Titanate Nanotubes Coated with Ag Nanoparticles: Effects of Annealing Temperature on Crystalline Structure, Morphology, and Photocatalytic Activity

Abstract

Silver nanoparticles (AgNPs) were initially incorporated into titanate nanotubes (Ag-TNTs) synthesized through a hydrothermal process, employing a photoreduction technique. The Ag/TNT samples were annealed between 70 and 350 ºC to optimize the properties and functionalities of the composite material. The Ag/TNTs were characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Fourier-transform infrared spectroscopy (FT-IR), UV–Visible spectroscopy (UV–Vis), and UV–Visible diffuse reflectance spectroscopy (UV–Vis DRS). XRD analysis revealed the formation of silver titanate through the exchange of extra-framework Na⁺ ions in TNTs with Ag⁺ ions. New peaks at 28.3° and 48.2° appeared in the XRD pattern of Ag/TNTs-350, indicating a partial transformation of the nanotabular TNTs to anatase nanotubes due to the temperature change. The interaction observed between the Ag and TNT particles is believed to be linked to a distinct medium band at 1384 cm⁻¹. This band possibly indicates that silver ions may interact with oxygen or nitrogen-containing groups on the surface of the TNT particles. HRTEM analysis revealed that the inclusion of AgNPs did not significantly alter the original nanotube morphology of TNTs, except in the Ag/TNTs-350 sample, which exhibited structural flaws and uneven walls. The bandgap reduction from 2.7 eV in TNTs-100 to 2.2 eV in Ag/TNTs-350 underscores the impact of AgNPs and annealing on modulating electronic properties and improving photocatalytic performance. The photocatalytic activity of Ag/TNTs-70-350 nanostructures was evaluated based on the degradation of MB dye in an aqueous solution under natural sunlight. The interaction of Ag-TNTs with titanate/titania crucially influences visible light activity, with degradation rates consistently increasing as annealing temperatures rise. The computational analysis revealed that the HOMO–LUMO gap and singlet–singlet excited state energies of the amorphous clusters closely resembled those of crystalline Ag/TNTs, indicating their potential for efficient dye removal.