Recent advances and perspectives of CeO2-based catalysts: Electronic properties and applications for energy storage and conversion

Theoretical investigations of structural, electronic, magnetic, and optical properties of group V (X = V, Nb, Ta) added CeO2-X materials for optoelectronic applications

CONTEXT

Depletion of natural resources, responsible for energy production, is a serious concern for researchers to develop alternate energy resources or materials. Scientists have proposed various energy materials which are based on semiconductors and their underlying physics. Cerium oxide (CeO2) is a versatile energy material which receives much attention owing to excellent photocatalytic, photonic, thermal stability, and optoelectronic applications. Even though CeO2 exhibited remarkable physical properties, but yet, they can be enhanced upon suitable doping. Focus on current research is to dope group V elements into CeO2 in order to enhance its electronic and optical response. The density of states (DOS) and band gaps of proposed materials are calculated, and significant improvement is noted after applying TB-mbj method. Optical absorption spectra of V/Nb/Ta-doped CeO2 show blueshift and decrease in reflectivity along with the presence of magnetism illustrate potential uses of these materials in future UV optoelectronics, spintronics, sensing, and energy harvesting devices.

METHODS

This research is based on computational work carried using Wien2k code where PBE-GGA approximation is used to approximate exchange and correlation potentials. Supercells of vanadium/niobium/tantalum-doped CeO2 are constructed, and spin-polarized density of states (DOS) along with optical constant are calculated. TB-mbj method is used to bring improvements in DOS and band gaps of proposed materials. Iterations are conducted using convergence criterion, and non-relativistic calculations are performed.

Modulating band structure through introducing Cu0/CuxO composites for the improved visible light driven ammonia synthesis

The photocatalytic performance of ceria-based materials can be tuned by adjusting the surface structures with decorating the transition-metal, which are considered as the important active sites. Herein, cuprous oxide-metallic copper composite-doped ceria nanorods were assembled through a simple hydrothermal reduction method. The photocatalytic ammonia synthesis rates exhibit an inverted "V-shaped" trend with increasing Cu0/CuxO mole ratio. The best ammonia production rate, approximately 900 or 521 µmol·gcal-1·h-1 under full-spectra or visible light, can be achieved when the Cu0/CuxO ratio is approximately 0.16, and this value is 8 times greater than that of the original sample. The absorption edge of the as-prepared samples shifted towards visible wavelengths, and they also had appropriate ammonia synthesis levels. This research provides a strategy for designing noble metal-free photocatalysts through introducing the metal/metallic oxide compositesto the catalysts.

Tailoring the Structural and Optical Properties of Cerium Oxide Nanoparticles Prepared by an Ecofriendly Green Route Using Plant Extracts

The present study explores an environmentally friendly green approach to obtain cerium oxide nanoparticles via a biomediated route using Mellisa officinalis and Hypericum perforatum plant extracts as reducing agents. The as-prepared nanoparticles were studied for their structural and morphological characteristics using XRD diffractometry, scanning electron microscopy, Raman, fluorescence and electronic absorption spectra, and X-ray photoelectron spectroscopy (XPS). The XRD pattern has shown the centered fluorite crystal structure of cerium oxide nanoparticles with average crystallite size below 10 nm. These observations were in agreement with the STEM data. The cubic fluorite structure of the cerium oxide nanoparticles was confirmed by the vibrational mode around 462 cm-1 due to the Ce-08 unit. The optical band gap was estimated from UV-Vis reflectance spectra, which was found to decrease from 3.24 eV to 2.98 eV. A higher specific area was determined for the sample using M. officinalis aqueous extract. The EDX data indicated that only cerium and oxygen are present in the green synthesized nanoparticles.

The Role of Cerium Valence in the Conversion Temperature of H2Ti3O7 Nanoribbons to TiO2-B and Anatase Nanoribbons, and Further to Rutile

CeO2-TiO2 is an important mixed oxide due to its catalytic properties, particularly in heterogeneous photocatalysis. This study presents a straightforward method to obtain 1D TiO2 nanostructures decorated with CeO2 nanoparticles at the surface. As the precursor, we used H2Ti3O7 nanoribbons prepared from sodium titanate nanoribbons by ion exchange. Two cerium sources with an oxidation state of +3 and +4 were used to obtain mixed oxides. HAADF-STEM mapping of the Ce4+-modified nanoribbons revealed a thin continuous layer at the surface of the H2Ti3O7 nanoribbons, while Ce3+ cerium ions intercalated partially between the titanate layers. The phase composition and morphology changes were monitored during calcination between 620 °C and 960 °C. Thermal treatment led to the formation of CeO2 nanoparticles on the surface of the TiO2 nanoribbons, whose size increased with the calcination temperature. The use of Ce4+ raised the temperature required for converting H2Ti3O7 to TiO2-B by approximately 200 °C, and the temperature for the formation of anatase. For the Ce3+ batch, the presence of cerium inhibited the conversion to rutile. Analysis of cerium oxidation states revealed the existence of both +4 and +3 in all calcined samples, regardless of the initial cerium oxidation state.