Self-assembled,-monodispersed,-flower-like-g-AlOOH

Self-assembled,monodispersed,flower-likeg-AlOOHhierarchicalsuperstructuresforefficientandfastremovalofheavymetalionsfromwaterSelf-assembled,monodispersed,uniform,andflower-likeg-AlOOHhierarchicalsuperstructureshavebeensynthesizedinhighyieldviaasimple,economicalandenvironmentallyfriendly,hydrothermalroute.TheproductpossessesalargeBETsurfaceareaof145.5m2g
1.ItisfoundthatPb(II)andHg(II)ionscanbequicklyremovedfromaqueoussolutionsbytheflower-likeg-AlOOH.Afteronly5min,theremovalrateforPb(II)andHg(II)ionsisover99.0%.Themaximaladsorptionisca.124.22mgg
1forPb(II)and131.23mgg
1forHg(II).Heavymetalions,suchasPb(II),Hg(II),Cd(II),andCu(II),arehighlytoxicwaterpollutantsandtheirefficientremovalfromwaterisofgreatimportance.Adsorptionisoneofthemosteffectiveandsimplestapproachesthathasbeendevelopedforthispurpose.Variousmetaloxideshavebeenextensivelyemployedastheadsor-bentsforremovingheavymetalions.1–3Amongthem,g-Al2O3,asanontoxicandlowcostadsorbent,playsanimportantroleinremovingheavymetalionsfromthenaturalenvironment.4–6g-Al2O3canusuallybeobtainedviadehydrationofg-AlOOH(boehmite).Duringheating,theg-AlOOHnanostructuresundergoanisomor-phoustransformationtonanocrystallineg-Al2O3,andtheproductscanretainthemorphologyoftheparentg-AlOOHnanostructures.7Uptonow,varioushierarchicallystructuredg-AlOOH,suchasnanowirebunches,7hierarchicallynanostructuredmicroflowers,8rotor-likeandcarambola-likemicro/nanoarchitectures,9hollowmicrospheres,10andspindle-likenanoarchitectures,6havebeenprepared.However,itstillremainsachallengetodevelopafacileandenvironmentallyfriendlymethodforobtainingmonodispersed,uniform,andflower-likehierarchicalg-AlOOHwiththinlamellarstructures.Inaddition,hierarchicalg-AlOOH,asamainprecursorfortheproductionofhierarchicalg-Al2O3,wasalsoseldomusedasanadsorbentfortheremovalofheavymetalions(divalentcation)inmodelwastewater.Herein,wedesignasimple,economicalandenvironmentallyfriendlyhydrothermalroutetosynthesizemono-dispersed,uniform,andflower-likeg-AlOOHat140Cfor10hinhighyield,employingdeionizedwaterasasolventandNaAlO2andureaasreagentsinthepresenceofpolyacrylicacidsodiumsalt(PAAS)assurfactant.Inthisself-assemblyprocessunderhydro-thermalconditions,inexpensivealuminiumsaltsandnontoxicreagentswereused.Theas-obtainedflower-likeg-AlOOHstructureshavealargeBETsurfaceareaandcanbeusedasadsorbentsforthefastremovalofhighlytoxicheavymetalPb(II)andHg(II)ionsfromwater.Inaddition,theproductalsohasahighadsorptioncapacityforthetwometalions.Fig.1SEMimages(aandb),TEMimages(c–e),EDXspectrum(f)andXRDpattern(g)oftheflower-likeg-AlOOH.ViewArticleOnlineScanningelectronmicroscopy(SEM)imageoftheproduct(Fig.1a)showsthattheproductconsistsofmanymonodispersed,uniform,andflower-likearchitectureswithadiameterofapproxi-mately700nm.Thedetailedmorphologyoftheflower-likestructuresisshowninFig.2b,whichrevealthatthestructureiscomposedofmanynanosheetswithathicknessof3–8nm.As-preparednano-flowersaremonodispersedandcanbewelldispersedinthesolutionwithoutlargeaggregation(Fig.1aandc).Theflower-likeproductisfurtherrevealedbythetransmissionelectronmicroscopy(TEM)imagesinFig.1candd,fromwhichitisapparentthatthestructureisassembledbysomenanosheets.Fig.1erevealsthehigh-resolutionTEMimagesofthenanosheet,whichclearlyindicatethatthenanosheetisacurvinglamellawithathicknessofca.3.6nm.Theenergy-dispersiveX-ray(EDX)spectrumofthenanoflowerindicatesthepresenceofAlandO,therebyprovingthepresenceofg-AlOOH(Fig.1f).AtypicalX-raydiffraction(XRD)patternoftheproductobtainedisshowninFig.1g.Allofthediffractionpeakscanbeindexedtotheorthorhombicg-AlOOH(JCPDS21-1307).Comparedwiththestandarddiffractionpatterns,nocharacteristicpeaksarefromimpurities.TheFouriertransforminfraredspectros-copy(FTIR)spectrumandroomtemperaturephotoluminescence(PL)spectrumalsoconfirmthestructureoftheflower-likeg-AlOOH(seeFig.S1,ESI†).Toinvestigatethegrowthmechanismoftheflower-likeg-AlOOH,time-dependentexperimentswereconductedtorevealthegrowthprocess.SEMandXRDwereemployedtomonitortheevolutionofthemorphologyandcrystallinestructureoftheprod-uctscollectedatdifferentreactiontimes.Fig.2a–dshowstheSEMimagesoftheproductsobtainedatdifferentreactiontimes.AsshowninFig.2a,after30minduration,theproductwithlamellarmorphologyappeared.Byincreasingthehydrothermaltimeto1h,theflower-likeproductwhichwasassembledbyfewerlamellarstructureswasprepared(Fig.2b).Furtherprolongingthereactionto5and10h,thelamellarstructuresincreasedclearly(Fig.2cand2d).Themorphologyoftheproductdidnotchangewithlongerreactiontime.ThecorrespondingXRDpatternsoftheintermediateproductsobtainedatdifferentreactiontimesareshowninFig.S2(ESI†).Onthebasisofthetime-dependentexperiments,theformationoftheflower-likeg-AlOOHcanberationallyexpressedasaselfassemblymechanismasshowninFig.2e.First,athightemperature,aluminateanionsundergothefollowingchemicalreactions:AlO2
+2H2O/Al(OH)3+OH
(1)Al(OH)3/g-AlOOH+H2O(2)Whenthesolutionwasheatedat140C,Al(OH)3wouldbeformedonthebasisofthehydrolysisreactionthrougheqn(1);thenewlyformedAl(OH)3wasactiveandunstable,whichwouldfurtherdehydrateandconvertintog-AlOOHthrougheqn(2