Nanocatalysts with Tunable Properties Derived from Polystyrene-b-poly(vinyl pyridine) Block...

Nanocatalysts with Tunable Properties Derived from Polystyrene-b-poly(vinyl pyridine)Block Copolymer Templates for Achieving Controllable Carbon Nanotube Synthesis

Jennifer Q. Lu†

School of Engineering, UniVersity of California, Merced; Agilent Technologies,3500 Page Mill Road, California, 95034

ReceiVed: September 17, 2007; ReVised Manuscript ReceiVed: January 6, 2008

Chemical vapor deposition, one of major carbon nanotube synthesis techniques, has demonstrated great promisefor substrate-based carbon nanotube device applications. To understand carbon nanotube growth mechanismsfor achieving controllable synthesis, thereby producing carbon nanotubes with consistent and predefinedbehavior for materializing their highly touted properties, it is imperative to develop a methodology to createnanocatalyst systems controllably with great tunability. In this paper, a comparative study of polystyrene-b-poly(4-vinyl pyridine) versus polystyrene-b-poly(2-vinyl pyridine) as templates for creating nanocatalysts ispresented. Creating uniform and periodically ordered nanocatalyst arrays with tunable size and spacing isdemonstrated. A scaling relationship between nanocatalyst spacing and polystyrene block length for polystyrene-b-poly(2-vinyl pyridine) is elicited. The potential of synthesizing nanocatalysts with adjustable compositionis discussed. The ability to tailor nanocatalyst size and spacing by varying block lengths has been demonstratedto promote controllable synthesis of carbon nanotubes with adjustable diameter and density. Finally, an exampleof using such highly controlled nanocatalysts derived from the block copolymer template approach, to studythe effect of substrate on carbon nanotubes synthesis, is described.

IntroductionOne-dimensional (1D) nanomaterials such as carbon nano-

tubes (CNTs) exhibit many exciting properties that are unat-tainable by their micro- and macroscopic counterparts. Becauseof extreme miniature size, quantum mechanical effects, whichare highly sensitive to tube diameter and the geometricarrangement of carbon atoms on a surface, determine theelectronic structure of a CNT.1 Such unusually strong correlationbetween properties and size and spatial arrangement of carbonatoms offers rich and diverse phenomena on the one hand, buton the other hand it imposes a great challenge to synthesizeCNTs with desirable and consistent properties. Among the mostcommon synthesis methods—arc discharge,1–3 laser ablation,4

and chemical vapor deposition (CVD).5–10 CVD is the mostdesirable approach for substrate-based device applications. Itis generally believed that nanocatalysts are responsible forfacilitating the decomposition of carbon feed stock, catalyzingthe formation of carbon hexagons, and assisting the diffusionof carbon along the tube wall for continuing growth. Therefore,the characteristics of nanocatalysts dictate the properties ofCNTs. To achieve controllable synthesis, it is essential to createsynthetic methods that are able to generate nanocatalysts withpredetermined and reproducible characteristics.

The most common catalyst systems are derived either directlyfrom transition metal compounds or thin metal films, neitherof which provides a means to control nanocatalyst size andsubsequent CNT diameter. In addition, these methods do nothave the ability to control the composition and density ofnanocatalysts. To reveal the CNT growth mechanisms forachieving controllable synthesis, a comprehensive investigationto understand the effect of each parameter of a catalyst system,including size, spacing, composition, and the underlying sup-porting material, on the properties of as-synthesized CNTs is

essential. Thus, the prerequisite for synthesizing CNTs withpredetermined, consistent, and reproducible properties is theability to create catalyst systems with controllable and adjustableproperties.

Employing self-assembled morphologies of block copolymershas been successfully used as templates or scaffolds to producenanoscale structures that otherwise cannot be attained by top-down conventional lithography.11–16 One can employ self-assembled morphologies created by catalyst-containing blockcopolymers to produce nanocatalysts. There are two generalapproaches to synthesize metal-containing block copolymers.One involves direct polymerization of a nonmetal-containingmonomer followed by a metal-containing monomer.17–20 Theother involves selectively binding metal species onto one ofthe blocks of a pre-existing block copolymer through eithercoordination or ionic bonds.21–25 Because only a few transitionmetal coordinated monomers undergo living polymerizationwhereas a variety of metals can be potentially incorporated ontothe preexisting polymer chains, the latter approach has thepotential to produce nanocatalysts with tunable composition.Recently, group VIIIA metal nanoparticles have been success-fully synthesized using this approach. These metal nanoparticleshave been demonstrated to be excellent nanocatalysts forsynthesizing high purity and low defect CNTs.22,24–27

In this paper, a comparative study of polystyrene-b-poly(4-vinyl pyridine) (PS-b-P4VP) versus polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) as templates for nanocatalyst synthesisis first presented. The potential of tuning nanocatalyst composi-tion using this approach is discussed. The ability to adjust sizeand spacing by varying the block length of each segment isdemonstrated. A scaling relationship between the spacing ofnanocatalysts and block length is elucidated. The ability oftuning CNT diameter and density by adjusting size and spacingis demonstrated. The potential of using such highly engineered† E-mail: [email protected]; phone and fax: 1-209-228-4149.

J. Phys. Chem. C 2008, 112, 10344–1035110344

10.1021/jp0774589 CCC: $40.75 2008 American Chemical SocietyPublished on Web 06/20/2008

catalyst systems to study the CNT growth mechanisms ispresented in the end.

The rationale for using this system is that block copolymerscomposed of polystyrene (PS), a hydrophobic block, andpoly(vinyl pyridine) (PVP), a hydrophilic block, are known toself-assemble into periodically ordered discrete morphologieson the nanometer scale in both solution and solid.28–30 Anothervery important aspect of this system is that the nitrogen in thepoly(2-vinyl pyridine) (P2VP) and poly(4-vinyl pyridine)(P4VP) repeat unit is very reactive can localize metal speciesvia complexation or protonation. Researchers have demonstrateduse of the P2VP core as a nanoreactor to prepare CdS and Aunanoparticles via complexation and protonation respectively.31,32

We have already shown that a transition metal such as iron,cobalt, or nickel, can be incorporated onto the pyridine unit viacomplexation.22 The morphologies formed by self-assemblingmetal-modified block copolymers can thus be used as templatesto produce periodically ordered arrays of transition metalnanoparticles, which are catalytically active to promote thegrowth of CNTs.22

Experimental Methods

Self-assembly of Pure Block Copolymers in Toluene:Homopolymers, P4VP and P2VP, and block copolymers, PS-b-P4VP and PS-b-P2VP, were used as received from PolymerSource. The block copolymers were first dissolved in toluene(Aldrich) to form 0.25 wt % solutions. Spin-coating was usedto deposit a polymer solution onto 5000 Å coated thermal siliconoxide surfaces. A DI-500 Digital Instrument atomic forcemicroscope was used to analyze surface morphology.

Synthesis of Metal Nanoparticles. CoCl2 · 6H2O, FeCl2 ·4H2O, NiCl2 ·6H2O, and MoCl5 were purchased from Aldrich.Ethanol (Aldrich) was used as solvent for metal salts. To formmetal nanoparticles, the molar ratio of metal to pyridine wasadjusted to 0.25. The solutions were mixed at 50 °C for 8 hand then spin-coated on silicon oxide surfaces. The blockcopolymer templates were removed by UV ozone, leaving thenonvolatile inorganic metal nanoparticles behind. Surfacemorphology analysis of the resulting nanoparticles was per-formed on the DI-500 Digital Instrument atomic force micro-

scope. A Quantum 2000 X-ray photoemission spectrometer withan aluminum source was used to determine the chemicalcomposition of these synthesized nanoparticles.

CNT Growth. After thermolysis of metal nanoparticles at700 °C in air, the substrates were heated to 900 °C under H2.Under this reducing atmosphere, metal oxide nanoparticles werereduced to their metallic state. Methane was subsequently addedto the gas flow to initiate CNT growth. After the growth, thecarbon feed stock was switched off, and the furnace was thencooled down to room temperature under a flow of H2 gas. CNTgrowth yield was characterized by a Hitachi S-4500 SEM. CNTdiameter and purity were determined by a Renishaw InViaRaman microscope. The laser illumination area was 1 µm2. Theintegrated time was 1 min. The wavelength of the laser usedwas 632.8 nm, and the laser power was around 1 mW. CNTdiameter was also estimated based on the AFM height analysis.The Quantum 2000 X-ray photoemission spectrometer wasemployed to compare the chemical state and concentration ofmetal nanoparticles before and after growth on various sup-porting materials.

Results and Discussions

A. Self-assembly of PS-b-P2VP and PS-b-P4VP. Tolueneis a good solvent for PS and is a poor solvent for both P2VPand P4VP. To investigate the micellization of the blockcopolymers in toluene solution, PS319-b-P2VP76 and PS323-b-P4VP78 were dissolved in toluene to afford 0.25 wt % solutions.The solutions were then spin-coated on thermally grown siliconoxide surfaces. Figure 1 is a set of AFM phase images ofsurfaces formed by PS319-b-P2VP76 and PS323-b-P4VP78. Peri-odically ordered surface micelles were detected on the surfaceformed by the PS323-b-P4VP78 solution. On the contrary, noordered morphology was observed on the film surface formedby the PS319-b-P2VP76 solution. P2VP (MW ) 9000 g/mol)dissolves in chloroform and dioxane, but P4VP (MW ) 8800g/mol) does not. Thus, P4VP is more polar than P2VP. This isbecause nitrogen, the electronegative element bearing a partialnegative charge in P4VP, is in a better position to interact witha polar solvent. On contrary, the nitrogen in P2VP is partiallyscreened by the polymer backbone. As a result, P2VP can bedissolved in not very polar solvents such as acetone and dioxane,

Figure 1. AFM phase images of PS-b-PVP thin films (1 × 1 µm scan size, 40° in phase).

Nanocatalysts with Tunable Properties J. Phys. Chem. C, Vol. 112, No. 28, 2008 10345

whereas P4VP is insoluble in either of them. The larger chemicaldissimilarity between PS and P4VP blocks causes the formationof PS-b-P4VP spherical micelles in toluene. The association ofsolvent-unfavorable P4VP segments forms the core, and solvent-favorable PS chains compose the corona. In contrast, chemicaldissimilarity between PS and P2VP is insufficient to self-organize into micelles in toluene; thus, no ordered nanostructureswere detected on the surface formed by the PS-b-P2VP solution.

The PS-b-P2VP block copolymer used in this experimentcontains less than 25% P2VP, while others, such as Moller’searlier work, used a symmetrical polymer with the volumefraction of P2VP close to 50%. The increase micellizationtendency with higher volume fraction of P2VP coupled withthe use of a higher concentration solution (>0.5 wt %) explainsthe discrepancy in the observation.32

B. Synthesis of Metal Nanoparticles.B.1. PS-b-P4VP. PS-b-P4VP block copolymers, whose char-

acterization data is tabulated in the table in Figure 3, weredissolved in toluene. Spherical solution micelles were generatedwith coronas formed by PS and cores comprised of P4VP. Bydepositing the solution micelles onto silicon oxide surfaces,surface micelles were thus obtained. Figure 2a is a set of AFMheight images of highly ordered surface micelles produced byself-assembled PS413-b-P4VP79 and PS4137-b-P4VP667. To usethem as templates to generate metal nanoparticles, FeCl2 ·4H2Owas introduced into these two polymer solutions to attach Fe(II)onto the P4VP segments. Figure 2b is the corresponding ironnanoparticles produced from these block copolymer templates,clearly demonstrating that highly ordered nanoparticles have

Figure 2. AFM height images of polymer surface micelles (a) and corresponding iron nanoparticles (b) obtained from PS413-b-P4VP79 and PS4137-b-P4VP667 (1 × 1 µm scan size).

Figure 3. Panels a and b: correlation between surface micelles and resulting iron nanoparticles. The table below tabulates the number of repeatunits of block copolymer systems.

10346 J. Phys. Chem. C, Vol. 112, No. 28, 2008 Lu

been obtained using this approach. The nanoparticle size andspacing can be adjusted by tuning block lengths as depicted inFigure 2.

Figure 3, panels a and b, summarizes AFM analysis resultsof surface micelles formed by pure block copolymers andcorresponding iron nanoparticles derived from these templates.The close correlation of size and spacing between polymersurface micelles and associated nanoparticles indicates that thepolymer micelle properties, such as the aggregate number, donot noticeably change upon metal incorporation. Therefore,nanoparticle size and spacing can be predictably tailored usingblock copolymers.

In general, it is difficult to use PS-b-P4VP templates toreproducibly produce highly ordered, uniformly sized cobalt andnickel nanoparticles. Hydrated cobalt(II) and nickel(II) arehighly reactive, and as a result they tend to complex with morethan one 4-pyridine group.33 Thus, the inter- and intramolecularbond formation caused by multiple complexation can hinderthe self-assembly process. However, Fe(II) is less likely to formcomplexes with more than one ligand. Hence, nanoparticles withreasonable periodicity and size control can be repeatedly attainedusing PS-b-P4VP templates. The low stability constants of Co(II)and Ni(II) with 2-methylpyridine, due to the steric hindranceeffect of the 2-methyl group, suggests that these metals do nothave the tendency to react with more than one pyridine group.33

Thus, PS-b-P2VP is a better candidate to generate highly orderedtransition metal nanoparticles.

B.2. PS-b-P2VP. Metal-induced Micellization. The immis-cibility of PS and P2VP is not sufficient for PS-b-P2VP to self-assemble in toluene to give rise to micelles. However, the filmformed by the iron-complexed PS-b-P2VP solution is highlyordered, as displayed in Figure 4b. This suggests that the metalcomplexation increases the polarity of the P2VP segment sothat the dissimilarity of PS and metal-modified P2VP issufficient so that the metal-modified block copolymer self-assembles into micelles, as illustrated in Figure 4a. Figure 4cis the AFM height image showing periodically ordered ironnanoparticles after removal of the polymer template. The inset

2D Fourier transform spectrum shows that nanoparticles arearranged in a hexagonal lattice.

Bimetallic Catalyst Nanoparticles. Analogous to the approachdescribed in the preceding section, Ni and Co nanoparticles canalso be generated. Furthermore, bimetallic nanoparticles suchas Fe/Co, Fe/Ni, and Ni/Co can also be produced. It has beenwidely reported that bimetallic catalyst systems such as Fe/Mo,Fe/Ni, Fe/Co, and Co/Mo are capable of producing high-yieldand high-purity single-walled CNTs. It has been hypothesizedthat one metal is responsible for nucleation and the other isresponsible for repair.34 It has also been suggested that one metalwith higher melting temperature, Mo as an example, may actas a matrix to stabilize the catalytically active metal, Co, fromaggregation at the high growth temperature.35 Figure 5 is anexample demonstrating that Ni/Fe bimetallic nanoparticles canbe readily generated. A 0.25 wt % PS794-b-P2VP139 in toluenesolution was used for this study. For the preparation of singlemetallic nanoparticles, the amount of iron was added to obtaina molar ratio of iron to VP of 0.25. For the formation ofbimetallic nanoparticles, Fe(II) was added to obtain the molar

Figure 4. Metal-induced micellization of PS-b-P2VP. (a) Schematic representation of metal induced micellization; (b) AFM height image ofsurface micelles of iron-complexed PS319-b-P2VP76 (1 × 1 µm scan size); (c) AFM height image of the corresponding iron nanoparticles aftertemplate removal (1 × 1 µm scan size); The inset is a 2D Fourier transform spectrum.

Figure 5. XPS spectra of Fe (top) and NiFe (bottom) nanoparticlesderived from PS794-b-P2VP139. Insets are AFM height images (0.25 ×0.25 µm scan size, 10 nm height).


ratio of iron to VP of 0.125. After stirring the solution overnight,hydrated Ni(II) chloride was then introduced into the solution,with the amount of the nickel salt adjusted for the molar ratioof nickel to VP at 0.125. The XPS spectra shown in Figure 5confirm the formation of bimetallic nanoparticles. The XPSspectrum of the nanoparticles prepared from the polymersolution with both nickel and iron salts has the peaks corre-sponding to Fe2p3/2 at 711 eV and Ni2p3/2 at 856.4 eV,consistent with the binding energy of nickel and iron in theirhighest oxidation state, respectively,36 whereas the XPS spec-trum of iron nanoparticles shows only one peak associated withFe(III). It is evident, according to the inserted AFM height

images of nanoparticles, that size and spacing are independentof metal sources, but are dictated by the characteristics of theblock copolymer template.

However, using the same approach, the Co/Mo nanoparticlesproduced are not uniform in size, as displayed in the insertedAFM height images in Figure 6. The XPS result indicates that,even though equimolar Co and Mo were mixed with the polymersolution, the molar ratio of derived Co/Mo was 6. This resultindicates that Mo, a group VI metal, cannot effectively interactwith a 2-pyridine unit. Different block copolymer systems areunder investigation for effective incorporation of Mo. Neverthe-less, the potential of using the block copolymer approach togenerate highly ordered catalyst nanoparticles with adjustablecomposition has been demonstrated. The adjustable compositioncan serve as an excellent tool to investigate the catalytic activityas a function of composition.

Size and Spacing. Using PS-b-P2VP block copolymers withvarying block lengths, the size and spacing of nanoparticles canbe adjusted. Figure 7 is a set of AFM height images of ironnanoparticles demonstrating the ability of tailoring size andspacing by this block copolymer approach. The corresponding2D Fourier transform spectra of iron nanoparticles indicate thatnanoparticles are highly ordered and are arranged in a hexagonallattice. Not only does the catalyst size determine carbonnanotube diameter, but it also can significantly affect the growth.According to simulation, melting temperature and on-set ofgraphitization temperature are substantially reduced with shrink-ing nanoparticle size.37,38 Moreover, the eutectic point forcarbon-metal occurs at a lower carbon concentration.39 It isbelieved that the density of catalyst will also affect CNT growthyield and orientation. On one hand, excessively high catalystdensity results in low yield due to insufficient carbon concentra-tion.40 On the other hand, if the catalyst density is too low, theresulting CNT density is too to form vertically aligned CNTs.25,41

The ability of tuning size and spacing of catalyst nanoparticles

Figure 6. XPS spectra of Co (top) and Co/Mo (bottom) nanoparticlesderived from PS794-b-P2VP139. Insets are AFM height images (0.25 µmscan size, 10 nm height).

Figure 7. AFM height images of iron nanoparticles produced from various PS-b-P2VP polymer templates (1 × 1 µm scan size, 10 nm in height).2D Fourier transform spectrum and size and spacing of nanoparticles are displayed next to each image.


will pave the path to conduct a parametrized investigation ofthe effect of catalyst size and density on CNT growth.

Over the past two decades, extensive theoretical and experi-mental research on solution micelles has been fueled bytechnological promises in applications as surfactants and drugdelivery vesicles. The scaling relationship between the coronasize in solution and block lengths have been predicted by boththe scaling theory using the star copolymer model and self-consistent mean-field theory.42–44 An empirical model deducedfrom experimental results has also been established.15 To thebest of our knowledge, the correlation between the interparticleseparation and block lengths of the corresponding block polymertemplate has not yet been documented. The interparticle distancewas deduced from the 2D Fourier transfer spectrum of an AFMheight image. Assuming the micelles are spherical in shape,the interparticle distance, which is the center-to-center separa-tion, periodicity, minus the micelle height, can be obtained. Onthe basis of the experimental data from this study, a mathemati-cal fit yields a scaling relationship of interparticle separation, d∼2 × N PS

0.58 as depicted in Figure 8b with the exclusion of thedata from polymer No. 5. This scaling relationship indicatesthat PS chains are well-extended on a surface, behaving like ina good solvent where the polymer chain length is proportionalto N0.59. This result further indicates that the distance betweennanoparticles is solely controlled by the PS chain length, whenthe PS chain length is substantially longer than the P2VP chainlength. In this circumstance, the P2VP chain length has littleeffect on interparticle spacing. Therefore, by adjusting the degreeof polymerization of PS, the interparticle spacing can be tailoredaccordingly. In the case of polymer No. 5, where the numberof repeat units of P2VP is almost equivalent to that of PS, the

spacing is no longer solely dependent on the PS chain length.One can view a surface micelle as a hemisphere of a star-likepolymer molecule on a surface with brushes composed of PSchains and inner cores formed by P2VP chains as illustrated inFigure 8c. With increasing P2VP chain length, a larger portionof the PS chain is located at the interface, as predicted byZhulina.43 Therefore, as interface curvature increases as a resultof large PVP volume fraction, and the interparticle distancebecomes shorter. Therefore, nanoparticles produced from usingpolymer No. 5 as a template have narrower interparticle distance.According to this result, it can be deduced that when the ratioof NPVP and NPS is less than 0.35, then the interparticle spacingis solely controlled by the effective PS chain length in a goodsolvent.

C. Carbon Nanotubes Growth.C.1. Diameter and Density Controlled Growth. By varying

size and density of catalyst nanoparticles, CNT diameter anddensity can be adjusted. Figure 9a is an example showing thathighly ordered cobalt nanoparticles with an average diameterof 2.4 and 4.2 nm have been derived from PS475-b-P2VP141andPS794-b-P4VP139 respectively. Smaller and denser cobalt nano-particles produced from PS475-b-P2VP141 result in highly densedCNT mat. The summary of CNT diameter displayed in Figure9a is based on AFM height estimation of 30 CNTs. The rangeof CNTs from AFM height analysis is in good agreement withRaman spectroscopic analysis. This set of results points out thatsmaller catalyst nanoparticles produce smaller diameter CNTs.Using 2.4 nm iron nanoparticles, CNTs have an averagediameter of 1.1 nm. The average CNT diameter increases to1.7 nm when 4.2 nm nanoparticles are used. Figure 9b is a setof typical Raman spectra showing that these kinds of catalyst

Figure 8. Relationship of interparticle separation and chain lengths. (a) Summary of the interparticle spacing of iron nanoparticles prepared fromPS-b-P2VP polymer templates (NPS and NP2VP are the repeat units of PVP and PS blocks); (b) the number of PS repeat units as a function ofinterparticle spacing; (c) illustration to depict that larger portion of PS segment are located at the interface with increasing volume fraction of PVP.


nanoparticles are capable of producing small-diametered, defect-free CNTs. Therefore the block copolymer template approachprovides a rational method for adjusting the diameter and densityof CNTs by tailoring the block lengths. Electronic propertiesof a random CNT network are highly dependent on CNTdensity.45 Thus, adjusting catalyst nanoparticle spacing bymodifying block chain lengths offers a rational means to controlCNT density and, consequently, their electronic properties.

C.2. Growth Yield on Different Substrates and Implications.The role of catalyst-support on CNT growth has beeninvestigated. Iron nanoparticles derived from iron complexedPS475-b-P2VP141 was used in this investigation. The AFM heightimages in Figure 10 indicate that nanoparticle properties, interms of size and periodicity, are similar on all the substrates.However, the growth results differ. Smaller diameter and longerCNTs with low amorphous carbon content and low defect

Figure 9. (a) SEM images of CNT mats; Insets are AFM height images of cobalt nanocatalysts (1 × 1 µm scan size); the table summarizes cobaltnanoparticle size and corresponding CNT diameter. (b) Representative Raman spectra at the frequency regions where CNTs oscillate radially andcircumferentially.

Figure 10. AFM height images of iron nanoparticles derived from PS475-b-P2VP141 (scan size: 1 × 1 µm, scan height: 10 nm) and inserts are thecorresponding SEM images of CNTs (bar is 2 µm), XPS elemental analysis is summarized in the table.


density have been produced on a silicon oxide surface, whereaslow-density, shorter CNTs have been generated on a Mo surface,and no growth was observed by AFM and Raman analysis usingthe identical nanoparticles on a silicon nitride support. XPSanalysis indicated that Fe(III) nanoparticles on the silicon nitridesupport failed to reduce from Fe(III) to Fe(0). Less Fe presenceon Mo and silicon nitride surfaces after CNT growth impliesthat either the adhesion of nanoparticles on these surfaces mightbe inadequate, or Fe diffused into the supporting materials atelevated temperature. This result indicates that the surface, onwhich nanocatalysts reside, exerts great influence on CNTgrowth results. The ability of forming catalyst nanoparticles ondifferent surfaces using this block copolymer approach opensthe door for a systematic investigation of the role of a supportingmaterial on CNT growth.

Conclusion

Comparing PS-b-P4VP with PS-b-P2VP, PS-b-P4VP can beself-assembled in toluene but PS-b-P2VP cannot. However,metal attachment renders the P2VP more hydrophilic, leadingto self-assemble in toluene. The PS-b-P2VP template is capableof producing a variety of catalyst nanoparticles with a higherdegree of order and narrower size distribution. This is becauseof the steric hindrance effect. Metal species, Lewis acids, cannotcoordinate with more than one pyridine ligand in the case ofP2VP, thus preventing intermolecular cross-linking, which canbe detrimental to the self-assembly process.

The potential to synthesize bimetallic nanoparticles withtunable composition by the block copolymer micelle approachhas been demonstrated. It has been confirmed that surfacemicelle size and spacing and resultant catalyst nanoparticle sizeand interparticle distance are dictated by block lengths for agiven block copolymer system. For PS-b-P2VP, when the ratioof NPVP to NPS is less than 0.35, the interparticle spacing is solelycontrolled by the PS chain length. CNT diameter and densitycan be rationally tailored by adjusting catalyst nanoparticle sizeand distance through the control of block lengths.

The ability to tailor the catalyst properties (size, spacing, andcomposition) provides the pathway to conduct a comprehensivestudy of CNT growth. For example, the effect of catalyst-supporthas been investigated using identical catalyst nanoparticles ondifferent supporting materials, revealing the vital role of thesupport material. Highly engineered nanocatalysts with adjust-able properties will enable the fundamental study of the CNTgrowth mechanisms and facilitate controllable synthesis of CNTswith desirable properties, consequently promoting the realizationof their highly touted properties.

Acknowledgment. The author would like to thank Jie Liu’sgroup at the Chemistry department at Duke University forcarbon nanotube synthesis and Grant Girolami and DanielleChamberlin at Agilent Technologies for scanning electronmicroscope inspection and Raman analysis.

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