Intersheet Distance Research Articles

Tunable graphene oxide inter-sheet distance to obtain graphene oxide–silver nanoparticle hybrids

An improved capacitor using silver nanoparticles incorporated within sheets of graphene oxide. 90 times higher specific capacitance than bare graphene oxide.

Tough Photocrosslinked Silk Fibroin/Graphene Oxide Nanocomposite Hydrogels.

The development of protein-based hydrogels for tissue engineering applications is often limited by their mechanical properties. Herein, we present the facile fabrication of tough regenerated silk fibroin (RSF)/graphene oxide (GO) nanocomposite hydrogels by a photochemical cross-linking method. The RSF/GO composite hydrogels demonstrated soft and adhesive properties during initial stages of photocrosslinking (<2 min), which is not observed for the pristine RSF hydrogel, and rendered a tough and nonadhesive hydrogel upon complete cross-linking (10 min). The composite hydrogels exhibited superior tensile mechanical properties, increased β-sheet content, and decreased chain mobility compared to that of the pristine RSF hydrogels. The composite hydrogels demonstrated Young's modulus as high as ∼8 MPa, which is significantly higher than native cartilage (∼1.5 MPa), and tensile toughness as high as ∼2.4 MJ/m3, which is greater than that of electroactive polymer muscles and at par with RSF/GO composite membranes fabricated by layer-by-layer assembly. Small-angle scattering study reveals the hierarchical structure of photocrosslinked RSF hydrogels to comprise randomly distributed water-poor (hydrophobic) and water-rich (hydrophilic) regions at the nanoscale, whereas water pores and channels exhibiting fractal-like characteristics at the microscale. The size of hydrophobic domain (containing β-sheets) was observed to increase slightly with GO incorporation and/or alcohol post-treatment, whereas the size of the hydrophilic domain (intersheet distance containing random coils) was observed to increase significantly, which influences/affects water uptake capacity, cross-link density, and mechanical properties of hydrogels. The presented results have implications for both fundamental understanding of the structure-property relationship of RSF-based hydrogels and their technological applications.

Polymer nanocomposites from natural clay: understanding clay-PEG interactions and their effect on spacing between clay-plates

The interactions between clay-plates and hydrophilic polymer are investigated assuming that the polyethylene glycol (PEG) chains are grafted onto face-to-face clay-plates. Besides the usual van der Waals attractive interaction, the clay-plates experience a repulsive effective interaction, due to the excluded volume force between monomers along the grafted PEG chains. The face-to-face clay-plates then play the role of polymer brushes. The free energy (per unit area) of a clay-plate pair is the sum of these interactions, and from its expression, we determine the minimal inter-sheet distance, after intercalation, which corresponds to a critical percentage of PEG. Finally, our result is in good agreement with some recent experimental work.

Wettability of vertically-oriented graphenes with different intersheet distances

The dependence of vertically-oriented graphenes' wettability on their surface morphologies is investigated with experiments and numerical simulations.

Behavior of protruding lateral plane graphene sheets in liquid dodecane: molecular dynamics simulations

Molecular dynamics simulations are used to investigate the behavior of two parallel graphene sheets fixed on one edge (lateral plane) in liquid dodecane. The interactions of these sheets and dodecane molecules are studied with different starting inter-sheet distances. The structure of the dodecane solvent is also analyzed. The results show that when the distance between the two graphene sheets is short (less than 6.8 A), the sheets will expel the dodecane molecules between them and stack together. However, when the distance between two sheets is large (greater than 10.2 A), the two sheets do not come together, and the dodecane molecules will form ordered layers in the interlayer spacing. The equilibrium distance between the graphene sheets can only take on specific discrete values (3.4, 7.8, and 12.1 A), because only an integer number of dodecane layers forms between the two sheets. Once the graphene sheets are in contact, they remain in contact; the sheets do not separate to allow dodecane into the interlayer spacing.

Intersheet Distance Modeling and Thermal Characterization of New Micro/Nano-Composites Composed by Clay Matrix and PEG 6000 Addition

Nowadays, the energetic efficiency becomes one of the major interests of the global society. Thus, the energetic challenges of the new century enforce the scientific and industrial environment to the development of new efficient materials, which present more than the classical thermal properties, according to the energy storage, energy consumption and other specific needs. In this context, the present work constitutes the third step of the development of a new kind of composite materials (micro-composites and nano-composites), using natural marl (clay) clay and biodegradable polymer, which is the PolyEthylene Glycol 6000 (PEG 6000). This step corresponds to characterization of the variation of the specific heat (denoted Cp) of the materials elaborated. So, in order to estimate the capacity of thermal energy adsorption, we utilized a SHIMATZU-DSC 60 Differential Scanning Calorimeter. The main results present the evolution of the Cp according to the PEG 6000 doping and also the specific melting enthalpy of the polymer within the natural clay matrix; by the way this enthalpy constitutes the specific heat stocked in the materials.

The high pressure structure and equation of state of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) up to 20 GPa: X-ray diffraction measurements and first principles molecular dynamics simulations.

Recent theoretical studies of 2,6-diamino-3,5-dinitropyrazine-1-oxide (C4H4N6O5 Lawrence Livermore Molecule No. 105, LLM-105) report unreacted high pressure equations of state that include several structural phase transitions, between 8 and 50 GPa, while one published experimental study reports equation of state (EOS) data up to a pressure of 6 GPa with no observed transition. Here we report the results of a synchrotron-based X-ray diffraction study and also ambient temperature isobaric-isothermal atomistic molecular dynamics simulations of LLM-105 up to 20 GPa. We find that the ambient pressure phase remains stable up to 20 GPa; there is no indication of a pressure induced phase transition. We do find a prominent decrease in b-axis compressibility starting at approximately 13 GPa and attribute the stiffening to a critical length where inter-sheet distance becomes similar to the intermolecular distance within individual sheets. The ambient temperature isothermal equation of state was determined through refinements of measured X-ray diffraction patterns. The pressure-volume data were fit using various EOS models to yield bulk moduli with corresponding pressure derivatives. We find very good agreement between the experimental and theoretically derived EOS.

Further insights into the structure and chemistry of the Gilsonite asphaltene from a combined theoretical and experimental approach

Abstract Gilsonite is a natural fossil resource, similar to a petroleum asphalt high in asphaltenes. Asphaltenes are a class of organic compounds operationally defined based on their solubility in organic solvents, and as a result there is wide range of potential compositions and structures that can fit into this class. Specific compounds are challenging to propose due to its complexity. A sample of the asphaltene derived from the Gilsonite deposit was characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, and high-resolution transmission electron microscopy. The high intensity of the 1600 cm−1 infrared peak, which corresponds to a C C stretching vibrational mode of the aromatic carbons also found prevalent in other asphaltenes, is likely a characteristic asphaltene feature. The high intensity can be explained by the stack structure and/or by polycyclic aromatic infrared transitions with a high dipole moment derivative. The nanosize stack structure was validated by the electron microscope and diffraction patterns, giving inter-sheet distances of 2.54 and 3.77 A. Complementary calculations using density functional theory suggest a specific island-type polycyclic aromatic molecular model, with the calculated vibrational modes consistent with all of the characteristic peaks in the infrared spectrum. The method combining theoretical and experimental can be extended for more specific asphaltene molecular structure identifications.

A novel sensing mechanism for resistive gas sensors based on layered reduced graphene oxide thin films at room temperature

In order to investigate the gas sensing mechanism of chemiresistors based on reduced graphene oxide (RGO), layered reduced graphene oxide was airbrushed onto interdigital electrodes (IDEs). Sensitive performances of all sensors with RGO sensitive films at room temperature were studied. Results showed that the total flow rate had a significant effect on the initial electric resistance of all sensors and their sensing responses to target gases as well. Besides, sensors with different quantities of deposited RGO solution were utilized to study the relationship among film thickness, sensing response and sensitivity. It was found that appropriate quantity of deposited RGO solution was critical for sensors’ sensing response and sensitivity. Furthermore, a hybrid sensing mechanism including intra-sheet semiconducting characteristics and inter-sheet distance variation was proposed to explain some interesting phenomena. The proposed sensing mechanism can potentially enrich the applicable transduction mechanisms.

Ultrathin Sheets of Metal or Metal Sulfide from Molecularly Thin Sheets of Metal Thiolates in Solution

Materials that exist as single molecule thick two-dimensional sheets are in great demand because they hold promise as precursors for synthesis of layered functional materials. We demonstrate that metal thiolates, that exist as lamellar assemblies in the neat state, can be disassembled into individual molecular sheets simply by dilution in apolar organic solvents and that these can form ultrathin metallic layers on substrates upon heat treatment. We establish the pathway to the disassembly of metal thiolates in solution using a combination of techniques, including X-ray diffraction, light scattering, FTIR, and TEM. Our results indicate that the lamellar structure of Pd-thiolates is preserved in toluene up to a concentration of 300% w/v and the average intersheet distance is unchanged. Interestingly, the dynamics of the Pd-thiolate sheets remain correlated even on diluting them up to 30% w/v, though the disorder within the lamellar stacks increases with a decrease in their coherence length. Finally, at dilutions less than about 5% w/v, individual sheets of these structures can be accessed that are isolated and directly observed using TEM. Heat treatment of the ultrathin films of metal thiolates deposited on appropriate substrates resulted in the formation of metal or metal sulfides with retention of sheetlike morphologies.

Graphenated Carbon Nanotubes for Enhanced Nucleation of Manganese Oxide Electrodeposits in High Performance Composite Electrodes

Carbon nanotubes (CNTs) and graphenes are emerging electrode materials currently being investigated for supercapacitor applications due to their unique properties that offer advantages over traditional activated carbons including ultra-low resistivity (~ 10e-6 Ω cm) (1), high specific surface area (2630 m2/g for single graphene sheet (2)), and tailorable mesopore size distributions. In particular, the vertically oriented forms deposited by plasma enhanced chemical vapor deposition (PECVD) have received strong attention recently due to the preponderance of exposed graphitic edge-planes, the ability for direct growth of nanostructures perpendicular to metal current collectors, and increases in capacitance by fine-tuning the inter-tube or inter-sheet distance. These attributes are important since the graphitic edge-plane sites are known to have 20x greater double-layer capacitance (50-70 μF/cm2) than the basal plane sites (3 μF/cm2) (3), thus improving specific energy density. Likewise, growth directly on metallic substrates can significantly reduce contact resistance with current collectors and negate inter-particle resistances in the electrode, thereby lowering equivalent series resistance (ESR) and improving power densities. Furthermore, CNTs, graphenes, and related hybrid materials serve as highly conductive nanostructured platforms for battery materials or metal oxides that provide significant psuedocapacitance for use in asymmetric or hybrid supercapacitors. Recently, manganese oxides (MnOx) have gained attention as a preferred battery material in aqueous electrolytes due to their high theoretical capacity (~ 1100 F/g) (4) and relatively lower cost. Recently, we have developed a single PECVD process (5) to grow vertically aligned carbon nanotubes with few-layered graphene protruding orthogonally from the sidewalls, referred to as graphenated carbon nanotubes (g-CNTs). The fundamental advantage of g-CNTs is the 3D high volume-density framework of the CNT forest coupled with the high charge density of the 2D graphene edges. We previously demonstrated that electrodes made from g-CNTs achieved a 7x improvement in specific capacitance relative to a CNT electrode without graphitic foliates when charged with high frequency pulses (6).Here, we demonstrate that g-CNTs are an ideal platform for formation of composite electrodes with manganese oxides for energy storage applications due to their superior performance. Preliminary results show that a g-CNT/MnOx electrode had up to 5x greater specific capacitance than a CNT/MnOx electrode (Figure 1) formed under an identical electrodeposition process. The key factor in the improved performance is that the graphene foliates appear to serve as nucleation sites for nanostructured MnOx deposits that form along individual g-CNTs (Figure 2). This is in contrast to results from REF (7) which indicate that MnOx nanoflowers only form at the junctions of overlapping or intersecting CNTs with no deposits on an isolated nanotube. Furthermore, the ability to vary the density of reactive graphene foliates on g-CNTs by increasing growth time (Figure 3) enables one to control or optimize the MnOx loading and utilization efficiency in the composite structure for maximum specific capacitance at a particular current density (Figure 4). Overall these results which are yet to be optimized have revealed important benefits and promise for potential applications using this exciting new material.

Evidence of ammonium ion-exchange properties of natural bentonite and application to ammonium detection

Ammonium exchange with hybrid PVC–bentonite (mineral montmorillonite clay) thin film was revealed using FTIR spectroscopy, EDX, cyclic voltammetry and electrochemical impedance spectroscopy. The effect of ammonium exchange on the charge transfer resistance of PVC–bentonite hybrid thin film was attributed to a modification of the intersheet distance and hydration of bentonite crystals.The obtained impedimetric ammonium sensor shows a linear range of detection from 10−4M to 1M and a detection limit around 10−6M.

Comblike Ionic Complexes of Poly(γ-glutamic acid) and Alkanoylcholines Derived from Fatty Acids

A series of ionic complexes with almost stoichiometric composition have been “synthesized” from bacterial poly(γ-glutamic acid) and alkanoylcholines derived from fatty acids with 12, 14, 16, and 18 carbon atoms. The complexes were stable up to temperatures close to 200 °C and were non-water-soluble but readily soluble in organic solvents. In the solid state they adopted the biphasic structure typical of comblike systems with the polypeptide chains arranged in sheets and periodically separated by a paraffinic phase which was partially crystallized for C14, C16, and C18. The window width of the layered structure was estimated by SAXS to be within the 3.6–4.5 nm range, and WAXS showed that the alkyl tails were crystallized in a hexagonally packed lattice with a d100 spacing of 0.41 nm. These complexes displayed reversible melting of the paraffinic phase in the 40–65 °C range at temperatures with values increasing with the length of the alkanoyl group. A slight expansion of the intersheet distance occurred at melting followed by contraction upon heating at higher temperatures but without significant alteration of the layered structure. The 13C CP-MAS NMR analysis revealed the underlying polymethylene anti-to-gauche conformational conversion that takes place at the thermal transition. An energy-based simulation study of the crystallization process afforded a molecular picture of the complex and evidenced the favoring effect of the choline structure on the packing of the alkyl side chains.

Intercalating Oleylamines in Graphite Oxide

Graphite oxide has been synthesized from raw graphite particles and been treated with various mass amounts of oleylamine as intercalants to form intercalation compounds. X-ray diffraction patterns reveal that the inter-sheet distances strongly depend on the graphite oxide to oleylamine mass ratios. The equilibrium-like behavior implies diffusion-dominated oleylamine adsorption on graphite oxide in solution and excluded volume intercalations among oleylamine-adsorbed graphite oxide during restacking. The intercalation compounds are soluble in organic solvents, and their applications in the fabrication of transparent and conductive coatings have been demonstrated.

Spontaneous Formation of Twisted Aβ16-22 Fibrils in Large-Scale Molecular-Dynamics Simulations

Protein aggregation is associated with fatal neurodegenerative diseases, including Alzheimer's and Parkinson's. Mapping out kinetics along the aggregation pathway could provide valuable insights into the mechanisms that drive oligomerization and fibrillization, but that is beyond the current scope of computational research. Here we trace out the full kinetics of the spontaneous formation of fibrils by 48 Aβ16-22 peptides, following the trajectories in molecular detail from an initial random configuration to a final configuration of twisted protofilaments with cross-β-structure. We accomplish this by performing large-scale molecular-dynamics simulations based on an implicit-solvent, intermediate-resolution protein model, PRIME20. Structural details such as the intersheet distance, perfectly antiparallel β-strands, and interdigitating side chains analogous to a steric zipper interface are explained by and in agreement with experiment. Two characteristic fibrillization mechanisms—nucleation/templated growth and oligomeric merging/structural rearrangement—emerge depending on the temperature.

Doped Graphene Sheets As Anode Materials with Superhigh Rate and Large Capacity for Lithium Ion Batteries

One great challenge in the development of lithium ion batteries is to simultaneously achieve high power and large energy capacity at fast charge and discharge rates for several minutes to seconds. Here we show that nitrogen- or boron-doped graphene can be used as a promising anode for high-power and high-energy lithium ion batteries under high-rate charge and discharge conditions. The doped graphene shows a high reversible capacity of >1040 mAh g(-1) at a low rate of 50 mA g(-1). More importantly, it can be quickly charged and discharged in a very short time of 1 h to several tens of seconds together with high-rate capability and excellent long-term cyclability. For example, a very high capacity of ∼199 and 235 mAh g(-1) was obtained for the N-doped graphene and B-doped graphene at 25 A g(-1) (about 30 s to full charge). We believe that the unique two-dimensional structure, disordered surface morphology, heteroatomic defects, better electrode/electrolyte wettability, increased intersheet distance, improved electrical conductivity, and thermal stability of the doped graphene are beneficial to rapid surface Li(+) absorption and ultrafast Li(+) diffusion and electron transport, and thus make the doped materials superior to those of pristine chemically derived graphene and other carbonaceous materials.

Ab Initio Studies of Cellulose I: Crystal Structure, Intermolecular Forces, and Interactions with Water

We have studied the structural, energetic, and electronic properties of crystalline cellulose I using first-principles density functional theory (DFT) with semiempirical dispersion corrections. The predicted crystal structures of both Iα and Iβ phases agree well with experiments and are greatly improved over those predicted by DFT within the local and semilocal density approximations. The cohesive energy is analyzed in terms of interchain and intersheet interactions, which are calculated to be of similar magnitude. Both hydrogen bonding and van der Waals (vdW) dispersion forces are found to be responsible for binding cellulose chains together. In particular, dispersion corrections prove to be indispensable in reproducing the equilibrium intersheet distance and binding strength; however, they do not improve the underestimated hydrogen bond length from DFT. The computed energy gaps of crystalline cellulose are 5.7 eV (Iα) and 5.4 eV (Iβ), whereas localized surface states appear within the gap for surfaces. The interaction of cellulose with water is studied by investigating the adsorption of a single water molecule on the hydrophobic Iβ(100) surface. The formation of hydrogen bond at the water/cellulose interface is shown to depend sensitively on the adsorption site for example above the equatorial hydroxyls or the CH moieties pointing out of the cellulose sheets. VdW dispersion interactions also contribute significantly to the adsorption energy.

Vibrational Circular Dichroism as a Probe of Fibrillogenesis: The Origin of the Anomalous Intensity Enhancement of Amyloid-like Fibrils

Amyloid fibrils are affiliated with various human pathologies. Knowledge of their molecular architecture is necessary for a detailed understanding of the mechanism of fibril formation. Vibrational circular dichroism (VCD) spectroscopy has recently shown sensitivity to amyloid fibrils [Ma et al. J. Am. Chem. Soc. 2007, 129, 12364 and Measey et al. J. Am. Chem. Soc. 2009, 131, 18218]. In particular, amyloid fibrils give rise to an intensity enhanced signal in the amide I band region of the corresponding VCD spectrum, offering promise of utilizing such a method for probing fibrillogenesis and the chiral structure of fibrils. Herein, we further investigate this phenomenon and demonstrate the use of VCD to probe the fibril formation kinetics of a short alanine-rich peptide. To elucidate the origin of the anomalous VCD intensity enhancement, we use an excitonic coupling model to simulate the VCD spectrum of stacked β-sheets containing one (Ising-like model) and two amide I oscillators per strand, as models for the underlying amyloid-fibril secondary structure. With this simple model, we show that the VCD intensity enhancement of amyloid-like fibrils results from intrasheet and, to a more limited extent, also from intersheet vibrational coupling between stacked β-sheets. The enhancement requires helically twisted sheets and is most pronounced for arrangements with parallel-oriented strands. Both the intersheet distance and the orientation of the amide I transition dipole moments of neighboring sheets are found to modulate the intensity enhancement of the amide I VCD signal. Moreover, our simulations suggest that, depending on the three-dimensional arrangement of the β-strands, the sign of the VCD signal of amyloid-like fibrils can be used to distinguish between right- and left-handed helical twists of parallel-oriented β-sheets. We compare the results of our simulation to experimental spectra of two short peptides, GNNQQNY, the N-terminal peptide fragment of the yeast prion protein Sup35, and an amyloidogenic alanine-rich peptide, AKY8. Our results demonstrate the advantages of using VCD spectroscopy to probe the kinetics of peptide and protein aggregation as well as the chirality of the resulting supramolecular structure.

Simulation of the Amide I Absorption of Stacked β-Sheets

Aggregated β-sheet structures are associated with amyloid and prion diseases. Techniques capable of revealing detailed structural and dynamical information on β-sheet structure are thus of great biomedical and biophysical interest. In this work, the infrared (IR) amide I spectral characteristics of stacked β-sheets were modeled using the transition dipole coupling model. For a test set of β-sheet stacks, the simulated amide I spectrum was analyzed with respect to the following parameters; intersheet distance, relative rotation of the sheets with respect to each other and the effect of number of sheets stacked. The amide I maximum shifts about 5 cm(-1) to higher wavenumbers when the intersheet distance between two identical β-sheets decreases from 20 to 5 Å. Rotation around the normal of one of the sheets relative to the other results in maximum intersheet coupling near 0° and 180°. Upon of rotation from 0° to 90° at an intersheet distance of 9 Å, the amide I maximum shifts about 3 cm(-1). Tilting of one of the sheets by 30° from the normal results in a shift of the amide I maximum by less than 1 cm(-1). When stacking several β-sheets along the normal, the amide I maximum shifts to higher wavenumbers with increasing stack size. The amide I maximum shifts about 6 cm(-1) when stacking four sheets with an intersheet distance of 9 Å. The study provides an aid in the interpretation of the IR amide I region for experiments involving β-sheets and creates awareness of the many effects that determine the spectrum of β-sheet structures.

Intercalation of P atoms in Fullerene-like CPx

The energy cost for P atom intercalation and corresponding structural implications during formation of Fullerene-like Phosphorus carbide (FL-CPx) were evaluated within the framework of Density Functional Theory. Single P atom interstitial defects in FL-CPx are energetically feasible and exhibit energy cost of 0.93–1.21eV, which is comparable to the energy cost for experimentally confirmed tetragon defects and dangling bonds in CPx. A single P atom intercalation event in FL-CPx can increase the inter-sheet distance from 3.39–3.62Å to 5.81–7.04Å. These theoretical results are corroborated by Selected Area Electron Diffraction characterization of FL-CPx samples.