Universality in adsorbate ordering on nanotube surfaces

Numerically efficient transfer matrix technique for studying statistics of coherent adsorbates on small nanotubes has been developed. In the framework of a realistic microscopic model fitted to the data of ab initio calculations taken from literature sources, the ordering of potassium adsorbate on (6,0) single-walled carbon nanotube has been studied. Special attention has been payed to the phase transition-like abrupt changes seen in the adsorption isotherms at low temperature. It has been found that the behavior during the transitions conforms with the universality hypothesis of the theory of critical phenomena and is qualitatively the same as in the one dimensional Ising model. Quantitatively the critical behavior can be fully described by two parameters. Their qualitative connection with the properties of interphase boundaries is suggested but further research is needed to develop a quantitative theory.Comment: 11 pages, 6 figures; some typos correcte

Spectroscopic and Microscopic Characterisation of Carbon Nanostructures

Currently, carbon nanotubes (CNTs) are produced using a variety of techniques which yield CNT materials with wide ranging levels of chemical purity and structural perfection. Consequently, characterising CNT materials accurately is of utmost importance if potential applications of CNTs are to be realised on a large scale. In this work four commercially available CNT samples are characterised using a number of techniques, namely: scanning electron microscopy (SEM); high resolution transmission microscopy (HRTEM); energy dispersive x-ray analysis (EDX); Auger electron spectroscopy (AES); low-loss electron energy loss spectroscopy (low-loss EELS); ultra-violet photoemission spectroscopy (UPS); x-ray photoemission spectroscopy (XPS) and Raman spectroscopy. The information provided by these techniques is assessed in their ability to characterise different CNT materials. The definition of CNT ‘quality’ is also discussed and the ability of these techniques to determine such a property is considered. A significant part of ascertaining the ‘quality’ of a CNT sample lies in understanding the nature and number of defects in the walls of these materials. In this work, defects are introduced into the lattice of different CNT species using 1.5 keV Ar+ ions and the effects are monitored using XPS. In particular, the resultant reactivity of irradiated CNTs to ambient atmospheric oxygen is investigated, which is found to be markedly enhanced for CNTs with one wall when compared to those with multiple walls. It is also demonstrated that the type of incident ion and irradiation dose can be used to selectively control the level and nature of the surface composition of oxygen functionalised SWCNTs. Many applications of fullerenes require detailed understanding of how these molecules interact with surfaces and the perturbations this induces. In this work the interaction of C60 with highly ordered pyrolytic graphite (HOPG) and Ni(110) is studied using scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), XPS and UPS. Investigation focuses on a novel two-dimensional solid-vapour phase in the C60-HOPG system and the C60-induced reconstruction of the Ni(110) surface

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

Hydrogen storage methods

Hydrogen exhibits the highest heating value per mass of all chemical fuels. Furthermore, hydrogen is regenerative and environmentally friendly. There are two reasons why hydrogen is not the major fuel of today's energy consumption. First of all, hydrogen is just an energy carrier. And, although it is the most abundant element in the universe, it has to be produced, since on earth it only occurs in the form of water and hydrocarbons. This implies that we have to pay for the energy, which results in a difficult economic dilemma because ever since the industrial revolution we have become used to consuming energy for free. The second difficulty with hydrogen as an energy carrier is its low critical temperature of 33K (i.e. hydrogen is a gas at ambient temperature). For mobile and in many cases also for stationary applications the volumetric and gravimetric density of hydrogen in a storage material is crucial. Hydrogen can be stored using six different methods and phenomena: (1) high-pressure gas cylinders (up to 800bar), (2) liquid hydrogen in cryogenic tanks (at 21K), (3) adsorbed hydrogen on materials with a large specific surface area (at T<100K), (4) absorbed on interstitial sites in a host metal (at ambient pressure and temperature), (5) chemically bonded in covalent and ionic compounds (at ambient pressure), or (6) through oxidation of reactive metals, e.g. Li, Na, Mg, Al, Zn with water. The most common storage systems are high-pressure gas cylinders with a maximum pressure of 20MPa (200bar). New lightweight composite cylinders have been developed which are able to withstand pressures up to 80MPa (800bar) and therefore the hydrogen gas can reach a volumetric density of 36kg·m−3, approximately half as much as in its liquid state. Liquid hydrogen is stored in cryogenic tanks at 21.2K and ambient pressure. Due to the low critical temperature of hydrogen (33K), liquid hydrogen can only be stored in open systems. The volumetric density of liquid hydrogen is 70.8kg·m−3, and large volumes, where the thermal losses are small, can cause hydrogen to reach a system mass ratio close to one. The highest volumetric densities of hydrogen are found in metal hydrides. Many metals and alloys are capable of reversibly absorbing large amounts of hydrogen. Charging can be done using molecular hydrogen gas or hydrogen atoms from an electrolyte. The group one, two and three light metals (e.g. Li, Mg, B, Al) can combine with hydrogen to form a large variety of metal-hydrogen complexes. These are especially interesting because of their light weight and because of the number of hydrogen atoms per metal atom, which is two in many cases. Hydrogen can also be stored indirectly in reactive metals such as Li, Na, Al or Zn. These metals easily react with water to the corresponding hydroxide and liberate the hydrogen from the water. Since water is the product of the combustion of hydrogen with either oxygen or air, it can be recycled in a closed loop and react with the metal. Finally, the metal hydroxides can be thermally reduced to metals in a solar furnace. This paper reviews the various storage methods for hydrogen and highlights their potential for improvement and their physical limitation

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

Quasiparticle dynamics in graphene

The effectively massless, relativistic behaviour of graphene's charge carriers—known as Dirac fermions—is a result of its unique electronic structure, characterized by conical valence and conduction bands that meet at a single point in momentum space (at the Dirac crossing energy). The study of many-body interactions amongst the charge carriers in graphene and related systems such as carbon nanotubes, fullerenes and graphite is of interest owing to their contribution to superconductivity and other exotic ground states in these systems. Here we show, using angle-resolved photoemission spectroscopy, that electron–plasmon coupling plays an unusually strong role in renormalizing the bands around the Dirac crossing energy—analogous to mass renormalization by electron–boson coupling in ordinary metals. Our results show that electron–electron, electron–plasmon and electron–phonon coupling must be considered on an equal footing in attempts to understand the dynamics of quasiparticles in graphene and related systems

Synthesis, characterization and evaluation of ionic liquids and polymeric ionic liquids/functionalized multiwalled carbon nanotubes for Cr(VI) adsorption.

Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.In this study, a series of imidazolium and pyridinium-based ionic liquids (ILs), polymeric ionic liquids (PILs), and their carbon nanotubes-functionalized composites were synthesized, characterized and applied as potential adsorbents for hexavalent Cr(VI). Polymeric ionic liquids of different polymerizable moieties (vinyl and styrenic moieties) were studied. Furthermore, multi-walled carbon nanotubes (MWCNTs) were synthesized, characterized and dispersed on both imidazolium and pyridinium-based ILs and PILs, respectively. Thermal studies revealed that vinyl pyridinium PILs possess good thermal stability than the vinyl imidazolium counterparts. The size of the counter-anions bromide (Br-), hexafluorophosphate (PF6-), and bis(trifluoromethanesulfonyl) imide (TFSI-) and the charge delocalization in cationic rings greatly influenced the glass transition temperatures of PILs. Expectedly, pyridinium and imidazolium-based PILs with hexafluorophosphate ions showed poor solubility in polar protic solvents (water, methanol) and good solubility in polar aprotic solvents (DMSO, DMF, THF) except acetone (a dipolar aprotic solvent). The as-synthesized ILs/MWCNT composites were characterized using FTIR spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and thermal analysis. The results obtained indicate that the pyridinium-based ILs exhibited higher decomposition temperatures (above 400 °C) compared to imidazolium-based ILs counterparts (onset decomposition at 250 °C) with poor water-solubility and their glass transition temperatures were dependent on ion mobility. The effect of the alkyl lateral chain (propyl and isopropyl) at the first and third position of imidazolium and N-position of pyridinium cationic rings towards their thermal stability, conductivity, and solubility of the ionic liquids was investigated. Their solubilities in different polar and non-polar solvents were also investigated. Spectroscopic and microscopic analyses confirmed the formation of the ILs/MWCNT composites with new functionalities and unaltered surface morphology of carbon nanotubes. Pyridinium and imidazolium-based PILs/MWCNT composites were characterized by thermal, spectroscopic, and electron microscopy techniques. It was observed that the composites were thermally stable compared to the corresponding precursors and were insoluble in polar aprotic solvents. For application, solid-liquid adsorption process was used in the adsorption of Cr(VI) from aqueous solution using the as-synthesized ILs/MWCNT and PILs/MWCNT composites as adsorbents. Under batch adsorption experiments, the effect of solution pH, contact time and initial concentration of Cr(VI) were investigated. It was established that the adsorption of Cr(VI) took place under acidic conditions (pH=2-3), thereby confirming significant adsorption of dichromate (Cr2O7-) and hydrochromate (HCrO4-) anions. At lower pH values, the ionic and π-anionic electrostatic interactions between the positively-charged regions of the composites and Cr(VI) were believed to have facilitated the adsorption of anionic (Cr2O7-) and (HCrO4-). Adsorption results obtained based on contact time showed that increase in contact time gradually increases the adsorption of Cr(VI) within 2 h. However, further increase in experimental contact time above 2 h insignificantly affected the adsorption of Cr(VI) due to early or quick oversaturation of the surface active sites on the adsorbents. The adsorption of Cr(VI) onto ILs/MWCNT and PILs/MWCNT composites fitted well into both Langmuir and Freundlich adsorption isotherms. However, the homogeneity/heterogeneity nature of the adsorbents relied on the diversified nature of the composites, which includes bulky pyridinium and imidazolium organic cations with delocalized charges, some large counter anions and the graphitic functional carbon groups. In order to understand the mechanisms of the adsorption of Cr(VI) onto ILs/MWCNT and PILs/MWCNT composites, pseudo-second-order kinetic model was employed. The results obtained showed that the calculated maximum adsorption capacities (qecal) and experimental maximum adsorption capacities (qe.exp) depict high correlation co-efficiencies (R2>0.99) confirming the applicability and feasibility of pseudo-second-order model on the adsorption of Cr(VI) in this study.Abstract available in pdf

High Pressure Hydrogen Storage on Carbon Materials for Mobile Applications

Recognising the difficulties encountered in measuring the adsorption of hydrogen at high pressure, a reliable volumetric differential pressure method of high accuracy and good repeatability has been developed for measurement up to ca 100 bar. The apparatus used has two identical limbs, a sample and a blank limb, between which a high accuracy differential pressure cell measures changes in pressure. By simultaneously expanding the two limbs and closely controlling the temperature of the entire system, many of the errors due to expansion of the gas can be avoided. In addition, helium blank measurements are used as a base line correction, which substantially reduces the effects caused by the rapid expansion of gas through a small port. Using this method, the hydrogen storage capacities of relatively small samples (1.0-2.5 g) of a selection of carbon materials have been accurately measured to a conservative limit of detection of 0.05 wt% and an accuracy of +/-0.02 wt%. The accuracy of the apparatus has been proven using lanthanide nickel (LaNi5), which has a known hydrogen storage capacity of 1.5 wt%, as a standard. The method has also been developed in order to analyse samples at elevated temperatures of up to 270 C. This has been demonstrated using lithium nitride (Li3N) compounds. The carbon materials studied include a series of activated carbons, carbon nanofibres (CNF) and carbon nanotubes (CNT). The activated carbons have displayed almost instantaneous hydrogen uptake independent of the degas method used, which indicates that sorption occurs via a physisorption mechanism. The series of powdered activated carbons have displayed direct correlation between the BET surface area and the hydrogen sorption capacity. The largest hydrogen sorption capacity observed for activated carbons was for a chemically activated carbon with a surface area of 3100 m2 g-1, achieving an uptake of 0.6 wt%. The preparation of CNF, grown from ethylene over mixed copper, iron and nickel alloy catalysts, has been extensively investigated. Control of the parameters of preparation has allowed the formation of CNF with surface areas of 10 - 500 m2 g-1, diameters of 100 - 1000 nm, lengths of 1-10s nm, gas conversions of 0-90 % and the formation of herringbone and platelet CNF structures. The CNF studied have been observed to be capable of adsorbing a maximum of 0.5 wt% hydrogen at 100 bar and ambient temperature. Only one of the materials studied was observed to break by a significant amount the trend of surface area vs hydrogen sorption capacity, observed for the activated carbons. This was a single-walled nanotube (SWNT) sample which achieved ca 1.6 wt% after slow carbon dioxide activation at low temperature. This larger sorption is hypothesised to result from the hydrogen slowly diffusing into the SWNT through defects in the structure and between the graphite planes in the CNF

Graphene-Fibers Hybrid Structures as Adsorbents for Heavy Metal Ions in Aqueous Solutions

The project focuses on the study of graphene-fiber hybrid structures for adsorption of heavy metal ions in aqueous solutions. Polyvinyl alcohol-based graphene–fiber structures were created using centrifugal spinning and a carbonization process. Characterization methods of the graphene–fiber hybrid structures (GFHS) include SEM, FTIR, TGA, and EDX. Single-step batch-type adsorption studies were performed to analyze the interaction of Cu (II) and Pb (II) ions onto GFHS surface. Different heavy metal ion concentrations were used, as well as a variation of pH values. Elemental analysis of the adsorbent’s surface after filtration experiments was studied by EDX and spectroscopy to verify the presence of the metal ions on the surface. Spectrometry through AAS was conducted on the liquid phase after adsorption of Pb(II) ions at pH 5, results showed a 53% Pb(II) removal