Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z26: Surfaces, Interfaces, Colloids and Catalysis II |
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Sponsoring Units: DCP Chair: David Chandler, Sandia National Laboratories Room: 204A |
Friday, March 6, 2015 11:15AM - 11:27AM |
Z26.00001: Band offsets across solid-liquid interfaces from continuum solvation methods Ravishankar Sundararaman, Yuan Ping, Giulia A. Galli, William A. Goddard III The band edge positions of photo-electrodes relative to water redox potentials play an important role in determining the efficiency of the photo-electrochemical cell. Direct theoretical calculations of solid-liquid interfaces are expensive and simplified models are desirable for rapid theoretical screening of new materials. However, traditional solvation models are extensively fit to describe organic solutes and hence extrapolate poorly to highly-polar inorganic surfaces. We develop minimally-empirical continuum solvation models suitable for treating such surfaces\footnote{R. Sundararaman, K. Schwarz, K. Letchworth-Weaver and T.A. Arias, {\it arXiv}:{\bf 1410}.2273}$^,$\footnote{R. Sundararaman and W.A. Goddard III, {\it arXiv}:{\bf 1410}.2922} and present theoretical predictions of the band positions of rutile TiO2 (110) and WO3 (001) surfaces in water. We obtain non-negligible solvation effects $\sim$ 1-2 eV, in good agreement with experimental results.\footnote{G. Xiong et al., {\it Adv. Funct. Mater.} {\bf 17}, 2133, (2007)}$^,$\footnote{M. G. Walter et al., {\it Chem. Rev.} {\bf 110}, 6446, (2010)}$^,$\footnote{J. Meyer et al., {\it Appl. Phys. Lett} {\bf 96}, 193302 (2010)}$^,$\footnote{J. Yourey et al., {\it J. Mat. Chem.} {\bf 21}, 7651, (2011)} [Preview Abstract] |
Friday, March 6, 2015 11:27AM - 11:39AM |
Z26.00002: Friction boosted by spontaneous epitaxial rotations Davide Mandelli, Andrea Vanossi, Nicola Manini, Erio Tosatti It is well known in surface science that incommensurate adsorbed monolayers undergo a spontaneous, energy-lowering epitaxial rotation from aligned to misaligned relative to a periodic substrate. We show first of all that a model 2D colloidal monolayer in an optical lattice, of recent importance as a frictional model, also develops in full equilibrium a small rotation angle, easy to detect in the Moir\'{e} pattern. The colloidal monolayer misalignment is then shown by extensive sliding simulations to increase the dynamic friction by a considerable factor over the aligned case. More generally, this example suggests that spontaneous rotations are rather ubiquitous and should not be ignored in all tribological phenomena between mismatched lattices. [Preview Abstract] |
Friday, March 6, 2015 11:39AM - 11:51AM |
Z26.00003: Surface Electric Potential of Macroions between the Limits of Small Ions and Charged Nanocolloids Benxin Jing, Y. Elaine Zhu The surface electric potential of macroions in the size of 1-10 nm in aqueous solutions is critical to understand the supramolecular assembly involving biomacromolecules, charged nanoparticles and nanoclusters and their resulting material properties. However, the electric potential of these macroions could not be accurately determined because their sizes fall in between the limits of small ions and plain charged nanocolloids, while solving the non-linear Possion-Boltzmann equation remains a grand challenge to date. In this work, we investigate polyhedral oligomeric silsesquioxane (POSS) with 8 amine terminal groups as a model macroion. We employ a single molecule fluorescence technique, fluorescence correlation spectroscopy (FCS), combined with photon counting histogram (PCH) to quantitatively measure the local proton concentration, which is the local co-ion concentration in vicinity of POSS with 1.5 nm in diameter. By changing the ionic strength of aqueous solution and the distance between pH-sensitive fluorescence probe and POSS, we quantitatively determine the proton concentration gradient. The distance dependent local pH can be simply analyzed to obtain the surface electric potential of the POSS macroion without the necessity to solve the non-linear Possion-Boltzmann equation. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z26.00004: X-ray absorption spectroscopy of lithium sulfur battery reaction intermediates Kevin Wujcik, Tod Pascal, David Prendergast, Nitash Balsara Lithium sulfur batteries have a theoretical energy density nearly five times greater than current lithium ion battery standards, but questions still remain regarding the reaction pathways through which soluble lithium polysulfide (Li2Sx, ``x'' ranging from 2 to 8) reaction intermediates are formed. Complicating spectroelectrochemical approaches to elucidate redox pathways is the challenge of obtaining spectral standards for individual Li2Sx species. Lithium polysulfides cannot be isolated as individual component and exist only in solution as a distribution of different Li2Sx molecules formed via disproportionation reactions (e.g. 2Li2S4 goes to Li2S3 + Li2S5). X-ray absorption spectroscopy (XAS) at the sulfur K-edge has recently been employed as a technique to study Li-S chemistry. We have recently obtained XAS standards for individual Li2Sx species via first principles DFT simulations and the excited electron and core hole approach. Here, experimental sulfur K-edge XAS of Li2Sx species dissolved in poly(ethylene oxide) are compared to spectra obtained from analogous theoretical calculations. The impact that polysulfide solution concentration and the presence of other lithium salts (e.g. LiNO3) have on X-ray spectra of Li2Sx species is explored via experiment and theory. [Preview Abstract] |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z26.00005: Depth profile of halide anions under highly charged biological membrane Woongmo Sung, Wenjie Wang, Jonggwan Lee, David Vaknin, Doseok Kim Halide ion (Cl$^{\mathrm{-}}$ and I$^{\mathrm{-}})$ distribution under a cationic Langmuir monolayer consisting of 1,2-dipalmitoyl-3 trimethylammonium-propane (DPTAP) molecules was investigated by vibrational sum-frequency generation (VSFG) and X-ray spectroscopy. From VSFG spectra, it was observed that large halide anions (I$^{\mathrm{-}})$ screen surface charge more efficiently so that interfacial water alignment becomes more randomized. On the other hand, number density of ions directly measured by X-ray fluorescence spectroscopy at grazing incidence angle reveals that the ion densities within 6 $\sim$ 8 nm are the same for both I$^{\mathrm{-}}$ and Cl$^{\mathrm{-}}$. Since the observed ion densities in both cases are almost equal to the charge density of the DPTAP monolayer, we propose that larger halide anions are attracted closer to the surface making direct binding with the charged headgroups of the molecules in the monolayer, accomplishing charge neutrality in short distance. This direct adsorption of anions also disturbs the monolayer structure both in terms of the conformation of alkyl chains and the vertical configuration of the monolayer, with iodine having the stronger effect. Our study shows that the length scale that ions neutralize a charged interface varies significantly and specifically even between monovalent ions. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z26.00006: Electrochemical Growth of Ag Junctions and Diffusion Limited Aggregate (DLA) Fractal Simulation Zak Olson, Sam Tuppan, Woo-Joong Kim We attempt construction of a single atom connection between two copper wires. By applying a DC voltage across the wires when immersed in a silver nitrate solution, we deposit silver until a junction is formed. The deposited silver forms a fractal structure that can be simulated with a diffusion limited aggregation model. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z26.00007: Single crystal growth and properties of two layered oxytellurides Tiglet Besara, Daniel Ramirez, Theo Siegrist, Jifeng Sun, Jeffrey Whalen, Takahisa Tokumoto, Stephen McGill, Ryan Stillwell, Stanley Tozer, David Singh We report on the synthesis, structure, and physical properties of two layered oxytellurides: Ba$_{3}$Yb$_{2}$O$_{5}$Te and Ba$_{2}$TeO. Both compound were grown in single crystalline form using a molten metal flux, and crystallize in a tetragonal space group: $P$4$/mmm$ for Ba$_{3}$Yb$_{2}$O$_{5}$Te and $P$4$/nmm$ for Ba$_{2}$TeO. Ba$_{3}$Yb$_{2}$O$_{5}$Te consists of Ba$_{2}$Yb$_{2}$O$_{5}$ perovskite double layers separated by a CsCl-type BaTe slab, while Ba$_{2}$TeO consists of an inverse PbO-type BaO layer separated by an NaCl-type BaTe slab. Ba$_{3}$Yb$_{2}$O$_{5}$Te displays short range 2D magnetic ordering below 4 K, and a sharp optical absorption feature at 1.27 eV consistent with a $^{2}F_{7/2}\rightarrow ^{2}F_{5/2}$ transition of Yb$^{3+}$. For Ba$_{2}$TeO, optical measurements display a sharp increase in absorbance, a manifest of a band edge. DOS corroborates the band gap, at 2.93 eV, indicating semiconducting behavior. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z26.00008: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z26.00009: Superhydrophilic - Superhydrophobic Transition in Vertically Aligned Titania Nanotubes Oomman Varghese, Ram Neupane, Maggie Paulose Both hydrophilic (wetting) and hydrophobic (non-wetting) surfaces find applications in a variety of technological areas. For example, hydrophilic surfaces are used in microfluidic devices to provide antifogging and antifouling functions whereas hydrophobic coatings are used in clothes to attribute stain resistance. While in superhydrophilic surfaces the contact angle that water droplets make with the surface is nearly zero, the surfaces that make contact angles greater than about 120$^{\circ}$ are considered superhydrophobic. Oxide ceramics generally exhibit hydrophilic behavior. Surface texturing or organic coatings are often used to make the surface hydrophobic or superhydrophobic. We prepared highly ordered titania nanotube arrays on titanium foils using anodic oxidation that showed superhydrophilic behavior upon fabrication. We noticed a strong correlation between fabrication conditions and the wettability. We have become successful in converting such a superhydrophilic nanostructure into superhydrophobic without modifying the surface with organic molecules or texturing. Contact angles in excess of 145$^{\circ}$ have been obtained. We will present these results and discuss the physiochemical processes that decide wetting properties of oxide nanostructures. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z26.00010: Size-dependent melting of single and stacked silver alkanethiolate layers: experiment and phenomenological model Zichao Ye, Lito de la Rama, Liang Hu, Mikhail Efremov, Leslie Allen We report a systematic study of melting of silver alkanethiolate (AgSCn) lamellar crystals. A new synthesis method enables us to control their thickness by either modulating alkanethiol chain length (n $=$ 7-18) or stacking them to a specific layer number (m $=$ 1-10). Nanocalorimetry shows stepwise increase in the melting point, T$_{\mathrm{m}}$, of single layer AgSCn as an increment of chain length. Layer stacking also results in a size-dependent melting. An odd/even alternation is observed in the T$_{\mathrm{m}}$ of 2, 3, and 4-layer lamellae, but absent in that of single and multilayer samples. We develop a phenomenological model for lamellae melting based on the cumulative excess free energy contributions of four spatially separate regions in AgSCn crystal: free surface, Ag$-$S central plane, substrate interface, and interlayer interface. Surface excess free energy is revealed to be independent of chain length. The selective appearance of the odd/even effect is due to the significant stabilization of interlayer interfaces of odd-chain samples, possibly due to registration/packing. Such interface stabilization occurs most significantly for 2-layer samples. XRD results support the model as the measured van der Waals gap is smaller for crystals with odd chains. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z26.00011: Control and expression of --NH$_{2}$, --SH, --COOH and SiO$_{2}$ on the surface of silicon carbide quantum dots Munuve Mwania, Susana Aguirre-Medel, Peter Kroll We present simple protocols for reliably tailoring the surfaces of \textit{zinc blende} silicon carbide quantum dots ($\beta $-SiC QDs). The SiC QDs are synthesized via photo-assisted electrochemical corrosion of bulk powders at different temperatures and time scales. After washing the residual acid and resuspending in H$_{2}$O, the surfaces of SiC QDs were controllably coated with four different functional groups, specifically --NH$_{2}$, --SH, and --COOH and --SiO$_{2}$. We began by covalently attaching primary amines (-NH$_{2})$ to the QD surface. The amine terminations were then converted to amine/thiolate (-NH$_{2}$/SH) and amine/carboxylate (-NH$_{2}$/COOH) functional groups. SiO$_{2}$ shells around SiC QDs (\textit{to create SiC@SiO}$_{2}$\textit{ nano-structures}) were grown using a TEOS-mediated St\"{o}ber method. The presence of amine and thiol groups was confirmed by fluoresceamine assay test, X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FTIR). While a negative fluoresceamine assay test confirmed the replacement of amine groups by thiol groups, the thiolation of the surface was also confirmed through Ellman's assay, XPS and FTIR. The presence of the SiO$_{2}$ shells was examined using transmission electron microscopy and XPS. Our results open up possibilities to manipulate SiC QDs for various applications. [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z26.00012: Effects of Surface Treatments on Nylon 6,6 via Non-thermal Atmospheric Plasma for Thermoplastic Adhesives Chi-Chin Wu, Andres Bujanda, John Demaree, Jason Robinette, Amanda Weerasooriya, David Flanagan This work aims to modify the properties of Nylon 6,6 surfaces for attaining improved interfacial adhesion to thermoplastic composites utilizing atmospheric non-thermal plasma treatments followed by silane treatments using 3-aminopropyltriethoxysilane (APS) in some cases. An L-shaped dielectric barrier discharge configuration was employed to expose nylon substrates to oxygen-containing gas plasmas such as He/O$_{2}$ and He/H$_{2}$O, respectively, at room temperature. The chemically-modified surface of the substrate after plasma exposure was immediately examined by static water contact angle wettability measurements and X-ray photoelectron spectroscopy. It was found that the surface hydrophilicity was substantially enhanced and the amount of surface oxygen was significantly increased after a three-minute plasma exposure due to the increased surface energy and additional O-H bonds. The enhancements on interfacial adhesion were evaluated with lap shear tests using three types of adhesives: EPON 825/D230, EPON 825/D2000 and sikaflex252, respectively. The results of tensile tests on the adhesive joints showed an almost $\sim$ 300{\%} increase in interfacial adhesive strength for EPON 825/D230 bonds after plasma treatments. Finite element modeling of adhesive joints for bond strength is underway to compare with experimental results and study the quantitative relations between the mechanical properties within the bond and at interfaces. [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z26.00013: Theoretical evidence for unexpected O-rich phases at corners of MgO surfaces Daniel Berger, Sergey V. Levchenko, Saswata Bhattacharya, Karsten Reuter, Luca M. Ghiringhelli, Matthias Scheffler Introducing charge carriers into MgO via $p$ doping greatly reduces formation energy of an O-vacancy in the bulk and at the (100) surface $[1]$. In this work, we use hybrid density functional theory to explore O-vacancy and O/O$_2$-ad-species defects at corners of MgO surfaces. The defects are modelled using MgO clusters embedded into a field of norm-conserving pseudopotentials and point charges. The long-range response of the oxide to the charge carriers trapped at the defects is taken into account using a polarizable force field. The low-energy defect atomic structures are found using an {\em ab initio} genetic algorithm $[2]$. Concentrations of O-vacancies and O-ad-species at realistic temperatures and pressures are obtained with {\em ab initio} atomistic thermodynamics. Unexpectedly, we find that O-ad-species rather than O-vacancies are dominating defects at realistic conditions. The stability of the O-ad-species over O-vacancies and pristine corners is explained by an interplay between bond-breaking, bond-making, and charge-carrier trapping. $-$ $[1]$ N. Richter {\em et al.}, Phys. Rev. Lett. {\bf 111}, 045502 (2013); $[2]$ S. Bhattacharya {\em et al.}, New J. Phys., in press (2014) [Preview Abstract] |
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