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Amine-free preparation of SnSe nanosheets with high crystallinity and their lithium storage properties
Colloids and Surfaces A: Physicochem. Eng. Aspects 406 (2012) 1–5
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Amine-free preparation of SnSe nanosheets with high crystallinity and their lithium storage properties Shi-Zhao Kang a , Ladi Jia a , Xiangqing Li a , Yaxia Yin b , Liang Li a , Yu-Guo Guo b , Jin Mu a,∗ a b
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, China Institute of Chemistry, Chinese Academy of Science, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 23 March 2012 Received in revised form 24 April 2012 Accepted 26 April 2012 Available online 4 May 2012 Keywords: SnSe Nanosheet Solvothermal process Anode material Lithium-ion battery
a b s t r a c t In the present work, SnSe nanosheets were prepared via a reaction at the oil–water interface in the solvothermal process at low temperature (130 ◦ C), which did not involve any templates and amines such as oleylamine. X-ray diffraction, transmission electron microscope, absorption spectrum and nitrogen adsorption were used to characterize the products. The results indicated that the SnSe nanosheets obtained adopted a square-like morphology with lateral dimensions of approximately 120 nm × 120 nm and possessed high crystallinity. Moreover, their electrochemical performance, as an anode material, was evaluated by galvanostatic discharge–charge tests, showing that the as-prepared SnSe nanosheets exhibited a high initial discharge capacity of 1009 mA h g−1 as a potential energy storage material. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Because of layered structures and interesting properties, SnSe is a promising candidate in the field of solar cells [1], memory switching devices [2], holographic recording systems [3], infrared devices [4], lithium ion batteries [5], etc. Therefore, the synthesis of nanostructured SnSe has attracted considerable attention and a series of SnSe nanomaterials have been prepared, such as nanoparticles [6], nanocrystals [7], nanosheets [8], nanotubes [9], nanowires [9,10] and core–shell nanorods of SnSe-C [11]. However, the preparation of nanostructured SnSe with high phase-purity and high crystallinity is still a challenge due to its layered structure. There are very few reports about this up to now. Currently, the synthetic routes to SnSe mainly include Bridgman method [12], solid state reaction at high temperature [13], precipitation by using H2 Se [14], organometallic precursor method [15], hot-injection method [1,7], amine-assisted route [14,16], hydrothermal method [17], solvothermal method [18,19], etc. Therein, the hot-injection method, solvothermal method and amine-assisted route are frequently used to prepare nanostructured SnSe. However, these synthetic routes often involve the use of ethylenediamine, oleylamine or other amines as solvents. It can be deduced that amines on the nanostructured SnSe might induce
∗ Corresponding author. Tel.: +86 21 60873162; fax: +86 21 60873567. E-mail address: [email protected] (J. Mu). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.04.025
some adverse effects in applications which involve electron transfer, such as catalysis, solar cells and lithium ion batteries. Therefore, it is worthwhile to find a simple amine-free method for the preparation of SnSe nanomaterials. It is well-known that the interface between oil and water possesses some special functions. Taking advantages of these special properties, it is possible to prepare nanomaterials having some interesting structures at the oil–water interface. Recently, a twophase solvothermal process was developed and a series of CdS, core–shell CdSe/CdS and TiO2 nanocrystals were prepared [20–23]. Our previous experimental results also indicated that the high crystalline SnO2 nanoparticles could be obtained via oxidation of Sn2+ ions at the oil–water interface in a solvothermal process [24]. Because SnSe possesses a layered crystal structure, it can be expected that the two-phase solvothermal process might be a promising candidate for the preparation of SnSe nanomaterials with high crystallinity, and the oil–water interface would be of benefit to formation of SnSe nanosheets. Unfortunately, to the best of our knowledge, there are still few reports on the preparation of SnSe nanosheets with high crystallinity in a two-phase solvothermal process so far. In this paper, we aim to report the preparation of high crystalline SnSe nanosheets at the oil–water interface in a solvothermal process. A possible mechanism for the formation of SnSe nanosheets is proposed. In addition, the as-prepared SnSe nanosheets are investigated as anode material in order to assess their potential in the field of lithium ion battery.
2. Experimental
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2.2. Characterization The powder X-ray diffraction (XRD) analysis was made on a PANalytical Xpert Pro MRD X-ray diffractometer (Netherlands) using Cu K␣ radiation ( = 0.154056 nm). The morphology of the sample was characterized on a JEM-1400 transmission electron microscope (JEOL, Japan). High resolution transmission electron microscopy (HRTEM) image and electron diffraction analysis were recorded using a JEOL JEM-2100 transmission electron microscope (Japan). The UV–vis spectrum of the solid sample was measured with a Varian Cary 100 spectrophotometer with an attachment of integrating sphere (USA). The sample of about 2 mg was mixed with BaSO4 of about 200 mg, in which BaSO4 was used as a reflectance standard. The N2 adsorption and desorption isotherm
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SnCl2 ·2H2 O, toluene, sodium citrate, hydrazine hydrate and dimethyl diselenide are commercially available and used as received. Deionized water was used as solvent. In a typical procedure, (1) sodium citrate (3.0 mmol) was added into a beaker containing 15 mL of distilled water and stirred until a transparent solution was formed. After addition of SnCl2 ·2H2 O (1.5 mmol) and hydrazine hydrate (0.5 mL, 1.2%) under stirring, the pH value of the solution was adjusted to 11 using NaOH aqueous solution. Then, the solution was poured into a 50 mL Teflon liner stainless steel autoclave and deaerated using nitrogen for 20 min. (2) Dimethyl diselenide (0.15 mmol) was dissolved in toluene (15 mL) to prepare the oil phase. (3) The oil phase solution was added into the autoclave containing the aqueous solution above to form a two phase reaction system. After deaerated using nitrogen for another 20 min, the autoclave was sealed and heated at 130 ◦ C for 12 h. (4) The autoclave was naturally cooled to ambient temperature. Subsequently, the resulting black product was collected by centrifugation, washed with distilled water and ethanol for several times, respectively. After dried in a vacuum desiccator at room temperature, the SnSe nanosheets were obtained.
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was measured on a Micromeritics ASAP-2020 nitrogen adsorption apparatus (USA). 2.3. Electrochemical measurements Electrochemical properties of the samples were investigated using Swagelok-type cells assembled in an argon-filled glovebox. The copper foil coated with a slurry that consisted of 80 wt% active material (SnSe nanosheets), 10 wt% poly(vinylidene fluoride) (PVDF) and 10 wt% carbon black was used as the working electrode. The loading mass of SnSe nanosheets was about 10 mg cm−2 . Lithium foil and a glass fiber from Whatman were used as the counter electrode and the separator, respectively. The electrolyte solution was 1 M solution of LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1 in wt%) containing 2 wt% vinylene carbonate (VC). Galvanostatic cycling tests were carried out with an Arbin BT2000 system in the voltage range of 0.01–3.0 V (vs Li+ /Li) under a current density of 50 mA g−1 .
Fig. 2. TEM images (A and B), electron diffraction patterns (C and D), size distribution histogram (E) and UV–vis spectrum (F) of SnSe nanosheets.
S.-Z. Kang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 406 (2012) 1–5
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Fig. 3. TEM images of SnSe nanosheets after solvothermal treatment for 8 h (A), 12 h (B), 16 h (C), 24 h (D) and 32 h (E), respectively. HRTEM image of SnSe nanosheets after solvothermal treatment for 12 h (F, G).
3. Results and discussion The XRD pattern of the as-prepared sample is shown in Fig. 1. It can be observed from Fig. 1 that all diffraction peaks in the pattern can be indexed to the orthorhombic phase of SnSe (JCPDS file No. 48-1224). The lattice parameters calculated from the diffraction ˚ b = 4.15 A˚ and c = 4.44 A, ˚ which are in good pattern are a = 11.50 A, agreement with the reported values [1]. In addition, the diffraction peaks are fairly strong and sharp and no evidence of crystalline impurities, such as SnO, SnO2 , SnSe2 and Se, is found in the pattern. These results indicate that the orthorhombic SnSe with high phase-purity and high crystallinity can be obtained in the twophase solvothermal process. In order to substantiate the formation of nanosheets, we measured the morphology of the sample using TEM (Fig. 2A and B). From these TEM images, we clearly observe the formation of SnSe nanosheets which adopt a square-like morphology with lateral dimensions of approximately 120 nm × 120 nm. Moreover, the selected-area electron diffraction pattern of the sample (Fig. 2C and D) shows a spot pattern, suggesting that the as-prepared SnSe
nanosheets are well crystallized. Combined with the result of XRD, it is confirmed that the orthorhombic SnSe nanosheets with high crystallinity can be prepared according to the procedure described above. Fig. 2F shows the UV–vis spectrum of the as-prepared SnSe nanosheets. As can be seen from Fig. 2F, the SnSe nanosheets exhibit a strong absorption through the visible region, resulting in the black color of the sample. It was reported previously that the SnSe nanosheets possess a direct bandgap of 1 eV and an indirect bandgap of 0.90 eV [8]. Therefore, we speculate that the as-prepared SnSe nanosheets would possess a fairly narrow band gap, which is favorable to the application in the field of solar cells. In order to understand the interface reaction, the morphologies of products were monitored by TEM at certain time intervals (Fig. 3). As can be seen from Fig. 3, the nanosheets with average edge length of approximate 100 nm are formed after solvothermal treatment for 8 h (Fig. 3A). With reaction time prolonging, the nanosheets gradually grow up in lateral dimensions so that the nanosheets with lateral dimensions of approximate 150 nm × 150 nm are observed as the primary product (Fig. 3B and C). When reaction time is
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further prolonged, the nanosheets begin to merge together (Fig. 3D), and then change to the aggregation of nanoparticles (Fig. 3E). Moreover, the HRTEM image of nanosheets obtained after solvothermal treatment for 12 h (Fig. 3F and G) indicates that they consist of nanoparticles. Based on these experimental results and the nanoparticle coalescence pathway of SnSe nanosheets reported in Ref. [8], we presume a possible mechanism for the formation of SnSe nanosheets. Firstly, Sn2+ ions react with Se2− from dimethyl diselenide at the oil–water interface. The SnSe nanoparticles are formed in this stage. Then, the nanoparticles coalesce and grow into two-dimensional nanosheets. Subsequently, the continuous addition of nanoparticles leads to the merger and vertical growth of nanosheets. At last, the aggregation of nanoparticles emerges. In this process, the oil–water interface may act as a template for the growth of nanosheets. The interfacial tension might be the drive force which induces the lateral coalescence of nanoparticles with a preference to their vertical aggregation. Fig. 4 shows the first discharge–charge voltage profile and the cycling behavior of the as-prepared SnSe nanosheets. As can be seen from Fig. 4A, the first discharge capacity of SnSe nanosheets is 1009 mA h g−1 under 50 mA g−1 , which is much larger than that of SnSe nanocrystals prepared from oleylamine solution (810 mA h g−1 ) [5]. Because the BET measurement indicates that the specific surface area of SnSe nanosheets is only 0.9 m2 g−1 , it can be deduced that the specific surface area is not an important factor in this study. The higher discharge capacity suggests that the asprepared SnSe nanosheets have a higher electrochemical activity to lithium storage compared with the SnSe nanocrystals prepared from oleylamine solution, in which the impurity of oleylamine in active materials may be unfavorable for Li storage. Moreover, a long plateau at about 1.3 V is observed in the first cycle, indicating that the SnSe nanosheets are a potential anode material for lithium ion batteries. However, the Coulombic efficiency of SnSe nanosheets in the first charge–discharge process is only 41.3% (Fig. 5). And the specific capacity continuously decreases and reaches 73 mA h g−1 after 20 cycles (Fig. 4B) although the Coulombic efficiency can reach more than 90% in the following cycle, indicating poor capacity
retention. The results reported previously indicate that the formation of the irreversible Li2 Se is the main reason for the initial capacity decay of SnSe nanocrystals [5]. Furthermore, one possible explanation is that the nanosheet might be gradually destroyed because the alloy/dealloy reaction of Sn with Li+ accompanies with large volume change [25]. Therefore, it is a key to design the novel anode materials based on SnSe nanosheets to accommodate the volumetric change during cycles and to facilitate their application in the field of lithium-ion battery. The further efforts are currently being undertaken. 4. Conclusions The highly crystalline SnSe nanosheets have been successfully synthesized via the reaction at the oil–water interface in the solvothermal process. The oil–water interface plays an important role in the formation of SnSe nanosheets. This route offers a simple, amine-free approach for the synthesis of highly crystalline nanomaterials with layered crystal structure. In addition, the SnSe nanosheets show high electrochemical activity towards lithium storage. Further efforts are currently being undertaken to understand the mechanism of Li storage, as well as to improve the electrochemical performance. Acknowledgements This work was financially supported by the Key Project of the National Natural Science Foundation of China (No. 20933007) and the Key Discipline Development Program of Shanghai Municipal Education Commission (No. J51503). References [1] M.A. Franzman, C.W. Schlenker, M.E. Thompson, R.L. Brutchey, Solution-phase synthesis of SnSe nanocrystals for use in solar cells, J. Am. Chem. Soc. 132 (2010) 4060–4061. [2] K.-M. Chung, D. Wamwangi, M. Woda, M. Wuttig, W. Bench, Investigation of SnSe, SnSe2 , and Sn2 Se3 alloys for phase change memory applications, J. Appl. Phys. 103 (2008) 083523. [3] G. Valiukonis, D.A. Guseinova, G. Krivaite, A. Sileika, Optical spectra and energy band structure of layer-type AIV BVI compounds, Phys. Status Solidi B 135 (1986) 299–307. [4] H. Maier, D.R. Daniel, SnSe single crystals: sublimation growth, deviation from stoichiometry and electrical properties, J. Electron. Mater. 6 (1977) 693–704. [5] J. Ning, G. Xiao, T. Jiang, L. Wang, Q. Dai, B. Zou, B. Liu, Y. Wei, G. Chen, G. Zou, Shape and size controlled synthesis and properties of colloidal IV–VI SnSe nanocrystals, CrystEngComm 13 (2011) 4161–4166.
S.-Z. Kang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 406 (2012) 1–5 [6] S. Schlecht, M. Budde, L. Kienle, Nanocrystalline tin as a preparative tool: synthesis of unprotected nanoparticles of SnTe and SnSe and a new route to (PhSe)4 Sn, Inorg. Chem. 41 (2002) 6001–6005. [7] W.J. Baumgardner, J.J. Choi, Y.-F. Lim, T. Hanrath, SnSe nanocrystals: synthesis, structure, optical properties, and surface chemistry, J. Am. Chem. Soc. 132 (2010) 9519–9521. [8] D.D. Vaughn II, S.-I. In, R.E. Schaak, A precursor-limited nanoparticle coalescence pathway for tuning the thickness of laterally-uniform colloidal nanosheets: the case of SnSe, ACS Nano 5 (2011) 8852–8860. [9] L. Zhao, M. Yosef, M. Steinhart, P. Goring, H. Hofmeister, U. Gosele, S. Schlecht, Porous silicon and alumina as chemically reactive templates for the synthesis of tubes and wires of SnSe, Sn, and SnO2 , Angew. Chem. Int. Ed. 45 (2006) 311–315. [10] G. Shen, D. Chen, X. Jiang, K. Tang, Y. Liu, Y. Qian, Rapid synthesis of SnSe nanowires via an ethylenediamine-assisted polyol route, Chem. Lett. 32 (2003) 426–427. [11] V.G. Pol, S.V. Pol, A. Gedanken, Core–shell nanorods of SnS-C and SnSe-C: synthesis and characterization, Langmuir 24 (2008) 5135–5139. [12] V.P. Bhatt, K. Gireesan, G.R. Pandya, Growth and characterization of SnSe and SnSe2 single crystals, J. Cryst. Growth 96 (1989) 649–651. [13] A.L. Seligson, J. Arnold, Synthesis, structure, and reactivity of homoleptic tin(II) and lead(II) chalcogenolates and conversion to metal chalcogenides. X-ray crystal structures of {Sn[TeSi(SiMe3 )3 ]2 }2 and (PMe3 )Sn[TeSi(SiMe3 )3 ]2 , J. Am. Chem. Soc. 115 (1993) 8214–8220. [14] Y. Li, Z. Wang, Y. Ding, Room temperature synthesis of metal chalcogenides in ethylenediamine, Inorg. Chem. 38 (1999) 4737–4740. [15] P. Boudjouk, S. Seidder, Bis(triphenyltin) chalcogenides as convenient precursors to phase-pure binary semiconductors, Chem. Mater. 6 (1994) 2108–2112.
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[16] W. Wang, Y. Geng, P. Yan, F. Liu, Y. Xie, Y. Qian, A novel mild route to nanocrystalline selenides at room temperature, J. Am. Chem. Soc. 121 (1999) 4062–4063. [17] C. Wang, Y.D. Li, G.H. Zhang, J. Zhuang, G.Q. Shen, Synthesis of SnSe in various alkaline media under mild conditions, Inorg. Chem. 39 (2000) 4237–4239. [18] B. Li, Y. Xie, J. Huang, Y. Qian, Solvothermal route to tin monoselenide bulk single crystal with different morphologies, Inorg. Chem. 39 (2000) 2061–2064. [19] W. Wang, Y. Geng, Y. Qian, C. Wang, X. Liu, A convenient, low temperature route to nanocrystalline SnSe, Mater. Res. Bull. 34 (1999) 403–406. [20] Q. Wang, D.-C. Pan, S.-C. Jiang, X.-L. Ji, L.-J. An, B.-Z. Jiang, A new twophase route to high-quality CdS nanocrystals, Chem. Eur. J. 11 (2005) 3843–3848. [21] D.-C. Pan, S.-C. Jiang, L.-J. An, B.-Z. Jiang, Controllable synthesis of highly luminescent and monodisperse CdS nanocrystals by a two-phase approach under mild conditions, Adv. Mater. 16 (2004) 982–985. [22] D.-C. Pan, Q. Wang, S.-C. Jiang, X.-L. Ji, L.-J. An, Synthesis of extremely small CdSe and highly luminescent CdSe/CdS core–shell nanocrystals via a novel twophase thermal approach, Adv. Mater. 17 (2005) 176–179. [23] D.-C. Pan, N.-N. Zhao, Q. Wang, S.-C. Jiang, X.-L. Ji, L.-J. An, Facile Synthesis and characterization of luminescent TiO2 nanocrystals, Adv. Mater. 17 (2005) 1991–1995. [24] S.-Z. Kang, Y. Yang, J. Mu, Solvothermal synthesis of SnO2 nanoparticles via oxidation of Sn2+ ions at the water–oil interface, Colloids Surf. A 298 (2007) 280–283. [25] Z.-M. Cui, L.-Y. Jiang, W.-G. Song, Y.-G. Guo, High-yield gas–liquid interfacial synthesis of highly dispersed Fe3 O4 nanocrystals and their application in lithium-ion batteries, Chem. Mater. 21 (2009) 1162–1166.
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Amine-free preparation of SnSe nanosheets with high crystallinity and their lithium storage properties Shi-Zhao Kang a , Ladi Jia a , Xiangqing Li a , Yaxia Yin b , Liang Li a , Yu-Guo Guo b , Jin Mu a,∗ a b
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, China Institute of Chemistry, Chinese Academy of Science, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 23 March 2012 Received in revised form 24 April 2012 Accepted 26 April 2012 Available online 4 May 2012 Keywords: SnSe Nanosheet Solvothermal process Anode material Lithium-ion battery
a b s t r a c t In the present work, SnSe nanosheets were prepared via a reaction at the oil–water interface in the solvothermal process at low temperature (130 ◦ C), which did not involve any templates and amines such as oleylamine. X-ray diffraction, transmission electron microscope, absorption spectrum and nitrogen adsorption were used to characterize the products. The results indicated that the SnSe nanosheets obtained adopted a square-like morphology with lateral dimensions of approximately 120 nm × 120 nm and possessed high crystallinity. Moreover, their electrochemical performance, as an anode material, was evaluated by galvanostatic discharge–charge tests, showing that the as-prepared SnSe nanosheets exhibited a high initial discharge capacity of 1009 mA h g−1 as a potential energy storage material. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Because of layered structures and interesting properties, SnSe is a promising candidate in the field of solar cells [1], memory switching devices [2], holographic recording systems [3], infrared devices [4], lithium ion batteries [5], etc. Therefore, the synthesis of nanostructured SnSe has attracted considerable attention and a series of SnSe nanomaterials have been prepared, such as nanoparticles [6], nanocrystals [7], nanosheets [8], nanotubes [9], nanowires [9,10] and core–shell nanorods of SnSe-C [11]. However, the preparation of nanostructured SnSe with high phase-purity and high crystallinity is still a challenge due to its layered structure. There are very few reports about this up to now. Currently, the synthetic routes to SnSe mainly include Bridgman method [12], solid state reaction at high temperature [13], precipitation by using H2 Se [14], organometallic precursor method [15], hot-injection method [1,7], amine-assisted route [14,16], hydrothermal method [17], solvothermal method [18,19], etc. Therein, the hot-injection method, solvothermal method and amine-assisted route are frequently used to prepare nanostructured SnSe. However, these synthetic routes often involve the use of ethylenediamine, oleylamine or other amines as solvents. It can be deduced that amines on the nanostructured SnSe might induce
∗ Corresponding author. Tel.: +86 21 60873162; fax: +86 21 60873567. E-mail address: [email protected] (J. Mu). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.04.025
some adverse effects in applications which involve electron transfer, such as catalysis, solar cells and lithium ion batteries. Therefore, it is worthwhile to find a simple amine-free method for the preparation of SnSe nanomaterials. It is well-known that the interface between oil and water possesses some special functions. Taking advantages of these special properties, it is possible to prepare nanomaterials having some interesting structures at the oil–water interface. Recently, a twophase solvothermal process was developed and a series of CdS, core–shell CdSe/CdS and TiO2 nanocrystals were prepared [20–23]. Our previous experimental results also indicated that the high crystalline SnO2 nanoparticles could be obtained via oxidation of Sn2+ ions at the oil–water interface in a solvothermal process [24]. Because SnSe possesses a layered crystal structure, it can be expected that the two-phase solvothermal process might be a promising candidate for the preparation of SnSe nanomaterials with high crystallinity, and the oil–water interface would be of benefit to formation of SnSe nanosheets. Unfortunately, to the best of our knowledge, there are still few reports on the preparation of SnSe nanosheets with high crystallinity in a two-phase solvothermal process so far. In this paper, we aim to report the preparation of high crystalline SnSe nanosheets at the oil–water interface in a solvothermal process. A possible mechanism for the formation of SnSe nanosheets is proposed. In addition, the as-prepared SnSe nanosheets are investigated as anode material in order to assess their potential in the field of lithium ion battery.
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2.2. Characterization The powder X-ray diffraction (XRD) analysis was made on a PANalytical Xpert Pro MRD X-ray diffractometer (Netherlands) using Cu K␣ radiation ( = 0.154056 nm). The morphology of the sample was characterized on a JEM-1400 transmission electron microscope (JEOL, Japan). High resolution transmission electron microscopy (HRTEM) image and electron diffraction analysis were recorded using a JEOL JEM-2100 transmission electron microscope (Japan). The UV–vis spectrum of the solid sample was measured with a Varian Cary 100 spectrophotometer with an attachment of integrating sphere (USA). The sample of about 2 mg was mixed with BaSO4 of about 200 mg, in which BaSO4 was used as a reflectance standard. The N2 adsorption and desorption isotherm
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SnCl2 ·2H2 O, toluene, sodium citrate, hydrazine hydrate and dimethyl diselenide are commercially available and used as received. Deionized water was used as solvent. In a typical procedure, (1) sodium citrate (3.0 mmol) was added into a beaker containing 15 mL of distilled water and stirred until a transparent solution was formed. After addition of SnCl2 ·2H2 O (1.5 mmol) and hydrazine hydrate (0.5 mL, 1.2%) under stirring, the pH value of the solution was adjusted to 11 using NaOH aqueous solution. Then, the solution was poured into a 50 mL Teflon liner stainless steel autoclave and deaerated using nitrogen for 20 min. (2) Dimethyl diselenide (0.15 mmol) was dissolved in toluene (15 mL) to prepare the oil phase. (3) The oil phase solution was added into the autoclave containing the aqueous solution above to form a two phase reaction system. After deaerated using nitrogen for another 20 min, the autoclave was sealed and heated at 130 ◦ C for 12 h. (4) The autoclave was naturally cooled to ambient temperature. Subsequently, the resulting black product was collected by centrifugation, washed with distilled water and ethanol for several times, respectively. After dried in a vacuum desiccator at room temperature, the SnSe nanosheets were obtained.
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was measured on a Micromeritics ASAP-2020 nitrogen adsorption apparatus (USA). 2.3. Electrochemical measurements Electrochemical properties of the samples were investigated using Swagelok-type cells assembled in an argon-filled glovebox. The copper foil coated with a slurry that consisted of 80 wt% active material (SnSe nanosheets), 10 wt% poly(vinylidene fluoride) (PVDF) and 10 wt% carbon black was used as the working electrode. The loading mass of SnSe nanosheets was about 10 mg cm−2 . Lithium foil and a glass fiber from Whatman were used as the counter electrode and the separator, respectively. The electrolyte solution was 1 M solution of LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1 in wt%) containing 2 wt% vinylene carbonate (VC). Galvanostatic cycling tests were carried out with an Arbin BT2000 system in the voltage range of 0.01–3.0 V (vs Li+ /Li) under a current density of 50 mA g−1 .
Fig. 2. TEM images (A and B), electron diffraction patterns (C and D), size distribution histogram (E) and UV–vis spectrum (F) of SnSe nanosheets.
S.-Z. Kang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 406 (2012) 1–5
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Fig. 3. TEM images of SnSe nanosheets after solvothermal treatment for 8 h (A), 12 h (B), 16 h (C), 24 h (D) and 32 h (E), respectively. HRTEM image of SnSe nanosheets after solvothermal treatment for 12 h (F, G).
3. Results and discussion The XRD pattern of the as-prepared sample is shown in Fig. 1. It can be observed from Fig. 1 that all diffraction peaks in the pattern can be indexed to the orthorhombic phase of SnSe (JCPDS file No. 48-1224). The lattice parameters calculated from the diffraction ˚ b = 4.15 A˚ and c = 4.44 A, ˚ which are in good pattern are a = 11.50 A, agreement with the reported values [1]. In addition, the diffraction peaks are fairly strong and sharp and no evidence of crystalline impurities, such as SnO, SnO2 , SnSe2 and Se, is found in the pattern. These results indicate that the orthorhombic SnSe with high phase-purity and high crystallinity can be obtained in the twophase solvothermal process. In order to substantiate the formation of nanosheets, we measured the morphology of the sample using TEM (Fig. 2A and B). From these TEM images, we clearly observe the formation of SnSe nanosheets which adopt a square-like morphology with lateral dimensions of approximately 120 nm × 120 nm. Moreover, the selected-area electron diffraction pattern of the sample (Fig. 2C and D) shows a spot pattern, suggesting that the as-prepared SnSe
nanosheets are well crystallized. Combined with the result of XRD, it is confirmed that the orthorhombic SnSe nanosheets with high crystallinity can be prepared according to the procedure described above. Fig. 2F shows the UV–vis spectrum of the as-prepared SnSe nanosheets. As can be seen from Fig. 2F, the SnSe nanosheets exhibit a strong absorption through the visible region, resulting in the black color of the sample. It was reported previously that the SnSe nanosheets possess a direct bandgap of 1 eV and an indirect bandgap of 0.90 eV [8]. Therefore, we speculate that the as-prepared SnSe nanosheets would possess a fairly narrow band gap, which is favorable to the application in the field of solar cells. In order to understand the interface reaction, the morphologies of products were monitored by TEM at certain time intervals (Fig. 3). As can be seen from Fig. 3, the nanosheets with average edge length of approximate 100 nm are formed after solvothermal treatment for 8 h (Fig. 3A). With reaction time prolonging, the nanosheets gradually grow up in lateral dimensions so that the nanosheets with lateral dimensions of approximate 150 nm × 150 nm are observed as the primary product (Fig. 3B and C). When reaction time is
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further prolonged, the nanosheets begin to merge together (Fig. 3D), and then change to the aggregation of nanoparticles (Fig. 3E). Moreover, the HRTEM image of nanosheets obtained after solvothermal treatment for 12 h (Fig. 3F and G) indicates that they consist of nanoparticles. Based on these experimental results and the nanoparticle coalescence pathway of SnSe nanosheets reported in Ref. [8], we presume a possible mechanism for the formation of SnSe nanosheets. Firstly, Sn2+ ions react with Se2− from dimethyl diselenide at the oil–water interface. The SnSe nanoparticles are formed in this stage. Then, the nanoparticles coalesce and grow into two-dimensional nanosheets. Subsequently, the continuous addition of nanoparticles leads to the merger and vertical growth of nanosheets. At last, the aggregation of nanoparticles emerges. In this process, the oil–water interface may act as a template for the growth of nanosheets. The interfacial tension might be the drive force which induces the lateral coalescence of nanoparticles with a preference to their vertical aggregation. Fig. 4 shows the first discharge–charge voltage profile and the cycling behavior of the as-prepared SnSe nanosheets. As can be seen from Fig. 4A, the first discharge capacity of SnSe nanosheets is 1009 mA h g−1 under 50 mA g−1 , which is much larger than that of SnSe nanocrystals prepared from oleylamine solution (810 mA h g−1 ) [5]. Because the BET measurement indicates that the specific surface area of SnSe nanosheets is only 0.9 m2 g−1 , it can be deduced that the specific surface area is not an important factor in this study. The higher discharge capacity suggests that the asprepared SnSe nanosheets have a higher electrochemical activity to lithium storage compared with the SnSe nanocrystals prepared from oleylamine solution, in which the impurity of oleylamine in active materials may be unfavorable for Li storage. Moreover, a long plateau at about 1.3 V is observed in the first cycle, indicating that the SnSe nanosheets are a potential anode material for lithium ion batteries. However, the Coulombic efficiency of SnSe nanosheets in the first charge–discharge process is only 41.3% (Fig. 5). And the specific capacity continuously decreases and reaches 73 mA h g−1 after 20 cycles (Fig. 4B) although the Coulombic efficiency can reach more than 90% in the following cycle, indicating poor capacity
retention. The results reported previously indicate that the formation of the irreversible Li2 Se is the main reason for the initial capacity decay of SnSe nanocrystals [5]. Furthermore, one possible explanation is that the nanosheet might be gradually destroyed because the alloy/dealloy reaction of Sn with Li+ accompanies with large volume change [25]. Therefore, it is a key to design the novel anode materials based on SnSe nanosheets to accommodate the volumetric change during cycles and to facilitate their application in the field of lithium-ion battery. The further efforts are currently being undertaken. 4. Conclusions The highly crystalline SnSe nanosheets have been successfully synthesized via the reaction at the oil–water interface in the solvothermal process. The oil–water interface plays an important role in the formation of SnSe nanosheets. This route offers a simple, amine-free approach for the synthesis of highly crystalline nanomaterials with layered crystal structure. In addition, the SnSe nanosheets show high electrochemical activity towards lithium storage. Further efforts are currently being undertaken to understand the mechanism of Li storage, as well as to improve the electrochemical performance. Acknowledgements This work was financially supported by the Key Project of the National Natural Science Foundation of China (No. 20933007) and the Key Discipline Development Program of Shanghai Municipal Education Commission (No. J51503). References [1] M.A. Franzman, C.W. Schlenker, M.E. Thompson, R.L. Brutchey, Solution-phase synthesis of SnSe nanocrystals for use in solar cells, J. Am. Chem. Soc. 132 (2010) 4060–4061. [2] K.-M. Chung, D. Wamwangi, M. Woda, M. Wuttig, W. Bench, Investigation of SnSe, SnSe2 , and Sn2 Se3 alloys for phase change memory applications, J. Appl. Phys. 103 (2008) 083523. [3] G. Valiukonis, D.A. Guseinova, G. Krivaite, A. Sileika, Optical spectra and energy band structure of layer-type AIV BVI compounds, Phys. Status Solidi B 135 (1986) 299–307. [4] H. Maier, D.R. Daniel, SnSe single crystals: sublimation growth, deviation from stoichiometry and electrical properties, J. Electron. Mater. 6 (1977) 693–704. [5] J. Ning, G. Xiao, T. Jiang, L. Wang, Q. Dai, B. Zou, B. Liu, Y. Wei, G. Chen, G. Zou, Shape and size controlled synthesis and properties of colloidal IV–VI SnSe nanocrystals, CrystEngComm 13 (2011) 4161–4166.
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