Hydrophobic Inorganic–Organic Composite Nanosheets Based on

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Hydrophobic Inorganic−Organic Composite Nanosheets Based on Monolayers of Transition Metal Oxides Masashi Honda, Yuya Oaki,* and Hiroaki Imai* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan S Supporting Information *

ABSTRACT: Hydrophobic inorganic−organic composite nanosheets based on manganese and titanium oxide monolayers were obtained in a nonpolar organic medium. In general, monolayered materials of transition metal oxides were prepared and dispersed in aqueous and polar organic media. Here we report on a simple and generalizable approach for syntheses of the hydrophobic composite nanosheets consisting of the transition metal−oxide monolayers and the surface hydrophobic organic layers. The composite nanosheets were dispersed in a nonpolar organic medium. The resultant composite nanosheets based on the manganese oxide show the improved catalytic activity for oxidation of an alcohol in toluene. The large blueshift of the bandgap energy was observed on the composite nanosheets based on the titanium oxide. The present approach can be applied to syntheses of hydrophobic composite nanosheets from a variety of layered compounds. The hydrophobic composite nanosheets have potentials for a wide range of applications based on the composite structures.

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INTRODUCTION A variety of inorganic−organic composite materials have been studied in recent years.1 Control of the low-dimensional nanostructures is an important factor for generation of functional composite materials.2 Here we report on syntheses and properties of a two-dimensional (2D) nanocomposite based on monolayers of transition metal oxides. The composite nanosheets are composed of the transition metal oxide monolayers and the surface organic layers. Therefore, the hydrophobic composite nanosheets can be dispersed in a nonpolar organic medium. The composite nanosheets showed the properties originating from the formation of the composite structure. Syntheses, assemblies, and properties of monolayered materials have been widely studied in recent years.3,4 Delamination of the layered compounds leads to the formation of monolayers several micrometers in lateral size and single nanometer in thickness. In general, monolayers of transition metal oxides are prepared and dispersed in aqueous and polar organic media.3,4 Our group has focused on structure tuning of monolayered materials, such as the lateral size.5 The monolayered nanodots of the transition metal oxides as the ultrathin tiny objects were synthesized in our previous work. The next challenge here is to achieve the dispersion of the transition metal oxide monolayers in nonpolar media. A typical method for delamination of the layered compounds into the monolayers is introduction of bulky organic ions such as tetraalkylammonium into the interlayer space. The hydrophilic monolayers were formed in the colloidal liquid of aqueous and © XXXX American Chemical Society

polar organic media. Syntheses and properties of the monolayer-based materials with the hydrophobic nature were not fully studied in previous reports. A couple of previous studies focused on the hydrophobic nanosheets based on monolayers.6−12 The silicon monolayers modified with an alkyl group were synthesized and dispersed in nonpolar organic media such as hexane.6 Layered double hydroxides (LDHs) intercalated with surfactants bearing a long alkyl chain were exfoliated in organic media.7 The surfacemodified monolayers of molybdenum and tungsten disulfides were directly prepared in the solution phase.8 However, these monolayers with the surface modification were not stable in nonpolar organic media. 7,8 The delamination and the simultaneous polymerization in the monomer media have been studied to form the hybrid materials.9−11 Recently, the exfoliation of the layered octosilicate was achieved in pentane with ultrasonication.12 However, hydrophobic composite nanosheets based on transition metal oxide monolayers have not been prepared in nonpolar organic media. In addition, the properties and the potential applications of the hydrophobic nanosheet materials were not fully proposed in previous works. In a previous paper, the nanocomposite materials of titanate monolayer and grapheme oxide were prepared in aqueous media.1n,4m If the hydrophobic composite nanosheets are obtained, a variety of composite materials with functional Received: April 11, 2014 Revised: May 9, 2014

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prepared with typically 200 cm3 of purified water in a polypropylene sample bottle. After these materials were dissolved, an equal volume of 200 mmol dm−3 NaOH aqueous solution was added into the solution containing MnCl2 and EDTA under stirring. The mixed solution was maintained at 25 °C for 5 days. The precipitates were collected by centrifugation and then dried at room temperature. The interlayer sodium cations were exchanged to alkyl amines (C n −NH2 , CnH2n+1NH2, n = 10, 14, 16, 18), such as decylamine (C10−NH2, C10H21NH2, TCI, 98.0%), tetradecylamine (C14−NH2, C14H29NH2, TCI, 95.0%), hexadecylamine (C16−NH2, C16H33NH2, TCI, 90.0%), and stearylamine (C18−NH2, C18H37NH2, TCI, 80.0%). The procedures of the ion-exchange were described in a previous report.14 The pH values of the aqueous solutions containing an excess amount of these alkyl amines were adjusted to 7.0 by addition of the concentrated and diluted hydrochloric acid (HCl). The corresponding ammonium salts (Cn−NH3+, n = 10, 14, 16, 18), typically 0.1−1 mol dm−3, were contained in the aqueous solution. The powder of the resultant Na−MnO2, typically 0.2−0.3 g, was dispersed into the solution containing the ammonium salts, such as Cn−NH3+ (n = 10, 14, 16, 18). The dispersion liquid was maintained at 25 °C for a day. The collected precipitates were washed by ethanol and purified water. In this way, the precursor composites of manganese oxide and Cn− NH3+ (Cn−MnO2, n = 10, 14, 16, 18) were obtained. Syntheses of the Composite Nanosheets through the Exfoliation of the Cn−MnO2. The resultant Cn−MnO2 composites, typically ca. 0.05 g, were dispersed in 20 cm3 of toluene (Kanto, 99.5%) at 60 °C for 5 days. After centrifugation at 6000 rpm for 30 min, the dispersion liquid as the supernatant and the precipitates at the bottom of the sample bottle were separately obtained. The precipitates were dried at room temperature. The dispersion liquid was kept without drying. Catalytic Activity for Oxidation of an Alcohol. The dispersion liquid of the hydrophobic composite nanosheets derived from C16− MnO2 was prepared by 10 cm3 of toluene. The methods were the same as mentioned above. The powder of the Na−MnO2 was dispersed in 10 cm3 of toluene. These dispersion liquids were prepared in a two-necked round flask. The concentration of manganese was set at 2.5 mmol in both cases. Then, 1 mmol of benzyl alcohol (Kanto, 99.0%) was dissolved in the toluene. The reaction was performed at 110 °C for 2 h in air under stirring. The filtrated toluene solutions were analyzed by gas chromatography (GC, Shimadzu, GC-2014) equipped with a capillary column (Shimadzu, ZB-WAX). Syntheses of the Precursor Composite Based on Titanium Oxide. The pristine layered compound of Cs−TiO2 was synthesized by a solid-state synthesis reported in a previous paper.19 The powders of rutile titanium dioxide (TiO2, Junsei, 99%) and cesium carbonate (CsCO3, Junsei, 99%) were mixed and calcined at 800 °C for 20 h. The molar ratio of TiO2 and CsCO3 was set at 5.2:1.0. The intercalated cesium ions were exchanged to protons by the acid treatment. The resultant Cs−TiO2, typically 5 g, was immersed in 500 cm3 of 1 mol dm−3 HCl at 25 °C for 3 days. The protonated titanate (H−TiO2) was washed by purified water. The interlayer proton was then exchanged to Cn−NH3+ (n = 10, 14, 16, 18) through the immersion in the aqueous solution containing (Cn−NH2, n = 10, 14, 16, 18) for 10 days. The pH of the aqueous solutions containing an excess amount of these alkyl amines were adjusted to 7.0 by using the concentrated and diluted HCl. The corresponding ammonium salts (Cn−NH3+, n = 10, 14, 16, 18), typically 0.1−1 mol dm−3, were contained in the aqueous solution. The powder of the resultant H− TiO2, typically 0.2−0.3 g, was dispersed into the solution containing the ammonium salts, such as Cn−NH3+ (n = 10, 14, 16, 18). The dispersion liquid was maintained at 25 °C for a day. The collected precipitates were washed by ethanol and purified water. However, the C10−NH3+ was not intercalated in the interlayer. The precursor composites of manganese oxide and Cn−NH3+ (Cn−TiO2, n = 14, 16, 18) were obtained. Syntheses of the Composite Nanosheets through the Exfoliation of the Cn−TiO2. The exfoliation procedure was the same as that of the Cn−MnO2. The resultant Cn−TiO2 composites, typically ca. 0.05 g, were dispersed in 20 cm3 of toluene (Kanto,

organic molecules can be synthesized. Our intention here is synthesis and application of hydrophobic composite nanosheets based on the monolayers of transition metal oxides dispersed in a nonpolar organic medium. Figure 1 shows the schematic illustration of an approach to obtain the hydrophobic composite nanosheets based on the

Figure 1. Schematic illustrations of the approach for synthesis of the hydrophobic composite nanosheets based on the monolayers. (a) The pristine layered compound with the interlayer space of d0; (b) the precursor composite with the intercalation of the long chain alkylammonium ions in the interlayer space; and (c) the delamination into toluene through the hydrophobic interaction and the formation of the composite nanosheets on a silicon substrate. The d(001) and t indicate the interlayer distance of the precursor composite and the thickness of the composite nanosheets, respectively.

monolayers. The interlayer cations of the pristine layered compounds are exchanged from the alkaline ions to long-chain alkyl ammonium ions (Figure 1a,b). The resultant precursor composites are dispersed in a nonpolar medium to induce the exfoliation through the hydrophobic interaction between the alkyl chain and the medium (Figure 1b,c). The composite nanosheets with the surface organic layers are obtained in the colloidal liquid and on a substrate (Figure 1c). In the present study, the composite nanosheets based on the monolayers of manganese and titanium oxides were synthesized (Figures 2 and 3). In addition, the potential applications and the properties were studied on the resultant composite nanosheets

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EXPERIMENTAL SECTION

All the reagents were used as purchased without purification. Syntheses of the Precursor Composite Based on Manganese Oxide. The birnessite-type sodium manganite (Na−MnO2) nanosheets were synthesized by the method reported in our previous work.13 A chelating agent as a ligand, namely, disodium dihydrogen ethylenediamine tetraacetate (EDTA), was used for the direct synthesis of the Na−MnO2. The presence of EDTA inhibited the precipitation of a manganese oxide, namely, Mn3O4, containing low oxidation state of manganese. An aqueous solution containing 20 mmol dm−3 of manganese chloride tetrahydrate (MnCl2·4H2O, Kanto Chemical, 99.0%) and 5.0 mmol dm−3 of EDTA (Kanto, 99.5%) was B

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99.5%) at 60 °C for 5 days. After centrifugation at 6000 rpm for 30 min, the dispersion liquid as the supernatant and the precipitates at the bottom of the sample bottle were separately obtained. The precipitates were dried at room temperature. The dispersion liquid was kept without drying. Characterization. The crystal structures were analyzed by X-ray diffraction (XRD, Rigaku Mini Flex II and Bruker D8 Advance with Cu Kα radiation). The morphologies of the resultant materials were observed by field-emission scanning electron microscopy (FESEM, FEI Sirion and Hitachi S-4700 operated at 5.0 kV), field-emission transmission electron microscopy (FETEM, FEI Tecnai F20 operated at 200 kV), and atomic force microscopy (AFM, Shimadzu, SPM9600). For FETEM observation, a copper grid supported by a collodion membrane was immersed in the dispersion liquid and then dried after withdrawing. The thickness of monolayers was analyzed by AFM. A silicon (Si) wafer was immersed in a mixed solution of HCl/ methanol (Junsei, CH3OH, 99.8%) (1:1 by volume) and then transferred to a concentrated H2SO4 solution for 30 min each. The cleaned Si substrate was immersed in the dispersion liquid and then dried at room temperature. The chemical formulas of the pristine Na− MnO2 and Cn−MnO2 composites were estimated by the energy dispersive X-ray analysis (EDX, Bruker, Quantax 1.7), thermogravimetry analysis (TG, Seiko, TG-DTA 7000), and titration experiments. The molar ratio of manganese in the layer and sodium in the interlayer was quantified by using EDX on the FESEM observations. The amounts of the manganese and the incorporated water were measured by TG. The mean valence of manganese in the birnessite-type manganate nanocrystals was determined by a redox titration. A specified amount of the manganese oxide (∼0.10 g) was dissolved in an aqueous solution containing sodium oxalate (Na2C2O4, Kanto, 99.5%) and sulfuric acid (Junsei, H2SO4, 95.0%) at 90 °C. All the manganese ions are reduced to the divalent state by Na2C2O4. Then, the excess amount of Na2C2O4 was titrated by an aqueous solution of potassium permanganate (KMnO4, Kanto, 99.3%). Bandgap Energy of the Composite Nanosheets Derived from C14−TiO2. The bandgap energies were estimated from the UV− vis spectroscopy (JASCO V-670). The powdered sample of the C14− TiO2 before the delamination was measured by a diffuse-reflectance mode with an integrating sphere. Magnesium oxide (MgO) was used as the reference. After the delamination, the composite nanosheets were deposited on a silica glass substrate. The spectrum was obtained by a transmittance mode.

Figure 2. XRD patterns (a) and the FESEM images (b−e) of the pristine Na0.33MnO2·xH2O (x < 0.62) (b,d) and the C16−MnO2 composite (c,e). The XRD patterns (i)−(iii) correspond to those of the pristine Na0.33MnO2·xH2O (x < 0.62), the C16−MnO2 composite, and the collected precipitates after the immersion in toluene, respectively.15

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RESULTS AND DISCUSSION Synthesis of the Precursor Composite Based on Manganese Oxide. The layered manganese oxide was synthesized by the chelation-mediated approach in an aqueous solution.13 The formation of layered compounds consisting of the manganate layers and the interlayer sodium ions, namely, birnessite-type sodium manganate, was confirmed by the XRD analysis (Figure 2a). The sodium manganate showed the sheetlike morphologies ca. 50 nm in thickness and less than 5 μm in width (Figure 2b,c). The chemical formula of the resultant material was estimated to be Na0.33MnO2·xH2O (x < 0.62) (Na−MnO2) (Figure S1 in the Supporting Information). The ion exchange of the interlayer sodium cations was performed in an aqueous solution containing alkyl amines (Cn−NH2; CnH2n+1NH2, n = 10, 14, 16, 18). The XRD patterns indicate the formation of the precursor composite (Cn−MnO2, n = 10, 14, 16, 18) through the intercalation of the corresponding alkyl ammonium cations (Cn−NH3+) (Figure 2a and Figure S2 and Table S1 in the Supporting Information). For example, the interlayer distance (d(001)) was increased from 0.713 to 3.09 nm after the intercalation of C16−NH3+ (Figure 2a). The chemical composition of the C16−MnO2 was estimated to (C16− NH3)0.13MnO2·xH2O (x < 0.53) (Figure S1 in the Supporting Information). According to previous reports,14 these alkyl

ammonium cations generally form the bilayer structures in the interlayer space (Figure 1b). The thickness of the platy morphologies was increased after the intercalation (Figure 2b− e). The similar changes of the XRD patterns and FESEM images were observed on the other Cn−MnO2 (n = 10, 14, 18) (Figure S2 and Table S1 in the Supporting Information). Synthesis of the Hydrophobic Composite Nanosheets Based on Manganese Oxide Monolayers. The precursor composites of Cn−MnO2 (n = 10, 14, 16, 18) were dispersed in toluene at 60 °C for 5 days under stirring. The composite nanosheets were formed through the delamination in the dispersion liquid and on substrates (Figure 3). The colloidal liquid exhibiting Tyndall light scattering was obtained from the C16−MnO2 (Figure 3a). When the precipitates were observed in the dispersion liquid after several days, the original dispersion liquid recovered with an ultrasonication. The colloidal liquid was dropped on a silicon wafer and a collodion membrane. The sheet-like objects with the thickness around 3.3 nm and lateral size around 200 nm were observed on the images of AFM and FETEM (Figure 3b−d). In previous reports, the thickness of the monolayered manganese oxide formed in the aqueous C

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nanosheets 2.5−3.5 nm in thickness were obtained from the other Cn−MnO2 composite (n = 10, 14, 18) (Figure S3 in the Supporting Information). The t and d(001) were linearly increased with an increase in the n in the Cn−NH3+ (Figure 3e). On the basis of these results, the hydrophobic composite nanosheets of the manganese oxide monolayers modified with the Cn−NH3+ were obtained by the present simple approach (Figure 1). The precursor composites of the Cn−MnO2 were formed by the intercalation of Cn−NH3+ with the chain length longer than n = 10. Catalytic Properties of the Hydrophobic Composite Nanosheets Based on Manganese Oxide Monolayers. Recent reports showed that the birnessite-type layered manganese oxides acted as a catalyst for selective oxidation of benzylic and allylic alcohols to the corresponding aldehydes in organic media.16−19 The tetravalent manganese is reduced to the trivalent state through the oxidation of the alcohol. Then, the reduced trivalent manganese is oxidized to the tetravalent one by oxygen in the presence of the reaction medium.19 In previous works, the catalytic activities were studied in the bulk powder states.18,19 In the current work, the resultant hydrophobic composite nanosheets were used as the catalyst for the selective oxidation reaction of benzyl alcohol (1) to benzaldehyde (2) in toluene at 110 °C for 2 h in air (Scheme 1). When the hydrophobic composite nanosheets derived from Scheme 1. Selective Oxidation from Benzyl Alcohol (1) to Benzaldehyde (2) by Using the Two Catalysts, Such as the Na−MnO2 and the Resultant Hydrophobic Composite Nanosheets Derived from the C16−MnO2 Figure 3. Formation of the composite nanosheets derived from the C16−MnO2 in toluene. (a) The dispersion liquid exhibiting Tyndall light scattering; (b,c) AFM image on a Si substrate and its height profile of the lines A−B and C−D; (d) FETEM image; (e) the schematic model of the composite nanosheet on a silicon substrate; (f) the relationship between the d(001) (triangles) and t (squares with the error bars) with an increase in the n in the Cn−MnO2 estimated from the XRD patterns and the AFM observations, respectively.

the C16−MnO2 were used as the catalyst, the aldehyde 2 was selectively obtained from the alcohol 1 in 62% yield. After the reaction stopped, 98% of the initial benzyl alcohol was reacted. However, the gas chromatography analysis indicated the presence of the byproducts. It is inferred that the excess C16−NH3+ adsorbed on the C16−MnO2 was desorbed and then directly reacted with the 1 and/or 2. In contrast, the bulk powder of Na−MnO2 without the exfoliation selectively provided the aldehyde 2 in 24% yield. After the reaction stopped, 29% of the initial benzyl alcohol was reacted. The active sites of the catalyst for the reaction are increased by the exfoliation. Since the alkyl chains are grafted on the surface with an appropriate density, both the hydrophobicity and the improved catalytic activity are achieved by the composite material. For example, based on the composition (C16− NH3)0.13MnO2·xH2O (x < 0.53), the graft density of C16− NH3+ can be illustrated as shown in Figure 1c. The flexible alkyl chains fluctuate in toluene. Therefore, the reactants can be approached to the surface of the MnO2 monolayer. The hydrophobic interaction between the alkyl chains and the reactants facilitates the approach to the catalyst surface. The present study implies that the hydrophobic nanosheets based on the monolayers have potentials for catalysts in nonpolar media. Synthesis of the Precursor Composite Based on Titanium Oxide. The similar approach was applied to

medium was estimated to be 0.52 nm.4j The difference in the thickness suggests that the surface of the anionic manganate monolayers was modified by the C16−NH3+ (Figure 3e). These results suggest that the composite nanosheets of the monolayers and the organic layers were obtained by the exfoliation in toluene. The dispersed objects in toluene were collected by a centrifugation. The XRD pattern of the collected precipitates was changed from that of the C16−MnO2 precursor composite (Figure 2a). The characteristic peaks originating from the layered structure in the lower angle than 2θ = 30° disappeared after the dispersion in toluene. The changes of the XRD pattern indicate that the laminated structures with the periodicity disappeared after the exfoliation with the immersion in toluene. The thickness of the composite nanosheets on the AFM image (t = 3.3 nm) was larger than the interlayer distance of the C16−MnO2 precursor composite based on the XRD pattern (d(001) = 3.09 nm) (Figure 3e,f). The difference between the t and d(001) is ascribed to the interdigitation of the alkyl chains in the interlayer space (Figure 1b). On the basis of the relationship between t and n in the Cn−NH3+, the tilted angle of the alkyl chains to the layer was estimated to be around 45° for the C16−MnO2 composite (Figure 3f). The graft density of C16−NH3+ was calculated to be 1.06 molecules on the MnO2 monolayer 1 nm2 in the area. The similar composite D

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than n = 10 and n = 14, respectively. In other words, C6−MnO2 and C10−TiO2 were not obtained. The electrostatic interaction between the layers and the interlayer ions form the layered structures. The intercalated compounds are not obtained by using the shorter alkyl chains because the interaction becomes weakened by the intercalation. In contrast, the intercalation proceeds with the alkyl chains longer than the critical lengths. The hydrophobic interaction between the intercalated alkyl chains facilitates the formation of the layered composite structure. The differences in the critical chain length are ascribed to the charge of the layers depending on the compounds. Synthesis of the Hydrophobic Composite Nanosheets Based on Titanium Oxide Monolayers. The precursor composite of the Cn−TiO2 was dispersed in toluene to induce the exfoliation. For example, the colloidal liquid exhibiting Tyndall light scattering was obtained after the dispersion of the C14−TiO2 in toluene (Figure 5a). The composite nanosheets around 1.5 nm in the thickness and the 0.5−1 μm in the width were generated after the dispersion (Figure 5b−e). In previous reports, the thickness of the monolayered titanium oxide formed in the aqueous medium was estimated to be 0.70 nm.4e−g The difference in the thickness indicates the formation of the composite nanosheets with the surface modification by C14−NH3+ (Figure 5e). The similar results were obtained on the other Cn−TiO2 composites (n = 16, 18) (Figure S4 in the Supporting Information). The t and d(010) were linearly increased with an increase in the n in the Cn−NH3+ (Figure 5f). These results suggest the formation of the composite nanosheets based on the titanate monolayers with the surface modification by Cn−NH3+ (Figure 5e). Bandgap Energy of the Hydrophobic Composite Nanosheets Based on Titanium Oxide Monolayers. The bandgap energy (Eg) of the composite nanosheet derived from the C14−TiO2 was measured by UV−vis spectroscopy (Figure 5g). The UV−vis spectrum was obtained from the composite nanosheets on a silica glass substrate. The Eg of resultant composite nanosheet showed 4.06 eV (the spectrum (i) in Figure 5g), whereas the Eg of the precursor layered materials was 3.42 eV (the spectrum (ii) in Figure 5g). The remarkable blueshift of the absorption edge was observed on the composite nanosheets. The effective mass (me*) of the composite nanosheets based on TiO2 monolayer was calculated to be me* = 0.936me as follows, where me is the mass of an electron (= 9.11 × 10−31 kg). The blue shift of the bandgap energy (ΔEg) was calculated by eq 1.21 The smaller terms were ignored.

synthesis of the composite nanosheets based on the titanium oxide monolayers. Cesium titanate (Cs−TiO2, CsxTi2‑x/4□x/4O4) as the pristine layered compound was prepared by a solid state synthesis.20 The precursor composite of the layered titanate and the Cn−NH3+ (Cn−TiO2, n = 14, 16, 18) was formed by the ion exchange to proton and the subsequent intercalation (Figure 4). In contrast, the inter-

Figure 4. XRD patterns (a) and the FESEM images (b−e) of the pristine Cs−TiO2 (b,c) and C14−TiO2 composite precursor (d,e). The XRD patterns (i)−(iv) correspond to those of the pristine Cs−TiO2, its protonated structure, the C14−TiO2 composite precursor, and the collected precipitates after the immersion in toluene, respectively. The filled circles on the peaks indicate the interlayer spacings corresponding to the (0k0) planes.

ΔEg =

calation was not achieved by the C10−NH3+. After the intercalation of C14−NH3+, the peak position of the interlayer spacing, namely, the (0k0) plane, was shifted to the lower angle region (Figure 4a). The interlayer spacing was changed from d(001) = 0.861 nm for the Cs−TiO2 to d(010) = 2.88 nm for the C14−TiO2. The thickness of the platy morphologies was increased after the intercalation (Figure 4b−e). After the dispersion in toluene, the collected precipitates showed no remarkable peak on the XRD pattern (the spectrum iv in Figure 4a), even though the weak peaks originating from the C14− TiO2 remained. The Cn−MnO2 and Cn−TiO2 were obtained by the intercalation of Cn−NH3+ with the alkyl chain length longer

h2 h2 + 4μxy Lxy 2 8μz Lz 2

(1)

The Lz of the nanosheet is much smaller than the Lxy. Therefore, the first term of eq 1 can be ignored in the present calculation. μz =

h2 8ΔEg Lz 2

(2)

The thickness of the titanate monolayer was set to Lz = 0.7 nm. The ΔEg was estimated to be 0.82 eV by the difference between the bulk protonated titanate and the composite nanosheet. E

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The blueshift of the Eg, namely, ΔEg = 0.64 eV, is ascribed to the quantum size effect originating from the delamination into the monolayered structures. In aqueous media, ΔEg = 0.60 eV was observed by the exfoliation from the protonated titanate (Eg = 3.24 eV) to the monolayers (Eg = 3.84 eV).4f,5 Based on these facts, the exfoliation and the grafting of the alkyl ammonium induce about ΔEg = 0.6 eV and ΔEg = 0.2 eV, respectively (Figure 6). Therefore, the widened Eg = 4.06 eV is

Figure 6. Schematic representation for the changes of Eg with the modification by C14−NH3+ (blue colors) and after the exfoliation in a medium (red colors).

ascribed to the formation of the composite nanosheet structure. The Eg of the composite nanosheets with the ΔEg = 0.88 eV to bulk anatase was the largest value in titanium-oxide related materials.5,22

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CONCLUSIONS The hydrophobic composite nanosheets based on the monolayers of transition metal oxides were synthesized in a nonpolar medium. The precursor composites of the layered inorganic compounds and the interlayer Cn−NH3+ were prepared through the intercalation. The dispersion of the precursor composites in toluene led to the generation of the composite nanosheets based on the monolayers with the surface organic layers, namely, Cn−NH3+. The hydrophobic interaction between the surface organic layer and nonpolar dispersion media facilitated the delamination into the composite nanosheets in nonpolar media. The present simple approach can be applied to a large-scale synthesis and a variety of other layered compounds. If the intercalated molecules are designed and prepared by organic synthesis, composite nanosheets with a variety of the designed functions can be generated by the present approach. The resultant composite nanosheets based on MnO2 monolayers showed the improved catalytic activity. The composite nanosheets based on the TiO2 monolayers showed the large blueshift of Eg, namely, Eg = 4.06 eV. The emergence of these properties is ascribed to the formation of the composite nanosheet structures. Development of the hydrophobic composite nanosheets has potentials for a variety of applications and emergence of unprecedented properties.

Figure 5. Formation of the composite nanosheets derived from the C14−TiO2 in toluene. (a) The dispersion liquid exhibiting Tyndall light scattering; (b,c) AFM image on a Si substrate and its height profile of the lines A−B and C−D; (d) FETEM image; (e) the schematic model of the composite nanosheet on a silicon substrate; (f) the relationship between the d(010) (triangles) and t (squares with the error bars) with an increase in the n in the Cn−TiO2 estimated from the XRD patterns and the AFM observations, respectively; and (g) UV−vis spectra and their Tauc plots of the composite nanosheet (i) and the precursor C14−TiO2 composite (ii).

μz =

(6.63 × 10−34 [J ·s])2 8 × (0.82 × 1.60 × 10−19 [J]) × (0.7 × 10−9 [m])2

= 8.525 × 10−31 [kg]

(3)

Therefore, the me* of the composite nanosheet was calculated to me* = 0.936me. The me* of the hydrophilic monolayers dispersed in aqueous media was reported to be me* = 1.28me. The me* of the hydrophilic monolayer was decrease from that of bulk anatase, namely, me* = 1.63me. The effective mass was remarkably decreased by the formation of the composite nanosheets.

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving detailed data, including XRD patterns, SEM images, AFM images, and TG analyses. This material is available free of charge via the Internet at http://pubs.acs.org. F

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.O.). *E-mail: [email protected] (H.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grant-in-Aid for Scientific Research (No. 22107010) on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (No. 2206) (H.I.) from the Ministry of Education, Culture, Sports, Science and Technology and for Young Scientist (A, No. 22685022) (Y.O.) and Challenging Exploratory Research (No. 24655199) (Y.O.) from Japan Society of the Promotion of Science.

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dx.doi.org/10.1021/cm5012982 | Chem. Mater. XXXX, XXX, XXX−XXX