CdS Nanoparticles in Block Copolymer


Oct 11, 2007 - Location Control of Au/CdS Nanoparticles in Block Copolymer Micelles ... Containing Coumarin Units and a Poly(ethylene oxide) Short Cha...
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Langmuir 2007, 23, 11425-11429

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Location Control of Au/CdS Nanoparticles in Block Copolymer Micelles Haeng-Deog Koh, Nam-Goo Kang, and Jae-Suk Lee* Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea ReceiVed August 6, 2007. In Final Form: September 20, 2007 Micellar core-embedded Au or CdS nanoparticles (NPs), in which the number of NPs was controlled by a solid type or a solution type of metallic precursors and by their amounts, were constructed using a block copolymer as a template. The location of NPs located at the micellar core was dramatically changed to the corona by the solventinduced micellar core-corona inversion. By mixing the synthetic methods demonstrated, harmonious Au/CdS NPs with different particle sizes, numbers, and positions in the micellar core were also prepared.

Introduction In the past few decades, a variety of approaches for synthesizing metallic or semiconductor nanoparticles (NPs) using unique block copolymer micelles as nanoreactors have been attempted as a means of modifying the unique functions of optical, magnetic, electronic, and catalytic properties.1 With the mounting demand for these nanomaterials, a simple method for preparing functional metallic-block-copolymer nanocomposites is also required. Specifically, to enhance the physical properties of NPs, selective loading is an essential condition, in which the number of NPs is controlled in the desirable domains of the micellar core or the corona. Also required is the well-defined control of the size and shape of NPs, because physical and chemical properties are mainly determined by how metallic NPs are dispersed in the designed polymer structures.2 Recently, there has been an increase in the number of studies pertaining to the functional organization of two kinds of NPs self-assembled by organic molecule intermediates with functional groups such as -SH and -NH2.3 When two kinds of NP systems can be constructed in harmony, there is potential for a number of advanced applications such as for photoelectrochemical cells,4 electrochromic devices,5 light-emitting diodes,6 and sensors7 to

be brought into realization. Keeping pace with recent trends, the effort to produce a desirable self-assembly of two types of metallic NPs using a block copolymer micellar template was first made by Sohn et al., although the demonstrated nanocomposite consisted of only one NP in the micellar core and the other NPs decorating the periphery of each micellar shell.8 Although simple in design, this methodology suggested a good example of how different types of functional metal NPs can be reorganized on nanostructured block copolymer templates into specific arrangements by using a self-assembling procedure. We describe an in situ synthetic method for constructing various metallic/semiconductor-block-copolymer nanocomposites. Here, one big micellar-core-embedded Au NP or dozens of smaller Au NPs are prepared by changing the condition of the metallic precursor. In addition, the formation of structurally inverse coronaembedded NPs is also demonstrated using solvent tuning. Going one step beyond simple NP formation, a novel preparation method for Au/CdS NPs with well-defined harmonious location control enables the formation of one big NP surrounded by several other smaller NPs in the micellar core; the first is demonstrated by an in situ reaction. The selective loading of two types of NPs with well-defined control is incommensurably remarkable. Experimental Section

* Corresponding author. Address: Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryongdong, Buk-gu, Gwangju 500-712, Korea. Tel: +82 62 970 2306. Fax: +82 62 970 2304. E-mail: [email protected] (1) (a) Fo¨rster, S.; Antonietti, M. AdV. Mater. 1998, 10, 195-217. (b) Mo¨ssmer, S.; Spatz, J. P.; Mo¨ller, M. Macromolecules 2000, 33, 4791-4798. (c) Lui, X.; Go¨ring, P.; Pippel, E.; Steinhart, M.; Kim, D.-H.; Knoll, W. Macromol. Rapid Commun. 2005, 26, 1173-1178. (d) Li, C.-P.; Wei, K.-H.; Huang, T. Y. Angew. Chem., Int. Ed. 2006, 45, 1449-1453. (e) Wang, D.; Cao, Y.; Zhang, X.; Liu, Z.; Qian, X.; Ai, X.; Liu, F.; Wang, D.; Bai, Y.; Li, T.; Tang, X. Chem. Mater. 1999, 11, 392-398. (f) Qi, L.; Co¨lfen, H.; Antonietti, M.; Nano Lett. 2001, 1, 61-65. (g) Lee, Y.-K.; Hong, S.-M.; Kim, J.-S.; Im, J.-H.; Min, H.-S.; Subramanyam, E.; Huh, K.-M.; Park, S.-W. Macromol. Res. 2007, 15, 330-336. (2) (a) Zhao, H.; Douglas, E. P.; Harrison, B. S.; Schanze, K. S. Langmuir 2001, 17, 8428-8433. (b) Zhao, H.; Douglas, E. P. Chem. Mater. 2002, 14, 1418-1423. (c) Kang, Y.-J.; Taton, T. A. Angew. Chem., Int. Ed. 2005, 44, 409-412. (d) Kim, B.-S.; Qiu, J.-M.; Wang, J.-P.; Taton, T. A. Nano Lett. 2005, 5, 1987-1991. (e) Sohn, B.-H.; Yoo, S.-I.; Soo, B.-W.; Yun, S.-H.; Park, S.-M. J. Am. Chem. Soc. 2001, 123, 12734-12735. (3) (a) Sheeney-Haj-Ichia, L.; Pogorelova, S.; Gofer, Y.; Willner, I. AdV. Funct. Mater. 2004, 14, 416-424. (b) Fujihara, H.; Nakai, H. Langmuir 2001, 17, 63936395. (c) Sudeep, P. K.; Ipe, B. I.; Thomas, K. G.; George, M. V. Nano Lett. 2002, 2, 29-35. (4) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P. Nature 1997, 389, 699-701. (5) Bechinger, C.; Ferrer, S.; Zaban, A.; Sprague, J.; Gregg, B. A. Nature 1996, 383, 608-610. (6) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S. H.; Banin, U. Science 2002, 295, 1506-1508.

Metallic (or Semiconductor) Precursors and Reductants. Hydrogen tetrachloroaurate (III) (HAuCl4, 99%) and cadmium acetate (Cd(OAC)2, 99%) purchased from Aldrich were used without any treatments. Monohydrated hydrazine (N2H4‚H2O, 80%) and sodium sulfide (Na2S, 99.99%) were purchased from TCI and Aldrich. The different metallic precursors of HAuCl4 solid salt (P1), HAuCl4 dissolved in dilute HCl solution (30 wt %) (P2, pH ) 5), Cd(OAc)2 solid salt (P3), and Cd(OAc)2 dissolved in dilute HCl solution (30 wt %) (P4, pH ) 5) were used. N2H4‚H2O and Na2S (30 wt %) were dispersed in deionized (DI) water in order to be used as reductants. Preparation of Au/CdS NPs Using a Block Copolymer as a Template. To prepare a core-shell-type polymeric NP template,9 a polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer with a molecular weight of 72 kg mol-1, a polydispersity index [the weight-average molecular weight (Mw) divided by the number-average molecular weight (Mn), Mw/Mn] of 1.06, and a P2VP mole fraction of 30% was prepared using living anionic polymerization.10 PS-b-P2VP was dispersed in toluene at 5 mg/mL, followed (7) Bruchez, M.; Moronne, M.; Giu, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (8) Sohn, B.-H.; Choi, J.-M.; Yoo, S.-I.; Yun, S.-H.; Zin, W.-C.; Jin, J.-C.; Kanehara, M.; Hirata, T.; Teranish, T. J. Am. Chem. Soc. 2003, 125, 6368-6369. (9) Cho, Y.-H.; Cho, G.-J.; Lee, J.-S. AdV. Mater. 2004, 16, 1814-1817.

10.1021/la702385q CCC: $37.00 © 2007 American Chemical Society Published on Web 10/11/2007

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Figure 1. (a) TEM image of PS-b-P2VP NPs dispersed in toluene (5.0 mg/mL). The specimen was prepared by dropping the PS-b-P2VP solution onto a carbon-coated copper grid followed by staining the P2VP cores with I2. (b) Magnified TEM image showing a hexagonal array of PS-b-P2VP NPs with a particle size of 35 ( 3 nm after drying toluene. (c) Particle size distribution with 〈Dh〉 ) 40 nm measured by DLS in toluene.

by 24 h of stirring to obtain micellar structures with a P2VP core and a PS shell. For preparing Au and CdS NPs in the block copolymer micellar core, 0.1-1.0 equiv of HAuCl4 and Cd(OAc)2 from the P1-P4 metallic precursors per pyridine unit were dispersed into the micellar solution and stirred for 24 h at room temperature. The HAuCl4-coordinated micellar solutions were reduced by the prepared H2O‚N2H4 solution (30 wt %) in DI water. The Cd(OAc)2-coordinated micellar solutions were reduced by the Na2S solution (30 wt %). HAuCl4 and Cd(OAc)2 were reduced by the equivalent molar ratio of N2H4‚H2O and Na2S as reducing agents, respectively. No removal of excess reducing agents was required after reduction. The resultant solution with 5 mg/mL was diluted to 2.5 mg/mL with toluene for the morphological study of core-embedded NPs. In order to induce the inversion of the micellar core-corona, the resultant solution was also diluted to 1.0 mg/mL with methanol. Next, harmonious Au and CdS NPs with different particle sizes positioned were synthesized at the micellar core by a two-step reaction. First, 0.5 equiv of HAuCl4 or Cd(OAc)2 from P1 or P3 (solid salt type) was dispersed into the micellar solution and stirred for 24 h. After reduction by H2O‚N2H4 or Na2S in DI water (30 wt %), 0.5 equiv of HAuCl4 or Cd(OAc)2 from P2 or P4 (solution type) was again dispersed into the resultant solutions and stirred for 24 h. The final solutions were stirred for 12 h to complete the reduction and centrifuged for 5 min at 3000 rpm to remove the salts. Characterization. For a morphological study, the samples were drop-coated on carbon-coated copper grids for an energy-filtering transmission electron microscope [EF-TEM, EM 912 OMEGA (ZEISS, S-4700)] study. After the specimens were prepared by dropping the solutions onto carbon-coated copper grids and drying solvent and subsequently stained with I2, transmission electron microscopy (TEM) was used to analyze the specimens. Elemental analysis was performed by an energy-dispersive X-ray (EDX) spectrometer attached to the EF-TEM. A particle size analysis was then performed using dynamic light scattering (DLS) at 25 °C (Malvern Instruments, PCS). A UV-spectrophotometer (CARY 1E) was used to verify the absorbance. (10) (a) Shin, Y.-D.; Han, S.-H.; Samal, S.; Lee, J.-S. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 607-615. (b) Ahn, J.-H.; Shin, Y.-D.; Nath, G. Y.; Park, S.-Y.; Rahman, M. S.; Samal, S.; Lee, J.-S. J. Am. Chem. Soc. 2005, 127, 41324133. (c) Hirao, A.; Tsunoda, Y.; Matsuo, A.; Sugiyama, K.; Watanabe, T. Macromol. Res. 2006, 14, 272-286. (d) Shin, Y.-D.; Kim, S.-Y.; Ahn, J.-H.; Lee, J.-S. Macromolecules 2001, 34, 2408-2410.

Results and Discussions When the PS-b-P2VP (5 mg/mL) is solvated in toluene, a selective solvent for a PS block, polymeric NPs with a P2VP core and a PS shell were formed. On the whole, the particle size verified by DLS in solution was larger (by approximately 5 nm) than that measured by TEM after drying toluene. Polymeric NPs with a diameter of 35 ( 3 nm and a relatively narrow size distribution could be observed in the TEM images shown in Figure 1a,b. The TEM specimen was prepared by dropping the PS-b-P2VP solution onto a carbon-coated copper grid and removing the toluene by heating to 40 °C for 12 h, followed by staining the P2VP core with I2. An average particle size 〈Dh〉 of 40 nm was confirmed by DLS (Figure 1c). Various nanostructures of Au and/or CdS NPs embedded in the block copolymer micelles, in which the size, the number, and the location of NPs were distinctly controlled, were prepared. Here, the adoption of solid salt or solution precursors (P1, P2, P3, and P4) resulted in a different size and number of NPs after reduction. There has been a gradual increase in the number of reports pertaining to the synthetic method of forming Au NPs using P1.11 On the basis of this methodology, a single coreembedded Au NP or a single CdS NP was prepared by coordinating 0.5 equiv of P1 and P3 per pyridine unit of a block copolymer followed by reduction. Figure 2a,b shows the TEM and UV-vis absorption spectra of the single core-embedded Au NP and the single CdS NP, respectively. Au and CdS NPs (one NP per micelle) with similarly narrow size distributions (10 ( 3 nm) are distinctly observed in Figure 2a,b, respectively. The approximate diameter of the Au micelle including the PS shell was estimated to be 40 ( 4 nm. The Au and CdS NPs synthesized in the micellar core were also characterized by UV-vis absorption spectra in room temperature as shown in Figure 2c. The maximum absorption wavelength (λmax), indicating the surface plasmon resonance of Au NPs, and the optical transition of the first (11) (a) Spatz, J. P.; Mo¨ssmer, S.; Hartmann, C.; Mo¨ller, M. Langmuir 2000, 16, 407-415. (b) Selvan, S. T.; Harakawa, T.; Nogami, M.; Mo¨ller, M. J. Phys. Chem. B 1999, 103, 7441-7448. (c) Dou, H. J.; Jiang, M.; Peng, H. S.; Chen, D. Y.; Hong, Y. Angew. Chem., Int. Ed. 2003, 42, 1516-1519. (d) Chen, D.; Jiang, M. Acc. Chem. Res. 2005, 38, 494-502.

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Figure 2. (a) TEM image of a single core-embedded Au NP. (b) TEM image of a single core-embedded CdS NP. Au and CdS NPs were synthesized using the solid-salt type P1 and P3 precursors. The TEM specimen was prepared by dropping the PS-b-P2VP solution onto a carbon-coated copper grid followed by staining the P2VP cores with I2. (c) UV-vis absorption spectra of Au and CdS NPs embedded into the P2VP micellar core. Au (dotted line, λmax ) 545 nm) and CdS (solid line, λmax ) 410 nm).

excitation state of CdS NPs were clearly shown at 545 and 410 nm, respectively. In contrast, the solution type of precursors (P2 and P4) were first used in our case. When P2 was used as a metallic precursor, dozens of smaller Au NPs with a particle size of 4 ( 1 nm embedded in the micellar core instead of one bigger Au NP (10 ( 3 nm), as resulted from P1, were synthesized by our methodology as shown in Figure 3a-c. When the HAuCl4 of P2 is coordinated with a pyridine unit of the P2VP domain, HCl and H2O moieties are also adsorbed into the micellar cores. This environment can induce micellar swelling by the penetration of HCl and H2O moieties and obstructs the further agglomeration of small Au NPs.11 Consequently, dozens of smaller coreembedded Au NPs were formed in the micellar core after reduction, as shown in Figure 3a-c. Interestingly, the position of Au NPs positioned at the micellar core (in toluene) was dramatically changed to the micellar corona when the polymeric concentration was diluted to 1 mg/mL with methanol (a selective solvent for a P2VP block). It resulted in the inversion of a P2VP core and a PS corona. Thus, Au NPs embedded in the P2VP block moved the micellar corona after the solvent tuning, as shown in Figure 3d-f. The number of smaller NPs positioned in the micellar core and the corona was well controlled by varying the amount of loaded metallic precursors in the synthetic process. Figure 3a-c shows the synthesized Au NPs obtained by loading 0.1, 0.3, and 0.5 equiv of HAuCl4 (P2) in the micellar solution, respectively, followed by reduction. The corresponding average numbers (Nave) of core-embedded Au NPs (4 ( 1 nm) were estimated to be 12, 20, and 53 by TEM.2d After core-corona inversion by solvent tuning, the Nave of Au NPs embedded in the corona was similarly estimated to be 8, 15, and 42 for 0.1, 0.3, and 0.5 equiv of HAuCl4, respectively. Figure 3g,h indicates the particle size distributions of the core-embedded Au NPs (0.5 equiv of HAuCl4 loaded) and the corona-embedded

NPs. Compared with the pure block copolymer NPs (〈Dh〉 ) 40 nm), increased particle sizes were observed in both cases. Specifically, an increased overall particle size was observed in the corona-embedded Au NPs (〈Dh〉 ) 70 nm) as compared to the core-embedded NPs (〈Dh〉 ) 56 nm) by DLS because the PS core could be swelled by adsorbing the toluene already exited after the core-corona inversion with a larger volume of methanol. On the basis of the methodology demonstrated, the number control of synthesized Au NPs by varying the amount of metallic precursor, and the location control of Au NPs between the micellar core and the corona were successfully achieved. The advanced novelty of our approach originates from mixing the synthetic methods of preparing various Au/CdS-blockcopolymer nanocomposites. After synthesizing a single coreembedded Au NP by coordinating 0.5 equiv of HAuCl4 of P1 (a solid salt) per pyridine unit of block copolymer dispersed in toluene followed by reduction, the coordination of 0.5 equiv of HAuCl4 of P2 (a solution precursor) with the rest of the pyridine units is again attempted. Here we can anticipate the formation of harmonious Au NPs with two different sizes: one big coreembedded Au NP surrounded by dozens of smaller Au NPs after a second reduction with H2O‚N2H4. Figure 4a,b shows the corresponding TEM images consistent with the Au-blockcopolymer nanocomposites we designed. A single, large coreembedded Au NP (11 ( 2 nm) and dozens of smaller Au NPs (4 ( 1 nm) distinctly coexisted in the micellar core following a first-step reaction with P1 and a second-step reaction with P2, sequentially. The total particle size of an Au-block-copolymer was estimated to be 85 ( 5 nm after drying the toluene, while 〈Dh〉 ) 89 nm was confirmed by DLS (Figure 4c) in the solution. The methodology for synthesizing one big core-embedded Au NP surrounded by dozens of smaller Au NPs was extended to the formation of novel nanocomposites comprising different kinds of harmonious metallic/semiconducting NPs in the micellar

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Figure 3. (a-c) TEM images of core-embedded Au NPs dispersed in toluene. (d-f) TEM images of corona-embedded Au NPs obtained by diluting the resultant solution with methanol. The dilution induced the inversion of a P2VP core and a PS corona. The different average number was controlled by varying the amount of the solid type of the Au precursor (P2) in the synthetic process: (a) 0.1 equiv of HAuCl4 per pyridine unit and (d) the corresponding result after the core-corona inversion; (b) 0.3 equiv and (e) after the core-corona inversion; (c) 0.3 equiv and (f) after the core-corona inversion. The average numbers of Au NPs (N) in corresponding histograms (insets of a-f) were averaged for each structure (over 50 counts).

Figure 4. (a) TEM image of one big core-embedded Au NP surrounded by dozens of smaller Au NPs synthesized by a two-step reaction with P1 (the Au solid salt) and P2 (the solution type of the Au precursor), in sequence. (b) Magnified TEM image showing the particle size of 85 ( 5 nm, including the P2VP core/PS shell and Au NPs after drying the toluene. (c) Particle size distribution with 〈Dh〉 ) 89 nm measured by DLS in solution. The TEM specimen was prepared by dropping the polymeric solutions onto carbon-coated copper grids followed by I2 staining.

core. When P1 and P4 are sequentially used as precursors, one big core-embedded Au NP surrounded by dozens of smaller CdS

NPs can be formed after the reduction process. On the other hand, one big inversely core-embedded CdS NP surrounded by

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Figure 5. (a) TEM image of one big core-embedded Au NP surrounded by smaller CdS NPs (Au-CdS) synthesized by a two-step reaction with P1 and P4, in sequence. (b) TEM image of a bigger core-embedded CdS NP surrounded by smaller Au NPs (CdS-Au) synthesized by a two-step reaction with P3 and P2, in sequence. The TEM specimens were prepared by dropping the polymeric solutions onto carboncoated copper grids followed by I2 staining. (c) UV-vis absorption spectra of Au-CdS and CdS-Au. (d) EDX spectra of CdS-Au measured by focusing on CdS and Au NPs marked by white solid circles and white dotted circles, respectively, in panel b. The black dotted circles in panels a and b are the diameters of the total particles, including the P2VP core/PS shell and NPs.

dozens of smaller Au NPs can be also synthesized using P3 and P2 as precursors in sequence. Figure 5a,b shows the harmoniously location-controlled Au/CdS NPs. One big P2VP-core-embedded Au NP surrounded by dozens of small CdS NPs (Au-CdS) and, inversely, one bigger CdS surrounded by several small Au NPs (CdS-Au) were clearly observed, respectively. Bigger Au and CdS NPs positioned at the ultimate core of micelles were similarly estimated to be 11 ( 3 nm, and smaller Au and CdS NPs surrounding bigger NPs were continuously estimated to be 4 ( 1 nm. The harmonious core-embedded Au/CdS NP architectures (Au-CdS or CdS-Au) are then characterized by UV-vis absorption and an elemental analysis using an EDX spectrometer attached to a TEM. As shown in Figure 5c, the diagnostic peaks for CdS and Au NPs were clearly verified by λmax ) 350-450 nm and λmax ) 500-550 nm, respectively. In detail, the absorbance for several smaller CdS NPs (4 ( 1 nm) in Au-CdS was observed at λmax ) 400 nm, whereas the absorbance for the one big CdS NP (11 ( 3 nm) in CdS-Au was shown at the red-shifted λmax ) 407 nm because of the increased particle size.2a In contrast, the absorbance of the one big Au NP in AuCdS was observed at λmax ) 547 nm, whereas dozens of smaller Au NPs (4 ( 1 nm) in CdS-Au were shown in the shorter λmax ) 510 nm.2c The EDX spectrum of CdS-Au, as shown in Figure 5d, obviously confirms the coexistence of CdS (S(K): 2.3 keV; Cd(L): 3.2 and 3.4 keV)12 and Au(L) (9.7 and 11.4 keV).8 Similar EDX results were observed in Au-CdS. Harmoniously location(12) (a) Zhou, Y.; Ji, Q.; Masuda, M.; Kamiya, S.; Shimizu, T. Chem. Mater. 2006, 18, 403-406. (b) Mandal, S.; Rautaray, D.; Sanyal, A.; Sastry, M. J. Phys. Chem. B 2004, 108, 7126-7131.

controlled Au/CdS NPs in the micellar core will be further studied for photoelectrochemical cells or censors.3a

Conclusions One big micellar core-embedded Au with a particle size of 10 ( 3 nm and dozens of smaller Au NPs with 4 ( 1 nm particle sizes were prepared using a block copolymer template. The different sizes and numbers of NPs formed in the micellar cores were controlled by different types of Au precursors, either a solid salt type or a solution type. Diluting the solutions of the core-embedded NPs inversely provided us with corona-embedded NPs capable of maintaining a stable compatibility between NPs and block copolymer micelles. In all cases, the Au NPs were distinctly positioned at the selective domains we designed. Furthermore, a harmonious Au/CdS NP system, in which the size, the number, and the location of Au and CdS were respectively controlled, was also prepared in the micellar cores by using a two-step reaction. The in situ methodology demonstrates that the location control of NPs in block copolymer micelles is sincerely meaningful and extends the application scope of NPs into areas such as optical, electronic, catalytic, and photoelectrochemical devices or sensors. Acknowledgment. This work was supported by the Korean Science and Engineering Foundation (R01-2004-000-10143-0) and the Program for Integrated Molecular Systems, GIST. We thank the Korea Basic Science Institute (KBSI) for EF-TEM analysis. LA702385Q