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

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

Reduced-graphene oxide (rGO) sheets have been functionalized by covalently linking beta-cyclodextrin (beta CD) cavities to the sheets via an amide linkage. The functionalized beta-CD:rGO sheets, in contrast to rGO, 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 rGO sheet as well as nonplanar molecules included in the tethered beta-CD cavities have their fluorescence effectively quenched by the beta-CD:rGO sheets. The beta-CD:rGO sheets combine the hydrophobicity associated with rGO along with the hydrophobicity of the cyclodextrin cavities in a single water-dispersible material

Understanding Aqueous Dispersibility of Graphene Oxide and Reduced Graphene Oxide through pK(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 pK value (8.0) while GO sheets have groups that are more acidic (pK = 4.3), in addition to groups with pK 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 pK(a) value by stabilizing the carboxylate anion, resulting in superior water dispersibility

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

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

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

Spectral Migration of Fluorescence in Graphene Oxide Aqueous Dispersions: Evidence for Excited-State Proton Transfer

Aqueous dispersions of graphene oxide (GO) exhibit strong pH-dependent fluorescence in the visible that originates, in part, from the oxygenated functionalities present. Here we examine the spectral migration on nanosecond time-scales of the pH dependent features in the fluorescence spectra. We show, from time-resolved emission spectra (TRES) constructed from the wavelength dependent fluorescence decay curves, that the migration is associated with excited state proton transfer. Both ‘intramolecular’ and ‘intermolecular’ transfer involving the quasi-molecular oxygenated aromatic fragments are observed. As a prerequisite to the time-resolved measurements, we have correlated the changes in the steady state fluorescence spectra with the sequence of dissociation events that occur in GO dispersions at different values of pH

Resonance Raman Detection and Estimation in the Aqueous Phase Using Water Dispersible Cyclodextrin: Reduced-Graphene Oxide Sheets

Resonance Raman spectroscopy is a powerful analytical tool for detecting and identifying analytes, but the associated strong fluorescence background severely limits the use of the technique. Here, we show that by attaching β-cyclodextrin (β-CD) cavities to reduced graphene-oxide (<i>r</i>GO) sheets we obtain a water dispersible material (β-CD: <i>r</i>GO) that combines the hydrophobicity associated with <i>r</i>GO with that of the cyclodextrin cavities and provides a versatile platform for resonance Raman detection. Planar aromatic and dye molecules that adsorb on the <i>r</i>GO domains and nonplanar molecules included within the tethered β-CD cavities have their fluorescence effectively quenched. We show that it is possible using the water dispersible β-CD: <i>r</i>GO sheets to record the resonance Raman spectra of adsorbed and included organic chromophores directly in aqueous media without having to extract or deposit on a substrate. This is significant, as it allows us to identify and estimate organic analytes present in water by resonance Raman spectroscopy

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

Co<SUB>3</SUB>O<SUB>4</SUB>@Co/NCNT nanostructure derived from a dicyanamide-based metal-organic framework as an efficient bi-functional electrocatalyst for oxygen reduction and evolution reactions

There has been growing interest in the synthesis of efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), for their potential use in a variety of renewable energy technologies, such as regenerative fuel cells and metal-air batteries. Here, a bi-functional electrocatalyst, derived from a novel dicyanamide based nitrogen rich MOF {[Co(bpe)<SUB>2</SUB>(N(CN)<SUB>2</SUB>)]⋅(N(CN)<SUB>2</SUB>)⋅(5 H<SUB>2</SUB>O)}<SUB>n</SUB> [Co-MOF-1, bpe=1,2-bis(4-pyridyl)ethane, N(CN)2<SUP>−</SUP>=dicyanamide] under different pyrolysis conditions is reported. Pyrolysis of the Co-MOF-1 under Ar atmosphere (at 800 &#x00B0;C) yielded a Co nanoparticle-embedded N-doped carbon nanotube matrix (Co/NCNT-Ar) while pyrolysis under a reductive H<SUB>2</SUB>/Ar atmosphere (at 800 &#x00B0;C) and further mild calcination yielded Co<SUB>3</SUB>O<SUB>4</SUB>@Co core–shell nanoparticle-encapsulated N-doped carbon nanotubes (Co<SUB>3</SUB>O<SUB>4</SUB>@Co/NCNT). Both catalysts show bi-functional activity towards ORR and OER, however, the core–shell Co<SUB>3</SUB>O<SUB>4</SUB>@Co/NCNT nanostructure exhibited superior electrocatalytic activity for both the ORR with a potential of 0.88 V at a current density of −1 mA cm<SUP>−2</SUP> and the OER with a potential of 1.61 V at 10 mA cm<SUP>−2</SUP> , which is competitive with the most active bi-functional catalysts reported previously