Formation of CeO2 nanotubes through different conditions of hydrothermal synthesis

Abstract

The synthesis control in obtaining CeO2 nanostructures is of fundamental importance for obtaining materials with desired properties. However, progress in the synthesis and control of the properties of these unidirectional (1D) nanomaterials remains a major challenge. In this work, alkaline hydrothermal synthesis without the use of a template was performed, under different reaction conditions (NaOH concentration, temperature and time of synthesis), and the influence on the morphological, structural and optical properties of CeO2 nanotubes (CeNTs) formed was discussed. In some cases, hydrated CeNTs, organized in a cubic fluorite structure, were obtained, with varying sizes, and varying quantity of oxygen vacancies, according to the condition of syntheses used, thus generating materials with different properties. It was observed that the external diameter (De) of the obtained nanotubes and oxygen vacancies increased the higher the NaOH concentration, the temperature and the time used in the reaction. Only the synthesis conditions of 5 M NaOH and 150°C do not promote the morphology of interest, with the formation of prisms and a mixture of nanotubes/nanocubes. The best synthesis conditions were obtained when using an alkaline concentration of 10 mol.L−1 at 125 °C for a reaction time of 72 h, since, under these conditions, a higher yield of nanostructures was obtained over an interesting size range (CeNTs with internal and external diameters of 7 and 24,6 nm, respectively). The band gap value of 2.60 eV, which is possibly responsible for the red shifting of band gap, indicates that these materials are photosensitive in the visible region. The mechanism involves the process of cerium salt dissolution and recrystallization of CeO2 nanotubes. This work shows, for the first time, that control of the formation, size and different quantities of Ce4+/Ce3+ of the CeNTs that can be reached by adjusting the synthesis parameters. Control in obtaining these nanostructures, combined with a narrow gap energy, it may show a differential performance for applications in the photocatalysis, sensors, nanomedicine and advanced electronic devices areas.