  In this study, we investigate noble metal free photocatalytic water splitting
on natural anatase single crystal facets and on wafer slices of the [001] plane
before and after these surfaces have been modified by high pressure
hydrogenation (HPH) and hydrogen ion-implantation. We find that on the natural,
intact low index planes photocatalytic H$_2$ evolution (in absence of noble
metal co-catalyst) can only be achieved when the hydrogenation treatment is
accompanied by the introduction of crystal damage, such as simple scratching,
miscut in the wafer or by implantation damage. X-ray reflectivity (XRR), Raman,
and optical reflection measurements show that plain hydrogenation leads to a ~
1 nm thick black titania surface layer without activity, while a colorless,
density modified and ~ 7 nm thick layer with broken crystal symmetry is present
in the ion implanted surface. These results demonstrate that i) the H-treatment
of an intact anatase surface needs to be combined with defect formation for
catalytic activation, and ii) activation does not necessarily coincide with the
presence of black color.
