Ag_ZnO Heterostructure Nanocatalyst_

Photocatalytic Activity of Ag/ZnO Heterostructure Nanocatalyst:Correlation between Structure and PropertyYuanhui Zheng,*,†Chongqi Chen,†Yingying Zhan,†Xingyi Lin,†Qi Zheng,*,†Kemei Wei,†and Jiefang Zhu ‡National Engineering Research Center of Chemical Fertilizer Catalyst,Fuzhou Uni V ersity,Gongye Road 523,Fuzhou 350002,Fujian,China,Department of Applied Physics,Chalmers Uni V ersity of Technology,Fysikgrand 3,Goteborg SE 41296,SwedenRecei V ed:March 30,2008;Re V ised Manuscript Recei V ed:May 5,2008Ag/ZnO heterostructure nanocatalysts with Ag content of 1wt %are successfully prepared through three different simple methods,where chemical reduction and photolysis reaction are adopted to fabricate the heterostructure.The dispersity of Ag clusters and/or nanoparticles in Ag/ZnO nanocatalyst is investigated by EDX mapping and XPS techniques.The experimental results show that deposition -precipitation is an efficient method to synthesize Ag/ZnO nanocatalyst with highly dispersed Ag clusters and/or nanoparticles;the photocatalytic activity of Ag/ZnO photocatalysts mainly depends on the dispersity of metallic Ag in Ag/ZnO nanocatalyst;the higher the dispersity of metallic Ag in Ag/ZnO nanocatalyst is,the higher the photocatalytic activity of Ag/ZnO photocatalyst should be.In addition,it is also found that the dispersity of Ag/ZnO photocatalyst in the dye solution is another key factor for liquid-phase photocatalysis due to the UV-light utilizing efficiency.The higher the UV-light utilizing efficiency is,the higher the photocatalytic activity of Ag/ZnO heterostructure photocatalyst should be.1.IntroductionSemiconductor-based heterostructures with desired composi-tions and/or morphologies can modulate the properties of materials and find potential applications in biomedicine,pho-tocatalysis,and nanodevices.1–12Stimulated by these applica-tions,significant advances have been made to design various kinds of semiconductor-based heterostructures,such as core/shell and anisotropic (e.g.,dimer/trimer-type and hierarchical composite materials)heterostructures 13–17in recent years.So far,metal/semiconductor is one of the most popular hetero-structures and has been extensively studied because of its excellent catalytic activity.For example,recently,Ag/ZnO heterostructure photocatalyst with high catalytic activity has attracted much research attention.18–21However,the relationship between the structure and the photocatalytic property of Ag/ZnO heterostructure photocatalyst is still not clear.Considering the relatively high price of Ag species and the promising application of Ag/ZnO photocatalyst,we expect to find an effective method to decrease its cost (i.e.,decrease the Ag content of Ag/ZnO photocatalyst)and understand the effect of the dispersity of Ag clusters and/or nanoparticles in Ag/ZnO photocatalyst on its photocatalytic performance.It is well-known that material properties are determined by structure,such as size,morphology,pore,defect,and composi-tion.18,22–25With the photocatalytic performance of semiconduc-tor-based heterostructure photocatalyst as an example,it is mainly dependent on the concentration of heterostructure interface and defect which can increase the separation efficiency of photogenerated electron -hole pairs.As an example,in our previous study,18it is found that Ag nanoparticles and oxygenvacancy defects (V o ··)on the surface of ZnO nanocrystals benefit the separation of photogenerated electron -hole pairs,thus enhancing the photocatalytic activity.It has also been reported that there are two different pathways to transfer the photogenerated electrons from ZnO semiconductor to the dye for Ag/ZnO photocatalyst (see Scheme 1)in this literature,when the catalyst is irradiated by UV light.As shown in Scheme 1,the photogenerated electrons can be transferred to the dye through the Ag-ZnO interface (pathway I)and the oxygen vacancy defects on the surface of the ZnO semiconductor (pathway II).As to pathway I,the photogenerated electrons should reach the surface of Ag nanopaticles via the Ag-ZnO interface before they are transferred to the dye.So,it is believed that the dispersity of Ag clusters and/or nanoparticles in Ag/ZnO nanocrystals (i.e.,the concentration of Ag-ZnO interface)*Corresponding authors.E-mails:yuanhui.zheng@ and zhengqi@,Phone:+86-591-8373-1234-8112,Fax:+86-591-8373-8808.†Fuzhou University.‡Chalmers University of Technology.SCHEME 1:Photogenerated Electron Transfer in Ag/ZnO Nanocatalyst during the Catalytic Process aaø:work function;E f :Fermi level;V o ··:oxygen vacancy;CB:conduction band;VB:valence band;vac:vacuum level;m:metal;and s:semiconductor.18J.Phys.Chem.C 2008,112,10773–107771077310.1021/jp8027275CCC:$40.75 2008American Chemical SocietyPublished on Web06/26/2008should affect the photocatalytic activity of the Ag/ZnO hetero-structure photocatalyst:the higher the dispersity of Ag clusters and/or nanoparticles on the surface of ZnO nanocrystals is,the higher the photocatalytic activity of the Ag/ZnO heterostruc-ture photocatalyst should be.Therefore,it is necessary to improve the dispersion of metallic Ag in Ag/ZnO photocatalyst and use simple techniques to characterize the dispersion of metallic Ag in the catalyst.So far,metal dispersion in the catalysts has been extensively measured by chemisorption,26–30 and only a little work has been done by energy-dispersive X-ray (EDX)mapping.30,31In this paper,three facile synthetic methods are employed to fabricate the Ag/ZnO heterostructure photo-catalyst with Ag content of1.0wt%.EDX mapping and X-ray photoelectron spectroscopy(XPS)are adopted to investigate the dispersity of Ag clusters and/or nanoparticles in Ag/ZnO photocatalyst(i.e.,the dispersion of metallic Ag on the surface of ZnO nanocrystals).The relationship between the structure and the photocatalytic performance of these catalysts is inves-tigated in detail.The experimental results show that the photocatalytic activity of Ag/ZnO photocatalysts depends not only on the dispersity of Ag clusters and/or nanoparticles in Ag/ZnO photocatalyst but also on Ag/ZnO photocatalyst in the dye solution.2.Experimental Section2.1.Preparation of Ag/ZnO Heterostructure Nanocata-lysts.2.1.1.Materials.Zinc acetate,silver acetate,sodium hydroxide,and alcohol are all analytical grades and used without further purification.2.1.2.Synthesis.Ag/ZnO heterostructure nanocatalysts have been synthesized through the following three methods:For the deposition-precipitation(DP)method,0.037mmol of CH3COOAg and5mmol of fresh ZnO nanocrystals(named Z5in ref25)synthesized through the solvothermal method25were dispersed in150mL of ethanol.Then,1mL of NaOH ethanol solution(0.17M)was added into the above mixture and stirred under natural light at room temperature for two days.For the coprecipitation(CP)method,150mL of NaOH ethanol solution (0.17M)was added into the mixture of5mmol of Zn (Ac)2·2H2O and0.037mmol of CH3COOAg with agitation under natural light at room temperature for two days.For the solvothermal(ST)method,150mL of NaOH ethanol solution (0.17M)was added into the mixture of5mmol of Zn(Ac)2·2H2O and0.037mmol of CH3COOAg with agitation for30min in the dark,then turned intofive Teflon-lined stainless steel autoclaves of50mL capacity equally.The sealed tanks were put into an oven and heated at160°C for one day.The precipitates were collected byfiltration,washed with deionized water and ethanol several times,andfinally dried in the air at 60°C for10h.The obtained samples with Ag content of1.0 wt%were named Ag/ZnO-DP,Ag/ZnO-CP,and Ag/ZnO-ST, respectively.2.2.Characterizations.The powder X-ray diffraction(XRD) patterns of the samples were recorded by a Panalytical X′Pert Pro diffractometer using Co K R radiation(λ)0.179nm)at a scanning rate of0.12°/min.The surface areas were measured by the Brunauer-Emmett-Teller(BET)method by using N2 adsorption at77K on a Quantachrome NOVA4200e apparatus. The dispersity of Ag element in the as-synthesized samples was characterized by afield emission scanning electron microscope (SEM,JSM-6700F)equipped with an energy-dispersive X-ray spectroscopy(EDXS)system at15kV(penetration depth of electrons:∼2.2µm).The X-ray photoelectron spectroscopy (XPS)measurements were performed on a Phi Quantum2000 spectrophotometer with Al K R radiation(1486.6eV)(penetra-tion depth of X-ray:<10nm).UV-vis diffuse-reflectance spectra of the as-synthesized samples were measured on a UV-vis-NIR spectrometer(Cary500).Room-temperature fluorescent characterization was performed on an Edinbursh F900with an excitation wavelength of365nm.For photocata-lytic measurement,30mg of each catalyst was suspended in 90mL of methyl orange(MO)aqueous solution(5.0×10-5 M),and then the mixture was put in quartz tubes and agitated overnight in the absence of light to attain equilibrium adsorption on the catalyst surface.UV irradiation was carried out by using a4×4Wfluorescent Hg-lamp(Philips TUV4W,the strongest emission at254nm).After a given irradiation time,about3.5 mL of the mixture was withdrawn and the catalysts were separated from the suspensions byfiltration through0.22µm cellulose membranes.The photocatalytic degradation process was monitored by UV-vis spectrophotometer(UV-2450) (measuring the absorption of MO at463nm).3.Results and Discussion3.1.Synthetic Principle of Ag/ZnO Heterostructure Nano-catalysts.As described in the Experimental Section,Ag/ZnO heterostructure nanocatalysts are synthesized by three different methods,where chemical reduction18and photolysis reaction are adopted to fabricate the heterostructure.When a sodium hydroxide solution is added into a mixture of acetic zinc(or fresh prepared ZnO nanocrystals25)and silver acetate drop by drop with agitation,the brown-yellow precipitation is formed gradually.The chemical reaction processes for the formation of Ag/ZnO can be formulated as2ZnO+2OH-+H2O)ZnO⁄Zn(OH)42-(1a)Zn2++4OH-)Zn(OH)42-(1b)Ag++2OH-)Ag(OH)2-(2)ZnO⁄Zn(OH)42-+2Ag(OH)2-f Ag2O⁄ZnO+H2O+OH-(3a)Zn(OH)42-+2Ag(OH)2-)Ag2O⁄ZnO+2H2O+4OH-(3b)Ag2O⁄ZnO98hνAg⁄ZnO+O2(4a)Ag2O⁄ZnO98ethanolsolvothermalAg⁄ZnOx·(1-x)V(x<1)(4b) For the deposition-precipitation method,reactions1a,2,3a, and4a should happen;for the coprecipitation method,reactions 1b,2,3b,and4a should occur;and for the solvothermal method, reactions1b,2,3b,and4b should take place.18Different reaction processes should result in different structures of Ag/ZnO nanocatalyst,such as the dispersity of metallic Ag in Ag/ZnO nanocatalysts and the type/concentration of oxygen defects in ZnO nanocrystals,as discussed later(see sections3.3and3.4, respectively).3.2.Crystal Structure and BET Surface Area of Ag/ZnO Heterostructure Nanocatalysts.The XRD patterns of the as-synthesized samples through different methods are shown in Figure1.It is found that there are two sets of diffraction peaks for each sample,indicating that the as-synthesized samples are composite materials.Those marked with“#”can be indexed to10774J.Phys.Chem.C,Vol.112,No.29,2008Zheng et al.hexagonal wurtzite ZnO (JCPDS file no.36-1451),while the others marked with “*”can be indexed to face-centered-cubic (fcc)metallic Ag (JCPDS file no.04-0783).Comparing the half-peak width of the as-synthesized samples,we can see that the grain size of ZnO nanocrystals in Ag/ZnO-CP is much smaller than those of Ag/ZnO-DP and Ag/ZnO-ST.In addition,there is no remarkable shift of all diffraction peaks,implying that no Zn 1-x Ag x O solid solution is formed in the as-synthesized samples.The BET surface areas of Ag/ZnO-DP,Ag/ZnO-CP,and Ag/ZnO-ST are 19.2,16.0,and 23.3m 2/g,respectively.3.3.Dispersity of Metallic Ag on the Surface of ZnO Nanocrystals.In order to obtain detailed information about Ag distributions in the as-synthesized samples,SEM observations and EDX mapping measurements are carried out.Figure 2shows the low-magnification SEM images and Ag distributions in the as-synthesized samples with Ag content of 1.0wt %.It is easy to find that the coverage of blue color decreases from Figure 2a to c (Ag/ZnO-DP >Ag/ZnO-CP >Ag/ZnO-ST),indicating that the uniformity of metallic Ag in the as-synthesized samples decreases from Ag/ZnO-DP to Ag/ZnO-ST.This means that the concentration of Ag-ZnO interface decreases from Ag/ZnO-DP to Ag/ZnO-ST.A typical HRTEM image of an individual Ag/ZnO heterostructure (the inset in Figure 2a,c)reveals that the metallic Ag nanoparticle attaches on the surface of ZnO nanocrystal (i.e.,the formation of a dimer-type heterostructure).The composition of the as-synthesized samples is further characterized by EDXS and the results are shown in Figure 2d.All of the peaks on the curves are ascribed to Zn,Ag,O,Cu,and C elements,and no peaks of other elements are observed.The copper and carbon signals are from the sample holder with conductive tape.Therefore,it is concluded that the as-synthesized samples are composed of Zn,Ag,and O elements,which is in good agreement with the above XRD result.In addition,EDXS analysis reveals that the Ag content of the as-synthesized samples (listed in Figure 2d)approximates the theoretical value (1.0wt %).The surface Ag content of the as-prepared samples is also investigated by XPS,and the results are shown in Figure 3.The binding energies in the XPS spectra presented in Figure 3are calibrated by C 1s (284.8eV).The peaks appearing in Figure 3are attributed to Ag 3d 5/2(367.2eV)and Ag 3d 3/2(373.2eV),respectively.It is worthwhile to mention that the peak positions of Ag 3d shift remarkably to the lower binding energy compared with that of bulk Ag (Ag 3d 5/2,368.3eV;and Ag 3d 3/2,374.3eV 32),which is ascribed to the electron transfer from metallic Ag to ZnO nanocrystals (i.e.,the formation of monovalent Ag).18The shift of the binding energy of metallic Ag indicates that there is a strong interaction between metallic Ag and ZnO nanocrystals.In addition,it is found that the intensity of Ag 3d peaks decreases from Ag/ZnO-DP to Ag/ZnO-ST and the calculated surface Ag contents of the corresponding samples are 0.89,0.68,and 0.35wt %,implying the decrease of the content of surface Ag from Ag/ZnO-DP to Ag/ZnO-ST.In combination with the EDX mapping results,it can be concluded that Ag/ZnO-DP possesses the highest dispersity of metallic Ag among the as-synthesized samples.3.4.Optical Properties of Ag/ZnO Heterostructure Nano-crystals.The UV -vis diffuse-reflectance and photolumines-cence (PL)spectra of the as-synthesized Ag/ZnO heterostructure nanocrystals are shown in Figure 4.In Figure 4a,two prominent absorption bands are observed in the range 200-800nm.The former is assigned to the absorption of the ZnO semiconductor,and the corresponding absorption edge locates at around 375nm.The absorption edge of ZnO nanocrystals for Ag/ZnO-CP is slightly smaller than that of ZnO nanocrystals for Ag/ZnO-DP and Ag/ZnO-ST,indicating that the size of ZnO nanocrystals in Ag/ZnO-CP is smaller,which is in good agreement with the above XRD result.The latter is attributed to the characteristic absorption of surface plasmon resulting from the metallic Ag clusters and/or nanoparticles in the Ag/ZnO heterostructures.18The appearance of two kinds of characteristic absorption bands also confirms that the as-synthesized samples are composed of zerovalent Ag and paring with the position and intensity of the metallic Ag absorption in Figure 4a,one can see that the position blue-shifts for Ag/ZnO-DP (highlighted by an arrow),indicating that this sample has the smallest size of Ag clusters and/or nanoparticles.It is also found that for the Ag/ZnO-ST sample the intensity is much lower,which should result from the worst Ag distributions in the sample.In Figure 4b,there is a broad emission peak in the range 400-800nm for all samples.It is obvious that the emission peak of the as-synthesized samples shifts from 527to 585nm (Ag/ZnO-ST f Ag/ZnO-DP f Ag/ZnO-CP:red shift)and the corresponding intensity also changes.On the basis of our previous study,25the green and orange emissions at about 527and 605nm should be ascribed to oxygen vacancy (V o ··)and interstitial oxygen (O i ′′)defects,respectively.The red shift of emission peak indicates that the concentration of O i ′′defects increase from Ag/ZnO-ST to Ag/ZnO-CP (the concentration of O i ′′defects:Ag/ZnO-CP >Ag/ZnO-DP >Ag/ZnO-ST).Moreover,the con-centration of surface oxygen defects (including V o ··and O i ′′defects)of Ag/ZnO-DP and Ag/ZnO-CP is lowest and highest among that of the as-synthesized samples (the concentration of surface oxygen defects:Ag/ZnO-CP >Ag/ZnO-ST >Ag/ZnO-DP).183.5.Photocatalytic Activities of Ag/ZnO Heterostructure Nanocatalysts.MO is presently adopted as a representative organic pollutant to evaluate the photocatalytic performance of the as-synthesized Ag/ZnO heterostructure nanocatalysts.In the experiments,the commercial TiO 2(Degussa P-25)is used as a photocatalytic reference to qualitatively understand the photo-catalytic activity of Ag/ZnO catalysts.The photocatalytic activities of the as-prepared samples with Ag content of 1.0wt %and Degussa P-25are shown in Figure 5.C 0and C in Figure 5are the initial concentration after the equilibrium adsorption and the reaction concentration of MO,respectively.As seen in Figure 5,the photocatalytic activity of Ag/ZnO-DP and Ag/ZnO-ST is much higher than that of P-25,for exam-ple,and the required time for an entire decolorization of MO over Ag/ZnO-DP and Ag/ZnO-ST (Figure 5a,c)is less than orFigure 1.XRD patterns of the as-synthesized samples:(a)Ag/ZnO-DP,(b)Ag/ZnO-CP,and (c)Ag/ZnO-ST.The insets are the enlarged diffraction peaks of Ag (111).Photocatalytic Activity of Ag/ZnO Nanocatalyst J.Phys.Chem.C,Vol.112,No.29,200810775equal to 40min,much shorter than the corresponding value of P-25catalyst.However,the MO degradation efficiency is about 60%for Ag/ZnO-CP at 40min (Figure 5b),which is much smaller than that of P-25(92%).Moreover,it is obvious that Ag/ZnO-DP shows the highest photocatalytic activity among all these samples.The photograph of the as-synthesized samples (the inset of Figure 5)in MO solution shows that Ag/ZnO-DP and Ag/ZnO-ST samples are much more easily dispersed than Ag/ZnO-CP sample.It has been reported that the higher the concentration of oxygen defects on the surface of ZnO nanocrystals,the higherthe photocatalytic activity should be.18However,based on the above results,despite lowest concentration of oxygen defects on the surface of ZnO nanocrystals for Ag/ZnO-DP sample,its photocatalytic activity is remarkably better than that of the other two samples (Ag/ZnO-CP and Ag/ZnO-ST).According to the EDX mapping and XPS results,this phenomenon should be attributed to the highest dispersity of metallic Ag on the surface of ZnO nanocrystals for Ag/ZnO-DP sample.It has been reported that Ag clusters and/or nanoparticles on the surface of ZnO nanocrystals act as a sink for the electrons,promote interfacial charge-transfer kinetics between the metal and the semiconductor,improve the separation of photogenerated electron -hole pairs,and thus enhance the photocatalytic activity of Ag/ZnO photocatalysts.18Therefore,the higher the dispersity of Ag clusters and/or nanoparticles on the surface of ZnO nanocrystals is,the higher the photocatalytic activity of Ag/ZnO heterostructure photocatalyst should be.However,as presented in Figure 5,the photocatalytic efficiency of Ag/ZnO-CP is much lower than that of Ag/ZnO-ST,which should be related to the dispersity of Ag/ZnO photocatalyst in the dye solution.It is found that Ag/ZnO-CP cannot be well-dispersed in the dye solution (the inset of Figure 5),so the UV-light utilizing efficiency of this sample is very low.The higher the UV-light utilizing efficiency is,the higher the photocatalytic activity of the Ag/ZnO heterostructure photocatalyst should be.33Considering all the above factors,it is reasonable that Ag/ZnO-DP and Ag/ZnO-CP exhibit the highest and lowest photocata-lytic activity,respectively.Finally,the experimental results presented in this paper should be valuable for the synthesis of other metal/semiconductor catalysts with high catalytic activity.Figure 2.Low-magnification SEM images and EDX mapping of the as-synthesized samples:(a)Ag/ZnO-DP,(b)Ag/ZnO-CP and (c)Ag/ZnO-ST;(d)EDXS spectra recorded from the corresponding rectangular region of the as-synthesized samples in a -c.The coverage of blue color in a -c reflects Ag distributions in the as-synthesized samples,and the inset in a and c is the corresponding HRTEM image of a single Ag/ZnO heterostructure nanocrystal.Figure 3.Ag 3d XPS spectra of the as-synthesized samples:(a)Ag/ZnO-DP,(b)Ag/ZnO-CP,and (c)Ag/ZnO-ST.10776J.Phys.Chem.C,Vol.112,No.29,2008Zheng etal.4.ConclusionAg/ZnO heterostructure nanocatalysts were successfully prepared through three simple methods,where chemical reduc-tion and photolysis reaction are adopted to fabricate the heterostructure.It is found that deposition-precipitation is an efficient method to synthesized Ag/ZnO nanocatalyst with highly dispersed metallic Ag on the surface of ZnO nanocrystals;the photocatalytic activity of Ag/ZnO photocatalysts depends on the dispersity of Ag clusters and/or nanoparticles in Ag/ZnO photocatalyst and Ag/ZnO photocatalyst in the dye solution. The higher the dispersities of metallic Ag in Ag/ZnO photo-catalyst and Ag/ZnO catalyst in the dye solution are,the higher the photocatalytic activity of Ag/ZnO heterostructure photo-catalyst should be.Acknowledgment.The authors acknowledge thefinancial support from the Department of Science of the People’s Republic of China(20771025),and the Department of Science &Technology of Fujian Province(2005H201-2). 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