Gamma Ray Radiation Effect on Bi2WO6Photocatalyst

Qiang ZhangZhi-wen JiangMo-zhen WangXue-wu Ge

CAS Key Laboratory of Soft Matter Chemistry,Department of Polymer Science and Engineering,University of Science and Technology of China,Hefei 230026,China

Key words:Bi2WO6,Gamma ray,Oxygen vacancy,Visible-light photocatalyst

I.INTRODUCTION

Bi2WO6,the simplest Aurivillius-type oxide with a narrow band gap of about 2.8 eV[1,2],is becoming more and more enthusiastic in view of their promising application in the photocatalytic degradation of organic pollutants[3–6].However,the photocatalytic performance of Bi2WO6remains to be improved due to its low light absorption efficiency and electron-hole separation efficiency[7–9].Thus,many methods have been developed to improve the photocatalytic activity of Bi2WO6,including the regulation of morphology[10–13],doping[14–16],or the preparation of composite materials[17,18].For example,Zhouet al.[19]directly prepared three dimensional highly hierarchical flower-like Bi2WO6microspheres which were formed by the selfaggregation of nanoplates in a hydrothermal way at low temperature.Zhuet al.[20]prepared F?substituted Bi2WO6(Bi2WO6?xF2x)photocatalysts with high activity by a two-step process.Sunet al.[21]synthesized hollow sphere shaped Bi2WO6/reduced graphene oxide(RGO)composites by a facile and cost-effective route and evaluated their photocatalytic activities by the degradation of five different kinds of pollutants under natural sunlight irradiation.

On the other hand,researches show that the oxygen vacancies in the Bi2WO6crystals can also improve their photocatalytic performance due to the reduction of the recombination of photo-generated carriers[22-25].For example,Zhuet al.[24]reported Bi2WO6?xnanoplates with wide-range-visible photoresponse by introducing surface oxygen vacancies through the controllable hydrogen reduction method.Chaiet al.[25]prepared Bi2WO6with oxygen vacancies(Bi2WO6-OV)by simple assisted solvothermal method using ethylene glycol as the reducing agent.However,as far as we know,there are no reports about the creation of oxygen vacancies in Bi2WO6under high energy radiation,although it is well-known that high energy radiation such asγ-ray can interact with semiconductors and generate defects[26–29],and be widely used for the synthesis of various inorganic or polymeric materials[30–34].

Herein,in this work,theγ-ray radiation effect on the crystal structure and light absorption characteristics of Bi2WO6was studied by using60Co asγ-ray source.The visible light photocatalytic degradation of MB in aqueous solution was also carried out to investigate the photocatalytic performance of the irradiated Bi2WO6.The results show that the oxygen vacancies can be created in Bi2WO6nanocrystals underγ-ray radiation at a high absorbed dose,which improves the visible light photocatalytic performance of Bi2WO6.

II.EXPERIMENTS

A.Materials

All reagents used in this work including Bi(NO3)3·5H2O,Na2WO4·2H2O,hexadecyl trimethyl ammonium bromide(CTAB),and methylene blue(MB)werepurchasedfrom Sinopharm Chemical Reagents Co.,Ltd.(Shanghai,China).The reagents were all analytical grade except that MB was BS grade.Deionized water was used in all experiments.

B.Synthesis of Bi2WO6nanosheets

The synthesis of Bi2WO6nanosheets was via a simple hydrothermal method[35].In a typical procedure,0.48 g of Bi(NO3)3·5H2O,0.16 g of Na2WO4·2H2O,and 0.025 g of CTAB were dissolved in 40 mL of deionized water with magnetic stirring for 30 min.Next,the mixed solution was transferred into a 50 mL Te flonlined stainless steel autoclave and stood for 24 h in an oven at 120oC.Then,the autoclave was cooled to room temperature naturally.The obtained product was collected by centrifugation,washed by water and absolute alcohol respectively for three times,and finally dried in an oven at 60oC for 12 h.To remove the remaining surfactant CTAB,the dried sample was heated to 600oC in air at a rate of 5oC/min in a muffle furnace and calcinated for 5 h.

C.Gamma ray radiation on Bi2WO6nanosheets

The as-prepared powder of Bi2WO6nanosheets(100 mg)was put in a 10 mL of plastic centrifuge tube and irradiated by60Coγ-ray(1.37×1015Bq,located in USTC,China)at a dose rate of 5.28 kGy/h for different total absorbed doses(127,253,380,and 507 kGy).The dose rate was calibrated using alanine/EPR standard dosimeter.

D.Characterization

X-ray diffraction(XRD)spectra were obtained on a TTRIII diffractometer(Rigaku,Japan)using Cu Kαradiation source(λ=1.5418 ?)at a scanning rate of 8o/min in the range of 10o?70o.The field-emission scanning electron microscopy(FESEM,JEOL JSM-6700F,5 kV)was used to observe the morphologies of Bi2WO6nanosheets.UV-Vis diffuse-re flectance spectroscopy was performed on Shimadzu SOLID 3700 UV-Vis spectrophotometer.The X-ray photoelectron spectroscopy(XPS)was carried out with Thermo ESCALAB 250 using monochromatic Al Kαradiation.Photoluminescence spectra were obtained using steadystate transient fluorescence spectrometer(Horiba JY Fluorolog-3-tau)with the excitation laser irradiation of 250 nm.

E.Photocatalytic performance of the irradiated Bi2WO6 nanosheets

The photocatalytic performance of the irradiated Bi2WO6was evaluated using the decomposition of MB under the exposure of simulated visible light irradiation as the model reaction.A 300 W Xe lamp(Perfectlight PLS-SXE300/300UV)with a cut-offfilter(λ>420 nm)was used as the light source.The powder of Bi2WO6nanosheets(40 mg)was dispersed into 40 mL of the aqueous solution of MB(5 mg/L).The suspension was placed in the dark environment and stirred vigorously for 1 h to establish an adsorption-desorption equilibrium between Bi2WO6and MB.Then,the system was exposed under the light.One milliliter of the suspension was sampled out at every 30 min,and Bi2WO6particles were removed by centrifugation.The concentration of MB in the supernatant was measured by UVVis spectrometry(UV-2401 PC,Shimadzu).The light absorbance of MB at the wavelength of 664 nm was recorded.The decomposition rate of MB(DMB)was calculated according to the following equation:

whereC0is the concentration of MB at the adsorptiondesorption equilibrium in dark environment,andCtis the concentration of MB after being irradiated by light for a time period oft.

After the decomposition of MB,the irradiated Bi2WO6nanosheets were recycled by centrifugation and washed by absolute alcohol to remove MB.After being dried at 60oC,the recycled Bi2WO6nanosheets were used for the next cycle of photocatalytic experiment.

III.RESULTS AND DISCUSSION

A.Effect of gamma ray radiation on the crystal structure of Bi2WO6nanosheets

Disk-like Bi2WO6nanosheets with a diameter of 400?800 nm and a thickness of about 100 nm can be facilely prepared by a solvothermal method[35],as shown in FIG.1(a).After being irradiated byγ-ray,the morphology of these disk-like Bi2WO6nanosheets has little change when observed by SEM even at an absorbed dose as high as 507 kGy,as exhibited in FIG.1(b)?(e).However,the color of Bi2WO6powders obviously changes from light yellow to light blue afterγ-ray radiation with an absorbed dose above 380 kGy,as shown in the insets of FIG.1(a)?(e),which means that there are some changes occurring in the physical structure of Bi2WO6nanocrystals after high-doseγ-ray radiation although the morphology has not been destroyed.

FIG.1 SEM images of Bi2WO6before(a)and after being irradiated by γ-ray at different absorbed doses:(b)127 kGy,(c)253 kGy,(d)380 kGy,and(e)507 kGy.The insets are the corresponding digital photos of the appearance of the samples.The dose rate was 5.28 kGy/h.

FIG.2 The XRD spectra(a)and the corresponding magnified views(b)of Bi2WO6nanosheets before and after being irradiated by γ-ray at different absorbed doses.

FIG.2displaystheXRD spectraofBi2WO6nanosheets after being irradiated at different absorbed doses,compared with that of the unirradiated sample.Basically,the distinct peaks at 2θvalues of 28.3o(113),32.9o(020),47.2o(220),55.8o(313),and 58.5o(226)(FIG.2(a)),which are indexed to the orthorhombic phase of Bi2WO6(JCPDS card No.73-1126),have been detected for both primary and the irradiated samples.It means that the unit cell of Bi2WO6remains unchanged underγ-ray radiation.However,the corresponding magnified views of the peak at(113)of Bi2WO6samples(FIG.2(b))show a regularly shift toward high angle with the increase of the absorbed dose.The similar shift has been reported in previous studies,which would be caused by shrinkage of the lattice spacing induced by the defects[23,36].

In order to con firm the weak change in the crystal structure of Bi2WO6nanosheets induced byγ-ray radiation,XPS spectra of the primary and irradiated Bi2WO6nanosheets have been measured,as shown in FIG.3.The peaks at 164.5 and 159.2 eV in FIG.3(a)are ascribed to levels of Bi 4f5/2and Bi 4f7/2spin-orbit splitting photoelectrons in the Bi3+chemical state of unirradiated Bi2WO6,respectively.However,these two peaks have a 0.1 eV shift to a higher binding energy,which can be contributed to the formation of neighbouring oxygen vacancies with higher electron attracting ability[25].The similar change also occurs for the peaks at 35.45 and 37.6 eV which are assigned respectively to W 4f7/2and W 4f5/2spin-orbit doublets in the W6+chemical state(FIG.4(b)).There is a 0.15 eV shift to a higher binding energy for the irradiated Bi2WO6.Correspondingly,the binding energy of O 1s at 530.10 eV shifts to 530.25 eV afterγ-ray irradiation,and splits into two peaks at 530.04 eV(Bi?O)and 530.66 eV(W?O)[24].The decrease in the relative intensity of the peak for Bi?O to that for W?O reveals that oxygen vacancies are formed due to the removal of O atoms connected with Bi atoms.

FIG.3(a)Bi 4f,(b)W 4f,and(c)O 1s XPS spectra of primary and irradiated Bi2WO6nanosheets(the absorbed dose:507 kGy).

The UV-Vis spectra of the primary and irradiated Bi2WO6nanosheets are shown in FIG.4. It is seen that both primary and irradiated Bi2WO6nanosheets can absorb the light with a wavelength of between 300 and 450 nm. The corresponding band gaps(Eg)of the samples estimated according to the equation(αhν)2=A(hν?Eg)are 2.83(primary),2.83 eV(127 kGy),2.82 eV(253 kGy),2.77 eV(380 kGy),and 2.80 eV(507 kGy),showing a slight drop of theEgof Bi2WO6nanosheets with the increase of the absorbed dose,which means that the irradiated Bi2WO6nanosheets theoretically have a better visible light absorption efficiency and photocatalytic performance than primary Bi2WO6.

FIG.4(a)UV-Vis diffuse re flectance spectra(b)and the corresponding plots of(αhν)2vs.hν of primary and irradiated Bi2WO6nanosheets.

It is known that the intensity of the steady-state photoluminescence(PL)spectrum can re flect the recombination rate of photogenerated carriers in semiconductor materials[37].FIG.5 displays the PL spectra of the primary and irradiated Bi2WO6nanosheets under the excitation of 250 nm laser.All of the Bi2WO6samples present broad-band emission spectra.The emission intensity of Bi2WO6has no change until the absorbed dose reaches 380 kGy,but it drops significantly when the absorbed dose rises up to 507 kGy.The results indicate that the oxygen vacancy defects in Bi2WO6generated under high-doseγ-ray radiation will make the charge carriers transfer fast and retard their recombination[38].This manifests that Bi2WO6nanosheets should have an improved photocatalytic performance after the exposure under high-doseγ-ray radiation.

B.The photocatalytic performance of irradiated Bi2WO6 nanosheets

FIG.5 Steady-state PL spectra of primary and irradiated Bi2WO6nanosheets.

The photocatalytic performances of Bi2WO6nanosheets irradiated byγ-ray at different absorbed dose have been investigated using the decomposition of MB in aqueous solution under simulated visible light irradiation(λ>420 nm)as the model reaction.FIG.6(a)shows a typically time evolution of the UVVis spectra of the aqueous solution of MB(5 mg/L)at the presence of the irradiated Bi2WO6nanosheets irradiated at an absorbed dose of 507 kGy.It is found that the absorbance of MB decreases gradually with the visible light exposure time.MB was decomposed mostly after 150 min. The similar decomposition behaviors of MB catalyzed by primary and other irradiated Bi2WO6nanosheets under the same conditions are displayed in FIG.S1 in supplementary materials.The real-time relative concentration of MB(C/C0)at the presence of different Bi2WO6nanosheets under the exposure of visible light is presented in FIG.6(b)according to FIG.6(a)and FIG.S1.It shows that the photocatalytic activity of Bi2WO6nanosheets is improved gradually with the increase of the absorbed doses,which is in accord with the above results,i.e.,the generation of oxygen vacancy defects and the lowering ofEgand the peak intensity of photo-induced fluorescence.The maximum DMB at the presence of Bi2WO6nanosheets irradiated at an absorbed dose of 507 kGy can reach 99.2%.

The cyclic photocatalytic activity ofBi2WO6nanosheets irradiated at an absorbed dose of 507 kGy on the decomposition of MB is shown in FIG.7.The catalytic performance of the recycled irradiated Bi2WO6nanosheets seems to be little changed after three cycles,indicating a good stability of the oxygen vacancy defects existing in the irradiated Bi2WO6nanosheets.

IV.CONCLUSION

FIG.6(a)The time evolution of the UV-Vis spectra of the aqueous solution of MB(5 mg/L)under visible light irradiation(λ>420 nm)at the presence of Bi2WO6nanosheets irradiated at an absorbed dose of 507 kGy.(b)The realtime relative concentration of MB(C/C0)at the presence of different Bi2WO6nanosheets.

FIG.7 The real-time relative concentration of MB(C/C0)under visible light irradiation at the presence of primary(1st cycle)and recycled(2nd and 3rd cycles)Bi2WO6nanosheets irradiated at an absorbed dose of 507 kGy.

In this work,the effect ofγ-ray radiation on the crystal structure of Bi2WO6nanosheets was first studied.The color of Bi2WO6nanosheets changed significantly after a high-doseγ-ray radiation up to 507 kGy.Although the morphologies of the irradiated Bi2WO6nanosheets observed by SEM have no change,the XPS and XRD spectra con firm the generation of oxygen vacancy defects and the resulted shrinkage of the lattice spacing in the irradiated Bi2WO6nanocrystals.The existence of oxygen vacancy defects loweredEgand the peak intensity of photo-induced fluorescence,which fa-vors to improve the photocatalytic activity of the irradiated Bi2WO6nanosheets.The decomposition ratio of MB in aqueous solution at the presence of the Bi2WO6nanosheets irradiated at an absorbed dose of 507 kGy can reach 99.2%under the exposure of visible-light for 150 min.The recycled irradiated Bi2WO6nanosheets also can remain the excellent photocatalytic activity,indicating a good stability of the generated oxygen vacancy defects underγ-ray radiation.This work provides a new simple way to improve photocatalytic performance of Bi2WO6through creating oxygen vacancy defects in the crystal structure byγ-ray radiation.

Supplementary materials:The photocatalytic performance of irradiated Bi2WO6nanosheets with different absorbed doses and the working curve for the determination of MB concentration are given.

V.ACKNOWLEDGMENTS

This work was supported by the National NaturalScience Foundation ofChina (No.51473152,No.51573174, and No.51773189), Science Challenge Project(No.TZ2018004),and the FundamentalResearch Fundsforthe CentralUniversities(WK3450000001 and WK3450000004).

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