Nanocasting synthesis of ordered mesoporous crystalline WSe2 as anode material for Li-ion batteries

Nanocasting synthesis of ordered mesoporous crystalline WSe2 as anode material for Li-ion batteries

Materials Letters 136 (2014) 191–194 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet N...

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Materials Letters 136 (2014) 191–194

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Nanocasting synthesis of ordered mesoporous crystalline WSe2 as anode material for Li-ion batteries Fujie Chen a, Jun Wang b, Bin Li b, Chaohua Yao b, Haifeng Bao c,n, Yifeng Shi b,n a

School of Chemistry and Chemical Engineering, Wuhan Textile University, Hubei 430200, China College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou, Zhejiang 310036, China c School of Materials Science and Engineering, Wuhan Textile University, Hubei 430200, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 13 May 2014 Accepted 9 August 2014 Available online 19 August 2014

An ordered mesoporous WSe2 material with crystalline frameworks is synthesized by a nanocasting method using mesoporous silica SBA-15 as a hard template. Phosphotungstic acid (H3PW12O40) and selenium powder are used as W and Se sources, respectively. The phosphotungstic acid precursor is impregnated inside the template and converted to WSe2 by H2Se, which is formed inside the tube furnace by the reaction between Se and H2 gas flow. The mesoporous WSe2 product has a highly ordered mesostructure and possesses rod-like particle morphology. The mesoporous WSe2 material displays good electrochemical properties with high reversible capacity (530 mA h/g) and stable cycling performance. The results suggest that the mesoporous WSe2 material may be a promising candidate for reversible storage of lithium ions. & 2014 Elsevier B.V. All rights reserved.

Keywords: Tungsten selenide Porous materials Nanocasting Energy storage and conversion

1. Introduction Mesoporous material shows promising potentials in various research areas, including catalysis, sorption, separation, sensing, and drug delivery, etc. [1]. Their distinctive advantages come from the combination of their uniform mesoporosity and the multifunctional properties of their nano-sized frameworks [2]. Among them, mesoporous metal chalcogenide semiconductors (sulfide, selenide, and telluride) are outstand themselves because of their promising potentials in photocatalysis, lithium battery, solar cell, and thermoelectric devices, etc., which can be attributed to their semiconductor nature and unique electrochemistry properties [3]. However, the synthesis of highly ordered mesoporous chalcogenide with crystalline frameworks is still a great challenge, especially for selenide and tellurides [2,3]. Ordered mesostructured metal chalcogenides, like CdSe and CdTe, have been fabricated by using organic surfactants as structuredirecting agents via solution synthesis approach [3]. However, the obtained products are typically made of individual nanocrystals that supported by the surfactant micelle array. Therefore, their ordered mesostructures collapsed if the organic surfactants were entirely removed either by calcination or by solvent extraction [3]. In addition, the non-stoichiometric and amorphous status of the framework further limited their applications. Recently, Zhao and Shi reported the nanocasting synthesis of several mesoporous crystalline metal sulfides, including CdS, ZnS, MoS2 and WS2 [2]. Very recently,

n

Corresponding authors. E-mail addresses: [email protected] (H. Bao), [email protected] (Y. Shi).

http://dx.doi.org/10.1016/j.matlet.2014.08.050 0167-577X/& 2014 Elsevier B.V. All rights reserved.

their method has been improved for the synthesis of ordered mesoporous MoSe2 material, which shows promising performance in photocatalytic decomposition of dyes and lithium-ion batteries [4]. WSe2 has a similar crystal structure with MoSe2, and several interesting properties of WSe2 materials have been reported in literature. A photoelectrochemical cell efficiency of 17.1% over WSe2 material was reported by Prasad and Srivastava [5]. WSe2 material is also famous for its super-low thermal conductivity discovered by Cahill et al. [6]. Very recently, it has been found that WSe2 can be used as a cathode material for Mg-ion batteries [7]. These results indicate that if mesoporous WSe2 material can be synthesized, it may also be very useful in photocatalysis and energy storage. In this work, we synthesized a highly ordered mesoporous crystalline WSe2 material via nanocasting strategy. Mesoporous silica SBA-15 with rod-like particle morphology was used as a hard template. Phosphotungstic acid (PTA) and selenium powder were used as W and Se sources, respectively. The obtained mesoporous WSe2 product has a highly ordered mesostructure with 2D hexagonal symmetry. The lithium-ion storage behavior of the synthesized mesoporous WSe2 material was also investigated. Primary results show that the mesoporous WSe2 displays good electrochemical properties with high reversible capacity (530 mA h/g) and stable cycling performance.

2. Experimental Mesoporous silica SBA-15 was prepared according to the literature report [8]. The hydrothermal treatment temperature and the calcination temperature used in the synthesis are 110

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and 400 1C, respectively. Then, 40 g of PTA precursor was mixed with 10 g of SBA-15 hard template and 100 mL ethanol under stirring in an open crucible on the top of a hotplate. After the ethanol was evaporated up at 40 1C, the obtained [email protected] nanocomposite was loaded in a quartz boat placed in the middle of a quartz tube furnace. Another quartz boat with 45 g of selenium powder was placed in the upstream direction just by the side of the former one. Then the furnace was heated up to 600 1C with a ramp of 2 1C/min under a hydrogen gas flow (400 mL/min). After keeping in this temperature for 8 h, the furnace was cooled down to room temperature. The silica template was removed by 4% HF aqueous solution. The final product was collected after the sample was washed with water and ethanol, and dried in a vacuum oven at 60 1C over night. The working electrode for electrochemical measurements was prepared by casting a slurry made of the active material (WSe2, 80 wt%), conductive material (carbon black, 10 wt%) and adhesive (polyvinylidene fluoride, 10 wt%), dissolved in N-methylpyrrolidone, on a clean copper foil with a loading amount of  1 mg/cm2. Button-type test cells were assembled in an Ar-filled glove box by using fresh Li foil as the counter electrode and 1.0 M LiPF6 in diethylcarbonate, ethyl methyl carbonate and ethylene carbonate (DMC:EMC:EC ¼ 1:1:1w/w/w) as electrolyte solution. The electrochemical performances were characterized on battery tester LAND CT2001A (Wuhan, China) at room temperature. X-ray diffraction (XRD) patterns were recorded with a Cu Kα radiation source on a Bruker D8 diffractometer. Transmission electron microscopy (TEM) images were taken on a FEI Tecnai T20 Sphera electron microscope operating at 200 keV. Scanning electron microscopy (SEM) images were acquired on a FEI XL30 Sirion FEG Digital Scanning electron microscope. Nitrogen sorption isotherms were measured at 77 K on a Micromeritics Tristars 3000 analyzer.

3. Result and discussion Mesoporous silica SBA-15 template with 2D hexagonal patterned cylinder mesopores was prepared according to the literature report and used as the hard template in this work [8]. Three strong diffraction peaks can be found in the small-angle X-ray diffraction (XRD) pattern of the SBA-15 template (Fig. 1a), which clearly demonstrated the highly ordered mesostructure of the template [1]. Nitrogen sorption analysis reveals that it has a pore

volume of 1.1 cm3/g and a specific surface area of 710 m2/g. The mean pore size calculated from the adsorption branch of the isotherms is 9.3 nm. PTA precursor was filled inside the SBA-15 template via a solvent evaporation induced capillary condensation process [9]. Then it was heated up to 600 1C with selenium powder under a hydrogen gas flow in a tube furnace. During this process, selenium was melted and then gasified by reaction with H2 to form H2Se [4]. At the same time, the H2Se further reacted with PTA precursor to form WSe2 inside the mesopore of the SBA-15 template. Mesoporous WSe2 product was obtained after the removal of silica template. Small-angle XRD pattern of the mesoporous WSe2 product shows an narrow and intensive diffraction peak at 2 theta value around 11 and a broad peak at 2 theta value around 1.71, corresponding to the (1 0 0) and (1 1 0) Bragg diffraction peaks of the p6mm mesostructure (Fig. 1a) [1]. This result indicates that the mesoporous WSe2 product well replicated the ordered mesostructure of SBA-15 template. The diffraction peaks of mesoporous WSe2 is slightly shifted to a higher angle due to the mesostructure shrinkage caused by the high temperature treatment during the synthesis. Wide-angle XRD pattern of the mesoporous WSe2 product shows more than six intensive diffraction peaks at 2 theta value from 10 to 701, and all of them can be indexed to the hexagonal phase WSe2 (JCPDS: 89-5257) (Fig. 1b). These results suggest that PTA precursor was successfully converted to WSe2 and the framework was well crystalized. SEM observation reveals that the particle morphology of the mesoporous WSe2 product is rod-like particles, 1.0–1.5 μm in length and 300–600 nm in diameter, indicating that it well copied the particle morphology of SBA-15 template (Fig. 2a and b). Highresolution SEM images show that each mesoporous WSe2 particle is composed of an ordered nanowires array (Fig. 2c). The diameters of these WSe2 nanowires are estimated to be approximately 8–10 nm (Fig. 2d), in agreement with the mean pore size value (9.3 nm) of SBA-15 template’s cylindrical mesopores estimated by nitrogen sorption analysis. TEM observation also confirms that the mesoporous WSe2 product possesses a highly ordered mesoporous structure with rod-like particle morphology (Fig. 2e and f). The HRTEM image (Fig. 2g) clearly shows the crystalline nature of the mesoporous WSe2 product, as well as the layered crystal structure with an interlayer distance about 0.65 nm, corresponding to the interplanar distance between the Se–W–Se trilayers. More than four concentric cycles can be found in the selected area electron

Fig. 1. Powder XRD patterns of SBA-15 template and mesoporous WSe2 product.

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Fig. 2. (a)–(d) SEM images, (e)–(g) TEM images, (d) (inset) EDX and (h) SAED patterns of mesoporous WSe2 materials.

Fig. 3. (a) The discharge/charge profiles and (b) the cycle performance of mesoporous WSe2 material in the range of 0.01–3.00 V.

diffraction (SAED) pattern of mesoporous WSe2 (Fig. 2h), which can be indexed to the (1 0 0), (1 0 3), (1 0 5) and (1 1 0) diffraction rings. This result suggests that the framework of mesoporous WSe2 is nanocrystalline with random orientation. Energy dispersive X-ray (EDX) analysis reveals that the mesoporous WSe2 product is only composed by tungsten and selenium without oxygen or silicon atoms (Fig. 3d inset), indicating that PTA was completely converted to WSe2 and silica template was entirely removed. All these results demonstrate that an ordered mesoporous crystalline WSe2 material with 2D hexagonal patterned mesostructure has been successfully synthesized. Large amount of PTA precursor can be easily filled inside the mesoporous silica template, which can be attributed to the high solubility and high density. 40 g of PTA precursor can be impregnated into the mesopores space of 10 g of SBA-15 hard template by one cycle of loading process in this work. In addition, PTA precursor contains more than 76 wt% tungsten. As a result, 40 g of PTA precursor leads to the formation of 57 g of mesoporous WSe2 product in theory. In our synthesis, 55.2 g of final product was collected, which is a very high yield for nanocasting synthesis. In general, 1 g of mesoporous silica template only leads to less than 0.5 g product in the nanocasting synthesis of metal oxides and carbons, in which the yields are far lower than that in our synthesis. Galvanostatic discharge–charge experiments were taken to investigate the reversible electrochemical lithium storage performance of the ordered mesoporous WSe2 product. Fig. 3a shows the first ten discharge/charge profiles of the mesoporous WSe2 electrode at a current rate of 0.1 C (Note that 0.1 C refers to 4 mol Li uptake into MoSe2 per formula unit in 10 h) between 0.01 and 3.00 V. The record discharge/charge profiles is quite similar to

those of mesoporous MoSe2 and MoS2 materials, which can be attributed to their similar chemistry properties as well as their layered crystal structures [4]. In the first cycle, the discharge and charge capacities of mesoporous WSe2 material are 762 and 555 mA h/g, respectively. The large irreversible capacity of 207 mA h/g may come from the formation of a solid electrolyte interface (SEI) layer [2]. After the first cycle, the discharge/charge curves are all overlapping together (Fig. 3a), indicating a highly reversible lithium ion storage behavior. Similar with the result of mesoporous MoSe2, both of the discharge and charge capacities are much higher than the expected theoretical capacity of 314 mA h/g (assuming an uptake of 4 mol of Li into 1 mol of WSe2 species). It has been widely reported that ultra-high capacities were recorded for the nanostructured metal chalcogenide materials with layered crystal structures. Foe example, the theoretical capacity of MoS2 is 670 mA h/g, while several different groups reported that their MoS2 materials with different nanostructured all show reversible capacities even higher than 900 mA h/g [10–12]. In case of mesoporous MoSe2, it shows a reversible capacity of 630 mA h/g, which is also much higher than its theoretical value (422 mA h/g) [4]. It might because of the interfacial Li storage mechanism proposed by Maier and/or other reasons which are not clear at this moment [13]. Although the discharge/charge capacity of mesoporous WSe2 is much higher than its theoretical capacity value, it still possesses an excellent cycling performance as revealed by the charge/discharge profiles showed in Fig. 3a and the cycling performance data in Fig. 3b. The coulombic efficiency increases to 93.6% in the second cycle and keeps higher than 95% after that. Therefore, the reversible capacity is stable at 530 mA h/g after 30 cycles (Fig. 3b). The lithium storage capacity of this material is not as

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high as that of silicon-based anode material and the mesoporous MoSe2 material. However, WSe2 has a very high density value (9.32 g/cm3), which makes is has a high specific capacity in volume. In addition, the layered crystal structure makes it can be used for the storage of ions with larger size, like Mg2 þ and Na þ . The Mg and Na ion storage behavior of our material is under testing. 4. Conclusions A highly ordered mesoporous crystalline WSe2 materials with 2D hexagonal mesostructure was synthesized by using SBA-15 as hard template via nanocasting strategy. The mesoporous WSe2 material possess a reversible lithium storage capacity of 530 mA h/g for up to 20 cycles without any significance decrease, making it may be used as anode candidate for lithium-ion battery. Acknowledgments This work was supported by funding from the Program for New Century Excellent Talents in University (NCET-12-1083), NSFC

(21003035, 21103038), Key Project of Chinese Ministry of Education (211066) and the Special Funds for key innovation team of Zhejiang Province (2010R50017).

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