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Enhanced optical absorption of nanosized TiO2 by composition with zeolite
Materials Chemistry and Physics xxx (2015) 1e4
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Enhanced optical absorption of nanosized TiO2 by composition with zeolite Satoru Fukugaichi a, *, Naoto Matsue b a b
Ehime Institute of Industrial Technology, 487-2 Kumekubota, Matsuyama 791-1101, Japan Faculty of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
TiO2-zeolite composite was synthesized by using nanosized TiO2. The composite had higher UV light absorptivity than a corresponding mixture. The increased absorptivity increased the photocatalytic activity of the composite.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2014 Received in revised form 4 March 2015 Accepted 6 April 2015 Available online xxx
Enhancement of the photocatalytic activity of TiO2 and its mechanism are important subjects in the field of catalysis. Under the coexistence of TiO2 with a diameter of about 20 nm, we synthesized faujasite Xtype zeolite, and the product was made up of the composite of TiO2 and faujasite X-type zeolite. The composite had higher acetaldehyde photocatalytic decomposition activity than a corresponding mixture, and the activity of the composite was higher than that of TiO2 on TiO2 mass basis. The composite had higher UV light absorptivity than the corresponding mixture, and their absorption of UV light was mainly due to TiO2. The increased UV light absorptivity of the composite correlated to its increased acetaldehyde photocatalytic decomposition activity. © 2015 Elsevier B.V. All rights reserved.
Keywords: Composite materials Chemical synthesis Optical properties Oxidation
1. Introduction Titanium dioxide (TiO2) photocatalysts have been widely studied as purification materials for toxic organic substances in water and air [1,2]. Numerous strategies have been performed to increase their photocatalytic activity by reducing their particle size to nanometer level [3,4], and by hybridizing TiO2 with various
* Corresponding author. Present address: Paper Industrial Innovation Center of Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan. E-mail address: [email protected] (S. Fukugaichi).
adsorbents. Among the adsorbents, zeolites have been most frequently used, and some methods for the hybridization (or composition) of TiO2 with zeolites have been reported. They are precipitation of TiO2 on the surface of zeolites [5e10], cation exchange of Ti4þ into zeolite cavities followed by the formation of TiO2 [11e14], physical mixing [15,16], solid state dispersion [17e19] and zeolite synthesis under the coexistence of TiO2 with a diameter of about 150 nm [20]. Various reasons have been proposed for the increase in the photocatalytic activity of TiO2 with the composition. Takeda et al. [6] reported that photodecomposition kinetics depended on the apparent diffusion coefficient of adsorbed substrates from adsorbents to TiO2. Vaisman et al. [8] mentioned that
http://dx.doi.org/10.1016/j.matchemphys.2015.04.002 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Fukugaichi, N. Matsue, Enhanced optical absorption of nanosized TiO2 by composition with zeolite, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.04.002
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S. Fukugaichi, N. Matsue / Materials Chemistry and Physics xxx (2015) 1e4
the scattering of UV light by zeolite particles would help TiO2 to absorb photons. Sasikala et al. [10] reported that the optical absorptivity of TiO2 was improved by support materials such as ZrO2 and zeolite, but no correlation between increase in the optical absorptivity and increase in the photocatalytic activity was shown. Objectives of this study were to compare the optical absorptivity and the photocatalytic activity of a composite of TiO2 and zeolite with those of a corresponding mixture, and to synthesize the composite using nanosized TiO2.
by a gas chromatography (GC-390B, GL Sciences Inc.) with a flame ionization detector equipped with a metanizer (MTN-1, Shimadzu Scientific Instruments). After the concentration of acetaldehyde in the chamber reached adsorption equilibrium in the dark, UV light (254 nm or 365 nm) was irradiated on the sample from above the chamber. The intensity of the UV light on the sample was adjusted to 55 mW cm2. The UV light intensity was measured by an UV power meter (UVA-254 for lmax ¼ 254 nm and UVA-365 for lmax ¼ 365 nm, Custom co.).
2. Experimental
2.4. Evaluation of photocatalytic activity
2.1. Preparation of samples
To evaluate the photocatalytic activity of the samples, apparent quantum efficiency (QE) was calculated by using equations below [21].
To synthesize faujasite X-type zeolite under the coexistence of nanosized TiO2, 3.0 g of sodium aluminate (Wako Pure Chemical Industries, Ltd., Japan), 10.9 g of sodium hydroxide (Wako Pure Chemical Industries, Ltd., Japan) and 153 g of distilled water were put into a 250 mL Teflon vessel with a lid. The vessel was shaken by hand to dissolve sodium aluminate and sodium hydroxide. After dissolution of the contents, 19.3 g of water glass (SiO2: 28 mass%, Na2O: 9 mass%; Mitsuwa Chemical Co., Ltd., Japan) was gradually added into the vessel, and the vessel was vigorously shaken by hand to dissolve water glass. Then 0 g, 0.5 g, 1.0 g, 2.0 g or 5.0 g of TiO2 (P25; Nippon Aerosil Co., Ltd., Japan) was added into the vessel, and shaken by hand. The contents were aged for 24 h at room temperature, and then heated in an oven at 323 K for 7 days. The contents were washed thrice with 200 mL of distilled water, heated in an oven at 378 K for 12 h, and used as samples. The sample without the addition of TiO2 is hereafter abbreviated as ZE, and the samples with the addition of TiO2 are abbreviated as T/Z-N, where T/Z indicates the composite of TiO2 and zeolite, and N is mass% of TiO2 in the T/Z. The N value was 8.8 with the addition of 0.5 g of TiO2; N was 16 with 1.0 g of TiO2; N was 27 with 2.0 g of TiO2; N was 48 with 5.0 g of TiO2. As reference materials, physical mixtures of TiO2 and ZE were prepared by a mechanical mixing of TiO2 and ZE to give TiO2 contents of 8.8 mass% (MIX-8.8), 16 mass% (MIX-16), 27 mass% (MIX-27) and 48 mass% (MIX-48). 2.2. Characterization of samples Morphology of the samples was observed with a scanning electron microscope (SEM: JSM-6335F, Jeol Ltd.). Specific surface area (BET; p/p0 ¼ 0.05e0.30) was determined by the nitrogen adsorption method at 77 K by using a volumetric adsorption equipment (Autosorb1, Quantachrome Instruments). Prior to the determination, the samples were heated at 350 C for 1 h in order to remove water in the samples. FT-IR spectra (Nicolet 6700, Thermo Fisher Scientific Inc.) of the samples were measured with the KBr pellet method. Diffuse reflectance spectroscopy (DRS) was performed by using an integrated sphere (V-570, Jasco, Co.), where 0.2 g of the sample was spread on a circular quartz glass window of 3.14 cm2. 2.3. Acetaldehyde decomposition experiment A 10 mg of the sample was spread on a glass plate (28 cm2), and the glass plate was then exposed to UV irradiation with a UV lamp (UVGL-25, UVP Inc., UV intensity: 1 mW cm2) for 72 h to remove organic substances. The chamber was covered with a lid made of quartz, evacuated for 10 min, and the chamber was filled with synthetic air (21% O2 and 79% N2) passed through a water tank kept at 277 K. Then acetaldehyde gas was introduced into the chamber to give an acetaldehyde concentration of 160 ppm (vol/vol). The concentrations of acetaldehyde and carbon dioxide were analyzed
41 ¼ 2Q1/P
(1)
42 ¼ 5Q2/P
(2)
4total ¼ 41 þ 42
(3)
It is known that acetaldehyde is oxidized to acetic acid, and finally to carbon dioxide in the photocatalytic reaction with TiO2 [18]. Where 41 and 42 are QEs of oxidization reactions from acetaldehyde to acetic acid and from acetic acid to carbon dioxide, respectively, and Q1 and Q2 are the numbers of disappeared acetaldehyde molecules and evolved carbon dioxide molecules after 60 min from the UV irradiation, respectively. Whereas P is the number of incident photons irradiated within 60 min, and the value of P was calculated from the value of incident intensity of UV: 55 mW cm2. Total QE (4total) was obtained by equation (3). 3. Results and discussion The XRD analyses showed that TiO2 used in this study was composed of anatase and rutile phase, and zeolite species formed in all cases was faujasite X-type. The peaks of both TiO2 and faujasite X-type zeolite were identified in the XRD patterns of MIXs and T/Zs, and no significant difference in the XRD patterns between MIXs and T/Zs was observed (data not shown). FT-IR spectra of MIXs and T/Zs were also similar to each other, and a shoulder peak around 950 cm1 assigned to the anti-symmetric stretching vibration of TieOeSi bonds [22] was not observed (data not shown). These indicated that the coexistence of TiO2 did not affect the formation of faujasite X-type zeolite in the synthesis of T/Zs. Fig. 1 shows SEM images of TiO2 (TO), ZE, MIX-27 and T/Z-27. The shape of TiO2 particles was spherical and their diameter was about 20 nm (TO, Fig. 1a). The crystals of faujasite X-type zeolite were belt-like shape, and were quite larger than the TiO2 particles (ZE, Fig. 1b). In SEM images of MIXs, the TiO2 particles and the faujasite X-type zeolite crystals mostly existed separately from each other, and the separation became clear with increasing the content of TiO2. An example of the SEM images of MIXs is shown in Fig. 1c (MIX-27). In contrast, the TiO2 particles contained in T/Zs were closely attached to the surface of faujasite X-type zeolite crystals, and the faujasite X-type zeolite crystals were wrapped by the TiO2 particles (Fig. 1d, T/Z-27). In addition, the aggregated TiO2 observed in MIXs hardly observed in T/Zs: in SEM images of T/Zs with lower magnifications, no TiO2 particle seemed to be included in the samples (not shown). The BET surface area (m2 g1) of TO was 51, and that of ZE was 944. The surface area of both MIXs and T/Zs decreased with increasing the content of TiO2. The surface area (m2 g1) of MIX-8.8, MIX-16, MIX-27 and MIX-48 was 816 (865), 737 (801), 632 (703)
Please cite this article in press as: S. Fukugaichi, N. Matsue, Enhanced optical absorption of nanosized TiO2 by composition with zeolite, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.04.002
S. Fukugaichi, N. Matsue / Materials Chemistry and Physics xxx (2015) 1e4
3
Fig. 1. SEM images of (a) TO (100,000), (b) ZE (50,000), (c) MIX-27 (50,000), and (d) T/Z-27 (50,000).
Fig. 2. DRS spectra of (a) MIX and (b) T/Z.
and 426 (515), respectively. Where the value in the parenthesis indicates theoretical surface area of each MIX, which was calculated as weighted average by using the surface areas of TO and ZE and their contents in each MIX. In all cases, the surface area of MIX was smaller than the theoretical value. This indicated that TiO2 and faujasite X-type zeolite particles attached each other to some extent, decreasing the adsorption of nitrogen molecules. The surface area (m2 g1) of T/Z-8.8, T/Z-16, T/Z-27 and T/Z-48 was 772, 632, 483 and 271, respectively. The lower surface area of T/Zs than the corresponding MIXs is ascribed to the closer attachment between TiO2 and faujasite X-type zeolite particles as observed in the SEM images. Above observations confirmed the formation of the composites of nanosized TiO2 and faujasite X-type zeolite, and this is the first to report the synthesis of the composite of TiO2 and zeolite through the synthesis of zeolite in the presence of nanosized crystalline TiO2. Fig. 2 shows DRS spectra of MIXs and T/Zs together with those of TO and ZE. The absorbance of MIXs and T/Zs between 200 and
400 nm increased with increasing the content of TiO2. For both MIXs and T/Zs, the increase in the absorbance was more than that expected from their content of TiO2. This suggests that the dispersion state of TiO2 particles in both MIXs and T/Zs was better than
Table 1 Apparent quantum efficiency from acetaldehyde decomposition experiment. TiO2 mass %
MIX
T/Z
8.8 16 27 48 100 8.8 16 27 48 100
254 nm
365 nm
41
42
4total
41
42
4total
0.20 0.15 0.17 0.17 0.38 0.17 0.34 0.37 0.30 0.38
0.24 0.18 0.38 0.24 1.04 0.22 0.39 0.72 0.35 1.04
0.44 0.32 0.56 0.41 1.41 0.39 0.72 1.09 0.65 1.41
0.04 0.05 0.10 0.09 0.25 0.05 0.14 0.24 0.18 0.25
0.09 0.12 0.12 0.11 0.53 0.12 0.16 0.30 0.21 0.53
0.13 0.17 0.22 0.20 0.78 0.17 0.30 0.55 0.39 0.78
Please cite this article in press as: S. Fukugaichi, N. Matsue, Enhanced optical absorption of nanosized TiO2 by composition with zeolite, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.04.002
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S. Fukugaichi, N. Matsue / Materials Chemistry and Physics xxx (2015) 1e4
Fig. 3. Comparison of DRS absorbance and acetaldehyde photocatalytic activity between MIXs and T/Zs. The absorbance and the activity of MIX was subtracted from those of T/Z, and shown as DA and D4total. (a): at 365 nm, (b): at 254 nm.
that of TO. Fig. 2 also indicates better dispersion of TiO2 particles in T/Zs than in MIXs, as had been shown in the SEM images (Fig. 1c and d). The amount of adsorption of acetaldehyde by MIXs and T/Zs was negligible before the irradiation of UV light in the acetaldehyde decomposition experiment. This is ascribed to smaller amounts of MIXs and T/Zs as compared to the amount of acetaldehyde introduced, and also ascribed to the existence of water molecules adsorbed on the surface of zeolite particles, because wet-air was introduced into the reaction chamber before the introduction of acetaldehyde. The QE values obtained are shown in Table 1. In all cases, 4total of MIXs and T/Zs reached a maximum at 27 mass% of TiO2, and decreased at 48 mass%. The reason for the decrease of 4total at 48 mass% is explained as follows. In the photocatalytic decomposition experiment, the sample was put on a glass plate, and UV light was irradiated from the above. The particle size of TiO2 is much smaller than that of zeolite, therefore isolated TiO2 particles might sink to the bottom of the glass plate, and some zeolite particles were located onto the TiO2 particles. As a result, some of the TiO2 particles could not absorb UV light due to the zeolite particles which covered the TiO2 particles. Since the rate of the isolated TiO2 particles increased with increasing the content of TiO2, the above covering effect was remarkable at 48%. With increasing the content of TiO2 beyond 48%, the content of zeolite decreased, the covering effect became small, and the photocatalytic activity increased again. In our previous study [20], in a similar way, we obtained a composite of TiO2 and faujasite X-type zeolite with TiO2 content of 22.6 mass% by using TiO2 with larger particle size (about 150 nm). The previous composite showed the adsorption of acetaldehyde, and had higher photocatalytic activity than bare TiO2 on sample mass basis. In this study, the photocatalytic decomposition activities of T/Z were higher than those expected from their TiO2 content, but the activity of T/Z was lower than that of TO (Table 1). These indicate that particle size of TiO2 and the adsorption of acetaldehyde onto zeolite are important factors in the enhancement of photocatalytic activity of composites of TiO2 and zeolite. To compare UV light absorptivity and QE values between MIX and T/Z, DA (absorbance of T/Z minus that of MIX) and D4total (QE of T/Z minus that of MIX) was calculated, and plotted against the content of TiO2 (Fig. 3). Except for D4total at TiO2 content of 8.8 mass % with 254 nm irradiation, DA and D4total were positive values, and at both wavelengths of 365 and 254 nm, DA and D4total showed similar pattern: the values of DA and D4total reached the maximum at TiO2 content of 27 mass%. These suggested that the increase in the QE of T/Z as compared to that of MIX was strongly dependent on the increase in the UV light absorptivity, and the higher optical
absorptivity of T/Z resulted in the higher photocatalytic activity of T/Z than that of MIX. 4. Conclusion The composite of nanosized TiO2 and faujasite X-type zeolite was obtained through the synthesis of faujasite X-type zeolite under the coexistence of nanosized TiO2. The composite had higher acetaldehyde photocatalytic decomposition activity than the corresponding mixture. The composite showed higher dispersion state of TiO2 particles on the surface of zeolite crystals than the corresponding mixture. This higher dispersibility provided higher UV light absorptivity, and the increase in the absorptivity was well correlated to the increase in the acetaldehyde photocatalytic decomposition activity of the composite. In addition to the improvement of dispersibility of TiO2, the adsorption of a substrate to zeolite and subsequent diffusion of the substrate from zeolite to TiO2 may increase the photocatalytic activity of the composite of TiO2 and zeolite. Optimization of the factors is necessary for the preparation of the composite with higher photocatalytic activity. References [1] A. Wold, Chem. Mater. 58 (1993) 280. [2] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem, Photobiol. C Photochem. Rev. 1 (2000) 1. [3] Y. Murakami, T. Matsumoto, Y. Takasu, J. Phys. Chem. B 103 (1999) 1836. [4] S.T. Aruna, S. Tirosh, A. Zaban, J. Mater. Chem. 10 (2000) 2388. [5] Y. Xu, C.H. Langford, J. Phys. Chem. 99 (1995) 11501. [6] N. Takeda, M. Ohtani, T. Torimoto, S. Kuwabata, H. Yoneyama, J. Phys. Chem. B 101 (1997) 2644. [7] Y. Xu, C.H. Langford, J. Phys. Chem. B 101 (1997) 3115. [8] E. Vaisman, R.L. Cook, C.H. Langford, J. Phys. Chem. B 104 (2000) 8679. [9] H. Yoneyama, T. Torimoto, Catal. Today 58 (2000) 133. [10] R. Sasikalaa, A.R. Shirolea, V. Sudarsana, V.S. Kamblea, C. Sudakarb, R. Naikb, R. Raoc, S.R. Bharadwaj, Appl. Catal. A 390 (2010) 245. [11] H. Chen, A. Matsumoto, N. Nishimiya, K. Tsutsumi, Colloids Surf. A Physicochem. Eng. Asp. 157 (1999) 295. [12] Y. Kim, M.Y. Mol, J. Catal. A Chem. 168 (2001) 257. [13] M. Takeuchi, T. Kimura, M. Hidaka, D. Rakhmawaty, M. Anpo, J. Catal. 246 (2007) 235. [14] S. Yoon, Y.H. Lee, W. Cho, I. Koh, M. Yoon, Catal. Commun. 8 (2007) 1851. [15] J. Mo, Y. Zhang, Q. Xu, R. Yang, J. Hazard. Mater. 168 (2009) 276. [16] K. Yamaguchi, K. Inumaru, Y. Oumi, T. Sano, S. Yamanaka, Microporous. Mesoporous. Mater. 117 (2009) 350. [17] M.V.P. Sharma, V. Durgakumari, M. Subrahmanyam, J. Hazard. Mater. 160 (2008) 568. [18] J. Mo, Y. Zhang, Q. Xu, R. Yang, J. Hazard. Mater. 168 (2009) 276. [19] C. Zhaoa, H. Denga, Y. Li, Z. Liub, J. Hazard. Mater. 176 (2010) 884. [20] S. Fukugaichi, T. Henmi, N. Matsue, Catal. Lett. 143 (2013) 1255. [21] I. Sopyan, M. Watanabe, S. Murasawa, K. Hashimoto, A. Fujishima, J. Photochem. Photobiol. A Chem. 98 (1996) 79. [22] M. Huang, C. Xu, Z. Wu, Y. Huang, J. Lin, J. Wu, Dyes Pigments 77 (2008) 327.
Please cite this article in press as: S. Fukugaichi, N. Matsue, Enhanced optical absorption of nanosized TiO2 by composition with zeolite, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.04.002
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Enhanced optical absorption of nanosized TiO2 by composition with zeolite Satoru Fukugaichi a, *, Naoto Matsue b a b
Ehime Institute of Industrial Technology, 487-2 Kumekubota, Matsuyama 791-1101, Japan Faculty of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
TiO2-zeolite composite was synthesized by using nanosized TiO2. The composite had higher UV light absorptivity than a corresponding mixture. The increased absorptivity increased the photocatalytic activity of the composite.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2014 Received in revised form 4 March 2015 Accepted 6 April 2015 Available online xxx
Enhancement of the photocatalytic activity of TiO2 and its mechanism are important subjects in the field of catalysis. Under the coexistence of TiO2 with a diameter of about 20 nm, we synthesized faujasite Xtype zeolite, and the product was made up of the composite of TiO2 and faujasite X-type zeolite. The composite had higher acetaldehyde photocatalytic decomposition activity than a corresponding mixture, and the activity of the composite was higher than that of TiO2 on TiO2 mass basis. The composite had higher UV light absorptivity than the corresponding mixture, and their absorption of UV light was mainly due to TiO2. The increased UV light absorptivity of the composite correlated to its increased acetaldehyde photocatalytic decomposition activity. © 2015 Elsevier B.V. All rights reserved.
Keywords: Composite materials Chemical synthesis Optical properties Oxidation
1. Introduction Titanium dioxide (TiO2) photocatalysts have been widely studied as purification materials for toxic organic substances in water and air [1,2]. Numerous strategies have been performed to increase their photocatalytic activity by reducing their particle size to nanometer level [3,4], and by hybridizing TiO2 with various
* Corresponding author. Present address: Paper Industrial Innovation Center of Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan. E-mail address: [email protected] (S. Fukugaichi).
adsorbents. Among the adsorbents, zeolites have been most frequently used, and some methods for the hybridization (or composition) of TiO2 with zeolites have been reported. They are precipitation of TiO2 on the surface of zeolites [5e10], cation exchange of Ti4þ into zeolite cavities followed by the formation of TiO2 [11e14], physical mixing [15,16], solid state dispersion [17e19] and zeolite synthesis under the coexistence of TiO2 with a diameter of about 150 nm [20]. Various reasons have been proposed for the increase in the photocatalytic activity of TiO2 with the composition. Takeda et al. [6] reported that photodecomposition kinetics depended on the apparent diffusion coefficient of adsorbed substrates from adsorbents to TiO2. Vaisman et al. [8] mentioned that
http://dx.doi.org/10.1016/j.matchemphys.2015.04.002 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Fukugaichi, N. Matsue, Enhanced optical absorption of nanosized TiO2 by composition with zeolite, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.04.002
2
S. Fukugaichi, N. Matsue / Materials Chemistry and Physics xxx (2015) 1e4
the scattering of UV light by zeolite particles would help TiO2 to absorb photons. Sasikala et al. [10] reported that the optical absorptivity of TiO2 was improved by support materials such as ZrO2 and zeolite, but no correlation between increase in the optical absorptivity and increase in the photocatalytic activity was shown. Objectives of this study were to compare the optical absorptivity and the photocatalytic activity of a composite of TiO2 and zeolite with those of a corresponding mixture, and to synthesize the composite using nanosized TiO2.
by a gas chromatography (GC-390B, GL Sciences Inc.) with a flame ionization detector equipped with a metanizer (MTN-1, Shimadzu Scientific Instruments). After the concentration of acetaldehyde in the chamber reached adsorption equilibrium in the dark, UV light (254 nm or 365 nm) was irradiated on the sample from above the chamber. The intensity of the UV light on the sample was adjusted to 55 mW cm2. The UV light intensity was measured by an UV power meter (UVA-254 for lmax ¼ 254 nm and UVA-365 for lmax ¼ 365 nm, Custom co.).
2. Experimental
2.4. Evaluation of photocatalytic activity
2.1. Preparation of samples
To evaluate the photocatalytic activity of the samples, apparent quantum efficiency (QE) was calculated by using equations below [21].
To synthesize faujasite X-type zeolite under the coexistence of nanosized TiO2, 3.0 g of sodium aluminate (Wako Pure Chemical Industries, Ltd., Japan), 10.9 g of sodium hydroxide (Wako Pure Chemical Industries, Ltd., Japan) and 153 g of distilled water were put into a 250 mL Teflon vessel with a lid. The vessel was shaken by hand to dissolve sodium aluminate and sodium hydroxide. After dissolution of the contents, 19.3 g of water glass (SiO2: 28 mass%, Na2O: 9 mass%; Mitsuwa Chemical Co., Ltd., Japan) was gradually added into the vessel, and the vessel was vigorously shaken by hand to dissolve water glass. Then 0 g, 0.5 g, 1.0 g, 2.0 g or 5.0 g of TiO2 (P25; Nippon Aerosil Co., Ltd., Japan) was added into the vessel, and shaken by hand. The contents were aged for 24 h at room temperature, and then heated in an oven at 323 K for 7 days. The contents were washed thrice with 200 mL of distilled water, heated in an oven at 378 K for 12 h, and used as samples. The sample without the addition of TiO2 is hereafter abbreviated as ZE, and the samples with the addition of TiO2 are abbreviated as T/Z-N, where T/Z indicates the composite of TiO2 and zeolite, and N is mass% of TiO2 in the T/Z. The N value was 8.8 with the addition of 0.5 g of TiO2; N was 16 with 1.0 g of TiO2; N was 27 with 2.0 g of TiO2; N was 48 with 5.0 g of TiO2. As reference materials, physical mixtures of TiO2 and ZE were prepared by a mechanical mixing of TiO2 and ZE to give TiO2 contents of 8.8 mass% (MIX-8.8), 16 mass% (MIX-16), 27 mass% (MIX-27) and 48 mass% (MIX-48). 2.2. Characterization of samples Morphology of the samples was observed with a scanning electron microscope (SEM: JSM-6335F, Jeol Ltd.). Specific surface area (BET; p/p0 ¼ 0.05e0.30) was determined by the nitrogen adsorption method at 77 K by using a volumetric adsorption equipment (Autosorb1, Quantachrome Instruments). Prior to the determination, the samples were heated at 350 C for 1 h in order to remove water in the samples. FT-IR spectra (Nicolet 6700, Thermo Fisher Scientific Inc.) of the samples were measured with the KBr pellet method. Diffuse reflectance spectroscopy (DRS) was performed by using an integrated sphere (V-570, Jasco, Co.), where 0.2 g of the sample was spread on a circular quartz glass window of 3.14 cm2. 2.3. Acetaldehyde decomposition experiment A 10 mg of the sample was spread on a glass plate (28 cm2), and the glass plate was then exposed to UV irradiation with a UV lamp (UVGL-25, UVP Inc., UV intensity: 1 mW cm2) for 72 h to remove organic substances. The chamber was covered with a lid made of quartz, evacuated for 10 min, and the chamber was filled with synthetic air (21% O2 and 79% N2) passed through a water tank kept at 277 K. Then acetaldehyde gas was introduced into the chamber to give an acetaldehyde concentration of 160 ppm (vol/vol). The concentrations of acetaldehyde and carbon dioxide were analyzed
41 ¼ 2Q1/P
(1)
42 ¼ 5Q2/P
(2)
4total ¼ 41 þ 42
(3)
It is known that acetaldehyde is oxidized to acetic acid, and finally to carbon dioxide in the photocatalytic reaction with TiO2 [18]. Where 41 and 42 are QEs of oxidization reactions from acetaldehyde to acetic acid and from acetic acid to carbon dioxide, respectively, and Q1 and Q2 are the numbers of disappeared acetaldehyde molecules and evolved carbon dioxide molecules after 60 min from the UV irradiation, respectively. Whereas P is the number of incident photons irradiated within 60 min, and the value of P was calculated from the value of incident intensity of UV: 55 mW cm2. Total QE (4total) was obtained by equation (3). 3. Results and discussion The XRD analyses showed that TiO2 used in this study was composed of anatase and rutile phase, and zeolite species formed in all cases was faujasite X-type. The peaks of both TiO2 and faujasite X-type zeolite were identified in the XRD patterns of MIXs and T/Zs, and no significant difference in the XRD patterns between MIXs and T/Zs was observed (data not shown). FT-IR spectra of MIXs and T/Zs were also similar to each other, and a shoulder peak around 950 cm1 assigned to the anti-symmetric stretching vibration of TieOeSi bonds [22] was not observed (data not shown). These indicated that the coexistence of TiO2 did not affect the formation of faujasite X-type zeolite in the synthesis of T/Zs. Fig. 1 shows SEM images of TiO2 (TO), ZE, MIX-27 and T/Z-27. The shape of TiO2 particles was spherical and their diameter was about 20 nm (TO, Fig. 1a). The crystals of faujasite X-type zeolite were belt-like shape, and were quite larger than the TiO2 particles (ZE, Fig. 1b). In SEM images of MIXs, the TiO2 particles and the faujasite X-type zeolite crystals mostly existed separately from each other, and the separation became clear with increasing the content of TiO2. An example of the SEM images of MIXs is shown in Fig. 1c (MIX-27). In contrast, the TiO2 particles contained in T/Zs were closely attached to the surface of faujasite X-type zeolite crystals, and the faujasite X-type zeolite crystals were wrapped by the TiO2 particles (Fig. 1d, T/Z-27). In addition, the aggregated TiO2 observed in MIXs hardly observed in T/Zs: in SEM images of T/Zs with lower magnifications, no TiO2 particle seemed to be included in the samples (not shown). The BET surface area (m2 g1) of TO was 51, and that of ZE was 944. The surface area of both MIXs and T/Zs decreased with increasing the content of TiO2. The surface area (m2 g1) of MIX-8.8, MIX-16, MIX-27 and MIX-48 was 816 (865), 737 (801), 632 (703)
Please cite this article in press as: S. Fukugaichi, N. Matsue, Enhanced optical absorption of nanosized TiO2 by composition with zeolite, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.04.002
S. Fukugaichi, N. Matsue / Materials Chemistry and Physics xxx (2015) 1e4
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Fig. 1. SEM images of (a) TO (100,000), (b) ZE (50,000), (c) MIX-27 (50,000), and (d) T/Z-27 (50,000).
Fig. 2. DRS spectra of (a) MIX and (b) T/Z.
and 426 (515), respectively. Where the value in the parenthesis indicates theoretical surface area of each MIX, which was calculated as weighted average by using the surface areas of TO and ZE and their contents in each MIX. In all cases, the surface area of MIX was smaller than the theoretical value. This indicated that TiO2 and faujasite X-type zeolite particles attached each other to some extent, decreasing the adsorption of nitrogen molecules. The surface area (m2 g1) of T/Z-8.8, T/Z-16, T/Z-27 and T/Z-48 was 772, 632, 483 and 271, respectively. The lower surface area of T/Zs than the corresponding MIXs is ascribed to the closer attachment between TiO2 and faujasite X-type zeolite particles as observed in the SEM images. Above observations confirmed the formation of the composites of nanosized TiO2 and faujasite X-type zeolite, and this is the first to report the synthesis of the composite of TiO2 and zeolite through the synthesis of zeolite in the presence of nanosized crystalline TiO2. Fig. 2 shows DRS spectra of MIXs and T/Zs together with those of TO and ZE. The absorbance of MIXs and T/Zs between 200 and
400 nm increased with increasing the content of TiO2. For both MIXs and T/Zs, the increase in the absorbance was more than that expected from their content of TiO2. This suggests that the dispersion state of TiO2 particles in both MIXs and T/Zs was better than
Table 1 Apparent quantum efficiency from acetaldehyde decomposition experiment. TiO2 mass %
MIX
T/Z
8.8 16 27 48 100 8.8 16 27 48 100
254 nm
365 nm
41
42
4total
41
42
4total
0.20 0.15 0.17 0.17 0.38 0.17 0.34 0.37 0.30 0.38
0.24 0.18 0.38 0.24 1.04 0.22 0.39 0.72 0.35 1.04
0.44 0.32 0.56 0.41 1.41 0.39 0.72 1.09 0.65 1.41
0.04 0.05 0.10 0.09 0.25 0.05 0.14 0.24 0.18 0.25
0.09 0.12 0.12 0.11 0.53 0.12 0.16 0.30 0.21 0.53
0.13 0.17 0.22 0.20 0.78 0.17 0.30 0.55 0.39 0.78
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S. Fukugaichi, N. Matsue / Materials Chemistry and Physics xxx (2015) 1e4
Fig. 3. Comparison of DRS absorbance and acetaldehyde photocatalytic activity between MIXs and T/Zs. The absorbance and the activity of MIX was subtracted from those of T/Z, and shown as DA and D4total. (a): at 365 nm, (b): at 254 nm.
that of TO. Fig. 2 also indicates better dispersion of TiO2 particles in T/Zs than in MIXs, as had been shown in the SEM images (Fig. 1c and d). The amount of adsorption of acetaldehyde by MIXs and T/Zs was negligible before the irradiation of UV light in the acetaldehyde decomposition experiment. This is ascribed to smaller amounts of MIXs and T/Zs as compared to the amount of acetaldehyde introduced, and also ascribed to the existence of water molecules adsorbed on the surface of zeolite particles, because wet-air was introduced into the reaction chamber before the introduction of acetaldehyde. The QE values obtained are shown in Table 1. In all cases, 4total of MIXs and T/Zs reached a maximum at 27 mass% of TiO2, and decreased at 48 mass%. The reason for the decrease of 4total at 48 mass% is explained as follows. In the photocatalytic decomposition experiment, the sample was put on a glass plate, and UV light was irradiated from the above. The particle size of TiO2 is much smaller than that of zeolite, therefore isolated TiO2 particles might sink to the bottom of the glass plate, and some zeolite particles were located onto the TiO2 particles. As a result, some of the TiO2 particles could not absorb UV light due to the zeolite particles which covered the TiO2 particles. Since the rate of the isolated TiO2 particles increased with increasing the content of TiO2, the above covering effect was remarkable at 48%. With increasing the content of TiO2 beyond 48%, the content of zeolite decreased, the covering effect became small, and the photocatalytic activity increased again. In our previous study [20], in a similar way, we obtained a composite of TiO2 and faujasite X-type zeolite with TiO2 content of 22.6 mass% by using TiO2 with larger particle size (about 150 nm). The previous composite showed the adsorption of acetaldehyde, and had higher photocatalytic activity than bare TiO2 on sample mass basis. In this study, the photocatalytic decomposition activities of T/Z were higher than those expected from their TiO2 content, but the activity of T/Z was lower than that of TO (Table 1). These indicate that particle size of TiO2 and the adsorption of acetaldehyde onto zeolite are important factors in the enhancement of photocatalytic activity of composites of TiO2 and zeolite. To compare UV light absorptivity and QE values between MIX and T/Z, DA (absorbance of T/Z minus that of MIX) and D4total (QE of T/Z minus that of MIX) was calculated, and plotted against the content of TiO2 (Fig. 3). Except for D4total at TiO2 content of 8.8 mass % with 254 nm irradiation, DA and D4total were positive values, and at both wavelengths of 365 and 254 nm, DA and D4total showed similar pattern: the values of DA and D4total reached the maximum at TiO2 content of 27 mass%. These suggested that the increase in the QE of T/Z as compared to that of MIX was strongly dependent on the increase in the UV light absorptivity, and the higher optical
absorptivity of T/Z resulted in the higher photocatalytic activity of T/Z than that of MIX. 4. Conclusion The composite of nanosized TiO2 and faujasite X-type zeolite was obtained through the synthesis of faujasite X-type zeolite under the coexistence of nanosized TiO2. The composite had higher acetaldehyde photocatalytic decomposition activity than the corresponding mixture. The composite showed higher dispersion state of TiO2 particles on the surface of zeolite crystals than the corresponding mixture. This higher dispersibility provided higher UV light absorptivity, and the increase in the absorptivity was well correlated to the increase in the acetaldehyde photocatalytic decomposition activity of the composite. In addition to the improvement of dispersibility of TiO2, the adsorption of a substrate to zeolite and subsequent diffusion of the substrate from zeolite to TiO2 may increase the photocatalytic activity of the composite of TiO2 and zeolite. Optimization of the factors is necessary for the preparation of the composite with higher photocatalytic activity. References [1] A. Wold, Chem. Mater. 58 (1993) 280. [2] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem, Photobiol. C Photochem. Rev. 1 (2000) 1. [3] Y. Murakami, T. Matsumoto, Y. Takasu, J. Phys. Chem. B 103 (1999) 1836. [4] S.T. Aruna, S. Tirosh, A. Zaban, J. Mater. Chem. 10 (2000) 2388. [5] Y. Xu, C.H. Langford, J. Phys. Chem. 99 (1995) 11501. [6] N. Takeda, M. Ohtani, T. Torimoto, S. Kuwabata, H. Yoneyama, J. Phys. Chem. B 101 (1997) 2644. [7] Y. Xu, C.H. Langford, J. Phys. Chem. B 101 (1997) 3115. [8] E. Vaisman, R.L. Cook, C.H. Langford, J. Phys. Chem. B 104 (2000) 8679. [9] H. Yoneyama, T. Torimoto, Catal. Today 58 (2000) 133. [10] R. Sasikalaa, A.R. Shirolea, V. Sudarsana, V.S. Kamblea, C. Sudakarb, R. Naikb, R. Raoc, S.R. Bharadwaj, Appl. Catal. A 390 (2010) 245. [11] H. Chen, A. Matsumoto, N. Nishimiya, K. Tsutsumi, Colloids Surf. A Physicochem. Eng. Asp. 157 (1999) 295. [12] Y. Kim, M.Y. Mol, J. Catal. A Chem. 168 (2001) 257. [13] M. Takeuchi, T. Kimura, M. Hidaka, D. Rakhmawaty, M. Anpo, J. Catal. 246 (2007) 235. [14] S. Yoon, Y.H. Lee, W. Cho, I. Koh, M. Yoon, Catal. Commun. 8 (2007) 1851. [15] J. Mo, Y. Zhang, Q. Xu, R. Yang, J. Hazard. Mater. 168 (2009) 276. [16] K. Yamaguchi, K. Inumaru, Y. Oumi, T. Sano, S. Yamanaka, Microporous. Mesoporous. Mater. 117 (2009) 350. [17] M.V.P. Sharma, V. Durgakumari, M. Subrahmanyam, J. Hazard. Mater. 160 (2008) 568. [18] J. Mo, Y. Zhang, Q. Xu, R. Yang, J. Hazard. Mater. 168 (2009) 276. [19] C. Zhaoa, H. Denga, Y. Li, Z. Liub, J. Hazard. Mater. 176 (2010) 884. [20] S. Fukugaichi, T. Henmi, N. Matsue, Catal. Lett. 143 (2013) 1255. [21] I. Sopyan, M. Watanabe, S. Murasawa, K. Hashimoto, A. Fujishima, J. Photochem. Photobiol. A Chem. 98 (1996) 79. [22] M. Huang, C. Xu, Z. Wu, Y. Huang, J. Lin, J. Wu, Dyes Pigments 77 (2008) 327.
Please cite this article in press as: S. Fukugaichi, N. Matsue, Enhanced optical absorption of nanosized TiO2 by composition with zeolite, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.04.002