Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B34: Photocatalysis, Proton Conductors and Other Energy Topics |
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Sponsoring Units: GERA Chair: Zhigang We, Colorado School of Mines Room: 704 |
Monday, March 3, 2014 11:15AM - 11:27AM |
B34.00001: ZnO/PbTiO$_{3}$ as a Novel Catalyst for CO$_{2}$ Conversion Babatunde Alawode, Alexie Kolpak Due to its role in climate change, there is a great interest in finding ways to take advantage of the vast amount of waste CO$_{2}$ we produce by its conversion to useful substances. This is currently impractical due to the high temperatures and pressures generally required for the synthesis of compounds using CO$_{2}$ as a precursor. To make direct CO$_{2}$ capture and conversion economically viable, new materials able to catalyze the conversion reactions at significantly milder conditions will be essential. In this work, we use DFT computations to design a dynamically tunable ferroelectric oxide-supported thin film catalyst that can capture CO$_{2}$ directly from the emission stream and convert it into methanol or cyclic carbonates. One promising candidate for a dynamically tunable catalyst of this type is Zn$_{\mathrm{x}}$O$_{\mathrm{y}}$/PbTiO$_{3}$. We demonstrate that switching the polarization of the ferroelectric substrate substantially changes the surface atomic and electronic properties of the heterostructure, thereby enabling tunable absorption. We investigate reaction pathways on unsupported and supported ZnO for common CO$_{2}$ reactions. Our approach may lead not only to new technologies for reducing emissions, but also to novel catalysts that could decrease energy consumption for industrial-scale synthetic processes. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B34.00002: Dynamics of Interfacial Charge Transfer in Semiconductor Crystalline Film-Assisted Photocatalyst Wei Sea Chang, Chien Nguyen Van, Ying-Hao Chu Semiconductors are by far the most intensively studied photocatalytic materials due to their favorable electronic and optical properties. Many have reported on semiconductor particles for their size quantization effect. The discrepancies about particle sizing, however, have always been questioned as to what is the optimum particle size for quantization effects to be observed. Here, we offer a novel semiconductor/metal architecture to understand the mechanism of interfacial charge transfer in a visible-light driven redox reaction. Semiconductor BiFeO$_{3}$ (BFO) film with preferred orientation were synthesized by using pulsed laser deposition. Au nanoparticles were produced by post-thermal annealing under oxygen atmosphere to distribute over the semiconductor film surface. High resolution X-ray diffraction and transmission electron microscopy results indicate that the Au (111) nanoparticles are partially embedded on the (100)-, (110)-, and (111)-oriented BFO film. A comprehensive study of the electronic properties has been performed by ultrafast pump-probe and X-ray photoelectron spectroscopy. We seek to answer how oriented-semiconductor films with Au nanoparticles distributed uniformly over the film would affect the dynamics of photocatalytic reactions. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B34.00003: Fe doping and anion defects in bismuth pyrochlore photocatalysts Cedric Mayfield, David Barker, Vaidyananthan Subramanian, Muhammad Huda To understand the change in photocatalytic properties of Bi$_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7\thinspace }}$after incorporating localized Fe 3d electrons, the electronic properties and formation energies of anion defects and cation substitutions have been systematically studied by first principles density functional theory. We have found for each type of doping, intrinsic or extrinsic, structural distortions are localized to the defect site. For the intrinsic defects, O vacancies (O$_{\mathrm{vac}})$ are relatively shallow donors compared to O interstitials (O$_{\mathrm{int}})$. For the extrinsic defects, Fe substitutions at the Bi sites (Fe$_{\mathrm{Bi}})$ are more stable than Fe substitutions at the Ti sites (Fe$_{\mathrm{Ti}})$, however they both promote the acceptor defect levels which are critical for band gap engineering. Complex doping (combined intrinsic and extrinsic doping) was also considered to examine the defect correlations at first nearest neighbor to third nearest neighbor distances. A detailed electronic structure analysis will be presented for both pristine and doped Bi$_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B34.00004: Raising Power Output in an Acoustic Energy Harvester Michael Primrose, Orest Symko A promising approach for the conversion of heat to electricity consists of coupling a thermoacoustic heat engine to a piezoelectric device. When heated, this unit resonates at high audible frequencies which are converted to electricity. Being compact and small its power output is limited. To overcome this, several piezoelectric devices can be coupled to the acoustic engine thereby generating more electrical power at the expense of increasing the load on the engine. In the prototype studied, three PZT unimorph piezoelectric devices converted the heat-generated sound at 2.5 kHz in the engine to electrical signals which were rectified and sent to a resistive load matched to the unimorphs. Within variations in device characteristics, results show a three-fold increase in power to the load, effectively raising the power density of the converter. Such approach with multiple piezoelectric elements provides increased power output within impedance limitations of the engine. The technique, based on a device that has essentially no moving parts and is simple, shows much promise for the conversion of heat to electricity in many applications. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B34.00005: Digital superlattice model for the measurement of strain and interfacial intermixing by X-ray diffraction Yifei Meng, Honggyu Kim, Jian-Min Zuo We have developed a digital superlattice model that describes the discrete lattice fluctuation in high quality GaSb/InAs type II superlattices (T2SL) grown by molecular beam epitaxy. T2SLs have attracted considerable attention as a candidate for middle-wavelength and long-wavelength infrared light detection. However, so far the performance of T2SL materials has been limited by short carrier lifetime below the theoretical predictions. Interfacial defects have suggested as possible cause. To quantify the T2SL structure, we extract interfacial strain and composition profile at atomic monolayer scale using a combination of direct 2theta-omega scan and model fitting. The digital superlattice model we developed describes the discrete fluctuation in T2SL, which enables accurate simulation of peak widths, positions and intensities. The simulation results indicate more cation intermixing compared with anion. Also strong evidence of interfacial strain is revealed in the X-ray diffraction data. The development of this technique allows a systematic study of interfacial treatments and their influences on atomic structure of T2SL. The detailed structure information is extremely helpful for optimizing the growth and refining existing energy band calculation model. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B34.00006: Gold plasmonic effects on charge transport through single molecule junctions Olgun Adak, Latha Venkataraman We study the impact of surface plasmon polaritons, the coupling of electromagnetic waves to collective electron oscillations on metal surfaces, on the conductance of single-molecule junctions. We use a scanning-tunneling microscope based break junction setup that is built into an optical microscope to form molecular junctions. Coherent 685nm light is used to illuminate the molecular junctions formed with 4,4'-bipyridine with diffraction limited focusing performance. We employ a lock-in type technique to measure currents induced by light. Furthermore, the thermal expansion due to laser heating is mimicked by mechanically modulating inter-electrode separation. For each junction studied, we measure current, and use AC techniques to determine molecular junction resonance levels and coupling strengths. We use a cross correlations analysis technique to analyze and compare the effect of light to that of the mechanical modulation. Our results show that junction transmission characteristics are not altered under illumination, within the resolution of our instrument. We argue that photo-currents measured with lock-in techniques in these kinds of structures are due to thermal effects. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B34.00007: Increased Efficiency of a Permanent Magnet Synchronous Generator through Optimization of NdFeB Magnet Arrays Helena Khazdozian, Ravi Hadimani, David Jiles The United States is currently dependent on fossil fuels for the majority of its energy needs, which has many negative consequences such as climate change. Wind turbines present a viable alternative, with the highest energy return on investment among even fossil fuel generation. Traditional commercial wind turbines use an induction generator for energy conversion. However, induction generators require a gearbox to increase the rotational speed of the drive shaft. These gearboxes increase the overall cost of the wind turbine and account for about 35 percent of reported wind turbine failures. Direct drive permanent magnet synchronous generators (PMSGs) offer an alternative to induction generators which eliminate the need for a gearbox. Yet, PMSGs can be more expensive than induction generators at large power output due to their size and weight. To increase the efficiency of PMSGs, the geometry and configuration of NdFeB permanent magnets were investigated using finite element techniques. The optimized design of the PMSG increases flux density and minimizes cogging torque with NdFeB permanent magnets of a reduced volume. These factors serve to increase the efficiency and reduce the overall cost of the PMSG. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B34.00008: Preperation and electrochemical characterization of Sm and Gd co-doped ceria/carbonate composite electrolytes for IT-SOFC applications Sibel Dikmen, Rabia Ozsakarya, Erdal Dikmen Sm and Gd co-doped ceria based composite electrolytes were prepared by mixing nanosized powders of Ce$_{\mathrm{0.8}}$Sm$_{\mathrm{0.1}}$Gd$_{\mathrm{0.1}}$O$_{\mathrm{2-\delta }}$ (SGDC) and alkaline carbonates (Na-Li)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, (Li-K)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, and (Na-K)$_{\mathrm{2}}$CO$_{\mathrm{3}}$ at a weight ratio of 4:1. Structure of the samples was characterized by powder X-ray diffraction. The microstructure and morphology were examined by SEM. Impedance spectroscopy was used to perform electrochemical characterization. The conductivities of the samples increase as the temperature increases and for the composite electrolytes SGDC(Na-Li)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, and SGDC(Li-K)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, there is a sharp increase in conductivity at around 475 and 450$^{\mathrm{o}}$C, respectively. This sudden change in the conductivity refers to superionic phase transition in the interfaces between SGDC phase and salt phase. The single cell power density reached a maximum of 1056, 826, and 565 mWcm$^{\mathrm{-2}}$ for SGDC/ (Na-Li)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, SGDC/(Li-K)$_{\mathrm{2}}$CO$_{\mathrm{3}}$, and SGDC/ (Na-K)$_{\mathrm{2}}$CO$_{\mathrm{3}}$ as the electrolytes, respectively. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B34.00009: Electronic structure and excitations in oxygen deficient CeO$_{2-\delta}$ from DFT calculations C. Lane, T. Jarlborg, B. Barbiellini, Yung Jui Wang, R.S. Markiewicz, Zhi Liu, Zahid Hussain, A. Bansil Mixed valent cerium oxides (ceria) are technologically important materials with remarkable properties useful for applications in heterogeneous chemical and electrochemical catalysis. We investigated the equilibrium electronic structures of supercells of CeO$_{2-\delta}$ within the Density Functional Theory (DFT), wherein properties such as lattice constants, bulk moduli and magnetic moments were well reproduced by the generalized gradient approximation (GGA) without the need to introduce the Hubbard U parameter. The chemical expansion and magnetic moment were calculated for Ce$_{4}$O$_{8-N}$ as a function of $N$, which for $N=8$, removing all the oxygen atoms, the fcc non-magnetic $\alpha$-phase of Ce was recovered. In the ground state of defective ceria, the Ce-\textit{f} majority band resides near the Fermi level, but appears at 2 eV below the Fermi level in photoemission spectroscopy experiments. We have demonstrated that x-ray photoelectron spectroscopy (XPS) relaxation effects yield a renormalization of \textit{f}-levels away from the Fermi level for electron excitation spectroscopies, which is also consistent with Ce-M and O-K x-ray absorption spectroscopy. Work supported in part by the US Department of Energy. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B34.00010: Density functional theory study of defect energies and space charge distribution at a solid-oxide electrolyte surface Chu Han, Angelo Bongiorno Yttrium-doped barium zirconate (BZY) is a proton conducting electrolyte forming a class of novel materials for new generation of solid oxide fuel cells, for hydrogen separation and purification, and for electrolysis of water. Here we use density functional theory calculations to compute the energy of protons and oxygen vacancies at the surface and in the bulk of lightly Y-doped BZY materials. We found that protons are energetically more stable at the surface termination than in the bulk of BZY by about 1 eV. In contrast, doubly-positively charged oxygen vacancies are found to form iso-energetic defects at both the terminal surface layer and in the bulk of BZY, while in the sub-surface region the defect energy raises by about 1 eV with respect to the value in the bulk. The energetic behavior of protons and oxygen vacancies in the near surface region of BZY is attributed to the competition of strain and electrostatic effects. Lattice model representations of BZY surfaces are then used in combination with Monte Carlo simulations to solve the Poisson-Boltzmann equation and investigate the implication of the results above on the structure of the space charge region at the surface of BZY materials. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B34.00011: DFT calculations of XAS and XPS processes in Ceria cells B. Barbiellini, T. Jarlborg, C. Lane, Yung Jui Wang, R.S. Markiewicz, Liu Zhi, Zahid Hussain, A. Bansil Final-state effects in X-ray absorption spectroscopy (XAS) and X-ray photoemission(XPS) have been calculated around oxygen vacancies in ceria (CeO$_2$). The method considers final-state total energies calculated using the constrained density functional theory. In the XAS final state, a core electron is extracted and then added to the valence electrons. The electronic structure is carried out self-consistently under these conditions. After the system has relaxed, one considers the total energy difference between the unperturbed state and the relaxed state to determine the XAS threshold energy. In the XPS final state, one electron is transferred from the ground-state density of states to a homogeneous plane-wave single-particle state. The total energy of the final state is constructed by using an average Kohn-Sham energy corresponding to the hole energy level and the hole density $\rho_h$. The present scheme is able to capture important XAS and XPS features observed in experiments when oxygen vacancies are created in ceria. Work supported by the US DOE. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B34.00012: Proton Diffusion in Tungsten Oxide Dihydrate: Hints from ab initio Calculations Hao Lin, Fei Zhou, Chi-Ping Liu, Vidvuds Ozolins Knowledge of proton diffusion mechanisms in tungsten oxide (WO$_{3}$) and its hydrates (WO$_{3}$${\cdot}$H$_{2}$O,WO$_{3}$${\cdot}$2H$_{2}$O) is essential for designing fuel cell membranes, electrochromics, energy storage materials and gas sensors. It is generally believed that tungsten oxide dihydrate is a better proton conductor than anhydrous tungsten oxide and monohydrate, due to the existence of fast diffusion pathways through the interlayer structural water. Aiming to test this assumption, we performed density functional theory calculations and surprisingly found that the interlayer structural water in dihydrate does not contribute to proton diffusion and that proton diffusion mechanisms are similar in dihydrate and tungsten oxide. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B34.00013: Understanding cation ordering and oxygen vacancy site preference in Ba$_{3}$CaNb$_{2}$O$_{9}$ from first-principles Hepeng Ding, Anil Virkar, Feng Liu We investigate the physical mechanism underlying the formation of the B-site cation ordering and the oxygen vacancy site selection in Ba$_{3}$CaNb$_{2}$O$_{9}$ using density functional theory calculations. We found that either cation site exchange or oxygen vacancy formation induces negligible lattice strain. This implies that the ionic radius plays an insignificant role in governing these two processes. Furthermore, the electrostatic interactions are found dominant in the ordering of mixed valence species on one or more sites, the ionic bond strength is identified as the dominant force in governing both the 1:2 B-site cation ordering along the \textless 111\textgreater direction and the oxygen vacancy site preference in Ba$_{3}$CaNb$_{2}$O$_{9}$. Specifically, the cation ordering can be rationalized by the increased mixing bonding energy of the Ca-O-Nb bonds over the Ca-O-Ca and Nb-O-Nb bonds, i.e., 1/2(Ca-O-Ca $+$ Nb-O-Nb) \textless Ca-O-Nb; while oxygen vacancy prefers a site to minimize the electrostatic energy and to break the weaker B-O-B bond. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B34.00014: QENS Investigation of Proton Diffusion in Sr/Ca-doped LaPO$_{4}$ Amal al-Wahish, N. Jalarvo, Z. Bi, C. Bridges, M.P. Paranthaman, A. Huq, K. Herwig, D. Mandrus We have investigated the diffusion dynamics of protons in hydrated~La$_{0.958}$Sr$_{0.042}$LaPO$_{4}$ and La$_{0.958}$Ca$_{0.042}$LaPO$_{4}$. These materials contain networks of tetrahedra rather than octahedra and relatively little is known about the mechanisms of proton transport in such systems. The samples were characterized by X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), and neutron powder diffraction (NPD) from room temperature to 800 $^{\circ}$C. The macroscopic and microscopic dynamics were studied using electrochemical impedance spectroscopy (EIS) and quasielastic neutron scattering (QENS). The activation energy characterizing the proton diffusion was determined in the temperature range~500-700 $^{\circ}$C using both QENS and EIS. For~La$_{0.958}$Ca$_{0.042}$LaPO$_{4}$, QENS~reveals a dynamical process that was not detected by EIS. The QENS activation energy was determined to be 0.09 eV, roughly an order of magnitude lower than the $\sim$ 1 eV inferred from EIS measurements. We will present results from the EIS, NPD and QENS analysis of the samples. [Preview Abstract] |
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