Characterization and Applications of Multiwalled Carbon Nanotubes

Multiwalled carbon nanotubes (MWNTs) have attracted great interest during thelast decade due to their possession of a unique set of properties. In addition totheir strength, MWNTs have well defined morphologies, with large aspect ratiosand pores in the meso range, and intriguing transport properties, such as highelectrical and thermal conductivity.We are interested in how variations in the MWNT morphology affect areas ofpossible engineering applications. We have identified morphology as a criticalelement for the performance of MWNTs in engineering applications. Specificareas studied and reported here spans from surface adsorption and capillarycondensation, to dispersion and dispersion processes, and transport propertiesin relation to MWNT aspect ratio. This wide range of exploration is typicallyneeded for evaluating opportunities for new materials.MWNTs can be used in different types of adsorption systems and it should bepossible to tailor the MWNT morphology to suit a specific adsorption process.We found that the major part of butane, our model gas, adsorbs on the externalMWNT and only a small fraction ends up in the pores.The unusually large aspect ratio makes MWNTs ideal as fillers in polymermatrixes. Since MWNTs are electrically conductive, it is possible to align theMWNTs in the matrix before curing. We investigated the effect of AC-fields onaqueous MWNT dispersions and the possibility to align MWNTs in an electricalfield.It is also necessary to develop suitable dispersion methods, to enable theproduction of homogeneous dispersions and composites. We studied a numberof different mechanical dispersion methods and their effect on the MWNTmorphology

Encapsulation of gases in powder solid matrices and their applications: A review

Gas encapsulation in solid matrices can be an important means to sequester harmful or greenhouse gases and to store useful gases for their subsequent release for a targeted application. In this review, recent developments, the characteristics and gas adsorption capacity of non-organic and organic solid powder matrices (e.g. activated carbons, carbon nanotubes, zeolites, metal-organic frameworks, and cyclodextrins); and potential applications of their complexes in various fields (energy, environment protection, nano-device production, medicine, and food and agriculture productions) are described

Modeling of gas adsorption on graphene nanoribbons

We present a theory to study gas molecules adsorption on armchair graphene nanoribbons (AGNRs) by applying the results of \emph{ab} \emph{initio} calculations to the single-band tight-binding approximation. In addition, the effect of edge states on the electronic properties of AGNR is included in the calculations. Under the assumption that the gas molecules adsorb on the ribbon sites with uniform probability distribution, the applicability of the method is examined for finite concentrations of adsorption of several simple gas molecules (CO, NO, CO$_2$, NH$_3$) on 10-AGNR. We show that the states contributed by the adsorbed CO and NO molecules are quite localized near the center of original band gap and suggest that the charge transport in such systems cannot be enhanced considerably, while CO$_2$ and NH$_3$ molecules adsorption acts as acceptor and donor, respectively. The results of this theory at low gas concentration are in good agreement with those obtained by density-functional theory calculations.Comment: 7 pages, 6 figure

INVESTIGATION OF ADSORPTION, REACTION AND CONFINEMENT OF MOLECULES IN SINGLE WALL CARBON NANOTUBES

Adsorption of simple molecules (CF4, Xe, CO2, NO and H2O) inside single wall carbon nanotubes has been investigated by means of infrared spectroscopy. It was demonstrated that confinement has a profound effect of the IR spectra of the internally adsorbed species. The spectral changes relate to the enhanced binding of the adsorbates to the nanotube interior walls and to the spatial limitations that prohibit formation of bulk-like structures.It was found that CF4 exhibits a 15 cm-1 redshift in its í3 symmetric stretching mode when adsorbed on the exterior surface of closed SWNTs. Adsorption on the nanotube is accompanied by adsorption in the interior in the case of opened SWNTs and the í3 mode is redshifted 35 cm-1. In addition it was shown that confined CF4 does not exhibit LO-TO splitting observed in the bulk phase. Physisorption of NO inside of carbon nanotubes results in cis-(NO)2 dimer formation for almost all adsorbed NO, indicating that confinement shifts the equilibrium according to Le Chatelier's Principle. In all cases Xe was used as a displacing agent to verify the internal adsorption. It was shown that Xe preferentially adsorbs inside nanotube displacing high coverage CF4 molecules. The externally bound adsorbates do not form a full monolayer and therefore Xe adsorbs non-competitively on empty external sites. Confinement of H2O in the nanotube interior leads to appearance of a sharp mode at 3507 cm-1 that is indicative of a weaker hydrogen bond relative to hydrogen bonding in bulk ice. Molecular simulations show that the confined water forms stacked ring structures with bulk-like intra-ring and weaker inter-ring hydrogen bonds. The spectroscopy studies of adsorption in nanotubes were accompanied by nitrogen volumetric adsorption studies of bulk nanotubes. It was demonstrated that n-nonane can be utilized as a nanotube interior blocking agent. The oxidation of SWNTs by ozone, followed by heating to remove oxidized carbon atoms as carbon oxides occurs preferentially on the outer surface of bulk samples of nanotubes. The high surface reactivity of O3 at the outer surface of a bulk nanotube sample causes this effect.It was found that CF4 exhibits a 15 cm-1 redshift in its í3 symmetric stretching modewhen adsorbed on the exterior surface of closed SWNTs. Adsorption on the nanotube isaccompanied by adsorption in the interior in the case of opened SWNTs and the í3 mode isredshifted 35 cm-1. In addition it was shown that confined CF4 does not exhibit LO-TO splittingobserved in the bulk phase.Physisorption of NO inside of carbon nanotubes results in cis-(NO)2 dimer formation foralmost all adsorbed NO, indicating that confinement shifts the equilibrium according to LeChatelier's Principle.In all cases Xe was used as a displacing agent to verify the internal adsorption. It wasshown that Xe preferentially adsorbs inside nanotube displacing high coverage CF4 molecules.The externally bound adsorbates do not form a full monolayer and therefore Xe adsorbs noncompetitivelyon empty external sites

Statistical Mechanical and Quantum Mechanical Modeling of Condensed Phase Systems

Understanding adsorption in nanoporous media such as carbon nanotubesare vital to improving fluid storage and separations processes. Onemajor objective of this research is to shed light on an on-goingcontroversy in literature over where gases adsorb on single walledcarbon nanotube bundles. Grand-canonical Monte Carlo simulations havebeen performed using models of carbon nanotube bundles composed oftubes of all the same diameter (homogeneous) and tubes of differentdiameters (heterogeneous). We use three metrics with which we compareour simulation results to those found in experiments on HiPconanotubes: the specific surface area, the isosteric heat ofadsorption, and adsorption capacity. Simulations of classicallybehaved fluids Ar, CH$_4$, and Xe indicate that nanotubes prepared bythe HiPco process are best described by a model consisting ofheterogeneous bundles with $sim11\%$ of the nanotubes opened. Nerequires additional considerations to describe the quantum effects atthe temperatures of interest. Simulation results from Ne simulationsare consistent with those from classical fluids. However, Nesimulations strongly indicate that the small interstitial channelsformed by exactly three nanotubes are closed. Combined with previousstudies on classically behaved fluids Ar, CH$_4$, and Xe, experimentaldata including Ne adsorption are best matched by hetergeneous bundleswith $sim11\%$ open nanotubes.The development of a heterogeneous Co/C/O reactive force field(ReaxFF) potential has also been a major objective of this research.ReaxFF provides a method to describe bond-breaking and bond-formingevents that can be applied to large-scale molecular dynamicssimulations. The many-bodied semi-empirical potential has been trainedfrom emph{ab initio} density functional theory calculations. Thetraining set originally included description of bulk and surfacecondensed phase cobalt systems, but was later expanded to includebinary (Co/C, Co/O) and tertiary (Co/C/O) heterogeneous interactions.We have tested these parameters against additional DFT calculationsnot included in the fitting data set. The parameter optimization hasproduced a force field capable of describing new configurations withsame accuracy as those used in the fitting procedure. The optimizedparameters have been used to predict the melting point and diffusioncoefficients of bulk fcc cobalt. Large-scale simulations of a Co$_2$Cphase nanoparticle show segregation on short time scales (less than300 ps), with all C atoms forming graphene precursors on the surfaceof a Co nanoparticle core. ReaxFF has also been used to showdiffusion of Co is more energetically favorable than oxygen through acobalt oxide crystal. This is consistent with experimentalobservations that oxidized cobalt nanoparticle form hollow cobaltoxide nanospheres. These two binary applications demonstrate thatReaxFF is transferable to heterogeneous systems and a computationallyinexpensive means by which transition metal surface reactions can beexplored

Application of Various Adsorbents to Remove Micro-Pollutants in Aquatic System

Untreated or insufficiently treated pharmaceuticals and endocrine disrupting compounds (EDCs) as well as heavy metals have influenced the ecosystem and their exposures in the water system have threatened human health; causing cancers and adverse health effect to immune system. While various water treatment techniques have been applied to solve this problem, adsorption has been considered as one of the most efficient and manageable water purification techniques. Advanced analysis methods for aqueous contaminants have improved comprehension, allowing proficiency about the fate of trace leveled emerging contaminants, thus allowed to reveal the adsorption mechanisms of each pollutant. This dissertation focuses on the investigation of adsorption for micro-pollutants in aquatic environment via the application of different types of carbonaceous (powdered activated carbon, carbon nanotubes, and biochars) and biodegradable (chitosan) adsorbent. The effect of water chemistry conditions such as pH, concentration of ionic strength induced species and natural organic matters were considered as significant factors to increase or decrease the adsorption capacity of each adsorbent. This study also illuminates the use of biochar, byproduct of bio-oil, with simple chemical activation as an efficient adsorbent for pharmaceutical and EDCs removal. In-depth analysis about adsorption between these micro-pollutants and biochars was performed by characterization of physicochemical properties by nuclear magnetic resonance analysis in conjunction with molecular modeling subsequently interpreting the binding energy. Aromaticity and composition of carbonaceous structure of adsorbent controlled the adsorption capacity, while hydrophobicity of adsorbates influenced the adsorption affinity toward the adsorbents. More specifically, the presence of adsorption competitors resulted in less effective binding due to a combination of less favorable binding energy, polarity, and π-energy with the adsorbent and electrostatic repulsion from the cosolutes that occupied adsorption sites

Predicting Adsorption Affinities of Small Molecules on Carbon Nanotubes Using Molecular Dynamics Simulation

Citation: Comer, J., Chen, R., Poblete, H., Vergara-Jaque, A., & Riviere, J. E. (2015). Predicting Adsorption Affinities of Small Molecules on Carbon Nanotubes Using Molecular Dynamics Simulation. ACS Nano, 9(12), 11761–11774. https://doi.org/10.1021/acsnano.5b03592Computational techniques have the potential to accelerate the design and optimization of nanomaterials for applications such as drug delivery and contaminant removal; however, the success of such techniques requires reliable models of nanomaterial surfaces as well as accurate descriptions of their interactions with relevant solutes. In the present work, we evaluate the ability of selected models of naked and hydroxylated carbon nanotubes to predict adsorption equilibrium constants for about 30 small aromatic compounds with a variety of functional groups. The equilibrium constants determined using molecular dynamics coupled with free-energy calculation techniques are directly compared to those derived from experimental measurements. The calculations are highly predictive of the relative adsorption affinities of the compounds, with excellent correlation (r ? 0.9) between calculated and measured values of the logarithm of the adsorption equilibrium constant. Moreover, the agreement in absolute terms is also reasonable, with average errors of less than one decade. We also explore possible effects of surface loading, although we demonstrate that they are negligible for the experimental conditions considered. Given the degree of reliability demonstrated, we move on to employing the in silico techniques in the design of nanomaterials, using the optimization of adsorption affinity for the herbacide atrazine as an example. Our simulations suggest that, compared to other modifications of graphenic carbon, polyvinylpyrrolidone conjugation gives the highest affinity for atrazine—substantially greater than that of graphenic carbon alone—and may be useful as a nanomaterial for delivery or sequestration of atrazine

ENVIRONMENTALLY RELEVANT ADSORPTION ON CARBONACEOUS SURFACES STUDIED BY OPTICAL DIFFERENTIAL REFLECTANCE AND TEMPERATURE PROGRAMMED DESORPTION

This study evaluated the application of a versatile optical technique to study the adsorption and desorption of model adsorbates representative of volatile polar (acetone) and non-polar (propane) organic compounds on a model carbonaceous surface under ultra high vacuum (UHV) conditions. The results showed the strong correlation between optical differential reflectance (ODR) and adsorbate coverage determined by temperature programmed desorption (TPD). The ODR technique was found to be a powerful tool to investigate surface adsorption and desorption from UHV to high pressure conditions. The effects of chemical functionality and surface morphology on the adsorption/desorption behavior of acetone, propane and mercury were investigated for two model carbonaceous surfaces, namely air-cleaved highly oriented pyrolytic graphite (HOPG) and plasma-oxidized HOPG. Oxygen-containing functional groups exist on both air-cleaved and plasma-oxidized HOPG. They can be removed by thermal treatment (> 500 K). The presence of these groups almost completely suppresses propane adsorption at 90 K and removal of these groups leads to a dramatic increase in adsorption capacity. The amount of acetone adsorbed is independent of surface heat treatment and depends only on total exposure. The effect of morphological heterogeneity is evident for plasma-oxidized HOPG as this substrate provides greater surface area, as well as higher energy binding sites. Mercury adsorption at 100 K on HOPG surfaces with and without chemical functionalities and topological heterogeneity created by plasma oxidation occurs through physisorption. The removal of chemical functionalities from HOPG surface enhances mercury physisorption. Plasma oxidation of HOPG provides additional surface area for mercury adsorption. Mercury adsorption by activated carbon at atmospheric pressure occurs through two distinct mechanisms, physisorption below 348 K and chemisorption above 348 K. No significant impact of oxygen functionalities was observed in the chemisorption region. The key findings of this study open the possibility to apply scientific information obtained from studies with simple surfaces like HOPG under ideal conditions (UHV) to industrial sorbents under realistic process conditions. HOPG surfaces can be modified chemically and topologically by plasma oxidation to simulate key features of activated carbon adsorbents

Removal of Heavy Metals Using Adsorption Processes Subject to an External Magnetic Field

Adsorption is a broadly used process for the removal of heavy metals and the world trend is directed to the application of new technologies to intensify existing processes. The properties of the magnetic field (intensity and arrangement) and the intrinsic magnetic properties of the adsorbent and the adsorbate are decisive for satisfactory results. The intensity of the magnetic field is important, because this implies that the greater number of spins present will align with the magnetic field according to the magnetic nature present, allowing the mobility of the adsorbate and generating heterogeneity on the surface of the adsorbent. Similarly, the arrangement of the magnetic field will determine the direction of the magnetic field lines. The application of a magnetic field as an alternative for the intensification of the adsorption process based on the consideration that the magnetic field is safe, environmentally friendly and economic