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Synergy effects of the complexation of a titania and a smectite on the film formation and its photocatalyst' performance
Applied Clay Science 169 (2019) 129–134
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research Paper
Synergy effects of the complexation of a titania and a smectite on the film formation and its photocatalyst' performance Siwada Deepracha, Sareeya Bureekaew, Makoto Ogawa
T
⁎
School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), 555 Moo 1 Tumbol Payupnai, Amphoe Wangchan, Rayong 21210, Thailand
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalyst Smectite Titania Film
A clay mineral (synthetic hectorite) was complexed with titanium dioxide (anatase with the average particle size of 20 nm) as a film on a glass substrate for photocatalyst application. The mass ratio of the hectorite to the anatase was optimized to be 1(hectorite):10(anatase) to obtain homogeneous coatings. The photocatalytic decolorization of organic dyes (methyl orange and methylene blue) by UV irradiation was examined as a model reaction to find a synergy effect of the hybridization, an efficient methylene blue decomposition in an acidic solution.
1. Introduction Heterogeneous photocatalysts are useful for environmental purification, organic syntheses, and artificial photosyntheses. Photocatalytic reactions using semiconductor photocatalysts have been conducted in aqueous dispersion, where reactions are easily conducted using desired amount of photocatalyst with no mass transfer limitation. Semiconductor photocatalysts with smaller particle size have been developed in order to improve the reaction efficiency by the larger reactive surfaces, while there are drawbacks when nano-size semiconductor particles are employed; (1) separation of nanoparticles from the dispersion after the reaction is not easy, (2) nanoparticles tend to aggregate in dispersion especially when large amounts of nanoparticles are used. The immobilization of semiconductor nanoparticles on solid supports is a possible solution to overcome these limitations. Titanium dioxide (TiO2) is one of the heterogeneous photocatalyst used most extensively (Hashimoto et al., 2005). TiO2 coatings with varied thickness and transparency have been obtained by several wet and dry processes (Ling et al., 2004; Bosc et al., 2005; Shan et al., 2010). Self-cleaning (Noguchi et al., 1998), antibacterial (Yu et al., 2003) and anti-fogging functions of TiO2 coating due to the photocatalytic property and photo-induced superhydrophilicity have been reported (Wang et al., 1997, 1998; Nakajima et al., 2000). The preparation of TiO2 coatings with varied thickness, microstructures (roughness and porosity), and homogeneity is worth investigating in order to optimize the materials' performances as well as to find new applications.
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In the present study, the hybridization of TiO2 with smectite is examined to obtain photocatalyst films. Smectite is a group of layered clay minerals consisting of negatively charged silicate layer and the charge compensating interlayer cations (Bergaya et al., 2006). Due to the swelling property in water and the useful rheological characteristics of the resulting aqueous dispersion, smectite has been used in many practical uses such as cosmetics, paints, civil engineering and so on (Theng, 1974). Recently, the studies on the complexation of TiO2 with clays and clay minerals were summarized from the aspects of preparation, structural characterstics and the structure-property relationship (Deepracha et al., 2018). As an example of the applications, the TiO2-smectite hybrids have been used as photocatalysts (Yamanaka et al., 1987; Mogyorosi et al., 2003; Pernyeszi and Dékány, 2003; Ilisz et al., 2003; Kun et al., 2006; Ménesi et al., 2008). It is also known that thin films of smectite have been obtained by drying the dispersion on substrates, and also by more sophisticated methods as Langmuir-Blodgett (LB) and Layer-by-Layer deposition techniques (Ogawa et al., 1994a, 1994b; Kotov et al., 1997; Ariga et al., 1999; Inukai et al., 2000; Zhou et al., 2002; Ras et al., 2004, 2007; Hornok et al., 2006; Okada et al., 2008). Here, taking advantages of the characteristic features of smectite, the immobilization of TiO2 on substrate was investigated by casting the dispersion containing smectite and TiO2, and possible photocatalytic application of the product was examined.
Corresponding author. E-mail address: [email protected] (M. Ogawa).
https://doi.org/10.1016/j.clay.2018.12.005 Received 19 September 2018; Received in revised form 24 November 2018; Accepted 8 December 2018 Available online 04 January 2019 0169-1317/ © 2018 Elsevier B.V. All rights reserved.
Applied Clay Science 169 (2019) 129–134
S. Deepracha et al.
The photographs of the cast films prepared at different hectorite: anatase ratios are shown in Fig. 3. The homogeneity of the hybrid films was affected by the composition. SEM images of the hybrid film (1:10 of hectorite: anatase) and the anatase film are shown in Fig. 4. As shown by the cross sectional SEM image (Fig. 4a), when hectorite was not added, the film was not homogeneous and the anatase particles aggregated. On the other hand, relatively homogeneous film with the smooth surface and the thickness of ca. 2 μm was obtained for the hybrid (1:10 of hectorite: anatase) as shown in Fig. 4b. Thus, the complexation of the hectorite and the anatase led the homogeneous coating by suppressing the aggregation of anatase, and the coating homogeneity is governed by the composition of hectorite and anatase. In order for the film to be applied as photocatalysts, it is favorable that the amount of the additive (smectites) is smaller. Accordingly, further increase of the hectorite was not examined in the present study. The optimum hectorite amount may vary depending on the size of anatase. In our separate study on the preparation of hybrid composed of 5 nm anatase particles with a synthetic saponite, the saponite to the anatase mass ratio of 2:9 was the best in terms of homogeneity of the dispersion and the coating (Goto and Ogawa, 2015, 2016). Based on the homogeneity, the hybrid film (hectorite to anatase mass ratio of 1:10) for the uses for the photocatalytic reactions in aqueous phase was examined by immersing the films into aqueous solutions of HCl or NaOH (pH of 3, 7, and 10) at room temperature for 1 day and the results were compared with the anatase film prepared without hectorite. The change of the mass of the films was measured to examine the leaching or liberation of the hybrid from the substrate. There was no mass loss in neutral water (pH = 7). The mass of the anatase film decreased after immersing in acidic and basic solutions, on the contrary, no mass change was seen for the hybrid film. Based on the results, the hybrid film is applicable for the photocatalytic reactions in aqueous phase over a wide pH range of 3 to 10. The photocatalytic decolorization of MO was examined at initial concentrations of 10 with pH of solution (pH = 6), as shown in Fig. 5a. There was no adsorption of MO on the films (hectorite, anatase and hybrid) in the dark. The photocatalytic decolorization was observed and the concentration of MO was decreased from 10 ppm to 2 ppm for both hybrid film and anatase film after the UV irradiation for 3 h. The rate constant of the decolorization was derived from the pseudo first order plots (Fig. 5b) to be 0.5 and 0.47 h−1 for the hybrid film and the anatase film, respectively. Even though the anatase particles were not adhered strongly on the borosilicate glass substrate, the reaction was possible without the loss of the activity as shown by the first order plot (Fig. 5b). There was no negative effect of the hectorite on the photocatalytic decolorization of MO by the anatase. The photocatalytic decolorization of MB in the acidic solution was conducted at pH = 3. Under the acidic condition (pH = 3), the anatase showed a positive charge on surface as seen by zeta potential in the Fig. S1. The MB was not adsorbed on anatase surface due to same positive surface charge. The change in the concentration is shown in Fig. 6. Before the irradiation, the concentration of MB was decreased from 15 ppm to 13 ppm for the hectorite film and the hybrid film due to the adsorption of MB by the ion exchange with the hectorite. The cation exchange of the hectorite with proton may compete the exchange with MB in acidic solution. However, the adsorbed amount of MB (ca. 0.06 μmol) was equal to the cation exchange capacity of the hectorite in the film (0.7 mequiv/g clay and 0.08 mg of clay, meaning 0.058 μmol of MB can be adsorbed). The cation exchange with MB is thought to prevent re-swelling of hectorite. After the UV irradiation, the decolorization of MB was seen for the anatase film and the hybrid film, while MB in the solution without catalyst or with the hectorite film was not decomposed. The rate constant of the decolorization catalyzed by the hybrid film was 0.5 h−1, which is higher than that by the anatase film (0.1 h−1) as derived from the pseudo first order plots (Fig. 6b). The significant difference in the photocatalytic decolorization rate between the hybrid film and the anatase film without hectorite is observed under
2. Experimental methods 2.1. Materials Anatase (ST-21) was kindly donated from Ishihara Sangyo Kaisha Ind. Co., Ltd. A synthetic sodium-hectorite (Sumecton SWF) was kindly donated from Kunimine Industrial Co., Ltd. The cation exchange capacity of the hectorite is 70 mequiv/100 g of clay. Methylene blue (abbreviated as MB) was purchased from Merck Ltd. Methyl orange (abbreviated as MO) was purchased from Tokyo Chemical Industry Co., Ltd. All chemicals were used without further purification. 2.2. Preparation of hybrid films Hybrid films were prepared by casting the aqueous dispersion containing the anatase and the hectorite onto a borosilicate substrate and dried at 30 °C. The mass ratios of hectorite to anatase at 0.25:10, 0.5:10, and 1:10 were investigated. The mass ratio of hectorite to anatase was varied by changing the concentration (2.5, 5, and 10 mg in 75 ml of water) of the hectorite in the mixture with the fixed amount (100 mg in 25 ml of water) of the anatase. 2.3. Photocatalytic decolorization of organic dyes The photocatalytic decolorization of organic dyes (MO and MB) was investigated as a model reaction to examine photocatalytic activity of the hybrid film. The reaction was carried out by putting the film in an aqueous solution of dyes and irradiated by a UV LED light (USHIO SPL2, 365 nm). The light intensity was 0.15 W/cm2. The concentration of the dyes was determined by UV–Vis spectroscopy (absorbance at 464 and 664 nm for MO and MB, respectively) and the changes of the concentration of the dyes during the irradiation were followed. 3. Results and discussion A commercially available anatase (ST-21, particle size of 20 nm, supplied from Ishihara Sangyo Kaisha Ind. Co., Ltd.) and a synthetic hectorite (Sumecton SWF, supplied from Kunimine Ind. Co., Ltd.) were used for the fabrication of photocatalytic film in the present study. The particle morphology and the crystal structure of the anatase and the hectorite were characterized by scanning electron microscope (SEM) and X-ray powder diffraction pattern (XRD) as shown in Fig. 1. The synthetic hectorite was used among available smectites from the following reasons; (1) phase and elemental purity if compared with natural smectites, (2) finite particle size (ca. 20 nm to 100 nm as seen in Fig. 1b and DLS data, not shown), (3) transparency of the dispersion and the film in UV to visible wavelength region thanks to the elemental purity (1) and finite particle size (2). The photographs of an aqueous dispersion (0.01%g/v in water) and film (prepared by casting the dispersion, the thickness is 50 nm) of the hectorite are shown in Fig. 1d. Fig. 2 shows the photographs of the anatase (ST-21, particle size of 20 nm) added in water with the absence and the presence of the hectorite at different mass ratios of hectorite to anatase of 0.25:10, 0.5:10, and 1:10. The aqueous dispersion of the anatase is not stable and sedimentation was observed after the storage of the aqueous mixture of the anatase without agitation. By adding the anatase into an aqueous hectorite dispersion, stable dispersion was obtained thanks to the swelling of the hectorite. Stable dispersion of anatase particles (with the size of 25 nm to 260 nm) were obtained by employing surfactants or organic solvents (Hsu and Chang, 2000; Tkachenko et al., 2006; Iijima et al., 2009; Liu et al., 2015), together with additives which were removed during or after the film fabrication for the photocatalyst's application. The present methodology using smectite, inorganic solids, is advantageous from the viewpoint of eco-friendly operation (no organic solvents or additives) without the needs of the additional process to remove additives. 130
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Fig. 1. SEM images of (a) the anatase and (b) the hectorite, (c) XRD patterns of the anatase and the hectorite, and (d) the photographs of an aqueous dispersion and a cast film of the hectorite.
benzene by a commercially available titania (P25) was reported (Ide et al., 2012). Efforts are being made in our laboratory to find a new phenomenon by the hybrid photocatalysts as well as to explain the role of smectites using various smectites and titanium dioxides. Porous TiO2 films (Grosso et al., 2001; Stathatos et al., 2001; Zhang et al., 2003; Choi et al., 2006; Chen and Dionysiou, 2008; Nursam et al., 2015) and monoliths (Konishi et al., 2006; Hasegawa et al., 2010; Wan et al., 2018), and TiO2 deposited on a zeolite monolith (Jing et al., 2016) have been prepared in order to achieve higher efficiency by increasing exposure of TiO2 surface. The present strategy for the immobilization of semiconductor particles on solid support using smectite can be combined with the hierarchical structural design for further
the same reaction conditions, which was resulted by the MB adsorption on hectorite. The photographs of the films were taken after removing the films from the solution, as shown in Fig. 6c. The decreasing activity of MB degradation for the anatase film without the hectorite was probably affected by losing the anatase particles. The higher rate of MB degradation by the hybrid film over pure anatase resulted from a synergy effect and improved stability by complexation of anatase with smectite. A synergetic effect of the complexation of the anatase with the hectorite was demonstrated by the efficient decomposition of MB, which cannot be explained by the sum of the adsorption of MB on the hectorite and the photocatalytic decomposition of MB on the anatase. Another un-explained role of clays on the photocatalytic oxidation of
Fig. 2. Photographs of the anatase (ST-21) in water in the absence and the presence of the hectorite (at different mass ratios of hectorite to anatase of 0.25:10, 0.5:10, and 1:10). The photographs were taken after storing the stirred dispersion for a week without agitation. 131
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Fig. 3. Photographs of the products obtained by casting the dispersions containing the anatase and the hectorite onto glass substrate. From the left: anatase, hectorite to anatase mass ratios of 0.25:10, 0.5:10, and 1:10.
Fig. 4. SEM images of (a) anatase film on the substrate and (b) the hybrid film (at 1:10 hectorite to anatase mass ratio) on the substrate.
the anatase film. Acknowlegment This work was supported by the Research Chair Grant 2017 (grant number FDA-CO-2560-5655) from the National Science and Technology Development Agency (NSTDA), Thailand. One of the author (Deepracha, S.) acknowledges Vidyasirimedhi Institute of Science and Technology for the scholarship to his Ph.D. study.
optimization of photocatalysts' performances. 4. Conclusion The photocatalytic hybrid film composed of an anatase (20 nm size of ST-21, Ishihara Sangyo Kaisha Ind. Co., Ltd.) and a synthetic hectorite (Sumecton SWF, Kunimine Ind. Co., Ltd.,) was successfully fabricated. Homogeneous coating was obtained on a glass substrate when the hectorite to anatase mass ratio was 1:10. The film was adhered on the substrate stably enough to be applied for the photocatalytic reactions in aqueous phases even in acidic solutions. The photocatalytic decolorization of methylene blue and methyl orange in aqueous solutions was possible. A synergy effect of the hybridization was seen for the photocatalytic decomposition of methylene blue in an acidic solution, where the reaction was much faster by the hybrid film than that by
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2018.12.005.
Fig. 5. (a) Photocatalytic decolorization of MO at pH solution of 6 by the hybrid film (hectorite:anatase = 1:10), and (b) apparent pseudo first order plot of MO decolorization for the anatase film and the hybrid film. 132
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Fig. 6. (a) Photocatalytic decolorization of MB by the UV irradiation under an acidic condition (pH = 3), (b) apparent pseudo first order plot of MB decolorization for the anatase film and the hybrid film, and (c) photographs of MB solution, the anatase film and the hybrid film after the reaction.
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Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research Paper
Synergy effects of the complexation of a titania and a smectite on the film formation and its photocatalyst' performance Siwada Deepracha, Sareeya Bureekaew, Makoto Ogawa
T
⁎
School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), 555 Moo 1 Tumbol Payupnai, Amphoe Wangchan, Rayong 21210, Thailand
A R T I C LE I N FO
A B S T R A C T
Keywords: Photocatalyst Smectite Titania Film
A clay mineral (synthetic hectorite) was complexed with titanium dioxide (anatase with the average particle size of 20 nm) as a film on a glass substrate for photocatalyst application. The mass ratio of the hectorite to the anatase was optimized to be 1(hectorite):10(anatase) to obtain homogeneous coatings. The photocatalytic decolorization of organic dyes (methyl orange and methylene blue) by UV irradiation was examined as a model reaction to find a synergy effect of the hybridization, an efficient methylene blue decomposition in an acidic solution.
1. Introduction Heterogeneous photocatalysts are useful for environmental purification, organic syntheses, and artificial photosyntheses. Photocatalytic reactions using semiconductor photocatalysts have been conducted in aqueous dispersion, where reactions are easily conducted using desired amount of photocatalyst with no mass transfer limitation. Semiconductor photocatalysts with smaller particle size have been developed in order to improve the reaction efficiency by the larger reactive surfaces, while there are drawbacks when nano-size semiconductor particles are employed; (1) separation of nanoparticles from the dispersion after the reaction is not easy, (2) nanoparticles tend to aggregate in dispersion especially when large amounts of nanoparticles are used. The immobilization of semiconductor nanoparticles on solid supports is a possible solution to overcome these limitations. Titanium dioxide (TiO2) is one of the heterogeneous photocatalyst used most extensively (Hashimoto et al., 2005). TiO2 coatings with varied thickness and transparency have been obtained by several wet and dry processes (Ling et al., 2004; Bosc et al., 2005; Shan et al., 2010). Self-cleaning (Noguchi et al., 1998), antibacterial (Yu et al., 2003) and anti-fogging functions of TiO2 coating due to the photocatalytic property and photo-induced superhydrophilicity have been reported (Wang et al., 1997, 1998; Nakajima et al., 2000). The preparation of TiO2 coatings with varied thickness, microstructures (roughness and porosity), and homogeneity is worth investigating in order to optimize the materials' performances as well as to find new applications.
⁎
In the present study, the hybridization of TiO2 with smectite is examined to obtain photocatalyst films. Smectite is a group of layered clay minerals consisting of negatively charged silicate layer and the charge compensating interlayer cations (Bergaya et al., 2006). Due to the swelling property in water and the useful rheological characteristics of the resulting aqueous dispersion, smectite has been used in many practical uses such as cosmetics, paints, civil engineering and so on (Theng, 1974). Recently, the studies on the complexation of TiO2 with clays and clay minerals were summarized from the aspects of preparation, structural characterstics and the structure-property relationship (Deepracha et al., 2018). As an example of the applications, the TiO2-smectite hybrids have been used as photocatalysts (Yamanaka et al., 1987; Mogyorosi et al., 2003; Pernyeszi and Dékány, 2003; Ilisz et al., 2003; Kun et al., 2006; Ménesi et al., 2008). It is also known that thin films of smectite have been obtained by drying the dispersion on substrates, and also by more sophisticated methods as Langmuir-Blodgett (LB) and Layer-by-Layer deposition techniques (Ogawa et al., 1994a, 1994b; Kotov et al., 1997; Ariga et al., 1999; Inukai et al., 2000; Zhou et al., 2002; Ras et al., 2004, 2007; Hornok et al., 2006; Okada et al., 2008). Here, taking advantages of the characteristic features of smectite, the immobilization of TiO2 on substrate was investigated by casting the dispersion containing smectite and TiO2, and possible photocatalytic application of the product was examined.
Corresponding author. E-mail address: [email protected] (M. Ogawa).
https://doi.org/10.1016/j.clay.2018.12.005 Received 19 September 2018; Received in revised form 24 November 2018; Accepted 8 December 2018 Available online 04 January 2019 0169-1317/ © 2018 Elsevier B.V. All rights reserved.
Applied Clay Science 169 (2019) 129–134
S. Deepracha et al.
The photographs of the cast films prepared at different hectorite: anatase ratios are shown in Fig. 3. The homogeneity of the hybrid films was affected by the composition. SEM images of the hybrid film (1:10 of hectorite: anatase) and the anatase film are shown in Fig. 4. As shown by the cross sectional SEM image (Fig. 4a), when hectorite was not added, the film was not homogeneous and the anatase particles aggregated. On the other hand, relatively homogeneous film with the smooth surface and the thickness of ca. 2 μm was obtained for the hybrid (1:10 of hectorite: anatase) as shown in Fig. 4b. Thus, the complexation of the hectorite and the anatase led the homogeneous coating by suppressing the aggregation of anatase, and the coating homogeneity is governed by the composition of hectorite and anatase. In order for the film to be applied as photocatalysts, it is favorable that the amount of the additive (smectites) is smaller. Accordingly, further increase of the hectorite was not examined in the present study. The optimum hectorite amount may vary depending on the size of anatase. In our separate study on the preparation of hybrid composed of 5 nm anatase particles with a synthetic saponite, the saponite to the anatase mass ratio of 2:9 was the best in terms of homogeneity of the dispersion and the coating (Goto and Ogawa, 2015, 2016). Based on the homogeneity, the hybrid film (hectorite to anatase mass ratio of 1:10) for the uses for the photocatalytic reactions in aqueous phase was examined by immersing the films into aqueous solutions of HCl or NaOH (pH of 3, 7, and 10) at room temperature for 1 day and the results were compared with the anatase film prepared without hectorite. The change of the mass of the films was measured to examine the leaching or liberation of the hybrid from the substrate. There was no mass loss in neutral water (pH = 7). The mass of the anatase film decreased after immersing in acidic and basic solutions, on the contrary, no mass change was seen for the hybrid film. Based on the results, the hybrid film is applicable for the photocatalytic reactions in aqueous phase over a wide pH range of 3 to 10. The photocatalytic decolorization of MO was examined at initial concentrations of 10 with pH of solution (pH = 6), as shown in Fig. 5a. There was no adsorption of MO on the films (hectorite, anatase and hybrid) in the dark. The photocatalytic decolorization was observed and the concentration of MO was decreased from 10 ppm to 2 ppm for both hybrid film and anatase film after the UV irradiation for 3 h. The rate constant of the decolorization was derived from the pseudo first order plots (Fig. 5b) to be 0.5 and 0.47 h−1 for the hybrid film and the anatase film, respectively. Even though the anatase particles were not adhered strongly on the borosilicate glass substrate, the reaction was possible without the loss of the activity as shown by the first order plot (Fig. 5b). There was no negative effect of the hectorite on the photocatalytic decolorization of MO by the anatase. The photocatalytic decolorization of MB in the acidic solution was conducted at pH = 3. Under the acidic condition (pH = 3), the anatase showed a positive charge on surface as seen by zeta potential in the Fig. S1. The MB was not adsorbed on anatase surface due to same positive surface charge. The change in the concentration is shown in Fig. 6. Before the irradiation, the concentration of MB was decreased from 15 ppm to 13 ppm for the hectorite film and the hybrid film due to the adsorption of MB by the ion exchange with the hectorite. The cation exchange of the hectorite with proton may compete the exchange with MB in acidic solution. However, the adsorbed amount of MB (ca. 0.06 μmol) was equal to the cation exchange capacity of the hectorite in the film (0.7 mequiv/g clay and 0.08 mg of clay, meaning 0.058 μmol of MB can be adsorbed). The cation exchange with MB is thought to prevent re-swelling of hectorite. After the UV irradiation, the decolorization of MB was seen for the anatase film and the hybrid film, while MB in the solution without catalyst or with the hectorite film was not decomposed. The rate constant of the decolorization catalyzed by the hybrid film was 0.5 h−1, which is higher than that by the anatase film (0.1 h−1) as derived from the pseudo first order plots (Fig. 6b). The significant difference in the photocatalytic decolorization rate between the hybrid film and the anatase film without hectorite is observed under
2. Experimental methods 2.1. Materials Anatase (ST-21) was kindly donated from Ishihara Sangyo Kaisha Ind. Co., Ltd. A synthetic sodium-hectorite (Sumecton SWF) was kindly donated from Kunimine Industrial Co., Ltd. The cation exchange capacity of the hectorite is 70 mequiv/100 g of clay. Methylene blue (abbreviated as MB) was purchased from Merck Ltd. Methyl orange (abbreviated as MO) was purchased from Tokyo Chemical Industry Co., Ltd. All chemicals were used without further purification. 2.2. Preparation of hybrid films Hybrid films were prepared by casting the aqueous dispersion containing the anatase and the hectorite onto a borosilicate substrate and dried at 30 °C. The mass ratios of hectorite to anatase at 0.25:10, 0.5:10, and 1:10 were investigated. The mass ratio of hectorite to anatase was varied by changing the concentration (2.5, 5, and 10 mg in 75 ml of water) of the hectorite in the mixture with the fixed amount (100 mg in 25 ml of water) of the anatase. 2.3. Photocatalytic decolorization of organic dyes The photocatalytic decolorization of organic dyes (MO and MB) was investigated as a model reaction to examine photocatalytic activity of the hybrid film. The reaction was carried out by putting the film in an aqueous solution of dyes and irradiated by a UV LED light (USHIO SPL2, 365 nm). The light intensity was 0.15 W/cm2. The concentration of the dyes was determined by UV–Vis spectroscopy (absorbance at 464 and 664 nm for MO and MB, respectively) and the changes of the concentration of the dyes during the irradiation were followed. 3. Results and discussion A commercially available anatase (ST-21, particle size of 20 nm, supplied from Ishihara Sangyo Kaisha Ind. Co., Ltd.) and a synthetic hectorite (Sumecton SWF, supplied from Kunimine Ind. Co., Ltd.) were used for the fabrication of photocatalytic film in the present study. The particle morphology and the crystal structure of the anatase and the hectorite were characterized by scanning electron microscope (SEM) and X-ray powder diffraction pattern (XRD) as shown in Fig. 1. The synthetic hectorite was used among available smectites from the following reasons; (1) phase and elemental purity if compared with natural smectites, (2) finite particle size (ca. 20 nm to 100 nm as seen in Fig. 1b and DLS data, not shown), (3) transparency of the dispersion and the film in UV to visible wavelength region thanks to the elemental purity (1) and finite particle size (2). The photographs of an aqueous dispersion (0.01%g/v in water) and film (prepared by casting the dispersion, the thickness is 50 nm) of the hectorite are shown in Fig. 1d. Fig. 2 shows the photographs of the anatase (ST-21, particle size of 20 nm) added in water with the absence and the presence of the hectorite at different mass ratios of hectorite to anatase of 0.25:10, 0.5:10, and 1:10. The aqueous dispersion of the anatase is not stable and sedimentation was observed after the storage of the aqueous mixture of the anatase without agitation. By adding the anatase into an aqueous hectorite dispersion, stable dispersion was obtained thanks to the swelling of the hectorite. Stable dispersion of anatase particles (with the size of 25 nm to 260 nm) were obtained by employing surfactants or organic solvents (Hsu and Chang, 2000; Tkachenko et al., 2006; Iijima et al., 2009; Liu et al., 2015), together with additives which were removed during or after the film fabrication for the photocatalyst's application. The present methodology using smectite, inorganic solids, is advantageous from the viewpoint of eco-friendly operation (no organic solvents or additives) without the needs of the additional process to remove additives. 130
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Fig. 1. SEM images of (a) the anatase and (b) the hectorite, (c) XRD patterns of the anatase and the hectorite, and (d) the photographs of an aqueous dispersion and a cast film of the hectorite.
benzene by a commercially available titania (P25) was reported (Ide et al., 2012). Efforts are being made in our laboratory to find a new phenomenon by the hybrid photocatalysts as well as to explain the role of smectites using various smectites and titanium dioxides. Porous TiO2 films (Grosso et al., 2001; Stathatos et al., 2001; Zhang et al., 2003; Choi et al., 2006; Chen and Dionysiou, 2008; Nursam et al., 2015) and monoliths (Konishi et al., 2006; Hasegawa et al., 2010; Wan et al., 2018), and TiO2 deposited on a zeolite monolith (Jing et al., 2016) have been prepared in order to achieve higher efficiency by increasing exposure of TiO2 surface. The present strategy for the immobilization of semiconductor particles on solid support using smectite can be combined with the hierarchical structural design for further
the same reaction conditions, which was resulted by the MB adsorption on hectorite. The photographs of the films were taken after removing the films from the solution, as shown in Fig. 6c. The decreasing activity of MB degradation for the anatase film without the hectorite was probably affected by losing the anatase particles. The higher rate of MB degradation by the hybrid film over pure anatase resulted from a synergy effect and improved stability by complexation of anatase with smectite. A synergetic effect of the complexation of the anatase with the hectorite was demonstrated by the efficient decomposition of MB, which cannot be explained by the sum of the adsorption of MB on the hectorite and the photocatalytic decomposition of MB on the anatase. Another un-explained role of clays on the photocatalytic oxidation of
Fig. 2. Photographs of the anatase (ST-21) in water in the absence and the presence of the hectorite (at different mass ratios of hectorite to anatase of 0.25:10, 0.5:10, and 1:10). The photographs were taken after storing the stirred dispersion for a week without agitation. 131
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Fig. 3. Photographs of the products obtained by casting the dispersions containing the anatase and the hectorite onto glass substrate. From the left: anatase, hectorite to anatase mass ratios of 0.25:10, 0.5:10, and 1:10.
Fig. 4. SEM images of (a) anatase film on the substrate and (b) the hybrid film (at 1:10 hectorite to anatase mass ratio) on the substrate.
the anatase film. Acknowlegment This work was supported by the Research Chair Grant 2017 (grant number FDA-CO-2560-5655) from the National Science and Technology Development Agency (NSTDA), Thailand. One of the author (Deepracha, S.) acknowledges Vidyasirimedhi Institute of Science and Technology for the scholarship to his Ph.D. study.
optimization of photocatalysts' performances. 4. Conclusion The photocatalytic hybrid film composed of an anatase (20 nm size of ST-21, Ishihara Sangyo Kaisha Ind. Co., Ltd.) and a synthetic hectorite (Sumecton SWF, Kunimine Ind. Co., Ltd.,) was successfully fabricated. Homogeneous coating was obtained on a glass substrate when the hectorite to anatase mass ratio was 1:10. The film was adhered on the substrate stably enough to be applied for the photocatalytic reactions in aqueous phases even in acidic solutions. The photocatalytic decolorization of methylene blue and methyl orange in aqueous solutions was possible. A synergy effect of the hybridization was seen for the photocatalytic decomposition of methylene blue in an acidic solution, where the reaction was much faster by the hybrid film than that by
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2018.12.005.
Fig. 5. (a) Photocatalytic decolorization of MO at pH solution of 6 by the hybrid film (hectorite:anatase = 1:10), and (b) apparent pseudo first order plot of MO decolorization for the anatase film and the hybrid film. 132
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Fig. 6. (a) Photocatalytic decolorization of MB by the UV irradiation under an acidic condition (pH = 3), (b) apparent pseudo first order plot of MB decolorization for the anatase film and the hybrid film, and (c) photographs of MB solution, the anatase film and the hybrid film after the reaction.
References
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