Surface Modification of Layered Perovskite Nanosheets with a

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Surface modification of layered perovskite nanosheets with a phosphorus coupling reagent in a biphasic phases system Takeshi Sugaya, Masahiko Ozaki, Régis Guégan, Naokazu Idota, and Yoshiyuki Sugahara Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03923 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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Surface modification of layered perovskite nanosheets with a phosphorus coupling reagent in a biphasic phases system AUTHOR NAMES Takeshi Sugaya† Masahiko Ozaki† Régis Guégan‡ Naokazu Idota,§ Yoshiyuki Sugahara*,†,∥

AUTHOR ADDRESS †

Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan ‡

Global Center for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan §

Department of Chemical Science and Technology, Faculty of Bioscience and Applied Chemistry, Hosei University, 3-7-2 kajino-cho, Koganeishi, Tokyo 184-8584, Japan ∥ Kagami

Memorial Research Institute for Materials Science and Technology, Waseda

University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan

KEYWORDS

ACS Paragon Plus Environment

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Layered perovskite nanosheets, Oleyl phosphate, Surface modification, Phase transfer, Liquid-liquid biphasic system

ABSTRACT

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, prepared via 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 values of 2 or 4. Both infrared and solid-state

13C

cross-polarization and magic-angle spinning (CP/MAS)

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, former of which indicates the formation of Nb-O-P bonds. These whole data set obtained by complementary techniques clearly pointed out the modification of the HLaNb nanosheets surface by OP moieties causing phase transfer. OP_HLaNb nanosheets showed higher dispersibility in cyclohexane than 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 due to use of a liquid-liquid biphasic system.


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Introduction Nanosheets are two-dimensional nanomaterials obtained by the delamination/exfoliation of layered materials1. Some layered materials, such as layered perovskites, are negatively charged, and unexfoliated layered structures composed of charged layers and interlayer ions, and Coulomb interactions could be involved between them2. Intercalation of bulky organic cations, such as tetramethylammonium ion (TMA+) and tetrabutylammonium ion (TBA+), between their layers leads to preparation of nanosheets through the multisteps: ion-exchange, osmotic swelling and delamination3,4. Also, grafting reactions of long chain organic groups on the interlayer surface5 decrease the attractive interactions between host layers, leading to their exfoliation. Since nanosheets exhibit various properties, including large surface areas, they can be used in several applications: catalysts6,7, electrode materials7,8 and nanofillers in polymer nanocomposites9,10. Nanosheet-polymer hybrids with the use of nanosheets display, for example, improved mechanical properties, thermal stabilities and gas barrier properties11. The proper dispersion of the nanosheets mainly depends on their hydrophobicity, however. If nanosheets show highly hydrophilic behavior, their aggregation occurs due to low affinity between the nanosheets and the polymers during the preparation of hybrids. Thus, it is necessary to enhance the affinity between the nanosheets and the polymers by surface modification with organic compounds is consequently required12. For surface modification of metal-oxide nanoparticles, hydrophobic surfaces can be prepared by grafting reactions between hydrophilic surfaces bearing M-OH groups and coupling agents13. Carboxylic acid14, silane coupling agents14 and phosphorus coupling agents15 have mainly been employed as coupling agents for surface modification of metaloxide nanoparticles. To prepare nanosheets modified with organic compounds, chemicals bearing long alkyl chains are frequently immobilized on the interlayer surface. When the


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interlayer surface is modified by coupling agents bearing long-chain groups, a simple sonication process leads to the exfoliation of layered materials1,16. Generally, modification of the interlayer surfaces of layered materials is achieved by reaction with coupling agents17,18,19,20,21, and the use of microwaves reduces the reaction period20,21. In this method, however, multiple steps are generally required. In contrast, simplification of the experimental procedure can be expected2 with a modification with the coupling agents after delamination of layered materials by intercalation of bulky organic cations. Graft reactions on the interlayer surface have been developed using several surface modifying agents. The first report in 1980 on the grafting reaction of an interlayer surface was done by Ruiz-Hitzky et al. who used trimethylchlorosilane with layered silicic acid, magadiite22. Modification by alcohols18,23, silane coupling agents17,24 and phosphorus coupling agents19 were then reported in literature. When alcohols are used as coupling agents, instability of the M-O-C bonds with respect to hydrolysis is a concern25. When silane coupling agents are used, on the contrary, they frequently form multi-molecular layers by homocondensation26. When phosphorus coupling agents are used, on the other hand, organic groups can be immobilized on the surface as monolayers, because M-O-P bonds are stable with respect to hydrolysis and no homocondensation reactions occur under mild conditions15. Among phosphorus coupling agents, monoester and diester of phosphoric acid bearing P-OH groups were recognized to be efficient for the surface modification. Oleyl phosphate is known as an interesting hydrophobic modifier for nanoparticles27,28,29. Recently surface modification of inorganic materials based on liquid-liquid biphasic systems have been developed30. With this technique where inorganic materials react at the water-nonpolar solvent interface, nanoparticles31,32 and nanoparticle dispersions can be prepared through phase transfer30. Moreover, through the use of liquid-liquid biphasic


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system, surface modification of TiO228,33,34 and Fe3O435,36 nanoparticles could be achieved. In the conventional method for modifying the surface of nanoparticles and nanosheets using organic solvents, modified nanoparticles and nanosheets are obtained as colloidal dispersions of organic solvents. In this method, however, the centrifugation of the dispersion was required. With this method, it was frequently observed that dispersibility could not be fully recovered upon redispersion because of the formation of aggregates33. In contrast, in liquidliquid biphasic systems, the aggregation is prevented because organically-modified nanoparticles and nanosheets dispersed in organic solvents can be obtained through transfer to organic solvents. In this study, we report surface modification of HLaNb2O7·xH2O (HLaNb) nanosheets in a liquid-liquid biphasic system consisting of an HLaNb nanosheets/water dispersion and an oleyl phosphate (OP)/cyclohexane solution. We prepared the HLaNb nanosheets/water dispersion by delamination of HLaNb through intercalation of TBA+ via an ion-exchange mechanism, leading to an increase in its specific surface area. Surface modification of HLaNb nanosheets with OP in a liquid-liquid biphasic system can increase the hydrophobicity, leading to phase transfer of the HLaNb nanosheets from an aqueous phase to a cyclohexane phase. We compared the OP-modified HLaNb nanosheets in a liquid-liquid biphasic system with the OPmodified HLaNb nanosheets (OP_HLaNb_interlayer nanosheets) which were prepared by modification of the interlayer surface of HLaNb by a grafting reaction and subsequent exfoliation by the ultrasonication. Experimental procedure Instrumentation


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X-ray diffraction (XRD) patterns were obtained with a Rigaku RINT-1000 (FeK radiation) diffractometer. Inductively coupled plasma (ICP) emission spectrometry was performed with a Varian VISTA-MPX spectrometer. Samples were dissolved in a mixture of (NH4)2SO4, H2SO4 and HNO3 with stepwise heating at 200, 250, 300 and 350ºC each for 30 min. After the addition of HCl and water, the solution was heated further at 150ºC for 20 min. Transmission electron microscope (TEM) images were obtained with a JEOL JEM-1011 microscope operating at 100 kV. Infrared (IR) spectra were recorded on a JASCO FT/IR-460 Plus spectrometer using the KBr disk method and the liquid film method. Phosphorus-31 NMR spectra were obtained with a JEOL ECZ-500R spectrometer (solvent, CDCl3; frequency, 202.46 MHz). Solid-state 31P MAS (magic-angle spinning) NMR spectra were recorded on a JEOL ECX-400 spectrometer (spinning rate, 8.0 kHz; frequency, 160.25 MHz; pulse delay, 5.0 s). A 13C NMR spectrum was obtained with a JEOL ECZ-500R spectrometer (solvent, CDCl3; frequency, 125.65 MHz). Solid-state 13C CP/MAS (cross-polarization and magic-angle spinning) NMR spectra were recorded on a JEOL ECX-400 spectrometer (spinning rate, 8.0 kHz; frequency, 99.55 MHz; pulse delay, 5.0 s; contact time, 1.5 ms). CHN analysis was performed with a Perkin Elmer PE2400II instrument. Atomic force microscope (AFM) observation was performed with a Digital Instrument Nanoscope III microscope with a tapping mode. The zeta potentials were recorded on an Otsuka electronics ELS-Z2 instrument.

Delamination of HLaNb2O7·xH2O (HLaNb) HLaNb was prepared by the method reported previously23. To obtain a suspension of HLaNb nanosheets, ion-exchanged water (100 mL), HLaNb (0.4 g) and 40.0 wt% TBAOH solutions (598 L) were mixed and stirred for a week for delamination. After a centrifugation


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process (4000 rpm, 5 min) to remove undelaminated HLaNb as a precipitate, an aqueous dispersion of HLaNb nanosheets was prepared. The delamination ratio was calculated based on the difference between the initial and undelaminated HLaNb masses. Preparation of OP-modified HLaNb nanosheets (OP_HLaNb nanosheets) in a liquid-liquid biphasic system To prepare OP_HLaNb nanosheets, a liquid-liquid biphasic system consisting of an aqueous dispersion of HLaNb nanosheets with their pH adjusted to 2, 4, 6, 8 or 10 (1.5 mg/mL, 15 mL) and a cyclohexane solution of OP with HLaNb:OP =1:4 (in a molar ratio) (15mL) was prepared and stirred at room temperature for 1, 3, 6 or 7 days. We utilized a relatively large excess of OP because phase transfer did not complete after a week in the experiments with the HLaNb:OP ratios of 1:1 and 1:2. Special attention was paid to avoid the formation of bubbles. After centrifugation of a cyclohexane phase (4000 rpm, 5 min) to remove the precipitate, OP_HLaNb nanosheets_x/cyclohexane dispersion (x represents a pH value, x=2, 4, 6, 8 or 10) was obtained. After the OP_HLaNb nanosheets were separated by ultracentrifugation (14000 rpm, 10 min), washed by dispersing in excess cyclohexane and subsequent ultracentrifugation (14000 rpm, 10 min) and dried, OP_HLaNb nanosheets_x was prepared. Also, for the investigation of dispersibility, the OP_HLaNb nanosheets_x/cyclohexane dispersion was allowed to stand for 3 months. Phase transfer ratios were calculated based on the amounts of Nb in the aqueous phase before and after reaction. Modification ratios were determined as molar ratios of the OP moiety to the [LaNb2O7] unit (the modification ratios were 1.0 at maximum) based on the ICP results.


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To estimate possible contribution of TBA+ to phase transfer, a liquid-liquid biphasic system consisting of an aqueous dispersion of HLaNb nanosheets (15 mL) with pH adjusted to 10 and cyclohexane (15mL) without OP and stirred at room temperature for 7 days. For investigating the formation of salts between anions from OP and TBA+, a liquid-liquid biphasic system consisting of water (15 mL) containing TBAOH (89.7 L) and HNO3 (adjusted a pH to 6) and cyclohexane (15 mL) in which OP (16 mg) was dissolved was prepared and stirred for 3 days. OP_TBA_no_HLaNb was obtained after evaporation of the volatiles. For washing the salts between anions from OP and TBA+, a liquid-liquid biphasic system consisting of an OP_HLaNb nanosheets_6/cyclohexane dispersion after phase transfer and an acetic acid aqueous solution with a pH of 4 (15 mL) was prepared and stirred for 1 week. After OP_HLaNb nanosheets were separated by ultracentrifugation (14000 rpm, 10 min), washed by dispersion in excess cyclohexane, subjected to subsequent ultracentrifugation (14000 rpm, 10 min) and dried, OP_HLaNb nanosheets_6_acetic acid were prepared. Preparation of OP_HLaNb nanosheets by exfoliation of OP-modified HLaNb (OP_HLaNb_interlayer nanosheets) For comparison, OP_HLaNb_interlayer nanosheets were prepared by exfoliation of OPmodified HLaNb whose interlayer surface was modified with OP. A propoxy derivative of HLaNb (C3-HLaNb) and a decoxy derivative of HLaNb (C10-HLaNb) were prepared by the method previously reported15. C10-HLaNb (0.2 g), 2-butanone (40 mL) and OP (0.67 mL) were sealed in a glass ampoule and heated at 80ºC for 3 or 6 days. The product was washed with excess cyclohexane and an OP_HLaNb_interlayer_y (y=3 or 6 days) was obtained. To exfoliate the OP_HLaNb_ interlayer, the OP_HLaNb_interlayer was dispersed in


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cyclohexane and sonicated for 1 h. After centrifugation (4000 rpm, 5 min), an OP_HLaNb_interlayer_y nanosheets/cyclohexane dispersion was obtained. Results and discussion Delamination of HLaNb An aqueous dispersion of HLaNb nanosheets exhibited Tyndall scattering (Fig. S1). A TEM image of aqueous dispersion of HLaNb nanosheets revealed a plate-like morphology with a lateral size of about 3 m and an electron diffraction (ED) pattern that was indexed as a tetragonal cell with a = 0.39 nm (Fig. S2), which is consistent with the a lattice parameter of HLaNb (0.389 nm)37. Thus, the lateral structure of HLaNb was unchanged after delamination. The delamination ratio was estimated at 37.6%. These results show that ionexchange between H+ and TBA+ for delamination of HLaNb were proceeded. The zeta potential of an aqueous dispersion of HLaNb nanosheets with their pH adjusted to 2, 4, 6, 8 or 10 was 3.62 mV (pH 2), -7.65 mV (pH 4), -19.5 mV (pH 6), -35.0 mV (pH 8) or 43.6 mV (pH 10). The isoelectric point was 2.6 and the absolute value of the zeta potential decreased as the pH of the aqueous phase was lowered.

OP-modified HLaNb nanosheets in a liquid-liquid biphasic system OP-modified HLaNb nanosheets were prepared using a liquid-liquid biphasic system consisting of an aqueous phase containing HLaNb nanosheets and a cyclohexane phase in which OP was dissolved. Fig. 1 shows photographs of the systems before (0 day) and after surface modification (1-6 days). When the pH of the aqueous phase was 2, 4 or 6, the initially turbid aqueous phase became transparent and the initially transparent cyclohexane phase


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became turbid. When the pH of the aqueous phase was 8 or 10, on the contrary, appearance of aqueous and cyclohexane phases did not change. Fig. 2 shows TEM images and ED patterns of nanosheets obtained from an HLaNb nanosheets/cyclohexane dispersions. TEM images showed plate-like morphologies with lateral sizes of about 2-3 m and ED patterns were assignable to a tetragonal cell with a = 0.39 nm. ED patterns corresponding to HLaNb were observed when the pH of the aqueous phase was 2, 4 and 6. Taking change in appearance into account, it is concluded that phase transfer of HLaNb nanosheets from the aqueous phase to the cyclohexane phase occurred at pH≤6. No Tyndall scattering was observed for the cyclohexane phases, on the other hand, when the pH>6 (i.e. 8 and 10), indicating the absence of any phase transfer in these cases. Fig. 3a shows IR spectrum of OP and Figs. 3b-d show IR spectra of OP_HLaNb nanosheets_x (x=2, 4 and 6). In Figs. 3b-d, absorption bands assignable to oleyl groups ( (C=C): 3003 cm-1,  (C-H): 2852, 2923 cm-1,  (CH2): 1465 cm-1) were observed19, indicating the presence of oleyl group. The  (P=O) band, which was observed at 1217 cm-1 in the spectrum of OP, was shifted to 1155 cm-1 and observed as shoulders of the broad bands, which were assignable to the  (P-O) mode. The profile of the  (P-O) absorption band region was reported to be affected by the modification of HLaNb with organophosphonic acids in a previous study19. Figs. 4a-c show solid-state 13C CP/MAS NMR spectra of OP_HLaNb nanosheets_x (x=2, 4 and 6). Compared to the 13C NMR spectrum of OP (Fig. 4e), signals assignable to oleyl groups at 130, 68, 33, 31, 28, 27, 24 and 15 ppm were observed22, indicating the presence of oleyl group, and their assignments are shown as an inset. Signals assignable to N[CH2CH2CH2CH3]4 of TBA+ at 58 ppm and N[CH2CH2CH2CH3]4 of TBA+ at 20 ppm (both marked with asterisks) were also observed38. Signals assignable to N[CH2CH2CH2CH3]4 and


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N[CH2CH2CH2CH3]4 were likely to be overlapped with those assigned to OP at 24 and 15 ppm38. No absorption bands assignable to TBA+ were observed in the IR spectra, probably due to overlapping with absorption bands of OP. These results therefore show the presence of OP or related species and TBA+ on the HLaNb nanosheet surfaces. Figs. 5a-c show solid-state 31P MAS NMR spectra of OP_HLaNb nanosheets_x (x=2, 4 and 6). Signals at around -2 ppm, which were shifted from those of bulk OP at 2.4 and 1.4 ppm (Fig. 5g), were observed in all the spectra of OP_HLaNb nanosheets_x (x=2, 4 and 6). In addition, sharp signals at 0 ppm were observed in the spectra of OP_HLaNb nanosheets_x (x=2, 4 and 6). The two signals suggest the presence of two P environments. In order to clarify these two environments, the following two experiments, the acid treatment of OP_HLaNb_6 (OP_HLaNb nanosheets_6_acetic acid) and the reaction between TBA+ and OP (OP_TBA_no_HLaNb) were performed. In the solid-state 31P MAS NMR spectra of OP_HLaNb nanosheets_6_acetic acid (Fig. 5e), the intensity of the sharp signal at 0 ppm was decreased. In the solid-state 31P MAS NMR spectrum of OP_TBA_no_HLaNb (Fig. 5f), a signal at 0 ppm was observed. In addition, ICP and CHN analyses (Table 1) revealed that the relative molar amounts of P and N to 2 Nb were decreased after washing of OP_HLaNb nanosheets_6 with acetic acid. Formation of salts between anions derived from OP, which was not immobilized on the HLaNb nanosheets, and TBA+, whose presence was demonstrated by solid-state 13C CP/MAS NMR and CHN analysis, was therefore suggested. The salts, which were not grafted to HLaNb nanosheets, caused sharp signals due to high mobility in the solid-state 31P MAS NMR of OP_HLaNb nanosheets. Among OP_HLaNb nanosheets_2, 4 and 6, in the solid-state 31P MAS NMR (Figs. 5a-c), the intensity of the sharp signal decreased when a pH was decreased. This occurred because the amounts of TBA+, remaining on the HLaNb nanosheet surfaces, was reduced with the decrease in pH and the


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amounts of the salts consequently lowered. It is considered that washing with acetic acid led to the removal of the salts from the HLaNb nanosheet surfaces. The signal at around -2 ppm, on the other hand, which shows upfield shifts from the signals at 2.4 and 1.4 ppm in the spectra of OP, suggests formation of Nb-O-P bonds13. Since a monodentate environment was proposed for the surface modification of HLaNb with organophosphonic acids, a similar monodentate environment is likely in the present study. Based on the spectroscopic results, surface modification was achieved when pH of the aqueous phase was 2, 4 or 6. Correspondingly, judging from the appearance (Fig. 1) and TEM images (Fig. 2), phase transfer from the aqueous phase to the cyclohexane phase occurred when the pH of the aqueous phase was 2, 4 or 6. When the pH of the aqueous phase was 8 or 10, however, no phase transfer was observed. From the results of the zeta potentials, when the pH of the nanosheet aqueous dispersion was 10, the surface charge was a large negative value (-43.6 mV), because essentially all the -OH groups on the surface of the nanosheets were dissociated to form -O- groups. TBA+ can therefore interact with -O- groups on the surface of nanosheets. It is considered that no grafting reaction proceeded because OH groups on the OP and HLaNb nanosheet surfaces were dissociated into -O- group interacting with TBA+. When the pH of the aqueous phase was lowered by HNO3, TBA+ was replaced with H+ by approaching to an ion-exchange equilibrium, because the amounts of H+ increased in the dispersion. The amounts of -OH groups were therefore increased and the zeta potentials approached to zero. As concerns the remaining TBA+ (Table 1), the amounts of N decreased as the pH was lowered, indicating that TBA+ was removed via ion-exchange with H+. TBA+ was not removed completely, however, and the remaining TBA+ formed salts with OP-related anions which were not grafted to the HLaNb nanosheets, as shown by the presence of sharp signals at 0 ppm. It was considered that phase transfer occurred when hydrophobicity caused not only by oleyl groups of the OP moiety, which was immobilized on


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the HLaNb nanosheets, but also by hydrophobicity of the salts remaining on the HLaNb nanosheet surfaces is sufficient. These results showed that, when the pH of the aqueous phase was 2, 4 or 6, phase transfer occurred due to hydrophobicity caused by the OP moiety which was immobilized on the HLaNb nanosheets and salts between anions from OP and TBA+. When the pH of the aqueous phase was 8 or 10, on the other hands, phase transfer did not occur. In the experiment with a liquid-liquid biphasic system consisting of an aqueous dispersion of HLaNb nanosheets with pH adjusted to 10 and a cyclohexane solution without OP, no change in appearances was observed in both the phases, indicating that the hydrophobicity of the HLaNb nanosheet surface covered only with TBA+ was insufficient for phase transfer and modification with OP was required for phase transfer. Fig. 6 shows the variations in phase transfer ratios with the reaction period. Among three phase transfer ratios, that at pH 2 was the highest after 1 day, while the phase transfer ratio at pH 4 was the highest after 3 and 6 days. When the pH of the aqueous phase was low, modification with OP was more likely to occur because the amounts of OH groups on the HLaNb surface increased due to removal of TBA+ via an ion-exchange with H+. Phase transfer was therefore rapid, and the phase transfer ratio at pH 2 was the highest after 1 day. On the other hand, the phase transfer ratio at pH 4 was the highest after 3 and 6 days. When the pH of the aqueous phase was 6, phase transfer proceeded only slightly before 1 day and the transfer ratio after 1 day was low compared to those with pH 2 and 4. It is likely that the low degree of the ion-exchange due to the low concentration of H+ led to the insufficient modification with OP to cause a low transfer ratio. On the other hand, the induction period was caused by molecular structure of TBA+ and OP. The TBA+ can be estimated as a sphere whose diameter is 1.00 nm39, while the oleyl group of OP can be estimated as a cylinder whose diameter is 0.68 nm and height is 2.0 nm40. Since OP has one or two bulky oleyl


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groups, it is likely that ion-exchange at neighboring sites occupied by TBA+ was required for the modification with OP. Thus, the reaction with OP became possible after the ion-exchange proceeded to a certain extent, leading to the appearance of the induction period. Fig. 7 shows AFM images of nanosheets obtained from the OP_HLaNb nanosheets_2/cyclohexane dispersion. In the AFM observation, plate-like morphologies with a thickness of 5.2, 10.4, 16.6 and 22.1 nm were observed, suggesting that OP_HLaNb nanosheeets were present as monolayers whose thickness was about 5 nm (estimated from the repeating distance of stacked OP-modified nanosheet, OP_HLaNb_interlayer, 3.39 nm (see the next section) and the diameter of TBA+39, 1.0 nm; 3.4+1.0×2≈5 nm) as well as multilayers whose thicknesses were about 10 nm, 17 nm and 22 nm. Thus, the removal of TBA+ was likely to cause restacking partly to form HLaNb particles, which could not undergo modification of OP in the interlayer space, and such HLaNb particles seemed to be removed as a precipitate during the preparation of OP_HLaNb nanosheets_2/cyclohexane dispersions via centrifugation and to cause reduction of the possible maximal phase transfer ratio. Table 1 shows the relative molar amounts of P and N to 2 Nb in OP_HLaNb nanosheets_x (x=2, 4 and 6) reacted for 7 days. The C/P ratios were calculated for OP moieties as 19.1 (pH 2), 21.0 (pH 4) and 21.0 (pH 6). From the C/P ratio, it was considered that most of the OP, which was grafted to HLaNb, was monoester (C/P=18). Fig. 8 shows relations between the reaction period and the modification ratio. It should be noted that the modification ratios were slightly overestimated by the presence of the salts between anions derived OP and TBA+. The result showed no drastic increase in the modification ratio with an increase in the reaction period, indicating that modification with OP was almost completed when phase transfer occurred, and further surface modification was limited in cyclohexane dissolving excess OP.


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In the course of the preparation of OP_HLaNb nanosheets_x/cyclohexane (x=2, 4 and 6) dispersions, precipitation occurred after centrifugation. When OP_HLaNb nanosheets_x/cyclohexane (x=2, 4 and 6) dispersions were allowed to stand for three months, Tyndall scattering was still observed, indicating a relatively good stability (Fig. S3). Concentration of OP_HLaNb nanosheets_2/cyclohexane was calculated as 0.95 mg/mL from the weight of the precipitate after the centrifugation. One day after evaporated from 15 mL to 7mL, Tyndal scattering was observed and precipitates formed, indicating a part of nanosheets aggregated. The dispersion, however, was formed again upon sonication.

Preparation of OP_HLaNb_interlayers by grafting reaction and their exfoliation to nanosheets After a grafting reaction with OP, the interlayer distance was expanded from that of C10HLaNb (2.75 nm) to 3.39 nm (Fig. S4), suggesting the change in the interlayer environment. In the XRD patterns of C10-HLaNb and OP_HLaNb_interlayer, A (110) reflection of HLaNb at 241.2º was observed in the XRD pattern of OP_HLaNb_interlayers at the same angle, indicating that the lateral structure of HLaNb was unchanged after modification. Fig. 3e shows IR spectrum of the OP_HLaNb_interlayers. The absorption bands observed in the spectra of OP_HLaNb nanosheets, which were prepared in the liquid-liquid biphasic system, ( (C=C),  (C-H),  (CH2)) were observed, indicating the presence of oleyl groups. Also, a change in a profile of the  (P=O) absorption band region, where the  (P=O) band was shifted and observed as a shoulder of the  (P-O) band, was observed, indicating a reaction between HLaNb and OP19.


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Fig. 5d shows solid-state 31P MAS NMR spectrum of the OP_HLaNb_interlayers. A signal at -3 ppm which was shifted from those of OP at 2.4 and 1.4 ppm was observed, indicating the formation of Nb-O-P bonds12. Fig. 4d shows solid-state 13C CP/MAS NMR spectrum of the OP_HLaNb_interlayers. Signals assignable to the oleyl group were observed at 130, 68, 33, 31, 28, 27, 24 and 15 ppm22, and their assignments are shown as an inset. These IR and 31P MAS NMR spectra showed that OP reacted with HLaNb to form Nb-O-P bonds, and absorption bands assignable to the oleyl group in the IR spectrum and signals assignable to oleyl groups in the 13C CP/MAS NMR spectrum of OP_HLaNb_interlayers showed maintenance of the structure of the oleyl groups upon interlayer modification. The C/P ratio was calculated as 18.3 based on ICP and CHN analyses, and the modification rate was 0.60. The C/P ratio (18.3) indicates that almost all the OP that modified HLaNb was an OP monoester. These results show no spectroscopic difference between the OP_HLaNb nanosheets_x (x=2, 4, and 6) that was obtained by surface modification in a liquid-liquid biphasic system and the OP_HLaNb_interlayers that was obtained by interlayer modification. Thus, the reaction of OP in the interlayer space of C10-HLaNb and the reaction of OP on the HLaNb nanosheet delaminated by TBA+ led to essentially the same type of surface modification. When the OP_HLaNb_interlayer nanosheets/ cyclohexane dispersion was allowed to stand for one week (Fig. S3), precipitation occurred and no Tyndal scattering was observed, indicating relatively poor stability. Comparison of OP_HLaNb nanosheets and OP_HLaNb_interlayer nanosheets


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We compare OP_HLaNb nanosheets prepared in a liquid-liquid biphasic system and OP_HLaNb_interlayer nanosheets. The modification ratio of OP_HLaNb nanosheets at pH 2, where the highest modification ratio (0.47) in a liquid-liquid biphasic system was achieved, was smaller than that of the OP_HLaNb_interlayer nanosheets (0.60). It is considered that a limited number of the reactive sites on the HLaNb surface were reacted due to the presence of Nb-O-TBA+ groups in a biphasic system. As results of the dispersibility investigation, OP_HLaNb nanosheets_x (x=2, 4 or 6)/cyclohexane dispersion showed higher dispersibility than the OP_HLaNb_interlayer nanosheets/cyclohexane dispersion. TBA+ on the HLaNb nanosheets and use of a liquid-liquid biphasic system contributed to the results, possibly because the amounts of P-OH groups were decreased via the formation of P-O- TBA+ groups, leading to more hydrophobic surface in the OP_HLaNb nanosheets_x. Thus, OP_HLaNb nanosheets/cyclohexane dispersion in liquid-liquid biphasic system exhibited higher dispersibility despite the smaller number of OP moieties with the aid of TBA+ on the HLaNb nanosheets. Conclusions The present study demonstrates that surface modification of HLaNb nanosheets with OP can be achieved in a liquid-liquid biphasic system using phase transfer consisting of the aqueous and cyclohexane phases. When the pH of the aqueous phase was 2 or 4, essentially all the HLaNb nanosheets in the aqueous phase were transferred to the cyclohexane phase within 3 days. An OP_HLaNb nanosheets_x (x=2, 4 or 6)/cyclohexane dispersion showed higher stability than an OP_HLaNb_interlayer nanosheets/cyclohexane dispersion. Since it was reported that TiO2 nanoparticles whose surface was modified by a liquid-liquid biphasic system can be employed for nanoparticles-polymer hybrids28, it is expected that the surface-


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modified HLaNb nanosheets prepared in the present study could be used for nanosheetspolymer hybrids with high dispersibility. Since surface modification of HLaNb nanosheets in the liquid-liquid biphasic system was successful in the present study, furthermore, such biphasic systems are likely to be applicable to preparation of other nanosheets with covalent surface modification.


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TABLE Table 1 Molar ratios of P and N and C/P values. Sample

Nb* / -

N/-

P/-

C/P** / -

OP_HLaNb nanosheets_pH 2

2

0.09

0.47

19.1

OP_HLaNb nanosheets_pH 4

2

0.11

0.37

21.0

OP_HLaNb nanosheets_pH 6 OP_HLaNb nanosheets_pH 6_acetic acid OP_HLaNb _interlayer

2

0.13

0.38

21.0

2

0.05

0.29

21.0

2

-

0.60

18.3

　　　

*set to 2 **calculate for OP moieties only by excluding carbon in TBA+

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by JSPS KAKENHI Grant Number 18H02062. ACKNOWLEDGMENT The authors thank Sakai Chemical Industry Co., Ltd., for donating oleyl phosphate. REFERENCES 1) Nicolosi, V.; Chhowalla, M.; Kanatzidis, G.; Strano S.; Coleman, N. Liquid Exfoliation of Layered Materials. Science 2013, 340, 1226419. 2) Akbarian-Tefaghi, S.; Rostamzadeh, T.; Brown, T. T.; Davis-Wheeler, C.; Wiley, J. B. Rapid Exfoliation and Surface Tailoring of Perovskite Nanosheets via Microwave-Assisted Reactions. ChemNanoMat 2017, 3, 538-550.


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18) Mitamura, Y.; Komori, Y.; Hayashi, S.; Sugahara, Y.; Kuroda, K. Interlamellar Esterification of H-Magadiite with Aliphatic Alcohols. Chem. Mater. 2001, 13, 3747-3753. 19) Shimada, A.; Yoneyama, Y.; Tahara, S.; Mutin, P. H.; Sugahara, Y. Interlayer Surface Modification of the Protonated Ion-exchangeable Layered Perovskite HLaNb2O7 ･ xH2O with Organophosphonic Acids. Chem. Mater. 2009, 21, 4155-4162. 20) Boykin, J. R.; Smith, L. J. Rapid Microwave-Assisted Grafting of Layered Perovskites with n–Alcohols. Inorg. Chem. 2015, 54, 4177-4179. 21) Akbarian-Tefaghi, S.; Veiga, T.; Amand, G.; Wiley, J. B. Rapid Topochemical Modification of Layered Perovskites via Microwave Reactions. Inorg. Chem. 2016, 55, 1604-1612. 22) Ruiz-Hitzky, E.; Rojo, J. M. Intracrystalline Grafting on Layer Silicic Acid. Nature 1980, 287, 28-30. 23) Suzuki, H.; Notsu, K.; Takeda, Y.; Sugimoto, W.; Sugahara, Y. Reactions of Alkoxyl Derivatives of a Layered Perovskite with Alcohols: Substitution Reactions on the Interlayer Surface of a Layered Perovskite. Chem., Mater. 2003, 15, 636-641. 24) Varadwaj, G. B. B.; Parida, K.; Nyamori, V. O. Transforming Inorganic Layered Montmorillonite into Inorganic–Organic Hybrid Materials for Various Applications. Inorg. Chem. Front. 2016, 3, 1100 -1111. 25) Liao, K. C.; Anwar, H.; Hill, I. G.; Vertelov, G. K.; Schwartz, J. Comparative Interface Metrics for Metal-Free Monolayer-Based Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2012, 4, 6735-6746. 26) Marcinko, S.; Fadeev, A. Y. Hydrolytic Stability of Organic Monolayers Supported on TiO2 and ZrO2. Langmuir 2004, 20, 2270-2273. 27) Fujita, M.; Idota, N.; Matsukawa, K.; Sugahara, Y. Preparation of Oleyl PhosphateModified TiO2/Poly (methyl methacrylate) Hybrid Thin Films for Investigation of Their Optical Properties. J. Nanomater. 2015, 2015, 297197. 28) Takahashi, S.; Hotta, S.; Watanabe, A.; Idota, N.; Matsukawa, K.; Sugahara, Y. Modification of TiO2 Nanoparticles with Oleyl Phosphate via Phase Transfer in the Toluene−Water System and Application of Modified Nanoparticles to Cyclo-OlefinPolymer-Based Organic−Inorganic Hybrid Films Exhibiting High Refractive Indices. ACS Appl. Mater. Interfaces 2017, 9, 1907-1912. 29) Iijima, M.; Tajima, S.; Yamazaki, M.; Kamiya, H. Redispersion Property of TiO2 Nanoparticles Modified with Oleyl-group. J. Soc. Powder Technol., Jpn. 2012, 49, 20-27. 30) Yang, J.; Lee, J. Y.; Ying, J. Y. Phase Transfer and its Applications in Nanotechnology. Chem. Soc. Rev. 2011, 40, 1672-1696. 31) Zhao, P.; Li, N.; Astruc, D. State of the Art in Gold Nanoparticle Synthesis. Coordin. Chem. Rev. 2013, 257, 638-665.


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Table of contents 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 to a cyclohexane phase containing oleyl phosphate (OP).


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Fig. 1 Photographs in a liquid-liquid biphasic system, consisting of a cyclohexane phase (upper) and an aqueous phase (lower). 84x125mm (300 x 300 DPI)


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Fig. 2 TEM images and ED patterns of OP_HLaNb nanosheets_x/cyclohexane dispersion, (a) pH 2, (b) pH 4 and (c) pH 6. 84x27mm (300 x 300 DPI)


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Fig. 3 IR spectra of OP, OP_HLaNb_interlayer, OP_HLaNb nanosheets_x (x=2, 4 or 6) and HLaNb, (a) OP, (b)OP_HLaNb nanosheets_2, (c) OP_HLaNb nanosheets_4, (d) OP_HLaNb nanosheets_6, (e) OP_HLaNb_interlayers and (f) HLaNb. IR spectrum of OP was measured by the liquid film method and those of the rest were measured by the KBr disk method. 94x89mm (600 x 600 DPI)


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Fig. 4 Solid-state 13C CP/MAS NMR spectra of OP_HLaNb_interlayer and OP_HLaNb nanosheets_x (x=2, 4 or 6), (a) OP_HLaNb nanosheets_2, (b) OP_HLaNb nanosheets_4, (c) OP_HLaNb nanosheets_6, (d) OP_HLaNb_interlayers and liquid-state 13C NMR spectrum of (e) OP (Assignments are shown as an inset.). 94x68mm (600 x 600 DPI)


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Fig. 5 Solid-state 31P MAS NMR spectra of OP_HLaNb_interlayer, OP_HLaNb nanosheets_x (x=2, 4 or 6), OP_HLaNb nanosheets_6_acetic acid and OP_TBA_no_HLaNb, (a) OP_HLaNb nanosheets_2, (b) OP_HLaNb nanosheets_4, (c) OP_HLaNb nanosheets_6, (d) OP_HLaNb_interlayers, (e) OP_HLaNb nanosheets_6_acetic acid (f) OP_TBA_no_HLaNb and liquid-state 31P NMR spectrum of (g) OP. 94x101mm (600 x 600 DPI)


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Fig. 6 Relations between phase transfer ratio and reaction times. 94x88mm (600 x 600 DPI)


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Fig. 7 AFM images of nanohseets obtained from the OP_HLaNb nanosheets_2/cyclohexane dispersion. 94x137mm (600 x 600 DPI)


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Fig. 8 Relations between modification ratio and reaction time. 94x85mm (600 x 600 DPI)


Surface Modification of Layered Perovskite Nanosheets with a

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