Aqueous hydrothermal techniques for ZnO nanorod crys-tal growthcan proceedrapidly atrelatively mild temperatures... 2.Scanning electron microscopy images of silicon wafers after ZnO hydr
Trang 1Applied Catalysis A: General
jo u r n al h om ep a g e :w w w e l s e v i e r c o m / l o c a t e / a p c a t a
Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695, USA
Article history:
Received 29 March 2011
Received in revised form 26 August 2011
Accepted 28 August 2011
Available online 3 September 2011
Keywords:
Atomic layer deposition
Nonwoven fiber
Diethyl zinc
Zinc oxide
Nanocrystals
Nanorods
Hydrothermal
Photocatalytic
a b s t r a c t Photocatalyticallyactivezincoxidenanocrystallinerodsaregrownonhighsurfaceareapolybutylene terephthalate(PBT)polymerfibermatsusinglowtemperaturesolutionbasedmethods,wheretheoxide crystalnucleationisfacilitatedusingconformalthinfilmsformedbylowtemperaturevapor phase atomiclayerdeposition(ALD).Scanningelectronmicroscopy(SEM)confirmsthathighlyoriented sin-glecrystalZnOnanorodcrystalsaredirectednormaltothestartingfibersubstratesurface,andthe extentofnanocrystalgrowthwithinthefibermatbulkisaffectedbytheoverallthicknessoftheZnO nucleationlayer.Thehighsurfaceareaofthenanocrystal-coatedfibersisconfirmedbynitrogen adsorp-tion/desorptionanalysis.Anorganicdyeinaqueoussolutionincontactwiththecoatedfiberdegraded rapidlyuponultravioletlightexposure,allowingquantitativeanalysisofthephotocatalyticpropertiesof fiberswithandwithoutnanorodcrystalspresent.Thedyedegradesnearlytwiceasfastincontactwith theZnOnanorodcrystalscomparedwithsampleswithonlyanALDZnOlayer.Additionally,thecatalyst
onthepolymerfibermatcouldbereusedwithoutneedforaparticlerecoverystep.Thiscombinationof ALDandhydrothermalprocessescouldproducehighsurfaceareafinishesoncomplexpolymersubstrates forreusablephotocatalyticandothersurface-reactionapplications
© 2011 Elsevier B.V All rights reserved
The large band gap and strong exciton binding energy of
zinc oxidemake it a valuable semiconductor for many
micro-electronic and optoelectronic devices including solar cells [1],
photo-detectors [2]and light emittingdiodes[3,4].In addition,
ZnOisoneofmanynaturallyoxygendeficientmetaloxidesthat
willphotocatalyticallydecomposecomplexorganicmoleculesin
thepresenceofUVillumination[5–7].NanostructuredZnO
crys-talsareparticularlyinterestingforphotocatalysisbecauseoftheir
highsurfaceareawhichincreasesthecrystal/solutioncontactarea
Recently,researchershavedefinedmethodstocreatecrystalline
ZnOnanowires[1,8,9],nanorods[10],nanotubes[11],nanobelts
[12,13],nanotowers[14],dendritichierarchicalstructures[15]and
an assortment of other structures [16] However, few of these
studiesaddressedissuesinphotocatalysis.Oneproblemwith
free-standingZnOnanostructuresisthattheycouldreadilyaggregate
inaqueoussolution.Itisalsoachallengetorecycleandregenerate
thesenanostructuresfromthesolution.Catalyticallyactive
parti-cleswithmagneticattractionshowsomepromiseinthisregard
[17].AnotherpromisingapproachistoattachZnOnanostructures
ontoathree-dimensional(3D)highsurfaceareasupport.Polymer
∗ Corresponding author.
E-mail address: parsons@ncsu.edu (G.N Parsons).
fibermatsareespeciallyattractiveassupportsbecausetheyare inexpensive,readilyavailable,and theyareflexible andeasy to use
Aqueous hydrothermal techniques for ZnO nanorod crys-tal growthcan proceedrapidly atrelatively mild temperatures (<150◦C),andtheprocessingpermitssurface-selectivegrowththat drivesnanostructureevolution[18].Formosthydrothermal meth-ods,anoxideseedlayerisessentialtoinitiateandcontinuecrystal evolution.Theseedlayerpresentsnucleationsites,loweringthe thermodynamicbarrierforZnOnano-and micro-crystalgrowth andfurtherenhancingthegrowthdirectionselectivityandaspect ratio[14,15].Previousresearchersformnucleationsitesby apply-ing ZnO particlesor a nanocrystalline filmby dipcoating,spin coating [15] orsputtering [19].These approaches canworkfor depositiononplanarsurfaces,butforcomplex3Dsubstrates,these methodsarenotexpectedtoyielduniformseedlayersand homo-geneousseedlayerdistribution
Atomiclayerdeposition(ALD)isavaporphasethinfilm deposi-tiontechniquewhichcandepositmaterialsuniformlyoncomplex 3Dsurfaces.IntheALDprocess,twoco-reactants(e.g.diethylzinc andwaterforZnO formation)areintroducedontothesubstrate alternatively, separatedbyaninertgaspurgestep,allowingthe surfacetoreactwitheachreagentinaseriesofself-limiting adsorp-tion/reactionsteps[16,20–23].Repeatingthissequencebuildsupa coatingwithdesiredthicknessonthesubstrate.Severalresearch groups recently showedthat this process yields uniformmetal
0926-860X/$ – see front matter © 2011 Elsevier B.V All rights reserved.
Trang 2212 B Gong et al / Applied Catalysis A: General407 (2011) 211– 216
Fig 1. Schematic view of the viscous flow ALD reactor used for these studies In
one ALD cycle, two co-reactants (e.g diethyl zinc and water for ZnO formation) are
introduced alternatively, with an inert gas purge step in between, allowing
forma-tion of one atomic layer of ZnO Desired thickness could be achieved by repeating
the ALD cycles.
oxidethin filmcoatingsonhighaspectratiopolymerfiber
sub-strates[20–26].Someofthesereportsalsoshowphotocatalytic
performanceoftheresultingpolymer/oxidestructures[24,25].For
thisstudy,weshowthattheALDcoatingprovidesanidealseed
layerforhydrothermal growthofZnO nanorodcrystalsonfiber
substrates,andthatthesenanocrystal-coatedfibersshowhigh
pho-tocatalyticactivitycomparedtopreviousstructures
Inparticular,wedescribeanALDprocesstodepositathinlayer
ofZnOontoa polybutyleneterephthalate(PBT)nonwovenfiber
mat,wheretheZnOlayeristhenusedasaseedlayerforlow
tem-peratureZnOnanorodhydrothermalgrowth[16].Thissequence
creates a hierarchical fiber/nanorod crystal composition with
surface-normalZnOnanorodsonthecylindricalfibertemplate.The
finalfibercross-sectionwasimagedandphysicallycharacterized,
andthephotocatalyticpropertiesofthefiber/nanorod
construc-tionweretestedandcomparedtouncoatedfibersandtofibers
uniformlycoated withZnO ALD (i.e.without thehydrothermal
growthstep).Thehierarchicalstructureshowssuperior
photocat-alyticperformance,consistentwiththeexpectedenhancedsurface
area
2.1 ZnOseedlayerdepositionbyALD
ThesubstrateforZnOnanocrystalgrowthwasamultilayered
nonwovenPBTfibermatacquiredfromtheNonwovenCooperative
ResearchCenter(NCRC)atNCStateUniversity.Electronmicroscopy
imagesofthePBTmatsshowedthattheywereamassofindividual
fibers(2–3mindiameter)withatotalmatthicknessof∼0.5mm
[27].WemonitoredALDgrowthbydepositingsimultaneouslyonto
polishedsiliconwaferpieces.Fig.1displaysaschematicdrawingof
thehomemadeviscousflowhotwallvacuumreactorusedforzinc
oxideALD[28].Thereactionsystemiscomposedofstainlesssteel
tube∼3.5cmindiameter,surroundedbyaheatingjacketto
con-trolthereactortemperature(100◦Cforthesestudies).Thecarrier
gaswasultrahigh-purityAr(99.999%NationalWelders)flowing
at∼200standardcubiccentimetersperminute(sccm).The
reac-tionsystemwaspumpedusinga rotarymechanical pump,and
thesteady-stateprocesspressurewas∼1.0Torr,asmonitoredbya
Baratronpressuregauge(MKSInstrumentInc.).OneZnOALDcycle
consistedofa2sexposuretodiethylzinc(DEZ,98%Strem
Chemi-cal)followedbya60sArpurge,a2swaterexposure,andanother
60sArpurge(thesequenceisdenotedas2/60/2/60s).Thereactant
pulseproducedapressureincreaseof50mTorrinthereactor.The
seedlayersweredepositedusingeither100or200ZnO
deposi-tioncycles,whichproduce∼20or40nmthickfilms,respectively,
onplanarsiliconsubstrates.Refractiveindexandfilmthickness
onsiliconwasmeasuredbyvariable-anglealpha-SEspectroscopic
ellipsometry(J.A.WoollamCo.,Inc.)
2.2 HydrothermalgrowthofZnOnanorodcrystalsonseedlayer AfterALDcoating,thePBTfibersandsiliconcontrolwaferwere transferredintoateflonvesselcontaining30mlaqueoussolutionof equimolar(20mM)zincnitratehexahydrate(Zn(NO3)2·6H2O,99% Aldrich)andhexamethylenetetramine(C6H12N4,99%Aldrich).The vesselwasleftopenandheldinanovenat80◦Cfor6hresulting
inthegrowthofZnOnanorodcrystalsontheZnOcoatedsilicon andPBTsubstrates.Thesiliconwaferwasheldface-downinthe solutiontopreventtheprecipitationofanyZnOparticlesthatmay haveformedinthesolutionbulk.Aftergrowth,thePBTfibermat andSiwaferwererinsedwithdeionizedwaterfor2min,andthen driedinN2 flowatroomtemperature.Seedlayerthicknessesof
∼20and40nmwereinvestigated
2.3 Microscopyandsurfaceanalysis Themicrostructureofthemodifiedfiberswasanalyzedusing
anFEIXL30ScanningElectronMicroscope(SEM)operatingat7kV withaworkingdistanceof5mm.BeforeSEMimaging, samples sputter-coated with5nm of Au/Pd to reduce surface charging TransmissionElectronMicroscope(TEM)imagesofZnOnanorod crystalsonpolymerfibermatswerecollectedusingaHitachiHF coldfieldemissionTEMoperatedat200kVwith0.2nmpoint res-olution.TheTEMsampleswerepreparedbyheatingthetreated fiberat400◦Cinairfor24h,resultingincalcinationofthe poly-mer.Aftercalcination,theresultingmaterialsweredispersedin methanol,sonicatedfor1min,andthentransferredbypipetteonto carbonfilm-coatedTEMgrids(TedPella,Inc.)
Thestaticwatercontactangleonthestartingandmodified sur-faceswascollectedusinga Model200RameHartcontactangle goniometerequippedwithaCCDcamera.Wemeasuredatleastfive differentpointsoneachsampleandtheaveragevalueisreported
AQuantachromeAutosorb-1Csurfaceareaandporesize ana-lyzerprovidedinformationontheBrunauerEmmettTeller(BET) surfaceareaofthematerialsbeforeandafterprocessing.Before eachanalysis,sampleswereheatedundervacuumat100◦Cforat least4htoremoveresidualandmoistureadsorbed.Therecorded datawascollectedfrom∼200mgsamplesusingasevenpointBET (P/P0rangefrom0.05to0.35)analysis
2.4 Photocatalyticcharacterization FibersampleswithZnO ALD coating,ZnO coatingwith sub-sequenthydrothermalnanocrystalgrowth,aswellasuntreated fiberswereallcutintouniformsamplepieces(1.8cm×1.8cm)and placedintothreeglassvials,eachcontaining25mlofdeionized waterwithequalconcentrations(3×10−4vol.%)ofcommercially availableazoacidred40dye.Wethenexposedthevial(uncapped)
to UV radiation froma shuttered Intell-Ray 400Uvitron Inter-national UV floodlight (320–390nm) providing 79mW/cm2 of energyfluximpingingfromthetop.Theincidentpowerdensity wasdeterminedusinga1916-CNewportopticalpowermeter.By monitoringtheconcentrationofthedyeinthevesselbyUV–vis absorbance(measuredbyaThermoScientificEvolution300 UV-Visspectrophotometer) asa function of time, we wereable to quantifytherelativerateofdyedegradationand henceanalyze theeffectivephotocatalyticactivityofthedifferentprepared sam-ples
Fig.2presentsSEMimagesofsiliconwafersafter hydrother-malZnOnanorodcrystalgrowth.Thesampleinpanels(a)and(b)
ispreparedbyhydrothermalgrowthdirectlyonthesiliconwafer
Trang 3Fig 2.Scanning electron microscopy images of silicon wafers after ZnO hydrothermal growth Images (a) and (b) were collected from samples without an ALD ZnO nucleation layer Alternatively, images (c) and (d) were from silicon samples that were coated with 100 cycles of ALD ZnO before hydrothermal ZnO nanorod crystal growth.
(i.e.withouttheZnOALD seedlayer),andtheimagesinpanels
(c)and(d)werecollectedfromasiliconwaferwiththeALDZnO
seedlayer.Withouttheseedlayer,onlysmallamountofsparsely
distributedZnOnanocrystalsarepresent.Theyarealsorelatively
large(∼1mindiameterand∼3–5mlong).Whenthesubstrate
is pre-coatedwith100ZnOALD cycles(producinga seedlayer
∼20nmthick,asdeterminedbyellipsometry),thehydrothermal
growthstepyieldscompletecoverageofZnOnanorodcrystalswith
uniformsizeof∼50nmdiameterand∼500nmlong.Wealsonote
thatthenanorodsshowpredominantlysurface-normalorientation,
whereasmorerandomorientationisproducedwithouttheseed
layer.TheALDZnOprovidesagoodseedlayerforthe
hydrother-malgrowthofZnOnanocrystals.ThedetailedALDconditioncould
changethesurfaceroughnessofthePBTfibermat,andfurtheraffect
themorphologyofcoatedZnOnanorods
TheeffectsofZnOALDseedingwerealsotestedonpolymerfiber
mat.Fig.3presentsSEMimagesofPBTnonwovenfibermatsafter
ZnOnanorodcrystalgrowth.ForthebarePBTfibermat,theimages
inFig.3(a)and(b)showonlysparseandrelativelylargeZnO
clus-ters,similartogrowthonuntreatedsiliconwafer.Fig.3(c)and(d)
showsaPBTfibermatafter20nm(100cycles)ofALDZnOfollowed
byhydrothermalgrowth.Interestingly,ZnOnanocrystalsonlygrow
ontheoutersurfaceofthesubstratemat,andfibersinthemiddle
layersofthesubstrateshowalmostnonanocrystalgrowth.This
non-uniformityisparticularlyvisibleinFig.3(d),inwhichfibersat
thetopofthematappeartohaveamuchlargerdiameterbecause
ofthenanorodcrystals
To understand this non-uniformity in nanorod growth, we
examinedwaterdropletcontactangleandwaterpenetrationinto
thenonwovenfibermataftertheALDcoating[22].Asreceived,the
PBTfibersappearhydrophobic.Awaterdropletplacedonthefiber
matdidnotabsorbandtheaveragestaticwatercontactanglewas
∼120◦.Aftercoatingthematwith100cyclesofZnOALD,water
stilldidnotreadilypenetrate,andthecontactanglewas∼100◦.
We believethatthehydrophobicnatureofthecoatedPBTfiber matlimitsthepenetrationoftheaqueoushydrothermalprocess solutionintothemat,resultinginhydrothermalgrowthprimarily
ontheouterfibers,asshowninFig.3(c)and(d).Wefind,however, thatafter200cyclesofZnOALD,thePBTfibermatbecame com-pletelywetting(contactangle∼0◦),whichwillreadilyallowthe
aqueoushydrothermalsolutiontopenetrateintothematrix.This wettingtransitionforALDcoatedpolymerfibershasbeen previ-ouslyobserved,anditisunderstoodtoresultfromacombination
ofchangesinsurfacechemicalterminationandsurfaceroughness [22].AsdemonstratedinFig.3(e)and(f)PBTfibersamplescoated with200cyclesALDZnOasaseedlayeryieldedauniformcoating
ofZnOnanorodcrystalsdeeperintothefibermat.Severalsample fibersextractedatrandomfromthebulkofthematwereexamined
bySEM,andallshowedgoodcoverageofZnOnanocrystalsafterthe hydrothermalgrowthwithsmallvariationinnumberanddensity
ofthecrystallites
HighresolutionTEMimagespresentedinFig.4shownanorod crystals grownonPBT usingthe 200cyclesZnO ALD seed lay-ers The PBT fiber has been removed by a calcination step at
400◦C for 24h Fig 4(a) clearly shows both the oriented ZnO nanorod crystals and the ZnO shell layer The lattice fringe spacing of ∼0.32nm measured in Fig 4(b) confirms the ZnO wurtzite structure The hydrothermal process likely produces zincite [29] which transforms to wurtzite during the relative hightemperaturecalcinationstep.Theparticularsampleshown
in Fig 4 reveals a smaller number of nanocrystals This could result from damage during sonication for the TEM sample
Trang 4214 B Gong et al / Applied Catalysis A: General407 (2011) 211– 216
Fig 3. Scanning electron micrographs obtained from: (a) and (b) untreated PBT fibers after hydrothermal ZnO nanorod crystal growth; (c) and (d) PBT fibers after 100 ALD cycles of ZnO (∼20 nm thick), followed by hydrothermal ZnO nanorod growth Nanorod crystals are visible primarily on the top-most fibers in the fiber mat Panels (e) and (f) show PBT fibers after 200 cycles (∼40 nm) of ALD ZnO, followed by ZnO nanorod growth Nanorod growth is visible on all the fibers In panel (b) a circle highlights a large crystal, similar in size to the one shown in Fig 2 (b), formed on the untreated fiber.
preparation,orsomenon-uniformityinthehydrothermalgrowth
step
ThesurfaceareaiscriticalforthecatalyticperformanceofZnO
structures.TheBETsurfaceareameasuredbynitrogenadsorption/
desorptionanalysiswas∼0.73m2/gfortheuntreatedPBTfibermat, withafactorof2–3increaseinsurfaceareato∼1.79m2/g,afterthe ZnOseedlayerandhydrothermalgrowth.Thisincreaseisrather modestonapermassbasis.However,wenotethatafter
hydrother-Fig 4.Transmission electron microscopy images obtained from ZnO nanorod crystals on PBT fibers where the polymer was removed by calcination before imaging In image (a), the nanorods are visible protruding from the ZnO thin film layer that remains after calcination The arrow on the left in image (a) points to a region of ALD ZnO coating without nanorod crystal growth The image in (b) was collected from the tip of a nanocrystal rod, as indicated by the region circled in (a) The HRTEM image shows the lattice
Trang 5Fig 5.Normalized absorbance of organic dye at 525 nm plotted versus UV radiation
exposure time PBT fiber substrates with various surface treatments were immersed
in the aqueous solution containing the azo dye (acid red 40), and illuminated using a
UV lamp The fibers with ALD ZnO and ZnO nanorod crystals produced the most rapid
photocatalytic dye degradation The inset shows a photograph of the dye solutions
in contact with the different substrates after 2 h of illumination The red dye is nearly
completely removed from the solution in contact with the nanorod-coated fibers.
malgrowth,thenetmass(percm2offibermatsample)increased
byafactoroffourcomparedtothesamplewithALDZnOcoating,
whichverifiesasignificantamountofhydrothermalZnO
deposi-tion.Theincreaseinmass,combinedwithanincreaseinsurface
areaperunitmassbasismeansthatonapersamplebasis(i.e.for
afixedfibermatsamplesize),thesurfaceareaofthefibermat
increasesbyatleastafactorof10comparedtothestartingsample
Anevenlargerincreaseinsurfaceareacouldbeexpectedifafiber
matsupportwithfinerfiberswasused,orifthinnerand/orlonger
nanorodscouldbegrown.Thedensityandporosityofthefibermat
alsolikelyplayaroleindeterminingtheoptimumconditionsto
achieveuniformnanocrystalgrowthandhighsurfacearea
Anorganicdyeinaqueoussolutionwasusedtotestthe
pho-tocatalyticperformanceofZnOfunctionalizedPBTfibermats.The
photocatalyticdecompositionoforganicmaterialsinaqueous
solu-tionis generally believedto beinitiatedby photo-excitationof
ZnO,producing hydroxylradicalsand holeswithhighoxidative
potential,permittingrapidoxidationoforganicsincontactwith
the surface [5,7] Fig 5 shows the photocatalytic performance
ofZnO treatedfibermatsampleswheretheUV–visabsorbance
measured at 525nm, normalized to the starting absorbance of
each dye solution sample, is plotted versus UV exposuretime
Upon UV irradiation,the dyedegraded in allsample vials, but
thesamplevialcontainingthenanocrystal-coatedfibersin
con-tactwiththesolutionshowedasubstantiallyfasterdegradation
ratecomparedwiththeothersamples.Inaddition,weperformed
acontrolexperimentwithoutUVexposurewhereasimilarsized
nanocrystal-coatedPBTfibermatwasplacedintothedyesolution
andkeptindarkfor2h.Asexpected,negligibleUV–visabsorbance
changewasobservedfromthedyesolution,whichconfirmedthat
thedecomposition is photocatalytic.Additionally, dyesolutions
withandwithouttheuntreatedPBTfibermatshowedonly
lim-itedabsorbancechangeunderUVexposure,confirmingthatthe
fibersthemselvesdonotleadtodyedegradation[30].However,we
findthattheconformalZnOcoatingonthefibers(withoutnanorod
growth)issufficienttocatalyzesomeUVdegradationofthedye
Theinsetincludesimagesofthree solutionvials aftera totalof
2hUVexposure.ThevialcontainingthecontrolPBTfibershows
littledegradation,andthevialwithALD ZnOcoatedPBTshows
improveddegradationcomparedtothevialwiththeuncoatedPBT
substrate.ThevialcontainingthePBTwithALDZnOandnanorod
crystalsshowedthebestperformance,degrading∼90%ofthedye
Fig 6.Reusability of ZnO treated PBT fiber mat for photocatalytic dye degradation For the PBT fiber mats coated with ALD ZnO and with ALD ZnO + nanorods both showed repeatable photocatalytic degradation performance over three consecutive 2-h exposure runs The slight decrease in photocatalytic efficiency for each sample
is ascribed to surface contamination that accumulated during testing.
within2h.Thissuperiorperformanceisascribedtothelarger solu-tion/photocatalystcontactareafortheALD/hydrothermalprepared materials
ThereusabilityoftheZnOcoatedPBTfibermatsfor photocat-alytic dyedecompositionwasalsotested.Fig.6 displaysresults
ofthreedegradationtestsperformedinsequenceusingALD ZnO-coatedPBTfibers,andusingsimilarsamplescoatedfurtherwith ZnO nanorods.bothtypesofsamplesshowedrepeatable photo-catalyticactivitytowardsaciddyedegradation,whereagain,the sampleswithnanorodsshowmorerapiddyedissociation.Wenote thataftereachrun,samplesweretransferreddirectlyintoafresh fluidsamplewithoutsurfacecleaningorothertreatment,sothe decreasedreactionrateduringthesecondandthirdrunsislikely duetosurfacecontaminationaccumulatedduringtheprevioustest
Wealsoperformedside-by-sidecomparisonsofthesame mate-rialsetsusingsunlightilluminationinplaceofthelaboratoryUV lamp.While degradationundersunlightwaslessrapidthanfor theUV lamp,theexperimentproduced thesametrendin pho-tocatalyticperformance.Thefiberswithnanorodcrystalspresent showed substantially more degradation with the same expo-suretime TheintegratedALD/hydrothermaldepositionmethod describedheredemonstratedanefficientwaytofurtherimprove photocatalytic materials, and it would be a viable method to enhanceotherphotoactivesurfaceprocesses
PhotocatalyticallyactiveZnOnanorodcrystalsarereadilygrown usinga lowtemperaturehydrothermal procedureonPBT fibers mats, when the fibers are first coated with a conformal ZnO nucleationlayerusingatomiclayerdeposition.TheALDefficiently functionalizedthepolymerfibertofacilitatehydrothermalnanorod crystalnucleationandsubsequentgrowth.Theprocessproduces fiberswith∼10×enhancementintotalsurfacearea(determined from BET analysis) on a per sample size (cm2/cm2) basis We demonstratedthattheZnOfilm/nanorodcompositewill photocat-alyticallydegradeanazoorganicdye(acidred40)inaqueousmedia
ataratethatisnearly2×fasterthanasimilarfiberwithonlythe ZnOfilmcoating.This2×rateenhancementislessthanthe10× sur-faceareaincrease,probablybecauseofshadowingeffectsduring illumination.Moreimportantly,thefunctionalizedpolymerfiber matcouldbereusedveryeasily,andnoadditionalparticle recov-eryprocessisneeded.ThiscombinationofALDandhydrothermal
Trang 6216 B Gong et al / Applied Catalysis A: General407 (2011) 211– 216
processesmayalsobeusefulforothercrystalgrowthsystems,such
asTiO2,Fe2O3,SnO2andV2O5,wherehighareaandreadysolution
accessaredesired
Acknowledgement
WeacknowledgesupportforthisworkfromtheUSNational
ScienceFoundationundergrantCBET-1034374
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