Formation of giant solvation shells around fulleride anions in liquid ammonia

The local ordering of polar solvents around crystalline carbon nitride nanosheets in solution

The crystalline graphitic carbon nitride, poly-triazine imide (PTI) is highly unusual among layered materials since it is spontaneously soluble in aprotic, polar solvents including dimethylformamide (DMF). The PTI material consists of layers of carbon nitride intercalated with LiBr. When dissolved, the resulting solutions consist of dissolved, luminescent single to multilayer nanosheets of around 60-125 nm in diameter and Li+ and Br- ions originating from the intercalating salt. To understand this unique solubility, the structure of these solutions has been investigated by high-energy X-ray and neutron diffraction. Although the diffraction patterns are dominated by inter-solvent correlations there are clear differences between the X-ray diffraction data of the PTI solution and the solvent in the 4-6 Å-1 range, with real space differences persisting to at least 10 Å. Structural modelling using both neutron and X-ray datasets as a constraint reveal the formation of distinct, dense solvation shells surrounding the nanoparticles with a layer of Br-close to the PTI-solvent interface. This solvent ordering provides a configuration that is energetically favourable underpinning thermodynamically driven PTI dissolution. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Synthesis of pyrrolo[3,4-c]carbazole-1,3(2H,6H)-diones via addition/cyclization reactions between 3-chloro-4-indolylmaleimides and alkynes

3-Chloro-4-indolylmaleimides and two different alkynes are used as the starting materials in a novel and highly effective Pd-catalyzed addition/C–H activation/cyclization sequence for the synthesis of pyrrolo[3,4- c]carbazole-1,3(2 H,6 H)-diones. The desired products are obtained in moderate to excellent yields. Such compounds show a wide range of biological activities.

Adsorption of common solvent molecules on graphene and MoS2 from first-principles

Solvents are an essential element in the production and processing of two-dimensional (2D) materials. For example, the liquid-phase exfoliation of layered materials requires a solvent to prevent the resulting monolayers from re-aggregating, while solutions of functional atoms and molecules are routinely used to modify the properties of the layers. It is generally assumed that these solvents do not interact strongly with the layer and so their effects can be neglected. Yet experimental evidence has suggested that explicit atomic-scale interactions between the solvent and layered material may play a crucial role in exfoliation and cause unintended electronic changes in the layer. Little is known about the precise nature of the interaction between the solvent molecules and the 2D layer. Here, we use density functional theory calculations to determine the adsorption configuration and binding energy of a variety of common solvent molecules, both polar and non-polar, on two of the most popular 2D materials, namely, graphene and MoS₂. We show that these molecules are physisorbed on the surface with negligible charge transferred between them. We find that the adsorption strength of the different molecules is independent of the polar nature of the solvent. However, we show that the molecules induce a significant charge rearrangement at the interface after adsorption as a result of polar bonds in the molecule.

Single Crystal, Luminescent Carbon Nitride Nanosheets Formed by Spontaneous Dissolution

A primary method for the production of 2D nanosheets is liquid-phase delamination from their 3D layered bulk analogues. Most strategies currently achieve this objective by significant mechanical energy input or chemical modification but these processes are detrimental to the structure and properties of the resulting 2D nanomaterials. Bulk poly(triazine imide) (PTI)-based carbon nitrides are layered materials with a high degree of crystalline order. Here, we demonstrate that these semiconductors are spontaneously soluble in select polar aprotic solvents, that is, without any chemical or physical intervention. In contrast to more aggressive exfoliation strategies, this thermodynamically driven dissolution process perfectly maintains the crystallographic form of the starting material, yielding solutions of defect-free, hexagonal 2D nanosheets with a well-defined size distribution. This pristine nanosheet structure results in narrow, excitation-wavelength-independent photoluminescence emission spectra. Furthermore, by controlling the aggregation state of the nanosheets, we demonstrate that the emission wavelengths can be tuned from narrow UV to broad-band white. This has potential applicability to a range of optoelectronic devices.

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.

Dynamic solvation shell and solubility of C60 in organic solvents

The notion of (static) solvation shells has recently proved fruitful in revealing key molecular factors that dictate the solubility and aggregation properties of fullerene species in polar or ionic solvent media. Using molecular dynamics schemes with carefully evaluated force fields, we have scrutinized both the static and the dynamic features of the solvation shells of single C60 particle for three nonpolar organic solvents (i.e., chloroform, toluene, and chlorobenzene) and a range of system temperatures (i.e., T = 250-330 K). The central findings have been that, while the static structures of the solvation shell remain, in general, insensitive to the effects of changing solvent type or system temperature, the dynamic behavior of solvent molecules within the shell exhibits prominent dependence on both factors. Detailed analyses led us to propose the notion of dynamically stable solvation shell, effectiveness of which can be characterized by a new physical parameter defined as the ratio of two fundamental time constants representing, respectively, the solvent relaxation (or residence) time within the first solvation shell and the characteristic time required for the fullerene particle to diffuse a distance comparable to the shell thickness. We show that, for the five (two from the literature) different solvent media and the range of system temperatures examined herein, this parameter bears a value around unity and, in particular, correlates intimately with known trends of solubility for C60 solutions. We also provide evidence revealing that, in addition to fullerene-solvent interactions, solvent-solvent interactions play an important role, too, in shaping the dynamic solvation shell, as implied by recent experimental trends.

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.

Scalable method for the reductive dissolution, purification, and separation of single-walled carbon nanotubes

As synthesized, bulk single-walled carbon nanotube (SWNT) samples are typically highly agglomerated and heterogeneous. However, their most promising applications require the isolation of individualized, purified nanotubes, often with specific optoelectronic characteristics. A wide range of dispersion and separation techniques have been developed, but the use of sonication or ultracentrifugation imposes severe limits on scalability and may introduce damage. Here, we demonstrate a new, intrinsically scalable method for SWNT dispersion and separation, using reductive treatment in sodium metal-ammonia solutions, optionally followed by selective dissolution in a polar aprotic organic solvent. In situ small-angle neutron scattering demonstrates the presence of dissolved, unbundled SWNTs in solution, at concentrations reaching at least 2 mg/mL; the ability to isolate individual nanotubes is confirmed by atomic force microscopy. Spectroscopy data suggest that the soluble fraction contains predominately large metallic nanotubes; a potential new mechanism for nanotube separation is proposed. In addition, the G/D ratios observed during the dissolution sequence, as a function of metal:carbon ratio, demonstrate a new purification method for removing carbonaceous impurities from pristine SWNTs, which avoids traditional, damaging, competitive oxidation reactions.

How hydrophobic buckminsterfullerene affects surrounding water structure

The hydrophobic hydration of fullerenes in water is of significant interest as the most common Buckminsterfullerene (C60) is a mesoscale sphere; C60 also has potential in pharmaceutical and nanomaterial applications. We use an all-atom molecular dynamics simulation lasting hundreds of nanoseconds to determine the behavior of a single molecule of C60 in a periodic box of water, and compare this to methane. A C60 molecule does not induce drying at the surface; however, unlike a hard sphere methane, a hard sphere C60 solute does. This is due to a larger number of attractive Lennard-Jones interactions between the carbon atom centers in C60 and the surrounding waters. In these simulations, water is not uniformly arranged but rather adopts a range of orientations in the first hydration shell despite the spherical symmetry of both solutes. There is a clear effect of solute size on the orientation of the first hydration shell waters. There is a large increase in hydrogen-bonding contacts between waters in the C60 first hydration shell. There is also a disruption of hydrogen bonds between waters in the first and second hydration shells. Water molecules in the first hydration shell preferentially create triangular structures that minimize the net water dipole near the surface near both the methane and C60 surface, reducing the total energy of the system. Additionally, in the first and second hydration shells, the water dipoles are ordered to a distance of 8 A from the solute surface. We conclude that, with a diameter of approximately 1 nm, C60 behaves as a large hydrophobic solute.