Adsorption induced dye stability of cationic dyes on clay nanosheet

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Adsorption induced dye stability of cationic dyes on clay nanosheet Aranee Pleng Teepakakorn, Sareeya Bureekaew, and Makoto Ogawa Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02978 • Publication Date (Web): 28 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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Adsorption induced dye stability of cationic dyes on clay nanosheet Aranee (Pleng) Teepakakorn1, Sareeya Bureekaew2 and Makoto Ogawa2* 1. School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), 555 Moo 1 Payupnai, Wangchan, Rayong 21210, Thailand 2. School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), 555 Moo 1 Payupnai, Wangchan, Rayong 21210, Thailand *E-mail: [email protected].

ABSTRACT: Rhodamine 6G was adsorbed on smectite clays, a natural montmorillonite, a synthetic saponite and synthetic hectorites, and the decolorization of the dyes upon visible light irradiation was examined for aqueous suspensions and cast films. Excellent dye stability was achieved when the natural montmorillonite was used. Not only for the Rhodamine 6G, better photostability of tris(2,2'-bipyridine)ruthenium(II) complex was found when adsorbed on a natural montmorillonite.

The excited state of the dye was quenched efficiently by the impurities in the

natural montmorillonite. From the relationship between the excited state quenching (as derived from photoluminescence quantum efficiency and photoluminescence intensity) and the decolorization rate constants, the quenching of the excited state of the dye adsorbed on the natural

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montmorillonite was proposed as the important mechanism for the stabilization of dye upon the photoirradiation.

Introduction Dyes are used as colorant in such application as textile, food, cosmetic, plastic, paper and photographic sensitization.1,2 The degradation and the leaching of dyes limit their long-term for the colorant uses. Not only for the simple colorant uses, advanced materials application of dyes for optical devices (lighting and displays) and energy (dye sensitized solar cell and photocatalyst) requires higher stability.3-5 In order to improve the stability, the molecular design of dyes for each application and the device design using gas barrier film have been topics of interests in the current materials chemistry. The immobilization of dyes on/in solids is a way to control the photochemical and photophysical properties of dyes6,7 and the resulting compounds can be regarded as a pigment, where the color is originated from the immobilized dye and tuned to some extent by the host-guest interactions. Such inorganic nanoporous materials as mesoporous silicas,8 zeolites,9 layered double hydroxides,10 and smectites11,12 have been applied to host dyes and the photofunctions of the resulting hybrids have been reported. Motivated by the outstanding stability of “Maya Blue”, which is a hybrid of a dye (indigo) and a clay mineral (palygorskite),13,14 the preparation and the application of “Maya Blue” type hybrid pigments composed of dyes and nanoporous solids have been studied toward improved stability.15-25

Among available inorganic hosts, smectite is

characterized by the expandable interlayer space and the possible organic modification to host various cationic and ionically neutral dyes. Smectite is a 2:1-type layered clay mineral consisting


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of negatively charged silicate layers and charge compensating interlayer cations (the schematic structure is shown in Figure 1A) and possess such attractive features as swelling behavior, large surface area and ion-exchange properties.26 Thanks to the materials variation and possible modification, the states of the adsorbed dyes in/on smectites have been controlled by layer charge density, type of isomorphous substitution and co-adsorbing species as well as particle size of clays.27-38 Photofunctional hybrids based on smectites and dyes have been prepared11,12 and the dye stability has been examined for the practical application.

The host-guest interactions

(electrostatic and ion-dipole interactions) were reported to be responsible for the stability toward the decomposition and the leaching.16,19,22 The limited oxygen diffusion by the host was proposed to suppress the oxidative decomposition of dye.16,19,20,22

Figure 1. (A) Schematic structure of smectites. (B) Scanning electron micrographs of a synthetic hectorite (SWF) (top) and a natural montmorillonite (KF) (bottom) and photographs of aqueous suspensions (50 mg smectites in 5 ml of water) of SWF (top) and KF (bottom). In the present study, excellent stability was found for cationic dyes (organic and organometallic) complexed with a natural smectite. The dye stability was discussed using two


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dyes (rhodamine 6G and tris(2,2'-bipyridine)ruthenium(II)) and several smectites both synthetic and natural ones to propose the important role of the photoexcited states and the quenching.

Experimental Materials A natural montmorillonite (Kunipia F, from Kunimine Ind. Co., Ltd., abbreviated as KF), a synthetic saponite (Sumecton SA, from Kunimine Ind. Co., Ltd., abbreviated as SA), and synthetic hectorites (Sumecton SWF, from Kunimine Ind. Co., Ltd., abbreviated as SWF and laponite RD from Laporte Ind., Ltd., abbreviated as LPN) were used. The SEM images of KF and SWF are shown in Figure 2B together with the photographs of their aqueous suspension. Rhodamine 6G (abbreviated as R6G) and tris(2,2'-bipyridine)ruthenium(II) complex (abbreviated as [Ru(bpy)3]2+) from Tokyo Chemical Industry Co., Ltd. were used without further purification. Sample preparation The intercalation of rhodamine 6G into smectites; KF, SA, SWF and LPN was done by mixing an aqueous clay suspension (0.25 g in 15 ml of DI water) with an aqueous solution of R6G (15 ml of 0.167 mM R6G solution, corresponding to 1 mmol of R6G to 100 g of clay) in an amber bottle at room temperature for 1 day. The amount of the dye was 1 mmol of R6G to 100 g of clay in order to obtain homogeneous suspension and film by keeping the swelling ability of sodium type smectites.

The film sample was prepared by casting the suspensions (150 µL) on a

borosilicate plate (25 x 12 x 1 mm) and dried at room temperature. The intercalation of [Ru(bpy)3]2+ (1 mmol/100 g clay) into SWF and KF was also done using the same procedure used for the incorporation of R6G into smectite.24,39


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Photodecolorization The photodecolorization of the dye in both suspension and film under visible light irradiation was examined using a 100 W Xe lamp, ABET Technologies SunliteTM Solar Simulator. The changes in the absorption spectrum upon the irradiation was monitored.

The

photodecolorization test was done under ambient condition without any atmospheric replacement. The photodecolorization of the dye was examined for SWF-R6G and KF-R6G films in a glove box in N2 and compared with that in air. A Xe lamp (Hamamatsu Photonics K.K., Type No. L2482, Hamamatsu, Japan) was used as the light source and the intensity at the sample position was 3.95 W/m2. Characterization UV-Vis absorption spectra were recorded on a Hitachi U2900 spectrophotometer (Hitachi, Japan). The photoluminescence spectra (in the wavelength range of 500–800 nm for R6G system and 440-850 nm for [Ru(bpy)3]2+ system) were recorded on a Fluorescence spectrophotometer (FLS980, Edinburgh Instruments, England) with the excitation at 490 nm and 430 nm for R6G and [Ru(bpy)3]2+, respectively. The photoluminescence quantum efficiency was determined using an integrating sphere with the excitation at 490 nm and 430 nm for R6G and [Ru(bpy)3]2+, respectively. Scanning electron micrographs (SEM) were obtained on a JEOL (Tokyo, Japan) JSM-7610F field emission scanning electron microscope. Prior to the measurements, the samples were coated with platinum with the thickness of 7-10 nm. The chemical composition of the used clay was determined by X-ray fluorescence spectrometer (S8 Tiger, Bruker).


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Results and discussion The UV-Vis spectra of R6G-SWF, SA, LPN and KF suspensions (Figure S1) exhibited the absorption ascribable to R6G at 500–560 nm. The band ascribable to H-aggregates (455-470 nm) was observed for KF and that due to monomer was observed for other smectites, which is consistent with those reported for R6G adsorbed on smectites.40,41 It has been proposed that the higher charge density and larger particle size of a natural smectite (KF) when compared with those of synthetic smectites (Figure 1B) are responsible for the aggregate formation.40-42 The appearance of KF-R6G and SWF-R6G suspensions before and after the irradiation for 3 h is shown in Figure 2A, where decolorization is seen for SWF system. The decolorization was followed by the change in the UV-Vis absorption spectra (Figure 2B). The changes in the relative absorbance (A/A0; relative absorbance to the initial absorbance) at the absorption maximum at 535, 540, 537 and 542 nm for SWF-R6G, SA-R6G, LPN-R6G and KF-R6G, respectively, with the irradiation time is summarized in Figure 2C. The A/A0 of SWF-R6G, LPN-R6G and SA-R6G decreased to 20%, 25% and 30%, respectively, after the irradiation for 3 h, while the change was smaller (15%) for KF-R6G. The photodecolorization rate constant, which was derived from pseudo first order reaction, is summarized in Table 1.


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Figure 2. Photographs (A) and the changes in the UV-Vis absorption spectra (B) of SWF-R6G (bottom) and KF-R6G (top) suspensions before and after the irradiation for 3 h. (C) The changes in the relative absorbance of SWF-R6G (black), SA-R6G (red), LPN-R6G (blue) and KF-R6G (green) with the irradiation time.


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Table 1. The photodecolorization rate constant, normalized photoluminescence intensity and photoluminescence quantum efficiency of SWF-R6G, SA-R6G, LPN-R6G and KF-R6G suspensions and films.

Samples

Photodecolorization rate constant (h-1)

Normalized photoluminescence intensity

Photoluminescence quantum efficiency (%)

Suspension

Film

Suspension

Film

Suspension

Film

SWF-R6G

0.56 (R2 = 0.952 a)

0.35 (R2 = 0.929 a)

1

1

12.4

4.29

LPN-R6G

0.49 (R2 = 0.985 a)

0.38 (R2 = 0.953 a)

0.83

0.98

13.9

3.22

SA-R6G

0.43 (R2 = 0.983 a)

0.33 (R2 = 0.961 a)

0.58

0.52

13.1

4.24

KF-R6G

0.05 (R2 = 0.965 a)

0.007 (R2 = 0.937 a)

0.006

0.061

0.2

0.16

a

Values in the parentheses are the R2 value, which was derived from the linear correlation of pseudo first order reactions (Figure S2 and S4).

The photoluminescence spectra of SWF-R6G, SA-R6G, LPN-R6G and KF-R6G suspensions are shown in Figure 3A. Depending on the clays, the photoluminescence intensity varied, even though the concentration of R6G was the same. The photoluminescence intensity was normalized by the intensity of SWF-R6G as 1 and the values are summarized in Table 1. It is known that the luminescence of some dyes is quenched when adsorbed on smectites that contain iron in the silicate layer.42-45 The amount of iron in KF, SA, LPN and SWF was evaluated from X-ray fluorescence spectrometer to be 3.14, 0.038, 0.048 and 0.025 wt%, respectively. The photoluminescence quantum efficiency of KF-R6G was lower than those of SWF-R6G, SA-R6G, LPN-R6G suspensions (Table 1) which is consistent with the normalized photoluminescence intensity. As to the smectites used in the present study, we have reported that the luminescence of


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the intercalated species was weaker for KF if compared with that for SA due to the excited state quenching.45 It was proposed that the energy transfer from the photoexcited pesticide to the iron in a natural montmorillonite is a possible mechanism for the stabilization of the pesticide upon photoirradiation.46,47

In the present study, from the linear relationship between the

photodecolorization rate constant and the normalized photoluminescence intensity as well as the correlation between the photodecolorization rate constant and the photoluminescence quantum efficiency (Figure 3B), it was suggested that the luminescence quenching played an important role for the dye decolorization. The oxidation of the dyes from the photoexcited state is thought to be the main pathway for the decolorization by the photoirradiation. By quenching the excited state, the probability of the oxidation was suppressed. It should be noted here that, in addition to the quenching by clay, the self-quenching from the dye aggregation may affect the photodecolorization of the dye. In the present study, in order to minimize the aggregation and the absorption density of dye, the amount of the added dye was low (1 mmol/ 100 g) to keep the suspension homogeneous and to obtain films with high optical quality.


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Figure 3. Photoluminescence spectra (A) and the relationship between the photodecolorization rate constant and the normalized photoluminescence intensity (black) and the photoluminescence quantum efficiency (red) (B) of SWF-R6G, SA-R6G, LPN-R6G and KF-R6G suspensions. Figure 4A shows the XRD patterns of the SWF-R6G and KF-R6G films before and after the irradiation with the simulated sunlight. The reflections due to the basal spacings (d001) of smectite are seen for SWF-R6G and KF-R6G films before and after the irradiation, suggesting the silicate layers are stacked parallel to the substrate.30,48,49 The d001 which was determined by Bragg’s equation from the reflection at the lowest 2 theta region, are 1.37 and 1.26 nm for SWFR6G and KF-R6G, respectively. The gallery height was calculated by subtracting the thickness of the silicate layer (1 nm) from the observed basal spacings (d001) to be 0.37 nm for SWF-R6G and 0.26 nm for KF-R6G. Since the amount of R6G was small (only 1% of cation exchange capacity of the clays), the diffraction patterns are quite similar to those of the starting materials (SWF and KF). The hydrated sodium ion occupied major part of the interlayer space and the different degree of hydration is suggested depending on smectites.


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The appearance of KF-R6G and SWF-R6G films before and after the irradiation for 3.5 h is shown in Figure 4B, where decolorization is seen for SWF-R6G film. The stability of R6G in the cast films was also different depending on the smectites used. The decolorization was followed by the change in the UV-Vis absorption spectra (Figure 5A). The changes in the relative absorbance (A/A0; relative absorbance to the initial absorbance) at the absorption at 549, 553, 552 and 554 nm for SWF-R6G, SA-R6G, LPN-R6G and KF-R6G, respectively, with the irradiation time is summarized in Figure 5B. Being same as the results for the suspensions (Figure 2C), the R6G in KF-R6G film was more stable than those in SWF-R6G, SA-R6G and LPN-R6G films.

Figure 4. (A) XRD patterns of SWF-R6G (black) and KF-R6G (red) before (a) and after (b) irradiation for 3.5 h. (B) Photographs of SWF-R6G (bottom) KF-R6G (top) films before and after irradiation.


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Figure 5. (A) The changes in the UV-Vis absorption spectra of SWF-R6G (bottom) and KF-R6G (top) films before and after the irradiation for 3.5 h. (B) The changes in the relative absorbance of SWF-R6G (black), SA-R6G (red), LPN-R6G (blue) and KF-R6G (green) with the irradiation time. The photoluminescence spectra of the films are shown in Figure 6A. The normalized photoluminescence intensity as well as the photoluminescence quantum efficiency are summarized in Table 1. Being same as the suspensions, the normalized photoluminescence intensity and the photoluminescence quantum efficiency of the films varied depending on the smectites. The photoluminescence quantum efficiency of the films was lower than the corresponding suspension as summarized in Table 1, suggesting self-quenching is also concerned. The average distance between adjacent R6G in the suspension is much larger than that in the film, which is thought to affect the self-quenching efficiency. The relationships between the photodecolorization rate constant and the normalized photoluminescence intensity / the photoluminescence quantum efficiency for the films are shown in Figure 6B. The photodecolorization rate was higher when the luminescence quenching was


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more efficient, confirming that the quenching of the excited state of R6G by the impurity in KF played an important role for the dye decolorization. The photodecolorization rate constant of R6G in the cast films was smaller than those in the suspensions (Table 1). In the film, the silicate layers were stacked in the direction parallel to the substrate as shown by the cross-sectional SEM images (in Figure S5). Due to the stacking, O2 diffusion in the interlayer space, where the dye was adsorbed, was thought to be suppressed as proposed for the gas barrier properties of the claypolymer films (tortuous path effect).50-55 The suppressed O2 diffusion is a possible reason for the slower photodecolorization rates seen for the film than those for the suspension. As to the difference in the photodecolorization rate constant between film and suspension, there are many parameters (distance between the adjacent dye, mobility of the dye and the amount) to be concerned in addition to O2 diffusion. The states of the dye in the film and the suspension also affect the photodecolarization, because absorption coefficient, absorption wavelength and the nature of the deactivation process are different depending on the states. In the present study, in order to avoid sedimentation of dye-clay during the irradiation and to obtain oriented films, the dye loading of 1 mmol/100 g clay was employed. The aggregation is less plausible especially in SWF system thanks to the low loading amount of the dye. However, the possibility of the aggregation cannot be excluded, and the aggregation during film preparation is possible.42 Even though the absorption due to the aggregates was not clearly seen in Figure 5, dye aggregation may be concerned on the difference in the photoluminescence quantum efficiency between the suspension and the film.

The photodecolorization of the dye without specific aggregates

([Ru(bpy)3]2+ in Figure 7) is a simple example, where the excited state quenching from clay nanosheet were clearly seen in the present study. The stability of aggregates is also worth investigating, since some of the aggregates are quite important for the application.56,57 In order to


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discuss the difference quantitatively, the decolorization quantum efficiency under varied conditions (dye loading, concentration of the suspension, thickness of the film, atmosphere, etc.) is worth investigating.

Figure 6. Photoluminescence spectra (A) and the relationship between the photodecolorization rate constant and the normalized photoluminescence intensity (black) and the photoluminescence quantum efficiency (red) (B) of SWF-R6G, SA-R6G, LPN-R6G and KF-R6G films.

The decolorization in N2 atmosphere was also studied for SWF-R6G and KF-R6G films. The changes in the UV-Vis absorption spectra are shown in Figure S6 (in the supporting information). The photodecolorization rate constants derived from Figure S6 are summarized in Table 2. For SWF-R6G film, the decolorization was slower when the irradiation was done in N2 atmosphere if compared with that in air, confirming the role of atmospheric oxygen on the


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decolorization (oxidative decomposition). On the other hand, the effect of the atmosphere was not clear for KF-R6G film, since the reactions between the excited state and atmospheric oxygen were less plausible due to the efficient quenching of the excited state as discussed.

Table 2. The photodecolorization rate constant determined for SWF-R6G and KF- R6G films in N2 and air. Photodecolorization rate constant (h-1) Samples in N2

in Air

SWF-R6G

0.92 (R2 = 0.889 a)

2.74 (R2 = 0.946 a)

KF-R6G

0.08 (R2 = 0.868 a)

0.09 (R2 = 0.863 a)

a

Values in the parentheses are the R2 value, which was derived from the linear correlation of pseudo first order plot.

The proposed strategy to stabilize dye by the complexation with smectites through excited state quenching was applied to tris(2,2'-bipyridine)ruthenium(II) complex (abbreviated as [Ru(bpy)3]2+). The change in the UV-Vis absorption spectra of the aqueous suspension of the smectites containing [Ru(bpy)3]2+ upon the irradiation is shown in Figure 7A and 7B. The photoluminescence of [Ru(bpy)3]2+ was quenched in KF as shown by the normalized photoluminescence intensity and the photoluminescence quantum efficiency (using the excitation wavelength at 430 nm). The decolorization rate constants were lower for KF-[Ru(bpy)3]2+ than for SWF-[Ru(bpy)3]2+ depending on the photoluminescence quenching (Figure 7C).


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Figure 7. The changes in the absorption spectra of SWF-[Ru(bpy)3]2+ (A) and KF-[Ru(bpy)3]2+ (B) suspension. (C) The relationship between the photodecolorization rate constant and the normalized photoluminescence intensity (black) and photoluminescence quantum efficiency (red) of SWF-[Ru(bpy)3]2+ and KF-[Ru(bpy)3]2+ suspensions.

Conclusions In summary, the remarkable improvement of the stability of the dyes was achieved by the complexation with a natural montmorillonite. The mechanism of the stabilization was explained by the quenching of the photoexcited state of the dyes by the impurities in the natural montmorillonite. The mechanism is worth discussing for the dye stability in other host-guest systems and the strategy (improve dye stability by the quenching of the photoexcited state from the host) is applicable to construct functional pigments of various colors based on host-guest complexes.


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Acknowledgments This work was supported by the Research Chair Grant 2017 (grant number FDA-CO-25605655) from the National Science and Technology Development Agency (NSTDA), Thailand. One of the author (Teepakakorn, A. P.) acknowledges Vidyasirimedhi Institute of Science and Technology for the scholarship to her Ph.D. study. Ltd. for the donation of the clay samples.

The authors appreciate Kunimine Ind. Co.,

The support from Kamnoetvidya Science Academy

(KVIS), Rayong, Thailand is appreciated for the use of UV-Vis spectrophotomerter (Hitachi U2900 spectrophotometer from Hitachi, Japan).

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Supporting information The changes in the absorption spectra and photographs of suspensions and films by the simulated sunlight irradiation, The plotted relationship between In A and irradiation time of suspensions and films which was derived from pseudo first order reaction, Scanning electron micrographs of the cross-section of films, The changes in the absorption spectra SWF-R6G and KF-R6G films in N2 atmosphere and in air using Xe lamp (Hamamatsu Photonics K.K.) as the light source and The relationship between In A and the irradiation time for SWF-R6G and KF-R6G films in N2 atmosphere and in air.


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Table of Contents


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Adsorption induced dye stability of cationic dyes on clay nanosheet

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