**3.5 Hydrogen doping**

It has occasionally been noted that hydrogenated black titania NPs possess Ti-H groups. 106,117,134,137 Hydrogenated titania nanowire microspheres obtained *via* treatment at 500°C for four hours under a flow of H2 (5% in N2, 300 sccm) and hydrogenated titania NPs treated with hydrogen plasma at 500°C for 4–8 hours, a satellite peak at about 457.3 eV in the Ti 2p XPS spectrum was observed, which refers to the presence of surface Ti-H bonds [27]. The surface of hydrogenated titania nanosheets produced at 400°C for two hours under a 10-bar pure H2 atmosphere was hypothesized to be converted to Ti-H bonds [18]. The diffraction peak at about 59.281 in the X-ray diffraction (XRD) pattern of hydrogenated titania NTs obtained *via* treatment in ultrapure H2 atmosphere at 200–600°C for one hour also referred to the presence of the Ti-H bond [28].

## **3.6 Valence band edge modifications**

The black titania nanostructures sometime exhibit a shift in the valence band. For instance, when hydrogenated titania was obtained *via* treatment at 200°C for four days under 20 bar H2 [11] or heated at 500°C for one hour under a hydrogen atmosphere [29], a redshift of the valence band was observed as indicated from the valence band XPS spectrum. A similar shift was observed in the black brookite titania NPs, rutile titania NPs, and titania NTs prepared with the Al reduction method, in the black anatase titania NTs obtained through NaBH4 reduction, in the black titania NPs fabricated *via* electron beam treatment, in the hydrogenated titania NTs treated

at 450°C for an hour in a reducing 5% H2 and 95% Ar environment and in the electrochemically hydrogenated black titania NTs [19]. It was discovered that Ti3+ ions did not contribute to the extra bandgap states of the hydrogenated TiO2 nanocrystals because when Ti3+ appeared, these mid-gap states vanished [21]. On the other hand, pure and hydrogenated TiO2 NWs treated for three hours at 200–550°C in an ultrapure hydrogen atmosphere (ambient pressure) showed similar valence-band structures. Hydrogenated titania nanosheets exhibit similar valence band shifts.
