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Project 3, Zirconium oxide model systems
Abstract:
Acidic aqueous solutions of Zr(SO4)2 * 4 H2O can be used to deposit nanocrystalline zirconium oxide films on functionalized surfaces. Because zirconium hydrolyzes easily, such solutions are potentially unstable towards colloid formation and precipitation. Particle growth (conditions: 2 or 4 mM Zr(SO4)2, 0.4 or 0.6 N HCl, T = 323, 328, 333, or 343 K) was investigated using dynamic light scattering (DLS, in situ), and analytical ultracentrifugation (AUC, ex situ after quenching to 77 K and re-thawing to 298 K).
The AUC measurements revealed three stages of growth (all dimensions given are hydrodynamic radii). 1. Several discrete polynuclear complexes with rh = 0.43 to 2.29 nm coexisted; 2. particle size distribution with single sharp maximum; 3. above rh ≈ 260 nm rapid transition to polydisperse medium with particles up to 100 µm.
The DLS measurements revealed a linear increase of the hydrodynamic radii (z-average of particle population) from 5 nm to 1000 - 2500 nm with rates of 0.01 to 0.6 nm*s-1. The rates were proportional to the Zr(SO4)2-concentration, while the increase of the HCl concentration slowed or even inhibited growth. The apparent activation energy for this step was 136 kJ*mol-1. From the induction period before detection of first particles initial growth rates (rh < 5 nm) were calculated to ≈ 1*10-4 to 5*10-3 nm*s-1. Independent of the conditions, the two reaction rates were always proportional to each other, indicating linked rate laws.
A 4 mM Zr(SO4)2 in 0.4 N HCl solution exhibited no particle growth at 323 K but complexes of a constant radius (after 6, 12, and 24 h) of 1.16 nm were detected (AUC). Under these conditions, films were deposited, and their thickness increased linearly with time, specifically by 2.1*10-4 nm s-1. This rate corresponds to the initial growth rate in solution. In contrast to films grown from media with significant particle growth, these films showed surfaces free of attached particles, cracks, and holes.
