TiO2 nanocomposites

TiO2 nanocomposites

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CERAMICS INTERNATIONAL

Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint

Efficient photocatalytic degradation of methylene blue over BiVO4/TiO2 nanocomposites Natda Wetchakuna,n, Surachai Chaineta, Sukon Phanichphantb, Khatcharin Wetchakunc a

Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand b Materials Science Research Centre, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand c Program of Physics, Faculty of Science, Ubon Ratchathani Rajabhat University, Ubon Ratchathani 34000, Thailand Received 19 December 2014; received in revised form 6 January 2015; accepted 8 January 2015

Abstract BiVO4/TiO2 nanocomposites were successfully synthesized by coupling the modified sol-gel method with hydrothermal method. The samples were physically characterized X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer, Emmett and Teller (BET)-specific surface area, UV–vis diffuse reflectance spectrophotometry, zeta potential, and photoluminescence techniques. The BiVO4/TiO2 nanocomposites exhibited good photocatalytic activity in degradation of methylene blue under simulated solar light irradiation. The photodegradation of methylene blue demonstrated that 0.5BiVO4/0.5TiO2 photocatalyst exhibited much enhanced photoactivity than pure BiVO4 and TiO2. Based on the obtained results, the as-prepare BiVO4/ TiO2 nanocomposite possessed great adsorptivity of methylene blue, extended light adsorption range, and efficient charge separation properties. Overall, this work could provide new insights into the fabrication of a BiVO4/TiO2 composite as high performance photocatalyst and promise as a solar light photocatalyst for dye wastewater treatment. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: BiVO4/TiO2; Degradation; Nanocomposite; Photocatalytic activity

1. Introduction In recent years, photocatalysis processes have been intensively investigated for wastewater treatment. The photocatalytic oxidation has also been proposed as an effective method for treatment of toxic and polluted water [1,2]. Titanium dioxide (TiO2) is the most popular material used in heterogeneous photocatalysis for its excellent properties, such as high stability, chemical inertness, non-toxicity, and low cost [3,4]. However, a major drawback of TiO2 is that only UV in the solar spectrum (about 3–5%) can be utilized to initiate the photocatalytic redox processes because of the large band gap of anatase TiO2 (3.2 eV) and rutile TiO2 (3.0 eV) [5–7]. Moreover, individual TiO2 photocatalyst give low separation efficiency of electron-hole pairs. The above problems can be solved, in part, if TiO2 is coupled by a smaller band gap n

Corresponding author. Tel.: þ66 84 0459424; fax: þ 66 53 892270. E-mail address: [email protected] (N. Wetchakun).

semiconductor with a higher conduction band (CB) than that of TiO2 [8–10]. Therefore, it is essential to develop the TiO2 photocatalyst by coupling with another metal oxide semiconductor with appropriate band potentials to form a heterojunction and thus to improve the separation of photogenerated electron-hole pairs and increase the charge carrier lifetime. For instance, TiO2 coupled by BiVO4 have been extensively alternated way for reduction electron-hole pair recombination and made the high photocatalytic performance. BiVO4 has been chosen as a sensitizer due to its high visible-light absorption ability. There are three crystalline phases reported for synthetic BiVO4, namely, a monoclinic scheelite, a tetragonal zircon and a tetragonal scheelite structure [11,12]. Among these phase structures, the monoclinic scheelite structure of BiVO4 possesses the best photocatalytic performance under visible-light irradiation due to its relatively narrow band gap of 2.4 eV, compared to the two tetragonal phases with the band gap energy of 3.1 eV [13]. Therefore, the formation of monoclinic BiVO4 phase is preferable with

http://dx.doi.org/10.1016/j.ceramint.2015.01.040 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: N. Wetchakun, et al., Efficient photocatalytic degradation of methylene blue over BiVO4/TiO2 nanocomposites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.01.040

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an expectation to shift the absorption into visible-light region and thus enhanced photocatalytic efficiency could be obtained. To date, BiVO4/TiO2 nanocrystalline heterostructure have been successfully prepared by a sol-gel method. The results of this catalyst possessed higher photocatalytic activity than pure BiVO4 and pure TiO2 towards the removal of benzene [8]. Zhang et al. [14] reported microwave hydrothermal synthesis was employed to prepare BiVO4/TiO2 heterojunction structure. The results of photocatalytic activities indicate that 20%TiO2/ BiVO4 shows the best photocatalytic activity for rhodamine B (RhB) degradation under UV light and simulated sunlight irradiation. This possibly due to its high crystallinity, narrow band gap, and most importantly, the hierarchical heterostructure which can effectively separate photoinduced electron–hole pairs on the surface of BiVO4/TiO2 photocatalysts. Moreover, the photocatalytic activity of the BiVO4/TiO2 prepared by coprecipitation-based reactions followed by either thermal or hydrothermal treatment was tested for the degradation of isopropanol in the gas phase under indoor illumination conditions [15]. The BiVO4/TiO2 shows the highest photocatalytic activity. However, the modified synthesis method (coupling the modified sol-gel method with hydrothermal method) with the new physicochemical properties of BiVO4/ TiO2 nanocomposites towards methylene blue degradation under simulated solar light irradiation have never been reported in other publication papers. To the best of our knowledge, this composite material was compared and discussed to illustrate the physicochemical properties on the apparent photoactivity. In the present work, BiVO4/TiO2 composites with better crystallinity were synthesized by coupling the modified sol-gel method with hydrothermal method. The effects of morphology, crystal structure, specific surface area and optical response on the photocatalytic activity of BiVO4/TiO2 composites were investigated in detail. Their photocatalytic activities were further evaluated by the photocatalytic degradation of methylene blue (MB) under simulated solar light irradiation. The possible photocatalytic mechanism in BiVO4/TiO2 system was proposed. 2. Experimental 2.1. Photocatalyst preparation BiVO4/TiO2 nanocomposite catalysts with different mole ratios between BiVO4 and TiO2 have been prepared by the combination of the modified sol-gel method and hydrothermal method. Firstly, pure TiO2 was synthesized by the modified sol-gel method using titanium tetraisopropoxide (TTIP) as the Ti-precursor. 20 ml titanium tetraisopropoxide was dissolved in 250 ml of nitric acid solution (6 M) and mixed until a homogeneous solution was obtained. The mixture of TTIP and nitric acid solution was loaded into a cellophane membrane and suspended for 1 h in a clear solution containing 0.001 M of ethylenediaminetetraacetic acid (EDTA) and 7 ml of ammonia solution (25%). Then, the suspension was centrifuged at

5000 rpm for 10 min, washed with deionized water and then dried in an oven at 60 1C for 24 h. The obtained powder was finally calcined in a furnace at a temperature of 400 1C for 3 h. Then, the as-synthesized TiO2 powders were subsequently added to the pre-prepared mixture solution of bismuth nitrate hexahydrate (Bi(NO3)3  6H2O) in nitric acid and ammonium vanadate (NH4VO3) in ammonia solution (1:1 mol ratio) to form suspensions with different BiVO4:TiO2 mole ratios, 0.2:0.8, 0.4:0.6, 0.5:0.5, 0.6:0.4, and 0.8:0.2.Then, the pH of the prepared suspension was adjusted to 7 by slowly adding 0.1 M of NH3  H2O solution. After that, the suspension was transferred into a teflon-lined stainless steel autoclave and the hydrothermal reaction was carried out at 120 1C for 6 h. Finally, BiVO4/TiO2 composite was obtained by filtration and drying at 80 1C for 24 h. For control experiments, pure BiVO4 photocatalyst was also prepared by the procedure described above. 2.2. Photocatalyst characterization The crystalline phases of pure BiVO4, pure TiO2 and BiVO4/TiO2 were determined by powder X-ray diffraction analysis (XRD, JEOL JDX-3530). The diffraction patterns were recorded in the range of 2θ ¼ 20 to 801 using Cu Kα radiation (λ ¼ 0.15406 nm) with a step scan of 0.51/min. Morphologies and microstructures of the as-prepared samples were examined by transmission electron microscopy (TEM, JEOL JEM-2010). UV–vis diffuse reflectance spectra of the photocatalyst particles were recorded at room temperature in the range of 300–700 nm using a Lambda 950 UV–vis spectrophotometer. Isoelectric point (IEP) of BiVO4and TiO2 particles was also examined by zetasizer nano instrument (ZS Malvern). The photoluminescence (PL) spectra were recorded with a AvaSpec-2048TEC-USB2-2 spectrophotometer excited by LED (Oceans optics, LLS-345) as a light source with a wavelength of 345 nm. 2.3. Photocatalytic activity The photocatalytic activity of BiVO4, TiO2, and BiVO4/ TiO2 composite powders for the decomposition of methylene blue (MB) was evaluated under irradiation of a 50 W with halogen lamp (Essential MR, Philips (Thailand)) as the simulated solar light, providing a light intensity of 185 mW cm  2 at the natural pH value. The lamp located at 10 cm away from the reaction solution. The initial concentration of MB was 2  10  5 M with a catalyst loading of 1 g/L. Prior to the illumination, the suspension was agitated for 1 h in the absence of light to achieve the equilibrium adsorption. Each time before the absorption measurement, the sample solution was centrifuged at 5000 rpm for 10 min in order to separate the catalyst particles from the solution. The degradation process was monitored by a UV–vis spectrophotometer (Shimadzu UV-1800) and the concentration of the residual MB was analyzed quantitatively by measuring the maximum absorption at 664 nm.

Please cite this article as: N. Wetchakun, et al., Efficient photocatalytic degradation of methylene blue over BiVO4/TiO2 nanocomposites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.01.040

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Fig. 1. XRD patterns of pure BiVO4, pure TiO2, and BiVO4/TiO2 varying mole ratio.

3. Results and discussion 3.1. XRD analysis The curve a in Fig. 1 revealed that the crystal phase of TiO2 nanoparticles was anatase and rutile phases with the diffraction peaks at about 2θ¼ 25.521, 48.211 and 2θ ¼ 27.51, 36.151, 41.31, 44.181, 48.211, 54.391, 56.851 which could be perfectly indexed to the (1 0 1), (2 0 0) and (1 1 0), (1 0 1), (1 1 1), (2 1 0), (2 0 0), (2 1 1), (2 2 0) crystal faces of anatase (JCPDS 21-1272) and rutile (JCPDS 21-1276) TiO2, respectively. In addition, diffraction peaks of BiVO4 with 2θ values of 28.831, 30.511, 34.461, 35.221, 42.471, 46.711, 47.271, and 53.31 appeared, which could be perfectly indexed to the ( 1 2 1), (0 4 0), (2 0 0), (0 0 2) (0 5 1), (2 4 0), (0 4 2), and (1 6 1) crystal planes of monoclinic BiVO4 (JCPDS 14-0688). No characteristic peaks for impurity were observed, suggesting that the composition of the above nanocomposites was BiVO4 and TiO2. The XRD patterns of BiVO4/TiO2 heterojunction photocatalyst exhibited characteristic diffraction peaks of both BiVO4 and TiO2 crystalline phases. The diffraction peaks of the BiVO4/TiO2 composites are sharp and high intensity, indicating that the catalysts are well-crystallized.

Fig. 2. (a) KM absorbance plot and (b) band gap energy of pure BiVO4, pure TiO2

Thus, it can be indicated that the enhanced light absorbing property was the precondition of effective photocatalytic degradation. 3.3. Morphology characterization The morphology of pure BiVO4 (Fig. 3(a)) showed coral-shaped with approximately 100 nm dimensions, whereas pure TiO2 from TEM analysis (Fig. 3(b)) was found to be spherical-shaped nanoparticles with a diameter in the range of 20–30 nm. As shown in Fig. 3(c), it clearly shows two different morphologies of the composite, which could be assigned to TiO2 and BiVO4, suggesting a high degree of crystallinity. This confirmed that BiVO4/TiO2 nanocomposites were successfully prepared with TiO2 nanoparticles covering the BiVO4 surface.

3.2. Optical properties The UV–vis diffuse reflectance spectra of pure BiVO4, pure TiO2 and BiVO4/TiO2 were shown in Fig. 2(a). The optical band gap (Eg) can be determined from the plot between E ¼ 1240/λAbsorp. Edge and [F(R1)hυ]n, where n ¼ 1/2 is indirect band gap TiO2 [16] and n ¼ 2 is direct band gap BiVO4 [17,18] as shown in Fig. 2(b). Pure TiO2 showed the absorption onset at 408 nm, which corresponded to the band gap energy of 2.96 eV. The absorption edge of BiVO4/TiO2 nanocomposites showed a shift towards the visible region.

3.4. Photocatalytic activity for methylene blue (MB) degradation The photocatalytic activities of all samples were evaluated by measuring the degradation of MB under simulated solar light irradiation as shown in Fig. 4. The concentrations (C) of MB were determined from UV–vis absorption studies by using the Beer-Lambert law relation at the maximum wavelength (λmax) of 664. Variations of MB concentration (C/C0) with simulated solar light irradiation time over different catalysts

Please cite this article as: N. Wetchakun, et al., Efficient photocatalytic degradation of methylene blue over BiVO4/TiO2 nanocomposites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.01.040

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Fig. 3. TEM images of (a) pure BiVO4, (b) pure TiO2, (c) 0.5BiVO4:0.5TiO2 nanocomposites.

Fig. 4. (a) Comparison of photodegradation efficiency of MB using all photocatalysts; (b) Cycling runs of photodegradation of MB using 0.5BiVO4:0.5TiO2 (initial concentration of MB 2  10  5 M and catalyst loading 1 g/L).

were presented in Fig. 4(a), respectively. As a comparison, direct photolysis of MB as well as the photocatalytic degradation over pure BiVO4 and TiO2 was also performed under the

identical conditions. As seen from Fig. 4(a), the mole ratio of BiVO4/TiO2 at 0.5:0.5 provided the highest photocatalytic activity where the highest MB degradation of 84% was obtained within 120 min of irradiation. Lower or higher concentration than that of 0.5BiVO4:0.5TiO2 composite would lead to a decreased photocatalytic performance. The values of BET surface areas (SBET) for TiO2 and pure BiVO4 were 75.0 and 45.43 m2 g  1, respectively. The composite samples generally had a larger BET surface area than the pure BiVO4, which is attributed to the conversion of spherical TiO2 to BiVO4 with coral structures. Moreover, the SBET value of the heterojunction catalysts increased gradually from 25.12 to 50.15 m2 g  1 as the BiVO4/TiO2 mole ratio was increased from 0.2 to 0.8. A large surface area with nanoparticles might be promoted adsorption, desorption and diffusion of reactants, which is favorable for high photocatalytic activity [19,20]. Stability of photocatalytic MB degradation using the selected 0.5BiVO4/0.5TiO2 nanocomposite was also investigated as shown in Fig. 4(b). It was found that the 0.5BiVO4/ 0.5TiO2 catalyst exhibited a highly stable photocatalytic performance towards MB degradation since no significant loss in activity was observed. It only displays less than 5% deactivation after five cycles. Fig. 5 shows the photocatalytic mechanism of the BiVO4/ TiO2 heterojunction photocatalyst. The conduction band (CB) and valence band (VB) potentials of BiVO4 and TiO2 at the point of zero charge can be calculated by the following equation [21,22]: E0CB=χ–EC-1/2Eg, where χ the absolute electronegativity of the semiconductor (χ is 5.81 eV [23] and 6.04 eV [24] for TiO2 and BiVO4, respectively). EC is the energy of free electrons on the hydrogen scale (4.5 eV) and Eg is the band gap of the semiconductor. The calculated CB and VB of BiVO4 were 0.25 and 1.54 eV, and of TiO2 were -0.17 and 1.31 eV, respectively. The band gap energies of BiVO4 and TiO2 were found to be 2.58 eV and 2.96 eV, respectively. EVB values can be obtained by EVB ¼ ECB þ Eg. The positions of the conduction and the valence bands of monoclinic BiVO4 crystallites and TiO2 crystallites were shown in Fig. 5. As shown in Fig. 5, the injected electrons and photoinduced electrons easily flow into conduction band of BiVO4 under

Please cite this article as: N. Wetchakun, et al., Efficient photocatalytic degradation of methylene blue over BiVO4/TiO2 nanocomposites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.01.040

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Fig. 5. Schematic diagram for methylene blue degradation over BiVO4/TiO2 nanocomposites under simulated solar light irradiation

simulated sunlight irradiation. Dissolved oxygen molecules react with the surface of the BiVO4 electrons (e  ) to yield superoxide radical anions, O2  , which on protonation generate the hydroperoxy, HO2 , radicals, producing hydroxyl radical OH, which was a strong oxidizing agent to decompose the organic dye. In addition, the hole reacts with water and hydroxide ion molecules to finally also form hydroxyl radicals. Moreover, the photocatalytic degradation of methylene blue depended on adsorption ability of BiVO4/TiO2. It could be explained by the different surface charges characterized by zeta potential. The isoelectric point (no net charge) of BiVO4 and TiO2 were found at the pH of 4.56 and 5.15, respectively, suggesting that, under the photocatalytic conditions used in this study at pH 6.5, BiVO4 and TiO2 possessed negative charges on the surface. Since BiVO4 and TiO2 have a net surface negative charge, the cationic methylene blue molecules preferably adsorbed on these particles, hence improving the photocatalytic performance. Furthermore, the better separation of photogenerated electrons and holes in the BiVO4/TiO2 nanocomposite was confirmed by PL emission spectra. Generally, the photoluminescence emissions on semiconductor materials is directly related to the radiative recombination of photo-generated electrons and holes, therefore, the lowest PL intensity of BiVO4/TiO2 indicates the lowest recombination of photogenerated electron and hole pairs. In Fig. 6, it was indicated that the BiVO4/TiO2 exhibited much lower emission intensity than BiVO4 and TiO2, indicating that the recombination of the photogenerated charge carrier was inhibited greatly in the BiVO4/TiO2. This is the reason for the excited electron transference from conduction band of TiO2 to the conduction and of BiVO4, which also decreased the PL intensity. Moreover, the synthetic process provided the defects formed in our system were likely to be due an oxygen vacancy and interstitial oxygen. The two different oxygen defects can be enhanced the electron hole pair separation rate in BiVO4/TiO2 composite. As the redox reactions might occur on the surface of oxygen vacancies and interstitial oxygen defects, the oxygen defects can be considered to be the active sites of the BiVO4/TiO2 composite [25].

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Fig. 6. PL emission spectra of pure BiVO4, pure TiO2, and 0.5BiVO4/0.5TiO2.

4. Conclusions A series of BiVO4/TiO2 nanocomposites with different mole ratios have been successfully synthesized by hydrothermal method coupling with the modified sol-gel method. The mole ratio of BiVO4 to TiO2 has a significant influence on the activity of the photocatalysts. When the system was irradiated with simulated solar light for 2 h, the efficiency of the degradation of MB with the most active 0.5BiVO4/0.5TiO2 nanocomposites was 85%. Moreover, the efficient simulated solar light radiation-activated BiVO4/TiO2 photocatalyst remained stable after five irradiation cycles. Enhancement of the decomposition of MB using BiVO4/TiO2 photocatalysts is attributed to the formation of composite between BiVO4 and TiO2, leading to an effective separation of photogenerated electrons and holes. Moreover, with further improvement and optimization, the BiVO4/TiO2 composite photocatalysts might be promise as a photocatalyst for dye wastewater treatment. Acknowledgments The authors would like to gratefully acknowledge the Thailand Office of the Higher Education Commission for providing financial support through the National Research University (NRU) Project for Chiang Mai University and the Centre of Excellence in Materials Science, Chiang Mai University. We also acknowledge the National Research Council of Thailand and the Graduate School Chiang Mai University for providing financial support. References [1] M. Alvaro, C. Aprile, M. Benitez, E. Carbonell, H. García, Photocatalytic activity of structured mesoporous TiO2 materials, J. Phys. Chem. B. 110 (13) (2006) 6661–6665. [2] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Prog. Solid State Chem. 32 (2004) 42–90. [3] S. Pal, A.M. Laera, A. Licciulli, M. Catalano, A. Taurino, Biphase TiO2 microspheres with enhanced photocatalytic activity, Ind. Eng. Chem. Res. 53 (19) (2014) 7931–7938.

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Please cite this article as: N. Wetchakun, et al., Efficient photocatalytic degradation of methylene blue over BiVO4/TiO2 nanocomposites, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.01.040