Fabrication of Three-Layer-Component Organoclay Hybrid Films with

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Fabrication of Three-Layer-Component Organoclay Hybrid Films with Reverse Deposition Orders by a Modified Langmuir−Schaefer Technique and Their Pyroelectric Currents Measured by a Noncontact Method Masanari Hirahara and Yasushi Umemura* Department of Applied Chemistry, National Defense Academy, Hashirimizu 1-10-20, Yokosuka, Kanagawa 239-8686, Japan S Supporting Information *

ABSTRACT: In an aqueous clay mineral (montmorillonite) dispersion at a low concentration, isolated clay nanosheets with negative charges were suspended. When a solution of amphiphilic octadecylammonium chloride (ODAH+Cl−) was spread on an air-dispersion interface, the clay nanosheets were adsorbed on the ODAH+ cations at the interface to form a stable ultrathin floating film. The floating film was transferred onto a substrate by the Schaefer method, and then the film was immersed in a [Ru(dpp)3]Cl2 (dpp = 4,7-diphenyl-1,10phenanthroline) solution. The Ru(II) complex cations were adsorbed on the film surface because the film surface possessed a cation-exchange ability. The layers of ODAH+, clay nanosheets, and [Ru(dpp)3]2+ were deposited in this order. By repeating these procedures, three-layer-component films were fabricated (OCR films). In a similar way, three-layer-component films in which the layers of [Ru(dpp)3]2+, clay nanosheets, and ODAH+ were deposited in the reverse order (RCO films) were prepared by spreading a [Ru(dpp)3](ClO4)2 solution and immersing the films in an ODAH+Cl− solution. Both OCR and RCO films were characterized by surface pressure−molecular area (π−A) curve measurements, IR and visible spectroscopy, and the XRD method. The OCR and RCO film systems possessed nearly the same properties in the densities of ODAH+ and [Ru(dpp)3]2+ and the tilt angle of the Ru(II) complex cation, although the layer distance for the RCO film was a little longer than that for the OCR film and the layered structure for the RCO film was less ordered than that for the OCR film. Pyroelectric currents for the films were measured by a noncontact method using an 241Am radioactive electrode. When the films were heated, the pyroelectric currents were observed and the current directions for the OCR and RCO films were different. This was clear evidence that the layer order in the OCR film was reverse of that in the RCO film.

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INTRODUCTION Montmorillonite, a clay mineral, is a typical layered material and it exfoliates into the isolated and negatively charged layers (termed “clay nanosheets”, hereafter) with a thickness of about 1 nm in a dilute aqueous dispersion.1−4 When a chloroform solution of an amphiphilic salt such as octadecylammonium chloride (ODAH+Cl−) is spread at an air−clay dispersion interface, the clay nanosheets are adsorbed electrostatically on a floating film of the amphiphilic cations and an organoclay hybrid monolayer is formed.5−22 The hybrid monolayers are transferred onto a substrate surface by a horizontal lifting method (Schaefer method) to fabricate a multilayer film.23−26 We have studied this type of ultrathin hybrid film.19−21,27−33 The hybrid films have some interesting features. First, the layer structures of the hybrid films are very stable, which is different from the structural stability of conventional Langmuir− Blodgett (LB) or Langmuir−Schaefer (LS) films.19,20,27,33 Second, the densities of the amphiphilic cations in the films are controllable by changing clay concentrations in the dispersions; in a high-concentration dispersion, the adsorption © 2015 American Chemical Society

rate of the clay is high so that the clay nanosheets are adsorbed on a floating film of the amphiphilic cations before the cations form closely packed domains through hydrophobic interaction.19,20,27 Third, the cation-exchange capacity remains on the clay nanosheet surface of the hybrid film because the negative charge density of the clay nanosheet layer is higher than the positive charge density of the amphiphilic cation layer in the film. Hence, one can form another cationic layer (as the third layer component) by immersing the film surface in a corresponding salt solution after every deposition of the organoclay hybrid monolayer from the air-dispersion interface.20,27,33 The density of the third cationic component in the film is dependent on the charge balance between the clay and amphiphilic cation layers. It is noteworthy that the hybrid three-layer-component films showed second harmonic generation activities due to their noncentrosymmetric layer Received: April 27, 2015 Revised: July 6, 2015 Published: July 21, 2015 8346

DOI: 10.1021/acs.langmuir.5b01401 Langmuir 2015, 31, 8346−8353

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Langmuir order.20,27,32 The hybrid films providing well-defined 2-D molecular array were studied toward applications to, for example, platforms for energy transfer,9 optical and magnetic devices,13,14,26,28,32 electrode modifiers,16,17 sensors,18,29 and catalysts.31 It is well known that conventional Langmuir films, molecular single layers floating on water surfaces, are formed with waterinsoluble amphiphilic substances. Contrary to the conventional Langmuir films, floating hybrid films are possibly prepared by spreading water-soluble amphiphilic cations on clay dispersions.19,30 This is another feature of the organoclay hybrid films. The solubility in water for a ruthenium(II) complex salt of [Ru(dpp)3](ClO4)2 (dpp = 4,7-diphenyl-1,10-phenanthroline) is very low. When a chloroform solution of [Ru(dpp)3](ClO4)2 is spread on the surface of an aqueous solution of a salt such as NaClO4, a stable Langmuir film is formed.34−36 However, when the Ru(II) complex salt solution was spread on a water surface at a neutral pH (without any salts), the floating Langmuir film was formed, but it was unstable (Figure S1, 0 ppm). If the Ru(II) complex salt solution was spread on an aqueous clay nanosheet dispersion, then the [Ru(dpp)3]2+ Langmuir film was stabilized by hybridization with the clay nanosheets. It was expected that the hybrid floating film of the Ru(II) complex cations and the clay nanosheets could be transferred on a substrate, and then the film surface was dipped in a solution of the alkylammonium salt, ODAH+Cl−, to form a three-layercomponent film composed of the [Ru(dpp)3]2+ cations, the clay nanosheets, and the ODAH+ cations in this order from the substrate surface. The three-layer-component films with this layer order were actually prepared in this study, which were called RCO films (Figure 1a).

A noncentrosymmetric system of a dielectric substance exhibits a pyroelectric property: a change in spontaneous polarization due to temperature change.37 The pyroelectric properties of noncentrosymmetric LB and LS films have been investigated38−43 since the early studies of azobenzene multilayers and merocyanine/stearylamine alternating multilayers.44,45 The application of LB and LS films to pyroelectric devices is an attractive idea because the noncentrosymmetric LB and LS films can be designed at molecular-layer levels.39,43 In this work, we actually prepared the three-layer-component hybrid films of [Ru(dpp)3]2+ cations, clay nanosheets, and ODAH+ cations with layer orders of RCO and OCR. The layered structures of these films were characterized by IR and visible (vis) spectroscopy and an X-ray diffraction (XRD) method. The reverse-ordered layers in the RCO and OCR systems were confirmed by observing their pyroelectric currents in directions opposite to each other. To avoid damaging the film surfaces during the pyroelectric measurements, we employed a noncontact method by using a radioactive electrode.

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EXPERIMENTAL SECTION

Film Preparation. An amphiphilic monoalkylammonium salt of octadecylammonium chloride [CH3(CH2)17NH4Cl:ODAH+Cl−] was dissolved in a mixed solvent of chloroform and ethanol (4:1 by volume) at 2 × 10−4 mol dm−3 (spreading solution for OCR film preparation) and also dissolved in a water/ethanol mixed solvent (9:1 by volume) at 1 × 10−5 mol dm−3 (for RCO film preparation). A ruthenium(II) complex salt of [Ru(dpp)3](ClO4)2 (dpp = 4,7diphenyl-1,10-phenanthroline, Figure 1c) was synthesized and purified according to the literature.36,46,47 A part of the perchlorate was changed to the corresponding chloride by using an anion-exchange resin. The perchlorate was dissolved in chloroform at 2 × 10−4 mol dm−3 (spreading solution for RCO film preparation), while the chloride was dissolved in water at 1 × 10−5 mol dm−3 (for OCR film preparation). An LB trough (KSV minitrough, Finland) was filled with the aqueous montmorillonite dispersion at 50 ppm (ppm = mg dm−3). The ODAH+Cl− spreading solution was dropped onto the dispersion surface. After waiting 30 min (for the adsorption of the clay nanosheets on the floating ODAH+ film), the floating film was compressed with barriers up to a surface pressure of 10 mN m−1. The floating film of ODAH+/clay was transferred on a substrate by a horizontal lifting method (Schaefer method). Subsequently, the film surface was immersed in the aqueous [Ru(dpp)3]Cl2 solution for 30 s, and then the surface was rinsed with water. In this way, one set of ODAH+, clay nanosheet, and [Ru(dpp)3]2+ (OCR) layers was deposited. (One set of the three layers will be called one hybrid layer hereafter.) By repeating these procedures, multilayer films were prepared (Figure 1b). The reverse-ordered three-component multilayers (the order of [Ru(dpp)3]2+, clay nanosheet, and ODAH+ (RCO) layers) were prepared in a similar way. The [Ru(dpp)3](ClO4)2 solution was spread on the clay dispersion surface, and after 30 min of waiting, the floating film was compressed up to 10 mN m−1. The film was transferred on a substrate by the horizontal lifting method, and then the film surface was immersed in the ODAH+ solution (in water/ethanol mixed solvent) for 30 s (Figure 1a). The RCO multilayers were fabricated by repeating these procedures. Measurements. Surface pressure (π) was measured by the Wilhelmy method. Transmission absorption spectra in the infrared (IR) and visible (vis) regions were recorded with a PerkinElmer Spectrum One Fourier transform IR spectrometer and a Hitachi U4100 UV−visible−NIR spectrophotometer, respectively. XRD patterns were taken with a PANalytical X’Pert MRD using a Cu Kα line. Pyroelectric currents were measured by a noncontact method using an 241Am radioactive electrode (Figure 2). The α beam from the

Figure 1. Film preparation for (a) RCO and (b) OCR films and (c) the molecular structure of a ligand (4,7-diphenyl-1,10-phenanthroline: dpp) in the Ru(II) complex cation.

On the other hand, because [Ru(dpp)3]Cl2 is soluble in water, it was possible to prepare another type of three-layercomponent film by spreading the ODAH+Cl− chloroform solution on the aqueous clay dispersion and then dipping the transferred hybrid film in an aqueous [Ru(dpp)3]Cl2 solution.33 The layer order of this three-component film should be the ODAH+ layer, the clay nanosheet layer, and the [Ru(dpp)3]2+ layer from the substrate (Figure 1b). The films with this order were called OCR films. It was interesting that one would prepare a couple of the three-layer-component hybrid film systems with the reverse layer orders to each other by the modified LS technique. 8347

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Langmuir

Figure 2. Experimental setup for pyroelectric current measurements. 241

Am electrode ionized the air to keep the electrical conductivity between the electrode and the film surface. The connection was set so that the positive current direction was radioactive electrode → electrometer → gold wire (bare ITO electrode). The film sample, the Peltier module, and the radioactive electrode were shielded in a grounded metal box in which the humidity was kept over 50% during the measurements.

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RESULTS AND DISCUSSION Film Deposition. For the OCR and RCO film preparation, the ODAH+Cl− and [Ru(dpp)3](ClO4)2 solutions, respectively, were spread at air−clay dispersion interfaces, and then the floating films adsorbed by the clay nanosheets were compressed with the Langmuir barriers. Upon compression, surface pressure (π) was measured as a function of molecular area. (A is the film area/number of ODAH+ or [Ru(dpp)3]2+ cations.) π−A isotherm curves for ODAH+ and [Ru(dpp)3]2+ on the clay nanosheet dispersions are shown in Figure 3. The

Figure 4. (a) IR and (b) vis spectra for multilayers of OCR films and the change in absorbance at 2924 cm−1 and 469 nm, respectively, as a function of layer number (insets).

cm−1 were due to the alkyl chains of ODAH+ in the films; these peaks were assigned to the symmetric −CH2− stretching, antisymmetric −CH2− stretching, and antisymmetric −CH3 stretching modes, respectively.50 The peak position of the antisymmetric −CH2− stretching mode at 2924 cm−1 was indicative of gauche rotomer formation or disordered alkyl chains. (For an all-trans alkyl chain, the corresponding peak would appear at ∼2918 cm−1.33,50) The absorption intensities for these peaks increased as the number of hybrid layers increased. A linear relation between the layer number and the absorption intensity at 2924 cm−1 was given as indicated in the inset in Figure 4a. This implied the layer-by-layer deposition of ODAH+ for the OCR multilayers. An area per ODAH+ cation in the film could be estimated from the slope of the line (absorbance per layer) in the inset and the molar extinction coefficient of ODAH+ for the antisymmetric −CH2− stretching mode.19,30,31,33,51 The estimated value for the area per ODAH+ was 1.4 nm2 molecule−1; this value was close to the molecular area at 10 mN m−1 (1.4 nm2 molecule−1) on the π−A isotherm curve (Figure 3a). Visible spectra for the OCR films showed an absorption peak at 469 nm due to the metal-to-ligand charge-transfer (MLCT) bands of [Ru(dpp)3]2+ in the films,36,52 and the absorption intensity increased with the increase in the layer number (Figure 4b). A linear relation between the layer number and the absorption intensity at 469 nm (Figure 4b, inset) implies the layer-by-layer deposition in the OCR films as well as the IR spectral results. The averaged area per [Ru(dpp)3]2+ in the OCR films was estimated from the slope of the linear relation and the extinction coefficient of the Ru(II) complex cation (2.95 × 10 4 (mol dm−3) −1 cm−1) 52,53 to be 0.85 nm2 molecule−1. Similar results had been obtained in our previous work.33 The area per [Ru(dpp)3]2+ in the film (0.85 nm2) was too small for the complex,34−36 which had been discussed in our previous work. There would be large overlapping of the ligands due to the hydrophobic interaction between the adjacent complex cations.

Figure 3. π−A isotherm curves for (a) ODAH+ and (b) [Ru(dpp)3]2+ on clay dispersions at 50 ppm.

isotherm curve for ODAH+ (Figure 3a) rose up around 1.7 nm2 molecule−1, and as the floating film was compressed, the surface pressure increased up to 23 mN m−1, showing the collapse of the floating monolayer film. Considering the cross-sectional area of an alkyl chain (ca. 0.2 nm2),48,49 the ODAH+ cation density on clay nanosheets was very low. The floating ODAH+/ clay films were deposited at 10 mN m−1, where the ODAH+ density in the film was about 1.4 nm2 molecule−1. The π−A isotherm curve when the [Ru(dpp)3](ClO4)2 solution was spread on the clay dispersion is shown in Figure 3b. The surface pressure lifted off from zero pressure at about 1.0 nm2 molecule−1 and then rose steeply. The rising rate changed over 20 mN m−1, and no clear collapse point was observed. The [Ru(dpp)3]2+ cation density at 10 mN m−1, where the [Ru(dpp)3]2+/clay films were transferred, was about 0.9 nm2 molecule−1. After every deposition of the floating ODAH+/clay film on a substrate, the film was immersed in the [Ru(dpp)3]Cl2 solution in order to prepare the OCR multilayers. IR spectra (in the C− H stretching region) for the OCR multilayers are shown in Figure 4a. The absorption band peaks at 2852, 2924, and 2959 8348

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Langmuir For the preparation of RCO films, [Ru(dpp)3]2+/clay films floating on the clay dispersions were transferred on substrates, and then the transferred film surfaces were immersed in the ODAH+Cl− solution. IR and vis spectra for the RCO films are shown in Figure 5a ,b, respectively. These spectra gave

structure of the RCO film would be less ordered than that of the OCR film. Out-of-Plane XRD Patterns and Polarized Spectra. Figure 6a,b shows out-of-plane XRD patterns for the OCR and

Figure 5. (a) IR and (b) vis spectra for multilayers of RCO films and change in absorbance at 2924 cm−1 and 469 nm, respectively, as a function of layer number (insets).

Figure 6. Out-of-plane XRD patterns for (a) OCR and (b) RCO films (seven hybrid layers) measured at 20, 40, and 60 °C.

RCO films (seven hybrid layers), respectively, measured at 20, 40, and 60 °C. (The XRD patterns measured at 40 and 60 °C will be discussed later.) The XRD pattern measured at 20 °C for the OCR film gave peaks at 2θ = 3.49 and 6.96° (Figure S2), which could be assigned to (001) and (002) diffractions, respectively. The basal spacing calculated from the (001) peak was 2.53 nm. Because the thickness of the clay nanosheet layer was 0.96 nm,2,54 the thickness of the ODAH+ and Ru(II) complex cation layers was 1.57 nm. In the XRD pattern of the RCO film (Figure 6b and Figure S2), the peaks at 2θ = 3.19 and 6.71° were assigned to (001) and (002) diffractions, respectively. The basal spacing estimated from the (001) peak was 2.77 nm, and the total thickness of the Ru(II) complex cation and ODAH+ layers was 1.81 nm. If the alkyl chain of ODAH+ is all-trans conformation, then its length is about 2.4 nm (estimated from a molecular model). Taking the Ru(II) complex cation size (>1.5 nm) into consideration, the alkyl chains would be folded and occupy the empty spaces between the complex cations in both film systems.33 This was consistent with the IR spectral data (gauche rotomer formation) described above. There was a little difference between the diffraction peak widths for the OCR and RCO films. Similar to the IR and vis spectral results, the width for the RCO film was broader than for the OCR film, suggesting a less ordered layer structure of the RCO film than that of the OCR film. To get information about tilt angles of [Ru(dpp)3]2+ in the films, their vis spectra were measured with s- and p-polarized beams at various incident angles.33,50 The ratio of absorbance measured with the p-polarized beam to that measured with the s-polarized one at 469 nm was dependent on the incident angle from the surface normal of the film (Figures S3 and S4). By

absorption peaks due to ODAH+ (Figure 5a) and [Ru(dpp)3]2+ (Figure 5b), and these absorptions became intense with the increase in the layer number. Areas per ODAH+ and [Ru(dpp)3]2+ in the RCO films evaluated from the linear increase in the absorption intensity as a function of layer number (insets in Figure 5a,b) are listed in Table 1, together Table 1. IR and Visible Spectral Results for ODAH+/Clay/ [Ru(dpp)3]2+ (OCR) and [Ru(dpp)3]2+/Clay/ODAH+ (RCO) Films IR spectral data

OCR RCO

vis spectral data

slope/ abs layer−1

area per ODAH+ /nm2 molecule−1a

slope /abs layer−1

area per [Ru(dpp)3]2+ 2 /nm molecule−1b

1.3 × 10−4 1.3 × 10−4

1.4 1.4

5.8 × 10−3 6.2 × 10−3

0.85 0.80

a

The extinction coefficient for ODAH+ used in this calculation was 1.06 × 103 (mol dm−3)−1 cm−1.51 bThe extinction coefficient for [Ru(dpp)3]2+ used in this calculation was 2.95 × 104 (mol dm−3)−1 cm−1.52

with the results for the OCR films. The area per [Ru(dpp)3]2+ (0.80 nm2 molecule−1) calculated for the RCO film was consistent with the π−A isotherm data (0.9 nm2 molecule−1 at 10 mN m−1, Figure 3b). Comparing the results for the OCR and RCO films in Table 1, the densities of both ODAH+ and [Ru(dpp)3]2+ were very close to each other. On the contrary, the absorption peak widths in the IR and vis spectra for the RCO film system were broader than those for the OCR film system. This suggested that the molecular packing or layer 8349

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Langmuir curve fitting analysis, the average tilt angles for [Ru(dpp)3]2+ in the OCR and RCO films were obtained to be 34 ± 1 and 35 ± 1°, respectively. No remarkable difference in the tilt angle was seen between the OCR and RCO films. (See the detailed description in the Supporting Information.) The polarized IR spectra of the films were measured and analyzed similarly to the vis spectral data (Figures S5 and S6). The resultant tilt angles of ODAH+ were 43 ± 1 and 44 ± 1° for the OCR and RCO films, respectively. These results were consistent with the above discussion about the gauche rotomer formation in ODAH+; the random orientation of the alkyl chain gave rise to the tilt angle of 45° on average. Pyroelectric Current Measurements. We could prepare a pair of three-layer-component film systems with reverse deposition orders to each other: the OCR and RCO film systems. The ODAH+ and [Ru(dpp)3]2+ densities and the [Ru(dpp)3]2+ tilt angle were almost the same between these systems, although the basal spacing was rather different. To confirm the reverse-ordered layer structures in the OCR and RCO film systems, their pyroelectric currents were measured. In a three-layer-component film, the centers of mass for positive and negative charges do not coincide so that spontaneous polarization appears on the film surfaces. When the temperature of the film changes, the amount of spontaneous polarization also changes, which causes pyroelectricity. In this work, the pyroelectricity was observed as a pyroelectric current by measuring the current when both film surfaces were short-circuited. The pyroelectric current (I) is therefore proportional to the rate of temperature change (dT/ dt).37−39,44

Figure 7. Current change for a bare ITO glass electrode upon (a) increasing and (b) decreasing temperature for a OCR film (five hybrid layers) upon (c) increasing and (d) decreasing temperature and for a five-layer RCO film (five hybrid layers) upon (e) increasing and (f) decreasing temperature (together with the rate of temperature change).

I = pA(dT /dt )

to the temperature change rate (dT/dt). The current change for the RCO film (Figure 7f) showed a similar behavior to that for the OCR film, not being proportional to the temperature change rate. Additionally, the currents for both films changed in the same direction, increasing and then decreasing upon temperature lowering. Considering these results, the threelayer-component films did not indicate the pyroelectricity when the temperature was lowered. The current changes generated with increasing temperature for 9 and 15 hybrid layers of the OCR and RCO films (Figures S7 and S8) were indicative of the pyroelectricity. The values of the pyroelectric currents for the 9- and 15-layer OCR films were close to that for the 5-layer OCR film, and the pyroelectric current values for the RCO films (5, 9, and 15 layers) were also close to each other. Similar to the results for the 5-hybrid-layer films of the OCR and RCO films, the 9- and 15-layer films did not show pyroelectricity when the temperature was lowered (data not shown). It was noteworthy that relatively high humidity (more than about 50%) around the samples and long intervals (more than 20 min) between measurements were necessary to observe the pyroelectric currents repeatedly with increasing temperature. This suggested that the pyroelectric currents would be ascribed to water desorption from the films when they were heated. No observation of the pyroelectric currents upon temperature lowering (for ∼20 s) could be explained according to this idea; it was too short to adsorb water from the air into the films during the temperature change. IR spectra for the OCR and RCO films (seven hybrid layers) on glass plates were measured at 20 (before heating), 40, and 60 °C and then 20 °C after being cooled spontaneously (in a few minutes). The results for the OCR film are shown in Figure

p is the pyroelectric coefficient, and A is the surface area of the radioactive electrode. Figure 7a,b exhibited the results for a bare ITO glass electrode (substrate alone). With rising temperature from 30 to 60 °C in about 10 s, the current decreased (Figure 7a). However, the current was not proportional to the rate of temperature change (dT/dt). With decreasing temperature (Figure 7b), the current increased but it was not proportional to the temperature change rate either. Namely, the bare ITO glass electrode did not show pyroelectricity. For an OCR film (five hybrid layers) deposited on an ITO electrode (Figure 7c), the current which was reversely proportional to the rate of temperature change was observed with the rise in temperature. On the other hand, the current for an RCO film (five hybrid layers, Figure 7e) changed proportionally to the rate of rising temperature change. The three-layer-component OCR and RCO films showed pyroelectric currents, the directions of which were opposite to each other. These opposite current directions indicated the reverse layer orders in the OCR and RCO film systems. The pyroelectric current for the OCR film was larger than that for the RCO film.55 As described above, the layered structure in the RCO film was less ordered than that in the OCR film. The difference in the layer ordering might bring about a difference in the pyroelectric current value, but there was no further data supporting this idea. With decreasing temperature, no pyroelectric currents were observed for either OCR or RCO films (Figure 7d,f). The current for the OCR film (Figure 7d) increased steeply and then decreased gradually when the temperature was lowered from 60 to 30 °C, but the current change was not proportional 8350

DOI: 10.1021/acs.langmuir.5b01401 Langmuir 2015, 31, 8346−8353

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Langmuir 8a. The spectrum measured at 20 °C before heating gave a broad absorption band due to the hydrogen-bonding O−H

enough for the complete recovery of the sample conditions (water adsorption). IR spectra and XRD patterns for the RCO film (seven hybrid layers) exhibited similar behavior to that for the OCR film. The absorption intensity of the broad band around 3460 cm−1 (Figure 8b) due to water molecules in the film being weakened as the measurement temperature increased from 20 to 60 °C, and the absorption intensity did not recover even when the measurement temperature decreased to 20 °C (a few minutes after the 60 °C measurement). The (001) diffraction peaks in the XRD patterns of the RCO film measured at 20, 40, and 60 °C (Figure 6b) were observed at 2θ = 3.19 (2.77), 3.27 (2.70), and 3.34° (2.64 nm), respectively, indicating the decrease in the interlayer distance with the increase in the temperature. The IR and XRD data for the OCR and RCO films elucidated the mechanism for the pyroelectricity of the films. With increasing temperature, the water molecules in the films were desorbed and the interlayer distances were decreased, which caused the change in the spontaneous polarization on the film surfaces.

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CONCLUSIONS One of the interesting features of the organoclay hybrid films is that even water-soluble amphiphilic cations spread on an aqueous clay dispersion form a stable floating film by adsorption of the clay nanosheets. By applying this feature, we prepared the two types of three-layer-component films with the reverse deposition orders to each other: the layer order of ODAH+, clay nanosheet, and [Ru(dpp)3]2+ (OCR) from a substrate and the order of [Ru(dpp)3]2+, clay nanosheet, and ODAH+ (RCO). The OCR and RCO film systems prepared in this work possessed almost the same characteristics in the ODAH+ and [Ru(dpp)3]2+ densities and the [Ru(dpp)3]2+ tilt angle. Some differences were found between the OCR and RCO film systems; the basal spacing (interlayer distance) for the RCO film was a little longer than that for the OCR film, and the layered structure of the RCO film was less ordered than that of the OCR film. Pyroelectric currents for the OCR and RCO films were measured by a noncontact method using a radioactive electrode. The pyroelectric currents were observed with increasing temperature, and their current polarities revealed that the layer order in the OCR film was reverse that in the RCO film. The IR and XRD data elucidated the mechanism of the pyroelectricity. When the film was heated, water molecules were desorbed from the film, and the layer distance was decreased so that the spontaneous polarization on the film surfaces was changed to give rise to the pyroelectricity.

Figure 8. IR spectra for (a) OCR and (b) RCO films (seven hybrid layers) measured at 20 (before heating), 40, 60, and 20 °C (after heating).

stretching vibration around 3460 cm −1, together with absorption peaks assigned to the C−H stretching vibrational modes (2800−3000 cm−1, due to ODAH+) and the isolated O−H stretching vibrational mode (3627 cm−1, due to the clay nanosheets).56 The appearance of the broad absorption band around 3460 cm−1 implied the existence of water molecules in the film (probably in the interlayer spaces). As the sample temperature increased, the broad absorption band around 3460 cm−1 became weak at 40 °C and disappeared at 60 °C. These spectral changes indicated water desorption from the OCR film with the increase in temperature. XRD patterns for the OCR film were measured at 20, 40, and 60 °C (Figure 6a). The peaks assigned to the (001) diffraction were observed at 2θ = 3.49, 3.55, and 3.59° (basal spacings: 2.53, 2.49, and 2.46 nm), respectively. Hence the desorption of water molecules from the OCR film by heating gave rise to the decrease in the basal spacing, which caused the change in the spontaneous polarization on the film surfaces, or the pyroelectricity. After the IR spectral measurement for the OCR film at 60 °C, the film sample was cooled spontaneously down to 20 °C (in a few minutes, under circumstances of >50% humidity) and its IR spectrum was recorded (Figure 8a). The broad absorption around 3460 cm−1 was restored although the absorption intensity was weaker than that measured before the heating. As described above, no pyroelectric current was observed with the temperature lowering (∼20 s), and long intervals (more than 20 min) between measurements were necessary to observe the pyroelectric currents repeatedly with increasing temperature. The IR spectral results revealed the reason that no pyroelectric currents were observed upon the temperature lowering; even a few minutes were not long

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ASSOCIATED CONTENT

S Supporting Information *

Experimental section in detail, π−A isotherms of [Ru(dpp)3]2+ on water and clay dispersions, out-of-plane XRD patterns, vis and IR spectra measured with polarized beams, plots of dichroic ratio against incident angle, and pyroelectric current generation upon increasing temperature for the OCR and RCO films. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01401.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 8351

DOI: 10.1021/acs.langmuir.5b01401 Langmuir 2015, 31, 8346−8353

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Langmuir

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Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b01401 Langmuir 2015, 31, 8346−8353

Article

Langmuir

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DOI: 10.1021/acs.langmuir.5b01401 Langmuir 2015, 31, 8346−8353