A comparative assessment of the 48-h acute toxicity of aqueous nanoparticles synthesized using the same methodology, including Au, Ag, and Ag-Au bimetallic nanoparticles, was conducted to determine their ecological effect in freshwater environments through the use of Daphnia magna, using their mortality as a toxicological endpoint. D. magna are one of the standard organisms used for ecotoxicity studies due to their sensitivity to chemical toxicants. Particle suspensions used in toxicity testing were well-characterized through a combination of absorbance measurements, atomic force or electron microscopy, flame atomic absorption spectrometry, and dynamic light scattering to determine composition, aggregation state, and particle size. The toxicity of all nanoparticles tested was found to be dose and composition dependent. The concentration of Au nanoparticles that killed 50% of the test organisms (LC(50)) ranged from 65-75 mg/L. In addition, three different sized Ag nanoparticles (diameters = 36, 52, and 66 nm) were studied to analyze the toxicological effects of particle size on D. magna; however, it was found that toxicity was not a function of size and ranged from 3-4 μg/L for all three sets of Ag nanoparticles tested. This was possibly due to the large degree of aggregation when these nanoparticles were suspended in standard synthetic freshwater. Moreover, the LC(50) values for Ag-Au bimetallic nanoparticles were found to be between that of Ag and Au but much closer to that of Ag. The bimetallic particles containing 80% Ag and 20% Au were found to have a significantly lower toxicity to Daphnia (LC(50) of 15 μg/L) compared to Ag nanoparticles, while the toxicity of the nanoparticles containing 20% Ag and 80% Au was greater than expected at 12 μg/L. The comparison results confirm that Ag nanoparticles were much more toxic than Au nanoparticles, and that the introduction of gold into silver nanoparticles may lower their environmental impact by lowering the amount of Ag which is bioavailable.
The compounds ([SnSe]1+δ) m (NbSe2)1, where 1 ≤ m ≤ 10, were prepared from a series of designed precursors. The c-axis lattice parameter systematically increases by 0.577(5) nm as the value of m is increased, which indicates that an additional bilayer of rock salt structured SnSe is inserted for each unit of m. The in-plane structure of both constituents systematically changes as the thickness of SnSe increases. Both X-ray diffraction and electron microscopy studies show the presence of turbostratic disorder between the different constituent layers. The electrical resistivity and Hall coefficient increase systematically as a function of m stronger than would be expected for noninteracting metallic NbSe2 and semiconducting SnSe layers, suggesting the presence of charge transfer between the layers. The temperature dependence of the resistivity changes from metallic behavior for m < 4 to weakly increasing, for higher m, as temperature decreases. Compounds with m > 3 show an upturn in the resistivity below 50 K and a corresponding increase in the Hall coefficient, which both become more pronounced as m increases. This suggests localization of carriers, which is expected in two-dimensional crystals. The extent of charge transfer in ([SnSe]1+δ) m (NbSe2)1 can be tuned as a function of SnSe thickness and spans over the same range as reported in the literature for various NbX2 based intercalated and misfit layer compounds.
PbSe) 1.14 ] m (NbSe 2 ) n compounds with 1 ≤ m ≤ 6 and n = 1 were synthesized using the modulated elemental reactants (MER) method. X-ray diffraction patterns (XRD) showed that the desired compounds self-assembled during annealing of the precursors with their c-axis crystallographically aligned normal to the substrate. The c-axis lattice parameter increased by 0.612 nm as m, the number of PbSe bilayers, increased by one. Analysis of the in-plane diffraction patterns indicated that the a-lattice parameters remained constant as m was varied. Reciprocal space maps along hkl (h, k ≠ 0; l ≠0) indicate very short coherence lengths in mixed-index directions, consistent with the rotational disorder between layers observed in electron microscopy cross sections. The in-plane electrical resistivity and Hall coefficients were measured for each ferecrystal from 22 to 295 K. The resistivity systematically increased as m increased, but the magnitude of the increase is greater than predicted assuming independent layers. Assuming the metallic conduction results from a single band in the NbSe 2 layer, the carrier concentrations determined from the Hall coefficients decreases as m increases, suggesting increased charge transfer from PbSe to NbSe 2 with increasing values of m. First-principles electronic-structure calculations based on the generalized gradient approximation to density functional theory suggest that the PbSe valence band overlaps the empty bands in NbSe 2 , supporting the idea of interlayer charge transfer from PbSe to NbSe 2 .
A new polytype of the misfit layer compound ([SnSe] 1.16) 1 (NbSe 2) 1 with extensive rotational disorder was prepared from designed modulated elemental reactants. This polytype, previously referred to as a ferecrystal due to the extensive rotational disorder, formed over a range of compositions and precursor thicknesses and the resulting c-axis lattice parameters ranged from 1.2210(4) to 1.2360(4) nm. These values bracket the value published for the crystalline misfit compound prepared at high temperature. The a-and b-axis in-plane lattice parameters of both the SnSe and NbSe 2 constituents were incommensurate, which differs from the misfit layered compound formed via high temperature reaction that has a common b-axis lattice parameter for the two constituents. The in-plane area per unit cell of the ferecrystal is 1-2% larger than the compound formed at high temperature. The ferecrystalline ([SnSe] 1.16) 1 (NbSe 2) 1 compound is 1.6 times more conductive than the misfit layer compound. Hall effect measurements indicate that the ferecrystal is a p-type metal and that the higher conductivity is a consequence of higher mobility of carriers in the ferecrystalline compound.
This work provides a review on the in-plane structure of a recently established subgroup of misfit layer compounds, namely ferecrystals. While misfit layer compounds can be described as layered intergrowths of two distinct substructures in which only one axis of the subunits remains incommensurate, for ferecrystals both constituents retain independent in-plane crystalline structures. This is accompanied by extensive rotational disorder, a key feature of ferecrystals. Our comparison of the in-plane structures of misfit layer compounds and ferecrystals suggests that the weaker registration between the subunits in ferecrystals allows the synthesis of a wider variety of compounds, varying both the range of constituents, their mismatch and their thicknesses. The in-plane structures of the individual constituents in ferecrystalline compounds exhibit interesting features such as 2D-symmetry, size induced structural transformation, misfit parameters that depend on the component thicknesses, and pairing distortions of constituent layers.
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