Strong structuring arising from weak cooperative O-H···π and C-H···O hydrogen bonding in benzene-methanol solution.

Weak hydrogen bonds, such as O-H···π and C-H···O, are thought to direct biochemical assembly, molecular recognition, and chemical selectivity but are seldom observed in solution. We have used neutron diffraction combined with H/D isotopic substitution to obtain a detailed spatial and orientational picture of the structure of benzene-methanol mixtures. Our analysis reveals that methanol fully solvates and surrounds each benzene molecule. The expected O-H···π interaction is highly localised and directional, with the methanol hydroxyl bond aligned normal to the aromatic plane and the hydrogen at a distance of 2.30 Å from the ring centroid. Simultaneously, the tendency of methanol to form chain and cyclic motifs in the bulk liquid is manifest in a highly templated solvation structure in the plane of the ring. The methanol molecules surround the benzene so that the O-H bonds are coplanar with the aromatic ring while the oxygens interact with C-H groups through simultaneous bifurcated hydrogen bonds. This demonstrates that weak hydrogen bonding can modulate existing stronger interactions to give rise to highly ordered cooperative structural motifs that persist in the liquid phase.

Weak Interactions in Dimethyl Sulfoxide (DMSO)-Tertiary Amide Solutions: The Versatility of DMSO as a Solvent.

The structures of equimolar mixtures of the commonly used polar aprotic solvents dimethylformamide (DMF) and dimethylacetamide (DMAc) in dimethyl sulfoxide (DMSO) have been investigated via neutron diffraction augmented by extensive hydrogen/deuterium isotopic substitution. Detailed 3-dimensional structural models of these solutions have been derived from the neutron data via Empirical Potential Structure Refinement (EPSR). The intermolecular center-of-mass (CoM) distributions show that the first coordination shell of the amides comprises ∼13-14 neighbors, of which approximately half are DMSO. In spite of this near ideal coordination shell mixing, the changes to the amide-amide structure are found to be relatively subtle when compared to the pure liquids. Analysis of specific intermolecular atom-atom correlations allows quantitative interpretation of the competition between weak interactions in the solution. We find a hierarchy of formic and methyl C-H···O hydrogen bonds forms the dominant local motifs, with peak positions in the range of 2.5-3.0 Å. We also observe a rich variety of steric and dispersion interactions, including those involving the O═C-N amide π-backbones. This detailed insight into the structural landscape of these important liquids demonstrates the versatility of DMSO as a solvent and the remarkable sensitivity of neutron diffraction, which is critical for understanding weak intermolecular interactions at the nanoscale and thereby tailoring solvent properties to specific applications.

Intermediate Range Order in Metal-Ammonia Solutions: Pure and Na-Doped Ca-NH3.

The local and intermediate range ordering in Ca-NH3 solutions in their metallic phase is determined through H/D isotopically differenced neutron diffraction in combination with empirical potential structure refinements. For both low and high relative Ca concentrations, the Ca ions are found to be octahedrally coordinated by the NH3 solvent, and these hexammine units are spatially correlated out to lengthscales of ∼7.4-10.3 Šdepending on the concentration, leading to pronounced ordering in the bulk liquid. We further demonstrate that this liquid order can be progressively disrupted by the substitution of Ca for Na, whereby a distortion of the average ion primary solvation occurs and the intermediate range ion-ion correlations are disrupted.

High-Performance Zinc-Air Batteries with Scalable Metal-Organic Frameworks and Platinum Carbon Black Bifunctional Catalysts.

Metal-organic framework (MOF)-related derivatives have generated significant interest in numerous energy conversion and storage applications, such as adsorption, catalysis, and batteries. However, such materials' real-world applicability is hindered because of scalability and reproducibility issues as they are produced by multistep postsynthesis modification of MOFs, often with high-temperature carbonization and/or calcination. In this process, MOFs act as self-sacrificial templates to develop functional materials at the expense of severe mass loss, and the resultant materials exhibit complex process-performance relationships. In this work, we report the direct applicability of a readily synthesized and commercially available MOF, a zeolitic imidazolate framework (ZIF-8), in a rechargeable zinc-air battery. The composite of cobalt-based ZIF-8 and platinum carbon black (ZIF-67@Pt/CB) prepared via facile solution mixing shows a promising bifunctional electrocatalytic activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), the key charge and discharge mechanisms in a battery. ZIF-67@Pt/CB exhibits long OER/ORR activity durability, notably, a significantly enhanced ORR stability compared to Pt/CB, 85 versus 52%. Interestingly, a ZIF-67@Pt/CB-based battery delivers high performance with a power density of >150 mW cm-2 and long stability for 100 h of charge-discharge cyclic test runs. Such remarkable activities from as-produced ZIF-67 are attributed to the electrochemically driven in situ development of an active cobalt-(oxy)hydroxide nanophase and interfacial interaction with platinum nanoparticles. This work shows commercial feasibility of zinc-air batteries as MOF-cathode materials can be reproducibly synthesized in mass scale and applied as produced.

Size-Related Electrochemical Performance in Active Carbon Nanostructures: A MOFs-Derived Carbons Case Study.

Metal-organic framework-derived carbon nanostructures have generated significant interest in electrochemical capacitors and oxygen/hydrogen catalysis reactions. However, they appear to show considerably varied structural properties, and thus exhibit complex electrochemical-activity relationships. Herein, a series of carbon polyhedrons of different sizes, between 50 nm and µm, are synthesized from zeolitic imidazolate frameworks, ZIF-8 (ZIF-derived carbon polyhedrons, ZDCPs) and their activity is studied for capacitance and the oxygen reduction reaction (ORR). Interestingly, a well-correlated performance relationship with respect to the particle size of ZDCPs is evidenced. Here, the identical structural features, such as specific surface area (SSA), microporosity, and its distribution, nitrogen doping, and graphitization are all strictly maintained in the ZDCPs, thus allowing identification of the effect of particle size on electrochemical performance. Supercapacitors show a capacity enhancement of 50 F g-1 when the ZDCPs size is reduced from micrometers to ≤200 nm. The carbonization further shows a considerable effect on rate capacitance-ZDCPs of increased particle size lead to drastically reduced charge transportability and thus inhibit their performance for both the charge storage and the ORR. Guidelines for the capacitance variation with respect to the particle size and SSA in such carbon nanostructures from literature are presented.

A novel ammonium pentaborate - poly(ethylene-glycol) templated polymer-inclusion compound.

A new hydrogen-bonded supramolecular framework is reported, consisting of ammonium pentaborate, containing poly(ethylene-glycol) chains extending down tubular cavities in the structure. The crystal architecture is templated by the presence of the polyether chains, analogous to template synthesis of zeolites and metal organic frameworks. The ammonium pentaborate is formed by the thermolysis of ammonia borane, followed by hydrolysis of the dehydrogenation products by ambient water. This structure represents the first known example of a borate-based polymer inclusion compound.

Solvation of Na? in the Sodide Solution, LiNa?10MeNH2.

Alkalides, the alkali metals in their ?1 oxidation state, represent some of the largest and most polarizable atomic species in condensed phases. This study determines the solvation environment around the sodide anion, Na?, in a system of co-solvated Li+. We present isotopically varied total neutron scattering experiments alongside empirical potential structure refinement and ab initio molecular dynamics simulations for the alkali?alkalide system, LiNa?10MeNH2. Both local coordination modes and the intermediate range liquid structure are determined, which demonstrate that distinct structural correlations between cation and anion in the liquid phase extend beyond 8.6 ?. Indeed, the local solvation around Na? is surprisingly well defined with strong solvent orientational order, in contrast to the classical description of alkalide anions not interacting with their environment. The ion-paired Li(MeNH2)4+?Na? species appears to be the dominant alkali?alkalide environment in these liquids, whereby Li+ and Na? share a MeNH2 molecule through the amine group in their primary solvation spheres.

Production of phosphorene nanoribbons.

Phosphorene is a mono-elemental, two-dimensional (2D) substance with outstanding, highly directional properties and a bandgap that depends on the number of layers of the material1-8. Nanoribbons, meanwhile, combine the flexibility and unidirectional properties of one-dimensional nanomaterials, the high surface area of 2D nanomaterials and the electron-confinement and edge effects of both. The structures of nanoribbons can thus lead to exceptional control over electronic band structure, the emergence of novel phenomena and unique architectures for applications5,6,9-24. Phosphorene's intrinsically anisotropic structure has motivated numerous theoretical calculations of phosphorene nanoribbons (PNRs), predicting extraordinary properties5,6,12-24. So far, however, discrete PNRs have not been produced. Here we present a method for creating quantities of high-quality, individual PNRs by ionic scissoring of macroscopic black phosphorus crystals. This top-down process results in stable liquid dispersions of PNRs with typical widths of 4-50 nm, predominantly single-layer thickness, measured lengths of up to 75 µm and aspect ratios of up to 1,000. The nanoribbons are atomically flat single crystals, aligned exclusively in the zigzag crystallographic orientation. The ribbons have remarkably uniform widths along their entire lengths, and are extremely flexible. These properties-together with the ease of downstream manipulation via liquid-phase methods-should enable the search for predicted exotic states6,12-14,17-19,21, and an array of applications in which PNRs have been predicted to offer transformative advantages. These applications range from thermoelectric devices to high-capacity fast-charging batteries and integrated high-speed electronic circuits6,14-16,20,23,24.

Local Structure and Polar Order in Liquid N-Methyl-2-pyrrolidone (NMP).

N-Methyl-2-pyrrolidone (NMP) is an exceptional solvent, widely used in industry and for nanomaterials processing. Yet despite its ubiquity, its liquid structure, which ultimately dictates its solvation properties, is not fully known. Here, neutron scattering is used to determine NMP's structure in unprecedented detail. Two dominant nearest-neighbor arrangements are found, where rings are parallel or perpendicular. However, compared with related solvents, NMP has a relatively large population of parallel approaches, similar only to benzene, despite its nonaromaticity and the presence of the normally structure-reducing methyl group. This arrangement is underpinned by NMP's dipole moment, which has a profound effect on its structure: nearest-neighbor molecules arrange in an antiparallel but offset fashion. This polar-induced order extends beyond the first solvation shell, resulting in ordered trimers that reach the nanometer range. The degree of order and balance of interactions rationalize NMP's high boiling point and versatile capabilities to solvate both charged and uncharged species.

Charged Carbon Nanomaterials: Redox Chemistries of Fullerenes, Carbon Nanotubes, and Graphenes.

Since the discovery of buckminsterfullerene over 30 years ago, sp2-hybridised carbon nanomaterials (including fullerenes, carbon nanotubes, and graphene) have stimulated new science and technology across a huge range of fields. Despite the impressive intrinsic properties, challenges in processing and chemical modification continue to hinder applications. Charged carbon nanomaterials (CCNs), formed via the reduction or oxidation of these carbon nanomaterials, facilitate dissolution, purification, separation, chemical modification, and assembly. This approach provides a compelling alternative to traditional damaging and restrictive liquid phase exfoliation routes. The broad chemistry of CCNs not only provides a versatile and potent means to modify the properties of the parent nanomaterial but also raises interesting scientific issues. This review focuses on the fundamental structural forms: buckminsterfullerene, single-walled carbon nanotubes, and single-layer graphene, describing the generation of their respective charged nanocarbon species, their interactions with solvents, chemical reactivity, specific (opto)electronic properties, and emerging applications.

Dihydrogen vs. hydrogen bonding in the solvation of ammonia borane by tetrahydrofuran and liquid ammonia.

The solvation structures of two systems rich in hydrogen and dihydrogen bonding interactions have been studied in detail experimentally through neutron diffraction with hydrogen/deuterium isotopic substitution. The results were analysed by an atomistic Monte Carlo simulation employing refinement to the experimental scattering data. The systems studied were the hydrogen storage material ammonia borane (NH3BH3, AB) dissolved in tetrahydrofuran (THF), and liquid ammonia (NH3), the latter in which AB shows unusually high solubility (260 g AB per 100 g NH3) and potential regeneration properties. The full orientational and positional manner in which AB-AB, AB-THF and AB-NH3 pairs interact with each other were successfully deciphered from the wide Q-range total neutron scattering data. This provided an unprecedented level of detail into such highly (di)hydrogen bonding solute-solvent interactions. In particular this allowed insight into the way in which H-B acts as a hydrogen bond acceptor. The (di)hydrogen bonding was naturally determined to dictate the intermolecular interactions, at times negating the otherwise expected tendency for polar molecules to align themselves with anti-parallel dipole moments. Several causes for the extreme solubility of AB in ammonia were determined, including the ability of ammonia to (di)hydrogen bond to both ends of the AB molecule and the small size of the ammonia molecule relative to AB and THF. The AB B-H to ammonia H dihydrogen bond was found to dominate the intermolecular interactions, occurring almost three times more often than any other hydrogen or dihydrogen bond in the system. The favourability of this interaction was seen on the bulk scale by a large decrease in AB clustering in ammonia compared to in the dihydrogen bond-less THF.

Formation of Methane Hydrate in the Presence of Natural and Synthetic Nanoparticles.

Natural gas hydrates occur widely on the ocean-bed and in permafrost regions, and have potential as an untapped energy resource. Their formation and growth, however, poses major problems for the energy sector due to their tendency to block oil and gas pipelines, whereas their melting is viewed as a potential contributor to climate change. Although recent advances have been made in understanding bulk methane hydrate formation, the effect of impurity particles, which are always present under conditions relevant to industry and the environment, remains an open question. Here we present results from neutron scattering experiments and molecular dynamics simulations that show that the formation of methane hydrate is insensitive to the addition of a wide range of impurity particles. Our analysis shows that this is due to the different chemical natures of methane and water, with methane generally excluded from the volume surrounding the nanoparticles. This has important consequences for our understanding of the mechanism of hydrate nucleation and the design of new inhibitor molecules.

Switchable changes in the conductance of single-walled carbon nanotube networks on exposure to water vapour.

We have discovered that wrapping single-walled carbon nanotubes (SWCNTs) with ionic surfactants induces a switch in the conductance-humidity behaviour of SWCNT networks. Residual cationic vs. anionic surfactant induces a respective increase or decrease in the measured conductance across the SWCNT networks when exposed to water vapour. The magnitude of this effect was found to be dependent on the thickness of the deposited SWCNT films. Previously, chemical sensors, field effect transistors (FETs) and transparent conductive films (TCFs) have been fabricated from aqueous dispersions of surfactant functionalised SWCNTs. The results reported here confirm that the electrical properties of such components, based on randomly orientated SWCNT networks, can be significantly altered by the presence of surfactant in the SWCNT layer. A mechanism for the observed behaviour is proposed based on electrical measurements, Raman and UV-Vis-NIR spectroscopy. Additionally, the potential for manipulating the sensitivity of the surfactant functionalised SWCNTs to water vapour for atmospheric humidity sensing was evaluated. The study also presents a simple method to establish the effectiveness of surfactant removal techniques, and highlights the importance of characterising the electrical properties of SWCNT-based devices in both dry and humid operating environments for practical applications.

Chemical routes to discharging graphenides.

Chemical and electrochemical reduction methods allow the dispersion, processing, and/or functionalization of discrete sp2-hybridised nanocarbons, including fullerenes, nanotubes and graphenes. Electron transfer to the nanocarbon raises the Fermi energy, creating nanocarbon anions and thereby activating an array of possible covalent reactions. The Fermi level may then be partially or fully lowered by intended functionalization reactions, but in general, techniques are required to remove excess charge without inadvertent covalent reactions that potentially degrade the nanocarbon properties of interest. Here, simple and effective chemical discharging routes are demonstrated for graphenide polyelectrolytes and are expected to apply to other systems, particularly nanotubides. The discharging process is inherently linked to the reduction potentials of such chemical discharging agents and the unusual fundamental chemistry of charged nanocarbons.

Ionic solutions of two-dimensional materials.

Strategies for forming liquid dispersions of nanomaterials typically focus on retarding reaggregation, for example via surface modification, as opposed to promoting the thermodynamically driven dissolution common for molecule-sized species. Here we demonstrate the true dissolution of a wide range of important 2D nanomaterials by forming layered material salts that spontaneously dissolve in polar solvents yielding ionic solutions. The benign dissolution advantageously maintains the morphology of the starting material, is stable against reaggregation and can achieve solutions containing exclusively individualized monolayers. Importantly, the charge on the anionic nanosheet solutes is reversible, enables targeted deposition over large areas via electroplating and can initiate novel self-assembly upon drying. Our findings thus reveal a unique solution-like behaviour for 2D materials that enables their scalable production and controlled manipulation.

Electron Solvation and the Unique Liquid Structure of a Mixed-Amine Expanded Metal: The Saturated Li-NH3 -MeNH2 System.

Metal-amine solutions provide a unique arena in which to study electrons in solution, and to tune the electron density from the extremes of electrolytic through to true metallic behavior. The existence and structure of a new class of concentrated metal-amine liquid, Li-NH3 -MeNH2 , is presented in which the mixed solvent produces a novel type of electron solvation and delocalization that is fundamentally different from either of the constituent systems. NMR, ESR, and neutron diffraction allow the environment of the solvated electron and liquid structure to be precisely interrogated. Unexpectedly it was found that the solution is truly homogeneous and metallic. Equally surprising was the observation of strong longer-range order in this mixed solvent system. This is despite the heterogeneity of the cation solvation, and it is concluded that the solvated electron itself acts as a structural template. This is a quite remarkable observation, given that the liquid is metallic.

Controlling the Cross-Sensitivity of Carbon Nanotube-Based Gas Sensors to Water Using Zeolites.

Carbon nanotube-based gas sensors can be used to detect harmful environmental pollutants such as NO2 at room temperature. Although they show promise as low-powered, sensitive, and affordable monitoring devices, cross-sensitivity of functionalized carbon nanotubes to water vapor often obscures the detection of target molecules. This is a barrier to adoption for monitoring of airborne pollutants because of the varying humidity levels found in real world environments. Zeolites, also known as molecular sieves because of their selective adsorption properties, are used in this work to control the cross-sensitivity of single-walled carbon nanotube (SWCNT)-based sensors to water vapor. Zeolites incorporated into the sensing layer are found to reduce interference effects that would otherwise obscure the identification of NO2 gas, permitting repeatable detection over a range of relative humidities. This significant improvement is found to depend on the arrangement of the SWCNT-zeolite layers in the sensing device, as well as the hydrophilicity of the chosen zeolite.

Questioning Antiferromagnetic Ordering in the Expanded Metal, Li(NH3)4: A Lack of Evidence from µSR.

We present the results of a muon spin relaxation study of the solid phases of the expanded metal, Li(NH3)4. No discernible change in muon depolarization dynamics is witnessed in the lowest temperature phase (≤25 K) of Li(NH3)4, thus suggesting that the prevailing view of antiferromagnetic ordering is incorrect. This is consistent with the most recent neutron diffraction data. Discernible differences in muon behavior are reported for the highest temperature phase of Li(NH3)4 (82-89 K), attributed to the onset of structural dynamics prior to melting.

Probing the charging mechanisms of carbon nanomaterial polyelectrolytes.

Chemical charging of single-walled carbon nanotubes (SWCNTs) and graphenes to generate soluble salts shows great promise as a processing route for electronic applications, but raises fundamental questions. The reduction potentials of highly-charged nanocarbon polyelectrolyte ions were investigated by considering their chemical reactivity towards metal salts/complexes in forming metal nanoparticles. The redox activity, degree of functionalisation and charge utilisation were quantified via the relative metal nanoparticle content, established using thermogravimetric analysis (TGA), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS). The fundamental relationship between the intrinsic nanocarbon electronic density of states and Coulombic effects during charging is highlighted as an important area for future research.

Electrochemical processing of discrete single-walled carbon nanotube anions.

The dissolution of single-walled carbon nanotubes (SWCNTs) remains a fundamental challenge, reliant on aggressive chemistry or ultrasonication and lengthy ultracentrifugation. In contrast, simple nonaqueous electrochemical reduction leads to spontaneous dissolution of individualized SWCNTs from raw, unprocessed powders. The intrinsic electrochemical stability and conductivity of these nanomaterials allow their electrochemical dissolution from a pure SWCNT cathode to form solutions of individually separate and distinct (i.e., discrete) nanotube anions with varying charge density. The integrity of the SWCNT sp² framework during the charge/discharge process is demonstrated by optical spectroscopy data. Other than a reversible change in redox/solvation state, there is no obvious chemical functionalization of the structure, suggesting an analogy to conventional atomic electrochemical dissolution. The heterogeneity of as-synthesized SWCNT samples leads to the sequential dissolution of distinct fractions over time, with fine control over the electrochemical potential. Initial preferential dissolution of defective nanotubes and carbonaceous debris provides a simple, nondestructive means to purify raw materials without recourse to the usual, damaging, competitive oxidation reactions. Neutral SWCNTs can be recovered either by electroplating at an anode or by reaction with a suitable electrophile.