Oscillating Structural Transformations in the Electrochemical Synthesis of Graphene Oxide from Graphite.

Electrochemical synthesis of graphene oxide (GO) is known to occur with potential oscillations, but the structural changes underlying these oscillations have remained unclear. In situ time-resolved synchrotron radiation X-ray diffraction demonstrates that the electrochemical synthesis of GO in aqueous H2SO4 can be described as an oscillating reaction. The transformation from graphite to GO proceeds through periodic structural oscillations that correlate with potential cycles. Stage-1 graphite intercalation compound (GIC) is found only at the peak of the potential cycle, but not at the bottom of the cycle. Stage-1 GIC is formed in the first half-cycle from stage-2 GIC and then transforms into "pristine graphite oxide" (PGO) on the lower side of the potential cycle, after which the cycle restarts with the formation of a new portion of stage-1 GIC. Water-washing results in the transformation of PGO into water-swollen GO with d(001) ~11 Å. These periodic structural changes can be considered a new type of oscillating reaction. The presented results provide broad insights into the oscillating structural changes occurring during the anodic graphite oxidation in aqueous H2SO4 and allow for an update of the mechanism of GO electrochemical formation.

Critical Role of Functional Groups Containing N, S, and O on Graphene Surface for Stable and Fast Charging Li-S Batteries.

Lithium-sulfur (Li-S) batteries are considered one of the most promising energy storage technologies, possibly replacing the state-of-the-art lithium-ion (Li-ion) batteries owing to their high energy density, low cost, and eco-compatibility. However, the migration of high-order lithium polysulfides (LiPs) to the lithium surface and the sluggish electrochemical kinetics pose challenges to their commercialization. The interactions between the cathode and LiPs can be enhanced by the doping of the carbon host with heteroatoms, however with relatively low doping content (<10%) in the bulk of the carbon, which can hardly interact with LiPs at the host surface. In this study, the grafting of versatile functional groups with designable properties (e.g., catalytic effects) directly on the surface of the carbon host is proposed to enhance interactions with LiPs. As model systems, benzene groups containing N/O and S/O atoms are vertically grafted and uniformly distributed on the surface of expanded reduced graphene oxide, fostering a stable interface between the cathode and LiPs. The combination of experiments and density functional theory calculations demonstrate improvements in chemical interactions between graphene and LiPs, with an enhancement in the electrochemical kinetics, power, and energy densities.

Carboxyl groups do not play the major role in binding metal cations by graphene oxide.

In this study, we investigate the chemical interactions of Mn2+ ions with graphene oxides, prepared by Hummers' (HGO) and Brodie's (BGO) methods in aqueous solutions by means of NMR relaxation. Carboxyl groups, which are always present in HGO in significant quantities, are often considered as the main binding sites for metal ions. Here we demonstrate that metal ions are bound efficiently by BGO, containing a negligibly small quantity of carboxyl groups. The difference in the shape of the relaxation curves is due mostly to the difference in the solubility and exfoliation degree of the two GO samples in aqueous media. HGO binds Mn2+ in the broad pH range, including highly acidic solutions, while BGO binds only at pH > 6, since it is not dispersible in water at lower pH values. The ability of BGO to chemically bind Mn2+ despite lacking sulfate and carboxyl groups, coupled with our earlier published findings, strongly suggests that carboxyl groups do not play the main role in binding metal ions by GO, as is commonly believed. We propose that metal ions initiate a significant transformation in the GO structure to attain the most efficient coordination of metal ions. This reorganization might involve the metal cation induced C-C bond cleavage with the formation of enols at the newly formed edges.

Acetylation of graphite oxide.

Unlike many methods of chemical modification of Graphite Oxide (GO) reported during 1930-1960 and re-studied in much detail over the last decade, acetylation somehow escaped attention and remained almost completely unexplored. Acetylated Graphite Oxide (AcGO) was prepared using a reaction with acetic anhydride. Successful acetylation is evidenced by an increase in the average interlayer distance from d(001) = 7.8 Å in the precursor GO to 10 Å in AcGO. The amount of oxygen in AcGO significantly decreased compared to the precursor GO (C/O = 2.2), reflecting partial reduction of GO in the process of acetylation and resulting in a scarcely functionalized material with C/O = 6.2. A theoretical model of the complete acetylation of GO results in a non-porous close packed molecular structure with an interlayer distance of ∼10 Å, in good agreement with experiment. Remarkably, AcGO shows significant swelling despite the oxidation degree being comparable to that of reduced GO, which does not swell in polar solvents. Moreover, AcGO shows swelling in acetonitrile similar to that of the precursor GO but not in water, thus providing an example of selectivity in the sorption of common polar solvents. The low oxidation degree combined with selective swelling properties makes AcGO a promising material for membrane applications.

Covalent Organic Framework (COF-1) under High Pressure.

COF-1 has a structure with rigid 2D layers composed of benzene and B3 O3 rings and weak van der Waals bonding between the layers. The as-synthesized COF-1 structure contains pores occupied by solvent molecules. A high surface area empty-pore structure is obtained after vacuum annealing. High-pressure XRD and Raman experiments with mesitylene-filled (COF-1-M) and empty-pore COF-1 demonstrate partial amorphization and collapse of the framework structure above 12-15 GPa. The ambient pressure structure of COF-1-M can be reversibly recovered after compression up to 10-15 GPa. Remarkable stability of highly porous COF-1 structure at pressures at least up to 10 GPa is found even for the empty-pore structure. The bulk modulus of the COF-1 structure (11.2(5) GPa) and linear incompressibilities (k[100] =111(5) GPa, k[001] =15.0(5) GPa) were evaluated from the analysis of XRD data and cross-checked against first-principles calculations.

Activated graphene as a material for supercapacitor electrodes: effects of surface area, pore size distribution and hydrophilicity.

Activated reduced graphene oxide (a-rGO) is a material with a rigid 3D porous structure and high specific surface area (SSA). Using variation of activation parameters and post-synthesis mechanical treatment we prepared two sets of materials with a broad range of BET (N2) SSA ∼1000-3000 m2 g-1, and significant differences in pore size distribution and oxygen content. The performance of activated graphene as an electrode in a supercapacitor with KOH electrolyte was correlated with the structural parameters of the materials and water sorption properties. a-rGO is a hydrophobic material as evidenced by the negligibly small BET (H2O) SSA determined using analysis of water vapor sorption isotherms. However, the total pore volume determined using water vapor sorption and sorption of liquid water is almost the same as the one found by analysis of nitrogen sorption isotherms. Ball milling is found to provide an improved bulk density of activated graphene and collapse of all pores except the smallest ones (<2 nm). A decrease in the activation temperature from 850 °C to 550 °C is found to result in materials with a narrow micropore size distribution and increased oxygen content. Elimination of mesopores using ball milling or a lower activation temperature provided materials with better specific capacitance despite a significant decrease (by ∼30%) of the BET (N2) SSA. The best gravimetric and volumetric capacitances in KOH electrolyte were achieved not for samples with the highest value of the BET (N2) SSA but for materials with 80-90% of the total pore volume in micropores and an increased BET (H2O) SSA. Comparing the performance of electrodes prepared using rGO and a-rGO shows that a more hydrophilic surface is favorable for charge storage in supercapacitors with KOH electrolyte.

Gravimetric tank method to evaluate material-enhanced hydrogen storage by physisorbing materials.

The most common methods to evaluate hydrogen sorption (volumetric and gravimetric) require significant experience and expensive equipment for providing reproducible results. Both methods allow one to measure excess uptake values which are used to calculate the total amount of hydrogen stored inside of a tank as required for applications. Here we propose an easy to use and inexpensive alternative approach which allows one to evaluate directly the weight of hydrogen inside a material-filled test tank. The weight of the same tank filled with compressed hydrogen in the absence of loaded material is used as a reference. We argue that the only parameter which is of importance for hydrogen storage applications is by how much the material improves the total weight of hydrogen inside of the given volume compared to compressed gas. This parameter which we propose to name Gain includes both volumetric and gravimetric characterization of the material; it can be determined directly without knowing the skeletal volume of the material or excess sorption. The feasibility of the Gravimetric Tank (GT) method was tested using several common carbon and Metal Organic Framework (MOF) materials. The best Gain value of ∼12% was found for the Cu-BTC MOF which means that the tank completely filled with this material stores a 12% higher amount of hydrogen compared to H2 gas at the same P-T conditions. The advantages of the GT method are its inexpensive design, extremely simple procedures and direct results in terms of tank capacity as required for industrial applications. The GT method could be proposed as a standard check for verification of the high hydrogen storage capacity of new materials. The GT method is expected to provide even better accuracy for evaluation of a material's performance for storage of denser gases like e.g. CO2 and CH4.

A Molecular Pillar Approach To Grow Vertical Covalent Organic Framework Nanosheets on Graphene: Hybrid Materials for Energy Storage.

Hybrid 2D-2D materials composed of perpendicularly oriented covalent organic frameworks (COFs) and graphene were prepared and tested for energy storage applications. Diboronic acid molecules covalently attached to graphene oxide (GO) were used as nucleation sites for directing vertical growth of COF-1 nanosheets (v-COF-GO). The hybrid material has a forest of COF-1 nanosheets with a thickness of 3 to 15 nm in edge-on orientation relative to GO. The reaction performed without molecular pillars resulted in uncontrollable growth of thick COF-1 platelets parallel to the surface of GO. The v-COF-GO was converted into a conductive carbon material preserving the nanostructure of precursor with ultrathin porous carbon nanosheets grafted to graphene in edge-on orientation. It was demonstrated as a high-performance electrode material for supercapacitors. The molecular pillar approach can be used for preparation of many other 2D-2D materials with control of their relative orientation.

Hydration of bilayered graphene oxide.

The hydration of graphene oxide (GO) membranes is the key to understand their remarkable selectivity in permeation of water molecules and humidity-dependent gas separation. We investigated the hydration of single GO layers as a function of humidity using scanning force microscopy, and we determined the single interlayer distance from the step height of a single GO layer on top of one or two GO layers. This interlayer distance grows gradually by approximately 1 Å upon a relative humidity (RH) increase in the range of 2 to ∼80%, and the immersion into liquid water increases the interlayer distance further by another 3 Å. The gradual expansion of the single interlayer distance is in good agreement with the averaged distance measured by X-ray diffraction on multilayered graphite oxides, which is commonly explained with an interstratification model. However, our experimental design excludes effects connected to interstratification. Instead we determine directly if insertion of water into GO occurs strictly by monolayers or the thickness of GO layers changes gradually. We find that hydration with up to 80% RH is a continuous process of incorporation of water molecules into single GO layers, while liquid water inserts as monolayers. The similarity of hydration for our bilayer and previously reported multilayered materials implies GO few and even bilayers to be suitable for selective water transport.

Coronene encapsulation in single-walled carbon nanotubes: stacked columns, peapods, and nanoribbons.

Encapsulation of coronene inside single-walled carbon nanotubes (SWNTs) was studied under various conditions. Under high vacuum, two main types of molecular encapsulation were observed by using transmission electron microscopy: coronene dimers and molecular stacking columns perpendicular or tilted (45-60°) with regard to the axis of the SWNTs. A relatively small number of short nanoribbons or polymerized coronene molecular chains were observed. However, experiments performed under an argon atmosphere (0.17 MPa) revealed reactions between the coronene molecules and the formation of hydrogen-terminated graphene nanoribbons. It was also observed that the morphology of the encapsulated products depend on the diameter of the SWNTs. The experimental results are explained by using density functional theory calculations through the energies of the coronene molecules inside the SWNTs, which depend on the orientation of the molecules and the diameter of the tubes.

Swelling of Ti3 C2 Tx MXene in Water and Methanol at Extreme Pressure Conditions.

Pressure-induced swelling has been reported earlier for several hydrophilic layered materials. MXene Ti3C2Tx is also a hydrophilic layered material composed by 2D sheets but so far pressure-induced swelling is reported for this material only under conditions of shear stress at MPa pressures. Here, high-pressure experiments are performed with MXenes prepared by two methods known to provide "clay-like" materials. MXene synthesized by etching MAX phase with HCl+LiF demonstrates the effect of pressure-induced swelling at 0.2 GPa with the insertion of additional water layer. The transition is incomplete with two swollen phases (ambient with d(001) = 16.7Å and pressure-induced with d(001) = 19.2Å at 0.2 GPa) co-existing up to the pressure point of water solidification. Therefore, the swelling transition corresponds to change from two-layer water intercalation (2L-phase) to a never previously observed three-layer water intercalation (3L-phase) of MXene. Experiments with MXene prepared by LiCl+HF etching have not revealed pressure-induced swelling in liquid water. Both MXenes also show no anomalous compressibility in liquid methanol. The presence of pressure-induced swelling only in one of the MXenes indicates that the HCl+LiF synthesis method is likely to result in higher abundance of hydrophilic functional groups terminating 2D titanium carbide.

Activated carbons with extremely high surface area produced from cones, bark and wood using the same procedure.

Activated carbons have been previously produced from a huge variety of biomaterials often reporting advantages of using certain precursors. Here we used pine cones, spruce cones, larch cones and a pine bark/wood chip mixture to produce activated carbons in order to verify the influence of the precursor on properties of the final materials. The biochars were converted into activated carbons with extremely high BET surface area up to ∼3500 m2 g-1 (among the highest reported) using identical carbonization and KOH activation procedures. The activated carbons produced from all precursors demonstrated similar specific surface area (SSA), pore size distribution and performance to electrodes in supercapacitors. Activated carbons produced from wood waste appeared to be also very similar to "activated graphene" prepared by the same KOH procedure. Hydrogen sorption of AC follows expected uptake vs. SSA trends and energy storage parameters of supercapacitor electrodes prepared from AC are very similar for all tested precursors. It can be concluded that the type of precursor (biomaterial or reduced graphene oxide) has smaller importance for producing high surface area activated carbons compared to details of carbonization and activation. Nearly all kinds of wood waste provided by the forest industry can possibly be converted into high quality AC suitable for preparation of electrode materials.

Super-oxidized "activated graphene" as 3D analogue of defect graphene oxide: Oxidation degree vs U(VI) sorption.

Porous carbons are not favorable for sorption of heavy metals and radionuclides due to absence of suitable binding sites. In this study we explored the limits for surface oxidation of "activated graphene" (AG), porous carbon material with the specific surface area of ∼2700 m2/g produced by activation of reduced graphene oxide (GO). Set of "Super-Oxidized Activated Graphene" (SOAG) materials with high abundance of carboxylic groups on the surface were produced using "soft" oxidation. High degree of oxidation comparable to standard GO (C/O=2.3) was achieved while keeping 3D porous structure with specific surface area of ∼700-800 m2/. The decrease in surface area is related to the oxidation-driven collapse of mesopores while micropores showed higher stability. The increase in the oxidation degree of SOAG is found to result in progressively higher sorption of U(VI), mostly related to the increase in abundance of carboxylic groups. The SOAG demonstrated extraordinarily high sorption of U(VI) with the maximal capacity up to 5400 µmol/g, that is 8.4 - fold increase compared to non-oxidized precursor AG, ∼50 -fold increase compared to standard graphene oxide and twice higher than extremely defect-rich graphene oxide. The trends revealed here show a way to further increase sorption if similar oxidation degree is achieved with smaller sacrifice of surface area.

Synthesis of graphene nanoribbons encapsulated in single-walled carbon nanotubes.

A novel material, graphene nanoribbons encapsulated in single-walled carbon nanotubes (GNR@SWNT), was synthesized using confined polymerization and fusion of polycyclic aromatic hydrocarbon (PAH) molecules. Formation of the GNR is possible due to confinement effects provided by the one-dimensional space inside nanotubes, which helps to align coronene or perylene molecules edge to edge to achieve dimerization and oligomerization of the molecules into long nanoribbons. Almost 100% filling of SWNT with GNR is achieved while nanoribbon length is limited only by the length of the encapsulating nanotube. The PAH fusion reaction provides a very simple and easily scalable method to synthesize GNR@SWNT in macroscopic amounts. First-principle simulations indicate that encapsulation of the GNRs is energetically favorable and that the electronic structure of the encapsulated GNRs is the same as for the free-standing ones, pointing to possible applications of the GNR@SWNT structures in photonics and nanoelectronics.

Temperature-dependent swelling transitions in MXene Ti3C2Tx.

Swelling is a property of hydrophilic layered materials, which enables the penetration of polar solvents into an interlayer space with expansion of the lattice. Here we report an irreversible swelling transition, which occurs in MXenes immersed in excess dimethyl sulfoxide (DMSO) upon heating at 362-370 K with an increase in the interlayer distance by 4.2 Å. The temperature dependence of MXene Ti3C2Tx swelling in several polar solvents was studied using synchrotron radiation X-ray diffraction. MXenes immersed in excess DMSO showed a step-like increase in the interlayer distance from 17.73 Å at 280 K to 22.34 Å above ∼362 K. The phase transformation corresponds to a transition from the MXene structure with one intercalated DMSO layer into a two-layer solvate phase. The transformation is irreversible and the expanded phase remains after cooling back to room temperature. A similar phase transformation was observed also for MXene immersed in a 2 : 1 H2O : DMSO solvent ratio but at a lower temperature. The structure of MXene in the mixed solvent below 328 K was affected by the interstratification of differently hydrated (H2O)/solvated (DMSO) layers. Above the temperature of the transformation, the water was expelled from MXene interlayers and the formation of a pure two-layer DMSO-MXene phase was found. No changes in the swelling state were observed for MXenes immersed in DMSO or methanol at temperatures below ambient down to 173 K. Notably, MXenes do not swell in 1-alcohols larger than ethanol at ambient temperature. Changing the interlayer distance of MXenes by simple temperature cycling can be useful in membrane applications, e.g. when a larger interlayer distance is required for the penetration of ions and molecules into membranes. Swelling is also very important in electrode materials since it allows penetration of the electrolyte ions into the interlayers of the MXene structure.

High-surface-area activated carbon from pine cones for semi-industrial spray deposition of supercapacitor electrodes.

High surface area carbons are so far the best materials for industrial manufacturing of supercapacitor electrodes. Here we demonstrate that pine cones, an abundant bio-precursor currently considered as a waste in the wood industry, can be used to prepare activated carbons with a BET surface area exceeding 3000 m2 g-1. It is found that the same KOH activation procedure applied to reduced graphene oxide (rGO) and pine cone derived biochars results in carbon materials with a similar surface area, pore size distribution and performance in supercapacitor (SC) electrodes. It can be argued that "activated graphene" and activated carbon are essentially the same kind of material with a porous 3D structure. It is demonstrated that the pine cone derived activated carbon (PC-AC) can be used as a main part of aqueous dispersions stabilized by graphene oxide for spray deposition of electrodes. The PC-AC based electrodes prepared using a semi-industrial spray gun machine and laboratory scale blade deposition of these dispersions were compared to pellet electrodes.

Facile fabrication of graphene-based high-performance microsupercapacitors operating at a high temperature of 150 °C.

Many industry applications require electronic circuits and systems to operate at high temperatures over 150 °C. Although planar microsupercapacitors (MSCs) have great potential for miniaturized on-chip integrated energy storage components, most of the present devices can only operate at low temperatures (<100 °C). In this work, we have demonstrated a facile process to fabricate activated graphene-based MSCs that can work at temperatures as high as 150 °C with high areal capacitance over 10 mF cm-2 and good cycling performance. Remarkably, the devices exhibit no capacitance degradation during temperature cycling between 25 °C and 150 °C, thanks to the thermal stability of the active components.

Intercalation of Dyes in Graphene Oxide Thin Films and Membranes.

Intercalation of dyes into thin multilayered graphene oxide (GO) films was studied by neutron reflectivity and X-ray diffraction. Methylene blue (MB) penetrates the interlayer space of GO in ethanol solution and remains intercalated after the solvent evaporation, as revealed by the expansion of the interlayer lattice and change in chemical composition. The sorption of MB by thin GO films is found to be significantly stronger compared to the sorption of Crystal violet (CV) and Rose bengal (RB). This effect is attributed to the difference in the geometrical shape of planar MB and essentially nonflat CV and RB molecules. Graphite oxides and restacked GO films are found to exhibit different methylene blue (MB) sorptions. MB sorption by precursor graphite oxide and thin spin-coated films of GO is significantly stronger compared to freestanding micrometer-thick membranes prepared by vacuum filtration. Nevertheless, the sorption capacity of GO membranes is sufficient to remove a significant part of the MB from diluted solutions tested for permeation in several earlier studies. High sorption capacity results in strong modification of the GO structure, which is likely to affect permeation properties of GO membranes. Therefore, MB is not suitable for testing size exclusion effects in the permeation of GO membranes. It is not only hydration or solvation diameter but also the exact geometrical shape of molecules that needs to be taken into account considering size effects for penetration of molecules between GO layers in membrane applications.

Cation size and anion anisotropy in structural chemistry of metal borohydrides. the peculiar pressure evolution of RbBH(4).

The pressure evolution of RbBH(4) has been characterized by synchrotron powder X-ray diffraction and Raman spectroscopy up to 23 GPa. Diffraction experiments at ambient temperature reveal three phase transitions, at 3.0, 10.4, and 18 GPa (at 2.6, 7.8, and approximately 20 GPa from Raman data), at which the space group symmetry changes in the order Fm-3m(Z=4) --> P4/nmm(2) --> C222(2) --> I-42m(4). Crystal structures and equations of state are reported for all four phases. The three high-pressure structure types are new in the crystal chemistry of borohydrides. RbBH(4) polymorphs reveal high coordination numbers (CNs) for cation and anion sites, increasing with pressure from 6 to 8, via an intermediate 4 + 4 coordination. Different arrangements of the tetrahedral BH(4) group in the Rb environment define the crystal symmetries of the RbBH(4) polymorphs. The structural evolution in the MBH(4) series is determined by the cation's size, as it differs drastically for M = Li (CNs = 4, 6), Na (CN = 6), and Rb. The only structure common to the whole MBH(4) family is the cubic one. Its bulk modulus linearly decreases as the ionic radius of M increases, indicating that the compressibility of the material is mainly determined by the repulsive BH(4)...BH(4) interactions.

Swelling properties of graphite oxides and graphene oxide multilayered materials.

Graphite oxide (GtO) and graphene oxide (GO) multilayered laminates are hydrophilic materials easily intercalated by water and other polar solvents. By definition, an increase in the volume of a material connected to the uptake of a liquid or vapour is named swelling. Swelling is a property which defines graphite oxides and graphene oxides. Less oxidized materials not capable of swelling should be named oxidized graphene. The infinite swelling of graphite oxide yields graphene oxide in aqueous dispersions. Graphene oxide sheets dispersed in a polar solvent can be re-assembled into multilayered structures and named depending on applications as films, papers or membranes. The multilayered GO materials exhibit swelling properties which are mostly similar to those of graphite oxides but not identical and in some cases surprisingly different. Swelling is a key property of GO materials in all applications which involve the sorption of water/solvents from vapours, immersion of GO into liquid water/solvents and solution based chemical reactions. These applications include sensors, sorption/removal of pollutants from waste waters, separation of liquid and gas mixtures, nanofiltration, water desalination, water-permeable protective coatings, etc. Swelling defines the distance between graphene oxide sheets in solution-immersed GO materials and the possibility for penetration of ions and molecules inside of interlayers. A high sorption capacity of GO towards many molecules and cations is defined by swelling which makes the very high surface area of GO accessible. GtO and GO swelling is a surprisingly complex phenomenon which is manifested in a variety of different ways. Swelling is strongly different for materials produced using the most common Brodie and Hummers oxidation procedures; it depends on the degree of oxidation, ad temperature and pressure conditions. The value of the GO interlayer distance is especially important in membrane applications. Diffusion of solvent molecules and ions is defined by the size of "permeation channels" provided by the swelled GO structure. According to extensive studies performed over the last decade the exact value of the inter-layer distance in swelled GO depends on the nature of solvent, temperature and pressure conditions, and the pH and concentration of solutions and exhibits pronounced aging effects. This review provides insight into the fundamental swelling properties of multilayered GO and demonstrates links to advanced applications of these materials.