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Sol-Gel Template Synthesis of SemiconductorNanostructures

B r i nda B . L a k s hm i, P e te r K . D or hout , and C ha r l es R . M ar t i n*

Dep a r tm en t of Ch emi str y, Co l or a d o S ta te Un i ver si ty, F or t Col l i n s, Co l or a d o 8 0 52 3

Received October 31, 1996. Revised M anu scri pt Received J anua ry 17, 1997X

The templat e method for preparing na nostructures enta ils synthesis of the desired mat erialw ithin t he pores of a na noporous membra ne or other solid. A nanofibril or t ubule of thedesired mat erial is obta ined w ithin each pore. Methods used previously to deposit ma teria lswithin the pores of such membranes include electrochemical and electroless deposition andin situ polymerizat ion. This paper describes the first use of sol-gel chemistry to preparesemiconductor na nofibrils and t ubules w ithin t he pores of an a lumina t empla te membra ne.TiO 2, WO3, and ZnO nanostr uctures have been prepared. TiO2 nanofibrils with diametersof 22 nm w ere found t o be single crysta ls of an a ta se with the c-axis oriented along the fibrilaxis . B u n d le s of th e se f ibri ls we re also fou n d to b e sin g le crysta l lin e, su g g estin g th a t th eindiviual f ibrils are arr a nged in a highly orga nized fashion within th e bundle. Fina lly, 200nm diameter TiO2 fibrils were used as photocatalysts for the decomposition of salicylic acid.

Introduction

We ha ve been exploring a general m ethod for prepar -ing nanomat erials tha t enta ils synthesis of the desiredma terial w ithin the pores of a na noporous membran e.1-3

The membranes used contain cylindrical pores withmon odis pe rse dia me t ers , a n d a n a n o s cop ic f ibr i l ortubule of the desired mat erial is synthesized within eachpore. This method has been used to make tubules andfibrils composed of polymers, metals, semiconductors,carbon, and Li ion intercalat ion ma terials.1-3 Methodsused to synthesize such materials within the pores ofthe template membranes include electroless metal depo-sit ion, electrochemica l meth ods, and in situ polym-

erizat ion.1-3

S ol-gel chemistry h as recently evolved into a generaland powerful a pproach for preparing inorgan ic materi-a ls .4,5 This method typically entails hydrolysis of asolution of a precursor molecule to obtain first a suspen-s ion of col loida l p a rt icles (t h e s ol) a n d t h e n a g e lcomposed of a ggregated sol pa rt icles. The gel is thent h er m a l ly t r ea t e d t o y i el d t h e d es ir ed m a t e r ia l . I to c c u rre d t o u s t h a t s o l-gel chemistry could be donewithin the pores of the nanoporous template membranest o o bt a in t u bu le s a n d f ibr i ls of a v a r ie t y o f in org a n icma terials. We have recently used th is combination ofsol-g e l a n d t e mp la t e me t h o ds t o p re p a re f ibr i ls a n dtubules of a variety of inorga nic semiconducting m at eri-

als including TiO2, ZnO, and WO3. We have found tha tsingle-crystal anatase-phase TiO 2 nanostructures canbe obtained via this approach and that these nanostruc-t u re s c a n be u s e d a s e ff icien t p h ot o ca t a ly s t s . Th eresults of these investigat ions a re described here.

Experimental Section

Materials. Titanium isopropoxide, zinc acetate, LiOH H 2O,WCl 6, 2,4-pentanedione, and salicylic acid (all from Aldrich),ethanol (McCormick Disti l l ing Co.), and concentrated HCl(M al l inc kr odt ) w e r e use d a s r e ce ive d. P ur i f ie d w at e r w a sobtained by passing house-disti l led w at er th rough a Mill i-Q(Mill ipore) wat er purification system. The alumina templatemembranes w ere either obtained commercially (Wha tma nAnopore fi l ters, Fisher) or prepared in-house. 6,7 The com-mercial membranes had 200 nm diameter pores, and the in-house-prepar ed membranes ha d 22 nm diameter pores. Thepor e d i am e t e r of t he i n-house pr epar e d m e m b r ane s w a sobtained from a calibra tion curve of pore diameter vs voltageused during membrane synthesis.7

Synthetic Methods. TiO 2 tubules and fibri ls were pre-

par e d usi ng a sol-

g e l m e t hod si m i l ar t o t hat d e sc r i b e d b yH a m asa ki e t a l .8 Tita nium isopropoxide (5 mL) wa s a dded to25 mL of ethanol, and the resulting solution wa s stirred in anice bat h. To a second 25 mL portion of ethanol were a dded0 .5 m L o f w a t e r a n d 0 . 5 m L o f 0 .1 M H C l . Th e t i t a n i umisopropoxide solution w as removed from t he ice bath an d t heetha nol/HC l/wa ter solution wa s slowly added; unless otherw isenot ed , t he t e m pe r at ur e w a s m ai nt a i ned at 15 C .

After ca. 60 s the resulting mixture turned milky white (solfor m at i on). The a l um ina t e m pl at e m e m b r ane w as i m m ed i -at ely dipped into this solution for an immersion t ime tha t w asva ried betw een 5 a nd 60 s. After the desired immersion time,the membrane was removed from the sol and dried in air for30 m in a t r oom t e m pe r at ur e . The m e m b r ane s w e r e t he npl ac e d i n a t ub e fur nac e ( i n ai r ) , and t he t e m pe r at ur e w asramped (50 C h-1) t o 400 C . The m e m b rane s w e r e heat e d

at this temperature for 6 h, and the temperature was rampedb a c k d o w n ( 3 0 C h-1) t o r oom t e m pe r at ur e . A l it e r at ur esearch produced one recent reference to the production of TiO 2f ib r il s i n m e m br ane s of t hi s t y pe; how e ver , a m uch m or edifficult electrosynthetic method wa s used.9

ZnO fibers were prepared using a method similar t o tha t ofS akhor a e t a l .10 To 20 mL of etha nol wa s a dded 0.35 g of zincac e t at e , and t he r e sul t i ng m i xt ur e w as b oi l e d unt i l a c l e ar

* Corresponding au thor. E-mail: crmart in@lama r.colostate.edu.X Abstract published in Ad vanceACS Abstracts,Februa ry 15, 1997.(1 ) Ma rt i n, C . R. Science1994, 26 6, 1961.(2 ) Ma rt i n, C . R. Acc. Chem. Res. 1995, 28, 61.(3 ) Ma rt i n, C . R. Chem. M ater. 1996, 8, 1739.(4) Br inker, C. J .; Scherer, G . W. S ol-gel Science; Acad emic Press,

Inc.: New York, 1990. Hench, L. L.; West, J . K. Chem. Rev. (Wash-ington, D.C.) 1990, 90, 33.

(5) ORegan , B .; Moser, J .; Anderson, M.; G ra t zel, M.J . Phys. Chem.1990, 94, 8720; N a t u r e 1991, 335, 737.

(6) Foss, C. A.; Tierney, M. J .; Mar tin, C . R. J . Phys. Chem. 1992,96, 9001.

(7 ) Horny ak, G . L . ; Mart i n, C . R. J . Phys. Chem. , submitted.(8 ) H ama saki , Y. ; O hkubo, S. ; Murakami , K. ; Se i , H. ; Nog ami , G.

J. Electrochem. Soc. 1994, 141, 660.(9) Hoyer, P. L a n g m u i r 1996, 12, 1411.(10) Sa khora, S .; Tickan en, L. D.; Anderson, M. A.J . Phys. Chem.

1992, 96, 11087.

857Chem. M ater. 1997, 9, 8 57-862

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solution wa s obtained (ca . 30 min). The volume wa s return edto 20 mL with ethanol, and 0.06 g of LiOH H 2O w a s a d d e d.Th e r es u lt i n g s ol ut i on w a s u lt r a s o ni ca t e d u n t il a w h i t esuspension wa s obtained (ca. 1 h). The alumina membranewas immersed into this sol for 1 min, removed, and al lowedto dry in air at room temperature for 30 min. The membranew as t he n he at e d i n ai r a t 120 C for 6 h.

WO3 f ibers were prepared using a method similar to thatof Nishide et al .11 E t ha nol (10 m L ) w as pur ge d of a i r usi ngan Ar bubbler, and 1 g of WCl 6 was added with continued Ar

bubbling. 2,4-Pent an edione (2 mL) and H 2O (0.05 mL) wereadded, an d the resulting mixture turned blue in color. Thealumina membrane wa s immersed into this mixture for 1 min,removed and dr ied in air a t room tempera tur e for 30 min. Themembrane was then placed in a furnace and the temperaturewa s ramped (as before) to 550 C. The membrane wa s heatedat t hi s t e m per at ur e for 6 h, a f t e r w hi ch t he t e m pe r at ur e w a sramped back down to room temperature.

These sol-gel synth etic methods yielded either tubules orfibri ls of the desired semiconductor within the pores of thet e m pl at e m e m br ane . I n a d d i t ion, t hin f i l m s of t he m a t e r ialwere deposited on both fa ces of the membran e. One or bothof t he se sur fac e f i lm s w e r e r e m ove d pr i or t o a nal y si s oft he t ub ule s or f ib r il s . Thi s w as ac com pl ishe d b y si m pl ypol i shi ng t he sur fac e (s) of t he m e m b r ane w i t h sand pape r(1500 gr it).

Electron Microscopy. S canning electron microscopic(SE M) images of the 200 nm dia meter tubules a nd fibrils wereobtained as follows: One surface layer wa s removed, an d themembrane was glued (using Torr-Seal Epoxy, Varian) to apie ce of paper t ow e l. The m e m b r ane w a s g l ue d w i t h t hepolished face up. The resulting composite wa s immersed into6 M a q u e ou s N a O H f or 1 0 m in i n o r d er t o d i s so lv e t h ealum ina . This yielded an ensemble of semiconductor tubulesor fibrils tha t protruded from t he epoxy surfa ce like the bristlesof a brush. This sample wa s att ached to an SE M sample stubwith si lver paint (Spi), and 10 nm of Au-P d w as sput t e r e don t o t h e s u rf a ce u s in g a n An a t e ch s pu t t er coa t e r . Th eresulting sample was imaged using a Phillips 505 microscope.

Tra nsmission electron microscopic (TEM) images of thesemiconductor na nostructures were obta ined as follows: Bothsur face l ay e r s w e r e r e m ove d, a nd a pie ce of t he r e sul t ing

membrane was placed onto a carbon-fi lm-coated TEM grid.The 6 M aq ue ous Na O H sol ut ion w a s t he n a ppl ie d t o t hem em b r a n e i n or d er t o d is s ol ve t h e a l u m in a . Th e f r ee dnanost r uc t ur e s ad he r e d t o t he T E M g r i d and w e r e i m ag e dusing a J EOL 2000 microscope. Electron diffraction data werealso obta ined using this instrument. The accelerating volta geof the electron beam was 100 kV and the camera length was120 cm. A gold single crystal wa s used as a sta nda rd to checkthe camera length.

UV-Visible Spectroscopy. B ot h s u r fa c e l a y e rs w e r eremoved, and t he alumina membrane w ith th e semiconductornan ostructures w ithin the pores was placed in a custom-madesample holder. The sample holder wa s placed in the sa mple

chamber of an Hita chi spectrophotometer. A simple absorp-t i on m od e s p ect r u m w a s ob t a i n ed u s in g a b a r e a l um in am e m br ane a s t he r e fer e nce . B e c ause t he al um ina t e m pl at em e m b r ane s ar e t r anspar e nt t hr oug hout t he vi si b l e and UVregions, these spectra showed the absorption band edge of thesemiconductor n an ostructures within the pores.

Photocatalysis. TiO2 i s know n t o b e a g ood cat a l y st forphotodecomposition of organic molecules. 12-19 The photocata -l yt i c a ct i vi t y of t h e s ol-gel-synt hesized TiO2 f ib r il s w a sevaluated using salicylic acid as the organic molecule to be

decomposed. Sa mples in which the TiO2 fibrils protruded froman e poxy sur fac e w e r e pr e par e d as d e sc r i b e d for t he S E Man alyses. The ar ea of epoxy surfa ce used for these photoca-talysis studies was 1 cm 2; this area contained ca. 109 200 nmdiam eter TiO2 f ibri ls. After removal of the alumina t emplat em em b ra n e i n N a O H , t h e Ti O2 f ib r il s w e r e r i nse d i n f i veportions of water to ensure complete removal of the NaOH.The fibri ls were then immersed (overnight and in the dark)in 15 mL of 0.5 mM aqueous salicylic acid in order to al lowor g a n ics p r es en t i n t h e e pox y t o l ea c h o ut p ri or t o t h ephotocata lysis experiments.

The sa m ple s pr e t r eat e d i n t hi s w ay w e r e t he n i m m er se di nt o a b eake r c ont ai ning 15 m L of fr esh 0.5 m M aq ue oussal i cy l ic ac i d. A q uar t z l id w as plac e d ove r t he t op of t hebeaker, and the beaker was placed on the roof of the chemistrybuilding in direct Colorado sun light. Two controls w ere run

at the sa me time and place as t he fibri l lar TiO2 samples. Thefirst consisted of an identical beaker with the same amountof salicylic acid solution; however, a thin film (200 nm thick)of TiO2, prepared under identical conditions as the fibri l larsample, was used a s the photocat alysts. The second controlwas also identical but contained no photocatalyst .

The concentra tion of sa licylic acid in the t hree solutions wa sdetermined every 5 min for a tota l of 30 min. The sa licylicacid concentra tion wa s determined from its char acteristic UVabsorbance (296 nm), using a calibration curve obtained fromsolutions of known concentra tion.

Results and Discussion

TiO2 Tubules and Fibrils. F ig u re 1 s h o ws SE Mimages of TiO

2 t u bu les a n d f ibr i ls p rep a re d in t h e

alumina membrane w ith 200 nm diameter pores. Tu-bules were obtained if the membrane w as immersed intothe sol for a brief period (5 s, Figure 1a), wherea s solidTiO 2 f ibrils w ere obta ined after long imm ersion t imes

(11) Nishide, T.; Yama guchi, H.; Mizukami, F. J. Mater. Sci.1995,30, 4946.

(12) Fujishima, A.; Honda, K. N a t u r e 1972, 37, 238.(13) Matthew, R. W. J . Phys. Chem. 1987, 91, 3328.(14) Kra eutler, B.; B ar d, A. J . J. Am. Chem. Soc. 1978, 100, 5985.(15) Ollis, D. F.; Hs iao, C.; Budima n, L.; Lee, C.J . C a t a l .1984, 8 8,

89.(1 6) O kamoto, K. ; Yama moto, Y. ; Tana ka, H .; Tana ka, M.; I t ay a ,

A. Bu l l . C h em . So c. J p n . 1985, 58, 2015.(17) Mathews, R. W. J . C a t a l . 1987, 97, 565.(18) Spa nel, L.; Anderson, M. A.J. Am. Chem. Soc.1991, 1 13, 2826.(19) Goldstein, S.; Cza pski, G.; Raba ni, J .J. Phys. Chem. 1994, 9 8,

6586.

Figure 1. SEM images of TiO2tubules and fibrils prepared in the a lumina membra ne with 200 nm dia meter pores. The sol wasmaint ained a t 15 C , an d the immersion t ime w as varied from 5 to 60 s. (a) Immersion t ime ) 5 s; remna nts of the TiO2surfacelayer can be seen in this image. (b) Immersion t ime ) 25 s. (c) Immersion time )60 s.

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(60 s, Figure 1c). Int ermedia te immersion times yieldtubules with very thick wa lls (25 s, Figure 1b). In a llc a s e s t h e t u bu le s a n d f ibr i ls we re 50 m lon g (t h ethickness of the t emplat e membrane) and had outsidediameters of 200 nm (the diameter of the pores).

Whether tubules or fibrils are obtained is also deter-mined by the temperature of the sol. The structuresshown in Figure 1 were obtained by immersion of thea lu min a t e mp la t e me mbra n e in t o a s ol ma in t a in ed a t15 C. When the sol wa s 5 C, thin-wa lled tubules were

obt a in e d e v en a t lon g immers ion t imes (1 min ). I ncon t ra s t , w h e n t h e s ol wa s ma in t a in ed a t 20 C, s ol idTiO 2 f ibrils were obtained even after very brief (5 s)immers ion t ime s. Th e se re s u lt s s h ow t h a t t h ro ug hcontrol of temperature and immersion t ime, both tu-bules a nd fibrils can be prepared; in addit ion, the wa llt h ic k n e s s o f t h e t u bu le s c a n be v a rie d a t wi l l ( e . g . ,Figure 1a,b).

The mechanism of formation of TiO 2 from acidifiedtitanium alkoxide solutions is well documented. 20,21 I nthe early sta ges, sol part icles held together by a networkof -Ti-O- bonds a re obtained. These part icles ult i-mat ely coalesce t o form a three-dimensional infiniten e t work , t h e g e l . Th a t t u bu le s a re in it ia l ly o bt a in e d

wh e n t h is p ro c e s s is do n e in t h e a lu min a me mbra n eindicates that the sol particles adsorb to the pore walls.This is not surprising since the pore walls are negativelycharged a nd t he part icles are posit ively char ged.22 Anana logous situat ion occurs when posit ively cha rgedelectronically conductive polymers are synthesized withinmembranes containing anionic sites on their pore walls. 2

In the conductive polymer case, 2 the rate of polym-erization within the pore is faster than in bulk solutiondue to enhanced local concentrat ion of oligomers ad-sorbed to the pore wa ll. A similar situat ion occurs inthe sol-gel TiO2 case. When a lower concentra tion oftit a nium isopropoxide (6 v/v % a s opposed t o th e 20 v/v%used in Figure 1) wa s used to ma ke the sol, gelat ion

in bu lk s olu t ion wa s e xt re mely s low, e ve n a t roomt e mpe ra t u re . H o we ve r , wh en t h e a lu min a me mbra n ewa s dipped into t his sol, solid fibrils of TiO2ar e obta inedin the pores, even at s hort (5 s) immer sion times. Theseresults show tha t gelat ion occurs within t he pores underconditions w here gelat ion in bulk solution is negligiblyslow. By an alogy to the conductive polymer case2 thisis u n do u bt e dly du e t o t h e e n h a n c e me n t in t h e lo c a lconcentration of the sol particles due to adsorption onthe pore wall.

Figure 2 shows an absorption spectrum for an alu-min a me mbra n e con t a in in g 200 n m dia me t er TiO2f ibrils within t he pores. An a brupt increase in a bsor-bance is observed a t w avelengths below 389 nm. This

corre sp on ds t o t h e ba n dg a p of t h e TiO2, a n d b u l ksamples of TiO2 show an analogous absorption spec-t ru m.23 These fibrils (and even the 22 nm diameterfibrils to be discussed below) are too large in diameterto expect to see evidence for quantum confinement inthe absorption spectrum.23 However, preparing fibrilswith diameters sma ll enough t o see evidence for q uan-tum confinement is a goal of this research effort .

Electron Diffraction Data for TiO2 Fibrils Pre-pared in the Membrane with 22 nm DiameterPores. F ig u re 3a s h ows a TE M ima g e o f t h e TiO2nanostructures obtained after dissolving away the 22nm pore diameter membra ne. Numerous images of thistype were obta ined, and in a ll ca ses bundles of the TiO2nan ostructures w ere observed. The bundle sizes ob-s erv ed ra n g ed f ro m a s s ma ll a s 2-4 fibrils to as largeas 10 or more fibrils. The ma in feature in Figure 3at h a t ru n s dia g o n a l ly a c ros s t h e ima g e c on s ist s of t wobu n dle s o f f ibr ils , o ne o n t h e r ig h t e dg e o f t h e ma inf ea t u r e a n d on e o n t h e l ef t e d g e. I n t h i s ca s e t h ebu n dle s con s ist of a p p rox ima t e ly 3-4 f ibr i ls . Th eelectron diffraction data to be discussed below (Figure3b) were obtained from a port ion of the bundle on t hel ef t s id e o f t h i s m a i n f e a t u r e. A s econ d s et of t w obundles is observed below this main feature; this secondset of bundles also proceeds diagona lly a cross the ima ge

bu t a t a s ma ller a n g le.F ig ure 3b s h ows t h e in dex ed e le ct ro n di ff ra c t ionpattern obtained from the center of the fibril bundle ont h e l ef t s id e o f t h e m a i n f ea t u r e i n F i gu r e 3a . Th eorien t a t ion of t h e ima g e s in F ig ure 3a , b is t h e s a me ;t h a t is , t h e c* axis in Figure 3b is par allel to the f ibrilbu n dle a x is in F igu re 3a . Th e se da t a s h ow t h a t t h efibrils a re highly crystalline an at ase-phase TiO2, wi t ht h e c* axis of the anatase oriented along the long axisof the fibril. Sma ll fibril bundles throughout the sampledisp la y e d t h e s a me cry s t a l l in e orien t a t ion ; i .e ., t h ereciprocal la ttice direction [110] is a lmost a lwa ys pa ra l-lel t o t h e e lect ro n be a m, a n d t h e c* a x is is a lon g t h efibril a xis. We can conclude from these observat ions

t h a t t h e f ibr i ls c ry st a l lize a s lon g , p ris ma t ic c ry st a lswit h t h e ra re , a n d me t a s t a ble , a n a t a s e min e ra lo g ic a lorientat ion [001] with {110}.24

J ust as interest ing, such small f ibril bundles do notd em on s t r a t e a g r ea t d ea l of m os a i c s pr ea d i n t h ediffracted electron beam, as one would expect for poly-crysta lline materia ls or oriented polymer fiber samples.25

L a rg e r bu n dle s o f c ry s t a l l in e f ibr i ls do s h o w mo s a icspreading of the reflect ions, but t hey rema in oriented

(2 0) L e e , B . I . ; Pope, E . J . A. Chemical processing of ceram ics;Mar cel Dekker, Inc.: New York, 1994.

(2 1) L i vag e , J . ; H e nry , M.; S anc he z , C . Prog. Solid. State Chem.1988, 18, 259.

(22) Bischoff, B. L.; Anderson, M. A. Chem. M ater. 1995, 7, 1772.(2 3) E nri g ht , B . ; Fi tz mauri c e, D . J . Phys. Chem. 1996, 100, 1027.

(2 4) Da na, J . D. (re wri t te n by P olac he , C . ; Be rman , H.; Frondel ,C .) The system of mineralogy; J ohn Wiley: New York, 1955.

(25) Klug, H. P .; Alexander, L. E. X-ray Di ffraction procedur es for p ol y cr y s t al l i n e a n d a m o r p h ou s m a t er i a l s ; Wiley Interscience: NewYork, 1974.

Figure2. U V-visible spectru m of th e 200 nm diam eter TiO2fibrils.

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a lon g t h e ir [001] f iber a x is . I n a ddit ion , da rk -f ieldimages of the fibril bundles indicate that the crystallinedoma in s a re v e ry la rg e . F or e xa mp le, bu n dle s wit hdia me t ers a s la rg e a s 1 m and lengths correspondingto the entire thickness of the template m embrane (30m) are single crystals.

Th e se da t a s u g ge s t t h a t a f t e r t h e a lu min a t e mpla t eis removed, the individual fibrils are drawn together inan ordered fashion, like stacked 4 4 lumber. Thiss t a c kin g ma y be f a c il it a t e d by v a ca n t or la bi le l ig a n dsites on the Ti atoms, which sit near the (110) faces(F ig u re 4). Th e L e wis a c idic s u rfa c e s i t e s ma y bea t t ra c t e d t o t h e ox ide (or h y drox ide ) s it e s on ot h e r

fibrils; these interact ions could hold the fibrils in anoriented bundle that diffracts like a lar ger, single crystal(Figure 3b).

Highly oriented crystalline domains of other meta-sta ble pha ses have been shown t o grow in an internalma t r ix , in dic a t in g t h a t a l t h o u g h o u r o bs e rv a t io n s o foriented TiO2are unusua l, they ar e not without prece-dent.26 Furth ermore, it is known tha t sol-gel synthesescan result in crystallizat ion of metastable phases be-

cause of the lower t emperatu res often employed in suchsyntheses.27 In addition, it has been previously shownthat sol-gel synthesis can yield single-crystalline ma te-r i a l , i f t h e c r y s t a l s a r e g r o w n a t v e r y l o w t e m p e r a -tures.28 Single-crystalline oriented MgO na norods ha vealso recently been reported, although this was not a sol-gel synthesis.29 Oriented carbon nanofibers have beenprepar ed by template syn thesis.30

Finally, electron diffract ion data from the 200 nmdiameter TiO2 f ibr i ls s h o w t h a t t h e s e f ibr i ls a re a ls ohighly crystalline ana ta se. However, the dat a suggesttha t th ese lar ger-diameter fibrils ar e not single crystals.

Photocatalysis with the TiO2 Fibrils. TiO 2 ca nb e u s ed a s a p h ot oca t a l y st t o d ecom p os e or g a n i c

molecules.12-

16 The mechanism is believed to involvea bs orp t ion of a U V p h ot o n by TiO2 t o p ro du c e a nelectron-h ole pa ir . Th e se re a c t wi t h wa t e r t o y ie ldh y drox y l a n d s u pe rox ide ra dica ls wh ich ox idize t h eorganic molecule. The ultima te products of these rea c-t ions ar e CO2a n d w a t er .18,19 Most investigations of thephotocatalytic activity of TiO2 h a v e u s ed U V la mp s a st h e s ou rce a n d TiO2 t h in f il m s o r p ow d e r s a s t h eca t a ly st . We re port h e re t h e u s e of s u n lig h t a s t h esource and immobilized template-synthesized TiO2fibrilsa s t h e c a t a ly s t .

Figure 5 shows a plot of salicylic acid concentrationvs t ime of exposure of the solution to sunlight . Theupper curve is for the solution that contained no TiO 2

photocat alyst . No loss of salicylic acid from the solutionwa s o bse rve d. I n de ed, t h e re wa s a s l ig h t in cre a s e inconcentrat ion with t ime, due to evaporat ion of water;tha t evapora t ion occurred is proven by the a ppeara nceof w a t e r drop s on t h e q u a rt z l id . Th e lowe r c u rv e in

(26) (a) Lin, J .; Cates, E .; Bia nconi, P . A.J. Am. Chem. Soc. 1994,116, 4738. (b) Bianconi, P. A.; Lin, J .; Strzelecki, A. N a t u r e 1991, 3 49,315.

(27) Lange, F. F. Science1996, 273, 903.(28) Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 11 3,

2826.(29) (a) Yang, P.; Leiber, C. M. Science1996, 273, 1836. (b) Itoh,

H.; U ta mapany a , S. ; Sta rk, J . V. ; Kl abunde , K. J .; Sc hl up, J . R.Chem.M ater. 1993, 5, 71.

(30) Kyotani, T.; Tsai, L.; Tomita, A. Chem. M ater. 1996, 8, 2109.

Figure 3. (a, t op) TEM ima ge of a bun dle of 15 nm dia meterTiO 2 f ibri ls. (b, bottom) Corresponding electron diffractionpat t e r n.

Figure 4. Vie w d ow n t he c- a x i s o f a n a t a s e s h o w i n g t h epossible labile oxygen sit es on th e (110) fa ce. TiO6 polyhedrawith filled circles (Ti) and open circles (O) are represented.

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Figure 5 is for the solution t ha t conta ined the fibrillarTiO 2 photocat alyst . A rapid (relat ive to the t hin-filmcatalyst , middle curve) decrease in salicylic acid con-centra t ion wa s observed. Indeed, the rate of photode-comp os it ion f or t h e f ibr i lla r ca t a ly s t is a n orde r ofmagnitude higher than for the thin-film catalyst (seekinetic analysis below).

B o t h t h e t h in f i lm a n d f ibr i l la r c a t a ly s t we re s u p -ported on an epoxy surfa ce with a n a rea of 1 cm 2. Theenhancement in rate for the f ibrillar catalyst is due tothe higher t otal surfa ce ar ea of TiO2exposed t o solution.Indeed, if it is assumed tha t th e surface area of the thin-film catalyst is 1 cm 2 and tha t a ll of the surfaces of the

fibrils are catalyt ically act ive, an enhancement in rateof 315 would be predicted for the fibrillar ca ta lyst. Thiscalculated enhancement arises because t he tota l calcu-lat ed surfa ce a rea of the fibrils is 315 cm2 of TiO2surfacea re a /cm2 of substra te epoxy surface area . This corre-sponds to 5.1 m 2 of TiO2surfa ce a rea /g of a na ta se fibrils.Even higher surface areas would be expected for tubulesor smaller dia meter f ibrils.

That the experimental enhancement in rate is lessis not surprising. First , the surfa ce of the TiO2 thin-f ilm ca t a ly s t is u n dou bt edly n ot a t o mica l ly s moot h ,me a n in g t h a t i t s t ru e c a t a ly t ic s u rfa c e a re a is g rea t e rt h a n t h e a s s u me d 1 c m 2. Se con d, SE M ima g e s s h owtha t the fibrils do not stand str aight up but ra ther lean

a g a in s t e a c h o t h e r t o f orm clu mps o r bu n c h es . As aresult , large port ions of the f ibril surfaces are shadedfrom the sunlight . Hence, this prototype fibrillar cata -ly s t is n ot op t ima l . Va ria ble s t o be c on s idere d f o roptimizat ion include diameter and aspect rat io of thefibrils ( length divided by diameter) and distance be-tw een fibrils.

Th e da t a in F ig u re 5 c a n be u s ed t o q u a n t i t a t iv elyevalua te the ra te of photodecomposition. The kineticsof photodecomposition of orga nics on TiO2 h a s be e nexplained in terms of a Langmuir model.13,15-17 At alow concentration of substrate (i.e. , the molecule to bedecomposed) this model predicts simple pseudo-first-

order kinetics with respect to the substrat e concentra -t ion . Wh e n t h e da t a in F ig u re 5 a re p lot t e d a ccrdin gto this model, straight-line plots were obtained and thedecomposition rate constant can be calculated from theslope. The fibrillar cata lyst shows a r at e constant of0.03 min-1, as opposed to a rate constant of 0.003 min-1

for t he thin-film cata lyst .

ZnO and WO3 Fibrils. Figure 6 shows S EM ima gesof solid ZnO fibrils obtained within the pores of the200 nm diameter alumina t emplat e membra nes. Thesurface layer is not completely removed which m akest h e f ibers s t ic k t o g et h e r . As w a s t h e c a s e f o r TiO2,these fibrils are 200 nm in diameter and 50 m long.These solid f ibrils were obtained using a sol at 20 C

Figure 5. (a ) Photodecomposition of sa licylic acid in sunlight .Data for no photocatalyst , the thin-fi lm TiO 2 photocatalyst ,and the fibrillar (200 nm) TiO2 photocatalyst are shown.

Figure 6. SEM image of 200 nm dia meter ZnO fibrils.

Figure 7. U V-visible spectrum of th e 200 nm dia meter ZnOfibrils.

Figure 8. SE M image of 200 nm diameter WO3 fibrils.

Syn t h esi s of Sem icon d u ct or N an ost r uct u r es Ch em . M at er ., V ol . 9, N o. 3, 1997 861

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an d an imm ersion time of 1 min. By an alogy to the TiO2case, hollow ZnO tubules are obtained if lower temper-a t u r es a n d s h or t er i m me rs ion t i m es a r e u s ed . Anabsorption spectrum for a composite membra ne con-ta ining these ZnO fibrils is shown in Figure 7. Again,the bandgap absorption for ZnO is observed.10,18 F i-n a l ly , w h e n e xp os ed t o 366 n m l ig h t , t h e s e f ibr i lsshowed t he chara cterist ic10 greenish-yellow bulk lum i-nescence of ZnO.

F i g u r e 8 s h o w s S E M i m a g e s o f s o l i d W O 3 fibrils

ob t a i n ed w i t h in t h e p or es of t h e 200 n m d ia m e t era lu min a t e mp la t e me mbra n e . As wa s t h e c a s e for t h eother materials, these fibrils are 200 nm in diametera n d 50 m long. The SEM picture shows tha t some ofthe surfa ce layer is not removed as in the case of ZnO.These solid f ibrils w ere obtained using a n immersiont im e o f 1 m in a n d t em per a t u re o f 2 0 C . We a r eespecially interested in exploring the electrochemistryof these WO3 fibrils, to obtain the corresponding tung-sten bronzes.

Conclusions

We have shown that sol-gel synthetic methods canbe used with the templat e-based a pproa ch for preparingnan omaterials to yield semiconductor nan ostructures.This ma rria ge of sol-gel and template methods shouldb e a p p li ca b l e t o a l a r g e n u m be r o f m a t e r i a ls . B o t hfibrillar and tubular nanostructures are possible, andsingle-crystalline nanostructures of the metasta ble ana -ta se pha se ca n be obta ined. We are current ly exploring

applications of these nanostructures in photocatalysis,electrochemistry, bat tery research and development,photoelectrochemistry, and enzyme immobilization.

Acknowledgment. Financial support for this workwas provided by the U.S. Department of Energy (DE-FG03-95ER14576) and by the National Science Founda-tion (CTS-9423090).

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