  A major goal of energy research is to use visible light to cleave water
directly, without an applied voltage, into hydrogen and oxygen. Since the
initial reports of the ultraviolet (UV) activity of TiO2 and SrTiO3 in the
1970s, researchers have pursued a fundamental understanding of the mechanistic
and molecular-level phenomena involved in photo-catalysis. Although it requires
UV light, after four decades SrTiO3 is still the gold standard for splitting
water. It is chemically stable and catalyzes both the hydrogen and the oxygen
reactions without applied bias. While ultrahigh vacuum (UHV) surface science
techniques have provided useful insights, we still know relatively little about
the structure of electrodes in contact with electrolytes under operating
conditions. Here, we report the surface structure evolution of a SrTiO3
electrode during water splitting, before and after training with a positive
bias. Operando high-energy X-ray reflectivity measurements demonstrate that
training the electrode irreversibly reorders the surface. Scanning
electrochemical microscopy (SECM) at open circuit correlates this training with
a tripling of the activity toward photo-induced water splitting. A novel
first-principles joint density-functional theory (JDFT) simulation constrained
to the X-ray data via a generalized penalty function identifies an anatase-like
structure for the more active, trained surface.
