J Mater Sci: Mater Electron DOI 10.1007/s10854-016-4883-9

Synthesis and characterization of Cu–BiVO4/MCM-41 composite catalysts with enhanced visible light photocatalytic activities Yulin Xing1 • Junxia Wang1 Chen Wang1 • Enqi Zhong1

•

Long Chen1 • Anqi Wang1 • Fan Li1

•

Received: 27 March 2016 / Accepted: 21 April 2016 Ó Springer Science+Business Media New York 2016

Abstract Cu–BiVO4/MCM-41 photocatalysts with tunable Cu content were synthesized via a simple sol–gel method using citric acid as chelating agent. The structure, morphology and optical properties of the composites were characterized by means of X-ray diffraction, field emission scanning electron microscope and ultraviolet–visible spectroscopy. The photocatalytic activity was investigated by the photocatalytic degradation of methylene blue (MB) under visible light. The experimental results showed that all as-prepared catalysts belonged to the monoclinic scheelite phase, and slight portion of Cu2? incorporation might lead to lattice expansion of BiVO4. Furthermore, loading BiVO4 with MCM-41 molecular sieve could suppress the aggregation of pure catalyst particles effectively. Among all as-prepared samples, 5 %Cu–BiVO4/MCM-41 exhibited the best photocatalytic activity with a 97 % of MB removal in 3 h, moreover, its pseudo-first-order reaction rate constant was 3 times higher than that of pure BiVO4. The enhanced photocatalytic ability might be ascribed to the excellent absorption performance, low recombination of photogenerated charge carriers and enhancement of light adsorption.

1 Introduction Semiconductor-based photocatalysis has received dramatically attention over the past decades due to its prominent ability of water decomposition and photodegradation of & Junxia Wang [email protected] 1

Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, China

organic pollutants [1–4]. To date, TiO2 and TiO2-based catalysts have been studied worldwide because of their superior photocatalytic ability, photo-stability, non-toxicity, cheapness and so on [5, 6]. Nevertheless, the large band gap (ca. 3.2 eV) of TiO2 makes it only active under ultraviolet light, which only occupies 4 % of the whole solar spectrum. Consequently, the low utilization efficiency of sunlight partly limits the practical applications of TiO2 photocatalyst [7]. In order to make better use of solar energy, many investigations have been undertaken to exploit semiconductor materials with visible light response. In recent years, some photocatalysts like BiVO4 [8, 9], Bi2WO6 [10], NaTaO3 [11], LaNiO3 [12] and InNbO4 [13] have been reported to show visible light driven photocatalytic ability. As a non-titania based photocatalyst, BiVO4 possesses much attraction in photocatalytic reactions due to its highlighted advantages of narrow band gap (ca. 2.4 eV) and effective photocatalytic activities under visible light irradiation [14, 15]. However, there are some limitations in using BiVO4 photocatalyst in powder form [16]: (1) catalyst powder is easy to agglomerate during photodegradation process and thus affects the catalytic performance; (2) the floated powder is difficult to separate after action. Besides, BiVO4 photocatalyst with a small surface area is not beneficial for photocatalysis. To overcome above disadvantages, special efforts have been devoted to loading BiVO4 on a support. So far, various kinds of supports for BiVO4 have been studied including bentonite [17], carbon spheres [18], and fly ash cenospheres [19], et al. Recently, Ma et al. and Han et al. [20, 21] have found that MCM-41 supported composite catalysts showed outstanding advantages of high specific surface area, high adsorption ability and excellent photocatalytic activities. Considering the large specific surface area, which is desirable to disperse

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and stabilize the inorganic nanoparticles, the combination of BiVO4 and MCM-41 may effectively inhibit the aggregation of these powder particles and consequently give a higher photocatalytic performance. However, the high recombination efficiency of photogenerated charge carriers is also a key factor to hinder the further enhancement of photocatalytic ability for pure BiVO4 [22, 23]. Among different modification methods, BiVO4 modified with Cu ions is a feasible route for suppressing the recombination of photogenerated charge carriers and thus improving the catalytic abilities [24–26]. In this paper, Cu–BiVO4/MCM-41 composite photocatalysts were prepared by loading BiVO4 on MCM-41 and modifying with Cu. Methylene blue (MB) was used as a model dye to evaluate the photocatalytic activity of the catalysts under visible light irradiation. The photocatalytic activity of Cu–BiVO4/MCM-41 composite photocatalysts with different Cu content was discussed. The aim of this work was to obtain the information analyzed by various techniques (XRD, FE-SEM and UV–Vis), as well to explain the relationship between the structure of the catalysts and photocatalytic activities. In addition, a possible mechanism for photocatalytic degradation of MB dyes was also proposed.

2 Experimental 2.1 Preparation of photocatalyst MCM-41 molecular sieve was prepared via a hydrothermal method as described in the Refs [27]. Cetrimonium bromide (CTAB) and tetraethyl orthosilicate (TEOs) was used as template and silicon source, respectively. The pH value of the system was adjusted to 9 using ammonia solution. After 48 h of hydrothermal crystallization process, the filtered precipitate was dried at 80 °C for 12 h and then calcined at 550 °C for 6 h. Cu–BiVO4/MCM-41 composite photocatalysts were synthesized as follows: 2 mmol NH4VO3 and 2 mmol citric acid were dissolved in ammonia solution and magnetically stirred to obtained solution A. Meanwhile, 2 mmol Bi(NO3)35H2O, 2 mmol citric acid, 1 mmol urea and a certain amount of Cu(NO3)33H2O were dissolved in 4 M HNO3 solution and stirred for a while to obtained solution B. Then, solution B was added dropwise to solution A, and the mixed solution was stirred for 30 min. After that, a certain amount of MCM-41 was added into the above mixture and stirred for 60 min to get a homogeneous solution. The pH of the mixed solution was adjusted to 9 using ammonia solution. After stirring for another 30 min, the obtained sol was dried at 80 °C overnight. Finally, the gel was calcined in the muffle furnace in air at 500 °C for

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2 h with a heating rate of 2 °C/min from room temperature to 300 and 3 °C/min from 300 to 500 °C. The wt% ratio of Cu to BiVO4 in composites was 0, 0.5, 1.0, 3.0, 5.0, 7.0, respectively. 2.2 Characterizations X-ray powder diffraction (XRD) analysis were performed on a Bruker AXS D8 Focus diffractometer, equipped with ˚ ). Ni-filtered and Cu Ka radiation source (k = 1.5406 A Surface morphology was observed with an SU8010 (Hitachi, Japan) field emission scanning electron microscopy (FE-SEM). The UV–Vis absorption spectra were obtained with a Shimadzu UV-2550 spectrophotometer equipped with an ISR-2550 integrating sphere attachment. 2.3 Measurement of photocatalytic activity The photocatalytic activity of composites was investigated by the photocatalytic degradation of MB solution under visible light irradiation. In a typical photocatalytic process, 40 mg catalyst was dispersed into 50 mL of 10 mg/L MB solution. The suspension was first stirred for 30 min in dark to establish adsorption–desorption equilibrium between MB dyes and photocatalyst, and then exposed to the visible light irradiation (500 W Xe lamp). At an irradiation interval of every 30 min, 5 mL solution was taken out and centrifuged at a speed of 3500 rpm for 10 min to remove the photocatalyst particles. The absorbance intensity of the centrifuged solution was measured by using UV-1081 spectrophotometer at 664 nm.

3 Results and discussion 3.1 X-ray diffraction Figure 1a presents the wide-angle XRD patterns of the asprepared samples. All the diffraction peaks corresponding to the (110), (011), (-121), (040), (200), (002), (211), (051), (240), (042), (202) and (161) planes, can be indexed to the monoclinic scheelite BiVO4 (JCPDS card No. ˚ , b = 11.701 A ˚, 14-0688, unit-cell parameters a = 5.195 A ˚ c = 4.092 A, b = 90.38°). Meanwhile no diffraction peaks of any other phases or impurities are observed, indicating that all of the samples are single phase. The splitting peaks at 2h = 18.9°, 35° and 47° can further demonstrate this observation. Moreover, the characteristic diffraction peaks of Cu species are not detected in all composite samples might be due to their low concentration or crystallization [28, 29]. In order to further study the effects of Cu modification on crystal structure, the details of XRD patterns of Cu modified samples in the range of 2h from 26° to 32° are

J Mater Sci: Mater Electron Fig. 1 a Wide-angle XRD patterns of as-prepared samples, b the details of XRD patterns of Cu modified BiVO4/MCM-41 samples in the range of 2h from 26° to 32°, c low-angle XRD patterns of MCM-41 and 5 %Cu–BiVO4/MCM-41

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shown in Fig. 1b. The positions of (-121) and (040) planes show a little shift to lower angle with the increased content of Cu, which might be ascribed to the lattice expansion due to the slight portion of Cu2? incorporation [30, 31]. Figure 1c shows the low-angle XRD patterns of as-prepared MCM-41 molecular sieve and 5 %Cu–BiVO4/MCM-41. Three characteristic diffraction peaks corresponding to (100), (110), (200) planes are observed in MCM-41 molecular sieve, revealing the long-range ordered hexagonal mesostructures of MCM-41. This observation clearly demonstrates that the as-prepared support is well-ordered MCM-41 in our experiment. Furthermore, the characteristic diffraction peak of (100) plane is also found in 5 %Cu– BiVO4/MCM-41 sample, which suggests that the pore channel structure of MCM-41 is not destroyed by loading BiVO4. In comparison to pure MCM-41, the much weaker intensity of (100) plane in 5 %Cu–BiVO4/MCM-41 sample might be resulted from filling of BiVO4 particles in the mesopore channels of MCM-41 [32]. 3.2 SEM Figure 2 shows the FE-SEM images of MCM-41, pure BiVO4 and 5 %Cu–BiVO4/MCM-41 samples. The as-prepared MCM-41 molecular sieve with sponge-like structure is presented in Fig. 2a, b. This rough surface of the support is convenient for catalyst particles to load. As shown in Fig. 2c, d, the uniform spherical BiVO4 sample composed of nanoparticles is obtained via simple sol–gel method, and the size of pure BiVO4 particle is about 2–3 um. The formation mechanism of spherical BiVO4 sample can be properly explained according to the model proposed by Kakihana [33]. It is obvious that the aggregation of catalyst particles has an important effect on the surface area and then catalytic activity. Thus, the aggregation of pure BiVO4 sample may be one key factor to its low catalytic activities. By comparison, the aggregation of pure BiVO4 particles can be effectively suppressed by loading MCM41. As shown in Fig. 2f, the morphology of 5 %Cu– BiVO4/MCM-41 sample is greatly affected by the original structure of MCM-41. Besides, the catalyst particles distribute equally on the surface of MCM-41 rather aggregate with each other, which should be responsible for the enhanced photocatalytic ability. 3.3 UV–Vis adsorption spectra The UV–Vis adsorption spectra of pure BiVO4 and 5 %Cu–BiVO4/MCM-41 samples are displayed in Fig. 3a. In comparison to pure BiVO4, the composite photocatalyst exhibits enhanced light absorption ability within the region of 510–800 nm ascribed to the Cu modification [34, 35]. Furthermore, the absorption edge of the composite catalyst

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shows a little red shift as compared to pure BiVO4. This observed red-shift could be attributed to a charge-transfer transition between the metal ions and the BiVO4 conduction or valance band. Similar phenomenon has also been observed in many ions modified BiVO4 composites [24, 36, 37]. The optical band gap values of the as-prepared samples can be estimated by using the Kubelka–Munk equation: ahm ¼ Aðhm  Eg Þn

ð1Þ

where a, hm, A and Eg are absorption coefficient, photon energy, constant and the band gap energy, respectively. The value of n depends upon whether the characteristics of the transition in a semiconductor is direct (n = 0.5) or indirect (n = 2) [38]. For the direct band gap of BiVO4, n is equal to 0.5 in the Eq. (1). The band-gap energy is determined by extrapolating the linear part of (ahm)2 versus (hm) plot to the energy axis at a = 0. The band gap energies of pure BiVO4 and 5 %Cu–BiVO4/MCM-41 are estimated to 2.46 and 2.36 eV as displayed in Fig. 3b. The above results indicate that Cu doped BiVO4/MCM-41 catalyst would have better photocatalytic activities due to its enhanced visible-light absorption and narrower band-gap. 3.4 Photocatalytic activity The photocatalytic properties of catalysts are evaluated by photodegradation of MB under visible light irradiation. Figure 4a presents the photocatalytic degradation efficiency of MB. All as-prepared Cu modified BiVO4/MCM41 composite catalysts show better photocatalytic activities than that of pure BiVO4, indicating that introducing MCM41 and Cu species play an important role in the enhancement of photocatalytic activity. On the one hand, loading BiVO4 with MCM-41 molecular sieve can suppress the aggregation of pure BiVO4 particles, and thus results in larger surface area and more active sites. On the other hand, Cu modification reduces the recombination of photogenerated electron–hole pairs, which prolongs the life time of charge carriers. It also can be seen from Fig. 4a that the content of Cu in composite catalysts is also a key factor to photodegradation of MB. With the increase of Cu content, the degradation efficiency raised at first, then decreased. The highest photocatalytic activity with a 97 % MB removal efficiency within 180 min is obtained by 5 %Cu–BiVO4/MCM-41 composite. As the content is up to 7 %, the excess Cu species may act as a recombination center for photo-generated carriers or cover the active sites on the catalyst surface and thereby reduce the degredation efficiency [39]. It was demonstrated that the photocatalytic degradation of MB by BiVO4 photocatalysts obeyed the pseudo-firstorder reaction kinetics behavior when the pollutant was

J Mater Sci: Mater Electron

Fig. 2 FE-SEM images of as-prepared samples (a, b) MCM-41, (c, d) pure BiVO4, (e, f) 5 %Cu–BiVO4/MCM-41

within the millimolar concentration range [29, 40]. The pseudo-first-order reaction kinetics can be expressed as follows: lnðC0 =Ct Þ ¼ kt

ð2Þ

where C0 is the initial concentration of MB (after string in the dark for 30 min), Ct is the concentration at irradiation time t (min), and k is the first-order rate constant. The linear relationship of ln(C0/Ct) versus time for degradation of MB using different catalysts is shown in Fig. 4b. As listed in Table 1, the first-order rate constant k of 5 %Cu– BiVO4/MCM-41 is about 3 times than that of pure BiVO4. Figure 5 shows the temporal evolution of the spectral changes of MB during the photo-degradation process over pure BiVO4 and 5 %Cu–BiVO4/MCM-41 samples. After establishing adsorption–desorption equilibrium, the absorbance of MB solution over 5 %Cu–BiVO4/MCM-41 is

smaller than that of pure BiVO4, revealing that the absorption performance of BiVO4 catalyst can be improved effectively by introducing MCM-41. As shown in Fig. 5a, b, the characteristic absorption peak of MB at 664 nm shifts gradually to blue region during the photocatalytic degradation process. Such blue-shift is characteristic process of N-demethylation derivatives of MB [41]. Furthermore, the composite catalyst has a larger blue shift and larger decrease in the absorbance of MB solution than pure catalyst demonstrating its better photocatalytic activity. 3.5 The possible mechanism of photocatalytic degradation of MB Figure 6 illustrates the schematic diagram of photocatalytic degradation of MB over pure BiVO4 and Cu–BiVO4/MCM41composite photocatalyst. During the photocatalytic

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Fig. 3 a The UV–Vis absorption spectra and b the band gap energies of pure BiVO4 and 5 %Cu–BiVO4/MCM-41

Fig. 4 a Photocatalytic degradation efficiency of MB over as-prepared samples under visible light and b the relationship between ln(C0/Ct) and irradiation time over pure BiVO4 and 5 % Cu–BiVO4/MCM-41

Table 1 Rate constants of MB photodegradation and linear regression coefficients from a plot of ln(C0/Ct) = kt with different samples Photocatalysts

R2

Regression equation

K (min-1)

Pure BiVO4

y = 0.00675x - 0.00598

0.99937

0.00675

5 %Cu–BiVO4/MCM-41

y = 0.02062x ? 0.05454

0.99498

0.02062

degradation process, the particles of composite catalysts suspend in the solution to absorb organic substances effectively. On the contrary, pure BiVO4 particles aggregate and float on the solution surface. Under visible-light irradiation, the composite photo-catalyst with narrower band gap can utilize more light than pure BiVO4 to energize the valence

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band electrons. After the excited electrons moving from valence band to conduction band, the charged holes are left in the valence band. The photo-generated holes can migrate to the surface of catalyst as strong oxidizing agent to oxidize the dye directly and can also react with H2O to give OH radicals [29, 42]. Meanwhile, the photo-generated electrons can be

J Mater Sci: Mater Electron

Fig. 5 Temporal absorption spectral patterns of MB during the photodegradation process over a pure BiVO4 and b 5 %Cu–BiVO4/MCM-41

O2 pure BiVO4

CB

Reduction

e- e- e - e-

O2

MB (dye) products MB (dye)

OH composite catalyst

Oxidation

VB

-

OH / H2O

h+ h+ h+ h+

products

BiVO4 semiconductor Fig. 6 Schematic diagram of photocatalytic degradation of MB dye

captured by the adsorbed O2 on the surface of the catalyst to reduce superoxide anion radical O2-, which would further interact with H2O to produce OH radical [43]. Both O2- and OH are attributed to the degradation of MB [24]. In addition, the dispersed Cu2? can also become the effective traps for excitation electrons, and thus prevents the recombination of electron–hole pairs effectively and prolongs the life time of charge carriers [42]. The reaction includes the following: BiVO4 þ hm ! BiVO4 ðhþ Þ þ BiVO4 ðe Þ 

BiVO4 ðe Þ þ O2 ! BiVO4 þ

O 2

ð3Þ ð4Þ

BiVO4 ðhþ Þ þ H2 O ! BiVO4 þ OH þ Hþ

ð5Þ

O 2

ð6Þ

þ H2 O ! OH þ OH



hþ =  O 2 =  OH þ MBðdyesÞ ! degradation products ð7Þ

4 Conclusions In this study, Cu–BiVO4/MCM-41 composite photocatalysts with monoclinic scheelite phase have been successfully synthesized by a simple sol–gel method. According to the structural studies, the lattice expansion of BiVO4 might be ascribed to the slight portion of Cu2? incorporation. Meanwhile, the composite catalyst maintained pore channel structure as well as MCM-41 support, which played an important role in suppressing the aggregation of pure catalyst particles. Moreover, Cu modification not only narrowed down the band gap of catalyst but also improved light absorption ability in visible light region. The best photocatalytic activity was obtained by 5 %Cu–BiVO4/ MCM-41 with a 97 % of MB removal in 3 h. In this composite photocatalyst, MCM-41 as support to immobi-

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lize catalyst particle and enhance the adsorption ability and Cu ions as electron capture center to prevent the recombination of electrons and holes, thus resulting in improved photocatalytic ability. So, these findings shed light on the synthesis of other composite photocatalyst with enhanced catalytic performance. Acknowledgments The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 21203170) and the National College Students’ Innovative Training Program (No. 201410491024).

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