DTIC ADA525311: Tailoring Advanced Nanoscale Materials Through Synthesis of Composite Aerogel Architectures

9 Pages·0.97 MB·English

by  Defense Technical Information Center

Tailoring Advanced Nanoscale
R
E
V
I
E
Materials Through Synthesis of W
S
Composite Aerogel Architectures**
ByMichele L. Anderson,Rhonda M. Stroud,
Catherine A. Morris,Celia I. Merzbacher,
and Debra R. Rolison*
Introducing a desired solid guest to an about-to-gel silica sol prevents
complete encapsulation of the guest particles by the silica, such that the
composite retains the bulk and surface properties of each component on
the nanoscale. The transport- and density-dependent properties of the
composite aerogel can be tuned by varying the volume fraction of the
second solid, thereby increasing the design flexibility of these nanoscale materials for optical, chemical,
thermal, magnetic, and electronic applications. The chemical and physical properties of the composite
material can be further engineered at multiple points during sol–gel processing by modifying the host
solid, the guest solid, the composite gel, or thecomposite aerogel.
1.Introduction specialty optical, thermal, and/or electronic nanoscale mate-
rials.[2]
Using silica sol as a glue, we have developed a general
method to prepare composites of particulate guests and na-
noscalesilicathatarehighlyporous,havehighsurfaceareas,
–
and can be nanoscopically tailored to suit specific applica-
tions. The properties of the particulate guest are retained in [*] Dr. D.R. Rolison,Dr. M.L. Anderson,C. A.Morris
the composite, thereby creating a composite that unites the SurfaceChemistry(Code6170)
continuousmesoporousnetworkandhighsurfaceareaofsil- NavalResearchLaboratory
icagelnetworkswithpre-selectedchemical,physical,andop- Washington,DC20375(USA)
ticalproperties oftheguest solid.Theuseofsol–gelchemis- Email:[email protected]
trytocreatethesecompositesaffordsenormousflexibilityto Dr. R.M. Stroud
tailor them with solution- and gas-phase reagents at various SurfaceModification(Code6370)
stagesofthe wetgel preparation or aftersupercritically dry- NavalResearchLaboratory
ingthewetgeltoformtheaerogel,asshowninFigure 1. Washington,DC20375(USA)
Sol–gelsandsol–gelcompositescanbepreparedwithlow- Dr. C.I.Merzbacher
erdensitiesandhigherporositiesandsurfaceareasthancon- OpticalTechniques(Code5670)
ventional ceramics, which confers on them novel physical NavalResearchLaboratory
and chemical properties.[1] Sol–gel-derived materials, as pro- Washington,DC20375(USA)
ducedbythe hydrolysis andcondensation ofmetalalkoxide [**] The authors gratefully acknowledge funding from the Defense
precursors (M(OR) ), Equations 1 and 2, respectively, as AppliedResearchProjectsAgencyandtheU.S.OfficeofNaval
n
shown for generation of silica from a tetraalkoxysilane pre- Research.M.L.A.wasanASEEPostdoctoralAssociate(1997–
cursor, have been widely studied for applications requiring 2000).
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1. REPORT DATE 3. DATES COVERED
2000 2. REPORT TYPE 00-00-2000 to 00-00-2000
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Tailoring Advanced Nanoscale Materials Through Synthesis Of
5b. GRANT NUMBER
Composite Aerogel Architectures
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
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Andersonetal./TailoringAdvancedNanoscaleMaterialsThroughSynthesisofComposite
S (RO)3–Si–OR+H–OH(cid:222)(RO)3–Si–OH+R–OH (1) a wet gel affords a significant improvement in the surface
W areaandporosityofthedriedmaterial,[1,3]ascontrastedwith
E (cid:126)Si–OH+HO–Si(cid:126)(cid:222)(cid:126)Si–O–Si(cid:126)+H O (2) conventionalambient-dryingtechniquesthatyieldaxerogel,
2
I
V as illustrated in Figure2. Aerogels are highly porous solids
E Catalytic, electrocatalytic, and sensing applications, in (75–99%openspace)thatcontainmesopores(pores2–50nm
R
particular, are facilitated by high active surface areas con- insize)andmicropores(pores<2nm)andexpresshighsur-
tainedwithinahighlyporousmediumsothatrapidreactant/ faceareas(upto1000m2/g)andultra-lowdensities(downto
analyte throughput to the active sites occurs. The use of su- 0.003g/cm3).[2a,4]Thesephysicalattributesarealsopresentin
percritical-fluiddryingtechniquestoproduceanaerogelfrom compositeaerogelspreparedfromwetcompositegels.
Michele Anderson received a Ph.D. in physical chemistry in 1997 from the University of Arizona, where she
characterizedtheopticalandelectronicpropertiesofmaterialsusedinorganiclight-emittingdiodesandphoto-
voltaicdevices.Since1997,shehasworkedattheNavalResearchLaboratory(NRL),firstasapostdoctoralfel-
lowoftheAmericanSocietyforEngineeringEducation(1997–2000),thenas aseniorscientistwithGeo-Cen-
ters, Inc. While at NRL, she has developed composite aerogels for catalytic, electrocatalytic, and sensing
applications.
RhondaStroudjoinedthestaffoftheSurfaceModificationBranchoftheNRLin1998.ShereceivedherPh.D.
inPhysicsfromWashingtonUniversityinSt.Louis,wheresheinvestigatedthephasestabilityofTi-basedqua-
sicrystals. Her currentresearch focuseson analyzing the structure ofoxidematerials using transmission elec-
tronmircroscopy,andrelatingthatstructuretoeletronictransport,magneticandopitcalproperties.
CatherineMorrisreceivedherBachelors degreeinchemicalEngineeringfromCornell Universityin2000and
has since joined the staff at Intel Corporation in Albuquerque, NM. Her first research on aerogels began as a
hight school student participating in the NRL Science and Engineering Apprentice Program and continued
throughherundergraduatestudiesatCornellasshedevelopedcompositeaerogels.
CeliaMerzbacher(Ph.D.,1987,PennsylvaniaStateUniversity)joinedthestaffoftheOpticalSciencesDivision
attheNRLin1989.Herresearchhascenteredonthedevelopmentofnovel,mostlyamorphousmaterialsandthe
characterization of their structure/porperty relationships. Materials of interest include inorganic, organic, and
composite aerogels; longwave infrared transmitting glasses; photosensitive heavy metal fluoride glasses; and
radioactivewasteformglasses.
DebraRolison(Ph.D.,1980,UniversityofNorthCarolinaatChapelHill)currentlyheadstheAdvancedElec-
trochemical Materials section. Her program focuses on the influence of nanoscale domains on electron- and
charge-transferreactionsandthedesignandsynthesisofnewnanostructuredmaterialsandcompositesforcata-
lytic chemistries, energy storage and conversion (fuel cells, supercapacitors, batteries, thermoelectric devices),
andsensors.Dr.RolisonwasamemberoftheAdvisoryBoardforAnalyticalChemistryandisacurrentmember
of the Editorial Boards of the Journal of Electroanalytical Chemistry and Langmuir. She is a member of the
BoardofDirectorsfortheSocietyforEletroanalyticalChemistryandhasservedsince1997aseditoroftheso-
ciety’snewsletter:SEACCommunications.
482 ADVANCEDENGINEERINGMATERIALS2000,2,No.8
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Andersonetal./TailoringAdvancedNanoscaleMaterialsThroughSynthesisofComposite
sional network, and ii) guest–host R
materials,inwhichthewetsol–gelor E
V
dried gel serves as a host matrix for
I
particulateormolecularsolids. E
W
We use silica sol as a nanoglue to
S
prepare composite aerogels by dis-
persing a non-bonding solid (either
in colloidal or non-colloidal form) in
an about-to-gel silica sol.[9–11] By
minimizing the time for interaction
between the sol and particulate prior
to gelation, it is possible to avoid en-
capsulation of the guest by the silica
host. However, the guest solid, even
whensmallenoughtomovethrough
the mesopores of the gel, is suffi-
cientlyincorporatedintothethree-di-
mensional silica network that it does
not elute out either upon replacing
the pore-filling liquid or during su-
Fig.1.Aschematicdetailinghowthesol–gelprocessisusedtopreparecompositegelsandaerogels.Therequiredproper-
percritical drying. The particulate is
tiesofthecompositematerialcanbeselectedpriortogelationoftheSiO solonthebasisofwhichparticulateguestis
2
chosen.Additionalopportunitiesexisttonano-engineerthepropertiesofthecompositegeland/oraerogelwithsolution- therefore thoroughly immobilized,
orgas-phasemodifiers,asindicatedatpointsI,II,III,andIV.TMOS=tetramethoxysilane.SCF=supercriticalfluid. yet readily accessible via the meso-
SCFCO dryingconditions:T =31(cid:30)C,P =7.4MPa.
2 c c porousnetwork.
2.AlternateApproachestoFabricateGuest–Host
SilicaComposites
Our approach is distinct from previous approaches in
B
whichtheguestsolidiscovalentlylinkedtothethree-dimen-
Y
M sionalsilicanetworkduringgelation.[12]Covalentattachment
C of the guest is achieved either by mixing it with the sol–gel
precursors prior to gelation (to form a single hetero-net-
Fig.2.Representationsillustratingthedifferenceinthedegreeofshrinkageexperienced work)[12,13] or by infiltration of the guest after gelation fol-
bythenetworkofacompositegeldriedbysupercritical-fluidextraction(toproducean
aerogel)orbyambientevaporationofthepore-fillingliquid(toproduceaxerogel).The lowed bycovalent bonding to the surfacesof the wet gel.[13f]
compositegelthatisdriedtoformanaerogelischaracterizedbygreaterporosityand These methods are restricted to molecules and particulates
decreasedencapsulationofimmobilizedguestparticlesthanwhendriedtoformaxero-
forwhich(oftencomplex)syntheticroutescanbefoundtoat-
gel.
tach a functional group that covalently bonds to the silanols
Silicagels(composedof(cid:126)10nmSiO particles[5])andcom- (Si–OH)ofthesilicanetwork.
2
posite gels have been widely studied, along with synthetic Our method also differs from impregnation techniques
methods that retain the desired property of the guest that produce guest–host nanocomposites. Using these ap-
phase.[6,7] Due to the mild conditions employed during sol– proaches, reactive precursors are infused into a pre-formed
gelprocessing,temperature-sensitivemoieties(suchasmole- silicaorothermesoporousnetworkandreactedtoformasec-
culesandbiomolecules)canbeincorporatedasguests.Ambi- ondparticulatephasedispersedthroughthehost.[6,7,12c,14]The
entprocessingalsocompatiblyintegratessol–gels,whichare chemical composition and physical form of the impregnated
readily fashioned in assorted physical forms (e.g., thin film, phasearedeterminedbyprecursorchemistryandconcentra-
monolith, powder), with other materials to form multilayer tion, reaction conditions (e.g., temperature, means of mass
orcompositestructures.[2] transportofthereactants),andpost-treatmentofthecompos-
Sol–gel-derivedhostscontaininghighlydispersedparticu- ite.Impregnationtechniquesareoftenplaguedbydispersions
lateguestshavepreviouslybeenstudiedfortheiropticalnon- in final particle sizes and non-uniform particle distribution
linearity, catalytic, biocatalytic, electrocatalytic, and sensing throughthesilicamatrix.
capabilities.[7,8]Thesenanocompositesfallintotwogeneralca- Our method is unique in that the guest associates with
tegories: i) hybrid systems, in which a metal alkoxide and a the silica sol (host) for just seconds to a few minutes (rather
particulate precursor that contains alkoxide functionalities than hours) before gelation occurs. Earlier work on guest–
are reacted together to produce a single-phase, three-dimen- host nanocomposites, including aerogel-based composites,
ADVANCEDENGINEERINGMATERIALS2000,2,No.8 483
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Andersonetal./TailoringAdvancedNanoscaleMaterialsThroughSynthesisofComposite
S have involved extensive physical mixing of the solid with silica-based composite aerogels shown in Figure3 are inter-
W either the sol–gel precursors or sol to prevent settling of the mediatetothevaluesforthesilicaandguestparticulate.Pure
E solidpriortogelation.[13g,15]Frickeandco-workersfirstdem- silica aerogel ((cid:126)1% dense) has a pore volume of (cid:126)4.4cm3/g;
I
V onstrated this concept in their investigations of silica aero- silica-based composite aerogels still preserve pore volumes
E gels containing carbon (an opacifier) for thermal insula- characteristic of highly mesoporous materials. The density-
R
tion.[15b] Their subsequent work has shown that colloidal
ZnO–silica aerogel nanocomposites retain the characteristic
yellow-green photoluminescence of ZnO nanocrystals (also
illustratedbyDeng,etal.,forZnO–silicacompositescontain-
ingZnOmadebyreactingZnsaltsintroducedbyimpregna-
tion[16]).
Although certain properties are independent of mixing
times, we find that extensive mixing of the particulate guest
withthesilicasolpriortogelationcanleadtothelossofcriti-
calpropertiesforsomematerials,particularlytransportprop-
erties (which require intimate contact between guest parti-
cles) and chemical properties (which involve guest
Fig.3.Silica-basedcompositeaerogels(fromlefttoright):puresilicaaerogel[overlay-
interaction with molecules in the mesopores). For instance, ing Ti–V/Zr–Nb/Hf–Ta in the periodic table]; colloidal Pt–silica composite aerogel
(2nmPtsol);colloidalAu–silicacompositeaerogel(30nmAusol);carbonblack–silica
when we prepared carbon–silica composite gels using pro-
aerogel(Vulcancarbonblack,XC-72);poly(methylmethacrylate)–silicacompositeaero-
longed mixing times to ensure thorough homogenization of gel(polymerM (cid:126)15000,sievedto<44(cid:109)m);FeII(bpy)NaYzeolite–silicacomposite
w 3
the carbon and silica phases, electronic transport paths aerogel(0.1–1(cid:109)mtypeYzeolitecrystallitesmodifiedwithiron(II)tris-bipyridine);tita-
nia(aerogel)–silicacompositeaerogel((cid:109)m-sizedparticulatescomposedof(cid:126)15nmTiO
throughthecarbonweredisruptedinthedriedcomposite.[9] aerogel domains); and titania–silica composite aerogel (20–40nm Degussa P-252
Minimizing the interaction time between the same ratios of TiO2).Allsamplesshownare(cid:126)1cmdiametermonolithsofvaryinglengths.
carbontosilica,however,yieldsacompositeinwhichthecar-
bon is uniformly dispersed, yet electrically conducting. We
expectthedesirablephysical,transport,andchemicalproper-
tiesofothersolidstobeexpressedinthecompositewhenwe
limitexposureoftheguesttothesilicasol.
Applicationsinwhichgas-andliquid-phaseimmersionis C
employed(i.e.,catalysis)mayrequiregelswithhighmechan- M
icalandthermaldurability.[17–19]Wefindthatthepresenceof Y
B
the particulate guest often strengthens the nanoglued com-
posite aerogel relative to the mechanical durability of pure
silicaaerogel,evenwithoutthermallydensifying[16b]thecom-
posite.
3.ChemicalandPhysicalDiversityofComposite
Aerogels
We have demonstrated immense range in the size and
compositionoftheguestsolidinsilica-basedcompositeaero-
gels, as seen in Figure3. Guests whose sizes span over five
orders of magnitude (from several nanometers to sub-milli-
meter)andwhosechemicalcompositionsvaryfromcolloidal
metal to metal oxides, non-oxide semiconductors, carbon
blacks, and polymer particles have been immobilized in the
silicamatrix.Ingeneral,theknownattributesoftheguest,as
wellasthestructureofthesilicanetwork(Fig. 4),areretained
in the composite gel or aerogel.[9,11] The composite chemical,
Fig.4.(bottom)SEMofacolloidalPt–silicacompositeaerogel.Thesilicaparticlesap-
opticaland/orelectricalpropertiescanthereforebepredeter- peargrayandthemesoporesareblack.Thebeaded-necklacemorphologyischaracteris-
minedtosuitthedesiredapplication. ticofthefractalstructureofbase-catalyzedsilicaaerogel[5].Theguest,(cid:126)2nmPtpar-
ticles,isobscuredbythesurroundingsilicahost.(top)HRTEMofacolloidalPt–silica
Particulate–silica composite aerogels are especially suited
compositeaerogel.Themottledgrayregionistheamorphoussilicahost,whichencases
fortechnologiesthatrequirehighlydispersed,readilyaccessi- the1.8nmPtparticle.HRTEMstudiesofcompositescontaining5nmorlargergold
ble reactive sites (e.g., catalysts, sensors). The total pore vol- particlesindicatethatthenanoparticlesurfaceispartiallyexposedtothemesoporous
network,inagreementwiththefindingthatcolloidalgoldguestssizedat(cid:126)5nmcan-
umesandBrunauer–Emmett–Teller(BET)surfaceareasofthe notbechemicallymodified.
484 ADVANCEDENGINEERINGMATERIALS2000,2,No.8
Andersonetal./TailoringAdvancedNanoscaleMaterialsThroughSynthesisofComposite
andtransport-controlledpropertiesofcompositeaerogelscan shifted Au plasmon resonance,[8f,g] as do gold–silica compo- R
alsobetailoredbyadjustingthemassorvolumeratioofsilica sites derived from organically modified sol–gel monomers E
V
sol-to-guest particulate. For example, the density of zeolite– that are used to stabilize the dispersed Au particles.[8b,12a,c]
I
silica composite aerogel reflects a weighting of the densities Composites in which the metal particle size evolves over E
W
oftheindividualcomponents(silicaaerogelandzeolitepow- time,asoccursinthe surfactant-mediatedsynthesis ofgold–
S
der).[9] silica gels from gold-containing micelles, also exhibit a red
Theopticalpropertiesofcompositegelsandaerogelsgen- shiftrelativetotheAusol.[15c]
erally reflect those of the guest because base-catalyzed silica
aerogelsaretransparentandfeaturelessovermostoftheUV- 4.DesignerArchitectures
visible[11] and infrared (IR)[9] spectral regions. The range of
optically active guests includes even organic species given Particulate–silica composite aerogels are platforms that
the gentle processing conditions and known tolerance of or- provide opportunities to engineer a broad range of nano-
ganic molecules to supercritical CO .[20] Composite gels and scopicmaterialswithspecificpre-selectedproperties.Thegel
2
aerogels are readily characterized by optical spectroscopies: preparation scheme, Figure 1, offers multiple means to
for example, FeII(bpy) NaY zeolite–silica and poly(methyl- furthertailortheoptical,chemical,andphysicalpropertiesof
3
methacrylate)–silica composite aerogels exhibit the infrared the host solid, the guest solid, the composite wet gel or the
absorption features characteristic of the bipyridine and driedaerogelviasolution-orgas-phasemodification.
methylmethacrylatemoieties,respectively.[9] Additional tailoring of the composite gel architecture can
Theinnateopticalcharacterofcommerciallyrelevantmet- beachievedbymodifyingthesurfaceoftheparticulateguest
al oxide guests, such as photocatalytic titania, is also pre- priortogelation,asshowninFigure 1I.Wehavedemonstrat-
servedinthemetaloxide–silicacompositeaerogel.Opaqueti- ed that active sites that are introduced to the surface of the
tania(Degussa)whenincorporatedintoacompositeyieldsan guestparticles priortogelationremainaccessibletoexternal
aerogelthatexhibitsabroadUVabsorbancecharacteristicof reagents after supercritical drying. Carbon-supported metal
the large bandgap semiconducting guest. Titania (aerogel)– colloids in carbon–silica composite aerogels (produced by
silica composite aerogels (see Fig.3) have a UV absorbance combiningcolloidalmetal-modifiedVulcancarbonwithsilica
characteristicofpuretitaniaaerogel,whichistranslucentand sol)remainaccessibletoCOandMeOH,andhavebeenelec-
exhibits a spectrum more molecular in nature.[9] The im- tronically addressed within the aerogel to catalyze redox re-
proved photocatalytic activity of titania aerogel relative to actions.[24]
moreconventionalformshasalreadybeen noted,[21]buttita- High-surface-area carbon blacks aretypically used in fuel
niaaerogelsaredifficulttoprepareascrack-freemonolithsor cellstodispersethenanoscaleelectrocatalyst[25]andthenfab-
films.[22] The transparency of silica aerogel permits effective ricated into a fuel-cell electrode of the required geometry by
illumination of the titania component in the composite aero- combining with a porous binder, such as poly(tetrafluoro-
gel, and may improve practical photocatalysts by permitting ethylene). Composite aerogels should improve existing elec-
efficientilluminationoftheentirephotoactivematerialwithin trocatalytictechnologiesbecausetheirintegratedstructureof-
astructurethatfacilitatesreagentfluxto(andfrom)theactive fers multifunctionality by providing superior access of fuel
sitesthroughthecontinuousmesoporousnetwork. and oxidant to the dispersed, carbon-supported catalyst via
Colloidalgold–silicacompositegelsandaerogelsretainthe thecontinuous mesoporous network,while alsomaintaining
color and transparency of the native metal sol (see electronicconductivitythroughoutthecomposite.
Fig.3),[9,11,23]becausethe characteristic plasmon resonanceat Modified carbon–silica composite aerogels are potential
500–550nmofthenanoscalegoldparticlesisunobscuredby black optical materials as well. Neither ambient nor He–Ne
thetransparentsilicamatrix.Theplasmonresonancepeakpo- laser light is transmitted through a 1 cm monolithic carbon–
sition is identical for the Au sol and the colloidal Au–silica silicacompositeaerogel,despiteitshighporosity(Fig. 5).[9]In
compositegel,bothofwhichsurroundthecolloidalgoldwith contrast,nativesilicaexhibitsclaritythattransmitslightwith
acondensedfluid.Theplasmonresonancepeakisblue-shifted little scattering (see Fig.3). Opaque or low-reflectivity coat-
(relative to the sol) in the composite gold–silica aerogel, in ings and monoliths may be prepared and the wavelengths
whichthegoldparticlesarenowsurroundedbyamixtureof thatareabsorbedcanthenbeextendedbeyondthevisibleby
moreairthansilica.Theblue-shiftisinkeepingwiththerela- adsorbingmolecularmodifierstothecarbon.
tionshipbetweenmaximumabsorbanceandsolventrefractive The surface of the guest particulate may also be tailored
indexintheMieapproximationforcolloidalmetalparticles.[11] following gelation by adding solution-phase reagents to the
The retention of the metal particle’s size and its optical pore-fluid washes that are performed prior to supercritical-
properties in our colloidal gold–silica composite gels and fluiddrying,asshowninFigure 1II.Forexample,thesurface
aerogels distinguishes them from compositionally related ofgold colloidslargerthan(cid:126)20nmremainsaccessibletoex-
composites in which the gold particles are covalently at- ternal reagents via the three-dimensional mesoporous net-
tached to the silica network or are stabilized by surfactants workofthecompositegel.[11]Thebase-conjugateformofthe
duringgelation.Gold–silicacore–shellparticlesexhibitared- pH-sensitivedyemethylorangepreferentiallyadsorbs(from
ADVANCEDENGINEERINGMATERIALS2000,2,No.8 485
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Andersonetal./TailoringAdvancedNanoscaleMaterialsThroughSynthesisofComposite
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E
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E
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Fig.6.Microporeandmesoporesizedistributionsforsilicaaerogelannealedat500(cid:30)C
[—(cid:38)—](minimallyshrunkcomparedwithas-supercriticallydriedaerogel)anddensi-
fiedat900(cid:30)C[—(cid:42)—]toca.50%ofitsoriginalsize.Lossofbothmeso-andmicropor-
osityisobserved,butdirectanalysisofthetotalporevolumesineachoftheseregions
indicatesa40%lossofmesoporousvolumeanda>85%lossofmicroporousvolume
uponpartialdensification.
posite aerogels,[9,23] analogously to the specific adsorption of
Fig.5.He–Nelaser(fromleft)irradiatingair,SiO aerogel,andVulcancarbon–silica the dye in colloidal Au–silica composite wet gels, as de-
2
compositeaerogel.Nolightistransmittedthroughthecarbon–silicacompositeaerogel, scribedabove.[11]Thissurface-specificmodificationisconsis-
eventhoughitis80%openspace. C
tent with the retention of a continuous mesoporous network M
acetone solution) to the metal surface in colloidal Au–silica in silica-based composite aerogels, even after partial densifi- Y
composite wet gels, and not to the surface of the silica do- cation,asindicatedbyourN -physisorptionstudiescompar- B
2
mains. The UV-visible absorption spectrum of a methyl-or- ing as-prepared and partially densified aerogels (see Fig.6).
ange-modifiedcolloidalAu–silicagelexhibitsresolvedpeaks Onthebasisoftheseindependentmeasurementsofthetotal
forcolloidalAuandmethylorange.[11]Amorecomplexmod- sampleporevolumethatiscontributedbymicro-andmeso-
ification of the metal surface architecture using solution- pores,nearly60%ofthe500(cid:30)C-annealedaerogelmesoporos-
phase reagents can be conceived that customizes these com- ity is preserved in the 900(cid:30)C-partially densified aerogel,
positeswithmolecularrecognitioncentersforanalytespecifi- while<15%ofthemicroporousvolumeisretainedinthepar-
cityortailorsthecolloidalmetal-modifiedcarbon–silicacom- tiallydensifiedsample.
positesformoreefficientelectrocatalysis. We have also verified the feasibility of optical or colori-
Modificationofthecompositeaerogelfollowingsupercriti- metricsensingwithcompositegelsbyusingacombinationof
caldrying(Fig. 1IIIand1IV)mayalsobeemployed.Forcom- themodificationstepsillustratedinFigure1.Wedemonstrat-
posite aerogels that do not contain organic moieties, partial edthismultistepmodificationstrategybythermallydensify-
densification at elevated temperatures can be used to ing50nmcolloidalgold–silicacompositeaerogelsandmodi-
strengthenthesilicanetwork.[15,26,27]SilicaorcolloidalAu–sil- fying the colloidal Au guests with methyl orange by
ica composite aerogels heated to 900(cid:30)C shrink ((cid:126)50%reduc- immersionofthepartiallydensifiedcompositeaerogelintoa
tion in the sizeof the monolith), but the primary loss infree non-aqueoussolutionofthedye(Fig. 1IV).[9,11,23]Analogously
volume, as determined by N -physisorption measurements, tothewetcompositegelsdiscussedabove,resolvedpeaksfor
2
occurs by collapse of the micropores (pores <2 nm), while theAuplasmonresonanceandthemethylorange(base-con-
most of the mesoporosity (2–50nm pores) is preserved jugate form) absorbance areseen in the UV-visible spectrum
(Fig. 6). Preserving the mesoporous free volume means that of a methyl-orange-modified colloidal gold–silica composite
themostfacilemass-transportpathwaysthroughthecompos- aerogel that was thoroughly rinsed with acetone, then air-
ite aerogel for gas- or solution-phase reactants remain unal- dried.[9,23]Exposingthedye-modified,air-driedcompositeto
tered.Furthermore,thecompositeconstitutesarigidsolidar- HClvapor(i.e.,Fig.1III),producesared-shiftinthedye’sab-
chitecture, such that the silica aerogel structure and metal sorption,correspondingtoitsprotonation.
particle size distribution are retained in partially densified The gas-phase acid molecules may be detected either vi-
colloidalAu–silicacompositeaerogels.[23] suallyorbyinstrumentalcolorimetry.Visualdetectionispos-
Partiallydensifiedcompositeaerogelsaresufficientlydur- sible because although the surface coverage of the adsorbed
able that they remain intact upon reimmersion into liquids dye is quite low (typically <0.1 of a monolayer), the surface-
(Fig. 1IV).[17b,23,25] We have demonstrated this durability by to-volume ratio of the composite is enormous, which brings
preferentially adsorbing methyl orange from solution onto theeffectiveconcentrationofthedyeinthemodifiedcompos-
theAusurfaceinpartiallydensifiedcolloidalAu–silicacom- ite aerogel to millimolar levels. Color changes are rapid, be-
486 ADVANCEDENGINEERINGMATERIALS2000,2,No.8
Andersonetal./TailoringAdvancedNanoscaleMaterialsThroughSynthesisofComposite
causeofthe highporosity,andarereadilydiscerned byeye. a diverse range of applications. Because composite gels and R
Uponuptakeofmethylorange,thecolorofthecolloidalAu– aerogels prepared using silica nanoglue retain the surface E
V
silica composite aerogel changes from cranberry to peach and bulk properties of each component, improvements to
I
(again, no methyl orange is retained in partially densified current technologies may be readily achieved using existing E
W
pure silica), and a further color change from peach to bright materials. The ability to form an interconnected network of
S
pink occurs within seconds of exposure of the dye-modified thesecondaryorguestphaseallowsforionic,electronic,and
Au–silicacompositeaerogeltoHClvapor. thermaltransportpropertiesnotattainableinasilicasystem.
Composite aerogels are customizable platforms around
which advanced structural and chemical architectures may
5.Methods nowbe designed. Notonlyis the rangeofguest particulates
virtually limitless, but any primary guest phase can poten-
5.1.PreparationofCompositeAerogels
tiallybemodifiedpriortoincorporation.Forexample,weare
Ourrecipesforbase-andacid-catalyzedgelsarebasedon expanding this method to include other nanoscale materials,
published procedures,[10] and are described elsewhere in de- suchasthiolatedAucolloidsthatpossessspecificactivefunc-
tail.[11]Equalvolumepercentsofparticle suspensions(Auor tionalities (e.g., photo- or electroactive molecules). In addi-
Pt sol) and silica sol were combined, the mixture stirred for tion,eithertheparticulateguestorsilicahostmaybetailored
1 min, and then poured into molds (and sealed with Paraf- foraspecificapplicationbyreactionat,ormolecularadsorp-
ilm) prior to gelation. Volume percents ranging from tionto,theirsurfaces,whichremainaccessibletoexternalre-
1–90vol.-% solid were incorporated into particulate–silica agents via the continuous mesoporous network of the com-
composites. For guests at <50 vol.-%, the solid was rapidly positeaerogel.
added to the silica sol with stirring, mixed for 15s, and the
dispersion was then poured into molds seconds before gela-
–
tion. For >50vol.-% solid, a homogenous dispersion of the
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