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One-Pot Preparation of Titania-Nanoparticles through Hairy and Crew-Cut Type Micelle Templates Jinkee Hong† and Sang Wook Kang*,‡ School of Chemical & Biological Engineering, Seoul National UniVersity, Seoul 151-744, Republic of Korea, and Department of Chemistry, Sangmyung UniVersity, Seoul 110-743, Republic of Korea
A facile approach successfully produced block copolymer micelle templated titanium oxide nanoparticles via electrostatic interaction based adsorption between cationic polystyrene-block-poly(4-vinylpyridine) (PS-bP4VP) and negatively charged titania precursor, titanium(IV) bis(ammonium lactate)dihydroxide (TALH) complexes in aqueous solution. Both hairy and crew-cut types of block copolymer micelles were examined as a particle scaffold. The diameter of the titiania nanoparticles prepared were from 10 to 70 nm depending on the molecular weight or type of block copolymer micelle template. Also, mass production of titania nanoparticles on the Si-wafer was shown by a one-pot process. The size morphologies and distributions shown were confirmed by field emission scanning electron microscopy (FESEM). The results suggest that the versatile and environmentally friendly technique to manufacture titania nanoparticles, which could be easily widened to prepare electrostatic interaction based organic/inorganic complexes with further applications. 1. Introduction
2. Experimental Section
Titanium dioxide (TiO2) nanoparticles have attracted tremendous attention in the past decade because of their wide application potentials from energy to environmental issues such as photovoltaics, photocatalysis, and sensors. A variety challenges of chemical physical methods have been employed to prepare titania nanoparticles involved polymer template synthesis, sol-gel processing, self-assembly processes, and emulsion separation techniques.1-3 Among the several efforts, block copolymer template approaches have advantages including nanosized control, narrow size distribution, and wide further applications.4-7 Recently, J. L. Hedrick and S. H. Kim reported on the fabrication of complex titania nanostructures by a PDMAb-PLA block copolymer template.8 Also W. Knoll and X. Li suggested a highly dense array of titania nanoparticles using a reverse PS-b-P2VP micelle as a reaction scaffold.9
2.1. Materials. PS(Mw)18.6K)-b-P4VP(Mw)55.9K)(PS18.6K-b-P4VP55.9K), PS57.5K-b-P4VP18.6K, PS124K-b-PAA42K, PS480K-b-PAA145K block copolymers were purchased from Polymer Source, titanium(IV) bis(ammonium lactate)dihydroxide (TALH) (50 wt % in aqueous solution) was purchased from Sigma-Aldrich and used as received. The water used in all experiments was prepared in Milipore Milli-Q system and had a resistivity higher than 18.2 MΩ cm. 2.2. Preparation of Titanium Dioxide Nanoparticles. Preparation of block copolymer micelles were followed our previous papers.11,12 For the preparation of protonated PS-b-P4VP micelles in water, 100 mg of PS-b-P4VP block copolymer was first dissolved in 4 mL of N,N-dimethylformamide (DMF) and 96 mL of water (pH 2.0) was then added, resulting in spherical micelles composed of a hydrophobic PS core and a protonated P4VP corona shell. After forming PS-b-P4VP micelles at pH 2, the pH of PS-b-P4VP micelle solutions was adjusted to the desired solution pH using 1 M HCl. The 5 mL of positively charged PS-b-P4VP micelle solutions (1 mg · mL-1) were adjusted to pH 2, and 10 wt % negatively charged TALH aqueous solution was mixed dropwise to prepare PS-b-P4VP/ TALH complexes with stirring. After stirring for 3 h, the resulting solution was subjected to dialysis against Millipore water for over 24 h (Spectra/Por 4 Regenerated Cellulose Membrane, MWCO ) 12-14K) to remove any residual titania precursor. For the low-number density titania nanoparticles, the solution was spin-coated (1500 rpm) to the negatively charged silicon wafer and the samples for high-number density titania nanoparticles, and the solution was simply dropped onto the Si-wafer substrate. The substrates were pretreated by heating at 70 °C for 20 min in a 5:1:1 vol % mixture of water, hydrogen peroxide, and 29% ammonia solution. The PS-b-P4VP/TALH complexes on the silicon wafers were slowly heated to 450 °C and held at that temperature for 10 h to convert TALH into crystalline titanium dioxide nanoparticles.13-14 2.3. Characterization Equipments. The ζ-potentials of PSb-P4VP micelles were measured by an electrophoretic light scattering spectrophotometer (ELS-8000). The surface mor-
In this article, we suggest a very simple and environmentally friendly mass production approach to prepare titania nanoparticles in aqueous solution with use of a positive charged PSb-P4VP block copolymer micelle as a morphological template. Negatively charged TALH is a well-known titania precursor which can be adsorbed to positive polyelectrolytes.10 The titania nanoparticles were synthesized by two very simple steps: (1) preparation of polystyrene-block-poly(4-vinylpyridine) (PS-bP4VP) block copolymer micelles and titanium(IV) bis(ammonium lactate) dihydroxide (TALH) complexes based on electrostatic interaction and (2) calcination. In comparison to other block copolymer template reports, the primary advantages of our suggestion in this article are PS-b-P4VP/TALH complex fabrication is conducted from aqueous solutions, which remove the use of toxic organic solvents and possible extension of our approach to every charged nanoobject can be used as a template with a complementary charged precursor for inorganic structures with various sizes and morphologies. * To whom correspondence should be addressed. Tel.: +82 2 2287 5362. E-mail:
[email protected]. † Seoul National University. ‡ Sangmyung University.
10.1021/ie1010838 2010 American Chemical Society Published on Web 09/07/2010
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Figure 2. SEM and TEM (inset) images of low number density titania nanoparticles, templated by (a) hairy PS18.6-b-P4VP55.9k and (b) crew-cut PS57.5k-b-P4VP18.6k micelles.
Figure 1. The schematic drawing of PS-b-P4VP, TALH, and the example of a (PS-b-P4VP/TALH) complex case is shown. Table 1. ζ-Potential Experiments of TALH, PS-b-P4VP Micelles, and PS-b-P4VP/TALH Complexes Produced by Different Molecular Weights raw materials
ζ-potential (mV)
micelle/titania precursor complexes
ζ-potential (mV)
TALH PS18.6K-b-P4VP55.9K PS57.5K-b-P4VP18.6K PS124K-b-P4VP42K PS480K-b-P4VP145K
-44.14 48.25 43.06 45.71 50.11
PS18.6K-b-P4VP55.9K/TALH PS57.5K-b-P4VP18.6K/TALH PS124K-b-P4VP42K/TALH PS480K-b-P4VP145K/TALH
8.32 3.18 8.49 8.71
Figure 3. SEM images of high number density titania nanoparticles which were templated by (a) hairy PS18.6-b-P4VP55.9k and (b) crew-cut PS57.5k-bP4VP18.6k micelles.
phologies of the titania nanoparticles were examined by fieldemission SEM (JEOL JSM-7401F). 3. Results and Discussion Figure 1 illustrates the concept used to manufacture PS-bP4VP/TALH complexes via electrostatic interaction. PS-b-P4VP block copolymer micelles were prepared in pH 2 aqueous solution for their high charge density on the P4VP corona region. The pyridine groups are highly protonated in acidic conditions, and the charge density of PS-b-P4VP micelles were easily controlled by changing the solution pH.11 The negatively charged titania precursor of TALH is added to the positively charged PS-b-P4VP micelles and allowed to adsorb onto the micelle P4VP corona part through electrostatic interactions. Removal of the residual TALH process was followed through dialysis of the PS-b-P4VP/TALH complexes. The PS-b-P4VP/ TALH complexes resulted in the preparation of titania nanoparticles through a heat treatment process. The adsorbed titania precursor TALH onto PS-b-P4VP micelles is well-known to hydrolyze and condense at enhanced temperatures to form crystalline titanium oxide nanoparticles in the PS-b-P4VP micelle template. The ζ-potential measurements of PS-b-P4VP/TALH complexes indicate the coverage of the negatively charged TALH molecule to the positively charged corona part of PS-b-P4VP. The ζ-potential measurements of block copolymer micelles, titania precursor, and the complexes are summarized in Table 1. The ζ-potential of TALH was measured as -44.14 mV, and the PS18.6K-b-P4VP55.9K, PS57.5K-b-P4VP18.6K, PS124K-b-P4VP42K, and PS480K-b-P4VP145K micelles were checked at +48.25, +43.06, +45.71, +50.11 mV, respectively. The ζ-potential measurement of complexes, PS18.6K-b-P4VP55.9K/TALH, PS57.5Kb-P4VP18.6K/TALH, PS124K-b-PAA42K/TALH, and PS480K-bPAA145K/TALH show a weak positive charge, measured at +8.32, +3.18, +8.49, and +8.71 mV, respectively. These results
Figure 4. SEM images of titania nanoparticles: (a, b) crew-cut PS124-bP4VP42k and (c, d) crew-cut PS480k-b-P4VP145k micelle template based particles.
represent the superiority charge densities of positively charged PS-b-P4VP to adsorb TALH enough, however the complexes still maintain their charge-charge repulsion to produce evenly and uniformly distributed titania nanoparticles without any aggregation of block copolymer micelles. Figure 2 shows the scanning electron microscopy (SEM) image of two different types of PS-b-P4VP micelles and TALH complex based titania nanoparticles. The amount of adsorbed TALH to PS-b-P4VP micelles strongly depends on the charge density which could be controlled by changing the pH of the micelle solution and molecular weight of the used block copolymer. In this paper, every used micelle solution pH was adjusted to pH 2, which is a well-known condition for the fully charged corona shell of the PS-b-P4VP micelle for a secure enough titania precursor. To control the size of the titania nanoparticle, we utilize hairy (the hydrophilic outer corona part is relatively longer than compared to the hydrophobic part) PS18.6k-b-P4VP55.9k and crew-cut (hydrophobic core region is relatively larger than the hydrophilic shell region) PS57.5k-b-
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Figure 5. Titania nanoparticle size distribution at different molecular weight templates (a) hairy PS18.6-b-P4VP55.9k, (b) crew-cut PS57.5k-b-P4VP18.6k, (c) crew-cut PS124-b-P4VP42k, and (d) crew-cut PS480k-b-P4VP145k micelle templates collected from SEM images.
P4VP18.6k micelles as a template of adsorbed TALH precursor. Parts a and b of Figure 2 depict the PS-b-P4VP/TALH complexes derived titania nanoparticles from both hairy PS18.6kb-P4VP55.9k and crew-cut PS57.5k-b-P4VP18.6k micelles which are used as a building scaffold. The samples were prepared by spin coating of the PS-b-P4VP/TALH complex aqueous solution onto the negatively charged treated silicon wafer and calcination. The average size of the prepared titania nanoparticles were 16.8 ( 3.7 and 26.6 ( 3.1 nm calculated through SEM images, respectively. The morphology of titania nanoparticles based on crew-cut micelles are shown as more spherical than hairy micelle based particles due to their original micelle morphology, which has a relatively large hydrophobic portion. In addition, the mass production of titania nanoparticles were also shown for further possible various applications. In this case, the samples were prepared by dropping of the PS-b-P4VP/TALH complex aqueous solution onto the substrate. Calcination of the dropped PS-b-P4VP/TALH complex aqueous solution resulted in the formation of a high density of titania nanoparticles. As presented in Figure 3a,b, the morphology of titania nanoparticles based on crew-cut micelles has a relative sphere shape compared to hairy micelles. The difference of the sphere morphological void derived porosity of the titania nanoparticle coated films strongly support the different morphology and curvature of the titania nanoparticles. It is also interesting to note that the size of the titania nanoparticle is related to the template size due to the possibility of increase adsorbed TALH amount. The experiments with bigger molecular weight of block copolymer micelles revealed our hypothesis shown in Figure 4. Parts a,b and c,d of Figure 4 show the titania nanoparticles from crew-cut PS124k-bP4VP42k micelles and crew-cut PS480k-b-P4VP145k micelles, respectively. The averaged size from PS124k-b-P4VP42k micelles and crew-cut PS480k-b-P4VP145k micelles were 32.4 ( 4.7 and 35.5 ( 2.9 nm. The size distributions of titania nanoparticles were obtained by measuring the digitized micrographs as shown in Figure 5. The diameters were derived from an average of at least 130 particles from SEM images which were prepared in low number density. In the case of hairy micelle based particles shows
uniform size distribution, however the crew-cut types of micelle based particles show relatively wide size distributions compared with hairy micelle template, giving the particle size range of about 10-25 nm for hairy PS18.6k-b-P4VP55.9k micelles, 15-37 nm for crew-cut PS57.5k-b-P4VP18.6k micelles, 17-70 nm for crew-cut PS124k-b-P4VP42k micelles, and 25-53 nm for crewcut PS480k-b-P4VP145k micelles. 4. Conclusions In conclusion, we have obtained a facile and simple method to fabricate the titania nanoparticles by a block copolymer micelle templating strategy. Nanosized titania particles can be prepared by the heat treatment of the titania precursor TALH adsorbed sacrificial block copolymer micelle template approach. The titania nanoparticles have well distributed diameters and controlled particle size through changing the ratio of the hydrophilic/hydrophobic part and the molecular weight of the micelle templates used. We believe this approach may provide a new possibility for the preparation of other nanocomplex based nanostructures by charged inorganic precursors and complementarily charged various nano-objects. Literature Cited (1) Caruso, F.; Shi, X.; Caruso R., A.; Susha, A. Hollow Titania Spheres from Layered Precursor Deposition on Sacrificial Colloidal Core Particles. AdV. Mater. 2001, 13, 740–744. (2) Wu, C.-I.; Huang, J.-W.; Wen, Y.-L.; Wen, S.-B.; Shen, Y.-H.; Yeh, M.-Y. Preparation of TiO2 Nanoparticles by Supercritical Carbon Dioxide. Mater. Lett. 2008, 62, 1923–1926. (3) Zhou, W.; Cao, Q.; Tang, S. Effects on the Size of Nano-TiO2 Powders Prepared with Sol-Emulsion-Gel Method. Powder Technol. 2006, 168, 32–36. (4) Cheng, Y.-J.; Muller-Buschbaum, P.; Gutmann, J. S. Ultrathin Anatase TiO2 Films with Stable Vesicle Morphology Templated by PMMAb-PEO. Small 2007, 3, 1379–1382. (5) Kluson, P.; Luskova, H.; Solcova, O.; Matejova, L.; Cajthaml, T. Lamellar Micelles-mediated Synthesis of Nanoscale Thick Sheets of Titania. Mater. Lett. 2007, 61, 2391–2394. (6) Yuwono, A. H.; Zhang, Y.; Wang, J.; Zhang, X. H.; Fan, H.; Ji, W. Diblock Copolymer Templated Nanohybrid Thin Films of Highly Ordered TiO2 Nanoparticle Arrays in PMMA Matrix. Chem. Mater. 2006, 18, 5876– 5889.
Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010 (7) Kim, D. H.; Sun, Z.; Russell, T. P.; Knoll, W.; Gutmann, J. S. Organic-Inorganic Nanohybridization by Block Copolymer Thin Films. AdV. Funct. Mater. 2005, 15, 1160–1164. (8) Kim, S. H.; Park, O.-H.; Nederberg, F.; Topuria, T.; Krupp, L. E.; Kim, H.-C.; Waymouth, R. M.; Hedrick, J. L. Application of BlockCopolymer Supramolecular Assembly for the Fabrication of Complex TiO2 Nanoclusters. Small 2008, 4, 2162–2165. (9) Li, X.; Lau, H. A.; Kim, D. H.; Knoll, W. High-Density Arrays of Titania Nanoparticles Using Monolayer Micellar Films of Diblock Copolymers as Template. Langmuir 2005, 21, 5212–5217. (10) Shi, X.; Cassagneau, T.; Caruso, F. Electrostatic Interactions between Polyelectrolytes and a Titania Precursor: Thin Film and Solution Studies. Langmuir 2002, 18, 904–910. (11) Cho, J.; Hong, J.; Char, K.; Caruso, F. Nanoporous Block Copolymer Micelle/Micelle Multilayer Films with Dual Optical Properties. J. Am. Chem. Soc. 2006, 128, 9935–9942.
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ReceiVed for reView May 12, 2010 ReVised manuscript receiVed August 11, 2010 Accepted August 20, 2010 IE1010838
