CRYSTAL GROWTH & DESIGN
Double-Source Approach to In2S3 Single Crystallites and Their Electrochemical Properties Liu,†,‡
Yi
Huayun
Xu,†
and Yitai
Qian*,†
Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry and Department of Materials Science and Engineering, UniVersity of Science & Technology of China, Hefei 230026, People’s Republic of China, and Department of Chemistry, Zaozhuang UniVersity, Zaozhuang 277100, People’s Republic of China
2006 VOL. 6, NO. 6 1304-1307
ReceiVed August 19, 2005; ReVised Manuscript ReceiVed April 14, 2006
ABSTRACT: Novel cubic and tetragonal In2S3 hollow microspheres consisting of nanoflakes, as well as cubic In2S3 nanoflakes, were synthesized via a double-sulfur-source approach using InCl3‚4H2O as the indium source under solvothermal conductions. Cubic and tetragonal In2S3 hollow microspheres with an average diameter of 5-15 µm reveal flat and clean nanoflakes inside the microspheres; the In2S3 nanoflakes have an average thickness of 10 nm. The samples were characterized by XRD, FESEM, TEM, SAED, HRTEM, and XPS. Electrochemical tests show that these cubic In2S3 nanoflakes deliver an astoundingly large discharge capacity of 1150 mA h g-1 vs Li metal at 0.032 mA cm-2 (voltage window of 0-3.0 V) compared with that of ordinary cathode materials. The obtained sample might find promising application as an electrode material in lithium ion batteries. 1. Introduction As a typical III-VI main-group chalcogenide, due to its optoelectronic properties,1 electronic properties,2 optical properties,3,4 acoustic properties,5 and semiconductor sensitization,6 indium sulfide has been used as an n-type semiconductor with a band gap of 2-2.45 eV7 and as a photoconductor,8,9 and it has shown promise as a photovoltaic electric generator10 and in medical applications such as cancer diagnosis.11 In addition, it has also been used for the preparation of green and red phosphors and the manufacture of picture tubes for color televisions,12-14 dry cells,15 and so forth. To our knowledge, In2S3 exists in three phases: the defective cubic structure R-In2S3 (stable up to 693 K), a defect spinel structure called β-In2S3 (stable up to 1027 K), and a higher temperature layered structure, γ-In2S3 (above 1027 K).16-18 The defect spinel structure is obtained in either the expected cubic or tetragonal form; the regular sites of indium atoms are centers of both disturbed sulfur tetrahedra and octahedra, and the tetragonal form arises out of vacancy ordering in the tetrahedral cation sites, leading to formation of a supercell.19 It has a close relation between structure and properties. Almost all properties of indium sulfide rest with its defect structure. Figure 1 is a crystalline structure schematic diagram of all In2S3 crystal types. Usually, the particle sizes and shapes of materials have important effects on their physical properties, and their properties highly depend on their synthetic and processing methods.20-22 Several methods have been reported in the case of the synthesis of β-In2S3, such as direct reaction of the elements,23 heat treatment at high temperature,24,25 thermal decomposition of a single-source precursor,26 solution preparation methods, laserinduced synthesis,27 sonochemical synthesis,28 and the preparation of small In2S3 colloidal particles in reverse micelles.29 β-In2S3 has been synthesized by hydrothermal means at lower temperature or under mild solvothermal conditions by our group.30-33 Although the synthesis of β-In2S3 has been reported previously, there has been little presented on the fabrication of cubic In2S3 by adopting two sulfur sources via a solvothermal process. On the other hand, research on the electrochemical † ‡
University of Science & Technology of China. Zaozhuang University.
properties of metal sulfides as anodes has seldom been attempted; especially, electrochemical tests of cubic In2S3 have hardly been reported. Herein, a new approach to the preparation of cubic and tetragonal In2S3 crystallities with various morphologies is reported. The obtained sample delivers an attractive discharge capacity which is much greater than that of ordinary cathode materials, and it might find promising application as an electrode material in lithium ion batteries. 2. Experimental Section All of the analytical reagents have been purchased from Shanghai Chemical Reagents Co. and were used without further purification. The cubic β-In2S3 crystallites were prepared as follows: we chose InCl3‚ 4H2O as the indium source and CS2 and sulfur powder as sulfur sources. In a typical procedure, 1 mmol of InCl3‚4H2O and 3 mL of CS2 solution containing 1 mmol of sulfur powder were first dissolved in 20 mL of anhydrous ethanol in a Teflon liner of 60 mL capacity at room temperature; the ethanol solution was agitated until the InCl3‚4H2O was dissolved. After that, in different reactions, 2 mmol of tartaric acid (C4H6O6) and 1 mmol of EDTADS (C10H14N2O8Na2‚2H2O) as coordination agents were separately added to mixtures of indium and sulfur sources in ethanol solutions, anhydrous ethanol was added until 80% of the Teflon liner’s volume was filled, and then the liner was sealed in a stainless steel autoclave. The autoclave was maintained at 190 °C for 16 h and then cooled to room temperature naturally. Yellow-orange precipitates were filtered off, washed with distilled water and absolute ethanol several times, and dried under vacuum at 60 °C for 5 h. X-ray powder diffraction (XRD) patterns of the prepared samples were determined using a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite-monochromated Cu KR radiation (λ ) 1.541 874 Å). Field scanning electron microscopy (FESEM) images were taken with a JEOL-6700F scanning electronic microanalyzer. Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were characterized by a Hitachi H-800 transmission electron microscope (TEM) with a tungsten filament and an accelerating voltage of 200 kV. High-resolution transmission electron microscope (HRTEM) images were recorded on a JEOL 2010 microscope. The chemical composition of the prepared sample was obtained by X-ray photoelectron spectroscopy on a VGESCALAB MKII X-ray photoelectronic spectrometer, using nonmonochromated Mg KR radiation as the excitation source. The samples used for TEM and HRTEM characterization were dispersed in absolute ethanol and were ultrasonicated before observation. Thermal gravimetric analysis (TGA) was conducted using a Shimadzu TGA-50H analyzer (purified air stream 50 mL/min; heating rate 10 °C/min). The electrode laminate
10.1021/cg0504298 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/19/2006
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Figure 1. Crystal structures of In2S3: left, hexagonal phase; middle, tetragonal phase; right, cubic phase. Table 1. Summary of the Results on the Products Obtained with Different Sulfur Sources and Coordination Agents at 190 °C for 16 h, Using InCl3‚4H2O as Starting Material sample no. 1 2 3 4 5
coord agent
2 mmol of tartaric acid 1 mmol of EDTADS (C10H14N2O8Na2‚2H2O)
S source
product detected by XRD
CS2 S powder CS2 + S powder CS2 + S powder CS2 + S powder
nonexistence of In2S3 nonexistence of In2S3 cubic In2S3 cubic In2S3 tetragonal In2S3
for the electrochemical testing was prepared by casting a slurry consisting of active material powders (84 wt %), acetylene black (8 wt %), and poly(vinylidene fluoride) (PVDF; 8 wt %) dispersed in 1-methyl-2-pyrrolidinone (NMP) onto an aluminum foil. The laminates were then dried at 70 °C for 1 h. The In2S3/Li coin cells (2032 size) were made with 1 M LiPF6 in ethylene carbonate (EC): diethyl carbonate (DEC; 1:1 w/w) was the electrolyte. The cells were tested on a multichannel battery cycler (Shenzhen Neware Co. Ltd.) and subjected to charge-discharge cycles at 0.032 mA/cm2 between 0.0 and 3.0 V (vs Li metal).
3. Results and Discussion In our experiment, series of different solvothermal conditions were used. We found the product of In2S3 to be tetragonal phase, when 1 mmol of EDTADS (C10H14N2O8Na2‚2H2O) was added into ethanol solution as coordination agents. In the meantime, it is worth noting that if we substitute a single sulfur source (either CS2 or sulfur powder) for two sulfur sources (CS2 and sulfur powder) in the entire solvothermal reaction process, In2S3 was not found. The prepared solid product was simplex sulfur when sulfur powder was used as the sulfur source. No solidphase product could be obtained when CS2 was introduced as the sulfur source. In the meantime, a reaction temperature of 190 °C and a reaction time of 16 h are suitable for the present solvothermal conditions. The experimental results using InCl3‚ 4H2O and different sulfur sources (CS2, sulfur powder, and CS2 mixed with sulfur powder) as the starting materials are summarized in Table 1. Parts a and b of Figure 2 give typical XRD patterns of the prepared cubic In2S3. Part c of Figure 2 is the typical XRD pattern of the prepared tetragonal In2S3. All of the peaks in Figure 2a,b can be indexed as cubic In2S3 with lattice parameters of a ) 10.75 Å (Figure 2a) and a ) 10.76 Å (Figure 2b), which are in agreement with the reported value of a ) 10.77 Å (JCPDS Card No. 65-0459), and all of the peaks in Figure 2c can be indexed as tetragonal In2S3 with lattice parameters of a ) 7.621 Å and c ) 32.37 Å, which are in agreement with the reported values of JCPDS Card No. 73-1366. No other characteristic peaks can be detected in Figure 2. Microstructures of cubic and tetragonal In2S3 were revealed through low-magnification and high-magnification FESEM studies. Parts a and b of Figure 3 are the low- and highmagnification FESEM images of the prepared cubic In2S3 without any coordination agents. These images show that the product of cubic In2S3 exists as hollow microspheres with
Figure 2. XRD patterns of cubic and tetragonal In2S3 samples prepared under different initial conditions: (a) without any coordination agent; (b) 2 mmol of tartaric acid; (c) 1 mmol of EDTADS (C10H14N2O8Na2‚ 2H2O).
average diameters of up to several tens of micrometers, which consist of nanoflakes with a thickness of 10 nm; we can observe hollow microspheres consisting of clean and smooth nanoflakes. Parts c and d of Figure 3 give the low- and high-magnification FESEM images of the cubic In2S3 prepared by adding 2 mmol of tartaric acid as the coordination agent. These images reveal that the products of cubic In2S3 nanoflakes have widths and lengths in the range of 200-300 nm, with thicknesses of 5-15 nm. Parts e and f of Figure 3 give the low- and highmagnification FESEM images of the tetragonal In2S3 prepared by using EDTADS as the coordination agent. These images also show that the product of tetragonal In2S3 exists as hollow microspheres consisting of nanoflakes. The morphologies and microstructures of the prepared cubic In2S3 were further investigated using TEM. Figure 4 gives lowand high-magnification TEM images of prepared cubic and tetragonal In2S3 under different reaction conditions. The TEM
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Figure 3. Low- and high-magnification FESEM images of three In2S3 products under different initial conditions: (a, b) without any coordination agents; (c, d) 2 mmol of tartaric acid; (e, f) 1 mmol of EDTADS (C10H14N2O8Na2‚2H2O).
images further demonstrate that the obtained products have microstructures of nanoflakes. The results reveal that the cubic In2S3 microsphere and tetragonal microsphere products were composed of nanoflakes. The SAED patterns of representative cubic and tetragonal nanoflakes as well as HRTEM observations for individual nanoflakes provide additional insight into the structure of cubic In2S3. Typical selected area electron diffraction (SAED) patterns of cubic and tetragonal In2S3 single-crystal nanoflakes are given in parts a and b of Figure 5. Figure 5c is the HRTEM image taken from the single-crystal nanoflakes, which shows the clearly resolved lattice fringes; the interplanar spacing was measured to be 3.81 Å, corresponding to the (220) planar spacing of cubic In2S3. The lattice fringes in Figure 5c show the integral nature of the crystallinity of the nanoflake. All of these images further substantiate the notion that these nanoflakes are single crystals. Further evidence for the composition of the prepared cubic In2S3 was obtained by XPS analysis (not shown here). No peaks of elements other than C, S, and In are observed on a survey of the spectrum. The binding energies for In 3d5/2 and S 2p3/2 are 444.33 and 161.16 eV, respectively, which agree with the data reported for In2S3.32 The contents of In and S are quantified by In 3d and S 2p peak areas, and a molar ratio of 0.7:1 for In:S is given. No obvious peaks for other elements were observed. To depict the nature of the prepared In2S3 sample, thermogravimetric analysis (TGA) measurements were carried out. Figure 6 shows the weight loss of the sample as a function of temperature. TGA shows two distinct weight-loss steps in the temperature range of 400-600 °C, with a straightforward weight loss of 14.06%, which is approximately equal to the ideal weight loss of 14.72% when In2S3 is oxidized into In2O3 in air. The weight-loss process ceases at 550 °C, and the stable residue can be reasonably ascribed to In2O3. The dynamics mechanism of In2S3 inserted into small amounts of lithium was investigated using the precursors.34,35
Figure 4. Low- and high-magnification TEM images of prepared products: (a, b) cubic In2S3 (without any coordination agent); (c, d) cubic In2S3 (2 mmol of tartaric acid); (e, f) tetragonal In2S3 (1 mmol of EDTADS).
Figure 5. SAED patterns of prepared (a) cubic In2S3 and (b) tetragonal In2S3 and an HRTEM image of (c) a single cubic In2S3 nanoflake.
The electrochemical performance of the prepared cubic In2S3 in the cell configuration of Li/In2S3 was evaluated. Figure 7 shows the first discharge curves of cubic In2S3 with a cutoff potential of 0.0 V at a current density of 0.032 mA/cm2. The discharge plateaus corresponding to 1.58 and 1.35 V appeared
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Acknowledgment. Financial support from the National Natural Science Foundation of China and the 973 Project of China (Number 2005CB623601) is greatly appreciated. References
Figure 6. TGA curve of the prepared In2S3 sample.
Figure 7. Discharge curve of cubic In2S3 crystallites recorded with a current density of 0.032 mA/cm2.
in the first discharge process. In the whole discharge process, the electrochemical process may be ascribed to the reactions36
cathode: (1) In3+ + e f In2+; (2) In2+ + 2e f In anode: 3Li - 3e f 3Li+ Here, cathode reactions may present a discharge curve with two plateaus. The first discharge capacity of cubic In2S3 nanoflakes can reach about 1150 mA h g-1, corresponding to about 1.0 Li of intercalation per cubic In2S3, which is much higher than that of common cathode materials. The increased capacity may be attributed to the large surface area and the hollow microstructures consisting of nanoflakes. 4. Conclusion In summary, a double-source approach has been developed to fabricate cubic and tetragonal In2S3 single-crystal nanoflakes with various macroscopic morphologies using different coordination agents under solvothermal conditions. The samples were characterized by multiple techniques. Electrochemical tests show that cubic In2S3 nanoflakes deliver an astoundingly large first discharge capacity of 1150 mA h g-1 vs Li metal at 0.032 mA cm-2 (voltage window of 0.0-3.0 V) compared with that of ordinary cathode materials. The obtained sample might find promising application as an electrode material in lithium ion batteries.
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