The role of lanthanum in the enhancement of photocatalytic properties of TiO2 nanomaterials obtained by calcination of hydrogenotitanate nanotubes

Abstract

The influence of lanthanum on the thermal stability and photocatalytic activity of calcined hydrogenotitanate nanotubes (HNT) was herein evaluated. HNT samples were prepared through the hydrothermal treatment of TiO2 P25 in a concentrated NaOH solution (11.25M) at 130°C during 20h followed by acid washing. La-doped titanates nanotubes (La–HNT) photocatalysts with optimum 1wt% of La were then elaborated by incipient wetness impregnation method using lanthanum nitrate as precursor. La–HNT and HNT samples were then calcined at temperatures varying between 400°C and 700°C. Samples were characterized by means of nitrogen adsorption–desorption isotherms at 77K, X-ray diffraction (XRD), Raman spectroscopy, diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), transmission electron microscopy (TEM), ICP analysis and photocurrent experiments. The photocatalytic activities of La–HNT and HNT derived nanomaterials were then evaluated through the photocatalytic degradation of formic acid (FA).Main results reveal that lanthanum inhibits TiO2 crystallite growth and retards anatase transformation into the less active rutile phase. The addition of La ions to TiO2 results in a charge imbalance creating a high proportion of oxygen vacancies as evidenced by photocurrent, photoluminescence and Raman experiments.The photocatalytic experiments reveal that La-doped TiO2 helps to maintain a high photocatalytic activity level even after calcination at high temperatures contrary to La-free photocatalysts. The determination of kinetic parameters reveals that the maintaining of a high photocatalytic activity results from the synergetic effect between restriction of TiO2 crystallite size growth and formation of a high proportion of oxygen vacancies.