  Photoelectrochemical (PEC) water splitting is a promising method for
conversing solar energy into chemical energy stored in the form of hydrogen.
Nanostructured hematite ($\alpha$-Fe$_2$O$_3$) is one of the most attractive
materials for highly efficient charge carrier generation and collection due to
its large specific surface area and shortening minority carrier diffusion
length required to reach the surface. In the present work, PEC water splitting
performance of $\alpha$-Fe$_2$O$_3$ prepared by anodization of thin iron layers
on an FTO glass and subsequent annealing in low O$_2$-Ar ambient with only
0.03% O$_2$ was investigated. The key finding is that annealing the anodic
nanostructures with low oxygen concentration provides a strongly enhanced PEC
performance compared with classic air annealing. The photocurrent of the former
at 1.5 V vs. RHE results in 1.1 mA/cm2, being 11 times higher than that of the
latter. The enhancement of the PEC performance for $\alpha$-Fe$_2$O$_3$
annealed in low oxygen atmosphere can be attributed to controlled morphology,
Sn doping, and introduction of oxygen vacancies, which contribute to the
enhancement of the hole flux from the photogenerated site to the reactive
surface and additionally lead to an enhanced hole transfer at the interface
between the $\alpha$-Fe$_2$O$_3$ and the electrolyte. From the obtained
results, it is evident that low oxygen annealing is a surprisingly effective
method of defect engineering and optimizing $\alpha$-Fe$_2$O$_3$ electrodes for
a maximized PEC water splitting performance.
