NANO LETTERS
Polymer-Encapsulated Gold-Nanoparticle Dimers: Facile Preparation and Catalytical Application in Guided Growth of Dimeric ZnO-Nanowires
2008 Vol. 8, No. 9 2643-2647
Xinjiao Wang,† Gongping Li,‡ Tao Chen,† Miaoxin Yang,† Zhou Zhang,‡ Tom Wu,*,‡ and Hongyu Chen*,† DiVision of Chemistry and Biological Chemistry and DiVision of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, 21 Nanyang Link, Singapore 637371 Received March 21, 2008; Revised Manuscript Received June 26, 2008
ABSTRACT Rational assembly of nanoparticles is of vital importance for exploring fundamental electronic and optical properties and for constructing novel nanoscale devices. Through controlling aggregation kinetics, dimers and trimers of gold nanoparticles were generated and encapsulated with polymer by using a one-pot synthesis that involved simple heating and cooling. Dimers of gold nanoparticles were enriched from the resulting solution by centrifugation. The polymer shells maintain the stability of the nanoparticle organization, preventing aggregation and disintegration during subsequent purification, enrichment, and application. A typical enriched sample showed that the dimer population reached 61% among 989 nanoparticles surveyed. In a proof-of-concept application, the gold nanoparticle dimers were used as catalyst to guide the growth of dimeric zinc oxide nanowires. Nanowire dimers with unprecedented narrow spacing (20 to 60 nm) were achieved using a vapor transport growth method; dimeric nanowire population reached ∼25%.
Assembly of nanoparticles (NPs) has been a subject of intense studies because of interest in both fundamental sciences and potential applications.1-3 Despite successes on solid surfaces, organizing nanomaterials in solution remains a challenge. There are at least two approaches allowing this control. The commonly used method requires anisotropically functionalized NPs, which were previously obtained by solidstate synthesis4-8 or by purification of ligand-AuNP conjugates using electrophoresis.9 In addition, monofunctionalized gold compounds such as undecagold10 or commercially available Nanogold (Nanoprobes)11,12 can be used for NP organizations. The anisotropically functionalized NPs are often seen as “artificial molecules” that could undergo specific reactions to yield desired structures; organization of such NPs has been achieved in solution through the manipulation of the attached molecules.4-9,11-13 More recently, Stellacci and colleagues prepared bifunctionalized AuNPs by exploiting ligand organization on AuNPs.14 Crosslinking of these AuNPs led to soluble linear aggregates. In general, however, preparation of anisotropically functional* Corresponding author. E-mail:
[email protected] (H.C.) and
[email protected] (T.W.). † Division of Chemistry and Biological Chemistry. ‡ Division of Physics and Applied Physics. 10.1021/nl080820q CCC: $40.75 Published on Web 08/02/2008
2008 American Chemical Society
ized NPs and isolation of the NP organizations are nontrivial and often complicated by ligand exchange/dissociation.15 Alternatively, NP aggregates can be obtained by controlling aggregation kinetics. The general difficulty in this methodology is that, if the interactions between NPs are too weak, then the NP assembly may be lost during purification; if the interactions are too strong, then further aggregation may occur when the sample is concentrated or purified. Previously, Feldheim and co-workers prepared small clusters of AuNPs using rigid, multivalent thiol-linkers.16-18 Weaker capping ligands were used to reduce aggregation during purification. Putting shells on NPs can also resolve the dilemma; the shells prevent the dissociation of NPs and at the same time guard against further aggregation. More importantly, the shells provide secure anchoring sites for biofunctionalization and improve the long-term stability of the NP organization. Silica-coated AuNP aggregates were previously prepared by coalescence of template micelles,19,20 upon which the inorganic shells were generated. However, the inorganic shells cannot be readily removed, which limit subsequent applications. Here, we report the encapsulation of AuNP dimers by polystyrene154-block-poly(acrylic acid)60 (PS154-b-PAA60), where the polymer shells help to maintain the NP organiza-
Scheme 1. Illustration for the Preparation of Dimeric AuNPs and Dimeric ZnO NWsa
a A solution of citrate-stabilized AuNPs (a) is induced to aggregate (b) and simultaneously encapsulated by PS154-b-PAA60 (c). The resulting NPs were enriched by centrifugation (d) and then used as catalyst for growing ZnO NWs (e). The inset (f) shows the interactions between AuNP, ligand, and PSPAA.
tion during preparation, isolation, and storage but can be removed by oxidation when necessary. Previously, Taton and co-workers have shown that AuNPs,21,22 magnetic nanoparticles,23 or the hybrid aggregates of them24 can be encapsulated by shell-cross-linked polymer micelles. Using a modified method, we reported that small (∼5 nm) AuNPs can be singly encapsulated by diblock copolymer with long hydrophilic chains.25 In the present work, we found that addition of HCl can induce aggregation in our system, yielding small clusters of AuNPs within polymer shells. The long hydrophilic chains help to stabilize the core/shell structure, allowing purification by direct centrifugation. The facile synthesis and isolation provided large quantities of stable AuNP dimers and trimers, which were purified, enriched, and then used for applications. As a proof of concept, the dimers and trimers of AuNPs were used as catalysts to control nanowire (NW) growth. Proximate ZnO NWs with spacing of 20 to 60 nm were prepared using this new bottomup approach. In our method (Scheme 1),25 a thiol-ended hydrophobic ligand, 2-dipalmitoyl-sn-glycero-3-phosphothioethanol (sodium salt; 1), was used to render the AuNP surface hydrophobic, upon which an amphiphilic diblock copolymer (PS108PGA108, PS132PAA72, or PS154PAA60) attached to form an encapsulating shell. Adding HCl in the reaction mixture ([HCl]initial ) 1∼50 mM) induced aggregation of AuNPs but did not interrupt the encapsulation process. Briefly, citratestabilized AuNPs, PS154-b-PAA60, 1, and HCl in DMF/H2O (final volume ratio 4.5:1) were mixed in the given sequence; the mixture was heated to 110 °C for 2 h and then slowly cooled down. As the critical micelle concentration of the polymer decreased with temperature, it self-assembled into spherical micelles that included the hydrophobically functionalized NPs and their aggregates. Figure 1 shows the TEM image of a sample before removal of excess reactants and empty polymer micelles. With the long hydrophilic arms of the polymer, the aggregation process was slow enough that the reaction could be terminated at the early stages of aggregation. The product was a mixture of monomers and aggregates (dimers and larger) of AuNPs, resulting from the random encounters during aggregation. In previous studies 2644
Figure 1. TEM image and histogram (inset) of aggregated AuNPs (d ) 15 nm) before removal of empty micelles. Stain was used to improve contrast. See Supporting Information for larger-area views of aggregated samples.
Figure 2. Dependence of aggregation on the acidity of the initial solution, showing dimer % against log[HCl]initial. The dimer % was counted from TEM images of the respective samples. The pH of the solution (DMF/H2O) changes after reaction. 9 and b show two sets of experiments. The / shows when no HCl was added, dimer % < 0.5%.
where highly asymmetric diblock copolymers (PS250PAA13, PS100PAA13, and PMMA240PAA13) were used,21-24 Taton and co-workers found a general rule that large size AuNPs (d > 10 nm) act as surface templates where singly encapsulating micelles form, whereas small AuNPs (d < 10 nm) or magnetic NPs (d ∼ 10 nm) act as solutes that dissolve in the micelles as aggregates. The number of particles in the multiencapsulating micelles was found to roughly follow Gaussian distribution.23 In our system, the long hydrophilic chains of our polymers have helped to prevent aggregation and improved the probability of single-encapsulation in the absence of HCl.25 However, the addition of HCl has clearly led to a deviation from this general rule, giving aggregates of large size (d ∼ 15 nm) AuNPs. After the reaction mixture was cooled down, the polymercoated AuNP aggregates (abbreviated as [N × AuNPs]@polymer, N being an integer) were stable against further aggregation. No obvious change in composition was observed after a sample was stored for more than one month. The [N × AuNPs]@polymer were readily isolated by centrifugation. The sample was first diluted by water (14 times in volume) to lower the DMF concentration, trapping the polymer micelles in a kinetically and thermodynamically stable state.26 Nano Lett., Vol. 8, No. 9, 2008
Figure 3. (a) Sample enriched with dimers of AuNPs (d ) 15 nm), where dimers amounts to 61% out of 989 particles surveyed. (b) Histogram of the sample shown in (a). (c) A sample enriched with trimers and larger aggregates. Note that it is sometimes very difficult to distinguish tetrahedral tetramers (d) from triangular trimers (e).
Then the [N × AuNPs]@polymer were directly isolated by centrifugation without cross-linking the polymer shells. The centrifugation and resuspension were typically repeated once more, giving samples in which most empty micelles and DMF were removed. The resulting samples are very stable, and normally contain more multimers of AuNPs than the original sample. It is not immediately clear whether this enrichment effect was caused by the loss of monomers, by aggregation, or by the combined effect of both. We believe that aggregation during centrifugation should be negligible because similar polymer micelles were shown to be very stable in solutions with a high water content.26 We were able to enrich AuNP dimers to about 61% (vide infra); if aggregation happened at a significant level during the repeated centrifugation, larger aggregates should have appeared in large numbers. Nevertheless, after purification to remove DMF and empty micelles, the statistics obtained from the TEM images does not reflect the degree of aggregation in the original samples. Therefore, we prepared TEM samples before purification to investigate the extent of aggregation. Because monomers and dimers are normally the major species in our products, the number percentage of dimers (dimer %) is a convenient indicator for the degree of aggregation in a given sample. The extent of aggregation depends on the initial concentration of HCl in the reaction (Figure 2). Here, the prominent feature is a sharp rise of dimer % at [HCl]inital ∼ 2 mM. Above this concentration, the aggregation was significant (dimer % ) 12 ∼ 16%); below it few dimers formed (dimer % < 2%). When no HCl was added, less than 0.5% dimer was observed.25 Discussion of the pH effect is not appropriate because the pH of the DMF/H2O mixture solution changes during heating. For example, at DMF:H2O ) 4.5:1, heating of a pH ) 3.5 solution ([HCl]initial ) 50 mM) at 100 °C for 2 h shifted the pH to 5.9. Owing to this complication, it is difficult to determine the pH during aggregation. NevertheNano Lett., Vol. 8, No. 9, 2008
less, the dependence of aggregation on HCl provided a means where the aggregation of AuNPs can be controlled to give dimers and trimers, with a reasonably good yield of dimers (12 ∼ 16%). Among the preliminary results, the best yield of AuNP dimer was ∼19% (Figure 1, [AuNP] = 57 nM, [PS154-b-PAA60] ) 47 µM, [1] ) 0.22 mM, [HCl]initial ) 50 mM, heated in a DMF/H2O ) 4.5 mixture solution at 100 °C for 8 h). Our method is efficient considering that the highest theoretical yield based on a second order kinetics assumption is 25% dimer (see Supporting Information). Our facile preparative and isolation methods allowed the synthesis of AuNP dimers in large enough quantities such that enrichment became possible. During centrifugation, larger aggregates are expected to come down faster because of the larger centrifugal force and the smaller average Brownian force they experience. We found that AuNP dimers could be enriched to ∼60% by collecting partitions between 8000 and 11 000 rpm (5200 ∼ 9800 g). Figure 3a shows the TEM image of a sample prepared by this method; the dimers amount to 61% out of 989 particles in 5 randomly selected images. Enrichment of trimers was carried out by collecting partitions between 5000 and 8000 rpm (2000 ∼ 5200 g). It is more challenging owing to the small size and mass differences between dimers, trimers, and tetramers. The best sample we obtained so far contains roughly 35% trimers (Figure 3c). It should be noted that it is often very difficult to distinguish tetrahedral tetramers (Figure 3d) from triangular trimers (Figure 3e). While this miscounting should not affect dimer-enriched samples where very few tetramers exist, it likely distorts the survey of trimer-enriched samples. AuNPs are widely used to catalyze the growth of semiconductor NWs through a vapor-liquid-solid (VLS) process.27,28 Controlling the size, position, and order of catalytic AuNPs is an effective way to realize organized NW growth.29 In previous efforts, ordered NW arrays were created using methods such as e-beam lithography,30 nanosphere patterning,31-33 nanoimprint lithography,34 and microcontact printing.35 However, only moderate successes have been achieved in closely positioning NWs with a good control; an interwire spacing of less than 100 nm has not been demonstrated so far. The AuNP dimers inside polymer micelles provided us with a unique opportunity to use the organization of AuNP prepared by facile solution chemistry to guide the growth of ZnO NWs. With a setup similar to previous reports, a vapor transport method was used for the NW synthesis.36 The polymer encapsulated AuNPs were dispersed on sapphire substrates and the growth took place at ∼800 °C. ZnO NWs can be catalyzed by AuNPs and grow epitaxially on the crystallographically compatible sapphire substrates.37 Zn vapor was generated from the mixed powder of ZnO and graphite and carried downstream to the substrates by a carrier gas of argon containing 0.5% oxygen. The growth conditions, especially the growth temperature and the flow of the carrier gas, were optimized (see Supporting Information). No particular procedure was taken to remove the polymer shell; presumably, its removal was assisted by the oxygen in the carrier gas and occurred before the furnace reached the growth tem2645
Figure 4. TEM images of AuNP monomer (a), dimer (b), and trimer (c). SEM images of ZnO NW growth at three different stages (0, ∼1, and ∼5 min after temperature reached 800 °C) are shown (a1, a2, and a3) for monomer, (b1, b2, and b3) for dimer, and (c1, c2, and c3) for trimer. All of the scale bars represent 100 nm. (d) A typical SEM picture of ZnO nanowires grown by using an enriched sample of AuNP dimers.
perature. When a dimer-enriched (∼52%) sample of [N × AuNPs]@polymer was used as catalyst, the growth of ZnO NW monomers, dimers, and trimers were observed (Figure 4). Without extensive optimizations, an eventual ∼25% population of dimeric ZnO NWs was achieved (Figure 4d), corresponding to a fidelity of ∼50%. The individual TEM images of 15 nm AuNP monomers, dimers, and trimers were shown in Figure 4 a-c, respectively. When the growth was terminated by cooling immediately after the temperature reached the growth temperature, we observed that the Zn vapor was selectively absorbed around the AuNPs, which appeared as bright dots (Figure 4a1,b1,c1). The Zn absorption could contribute to the absence of coalescence in about half of the AuNP aggregates. In order to minimize the interface energy,38 AuNPs often migrated to the stable positions along the edges of the Zn droplets, which effectively separated the AuNPs and could further hinder their coalescence. Figure 4a2,b2,c2 depicts the initial stages of ZnO growth, that is, ∼1 min after the growth temperature reached the set point. Short NWs were observed on top of the eutectic alloy particles. The initial AuNPs of ∼15 nm in diameter led to ZnO NWs of ∼25 nm in diameter. After a growth of ∼5 min, length of the NWs reached ∼1 µm. The final edge-to-edge interwire distances are often between 20 and 60 nm, which are much larger than the original interparticle distances of ∼2 nm; a precise control of the interwire distance requires further studies. Nevertheless, preparing such proximately positioned NWs is still beyond the capability of conventional lithographic methods. In order to evaluate the effect of AuNP dimers, we compared their performance with that of pure monomers. Only individual ZnO NWs were obtained when AuNP monomers were used as catalysts (see Supporting Information). Therefore, we can conclude that the dimeric ZnO NWs were indeed induced by AuNP dimers, as opposed to accidentally aggregated monomeric AuNP@polymer. As far as we know, there has been no report on applying dimer/trimer NPs to prepare proximate NWs. Although further experiments are 2646
imperative to achieve better controls, we have demonstrated that the organized NPs can guide the growth of NWs. In conclusion, we have prepared polymer-encapsulated AuNP dimers. The long-term stability of NP organization is maintained by polymer shells, whereas in other systems it may be compromised by disintegration, aggregation, or ligand exchange/dissociation. The dimers were used to guide the ZnO NW growth, and interwire spacing of 20 to 60 nm was achieved. New electrical and optical properties may evolve from such dimeric semiconductor NWs. Acknowledgment. The authors thank Electron Microscopy Unit in NUS for TEM usage. H.Y.C. acknowledges the supports from NTU start-up (SUG 4/06) and MOE (ARC 27/07). T.W. thanks NTU start-up (SUG 20/06) and MOE (RG 46/07). Supporting Information Available: Theoretic calculation on the synthetic yield based on second-order kinetics; UV-visible spectra of AuNPs enriched in monomers, dimers and trimers; large views of TEM samples of dimers and trimers; and ZnO grown using monomeric AuNP@polymer as catalyst. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Qin, L.; Banholzer, M. J.; Millstone, J. E.; Mirkin, C. A. Nano Lett. 2007, 7, 3849–3853. (2) Huang, W.; Qian, W.; Jain, P. K.; El-Sayed, M. A. Nano Lett. 2007, 7, 3227–3234. (3) You, J. P.; Choi, J. H.; Kim, S.; Li, X.; Williams, R. S.; Ragan, R. Nano Lett. 2006, 6, 1858–1862. (4) Huo, F. W.; Lytton-Jean, A. K. R.; Mirkin, C. A. AdV. Mater. 2006, 18, 2304–2306. (5) Worden, J. G.; Shaffer, A. W.; Huo, Q. Chem. Commun. 2004, 518– 519. (6) Xu, X.; Rosi, N. L.; Wang, Y.; Huo, F.; Mirkin, C. A. J. Am. Chem. Soc. 2006, 128, 9286–9287. (7) Li, B.; Li, C. Y. J. Am. Chem. Soc. 2007, 129, 12–13. (8) Sardar, R.; Heap, T. B.; Shumaker-Parry, J. S. J. Am. Chem. Soc. 2007, 129, 5356–5357. (9) Loweth, C. J.; Caldwell, W. B.; Peng, X. G.; Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. 1999, 38, 1808–1812. Nano Lett., Vol. 8, No. 9, 2008
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