My current research interests are all concerned with the computer simulation of materials. In many cases it is at the surface of minerals that the interesting Chemistry and Physics occurs consequently the majority of my research is concerned with simulating the surfaces of materials. I am a contributor to the METADISE code which has been developed in Bath since the early 1990.s and is constantly being reviewed and updated to enable a wider range of systems be considered. Most recently functionality has been added to the code to allow the simulation of nano-particles and nano-tubes. Brief details of the projects I am currently working on are now detailed:
Atomic Structure and Self-Assembly of Nano particulates
Nanoparticles are becoming very attractive because of their size and shape-dependent properties. Consequently nanostructured materials have potential applications in many areas of Science. Using a method based on the well-known Wulff construction we are able to simulate nano-particles of minerals of many different symmetries and sizes, from simple rock-salt structures like MgO and to more complex minerals like calcite and apatite. It is in understanding how the factors that affect the stability of these particles and the mechanism by which they grow that forms by far the majority of my current research. For example, initial molecular dynamics simulations of small (18CaCO3) nano-particles showed that the particles became amorphous when run in a vacuum, whereas immersing the particle in water appears to slow this process, stabilizing the nano-particle and similar affects are seen when alternative solvents are considered. We are therefore currently investigating at what size these nano-particles become stable in vacuum, to what extent they are stabilized by various solvents and the effect of surface defects on the stability of the particles. We are also developing a reverse mapping technique, which will enable the results of modelling particles interacting with polymers on a meso-scale, using a lattice model, to be used as a starting point for atomistic, molecular dynamics calculations of nano-particles interacting with polymers.
Atomistic simulation of charged surfaces in contact with salt-water/solution
We are using Molecular Dynamics to study the effect of the surface charge on first, the interaction of the solution with a mineral surface and secondly, on the distribution of ions in solution. Initially we used the (100) surface of goethite, (.-FeOOH) as our model surface, in contact with a 0.9M NaCl solution. First the stable, stoichiometric, charge neutral, surface, terminated by hydroxyl groups was considered. After which charged surfaces were generated by either removing surface protons or hydroxyl groups. The results of the simulations showed clearly a layering of water at the mineral surface, in accord with the recent work from both experiment and simulation albeit on different minerals. We also found that unlike classical double layer models, the ion distribution also oscillated away from the surface. However, in keeping with classical double layer models, the ion distribution is controlled by the electrostatic distribution, perhaps suggesting that their failure at high ionic strengths is due to the neglect of electrostatic interactions of the solvent.
Currently we are considering the effect of salt solutions over a far greater range of concentrations and preliminary results suggest that the trends seen in our earlier work holds. Work is also ongoing in using the same methodology to consider other minerals and the effect the salt solution has on mineral growth and dissolution.
Ab initio simulation of Non-stoichiometric mineral surfaces
Traditional theoretical techniques for surface energy calculations usually are limited to considering stoichiometric surfaces. But there is no requirement that stoichiometric terminations are inherently more stable than their non-stoichiometric counterparts, especially if in contact with a realistic environment. We use an ab initio methodology to model nonstoichiometric surfaces of oxide minerals in equilibrium with a mixed vapour of oxygen, hydrogen or water. The stability of surfaces of different stoichiometry are compared in terms of general surface phase diagrams, function of the oxygen and hydrogen chemical potentials, which are dependent on temperature and partial pressures of oxygen and hydrogen. Thus, the stable structures and compositions can be evaluated at specific experimental conditions.
Using this approach, in collaboration with Dr A Marmier I have considered the non-polar surfaces of ZnO where it was it calculated that a surface covered in a mono-layer of water dominates the surface however the mode by which the water adsorbs differs between the (10.0) and (11.0) surface. The close proximity of water molecules on the (10.0) surface means hydrogen bonding can occur between adjacent chemi-adsorbed water molecules meaning there is little difference between the hydration and hydroxylation energies and indeed the most stable configuration is calculated as been a partially dissociated surface. In the case of the (11.0) surface it is only when dissociation has occurred that hydrogen bonds can form. Whilst the computational expense of DFT means only large defect concentrations can be considered, at low O partial pressure we calculated that a Zinc rich surface became stable, and that whilst seen at far lower defect concentrations, perhaps suggests that the colour change known to take place in ZnO, caused by the presence of Oxygen vacancies, is surface initiated.
Currently I am using the same methodology to model the surfaces of monoclinic ZrO2. This work was prompted by experimental worked performed by a colleague in the group, Dr D Eder, on the stoichiometry of hydrogen reduced zirconia and its influence on catalytic activity (PCCP 4(5):795-801 2002). A pleasant spin off to this work is also that we have developed a set of potential parameters that predict structures that are in excellent agreement with experiment and DFT for all three polymorphs of ZrO2.
Freeman C.L., Harding J.H., Cooke D.J., Elliott, J.A., Lardge, J.S. and Duffy D.M.
New Forcefields for Modeling Biomineralization Processes
published online DOI:10.1021/jp071887p 2007
Spagnoli D., Cooke D.J., Kerisit S. and Parker S.C.
Molecular dynamics simulations of the interaction between the surfaces of polar solids and aqueous solutions
J. Mater. Chem., 16, 1997 - 2006 (2006)
Marmier A, Spohr H, Cooke DJ, Kerisit S, Brodholt J, Wilson PB and Parker SC
Self Diffusion of Argon in Flexible, Single Wall, Carbon Nanotubes
MOL SIMULAT 31 (5): 385-389 2005
Kerisit S, Cooke DJ, Marmier, A and Parker, SC
Atomistic simulation of charged iron oxyhydroxide surfaces in contact with aqueous solution
CHEM COMM (24): 3027-3029 (2005)
Kerisit S, Cooke DJ, Spagnoli, D and Parker, SC
Molecular dynamics simulations of the interactions between water and inorganic solids
J MAT CHEM 15 (14): 1454-1462 2005
Parker SC, Cooke DJ, Kerisit S., Marmier A, Spagnoli D and Sayle DC
Atomistic simulation of the structure and stability of mineral interfaces
in Cecam Workshop on First-Principles Simulations: Perspectives and Challenges in Mineral Sciences, ed. M. Warren, A. Oganov, B. Winkler, Berichte aus Arbeitskreisen der Deutsche Gesellschaft für Kristallographie, 14 (2004)
Cooke DJ, Redfern SE, Parker SC
Atomistic simulation of the structure and segregation to the (00.1) and (01.2) surfaces of Hematite (alpha-Fe2O3)
PHYS CHEM MINERAL 31(8):507 - 517
Parker SC, Cooke DJ, Kerisit S, Marmier A, Taylor SL, Taylot SN
From HADES to PARADISE . Atomistic simulation of Defects in Minerals
J PHYSICS COND MATT 16:S2735-S2748 2004
Cooke DJ, Kerisit S, Parker SC
Computer modelling of the free energy of adsorption at the mineral aqueous interface
GEOCHIM COSMOCHIM AC 68(11S) A127
Parker SC, Cooke DC, Kerisit S, Marmier A
Computer modelling of cation segregation to oxide and mineral surfaces in the presence of water.
ABSTR PAP AM CHEM S 226: 113-GEOC Part 1 SEP 2003
Cooke DJ, Parker SC, Osguthorpe DJ
Calculating the vibrational thermodynamic properties of bulk oxides using lattice dynamics and molecular dynamics
PHYS REV B 67 (13): art. no. 134306 APR 1 2003
Cooke DJ, Parker SC
Segregation of isovalent impurities to the {00.1} and {01.2} surfaces of hematite (alpha-Fe2O3) using atomistic Simulations
GEOCHIM COSMOCHIM AC 66 (15A): A150-A150 Suppl. 1 AUG 2002
Cooke DJ, Refern SE, Osguthorpe DJ, Parker SC
Structure and stability of iron oxide surfaces and their reactivity with water
RADIAT EFF DEFECT S 156 (1-4): 75-79 2001
Parker SC, de Leeuw NH, Bourova E, Cooke DJ
Application of lattice dynamics and molecular dynamics techniques to minerals and their surfaces
REV MINERAL GEOCHEM 42: 63-82 2001
de Leeuw NH, Redfern SE, Cooke DJ, Osguthorpe DJ, Parker SC
Modelling Dynamic Properties of Mineral Surfaces
In 'Solid-Liquid Interface Theory' ed JW Halley ACS Symposium Series, 789, 2001
University of Cambridge Department of Materials Science and Metallurgy Modelling of the Biological Interface with Materials