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Panoscopic organization of anisotropic colloidal structures from photofunctional inorganic nanosheet liquid crystals† Teruyuki Nakato,*a Yoshihiro Nono,a Emiko Mouria and Munetaka Nakatab Colloidal liquid crystals of inorganic nanosheets with thickness of around 1 nm and lateral dimensions of several micrometers prepared by exfoliation of a layered niobate are converted to hierarchically organized arrays whose structures are controlled from the nano to macroscopic length scale through the growth of liquid crystalline domains called tactoids as the secondary building blocks followed by controlled application of external fields. Growth of the tactoids is attained by incubation of the liquid crystals at room temperature. The tactoids are then assembled into higher-order structures with characteristic lengths of sub-mm to mm under the simultaneous application of an ac electric field and gravity, whose directions determine the final textural motif of the arrays. Whereas a net-like texture is observed when applying the electric and gravitational forces in the same direction, a striped texture
Received 1st October 2013, Accepted 1st November 2013
where the nanosheets are unidirectionally aligned is observed when the electric field is applied in the
DOI: 10.1039/c3cp54140a
control. Since the niobate nanosheets have wide band-gap semiconducting nature, the nanosheet stripe arrays exhibit photocatalysis that reflected the alignment of the nanosheets with respect to the polarized
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direction of impinging light.
direction perpendicular to gravity. The use of well-grown tactoids is key to the macroscopic structural
Introduction Hierarchical soft structure is a key concept for novel intelligent materials, and such structures have been well realized in various organic systems exemplified by liquid crystals (LCs) and gels.1 However, panoscopic organization, the term which means structural control over all length-scales from micro-, meso-, to macroscopic,2 of soft matter has still insufficiently been developed because its building blocks are usually organic moieties which lack robustness for the retention of defined structures up to macroscopic scales. If we can use inorganic building blocks, panoscopic structures will be constructed for soft matter rather easily. In addition, versatile properties of inorganic crystals such as semiconducting, dielectric, and
a
Department of Applied Chemistry, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu-shi, Fukuoka 804-8550, Japan. E-mail: [email protected] b Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei-shi, Tokyo 184-8588, Japan † Electronic supplementary information (ESI) available: Schematic representation of the effects of external forces on the alignment of the colloid liquid crystals; the TEM image and size distribution of the nanosheets, optical and POM images of an as-prepared NSLC; the conoscope image of the just injected NSLC; additional POM and FOM images of the NSLC arrays; experimental setting and results of the control experiments of photocatalytic tests. See DOI: 10.1039/c3cp54140a
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(photo)catalytic functions, can be utilized in such materials. Nevertheless, inorganic soft structures, in particular panoscopic ones, remain almost unexplored, although organization of inorganic building blocks into flexible structures with the aid of organic components as soft moieties has attracted interest for developing novel advanced materials by mimicking biosystems.3,4 We expect that colloidal LCs of inorganic particles are promising systems for organizing inorganic panoscopic soft structures. Colloidal LCs are lyotropic systems giving ordered structures of anisotropic particles like rods and plates based on the excluded volume effect.5–11 They are very rare examples of the inorganic soft structures, where crystalline colloidal particles are mobile but oriented in the solvents. Among them, nanosheet LCs (NSLCs) are interesting end members because they are constructed using extremely thin 2D particles that can be obtained through exfoliation of broadly diverse inorganic layered compounds.12,13 The nanosheets are characterized by mm-sized mesogens with high anisotropy ensured by their thickness of around 1 nm. In fact, previous studies on NSLCs have shown generation of stable liquid crystalline phases from the nanosheets of semiconducting oxides,14–17 clays,18–22 phosphates,23,24 and graphenes.25–27 Because of the robustness of the inorganic crystallites and the mesogen size being much larger than that of conventional organic molecules, the NSLCs can be used to construct
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large-scale structures, which is impossible with the organic liquid crystals, with characteristic lengths of sub-mm to mm through panoscopic organization of the nanosheets. The nanosheets must have rich alternatives of arrangements for higher-order structures compared with 1D rods, because 2D particles have two different axes for regulation of their alignment while 1D particles are unidirectionally aligned with a single external force (Fig. S1, ESI†). Hence, we may obtain a wide variety of hierarchically structured arrays of NSLCs with careful orientational control. The NSLC arrays are low-density soft self-assemblies that can be remotely controlled by external stimuli based on the liquid crystallinity, being very different from conventional colloidal arrays where symmetrical particles are densely organized.28,29 However, no previous studies have yet achieved such a hierarchical and macroscopic, i.e., panoscopic structural control of the NSLCs. We report herein photocatalytically active panoscopic NSLC arrays of oxide nanosheets fabricated from layered niobate K4Nb6O17 known as a wide band-gap semiconductor through hierarchical organization of the anisotropic crystalline mesogens. The panoscopic nature is provided by a two-stage assembly of the nanosheets. The first stage is controlled growth of the LC domains called tactoids, the word which is used for colloidal domains of anisotropically associated rods or plates,30,31 by assembly of the nanosheets. This is the key step of panoscopic organization because the grown tactoids have the dimensions of sub-mm length and act as the secondary building blocks of the final structures, although the structural control by domains has not been reported in previous studies of NSLCs. Then, the tactoids are further organized into higher-order structures with external fields. Since external electric and magnetic forces can be used as tools for aligning the NSLCs in a homeotropic manner,21,23,24,32,33 we may arrange the nanosheets into hierarchically regulated panoscopic structures with the application of dual external forces such as electric field and gravity. Parallel and orthogonal applications of the external forces induce different array structures from the same tactoids because of the 2D nature of the LC mesogens. Furthermore, the semiconducting nature of the nanosheets34,35 allows the NSLC arrays to undergo the photocatalytic reaction, which is controlled by the arrangement of the nanosheets with respect to the impinging light. Semiconductor photocatalysts structurally regulated with liquid crystallinity have not been reported except for a solid film prepared from the liquid crystalline photocatalytic rods.36
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gradually cooled.15,37 K4Nb6O173H2O (1 g) was treated with a 0.2 mol L1 aqueous solution of propylammonium chloride (Tokyo Chemical Industry Co., Ltd., Japan) at 120 1C for a week to exchange the interlayer potassium ions for propylammonium ions.15,38 The reaction product was centrifuged at 11 000 rpm, washed, and dialyzed with water to yield the stock sample of niobate NSLC. The NSLC sample was diluted with water to 5 g L1 of the niobate concentration (indicated as the mass of [Nb6O17]4) for each measurement. The lateral lengths of the nanosheets obtained from TEM observations had a size distribution obeying a log-normal distribution15 to give an average size of 2.0 mm (Fig. S2, ESI†). Liquid crystallinity was indicated by the appearance of the Schlieren texture in the polarized optical microscope (POM) image (Fig. S3, ESI†). Samples for fluorescence microscope observations were prepared by adding an aqueous solution of tris(2,2 0 -bipyridyl)ruthenium(II) (Ru(bpy)32+) dichloride (Sigma-Aldrich Co., USA) to NSLC of the niobate followed by stirring for 1 h. The concentration of Ru(bpy)32+ was set to 5 105 mol L1. For the photocatalytic decomposition of the cyanine dye, an aqueous solution of 1,1 0 -diethyl-2,2 0 -cyanine (PIC) bromide (Hayashibara Biochemical Laboratories Inc., Japan) was added to NSLC, and mixed for 1 h. The concentration of PIC was set to 1 104 mol L1. The concentration of the niobate nanosheets was 5 g L1 for both of the experiments. Organization of the NSLC arrays The niobate NSLC sample was organized into the NSLC arrays by controlling three parameters: incubation for the growth of tactoids, external electric field, and gravity. A small aliquot of the NSLC sample was injected into a thin layer liquid crystalline (LC) cell, where two ITO-coated glass plates were faced with a 100 mm film spacer (Teijin DuPont Films Japan) and sealed with epoxy resin.32 The sample injected into the cell was incubated to allow the growth of tactoids by making the cell stand with the ‘‘flat’’ cell setting (i.e., at the horizontal position, Fig. 1, left) under ambient conditions. The incubation time, i.e., the period from the sample injection to the application of electric field is varied.
Experimental section Sample preparation The NSLC sample (5 g L1), where negatively charged niobate (Nb6O174) nanosheets accompanied by propylammonium ions acted as the countercations, was prepared.15 Single crystalline K4Nb6O17 was prepared by a flux method; a mixture of K2CO3 (Wako Pure Chemical Industries, Ltd., Japan) and Nb2O5 (Soekawa Chemicals Co., Ltd., Japan) was heated at 1150 1C and
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Fig. 1 Setup of the LC cell for incubation and ac voltage application to the niobate NSLC. The ac voltage is applied from the same direction as that of gravity with the ‘‘flat’’ cell setting while the voltage is applied from the direction perpendicular to that of gravity with the ‘‘upright’’ cell setting. The arrows indicate the direction of electric field (E) and gravity (g), respectively.
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Alternate current (ac) electric field (500 V cm1, 50 kHz) supplied by an NF BP4610 bipolar generator was applied to the NSLC sample in the ITO cell. These conditions were based on the results of our previous study, where the electrooptic response of the niobate NSLC was investigated.32 The frequency of 50 kHz was chosen because lower frequencies induce boiling of the colloid due to phoretic behavior of the nanosheets. The electric field strength of 500 V cm1 was found to be a minimal value that stationarily aligns the nanosheets with the ac application. We examined two cell settings, i.e., ‘‘flat’’ and ‘‘upright’’ settings (Fig. 1, right). While the ac electric field was applied in the direction perpendicular to the ITO substrate surface, the direction of gravity depends on the cell setting. The electric field and gravity were applied from the same direction with the flat cell setting, but they were in the directions perpendicular to each other with the upright setting. For the ac application to the cell with the upright setting, the cell incubated with the flat setting was turned to the upright position by hand, and then the application of ac voltage was started immediately. Microscope observations The polarized optical microscope (POM) and fluorescence optical microscope (FOM) observations were performed using an Olympus BX-51 optical microscope equipped with a BX2-FL-1 fluorescence unit using an Olympus U-MSWB2 mirror. Optical retardation of the birefringent areas that appeared in the POM images was determined by the Senarmont method4 using an Olympus U-CSE compensator under monochromatic radiation of 550 nm produced by an Olympus 43IF550W45 green bandwidth filter. The compensation was carried out for a typically birefringent area of 0.003 mm2. Conversion of the colored FOM images to the gray-scaled ones was carried out using Adobe Photoshop Elements 8.0 software. The average area of the tactoids was evaluated by pixels of more than 15 tactoids on gray-scaled FOM images. Photocatalytic decomposition of the PIC dye For the photocatalytic experiments, the PIC-added NSLC sample was injected into the ITO cell, and irradiated with UV light using an Ushio SX-UI-251-HQ high-pressure mercury lamp with the upright cell setting. The light was passed through a HOYA U340 bandpass filter to remove visible light and an OPTO-LINE PUV-32 polarizer to irradiate the sample with polarized UV light (Fig. 2). Irradiation with polarized UV light from a mercury lamp provided two optical setups: the polarizing direction of the impinging light was parallel or orthogonal to the direction of gravity applied to the NSLC array. These optical setups will be called the parallel and orthogonal geometries (see Fig. 2). Visible spectra of the NSLC sample were measured before and after the irradiation by using a Shimadzu UV-2450 spectrophotometer in the range of wavelength from 450 nm to 650 nm. PIC concentrations were estimated from spectral data after subtraction of the background spectrum to remove the effect of Rayleigh scattering from the nanosheets.
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Fig. 2 Experimental setup for the photocatalytic decomposition of the PIC dye in the stripe-patterned niobate NSLC array (5 g L1) under the ac voltage application. The light from the mercury lamp was polarized and then impinged on the NSLC. The reaction was carried out with two optical geometries called parallel and orthogonal geometries corresponding to the setups with the polarized direction of the UV light being parallel and perpendicular to gravity, respectively.
Results and discussion Tactoid growth in the NSLC with incubation The first stage of the panoscopic organization of the niobate NSLC is the growth of tactoids with the incubation. This step is clarified by the combined observations of POM and FOM using tris(2,2 0 -bipyridine)ruthenium(II) (Ru(bpy)32+) as a luminescent probe adsorbed on the nanosheets. The POM observations represent structural alteration of the NSLC, and the results are independent of the addition of Ru(bpy)32+. A dark POM image for the sample just after the injection into the cell indicates homogeneous orientation of the nanosheets with their in-plane direction parallel to the ITO substrate (Fig. 3a and p);32 a conoscope image showing an isogyre cross supports the presence of liquid crystalline ordering (Fig. S4, ESI†). In contrast, birefringent spots appear in the POM images of the incubated samples (Fig. 3b–e), indicating the presence of the nanosheets inclined or perpendicular to the ITO surface. Although the FOM images show an almost uniform red view irrespective of the incubation (Fig. 3f–j), suggesting homogeneous distribution of the Ru(bpy)32+-attached nanosheets, the gray-scaled FOM images provide evidence for the growth of tactoids (Fig. 3k–o). The gray-scaled image of the sample just injected is apparently uniform without distinguishable domains (Fig. 3k), but the incubated samples exhibit domain boundaries indicated by dark lines (Fig. 3l–o). Superimposition of the POM and gray-scaled FOM images indicates that the domain boundaries found with FOM correspond to the edge and disclination lines of the birefringent areas in the POM image (Fig. 4). Thus, the nanosheets are aligned parallel to the ITO surface inside the domains but inclined at the boundaries and defects, as illustrated in Fig. 3q. These results show that orientationally ordered nanosheets are associated during the incubation to form tactoids. Longer incubation provides larger tactoids, as shown in Fig. 5, indicating that the tactoid growth occurs through Ostwald ripening. A well-grown tactoid has a lateral size of up to 100 mm. The size is
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Fig. 3 POM (a–e), FOM (f–j), and gray-scaled FOM (k–o) images of the niobate NSLC (5 g L1) before (a, f and k) and after the incubation for 60 min (b, g and i), 120 min (c, h and m), 180 min (d, i and n), 360 min (e, j and o); schematic representation of the nanosheet arrangement in the NSLC before (p) and after (q) the incubation. The direction of gravity, indicated by broad arrows, is perpendicular to the POM images.
Formation of panoscopic structures through alignment of the grown tactoids
Fig. 4 Superimposed microscopic image (a) of the niobate NSLC (5 g L1) after the incubation for 120 min produced from POM (b) and gray-scaled FOM images (c).
Fig. 5 Increase in the average tactoid area estimated from the grayscaled FOM images as a function of the incubation time.
much larger than that of a single nanosheet (B2 mm), and appropriate for the organization of macroscopic structures.
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The NSLC tactoids are further organized to generate hierarchically structured macroscopic arrays under external forces. Application of the electric field and gravity from the same direction based on the flat cell setting yields NSLC arrays of net-like structures with characteristic lengths of sub-mm, as shown in Fig. 6. The ac voltage applied perpendicular to the substrate surface changes the nanosheet arrangement, from parallel to perpendicular with respect to the substrate, as evidenced by the increase in the birefringent area of the POM images (Fig. 6a–c), because the nanosheets are aligned with their surfaces parallel to the electric field.32 However, the texture of the electrically aligned NSLC samples depends on the incubation time. Upon application of
Fig. 6 POM and FOM images and schematic nanosheet arrangements of the niobate NSLC (5 g L1) with flat cell setting. The ac voltage was applied for 8 min from the direction parallel to gravity through the flat cell setting immediately after the sample injection (a, d and g), after the incubation for 60 min (b, e and h) and 120 min (c, f and i). The direction of gravity (g) is perpendicular to the POM images.
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ac voltage, the samples with short incubation times exhibit isolated birefringent dots, whereas the samples form nets if the incubation before the ac voltage application is extended. The longer the incubation time, the coarser the meshes become. This fact indicates that the tactoids work as the secondary building blocks of the NSLC arrays to give the final structures different from those with short incubation, as illustrated in Fig. 6g–i, although the nanosheets themselves are aligned parallel to the electric field irrespective of the incubation time. The FOM images indicate concentration fluctuation of the nanosheets in the sample induced by the tactoid growth (Fig. 6d–f). The luminescent areas are identical with those of birefringent. Thus, most of the nanosheets are localized in the tactoids, and the solvent (water) is present in the dark and nonfluorescent regions. This is in contrast with a sample without incubation, because its FOM image indicates almost homogeneous luminescence, which indicates small concentration fluctuations of the nanosheets in the colloid in the absence of the tactoids. Consequently, the results indicate that the niobate NSLC forms panoscopic colloidal arrays with careful growth of the tactoids. When dual external forces are applied from orthogonal directions, the NSLC samples form structurally different arrays characterized by unidirectional nanosheet orientation. The samples are incubated with the flat cell setting and then turned upright to direct the electric field and gravity perpendicular as soon as the start of ac application. The birefringent area increases with the ac application (Fig. 7a–c), and striped structures are obtained. The stripe texture reflects the unidirectional alignment. Each stripe has a characteristic length of sub-mm in thickness and mm in length, indicating the panoscopic nature of the array structure. This is quite different from the net-like textures obtained when the voltage and gravity are applied from the same direction. As observed with FOM
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(Fig. 7d–f), the luminescent areas correspond to the birefringent stripes for the incubated samples, indicating the concentration fluctuation of the nanosheets through association with the tactoids; i.e., most of the nanosheets are localized in the stripes as illustrated in Fig. 7g–i. In these NSLC arrays, the direction of stripes is the same as that of gravity. This rationalizes the unidirectional nanosheet orientation. The 2D particles need two different regulation axes to be aligned unidirectionally (see Fig. S1, ESI†). The electric field applied to the NSLC sets the direction of the nanosheets (and their tactoids) perpendicular to the ITO plate, and the in-plane direction with respect to the substrate surface is determined using the gravity applied from the direction perpendicular to the ac voltage. Although the stripe motif is common to the samples with different incubation periods, the growth of the nanosheet tactoids influences the quality of stripes. Thick stripes are generated in the samples with longer incubation, and optical retardation by the birefringent stripes estimated using a Senarmont method (Table 1) indicates that more birefringent stripes are formed in these cases. This is explained by the presence of well-grown tactoids in the enough incubated samples because more grown tactoids that assemble larger numbers of nanosheets give larger birefringence. Hence, the hierarchical organization via the tactoids growth is still important for the formation of NSLC arrays under the orthogonal external fields. POM observations of the evolution of the stripe texture indicate flow-induced orientation with gravity (Fig. 8). After application of the ac voltage for 1 min, i.e. just after switching of the cell setting from flat to upright, the sample exhibits a netlike texture. This indicates that the change in the direction of gravity induced by switching the cell setting within such a short period has no effect on the nanosheet alignment. Teardrop-like domains appear along the direction of gravity through continuous orthogonal application of the ac electric field and the gravity for several minutes, and then the teardrops are combined into stripes for longer application of the external fields. The results demonstrate coalescence of the tactoids to stripes induced by solvent (water) flow along gravity. Although this flow-induced alignment is essentially a transient state of the tactoids going to be settled, the arrayed structure is maintained for several tens of minutes or even longer as indicated by the
Table 1 Optical retardation of the stripe-patterned niobate NSLC arrays obtained at a constant ac voltage of 500 V cm1 for 60 min and various incubation periods
Fig. 7 POM and FOM images and schematic nanosheet arrangements of the niobate NSLC (5 g L1) with upright cell setting. The ac voltage was applied for 60 min from the direction perpendicular to gravity through the upright cell setting immediately after the sample injection (a, d and g), after the incubation for 60 min (b, e and h) and 120 min (c, f and i). The direction of the electric field (E) is perpendicular to the POM images. The direction of gravity (g), indicated by broad arrows, is parallel to the POM images.
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Incubation time/min
Optical retardationa/nm
0b 60 120 180
47 74 86 88
a
Determined by the Senarmont method. The measurement of optical retardation was started after 60 min of ac voltage application and was finished in 2 min. b Sample setting was changed to upright setting within 5 min after the sample injection, and the ac voltage application was started simultaneously with the turning of cell setting.
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Fig. 8 POM images of the niobate NSLC (5 g L1) to which the ac voltage was applied for 1, 4, 8, 16, 30, and 60 min (from (a) to (f)) with the upright cell setting after the incubation for 120 min. The image of the sample with 60 min of ac voltage application is shown in Fig. 3, but reproduced for comparison purpose. A and P indicate the directions of the analyzer and the polarizer, respectively. The direction of the electric field (E) is perpendicular to the POM images. The direction of gravity (g) is indicated by the arrow in the POM image.
POM image of the sample after the ac application with the upright setting for one hour (Fig. 8f). Photocatalysis by an NSLC array Unidirectionally aligned NSLC arrays with striped texture exhibit photocatalytic behavior that reflects the anisotropic structure of the array. The probe reaction is photocatalytic decomposition of PIC molecules which are adsorbed and laid flat on the niobate nanosheets.39 Addition of the dye to a typical striped NSLC array (incubation for 60 min) does not affect the array organization, and a characteristic absorption band of PIC forming a J-aggregate is displayed on the niobate nanosheets (Fig. 9 inset).39 The PIC molecules are decomposed upon irradiation with the polarized
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UV light of the NSLC array with both the parallel and orthogonal optical geometries shown in Fig. 2; the geometry indicates the direction of the polarization of impinging light being set parallel or perpendicular to the direction of gravity, that is, the direction of stripes. The reaction is ascribed to the photocatalysis of the niobate nanosheets, because the decomposition is negligible in the dark or without the nanosheets (Fig. S5, ESI†). The dye decomposition proceeds faster when the polarization direction of the impinging UV light is parallel to the stripes of the NSLC array rather than perpendicular (Fig. 9). This is explained by the matching of the directions of the light polarization and nanosheets. Photoexcitation of the semiconducting nanosheets should be more efficient when the impinging light is vibrating in the direction parallel to the nanosheet edges than that in the orthogonal direction. Thus, the photocatalytic reaction should proceed faster with the parallel geometry. The present results demonstrate the usefulness of the nanosheet ordering at the centimeter length scale provided by the panoscopic organization because such a macroscopic structure allows the reaction control by using a conventional light source. On the other hand, the time courses of the photocatalytic PIC dye decomposition indicate acceleration in the course of reaction. Table 2 compares the first-order rate constants of the reactions in the regions of before and after the acceleration for both the parallel and orthogonal geometries; the logarithmic plots of the reaction time courses for the calculation of rate constants are shown in Fig. S6 (ESI†). In the slow reaction region (before the acceleration), the reaction with the parallel geometry is much faster than that with the orthogonal geometry, as indicated by the difference in an order of the rate constants. However, after the acceleration, the rate constants for both of the geometries become close to each other although the reaction proceeds still somewhat faster with the parallel geometry. This behavior suggests a structural alteration of the NSLC array during the reaction. Optical microscope observations of the samples indicate partial disruption of the unidirectional nanosheet alignment during the reaction. Fig. 10 shows the POM images (magnified compared with previous images) of the striped NSLC array in the course of photocatalytic dye decomposition with the parallel geometry obtained at the beginning (0 min), and before (12 min) and after the acceleration (32 min) of the reaction. Observations
Table 2 First-order rate constants for the photocatalytic decomposition of the PIC dye in the striped niobate NSLC array
Geometry of the setupa
k1b/min1
k2c/min1
Parallel k(J) Orthogonal k(>) k(J)/k(>)d
0.0014 0.014 10
0.17 0.26 1.5
a
Fig. 9 Time courses of the photocatalytic decomposition of the PIC dye added to the stripe niobate NSLC array ([niobate] = 5 g L1, [PIC] = 0.1 mmol L1) upon the irradiation with polarized UV light for (a) parallel and (b) orthogonal geometries. The inset shows spectral changes for PIC with the parallel geometry.
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Parallel or orthogonal, indicating the relationship between the direction of the polarization of the impinging light and that of the stripes (see Fig. 2). b Rate constants before the acceleration of the reaction. c Rate constants after the acceleration of the reaction. d The ratio of the rate constant for the parallel geometry to that for the orthogonal geometry in the same reaction region, i.e., before or after the acceleration.
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findings provide seeds for applications of the NSLC arrays for remotely controlled photofunctional systems.
Acknowledgements This work was supported by MEXT KAKENHI Grant Number 25107728 and JSPS KAKENHI Grant Number 24350107. We thank Professor Kozo Kuchitzu (Tokyo University of Agriculture and Technology) for his valuable comments on English style. Fig. 10 POM images of the striped niobate NSLC array including the PIC dye ([niobate] = 5 g L1, [PIC] = 0.1 mmol L1) observed before the photocatalytic dye decomposition (0 min, a and d) and after the irradiation of polarized UV light with the parallel geometry for 12 min (b and e) and 32 min (c and f). Direction of the polarizer was inclined by 451 (a–c) or parallel (d–f) with respect to gravity.
with the direction of the polarizer inclined by 451 with respect to gravity, i.e., stripes (Fig. 10a–c) indicate that the thick stripe clearly observed at the beginning of the reaction is degraded with the progress of reaction whereas the degraded stripes basically retain their direction along with the gravity. However, observations with the direction of the polarizer parallel to the stripes (Fig. 10d–f) show a difference before and after the acceleration of the reaction. A rather large birefringent area is observed for the sample after the acceleration (Fig. 10f) although the images are almost dark until the acceleration. Because the nanosheets parallel to the stripes are non-birefringent with this setup, the birefringent area indicates the appearance of disordered nanosheets outside the stripes after the reaction passes the acceleration point. The disordering explains the small difference in the reaction rates between the parallel and orthogonal geometries. This structural alteration is rationalized by perturbation of the microenvironments around the nanosheets during the reaction through consumption of the reactants and generation of the products on the niobate nanosheets.
Conclusions In summary, the niobate NSLC is converted to hierarchically organized soft arrays. The incubation and the application of external forces under appropriate conditions permit structural variations in the arrays. The incubation of NSLC raises the nanosheet tactoids which act as the secondary building blocks of the arrays. These arrays are novel inorganic soft materials with macroscopic structures consisting of mobile crystalline oxide blocks. The large colloidal mesogens of the NSLC compared with the molecular mesogen of organic LCs enable panoscopic construction of the macroscopic structures characterized by a sub-mm to mm length scale. The use of semiconducting niobate nanosheets materializes photocatalysis of the arrays, and their reaction can be regulated by their macroscopic higher-order structures. Since the array structure can be modified with external forces regardless of the reaction, the present
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References 1 I. W. Hamley, Introduction to Soft Matter: Synthetic and Biological Self-Assembling Materials (Revised Edition), Wiley, Chichester, 2007. 2 G. A. Ozin, Chem. Commun., 2000, 419–432. 3 S. Mann, Nat. Mater., 2009, 8, 781–792. 4 T. Kato, T. Sakamoto and T. Nishimura, MRS Bull., 2010, 35, 127–132. 5 I. Langmuir, J. Chem. Phys., 1938, 6, 873–896. 6 L. Onsager, Ann. N. Y. Acad. Sci., 1949, 51, 627–659. 7 G. J. Vroege and H. N. W. Lekkerkerker, Rep. Prog. Phys., 1992, 55, 1241–1309. 8 J.-C. P. Gabriel and P. Davidson, Top. Curr. Chem., 2003, 226, 119–172. 9 A. S. Sonin, J. Mater. Chem., 1998, 8, 2557–2574. 10 P. Davidson and J.-C. P. Gabriel, Curr. Opin. Colloid Interface Sci., 2005, 9, 377–383. 11 H. N. Lekkerkerker and G. J. Vroege, Philos. Trans. R. Soc., A, 2013, 371, 20120263. 12 N. Miyamoto and T. Nakato, Isr. J. Chem., 2012, 52, 881–894. 13 T. Nakato and N. Miyamoto, Materials, 2009, 2, 1734–1761. 14 N. Miyamoto and T. Nakato, Adv. Mater., 2002, 14, 1267–1270. 15 N. Miyamoto and T. Nakato, J. Phys. Chem. B, 2004, 108, 6152–6159. 16 T. Nakato, N. Miyamoto and A. Harada, Chem. Commun., 2004, 78–79. 17 D. Yamaguchi, N. Miyamoto, T. Fujita, T. Nakato, S. Koizumi, N. Ohta, N. Yagi and T. Hashimoto, Phys. Rev. E, 2012, 85, 011403. 18 J.-C. P. Gabriel, C. Sanchez and P. Davidson, J. Phys. Chem., 1996, 100, 11139–11143. 19 L. J. Michot, I. Bihannic, S. Maddi, S. S. Funari, C. Baravian, P. Levitz and P. Davidson, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 16101–16104. 20 H. Hemmen, N. I. Ringdal, E. N. De Azevedo, M. Engelsberg, ´heust, J. O. Fossum and K. D. Knudsen, E. L. Hansen, Y. Me Langmuir, 2009, 25, 12507–12515. 21 E. Paineau, K. Antonova, C. Baravian, I. Bihannic, P. Davidson, ´ror-Clerc, P. Levitz, A. Madsen, F. Meneau I. Dozov, M. Impe and L. J. Michot, J. Phys. Chem. B, 2009, 113, 15858–15869. 22 N. Miyamoto, H. Iijima, H. Ohkubo and Y. Yamauchi, Chem. Commun., 2010, 46, 4166–4168. 23 J.-C. P. Gabriel, F. Camerel, B. J. Lemaire, H. Desvaux, P. Davidson and P. Batail, Nature, 2001, 413, 504–508.
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Panoscopic organization of anisotropic colloidal structures from photofunctional inorganic nanosheet liquid crystals† Teruyuki Nakato,*a Yoshihiro Nono,a Emiko Mouria and Munetaka Nakatab Colloidal liquid crystals of inorganic nanosheets with thickness of around 1 nm and lateral dimensions of several micrometers prepared by exfoliation of a layered niobate are converted to hierarchically organized arrays whose structures are controlled from the nano to macroscopic length scale through the growth of liquid crystalline domains called tactoids as the secondary building blocks followed by controlled application of external fields. Growth of the tactoids is attained by incubation of the liquid crystals at room temperature. The tactoids are then assembled into higher-order structures with characteristic lengths of sub-mm to mm under the simultaneous application of an ac electric field and gravity, whose directions determine the final textural motif of the arrays. Whereas a net-like texture is observed when applying the electric and gravitational forces in the same direction, a striped texture
Received 1st October 2013, Accepted 1st November 2013
where the nanosheets are unidirectionally aligned is observed when the electric field is applied in the
DOI: 10.1039/c3cp54140a
control. Since the niobate nanosheets have wide band-gap semiconducting nature, the nanosheet stripe arrays exhibit photocatalysis that reflected the alignment of the nanosheets with respect to the polarized
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direction of impinging light.
direction perpendicular to gravity. The use of well-grown tactoids is key to the macroscopic structural
Introduction Hierarchical soft structure is a key concept for novel intelligent materials, and such structures have been well realized in various organic systems exemplified by liquid crystals (LCs) and gels.1 However, panoscopic organization, the term which means structural control over all length-scales from micro-, meso-, to macroscopic,2 of soft matter has still insufficiently been developed because its building blocks are usually organic moieties which lack robustness for the retention of defined structures up to macroscopic scales. If we can use inorganic building blocks, panoscopic structures will be constructed for soft matter rather easily. In addition, versatile properties of inorganic crystals such as semiconducting, dielectric, and
a
Department of Applied Chemistry, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu-shi, Fukuoka 804-8550, Japan. E-mail: [email protected] b Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei-shi, Tokyo 184-8588, Japan † Electronic supplementary information (ESI) available: Schematic representation of the effects of external forces on the alignment of the colloid liquid crystals; the TEM image and size distribution of the nanosheets, optical and POM images of an as-prepared NSLC; the conoscope image of the just injected NSLC; additional POM and FOM images of the NSLC arrays; experimental setting and results of the control experiments of photocatalytic tests. See DOI: 10.1039/c3cp54140a
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(photo)catalytic functions, can be utilized in such materials. Nevertheless, inorganic soft structures, in particular panoscopic ones, remain almost unexplored, although organization of inorganic building blocks into flexible structures with the aid of organic components as soft moieties has attracted interest for developing novel advanced materials by mimicking biosystems.3,4 We expect that colloidal LCs of inorganic particles are promising systems for organizing inorganic panoscopic soft structures. Colloidal LCs are lyotropic systems giving ordered structures of anisotropic particles like rods and plates based on the excluded volume effect.5–11 They are very rare examples of the inorganic soft structures, where crystalline colloidal particles are mobile but oriented in the solvents. Among them, nanosheet LCs (NSLCs) are interesting end members because they are constructed using extremely thin 2D particles that can be obtained through exfoliation of broadly diverse inorganic layered compounds.12,13 The nanosheets are characterized by mm-sized mesogens with high anisotropy ensured by their thickness of around 1 nm. In fact, previous studies on NSLCs have shown generation of stable liquid crystalline phases from the nanosheets of semiconducting oxides,14–17 clays,18–22 phosphates,23,24 and graphenes.25–27 Because of the robustness of the inorganic crystallites and the mesogen size being much larger than that of conventional organic molecules, the NSLCs can be used to construct
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large-scale structures, which is impossible with the organic liquid crystals, with characteristic lengths of sub-mm to mm through panoscopic organization of the nanosheets. The nanosheets must have rich alternatives of arrangements for higher-order structures compared with 1D rods, because 2D particles have two different axes for regulation of their alignment while 1D particles are unidirectionally aligned with a single external force (Fig. S1, ESI†). Hence, we may obtain a wide variety of hierarchically structured arrays of NSLCs with careful orientational control. The NSLC arrays are low-density soft self-assemblies that can be remotely controlled by external stimuli based on the liquid crystallinity, being very different from conventional colloidal arrays where symmetrical particles are densely organized.28,29 However, no previous studies have yet achieved such a hierarchical and macroscopic, i.e., panoscopic structural control of the NSLCs. We report herein photocatalytically active panoscopic NSLC arrays of oxide nanosheets fabricated from layered niobate K4Nb6O17 known as a wide band-gap semiconductor through hierarchical organization of the anisotropic crystalline mesogens. The panoscopic nature is provided by a two-stage assembly of the nanosheets. The first stage is controlled growth of the LC domains called tactoids, the word which is used for colloidal domains of anisotropically associated rods or plates,30,31 by assembly of the nanosheets. This is the key step of panoscopic organization because the grown tactoids have the dimensions of sub-mm length and act as the secondary building blocks of the final structures, although the structural control by domains has not been reported in previous studies of NSLCs. Then, the tactoids are further organized into higher-order structures with external fields. Since external electric and magnetic forces can be used as tools for aligning the NSLCs in a homeotropic manner,21,23,24,32,33 we may arrange the nanosheets into hierarchically regulated panoscopic structures with the application of dual external forces such as electric field and gravity. Parallel and orthogonal applications of the external forces induce different array structures from the same tactoids because of the 2D nature of the LC mesogens. Furthermore, the semiconducting nature of the nanosheets34,35 allows the NSLC arrays to undergo the photocatalytic reaction, which is controlled by the arrangement of the nanosheets with respect to the impinging light. Semiconductor photocatalysts structurally regulated with liquid crystallinity have not been reported except for a solid film prepared from the liquid crystalline photocatalytic rods.36
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gradually cooled.15,37 K4Nb6O173H2O (1 g) was treated with a 0.2 mol L1 aqueous solution of propylammonium chloride (Tokyo Chemical Industry Co., Ltd., Japan) at 120 1C for a week to exchange the interlayer potassium ions for propylammonium ions.15,38 The reaction product was centrifuged at 11 000 rpm, washed, and dialyzed with water to yield the stock sample of niobate NSLC. The NSLC sample was diluted with water to 5 g L1 of the niobate concentration (indicated as the mass of [Nb6O17]4) for each measurement. The lateral lengths of the nanosheets obtained from TEM observations had a size distribution obeying a log-normal distribution15 to give an average size of 2.0 mm (Fig. S2, ESI†). Liquid crystallinity was indicated by the appearance of the Schlieren texture in the polarized optical microscope (POM) image (Fig. S3, ESI†). Samples for fluorescence microscope observations were prepared by adding an aqueous solution of tris(2,2 0 -bipyridyl)ruthenium(II) (Ru(bpy)32+) dichloride (Sigma-Aldrich Co., USA) to NSLC of the niobate followed by stirring for 1 h. The concentration of Ru(bpy)32+ was set to 5 105 mol L1. For the photocatalytic decomposition of the cyanine dye, an aqueous solution of 1,1 0 -diethyl-2,2 0 -cyanine (PIC) bromide (Hayashibara Biochemical Laboratories Inc., Japan) was added to NSLC, and mixed for 1 h. The concentration of PIC was set to 1 104 mol L1. The concentration of the niobate nanosheets was 5 g L1 for both of the experiments. Organization of the NSLC arrays The niobate NSLC sample was organized into the NSLC arrays by controlling three parameters: incubation for the growth of tactoids, external electric field, and gravity. A small aliquot of the NSLC sample was injected into a thin layer liquid crystalline (LC) cell, where two ITO-coated glass plates were faced with a 100 mm film spacer (Teijin DuPont Films Japan) and sealed with epoxy resin.32 The sample injected into the cell was incubated to allow the growth of tactoids by making the cell stand with the ‘‘flat’’ cell setting (i.e., at the horizontal position, Fig. 1, left) under ambient conditions. The incubation time, i.e., the period from the sample injection to the application of electric field is varied.
Experimental section Sample preparation The NSLC sample (5 g L1), where negatively charged niobate (Nb6O174) nanosheets accompanied by propylammonium ions acted as the countercations, was prepared.15 Single crystalline K4Nb6O17 was prepared by a flux method; a mixture of K2CO3 (Wako Pure Chemical Industries, Ltd., Japan) and Nb2O5 (Soekawa Chemicals Co., Ltd., Japan) was heated at 1150 1C and
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Fig. 1 Setup of the LC cell for incubation and ac voltage application to the niobate NSLC. The ac voltage is applied from the same direction as that of gravity with the ‘‘flat’’ cell setting while the voltage is applied from the direction perpendicular to that of gravity with the ‘‘upright’’ cell setting. The arrows indicate the direction of electric field (E) and gravity (g), respectively.
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Alternate current (ac) electric field (500 V cm1, 50 kHz) supplied by an NF BP4610 bipolar generator was applied to the NSLC sample in the ITO cell. These conditions were based on the results of our previous study, where the electrooptic response of the niobate NSLC was investigated.32 The frequency of 50 kHz was chosen because lower frequencies induce boiling of the colloid due to phoretic behavior of the nanosheets. The electric field strength of 500 V cm1 was found to be a minimal value that stationarily aligns the nanosheets with the ac application. We examined two cell settings, i.e., ‘‘flat’’ and ‘‘upright’’ settings (Fig. 1, right). While the ac electric field was applied in the direction perpendicular to the ITO substrate surface, the direction of gravity depends on the cell setting. The electric field and gravity were applied from the same direction with the flat cell setting, but they were in the directions perpendicular to each other with the upright setting. For the ac application to the cell with the upright setting, the cell incubated with the flat setting was turned to the upright position by hand, and then the application of ac voltage was started immediately. Microscope observations The polarized optical microscope (POM) and fluorescence optical microscope (FOM) observations were performed using an Olympus BX-51 optical microscope equipped with a BX2-FL-1 fluorescence unit using an Olympus U-MSWB2 mirror. Optical retardation of the birefringent areas that appeared in the POM images was determined by the Senarmont method4 using an Olympus U-CSE compensator under monochromatic radiation of 550 nm produced by an Olympus 43IF550W45 green bandwidth filter. The compensation was carried out for a typically birefringent area of 0.003 mm2. Conversion of the colored FOM images to the gray-scaled ones was carried out using Adobe Photoshop Elements 8.0 software. The average area of the tactoids was evaluated by pixels of more than 15 tactoids on gray-scaled FOM images. Photocatalytic decomposition of the PIC dye For the photocatalytic experiments, the PIC-added NSLC sample was injected into the ITO cell, and irradiated with UV light using an Ushio SX-UI-251-HQ high-pressure mercury lamp with the upright cell setting. The light was passed through a HOYA U340 bandpass filter to remove visible light and an OPTO-LINE PUV-32 polarizer to irradiate the sample with polarized UV light (Fig. 2). Irradiation with polarized UV light from a mercury lamp provided two optical setups: the polarizing direction of the impinging light was parallel or orthogonal to the direction of gravity applied to the NSLC array. These optical setups will be called the parallel and orthogonal geometries (see Fig. 2). Visible spectra of the NSLC sample were measured before and after the irradiation by using a Shimadzu UV-2450 spectrophotometer in the range of wavelength from 450 nm to 650 nm. PIC concentrations were estimated from spectral data after subtraction of the background spectrum to remove the effect of Rayleigh scattering from the nanosheets.
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Fig. 2 Experimental setup for the photocatalytic decomposition of the PIC dye in the stripe-patterned niobate NSLC array (5 g L1) under the ac voltage application. The light from the mercury lamp was polarized and then impinged on the NSLC. The reaction was carried out with two optical geometries called parallel and orthogonal geometries corresponding to the setups with the polarized direction of the UV light being parallel and perpendicular to gravity, respectively.
Results and discussion Tactoid growth in the NSLC with incubation The first stage of the panoscopic organization of the niobate NSLC is the growth of tactoids with the incubation. This step is clarified by the combined observations of POM and FOM using tris(2,2 0 -bipyridine)ruthenium(II) (Ru(bpy)32+) as a luminescent probe adsorbed on the nanosheets. The POM observations represent structural alteration of the NSLC, and the results are independent of the addition of Ru(bpy)32+. A dark POM image for the sample just after the injection into the cell indicates homogeneous orientation of the nanosheets with their in-plane direction parallel to the ITO substrate (Fig. 3a and p);32 a conoscope image showing an isogyre cross supports the presence of liquid crystalline ordering (Fig. S4, ESI†). In contrast, birefringent spots appear in the POM images of the incubated samples (Fig. 3b–e), indicating the presence of the nanosheets inclined or perpendicular to the ITO surface. Although the FOM images show an almost uniform red view irrespective of the incubation (Fig. 3f–j), suggesting homogeneous distribution of the Ru(bpy)32+-attached nanosheets, the gray-scaled FOM images provide evidence for the growth of tactoids (Fig. 3k–o). The gray-scaled image of the sample just injected is apparently uniform without distinguishable domains (Fig. 3k), but the incubated samples exhibit domain boundaries indicated by dark lines (Fig. 3l–o). Superimposition of the POM and gray-scaled FOM images indicates that the domain boundaries found with FOM correspond to the edge and disclination lines of the birefringent areas in the POM image (Fig. 4). Thus, the nanosheets are aligned parallel to the ITO surface inside the domains but inclined at the boundaries and defects, as illustrated in Fig. 3q. These results show that orientationally ordered nanosheets are associated during the incubation to form tactoids. Longer incubation provides larger tactoids, as shown in Fig. 5, indicating that the tactoid growth occurs through Ostwald ripening. A well-grown tactoid has a lateral size of up to 100 mm. The size is
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Fig. 3 POM (a–e), FOM (f–j), and gray-scaled FOM (k–o) images of the niobate NSLC (5 g L1) before (a, f and k) and after the incubation for 60 min (b, g and i), 120 min (c, h and m), 180 min (d, i and n), 360 min (e, j and o); schematic representation of the nanosheet arrangement in the NSLC before (p) and after (q) the incubation. The direction of gravity, indicated by broad arrows, is perpendicular to the POM images.
Formation of panoscopic structures through alignment of the grown tactoids
Fig. 4 Superimposed microscopic image (a) of the niobate NSLC (5 g L1) after the incubation for 120 min produced from POM (b) and gray-scaled FOM images (c).
Fig. 5 Increase in the average tactoid area estimated from the grayscaled FOM images as a function of the incubation time.
much larger than that of a single nanosheet (B2 mm), and appropriate for the organization of macroscopic structures.
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The NSLC tactoids are further organized to generate hierarchically structured macroscopic arrays under external forces. Application of the electric field and gravity from the same direction based on the flat cell setting yields NSLC arrays of net-like structures with characteristic lengths of sub-mm, as shown in Fig. 6. The ac voltage applied perpendicular to the substrate surface changes the nanosheet arrangement, from parallel to perpendicular with respect to the substrate, as evidenced by the increase in the birefringent area of the POM images (Fig. 6a–c), because the nanosheets are aligned with their surfaces parallel to the electric field.32 However, the texture of the electrically aligned NSLC samples depends on the incubation time. Upon application of
Fig. 6 POM and FOM images and schematic nanosheet arrangements of the niobate NSLC (5 g L1) with flat cell setting. The ac voltage was applied for 8 min from the direction parallel to gravity through the flat cell setting immediately after the sample injection (a, d and g), after the incubation for 60 min (b, e and h) and 120 min (c, f and i). The direction of gravity (g) is perpendicular to the POM images.
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ac voltage, the samples with short incubation times exhibit isolated birefringent dots, whereas the samples form nets if the incubation before the ac voltage application is extended. The longer the incubation time, the coarser the meshes become. This fact indicates that the tactoids work as the secondary building blocks of the NSLC arrays to give the final structures different from those with short incubation, as illustrated in Fig. 6g–i, although the nanosheets themselves are aligned parallel to the electric field irrespective of the incubation time. The FOM images indicate concentration fluctuation of the nanosheets in the sample induced by the tactoid growth (Fig. 6d–f). The luminescent areas are identical with those of birefringent. Thus, most of the nanosheets are localized in the tactoids, and the solvent (water) is present in the dark and nonfluorescent regions. This is in contrast with a sample without incubation, because its FOM image indicates almost homogeneous luminescence, which indicates small concentration fluctuations of the nanosheets in the colloid in the absence of the tactoids. Consequently, the results indicate that the niobate NSLC forms panoscopic colloidal arrays with careful growth of the tactoids. When dual external forces are applied from orthogonal directions, the NSLC samples form structurally different arrays characterized by unidirectional nanosheet orientation. The samples are incubated with the flat cell setting and then turned upright to direct the electric field and gravity perpendicular as soon as the start of ac application. The birefringent area increases with the ac application (Fig. 7a–c), and striped structures are obtained. The stripe texture reflects the unidirectional alignment. Each stripe has a characteristic length of sub-mm in thickness and mm in length, indicating the panoscopic nature of the array structure. This is quite different from the net-like textures obtained when the voltage and gravity are applied from the same direction. As observed with FOM
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(Fig. 7d–f), the luminescent areas correspond to the birefringent stripes for the incubated samples, indicating the concentration fluctuation of the nanosheets through association with the tactoids; i.e., most of the nanosheets are localized in the stripes as illustrated in Fig. 7g–i. In these NSLC arrays, the direction of stripes is the same as that of gravity. This rationalizes the unidirectional nanosheet orientation. The 2D particles need two different regulation axes to be aligned unidirectionally (see Fig. S1, ESI†). The electric field applied to the NSLC sets the direction of the nanosheets (and their tactoids) perpendicular to the ITO plate, and the in-plane direction with respect to the substrate surface is determined using the gravity applied from the direction perpendicular to the ac voltage. Although the stripe motif is common to the samples with different incubation periods, the growth of the nanosheet tactoids influences the quality of stripes. Thick stripes are generated in the samples with longer incubation, and optical retardation by the birefringent stripes estimated using a Senarmont method (Table 1) indicates that more birefringent stripes are formed in these cases. This is explained by the presence of well-grown tactoids in the enough incubated samples because more grown tactoids that assemble larger numbers of nanosheets give larger birefringence. Hence, the hierarchical organization via the tactoids growth is still important for the formation of NSLC arrays under the orthogonal external fields. POM observations of the evolution of the stripe texture indicate flow-induced orientation with gravity (Fig. 8). After application of the ac voltage for 1 min, i.e. just after switching of the cell setting from flat to upright, the sample exhibits a netlike texture. This indicates that the change in the direction of gravity induced by switching the cell setting within such a short period has no effect on the nanosheet alignment. Teardrop-like domains appear along the direction of gravity through continuous orthogonal application of the ac electric field and the gravity for several minutes, and then the teardrops are combined into stripes for longer application of the external fields. The results demonstrate coalescence of the tactoids to stripes induced by solvent (water) flow along gravity. Although this flow-induced alignment is essentially a transient state of the tactoids going to be settled, the arrayed structure is maintained for several tens of minutes or even longer as indicated by the
Table 1 Optical retardation of the stripe-patterned niobate NSLC arrays obtained at a constant ac voltage of 500 V cm1 for 60 min and various incubation periods
Fig. 7 POM and FOM images and schematic nanosheet arrangements of the niobate NSLC (5 g L1) with upright cell setting. The ac voltage was applied for 60 min from the direction perpendicular to gravity through the upright cell setting immediately after the sample injection (a, d and g), after the incubation for 60 min (b, e and h) and 120 min (c, f and i). The direction of the electric field (E) is perpendicular to the POM images. The direction of gravity (g), indicated by broad arrows, is parallel to the POM images.
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Incubation time/min
Optical retardationa/nm
0b 60 120 180
47 74 86 88
a
Determined by the Senarmont method. The measurement of optical retardation was started after 60 min of ac voltage application and was finished in 2 min. b Sample setting was changed to upright setting within 5 min after the sample injection, and the ac voltage application was started simultaneously with the turning of cell setting.
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Fig. 8 POM images of the niobate NSLC (5 g L1) to which the ac voltage was applied for 1, 4, 8, 16, 30, and 60 min (from (a) to (f)) with the upright cell setting after the incubation for 120 min. The image of the sample with 60 min of ac voltage application is shown in Fig. 3, but reproduced for comparison purpose. A and P indicate the directions of the analyzer and the polarizer, respectively. The direction of the electric field (E) is perpendicular to the POM images. The direction of gravity (g) is indicated by the arrow in the POM image.
POM image of the sample after the ac application with the upright setting for one hour (Fig. 8f). Photocatalysis by an NSLC array Unidirectionally aligned NSLC arrays with striped texture exhibit photocatalytic behavior that reflects the anisotropic structure of the array. The probe reaction is photocatalytic decomposition of PIC molecules which are adsorbed and laid flat on the niobate nanosheets.39 Addition of the dye to a typical striped NSLC array (incubation for 60 min) does not affect the array organization, and a characteristic absorption band of PIC forming a J-aggregate is displayed on the niobate nanosheets (Fig. 9 inset).39 The PIC molecules are decomposed upon irradiation with the polarized
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UV light of the NSLC array with both the parallel and orthogonal optical geometries shown in Fig. 2; the geometry indicates the direction of the polarization of impinging light being set parallel or perpendicular to the direction of gravity, that is, the direction of stripes. The reaction is ascribed to the photocatalysis of the niobate nanosheets, because the decomposition is negligible in the dark or without the nanosheets (Fig. S5, ESI†). The dye decomposition proceeds faster when the polarization direction of the impinging UV light is parallel to the stripes of the NSLC array rather than perpendicular (Fig. 9). This is explained by the matching of the directions of the light polarization and nanosheets. Photoexcitation of the semiconducting nanosheets should be more efficient when the impinging light is vibrating in the direction parallel to the nanosheet edges than that in the orthogonal direction. Thus, the photocatalytic reaction should proceed faster with the parallel geometry. The present results demonstrate the usefulness of the nanosheet ordering at the centimeter length scale provided by the panoscopic organization because such a macroscopic structure allows the reaction control by using a conventional light source. On the other hand, the time courses of the photocatalytic PIC dye decomposition indicate acceleration in the course of reaction. Table 2 compares the first-order rate constants of the reactions in the regions of before and after the acceleration for both the parallel and orthogonal geometries; the logarithmic plots of the reaction time courses for the calculation of rate constants are shown in Fig. S6 (ESI†). In the slow reaction region (before the acceleration), the reaction with the parallel geometry is much faster than that with the orthogonal geometry, as indicated by the difference in an order of the rate constants. However, after the acceleration, the rate constants for both of the geometries become close to each other although the reaction proceeds still somewhat faster with the parallel geometry. This behavior suggests a structural alteration of the NSLC array during the reaction. Optical microscope observations of the samples indicate partial disruption of the unidirectional nanosheet alignment during the reaction. Fig. 10 shows the POM images (magnified compared with previous images) of the striped NSLC array in the course of photocatalytic dye decomposition with the parallel geometry obtained at the beginning (0 min), and before (12 min) and after the acceleration (32 min) of the reaction. Observations
Table 2 First-order rate constants for the photocatalytic decomposition of the PIC dye in the striped niobate NSLC array
Geometry of the setupa
k1b/min1
k2c/min1
Parallel k(J) Orthogonal k(>) k(J)/k(>)d
0.0014 0.014 10
0.17 0.26 1.5
a
Fig. 9 Time courses of the photocatalytic decomposition of the PIC dye added to the stripe niobate NSLC array ([niobate] = 5 g L1, [PIC] = 0.1 mmol L1) upon the irradiation with polarized UV light for (a) parallel and (b) orthogonal geometries. The inset shows spectral changes for PIC with the parallel geometry.
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Parallel or orthogonal, indicating the relationship between the direction of the polarization of the impinging light and that of the stripes (see Fig. 2). b Rate constants before the acceleration of the reaction. c Rate constants after the acceleration of the reaction. d The ratio of the rate constant for the parallel geometry to that for the orthogonal geometry in the same reaction region, i.e., before or after the acceleration.
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findings provide seeds for applications of the NSLC arrays for remotely controlled photofunctional systems.
Acknowledgements This work was supported by MEXT KAKENHI Grant Number 25107728 and JSPS KAKENHI Grant Number 24350107. We thank Professor Kozo Kuchitzu (Tokyo University of Agriculture and Technology) for his valuable comments on English style. Fig. 10 POM images of the striped niobate NSLC array including the PIC dye ([niobate] = 5 g L1, [PIC] = 0.1 mmol L1) observed before the photocatalytic dye decomposition (0 min, a and d) and after the irradiation of polarized UV light with the parallel geometry for 12 min (b and e) and 32 min (c and f). Direction of the polarizer was inclined by 451 (a–c) or parallel (d–f) with respect to gravity.
with the direction of the polarizer inclined by 451 with respect to gravity, i.e., stripes (Fig. 10a–c) indicate that the thick stripe clearly observed at the beginning of the reaction is degraded with the progress of reaction whereas the degraded stripes basically retain their direction along with the gravity. However, observations with the direction of the polarizer parallel to the stripes (Fig. 10d–f) show a difference before and after the acceleration of the reaction. A rather large birefringent area is observed for the sample after the acceleration (Fig. 10f) although the images are almost dark until the acceleration. Because the nanosheets parallel to the stripes are non-birefringent with this setup, the birefringent area indicates the appearance of disordered nanosheets outside the stripes after the reaction passes the acceleration point. The disordering explains the small difference in the reaction rates between the parallel and orthogonal geometries. This structural alteration is rationalized by perturbation of the microenvironments around the nanosheets during the reaction through consumption of the reactants and generation of the products on the niobate nanosheets.
Conclusions In summary, the niobate NSLC is converted to hierarchically organized soft arrays. The incubation and the application of external forces under appropriate conditions permit structural variations in the arrays. The incubation of NSLC raises the nanosheet tactoids which act as the secondary building blocks of the arrays. These arrays are novel inorganic soft materials with macroscopic structures consisting of mobile crystalline oxide blocks. The large colloidal mesogens of the NSLC compared with the molecular mesogen of organic LCs enable panoscopic construction of the macroscopic structures characterized by a sub-mm to mm length scale. The use of semiconducting niobate nanosheets materializes photocatalysis of the arrays, and their reaction can be regulated by their macroscopic higher-order structures. Since the array structure can be modified with external forces regardless of the reaction, the present
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