RESUMO
Ceria nanomaterials with facile CeIII/IV redox behavior are used in sensing, catalytic, and therapeutic applications, where inclusion of CeIII has been correlated with reactivity. Understanding assembly pathways of CeO2 nanoparticles (NC-CeO2) in water has been challenged by "blind" synthesis, including rapid assembly/precipitation promoted by heat or strong base. Here, we identify a layered phase denoted Ce-I with a proposed formula CeIV(OH)3(NO3)·xH2O (x ≈ 2.5), obtained by adding electrolytes to aqueous cerium ammonium nitrate (CAN) to force precipitation. Ce-I represents intermediate hydrolysis species between dissolved CAN and NC-CeO2, where CAN is a commonly used CeIV compound that exhibits unusual aqueous and organic solubility. Ce-I features Ce-(OH)2-Ce units, representing the first step of hydrolysis toward NC-CeO2 formation, challenging prior assertions about CeIV hydrolysis. Structure/composition of poorly crystalline Ce-I was corroborated by a pair distribution function, Ce-L3 XAS (X-ray absorption spectroscopy), compositional analysis, and 17O nuclear magnetic resonance spectroscopy. Formation of Ce-I and its transformation to NC-CeO2 is documented in solution by small-angle X-ray scattering (SAXS) and in the solid-state by transmission electron microscopy (TEM) and powder X-ray diffraction. Morphologies identified by TEM support form factor models for SAXS analysis, evidencing the incipient assembly of Ce-I. Finally, two morphologies of NC-CeO2 are identified. Sequentially, spherical NC-CeO2 particles coexist with Ce-I, and asymmetric NC-CeO2 with up to 35% CeIII forms at the expense of Ce-I, suggesting direct replacement.
RESUMO
Bismuth metal-organic frameworks and coordination polymers (CP) are challenging to synthesize, given the poor solubility of bismuth precursors and asymmetric and labile ligation of Bi3+ due to its intrinsic lone pair. Here, we synthesize and structurally characterize three Bi3+-CPs, exploiting a tetrafluoroterephtalate (F4BDC) linker to determine the effect of high acidity on these synthesis and coordination challenges. Single-crystal X-ray diffraction characterization showed that pi-pi stacking of linkers directs framework arrangement and generally deters open porosity in the three structures, respectively featuring Bi chains (Bi chain -F 4 BDC), Bi dimers (Bi 2 -F 4 BDC) linked into chains, and Bi tetramers (Bi 4 -F 4 BDC). Powder X-ray diffraction and microscopic imaging show the high purity and stability of these compounds in water. Naphthalenedisulfonate (NDS) was used as a mineralizer in the synthesis of (Bi chain -F 4 BDC) and (Bi 4 -F 4 BDC), and studies of its role in assembly pathways yielded two additional structures featuring mixed NDS and F4BDC, respectively, linking monomer and octamer Bi nodes, and confirmed that F4BDC is the preferred (less labile) linker. Methylene blue (MB) adsorption studies show differing efficacies of the three Bi-F4BDC phases, attributed to surface characteristics of the preferential growth facets, while generally most effective adsorption is attributed to the hydrophobicity of fluorinated ligands. Finally, thermogravimetric analysis of all three Bi-F4BDC phases indicates simultaneous ligand degradation and in situ formation of volatile Bi compounds, which could be exploited in the chemical vapor deposition of Bi-containing thin films.
