Atomistic details of copper oxide surfaces and surface oxidation
The oxidation and corrosion of metals are issues which affect many of our everyday objects, from water pipes to electronic devices. They eventually lead to material failure and are extremely costly to businesses and individuals. These important problems have been studied for a long time. Copper is one of the most studied metals and it has become a model to understand oxidation and corrosion. In this review we show that now now have a good atomistic understanding of the physical characteristics of copper oxides and of some key processes of their formation . However a number of challenges remain. For example, the atomistic details of the nucleation of the oxide is still unknown. However we believe that recent advances in experimental techniques, bringing greater temporal and spatial resolution, along with improvements in the accuracy, realism and timescales achievable with computational approaches make it possible for these questions to be answered in the near future.
This study has been published in Surface Science Reports
Journal link:Surface Science Reports 70, 424 (2015)
DFT structure of the radioactive film (left; I in purple and Te in green) and DFT-based simulated STM image (right). Te atoms bind stronger to the substrate than their I neighbours.
Nuclear decay is one of the most extreme processes and is central to a range of fields including energy, medicine, imaging, labelling, archaeology and sensing. Low-energy electrons from radioactivity decay are biologically active. However, it is challenging to generate them locally. In this work, a new radioactive film that enables enhanced low-energy electron emission has been synthesized. Scanning tunnelling microscopy, supported by electronic structure simulations explained the strong stabilty of the film as it underwent radioactive decay. This new radioactive film may have future applications in cancer therapies using nanoparticles.
This study has been published in Nature Materials
Journal link:Nature Materials 14, 904 (2015)
Very similar water structure VS. Very different friction
Friction is one of the main sources of dissipation. For instance, about one third of the world mechanical energy is dissipated into friction . Understanding nanoscale friction at the interface between a liquid and a solid is also crucial for the development of efficient membranes for water desalination and power harvesting. Researchers at UCL in a collaboration study with Laurent Joly, from the University of Lyon, have investigated the friction properties of liquid water at the interface with graphene and with an hexagonal boron nitride sheet, using ab initio molecular dynamics for the first time. They found the striking result that the friction coefficient on boron nitride is ≈3 times larger than that on graphene although the structure of the water layers on the two sheets is almost identical.
This study (recently published in Nano Letters) has recently appeared on the Research Highlights of Nature Nanotechnology.
Journal link:Nano Lett. 14, 6872–6877 (2014)
Research Highlights in Nature Nanotechnology: http://www.nature.com/nnano/reshigh/2015/0115/full/nnano.2014.330.html
 Szeri, A. Z. Tribology: Friction, Lubrication, and Wear (Hemisphere, 1980).
A snapshot from the ab-initio path-integral molecular dynamics simulation where a metallic liquid phase is found at 900 GPa and 50 K. Accounting for the quantum nature of the nuclei is essential for the appearance of this low-temperature metallic phase.
The nature of dense hydrogen is a central problem in physics and its abundance, for example, in gas giants such as Jupiter and Saturn means that it is critical to our understanding of the universe. In spite of the tremendous progress made over the last 80 years, important gaps in our understanding of the hydrogen phase diagram remain, with arguably the most challenging issue being the solid to liquid melting transition at ultra-high pressures.
This study, involving a group of researchers from the Thomas Young Centre at UCL, as well as from Peking University, Cambridge and York, presents a fundamental advance in the understanding of dense hydrogen which has far reaching implications for a wide range of scientific fields.
The scientists from the four universities looked, in an international effort, at the melting of hydrogen by computer simulation of the coexistence of the solid and liquid phases, for the first time taking the quantum motion of the protons into account explicitly. The findings show a low-temperature metallic atomic liquid phase of hydrogen at pressures 900 GPa and above, down to 50K, the lowest temperature that can be reliably simulated. The existence of this low temperature liquid is associated with a negative slope of the melting line between atomic liquid and solid phases at pressures between 500 and 800 GPa. These results are highly quantum in nature, with classical simulations demonstrating completely different behaviour, with the simulations showing considerably higher melting points. This study confirms the existence of this phase in simulations and shows how the quantum motion of the protons plays a critical role in its stabilisation.
This work has been published in Nature Communications
Journal link:Nature Communications
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.
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 there is dispute about the temperature at which this layer first occurs.
Understanding the structure of the layer and its temperature dependence is important in the context of atmospheric heterogeneous catalysis, because the surface of ice particulates facilitate reactions of radicals and trace gases in the atmosphere.
In a recent paper in Nature Materials, TYC researchers at UCL (Matt Watkins, Angelos Michaelides and Ben Slater) in collaboration with researchers from University of Zurich, Peking University and the Chinese Academy of Sciences have discovered unexpected properties of ice at the nanoscale that relate to Faraday’s experiments. Using density functional theory calculations, they discovered each molecule is bound to the surface by a difference force, unlike most crystalline materials where each surface molecule has an identical binding energy. A fraction of the surface molecules are so weakly bound that they are easily displaced to form an overlayer, leading to less crystalline surface layers. The figure illustrates the variation in binding energy, which arises from the interaction of the water’s dipole within a geometrically frustrated array of neighbouring dipole moments.
This work has been published in Nature Materials
Journal link:Nature Materials.
A partially melted ice nanoparticle at about -100 degrees Celsius
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 central role in a wide variety of chemical processes, and is particularly relevant to environmental and atmospheric chemistry. However, despite being an everyday process, melting is not as well understood as one might have thought, particularly on the nanoscale.
Ice cubes, like those you might put in a gin and tonic melt at zero degrees Celsius, but at what temperature do ice nanoparticles (that is particles about 0.000000001 metre large) melt? An international team of researchers from the Thomas Young Centre, the London Centre for Nanotechnology, UCL Chemistry Department and Peking University set about to answer this question with computer simulation techniques and some of the most powerful computers in the UK (the HECToR and Legion Supercomputers). The answer they got is around a cool minus 100 degrees Celsius. That is, on the nanoscale the melting point of ice particles is about 100 degrees less than it is for macroscopic (everyday) ice cubes.
The strong dependence of melting temperature on particle size sounds remarkable but is not that surprising: it is due to the large surface to volume ratio of the nanoparticles and can be explained pretty well with textbook thermodynamic theories that relate melting temperatures with particle size.
For more information see the article by Pan et al. in ACS Nano.