Paul J. Franklyn

Symmetric pseudocapacitors based on molybdenum disulfide (MoS2)-modified carbon nanospheres: correlating physicochemistry and synergistic interaction on energy storage

Molybdenum disulfide-modified carbon nanospheres (MoS2/CNS) with two different morphologies (spherical and flower-like) have been synthesized using hydrothermal techniques and investigated as symmetric pseudocapacitors in an aqueous electrolyte. The physicochemical properties of these MoS2/CNS layered materials have been investigated using surface area analysis (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman, Fourier transform infrared (FTIR) spectroscopy, and advanced electrochemistry, including cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL), long-hour voltage-holding tests, and electrochemical impedance spectroscopy (EIS). The two different MoS2/CNS layered materials exhibit unique differences in morphology, surface area, and structural parameters, which have been correlated with their electrochemical capacitive properties. The flower-like morphology (f-MoS2/CNS) shows lattice expansion (XRD), large surface area (BET analysis), and small-sized nanostructures (corroborated by the larger FWHM of the Raman and XRD data). In contrast to the f-MoS2/CNS, the spherical morphology (s-MoS2/CNS) shows lattice contraction and small surface area with relatively large-sized nanostructures. The presence of CNS on the MoS2 structure leads to slight softening of the characteristic Raman bands (E12g and A1g modes) with larger FWHM. MoS2 and its CNS-based composites have been tested in symmetric electrochemical capacitors in an aqueous 1 M Na2SO4 solution. CNS improves the conductivity of the MoS2 and synergistically enhances the electrochemical capacitive properties of the materials, especially the f-MoS2/CNS-based symmetric cells (most notably, in terms of capacitance retention). The f-MoS2/CNS-based pseudocapacitor shows a maximum capacitance of 231 F g−1, with high energy density 26 W h kg−1 and power density 6443 W kg−1. For the s-MoS2/CNS-based pseudocapacitor, the equivalent values are 108 F g−1, 7.4 W h kg−1 and 3700 W kg−1. The high-performance of the f-MoS2/CNS is consistent with its physicochemical properties as determined by the spectroscopy and microscopy data. These findings have opened doors for further exploration of the synergistic effects between MoS2 graphene-like sheets and CNS for energy storage.

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.

The effect of CO2 on the CVD synthesis of carbon nanomaterials using fly ash as a catalyst

The efficient use of fly ash is a worldwide issue due to its high production and harmful effects on the environment. In this work the synthesis of carbon nanomaterials (CNMs) via the chemical vapour deposition (CVD) method, using fly ash as a catalyst and CO2 as an alternate carbon source, was investigated. Here CO2 was employed in three different ways: (1) as a sole carbon source, (2) as an additive to C2H2 and (3) as a carbon source prior to the reaction of C2H2 with fly ash. SEM, TEM and laser Raman spectroscopy confirmed that CNMs were formed in all three cases. In the first case, when CO2 was used as a sole carbon source, CNMs were formed in low yields with a considerable amount of amorphous carbon. However, in the second case when CO2 was used as an additive to C2H2, a drastic increase in CNM formation was observed. In both cases optimal yields were observed at 600 °C. However in the third case, when CO2 was used as a carbon source prior to the reaction with C2H2, uniform-sized nanofibers of the highest yields of all three cases were formed. Likewise these CNMs were found to be the most thermally stable. Hence this study has shown that the use of waste materials such as fly ash as a catalyst and CO2 as a carbon source prior to the reaction with C2H2, results in a very simple and cost efficient process to make uniformally shaped, thermally stable CNMs.

Morphological and crystallinity differences in nitrogen-doped carbon nanotubes grown by chemical vapour deposition decomposition of melamine over coal fly ash

Millions of tons of coal fly ash (CFA) are produced each year in thermoelectric coal powered stations as a waste-product. Until recently few researchers have endeavored to use CFA as a catalyst in the formation of carbon nanomaterials (CNMs). In this study a two-stage tube furnace was used to synthesize N-doped carbon nanotubes (NCNTs) by chemical vapor deposition using melamine as the source of nitrogen and carbon, with CFA as a catalyst, at temperatures ranging from 800 to 900 °C. The masses of the NCNTs formed were found to have increased with increased synthesis temperature. The morphology and crystallinity of the NCNTs along with the amount of nitrogen incorporated into these were found to vary with the synthesis temperature. NCNTs synthesized at 800 °C were found to be typical multiwalled carbon nanotubes by transmission electron microscopy, whereas those at 850 and 900 °C were found to be chain-like and bamboo-like compartmentalised nanotubes respectively. The NCNTs synthesized at 800 °C were found to contain the least incorporation of nitrogen by elemental analysis and were the most crystalline (as determined by using the IG/ID ratio and the G-band position from laser Raman spectroscopy), whereas those at 850 °C were the least crystalline but had the highest incorporation of nitrogen. NCNTs synthesised at 800 °C were the most thermally stable, whereas those synthesized at 850 °C were the least stable. NCNTs synthesized at 900 °C had a crystallinity, thermal stability and nitrogen content which lay between the other two.

Novel synthesis of Ag decorated TiO2 anchored on zeolites derived from coal fly ash for the photodegradation of bisphenol-A

The disposal of millions of tons of coal fly ash (CFA) threatens the environment, hence means to reuse CFA are highly sought after. In this study, CFA was reused to make materials which were tested for water purification. Zeolitic material (CFA_Zeo) was derived from CFA by a 2-step alkali-fusion hydrothermal method and then composited with TiO2 nanoparticles using a novel resin-gel technique. CFA_Zeo loadings were 15 and 30 wt% in the resulting TiO2/CFA_Zeo composites. These composites were then loaded with 1 wt% Ag nanoparticles by a deposition–precipitation technique using NaOH and urea. CFA_Zeo rods (morphology confirmed by TEM) were confirmed by PXRD to be sodium aluminum silicate hydrate. TEM analyses of the CFA_Zeo rods in the composites revealed them to be completely coated with TiO2 nanoparticles that had Ag nanoparticles on their surfaces. The photoluminescence emission peak of TiO2 was found to be significantly higher than that of TiO2/CFA_Zeo composites, with the TiO2/CFA_Zeo composites that were loaded with Ag having even lower emission intensities. UV-vis DRS spectra showed that CFA_Zeo had no effect on the band gap of TiO2, while composites that contained Ag had a wide absorption band in the visible region. The photocatalytic efficiency of these materials was then determined using bisphenol-A (BPA) as a model compound under both UV and visible light. Except for the 30 wt% TiO2/CFA_Zeo composites without Ag, all of the composites had superior photoactivity to uncomposited TiO2 under both UV and visible light. On the other hand, composites with Ag nanoparticles showed the best photoactivities. The superior photoactivities of these composites under UV-light were mainly attributed to the separation of charge carriers, whereas under visible light it was attributed to the ability of silver to harvest visible light through surface plasmon resonance (SPR).

Ion-Beam-Synthesized Colloidal Silver Nanoclusters in Crystalline Sapphire as Third-Order Optical Material

Silver ion implantation in single-crystalline sapphire has given rise to the formation of silver nanoparticle-sapphire composites, which have been imaged using transmission electron microscopy, and confirmed using linear optical absorption and Rutherford backscattering spectrometry. Nonlinear refractive index and two-photon absorption of these nanocomposites have been observed using Z-scan and Anti-resonant Interferometric Nonlinear Spectroscopy (ARINS) in the close proximity of surface plasmon resonance (SPR) wavelength of silver nanoclusters (~400 nm) and at ~807 nm, respectively. Both sign and value of the nonlinear parameters were determined, and the third-order optical susceptibility (χ(3)) of the composites has been found to be significant. Such metal nanocomposites in glasses and sapphires having appreciable χ(3) with temporal responses in picosecond to femtosecond time domain have great relevance to futuristic switching materials in nanophotonics.

TiO2 composited with carbon nanofibers or nitrogen-doped carbon nanotubes synthesized using coal fly ash as a catalyst: bisphenol-A photodegradation efficiency evaluation

Coal fly ash (CFA) was used as a catalyst for the synthesis of nitrogen-doped carbon nanotubes (NCNTs) and carbon nanofibers (CNFs) by chemical vapor deposition. Carbon nanomaterials (CNMs) were successfully purified by sequential treatment in 5% HF and then in a dil. HNO3/H2SO4 mixture, as was shown by SEM, TGA, and XRD. The purified NCNTs and CNFs were composited with TiO2 nanoparticles at varying loadings (i.e. 1, 5 and 20% CNF/NCNT loadings) by a surfactant wrapping sol–gel/hydrothermal method and used for the photodegradation of bisphenol-A (BPA) in water with light being sourced from a solar simulator. It was shown by TEM that the CNMs were completely coated with TiO2 nanoparticles and interactions between the CNMs and TiO2 were demonstrated using PXRD and laser Raman spectroscopy. Photoluminescence measurements showed that compositing TiO2 with CNMs, especially NCNTs significantly reduced its emission intensity suggesting a reduced electron/hole recombination rate. Unbound TiO2 was used to optimize the photodegradation experimental conditions, i.e. solution pH, the mass of the photocatalyst, the initial concentration of BPA and solution temperature. The photocatalytic efficiency of the various TIO2 and CNFs/NCNTs was assessed using the optimized conditions where it was observed that the composites containing 1 and 5% loadings of CNMs outperformed TiO2. The photocatalytic efficiency of the NCNT based composites was higher than that of the CNF based composites. This work shows that CFA, a toxic material, could be used to synthesize materials useful for cleaning water.

Variable Temperature Study of Au and Au-Pt Nanoparticles on Selected Oxide Supports

We report on the size relationship of Au and Au-Pt nanoparticles that were synthesised on silica and anatase phase titania supports. Deposition-precipitation (DP) of metal chlorides with the addition of urea and ammonium hydroxide was used to produce the nanoparticles. The relative particle size relationship of the Au and Au-Pt nano particles (NPs) was investigated, relating the Pt concentration and the support polymorph over a temperature range. It was found, with the use of in-situ variable temperature powder X-ray diffraction (VT-PXRD) and transmission electron microscopy (TEM), that the addition of Pt to the Au system corresponded to a reduction in particle size over a broad temperature range.