CRYSTAL GROWTH & DESIGN
Shape-Controlled Synthesis and Assembly of Copper Sulfide Nanoparticles
2008 VOL. 8, NO. 6 2032–2035
Xu-Sheng Du,† Maosong Mo,† Rongkun Zheng,‡ Szu-Hui Lim,† Yuezhong Meng,§ and Yiu-Wing Mai*,† Centre for AdVanced Materials Technology (CAMT), School of Aerospace Mechanical and Mechatronic Engineering J07, UniVersity of Sydney, NSW 2006, Australia, Australian Key Centre for Microanalysis and Microscopy, UniVersity of Sydney, NSW 2006, Australia, and State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics & Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, China ReceiVed NoVember 21, 2007; ReVised Manuscript ReceiVed March 6, 2008
ABSTRACT: Cu2S nanocrystals and superlattices were synthesized by a simple solution-phase thermolysis of the copper dodecanethiolate polymer precursor. X-ray diffraction (XRD), selected area electron diffraction (SAED), and high resolution transmission electron microscopy (HRTEM) confirmed the chalcocite crystal structure of the nanocrystals. Monodispersed Cu2S nanodisks, spheres and hexagon nanoplates could be fabricated with this method. Further, copper sulfide nanoarrays assembled by nanoparticles of different shapes were obtained by adjusting the experimental conditions. The growth of the nanocrystals and their self-assembly structures were studied and believed to be associated with the lamellar structure of the copper thiolate precursor. Introduction Recently, the synthesis and assembly of nanocrystals have attracted much attention because these mesostructures present new physical properties.1 The technologically useful nanomaterials and their applications depend not only on the quality of the nanoparticles (e.g., size and shape) but also on their assembly structure (spatial orientation and arrangement).2 So developing new methodology for tailoring the structure of materials with novel assembly structure becomes more and more interesting in advanced materials science. It is well-known that Cu2S is a functional material for its semiconducting and photovoltaic capabilities, and it has been extensively investigated and applied in solar cells, field emission, nanoscale electronic devices and sensor fields.3 Research on the synthesis of organized arrays of Cu2S nanocrystals have been reported in recent years. Nanowire arrays of Cu2S were grown on Cu foil substrates.4 The nanowire-, nanotube- and nanovesiclelike copper sulfide assembly structures were prepared by an organic amine-assisted hydrothermal method.5 Qian et al. synthesized various metal sulfide nanocrystal superlattices through the reaction between metal salts and thioacetamide.6 Also, the superlattices assembly by Cu2S hexagon nanoplates was obtained with copper salt and sulfur in oleylamine.7 The approach of solventless thermolysis of the copper thiolate precursor was recently applied to prepare Cu2S nanoplates by Korgel’s group, and nanoplate assembly into a chainlike superstructure was observed and studied.8 Our previous studies on Cu2S revealed that the uniform nanodisks, spheres, hexagon plates and its smectic ordered assembly structure could be obtained by carefully adjusting the experimental parameters.9 However, there are two thiolates (monothiolate and dithiolate) in the reaction system, and it is difficult to determine which should be used as the primary capping ligand to control the shape and assembly of the resulting Cu2S nanoparticles. * Corresponding author. Fax: +61-20-93513760. E-mail:
[email protected]. † Centre for Advanced Materials Technology (CAMT), School of Aerospace Mechanical and Mechatronic Engineering J07, University of Sydney. ‡ Australian Key Centre for Microanalysis and Microscopy, University of Sydney. § Sun Yat-Sen University.
Therefore, it is necessary to study the case of using a single thiolate as precursor. To our surprise, both long ordered superstructure assembly with Cu2S nanodisks and hexagon platelets were observed under such circumstances. In this paper, we will give details of the synthesis of such interesting assembly structures and discuss the corresponding formation mechanisms. Experiment Section Materials. In a typical synthesis procedure, 15 mL of dodecanethiol (Aldrich 98%) was added slowly to a 50 mL ethanol solution containing 4 g of CuCl2. During addition, the system becomes a slurry caused by thiolate precipitation. After being stirred slowly for 2 h, the metal thiolates were separated by filtration and washed several times with ethanol. The material was then dried under vacuum at 70 °C for 24 h, and 13.5 g of waxy powder was obtained. The reaction yield was almost 100% because of the low thiolate solubility in the alcoholic medium. The copper thiolate precursor was added to dodecanethiol and heated in an oil bath at 200 °C for 0.5 h. After the reaction, ethanol was added and the brown solid product was collected and washed with ethanol to remove the excess dodecanethiol. Characterization. X-ray diffraction (XRD) analysis was performed using a Siemens D5000 X-ray diffractometer with Ni-filtered Cu KR radiation (λ ) 1.54 Å). The scans were conducted at ambient temperature with 2θ varying from 2° to 60°. Transmission electron microscope (TEM) images were recorded on a Philips CM12 instrument at an accelerating voltage of 120 kV. High-resolution TEM images were also obtained and selected-area electron diffraction patterns performed with a JEOL 3000F TEM at an acceleration voltage of 300 kV. Images were acquired digitally using a Gatan multipole scanning CCD camera with an iTEM imaging software. TEM samples were prepared by drop casting nanocrystals dispersed in ethanol onto TEM grids. Differential scanning calorimetry (DSC) data were taken with a TA modulated DSC 2920 instrument in a nitrogen atmosphere. Results and Discussion Typical TEM images of the product are shown in Figure 1a, in which the nanorod crystals are seen to self-assemble into an
10.1021/cg701145q CCC: $40.75 2008 American Chemical Society Published on Web 05/16/2008
Copper Sulfide Nanoparticles
Crystal Growth & Design, Vol. 8, No. 6, 2008 2033
Figure 2. Low- and high-magnification TEM images of Cu2S nanocrystals formed at 200 °C for (a, b) 0.5 h and (c, d) 1 h.
Figure 1. (a-d) Typical TEM images of self-assembled Cu2S nanoplates and images with different tilt angle (b) -15°, (c) 0°, (d) +15°. (e) Proposed model for self-assembly of Cu2S nanoplates.
ordered column structure with a length of 2 µm and a diameter of 200 nm. In addition to the intact superlattices, broken columns with interrupted ordered structures are also observed (see Supporting Information, S1a), in which the assembly structure of the crystals in the broken parts still remains. By tilting TEM analysis of the broken parts of the assembly structure (see Supporting Information, S1a,b), it is found that the rodlike particles are actually nanoplatelets. From the high magnification image (see inset in Figure 1a), the particles are really hexagon nanoplates with an edge length of 20 nm. Obviously, the size of the hexagon nanoplates is much larger than that prepared by reaction between copper salt and sulfur,7 where the hexagon nanoplate has an edge length only ∼9 nm. Also, the assembly mode is different as all superlattices we obtained are wormlike and the stacking direction of the nanoparticles is parallel to the substrate. It is clear that the assembly structure in the TEM images is multilayered as seen from different contrast among the nanoparticles in the ordered stacks, which is further confirmed by TEM tilting experiments. Figure 1(b-d) shows a series of TEM images of an array of nanoplatelets stacked side by side (corresponding to the end portion of the nanorod marked in Figure 1a) taken after tilting by -15°, 0°, and +15°. When the tilt angle is changed from -15° (Figure 1b) to 0° (Figure 1c), the length of the platelets in the rectangle area is decreased and becomes almost negligible when the tilt angle is further increased to +15° (Figure 1d). These results confirm the multilayered columnar assembly structure of the nanocrystals, and the proposed model of the self-assembly structure is shown in Figure 1e. Although previous studies demonstrated that nanoparticles can organize themselves into a variety of ordered structures including chains,8 rings,10a tubes and vesicles,5 and even free-floating sheets,10b such long wormlike structure has only been observed in a few cases. Further, all our microscopic observations have shown that the columnar assembly structure
is a common phenomenon in the product, as shown in Figure 2. By increasing the reaction time to 1 h, slightly larger nanoplates are obtained without changing the self-assembly structure (Figure 2 c,d). It is noted that the thermolysis temperature has a major effect on the fabrication and morphology of the Cu2S nanocrystals. The waxy copper thiolate precursor is difficult to dissolve in the solvent at room temperature due to its macromolecular structure, but can easily do so at elevated temperature (above 120 °C in the experiments) and became a yellow clear solution. With increasing heating time, the solution varies from a transparent yellow, to deep orange, and then brown slurry. At 200 °C, the slurry appears after only about 0.2 h, while this happens after 0.3 h at 180 °C, indicating that higher temperature accelerates the thermolysis reaction. TEM images shown in Figure 3 illustrate how the morphology evolves with increased reaction time at 180 °C. Small particles start to form after 0.3 h (see Supporting Information, S2). After 1 h, the nanocrystals become disklike, and they tend to stack together into extended chains and self-assemble into columnar structure with a diameter of about 100 nm (Figure 3a). Similar to the assembly columns formed at 200 °C, broken nanocolumns can also be observed. From the particles lying flat and those standing on their edges on the substrate (see high magnification images in inset of Figure 3a) it is clearly shown that the nanocrystals are really nanodisks with 4–6 nm thickness and 8–12 nm diameter and fairly uniform in shape. At longer reaction time (4 h), the disks change into monodispersed spherical particles (Figure 3b, c). When the reaction time is longer than 6 h, the products are mainly spherical particles with a broad size distribution (Figure 3d, e), indicating that an Ostwald ripening process has occurred. However, the columnar assembly structure is roughly retained despite the change of the nanoparticle shape. HRTEM was used to examine the crystal structure of the nanodisks and hexagon nanoplates obtained. Figure 4a shows corresponding lattice fringes with a measured interfringe distance of 3.4 Å, which is attributed to the lattice spacing of the (002) planes of chalcocite Cu2S fabricated at 180 °C. The lattice fringes with an interfringe distance of 2.0 Å in Figure 4b are due to its (110) planes. HRTEM images of the hexagon nanoplates produced at 200 °C (Figure 4c) show clearly the lattice spacing of the (002) planes of chalcocite Cu2S, indicating the same crystal structure of the products prepared at different temperature. Moreover, the respective rings in the SAED pattern
2034 Crystal Growth & Design, Vol. 8, No. 6, 2008
Du et al.
Figure 4. (a-c) HRTEM images of Cu2S nanocrystals standing on their edges perpendicular to the substrate with the electron beam perpendicular to the (001) direction, and (d) SAED pattern of Cu2S nanocrystals.
Figure 3. Low- and high-magnification TEM images of samples formed at 180 °C for (a)1 h, (b, c) 4 h, and (d, e) 6 h.
(Figure 4d) can also be indexed as hexagonal phase Cu2S (JCPDS card, No. 84-0207). All these results confirm that the nanocrystals are chalcocite copper sulfide, which is the same as those prepared by solventless thermolysis of copper thiolate in the presence of octanoate.8 XRD was further used to examine the as-prepared samples. The sharp diffraction peaks seen at low 2θ (
