High-temperature hydrogenation of pure and silver-decorated titanate nanotubes to increase their solar absorbance for photocatalytic applications

Titanate nanotubes (TiNTs) and silver-decorated titanate nanotubes (TiNTs@Ag) were synthesized using the hydrothermal method. In the decorated nanotubes the silver particles were obtained by the photoreduction of AgNO3 under UV light. Pure and Ag-decorated nanotubes were high-temperature heat treated at 550 °C in a hydrogen atmosphere and the “core–shell”-structured TiO2 nanoparticles were formed. For the structural characterization of all the titanate nanostructures we used conventional and analytical transmission electron microscopy (TEM) techniques, X-ray diffraction (XRD) and Raman spectroscopy. The Ag-decorated titanate nanostructures were additionally studied by X-ray photo-electron spectroscopy (XPS), with the aim being to investigate the surface chemistry. Using UV–ViS-NIR spectroscopy we studied the photocatalytic activity on caffeine for the TiNT and TiNT@Ag samples that were high-temperature heat treated at 550 °C in a hydrogen atmosphere. We found that hydrogenation at high temperature of the TiNTs and TiNTs@Ag is essential to increase the absorption of light in the visible and near-IR regions. In the case of the hydrogenated TiNTs@Ag the absorption is even stronger in the visible region than in the UV, which was not observed in TiO2-based nanostructures up until now, to the best of our knowledge. The hydrogenated TiNTs@Ag sample had a two-times higher photodegradation impact on the caffeine than the hydrogenated TiNT samples, which is a consequence of the increased absorption of visible light and the synergetic effects between the silver and the TiO2 nanoparticles that increase the efficiency of the formation of electron–hole pairs and the charge transfer to the surface of the nanoparticles.