Research

Ab Initio Simulations of Catalytic and Environmental Interfaces


Our research aims at understanding important phenomena in surface- materials- and nano-science. Using concepts from quantum mechanics to statistical mechanics, we apply and develop methods and computer simulations to study, for instance, chemical reactions
at surfaces and processes of environmental relevance. Much of our research is carried out with leading experimentalists across the world. It is of both fundamental and applied interest. Water is a major focus of our work.

More information on our interests can be found below or by checking out some of our recent science highlights. A poster summarising some of our old work can be found here.

Water at metal surfacesIceIce NucleationHeterogeneous catalysisMachine Learning PotentialsSolid-liquid interfacesBoosting accuracy in computer simulationsImproving & Benchmarking DFTQuantum nuclear effectsOil/Water and Oil/Solid Interfaces

Water at metal surfaces

Water covers almost all solid surfaces under ambient conditions. From heterogenous ice nucleation on aerosol particles to waste water treatment, interfacial water is of crucial importance to an endless list of problems in the physical and chemical sciences. A prerequisite to understand these varied phenomena is the seemingly simple task of establishing what the water overlayer structure is. However, characterizing water overlayer structures is a challenging task and despite thousands of publications on the chemical physics of water at interfaces only a handful of determinations have been accomplished to date.

Some references (more)

N. Gerrard, C. Gattinoni, F. McBride, A. Michaelides, and A. Hodgson, Strain Relief during Ice Growth on a Hexagonal Template J. Am. Chem. Soc. 141, 8599-8607 (2019)

M. L. Liriano, C. Gattinoni, E. A. Lewis, C. J Murphy, E. C. H. Sykes, and A. Michaelides, Water-ice analogues of polycyclic aromatic hydrocarbons: Water nanoclusters on Cu(111) J. Am. Chem. Soc. 139, 6403-6410 (2017)

 Ice

1One reason the group is called the ICE group is we do a lot of work trying to understand the structure and properties of ice under various conditions. This includes ice at very high pressure, the ice surfaces, and ice nanoparticles. 

An example of something we investigate is the surface premelting of ice, which is the phenomenon of there existing a thin film of liquid water covering ice surfaces even at temperatures well below freezing. This was first postulated all the way back in 1859 by Michael Faraday yet it is still an active research topic.

1
Another example is understanding the properties of low-dimensional ice. Delicate balancing between forces means it has different properties to the bulk ice, and it is known to form a variety of structures. Confined and interfacial ice is ubiquitous in nature, and it it relevant to areas as diverse as cloud microphysics and tribology. Despite this major gaps in the understanding of the structures and phase transitions of this remain.

 

Some references (more)

Slater, B.; Michaelides, A. Surface Premelting of Water Ice. Nat. Rev. Chem.,3(3), 172–188 (2019)

Chen, J.; Zen, A.; Brandenburg, J. G.; Alfè, D.; Michaelides, A. Evidence for Stable Square Ice from Quantum Monte Carlo. Phys. Rev. B, 94(22), 220102 (2016)

Chen, J.; Schusteritsch, G.; Pickard, C. J.; Salzmann, C. G.; Michaelides, A. Two Dimensional Ice from First Principles: Structures and Phase Transitions. Phys. Rev. Lett., 116(2), 25501 (2016)

M. Watkins, D. Pan, E. G. Wang, A. Michaelides, J. VandeVondele and B. Slater Large variation of vacancy formation energies in the surface of crystalline ice Nature Mater. 10, 794-798 (2011)

D. Pan, L. M. Liu, B. Slater, A. Michaelides and E. Wang Melting the Ice: On the Relation between Melting Temperature and Size for Nanoscale Ice Crystals ACS Nano. 5, 4562 (2011)

Ice nucleation

cr-2015-007443_0001How ice forms is a poorly understood phenomenon: although it may seem trivial to make ice by putting a bottle of the water in a domestic freezer, the liquid form can exist to temperatures below 0oC (this is known as supercooling and you can see a demonstration here). When ice forms at temperatures close to the melting point, it is almost always due to the presence solid impurities that act as ice nucleating agents. This is called heterogeneous ice nucleation.

This ubiquitous phenomenon has a huge impact on the world around us, ice on Earth provides habitats such as the Arctic ice sheets for Polar Bears, and affects geology (e.g. glaciers eroding mountain ranges, freeze-thaw weathering of rocks). The climate is strongly influenced by the formation of ice in the atmosphere. For instance atmospheric ice nucleation on aerosols leads to the formation of ice-rich clouds, which prevents too much solar radiation reaching us. Ice formation is part of the overall water cycle, thus understanding it is crucial for weather prediction. Moreover, ice formation can lead to catastrophic consequences for vehicles operating in harsh-weather conditions. Examples include the emergency landing of a flight from Beijing to London in 2008 due to ice formation in the fuel system, and the recent explosion of a SpaceX rocket when attempting connection with an iced anchor upon landing. All these examples would benefit from a comprehensive understanding of the ice nucleation process at the molecular scale.

nucleusOur research aims to further our understanding of ice 
nucleation by implementing a range of computational techniques, from DFT studies of small water clusters and layers at different surfaces, through to large-timescale molecular dynamics simulations where we directly probe the nucleation mechanism. We have investigated the nucleation ability of substances known to effectively nucleate ice in the atmosphere such as silver iodide, kaolinite, feldspar and organic molecules including steroids and amino acids. Analysing more complex case studies like these and looking at simpler model templates to focus on certain phenomena, has afforded great insight into the properties of a substance that make it effective at nucleating ice.

We also investigate homogeneous nucleation (absence of any foreign particle) as it has a more fundamental importance to our understanding of the phenomena. Supercooled water exhibits dynamical heterogeneity, with both immobile and mobile regions of molecules present. There exist strong counter arguments for ice preferentially forming in the mobile and immobile regions, and we recently cleared this debate by showing that it occurs in the latter.

Some references (more)

Fitzner, M.; Sosso, G. C.; Cox, S. J.; Michaelides, A. Ice Is Born in Low-Mobility Regions of Supercooled Liquid Water. PNAS. 

Kiselev, A.; Bachmann, F.; Pedevilla, P.; Cox, S. J.; Michaelides, A.; Gerthsen, D.; Leisner, T. Active Sites in Heterogeneous Ice Nucleation—the Example of K-Rich Feldspars. Science (80-. ).2017, 355(6323), 367–371.

Pedevilla, P.; Fitzner, M.; Michaelides, A. What Makes a Good Descriptor for Heterogeneous Ice Nucleation on OH-Patterned Surfaces. Phys. Rev. B 2017, 96 (11), 115441.

S. J. Cox, S. M. Kathmann, J. A. Purton, M. J. Gillan, A. Michaelides Non-hexagonal ice at hexagonal surfaces: the role of lattice mismatch Phys. Chem. Chem. Phys. 14, 7944 (2012)

D. Pan, L. M. Liu, G. A. Tribello, B. Slater, A. Michaelides and E. Wang Surface energy and surface proton order of the ice Ih basal and prism surfaces  J. Phys.: Condensed Matter 22, 074209 (2010)

Heterogeneous catalysis

Our group has many years experience in the theory and simulation of chemical processes at surfaces, particularly those of relevance to heterogeneous catalysis. Currently, we are interested in highly dilute, so-called single atom alloys (SAAs).

SAAs are a new promising class of catalysts, which deviate from classical transition metal catalysts in that the commonly used linear scaling relations are broken for many adsorption energies and transition states. Hence they enable the possibility for higher activity and/or selectivity in useful chemical reactions, such as C-H activations. 

Some references (more)

Matthew T. Darby, Romain Réocreux, E. Charles H. Sykes, Angelos Michaelides, and Michail Stamatakis Elucidating the Stability and Reactivity of Surface Intermediates on Single-Atom Alloy Catalysts ACS Catal. 8, 5038 (2018)

Matthew D. Marcinkowski, Matthew T. Darby, Jilei Liu, Joshua M. Wimble, Felicia R. Lucci, Sungsik Lee, Angelos Michaelides, Maria Flytzani-Stephanopoulos, Michail Stamatakis and E. Charles H. Sykes Pt/Cu single-atom alloys as coke-resistant catalysts for efficient C–H activation Nature Chem. 10, 325 (2018)

skmap01.26.34_2

It is well documented that there are a number of instances in which the accuracy of empirical potentials falls short of our requirements – this compromise must often be accepted if we are to study systems any larger than a few hundred atoms. When greater accuracy is mandatory, we turn to ab initio molecular dynamics – almost always relying on density functional theory (DFT) for the determination of energies and forces. In such cases, however, we are often limited by the computational cost of these simulations to studying systems with no more than a few hundred atoms. Machine learning offers a way to perform simulations with the accuracy of ab initio methods, but with a cost which is much closer to that of traditional empirical potentials. We are interested in applying these machine learning potentials to understanding the behaviour of carbon, and the interface between water and carbonaceous systems. One particular case in which machine learning models may be useful is the case of water on graphene and hexagonal Boron-Nitride, two similar materials where error in the binding energy of just a few tens of meV can change their behaviour of from hydrophilic to hydrophobic. In this situation, machine learning potentials have the possibility to provide the accuracy which is required for reliable simulations, while doing so efficiently enough that long simulations with tens of thousands of atoms can be performed.

P. Rowe, G. Csányi, D. Alfè, A. Michaelides, Development of a Machine Learning Potential for Graphene, Phys. Rev. B., 97, 054303 (2018).

Solid-liquid interfaces

jpcl_2018_4865_abstract

Under ambient conditions, most surfaces are covered with a thin film of water. As such solid-water interfaces are of relevance to a huge array of scientific and technologic areas. Exciting recent advances in computational algorithms and hardware mean it is now possible to examine in intimate details structures and dynamics at solid-liquid interfaces entirely from first principles. Over the last few years we have studied a variety of systems such as water on salt, the controversial water/TiO2 interfaces and water on ZnO.

Some references (more)

Chi M. Yim, Ji Chen, Yu Zhang, Bobbie-Jean Shaw, Chi L. Pang, David C. Grinter, Hendrik Bluhm, Miquel Salmeron, Christopher A. Muryn, Angelos Michaelides, and Geoff Thornton, Visualization of Water-Induced Surface Segregation of Polarons on Rutile TiO2(110) J. Phys. Chem. Lett.9, 4865-4871 (2018)

H. Hussain, G. Tocci, T. Woolcot, X. Torrelles, C. L. Pang, D. Humphrey, C. Yim, D. Grinter, G. Cabailh, O. Bikondoa, R. Lindsay, J. Zegenhagen, A. Michaelides, and G. Thornton, Structure of a model TiO2 photocatalytic interface Nat. Mater. 16, 461-466 (2016)

Boosting accuracy in computer simulations

Computational approaches based on the fundamental laws of quantum mechanics are now integral to almost all materials design initiatives in academia and industry. If computational materials science is genuinely going to deliver on its promises, then an electronic structure method with consistently high accuracy is urgently needed.
We have contributed to improve and systematically employ highly accurate methods for electronic structure calculations, such as diffusion quantum Monte Carlo.

Some references (more)

A. Zen, J. G. Brandenburg, A. Michaelides, and D. Alfè A new scheme for fixed node diffusion quantum Monte Carlo with pseudopotentials: improving reproducibility and reducing the trial-wave-function bias J. Chem. Phys. 151, 134105 (2019)

J. G. Brandenburg, A. Zen, M. Fitzner, B. Ramberger, G. Kresse, T. Tsatsoulis, A. Grüneis, A. Michaelides, and D. Alfè Physisorption of Water on Graphene: Subchemical Accuracy from Many-Body Electronic Structure Methods J. Phys. Chem. Lett. 10, 358 (2019)

A. Zen, J. G. Brandenburg, J. Klimeš, A. Tkatchenko, D. Alfè, and A. Michaelides Fast and accurate quantum Monte Carlo for molecular crystals P. Natl. Acad. Sci. Usa 115, 1724 (2018)

A. Zen, S. Sorella, M. J. Gillan, A. Michaelides, and D. Alfè Boosting the accuracy and speed of quantum Monte Carlo: Size consistency and time step Phys. Rev. B 93, 241118(R) (2016)

Improving and Benchmarking DFT

TOCMost of the "first principles" simulations we do are with a theory known as density-functional theory (DFT). In principle it is exact but in practice it relies on an approximation for how electrons interact with each other. We are tackling the issue of the accuracy of DFT through extensive series of studies of small gas phase complexes, molecular crystals, and molecules at solid interfaces. These benchmark studies with techniques such as quantum Monte Carlo and coupled cluster come with extreme computational burdens. However, these benchmarks are essential to establish the accuracy of more traditional methods such as DFT, and help to ensure that the numbers we produce stand the test of time and experiment.

A major challenge for DFT is the accurate description of van der Waals interactions, and London dispersion in particular. London dispersion interactions are ubiquitous in nature contributing to the binding of biomolecules such as DNA, molecular crystals, and adsorption of molecules on surfaces. Many schemes have been developed that allow dispersion to be accounted for within DFT in a more or less approximate manner. One of the most promising and rigorous method is the nonlocal van der Waals density functional (vdW-DF) proposed by Langreth and Lundqvist and co-workers (M. Dion et al., Phys. Rev. Lett. 92, 246401, 2004). We have been working on developing improved versions on the vdw-DF approach, and in particular have developed optB88-vdW, optPBE-vdW, and optB86b-vdW functionals.

Some references (more)

J. G. Brandenburg, A. Zen, D. Alfè, and A. Michaelides Interaction between water and carbon nanostructures: How good are current density functional approximations? J. Chem. Phys. (accepted). arXiv:1907.09525

Y. S. Al-Hamdani et al. Properties of the water to boron nitride interaction: From zero to two dimensions with benchmark accuracy J. Chem. Phys. 147, 044710 (2017)

B. Vlaisavljevich et al. Performance of van der Waals Corrected Functionals for Guest Adsorption in the M2(dobdc) Metal−Organic Frameworks J. Phys. Chem. A 121, 4139 (2017)

Y. S. Al-Hamdani, D. Alfè, and A. Michaelides How strongly do hydrogen and water molecules stick to carbon nanomaterials? J. Chem. Phys. 146, 094701 (2017)

J. Chen, A. Zen, J. G. Brandenburg, D. Alfè, and A. Michaelides Evidence for stable square ice from quantum Monte Carlo Phys. Rev. B 94, 220102(R) (2016)

A. Zen, L. M. Roch, S. J. Cox, X. L. Hu, S. Sorella, D. Alfè, and A. Michaelides Toward Accurate Adsorption Energetics on Clay Surfaces J. Phys. Chem. C 120, 26402 (2016)

M. J. Gillan, D. Alfè and A. Michaelides Perspective: How good is DFT for water? J. Chem. Phys. 144, 130901 (2016)

Y. S. Al-Hamdani, M. Ma, D. Alfè, O. A. von Lilienfeld, and A. Michaelides Communication: Water on hexagonal boron nitride from diffusion Monte Carlo J. Chem. Phys. 142, 181101 (2015)

Y. S. Al-Hamdani, D. Alfè, O. A. von Lilienfeld, and A. Michaelides Water on BN doped benzene: A hard test for exchange-correlation functionals and the impact of exact exchange on weak binding J. Chem. Phys. 141, 18C530 (2014)

B. Santra, J. Klimeš, A. Tkatchenko, D. Alfè, B. Slater, A. Michaelides, R. Car, and M. Scheffler On the accuracy of van der Waals inclusive density-functional theory exchange-correlation functionals for ice at ambient and high pressures J. Chem. Phys. 139, 154702 (2013)

J. Klimeš, and A. Michaelides Perspective: Advances and challenges in treating van der Waals dispersion forces in density functional theory J. Chem. Phys. 137, 120901 (2012)

J. Klimeš, D. R. Bowler and A. Michaelides Van der Waals density functionals applied to solids Phys. Rev. B 83, 195131 (2011)

J. Klimeš, D. R. Bowler and A. Michaelides A critical assessment of theoretical methods for finding reaction pathways and transition states of surface processes J. Phys.: Condensed Matter. 22, 074203 (2010)

B. Santra, J. Klimeš, D. Alfè, B. Slater, A. Michaelides, R. Car and M. Scheffler Hydrogen bonds and van der Waals forces in ice at ambient and high pressures Phys. Rev. Lett. 107, 185701 (2011)

B. Santra, A. Michaelides and M. Scheffler Coupled cluster benchmarks of water monomers and dimers extracted from DFT liquid water: the importance of monomer deformations J. Chem. Phys. 131, 124509 (2009)

Quantum nuclear effects

qhb

Quantum nuclear effects are important but generally poorly understood. For this reason, we are working to understand them by developing and applying state-of-the art path integral techniques. The types of systems we are looking at include processes at surfaces(chemical reactions and adsorption) and on the fundamental nature of the hydrogen bond. Read more about Quantum Nuclear effects.

Some references (more)

W. Fang, J. Chen, Y. Feng, X.-Z. Li, and A. Michaelides, The quantum nature of hydrogen Int. Rev. Phys. Chem. 38, 35-61 (2019)

W. Fang, J. O. Richardson, J. Chen, X.-Z. Li, and A. Michaelides, Simultaneous Deep Tunneling and Classical Hopping for Hydrogen Diffusion on Metals Phys. Rev. Lett. 119, 126001 (2017)

Oil/Water and Oil/Solid Interfaces

Understanding behaviours of ions, molecules at oil/water and oil/solid substrate interfaces is crucial for various applications in oil and gas industry. For example, in many enhanced oil recovery techniques surface active substances (e.g. surfactants) are added into the system to lower the interfacial tension between water and oil which help reduce the capillary force and promote the detachment of oil or gas molecules from pore surfaces. We conduct extensive molecular dynamics simulations to obtain molecular level insights into the factors affecting the water/oil interfacial tension. The results could be used to improve the design of effective surfactant formulation at specific system conditions (e.g. temperature, salinity).

Gas hydrate formation/agglomeration is another area that highlights the importance of understanding phenomena at oil/water or oil/solid substrate interfaces. The transport of guest molecules (methane, ethane, propane ...) through the interfacial film governs the growth rate of gas hydrates. The presence of natural surfactants (surface active compounds in crude oil) or the addition of surfactants/inhibitors strongly affects the transport mechanism and rate. Classical molecular dynamics simulations can provide useful information about how gas molecules diffuse through the interfacial film as well as how molecular features of the interfacial layer could hinder or enhance the diffusivity of the gas molecules. Coupling these information with experimental data could be beneficial to the synthesis of new effective hydrate inhibitors and the development of kinetic models to predict the formation of gas hydrates inside gas and oil pipelines.

 

Recent Highlights

A partially melted ice nanoparticle at about -100 degrees Celsius

Nano ice melts at -100 degrees!

25th March 2011

Computer simulations provide a molecule’s eye view of the melting of ice nanoparticles, predicting melting at very low temperatures. The melting of ice is a very familiar process but its ubiquity belies its importance. It plays a ...

A view of the ice surface illustrating weakly, (red), intermediate (white) and strongly bound water molecules (blue). White molecules are at the external surface, grey lie sub-surface.

Large variation in ice surface vacancies

25th March 2012

Ice exhibits a phenomenon known as pre-melting which was first alluded to by Michael Faraday in his ‘regelation’ experiments at the Royal Institution in the 1850’s. A liquid like layer forms at the surface of ice, but ...

Figure showing friction in graphene vs hBN

Simulating water slippage

25th January 2015

Friction is one of the main sources of dissipation. For instance, about one third of the world mechanical energy is dissipated into friction [1]. Understanding nanoscale friction at the interface between a liquid and a solid is also ...

Snapshots of a water nanodroplet diffusing on graphene with large (about 0.7 nm)-amplitude ripples.

Nanodroplets surfing on graphene

25th January 2016

The motion of atoms, molecules and clusters across the surfaces of materials is of critical importance to an endless list of phenomena.ion across surfaces generally involves motion on a vibrating but otherwise stationary substrate. H ...

QMC on 2D ice paper published at Phys. Rev. B

7th December 2016

In this work, Ji, Andrea and Gerit have worked together in re-evaluating the stability of so-called two-dimensional (2D) ice, one of the most interesting and controversial topics about ice in recent years. Recent experiments on ice ...