Surface Modification of Layered Perovskite Nanosheets with a Phosphorus Coupling Reagent in a Biphasic System

Oleyl phosphate-modified HLaNb₂O₇· xH₂O nanosheets (OP_HLaNb nanosheets) were prepared via phase transfer from an aqueous phase, comprising a dispersion of HLaNb₂O₇· xH₂O (HLaNb) nanosheets, formed through the intercalation of tetrabutylammonium ion (TBA⁺) in the interlayer space of HLaNb and subsequent delamination, to a cyclohexane phase containing oleyl phosphate (OP, a mixture of monoester and diester). The modification of HLaNb nanosheets with OP was essentially completed within 3 days at a pH value of 2 or 4. Both infrared and solid-state ¹³C cross-polarization and magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of the OP_HLaNb nanosheets showed the presence of OP and/or related species and TBA⁺ on the HLaNb nanosheet surface. The solid-state ³¹P MAS NMR spectra of OP_HLaNb nanosheets exhibited new signals at -2 and 0 ppm, the former of which indicates the formation of Nb-O-P bonds. These whole data set obtained by complementary techniques clearly point out the modification of the HLaNb nanosheet surface by OP moieties causing a phase transfer. OP_HLaNb nanosheets showed higher dispersibility in cyclohexane than the OP_HLaNb_interlayer nanosheets, which were prepared via stepwise substitution reactions in the interlayers of HLaNb to achieve surface modification with OP and subsequent exfoliation in cyclohexane. The presence of TBA⁺ on the HLaNb nanosheets and the use of a liquid-liquid biphasic system were likely to improve the dispersibility. These results show that the preparation of OP-modified HLaNb nanosheets which could be well-dispersed in the cyclohexane phase was successful because of the use of a liquid-liquid biphasic system.