Doped oxide surfaces: ceria
We have been studying the reactivity of cerium dioxide (CeO2) using DFT+U (density functional theory corrected for on-site Coulomb interactions). Using U = 5 eV, we showed that formation of oxygen vacancies leads to formation of localised Ce3+ ions. The formation of oxygen vacancies is key to applications of ceria. For example, in oxidation reactions, such as CO oxidation to CO2 oxygen from the lattice is used. Therefore, the ease with which oxygen can be removed is important.
More recently, we have applied hybrid DFT, in the shape of the screened exchange HSE06 functional to oxygen vacancies in the (110) and (100) surfaces of ceria.
Figure 1. (a): Oxygen vacancy in Ti-doped CeO2 (110) surface. (b): CO adsorption at Ti-doped CeO2.
With Ti and Zr as dopants, the energy cost to form an oxygen vacancy is substantially reduced and the surface is more reactive to CO adsorption, forming a mono-dentate adsorbate that easily breaks up to form CO2.
Figure 2: (a)Spin density of the O- state in La-CeO2 (110) surface.The green isosurfaces show the distribution of the hole, which is primarily localised on an oxygen neighbouring the dopant site. (b) the spin density for La-CeO2 when the oxygen vacancy is formed showing that a reduced Ce3+ ion and an oxygen hole are present.
La doping of the (111) and (110) surfaces of CeO2, we identified an unexpected defect. With La doping, a 3+ ion substitutes for the 4+ Ce ion, leaving behind an oxygen "hole", i.e. an O- ion. The O- ion is localised on an oxygen near the dopant, as in the image. In metal oxides with lower valent dopants, it is generally found that the stable defect is on in which two dopants are accompanied by an oxygen vacancy. The oxygen vacancy acts to compensate the two oxygen holes, by donating two electrons to the oxide. This is well known for the case of Li-doped MgO.
For La-CeO2, we investigated the anion vacancy compensation mechanism by doping with 2 La ions and removing an oxygen vacancy. We found (i) the oxygen vacancy has an energy cost of +0.64 eV, so that it will not necessarily form spontaneously and (ii) upon oxygen vacancy formation we form a Ce3+ ion and an oxygen hole. This is in contrast to the expected situation in which the two oxygen holes are compensated. Experimental efforts are underway to establish the presence of this defect centre.
For other 3+ dopants, namely, Al, Sc, In and Y, we have applied hybrid DFT. The polaron description of a single dopant is found with both DFT+U and hybrid DFT. For dopant compensation, we find that Al behaves classically, while the other dopants are similar to La. There is thus a dopant ionic radius effect in trivalent doping.