  Here we investigate the band-level energetics of "black" hydrogenated titania
in different polymorphs using in-situ photoelectrochemical measurements and XPS
valence band measurements. We find that the conduction band of black rutile is
higher in energy than in black anatase by 0.4 eV. For photocatalytic hydrogen
generation, in a polymorph hetero-junction such as in black Degussa P25, thus
black rutile can act as a photosensitizer while black anatase provides
charge-mediation catalysis onto H2O to generate H2. By optimizing the thermal
reduction conditions of black anatase/rutile junctions the H2 production can be
significantly increased.
