Characterization of Supported Solid Thin Films of Laponite Clay

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Characterization of Supported Solid Thin Films of Laponite Clay. Intercalation of Rhodamine 6G Laser Dye V. Martı´nez Martı´nez, F. Lo´pez Arbeloa,* J. Ban˜uelos Prieto, T. Arbeloa Lo´pez, and I. Lo´pez Arbeloa Departamento de Quı´mica Fı´sica, Universidad Paı´s Vasco-EHU, Apartado 644, 48080-Bilbao, Spain Received February 6, 2004. In Final Form: April 27, 2004 The morphology of thin films of Laponite (Lap) clay elaborated by the evaporation method and spincoating technique was analyzed by atomic force microscopy and scanning electron microscopy, indicating a better quality film for the latter procedure. Rhodamine 6G (R6G) laser dye was intercalated into these films by ion exchange mechanism, performed by immersing the clay film into adequate dye solutions in which the effect of the dye concentration, immersion time, and nature of the solvent on the adsorption process were checked. The adsorption of R6G at the interlayer space of Lap was analyzed by the X-ray diffraction technique, and the presence of several R6G species (monomers and aggregates) was characterized by absorption and fluorescence spectroscopies. Less viscous solvents lead to higher dye loadings, suggesting a diffusional process for the intercalation of the dye in the interlayer spaces of Lap, and polar solvents favor the swellability of the interlayer space giving rise to a more homogeneous distribution of R6G molecules through the film and decreasing the dye aggregation. With the aging of the samples, the dye molecules can migrate through the interlayer spaces, leading to a more expanded distribution of R6G molecules and to the dye deaggregation.

Introduction In recent decades, an increasing number of studies related to optical properties of organic molecules doped into an inorganic matrix have arisen because of the advantages of these materials with respect to organic matrixes, such as more rigid environments, better transparency, and thermal stability.1,2 In particular cases, intercalation of dyes into inorganic host materials can provide several photofunctional applications, for instance, chemical sensors, nonlinear optical glasses, optical storage devices, and solid-state tunable lasers.3 The immobilization of photoactive species into ordered nanostructures, which supply a rigid environment, is of a great interest in nanotechnology. Inorganic materials offer a multitude of different mono-, bi-, and tridimensional structures to accommodate organic molecules.4-6 Nowadays, several research projects into the preparation of ordered nanocomposites are being carried out for the design of solid-state samples,7-23 which can improve the * Corresponding author. Phone: +34 94 601 59 71. Fax: +34 94 601 35 00. E-mail: [email protected]. (1) Ye, C.; Lam, K. S.; Chik, K. P.; Lo, D.; Wong, K. H. Appl. Phys. Lett. 1996, 69, 3800-3802. (2) Wu, S.; Zhu, C. Opt. Mater. 1999, 12, 99-103. (3) Schulz-Ekloff, G.; Wo¨hrle, D.; Van Duffel, B.; Schoonheydt, R. A. Microporous Mesoporous Mater. 2002, 51, 91-138 and references therein. (4) Ogawa, M.; Kuruda, K. Chem. Rev. 1995, 95, 399-438. (5) Lourie, O. R.; Jones, C. R.; Bartlett, B. M.; Gibbons, P.; Ruoff, R. S.; Buhro, W. E. Chem. Mater. 2000, 12, 1808-1810. (6) Ganesan, V.; Ramaraj, R. J. Lumin. 2001, 92, 167-173. (7) Edelteins, A. S.; Cammarata, R. C. Nanomaterials: Synthesis, Properties and Applications; Inst. Physics: Bristol, 1996. (8) Nalwa, H. S., Ed. Handbook of Advanced Electronic and Photonic Materials and Devices; Academic Press: San Diego, CA, 2001. (9) Ramamurthy, V., Ed. Photochemistry in Organized and Constrain Media; VCH: New York, 1991. (10) Thomas, J. K. Chem. Rev. 1993, 93, 301-320. (11) Ramamurthy, V.; Eaton, D. F. Chem. Mater. 1994, 6, 11281136. (12) Van Duffel, B.; Verbiest, T.; Van Elshocht, S.; Persoons, A.; De Schryver, F. C.; Schoonheydt, R. A. Langmuir 2002, 17, 1243-1249.

optical properties and the chemical capacities of the organic-inorganic hybrid materials. A group appertaining to the numerous varieties of ordered and constrained host systems is layered materials such as clay minerals, layered double hydroxides, zirconium phosphate, and so forth.4,24,25 The lamellar solids can accommodate a great diversity of guest molecules because of their ionic exchange capacity and the expandability of their interlayer spaces while maintaining the other structural features. At this point, clay minerals possess a two-dimensional interlayer space due to the stacking of an octahedral (O) Al2O3 or MgO sheet between two tetrahedral (T) SiO2 sheets, the so-called TOT layer. Platelets have a negative charge because of isomorphic substitution of Si4+ and/or Al3+ by other cations with lower valence, which is neutralized by inorganic exchangeable cations located in the interlayer space.26 The swelling of the interlayer space (13) Miyamoto, N.; Kawai, R.; Kuruda, K.; Ogawa, M. Appl. Clay Sci. 2001, 19, 39-46. (14) Calzaferri, G.; Huber, S.; Maas, H.; Minkowski, C. Angew. Chem. Int. Ed. 2002, 42, 3732-3758. (15) Era, M.; Miyake, K.; Yoshida, Y.; Yase, K. Thin Solid Films 2001, 393, 24-27. (16) Iyi, N.; Kurashima, K.; Fujita, T. Chem. Mater. 2002, 14, 583589. (17) Sasai, R.; Otto, H.; Shindachi, I.; Shichi, T.; Takagi, K. Chem. Mater. 2001, 13, 2012-2016. (18) Sonobe, K.; Kikuta, K.; Takagi, K. Chem. Mater. 1999, 11, 10891093. (19) Hussein, B. M. Z.; Zainal, Z.; Yahaya, A. H.; Ası´s, A. B. A. Mater. Sci. Eng. 2002, B88, 98-102. (20) Ray, K.; Nakahara, H. J. Phys. Chem. B 2002, 106, 92-100. (21) Hagerman, M. E.; Salamone, S. J.; Herbst, R. W.; Payeur, A. L. Chem. Mater. 2003, 15, 443-450. (22) Giannelis, E. P. Chem. Mater. 1990, 2, 627-629. (23) Shichi, T.; Takagi, K. J. Synth. Org. Chem. Jpn. 2002, 60, 10761086. (24) Constantino, U.; Coletti, N.; Nocchetti, M. Langmuir 1999, 15, 4454-4460. (25) Ogawa, M.; Kuruda, K. Bull. Chem. Soc. Jpn. 1997, 70, 25932618. (26) Newmann, A. C. D. Chemistry of Clays and Clay Minerals; Longman Science Technology Mineral Society: London, 1987.

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of clays depends, apart from other factors, on the nature of the isomorphic substitution, on the negative charge density, and on the hydration degree of the inorganic cations.26,27 The sodium forms of smectite-type clays,28 with a moderate negative charge per unit formula in the 0.5-1 range (50-100 mequiv/100 g of cationic exchangeable capacity, CEC), present the best swelling capacity to accommodate a large amount of cationic or polar molecules. Moreover, the incorporation of cationic surfactant with long hydrocarbon chains gives rise to hydrophobic clays, which permit the adsorption of nonionic compounds.25,29-32 Adsorption of cationic organic dyes in smectite clays has been extensively studied, mainly in aqueous colloid suspensions.33-37 The clay surface provides a rigid environment for the dye molecules and induces the dye aggregation as a consequence of the augmentation of the local dye concentration. However, the technological interest of dye/clay systems has recently been focused on obtaining ordered dye/clay systems in the solid state. In this sense, clay films can be employed to fabricate environmentally stable optoelectronic devices and, more extensively, to improve the conductivity of a clay-modified electrode in electrochemical systems.38-42 To obtain ordered dye/clay systems on the macroscopic scale, two different organizations are necessary. On one hand is the organization of the clay layers into a regular macroscopic distribution; to this end, thin films were obtained to achieve a planar assembly of clays particles parallel distribution to the plate.3,12,36 On the other hand is a preferential orientation of adsorbed dye molecules in the clay surface, which depends on several factors such as the constraint of the interlayer space of clays and hostguest and guest-guest interactions; the balance of all interactions determines the arrangement of the dye molecules in the clay surface.3,25,36,37 In this work, the adsorption of Rhodamine 6G dye (R6G) in thin film of Laponite (Lap) is studied. R6G is probably the most commonly used laser dye,43 whereas Lap clay is a synthetic smectite44 with a high chemical purity, small particle size (1 µm).48 Besides, the hydration degree of the film can also contribute because cracked films were observed by the naked eye after strong dry conditions (overnight in a vacuum at 40° C). The better distribution of Lap particles through the substrate by the spin-coating technique is confirmed by the XRD technique since the 001 band for the coated film is narrower than the peak for a cast film (data not shown). Probably, an irregular arrangement of the clay particles into the substrate should be obtained by this method because of modifications in the contact angle between the clay particles and the plate surface during the solvent evaporation. Because of the good transparency and parallel distribution of the Lap particles in coated films, the spin-coating technique is recommended to spectroscopically characterize the adsorption of rhodamine dyes in thin films of Lap clay, similar to those used in electrochemical systems to get clay-modified electrodes.49-51 The disadvantage of this method is that it provides films with a nonuniform thickness over the substrate area,52 as is confirmed by AFM images (Figure 3). The thickness of the film can be determined from AFM images by the depth of a groove performed by a metallic pointer in the Lap film just before the film-drying process. From the height distribution of the AFM cantilever obtained in the present work (Figure 3, right), it can be concluded that the Lap film is thinner in the edge than in the center of the support plate. This decrease can be attributed to the augmentation in the centrifugal force with the spinning radius. Other methods to get solid thin films, such as Langmuir-Blodgett films and electrophoretic depositions, can provide clay films with regular width,20,36,53,54 but in these cases the thickness of the films ( 3.0). Higher percent CEC values were obtained by immersing a thinner film (around 70 nm) obtained by coating a less viscous Lap aqueous suspension (1 g L-1) into a 10-3 M R6G solution. In this case, relatively high loadings (up to 65% CEC) were obtained after 2 days of immersion without the saturation of the absorbance signal (Amax ≈ 1.0). The adsorption of R6G on the Lap film is thought to be mainly performed by the cation exchange mechanism,46 although other forces such as dye-dye hydrophobic interactions can also contribute to the adsorption of the cation dyes in the interlayer space of the clays.58 Moreover, taking into account that the adsorption procedure is from immersion of the Lap film into R6G solutions, other interactions such as solvent-clay and solvent-dye can also affect the extent of the adsorption.36 To study the solvent effect on the adsorption of R6G into Lap film, three solvents, acetone, ethanol, and a water/ethanol (xw ) 0.8) mixture, were considered for the dye solutions. Pure water was excluded because experimental observations show that the immersion of Lap films into R6G aqueous solutions leads to the Lap film becoming completely unstuck from the plate. This process is induced by the dye molecules in the aqueous medium, because pure water (without dye) leads to a less appreciable flocculation of the clay particles of the film into the solution. (58) Yamagishi, A.; Soma, M. J. Phys. Chem. 1981, 85, 3090-3092.

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Figure 5. Absorbance of a R6G/Lap film against the immersion time into a 10-5 M solution of R6G in different solvents: water/ ethanol (xwater ) 0.8) mixture (9); pure ethanol (b); and pure acetone (2). Dashed area symbolizes the saturation absorbance value for a CARY 4E spectrophotometer.

The adsorption of R6G on the Lap film depends on the nature of the solvent used in the preparation of the immersion R6G solutions. Figure 5 shows an important increase in the absorbance of the sample obtained from the R6G/acetone solution with respect to the other solvents for a common dye concentration and Lap film preparation. Generally, the loading is proportional to 1:1.5:5 for the water-ethanol mixture/ethanol/acetone, respectively. This augmentation inversely correlates with the solvent viscosity (η), 2.83:1.20:0.33, respectively. These results suggest that the diffusion of the solvent molecules through the interlayer space of supported films plays an important role in the incorporation of the R6G dye into solid thin films of Lap. However, the amount of adsorbed R6G is not exactly proportional to the inverse of solvent viscosity (1: 2.4:8.5), suggesting that other solvent parameters have to be considered to explain the solvent effect on the adsorption capacity of Lap films for R6G molecules. Because the swelling depends on the solvation degree of the inorganic exchangeable cations,28,59 the polarity of the solvent probably contributes to the swellability of the interlayer space of clay films. Consequently, polar solvents would favor the accessibility of the interlayer spaces with respect to apolar solvents, mainly those interlayer spaces placed in the inner part of the films (close to the substrate). Other authors have also proposed that the polarity of the solvent plays an important role in the diffusion process into porous dielectric films.60 This is confirmed by XRD data. The diffraction band of pure Lap films (Figure 6) is placed at a 2θ value equal to 6.70, giving rise to the 001 space of 13.0 Å. Considering the thickness of a TOT clay layer to be around 9.5 Å,61 this leads to an interlayer space of 3.5 Å. The incorporation of the R6G molecules on the Lap film shifts the diffraction band to shorter 2θ values (Figure 6b), confirming the adsorption of R6G molecules in the interlayer space of Lap films. The R6G/Lap film obtained by the immersion in the R6G water/ethanol mixture gives a sharp peak at 2θ ) 4.3 (Figure 6b), in which an increase of 7.5 Å takes place in the basal spacing after the R6G intercalation with respect to the pure Lap film. A broader 001 band is observed from R6G/Lap films (59) Cenens, J.; Schoonheydt, R. A. Clays Clay Miner. 1988, 36, 214224. (60) Shamiryan, D.; Maex, K. Mater. Res. Soc. Symp. Proc. 2003, 766, 229-234. (61) Chen, G.; Iyi, N.; Sasai, R.; Fijuta, T.; Kitamura, K. J. Mater. Res. 2002, 17, 1035-1040.

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Figure 6. XRD patterns of coated films: pure Lap film (a) and R6G/Lap films prepared by immersion into a 10-3 M solution of R6G for 2 days in a water/ethanol (xw ) 0.8) mixture (b), pure ethanol (c), and pure acetone (d). Table 1. Interlayer Space Distances of the R6G/Lap System Obtained by the XRD Technique system

2θ (deg)

d001 (Å)

dbasal (Å)

pure Lap R6G/Lap water-ethanol mixture R6G/Lap ethanol R6G/Lap acetone

6.7 4.4:7.6

13.0 20.5:11.9

3.5 11.0:2.4

4.7:7.0

18.8:12.6

9.3:3.1

4.7:7.3

18.8:12.1

9.3:2.6

obtained with acetone (Figure 6d) and ethanol (Figure 6c) as solvents, and a distance between layers of 18.8 Å is obtained, which implies an increase of 5.8 Å in the interlayer space upon R6G adsorption. All these results are summarized in Table 1. These results suggest a more constrained interlayer space of Lap films in less polar environments. Besides, the X-ray profiles of nonaqueous solvents, with two more or less overlapping diffraction bands (Figure 6c-d), indicate a less homogeneous distribution of dye molecules through the interlayer space of Lap films,62 owing to a lower swellability and more restricted space in such a kind of environment, as was discussed previously. However, as a result of the low viscosity of acetone, R6G molecules are more extensively adsorbed in Lap film for identical films and immersion times (Figure 3). The adsorption of more R6G molecules in a more limited and less accessible interlayer space obtained for R6G solution in acetone favors the dye aggregation at the clay surface, as is discussed in the following. Dye Aggregation: Loading Effect. The loading of the R6G dye in Lap films not only increases the absorbance of the sample (Figure 3) but also modifies the shape of the absorption spectra. The main absorption band, placed at 528 nm for low loadings, shifts to lower energies (Figure 7A); the absorption band intensity at the shoulder (around 500 nm) increases with respect to the main absorption band. These changes are analyzed by the R ) Amax/A500 parameter (ratio between the maximum absorbance and the absorbance in the shoulder), because this parameter represents the shape of the absorption spectrum, which decreases with the dye coverage (Figure 7B). This metachromatic effect (the replacement of the main absorption band for other absorption bands placed at lower wavelengths) is also observed in aqueous solution by (62) Endo, T.; Nakada, N.; Sato, T.; Shimada, M. J. Phys. Chem. Solids 1989, 50, 133-137.

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Figure 7. Evolution of the spectroscopic properties of R6G/Lap films with the dye loading obtained for different immersion times in 10-5 M solution of R6G: (A) absorption (9) and fluorescence (b) wavelengths in a water/ethanol (xw ) 0.8) mixture; (B) absorption R ) Amax/A500; (C) fluorescence intensity corrected by the absorbance at the excitation wavelength.

increasing the dye concentration, and it is ascribed to the dye aggregation.63,64 The 528-nm absorption band (observed mainly in low loading R6G/Lap samples) is close to that observed in diluted aqueous solutions, which is ascribed to the R6G monomer adsorption on Lap surfaces. The increase in the absorbance at 500 nm in detriment of the monomer band would correspond to the absorption of the H band of a sandwich aggregate, probably a dimer.65,66 The dye aggregation phenomenon of R6G in Lap films was confirmed by fluorescence spectra (steady state). Because the fluorescence intensity of a sample depends on others factors, on the excited molecules, the fluorescence efficiency is better analyzed by the ratio between the fluorescence intensity (determined from the area under the curve) and the absorbance of the sample at the excitation wavelength, Iflu/Aexc. The evolution of this parameter with the loading of R6G in Lap films is shown in Figure 7C. The drastic decrease observed in the fluorescent efficiency (more than 2 orders of magnitude) by increasing the loading up to 65% CEC is ascribed to the dye aggregation, because H aggregates do not emit and they are efficient quenchers of monomer emission.65,66 These results would confirm that the R6G aggregation in Lap films leads to H-type aggregates. On the other hand, Figure 7A shows that the fluorescence band shifts to lower energies by increasing the dye loading. This shift (around 25 nm) is much greater than that observed in the main absorption band (around 5 nm). Because the dipole moment of R6G in the excited state is similar to the ground state, as is confirmed by the low Stokes shift observed in low loading samples, the important bathochromic shift in the fluorescence band cannot be exclusively attributed to changes in the environment polarity caused by the presence of organic molecules in the interlayer space.67,68 Indeed, reabsorption/reemission for high absorption samples, that is, in high loading R6G/ Lap films, can reduce the recorded fluorescence intensity and shift the fluorescence band to lower energies,69,70 but (63) Lo´pez Arbeloa, F.; Llona Gonza´lez, I.; Ruiz Ojeda, P.; Lo´pez Arbeloa, I. J. Chem. Soc., Faraday Trans. 2 1982, 78, 989-994. (64) Lo´pez Arbeloa, F.; Ruiz Ojeda, P.; Lo´pez Arbeloa, I. J. Chem. Soc., Faraday Trans. 2 1988, 84, 1903-1912. (65) McRae, E. G.; Kasha, M. Physical Process in Radiation Biology; Academic Press: New York, 1964. (66) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371-392. (67) Van der Auweraer, M.; Verschuere, B.; De Schryver, F. C. Langmuir 1988, 4, 583-588. (68) Pevenage, D.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1999, 15, 8465-8473. (69) Lo´pez Arbeloa, I. J. Photochem. 1982, 18, 161-168. (70) Rohatgi-Mukherjee, K. K. Ind. J. Chem. 1992, 31A, 500-511.

these effects cannot explain such an extensive bathochromic shift. Moreover, because a J aggregate can emit,71,72 we do not exclude the possibility that the bathochromic shift and low fluorescence intensity observed for high loading samples could be due to the emission from J aggregates, where the rigidity of the interlaminar space could improve their fluorescence efficiency. In any case, the fluorescence efficiency of this possible J aggregate should be very low. New methods to spectroscopically characterize R6G aggregates on Lap films are now in progress. Dye Redistribution: Aging Effect. As is mentioned, the adsorption of cation R6G dye in a Lap thin film is not homogeneous through all the interlayer spaces. Depending on the immersion time of Lap films into R6G solutions, the adsorption is mainly performed in the most accessible interlayer spaces of the clay; in other words, the most externally disposed layers in the Lap thin films. Several experimental evidences justify this assumption for sufficiently thick films: (i) For a common dye solution and low immersion time (tim < 20 min), the loading of R6G in Lap film is independent of the number of accumulation films in successive spin-coating processes (1, 2, and 4 times). Consequently, the pile-up of interlayer spaces at the inner part of the supported film (adjacent to the glass plate) does not enhance the adsorption capacity of Lap films for R6G molecules during low immersion times. (ii) For short immersion times (tim < 20 min), the duplication of the clay surface in contact with the R6G solution (i.e., by two spin-coated procedures on both sides of the glass plate versus two accumulated spin-coating processes on the same side of the substrate) doubles the absorbance of the sample, suggesting that the first steps of the adsorption take place in the most externally exposed interlayer surface. (iii) The nonhomogeneous distribution of the R6G molecules in the interlayer space is confirmed by the solvent dependence of R6G aggregation in Lap films. For the same films and the same loading at short immersion times (Figure 7B), the R ) Amax/A500 parameter decreases (dye aggregation increases) in the order water/ethanol mixture, pure ethanol, and acetone. This enhancement in the dye aggregation is consistent with the effect of the solvent on the adsorption of R6G in Lap films, discussed above. A low viscous solvent (enhancing the adsorption of (71) Vuorimaa, E.; Ikonen, M.; Lemmetyien, H. Chem. Phys. 1994, 188, 289-302. (72) Vuorimaa, E.; Belovolova, L. V.; Lemmetyien, H. J. Lumin. 1997, 71, 57-63.

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Figure 8. Evolution of the R ) Amax/A500 parameter (9) and the absolute fluorescent intensity (b) with the aging time for a 1% CEC R6G/Lap sample (tim ) 20 min in 10-5 M in a water/ ethanol (xw ) 0.8) mixture.

R6G molecules) and a nonaqueous environment (reducing the accessibility of the interlayer spaces), such as acetone, favor the dye aggregation at the outer interlayer spaces (externally exposed) of the Lap films, even for low R6G loading samples. All these results suggest that the interlayer spaces of Lap film in contact with the R6G solution (externally exposed) are more accessible than those in contact with the plate, at least for relatively low immersion times (tim < 30 min). (iv) In moderated R6G/Lap loading samples, an important increase in the fluorescence intensity was observed by turning 180° the supported films in the sample compartment of the spectrofluorimeter for direct excitation at the inner part of the Lap interlayer spaces (i.e., the excitation was performed on the substrate). This observation is consistent with a higher fraction of R6G monomeric units in the inner part of the interlayer (inhomogeneous distribution), which implies a greater proportion of monomer molecules directly excited and a reduction of the fluorescence quenching by the R6G aggregates. However, this inhomogeneous adsorption tends to be reduced by the aging time after sample preparation. In fact, the evolution in absorption and fluorescence spectra with the aging of R6G/Lap thin films suggests a redistribution of the adsorbed R6G molecules (Figure 8); thus, the increase in the R ) Amax/A500 parameter with the aging time (Figure 8a) indicates a diminution of the dye aggregation, which is coherent with the increasing in the fluorescence intensity with the aging time (Figure 8b) owing to a reduction in the quenching of the monomeric emission by the aggregates.65,66 These results confirm that the adsorbed R6G species are not static and tend to migrate to the inner (close to the substrate) interlayer spaces of the Lap films. This redistribution of the adsorbed R6G molecules in all interlayer spaces of the whole thin film leads to a reduction of the dye aggregation (for low immersion time