Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P38: Focus Session: Nanomaterials for Energy Applications I |
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Sponsoring Units: DCP Chair: Nathaniel Rosi, University of Pittsburgh Room: 410 |
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P38.00001: Nanoscale Architectures for Energy Applications Invited Speaker: In my group, we have developed a number of different potential architecture systems for gaining insights into energy storage and photovoltaics. In one manifestation of our efforts, generating a heterojunction comprising nanotubes and nanocrystals, externally bound and connected, has been significant. The unique, innovative, and important aspect of this particular nanoscale architecture is that it takes advantage of the tunability, in terms of size, shape, and chemistry, of nanotubes and nanocrystals, to create a sharp junction interface, whose properties are inherently manipulable, tailorable, and hence, predictable. For example, the electrical resistance of nanotube-nanoparticle networks is dependent on the nanoscale junctions that exist between these constituent nanomaterials as well as on microscale and macroscale connectivity. Thus, rational design of these nanomaterials is critical to a fundamental understanding of charge transport in single molecules and the determination of their conductance. Results on these systems can therefore be used to increase understanding of intrinsic factors affecting carrier mobility, such as electronic structure, carrier trapping, and delocalization. In a second manifestation, three-dimensional, dendritic micron- scale spheres of alkali metal hydrogen titanate 1D nanostructures (i.e.: nanowires and nanotubes) have been generated using a modified hydrothermal technique in the presence of hydrogen peroxide and an alkali metal hydroxide solution. Sea-urchin-like assemblies of these 1D nanostructures have been transformed into their hydrogen titanate analogues by neutralization as well as into their corresponding semiconducting, anatase titania nanostructured counterparts through a moderate high-temperature annealing dehydration process without destroying the 3D hierarchical structural motif. The as-prepared hollow spheres of titanate and titania 1D nanostructures have overall diameters, ranging from 0.8 $\mu$m to 1.2 $\mu$m, while the interior of these aggregates are vacuous with a diameter range of 100 to 200 nm. We have demonstrated that these assemblies are useful for example as active photocatalysts for the degradation of synthetic Procion Red dye under UV light illumination. In a third set of experiments, a size- and shape-dependent morphological transformation was demonstrated during the hydrothermal soft chemical transformation, in neutral solution, of titanate nanostructures into their anatase titania counterparts. Our results indicate that as-synthesized titania nanostructures possessed higher photocatalytic activity than the commercial titania precursors from whence they were derived. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P38.00002: High Capacity Hydrogen Sorption in Nanoscale Transition Metal Based Organometallic Complexes Invited Speaker: Using a highly sensitive nanobalance we have discovered high capacity hydrogen absorption in a range of transition-metal(TM) based organometallic complexes prepared using physical vapor deposition techniques. Hydrogen absorption upto 14 wt{\%} has been measured in titanium ethylene complexes and confirmed by mass doubling when deuterium is employed instead of hydrogen. I will present results of comprehensive measurements on other TM-complexes of alkenes, alkanes and ring compounds. I will also discuss these measurements in the context of theoretical calculations based on first principles quantum mechanics that have appeared in the recent literature. Finally, I will discuss the prospects for practical applications of these materials and the problems that might be encountered. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P38.00003: Electron Dynamics in Nanocrystalline TiO2 and ZnO Measured by Terahertz Spectroscopy Charles Schmuttenmaer, Jason Baxter Understanding the microscopic details of carrier transport in nanocrystalline colloidal thin films is required for complete understanding of a variety of photochemical and photoelectrochemical cells utilizing interpenetrating networks. Measuring the photoconductivity in these materials, however, is a challenging problem because of the inherent difficulty of attaching wires to nanometer-sized objects. Furthermore, picosecond carrier dynamics play an important role in efficient charge separation and transport, but the low temporal resolution of traditional methods used to determine their photoconductivity precludes their use in studying sub-ps to ps dynamics. This talk will present recent advances utilizing THz spectroscopy to investigate and elucidate the microscopic behavior of carrier dynamics within the context of materials for energy applications such as dye-sensitized solar cells and solar-driven cells for catalytic chemistry. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P38.00004: Theoretical investigation of the high energy excitations in silicon nanocrystals Adam Gali, Marton Voros, Dario Rocca, Gerely Zimanyi, Giulia Galli Recently, efficient multi-exciton generation (MEG) has been reported for several nanoclusters including silicon nanocrystals (SiNC), too. However, the existence of MEG has been disputed in the literature. The reported bi-exciton states in SiNC involve high energy empty states of SiNC. These states are expected to be very delocalized, and thus easily modified by the environment surrounding the SiNCs. In addition, the SiNCs are fabricated in a solution that usually contains CnHm molecules, e.g. hexane, that may bind to the surface of SiNCs, and modify their absorption spectrum. We have studied the absorption spectra of hydrogenated SiNCs by first principles calculations. The geometry was optimized within density functional theory (DFT), while absorption spectra were determined by time-dependent DFT. The effect of the environment on the SiNC was modeled by i) varying the distance between the nanoparticles ii) allowing for surface reconstruction and iii) monitoring the effect of absorption of CnHm groups on absorption spectra. We found that the high energy spectrum of SiNCs strongly depends on the environment. Our findings indicate that taking into account effects of surface states and SiNCs concentration in solution is crucial, in order to understanding multi exciton generation. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P38.00005: Fast exciton relaxation and multiple exciton generation (MEG) in semiconductor nanocrystals: the role of defects Christophe Delerue, Guy Allan Recent works have concluded that a single high-energy photon could generate multiple excitons in semiconductor nanocrystals but these results are debated and are not well understood theoretically. More generally, the physics of the relaxation of excitations in semiconductor nanocrystals receives growing interest. We show that surface defects must play an important role in these processes. We calculate the rate for the relaxation of hot carriers by impact ionization and we show that the presence of surface defects leads to an increase of the relaxation rate at lower excitation energy. We present simulations of the carrier multiplication in Si nanocrystals and we discuss the results of recent experiments in light of these results. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P38.00006: First-principles study of LaSn$_{3 }$ as an anode for lithium-ion batteries Dongwon Shin, Christopher Wolverton, John Vaughey, Michael Thackeray Using both density functional theory (DFT) calculations and experiment, we investigate the tin-rich intermetallic compound LaSn$_{3 }$as a possible anode for lithium-ion batteries. We use DFT calculations to compare the relative energies of hypothetical insertion- and displacement-type reactions in an effort to elucidate the energetically-preferred reaction mechanism of Li with LaSn$_{3}$. From our DFT calculations, we find: (i) lithium insertion reactions with LaSn$_{3}$ are predicted to be energetically unfavorable and highly unlikely to occur; (ii) in contrast, the energetically preferred reaction is a displacement reaction in which La is partially displaced from LaSn$_{3}$ to yield La$_{3}$Sn$_{5}$ and Li reacts with the residual Sn to form Li$_{17}$Sn$_{4}$, corresponding to an electrochemical capacity of 307 mAh/g (iii) this partial displacement reaction is preferred relative to the complete displacement and lithiation of Sn; and (iv) the lithiated-tin compound, Li$_{17}$Sn$_{4}$, is energetically more favored than the commonly reported Li$_{22}$Sn$_{5}$ composition. Electrochemical and structural data largely confirm the DFT predictions; they demonstrate that lithium reacts with LaSn$_{3}$ via a displacement reaction to provide a reversible specific capacity of 200-250 mAh/g. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P38.00007: Ab Initio Prediction of the Size-Dependence of Nano-scale Platinum Dissolution in Water Kristin Persson, Byungchan Han, Gerbrand Ceder In low-temperature fuel cells, the mechanism behind the observed performance loss of the platinum catalyst is not well understood. Using ab initio methods, we calculate 0.5 - 2 nm diameter Pt nanoparticles with varying degrees of O and OH surface absorbates, optimized by site and particle surface structure. In fuel cells, the oxidation of the particle surface origins from the breakup of water molecules. To mimic these conditions we employ a grand canonical ensemble treatment of water as a source of O and H. Additionally, pH effects and dissolved species (from experiments) are incorporated, the latter by changing the experimental element reference state to that of calculated solids. This formalism allows us to determine the stability regions of nanoparticle Pt in equilibrium with water, as a function of particle size, potential and pH. As a result we find enhanced dissolution for the smaller Pt nano-particles, compared to the larger. Furthermore, surface passivation effects from O and OH adsorption do not significantly increase the stability of the nano-particle phases in the potential-pH region relevant for fuel cell operating conditions. Thus, we can identify size-dependent dissolution as a mechanism which will promote the growth of larger particles at the expense of smaller ones and ultimately cause a degradation in the nanoparticle Pt catalyst performance. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P38.00008: First-principles theory of capacitive and electrochemical energy storage Joongoo Kang, Yong-Hyun Kim Recently there has been much interest in development of new electrochemical capacitors to meet high-power and high-energy applications. Pseudo-capacitors using fast surface redox reactions can store electrical energy of 10 to 100 times larger than supercapacitors and still exhibit fast and reversible charge-discharge responses in contrast to batteries. Yet, energy storage mechanisms in super- and pseudo-capacitors have not been fully understood at the level of electrons. Here we have performed first-principles calculations for electrical double layers of a TiO$_{2}$ (101) electrode and solvated lithium ions on the surface, with the ethylene carbonates (EC) as solvent molecules. As Li ions are desolvated from Li-EC$_{4}$ to Li-EC$_{3}$ and bare Li ions, the capacitance gets larger due to the reduced distance between the Li ions and the electrode. When Li ions are intercalated into the subsurface of the TiO$_{2}$ electrode as supposed in pseudocapacitors, the electrostatic energy due to charge separation is reduced for a given stored charge, but the electrochemical reaction starts to occur causing a large increase in the capacitance. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P38.00009: Steps in hydrogen production from methanol on sub-nanometer palladium clusters Faisal Mehmood$^{1}$, Jeffrey P. Greeley$^{2}$, Peter Zapol$^{1}$, Larry A. Curtiss$^{1,2}$ Extensive experimental and theoretical work has been done to understand the decomposition of methanol on various metal and metal oxide nanoparticles for hydrogen production. The activity of sub-nanometer sized particles $<$ 1nm however is not very well known, primarily because of technical challenges involved in preparation and stabilization of the clusters. To explore the properties of the Pd clusters computationally, we have carried out density functional calculations for the methanol decomposition reaction on Pd$_{4}$ and Pd$_{8}$ clusters. The thermodynamics and kinetics of three decomposition routes involving C--O, C--H and O--H scission were investigated; activation energy barriers were determined with the nudged elastic band method. A detailed analysis of the PES for methanol decomposition shows C--O activation to be the least favorable step. In addition, all possible reaction paths for the Pd$_{4}$ cluster are much lower in comparison to single crystal surface and large nanoparticles. To understand how particle size affects the elementary reaction steps, we also present a comparison of methanol decomposition on Pd$_{4}$ with Pd$_{8}$ clusters. Finally, we will discuss the implication of a linear correlation between the transition state and final state energies that is followed for all elementary reaction steps on Pd$_{4}$ and Pd$_{8}$ clusters. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P38.00010: Two-Dimensional Boron-Nitride Layers as Flexoelectric Nanogenerators I. Naumov, A. Bratkovsky, V. Ranjan The direct conversion of ambient motion into electrical energy, especially at nanoscale, is fundamental and technological challenge. Boron-Nitride non-centrosymmetric monolayers are piezoelectrics that can sustain much larger structural and produce very large (a few Volts) voltage drop across flexed nanostrips. We show, with the use of ab-initio calculations, the existence of giant nonlinear flexoelectric effect in BN 2D strips. The induced polarization is quadratic in amplitude of atomic displacements $A$, yet the dipole moment per unit cell is about four times larger compared to PbZrTiO3 [1]. The resulting voltage drop across the BN nanostrip is set by bandgap in material $E_g/q\sim 5$ Volts and nearly independent of the strip width. The large voltage produced by this inert bio-compatible material may find a variety of applications and, in particular, as nanogenerators and sensors powered by an ambient motion or agitation. Prior alternatives, like ZnO, GaN and CdS, are leaky, generate much smaller voltage, and impractical [2]. [1] I.Naumov, A.Bratkovsky, V.Ranjan, arXiv:0810.1775 (2008). [2] Y. Qin, X. Wang, Z.L. Wang, Nature {\bf 451}, 809 (2008); M.A. Schubert et al, Appl. Phys. Lett. {\bf 316}, 122904 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P38.00011: Electrochemical Double Layer Capacitors Using Aligned Multiwall Carbon Nanotubes Grown Directly on Conductive Substrates Rakesh Shah, Xianfeng Zhang, Saikat Talapatra We report on the properties of Electrochemical Double Layer Capacitor (EDLCs) electrodes fabricated using aligned multiwall carbon nanotubes (MWNT) grown on Inconel sheets. Air assisted chemical vapor deposition technique was employed to synthesize the aligned MWNT on these substrates. The capacitive behavior of the EDLC's fabricated using different lengths of aligned MWNTs was examined using cyclic voltammetry, constant current charge/discharge, and impedance spectroscopy. These measurements show that the charge storage phenomenon was non-Faradic with equivalent series resistance in the range of 0.13-0.4$\Omega $. The maximum values of specific capacitance of the carbon nanotube materials used in these devices were in the range of 14.6-21.57 F/g. The maximum value of power density and energy density of the whole supercapacitor devices were 1.48 Wh/Kg and 2.7 KW/Kg, respectively. These results show that the multiwall carbon nanotubes grown directly on conductive substrates are promising candidates as electrodes for electrochemical energy store device applications. [Preview Abstract] |
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