Abstract
•Na retards the dehydration process during the transformation of titanate into TiO2.•Presence of Na after calcination strongly impacts negatively photocatalysis.•Calcined Na-free titanate leads to highly active TiO2 in formic acid degradation.•The enhancement of photoactivity results from improved textural properties.
The influence of the sodium content on thermal stability and photocatalytic activity of calcined titanate nanotubes (TNT) was herein evaluated by preparing different samples through hydrothermal treatment of TiO2 powder (P25) in a concentrated NaOH solution (11.25M) at 130°C during 20h followed by acid washing steps. Titanate nanotubes samples with sodium (Na-TNT) and sodium-free (H-TNT) were then obtained using different concentrations of HCl aqueous solutions namely 0.1M and 1M respectively. As synthesized nanomaterials with different Na percentages were then calcined at temperatures varying between 400°C and 700°C. Samples were characterized by means of nitrogen adsorption-desorption isotherms at 77K, thermal analysis (DTA, TGA), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and diffuse reflectance spectroscopy (DRS). The photocatalytic activities of Na-TNT and H-TNT derived nanomaterials were then evaluated through the photocatalytic degradation of formic acid (FA).
Results show that the presence of sodium retards the dehydration process during the transformation of titanate into TiO2 shifting the formation of anatase phase to higher temperatures. However, sodium is not necessary to preserve the nanotubular morphology. The presence of sodium after calcination strongly impacts negatively the photocatalytic properties. If sodium is completely removed from the initial titanate orthorhombic phase, calcination leads to TiO2 anatase materials with enhanced photocatalytic properties compared to P25 in the degradation of formic acid, particularly if nanotubular morphology is preserved. The highest activity was therefore achieved for the H-TNT sample calcined at 400°C. The photodegradation activity of formic acid depends on the specific surface areas and TiO2 crystallinity. In pure anatase nanoparticles the activity strongly decreases with coherent crystallographic domains ≥10nm.