Study of PE and iPP orientations on the surface of carbon nanotubes by using molecular dynamic simulations

In this study, the early stage of interfacial crystallisation behaviour of low molecular weight polyethylene (PE) and isotactic polypropylene (iPP) oligomer on the surface of carbon nanotubes (CNTs) with different diameters, chiralities and topography structures was studied using MD simulations. We started to simulate the effect of CNTs chirality and diameter on PE molecular chain orientation, and then the effect of CNTs topography structure on PE and iPP molecular chain orientation was investigated. Finally, some experiments were carried out to prove the simulated results. Our study shows that for CNTs with a diameter comparable with the radius of gyration (R_g) of a polymer chain, an easy orientation of PE chains along CNTs axis is observed for all the systems of the CNTs with different chiralities due to a geometric confinement effect. For CNTs with a much larger diameter, multiple orientation of PE chains is induced on its surface due to the lattice matching between graphite lattice and PE molecular chains. In this case, the chirality of CNTs dominates the orientation of graphite lattice, which determines the orientation of PE chains arrangement on CNTs surface. More importantly, it was found that the groove structure formed by CNT bundles is very useful for the stabilisation of polymer chain, and thus facilitates the orientation of molecular chain along the long axis of CNTs. As a result, a novel nanohybrid shish–kebab (NHSK) structure with CNTs acting as central shish while polymer lamellae as kebab can be successfully obtained for both PE with zigzag conformation and iPP with helical conformation. This simulation result was well supported by the experimental observation. Our study could provide not only a deep understanding of the origin of the polymer chain orientation on CNTs surface but also the guidance for the preparation of polymer/CNTs nanocomposites with novel NHSK structure.     

Thermal behaviour of Pd clusters inside carbon nanotubes: insights into the cluster-size,tube-size and metal–tube interaction effects

Molecular dynamics simulations were used to investigate the cluster-size, tube-size and metal–tube interaction effects on the melting of Pd clusters encapsulated inside carbon nanotubes (CNTs). The second moment approximation to the tight-binding potential was used to model Pd–Pd metal–metal interaction and the Tersoff potential was used for C–C interactions. Pd–C interaction was modelled by the typical weak van der Waals Lennard-Jones (VDW-LJ) potential to understand the cluster-size and tube-size effects on the thermal behaviour of supported Pd clusters. Linear decrease in cluster melting point with the inverse in cluster diameter is predicted for the CNT containing Pd clusters, well known as Pawlow's law. It is also found that the melting temperature of the supported Pd cluster is much lower than that of free one, and the rearrangement and transformation of the cluster at higher temperatures before melting are responsible for this lowering. In this case, the downward shift is independent of the CNT diameter for the same Pd cluster. In addition, the Pd–C interaction was redefined to assess the metal–tube interaction effect on the thermal evolution of the CNT-containing Pd clusters by fitting to first-principle calculations. Using the fitted strong density functional theory-Morse Pd–C potential, deformation for the CNT and structural transformation from the icosahedral to the stacked for the Pd cluster inside the CNT are found, which is not shown by using the VDW-LJ potential.     

Silicon–doping in carbon nanotubes: formation energies, electronic structures, and chemical reactivity

By carrying out density functional theory (DFT) calculations, we have studied the effects of silicon (Si)-doping on the geometrical and electronic properties, as well as the chemical reactivity of carbon nanotubes (CNTs). It is found that the formation energies of these nanotubes increase with increasing tube diameters, indicating that the embedding of Si into narrower CNTs is more energetically favorable. For the given diameters, Si-doping in a (n, 0) CNT is slightly easier than that of in (n, n) CNT. Moreover, the doped CNTs with two Si atoms are easier to obtain than those with one Si atom. Due to the introduction of impurity states after Si-doping, the electronic properties of CNTs have been changed in different ways: upon Si-doping into zigzag CNTs, the band gap of nanotube is decreased, while the opening of band gap in armchair CNTs is found. To evaluate the chemical reactivity of Si-doped CNTs, the adsorption of NH₃ and H₂O on this kind of material is explored. The results show that N–H bond of NH₃ and O–H bond of H₂O can be easily split on the surface of doped CNTs. Of particular interest, the novel reactivity makes it feasible to use Si-doped CNT as a new type of splitter for NH₃ and H₂O bond, which is very important in chemical and biological processes. Future experimental studies are greatly desired to probe such interesting processes.      

Effect of the Si,Al and B doping on the sensing behaviour of carbon nanotubes toward ethylene oxide: a computational study

ABSTRACT

Exo– and endo–adsorption of ethylene oxide (EO) on pristine (9,0) (zigzag) carbon nanotube (CNT) and its doped forms with silicon (Si–CNT), aluminum (Al–CNT) and boron (B–CNT) were investigated using density functional theory (DFT) at M06–2X/6–311++G** level. The natural bond orbital (NBO) and the quantum theory of atoms in molecules (QTAIM) analyses were also performed by using the same level of theory. The effect of the doping on sensing behaviour of the CNT toward EO molecule was investigated through intermolecular interactions studies by calculation of total and partial density of states (DOS, PDOS). The enhanced sensitivity of doped–CNTs towards EO molecule associated with adsorption energies (E_ads) and the changes in geometric and electronic structures was examined and the global chemical reactivity parameters were calculated and comprehensively analysed. The thermodynamic property changes were calculated and compared. The results indicated that the EO adsorption on the pristine and doped CNTs was an exothermic spontaneous process. Moreover, based on the calculated E_g change (ΔE_g) and E_ads values, Al–CNT with superior sensitivity for sensing of EO molecule, indicates promising perspectives for its use in fabrication of new EO gas–sensing devices.     

Effect of chirality,length and diameter of carbon nanotubes on the adsorption of 20 amino acids: a molecular dynamics simulation study

We carried out molecular dynamics simulations to study the adsorption of all the 20 amino acids (AAs; aromatic, polar and non-polar) on the surface of chiral, zigzag and armchair single-walled carbon nanotubes. The adsorption was occurring in all systems. In the aromatic AAs, the π–π stacking and the semi-hydrogen bond formation cause a strong interaction with the carbon nanotubes (CNTs). We also investigated the chirality, length and diameter dependencies on adsorption energies. We found that all AAs have more tendency to adsorption on the chiral and zigzag CNTs over the armchair. The results show that increasing both the diameter and the length causes the enhancement of the adsorption energy. But, the effect of the length is more than of the diameter. For example, the adsorption energy of Trp on the surface of CNT (4,1), with 2 nm length, is 20.4 kcal/mol. When the length of CNT becomes twice, the adsorption energy increases by 24 ± 0.3%. But by doubling the diameter, the adsorption energy increased only by 9.8 ± 0.25%.     

Buckling analysis of defective cross-linked functionalized single- and double-walled carbon nanotubes with polyethylene chains using molecular dynamics simulations

Functionalized carbon nanotubes (CNTs) can be used for improving the mechanical properties and load transfer in nanocomposites. In this research, the buckling behavior of perfect and defective cross-linked functionalized CNTs with polyethylene (PE) chains is studied employing molecular dynamics (MD) simulations. Two different configurations with the consideration of vacancy defects, namely mapped and wrapped, are selected. According to the results, critical buckling force of cross-linked functionalized CNTs with PE chains increases as compared to pure CNTs, especially in the case of double-walled carbon nanotubes (DWCNTs). By contrast, it is demonstrated that critical strain of cross-linked functionalized CNTs decreases as compared to that of pristine CNTs. Also, it is observed that increasing the weight percentage leads to the higher increase and the decrease in critical buckling force and strain of cross-linked functionalized CNTs, respectively. Moreover, the presence of defect considerably reduces both critical buckling force and strain of cross-linked functionalized CNTs. Finally, it is shown that the critical buckling strain is more sensitive to the presence of defects as compared to critical buckling force.     

A Stretchable Polymer–Carbon Nanotube Composite Electrode for Flexible Lithium‐Ion Batteries: Porosity Engineering by Controlled Phase Separation

Flexible energy‐storage devices have attracted growing attention with the fast development of bendable electronic systems. However, it still remains a challenge to find reliable electrode materials with both high mechanical flexibility/toughness and excellent electron and lithium‐ion conductivity. This paper reports the fabrication and characterization of highly porous, stretchable, and conductive polymer nanocomposites embedded with carbon nanotubes (CNTs) for application in flexible lithium‐ion batteries. The systematic optimization of the porous morphology is performed by controllably inducing the phase separation of polymethylmethacrylate (PMMA) in polydimethylsiloxane (PDMS) and removing PMMA, in order to generate well‐controlled pore networks. It is demonstrated that the porous CNT‐embedded PDMS nanocomposites are capable of good electrochemical performance with mechanical flexibility, suggesting these nanocomposites could be outstanding anode candidates for use in flexible lithium‐ion batteries. The optimization of the pore size and the volume fraction provides higher capacity by nearly seven‐fold compared to a nonporous nanocomposite.     

Effect of temperature on elastic properties of CNT-polyethylene nanocomposite and its interface using MD simulations

This paper investigates the effect of temperature on the elastic modulus of carbon nanotube-polyethylene (CNT-PE) nanocomposite and its interface using molecular dynamics (MD) simulations, by utilizing the second-generation polymer consistent force field (PCFF). Two CNTs—armchair and zigzag—were selected as reinforcing nano-fillers, and amorphous PE was used as the polymer matrix. For atomistic modelling of the nanocomposite, the commercially available code Materials Studio 8.0 was used and all other MD simulations were subsequently performed using the open source code Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). To obtain the elastic modulus of the nanocomposite, stress-strain curves were drawn at different temperatures by performing uniaxial deformation tests on the nanocomposite material, whereas the curvatures of the interfacial interaction energy vs. strain curves were utilized to obtain Young’s modulus of the interface. In addition, the glass transition temperatures of the polymer matrix and nanocomposites were also evaluated using density-temperature curves. Based on the results, it is concluded that, irrespective of temperature condition, a nanocomposite reinforced with CNT of larger chirality (i.e., armchair) yields a higher value of Young’s modulus of the nanocomposite and its interface. It was also found that, at the phase transition (from a glassy to a rubbery state) temperature (i.e., glass transition temperature), Young’s moduli of the polymer matrix, nanocomposite, and its interface drop suddenly. The results obtained from MD simulations were verified with results obtained from continuum-based rule-of-mixtures.     

In Situ TEM Observation of Electrochemical Lithiation of Sulfur Confined within Inner Cylindrical Pores of Carbon Nanotubes

Lithium insertion into sulfur confined within 200 nm cylindrical inner pores of individual carbon nanotubes (CNTs) was monitored in situ in a transmission electron microscope (TEM). This electrochemical reaction was initiated at one end of the S‐filled CNTs. The material expansion during lithiation was accommodated by the expansion into the remaining empty pore volume and no fracture of the CNT walls was detected. A sharp interface between the initial and lithiated S was observed. The reaction front was flat, oriented perpendicular to the confined S cylinder, and propagated along the cylinder length. Lithiation of S in the proximity of conductive carbon proceeded at the same rate as the one in the center of the pore, suggesting the presence of electron pathways at the Li₂S/S interface. Density of states calculations further confirmed this hypothesis. In situ electron diffraction showed a direct phase transformation of S into nanocrystalline Li₂S without detectable formation of any intermediates, such as polysulfides and LiS. These important insights may elucidate some of the reaction mechanisms and guide the improvements in the design of C–S nanocomposites for high specific energy Li–S batteries. The proposed use of conductive CNTs with tunable pore diameter as cylindrical reaction vessels for in situ TEM studies of electrochemical reactions proved to be highly advantageous and may help to resolve the ongoing problems in battery technology.     

Carbon nanotubes in blends of polycaprolactone/thermoplastic starch

Despite the importance of polymer–polymer multiphase systems, very little work has been carried out on the preferred localization of solid inclusions in such multiphase systems. In this work, carbon nanotubes (CNT) are dispersed with polycaprolactone (PCL) and thermoplastic starch (TPS) at several CNT contents via a combined solution/twin-screw extrusion melt mixing method. A PCL/CNT masterbatch was first prepared and then blended with 20 wt% TPS. Transmission and scanning electron microscopy images reveal a CNT localization principally in the TPS phase and partly at the PCL/TPS interface, with no further change by annealing. This indicates a strong driving force for the CNTs toward TPS. Young's model predicts that the nanotubes should be located at the interface. X-ray photoelectron spectroscopy (XPS) of extracted CNTs quantitatively confirms an encapsulation by TPS and reveals a covalent bonding of CNTs with thermoplastic starch. It appears likely that the nanotubes migrate to the interface, react with TPS and then are subsequently drawn into the low viscosity TPS phase. In a low shear rate/low shear stress internal mixer the nanotubes are found both in the PCL phase and at the PCL/TPS interface and have not completed the transit to the TPS phase. This latter result indicates the importance of choosing appropriate processing conditions in order to minimize kinetic effects. The addition of CNTs to PCL results in an increase in the crystallization temperature and a decrease in the percent crystallinity confirming the heterogeneous nucleating effect of the nanotubes. Finally, DMA analysis reveals a dramatic decrease in the starch rich phase transition temperature (∼26 °C), for the system with nanotubes located in the TPS phase.     

Preparation and properties of chitosan/carbon nanotube nanocomposites using poly(styrene sulfonic acid)-modified CNTs

Poly(styrene sulfonic acid)-functionalized carbon nanotubes (CNT-PSSA), which was obtained with atom transfer radical polymerization (ATRP), was utilized in preparation of chitosan/CNT nanocomposites (CH/CNT-PSSA). Chemical linkages between chitosan and CNTs form in the nanocomposites through the reaction between the sulfuric acid groups of CNT-PSSA and the amino groups of chitosan, to warrant the homogenous dispersion of CNTs. The CH/CNT-PSSA nanocomposites were superior to the neat chitosan polymer in thermal and mechanical properties, water and solvent uptakes, bond water ratios, and electrical conductivity. The attractive property of the CH/CNT-PSSA nanocomposites also implied their application potentials for separation membranes and sensor electrodes.     

Molecular dynamics study of core–shell structure from carbon nanotube and platinum nanowire

The interaction between the carbon nanotubes (CNTs) and platinum (Pt) nanowires (NWs) was investigated using forced field-based molecular dynamics (MD) simulations. Our results display that the Pt NW can induce the self-assembly of the CNTs to form a shell-core structure, this is because of the van der Waals interaction and the offset face-to-face π–π stacking interaction. The diameter of the CNT plays a major role in the formation of shell–core structure. Furthermore, the position of the CNT on the Pt NW also affects the formation of shell–core configuration, whereas the cross section of the NWs has a negligible effect on the fabrication process. Moreover, the interaction between multi-wall carbon nanotube and Pt nanowires was also discussed in detail, it is worth noting that the formation conformation of the CNT is also much more stable.     

All‐Nanomat Lithium‐Ion Batteries: A New Cell Architecture Platform for Ultrahigh Energy Density and Mechanical Flexibility

The ongoing surge in demand for high‐energy/flexible rechargeable batteries relentlessly drives technological innovations in cell architecture as well as electrochemically active materials. Here, a new class of all‐nanomat lithium‐ion batteries (LIBs) based on 1D building element‐interweaved heteronanomat skeletons is demonstrated. Among various electrode materials, silicon (Si, for anode) and overlithiated layered oxide (OLO, for cathode) materials are chosen as model systems to explore feasibility of this new cell architecture and achieve unprecedented cell capacity. Nanomat electrodes, which are completely different from conventional slurry‐cast electrodes, are fabricated through concurrent electrospinning (for polymeric nanofibers) and electrospraying (for electrode materials/carbon nanotubes (CNTs)). Si (or rambutan‐shaped OLO/CNT composite) powders are compactly embedded in the spatially interweaved polymeric nanofiber/CNT heteromat skeletons that play a crucial role in constructing 3D‐bicontinuous ion/electron transport pathways and allow for removal of metallic foil current collectors. The nanomat Si anodes and nanomat OLO cathodes are assembled with nanomat Al₂O₃ separators, leading to the fabrication of all‐nanomat LIB full cells. Driven by the aforementioned structural/chemical uniqueness, the all‐nanomat full cell shows exceptional improvement in electrochemical performance (notably, cell‐based gravimetric energy density = 479 W h kg_Cell^?1) and also mechanical deformability, which lie far beyond those achievable with conventional LIB technologies.     

A general and efficient cantilever functionalization technique for AFM molecular recognition studies

Atomic force microscopy (AFM) is a versatile technique for the investigation of noncovalent molecular associations between ligand–substrate pairs. Surface modification of silicon nitride AFM cantilevers is most commonly achieved using organic trialkoxysilanes. However, susceptibility of the Si? O bond to hydrolysis and formation of polymeric aggregates diminishes attractiveness of this method for AFM studies. Attachment techniques that facilitate immobilization of a wide variety of organic and biological molecules via the stable Si? C bond on silicon nitride cantilevers would be of great value to the field of molecular recognition force spectroscopy. Here, we report (1) the formation of stable, highly oriented monolayers on the tip of silicon nitride cantilevers and (2) demonstrate their utility in the investigation of noncovalent protein–ligand interactions using molecular recognition force spectroscopy. The monolayers are formed through hydrosilylation of hydrogen‐terminated silicon nitride AFM probes using a protected α‐amino‐ω‐alkene. This approach facilitates the subsequent conjugation of biomolecules. The resulting biomolecules are bound to the tip by a strong Si? C bond, completely uniform with regard to both epitope density and substrate orientation, and highly suitable for force microscopy studies. We show that this attachment technique can be used to measure the unbinding profiles of tip‐immobilized lactose and surface‐immobilized galectin‐3. Overall, the proposed technique is general, operationally simple, and can be expanded to anchor a wide variety of epitopes to a silicon nitride cantilever using a stable Si? C bond. © 2012 Wiley Periodicals, Inc. Biopolymers 97: 761–765, 2012.     

A theoretical study on the stability of CNT encased cyclic peptide beyond hydrogen bond cut-off

The Carbon nanotubes (CNT) are potential candidate for many biomedical applications especially in targeted drug delivery for cancer diseases. However, the use of CNT has limitations due to its insolubility in aqueous media. The self-assembly of cyclic peptide encased on the CNT has enhanced its dispersion in aqueous medium which extend their applications as antibacterial and drug delivery agents. To understand this process, an attempt has been made to investigate the dynamics and stability of trimer cyclic peptide encasing with CNT using classical molecular dynamics. The model cyclic peptide monomer constitutes 14 series of amino acids viz.; (cyclo-[(D-ARG-L-VAL-D-ARG-L-THR-D-AGR-L-LYS-D-GLY-L-ARG-D-ARG-L-ILE-D-ARG-L-ILE-D-PRO-L-PRO)]). Each cyclic peptide in the assembly stacking far apart at approximately 15 Å each other beyond hydrogen bond cut-off distance. The trimer was observed to be stable only over 10 ns of entire MD trajectory. But when there is electrostatic interaction between cyclic peptides at 6.5 Å distance then assembly is stable for entire 50 ns. Our result reveals that for a stable assembly, beyond the hydrogen bond cut-off distance, the electrostatic interaction plays significant role.     

Carbon nanotubes promote Cr(VI) reduction by alginate-immobilized Shewanella oneidensis MR-1

Bioreduction of hexavalent chromium (Cr(VI)) into trivalent one (Cr(III)) based on microbial immobilization techniques has been recognized as a promising way to remove Cr contaminants from wastewater. However, such a bioreduction process is inefficient due to limited electron transfer through the immobilization matrix. In this study, a modified immobilization process was proposed by impregnating carbon nanotubes (CNTs) into Ca-alginate beads, which were then used to immobilize Shewanella oneidensis MR-1 for enhanced Cr(VI) reduction. Compared with the free cells and the beads without CNTs, the AL/CNT/cell beads showed up to 4 times higher reduction rates, mainly attributed to an enhanced electron transfer by the CNTs. In addition, the dose of CNTs greatly improved the stability of beads, suggesting a high feasibility of the AL/CNT/cell beads for repeated use. The optimized CNT concentration, temperature and pH for Cr(VI) reduction by the AL/CNT/cell beads were 0.5%, 30 °C and 6.0–7.0, respectively.     

A dissipative particle dynamics model of carbon nanotubes

A mesoscale dissipative particle dynamics model of single wall carbon nanotubes (CNTs) is designed and demonstrated. The coarse-grained model is produced by grouping together carbon atoms and by bonding the new lumped particles through pair and triplet forces. The mechanical properties of the simulated tube are determined by the bonding forces, which are derived by virtual experiments. Through the introduction of van der Waals interactions, tube–tube interactions were studied. Owing to the reduced number of particles, this model allows the simulation of relatively large systems. The applicability of the presented scheme to model CNT based mechanical devices is discussed.     

Binding of nucleobases with graphene and carbon nanotube: a review of computational studies

Functionalized carbon nanotubes (CNTs) constitute a new class of nanostructured materials that have vast applications in CNT purification and separation, biosensing, drug delivery, etc. Hybrids formed from the functionalization of CNT with biological molecules have shown interesting properties and have attracted great attention in recent years. Of particular interest is the hybridization of single- or double-stranded nucleic acid (NA) with CNT. Nucleobases, as the building blocks of NA, interact with CNT and contribute strongly to the stability of the NA–CNT hybrids and their properties. In this work, we present a thorough review of previous studies on the binding of nucleobases with graphene and CNT, with a focus on the simulation works that attempted to evaluate the structure and strength of binding. Discrepancies among these works are identified, and factors that might contribute to such discrepancies are discussed.     

Macroporous Catalytic Carbon Nanotemplates for Sodium Metal Anodes

Because of its remarkably high theoretical capacity and favorable redox voltage (?2.71 V vs the standard hydrogen electrode), Na is a promising anode material for Na ion batteries. In this study, macroporous catalytic carbon nanotemplates (MC‐CNTs) based on nanoweb‐structured carbon nanofibers with various carbon microstructures are prepared from microbe‐derived cellulose via simple heating at 800 or 2400 °C. MC‐CNTs prepared at 800 °C have amorphous carbon structures with numerous topological defects, and exhibit a lower voltage overpotential of ≈8 mV in galvanostatic charge/discharge testing. In addition, MC‐CNT‐800s exhibit high Coulombic efficiencies of 99.4–99.9% during consecutive cycling at current densities ranging from 0.2 to 4 mA cm^?2. However, the carbon structures of MC‐CNTs prepared at 800 °C are gradually damaged by cycling. This results in significant capacity losses after about 200 cycles. In contrast, MC‐CNTs prepared at 2400 °C exhibit well‐developed graphitic structures, and maintain predominantly stable cycling behaviors over 1000 cycles with Coulombic efficiencies of ≈99.9%. This study demonstrates the superiority of catalytic carbon nanotemplates with well‐defined pore structures and graphitic microstructures for use in Na metal anodes.     

Interfacial mechanical behaviour of protein–mineral nanocomposites: A molecular dynamics investigation

Biological composite materials, such as bone, tooth and nacre, are comprised of a mixture of nano-sized hard components (e.g. mineral platelets) and soft components (e.g. protein molecules). Their mechanical behaviour greatly depends on the protein–mineral interfaces. This paper investigates the effects of mineral surface nanostructures on the interfacial interaction and mechanical behaviour of protein–mineral nanocomposites. Interfacial shear between osteopontin (OPN) and hydroxyapatite (HA) mineral layers with surface nanostructures is investigated using the atomistic molecular dynamics (MD) simulations. The results indicate that the OPN residues can be attached to HA surfaces but the surface nanostructures greatly affect the interfacial interaction and mechanical behaviour. The HA layers with a higher number of nano-sized grooves (defects) increase the surface roughness but reduce the pulling force and energy dissipation.