Synthesis and structural features of black TiO2 nanotubes after annealing in hydrogen

TiO2 nanotubes (TiO2 NTs) with a diameter of 60 nm, a wall thickness of 12 nm, and a length of 13 μm were synthesized by anodic oxidation of Ti plates in ethylene glycol with the addition of NH4F and H2O. The intense black color of TiO2 NTs is acquired after heating in hydrogen at 500 °C. The carbon content on the surface of the black TiO2 TNs is 20.1 ± 1 at.%, of which 13.8 at.% is graphite carbon. It was found that only 2.4 at.% of the titanium ions exist as Ti3+ ions. It has been concluded that carbon contributes to the black color of TiO2 NTs. The ordering in the arrangement of carbon networks of amorphous carbon in the black TiO2 NTs is revealed. There is an oriented arrangement of carbon rings along the walls of the black TiO2 NTs, which is similar to the formation of carbon nanotubes inside the pores of TiO2 NTs. The formation of graphitized carbon in a regular TiO2 nanoporous structure makes it possible to obtain intensely colored films.