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

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

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

Cobalt oxide 2D nanosheets formed at a polarized liquid|liquid interface toward high-performance Li-ion and Na-ion battery anodes

Cobalt oxide (Co3O4)-based nanostructures have the potential as low-cost materials for lithium-ion (Li-ion) and sodium-ion (Na-ion) battery anodes with a theoretical capacity of 890 mAh/g. Here, we demonstrate a novel method for the production of Co3O4 nanoplatelets. This involves the growth of flower-like cobalt oxyhydroxide (CoOOH) nanostructures at a polarized liquid|liquid interface, followed by conversion to flower-like Co3O4 via calcination. Finally, sonication is used to break up the flower-like Co3O4 nanostructures into two-dimensional (2D) nanoplatelets with lateral sizes of 20−100 nm. Nanoplatelets of Co3O4 can be easily mixed with carbon nanotubes to create nanocomposite anodes, which can be used for Li-ion and Na-ion battery anodes without any additional binder or conductive additive. The resultant electrodes display impressive low-rate capacities (at 125 mA/g) of 1108 and 1083 mAh/g, for Li-ion and Na-ion anodes, respectively, and stable cycling ability over >200 cycles. Detailed quantitative rate analysis clearly shows that Li-ion-storing anodes charge roughly five times faster than Na-ion-storing anodes</p

Metallic NiPS₃@NiOOH Core–Shell Heterostructures as Highly Efficient and Stable Electrocatalyst for the Oxygen Evolution Reaction

We report metallic NiPS₃@NiOOH core–shell heterostructures as an efficient and durable electrocatalyst for the oxygen evolution reaction, exhibiting a low onset potential of 1.48 V (vs RHE) and stable performance for over 160 h. The atomically thin NiPS₃ nanosheets are obtained by exfoliation of bulk NiPS₃ in the presence of an ionic surfactant. The OER mechanism was studied by a combination of SECM, in situ Raman spectroscopy, SEM, and XPS measurements, which enabled direct observation of the formation of a NiPS₃@NiOOH core–shell heterostructure at the electrode interface. Hence, the active form of the catalyst is represented as NiPS₃@NiOOH core–shell structure. Moreover, DFT calculations indicate an intrinsic metallic character of the NiPS₃ nanosheets with densities of states (DOS) similar to the bulk material. The high OER activity of the NiPS₃ nanosheets is attributed to a high density of accessible active metallic-edge and defect sites due to structural disorder, a unique NiPS₃@NiOOH core–shell heterostructure, where the presence of P and S modulates the surface electronic structure of Ni in NiPS₃, thus providing excellent conductive pathway for efficient electron-transport to the NiOOH shell. These findings suggest that good size control during liquid exfoliation may be advantageously used for the formation of electrically conductive NiPS₃@NiOOH core–shell electrode materials for the electrochemical water oxidation

Liquid processing of interfacially grown iron-oxide flowers into 2D-platelets yields lithium-ion battery anodes with capacities of twice the theoretical value

Iron oxide (Fe2O3) is an abundant and potentially low-cost material for fabricating lithium-ion battery anodes. Here, the growth of α-Fe2O3 nano-flowers at an electrified liquid–liquid interface is demonstrated. Sonication is used to convert these flowers into quasi-2D platelets with lateral sizes in the range of hundreds of nanometers and thicknesses in the range of tens of nanometers. These nanoplatelets can be combined with carbon nanotubes to form porous, conductive composites which can be used as electrodes in lithium-ion batteries. Using a standard activation process, these anodes display good cycling stability, reasonable rate performance and low-rate capacities approaching 1500 mAh g−1, consistent with the current state-of-the-art for Fe2O3. However, by using an extended activation process, it is found that the morphology of these composites can be significantly changed, rendering the iron oxide amorphous and significantly increasing the porosity and internal surface area. These morphological changes yield anodes with very good cycling stability and low-rate capacity exceeding 2000 mAh g−1, which is competitive with the best anode materials in the literature. However, the data implies that, after activation, the iron oxide displays a reduced solid-state lithium-ion diffusion coefficient resulting in somewhat degraded rate performance.</p

Influence of the Fe:Ni Ratio and Reaction Temperature on the Efficiency of (Fe_<i>x</i>Ni_1–<i>x</i>)₉S₈ Electrocatalysts Applied in the Hydrogen Evolution Reaction

Inspired by our recent finding that Fe_4.5Ni_4.5S₈ rock is a highly active electrocatalyst for HER, we set out to explore the influence of the Fe:Ni ratio on the performance of the catalyst. We herein describe the synthesis of (Fe_<i>x</i>Ni_1–<i>x</i>)₉S₈ (<i>x</i> = 0–1) along with a detailed elemental composition analysis. Furthermore, using linear sweep voltammetry, we show that the increase in the iron or nickel content, respectively, lowers the activity of the electrocatalyst toward HER. Electrochemical surface area analysis (ECSA) clearly indicates the highest amount of active sites for a Fe:Ni ratio of 1:1 on the electrode surface pointing at an altered surface composition of iron and nickel for the other materials. Specific metal–metal interactions seem to be of key importance for the high electrocatalytic HER activity, which is supported by DFT calculations of several surface structures using the surface energy as a descriptor of catalytic activity. In addition, we show that a temperature increase leads to a significant decrease of the overpotential and gain in HER activity. Thus, we showcase the necessity to investigate the material structure, composition and reaction conditions when evaluating electrocatalysts