Research

The goal of this project is to explore the microscopic dynamics of hydrogen ions in proton-conducting oxide materials. Proton conduction is a fundamental process that has attracted considerable attention based on important developments and applications in hydrogen energy research; therefore experimental characterization of transition rates and migration mechanisms is highly valuable. We employ an optical picosecond pump-probe technique that directly measures the vibrational lifetimes of hydrogen-related defects. These measurements reveal that proton migration is highly coupled to infrared photon absorption and suggest a means to enhance proton conduction in important device applications. 

Important Results:

1. Proton Tunneling in KTaO3 

We have found that the vibrational lifetimes in the perovskite oxide KTaO3 are extremely long-lived and, unlike the semiconductors, the stretch mode decays by tunneling to the next near neighbor oxygen ion. The tunneling process is found to be phonon assisted-the vibration of the surrounding lattice alters the barrier shown in the figure to promote a higher tunnel probability. 

The excited state proton-tunneling rate (kpt) has been extracted from the vibrational lifetime and shows a rate constant of about 1010 s-1 at room temperature. This excited-state tunneling rate is found to be 7 orders of magnitude large than the ground state in a similar proton conduction oxide [Phys. Rev. B 60, R3713 (1999)]

References:

1. E. J. Spahr, L. Wen, M. Stavola, L. A. Boatner, L. C. Feldman, N. H. Tolk, and G. Lüpke, Proton Tunneling: A Decay Channel of the O-H Stretch Mode in KTaO3 , Phys. Rev. Lett. 102, 075506 (2009)

Funding: NSF, DoE, Jeffress Foundation

In this study the vibrational lifetimes of the O-H and O-D stretch mode in rutile TiO2 show that the proton hops over the potential barrier between oxygen ions when a resonant IR photon is absorbed. The rutile sublattice (a) and potential energy surface (b) are shown in the above figure. Arrows indicate the migration pathway of the proton 

The vibrational lifetimes, shown in the figure are found to be only a few picoseconds indicating that the hopping transport process is very efficient. The photon-stimulated hopping rate is found to be approximately 1 THz at room temperature. In contrast, the thermal diffusion rate at room temperature is only 103 s-1, indicating a giant enhancement of 9 orders of magnitude! This photo-enhanced hydrogen transport effect is significant for potential. 

References:

1. E. J. Spahr, L. Wen, M. Stavola, L. A. Boatner, L. C. Feldman, N. H. Tolk, and G. Lüpke, Giant Enhancement of Hydrogen Transport in Rutile TiO2  at Low Temperatures, Phys. Rev. Lett. 104, 205901 (2010).

Funding: NSF, DoE, Jeffress Foundation