Synthesis and properties of Co-doped titanate nanotubes and their optical sensitization with methylene blue

• New hydrothermal chemical route to synthesize cobalt-doped titanate nanotubes.
• The Co-doping stabilizes the TNTs morphology in a temperature range of 160–200 °C.
• Optical band gaps of CoTNTs are red shifted compared to the TNT samples.
• Methylene blue sensitized CoTNTs are highly stable under UV–vis irradiation.

Here we report on a novel chemical route to synthesize homogenous cobalt-doped titanate nanotubes (CoTNT), using an amorphous Co-doped precursor. The influence of the synthesis temperature, autoclave dwell time and metal doping on the structural and microstructural as well as on the optical properties of the synthesized titanate nanotubes is studied and discussed. The optical band gaps of the CoTNT samples are red shifted in comparison with the values determined for the undoped samples, such red shifts bringing the absorption edge of the CoTNT samples into the visible region. CoTNT materials also demonstrate particular high adsorption ability for methylene blue, the amount of the adsorbed dye being higher than the one predictable for a monolayer formation. This suggests the possibility of intercalation of the dye molecule between the TiO6 layers of the TNT structure. It is also shown that the methylene blue sensitized Co-doped nanostructures are highly stable under UV radiation and present a strong and broad absorption in the visible region.