Hierarchical structure of niobate nanosheets in aqueous solution

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.