Research
Clean energy technologies will play a critical role in our future. As we work towards developing high efficiency batteries, solar cells, solar fuel devices, and other systems for generating renewable energy, we must understand the chemistry and dynamic evolution of the complex chemical environments central to these technologies. With a 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 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. In addition, generating fuel from the reduction of CO2 directly addresses the rising levels of CO2 emissions which are a large driver of climate change. CO2 reduction chemistry is challenging, however, due to the need to control selectivity for desired products. We conduct a range of in situ and operando studies using X-ray spectroscopy, diffraction, and scattering techniques to understand the catalytic interface during aqueous or vapor-fed CO2 reduction, in order to identify the active sites for specific products and how they evolve 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.
Local structure effects in organo-lead halide perovskite photovoltaics
Organo-lead halide perovskites are promising materials for photovoltaic applications, with synthetically tunable compositions that dictate band gaps and stability. We use high resolution hard X-ray spectroscopy to probe a ligand field splitting at the lead sites in these materials, which is highly sensitive to the local structure. By coupling to first principles calculations, we interpret our spectra in terms of hybridized lead-halide d orbitals which respond to octahedral tilts in the material. These tilts have a large impact on the band gaps. We are investigating sensitivities to the cations in the materials, as well as how the reduced dimensionality of 2D perovskites alters these electronic couplings.
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 a team of scientists, we made the first demonstration of second harmonic generation 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. We have subsequently demonstrated soft X-ray SHG spectra of a model buried interface and continue to develop this method towards the study of functional interfaces for energy applications.