PARADIM Highlight #69—External User Project (2023)
T.A. Pascal and D.P. Fenning (University of California—San Diego), D.G. Schlom (Cornell University), and V. Rose (Argonne National Laboratory)
Ferroelectric materials are widely utilized in nonvolatile memory, sensors, and actuators. But as a PARADIM user has recently demonstrated, the switchable structure at the surface of a ferroelectric can alter its electronic and interface properties— providing an excellent opportunity to modulate catalytic activity.
Figure 1: Journal covers highlighting the recent work by Pedram Abbasi, David P. Fenning, and collaborators on epitaxially thin films of barium titanate (BaTiO3)—a ferroelectric model compound—and the impact of polarization on the hydrogen evolution reaction [1, 2].
With the help of PARADIM, Fenning’s group at UCSD has explored the use of MBE-grown epitaxial thin films of BaTiO3—a ferroelectric model compound—to study the role of polarization on the hydrogen evolution reaction (HER) by surface spectroscopy and ab initio DFT+U calculations . The work indicates that an upward-polarized (001) surface reduces the work function relative to the downward polarization leading to a smaller HER barrier, in agreement with higher catalytic activity observed experimentally.
To further elucidate the effect of polarization switching on surface structure and chemistry the researchers teamed up with scientists from Argonne National Lab to study the BaTiO3 thin films by synchrotron X-ray scanning tunneling microscopy (SX-STM), a unique method that integrates nanoscale surface imaging and chemically sensitive spectroscopy .
In combination with ab initio calculations a stronger binding strength of a model reactant (here O2) to the upward-polarized surface is observed. The work advances the understanding of the surface chemistry and electronic structure of ferroelectrics.
Ferroelectric nanomaterials are of interest in catalysis, nonvolatile memory, and neuromorphic computing among other applications because of their switchable structure that can alter the electronic and interface properties of a single material. The investigation of the role of polarization on the surface structure and chemistry of ferroelectric nanomaterials is a longstanding challenge, as it ideally requires a combination of both nanoscale imaging and chemical spectroscopy. In this work, we study a model ferroelectric BaTiO3 thin film by synchrotron X-ray scanning tunneling microscopy (SX-STM), a unique method that integrates nanoscale surface imaging and chemically sensitive spectroscopy. We find that polarization switching from downward to upward in (001) single-crystalline BaTiO3 thin films increases the intensity of X-ray absorption across Ba M, Ti L, and O K edges. Chemical mapping of nanometer-sized domains further demonstrates the modulation of surface structures upon polarization switching, as well as confirming the trends observed in single-point experiments across the surface. We complement these measurements with ab initio computational absorption spectroscopy to elucidate the effect of polarization switching on the core–hole excitations using the Bethe–Salpeter equation approach. Our experimental and theoretical results thus confirm a stronger binding strength for the upward-polarized surface with molecular O2 as a model reactant, offering mechanistic evidence that supports previous reports. This work advances the understanding of the surface chemistry and electronic structure of ferroelectrics, which can ultimately aid strategies to design interfaces with tailored properties.
An emerging opportunity for ferroelectrics is to modulate catalytic activity. Such behavior has been reported in a prior PARADIM publication by this same user utilizing MBE-grown epitaxial BaTiO3 thin films with atomically sharp interfaces as model surfaces to demonstrate the effect of ferroelectric polarization on the electronic structure, intermediate binding energy, and electrochemical activity toward the hydrogen evolution reaction (HER). In the prior work [P. Abbasi, M.R. Barone, Ma. de la Paz Cruz-Jáuregui, D. Valdespino-Padilla, H. Paik, T. Kim, L. Kornblum, D.G. Schlom, T.A. Pascal, and D.P. Fenning, "Ferroelectric Modulation of Surface Electronic States in BaTiO3 for Enhanced Hydrogen Evolution Activity," Nano Lett. 22(2022) 4276-4284] surface spectroscopy and ab initio DFT+U calculations of the well-defined (001) surfaces indicate that an upward polarized surface reduces the work function relative to downward polarization and leads to a smaller HER barrier, in agreement with the higher activity observed experimentally. In this work synchrotron methods in combination with theory are used to elucidate the effect of polarization switching on the core–hole excitations and confirm a stronger binding strength for the upward-polarized surface with molecular O2 as a model reactant.
This achievement has both scientific and social components. The catalysis community typically studies materials in powder form because of their high surface area and practical nature. The epitaxial ferroelectric film community is well isolated from the catalytic community. The idea for this study (and the one before it) came from a graduate student working in the area of catalysis who attended the PARADIM summer school on MBE+ARPES and wondered about applying these techniques to his research. The result was the creation of a model system (an epitaxial BaTiO3 heterostructure grown in the PARADIM MBE) to answer important open questions in the catalysis community.
The work was initialized by researchers from the University of California San Diego and involved members of PARADIM’s in-house research team as well as members of the Synchrotron X-ray Scanning Probe Microscopy team at Argonne National Laboratory.
- P. Abbasi, M.R. Barone, Ma. de la Paz Cruz-Jáuregui, D. Valdespino-Padilla, H. Paik, T. Kim, L. Kornblum, D.G. Schlom, T.A. Pascal, and D.P. Fenning, "Ferroelectric Modulation of Surface Electronic States in BaTiO3 for Enhanced Hydrogen Evolution Activity," Nano Letters 22, 4276-4284 (2022).
- P. Abbasi, N. Shirato, R.E. Kumar, I.V. Albelo, M.R. Barone, D.N. Cakan, Ma. de la Paz Cruz-Jáuregui, S. Wieghold, D.G. Schlom, V. Rose, T.A. Pascal, and D.P. Fenning, "Nanoscale Surface Structure of Nanometer-Thick Ferroelectric BaTiO3 Films Revealed by Synchrotron X-ray Scanning Tunneling Microscopy: Implications for Catalytic Adsorption Reactions," ACS Applied Nano Materials 6, 2162-2170 (2023).
This research was primarily supported by the NSF through the UC San Diego Materials Research Science and Engineering Center (UCSD MRSEC), Grant No. DMR-2011924. T.A.P. and D.P.F. acknowledge start-up funding from the Jacob School of Engineering, UCSD. Materials synthesis was performed in a facility supported by the National Science Foundation [Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM)] under Cooperative Agreement No. DMR-2039380. M.R.B. and D.G.S. also acknowledge support from the NSF under Cooperative Agreement No. DMR-2039380. Substrate preparation was performed in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the NSF (Grant No. NNCI-2025233). PFM studies were conducted under Grant CONACYT A1-S-14758 and technical assistance from D. Valdespino and E. Murillo. This work used the Extreme Science and Engineering Discovery Environment (XSEDE) resources Expanse at the San Diego Super Computing Center (SDSC) through allocation CSD622 and DDP381. The use of the Advanced Photon Source, a U.S. Department of Energy Office of Science User Facility, was supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. The authors would also like to thank Dr. David Prendergast from Lawrence Berkley National Lab (LBNL) for helpful discussions on XAS calculations.
Access to data associated with BaTiO3 thin film growth is provided through the PARADIM Data Collective at DOI: 10.34863/80nw-gm95.