合成微球AlOOH@Fe3O4纳米复合物

Particuology10 (2012) 751–758Synthesis of mesoporous␥-AlOOH@Fe3O4magnetic nanomicrospheresYuanyuan Zheng,Shengfu Ji∗,Hongfei Liu,Ming Li,Hao YangState Key Laboratory of Chemical Resource Engineering,Beijing University of Chemical Technology,Beijing100029,Chinaa r t i c l e i n f oArticle history:Received9November2011Received in revised form13April2012Accepted16April2012Keywords:Magnetic nanomicrospheresFe3O4␥-AlOOH shellMesoporousHydrolysisa b s t r a c tthe mesoporous␥-AlOOH@Fe3O4magnetic nanomicrospheres consisted of a mesoporous␥-AlOOH shelland a Fe3O4magnetic core.The diameter of␥-AlOOH@Fe3O4nanomicrospheres was about200nm,thethickness of mesoporous␥-AlOOH shell was about5nm and the average pore size was3.8nm.The thick-ness of the mesoporous␥-AlOOH shell could be controlled via layer-by-layer coating times.The formationmechanism of the mesoporous␥-AlOOH shell involved a“chemisorption–hydrolysis”process.© 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy ofSciences. Published by Elsevier B.V. All rights reserved.1.IntroductionMagnetic nanosized materials with superior magnetic sepa-ration performance have found great potential for applicationin variousfields such as adsorption separation(Deng,Qi,Deng,Zhang,&Zhao,2008),drug delivery(Huang et al.,2008),mag-netic recycling nanocatalysts(Laurent et al.,2008)and moleculeimmobilization(Bromberg et al.,2011).Among them,Fe3O4nanospheres are a kind of for their uniquemagnetic response and(Mori,Kondo,Morimoto,&Yamashita,2008;Park et al.,2006;Zhu,Yuan,&Dai,2008).However,Fe3O4nanospheres still have some disad-vantages in thefields of magnetic recycling catalysis.For example,Fe3O4nanospheres serving as catalyst support can hardly adsorbactive components due to lack of functional groups on their surfaces(Kong,Lu,Bian,Zhang,&Wang,2011).To overcome the above dis-advantages,magnetic nanosized materials with Fe3O4as the coreand other materials such as carbon,silica,alumina,TiO2and var-ious polymers as the shell have been extensively proposed(Lang,Wang,Xing,Zhang,&Liu,2008;Li et al.,2008;Peng,Liang,&Qiu,2011;Wu,He,&Jiang,2008).Boehmite(␥-AlOOH)is an aluminum oxyhydroxide,which hasbeen considered as one of the best candidates owing to its low cost,excellent biocompatibility,good chemical stability,high surface∗Corresponding author.E-mail address:jisf@(S.Ji).area,relatively good conductivity,and controllable synthesis pro-tocols(Feng et al.,2008;Yuan et al.,2008).In consideration of theadvantages of the␥-AlOOH,magnetic␥-AlOOH nanoparticles pre-pared by coating Fe3O4nanospheres with␥-AlOOH,which not onlyovercome the disadvantages of Fe3O4nanospheres,but also pos-sess superior magnetic response,large surface area and modifiablechemical surfaces,have potential applications in catalysis,adsorp-tion and separation.At present,some researchers have reported thesynthesis of magnetic␥-AlOOH nanoparticles.Kwon,Park,Jang,Lee,and Park(2007)prepared magnetic␥-AlOOH catalyst by asol–gel method with aluminum tri-sec-butoxide as the aluminumsource and Fe3O4as the magnetic material.Xuan et al.(2011)reported the fabrication of hierarchical␥-AlOOH/SiO2/Fe3O4mag-netic microspheres by a hydrothermal method at190◦C usingSiO2-coated Fe3O4as magnetic core.However,these obtained mag-netic␥-AlOOH particles either show poor magnetic response orcall for complex preparation process.Therefore,it is necessary todevelop a simpler method to prepare magnetic␥-AlOOH particleswith high magnetic response.In this paper,the core–shell structural mesoporous␥-AlOOH@Fe3O4magnetic nanomicrospheres were prepared by thehydrolysis of aluminum isopropoxide with Fe3O4nanospheres syn-thesized by a solvothermal method as the magnetic core.Theeffects of the hydrolysis time of AIP,the concentration of AIPand the number of coating layer on the structure of magneticnanomicrospheres were investigated.The formation mechanismof mesoporous␥-AlOOH shell on the surfaces of magnetic nanomi-crospheres was also analyzed.1674-2001/$–see front matter© 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. /10.1016/j.partic.2012.04.003752Y.Zheng et al./Particuology10 (2012) 751–7582.Experimental2.1.MaterialsFerric chloride hexahydrate(FeCl3·6H2O),Polyvinylpyrrolidone (PVP),anhydrous sodium acetate(NaAc),ethylene glycol(EG),alu-minum isopropoxide(AIP),and absolute ethyl alcohol were of analytical grade and used without further purification.Deionized water was used throughout the experiments.2.2.Synthesis of Fe3O4nanospheresFe3O4nanospheres were prepared through a solvothermal method.Briefly,FeCl3·6H2O(2.7g),PVP(7.5g,surfactant)and NaAc (7.5g)were dissolved in ethylene glycol(100mL)under vigorous magnetic stirring.After stirring for30min at room temperature, the obtained homogeneous orange solution was transferred to a Teflon-lined stainless-steel autoclave(100mL capacity).The auto-clave was sealed,settled into a muffle furnace and heated at200◦C for12h under stirring at400rpm.After reaction for12h,the autoclave was cooled down to room temperature naturally.The obtained black product was separated with a permanent magnet, washed several times with deionized water and ethanol,and then dried in vacuum at50◦C for12h.2.3.Synthesis of -AlOOH@Fe3O4magnetic nanomicrospheresThe obtained Fe3O4particles(0.1g)were dispersed in an AIP ethanol solution(60mL,0.008–0.032mol/L)of certain concentra-tion under ultrasonication.After30min,the above solution was transferred to a three-neckflask(250mL)and kept at45◦C.Then the solution was stirred for12h to obtain saturated adsorption of AIP on the surface of Fe3O4particles.Subsequently,ethanol/water (5/1,v/v,50mL)was added into the solution,with continued stir-ring for a specific time to allow the hydrolysis of AIP.Then,the mixture solution was transferred to a Teflon-sealed autoclave and heated at80◦C for20h.The obtained particles were separated with a permanent magnet,washed several times with deionized water and ethanol,and then dried in vacuum at50◦C for12h(denoted as1-␥-AlOOH@Fe3O4).The obtained1-␥-AlOOH@Fe3O4magnetic particles dispersed in AIP ethanol solution,then the whole coatingwas repeated to coat another monolayer of␥-AlOOH on the sur-face of1-␥-AlOOH@Fe3O4.The coating was repeated up to three times,to obtain magnetic particles with three layers of ␥-AlOOH(denoted as3-␥-AlOOH@Fe3O4).2.4.CharacterizationThe X-ray diffraction(XRD)patterns were recorded using a D/Max2500VB2+/PC diffractometer with Cu K␣irradia-tion( =0.15418nm),to characterize the crystal structures of the as-prepared samples.HRTEM and SEM images that contain information about the surface morphology and particle size of the as-prepared samples were obtained using JEOL(JEM2100) transmission emission microscope and Zeiss SUPRA55scanning electron microscope.Magnetic properties of the samples were measured using a vibrating sample magnetometer(VSM;Lake Shore Model7400)under magneticfields up to20kOe.Nitro-gen adsorption–desorption analysis was tested on ASAP2020M automatic specific surface area and aperture analyzer.Before mea-surements,the samples were pretreated at300◦C for4h.The adsorption data were used to calculate the specific surface area (S BET)using Brunauer–Emmett–Teller(BET)method.The pore vol-ume and pore size distribution were derived from the desorption Fig. 1.XRD patterns of samples:(a)Fe3O4,(b)1-␥-AlOOH@Fe3O4,(c)3-␥-AlOOH@Fe3O4and(d)␥-AlOOH(concentration of AIP=0.016mol/L,hydrolysis time of AIP=1h).Insert corresponds to partially enlarged XRD patterns.Fig.2.Nitrogen adsorption–desorption isotherms of samples:(a)Fe3O4,(b)1-␥-AlOOH@Fe3O4and(c)3-␥-AlOOH@Fe3O4(concentration of AIP=0.016mol/L, hydrolysis time of AIP=1h).branches of isotherms using the Barrett–Joyner–Halanda(BJH) method.3.Results and discussion3.1.XRD and BET of the samplesFig.1shows the large-angle XRD patterns of the samples.In pattern(a),the typical peaks can be indentified clearly as cubic spinel structural Fe3O4(JCPDS No.88-0866).Pattern(b)is the Table1BET surface area,pore volume and average pore size of samples.Sample S BET(m2/g)Pore volume(cm3/g)Average poresize(nm)Fe3O4310.14 1.91-␥-AlOOH@Fe3O4620.16 3.83-␥-AlOOH@Fe3O41790.50 3.8Y.Zheng et al./Particuology 10 (2012) 751–758753Fig.3.SEM images of samples:(a)and (b)Fe 3O 4;(c)and (d)1-␥-AlOOH@Fe 3O 4(concentration of AIP =0.016mol/L,hydrolysis time of AIP =1h).XRD pattern of 1-␥-AlOOH@Fe 3O 4,showing the diffraction peaks of Fe 3O 4though the peaks of ␥-AlOOH are hardly observable,probably due to the thin-skinned ␥-AlOOH shell.Pattern (c)is the XRD pattern of 3-␥-AlOOH@Fe 3O 4,in which,besidestheFig.4.EDX spectrum of mesoporous ␥-AlOOH@Fe 3O 4magnetic microspheres (con-centration of AIP =0.016mol/L,hydrolysis time of AIP =1h).diffraction peaks of Fe 3O 4,three additional peaks at 12.4◦,27.6◦,and 49.4◦can be indexed to orthorhombic ␥-AlOOH (JCPDS No.21-1307).To better observe the diffraction peaks of ␥-AlOOH,a partially magnified XRD pattern is shown in Fig.1topright.Fig. 5.Room-temperature magnetization curves of samples:(a)Fe 3O 4,(b)1-␥-AlOOH@Fe 3O 4and (c)3-␥-AlOOH@Fe 3O 4(concentration of AIP =0.016mol/L,hydrolysis time of AIP =1h).754Y.Zheng et al./Particuology10 (2012) 751–758Fig.6.TEM images of1-␥-AlOOH@Fe3O4magnetic nanomicrospheres at different hydrolysis times:(a)and(b)uncoated Fe3O4nano-particles,(c)and(d)0.5h,(e)and(f) 1h,(g)and(h)2h,and(i)and(j)6h(concentration of AIP=0.016mol/L).Y.Zheng et al./Particuology10 (2012) 751–758755Nitrogen adsorption–desorption isotherms of all samples are shown in Fig.2.The isotherms b and c present type IV(definition by IUPAC)which is characteristic of mesoporous materials.And the appearance of type3-H hysteresis loops in isotherms b and c indicates the presence of“slit-like”type pores in␥-AlOOH@Fe3O4 magnetic particles(Gregg&Sing,1982).Table1lists the textu-ral parameters of all samples.It can be seen that the BET surface area,pore volume and average pore size of1-␥-AlOOH@Fe3O4are 62m2/g,0.16cm3/g and3.8nm,respectively.Obviously,the3-␥-AlOOH@Fe3O4particles have a larger BET value of179m2/g and a larger pore volume of0.5cm3/g than1-␥-AlOOH@Fe3O4,but retain the average pore size of3.8nm unchanged.3.2.SEM and VSM of the samplesFig.3(a)and(b)shows the SEM images of Fe3O4particles, showing in Fig.3(a)that Fe3O4particles are spherical with an aver-age diameter of200nm.Fig.3(b)shows that Fe3O4nanospheres were formed by self-assembly of many small Fe3O4nanoparticles (Zhu,Zhao,Chen,&Shi,2007).Fig.3(c)and(d)shows that1-␥-AlOOH@Fe3O4nanoparticles are also spherical with uniform size, with diameters slightly larger than those of Fe3O4nanospheres. The highly magnified SEM image(Fig.3(d)),shows the1-␥-AlOOH@Fe3O4nanospheres to have a coarser surface,because AIP does not dissolve in absolute ethanol(Kinoshita et al.,2011).In Fig.4,the energy-dispersive X-ray(EDX)spectrum(using Si-wafer)of mesoporous␥-AlOOH@Fe3O4magnetic nanomicro-spheres indicates the presence of O,Al and Fe,in agreement with the XRD results(Fig.1),thereby supporting the ele-mental composition of the magnetic nanomicrospheres.The elemental compositions of specimens of3-␥-AlOOH@Fe3O4and1-␥-AlOOH@Fe3O4are shown in Fig.4insert table.The atomic ratio of Al:Fe of3-␥-AlOOH@Fe3O4is found to be2.5:1,which is close to three times of that of1-␥-AlOOH@Fe3O4.The magnetization curves of the samples at room temperature are shown in Fig.5.The saturation magnetization(Ms)values of Fe3O4,1-␥-AlOOH@Fe3O4and3-␥-AlOOH@Fe3O4are77.1,71.2 and46.6emu/g,respectively,with relatively low remanence and coercivity values.The results reveal that the Ms values of sam-ples diminish with increasing thickness of the␥-AlOOH shell on the pared to literature(Kwon et al.,2007), the␥-AlOOH@Fe3O4magnetic nanomicrospheres prepared by the above experiment possess excellent magnetic response to external magneticfield.3.3.Effect of reaction parameters on the structure3.3.1.Hydrolysis time of AIPTo investigate the effects of hydrolysis time of AIP on samples, a set of experiments were performed by varying hydrolysis times from0.5h to6h.Fig.6(a)and(b)shows the TEM images ofuncoated Fig.7.TEM images of1-␥-AlOOH@Fe3O4magnetic nanomicrospheres for different AIP concentrations:(a)0.008mol/L,(b)0.016mol/L,(c)0.024mol/L and(d)0.032mol/L (hydrolysis time=1h,number of coating layer=1).756Y.Zheng et al./Particuology 10 (2012) 751–758Fig.8.TEM images of ␥-AlOOH coated magnetic nanomicrospheres for different coating layer numbers:(a)one,(b)two and (c)three (hydrolysis time =1h,concentration of AIP =0.016mol/L).Fe 3O 4nanospheres.It can be seen that Fe 3O 4nanospheres are spherical with coarse surfaces.Fig.6(c)–(j)shows the TEM images of 1-␥-AlOOH@Fe 3O 4at different hydrolysis times.For hydrolysis time of only 0.5h,Fe 3O 4nanospheres were partially coated by the ␥-AlOOH,forming a discontinuous ␥-AlOOH shell layer.When the hydrolysis time was prolonged to 1h,as shown in Fig.6(e)and (f),a continuous and dense ␥-AlOOH shell with an average thickness of 5nm was formed on the surfaces of Fe 3O 4nanospheres.Extend-ing the hydrolysis time to 2h and 6h,the ␥-AlOOH shell on the surfaces of Fe 3O 4nanospheres was still continuous,and its thick-ness remained constant.These results show that the formation of ␥-AlOOH shell is effected by the hydrolysis plete hydrol-ysis of AIP can be achieved within 1h,and further extending the hydrolysis time is unnecessary.3.3.2.Concentration of AIPFig.7shows the TEM images of 1-␥-AlOOH@Fe 3O 4prepared with different AIP concentrations from 0.008to 0.032mol/L.For AIP concentration of 0.008mol/L,as shown in Fig.7(a),only a thin ␥-AlOOH shell was formed on the surfaces of Fe 3O 4nanospheres.In Fig.7(b),when the concentration of AIP was increased to 0.016mol/L,a continuous and dense filmy ␥-AlOOH shell with an average thickness of 5nm was formed on the surface of Fe 3O 4nanospheres.While AIP concentration was raised to 0.024and0.032mol/L,as shown in Fig.7(c)and (d),floccular ␥-AlOOH appeared around the Fe 3O 4nanospheres.And the floccular ␥-AlOOH increased with increasing AIP concentration,while the morphology of the ␥-AlOOH shell changed from filmy to floccular.Meanwhile,the prepared 1-␥-AlOOH@Fe 3O 4nanomicrospheres aggregated severely.These results reveal that the thickness of the ␥-AlOOH shell can be increased by increasing the concentration of AIP.But beyond AIP concentration of 0.016mol/L,continued increase of AIP concentration caused the formation of floccular ␥-AlOOH shell,thereby leading to non-uniform shell thickness.3.3.3.Coating timesFig.8shows the TEM images of ␥-AlOOH@Fe 3O 4with different numbers of coating layer.The insets reveal that all samples present a core–shell structure.From the partially enlarged figures,all sam-ples show a filmy mesoporous ␥-AlOOH shell with thicknesses of 5,10and 15nm,respectively.The results show that the thickness of mesoporous ␥-AlOOH shell increases with the number of coat-ing layers,though the morphology remains the same.By adding a layer of ␥-AlOOH shell,the thickness of ␥-AlOOH shell is increased by about 5nm.These results also explain why the diffraction peaks of ␥-AlOOH are stronger in pattern (c)of Fig.1,and the S BET of 3-␥-AlOOH@Fe 3O 4is much larger in Table 1.Y.Zheng et al./Particuology10 (2012) 751–758757Fig.9.Schematic of the formation mechanism of␥-AlOOH shell coated Fe3O4core.3.4.Formation mechanismIn general,the formation of boehmite(␥-AlOOH)can be envis-aged by the following hydrolysis and alcohol condensation reaction (Brinker&Scherer,1990):((CH3)2CHO)3Al+2H2O=AlO(OH)+3(CH3)2CHOH.Izumi,Hiroyuki,and Toyoki(1997)reported the preparation offilms of metal oxides by the means of stepwise adsorption of alkoxides.The process was composed of four steps:chemisorp-tion of alkoxides,rinsing,hydrolysis of chemisorbed alkoxides and drying.From the above literature and our results,the forma-tion mechanism of␥-AlOOH shell coated Fe3O4core could be a “chemisorptions–hydrolysis”process,as shown schematically in Fig.9.Step one is the chemisorption process,initially resulting in many uncoordinated carbonyl groups on the surface of Fe3O4 nanospheres prepared by the solvothermal process using PVP as the surfactant(Lu,Niu,Qiao,&Gao,2008).After stirring for12h,these uncoordinated carbonyl groups were adsorbed by AIP in the solu-tion,leading to AIP adsorption on the surface of Fe3O4nanospheres (Hicks&Pinnavaia,2003;Pattanaik&Bhaumik,2000).When the concentration of AIP reached0.016mol/L,the adsorption of AIP on the surface of Fe3O4nanospheres became saturated.Following the above was the hydrolysis process.After adding the ethanol/water mixture,the AIP on the surface of Fe3O4nanospheres was hydrolyzed into␥-AlOOH,resulting in the formation offilmy mesoporous␥-AlOOH shell on the surface of Fe3O4nanospheres. Coating a monolayer of␥-AlOOH resulted in a large amount of hydroxyl groups on the surface of the mesoporous␥-AlOOH shell(Hicks&Pinnavaia,2003).Repetition of the coating pro-cess resulted in the adsorption of AIP from the solution(Izumi et al.,1997),thereby increasing the thickness of the mesoporous ␥-AlOOH shell.According to Kuiry,Megen,Patil,Deshpande,and Seal(2005), the water content in the present synthesis was less,and there-fore,the hydrolysis was incomplete and the parallel polymerization might possibly lead to the formation offloccular␥-AlOOH in the solution.In our work,when the concentration of AIP was more than0.016mol/L,there was a large number of dissociative AIP in the solution.During the hydrolysis,␥-AlOOH shell on the surface of Fe3O4nanospheres could react withfloccular␥-AlOOH(Kim,Lee, Jun,Park,&Potdar,2007)in the solution,to result in the morphol-ogy transition of␥-AlOOH shell fromfilmy tofloccular(Fig.7(d)).4.ConclusionsMesoporous␥-AlOOH@Fe3O4magnetic nanomicrospheres with a shell–core structure were successfully prepared with hydrolysis time of1h and AIP concentration of0.016mol/L.The magnetic nanomicrospheres are composed of Fe3O4nanospheres withfilmy mesoporous(3.8nm,average pore size)␥-AlOOH shell.The thickness of the mesoporous␥-AlOOH shell can be controlled by the layer-by-layer coating process.The formation mechanism of mesoporous␥-AlOOH shell can be interpreted as a “chemisorption–hydrolysis”process.Among prepared samples,3-␥-AlOOH@Fe3O4nanomicrospheres with three layers of␥-AlOOH shell show higher saturation magnetization(46.6emu/g)and larger S BET(178.9m2/g),promising a wide range of potential applications in catalysis and adsorption separation.AcknowledgmentsFinancial funds from the National Natural Science Founda-tion of China(Grant Nos.21173018and21136001)are gratefully acknowledged.ReferencesBrinker,C.J.,&Scherer,G.W.(1990).Sol–gel sciences:The physics and chemistry of sol–gel processing.San Diego:Academic Press.Bromberg,L.,Chang, E.P.,Hatton,T. 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