RESEARCH

Photo-driven heterogeneous catalysis (at the solid/liquid interface) spans many time scales from the picosecond to the millisecond—from the point of charge transfer at the solid/liquid interface to the final steady state evolution of the product. Experimental setups that follow the reaction from the initial transfer of charge at the interface can uniquely address how the disparate time scales are involved in yielding the overall, steady state rate of fuel production. We combine ultrafast transient spectroscopy with in-situ photo-electrochemistry in targeted sunlight to fuel systems that allow access to these initial photo-excited states of the reaction. In order to do this, the material, the integrated device that controls charge flow, and the ultrafast transient spectroscopy must be carefully chosen and combined. The current focus is on the multi-step water oxidation reaction, critical to any solar to fuel process and whose mechanism has been, from the steady state perspective, the focus of decades of research. Ultrafast optical, mid-to-far infrared, and x-ray spectroscopies are applied to address both sides of the solid/aqueous interface: the charge carrier in the solid and the surface-bound molecular intermediates of the cycle. By understanding catalysis dynamically, we are setting the stage for how to harness the fundamental principles behind catalysis in fuel based storage systems. The catalysts and experimental setups currently under investigation are described below.

Charge Transfer Reaction Kinetics at Semiconductor/Electrolyte Interfaces

An n-type-semiconductor/aqueous interface can efficiently drive the water oxidation reaction with high energy photons. With n-SrTiO3, the efficiency of photon-to-product conversion can be as high as 90%, even with an ultrafast laser pulse initiating the reaction. With this selectivity, surface sensitive ultrafast optical spectroscopy can probe how fast charges generated in the semiconductor transfer to water species at the surface. The work highlighted below mapped out the activation barrier of the earliest step of the water oxidation reaction directly with a time and spectrally resolved probe.

diagram

M.M. Waegele, X. Chen, D.M. Herlihy, and T. Cuk, "How Surface Potential Determines the Kinetics of the First Hole Transfer of Photo—catalytic Water Oxidation" J. Am. Chem. Soc. 2014, 136, 10632.

Transformation of Charge into Catalytic Intermediates

The same n-SrTiO3/aqueous interface is investigated for the surface-bound molecular intermediates of hole transfer with ultrafast mid-to-far infrared spectroscopy. Probing a single crystal surface by a polarized, evanescent wave in attenuated total reflection is found to be especially surface sensitive. Recently, the transformation of charge into an oxyl radical—thought to be the catalytic intermediate for water oxidation on titania surfaces—was molecularly detected by our experiments, aided by high-end first principles calculations.

diagram

D.M. Herlihy, M.M. Waegele, X. Chen, C. D. Pemmarju, D. Prendergast and T. Cuk, " Uncovering the Oxyl Radical of Photocatalytic Water Oxidation by its Sub-Surface Vibration" Nature Chemistry 2016, 8, 549. Featured in News and Views.

Interfacial Charge Carrier Mobility of Catalytic Surfaces

Charge transport to the surface and surface reactivity, while both important to catalysis, are usually thought of as independent phenomena. Yet, charge transport within the solid, especially in regards to the lateral mobility along the surface, could be highly dependent on reaction intermediates that hop along the interface. We have applied transient optical grating spectroscopy to measure this lateral mobility at undoped and n-GaN/aqueous interfaces; interestingly, the lateral hole mobility increases from air by a factor > 2 uniquely at the n-GaN/aqueous interface, where equilibration with water oxidation traps reaction intermediates at the interface.

diagram

H.Q. Doan, K.L. Pollock, T. Cuk, "Transient Optical Diffraction of GaN/Aqueous Interfaces: Interfacial Carrier Mobility Dependence on Surface Reactivity" Chem. Phys. Lett. (Frontiers) 2016, 649, 1 (Invited, Cover Article).

Charge Localization in Transition Metal Oxides

3d transition metal oxides are considered some of the best water oxidation catalysts and yet their excited states have proven the subject of much debate. While the localization of carriers in 3d electronic levels makes computation difficult, charge localization to well-defined d-orbitals can be a tool for spectroscopy. Transient optical spectroscopy of d-d excitations of these transition metal oxides, in both molecular and solid state form, has the potential to define molecular properties within delocalized charge transfer pathways. An example where on-site d-d excitations followed by transient optical spectroscopy and a successful mapping to theory was made is shown below.

diagram

S.N. Choing, A.J. Francis, G. Clendenning, M.S. Schuurman, R.D. Sommer, I. Tamblyn, W.W. Weare, and T. Cuk, "Long—Lived LMCT in a d0 Vanadium (V) Complex by Internal Conversion to a State of 3dxy Character" J. Phys. Chem. C 2015, 119, 17029 (Cover article).

Fast Interfacial Charge Transfer Processes for Battery Applications

Another area in the laboratory exploits fast interfacial charge transfer processes at transition metal oxide electrodes to create highly efficient and reversible metal deposition for battery applications; applications to flow cell batteries are especially relevant.

N. Tran, A. Singh and T. Cuk, "Highly Reversible Transition Metal Deposition and Oxidation on Symmetric RuO2 Electrodes for Battery Applications" J. of Electrochem. Soc. 2016, 163, A286.