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Synthesis and Characterization ofCarbon NanotubeNanocrystalHeterostructures

Sarbajit Banerjee and Stanislaus S. Wong*,,

Department of Chemistry, SUNY at Stony Brook, Stony Brook, New York 11794, and

Materials and Chemical Sciences Department, BrookhaVen National Laboratory,

Building 480, Upton, NY 11973

Received October 25, 2001; Revised Manuscript Received December 17, 2001

ABSTRACT

Oxidized single-walled carbon nanotubes (SWNTs) have been reacted with cadmium selenide (CdSe) nanocrystals, capped with mercaptothiol

derivatives, as well as with titanium dioxide (TiO2) nanocrystals, functionalized with 11-aminoundecanoic acid to form nanoscale heterostructures,characterized by transmission electron microscopy and infrared spectroscopy. The reaction with acid-terminated CdSe nanocrystals and

acid-terminated tubes was facilitated with the aid of intermediary linking agents, such as ethylenediamine and semicarbazide, in an amide-

forming reaction in the presence of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride, EDC. Based on electronic absorption

spectroscopy, charge transfer is thought to proceed from the nanocrystal to the nanotube in the CdSe nanotube system, whereas in the

TiO2nanotube system, charge transfer is expected to occur from the nanotube to the nanocrystal.

Single-walled carbon nanotubes (SWNTs)1 have been thefocus of intensive study due to their unique structure-dependent electronic and mechanical properties.2,3 They arethought to have potential applications as catalyst supportsin heterogeneous catalysis,4 as high strength engineeringfibers,5 and as molecular wires for the next generation ofelectronics devices.6 SWNTs have been used as molecularwires, due to their long electron mean free paths and theirballistic transport properties.7 Depending on their helicityand diameter, carbon nanotubes can be either metallic orsemiconducting. It is also predicted that the band gap ofsemiconducting nanotubes decreases with increasing diam-eter.8 Moreover, a single defect can change the structure ofa tube from a metallic to a semiconducting variety.

A novel strategy of altering the electronic properties ofnanotubes is to chemically functionalize9 them with a moietyor structure whose intrinsic properties are electronicallyconfigurable. One such structure is the family of semicon-

ductor nanocrystals,10

such as CdS and CdSe, alternativelyknown as quantum dots, which exhibit strongly size-dependent optical and electrical properties. The high lumi-nescence yield of these materials11 as well as the potentialof adjusting emission and absorption wavelengths by select-ing for nanocrystal size make quantum dots attractive forconstructing optoelectronic devices, for instance, with tailored

properties. In fact, the electronic structure of semiconductornanocrystallites exhibits distinctive quantum effects;12 theband gap of these materials increases with decreasing particlesize.

In this letter, we report the synthesis and characterization

of SWNTs covalently joined to CdSe and TiO2 semiconduc-tor nanocrystals by short chain organic molecule linkers. Ineach case, purified, oxidized nanotubes were reacted withnanocrystals, derivatized with either amine or acid terminalgroups (Figures 1 and 2). By judiciously varying the natureof the organic capping groups on the nanocrystal surface andthe organic bifunctional linkers, it should be possible tomodulate interactions between the nanotubes and the nano-crystals, with implications for self-assembly. Such compos-ites are expected to be useful for applications as diverse asmolecular electronics, photocatalysis, solar energy conver-sion, and as probes for scanning force microscopy.

The optical and physical properties of these nanocompos-ites are currently being actively investigated and will besubsequently reported. Starting materials and reagents werecharacterized by UV-visible spectroscopy (Beckmann CoulterDU600 and ThermoSpectronics UV1), transmission electronmicroscopy (TEM, Philips CM12), scanning electron mi-croscopy (SEM, Leo 1550 with a field emission column),and energy-dispersive X-ray spectroscopy (EDS). For mi-croscopy analyses, a portion of the solid was dispersed inethanol by mild sonication and evaporated over a 300 meshCu grid, coated with a lacey carbon film. The nanotube-

* Corresponding author. Phone: 631-632-1703; 631-344-3178. E-mail:sswong@ms.cc.sunysb.edu; sswong@bnl.gov.

SUNY at Stony Brook. Brookhaven National Laboratory.

NANO

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10.1021/nl015651n CCC: $22.00 2002 American Chemical SocietyPublished on Web 01/12/2002

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nanocrystal adduct was analyzed by TEM, FT-IR spectros-copy (Mattson Galaxy FT-IR 3000), and UV-visible spec-troscopy.

Raw SWNT bundles (Carbolex; length range of 0.8-1.2m and widely varying width; SEM images in Figure S1(Supporting Information)) were thoroughly oxidized accord-ing to existing procedures involving acidic potassium per-

manganate solution and hydrochloric acid.13 This treatment14has been shown to coat the nanotube sidewalls with keto,carboxylic, aldehyde, and alcoholic groups. TEM analysesindicated that the oxidation process not only removedamorphous carbon but also the majority of metal particles(Figure 3a) as well. The purified tubes were then dried in avacuum oven at 180 C. Carboxylic acid groups are expectedto be the predominant group along the sidewalls.14 This wasfollowed by acid etching in 2.6 M HNO3 to further shortenthe tubes. TEM analysis showed that oxidation and etchingremoved most of the amorphous carbon and metal catalystparticles. The remaining tubes obtained, for the most part,were relatively free of impurities, such as particulates, and

either one or both of the tube caps were opened. The tubeswere also substantially shortened after acid etching, rangingin length from 100 to 200 nm.

Wurtzite CdSe nanocrystals capped with trioctylphosphineoxide (TOPO) were synthesized by existing procedures, usingpurified dimethylcadmium as a precursor.15 After dissolutionin hexane and ethanol, a sharp low energy exciton wascentered at 550 nm in the absorption spectrum (Figure 4b)of the orange-red sample. The sample prepared was relativelymonodisperse with an average particle size of 32 ( 3 , afigure derived from comparison of our data with experimen-

tal, room-temperature optical absorption spectra from previ-ous reports15,16 and confirmed by TEM data (Figure S2,

Figure 1. Schematic of the addition of titanium dioxide nanocrystals to oxidized single-walled carbon nanotubes (OCNTs). (a) Raw SWNTs(I) were treated in a permanganate-sulfuric acid mixture to create opened, oxidized carbon nanotubes [SWNT-COOH] (II). Amine-terminated TiO2 nanoparticles (III) were prepared by the forced hydrolysis of titanium(IV) isopropoxide in the presence of 11-aminoundecanoicacid. (b) (II) and (III) were linked in the presence of EDC to form the adduct ( IV). The bonds at the interfaces are not drawn to scale.

Figure 2. Schematic of the addition of CdSe nanocrystals toOCNTs. TOPO capped nanocrystals (V) were prepared by estab-lished methods using organometallic precursors. (a) TOPO cappingwas substituted by a thiol ligand to form an acid-terminated CdSenanocrystal (VI). Substituted thiocarboxylic acids used included

p-mercaptobenzoic acid, thioglycolic acid, and 3-mercaptopropionicacid, as represented by different R groups shown. (b) (VI) waslinked to OCNTs (II) by an ethylenediamine linker (VII) in thepresence of EDC to form the adduct (VIII). The bonds at theinterfaces are not drawn to scale.

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Supporting Information). The presence of TOPO capping wasdemonstrated by a strong phosphorus signal in the energydispersive spectrum. However, TOPO molecules are notstrongly bound to the nanocrystals themselves, as they canbe washed away by polar solvents such as methanol.15

To create a relatively robust ligand shell, CdSe quantumdots were further derivatized with several other moieties,prepared through a ligand exchange reaction. Thioalcoholsand thio acids have been shown to be effective size-regulatingand stabilizing agents for cadmium chalcogenide nanopar-ticles.17,18 Moreover, they strongly attach to the CdSe surface,

yielding a uniform surface coverage of passivating caps;indeed, we have taken advantage of the richness of thiolchemistry to vary terminal functional groups at the nano-crystal surface, which is crucial for chemical manipulationof these nanostructures.19-21

To demonstrate the generalizability of our derivatizationmethodology, we chose a range of different mercaptocar-boxylic ligands, of different hydrocarbon chain lengths andcomposition, to ensure a certain level of solubility in DMF,the reaction medium. These included22 p-mercaptobenzoicacid, thioglycolic acid, and 3-mercaptopropionic acid (Al-

drich). The UV-visible spectrum in methanol showed asmall blue shift upon capping with thiols in the ligandexchange reaction; the peaks of the optical features were alsosomewhat broadened, especially with samples capped withthe aliphatic mercaptoacid derivatives (Figure 4b). While theformer situation indicated a slight decrease in the averagesize of the crystallites (5-10%) presumably due to thesmaller size of these thiols with respect to the starting TOPOmolecules, the latter observation indicates an increasedbroadness in size distribution (standard deviation in size from8% to 15%), which has been attributed to a loss of some

surface Cd and Se species.15The success of the capping reactions was further confirmed

by the elemental signature of sulfur in the EDS spectrum(Figure S3, Supporting Information); the size distribution ofthe particles, deduced by optical means, was further con-firmed by TEM. We attempted capping of CdSe with longerchain mercaptothiols as well, most notably with 11-mercapto-1-undecanol and 16-mercaptohexadecanoic acid. These,however, were found not to link particularly well tonanotubes, potentially because of steric and configurationalconsiderations involving the lower numbers of as well as

Figure 3. Transmission electron micrographs taken at 120 kV on a 300 mesh Cu grid with a lacey carbon film. (a) A purified single-walled carbon nanotube bundle. Scale bar denotes 30 nm. (b) Oxidized carbon nanotube bundle circumscribed by capped TiO2 particles.Scale bar represents 45 nm. (c) Oxidized tubes linked to CdSe nanocrystals capped with 4-mercaptobenzoic acid. The crystallites areconcentrated at the functionalized open ends, with some coverage on the sidewalls. Scale bar is 150 nm. (d) Oxidized nanotube linked toCdSe nanoparticles capped with 3-mercaptopropionic acid. Image at high magnification shows prolate nanocrystals scattered along the

length of the tube. Scale bar signifies 20 nm.

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groups adsorb strongly onto bare titanium dioxide surfaces.31

Hence, larger numbers of these particles can interact withnanotubes and in effect, amide bond formation may not berequired for an underivatized, uncapped titanium oxide-OCNT adduct to be directly produced. Indeed, we postulatethat this is the reason for enhanced coverage with TiO2particles.

EDS analysis (Figure S3, Supporting Information) on bothnanocomposite systems was consistent with the presence of

appropriately functionalized nanocrystals attached to nano-tubes as proposed. Estimating the precise stoichiometry ofthe reactions, though, is difficult due to a lack of knowledgeabout the exact spatial location of the carboxylic groups onthe nanotubes as well as variations in the numbers and typesof defect sites, specific to each individual tube. All of thesefactors do have an influence on the position and configurationof nanocrystals in the resulting nanocomposites. In light ofthe above, we have at times overestimated the quantity ofnanocrystallites required to react with the nanotubes, and thesamples show some amount of dispersed unreacted nano-crystals. These patterns of nanocrystal clustering on thenanotubes were not observed in control experiments, withoutthe use of EDC, where the tubes themselves were found toremain totally separate from the nanocrystals.

FT-IR spectra provide further evidence for bonding (FigureS4, Supporting Information). The amide bond region showsseveral sharp and distinctive peaks. The peaks are numerousbut sharp and unbroadened and likely correspond to distinc-tive modes of bonding between the OCNTs and the nano-crystals. All the spectra (KBr pressed pellet) show a peak at1581 cm-1, corresponding to the IR active phonon mode ofnanotubes that has been previously reported.32 In addition,the sharpest peak in all the spectra occurs at 1638 cm-1,located in the amide bond region. We postulate that this does

in fact correspond to an amide bond, potentially correlatingwith unreacted amine groups attached to the modifiednanotube ends as well as with amide bonds of the linkergroups associated with the capping ligands. Previous obser-vations of amide bonds in carbon nanotubes linked to amineshave been noted at 1663 and 1642 cm-1.33 The region ofthe mid-IR around 1615-1580 cm-1 also has a number ofdistinctive peaks, which leads us to believe that at least someof the bonding may occur through an additional ionic,noncovalent COO-NH3+ interaction, since previous work hasattributed CO in this region to carboxylate anion stretchmodes in such interactions.34

Peaks observed in the 3000-2850 cm-1 range are likely

a result of C-H stretches within the intermediary linkersused as well as within the aliphatic chains, when aliphaticthiocarboxylic acids were used as capping groups. Thisassignment is confirmed by the disappearance of these peaksin the specific example of mercaptobenzoic acid, associatedwith the SWNT-CONH-C6H4S-CdSe adduct, where nei-ther en nor sc was used and the CdSe nanocrystal in thatcase was effectively capped by an aromatic thioamide.Aliphatic C-H stretches are shifted 10-15 cm-1 from C-Hstretches in neat en or sc. Similarly, the C-H bends, notedin en at 1459 cm-1 and in sc at 1438 and 1484 cm-1, are in

a range from 1422 to 1469 cm-1 in the adduct. A shift uponproduct formation is also seen for C-C aromatic stretchesin the mercaptobenzoic acid ligand from an initial value of1554 cm-1 to a range of 1543 to 1578 cm-1 depending onthe mode of linkage to the nanotube. These results areconsistent with a similar type of interpretation of IR spectrafor derivatized SWNTs, which has been previously re-ported.35

We expect that OCNTs may be able to act as hole

scavengers for titanium oxide nanocrystals in an analogousway as other derivatized carboxylic acids have been shownto do (Figure 4a).36 This suggests that photoactive modulationof the band gap on the nanotube should be possible, whichcould then facilitate reductive processes on the titaniasurfaces. The absorption spectrum for OCNT is featurelessin the 200-800 nm region, while TiO2 nanocrystals showabsorption below 400 nm with a peak at 259 nm. The adductproduct shows peaks at 227, 239, 249, and 288 nm. Whilethe sharp peak, located at 249 nm, might correspond tounreacted TiO2, we postulate that the broad band centeredabout 288 nm, coupled with depletion of the band at 259

nm, corresponds to a charge-transfer transition involvingelectron transfer to the titanium dioxide surface and lattice,thereby suggesting that charge separation has been achieved.

For our system, in contrast to titanium dioxide, which hasa large band gap and a low lying conduction band, CdSehas a high lying conduction band and a low band gap; hence,it may be expected that charge injection occurs from thenanocrystal to the nanotube in that case.25,31 Evidence forthis hypothesis (Figure 4b) comes from the disappearanceof the lowest energy exciton in the adduct at around 550nm, corresponding to the band gap excitation in the initialnanocrystal reagent solution. Indeed, for the adduct, distinc-tive, intense, and sharp charge transfer bands appear at lowerwavelengths (

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demonstrated from nanocrystal to nanotube (in the case ofCdSe) or from nanotube to nanocrystal (with TiO2).

Future work will take advantage of the tunability, in termsof size, shape, and chemistry, of nanotubes and nanocrystals,to create a sharp junction interface, whose properties aremanipulable and hence, predictable. For instance, single-electron charging effects and energy-level or conductancequantization can be controlled to varying degrees at roomtemperature, and the results can be used to increase under-

standing of factors affecting carrier mobility, such aselectronic structure, carrier trapping, and delocalization.

Acknowledgment. We acknowledge support of this workthrough startup funds provided by the State University ofNew York at Stony Brook as well as Brookhaven NationalLaboratory. Acknowledgment is also made to the donors ofthe Petroleum Research Fund, administered by the AmericanChemical Society, for support of this research. We also thankDr. James Quinn for his guidance and help with the SEMand TEM work.

Supporting Information Available: (a) SEM images of

raw nanotubes, (b) TEM images of unreacted CdSe nano-crystals, (c) EDX data correlating with TEM images (Figure3) of nanocrystal-nanotube composites observed and withunreacted nanocrystals, and (d) representative FT-IR spectraof these composites are presented. This material is availablefree of charge via the Internet at http://pubs.acs.org.

References

(1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603.(2) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of

Fullerenes and Carbon Nanotubes; Academic Press: New York,1996.

(3) Odom, T. W.; Huang, J.-L.; Kim, P.; Lieber, C. M. Nature 1998,391, 62.

(4) Planeix, J. M.; Coustel, N.; Coq, B.; Ajayan, P. M. J. Am. Chem.Soc. 1994, 116, 7935.

(5) Calvert, P. Nature 1992, 357, 365.(6) Saito, S. Science 1997, 278, 77.(7) White, C. T.; Todorov, T. N. Nature 1998, 393, 240.(8) Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, P. Appl.

Phys. Lett. 1998, 73, 2447.(9) Banerjee, S.; Wong, S. S. Nano Lett. 2002, 2, 49.

(10) Brus, L. E. Appl. Phys. A 1991, 53, 463.(11) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.;

Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem.B 1997, 101, 9463.

(12) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226.(13) Hiura, H.; Ebbesen, T. W.; Tanigaki, K. AdV. Mater. 1995, 7, 275.(14) Bond, A. M.; Miao, W.; Raston, C. Langmuir 2000, 16, 6004.(15) Murray, C. B.; Norris, D. J.; Bawendi, M. B. J. Am. Chem. Soc.

1993, 115, 8706.(16) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998,

120, 5343.(17) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner,

K.; Chemseddine, A.; Eychmuller, A.; Weller, H. J. Phys. Chem.1994, 98, 7665.

(18) Rogach, A. L.; Kornowski, A.; Gao, M.; Eychmuller, A.; Weller, H.J. Phys. Chem. B 1999, 103, 3065.

(19) Swayambunathan, V.; Hayes, D.; Schmidt, K. H.; Liao, Y. X.; Meisel,D. J. Am. Chem. Soc. 1990, 112, 3831.

(20) Peng, X.; Wilson, T. E.; Alivisatos, A. P.; Schultz, P. G. Angew.Chem., Int. Ed. Engl. 1997, 36, 145.

(21) Eychmuller, A.; Rogach, A. L. Pure Appl. Chem. 2000, 72, 179.(22) Chen, C.-C.; Yet, C.-P.; Wang, H.-N.; Chao, C.-Y. Langmuir1999,

15, 6845. In a typical reaction, carried out on a standard Schlenkapparatus under an inert atmosphere, a few mg of TOPO cappednanocrystallites and 20 mg of the chosen mercaptocarboxylic acidwere added together in methanol with stirring, after which an aliquot

of 0.1 N tetramethylammonium hydroxide in propanol was drippedin carefully until the pH of the resulting solution was close to 10.Thereafter, the solution was heated at 57 C for 6-12 h until itbecame optically clear. The nanocrystals were precipitated by gradualdropwise addition of THF, purified by repeated dissolution inmethanol, and then, reprecipitated in THF. The resulting solid wasobtained by filtering over a 0.2 m nylon 6,6 membrane.

(23) Cassagneau, T.; Fendler, J. H.; Mallouk, T. E. Langmuir 2000, 16,241. Specifically, a millimolar ethylene glycol solution of 11-aminoundecanoic acid was heated to 70 C for a few minutes, afterwhich a millimolar solution of 2-propanol containing titanium(IV)isopropoxide was added under vigorous stirring. Distilled water (0.5mL) was added slowly to complete the reaction; stirring wascontinued at room temperature for a further hour. The carboxylicacid moiety of 11-aminoundecanoic acid tends to strongly interactwith the positively charged surface of TiO2 during the hydrolysis

and condensation reaction, yielding amine-terminated titania particles.The colloidal suspension was filtered over a 0.2 m nylon 6,6membrane; the yield for the reaction was initially low, as the sourceof titanium was not converted into well-dispersed TiO2 nanoparticles.However, allowing the supernatant to settle for a week followed bycentrifugation at 9000 rpm for 40 min permitted the isolation of avery stable colloidal dispersion of titanium dioxide.

(24) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99,13065.

(25) Hao, E.; Yang, B.; Zhang, J.; Zhang, X.; Sun, J.; Shen, J. J. Mater.Chem. 1998, 8, 1327.

(26) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc.1992, 114, 5221.

(27) March, J. AdVanced Organic Chemistry; John Wiley and Sons: NewYork, 1992.

(28) Ajayan, P. M.; Ebbesen, T. W.; Ichihashi, T.; Iijima, S.; Tanigaki,

K.; Hiura, H. Nature 1993, 361, 333.(29) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul,P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E.Science 1998, 280, 1253.

(30) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber,C. M. Nature 1998, 394, 52.

(31) Lawless, D.; Kapoor, S.; Meisel, D. J. Phys. Chem. 1995, 99, 10329.(32) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T., Jr.;

Liu, J.; Smalley, R. E. J. Am. Chem. Soc. 2000, 122, 2383.(33) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P.

C.; Haddon, R. C. Science 1998, 282, 95.(34) Chen, J.; Rao, A. M.; Lyuksyutov, S.; Itkis, M. E.; Hamon, M. A.;

Hu, H.; Cohn, R. W.; Eklund, P. C.; Colbert, D. T.; Smalley, R. E.;Haddon, R. C. J. Phys. Chem. B. 2001, 105, 2525.

(35) Hamon, M. A.; Chen, J.; Hu, H.; Chen, Y.; Itkis, M. E.; Rao, A. M.;Eklund, P. C.; Haddon, R. C. AdV. Mater. 1999, 11, 834.

(36) Thurnauer, M. C.; Rajh, T.; Tiede, D. M. Acta Chem. Scand. 1997,51, 610.

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