  In this work, single crystalline $\alpha$-Fe$_2$O$_3$ nanoflakes (NFs) are
formed in a highly dense array by Au seeding of a Fe substrate by a thermal
oxidation technique. The NFs are conformally decorated with a thin FeOOH
cocatalyst layer. Photoelectrochemical (PEC) measurements show that this
photoanode with the $\alpha$-Fe$_2$O$_3$/FeOOH NFs rooted on the Au/Fe
structure exhibits a significantly enhanced PEC water oxidation performance
compared to the plain $\alpha$-Fe$_2$O$_3$ nanostructure on the Fe substrate.
The $\alpha$-Fe$_2$O$_3$/FeOOH NFs on Au/Fe photoanode yields a photocurrent
density of 3.1 mA cm-2 at 1.5 VRHE, and a remarkably low onset potential of
0.5-0.6 VRHE in 1 M KOH under AM 1.5G (100 mW cm-2) simulated sunlight
illumination. The enhancement in PEC performance can be attributed to a
synergistic effect of the FeOOH top decoration and Au under-layer. While FeOOH
facilitates hole transfer at the interface of electrode/electrolyte, the Au
layer provides a sink for the electron transport to the back contact: this
leads overall to a drastically improved charge-separation efficiency in the
single crystalline $\alpha$-Fe$_2$O$_3$ NF photoanode.
