Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

Kinetics of charge transfer in a heterogeneous catalyst‐reactant system: the
interplay of solid state and molecular properties
Tanja Cuk
Chemical Sciences Division, Lawrence Berkeley National Laboratory
D46 Hildebrand, Department of Chemistry, University of California Berkeley, Berkeley, CA
94720
tanjacuk@berkeley.edu
Program Scope: One of the greatest challenges in the design of efficient and selective catalysis
technologies is the limited fundamental understanding of how interfacial properties at solid
state/reactant interfaces guide catalytic reactions. The central research question is to determine how
molecular properties of reactants influence charge dynamics in solid state catalysts, and conversely,
how these charge dynamics induce molecular changes; likely, it is this synergy that results in catalytic
efficiency. There are two perspectives on the solid state catalyst/reactant interface. From the solid
state perspective, interfacial properties are described by band alignment and Fermi levels; from the
molecular perspective, interfacial properties are described by oxidation states and reactant adsorption.
The characterization of catalytic materials has either focused on molecular changes such as adsorption,
dissociation, and new bond formations occurring on the reactant side of the interface or the changes in
interfacial band alignment and passivation of surface states on the solid state side. While these
properties are useful for understanding catalytic performance, a causal link between the changes
observed on each side of the interface is lacking. What is needed is to directly follow the trajectory of
how charge carriers in solid state catalysts initiate interfacial charge capture by reactant molecules—
from the creation of the charge carrier in the bulk, to its accumulation at the interface, and to the
moment at which a reactant molecule transforms and, in the process, consumes the charge carrier.
Recent Progress: Recent progress has been made in two areas related to the study of water oxidation
on Co3O4. This catalyst has proven efficient for O2 evolution from H2O when activated by visible light
absorbing Ru(bpy)3. Co3O4 has two highly absorbing band gaps in the visible range, though it alone has
not been shown to initiate a reaction. We have characterized the transient optical spectroscopy of
these two band gaps for the first time. The transient spectrum provides insight into the dispersion of
the valence and conduction bands that initiate reactions at the interface. Further, the transient kinetics
provide the time scales of thermalization and electron hole recombination that compete with charge
capture by reactant molecules. Secondly, we have attempted several p‐n junctions with Co3O4 in order
to separate the photogenerated electron hole pairs and initiate molecular reactions at the interface.
The successful formation of such a junction will allow for ultrafast, laser initiated charge injection into
the catalyst, important for the larger goal of this program. The transient optical and infrared
spectroscopy of already well‐separated charges will describe the detailed balance between the flow of
charge carriers to catalyst‐reactant interfaces and the dynamics of interfacial charge capture by reactant
molecules.
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