银合金的制备及其可见光催化活性

Preparation of Ag/AgCl/BiMg2VO6composite and its visible-light photocatalytic activityRui Guo,Gaoke Zhang*,Jiu LiuSchool of Resources and Environmental Engineering,Wuhan University of Technology,122Luoshi Road,Wuhan430070,PR China1.IntroductionNowadays,environmental problems associated with harmfulorganic pollutants in water are driving the impetus tofind effectiveapproaches for environmental remediation.Semiconductor photo-catalysis is commonly described as an ideal‘‘green’’technology forthe conversion of solar energy and photo-oxidation of organicpollutants,especially some azo dyes[1].To date,the semiconduc-tor TiO2has undoubtedly been proven to be one of the mostexcellent photocatalysts for oxidative decomposition of manyorganic compounds[2,3].However,TiO2is only responsive toultraviolet or near-ultraviolet irradiation because of its wide band-gap(3.2eV),which occupies only about4%of the solar lightspectrum[4].From the viewpoint of efficiently utilizing solar lightin the visible region(l>400nm),the development of efficientvisible-light-induced photocatalysts for the photodegradation oforganic pollutants has been an urgent issue.It is well known that noble metal nanoparticles(NPs)showstrong UV–vis absorption due to their surface plasmon resonance(SPR),which is produced by the collective oscillations of surfaceelectrons[5,6].In particular,Ag NPs show efficient plasmonresonance in the visible region,which have been used insemiconductor photocatalysis[7–9].Moreover,silver halides(AgX,X=Cl,Br,I)[10,11],known as widely applied photosensitivematerials,werefirstly reported as photocatalysts for watersplitting in1999.The high efficient plasmonic Ag/AgX photo-catalysts have been developed and aroused broad interesting andconcerning[12].Zhu et al.prepared Ag/AgCl by the ion-exchangereaction between hydrochloric acid and the as-prepared silvermolybdate[13].Wang et al.synthesized H2WO4ÁH2O/Ag/AgClcomposite nanoplates by a one-step ionic reaction betweenAg8W4O16/Ag nanorods and HCl aqueous solution[14].Yu et al.acquired visible-light-driven plasmonic photocatalyst Ag/AgCl/TiO2nanotube arrays(NTs)by depositing AgCl nanoparticles(NPs)into the self-organized TiO2NTs,and then reducing partial Ag+ionsto Ag0species under irradiation[15].Wang and Huang et al.obtained AgCl or AgBr particles with Ag NPs deposited on theirsurface(Ag@AgCl,Ag@AgBr),byfirst treating Ag2MoO4with HClor HBr in an ion-exchange reaction to form AgCl or AgBr powderand then irradiating them to reduce some Ag+ions to Ag0species[16–18].Zhang et al.reported a simple strategy for constructing anall-solid-state AgBr-Ag-Bi2WO6nanojunction by a facile deposi-tion-precipitation method with Bi2WO6as the substrate[19].Allof these reported Ag/AgX photocatalysts exhibited improvedphotocatalytic activities for the destruction of bacteria or thedegradation of organic dye than the bare semiconductors.Theresults indicate that Ag/AgX is not only a good photocatalyst,butalso a good co-catalyst.Recently,many of Bi-based photocatalysts have also attractedgreat attentions for their high visible-light photocatalytic proper-ties,including BiWO6[20–22],BiVO4[23–25],Sr6Bi2O9[26],Bi3Nb x Ta1Àx O7[27],Bi3TiNbO9[28].Most of them exhibitedMaterials Research Bulletin48(2013)1857–1863A R T I C L E I N F OArticle history:Received17October2012Received in revised form1January2013Accepted7January2013Available online23January2013Keywords:A.NanostructuresA.SemiconductorsB.Plasma depositionC.Electron microscopyD.Catalytic propertiesA B S T R A C TA novel composite photocatalyst Ag/AgCl/BiMg2VO6was synthesized by depositing Ag/AgClnanoparticles on BiMg2VO6substrate via a precipitation–photoreduction method and characterizedby X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),energy dispersive X-ray analysis(EDXA),X-ray photoelectron spectroscopy(XPS)and UV–visdiffuse reflectance spectrophotometer(UV–vis DRS).The photocatalyst showed high and stablephotocatalytic activity for photocatalytic degradation of acid red G under visible-light irradiation(l>420nm).In addition,the active O2Àand h+,as main reactive species,played the major roles duringthe reaction process.The high photocatalytic activity of the composite may be related to the efficientelectron–hole pairs separation at the photocatalyst interfaces,as well as the surface plasmon resonanceof Ag nanoparticles formed on AgCl particles in the degradation reaction.ß2013Elsevier Ltd.All rights reserved.*Corresponding author.Tel.:+862787651816;fax:+862787887445.E-mail address:gkzhang@(G.Zhang).Contents lists available at SciVerse ScienceDirectMaterials Research Bulletinj o u rn a l h om e p a ge:w w w.e l s e v i e r.c o m/l o c a t e/m a t r e s b u0025-5408/$–see front matterß2013Elsevier Ltd.All rights reserved./10.1016/j.materresbull.2013.01.019excellent visible-light photocatalytic activity,owing to their large surface area,specific structure and narrow band gaps.On the other hand,in spite of its heavy metal status,bismuth is considered to be safe,as it is non-toxic and noncarcinogenic[29].Moreover, bismuth compounds have been widely used in cosmetics and in the clinic for centuries because of their high effectiveness and low toxicity in the treatment of a variety of microbial infections, including syphilis,diarrhea,gastritis and colitis[30].Therefore,it is important to develop new Bi-based photocatalysts with visible-light photocatalytic activity.Considering the outstanding properties of Bi-based photocata-lysts,in this paper,we prepared a new Bi-based photocatalyst BiMg2VO6by a sol–gel process.The orthorhombic BiMg2VO6 microparticle can be facilely obtained,however,its activity is restricted by wide band gap(3.4eV).Therefore,BiMg2VO6was employed as a support for the highly active Ag/AgCl NPs structure and the novel visible-light composite Ag/AgCl/BiMg2VO6was synthesized by depositing Ag/AgCl NPs into the BiMg2VO6powder via a precipitation–photoreduction method.The photocatalytic activity of the as-prepared Ag/AgCl/BiMg2VO6samples was evaluated by the degradation of acid red G(ARG)aqueous solution under visible-light irradiation(l>420nm).2.Experimental2.1.MaterialsBismuth(III)nitrate pentahydrate(Bi(NO3)3Á5H2O),silver nitrate(AgNO3),magnesium nitrate hexahydrate (Mg(NO3)2Á6H2O),ammonium metavanadate(NH4VO3),citric acid monohydrate(C6H8O7ÁH2O,CA),hydrochloric acid(HCl),ammonia water(NH3ÁH2O)and ethylene diamine tetraacetic acid(EDTA) were chosen as starting chemical reagents.All of the reagents were analytical grade and were used without further purification.Citric acid was used as the chelate reagent and deionized water was used in the whole experiment.2.2.Preparation of photocatalystThe BiMg2VO6samples were prepared by a sol–gel method. Firstly,0.072mol C6H8O7ÁH2O(CA)and0.018mol Bi(NO3)3Á5H2O were added into30mL distilled water at808C with magnetic stirring,and then EDTA–ammonia solution with the matching ratio of0.048mol EDTA:30mL ammonia water was added slowly into the solution under continuous stirring until Bi(NO3)3Á5H2O was absolutely dissolved.The Bi–CA solution was obtained.Secondly, 0.036mol Mg(NO3)2Á6H2O and0.018mol NH4VO3in a molar ratio of2:1:1of Mg/V/Bi were added into30mL distilled water under stirring,respectively.Then the Mg(NO3)2Á6H2O solution and NH4VO3solution were added into the Bi–CA solution respectively to prepare a homogeneous brilliant blue aqueous solution.In order to prevent precipitation,the precursor solution was prepared according to the molar ratio approximately1:4of Bi-Mg-V/CA,3:2 of CA/EDTA.Thirdly,the as-obtained solution was stirred at808C until it became amorphous colloidal,and then heated at200–3008C on electronic furnace until the amorphous polymeric precursors were formed.The precursors were calcined at3008C for 2h and at3508C for2h respectively with a heating rate of 108C minÀ1to promote the decomposition of organic matters,and then were ground and sintered at8008C for4h(the heating rates of108C minÀ1from room temperature to7008C and58C minÀ1 from7008C to8008C were used,respectively).Finally,the light yellow BiMg2VO6powders were obtained.Ag/AgCl NPs were deposited on the as-prepared BiMg2VO6 samples by a precipitation–photoreduction reaction.Typically,the synthesized BiMg2VO6(0.5g)powders were dispersed in30mL deionized water and sonicated for10min.Then10mL of0.1mol/L AgNO3solution was added into the BiMg2VO6suspension and sonicated for10min.After stirring magnetically for20min,20mL of0.1mol/L HCl aqueous solution was added,sonicated for10min and stirred magnetically for20min.Subsequently,the resulting suspension was irradiated by a20W ultraviolet lamp for30min to reduce partial Ag+ions in the AgCl particles to Ag0species by photochemical decomposition of AgCl.The whole reaction process was kept at room temperature.Finally,the dark yellow Ag/AgCl/ BiMg2VO6powders were collected,washed for3times,filtered and dried at808C for several hours.2.3.CharacterizationThe structure and phase composition of the as-synthesized samples were characterized using X-ray diffraction(XRD)on a D/ MAX-RB X-ray diffractometer(Rigaku,Japan)equipped with Cu K a radiation(l=0.1540558nm)and recorded with2u ranging from 208to708,while the accelerating voltage and the applied current were held at40kV and50mA,respectively.Scanning electron microscopy(SEM,JSM-5610LV)at20kV was used to observe the morphologies of the products.The morphologies and microstruc-tures of the as-obtained samples were further examined by transmission electron microscopy(TEM)and high-resolution transmission electron microscopy(HRTEM)with a JEM-2100F electron microscope(JEOL,Japan),using a200kV accelerating voltage.Chemical analysis of the photocatalyst was performed by Energy dispersive X-ray analysis(EDXA)joined a JEM-561LV SEM. X-ray photoelectron spectroscopy(XPS)measurements were carried out on a Thermo VG Multilab2000spectrometer with the degree of vacuum in analysis room excelled3Â108Pa.All the binding energies were referenced to the C1s peak at284.8eV of the surface adventitious carbon.The absorption edges of the BiMg2VO6and Ag/AgCl/BiMg2VO6samples were measured using a UV–vis spectrophotometer(UV-2550,Shimadzu,Japan)in the range of200–800nm.BaSO4was used as a reflectance standard in the UV–vis diffuse reflectance experiment.The OH trapping fluorescence spectra were taken on afluorescence spectropho-tometer(Shimadzu RF-5300PC).2.4.Evaluation of photocatalytic activityPhotocatalytic activity of the as-prepared phtotocatalysts was evaluated by the degradation of acid red G(ARG)aqueous solution under visible-light irradiation.A300W Dy lamp was used as a light source with a420nm cutofffilter to ensure complete removal of irradiation below420nm.For the degradation of ARG,0.05g of the as-prepared catalysts was added into100mL of ARG aqueous solution with a concentration of50mg/L.In each experiment,the suspensions were stirred in the dark for30min to fully disperse the catalyst prior to irradiation.At given time intervals,about5mL dispersions were collected and centrifuged to remove the photocatalyst particles.The concentration of ARG aqueous solution was determined by its absorbance at505nm using a UV–vis spectrophotometer(UV751GD,China).The examination experiment process of reactive species is similar to the photodegradation experiment.A quantity of scavengers were introduced into the ARG solution prior to addition of the catalyst.Terephthalic acid photoluminescence probing technique(TA-PL)is used in the detection of hydroxyl radical ( OH).Terephthalic acid readily reacts with OH to produce highly fluorescent product,2-hydroxyterephthalic acid.The PL intensity of2-hydroxyterephtalic acid is proportional to the amount of OH radicals produced in water[31–33].In the detection experiment,a basic TA solution was added to the reactor instead of ARG and the concentration of TA was set at5Â10À4M in2Â10À3M NaOHR.Guo et al./Materials Research Bulletin48(2013)1857–1863 1858aqueous solution.After visible-light irradiation for given time,the reaction solution was used to measure the increase of the PL intensity at around426nm excited by its own312nm absorption band[34].In the anaerobic study,the solution was deaerated by bubbling high pure N2during the decomposition process.3.Results and discussion3.1.Morphology and phase structuresPowder XRD study was carried out to confirm the crystalline structures of the prepared catalysts.Fig.1shows the XRD patterns of as-synthesized BiMg2VO6and Ag/AgCl/BiMg2VO6samples.In Fig.1a,it is observed that the main diffraction peaks of pure BiMg2VO6can be well indexed with orthorhombic BiMg2VO6(JCPDS No.48-0195).The strongest diffraction peaks at2u=21.88,22.48, 29.68,31.58,48.68and51.58,which are marked with‘‘&’’,can be assigned to(021),(200),(131),(221),(312)and(152)planesof BiMg2VO6,respectively.After the deposition and photoreduction process,apparent diffraction peaks corresponding to AgCl and Ag can also be observed.The typical diffraction peaks at2u=27.88,32.28 and46.28,which are marked with‘‘*’’,can be assigned to(111), (200)and(220)planes of the cubic phase of AgCl,respectively (JCPDS No.31-1238);while peaks at2u=38.18,44.38and64.48, which are marked with‘‘~’’,can be assigned to(111),(200)and (220)crystal orientations of the cubic phase of Ag,respectively (JCPDS No.65-2871).Moreover,it is found that the characteristic peaks of BiMg2VO6did not change significantly in Fig.1b.Fig.2displays the morphologies of the as-prepared BiMg2VO6 and Ag/AgCl/BiMg2VO6catalysts.Fig.2a reveals that the synthe-sized BiMg2VO6particles are of anomalous shape generally.After deposition and photoreduction process,as shown in Fig.2b,some particles speculated to Ag/AgCl NPs are present on the surface of the BiMg2VO6.The microstructure and interfacial region of the prepared Ag/ AgCl/BiMg2VO6catalyst were further examined using TEM and HRTEM.Fig.3a indicates that the particle size of the Ag/AgCl/ BiMg2VO6catalysts is about50–200nm.Fig.3b exhibitsdifferent Fig.3.(a)TEM image of Ag/AgCl/BiMg2VO6sample,(b)and(c)HRTEM images of Ag/AgCl/BiMg2VO6sample.Intensity(a.u.)2 Theta (d egree)Fig.1.XRD patterns of(a)BiMg2VO6,(b)the fresh Ag/AgCl/BiMg2VO6and(c)theused Ag/AgCl/BiMg2VO6after5recycling runs.Fig.2.SEM micrographs of the samples(a)BiMg2VO6and(b)Ag/AgCl/BiMg2VO6.R.Guo et al./Materials Research Bulletin48(2013)1857–18631859fringes with lattice spacing of 0.302nm and 0.196nm,whichcorrespond to the (131)plane of BiMg 2VO 6and the (220)plane of AgCl,respectively.Additionally,in Fig.3c,the lattice fringes with interplanar spacing of 0.235nm on the edge of the sample were also observed,corresponding to the (111)plane of metal Ag [35].All of these results confirm that Ag and AgCl particles were formed on the surface of BiMg 2VO 6catalysts.3.2.Chemical compositionThe chemical composition and the valence states of theprepared photocatalysts were determined by EDXA and XPS analysis.Fig.4shows a typical EDXA spectrum of the obtained sample.From the Fig.4,peaks associated with Bi,Mg,V,Ag and Cl are observed.Bi,Mg,and V peaks result from the BiMg 2VO 6catalysts,while Ag and Cl elements are from Ag/AgCl NPs.Furthermore,the chemical composition can be obtained from the EDXA analysis,as shown in Table 1.On the other hand,the valence states of elements of the Ag/AgCl/BiMg 2VO 6sample have been determined by XPS (Fig.5).The typical survey-scan XPS spectrum indicates that the elements of Bi,Mg,V,O,Cl and Ag were detected in the Ag/AgCl/BiMg 2VO 6composite.No other impurity peaks were detected,evidencing the high purity of the resulting sample.The XPS peak for C 1s (284.8eV)is ascribed to sample handling or instrument background.A typical high-resolution XPS spectrum of Ag 3d is shown in Fig.5b.The peaks at 367and 373eV are assigned to Ag 3d 5/2and Ag 3d 3/2,respectively.The Ag 3d 5/2peak is further divided into two different peaks at 367.65and 368.44eV and Ag 3d 3/2peak is also divided into two different peaks at 373.65and 374.32eV.The peaks at 368.44and 374.32eV can be attributed to metal Ag 0,whereas the peaks at 367.65and 373.65eV can be attributed to Ag +of AgCl [36].Moreover,the binding energies of Cl 2p 1/2and Cl 2p 3/2deriving from Cl À(Fig.5c,AgCl)are 199.4and 197.8eV,respectively [37].These results confirm that there are both Ag and AgCl species in the as-prepared catalysts.The UV–vis diffuse reflectance spectra (DRS)of the samples are represented in Fig.6.As can be seen in the Fig.6,BiMg 2VO 6is only1 2 3 4 5 6 7 8 9 10 1112Ato mic ra tio Ag:Cl: Bi: Mg:V = 25.39: 23. 06: 26. 69:8. 67: 16. 19BiVAgClBiMgI n t e n s i t y (a .u .)Ene rgy (KeV)Fig.4.EDXA spectrum of Ag/AgCl/BiMg 2VO 6sample.Table 1Weight percentage (%)of each component in the photocatalyst.ComponentAgAgClBiMg 2VO 6Weight percentage (%) 2.0527.0470.91aR e l a t i v e i n t e n s i t y (a .u .)Binding energy(eV)bR e l a t i v e i n t e n s i t y (a .u .)Binding ene rgy (eV )19419 619 820 020 2204199.4197.8cCl 2p 1/2Cl 2p 3/2Cl 2pR e l a t i v e i n t e n s i t y (a .u .)Binding ene rgy (eV )Fig. 5.(a)XPS survey spectrum of the Ag/AgCl/BiMg 2VO 6sample,(b)thecorresponding high-resolution XPS spectrum of Ag 3d region and (c)Cl 2p region.(For interpretation of the references to color in figure legend,the reader is referred to the web version of the article.)R.Guo et al./Materials Research Bulletin 48(2013)1857–18631860responsive to ultraviolet irradiation with an absorption edge at about 365nm whereas Ag/AgCl/BiMg 2VO 6composite displays clear optical response in the visible region with an adsorption edge of approximate 550nm,which may result from the existence of Ag NPs produced by photochemical decomposition or photocatalytic reduction of AgCl.Thus,the incident photon frequency is resonant with the collective excitation of conduction electrons of the Ag NPs,called the localized surface plasmon resonance (LSPR)[15,38].The LSPR effect has a prominent contribution to the enhanced photocatalytic activity.For a semiconductor,the band gap energy of a semiconductor can be calculated by the following formula [39–41]:a h n ¼A ðh n ÀE g Þn =2(1)where a ,h ,n ,E g and A are absorption coefficient,Planck constant,light frequency,band gap energy,and a constant,respectively.Aside from them,n is determined by the type of optical transition of a semiconductor (i.e.,n =1for direct transition and n =4for indirect transition).The values of n and E g were determined by the following steps [27]:(1)plot ln(a h n )vs.ln(h n –E g ),using an approximate value of E g ,and then determine the value of n with the slope of the straightest line near the band edge;(2)plot (a h n )2/n vs.h n and then evaluate the band gap E g by extrapolating the straightest line to the h n axis intercept.Following this method,the values of n for BiMg 2VO 6and Ag/AgCl/BiMg 2VO 6were both estimated to be 2.This means that the optical transitions for the present compounds are both indirectly allowed.Therefore,E g of BiMg 2VO 6and Ag/AgCl/BiMg 2VO 6was determined from a plot of a h n versus energy (h n )(Fig.7)and was found to be around 3.40and 2.21eV,respectively.The obvious change of E g for the substrate and composite could be due to the deposition of Ag/AgCl NPs.3.3.Photocatalytic activityThe photocatalytic activities of the as-prepared samples wereevaluated by the degradation of ARG under visible-light irradiation for 120min.Fig.8displays the photodegradation efficiencies of ARG under different conditions.It is found that the degradation of ARG hardly occurred in the dark or under visible-light irradiation without the Ag/AgCl/BiMg 2VO 6catalyst.Moreover,the degrada-tion efficiencies of ARG were 95.90%,41.40%and 0.00%for the Ag/AgCl/BiMg 2VO 6,Ag/AgCl and BiMg 2VO 6samples,respectively,which indicates that ARG can be degraded efficiently by the Ag/AgCl/BiMg 2VO 6catalyst than the Ag/AgCl NPs or BiMg 2VO 6samples.Meanwhile,the temporal evolution of the absorption spectral changes during the photocatalytic degradation of ARG by the Ag/AgCl/BiMg 2VO 6catalyst is shown in Fig.9.It is showed that the absorption peaks of the solution decreased fast and dis-appeared after visible-light irradiation for 120min,which indicates that the ARG molecular was indeed decomposed in the reaction process.As a photocatalyst,its stability is also very important for its application.The stability of the Ag/AgCl/BiMg 2VO 6composite was further evaluated by reusing the catalyst for the degradation of ARG under visible-light irradiation (Fig.10).After five cycles of the photocatalytic degradation of ARG,it can be seen from Fig.10that the Ag/AgCl/BiMg 2VO 6catalyst did not exhibit significant loss of activity,confirming that the Ag/AgCl/BiMg 2VO 6composite is effective and stable during the photocatalytic reaction.As shown in Fig.1c,the obvious diffraction peaks of metallic Ag can be found and the intensity of diffraction peaks of the AgCl decreased after five recycling experiments,indicating that Ag nanoparticles on the surface of the catalysts might become larger due to the reduction of AgCl with the increase of the irradiation time.This result is similar20030400506007008000.00.20.40.60.81.01.21.4 BiMg 2VO 6Ag/AgCl/BiMg 2VO 6Wavelength (nm)A b s o r b a n c e (a . u .)Fig.6.UV–vis diffuse reflectance spectra of BiMg 2VO 6and Ag/AgCl/BiMg 2VO 6.h (e V )h (eV)Fig.7.Plots of a h n versus energy (hn )for the band gap energy of BiMg 2VO 6and Ag/AgCl/BiMg 2VO 6.0.00.20.40.60.81.0C / C 0Irradiation time (min)Fig.8.The photodegradation efficiencies of ARG under different conditions.R.Guo et al./Materials Research Bulletin 48(2013)1857–18631861to the reported paper [15,42,43].However,the photocatalytic performance of the Ag/AgCl/BiMg 2VO 6composite did not decrease.3.4.Possible photocatalytic mechanismThe excellent photocatalytic performance of the as-prepared Ag/AgCl/BiMg 2VO 6motivated us to further investigate the photo-catalytic mechanism.It is generally accepted that the dyes and organic pollutants can be photodegraded via photocatalytic oxidation process.During this process,electron–hole pairs are directly produced by photocatalyst after illumination.A large number of photoinduced reactive species including h +, OH andO 2Àare suspected to be involved in the photocatalytic reaction [13,44].As mentioned above,TA-PL has been widely used in the detection of OH radicals.The PL emission spectra excited at 312nm from TA solution were measured at given time of illumination and shown in Fig.11.It can be seen that the intensity of the PL emission spectra at about 426nm slowly increased with increasing irradiation time.This result indicates that OH radicals were not the dominant reactive species.To further understand the effect of reactive species,a series of quenchers were developed to scavenge the relevant reactive species [45–47].The comparison experiments were also per-formed without quenchers under identical conditions.KI was introduced as the scavenger of h +[48,49].1,4-benzoquinone (BQ,C 6H 4O 2)was adopted to quench O 2À[44,46].Fig.12shows the effects of scavengers KI and BQ on the degradation efficiencies of ARG.It can be observed that KI and BQ affected the degradation rate apparently.These results indicate that the h +and O 2Àwere produced and acted as the main reactive species in the photo-catalytic degradation of ARG under visible-light irradiation.Moreover, O 2Àcould be generated by photogenerated electron reacting directly with O 2adsorbed on the surface of the catalyst Ag/AgCl/BiMg 2VO 6.Thus,the anaerobic experiment was conducted to further investigate the effect of oxygen [26,44,50].As shown in Fig.13,the degradation efficiency of ARG under the anaerobic suspension (N 2-saturated condition)was much lower than that of initial air-equilibrated solution.This result indicates that the presence of oxygen was responsible for the significant degradation of ARG,and acted as an efficient electrons trap,bringing about theA b s o r b a n c e (a .u .)Waveleng th (nm)Fig.9.Absorption spectral changes of ARG aqueous solution at room temperatureunder visible-light irradiation by Ag/AgCl/BiMg 2VO 6sample.(For interpretation of the references to color in figure legend,the reader is referred to the web version of the article.)C / C 0Irradiation time (min)Fig.10.Cycling degradation curves for Ag/AgCl/BiMg 2VO 6sample.F l u o r e s c e n c e i n t e n s i t yWavelength (nm)Fig.11. OH trapping PL spectra of Ag/AgCl/BiMg 2VO 6on TA solution under visible-light irradiation.(For interpretation of the references to color in figure legend,the reader isreferred to the web version of the article.)C / C 0Irradiation time (min)Fig.12.The degradation efficiencies of ARG under visible-light irradiation without or with different scavengers:(&)no quencher;(*)2mM KI;(!)2mM BQ.R.Guo et al./Materials Research Bulletin 48(2013)1857–18631862generation of O 2Àand preventing the recombination of electrons and holes [26,27].Under visible-light irradiation,the SPR produced by the collective oscillations of surface electrons on Ag NPs could induce enhancement of the local inner electromagnetic field [51].Due to the local electromagnetic field and excellent visible-light absorp-tion capability of Ag NPs,the excited electrons can be transferred quickly and induced away from Ag NPs to the conduction band (CB)of AgCl as far as possible,which is beneficial to the stabilization of AgCl [12,43].And these transferred electrons can further react with O 2adsorbed on the surface of AgCl to generate O 2Àthat successively decompose the ARG to the final products [12,16].However,in the case of BiMg 2VO 6,it could not be stimulated to produce electron–hole pairs by visible light for its wide gap energy (E g =3.40eV).In spite of this,the h +in the VB of AgCl may transfer to the VB of BiMg 2VO 6[13].The above interaction may facilitate electron–hole separation and interfacial charge transfer.Simulta-neously,the h +on the surface of Ag NPs can also result in the oxidation of ARG [15,16,18,52].4.ConclusionsIn summary,the novel visible-light composite photocatalyst Ag/AgCl/BiMg 2VO 6was synthesized through a precipitation–photoreduction method.The as-prepared Ag/AgCl/BiMg 2VO 6photocatalyst exhibited excellent performance for the degradation of ARG,and displayed much higher photocatalytic activity than the Ag/AgCl NPs or BiMg 2VO 6samples under visible-light irradiation (l >420nm).After five cycling tests,the photocatalytic activity of the composite still remained stable.The visible-light photocata-lytic activity of Ag/AgCl/BiMg 2VO 6photocatalyst can be attributed to the effective electron–hole separation at the photocatalyst interfaces and the localized surface plasmon resonance of Ag NPs.O 2Àand h +are the dominant reactive species that promoted the transfer of the photoinduced carriers during the photocatalytic oxidation process.AcknowledgmentsThis work was supported by National Program on Key Basic Research Project of China (973Program)2013CB632402,SRFDP(20110143110015),Program of Wuhan Subject Chief Scientist (201150530147)and the Wuhan Technologies R &D Program.References[1]M.R.Hoffmann,S.T.Martin,W.Choi,D.W.Bahnemann,Chem.Rev.95(1995)69–96.[2]M.Fujihira,Y.Satoh,T.Osa,Nature 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