Research in the Baker group seeks to understand the critical role that surface electron dynamics and interfacial solvation have on the selectivity and efficiency of catalytic energy conversion processes.  Toward this goal our group has developed XUV reflection-absorption spectroscopy as a surface sensitive probe of ultrafast electron dynamics.  We also utilize sum frequency generation vibrational spectroscopy to understand how solvation structures and electric fields present inside the electrochemical double layer control the activity and selectivity of electrocatalytic reactions such as CO₂ reduction.  See below for descriptions of specific projects.

At the heart of many energy conversion technologies is the need to control charge and spin transport in systems that are far from equilibrium.  This can never be accomplished without the ability to probe these dynamics on the relevant scales of time and space.  Just like taking a photograph of a fast-moving object requires a short shutter time to produce a crisp image, resolving ultrafast electron and nuclear dynamics requires the use extremely short pulses of light to act as an ultrafast shutter for spectroscopic measurements.

High harmonic generation enables the production of extreme ultraviolet (XUV) light with femtosecond to attosecond pulse durations.  Like x-ray absorption, XUV spectroscopy is element specific, providing chemical details such as oxidation state, spin state, and coordination environment of individual elements in molecules and materials.  Consequently, using a tabletop XUV light source based on high harmonic generation, it is possible to observe charge and spin dynamics in chemical detail with ultrafast time resolution.

To extend this exciting tool to understand electron dynamics at catalytic interfaces, the Baker group has pioneered ultrafast XUV spectroscopy in a reflection-absorption geometry to serve as a surface sensitive analog of x-ray transient absorption.  This technique now enables direct observation of charge and spin transport at surfaces with nanometer surface sensitivity, femtosecond time resolution, and unprecedented chemical state specificity.  Using this method, our group studies electron dynamics at semiconductor surfaces with applications for photocatalysis, photovoltaics, and ultrafast information processing.

Examples of our work in this area include:

S. Bandaranayake, A. Patnaik, E. Hruska, Q. Zhu, S. Das, and L.R. Baker. "Effect of Surface Electron Trapping and Small Polaron Formation on the Photocatalytic Efficiency of Copper(I) and Copper(II) Oxides", ACS Applied Materials & Interfaces, 2024, 16, 31, 41616–41625.

E. Hruska, J. Husek, S. Bandaranayake, and L.R. Baker, "Visible Light Absorption and Hot Carrier Trapping in Anatase TiO₂: The Role of Surface Oxygen Vacancies" Journal of Physical Chemistry C, 2022, 126, 26, 10752–10761.

S. Biswas, and L. R. Baker, "Extreme Ultraviolet Reflection–Absorption Spectroscopy: Probing Dynamics at Surfaces from a Molecular Perspective," Accounts of Chemical Research, 2022, 55, 6, 893–903.

J. Husek, A. Cirri, S. Biswas, and L. R. Baker, “Surface Electron Dynamics in Hematite (a-Fe2O3): Correlation Between Ultrafast Surface Electron Trapping and Small Polaron Formation,” Chemical Science, 2017, 8, 8170-8178.

It is understood that photochemical complexes in nature carefully control both the charge and spin states of reaction intermediates involved during solar to chemical energy conversion.  Control of spin states is one reason why nature prefers to operate in chiral environments and continues to display catalytic selectivity far exceeding the best artificial systems.  Despite this understanding, controlling spin dynamics at photochemical interfaces has been largely neglected in the field of semiconductor photocatalysis.  This is largely due to the inability to measure spin dynamics at interfaces with the required time resolution and state sensitivity.

XUV spectroscopy with linearly polarized light is insensitive to the absolute orientation of electron spins, which are necessary to generate magnetization or to drive spin currents in materials.  In contrast, circularly polarized XUV light can resolve the spin orientation of electrons (i.e., m_j states).  Using femtosecond pulses circularly polarized XUV light, our group has developed the ability to directly observe spin polarized electron dynamics at surfaces.  This new ability will provide fundamental understanding of the ultrafast dynamics governing important processes such as magnetization switching for ultrafast information processing, spin selective catalysis, and chiral induced spin selectivity (CISS).

Here is an example some of our first work in this new area:

H. Gajapathy, S. Bandaranayake, E. Hruska, A. Vadakkayil, B. P. Bloom, S. Londo, J. McClellan, J. Guo, D. Russell, F. M. F. de Groot, F. Yang, D. H. Waldeck, M. Schultze, and L. R. Baker, "Spin Polarized Electron Dynamics Enhance Water Splitting Efficiency by Yttrium Iron Garnet Photoanodes: A New Platform for Spin Selective Photocatalysis," Chemical Science, 2024, 15, 3300-3310.

Electrochemical CO₂ reduction has potential to close the carbon cycle by utilizing CO₂ as a feedstock for renewable fuels and chemical synthesis.  However, achieving this goal requires a better understanding of the physical processes occurring inside the electrochemical double layer.  For example, it is known that specific ions strongly influence kinetics of CO₂ activation, although mechanistic understanding of specific ion effects remains limited.  This calls for a molecular-level understanding of ion pairing and interfacial solvation, which is necessary for optimizing efficiency and selectivity of many electrochemical processes.

Sum frequency generation (SFG) is a spectroscopic technique that provides a molecular view of buried solid-liquid interfaces.  The Baker group uses this tool to study surface reaction mechanisms, interfacial electric fields, and ion solvation during electrochemical energy conversion.  Electrochemical CO₂ reduction is a complex reaction that is particularly sensitive to local electric fields and interfacial solvation.  Using SFG, our group studies how electrolyte cations interact at active versus inactive sites on a catalyst surface, how cation solvation controls the interfacial electric field, and how these effects mediate the activity and selectivity of CO₂ reduction.

Examples of our work in this area include:

Q. Zhu, C.L. Rooney, H. Shema, C. Zeng, J.A. Panetier, E. Gross, H. Wang, and L.R. Baker, "The solvation environment of molecularly dispersed cobalt phthalocyanine determines methanol selectivity during electrocatalytic CO2 reduction", Nature Catalysis, 2024, 7, 987–999.

J. Rebstock, Q. Zhu, and L.R. Baker, "Comparing interfacial cation hydration at catalytic active sites and spectator sites on gold electrodes: understanding structure sensitive CO₂ reduction kinetics," Chemical Science, 2022, 13, 7634-7643.

Q. Zhu, S. Wallentine, G. Deng, J. Rebstock, and L. R. Baker, "The Solvation-Induced Onsager Reaction Field Rather than the Double-Layer Field Controls CO₂ Reduction on Gold," Journal of American Chemical Society Au, 2022, 2, 2, 472–482.

We design and evaluate catalysts for electrochemical energy conversion.  This includes studies of colloidally synthesized nanoparticles, where it is not only possible to tune the size, shape, and composition of the catalyst, but the surface ligand present on the colloidal particle functions as an important part of the catalyst system.  Traditionally, the surface ligand present during synthesis is removed prior to catalysis to increase the active surface area; however, we show that the ligand can actually enhance catalytic function, much like an enzyme by controlling the reaction environment around an active site.  For example, certain ligands significantly increase the rate of CO₂ reduction on gold by acting as a selectively permeable membrane, which allows efficient CO₂ transport while blocking transition metals impurities that otherwise poison catalytic active sites.  We also study catalysts for direct electrochemical conversion of CO₂ capture species, where integrating the steps of capture and conversion reduces the energy required for CO₂ release and sorbent recovery.  We recently demonstrated the ability to convert amine capture solutions directly to methane with high selectivity using a single atom nickel catalyst.  These efforts benefit from extensive collaborations with synthetic and theoretical chemists, who complement our expertise in spectroscopic characterization.

Examples of our recent work in this area include:

T. Neves-Garcia, M. Hasan, Q. Zhu, J. Li, Z. Jiang, Y. Liang, H. Wang, L.M. Rossi, R.E. Warburton, L.R. Baker. " Integrated Carbon Dioxide Capture by Amines and Conversion to Methane on Single-Atom Nickel Catalysts", Journal of the American Chemical Society, 2024, 146, 46, 31633–31646.

Q. Zhu, C. J. Murphy, and L. R. Baker, "Opportunities for Electrocatalytic CO₂ Reduction Enabled by Surface Ligands," Journal of American Chemical Society, 2022, 144, 7, 2829–2840.

H. Shang, D. Kim, S. Wallentine, M. Kim, D. Hofmann, R. Dasgupta, C. J. Murphy, A. Asthagiri, and L. R. Baker, "Ensemble effects in Cu/Au ultrasmall nanoparticles control the branching point for C1 selectivity during CO₂ electroreduction," Chemical Science, 2021, 12, 9146-9152.

H. Shang, S. Wallentine, D. M. Hofmann, Q. Zhu, C. J. Murphy, and L. R. Baker, "Effect of Surface Ligands on Gold Nanocatalysts for CO₂ Reduction," Chemical Science, 2020, 11, 12298-12306.

In addition to the research programs described above, Prof. Baker serves as Director of the NSF NeXUS Facility.  NeXUS is an open-access user facility designed to put state-of-the-art capabilities enabled by ultrafast x-rays into the hands of a diverse community of scientific users.  Located on Ohio State campus and supported by the National Science Foundation, NeXUS is accessible to researchers worldwide.

NeXUS provides the ability to study chemical dynamics in molecules and materials on the time scale of electron motion (attosecond) and the length scale of individual atoms (angstrom).  At the heart of NeXUS is a kilowatt-class ultrafast laser, making NeXUS the first facility to translate technology recently developed under the European Extreme Light Infrastructure project to the United States.  This laser is used to produce extreme ultraviolet (XUV) and soft x-ray pulses by high harmonic generation with 100 kHz to MHz repetition rates.  This light is then delivered into a variety of beamlines and experimental end stations for time-resolved x-ray absorption/reflection spectroscopy (XAS/XRS), angle-resolved photoelectron spectroscopy (ARPES), element-specific scanning tunneling microscopy (STM), and attosecond science (ATTO).

The combination of attosecond pulses, XUV and soft x-ray photon energies, high repetition rate, and suite of molecular and material characterization stations enables measurements at NeXUS that can only be performed at a handful of places worldwide.  Accordingly, NeXUS fills a strategic gap in the US research infrastructure and represents a focal point of interdisciplinary collaboration serving the entire scientific community.

To learn more about the NeXUS Facility, including how to submit a user proposal, visit the website:  https://nsf-nexus.osu.edu.