Research

Emerging technologies increasingly rely on complex chemical environments to achieve new levels of selectivity and efficiency. Understanding and controlling these systems requires fundamental knowledge of the chemistry and dynamic evolution of these complex environments. With molecular-scale knowledge of the chemical processes that control performance, we can guide discovery efforts for new, highly efficient systems. Characterizing functional materials and their interfaces with gases or condensed phases presents an experimental challenge, however, especially during operation.

We use in situ and operando X-ray spectroscopy and scattering, coupled with first principles calculations, to understand the chemical processes central to clean energy technologies. X-ray methods have the high penetration necessary to study functioning systems, while providing atomic and electronic structure information, often in an element-specific manner. These measurements can then be compared to first principles theory in order to yield molecular-scale models of the chemistry. Our experiments use beamlines at the Advanced Light Source at LBNL, as well as the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory and multiple synchrotron and X-ray free electron laser facilities around the world. We often work collaboratively within large teams, benefiting from colleagues with synthesis expertise and providing complementary information to other characterization studies.

Gas adsorption in porous materials

Chemical separations account for 10-15% of energy costs worldwide. We aim to discover new ways to efficiently separate gases using porous materials such as metal-organic frameworks (MOFs). These materials have a high potential for chemical customization, creating specific binding sites for desired gas molecules. Our work helped discover a means for cooperative gas adsorption, in which binding at one site in the MOF promotes binding at the neighboring site, resulting in high efficiency adsorption. We study interactions of gases with MOF binding sites using in situ X-ray spectroscopy in a custom built gas cell at the Advanced Light Source, and couple to first principles calculations to understand interactions in terms of specific molecular orbitals, binding geometries, and symmetries.

Solar fuel generation

Inspired by photosynthesis in plants, the generation of storable fuels using sunlight is a pathway to clean energy that can be readily integrated with our current energy infrastructure. CO2 reduction chemistry is challenging, however, due to the need to control selectivity for desired products, and generally low durability of current systems. We conduct a range of in situ and operando studies using X-ray spectroscopy, diffraction, and scattering techniques to understand the chemistry and dynamics of catalysts and light absorbers during operation. A major aspect of these studies is the design of electrochemical cells that can achieve the current densities and mass transport rates needed for effective CO2 reduction catalysis while being probed by X-rays.

Nonlinear soft X-ray spectroscopy

Nonlinear spectroscopy techniques are used to collect infrared or Raman spectra with a surface sensitivity of just a few atomic layers, much better than most surface science techniques. The requirement for high spatial and temporal coherence, however, means that ultrafast lasers are required. Until recently, X-ray sources lacked sufficient coherence to exploit similar nonlinear effects. Working with an international team of scientists, we made the first demonstration of soft X-ray second harmonic generation (SXSHG) in the soft X-ray regime at an X-ray free electron laser facility, demonstrating sensitivity to the first 1-3 atomic layers of graphite. This approach has many advantages, as it directly probes the electronic structure of the sample with elemental selectivity, and the high penetration of X-rays allows the study of buried interfaces, which is not possible using optical nonlinear spectroscopy. Currently, we are applying SXSHG towards studies of electronic structure at solid-solid junctions, and the structure and dynamics of liquid surfaces.

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