Oxygen Defects at Reducible Oxide Surfaces: The Example of Ceria and Vanadia

Abstract

Cerium and vanadium oxide-based systems play a major role in a variety of technological applications, with the reducibility of the systems being crucial to their functionality in the applications. The in-depth understanding and control of the type, density, and distribution of oxygen vacancies provide a means to influence the electronic structure and to tailor the systems' functionality. Hence, a great deal of experimental and theoretical work has been devoted to the study of partially reduced ceria and vanadia, both surfaces and bulk. Here, theoretical data for structural and electronic properties and energetic quantities related to the formation and interaction of neutral oxygen vacancies at the CeO 2 (111) and V 2 O 5 (001) surfaces are reviewed, discussed and compared. Experimental findings on oxygen defects in ceria and vanadia are briefly reported. Special attention is given to the fate of the electrons left in the system upon vacancy formation, the vacancy-induced lattice relaxation, whether vacancies agglomerate or repel each other, and the ability of state-of-the-art quantum-mechanical methods to provide an accurate decription of the geometric and electronic structures of the partially reduced oxide systems as well as reliable oxygen defect formation energies. IntroductionReducible metal oxides are extremely functional solid state compounds exhibiting a rich chemistry related to changes of the metal oxidation state [1]. As complex materials such as surfaces, supported particles and films, porous networks, they play a predominant role in applications in advanced technologies like catalysis, sensors, fuel cells, and microelectronics. Point defects in oxides such as vacancies and interstitials account for the transport properties of ionic solids. Defect sites on oxide supports of metal catalyst particles not only act as anchoring and nucleation centers

Facet-Dependent Stability of Near-Surface Oxygen Vacancies and Excess Charge Localization at CeO2 Surfaces

“…When a surface (subsurface) oxygen vacancy is created without allowing for lattice relaxations, the excess charge is then distributed among 2 (4), 3 (4) and 3 (4) adjacent Ce atoms in the (100), ( 110) and (111) surfaces, respectively (see Figure S2). Moreover, we recall that reduced Ce ions are larger than the Ce 4+ , e.g., the average bond length of the eightfold coordinated Ce 4+ ions in CeO2 is 2.37 Å, whereas that of the sevenfold coordinated Ce 3+ ions in Ce2O3 is 2.50 Å [43,50] This implies that in the unrelaxed structures, the larger Ce (4-)+ O bonds are compressed, which increases lattice strain, destabilizing the structures. We here find a correlation between the number of compressed Ce (4-)+ O bonds (NB) and the vacancy formation energy ( ), i.e., the larger NB is, the larger is .…”

Facet-Dependent Stability of Near-Surface Oxygen Vacancies and Excess Charge Localization at CeO2 Surfaces

“…When a surface (subsurface) oxygen vacancy is created without allowing for lattice relaxations, the excess charge is then distributed among 2 (4), 3 (4) and 3 (4) adjacent Ce atoms in the (100), ( 110) and (111) surfaces, respectively (see Figure S2). Moreover, we recall that reduced Ce ions are larger than the Ce 4+ , e.g., the average bond length of the eightfold coordinated Ce 4+ ions in CeO2 is 2.37 Å, whereas that of the sevenfold coordinated Ce 3+ ions in Ce2O3 is 2.50 Å [43,50] This implies that in the unrelaxed structures, the larger Ce (4-)+ O bonds are compressed, which increases lattice strain, destabilizing the structures. We here find a correlation between the number of compressed Ce (4-)+ O bonds (NB) and the vacancy formation energy (𝐸 ), i.e., the larger NB is, the larger is 𝐸 .…”

“…We here further inspect the unrelaxed structures and consider the distribution of the two excess electrons. As mentioned above, lattice relaxations are crucial to the formation of 2Ce 3+ , which leads to the structure stabilization [43,50]. However, in the unrelaxed structures, the excess charge is "delocalized'' over all nearest-neighbor cations to the vacancies, which are partially reduced, Ce (4-)+ .…”

“…As already mentioned, the reduced (111) facet has been the most studied, and the relative stability of near-surface oxygen vacancies and the excess charge localization have been addressed . Here, we briefly review the current understanding, as far as relevant for comparison with the reduced (110) and (100) surfaces, and point the reader to works that have compiled most results in the literature [48,50,55,56]. The most recent works in the literature (after 2009), independently of the specific implementation of periodic DFT calculations (different unit cell sizes, underlying exchange-correlation functional, U value in DFT+U, etc), generally agree that the subsurface vacancy is more stable than the superficial one and that the Ce 3+ ions prefer sites that are not adjacent to the vacancies, preferably in the outermost cationic layer.…”

“…We observed that a wide spread of values has been reported in the literature for the vacancies formation energies, indicating a strong dependence of the results on the details of the computations (cf. Table S2) [48,50,55,56,82].…”