Evolution and atomistic structure of dislocations defects and clusters within CeO 2 supported on ZrO 2 S. Andrada Maicaneanu, a Dean C. Sayle* a and Graeme W. Watson b a Department of Environmental and Ordnance Systems, Cranfield University, Royal Military College of Science, Shrivenham, Swindon, UK SN6 8LA. E-mail: sayle@rmcs.cranfield.ac.uk b Department of Chemistry, Trinity College, Dublin 2, Ireland Received (in Cambridge, UK) 8th November 2000, Accepted 20th December 2000 First published as an Advance Article on the web 23rd January 2001 ‘Simulated amorphisation and recrystallisation’ was em- ployed to explore the structural features that evolve within ZrO 2 (111) supported CeO 2 , including epitaxial relation- ships, screw and screw-edge dislocations, vacancies and surface clusters. Ceria and ceria containing materials are used as catalysts and promoters in several heterogeneous catalytic reactions 1 and comprise a major component in three-way catalysts (TWC), which are used for the treatment of automobile exhaust gases. The oxygen storage capacity (OSC), due to the ability of cerium to shift between Ce 4+ and Ce 3+ , is one of the key properties of these materials. Accordingly, ceria based catalysts can work in both oxidizing and reducing conditions, converting carbon monoxide, nitrogen oxides and hydrocarbons to non-toxic products. It has been shown experimentally that ceria films, vapour deposited on zirconia and zirconia based substrates such as yttrium-stabilized zirconia (YSZ), are more easily reduced than films supported on a-Al 2 O 3 . 2 Here we employ a simulated amorphisation and recrystallisa- tion methodology 3,4 to explore the structural changes that evolve within ZrO 2 (111) supported CeO 2 . Since elucidation of the atomistic structure, particularly for ultra-thin supported materials is difficult or even intractable experimentally, the simulation provides an invaluable complement. Simulated amorphisation and recrystallisation 3,4 in this present study involves straining the CeO 2 thin film under considerable pressure and placing it on top of a ZrO 2 support. Dynamical simulation is then applied to the system at high temperature upon which the CeO 2 amorphises. Under pro- longed dynamical simulation, the CeO 2 recrystallises revealing a wealth of structural modifications that evolve as the system endeavours to accommodate the lattice misfit, whilst maximis- ing interfacial interactions. Crucially, by ensuring that the CeO 2 thin film recrystallises from an amorphous structure, no influence on the compromise between minimising the lattice misfit whilst maximising the interfacial interactions is in- troduced artificially into the simulation. Central to this methodology is that dynamical simulation, as applied to an amorphous structure, allows a more compre- hensive exploration of the configurational space due to the high energy amorphous starting point and the conformational freedom this gives rise to. In addition, a single mesoscale simulation has been performed in which a multitude of structural features are present within this simulation cell (in contrast to performing many smaller simulations comprising fewer structural features). Previous simulations on the SrO/ MgO(001) system 4 using different simulation cells revealed equivalent thin film energies, epitaxial relationships, disloca- tion densities and structural configurations suggesting that a single very large simulation cell is sufficiently representative for an initial investigation of the CeO 2 /ZrO 2 system. In addition, during experimental fabrication using for example vapour deposition 2 the thin film will endeavour to crystallise into as low an energy structure as possible. Our method is designed to generate low energy structures via recrystallisation from an amorphous material and will reflect therefore the structural characteristics present within the experimental system. The calculations presented in this study are based on the Born model for ionic solids, with potential parameters taken from Lewis and Catlow 5 and Dwivedi and Cormak. 6 These potentials have been employed to model lattice parameters, 7 thermal expansivities, 8 conductivity and diffusion properties 8 for CeO 2 and ZrO 2 solid solutions, in accord with experiment. In addition, a rigid ion model was used to reduce the computa- tional expense. The dynamical simulations, which employ three-dimensional periodicity, were performed using the DL_ POLY code, 9 and therefore a void normal to the surface is included to represent the free surface. The simulation cell contains ions distributed in two regions: region I comprises the CeO 2 thin film and the first six repeat units of the ZrO 2 (111) support, and ions within this region are allowed to move under the dynamical regime, while region II comprises a fixed region (four repeat ZrO 2 units) of the support and is included to ensure the correct crystalline environment. In this preliminary study we consider a model system, that of CeO 2 supported on cubic zirconia, as a first step in exploring CeO 2 supported on yttrium stabilised zirconia (YSZ), which will be considered in future studies; it has been shown experimentally that ceria grows epitaxially on YSZ. 10,11 To generate the CeO 2 /ZrO 2 (111) interface, two CeO 2 (111) repeat units (thick) were placed directly on top of ten repeat units of the ZrO 2 (111) support using a ‘cube-on-cube’ method- ology. 3 In particular, a 27 3 27 (which corresponds to 54 cerium atoms or 27 CeO 2 units for each side of the simulation cell) CeO 2 thin film was placed directly above a 20 3 20 ZrO 2 (111) support, giving an interfacial area of 10 305 Å 2 and 65 496 ions within the simulation cell. The lattice misfit associated with the system is +36%; the CeO 2 is therefore constrained initially under considerable pressure. Dynamical simulation was then applied to the system for 115 ps at 3400 K, 55 ps at 2500 K, 5 ps at 2000, 1500 and 1000 K, 40 ps at 500 K, 10 ps at 100 K and 20 ps at 0 K; the latter acts effectively as an energy minimisation. During the initial dynamical simulation step, the considerable strain within the CeO 2 results in its amorphisation, which, upon prolonged dynamical simulation, recrystallises. That the CeO 2 undergoes an amorphous transi- tion eliminates all ‘memory’ of the starting configuration enabling the CeO 2 to evolve structurally in response solely to the lattice misfit and underlying ZrO 2 . Inspection of the final structure for the CeO 2 /ZrO 2 (111) system [Fig. 1(a)] reveals that the CeO 2 thin film has recrystallized into the fluorite structure. The success of the simulated amorphisation and recrystallisation methodology in generating the CeO 2 structure from an amorphous solid suggests that the methodology is applicable to study supported fluorite-structured systems in addition to the supported rocksalt- structured systems considered previously. 3 The final CeO 2 thin film structure exposes the (111) plane at both the interface and surface and comprises ca. five CeO 2 repeat units with an incomplete (ca. 25% occupancy) surface layer (layer five), which comprises small clusters ranging from, This journal is © The Royal Society of Chemistry 2001 DOI: 10.1039/b008979f Chem. Commun., 2001, 289–290 289