Surface Science Approach Research Articles

Photoemission Study on the Interaction Between LiCoO2 Thin Films and Adsorbed Water

Surface layers at the cathode-electrolyte interface strongly affect the performance of the Li-ion cell. Such surface layers are found during manufacturing and operation of the battery. As water can never be fully excluded from the manufacturing chain, it will form an additional surface layer situated between cathode and electrolyte. In this contribution, we investigate the interaction between the LiCoO2 electrode and H2O by a surface science approach. H2O was adsorbed stepwise onto a LiCoO2 thin film, and intermediate X-ray photoelectron spectroscopy analysis was performed after every step. Adsorption results in the formation of a Li2O/LiOH-type reaction layer on top of the electrode and downward band bending attributed to a Li+-ion transfer out of the electrode.

Surface Science Approaches for the Preparation of Alumina‐Supported Hydrotreating Catalysts

AbstractNew developments and breakthroughs in the design of hydrotreating (HDT) catalysts for complying with fuel specifications and reduced sulfur contents ask for a continuous improvement in the molecular scale description of the active phase both at the oxide and sulfide states. HDT catalysts are mainly based on alumina‐supported molybdenum nanophases and the present contribution is dedicated to review the surface science approaches that have helped gain a molecular‐scale insight into the structure of the active‐phase (mainly molybdenum) and the role of metal‐support (mainly alumina) interactions through the use of well‐defined surfaces and surface sensitive analysis techniques. The main focus of this Review is placed on model systems that can bridge the gap with industrial catalysts, that is, oxide‐supported active phases prepared in aqueous conditions, as opposed to more traditional surface‐science studies that consider metal‐supported nanophases prepared in UHV conditions. The synthesis and structure of well‐defined alumina surfaces (thin films and single crystal wafers) is first presented before discussing aqueous‐deposition of Mo precursors on flat surfaces. The detailed role of the oxide support is then addressed with an emphasis on the genesis and structure of the MoS2 phase namely sulfidation rate, stacking, and orientation.

Investigation of moisture damage resistance of GTR-modified asphalt binder by static contact angle measurements

More than 300 million scrap tires are being generated every year in the United States and their proper disposal has been a major concern from environmental safety and economical point of views. Using these scrap tires as ground tire rubber (GTR) in asphalt mixes is becoming a common practice now a days. A number of research studies have been conducted to characterize the mechanistic properties of GTR modified asphalt. However, a very limited research has been conducted to evaluate their moisture susceptibility, particularly through fundamental science approaches. Moisture damage or stripping, the loss of strength and stiffness of asphalt mixes due to the existence of moisture, of asphalt pavements is one of the major concerns to the state and federal highway agencies for many years. The objective of this research was to evaluate moisture susceptibility of GTR modified asphalt binders through a surface science approach, in which surface free energy (SFF) components were estimated by following the sessile drop method. In the experimental plan, a commonly used performance grade (PG) binder (PG 64-22) modified with different dosages (0%, 10%, 15%, and 20%) of GTR were tested for static contact angles. Three different probe liquids, namely: water, formamide and ethylene glycol were used to determine contact angle with binder samples using an optical contact angle analyzer (OCA) device. The SFE of five different aggregates, namely: sandstone, gravel, granite, basalt and limestone, were used to evaluate the work of adhesion and compatibility ratio (CR). The SFE results of unmodified and modified binders showed that SFE increased due to the addition of GTR in binder. The work of adhesion of a binder aggregate system also increased with an increase of the GTR content when measured in the absence of water, but it decreased gradually in the presence of water. Granite and basalt showed lowest work of adhesion, whereas sandstone showed the highest work of adhesion. The CR data showed that granite and basalt were the most compatible aggregates with GTR modified binders. The CR value also had an increasing trend with an addition of higher GTR content, indicating an improved moisture resistance. The CR values of all asphalt aggregate systems were found to be greater than 1.5 with 20% GTR, indicating “good” moisture susceptibility.

Analysis of All-Solid Li-Ion Batteries By Photoelectron Spectroscopy: Experimental Procedures, Interpretation and Results

Electrochemical devices such as lithium-ion secondary batteries consist of complex layer systems with interfaces and interphases that play a crucial role in cell performance and degradation. The understanding of the energy level alignment and double layer formation especially at electrode-electrolyte interfaces is therefore mandatory. Photoelectron Spectroscopy (PES) is a reliant tool and widely accepted method in surface science to probe the electronic structure of thin, electronically conducting layer systems on a nanometer scale, e.g. of semiconductor devices. For such systems, an aligned Fermi level (i.e. electrochemical potential of electrons) can usually be presumed resulting in well-defined experimental conditions for PES. For ionic systems involving (solid) electrolytes, the experimental conditions are less clear due to the lack of sufficient electronic conductivity. Nevertheless, also in such cases PES proofs to be a useful tool, i.e. for analysis of solid electrolytes or characterization of full cells and interfaces [1]. In this contribution, we focus on the different electrolyte- and ionic-interface related effects, such as charging and Fermi level misalignment, and discuss experimental strategies and interpretation of PES measurements. By example of a LiCoO2|LiPON|Li thin film battery, we demonstrate how interface experiments and analysis of the top electrode of a layer stack can be performed and interpreted. Key factors taken into consideration are the electrochemical equilibrium of the lithium ions at the interfaces as well as the ability of the electrolyte to sustain an electric (ionic) current, given corresponding (de)intercalation reactions are triggered. Next to information on electron energy level alignment and double layer formation, information on ion energy levels, ionic and electronic contribution to cell voltage, formation of reaction layers as well as electronic structure of intercalation electrodes can be obtained from suitable experiments (see e.g.[1-3]). 1. Hausbrand, R., D. Becker, and W. Jaegermann, A surface science approach to cathode/electrolyte interfaces in Li-ion batteries: Contact properties, charge transfer and reactions. Progress in Solid State Chemistry, 2014. 42(4): p. 175-183. 2. Tonti, D., C. Pettenkofer, and W. Jaegermann, In-Situ Photoelectron Spectroscopy Study of a TiS2 Thin Film Cathode in an Operating Na Intercalation Electrochemical Cell. Ionics, 2000. 6(3-4): p. 196-202. 3. Schwöbel, A., R. Hausbrand, and W. Jaegermann, Interface reactions between LiPON and lithium studied by in-situ X-ray photoemission. Solid State Ionics, in press.

Surface Chemistry of Electrocatalysts for CO2 Reduction to Fuels

The surface chemistry of electrocatalysts for the production of fuels by CO2 reduction was explored by surface science model studies using high-resolution spectroscopy and atomic scale imaging, as well as by electrochemical analysis of active electrodes. Using an experimental surface science approach, ultrahigh vacuum conditions were used to facilitate the fabrication of highly characterizable electrode-adsorbate systems. We have utilized a range of techniques for surface analysis to determine the structure and properties of single-crystal surfaces and unique alloys formed by vapor deposition under controlled conditions. Specifically, XPS, LEIS, LEED and STM were used to characterize the composition and structure of III-V semiconductors and bimetallic model catalysts. TPD, UPS, and vibrational spectrocopy using HREELS and IRAS were used to study the binding and electronic structure of adsorbates. Many of our experiments are designed to explain the role of the electrode surface in the high selectivity observed in pyridine and other N-heterocycle-catalyzed CO2 reduction systems. We examined the preferential adsorption sites, bonding interactions, and reactivities of these molecular catalysts and other relevant adsorbates on the surfaces of photoactive electrode materials. Ambient pressure photoemission spectroscopy (APPES), STM, and vibrational spectroscopies were used to study the interaction of water and CO2 reduction catalysts with the surfaces of III-V semiconductors. We have applied these techniques to also study bimetallic alloy electrocatalysts. Bimetallic alloys have been shown to exhibit lower overpotentials for CO2 reduction to fuels when compared to their metal constituents. By tuning the composition of the alloys, one can influence the binding of adsorbates and therefore reaction selectivity. We synthesized bimetallic surface alloys in controlled UHV environments to investigate the relationship between surface composition and electronic structure. We studied the interaction of relevant adsorbed species with the bimetallic alloy surfaces using TPD, IRAS, HREELS, and UPS to begin to understand the surface chemistry of these electrodes. In parallel to these surface science investigations, we studied the electrochemical behavior of single crystals and bimetallic alloys. The purpose of these studies is to correlate CO2 reduction product selectivity to the quantitative measurements of surface chemistry made in UHV discussed above. Specifically we examined the relationship between electrochemical activity, surface composition, and the binding strengths of intermediates along CO2 reduction pathways. This work aids development of a molecular-level understanding of the heterogeneous processes involved the reaction mechanisms of efficient electrocatalytic fuel generation. This material is based upon work supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012455.

Influence of support morphology on the bonding of molecules to nanoparticles

Supported metal nanoparticles form the basis of heterogeneous catalysts. Above a certain nanoparticle size, it is generally assumed that adsorbates bond in an identical fashion as on a semiinfinite crystal. This assumption has allowed the database on metal single crystals accumulated over the past 40 years to be used to model heterogeneous catalysts. Using a surface science approach to CO adsorption on supported Pd nanoparticles, we show that this assumption may be flawed. Near-edge X-ray absorption fine structure measurements, isolated to one nanoparticle, show that CO bonds upright on the nanoparticle top facets as expected from single-crystal data. However, the CO lateral registry differs from the single crystal. Our calculations indicate that this is caused by the strain on the nanoparticle, induced by carpet growth across the substrate step edges. This strain also weakens the CO-metal bond, which will reduce the energy barrier for catalytic reactions, including CO oxidation.

The Interaction of Formic Acid with Zinc Oxide: A Combined Experimental and Theoretical Study on Single Crystal and Powder Samples

We present azimuth- and polarization-dependent infrared spectroscopy results obtained under ultra-high vacuum conditions on surface species formed by the interaction of formic acid with the mixed-terminated ZnO(10\(\bar{1}\)0) surface. Since there are no previous IRRAS data for formic-acid derived species on any ZnO single crystal surfaces, we have carried out calculations using density function theory to aid the interpretation of the results. From our combined experimental and theoretical data we conclude that two different formate species are formed. The more strongly bound species is a bidentate with the formate molecular plane oriented along the [1\(\bar{2}\)10] direction. The less strongly bound species is a quasi-bidentate with its molecular plane oriented along the [0001] direction. This second species is characterized by a strong hydrogen bond between a surface OH species and the formate. In addition, IR data were recorded for the same molecule adsorbed on commercial ZnO nanoparticles. The different bands of the powder IR-data are assigned on the basis of the experimental and theoretical results obtained for the single crystal surface. This study demonstrates the importance of the Surface Science approach to heterogeneous catalysis also for ZnO, an important catalyst for the conversion of syngas to methanol.

ChemInform Abstract: Understanding the Electron‐Stimulated Surface Reactions of Organometallic Complexes to Enable Design of Precursors for Electron Beam‐Induced Deposition

AbstractReview: 77 refs.

Evaluation of Moisture Susceptibility of Nanoclay-Modified Asphalt Binders through the Surface Science Approach

AbstractDue to an increasing rate of traffic volume and truckloads in recent years, asphalt binders are often modified with synthetic polymers to increase stiffness and sustain excessive heat during hot summer days. However, the cost of polymer-modified binders (PMBs) is significantly higher than unmodified binders. Nanoclays, on the other hand, are relatively inexpensive and naturally abundant, and have favorable intrinsic properties, such as nanoscopic size and high surface area. To this end, the research reported in this paper investigated moisture resistance properties of a commonly used Performance Grade (PG) 64-22 binder modified with different dosages of two selected nanoclays [(1) Cloisite 15 A, and (2) Cloisite 11B]. The state of dispersion of the nanoclay in the binder was examined using a scanning electron microscope and small-angle x-ray diffraction. The morphological analyses of scanning electron microscope scans and inter-gallery distances (d-spacing values) of small-angle x-ray diffraction ...

Water oxidation catalysis: effects of nickel incorporation on the structural and chemical properties of the α-Fe₂O₃(0001) surface.

Photoelectrochemical solar fuel synthesis devices based on photoactive hematite (α-Fe2O3) anodes have been extensively investigated, yet a fundamental understanding regarding its associated water oxidation surface reaction mechanism is still lacking. To help elucidate detailed reaction mechanisms, we studied water chemisorption and reaction as well as structural changes induced by Ni incorporation into the α-Fe2O3(0001) surface. Investigation by scanning probe and electron diffraction techniques show that vapor deposition of Ni and subsequent annealing to 700 K leads to the interdiffusion and incorporation of Ni into the near-surface region of hematite and changes the structure of the (0001) surface by the formation of FeO-like domains on the topmost layer. These results are discussed in the context of a proposed water oxidation mechanism on this surface in which Ni doping facilitates water oxidation by increasing O hole concentrations and forms less negatively charged O anions (*O) and *O···OH species [ Liao, P. L.; Keith, J. A.; Carter, E. A. J. Am. Chem. Soc. 2012 , 134 , 13296 - 13309 . ]. Consistent with predictions from this theory, electrochemical measurements using cyclic voltammetry carried out on the ultrahigh vacuum-prepared surfaces demonstrated that Ni incorporation leads to higher current density and lower onset potential than the unmodified α-Fe2O3 surface. Our work utilizing a surface science approach helps to connect such theoretical predictions of reaction thermodynamics on well-defined structures and the performance of modified hematite model electrocatalysts for water oxidation.

Interface reactions between LiPON and lithium studied by in-situ X-ray photoemission

Lithium phosphorus oxynitride (LiPON) is an amorphous solid lithium ion conductor commonly used in all solid state thin film batteries (TFBs) with LiCoO2 as cathode- and lithium as anode-material. TFBs exceed conventional Li ion batteries with respect to lifetime and safety but may suffer from high ion transfer resistances and interface reactions between the electrodes and the electrolyte. In this contribution we study interface layer formation between LiPON and metallic lithium using an in-situ X-ray photoemission spectroscopy (XPS) surface science approach.

Exploring Zeolite Chemistry with the Tools of Surface Science: Challenges, Opportunities, and Limitations

The complexity of catalysts that the surface science community has been able to address has increased substantially in a systematic manner, starting with metal and oxide single crystal surfaces and evolving to an atomistic description of clusters and nanoparticles on well-defined, planar supports. The next step in adding complexity is now to address surfaces of porous oxide materials, in particular of zeolites, which are the most extensively used catalysts in the industry. The recently reported successful fabrication of well-ordered thin films, consisting of planar arrangement of aluminosilicate polygonal prisms on a metal substrate counting with highly acidic bridging hydroxyl groups on the surface, represents the limiting case of infinitely large pore and cages in zeolites. This model system allows one to study reactions catalyzed by zeolites using the toolkit of surface science. In this Perspective, we describe the zeolitic model system, with its virtues and limitations, as well as the challenges, opportunities and expectations for the future in modelling porous catalysts by a surface science approach. .

Understanding the electron-stimulated surface reactions of organometallic complexes to enable design of precursors for electron beam-induced deposition

Standard practice in electron beam-induced deposition (EBID) is to use precursors designed for thermal processes, such as chemical vapor deposition (CVD). However, organometallic precursors that yield pure metal deposits in CVD often create EBID deposits with high levels of organic contamination. This contamination negatively impacts the deposit’s properties (e.g., by increasing resistivity or decreasing catalytic activity) and severely limits the range of potential applications for metal-containing EBID nanostructures. To provide the information needed for the rational design of precursors specifically for EBID, we have employed an ultra-high vacuum (UHV) surface science approach to identify the elementary reactions of organometallic precursors during EBID. These UHV studies have demonstrated that the initial electron-induced deposition of the surface-bound organometallic precursors proceeds through desorption of one or more of the ligands present in the parent compound. In specific cases, this deposition step has been shown to proceed via dissociative electron attachment, involving low-energy secondary electrons generated by the interaction of the primary beam with the substrate. Electron beam processing of the surface-bound species produced in the initial deposition event usually causes decomposition of the residual ligands, creating nonvolatile fragments. This process is believed to be responsible for a significant fraction of the organic contaminants typically observed in EBID nanostructures. A few ligands (e.g., halogens) can, however, desorb during electron beam processing while other ligands (e.g., PF3, CO) can thermally desorb if elevated substrate temperatures are used during deposition. Using these general guidelines for reactivity, we propose some design strategies for EBID precursors. The ultimate goal is to minimize organic contamination and thus overcome the key bottleneck for fabrication of relatively pure EBID nanostructures.

Activity-Stability Trends for the Oxygen Evolution Reaction on Monometallic Oxides in Acidic Environments.

In the present study, we used a surface-science approach to establish a functional link between activity and stability of monometallic oxides during the OER in acidic media. We found that the most active oxides (Au ≪ Pt < Ir < Ru ≪ Os) are, in fact, the least stable (Au ≫ Pt > Ir > Ru ≫ Os) materials. We suggest that the relationships between stability and activity are controlled by both the nobility of oxides as well as by the density of surface defects. This functionality is governed by the nature of metal cations and the potential transformation of a stable metal cation with a valence state of n = +4 to unstable metal cation with n > +4. A practical consequence of such a close relationship between activity and stability is that the best materials for the OER should balance stability and activity in such a way that the dissolution rate is neither too fast nor too slow.

Catalysis and Interfacial Chemistry in Lithium Batteries: A Surface Science Approach

Control of the interfacial chemistry of the electrodes in lithium batteries is vitally important to their safe and effective application. Water and virtually every organic solvent is thermodynamically unstable in the presence of metallic lithium. The electrode potential of a graphite electrode in a lithium-ion battery at the top of charge is at an equivalent chemical potential. In principle, the entire lithium or charged graphite electrode can be completely consumed by reaction with the solvent if the interfacial chemistry is not adventitious, i.e. does not form a reaction self-limiting passive film. A greater understanding of the reactions of the electrolyte and the nature of passivity will be essential to utilize metallic lithium or hosts like silicon that store equivalent amounts of lithium. A surface science approach, like that presented here but now out of fashion, may provide additional insight to approaches currently being used.

A surface science approach to cathode/electrolyte interfaces in Li-ion batteries: Contact properties, charge transfer and reactions

Reactions and charge transfer at cathode/electrolyte interfaces affect the performance and the stability of Li-ion cells. Corrosion of active electrode material and decomposition of electrolyte are intimately coupled to charge transfer reactions at the electrode/electrolyte interfaces, which in turn depend on energy barriers for electrons and ions. Principally, energy barriers arise from energy level alignment at the interface and space charge layers near the interface, caused by changes of inner electric (Galvani) potential due to interfacial dipoles and concentration profiles of electronic and ionic charge carriers. In this contribution, we introduce our surface science oriented approach using photoemission (XPS, UPS) to investigate cathode/electrolyte interfaces in Li-ion batteries. After an overview of the processes at cathode/electrolyte interfaces as well as currently employed analysis methods, we present the fundamentals of contact potential formation and energy level alignment (electrons and ions) at interfaces and their analysis with photoemission. Subsequently, we demonstrate how interface analysis can be employed in Li-ion battery research, yielding new and valuable insights, and discuss future benefits.

Chemical Warfare Agent Surface Adsorption: Hydrogen Bonding of Sarin and Soman to Amorphous Silica.

Sarin and soman are warfare nerve agents that represent some of the most toxic compounds ever synthesized. The extreme risk in handling such molecules has, until now, precluded detailed research into the surface chemistry of agents. We have developed a surface science approach to explore the fundamental nature of hydrogen bonding forces between these agents and a hydroxylated surface. Infrared spectroscopy revealed that both agents adsorb to amorphous silica through the formation of surprisingly strong hydrogen-bonding interactions with primarily isolated silanol groups (SiOH). Comparisons with previous theoretical results reveal that this bonding occurs almost exclusively through the phosphoryl oxygen (P═O) of the agent. Temperature-programmed desorption experiments determined that the activation energy for hydrogen bond rupture and desorption of sarin and soman was 50 ± 2 and 52 ± 2 kJ/mol, respectively. Together with results from previous studies involving other phosphoryl-containing molecules, we have constructed a detailed understanding of the structure-function relationship for nerve agent hydrogen bonding at the gas-surface interface.

Universal Activity Volcano for 2e- and 4e- Reduction of Oxygen on Metal Surfaces

Oxygen reduction reaction (ORR) has been a subject of major interest in electrochemistry owing to its technological and scientific importance. The fundamental understanding of this reaction has been largely based on the surface science approach to electrocatalysis.[1] The surface science approach has allowed the bridging of theory and experiment by enabling direct theoretical investigations of the reaction mechanism of oxygen reduction reaction on model systems. In this talk, we will address the issue of 2e− versus 4e− reduction of oxygen on metal surfaces. Using a detailed thermodynamic analysis based on density functional theory calculations, we show that to a first approximation an activity descriptor, ΔGOH*, the free energy of adsorbed OH*, can be used to describe trends for the 2e− and 4e− reduction of oxygen. We demonstrate the existence of a universal activity volcano shown in the Figure and find quite a remarkable agreement between the predictions of the model and experimental results spanning nearly 30 years.[2-4]References[1] Fuel Cell Catalysis: A Surface Science Approach, Marc Koper and Andrzej Wieckowski, (2009)[2] V. Viswanathan, H. A. Hansen, J. Rossmeisl, and J. K. Nørskov, ACS Cat., (2012) 2, 1654-1660.[3] V. Viswanathan, H. A. Hansen, J. Rossmeisl, and J. K. Nørskov, J. Phys. Chem. Lett., (2012) 3, 2948-2951.[4] H. A. Hansen, V. Viswanathan and J. K. Nørskov, (submitted).Fig 1: Activity volcanoes for the 2e− and 4e− reduction of oxygen are shown in red and blue respectively. The source of the experimental data for the 2e− and 4e− reduction of oxygen are listed in ref. [3]. The experimental value of the limiting potential, UL, is determined by the half-wave potentials for the 4e− reduction and by the onset of ring current for the 2e− reduction. The activity of (111) and (100) surfaces has been plotted in circles and squares, respectively.

Steady-state and unsteady-state kinetic approaches for studying reactions over three-way natural gas vehicle catalysts

Abstract Kinetics has been proven to be a powerful method to probe catalytic surfaces under reaction conditions in order to elucidate, at molecular level, complex chemical processes. Numerous techniques and methodologies have been already implemented (surface science approaches, TAP, SSITKA…) running in very different pressure ranges (pressure gap) which led to controversial statements regarding suggested mechanism schemes, especially for DeNOx reactions. Two typical reactions taking place over NGV catalysts have been selected to illustrate which information can be tackled from kinetic measurements. Both reactions occur in different temperature ranges and are sensitive to the structure of the catalyst, to the surface composition of bimetallic particles, with possible surface enrichments, and to the participation of the support. Hence, it will be tentatively demonstrated that kinetic combined with spectroscopic or theoretical methods can be appropriate to establish relevant correlations between kinetic parameters and the topology of the catalyst surface.

Adsorption of Diethyl Carbonate on LiCoO2 Thin Films: Formation of the Electrochemical Interface

Despite numerous efforts to elucidate interface-related phenomena of Li ion battery cathodes, the exact nature of cathode/electrolyte interfaces is still not fully resolved. Key factors for the properties of semiconducting ionic electrodes are band bending and energy level alignment at the interface, which have not been given much attention in the past. In this contribution, we investigate the formation of the electrochemical interface for a LiCoO2 electrode in contact with a solvent adsorbate phase by a surface science approach. Diethyl carbonate (DEC) was adsorbed stepwise onto a LiCoO2 thin film electrode and the electrode surface analyzed with X-ray photoelectron spectroscopy (XPS) after each adsorption step. Adsorption results in the formation of a charged layer in the electrode, which we attribute to the transfer of lithium ions from the electrode to the adsorbed phase. The offset between the LiCoO2 valence band and HOMO of the adsorbed DEC is large (4 eV) under the experimental conditions, which renders solvent oxidation unlikely.