Theoretical study of environmental dependence of oxygen vacancy formation in CeO2

Abstract

Density functional methods were used to study the environmental dependence of O vacancy formation in CeO2. It was found that an O vacancy in the 2+ charged state has the lowest formation energy for a wide range of Fermi energies (EF) from 0 to ∼1.9eV, while a neutral vacancy becomes the most stable at higher EF values. The O vacancy formation energy can be strongly affected by temperature (T) and the partial pressure of oxygen (PO2). The effect of T and PO2 on the equilibrium compositions of reduced ceria (CeO2−x) was also calculated and showed qualitative agreement with gravimetric experiments.

Disentangling the role of small polarons and oxygen vacancies in CeO2

“…The results of DFT+U studies are largely dependent on the U value, bringing some uncertainty to the results and raising questions on what value of U should be used to describe the properties of perfect and reduced CeO 2 . Furthermore, the band gap and the oxygen-vacancy formation energy derived from DFT+U calculations have noticeable differences from experimental data [43,44], with the reported oxygen-vacancy formation energies varying over a wide range, from 2.3 to 4.7 eV [10,13,16,[29][30][31][32][33][34][35][36][37][38][39].…”

“…For the oxygen vacancy, the excess electrons are localized on individual Ce atoms, turning two Ce 4+ into two Ce 3+ species, which, by Coulomb attraction, sit near to the vacancy [29][30][31][32][33][34][35][36][37][38][39][40][41][42]. The results of DFT+U studies are largely dependent on the U value, bringing some uncertainty to the results and raising questions on what value of U should be used to describe the properties of perfect and reduced CeO 2 .…”

“…For the oxygen vacancy in DFT-LDA or GGA, the excess electrons wind up occupying the narrow 4f band, erroneously leading to delocalized electrons due to the well-known self-interaction error [31][32][33]. The DFT+U method has also been employed to study the electronic properties of CeO 2 and the impact of oxygen vacancies [29,30,[33][34][35][36][37][38][39][40][41][42].…”

“…There have appeared numerous studies of bulk CeO 2 based on DFT calculations, many focusing on the oxygen vacancy [10][11][12][13][14][15][16][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42]. DFT calculations using the local density approximation (LDA) or the generalized gradient approximation (GGA) for the exchange-correlation term underestimate the separation between the top of the valence band (O 2p band) and the empty Ce 4f bands in CeO 2 by ~35 % [16,[31][32][33].…”

“…DFT calculations using the local density approximation (LDA) or the generalized gradient approximation (GGA) for the exchange-correlation term underestimate the separation between the top of the valence band (O 2p band) and the empty Ce 4f bands in CeO 2 by ~35 % [16,[31][32][33].…”