Hydrothermal synthesis and mechanism of triangular prism-like monocrystalline CeO2 nanotubes via a facile template-free hydrothermal route

Monocrystalline CeO2 tablet-like nanostructures and triangular prism-like nanotubes were synthesized by thermal conversion of cerium carbonate hydroxide (CeOHCO3) precursors prepared by a simple template-free hydrothermal method using Ce(NO3)3·6H2O as cerium source, CO(NH2)2 as both precipitator and carbon source and polyvinylpyrrolidone (PVP) as surfactant. X-ray diffractometer (XRD) images inferred that the as-synthesized Ce(CO3)(OH) has a hexagonal structure, and the CeO2 obtained by calcining the Ce(CO3)(OH) at 500 °C for 5 h has a cubic fluorite structure. Scanning electron microscope (SEM) was employed to reveal the transformation from tablet-like to triangular prism-like structures, and then to triangular prism-like nanotubes with the increase of temperature from 120 up to 200 °C. Monocrystalline structure was revealed by high resolution transmission electron microscope (HRTEM) and select area electron diffraction (SAED) patterns. The thermal decomposition process of the as-synthesized Ce(CO3)(OH) was investigated by thermo-gravimetric differential thermal analysis (TG-DTA) apparatus, and the possible formation mechanism of CeO2 has been discussed. The spectral properties were characterized by Fourier transform infrared spectrum (FT-IR), Raman scattering, Photoluminescence (PL) spectra and UV-vis spectroscopy. There is a red-shifting in the band gap of the material compared to bulk one, which is mainly attributed to the influences of the Ce3+ ions, oxygen vacancies and the change of morphology.