Prof Nora de Leeuw

Research Overview

Current research in the group is focused on the following themes:

Computer Modelling of Bio-Material Interfaces

One of our main areas of research is to employ computational techniques to study key aspects of the solid state chemistry of the major natural mammalian bone and teeth enamel constituent hydroxy-apatite, Ca10(PO4)6(OH)2 and its interactions with biological molecules and ceramic supports. As these minerals are promising candidates in the manufacture of artificial bones, we need to investigate the structure, properties and formation of the natural bone, which is grown on an organic matrix (bio-mineralisation). A similar application, which may be important for the acceptance by the body of surgical and dental implants, is to use ceramics and bio-glasses as a support for the crystallisation and layer growth of inorganic apatite, which can then integrate with the natural bone. Present research therefore includes the effect of fluoride on the dissolution of tooth enamel 1 and the investigation of the structures, stabilities and adhesive properties of the interfaces between apatite films and bio-glass surfaces, where we need to ensure that the simulated substrate surface is as realistic a model as possible. 2

Side view of a slab of hydroxy-apatite material in waterGeometry optimised structure of an apatite film grown on silica surface

(Left) Side view of a slab of hydroxy-apatite material in water with one surface OH - replaced by a F - ion, which remains in the structure while the surface OH - groups have dissolved into the solvent water (Ca = green, F = dark blue, O hydroxy = pale blue, O phosphate = red, O water = red, H = white, P = purple).

(Right) The geometry optimised structure of an apatite thin film grown onto a silica surface (Si = yellow, O quartz = dark blue, O apatite = red, F = pale blue, P = purple, Ca = green, PO 4 groups shown as tetrahedra)

Surface reactivity of complex oxides and mixed metal-oxide catalysts

Oxides are materials of major technological importance, used in sensors, superconductors, catalysts and as components in device fabrication. In these technologies, the nature of the surface is often crucial to the performance of the material. It is therefore essential to develop a detailed understanding of the elemental distribution, structure and electronic properties of the surface, which exerts a controlling influence in the case of sensors and catalysts. Mixed-metal oxides such as FeSbO4 are selective oxidation catalysts for a host of chemical feedstocks, including acrolein and acrylonitrile, where the mixed metal system provides optimal activity and selectivity. Areas of research include the distribution of the different cations in the bulk oxide structure, 3 the geometric and electronic structure of the surface and the effects of defects and doping on the chemical activity.

One way in which we probe the surface reactivity is by the adsorption of a water molecule and calculation of the adsorption energy, which gives a quantitative measure of the strength of adsorption and hence reactivity of particular surface sites, such as under-coordinated terrace, step and corner sites. In addition, reactivity of materials towards water is an interesting topic in itself, for example in the hydration of CaO, which is an integral component of cements. 4

Adsorption and dissociation of water at reactive low-coordinated surface sites of a CaO material

Adsorption and dissociation of water at reactive low-coordinated surface sites of the CaO material (Ca = green, O solid = red, O water = blue, H = white).

Formation of Ionic Thin Films at Metal Surfaces

In addition to metal oxide catalysts, metals such as silver and copper are also catalysts in their own right. Often, their reactivity is enhanced by the co-adsorption of other species, which are not part of the catalytic process themselves. For example, the co-adsorption of chlorine molecules at silver surfaces is found to significantly enhance the catalytic conversion of ethane to the epoxide, which is otherwise a slow process, where high dosages of chlorine are found to lead to the formation of a silver chloride layer. 5 In general, the interface between metals and ionic layers is of interest in a wide range of engineering applications, such as mechanical seals, metal/ceramic composite materials and corrosion control, where the formation of passivating ionic layers may protect the underlying metal from further oxidation. We employ both electronic structure calculations and larger-scale forcefield methods to investigate these metal/ionic interfaces

Crystal Nucleation, Growth and Dissolution

The field of dissolution, growth and inhibition of inorganic solids, often containing complex molecular ionic species such as carbonates, sulphates, phosphates or nitrates, is currently one of the major areas of interest in solid state research and we are interested in the growth and dissolution of a range of materials for different applications. Silicates are all pervasive in the geological environment as rock materials and as such weathering phenomena and the response to water are of interest in a number of fields, more recently also in nano-technology and engineering 6 .

Dissolution of a silica nano-tube

Dissolution of a silica nano-tube. The protons pointing towards the Si vacancy are indicated by arrows at the top of the figure (Si = orange, O = blue, H = white).

Carbonates are sedimentary minerals, which could be of use as sinks for environmental pollutants. However, they also form a major industrial problem due to their precipitation as scale in boilers and (oil) transportation pipes, hence any factors inhibiting their nucleation and growth are extensively investigated 7 . Finally, bio-degradable materials, which can be easily integrated with hard or soft tissue in the body play a major rôle in modern research into tissue engineering.

Experimental growth steps on the major calcite surface

Experimental growth steps on the major calcite surface (Ca = green, O carbonate = red, C = yellow, H = white, O water = blue).

Bar chart showing the growth inhibiting effect of foreign cations on the growing calcium carbonate lattice.

Bar chart showing the growth inhibiting effect of foreign cations on the growing calcium carbonate lattice.

The organic/inorganic interface

The interface between inorganic materials and organic molecules is important in many fields, from mineral separation techniques used for the concentration of mineral ores in the mining industry, treatment of (industrial) waste water and enrichment of pharmaceutical products, to biomimetics engineering and pollutant control. In order to understand the sorption of the organic material to the inorganic surface, we need to investigate the interactions between the two at the atomic level. We hence use computer simulation techniques to quantify molecular recognition of a range of organic functional groups 8 , also taking into account any competitive interactions with the surrounding solvent. 9,10

Co-adsorption of water and methanoic acid at a reactive step edge on the fluorite {111} surface

Co-adsorption of water and methanoic acid at a reactive step edge on the fluorite {111} surface (Ca = green, F = blue, O meth = purple, C = yellow, H = white, O water = red).

Selected Publications

  1. N.H. de Leeuw, Resisting the onset of hydroxy-apatite dissolution through the incorporation of fluoride, J. Phys. Chem. B 108 , 1809 (2004)
  2. N.H. de Leeuw and D. Mkhonto, Atomistic simulations of the effect of surface pre-relaxation on the adhesion of apatite thin films to the a -quartz (0001) surface, Chem. Mater 15 , 1567 (2003)
  3. R. Grau-Crespo, N.H. de Leeuw, C.R.A. Catlow, The distribution of cations in FeSbO 4 : A DFT study, Chem. Mater. 16 , 1954 (2004)
  4. N.H. de Leeuw and J.A. Purton, Density Functional Theory calculations of the interaction of protons and water at low-coordinated surface sites of calcium oxide, Phys. Rev. B 63 , 195417 (2001)
  5. N.H. de Leeuw, C.J. Nelson, C.R.A. Catlow, P. Sautet and W. Dong, Density Functional Theory calculations of the adsorption of chlorine at perfect and defective silver (111) surfaces, Phys. Rev. B 69 , 045419 (2004)
  6. Z. Du and N.H. de Leeuw, A combined density functional theory and interatomic potential-based simulation study of the hydration of nano-particulate silicate surfaces, Surf. Sci 554 , 193 (2004)
  7. N.H. de Leeuw, Molecular dynamics simulations of the growth inhibiting effect of Fe 2+ , Mg 2+ , Cd 2+ and Sr 2+ on calcite crystal growth, J. Phys. Chem. B 106 , 5241 (2002)
  8. N.H. de Leeuw and T.G. Cooper, A computer modeling study of the inhibiting effect of organic adsorbates on calcite crystal growth,Cryst. Growth & Design , 123 (2004)
  9. T.G. Cooper and N.H. de Leeuw, A computer modelling study of the competitive adsorption of water and organic surfactants at surfaces of the mineral scheelite, Langmuir 20 , 3984 (2004)
  10. T.G. Cooper and N.H. de Leeuw, Co-adsorption of surfactants and water at inorganic solid surfaces, Chem. Commun. 1502 (2002).