**Abstract**

Inorganic Janus nanosheets were successfully prepared using the difference in reactivity between interlayers I and II of layered hexaniobate K4Nb6O17·3H2O. Janus nanosheets exhibit the highest anisotropy among Janus compounds due to their morphology. It is therefore important to prepare Janus nanosheets with stable shapes in various solvents, robust chemical bonds between nanosheets and fuctional groups and high versatility due to surface functional groups. K4Nb6O17·3H2O, which possesses two types of interlayers and two types of organophosphonic acids that react with metal oxides to form robust covalent bonds, was employed to prepare Janus nanosheets for this study. Interlayer I was modified by octadecylphosphonic acid, followed by modification by carboxypropylphosphonic acid mainly at interlayer II. Preparation of Janus nanosheets with two organophosphonate moieties was confirmed by 31P MAS NMR. After these regioselective and sequential modifications, the products were exfoliated into single-layered nanosheets in THF. Two types of derivatives with different repeating distances were recovered from a dispersion containing nanosheets exfoliated by different processes, centrifugation, and solvent evaporation. AFM analysis of the exfoliated nanosheets revealed that the products were Janus compounds. There are high expectations for application of these types of Janus nanosheets in various fields and for design of various Janus nanosheets using this preparation method.

**Keywords:** Janus nanosheets, K4Nb6O17·3H2O, organophosphonic acid, grafting reaction, intercalation

### **1. Introduction**

A Janus compound has two surface properties, and each of these properties appears on one of two sides of the compound [1]. Janus compounds are expected to be applied as functional materials, including electronic paper [2], solid surfactants [3], optics materials [4], and drug delivery system (DDS) vectors [5]. The morphology of Janus compounds is classified into three categories: 0-dimensional compounds such as particles; one-dimensional compounds including cylinders, tubes, and rods; and two-dimensional compounds, typically sheets or discs [6–10].

There are various methods of preparing Janus nanoparticles. Regioselective surface modification of nanoparticles can produce Janus nanoparticles [11]. By forming nanoparticles with two raw materials, Janus nanoparticles with different compositions can be prepared [2]. Self-assembly and subsequent cross-linking of polymer chains can also produce Janus nanoparticles [12]. Janus rods can be prepared by forming silica using TEOS by the sol-gel method on Fe3O4 regioselectively [13]. Janus cylinders can be prepared by masking one side of a cylinder and conducting subsequent modification of the other side [14].

Janus nanosheets exhibit the highest anisotropy among Janus compounds due to their unique morphology. They are also useful as emulsifiers, because Janus nanosheets cannot rotate at the interfaces of micelles [15]. Most Janus nanosheets reported so far have consisted of polymers. Stupp et al. reported the preparation of Janus nanosheets by polymerization of oligomers with polymerizable groups [16]. Polymerization of oligomers led to the formation of sheet morphology, because the polymerizable groups were located at the center of an oligomer. Walther et al. used triblock copolymers, polystyrene-*block*-polybutadiene-*block*-poly(*tert*-butyl methacrylate), for preparing Janus nanosheets [17]. Janus nanosheets were prepared by cross-linking polybutadiene domains of the triblock copolymers, and the resulting Janus nanosheets had two types of surfaces, polystyrene, and poly(*tert*butyl methacrylate) moieties. These methods are based on selective polymerization or cross-linking. On the other hand, Janus nanosheets were prepared by dropping poly(*ε*-caprolactone) at the interface between water and pentyl acetate and evaporating solvents to crystallize polymers [10]. The properties of Janus compounds were realized by folding the polymer an odd number of times, which exposed carboxyl groups on one side of the nanosheets. Although polymer-based Janus nanosheets have been prepared, these Janus nanosheets became swollen and deformed in organic solvents because they were composed of polymers [18].

It is therefore obvious that the preparation of inorganic Janus nanosheets is an important issue. There are only a few reports of preparation of Janus nanosheets derived from inorganic compounds, however, comprising silica nanosheets prepared by the sol-gel method [19–21] and fluorohectorite, which is a layered clay mineral [22].

Silica-based Janus nanosheets were prepared by hydrolysis and condensation of two trifunctional organoalkoxysilanes. First, phenylalkoxysilane was assembled on the surface of oil in a W/O emulsion and reacted by the sol-gel method. The product, which formed a hollow shell, was further reacted with aminoalkyltrialkoxysilane. The resultant product had phenyl groups and amino groups on the inner and outer surfaces of hollow particles, respectively. Janus nanosheets were obtained by breaking the hollow silica particles using a colloid milling method. The resulting Janus nanosheets have a thickness of 65 nm and a curvature originating from the hollow particle morphology [19]. Another method of preparing silica-based Janus nanosheets using a CaCO3 template was also reported. First, 3-butyldianhydride mercaptopropyltrimethoxysilane was assembled on the surface of CaCO3 particles and reacted using a sol-gel process. The products were further reacted with octadecyltrichlorosilane. Janus nanosheets were obtained by removing the templates and crushing the resultant hollow particles. Janus nanosheets with single a nanometer thickness were prepared by this method [21]. These sol-gel preparation methods, which were developed by Yang et al., required adsorption of organosilanes on liquid-liquid interfaces or self-assembly on templates. Thus, these methods restricted the reaction system design and choices of molecules.

Another method of preparing Janus nanosheets using inorganic layered material was reported in which the Janus nanosheets consisted of a sheet of a clay mineral, fluorohectorite, and two cationic polymers. The interlayer of fluorohectorite was

**43**

*Janus Nanosheets Derived from K4Nb6O17·3H2O via Regioselective Interlayer Surface Modification*

selectively exfoliated into double-layered nanosheets. Both surfaces of the products, which had a negative charge, were modified with a cationic polymer, protonated polyethyleneimine-ethylene oxide. After that, the products were exfoliated into single-layered nanosheets, and one surface which had not been modified was reacted with another cationic polymer, protonated dendritic poly(amidoamine). The products had one cationic polymer on one side and the other polymer on the other side of the nanosheets [22]. This preparation method has the advantage that any cationic compound could be used as the modifier, regardless of its functional group. On the other hand, the products use in water was restricted due to the presence of ionic bonds between the surfaces of the clay sheets and the polymers.

Preparation of graphene-based Janus nanosheets has also been developed [23].

For example, two-step functionalization was carried in one report [24]. First, the graphene surface of the substrate was functionalized by a photochlorination reaction. The functionalized graphene was then peeled off the substrate to expose the unmodified side using PMMA film as a mediator. The exposed fresh side of the graphene was further functionalized by phenylation reaction. Graphene-based Janus nanosheets were also prepared using a Pickering emulsion [25]. A graphene oxide (GO) dispersion was mixed with hydrochloric acid and wax, and this mixture was ultrasonicated to prepare a Pickering emulsion. The micelles were then washed with a sodium hydroxide aqueous solution, and GO was adsorbed on the surface of micelles to form a monolayer. The exposed GO surface of the micelles was further modified with alkylamine. Finally, Janus nanosheets were obtained by dissolving wax in chloroform. In the preparation of graphene-based Janus nanosheets, it is necessary that a single layer of graphene be adsorbed on a substrate or at the liquid-

These earlier studies show that the following conditions are desirable: nanosheets should have covalent bonds with organic groups, the choice of functional groups should not be limited, and regioselective surface modification should be easily achieved. Another preparation method that satisfies the above conditions

Some inorganic layered materials have structures in which negatively charged nanosheets and metal cations are piled up alternately. Nanosheets are very thin, just a few atoms thick. On the other hand, their lateral sizes are large, possibly on a micrometer scale. Therefore, these materials have high aspect ratios. From the viewpoints of structure and composition, inorganic layered materials are classified into several categories, including clay minerals [26–28], layered silicate [29–31], and layered transition metal oxide [32–34]. These inorganic layered materials have been used as hosts for inorganic-organic hybrids. There are only two types of reaction for preparing inorganic-organic hybrids: intercalation reactions forming noncovalent bonds and grafting reactions forming covalent bonds. Grafting reactions that form covalent bonds between layer surfaces and organic groups utilize alcohols, carboxylic acids, silane coupling agents, and phosphorous coupling reagents such as organophosphonic acids as modifiers. Grafted organophosphonate moieties, in particular, are seldom eliminated from nanosheet surfaces because organophosphonate moieties form stable M–O–P bonds with nanosheet surfaces, except for Si–O–P bonds, which are known to become hydrolyzed under certain conditions [35]. Surface modification with monolayer can be easily achieved, moreover because homocondensation reactions do not occur between organophosphonic acids under mild conditions [36]. There have been many reports of surface modification by silane coupling agents for clay minerals [37], layered silicates [38–40], and layered transition metal oxides [41, 42]. In the case of surface modification by alcohol, there have been reports of polysilicates with ethylene glycol [43] and aliphatic alcohols [44], while layered perovskites have been modified with *n*-alcohol [45] and alcohol

liquid interphase to achieve regioselective functionalization.

should therefore be developed.

*DOI: http://dx.doi.org/10.5772/intechopen.84228*

#### *Janus Nanosheets Derived from K4Nb6O17·3H2O via Regioselective Interlayer Surface Modification DOI: http://dx.doi.org/10.5772/intechopen.84228*

selectively exfoliated into double-layered nanosheets. Both surfaces of the products, which had a negative charge, were modified with a cationic polymer, protonated polyethyleneimine-ethylene oxide. After that, the products were exfoliated into single-layered nanosheets, and one surface which had not been modified was reacted with another cationic polymer, protonated dendritic poly(amidoamine). The products had one cationic polymer on one side and the other polymer on the other side of the nanosheets [22]. This preparation method has the advantage that any cationic compound could be used as the modifier, regardless of its functional group. On the other hand, the products use in water was restricted due to the presence of ionic bonds between the surfaces of the clay sheets and the polymers.

Preparation of graphene-based Janus nanosheets has also been developed [23]. For example, two-step functionalization was carried in one report [24]. First, the graphene surface of the substrate was functionalized by a photochlorination reaction. The functionalized graphene was then peeled off the substrate to expose the unmodified side using PMMA film as a mediator. The exposed fresh side of the graphene was further functionalized by phenylation reaction. Graphene-based Janus nanosheets were also prepared using a Pickering emulsion [25]. A graphene oxide (GO) dispersion was mixed with hydrochloric acid and wax, and this mixture was ultrasonicated to prepare a Pickering emulsion. The micelles were then washed with a sodium hydroxide aqueous solution, and GO was adsorbed on the surface of micelles to form a monolayer. The exposed GO surface of the micelles was further modified with alkylamine. Finally, Janus nanosheets were obtained by dissolving wax in chloroform. In the preparation of graphene-based Janus nanosheets, it is necessary that a single layer of graphene be adsorbed on a substrate or at the liquidliquid interphase to achieve regioselective functionalization.

These earlier studies show that the following conditions are desirable: nanosheets should have covalent bonds with organic groups, the choice of functional groups should not be limited, and regioselective surface modification should be easily achieved. Another preparation method that satisfies the above conditions should therefore be developed.

Some inorganic layered materials have structures in which negatively charged nanosheets and metal cations are piled up alternately. Nanosheets are very thin, just a few atoms thick. On the other hand, their lateral sizes are large, possibly on a micrometer scale. Therefore, these materials have high aspect ratios. From the viewpoints of structure and composition, inorganic layered materials are classified into several categories, including clay minerals [26–28], layered silicate [29–31], and layered transition metal oxide [32–34]. These inorganic layered materials have been used as hosts for inorganic-organic hybrids. There are only two types of reaction for preparing inorganic-organic hybrids: intercalation reactions forming noncovalent bonds and grafting reactions forming covalent bonds. Grafting reactions that form covalent bonds between layer surfaces and organic groups utilize alcohols, carboxylic acids, silane coupling agents, and phosphorous coupling reagents such as organophosphonic acids as modifiers. Grafted organophosphonate moieties, in particular, are seldom eliminated from nanosheet surfaces because organophosphonate moieties form stable M–O–P bonds with nanosheet surfaces, except for Si–O–P bonds, which are known to become hydrolyzed under certain conditions [35]. Surface modification with monolayer can be easily achieved, moreover because homocondensation reactions do not occur between organophosphonic acids under mild conditions [36]. There have been many reports of surface modification by silane coupling agents for clay minerals [37], layered silicates [38–40], and layered transition metal oxides [41, 42]. In the case of surface modification by alcohol, there have been reports of polysilicates with ethylene glycol [43] and aliphatic alcohols [44], while layered perovskites have been modified with *n*-alcohol [45] and alcohol

*Functional Materials*

subsequent modification of the other side [14].

organic solvents because they were composed of polymers [18].

restricted the reaction system design and choices of molecules.

There are various methods of preparing Janus nanoparticles. Regioselective surface modification of nanoparticles can produce Janus nanoparticles [11]. By forming nanoparticles with two raw materials, Janus nanoparticles with different compositions can be prepared [2]. Self-assembly and subsequent cross-linking of polymer chains can also produce Janus nanoparticles [12]. Janus rods can be prepared by forming silica using TEOS by the sol-gel method on Fe3O4 regioselectively [13]. Janus cylinders can be prepared by masking one side of a cylinder and conducting

Janus nanosheets exhibit the highest anisotropy among Janus compounds due to their unique morphology. They are also useful as emulsifiers, because Janus nanosheets cannot rotate at the interfaces of micelles [15]. Most Janus nanosheets reported so far have consisted of polymers. Stupp et al. reported the preparation of Janus nanosheets by polymerization of oligomers with polymerizable groups [16]. Polymerization of oligomers led to the formation of sheet morphology, because the polymerizable groups were located at the center of an oligomer. Walther et al. used triblock copolymers, polystyrene-*block*-polybutadiene-*block*-poly(*tert*-butyl methacrylate), for preparing Janus nanosheets [17]. Janus nanosheets were prepared by cross-linking polybutadiene domains of the triblock copolymers, and the resulting Janus nanosheets had two types of surfaces, polystyrene, and poly(*tert*butyl methacrylate) moieties. These methods are based on selective polymerization or cross-linking. On the other hand, Janus nanosheets were prepared by dropping poly(*ε*-caprolactone) at the interface between water and pentyl acetate and evaporating solvents to crystallize polymers [10]. The properties of Janus compounds were realized by folding the polymer an odd number of times, which exposed carboxyl groups on one side of the nanosheets. Although polymer-based Janus nanosheets have been prepared, these Janus nanosheets became swollen and deformed in

It is therefore obvious that the preparation of inorganic Janus nanosheets is an important issue. There are only a few reports of preparation of Janus nanosheets derived from inorganic compounds, however, comprising silica nanosheets prepared by the sol-gel method [19–21] and fluorohectorite, which is a layered clay

Silica-based Janus nanosheets were prepared by hydrolysis and condensation of two trifunctional organoalkoxysilanes. First, phenylalkoxysilane was assembled on the surface of oil in a W/O emulsion and reacted by the sol-gel method. The product, which formed a hollow shell, was further reacted with aminoalkyltrialkoxysilane. The resultant product had phenyl groups and amino groups on the inner and outer surfaces of hollow particles, respectively. Janus nanosheets were obtained by breaking the hollow silica particles using a colloid milling method. The resulting Janus nanosheets have a thickness of 65 nm and a curvature originating from the hollow particle morphology [19]. Another method of preparing silica-based Janus nanosheets using a CaCO3 template was also reported. First, 3-butyldianhydride mercaptopropyltrimethoxysilane was assembled on the surface of CaCO3 particles and reacted using a sol-gel process. The products were further reacted with octadecyltrichlorosilane. Janus nanosheets were obtained by removing the templates and crushing the resultant hollow particles. Janus nanosheets with single a nanometer thickness were prepared by this method [21]. These sol-gel preparation methods, which were developed by Yang et al., required adsorption of organosilanes on liquid-liquid interfaces or self-assembly on templates. Thus, these methods

Another method of preparing Janus nanosheets using inorganic layered material was reported in which the Janus nanosheets consisted of a sheet of a clay mineral, fluorohectorite, and two cationic polymers. The interlayer of fluorohectorite was

**42**

mineral [22].

with fluoroalkyl groups [44, 46]. Also, layered perovskites were grafted with phenyl or *n*-alkylphosphonic acids using the aforementioned *n*-alkoxy derivatives as intermediates [47].

Layered hexaniobate (K4Nb6O17·3H2O) has a unique structure among layered transition metal oxides; K4Nb6O17·3H2O has two types of interlayers that are piled up alternately and exhibit different reactivities [48]. Interlayer I possesses hydrated water and shows high reactivity, which anhydrous interlayer II exhibits low reactivity. There have been a certain number of reports of reactions between K4Nb6O17·3H2O and organic molecules using the differences in reactivity between interlayer I and interlayer II [33].

Intercalation of small ammonium ions occurred sequentially, first in interlayer I and then in interlayer II [49]. On the other hand, bulky ammonium ions were intercalated only into interlayer I [50]. Compounds that were modified only in interlayer I were called A-type, and compounds that were modified in both interlayers I and II were called B-type. Kimura et al. modified the K4Nb6O17·3H2O surfaces with phenylphosphonic acid using A-type and B-type ion-exchanged intercalation compounds of K4Nb6O17·3H2O [51]. In their report, bulky dioctadecyldimethylammonium ions were intercalated into only interlayer I, and A-type phenylphosphonate derivatives were obtained using this A-type ammonium intercalation compound as an intermediate. On the other hand, dodecylammonium ions were intercalated into both interlayers I and II and a B-type phenylphosphonate derivative was obtained using this B-type ammonium intercalation compound as an intermediate. Thus, regioselective surface modification of K4Nb6O17·3H2O by organophosphonic acid was successfully achieved.

A variety of nanosheets have been obtained by exfoliation of layered materials [52], and various methods have been reported for their exfoliation. A simple method of exfoliation is dispersing layered materials in water. Water molecules can intercalate in the interlayer and promote exfoliation [53]. Bulky ammonium ions intercalated in the interlayer can expand the interlayer distance and decrease interactions between the negative charge and positive charge to cause exfoliation. [54]. Mechanical exfoliation using ultrasonication has also been employed [55]. On the other hand, *in situ* polymerization of organic monomers in the interlayer can also lead to exfoliation of layered materials. A modifier grafted onto the interlayer surface is reacted with monomers and generates polymer chains that expand the interlayer distance. This polymerization method is called the "grafting from" method and is often reported in the field of graphene [56]. Introducing small molecules, such as carboxyl acids, into the interlayer as an initiator group could cause polymerization from the surface of graphene [57, 58]. Another report of the "grafting from" method utilized a layered perovskite, HLaNb2O7·*x*H2O, which was modified with organophosphonic acid bearing an initiation group on the interlayer surface, and *N*-isopropylacrylamide (NIPAAm) was polymerized from the initiation group by atom transfer radical polymerization [59]. The interlayer distance was expanded by polymerization, and nanosheets dispersed in water were obtained. Because a thermos-responsive polymer, poly(*N*-isopropylacrylamide), PNIPAAm, was bound to the nanosheet surfaces, the nanosheets were hydrophilic and dispersed in water at below the lower critical solution temperature (LCST). On the contrary, the nanosheets become hydrophobic and aggregate at over the LCST in water.

In this research, the preparation of Janus nanosheets was achieved by taking advantage of the presence of two types of interlayers with different reactivities in K4Nb6O17·3H2O. Interlayer II of an A-type organophosphonic acid derivative of K4Nb6O17·3H2O was reacted with another type of organophosphonic acid. Both sides of niobate nanosheets were modified by two organophosphonic acids regioselectively, because organophosphonic acid could not undergo an exchange reaction and homocondensation. Janus nanosheets could be obtained by exfoliation of the

**45**

ODPA:C12N+

*Janus Nanosheets Derived from K4Nb6O17·3H2O via Regioselective Interlayer Surface Modification*

product into single-layer nanosheets in an appropriate solvent, THF. Lipophilic octadecylphosphonic acid (ODPA) and hydrophilic carboxypropylphosphonic acid (CPPA) were chosen as the organophosphonic acids. The properties of both sides of Janus nanosheets were explored by the AFM phase imaging technique. This report is based on a study first reported in Chemical Communications (**Figure 1**) [60].

An A-type alkylammonium intercalation compound, (2C182MeN)1.0(K, H)3 [Nb6O17] (2C182MeN = dioctadecyldimethylammonium ion), was prepared based on the previous report [49]. Octadecylphosphonic acid (ODPA) was synthesized as described elsewhere [61, 62]. Dodecyltrimethylammonium chloride, carboxypropylphosphonic acid (CPPA), 2-butanone, acetone, and tetrahydrofuran (THF) were

First, interlayer I of the A-type alkylammonium intercalation compound was modified by ODPA. The A-type alkylammonium intercalation compound (0.05 g) and ODPA (0.048 g) were used to adjust the Nb:ODPA molar ratio to 1:4 and reacted in 2-butanone (20 mL) at 150°C for 7 days. After the reaction, the crude product was centrifuged, washed with THF and HCl (pH = 3), and air-

, H+

(0.1 g) and dodecyltrimethylammonium chloride (0.19 g) were used to adjust the

After the reaction, the crude product was centrifuged, washed with water, and air-dried (ODPA\_C12N\_NbO). Then, interlayer II of ODPA\_C12N\_NbO was modified by CPPA. ODPA\_C12N\_NbO (0.05 g) and CPPA (0.05 g) were used to adjust the Nb:CPPA molar ratio to 1:10 before reaction in 2-butanone (10 mL) at 80°C for 3 days. After reaction, the crude product was centrifuged, washed with THF and HCl (pH = 3), and corrected as a precipitate (ODPA\_CPPA\_NbO). After centrifugation, the product dispersed in supernatant was corrected by slow evaporation of THF (ODPA\_CPPA\_NbO\_evaporation). TEM and AFM samples were prepared by stirring ODPA\_CPPA\_NbO in THF to exfoliate them into single-layer nanosheets.

molar ratio to 1:10 and reacted in water (10 mL) at 80°C for 3 days.

) in interlayer II were then exchanged

) to expand interlayer II. ODPA\_NbO

*DOI: http://dx.doi.org/10.5772/intechopen.84228*

**2. Experimental section**

*Preparation of Janus nanosheet.*

**Figure 1.**

used without further purification.

dried (ODPA\_NbO). The cations (K<sup>+</sup>

with the dodecylammonium ion (C12N+

*Janus Nanosheets Derived from K4Nb6O17·3H2O via Regioselective Interlayer Surface Modification DOI: http://dx.doi.org/10.5772/intechopen.84228*

#### **Figure 1.** *Preparation of Janus nanosheet.*

*Functional Materials*

as intermediates [47].

interlayer I and interlayer II [33].

with fluoroalkyl groups [44, 46]. Also, layered perovskites were grafted with phenyl or *n*-alkylphosphonic acids using the aforementioned *n*-alkoxy derivatives

transition metal oxides; K4Nb6O17·3H2O has two types of interlayers that are piled up alternately and exhibit different reactivities [48]. Interlayer I possesses hydrated water and shows high reactivity, which anhydrous interlayer II exhibits low reactivity. There have been a certain number of reports of reactions between K4Nb6O17·3H2O and organic molecules using the differences in reactivity between

K4Nb6O17·3H2O by organophosphonic acid was successfully achieved.

nanosheets become hydrophobic and aggregate at over the LCST in water.

In this research, the preparation of Janus nanosheets was achieved by taking advantage of the presence of two types of interlayers with different reactivities in K4Nb6O17·3H2O. Interlayer II of an A-type organophosphonic acid derivative of K4Nb6O17·3H2O was reacted with another type of organophosphonic acid. Both sides of niobate nanosheets were modified by two organophosphonic acids regioselectively, because organophosphonic acid could not undergo an exchange reaction and homocondensation. Janus nanosheets could be obtained by exfoliation of the

A variety of nanosheets have been obtained by exfoliation of layered materials [52], and various methods have been reported for their exfoliation. A simple method of exfoliation is dispersing layered materials in water. Water molecules can intercalate in the interlayer and promote exfoliation [53]. Bulky ammonium ions intercalated in the interlayer can expand the interlayer distance and decrease interactions between the negative charge and positive charge to cause exfoliation. [54]. Mechanical exfoliation using ultrasonication has also been employed [55]. On the other hand, *in situ* polymerization of organic monomers in the interlayer can also lead to exfoliation of layered materials. A modifier grafted onto the interlayer surface is reacted with monomers and generates polymer chains that expand the interlayer distance. This polymerization method is called the "grafting from" method and is often reported in the field of graphene [56]. Introducing small molecules, such as carboxyl acids, into the interlayer as an initiator group could cause polymerization from the surface of graphene [57, 58]. Another report of the "grafting from" method utilized a layered perovskite, HLaNb2O7·*x*H2O, which was modified with organophosphonic acid bearing an initiation group on the interlayer surface, and *N*-isopropylacrylamide (NIPAAm) was polymerized from the initiation group by atom transfer radical polymerization [59]. The interlayer distance was expanded by polymerization, and nanosheets dispersed in water were obtained. Because a thermos-responsive polymer, poly(*N*-isopropylacrylamide), PNIPAAm, was bound to the nanosheet surfaces, the nanosheets were hydrophilic and dispersed in water at below the lower critical solution temperature (LCST). On the contrary, the

Layered hexaniobate (K4Nb6O17·3H2O) has a unique structure among layered

Intercalation of small ammonium ions occurred sequentially, first in interlayer I and then in interlayer II [49]. On the other hand, bulky ammonium ions were intercalated only into interlayer I [50]. Compounds that were modified only in interlayer I were called A-type, and compounds that were modified in both interlayers I and II were called B-type. Kimura et al. modified the K4Nb6O17·3H2O surfaces with phenylphosphonic acid using A-type and B-type ion-exchanged intercalation compounds of K4Nb6O17·3H2O [51]. In their report, bulky dioctadecyldimethylammonium ions were intercalated into only interlayer I, and A-type phenylphosphonate derivatives were obtained using this A-type ammonium intercalation compound as an intermediate. On the other hand, dodecylammonium ions were intercalated into both interlayers I and II and a B-type phenylphosphonate derivative was obtained using this B-type ammonium intercalation compound as an intermediate. Thus, regioselective surface modification of

**44**

product into single-layer nanosheets in an appropriate solvent, THF. Lipophilic octadecylphosphonic acid (ODPA) and hydrophilic carboxypropylphosphonic acid (CPPA) were chosen as the organophosphonic acids. The properties of both sides of Janus nanosheets were explored by the AFM phase imaging technique. This report is based on a study first reported in Chemical Communications (**Figure 1**) [60].
