In a recently published Nature Reviews Chemistry article, titled “Surface premelting of water ice”, Ben Slater and Angelos review the current understanding of the quasi-liquid layer (QLL) that forms on the surface of ice. The review describes how advances in experimental and computational techniques furthered our understanding in the years since Faraday first postulated the existence of a QLL in the 1850s, while highlighting topics that still pose open questions, such as the QLL thickness.
Posts in category Highlights
The next issue of The Journal of Physical Chemistry Letters will feature cover art from the perspective article ‘Lonely Atoms with Special Gifts: Breaking Linear Scaling Relationships in Heterogeneous Catalysis with Single-Atom Alloys’ by Matthew T. Darby, Michail Stamatakis, Angelos Michaelides, and E. Charles H. Sykes. The cover depicts the atomic structure of a so-called single-atom alloy, which is bimetallic alloy with a low concentration of the catalytically active component. One of these active sites prominently features a methane molecule after C-H bond activation. Dispersing the active component offers potential for well-defined and enhanced catalytic performance.
Andrea’s and Gerit’s paper has been published in the Proceedings of the National Academy of Sciences of the United States of America. The work had important contributions also from Jiří, a previous member of the ICE group, and from Dario and Alexandre, longstanding Angelos’ collaborators.
In the paper we study molecular crystals with quantum Monte Carlo.
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 show that, thanks to recent algorithmic advances and the strategy developed in our manuscript, quantum Monte Carlo yields extremely accurate predictions for the lattice energies of materials at a surprisingly modest computational cost. It is thus no longer a technique that requires a world-leading computational facility to obtain meaningful results. While we focus on molecular crystals, the significance of our findings extends to all classes of materials.
Our collaborative paper with Charlie Sykes, Michail Stamatakis and other entitled “PtCu Single Atom Alloys as a Coke Resistant Strategy for Efficient C-H Activation” has been accepted for publication at Nature Chemistry. Well done everyone!
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 formed by water under nanoconfinement provide evidence for a two-dimensional (2D) “square ice” phase. However, the interpretation of the experiments has been questioned and the stability of square ice has become a matter of debate. In this paper we have carried out diffusion Monte Carlo (DMC), DFT and force field calculations. We find that at relatively high pressure, square ice is indeed the lowest enthalpy phase examined, supporting the initial experimental claim. Moreover, at lower pressures, a “pentagonal ice” phase (not yet observed experimentally) has the lowest enthalpy, and at ambient pressure, the “pentagonal ice” phase is degenerate with a “hexagonal ice” phase. We have also evaluated the accuracy of various density functional theory exchange-correlation functionals and force field models, and in doing so we extend the understanding of how such methodologies perform to challenging 2D structures presenting dangling hydrogen bonds.
The assembly of complex structures in nature is driven by an interplay between several intermolecular interactions, from strong covalent bonds to weaker dispersion forces. Scientists from Tufts University USA, University College London UK, and CIC Energigune Spain worked in collaboration to understand microscopically the role non-covalent interactions play in the adsorption and assembly of 2-butanol on the (111) surface of gold. 2-butanol has recently been shown to have interesting properties as a chiral modifier of surface chemistry.
Using high-resolution scanning tunneling microscopy (STM), they found that the chiral molecules acquire a second chiral center when adsorbed to the surface via dative bonding of one of the oxygen atom lone pairs. The evolution of these square units is surprising given that the underlying surface has a hexagonal symmetry.
DFT calculations reveal that the tetramers are stable entities that are able to associate with each other by weaker van der Waals interactions and tessellate in an extended square network. This study provides the first microscopic insight into the surface properties of this important chiral modifier and provides a well-defined system for studying the network’s enantio-selective interaction with other molecules.
This article has been selected as a 2016 Editors’ Choice article.
Scientists at UCL and Cambridge predict new two-dimensional ice structures on the basis of state-of-the-art computer simulations.
A systematic computer simulation study has led to predictions about how water molecules freeze into a single layer of ice. These simulations, published in Physical Review Letters, reveal several models for 2D ice, including a hexagonal, a Cairo tiling pentagonal, a square and a rhombic structure. The new 2D ice structures, obtained on the basis of first principles simulations and unbiased structure search methods, extend the knowledge of ice in nature and are potentially important in understanding phenomena such as cloud microphysics and tribology.
The authors also predict a sequence of phase transitions that happens as a function of pressure and confinement, leading to the determination of a phase diagram of 2D ice. Overall this work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures formed to the confining pressure and confinement width. The observation of the flat square structure supports recent experimental observations of square ice confined within graphene sheets. The authors also discuss how other structures such as the Cairo tiling pentagonal structure may be observed by slightly altering the conditions used so far in experiments.
This study has been published in Physical Review Letters
Journal link: Phys. Rev. Lett. 116, 025501 (2016)
Two dimensional radioactive films are a new and exciting system to study nuclear decay at the atomic level with applications in a variety of fields ranging from medical imaging to cancer therapy. Before these films can be used in real-world application however, their behaviour and stability under ambient conditions has to be understood. This study, which was done in collaboration between the experimental group of Prof. Sykes (Tufts University, USA) and the computational group of Prof. Michaelides (University College London, UK) addresses precisely that issue. Using X-ray photoelectron spectroscopy and scanning tunneling microscope experiments combined with density functional theory computations the authors were able to monitor the decay product, 125Te, over time with atomistic resolution. This work reveals not only that the radioactive films and the decay product are stable in air at ambient conditions, but also shows precisely what happens to these films over time. Freshly formed Te is bound very strongly to the gold substrate, even stronger than the radioactive iodine atoms, and oxidises to TeO2 in air. TeO2 units are able to diffuse through the films and tend to dimerize to (TeO2)2. The radioactive films as well as the decay products remain intact throughout these reactions. This crucial insight opens the door for a range of useful applications of two dimensional radioactive films on gold. Adsorbed on gold nanoparticles they could for example lead to highly targeted cancer therapy treatments.
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 Diffusion across surfaces generally involves motion on a vibrating but otherwise stationary substrate. Using molecular dynamics, scientist at UCL have discovered a new mechanism for surface diffusion on layered 2D materials such as graphene. The ripples on graphene can carry nanodroplets along when moving across the surface. For water nanodroplets, this mechanism allows exceedingly fast diffusion 2–3 orders of magnitude faster than the self-diffusion of water molecules in liquid water. The new mechanism for surface diffusion can open up the prospect of achieving fast and controllable motion of adsorbates across material surfaces generally.
This study has been published in Nature Materials
Journal link:Nature Materials 15, 66 (2016)
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)