Band engineering is thoroughly employed nowadays targeting technologically
scalable photoanodes for solar water splitting applications. Most often complex
and costly recipes are necessary, for average performances. Here we report very
simple photoanode growth and thermal annealing, with effective band engineering
results. Strongly enhanced photocurrent, of more than 200 %, is measured for
Ti-doped hematite nanorods grown from aqueous solutions and annealed under
Nitrogen atmosphere, compared to air annealed ones. Oxidized surface states and
increased density of charge carriers are found responsible for the enhanced
photoelectrochemical activity, as shown by electrochemical impedance
spectroscopy and synchrotron X-rays spectromicroscopies. They are found related
to oxygen vacancies, acting as n-dopants, and the formation of pseudo- brookite
clusters by surface Ti segregation. Spectro-ptychography is used for the first
time at Ti L3 absorption edge to isolate Ti chemical coordination arising from
pseudo-brookite clusters contribution. Correlated with electron microscopy
investigation and Density Functional Theory (DFT) calculations, our data
unambiguously prove the origin of the enhanced photoelectrochemical activity of
N2-annealed Ti-doped hematite nanorods. Finally, we present here a handy and
cheap surface engineering method beyond the known oxygen vacancy doping,
allowing a net gain in the photoelectrochemical activity for the hematite-based
photoanodes.Comment: 2 parts: first main manuscript with 39 pages, second supplementary
informations with 14 page