还原氧化石墨烯修饰Bi2WO6提高其在可见光下的光催化性能

[Article]

https://www.360docs.net/doc/538894135.html,

物理化学学报(Wuli Huaxue Xuebao )

Acta Phys.?Chim.Sin .2011,27(6),1482-1486

June

Received:January 18,2011;Revised:March 23,2011;Published on Web:May 9,2011.?

Corresponding authors.SHI Zu-Jin,Email:zjshi@https://www.360docs.net/doc/538894135.html,;Tel:+86-10-62751495.YANG Shang-Feng,Email:sfyang@https://www.360docs.net/doc/538894135.html,;Tel:+86-551-3601750.

The project was supported by the National Natural Science Foundation of China (20771010,20801052),National Key Basic Research Program of China (973)(2011CB932601),National High Technology Research and Development Program of China (863)(2007AA03Z311),and “100Talents Program of Chinese Academy of Sciences ”(A1010).

国家自然科学基金(20771010,20801052),国家重点基础研究发展规划项目(973)(2011CB932601)国家高技术研究发展计划项目(863)(2007AA03Z311)和中国科学院百人计划(A1010)资助

?Editorial office of Acta Physico ?Chimica Sinica

还原氧化石墨烯修饰Bi 2WO 6提高其在可见光下的光催化性能

应

红1,2王志永2郭政铎2施祖进2,*杨上峰1,*

(1中国科学技术大学材料科学与工程系,合肥微尺度物质科学国家实验室,中国科学院能量转换材料重点实验室,合肥

230026;

2

北京大学化学与分子工程学院,稀土材料化学与应用国家重点实验室,北京分子科学国家实验室,北京100871)摘要:

通过两步水热法合成了一种新型的还原氧化石墨烯(RGO)修饰的Bi 2WO 6(Bi 2WO 6-RGO),结果表明其

在可见光下的光催化性能得到了显著的提高.研究了RGO 在Bi 2WO 6-RGO 中的含量对其光催化性能的影响,从而确定出RGO 相对于Bi 2WO 6的最佳掺杂质量比值为1%.通过扫描电镜(SEM)研究发现,RGO 并没有改变Bi 2WO 6光催化剂的结构和形貌.Bi 2WO 6-RGO 在可见光下的光催化性能得以提高可以归功于RGO.其可能的

机理是石墨烯的存在有利于光生载流子(激子)的分离,从而导致产生更多的O 2●-用于有机染料污染物(如罗丹明

B (RhB))的降解.RhB 分子在石墨烯上的有效吸附可能也是导致Bi 2WO 6-RGO 光催化性能提高的另一原因.关键词:

Bi 2WO 6;石墨烯;水热法;光催化剂;有机染料污染物

中图分类号:

O643;O645

Reduced Graphene Oxide-Modified Bi 2WO 6as an Improved

Photocatalyst under Visible Light

YING Hong 1,2

WANG Zhi-Yong 2

GUO Zheng-Duo 2

SHI Zu-Jin 2,*

YANG Shang-Feng 1,*

(1Key Laboratory of Materials for Energy Conversion,Chinese Academy of Sciences,Hefei National Laboratory for Physical Sciences at Microscale,Department of Materials Science and Engineering,University of Science and Technology of China,Hefei 230026,P .R.China ;2Beijing National Laboratory for Molecular Sciences,State Key Laboratory of Rare Earth Materials Chemistry and Applications,College of Chemistry and Molecular Engineering,Peking University,Beijing 100871,P .R.China )

Abstract:A new and improved photocatalyst,reduced graphene oxide (RGO)-modified Bi 2WO 6(Bi 2WO 6-RGO),was synthesized by a two-step hydrothermal process.The effect of RGO content on photoactivity was investigated and the optimum mass ratio of RGO to Bi 2WO 6was determined to be 1%.Based on scanning electron microscopic study,RGO does not change the structure and morphology of the Bi 2WO 6photocatalyst.Therefore,the improvement in the photoactivity of the Bi 2WO 6-RGO composite is undoubtedly ascribed to RGO.The presence of graphene can facilitate the dissociation of photogenerated

excitons,which leads to more O 2●-to degrade dye pollutants like rhodamine-B (RhB).Moreover,the efficient adsorption of RhB molecules on graphene is another reason for the improved photoactivity.Key Words:Bi 2WO 6;Graphene;

Hydrothermal process;

Photocatalyst;

Dye pollutant

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YING Hong et al.:Reduced Graphene Oxide-Modified Bi2WO6as an Improved Photocatalyst under Visible Light No.6

1Introduction

In the past decade much attention have been paid to semicon-ductor as photocatalyst to degrade organic dye pollutant,1 which was difficult to be biodegraded.2Nowadays,titanium di-oxide(TiO2)has gained particular interest in this research field.3In spite of the success in converting light to chemical en-ergy,TiO2could not be used widely and practically in the solar environment because its activation is limited to UV light.To utilize solar energy sufficiently especially those within visible region,intensive research has been concentrated on photo-catalysts with high photocatalytic activity such as Bi2WO6,4-6 BiVO4,7and CaIn2O4,8etc.In particular,Bi2WO6,with a high degree of mineralization and layered by WO6and[Bi2O2]2+lay-ers,can lead to generation of active O2?-to decompose dye pol-lutant efficiently under visible light.4-6Bi2WO6with different structures,like three dimensional(3D)nanoparticles and two dimensional(2D)nanoplates,which could be synthesized through hydrothermal process by adjusting the pH value of the solution,shows diverse efficiency of photodegradation.9Lin et al.10reported that the efficiency of photodegradation was affect-ed by the separation of photoexcited electron-hole and adsorp-tion of organic molecules.In order to promote the separation of photogenerated electron-hole pairs,Zhu et al.11modified Bi2WO6with C60and found the synergetic effect of Bi2WO6 photocatalyst with C60could enhance photoactivity efficiently. Li et al.12modified Bi2WO6with low cost carbon and improved the photocatalytic performance due to the enhanced photogene-rated electron-hole separation and more RhB adsorption associ-ated with carbon.

Graphene,as a new member of carbon family,is a zero-bandgap semiconductor owing to its two-dimensional platelike structure.13In general,graphene has unique electronic proper-ties,large specific surface area,and high transparency.13-15The reducing of graphene oxide(GO)is an easy and widely-used route to obtain graphene sheets,16named reduced graphene ox-ide(RGO).Graphene has been incorporated with photocatalyst as enhanced photodegradation composite under visible light.17-20TiO2-RGO composites were obtained through a sim-ple hydrothermal process,and the excellent degradation of dye pollutants was ascribed to graphene?s adsorptivity,transparen-cy,and conductivity.17Graphene-Au exhibited high visible-light photocatalytic activity for dye degradation.18Recently, BiVO4was incorporated with RGO to enhance photoelectro-chemical water splitting reaction.19More recently,Wang et al.20 synthesized graphene-Bi2WO6composite via in situ hydrother-mal reaction and found the enhanced photocatalytic activity.

In this study,we have synthesized RGO-modified Bi2WO6 (Bi2WO6-RGO)as an improved photocatalyst via a two-step hy-drothermal process.A series of Bi2WO6-RGO with different loading ratios of RGO was prepared and their photocatalytic activity towards the photodegradation of rhodamine-B(RhB) under visible light(≥420nm)was investigated.2Experimental

2.1Preparation of Bi2WO6-RGO

2.1.1Synthesis of Bi2WO6

All chemical reagents were of analytical grade without fur-ther purification.Bi(NO3)3·5H2O,Na2WO4·2H2O,HNO3,and NaOH were used as the starting materials.Bi2WO6was synthe-sized by the hydrothermal method.Na2WO4·2H2O and Bi(NO3)3·5H2O(the molar ratio is1:2)were mixed together in deionized water(80mL),followed by ultrasonication for20min.After that,the pH value of the solution was adjusted to2by adding 0.5mol·L-1NaOH/HNO3.Then80mL of the final solution was transferred into a100mL Teflon-lined stainless steel auto-clave.The autoclave was sealed and maintained at180°C for 20h.After the reaction,yellowish product was collected via re-peated centrifugation and dispersion in water until pH value of the supernatant is7and then dried in an oven at80°C.

2.1.2Synthesis of GO

GO was prepared with the modified Hummers method.16 Briefly,graphite was mixed with NaNO3and H2SO4in an ice-water bath with stirring.KMnO4was added slowly into the pre-vious mixture with stirring for2h again.After that,stirring vig-orously for5days at room temperature was continued.With H2O2added into,the mixture was rinsed with a mixed aqueous solution of H2SO4/H2O2.Finally,the product was obtained after drying in an oven at80°C.

2.1.3Synthesis of Bi2WO6-RGO

Yellowish product Bi2WO6(1g)was dispersed in50mL de-ionized water and then ultrasonicated for1h.Different content of GO(0.5%,1%,2%,mass ratio of GO to Bi2WO6)was dis-persed rapidly in30mL ethanol with ultrasonic.Then the two solutions were mixed in a beaker under ultrasonic for30min and stirred for another3h to get a homogeneous suspension. Thereafter the80mL solution was transferred into the100mL Teflon-lined stainless steel autoclave and maintained in160°C for4h.Then the light gray product was collected by filtration, rinsed some times by deionized water,and dried in an oven at 80°C.For comparison,blank Bi2WO6(without RGO)for pho-tocatalytic experiment experienced this hydrothermal process again under the same condition without the addition of GO.

2.2Characterization

High-resolution transmission electron microscopy(HR-TEM)observations were carried out on Tecnai F20(America) with an accelerating voltage of300kV.The transmission elec-tron microscopy(TEM)images were obtained by Hitachi H-9000(Japan)instrument.X-ray photoelectron spectroscopy (XPS)data were obtained using an ESCALab250electron spec-trometer from Thermo Scientific Corporation(America)with monochromatic150W Al Kαradiation and the base pressure was about6.5×10-8Pa.The binding energies were referenced to the C1s line at284.8eV from alkyl or adventious carbon. Scanning electron microscope(SEM)images were carried out on FEI NanoSEM430instrument(America).The information of crystalline structures of Bi2WO6and Bi2WO6-RGO were ob-

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tained by an X-ray diffractometer (XRD)(Rigaku Dmax2000,Japan).UV-visible absorption spectra were recorded on a UV-Vis spectrophotometer (Hitachi UV-3100,Japan)in the wavelength range of 400-700nm.2.3Photocatalytic experiment

Photocatalytic activities of the Bi 2WO 6and Bi 2WO 6-RGO were evaluated by degradation of RhB under visible irradiation of a solar simulator (Newport 91160-1000,America)with a cal-ibrated illumination power density of 100mW ·cm -2.The visi-ble light was obtained through putting a cutoff filter (λ≥420nm)between the lamp and the solution.In this experiment,0.1g photocatalysts was added into 100mL RhB solution (5×10-6mol ·L -1).After stirring for 10min in dark,the solution was ex-posed to visible light irradiation with stirring.Then 3mL RhB solution was extracted each 30min for UV-Vis absorption spec-troscopic measurement.For comparison,the degradation of RhB irradiated without the addition of Bi 2WO 6-RGO under vis-ible light (λ≥420nm)and the degradation with the addition of Bi 2WO 6-RGO under dark condition were also measured as two reference experiments.

3Results and discussion

Fig.1shows the X-ray diffraction patterns of Bi 2WO 6with and without RGO,both of which exhibit characteristic peaks of Bi 2WO 6.Nearly identical peaks can be observed from the comparative figure of Bi 2WO 6and Bi 2WO 6-RGO (1%),indicat-ing that the crystalline structure of Bi 2WO 6does not change during the reduction of GO in 160°C for 4h.No any peak of RGO was observed in the XRD pattern of the Bi 2WO 6-RGO (1%)because of the small content of graphene used.

Fig.2shows the C 1s XPS spectra of GO and Bi 2WO 6-RGO (1%).C 1s XPS spectrum generally contains three characteris-tic peaks corresponding to nonoxygenated C ―C bond (284.5eV),C ―O (epoxy and hydroxyl)(286.6eV),and carboxylate C ＝O from carboxylic acid (288.9eV),respectively.19Fig.2in-dicates that GO,which was prepared with the modified Hum-mers method,has a considerable degree of graphene oxidation.After 160°C hydrothermal process for 4h in water and etha-nol,obvious decrease of the intensities of C ―O and C ＝O peaks of C 1s were observed for Bi 2WO 6-RGO (1%),indicat-

ing that efficient deoxygenation of GO occurred through the second hydrothermal process.

It has been reported that the photocatalytic activity of Bi 2WO 6for degradation of RhB under visible-light irradiation is dependent on the shape,size,and structure of the Bi 2WO 6.92D platelike structure could be changed to 3D flowerlike super-structure with the pH control or addition of surfactant.9From the low-magnification SEM image of Bi 2WO 6without RGO (Fig.3(a)),we can observe flowerlike spheres with coarse sur-face.The diameter of the flowerlike spheres is about 3μm.For comparison,SEM image of Bi 2WO 6-RGO (1%)was given in Fig.3(b).The similar shape,diameter,and structure of macro-spheres did not changed obviously from these images.Howev-er,while Bi 2WO 6precursor was transformed into crystal struc-ture in hydrothermal process,existence of GO may disturb the crystalline structure and lead to different photocatalytic activi-ty.6In our work,in order to exclude this additional influence,Bi 2WO 6with considerable photocatalytic activity would be ob-tained firstly and then modified with RGO for its excellent con-ductivity and transparency.According to the macroscopic shape of SEM images of these samples,the structure of Bi 2WO 6did not changed at all after RGO was attached to Bi 2WO 6.GO were adsorbed onto Bi 2WO 6plates or spheres first-ly and then transformed to RGO with the reduction of ethanol and water via hydrothermal process.As the content of RGO was very low,the platelike structure of the graphene could not be observed in these SEM images.

More structure details of the Bi 2WO 6-RGO were obtained from TEM and HRTEM observations (Fig.4).The “

graphene

1XRD patterns of Bi 2WO 6and Bi 2WO 6-RGO

(1%)2C 1s XPS spectra of GO and Bi 2WO 6-RGO

(1%)

Fig.3SEM images of Bi 2WO 6(a)and Bi 2WO 6-RGO (1%)(b)

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YING Hong et al .:Reduced Graphene Oxide-Modified Bi 2WO 6as an Improved Photocatalyst under Visible Light

No.6

cloth ”(RGO looks like a cloth)tangling with the Bi 2WO 6parti-cle was observed in the low magnification TEM image (Fig.4a).The high-magnification HRTEM image (inset of Fig.4b)exhibits a group of clear parallel crystal planes,which ascribe to (200)faces with interspacing of 0.272nm.And the poorly-crystallized carbon layer adhering to the surface of Bi 2WO 6particle is very clear in the HRTEM image (Fig.4b).From these TEM and HRTEM images we can infer that Bi 2WO 6was modified by RGO and the composite photocata-lyst was obtained.The presence of these “graphene cloth ”is important to improve the photoactivity because the excited electron of Bi 2WO 6in the visible irradiation could be trans-ferred to graphene rapidly.

The photocatalytic activity of Bi 2WO 6-RGO with different contents of RGO was evaluated by the degradation of RhB in aqueous solution under visible light.RhB showed a major ab-sorption band at 553nm.Before irradiation,the photocatalyst and RhB solution mixture were stirred for 10min,then the ad-sorption equilibrium of RhB on photocatalyst was reached.As the irradiation time increases,RhB is de-ethylated step by step.21During this process,the absorption intensity of 533nm peak decreases gradually and the characteristic peaks shifts to shorter wavelength with the color changing from pink to light green.Photocatalytic degradation of RhB by Bi 2WO 6with dif-ferent mass ratios of RGO to Bi 2WO 6(0%,0.5%,1%,2%)un-der visible irradiation was presented in Fig.5a.With the in-crease of the content of RGO,the photoactivity towards degra-dation of RhB is enhanced gradually at first for 0.5%-1%load-ing ratio but decreased with the higher loading ratio of 2%.With irradiation,Bi 2WO 6-RGO (1%)photocatalyst shows the highest velocity of photodegradation among different photocat-alysts under visible light.This result is similar to C 60-modified Bi 2WO 6,for which the optimum synergetic effect of C 60and Bi 2WO 6is reported at a mass ratio of 1.25%(C 60to Bi 2WO 6).11Fig.5b shows the UV-Vis absorption spectra of RhB during the photodegradation with Bi 2WO 6-RGO (1%)as the photocatalyst under visible light for different irradiation time.Graphene with appropriate content could accept the photogenerated elec-trons of Bi 2WO 6,and reduce O 2to O 2

?-rapidly which promotes the degradation of dyes.11

Meanwhile,graphene enhances the adsorption of RhB,which also improves the degradation of

dye pollutants.However,when the mass ratio of RGO to Bi 2WO 6reaches to 2%,the degradation of RhB slows down dramatically.This phenomena is interpreted by two reasons.First and mainly,thick and dense graphene layer could inhibit the inherent optical absorption of Bi 2WO 6and finally reduce the photoactivity.12Second,the excess GO could not be re-duced entirely during the hydrothermal process,and thus the ex-cited electron of Bi 2WO 6,which can reduce O 2to O 2?-,is used to reduce C ＝O and C ―O of the remnants GO.22

In order to identify the role of RGO in improving the photo-catalytic activity of Bi 2WO 6,the experiments should be carried out between pure Bi 2WO 6and Bi 2WO 6-RGO composite with identical crystal structure and macroscopic shape of Bi 2WO 6.It should be noted that,recently,Wang et al .20synthesized gra-phene-Bi 2WO 6composite through a similar hydrothermal meth-od and found much more enhanced photocatalytic activity to-wards photodegradation of RhB in terms of the shorter degra-dation time.The nano size of Bi 2WO 6is the main reason for shortening degradation time to several minutes.Nevertheless,the major difference between the work in Ref.20and our pres-ent work is that the Bi 2WO 6crystal in Ref.20is formed in the presence of GO (in situ ).It is known that the structure and

mor-

Fig.4TEM (a)and HRTEM (b)images of

Bi 2WO 6-RGO (1%)

Inset is the magnification of labeled

area.

5(a)Photocatalytic degradation of Bi 2WO 6-RGO with different mass ratios of RGO under visible light irradiation

C 0and C are the initial concentration of RhB under adsorption equilibrium and

the concentration of RhB at different irradiation time,respectively.For comparison,“dark ”is photodegradation curve of Bi 2WO 6-RGO (1%)in dark,“photolysis ”is photodegradation curve without the addition of any photocatalyst

under visible light.(b)UV-Vis absorption spectra of RhB during the photodegradation with Bi 2WO 6-RGO (1%)as the photocatalyst under visible

light for different irradiation time

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phology of Bi2WO6would be changed with an additive like sur-factant.9GO,as a water-soluble additive,might also change the structure and morphology of Bi2WO6and thereby lead to a dif-ferent photoactivity.Besides,Bi3+as metal ion would be ad-sorbed by carboxylic acid functional groups of the GO easily.23 Consequently,GO would be aggregated in the existence of Bi3+ between graphene layers.The aggregated graphene might de-crease light absorption of Bi2WO6.To exclude the possibility that different photoactivity is originated from different struc-ture of Bi2WO6instead of graphene modification,Bi2WO6was preferably prepared first and RGO was introduced in the second-step hydrothermal process in our work.Hence in this way the improvement of photoactivity of Bi2WO6-RGO can be undoubt-edly ascribed to RGO.

4Conclusions

Bi2WO6-RGO was synthesized by a two-step hydrothermal process:the first step is to obtain Bi2WO6with controllable mi-crostructure,and in the second step GO is reduced and at-tached to Bi2WO6.The application of Bi2WO6-RGO in photo-catalytic degradation of RhB is studied.The modification of RGO does not affect the structure and morphology of Bi2WO6 as confirmed from XRD and SEM studies,which is important for elucidating the role of RGO.Graphene accepts photogene-rated electrons of Bi2WO6efficiently under visible light and promotes the dissociation of photogenerated excitons of Bi2WO6.More electrons on the graphene lead to faster reduc-tion of O2to O2?-which promotes the degradation of RhB.More-over,the existence of RGO on the surfaces of Bi2WO6particles can enhance the adsorption of RhB.Effect of content of RGO on the photoactivity was discussed and the optimum loading ra-tio was determined to be1%(mass ratio of RGO to Bi2WO6). Nevertheless,the photoactivity decreases when the mass ratio of RGO to Bi2WO6is2%,which may be caused mainly by the decrease of optical absorption of photocatalyst under visible light.The RGO-modified Bi2WO6with improved photoactivity prepared in this work has a high potential as the photocatalyst in environment remediation.

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