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 [1]. 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. Ni_(0.9)Fe_(0.1)O_(x) 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” figure-of-merit as a function of thickness to inform photoelectrode design [2]. 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 [3]. We developed dual-electrode photoelectrochemical techniques to make direct electrical measurements on semiconductor-catalyst interfaces during operation and thus confirm the new theory [4]. (1) Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. Solution-cast metal oxide thin film electrocatalysts for oxygen evolution. J. Am. Chem. Soc. 2012, 134, 17253-17261. (2) Trotochaud, L.; Mills, T. J.; Boettcher, S. W. An optocatalytic model for semiconductor–catalyst water-splitting photoelectrodes based on in situ optical measurements on operational catalysts. J. Phys. Chem. Lett. 2013, 4, 931-935. (3) Mills, T. J.; Boettcher, S. W. Theory and simulations of electrocatalyst-coated semiconductor electrodes for solar water splitting. Submitted. 2013. (4) Lin, F.; Boettcher, S. W. Adaptive semiconductor-electrocatalyst junctions in water splitting photoanodes. Nat. Mater. 2013, Accepted.