Ben Slater

Modular and predictable assembly of porous organic molecular crystals

Nanoporous molecular frameworks are important in applications such as separation, storage and catalysis. Empirical rules exist for their assembly but it is still challenging to place and segregate functionality in three-dimensional porous solids in a predictable way. Indeed, recent studies of mixed crystalline frameworks suggest a preference for the statistical distribution of functionalities throughout the pores rather than, for example, the functional group localization found in the reactive sites of enzymes. This is a potential limitation for ‘one-pot’ chemical syntheses of porous frameworks from simple starting materials. An alternative strategy is to prepare porous solids from synthetically preorganized molecular pores. In principle, functional organic pore modules could be covalently prefabricated and then assembled to produce materials with specific properties. However, this vision of mix-and-match assembly is far from being realized, not least because of the challenge in reliably predicting three-dimensional structures for molecular crystals, which lack the strong directional bonding found in networks. Here we show that highly porous crystalline solids can be produced by mixing different organic cage modules that self-assemble by means of chiral recognition. The structures of the resulting materials can be predicted computationally, allowing in silico materials design strategies. The constituent pore modules are synthesized in high yields on gram scales in a one-step reaction. Assembly of the porous co-crystals is as simple as combining the modules in solution and removing the solvent. In some cases, the chiral recognition between modules can be exploited to produce porous organic nanoparticles. We show that the method is valid for four different cage modules and can in principle be generalized in a computationally predictable manner based on a lock-and-key assembly between modules.

Flexibility in a Metal–Organic Framework Material Controlled by Weak Dispersion Forces: The Bistability of MIL‐53(Al)

Breathtaking MOFs: DFT calculations reveal that the exceptional, thermally induced density change of the metal-organic framework MIL53(Al) is controlled by a competition between shortand long-range interactions and entropic factors. As shown in the picture (C green, Al cyan, O red, H white), dispersive interactions between the phenyl rings are responsible for stabilizing a narrow-pore form at low temperature. At 325-375 K, vibrational entropy causes the structure to expand markedly, permitting large volumes of light gases to be adsorbed.

Zeolitic imidazole frameworks: structural and energetics trends compared with their zeolite analogues

We use periodic DFT calculations to compute the total energy of known zeolitic imidazole frameworks (ZIFs) together with those of hypothetical porous ZIFs. We show that the total energy of ZIFs decreases with increasing density, in a similar fashion to the alumino-silicate zeolites, but with a more complex energy landscape. The computational evaluation of the stability of hypothetical ZIFs is useful in the search for viable synthesis targets. Our results suggest that a number of hitherto undiscovered nanoporous topologies should be amenable to synthesis (CAN, ATN) and that even the most open framework types might be obtained with appropriately substituted ligands.

The polymorphism of ice: five unresolved questions

Our recent discovery of three new phases of ice has increased the total number of known distinct polymorphs of ice to fifteen. In this Perspective article, we give a brief account of previous work in the field, and discuss some of the particularly interesting open questions that have emerged from recent studies. These include (i) the effectiveness of acid and base dopants to enable hydrogen-ordering processes in the ices, (ii) the comparison of the calorimetric data of some of the crystalline phases of ice and low-density amorphous ice, (iii) the disagreement between the experimental ice XV structure and computational predictions, (iv) the incompleteness of some of the hydrogen order/disorder pairs and (v) the new frontiers at the high and negative pressure ends of the phase diagram.

Experimental and computational study of the gas-sensor behaviour and surface chemistry of the solid-solution Cr2−xTixO3(x≤ 0.5)

The solid solution Cr2−xTixO3 is an excellent gas sensor material, with stability of performance over the short and long-term and minor influences of variations of humidity. It is the first new material to be successfully commercialised in large-volume manufacture for sensing of hydrocarbons, volatile organic compounds (VOC), hydrogen and carbon monoxide at low (ppm) concentrations in air since the introduction of SnO2 for this purpose in the 1960s. The phase limit is at x ≃ 0.3–0.4, above which a 2-phase mixture with CrTiO3 is found. Substitution of Ti strongly decreases the electrical conductivity of the porous bodies studied. Surface high-valency Cr, assumed to be CrVI, whose proportion is decreased by Ti substitution, is detected by XPS. This effect, and the surface segregation of Ti, control the gas sensor behaviour. Defect models of the (0001) and (102) surfaces have been assessed by computational modelling: in the absence of Ti, one stable defect is a CrVI–VCr‴ pair, which is surface segregated at (0001) and contributes to the relatively high p-type conductivity shown by finely porous bodies of Cr2O3 at elevated temperature; with Ti addition, a stable defect, also surface segregated, is the complex (Ti˙Cr)3VCr‴. Distortion of the arrangement of surface oxygen above the Cr vacancy creates a possible binding site; the high-valency surface cation creates another. It is suggested that the two sites act in concert to promote the dissociation of oxygen and the surface reaction needed for gas sensing.

Structure of the (104) surfaces of calcite, dolomite and magnesite under wet and dry conditions

Atomistic computer simulation methods have been employed to model the structure of the (104) surfaces of calcite (CaCO3), dolomite [CaMg(CO3)2] and magnesite (MgCO3). Our calculations show that, under anhydrous vacuum conditions, calcite undergoes the greatest degree of surface relaxation with rotation and distortion of the carbonate group accompanied by movement of the calcium ion. The magnesite surface is the least distorted of the three carbonates, with dolomite being intermediate to the two end members. When water molecules are placed on the surface to produce complete monolayer coverage, the surfaces of all three carbonate minerals are stabilized and the amount of relaxation in the surface layers substantially reduced. Of the three phases, dolomite shows the strongest and highest number of interfacial hydrogen bonds between water and the carbonate mineral surface. These calculations suggest that the equilibrium H2O + CO32−⇌HCO3− + OH− will favour the production of hydrogen carbonate ions most strongly for dolomite, less strongly for calcite, and least likely for magnesite.

The phase diagram of water at negative pressures: virtual ices.

The phase diagram of water at negative pressures as obtained from computer simulations for two models of water, TIP4P/2005 and TIP5P is presented. Several solid structures with lower densities than ice Ih, so-called virtual ices, were considered as possible candidates to occupy the negative pressure region of the phase diagram of water. In particular the empty hydrate structures sI, sII, and sH and another, recently proposed, low-density ice structure. The relative stabilities of these structures at 0 K was determined using empirical water potentials and density functional theory calculations. By performing free energy calculations and Gibbs-Duhem integration the phase diagram of TIP4P/2005 was determined at negative pressures. The empty hydrates sII and sH appear to be the stable solid phases of water at negative pressures. The phase boundary between ice Ih and sII clathrate occurs at moderate negative pressures, while at large negative pressures sH becomes the most stable phase. This behavior is in reasonable agreement with what is observed in density functional theory calculations.

Large variation of vacancy formation energies in the surface of crystalline ice

Resolving the atomic structure of the surface of ice particles within clouds, over the temperature range encountered in the atmosphere and relevant to understanding heterogeneous catalysis on ice, remains an experimental challenge. By using first-principles calculations, we show that the surface of crystalline ice exhibits a remarkable variance in vacancy formation energies, akin to an amorphous material. We find vacancy formation energies as low as ~0.1-0.2 eV, which leads to a higher than expected vacancy concentration. Because a vacancys reactivity correlates with its formation energy, ice particles may be more reactive than previously thought. We also show that vacancies significantly reduce the formation energy of neighbouring vacancies, thus facilitating pitting and contributing to pre-melting and quasi-liquid layer formation. These surface properties arise from proton disorder and the relaxation of geometric constraints, which suggests that other frustrated materials may possess unusual surface characteristics.

Proton ordering in cubic ice and hexagonal ice; a potential new ice phase—XIc

Ordinary water ice forms under ambient conditions and has two polytypes, hexagonal ice (Ih) and cubic ice (Ic). From a careful comparison of proton ordering arrangements in Ih and Ic using periodic density functional theory (DFT) and diffusion Monte Carlo (DMC) approaches, we find that the most stable arrangement of water molecules in cubic ice is isoenergetic with that of the proton ordered form of hexagonal ice (known as ice XI). We denote this potential new polytype of ice XI as XIc and discuss a possible route for preparing ice XIc.

Melting the Ice: On the Relation between Melting Temperature and Size for Nanoscale Ice Crystals

Although the melting of ice is an everyday process, important issues remain unclear particularly on the nanoscale. Indeed despite extensive studies into ice melting and premelting, little is known about the relationship between (pre)melting and crystal size and morphology, with, for example, the melting temperature of ice nanocrystals being unclear. Here we report extensive long-time force-field-based molecular dynamics studies of the melting of hexagonal ice nanocrystals in the ca. 2 to 8 nm size range. We show that premelting is initiated at the corners of the crystals, then the edges between facets, and then the flat surfaces; that is, the melting temperature is related to the degree of coordination. A strong size dependence of the melting temperature is observed, with the combination of small particle size and premelting leading nanosized ice crystals to have liquid-like surfaces as low as about 130 K below the bulk ice melting temperature. These results will be of relevance in understanding the size dependence of ice crystal morphology and the surface reactivity of ice particles under atmospheric conditions.