Cranfield Defence and Security

Ceria nanoparticles are one of the key abrasive materials for chemical-mechanical planarization of advanced integrated circuits. However, ceria nanoparticles synthesized by existing techniques are irregularly faceted, and they scratch the silicon wafers and increase defect concentrations. We developed an approach for large-scale synthesis of single-crystal ceria nanospheres that can reduce the polishing defects by 80% and increase the silica removal rate by 50%, facilitating precise and reliable mass-manufacturing of chips for nanoelectronics. We doped the ceria system with titanium, using flame temperatures that facilitate crystallization of the ceria yet retain the titania in a molten state. In conjunction with molecular dynamics simulation, we show that under these conditions, the inner ceria core evolves in a single-crystal spherical shape without faceting, because throughout the crystallization it is completely encapsulated by a molten 1- to 2-nanometer shell of titania that, in liquid state, minimizes the surface energy. The principle demonstrated here could be applied to other oxide systems.

We present periodic density functional theory (DFT) calculations of bulk ceria and its low index surfaces (1 1 1), (1 1 0) and (1 0 0). We find that the surface energies increase in the order (1 1 1) > (1 1 0) > (1 0 0), while the magnitude of the surface relaxations follows the inverse order. The electronic properties of the bulk and surfaces are analysed by means of the electronic density of states and the electron density. We demonstrate that the bonding in pure ceria is partially covalent and analysis of the resulting electronic states confirms the presence of localised Ce 4f states above the Fermi level. The surface atoms show only a small change in the charge distribution in comparison to the bulk and from the DOS the main differences are due to the changes in the oxygen 2p and cerium 5 d states. Investigation of the atomic and electronic structure of an oxygen vacancy on the (1 0 0) surface shows the problems DFT can have with the description of strongly localised systems, wrongly predicting electron delocalisation over all of the cerium atoms in the simulation cell. We demonstrate an improvement in the description of the strongly correlated cerium 4f states in partially reduced ceria by applying the DFT+U methodology, which leads to the appearance of a new gap state between the valence band and the empty Ce 4f band. Analysis of the partial charge density shows that these states are localised on the CeIII ions neighbouring the oxygen vacancy. In terms of classical defect chemistry, the vacancy is bound by two neighbouring CeIII ions, which have been reduced from CeIV, i.e. VO··+2CeCe′. The remaining Ce ions are in the CeIV oxidation state. The localisation of Ce 4f electrons modifies the predicted structure of the defective surface.

Ceria is an important component in three-way catalysts for the treatment of automobile exhaust gases owing to its ability to store and release oxygen, a property known as the oxygen storage capacity. Much effort has been focused on increasing the OSC of ceria, and one avenue of exploration is the ability to fabricate CeO(2)-based catalysts, which expose reactive surfaces. Here we show how models for a polycrystalline CeO(2) thin film, which expose the (111), (110), and dipolar (100) surfaces, can be synthesized. This is achieved by supporting the CeO(2) thin film on an yttrium-stabilized zirconia substrate using a simulated amorphization and recrystallization strategy. In particular, the methodology generates models which reveal the atomistic structures present on the surface of the reactive faces and provides details of the grain-boundary structures, defects (vacancies, substitutionals, and clustering), and epitaxial relationships. Such models are an important first step in understanding the active sites at the surface of a catalytic material.

... metall., 30, 1453. BRANDON, D. G., RALPH, B., RANGANATHAN, S., and WALD, MS, 1964, Acta metall., 12, 813. ... crystallogr, 30, 197. HARDING, JH, and HARKER, AH, 1982, AERE R 10425, Theoretical Physics Division, AERE LEWIS, G. V., and CATLOW, CRA 1985, J. Phys. ...

The effect of titanium, vanadium and cerium oxides on the activity and selectivity of Fe2O3-based catalysts for ethylbenzene (EB) dehydrogenation reaction has been examined in an integral fixed bed reactor. For all single- or double-promoted catalysts, the selectivity to styrene was higher than that for the unpromoted catalyst. For the V2O5-promoted catalyst, an optimum content of 3 wt.% was found with respect to the selectivity to styrene. Moreover, by promoting the standard catalyst with a combination of TiO2/CeO2 up to 6 wt.%, the activation energy decreased by 10 kJ/mol. The physicochemical properties of the promoted Fe2O3-Cr2O3-K2CO3 catalysts were examined using N2 adsorption, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. All catalysts were macroporous, 65–75% of pores having a radius larger than 150 Å. No diffraction patterns specific to TiO2 or V2O5 were observed. A CeO2 crystalline phase was present in the single promoted catalyst and in the double promoted with TiO2 as well. The average size of crystals was 48.6 nm for single and 21.4 nm for the double one. The XPS results confirmed the better dispersion of CeO2 in the presence of TiO2. Based on experimental data several kinetic models were proposed and kinetic parameters were estimated using a non-linear least squares optimization procedure. The Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetic model with surface reaction rate-determining step better described the experimental data.

We examine the defect chemistry of rutile TiO2 using computer simulation techniques. The stable valencies of dopant transition metal ions in TiO2 are calculated to be Nb(V), V(III), Fe(III) and Cr(III). We find that Cr3+, Al3+, Ga3+, V3+ and Fe3+ dissolve preferentially at substitutional sites in TiO2 with charge compensation by oxygen vacancies rather than Ti4+ interstitials, although experimentally both are observed. Nb5+ is calculated to dissolve preferentially at substitutional sites with charge compensation being facilitated via Ti(III) ions or titanium vacancies. The solution enthalpy of Nb5+ is reduced by the introduction of Al3+ into the lattice. The solution enthalpy is further reduced when we consider bound clusters.

Atomistic simulation has been employed to generate models with ever increasing structural complexity: often approaching those observed in real systems. Consequently, constructing such models ‘by hand’ becomes increasingly difficult and perhaps intractable. Accordingly, simulation methodologies, which facilitate some kind of structural evolution, are becoming more widespread. Central to these evolutionary methods is that the structures derived (potentially) depend solely on the interatomic potentials, rather than the intuition of the simulator. Here, we review some of the approaches developed.

The formation of nanorods, driven by the physicochemical phenomena during the freezing and after the aging of frozen ceria nanoparticle suspensions, is reported. During freezing of a dilute aqueous solution of CeO2 nanocrystals, some nuclei remain in solution while others are trapped inside micro- and nanometer voids formed within the growing ice front. Over time (2-3 weeks) the particles trapped within the nanometer-wide voids in the ice combine by an oriented attachment process to form ceria nanorods. The experimental observations are consistent with molecular dynamics simulations of particle aggregation in constrained environments. These observations suggest a possible strategy for the templated formation of nanostructures through self-assembly by exploiting natural phenomena, such as voids formed during freezing of water. This research suggests a very simple, green chemical route to guide the formation of one- and three-dimensional self-assembled nanostructures.

Static atomistic simulation techniques have been employed to identify a model for the active site configuration and its location within the NO decomposition catalyst, Cu-ZSM-5. We propose that the active site comprises a copper pair, bridged by OH and forming a six membered ring, specifically, {-O-Cu(II)-OH-Cu(I)-O-Al-}, within the zeolite framework. The six-membered ring arises from the strong association of both of the copper species with a single aluminium in the zeolite framework and consequently the ring is strained, reflected in the low (3.1 Å) intercopper distance in the cluster. Indeed, this Cu-Cu distance compares well with the experimentally determined value of 3.0. In addition, it is expected that the strain in the cluster influences the activity of the cluster, which we suggest may be responsible for its unique activity for NO decomposition.