Presented in this thesis are findings from investigations into the hydrothermal synthesis and growth sequence of (a) zinc oxide (ZnO) particles and (b) synthesis via normal evaporation drying and spray drying of sodium zirconate powders (Na2ZrO3) for CO2 capture applications. Particle characterisation involved the usage mainly of X-ray diffraction analysis, and scanning and transmission electron microscopy to acquire information on changes in particle properties as synthesis conditions were varied.
Zinc oxide particles were grown hydrothermally from suspensions produced by drop-wise mixing of a zinc acetate solution and aqueous sodium hydroxide. The precursor suspensions were heated in an acid digestion vessel at a steady rate (5 oC per min), to a dwell temperature of 120°C and then held for different dwell times. Analysis of characterisation results revealed that particle growth occurred through a hierarchical process, starting with zinc oxide nanocrystallites which self-assembled into ≤20 nm wide particles. Progression of hierarchical growth led to the emergence of hexagonal ZnO microrods through the stacking along the direction of hexagonal layers, (each ~50 nm thick), made up of the assembled ≤20 nm particles. The process ended with the formation of hexagonal double rods (≤1.8 µm and width ≤0.6 µm) through multiple mechanisms, including secondary growth of rods off the basal layers of fully developed pre-existing rods, end-to-end attachment of rods, and possibly growth via crystal twinning.
In another series of experiments, the sequential multistage growth process of high aspect ratio zinc oxide rods are presented for hydrothermal treatment of a precursor suspension obtained by titration of a solution of zinc acetate dissolved in distilled water against an ammonium hydroxide solution. In this case, particle growth commenced with the emergence of metastable octahedral wulfingite [Ɛ-Zn(OH)2] particles which transformed to high aspect ratio hexagonal ZnO microrods with increased reaction times.
Finally, sodium zirconate powders composed of nanoscale primary particles were synthesized via evaporation drying and spray drying of a mixture of sodium acetate and zirconium acetate in nitric acid for applications in CO2 capture. Loose agglomerates (~10 µm) of irregular shaped particles were obtained from evaporation drying, while < 5 µm porous and hollow granules with nanoparticle sub-structure were obtained from spray drying. Phase composition of calcined powders from both synthesis methods was mainly Na2ZrO3 with minor proportions of monoclinic ZrO2. Analysis of results from subjecting both powders to multiple carbonation (700 oC, 22 % CO2 atmosphere) and decarbonation (900 oC, 100 % N2 atmosphere) in thermogravimetric analysis (TGA) cycles showed that the spray dried powder had a superior CO¬2 uptake performance, providing ~ 80 % CO2 uptake efficiency (molar uptake/theoretical molar uptake) compared to ~ 45 % and ~ 15 % for the evaporation dried and commercial powders respectively. The improved performance of the spray dried sample was attributed to the morphology of the particles, as the relatively high porosity of the hollow granules enabled a faster rate of gas-solid reactions relative to the evaporation dried powder
