Photocatalytic hydrogen generation from water–methanol mixtures using “black” anatase obtained by annealing of titanate nanotubes

• Anatase is converted into brown titanate nanotubes by digestion with NaOH.
• Titanate nanotubes annealed under hydrogen are converted into “black” titanate.
• Black titania exhibits visible light photocatalytic activity for hydrogen generation.

Digestion for an extended period of a commercial nanoparticulated TiO2 sample under strong basic conditions (10 M NaOH) leads to the formation of titanate nanotubes with long aspect ratio. The most remarkable observation was the appearance of light-brown colour in the titanate sample that is reflected in the presence for this titanate nanotube sample of a continuous absorption in the visible region decreasing in intensity towards the red. Titanate nanotubes were annealed under hydrogen atmosphere at 350 and 500 °C. The last temperature leads to the reconstitution of anatase phase and an increase in the intensity of the black colour. The photocatalytic activity of the set of titanium samples was tested for hydrogen generation from water–methanol mixtures under monochromatic visible light (532 nm) or UV light (355 nm) or under simulated sunlight. It was determined that the most active sample in all conditions was the one that after NaOH digestion was submitted to hydrogen annealing at 500 °C. This sample exhibits even higher activity than that observed for “black” titania prepared as previously reported by direct treatment of nanoparticulated TiO2 with H2 without NaOH digestion.

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