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Self-assembled monolayers

DOI: 10.1002/smll.200801115

Application of Block-Copolymer Supramolecu-lar Assembly for the Fabrication of ComplexTiO2 Nanostructures**

Sung Ho Kim, Oun-Ho Park, Fredrik Nederberg,

Teya Topuria, Lesile E. Krupp, Ho-Cheol Kim, Robert

M. Waymouth, and James L. Hedrick*

Recent progress in TiO2 nanomaterials combined with new

synthetic methodologies has attracted a considerable amount

of attention towards a wide range of TiO2 structures, including

nanoparticles, nanorods, nanotubes, nanowires, and mesopor-

ous and photonic structures.[1] Continuing breakthroughs in

the synthesis of TiO2 nanostructures have brought new

properties and birthed new applications with improved

performance, ranging from energy to environmental areas

such as photocatalysis, photovoltaics, photo-/electrochromics,

and sensors.[2] Numerous efforts directed towards the self-

assembly of amphiphilic block copolymers with inorganic

precursors have obtained improvements in production cost

and feature size, and the ability to pattern large-area

substrates. Several successful synthetic routes to TiO2

nanoparticles using block-copolymer templates have been

reported including polystyrene-b-poly(methylmethacrylate)

(PS-b-PMMA),[3] polystyrene-b-poly(ethylene oxide) (PS-b-

PEO),[4] polystyrene-b-poly(vinylpyridine) (PS-b-PVP),[5]

and poly(methylmethacrylate)-b-poly(ethyleneoxide) (PMMA-

b-PEO).[6] When these block copolymers are dissolved in a

selective solvent at or above their critical micelle concentra-

tion (CMC), they form well-ordered inverse micelles with the

TiO2 precursor residing in the block that comprises the core of

micelles while the corona is titania-free. Subsequent hydrolysis/

condensation of inorganic precursors and removal of block

copolymers via thermolysis forms highly ordered arrays of

TiO2 nanodots. Reports of TiO2 nanostructures have largely

been limited to inverse micellar morphologies including

nanoparticles, nanowires, and porous linear network struc-

[�] Dr. J. L. Hedrick, Dr. S. H. Kim, Dr. O.-H. Park, Dr. F. Nederberg, Dr. T.

Topuria, L. E. Krupp, Dr. H.-C. Kim

IBM Almaden Research Center

650 Harry Road, San Jose, CA 95120 (USA)

E-mail: Hedrick@almaden.ibm.com

Prof. R. M. Waymouth

Department of Chemistry, Stanford University

Stanford, CA 94305 (USA)

[��] The authors acknowledge support from the NSF Center for PolymerInterfaces and Macromolecular Assemblies (CPIMA: NSF-DMR-0213618). F.N. thanks the Swedish research council (VR) forfinancial support. We thank Bill Legg for TEM specimen prep-aration.

: Supporting Information is available on the WWW under http://www.small-journal.com or from the author.

� 2008 Wiley-VCH Verlag GmbH & Co

tures, despite the considerable work that has been done in the

self-assembly of amphiphilic block copolymers in selective

solvents with supramolecular structures including spherical

micelles, worm micelles, vesicles, multicompartment micelles,

toroids, and helices.[7] Hollow micellar TiO2 nanostructures

prepared from supramolecular assemblies, such as spheres or

nanovesicles, and templated from block copolymers where the

corona of micelles is miscible with the TiO2 precursor have

rarely been reported, even though they have the potential to

significantly expand the range of accessible morphologies.

Exploiting the micellar core/shell morphological systems

allows not only selective encapsulation of chemical species

into micellar TiO2 shells but also increases the functional-

inorganic-nanostructure surface area available for interfacial

processes compared to solid nanoparticles.

Compared to well-studied examples of inorganic hybrid

nanomaterials, such as silica,[8] the difficulty in the control of

TiO2 nanostructures seems to be attributed to the lack of

interactions between block-copolymer templates and the

inorganic precursor. An additional challenge is the high

reactivity and sensitivity of typical TiO2 precursors, such as

titanium alkoxides or chlorides, to moisture. Recently, we

reported that amphiphilic poly(N,N-dimethylacrylamide-

block-lactide) copolymer (PDMA-b-PLA) could be success-

fully used for preparation of porous silicate nanostructures

from coassembly with a cross-linkable organosilicate pre-

cursor.[9] The use of the PDMA block stems from its polarity

and hydrogen-bonding capability, which promotes strong

interactions between the block copolymers and thermosetting

inorganic precursor. The PLA block with different solubility

compared to the PDMA block forms a core of micelles and

ultimately generates nanopores upon thermolysis of copoly-

mers. Here, we extend the use of PDMA-b-PLA block

copolymer as a structure-directing template for the fabrication

of TiO2 nanomaterials. As another crucial complementary

method, we use chemically modified titanium alkoxide

precursor followed by oligomerization of the precursor as

an effective way to prevent hydrolysis in the reaction.[10]

Figure 1a illustrates the general procedure used to produce

nanostructured TiO2 thin films and evaluate their resulting

morphologies. PDMA-b-PLA block copolymer and an

oligomeric TiO2 precursor (OT) of titanium acetylacetonate

were dissolved in propylene glycol propyl ether (PGPE), a

selective solvent for PDMA block. Then, TiO2 thin films were

prepared by spin casting the solution on silicon wafer followed

by TiO2 crystallization and polymer thermolysis. The

morphologies of the resulting TiO2 thin films were character-

ized with tapping-mode atomic force microscopy (AFM) and

transmission electron microscopy (TEM). Height and phase

AFM images of the TiO2 nanostructure prepared from a

0.1wt% solution of PDMA-rac-PLA/OT (60/40) after thermal

treatment at 450 8C for 4 h show the formation of toroidal

nanostructures with outer diameters of �90 nm and internal

voids of�30 nm (Figure 1b and c). The significant difference in

hardness of the protruded ring compared with the other areas

(including the inside and the outside of the toroids) suggests

the formation of micellar TiO2 nanostructures selectively

sequestered into the polar corona of block copolymers.

Analysis with TEM was performed to confirm the morpho-

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Figure 1. Micelle-templated TiO2 nanostructures. a) The nanostructures are prepared from

self-assembly of amphiphilic block copolymers and oligomeric titanate precursor (at 0.1wt%,

PDMA-rac-PLA/OT¼ 60/40 (w/w)) followed by block-copolymer thermolysis. b) Height and

c) phase-contrast AFM images of the resulting nanostructure. d) Selected-area electron-

diffraction (SAED) pattern of TiO2 formed from oligomeric TiO2 precursor (OT) after thermal

treatment, and TEM images at e) low and f) high magnification of the nanostructures show the

formation of anatase crystalline TiO2 toroids nanostructures.

logical structure and identity of the resulting TiO2. Selected-

area electron-diffraction (SAED) analysis for the TiO2

nanostructure formed from this OT precursor after thermal

treatment at 450 8C for 4h shows the formation of an anatase

crystalline structure, as indicated by the ring pattern in the

inset of Figure 1d.[10a] Successful formation of hollow TiO2

micelles is also observed from the TEM image in Figure 1e, in

which similarly sized spheres are surrounded with electron-

dense dark spots indicative of TiO2. The high-resolution image

in Figure 1f shows the formation of TiO2 nanocrystallites with

sizes of �5–10mm.

Nanoscale patterning with block copolymers offers a

simple strategy to tune and optimize the nanostructures by

controlling the self-assembly processes of the block copoly-

mers. First, we added some dichloroethane (DCE), a good

solvent for both blocks, into the deposition solution,

considering the fact that the shape and size of block-copolymer

micelles are typically influenced by changing easily accessible

experimental variables such as solvent quality or deposition

conditions. Figure 2 shows that the DCE addition reduces the

size of individual micelles, increases the number of micelles,

and eventually transforms isolated TiO2 toroids into con-

tinuous nanoporous thin films without loss of micellar

structure, suggesting an interesting connection of toroids with

nanoporous-thin-film morphology. Similar morphological

change with an increasing amount of DCE is also observed

small 2008, 4, No. 12, 2162–2165 � 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

in those micelles prepared from stereo-

structure-controlled block-copolymermix-

tures, although the distinctive physical and

chemical stabilities of the stereoisomer

mixtures accompanied the change of their

solubility in a specific solvent.

Next, the scope of possible morphol-

ogies of TiO2 toroidal micelles was further

tuned and expanded by using stereo-

controlled polylactide block copolymers.

Since Ikada et al. first reported a stereo-

complex formation from mixtures of L-

PLA and D-PLA in both the melt and

solution,[11a] numerous studies have been

performed on the formation of the stereo-

complex and its crystalline structure,

morphology, and physical structure.[11]

Recently, we showed from a study on

the effect of stereochemistry of core-

forming PLA block copolymers on tem-

plated inorganic SiO2 nanostructures that

this unique noncovalent interaction, dri-

ven by stereocomplexation from mixtures

of D- and L-lactides, could provide an

additional control over the hierarchical

assembly of supramolecular micelles as

characterized by distinctive vertical and/or

horizontal growth of toroidal SiO2 nanos-

tructures.[9a] Here, we investigate the

supramolecular morphologies from com-

binations of stereoisomer pairs; i) a

mixture of PDMA-PLA block copolymers

with a different lactide-block sterochem-

istry and ii) a mixture of an enantiomerically pure PDMA-

PLA block with a PLA homopolymer of the opposite

configuration. Stereo-regular PDMA-PLA block copolymers

(PDMA70-PLA150, Mn� 31 000–33 000 g mol�1, PDI¼ 1.06–

1.16) and PLA homopolymers (PLA150, Mn� 25,000 g mol�1,

PDI� 1.11–1.18) were synthesized by the ring-opening

polymerization (ROP) of lactide monomers (see the Support-

ing information). According to the stereosequence of poly-

lactides, the block copolymers and homopolymers are referred

to as the D-/L-/rac-block and D-/L-/rac-homo. The formation of

stereocomplexes from these stereomixtures was easily con-

firmed from differential scanning calorimetry (DSC), in which

the mixtures of L- and D-lactides prepared from our catalyst

showed a Tm of �210 8C, significantly higher than the

respective stereo-regular polymers (�150 8C; see Figure S1

in the Supporting information). Figure 3 shows the effect of

stereocomplexation of polylactides on the templated toroidal

TiO2 nanostructures. In the mixture of D-block and L-block,

the height of the TiO2 toroids increases (�2�) compared to

that of those from atactic block copolymer (rac-block;

Figure 3a–c). The stereocomplex core of the micelles formed

from the mixture of D-block and L-block induces a stacking of

smaller-unit toroidal micelles, either in the solution or during

the deposition process, while maintaining the size and shape of

core/shell micelles of block copolymer and titania precursor.

Under these preparation conditions, this leads to vertical

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communications

Figure 2. AFM images of TiO2 nanostructures prepared from 0.1wt % block copolymers/OT

(60/40) mixtures by changing the selectivity of solvent used: rac-block/OT solutions in a) a

pure PGPE and b) a mixed solvent (DCE/PGPE¼ 6/4 (w/w)), and (D-blockþ L-block)/OT in

mixed solvents with DCE/PGPE ratios of c) (8/2) and d) (9/1) (w/w). The scale bars are

500nm.

Figure 3. TiO2 toroids prepared from stereomixtures: top-view and side-view AFM images of

a,b) 0.1wt% of rac-block and c) (D-blockþ L-block) copolymers in the polymer/OT (60/40),

and D-block (0.1wt%)/L-homo (x wt%)/OT (2.0wt %) mixtures with x of d) 0.5wt%, e) 1.0wt%,

and f)2.0wt%.The fractions of DCE in themixed solvents were fixed to be 0.8 (w/w). The scale

bars are 500nm (a–c) and 1mm (d–f).

2164 www.small-journal.com � 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhe

growth of the resulting TiO2 nanostruc-

tures. When the stereoregular block copo-

lymer was mixed with the stereocomple-

mentary homopolymer, a significant

perturbation in the core/shell dynamics

was observed, due to selective swelling of

the PLA core owing to stereocomplexa-

tion, as shown in Figure 3d–f. The stereo-

complexation between D-block and L-homo

can produce different sizes of TiO2 toroids,

with diameters from 100 nm to several

hundred nanometers, simply by varying the

concentration and mixing ratio of L-homo

to D-block. In addition, it represents a new

method to encapsulate functional cargoes

in supramolecular assemblies through

modification with a stereocontrolled homo-

polymer and stereocomplexation with

block copolymer templates.

In summary, micellar TiO2 nanostruc-

tures with titania-free polymer block

comprising the core and TiO2-precursor-

containing block comprising the coronawere

successfully prepared from self-assembly of

amphiphilic block copolymer/oligomeric

titanate mixture followed by crystalline-

TiO2 formation and block-copolymer ther-

molysis. The resulting TiO2 nanostructures

were further tuned and expanded by i)

changes in the selectivity and solubility of

solvent used and ii) control of stereochem-

istry and stereocomplexation of polylac-

tides comprising the core of micelles. This

study demonstrates a simple method to

prepare well-defined and size-tunable

nanoporous TiO2 nanostructures, which

have potential applications photocatalysis,

photovoltaics, and material encapsulation.

Experimental Section

Materials: PLA block copolymers and

homopolymers were synthesized by the ring-

opening polymerization of lactide monomers

in a glove box using thiourea and sparteine

catalysts designed for bifunctional activation

of both monomer and alcohol through hydro-

gen bonding (see Scheme S1 and Table S1 in

the Supporting information).[9a] The PDMA70-

PLA150 block copolymers with degree of

polymerization (DP) of 70 for PDMA and 150

for PLA were prepared from a dual-headed

initiator containing an alkoxyamine and a

primary hydroxyl group through nitroxide-

mediated polymerization of dimethylacryla-

mide and subsequent ring-opening of lactide

monomers. The PLA150 homopolymer was

prepared using benzyl alcohol as an initiator.

The OT precursor used in this study was

im small 2008, 4, No. 12, 2162–2165

prepared from sol-gel reaction of titanium acetylacetonate (Tyzer

AA-75, DuPont) with 2M aqueous HCl solution (2eq), and diluted

with propylene glycol propyl ether (PGPE).[10a] The details of the

preparation of polymers and inorganic precursor can be found in

the Supporting information.

Nanostructured TiO2 Thin-Films: The block copolymer (and/or

homopolymer) and the OT were dissolved in pure PGPE or

dichloroethane (DCE)/PGPE mixed solvent. Thin films of hybrids

containing polymer and OT were prepared by spin casting the

solution on untreated silicon wafers at 3000 rpm for 30 s. Then, the

spin-coated wafers were heated to 450 -C at a rate of 10 -CminS1 for

the formation of the inorganic network, and then held at 450 -C for

4 h under ambient atmosphere to degrade the copolymer template

and produce TiO2 thin films. The surface morphologies of the

resulting thin films were investigated with tapping-mode AFM using

a standard silicon cantilever. HRTEM images were obtained using a

JEOL 2010F operated at 200kV. The specimen was plan-view

prepared by conventional mechanical polishing and ion milling and

C-coated (1 nm from one side) to prevent charging.

Keywords:TiO2

. nanostructures . self-assembly . stereocomplexes .polylactides

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Received: July 31, 2008Published online: October 30, 2008

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