Tailored synthesis of shaped carbon nanomaterials over supported Au, Ni, La and La-Ni novel radially aligned nanorutile: characterisation and investigation of properties
Date
2017
Authors
Mutambara, Farai Dziike
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Abstract
Radially aligned nanorutile (RANR), in the form of microspheres, were synthesised using a hydrothermal method. Parameters such as reaction time, reaction temperature, reactant concentrations, as well as the solution’s pH value result in RANR at various stages of formation. These consisted of densely packed and spherically aligned rutile phase nanorods which were oriented on a common crystallographic axis (110) that had independent terminating ends. The average diameter of these nanorods was between 2–12 nm, while their lengths ranged from 50–650 nm. Loadings ranging from 0.5 to 10 wt. % of metal nanoparticles of: Au, Ni, La and La-Ni onto the RANR support were achieved by using a deposition-precipitation method with urea. The choice of the metals was carefully done considering their positions on the periodic table. Ni is known to catalyse the synthesis of CNMs. Its behaviour was tested on the unique RANR support and compared to a higher transition metal, Au. Further testing was taken to a different metal group, La and effect of co-loading tested on La-Ni lanthanide-transition system. These metal nanoparticles (i.e. Au, Ni, La and La-Ni) that were deposited on RANR support were found to range in size from 2–3 nm, 1–6 nm, 2–8 nm and 2–10 nm respectively.
The PXRD pattern of the supported catalysts predominantly exhibited peaks of pure rutile with a major peak at 2θ value of 32.5° for the major rutile peak, and minor peaks at 42.5, 48, 52.1, 64.0, 66.5, 75.0 and 82° respectively. La showed major peaks occurring at 2θ values of 30° and 15° with minor peaks at 26.5° and 28.0°. Ni had peaks at 2θ values 28°, 38.5°, 58° and 76°. Au had peaks occurring at 2θ values 24.3°, 28°, 43.5° and 48°. The peak positions of both La and Ni occurred at their respective 2θ values in the co-loaded La-Ni/RANR catalysts. TPR analyses revealed different reduction temperatures that increased with increase in wt. % loading. Au, Ni and La had mean reduction temperatures of 90 °C, 500 °C and 600 °C respectively. However, the co-loaded La-Ni/RANR catalyst showed different reduction temperatures ranging 500 – 700 °C for loading of 1 – 10 wt. %. BET surface area analysis showed the Metal/RANR catalyst decreased from 50 to 33 m2 g-1 with increase in wt. % loading from 5 – 10 wt. %. This implied that the metal particles were occupying the pore sited on the surface of the RANR support thereby decreasing the BET surface of the Metal/RANR catalyst with increase in metal wt. % loading.
Parametric studies for the synthesis of shaped carbon nanomaterials (SCNMs), in particular carbon nanofibers (CNFs), via chemical vapour deposition (CVD) using La, Au, Ni and co-
loaded La-Ni nanoparticles supported on RANR were performed under varying conditions including: wt. % loadings, temperature, flow rate, time and the type of metal used. The synthesis of CNFs from the decomposition of acetylene and hydrogen (H2/C2H2) was targeted for the tailored synthesis of shaped carbon nanomaterials over supported Au, Ni, La and La-Ni on novel radially aligned nanorutile. Characterisation of the various metal/RANR catalysts and the CVD products was performed by: transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), powder X-ray diffraction spectroscopy (PXRD), electron probe microanalysis (EPMA), laser Raman spectroscopy (LRS), thermogravimetric analysis (TGA), ultraviolet–visible spectroscopy (UV-Vis), Fourier transform infra-red spectroscopy (FTIR) and Brunauer-Emmett-Teller (BET) surface area measurements.
La/RANR supported the synthesis of straight chain CNFs. The CNFs grew predominantly from the tips of the La nanoparticles in the catalyst. The La nanoparticles were observed on most of the tips of the CNFs suggesting that tip-growth was the mechanism of growth of the CNFs synthesised over La/RANR. During CNFs growth, the La nanoparticles were displaced from their positions on the RANR support. The dislodged La nanoparticles had a tendency of undergoing sintering to form large sized nanoparticles. Consequently, the sintered La nanoparticles catalysed the synthesis of thick CNFs with diameters ranging from 40–270 nm as compared to supported La nanoparticles with an average particle size of 6 nm. It was observed that the morphology of the La metal nanoparticles played a critical role in the control of the morphology of the CNFs and is closely related to the properties of the metal particles, the reaction conditions and the pre-treatment of the La/RANR catalyst. At 5 wt. % loadings, the average diameter of the CNFs synthesized by the La/RANR catalysts was 40 nm while the lengths ranged from 200 nm – 100 μm. Thus increased La wt. % loadings resulted in larger catalyst particle sizes which in turn caused larger fiber thicknesses.
The RANR was observed to be an actively interactive support that enhances the catalytic performance of the metal nanoparticles on its surface especially with Ni in the synthesis of CNFs over Ni/RANR catalysts. Variation of parametric conditions such as: wt. % loading, reaction temperature, gas flow-rate and reaction time had a significant effect on the morphology, mechanical properties and density of the CNFs grown on the Ni/RANR catalysts. The Ni/RANR catalysts were observed to support the synthesis of twisted CNFs in contrast to the straight chain CNFs synthesised on La/RANR catalysts. The twisted CNFs were synthesized by way of regular oriental nucleation of heptagons and pentagons along the
nanofibre body. CNFs growth time was found to be a crucial parameter that can be used to understand the growth process and tailor the artificial properties of the resulting carbon material assembly. There was a threshold time interval over which both growth of fibres was consistent and uniform coverage was achieved. Detailed studies on CNFs synthesised over 1, 8 and 10 wt. % Ni/RANR showed similar trends with time at various temperatures, wt. % loading and flow rates while the 0.5 wt. % Ni/RANR did not catalyse the synthesis of twisted CNFs except a few amorphous carbon deposited on the catalysts. It was also proved that the longer the time, the more graphitic the CNFs. Thus the carbon crystallisation on the Ni/RANR catalyst resulted in the formation of primary amorphous carbon. Then sufficient time of heating transformed the carbon into graphitic material while the catalyst shaped it into its specific twisted morphology.
Gold however, presented a different set of trends in its performance as a catalyst supported on RANR. The CVD products which formed over Au/RANR catalysts were observed to vary under different parametric reaction conditions. At low wt. % loading, catalysts catalysed the formation of amorphous carbon material. However, Au/RANR at higher wt. % loading was observed to catalyse the synthesis of both straight and coiled or twisted CNFs. Like La and Ni, Au also supported a tip-growth mechanism as Au particles were also observed on many of the tips of the CNFs as the Au particles were dislodged from the RANR support. Temperature facilitated enhanced sintering and the dislodged Au nanoparticles grew to larger particles of different sizes. The sintered nanoparticles determined the diameters of the CNFs synthesized. The fibres synthesized under different parametric conditions ranged from 10–100 nm in thickness with an average diameter of 50 nm. Detailed studies also showed that both the diameter and length distribution range increased drastically with increase in reaction temperature. The temperature enhanced decomposition of C2H2 and thus more carbon was available for forming the carbon matrix of the CNFs.
La and Ni were co-loaded on RANR to form a co-loaded La-Ni/RANR catalyst. Metal wt. % loadings ranging from 1-10 wt. % were achieved. The peaks identifying La may be ascribed to La(OH)3 and La2O3 phases while Ni peaks were due to NiO. The phases reduced to metallic La and Ni to give co-loaded La-Ni/RANR catalysts. Reduction peaks shifted to higher temperatures with an increase in metal nanoparticles wt. % loading. The shift in the peak positions to higher temperatures may be attributed to increased crystallinity of the metal oxides and mainly the increase in the oxygen in the metallic matrix of the catalysts. The Ni nanoparticles were characterised by a spherical morphology with a diameter range of 1–6 nm
with an average of 4 nm. The short rod structured nanoparticles were those of La with a diameter range of 2–8 nm and an average of 7 nm. The co-loaded La-Ni/RANR catalysts catalysed the synthesis of CNFs that were a mixture of both straight long stretching nanofibers and twisted or coiled nanofibers. The actual morphology of each individual CNF may have been directly attributed to the surface or end of the La-Ni nanoparticle over which it grew. The straight chain CNFs were catalysed by the La end of the co-loaded La-Ni/RANR catalyst, while the coiled or twisted CNFs, were catalysed by the Ni end of this catalyst. During CVD reaction, the respective metal nanoparticles also dislodged from the support, sintered and gave synthesised fibres with a diameter range of 50–500 nm with an average of 175 nm. It was observed that there were no synergistic effects as a result of co-loading La and Ni on the RANR support.
The supported catalysts, La/RANR, Ni/RANR, Au/RANR, and La-Ni/RANR showed exceptional performance as catalysts in the synthesis of CNFs. However, the plain RANR had no catalytic effect observed during the CVD synthesis of SCNMs. TiO2 is well known for its excellent photocatalytic behaviour. Therefore the supported catalysts and the plain RANR were compared as catalysts for photocatalytic degradation of methyl orange (MeO). Detailed studies of the preparation of the supported catalysts revealed that metal loading did not result in the blending of the nanoparticles species into the rutile phase lattice of the RANR. Both PXRD and LRS showed the RANR peaks dominating those of the supported nanoparticles. The supported metal nanoparticles caused slight shifting of the RANR peak positions. The low wt. % supported catalysts did not show any significant photocatalytic activity different from that of pure RANR. A 5–8 wt. % loading was observed to be ideal for the photodegradation of MeO. La/RANR showed the highest photocatalytic performance of all the supported catalysts. However, plain RANR exhibited excellent photocatalytic activity, with degradation efficiency that far exceeded that of the whole range of supported catalysts. The co-loaded La-Ni/RANR was the worst photocatalysts due to a combination of Ni inactivity and a tendency to inhibit the photocatalytic activity of both La and plain RANR. Photoluminescence studies exhibited reduced intensities for the supported catalysts indicating consequential effective prohibition of recombination of the electrons and respective holes. However, La/RANR sufficiently slowed the radiative recombination process of photogenerated electrons and holes in TiO2. This is because La loaded TiO2 has a tendency of expanding the wavelength response range as determined by diffuse reflectance UV-Vis spectroscopy.
Description
A thesis submitted to the Faculty of Science, School of Chemistry (Molecular Science Institute), University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy, 2017.