BiVO4 and its activities of various photocatalytic conditions

Journal of Solid State Chemistry 229 (2015) 141–149

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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc

Microwave assisted synthesis of sheet-like Cu/BiVO4 and its activities of various photocatalytic conditions Xi Chen a,b, Li Li a,b,c,n, Tingting Yi b, WenZhi Zhang a,b, Xiuli Zhang b, Lili Wang b a

College of Materials Science and Engineering, Qiqihar University, Qiqihar 161006, China College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar 161006, China c College of Heilongjang Province Key Laboratory of Fine Chemicals, Qiqihar University, Qiqihar 161006, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 1 April 2015 Received in revised form 14 May 2015 Accepted 27 May 2015 Available online 3 June 2015

The Cu/BiVO4 photocatalyst with visible-light responsivity was prepared by the microwave-assisted hydrothermal method. The phase structures, chemical composition and surface physicochemical properties were well-characterized via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV– vis diffuse reflectance absorption (UV–vis/DRS), scanning electron microscopy (SEM), and N2 adsorption– desorption tests. Results indicate that the crystal structure of synthetic composite materials is mainly monoclinic scheelite BiVO4, which is not changed with the increasing doping amount of Cu. In addition, the presence of Cu not only enlarges the range of the composite materials under the visible-light response, but also increases the BET value significantly. Compared to pure BiVO4, 1% Cu/BiVO4-160 performs the highest photocatalytic activity to degrade methylene blue under the irradiation of ultraviolet, visible and simulated sunlight. In addition, the capture experiments prove that the main active species was superoxide radicals during photocatalytic reaction. Moreover, the 1% Cu/BiVO4-160 composite shows good photocatalytic stability after three times of recycling. & 2015 Elsevier Inc. All rights reserved.

Keywords: Microwave-assisted hydrothermal synthesis method Cu/BiVO4 Visible-light response Photocatalysis Methylene blue

1. Introduction In today’s world, people are not only enjoying the increasing achievements brought about by economy and science, but also facing with environment pollution at the same time. Chemical pollution is one of the conspicuous environment pollutions at present. To solve this problem, many kinds of the traditional methods have been used, such as biological treatment, activated carbon adsorption, electrolytic route etc. However, these methods usually have some disadvantages such as treating incompletely, high cost, narrow application scope and so on [1–3]. In recent years, semiconductor photocatalytic technology has become a novel type of eco-friendly treatment for pollutant; it can completely degrade and mineralize organic pollutant into inorganic ions, CO2, and H2O under the action of light. At present, the catalytic materials (such as ZnO, TiO2, etc.) are usually used in semiconductor photocatalytic technology, because of their advantages in better catalytic performance, high stability, and low costs. Das et al. have synthesized the titanium silicate Ti-SBA-15 material which shows excellent catalytic activity in the n Corresponding author at: College of Materials Science and Engineering, Qiqihar University, Qiqihar 161006, China. E-mail addresses: [email protected], [email protected] (L. Li).

http://dx.doi.org/10.1016/j.jssc.2015.05.026 0022-4596/& 2015 Elsevier Inc. All rights reserved.

degradation of methylene blue [4]. However, a lot of defects also have been reported, for instance, low photocatalytic efficiency, wide band gap, and only perform the photochemical activity in ultraviolet region [5–7]. Since these catalysts cannot utilize the sunlight efficiently, while the UV light (under 400 nm) in solar spectrum is less than 5%, and the wavelengths of visible light (400–800 nm) are accounted for 43%. Therefore, it is great meaningful for researches to improve photocatalytic materials with visible-light-response in the practical application. The inorganic semiconductor material BiVO4 performs a high photocatalytic activity because of its narrow band gap, and the light response range can reach more than 500 nm which has been used gradually in visible light photocatalysis at present. Some relevant researches show that the photocatalytic performance of BiVO4 is closely related with its crystalline phases [8,9]. It is known that BiVO4 appears in three main crystalline phases: monoclinic scheelite, tetragonal zircon, and tetragonal scheelite. Among them, monoclinic BiVO4 performs the best visible-light-driven photocatalytic activity owing to its narrow band gap (2.4 eV) which not only results in high recombination efficiency of photoinduced electron–hole pairs during the photocatalytic process, but also decreases the photo-quantum yield of monoclinic BiVO4 to some extent [10]. Dong et al. reported that T-shaped BiVO4 displays a high photocatalytic activity to degrade methylene blue under visible light irradiation [11]. However, the ability of BiVO4 to

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degrade organic under visible light remains to be further improved indeed. For this purpose, people begin to focus in modifying BiVO4 by doping in order to reduce the recombination of electron–hole pairs [12,13]. In the current study, doping precious metals on BiVO4 is a great modification method which has a remarkable effect on the recombination of photoinduced electron–hole [14–16]. Of course, according to some reports, other metal dopings also have a similar effect. For example, Guillodo et al. found that Cu doping can change electronic distribution of the composite material, then improve the efficiency of light quantum [17]. In addition, the studies of microwave synthesis have gradually increased in recent years which has become a novel kind of synthesis technology due to its simple operation, rapid heating, high purity, high yield even better reliability and reproducibility, etc. Because of the advantages of microwave radiation heating, for instance, simple production, uniform nucleation, and precipitation can quickly dissolve in the aqueous solution become saturated solution, so more and more researchers begin to use microwave assisted method for synthesis of the samples [18–20]. Accordingly, through the microwave-assisted hydrothermal method, Cu element was doped on BiVO4 in this paper. On the one hand, hoping that by Cu doping, the absorption of light can be changed effectively and shifts to the visible light region in order to achieve an efficient utilization of visible light. On the other hand, Cu doping can change the way of electronic conduction to some extent, and decreases the recombination of photogenerated electron–hole pairs [21]; thus, improves the efficiency of photocatalytic reaction of the composite material. Meanwhile, in this paper we not only prepared Cu doped BiVO4 photocatalyst by the microwave hydrothermal method, but also altered the time of microwave hydrothermal treatment and amount of Cu doping to further investigate the effects of experimental conditions on the structure and catalytic performance of photocatalyst. Moreover, the photocatalytic activities have been investigated under different photocatalytic conditions, such as ultraviolet, visible light, microwave-assisted photocatalyst and simulated sunlight. Finally, the possible photocatalytic mechanism of Cu/BiVO4 was discussed.

100 mL. The volume of mixture was less than 50 mL. The microwave hydrothermal reactions were carried out at 200 °C for y min (y refers to 140, 160 and 180, respectively). Finally, the products were washed with deionized water and absolute ethanol three times, and dried at 60 °C for 12 h in air. After milling, these products were marked for 1% Cu/BiVO4-140, 1% Cu/BiVO4-160, 1% Cu/BiVO4-180, 0.5% Cu/BiVO4-160 and 1.5% Cu/BiVO4-160, respectively. 2.3. Characterization The X-ray diffraction (XRD) patterns of the as-prepared samples were recorded using an X-ray diffractometer (Bruker-AXS (D8)) equipped with Cu Kα X-ray radiation. The morphologies of the as-prepared samples were examined using a scanning electron microscope (SEM) (HitachiS-4300). The Brunauer–Emmett–Teller (BET) surface areas, pore volume, and pore size distribution were determined by nitrogen adsorption isotherms at 77 K using a Quantachrome Nova Win II instrument. X-ray photo-electron spectroscopy (XPS) was performed using an ESCALAB 250Xi spectrometer equipped with Al Kα radiation at 300 W. The optical absorbance spectra of the as-prepared samples were recorded using a UV–visible spectrophotometer (TU-1901) equipped with a diffuse reflectance accessory, and BaSO4 was used as the reflectance standard. 2.4. Photocatalytic activity test The photocatalytic test was processed in a specially made double-wall beaker made of quartz with different optical sources in it. A 125 W Hg lamp was used as the UV light source (125 W Hg lamp with the maximum emission at 313.2 nm). A 400 W Xe lamp was used as the visible light source with the maximum emission over 410 nm (used specially made glass to filter UV light). A 1000 W Xe lamp was used as the simulated sunlight source, which made the distance at 8.5 cm away from the reaction solution. The photocatalytic experimental facilities and photocatalytic reaction processes see our group previously reported [22].

2. Experiment section 3. Results and discussion 2.1. Materials and instruments 3.1. XRD analysis Bismuth nitrate (Bi(NO3)3
5H2O, 99%) was purchased from Guangfu Fine Chemical Research Institute, Tianjin, China. Copper nitrate (Cu(NO3)2
3H2O, 99.5%) was purchased from Tianli Chemical Reagent Co. Ltd., Tianjin, China. Analytical grade methylene blue (MB), sodium hydroxide (NaOH), and ethyl alcohol (EtOH) were used as received without further purification. Deionized water was obtained from local sources. MDS-8G microwave reactor was purchased from Xin Yi Co., Shanghai, China. BL-GHX-V photochemical reactions instrument was purchased from Bilang Biotechnology Co., Xian, China. 2.2. Synthesis of Cu/BiVO4 In a typical process, 0.01 mol Bi(NO3)3
5H2O was dissolved in 20 mL distilled and stirred for 30 min, and 0.01 mol NH4VO3 was dissolved in 20 mL of boiled distilled water, remained the heat for 30 min. After that, the NH4VO3 solution was added dropwise into the Bi(NO3)3
5H2O solution to form a salmon suspension. After stirring for 20 min, different mass fractions x (x refers to 0.5%, 1% and 1.5%, respectively) of copper nitrate were added and then 5 mL of NaOH solutions was added to adjust the pH values of the suspensions to 8.5. The mixture was stirred vigorously for 30 min and then transferred into a PTFE-lined autoclave with a capacity of

To investigate the crystal structure of Cu/BiVO4, the XRD patterns of as-prepared samples are shown in Fig. 1. From XRD patterns, the diffraction peaks of as-prepared Cu/BiVO4 at 2θ values of 18.7°, 18.9°, 28.9°, 30.6°, 34.5°, 35.2°, 46.7° and 47.3° correspond to the crystal planes of the (110), (011), (121), (040), (200), (002), (240) and (042), respectively (JCPD standard card no. 14-0688). Moreover, the diffraction peaks correspond to crystal planes of (101), (200), and (204) were split into two peaks, respectively [23]. In addition, the peaks at 2θ values which are greater than 60° have been assigned to the XRD pattern of BiV1.025O4 þ x (JCPD standard card no. 44-0081), illustrate that there also have another phase in synthetic products. Generally, the crystalline phase of synthetic samples can be mainly well indexed to be monoclinic phase of BiVO4 and the diffraction peaks of each crystal planes were sharp, thus, confirms a better crystallinity. Meanwhile, after Cu doping, the crystalline phase of BiVO4 has no changed with amount of Cu doping or time of microwave irradiation increased. It is noted that any other diffraction peak of Cu was not be observed from XRD patterns. This result can be attributed to the low amount of Cu doping which cannot be detected. The average crystallite size of the as-prepared samples have been obtained from the full width at half maximum (FWHM) of

X. Chen et al. / Journal of Solid State Chemistry 229 (2015) 141–149

( 321 ) ( 123 )

( 200 ) ( 002 )

( 220 ) ( 051 ) ( 240 ) ( 042 ) ( 202 ) ( 161 )

( 040 )

( 110 ) ( 011 )

( 020 )

( 121 )

the most intense peaks of the respective crystals using the Scherrer equation, as shown in Table 1. d ¼ Kλ/(B cos θ), where d is the average crystallite size, λ is the X-ray wavelength, θ is the Bragg diffraction angle, and B is the full width at half-maximum [24]. Different time of microwave hydrothermal treatment has little influence on crystal cell parameters of 1% Cu/BiVO4 with the same amount of Cu doping. As the microwave hydrothermal time

pure BiVO4 1% Cu/BiVO4-140 1% Cu/BiVO4-160 1% Cu/BiVO4-180 0.5% Cu/BiVO4-160

1.5% Cu/BiVO4-160

10

20

30

40

50

60

70

80

2Theta Degree Fig. 1. XRD patterns of the pure BiVO4 and different Cu/BiVO4 samples.

Table 1 The crystal cell parameters (Å), crystallite sizes (nm) and energy band gaps (eV) of BiVO4 with different amount of Cu doping. Sample

Pure BiVO4 1% Cu/BiVO4-140 1% Cu/BiVO4-160 1% Cu/BiVO4-180 0.5% Cu/BiVO4-160 1.5% Cu/BiVO4-160

Crystal parameters

crystallites sizes/nm

a/Å

b/Å

c/Å

(121)

(040)

5.1849 5.1806 5.1834 5.1786 5.1684 5.1659

11.6609 11.6357 11.7344 11.7215 11.6901 11.6681

5.0838 5.1101 5.1147 5.1110 5.1063 5.1179

30.39 39.59 39.78 36.55 35.44 33.67

52.22 53.60 59.89 59.03 59.46 60.34

Eg/eV

2.25 1.96 1.90 1.91 2.03 1.84

143

increased, the growth rates of (121) and (040) crystal planes show a trend of increased, then decreased. Compared to pure BiVO4, after Cu doping, the average crystallite sizes corresponded to (121) and (040) crystal planes were increased, and the growth of (040) crystal plane was accelerated. However, excessive amount of Cu doping would restrain the growth of (121) crystal plane, led to form large lump-like crystals with hierarchical structure (see SEM). In addition, as the amount of Cu doping increased, the crystal cell parameters of as-prepared samples enhanced first, and then decreased. This result can be explained by XPS analysis. Cu ions existing in 0.5% Cu/BiVO4-160 and 1.5% Cu/BiVO4-160 were hybrid valences, whereas Cu ions existing in 1% Cu/BiVO4-160 were bivalence. Compared to the ionic radius of Cu2 þ , the ionic radius of Cu ions with other valences was larger, and got closer to the ionic radius of Bi3 þ . Therefore, Cu ions with other valences were easier to substitute for Bi3 þ in the lattice and then resulted in lower lattice parameters. However, Cu2 þ would squeeze in the crystal lattice in the form of interstitially doping which led to the enlargement of lattice parameters [25]. Thus, the crystallite size of 1% Cu/BiVO4-160 was larger than that of the others obviously. 3.2. UV–visible diffuse reflectance spectroscopy To understand the optical absorption properties of undoped and doped BiVO4 samples, the diffuse reflectance spectra of asprepared samples are shown in Fig. 2. According to Fig. 2(a), microwave irradiation time only has a little effect on optical absorption properties of Cu/BiVO4. However, since Cu doping, the absorption band edge of BiVO4 shifts to a longer wavelength. As the amount of Cu doping increased, the appearance of red shift becomes more obvious which attributes to the d orbital electron of Cu ions jumps into conduction band of BiVO4 [26]. Fig. 2 (b) indicates the band gaps of undoped and doped BiVO4 samples are deduced from plot of (ahv)1/2 versus energy (hv) by using tangent method, and their values of energy gap are presented in Table 1 [27]. As shown in Table 1, as the amount of Cu doping increased, the band gap of Cu/BiVO4 gradually decreases which results in the absorption of visible light of Cu/BiVO4 enhanced and improves its photocatalytic activity. 3.3. Scanning electron microscopy The morphologies of BiVO4 undoped and doped Cu were

1.0

1.0

0.8

0.8 1.5% Cu/BiVO -160 1% Cu/BiVO -140

Abs

0.6

0.6 (ahv)

1% Cu/BiVO -180

0.4 1% Cu/BiVO -160

pure BiVO

1.5% Cu/BiVO -160

0.5% Cu/BiVO -160

0.4

1% Cu/BiVO -160

0.2

0.5% Cu/BiVO -160

0.2

pure BiVO

0.0 200

300

400

500

600

Wavelength(nm)

700

800

0.0 2.0

2.5

3.0

3.5

hv/eV

Fig. 2. UV–vis/DRS spectra (a); plot of (ahv)1/2 versus energy (hv) of pure BiVO4 and different Cu/BiVO4 samples (b) visible light of Cu/BiVO4 enhanced and improves its photocatalytic activity.

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5 μm

1 μm

500 nm

5 μm

1 μm

500 nm

5 μm

1 μm

500 nm

5 μm

1 μm

500 nm

Fig. 3. SEM images of the pure BiVO4 (a–c); 0.5% Cu/BiVO4-160 (d–f); 1% Cu/BiVO4-160 (g–i); 1.5% Cu/BiVO4-160 (j–l).

investigated by scanning electron spectroscopy (SEM), as shown in Fig. 3. Fig. 3 clearly indicates the evolution process of the BiVO4 crystals by varying the amount of Cu doping. During the synthesis, the particles grew along the different directions at different growth rates, as the reaction proceeded, the crystal plane with the higher growth rate would disappear while the crystal plane with the lower growth rate would increase in area [28]. Fig. 4 shows

that the intensity ratios of (121)/(040) of BiVO4 decreased as the amount of Cu doping increased. Since BiVO4 remained a higher growth rate of the (121) crystal plane, compared to (040) crystal plane. Therefore, it consisted of a small amount of long-rod-like particles with the trunk length of 1 μm and a large amount of irregular short-rod-like particles with the branch length of hundreds of nanometers (a–c). After 0.5% Cu doping, the growth rate

X. Chen et al. / Journal of Solid State Chemistry 229 (2015) 141–149

145

3.5. N2 adsorption–desorption isotherms

I(121) /I (040)

2.5

2.0

1.5

1.0 0

0.5%

1%

1.5%

The mass fraction of Cu doped Fig. 4. Plot of I(121)/I(040) versus doping content of copper.

of the (040) crystal plane became faster than that of (121) crystal plane, the area of the (040) crystal plane became smaller while the area of the (121) crystal plane became larger, resulting in the transformation of long-rod-like particles from apexes to smooth. A large number of short-rod-like crystal particles gradually dissolved. The particles were transformed from the small-size grain surface to the large-size particle surface (d–f). Continued to increase Cu doping to 1%, found that the growth rate of (040) crystal plane became more accelerated, a large number of crystals disappeared to form sheet-like structure and the crystal surface became rough. When Cu doped amount was 1.5%, particles with the length of hundreds of nanometers began to aggregate to form larger particles with hierarchical structure (j–l). The SEM results indicate that Cu doping not only can increase the growth rate of (040) crystal plane, but also enhance the degree of aggregation of particles. 3.4. X-ray photoelectron spectroscopy The chemical states of elements on the surface of Cu/BiVO4 were further studied by X-ray photoelectron spectroscopy (XPS), and the results are shown in Fig. 5. The typical X-ray photoelectron survey spectrum of Cu/BiVO4 indicates that these catalysts consist of Bi, O, V, Cu, and C elements (Fig. 5a); therein to, C element comes from the instrument itself or environmental disturbance [29]. The signals of Bi4f7/2 and Bi4f5/2 at 158.9 and 164.2 eV (Fig. 5b) reveal that bismuth is in the state of Bi3 þ [30]. The O1s profile is asymmetric and can be fitted to two peaks locating at 529.3 and 531.2 eV, respectively, indicates two different kinds of O species in the samples. One is lattice oxygen, the other comes from adsorbed water, and vanadium is in the state of V5 þ (Fig. 5c) [31– 33]. The peaks at binding energies of 933.9 and 953.4 eV are the split signals of Cu2p3/2 and Cu2p1/2. The “shake-up” is the characteristic peak of Cu2 þ which is related to O, corresponding to the Cu2 þ species in the crystal (Fig. 5d) [34,35]. The “shake-up” at 942.4 eV also appears in Fig. 5e and f which confirms the Cu2 þ both existed in 0.5% Cu/BiVO4-160 and 1.5% Cu/BiVO4-160 samples. However, the binding energies of Cu2p3/2 and Cu2p1/2 of 0.5% Cu/BiVO4-160 and 1.5% Cu/BiVO4-160 both have a little chemical shift, compares to those of 1% Cu/BiVO4-160. The results show that Cu ions exist in 0.5% Cu/BiVO4-160 and 1.5% Cu/BiVO4-160 are hybrid valences [36,37]. As for the reason why the different amount of Cu doping changed its valence state, we need to further research.

To investigate the influence of Cu doping on physicochemical properties of BiVO4, the surface areas and pore structures of the pure BiVO4 and 1% Cu/BiVO4-160 samples were detected by N2 adsorption–desorption analysis, as shown in Fig. 6. The N2 adsorption–desorption isotherms of the synthesized pure BiVO4 and 1% Cu/BiVO4-160 samples both exhibit a type IV isotherm with a H3 hysteresis loop according to the classification of IUPAC. The formation of this type of isotherm and hysteresis loop was attributed to the cohesion of capillaries and the aggregation of platelike grains, respectively. According to pore size distribution, the samples both possess a mesoporous distribution. Moreover, the pore size distribution of 1% Cu/BiVO4-160 presents narrow, because of Cu doping, most of the particles of BiVO4 have formed a high packing structure, and pore size distribution has become uniform. In addition, as shown in Table 2, the BET specific area of composites was decreased, and then enhanced as Cu doping amount increased. The reason might be that the Cu ions existing in 0.5% Cu/BiVO4-160 and 1.5% Cu/BiVO4-160 were hybrid valences. Cu ions with larger radius substituted for Bi3 þ in the lattice resulting in a lot of defects generated on the surface of samples. Therefore, the BET specific area of 0.5% Cu/BiVO4-160 and 1.5% Cu/BiVO4-160 are larger than that of 1% Cu/BiVO4-160. 3.6. Photocatalytic performance of Cu/BiVO4 To investigate the influence of microwave hydrothermal time and amount of Cu doping on photocatalytic performance of Cu/BiVO4, MB used as molecule model to accomplish the UV photocatalytic test, and the results as shown in Fig. 7(a–d). When the time of microwave hydrothermal synthesis was 160 min, Fig. 7 (a) shows the UV photocatalytic activity of 1% Cu/BiVO4-160 was the highest of all. Fig. 7(b) shows that the influence of different amount of Cu doping on photocatalytic performance of Cu/BiVO4 when the time of microwave-hydrothermal was 160 min. The results indicate that 1% Cu doping presents the best photocatalytic activity under UV irradiation. To discuss the degradation rate of MB by BiVO4 undoped and doped Cu, the analysis of kinetics-experiments is shown in Fig. 7(c). According to the experiment, the kinetics equation can be expressed as follows:

1/(Ct /C0 ) = kt + b where k is the rate constant (min  1), C0 is the original MB concentration (mg L  1), Ct represents the concentration at reaction time t (mg L  1), and b is the constant. It is seen from Fig. 7(c), the linear relationship between the 1/(Ct/C0) and the reaction time t illustrates that the photocatalytic reactions follow the pseudo-second-order kinetics model. After calculating, the rate constants of various samples as well as direct photolysis, pure BiVO4, 0.5% Cu/BiVO4-160, 1% Cu/BiVO4-160 and 1.5% Cu/BiVO4-160 by decomposing MB under UV irradiation are 0.0137, 0.0620, 0.0143, 0.2441 and 0.0332 min  1, respectively. Therefore, the photocatalytic activities of various samples under UV irradiation are: 1% Cu/BiVO4-1604pure BiVO4 41.5% Cu/BiVO4-16040.5% Cu/BiVO4-1604direct photolysis. This result can be attributed to: When the Cu doping amount was less than 1%, Cu ions supplied a limited number of captures which just have weak suppression of the recombination of photogenerated electron–hole pairs; When the Cu doping amount was more than 1%, partial Cu ions possibly evolved into the recombination centers of photogenerated electron–hole pairs, thus, decreased the photocatalytic activity. In addition, the size of Cu nanoparticles has changed as the doping amount increased. Owing to the quantum size effect, the Fermi level of Cu nanoparticles decreased as size of the nanoparticles increased. While the Fermi level of Cu nanoparticles

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X. Chen et al. / Journal of Solid State Chemistry 229 (2015) 141–149

80000

600000

O1s

158.9 eV

Cu2p

V2p

400000

Bi4f5/2

60000 Intensity/a.u.

Intensity/a.u.

Bi4f7/2

Bi4f

C1s 200000

164.2 eV

40000

20000

0 0

200

400

600

800

1000

0

1200

156

160

164

168

Eb/eV

Eb/eV

30000 70000

Cu2p3/2

529.3 eV

29000

50000

V2p3/2 516.1 eV

531.2 eV

40000

V2p1/2

20000 510

515

520

525

shake-up

Cu2p1/2 953.4 eV

28000

27000

523.9 eV

30000

933.9 eV

942.4 eV

O1s

Intensity/a.u.

Intensity/a.u.

60000

530

26000

535

930

940

950

960

Eb/eV

Eb/eV

32000

34000

shake-up 31000

shake-up 942.4 eV

Cu2p1/2

Cu2p3/2 32000

955.3 eV

934.2 eV

Intensity/a.u.

Intensity/a.u.

33000

942.4 eV

Cu2p1/2

Cu2p3/2

955.1 eV

934.8 eV

30000

29000

31000

930

940

950

960

Eb/eV

930

940

950

960

Eb/eV

Fig. 5. XPS spectra of 1% Cu/BiVO4-160 (a), Bi 4f (b), V 2p and O 1s (c), Cu 2p (d), Cu 2p of 0.5% Cu/BiVO4-160 (e), and Cu 2p of 1.5% Cu/BiVO4-160 (f).

matched with photocatalyst BiVO4, the electrons can transfer effectively thereby suppressing the recombination of photogenerated electron–hole pairs. Because of the Cu doping, photogenerated electrons transferred from the high Fermi level of BiVO4 to the low Fermi level of Cu during the photocatalytic reaction until their Fermi levels were equivalent, thus, suppressed the recombination of photogenerated electron–hole pairs and enhanced the photocatalytic activities. Therefore, the amount of Cu doping should be commanded in an appropriate scope. To investigate the universality of Cu/BiVO4 by decomposing different dyes, the dependence of the degradation rate of different dyes under UV irradiation using 1%

Cu/BiVO4-160 is plotted in Fig. 7(d). The decolorization rates of MB, Malachite Green (MG), Rhodamine B (RhB), Methyl orange (MO) and phenol (PH) via using 1% Cu/BiVO4-160 as photocatalyst were 99%, 87%, 71%, 49% and 10%, respectively, after exposuring to UV light for 120 min. In addition, there are a lot of differences among the structures of these five reactants which belong to different kinds of organics. It can be seen that Cu/BiVO4 has a good universality for the degradation of dyes and performed a favorable photocatalytic activity. Although the degradation of phenol was worse but still higher than that of direct photolysis under UV light (only 1%). Because phenol does not belong to dye, so there is no dye sensitization effect

X. Chen et al. / Journal of Solid State Chemistry 229 (2015) 141–149

147

Fig. 6. Nitrogen-adsorption isotherms and BJH pore size distributions (insets (a,b)) of Pure BiVO4 (a) and 1% Cu/BiVO4-160 (b). Table 2 BET surface areas, average pore diameters and pore volumes of different synthesized products. Sample

SBET/(m2 g  1)

D/nm

Vtotal/(cm3 g-1)

Pure BiVO4 1% Cu/BiVO4-160 0.5% Cu/BiVO4-160 1.5% Cu/BiVO4-160

4.01 15.37 17.66 43.56

15.5 17.3 15.6 15.7

0.011 0.020 0.020 0.044

during photocatalytic process. Moreover, phenol has a good stable structure. To investigate the multimodal photocatalytic activities of BiVO4 undoped and doped Cu, a study of their photocatalytic experiments was recorded under the irradiation of ultraviolet, visible and simulated sunlight, as shown in Fig. 7(e and f). Fig. 7(a) shows that after illuminating for 180 min, the direct degradation of MB solution only was 23%. The photocatalytic activity has improved rarely while adding the BiVO4 into MB solution. This proves that monoclinic phase of BiVO4 has certain visible light catalysis activity. When the amount of Cu doping was 0.5% or 1.5%, the visible light catalysis activity of BiVO4 would decrease obviously. While Cu doped amount was 1%, the visible light activity of BiVO4 would be the highest which could degrade MB to 45% after illuminating for 180 min under visible light, even better than that of Degussa P25. In Fig. 7(f), five different photocatalysts all perform the favorable photocatalytic activities in the field of microwave and the photocatalytic activities of BiVO4 undoped and doped Cu are higher than that of Degussa P25. In addition, it is noteworthy that both photocatalytic activities of 0.5% Cu/BiVO4-160 and 1.5% Cu/BiVO4-160 was higher than that of 1% Cu/BiVO4-160 in microwave field. This result was different with other conditions of photocatalytic experiments. We considered the Cu ions existing in 0.5% Cu/BiVO4-160 and 1.5% Cu/BiVO4-160 were in the form of hybrid valences. This type of Cu ions led the samples to generate more defects in microwave field to some extent, which generated more active centers to improve the photocatalytic activities of samples in microwave field. To evaluate the photocatalytic activities of BiVO4 undoped and doped Cu under the simulated sunlight, MB used as molecular model to accomplish the photocatalytic experiment, as shown in Fig. 7(g). After illuminating for 180 min under the simulated sunlight, the MB was degraded to 58% by using 1% Cu/BiVO4-160 which has the best photocatalytic activities of all, thus, also has excellent application value. To assess

the recyclability of Cu/BiVO4, three cycles of photocatalytic degradation of MB were carried out by using 1% Cu/BiVO4-160 under illumination of UV light for 120 min. As shown in Fig. 7(h), there is no significant change in the degradation efficiency of 1% Cu/BiVO4-160 after three runs, which still remained good stability. 3.7. The possible mechanism of photocatalytic reaction To find out the major active species of Cu/BiVO4 during photocatalytic reaction, the capture experiment of 1% Cu/BiVO4-160 has been carried out. Tertiary butanol (TBA), an efficient hydroxyl radical (OH) quencher, ethyl alcohol (EtOH), a scavenger for hole (h þ ), and p-benzoquinone (BQN), a superoxide radical (O2−) quencher were added into MB solution, respectively. As demonstrated in Fig. 8, after illuminating without scavenger for 120 min, MB was almost degradated completely. However, the degradation of MB was retarded to some extent when tertiary butanol, ethyl alcohol and p-benzoquinone were introduced into the reaction system, respectively, indicates that superoxide radical play a major role in the degradation of MB during the photocatalytic reaction. Based on the experimental results, the possible mechanism of photocatalytic reaction has been speculated, as shown in Fig. 9. When the energy of light was greater than or equal to the band gap of BiVO4, electrons moved from valence band (CB) to conduction band (VB), forming the photogenerated electrons, meanwhile, leaving behind the same amount of holes in the VB. Accordingly, dissolved oxygen can be reduced by electrons to form superoxide radical anions (O2−) while holes oxidized H2O molecules to form hydroxyl radicals (OH). Superoxide radical anion and hydroxyl radical both have strong oxidizing properties; even can oxidize most of organics to CO2 and H2O as being the end product. However, during the excitation process, photogenerated electrons and holes got close to each other due to the electric attraction, leading to the recombination of electrons and holes. The charge carriers were redistributed when the amount of Cu doping was appropriate. Since the work function of Cu was lower than that of the BiVO4, the energy band of semiconductor would bend downward to form a Schottky barrier. Then, electrons do not move from a high Fermi level of BiVO4 to a low Fermi level of metal until their Fermi levels were the same [38]. Cu2 þ became the effective traps for excitation electrons, resulting in the separation of photon-generated carriers, thus, preventing the recombination of electrons and holes, and increasing the activity of photocatalyst immensely.

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X. Chen et al. / Journal of Solid State Chemistry 229 (2015) 141–149

Fig. 7. UV photocatalytic degradation methylene blue profiles with different time of microwave hydrothermal Cu/BiVO4 (a); UV photocatalytic degradation methylene blue profiles with different Cu doped amount of Cu/BiVO4 (b); UV photocatalytic degradation methylene blue kinetics with different Cu doped amounts of Cu/BiVO4 (c); results of 1% Cu/BiVO4-160 UV photocatalytic degradation for different dyes (d); visible-light photocatalytic degradation methylene blue profiles with different Cu doped amount of Cu/BiVO4 (e); microwave-assisted photocatalytic degradation methylene blue profiles with different Cu doped amount of Cu/BiVO4 (f); simulated sunlight photocatalytic degradation methylene blue profiles with different Cu doped amount of Cu/BiVO4 (g); the recycling use results of 1% Cu/BiVO4-160 under UV photocatalytic degradation (h).

X. Chen et al. / Journal of Solid State Chemistry 229 (2015) 141–149

1.0 1% Cu/BiVO4 -160 EtOH TBA BQN

0.8

149

Research of Heilongjiang Province Education Department, China (12511592), Government of Heilongjiang Province Postdoctoral Grants, China (LBH-Z11108), Open Project of Green Chemical Technology Key of Laboratory of Heilongjiang Province College, China (2013 year) and Postdoctoral Researchers in Heilongjiang Province of China Research Initiation Grant Project (LBH-Q13172).

C t/C 0

0.6 References

0.4

0.2

0.0 0

30

60

90

120

t/min Fig. 8. Reactive species trapping experiments.

Fig. 9. The photocatalytic reaction mechanism of Cu/BiVO4 composite.

4. Conclusions A series of monoclinic scheelite Cu/BiVO4 photocatalysts with high crystallinity were prepared by microwave-assisted hydrothermal method under different microwave hydrothermal time and diverse amount of Cu dopings. In addition, the phase structure of BiVO4 has been changed with the time of microwave radiation and the amount of Cu doping increased. The presence of Cu increased the absorption strength of photocatalyst under the visiblelight-response, and in different degrees of red-shift in the absorption band. Meanwhile, enhanced the growth rate of the (040) facets and the aggregation of crystals which formed the massive sheet-like nanostructures in the end. The BET values and average pore sizes of the composites both increased significantly. Moreover, the amount of Cu doping has influences on the degradation of MB by using Cu/BiVO4. When Cu doping amount was 1%, BiVO4 displayed the highest photocatalytic activities under the irradiation of ultraviolet, visible and simulated sunlight, compared to pure BiVO4 and other Cu/BiVO4. After doping, Cu2 þ as an electron capture center prevented the recombination of electrons and holes, thus, enhanced the photocatalytic activities of composite catalysts.

Acknowledgments The project was supported by the National Natural Science Foundation of China (21376126), Natural Science Foundation of Heilongjiang Province, China (B201106, B201314), Scientific

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