The 2021 Emerging Leader in Atomic Spectroscopy: Using X-ray Spectroscopy to Understand Surface Science and Energy Conversion

L. Robert Baker is an associate professor at The Ohio State University in the Department of Chemistry & Biochemistry. His research focuses on X-ray spectroscopy, nonlinear and time-resolved spectroscopy, the chemistry of surfaces and interface science, and energy conversion and catalysis—work that may lead to better solar energy conversion materials. He is the winner of the 2021 Emerging Leader in Atomic Spectroscopy Award, which is presented by Spectroscopy magazine. This annual award, begun in 2017, recognizes the achievements and aspirations of a talented young atomic spectroscopist, selected by an independent scientific committee.

You are the lead scientist for the new National Science Foundation (NSF) National eXtreme Ultrafast Science (NeXUS) Facility. Would you explain to our readers the purpose of this exciting new facility and what you might hope to discover using its facilities?

This 5-year, $9.5M project is supported by the National Science Foundation through the Mid-scale Infrastructure program. Mid-scale infrastructure refers to facilities that are small enough to adapt quickly to rapidly evolving research needs but bigger than could be supported in a traditional academic laboratory. The NSF recently identified mid-scale infrastructure as one of the 10 big ideas that will shape the future of multidisciplinary basic science.

As the principal investigator (PI) and co-director of NeXUS, I am working with a talented team to build this national NSF user facility on the Ohio State campus. The combination of attosecond pulses, soft X-ray photon energies, and high repetition rate at NeXUS will enable measurements that currently cannot be made anywhere else in the United States. Accordingly, NeXUS is designed to fill a key strategic gap in the U.S. research infrastructure and will make OSU an international focal point serving a broad user community in ultrafast optical science.

In one of your 2020 publications, you used extreme ultraviolet reflection–absorption (XUV-RA) spectroscopy to study the surface electronic structure and carrier kinetics in photocatalytic transition metal oxides (1). Specifically, the dynamics of small polaron formation at hematite surfaces demonstrates that the rate of carrier self-trapping is slower than in the bulk due to a greater lattice reorganization energy required for surface polaron formation. Why is this research important? What were the major analytical hurdles you encountered when developing XUV-RA?

Polaron formation is one of the major obstacles to efficient solar energy conversion using earth-abundant metal oxide catalysts. A polaron can be thought of as a local defect that forms inside a solid in response to the excited electron. It traps the electron so it can only move by a slow, thermally activated hopping process.

In this work we learned two important things: First, we found that the dynamics of polaron formation and hopping are very different at a hematite surface compared to the bulk of the same material. Second, we showed that these dynamics can be controlled in a rational way by functionalizing the surface with tunable molecules. These findings provide a strategy for controlling electron dynamics at a catalyst surface in order to improve solar energy conversion efficiency by these earth-abundant materials.

The biggest challenge associated with this work was the development of XUV reflection–absorption spectroscopy that enables us to visualize electron motion with femtosecond time resolution and surface sensitivity. This is exciting because these findings are just one example of the discoveries that can be made by directly observing electron dynamics at surfaces.

You have also published recent work using in-situ, plasmon-enhanced vibrational sum frequency generation (SFG) spectroscopy to directly observe carbon dioxide electroreduction on gold surfaces (2). Infrared and near-infrared frequencies were used to observe the gold and electrolyte interface during active electrocatalysis, determining that the rate-determining step for this reaction is CO₂ adsorption. This type of spectroscopy provides the capability for direct observation of interfacial processes that govern the surface kinetics of electrocatalysis. Would you explain the significance and meaning of this work for the readers of Spectroscopy?

Electrocatalysis is an important process for turning electrical energy into useful chemicals. However, the ability to watch how a molecule reacts on an electrode surface is a real challenge. It requires the ability to measure molecular spectra with surface specificity and to do so with high sensitivity even under in situ reaction conditions. In this paper, we described a new iteration on a well-established technique known as sum frequency generation vibrational spectroscopy. We find that using the surface plasmon resonance of a gold electrode, we can couple light to an active electrode/electrolyte interface and measure vibrational spectra at the interface with detection limits less than 1% of a surface monolayer. This is possible even under conditions of active electrocatalysis. We hope that this method will provide a new window into the details of how reaction selectivity can be controlled in electrocatalytic systems.

In other work you have carried out fundamental studies in applying SFG vibrational spectroscopy to investigate strong metal–support interactions (SMSI) in Pt/TiO₂ catalysts (3). You and your colleagues were the first to report the use of SFG to study oxide–metal interfaces during catalytic reactions. The results of this study demonstrated that TiO₂ charge transfer from a Pt/TiO₂ catalyst occurs by an acid–base mechanism and controls the distribution of furfuraldehyde hydrogenation. How is this work important for laboratories studying catalysis? How do you envision SFG assisting a broader analytical community?

This result showed how acid-base chemistry on oxide surfaces represents the molecular basis for widely studied strong metal-support interactions in heterogenous catalysis. SFG was key to this discovery because it provided the ability to observe the reaction intermediates involved in this process. For this reason, SFG will continue to play an important role for communities that are interested in understanding molecular structure and chemical reactivity at interfaces.

What issues and problems would you define as previously ignored or neglected your field? What major developments do you see as important in new techniques and in energy conversion and catalysis? What is your vision for improvements that could be made in instrumentation for studying these phenomena and the materials involved?

The question of how electrons move at surfaces and control chemical reaction selectivity and energy conversion efficiency is a very important one. I don’t think this question has really been neglected; the challenge is just that it is still very difficult to make direct observation of these processes. This ability is needed to learn more about the material properties that control interfacial electron dynamics. Toward this goal, my research group at Ohio State has constructed an ultrafast XUV light source for the study of electron dynamics at photochemically relevant surfaces and interfaces. Using this instrument, we measure XUV spectra with femtosecond time resolution and surface sensitivity. Because XUV spectroscopy is element-specific, it is possible to track electron dynamics with site-selective resolution by measuring transient oxidation and spin state changes in real-time. We hope that this technique will contribute to a better understanding of electron dynamics so that eventually energy conversion at surfaces can be tuned with the same level of precision as natural photosynthetic systems.

What would you consider to be the most useful contributions of your work?

Although the scientific discoveries mentioned above are important, the most rewarding part of this work is the education and training of students who represent the next generation of the field. They will be faced with many challenges to solve and providing them with the training and experiences they will need to succeed is without question the most important and rewarding aspect of this work.

What are some key aspects that motivate you? Would you share some of your work and organizational habits that have helped you be productive and successful professionally?

It is always exciting to try to see something that has never been seen before. That is what we’re trying to accomplish when we build a new spectrometer or embark on a new experiment. It is the thrill of discovery and the opportunity to work with great students and colleagues that keeps me motivated. I have also benefited from the guidance of excellent mentors and the support of family. I also enjoy taking time off once in a while for a good run.

What can you share with our readers regarding your next area of interest for your research?

There are many exciting directions to go from here. One area we are enthusiastic about is utilizing XUV reflection–absorption spectroscopy to understand magnetization dynamics and spin transport at interfaces. This is a direction that builds naturally from studies of charge transport, but now the focus is placed on the spin rather than the charge of the electron. Developing SFG in the XUV spectral region is another exciting direction. This is a technique that has potential to reveal new details about dynamics at buried interfaces that are hard to probe by traditional surface spectroscopies.

References

(1) S. Bandaranayake, E. Hruska, S. Londo, S. Biswas, and L.R. Baker, Small Polarons and Surface Defects in Metal Oxide Photocatalysts Studied Using XUV Reflection–Absorption Spectroscopy. J. Phys. Chem. C 124(42), 22853–22870 (2020).

(2) S. Wallentine, S. Bandaranayake, S. Biswas, and L.R. Baker, Direct Observation of Carbon Dioxide Electroreduction on Gold: Site Blocking by the Stern Layer Controls CO₂ Adsorption Kinetics, J. Phys. Chem. Lett. 11(19), 8307–8313 (2020).

(3) (7) L.R. Baker, G. Kennedy, M.Van Spronsen, A. Hervier, X. Cai, S. Chen, L.W. Wang, and G.A. Somorjai, Furfuraldehyde hydrogenation on titanium oxide-supported platinum nanoparticles studied by sum frequency generation vibrational spectroscopy: acid–base catalysis explains the molecular origin of strong metal–support interactions, J. Am. Chem. Soc. 134(34), 14208–14216 (2012).

L. Robert Baker, the 2021 winner of the Emerging Leader in Atomic Spectroscopy Award, is an Associate Professor of chemistry at The Ohio State University where he leads research on X-ray spectroscopy, nonlinear and time-resolved spectroscopy, the chemistry of surfaces and interface science, and energy conversion and catalysis. Baker earned his BS degree in chemistry and his MS in physical chemistry from Brigham Young University and received his PhD in 2012 in physical chemistry from University of California, Berkeley under Professor Gabor A. Somorjai. Following postdoctoral research at University of California at Berkeley (mentored by Prof. Stephen R. Leone), he then became an assistant professor at The Ohio State University, Department of Chemistry and Biochemistry in 2014. In 2020, he was appointed an Associate Professor of Chemistry. He serves as principal investigator of the NSF National eXtreme Ultrafast Science (NeXUS) facility. His awards include the Camille Dreyfus Teacher-Scholar Award, Air Force Office of Scientific Research Young Investigator Award, Department of Energy Early Career Award, and the Young Innovator Award in NanoEnergy.