Osmotic Swelling of Layered Compounds as a Route to Producing

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Osmotic Swelling of Layered Compounds as a Route to Produce HighQuality 2D Materials. A Comparative Study by Tetramethylammoniumvs Tetrabutylammonium-Cation in a Lepidocrocite-Type Titanate Tosapol Maluangnont, Kazuaki Matsuba, Fengxia Geng, Renzhi Ma, Yusuke Yamauchi, and Takayoshi Sasaki Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm401409s • Publication Date (Web): 27 Jun 2013 Downloaded from http://pubs.acs.org on July 19, 2013

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Chemistry of Materials

Osmotic Swelling of Layered Compounds as a Route to Produce High-Quality 2D Materials. A Comparative Study by Tetramethylammonium- vs Tetrabutylammonium-Cation in a Lepidocrocite-Type Titanate

Tosapol Maluangnont, Kazuaki Matsuba, Fengxia Geng, Renzhi Ma, Yusuke Yamauchi, and Takayoshi Sasaki*

International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

Email: [email protected]

1 ACS Paragon Plus Environment


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Abstract Osmotic swelling/exfoliation behaviors in a lepidocrocite-type titanate H1.07Ti1.73O4·H2O were investigated upon reactions with tetramethylammonium (TMA+) and tetrabutylammonium (TBA+) cations. The reaction products in various physical states (suspension, wet aggregate, and deposited nanosheets) were characterized by several techniques including X-ray diffraction under controlled humidity, small angle X-ray scattering, particle size analysis, and atomic force microscopy. As the ratio of tetraalkylammonium ion in a solution to exchangeable proton in a solid decreased, the predominant product changed from the osmotically swollen phase, having interlayer spacing d of several tens of nanometers, to the exfoliated nanosheets. The different behaviors of two cations in the osmotic swelling were evident from the slope and the transition point in d vs C-1/2 plot, where C is the concentration of the cations. At a short reaction time, crystallites of a few stacks were obtained as a major product in the reaction with TMA+. On the other hand, a mixture of those crystallites and a significant portion of unilamellar nanosheets were obtained in the reaction with TBA+. In both cases, those stacks were ultimately thinned down at long reaction time to unilamellar nanosheets. The lateral size of the nanosheets could be controlled, depending on the type of the cations, the tetraalkylammonium-to-proton ratios, and the mode of the reaction (manual vs mechanical shaking). The nanosheets produced by TMA+ had larger lateral sizes up to tens of micrometers, and the suspension showed a distinctive silky appearance based on liquid crystallinity. Our work provides insights into the fundamentals of osmotic swelling and exfoliation, enabling a better understanding of the preparation of nanosheets, which are one of the most important building blocks in nanoarchitectonics. Keyword: Swelling, exfoliation, titanate nanosheets, lepidocrocite, tetraalkylammonium cation


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Introduction Layered compounds comprising of two-dimensional (2D) host layers stacking along a certain crystallographic direction are of great interest from both fundamental and application points of view.1 These host layers as a building block are composed of atoms strongly bonded with each other within planes, but are held together weakly between planes. The insertion (intercalation) of guest species into the weaklyheld interlayer region results in an expansion along the stacking direction, together with a modification of the physical properties of the host structure.2 The intercalation of appropriate species sometimes promotes the simultaneous introduction of solvents (typically water), where the electrical double layers are developed on both sides of the charged nanosheets.3,4 The inclusion of a large volume of electrolyte solution results in expansion as large as several tens of nanometers. Ultimately, 2D building blocks are separated into individual layers in a process called exfoliation or delamination. Such molecularly thin nanosheets can possess lateral sizes in the µm-range whilst being only a few nm-thick, giving a large aspect ratio (lateral-to-thickness). The high 2D anisotropy gives nanosheets unique or enhanced properties.5-7 Nanosheets also serve as functional building blocks for the synthesis of artificial nanostructured materials with tailored functionalities.8-10 The synthesis of nanosheets through the osmotic swelling/exfoliation process11,12 has advantages over other routes (e.g., mechanical cleavage,13 sonication in appropriate solvent14) as it provides a colloidal suspension of unilamellar nanosheets of high quality in good yield. Osmotic swelling is different from short-range crystalline swelling; the latter refers to the accommodation of certain molecular layers of water (0.24-0.28 nm per layer)15 as known in e.g. layered double hydroxides (LDH),15 clays,16 or layered titanates.17, 18 On the other hand, for non-clay materials, only a few examples of the osmotically swollen phase have been investigated (LDH,19 titanate,12, 20, 21 birnessite-type manganese oxide22,23), in contrast to many examples on the exfoliation9 of layered inorganic materials. Tetraalkylammonium (TAA+) cations 3 ACS Paragon Plus Environment


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are widely employed

as a reagent for nanosheets synthesis

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via the intercalation/osmotic

swelling/exfoliation route, mostly in the form of their hydroxide e.g., TAAOH. Examples include the use of tetrabutylammonium (TBA+) cation in exfoliating layered titanate,11,

12

α-zirconium phosphate,24 or

layered perovskite phases.25, 26 Exfoliation by tetramethylammonium (TMA+) cation is also feasible as in layered cobalt oxide,27, 28 but has received less attention. To the best of our knowledge, only one case of a birnessite-type manganese oxide where the osmotic swelling/exfoliation by either TBA+ 22, 23 or TMA+ 29-31 was reported. We note here the need to gain adequate understanding of the similarities/differences in the exfoliation and swelling behavior between TMA+ vs TBA+. Such in-depth understanding will be of valuable help in controlled synthesis of nanosheets of high quality. This lack of information is in sharp contrast to the case of intercalation, where a series of TAA+-intercalated compounds with different alkyl chain lengths is known in various layered host structures.32-35 Titanate nanosheet derived from the reaction between TBA+ and the protonated lepidocrocite-type HxTi2-x/4□x/4O4·H2O (□, vacancy; x = 0.7) was one of the first examples of inorganic nanosheets in the field.12 This protonated material was obtained from the acid exchange of the parent compound CsxTi2x/4□x/4O4.

The crystal structure36 comprises edge-shared TiO6 octahedra that form 2D layers along the ac-

and stack along the b-direction of the orthorhombic cell (the inset in Figure S1a). The presence of vacancies at the Ti-site gives rise to the negative charges on the layer, which are balanced by the exchangeable cations residing between the layers. Starting from HxTi2-x/4□x/4O4·H2O, the macromolecularlike nature of such exfoliated nanosheets,11 the swelling/exfoliation process,12 the liquid crystalline phase of the colloidal suspensions,37 and the fabrication into various nanostructures such as films,38 flakes39 or hollow shells40,

41

from those nanosheets have been reported. In this work, we select an isomorphous

compound H4x/3Ti2-x/3O4·H2O (x = 0.8),18 which is a protonated form of KxTi2-x/3Lix/3O4. This composition has the advantage of a larger crystal size (5-15 µm, the inset in Figure S1b) than that in Ref.12 (~1 µm), allowing us to explore chemical behaviors and structural changes in depth. 4 ACS Paragon Plus Environment

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In this article, we report the spontaneous osmotic swelling/exfoliation of the lepidocrocite-type titanate upon interaction with TMA+ and TBA+. The different behaviors of TMA+ vs TBA+ were compared through a combination of techniques, probing the nature of the reacted materials at different length scales. The structures of the products were analyzed through small angle X-ray scattering (SAXS) from the suspensions, and X-ray diffraction (XRD) under controlled humidity from the sediments after centrifugation. Some properties of the colloidal suspensions, including the size of the particles and the liquid crystalline nature, were also investigated. Insights into the kinetics of the reaction were disclosed by analyzing the dimensions of nanosheets using atomic force microscopy (AFM), as obtained from several images over a large area of a substrate on which the nanosheets were deposited. Experimental Section Synthesis of the Titanate. A potassium titanate KxTi2-x/3Lix/3O4 (x = 0.8) was produced by conventional solid state synthesis from an intimate mixture of K2CO3, Li2CO3, and TiO2.18 The protonated form H4x/3Ti2-x/3O4·H2O (x = 0.8) was derived through the repeated ion exchange (3 cycles) of the potassium form with 1 M HCl.18 The XRD patterns of the potassium- and protonated-form (Figure S1), including the respective unit cell parameters, agree with those reported previously.18 Osmotic Swelling and Exfoliation. A weighed amount (0.4 g) of the protonated form was equilibrated at room temperature with an aqueous solution (100 mL) of TMAOH [(CH3)4NOH] or TBAOH [(C4H9)4NOH]. The amount of TAAOH is expressed as the mole ratio of TAA+ over that of H+ available from the weighed amount of the titanate. This falls in the range TMA+/H+ = 0.5-50 and TBA+/H+ = 0.5-15, using commercially available reagents of 15% TMAOH and 10% TBAOH (Wako Chemicals). Milli-Q filtered water (Millipore Co., ρ > 15 MΩ·cm) was used throughout. The reaction mixture was either mechanically shaken (180 rpm) for 7 days, or manually shaken (2×10 strokes a day) for up to 18 weeks.



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Apparatus. XRD measurements were performed with a Rigaku Rint 2200HF diffractometer with a Cu Kα radiation at 2-30° 2θ (scan speed 1°/min). The colloidal suspension was centrifuged at a speed of 15,000 rpm for 30 min. The recovered sediment in wet state was placed on a horizontal sample stage in a chamber where the relative humidity was controlled at 95%. From different trials using the suspension at the same TAA+/H+, we find the standard deviation in the interlayer spacing, d, by XRD to be within 1 nm. The loosely-stacked nature of the specimen might be the intrinsic reason for this fluctuation, as judged from broad peaks observed for almost all aggregates except those at very high TAA+/H+. Another source of errors could come from sample handlings, especially the flatness of the wet sample on the XRD holder, which cannot be controlled precisely from specimen to specimen. Additionally, diffraction peaks at low 2θ are known to intrinsically contain large errors. SAXS measurements were performed on a Rigaku NANO-Viewer with Cu Kα radiation, covering the range 0.08 < Q/ nm-1 < 3, where Q = 4π sinθ/λ (λ =0.154 nm) is the magnitude of the scattering vector. Approximately 2 drops of the suspension were transferred to the holder, which was subsequently sealed with Scotch tape. The holder was allowed to rest for ~0.5 h after loading to remove the effect of injections. The typical exposure time per sample is 2 h unless indicated otherwise. The standard deviation in d by SAXS from specimen-to-specimen was found to be ~0.2 nm. AFM images of the nanosheets deposited on a Si substrate were taken with an SPA-400 system (SPA400, Seiko Instruments Inc.) in noncontact mode using Si probes with a force constant of 20 N·m–1. The nanosheet suspension obtained via mechanical shaking was diluted to a concentration of 0.08 g·L-1 before pH adjustment to 9. Nanosheets obtained via manual shaking had larger lateral sizes, requiring a reduction of the concentration to 0.004 g·L-1 to enable easy detection of individual, non-overlapping nanosheets. The nanosheets were deposited onto a polyethylenimine (PEI)-coated substrate as described previously.37 Ten images (usually for the area of 20×20 µm2) per one specimen of TAA+/H+ were taken, 6 ACS Paragon Plus Environment

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and the dimensions of the nanosheets including the thickness h, length l, and width w were analyzed to give the averaged value , and , respectively. Due to the irregular shape of the nanosheets, however, the latter two should be considered the upper limit. The total number of nanosheets counted N was 14-40 per specimen. Particle size analysis of the suspensions was performed using the instrument of Photal Otsuka Electronics, model ELSZ-2, capable of measuring the size in the range of 0.6 nm-7 µm. Values reported here were from three trials, after the reproducibility of both the average value and the distribution of the particle size was attained. Observation under polarized optical microscope was carried out using an Olympus BX51 system.

Results and Discussions XRD under 95% relative humidity on colloidal sediments recovered by centrifugation. The XRD patterns of the products after manual shaking for 3 weeks at various TMA+/H+ molar ratios are shown in Figure 1a. The colloidal sediment was obtained after high-speed centrifugation, and XRD measurements were performed under 95% humidity to suppress the drying during data acquisition. At low TMA+/H+ (0.54), a broad halo was observed, occasionally with weak peaks. The broad halo indicates the lack of the long range order. Its profile was previously shown to resemble the square of the structure factor of the titanate layer in the lepidocrocite-like structure.12 These findings suggest the delamination of the layered crystals into individual layers. At high TMA+/H+ ratios (5-50), a series of peaks can be observed, where the peak positions progressively shifted toward a smaller angular range as the TMA+/H+ ratio decreased. The spacing at each peak can be indexed to 0k0 where k is an integer, shown on the top of each peak. The resulting intersheet spacing d was 9.2, 5.8, 4.8, 4.0 and 2.6 nm at TMA+/H+ = 5, 10, 15, 20 and 50 7 ACS Paragon Plus Environment


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respectively. Compared to the intersheet spacing in the starting H1.07Ti1.73O4·H2O (d ~ 0.92 nm),18 this is equal to an expansion of ~3-10 times. The large separation cannot be accounted for by the mere inclusion of TMA+ (diameter ~0.44 nm).31 Instead, it is better ascribed to the osmotic swelling phase with a large separation between the sheets due to the inclusion of a hydrated structure. The sharpness of diffraction peaks at high TMA+/H+ ratios indicates the well-ordered structure along the stacking direction in the osmotically swollen phase. The peaks became broader as TMA+/H+ ratios decreased, suggesting the decrease in the coherent length to very limited number of stacked nanosheets.42 Eventually, there was almost no flat region at TMA+/H+ = 5. Such the oscillating profile without a flat region between the peaks was previously shown11 to originate from a pairwise association of nanosheets. This aspect clearly shows that the delamination process proceeds through the cleavage of the lamellar crystallites into thinner ones, along with the enlargement of the interlayer separation. Similar results were obtained when the titanate was mechanically shaken with TMAOH for 7 days (Table 1 and Figure S3a).


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Figure 1.


XRD patterns of the aggregate after centrifugation, from the manual shaking of

H1.07Ti1.73O4·H2O with (a) TMA+ and (b) TBA+ at TAA+/H+ molar ratios indicated. The numerals above the peak are the order of reflection. The traces have been offset along the Y-axis for clarity. The peak with * is at 1.71 nm, presumably the intercalation of TMA+ with a bilayer of water molecules (Figure S2).



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Table 1. The comparison of the interlayer spacing d (in nm) as obtained by XRD and SAXS, for H1.07Ti1.73O4·H2O reacting through the manual shaking or mechanical shaking route at various TAA+/H+ molar ratios. Cation TAA+/H+

Manual shaking Mechanical shaking XRD SAXS XRD SAXS TMA+ 0.5 Not observed Not observed Not observed Not observed 1 Not observed Not observed Not observed Not observed 2 Not observed 22.8 Not observed Not observed 3 Not observed 16.0 Not observed 16.6 4 Not observed 12.7 Not observed 13.5 5 9.3 10.7 8.4 10.9 10 5.8 6.5 6.3 6.5 15 4.8 5.0 4.9 5.2 20 4.0 4.1 4.0 4.1 50 2.6 2.6 2.7 2.6 TBA+ 0.5 Not observed 58.4a Not observed Not observed 1 Not observed 35.0a Not observed Not observed a 2 15.9 21.0 Not observed Not observed 3 Not observed 13.9a Not observed Not observed 4 Not observed 11.7 Not observed Not observed 5 7.5 10.0 8.1 Not observed 10 5.9 5.9 5.3 5.6 15 4.4 4.4 4.6 4.4 a Exposure time 12 h. Other values for SAXS were from the exposure time of 2 h.

The XRD patterns of H1.07Ti1.73O4·H2O manually reacted with TBA+ are shown in Figure 1b. The reaction sequence of TBA+, which proceeds from osmotic swelling to exfoliation as TAA+/H+ decreases, is in qualitative agreement to that with TMA+ mentioned above as well as our previous results of HxTi2x/4□x/4O4·H2O

(x = 0.7) with TBA+.12, 20

For both types of cations, the d values obtained by XRD from the manual shaking agreed reasonably with those from the mechanical shaking (Table 1 and Figure S3b). Additionally, we found that exfoliation by tetraethylammonium (TEA+) and tetrapropylammonium (TPA+) cations, whose sizes are between TMA+ and TBA+, was possible as well (unpublished). 10 ACS Paragon Plus Environment

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SAXS data on the colloidal suspensions. Figure 2a shows the scattering profiles of samples prepared via a manual shaking route with TMAOH, as the plot of log (IQ2 ) vs log Q, where I is the scattering intensity, Q is the magnitude of the scattering vector, and d = 2π/Q. The use of SAXS enables us to observe d values larger than those possible by normal XRD instruments, such as those at TMA+/H+ = 2-4. The spacing at each peak can also be indexed as 0k0 where

k is an integer. The d values ranged from 2.6

nm (TMA+/H+ = 50) to 22.8 nm (TMA+/H+ = 2) (Table 1). This is equal to an expansion by 3-25 times from the spacing in H1.07Ti1.73O4·H2O. Similar results were obtained when the titanate was mechanically shaken with TMAOH for 7 days (Figure S4a and Table 1). From the suspension manually shaken at TBA+/H+ = 0.5, a d value as large as 58 nm was detected (Figure 2b). The ordered structures with large d values were not easily observed through mechanical shaking as shown in Figure S4b.



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Figure 2. SAXS profiles shown as log (IQ2) vs log Q, as obtained from the suspension manually shaken with (a) TMA+, and (b) TBA+ at TAA+/H+ molar ratios indicated. The numerals above the peak are the order of reflection. The traces have been offset along the Y-axis for clarity.

Regardless of the mode of the reaction, the peaks became progressively broader as the TAA+/H+ ratios decreased, indicating a decrease in the coherent length along the stacking direction. Such a change in broadness of the peaks as a function of TAA+/H+ as observed by SAXS was qualitatively similar to that by XRD in the previous section. The d values by two analytical techniques (XRD vs SAXS) were in good agreement (Table 1). The difference between the two techniques was small at large TAA+/H+, and vice versa. The spacing of the osmotically swollen phase, which had already sedimented to the bottom of the 12 ACS Paragon Plus Environment

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flask, was insensitive to the centrifugation required by XRD measurements. On the other hand, such treatment which forces the exfoliated nanosheets to precipitate should have a stronger effect at low TAA+/H+. The effect of the forced centrifugation was absent in SAXS measurements, as the suspensions were analyzed without any pretreatment. Osmotic swelling and exfoliation by TMA+ vs TBA+. It has been reported that the degree of osmotic swelling is inversely proportional to the square root of electrolyte concentrations C (in mol·L-1 or molarity M)3, 12, 22, 43-45 Figure 3a is the plot of d vs C-1/2, showing the two linear relationships for both TMA+ and TBA+. The d values were taken from SAXS measurements, which had the advantage of being free from artifacts created by centrifugation. The osmotic swelling can be divided into two regions, with a change in slope (i.e., a transition) at TMA+/H+ ~ 6 and TBA+/H+ ~ 3. The genuine (equilibrium) osmotic swelling predominated at high TAA+/H+,12, 20, 22 resulting in phase separation to the bottom of the flask as will be shown in the photographs later. On the other hand, data at low TAA+/H+ might represent osmotic swelling through restacking over time, or the formation of liquid crystalline structure. The former might explain the observation of the spacing by SAXS (TBA+/H+ = 0.5, 1) at the exposure time of 12 h, but not at 2 h.46



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Figure 3.

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(a) d vs C-1/2. The numerals above each datum represent the ratio of TAA+/H+. Equations for

each plot at different sides of the transitions (denoted by a dashed line) are also shown. Note that each line was plotted on different scales on the Y-axis. (b) d-2 vs C, with the equations of each plot and R2 shown; and (c) the Debye screening length (κ-1, in nm) vs C. The inset in (b) and (c) are the zoom-in of the region


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at low C. The d values and other corresponding values were from SAXS measurements on the manuallyshaken samples.

So far, the effects of interlayer cations on the osmotic swelling have mostly been examined on clay minerals.4,

43-45, 47, 48

In this work, we compare different behaviors of TMA+ vs TBA+ in the

swelling/exfoliation in H1.07Ti1.73O4·H2O. The rates of change at high (low) TAA+/H+ were ~4 nm·M1/2 (7 nm·M1/2) for TMA+, and 5 nm·M1/2 (9 nm·M1/2) for TBA+. The transition points were also different (TMA+/H+ ~ 6 and TBA+/H+ ~ 3). We propose two explanations for these observations.49 In Explanation I, the slope might represent the change in the structure of the osmotically swollen phase, which in turn reflects either the structure of the electrical double layers, or the number of water molecules included between the negatively-charged titanate and the positively-charged TAA+. At high TAA+/H+ in the region of the genuine osmotic swelling, we note that the ratio of the slope for TBA+/TMA+ is 4.97/3.53 = 1.41, in good agreement with the hydrodynamic radius ratio50 of TBA+/TMA+ = 0.51/0.35 = 1.46. The agreement is not as good at low TAA+/H+ where a slope ratio is 1.18 (= 8.76/7.43), likely because the diffraction features in this range were from the restacked nanosheets. For both cases, the agreement is striking considering that water molecules, not TAA+, constitute a major part of the osmotically swollen phase. We speculate that this might suggest a structure-directing effect of the cation to some extent,4, 51, 52 especially at high TAA+/H+. In Explanation II, we consider an establishment of equilibrium concentration of water/TAA+ clusters between the external solution and the interlayer region inside the crystals. Recent work in our group53 on the osmotic swelling of a lepidocrocite titanate of the composition H0.8Ti1.2Fe0.8O4·H2O with 2(dimethylamino)ethanol (DMAE) has shown that there exists a concentration gradient of DMAE between those two regions. It is found53 that the plot of the equilibrated DMAE concentration (either external or internal) vs the starting concentration gives a linear relationship with different slopes. The internal 15 ACS Paragon Plus Environment


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concentration of DMAE is higher than that of the external until the transition at DMAE/H+ ~ 10, beyond which the trend is reversed. Based on these findings,53 we suggest that at low TAA+/H+ ratios, the majority of TAA+ cations could intercalate into the interlayer region, resulting in a fast-changing slope which is likely proportional to a TAA+ global concentration. On the other hand, the local concentration of TAA+ cations between the sheets could be substantially constant due to saturation at high TAA+/H+ ratios, resulting in a slow-changing slope when the global concentration of TAA+ cations is used in the calculation. The stoichiometry at the transition (TMA+/H+ ~ 6 and TBA+/H+ ~ 3) deserves further discussion. Garrett and Walker44 have proposed the formation of an “iceberg”, which is an ordered structure of water acting as a host and the n-butylammonium cations acting as a guest, to explain a discontinuity (i.e., the transition point) in the d vs C-1/2 plot in the swelling of vermiculite. Following this explanation,44 the likelihood that some partial arrangement of water would form especially at high TAA+/H+, is very likely dependent on the type of TAA+ cations, which is along the line of the Explanation I proposed above. Alternatively, the formation of an iceberg should depend on the relative concentration of TAA+ cations and water, as a result of the establishment of the equilibrium between the local and global concentration as well (Explanation II). The transition at TBA+/H+ ~ 3 for H1.07Ti1.73O4·H2O in this work can be compared to that at TBA+/H+ ~ 5 for H0.70Ti1.825□0.175O4·H2O.12 Such difference can be explained taking into account the number of exchangeable protons available. In the former, the number of TBA+ is 3×1.07 = 3.21 equivalent, while for the latter it is 5×0.70 = 3.50 equivalent. The two values are reasonably close to each other. The different d values at TBA+/H+ = 10 (~5.9 nm in H1.07Ti1.73O4·H2O vs 7.0 nm in H0.70Ti1.825□0.175O4·H2O) can also be explained. To obtain the same TBA+/H+ in the reaction mixture, the high charge density by H1.07Ti1.73O4·H2O requires a high concentration C of TBA+, resulting in a small d (as d is proportional to C1/2

).


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Recently, the exfoliation of H1.07Ti1.73O4·H2O by TBA+ has been reported by Besselink et al.54 Although qualitatively similar, the d values in the two works differ significantly (e.g., at TBA+/H+ = 2, d ~ 21 nm and ~11 nm in our work and in Ref.54, respectively). This may be explained based on the smaller solid-to-solution ratio in our work (4 g·L-1), resulting in a lower absolute concentration of TBA+ in the system. Following the linear relationship of d vs C-1/2, we then observed a larger d compared to the work of Besselink et al.54 who employed the solid-to-solution ratio of 5 g·L-1. We also carried out equilibration at 2 g·L-1, where we observed higher degree of swelling compared with the case at 4 g·L-1.55 Figure 3b shows the plot of d-2 vs C, which gives a straight line similar to the previous report.54 In this plot, a complete exfoliation can be defined at the concentration where d-2 = 0 nm-2. Using the fitting linear equations, this condition corresponds to the point where C = 0.045 M (TMA+) and 0.035 M (TBA+). At the present experimental conditions, this is equivalent to TMA+/H+ = 1.75, and TBA+/H+ = 1.34. These values are in the expected range based on XRD and SAXS results, which show a featureless pattern at TAA+/H+ = 1 and broad, low intensity peaks at TAA+/H+ = 2. The data for both sets did not cross the Xaxis at C = 0. Besselink et al.54 reported a similar finding in their plot with TBA+, and it was suggested that the interlayer distance was not solely determined by the electrical double layer. Alternatively, we feel that the d-2 vs C plot at low C values can be inaccurate from both d and C. The former inaccuracy is due to the restacking of nanosheets giving broad XRD patterns, while the latter is due to low concentration of TAA+ required during the reaction. We note from Figure 3a that at TMA+/H+ = 1.75 and TBA+/H+ = 1.34, the experimental d values were roughly 30 nm, giving d-2 = 0.001 nm-2. Therefore, the crossing at X-axis could be more or less reflecting the accuracy of the linear fitting, as is partly evidenced from the inset in Figure 3b. Figure 3c shows the Debye screening length (κ-1, in nm) vs C, calculated from the relation κ-1 = d/4 based on the electrostatic attraction theory.3, 47, 56 κ-1 can be considered as a thickness of the electrical 17 ACS Paragon Plus Environment


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double layer, and was in the range of 0.7-5.7 nm for TMA+/H+ = 50 to 2, and 1.1-14.8 nm for TBA+/H+ = 15 to 0.5. The increase of κ-1 as the concentration of the electrolyte (which is proportional to TAA+/H+) decreases is as expected. However, the larger κ-1 by TMA+ as compared to that by TBA+ is in contrast to theoretical calculations,57-59 which suggest that a small cation should show a smaller κ-1. Yet, recent experimental work60 with a surface-force measurement has reported the Debye screening length of 3.8 and 1.0 nm, for ionic liquids (the diameter of the cation in nm in the parentheses) of imidazolium (0.9 nm) or phosphonium (2.4 nm) type. Also, limited results by Williams et al. (Figure 3 in ref. 48) showed that at the same electrolyte concentration, d values of swollen n-propyl-vermiculite were larger than those of swollen n-butyl-vermiculite; these are in agreement with our results where larger d values are observed with cations of smaller size. Note that the magnitude of almost all values of κ-1 in our work is in the same order of magnitude to that in ref. 60. The discrepancy from the theoretical calculations vs the experimental works warrants further investigations. Particle size analysis. Figure 4 shows the “size”

61

of particles in the suspension as a function of

TAA+/H+. By mechanical shaking with TBA+, the “particle size” was ~0.2-0.4 µm (TBA+/H+ = 0.5-5), then went up to ~3 and 7 µm for TBA+/H+ = 10 and 15, respectively. In contrast, regardless of the TMA+/H+ ratios, the “particle size” was ~8 µm. This value is at the upper limit of the instrument and indicates that the size of the nanosheets could be larger. This technique has been previously employed62 in the study on the exfoliation of HCa2Nb3O10, where it was found that the “sizes” of particles ultrasonicated and stirred with TMA+ were larger than that with TBA+. Additionally, for the manually-shaken suspensions, the “particle sizes” for both cations fell in the range of ~8 µm.


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Figure 4.

“Particle size” in µm as a function of TAA+/H+ ratio, from the suspension prepared through

mechanical shaking with either TMA+ or TBA+. The error bar indicates ± one standard deviation.

Shown in Figure 5 are representative photographs of the suspension after mechanical shaking with TAAOH. A white suspension with bluish-tint as shown in Figure 5a and inset (TMA+/H+ = 0.5 and 1 and TBA+/H+ = 0.5, 1 and 2), was obtained at low TAA+/H+ where exfoliation dominated. This suspension was stable for at least 6 months. In contrast, at larger TAA+/H+ ratios (TMA+/H+ ≥ 5, and TBA+/H+ ≥ 10), a white suspension did not show the bluish tint. This may be a result of the intense light scattering associated with large sized dispersed objects. The separation into liquid and the precipitates (Figure 5b and 5c) was noticeable upon being left standing for a day from the suspensions at large TAA+/H+ (both TMA+ and TBA+). Combining results from the “particle size” analyses with those from the visual observation of the suspensions, it can be inferred that the colloidal dispersion stability of exfoliated nanosheets produced with TBA+ increased as the “particle size” decreased. On the other hand, it looked as if the colloidal dispersion stability of the exfoliated nanosheets produced with TMA+ was independent of the “particle size”. However, this finding could be inconclusive considering that the “particle size” of ~8 µm is at the upper limit of the instrument. The presence of precipitate in the flask corresponded well to the osmotically swollen phase as detected by both XRD and SAXS. The suspension mechanically shaken at TMA+/H+ = 0.5 exhibited a pronounced anisotropic stream as shown in Figure 5d. Such distinctive silky appearance is 19 ACS Paragon Plus Environment


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suggestive of the liquid crystallinity, which is reported in details in the next section. In general, the appearance of the suspensions greatly depended on the TAA+/H+ ratio, and was less affected by the mode of the reaction.

Figure 5.

Appearance of the reaction mixture prepared via a mechanical shaking route. (a) low

TAA+/H+ where the suspension had a bluish-tint and was stable for at least 6 months; (b) high TAA+/H+ immediately after the shaking; (c) high TAA+/H+ after left standing for a day; and (d) the suspension (e.g., TMA+/H+ = 0.5) where the anisotropic stream is greatly enhanced. The inset shows several drops of the suspension from (a) and (b) on the black plate, highlighting the bluish-tint found in (a) but not in (b).

Observation under polarized optical microscope. Figure 6 shows representative images from several drops of the suspension at TAA+/H+ = 0.5, 2 and 15, recorded with the polarized optical microscope. The formation of the nematic phase was evident from Schlieren textures in Figure 6a for the suspension mechanically shaken at TMA+/H+ = 0.5. This finding indicates the liquid crystalline behavior of the nanosheet colloids, where the exfoliated inorganic layers are not isotropically distributed but orientationally ordered to some extent. The proportion of such textures decreased as the TMA+/H+ increased.


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Figure 6.

Microscopic images of the nanosheet suspensions under cross polarizers at TAA+/H+ = 0.5

(left panel), 2 (middle panel) and 15 (right panel): (a) TMA+, mechanical shaking; (b) TMA+, manual shaking; (c) TBA+, mechanical shaking; and (d) TBA+, manual shaking. The scale bars are 50 µm.

In contrast to the case of TMA+ mentioned above, the Schlieren textures were not observed for the mechanically shaken suspensions at TBA+/H+ = 0.5 and 2 (Figure 6c), suggesting they were isotropic. The presence (absence) of such textures for nanosheets from the suspensions mechanically shaken with TMA+ (TBA+) can be understood by a large (small) lateral size of the nanosheets. In this work, the nominal concentration of the nanosheets was 4 g·L-1. The liquid crystalline nature of inorganic nanosheets depends on the aspect ratio of the nanosheets at a fixed concentration,63 where a higher concentration is required for smaller lateral size nanosheets to observe the liquid crystal nature.



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The mode of reaction affects greatly the liquid crystallinity of the nanosheets. At TBA+/H+ = 0.5 and 2, the nanosheet suspensions prepared manually showed liquid crystallinity (Figure 6d), whereas those prepared mechanically did not (Figure 6c). This can also be explained by the difference in the lateral sizes of nanosheets by two modes of the reaction. Fracturing is reduced by a gentle manual shaking, resulting in nanosheets with larger lateral sizes and consequently the presence of liquid crystallinity. That is, for exfoliated nanosheets prepared with TBA+, the use of manual shaking instead of mechanical shaking promotes liquid crystallinity. Our results at TBA+/H+ = 0.5 and 2 are in agreement with those by Nakato et al.37 who studied the liquid crystallinity of the suspension prepared by manual shaking of H1.07Ti1.73O4·H2O at TBA+/H+ ~4. Interestingly, at TMA+/H+ = 0.5, the suspension prepared by the mechanical shaking showed sharp Schlieren textures (Figure 6a), while that by the manual shaking showed limited liquid crystallinity together with solid crystallites (Figure 6b). At the same TAA+/H+, there was a higher proportion of solid crystallites for TMA+ than there was with TBA+ (compare Figure 6b with 6d). We suggest that this difference came from the slower reaction rate of TMA+. The solid crystallites observed can simply be starting crystals not fully reacted with TMA+. Our claim on the slow reaction rate with TMA+ is supported by statistical AFM analysis in the next section. At TAA+/H+ = 15 and regardless of the type of TAA+, the liquid crystallinity was just faintly observed for the samples prepared by mechanical shaking. Yet for the manual shaking route, mostly small crystallites (~5-15 µm) were found, the size of which were in agreement with the particle size of the protonated lepidocrocite crystals (the inset in Figure S1b). They should be osmotically-swollen crystals, i.e. the precipitate in the flask. Statistical AFM analysis. Figure 7 shows representative AFM images of unilamellar nanosheets. Regardless of the mode of the reaction, nanosheets prepared by TMA+ possessed larger lateral sizes than those by TBA+, which is consistent with the results from particle size analysis and optical microscopy 22 ACS Paragon Plus Environment

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reported in the previous sections; compare Figure 7a with 7b (for manual shaking), and 7c with 7d (for mechanical shaking). Obtaining nanosheets with larger lateral sizes through manual shaking is understandable, considering that manual shaking is milder than mechanical shaking. The effect of the type of TAA+ toward the lateral sizes of the nanosheets is more striking. In order to gain insight, we examined the dimensions of the nanosheets as a function of the reaction time. The analyses were performed using the manual-shaking suspension, as its slow reaction rate (compared with the mechanical-shaking suspension) enabled a collection of more accurate snapshots of the reaction products. Only reactions at low TAA+/H+ = 0.5 are presented here, with more details in Table S1 and Figure S7-S8.

Figure 7.

Representative AFM images of the unilamellar nanosheets at TAA+/H+ = 0.5: (a) TMA+,

manual shaking; (b) TBA+, manual shaking; (c) TMA+, mechanical shaking; and (d) TBA+, mechanical shaking. Images (a) and (b) are from 18-week manual shaking suspensions, whereas images (c) and (d) are from 1-week mechanical shaking suspensions. Note the different scales.

At the manual shaking time of 2 weeks, the different behaviors between TMA+ vs TBA+ are very clear, as shown by the distribution in the thickness h of the nanosheets shown in Figure 8a. Around 40% of the exfoliated products produced by the reaction with TMA+ was thicker than 10 nm. In contrast, a similar 23 ACS Paragon Plus Environment


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proportion of those with TBA+ was less than 2 nm-thick. On average, at 2 weeks was 8±5 and 6±6 nm for nanosheets prepared with TMA+ and TBA+, respectively. The large standard deviation is not uncommon in the field,64-67 likely reflecting the intrinsic inhomogeneity of fracturing especially at a short reaction time.

Figure 8.

Histograms of the dimensions of nanosheets prepared from TMA+ and TBA+ at different

reaction times by manual shaking at TAA+/H+ = 0.5: (a) h, 2 weeks; (b) h, 18 weeks; (c) l, 2 weeks; and (d)

l, 18 weeks. Inset in (b) and (d) shows the evolution of and , respectively, as a function of the manual shaking time. The error bar indicates ±one standard deviation.

Figure 8b shows that the distribution in the thickness of the nanosheets by two cations at 18 weeks became similar, giving = 1.3±0.3 nm (TMA+) and 1.2±0.3 nm (TBA+), indicating the formation of unilamellar crystallites. Although the crystallographic thickness along the b direction of the 2D 24 ACS Paragon Plus Environment

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lepidocrocite layer is ~0.75 nm,67,

68

the reported thickness of a unilamellar nanosheet is ~1.3-1.4 nm

probably due to the adsorption of water molecules and/or TAA+ on its surface.38, 67 As shown in the inset in Figure 8b, nanosheets became thinner as the reaction time increased. At 4 weeks, nanosheets from the reaction with TBA+ contained 1-2 layer stacks, whereas those from TMA+ contained 2-4 layers. Considering the unilamellar nanosheets as the final product, it is interesting that a longer reaction time was required by TMA+, while the reaction with TBA+ reached the final stage in a shorter reaction time. This is in contrary to what one would expect based on the limiting self-diffusion coefficients50 (1.15×10-9 m2s-1 for TMA+, 0.47×10-9 m2s-1 for TBA+), suggesting that the reaction with TMA+ should be faster. The diffusion of the TAA+ cations into the crystals appears not to be the rate determining steps in the exfoliation. Figure 8c shows the distribution in the length l of nanosheets prepared from two cations at 2 weeks. With TBA+, the nanosheets possessed a wide spectrum of length from 10 µm. In contrast, the minimum length of nanosheets made with TMA+ was ~4 µm. The average value was 10±5 µm from both cations. As with h, l decreased as the reaction time increased, giving = 6±3 µm (TMA+) and 4±3 µm (TBA+) at 18 weeks (the inset in Figure 8d). Particularly, the product by TMA+ contained large unilamellar nanosheets (some of them up to 10 µm as shown in Figure 7a), which are beneficial as a building block to construct well-defined nanostructures. Also, the distribution in the length at 18-week moved toward the smaller values compared to that at 2 weeks; compare Figure 8d with Figure 8c. Although we do not have statistical AFM analysis on the nanosheets prepared via mechanical shaking, it is clear (Figure 7c and 7d) that the nanosheets produced with TMA+ have much larger lateral sizes than those produced with TBA+. The large lateral size of the nanosheets made by TMA+ suggests that the inclusion of water/TMA+ clusters creates lower stress than the inclusion of water/TBA+ clusters. The statement implies a smaller size 25 ACS Paragon Plus Environment


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of water/TMA+ clusters compared to water/TBA+. However, XRD and SAXS measurements (Table 1) shows that at the same TAA+/H+ ratio, d values by TMA+ were larger than that by TBA+. We speculate that the mechanical stress imposed on the layer at the initial stage of the reaction could be different from those constituting an electrical double layer at equilibrium. At the very early stage, the edges of the layers are forced apart while the inner regions could be left unaffected. This period could be critical in determining the lateral size of the nanosheets. Considering that the ion size of TBA+ is larger than that of TMA+, the mechanical stress (which ultimately leads to fracturing of the crystals) due to TBA+ might be larger than that by TMA+. Once the water/TAA+ clusters have penetrated to the central part of the crystals, the swelling will be uniform and the stress could be reduced considerably. Our discussion here is similar to that reported previously by Garrett and Walker.44 From the plot of the lateral dimensions (width w or length l) vs h from all nanosheets counted at TAA+/H+ = 0.5, there seems to be a positive correlation for nanosheets prepared with TBA+ where the thick sheets were associated with large lateral sizes and vice versa (Figure 9b), while no such correlation is found for those produced with TMA+. This finding can be again related to the mechanical stress experienced by the crystals upon the introduction of TAA+ ions. Starting from the protonated layered crystals comprising of e.g. thousands of stacks of nanosheets, the intercalation of TAA+ ions can start randomly at equal probability at each gallery. Let us consider two typical extreme cases for reaction points at (i) the middle or (ii) close to the top (or bottom) of the stacks. In case (i), we could obtain two separated crystallites, each of which contains nearly half the number of stacks from the original crystals. Here, these crystallites are still thick, and the fracturing of such crystallites is rather suppressed. On the other hand for case (ii), we could produce one crystallite that is extremely thin (i.e., a few sheets) and another crystallite of almost the same thickness to the starting crystals. Such a thin crystallite may be liable to undergo further lateral fragmentation. Either situation (i) or (ii) can occur for both TMA+ and TBA+. However, considering that 26 ACS Paragon Plus Environment

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the TBA+ ions create a large mechanical stress, in the case (ii), the thinner crystallite tends to be fractured with TBA+. This results in a correlation of lateral dimensions vs h. On the other hand, such effect should be limited for the TMA+ ions that create a smaller mechanical stress. This gives rise to the lack of correlation of lateral dimensions vs h. Similar results for TAA+/H+ = 1 are shown in Figure S9. Such a correlation (or the lack thereof) was observed not only at the manual shaking time of 2 weeks but also 4 and 8 weeks. At the manual shaking time of 18 weeks (Figure S10), the plots of lateral dimensions vs h are similar for both TMA+ and TBA+, as the reaction had reached the final stage.

Figure 9.

w (or l) vs h plots of nanosheets prepared by manual shaking for 2 weeks at TAA+/H+ = 0.5

with (a) TMA+ and (b) TBA+.



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Conclusions Different behaviors of TMA+ vs TBA+ toward the osmotic swelling and exfoliation of a lepidocrocite-type titanate H1.07Ti1.73O4·H2O were studied. As TAA+/H+ decreased, the predominant product changed from the osmotically swollen phase to exfoliated nanosheets. For both types of the cations, there were two linear relationships in d vs C-1/2 plot, characteristics of the formation of the electrical double layer. Yet, there were differences on the slope and the transition point of the plot. Also for both cations, crystallites of a few stacks were obtained at the initial stage of the reaction in the region where exfoliation predominated. However, there was a significant portion of unilamellar nanosheets in case of the reaction with TBA+, while unilamellar nanosheets were scarcely found with TMA+.

At long reaction time,

unilamellar nanosheets were ultimately obtained as a final product for both TBA+ and TMA+. Evidences supporting the larger lateral size of nanosheets produced by TMA+, as compared to those by TBA+, were gathered from statistical AFM analysis on the deposited nanosheets, from particle size analysis on the suspension, and from the observation of the liquid crystallinity under a polarized optical microscope. The shaking of H1.07Ti1.73O4·H2O with TMA+ at low TMA+/H+ ratios gave high quality, molecularly thin nanosheets of lateral sizes up to 15 µm at a considerable concentration (nominally 4 g·L-1). Understandings of the fundamentals of osmotic swelling/exfoliation obtained in the present study could enable improved utilization of 2D nanosheets, which are one of the most important building blocks in nanoarchitectonics. Supporting Information XRD patterns, SAXS profiles, and AFM images and the corresponding statistical analyses (12 pages). This material is available free of charge via the Internet at http://pubs.acs.org.


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Acknowledgements This work was partly supported by the World Premier International Research Center Initiative on Materials Nanoarchitectonics (WPI-MANA), MEXT, Japan, and CREST of the Japan Science and Technology Agency (JST).

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the system HxTi2-x/4□x/4O4·H2O (x = 0.7), such possibility involving the sheet bending, i.e., that the sheets are being “less parallel”, has been previously excluded11; our present system should behave similarly. The presence of multiple higher order peaks suggests that such lattice distortion is negligible. The peak broadening involving such an effect is proportional to sinθ, meaning that rapid broadening or even disappearance takes place at a higher angular range. 43.

Walker, G. F. Nature 1960, 187, 312-313.

44.

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Restacking was also evident in the SAXS measurements where the exposure time was varied

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104, 295-306. 49. Williams et al.48 reported the osmotic swelling and the different slopes of d vs C-1/2 in vermiculites containing n-propyl- and n-butylammonium cations. Also in the same report, it was found that vermiculites containing methylammonium cations absorbed only a maximum of two layers of water. The authors48 suggested a delicate balance between hydrophobic and hydrophilic hydration as a reason for different behaviors of methylammonium vs n-butylammonium cations. 50.

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Another experiment (Figure S6) was performed by a mechanical shaking of H1.07Ti1.73O4·H2O with

TBA+ at TBA+/H+ = 10 and the solid-to-solution ratio of 2 g·L-1. The aggregate recovered from this suspension (where the total concentration c of TBA+ is 0.129 M) gave a d value of 7.4 nm. Compared to the suspensions reacted at the solid-to-solution ratio of 4 g·L-1, this value is close to that of 8.1 nm from the 32 ACS Paragon Plus Environment

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suspension having TBA+/H+ = 5 (c = 0.129 M), and is far from d = 5.3 nm from that having TBA+/H+ = 10 (c = 0.258 M). However, instead of using c in describing the experiments, we feel that the use of TAA+/H+ ratio can give a hint on some physical meaning of the process. As shown previously,12 the threshold of exfoliation at TBA+/H+ = 0.3-0.5 approximately corresponds to the stoichiometry of the TBA intercalated titanate, where TBA+ ions are effectively packed within the interlayer space. The use of c instead of TBA+/H+ would not easily lead to such the physical meaning. 56.

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Osmotic Swelling of Layered Compounds as a Route to Producing

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