Synthesis of layered metal hydroxide materials containing nickel, cobalt and copper
thesisposted on 08.06.2021, 15:55 by Kurt Lawson
The formation of different nickel, cobalt and copper layered hydroxide phases by a variety of solution and solid-state synthesis methods have been investigated. Initially, the preparative conditions were varied to generate single-phase products from metal(II) nitrate hexahydrate that were characterised by powder X-ray diffraction, vibrational spectroscopy and thermogravimetric analysis. As well as the β-M(OH)2 and α-M(OH)2 phases (where M = Ni, Co) two different hydroxynitrate phases with the generic formula M(OH)2-x(NO3)x materials were isolated with x = 0.67 and 1.0 (where M = Ni, Co). The reduction of symmetry of the nitrate anion from D3h to C2v can allow the alpha-phase to be distinguished from the two layered hydroxynitrate phases in both infrared and Raman spectroscopy. The symmetric N-O stretch enables the two hydroxynitrate phases to be distinguished through the sharp absorptions at ca. 1000 cm-1 (x = 0.67) and ca. 1050 cm-1 (x = 1.0). The TGA data of the pure phases showed key differences between the layered hydroxides, with anhydrous phases having singular weight losses over short temperature ranges while hydrated phases have multiple losses over larger ranges.
Short-range characterisation techniques were used to characterise the poorly crystalline alpha-phases of nickel and cobalt together with beta-phase copper hydroxide, and the results compared with the three copper hydroxide minerals malachite, azurite and georgeite. Two synthetic forms of georgeite were also analysed, prepared by supercritical antisolvent precipitation with carbon dioxide and sodium carbonate precipitation. The alpha-phases and georgeite were both shown through infrared spectroscopic analysis to have uncoordinated nitrate and carbonate with D3h symmetry incorporated into the structures. Synchrotron X-ray pair distribution function analysis showed only short-range order (<15 Å) in the alpha-phases with only intralayer atom pair distances present. Shorter atom pair distances were observed for georgeite phases (<5 Å), but were matched to intralayer bond distances expected of alpha-phase type layered hydroxides of 2.0 Å and 3.0 Å. This redefines georgeite as an alpha-phase hydroxide for the first time contrary to the literature definition of a hydroxysalt phase.
The disordering of beta-phase nickel hydroxide through the formation of stacking faults and interstratification was investigated by varying temperature and time using nickel(II) nitrate. Stacking faulted materials were formed at low temperature and identified by broadening of the 00l reflections while interstratified materials were identified through the splitting of the 001 into two lines. In contrast to interstratified materials presented in previous studies as alpha-phase and beta-phase layering, this work has shown through vibrational spectroscopy that layered hydroxysalts can also undertake the interstratification process. Standard mixtures of hydroxynitrate and beta-phase were prepared to investigate if the intensity of particular vibrational bands could be correlated with the proportion of the particular phases in mixtures. The intensity of the infrared bands at 990 cm-1 and the Raman bands at 1315 cm-1 were shown to correlate with content, theoretically providing a method to determine mixture compositions.