Synthesis and characterisation of carbon nanomaterials using South African coal fly ash and their use in novel nanocomposites

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2015.The synthesis and applications of carbon nanomaterials (CNMs) such as carbon nanofibres (CNFs), carbon nanotubes (CNTs) and carbon nanospheres (CNSs) have attracted a lot of attention due to their unique chemical and physical properties. For the synthesis of CNMs with desired morphology to occur, one needs to consider three components, namely, the catalyst, carbon source and source of power. However, the cost of the catalysts involved in making CNMs is one of the challenging factors. Due to properties such as high aspect ratio, CNM use as fillers in polymer nanocomposites has been on the forefront to improve the mechanical strength of polymer materials such as polyesters. Due to their hydrophobic nature, the interaction between the filler and matrix tends to be problematic. In this study, we investigated the use of a waste material, coal fly ash as a catalyst for the synthesis of CNMs using the chemical vapour deposition method. The use of CO2 and C2H2 as carbon sources, either independently or together, was also employed. A comparison of two different ashes was also investigated. Lastly, the use of these synthesized and acid treated CNMs as fillers was examined. The catalysts and synthesized CNMs were characterised using SEM, TEM, EDS, laser Raman spectroscopy, XRD, BET, TGA and Mössbauer spectroscopy. The mechanical properties were investigated by testing the tensile, flexural and impact properties. The synthesis of CNMs using fly ash as a catalyst without pre-treatment or impregnating with other metals was achieved. Optimum yields and uniform morphology was obtained at 650 oC, at a flow rate of 100 ml/min using H2 as a carrier gas and C2H2 as a carbon source. Mössbauer spectroscopy revealed that cementite (Fe3C) was the compound responsible for CNF formation. Further, CNMs were formed over fly ash as a catalyst, using CO2 as a sole carbon source, an additive and a carbon source before reacting with C2H2. Duvha was Page | iii found to be a better fly ash catalyst compared to Grootvlei and an optimum loading was achieved at 0.25%. Treating the CNFs with HCl/HNO3 resulted in the highest tensile, flexural and impact strengths. This studyGR 201

Plant and bacteria mediated synthesis of TiO2 NPs for dye degradation in water. A review

Researchers are concerned about the water contamination brought on by contaminants such as dyes and pharmaceuticals as a result of rising human activity, notably in the textile sector, hospitals, and homes. Many physical and chemical processes are frequently employed for the synthesis of nanoparticles (NPs). Green synthesis techniques have recently improved the simplicity, safety, and cost factors concerned with the synthesis of nanoparticles. In recent times, interest has moved towards environmentally friendly, reliable and efficient production of titanium dioxide nanoparticles (TiO2 NPs). This semiconductor material is known for its photocatalytic abilities in particular against the degradation of dye pollutants, though it suffers from several limitations. The use of reducing and capping agents such as plant and bacteria, has allowed for the synthesis of this safe material. Thus, it is imperative to review and understand the progress made in the synthesis of TiO2 materials using green methods

Synthesis of C. Benghanlensis NiO NPs for the degradation of malachite green and the removal of bacteria

An increase in environmental pollution has affected human lives, where water scarcity due to water pollution and climate change is on the rise. The increase in the usage of dyes such as malachite green in the textile industry and bacteria has increased the different types of pollutants found in the water streams, hence the need to remove these pollutants. NiO nanoparticles (NiONPs) were formed using Commelina benghalensis plant extract as a reducing agent. These materials were characterized using PL, XRD, VSM, TGA, FTIR, UV–Vis and BET. XRD identified the phases present in the material. The phytochemicals present were detected using FTIR. UV–vis was used to analyze the material's formation and optical properties, and the bandgap was calculated to be 3.8 eV The average pore size was confirmed to be 10.44 nm, indicating that the material is mesoporous in nature. From VSM, the NiO NPs were shown to have paramagnetism and through TGA, a high thermal stability of the nanomaterial was observed. At optimum conditions (pH 3, 30 mg and 5 ppm) a 95% degradation of MG was achieved and these materials were recyclable for more than 3 times whilst maintaining a degradation of 90% and above. Testing this material using real water samples sampled from the tap, pond and sewage, the NiO material was more potent against S.aureus compared E.coli bacterial strains. These results indicate the material can be used for multifunctional removal of various pollutants

Green synthesis of Zinc sulphide (ZnS) nanostructures using S. frutescences plant extract for photocatalytic degradation of dyes and antibiotics

Pollutants such as dyes and pharmaceuticals have become a problem in the environment, thus there is a need to find multifunctional materials that are safe and can be used for the removal of various pollutants. In this study, we report on the synthesis of Zinc sulphide (ZnS) nanostructures and their use as photocatalysts for the degradation of dyes and various antibiotics. Fourier transform infrared spectroscopy (FTIR) confirmed the functional groups found in plants and these were linked to the biomolecules identified through Liquid chromatography-mass spectrometry (LCMS). Ultraviolet-visible spectroscopy (UV–vis) and x-ray diffraction (XRD) confirmed the formation of the ZnS nanostructures. Thermal Gravimetric Analysis (TGA) and Brunner Emmet Teller (BET) confirmed the material was thermally stable up until 480 °C and mesoporous in nature, respectively. Scanning electron microscope (SEM) and transmission electron microscope (TEM) showed that the material is spherical in shape and energy dispersive spectroscopy (EDS) further corroborated their formation. From the degradation analysis, 90% of the malachite green (MG) dye could be degraded in 60 min at optimum conditions (pH 6, 25 mg and 10 mg l ^−1 ) and the holes were responsible for the degradation. Lastly, when tested against antibiotics, the ZnS material managed to degrade both the sulfisoxazole (SSX) and sulfamethoxazole (SMX). These results showed that the ZnS nanoparticles could be used as a multifunctional material for the degradation of various pollutants

Effect of calcination temperature on the synthesis of TiO2 nanoparticles from Sutherlandia frutescence for the degradation of Congo red dye and antibiotics ciproflaxin and sulfamethoxazole

Considering that pollutants such as dyes and pharmaceuticals are mutagenic and carcinogenic, they provide a major risk to aquatic life and humans, thereby making their removal to be crucial. The current study is focused on synthesizing TiO2 nanoparticles through the utilization of an aqueous extract from Sutherlandia frutescens. FTIR, XRD, SEM, UV–Vis and EDS techniques were employed for the analysis of the attributes of TiO2 nanoparticles. In accordance with FT-IR analysis, The S frutescens plant's unique functional groups were found to be deposited on the TiO2 nanoparticles which were synthesized. At wavelengths of 352 nm and 370 nm, substantial absorption peaks were seen in UV–Vis confirming the TiO2 formation and the bandgaps were calculated and found to be 3,19 eV, 3.26 eV and 3.29 eV for 400 °C, 450 °C and 500 °C respectively. XRD further confirmed, with the anatase peak of the TiO2 synthesis with a 10.17 average crystallite size. SEM showed the materials were spherical in shape, irregular and unevenly distributed on the surface. The synthesized nanoparticles were subjected to assessment for their photocatalytic performance while exposed to the dye Congo red (CR) and the antibiotics sulfamethoxazole (SMX) and ciprofloxacin (CIP). The highest degradation was reported for the TiO2 NPs @ 400 °C at the optimum dosage of 30 mg to be 87 % against 5 ppm CR dye after 120 mins. Upon reusing this material, its efficiency was reduced to 64 % after the first cycle followed by 61 % then 36 % for the second and third cycle respectively. Moreover, Electrons were the entities accountable for the deterioration of the dyes. Testing against the 10 ppm antibiotics, SMX and CIP had degradations of 82 and 94.6 %, respectively, after an exposure of UV (300 W) light for a period of 120 min. This investigation shows that these environmentally friendly materials can be utilized to degrade a variety of contaminants

Biosynthesized Bimetallic (ZnOSnO₂) Nanoparticles for Photocatalytic Degradation of Organic Dyes and Pharmaceutical Pollutants

The quest for eco-friendly synthetic routes that can be used for the development of multifunctional materials, in particular for water treatment, has reinforced the use of plant extracts as replacement solvents. In this study, bimetallic ZnOSnO2 nanoparticles of different ratios were synthesized using the Sutherlandia frutescens (S. frutescens) plant and tested for the degradation of methylene blue dye and the antibiotics sulfisoxazole and sulfamethoxazole. From the analysis, FTIR confirmed the formation of bimetallic nanoparticles in all ratios within the fingerprint region. SEM revealed homogenous and heterostructures of tubular and spherical structures, with the size distribution ranging from 5–60 nm, respectively. XRD confirmed the formation and the crystallinity of the bimetallic nanoparticles, UV-Vis confirmed the optical properties of the materials and the bandgap values were found between 3.08 and 3.3 eV. From the surface area analysis, type III isotherm and mesoporous structures were confirmed. The photocatalytic activity of these ratios was investigated against MB dye and the antibiotics SSX and SMX. The highest degradation of 88% for MB was obtained using the 50:50 loading ratio at 150 min with a fast kinetic rate of 0.0008 min−1. Furthermore, the holes were the species found to be responsible for the degradation of MB. The SSX and SMX antibiotics exhibited a 66% and 70% degradation, respectively. From this analysis, it can be noted that it is possible to synthesize environmentally safe materials that can be used to degrade various pollutants in our water streams

Sulphate Removal in Industrial Effluents Using Electrocoagulation Sludge as an Adsorbent

The high concentration of sulphates is detrimental to the infrastructure of wastewater treatment plants. Hence in this study, we present the application of electrocoagulation sludge as an adsorbent to remove sulphates from industrial effluents before they are released back to the environment. The sludge contains iron and aluminium cations and cationic complexes that precipitate sulphates in water. Corrugated iron sheet was used as a sacrificial electrode during electrocoagulation (EC) to generate sludge. FTIR, XRD, SEM, TEM, and Zeta Potential were used to characterize the sludge. The following parameters: contact time, pH, initial concentration, and adsorbent dosage were optimized to 120 min, 2, 100 mg/L and 150 mg, respectively. For the synthetic water, the sulphate removal was 99.1%, whereas for the real water it was found to be 98.7%. The adsorption capacity of the EC sludge was 66.76% for 2 h under acidic conditions. The Langmuir isotherm fitted better than the Freundlich isotherm. This confirmed the homogenous distribution of the active sites on the EC sludge. At different EC’s sludge, the pseudo-second order kinetic model produced the best fitting experimental results which confirmed the removal of sulphate ions by chemisorption. This approach (method) is useful for purifying industrial effluents before they are discharged into the environment

Green Synthesis of CuO-TiO₂ Nanoparticles for the Degradation of Organic Pollutants: Physical, Optical and Electrochemical Properties

CuO-TiO2 nanocomposites were successfully synthesized using the C. benghalensis plant extracts. The effect of the composition of CuO to TiO2 on the morphological, optical, electrochemical, and photodegradation efficiency in the composites was studied. SEM, XRD, UV-vis, FTIR, TGA, BET, and CV were used to characterize these materials. The XRD data reported the tenorite structure of the CuO and the anatase phase of the TiO2. SEM showed the spherical morphologies for all the CuO-TiO2 NPs, and these were also mesoporous in nature, as depicted by BET. The voltammogram of the CuO-TiO2 30/70 electrode showed a higher response current density compared to the other two samples, suggesting a higher specific capacitance. Upon testing the photocatalytic efficiencies of the CuO-TiO2 nanocomposites against methylene blue (MB), ciprofloxacin (CIP), and sulfisoxazole (SSX), the highest degradation of 94% was recorded for SSX using the CuO-TiO2 30/70 nanocomposites. Hydroxyl radicals were the primary species responsible for the photodegradation of SSX, and the material could be reused once. The most active species in the photodegradation of SSX has been identified as OH•. From this study, it can be noted that the CuO-TiO2 nanocomposites were more selective toward the degradation of antibiotics (sulfisoxazole and ciproflaxin) as compared to dyes (methylene blue)

Direct synthesis of carbon nanofibers from South African coal fly ash

Carbon nanofibers (CNFs), cylindrical nanostructures containing graphene, were synthesized directly from South African fly ash (a waste product formed during the combustion of coal). The CNFs (as well as other carbonaceous materials like carbon nanotubes (CNTs)) were produced by the catalytic chemical vapour deposition method (CCVD) in the presence of acetylene gas at temperatures ranging from 400°C to 700°C. The fly ash and its carbonaceous products were characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), laser Raman spectroscopy and Brunauer-Emmett-Teller (BET) surface area measurements. It was observed that as-received fly ash was capable of producing CNFs in high yield by CCVD, starting at a relatively low temperature of 400°C. Laser Raman spectra and TGA thermograms showed that the carbonaceous products which formed were mostly disordered. Small bundles of CNTs and CNFs observed by TEM and energy-dispersive spectroscopy (EDS) showed that the catalyst most likely responsible for CNF formation was iron in the form of cementite; X-ray diffraction (XRD) and Mössbauer spectroscopy confirmed these findings