Synthesis and study of carbon nanotubes and carbon spheres

ABSTRACT The synthesis of multi-walled carbon nanotubes (MWCNTs) and carbon spheres (CSs) was achieved using catalytic and non-catalytic chemical vapour deposition processes (CVD) respectively. Fe-Co bimetallic catalysts supported on CaCO3 were prepared by a wet impregnation (IMP), a deposition-precipitation (DP) and a reverse micelle method (RM). The sizes of the Fe and Co particles were not affected by the Fe and Co sources (nitrate, acetate) when the wet impregnation and deposition-precipitation methods were used. High quality ‘clean’ multi-walled carbon nanotubes (MWCNTs) were obtained from all three Fe-Co synthesis procedures under optimized reaction conditions. The CNTs produced gave yields ranging from 623% - 1215% in 1 h under the optimum conditions, with similar outer diameters (o.d.) of 20 - 30 nm and inner diameters (i.d.) ~ 10 nm. The Fe-Co catalyst formed in the wet impregnation method revealed that the yield, outer diameter and purity of the CNTs were influenced by C2H2/N2 ratios, time and temperature. All the methods gave high quality CNTs after short reaction times but the quality deteriorated as the synthesis time was increased from 5 - 360 min. Indeed, the influential parameter in controlling CNT purity, length and outer diameter was found to be the synthesis time. In order to control the i.d. of the CNTs, the three methods of catalyst preparation were employed with the aim of controlling the Fe-Co catalyst particle sizes. It was observed that the IMP and DP methods were less effective in controlling the size of the metal particles. A reverse micelle process was used to synthesize Fe-Co nanoparticles that were highly crystalline and uniform in size. The reverse micelle technique displayed the ability to prepare nanoparticles of controlled size (3, 6 and 13 nm) obtained by varying the concentrations of Fe and Co in the micelle. By using the RM method, smaller diameter CNTs could be obtained compared with the IMP and DP methods. The CNT i.d. was found to correlate with the size of the catalyst particle used. The effect of synthesis time on CNT widths was investigated for the first time. In this study the issue of carbon build up on the CNTs as a function of time was investigated. It was observed that both the CNT yield and the outer diameters increased with time. With increase in synthesis time, the tubes broke into small fragments. The use of excess C2H2 resulted in the deposition of carbon on the already formed CNTs and it is this deposited carbon that caused tube fragmentation. MWCNTs with unusual rough surfaces (including pits) were synthesized by the CVD of acetylene using a novel Ca3(PO4)2-CaCO3 support mixture. Mixtures of Ca3(PO4)2- CaCO3 (0/100 to 100/0) yielded tubes with very rough surfaces and the CNT yield increased as the amount of CaCO3 in the support mixture was increased. The inner walls of the CNTs possessed a regular orientation of crystalline graphite sheets (3 - 5 nm) while the outer surface of the CNTs had a thick, rough, compact layer (~ 30 nm) of carbon with a random orientation of graphite sheets. The production of pure carbon spheres (CSs) was achieved in the absence of a catalyst through the direct pyrolysis of acetylene and ethylene in a horizontal CVD reactor. The detailed experiments conducted with acetylene as a precursor indicated that the diameters of the CSs could be controlled by varying the pyrolysis conditions (e.g. temperature and synthesis time) and that the process could readily be scaled up for commercial production. This process thus provides a variant of the carbon black synthesis procedure. The effect of using oxygenates (alcohol C:O ratio dependence) on the CS morphology was also investigated. CSs were also synthesized in a vertical swirled floating catalytic chemical vapour deposition (SFCCVD) reactor for the first time. This process allowed for continuous and large scale production of these materials. The CSs were obtained by the direct pyrolysis of acetylene in an inert atmosphere without the use of a catalyst. The effect of pyrolysis temperatures and the flow rate of argon carrier gas on the size, quality and quantity of the synthesized carbon spheres were investigated. TEM analysis of the carbon materials revealed graphitic spheres with a smooth surface and uniform diameter that could be controlled by varying reaction conditions (size: 50 - 250 nm). The materials were spongy and very light. It was established that under controlled experimental parameters, sphere size is also regulated by the structural and bonding properties of a hydrocarbon source such as carbon/hydrogen (C:H) content, hybridization and isomerism

Mobility and Transformation of Inorganic Contaminants in Mining-impacted Groundwater

Mining often results in the contamination of groundwater by metal, sulphate and radionuclide ions following their percolation from tailings impoundments. This chapter discusses the processes by which elements within tailings are transformed and translocated to groundwater and the role of aquifer characteristics and colloids in these processes. The prevention and remediation of contaminated groundwater is also discussed, with particular attention given to the use of permeable reactive barriers and sulphate reducing bacteria

Spectroscopic determination of water salinity in brackish surface water in Nandoni Dam, at Vhembe District, Limpopo Province, South Africa

The problem of limited water supply in the Vhembe District (Limpopo Province, South Africa) is exacerbated by a preponderance of dissolved salts, which cause disagreeable taste and odour in the water as reported by the communities using this water for drinking. The water treatment plant that supplies the treated water to the communities in the District sources this raw water from the Nandoni Dam at the Luvuvhu river catchment. There are no scientific studies that have been reported in the literature that focused on determining the levels of water salinity from various water sources in the municipalities of the District. Water samples from various sites across the Nandoni Dam, a primary source of domestic water supply in the region, were collected through each season over a period of twelve months in order to ascertain the concentrations of dissolved salts in the dam. Onsite analyses of the water samples were conducted using the YSI ProDSS multimeter, while the laboratory water analyses were conducted using the spectroquant and atomic absorption spectrometers. Although salinity tests seem to indicate that the water sampled across most of the Nandoni Dam is brackish during all seasons of the year with the highest being 750 mg/L, water samples from the dam mid-outlet and the treatment plant are slightly below the World Health Organization (WHO) brackish water bracket of 500 mg/L with unfavourable taste for drinking. Results from this study indicate that the water sourced from the Nandoni Dam is not suitable for human consumption and therefore requires integrated water resource management, as well as robust and cost-effective water desalination treatment

Student evaluation of career readiness after completing the science education course at Nazarene Teachers College, Kingdom of Swaziland

This exploratory study sought to determine how the current science education curriculum at Nazarene Teachers College contributes to the final (third) year’s students’ career readiness from their own perspectives. A self-administered questionnaire was used for data collection. The target population was 150 third year students; 110 responded to the questionnaire which had a response rate of 79%. To describe the biographical information of the respondents, frequencies and percentages were used. Tables of frequency distribution were then used to deduce the respondents’ responses to the group of questionnaire questions that probe the various career readiness concepts. Scale reliability testing was also used to determine the reliability of career readiness concepts. An overall measure of career readiness was calculated for each respondent and the effect of biographical properties as part of evaluation of respondents. The empirical (quantitative survey)showed that the science education course at the Nazarene Teachers College indeed prepares the students for their career readiness with specific reference to the teaching profession although some aspects can be improved. Therefore, it was recommended that there should be increased time allocated to science education and its teaching approaches; that a suitable science laboratory with all the required equipment for experiential learning should be constructed; the current curriculum content with regards to the relevance for primary school science should be reviewed; and the lecturers should be workshopped on the use of interactive methods of teaching science to facilitate active engagement with students. This study reveals that the science education course at the Nazarene Teachers College strengthened the students’ attitudes towards teaching science; improved their skills and confidence to teach science and enabled them to focus on a learner-centred approach. Students indicated that the use of specific teaching methods and the use of teaching aids contributed in a significant way to their preparation as teachers in the classroom. However, the students rated the contribution science makes to society as of minor importanceCurriculum and Instructional StudiesM. Ed. (Natural Science Education

The Synthesis of Carbon Nanomaterials using Chlorinated Hydrocarbons over a Fe-Co/CaCO3 Catalyst

The effect of chlorine on the morphology of carbon nanotubes (CNTs) prepared from a Fe-Co/CaCO3 catalyst was investigated using chlorobenzene (CB), dichlorobenzene (DCB), trichlorobenzene (TCB), dichloroethane (DCE), trichloroethane (TCE) and tetrachloroethane (TTCE) as chlorine sources using a catalytic chemical vapour deposition (CCVD) method. Toluene was used as a chlorine-free carbon source for comparison. Multi-walled carbon nanotubes (MWCNTs) were successfully synthesized. The physicochemical properties of the CNTs were studied using transmission electron microscopy (TEM), Raman spectroscopy, thermal gravimetric analysis (TGA), energy-dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (PXRD) spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques. The inner and outer diameters of the MWCNTs increased with an increase in the number of chlorine atoms contained in the reactant. Chlorine incorporation into the MWCNTs was observed by EDS analysis for all reactants. Formation of ‘bamboo-like’ structures for the MWCNTs generated from TCE and TTCE was also observed, facilitated by the presence of the high percentage of chlorine in these reactants. Numerous MWCNTs revealed the presence of small carbon nanostructures that grew on top of the dominant CNTs, suggesting an unexpected secondary carbon growth mechanism.KEYWORDS Multi-walled carbon nanotubes, CVD, synthesis, chlorine, benzenes, ethanes

Water as the Pore Former in the Synthesis of Hydrophobic PVDF Flat Sheet Membranes for Use in Membrane Distillation

Although PVDF flat sheet membranes have been widely tested in MD, their synthesis and modifications currently require increased use of green and inexpensive materials. In this study, flat sheet PVDF membranes were synthesized using phase inversion and water as the pore former. Remarkably, the water added in the casting solution improved the membrane pore sizes; where the maximum pore size was 0.58 µm. Also, the incorporation of f-SiO2NPs in the membrane matrix considerably enhanced the membrane hydrophobicity. Specifically, the membrane contact angles increased from 96° to 153°. Additionally, other parameters investigated were mechanical strength and liquid entry pressure (LEP). The maximum recorded values were 2.26 MPa and 239 kPa, respectively. The modified membranes (i.e., using water as the pore former and f-SiO2NPs) were the most efficient, showing maximum salt rejection of 99.9% and water flux of 11.6 LMH; thus, indicating their capability to be used as efficient materials for the recovery of high purity water in MD

A review of shaped carbon nanomaterials

Materials made of carbon that can be synthesised and characterised at the nano level have become a mainstay in the nanotechnology arena. These carbon materials can have a remarkable range of morphologies. They can have structures that are either hollow or filled and can take many shapes, as evidenced by the well-documented families of fullerenes and carbon nanotubes. However, these are but two of the shapes that carbon can form at the nano level. In this review we outline the types of shaped carbons that can be produced by simple synthetic procedures, focusing on spheres, tubes or fibres, and helices. Their mechanisms of formation and uses are also described

Cyclodextrin-Based Nanofibers and Membranes: Fabrication, Properties and Applications

Cyclodextrin (CD)-based electrospun nanofibers have become critical role players in the water treatment arena due to their high porosities, small diameters, high surface area-to-volume ratio and other unique properties they exhibit. Investigations demonstrate that nanofibers containing CD molecules can be facially blended with other polymeric species and/or photocatalytic and magnetic nanoparticles to enhance their rates of adsorption, inclusion complexation and selective photodegradation. These properties make them excellent candidates for the removal of water pollutants. On the other hand, the electrospinning process has become the method of choice in the fabrication of various types of CD nanofibrous mats due to its versatility, cost-effectiveness and its potential for the mass production of uniform nanofibers. CDs and CD-derivatives have also found application in membrane technologies, particularly in mixed matrix and thin film composite membranes. CD-blended membranes display improved performances in terms of selectivity, rejection, permeation and flux with reduced fouling propensities and can be used for drinking water purification and removal of emerging micropollutants. This chapter critically reviews CD-based electrospun nanofibers looking at their production, characterization methods and various applications. The use of CDs as membrane materials and how they can be fully explored in water treatment are also investigated