**3. Analysis**

XRD analysis (convergence method) was carried out with a Rigaku RINT-1000 diffractometer (Mn-filtered FeK*α* radiation). XRD analysis (parallel beam method) was performed with a Rigaku SmartLab diffractometer (oblique incidence, FeK*α* radiation). IR analysis was conducted with a JASCO FT/IR-460 Plus spectrometer by the KBr method. Solid-state 31P magic angle spinning (MAS) NMR spectra were recorded on a JEOL JNM-ECX400 spectrometer. The measurement conditions were as follows: resonance frequency: 160.26 MHz; pulse angle: 90°; pulse delay: 30 s; and MAS frequency: 12 kHz. Triphenylphosphine (−8.4 ppm) was used as a reference. Solid-state 13C cross-polarization (CP)/MAS NMR spectra were recorded on a JEOL JNM-ECX-400 spectrometer. The measurement conditions were as follows: resonance frequency: 99.55 MHz; pulse delay: 5 s; contact time: 1.5 ms; and MAS frequency: 12 kHz. Hexamethylbenzene (17.4 ppm) was used as a reference. ICP-AES measurement was performed using a Thermo Jarrell Ash ICAP-574II instrument. Samples (about 10 mg) were dissolved by heating at 150°C overnight in HF (1 mL), HCl (3 mL), and HNO3 (4 mL). H3BO3 (70 mL) was added as a masking reagent for HF. HF (1 mL), HCl (3 mL), HNO3 (4 mL), and H3BO3 (70 mL) were added to each standard solution for matrix matching. The amounts of C, H, and N in the samples were measured by elemental analysis using a PerkinElmer PE2400II instrument. Transmission electron microscope (TEM) images were observed with a JEOL JEM-1011 microscope operating at 100 kV. A TEM sample was prepared by dropping drops of a dispersion on a Cu 150P grid and drying under reduced pressure. Atomic force microscope (AFM) images were observed with an Agilent 5500 AFM/SPM microscope in the acoustic AC mode under ambient conditions. An ordinary commercial silicon cantilever was used as an AFM tip (e.g., a RTESP-300 from Bruker: resonance frequency ≈ 300 kHz, and spring constant ≈ 40 N/m). Samples for AFM were prepared by spin coating of the dispersion on a Si wafer.

## **4. Results and discussion**

**Figure 2** shows 13C CP/MAS NMR spectra of the products. In the spectrum of ODPA\_NbO (**Figure 2a**), signals assignable to the octadecyl group were observed. It is likely that the ODPA moiety was introduced into interlayer I, since dioctadecyldimethylammonium ions, whose presence was required for interlayer modification with organophosphonic acids [51], were present only in interlayer I. In the spectrum of ODPA\_C12N\_NbO (**Figure 2b**), signals assignable to alkyl chains (octadecyl and dodecyl) were observed at 15–43 ppm [51]. In addition, a signal assignable to a carbon atom adjacent to a nitrogen atom was observed at 43 ppm [63], indicating the presence of C12N+ . Since C12N+ is known to be intercalated into both interlayer I and interlayer II [48], the intercalation of C12N+ into interlayer II was likely to occur. In the spectrum of ODPA\_CPPA\_NbO (**Figure 2c**), signals due to alkyl chains were observed at 15–36 ppm. On the other hand, a signal originating from C12N+ at 43 ppm disappeared and a signal due to C=O groups of CPPA was observed at 178 ppm [64]. These results suggest the removal of C12N+ and introduction of the CPPA moiety to ODPA\_CPPA\_NbO.

**Figure 3** shows IR spectra of the products. In the spectrum of ODPA\_NbO (**Figure 3a**), absorption bands due to *ν* (C–H), *σ*s (CH2), and *ν* (P–O) modes were observed at 2956–2849, 1468, and 1011 cm<sup>−</sup><sup>1</sup> , respectively [65], indicating that ODPA moiety was present in ODPA\_NbO. In the spectrum of ODPA\_C12N\_NbO (**Figure 3b**), an adsorption band at 1540 cm<sup>−</sup><sup>1</sup> assignable to the *σ* (N–H) mode was observed in addition to the aforementioned adsorption band, indicating that

**47**

**Figure 3.**

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

octadecylammonium ions were present in ODPA\_C12N\_NbO. In the spectrum of ODPA\_CPPA\_NbO (**Figure 3c**), a new adsorption band that was assignable to the

**Figure 2.** *13C CP/MAS NMR spectra of (a) ODPA\_NbO, (b) ODPA\_C12N\_NbO, and (c) ODPA\_CPPA\_NbO.*

the presence of the CPPA moiety in ODPA\_CPPA\_NbO. It was reported that an

[66], indicating

*σ* (C=O) mode of the CPPA moiety was observed at 1700 cm<sup>−</sup><sup>1</sup>

*IR spectra of (a) ODPA\_NbO, (b) ODPA\_C12N\_NbO, and (c) ODPA\_CPPA\_NbO.*

*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*

*Functional Materials*

**4. Results and discussion**

the presence of C12N+

from C12N+

. Since C12N+

I and interlayer II [48], the intercalation of C12N+

tion of the CPPA moiety to ODPA\_CPPA\_NbO.

observed at 2956–2849, 1468, and 1011 cm<sup>−</sup><sup>1</sup>

(**Figure 3b**), an adsorption band at 1540 cm<sup>−</sup><sup>1</sup>

XRD analysis (convergence method) was carried out with a Rigaku RINT-1000 diffractometer (Mn-filtered FeK*α* radiation). XRD analysis (parallel beam method) was performed with a Rigaku SmartLab diffractometer (oblique incidence, FeK*α* radiation). IR analysis was conducted with a JASCO FT/IR-460 Plus spectrometer by the KBr method. Solid-state 31P magic angle spinning (MAS) NMR spectra were recorded on a JEOL JNM-ECX400 spectrometer. The measurement conditions were as follows: resonance frequency: 160.26 MHz; pulse angle: 90°; pulse delay: 30 s; and MAS frequency: 12 kHz. Triphenylphosphine (−8.4 ppm) was used as a reference. Solid-state 13C cross-polarization (CP)/MAS NMR spectra were recorded on a JEOL JNM-ECX-400 spectrometer. The measurement conditions were as follows: resonance frequency: 99.55 MHz; pulse delay: 5 s; contact time: 1.5 ms; and MAS frequency: 12 kHz. Hexamethylbenzene (17.4 ppm) was used as a reference. ICP-AES measurement was performed using a Thermo Jarrell Ash ICAP-574II instrument. Samples (about 10 mg) were dissolved by heating at 150°C overnight in HF (1 mL), HCl (3 mL), and HNO3 (4 mL). H3BO3 (70 mL) was added as a masking reagent for HF. HF (1 mL), HCl (3 mL), HNO3 (4 mL), and H3BO3 (70 mL) were added to each standard solution for matrix matching. The amounts of C, H, and N in the samples were measured by elemental analysis using a PerkinElmer PE2400II instrument. Transmission electron microscope (TEM) images were observed with a JEOL JEM-1011 microscope operating at 100 kV. A TEM sample was prepared by dropping drops of a dispersion on a Cu 150P grid and drying under reduced pressure. Atomic force microscope (AFM) images were observed with an Agilent 5500 AFM/SPM microscope in the acoustic AC mode under ambient conditions. An ordinary commercial silicon cantilever was used as an AFM tip (e.g., a RTESP-300 from Bruker: resonance frequency ≈ 300 kHz, and spring constant ≈ 40 N/m). Samples for AFM were prepared by spin coating of the dispersion on a Si wafer.

**Figure 2** shows 13C CP/MAS NMR spectra of the products. In the spectrum of ODPA\_NbO (**Figure 2a**), signals assignable to the octadecyl group were observed. It is likely that the ODPA moiety was introduced into interlayer I, since dioctadecyldimethylammonium ions, whose presence was required for interlayer modification with organophosphonic acids [51], were present only in interlayer I. In the spectrum of ODPA\_C12N\_NbO (**Figure 2b**), signals assignable to alkyl chains (octadecyl and dodecyl) were observed at 15–43 ppm [51]. In addition, a signal assignable to a carbon atom adjacent to a nitrogen atom was observed at 43 ppm [63], indicating

occur. In the spectrum of ODPA\_CPPA\_NbO (**Figure 2c**), signals due to alkyl chains were observed at 15–36 ppm. On the other hand, a signal originating

**Figure 3** shows IR spectra of the products. In the spectrum of ODPA\_NbO (**Figure 3a**), absorption bands due to *ν* (C–H), *σ*s (CH2), and *ν* (P–O) modes were

ODPA moiety was present in ODPA\_NbO. In the spectrum of ODPA\_C12N\_NbO

was observed in addition to the aforementioned adsorption band, indicating that

observed at 178 ppm [64]. These results suggest the removal of C12N+

at 43 ppm disappeared and a signal due to C=O groups of CPPA was

is known to be intercalated into both interlayer

into interlayer II was likely to

, respectively [65], indicating that

assignable to the *σ* (N–H) mode

and introduc-

**3. Analysis**

**46**

**Figure 2.** *13C CP/MAS NMR spectra of (a) ODPA\_NbO, (b) ODPA\_C12N\_NbO, and (c) ODPA\_CPPA\_NbO.*

#### **Figure 3.**

*IR spectra of (a) ODPA\_NbO, (b) ODPA\_C12N\_NbO, and (c) ODPA\_CPPA\_NbO.*

octadecylammonium ions were present in ODPA\_C12N\_NbO. In the spectrum of ODPA\_CPPA\_NbO (**Figure 3c**), a new adsorption band that was assignable to the *σ* (C=O) mode of the CPPA moiety was observed at 1700 cm<sup>−</sup><sup>1</sup> [66], indicating the presence of the CPPA moiety in ODPA\_CPPA\_NbO. It was reported that an

adsorption band due to the *ν*as (CH2) of alkyl chain was shifted from 2924.7 cm<sup>−</sup><sup>1</sup> to a lower wavenumber by increasing the packing density of the alkyl chain [67]. In the case of the *all-trans* octadecyl alkyl chain, *ν*as (CH2) was observed at 2917.8 cm<sup>−</sup><sup>1</sup> [67, 68] and a *σ*s (CH2) band was observed at 1468 cm<sup>−</sup><sup>1</sup> [68]. In the spectrum of ODPA\_NbO, adsorption bands assignable to *ν*as (CH2) and *ν*s (CH2) modes were observed at 2918 and 2848 cm<sup>−</sup><sup>1</sup> , respectively, and a *σ*s (CH2) adsorption band was observed at 1468 cm<sup>−</sup><sup>1</sup> . Thus, the alkyl chain in ODPA\_NbO was likely to be in an *all-trans* conformation. On the other hand, *ν*as (CH2), *ν*s (CH2), and *σ*s (CH2) adsorption bands were observed at 2923, 2852, and 1456 cm<sup>−</sup><sup>1</sup> in the spectrum of ODPA\_CPPA\_NbO, respectively, indicating that the alkyl chain in ODPA\_CPPA\_ NbO was likely to contain *gauche-blocks*.

**Figure 4** shows 31P MAS NMR spectra of the products. A signal was observed at 28 ppm in the spectrum of ODPA\_NbO (**Figure 4a**). This signal was shifted upfield from the chemical shift of the ODPA molecule (33 ppm at 31P MAS NMR) by 5 ppm, indicating that interlayer surface modification by ODPA had proceeded and an Nb–O–P bond had been formed [47]. In the spectrum of ODPA\_C12N\_NbO (**Figure 4b**), a signal was observed at 25 ppm. This signal was shifted upfield from 28 ppm, the chemical shift of ODPA\_NbO, by 3 ppm. This shift suggests that C12N+ would change the electronic environment around the P atom by an ion exchange reaction with H+ of the P–OH group [47], although the details were not yet clarified. Thus, it is likely that C12N+ was intercalated not only in interlayer II, but probably also in interlayer I upon the reaction with ODPA\_NbO. In the spectrum of ODPA\_CPPA\_ NbO (**Figure 4c**), a new signal was observed at 31 ppm in addition to the signal at 28 ppm. The signal at 28 ppm was observed in the same position as that of the ODPA moiety of ODPA\_NbO, confirming maintenance of the ODPA moiety at interlayer I. Because a signal of a CPPA molecule was observed at 34 ppm, a signal at 31 ppm was assignable to the CPPA moiety. This signal was shifted upfield by 3 ppm, indicating

**Figure 4.** *31P MAS NMR spectra of (a) OPDA\_NbO, (b) ODPA\_C12N\_NbO, and (c) ODPA\_CPPA\_NbO.*

**49**

**Table 1.**

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

that the CPPA moiety was grafted onto the interlayer surface and a Nb–O–P bond was formed. The above results suggested that ODPA and CPPA formed covalent bonds

**Table 1** shows the molar ratio calculated from the ICP measurement and elemental analysis. The molar ratio of ODPA\_NbO was P:Nb = 1.3:6.0. On the other hand, the molar ratios of ODPA\_C12N\_NbO and ODPA\_CPPA\_NbO were P:Nb = 1.3:6.0 and

groups were observed in the IR spectrum of ODPA\_CPPA\_NbO, ODPA and CPPA

ODPA moiety in ODPA\_NbO, because the molar ratio of P and Nb of ODPA\_NbO did not change after reaction with a dodecylammonium chloride solution. Also, the molar ratio of P to 6 Nb in ODPA\_CPPA\_NbO increased by 2.2 (3.5 – 1.3), confirming grafting of the CPPA moiety. Assuming Nb = 6.0, the maximum modification amounts for interlayer I and II are 2.0 [51]. Since the Nb–O–P bond was stable with respect to hydrolysis and no homocondensation between two P–OH groups of phosphonic acid occurred under mild conditions [36], the amount of the ODPA moiety in interlayer I was estimated to be 1.3 (65% of the maximum modification amount), that of the CPPA moiety at interlayer I was in the range of 0.2–0.7 (10–35% of maximum modification amount), and that of the CPPA moiety in interlayer II was in the range of 1.5–2.0 (75–100% of the maximum modification amount). Thus, an organic derivative with interlayer I and interlayer II dominantly modified with hydrophobic ODPA and hydrophilic CPPA, respectively, were successfully prepared (**Figure 5**). Based on the nitrogen ratio of ODPA\_NbO, it seems that a small amount of unreacted A-type alkylammonium intercalation compound was present in ODPA\_NbO or a small number of released 2C182MeN ions were present in interlayer

C12N+ proceeded. Because no nitrogen was detected in ODPA\_CPPA\_NbO, C12N+ was completely removed from interlayer I and II after the reaction with CPPA.

**Figure 7** shows XRD patterns of the products. The *d* values of low-angle diffractions due to repeating distances were as follows: the *d* value of ODPA\_NbO, A-type derivative (**Figure 7a**), was 5.67 nm and the *d* value of ODPA\_C12N\_NbO

while maintaining an A-type stacking sequence, the *d* value of ODPA\_C12N\_NbO is likely to have increased from that of ODPA\_NbO. It is possible that a B-type stacking sequence was generated due to exfoliation and restacking during the reaction,

ODPA\_NbO 6.0 2.6 1.3 0.082 ODPA\_C12N\_NbO 6.0 0.58 1.3 1.8 ODPA\_CPPA\_NbO 6.0 0.49 3.5 —

synthesized while maintaining the crystal structure of the [Nb6O17]

A THF dispersion of nanosheets was easily attained by dispersing ODPA\_CPPA\_ NbO in THF. The resulting dispersion was cast on a TEM grid, and TEM observation was carried out (**Figure 6**). A sheet-like morphology with low contrast was observed. Spots observed in the electron diffraction (ED) pattern can be assigned to 200, 202, and 002 of the orthorhombic cells, and the lattice parameters were calculated to be *a* = 0.80 nm and *c* = 0.64 nm. This ED pattern was thus a *b*-axis incidence pattern of K4Nb6O17·3H2O [69]. Based on these results, ODPA\_CPPA\_NbO was

were likely to be in a monodentate environment on the surface of [Nb6O17]

<sup>4</sup><sup>−</sup> sheet surface. Since bands assignable to P–OH groups and P=O

<sup>4</sup><sup>−</sup> sheet.

proceeded without release of the

in interlayer II in ODPA\_C12N\_NbO

and H+ ions at interlayer II and

into interlayer II proceeded

**Nb/– K/– P/– N/–**

<sup>4</sup><sup>−</sup> nanosheets.

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

P:Nb = 3.5:6.0, respectively. Intercalation of C12N+

I *via* ion exchange. Since the amount of K+

decreased, an ion exchange reaction between K<sup>+</sup>

(**Figure 7b**) was 4.03 nm. If intercalation of C12N+

*Molar ratios of ODPA\_NbO, ODPA\_C12N\_NbO, and ODPA\_CPPA\_NbO.*

with the [Nb6O17]

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

that the CPPA moiety was grafted onto the interlayer surface and a Nb–O–P bond was formed. The above results suggested that ODPA and CPPA formed covalent bonds with the [Nb6O17] <sup>4</sup><sup>−</sup> sheet surface. Since bands assignable to P–OH groups and P=O groups were observed in the IR spectrum of ODPA\_CPPA\_NbO, ODPA and CPPA were likely to be in a monodentate environment on the surface of [Nb6O17] <sup>4</sup><sup>−</sup> sheet.

**Table 1** shows the molar ratio calculated from the ICP measurement and elemental analysis. The molar ratio of ODPA\_NbO was P:Nb = 1.3:6.0. On the other hand, the molar ratios of ODPA\_C12N\_NbO and ODPA\_CPPA\_NbO were P:Nb = 1.3:6.0 and P:Nb = 3.5:6.0, respectively. Intercalation of C12N+ proceeded without release of the ODPA moiety in ODPA\_NbO, because the molar ratio of P and Nb of ODPA\_NbO did not change after reaction with a dodecylammonium chloride solution. Also, the molar ratio of P to 6 Nb in ODPA\_CPPA\_NbO increased by 2.2 (3.5 – 1.3), confirming grafting of the CPPA moiety. Assuming Nb = 6.0, the maximum modification amounts for interlayer I and II are 2.0 [51]. Since the Nb–O–P bond was stable with respect to hydrolysis and no homocondensation between two P–OH groups of phosphonic acid occurred under mild conditions [36], the amount of the ODPA moiety in interlayer I was estimated to be 1.3 (65% of the maximum modification amount), that of the CPPA moiety at interlayer I was in the range of 0.2–0.7 (10–35% of maximum modification amount), and that of the CPPA moiety in interlayer II was in the range of 1.5–2.0 (75–100% of the maximum modification amount). Thus, an organic derivative with interlayer I and interlayer II dominantly modified with hydrophobic ODPA and hydrophilic CPPA, respectively, were successfully prepared (**Figure 5**).

Based on the nitrogen ratio of ODPA\_NbO, it seems that a small amount of unreacted A-type alkylammonium intercalation compound was present in ODPA\_NbO or a small number of released 2C182MeN ions were present in interlayer I *via* ion exchange. Since the amount of K+ in interlayer II in ODPA\_C12N\_NbO decreased, an ion exchange reaction between K<sup>+</sup> and H+ ions at interlayer II and C12N+ proceeded. Because no nitrogen was detected in ODPA\_CPPA\_NbO, C12N+ was completely removed from interlayer I and II after the reaction with CPPA.

A THF dispersion of nanosheets was easily attained by dispersing ODPA\_CPPA\_ NbO in THF. The resulting dispersion was cast on a TEM grid, and TEM observation was carried out (**Figure 6**). A sheet-like morphology with low contrast was observed. Spots observed in the electron diffraction (ED) pattern can be assigned to 200, 202, and 002 of the orthorhombic cells, and the lattice parameters were calculated to be *a* = 0.80 nm and *c* = 0.64 nm. This ED pattern was thus a *b*-axis incidence pattern of K4Nb6O17·3H2O [69]. Based on these results, ODPA\_CPPA\_NbO was synthesized while maintaining the crystal structure of the [Nb6O17] <sup>4</sup><sup>−</sup> nanosheets.

**Figure 7** shows XRD patterns of the products. The *d* values of low-angle diffractions due to repeating distances were as follows: the *d* value of ODPA\_NbO, A-type derivative (**Figure 7a**), was 5.67 nm and the *d* value of ODPA\_C12N\_NbO (**Figure 7b**) was 4.03 nm. If intercalation of C12N+ into interlayer II proceeded while maintaining an A-type stacking sequence, the *d* value of ODPA\_C12N\_NbO is likely to have increased from that of ODPA\_NbO. It is possible that a B-type stacking sequence was generated due to exfoliation and restacking during the reaction,


**Table 1.** *Molar ratios of ODPA\_NbO, ODPA\_C12N\_NbO, and ODPA\_CPPA\_NbO.*

*Functional Materials*

observed at 2918 and 2848 cm<sup>−</sup><sup>1</sup>

NbO was likely to contain *gauche-blocks*.

was observed at 1468 cm<sup>−</sup><sup>1</sup>

reaction with H+

Thus, it is likely that C12N+

adsorption band due to the *ν*as (CH2) of alkyl chain was shifted from 2924.7 cm<sup>−</sup><sup>1</sup>

a lower wavenumber by increasing the packing density of the alkyl chain [67]. In the case of the *all-trans* octadecyl alkyl chain, *ν*as (CH2) was observed at 2917.8 cm<sup>−</sup><sup>1</sup>

ODPA\_NbO, adsorption bands assignable to *ν*as (CH2) and *ν*s (CH2) modes were

in an *all-trans* conformation. On the other hand, *ν*as (CH2), *ν*s (CH2), and *σ*s (CH2)

ODPA\_CPPA\_NbO, respectively, indicating that the alkyl chain in ODPA\_CPPA\_

**Figure 4** shows 31P MAS NMR spectra of the products. A signal was observed at 28 ppm in the spectrum of ODPA\_NbO (**Figure 4a**). This signal was shifted upfield from the chemical shift of the ODPA molecule (33 ppm at 31P MAS NMR) by 5 ppm, indicating that interlayer surface modification by ODPA had proceeded and an Nb–O–P bond had been formed [47]. In the spectrum of ODPA\_C12N\_NbO (**Figure 4b**), a signal was observed at 25 ppm. This signal was shifted upfield from 28 ppm, the chemical shift of ODPA\_NbO, by 3 ppm. This shift suggests that C12N+ would change the electronic environment around the P atom by an ion exchange

in interlayer I upon the reaction with ODPA\_NbO. In the spectrum of ODPA\_CPPA\_ NbO (**Figure 4c**), a new signal was observed at 31 ppm in addition to the signal at 28 ppm. The signal at 28 ppm was observed in the same position as that of the ODPA moiety of ODPA\_NbO, confirming maintenance of the ODPA moiety at interlayer I. Because a signal of a CPPA molecule was observed at 34 ppm, a signal at 31 ppm was assignable to the CPPA moiety. This signal was shifted upfield by 3 ppm, indicating

**Figure 4.** *31P MAS NMR spectra of (a) OPDA\_NbO, (b) ODPA\_C12N\_NbO, and (c) ODPA\_CPPA\_NbO.*

[67, 68] and a *σ*s (CH2) band was observed at 1468 cm<sup>−</sup><sup>1</sup>

adsorption bands were observed at 2923, 2852, and 1456 cm<sup>−</sup><sup>1</sup>

to

[68]. In the spectrum of

in the spectrum of

, respectively, and a *σ*s (CH2) adsorption band

. Thus, the alkyl chain in ODPA\_NbO was likely to be

of the P–OH group [47], although the details were not yet clarified.

was intercalated not only in interlayer II, but probably also

**48**

#### **Figure 5.** *Proposed structure of ODPA\_CPPA\_NbO.*

resulting in a smaller repeating distance. Also, the *d* values of ODPA\_CPPA\_NbO (**Figure 7c**) and ODPA\_CPPA\_NbO\_evaporation (**Figure 7d**) were 2.41 and 4.74 nm, respectively. The stacking sequence would therefore be changed by reaction between ODPA\_C12N\_NbO and CPPA.

Here, the difference between these two *d* values is discussed. If ODPA\_CPPA\_NbO is a B-type derivative, the thickness of an organic moiety layer (sum of an ODPA monolayer and a CPPA monolayer) can be calculated by subtracting 0.82 nm, the niobate layer thickness, from 2.41 nm to make 1.59 nm [51]. The repeating distance of an A-type derivative could thus be estimated as the sum of a double niobate layer thickness and a double organic layer thickness. The repeating distance of an A-type derivative can therefore be estimated as follows: (1.59 nm × 2) + (0.82 nm × 2) = 4.82 nm. This value is approximately equal to *d* = 4.74 nm of ODPA\_CPPA\_NbO\_evaporation. From these estimations, it is proposed that ODPA\_CPPA\_NbO is a B-type derivative and ODPA\_CPPA\_NbO\_evaporation is an A-type derivative. As shown in **Figure 8**, a B-type derivative could be generated by forced restacking *via* centrifugation of exfoliated nanosheets (**Figure 8a** and **b**). On the other hand, an A-type derivative, in which hydrophilic groups faced each other and hydrophobic groups faced each other, was obtained by slow evaporation under mild conditions (**Figure 8c**).

The crystallite sizes calculated from diffraction of the repeating distances using Scherrer's formula were 3.67 and 7.71 nm for ODPA\_CPPA\_NbO and

**51**

**Figure 7.**

*ODPA\_CPPA\_NbO\_evaporation.*

**Figure 6.**

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

*TEM image of exfoliated ODPA\_CPPA\_NbO. The inset shows the corresponding ED pattern.*

*XRD patterns of (a) ODPA\_NbO, (b) ODPA\_C12N\_NbO, (c) ODPA\_CPPA\_NbO, and (d)* 

*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*

#### **Figure 7.**

*Functional Materials*

resulting in a smaller repeating distance. Also, the *d* values of ODPA\_CPPA\_NbO (**Figure 7c**) and ODPA\_CPPA\_NbO\_evaporation (**Figure 7d**) were 2.41 and 4.74 nm, respectively. The stacking sequence would therefore be changed by reac-

is a B-type derivative, the thickness of an organic moiety layer (sum of an ODPA monolayer and a CPPA monolayer) can be calculated by subtracting 0.82 nm, the niobate layer thickness, from 2.41 nm to make 1.59 nm [51]. The repeating distance of an A-type derivative could thus be estimated as the sum of a double niobate layer thickness and a double organic layer thickness. The repeating distance of an A-type derivative can therefore be estimated as follows: (1.59 nm × 2) + (0.82 nm × 2) = 4.82 nm. This value is approximately equal to *d* = 4.74 nm of ODPA\_CPPA\_NbO\_evaporation. From these estimations, it is proposed that ODPA\_CPPA\_NbO is a B-type derivative and ODPA\_CPPA\_NbO\_evaporation is an A-type derivative. As shown in **Figure 8**, a B-type derivative could be generated by forced restacking *via* centrifugation of exfoliated nanosheets (**Figure 8a** and **b**). On the other hand, an A-type derivative, in which hydrophilic groups faced each other and hydrophobic groups faced each other, was

The crystallite sizes calculated from diffraction of the repeating distances using Scherrer's formula were 3.67 and 7.71 nm for ODPA\_CPPA\_NbO and

obtained by slow evaporation under mild conditions (**Figure 8c**).

Here, the difference between these two *d* values is discussed. If ODPA\_CPPA\_NbO

tion between ODPA\_C12N\_NbO and CPPA.

*Proposed structure of ODPA\_CPPA\_NbO.*

**50**

**Figure 5.**

*XRD patterns of (a) ODPA\_NbO, (b) ODPA\_C12N\_NbO, (c) ODPA\_CPPA\_NbO, and (d) ODPA\_CPPA\_NbO\_evaporation.*

#### **Figure 8.**

*The estimated structures of ODPA\_CPPA\_NbO: Possible routes from (a) ODPA\_C12N\_NbO to (b) ODPA\_ CPPA\_NbO and (c) ODPA\_CPPA\_NbO\_evaporation.*

ODPA\_CPPA\_NbO\_evaporation, respectively. The crystallite size of ODPA\_CPPA\_ NbO\_evaporation was larger than that of ODPA\_CPPA\_NbO. It should be noted that underestimation could occur with use of lowest-angle diffractions due to the presence of strain [32]. The crystallite sizes could therefore reflect the average thickness of the particles in the stacking direction, making the number of stacked ODPA\_ CPPA\_NbO nanosheets lower than that of stacked ODPA\_CPPA\_NbO\_evaporation nanosheets. On the other hand, the estimated crystallite size could be interpreted as average thickness of a portion of the stacked sheets with an A-type or B-type stacking sequence. Based on this interpretation, ODPA\_CPAN\_NbO formed via forced restacking by centrifugation has lower stacking regularity or more random stacking than ODPA\_CPPA\_NbO\_evaporation nanosheets restacked under mild conditions.

**Figure 9** shows an AFM image of a sample prepared by spin coating of a THF dispersion of ODPA\_CPPA\_NbO on a Si wafer. It contained many nanosheets that showed a relatively uniform thickness in the range of 2.5–3.0 nm (**Figure 9A**). This thickness range is approximately equal to the *d* value of B-type ODPA\_CPPA\_NbO, indicating that ODPA\_CPPA\_NbO was exfoliated into single-layer nanosheets that were casted on a Si wafer.

As marked by the a and b arrows in **Figure 9**, two different colored nanosheet surfaces were observed in the phase image (**Figure 9B**). This indicates the presence of two different faces (36–37° and 38–39°) in each Janus nanosheet. This phase difference in the AFM phase image corresponds to the tapping phase gap in the vibration amplitude, and it was reported that the phase difference could occur with a difference in the crystallinity, viscosity, and adhesion of the sample surface [70, 71].

The Janus nanosheets consisted of a hydrophobic surface, which was dominantly covered with the ODPA moiety, and a hydrophilic surface, which was modified with the CPPA moiety. As a result, two chemically different surfaces gave different phases due to differences in the interactions between the apex of the AFM probe and the surfaces of the nanosheets and distinguished visually in the phase image. The origin of the phase contrast would be due to differences in viscosity and hydrophilicity/hydrophobicity. Since the apex of the AFM probe used in this measurement was hydrophilic, it is likely that the high-phase surface and a low-phase surface were assignable to the hydrophilic CPPA moiety and hydrophobic ODPA moiety, respectively.

**53**

**5. Conclusions**

**Figure 9.**

realized.

**Acknowledgements**

**Conflict of interest**

There are no conflicts to declare.

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

The c arrow in **Figure 9** marks the overlapping area of two nanosheets (a and b). Obviously, these area possessed double-layer thickness. The color of the phase image of this area (**Figure 9B**) indicates that nanosheet b partially overlapped nanosheet a. Thus, these results indicate that hydrophilic and lipophilic surfaces are facing each other. These results also indicate that the nanosheets prepared in this study exhib-

Janus nanosheets were successfully prepared by regioselective and sequential surface modification and exfoliation of K4Nb6O17·3H2O, whose interlayer I and interlayer II were dominantly modified by ODPA and CPPA, respectively. Since organophosphonic acids bearing various functional groups can be easily synthesized, Janus nanosheet surfaces can exhibit various properties in addition to hydrophobicity and hydrophilicity. The Janus nanosheets prepared by the present method can be dispersed in many solvents, moreover, because organophosphonic moieties are bound to niobate nanosheets by covalent bonds. The Janus nanosheets prepared in this study can be expected to be applied in surface chemistry research because of the hydrophobicity and hydrophilicity on opposing sides of the nanosheets. Also, by changing the functional groups of organophosphonic acids, novel two-dimensional materials with various functions with potential applications in various fields can be

This work was financially supported in part by a Grant-in-Aid for Scientific Research on Innovative Areas "New Polymeric Materials Based on Element-Blocks (No. 2401)" (JSPS KAKENHI Grant Numbers JP24102002), "Coordination Asymmetry (JP 23655205)," and Grant-in-Aid for Challenging Exploratory Research (JP17H05378). Reproduced from Ref. [60] with permission from the Royal Society of Chemistry.

ited hydrophobicity on one side and hydrophilicity on the other.

*Topographic (A) and phase (B) AFM images of ODPA\_CPPA\_NbO Janus nanosheets.*

*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*

**Figure 9.** *Topographic (A) and phase (B) AFM images of ODPA\_CPPA\_NbO Janus nanosheets.*

The c arrow in **Figure 9** marks the overlapping area of two nanosheets (a and b). Obviously, these area possessed double-layer thickness. The color of the phase image of this area (**Figure 9B**) indicates that nanosheet b partially overlapped nanosheet a. Thus, these results indicate that hydrophilic and lipophilic surfaces are facing each other. These results also indicate that the nanosheets prepared in this study exhibited hydrophobicity on one side and hydrophilicity on the other.
