aerogeles de oro coloidal

8/8/2019 Aerogeles de Oro Coloidal

1/8

C o ll o i d a l Go l d Ae r o g e l s : P r e p a r a t i o n , P r o p e r t i e s , a n d

C h a r a c t e r i z a t i o n

Michele L. Anderson, Cath erine A. Morr is, Rhonda M. Str oud,Celia I. Merzbacher, and Debra R. Rolison*,

S urface Chem istry Bran ch (Code 6170), S urface Modification Branch (Code 6670), an d OpticalPhysics Branch (Code 5610), Naval Research Laboratory, Washington, D.C. 20375

Received J un e 30, 1998. In Final Form: November 24, 1998

Colloidal meta l aerogels ar e composite nanoscale ma terials th at combine th e high su rface ar ea an dporosity of aerogels with the unique optical and physical properties of metal colloids. As such, they arebeing developed as a dvanced sensor, cata lytic, and electrocatalytic mat erials. We have prepa red colloidalgold-silica aer ogels contain ing gold colloids ra nging in size from 5 to 100 nm. The r esults p resen ted her einfocus on 5- and 28-nm Au-containing silica aerogels for t he initial characterization of the interactionbetween th e m etal colloid an d t he silica ma trix. A blue-shift of the Au plasmon r esonan ce for silica-immobilized Au colloids (relative t o the s am e colloids in a na tive Au sol) indicates a n in tera ction bet weenthe Au colloid and the nanoscale silica network. Transmission electron microscopy measurements havebeen used t o determin e the a verage size and dist ribut ion of th e colloidal Au par ticles, as well as to imagethe nanoscale silica environment supporting an immobilized Au colloid. Small-angle neutron scatteringmeasurements show no significant changes in the three-dimensional structures of either the base- oracid-catalyzed silica a erogels upon incorpora tion of small amoun ts (


2/8

xerogels for optimizing the colloidal metal for catalyticand chemical sensing applicat ions in that th e high porosityof an aerogel permits r apid an alyte flux to an d from themetal particles in a manner difficult to achieve with aguest-host str uctured xerogel.

The n an oscale fun ctiona lity of colloidal meta l aer ogelsmust be understood in order to optimize their propertiesas cata lysts, sensors, and advan ced mater ials.S pecifically,chan ges in th e gel stru ctur e as a fun ction of th e size andpresen ce oft he meta l colloid mu st be deter mined , as must

the effect of the surrounding gel-derived network on thephysical an d chemical propert ies ofth e immobilized meta lcolloid. Of pa rt icular import an ce for colloidal Au-silicaaerogels is t he a ccessible sur face ar ea of th e Au colloidsonce they are immobilized in the aerogel, in terms ofcatalytic activity and the area available for functionalderivat ization (e.g., modificat ion by direct ad sorption ofdyes to th e sur face or by self-assembly of rea gent-ta ggedthiols).

This paper addresses the effect ofimmobilizingcolloidalAu (sized at eith er 5 or 28 nm) in a silica aerogel stru ctur eon the n etwork an d propert ies of each component of thiscomposite nanoscale material. The optical properties ofthese ma terials ha ve been char acterized by UV-visibleabsorption spectroscopy, while str uctur al cha racterizationhas been achieved u sing a combination of contrast -mat ching sma ll-angle neut ron scattering (SANS), trans-mission electron microscopy (TEM), and physisorptionmeasurements. The appropriateness of each of thesetechniqu es for d escribing colloidal m eta l aerogels is alsodiscussed.

E x p e r i m e n t a l P r o c e d u r e s

C o ll o i d P r e p a r a t i on . A range of Au colloidal sols waspurchased (BBInternational), and nominally 10-nm Au colloidalsols were prepared by citrate (Na 3C6H 5O2H 2O, Alfa AESAR,99%) reduction of tetrachloroauric acid (HAuCl 4, 99%, AlfaAESAR, 49% Au).23 All glassware was cleaned in a base bathand rinsed copiously with 18 M cm water (Barnstead NAN-Opure) prior to preparation of or use with the Au sols. A 1%

citrat e solut ion (2.5 mL) was added (under flowing ar gon pur ifiedby passing through a Drierite/4- molecular sieve tower) to 200mL ofdea erat ed boiling water , immediat ely followed by additionof 2.0 mL of a 0.79% solution of HAuCl4 (transpa rent yellow incolor). This solution was refluxed for 35 min under flowing Arduring which it developed a purple-black color, followed by thedeep cranberry color characteristic of a 10-nm Au sol.24 Thiscolloidal sol was cooled to a mbient under flowing Ar an d t hentransferred to an amber reagent bott le for s torage. High-resolution transmission electron microscopy (described below)of the purchased 5-nm colloidal Au sol and the colloidal Au solprepar ed in-house has shown tha t th ese sols have mean par ticlediameter s of 5.6 ( 0.3 and 28 ( 5 nm , respectively.

A e r o g e l P r e p a r a t i o n . Acid- and base-catalyzed silica aero-gels were prepared by procedures similar to those previouslypublished.25-27 For base-catalyzed gels, two beakers were pre-

pared: one with 3.939 mL of tetr amet hoxysilane (TMOS, AlfaAESAR, 98%)a nd 4.519 mL ofm eth an ol (MeOH), th e oth er with4.514 mL of MeOH, 1.524 mL of H 2O, and 5.2 L of NH 4OH (30%in H 2O, Aldrich). The two solutions were combined a nd stirredfor 1 min, and the mixture was poured intocylindrical molds (13 57 mm, filled with ca. 3 mL of clear, tra nsparent sol) and

covered with Parafilm. Acid-catalyzed gels were prepared byadding 4.5 mL ofa pH 4.6 potas sium hydrogen phtha late/NaOHbuffer t o a beaker conta ining 2.621 mL of TMOS, 0.545 mL ofH 2O, and 0.035 g of 0.04 N HCl, which ha d been sonicat ed for10 min; the mixture was stirred for 1 min, poured into molds,and covered with Parafilm.

Both acid- and base-cat alyzed gels were aged in their moldsfor 1 day, then transferred to ca. 20-mL glass vials and rinsed8-12 times with a cetone over 3-4 days. The base-cata lyzed gelswere rinsed with ethanol at least 3 times before washing withacetone, which reacts with the base catalyst to form a yellow-

orange product (possibly due to base-catalyzed formation of theenol taut omer of acetone). The gels were then introduced into asupe rcrit ical dryer (Fisons Bio-Rad E3000), an d the acetone wasreplaced with liquid CO2, which was brought above its criticaltemperatur e and pressure (Tc ) 31 C; Pc ) 7.4 MPa) and thenreleased to dry the gels. Gel shrinka ge during the super criticaldrying step was minimal. The dried gels were sintered at 500 Cfor 2 h, also with minimal shrinkage. The base-catalyzed gelsand a erogels were almost tran sparen t, while the acid-cat alyzedaerogels were a translucent white.

Colloidal gold aerogels were ma de by prepa ring a n acid- orbase-cata lyzed silica sol, as described above, an d addin g a volumeof Au sol equivalent to th e volume of silica sol to produce a 50:50vol % sol of Au:SiO2. This colloidal mixtu re wa s st irred for ca.1 min an d th en pour ed int o molds an d treat ed as described above.The finished a erogel monoliths were identical in appea ran ce to

the pure silica gels (i.e., nearly transparent or translucent forbase-and acid-catalyzed silica sols,respectively),but with a pinkcolorat ion due to th e imm obilized colloidal Au. To ensur e t hatany observed chan ges tot he silica structur e and properties wereattributable to the presence of the colloidal Au and not due tovolume dilut ion of the st an dard silica sol, diluted silica blan kswere prepa red by rem oving a sm all aliquot of the about-to-gelsilica sol (prior to a ddition of the Au sol) and diluting with anequivalen t volume of H2O to produce a 50:50 vol % SiO2:H 2O sol(designat ed as diluted SiO2),which was then tr eated in the samemanner as the other samples.

P h y s i c a l C h a r a c t e r i z a t i o n . UV-visible absorpt ion spectr afor the colloidal gold sols and aerogel monoliths were obtainedusing an HP 8452 diode array single-beam spectrophotometerin th e spectrum /peaks mode. Powdered aer ogels brushed or Ausols evaporat ed ont o holey-Algrids were an alyzed using a HitachiH-9000 h igh-resolution t ran smission electron microscope. Ni-trogen physisorption measurements (at 77 K) of the variousaerogel monoliths were obtained using a Micromeritics ASAP2010 accelerated sur face area and porosimetr y system. Reportedsurface areas are based on a mult ipoint BET analysis , andaverage pore sizes and distr ibutions were determined using datareduction programs provided by Micromeritics, including theirDFTplus program. The data were best fit using the BJH equa tionwith a cylindrical pore geometr y.

SANS data were collected on a 30-m SANS spectrometer atthe National Institute of Standards and Technology (NIST,Gaithersburg, MD) using configurations and data analysisprocedures detailed elsewhere.26,28 Sam ples for the SANS studieswere aer ogel disks ca. 1-2 mm thick dry cut (with a diamondsaw) from cylindrical monoliths. In contrast-matching SANS,the aerogel pores were filled with a H 2O/D2O mixture thatmat ched the neut ron scatterin g properties of either SiO2 (42:58

vol % H 2O:D2O) or Au (26:74 vol % H 2O:D2O). The sinteredaerogels were rewetted by placing them in a satu rat ed environ-ment of the a ppropriate contra st-mat ching fluid at 85 C for a tleast 9 h prior t o immersing the sample in H 2O/D2O.

R e s u l t s a n d D i s c u s s i o n

Preparation ofa Colloidal Gold Aerogel. As detailedin Experimental Procedures, colloidal gold-silica com-posite aerogels are made by adding an aqueous sol of colloidal Au to an about-to-gel silica sol. As hydrolysisand condensation reactions contin ue in the silica sol toform a three-dimensional network of silica particles (or

(22) Ye, S.; Vijh, A. K.; Wang, Z.-Y.; Dao, L. H. Can. J. Chem. 1997,75 , 1666.

(23) Horisberger, M.; Rosset, J. J. Histochem. Cytochem. 1977, 25 ,296.

(24) Turkevich, J .;Ga rt on, G.;St evenson, P. C. J. Colloid S ci. S uppl.1954, 1, 26.

(25) Russo, R. E.; Hu nt, A. J. J. Non-Cryst. Solids 1986, 86, 219.(26) Merzbacher, C. I.; Barker, J. G.; Swider, K. E.; Rolison, D. R.

J. Non-Cryst. S olids 1998, 224 , 92.(27) Ellerby, L. M.; Nishida, C. R.; Nishida, F.; Yamana ka, S. A.;

Dunn, B.; Valentine, J. I.; Zink, J. I. S cience 1992, 225 , 1113.(28) Merzbacher, C. I.; Barker, J. G.; Swider, K. E.; Ryan, J. V.;

Bernstein, R. A.; Rolison, D. R. J. Non-Cryst. Solids 1998, 225 , 234.

Colloid al Gold A erogels L an gm u ir, Vol. 15, N o. 3, 1999 67 5


3/8

gel), th e met al colloids become incorpora ted int o the gel.Even under r epeated washings or dur ing the supercriticaldrying pr ocedur e, the met al colloids rema in immobilizedin the gel str uctu re an d do not wash out . In a guest-host

xerogel structur e, the collapse of the network around aguest provides a physical entrapment;3-8,14-20 however,th is mechanism is less obviously applied to a guest-hostaerogel stru ctu re.

We ha ve successfully form ed composite a erogels witha diverse chemical an d size range of suspended par ticu-lat es includin g: colloidal Au sized at 5, 10, 20, 30, 50, and100 nm; colloidal Pt sized at 2-3 nm; zeolite powders(sized at 0.1-1 m), colloidal TiO2 (Degussa P25); pow-dered TiO2 aerogel; powdered poly(met hyl meth acrylat e)(sieved t o


4/8

similar r esult s for t he sa me ra nge of colloid sizes, with ared-shift of ca. 6 nm in absorba nce peak position between5- and 30-nm Au sols, and a r an ge of fwhm values of 49-66 nm (increas ing as par ticle size decreases). Also, TEMmeasu remen ts on the Au sol used to prepa re the composite

aerogel in Figure 2 indicate a range of Au particle sizesfrom nearly sph eroidal (20 21 nm ( 1 nm ) to elliptical(26 40 nm ( 1 nm). On the basis of th ese results, theTEM measu rement s provide a m ore accura te indicationof th e colloidal Au pa rt icle size, sha pe, an d size distr ibu-tion.

S m a l l - A n g l e N e u t r o n S c a t t e r i n g ( S A N S ) . Small-angle scatt ering ha s been u sed to characterize aerogelsdue to its sensitivity to structura l featur es over a lengthscale of 1-200 nm.38,39 In the case of our colloidal Au-silica a erogels, th is length scale covers n ot only a critical

ran ge for the SiO2 stru cture but also the size of the Aupart icles incorporat ed into t he SiO2 structure.

The small-angle neutron scattering results for acid-catalyzed silica aerogels with and without colloidal Auare shown in Figure 3. The sca t te r ing curve for thestandard (undiluted) silica aerogel is similar to thosepublished previously.38,40 The curve for t he dilut ed silicaaerogel is largely similar t o th at of th e undilu ted sam ple,except at low scatter ing vectors, q, indicating that thestructures of the two materials are also similar. Thetra nsition to a slope of nearly zero at low q occurs at length

scales th at corr elate with the size oft he largest scat ter ers(q ) 2/length ).Th e shift in th is tr an sition to lower valuesof q for the diluted aerogel indicates that the fractalnetwork structure extends to longer lengths.

To probe the in fluen ce of the colloidal Au gu est on t hestr uctu re of th e silica network, th e pores were filled witha Au contr ast -matchin g fluid. We ha ve previously shownthat the pores of silica aerogel (e88% porosity) can berefilled with water without a ffecting t he n etwork stru c-ture .26 The slope of the curve for the Au-mat ched acid-catalyzed composite aerogel is essentially identical to th ecurve for t he dr y silica (see Figur e 3).41 The high-q cutoffoccurs a t slight ly lower valu es ofq because of incoher entbackground scattering from the hydrogen in the pore-filling liquid. Rath er th an a n ear-zero slope at low q, the

slope becomes steeper, which is a feature observed for allrewetted aerogels an d is att ributed t o the presence of afew large (micromet er s ize) bubbles.26,28,42

On t he basis of the similarity of th e scatt ering curvesoft he pure SiO2(ac), dr y Au:SiO2(ac) (not shown), an d theAu-mat ched Au:SiO2(ac)aer ogels, th e acid-cat alyzed silicastr uctu re is un affected by incorpora tion of sma ll am ount sof eith er 5-nm or 28-nm colloidal Au. However, we can not

(38) Schaefer, D. W.; Keefer, K. D. Phys. Rev. Lett. 1986, 56, 2199.(39) Craievich, A.; Aegert er, M. A.; dos San tos, D. I.; Woignier, T.;

Zarzycki, J. J. Non-Cryst. Solids 1986, 86, 394.

(40) E mmerling, A.; Gross, J.; Gerlach, R.; Goswin, R.;Reichenauer ,G.; Fricke, J.; Haubold, H.-G. J. Non-Cryst. Solids 1990, 12 5, 230.

(41) The scattered intensity from the Au-matched Au:SiO 2 samplesis less than t hat from the pur e SiO2 samples, due toth e smaller contrastbetween silica and gold vs silica and air.

(42) Merzbacher, C. I.; Barker, J. G.; Swider, K. E.; Rolison, D. R.Adv. Colloid Interface Sci. 1998, 76-77, 57.

F i g u r e 2 . Tran smission electr on micrograph s of a 28-nm Au:SiO2(bc) aerogel showing a Au pa rt icle (dar k spot) an d its SiO 2environment (top image), and a close-up of t he multiplecrystalline domain s of th e Au colloid amidst a morph ous silica(surr ounding gray mottled areas).

F i g u r e 3 . SANS of acid-catalyzed SiO2 and colloidal Au:SiO2composite aerogels.

http://dontstartme.literatumonline.com/action/showImage?doi=10.1021/la980784i&iName=master.img-002.png&w=213&h=261http://dontstartme.literatumonline.com/action/showImage?doi=10.1021/la980784i&iName=master.img-001.jpg&w=234&h=447


5/8

use SANS to addr ess the average str uctu re of th e colloidalAu in t hese composite aerogels. The scattered intensityfrom the Au component, expressed in the SiO2-matchedsample for both acid-catalyzed (Figure 3) an d base-cata lyzed (Figur e 4) 28-nm Au:SiO2 aerogels, is essent iallyat background levels (thus the noise at high q seen inFigures 3 and 4), which is reasonable due to the low Auconcentr ation in th ese samples.

SANS spectra for the analogous base-catalyzed silica(SiO2(bc)) an d colloidal Au-Si O2(bc) series ar e shown inFigure 4. Un like the acid-cata lyzed samp les, scatt eringfrom the stan dard an d dilut ed SiO2 sam ples does differ,both in th e sha pe of th e roll-off to near-zero slope an d inth e slope of th e near -linear region (-0.5 < q < 0.5 nm-1).These differences indicate that structural changes areindu ced by dilut ing th e base-cat alyzed SiO2 sol with pu rewater prior to gelation. A pure water dilution of thestan dard base-cata lyzed silica sol does not appear to bea good cont rol for th ese gels, most lik ely becau se th e base-catalyzed sol is unbuffered (whereas the acid-catalyzedsol was pr epar ed in a p H 4.6 buffer). We confirm ed th ata shift to lower pH does occur when base-catalyzed silicasol is diluted (50:50 vol %) with pure water.

The shape of the spectra for both t he dry an d th e Au-mat ched 28-nm Au:SiO2(bc) does, however, mim ic th at ofthe un diluted SiO2(bc) aerogel (except a t t he ext remes ofq, for the reasons s ta ted above) . Di lut ing the base-catalyzed silica sol with Au sol does n ot lower t he p H ofthe unbuffered silica sol to th e extent t hat pure water

does, so the kinetics of gelation are more comparable tothe un diluted silica sol.

P h y s i s o r p t i o n C h a r a c t e r i z a t i o n . Adsorption anddesorption isotherms determined from N 2 physisorptionmeasurements on acid- and base-catalyzed SiO2 andcolloidal Au:SiO2 aerogels are shown in Figure 5. All ofthe isotherms exhibit essentiallyt he same shape,includinga small amoun t of hyst eresis between the adsorpt ion an ddesorption isother ms. Th is behavior is characteristic ofmat erials with both m icro- and mesoporosity (pores 900m 2 /g, compared with ca. 750-820 m 2 /g for the acid-catalyzed gels. Surface areas for each of the aerogelsinvestigated are listed in Table 1. These results areconsistent with the current understanding that acid-catalyzed silica gels have a ram ified, bra nched str uctu re,while base-cat alyzed SiO2 consists ofa three-dimensionalnetwork ofcondensed par ticles with roughened su rfaces,47

and th ey are in agreement with reports in the litera tur eof higher surface areas for base-catalyzed than for acid-catalyzed silica aer ogels.39

The average pore diam eter an d total pore volume oft heaerogels can also be extra cted from the isoth erm s and arereport ed in Table 1. The average pore diamet er is ca. 4-5nm larger in t he base-catalyzed gels t han in th e acid-catalyzed ones, an d th e pore size upper limit (after whichthere is no significant pore density) is 46 nm for a ll ofthe acid-catalyzed gels and 162 nm for all of the base-catalyzed gels. The a verage pore size of 8.8 ( 0.9 nm inth e sta nda rd acid-catalyzed SiO2 is identical to tha t of th ediluted sample within m easuremen t error an d decreasesby


6/8

porat ion of 28-nm Au colloids, despite th eir low concen-tr at ion, resu lts in a nea rly 1-nm decrease in average porediameter for the base-catalyzed system, and a furtheraverage pore size decrease is observed for a nominally(man ufactur er-based) 30-nm Au:SiO2(bc) sample; seeTable 1. The t otal pore volume in st anda rd SiO2 and in28-nm Au:SiO2 is nea rly identical, while a decrease intotal pore volume is observed for the 30-nm Au:SiO2sample. These results a re consistent with a loss of porevolume relative to the pure silica aerogel resulting fromocclusion of pore space by Au colloids. It is expected thatcolloidal Au-SiO2 gels cont aining even lar ger diameterAu particles would show additional loss of pore volumeand a corresponding smaller average pore diameter.

The average pore diameters an d total pore volumesreported for th e base-cat alyzed aerogels reflect m easur e-ments on samples from two separa te ba tches . Thesignificant deviation in th e physisorpt ion char acter of th ewater-diluted silica samples as compared to th e other base-catalyzed aerogels (with a nd with out Au) ma y be relat edto th e stru ctur al differences we observed by SANS for th ewater-diluted SiO2(bc)aerogel. For unbuffered silica sols,the wa ter-diluted sample does not a ppear t o reflect t hegeneric base-cat alyzed stru ctur e eith er by SANS or by N2physisorption and is therefore not a good control fordilut ion effects.

Given the scale of the connected mesoporous networkin th ese aerogels, it is somewhat su rpr ising tha t 5-nm Au

F i g u r e 5 . N2 adsorption (+) and desorption (O) isotherm s for silica an d colloidal gold-silica a erogels.

http://dontstartme.literatumonline.com/action/showImage?doi=10.1021/la980784i&iName=master.img-004.png&w=437&h=547


7/8

particles remain in the SiO2 net work following gelat ionand do not wash out .48,49 While some of the 5-nm Aupar ticles ma y be tr app ed in pores from which they cann otescape, the fact th at no appa ren t loss of 5-nm Au occur supon washing or supercritical drying indicates that aunique structural composite has been formed. Base-

catalyzed silica aerogel is understood to consist of10-nm colloidal particles of SiO2 t ha t a r e a r r a nge d i n athr ee-dimensional pearl n ecklace structur e.38 Given th esimilarit y in s ize of the colloidal Au t o th e colloidal SiO2th e 5-nm Au par ticles ma y be incorpora ted into th e th ree-dimensional SiO2 structure. An average pore diameter of14.8 ( 0.9 nm was determined for a 5-nm Au:SiO 2(bc)aerogel from t he ph ysisorption m easuremen ts, which isseveral nanometers larger than that for standar d SiO2-(bc) gels. A correspondin g increase in tota l pore volume(from 2.73 cm 3/g for S iO2 to 3.04 cm 3 /g for 5-nm Au:SiO2)was a lso observed. In corpora tion of the 5-nm Au colloidsinto the SiO2 network is consistent with this result andwould explain why these sma ll par ticles rema in in the gelafter washing.

D e s ign ofC atalys ts ,Ele c tr oc atalys ts ,and Se ns or s .The efficient tra nsport of gas- an d liquid-phase speciesthrough the porous aerogel structure can be furtherexploited by chemically ta ilorin g t he silica-immobilizedmeta l colloids for specific applications. On t he bas is of th eaffinity of Au for thiolates,50 the formation of self-assembled monolayers (SAMs) of thiols on the Au colloidsurface is an attr active route to functionalizing t hesenan oscale composites for specificapp lications, par ticularlyin cases where direct adsorption of the sensing elementto the metal particle is not possible. Modification of thecolloida l Au-silica aer ogel with t hiols is facilitat ed by th ehigh porosity of the aerogel stru cture an d by th e variousopportunities to modify these composites.

As indicated in Figure 6, molecular modifiers (e.g.,

thiols) may be int roduced onto th e colloidal Au pr ior toimmobilization in the silica network. Thiols introducedat this stage can be used either to prevent aggregation ofth e colloidal Au (for p reconcent ra tion of th e Au sol priorto addition to th e silica sol)or to introduce a desired sensingelement , since gel-tr apped organ ics are kn own to sur vivethe washing and supercritical drying procedures withoutdegradation.51 Considerable interest in t he pr operties of

Au colloids a nd of SAM-modified Au par ticles ha s led t oa variety of preparations in the literature for thiolatedAu colloids which deter aggregation of the Au particlesand/or which contain thiols tagged with specific func-tionalities.52-56

Alternately, molecular modifiers can be introduced toa preformed colloidal Au-silica gel from solut ion dur ingwash ing or to a dr ied, sint ered aer ogel from th e solut ionor gas phase, as shown in Figure 6. Our preliminar y studieshave included the exposure of silica and colloidal Au-silica gels imm ers ed in acetone (i.e., not yet super criticallydried) to an acetone solution of methyl oran ge.

The pores in both gels visibly fill with th e meth yl orangesolution, based on color changes of clear to yellow andpink to oran ge for s ilica a nd colloidal Au-silica, respec-tively. Followed by extensive r insing with acetone, th emethyl orange completely leaches out of the silica gel(which becomes clear again) but is partially retained byth e colloidal Au-silica gel (which r emain s oran ge). UV-vis spectra of these gels in acetone (i.e., the pores arefilled with acetone) confirm these results, a s sh own inFigur e 7. Adsorpt ion to th e Au colloid su rface result s ina broadenin g oft he meth yl oran ge absorpt ion peak at 416nm a nd a red -shift of2 nm. A similar peak sh ift (4 nm)an d broaden ing ar e observed for t he colloidal Au plasmon

resonance.57 Lee et al.58 have also reported only minor

(48) This result is distinctly different from the case in which apreformed silica gel soaked in Au sol visibly takes up Au colloid but theabsorbed colloid visibly flushes out of th e gel by was hing.

(49) Swider, K. E.; Rolison, D. R. Unpublished results, 1996.(50) Nuzzo, R. G.; Zegarsk i, B. R.; Dubois, L. H. J . Am . Ch em. S o c.

1987, 109 , 733.(51) Morris, C. A.; Rolison, D. R. Unpublished results, 1997.

(52) Brust , M.; Walker , M.; Beth ell, D.; Schiffrin, D. J .; Whyma n, R. J. Chem. Soc., Chem. Commun. 1994, 801.

(53) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Nata n, M. J .Anal. Chem. 1995, 67, 735.

(54) Hostetler, M. J.;Gr een, S. J.; Stokes, J. J .;Mur ray, R. W. J . A m .Chem. S oc. 1996, 11 8, 4212.

(55) Nelles, G.; Schonh err , G.; J aschke, M.; Wolf, H .; Schau b, M.;Kuth er,J .;Tremel,W.;Bamberg,E.;Ringsdorf,H.;But t, H.-J. Langmuir1998, 14 , 808.

(56) Weisbecker, C. S.; Merrit t, M. V.; Whitesides, G. M. L a n g mu i r 1996, 12 , 3763.

(57) The absorbance peak for t he Au sol used in the modificationstu dy differs from t hose report ed above (i.e., it is r ed-shifted compa redwith the 5- and 28-nm sols) because the average colloid size for thisin-house pr epared sol is larger.

(58) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391.

Figure 6. Schematicoft he prepara tion ofcolloidal gold a erogelsshowing th e sol-gel processing meth od an d the steps a t whichcolloidal Au and thiol modifiers may be introduced.

F i g u r e 7 . UV-vis spectra for a colloidal Au sol, a colloidalAu:SiO2(bc) gel (pores filled with aceton e), an aceton e solut ionof methyl orange, and the colloidal Au:SiO2(bc) gel followingexposure to meth yl orange and copious rinsing with acetone.The UV-vis spectr um ofa SiO2(bc)gelexposed tomethylorangeand rinsed thoroughly exhibited no spectral features over thisregion.

680 L an gm u ir, V ol. 15, N o. 3, 1999 An d erson et al.
http://dontstartme.literatumonline.com/action/showImage?doi=10.1021/la980784i&iName=master.img-006.png&w=236&h=171http://dontstartme.literatumonline.com/action/showImage?doi=10.1021/la980784i&iName=master.img-005.png&w=228&h=146


8/8

changes in the absorption spectra for dyes adsorbed tometa l part icles in solution. On the basis of th ese results ,methyl oran ge adsorbs exclusively to t he Au surface inth e colloidal Au-silica a erogel.These preliminary resultsdemonstr at e th at incorpora tion of colloidal Au into a silicanetwork does not block molecular access tothe Au surfaceand that the nanoscale metal and silica components ofthe mesoporous material may be differentially modified.

C o n c l u s i o n s

Composit e aer ogels of colloidal Au-colloidal silica canbe produced in which nanoscale preformed Au colloidsare immobilized in both acid- and base-catalyzed silicaaerogel str uctu res and rema in tra pped following washingand supercritical drying. UV-vis measurement s show ablue-shift in the absorbance maximum of silica-im-mobilized Au colloids, indicat ive of a Au-SiO2 interaction.However, spectroscopicdet ermin at ion ofAu part icle sizesappears to give unreliable results for the relatively largepart icle sizes involved here, an d a more direct meth odsuch as TEM is requ ired for accura te size deter mina tion.

SANS studies detect no significant stru ctur al changesin a cid- or base-cata lyzed silica due to t he presence ofsmall amounts of Au colloid. However, physisorption

measur ements sh ow a decrease in BET sur face area a ndavera ge pore size for ba se-cat alyzed gels conta ining lar geAu colloids (i.e., Au par ticles J10 nm in diameter) ,indicatin g th at th e Au colloids occlude pore spa ce in th e

sta nda rd silica aerogel. The increase in avera ge pore sizefor colloidal 5-nm Au:SiO2(bc) compared with that forstan dard base-catalyzed silica is consistent with incor-por a t i on of t he 5-nm Au pa r t icl es i n t o t he t h r e e-dimensional SiO2 network and explains why th ese Aucolloids remain trapped in the silica structure despite itsextensive mesoporosity.

The surface of t he silica-immobilized Au colloid isaccessible for modification by externally introduced re-agents, as verified for the direct adsorption of methyl

orange from a pore-filling solut ion. Determ inat ion ofwha tfraction of th e tota l Au su rface area is t ru ly accessible iscurrentlyunderway,but these initialresults demonstratethe feasibility of tailoring colloidal metal aerogels toprodu ce novel sensin g, cat alytic, an d electr ode mater ials.

A c k n o w l e d g m e n t . This research was supported byth e Office of Nava l Research a nd DARPA. We th an k Dr.Karen Swider (ASEE /NRL Postdoctoral Associat e, 1993-1996) for preliminary experiments showing the incorpo-ra tion of colloidal gold int o acid-catalyzed silica gel, Dr.John Barker (NIST) for ass is tance with the SANSexperiments (supported by the Na tional Science Found a-tion under agreement No. DMR-943101), and ProfessorNicholas Leventis (University of MissourisRolla) for

helpful comments. M.L.A. also thanks the AmericanSociety for Engineering Education for postdoctoral support.

LA980784I


aerogeles de oro coloidal

Documents