Hierarchical structure of niobate nanosheets in aqueous solution

Abstract

The hierarchical structure of an aqueous dispersion of niobate nanosheets was explored by using a combined method of ultra‐small‐angle and small‐angle scattering of neutrons and X‐rays. The concentration of the sheets studied was in the range where the dispersion exhibits a liquid‐crystal phase as evidenced by observation between crossed polarizers in a previous report. The scattering data covered a wide q scale of more than four orders of magnitude [3 × 10−4≤q≤ 10 nm−1, where q = (4π/λ)sin(θ/2), λ and θ being the wavelength of the incident beam and the scattering angle, respectively], corresponding to the length scale l = 2π/q from ~1 nm to ~20 µm. The scattering analyses provided information on the hierarchical structural elements including: (i) single nanosheets as a structure element (hierarchy I), (ii) parallel stacks of the sheets (hierarchy II), and (iii) spatial arrangements of the stacks (hierarchy III), in order of increasing length scale. Hierarchy II is closely related to the liquid‐crystal nature of the dispersion in which the spacing and the persistence length, normal and parallel to the stack surface, respectively, were disclosed. Hierarchy III gives rise to the low‐q upturn in the scattering profile, which may be characterized by mass‐fractal‐like power‐law scattering behavior. This finding is a surprise from the viewpoint of the liquid‐crystal nature of the dispersion, a possible model of which is proposed in the text.

Aspect-ratio-dependent phase transitions and concentration fluctuations in aqueous colloidal dispersions of charged platelike particles

“…These findings also indicate that the large concentration fluctuations in the isotropic and Lay phases and that the concentration fluctuations are different for the two φ p ranges. The concentration fluctuations found in the Lay phase firmly supports our previous finding [1] that the domains poor in the particles are spatially distributed with a mass fractal structure.…”

“…That is, in both the upper isotropic and the lower Lay phases, as well as in the Lay single phase, the obtained D value from the scattering profile may reflect only the local concentration in grains, where the nanosheets are self-assembled more densely than the bulk (average) concentration, and these grains coexist with the nanosheet-poor matrix. It is important to note that this finding further verifies firmly our previous observation of the intriguing concentration fluctuations of the sheets observed in the Lay single phase [1]. It is noteworthy that the grain or cluster formation of the colloidal particles has been directly observed by optical microscopy in some other systems too [54,55,[68][69][70][71].…”

“…This work was motivated by our previous work [1], which elucidated the intriguing a mass-fractal-like concentration fluctuations existing in the (macroscopically uniform) lyotropic liquid crystalline phase (LC) composed of aqueous dispersions of exfoliated niobate nanosheets. The nanosheets, as one of the charged platelike colloidal dispersions, had a particle size characterized by d = 1.6 nm and L = 3.2 μm and at a particular particle volume concentration φ p = 0.032, where d and L are the thickness and the average lateral size (L) of the sheets, respectively.…”

Self-assembly of biaxial discorectangular lead carbonate nanosheets into stacked ribbons studied by SAXS and HAADF-STEM tomographic tilt series

“…25,26 So far only 1D assembly of the sheets into lamellar stacks has been observed, whereby SAXS has mainly been used to determine the spacing between the constituting nanosheets. 22,23,25,27 Here, we employ nonionic water-in-oil (w/o) microemulsions at low water content to synthesize well-defined lead carbonate nanosheets of biaxial shape that are characterised by a narrow width and thickness distribution and κ 1 and κ 2 values of about 2.7 and 5.5, respectively. (θ)/λ is the modulus of the scattering vector, 2θ is the scattering angle and λ is the wavelength).…”

Inorganic Nanosheet Liquid Crystals: Self-Assembled Structures in Dispersions of Two-Dimensional Inorganic Polymers

“…Analysis of the scattering profile of an aqueous dispersion of niobate nanosheets (0.32 vol.%) without salt. Reprinted with permission from reference 41) . a) The concentration where the colloid transits from isotropic to biphasic.…”