water reactivity. Two distinctive reaction pathways are unraveled depending on the whether H2O and OHt are on the same or adjacent Ti rows. The underlying reaction mechanisms involve proton transfer from H 2O to OHt leading to the formation of new H2O molecules, accompanied by O scrambling and along- or across-row apparent motion of OHt's. © 2010 American Chemical Society. Source

Wang C.-M.,Environmental Molecular science Laboratory | Li X.,Fundamental and Computational science Directorate | Wang Z.,Fundamental and Computational science Directorate | Xu W.,Pacific Northwest National Laboratory | And 10 more authors.
Nano Letters

It is well-known that upon lithiation, both crystalline and amorphous Si transform to an armorphous Li xSi phase, which subsequently crystallizes to a (Li, Si) crystalline compound, either Li 15Si 4 or Li 22Si 5. Presently, the detailed atomistic mechanism of this phase transformation and the degradation process in nanostructured Si are not fully understood. Here, we report the phase transformation characteristic and microstructural evolution of a specially designed amorphous silicon (a-Si) coated carbon nanofiber (CNF) composite during the charge/discharge process using in situ transmission electron microscopy and density function theory molecular dynamic calculation. We found the crystallization of Li 15Si 4 from amorphous Li xSi is a spontaneous, congruent phase transition process without phase separation or large-scale atomic motion, which is drastically different from what is expected from a classic nucleation and growth process. The a-Si layer is strongly bonded to the CNF and no spallation or cracking is observed during the early stages of cyclic charge/discharge. Reversible volume expansion/contraction upon charge/discharge is fully accommodated along the radial direction. However, with progressive cycling, damage in the form of surface roughness was gradually accumulated on the coating layer, which is believed to be the mechanism for the eventual capacity fade of the composite anode during long-term charge/discharge cycling. © 2012 American Chemical Society. Source

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