Difference in performance and mechanism for methylene blue when TiO2 nanoparticles are converted to nanotubes

There are few reports on the diversification in adsorption and photo-catalysis caused by morphological changes. In this study, TiO2 nanotubes (TiO2 NTs) were synthesized by alkaline hydro-thermal method using commercial TiO2 nanoparticles (TiO2 NPs) as raw materials. When TiO2 nanoparticles are converted to nanotubes, the difference in adsorption and photocatalytic performance was analyzed using methylene blue (MB) as a model pollutant. The results showed that TiO2 NPs exhibited low adsorption capacity and greater photocatalytic activity for MB. In contrast, TiO2 NTs showed dual functions of adsorption and photo-catalysis. It exhibited better adsorption performance for MB and the maximum uptake was 102.41 mg/g. However, the photo-catalytic activity changed because the hydro-thermal method altered the crystal structure and increased the gap energy of TiO2 NTs, which led to the partial reduction of photo-catalytic activity compared to TiO2 NPs. The dominating free radicals were ·OH, followed by ·O2− in the process of MB photodegradation by TiO2 NPs while ·OH and ·O2− were main radicals in the process of MB photodegradation by TiO2 NTs. Moreover, 99% MB was degraded in 24 min via adsorption and photo-catalysis synergy under the following optimal conditions: TiO2 NTs dose = 6 mg, pH = 9, and light intensity = 19 A. Compared with other photo-catalysts reported, TiO2 NTs exhibited a superior performance through adsorption and photo-catalysis synergy with advantages of faster degradation, less catalyst dose and high efficiency. This work hopes to provide reference for the design and synthesis of materials with dual functions of adsorption and photo-catalysis.