Gold-Based Hybrid Nanocrystals Through Heterogeneous Nucleation and Growth

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1 Gold-Based Hybrid Nanocrystals Through Heterogeneous Nucleation and Growth By Jie Zeng,* Jianliu Huang, Chi Liu, Cheng Hao Wu, Yue Lin, Xiaoping Wang,* Shuyuan Zhang, Jianguo Hou,* and Younan Xia Despite many decades of intense research and development, it remains a grand challenge to prepare nanocrystals (NCs) with complex structures and yet well-controlled sizes, shapes, compositions, and properties. [1] One of the effective approaches to this challenge is to take NCs of one material as the seeds and deposit a different material around or onto their surface in an effort to generate hybrid nanocrystals. There are two typical distinct outcomes for such a seeded growth process: i) the formation of a conformal, complete shell of the second material on the surface of the seed [2] and ii) the deposition and growth of the second material on specific site(s) of the seed. [3 6] In either case, the hybrid NCs are expected to display a combination of the properties associated with each material. In addition, it is also expected to observe new properties and capabilities due to a coupling between the two different materials. Over the past five years, a number of methods have been developed, including those based on selective anisotropic growth, [6] formation of micelles, [5a] oxidation-directed decomposition, [5d] and phase segregation induced by ripening. [3c,5c] Although all these methods are capable of generating high-quality materials, none of them seems to be effective over a broad range of different materials. Based on the principle of heterogeneous nucleation and growth, here we report a general strategy for the facile synthesis of Au-based hybrid NCs with a variety of compositions, including CdSe Au, PbSe Au, FePt Au, Cu 2 O AuAg, and FePt CdS Au. We started with CdSe NCs to demonstrate the capability of this approach in generating CdSe Au hybrid NCs. Wurtzite CdSe NCs with a size of 3.4 nm were synthesized using the procedure developed by Qu and Peng [7] with some modifications (see the Supporting Information, Fig. S1A). [8] Controllable CdSe Au hybrid NCs were obtained through the in situ reduction of a Au I organometallic complex on the surface of CdSe NCs. In a typical synthesis, a specific amount of Au I SR (Q Au ) was added to a toluene solution containing CdSe NCs (Cd: 1.3 mmol L 1 )at 30 8C under ambient conditions. Ethylene glycol (50 ml) was then [*] Dr. J. Zeng, Prof. X. Wang, Prof. J. Hou, Prof. S. Zhang, J. Huang, C. Liu, C. H. Wu, Y. Lin Hefei National Laboratory for Physical Sciences at the Microscale University of Science and Technology of China Hefei, Anhui (P. R. China) jzeng@mail.ustc.edu.cn; xpwang@ustc.edu.cn; jghou@ustc.edu.cn Prof. Y. Xia, Dr. J. Zeng Department of Biomedical Engineering Washington University St. Louis, MO (USA) DOI: /adma added under agitation to initiate the reaction. As the reaction proceeded, CdSe Au hybrid NCs formed and the size of the Au components could be easily tuned by varying Q Au. Figure 1 shows transmission electron microscopy (TEM) images of the products, where the CdSe component is lighter, in contrast, than the Au component due to their difference in electron density. X-ray photoelectron spectroscopy (XPS) reveals the increase of Au/Cd molar ratio as the amount of Q Au was increased: 0.5 (Q Au ¼ 2 mmol), 1.1 (Q Au ¼ 4 mmol), and 1.6 (Q Au ¼ 6 mmol). The histogram shown in Figure S3A summarizes the result of our statistical analysis of the configuration of the product obtained with Q Au ¼ 4 mmol, which reveals that the majority of the final NCs took a dimeric, hybrid structure (ca. 84.6%). Other species include individual CdSe NCs (ca. 3.3%) and trimeric, hybrid NCs (ca. 12.1%) with two Au particles growing on one CdSe seed. No individual Au nanoparticles were observed. The formation of trimeric NCs can be easily understood due to the simultaneous nucleation of Au on more than one site on the CdSe seed. The size distributions of CdSe (3.4 nm) and Au (2.0 nm) are also shown in Figure S3B and S3C, respectively. The corresponding energy dispersive X-ray (EDX) spectroscopy (Fig. S3D) of the sample for Q Au ¼ 4 mmol reveals a Au/Cd molar ratio of 1.0, which is consistent with the result of XPS. The high-resolution TEM image in Figure 1D indicates that the Au component was firmly attached to the hexagonal CdSe surface, with its {111} facet parallel to the {101} facet of wurtzite CdSe. The lattice spacings of Au and CdSe, determined to be 0.23 and 0.32 nm, respectively, are in agreement with the values for both the Au(111) planes (International Centre for Diffraction Data, no ) and the hexagonal CdSe(101) planes (International Centre for Diffraction Data, no ). Two points should be noted for the synthetic procedure. The first is on the use of a Au I instead of Au III organometallic complex as a precursor to elemental Au. Our previous studies have shown that Au nanostructures with controllable morphologies could be fabricated through reduction of a Au I oleylamine complex under mild reduction conditions. [9] These studies also suggest that the Au I complex could decompose and serve as an effective source of elemental Au. Here, a different Au I organometallic source is employed, which was formed via the reaction of Au III and mercaptan (RSH): [10] Au III þ 3RSH! Au I SR þ RS SR (1) The second point is on the role played by the surface of CdSe NCs. Without the precursor NC, the reduction of Au I is very difficult to obtain at room temperature due to the high reaction 1936 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22,

2 Figure 1. A C) TEM images of CdSe Au hybrid NCs obtained by changing Q Au from 2 to 4 to 6 mmol, respectively. The NCs are seen as dimeric units, all with a similar size of 3.4 nm for the CdSe component. The Au/Cd molar ratio was increased from 0.5, to 1.1 and 1.6. D) Highresolution TEM image and a schematic drawing of the CdSe-Au hybrid NC. barrier. However, thanks to the small curvature radius and high surface energy, the CdSe NC can serve as a reasonable catalytic site for the reduction of Au I as well as provide a nucleation site for the formation of Au 0 clusters. In a sense, the presence of CdSe seeds can reduce the reaction barrier for the reduction of Au I and facilitate the nucleation of Au. [11] Based on the above argument, we attribute the formation of CdSe Au hybrid NCs to the heterogeneous nucleation and in situ growth of gold on Cd seeds. The primary reaction in the preparation of CdSe Au hybrid NCs through this process involved the reduction of Au I SR complex by a weak reductant (ethylene glycol) on the surface of CdSe NCs in a solution. The surface of CdSe NCs acted both as the catalytic site for the reduction of Au I SR and as the initial point of heterogeneous nucleation and growth of Au. Once the nucleation sites for gold were established, the subsequent reduction of Au I SR would proceed only at these sites until the reaction was quenched or the Au I precursor was completely depleted. As a result, both the heterogeneous nucleation of Au and the following growth of Au led to the formation of CdSe Au hybrid NCs. Along with the reduction process, the RSH generated in situ could adsorb onto the surface of the hybrid NCs through Au-thiolate boning, giving the resultant hybrid NCs a hydrophobic surface. In this synthesis, two factors helped preventing the formation of a Au shell on the CdSe nanocrystal seed: i) the large lattice mismatch between CdSe and Au (50%) [2b] and ii) the self-catalytic reduction of Au I on the formed Au clusters. [11] Additionally, the active surface of the added CdSe NCs, which can lower the energy barrier for the reduction of Au I SR by ethylene glycol, also contributes to the elimination of isolated Au nanoparticles. Our control experiments indicate that, in the absence of CdSe NCs, ethylene glycol could not reduce the Au I SR precursor even at 80 8C. We have also demonstrated that the heterogeneous nucleation and growth approach could be extended to the synthesis of Au-based hybrid NCs with a large number of different seed materials. To this end, various kinds of NCs were prepared and used as the seeds, including semiconductors (PbSe), magnetic alloys (FePt), and metal oxides (Cu 2 O), as shown in Figure S1. For the PbSe Au case (Fig. 2A and 2B), the Au component only covered one side of the PbSe NC like a cap and the overall shape of the hybrid NC was similar to a Chinese tumbler. In the case of FePt Au, gourdlike heterodimers were formed, in which the size of the Au component was much larger than the FePt component (Fig. 2C and 2D). For both of these systems (unlike the CdSe Au case), highresolution TEM images indicate that the Au component was multicrystalline. Comparing with the above cases, the growth of Au particles on the Cu 2 O seeds was quite slow. Prior studies have shown that the addition of Ag content could facilitate the nucleation and growth of Au nanocrystals and promote the self-catalyzed reduction of Au ions. [12] Therefore, in order to accelerate the reaction, we introduced both the Au I and the Ag I stock solutions into the system to produce Cu 2 O AuAg hybrid NCs. Here, the shape of the resulting hybrid NCs is somewhat in between the Chinese tumbler and the gourd (Fig. 2E and 2F). The EDX data from PbSe Au, FePt Au, and Cu 2 O AuAg systems all confirm the presence of various elements for the hybrid NCs (Fig. S4 S6). Besides the binary systems, the general route based on a heterogeneous nucleation and growth could be further extended to fabricate NCs with multiple components. A typical example is the successful synthesis of FePt CdS Au ternary hybrid NCs with the use of FePt CdS heterodimers as the seeds (Fig. 3). Even though both FePt and CdS can act as the catalytic sites for the heterogeneous nucleation of Au in a solution, the Au atoms prefer to be deposited on the surface of the CdS portion rather than the FePt portion. This observation indicates that the coupling between two portions of the hybrid seeds can have a significant impact on the heterogeneous nucleation process. This coupling effect probably exists widely in binary and multicomponent material systems. Our studies of the heterogeneous nucleation mechanism involved in the above systems suggest that the well-defined Adv. Mater. 2010, 22, ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1937

3 would be the most-commonly observed nanostructures. However, when NC seeds with a non-spherical shape were used, such as the growth of CdS on FePt, some edge regions on the CdS surface might also provide additional nucleation sites to form NCs with multiple components (Fig. 3C, in this case, four patches of Au were formed on the same particle). To accurately position the Au clusters during the heterogeneous nucleation process, we exploited the use of cubic Cu 2 O NCs as seeds. The experimental observation, as shown by the TEM image in Figure 4, indicates that the AuAg clusters were formed at all corners of the Cu 2 O nanocube. This result further supports the heterogeneous nucleation mechanism described above. The controlled deposition of AuAg on Cu 2 O nanocubes should allow one to produce non-spherical hybrid NCs with welldefined morpholoogies and surface structures. In summary, we have demonstrated that both binary and ternary hybrid NCs could be produced by controlling the heterogeneous nucleation and growth of Au on NC seeds with different shapes and/or compositions. Most importantly, it is also feasible to accurately and selectively control the locations at which Au nucleation and growth would occur. This approach provides a versatile route to Au-based hybrid nanomaterials with tunable sizes, shapes, and compositions. These kinds of hybrid NCs not only provide a natural vehicle for integrating multiple functions but also may lead to the development of new properties when there is a strong coupling between different components of the hybrid structure. The hybrid NCs are expected to provide new opportunities for improving the efficiency of photoelectric devices, solar cells, photocatalysts, and NC-based synchronous biodetection and treatment. Figure 2. Extension of the heterogeneous nucleation and growth approach to the synthesis of various binary hybrid NCs. A) TEM and B) high-resolution TEM images of PbSe Au hybrid NCs. C) TEM and D) high-resolution TEM images of FePt Au hybrid NCs. E) TEM and F) highresolution TEM images of Cu 2 O AuAg hybrid NCs. structure of NC seeds has a great impact on the nucleation and growth of Au. We noticed that the site with the smallest curvature radius on the seed surface can act as the nucleation site for Au due to the highest activity at this site. Thanks to the self-catalytic effect of Au, once some Au had been deposited somewhere on the surface of a seed, the following reduction of the Au precursor would preferentially occur at this site instead of nucleating at new sites. As a consequence, when spherical seeds (including CdSe, PbSe, FePt, and Cu 2 O NCs) were used, the hybrid dimeric NCs Experimental Synthesis of the Organometallic Complex Au I SR: The Au I SR complex was prepared by reacting aau III organometallic compound with an alkanethiol. In a typical synthesis, tetraoctylamine bromide (200 mg) was dissolved in toluene (20 ml). Then, an aliquot of HAuCl 4 aqueous solution (20 ml, 10 mm) was added under magnetic stirring. The upper organic layer turned red after 20 min and was washed three times with distilled water (30 ml each) in a separation funnel. After separation of the mixture, dodecyl mercaptan (0.2 ml) was added to the red organic layer. After ultrasonication for about 15 min, the solution became transparent, indicating the formation of a Au I SR organometallic complex, which was then used as the Au I stock solution. Synthesis of the Ag I Stock Solution: The organic Ag I stock solution was prepared according to a reported two-phase extraction process [13]. In a typical procedure, distilled water (9 ml) and AgNO 3 (46 mg) were added to a mixture of CHCl 3 (6 ml) and tetraoctylamine bromide (656 mg). The 1938 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22,

4 Figure 3. High-resolution TEM images of FePt (A), FePt CdS (B), and FePt-CdS-Au (C) nanocrystals. All scale bars correspond to 1 nm. Figure 4. TEM image of Cu 2 O Au hybrid NCs synthesized with Cu 2 O nanocubes serving as the seeds. The inset shows a schematic of the hybrid NC, where the cube and spheres represent the Cu 2 O nanocube and the Au clusters, respectively. resulting mixture was stirred for 2 h and the CHCl 3 solution was then washed three times with distilled water (30 ml each) and separated, resulting in the organic Ag I stock solution. Synthesis of CdSe Au Hybrid NCs: CdSe NCs were synthesized and purified by the reported procedure [8]. For samples shown in Figure 1a d, 0, 0.2, 0.4, and 0.6 ml of the Au I stock solution (containing 0, 2, 4, and 6 mmol of Au I SR, respectively) was added to 2.5 ml of toluene containing CdSe NCs (the concentration of the Cd element was 1.3 mmol L 1 determined by inductively coupled plasma mass spectrometry (ICP-MS) at 30 8C under ambient conditions. Ethylene glycol (50 ml) was then added under agitation to initiate the reaction. The solution turned from red to brown after 0.5 h, indicating the formation of CdSe Au hybrid NCs. The products were precipitated with a copious amount of ethanol and collected by centrifugation and decantation. The Ar-dried precipitate was then dissolved in toluene and used directly for further characterizations. Synthesis of PbSe Au Hybrid NCs: PbSe NCs were synthesized and purified using a reported procedure [14]. The following is a typical procedure for PbSe Au hybrid NCs: the Au I stock solution (0.4 ml, containing 4 mmol of Au I SR), ethylene glycol (50 ml), and toluene (2.5 ml) containing PbSe NCs (the concentration of Pb element was 1.5 mmol L 1 determined by ICP-MS) were mixed together and heated at 50 8C for 2 h. The color of the solution changed from green brown to dark brown gradually, indicating the formation of PbSe Au hybrid NCs. When the reaction was completed, the products were precipitated and re-dissolved according to the procedure for the CdSe Au system. Synthesis of FePt Au Hybrid NCs: FePt NCs were synthesized and purified using a reported procedure [15]. A suspension of FePt NCs in toluene was diluted to light brown. Then the Au I stock solution (1.6 ml) and ethylene glycol (0.2 ml) were added to form FePt Au hybrid NCs. The mixture was slowly heated to 55 8C, and the solution became dark red after 0.5 h. Heating of the solution at 55 8C was continued in air for another 3.5 h. When the reaction was completed, the products were precipitated and re-dissolved according to the procedure for the CdSe Au system. Synthesis of Cu 2 O AuAg Hybrid NCs: Cu 2 O NCs were synthesized and purified using a reported procedure [16]. For the synthesis of Cu 2 O AuAg hybrid NCs, Au I stock solution (0.4 ml) and ethylene glycol (50 ml) were added to the light-green suspension of Cu 2 O NCs (2.5 ml, 1.5 mmol L 1 ). After stirring for 15 min at the room temperature, the Ag I stock solution (0.2 ml) was added. After another 15 min, the solution became purple. The reaction was further continued in air for 2 h and the products were precipitated and re-dissolved according to the procedure for the CdSe Au system. Synthesis of FePt CdS Au Hybrid NCs: FePt CdS dimeric NCs were synthesized and purified using a reported procedures [3c]. A suspension of FePt CdS NCs in toluene was diluted until the absorbance of the solution at 500 nm was about Then two drops of ethylene glycol and the Au I stock solution (0.4 ml) were added to ensure the concentration of Au element was 0.05 mm. The mixture was slowly heated to 45 8C and kept at this temperature for 2 h. When the reaction was completed, the products were precipitated and re-dissolved according to the procedure for the CdSe Au system. Characterization: For TEM, the nanoparticles were dispersed in toluene and dropped on carbon-coated copper grids. TEM and high-resolution images were collected using a JEOL 2010 instrument working at 200 kv, equipped with an EDX analyzer (Phoenix). A Hitachi UV-4100 instrument was used to record UV vis extinction spectra from various samples. The quantitative analysis and XPS measurements were carried out with a ESCALAB 250 instrument. Acknowledgements We thank W. Lu, Z. Hu, and G. Li for helpful discussions. This work was partially supported by grants from the National Key Basic Research Program of China (Grant No. 2006CB922002) and the National Natural Science Foundation of China (Grant Nos , , and ). J.Z. also acknowledges the financial support from the Chinese Academy of Sciences (CAS). Supporting Information is available online from Wiley InterScience or from the author. This article is part of a Special Issue on USTC Materials Science. Received: November 21, 2009 Published online: March 17, 2010 Adv. Mater. 2010, 22, ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1939

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