Colloidal Nanosheets

Miyamoto, Nobuyoshi; Ohsedo, Yutaka; Nakato, Teruyuki

doi:10.1007/978-4-431-56496-6_8

Nobuyoshi Miyamoto⁴,
Yutaka Ohsedo⁴ &
Teruyuki Nakato⁵

Part of the book series: Nanostructure Science and Technology ((NST))

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7 Citations

Abstract

This review emphasizes the importance of the nanosheets in the state of colloidal dispersion for many applications such as soft materials and nanofabrication. The two important aspects of the nanosheet colloids are highlighted: liquid crystal phase formation and rheological properties. The theories related to these aspects are also described. As important techniques for many applications, we also introduce macroscopic orientational control by external fields and formation of multicomponent nanosheet colloids.

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Assemblies and Superstructures of Inorganic Colloidal Nanocrystals

General in Colloidal Nanoparticles

Particles at fluid–fluid interfaces: From single-particle behavior to hierarchical assembly of materials

Article 12 December 2014

Keywords

1 Introduction

Nanosheet materials are obtained as colloidal dispersions through the exfoliation of a layered solid in a solvent. In many cases, colloidal dispersions of nanosheets are considered homogeneous and structureless dispersions that are just a passing point for the fabrication of many types of important solid-state nanomaterials, as introduced in other chapters of this book.

However, we emphasize that the nanosheet colloids themselves have controllable meso-scale structures and intriguing properties and functions. From the viewpoint of fundamental colloid sciences, nanosheet colloids are regarded as a very unusual colloidal system compared to other ordinary colloids because of the huge anisotropy of the particles. From the viewpoint of practical applications, nanosheet colloids serve as functional soft materials due to their interesting rheological properties, liquid crystallinity, and response to external stimuli. In addition, we can expect the development of smart soft materials by coupling the properties of nanosheet colloids with various functions inherent to the inorganic nanosheets, such as their electric, catalytic, magnetic, and optical properties. To obtain functional solid-state nanomaterials using nanosheets as nanomodules, it is also important to understand and control the structure and properties of the nanosheets in the state of colloidal dispersion because most of these solid materials are fabricated using colloidal dispersions of nanosheets as a precursor.

This chapter addresses colloidal nanosheets dispersed in a solvent. Starting with the fundamental aspects of the nanosheet colloids based on classical models, we then focus on the two important aspects of the nanosheet colloids: liquid crystal phase formation and rheological properties. Theories related to the colloid properties are also described. The studies on applications of the nanosheet colloids for soft materials and nanofabrication are also introduced.

2 Classical Theory of Colloids Applicable for Nanosheets

Let us first start with a general description of the classical models of colloids: the Stern model and DLVO theory [1]. In these models, the surface charge is important because it determines the stability and structure formation of a colloidal dispersion. Generally, colloid particles are charged in water, regardless of their chemical composition. Even if the particle has an electrostatically neutral chemical composition, the particle is normally charged due to dissociation or protonation of the surface functional groups. In the case of a particle of an oxide MO_x, where M is Si, Al, Fe, etc., the charge depends on pH because the hydroxy groups on the particle dissociate into M–O⁻ and H⁺ at higher pH, while it is protonated to form positively charged ${{{\text{M}}{-}{\text{OH}}}_{ 2}}^{ + }$ at lower pH. The charge becomes zero at intermediate pH, which is called the point of zero charge. The amount of charge on the surface is quantified by the charge density σ ₀ [C m⁻²] or surface potential ψ ₀ [V]. However, these values are difficult to directly measure experimentally. Hence, the zeta potential ζ [V], which is the potential at the slipping surface of the particle, is generally used. The zeta potential is slightly lower than the surface potential.

The charge of the particles should be compensated by the same amount of opposite charge of the counter ions. The Stern model is the standard model to depict the spatial distribution of the counter ions around the particle. According to this model, the particle is surrounded by an electric double layer composed of a Stern layer and a Gouy–Chapman layer (or diffuse layer). The Stern layer is the first layer in which the ions are strongly bound onto the particle surface due to the affinity of the ions to the surface. In the Gouy–Chapman layer, the ions are weakly bound to the particle and form an ionic cloud. The concentration of counter ions, or potential, decreases as the distance x from the surface increases. Under some assumptions and approximations, the potential at distance x is expressed as

$$\psi = \psi_{0} \exp ( - \kappa x).$$

(8.1)

Here, κ ⁻¹ is called as Debye screening length, which is usually regarded as a measure of the thickness of the diffuse layer and is expressed as

$$\kappa^{ - 1} = \sqrt {\frac{{\varepsilon_{\text{r}} \varepsilon_{0} kT}}{{2000N_{\text{A}} e^{2} I}}},$$

(8.2)

where $\varepsilon_{\text{r}}$ is the specific dielectric constant of the solvent, $\varepsilon_{0}$ is the dielectric constant of vacuum, T is temperature, k is Boltzmann’s constant, N _A is the Avogadro’s constant, e is the charge of an electron, and I is the ionic strength. An important point is that the thickness of the diffuse layer decreases with increasing ionic strength I.

The simplest theory on the interactions between colloid particles is the DLVO (Derjaguin–Landau–Verwey–Overbeek) theory [1]. In this theory, the potential energy V _T for a pair of colloid particles is expressed as the sum of van der Waals attraction V _A and the repulsion V _R due to overlap of the diffuse layers as

$$V_{\text{T}} = V_{\text{A}} + V_{\text{R}} .$$

(8.3)

The potential due to attraction between two parallel thin plates with thickness L, which are separated by distance h, is expressed as

$$\begin{aligned} & V_{\text{A}} = - \frac{A}{12\pi }\left\{ {\frac{1}{{h^{2} }} + \frac{1}{{(h + 2L)^{2} }} - \frac{2}{{(h + L)^{2} }}} \right\} \\ & \sim \frac{A}{2\pi }\frac{{L^{2} }}{{h^{4} }}({\text{if}}\,L \ll h) \\ \end{aligned},$$

(8.4)

where A is the Hamaker constant, which indicates the strength of the van der Waals interaction. The repulsion term is expressed as

$$V_{\text{R}} = \frac{64nkT}{\kappa }\left[ {\tanh \left( {\frac{{e\psi_{0} }}{4kT}} \right)} \right]\exp \;( - \kappa h).$$

(8.5)

Based on DLVO theory, we can discuss the stability of the dispersed state and the structure formation of a colloid. When the ionic strength is lower and the surface charge is higher, the repulsion V _R is dominant, giving a maximum on the V _T–h curve. In this case, the colloid dispersion is stable and the average distance between the particles is determined simply based on the particle concentration. When the ionic strength is higher and the surface charge is lower, the attraction force V _A dominates the system so that V _T is always negative and only the deep minimum appears on the curve. In this case, the colloid particles aggregate. With intermediate ionic strength and surface charge, a shallow, secondary minimum sometimes appears on the V _T–h curve as a result of the balancing of V _R and V _A. In this case, the particles may be loosely trapped at the position of the secondary minimum. These behaviors are qualitatively in accordance with many experiments, as shown in Chap. 3, while quantitative formulation is difficult with this simple theory.

To apply these theories to nanosheet colloids, Eqs. (8.1)–(8.5) can be valid, but the charge of the nanosheets should be considered. Most known nanosheets, such as clay minerals, metal oxides, metal phosphates, and graphene oxides, are negatively charged, while the rare example is layered double hydroxides that have permanent positive charges. The ideal charge density for each layered material can be calculated based on the chemical composition and crystal structure. Comparison by the unit of [e nm⁻²], charge per unit surface area, is useful for many purposes, while, in the case of clay minerals, the unit of [meq g⁻¹] is often used. Figure 8.1 compares the ideal layer charge of several nanosheet materials. The charge density ranges from 0.5 to 4.0 [e nm⁻¹], and the ion exchange capacity ranges from 0.5 to 8 [meq g⁻¹]. For a nanosheet surface with this range of layer charge, a surface potential as high as several hundreds of mV is expected. However, a zeta potential value of several tens of mV is often obtained experimentally [2] due to a low dissociation constant of the surface group and adsorption of counter ions in the Stern layer, which is largely affected by the type of counter cations. In contrast to most other materials, smectite-type clay minerals have permanent negative charges inside the layer, while the edge charge depends on the pH due to the presence of hydroxy group. The edges are normally negatively charged at higher pH, at which we usually address clay colloids. We note there are many publications in which the discussions are based on the misunderstanding that the edge is always positively charged.

3 Liquid Crystal Phases of Nanosheet Colloids

Because of the huge anisotropy of nanosheets, the nanosheet colloids form liquid crystal (LC) phases. In a dilute colloidal dispersion, nanosheets are moving around in translational and rotational manners, that is, Brownian motion. However, if the concentration of the nanosheets increases, the free rotational motion is restricted and the nanosheets cooperatively orient themselves along a certain direction to form an LC phase. We have named this type of liquid crystals as nanosheet liquid crystals and have been studying these materials as a new type inorganic LCs. Nanosheet LCs are classified as a lyotropic LCs, as distinguished from thermotropic LCs. A lyotropic LC is generally composed of a solvent and anisotropic dispersoids, and the LC phase formation depends on the concentration of the dispersoid. Many types of lyotropic LCs have been reported, as shown in Fig. 8.2: surfactant micelles, rigid polymers [3–5] (e.g., poly(paraphenylene terephthalamide) and DNA), rod-shaped particles (viruses [6], metals [7], oxides [8–10], and semiconductors [11]), nanoplates (gibbsite (Al(OH)₃) [12, 13], Ni(OH)₂ [14], and layered double hydroxides [15]), and nanosheets. Among them, nanosheets are regarded as ultimately anisotropic two-dimensional particles.

Although many nanosheet materials have been reported so far, only a few of them are reported as nanosheet liquid crystals for several reasons. One reason is that, in some cases, only a lower degree of exfoliation and lower yield of exfoliated single-layered nanosheets can be attained. In other cases, the colloids themselves are unstable so that they precipitate and aggregate within short time, preventing their use as a colloidal material. It is also a problem that, as often observed in layered clay mineral systems, the colloidal dispersions becomes viscose or gelated at low concentration, and the characterization as a fluid liquid crystal cannot be performed. Considering various applications, it is desirable that the system has a wider concentration range for the liquid crystal phase with high fluidity. Therefore, it is important to optimize various parameters, such as particle size, distribution of the particle size, and salt concentration, to improve the properties of the colloid to be suitable for nanosheet liquid crystal materials. Although we have a few numbers of the examples satisfying these conditions, Figs. 8.3 and 8.4 summarize the schematic structures of the reported nanosheet LCs and the phase diagram, respectively. In the following subsections, the individual research works of each material will be presented.

3.1 Layered Phosphates

In a 2001 article in Nature [16], Gabriel et al. reported phosphoric antimony nanosheets obtained from layered K₂Sb₃P₂O₁₄ as a nanosheet LC with a swollen lamellar structure. K₂Sb₃P₂O₁₄ is composed of SbO₆ octahedrons and PO₄ tetrahedrons combined with a shared apex (Fig. 8.3b). The layer has a thickness of 1.1 nm, and the negative charges are compensated by interlayer K⁺. The homogeneous, clear, and transparent nanosheet colloid was obtained by exchanging the interlayer K⁺ with H⁺, followed by washing and dialysis. In contrast to most other systems, this material does not require the addition of a stabilizer or exfoliating agent. Figure 8.5 shows the phase diagram of this system. In the range of the nanosheet concentration of 0.75–1.78 vol.%, this colloid possesses fluidity and birefringence. At more than 1.78 vol.%, it becomes a gel with birefringence. Below 0.75 vol.%, it enters a biphasic state, that is, the coexistence of birefringent and isotropic phases. After storing the biphasic sample, the dense birefringent phase and the dilute isotropic phase are macroscopically separated into the lower and upper parts of the sample, respectively, with a clear interface. Because the isotropic phase shows flow-induced birefringence, this upper phase is confirmed to be a colloid with dispersed nanosheets but not a simple solvent. Although not mentioned in the literature, at lower concentration, the system should transition to a purely isotropic phase. Thus, with increasing concentration, the system transitions from the isotropic phase to the complete liquid crystal phase, passing through the coexistence of the two phases. This is the general behavior of anisotropic colloids as theoretically predicted by Onsger [27], the theory of which is explained in detail in Sect. 4.1.

In this system, a swollen lamellar structure with the basal spacing of up to 225 nm was identified by small-angle X-ray scattering (SAXS). As shown in Fig. 8.6a, multiple narrow diffraction peaks appeared, indicating high structural order. Because of the large basal spacing, structural colors from blue to red were observed under white illumination. Of the several reported nanosheet colloid systems, there are only a few examples with the very large basal spacing, high structural order, and structural colors observed in this system. As shown in Fig. 8.6b, the basal spacing d increases with decreasing nanosheet concentration; it reaches the maximum of 225 nm. If the lamellar structure with layer thickness L undergoes ideal one-dimensional swelling, the basal spacing d follows the following formula,

$$d = L/\varphi^{ - 1} .$$

(8.6)

In the range of φ ⁻¹ < 100, the plot in Fig. 8.6b follows this formula. Extrapolation of the straight line gives the value L = 1.05 nm, and this value approximately coincides with the crystallographically estimated thickness of the nanosheet. By adding the salt to the colloid, phenomena such as flocculation, gelation, and decrease of basal spacing (Fig. 8.6c) occurred. However, these phenomena occurred only when a salt concentration was more than 10⁻² M. Thus, adding a small amount of salt does not greatly influence the state of the system.

As another phosphate nanosheet system, Cheng et al. investigated nanosheet colloids derived from layered zirconium phosphate α-ZrP [28]. In this report, powders of layered crystals of α-ZrP were synthesized by a hydrothermal method so that the average particle size and size distribution were precisely controlled by varying the reaction time and the concentration of the starting solution. Through exfoliation of these crystals, a series of the samples with different lateral size and size distribution were obtained. The phase transition concentrations ϕ _I (for the isotropic phase to biphase) and ϕ _N (for the biphase to liquid crystal phase) for these samples are shown in Fig. 8.7. Both ϕ _I and ϕ _N decrease with increasing average particle size. Although Miyamoto et al. had already reported the influence of particle size at the time [17], the importance of Cheng’s study is that they clarified the effect of the particle size distribution. As the particle size distribution increased, ϕ _N exceedingly increased, while ϕ _I did not change, resulting in widening of the concentration range for the biphase and the coexistence of the isotropic and LC phases. This result is in harmony with the theoretical prediction by the modified Onsager’s theory. Moreover, Cheng et al. reported the α-ZrP system, of which the nanosheet surface is chemically modified with thermosensitive poly(N-isopropylacrylamide). This system showed reversible liquid crystal-isotropic phase transition induced by temperature change.

Recently, by adsorbing the amphiphilic polyoxyalkyleneamine onto the surface of α-ZrP nanosheets, Wong et al. [29] prepared nanosheets dispersed in nonaqueous solvent, butyronitrile, stabilized by the steric effect of the adsorbed polymer layer. Nanosheet LCs dispersed in organic solvent are applicable for the fabrication of various inorganic/organic composites, although such a system has not been reported previously. Furthermore, in this system, similarly to the study of K₃Sb₃P₂O₁₄ reported by Gabriel [16], a lamellar structure with very large basal spacing of up to ca. 240 nm was confirmed, and structural colors attributable to this structure were observed (Fig. 8.8).

3.2 Layered Transition Metal Oxides

In 2002, Miyamoto and Nakato reported a liquid crystal nanosheet colloid prepared from the layered niobate, K₄Nb₆O₁₇ [30, 31]. They also confirmed the liquid crystallinity in the related oxide materials, such as KTiNbO₅ [18], KNb₃O₈ [18], and H_1.07Ti_1.73O₄ [19]. In 2004, they clarified the effect of the lateral size of the nanosheets on the formation of the LC phase for the first time. In that study, nanosheets with controlled large lateral size were obtained by exfoliation of a K₄Nb₆O₁₇ crystal of cm scale, synthesized by the flux method [32]. The obtained colloid was ultrasonicated to crush the nanosheets to a smaller average size, as controlled by the duration of the ultrasonication. A series of samples with average particle size ranging from 0.15 to 7.8 μm were obtained. The dependence of ϕ _I and ϕ _N on the average lateral size of the nanosheets is summarized in Table 8.1. The simple trend is found that ϕ _I increases as the average particle size increases. This trend roughly coincides with the prediction of Onsager theory. This niobate system is advantageous because the fluidic liquid crystal phase is obtained in a very large concentration range. Owing to the fluidity, macroscopic orientation by an external field is easy. The liquid crystal phase of the niobate nanosheets with an adsorbed organic dye was macroscopically aligned by gravitational field, providing a hierarchical organization of the dye structure.

Table 8.1 Mean lateral sizes and critical concentrations of the phase transitions

Full size table

Yamaguchi et al. conducted detailed structural analyses for the K₄Nb₆O₁₇ nanosheet LC system [33, 34]. Figure 8.9 shows the combined small-angle neutron/X-ray scattering patterns for the nanosheets with an average particle size of 3200 nm. The scattering profile roughly coincides with the theoretical form factor P(q) of a thin disc with radius R and thickness L. P(q) is represented as follows:

$$P(q) = \left( {\frac{2}{{q^{2} R^{2} }}} \right)\left[ {1 - \frac{{J_{1} (2qR)}}{qR}} \right]\frac{{\sin^{2} (qL/2)}}{{(qL/2)^{2} }}$$

(8.7)

Here, q is the scattering vector and J ₁ is the first-order Bessel function. This form factor is reduced as: P(q) ~ q ⁻² in the range of L ⁻¹ < q < R ⁻¹, P(q) ~ 1 in the range of q < R ⁻¹, and P(q) ~ q ⁻⁴ in the range of q > L ⁻¹. The solid line in Fig. 8.9 is the theoretical curve of P(q) calculated with sufficiently large R and with L = 1.6 nm. Because the theoretical curve coincides well with the experimental results at q > 10⁰, it is confirmed that there are mostly completely exfoliated nanosheets with the thickness of 1.6 nm in this colloid, rather than incompletely exfoliated layered crystals. In the range of 10⁻¹ < q < 100, several peaks appeared at the positions corresponding to q/2π = 1/d, 2/d, 3/d and so on. The peaks are ascribed to the regulated structure with the basal spacing d. This suggests that there exists not only the orientational order explained by Onsager theory but also positional order in the nanosheet colloid forming the LC phase. In the ultra-small-angle region (q < 10⁻² nm⁻¹), the scattering of q ^−2.9 is observed. This shattering is in excess compared to the q ⁻² expected for a two-dimensional object and may be ascribable to a fractal structure, which is composed of sparse and dense domains due to the inhomogeneous distribution of the nanosheets in the solvent. Several studies have suggested the existence of fractal structures in nanosheet colloids [35–40]. The sparse and dense structure is formed because the system is not dominated by repulsive force; attractive interaction among the nanosheets also exists.

On the other hand, Miyamoto et al. [20] reported the LC phase of the nanosheet colloid obtained from KCa₂Nb₃O₁₀ (Fig. 8.3i). KCa₂Nb₃O₁₀ is a type of Dion–Jacobson layered perovskite with the general formula M[A_n−1B_nO_3n+1] (M = K, Na, H, etc.; A = Ca, Ba, K, Sr, etc.; B = Nb, Ti, etc.) [41]. Layered perovskites are suitable for systematic basic research because a series of compounds with various chemical compositions and structures can be synthesized. In addition, the layered perovskites have intriguing functions, such as photocatalytic activity [42], catalytic activity as solid acids [43], and luminescent properties [44], so they are expected to be applicable to advanced soft materials.

3.3 Layered Clay Minerals

Layered clay minerals have been studied historically for the longest time of all layered materials and are very important materials for many types of industrial applications because of their low cost, abundance in earth soils, and low toxicity. The liquid crystallinity of a clay mineral was first reported by Langmuir in 1938 [45]. Langmuir found that the colloidal sol of bentonite clay produced in California had a birefringent texture, and he identified this as a liquid crystal phase. However, this observation was not reproduced and the liquid crystallinity of the clay mineral colloid was not reported for a long time afterward. One of the reasons is that most clay mineral colloids lose the fluidity at relatively low concentrations (at several wt%) and become physical gels. In many cases, the transition to a physical gel occurs below the concentration of the liquid crystal phase transition so that it is impossible to observe the emergence of the liquid crystal phase clearly. Even if birefringence is observed in the gel, it is difficult to confirm the permanent birefringence due to the liquid crystal phase, which should be distinguished by flow birefringence with very slow relaxation. In the late 1990, several research groups reported birefringent clay colloidal gels. In 1996, Gabriel et al. [46] carefully observed the gels of montmorillonite or hectorite and obtained birefringent gels at specific clay and salt concentrations. Mourchid et al. reported that hectorite formed a nematic liquid crystal phase at above 3 wt% [26]. Fossum et al. reported that the colloid of synthetic fluorohectorite was separated into three phases, the precipitate, birefringent gel, and isotropic, after allowing the colloid to stand for a period of time [47]. On the other hand, Michot et al. [25] reported that the montmorillonite colloid becomes a sol, gel, aggregate, or the partially birefringent gel depending on the particle size and salt concentration. However, the fluidity is lost in the nanosheet colloids in these reports, and it is difficult to confirm that these observations correspond to a true liquid crystal phase.

A clay mineral colloid with permanent birefringence and fluidity was reported by Michot et al. [24, 48, 49] in 2006 for the first time. The exfoliated nontronite nanosheet had a strip-like shape with a long side to short side ratio of 2.9–5.5, and the length of the long side was 2200 nm. Figure 8.10 shows the phase diagram with the particle size and ionic strength as the parameters. In the sample with small particle size (250 or 345 nm) and salt concentration less than 10⁻³ M, the phase transition from the isotropic phase to the liquid crystal phase occurred at a nanosheet concentration of 0.6–0.7 wt%. In contrast, in the sample with large average particle size, only the sol-gel transition was observed. In the relationship between the average particle-particle distance d determined by SAXS and the nanosheet concentration (Fig. 8.11), the system followed the law of one-dimensional swelling of Eq. (8.6) in the region of high nanosheet concentration. On the other hand, d was proportional to ϕ ^1/3 in the range of the lower nanosheet concentration, which corresponds to isotropic swelling. Whereas the ionic strength did not greatly influence the liquid crystal phase transition, very high ionic strength caused flocculation or gelation.

Michot et al. also reported the liquid crystal formation of the natural beidellite clay system [23, 50]. Using several clay mineral systems, they conducted an investigation of the rheological, osmotic pressure, and SAXS measurements. Based on these observations, they concluded that the electrostatic repulsive force between nanosheets dominates the rheological properties and liquid crystallinity, while the hydrodynamic capture effect may also be related [50–52].

On the other hand, Miyamoto et al. [22] reported that the synthetic fluorohectorite and fluorotetrasilicic mica with controlled particle size prepared by purification and sonication processes formed liquid crystal phases with very high fluidity. Figure 8.12 shows photos of these colloids under crossed polarizers. The sample with large particle size has a texture with interference colors ascribable to the liquid crystal phase at 1.0 wt%, while typical phase separation behavior occurs (Fig. 8.12d–g). In contrast, the sample with small particle size at 1.0 wt% is isotropic and only shows flow birefringence (Fig. 8.12b, i), and the permanent liquid crystal phase (Fig. 8.8c, j) is observed only at the higher concentration (2.0 wt%). Compared to other clay mineral systems, these systems are very fluid up to very high concentrations, which is suitable for various basic research and applications. In these systems, gelation did not occur even at 7.0 wt%. Fossum et al. also investigated synthetic fluorohectorite systems; they used a sample with impurities. The viscosity was high, and it was difficult to study the phase behavior [47, 53]. In addition to these fluorinated clays, Miyamoto et al. found that the natural montmorillonite produced in Tsukinuno and purified by a similar method showed liquid crystallinity.

3.4 Graphene and Grapheme Oxide

Graphene has received much attention due to its excellent electronic and mechanical properties, and it was selected as the subject for the Nobel Prize in 2010. However, it is difficult to obtain a large amount of completely exfoliated graphene, and the graphene reported in many papers is not a nanosheet but a “stacked graphene”. There are few reports on the liquid crystal phase of graphene; Dan et al. [54] obtained exfoliated graphene with high yield by dispersing in chlorosulfonic acid and observed the schlieren texture, which is characteristic of nematic liquid crystals.

In contrast to graphenes, it is easy to obtain single-layer graphene oxide nanosheets dispersed in a solvent, and many investigations have been carried out. However, graphene oxides have hydroxyl and/or carboxyl groups irregularly placed on their surface, and they have no defined chemical structure. The electronic properties characteristic of graphene are not retained in the graphene oxides, but the lost electronic properties can be recovered to some extent by post-reduction treatments. The liquid crystal phase of a graphene oxide was reported in 2001, followed by many additional reports. Kim et al. [55] obtained a single graphene oxide nanosheet with the thickness of 0.8 nm and the average particle size ranging from 0.75 to 1.65 μm. They observed an optical texture characteristic of a nematic phase by polarized optical microscopy, and the structure was confirmed for the freeze-dried sample by SEM (Fig. 8.13). In the system with the particle size of 0.75 μm, the phase transition concentration was 0.78 wt%. Aboutalebi et al. [56] prepared graphene oxide with the much larger average particle size of 33 μm, and the system had the ϕ _I of 0.1 wt%. Xu et al. [21] investigated the graphene oxide with the average particle size of 2.1 μm. This system had the ϕ _I of 0.025 wt% and the ϕ _LC of 0.5 wt%. When increasing the salt concentration by adding NaCl, ϕ _I and ϕ _LC tended to increase, while the colloid aggregated at higher salt concentrations. In the SAXS results, a peak attributed to the loose lamellar structure with basal spacing of 63 nm was observed at 2.5 wt%. The basal spacing was similar to that estimated from the one-dimensional swelling law. Because the graphene oxide nanosheet has strong luminescent properties, the characterization of the structure was carried out by direct observation by confocal laser scanning microscopy (Fig. 8.14).

Xu et al. [57] also reported that graphene oxide nanosheets form LC phases with lamellar structures or the twist-grain boundary phases of chiral liquid crystals with long-range chiral frustration. The thickness of the nanosheet was 0.8 nm, and the average particle size was 0.81 μm. By observation of the schlieren texture, the phase transition from the isotropic phase to the nematic phase at 0.23 vol.% was determined. At greater than 0.39 vol.%, the clear and oriented strip-shaped structure similar to the fingerprint-like structure of the cholesteric phase was observed. The chiral spatial arrangement of the graphene oxide nanosheet was verified by the strong signal of circular dichroism in the wide wavelength range from 380 to 800 nm. The graphene oxide nanosheet with liquid crystallinity was injected into an NaOH/methanol solution from a syringe, leading to coagulation of the nanosheets as a fiber, which showed high mechanical strength and electrical conductivity.

3.5 Other Related Materials

V₂O₅ nanoribbons are obtained by a sol-gel method from a homogeneous solution, not by exfoliation of a layered crystal. The shape is a thin ribbon with a uniform thickness rather than a nanosheet. However, they may be considered a type of nanosheet. The study of the liquid crystal phase of V₂O₅ colloid was first reported by Zocher in the 1920s [58]. In the 1990s, French research groups [59–64] conducted many investigations on the formation of the nematic phase and the response to electric and magnetic fields [62]. Application to anisotropic media for NMR measurement was also reported [63].

Layered double hydroxides are an important class of layered materials because of their anion exchange capability, in contrast to most of other layered materials with cation exchange capability. Because they are synthesized by a hydrothermal reaction from an aqueous solution of starting chemicals, the particle size and chemical composition are controllable. Although many reports on the exfoliation of layered double hydroxide (LDH) have been published [65–68], it is difficult to obtain fully exfoliated nanosheets with the thickness of 0.6 nm while retaining the large lateral size in high yield. For that reason, there is no report on the liquid crystallinity of exfoliated nanosheets of LDH. However, the liquid crystallinity was reported by several groups for aqueous colloids of unexfoliated Mg/Al-type LDH particles. In 2003, Liu et al. reported that the colloid of LDH microparticles (Mg/Al ratio = 2, average particle size 60 nm, thickness unknown) had a liquid crystal phase at greater than 27 wt% [15]. Wan et al. [69] investigated the LDH with Mg/Al ratio = 1, average particle size of 130 nm, and thickness of 5.5 nm. This system showed the liquid crystal/isotropic biphase at 10–25 wt%. At 13 wt%, a lamellar structure with basal spacing of 40 nm was identified by SAXS. In the system with Mg/Al ratio = 2, average particle size of 102 nm, and thickness of 7.4 nm, Zhang et al. [70] observed a liquid crystal/isotropic biphase at concentrations ranging from 16 to 30 wt%. It was also reported that the concentration range shifted to the higher side with increasing salt concentration. Mourad et al. [71] synthesized various LDH microparticles with Mg/Al ratio = 2 with controlled average particle size ranging from 46 to 146 nm. Although the liquid crystal phase was not observed in these systems, the liquid crystal phase appeared when polyisobutylene was added. Zhu et al. [72] and Luan et al. [73, 74] reported that, by adding polymers to LDH microparticles, phase separation to more than three phases was observed. Thus, the formation of the liquid crystal phase of LDH microparticle system requires a higher concentration than other true nanosheet systems (below several %) because they have a low anisotropic ratio of particles due to the low degree of exfoliation.

4 Theories for the Liquid Crystal Phase Formation of Nanosheet Colloids

4.1 Onsager Theory

A theory for the formation of the liquid crystal phase of anisotropic particles, such as rods and sheets, was presented in 1949 by Onsager [27]. In this theory, the formation of a nematic phase, that is, a liquid crystal phase with orientational order and no positional order, is explained by the excluded volume effect on particles. According to this theory, it is expected that the colloid changes from an isotropic phase to the nematic liquid crystal phase through isotropic-liquid crystal coexistence phase with increasing concentration. This behavior almost corresponds to the observations in experiments. Onsager’s theory uses a simple model that assumes only hard core repulsions between cylinder particles with diameter D and height L. Because a particle with L ≫ D is regarded as a rod and a particle with L ≪ D is regarded as a disc, this theory is applicable for both cases.

When an anisotropic particle is rotating freely by Brownian motion, the particle occupies a very large excluded volume b compared with its solid volume v _p (=π D ² L/4). The ratio of volumes is calculated as follows:

$$\frac{b}{{v_{\text{p}} }} = \frac{1}{{v_{\text{p}} }}\frac{1}{4}\pi D\left( {L^{2} + \frac{1}{2}(\pi + 3)DL + \frac{1}{4}\pi D^{2} } \right).$$

(8.8)

In the limit of L ≪ D, namely, in the infinitely thin plate particle limit, Eq. 8.8 reduces to:

$$\frac{b}{{v_{\text{p}} }} = \frac{\pi }{4}\frac{D}{L}.$$

(8.9)

In the case of the anisotropy ratio D/L = 100, b/v_p is 79. On the other hand, supposing that the particle is not rotating but is perfectly oriented, b/v _p = 4. Because of the large difference of b/v _p, large free volume for translational motion is gained if rotation is inhibited. In other words, translational entropy is gained if rotational entropy is lost. Therefore, in the situation that the particles have a large anisotropic ratio, the translational entropy gained by prohibited rotation of the particle, that is, orientation of the particle, should be larger than the rotational entropy lost. Thus, the nanosheets tend to be oriented and form a nematic liquid crystal phase.

Based on this simple theory, Onsager carried out numerical calculations of the transition concentration from the isotropic phase to the isotropic-liquid crystal coexistence phase, ϕ _I, and the transition concentration from the isotropic-liquid crystal coexistence phase-liquid crystal, ϕ _LC, and the results are as follows;

$$\phi_{\text{I}} = 3.3\left( {\frac{{v_{\text{p}} }}{b}} \right) = 3.3\left( {\frac{4L}{\pi D}} \right)$$

(8.10)

$$\phi_{\text{LC}} = 4.5\left( {\frac{{v_{\text{p}} }}{b}} \right) = 4.5\left( {\frac{4L}{\pi D}} \right)$$

(8.11)

As is obvious from these results, the concentration needed for the formation of the liquid crystal phase decreases as the anisotropic ratio D/L increases. Figure 8.15 summarizes the relationship between ϕ _I and D/L for the nanosheet and nanoplate systems reported so far. The trends found in the experimental results are mostly in accordance with the theoretically calculated results.

Onsager’s theory has been modified in various ways through theoretical and simulation approaches. The original Onsager’s theory is inaccurate in the case of extremely high concentration because it takes into consideration only two-particle interactions, but multiparticle interactions cannot be ignored at high concentrations. If the multiparticle interactions are taken into consideration, the concentrations of the phase transitions should decrease compared to the original theoretical value [77]. In many cases, nanosheet systems show a large polydispersity of particle size, and this has a great effect. Monte Carlo simulation was carried out on disc-shaped particle systems with polydispersity in particle size. It was shown that the gap between ϕ _I and ϕ _LC increases with increasing polydispersity [77]. This theoretical prediction was confirmed experimentally [28]. The flexible shape of nanosheets can also influence the liquid crystal properties. In the experimental and theoretical studies of solution systems of rigid polymers regarded as flexible and long rods, it was revealed that ϕ _I and ϕ _LC are larger for polymers with larger flexibility [5]. On the other hand, the emergence of the liquid crystal phase with one-dimensional or two-dimensional positional order, namely, a smectic phase or a columnar phase, was predicted by both theoretical and simulation studies [14]. Experimentally, Lekkerkerker et al. reported the emergence of the columnar phase in ideal experimental systems, such as rod-shaped boehmite particles and plate-shaped gibbsite particles [12]. However, in the nanosheet systems, columnar phases have not been reported due to their very large D/L ratio and large polydispersity.

4.2 Other Theories

Provided that the interaction between nanosheets is completely repulsive, considering only the simple Onsager theory is sufficient for understanding the nanosheet liquid crystal systems. However, in many cases, it is not enough. Whereas Onsager’s theory and its modified theory explain the emergence of nematic, smectic, and columnar phases, the theories do not explain the liquid crystal phases with swollen lamellar structure and high structural order, which have been observed in many nanosheet liquid crystal systems.

If the repulsive force between nanosheets is strong enough, we can easily guess that nanosheets in the nematic phase form a lamellar structure with the basal spacing predicted by the one-dimensional swelling law of Eq. (8.6). However, in many cases, a smaller basal spacing is observed compared to that predicted by the one-dimensional swelling law. These phenomena can be related to the fractal-like inhomogeneous structure composed of sparse and dense parts of nanosheets in nanosheet colloidal systems. The existence of the fractal structure in nanosheet colloidal systems was noted by many researchers. If the inhomogeneous structure exists, what is the origin of this structure? It is reasonable that relatively strong attractive forces, other than the weak van der Waals attractive force considered in DLVO theory, affect the formation of the fractal structure.

Depletion interaction is thought to be one of the origins of these attractive forces. This interaction is the entropic attractive force induced by the addition of smaller colloid particles to the original colloid. As the distance between two large colloid particles decreases, it becomes difficult for the small particles to be present in the narrow space between the two large colloid particles. As a result, the pressure from the small particles to the large particles works more strongly in the direction that is advantageous to drive the two large particles closer, that is, the attractive force is substantial. Because nanosheet systems have high polydispersity, it is possible that the attractive depletion force from the coexisting smaller nanosheets is working between the larger nanosheets.

The Ise-Sogami theory is also remarkable [78]. According to this theory, attractive forces are acting between the particles with the same type of charge (positive–positive or negative–negative) due to the attractive interaction between a colloidal particle and its counter ion cloud surrounding the particle. Although this theory is different from DLVO theory, which is known as the classical standard, and caused many controversies, this theory can explain the structural formation of spherical colloidal particles and the swelling behavior of layered clay minerals quantitatively. Smally et al. conducted a detailed study on the ideal system in which the single-crystal layered clay mineral vermiculite swelled in aqueous n-butylammonium solution. From the calculation based on DLVO theory, the basal spacing calculated from a secondary minimum did not coincide with the experimental results or a minimum did not appear. In contrast, a deep secondary minimum appeared in the potential curve in the calculation based on Ise-Sogami theory. The position of the secondary minimum d _min is represented by the following simple equation after some approximations, and the calculated values coincided well with the experimental results.

$$d_{\hbox{min} } = 4\kappa^{ - 1}$$

(8.12)

A similar explanation is also possible in the largely swelling layered ferrotitanate [79, 80] and layered perovskite [81], both reported by Sasaki et al.

The repulsive force originating from the waviness of nanosheets may also be considered. The waviness of layers in a lamellar phase induces an entropic repulsive force known as the Helfrich interaction [82]. In general, the inorganic crystals constructed by covalent bonds have a much higher modulus of elasticity than the well-studied molecular layers composed of surfactant. However, as suggested by the bent-shaped nanosheets formed by drying on a TEM grid [17] and the wrinkles in the nanosheets formed by the elongation and compression of cast nanosheets on a PDMS substrate [83], a nanosheet is not a completely rigid plate but has flexibility to some extent in a solvent. It is thought that the modulus of elasticity of nanosheets is influenced by the chemical composition, thickness of the layers of nanosheets, coexistence of solvent molecules, and counter ions, so that the extent of waviness of a nanosheet also changes depending on these factors. Considering Helfrich interaction, the basal spacing d is expected to show the difference from the one-dimensional ideal swelling behavior as:

$$d = \frac{L}{f}\frac{{A_{\text{s}} }}{{A_{\text{p}} }}$$

(8.13)

Here, A _s is the substantial area of the nanosheet and A _p is the area of a crumpled nanosheet projected from the top. If the nanosheet is crumpled, A _p is smaller than A _s, so the basal spacing is larger than the rigid plane.

5 Orientational Control Under External Fields

The colloidal nanosheets can be easily manipulated by external fields. Because of their nature of soft matter, they can change their orientation with weak external forces. This property allows us to construct various hierarchical structures up to macroscopic scale from the nanosheets. We can obtain the desired soft colloidal structures by the controlled application of external fields. Electric and magnetic fields, shear flows, and interaction at interfaces have been examined as external fields. The orientation under an external field is induced by the intrinsic shape anisotropy of the nanosheets. Since the shape anisotropy is accompanied by anisotropy of the dielectric constants and magnetic susceptibilities, the nanosheet orientation can be altered by the external forces even in isotropically dispersed states. However, the nanosheet colloids in the LC state often show specific structures because of the collective nature of LCs.

5.1 Orientation Under Electric Fields

The orientation of the nanosheets under electric fields is principally governed by their dielectric anisotropy derived from their 2D shape. Electric field-induced orientaional control is applicable for most colloidal nanosheets. The electric alignment of nanosheets was discovered in the 1930s as a birefringence change with the application of continuous AC or pulsed DC electric fields [84–86]. This phenomenon is termed electric birefringence. Because most of the nanosheets bear permanent electric charges, AC electric fields are usually utilized for the continuous application of electric voltages; continuous DC electric fields do not retain the nanosheets in a colloidal state but electrophoretically deposit them on electrodes [87].

5.1.1 Electric Birefringence

Electric birefringence is an electrooptic effect that occurs when anisotropic colloidal particles are electrically aligned. Because of their 2D shape, colloidal nanosheets induce large optical retardation to the light propagating along the in-plane direction (Fig. 8.16a). If the nanosheets are lamellarly oriented with a random arrangement with respect to their in-plane direction, the index ellipsoid of the nanosheet colloid can be drawn as optically uniaxial, as in Fig. 8.16b. Birefringence of the colloid consisting of the oriented nanosheets is observed, when the light is not parallel to the n _o axis; that is, the colloid acts as a medium with a uniform refractive index when the incidence of the light is perpendicular to the nanosheet surface [88].

Historically, electric birefringence had been investigated for clay colloids under continuous AC electric fields (70–500 V cm⁻¹, 50–10⁶ Hz [84–86]) to determine the orientation of colloidal anisotropic particles under external fields. Later, pulsed DC electric fields were utilized to extract detailed orientation mechanism by analyzing the temporal response of the nanosheets [89–95]. Because electrically charged nanosheets dispersed in water are accompanied by a huge amount of solvated countercations, the electrooptic response of the nanosheet colloids can be ascribed not only to the alignment of nanosheets but also to the electrically induced reorganization of the countercations around the clay nanosheets. Both of the movements occur upon the application of electric voltages with different time constants, and the opposite signs of the electric charges of the nanosheets and countercations lead to positive/negative inversion of the birefringence (Fig. 8.17) [94]. Because of the different diffusion rate of the nanosheet particles and the surrounding ionic species, their responses to an applied pulsed electric field have a time lag that causes time-dependent sign inversion of the birefringence. Based on such electrooptic behavior, the kinetics of the electric alignment of clay nanosheets has been discussed.

Because the colloidal clay nanosheets can adsorb cationic dyes, the electric alignment of clay nanosheets can be used to align the adsorbed dye molecules. This dye orientation is observed as electric dichroism of the optical absorption due to the supported dye molecules. This phenomenon can be applied for color modification of the colloids and to investigate the binding state of the dyes to the clay nanosheets [96–101].

5.1.2 Electrorheological Behavior

The electric response of colloidal particles leads to reversible alterations of the rheological properties of the system upon the application and removal of an electric voltage. This phenomenon is termed the electrorheological (ER) effect [102]. Colloidal systems showing great ER effects have attracted interest as novel smart fluids applicable for valves, clutches, dampers, and actuators. Layered materials, in particular clay minerals and recently graphene oxide (GO), have been examined as ER materials [103–107].

For ER applications, clay particles are usually not exfoliated but are hydrophobized and dispersed in silicone oil because the ER fluids are typically obtained as colloids of dielectric particles in insulating solvents. Interlayer modification by long-chain organic molecules or polymers is used for this purpose [103–105]. Modification of the external surfaces is also effective, as demonstrated by nanocomposites with inorganic dielectric oxide particles [108] or polymer nanospheres [109]. For GO particles, external surface modifications are mainly employed because they are exfoliated into stacked nanosheets [107, 110–112]. Under the application of an electric field (DC or AC), the modified clay or GO particles are polarized and aggregated into chain-like structures in parallel with the field. When the electric field is applied perpendicular to the flow direction, the chain-like aggregates suppress the flow of the colloid, which is a typical ER response.

However, dried clay particles with hydrophilic inorganic interlayer cations can also be dispersed in silicone oil to show ER effects [106, 113]. As shown in Fig. 8.18, chain-like structures are obtained for clay particles with inorganic interlayer cations. The difference in the structures compared to the interlayer cations indicates a critical role of the interlayer species in the ER properties of the clay minerals. Dried GO particles dispersed in silicone oil also exhibit ER behavior without hydrophobic additives [114].

The ER behavior of the exfoliated clay nanosheets has been examined in water. Aqueous clay colloids show reversible viscosity increase when applying DC or low-frequency (~10⁻³ Hz) AC voltages [115–117]. Their ER response is characterized by low threshold voltages of less than 100 V cm⁻¹. AC voltages with a frequency greater than 10⁻² Hz are ineffective. The viscosity increase has been ascribed to the formation of “house-of-cards”-type networks between the clay nanosheets whose structure is not fully relaxed, as indicated by incomplete recovery of the viscosity after the removal of the electric field (Fig. 8.19).

5.1.3 Electric Alignment of the Nanosheets in the LC State

Application of electric fields to aqueous nanosheet LCs gives macroscopically ordered nanosheet structures, which can be ascribed to the synergy of the electric and liquid crystalline alignment of the nanosheets. High-frequency AC voltages are generally utilized to align the electrically charged liquid crystalline nanosheets in aqueous systems. The first example of the electric alignment of the nanosheet LCs was reported for the colloids of beidellite clay [23]. When a colloidal LC sample in a 0.2-mm thick and 2-mm wide flat capillary is subjected to an AC voltage of 4000 V cm⁻¹ and 500 kHz between electrodes placed in parallel with 1 mm distance in the longitudinal direction of the capillary, the clay nanosheets are aligned in parallel to the electric field to give macroscopic homogeneous ordering of the nanosheets over the area of 2 mm × 1 mm (Fig. 8.20). AC voltage applications to isotropic and biphasic (coexistence of isotropic and nematic phases) colloids of the same clay mineral (beidellite) also align the nanosheets, and their ordering degree is unexpectedly high compared with those induced theoretically [118]. This behavior is explained by the strong coupling of the electrically charged clay nanosheets and the applied electric field.

The electric alignment of the colloidal LCs of the hexaniobate K₄Nb₆O₁₇ nanosheets was examined with AC electric fields of 500–2000 V cm⁻¹ and 50 kHz [88]. The electric field is applied to the LC sample placed in a 0.1-mm thick sandwich-type cell in the direction perpendicular to the cell surface. The nanosheets in the LC colloids are aligned parallel to the electric field, i.e., perpendicular to the cell surface, showing a strong electrooptic response (Fig. 8.21). However, in contrast to the beidellite clay system, the electrooptic response of isotropically dispersed nanosheets is much weaker (~1/1000) than that of the LC samples. This result indicates a contribution of the collective nature of the LC state to the nanosheet alignment under the external fields. The electrooptic behavior of the nanosheet LCs is characterized by a slow response and low threshold voltage; the nanosheet alignment shifts in a time range of seconds with a less than 1 V cm⁻¹ electric field. This is ascribed to the mesogen size of the nanosheet LCs, which is much larger than that of conventional molecular LCs. Retention of the electrically induced alignment, being a type of a memory effect, is observed for the LCs with high nanosheet concentrations. A macroscopic monodomain structure is obtained by repeated AC voltage applications, which reorient the nanosheets through backflow in the sample cell.

Graphene oxide (GO) nanosheet LCs also show electric alignment under AC electric fields of 10–200 V cm⁻¹ and 10 kHz [75] (Fig. 8.22). A 0.3-mm thick thin-layer cell is used with electrodes placed in parallel to the cell surface with a 0.5 mm distance. Similar to the niobate system, the GO nanosheet LCs show slow electric response and sensitivity to low electric fields. The electrooptic response also depends on the concentration of coexisting ionic species [119]. The electric response of the colloidal LCs were lost with 10⁻³ mol L⁻¹ of NaCl or HCl, while NaOH did not have large effect. This behavior is explained by the modification of the electric charges on the GO nanosheet surfaces in the presence of coexisting electrolytes, depending on their concentration and species; the surface charge alters the polarization of the nanosheets to change their electrooptic response.

5.2 Orientation Under Magnetic Fields

Similar to the electric fields, magnetic fields align colloidal nanosheets. Magnetic alignment is easily attained when the nanosheets contain magnetic elements. Nanosheet colloids of nontronite clay, which contains Fe(III) species in the aluminosilicate layers, show magnetic alignment [49, 120]. The nontronite nanosheets in the isotropic and LC states are both aligned under a magnetic field of >1 T, and their free and collective orientations are distinguished by ²H-NMR analysis. When the nanosheets are diamagnetic, more intense magnetic fields are required. Examples are nanosheet LCs of layered metal phosphate H₃Sb₃P₂O₁₄ and beidellite clay (Fig. 8.23), which are aligned under magnetic fields of 19 and 8 T, respectively [16, 23]. Platy particles of non-exfoliated hexaniobate K₄Nb₆O₁₇, as well as nanoscrolls obtained from its exfoliated nanosheets, are magnetically aligned at 12 T [121]. Because GO and reduced GO (rGO) have rather high magnetic susceptibilities among the diamagnetic materials due to their π-conjugated systems, their nanosheets can be aligned slowly under relatively weak magnetic fields of ~0.25 T [55, 122]. However, nanosheet LCs of synthetic fluorohectorite clay characterized by large nanosheet size and the absence of magnetic elements have been reported to show magnetic alignment at 2 T [123], which could be ascribed to the collective nature of the large-size nanosheets in the LC state.

Another technique for the magnetic alignment of the diamagnetic nanosheets is loading magnetic nanoparticles, such as Fe₃O₄, on the nanosheets. Partly restacked few-layer nanosheets of perovskite-type niobate HCa₂Nb₃O₁₀ are oriented under magnetic fields greater than 10⁻³ T [124, 125] when Fe₃O₄ nanoparticles are attached via covalent linkers. The 2D distribution of the magnetic nanoparticles on the nanosheets leads to biaxial superparamagnetic susceptibility of the colloidal particles, which are aligned parallel to the magnetic fields. GO and reduced GO nanosheets also become easily aligned magnetically with this method [55, 126].

5.3 Orientation by Shear Forces

The alignment of the colloidal nanosheets is also achieved under shear. The application of shear forces to the colloids induces nanosheet orientation along the shear direction. A typical example is observed during settlement of the colloidal nanosheets. Nanosheet alignment along gravity at a macroscopic (~ cm) scale has been reported for the colloidal LC of hexaniobate K₄Nb₆O₁₇ nanosheets placed in a test tube (1 cm in diameter) [30] (Fig. 8.24). This behavior is explained by the flow of the colloids induced by the settlement of the dispersed nanosheets with gravity.

The flow-induced orientation is observed more clearly by applying mechanical shear to the nanosheet colloids. When a test tube containing a hexaniobate nanosheet colloid is rotated, the colloid shows birefringence along the shear flow, indicating the shear-induced orientation of the nanosheets [17, 30]. The flow birefringence is observed for both isotropic and LC nanosheet colloids. The shear-induced alignment is temporal and gradually disappears after removal of the rotation. This phenomenon has also been observed for nanosheet LCs of montmorillonite clay [25] and GO [127]. The relaxation of the shear-induced alignment becomes slower at higher nanosheet concentrations.

The shear-induced alignment of the nanosheets has been applied for the preparation of practical materials. The preparation of self-supporting films is a known example. Casting the colloidal nanosheets on a flat substrate causes unidirectional deposition of the nanosheets, and self-supporting “paper” is obtained after removing the substrate. While “clay papers” are utilized as colorless transparent supports of functional dyes [129, 130], films of semiconducting oxide nanosheets work as “photocatalytic paper” [131]. “Graphene papers” prepared from GO or rGO nanosheets have been investigated as membranes for adsorption and separation (Fig. 8.25) [128, 132, 133]. On the other hand, fiber-spinning of mixtures of colloidal GO or rGO nanosheets with organic polymers yields inorganic–polymer composite fibers [134–136]. The strong shear forces applied during the spinning process align the nanosheets in the fibers to give strong mechanical strength and physicochemical properties such as electrical conductivity due to graphene.

5.4 Hierarchical Macroscopic Structures of Nanosheet LCs Under Dual External Fields

The 2D shape and large particle (mesogen) size of the nanosheet LCs enable the organization of various hierarchical nanosheet-based structures at the macroscopic scale. In contrast to 1D particles (rods) that are unidirectionally aligned with a single external force, 2D particles have two orthogonal axes for regulation of their alignment so that unidirectional alignment is only attained under dual external fields. The large particle (mesogen) size of the nanosheets at the μm level is advantageous for constructing macroscopic structures hierarchically, in comparison with conventional molecular LCs.

Based on this consideration, we have organized the hierarchical macroscopic structures in the colloidal LCs of hexaniobate K₄Nb₆O₁₇ nanosheets [137]. An important key for the hierarchical organization is the growth of LC domains, called tactoids, as the secondary building blocks followed by controlled application of external fields. The growth of the tactoids is achieved by keeping the nanosheet LCs stand at room temperature, a process called incubation, for several tens of minutes (Fig. 8.26). The tactoid growth is analogous to Ostwald ripening in crystal growth.

The tactoids are then assembled into macroscopic higher order structures with characteristic lengths of sub-mm to mm under simultaneous application of an AC electric field and gravity, whose directions determine the final structural motif. Although a net-like structure forms when applying the electric and gravitational forces in the same direction, a stripe structure, where the nanosheets are unidirectionally aligned, is obtained when the electric field is applied in the direction perpendicular to gravity (Fig. 8.27). The use of well-grown tactoids is a key to macroscopic structural control. Among these macroscopic structures of the photocatalytically active niobate nanosheet LCs, the stripe structure is applicable to control the photocatalytic reactions. The nanosheet stripes exhibit photocatalysis, which reflects the alignment of the nanosheets with respect to the polarized direction of the impinging light.

5.5 Orientation at the Interfaces

Although the inorganic nanosheets are well dispersed in aqueous and organic liquid phases when they are sufficiently hydrophilic or organophilic, respectively, the particles can be accumulated at heterogeneous interfaces if they have appropriate affinity to all the related phases. Typically, a small amount of long-chain organoammonium salt dissolved in an organic solvent is carefully spread on the surface of an aqueous nanosheet colloid to yield a nanosheet monolayer at the air–water interface [138, 139]. Another method of preparing an interfacial monolayer reported for synthetic saponite clay is intercalation of the organoammonium species and subsequent dispersion in an organic solvent, which is then spread on water [140]. Interfacial monolayer films have been fabricated into nanosheet multilayer films through the Langmuir–Blodgett technique.

Solid particles can stabilize interfaces when they are trapped there [141, 142]. This property is applicable to stabilizing liquid and gas droplets that are colloidally dispersed in continuous phases. Such colloidal systems of particle-stabilized droplets in continuous phases are termed Pickering emulsions (liquid in liquid) and foams (gas in liquids), and liquid marbles (liquid in air) [143, 144]. These colloidal systems provide broad practical applications in various fields, such as the petroleum, food, cosmetic, and pharmaceutical industries and are recognized as a type of multicomponent soft matter where particles are aggregated in a specific manner at structured interfaces. Inorganic particles can add unusual properties to such materials if the particles possess advanced optic, photochemical, catalytic, electric, and magnetic properties.

Particles with various morphologies have been examined as such “particle surfactants” [146]; 2D particles can effectively stabilize the interfaces if they are adsorbed in parallel to the interfaces (Fig. 8.28) [145, 147, 148]. In this regard, the exfoliation of layered crystals into nanosheets is not an essential requirement. In fact, non-exfoliated particles of layered crystals with platy morphologies have been employed for Pickering emulsions [145, 148–155] and foams [156–159], where moderately hydrophobic surfaces are required for droplet stabilization. The affinity to the droplet and continuous phases is adjusted by mixing with additives or intercalation of appropriate species into the interlayer spaces. Very recently, few-layer GO nanosheets were used to prepare both emulsions and foams after suitable modification of their surfaces (Fig. 8.29) [160–163]. α-ZrP nanosheets also stabilize Pickering emulsions and foams [147, 164].

5.6 Immobilization of Aligned Nanosheet Structures

Immobilization of the nanosheet colloid structure regulated by LC phase formation or external field is important for many applications. Inorganic nanosheets, especially clay minerals, have been immobilized in polymer matrix and utilized as nanofillers for plastic polymer materials to improve the thermal stability, gas-barrier properties, and mechanical properties. Recently, nanosheets were also utilized as the fillers for polymer gels, which are network-type polymers swollen by a solvent. The gel materials are applicable for biomaterials and soft actuators. In 2002, Haraguchi et al. [165] reported that the mechanical properties of polymer gels are largely improved by using synthetic hectorite nanosheets as the multiple physically crosslinking sites for polymer chains. Many studies with similar concepts have been reported [166–170]. The synthesis of composite gels was also carried out by photopolymerization, which allows easier control of the shape of the gels [167, 171–173].

Whereas isotropic nanosheet colloids were used in the above examples, composite materials with liquid crystalline nanosheets are expected to show better mechanical and anisotropic properties. Miyamoto et al. [174] reported the composite gel with anisotropic properties, which was synthesized using liquid crystalline clay nanosheets. An aqueous mixture of liquid crystalline fluorohectorite nanosheets, a monomer, a chemical crosslinker, and a radical initiator was flowed into glass capillary, allowing the nanosheets to be macroscopically ordered along the glass wall of the cell used for the synthesis. After polymerization and crosslinking in the capillary, the composite gel was obtained, which contained the nanosheets aligned in a tree-ring-like structure. In this report, the structural analysis during the synthetic process was carried out by SAXS (Fig. 8.30). Before the reaction started, the lamellar structure with the basal spacing of 35 nm was confirmed. The basal spacing decreased as the reaction proceeded, and it was 10 nm at the end of the reaction. After the reaction, the gel was placed in pure water and swelled macroscopically until the equilibrium swelling state. Corresponding to the macroscopic swelling, the basal spacing increased to more than 100 nm. Throughout these processes, the SAXS profile was kept anisotropic. These results confirmed that the polymerization proceeded, maintaining a well-dispersed, liquid crystal state of the nanosheets. Induced by the anisotropic structure, the gel showed anisotropic refractive index, modulus of elasticity, mass transport, and thermally induced phase transition behavior.

However, the tree-ring-like ordered structure in this system is not suitable for detailed evaluation of the anisotropic properties and for applications. Miyamoto et al. synthesized a sheet-shaped gel with a centimeter-scale ordered monodomain structure by applying alternating electric field during the synthesis. The orientation was controlled in both the out-of-plane and in-plane directions (Fig. 8.31) [173]. The gel sheet was easily printable with a tetravalent cationic dye at a resolution of tens of micrometers (Fig. 8.32a). When the gel was irradiated with light, only the colored part of the gel was heated, and the gel transformed into an asymmetric shape (Fig. 8.32b) through the thermally induced volume phase transition of PNIPA. This behavior is suitable for soft actuator applications.

Following the above reports, Mejia et al. [175] and Aida et al. [176, 177] reported anisotropic composite gels synthesized in a similar manner. Mejia et al. reported a composite of α-ZrP nanosheets and poly(acrylamide-co-N-isopropylacrylamide). They found that the equilibrium swelling ratio and the size of the oriented domain are controllable by tuning the synthetic conditions. Aida et al. used titanate and niobate nanosheets and magnetic field to obtain macroscopically aligned gels. Very large anisotropies in the optical, rheological, and deformation behavior were demonstrated.

6 Multicomponent Nanosheet Colloids

6.1 Phase Separation in Multicomponent Colloids

The addition of other particles to the nanosheet colloids to yield multicomponent colloids is an important strategy for extracting specific functions from nanosheet-based colloidal systems by utilizing physicochemical interactions between the colloidal particles. In general, multicomponent colloids containing more than two morphologically different particle species, such as plate–rod, plate–sphere, and rod–sphere [72, 74, 178–188], show phase separation, which is demixing of the morphologically different colloidal particles based on entropically driven depletion effects. The phase separation results in rich phase behavior, which is typically observed as the coexistence of plural LC phases [189]. The phase separation of multicomponent colloids involving nanosheets or plate-like particles has been studied theoretically rather than experimentally. Computational studies have clarified the multiphase coexistence in plate–rod and plate–sphere systems [190–196]. Additionally, there are theoretical studies of a plate–plate system [197–200], which is a model of the binary nanosheet LCs, indicating three-phase coexistence of one isotropic and two LC phases.

These theoretical predictions have been proved by experimental results, although the real systems are rather complicated because of the presence of particle size distributions and interparticle chemical interactions. For example, an experimental binary system of gibbsite (Al(OH)₃) plates and boehmite (AlOOH) rods shows five-phase coexistence (isotropic, rod-dominant nematic, plate-dominant nematic, unidentified nematic, and columnar phases), in accordance with theoretical considerations (Fig. 8.33) [179, 201]. The plate–polymer systems of gibbsite with poly(dimethylsiloxane) [202] and LDH with poly(ethylene glycol) [72] exhibit the coexistence of four to five phases (one isotropic and three to four LC). An LDH–polymer (poly(vinyl pyrrolidone) binary system shows six-phase existence (one liquid, two isotropic, two LC, and one sediment) [74].

With respect to multicomponent colloids involving exfoliated nanosheets, binary systems of the nanosheets and silica spheres have been examined, and their phase behavior is somewhat different from that of conventional plate–sphere systems. The beidellite clay–silica system shows only isotropic–nematic biphasic coexistence, similarly to the single beidellite system; however, introduction of silica particles accelerates the phase separation process [204]. In addition, the silica spheres suppress the sol-gel transition of the clay colloid to stabilize the nematic sol state. Binary colloids of α-ZrP nanosheets and silica particles show multiphase coexistence characterized by isotropic–isotropic demixing; isotropic–isotropic biphasic and isotropic–isotropic–nematic triphasic coexistence is observed (Fig. 8.34) [203]. Isotropic–isotropic phase separation is also observed for a binary system composed of synthetic hectorite clay (Laponite) nanosheets and Fe₂O₃ spherical nanoparticles [205]. A plate–rod system of montmorillonite clay nanosheets and sepiolite clay rods shows three-phase coexistence (one isotropic and two nematic) [206].

6.2 Phase Separation of Binary Nanosheet Colloids

In contrast to the plate–sphere and plate–rod systems, plate–plate binary colloids have not been thoroughly explored experimentally. Binary nanosheet colloids prepared from chemically different layered crystals are important members. In addition to the nanosheet systems, an example is a single-component colloid of gibbsite plates with a bimodal distribution of the particle thickness, which exhibits triphasic coexistence of an isotropic, nematic, and columnar phase, and the result is rationalized by computer simulation [200].

We have developed binary nanosheet colloids of niobate K₄Nb₆O₁₇ and smectite-type clay (natural montmorillonite or synthetic hectorite (Laponite)) nanosheets [207–209]. Although the binary nanosheet colloids are apparently homogeneous at the macroscopic (>sub-mm) scale and show liquid crystallinity, phase separation at the microscopic scale is evidenced by the SAXS analysis, which indicates compression of the LC phase due to the niobate nanosheets with the introduction of clay nanosheets. The niobate and clay nanosheets are separated and assembled into the microdomains of niobate and clay, respectively, as schematically shown in Fig. 8.35. Spectroscopic observations using a cationic organic dye as a probe also support demixing of the clay and niobate nanosheets at the microscopic level. Based on the result that the cationic dye molecules are selectively adsorbed on the clay nanosheets and the dye does not show spectral dichroism, the clay particles are isotropically dispersed in the binary colloids to be phase-separated from the LC niobate nanosheets.

However, the binary colloids of titanate nanosheets from lepidocrocite-type layered oxide H_1.07Ti_1.78O₄, used instead of the niobate nanosheets combined with the clay nanosheets, result in different phase separation behavior [210], although the niobate and titanate nanosheets have similar geometric shapes and chemical properties. The titanate–clay binary LCs show macroscopic phase separation with three-phase coexistence of one isotropic and two LC phases (Fig. 8.36). The difference between the titanate and niobate nanosheets is due to the different countercations that are introduced as exfoliating reagents for the mother layered crystals: propylammonium for the niobate and TBA⁺ for the titanate. Chemical interactions between the titanate and clay nanosheets, mediated by their countercations, destabilize the colloid. Although most of the nanosheets bear permanent surface charges accompanied by counterions, this fact is usually ignored in the simulations so that combination of the nanosheets with different counterions will modify the phase separation behavior.

Binary aqueous nanosheet colloids composed of synthetic hectorite (Laponite) and natural montmorillonite, which are both clay minerals with similar physicochemical properties and different particle sizes, show isotropic–isotropic biphasic coexistence in a mixed gel state [211]. The phase separation occurs at a microscopic scale, similarly to the niobate–clay system, due to kinetic arrest by the gel state (Fig. 8.37). DLS measurements suggest that the montmorillonite nanosheets form a matrix for the hectorite nanosheets by interacting with each other [212].

There are some other nanosheet–nanosheet binary systems with colloidal properties and applications. Reduced graphene (rGO) nanosheets are colloidally stabilized in the presence of montmorillonite clay nanosheets due to hydrogen-bonding interactions between the rGO and clay nanosheets [213]. The stabilized colloidal nanosheets are applicable for preparing highly oriented hybrid films. The addition of montmorillonite clay nanosheets to a colloidal LC of GO nanosheets enables preparation of graphene–clay hybrid fibers through wet spinning in the presence an appropriate organic polymer [214]. The hybrid fiber shows superior electric conductivity and thermal stability in air compared with neat graphene fibers due to the suppression of graphene oxidation by the clay nanosheets. Nanosheet–polymer hybrids with two nanosheet species have recently been explored for various applications, such as self-healable hydrogels, artificial nacres, and electrodes [215–217]. A binary system of rGO and manganate nanosheets was found to be stable at limited nanosheet concentrations and aggregated under other conditions [218], as observed for the titanate–clay system. Aggregation is a common phenomenon of binary colloids of nanosheets with opposite electric charges, such as the clay–LDH and GO–LDH systems [219–223].

6.3 Photochemical Applications of Niobate–Clay Binary Nanosheet Colloids

Niobate–clay binary nanosheet colloids that are phase separated at a microscopic scale can be applied for photofunctional systems. In such systems, photocatalytically active semiconducting niobate and physicochemically inert clay nanosheets are spatially separated but contact easily because the nanosheets are diffusive in the colloidal system and demixed only at a microscopic scale. Selective adsorption of cationic organic molecules onto the clay nanosheets leads to the existence of two spatially separated functional moieties that are photocatalytically active niobate nanosheets and organic dye molecules on the clay nanosheets.

If an electron-accepting organic species (methyl viologen; MV²⁺) is introduced to the niobate–clay colloids, photoinduced charge separation between the semiconducting niobate and the acceptor selectively adsorbed on the clay occurs (Fig. 8.38) [208, 209]. The photochemical behavior is characterized by controllability of both the yield and stability of the photoproduct (MV^+•) by the clay content. Figure 8.39 shows the amount of photoproduct in the colloids with various clay contents and their time-dependent decay after termination of the irradiation. The yield (maximum conversion) and lifetime of the product vary in the range of 8–70% and 10 min to 40 h, respectively, depending on the clay content, indicating that efficient and stable photoinduced charge separation is achieved under the optimum conditions.

On the other hand, the niobate–clay binary colloids exhibit photoinduced electron accumulation in the niobate nanosheets in the absence of an acceptor [224]. This process is also affected by the coexisting clay component, although the clay nanosheets do not directly contribute to the electron accumulation in the semiconducting nanosheets. In contrast, the photochemical reaction is insensitive to the addition of molecular species that can work as electron donors; they somewhat destabilize the stable electron accumulation achieved by the addition of clay nanosheets. The results indicate that the photochemical reactions in the nanosheet colloid systems are not simply controlled by interactions between the semiconductor nanosheets and the additives at the molecular level but are governed by indirect interactions between the colloidal components.

Moreover, photocatalytic decomposition of a cationic dye has been studied in this binary colloid, where the dye is selectively adsorbed on the clay nanosheets to be spatially separated from the photocatalytic nanosheets [225]. Upon UV irradiation, the dye is photocatalytically decomposed in the colloids containing the niobate nanosheets, but self-photolysis of the dye is observed in the colloids lacking the photocatalytic nanosheets. In the binary colloid, the degradation is retarded compared with the single-component niobate colloid. A greater clay content better stabilized the dye against decomposition. In contrast, irradiation of the colloids with visible light caused self-photolysis of the dye, even in the presence of niobate nanosheets, indicating the absence of electron transfer from the photoexcited dye to the photocatalytic nanosheets.

7 Rheological Properties

A colloid dispersion of smectite-type clays, such as hectorite and montmorillonite, in water loses its fluidity above a concentration of several wt%. The rheological behavior of clay colloids changes depending on the shear and the time elapsed after colloid preparation. These behaviors are referred to as thixotropy. This interesting rheological behavior is important in view of the practical applications of colloid dispersion. In particular, clay minerals have been used in a wide variety of application fields, such as paints and cosmetics, as rheology modifiers. Hence, many research works have been carried out in view of the technical aspects to understand the phenomena of rheological behavior. However, the fundamental studies are not sufficient, and research to obtain rational models to explain the rheological behavior of colloid dispersions and the control of their properties based on the obtained model is underway. In this section, the rheology of nanosheet colloids is reviewed, particularly focusing on clay mineral systems.

7.1 Theory and Models

According to the classical theory, anisotropic colloidal particles have a much higher viscosity than spherical colloidal particles. In a flow field, solid colloidal particles cause perturbation to the fluidic flow and flow resistance, namely, a viscosity increase, compared with an isotropic solution. In a colloid dispersion, the following viscosity equation is formulated in a dilute particle concentration:

$$\eta_{\text{r}} = 1 + k\phi,$$

(8.14)

where η _r is the relative viscosity, ϕ is the volume fraction of particles (equal to the concentration), and k is a parameter. Whereas k is 2.5 for spherical colloidal particles (Einstein equation for viscosity), on anisotropic colloidal particles, there are several cases governed by both the orientations of the particles by flow and disorder of the orientation by the rotational Brownian motion, and these relationships make k larger than 2.5.

Additionally, the particle size of nanosheets influences the viscosity. The relationship between the intrinsic viscosity of polymer solution [h] and molecular weight M is known as the Mark–Houwink–Sakurada equation, as follows:

$$[h] = k\,M^{a},$$

where k and a are constants. Applying this empirical formula, it can be shown that a clay nanosheet colloid (two-dimensional inorganic polymer solution) with a larger particle size (molecular weight) has a higher viscosity.

However, the complex rheological behavior on clay colloids cannot be explained by the above-mentioned simple theories because, as described in Sect. 8.4, various attractive forces (electrostatic attraction, van der Waals force, hydrogen bonding, and depletion force), repulsive forces (electrostatic repulsion and steric repulsion), and other interactions (entropy-induced orientation and phase separation) are intertwined. The rational elucidation of the rheological behavior of clay colloids using these forces is not easy, and it has become a controversial issue. At the moment, there are two probable elucidations for the aggregation and resulting gelation of clay colloids: one is the “card-house” model, which assumes the attractive force between particles as the dominant factor, and the other is the “colloidal glass” model, which assumes the repulsive force as the dominant factor.

The “card-house” model is a classical model for the gelation of clay colloids proposed by van Olphen [226]. In this model, the gel formation of a clay colloid is explained by the formation of a macroscopic network structure, like a “house built of playing cards” (Fig. 8.40a), which is composed of T-shaped connections of the plane (mainly negatively charged) and edge (mainly positively charged) of the clay sheet through electrostatic attraction. In a Monte Carlo simulation study, a card-house structure appeared in a particle dispersion system with a negatively charged plane and a positively charged edge [227]. Whereas this model was widely regarded as a standard model, there are some questions about this extreme house-of-cards structure, and few experimental demonstrations of this structure formation have been reported. In a recent precise simulation [228], the card-house structure appeared only in the extremely restricted conditions. When the ionic strength is less than 5 mM, the interaction of the system is governed by only repulsive forces. When the ionic strength is greater than 5 mM, the overlapped state of the fringes of the clays becomes stable (such as the state of “layered coins”). When the ionic strength is greater than 100 mM, the van der Waals force becomes dominant, and the clay aggregates. In the first place, the AlOH and SiOH groups on the edge of the clay are charged only in the lower pH conditions in which AlOH²⁺ and SiOH²⁺ are generated by protonation. In the general conditions for clay colloids (pH = 12–13), AlOH and SiOH dissociate into negatively charged AlO⁻ and SiO⁻, respectively. These dissociations are obvious from the zeta potential measurements [229].On the other hand, much of the experimental evidence suggests that the macroscopic network structure is formed in clay colloids through the attractive interaction between nanosheets. [35, 37, 39, 40, 230–233]. It is probable that the formation of a network structure similar to “layered coins” originates from the attractive forces between nanosheets, such as the hydrogen bonding and van der Waals forces through the AlOH and SiOH groups on the edge of the clay, as well as the attractive electrostatic interaction mediated by the countercations.

The other model, the colloidal glass model (or Wigner glass model) explains the gelation of nanosheet colloids as the restricted motion of negatively charged nanosheets or their clusters caused by electrostatic repulsion among them [26, 39, 234–239], as illustrated in Fig. 8.40b. Mourchid [26, 39, 235] determined the phase diagrams of the sol-gel transition semiquantitatively by rheological measurements and tried to explain the colloidal glass model based on osmotic pressure measurements. They found that the osmotic pressure decreased with increasing ionic strength and remained positive regardless of the concentration of the colloid and the ionic strength (Fig. 8.41). These results can be explained by the shielding of the electrostatic repulsion between particles due to the increased ionic strength and indicate that repulsive interaction was dominant in the system. The model is also supported by the phenomenon that the viscosity of the clay colloidal sol was decreased by adding a small quantity of salt [240–242] and the phenomenon that the onset concentration of the sol-gel transition increased with increasing ionic strength [243]. Michot et al. recently investigated the nontronite and montmorillonite systems and reported that the change of rheology by the colloid concentration, particle size, and ionic strength could be explained by the repulsive force only model [24, 25, 48]. However, as shown in the next section, the existence of a network structure in the gel was suggested by the structural analyses based on the results of the small-angle scattering [35, 37, 39, 40, 230–233]. These results contradict the colloid glass model.

7.2 Colloid Structure and Rheology

To clarify the gelation mechanism of nanosheet colloids, discussion of the colloid structure is very important. Whereas the formation of the anisotropic structure of the clay colloid in the liquid crystal state was described in the former section, detailed structural analyses by small-angle scattering methods (light, X-ray, and neutrons) have been carried out also for isotropic and gel phase clay colloids, and the relationship between the structure and its rheological behavior was discussed [35, 37–40, 244].

In small-angle scattering measurement of a dilute Laponite colloid, such as ca. 1%, the scattering curve almost coincided with the theoretical scattering curve for disc-like particles with 25 nm diameter and 1 nm thickness [36, 245–248]. In the region of the smaller scattering vector q, namely, the region that reflects the larger structure, excess scattering is sometimes observed, suggesting that the distribution of colloidal particles is not uniform. For example, Fig. 8.42 shows the results of the small-angle scattering of a Laponite colloid reported by Pignon et al. [37, 38]. The power laws of q ⁻³ and q ^−1.8 are observed. In general, a scattering curve with a power law of q ^−D is ascribed to a mass fractal structure with fractal dimension D. Similar scattering results were reported by several other groups. These results were, regardless of the true structure, ascribed to an inhomogeneous structure with fractal dimension of 2–3, and some estimated structures, such as that shown in Fig. 8.43, were demonstrated.

Furthermore, Pignon et al. revealed that the power law of the SAXS pattern of the Laponite colloid changes from q ⁻¹ to q ^−1.8 when increasing the concentration and ionic strength and as the gelation proceeds. The change of the fractal dimension (corresponding to the network structure) corresponded to a change of the yield stress so that it was concluded that the formation of a macroscopic inhomogeneous structure with up to micrometer scale dominated the rheological properties. However, the results of the small-angle scattering, ascribable to the fractal structure, were also observed in the niobate nanosheet system, showing an extremely lower viscosity compared to clay minerals [34]. Therefore, careful consideration is required to discuss the direct relationship between the structure and the high viscosity of the nanosheet colloid.

The structural changes of the colloid induced by shear [245] or with time [230, 249–256] were studied in detail, and important results to discuss the thixotropic behavior and gelation mechanism of the nanosheet colloids were obtained. Bonn and Tanaka obtained the phase diagram shown in Fig. 8.44 based on the time-course results of dynamic and static light scattering on a Laponite colloid system [239, 250, 254–256]. They claim that “clay colloids with no fluidity” can be classified as two different states, “gel” or “glass,” depending on the conditions. They define “gel” as a network structure formed by dominant attractive forces, such as van der Waals force, in the region where the electrostatic repulsion force is shielded at relatively high ionic strength. In this region, the growth of inhomogeneous clusters or network structures was observed with time. “Wigner glass” (repulsive colloid glass) is a typical colloidal glass that emerges when the electrostatic repulsive force is dominant in a lower salt concentration and when the sum of effective volume of colloid particles, including the Debye length, exceeds the volume of the system. In this type, the structure showed homogeneity and did not change with time. The motility of the particles was typical to colloidal glasses and was maintained with vibration in a small region, even after aging. The other phase, “attractive colloidal glass” appears in the intermediate region between the “gel” and the “Wigner glass.” This phase is composed of nanosheet clusters, not individual nanosheets, as the unit particle, so that an inhomogeneous structure grows with time and the particles show no motility after aging.

7.3 Control of the Rheological Properties of the Nanosheet Colloids

In this final chapter, the reports on controlling the rheological properties of nanosheet colloids are presented. Largely et al. investigated the viscosity change of montmorillonite colloid after adding various types of salts and acids [240–242]. Figure 8.45 shows the viscosity change of montmorillonite colloids with the addition of various salts. Whereas the addition of a small quantity of salt decreased the viscosity due to shielding of the electrostatic repulsion between nanosheet colloids, a large quantity of salt caused an abrupt increase in the viscosity due to attractive interaction between nanosheets and the formation of an aggregated structure, named as an attractive gel. The viscosity depended on the species of salt. When a small amount of phosphate or pyrophosphate was added to the clay colloid, the viscosity decreased greatly [241, 242]. The addition of polyelectrolytes, such as sodium polyacrylate [257], and nonionic polymers, such as poly(ethylene oxide), was also examined. In these cases, the viscosity decreased with the addition of a lower molecular weight polymer, whereas it increased with addition of a higher molecular weight polymer.

We found a pH-induced gelation of an exfoliated nanosheet colloid of layered niobate K₄Nb₆O₁₇ [258, 259]. When HCl or H₂SO₄ was added to the colloid (~10 gL⁻¹) and the pH was adjusted to 1.5, a physical gel was formed. Figure 8.46 shows the appearance of the colloid in the gel state. The change of pH changed the surface charge of the niobate nanosheet and influenced the electrostatic interaction between nanosheets. This process coincides with the gelation process of the clay colloid.

The lateral size of the nanosheet is also considered an important factor to control the viscosity. In the hexaniobate nanosheet system with controlled lateral size, colloids of the nanosheets with larger lateral size had higher viscosity [17]. However, in clay mineral systems, the opposite trend was reported for montmorillonite [25] and nontronite [24, 48]. Laponite, a synthetic hectorite with very small particle size of 25 nm [236], shows an extraordinarily high viscosity at low concentration. Thus we can not find any general trend for the relationship between the lateral size and the viscosity. Further systematic investigations are needed to understand the effect of nanosheet size on the viscosity.

8 Summary and Outlook

The fundamentals and applications of colloidal nanosheets were reviewed. Liquid crystal phases of nanosheet colloids and control of them by external field are emerging as important topics because the intriguing hierarchical structure of the nanosheet can be assembled. Although we still have fundamental issues such as control of stability and rheological property and understanding of the structure formation mechanisms, some of novel colloidal materials and composite materials were successfully reported already. Further development of the researches on colloidal nanosheets will lead to many applications such as optical and electronic devices, medical materials, composite materials with good mechanical and barrier properties, structural colors materials for sensors and cosmetics, and composite gels for soft actuators.

References

Verwey EJW, Overbeek JTG (1948) Theory of the stability of lyophobic colloids. Elsevier, Amsterdam
Google Scholar
Smalley M (2006) Clay swelling and colloid stability. CRC Press, New York
Google Scholar
Livolant F, Levelut AM, Doucet J, Benoit JP (1989) Nature 339:724
Article CAS Google Scholar
Strzelecka TE, Davidson MW, Rill RL (1988) Nature 331:457
Article CAS Google Scholar
Sato T, Teramoto A (1996) Adv Polymer Sci 126:85
Article CAS Google Scholar
Bawden FC, Pirie NW, Bernal JD, Fankuchen I (1936) Nature 138:1051
Article Google Scholar
Jana NR, Gearheart LA, Obare SO, Johnson CJ, Edler KJ, Mann S, Murphy CJ (2002) J Mater Chem 12:2909
Article CAS Google Scholar
Kajiwara K, Donkai N, Hiragi Y, Inagaki H (1986) Makromol Chem 187:2883
Article CAS Google Scholar
van Bruggen MPB, van der Kooij FM, Lekkerkerker HNW (1996) J Phys: Condens Matter 8:9451
Google Scholar
Lemaire BJ, Panine P, Gabriel JCP, Davidson P (2002) Europhys Lett 69:55
Article Google Scholar
Li L, Walda J, Manna L, Alivisatos AP (2002) Nano Lett 2:557
Article CAS Google Scholar
van der Kooij FM, Kassapidou K, Lekkerkerker HNW (2000) Nature 406:868
Article Google Scholar
van der Kooij FM, Lekkerkerker HNW (1998) J Phys Chem B 102:7829
Article Google Scholar
Brown ABD, Ferrero C, Narayanan T, Rennie AR (1999) Eur Phys J B 11:481
Article CAS Google Scholar
Liu S, Zhang J, Wang N, Liu W, Zhang C, Sun D (2003) Chem Mater 15:3240
Article CAS Google Scholar
Gabriel J-CP, Camerel F, Lemaire BJ, Desvaux H, Davidson P, Batail P (2001) Nature 413:504
Article CAS Google Scholar
Miyamoto N, Nakato T (2004) J Phys Chem B 108:6152
Article CAS Google Scholar
Nakato T, Miyamoto N, Harada A (2004) Chem Commun 78
Google Scholar
Nakato T, Yamashita Y, Kuroda K (2006) Thin Solid Filims 495:24
Article CAS Google Scholar
Miyamoto N, Yamamoto S, Shimasaki K, Harada K, Yamauchi Y (2011) Chem Asian J 6:2936
Article CAS Google Scholar
Xu Z, Gao C (2011) ACS Nano 5:2908
Article CAS Google Scholar
Miyamoto N, Iijima H, Ohkubo H, Yamauchi Y (2010) Chem Commun 46:4166
Article CAS Google Scholar
Paineau E, Antonova K, Baravian C, Bihannic I, Davidson P, Dozov I, Impéror-Clerc M, Levitz P, Madsen A, Meneau F, Michot LJ (2009) J Phys Chem B 113:15858
Article CAS Google Scholar
Michot LJ, Bihannic I, Maddi S, Baravian C, Levitz P, Davidson| P (2008) Langmuir 24:3127
Article CAS Google Scholar
Michot LJ, Bihannic I, Porsch K, Maddi S, Baravian C, Mougel J, Levitz P (2004) Langmuir 20:10829
Article CAS Google Scholar
Mourchid A, Lécolier E, van Damme H, Levitz P (1998) Langmuir 14:4718
Article CAS Google Scholar
Onsager L (1949) Ann NY Acad Sci 51:627
Article CAS Google Scholar
Mejia AF, Chang Y-W, Ng R, Shuai M, Mannan MS, Cheng Z (2012) Phys Rev E 85:061708
Article CAS Google Scholar
Wong M, Ishige R, Hoshino T, Hawkins S, Li P, Takahara A, Sue H-J (2014) Chem Mater 26:1528
Article CAS Google Scholar
Miyamoto N, Nakato T (2002) Adv Mater 14:1267
Article CAS Google Scholar
Miyamoto N, Kuroda K, Ogawa M (2004) J Phys Chem B 108:4268
Article CAS Google Scholar
Miyamoto N, Yamamoto H, Kaito R, Kuroda K (2002) Chem Commun 2378
Google Scholar
Yamaguchi D, Miyamoto N, Fujita T, Nakato T, Koizumi S, Ohta N, Yagi N, Hashimoto T (2012) Phys Rev E 85:011403
Article CAS Google Scholar
Yamaguchi D, Miyamoto N, Koizumi S, Nakato T, Hashimoto T (2007) J Appl Cryst 40:s101
Article CAS Google Scholar
Bhatia S, Barker J, Mourchid A (2003) Langmuir 19:532
Article CAS Google Scholar
Kroon M, Vos WL, Wegdam GH (1998) Phys Rev E 57:1962
Article CAS Google Scholar
Pignon F, Magnin A, Piau J-M (1997) Phys Rev Lett 4689:4689
Article Google Scholar
Pignon F, Magnin A, Piau J-M, Cabane B, Lindner P, Diat O (1997) Phys Rev E 56:3281
Article CAS Google Scholar
Mourchid A, Delville A, Lambard J, Lécolier E, Levitz P (1995) Langmuir 11:1942
Article CAS Google Scholar
Morvan M, Espinat D, Lambard J, Zemb T (1994) Colloids Surf A 82:193
Article CAS Google Scholar
Treacy MMJ, Rice SB, Jacobson AJ, Lewandowski JT (1990) Chem Mater 2:279
Article CAS Google Scholar
Ebina Y, Sasaki T, Harada M, Watanabe M (2002) Chem Mater 14:4390
Article CAS Google Scholar
Takagaki A, Sugisawa M, Lu D, Kondo JN, Hara M, Domen K, Hayashi S (2003) J Am Chem Soc 125:5479
Article CAS Google Scholar
Kudo A, Sakata T (1995) J Phys Chem 99:15963
Article CAS Google Scholar
Langmuir I (1938) J Chem Phys 6:873
Article CAS Google Scholar
Gabriel J-CP, Sanchez C, Davidson P (1996) J Phys Chem 100:11139
Article CAS Google Scholar
DiMasi E, Fossum JO, Gog T, Venkataraman C (2001) Phys Rev E 64:061704
Article CAS Google Scholar
Michot LJ, Baravian C, Bihannic I, Maddi S, Moyne C, Duval JFL, Levitz P, Davidson P (2009) Langmuir 25:127
Article CAS Google Scholar
Michot LJ, Bihannic I, Maddi S, Funari SS, Baravian C, Levitz P, Davidson P (2006) Proc Nat Acad Sci 103:16101
Article CAS Google Scholar
Paineau E, Michot LJ, Bihannic I, Baravian C (2011) Langmuir 27:7806
Article CAS Google Scholar
Paineau E, Bihannic I, Baravian C, Philippe A-M, Davidson P, Levitz P, Funari Ses, Rochas C, Michot LJ (2011) Langmuir 27:5562
Article CAS Google Scholar
Baravian C, Michot LJ, Paineau E, Bihannic I, Davidson P, ́ror-Clerc MI, Belamie E, Levitz P (2010) Europhys Lett 60:36005
Google Scholar
Hemmen H, Ringdal NI, Azevedo END, Engelsberg M, Hansen EL, Meheust Y, Fossum JO, Knudsen KD (2009) Langmuir 25:12507
Article CAS Google Scholar
Dan B, Behabtu N, Martinez A, Evans JS, Kosynkin DV, Tour JM, Pasquali M, Smalyukh II (2011) Soft Matter 7:11154
Article CAS Google Scholar
Kim JE, Han TH, Lee SH, Kim JY, Ahn CW, Yun JM, Kim SO (2011) Angew Chem Int Ed 50:3043
Article CAS Google Scholar
Aboutalebi SH, Gudarzi MM, Zheng QB, Kim J-K (2011) Adv Funct Mater 21:2978
Article CAS Google Scholar
Xu Z, Gao C (2011) Nat Commun 2:571
Article CAS Google Scholar
Zocher VH (1925) Anorg Allg Chem 147:91
Article CAS Google Scholar
Davidson P, Garreau A, Livage J (1994) Liq Cryst 16:905
Article CAS Google Scholar
Davidson P, Bourgaux C, Schoutteten L, Sergot P, Williams C, Livage J (1995) J Phys II France 5:1577
Article CAS Google Scholar
Pelletier O, Bourgaux C, Diat O, Davidson P, Livage J (1999) Eur Phys J B 12:541
Article CAS Google Scholar
Lamarque-Forget S, Pelletier O, Dozov I, Davidson P, Martinot-Lagarde P, Livage J (2000) Adv Mater 12:1267
Article CAS Google Scholar
Desvaux H, Gabriel J-CP, Berthault P, Camerel F (2001) Angew Chem Int Ed 40:373
Article CAS Google Scholar
Commeinhes X, Davidson P, Bourgaux C, Livage J (1997) Adv Mater 9:900
Article CAS Google Scholar
Adachi-Pagano M, Forano C, Besse J-P (2000) Chem Commun 91
Google Scholar
Hibino T, Jones WJ (2001) J Mater Chem 11:1321
Article CAS Google Scholar
Iyi N, Ebina Y, Sasaki T (2008) Langmuir 24:5591
Article CAS Google Scholar
Iyi N, Ebina Y, Sasaki T (2011) J Mater Chem 21:8085
Article CAS Google Scholar
Wang N, Liu S, Zhang J, Wu Z, Chenb J, Sun D (2005) Soft Matter 1:428
Article CAS Google Scholar
Zhang J, Luan L, Zhu W, Liu S, Sun D (2007) Langmuir 23:5331
Article CAS Google Scholar
Mourad MCD, Devid EJ, Schooneveld MMv, Vonk C, Lekkerkerker HNW (2008) J Phys Chem B 112:10142
Article CAS Google Scholar
Zhu W, Sun D, Liu S, Wang N, Zhang J, Luan L (2007) Colloids Surf A 301:106
Article CAS Google Scholar
Luan L, Liu S, Sun D (2009) J Solid State Chem 182:1462
Article CAS Google Scholar
Luan L, Li W, Liu S, Sun D (2009) Langmuir 25:6349
Article CAS Google Scholar
Shen T-Z, Hong S-H, Song J-K (2014) Nature Mater 13:394
Article CAS Google Scholar
Brown ABD, Clarke SM, Rennie AR (1998) Langmuir 14:3192
Google Scholar
Bates M, Frenkel D (1999) J Chem Phys 110:6553
Article CAS Google Scholar
Ise N, Sogami IS (2005) Structure formation in solution—ionic polymers and colloidal particles. Springer, Berlin
Google Scholar
Geng F, Ma R, Nakamura A, Akatsuka K, Ebina Y, Yamauchi Y, Miyamoto N, Tateyama Y, Sasaki T (2013) Nat Commun 4:1632
Article CAS Google Scholar
Geng F, Ma R, Ebina Y, Yamauchi Y, Miyamoto N, Sasaki T (2014) J Am Chem Soc 136:5491
Article CAS Google Scholar
Song Y, Iyi N, Hoshide T, Ozawa TC, Ebina Y, Ma R, Miyamoto N, Sasaki T (2015) Chem Commun 51:17068
Article CAS Google Scholar
Helfrich W (1978) Naturforsch 33a:305
CAS Google Scholar
Kunz DA, Feicht P, Godrich S, Thurn H, Papastavrou G, Fery A, Breu J (2013) Adv Mater 25:1337
Article CAS Google Scholar
Marshall CE (1930) Trans Faraday Soc 26:173
Article CAS Google Scholar
Mueller H (1939) Phys Rev 55:508
Article CAS Google Scholar
Mueller H (1939) Phys Rev 55:792
Article CAS Google Scholar
Sugimoto W, Terabayashi O, Murakami Y, Takatsu Y (2002) J Mater Chem 12:3814
Article CAS Google Scholar
Nakato T, Nakamura K, Shimada Y, Shido Y, Houryu T, Iimura Y, Miyata H (2011) J Phys Chem C 115:8934
Article CAS Google Scholar
Shah MJ (1963) J Phys Chem 67:2215
Article CAS Google Scholar
Yamaoka K, Sasai R, Ikuta N (1994) Chem Lett 563
Google Scholar
Sasai R, Yamaoka K (1995) J Phys Chem 99:17754
Article CAS Google Scholar
Shah MJ, Thompson DC, Hart CM (1963) J Phys Chem 67:1170
Article CAS Google Scholar
Yamaoka K, Tanigawa M, Sasai R (1994) J Chem Phys 101:1625
Article CAS Google Scholar
Sasai R, Ikuta N, Yamaoka K (1996) J Phys Chem 100:17266
Article CAS Google Scholar
Holzheu S, Hoffmann H (2002) J Phys Chem B 106:4412
Article CAS Google Scholar
O’Konski CT, Bergmann K (1962) J Chem Phys 37:1573
Article Google Scholar
Yamagishi A, Soma M (1981) J Phys Chem 85:3090
Article CAS Google Scholar
Yamagishi A, Taniguchi M, Takahashi M, Asada C, Matsushita N, Sato H (1994) J Phys Chem 98:7555
Article CAS Google Scholar
Yamaoka K, Sasai R, Takata N (2000) Colloids Surf A 175:23
Article CAS Google Scholar
Yamaoka K, Sasai R (2000) J Colloid Interface Sci 225:82
Article CAS Google Scholar
Yamagishi A (1982) Inorg Chem 21:1778
Article CAS Google Scholar
Winslow WM (1949) J Appl Phys 20:1137
Article CAS Google Scholar
Dürrschmidt T, Hoffmann H (1999) Colloids Surf A 156:257
Article Google Scholar
Kim JW, Kim SG, Choi HJ, Jhon MS (1999) Macromol Rapid Comm 20:450
Article CAS Google Scholar
Lu J, Zhao XP (2002) J Mater Chem 12:2603
Article CAS Google Scholar
Fossum JO, Méheust Y, Parmar KPS, Knudsen KD, Måløy KJ, Fonseca DM (2006) Europhys Lett 74:438
Article CAS Google Scholar
Zhang WL, Liu YD, Choi HJ (2011) J Mater Chem 21:6916
Article CAS Google Scholar
Xiang LQ, Zhao XP (2003) J Mater Chem 13:1529
Article CAS Google Scholar
Kim YJ, Liu YD, Choi HJ, Park S-J (2013) J Colloid Interface Sci 394:108
Article CAS Google Scholar
Zhang WL, Choi HJ (2011) Chem Commun 47:12286
Article CAS Google Scholar
Yin J, Wang X, Chang R, Zhao X (2012) Soft Matter 8:294
Article CAS Google Scholar
Yin J, Chang R, Kai Y, Zhao X (2013) Soft Matter 9:3910
Article CAS Google Scholar
Parmar KPS, Meheust Y, Schjelderupsen B, Fossum JO (2008) Langmuir 24:1814
Article CAS Google Scholar
Shin KY, Lee S, Hong S, Jang J (2014) ACS Appl Mater Interfaces 6:5531
Article CAS Google Scholar
Kimura H, Sugiyama T, Takahashi S, Tsuchida A (2013) Rheol Acta 52:139
Article CAS Google Scholar
Kimura H, Ueno M, Takahashi S, Tsuchida A, Kurosaka K (2014) Appl Clay Sci 99:160
Article CAS Google Scholar
Kimura H, Nakashima A, Takahashi S, Tsuchida A, Kurosaka K (2015) Appl Clay Sci 114:120
Article CAS Google Scholar
Dozov I, Paineau E, Davidson P, Antonova K, Baravian C, Bihannic I, Michot LJ (2011) J Phys Chem B 115:7751
Article CAS Google Scholar
Hong S-H, Shen T-Z, Song J-K (2014) J Phys Chem C 118:26304
Article CAS Google Scholar
Porion P, Faugere AM, Michot LJ, Paineau E, Delville A (2011) J Phys Chem C 115:14253
Article CAS Google Scholar
Eguchi M, Angelone MS, Yennawar HP, Mallouk TE (2008) J Phys Chem C 112:11280
Article CAS Google Scholar
Wu LL, Ohtani M, Takata M, Saeki A, Seki S, Ishida Y, Aida T (2014) ACS Nano 8:4640
Article CAS Google Scholar
Azevedo ENd, Engelsberg M, Fossum JO, Souza REd (2007) Langmuir 23:5100
Article CAS Google Scholar
Osterloh FE (2002) J Am Chem Soc 124:6248
Article CAS Google Scholar
Kim JY, Osterloh FE, Hiramatsu H, Dumas RK, Liu K (2005) J Phys Chem B 109:11151
Article CAS Google Scholar
Beydaghi H, Javanbakht M (2015) Ind Eng Chem Res 54:7028
Article CAS Google Scholar
Guo F, Kim F, Han TH, Shenoy VB, Huang J, Hurt RH (2011) ACS Nano 5:8019
Article CAS Google Scholar
Liu Z, Li Z, Xu Z, Xia Z, Hu X, Kou L, Peng L, Wei Y, Gao C (2014) Chem Mater 26:6786
Article CAS Google Scholar
Takagi S, Shimada T, Eguchi M, Yui T, Yoshida H, Tryk DA, Inoue H (2002) Langmuir 18:2265
Article CAS Google Scholar
Kawamata J, Suzuki Y, Tenma Y (2010) Phil Mag 90:2519
Article CAS Google Scholar
Miyamoto N, Kuroda K, Ogawa M (2004) J Mater Chem 14:165
Article CAS Google Scholar
Jalili R, Aboutalebi SH, Esrafilzadeh D, Konstantinov K, Moulton SE, Razal JM, Wallace GG (2013) ACS Nano 7:3981
Article CAS Google Scholar
Yang X, Qiu L, Cheng C, Wu Y, Ma ZF, Li D (2011) Angew Chem Int Ed 50:7325
Article CAS Google Scholar
Jalili R, Aboutalebi SH, Esrafilzadeh D, Shepherd RL, Chen J, Aminorroaya-Yamini S, Konstantinov K, Minett AI, Razal JM, Wallace GG (2013) Adv Funct Mater 23:5345
Article CAS Google Scholar
Seyedin MZ, Razal JM, Innis PC, Jalili R, Wallace GG (2015) Adv Funct Mater 25:94
Article CAS Google Scholar
Kim YS, Kang JH, Kim T, Jung Y, Lee K, Oh JY, Park J, Park CR (2014) Chem Mater 26:5549
Article CAS Google Scholar
Nakato T, Nono Y, Mouri E, Nakata M (2014) Phys Chem Chem Phys 16:955
Article CAS Google Scholar
Yamaki T, Asai K (2001) Langmuir 17:2564
Article CAS Google Scholar
Umemura Y, Yamagishi A, Schoonheydt R, Persoons A, Schryver FD (2002) J Am Chem Soc 124:992
Article CAS Google Scholar
Inukai K, Hotta Y, Taniguchi M, Tomura S, Yamagishi A (1994) J Chem Soc Chem Commun 959
Google Scholar
Binks BP (2002) Curr Opin Colloid Inter Sci 7:21
Article CAS Google Scholar
Binks BP, Horozov TS (2006) Colloidal particles at liquid interfaces. Cambridge University Press, Cambridge
Book Google Scholar
Aveyard R, Binks BP, Clint JH (2003) Adv Colloid Interface Sci 100–102:503
Article CAS Google Scholar
Fujii S, Murakami R (2008) KONA Powder Technol J 26:153
Article CAS Google Scholar
Nonomura Y, Kobayashi N (2009) J Colloid Inter Sci 330:463
Article CAS Google Scholar
Booth SG, Dryfe RAW (2015) J Phys Chem C 119:23295
Article CAS Google Scholar
Mejia AF, Diaz A, Pullela S, Chang Y-W, Simonetty M, Carpenter C, Batteas JD, Mannan MS, Clearfield A, Cheng Z (2012) Soft Matter 8:10245
Article CAS Google Scholar
Tsugita A, Takemoto S, Mori K, Yoneya T, Otani Y (1983) J Colloid Inter Sci 95:551
Article CAS Google Scholar
Ashby NP, Binks BP (2000) Phys Chem Chem Phys 2:5640
Article CAS Google Scholar
Abend S, Bonnke N, Gutschner U, Lagaly G (1998) Colloid Polym Sci 276:730
Article CAS Google Scholar
Bolhuis P, Frenkel D (1994) J Chem Phys 101:9869
Article CAS Google Scholar
Yang F, Liu S, Xu J, Lan Q, Wei F, Sun D (2006) J Colloid Inter Sci 302:159
Article CAS Google Scholar
Hirsemann D, Shylesh S, De Souza RA, Diar-Bakerly B, Biersack B, Mueller DN, Martin M, Schobert R, Breu J (2012) Angew Chem Int Ed 51:1348
Google Scholar
Nakato T, Ueda H, Hashimoto S, Terao R, Kameyama M, Mouri E (2012) ACS Appl Mater Interfaces 4:4338
Article CAS Google Scholar
Dong J, Worthen AJ, Foster LM, Chen Y, Cornell KA, Bryant SL, Truskett TM, Bielawski CW, Johnston KP (2014) ACS Appl Mater Inter 6:11502
Article CAS Google Scholar
Zhang S, Sun D, Dong X, Li C, Xu J (2008) Colloids Surf A 324:1
Article CAS Google Scholar
Liu Q, Luan L, Sun D, Xu J (2010) J Colloid Interface Sci 343:87
Article CAS Google Scholar
Tran DN, Whitby CP, Fornasiero D, Ralston J (2010) J Colloid Interface Sci 348:460
Article CAS Google Scholar
Binks BP, Sekine T, Tyowua AT (2014) Soft Matter 10:578
Article CAS Google Scholar
He Y, Wu F, Sun X, Li R, Guo Y, Li C, Zhang L, Xing F, Wang W, Gao J (2013) ACS Appl Mater Interfaces 5:4843
Article CAS Google Scholar
McCoy TM, Pottage MJ, Tabor RF (2014) J Phys Chem C 118:4529
Article CAS Google Scholar
Kim J, Cote LJ, Kim F, Yuan W, Shull KR, Huang J (2010) J Am Chem Soc 132:8180
Article CAS Google Scholar
Gudarzi MM, Sharif F (2011) Soft Matter 7:3432
Article CAS Google Scholar
Guevara JS, Mejia AF, Shuai M, Chang Y-W, Mannan MS, Cheng Z (2013) Soft Matter 9:1327
Article CAS Google Scholar
Haraguchi K, Li H-J, SOng L, Murata K (2007) Macromolecules 40:6973
Article CAS Google Scholar
Ma X, Li Y, Wang W, Ji Q, Xi Y (2013) Eur Polymer J 49:389
Article CAS Google Scholar
Liu M, Ishida Y, Ebina Y, Sasaki T, Aida T (2013) Nat Commun 4:2029
Google Scholar
Liu J, Song G, He C, Wang H (2013) Macromol Rapid Commun 34:1002–1007
Article CAS Google Scholar
Fan J, Shi Z, Lian M, Li H, Yin J (2013) J Mater Chem A 1:7433
Article CAS Google Scholar
Shen J, Yan B, Li T, Long Y, Li N, Ye M (2012) Soft Matter 8:1831
Article CAS Google Scholar
Haraguchi K, Takada T (2010) Macromolecules 43:4294
Article CAS Google Scholar
Paineau E, Dozov I, Bihannic I, Baravian C, Krapf M-EM, Philippe A-M, Rouziere SAh, Michot LJ, Davidson P (2012) ACS Appl Mater Interfaces 4:4296
Article CAS Google Scholar
Inadomi T, Ikeda S, Okumura Y, Kikuchi H, Miyamoto N (2014) Macromol Rapid Commun 35:1741
Article CAS Google Scholar
Miyamoto N, Shintate M, Ikeda S, Hoshida Y, Yamauchi Y, Motokawa R, Annaka M (2013) Chem Commun 49:1082
Article CAS Google Scholar
Mejia AF, Ng R, Nguyen P, Shuai M, Acosta HY, Mannan MS, Cheng Z (2013) Soft Matter 9:10257
Article CAS Google Scholar
Liu M, Ishida Y, Ebina Y, Sasaki T, Hikima T, Takata M, Aida T (2015) Nature 517:68
Article CAS Google Scholar
Kim YS, Liu M, Ishida Y, Ebina Y, Osada M, Sasaki T, Hikima T, Takata M, Aida T (2015) Nat Mater 14:1002
Article CAS Google Scholar
Adams M, Dogic Z, Keller SL, Fraden S (1998) Nature 393:349
Article CAS Google Scholar
van der Kooij FM, Lekkerkerker HNW (2000) Phys Rev Lett 84:781
Article Google Scholar
Oversteegen SM, Wijnhoven JGEJ, Vonk C, Lekkerkerker HNW (2004) J Phys Chem B 108:18158
Article CAS Google Scholar
Dogic Z, Purdy K, Grelet E, Adams M, Fraden S (2004) Phys Rev E 69:051702
Article CAS Google Scholar
Moulton SE, Maugey M, Poulin P, Wallace GG (2007) J Am Chem Soc 129:9452
Article CAS Google Scholar
Yasarawan N, van Duijneveldt JS (2010) Soft Matter 6:353
Article CAS Google Scholar
Kleshchanok D, Petukhov AV, Holmqvist P, Byelov DV, Lekkerkerker HN (2010) Langmuir 26:13614
Google Scholar
Doshi N, Cinacchi G, van Duijneveldt JS, Cosgrove T, Prescott SW, Grillo I, Phipps J, Gittins DI (2011) J Phys: Condens Matter 23:194109
CAS Google Scholar
Kleshchanok D, Meijer J-M, Petukhov AV, Portale G, Lekkerkerker HNW (2012) Soft Matter 8:191
Article CAS Google Scholar
Kleshchanok D, Meester V, Pompe CE, Hilhorst J, Lekkerkerker HN (2012) J Phys Chem B 116:9532
Article CAS Google Scholar
Zhang J, Lang PR, Meyer M, Dhont JKG (2013) Langmuir 29:4679
Article CAS Google Scholar
Harnau L (2008) Mol Phys 106:1975
Article CAS Google Scholar
Stroobants A, Lekkerkerker HNW (1984) J Phys Chem 88:3669
Article CAS Google Scholar
Bates MA, Frenkel D (2000) Phys Rev E 62:5225
Google Scholar
Varga S, Galindo A, Jackson G (2002) J Chem Phys 117:7207
Article CAS Google Scholar
Camp PJ, Allen MP (1996) Phys A 229:410
Article CAS Google Scholar
Varga S, Galindo A, Jackson G (2002) Phys Rev E 66:011707
Google Scholar
Oversteegen SM, Lekkerkerker HN (2004) J Chem Phys 120:2470
Article CAS Google Scholar
Harnau L, Dietrich S (2004) Phys Rev E 69:051501
Article CAS Google Scholar
Harnau L, Dietrich S (2002) Phys Rev E Stat Nonlin Soft Matter Phys 66:051702
Article CAS Google Scholar
Bier M, Harnau L, Dietrich S (2005) J Chem Phys 123 No. 114906
Google Scholar
Phillips J, Schmidt M (2010) Phys Rev E 81:041401
Article CAS Google Scholar
Verhoeff AA, Wensink HH, Vis M, Jackson G, Lekkerkerker HNW (2009) J Phys Chem B 113:13476
Article CAS Google Scholar
van der Kooij FM, Lekkerkerker HNW (2000) Langmuir 16:10144
Article CAS Google Scholar
van der Kooij FM, Vogel M, Lekkerkerker HNW (2000) Phys Rev E 62:5397
Article Google Scholar
Chen M, Li H, Chen Y, Mejia AF, Wang X, Cheng Z (2015) Soft Matter 11:5775
Article CAS Google Scholar
Landman J, Paineau E, Davidson P, Bihannic I, Michot LJ, Philippe AM, Petukhov AV, Lekkerkerker HN (2014) J Phys Chem B 118:4913
Article CAS Google Scholar
Cousin F, Cabuil V, Grillo I, Levitz P (2008) Langmuir 24:11422
Article CAS Google Scholar
Woolston P, van Duijneveldt JS (2015) Langmuir 31:9290
Article CAS Google Scholar
Miyamoto N, Nakato T (2003) Langmuir 19:8057
Article CAS Google Scholar
Miyamoto N, Yamada Y, Koizumi S, Nakato T (2007) Angew Chem Int Ed 46:4123
Article CAS Google Scholar
Nakato T, Yamada Y, Miyamoto N (2009) J Phys Chem B 113:1323
Article CAS Google Scholar
Nakato T, Yamashita Y, Mouri E, Kuroda K (2014) Soft Matter 10:3161
Article CAS Google Scholar
Pujala RK, Pawar N, Bohidar HB (2011) Langmuir 27:5193
Article CAS Google Scholar
Pujala RK, Bohidar HB (2012) Soft Matter 8:6120
Article CAS Google Scholar
Zhang C, Tjiu WW, Fan W, Yang Z, Huang S, Liu T (2011) J Mater Chem 21:18011
Article CAS Google Scholar
Fang B, Peng L, Xu Z, Gao C (2015) ACS Nano 9:5214
Article CAS Google Scholar
Zhang E, Wang T, Zhao L, Sun W, Liu X, Tong Z (2014) ACS Appl Mater Interfaces 6:22855
Article CAS Google Scholar
Wan SJ, Li YC, Peng JS, Hu H, Cheng QF, Jiang L (2015) ACS Nano 9:708
Article CAS Google Scholar
Ramphal IA, Hagerman ME (2015) Langmuir 31:1505
Article CAS Google Scholar
Lee YR, Kim IY, Kim TW, Lee JM, Hwang S-J (2012) Chem Eur J 18:2263
Google Scholar
Wan-Guo H, De-Jun S, Shu-Hua H, Chun-Guang Z, Guo-Ting W (1998) Colloid Polym Sci 276:274
Article Google Scholar
Li SP, Hou WG, Sun DJ, Guo PZ, Jia CX (2003) Langmuir 19:3172
Article CAS Google Scholar
Werner S, Lau VW, Hug S, Duppel V, Clausen-Schaumann H, Lotsch BV (2013) Langmuir 29:9199
Article CAS Google Scholar
Hu Q, Xu Z, Qiao S, Haghseresht F, Wilson M, Lu GQ (2007) J Colloid Interface Sci 308:191
Article CAS Google Scholar
Deák Á, Janovák L, Tallósy SP, Bitó T, Sebők D, Buzás N, Pálinko I, Dékány I (2015) Langmuir 31:2019
Article CAS Google Scholar
Nakato T, Yamada Y, Nakamura M, Takahashi A (2011) J Colloid Interface Sci 354:38
Article CAS Google Scholar
Nakato T, Inoue S, Hiraragi Y, Sugawara J, Mouri E, Aritani H (2014) J Mater Sci 49:915
Article CAS Google Scholar
van Olphen H (1977) An introduction to clay colloid chemistry, 2nd edn. Krieger, Malabar
Google Scholar
Dijkstra M, Hansen JP, Madden PA (1995) Phys Rev Lett 75:2236
Article CAS Google Scholar
Jönsson B, Labbez C, Cabane B (2008) Langmuir 24:11406
Article CAS Google Scholar
Tombácz E, Szekeres M (2004) Appl Clay Sci 27:75
Article CAS Google Scholar
Mongondry P, Tassin JF, Nicolai T (2005) J Colloid Interface Sci 283:397
Article CAS Google Scholar
Nicolai T, Cocard S (2001) Eur Phys J E 5:221
Article CAS Google Scholar
Pignon F, Magnin A, Piau J-M (1996) J Rheol 40:573
Article CAS Google Scholar
Schmidt G, Nakatani AI, Butler PD, Han CC (2002) Macromolecules 35:4725
Article CAS Google Scholar
Norrish K (1954) Discuss Faraday Soc 18:120
Article CAS Google Scholar
Mourchid A, Delville A, Levitz P (1995) Faraday Discuss 101:275
Article CAS Google Scholar
Avery RG, Ramsay JDF (1986) J Colloid Interf Sci 109:448
Article CAS Google Scholar
Rand B, Peken E, Goodwin JW, Smith RW (1980) J Chem Soc Faraday Trans 1(76):225
Article Google Scholar
Shalkevich A, Stradner A, Bhat SK, Muller F, Schurtenberger P (2007) Langmuir 23:3570
Article CAS Google Scholar
Bonn D, Kellay H, Tanaka H, Wegdam G, Meunier J (1999) Langmuir 15:7534
Article CAS Google Scholar
Abend S, Lagaly G (2000) Appl Clay Sci 16:201
Article CAS Google Scholar
Penner D, Lagaly G (2001) Appl Clay Sci 19:131
Article CAS Google Scholar
Permien T, Lagaly G (1994) Appl Clay Sci 9:251
Article CAS Google Scholar
Levitz P, Lecolier E, Mourchid A, Delville A, Lyonnard S (2000) Europhys Lett 49:672
Article CAS Google Scholar
Kroon M, Wegdam GH, Sprik R (1996) Phys Rev B 54:6541
Article CAS Google Scholar
Ramsay JDF, Lindner P (1993) J Chem Soc Faraday Trans 89:4207
Article CAS Google Scholar
Ramsay JDF (1986) J Colloid Interface Sci 109:441
Article CAS Google Scholar
Ramsay JDF, Swanton SW, Bunce J (1990) J Chem Soc Faraday Trans 86:3919
Article CAS Google Scholar
Balnois E, Durand-Vidal S, Levitz P (2003) Langmuir 19:6633
Article CAS Google Scholar
Ruzicka B, Zulian L, Ruocco G (2006) Langmuir 22:1106
Article CAS Google Scholar
Tanaka H, Meunier J, Bonn D (2004) Phys Rev E 69:031404
Article CAS Google Scholar
Ruzicka B, Zulian L, Ruocco G (2004) Phys Rev Lett 93: 258301
Google Scholar
Ruzicka B, Zulian L, Angelini R, Sztucki M, Moussaïd A, Ruocco G (2008) Phys Rev E 77:020402
Article CAS Google Scholar
Ruzicka B, Zulian L, Ruocco G (2004) J Phys: Condens Matter 16:S4993
CAS Google Scholar
Jabbari-Farouji S, Wegdam GH, Bonn D (2007) Phys Rev Lett 99:065701
Article CAS Google Scholar
Jabbari-Farouji S, Wegdam GH, Bonn D (2008) Phys Rev E 78:061405
Article CAS Google Scholar
Tanaka H, Jabbari-Farouji S, Meunier J, Bonn D (2005) Phys Rev E 71:021402
Article CAS Google Scholar
Labanda J, Sabaté J, Llorens J (2007) Colloids Surf A 301:8
Article CAS Google Scholar
Nakato T, Miyamoto N (2002) J Mater Chem 12:1245
Article CAS Google Scholar
Nakato T, Miyamoto N, Harada A, Ushiki H (2003) Langmuir 19:3157
Article CAS Google Scholar

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Acknowledgements

A part of this work was supported by Research Center for Materials and Energy Devices of Fukuoka Institute of Technology (FIT-ME) (Strategic Research Foundation Grant-Aided Project for Private University from MEXT) as well as by Canon Foundation, KAKENHI (#24104005 and #15K05657), and Network Joint Research Center for Materials and Devices (#201507 and #20166009).

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Department of Life, Environment, and Materials Science, Fukuoka Institute of Technology, 3-30-1 Wajiro-higashi, Higashi-ku, Fukuoka, 811-0295, Japan
Nobuyoshi Miyamoto & Yutaka Ohsedo
Department of Applied Chemistry, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu-shi, Fukuoka, 804-8550, Japan
Teruyuki Nakato

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Nobuyoshi Miyamoto
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Correspondence to Nobuyoshi Miyamoto .

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Kyushu Institute of Technology, Kitakyushu, Japan
Teruyuki Nakato
Yamaguchi University, Yamaguchi, Japan
Jun Kawamata
Tokyo Metropolitan University, Hachioji, Japan
Shinsuke Takagi

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Miyamoto, N., Ohsedo, Y., Nakato, T. (2017). Colloidal Nanosheets. In: Nakato, T., Kawamata, J., Takagi, S. (eds) Inorganic Nanosheets and Nanosheet-Based Materials. Nanostructure Science and Technology. Springer, Tokyo. https://doi.org/10.1007/978-4-431-56496-6_8

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DOI: https://doi.org/10.1007/978-4-431-56496-6_8
Published: 19 April 2017
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