Synthesis and properties of 1D Sm-doped CeO₂ composite nanofibers fabricated using a coupled electrospinning and sol-gel methodology

Ce_1-xSm_xO₂(x=0, 0.2, 0.5 and 0.8) nanofibers (NFs) were synthesized by coupling sol-gel with electrospinning and using poly-vinyl pyrrolidone (PVP) as the polymer medium, in an ethanol/water mixture. Control over the fabrication conditions was achieved through analysis of the most key synthetic factors, which include: (i) the applied field strength; (ii) the solution feed rate and (iii) the PVP content in the electrospinning solution. The optimum microstructural fiber morphology (high quality beeds-free fibers) was achieved using the following electrospinning parameters: an applied voltage of 18.5 kV, a 7 ml/h of solution feed rate and a 12% (w/w) of PVP composition. Morphological features of the resulting fibers were examined by scanning electron microscopy (SEM). The average fiber diameter was typically found to be in the range of 200-1100 nm and 50-300 nm, before and after calcination at 500 °C, respectively. X-ray diffraction (XRD) results showed that the fluorite cubic structure was preserved for the entire Ce_1-xSm_xO₂ compositional range studied, while elemental analysis using EELS and X-ray photoelectron spectroscopy (XPS) confirmed the purity of the bulk and surface composition of the fibers. Selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) proved that the NFs are highly crystalline. The thermal stability of the composite (polymer/inorganic nitrate salts) NFs was further investigated in an inert atmosphere (N₂) using thermogravimetric analysis (TGA), which allowed the transformation process of the NFs from composite to oxide to be monitored. The reducibility of the metal oxide NFs (mobility of oxygen species in the fluorite cubic lattice) as well as their thermal stability in successive oxidation-reduction cycles was evaluated using temperature-programmed reduction in a H₂ atmosphere (H₂-TPR). Acidic-basic features of the NFs and powder surfaces were studied through temperature programmed desorption (TPD) using NH₃ and CO₂ as probe molecules, where weak, medium and strong acid sites were successfully traced with profound differences depending on the morphology. The NFs' potential performance towards NH₃ oxidation was also evaluated. Two types of basic sites, hydroxyl groups and surface lattice oxygen are present on the NFs, as probed by CO₂ adsorption. Pyridine adsorption followed by infrared spectroscopy (Py-FT-IR) studies unveiled the more profound Lewis acid presence in Ce_0.5Sm_0.5O₂ NFs compared to bulk powder Ce_0.5Sm_0.5O₂.