Hidden ordered structure of clay nanosheets in binary colloids of niobate and clay nanosheets disclosed by small-angle neutron scattering

Abstract

Binary colloids of two different nanosheet species are promising systems for novel integrated materials because of their multicomponent and multiphase coexistence utilizable in novel smart materials. Among the binary colloids, niobate–clay binary nanosheet colloids are characterized by unusual photochemical functions and phase separation at a mesoscopic (∼several tens of micrometers) scale. The present study clarifies the colloidal state of the clay nanosheets in the mesostructured aqueous binary colloid of hexaniobate and fluorohectorite clay nanosheets by using small-angle neutron scattering (SANS) with the contrast variation technique. Compared to small-angle X-ray scattering (SAXS) that preferentially detects the niobate nanosheets, contrast variation SANS measurements can match out each nanosheet species to detect liquid crystalline ordering of each nanosheet species. The SANS results demonstrate the coexistence of two liquid crystalline phases induced by niobate and clay nanosheets, respectively, in the niobate–clay binary nanosheet colloids.

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Introduction

Colloids of inorganic nanosheets obtained by liquid phase exfoliation of inorganic layered crystals are promising colloidal systems for novel functional materials.¹ The high shape anisotropy of the colloidal nanosheets allows construction of various hierarchically ordered nanosheet-based structures ranging from 2D films to 3D porous solids.^2–4 In addition to such nanostructures in the solid state, colloidal nanosheets form lyotropic liquid crystals with their solvents to entropically stabilize the system.^5–10 The liquid crystallinity can be regarded as the basis for applying the colloidal nanosheets to advanced soft materials as well as for reconstructing them into a solid state.

The nanosheet colloids have been extended to binary systems, where two inorganic nanosheet species prepared by delamination of two different layered compounds are stably dispersed in a solvent. The binary nanosheet colloids are end members of multicomponent colloids consisting of different particles.^11–15 They generally cause phase separation in the absence of attractive interactions among particles on the basis of entropically driven demixing called depletion. Thus, the multicomponent nature of binary nanosheet colloids results in complicated phase behavior exemplified by multiphase coexistence.¹⁶ In addition, physicochemical properties of two nanosheet species can be integrated with unusual functions. We have investigated a binary nanosheet system composed of the hexaniobate nanosheets exfoliated from layered niobate (K₄Nb₆O₁₇) and clay nanosheets of smectite-type clay minerals such as hectorite, montmorillonite, and fluorohectorite,¹⁷ where the niobate is a photocatalytically active semiconductor^18,19 and the clay is a carrier of functional organic species.^20,21 In fact, unusual photochemical reactions have been discovered in the niobate–clay binary systems.^22–24

However, the structure of the niobate–clay binary nanosheet colloids has not been established. This is because of the occurrence of their phase separation at a mesoscopic scale to avoid the detection of the phases by the naked eye,²⁵ although the phase separation of multicomponent colloids usually occurs at an eye-detectable macroscopic scale.^26–28 We have recently carried out the characterization of the structure of niobate–clay binary nanosheet colloids by using small-angle X-ray scattering (SAXS) and confocal laser scanning microscopy (CLSM).²⁵ The study has clarified the liquid crystalline ordering of the niobate nanosheets in the binary colloids. The niobate nanosheets are assembled into liquid crystalline domains of several tens of micrometers. In contrast, the clay nanosheets are located in the voids between the niobate domains and are thought to be isotropically present on the basis of the SAXS results.

Nevertheless, this conclusion is not firm, because the clay nanosheets can form liquid crystalline phases when the lateral size of clay nanosheets is large enough. In our recent study of the niobate–clay binary colloids, where the clay nanosheets obtained from synthetic fluorohectorite are micrometer-sized, we observed that clay nanosheets aggregated and separated from the liquid crystalline domains of niobate nanosheets.²⁵ In contrast, we very recently discovered in the electric alignment study of these binary colloids that some of the clay nanosheets are aligned together with the niobate nanosheets.²⁹ The liquid crystallinity of these niobate–clay binary colloids as well as niobate and clay single-component colloids has been established by their fluidity and birefringence.^17,25,30,31

Based on these observations, in the present study, we attempted to unravel the ordered structure of the clay nanosheets in the niobate–clay binary colloids with large clay platelets. We employed the small-angle neutron scattering (SANS) technique. The hexaniobate nanosheet shows a smaller neutron scattering length density (3.26 × 10¹⁰ cm⁻², as [K₂Nb₆O₁₇]²⁻) than the fluorohectorite clay nanosheet (4.24 × 10¹⁰ cm⁻², as [Mg_2.6Li_0.46Si₄O₁₀F₂]^0.46−). Thus, the SANS technique detects the clay nanosheets better than the niobate nanosheets in H₂O. This is in contrast with the SAXS technique that more preferentially reflects the structure of the niobate nanosheets because the X-ray scattering length density of a niobate nanosheet (3.68 × 10¹¹ cm⁻² for [K₂Nb₆O₁₇]²⁻ (1306 electrons nm⁻³)) is higher than that of a fluorohectorite clay nanosheet (2.31 × 10¹¹ cm⁻² for [Mg_2.6Li_0.46Si₄O₁₀F₂]^0.46− (819 electrons nm⁻³)). Moreover, the contrast variation SANS measurements can selectively match out the niobate or clay nanosheets. Herein, we show the presence of an ordered structure of clay nanosheets in the binary nanosheet colloids of hexaniobate and fluorohectorite clay by using contrast variation SANS measurements.

Experimental

Sample preparation

A stock sample of niobate ([K₂Nb₆O₁₇]²⁻) nanosheet colloids was prepared at 60 g L⁻¹ nanosheet concentration in H₂O.²⁵ A niobate nanosheet colloid was prepared by the exfoliation of single-crystalline potassium hexaniobate (K₄Nb₆O₁₇).³⁰ Single-crystalline K₄Nb₆O₁₇ prepared by a flux method was treated with an aqueous propylammonium chloride solution at 120 °C for 7 d in a Teflon-lined autoclave. The solid product was centrifuged, re-dispersed, and stirred in water; the process was repeated twice. The washed precipitate was re-dispersed in water and dialyzed against water (5 h, 5 times and 24 h, twice). Niobate nanosheets with a lateral particle size of around 2 µm were finally obtained.³²

A stock sample of clay colloids was prepared at 30 g L⁻¹ nanosheet concentration in H₂O. Synthetic fluorohectorite clay mineral (ideal formula: Na_0.46[Mg_2.6Li_0.46Si₄O₁₀F₂])³³ provided as an aqueous suspension (Topy Industries Co., NHT sol) was centrifuged, by which the clay suspension was separated into three phases: upper supernatant, intermediate viscous sol, and lower sediment phases.³¹ The sol phase was picked up and dialyzed with water, and then dried at 60 °C. The average lateral particle size of the clay particles was 1–2 µm.³¹ A binary nanosheet colloid sample was prepared by simply adding a clay colloid to a niobate colloid. For the D₂O substitution, half of each stock sample was subjected to centrifugation and the deposit was diluted with D₂O, and this treatment was repeated only once in order to avoid severe structural change. Then, we set the nanosheet concentration at 60 and 30 g L⁻¹ for the niobate and clay nanosheet colloids, respectively, in D₂O.

These samples of the niobate or clay nanosheet colloids in D₂O and H₂O were mixed and diluted with water to prepare single-component niobate or clay nanosheet colloids with different D₂O : H₂O volume ratios. Niobate–clay binary nanosheet colloids with different D₂O : H₂O ratios were also prepared by appropriate mixing of the single-component nanosheet colloids of niobate and clay in D₂O and H₂O followed by dilution with D₂O or H₂O. The examined D₂O : H₂O ratios were 100 : 0, 85 : 15, 65 : 35, and 20 : 80. The concentration of niobate and clay nanosheets in the samples was set to 20 g L⁻¹.

Characterization

SANS measurements were carried out using a time-of-flight small and wide angle neutron scattering instrument (TAIKAN) on the BL15 beamport at the Materials and Life Science Experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC).³⁴ The neutron wavelength (λ) range was from 0.1 to 0.75 nm. The sample-to-detector distance was 5.65 m. The covered q-range was q ∼ 0.07–10 nm⁻¹, where the magnitude of the scattering vector q is defined by q = 4π sin(θ/2)/λ (θ representing the scattering angle). This q-range corresponds to the length scale (d ≡ 2π/q) from 0.6 to 90 nm in real space. The sample suspension was contained in a quartz cell with a 1 mm path length. The exposure time was around 1–5 h. All SANS data were normalized to an absolute scattering intensity using the SANS profile of a glassy carbon standard³⁵ after the necessary data corrections, such as detector efficiency, transmission, and incident neutron intensity. All data were presented after a smoothing calculation using moving average. The samples were not relaxed enough to remove the shear force applied during the sample preparation. Thus, we do not discuss the radial symmetry of the SANS data in relation to the particle orientation.

Results

Scattering by the single-component niobate and clay nanosheet colloids

The plots in Fig. 1A and 2A show SANS patterns of the single-component niobate and clay nanosheet colloids, respectively, with the nanosheet concentration of 20 g L⁻¹ and different D₂O : H₂O volume ratios. In all the profiles, the upturns toward low-q are observed while the profiles are almost flat in the q range of 1–10 nm⁻¹. Meanwhile, in the profiles of only the solvents without the nanosheets (the dashed lines in Fig. 1A and 2A), a structureless flat profile is observed at q < 10 nm⁻¹. Hence, the upturn in the low-q region is ascribed to the scattering by the nanosheets. In the flat region, incoherent scatterings of solvents and the nanosheets both contribute to the profile so that the intensity is stronger for the samples with less D₂O, which has less incoherent scattering than H₂O. For all the nanosheet colloids in the present study, we employ the averaged scattering intensities around q ∼ 2 nm⁻¹ as the q-independent experimental constant values of incoherent scattering for each nanosheet colloid so that these constant values are subtracted for further analyses.

Fig. 1 SANS patterns of a single-component hexaniobate nanosheet colloid (20 g L⁻¹) with different D₂O : H₂O ratios of the solvent. (A) Raw scattering patterns, (B) after subtracting the incoherent scattering of the nanosheets from the patterns in (A), and (C) after removing the form factor of the nanosheets from the patterns in (B).

Fig. 2 SANS patterns of a single-component fluorohectorite clay nanosheet colloid (20 g L⁻¹) with different D₂O : H₂O ratios of the solvent. (A) Raw scattering patterns, (B) after subtracting the incoherent scattering of the nanosheets from the patterns in (A), and (C) after removing the form factor of the nanosheets from the patterns in (B).

After subtracting the constant incoherent scattering, the SANS patterns are replotted as Fig. 1B and 2B. Here, the scattering intensity basically obeys q⁻², which is the form factor of isolated 2D nanosheets. Hence, we divide the scattering curves by q⁻² in order to extract the excess scattering reflecting nanosheet–nanosheet interactions, as shown in Fig. 1C and 2C. We discuss only the position of the most intense peak at the lowest q value in each pattern in the following discussion. Although the SANS patterns show several bumps at higher q values, their S/N ratios are too low to discuss quantitatively if we take into account error bars of the SANS patterns as indicated in Fig. S1 and S2 (SI).

All the samples show weak broad peaks accompanied by small numbers of higher-order peaks. Because of the fluidity and birefringence of the samples, they are liquid crystalline and the mesophase should be identified as nematic. We estimate the basal spacings from the main SANS peak of each sample fitted by Gauss functions, and the result is summarized in Table S1. The spacings can be recognized as the average inter-sheet spacings of the ordered nanosheets.

For the niobate nanosheet colloid, a broad peak is observed at q ∼ 0.12 nm⁻¹ (d ∼ 53 nm) (Fig. 1C). Although the SAXS pattern of the niobate nanosheet colloid at the same concentration measured in our previous study does not definitively show the peak,²⁵ the current SANS pattern indicates the presence of nematic ordering in the sample. If we employ the basal spacings detected in the SAXS patterns of the niobate nanosheet colloids with higher concentrations,²⁵ the basal spacing of the current sample is estimated as 152 nm based on the relationship between the nanosheet concentration and basal spacing (Fig. S2). Thus, we assign the d value of the SANS peak to the third-order peak with respect to the basal spacing. The upward curvature of the SANS pattern around the low-q limit indicates the presence of lower-order peaks. On the other hand, the clay colloid shows a peak at q ∼ 0.12 nm⁻¹ (d ∼ 53 nm) (Fig. 2C). This value is in harmony with the basal spacing (56 nm) of the clay nanosheet colloid estimated from the SAXS pattern at the same nanosheet concentration.²⁵ These scattering data indicate nematic ordering of each nanosheet in the single-component colloids.

On the other hand, Fig. 1 and 2 clearly show the effect of the D₂O/H₂O ratio of the solvent on the scattering intensities of the nanosheet colloids. The scattering intensity strongly depends on the D₂O/H₂O ratio. This is explained by considering that the neutron scattering intensity is proportional to the square of the difference between the neutron scattering length density of the scattering objects and the solvent, and that the neutron scattering length densities of D₂O (6.40 × 10¹⁰ cm⁻²) and H₂O (−0.56 × 10¹⁰ cm⁻²) are largely different. Since the neutron scattering length densities of the niobate (3.26 × 10¹⁰ cm⁻²) and clay (4.23 × 10¹⁰ cm⁻²) (see the SI for the calculation of these values) are in between the values of D₂O and H₂O, the scattering of the nanosheets is enhanced or matched out by varying the solvent composition, which is known as the contrast variation technique. The solvent composition for matching out the niobate or clay nanosheets is experimentally determined from the plot of the scattering amplitude (square root of the scattering intensity) s vs. D₂O ratio (Fig. 3). By fitting the plots using the following equation

s = a ([D₂O] – [D₂O]_matched)

where a and [D₂O]_matched are the free parameters, matching out points for the niobate and clay nanosheets are determined as [D₂O]_matched = 75% and 92%, respectively.

Fig. 3 The relationship between the scattering amplitude and the D₂O concentration in the (a) clay and (b) niobate nanosheet colloids (20 g L⁻¹). The solid lines were obtained by fitting the plots as in the main text.

These values are greater than the theoretical values (Table S1 in the SI) of 54% and 70% for the niobate and clay, respectively. The inconsistency would be due to incomplete replacement of H₂O for D₂O in the preparation of the stock samples. Because the counter cations of the negatively charged colloidal nanosheets (Na⁺ for clay and propylammonium for niobate) are hydrated, complete replacement of the strongly hydrated water molecules was difficult in our study. Slow hydration kinetics of clay minerals due to the large crystallite size of the mother crystals, which may prevent the samples from reaching equilibrium, also contribute to the discrepancy between the theoretical and experimental values. Moreover, the contribution of these counter cations is not considered in the calculation in Table S1.

However, we can say that when the scattering length densities of the solvent and nanosheets are matched, the scattering of each nanosheet mostly disappeared, which is useful for analyses of the mixture system. As shown in Fig. 2B and C, for the clay system, the scattering intensity after removing the incoherent scattering is the lowest with a D₂O ratio of 100% or 85%. In the niobate system, the lowest scattering intensity was observed with a D₂O ratio of 65% (Fig. 1B and C). These D₂O concentrations at the matching points are larger than the theoretical ones. This is ascribed to incomplete substitution of H₂O for D₂O because our experimental conditions for the substitution were moderate as written in the Experimental section.

SANS of the niobate–clay binary nanosheet colloids with contrast variation

The contrast variation measurements selectively match out the niobate or clay nanosheets in the niobate–clay binary nanosheet colloids. Fig. 4 shows scattering profiles of the niobate–clay binary nanosheet colloids with different D₂O : H₂O ratios. Unlike the single-component colloids of niobate or clay nanosheets, the niobate–clay binary colloids do not show the I ∝ q⁻² dependence. A downward slope is observed after subtraction of incoherent scattering and form factor of nanosheets (Fig. 4C). In addition, the scattering curve exhibits a broad scattering peak at around q ∼ 0.10 nm⁻¹ with low D₂O content (D₂O : H₂O = 20 : 80). The peak position is smaller by 0.02 nm⁻¹ than that of the single-component clay nanosheet colloid.

Fig. 4 SANS patterns of a binary colloid composed of hexaniobate (20 g L⁻¹) and fluorohectorite clay (20 g L⁻¹) nanosheets with different D₂O : H₂O ratios of the solvent. (A) Raw scattering patterns, (B) after subtracting the incoherent scattering of the nanosheets from the patterns in (A), and (C) after removing the form factor of the nanosheets from the patterns in (B).

The overall scattering intensity after subtraction of incoherent scattering (Fig. 4B and C) decreased with the increase of D₂O ratio, giving the minimum intensity at D₂O : H₂O = 85 : 15. This is explained by the fact that this condition is in the middle of the matching points of the niobate ([D₂O] = 75%) and clay ([D₂O] = 90%). It is notable that the scattering intensity and scattering profiles are mostly the same for D₂O : H₂O = 65 : 35 and 100 : 0, which are the closest to the matching out condition for the niobate and clay, respectively. The peak at q ∼ 0.10 nm⁻¹ is still visible under these conditions. These results disclose that both of the niobate and clay nanosheets contribute to the same nematic structure in the niobate–clay binary nanosheet colloids, as discussed later.

Discussion

Although our previous structural characterization of the liquid crystalline niobate–clay binary nanosheet colloids by using SAXS indicated the nematic ordering of the niobate nanosheets, the present SANS study clarifies that the ordering of the clay nanosheets of synthetic fluorohectorite characterized by large nanosheet size is involved in liquid crystalline domains of the binary colloid. In the previous study, we discovered that the niobate–clay binary nanosheet colloids show the SAXS patterns that are principally determined by the concentrations of the two nanosheet species irrespective of the clay source involving the synthetic hectorite (LAPONITE®) that hardly shows liquid crystallinity.²⁵ The results led to the structure of the binary colloids described by the coexistence of the liquid crystalline niobate nanosheet domains and isotropic clay nanosheets present among the niobate domains.

However, the present study clarifies the nematic ordering of clay nanosheets in the binary colloids hidden in their SAXS results. As described above, the matching points of niobate and clay are estimated as [D₂O] = 75% and 90%, respectively (Fig. 3). The scattering intensity of the peak due to the basal spacing of the nematic phase at [D₂O] = 65% (lower by 10% than the matching point of niobate) is the same as that of [D₂O] = 100% (higher by 10% than the matching point of clay). If the nematic structure is constructed only using the niobate nanosheets, the scattering intensity at [D₂O] = 65%, which is close to the matching point of niobate, must be lower than that at [D₂O] = 100%. The same scattering intensity at [D₂O] = 65 and 100% indicates that both of the niobate and clay nanosheets contribute to the nematic structure in the binary nanosheet colloids.

On the other hand, the SANS patterns of the niobate–clay binary nanosheet colloids show certain slope after dividing the SANS patterns by q⁻² to remove the form factor of 2D nanosheets. This indicates the existence of three-dimensional nature in the colloids. This would be mainly induced by clay nanosheets because the slope is most gentle for the sample in D₂O : H₂O = 85 : 15, which is close to the matching point of clay nanosheets. We assume the coexistence of aggregated clay nanosheets in the binary colloids.

Such a consideration leads to a revised structure of the niobate–clay binary nanosheet colloids. Although SAXS analysis has indicated the liquid crystalline phase composed of the niobate nanosheets, the SANS technique has disclosed the involvement of clay nanosheets in the liquid crystalline domains. Such multi-component liquid crystalline phases have often been observed in binary colloidal liquid crystals of anisotropic particles.^16,36 Also, the result is consistent with our very recent observation of the electric alignment of the niobate–clay binary colloids, where some of the clay nanosheets are involved in the electric response of the niobate nanosheets.²⁹

Conclusions

The present study demonstrates the effectiveness of the contrast-variation SANS measurements for determining the structure of the multicomponent colloids exemplified by the niobate–clay binary nanosheet colloids. The SANS measurements carried out on the aqueous colloid samples with different H₂O : D₂O ratios match out the scattering due to the niobate nanosheets in the binary colloid to unravel the nematic ordering of the clay nanosheets, which is hidden in the previously reported SAXS analysis of the binary nanosheet colloids. The results suggest that both of the niobate and clay nanosheets are involved in the nematic structure of the binary colloids. Integration of plural nanosheet species in colloidal liquid crystalline phases will open the future applications of the binary colloids to novel functional systems such as sensing, switchable permeation, and selective chemical reactions.

Author contributions

Teruyuki Nakato: conceptualization, supervision, data collection of SANS, writing, and editing. Riki Kato: data collection and analysis of SANS, and editing. Wataru Ishitobi: sample preparation. Emiko Mouri: sample preparation and editing. Hiroki Iwase: data collection and analysis of SANS, and editing. Nobuyoshi Miyamoto: supervision, data collection and analysis of SANS, and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. Requests should be directed to Teruyuki Nakato and should include a brief description of the intended use. Data will be preserved for at least 10 years after publication.

Supplementary information (SI): calculation details of neutron scattering length densities, basal spacings of the lamellar phase evolved in single-component niobate nanosheet colloids plotted against the nanosheet concentration determined by SAXS, SANS patterns of nanosheet colloids with error bars, Gauss fitting of the SANS peaks, basal spacings of the samples estimated by Gauss fitting of the SANS patterns. See DOI: https://doi.org/10.1039/d5sm01056j.

Acknowledgements

The SANS measurements were carried out with the approval of the Neutron Program Review Committee (Proposal No. 2018A0012).

Notes and references

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