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Posted: Mar 04, 2016
(Nanowerk News) Boundaries can set limits or offer opportunities. At the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory, studies of critical reactions at interfaces between solids and liquids provide insights into systems spanning all four of EMSL's Science Themes – Atmospheric Aerosol Systems, Biosystem Dynamics and Design, Energy Materials and Processes, and Terrestrial and Subsurface Ecosystems. With better understanding of interfacial events, scientists can improve predictive models and contribute solutions to real-world systems. These cross-cutting questions and impacts spur development of new instruments and techniques to better probe these complex surfaces. Pushing past these boundaries expands EMSL's frontiers, too.
Solid-liquid interface studies have a long history at EMSL, noted Don Baer, science theme lead for Energy Materials and Processes. A workshop report from the early days, when the facility being planned was called the Molecular Science Research Center, highlighted interfacial studies as "a critical area" with important roles in waste processing, detection and storage as well as contaminant transport in soils and groundwater.
Over time, research efforts expanded to include dynamic processes in energy materials, aerosols and biological systems. At the same time, EMSL developed new capabilities to enable understanding and control of chemical reactions at these interfaces. Among later goals that were set – and met – the user facility's scientists developed methods for in-situ studies, devised instruments that combined spectroscopy with microscopy to drill down to nanoscale levels, and enhanced experimental and theoretical techniques to understand and design increasingly complex materials.
"There are now a suite of capabilities getting us to the point where we can say we are really beginning to have tools to understand these complex interfaces and conditions," said Baer. "It's important to a wide range of technologies and natural processes, so there are many different research pipelines for interfacial research at EMSL."
Just Add Water
The foundation of solid-liquid interfacial research at EMSL really started with geosciences – with reactions that start when water hits the surface of minerals – according to Baer.
Such fundamental studies continue to drive EMSL research projects, among them are those related to "mineral trapping" for carbon sequestration. When gaseous CO2 gets pumped deep into suitable subsurface environs, high pressure and temperature conditions incite a phase change to "wet" supercritical CO2, or scCO2, a viscous substance that can react and form carbonate minerals. But it takes a thin film of water to kick-start the process.
"There's a lot of new science here," said John Loring, a geochemist at Pacific Northwest National Laboratory. He leads a Department of Energy Basic Energy Sciences project on mineral reactivity and transformations in adsorbed water films as it applies to scCO2. More specifically, Loring and his colleagues study forsterite, a basalt mineral component, to find optimal thresholds for the thickness of water films that coat the mineral and accelerate reactions that lead to solidified carbon. Among other things, they want to know why reactions slow down when water films are less than a nanometer thick.
Historically, infrared, or IR, spectroscopy has been the workhorse for studying these reactions. Recently, Loring developed a new sophisticated IR capability: high-pressure IR spectroscopic titrations. This uses mirrors to toggle between two optics in the cell: a transmission IR that precisely accounts for water sorbed onto the mineral surface, and a more surface-specific, attenuated total reflectance IR, which collects spectra as the mineral reacts. In addition, tiny aliquots of water can be titrated into the cell, at pressure.
Loring knows how much water was added to the cell, and he measures how much water remains dissolved in the scCO2 using the transmission method optics. The difference between the amount of water added to the cell and the amount that remains in the supercritical phase, is the amount of water sorbed onto the mineral in the cell, explained Loring. "In that way, we can tell something about reactivity of the mineral at a given amount of sorbed water," he said. "It's been a nice breakthrough."
The impact of water and scCO2 on the surface of clays is another aspect that factors into carbon sequestration. Fundamental, molecular scale processes related to clay-water-scCO2 interactions are investigated by longtime EMSL users and collaborators James Kirkpatrick, a professor of geological sciences and chemistry and dean of the College of Natural Science at Michigan State University, and Geoffrey Bowers, an associate professor of chemistry at Alfred University in New York.
"Clays are everywhere in the near-surface environment of Earth's crust," noted Kirkpatrick. "They have a layered, sheet-like structure with very large surface areas where interactions with cations and water can affect mineral surfaces."
With access to EMSL's helium-ion microscope and the high-pressure magic angle spinning nuclear magnetic resonance, or NMR, tools, Bowers and Kirkpatrick have been able to explore molecular scale events in situ to make better predictions about macroscopic systems.
Specifically, they've delved beyond water-clay interactions at pristine interfaces to examine more authentic, "dirty" interfaces where minerals and organic matter, or OM, interact. Their NMR studies help identify how ions and fluids are bound and how they move at these complex interfaces. The helium-ion microscope lets them visualize how clay and OM interact on a microscopic scale, guiding interpretation of NMR results.
In previous work they found that adding calcium to a clay-water-OM system resulted in microscopic structures strongly affected by pH. However, NMR probing on the molecular scale showed that OM had little influence on the binding and dynamics of a vast majority of calcium ions. "That's important information because data from pristine systems are often what gets into models," said Bowers. "If we saw substantial differences in ion behavior with OM present, then models wouldn't be predicting right answers."
Now, the two are looking at the effect of scCO2 on hydration and movement of molecules through clay and clay-OM systems. Although work is still in early phases, Bowers said: "Preliminary results suggest some nuclear waste material could become a solution for CO2 storage."
Another way to look at interfacial reactions is to modify solvents that mediate them.
In collaboration with EMSL, researcher Seema Singh, director of Biomass Pretreatment at the Joint BioEnergy Institute, or JBEI, is developing "designer" solvents to break down the protective lignin outer layer of cellulose.
The very things that make lignin useful to plants, a rigid structure that is hydrophobic to water and resistant to microbes, are the same factors that make it so intractable for breaking down into biofuels, Singh explained.
She's testing various ionic liquids – salts that are liquid at room temperature because they have cations too big to allow crystallization – under a range of pH conditions, to disassemble lignin into aromatic components that could be funneled into industrially useful products. Ultimately, this DOE/JBEI-funded research will lead to methods for converting lignin itself into "high-value chemicals."
Singh's efforts to precisely profile these reactions rely on a suite of EMSL instruments: mass spectrometry, NMR and electron paramagnetic resonance spectroscopy, and computer modeling capabilities.
Another innovative approach in atmospheric research is to use aerosols themselves as solvents.
In a recent paper, a team of scientists from the University of California at Irvine, the University of British Columbia and EMSL described using organic aerosols as solvents for studying chemical reactions of 2,4-Dinitrophenol, or DNP.
Under normal conditions, the aromatic pollutant, DNP dissolves into aerosols and can quickly diffuse throughout the particle, accessing both the surface and interior of the particle. However as temperature or relative humidity drops, aerosols become more viscous, and DNP become immobile. In this study, the team showed increasing viscosity made molecules so sluggish photodestruction reactions driven by solar radiation also slowed down.
This finding has implications for air-pollution models in areas of extreme cooling or dry climates. The National Oceanic and Atmospheric Administration and the National Science Foundation funded these studies.
Solid-Liquid Electrolyte Relations
Interfacial reactions are a major challenge in making long-lived lithium batteries, or finding other materials that might become better batteries.
With lithium, or any type of rechargeable battery, problems are three-fold: first, ions need to be able to travel easily through the electrolyte between the cathode and anode; second, that travel slows as electrolyte-electrode interactions degrade electrode surfaces over time; and third, the electrolyte and electrode age and form different chemicals that impact overall reactivity.
"Everything is dynamic so it's very difficult to get good resolution and see what's happening at these interfaces under operating conditions," said Chongmin Wang, EMSL scientist.
Despite that difficulty, Wang and his collaborators, with funding from Joint Center for Energy Storage Research, developed an in-situ approach within a transmission electron microscope at EMSL. This atomic-level imaging enabled researchers to follow subtle changes to the battery after rounds of charging and discharging.
More recently, with EMSL's atom probe tomography, Wang and his colleagues mapped the distribution of lithium ions and transition metal elements in cathodes, gleaning insight into ways ions are progressively lost during charge-discharge cycles.
In the future Wang wants to design battery components from "modern materials" that are more stable and exclude factors that interfere with battery longevity, or use crystals with tailored surface structures.
"No breakthroughs ... yet we are making progress," said Wang.
While some surfaces or interfaces cause undesired effects during reactions, Scott Chambers, a Wiley Research Fellow in EMSL, carefully constructs superlattices to put them to work.
The idea is to synthesize a lattice of repeating materials (A-B-A-B- …) with atomic layer control, so the resulting electronic structure separates electrons and "holes." In this way, wherever an electron-hole pair is created by light absorption, the electron goes to material A and the hole goes to material B. The surface material dictates which charged particle is on the surface. This kind of separation prevents electron-hole pair recombination, a process that keeps electrons and holes from doing useful chemical work.
"Our approach is on the fundamental side, trying to understand at an atomistic level how best to control this chemistry," said Chambers.
This approach has a great deal of promise for water splitting to make hydrogen and oxygen. Although simple in concept, water splitting is a complex process that requires considerable energy.
"Water electrolysis requires both an electron and a hole, each of sufficient energy to drive the two half-cell reactions. It is difficult to find one material with the right band structure to carry both particles at sufficient energy to do the job," Chambers said.
With advanced epitaxial oxide film deposition methods developed at EMSL over the past two decades, Chambers plans to synthesize different materials optimized to do each half-cell reaction. Then, the two could be combined into one cell, put under water and irradiated with sunlight, and used to generate O2 and H2 using nothing but the sun's enormous energy output, Chambers explained.
"If we can do this, it would go a long way toward solving some of our energy problems," said Chambers. "We'd have the perfect fuel: very combustible and with no carbon footprint."
Which end is up? The importance of orientation and conformation of molecules on surfaces plays out in every interfacial reaction. The trick is finding ways to visualize those structures and how they change during reactions – particularly as complexity scales up.
EMSL Scientist Hongfei Wang developed a high-resolution vibrational sum frequency generation spectroscopy technique that boosts spectral resolution by 10-20 times. By increasing the ability to see small structural differences, the instrument enhances research in many areas. For instance, catalysis relies on understanding how surface molecules are affected by solvents and absorbents. This method can also be particularly useful for studying biological interactions of cells or bacteria because surface membranes are usually "buried" in an environment that prevents applying other imaging techniques.
Still, Hongfei found room for improvement. He is now developing a high-resolution version of "second-order Mei scattering." This builds on the Mei scatter technique that – similar to radar, but using the shorter wavelength of light – analyzes the way light from one photon scatters after it interacts with a particle. The new version, based on Hongfei's doctoral degree studies, provides direct in-situ measurement of the surface of the particles because it scatters two photons and sums those reflected frequencies into one, thus providing a selective probe of the particle surface. With this higher resolution, Hongfei hopes to enhance research capabilities in energy, environmental, material and biological sciences by obtaining unprecedented molecular specificity.
Building on Strengths
In addition to innovations in instruments, EMSL's computational capabilities will play an increasingly important role in advancing research across the board, said Baer.
A recent workshop report noted NWChem was one of the user-facility's foundational strengths. This open source, high-performance computational chemistry package was created by a consortium of developers and is maintained at EMSL. Over the years, NWChem has become a key component in the integration of experiment and theory at EMSL.
Now, efforts are underway to expand NWChem to include increasingly complex environments, such as multi-scale simulations of cellular metabolites in biological systems. Then, engineering to integrate such data with more sophisticated instruments in development, will position EMSL at the leading edge of research.
EMSL, the Environmental Molecular Sciences Laboratory, is a national scientific user facility sponsored by the Department of Energy's Office of Science. Located at Pacific Northwest National Laboratory in Richland, Wash., EMSL offers an open, collaborative environment for scientific discovery to researchers around the world. Its integrated computational and experimental resources enable researchers to realize important scientific insights and create new technologies.
Source: By Elizabeth Devitt, Pacific Northwest National Laboratory