Vincent G. Gomes

Edge-enriched graphene quantum dots for enhanced photo-luminescence and supercapacitance

Graphene quantum dots (GQDs) with their edge-bound nanometer-size present distinctive properties owing to quantum confinement and edge effects. We report a facile ultrasonic approach with chemical activation using KOH to prepare activated GQDs or aGQDs enriched with both free and bound edges. Compared to GQDs, the aGQDs we synthesized had enhanced BET surface area by a factor of about six, the photoluminescence intensity by about four and half times and electro-capacitance by a factor of about two. Unlike their non-activated counterparts, the aGQDs having enhanced edge states emit enhanced intense blue luminescence and exhibit electrochemical double layer capacitance greater than that of graphene, activated or not. Apart from their use as part of electrodes in a supercapacitor, the superior luminescence of aGQDs holds potential for use in biomedical imaging and related optoelectronic applications.

Carbon functionalized TiO2 nanofibers for high efficiency photocatalysis

TiO2 nanofibers (30–50 nm diameter), fabricated by the electro-spinning process, were modified with organo-silane agents via a coupling reaction and were grafted with carbohydrate molecules. The mixture was carbonized to produce a uniform coating of amorphous carbon on the surface of the TiO2 nanofibers. The TiO2@C nanofibers were characterized by high resolution electron microscopy (HRTEM), x-ray diffraction (XRD), x-ray photoelectron (XPS), Fourier transform infrared (FTIR) and UV-vis spectroscopy. The photocatalytic property of the functional TiO2 and carbon nanocomposite was tested via the decomposition of an organic pollutant. The catalytic activity of the covalently functionalized nanocomposite was found to be significantly enhanced in comparison to unfunctionalized composite and pristine TiO2 due to the synergistic effect of nanostructured TiO2 and amorphous carbon bound via covalent bonds. The improvement in performance is due to bandgap modification in the 1D co-axial nanostructure where the anatase phase is bound by nano-carbon, providing a large surface to volume ratio within a confined space. The superior photocatalytic performance and recyclability of 1D TiO2@C nanofiber composites for water purification were established through dye degradation experiments.

High-yield aqueous phase exfoliation of graphene for facile nanocomposite synthesis via emulsion polymerization

Aqueous phase exfoliation was developed for producing high-yield graphene nanosheets from expanded graphite (EG). The process included ultrasonication with sodium dodecyl sulfate (SDS) emulsion in aqueous phase. The high throughput exfoliation process was characterized by UV-vis spectroscopy, transmission electron microscopy (TEM) and electrical impedance spectroscopy (EIS). Controlled sonication experiments revealed that optimum exfoliation corresponds to maxima in UV-vis spectra. TEM results showed that the exfoliated graphene comprised nanoflakes having ≤5 layers (~60%) and ≤10 layers for 90% of the product. The potential use of this highly dispersed graphene was demonstrated by one-pot synthesis of graphene/polymer composite via in situ emulsion polymerization with styrene. The integrated role of SDS included adsorption and exfoliation of graphite, dispersion of graphene produced and assisting with micelle formation in emulsion. The high surface area graphene nanosheets as dispersed phase in polymeric nanocomposites showed significant improvement in thermal stability and electrical conductivity.

Pressure swing adsorption for carbon dioxide sequestration from exhaust gases

Carbon dioxide removal using pressure swing adsorption (PSA) processes were investigated both theoretically and experimentally. CO2 is the more strongly adsorbed compared to nitrogen in a flue gas with suitable molecular sieve adsorbents. Zeolite 13X was found to be suitable for CO2 sequestration on testing several adsorbents for sorption-based separation. Numerical simulations indicate that the purity of nitrogen gas recovered can be increased from 30 to 90% with the help of PSA operation. The effects of feed flow rate, process cycle time, inert gas content and purge pressure were investigated. The optimal feed flow rate and cycle time were determined for the conditions employed in this work. The presence of inert gas was found to adversely affect the separation. The model predictions show good agreement with our laboratory experiments with promising separation performance.

Polymer brush synthesis on surface modified carbon nanotubes via in situ emulsion polymerization

In situ emulsion polymerization was employed for synthesizing carbon nanotube (CNT) composites in a colloidal system with poly(styrene) or PS to form nanostructured brush. CNTs were initially functionalized with oleic acid, followed by silanization with (3-aminopropyl) triethoxysilane to impart cross-linking properties. Styrene monomers were efficiently grafted to surface modified CNT via emulsion polymerization with variable CNT concentrations. FTIR analyses of the functionalized CNT and PS/CNT composites confirmed the bond formation and effectiveness of the developed experimental method. X-ray photoelectron spectroscopy confirmed the presence of the desired bonds and the composition of the composites. Structural properties of the composites characterized by TEM confirmed excellent deagglomeration and dispersion of CNTs in PS/CNT composite. Thermal characteristics from TGA and DSC data showed enhanced properties for the nanocomposites as a function of the CNT content. BET measurements indicated significant improvements in surface area and pore volume with enhancements in gas sorption for the polymer nanocomposites.

Activated carbon from chickpea husk by chemical activation with K2CO3: preparation and characterization

Activated carbon was prepared from chickpea husk by chemical activation with K2CO3. At 1073 K, the specific surface area of activated carbon prepared with an impregnation ratio of 1.0 yielded the maximum value of 1778 m2/g. From the results of the yield of the activated carbon and the reagent recovery ratio, it was concluded that the carbon involved in the husk char was removed as CO by reduction of K2CO3 above 1000 K. The fractal dimension changed slightly between 773 and 973 K, and it decreased rapidly between 973 and 1173 K. It was deduced that this decrease of the fractal dimension was due to the decomposition of the cross-linked structure and the small crystallite structure. The micropore volume and the specific surface area increased by the release of plugged pore due to the decomposition of the cross-linked structure. It was further deduced that the mesopore volume increased and the micropore volume decreased by combination of micropores due to the decomposition of small crystallites.

Operation of semi-batch emulsion polymerisation reactors: Modelling, validation and effect of operating conditions

A detailed dynamic model was developed for a styrene emulsion polymerisation semi-batch reactor to predict the evolution of the product particle size distribution (PSD) and molecular weight distribution (MWD) over the entire range of monomer conversion. A system exhibiting zero-one kinetics was employed, with the model comprising a set of rigorously developed population balance equations to predict monomer conversion, PSD and MWD. The modelling equations included diffusion-controlled kinetics at high monomer conversion where the transition from the zero-one regime to a pseudo-bulk regime occurs. The model predictions were found to be in good agreement with experimental results. Both particle growth and the PSD were found to be strongly affected by the monomer feedrate. Reactor temperature had a major influence on the MWD which was, however, insensitive to changes in the monomer feedrate. These findings were confirmed experimentally. As a result, it seems reasonable to propose that the use of the monomer feedrate to control the PSD and the reactor temperature to control the MWD are appropriate in practical situations. Consequently, an optimal monomer feed trajectory was developed off-line (using the validated reactor simulation) and verified experimentally by producing a polymer with specific PSD characteristics.

Selenium in sediments, pore waters and benthic infauna of Lake Macquarie, New South Wales, Australia

Measurements of selenium in sediments and benthic infauna of Lake Macquarie, an estuary on the east coast of Australia, indicate that sediments are a significant source of selenium in the lakes food web. Analysis of surficial sediment samples indicated higher selenium concentrations near what are believed to be the main industrial sources of selenium to the lake: a smelter and a power station. Sediment cores taken from sediments in Mannering Bay, near a power station at Vales Point, contained an average of 12 times more selenium in surficial sections than sediment cores from Nords Wharf, a part of the lake remote from direct inputs of selenium. The highest selenium concentration found in Mannering Bay sediments (17.2 μg/g) was 69 times the apparent background concentration at Nords Wharf (0.25 μg/g). Pore water concentrations in Mannering Bay were also high, up to 5 μg/l compared to those at Nords Wharf which were below detection limits (0.2 μg/l). Selenium concentrations in muscle tissues of three benthic-feeding fish species (Mugil cephalus, Platycephalus fuscus, Acanthopagrus australis) were significantly correlated (p<0.05) with surficial sediment selenium concentration. Selenium concentrations in polychaetes and molluscs of Mannering Bay were up to 58 times higher than those from Nords Wharf. Two benthic organisms, the eunicid polychaete Marphysa sanguinea and the bivalve mollusc Spisula trigonella, were maintained at different densities in selenium-spiked sediments. Both animals accumulated selenium from the spiked sediment, confirming that bioaccumulation from contaminated sediments occurs. Collectively, these data suggest that benthic food webs are important sources of selenium to the fish of Lake Macquarie.

Coalseam methane recovery by vacuum swing adsorption

Coalseam methane gas recovery using pressure and vacuum swing adsorption (PSA/VSA) processes were investigated both theoretically and experimentally. CO2 is more strongly adsorbed and also diffuses faster compared to methane and nitrogen in a carbon-based adsorbent. Hence such adsorbents are suitable for methane recovery. Carbon molecular sieve adsorbents were employed for testing the technical feasibility of the adsorption process. Numerical simulations indicate that the purity of methane gas recovered can be greater than 90% by switching from PSA to VSA operation. The effect of process cycle time, purge pressure and feed flow rates were investigated. Both model predictions and laboratory experiments show promising separation performance. This study shows that a relatively untapped source of methane from coalseam gas can be economically recovered for industrial applications.

Synthesizing activated carbons from resins by chemical activation with K2CO3

We prepared activated carbons from phenol–formaldehyde (PF) and urea–formaldehyde (UF) resins by chemical activation with K2CO3 with impregnation during the synthesis of the resins. The influence of carbonization temperature (773–1173 K) on the pore structure (specific surface area and pore volume) and the temperature range at which K2CO3 worked effectively as an activation reagent, were investigated. The specific surface area and micropore volume of PF–AC and UF–AC increased with an increase of carbonization temperature in the range of 773–1173 K. We prepared activated carbon with well-developed micropores from PF, and activated carbon with high specific surface area (>3000 m2/g) and large meso-pore volume from UF. We deduced the activation mechanism with thermogravimetry and X-ray diffraction. In preparing activated carbon from PF, K2CO3 was reduced by carbon in the PF char. The carbon was removed as CO gas resulting in increased specific surface area and pore volume above 1000 K. In preparing AC from UF, above 900 K the carbon in UF char was consumed during the K2CO3 reduction step.