Hierarchical MoS2 Hollow Architectures with Abundant Mo Vacancies

Apr 22, 2019 - The hollow structured HMF-MoS2 delivers an impressive specific capacity ... MoS2 and sodium, resulting in the promotion of sodium stora...
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Hierarchical MoS2 Hollow Architectures with Abundant Mo Vacancies for Efficient Sodium Storage Yang Li,†,‡,# Rupeng Zhang,§,# Wei Zhou,⊥,# Xin Wu,† Huabin Zhang,*,∥ and Jian Zhang*,†

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†

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ‡ College of Chemistry, Fuzhou University, Fuzhou 350108, China § Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, China ⊥ Department of Applied Physics, Faculty of Science, Tianjin University, Tianjin 300072, P. R. China ∥ School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459, Singapore S Supporting Information *

ABSTRACT: Achieving a molecular level understanding of surface performance of nanomaterials by modulating the electronic structure is important but challenging. Here, we have developed a hollow microcube framework constructed by Mo-defect-rich ultrathin MoS2 nanosheets (HMF-MoS2) through a zeolite-like-framework-engaged strategy. The hollow structured HMF-MoS2 delivers an impressive specific capacity (384.3 mA h g−1 after 100 cycles at 100 mA g−1) and cycle stability (267 mA h g−1 after 125 cycles at 1 A g−1) for sodium storage. As evidenced by experiments and density functional theory calculations, abundant Mo vacancies in MoS2 can greatly accelerate the charge transfer and enhance the interaction between MoS2 and sodium, resulting in the promotion of sodium storage. Kinetic analysis result reveals that the ultrafast sodium ion storage of HMF-MoS2 could be associated with the significant contribution of capacitive energy storage. This work highlights the detailed molecular level understanding of chemical reaction on MoS2 surface by defect and morphology engineering, which can be applied to other metal sulfides for energy storage devices. KEYWORDS: zeolite-like framework, MoS2, Mo defect, hollow architecture, sodium-ion battery

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architectures can not only enhance the energy density of SIBs duo to their higher weight fraction of active species, but also effectively alleviate the stress-induced structural variation.16−18 Moreover, the directional alignment of ultrathin MoS2 nanosheets in 3D hierarchical hollow architecture can reduce the sodium ion diffusion paths effectively.19 On the other hand, the adsorption of sodium ions is strongly related to electronic configuration of a material, especially the electron delocalization,20 and defect engineering has always been considered to be an effective approach to regulate the electronic structure.21−23 However, up to now, the rational design and facial synthesis of defect-rich oriented ultrathin

wo-dimensional (2D) layered-structure transitional metal dichalcogenides (e.g., MoS2, WS2, and SnS2) with analogous structure to graphene have shown great promise for energy-related applications.1−4 In particular, MoS2 with high specific capacity has been suggested as a promising anode material for sodium ion batteries (SIBs).5−9 The effective insertion and adsorption of sodium ions in the microstructure of MoS2 can largely facilitate conversion reaction, leading to the high capacity.10,11 Nevertheless, the conventional route of the reversible sodium-ion store in the bulk MoS2 electrode materials is diffusion-controlled and limited by the slow kinetics, resulting in an inferior rate capability and poor cyclability.12 Assembling defect-rich oriented ultrathin 2D MoS2 nanosheets into three-dimensional (3D) hierarchical hollow architectures would maximally expose individual MoS2 nanosheets and realize their full merits. 13−15 The hollow © XXXX American Chemical Society

Received: January 15, 2019 Accepted: April 17, 2019

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DOI: 10.1021/acsnano.9b00383 ACS Nano XXXX, XXX, XXX−XXX

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(Figures 2d and S4). The reduced particle size and wrinkled surface topology are considered the results of the etchinginduced shrinking. XRD patterns for different phases witness the conversion of HZIF-Zn/Mo into HMF-MoS2 (Figures 2e and S1). TEM is also performed to investigate the conversion process of the hybrids. As shown in Figures 3a and S5, the hollow feature of HMF-MoOx/ZnS is clearly demonstrated. Highresolution TEM (HRTEM) reveals the existence of a large quantity of ZnS nanoparticles with visible lattice fringes on the surface of HMF-MoOx/ZnS (Figure 3a, b). The measured interplanar distance and the diffraction rings of the selectedarea electron diffraction (SAED) analysis confirm the wellcrystallized ZnS nanoparticles (Figure 3c).26 Figure 3d,e shows the same free-standing HMF-MoS2 microcube with different orientations, which further confirms the hollow microcube structure. As shown in the magnified TEM image, the hierarchical edge of HMF-MoS2 is constructed from extensive MoS2 nanosheets with ultrathinness (Figures 3f and S6c,d). Figure 3g presents an enlarged view of the spot marked in Figure 3f, and also indicates the interface organized by MoS2 nanosheets. HRTEM image reveals parallel stacking and the layered boundary of MoS2 nanosheets (Figure 3h). SAED analysis further affords a pattern of several concentric circles, suggesting the polycrystalline nature of the ultrathin MoS2 nanosheets (Figure 3i). The lattice fringe is measured to be 0.27 nm. This value is identical to the (100) plane of MoS2 (Figure 3h,j). The interlayer distance of MoS2 in HMF-MoS2 is expanded to be 0.66 nm, which is larger than that of pristine MoS2 (0.62 nm) (Figure S7).27−29 Meanwhile, some Mo vacancies are observed and marked by the red polygon and circles in Figures 3k−m and S8. The high-angle annular darkfield scanning transmission electron microscopy (HAADFSTEM) image of HMF-MoS2 shows an almost transparent microcube framework constructed by ultrathin nanosheets. The elemental mapping images reveal the coexistence and even distribution of Mo and S elements in HMF-MoS2 (Figure 3n−p). Raman spectroscopy is performed to demonstrate the structure characters of HMF-MoS2 (Figure S9). Two distinct Raman signals at 380 and 405.1 cm−1 for HMF-MoS2 and pristine MoS2 correspond to E12g and A1g phonon vibration modes of MoS2, which are excited by terrace-terminated and edge-terminated MoS2 layer, respectively.30−32 The intensity ratio of E12g and A1g can reflect the edge-exposed degree of MoS2, in which a smaller value implies a higher edge exposure degree. The intensity ratio of E12g and A1g is 0.39 for HMFMoS2, and increases to 0.47 for pristine MoS2, revealing that more exposed sulfur edges and Mo defects have been generated in HMF-MoS2.33−36 The element composition and the surface electronic states of HMF-MoS2 are investigated by X-ray photoelectron spectroscopy (XPS). In the highresolution Mo 3d spectra, the binding energies centered at about 229.4 and 232.5 eV are attributed to the Mo4+ of MoS2 in HMF-MoS2. The S 2p XPS spectrum in HMF-MoS2 shows one doublet at 162.2 and 163.4 eV, which corresponds to the S 2p3/2 and S 2p 1/2 binding energy for S2− of MoS2, respectively.37−40 The existence of carbon residues in the HMF-MoS2 has also been confirmed as the presence of C 1s spectra. The residual carbon is further confirmed (ca. 8 wt %) by thermogravimetric analysis (TGA) (Figure S11), which can greatly enhance the electronic conductivity of HMF-MoS2.41 Figure S12 shows the N2 sorption isotherms of HMF-MoS2.

MoS2-based hollow architectures still remain a significant challenge. Here, we have developed an unusual two-time-sulfidation strategy coupled with a facile selective etching method to fabricate hollow microcube framework constructed by Modefect-rich ultrathin MoS2 nanosheets (denoted as HMFMoS2). This approach involves the initial formation of hollow amorphous-MoOx/ZnS microcube framework (HMF-MoOx/ ZnS) and the subsequent high-temperature sulfidation for the transformation of amorphous-MoOx into MoS2 (HMF-MoS2/ ZnS). In the high-temperature sulfidation process, the in situ confined sulfidation reaction restricts the oriented growth of MoS2, while the existence of ZnS facilitates the generation of MoS2 nanosheet. Thus, the HMF-MoS2 with accurately introduced Mo vacancy is obtained after the removing of ZnS by hydrochloric acid (HCl) solution. Density functional theory (DFT) calculation reveals that the Mo vacancies can not only accelerate the charge transfer, but also lead to strong binding affinity for sodium. As expected, with the merits of Mo-defect-rich oriented ultrathin MoS2 nanosheet to deliver high specific capacity, and the hollow porous structure to accommodate the large volume expansion upon cycling, the asobtained HMF-MoS2 compound produces excellent sodium storage performance.

RESULTS AND DISCUSSION The as-synthesized hybrid is constructed by an unusual twotime-sulfidation strategy coupled with a facile selective etching method. The synthetic process for HMF-MoS2 is schematically illustrated in Figure 1. First, the Zn, Mo-based hybrid zeolitic

Figure 1. Schematic illustration of the formation of HMF-MoS2 with abundant Mo vacancies.

imidazolate frameworks (denoted as HZIF-Zn/Mo) are dispersed in an ethanol solution, followed by the addition of thioacetamide (TAA). Thereafter, the mixture is refluxed at elevated temperature.24,25 During this sulfidation stage, HMFMoOx/ZnS is formed. Subsequently, calcination treatment in the presence of sulfur is applied to convert the Mo component to the MoS2 phase, while the crystallinity of the ZnS shell is further improved, resulting in the generation of HMF-MoS2/ ZnS. Finally, the HMF-MoS2 can be obtained after selective removal of the ZnS phase by hydrochloric (HCl) acid solution. The structure evolution of the products is thoroughly characterized by field-emission scanning electron microscopy (FESEM) (Figures 2, S1, S2, and S3). The surface of HMFMoS2 becomes wrinkly after the chemical etching process B

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Figure 2. FESEM images of HZIF-Zn/Mo (a); HMF-MoOx/ZnS (b) and (c); HMF-MoS2 (d); XRD patterns of HMF-MoOx/ZnS, HMFMoS2/ZnS, and HMF-MoS2 (e).

The specific surface area is confirmed to be about 264.44 m2 g−1, and the small hysteresis in the P/P0 range of 0.4−1.0 indicates mesoporous structure in HMF-MoS2. Motivated by the above structural and compositional design, the sodium storage property of the as-synthesized HMF-MoS2 compound is investigated in a coin-cell-type configuration. Cyclic voltammetry (CV) is first performed to qualitatively evaluate its electrochemical properties (Figure 4a). For the first cathodic scan, a wide shoulder peak located at about 1.0−1.3 V vs Na+/Na corresponds to the reversible intercalation of Na+ into the MoS2 lattice to form NaxMoS2.42,43 The other small but broader peak observed at 0.77 V can be ascribed to the irreversible formation of the solid-electrolyte interface (SEI) layer.44 A dominant peak below 0.4 V is associated with the formation of Na2S and metallic Mo via a conversion reaction.45−47 For the reverse anodic scan, a major peak at 0.45 V, a very tiny bump at about 1.41 V and a broad peak at 1.83 V are detected, resulting from the stepwise oxidation of Mo and MoS2 reconstruction.48 From the second scan onward, the CV curves show almost negligible changes in both amplitude and voltage positions, indicating the high reversibility and excellent cycling stability of the HMF-MoS2 anode in SIBs. The galvanostatic charging−discharging profiles (GCDs) of HMF-MoS2 electrode are also performed at 100 mA g−1 current density (Figure 4b). For the initial cycle, the discharge and charge capacities of HMF-MoS2 are 694.6 and 418.5 mA h g−1, respectively, with a Coulombic efficiency of 60.2%. The initial capacity loss can be attributed from the irreversible formation of the SEI layer and the electrolyte decomposition.49−51 Then, for the second and third cycles, the discharge capacities stabilize at 436.5 and 417.6 mA h g−1 with increased Coulombic efficiency of 92.7% and 93.8%, respectively. After 100 cycles, the HMF-MoS2 still remains high reversible capacities of 384.3 mA h g−1, delivering 88% of the second cycle and keeping almost 100% Coulombic efficiency (Figure 4b,c). Those values are superior to that of many other related

MoS2-based hybrids for SIBs (Table S1). For the control sample pristine MoS2, Figure 4c shows continue and progressive capacity decay during the whole cycle, which should be attributed to the large volume change-induced uncontrolled aggregation. As is shown in Figure 4d, the HMF-MoS2 electrode also demonstrates impressive rate capability. The average specific capacities for HMF-MoS2 are 412, and 226 mA h−1 at a stepped current density of 0.1 and 5 A g−1, respectively. The capacity retention for HMF-MoS2 is as high as 71.9% with an order of magnitude increase in the current density from 0.5 to 5.0 A g−1, demonstrating obvious advantage when comparing with the pristine MoS2 electrode. Furthermore, as the current density is reset to 0.1 A g−1, the reversible capacities and the voltage plateaus of HMF-MoS2 can fully recover and remain very stable for extended cycling, indicative of the high sustainability. All these results clearly demonstrate the superior structural tolerance of the designed architecture during the Na+ insertion/extraction. The cycling stability can be reflected from Figure 4e, where the HMF-MoS2 electrode is cycled at a large current density of 1.0 A g−1. It is worth noting that the HMFMoS2 exhibits a high reversible capacity of 283 mA h g−1 in the 75th cycles and finally stabilizes at around 267 mA h g−1 after 125 cycles, with a high capacity retention rate of 93.7% and almost 100% Coulombic efficiency. To unveil the electrochemical kinetics of HMF-MoS2 electrode, CV curves at different scan rates are further conducted (Figure 5a).52,53 The relation (i = avb) between current (i) and sweep rate (v) obeys the power law.54,55 From the obtained b value, the electrochemical reaction behaviors between the diffusion-controlled process and the surface capacitive process can be classified. As can be seen from the inset of Figure 5b, the calculated b values of the four redox peaks (O1, R1, R2, and O2) are 0.915, 0.868, 0.85, and 1.0, respectively, indicating that the current is predominantly controlled by the surface capacitive reaction. Besides, the b values of cathode and anode can be maintained in a range of C

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Figure 3. TEM image of an individual HMF-MoOx/ZnS (a). Low-magnification HRTEM image of outer shells of HMF-MoOx/ZnS and the corresponding SAED patterns (b and c). TEM images of the same free-standing of HMF-MoS2 microcube with different orientations (d and e). TEM (f and g), HRTEM (h and j), and the corresponding SAED patterns images of HMF-MoS2 (i). HRTEM and high-magnification view of the defects (k, l, and m). HAADF-STEM (n) and elemental mapping images of HMF-MoS2 (o and p).

ion and Mo-defect-rich ultrathin MoS2 nanosheets (Figure 5f).58,59 To further evaluate the effect of Mo vacancies on the electronic configuration and sodium storage performance of the HMF-MoS2, DFT calculations are performed. Total and partial charge density of states curves (TDOS and PDOS) show that the normal MoS2 has a direct band gap of 1.71 eV. Differently, after importing Mo vacancies, not only the energy gap between the valence band and conduction band is narrowed, but also the Fermi level is crossed, indicating an improved electrical conductivity (Figures 6, S14, and S15). It has been widely accepted that the sodium storage capacity of conversion compounds can be enhanced by the strong interaction between sodium and active components due to their reduced formation energy (Ef) of Na-based compounds. In comparison with normal MoS2, the sodium adsorption energy on the Mo-defected MoS2 is greatly changed from −1.65 eV to −3.02 eV, confirming that the Mo-defected MoS2 can more efficiently anchor sodium and facilitate the conversion reaction. Hence, the abundant Mo defects in HMF-MoS2 can promote sodium storage by enhancing interaction between MoS2 and sodium.

0.8−1.0 at a large voltage range, which further confirms the improved rate performance and cycling stability of HMF-MoS2 electrode, leading to a fast sodiation/de-sodiation (Figure 5b). More specifically, according to Dunn’s work,56 the ratios of stored charge contributed by capacitive contribution can be further quantified (Figure 5c). Based on the quantification, an increase of the capacitive contribution accompanying with an increase of scan rate and 82.5% capacitive contribution at a sweep rate of 0.8 mV s−1 can be observed (Figures 5d and S13). This value is comparable to the excellent Mo-base electrode.57 To further understand the quick redox reaction on the surface of HMF-MoS2 electrode, the electrochemical impedance spectrum (EIS) measurements are performed. Figure 5e shows the Nyquist plots of HMF-MoS2 electrode and pristine MoS2 electrode after 100 cycles at 0.1 A g−1, which contains a semicircle and an inclined line. Fitting the curves of Nyquist plots by a classic equivalent circuit model is shown in the inset of Figure 5e, where Rct is the charge-transfer resistance at the interfaces. By comparison, the Rct value of HMF-MoS2 (75.8 Ω) is lower than that of pristine MoS2 (95.3 Ω), indicating the faster interface kinetics of HMF-MoS2 electrode. A relative higher slope in the low-frequency region for HMF-MoS2 electrode indicates the enhanced interaction between sodium D

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Figure 4. CV curves of as-synthesized HMF-MoS2 compound electrode at a scan rate of 0.1 mV s−1 for the first four cycles (a). Galvanostatic charge−discharge curves of as-synthesized HMF-MoS2 compound electrode under a current density of 100 mA g−1 (b). Cycling performance of HMF-MoS2 and pristine MoS2 at a current density of 100 mA g−1 (c). Rate capability of the electrodes based on HMF-MoS2 and pristine MoS2 (d). Cycling performance of HMF-MoS2 at a current density of 1 A g−1 (e).

Figure 5. CV curves of HMF-MoS2 at various scan rates (a). b values vs battery voltage of HMF-MoS2 for cathodic and anodic scans. Inset: current vs scan rate of HMF-MoS2 at different voltages (b). Normalized contribution radio of capacitive- and diffusion-controlled capacities at different scan rates (c). Capacitive- and diffusion-controlled contribution to charge storage at 0.8 mV s−1 (d). Impedance plots and equivalent circuit (inset) used for the EIS analysis of the HMF-MoS2 and pristine MoS2 obtained after 100 cycles at 100 mA g−1 (e). The Z′ω−1/2 curves in the low-frequency region of the HMF-MoS2 and pristine MoS2 (f).

CONCLUSIONS In summary, we develop a facile self-templated method for preparing hollow microcube framework constructing by Modefect-rich ultrathin MoS2 nanosheets. The boosted sodium adsorption by Mo defect and the unique structural design is

fundamental for achieving enhanced sodium storage of HMFMoS2. The robust hierarchical hollow architecture with more exposed electrochemical active species endows HMF-MoS2 high specific capacity and large electrolyte/electrode contact area. Moreover, the directional alignment of the ultrathin MoS2 E

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the solvent is completely volatilized, the copper foil is cut into many copper pieces with a diameter of 12 mm. Finally, the fabricated working electrodes are assembled with sodium metal as a counter, polypropylene microporous membrane as the separator and 1.0 M NaClO4−EC/DEC (1:1 volume ratio) as the electrolyte in the glovebox filled with Ar atmosphere. Computational Methods. Vienna ab initio Simulation Package (VASP) was used for performing all the calculations. The exchangecorrelation energy was confirmed by the generalized gradient approximation. Besides, the D2 method proposed by Grimme was used for describing the van der Waals interactions. For eliminating interactions between the neighboring cells of slab models, a vacuum region of 20 Å was applied. Monkhorst k-point meshes (2 × 2 × 1) were applied for the Brillouin-zone integrations of slab supercell models with and without Ni decoration. An error of 1 × 10−5 eV atom−1 was adopted for the total energy convergence.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b00383. Additional information for the characterization of samples and the electrochemical data (PDF)

Figure 6. Charge density distribution of MoS2 with Mo vacancies (a). Total (b) and partial (c and d) charge density of states of MoS2 with Mo vacancies and pristine MoS2.

AUTHOR INFORMATION

nanosheets can facilitate sodium transfer and diffusion. DFT computations reveal that the Mo vacancies can effectively accelerate the charge transfer and enhance the binding between MoS2 and sodium. This work provides a good example for understanding the enhanced sodium storage properties at atomic and molecular levels.

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wei Zhou: 0000-0002-8004-3996 Huabin Zhang: 0000-0003-1601-2471 Jian Zhang: 0000-0003-3373-9621

EXPERIMENTAL SECTION Synthesis of HZIF-Zn/Mo Microcube. HZIF-Zn/Mo was prepared by solvothermal method. Typically, 0.09 g Zn(CH3COO)2·2H2O, 0.054 g 2-methylimidazole, 0.022 g molybdic acid, and 3.2 g polyvinylpyrrolidone were added to 60 mL N,Ndimethylformamide. Subsequently, the mixture was transferred to a 100 mL Teflon-lined airtight reactor, and then aged at 160 °C in a preheated oven for 48 h. The light green product was collected by centrifugation and washed several times with ethanol before drying at 60 °C overnight. Synthesis of HMF-MoOx/ZnS. The as-synthesized HZIF-Zn/Mo microcubes (120 mg) were dispersed in an ethanol solution, followed by the addition of thioacetamide (TAA). Thereafter, the mixture was refluxing at 120 °C for 12 h. Then, the formed precipitate was harvested by several rinse-centrifugation cycles with ethanol, and dried at 70 °C. Synthesis of HMF-MoS2. To synthesize the HMF-MoS2, sulfur powder (10 mg) and the as-prepared HMF-MoOx/ZnS (80 mg) were uniformly mixed with a mortar and pestle. Then, the mixture was placed in a porcelain boat and heated at 800 °C under nitrogen (N2) atmosphere for 120 min. After the temperature was reduced to room temperature under the N2 environment, the products were collected and then dispersed in 60 mL 1.0 M HCl solution for 12 h. The black product of HMF-MoS2 was rinsed with deionized water and ethanol and dried at 60 °C. Materials Characterization. XRD dates were obtained on MiniFlex II diffractometer. Raman dates were carried on LabRAM HR instrument with a 532 nm excitation laser. FESEM images were acquired on JSM6700-F microscope. TEM images were taken on Tecnai F20 microscope. XPS was performed on Thermo Fisher instrument. N2 sorption measurement was collected on ASAP 2010 analyzer. Electrochemical Measurements. The working electrode is prepared by spreading slurry made of active material (85%), carbon black (Super-P, 15%), and PVDF (5%) on a copper foil. Then the copper foil is transferred to the vacuum oven at 80 °C for 24 h. After

Author Contributions #

Y. L., R. Z., and W. Z. contributed equally. The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acsnano.9b00383 ACS Nano XXXX, XXX, XXX−XXX