Thermal tuning of the morphology of hydrothermally synthesized CeO2 nanotubes for photocatalytic applications

CeO2 nanotubes (NT) were synthesized via a hydrothermal method, employing two different alkaline bases, NaOH, and KOH, to obtain NTNaOH-CeO2 and NTKOH-CeO2. The as-produced NTNaOH-CeO2 and NTKOH-CeO2 were heated in air to study their morphological evolution and characterize the influence of morphology on their photocatalytic activities. Two morphological changes were observed, from NTNaOH-CeO2 and NTKOH-CeO2 nanotubes at room temperature to nanobars at temperatures starting from 375 °C, followed by a recrystallization of these at 550 °C and above. These transformations correspond to irreversible structural modifications that changed the morphological and optical properties as characterized by a high-resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), IR spectra, temperature-dependent Raman spectroscopy, thermo-gravimetric analysis (TGA), and photocatalytic activity. Higher temperatures produced significantly lower wavenumber shifts in the Raman band spectra of NTNaOH-CeO2 and NTKOH-CeO2. The dominant F2g phonon exhibited monotonic lower wavenumber shifts with increasing temperatures. A discontinuity on the phonon broadening for NTKOH-CeO2 was observed around 345 °C. The post-temperature HR-TEM and Raman analysis confirmed the changes in the morphology of both NTNaOH-CeO2 and NTKOH-CeO2, collapsing from nanotubes to nanobars (NB). The phonon softening's linewidths and lifetimes were analyzed using a model that considered four anharmonic processes. The study of the photocatalytic response of NTNaOH-CeO2 and NTKOH-CeO2 calcined at 850 °C (with the morphology transformation from NT to NB), which resulted in the prepared material presenting superior catalytic activity when compared with commercial non-stoichiometric CeO2, having activities up to two orders of magnitude faster than similar compounds reported in the literature. The so-prepared and calcined NTMOH-CeO2 (M = K or Na) NB has potential applications in water remediation to degrade textile dyes, which are found as contaminants in water wastage in this industry.