Regulate the crystal and optoelectronic properties of Bi2WO6 nanosheet crystals by Sm3+ doping for superior visible-light-driven photocatalytic performance

Journal Pre-proofs Full Length Article Regulate the crystal and optoelectronic properties of Bi2WO6 nanosheet crys-

tals by Sm3+ doping for superior visible-light-driven photocatalytic performance

Zhen Liu, Xingqiang Liu, Longfu Wei, Changlin Yu, Junhui Yi, Hongbing Ji PII: DOI: Reference:

S0169-4332(20)30065-9 https://doi.org/10.1016/j.apsusc.2020.145309 APSUSC 145309

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

21 October 2019 6 January 2020 6 January 2020

Please cite this article as: Z. Liu, X. Liu, L. Wei, C. Yu, J. Yi, H. Ji, Regulate the crystal and optoelectronic properties of Bi2WO6 nanosheet crystals by Sm3+ doping for superior visible-light-driven photocatalytic performance, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145309

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Regulate the crystal and optoelectronic properties of Bi2WO6 nanosheet crystals by Sm3+ doping for superior visible-light-driven photocatalytic performance Zhen Liu a,1, Xingqiang Liu b,1, Longfu Weia, Changlin Yua*, Junhui Yia, Hongbing Jia**

Affiliation: aSchool

of Chemical Engineering, Key Laboratory of Petrochemical Pollution Process and Control,

Guangdong Province, Guangdong University of Petrochemical Technology, Maoming 525000, Guangdong, China; bSchool

of Environmental Science and Engineering, Key Laboratory of Estuarine Ecological

Security and Environmental Health, Xiamen University Tan Kah Kee College, Zhangzhou 363105, Fujian, China

1These

authors contributed equally to this work

*Corresponding

Author:

Changlin Yu, Ph. D. Professor E-mail: [email protected] Hongbing Ji, Ph. D. Professor E-mail: [email protected]

1

Abstract: A series of novel Sm3+/Bi2WO6 nanosheets were successfully fabricated by a facile hydrothermal route. A number of physical chemistry characterizations and photocatalytic performance test showed that the doping of Sm3+ ions into Bi2WO6 caused the lattice contraction of Bi2WO6 crystal, which brought about the enhancement of specific surface area and surface OH groups. Moreover, Sm3+doping strengthened visible light absorption associated with oxygen vacancies. At the same time, oxygen vacancies generated by the substitution of Bi3+ with Sm3+ acted as the positive charge centers to trap the electrons, and consequently increased the life of photogenerated charge carriers. The obtained Sm3+/Bi2WO6 nanosheets were used to degrade different azo dyes, e.g. rhodamine B (RhB), methylene blue (MB), methyl orange (MO) and acid orange II (AOII). Sm3+(4at%)/Bi2WO6 sample showed the highest degradation activity, ~ 3 times of pristine Bi2WO6 with the degradation rate order of AOⅡ~ RhB > MB > MO. 100% AOⅡ and RhB can be degraded within 40 min visible light irradiation. Electron spin resonance and active species quenching experiments confirmed that superoxide radicals (·O2-) and h+ were the major radicals for dyes degradation in this Sm3+/Bi2WO6 system. Key words: Bi2WO6 nanosheet; Sm3+ doping; Lattice contraction; Oxygen vacancies; Dyes degradation

2

1. Introduction Nowadays, dyes and other organic chemicals are the main pollutants in wastewater in some countries with the fast development of dyes and chemical industries. The traditional water treatment technology is not easy to achieve complete removal of organic pollutants without producing secondary pollution. These organic pollutants are difficult to be eliminated by conventional wastewater treatment technologies [1, 2].

In the last few years, the rapid developing

photocatalysis technology has received great interest as a new green technology in degradation of all kinds of organic water pollutants [3-13]. Under the irradiation of light with appropriate wavelengths, most of the organic pollutants in water and air can be decomposed or mineralized via the photocatalytic reaction process into harmless or eco-friendly products, e.g. CO2, H2O, CO32-. In this process, the efficient photocatalysts are crucial to determine whether a photocatalysis technology can be put into practical process in water purification. In 1972, Fujishima and Honda [14] found that water can be decomposed to H2O and H2 on a TiO2 electrode. Then, TiO2 has become an extensive photocatalytical material for advanced oxidation of organic chemicals in water. However, the obvious defect of TiO2 with large band gap energy leads to a low utilization of solar spectrum and TiO2 only absorbs ultraviolet light with spectrum range of no longer than 387.5 nm [15-18]. Thus, it is important to explore other visible-light-driven (VLD) photocatalysts for taking full advantage of solar energy. Previous studies show that bismuth tungstate (Bi2WO6) semiconductor possesses VLD photoactivity [19-22]. Bi2WO6 is a n-type semiconductor with typically layered structure, being composed of the Bi2O2 layers and WO6 octahedral structure layers. Moreover, Bi2WO6 has a relatively small band-gap energy (2.6-2.8 eV), which allows it to be excited under visible light. But the recombination rate of photoexcited carriers in pristine Bi2WO6 is always high, and its photocatalytic activity is low, which largely limits its practical applications in environmental 3

remediation via solar light energy. Up to now, many efforts were devoted to improve the photocatalytic efficiency of Bi2WO6. The strategies include the morphologic control, e.g. the fabrication of nanocuboids [20], microsphere with layer structure [23], and microflowers [24], metal cations doping (Fe[25], Cu[26]), metal particles deposition (Ag[27, 28], Bi[29]), non-metal element doping(N[30], F[31,32]), semiconductor coupling (TiO2[33], ZnO[34], BiVO4[35], BiOI[36], Bi2S3[37], g-C3N4 [38]), carbon materials coupling (carbon quantum dots[39], graphene oxide [40,41], carbon nanotubes [42]), and etc. Among these strategies, metal ions doping is an effective route to enhance the photocatalytic performance. Hereinto, rare earth ions have rich 4f electronic configuration that could benefit the photochemical reaction process due to that the reactants could form complexes with their f-orbital, promoting the adsorption of reaction molecules [43, 44]. Also, the rich electrons favor the adsorption of O2, making more O2 transform into superoxide radicals by reduction reaction with captured electrons. Moreover, rare earth cations (Gd3+[45], Y3+[46] and Eu3+[47]) can replace the position of Bi3+ in Bi2WO6 crystal lattice, resulting in the improvement in crystal /texture property or light harvesting. For example, Luo et al. [48] have synthesized Sm3+ doped BiVO4 crystals by microwave hydrothermal method. They discovered that the doping of Sm3+ were beneficial for the separation of carriers. Tian et al. [45] found that the doping of Gd3+ ions with moderate amount into Bi2WO6 could significantly enhance the photocatalytic activity because of the rapid separation of electron-hole pairs via the electron shallow-trapping mechanism. Wang et al. [49] found that La and B co-doped BiVO4 exhibited much higher photocatalytic efficiency than that of single doped samples or pure BiVO4. In a recent research, Wang et al [50] reported that in the samarium and nitrogen co-doped Bi2WO6 nanosheets, samarium doping slightly improved the visible-light absorption of Bi2WO6 and N-codoping

strengthened

its

visible-light

absorption.

Under

visible-light

illumination,

Sm-N-Bi2WO6 showed obviously enhanced photocatalytic activity for degradation of RhB, which could be mainly attributed to that the existed Sm3+/Sm2+ pair redox centers can promote the charge

4

separation. In another samarium doped Bi2WO6, Zhang et al [51] found that doping of 0.5% Sm into -Bi2WO6 brought about the highest photocatalytic activity for RhB decomposition, which was ascribed to the good optical absorption and the large surface area. It is well known that the photocatalytic performance was influenced by multi-factors, e.g. the catalyst preparation parameters, the interactions between the reacted substrates. Here, with the aim to design the efficient solar energy driven Bi2WO6 photocatalysts by utilizing the rare earth Sm3+ as the dopant, a series of Sm3+ doped Bi2WO6 nanosheet photocatalysts were synthesized via a hydrothermal route. The effects of Sm doping with different concentration and hydrothermal condition on the texture and crystal property, optical absorption and photocatalytic activity were optimized. Their photocatalytic activities for the decomposition of azo dyes with different molecular structures under visible light illumination and photocatalytic mechanism were also intensively explored. 2. Experimental 2.1. Catalysts preparation All reagents used in experiment were of analytical grade. Sm doped Bi2WO6 crystals were fabricated using a hydrothermal process. A certain quantity of Bi(NO3)3·5H2O (10.0 mmol) and stoichiometric Sm(NO3)3·6H2O were added in 5 mL of 65% HNO3 and 15 mL deionized (DI) water and this solution was defined as solution A. Meanwhile, 5.0 mmol Na2WO4·2H2O was dissolved in 20 mL DI water and this solution was defined as solution B. Under magnetic stirring, the mixture was obtained by mixing the solution B and A. The pH value of the mixture solution was adjusted to ~ 7 using 2 M NaOH solution and amount of cetyl trimethyl ammonium bromide (0.1 g, CTAB) was finally added. After stirring for 2 h, the final mixture was placed in a 100 mL Teflon autoclave. The hydrothermal treatment temperature at 180 oC and the reaction time was 20 h, and then cooled until room temperature. The produced precipitated powder was filtered and washed several times with DI and ethanol, then dried at 60 oC for 12 h. The final amount of Sm in Bi2WO6 was obtained

5

from X-ray fluorescence analysis (Magix 601). The sample doped with x at% Sm was named as x% Sm/Bi2WO6. A series of Sm3+-doped Bi2WO6 samples (0%, 1%, 2%, 3%, 4%, 5% and 8%) were prepared. At the same time, 4% Sm3+/Bi2WO6 was treated at different hydrothermal temperatures (80 oC, 100 oC, 120 oC, 140 oC, 160 oC, 180 oC and 200 oC). 2.2. Catalysts characterization XRD patterns were tested by X-ray diffractometer (Bruker, D8, Advance) with Cu Kα (λ=0.15418 nm) radiation under 35 kV and 20 mA. The surface areas of the samples were measured by N2 adsorption data using an automatic analyzer (ASAP 2020). Scanning electron microscopy (SEM) of sample was measured with FLA650F type of the FEI company in the United States. The prime particle morphology of the samples was measured with transmission electron microscope (TEM) (CM-120, Holland Philips) with an energy dispersive X-ray spectrometry (EDX). X-ray photoelectron spectroscopy (XPS) was conducted on a physical electronics quantum 2000 photoelectron spectroscope by using Al KR radiation. The light absorption of the sample was tested by UV-Vis spectrophotometer (UV-2550, Japan) with the BaSO4 as the reference, and the scanning range was 250-700 nm. Fourier transform infrared spectra (FT-IR) of the samples were recored on infrared spectrometer (Nicolet-470) using KBr disk. The photocurrent density was analyzed by electrochemical workstation (CHI660D). With the graphite electrode, the standard calomel electrode and the samples were as the counter electrode, the reference electrode and the working electrode respectively, and the electrolyte was 0.1 M Na2SO4 solution. The electron spin resonance (ESR) spectra were recorded on an electron spin resonance spectrometer (Bruker ER200-SLC, Germany) with 5,5-dimethyl-1-pyrrolineNoxide (DMPO: 50 mM, 0.2 mL) in the aqueous or methanol solution with suspended photocatalyst. The operation parameters for ESR spectrometer were as follows: sweep width = 5 mT, center field =323.467 mT, microwave frequency = 9069 MHz, and microwave power = 0.998 mW. 2.3. Photocatalytic activity test

6

Different organic dyes such as rhodamine B (RhB), methylene blue (MB), methyl orange (MO) and acid orange II (AOII) were employed as the degradation target pollutant in aqueous solution under visible light irradiation by using 400 W metal halide lamp (XPA XuJiang Electromechanical Plant, Nanjing). 0.03 g catalyst was added to 80 mL of 20 mg·L-1 dyes. Before the photodegradation experiment, the suspension was treated for 10 min in an ultrasonic cleaner and then was magnetically stirred in the dark for 30 min. During the reaction process, the reactor was circulated by water to maintain at room temperature. After fixed irradiation intervals, ~2 mL aliquots of suspension were sampled, and the photocatalyst particles in solution were removed by high speed centrifugation. The dyes concentration was measured by UV-Vis spectrophotometer.

3. Results and discussion 3.1. Effect of Sm3+doping on crystal and texture properties XRD was applied to characterize the crystal properties of the obtained pure Bi2WO6 and Sm3+ doped Bi2WO6. Fig. 1(a) shows the XRD patterns of Sm3+ doped Bi2WO6 products with different Sm concentration synthesized at 180 oC. From it, it can be seen that the crystal phases of Bi2WO6 and Sm3+/Bi2WO6 are attributed to the orthorhombic crystal phase of Bi2WO6 (JCPDS 39-0256). The obvious diffraction peaks at 2θ of 28.30°, 32.79°, 47.15°, 55.82°, 58.54° and 76.07° are assigned to the crystal planes of (131), (002), (202), (133), (262) and (333), respectively. The strong peaks suggest the high crystallinity of these samples. Moreover, the doping Sm does not result in any new phases. To investigate the effect of Sm3+ doping on the Bi2WO6 crystal, we enlarged the main diffraction peak at 28.30° which is corresponding to (131) plane. As shown in Fig. 1(b), we found that the peak position of (131) diffraction peaks in the range of 2θ =27.5–29.5° shifts slightly toward a lower 2θ value with the increase of Sm contents from 1-5 at%. However, even high concentration of 8 at% did not cause further shift. Basing on Bragg’s equation [52, 53], the increase in lattice parameters (d(131) value) was leaded to the decrease of 2θ value. Due to the ionic radius follows the order of W6+ (0.062 nm)﹤Sm3+ (0.096 nm)﹤Bi3+ (0.108 nm), the shift of diffraction 7

peak toward higher 2θ value may be caused by the substitution of Bi3+ by Sm3+. However, in our case the diffraction peak shift to lower angles with Sm3+ doping could be due to that with relatively high concentration of doped Sm3+(over 1%), Sm3+ ions were also inserted into the interstices of the Bi2WO6 lattice, which results in the expansion of the crystalline lattice, as observed in the Mn2+ doped CsPbI2Br [54]. Hydrothermal temperature in the synthesis is an important parameter which influences the crystallization process. Fig. 1(c) gives the XRD patterns of 4 at% Sm3+/Bi2WO6 samples prepared at different temperatures. As seen from this figure, when the hydrothermal temperature increased, the diffraction peak of (131) also increased gradually. At 80 oC, almost no characteristic diffraction peaks of Bi2WO6 was found, which was unfavourable for the crystallization of Bi2WO6. When the temperature is below the boiling point of water (100 oC), there is no enough pressure and energy for the crystal growth of Bi2WO6 in the autoclave. At the temperatures from 160 to 200 oC, the Bi2WO6 shows almost the same strong diffraction peaks, showing that the temperature of 160 oC could completely achieve the crystallization of Bi2WO6 monoclinic phase. 8%Sm /Bi 2 W O 6

(333)

(262)

(133)

3+

Intensity (counts)

(202)

(002)

(131)

(a)

3+

5%Sm /Bi 2 W O 6 3+

4%Sm /Bi 2 W O 6 3+

3%Sm /Bi 2 W O 6 3+

2%Sm /Bi 2 W O 6 3+

1%Sm /Bi 2 W O 6 Bi 2 W O 6 10

20

30

40

50 60 2Theta (deg.)

70

80

90

8

(131)

(b) 3+

8%Sm /Bi2WO 6 3+

Intensity (counts)

5%Sm /Bi2WO 6 3+

4%Sm /Bi2WO 6 3+

3%Sm /Bi2WO 6 3+

2%Sm /Bi2WO 6 3+

1%Sm /Bi2WO 6 Bi2WO 6 28.2 28.4 28.6 2Theta (deg.)

In te n sity (c o u n ts)

29.0

29.2

29.4

(3 3 3 )

200℃

28.8

(1 3 3 ) (2 6 2 )

(1 3 1 )

(c)

28.0

(2 0 2 )

27.8

(0 0 2 )

27.6

180℃

160℃ 140℃ 120℃ 100℃ 80℃

10

20

30

40 50 60 2 T h etal (d eg.)

70

80

90

Fig.1 XRD patterns of Bi2WO6 and Sm3+/Bi2WO6 samples. (a) Sm3+/Bi2WO6 samples with different Sm3+ concentrations synthesized at 180 oC; (b) the enlarged diffraction peak at 28.30° of the Sm3+/Bi2WO6 samples with the different Sm3+ concentrations; (c) 4 at% Sm3+/Bi2WO6 samples prepared at different hydrothermal temperatures. Basing on the (131) plane, the average crystallite sizes of the samples were estimated by Scherrer equation: D=0.89λ/(βcosθ) to evaluate the influence of the doping Sm3+ on the crystal nucleation and growth and the results are presented in Table S1. Table S1 shows that the doping of Sm3+ with the concentration of 1 to 5% obviously decrease the average crystallite size (23.96-35.07 nm) compared with that of the bare Bi2WO6 (around 41.34 nm). However, the average particle size 9

of 8%Sm3+/Bi2WO6 is only slightly smaller than that of pristine Bi2WO6. The average crystallite size of 4%Sm3+/Bi2WO6 samples obtained at different temperature showed a trend of slight increase with the increase of temperature. N2 physical adsorption-desorption was used to examine the texture properties of the fabricated photocatalysts. Fig.S1 suggests the N2 adsorption/desorption isotherms and the pore size distributions for the typical Bi2WO6 and 4%Sm3+/Bi2WO6 (100 oC) samples. The isotherms in Fig. S1(a) can be identified as IV-type, so the sample belongs to the mesoporous (2-50 nm) materials. From Fig. S1(b) we can find that a majority of the pores of 4%Sm3+/Bi2WO6 are smaller than 26 nm, but the pore size of the pristine Bi2WO6 are smaller than 18 nm. 4%Sm3+/Bi2WO6 (100 oC) at P/Po= 0.991 has a larger pore volume of single point adsorption (0.17 cm-3·g-1) than that of the pristine Bi2WO6 (0.12 cm-3·g-1). Therefore, the Sm3+ doping enlarged the pore size and increased the pore volume of Bi2WO6. The obtained specific surface areas of all samples were shown in Table S2. We can see that the specific surface areas of Sm3+ doped Bi2WO6 samples prepared at 180 oC are larger than that of pristine Bi2WO6 and 4%Sm3+/Bi2WO6 shows the biggest surface area (12.32 m2/g). This could be due to the decrease of the average crystallite size induced by Sm3+ doping. As for the influence of hydrothermal temperature on the surface area of 4%Sm3+/Bi2WO6 samples, the total trend is that the increase in hydrothermal temperature induces a decrease in surface area. The reason is that at low temperature (80 oC), Bi2WO6 is in the amorphous state with a large suface area. But the continuous increase in temperature decreased the surface area. 3.2. Effect of Sm3+doping on morphologic structure Fig.2 shows the SEM photographs of the three typical samples. Fig. 2(a) suggests that Bi2WO6 sample was consisted of regular nanosheets with the diameter of about 50-200 nm. Fig. 2(c) was the SEM images of the 4%Sm3+/Bi2WO6 fabricated at 180 oC indicated that the regular nanosheets of Bi2WO6 were changed into irregular particles with smaller size. It should be noted that the doping

10

of Sm3+ had great influence on the total morphology of Bi2WO6. Fig. 2(b), the image of the 4%Sm3+/Bi2WO6 fabricated at 100 oC indicated these small particles further aggregated and formed small and loose particles, which could have bigger surface area than that of large nanosheets with smooth surface confirmed by the former BET test. With respect to 4%Sm3+/Bi2WO6 (180 oC) as shown in Fig. 2(c), the decrease in hydrothermal temperature caused the aggregated microspheric particles to be well aggregated.

Fig.2 The SEM images of the typical fabricated samples. (a) Bi2WO6 sample fabricated at 180 oC; (b) 4 at% Sm3+/Bi2WO6 fabricated at 100 oC; (c) 4 at% Sm3+/Bi2WO6 fabricated at 180 oC. The microstructure and morphology of the Bi2WO6 and 4%Sm3+/Bi2WO6 (100 oC) were further analyzed by TEM. Fig.3 displays the low magnification images and high resolution images. Fig.3(a) shows that Bi2WO6 appears perfect nanosheet morphology with the prime particles size of 20-50 nm. Obviously, 4%Sm3+/Bi2WO6 (100 oC) sample in Fig. 3(b) is no such regular morphology and the particles size is much smaller than that of Bi2WO6.The clear lattice spacings in Fig. 3(c) and Fig. 3(d) show that both Bi2WO6 and 4%Sm3+/Bi2WO6 have good crystallinity. ～ 0.313 nm is corresponding to the interplanar spacing of (131) crystal plane of Bi2WO6 (0.315 nm, JCPDS cards NO: 39-0256) and 4%Sm3+/Bi2WO6 displays the lattice spacing of ～0.316 nm. The larger lattice parameter could be induced by the substitution of Bi3+ by Sm3+. The EDX spectroscopy of 4%Sm3+/Bi2WO6 (100℃) sample is shown in Fig.4. Four elements (Bi, W, O and Sm) were obviously observed, which further confirmed the successful incorporation Sm3+ into Bi2WO6 nanosheets crystal. 11

Fig.3. The low magnification TEM images: (a) Bi2WO6, (b) 4%Sm3+/Bi2WO6 (100 oC); high resolution TEM images:(c) Bi2WO6, (d) 4%Sm3+/Bi2WO6 (100 oC).

Intensity(counts)

W

Bi

Bi Cu W O

Bi Sm

Sm 0

W

2

4

6

8 10 Energy(k eV)

12

14

16

18

20

Fig.4 EDX spectroscopy of 4%Sm3+/Bi2WO6 (100 oC) sample. 3.3. Surface composition and valence state of element in catalyst XPS was used to further detect the presence of Sm3+ and the valence state of each element on 12

pure Bi2WO6 and Sm3+-Bi2WO6. In the survey spectroscopy in Fig. 5(a), there are the characteristic peaks of Bi, W, O, Sm, and C elements in 4%Sm3+/Bi2WO6. The C 1s peak could come from the adsorption of CO2 on the sample surface or the polluted organic carbon instrument. The atomic percentage of Bi, W, O, Sm and C was 0.5, 0.5, 79.96, 1.24 and 18.7 from the XPS result. Fig. 5(b-c) shows the enlarged XPS spectroscopy of Bi 4f and W 4f, the peaks with binding energies of 158.3, 163.7 eV and 37.6 eV are attributed to the Bi3+ 4f7/2, Bi 4f5/2 and W 4f5/2 region for Bi2WO6, respectively [55]. In Fig. 5(d), the O 1s region can be divided into three peaks at 532.2, 532.7 and 533.4 eV, respectively. The peak at 532.2 eV would be representative of the O-vacancies that will be created in the lattice of the Bi2WO6 and the peak at 532.7 eV is indexed to lattice oxygen [56]. The third peak at 533.4 eV could be assigned to the hydroxyl groups on the surface of sample, respectively. The XPS spectroscopy of Sm 3d was shown in Fig. 5(e). Two weak peaks of Sm 3d were displayed at 1110.2 eV and 1083.3 eV and belong to Sm 3d3/2 and Sm 3d5/2, respectively, which confirmed that Sm3+ ions were doped into Bi2WO6.

Intensity(counts)

(a)

O 1s

Bi 4d Sm 3d Bi 4s

C 1s W 4f Bi 4f

1200

1000

800

600

400

200

0

Binding Energy (eV)

13

(b)

(c)

Bi 4f5/2

W 4f

Intensity(counts)

Intensity(counts)

Bi 4f7/2

168

166

164

162

160

158

156

154

39

Binding Energy (eV)

(d)

38

37 36 Binding Energy (eV)

(e)

34

Sm 3d5/2

Sm 3d3/2

Intensity(counts)

Intensity(counts)

35

535

534

533

532

531

Binding Energy (eV)

530

1115

1110

1105

1100

1095

1090

1085

1080

Binding Energy (eV)

Fig.5 XPS spectra of 4%Sm3+/Bi2WO6 sample. (a) XPS survey spectrum; (b) Bi 4f; (c) W 4f; (d) O 1s; (e) Sm 3d. 3.4. Effect of Sm3+doping on optical property The UV-Vis DRS of Bi2WO6 and Sm/Bi2WO6 are displayed in Fig.6. Fig. 6(a) suggests that the light absorption edge (λg) of Bi2WO6 is at ～428 nm and Sm3+ doping brings about the different degrees of shift of Bi2WO6 toward long wavelength direction. The increase in the intensity of visible light absorption induced by Sm3+ doping could be due to the excitations of the trapped electrons in localized states which are associated with oxygen vacancies just below the conduction band minimum [57]. The ahead of XRD analysis shows that the substitution of Bi3+ by Sm3+ caused the diffraction peak toward smaller angles. The oxygen vacancies could as positive charges centers easy trapped electrons [58, 59]. The electrons excited from local states to the conduction band can

14

cause the stronger visible light absorbance. Thus, the doping of suitable content of Sm3+ could create a number of oxygen vacancies and make the samples have better optical absorption properties. Fig.6(b) displays the UV-Vis DRS spectra of 4%Sm3+/Bi2WO6 samples synthesized at different hydrothermal temperatures. With respect to 4%Sm3+/Bi2WO6 (80 oC), the increase in hydrothermal temperatures could cause the enhancement of light absorption and obvious red-shift. That is probably due to that the sample prepared at 80 oC has poor crystallinity and no good monoclinic phase of Bi2WO6. Moreover, at very low temperature, Sm3+ is hard to be doped into the lattice of Bi2WO6, which could influence its light absorption. The increase in hydrothermal temperature could improve the crystallinity and the visible light absorption. The band gap energy (Eg) of the catalyst was estimated with Eg=1240/λg (eV), and the results are shown in Table S3 which indicates that Sm3+ doping obviously narrowed the band gap energy. Also, hydrothermal temperature also influenced the band gap energy and 4%Sm3+/Bi2WO6(100 oC) displayed the smallest band gap energy (2.73 eV). (a) 1.4 Bi2WO6

Intensity(counts)

1.2

3+

1% Sm /Bi2WO6 3+

2% Sm /Bi2WO6

1.0

3+

3% Sm /Bi2WO6

0.8

3+

4% Sm /Bi2WO6 3+

5% Sm /Bi2WO6

0.6

3+

8% Sm /Bi2WO6 0.4 0.2 0.0 250

300

350

400 450 Wavelength(nm)

500

550

600

15

Intensity(counts)

(b)

1.6

3+

o

4% Sm /Bi2WO6 (80 C) 3+

o

3+

o

3+

o

3+

o

3+

o

3+

o

1.4

4% Sm /Bi2WO6(100 C)

1.2

4% Sm /Bi2WO6(120 C) 4% Sm /Bi2WO6(140 C)

1.0

4% Sm /Bi2WO6(160 C)

0.8

4% Sm /Bi2WO6(180 C) 4% Sm /Bi2WO6(200 C)

0.6 0.4 0.2 0.0 300

400 500 Wavelength(nm)

600

700

Fig.6 The UV-Vis DRS spectra of Sm3/Bi2WO6 samples. (a) Samples with different Sm concentrations synthesized at 180

oC;

(b) 4%Sm3+/Bi2WO6 samples prepared at different

hydrothermal temperatures.

3.5. Surface properties of the catalysts The photocatalytic performance of catalyst is associated with their surface properties. FT-IR spectra were adopted to analyze the effect of Sm3+ doping on the surface chemical bond property of Bi2WO6. As shown in Fig.S2, over all spectra, two main kinds of peaks are identified. The absorption bands at 400-820 cm-1 are assigned to Bi–O, W–O stretching and W–O–W bridging stretching modes [60]. Moreover, the peak at 817 cm-1 belongs to the Bi–O and another peak at 734 cm-1 is ascribed to the W–O–W [60]. The two strong absorption peaks at 3430 cm-1 and 1630 cm-1 are attributed to the bending and stretching modes of the surface OH groups and absorbed water, respectively. The absence of absorption band at 400-800 cm-1 over 4%Sm3+/Bi2WO6 obtained at 80 oC

further confirmed no good Bi2WO6 crystal. In photocatalytic reactions, the OH groups of catalyst

can largely influence the activity, which can easily capture the photo-generated holes to generate hydroxyl radicals (·OH).

Hydroxyl radicals can effectively decompose organic dyes [61-63]. The

careful observation shows that pure Bi2WO6 and 1%Sm3+/Bi2WO6 samples display relatively weak

16

peaks at 3430 cm-1. However, these peaks for Sm3+/Bi2WO6 samples with the Sm concentration of 2-5at% become relatively stronger and broader, indicating that richer -OH groups could be produced because of the larger surface area or the improved property of the surface. As a result, the suitable content Sm3+ doping could effectively promote the photocatalytic activity. The influence of hydrothermal temperatures on the surface OH groups is not as obvious as the doping Sm3+ content. 3.6. Effect of Sm3+doping on photoelectrochemical properties How to analyze the separation of the photogenerated charge carriers is an important issue for photocatalytic process. Under xenon lamp irradiation, the charges separation can be analyzed indirectly from the photocurrent density and the strong photocurrent suggests the high charges separation probability [64-66]. The photocurrent-time curves of the samples under the intermittent xenon lamp light irradiation are shown in Fig.7. Once light was turned on, fast photocurrent responses occured over the three samples and the photocurrent rapidly decayed when light was turned off. It should pay attention to that the photocurrent density of Sm3+ doped Bi2WO6 is much higher than that of pristine Bi2WO6. Therefore, we can understand that Sm3+ doping can benefit the effective separation of photogenerated charges. Also, the photocurrent of 4%Sm3+/Bi2WO6 (100 oC) is slightly higher than that of 4%Sm3+/Bi2WO6 (180 oC).This enhancement of the photocurrent response could provide useful information for explaining the variations in photocatalytic degradation activity between pure Bi2WO6 and Sm3+ doped Bi2WO6.

17

o

3+

o

4% Sm /Bi2WO 6(180 C) o

4% Sm /Bi2WO 6(100 C) Light On

1.0 Current(uA)

3+

Bi2WO 6 (180 C)

1.2

Light Off

0.8 0.6 0.4 0.2 0.0

80

120

160 200 Irradiation times(sec)

240

Fig.7 The photocurrent-time curves of the Bi2WO6 and Sm3+/Bi2WO6 under the simulated solar light illumination. 3.7. Effect of Sm3+doping on photocatalytic performance The photocatalytic activity of the pristine Bi2WO6 and the as-prepared Sm3+ doped Bi2WO6 were measured by the decomposition of RhB under visible light irradiation. Fig.8(a) shows that all Sm3+ doped Bi2WO6 samples exhibit obviously higher photodegradation rates than pristine Bi2WO6, and the photodegradation rate of 4%Sm3+/Bi2WO6 is 3.3 times of pristine Bi2WO6 after 75 min light irradiation. Fig.8(b) demonstrates that the hydrothermal temperature has large influence on the photodegradation activity of 4%Sm3+/Bi2WO6. The 4%Sm3+/Bi2WO6 prepared at 100 oC displays the best photodegradation effect. Fig.8(c) displays the typical absorption variations of RhB over 4%Sm3+/Bi2WO6 (100 oC) under light irradiation. The main absorption peak declined sharply with time prolonging, indicating that RhB molecules were quickly decomposed [67]. The photodegradation rate has reached up to 100% within 40 min irradiation. To evaluate the photodegradation ability of 4%Sm3+/Bi2WO6 for other dyes, MB, MO and AO II were applied as the degradation target pollutant at the same condition {seen in Fig. 8(d)}, and 4%Sm3+/Bi2WO6 also displays the powerful decomposition abilities. The results of the degradation rate of dyes over all fabricated samples was summarized in Table S4 and the distinct role of Sm3+ doping in the

18

enhancement of photoctalytic performance of Bi2WO6 can be observed. (a) 1.0 blank test 0.8

0.6 Cx/Co

Bi2WO6 3+

1% Sm /Bi2WO6 3+

2% Sm /Bi2WO6

0.4

3+

3% Sm /Bi2WO6 3+

4% Sm /Bi2WO6 3+

5% Sm /Bi2WO6

0.2

3+

8% Sm /Bi2WO6 0

15

30 45 Time(min)

60

75

40

50

(b) 1.0

0.8

Cx/Co

0.6 3+

4% Sm /Bi2WO 80 oC 0.4 o 100 C o 120 C o 140 C 0.2 o 160 C o 180 C o 200 C 0.0 0 10

20

30

Time(min)

(c) 3.0

Absorbency(counts)

2.5 0min 10min 20min 30min

2.0 1.5 1.0 0.5 0.0 400

450

500

550 600 Wavelength(nm)

650

700

19

(d) 1.0

3+

o

4% Sm /Bi2WO6(100 C) 20 mg/L RhB 20 mg/L MB 20 mg/L MO 20 mg/L Acid Orange Ⅱ

0.8

Cx/Co

0.6

0.4

0.2

0.0 0

10

20

30

40

50

Time(min)

Fig.8 The photoactivity activities of the fabricated Bi2WO6 and Sm3+/Bi2WO6 samples. (a) Sm3+/Bi2WO6 samples with different Sm concentrations synthesized at 180 oC; (b) 4%Sm3+/Bi2WO6 samples fabricated at different hydrothermal temperatures; (c) UV–vis spectral variations of RhB over 4%Sm3+/Bi2WO6 (100 oC); (d) The decomposition of different dyes (RhB, MB, MO, AO II) over 4%Sm3+/Bi2WO6 (100 oC).

3.8. Discussions for the enhanced photocatalytic performance Generally, hole (h+), ·OH and ·O2- radicals were considered as the main reactive species in dyes decomposition reactions [68,69]. To reveal the photocatalytic mechanism of Sm3+ doped Bi2WO6, the trapping experiments of active species were carried out. Three different quenchers of p-benzoquinone (BZQ as ·O2- radicals scavenger), disodium ethylenediaminetetraacetate (Na2-EDTA as h+ scavenger), and tert-butyl alcohol (TBA as scavenger for ·OH radicals) were utilized. The experimental results in Fig.9 indicates that once 1 mM BZQ or Na2-EDTA was added, a fast deactivation over the 4%Sm3+/Bi2WO6 was observed, reducing the degradation rate of RhB from ≈100% to 51% (BZQ) and 26% (Na2-EDTA). The deactivation could be explained that the EDTA anions can act as holes trapper by adsorbed over the catalysts, so as to inhibit the photocatalytic activity of the 4%Sm3+/Bi2WO6 photocatalyst. The addition of BZQ into

20

4%Sm/Bi2WO6 can cause the capture and the decrease of active ·O2- radicals for photocatalytic reactions. Moreover, it can be also observed that the addition of TBA causes a slight decrease in activity, implying that ·OH radicals maybe the minor active species in the photocatalytic reaction. Therefore, in this 4%Sm3+/Bi2WO6 system, h+ and ·O2- are the main reaction radicals and ·OH plays a secondary role. 1.0

Cx/Co

0.8

0.6 3+

o

3+

o

4% Sm /Bi2WO6(100 C ) 0.4

4% Sm /Bi2WO6(100 C )+TBA 3+

o

3+

o

4% Sm /Bi2WO6(100 C )+BQ 4% Sm /Bi2WO6(100 C )+EDTA

0.2

0.0 0

10

20

30 Time(min)

40

50

Fig.9 The effects of the addition of quenchers on the degradation activity of RhB over 4%Sm3+/Bi2WO6 (100 oC) photocatalysts. Tert-butyl alcohol (TBA), benzoquinone (BZQ) and disodium ethylenediaminetetraacetate (EDTA). To further elucidate this conclusion, the ESR signals of the typical samples in aqueous solution or mehanol were obtained by DMPO spin-trapped ESR spectroscopy and the results are shown in Fig.10. Fig.10(a) shows that there is no ESR signals in the dark for the Sm3+/Bi2WO6 suspension, but the strong ESR signals of ·O2- and ·OH in aqueous solution and mehanol can be observed under visible light illumination for 12 min. In DMPO/methanol, the ESR signal of ·O2- radicals over 4%Sm3+/Bi2WO6 (100 oC) is obviously stronger than that of bare Bi2WO6, demonstrating that Sm doping brings about more ·O2- radicals under visible light irradiation. Moreover, the typical four-line EPR peaks assigned to the DMPO-·OH were also detected. Because the monolayer Bi2WO6 has the stronger OH radicals signal compared to the nanocrystals under visible light 21

illumination. Fig. 10(b) shows that compared to Bi2WO6, a large enhancement of the four-line EPR peaks was observed on 4%Sm3+/Bi2WO6(100 oC), demonstrating more ·OH radicals were generated over 4%Sm3+/Bi2WO6. Therefore, the DMPO spin-trapping ESR test gives the direct evidence for the role of Sm3+ doping in boosting the production of •OH and·O2- radicals. (a)

-

DMPO-¡¤O2

o

3+

4% Sm /Bi2WO6(100 C)

Intensity (counts)

12 min

In dark Bi2WO6

12 min

In dark

DMPO- ¡¤OH

3+

4% Sm /Bi2WO6(100℃)

12 min

Intensity (counts)

(b)

319.0

318.9

318.8 318.7 Magnetic Field (mT)

318.6

318.5

In dark

12 min

Bi2WO6

In dark 318.0

318.1

318.2

318.3

318.4

318.5

318.6

Magnetic Field (mT)

Fig.10 ESR spectra of (a) DMPO-·O2- and (b) DMPO-·OH of Bi2WO6 and 4%Sm3+/Bi2WO6 (100 oC).

According to above results, the role of Sm3+ in enhancing photocatalytic activity was proposed 22

in Fig.11. Firstly, the doping of Sm improved the texture property of Bi2WO6. N2 physical adsorption, and SEM tests confirmed that Sm doping decreased the particles size, increased surface areas and enriched surface -OH groups, benefiting particle dispersion in aqueous suspended system and dyes adsorption. Secondly, DRS and photocurrent measurements show that Sm doping resulted in remarkably improved visible light absorption and promoted the separation for photogenerated charge carriers. Thirdly, the doping of Sm could result in the generation of oxygen vacancies (VO••). The oxygen vacancies could promote the adsorption of oxygen [70] and trap temporarily the photogenerated electrons to restrain the recombination of charge carriers [71, 72]. The trapped photogenerated electrons can reacted with the adsorbed O2 to generate superoxide radicals (·O2-), which are powerful radicals to destroy the dyes molecules. The separated holes (h+) could also directly decompose the dyes. According to previous literatures [73, 74], we can roughly estimate the oxidation potential of the photogenerated holes (Bi5+) in the Bi2WO6 photocatalyst [75]. The standard redox potential of Bi5+/Bi3+ (E° = 1.59 V at pH 0) is more negative than that of OH•/OH- (+1.99) [76], suggesting that the photogenerated holes on the surface of Bi2WO6 is hard to react with OH-/H2O to yield •OH. But the monolayer Bi2WO6 has stronger OH radicals signal compared to the nanocrystals under visible light illumination [77]. Therefore, as shown in Fig.11, one possible reaction route would be that the Sm3+/Bi2WO6 can be excited to generate the photogenerated charge carriers under visible light irradiation. Subsequently, the electrons over the surface Sm3+/Bi2WO6 could be trapped by absorbed O2 and H2O to form •OH [78]. Another possible route for •OH generation could be that Sm doping change the energy structure of Bi2WO6 and the potential of Bi5+/Bi3+ could be promoted, which is in favor of more ·OH production. The generated ·OH radicals have strong oxidation ability to destroy

23

the chemical bond of dye molecules into H2O, CO2, and etc.

Fig.11 Proposed photocatalytic mechanism for Sm3+/Bi2WO6 system

4. Conclusions In this paper, a series of novel Sm3+/Bi2WO6 nanosheet photocatalysts have been synthesized via a hydrothermal route. The effects of Sm doping with different concentration and hydrothermal temperature on the crystal and optical property of Bi2WO6 were intensively explored. The Sm doped Bi2WO6 nanosheets, especially with the atomic ratio 4%, exhibited excellent photocatalytic performance in the decomposion of different dyes under the visible light illumination. Sm doping caused the lattice contraction of Bi2WO6 crystal, which brought about the enhancement of surface hydroxy, light absorbance and oxygen defects, and the suppression of recombination probability of photogenerated e- and h+. More h+, ·OH and ·O2- were available over Sm3+/Bi2WO6 for the dyes degradation reactions. This work indicates that the doping of rare earth Sm is a useful method for the development of the semiconductors with narrow band gap as efficient visible-light-driven photocatalystic materials.

Acknowledgments We acknowledged the funding from the National Natural Science Foundation of China (21567008, 21707055, 21938001, 219116074, 21425627), Project Supported by Guangdong 24

Province Universities and Colleges Pearl River Scholar Funded Scheme (2019), Guangdong Basic and Applied Basic Research Foundation (2019A1515011249, 2019A1515012130), the National Natural Science Foundation of China-SINOPEC Joint Fund (U1663220), Key-Area Research and Development Program of Guangdong Province (2019B110206002) and the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01C102), the Program for New Century Excellent Talents in Fujian Province University, Yangfan Talents Project of Guangdong province and Academic and Technical Leaders of the Main Disciplines in Jiangxi Province (20172BCB22018).

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 1These authors contributed equally to this work (match statement to author names with a symbol). The specific contribution is as following: Zhen Liu and Xingqiang Liu did the initial and supplemented experiment and collected the data, Longfu Wei and Junhui Yi analyzed the data and discussed the mechanism. Changlin Yu and Hongbing Ji conceived and designed the research project and wrote the paper.

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Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Professor Changlin Yu October 21, 2019

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Graphical abstract

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Highlights ► Doping of Sm3+ into Bi2WO6 induces the lattice contraction of Bi2WO6 nanosheet. ► Sm3+ lagely improves the texture propetry and visible light absorption of Bi2WO6 ► Oxygen vacancies trap the electrons, and suppressed the recombination of electrons and holes ► Sm3+ doping brought ~ 3 times increase in activity

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