Hierarchical structure of niobate nanosheets in aqueous solution
TL;DR: In this article, 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.
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.
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TL;DR: In this paper, the authors describe the aspects related to layered niobate exfoliation and the new possibilities that arises from the use of Niobate nanosheets in the manufacturing of thin films, layer-by-layer assemblies, hybrid structures, sensors and other materials.
Abstract: Layered materials with intracrystalline reactivity undergo intercalation and pillaring reactions to produce materials with useful properties for catalysis, electrodes for Li batteries and adsorbents. New possibilities for the use of layered inorganic solids came out from the layered structures capable of delamination. The exfoliated particles are considered a new class of nanomaterial based on single crystal nanosheets. Due to their unique morphological features and properties, these nanosheets can be used as building blocks for nanomaterials with innovative properties. In this feature article we describe the aspects related to layered niobate exfoliation and the new possibilities that arises from the use of niobate nanosheets in the manufacturing of thin films, layer-by-layer (LbL) assemblies, hybrid structures, sensors and other materials.
184 citations
TL;DR: In this article, the recent progress of emerging new materials of inorganic nanosheet liquid crystals is reviewed with a focus on the behaviors of each system, alignment by external field, and theoretical aspects.
Abstract: Inorganic layered crystals such as clay minerals, layered niobates, and graphite are exfoliated in solvents to form colloidal dispersions of extremely thin inorganic nanosheets. Recently, the liquid crystal phases of these “nanosheet colloids” have been rediscovered and are attracting interest as new types of inorganic liquid crystals. The huge anisotropy of the mesogenic nanosheets compared to other liquid crystal systems is an important feature of the nanosheet liquid crystals for fundamental studies in the fields of colloid science and soft matter physics. In addition, the rich functionalities intrinsic to inorganic materials open a variety of applications such as smart colloids and composite materials with structural regularity. In this article, the recent progress of the emerging new materials of inorganic nanosheet liquid crystals is reviewed with a focus on the behaviors of each system, alignment by external field, and theoretical aspects.
61 citations
TL;DR: Size-controlled nanosheet colloids of fluorohectorite and fluortetrasilicic mica were prepared in high yield and their transitions to fluid liquid crystal phases with highly ordered lamellar structures were identified over a wide concentration range, which is a rare case for clay mineral systems.
61 citations
TL;DR: The yield and stability of the MV*+ species were found to depend strongly on the clay content of the colloid: from a few mol % to approximately 70 mol % of the yield and several tens of minutes to more than 40 h of the lifetime.
Abstract: We investigated photoinduced charge separation occurring in a multicomponent colloidal system composed of oxide nanosheets of photocatalytically active niobate and photochemically inert clay and electron accepting methylviologen dications (MV2+). The inorganic nanosheets were obtained by exfoliation of layered hexaniobate and hectorite clay. The niobate and clay nanosheets were spatially separated in the colloidally dispersed state, and the MV2+ molecules were selectively adsorbed on the clay platelets. UV irradiation of the colloids led to electron transfer from the niobate nanosheets to the MV2+ molecules adsorbed on clay. The photoinduced electron transfer produced methylviologen radical cations (MV•+), which was characterized by high yield and long lifetime. The yield and stability of the MV•+ species were found to depend strongly on the clay content of the colloid: from a few mol % to ∼70 mol % of the yield and several tens of minutes to more than 40 h of the lifetime. The contents of the niobate nan...
24 citations
TL;DR: It is reported that colloidal nanosheets of a Dion–Jacobson type layered perovskites, Ca2Nb3O10, adopt a LC phase with a mesoscale structure by condensation after the conventional preparation of the nanosheet colloid.
Abstract: Three nanosheets to the wind: A liquid‐crystal phase in the condensed colloids of semiconductor nanosheets of a layered perovskite Ca2Nb3O10− was synthesized and characterized. Their properties and functionalities are tunable for further fundamental studies and potential applications as smart soft materials.
19 citations
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TL;DR: In this paper, an exfoliation of acid-exchanged K4Nb6O17 with tetra(n-butyl)ammonium hydroxide in water produces a colloidal suspension of individual sheets, which roll into loosely bound tubular structures.
Abstract: The exfoliation of acid-exchanged K4Nb6O17 with tetra(n-butyl)ammonium hydroxide in water produces a colloidal suspension of individual sheets, which roll into loosely bound tubular structures The tubule shape can be made permanent via precipitation of the colloid with alkali cations Atomic force microscopy and transmission electron micrographs reveal that the tubules have outer diameters ranging from 15 to 30 nm and that they are 01 to 1 μm in length The observed curling tendency, preferential folding, and cleavage angles of the individual sheets are interpreted in terms of the crystal structure of the parent solid, K4Nb6O17 The driving force for tubule formation appears to be relief of strain that is inherent in the asymmetric single sheets This driving force is absent in bilayer colloids formed early in the exfoliation process, which are found only as flat sheets Tubules in colloidal suspensions that have been subjected to turbulence have a tendency to unroll into flat sheets on surfaces, indica
241 citations
"Hierarchical structure of niobate n..." refers background in this paper
...The layered crystals are believed to be quantitatively delaminated into bilayers via cation exchange of interlayer K+ ions for propylammonium cations, which makes the thickness of the nanosheets monodisperse (Saupe et al., 2000; Miyamoto et al., 2002)....
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TL;DR: In this article, the fractal properties of colloidal suspensions of synthetic clay were studied by using a combination of small-angle neutron and x-ray scattering and static light scattering, and it is suggested that the structure of the dispersions at rest is composed of subunits measuring a few tens of nanometers that combine to form dense aggregates measuring about 1 mm.
Abstract: The characteristic length scales of the structure and fractal behavior of a thixotropic colloidal suspension of synthetic clay were studied by using a combination of small-angle neutron and x-ray scattering and static light scattering. At the same time, macroscopic mechanical behavior at rest was characterized by means of rheometric measurements. Two characteristic length scales were detected in these yield stress suspensions of discotic texture. The first, measuring several tens of nanometres, is linked to a fractal dimension of 3. The second, of the order of 1 mm, is linked to a fractal behavior of dimension D that increases with the particle volume fraction. Consequently, it is suggested that the structure of the dispersions at rest is composed of subunits measuring a few tens of nanometers that combine to form dense aggregates measuring about 1 mm. At larger length scales, these micrometer-sized aggregates are rearranged to form a continuous three-dimensional isotropic structure that has a fractal behavior of dimension D, which gives the gels their texture. The increase of this fractal dimension with the particle volume fraction, the ionic strength, and the gelation time is correlated to a hardening of the mechanical properties of the gels at rest. The gel state is reached above a volume fraction f v for a given ionic strength and gelation time. In the gel phase, a critical volume fraction f vc separates two domains. Gels belonging to the domain f v,f v,f vc have a fractal behavior of dimension D5160.05, suggesting an alignment of the micrometer-sized aggregates that leads to the formation of a mechanically weak fibrous structure. Gels belonging f v.f vc have a fractal dimension D51.860.01, corresponding to a mechanically stronger structure consisting of zones of high and lower particle density. A scaling law enabled these fractal dimensions to be correlated with the effect of the volume fraction on the yield stress. In contrast to what is commonly assumed in relation to clay suspensions, it is suggested here that it is the large length scales, of the order of 1 mm, associated with a fractal arrangement that governs the macroscopic mechanical behavior.
222 citations
"Hierarchical structure of niobate n..." refers background in this paper
...A huge aspect ratio like this has never been obtained for other kinds of inorganic nanosheets, such as those of clays (Pignon et al., 1997)....
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TL;DR: In this article, a set of precalibrated strongly scattering standards for the SAXS facilities of the National Center for Small-Angle Scattering Research (Oak Ridge) were developed.
Abstract: Absolute calibration forms a valuable diagnostic tool in small-angle scattering experiments and allows the parameters of a given model to be restricted to the set which reproduces the observed intensity. General methods which are available for absolute scaling of small-angle X-ray scattering (SAXS) data are reviewed along with estimates of the degree of internal consistency which may be achieved between the various standards. In order to minimize the time devoted to calibration in a given experimental program, emphasis is placed on developing a set of precalibrated strongly scattering standards for the SAXS facilities of the National Center for Small-Angle Scattering Research (Oak Ridge). Similar standards have been developed previously for calibration of small-angle neutron scattering (SANS) data. Particular attention is given to standards which can be used for either SAXS or SANS experiments where each sample has been independently calibrated for both types of radiation. These calibrations have been tested via the theoretical relationships between the two cross sections. It has been found that specimens best suited for such intercalibration purposes are a glassy carbon specimen where the scattering arises from voids in a carbon matrix and a perdeuterated polyethylene where the scattering arises from periodic arrangement of the crystalline lamellae. In only these two cases could the identical specimen be used for both the neutron and X-ray scattering experiments.
166 citations
"Hierarchical structure of niobate n..." refers methods in this paper
...After these corrections, the absolute scattering intensity was obtained by using a pre-calibrated secondary standard of irradiated aluminium (Schelten & Hendricks, 1975; Russell et al., 1988)....
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TL;DR: The phase transition concentrations decreased with increasing aspect ratio (lateral-to-thickness ratio) of the nanosheets, almost in accordance with the prediction by Onsager theory, indicating that the liquid crystallinity is explained basically by excluded-volume effect between the nanOSheets.
Abstract: Colloidally dispersed niobate nanosheets with the thickness of 1.8 nm and controlled mean lateral sizes of 0.15-7.8 mum were prepared and their liquid crystallinity was examined. The nanosheet colloids with different lateral sizes were obtained by exfoliation of single crystals of layered niobate K4Nb6O17 and subsequent ultrasonication. Naked-eye and microscope observations of the nanosheet colloids between crossed polarizers revealed liquid crystallinity of the sols characterized by birefringence as functions of the lateral sizes and concentration of the nanosheets. The nanosheet colloids with smaller lateral sizes (0.15-1.9 mum) varied from isotropic to biphasic (isotropic + liquid crystalline), and finally to fully liquid crystalline states as the colloid concentration increased. The phase transition concentrations (from isotropic to biphasic and biphasic to liquid crystalline) decreased with increasing aspect ratio (lateral-to-thickness ratio) of the nanosheets, almost in accordance with the prediction by Onsager theory, indicating that the liquid crystallinity is explained basically by excluded-volume effect between the nanosheets. On the other hand, the colloids with larger lateral sizes (6.2 and 7.8 mum) stably kept liquid crystalline state even at very low concentration (5.1 x 10-6 in volume fraction), which was much lower than that expected from the theory.
104 citations
"Hierarchical structure of niobate n..." refers background in this paper
...The definitions of L and are also given by Miyamoto & Nakato (2004)....
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...The layered crystal is easily delaminated into unit layers and the exfoliated [Nb6O17] 4 sheets (hereafter called ‘nanosheets’) have an outstanding structural feature (Miyamoto et al., 2002; Miyamoto & Nakato, 2004)....
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...The huge nanosheets are ordered to form a liquid-crystalline phase, which is basically explained by Onsager’s theory (Onsager, 1949), as evidenced by observation between crossed polarizers (Miyamoto & Nakato, 2004)....
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...For details of K4Nb6O17 single-crystal formation and the subsequent exfoliation process see Miyamoto & Nakato (2004)....
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TL;DR: Clastre, J., ALLI~AUME, M. C., and BouzoN, C. (1965). Acta Cryst. 18, 731.
Abstract: ALLI~AUME, M. & DEcAP, J. (1965). Acta Cryst. 18, 731. CLASTRE, J., ALLI~AUME, M. & BouzoN, C. (1964). Bull. Soc. Fran~. Mindr. Crist. 87, 434. CRUICKSHANK et aL (1961). Computing Methods and the Phase Problem in X-ray Crystal Analysis. Paper 6, 32. O'CONNOR, B. H. & MASLEN, E. N. (1965). Acta Cryst. 18, 363. PENVOLD, B. R. & WHITE, J. C. B. (1959). Acta Cryst. 12, 130. WATANABE, A. (1941). Naturwissenschaften, 29, 116. WATANABE, A. (1942). J. Pharm. Soc. Japan, 62, 503.
82 citations
"Hierarchical structure of niobate n..." refers methods in this paper
...All the scattering profiles were measured at ambient temperature and corrected for the background scattering and slit smearing (desmearing; Schmidt, 1965)....
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