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A general strategy for nanocrystal synthesis


Vol 437|1 September 2005|doi:10.1038/nature03968

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A general strategy for nanocrystal synthesis
Xun Wang1,2, Jing Zhuang1,2, Qing Peng1,2 & Yadong Li1,2
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w strategies for materials fabrication are of fundamental importance in the advancement of science and technology1–12. Organometallic13,14 and other organic solution phase15–17 synthetic routes have enabled the synthesis of functional inorganic quantum dots or nanocrystals. These nanomaterials form the building blocks for new bottom-up approaches to materials assembly for a range of uses; such materials also receive attention because of their intrinsic size-dependent properties and resulting applications18–21. Here we report a unied approach to the synthesis of a large variety of nanocrystals with different chemistries and properties and with low dispersity; these include noble metal, magnetic/ dielectric, semiconducting, rare-earth uorescent, biomedical, organic optoelectronic semiconducting and conducting polymer nanoparticles. This strategy is based on a general phase transfer and separation mechanism occurring at the interfaces of the liquid, solid and solution phases present during the synthesis. We believe our methodology provides a simple and convenient route to a variety of building blocks for assembling materials with novel structure and function in nanotechnology13–29. We chose noble metals as an example to demonstrate the effectiveness of this method in yielding high quality nanocrystals. Uniform noble metal quantum dots, or nanocrystals, can be obtained through the reduction of noble metal ions by ethanol at a temperature of 20 to 200 8C under hydrothermal or atmospheric conditions. In a typical synthesis, 20 ml of aqueous solution containing noble metal salts (for example, 0.5 g of AgNO3, HAuCl4 or other soluble chlorides), 1.6 g sodium linoleate, 10 ml ethanol and 2 ml linoleic acid were added to a 40 ml autoclave tube under agitation. The reactions were controlled at different temperatures for specic metals, for example, 80 to 200 8C for Ag, 20 to 200 8C for Ru, Rh and Ir, 20 to 100 8C for Au, Pd and Pt. The system was sealed and treated at the designated temperature for 10 hours. After the reaction was cooled to room temperature, the products were collected at the bottom of the vessel. Based on the same synthetic process, other fatty acid and corresponding salt systems can play the same roles as the linoleic acid system. Figure 1a shows transmission electron microscope (TEM) images of typical samples of Ag, Au, Rh and Ir nanocrystals and indicates the large quantity and good uniformity (see Supplementary Information Part I and II) that were achieved using this approach. The Ag and Au nanocrystals are usually in round shapes with smooth surfaces, and self-assemble into ordered two-dimensional (2D) arrays on the surface of the TEM grid (Fig. 1a). The diameters of the nanocrystals can be reasonably tuned from about 4 to 15 nm by altering temperature, the mole ratio of the protecting reagents to noble metal ions or the chain length of the fatty acid (see Supplementary Information Part III). This approach has also been shown to yield the nearly monodisperse ultrane metal nanocrystals of Ru, Rh, Ir, Pd and Pt with diameters of approximately 3 nm or less. Thorough high resolution (HR) TEM characterizations revealed the highly crystalline nature of these nanocrystals. Typical HRTEM images of Ir nanocrystals with diameters ,1.7 nm show an interplanar spacing
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of ,0.22 nm, which corresponds to the (111) planes of face-centred cubic Ir. EDS (energy dispersive spectroscopy) microanalysis and powder XRD (X-ray diffraction) (Fig. 2a) measurement have proven the successful synthesis of face-centred cubic structured Ag (JCPDS 4-783), Au (JCPDS 4-784), Pd (JCPDS 46-1043), Pt (JCPDS 4-802), Rh (JCPDS 5-685), Ir (JCPDS 46-1044) and hexagonal Ru (JCPDS 6-663). The primary reaction in the preparation of noble metal nanocrystals through this liquid–solid–solution (LSS) process involved the reduction of noble metal ions by ethanol at the interfaces of metal linoleate (solid), ethanol–linoleic acid liquid phase (liquid) and water–ethanol solutions (solution) at different designated temperatures (Fig. 3). After the aqueous solution of noble metal ions, sodium linoleate (or another sodium stearate) and the mixture of linoleic acid (or another fatty acid) and ethanol were added into the vessel in order. Three phases formed in this system: sodium linoleate (solid), the liquid phase of ethanol and linoleic acid (liquid), and the water– ethanol solution containing noble metal ions (solution). A phase transfer process of the noble metal ions occurred spontaneously across the interface of sodium linoleate (solid) and the water–ethanol solution (solution) based on ion exchange, which led to the formation of noble metal linoleate and the entering of the sodium ions into the aqueous phases. Then at a designated temperature, the ethanol in the liquid and solution phases reduced the noble metal ions at the liquid–solid or solution–solid interfaces. Along with the reduction process, the in-situ generated linoleic acid absorbed on the surface of the noble metal nanocrystals with the alkyl chains on the outside, through which the produced metal nanocrystals will gain hydrophobic surfaces. A spontaneous phase-separation process then occurred because of the weight of the metal nanocrystals and the incompatibility between the hydrophobic surfaces and their hydrophilic surroundings, and the noble metal nanocrystals can be easily collected at the bottom of the container. This LSS phase transfer and separation process can generate nanocrystals with a variety of properties such as, semiconducting, uorescent, magnetic and dielectric. The phase transfer process can occur for nearly all the transitional or main group metal ions, which gives exibility to the reactions at the interfaces (see Supplementary Information Part IV). After the phase transfer process of the metal ions from aqueous solution to the solid phase of (RCOO)nM, under designated reaction conditions, the Mn dehydrates into oxides (to yield for example, TiO2, CuO, ZrO2, SnO2 or ZnO) and/or composite oxides (to yield for example, MFe2O4 (M represents Fe, Co, Mg, Zn or Mn) and MTiO3 (M represents Ba or Sr) through co-precipitation). Alternatively, Mn might react with other anion species such as S22 (S22 was supplied by Na2S or (NH4)2S, to yield for example CdS, MnS, PbS, Ag2S, CuS or ZnS), Se22 (Se22 was generated by the reduction of SeO322 by N2H4, to yield for example CdSe or ZnSe) or F2 (F2 was provided from NaF or NH4F, to yield for example YF3, LaF3 orNaYF4) to yield various functional nanocrystals. Nearly all the bandgap semiconductors can be effectively prepared through this simple LSS phase transfer and separation method, such

Department of Chemistry, Tsinghua University, 2National Center for Nanoscience and Nanotechnology, Beijing, 100084 China.

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as TiO2, CuO, ZrO2, SnO2, CdS, Ag2S, ZnS, PbS, MnS, ZnSe and CdSe. Representative TEM images of typical semiconductors of Ag2S, PbS, ZnSe, CdSe and TiO2 show the successful synthesis of various uniform semiconductor nanocrystals through this LSS approach (Fig. 1b, c). Two-dimensional assembly of PbS, Ag2S, CuO, ZnSe and CdSe nanocrystals occurred spontaneously on the copper TEM grids after the evaporation of the solvents, indicating the regular shapes and narrow size distributions of these nanocrystals. Similar to the synthesis of noble metal nanocrystals, the size of the semiconductor nanocrystals can be tuned through several factors including temperature, mole ratio and the length of alkyl chains (see Supplementary Information Part III), however, for the synthesis of selenides, the temperature was controlled above 120 8C to ensure the complete reduction of SeO22 by N2H4 (Fig. 2b). As mentioned 3 above, the phase transfer process and control of the reactions at the different interfaces enabled the monodispersity and variability of the semiconductor nanocrystals obtained. By adopting bi-metal precursors in a certain mole ratio, composite oxide nanocrystals such as magnetic MFe2O4 (M represents Fe, Co, Mg, Zn or Mn) and dielectric MTiO3 (M represents Ba or Sr) can be effectively prepared through co-precipitation reactions following this

LSS phase transfer and separation method. Uniform nanocrystals of magnetic spinel MFe2O4 could be prepared through the coprecipitation of Fe2 ions and Fe3, Co2, Mg2, Mn2 and Zn2 ions. As shown in Fig. 1c, magnetic nanocrystals of Fe3O4 and CoFe2O4 with diameters ,10 nm formed 2D patterns on the TEM grids and showed good uniformity, which will be useful in biological labelling elds. In a similar way, the reaction between Ti4 and Ba2 and/or Sr2 under strong alkali conditions can be used to prepare uniform nanocrystals of BaTiO3 and SrTiO3. Typical TEM and XRD analyses are shown in Fig. 1c and Fig. 2c, respectively, and show the formation of uniform nanocrystals of tetragonal BaTiO3 (JCPDS 74-1960) with diameters ,17 nm. Our LSS phase transfer and separation approach can also be used in generating nearly monodisperse rare earth uorescent nanocrystals with up-conversion or down-conversion emission properties. These nanocrystals can also be prepared by tuning the reaction at the interfaces of the different phases. For example, after the phase transfer process of the rare earth ions, the reaction between NaF and the (RCOO)nLn generates LnF3 (NaYF4 in the case of Y; the reaction between NH4F and Y3 yields YF3), whereas the reaction between OH2 and (RCOO)nLn generates Ln(OH)3 nanocrystals (Fig. 2d).

Figure 1 | TEM images of nanocrystals. a, Noble metal nanocrystals: Ag (6.1 ^ 0.3 nm; 90 8C), Au (7.1 ^ 0.5 nm; 50 8C), Rh (2.2 ^ 0.1 nm; 120 8C) and Ir (1.7 ^ 0.09 nm; 120 8C). b, Semiconductor nanocrystals: Ag2S (7.3 ^ 0.4 nm; 120 8C; Ag:S22, 2:1;), PbS (5.7 ^ 0.2 nm; Pb2:S22, 1:1), ZnSe (8.2 ^ 0.9 nm; Zn2:SeO322, 1:1; 180 8C) and CdSe (7.1 ^ 0.8 nm; Cd2:SeO322, 1:1; 180 8C). c, Magnetic and dielectric nanocrystals: Fe3O4 (9.1 ^ 0.8 nm; Fe2:Fe3, 1:2; 160 8C), CoFe2O4 (11.5 ^ 0.6 nm; Co2:Fe2, 1:2; 180 8C), BaTiO3 (16.8 ^ 1.7 nm; 7 g NaOH for 0.5 g Ba(NO3)2 and equal amount of TiCl3 (in mole ratio); 180 8C; Ti3 have been adopted as Ti
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sources because of the relative stability of Ti3 to Ti4 under aqueous conditions, which will be oxidized into Ti4 under hydrothermal conditions) and TiO2 (4.3 ^ 0.2 nm; 1 ml 30% TiCl3 solution for 40 ml vessel; 180 8C). d, Rare earth uorescence nanocrystals: NaYF4 (10.5 ^ 0.7 nm; NaF:Y3, 4:1, 180 8C), YF3 (NH4F:Y3, 3:1, 180 8C), LaF3 (8.0 ^ 0.3 nm; F2:La3, 3:1, 180 8C), YbF3(9.5 ^ 0.6 nm; F2:Yb3, 3:1, 180 8C). e, TEM images of Ca10(PO4)6(OH)2, Ppy (4.2 ^ 0.5 nm), PAn (3.3 ^ 0.5 nm) and copper phthalocyanine (0.8 ^ 0.1 nm) nanocrystals.

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for wide research areas including biocompatible materials, organic optoelectronic semiconductors, nanomedicine as well as conducting polymers. For example, the monodisperse biocompatible hydroxyapatite (Ca10(PO4)6(OH)2) nanorods (Fig. 1e, bottom left), which as a raw material may nd an application in the preparation of articial bone grafts because of its uniformity and nanometre size. Also, the typical conducting polymer nanocrystals of PPy and PAn (Fig. 1e, right), may provide an ideal model for the investigation of nano-size effects in the conducting polymer eld. Finally, metal (copper in this case) phthalocyanine nanocrystals (Fig. 1e, top left) obtained following this LSS procedure, can be further investigated as an optoelectronic nanomaterial. Therefore, by properly tuning the chemical reactions at the interfaces, we believe that more interesting and important new-type nanocrystals can be obtained. All the nanocrystals (such as noble metal, magnetic, dielectric, semiconducting and rare earth uorescence), and other new type (such as, monodisperse biomedical, organic optoelectronic semiconductors and conducting polymers) nanoparticles, can be easily dispersed in nonpolar solvents (such as, cyclohexane or chloroform) to form homogenous colloidal solutions (Fig. 4), which are usually stable for months. By dropping the solution on the surface of a
Figure 2 | XRD patterns of nanocrystals. a, Noble metal nanocrystals: Au, Ag, Rh and Ir. b, Semiconductor nanocrystals: monoclinic Ag2S (JCPDS 14-0072), face-centred cubic PbS (JCPDS 5-592), ZnSe (JCPDS 80-21) and face-centred cubic CdS (JCPDS 75-0581). c, Magnetic and dielectric nanocrystals: Fe3O4 (JCPDS 76-1849), CoFe2O4 (JCPDS 79-1744), tetragonal BaTiO3 (JCPDS 74-1960) and ZrO2. d, Rare earth uorescence nanocrytals: NaYF4 (JCPDS 77-2042), YF3 (JCPDS 74-911), LaF3 (JCPDS 72-1435) and La(OH)3 (JCPDS 36-1481).

Figure 1d shows the TEM gures of NaYF4, YF3, LaF3 and YbF3 nanocrystals. NaYF4, YbF3 and LaF3 are approximately round, with a diameter in the range of 4–12 nm (that varies with temperature), whereas YF3 is characterized as having a rice-like shape with a diameter ,100 nm and length ,500 nm (composed of uniform nanocrystals with a diameter ,5 nm). Ln(OH)3 products are usually composed of uniform nanorods with a diameter 3–15 nm (that varies with temperature). By doping different rare earth ions such as Eu3, Tb3 or Yb/Er these nanocrystals were functional as uorescence nanocrystals (see Supplementary Information part V). Along with the series of functional nanocrystals mentioned above, this LSS strategy also shows great potential in the synthesis of a broad range of new-type nanocrystals and/or nanoparticles (see Supplementary Information part VI). This will provide new materials

Figure 3 | Scheme of liquid–solid–solution (LSS) phase transfer synthetic strategy.

Figure 4 | Cyclohexane solutions of nanoparticles with a typical concentration of 2%. a–d, Cyclohexane solutions of noble metal (a), semiconductors (b), rare earth uorescence (c) and magnetic nanocrystals (d). d, The separation of CoFe2O4 nanocrystals from the bulky solution by magnetic force.
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substrate or through a dip-coating technique, monolayer lms of functional nanocrystals could be easily obtained, which will greatly increase their application in nanoscience and technology. These nanocrystals could also be re-precipitated and separated by adding an appropriate amount of ethanol to the bulky nanocrystals solutions or by force due to an external eld and show advantages in processing. All these disperse/separation characteristics of the functional nanocrystals obtained through this LSS approach will provide the building blocks for the bottom-up approach to nanoscale fabrication in nanosciences and nanotechnologies.
Received 18 May; accepted 23 June 2005.
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Supplementary Information is linked to the online version of the paper at www.nature.com/nature. Acknowledgements This work was supported by NSFC, the Foundation for the Author of National Excellent Doctoral Dissertation of China and the State Key Project of Fundamental Research for Nanomaterials and Nanostructures. Author Information Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing nancial interests. Correspondence and requests for materials should be addressed to L.Y. (ydli@tsinghua.edu.cn).

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