**5. Conclusions**

*Functional Materials*

**Figure 8.**

*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

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

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.

**52**

were casted on a Si wafer.

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 realized.
