A facile microwave-hydrothermal method to fabricate B doped ZnWO4 nanorods with high crystalline and highly efficient photocatalytic activity

Materials Research Bulletin 94 (2017) 298–306

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A facile microwave-hydrothermal method to fabricate B doped ZnWO4 nanorods with high crystalline and highly efficient photocatalytic activity Zhen Liua , Jian Tiana , Debing Zenga , Changlin Yua,b,d,* , Lihua Zhua , Weiya Huanga , Kai Yangc , Dehao Lid a

School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, Jiangxi, China School of Chemistry and Environmental Engineering, Wuyi University, Jiangmen 529020, Guangdong, China c State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, 350002, China d Faculty of Environmental and Biological Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, Guangdong, China b

A R T I C L E I N F O

Article history: Received 18 December 2016 Received in revised form 15 June 2017 Accepted 15 June 2017 Available online 16 June 2017 Keywords: Microwave-hydrothermal ZnWO4 nanorods Photocatalysis B doping High crystalline

A B S T R A C T

A series length of 50200 nm B doped ZnWO4 nanorods with high crystallinity were fabricated via a fast microwave-hydrothermal method. The effects of B doping on the physical properties of ZnWO4 nanorods were intensively characterized by N2 physical adsorption, XRD, TEM, FT-IR, XPS, ultraviolet-visible diffuse reflection spectrum (UV–vis DRS), photolunminescence spectrum (PL), transient photocurrent response (TPR). The results indicated that the doped B could replace the lattice tungsten ion in ZnWO4 crystals, which caused the lattice spacing contraction, particle size decrease, specific surface area increase. More importantly, the separation of the photogenerated electrons and holes in ZnWO4 was significantly enhanced by B doping. With doped optimum B concentration (2.42 wt%), B/ZnWO4 displayed super photocatalytic performance in dyes decomposition. The degradation rate constant (0.065 h 1) of Rhodamine B over B (2.42 wt%)/ZnWO4 is 4 times of that over ZnWO4 (0.016 h 1). © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, semiconductor photocatalysis has aroused great interest in both environment purification and converting light energy into chemical energy [1–9]. Especially in pollutants degradation, photocatalysis technology possesses some advantages, such as large degradation rate, high mineralization efficiency and low toxicity, ideally producing CO2 and H2O as the endproducts. Therefore, recent years have witnessed extensive work on the design and preparation of semiconductor photocatalytic materials [10–15]. The optical absorption properties and catalytic performances of semiconductor photocatalysts are closely related to their microstructures at the nanoscale (morphology structure, crystallinity, crystal phase, crystal plane, and etc). As one of important semiconductors, ZnWO4 has been widely studied as photocatalyst for mineralization of organic pollutants

* Corresponding author. School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, Jiangxi, China. E-mail address: [email protected] (C. Yu). http://dx.doi.org/10.1016/j.materresbull.2017.06.021 0025-5408/© 2017 Elsevier Ltd. All rights reserved.

under UV light irradiation [16–21]. ZnWO4 has a monoclinic wolframite structure, in which there are two coordination structures of [ZnO6] and [WO6] and the layered structure was formed. The unique interlayer of the photocatalyst could generate an electric field which is beneficial for the separation of the electrons and the holes [20–22]. However, up to now its application in industrial scale is rare because the photocatalytic activity of ZnWO4 is not high enough for the requirements of practical application. Therefore, different strategies, e.g. element doping, noble metal deposition, and semiconductor coupling, were developed to enhance the photocatalytic activity of ZnWO4. For example, Zhu and co-workers [23] found that doping fluorine ions could enhance the photocatalytic activity of ZnWO4 powder. Our previous investigation showed that Ag nanoparticles deposition can improve the photocatalytic performance of ZnWO4 in degradation of rhodamine-B (RhB) under UV light irradiation, which could be due to that the increased transfer rate of photogenerated electrons in the Ag/ZnWO4 photocatalyst [19]. After the semiconductor coupling, the produced ZnWO4 composites, such as ZnO-ZnWO4 [24], ZnWO4-CdS [25], g-C3N4/ZnWO4 [26], WO3/ZnWO4 [27], SnO2-ZnO-ZnWO4

Z. Liu et al. / Materials Research Bulletin 94 (2017) 298–306

(-113) (-132)

(202)

(021) (200) (-121)

(011) (110)

(100)

(a)

(-111)

[17], and etc. usually exhibit higher photocatalytic activity than that of pure ZnWO4. Moreover, our previous investigation indicated that B can be doped into TiO2 crystal lattice, the produced TiO2-xBx showed high activity in degradation of acid orange II [28]. However, up to now, no report has been made about the B doping effects on the physical structure and photocatalytic performance of ZnWO4 crystals. In this work, a series of B doped ZnWO4 nanorods with high crystallinity were successfully fabricated via a facile microwavehydrothermal process and the obtained B/ZnWO4 nanorods showed high photocatalytic activity.

Intensity(a.u)

B(3.11wt%)/ZnWO 4 B(2.42wt%)/ZnWO 4 B(1.73wt%)/Zn WO 4 B(1.04wt%)/ZnWO 4

299

2. Experimental 2.1. Synthesis of B doped ZnWO4 nanorods All chemicals including zinc acetate (Zn(Ac)2
2H2O), sodium tungstate (Na2WO4
2H2O) and hexadecyl trimethyl ammonium bromide (CTAB) were of analytical grade and used without further purification. A typical procedure for the preparation of the ZnWO4 nanorods is as follows. 0.024 g CTAB was dissolved in 30 mL distilled water (DI) and then 4 mmol Zn(Ac)2
2H2O was added. After stirring for 10 min, solution A was obtained. 4 mmol Na2WO4
2H2O and 10 mL DI water mixed, obtained solution B. Solution B was slowly added to solution A. After stirring for 30 min, the suspension was transferred to a 70 mL Teflon-lined autoclave and the autoclave was maintained at 180  C for 1 h in the microwave-hydrothermal oven. When the microwave-hydrothermal reactor was gradually cooled to room temperpure, the products were precipitated by centrifugation, and washed with distilled water and ethanol and finally dried at 100  C for 8 h. B doped ZnWO4 samples were prepared by the same method. Different concentrations of borax were dissolved in 10 mL DI water under sonication irradiation. Then the produced borax solution was slowly added to the above ZnWO4 suspension before microwave-hydrothermal treatment. The microwave-hydrothermal temperature and time are the same as the fabrication of ZnWO4. The amount of B doped in ZnWO4 was determined as 0.35,

B(0.35wt%)/ZnWO 4 pure ZnWO 4 10

20

30

40 2 θ(°)

50

60

70

80

(a)

B(3.11wt%)/ZnWO 4

(b) B(3.11wt%)/ZnWO4

Intensity(a.u)

B(2.42wt%)/ZnWO4 B(1.73wt%)/ZnWO4

Transmittance(%)

B(2.42wt%)/Zn WO 4

B(1.73wt%)/ZnWO 4 B(1.04wt%)/ZnWO 4 B(0.35w t%)/ZnWO 4

B(1.04wt%)/ZnWO4

pure Zn WO 4

B(0.35wt%)/ZnWO4

4000

3500

3000

250 0

29.5

30.0

30.5

31.0

31.5

1500

1000

500

-1

Wavenumber(cm )

pure ZnWO4 29.0

200 0

32.0

(b) B(3.11wt%)/ZnWO 4

2θ(°)

Table 1 The average crystal sizes and specific surface area of all samples. Sample

Crystal size (nm)

Specific surface area (m2/g)

Pure ZnWO4 B(0.35 wt%)/ZnWO4 B(1.04 wt%)/ZnWO4 B(1.73 wt%)/ZnWO4 B (2.42 wt%)/ZnWO4 B(3.11 wt%)/ZnWO4

12.5 12.3 11.9 10.9 9.3 7.7

21.3 22.0 22.0 30.3 38.2 47.2

B(2.42wt%)/Zn WO 4 Transmittance(%)

Fig. 1. X-ray diffraction patterns of the ZnWO4 and B doped ZnWO4 samples prepared by microwave-hydrothermal method. (a) Full diffraction patterns; (b) The enlarged (-111) peak.

B(1.73 wt%)/ZnWO 4 B(1.04 wt%)/ZnWO 4 B(0.35wt%)/Zn WO 4 pure ZnWO 4

1000

800

600

400

-1

Wavenumb er(cm ) Fig. 2. FT-IR spectra of the ZnWO4 and B doped ZnWO4 samples. (a) Full spectrum; (b) The enlarged absorption bands from 400 cm 1 to 1000 cm 1.

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Fig. 3. The TEM image of the catalysts. (a) ZnWO4; (b) B(2.42 wt%)/ZnWO4; (c) High resolution TEM image combined with a fast two-dimensional Fourier transform (FFT) pattern of B(2.42 wt%)/ZnWO4.

further analyzed by X-ray photoelectron spectroscopy on a PHI Quantum 2000 XPS system with a monochromatic AlKa source and a charge neutralizer. All the binding energies were referenced to the C1 s peak at 284.8 eV of the surface adventitious carbon. To investigate the recombination of photogenerated electrons/holes in the photocatalysts, the photoluminescence (PL) emission spectra of the samples were recorded. A 280 nm He-Cd laser was used as an excitation light source. The emission from the sample was measured by a spectrometer (Spex 500 M, USA) equipped with a photon counter (SR400, USA). Photocurrent measurements and Mott Schottky measurements were performed on a CHI 660E electrochemical work station (Chenhua Instrument, China) in a conventional three electrode configuration with a Pt foil as the counter electrode and an Ag/AgCl (saturated KCl) as the reference electrode. A 500-W Xe lamp served as a light source. 0.1 mol L 1 Na2SO4 aqueous solution was used as the electrolyte. The working electrodes were prepared as follows: 10 mg photocatalyst and 0.5 mL Nafion dispersing reagent were added in 5 mL absolute ethanol, and sonicated for 30 min. The slurry was then spread on a 1.0 cm  1.0 cm indium-tin oxide (ITO) glass substrate and dried in air. The photoresponse of the samples, as light on and light off, were measured at 0.0 V.

1.04, 1.73, 2.42 and 3.11 wt% by ICP-AES (Thermo ICAP 6300), and denoted as B (x wt%)/ZnWO4. 2.2. Characterizations The Brunauer-Emmett-Teller (BET) surface areas of the sample were obtained from N2 adsorption/desorption isotherms determined at liquid nitrogen temperature on an automatic analyzer (Micromeritics, ASAP 2010). The samples were outgassed for 2 h under vacuum at 180  C prior to adsorption. The crystal property of the fabricated products were characterized by powder X-ray diffraction (XRD) on a Bruker D8-Advance X-ray diffractometer at 40 kV and 40 mA for monochromatized Cu Ka (l=1.5418 Å) radiation. Fouriertranslation infrared spectroscopy (FT-IR) spectrum was recorded on a Nicolet 560 FT-IR spectrometer (USA). Samples were pressed by a KBr disk preparation apparatus. The optical property of the samples was analyzed by UV–vis spectrophotometer (UV-3600, Shimadzu, Japan). The transmission electron microscopy (TEM) images were recorded on a CM-120 microscope (Philips, 120 kV). Samples were ultrasonically dispersed in ethanol and deposited on thin amorphous carbon films supported by copper grids. The composition of the samples was

0.7 0.6 0.5

150

dV/dlogD

3

Volume adsorbed(cm /g,STP)

200

adsorption desorption

0.4 0.3 0.2

100

0.1 0.0

50

0

100

200

300

400

Pore width(nm)

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0) Fig. 4. N2 adsorption-desorption isotherms and corresponding pore size distribution curve of B(2.42 wt%)/ZnWO4.

Z. Liu et al. / Materials Research Bulletin 94 (2017) 298–306

(a)Global spectrum Zn2p

301

(b)

Zn2p3/2

Zn2s W4s W4f

Zn2p1/2

Intensity(a.u)

Intensity(a.u)

O1s

C1s W5s B1s

1200

1050

900

750 600 450 Binding energy(eV)

300

150

0

1052

(c)

1048

1044

1040 1036 1032 1028 Binding energy(eV)

(d)

W4f7/2

1024

1020

527

526

O1s

Intensity(a.u)

Intensity(a.u)

W4f5/2

48

46

44

42

40 38 36 34 Binding energy(eV)

32

30

28

533

532

531 530 529 528 Binding energy(eV)

(e)

Intensity(a.u)

B1s

196

194

192

190

Binding energy(eV) Fig. 5. XPS spectra of B(2.42 wt%)/ZnWO4 catalyst. (a) Full spectrum; High resolution XPS spectra:(b) Zn 2p; (c) W 4f; (d) O 1s; (e) B 1s.

2.3. Photocatalytic activity measurement Under UV light (300 W mercury lamp with main wavelength at 253.7 nm) irradiation, the photocatalytic performance of the prepared bare ZnWO4 and B-ZnWO4 were evaluated in degradation

of rhdamine-B(RhB), Acid orange-II(AO-II), Methylene Blue (MB) and methyl orange (MO) in an aqueous solution, respectively. For example, amount of photocatalyst (0.02 g) was suspended in 50 mL aqueous solution with RhB concentration of 0.010 g/L. Before lamp was turned on, the suspension was dispersed with sonication

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irradiation and then stirred in the dark for 30 min. During photocatalytic reaction process, the suspension was vigorously stirred and the temperature of suspension was maintained at 22  2  C by circulation of water through an external cooling coil. At given intervals of illumination, the sample of suspension was taken out and centrifuged. The clear upper layer solution was analyzed by using a UV–vis spectrometer. The dye concentration was measured at l = 552 nm, the maximum absorption wavelength for RhB. And the maximum absorption wavelengths for AO-II, MB and MO occurred at 483 nm, 661 nm and 462 nm, respectively. 3. Results and discussion

WO6 octahedra. The bands at 610 and 532 cm 1 were due to the symmetrical vibrations of bridge oxygen atoms of the Zn O W groups. The absorption bands at 473 and 430 cm 1 can be assigned to symmetric and asymmetric deformation modes of W–O bonds and Zn–O bonds in WO6 and ZnO6 octahedra, respectively [30,31]. Although these bands at 430–1000 cm 1 of B/ZnWO4 is similar to ZnWO4, the intensity of these peaks over B/ZnWO4 is strong. From Fig. 2(b), we can see that B doping caused the band at 600 cm 1 slightly shift to low wavenumber. Therefore, the boron doping can affect Zn-O-W bond. Moreover, the wide band in the 3400 cm 1 is attributed to OH stretching vibration [32]. It is evident that the prepared samples contain a significant amount of surface OH groups over the catalyst [15].

3.1. Crystal property 3.3. Morphology structure Fig. 1(a) shows XRD patterns of pure ZnWO4 and B doped ZnWO4 catalysts. The XRD patterns exhibit similar diffraction peaks, which could be easily indexed as monoclinic wolframite tungstate phase according to the standard card (JCPDS Card number: 73-0554), as indexed in Fig. 1(a). In pure ZnWO4, all the diffraction peaks at 18.84 , 23.68 , 24.42 , 30.46 , 36.18 , 41.08 and 53.54 were indexed to (100), (011), (110), (-111), (021), (-121), and (202) crystal plane, respectively. With the increase of B concentration, the intensity of diffraction peaks decreased gradually. In Fig. 1(b), careful observation indicates that B doping brought about a slight shift in the diffraction peaks to high angle. The atom radius for B ion (0.027 nm) is smaller than that for W ion (0.060 nm). According to Bragg’s law, d(hkl) = nl/(2sinu ),the diffraction angle u is inversely proportional to interplanar distance d; the increase in the value of u should result in a decrease in lattice parameters. Therefore, the diffraction peaks slightly shift to high angle. Based on the strongest (-111) diffraction peak, the average crystal sizes of all samples were estimated by Scherrer equation [29] and the results were summarized in Table 1. The result indicates that the average crystal size of B/ZnWO4 samples were smaller than that of pure ZnWO4, which could be due that B3+ radius (0.027 nm) is less than W6+ (0.060 nm), when boron ion occupied the position of lattice tungsten, a decrease in unit cell volume could take place. 3.2. FT-IR spectra analysis Fig. 2 shows the FT-IR spectra of ZnWO4 and B/ZnWO4 catalysts with different boron concentration. Pure ZnWO4 shows six main absorption bands at 430–1000 cm 1. The absorption bands at 834 and 877 cm 1 can be attributed to the stretching modes of W O in

B(0.35wt%)/ZnWO4 B(1.04wt%)/ZnWO4 B(1.73wt%)/ZnWO4 Absorption(a.u)

B(2.42wt%)/ZnWO4 B(3.11wt%)/ZnWO4

300

400 500 Wavelength(nm)

600

Fig. 6. UV–vis DRS of the ZnWO4 and B doped ZnWO4 samples.

3.4. Pore structure and specific surface area Fig. 4 shows the N2 adsorption-desorption isotherms and the corresponding pore size distribution curve (inset) calculated from the desorption branch of the N2 isotherm by the BJH method for B (2.42 wt%)/ZnWO4. The N2 adsorption-desorption isotherms of the sample show type IV according to Brunauer-Deming–Deming -Teller (BDDT) classification, indicating the existence of mesopores [33].The hysteresis loops can be classified as type H3[34,35]. The pore diameter concentrated in 30 nm to 50 nm. In addition, the specific surface areas of these samples are also listed in Table 1. With the decrease of average crystal size, the surface area of samples increased gradually. 3.5. Chemical states of B-doped ZnWO4 nanorods

pure ZnWO4

200

The morphologies of the as-prepared ZnWO4 and B (2.42 wt %)/ZnWO4 samples were further studied by TEM. Fig. 3(a) and Fig. 3(b) show the low magnification TEM images of the two samples. It can be clearly found that these two samples display similar morphology. Products are composed of perfect nanorods with a diameter of about 58 nm and length of 50200 nm. Fig. 3(c) is the high resolution TEM image (HRTEM) of one nanorod combined with a fast two-dimensional Fourier transform (FFT) analysis. The FFT pattern in Fig. 3(c) shows the spatial arrangement of the spots; it suggests that the set of lattice planes was derived from a single monoclinic crystal. The HRTEM image and FFT pattern demonstrate that the ZnWO4 nanorod grew along the (100) direction. The clear lattice boundary in the HRTEM image illustrates the high crystallinity of the nanorods. Three distinct lattice spacings (0.468 nm, 0.359 nm and 0.566 nm) are corresponding to the(100), (110) and (010) crystal planes, respectively.

700

X-ray photoelectron spectroscopy (XPS) was performed to analyze the element composition and state over the typical B (2.42 wt%)/ZnWO4 sample. Fig. 5(a) shows its XPS survey spectrum and indicates that this sample is mainly composed by Zn, W, O, C and B elements. The high-resolution XPS spectrum of Zn 2p3/2 and Zn 2p1/2 (Fig. 5(b)) were detected at the binding energies of 1022.67 eV and 1045.84 eV, respectively, indicating the Zn2+ in ZnWO4 [36,37]. In Fig. 5(c), the XPS spectrum of W 4f shows two peaks at the binding energies of 34.81 eV and 35.78 eV, corresponding to W 4f7/2 and W 4f5/2, respectively. The binding energy of the O 1 s peak at 530.6 eV might be due to the formation of (O-H) in Fig. 5 (d) [38] and the binding energy of O 1 s at 530 eV and 529.4 eV were attributable to Zn-O-B [39] and Zn-O [40]. Fig. 6(e) showed the B 1 s peak at 191.75 eV, indicating the B3+ in ZnWO4, which confirmed that B was successfully doped into ZnWO4. Binding energies of two peaks located at 192.64 eV and 191.13 eV

Z. Liu et al. / Materials Research Bulletin 94 (2017) 298–306

Table 2 Band gap energy of the ZnWO4 and B doped ZnWO4 samples.

ZnWO4 B(0.35 wt%)/ZnWO4 B(1.04 wt%)/ZnWO4 B(1.73 wt%)/ZnWO4 B(2.42 wt%)/ZnWO4 B(3.11 wt%)/ZnWO4

Eg (eV)

lg

3.44 3.42 3.54 3.56 3.62 3.55

360 362 350 348 343 349

0.15

(nm)

were attributable to B-O [41] and Zn-B [42].So we can infer that B could replace W in the ZnWO4 lattice.

Photocurrent(μA)

Sample

303

pure ZnWO4 B(0.35wt%)/ZnWO4 B(1.04wt%)/ZnWO4

B(1.73wt%)/ZnWO4 B(2.42wt%)/ZnWO4 B(3.11wt%)/ZnWO4

0.10

0.05

3.6. Light absorption 250

Ultraviolet-visible diffuse reflection spectrum (UV–vis DRS) was applied to analyze the enfluence of B doping on the light absorption of ZnWO4 nanorods. Fig. 6 shows the obtained UV–vis spectra of the fabricated samples. Pure ZnWO4 nanorods display the main light absorption in the UV light region at 380 nm. B doping obviously affected the light absorption property of ZnWO4. On the one hand, B doping with the concentration higher than 1.04 wt% slightly shifted the light absorption edge to blue region, which is more obvious when the B concentration is 2.42 wt%. On the other hand, a small new absorption peak at 370390 nm appears, which could be considered as the new absorption peak generated by B instead of W in the crystal lattice. To evaluate the effect of B doping on the band gap energy of ZnWO4 nanorods, the equation Eg = 1240/lg (eV) was applied to calculate the band gap energy of the obtained photocatalysts [43]. lg is the absorption edge, which was obtained from the intercept between the tangent of the absorption curve and the abscissa. The calculated band gap energies for the different samples are shown in Table 2. The band gap energy was estimated to be 3.44, 3.42,3.54, 3.56, 3.62 and 3.55 eV for ZnWO4, B(0.35 wt%)/ZnWO4, B(1.04 wt %)/ZnWO4, B(1.73 wt%)/ZnWO4, B(2.42 wt%)/ZnWO4 and B(3.11 wt %)/ZnWO4, respectively. We can see that B doping can slightly adjust the band structure of ZnWO4. With the increase of B content, the band gap of ZnWO4 first increases and the maximum band gap is 3.62 eV for the B(2.42 wt%)/ZnWO4. This is due to the BursteinMoss shift [44,45]. At low content, B3+ did not replace W6+ in B (0.35 wt%)/ZnWO4, but can dope into the ZnWO4 lattice to form

55

donor defect. When the content increased to 1.04 wt%, the level of hole defect began to produce by B3+ instead of W6+. Therefore, a small new absorption peak at 370390 nm appears in Fig. 6. However, the carrier concentration (ne) is still less than Mott transition critical concentration (nMott), and the defect energy level in the forbidden band is discrete [46,47]. Therefore, body effect can be ignored [48,49]. Burstein-Moss shift resulted in the band gap blue shift. When B content further increased to 3.11 wt%, the band gap decreased to 3.55 eV. The carrier concentration of ne is greater than nMott due to many-body effects and the valence band shifts. The perturbation of the valence band has a great influence on the band gap shrinkage [50]. At this moment, band gap shrinkage is greater than the B-M shift and a red shift occured. 3.7. PL properties and transient photocurrent response analysis In order to further investigate the effect of B doping on the seperation of the photogenerated electron-hole pairs in ZnWO4, photoluminescence (PL) analysis was applied to reveal the seperation and recombination process of photogenerated electron-hole pairs [51,52]. Fig. 7 presents the PL spectra of the ZnWO4 and B/ZnWO4 nanorods using a 465 nm excitation wavelength. Under the same excitation wavelength, B doped ZnWO4 and pure

B(1.04wt%)/ZnWO4 B(1.73wt%)/ZnWO4

40

B(2.42wt%)/ZnWO4 B(3.11wt%)/ZnWO4

30 25 20

5 2 -2 -9 1/C (F )*10

Intensity(a.u.)

450

6

45

35

400

Fig. 8. Transient photocurrent response of pure ZnWO4 and B doped ZnWO4 samples.

pure ZnWO4

50

300 350 Irradiation time(s)

4 pure ZnWO4 3 2

B(2.42wt%)/ZnWO4

-0.54 eV

15

1

10

-0.53 eV

5 0 500

520

540 560 Wavelength(nm)

580

600

Fig. 7. The photoluminescence (PL) spectra of the ZnWO4 and B doped ZnWO4 samples.

0 -0.8

-0.6

-0.4

-0.2

0.0

Potential(V) vs. Ag/AgCl Fig. 9. Mott Schottky (MS) plots of the pure ZnWO4 and B(2.42 wt%)/ZnWO4.

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(a)

(c)

1.0 0.9

pure ZnWO4 B(0.35wt%)/ZnWO4

3.0

B(1.04wt%)/ZnWO4

dark

0.8

B(1.73wt%)/ZnWO4

2.5

0.7 0.6 blank

0.5

pure ZnWO4

0.4

B(2.42wt%)/ZnWO4

2.0

ln(C0/C)

C/C0

3.5

B(3.11wt%)/ZnWO4

1.5

B(0.35wt%)/ZnWO4

0.3

B(1.04wt%)/ZnWO4

0.2

B(1.73wt%)/ZnWO4

0.1

B(2.42wt%)/ZnWO4

0.0

B(3.11wt%)/ZnWO4

-30

-20

-10

1.0 0.5 0.0 0

10 t(min)

20

30

40

0

50

20

t(min)

30

40

50

1.0

(d)

(b) 100

10

0.9

Degradation rate(%)

60

40

20 e pur

w .35 B( 0 O4 ZnW

t

Zn %)/

WO 4

0.8

C/C0

O4 nW )/Z % t O4 w nW .73 )/Z B(1 % O4 wt nW O4 .42 )/Z B(2 ZnW % / t ) w wt% .11 .0 4 B(3 B( 1

80

0.7 0.6 Acid Orange II

Methylene blue

0.5 0.4

blank

Methyl orange

pure Zn WO 4

B(2.42wt%)/ZnWO 4

0

0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 Time (min)

Sample

Fig. 10. Decomposition of rhodamine-B (RhB) test under UV light irradiation. (a) RhB concentration changes; (b) Degradation rate of RhB over different samples; (c) Pseudofirst-order rates of RhB degradation over different samples;(d) Acid orange-II(AO-II), Methylene Blue (MB) and methyl orange (MO) concentration changes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

ZnWO4 exhibited similar PL spectra excepting the PL peaks slightly shifted to the low wavelength direction. B doping only affected the intensity of the PL peak. Compared to pure ZnWO4, the emission intensity of 0.35 wt%, 1.04 wt%, 1.73 wt%, 2.42 wt%, and 3.11 wt% B doped ZnWO4 was significantly decreased. It should be noted that B(2.42 wt%)/ZnWO4 shows the weakest PL peak. Therefore, PL analysis indicated that the separation efficiency of photogenerated electrons and holes over ZnWO4 after B doping. The effective charges separation can play important role in promoting photocatalytic activities [53]. To furher illuminate the effect of B doping on the separation of the photogenerated electron-hole pairs, photocurrent response analysis was carried out under light irradiation. Fig. 8 shows the photocurrent responseses of pure ZnWO4 and B doped ZnWO4 samples under light irradiation. Compared with pure ZnWO4, B doped ZnWO4 composites exhibited an obviously enhanced photocurrent response. It should be pointed out that B(2.42 wt %)/ZnWO4 appears the strongest photocurrent intensity. The photocurrent response result further demonstrated that under light irradiation more efficient separation of photogenerated electron-hole pairs occurred in B(2.42 wt%)/ZnWO4. The analysis of photocurrent is in agreement with the results in PL analysis.

3.8. Mott schottky analysis To futher inverstigate the position of the conduction band and valence band and their absolute energies with respect to the reduction and oxidation levels, Mott Schottky (MS) measurements were performed in the darkness by using electrochemical work station [54] (Fig. 9). It has been found that the conduction band minimum in many n-type semiconductors is more negative (0.1 V) than the flat band potential [55].The flat band potential of pure ZnWO4 and B(2.42 wt%)/ZnWO4 electrodes were 0.53 V and 0.54 V vs Ag/AgCl, respectively. Based on the band gap (Table 2), the estimated position of valence band maximum were 2.81 V and 2.98 V vs Ag/AgCl for pure ZnWO4 and B(2.42 wt %)/ZnWO4, respectively. The position of valence band for B(2.42 wt Table 3 The degradation rate constant of RhB over diffrent samples. 1

Sample

k/h

ZnWO4 B(0.35 wt%)/ZnWO4 B(1.04 wt%)/ZnWO4 B(1.73 wt%)/ZnWO4 B(2.42 wt%)/ZnWO4 B(3.11 wt%)/ZnWO4

0.016 0.022 0.026 0.038 0.065 0.046

R2 0.99 0.99 0.99 0.99 0.99 0.99

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%)/ZnWO4 is more positive than ZnWO4
Therefore, the holes oxidation ability of B(2.42 wt%)/ZnWO4 is stronger than ZnWO4 [56,57]. The position of conduction band and valence band change is due to substitution of B lattice W ion [58]. 3.9. Photocatalytic activity The influence of B doping on the photocatalytic activity of ZnWO4 nanorods were evaluated by measuring the degradation of RhB in an aqueous solution under UV light irradiation (365 nm). Fig. 10 (a) shows RhB concentration changes during the photocatalytic reaction. The adsorption of RhB in the dark was gradually enhanced with the increase of B concentration, which could due to the increase in specific surface area induced by B doping. Fig. 10 (b) gives the degradation rate of RhB over different photocatalyst after 50 min light irradiation. Obviously, the photocatalytic activity of B doped ZnWO4 nanorods is much higher than that of pure ZnWO4. The degradation rate of RhB over ZnWO4 was 56.5%. But over B (2.42 wt%)/ZnWO4 the degradation rate of RhB reached to 96.13%. The kinetics of degradation of RhB were also investigated. A pseudo-first-order kinetic model ln(C0/C) = kt was used, where k is the apparent reaction rate constant, C0 is the initial concentration of RhB after the adsorption–desorption equilibrium, and C is the concentration of RhB at different light illumination interval. Fig. 10(c) revealed that the plot between ln(C0/C) and reaction time is approximate linear, indicating that the photocatalytic degradation process of RhB followed the first-order kinetics model. The degradation rate constant were calculated and listed in Table 3 revealed that the degradation rate constant (0.065 h 1) over B (2.42 wt%)/ZnWO4 is 4 times of that over ZnWO4 (0.016 h 1). Fig. 10 (d) shows other organic dyes concentration changes during photocatalytic reaction in 50 min. It can be found that ZnWO4 doped with boron also obviously improved the degradation activity for other dyes. The reason for the enhanced activity induced by B doping can be explained by the following aspects. Firstly, B doping obvious decreased the average crystal size and increased the specific surface area of ZnWO4 nanorods. The decrease in crystal size can reduce the transfer distance of charges, which could benefit the seperation of photogenerated electrons and holes. The increase in surface area of ZnWO4 nanorods could adsorb more dye molecules and harvest more light. Secondly, both PL and photocurrent response analysis suggested that under light irradiation, B doping induced more efficient separation of photogenerated electron-hole pairs in ZnWO4 nanorods. However, excess B doping might act as recombination centers for the photogenerated electrons and holes and reduced the effect on promoting the separation of photogenerated electron-hole [59]. 4. Conclusions A facile microwave-hydrothermal method was successfully developed to fabricate B doped ZnWO4 nanorods with high crystallinity and highly efficient photocatalytic activity. Under microwave heating condition, when the reaction temperature is 180  C and the reaction time is 1 h, B can be easy doped into ZnWO4 nanorods. The doped B could replace the tungsten ion in the lattice, which will lead to the decrease of particle size and increase in specific surface area. More importantly, the separation of the photogenerated electrons and holes was significantly enhanced. With doped optimum B concentration (2.42 wt%), B/ZnWO4 displayed super photocatalytic performance. The degradation rate constant (0.065 h 1) of RhB over B(2.42 wt%)/ZnWO4 is 4 times of that over ZnWO4 (0.016 h 1).

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