Strontium-Modified TiO₂ Nanotubes for Efficient Photodegradation of Formic Acid

This study explores the enhancement of photocatalytic activity in one-dimensional TiO₂ nanotubes (TiNT) through strontium (Sr) doping, using the incipient impregnation method to introduce Sr concentrations ranging from 0.2 to 1.0 wt%. The photocatalytic performance of the resulting xSr-TiNT composites was evaluated by examining the degradation of formic acid under UV irradiation. The materials were characterized using N₂ physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis, Raman spectroscopy, photoluminescence (PL), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effective surface area was determined by XPS, revealing a significant increase in surface area with Sr loading, which correlates with enhanced photocatalytic performance. This enhancement in surface area is in agreement with the observed expansion of the unit cell volume in presence of Sr. Results show that a small amount of Sr (0.2 wt%) enhances the generation of oxygen vacancies and Ti³⁺ species, leading to increased photocatalytic activity. The degradation followed the Langmuir–Hinshelwood model, with the highest degradation rate observed at 0.8 wt% Sr, showing a 3.36-fold increase compared to pristine TiNT. At higher Sr concentrations, the proportion of oxygen vacancies and Ti³⁺ defects decreased due to the formation of Ti–O–Sr moieties. Additionally, the polarization effect induced by Sr doping improved charge separation, reducing electron–hole recombination and further enhancing photocatalytic efficiency. These findings emphasize the critical role of Sr concentration, effective surface area, and unit cell volume expansion in tuning the photocatalytic properties of TiO₂ nanotubes, with an optimum Sr loading of 0.8 wt% leading to improved photocatalytic activity.