Size Control of Monodisperse Copper Sulfide Faceted Nanocrystals

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical .... Figure 3 X-ray diffraction patters of the faceted copper sulfides nanocr...
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9658

J. Phys. Chem. C 2007, 111, 9658-9663

Size Control of Monodisperse Copper Sulfide Faceted Nanocrystals and Triangular Nanoplates Wenjing Lou,†,‡ Miao Chen,*,† Xiaobo Wang,† and Weimin Liu† State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, People’s Republic of China, and Graduate School of Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China ReceiVed: January 9, 2007; In Final Form: April 19, 2007

High-quality faceted nanocrystals and triangular nanoplates of copper sulfide have been successfully synthesized by using the single source precursor method in the presence of oleylamine. The obtained faceted copper sulfide nanocrystals have an average size of 9.8 ( 0.3 nm, showing the regularly hexagonal closely packed nanostructure. The resulting triangular copper sulfide nanoplates have an average side length of 12.3 ( 0.6 nm, thickness of 0.8 nm, forming the six-leave flowerlike nanostructure due to their size uniformity. We also demonstrate that the shape, size, and even the phase forms of copper sulfide nanocrystals can be controlled systematically by adjusting certain reaction parameters, such as the carbon number of the substitute alkyl (n), the reaction temperature, and the concentration of the precursor.

Introduction High-quality nanocrystals have been of great interest in both fundamental and technological fields due to their distinguished properties as compared with the corresponding bulk counterparts.1 While many studies have focused on the control of the size of nanoparticles for their quantum-confined properties, the ability to control nanocrystal shape and produce anisotropic structures has attracted much more attention.2 The synthesis of anisotropic nanocrystals such as nanoribbons,3 nanorods,2c,4 nanowires,5 nanotubes,6 nanoplates,7 nanoprisms,8 and isotropic nanocubes9 is of great importance to the applications ranging from chemical and biological sensing, separation, and catalysis to lasers and LEDs.10 In particular, nanoplates and nanoprisms are studied widely since they are superior over spherical nanocrystals as building blocks for constructing nanodevices with crystal orientation controlled by a “bottom-up” method owing to their anisotropic structures.8c,11 Although various chemical methods have been developed to prepare nanostructured materials with different plate-like shapes so far, these methods mainly focus on the fabrication of metal nanostructured materials, such as Ag nanoplates,8c,12 Ag nanodisks,13,14 Au nanodisks,15 and Au nanoplates.16 Transition-metal chalcogenide nanocrystals have been extensively investigated for the potential applications to catalyst, solar cell, photoluminescence, and optical devices. As a typical representative, copper sulfide nanocrystals, which are expected to be a prominent candidate for optical devices due to their high third-order nonlinear optical susceptibility (about 10-7esu), have recently drawn significant interest.17 This chalcocite group (Cu2-xS) involves various compounds such as Cu2S (γ, β-chalcocite), Cu1.96S (djurleite), and Cu1.8S (digenite), which show stoichiometry-dependent optoelectric properties.18 Cu1.96S, Cu1.9S, and Cu1.8S are the direct band gap materials, while Cu2S is the * Corresponding author. E-mail: [email protected] † Lanzhou Institute of Chemical Physics. ‡ Graduate School of Chinese Academy of Sciences.

indirect band gap one. Besides being an excellent semiconductor, CuS exhibits its commercial importance as pigment, catalyzer, colored indicator of nigrosine and so on.19 Until now, nanowires, nanorods, and nanodisks of copper sulfides have been prepared by means of various methods.20 However, there are few reports on the preparation of copper sulfide nanoplates or triangular nanocrystals. Over the past decade, some solution-phase synthetic strategies for the preparation of semiconductor nanoparticles have been employed, including the largely reported synthetic process characterized by high-temperature thermolysis of these airsensitive organometallic compound.21 More recently, several groups have proposed alternative procedures by utilizing single source precursors (SSPs) method for the preparation of semiconductor nanoparticles, which is known as the narrow size distribution and self-assembly structures of nanocrystals.22 It is therefore of great interest to find an easily accessible precursor which would be decomposed under mild condition. In this paper, we present an air-stable precursor copper (II) O,O′-dialkyldithiophosphates (Cu[S2P(OCnH2n+1)2]2), which can meet these requirements above.23 As far as know, the thermal decomposition of metal O,O′-dialkyldithiophosphates has been shown to be an autocatalytic reaction, resulting in olefins, mercaptans, hydrogen sulfide, and polymeric residue, and the temperature of decomposition is dependent on the structure of the alkyl groups and on the size of the metal cation.24 For example, the decomposition temperature of CuC12DTP is as low as 120 °C in the presence of some certain solvents. In this paper, we propose that well-defined copper sulfide nanocrystals can be obtained by thermal decomposition of CuCnDTP in the presence of oleylamine in the temperature range of 120-200 °C, and their size and shape can be controlled by adjusting the reaction conditions. Several reaction parameters, such as the carbon number of substitute alkyls, the reaction temperatures, and the concentration of the precursors, have been also investigated in this paper.

10.1021/jp070166n CCC: $37.00 © 2007 American Chemical Society Published on Web 06/15/2007

Copper Sulfide Nanocrystals and Nanoplates

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Figure 1. TEM images and SAED patterns of copper sulfide nanocrystals (A) synthesized with CuC8DTP and (B) synthesized with CuC12DTP at 140 °C for 5 h.

Experimental Section Materials and Instructions. Analytically pure copper chloride (CuCl2‚5H2O), ammonium O,O′-dialkyldithiophosphates and oleylamine were purchased commercially and were used without further purification. The powder X-ray diffraction (XRD) experiments were measured on a RigaKu D/max-RB diffractometer with Ni-filtered graphite monochromatized Cu KR radiation (λ ) 1.54056 Å) under 40 kV and 30 mA and scanning between 10° and 90° (2θ). Transmission electron microscopy (TEM) photographs were taken on a JEOL JEM 1200 transmission electron microscope with an accelerating voltage of 100 kV, and high-resolution transmission electron microscopy (HRTEM) studies were carried out on a JEOL JEM 2010 transmission electron microscope with an accelerating voltage of 200 kV. Samples were prepared by first dispersing the products in n-hexane by ultrasonication, then placing a drop of the suspension on a copper grid and dried in air for observation. The average diameter and the standard deviation of the prepared nanoparticles were calculated from the TEM images involving at least 50 individual particles. A Digital Instruments Nanoscopy 3A scanning probe microscope (SPM) operated in tapping mode at a 1 Hz scan rate and 256 lines per image was used to acquire atom force microcopy (AFM) images. The hexane solution of these triangular nanocrystals was dropped onto a fresh surface of mica. Synthesis of Single Source Precursors. The metal complexes Cu[S2P(OCnH2n+1)2]2 (n ) 8, 12) were prepared by the reaction

of copper chloride with ammonium O,O’-dialkyldithiophosphates in the mixture solution of water and ethanol in air at room temperature according to the literature.23 Green loose solid of Cu[S2P(OC12H25)2]2 and light-green solid of Cu[S2P(OC8H17)2]2 were obtained, respectively, which both did not disperse in ethanol. Typical Synthetic Process of Copper Sulfide Nanocrystals. A 0.20 mmol sample of Cu[S2P(OC12H25)2]2 and 5 g of oleylamine were added to a glass flask. After shaking, a glaucous transparent solution was achieved. Following, the mixed solution was rapidly heated up to 140 °C by immersing the glass flask into a bath with hot oil and then kept for 5 h in order to ensure the completeness of the reaction. During the process, the color of the solution turned from brown to puce to dark gray finally, indicating the formation of copper sulfides ultimately. After that, the resulting solution was then cooled to room-temperature naturally, and mixed with an excess amount of acetone which was used to precipitate the formed copper sulfide nanomaterials from the resulting solution. The precipitated black flocculant was collected by centrifugation and thoroughly washed with absolute ethanol. The obtained black solid was the product. All manipulations in this preparation process were performed in air. A 0.20 mmol sample of Cu[S2P(OC8H17)2]2 was dissolved in 5 g of oleylamine to form a bottle-green solution. The solution was heated to 140 °C and kept at this temperature for 5 h. The color of the reaction solution also turned to dark gray after

Figure 2. AFM images of triangular copper sulfides nanocrystals corresponding to Figure 1B measured in a 400 nm2 area. The left upper panel is a flattened image; the right upper panel is 3D image. The black line in the flattened image indicates the linear regions where the section analysis shown at the bottom of the flattened image was performed. The arrows indicate platelet-like crystals profiles. The right lower panel is the scheme of the triangular nanocrystals.

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Figure 3. X-ray diffraction patters of the faceted copper sulfides nanocrystals (A) and the triangular copper sulfides nanoplates (B) at 140 °C for 5 h. The * symbol indicates CuS peaks, and the hollow squre symbol indicates Cu9S5 peaks in Figure 2B.

Figure 4. HRTEM images of copper sulfides nanocrystals prepared by thermal decomposition of CuC8DTP at 140 °C for 5 h.

Figure 5. HRTEM images of copper sulfides triangular nanoplates prepared by thermal decomposition of CuC12DTP at 140 °C for 5 h.

heating 1 h, indicating the formation of copper sulfides. The copper sulfide nanocrystals were collected in the same manner as above. Synthesis of Copper Sulfide Nanocrystals under Various Reaction Conditions. A series of samples prepared at various reaction temperature were achieved by heating the solution of 0.20 mmol of Cu[S2P(OC12H25)2]2 and 5 g of oleylamine for 5 h at 120, 140, 160, 180, and 200 °C, respectively. Samples prepared at different precursor concentration were achieved by dissolving 0.40, 0.30, 0.25, and 0.20 mmol of Cu[S2P(OC12H25)2]2 in 5 g of oleylamine respectively, then heating them to 140 °C and keeping them at this temperature about 5 h. When the reaction finished, these resulting solutions were cooled to room-temperature naturally and mixed with an excess amount of acetone. The aimed products were achieved by

centrifugation of the precipitated flocculants and washed with absolute ethanol. Results and Discussion Various shapes of copper sulfide nanocrystals, sometimes as the certain exclusive form, can be readily fabricated by varying the carbon number of substitute alkyls and the reaction temperature. When the substitute alkyl is n-octyl and the reaction temperature is 140 °C, faceted copper sulfide nanocrystals with a uniform size have been obtained and their corresponding TEM image of these nanocrystals (Figure 1A) reveals a self-assembled hexagonal closed-packed array. The average particles size and the standard deviation of these nanocrystals is 9.8 ( 0.3 nm. Their SAED pattern (inserted in Figure 1A) exhibits polycrystalline diffraction rings with many obvious diffraction spots, indicating the high-degree crystallization of the product. While

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Figure 6. TEM images of copper sulfides nanocrystals synthesized from CuC12DTP at the concentration of 0.2 mmol/5 g under the different reaction temperature: (A) 120, (B) 140, (C) 160, (D) 180, and (E) 200 °C for 5 h.

the substitute alkyl is replaced by n-dodecyl, the morphologies of the copper sulfide nanocrystals are almost equilateral triangle (Figure 1B). Their average side length is 12.3 ( 0.6 nm, showing six-leave flowerlike clusters partially (signed with white circles in Figure 1B). Their SAED pattern (inserted in Figure 1B) also exhibits faintish polycrystalline diffraction rings. Since TEM image displays only the projection of the particles onto the observation plane, an immediate question arises: are the particles in form of triangular nanoplates or pyramids? To answer this question, SPM analysis was preformed and the representative results were shown in Figure 2. In order to ensure the reliability of data, a fresh surface of mica was used as the substrate. From the corresponding three-dimensional image in Figure 2, no sharp peaks for pyramids particles are observed and there are only many flat-roofed peaks. According to the section analysis, it can be found out that the average height of these nanocrystals is only 0.8 nm (details can be found in Figure S1 in Support Information). These results indicate that these triangular nanoparticles are nanoplates in nature. It is not difficult to understand the formation of the six-leave flowerlike structures. On the basis of the above test and analysis, we display the schematic illustration of the single triangular nanoplate and their self-assembly structure in Figure 2, too. The XRD patterns of the product prepared by Cu[S2P(OC8H17)2]2 and Cu[S2P(OC12H25)2]2 at 140 °C for about 5 h are shown in panels A and B, respectively, of Figure 3. In Figure 3A, the pattern shows a series of broadening peaks which are typical for nanocrystallites. The diffraction peaks can be completely indexed to the orthorhombic phase of Cu2S (chalcocite, cell constants a ) 13.50 Å, b ) 27.32 Å, c ) 11.85 Å; JCPDS card file No. 23-0961). In Figure 3B, the pattern can be indexed to the rhombohedral structure Cu9S5 (JCPDS card file No. 26-0476) and hexagonal structure CuS (JCPDS card file No. 03-0724). The subtle structures of the faceted nanocrystals and triangular nanoplates can be further confirmed by HRTEM images shown in Figure 4 and 5, respectively. It is clear that the sizes of both copper sulfide nanocrystals are consistent with the analytical results of TEM images above, and each nancrystal has clear

Figure 7. Shape and size of copper sulfides nanocrystals obtained at the same precursor concentration in oleylamine. Effects of n and temperature are illustrated, and typical TEM images of copper sulfide nanocrystals when n ) 8 are shown. The nanorods size is not included in the graph. x ) particle size or side length of triangular plates.

crystal lattice fringes with free from dislocation and stacking faults. In Figure 4, HRTEM images of the faceted nanocrystals reveal they are composed solely of Cu2S, exhibiting lattice planes with spacings of 2.72 and 2.88 Å corresponding to the d spacings of the (044) and (191) planes of Cu2S. From the analysis of the triangular nanoplates’ XRD pattern, we can make a conclusion that they are a mixture with two kinds of copper sulfides, CuS and Cu9S5. The characteristic HRTEM images of the CuS and Cu9S5 triangular nanoplates materials in the product are shown in Figure 5. It can be also seen that the triangular nanoplates in Figure 5 A and B exhibit crystalline domains with lattice spacings of 2.84 Å and 1.97 Å, corresponding to the d

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Figure 8. Powder XRD patterns of copper sulfide triangular nanoplates prepared by thermal decomposition of CuC12DTP at (A-D) 140, 160, 180, and 200 °C, respectively. The bottom line patterns are the standard patterns of the CuS and Cu9S5 pure bulk phases.

spacings of the (103) plane of CuS and the (110) plane of Cu9S5, respectively. In colloidal synthesis, several parameters (e.g., ligands, precursor concentration, and reaction temperature) have been chosen to affect the kinetics and thermodynamics in the nucleation and growth of nanocrystals in order to tailor the nanorystals size and shape. First, the effect of the reaction temperature has been explored. A series of preparation experiments at different reaction temperature were conducted by heating the solution of CuC12DTP or CuC8DTP in oleylamine for around 5 h at 120, 140, 160, 180, and 200 °C, respectively. For the reaction process of CuC12DTP, the transformation of the solution color had two stages: (a) from transparent glaucous to brown and (b) from brown to dark gray. When the reaction temperature was about 120 °C, stage a was carried out slowly keeping about half an hour, and the accomplishment of the stage b would spend much more time. At higher temperature, the two stages were all greatly accelerated. For example, stage a spent about 20 min at 140 °C, 10 min at 160 °C, 3 min at 180 °C, and almost very short at 200 °C, respectively, indicating that the efficient preparation could be obtained under an appropriate

Lou et al. high reaction temperature. TEM photos of the corresponding products are shown in Figure 6. At low temperature (120 °C), the mixtures of spherical and triangular copper sulfides nanocrystals with an average size of 9.8 ( 0.7 nm are obtained while n are 12. Due to the uniformity of the products’ size and shape, they have had the tendency to self-assembling into regularly arrays (Figure 6A). When the reaction temperature increases up to 140 °C, copper sulfides nanocrystals fully transform from sphere to triangle and their average side length increases to 12.3 ( 0.6 nm (Figure 6B). With the increasing of the reaction temperature, up to 160 °C, the average side length of copper sulfide nanoplates continues to increase to 15.6 ( 0.5 nm (Figure 6C). At higher reaction temperature (g180 °C), the copper sulfides nanocrystals keep the shape of triangular with a larger size and a very broad size distribution, and the tendency of self-assembly has disappeared (Figure 6D and E). As for CuC8DTP, self-assembled faceted copper sulfide nanocrystals are obtained at the different reaction temperature (Figure 7A-C and Figure 1A). It is clear that the faceted nanocrystals with the average diameter of 8.9 ( 0.7 nm come forth at 120 °C (Figure 7A). While the reaction temperatures are about 140 °C and 160 °C, both the obtained copper sulfide nanocrystals are spherical with the average sizes of 9.8 ( 0.3 nm and 11.4 ( 0.2 nm, respectively (Figure 1A and 7B). At 180 °C, the mixture of faceted nanocrystals with the average diameter of 13.1 ( 0.5 nm and a few nanorods with aspect ratio of about 4.5 can be achieved (Figure 7C). On the basis of the above analysis, it is clear that the carbon number of the substituted alkyl and the reaction temperature play an important role in the preparation and shape-controlling of copper sulfides nanocrystals. Their effects are illustrated and summarized in Figure 7 and Table S1 in Supporting Information. When n is 8, the shape of nanocrystals gives priority to faceted nanoparticles; when n changes to 12, the shape of nanocrystals shows mainly triangle nanoplates. The sizes of two kinds of copper sulfide nanocrystals equably increase with the increase of the reaction temperature. At high reaction temperature, both the size and shape show a disordered change. When the reaction temperature is too low or too high, it is difficult to acquire the narrow size distribution. As a result, the reaction temperatures ranging from 140 to 160 °C are optimal to obtain the narrow size distribution and self-assembled nanostructures. As a representative example, the evolution of the powder XRD profiles for copper sulfide nanoplates prepared by CuC12DTP at different reaction temperature is shown in Figure 8, and the corresponding standard line patterns of the CuS and Cu9S5 pure bulk phases are also appended in the bottom. Their XRD peak positions for CuS and Cu9S5 can be found in Table S2 in support information. In all cases, the characteristic line broadening peaks point to the nanosized crystal domains. It can be found

Figure 9. TEM images of copper sulfides nanocrystals synthesized from CuC12DTP at 140 °C at the precursor’s concentration of (A) 0.40, (B) 0.30, (C) 0.25, and (D) 0.20 mmol dispersed in 5 g of oleic amine for 5h.

Copper Sulfide Nanocrystals and Nanoplates that the reaction temperatures have great effect on the crystalline forms of the products. When the reaction temperatures are lower (Figure 8A and B), these products both are composed of two kinds of copper sulfides crystalline forms. The intensity of the peaks that belong to the phase of hexagonal structure CuS decrease along with the increase of the reaction temperature. When the reaction temperature increases to 180 °C or 200 °C, the peaks of the CuS have disappeared completely and these products have been composed solely of Cu9S5 (Figure 8C and D). As for CuC8DTP, only one kind of copper sulfides, Cu2S, can be gained in the whole investigated temperature range. Based on the results of these experiments, it can be concluded that a reductive reaction must exist during the process of the solvothermal decomposition of copper complexes, leading to changing Cu(II) into Cu(I). The reductive degree is affected by not only the reaction temperature but also the structure of the precursors. The detailed mechanism of copper sulfide nanocrystals has been carrying through. In addition, the effect of the concentration of the precursor on the shape and the size of the product has been examined. Different products were prepared by thermo-decomposing CuC12DTP/oleylamine solution at the different concentrations, and their corresponding morphologies were observed by means of TEM in Figure 9. It is seen clearly that the average size increases with the decrease of the concentration of the precursor (details are provided in Table S1 in support information). It is reasonable since higher concentration of the precursor can easily generate transient supersaturation in monomers, inducing a nucleation burst at the same reaction temperature, and affording more “growing seeds” during the earlier moment of reaction process which has the coequal chance to grow into large clusters or be depleted by others. It is noted that the concentration of the precursor has no obvious effect on the nanocrystals shape, which are all triangular. Conclusions In conclusion, the uniform copper sulfide faceted nanocrystals and triangular nanoplates have been synthesized by means of the solvothermal decomposition of copper dialkyldithiophosphates under mild conditions. The results of this paper show copper sulfides nanocrystals of predictable size and shape can be obtained by adjusting some critical parameters, such as the carbon number of the substitute alkyl, the reaction temperature and the concentration of the precursor. These findings are valuable since triangular particles are generally difficult to obtain due to their higher energies, compared to spherical particles. The shape anisotropy may be related to significantly different interaction of surfactant molecules with different crystallographic faces. However, it also has a close relationship with the stereostructure of the precursors according to our experiments. Now we are devoting ourselves to studying the mechanism of the decomposition of the precursors in the presence of long alkyl amine and the formation of the triangular nanoplates. We believe that these materials will be used as ideal model systems for the study of size- and shape-dependent structural phase behavior. Acknowledgment. This work is supported by the National Nature Science Foundation of China (Grant No. 20473106), the MinistryofScienceandTechnology(973GrantNo.2007CB607601), and Technology and Innovation Group (Grant No. 50421502). Supporting Information Available: The contents of Supporting Information may include the following: (1) AFM image and their section analyses of copper sulfide nanoplates, (2) Table S1 with the summary of the size and shape of copper

J. Phys. Chem. C, Vol. 111, No. 27, 2007 9663 sulfide nanocrystals obtained under various reaction conditions, and (3) Table S2 of the detailed XRD peak positions for CuS and Cu9S5 in the XRD pattern in Figure 8. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (b) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (c) Michalet, X. Science 2005, 307, 538. (2) (a) EI-Sayed, M. A. Acc. Chem. Res. 2004, 37, 326. (b) Yong, K.; Sahoo, Y.; Choudhury, K. R.; Swihart, M. T.; Minter, J. R.; Prasad, P. N. Nano Lett. 2006, 6, 709. (c) Sigman, M. B.; Ghezelbash, A.; Hanrath, T.; Saunders, A. E.; Lee, F.; Krogel, A. J. Am. Chem. Soc. 2003, 125, 16050. (d) Cozzoli, P. D.; Manna, L.; Curri, M. L.; Kudera, S.; Giannini, C.; Striccoli, M.; Agostiano, A. Chem. Mater. 2005, 17, 1296. (3) (a) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. 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