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47213-G5
Active Nanostructures for Clean Hydrogen Production

Milen K. Kostov, Florida State University

Progress report (09/01/07-08/31/08)
Since we expect this project to be computationally very demanding we started this investigation with the case of 50:50 mixture of CeO₂-ZrO₂, for which we know that ordering of the metal sublattice occurs. This will allow us to use smaller supercells in our calculations. The conceptually simplest structure of this composition is the L1₀ ordered structure in which the metal atoms occupy alternating planes, which is the choice structure in this preliminary work.
Pure CeO₂ has the fluorite crystal structure, in which the metal atoms occupy the fcc sites and the O atoms occupy the tetrahedral sites. To obtain the L1₀ Ce_0.5Zr_0.5O₂, we replaced one of the planes of Ce atoms with Zr, as illustrated in Figure 2. We performed full relaxation of the crystal structure [[i]].  The equilibrium lattice constant is 5.296Å, which is between that of CeO₂ (5.41Å) and cubic stabilized ZrO₂ (5.27Å), with a slight tetragonal distortion. The O planes move approximately 0.07Å closer to the smaller Zr atoms.
Next, we cleaved the low index surfaces of L1₀ Ce_0.5Zr_0.5O₂. Ceria and zirconia have ionic bonds in which the metal ions donate all four valence electrons to the O-p shell i.e. Ce⁴⁺O₂^2- and Zr⁴⁺O₂^2-. Therefore, the (100) and (001) surfaces, which consists of alternating metal and oxygen planes, are polar and very expensive energetically. In addition, they terminate either in pure O or pure metal layer, which would reduce their usefulness to the water splitting problem. The (011)/(110) and (111) surfaces, shown in Fig. 3, are neutral. In the (011) orientation, we have stoichiometric Ce_0.5Zr_0.5O₂ layers. In the (110) orientation, we have alternating stoichiometric CeO₂/ZrO₂ layers. In this orientation, there are two distinct surface terminations possible. In the (111) orientation, we have sets of three planes – two O planes sandwiching one plane of metal atoms, which taken together are stoichiometric.

 We constructed surface supercells by taking 6 atomic layers in the case of (011)/(110) and 9 layers in the (111) orientation and added 12Å of vacuum in order to decouple the two surfaces (for details see Objective 1.2). Then, the supercells were completely relaxed. The chosen thicknesses are enough to ensure that the atoms in the middle are bulk-like (forces are zero). The obtained surface energies are reported in Tab. 1. The calculated values are generally lower than the values obtained for pure CeO₂. As expected, the (111) surface has the lowest surface energy.
The (110) surface, which has one CeO₂ and one ZrO₂ termination has the largest surface energy which suggests that the presence of Zr in the proximity of Ce helps to decrease it.

	(110)	(011)	(111)
(J/m2)	1.076	1.035	0.683
for CeO2 [36]	1.057	1.057	0.720

Table 1: Surface energies for the most stable Ce_0.5Zr_0.5O₂ surfaces.
Finally, one O atom was removed from the relaxed surfaces. We consider four cases: (011) surface, (110) CeO₂ and ZrO₂ terminated surface; and (111) surface. Although, the metal layer in the (111) orientation is slightly below the topmost O layer it could still be active in the water splitting reaction. After relaxation of the ions around the vacancy, the formation energy of the vacancy will be obtained.
 

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