Engineering a Water-Dispersible, Conducting, Photoreduced Graphene Oxide

A critical limitation that has hampered widespread application of the electrically conducting reduced graphene oxide (<i>r</i>-GO) is its poor aqueous dispersibility. Here we outline a strategy to obtain water-dispersible conducting <i>r</i>-GO sheets, free of any stabilizing agents, by exploiting the fact that the kinetics of the photoreduction of the insulating GO is heterogeneous. We show that by controlling UV exposure times and pH, we can obtain <i>r</i>-GO sheets with the conducting sp²-graphitic domains restored but with the more acidic carboxylic groups, responsible for aqueous dispersibility, intact. The resultant photoreduced <i>r</i>-GO sheets are both conducting and water-dispersible

Covalently Linked, Water-Dispersible, Cyclodextrin: Reduced-Graphene Oxide Sheets

Reduced-graphene oxide (<i>r</i>GO) sheets have been functionalized by covalently linking β-cyclodextrin (β-CD) cavities to the sheets via an amide linkage. The functionalized β-CD:<i>r</i>GO sheets, in contrast to <i>r</i>GO, are dispersible over a wide range of pH values (2–13). Zeta potential measurements indicate that there is more than one factor responsible for the dispersibility. We show here that planar aromatic molecules adsorbed on the <i>r</i>GO sheet as well as nonplanar molecules included in the tethered β-CD cavities have their fluorescence effectively quenched by the β-CD:<i>r</i>GO sheets. The β-CD:<i>r</i>GO sheets combine the hydrophobicity associated with <i>r</i>GO along with the hydrophobicity of the cyclodextrin cavities in a single water-dispersible material

Accommodating Unwelcome Guests in Inorganic Layered Hosts: Inclusion of Chloranil in a Layered Double Hydroxide

The host–guest chemistry of most inorganic layered solids is limited to ion-exchange reactions. The guest species are either cations or anions to compensate for the charge deficit, either positive or negative, of the inorganic layers. Here, we outline a strategy to include neutral molecules like <i>ortho</i>- and <i>para</i>-chloranil, that are known to be good acceptors in donor–acceptor or charge-transfer complexes, within the galleries of a layered solid. We have succeeded in including neutral <i>ortho-</i> and <i>para</i>-chloranil molecules within the galleries of an Mg-Al layered double hydroxide (LDH) by using charge-transfer interactions with preintercalated <i>p</i>-aminobenzoate ions as the driving force. The <i>p</i>-aminobenzoate ions are introduced in the Mg-Al LDH via ion exchange. The intercalated LDH can adsorb <i>ortho-</i> and <i>para</i>-chloranil from chloroform solutions by forming charge-transfer complexes with the <i>p</i>-aminobenzoate anions present in the galleries. We use X-ray diffraction, spectroscopy, and molecular dynamics simulations to establish the nature of interactions and arrangement of the charge-transfer complex within the galleries of the layered double hydroxide

Graphene–Solvent Interactions in Nonaqueous Dispersions: 2D ROESY NMR Measurements and Molecular Dynamics Simulations

The liquid phase exfoliation of graphite by sonication in nonaqueous solvents like <i>N</i>-methyl-2-pyrrolidone (NMP) provides a simple and scalable route to dispersions of defect-free graphene sheets. The role of the solvent is crucial to the process; it is the interactions of the solvent with the graphene sheets that prevent agglomeration and stabilize the dispersion. Here, we show that the two-dimensional solution nuclear magnetic resonance (NMR) technique, rotating frame Overhauser effect spectroscopy (ROESY), provides a molecular signature of these interactions in graphene–NMP dispersions. Significant differences are observed in the spectra of the dispersions as compared to the pure solvent. Using classical molecular dynamics simulations, we show that these differences arise because of the induced layering of solvent molecules with reduced rotational mobility in the vicinity of the graphene sheets. The reduced mobility of solvent molecules in the dispersion as compared to the bulk solvent are reflected as differences in their two-dimensional ROESY NMR

Glass, Gel, and Liquid Crystals: Arrested States of Graphene Oxide Aqueous Dispersions

Colloidal systems with competing interactions are known to exhibit a range of dynamically arrested states because of the systems’ inability to reach its underlying equilibrium state due to intrinsic frustration. Graphene oxide (GO) aqueous dispersions constitute a class of 2D-anisotropic colloids with competing interactionslong-range electrostatic repulsion, originating from ionized groups located on the rim of the sheets, and weak dispersive attractive interactions originating from the unoxidized graphitic domains. We show here that aqueous dispersions of GO exhibit a range of arrested states, encompassing fluid, glass, and gels that coexist with liquid-crystalline order with increasing volume fraction. These states can be accessed by varying the relative magnitudes of the repulsive and attractive forces. This can be realized by changing the ionic strength of the medium. We observe at low salt concentrations, where long-range electrostatic repulsion dominates, the formation of a repulsive Wigner glass, while at high salt concentrations, when attractive forces dominate, the formation of gels exhibits a nematic to columnar liquid-crystalline transition. The present work highlights how the chemical structure of GOhydrophilic ionizable groups and hydrophobic graphitic domains coexisting on a single sheetgives rise to a rich and complex array of arrested states

Understanding Aqueous Dispersibility of Boron Nitride Nanosheets from ¹H Solid State NMR and Reactive Molecular Dynamics

Stable aqueous dispersions of BN nanosheets can be obtained by the sonication assisted exfoliation of hexagonal BN powders in water without the need of stabilizers: a phenomenon not observed for the isoelectronic graphite, of comparable hydrophobicity. We show here that the aqueous dispersions are stabilized by electrostatic repulsive interactions and establish from ζ potential measurements that the BN nanosheets are positively charged on delamination with the medium turning increasingly basic as sonication proceeds. We have investigated how charge develops on the sheets by reactive force-field (ReaxFF) molecular dynamics simulations of the interaction of water with BN nanosheets and independently identified the major chemical species present on the nanosheets from ¹H and ¹¹B solid-state NMR measurements. Charges develop on the sheets as a consequence of the dissociation of water molecules at the edges of the sheet that leave the nitrogen edge atoms protonated and the release of hydroxyl groups into the bulk leading to an increased basicity of the medium

Probing Graphene–Surfactant Interactions in Aqueous Dispersions with Nuclear Overhauser Effect NMR Spectroscopy and Molecular Dynamics Simulations

Sonication-assisted exfoliation of graphite in aqueous solutions of ionic surfactants is an attractive, scalable route for the production of defect-free graphene. The interaction of surfactant chains with graphene is crucial both to the process and to the stability of the dispersion. We use ¹H nuclear Overhauser effect (NOE) NMR techniques and classical molecular dynamics (MD) simulations to probe the molecular nature of these interactions in graphene dispersions stabilized by the cationic surfactant cetyltrimethylammonium bromide. We show that the surfactant chains are quasi-bound to the graphene sheets undergoing rapid exchange with the free surfactant ligands in the bulk, but what is surprising is the observation of NOE interactions between groups that are separated by more than 5 Å along the chain. MD simulations provide the key to interpreting these observations; these interactions are a consequence of the arrangement of the quasi-bound surfactant chains that allows segments of different chains to come into spatial proximity on the graphene sheet

Cyclodextrin-Functionalized Fe₃O₄@TiO₂: Reusable, Magnetic Nanoparticles for Photocatalytic Degradation of Endocrine-Disrupting Chemicals in Water Supplies

Water-dispersible, photocatalytic Fe₃O₄@TiO₂ core–shell magnetic nanoparticles have been prepared by anchoring cyclodextrin cavities to the TiO₂ shell, and their ability to capture and photocatalytically destroy endocrine-disrupting chemicals, bisphenol A and dibutyl phthalate, present in water, has been demonstrated. The functionalized nanoparticles can be magnetically separated from the dispersion after photocatalysis and hence reused. Each component of the cyclodextrin-functionalized Fe₃O₄@TiO₂ core–shell nanoparticle has a crucial role in its functioning. The tethered cyclodextrins are responsible for the aqueous dispersibility of the nanoparticles and their hydrophobic cavities for the capture of the organic pollutants that may be present in water samples. The amorphous TiO₂ shell is the photocatalyst for the degradation and mineralization of the organics, bisphenol A and dibutyl phthalate, under UV illumination, and the magnetism associated with the 9 nm crystalline Fe₃O₄ core allows for the magnetic separation from the dispersion once photocatalytic degradation is complete. An attractive feature of these “capture and destroy” nanomaterials is that they may be completely removed from the dispersion and reused with little or no loss of catalytic activity

Understanding Aqueous Dispersibility of Graphene Oxide and Reduced Graphene Oxide through p<i>K</i>_a Measurements

The chemistry underlying the aqueous dispersibility of graphene oxide (GO) and reduced graphene oxide (r-GO) is a key consideration in the design of solution processing techniques for the preparation of processable graphene sheets. Here, we use zeta potential measurements, pH titrations, and infrared spectroscopy to establish the chemistry underlying the aqueous dispersibility of GO and r-GO sheets at different values of pH. We show that r-GO sheets have ionizable groups with a single p<i>K</i> value (8.0) while GO sheets have groups that are more acidic (p<i>K</i> = 4.3), in addition to groups with p<i>K</i> values of 6.6 and 9.0. Infrared spectroscopy has been used to follow the sequence of ionization events. In both GO and r-GO sheets, it is ionization of the carboxylic groups that is primarily responsible for the build up of charge, but on GO sheets, the presence of phenolic and hydroxyl groups in close proximity to the carboxylic groups lowers the p<i>K</i>_a value by stabilizing the carboxylate anion, resulting in superior water dispersibility

Conformation of Ethylene Glycol in the Liquid State: Intra- versus Intermolecular Interactions

Ethylene glycol is a typical rotor molecule with the three dihedral angles that allow for a number of possible conformers. The geometry of the molecule in the liquid state brings into sharp focus the competition between intra- and inter-molecular interactions in deciding conformation. Here, we report a conformational analysis of ethylene glycol in the liquid state from ab initio molecular dynamics simulations. Our results highlight the importance of intermolecular hydrogen bonding over intramolecular interactions in the liquid, with the central OCCO linkage adopting both gauche and trans geometries in contrast to the gas phase, wherein only the gauche has been reported. The influence of intermolecular interactions on the conformation of the terminal CCOH moieties is even more striking, with certain regions of conformational space, wherein the ethylene glycol molecule cannot participate with its full complement of intermolecular hydrogen bonds, excluded. The results are in agreement with Raman and NMR spectroscopic studies of liquid ethylene glycol, but at the same time they are able to provide new insights into how intermolecular interactions favor certain conformations while excluding others