Controlled synthesis of copper sulfide 3D nanoarchitectures through a facile hydrothermal route

Journal of Alloys and Compounds 492 (2010) L44–L49

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Letter

Controlled synthesis of copper sulfide 3D nanoarchitectures through a facile hydrothermal route Zhiguo Cheng, Shaozhen Wang, Dajie Si, Baoyou Geng ∗ College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Laboratory of Molecular-Based Materials, Anhui Normal University, Wuhu 241000, PR China

a r t i c l e

i n f o

Article history: Received 21 October 2009 Received in revised form 17 November 2009 Accepted 18 November 2009 Available online 24 November 2009 Keywords: Hydrothermal method Hexagonal nanoplates Nanoarchitectures Metal sulfides

a b s t r a c t CuS three-dimensional nanoarchitectures have been successfully prepared in the presence of surfactant Poly(N-vinyl-2-pyrrolidone) by hydrothermally treating the solution of CuSO4 ·5H2 O and CS2 at 140 ◦ C for 18 h. X-ray diffraction, scanning electron microscope, transmission electron microscope, and ultraviolet–visible spectrophotometer were used to characterize the CuS. The assembled threedimensional nanoarchitecture, with a diameter of about 100 nm, was composed of hexagonal CuS nanoplates with a thickness of 20 nm. The influences of synthetic parameters, such as Poly(N-vinyl2-pyrrolidone), reaction temperature and reaction time, on the morphologies of the products have been investigated. The possible mechanism of forming three-dimensional flower-like nanoarchitecture was discussed. © 2009 Elsevier B.V. All rights reserved.

1. Introduction In recent years, materials with three-dimensional (3D) architectures have attracted much attention due to their attractive chemical and physical properties [1]. Up to date, a wide variety of 3D architectures materials, including metal sulfide [2], metal oxide [3], and other inorganic materials [4,5], have been successfully prepared with optical and electrical properties. However, it is still a great challenge to control the construction of 3D architectures from nanobuilding blocks via chemical methods because the control of nucleation and growth process of nanomaterials is difficult [6]. Recently, self-assembly has become an important technique for exquisite fabrication of sophisticated architectures [7,8]. Self-assembled nanoarchitectures have been attracting growing interest because of their highly specific morphologies, novel properties, and great application potential in many fields [8]. Therefore, further research is warranted to realize the advantages of this technique. Metal sulfide, such as CdS, ZnS, Bi2 S3 , and CuS, has been widely studied, owing to their excellent physical and chemical properties [9–12]. Among them, copper monosulfide (CuS) as an important semiconductor has been the focus of intense interest because of its excellent optical, electronic, and other physical and chemical properties [13]. It has great potential application in many fields,

∗ Corresponding author. Tel.: +86 553 3869303; fax: +86 553 3869303. E-mail address: [email protected] (B. Geng). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.132

such as gas sensors, photocatalysts, and as nonlinear optical material, and so on [14–17]. Copper sulfide has been synthesized via different approaches, for example, wet chemical methods, solution methods, hydrothermal or solvothermal methods, and thermolysis methods [18–21]. Recently, many efforts have been devoted to the synthesis of CuS plate-based nanoarchitectures. For example, Yan et al. [22] have synthesized CuS nanoarchitectures composed of intersectional nanoplates with an average edge length of ca. 350 nm by a micro-interface reaction method. Yu’s group [23] has prepared well-defined concaved cuboctahedrons of CuS crystals composed of hexagonal plates with an edge length of 1–1.5 ␮m and a thickness of ca. 200 nm by a solution reaction in ethylene glycol (EG). Li et al. [24] reported solvothermal synthesis of CuS architectures which were composed of intersectional hexagonal nanoplates with a mean edge length of ca. 1 ␮m and an average thickness of ca. 100 nm. Shen et al. [25] have fabricated flower-like CuS microspheres with a diameter of about 2–3 ␮m composed of CuS nanoflakes with a thickness of about 200 nm via the solvothermal treatment of a single-source precursor of Cu(S2 CNEt2 )2 . Although various CuS micro-architectures composed of hexagonal nanoplates have been synthesized in solution-based processes, there have rarely been reports on the selectively controllable synthesis of CuS nanoarchitectures. Hence, designing and developing new and simple solution-based methods to synthesize CuS nanoarchitecture and other similar semiconductors is still a great challenge at present. In this paper, we report on the simple method for the large-scale synthesis of CuS 3D flower-like nanoarchitectures composed of CuS nanoplates via the hydrothermal treatment of CuSO4 ·5H2 O and

Z. Cheng et al. / Journal of Alloys and Compounds 492 (2010) L44–L49

L45

CS2 at 140 ◦ C for 18 h, and the influence of the surfactant, reaction temperature and reaction time on the formation of CuS nanoarchitecture is discussed. On the basis of the evolution of the structure and the morphology with increasing reaction time, a growth mechanism of the 3D flower-like CuS nanoarchitecture is proposed. Our work may shed light on the designed fabrication of complex 3D architectures of other materials. 2. Experimental 2.1. Materials Copper sulfate pentahydrate [CuSO4 ·5H2 O, analytic reagent, 95%], carbon disulfide [CS2 , analytic reagent], Poly(N-vinyl-2-pyrrolidone) [PVP, technical grade], sodium hydroxide [NaOH, analytic reagent, >93%], and ethanol [CH3 CH2 OH, analytic reagent, >99.9%] were used as received without further purification. 2.2. Synthesis In a typical procedure, 0.25 g of CuSO4 ·5H2 O and 0.20 g of PVP were dissolved in 25 mL of deionized water. Then, 2.0 mL of 0.50 M NaOH and 1.20 mL CS2 solution was added dropwise into the above-mentioned solution under constant stirring to form blue slurry. After 10 min, the reaction mixture was transferred into a Teflon-lined stainless steel autoclave of 35 mL capacity. The autoclave was sealed and maintained at 140 ◦ C for 18 h, and then allowed to cool to room temperature naturally. Finally the as-formed precipitate was centrifuged, washed sequentially with deionized water and absolute ethanol, dried at 60 ◦ C for 6 h in vacuum. 2.3. Characterization The morphology and composition of the product were characterized by a (Hitachi S-4800 FE-SEM) field-emission scanning electron microscope operated at an acceleration voltage of 5.0 kV and equipped with an energy-dispersive X-ray spectroscopy (EDS). X-ray powder diffraction (XRD) was carried out on an XRD-6000 (Japan) X-ray diffractometer with Cu K␣ radiation ( = 1.54060 Å) at a scanning rate of 0.05◦ s−1 . Transmission electron microscopy (TEM) was performed using JEM 2010 F microscopes operated at optimum defocus with accelerating voltages of 200 kV. An

Fig. 1. XRD pattern of the as-prepared CuS 3D nanoarchitectures obtained at 140 ◦ C for 18 h.

ultraviolet–visible (UV–vis) spectrophotometer (U-3010 Spectrophotometer) was used to carry out the optical measurements of sample dispersed in distilled water.

3. Results and discussion Fig. 1 shows the XRD pattern of the as-prepared sample of CuS 3D flower-like nanoarchitecture. The diffraction peaks can be indexed to the hexagonal CuS with a = 3.792 Å and c = 16.344 Å, which are in agreement with the reported data for CuS (JCPDS No. 06-0464). No characteristic XRD peaks arising from possible impu-

Fig. 2. SEM, TEM images, and EDS spectrum of the as-prepared CuS 3D nanoarchitectures prepared at 140 ◦ C for 18 h. SEM images: (a) low-magnification SEM image; (b) high-magnification SEM image; (c) TEM image; (d) EDS spectrum.

L46

Z. Cheng et al. / Journal of Alloys and Compounds 492 (2010) L44–L49

rities, such as CuO, Cu2 S and CuCl2 , are detected, indicating that pure CuS is produced under the experimental conditions. The morphologies of CuS samples are investigated by FE-SEM. A low-magnification image of CuS samples prepared at 140 ◦ C for 18 h is shown in Fig. 2a, which clearly exhibits that the samples are well dispersed CuS flower-like nanoarchitecture. High-magnification image in Fig. 2b indicates that the CuS flower-like nanoarchitectures are composed of intersectional hexagonal nanoplates with a mean edge length of ca. 100 nm and an average thickness of ca. 20 nm. Among these CuS flower-like nanoarchitectures, some are constructed by only two perpendicular hexagonal nanoplates, and some are made up of several similar hexagonal nanoplates. These CuS nanoarchitectures can be described as the combination of several similar hexagonal nanoplates. The hexagonal shape may be related to the hexagonal phase of CuS. To further reveal the microstructure of the nanoplates, TEM images are recorded as shown in Fig. 2c. The average edge length of ca. 100 nm of the hexagonal nanoplates was confirmed, which agrees well with those measured from the SEM images. The dark stripes present in the TEM images are the corresponding inter-growing hexagonal nanoplates, which are perpendicular to the base plate. A typical EDX spectrum is shown in Fig. 2d, which indicates the presence of Cu and S in the products. Based on the calculation of the peak areas, the atomic ratio of Cu/S was about 1.06:1, nearly consisting with the stoichiometry of CuS. The effect of the reaction conditions on the final products was systematically investigated. It is found that CuS flower-like nanoarchitectures can only be obtained under proper experimental conditions. Fig. 3 shows the SEM images and XRD patterns of the products prepared under different reaction temperatures. The results show that the reaction temperature plays an important role

in fabricating these CuS nanoarchitectures. When the reaction happened at 100 ◦ C, only CuS hexagonal nanoplates were obtained, as is clearly shown in Fig. 3a. At this low temperature (100 ◦ C), the nanoplates could not be converted into flowers. When the temperature increased to 120 ◦ C, the nanoplates began to intersect each other to form this novel CuS flower-like nanoarchitecture as shown in Fig. 3b. Fig. 3c shows the images of the final products at 140 ◦ C for 18 h. It is found that almost all the nanoplates assemble into flowerlike nanoarchitectures. So, increasing temperature is favorable for the formation of the flower-like nanoarchitectures. It is clearly seen from XRD patterns shown in Fig. 3d that all samples prepared under different reaction temperatures are the covellite CuS. In order to understand the formation process of the CuS flower-like nanoarchitectures, we carried out time-dependent experiments during which samples were collected at different time intervals. As shown in Fig. 4a, at the early stage, the sample was composed of nanoparticles, trigonal nanoplates and a small quantity of hexagonal nanoplates. After an additional 3 h of reaction, a mass of CuS hexagonal nanoplates are formed, as illustrated in Fig. 4b. As the reaction proceeded (Fig. 4c and d), almost all CuS samples are in the morphology of hexagonal nanoplates with the size consistent. At the same time, A small quantity of nanoplates will meet with each other in such modes as one perpendicular to another, one intersecting with two, or three, et al. Eventually no single nanoplates remained and the sample was composed entirely of the 3D flower-like nanoarchitectures, as shown in Fig. 2. On the basis of the SEM and TEM observations, along with crystal structure and chemical composition analyses, it can be concluded that the formation of such CuS 3D nanoarchitectures are achieved via a self-assembly process. The possible formation and evolution of such CuS flower-like nanoarchitectures could be described as

Fig. 3. SEM images and XRD patterns of products prepared for 18 h at different reaction temperatures. SEM images: a: 100 ◦ C, b: 120 ◦ C, c: 140 ◦ C, d: XRD patterns of the samples obtained at I: 100 ◦ C, II: 120 ◦ C, III: 140 ◦ C.

Z. Cheng et al. / Journal of Alloys and Compounds 492 (2010) L44–L49

L47

Fig. 4. SEM images of the products recorded at various reaction times. a: 3 h; b: 6 h; c: 9 h; d: 12 h.

shown in Scheme 1. In our study, the formation of CuS nanoplates was a typical Ostwald ripening process. At first, the generation of tiny crystalline nuclei in a supersaturated solution occurred and then followed by crystal growth. The further crystal growth for the formation of CuS nanoplates was strongly related to the hexagonal crystal structure of CuS. After 3 h of reaction, CuS hexagonal nanoplates were attained. In the subsequent self-assembly process, PVP played an important role in the formation of integrated nanoplates and flower-like nanoarchitectures. The surfactant protective layer limits the size of the nanoparticles and protects them from further aggregation. Very recently, PVP has been applied as an important surfactant for the synthesis of nanomaterials, and various nanoarchitectures were successfully fabricated with the assistance of PVP [26,27]. Our experimental results indicated that PVP as a capping reagent, the hydrophilic polymer, played an

Scheme 1. Schematic illustration of the growth process of CuS 3D nanoarchitecture.

important role in the process of assembling nanoplates into flowerlike nanoarchitectures. Fig. 5 shows the morphologies of the products synthesized at 140 ◦ C in the presence of PVP aqueous solutions with different amounts. When there was no PVP used, the tiny crystalline nuclei were random aggregation, therefore, formless nanoarchitectures were formed and hexagonal nanoplates could not be detected (Fig. 5a). With PVP, the polymeric molecules would adsorb preferentially on the nuclei surface to inhibit aggregation by steric hindrance mechanism. When the amount of PVP is low (Fig. 5b), the nanoplates cannot be uniformly dispersed with different crystalline diameters which confirms the illustration of the capping mechanism of PVP in the growth process of CuS flower-like nanoarchitectures. The images indicate that the products formed under the amount of PVP (0.20 g) are composed of homogeneous flower-like nanoarchitectures with narrow diameter distributions of about 90–110 nm. These flower-like nanoarchitectures composed of hexagonal nanoplates are dispersed with good dispersity (Fig. 5c). With an increase of the amount of PVP to 0.40 g, flowerlike nanoarchitectures were still obtained (Fig. 5d), indicating that the capping behavior of PVP molecules to CuS crystallographic faces did not change and the higher PVP concentration was not necessary. These results directly confirmed that appropriate amount of PVP could promote the delicate assembly. The XRD results of the products obtained by using PVP are the same to Fig. 3d, from which a pure hexagonal phase of CuS can be identified. UV–vis spectrophotometry was used to characterize the absorption properties of the obtained samples. Fig. 6 shows the UV–vis absorption spectrum of the products obtained at 140 ◦ C for 18 h and dispersed in distilled water. Absorption spectra of the other samples obtained at 100 ◦ C and 120 ◦ C are of similar nature (not shown here). The spectrum shows that the samples absorb in the region of 300–500 nm. Compared with the absorption peak of bigger

L48

Z. Cheng et al. / Journal of Alloys and Compounds 492 (2010) L44–L49

Fig. 5. SEM images of the products prepared at 140 ◦ C for 18 h in the presence of PVP aqueous solutions with different amounts. (a) 0 g; (b) 0.10 g; (c) 0.20 g; (d) 0.40 g.

intersectional hexagonal nanoplates with a mean edge length of ca. 100 nm and an average thickness of ca. 20 nm. The effect of the surfactant, reaction temperature and reaction time on the formation of the flower-like CuS nanoarchitectures has been investigated systematically. It is found that PVP play an important role in the formation of flower-like nanoarchitectures. Finally, a possible mechanism for the formation of CuS nanoarchitectures is proposed, and an optical study has also been carried out. Acknowledgments This work was supported by the National Natural Science Foundation of China (20671003, 20971003), the Key Project of Chinese Ministry of Education (209060), the Education Department of Anhui Province (2006KJ006TD) and the Program for Innovative Research Team in Anhui Normal University. Fig. 6. UV–vis absorption spectrum of CuS 3D nanoarchitecture in distilled water.

CuS flakes at 630 nm and 603 nm [28], those of CuS 3D flowerlike nanoarchitectures were blue-shifted and confirmed a smaller crystal size, which could be attributed to the strong quantum confinement of the excitonic transition for flower-like structures [8]. In addition, the spectral nature shows another broadband in the near-IR region, which is the characteristic band for covellite CuS [29,30]. 4. Conclusions In summary, 3D flower-like CuS nanoarchitectures have been prepared through a facile hydrothermal method at a low temperature. The flower-like nanoarchitectures are composed of

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

P. Yu, X. Zhang, D.L. Wang, L. Wang, Y.W. Ma, Cryst. Growth Des. 9 (2009) 528. L. Shi, Y.M. Xu, Q. Li, Cryst. Growth Des. 9 (2009) 2214. L. Shi, K.Y. Bao, J. Cao, Y.T. Qian, CrystEngComm. 11 (2009) 2009. L. Shi, Y.M. Xu, Q. Li, J. Phys. Chem. C 113 (2009) 1795. L. Shi, Y.M. Xu, S.K. Hark, Y. Liu, S. Wang, L.M. Peng, K.W. Wong, Q. Li, Nano Lett. 7 (2007) 3559. H.X. Dong, Z.H. Chen, L.X. Sun, L. Zhou, Y.J. Ling, C.Z. Yu, H.H. Tan, C.P. Jagadish, X.C. Shen, J. Phys. Chem. C 113 (2009) 10511. Z.Y. Huo, C. Chen, Y.D. Li, Chem. Commun. 33 (2006) 3522. L.Y. Chen, Z.D. Zhang, W.Z. Wang, J. Phys. Chem. C 112 (2008) 4117. H.M. Wang, P.F. Fang, Z. Chen, S.J. Wang, J. Alloys Compd. 461 (2008) 418. H.M. Wang, Z. Chen, Q. Cheng, L.X. Yuan, J. Alloys Compd. 478 (2009) 872. Q.F. Han, Y. Sun, X. Wang, L. Chen, X.J. Yang, L.D. Lu, J. Alloys Compd. 481 (2009) 520. G.X. Chen, M.T. Niu, L.F. Cui, F. Bao, L.H. Zhou, Y.S. Wang, J. Phys. Chem. C 113 (2009) 7522. Y. Wan, C.Y. Wu, Y.L. Min, S.H. Yu, Langmuir 23 (2007) 8526.

Z. Cheng et al. / Journal of Alloys and Compounds 492 (2010) L44–L49 [14] X.L. Yu, Y. Wang, H.L.W. Chan, C.B. Cao, Micropor. Mesopor. Mater. 118 (2009) 423. [15] A.A. Sagade, R. Sharma, Sens. Actuators B 133 (2008) 135. [16] T.Y. Ding, M.S. Wang, S.P. Guo, G.C. Guo, J.S. Huang, Mater. Lett. 62 (2008) 4529. [17] A.M. Malyarevich, K.V. Yumashev, N.N. Posnov, V.P. Mikhailov, J. Appl. Phys. 87 (2000) 212. [18] P. Roy, K. Mondal, S.K. Srivastava, Cryst. Growth Des. 8 (2008) 1530. [19] X.S. Du, M.S. Mo, R.K. Zheng, S.H. Lim, Y.Z. Meng, Y.W. Mai, Cryst. Growth Des. 8 (2008) 2032. [20] S.H. Choi, K. An, E.G. Kim, J.H. Yu, J.H. Kim, T. Hyeon, Adv. Funct. Mater. 19 (2009) 1. [21] L.F. Chen, W. Yu, Y. Li, Powder Technol. 191 (2009) 52.

[22] [23] [24] [25] [26] [27] [28] [29] [30]

L49

H.J. Yan, W.Z. Wang, H.L. Xu, J. Cryst. Growth 310 (2008) 2640. C.Y. Wu, S.H. Yu, M.K. Antonietti, Chem. Mater. 18 (2006) 3599. F. Li, W.T. Bi, T. Kong, Q.H. Qin, Cryst. Res. Technol. 44 (2009) 729. X.P. Shen, H. Zhao, H.Q. Shu, H. Zhou, A.H. Yuan, J. Phys. Chem. Solids 70 (2009) 422. J.W. Wang, X. Wang, Q. Peng, Y.D. Li, Inorg. Chem. 43 (2004) 7552. Z.J. Luo, H.M. Li, H.M. Shu, K. Wang, J.X. Xia, Y.S. Yan, Cryst. Growth Des. 8 (2008) 2275. P. Zhang, L. Gao, J. Mater. Chem. 13 (2003) 2007. S.K. Haram, A.R. Mahadeshwar, S.G. Dixit, J. Phys. Chem. 100 (1996) 5868. P. Roy, S.K. Srivastava, Mater. Lett. 61 (2007) 1693.