Langmuir 2006, 22, 8623-8626
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Mineralization of Monodispersed CdS Nanoparticles on Polyelectrolyte Superstructure Forming an Electroluminescent “Necklace-of-Beads” Vivek Maheshwari and Ravi F. Saraf* Department of Chemical Engineering, UniVersity of NebraskasLincoln, Lincoln, Nebraska 68588 ReceiVed May 7, 2006. In Final Form: August 10, 2006 We report a nonmicellar method to synthesize monodisperse semiconducting nanoparticles templated on polymer chains dissolved in solution at high yield. The monodispersity is achieved due to the beaded necklace morphology of the polyelectrolyte chains in solution where the beads are nanometer-scale nodules in the polymer chain. The resultant structure is a nanoparticles studded necklace where the particles are imbedded in the beads. Multiple cycles of synthesis on the polymer template yield nanoparticles of identical size, resulting in a nanocomposite with high particle fraction. The resultant nanocomposite has beaded-fibrilar morphology with imbedded nanoparticles and can be solution-casted to make electroluminescent thin film device.
Mineralization using macromolecules as a template has attracted great interest to synthesize nanoparticles and concomitantly assemble them to make well-formed micro/macro structures.1 Biological and synthetic macromolecules such as polynucleotides,2,3 proteins,4 block copolymers,5-7 polyelectrolytes,8-10 amphiphiles,11,12 and lipids13,14 have been shown as molecular scaffolds to mineralize and self-assemble nanoparticles into ordered lattices,15,16 wires,17-19 capsules,9 and helical ribbons.20 In particular, the attraction in using polyelectrolytes lies in their ease of processing and assembly into structures such as multilayer films,8 capsules,9 and wires21 and their biocom* To whom correspondence should be addressed. E-mail: rsaraf@ unlnotes.unl.edu. (1) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273 (5277), 892. (2) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382 (6592), 607. (3) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382 (6592), 609. (4) Shenton, W.; Pum, D.; Sleytr, U. B.; Mann, S. Nature 1997, 389 (6651), 585. (5) Qi, L. M.; Colfen, H.; Antonietti, M. Nano Lett. 2001, 1 (2), 61. (6) Colfen, H.; Antonietti, M. Langmuir 1998, 14, 4 (3), 582. (7) Zhao, H.; Douglas, E. P.; Harrison, B. S.; Schanze, K. S. Langmuir 2001, 17, 8428. (8) Tao, C.; Zheng, S.; Mohwald, H.; Li, J. Langmuir 2003, 19, 9039. (9) Shchukin, D. G.; Ustinovich, E.; Sukhorukov, G. B.; Mohwald, H.; Sviridov, D. AdV. Mater. 2005, 17 (4), 468. (10) Minko, S.; Kiriy, A.; Gorodyska, G.; Stamm, M. J. Am. Chem. Soc. 2002, 124 (34), 10192. (11) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294 (5547), 1684. (12) Zhang, M. F.; Drechsler, M.; Muller, A. H. E. Chem. Mater. 2004, 16 (3), 537. (13) Jung, J. H.; Ono, Y.; Hanabusa, K.; Shinkai, S. J. Am. Chem. Soc. 2000, 122 (20), 5008. (14) Rabatic, B. M.; Pralle, M. U.; Tew, G. N.; Stupp, S. I. Chem. Mater. 2003, 15 (6), 1249. (15) Braun, P. V.; Osenar, P.; Tohver, V.; Kennedy, S. B.; Stupp, S. I. J. Am. Chem. Soc. 1999, 121 (32), 7302. (16) Lee, S. W.; Mao, C. B.; Flynn, C. E.; Belcher, A. M. Science 2002, 296 (5569), 892. (17) Scheibel, T.; Parthasarathy, R.; Sawicki, G.; Lin, X. M.; Jaeger, H.; Lindquist, S. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100 (8), 4527. (18) Richter, J.; Mertig, M.; Pompe, W.; Monch, I.; Schackert, H. K. Appl. Phys. Lett. 2001, 78 (4), 536. (19) Yan, H.; Park, S. H.; Finkelstein, G.; Reif, J. H.; Labean, T. H. Science 2003, 301 (5641), 1882. (20) Sone, E. D.; Zubarev, E. R.; Stupp, S. I. Angew. Chem., Int. Ed. 2002, 41 (10), 1706. (21) Kiriy, A.; Minko, S.; Gorodyska, G.; Stamm, M. Nano Lett. 2002, 2 (8), 881.
patibility. The morphology of the polyelectrolyte template dictates the shape of both the nanoscale mineralized product and the mineralized template. In this report, we illustrate a macromoleculemediated nonmicellar mineralization process that results in synthesis of monodispersed nanoparticles. The template for mineralization is the “necklace-of-bead” morphology that polyelectrolytes spontaneously form under certain conditions in dilute solution, where the chains agglomerate to form microns-long nanofibrils composed of nodular nanosized beads sown together by polymer strands22-24 (see Figure 1a). In our process, the nanoparticles are mineralized and stabilized in these beads. Interestingly, the mineralization process can be repeated over three cycles by recharging the source compounds followed by precipitation in the beads without changing the polymer solution. The size of the particles after each precipitation process is identical. The final product is an electrically percolating nanofibrilar composite which is stable in solution and is electroluminescent when deposited on a solid substrate. The process is particularly robust as the size of the nanoparticles for a given polymer/solvent system is invariant over a broad range of, concentration of the source salt relative to the polymer, molecular weight of the polymer, and reaction temperature. The particle size can be changed by using water/organic mixtures as the solvent. At pH of 12.0, the polyelectrolyte chains do not form necklace-of-bead because they are neutral and highly charged, respectively; under these conditions, the mineralized CdS particles are in the micron range and highly polydispersed. Cd(ClO4)2‚H2O is added to a 0.1% solution of poly(styrene sulfonate) (PSS) of molecular weight 5 × 105 Da, at a molar ratio of 0.25 Cd2+ relative to -SO3-1, followed by stirring to form a clear solution. Because, Cd(ClO4)2 has very low solubility in water, its dissolution in the presence of PSS indicates strong interaction between Cd2+ and -SO3-1 groups on the PSS chains. CdS is formed by adding Na2S at a relative molar ratio of 0.7 to the solution of PSS-Cd in the dark and under a nitrogen atmosphere. The solution immediately turns light yellow indicat(22) Kiriy, A.; Gorodyska, G.; Minko, S.; Jaeger, W.; Stepanek, P.; Stamm, M. J. Am. Chem. Soc. 2002, 124 (45), 13454. (23) Kirwan, L. J.; Papastavrou, G.; Borkovec, M.; Behrens, S. H. Nano Lett. 2004, 4 (1), 149. (24) Dobrynin, A. V.; Rubinstein, M.; Obukhov, S. P. Macromolecules 1996, 29 (8), 2974.
10.1021/la061273w CCC: $33.50 © 2006 American Chemical Society Published on Web 09/09/2006
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Figure 1. AFM and TEM analysis of CdS mineralization on PSS at various stages, with schematics to aid the explanation of observed structure. (a) AFM image of pure PSS chains deposited from solution. (b) AFM image after adding Cd(ClO4)2 salt to PSS. (c) AFM image after addition of Na2S to form CdS nanoparticles. (d) TEM images of PSS-CdS solution deposited on a carbon-coated copper grid. All the main AFM images are 2 × 2 µm and the magnified inset as indicated. For the schematic, the “red” dots in panel b are Cd2+ and “gold” dots in panel c are CdS particles.
ing the formation of CdS nanoparticles. Further cycles of CdS synthesis are performed by repeating the procedure by addition of fresh Cd(ClO4)2‚H2O to the same solution. Figure 1a is an atomic force microscopy (AFM) phase image of pure PSS deposited on a silica chip by dipping in a 0.1% PSS solution for 15 min, followed by immediate washing and airdrying to kinetically capture the morphology of the polymer chains in the solution. The PSS chains form a necklace-of-beads, of typically 8-12 nm beads “stitched” together by polymer “tie” segments seen as fibrils between the beads, as marked in the AFM image (see Figure 1a). Figure 1a also shows a schematic of the observed beaded structure morphology. The beaded morphology arises due to interactions between the hydrophobic parts of the chains and the long-range columbic interaction among the ions on the chain and the counterions in the solution.22-25 On addition of Cd(ClO4)2 at half the stoichiometric ratio relative to -SO3- in PSS, the open necklace structure collapses to a densely packed agglomeration of beads, as observed by AFM shown in Figure 1b. The collapsed structure occurs due to crosslinking of PSS chains within and among the beads by electrovalent bonding between the divalent Cd2+ and -SO3- on the polyelectrolyte, similar to the binding of Cd2+ with -COOH- in poly(acrylic acid).26 In contrast, addition of a nonchelating ion (such as monovalent Na+) only shields the negative charge on the PSS chain, resulting in a collapsed nonbeaded structure.22,27,28 On addition of Na2S to the Cd-PSS salt solution, the necklace morphology reappears (see Figure 1c) similar to Figure 1a. However, the beads are not as discrete but form a continuous thick nanofibril of width ∼12 nm. Transmission electron microscopy (TEM) (see Figure 1d), where only the high-Z nanoparticles are visible, as CdS has a higher electron contrast (25) Liao, Q.; Dobrynin, A. V.; Rubinstein, M. Macromolecules 2006, 39 (5), 1920. (26) Duxin, N.; Liu, F. T.; Vali, H.; Eisenberg, A. J. Am. Chem. Soc. 2005, 127 (28), 10063. (27) Solis, F. J.; de la Cruz, M. O. Macromolecules 1998, 31 (16), 5502. (28) Lyulin, A. V.; Dunweg, B.; Borisov, O. V.; Darinskii, A. A. Macromolecules 1999, 32 (10), 3264.
than PSS (composed of predominantly carbon and hydrogen) due to its higher atomic number, indicates that particles are imbedded in the nanofibrils forming a necklace structure as schematically shown in Figure 1c. The estimated diameter of the fibrils from TEM and AFM is 10-12 nm, whereas their length is in microns. As the Cd2+ forms CdS nanoparticles, the ionic cross-linking density is diminished by more than 1 order of magnitude, causing the agglomerated beaded structure to exfoliate. Concomitantly, the -SO3- ions that were electrovalently bound to Cd2+ are released on formation of CdS. On further addition of Cd(ClO4)2, the unbounded -SO3- serve as synthesis sites for the next cycle of CdS nanoparticles. To obtain a clear image of the nanoparticles, the CdS particles are extracted by adding dodecanethiol to the nanofibril CdS/PSS composite suspension followed by 1 h of sonication and subsequent removal of the particles by centrifuge-separation and re-suspension in heptane. As shown in Figure 2a in the TEM image, the particles have a uniform size distribution, with a diameter of ∼4 nm. Figure 2b shows a sharp excitonic peak in the UV-Vis absorption spectrum of the nanofibril PSS-CdS composite, indicating a narrow size distribution of the CdS nanoparticles. The excitonic peak in Figure 2b is invariant over first three cycles indicating that the size of the nanoparticles is invariant (as also confirmed by TEM). In the fourth cycle, the absorption spectrum shifts to higher wavelengths and broadens, indicating formation of larger size particles with a broad size distribution. The highly blue-shifted excitonic peak for synthesis in 95% DMF in water will be discussed later. For quantitative comparison of absorption spectra, all of the samples are diluted to identical Cd2+ concentrations. The sharp exciton peak at 412 nm is characteristic of a narrow size distribution for semiconducting nanoparticles. The absorption edge at 435 nm using the effective mass approximation29 provides an estimated size of ∼4 nm for the particles. For a dilute (29) Brus, L. E. J. Chem. Phys. 1984, 80 (9), 4403.
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Figure 2. Structural characterization of the mineralized CdS particles by TEM and UV-vis spectroscopy. (a) TEM image of the extracted CdS nanoparticles from the nanofibrilar composite shows uniform size distribution. (b) Absorption spectra of the nanofibrilar PSSCdS composite after 1st to 4th cycle of synthesis in water and synthesis in 95% DMF. All of the spectra are taken on solution with the same Cd2+ concentrations. (c) The calculated size distributions from the absorption spectra of the 1st cycle of synthesis in water and synthesis in 95% DMF for PSS-CdS.
suspension of nanoparticles, Figure 2c shows the calculated size distribution from the absorption spectra.30 Although the synthesis is in “bulk” with no micellar morphology, the distribution in Figure 2c is remarkably narrow, and the mean size of ∼4 nm is consistent with TEM image in Figure 2a. The second distribution in Figure 2c corresponding to synthesis in DMF (95%) is discussed later. The ultimate yield of CdS after 3 cycles is about 0.75 mole fractions to Sulfonate groups in PSS. The robustness of the process in producing monodisperse particles is attributed to the following characteristics: (i) Decreasing the initial mole ratio of Cd2+ relative to -SO3 from 0.25 to 0.025 does not change the size and distribution of the particles. (ii) Synthesis using PSS with 7-fold smaller molecular weight (i.e., 7 × 104 Da) with similar necklace-of-bead morphology yields identical results. (iii) Decreasing the reaction temperature in the range of 24 to 6 °C with varying reaction times does not affect the nanoparticle distribution or size. (30) Pesika, N. S.; Stebe, K. J.; Searson, P. C. AdV. Mater. 2003, 15 (15), 1289.
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The critical role of beaded morphology is confirmed by two sets of experiments: (i) Synthesis using identical concentration and ratio of styrene sulfonate monomer (SSM)/Cd(ClO4)2 as PSS/Cd(ClO4)2 yields bulk CdS. (ii) Changing the pH of the PSS solution from 8 to 12 which disrupts the formation of the necklace-of-bead morphology results in highly polydispersed microparticles of CdS. We conjecture that the size control and narrow distribution of CdS nanoparticles in the PSS template results from its nanoscale necklace-of-bead morphology. On addition of Cd(ClO4)2, the Cd2+ ions diffuse uniformly in the beads, reaching an equilibrium concentration. Subsequently, on addition of Na2S, its rapid diffusion into the nanoscale beads initiates a perfect heterogeneous nucleation of CdS from the Cd2+ ions present in the beads. The chelation of Cd2+ to -SO3 groups on the polymer chain reduces the ion diffusion preventing large agglomeration of CdS. Each bead thus acts as a batch-reactor, leading to the uniform size of CdS nanoparticles and the beaded-necklace morphology. The reappearance of beaded-chain morphology in PSS-CdS nanocomposite at the end of synthesis allows for an identical second (and third) cycle of nanoparticle synthesis. As noted above, an interesting characteristic of the mineralization process is its invariance of particle size distribution. To change the particle size, the morphology of the polyelectrolyte superstructure needs to be altered significantly. As the necklaceof-bead morphology is highly sensitive to the quality of solvent,24,31 we decrease the hydrophilicity of the solvent by replacing water with a 95% aqueous solution of dimethylformamide (DMF), in the initial PSS solution. The result is 3.4 nm CdS nanoparticles as seen by the blue-shift in the absorbance spectrum in Figure 2b, whereas the size distribution is still narrow as seen in Figure 2c. The necklace-of-bead morphology for the DMF-based system is highly collapsed with more dense beads due to decreased hydrophilicity of the solvent.29 The electrooptical response of the nanofibril CdS/PSS composite is measured by deposition using the same process used to make the AFM samples in Figure 1. The nanofibril network is deposited on a SiO2(100 nm)/Si substrate with ∼100 nm thick Au electrodes 50 µm apart. On application of an 80 V bias across the electrodes, Figure 3a shows the electroluminescence recorded on a CCD camera. The electroluminescence spectrum is redshifted by ∼25 nm compared to the 545 nm peak in the photoluminescence spectrum for the necklace in solution. The electroluminescent intensity has patches of intense spots as observed in the scan shown in Figure 3b. The patches are attributed to the nonuniform deposition of the nanofibril network. The presence of strong electroluminescence suggests that the nanofibril network is electrically percolating and the conduction through the PSS/CdS composite is electronic with significant minority (i.e., holes) current as opposed to purely ionic conduction. In this report, we have presented a robust and simple nonmicellar route for mineralizing monodisperse CdS nanoparticles on a nanoscale superstructure (i.e., necklace-of-bead) of polyelectrolyte formed in dilute aqueous solutions. The size of particles synthesized in the nanosized beads is unchanged for a fixed polymer/solvent system, for a broad range of conditions. The size of the nanoparticles can be altered by changing the hydrophilicity of the solvent, such as by addition of a polar organic solvent. On replacing pure water with 95% DMF, the diameter of CdS particles is reduced from ∼4 to 3.4 nm. The resultant mineralized nanofibrilar PSS/CdS composite is a stable suspension in solvent for over a week when stored in the dark. (31) Waigh, T. A.; Ober, R.; Williams, C. E.; Galin, J. C. Macromolecules 2001, 34 (6), 1973.
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Figure 3. Electrooptical characteristics of the nanofibril CdS/PSS composite deposited between Au electrodes 50 µm apart on a SiO2(100 nm)/Si substrate. The applied potential is 80 V. (a) The illumination due to electroluminescence from CdS nanoparticles as imaged by a CCD camera is observed in the 50 µm gap. (b) The line scan (indicated in panel a) of the intensity profile across the electroluminescence image shows an intensity well over the noise levels. The arrows indicate few patches on the sample with a low level of electroluminescence.
On deposition on solid substrate the nanofibrils are electrically percolating and electroluminescent, indicating electronic charge transport through CdS nanoparticles. The process is particularly robust to synthesize identical size monodispersed nanoparticles over three cycles of mineralization on the same polyelectrolyte solution to obtain highly filled nanofibrilar composite with a CdS to PSS-monomer molar ratio of ∼0.75. The stable nanofibrilar composite suspension in water can serve as an ink to make electroluminescent coating or electro-spun fibers by standard polymer solution processing.
Acknowledgment. We thank the Nebraska Tobacco Settlement Fund and the National Science Foundation. The authors acknowledge Dr. X. Z. Li at the electron microscopy center, University of Nebraska, Lincoln for help with TEM. Supporting Information Available: Additional information and figures. This material is available free of charge via the Internet at http://pubs.acs.org. LA061273W
