Heterogeneous photocatalysis of methylene blue over titanate nanotubes: Effect of adsorption

Heterogeneous photocatalysis of methylene blue over titanate nanotubes: Effect of adsorption

Journal of Colloid and Interface Science 356 (2011) 211–216 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.e...

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Journal of Colloid and Interface Science 356 (2011) 211–216

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Heterogeneous photocatalysis of methylene blue over titanate nanotubes: Effect of adsorption Lin Xiong a, Weiling Sun a, Ye Yang b, Cheng Chen a, Jinren Ni a,⇑ a b

Department of Environmental Engineering, Peking University, The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, China

a r t i c l e

i n f o

Article history: Received 13 June 2010 Accepted 17 December 2010 Available online 22 December 2010 Keywords: Titanate nanotubes Methylene blue Adsorption Photodegradation Sedimentation

a b s t r a c t Titanate nanotubes were synthesized with hydrothermal reaction using TiO2 and NaOH as the precursors and subsequent calcination at 400 °C for 2 h. The products were characterized with SEM and XRD. Adsorption and photocatalysis of methylene blue over titanate nanotubes and TiO2 were investigated. The results indicated that titanate nanotubes exhibited a better photocatalytic degradation of methylene blue in a simultaneous adsorption and photodegradation system than that in equilibrium adsorption followed by a photodegradation system, whereas TiO2 showed no significant differences in photocatalytic activity in the two systems. The methylene blue overall removal efficiency over TNTs in the first system even exceeded that over TiO2. The different catalytic performances of titanate nanotubes in the two systems were tentatively attributed to different effects of adsorption of methylene blue, i.e., the promoting effect in the former and the inhibition effect in the latter. Decantation experiments showed that the titanate nanotube photocatalyst could be easily separated from the reaction medium by sedimentation. Thus titanate nanotubes with high sedimentation rates and concurrent adsorption represent a new catalyst system with a strong potential for commercial applications. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction TiO2 is an extensively investigated photocatalyst owing to its attractive properties such as strong oxidizing power and excellent chemical inertness [1,2]. However, due to the complicated separation of suspended ultrafine solid catalysts from the reaction medium, its practical application appears to be difficult so far [3,4]. Immobilization of TiO2 is an effective strategy to overcome the problem of posttreatment separation [5,6], but it usually diminished photocatalytic activity because of the reduction in available surface area and active surface sites of the catalyst [7]. Consequently, there has been increased interest in developing an alternative photocatalyst which possesses both high photocatalytic activity and good separation performance. Since the innovative work of Kasuga et al. [8], titanate nanotubes (labeled as TNTs) have recently gained promising and important prospects due to a variety of advantages, including fascinating microstructures, large specific surface areas, and high pore volumes. Via a simple hydrothermal treatment of crystalline TiO2 nanoparticles and highly concentrated aqueous NaOH solution, high yield of TNTs with uniform diameters can be obtained [9,10]. Several reports have demonstrated that the nanotubes show

⇑ Corresponding author. Fax: +86 10 6275 6526. E-mail address: [email protected] (J. Ni). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.12.059

favorable photocatalytic activity after controlled calcination posttreatment [11–13]. In addition, because their lengths are at a scale of several hundreds of nanometers, this class of materials may be readily separated from solution by filtration or sedimentation [14]. Potentially, the TNTs may be utilized in commercial applications of photocatalysis. Photocatalytic degradation is a process occurring on the surface of the catalyst. It is often believed that preliminary adsorption of organics on the catalyst surface is a requisite for highly efficient photodegradation [15,16]. Previous studies also concluded that high adsorption could enhance the photodegradation rate [17– 19], whereas some authors indicated that the adsorbed compound might have an adverse effect on the degradation process [20]. Taking into account the large specific surface area and high pore volume of TNTs, they may possess a higher adsorption ability to certain contaminants, and adsorption may also play a significant role in the overall reaction. Nevertheless, as far as we know, very few works have been done with the effect of actual adsorption of the organic contaminants in photocatalysis mediated with TNTs. In this work, TNTs were synthesized by a hydrothermal method and characterized by electron microscopy and X-ray diffraction. Our goal was to investigate the potential applications of TNTs as a photocatalyst for the removal of contaminants in water, with a focus on the effect of adsorption in TNT photocatalysis. Thus adsorption and photocatalytic degradation over TNTs were examined. Dyestuffs were often selected as model pollutants in

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Fig. 1. Molecular structure of MB.

photocatalysis because of their severe environmental impact and strong resistance to biochemical oxidation [21,22]. Hence, methylene blue (MB), a typical textile dye, was used as the model compound here, as shown in Fig. 1. In particular, the cooperative action between adsorption of MB and its photodegradation is discussed in detail, and then the effect of adsorption was analyzed. Additionally, the separation performance of TNTs was evaluated. For comparison, MB adsorption and photocatalytic degradation over the TiO2 precursors were also conducted. 2. Materials and methods 2.1. Materials MB was purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China), and used without further purification. TiO2 was supplied by Degussa (P25, 90% anatase, 10% rutile, and an average particle size of 30 nm). Other reagents utilized were of analytical grade. All the solutions used in the test were prepared with deionized water. 2.2. Preparation and characterization of TNTs Commercial TiO2 powder (P25, Degussa) was used without any purification treatment as the starting material to prepare TNTs by hydrothermal treatment [23]. The amount of 0.3 g of TiO2 was mixed with 16.5 mL of 10 M NaOH aqueous solution through thor-

ough stirring, and then the mixture was taken into a Teflon-lined autoclave at 130 °C for 72 h. The products were separated, washed with ethanol until the pH value of the rinsing solution reached 7, and dried at 80 °C for about 4 h. The dried powder was annealed at 400 °C in air for 2 h. The morphology was observed by scanning electron microscopy (SEM) with a FEI XL30F microscope operating at 20 kV. The crystal phase and crystallinity were determined by powder X-ray diffraction (XRD) on a Rigaku Dmax/2400 diffractometer with Cu Ka radiation (k = 1.5418 Å) at an accelerating voltage of 40 kV, an emission current of 100 mA, and a scan rate (2h) of 8°/min. 2.3. Adsorption and photocatalytic experiments Adsorption and photocatalytic degradation of MB in catalyst suspensions were performed in a 300 mL square photoreactor (quartz, length 50 mm, width 50 mm, height 120 mm) as shown in Fig. 2. A 150 W medium-pressure mercury lamp (Beijing Electric Light Sources Research Institute, China) was placed outside the reactor and 10 cm away from the left of it. The average light intensity striking inside the reactor, measured by a UV-A radiometer (Photoelectric Instrument Factory of Beijing Normal University, China), was 1.85 mW/cm2 with a peak wavelength of 365 nm. A fan was set up above the reactor to generate cooling air and to maintain the temperature about 20 ± 2 °C, preventing excessive heating of the slurry. Prior to illumination, the suspension containing 0.075 g of catalysts and 150 mL of MB with initial concentration 100 mg/L was stirred for 4 h with a magnetic stirrer in the dark. According to our previous study [23], a 4-h contact time was sufficient for MB to reach adsorption–desorption equilibrium with catalysts. Then the suspension was irradiated under continuous stirring. At specific time intervals, samples were withdrawn and centrifuged at 4000 rpm to separate the catalyst particles. The concentrations of MB in supernatant were determined by using an UV–visible spectrophotometer (Specord 200, Analytik Jena AG, Germany) at 660 nm, corresponding to the maximum absorption wavelength of MB. All measurements were undertaken in triplicate with errors below 5% and average values were reported.

Fig. 2. Schematic diagram of the experimental setup for adsorption and photocatalytic degradation.

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3. Results and discussion 3.1. Morphology and crystalline phase of TNTs Fig. 3a shows the typical SEM micrographs of TNTs and TiO2. A large amount of randomly tangled nanotubes can be clearly seen after the alkali treatment, and they were observably different from the starting material comprising sphere-like TiO2 nanoparticles. The diameters of the obtained nanotubes were about 10 nm and the lengths were several hundreds of nanometers. Fig. 3b shows the powder XRD patterns of two samples. The pattern of TNTs could not correspond to anatase, rutile, or their mixtures, but was indexed as trititanate [24]. The characteristic peak at 2h  10° was attributed to the layered structure of titanate, and the other peaks at 2h  24°, 28°, and 48° were assigned to the diffraction of titanate containing sodium [25,26], in accordance with our previous reports [23]. In contrast, the anatase and rutile phases of the starting material were not found in the obtained nanotubes. This clearly indicated that the TiO2 precursors were completely transformed to TNTs after the hydrothermal treatment. 3.2. Photocatalytic degradation of MB Fig. 4 illustrates the results of photocatalytic degradation of MB over TNTs. Direct photolysis in the absence of TNTs showed insignificant conversion of MB. The change of MB concentration using TNTs in the dark after equilibrium adsorption was also negligible. However, the decrease in the concentration of MB was observed in the presence of both illumination and TNTs. These experiments obviously indicated that the degradation of MB occurred by photocatalysis mediated with TNTs. Even so, the photocatalytic activity evaluation of TNTs was quite different from that of common

Fig. 4. Time profiles of MB degradation in the TNT suspension under various reaction conditions. Experimental conditions: initial dye concentration = 100 mg/L, catalyst dosage = 0.5 g/L, temperature = 20 °C.

photocatalysts. This was because MB had a high adsorption onto TNTs, which was observed by our study on the adsorption behavior of methylene blue onto titanate nanotubes [23]. Thus the photodegradation of the dye should consist of that in bulk solution and that on the catalyst surface [27,28]. Hence, the decrease of the amount of the adsorbed MB on the TNT surface should also be considered in the photocatalytic activity evaluation of TNTs. According to the report by Xu and Langford [20], the amount of MB adsorbed onto TNTs, Qs (mg/g), and the MB concentration on the catalyst surface, Cs (mg/L), could be simply calculated by Eqs. (1) and (2), respectively

Fig. 3. SEM images (a) and XRD patterns (b) of TNTs and TiO2.

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Qs ¼

Q max KC b ; 1 þ KC b

ð1Þ

Cs ¼

Q sm ; V

ð2Þ

where Cb (mg/L) is MB concentration in bulk solution, m (g) is the mass of the TNTs catalyst, V (L) is the volume of the suspension, K (L/mg) is the equilibrium constant of Langmuir, and Qmax (mg/g) is the maximum adsorption capacity of TNTs. The values of K and Qmax are 1.06 L/mg and 133.33 mg/g, respectively, as obtained from our previous investigation [23]. Then the real photodegradation of MB could be evaluated by the decrease of total MB concentration C = Cb + Cs. As indicated in Fig. 4, considering the decreased concentration both in bulk solution and on the catalyst surface, totally 27.0% of the dye was photodegraded in 3 h over TNTs. This result demonstrated that the photocatalytic activity of as-prepared TNTs without the anatase phase of titania was significant, which was similar to the results of Zhu et al. [14] for the photocatalytic oxidation of sulforhodamine on the hydrogen titanates and Lee et al. [12] for the photocatalytic oxidation of basic violet 10 on the titanate nanotubes treated with acid concentration 0.1 and 0.01 N. Nevertheless, Yu et al. [11] argued that hydrogen titanate nanotubes by the hydrothermal method showed almost no photocatalytic activity for the photocatalytic degradation of acetone. The difference in photocatalytic activity of the as-obtained nanotubes might be ascribed to different experimental conditions. 3.3. Effect of adsorption on photodegradation of MB

was studied, as illustrated in Fig. 6. Removal efficiencies of MB in different reaction systems were also calculated. The results are summarized in Table 1. As shown in Fig. 5 and Table 1, in the MB simultaneous adsorption and photodegradation process, 91.6% of the dye in bulk solution was removed under UV irradiation for 3 h, while a blank experiment with dark adsorption for 3 h can only lead to the reduction of 43.4% in MB concentration, indicating a significant photocatalytic degradation of MB in the A + P system. Furthermore, it can be clearly observed that the kinetics of MB disappearance over TNTs in A + P reaction system was different from that in A/P system. The MB overall removal efficiency in the TNT A + P system (91.6%) was unexpectedly higher than that in the TNT A/P system (76.3%). However, MB removal due to adsorption in the TNT A + P system was undoubtedly no more than that in the TNT A/P system, since there were shorter contact times and lower dye concentrations in aqueous solution in the former system. Considering that MB overall removal consisted of the removal by adsorption and that by photodegradation, it could be concluded that photodegradation of MB in the TNT A + P system was more rapid than that in the TNT A/P system. In contrast, MB showed quite similar disappearance kinetics over TiO2 in A + P and A/P reaction systems (Fig. 6). The variation of MB concentration during the course had little difference between TiO2 A + P and A/P systems. The same trend for MB overall removal efficiency over TiO2 was also observed in two reaction systems (85.1% and 82.5%, respectively). Moreover, it was noteworthy that the removal of MB by dark adsorption for two reaction systems was just 2.6% and 1.2%, respectively (Table 1), indicating

To study the effect of adsorption in the photocatalysis of TNTs, besides the experiments described in Section 2.3, one more set of tests was also conducted in a dissimilar reaction system, where the reaction solutions in the tests did not achieve equilibrium adsorption but were subjected to adsorption and photodegradation simultaneously. By contrast, a relatively lower adsorption of MB on the catalyst surface was achieved in this case. For convenience, the two reaction systems, i.e., equilibrium adsorption followed by photodegradation, and simultaneous adsorption and photodegradation, were denoted as A/P and A + P, respectively, unless otherwise specified. Fig. 5 is a comparison of the variation of MB concentration as a function of time in A/P and A + P systems mediated with TNTs. For comparison, the variation of MB concentration as a function of time in A/P and A + P systems mediated with TiO2

Fig. 6. Variation of MB concentration as a function of time in A/P and A + P systems over TiO2. Experimental conditions: initial dye concentration = 100 mg/L, catalyst dosage = 0.5 g/L, temperature = 20 °C.

Table 1 Removal efficiencies of MB in the aqueous solution in terms of different reaction systems.

Fig. 5. Variation of MB concentration as a function of time in A/P and A + P systems over TNTs. Experimental conditions: initial dye concentration = 100 mg/L, catalyst dosage = 0.5 g/L, temperature = 20 °C.

Reaction systems

Overall removal efficiency, Ro (%)a

Removal efficiency due to dark adsorption, Ra (%)b

TNTs A/P TNTs A + P TiO2 A/P TiO2 A + P

76.3 91.6 85.1 82.5

43.6 43.4 2.6 1.2

a Ro = (1  Co/C0)  100%, where C0 (mg/L) is initial MB concentration and Co (mg/ L) is MB concentration when the irradiation was finished. b Ra = (1  Ca/C0)  100%, where Ca (mg/L) is MB concentration in treatment with dark adsorption at time 4 h (for A/P) and 3 h (for A + P), respectively.

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the dye adsorption onto TiO2 was almost negligible. Accordingly, TiO2 showed similar photocatalytic activity in the two systems. Additionally, as presented in Table 1, the MB overall removal efficiency in the TNT A + P system was the highest when compared with that in three other systems. Our results for adsorption and photodegradation over TNTs were consistent with the reports by Shon et al. [29]. In their study, it was observed that simultaneous powdered activated carbon adsorption and TiO2 photodegradation led to a superior dissolved organic matter removal compared to powdered activated carbon adsorption followed by TiO2 photodegradation. Generally, it is believed that adsorption of the reactant results in the enhancement of its photocatalytic degradation [17–19]. However, our studies revealed that the role of adsorption might be complicated for TNTs, as illustrated in Fig. 5. A reasonable interpretation for the unexpected photocatalytic degradation behavior over TNTs could be ascribed to the extent of adsorption of the dye. From Table 1, it can be clearly seen that TNTs had a far higher adsorption of MB than TiO2. This difference in adsorption capacity between TNTs and TiO2 might be ascribed to different surface charges and surface areas. The TiO2 catalyst was well characterized in the literature with the point of zero charge typically in the pH range of 6.5– 6.9 [30–33], which was extremely close to the solution pH, so the TiO2 surface should possess almost no net charge in the ambient experiments. But the surface of TNTs was negatively charged in the pH 2.5–11 range [23]; thus the adsorption of cationic MB molecules onto TNTs was far easier than that onto TiO2 by charge attraction. On the other hand, according to our previous studies [23], the BET surface area of TNTs was 157.9 m2/g, which was increased by a factor of 3.4 compared with TiO2 (46.9 m2/g). Thus the higher surface area made it possible to adsorb more dye molecules. During equilibrium adsorption followed by the photodegradation process, once the equilibrium adsorption was reached, derived from too high adsorption of MB onto TNTs, the catalyst particle was highly covered by the adsorbed dye molecules. In accordance with our previous study [23], the adsorption behavior of MB onto TNTs was well described by the Langmuir model. Thus the coverage of MB on the surface of TNTs could be given as



Qs KC b ¼ : Q max 1 þ KC b

ð3Þ

According to Eq. (3), the calculated surface coverage of TNTs was up to 96–98% in the subsequent photoreaction. Such high coverage by the adsorbed MB on the surface might cut off part of the UV light and impair the yield and diffusion of photoinduced hole/electron pairs, slowing down the photocatalytic degradation rate of the catalyst [20]. As to the simultaneous adsorption and photodegradation process, the instant amount of MB adsorbed onto TNTs at each time was not too much since it had avoided excessive uptake of MB due to equilibrium adsorption, which weakened the screening effect to UV light and offered adequate active sites to generate valence-band holes and conduction-band electrons. In the meantime the adsorbed MB molecules could be photodegraded rapidly in concurrent photocatalysis. This timely photodegradation of adsorbed MB molecules could increase the UV light transmitted to the photocatalyst surface and improve the degradation efficiency of MB enormously. As a result, the adsorption of MB promoted the accompanying photodegradation in this case. However, the adsorption of MB onto TiO2 was very poor in either system. The screening effect on the surface of TiO2 could be ignored and the available active sites were sufficient, so adsorption had little effect on MB photodegradation over TiO2. Thus there was only negligible distinction in MB overall removal efficiency in the TiO2 A + P and A/P systems.

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3.4. Catalyst decantation property The separation and recovery of the photocatalyst are very important factors in addition to photocatalytic performance for water treatment toward industrial photocatalysis. As well known, the use of common photocatalyst such as titania at a commercial scale has just been hindered seriously by the high cost of the catalyst separation process. Accordingly, the decantation property of as-prepared TNTs was also evaluated. Fig. 7 is a comparison of the sedimentation in aqueous TNTs and TiO2 suspensions. The catalyst concentration was 0.5 g/L. It can be obviously seen that TNTs sedimentated from the aqueous suspension rapidly, with the turbidity drop of the aqueous suspension around 72.0% within 180 min. In contrast, the turbid of TiO2 suspension changed insignificantly. These observations indicated that TNTs can be more easily separated from the reaction system than TiO2 by sedimentation. The outstanding separation performance of TNTs might greatly promote their industrial application in photocatalysis to clear up pollutants in wastewater. 4. Summary TNTs were prepared by hydrothermal treatment using TiO2 and NaOH as precursors followed by calcination at 400 °C for 2 h. Adsorption and photocatalytic degradation of MB over TNTs and TiO2 were studied comparatively. TNTs showed much higher adsorption ability to MB than TiO2. As unexpected, in the A + P system with relatively lower adsorption onto the photocatalyst, TNTs were observed to possess a higher MB overall removal efficiency than that in the A/P system with too high adsorption, suggesting that TNTs exhibited better photocatalytic activity in the A + P system. Nevertheless, TiO2 showed similar photocatalytic activity in A + P and A/P systems. The unexpected photodegradation behavior over TNTs could be explained by the different effects of adsorption of MB, namely the promoting effect under relatively lower adsorption and the inhibition effect under too high adsorption. Additionally, TNTs exhibited better sedimentation properties than TiO2 in aqueous dispersions and showed promising prospects as photocatalysts for recycle use. In summary, TNTs with the property of easy separation will have favor in future industrial applications, and not too high adsorption is necessary to improve their photocatalytic activity. To our knowledge, it is the first time this observation on the effect of adsorption in TNTs photocatalysis has been reported. However, further investigations about the adsorption and photocatalysis behaviors of TNTs should be made to obtain deeper insight into the cooperative action mechanism between them.

Fig. 7. The turbidity of TNTs and TiO2 suspensions as a function of sedimentation time. Experimental conditions: catalyst dosage = 0.5 g/L, temperature = 20 °C.

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Acknowledgments The present work was supported by the Research Fund for the Doctoral Program of Higher Education with Grant 20070001045. Also we thank Prof. Q. Chen from the Department of Electronics, Peking University, for some suggestions to improve the synthesis of titanate nanotubes. The two anonymous reviewers are also gratefully acknowledged for their constructive comments and suggestions. References [1] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [2] K. Kabra, R. Chaudhary, R.L. Sawhney, Ind. Eng. Chem. Res. 43 (2004) 7683. [3] R.S. Yuan, R.B. Guan, W.Z. Shen, J.T. Zheng, J. Colloid Interface Sci. 282 (2005) 87. [4] A.N. Okte, O. Yilmaz, Appl. Catal., A 354 (2009) 132. [5] P. Pucher, M. Benmami, R. Azouani, G. Krammer, K. Chhor, J.F. Bocquet, A.V. Kanaev, Appl. Catal., A 332 (2007) 297. [6] D.L. Jiang, S.Q. Zhang, H.J. Zhao, Environ. Sci. Technol. 41 (2007) 303. [7] F.Y. Wei, H.L. Zeng, P. Cui, S.C. Peng, T.H. Cheng, Chem. Eng. J. 144 (2008) 119. [8] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998) 3160. [9] G.H. Du, Q. Chen, R.C. Che, Z.Y. Yuan, L.M. Peng, Appl. Phys. Lett. 79 (2001) 3702. [10] A. Thorne, A. Kruth, D. Tunstall, J.T.S. Irvine, J. Phys. Chem. B 109 (2005) 5439. [11] J.G. Yu, H.G. Yu, B. Cheng, C. Trapalis, J. Mol. Catal. A: Chem. 249 (2006) 135. [12] C.K. Lee, C.C. Wang, M.D. Lyu, L.C. Juang, S.S. Liu, S.H. Hung, J. Colloid Interface Sci. 316 (2007) 562.

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