Visible-light responsive carbon–anatase–hematite core–shell microspheres for methylene blue photodegradation

Materials Science in Semiconductor Processing 27 (2014) 950–957

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Visible-light responsive carbon–anatase–hematite core–shell microspheres for methylene blue photodegradation JunQi Li n, ZhenXing Liu, DeFang Wang, ZhenFeng Zhu School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, PR China

a r t i c l e in f o

Keywords: C@TiO2@Fe2O3 microspheres Impregnating-calcination method Photocatalytic activity Visible-light

abstract Core shell carbon–anatase–hematite (C@TiO2@α-Fe2O3) microspheres with sandwich-like structures were synthesized by an effective three-step approach. Specifically, using carbonaceous saccharide microspheres as template directs the sequential deposition of TiO2 layer by the modified Stöber method and subsequent α-Fe2O3 layer via the impregnating-calcination process. The samples were characterized by SEM, TEM, XRD, and UV–vis diffuse reflectance spectroscopy. The photocatalytic activity towards degradation of methylene blue aqueous solution under visible light irradiation was significantly improved, as compared to that of the TiO2 hollow microspheres and TiO2@α-Fe2O3 hollow microspheres counterparts. This enhancement in photoactivity is mainly attributed to two positive effects of the coupling TiO2 between the C core and α-Fe2O3 shell. Firstly, a matching heterostructure between TiO2 and α-Fe2O3 could dramatically improve the separation of photoinduced electron–hole pairs. Secondly, C-core could enhance the absorption in the visible-light region, serve as good adsorbent towards dye molecules, and provide a network to rapidly transfer the photoexcited electrons from the conduction band of TiO2 during the photocatalytic process. The study also provides a general and effective method in the fabrication of core–shell composites with sound heterojunctions that may show a variety of applications. & 2014 Elsevier Ltd. All rights reserved.

1. Introduction Recently, the photocatalytic degradation of various kinds of organic using TiO2 powders as photocatalysts has been extensively studied [1–4], owing to their nontoxic, chemically stable, inexpensive and relatively high photocatalytic activity [5,6]. However, the application of pure TiO2 is limited because it is only active in the ultraviolet (UV) region due to its wide band gap energy (Eg ¼3.2 eV), and the fast recombination of photogenerated electron and hole. These fundamental problems prevent TiO2 from practical application [7,8]. To activate

n

Corresponding author. Tel.: þ 86 29 86177018; fax: þ 86 29 86177018. E-mail address: [email protected] (J. Li).

http://dx.doi.org/10.1016/j.mssp.2014.08.038 1369-8001/& 2014 Elsevier Ltd. All rights reserved.

the photocatalysts with higher efficiency and longer wavelength, a number of strategies have been adopted [9–12]. One of those strategies is to couple TiO2 with other semiconductor with appropriate conduction band (CB) and valence band (VB) gaps [13]. Among all reported available candidates, Fe2O3 and carbonaceous materials are the best for the preparation of photocatalytic composites. Fe2O3 is suitable for industrial applications considering its low cost, non-toxicity, high chemical stability and easy preparation. Moreover, Fe2O3 can be used as visible light photocatalyst, due to its narrow band gap of 2.2 eV. And its photocatalytic properties have been studied in water splitting [14,15] and photodegradation of organic dye [16,17], which all are with good catalytic activity. As well known, coupling with functional carbonaceous materials has proved to be one of

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the promising strategies [18,19]. Carbonaceous materials, with high adsorption capacity and excellent electrical conductivity, could enhance the adsorbability of organic pollutant, and facilitates the efficient separation of photoinduced electron–hole pairs, which all could increase the rate of photocatalytic reaction [20]. Many efforts have focused on the research on coupling TiO2 with Fe2O3 [21,22] or TiO2 with C-core [23,24], but few reports have so far been found regarding coupling TiO2 with Fe2O3 and C core. In this study, we synthesized C@TiO2@α-Fe2O3 core–shell microspheres by integrating the Stöber method and impregnating-calcination method. These results of MB degradation under visible light irradiation are compared with α-Fe2O3 hollow microspheres, C@TiO2 core–shell microspheres, TiO2 hollow microspheres and TiO2@α-Fe2O3 hollow microspheres. The relationship of C@TiO2@α-Fe2O3 core–shell microspheres with high photocatalytic activity between α-Fe2O3 and C core is also discussed. 2. Experimental section 2.1. Materials Tetrabutyl titanate was purchased from Tianjin kermel Chemical Co., Ltd. Iron nitrate (Fe(NO3)3  9H2O) of analytical reagent grade was obtained from Tianjin Fuchen Chemical Corporation. Glucose (D-( þ)–C6H12O6  H2O) of analytical reagent grade was obtained from Sinopharm Chemical Reagent Co., Ltd. Ethanol (EtOH, 99.7%) and sodium chloride were both purchased from Xi'an Chemical Co., Ltd. All reagents were used as received without further purification. Deionized water was used in all experiments. 2.2. Synthesis of TiO2 hollow microspheres, TiO2@α-Fe2O3 hollow microspheres and C@TiO2@α-Fe2O3 core–shell microspheres The fabrication of TiO2 hollow microspheres (THS), C@TiO2 core–shell microspheres (TCS), TiO2@α-Fe2O3 hollow microspheres (FTHS) and C@TiO2@α-Fe2O3 core–shell microspheres (FTCS) can be schematically described as shown in Scheme 1. It can be seen that the synthesis process mainly includes three steps, as follows: (1) Synthesis of carbonaceous saccharide microspheres. The monodisperse carbonaceous saccharide microspheres were synthesized by the traditional hydro-

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thermal method according to a reported process with some modifications [25]. Typically, 6 g of glucose was dissolved in 40 mL of water to form a clear solution. The solution was then sealed in a 50 mL Teflon-lined autoclave and maintained at 180 1C for 4 h. The samples were then centrifuged (5000 r/min), washed in water and ethanol for three cycles, respectively. The obtained carbonaceous saccharide microspheres were then dried at 50 1C for 24 h in air. (2) Synthesis of THS. In a typical procedure, the starting solution was synthesized by mixing 2 mL Ti(OBu)4 and 50 mL ethanol. Then, 0.2 g of carbonaceous saccharide microspheres was added into the solution and stirred vigorously for 1 h in a sealed beaker. Next, the samples were centrifugated and washed in ethanol for three cycles. After being placed at 50 1C for 24 h, C@TiO2 core–shell microspheres precursor was formed. For THS (or TCS), C@TiO2 core–shell microspheres precursor was heated in air (or in N2) at 1 1C min  1 up to 500 1C, kept at this temperature for 3 h, and was allowed to cool to room temperature naturally. (3) Synthesis of FTHS and FTCS. Briefly, as-prepared C@TiO2 spheres (0.5 g) were dispersed in 20 mL 0.1 M Fe(NO3)3  aqueous solution, then magnetic stirred for 10 min and ultrasonic dispersed for 30 min. With continuous stirring the mixture dried in air at 60 1C. Finally, the powder was divided into two parts, and one part was heated in air at 1 1C min  1 up to 500 1C, kept at this temperature for 3 h, and was allowed to cool to room temperature naturally. The other part was heated in N2 at 500 1C for 3 h. In this way, the yellow composite photocatalysts of FTHS and FTCS were synthesized. α-Fe2O3 hollow microspheres (FHS) were synthesized by changing C@TiO2 microspheres as carbonaceous saccharide microspheres.

2.3. Characterization Morphologies of the samples were observed by using a high-resolution field emission environmental scanning electron microscope (FESEM) (S-4800). All the images were obtained under high vacuum mode with Pt sputter coating. The crystalline structure was identified by transmission electron microscopy (TEM) (JEM-2100). X-ray diffraction (D/max-2200, Diffractometer with Cu Kα radiation) was used to verify crystal phase, estimate the crystal sizes. Absorption spectrum was measured using a UV–vis spectrophotometer (UV-2550) in the wavelength range of 200–800 nm, and the photoluminescence (PL) spectra were measured with a fluorescence spectrophotometer (HITACHI F-4500) using the 260 nm line of a Xe lamp as the excitation source at room temperature. 2.4. Photocatalytic measurement

Scheme 1. synthesis of THS, FTHS and FTCS.

The visible-light photocatalytic activity experiments on the THS, FTHS and FTCS were performed by the degradation of the methylene blue (MB) dye (20 mg/L) in a BL-GHX-V multifunctional photochemical reactor (Shanghai Bilon Experiment Equipment Co., Ltd., Shanghai, China). A 500 W Xe lamp equipped with a UV cutoff filter

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(λ4 420 nm) was used as a light source to cool the lamp. The volume of the reaction solution was 330 mL (11 test tubes of 30 mL) into which 330 mg of photocatalyst was added and stirred for 20 min. The solution was dispersed by sonication, and then transferred to test tubes. Stirring was performed at all the times during the reaction. Before the irradiation, the suspension was maintained in dark for 30 min to reach complete adsorption–desorption equilibrium. Sampling was also performed at regular intervals. The residual concentration of MB was determined by measuring its absorbance at 665 nm using an UV–vis spectrophotometer (UV-2550). 3. Results and discussions Fig. 1 dramatically displays morphology changes before and after α-Fe2O3 deposition on the TiO2 hollow spheres and C@TiO2 core–shell spheres. Fig. 1a shows typical SEM image of the as-synthesized carbonaceous saccharide microspheres. It can be clearly seen that carbon spheres are monodispersed with an average diameter of about 300 nm, and their surfaces are smooth. After coating of the titania layer, the surfaces of carbon core become rougher (Fig. 1b) and the diameter of the C@TiO2 core–shell spheres increased to 350–360 nm. The major driving force for this process could be attributed to the chelating effect of the deprotonated hydroxyl groups on carbon spheres surfaces to unprotected Ti4 þ sites on TiO2 NPs [26]. Hence, the resulting C@TiO2 samples avoid the formation of second titania particles and are free of unsupported TiO2 NPs. Fig. 1c shows the C@TiO2 core–shell spheres after dipping in Fe(NO3)3 aqueous solution. Obviously, some nanoparticles are deposited onto the surfaces of the spheres. Fig. 1d shows that C@TiO2 composite particles are calcined at 500 1C for 3 h in air, and THS are obtained.

It can be seen that THS are monodispersed with an average diameter of about 300 nm; although their surfaces became rougher, there were no nanoparticles deposited. When dipping the C@TiO2 spheres in Fe(NO3)3 aqueous solution, Fe3 þ will be adsorbed onto the surface of the TiO2 microspheres. As the deposition reaction proceeded, some Fe3 þ ions gradually hydrolyzed into Fe(OH)3 colloid adhered to the surfaces of the spheres. During the annealing process, Fe(OH)3 is dehydrated into Fe2O3. And FTHS and FTCS are finally formed. Fig. 2a shows the TEM image of FTHS. The strong contrast between the dark edges and bright centers indicates that the hollow structure of FTHS is obtained after calcined at 500 1C for 3 h in air. FTHS exhibit an inner diameter of 300 nm which is very close to that of the carbon spheres, indicating that the carbon spheres templates are completely removed by calcination. Fig. 2d shows TEM image of FTCS obtained by calcination at 500 1C for 3 h in nitrogen. It can be clearly seen that carbon spheres templates are still existed in the centre of TiO2 spheres. HRTEM is used to identify the crystal structure of iron oxide nanoparticles. As we can see from Fig. 2b and e, the lattice fringes of 0.352 nm correspond to the (101) plane of anatase TiO2, and the lattice fringes of 0.272 nm correspond to the (104) plane of α-Fe2O3 (hematite) [27], confirming that iron oxide nanoparticles are crystallized as α-Fe2O3. And iron oxide nanoparticles are successfully deposited on the surfaces of both TiO2 hollow spheres and C@TiO2 core–shell spheres. In addition, α-Fe2O3 has its roots inside TiO2, and its interfaces are tightly contacted. It also can be seen that the shell thickness of FTHS and FTCS is around 50 nm. And the iron oxide nanoparticles with shell thickness of 5–10 nm are uniformly deposited on the surfaces of the TiO2 hollow sphere (Fig. 1e) and C@TiO2 core–shell spheres (Fig. 1f).

Fig. 1. SEM images of carbonaceous saccharide microspheres (a), C@TiO2 core–shell spheres (b), C@TiO2 core–shell spheres after dipping in Fe(NO3)3 aqueous solution (c), THS (d), FTHS (e), and FTCS (f).

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Fig. 2. Low-magnification TEM images of the FTHS (a) and FTCS (d), (b) HRTEM image of the square area in (a), (e) HRTEM image of the square area in (d), (c) EDS pattern of FTHS, (f) EDS pattern of FTCS.

Fig. 3. (a) XRD patterns and (b) regional XRD patterns of THS, FTHS and FTCS.

What is more, EDX exhibited the presence of Fe element besides Ti, O and C (Fig. 2c and f). It is worth noting that the C content was dramatically different which was due to carbon spheres templates existing or not. These images further verified that well-defined FTHS and FTCS are effectively formed by calcination at 500 1C for 3 h in different atmosphere. As a conventional measurement, XRD is used to identify and determine the phase structure, crystallite size and relative crystallinity of the samples. Fig. 3a displays the

XRD patterns of the THS, FTHS and FTCS. As shown in the figure, the THS particles had formed anatase phase since the characteristic diffraction peaks of anatase (major peaks: 25.21, 37.81, 48.01, 53.81, 54.91 and 62.581 are the diffractions of the (101), (004), (200), (105), (211) and (204) planes of anatase) are evident in the samples. No other peaks indicate that the titania hollow spheres are of high purity. The calcination at 500 1C for 3 h in air or in N2 not only removed the template or not, but also made the formation of anatase phase. In XRD pattern of FTHS and

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FTCS, additional peaks of α-Fe2O3 are found. After α-Fe2O3 incorporated into THS, six additional diffraction peaks are found at 24.11, 33.31, 35.81, 40.81, 49.51, and 63.91, in well accordance with the (012), (104), (110), (113), (024) and (300) planes of hematite structure of α-Fe2O3 (JCPDS no. 33-0664). The results also shows that the relative (101) peaks are widened and the intensity decreased after the α-Fe2O3 and C core incorporated, indicating that α-Fe2O3 and C core could form heterojunction with TiO2. Fig. 3b shows the enlargement of the XRD patterns of the samples in the range from 2θ¼23.51 to 26.51. Compared with THS, the sample FTHS and FTCS show the anatase diffraction peak at 2θ¼25.11. The 0.41 left shift of the diffraction peaks may be ascribed to the fact that Fe3 þ is doped into TiO2 lattice and formed the Fe–O–Ti bond. Accordingly, after impregnating and annealing, the surface of the TiO2 microspheres is modified by α-Fe2O3 nanoparticles, and in the meantime, the TiO2 microspheres form heterojunction with α-Fe2O3 [28]. The UV–vis diffuse reflectance spectrums of THS, FTHS and FTCS are presented in Fig. 4. The onset of the absorption for THS spheres is at 390 nm, which is consistent with the intrinsic bandgap absorption of anatase TiO2 (  3.2 eV). For FTHS, the bandgap edge is shifted toward more the visible-light region (red-shift), and the absorption in the visible-light region is significantly enhanced after the α-Fe2O3 incorporation. FTCS exhibit a strong absorption bands from 400 to 800 nm due to the effective light absorption property of the functional carbonaceous species in the C–TiO2–Fe2O3 composites. The carbonaceous materials can introduce a sensitization effect to extend the response of FTHS into the visible-light range of the solar spectrum. The inset in Fig. 4 shows plots of the Kubelka– Munk remission function (i.e., relationship of [ahv]1/2 versus photon energy) corresponding to each spectrum, which indicates that the bandgap of bare THS was 3.15 eV. While the bandgap of FTHS was significantly reduced to 2.40 eV, this phenomenon should occur due to the direct interaction between TiO2 and α-Fe2O3 on the surface during the calcination process. It is well known that photocatalytic activity is closely related with the lifetime of photogenerated electrons and

holes. The separation and recombination processes of photogenerated charge carriers in nano-sized semiconductor materials could be reflected by PL spectrum. Thus, PL spectrum could provide a firm foundation for a quickly evaluation of the photocatalytic activity of semiconductor samples [29]. To confirm the photogenerated charge separation and recombination behaviors in THS, FTHS and FTCS, the PL emission spectra of different samples excitation at 260 nm were examined in the wavelength range of 300–500 nm, as shown in Fig. 5. It can be observed that all the samples exhibited obvious PL signals with similar curve shape while the intensities are different, suggesting that the amount of α-Fe2O3 and C core is not enough to generate a new PL signal. Usually, for anatase TiO2 materials, there are three types of physical origins: self-trapped excitons, oxygen vacancies and surface state [30]. In the present work, both samples displayed a broadband emission from 400 to 500 nm with two weak shoulder peaks of 455 and 474 nm and an intensive peak of 417 nm. The peak position of the 417 nm band can be attributed to radiative recombination of self-trapped excitons. Furthermore, the weak peak at 455 nm and at 474 nm can be attributed to oxygen vacancies. The PL intensity of the samples is declined after the α-Fe2O3 incorporated, demonstrating that the separation and transfer of photogenerated electrons trapped in THS have been enhanced by modified α-Fe2O3. This may due to the reason that when α-Fe2O3 located on the surfaces of TiO2 hollow spheres and through coupling effect a part of photoproduced electron transmit into α-Fe2O3 particles, which facilitates the efficient separation of photoinduced electron–hole pairs. What is more, it is worth noting that FTCS have no obvious PL signals, which attributed to the absorption of C core. The introduced C core provides a network to collect and rapidly transfer the photoexcited electrons from the conduction band of TiO2 to C core, which reduces the recombination probability between photoexcited electrons and holes. The photocatalytic activities of FHS, TCS, THS, FTHS and FTCS were evaluated by photocatalytic methylene blue under visible-light irradiation, as shown in Fig. 6. The blank experiments without catalysts and with carbon

Fig. 4. UV–vis diffuse reflectance spectra of the THS (a), FTHS (b), carbon spheres (c) and FTCS (d).

Fig. 5. Room temperature PL spectra of different samples excitation at 260 nm. (a) THS, (b) FTHS, and (d) FTCS.

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Fig. 6. (a) Photodegradation of MB in the presence of different samples under visible light irradiation and (b) the corresponding pseudo-first-order kinetic rate plot.

Fig. 7. Schematic diagram showing the process for enhanced catalytic activity of C@TiO2@Fe2O3 spheres.

spheres were also investigated. For the blank experiments, the value can be neglected with about 1% of conversion after 240 min illumination. Moreover, for the experiment with carbon spheres, the value was down to 73% after the system obtains adsorption–desorption equilibrium within 30 min, then the concentration of MB keeps unchanged. Fig. 6a shows the results on the photocatalytic decomposition of MB. It was found that 11.2%, 26.6%, 45.8%, 69.4% and 80.8% of MB were degraded in the presence of sample FHS, THS, TCS, FTHS and FTCS under visible light irradiation for 240 min, respectively. FHS is almost photocatalytic inactive and only 11.2%. The THS also show visible-light photocatalytic activity. The possibility contributed to this is that MB can be excited, and electrons from the excited MB flow to the conduction band of TiO2, which can enhance photocatalytic performance. In addition, FTCS performed

better under visible light than FTHS and THS. The result makes clear that heterojunction construction, coupling TiO2 with Fe2O3 and C core, plays an important role in the enhanced photocatalytic activity. The apparent rate constant (κapp) has been chosen as the basic kinetic parameter for the different photocatalysts, since it enables one to determine photocatalytic activity independent of the previous adsorption period in the dark and the concentration of MB remaining in the solution. The apparent first order kinetic equation lnðC=C 0 Þ ¼  κapp  t is used to fit experimental data in Fig. 6a, where C is the concentration of MB remaining in the solution at t, and C0 is the initial concentration at t¼0 [31]. The variations in ln(C/C0) as a function of irradiation time are given in Fig. 6b. The linear transforms show that the degradation kinetics of MB by both photocatalysts is apparent first order. κapp

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data for FTCS, FTHS and THS are 0.007, 0.005 and 0.0012 min  1, respectively. It can be observed that the κ value of FTHS and FTCS is higher than that of THS, and is over four times and five times faster than that of THS, which is in accordance with the curves of the catalysts' photocatalytic activities. The increased photocatalytic activity of the FTHS and FTCS compared to THS is primarily attributed to exist in heterojunction, coupling TiO2 with α-Fe2O3, which plays an important role in the separation of photogenerated electron–hole pairs. As shown in Fig. 7, when α-Fe2O3 nanoparticles deposited on the surfaces of TiO2, the Fermi levels of TiO2 and α-Fe2O3 must align in equilibrium due to the presence of the α-Fe2O3/TiO2 heterojunction [32,33]. α-Fe2O3 could be easily activated and yield charge carriers under visible light irradiation. Subsequently, the photogenerated electrons are immigrated from the conduction band of α-Fe2O3 to the conduction band of TiO2 under the action of built-in electric field and the concentration gradient of electrons. The electrons on the conduction band of TiO2 can be further transferred to dissolved  oxygen molecules to form O2 , while the accumulated holes on the valence band of α-Fe2O3 could be consumed by participating in reaction with OH  in the MB solution to produce  OH [34]. These active species significantly promoted the photocatalytic oxidation process. Dye's sensitization and the thin shell nature of the asprepared FTHS and FTCS spheres are also responsible for its superior photocatalytic performance. MB can be excited, and electrons from the excited MB flow to the conduction band of TiO2, which can enhance photocatalytic performance. Moreover, in photocatalysis, such a thin shell structure only 50 nm can serve as the transport paths for small molecules or allow the transmission and multiflections of visible light within their interior cavities, and also reduce the recombination opportunities of the photogenerated electron–hole pairs, which could move effectively to the surfaces to degrade the absorbed MB molecules. The photocatalytic activity of the FTCS is higher than that of FTHS, which most likely results from the existence in C core, as follows reasons: firstly, it is evident that the coated carbon spheres enhance the absorption in the visible-light region, and more photons can be absorbed and be utilized for the photocatalytic reaction. Secondly, since the amorphous carbon possess good adsorbability, the adsorption capacity of FTCS for MB is significantly enhanced, resulting in an increase of the rate of photocatalytic reaction. Finally, carbonaceous materials could be considered a competitive candidate for the electron acceptor material and rapidly transfer the photoexcited electrons from the conduction band of TiO2 during the photocatalytic process due to its excellent electrical conductivity [35]. The introduced carbon spheres could provide a network to collect and rapidly transfer the photoexcited electrons from the conduction band of TiO2 during the photocatalytic process, which reduces the recombination probability between photoexcited electrons and holes and leaves more holes for the oxidization reaction of the adsorbed MB. Therefore, it is supposed that the introduction of carbon can facilitate the separation of photogenerated electron–hole pairs and enhance the photocatalytic degradation efficiency (Fig. 8).

Fig. 8. Reuse of the C@TiO2@α-Fe2O3 core–shell microspheres.

In view of practical application, the photocatalyst should be chemically and optically stable after several repetitive tests. The circulating runs in the photocatalytic degradation of MB in the presence of FTCS under visible light were checked, as shown in Fig. 7. After five recycles, the photocatalytic efficiency of the FTCS decreased by only 2.9%, which indicates that the obtained FTCS are highly stable.

4. Conclusions In summary, FTHS and FTCS with visible-light activity are easily prepared through a two-step sol–gel method combined impregnating-calcination process using the C@TiO2 spheres as a precursor. Shell of α-Fe2O3 with a thickness of 5–10 nm is deposited on the TiO2 surfaces. Fe2O3 incorporation induced the red-shift of the absorption edge of TiO2 into the visible-light range. In addition, as revealed by PL, Fe2O3 incorporation effectively promoted the separation and transfer of photogenerated charge carriers, which can improve the photocatalytic activity of the samples. The enhanced photocatalytic activity of FTHS and FTCS can be attributed to their high surface area, heterojunction and the C-core existence in FTCS. The study provides an easy and effective method in the fabrication of composition with a sound heterojunction that may be useful to design novel high-performance photocatalysts.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51202136), Special Fund from Shaanxi Provincial Department of Education (2013JK0939), the Academic Backbone Cultivation Program of Shaanxi University of Science & Technology (XSGP201202) and the Postgraduate Innovation Fund of Shaanxi University of Science and Technology.

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