Joint Center For Artificial Photosynthesis Seminar

The conversion and storage of solar energy through water-splitting requires interfacing high-quality
semiconductors that absorb sunlight with efficient electrocatalysts that facilitate the multi-electron H2 and
O2 evolution reactions. I first present the solution synthesis, structural/compositional characterization, and
oxygen-evolution-reaction (OER) electrocatalytic properties of ~2-3 nm-thick films of a range of
transition metal oxides. The thin-film geometry enables the use of quartz-crystal microgravimetry,
voltammetry, and steady-state Tafel measurements to study the intrinsic activity and electrochemical
properties of the OER catalyst films. Ni0.9Fe0.1Ox was the most active catalyst, which is attributed to the
in-situ formation of layered Ni0.9Fe0.1OOH oxyhydroxide with nearly every Ni atom electrochemically
active.
In addition to OER activity, the optical properties of the catalyst and the electronic properties of the
catalyst-semiconductor interface govern the overall photoanode response. We use spectroelectrochemistry
to quantify the optical properties of catalysts in-situ. We present a simple Schottky-diode photoelectrode
model that accounts for parasitic optical absorption in the catalyst and calculate a "optocatalytic" figureof-
merit as a function of thickness to inform photoelectrode design. Real semiconductor-catalyst
interfaces, however, have electrical properties that are more complex than simple metal-semiconductor
Schottky diodes, because the catalyst charges and changes oxidation state (work function) during
operation. A new theory of adaptive junctions is proposed and applied via numerical simulation to
understand this behavior. We developed dual-electrode photoelectrochemical techniques to make direct
electrical measurements on semiconductor-catalyst interfaces during operation and thus confirm the new
theory.