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1、Synthesis,characterization and evaluation of the photocatalytic performanceof Ag-CdMoO4solar light driven plasmonic photocatalystRajesh Adhikaria,Shova Mallaa,Gobinda Gyawalia,Tohru Sekinob,Soo Wohn Leec,*aResearch Center for Eco-Multifunctional Nanomaterials,Sun Moon University,Republic of KoreabIn
2、stitute of Multidisciplinary Research for Advanced Materials,Tohoku University,JapancDepartment of Environmental Engineering,Sun Moon University,Asan Si,Chungnam 336-708,Republic of Korea1.IntroductionSynthesis and tailoring of the photocatalyst based on binarymetal oxide semiconductors have receive
3、d considerable attentiondue to their ability to transform solar energy into the chemicalenergy.They provide the safe and green pathway to utilize solarenergy for various purposes such as degradation of organicpollutants and hydrogen evolution from water 1,2.The oxidationof organic contaminants of wa
4、ste water appears to rely on a surfacephenomenon to achieve mineralization.This phenomenon hasbeen studied by heterogeneous photocatalysis that focuses on thestudy of photo assisted reactions using an irradiated nanostruc-tured semiconductor as a photocatalyst 3.Among the variousphotocatalyst,TiO2ha
5、s been widely studied due to its appropriateband gap,stability and low cost.However,this is active only underUV light which covers only 35%of the solar spectrum 4.In recent years,growing interest has also been focused onternary inorganic photocatalyst in order to solve the drawbacks ofthe binary com
6、pounds such as slow reaction rate and poor solarefficiency.As an important member of scheelite structures,metalmolybdates(MMoO4,M=Ca,Sr,Ba,Cd,Pb,etc.)have attractedconsiderable research interest due to their promising technologicalimportance in a broad range of applications such as photolumi-nescenc
7、e,scintillator materials,humidity sensors and catalysis 5.Longo et al.6 synthesized the hierarchical assembly of CaMoO4nano-octahedrons and their photoluminescence properties.Lacomba-Perales et al.7 investigated the optical properties ofsome tungstate family of scheelite structures such as SrWO4,CaW
8、O4,CdWO4,ZnWO4,PbWO4and CuWO4and reported theband gap energy.Cavalcante et al.8 recently studied the clustercoordination and photoluminescence properties of a-Ag2WO4microcrystals synthesized by sonochemistry(SC),coprecipitation(CP),and conventional hydrothermal(CH).They also studied thegrowth mechan
9、ism and photocatalytic properties of SrWO4microcrystals synthesized by injection of ions into a hot aqueoussolution 9.Among these metal tungstates and molybdates,cadmium molybdate is an important material owing to itsexcellent optical and chemical properties,electronic structureand relatively low ba
10、nd gap energy as compared to above scheelitestructures.This unique combination of physical and chemicalproperties of CdMoO4in term of molecular and electronicversatility,reactivity and stability suggests that a CdMoO4maybe a promising photocatalyst 10.Therefore,it is of greatimportance to study the
11、photocatalytic properties of this materialfor potential applications.Cadmium molybdate is one of the metallic molybdatecompounds with a scheelite structure and has a body-centerorthorhombic unit cell with C64hspace group.Each site of Cd isMaterials Research Bulletin 48(2013)33673373A R T I C L E I N
12、 F OArticle history:Received 31 January 2013Received in revised form 1 May 2013Accepted 7 May 2013Available online 15 May 2013Keywords:A.NanostructuresC.X-ray diffractionC.Electron microscopyD.Catalytic propertiesA B S T R A C TAg-CdMoO4plasmonic photocatalyst was synthesized in ethanol/water mixtur
13、e by photo assisted co-precipitation method at room temperature.As synthesized powders were characterized by X-raydiffraction(XRD),transmission electron microscopy(TEM),UVVis diffuse reflectance spectroscopy(DRS),X-ray photoelectron spectroscopy(XPS)and BrunauerEmmettTeller(BET)surface area analyzer
14、.Photocatalytic activity was evaluated by performing the degradation experiment over methylene blue(MB)and indigo carmine(IC)as model dyes under simulated solar light irradiation.The results revealedthat the Ag-CdMoO4showed the higher photocatalytic performance as compared to CdMoO4nanoparticles.Dis
15、persion of Ag nanoparticles over the surface of CdMoO4nanoparticles causes the surface plasmonresonance(SPR)and enhances the broad absorption in the entire visible region of the solar spectrum.Hence,dispersion of Ag nanoparticles over CdMoO4nanoparticles could be the better alternative to enhance th
16、eabsorption of visible light by scheelite crystal family for effective photocatalysis.?2013 Elsevier Ltd.All rights reserved.*Corresponding author.Tel.:+82 41 530 2882;fax:+82 41 530 2840.E-mail address:swleesunmoon.ac.kr(S.W.Lee).Contents lists available at SciVerse ScienceDirectMaterials Research
17、Bulletinjo u rn al h om ep age:ww w.els evier.c o m/lo c ate/mat res b u0025-5408/$see front matter?2013 Elsevier Ltd.All rights reserved.http:/dx.doi.org/10.1016/j.materresbull.2013.05.019surrounded by eight oxygen(octahedron)and Mo is surrounded byfour equivalents oxygen(tetrahedron)11.A complete
18、polyhe-dron structure has been depicted in Fig.1.Noble metal nanoparticles(NPs)show strong visible lightabsorption because of their size and shape dependent plasmonresonance which has wide variety of applications such ascolorimetric centers,photovoltaic devices,photochromic devicesand photocatalysts
19、.In particular,silver NPs show efficient plasmonresonance in the visible light region which has been utilized todevelop a plasmonic photocatalyst.Recently,Hayato et al.12fabricated Au nanoparticles on TiO2for the photocatalytichydrogen under visible light.Wang et al.13 synthesized AgAgClplasmonic ph
20、otocatalysts through ion exchange and photoreduc-tion method,which showed excellent photocatalytic activitiesunder visible light irradiation.It is reported that silver halides aregenerally considered as photosensitive materials and can coexiststably with Ag NPs during the whole photocatalytic proces
21、s.Kakuta et al.14 also observed that Ag NPs could contribute to thesmooth separation of electronhole pairs on the AgBr/SiO2NPs andenable it to catalyze H2production from alcohol radicals.Liu et al.15 studied one pot pyridine assisted synthesis of Ag/Ag3PO4as avisible light driven photocatalyst with
22、enhanced photocatalyticactivity.Xu et al.16 reported the enhanced photocatalyticactivity of Ag/AgCl/ZnO photocatalyst.Wang et al.17,18synthesized composite semiconductor H2WO4?H2O/AgCl as anefficient and stable photocatalyst under visible light and focusedon charge transfer process between small ban
23、d gap semiconduc-tors(SBG)coupled with AgCl.Yu et al.19 reported the fabricationand characterization of Ag/AgCl/TiO2nanotube arrays.In ourprevious works 20,21,Ag/AgCl/WO3and Ag0-PbMoO4weresynthesized photocatalyst for the treatment of aqueous hazardouspollutants.All these literatures mentioned above
24、 have proved thatthe Ag supported with metal oxide semiconductor have enhancedphotocatalytic activity under visible or simulated solar light.However,to our knowledge,no work has been reported on thefabrication of Ag-CdMoO4nanocomposite and their photocatalyticactivities.Here,we report the Ag-CdMoO4s
25、olar light drivenphotocatalyst by photo assisted co-precipitation method and itsperformance for the degradation of dyes under simulated solarlight irradiation based on surface plasmon resonance effect.2.Materials and methods2.1.Experimental procedureAll the chemicals used in this experiment were of
26、analyticalgrade,and were received from Sigma Aldrich and used without anyfurther purification.Ag-CdMoO4photocatalysts were synthesizedby photo assisted co-precipitation method in ethanol/watermixture and the procedure is briefly described as follows:firstly,5.66 g of cadmium nitrate tetra hydrate(Cd
27、(NO3)2?4H2O)(99.99%)and 2.97 g of molybdic acid(H2MoO4)(99.99%)were mixed in a50 ml ethanol/water mixture(volume ratio:4:1)in a separatebeaker(a).Similarly,1 wt%silver nitrate solution was prepared in10 ml double distilled water in another beaker(b)and added to theabove mixture(a).Final pH of the mi
28、xture was adjusted to 6 byusing hydrochloric acid(Extra pure grade,Duksan Chemicals)andstirred continuously for 2 h.After mixing all the precursor solution,whole mixture was then treated in UV light irradiation(UVA andUVB)for the photoreduction of silver nanoparticles at 1 h.Finally,the precipitate
29、was washed with distilled water several times,dried in air at 70 8C for 24 h and finally calcined at 300 8C for 4 h.Inthe same manner,different concentration of silver nitrate solution,0 wt%,2 wt%,3 wt%,4 wt%and 5 wt%,respectively were preparedand followed the similar procedure mentioned above in or
30、der tosynthesize other samples.Hence,six samples were prepared forcomparative purposes and each sample so obtained is representedas S-0,S-1,S-2,S-3,S-4,and S-5,respectively for convenience.2.2.Photocatalytic experimentIn the photocatalytic experiment,0.1 g of the catalyst powderwas placed in a Pyrex
31、 glass cell.Fifty milliliters of aqueousmethylene blue dye(10 mg/L)was slowly poured in the cell andthen dispersed well by using magnetic stirrer.The solution withdispersed catalyst particles was then kept in dark to achieveadsorptiondesorption equilibrium.After equilibration,the suspen-sion was irr
32、adiated with continuous stirring under simulated solarlight by using solar simulator(PEC-L01,Pecell,Am 1.5G).Eachaliquot was withdrawn at every 30 min,centrifuged to remove thenanoparticles and the absorbance of each aliquot was measured byUV-Vis spectrophotometer(MECASYS Optizen 2120).Similarmethod
33、 was applied for the degradation of indigo carmine dyeexcept that the initial concentration of dye was taken to be 20 mg/L.2.3.CharacterizationThe crystal structure of the as synthesized catalysts wascharacterized by powder X-ray diffraction(XRD)method by usingX-ray diffractometer(Rigaku).The morpho
34、logies and sizes of thesamples were observed by transmission electron microscopy(TEM),JEM-2010,JEOL Ltd.,Japan.UVVis DRS spectra wererecorded by UV-Vis spectrophotometer(JASCO).XPS measurementwas performed by PHI 5000 C ESCA system with Mg Ka sourceoperating at 14.0 kV and 25 mA.Photocatalytic exper
35、iments wereperformed by UV-Vis spectrophotometer(Mcsays)and portablesolar simulator.BET surface area was determined by BET analyzer(Micrometrics,ASAP 2020).3.Results and discussionFig.2 shows the XRD patterns of Ag-CdMoO4samples synthe-sized under identical conditions.All the diffraction peaks are w
36、ellindexed to the tetragonal scheelite structure of CdMoO4with latticeparameters of a=5.155 Aand c=11.194 A(JCPDS file no.07-0209).Moreover,these parameters are in close proximity with the latticeparameters obtained by angle dispersive X-ray diffraction(ADXRD)Fig.1.Tetragonal scheelite crystal struc
37、ture of CdMoO4.R.Adhikari et al./Materials Research Bulletin 48(2013)336733733368reported by Errandonea et al.22.No additional peaks caused byany other impurities have been detected.This confirms that all thesamples possess good crystallinity.Intensity of the peaks for allsamples is almost similar w
38、hich indicates that the degree ofcrystallinity is similar for all samples prepared under identicalconditions.Furthermore,we calculated the crystallite size of eachsample by using Scherrer equation:D Klb cos u(1)where K(=0.9)is a shape factor for spherical particles,l is thewavelength of the incident
39、 radiation,b is the line width at half-maximum height,u is Braggs angle and D is the crystallite size.Although the application of this equation depends on severalfactors such as dislocations,stacking faults,twinning,micro strain,grain boundaries,and sub-boundaries.However,we assumed thatthe synthesi
40、zed samples were homogenous and possess noimperfections in CdMoO4crystal and hence these factors wereneglected.Following these considerations,the crystallite size wasfound to be 35.2 nm,36.8 nm,37.1 nm,37.4 nm 37.8 nm and38.3 nm,for S-0,S-1,S-2,S-3,S-4 and S-5,respectively.Moreover,no peaks were det
41、ected for Ag nanoparticles which are due to thevery low concentration of silver precursor used during the photoreduction process.TEM images of samples,S-0 and S-4 are presented in Fig.3.Fig.3a shows the TEM image of S-0 which reveals that the CdMoO4nanoparticles possessing spherical morphology 23.Fi
42、g.3b showsthe particle size distribution of CdMoO4particles having averageparticle size of 3040 nm which is in good agreement with XRDanalysis.Fig.3c shows that the Ag nanoparticles are well dispersedon the surface of CdMoO4particles and possess sphericalmorphology(Fig.3d).Average particle size of A
43、g was found tobe 1015 nm indicating that the synthesis method reported in thepresent work is efficient for the preparation of nanoparticles withuniform particle size distribution.Fig.4 shows the XPS spectra to analyze the surface compositionand different valance state of the component of as synthesi
44、zedmaterial.Wide scanning spectrum of S-4(Fig.4a)indicates that theCd,Mo,O and Ag are present in the sample and no any otherimpurities are found.The elements Cd,Mo and O belong to theCdMoO4nanoparticles and the element Ag belongs to the silvernanoparticles dispersed on the surface of CdMoO4nanoparti
45、cles asa result of photoreduction of AgNO3.Two prominent peaksFig.2.XRD patterns of different samples synthesized under identical conditions.Fig.3.TEM image of S-0(a)and S-4(c),histogram of particle size distribution of CdMoO4particles(b)and HRTEM image of Ag particle(d).R.Adhikari et al./Materials
46、Research Bulletin 48(2013)33673373 3369Fig.4.XPS spectra of S-4(a)general survey spectrum and(b)XPS band for Ag,O,Cd and Mo.R.Adhikari et al./Materials Research Bulletin 48(2013)336733733370corresponding to 368.1 eV and 374.5 eV(Fig.4b)confirm thepresence of metallic Ag nanoparticles.The peak for C
47、1s at 284.8 eVis due to the hydrocarbon from the XPS instrument itself.Peaks at142.7 and 137.8 eV are ascribed to Cd which is originated fromCdm+ion of the CdMoO424.Mo 3d peaks at 234.6 and 231.6 eVare originated from the Mon+states of MoO325,and the splittingof the 3d doublet(3d3/2)is approximately
48、 at 3.0 eV.The Ag 3dpeaks appear at a binding energy of 368.1 eV(3d5/2)and thesplitting of the 3d doublet(3d3/2)is approximately at 6.0 eV.Thesebinding energies indicate principally the existence of reducedsilver particles on the CdMoO4surface 2628.Hence,from theXPS analysis,it can be proved that th
49、e photocatalyst consists of Agand CdMoO4.BET surface area of the different samples wasmeasured by using BET surface area analyzer under identicalconditions.All the samples had an identical isotherm plots(notshown).Table 1 presents the BET surface area of different samplesand the cadmium molybdate di
50、spersed with Ag particles possessedslightly lower surface area as compared to S-0 sample.UVVis diffuse reflectance spectra of the as prepared CdMoO4and Ag-CdMoO4photocatalysts with different Ag contents areshown in Fig.5.According to a previous report,the optical bandgap(Eg)for CdMoO4is of the direc