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Reversible conversion of MoS2 upon sodium extraction
Nano Energy 41 (2017) 217–224
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Full paper
Reversible conversion of MoS2 upon sodium extraction a,b
Shuai Hao
b,c
a,b
, Xi Shen , Meng Tian
b,c
, Richeng Yu , Zhaoxiang Wang
a,b,⁎
MARK a,b
, Liquan Chen
a
Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China b School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China c Laboratory for Advanced Materials & Electron Microscopy, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 100190, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Structure Reversibility Conversion NaMoS2 Molybdenum disulfide
Intercalation and conversion are two of the most popular forms of lithium (Li) or sodium (Na) storage. Conversion reactions usually involve transference of more than one electron. Therefore, the electrode materials that store Li- and Na-ions by way of such reactions theoretically deliver specific capacities a few times higher than those of the intercalation materials. Intercalation and conversion reactions can occur in sequence upon Na storage in MoS2, but the reversibility of the latter upon Na extraction is still controversial. In this article, we clarify that the conversion reaction of nano-MoS2 can be reversible in a carbon matrix and the extent of reversibility is highly dependent on the dynamic properties of the composite. In addition, hexagonal NaMoS2 is recognized as a new phase in the early stage of Na extraction from the reduction products of MoS2, Mo+Na2S, on the basis of high-resolution transmission electron microscopy (HRTEM) characterization and first-principles calculations. These findings enrich the understanding of reaction mechanism of MoS2 upon Na storage and removal and are helpful to the design and applications of the transition metal sulfides.
1. Introduction Conversion and intercalation are two of the most important storage ways of lithium, sodium and other small atoms. A conversion reaction usually corresponds to a high specific capacity for the active material since it involves multi-electron reaction in most cases, beneficial for obtaining secondary batteries with high energy densities. Tarascon and coworkers [1] realized reversible conversion reactions in nano-sized CoO and NiO in 2000 and obtained reversible capacities up to 650 and 600 mAh g−1, respectively, roughly twice that of graphitic carbons that store lithium (Li) by way of intercalation reaction (LiC6). The reversible conversion reaction draws extensive and intense interest since its end products include transition metals and it is the basis for constructing high energy-density secondary batteries [2]. However, all conversion reactions are not reversible, especially for materials composed of high-valence transition metals and robust polyanions such as PO43- [3], SO42- or anions with weak electronegativity (e.g. S2-). For example, although Li insertion in nano-sized MnO is reversible, the Li-ions inserted in nano-Mn3O4, nano-Mn2O3 or nano-MnO2 cannot be completely extracted up to 3.0 V (vs Li+/Li). Rather, only Mn2+ (in MnO) can be obtained as the highest oxidation state of recharge [4]. In contrast, by constructing a nano-composite
with conducting (e.g. nano-carbon) [5] or catalyzing (e.g., nano-Ni) [6] species, we successfully oxidized the metallic Sn that was previously believed inactive. Therefore, we believe that the reversibility of the conversion reactions is controlled to some extent by the dynamic properties of the material. Layer-structured disulfide (such as MoS2, FeS2, TiS2) has found extensive applications in electrical and optoelectronic devices [7–10]. The large space between the S-M-S slabs (M for Mo, Fe, Ti, etc.) and the electrostatic stability of the negatively charged S2- ions permit intercalation of guest atoms such as alkali metals [11,12] and even molecules [13] therein, and intercalation compounds are formed [14]. As an anode material for the secondary lithium or sodium batteries, MoS2 can transfer four electrons, corresponding to a specific capacity of 669 mAh g−1. Combined with carbons, MoS2 with various morphologies such as balls [15,16], tubes [17,18], flowers [19,20], sheets [21–23], etc. have been prepared and showed improved cycling and rate performances as well as specific capacities close to its theoretical value. Wang et al. [24] in this group comprehensively investigated the structural transition of micro-sized MoS2 and observed a series of phase transitions during sodium storage, 2H-MoS2→2H-Na0.5MoS2→1TNa0.5MoS2→1T-NaMoS2→Na2S(+Mo). Earlier the Li-ion storage process in MoS2 was found to be MoS2→LixMoS2→Li2S (+Mo) [25]. These
⁎ Corresponding author at: Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. E-mail address: [email protected] (Z. Wang).
http://dx.doi.org/10.1016/j.nanoen.2017.09.039 Received 5 August 2017; Received in revised form 3 September 2017; Accepted 19 September 2017 Available online 21 September 2017 2211-2855/ © 2017 Elsevier Ltd. All rights reserved.
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indicate the presence of nano-scaled 2H-MoS2 (JCPDS Card No. 371492) and the amorphous feature of the carbon (broad diffraction peak between 2θ = 22 and 27°). With increasing content of MoS2 in the composite, the diffraction intensity of the MoS2 (002) plane at 14.3° increases, demonstrating that more layers of S-Mo-S slabs are stacked in each MoS2 slice. Raman spectroscopy (Fig. 1b) is sensitive to the amorphous species and was applied to detect the purity of the MoS2/C composite. Each composite presents two Raman peaks at 382 cm−1 (E21g for the opposite vibrations of the S atoms against the Mo atom in 2H-MoS2) and 405 cm−1 ( A1g for the out-of-plane vibration of the S atoms), respectively [30]. No signals for MoO3 or MoO2 (characteristic bands between 250 and 380 cm−1) [31] were detected, demonstrating the absence or negligible content of the MoOx in the composite. The broad peaks at ∼ 1360 cm−1 (the D band) and ∼ 1600 cm−1 (the G band) are characteristic of the amorphous feature of the carbon in the composite. The SEM imaging shows that the pure carbon contains flower-like 3D networks with well-developed sub-micrometer pores (Fig. S1a). The 3D networks are composed of ultrathin corrugated nano-sheets (Fig. S1b). Loading of different contents of MoS2 does not influence the morphology of the carbon nano-sheets (Fig. S1a, b, c, g and h) except that the thickness of the nano-sheet increases slightly with increasing MoS2 content (Fig. S1d, e, f, j and k). Energy dispersive X-ray (EDX) mappings (Fig. S2) show that the MoS2 grains are distributed uniformly on the carbon sheets. The architecture of the MoS2/C composites was characterized by TEM imaging (Fig. 2c and d and Fig. S3). Some dark dots are distributed on the grey background (Fig. 2c and S3a). They become larger with increasing MoS2 content in the composite. With the help of the above SEM imaging, these dots are attributed to MoS2 nano-grains while the grey background is for the carbon sheets. The lattice fringes of the MoS2 can be recognized in the HRTEM images (Fig. 2d and Fig. S3b). The fringes with spacings of 0.65 nm and 0.27 nm on the surface and at the curled edge of grains belong to the MoS2 (002) and (100) planes, respectively. That is, the a-b plane of the MoS2 slices is parallel to the carbon sheets and the S-Mo-S slabs of about 3–6 layers stack along the c-direction (Fig. 2d). In addition, the size of the MoS2 slices is a few tens of nanometers or even smaller. The lattice fringe of the MoS2 (002) planes is bent to some extent when corrugations appear. Moreover, the MoS2 nano-slices were not separated from the carbon sheet host upon ultrasonic treatment for preparing the TEM samples, indicating the strong contact of the MoS2 slices with the carbon nano-sheets, beneficial for improving the electric conductivity and the dynamic properties of the composite. Fig. 3a exhibits the galvanostatic charge-discharge profiles of the MoS2/C composite between 0.01 and 3.00 V at 10 mA g−1 in the first
studies well explain the high discharge capacity of MoS2 as an anode material for the secondary batteries. In comparison with the electrochemical reduction of MoS2, little is known about the oxidation process of these reduction products, especially when a conversion reaction takes place and the MoS2 is transformed from crystalline to amorphous. It was reported that the Li-insertion product of MoS2 (Mo+Li2S) cannot be oxidized to MoS2, probably due to the covalent properties of the Li2S. Rather the Li2S is delithiated at a higher potential (2.3 V vs. Li+/Li) and that the original Li/MoS2 cell works like a Li/S or Li/Li2S cell [26]. Similarly, Xie et al. [16] reported that Na2S is oxidized to S rather than restored to MoS2, on the basis of the cyclic voltammetry (CV) analysis. However, Zhang et al. [27] and Wang et al. [28] claimed to have detected MoS2 in the charged sample after fully discharging MoS2 to Na2S and Mo. Nevertheless, the low signal-to-noise ratio of their XRD patterns made their recognition less persuasive and the reversibility of the conversion reaction remains unclear. There are even authors who supposed the reversibility of the conversion without any structural characterization [19,23,29]. Considering the wide applications of MoS2 and the impacts of reaction pathways on the design of the secondary batteries, a comprehensive understanding and persuasive clarification of the reaction pathway of MoS2 is essential. In this article, a series of MoS2 nano-slices embedded in amorphous carbon nano-sheets were prepared in order to find out the reversibility of MoS2 and its dependence on the dynamic properties of the nanocomposite. On the basis of high-resolution transmission electron microscopy (HRTEM) and other characterizations, we show solid evidence that the discharge product of MoS2 (Mo+Na2S) can be oxidized to MoS2, clarifying the structural reversibility of MoS2 after a complete reduction (an electrochemical conversion reaction). In addition, NaMoS2 was recognized at the atomic scale at the early stage of (Mo +Na2S) oxidation. Finally, the completeness of the oxidation and the reaction pathway are found highly dependent on the carbon content in the composite. The feasibility of this oxidation was supported with the first-principles calculated formation energy. These findings enrich our understanding to the reversible conversion reaction of MoS2. It will help to control the conversion process by material design and enlighten more research on other materials of conversion reactions and promote their applications in secondary batteries and other areas. 2. Results and discussion The content of MoS2 in the MoS2/C composite was determined to be 46.7, 55.9, 65.9 and 84.5 wt%, respectively. Therefore, these composites were named as MoS2/C-47, MoS2/C-56, MoS2/C-66 and MoS2/C85, respectively. The XRD patterns of the Mo2S/C composite (Fig. 1a)
Fig. 1. The XRD patterns (a) and Raman spectra (b) of the MoS2/C composites.
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Fig. 2. Typical morphology (SEM images; a, b) and architecture (TEM images; c, d) of the MoS2/C (MoS2/C-66) composites at various magnifications.
cycle. Since the MoS2 slices in the composite are nano-sized, the discharge plateaus that are usually observed in well-crystallized MoS2 are replaced with a series of slopes in the nano-composite. This evolution process excludes the concern that pseudo-capacitance that usually occurs on or near the surface of an electrode material contribute the major part of the observed capacity. In addition, for most part of the discharge profile, the discharge potential of MoS2/C-47 is higher than that of the
other samples due to its higher electric conductivity and the resultant lower polarization. The reversible capacity of the MoS2/C-47, MoS2/C56, MoS2/C-66 and MoS2/C-85 is 499, 538, 556 and 563 mAh g−1 (all calculated by taking both the MoS2 and the carbon host as the active material), respectively. It increases with the MoS2 content in the composite because the theoretical specific capacity of MoS2 (669 mAh g−1) is higher than that of the carbon host (inset of Fig. 3a). Meanwhile, the
Fig. 3. The galvanostatic discharge/recharge profiles of the MoS2/C composite between 0.01 and 3.00 V at 10 mA g−1 in the first cycle (a; the inset is for that of the pure carbon) and their cyclic voltammograms at a scanning rate of 0.1 mV s−1 (b; the inset is for an overview of the composite and pure carbon).
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with lattice spacing of 0.74 nm and of about 2–7 stacking layers thick come into view (Fig. 4c and S6c). Meanwhile, the number of domains for metallic Mo (Fig. 4c) and for Na2S (Fig. S6c) decreases. As the cell is recharged to 1.50 V, the domains with a lattice spacing of 0.74 nm grow larger; each domain contains more than 10 atomic layers (Fig. 4d and S6d). The increased grain size and the enhanced regularity of the crystal lattice make it easier to observe more of their structural details. It is seen that each light-grey lattice fringe is sandwiched between two dark-grey fringes. The corresponding line scan profile of the image contrast along the blue crossing line (inset of Fig. 4d) makes these features more significant. Considering the lattice spacing value and the alternately changing image contrast, these grains are indexed to “NaxMoS2”, a phase usually observed in Na-intercalated well-defined MoS2 [24] but never reported in the recharge product of fully discharged MoS2. The dark fringes are attributed to the S-Mo-S slabs while the light ones are for the Na layers. These observations demonstrate that NaxMoS2 is formed below 0.50 V and more Na2S is consumed to form more NaxMoS2 above 0.5 V. This process continues in a wide potential range. As the composite is recharged to 3.0 V, the lattice spacing shrinks to 0.61–0.68 nm, with no light lattice fringes observable between the dark ones (Fig. 4e and S6e). This means that the Na-ions are extracted from NaxMoS2 and MoS2 becomes the dominant sulfide; the shrunk fringes are for the MoS2 (002) plane. Meanwhile, domains for metallic Mo are still available but Na2S is absent. These observations and recognitions are supported with the X-ray photoelectron spectroscopic investigation of these materials as well (Fig. S8). Similar evolution process of the crystal lattice fringe is observed in the composite with lower MoS2 contents (Fig. S7 for MoS2/C-47). The slight difference between these composites lies in the fact that MoS2 is observed at a recharge potential as low as 0.5 V. It becomes the dominating phase in the 1.5 V recharged sample while very few NaxMoS2 domains can be observed at and above that potential. This means that de-intercalation of the Na-ions from NaxMoS2 (or transformation from NaxMoS2 to MoS2) can be finished at a lower potential in a composite with better electric conduction. Another point that deserves notice is that metallic Mo can be observed in all the recharged samples though their crystallinity is much lower than that of MoS2 and NaxMoS2. However, Na2S or elemental S was not observed in any of these recharged samples, probably due to their even lower crystallinity than in the initially discharged samples. We tried to characterize the recharge products of these composite with XRD technique but did not receive any viable information, probably due to the poor crystallinity of these samples. In order to have an in-depth understanding of the reversible conversion reaction, first-principles calculations were applied to compare the formation energy of different oxidation products (Table 1). As stated in Section 1 of this article, the Li-inserted MoS2 (final reduction products, Li2S+Mo) was believed unable to be reversibly converted to MoS2. Rather, Li2S was delithiated as the cell is recharged to about 2.3 V (vs. Li+/Li) while the metallic Mo remains untouched in this process [34]. Therefore, de-sodiation from Na2S (Path 3 in Table 1) is considered to be a possible oxidation path as well as the other two paths. It shows that the formation energy of NaxMoS2 (Path 1) is generally lower than (but close to) that of MoS2 (Path 2) as x < 3.0. Actually, as the valence of Mo is ≤ +1 and the valence of S in a sulfide is −2, the x value in NaxMoS2 cannot be ≥ 2. Therefore, the first-principles calculations indicate that both NaxMoS2 (x < 2) and MoS2 can be the oxidation product of (Na2S+Mo) thermodynamically, but the formation of NaxMoS2 is energetically more beneficial than that of MoS2. In addition, the minimal formation energy (3.85 eV) appears at x = 1.0. Therefore, NaMoS2 is the most possible oxidation product. This agrees well with the experiment and explains the presence of NaMoS2 and MoS2 at the very early stage of recharge (as low as 0.44 V). In contrast, much higher energy is required to remove the Na-ions from Na2S (the formation energy of S is ca. 6.896 eV and the oxidation peak observed
coulombic efficiencies of these composites decrease with the reducing content of MoS2 in it. The high carbon content in the composite are supposed to be responsible for the low coulombic efficiency. On the one hand, the specific surface area of the carbon nano-sheets is very high. The defect sites on the basal plane and edge of the carbon will adsorb Na-ions irreversibly. On the other hand, the high specific surface area and the high contents defects on the nano-carbon surface deteriorate the electrolyte decomposition, especially in the first discharge. Another interesting observation is that the recharge profile above 1.60 V is bent towards the high capacity side with the increasing MoS2 content. This bending is related to the enhanced oxidation peak at ca. 1.82 V in the CV profile (Fig. 3b) and will be discussed in detail in this article. The inset of Fig. 3b shows an overview of the CV profiles of the pure carbon and the composite in the initial cycle. The strong and obvious cathodic and anodic peaks exclude the suspicion that the nano-MoS2 contribute much pseudo-capacitance to the composite capacity as these redox peaks become more obvious with decreasing content of MoS2 in the composite. Each composite shows three reduction peaks. The two peaks above 0.50 V are for the Na-ion intercalation in MoS2 (formation of Na0.5MoS2 and NaMoS2, respectively) [24]. These intercalation reactions are believed reversible in the subsequent recharge processes [24]. Further discharge leads to decomposition of the intercalation product (Na1+xMoS2) and a conversion reaction takes place below 0.15 V, Na1+xMoS2+(3-x)Na++(3-x)e- → 2Na2S+Mo [24]. This reduction and the electrolyte decomposition correspond to a strong and broad (the 3rd) reduction peak in the CV profile. Obvious oxidation peaks appear at around 0.44, 1.00 and 1.82 V as the cell is recharged (Fig. 3b). Some authors observed peaks at ca. 1.00 and 1.82 V [21,32] and some others only reported the peak at ca. 1.82 V [15,19,28,33]. To our knowledge, no authors have ever reported the oxidation peak at ca. 0.44 V. Moreover, it is interesting that the relative intensity of the 1.82 V peak increases significantly but the relative intensity of the other two peaks roughly keeps constant, with increasing MoS2 content in the composite. As the pure carbon does not have oxidation peaks in this region (inset of Fig. 3b and Fig. S4a) and that the carbon host enhances the electric conductivity of the MoS2/C composites, we suppose that the distinct features of these CV profiles actually mean that the oxidation path is related to dynamic properties of the composite. That is, which of the oxidation reactions contribute more recharge capacity (integrated area of the oxidation peaks in the CV profile) to the cell is highly dependent on the electric conductivity of the composite. Fig. S4 shows the CV profiles of the MoS2/C composite in the first three cycles. The re-appearance of the oxidation peaks illustrates the good reversibility of these electrochemical reactions though the intensity of the peaks decreases and the polarization becomes severe. HRTEM imaging was conducted in order to recognize the oxidation products and to find out the recharge process of the discharge (0.01 V) products. Fig. 4 and Fig. S6 show the HRTEM images of MoS2/C-66 at various states. As it is discharged to 0.01 V, small (1–3 nm) and large (5–10 nm) grains are observed (Fig. 4a and Fig. S6a). Their lattice spacings are indexed to the Mo(200) (spacing 0.20 nm) and Na2S(220) (spacing 0.23 nm) planes, respectively, agreeing with the previous reports [24,32] and our XRD patterns (Fig. S5). These observations and indexing also indicate that a complete conversion reaction (reduction) occurs as the composite is discharged to 0.01 V. They also demonstrate that we were able to avoid the possible air contamination during sample preparation (solvent rinsing and drying of the electrode sheets, and ultrasonic dispersing) and sample transference (from the glove-box into the vacuum chamber of the instruments). The fact that the MoS2 nano-slices are embedded in a conducting carbon matrix facilitates the oxidation of the ultrafine discharge products of MoS2 (nano-sized Na2S and metallic Mo). When MoS2/C-66 is recharged to 0.50 V, grains with a lattice spacing of 0.74 nm appear but the above discharge products (Mo+Na2S) are still dominant in the view field (Fig. 4b and S6b). As the cell is charged to 0.75 V, more grains 220
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Fig. 4. The HRTEM images of MoS2/C-66 discharged to 0.01 V (a), recharged to 0.50 V (b), 0.75 V (c), 1.50 V (d), and 3.00 V (e) after initially discharged to 0.01 V. The insets in d and e are for the corresponding line profile of contrast along the blue line. (f) Schematic crystal structure of NaxMoS2 and MoS2 from [110] direction (the yellow, blue and grey balls are for the S, Mo and Na atoms, respectively).
Table 1 Formation energies of various oxidation products. ΔE = Eproduct-Ereactant (eV)
Reaction path
Path 1: Mo + 2Na2S → NaxMoS2 + (4-x)Na + (4-x)e Path 2: Mo + 2Na2S → MoS2 + 4Na+ + 4ePath 3: Mo + 2Na2S → Mo + S + 4Na+ + 4e+
-
x = 0.5
x = 1.0
x = 1.5
x = 2.0
x = 3.0
x = 4.0
4.048 4.165 6.896
3.852
4.018
4.009
4.683
5.436
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Fig. 5. The annular bright-field (ABF) images of MoS2/C-66 along the [001] zone axis and partial enlarged views (insets): (a) as-prepared, (b) recharged to 1.5 V after initially discharge to 0.01 V. The blue, yellow, blue/grey and yellow/grey balls are overlaid in the image for Mo, S, Mo/Na and S/Na atoms, respectively.
at 1.82 V). As NaxMoS2 is a phase that has not been reported before as a recharge product, we hereby provide more information on its structure at the atomic scale by STEM imaging. No atom is observed in the center of the hexagonal lattice of the as-prepared MoS2/C-66 (Fig. 5a), typical of the 2H-MoS2. As it is recharged to 1.5 V after the initial complete discharge, the center of the hexagonal lattice of NaMoS2 is occupied (Fig. 5b), similar to that of 1T-MoS2 [24,35,36]. As each spot in an STEM image is for a column of atoms, we suppose that the Na-ions in NaMoS2 are located right under the S or Mo atoms in the interlayer of 1T-MoS2 when viewed in the [001] direction. It was reported that MoS2 transits from 2H to metastable 1T phase as the alkali metals (e.g. Li+10,42, Na+26) or other small ions (e.g. NH4+13) are intercalated in its interlayer, enlarging its interlayer distance and releasing the resultant stress. Here, we recognize that the re-arrangement of NaMoS2 during charging is also in a structure of 1T phase. This structure is favorable in energy and internal stress release. As a result, after Na-ion extraction from NaMoS2, 1T-MoS2 is a preferential phase in fully recharged state. Therefore, the above characterization, recognition and calculations demonstrate the reversibility of MoS2 after the initial conversion reaction. With these, the conversion process of MoS2 during Na-ion insertion and extraction can be summarized as follows (Fig. 6). Insertion of the Na-ions in layered 2H-MoS2 leads to translation of one of its sulfur planes along an intra-layer atomic plane and results in the formation of 1T-NaxMoS2 (x = 0–1.5) [24]. As more Na-ions are inserted, it is reduced to Na2S and metallic Mo [32]. In the early stage of recharge, 1TNaxMoS2 (ca. x = 1) is firstly re-constructed by way of direct reaction of Mo+Na2S at ca 0.44 V due to its lower formation energy than MoS2. Right after that, hexagonal 1T-MoS2 is formed (Eq. (2a)) at ≤ 1.00 V because its formation energy is very close to that of NaMoS2. MoS2 can also be obtained by extracting Na-ions from NaMoS2 at around 1.0 V and above (Eq. (2b)). The higher is the electric conductivity of the composite, the better will these processes be separated. For example, many NaMoS2 domains can be observed in MoS2/C-66 charged to 1.5 V but very few NaMoS2 domains can be detected in MoS2/C-47 recharged to the same potential. However, in a real MoS2 or MoS2-based composite, these three processes may be overlapped in some potential ranges due to dynamic reasons. Therefore, more Na2S is consumed to form 1T-NaxMoS2 and MoS2 as the recharge process continues. The following reactions are typical of MoS2/C-47 or MoS2 composites with better dynamic properties.
Fig. 6. Schematic illustration of the reversible intercalation and conversion in MoS2 composite.
2Na2S + Mo → 1T-NaxMoS2 + (4-x)Na+ + (4-x)e- (x ~ 1) (≥ 0.44 V) (1) Mo + 2Na2S → MoS2 + 4Na+ + 4e- (≤ 1.00 V)
(2a)
1T-NaxMoS2 → 1T-MoS2 + xNa
(2b)
+
-
+ xe (≥ 1.00 V)
Whether or not the 1T-NaxMoS2 and MoS2 can be re-formed depends to a great extent on the dynamic properties, including particle size and electric conductivity of the composite. For MoS2 with poor dynamic properties (MoS2 overloaded and micro-MoS2), oxidation of the metallic Mo becomes difficult. In this case, the Na-ions are directly removed from Na2S and amorphous S is formed at ca. 1.82 V or above. The metallic Mo in the composite remains untouched. This is what happens in most, especially the micro-sized, MoS2 electrode materials,
222
Mo + Na2S → Mo + S + 2Na+ + 2e- (ca. 1.82 V)
(3)
This reaction corresponds to the very strong anodic peak at 1.82 V in the CV profile of MoS2/C-85 (Fig. 3b). In comparison, anodic peaks for the formation of NaxMoS2 at ca. 0.44 V and for formation of MoS2 at ca. 1.00 V become negligible. Clearly the metallic Mo can be directly oxidized to NaxMoS2
ca. the the (≥
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line. The X-ray photoelectron spectra (XPS) were recorded on an Escalab 205Xi spectrometer. The binding energy was calibrated with C1 s spectrum of the contaminated carbon (at 284.8 eV) in the vacuum chamber.
0.44 V) or MoS2 (ca. 1.00 V) for a fully discharged MoS2 composite with good dynamic properties. For the former case, a typical reversible conversion occurs and 1T-MoS2 is formed. In any of these two cases, MoS2 is expected to be one of the final recharge products (cutoff at 3.0 V). As for the case of MoS2 with poor dynamic properties, only elemental S and Mo are expected to be present in the final recharge product. As the dynamic properties of the MoS2 materials are usually somewhere between the “good” and the “poor”, MoS2 and elemental Mo and S can be detected in the final recharge products. That is just what we have observed in this work. Although theoretically different reaction paths of MoS2 lead to the same total specific capacity (a total of 4 electrons are transferred per formula of MoS2, ~ 669 mAh g−1), different specific energies will be delivered as the Na-ions are extracted at different potentials in different reaction paths. Therefore, controlling of the reaction paths by way of designing the material is critical in tuning the battery performances.
4.2. Electrochemical evaluation Button-type (CR2032) test cells were assembled in an Ar-filled glove-box (O2 ≤ 0.5 ppm, H2O ≤ 0.5 ppm), with glass fiber as the separator and 1 mol L−1 NaClO4 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 v/v) as the electrolyte. Galvanostatic discharge/recharge cycling was performed on a Land CT2001A battery tester between 0.01 and 3.00 V (vs Na+/Na) at room temperature. Cyclic voltammetry (CV) was carried out on a CHI600D electrochemistry workstation at a scan rate of 0.1 mV s−1. 4.3. Computational method
3. Conclusions The first-principles calculations were performed in the Vienna ab initio Simulation Package (VASP) code [37]. The generalized gradient approximation (GGA) was employed to describe the exchange-correlation interactions. The Hubbard U correction was taken to calibrate the strong correlation of the d-electrons of the Mo atom [38]. The Hubbard U for the Mo-ion was set to 3.0 eV, following the previous report [39]. The cutoff energy for the wave function was 500 eV. The spin polarization was considered throughout the calculations. Both the cell parameters and the ionic coordinates were fully relaxed, using 11 × 11 × 3 Γ- centered κ- meshes. In comparison with the energy of ferromagnetic configuration of NaMoS2, the anti-ferromagnetic state is believed more stable for its lower total energy.
In summary, a series of composites of MoS2 nano-slices embedded in carbon nano-sheets (MoS2/C) were prepared. Embedding the MoS2 nano-slices in conducting carbon sheets improves the dynamic properties of MoS2. That permits us to draw a complete picture of the reactions, reversible and irreversible, intercalation and conversion, during recharge of the fully discharged MoS2/C composite, including the evolution of the oxidation products and the dependence of the reversibility on the dynamic properties of the composite. On the basis of comprehensive structural characterization and first-principles calculations, the reversibility of the conversion reaction in fully discharged MoS2 is clarified. Either as an intermediate product or as general recharge product, NaMoS2 is detected and recognized as a new phase in the recharged composite. As the oxidation of metallic Mo may occur at different potentials and improving the dynamic properties of the composite is beneficial for promoting the direct or indirect formation of MoS2, designing the structure of the MoS2 composite is critical in enhancing the energy density of the cell though the reaction path does not change the total specific capacity of the material. These findings enrich the understanding of reversible conversion reaction of MoS2 in Na-ion batteries. It could enlighten more research on other material of conversion reaction and promote their application in sodium ion batteries.
Notes The authors declare no completing financial interest. Acknowledgments This work was financially supported by the National 973 Program of China (2015CB251100). Appendix A. Supporting information
4. Experimental section Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.nanoen.2017.09.039.
4.1. Synthesis of nano MoS2/C composites
References
(NH4)6Mo7O24·4H2O, CH4N2S and citric acid were used as the sources for molybdenum (Mo), sulfur (S) and carbon (C), respectively. These reagents were dissolved in 500 mL deionized water by magnetic stirring. The obtained solution was frozen-dried into powder. The powder was then heated and kept at 750 °C for 1 h under argon (Ar) atmosphere in a tube furnace for the MoS2/carbon composite. By tuning the molar ratio of these chemicals, the content of MoS2 in the composite was controlled, resulting in a series of Mo2S/C composites. Pure carbon and pure MoS2 were prepared for comparison. The structure of the Mo2S/C composite was characterized on a X-ray diffractometer (Bruker D8 Advance) equipped with Cu Kα radiation (λ = 1.54 Å). The morphology and microstructure of the sample were examined on a Hitachi S-4800 scanning electron microscope (SEM) and a Tecnai G2 F20 U-TWIN high-resolution transmission electron microscope (HRTEM), respectively. Aberration-corrected scanning transmission electron microscopy (STEM) was performed on a JEOL ARM 200 F transmission electron microscope equipped with double Cs correctors for the condenser lens and objective lens. Annular bright-field (ABF) and high-angle annular dark-field (HAADF) images were acquired at acceptance angles of 11.5–23.0 and 90–370 mrad, respectively. Raman spectroscopy was conducted on inVia-Reflex using the 532 nm laser
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Full paper
Reversible conversion of MoS2 upon sodium extraction a,b
Shuai Hao
b,c
a,b
, Xi Shen , Meng Tian
b,c
, Richeng Yu , Zhaoxiang Wang
a,b,⁎
MARK a,b
, Liquan Chen
a
Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China b School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China c Laboratory for Advanced Materials & Electron Microscopy, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 100190, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Structure Reversibility Conversion NaMoS2 Molybdenum disulfide
Intercalation and conversion are two of the most popular forms of lithium (Li) or sodium (Na) storage. Conversion reactions usually involve transference of more than one electron. Therefore, the electrode materials that store Li- and Na-ions by way of such reactions theoretically deliver specific capacities a few times higher than those of the intercalation materials. Intercalation and conversion reactions can occur in sequence upon Na storage in MoS2, but the reversibility of the latter upon Na extraction is still controversial. In this article, we clarify that the conversion reaction of nano-MoS2 can be reversible in a carbon matrix and the extent of reversibility is highly dependent on the dynamic properties of the composite. In addition, hexagonal NaMoS2 is recognized as a new phase in the early stage of Na extraction from the reduction products of MoS2, Mo+Na2S, on the basis of high-resolution transmission electron microscopy (HRTEM) characterization and first-principles calculations. These findings enrich the understanding of reaction mechanism of MoS2 upon Na storage and removal and are helpful to the design and applications of the transition metal sulfides.
1. Introduction Conversion and intercalation are two of the most important storage ways of lithium, sodium and other small atoms. A conversion reaction usually corresponds to a high specific capacity for the active material since it involves multi-electron reaction in most cases, beneficial for obtaining secondary batteries with high energy densities. Tarascon and coworkers [1] realized reversible conversion reactions in nano-sized CoO and NiO in 2000 and obtained reversible capacities up to 650 and 600 mAh g−1, respectively, roughly twice that of graphitic carbons that store lithium (Li) by way of intercalation reaction (LiC6). The reversible conversion reaction draws extensive and intense interest since its end products include transition metals and it is the basis for constructing high energy-density secondary batteries [2]. However, all conversion reactions are not reversible, especially for materials composed of high-valence transition metals and robust polyanions such as PO43- [3], SO42- or anions with weak electronegativity (e.g. S2-). For example, although Li insertion in nano-sized MnO is reversible, the Li-ions inserted in nano-Mn3O4, nano-Mn2O3 or nano-MnO2 cannot be completely extracted up to 3.0 V (vs Li+/Li). Rather, only Mn2+ (in MnO) can be obtained as the highest oxidation state of recharge [4]. In contrast, by constructing a nano-composite
with conducting (e.g. nano-carbon) [5] or catalyzing (e.g., nano-Ni) [6] species, we successfully oxidized the metallic Sn that was previously believed inactive. Therefore, we believe that the reversibility of the conversion reactions is controlled to some extent by the dynamic properties of the material. Layer-structured disulfide (such as MoS2, FeS2, TiS2) has found extensive applications in electrical and optoelectronic devices [7–10]. The large space between the S-M-S slabs (M for Mo, Fe, Ti, etc.) and the electrostatic stability of the negatively charged S2- ions permit intercalation of guest atoms such as alkali metals [11,12] and even molecules [13] therein, and intercalation compounds are formed [14]. As an anode material for the secondary lithium or sodium batteries, MoS2 can transfer four electrons, corresponding to a specific capacity of 669 mAh g−1. Combined with carbons, MoS2 with various morphologies such as balls [15,16], tubes [17,18], flowers [19,20], sheets [21–23], etc. have been prepared and showed improved cycling and rate performances as well as specific capacities close to its theoretical value. Wang et al. [24] in this group comprehensively investigated the structural transition of micro-sized MoS2 and observed a series of phase transitions during sodium storage, 2H-MoS2→2H-Na0.5MoS2→1TNa0.5MoS2→1T-NaMoS2→Na2S(+Mo). Earlier the Li-ion storage process in MoS2 was found to be MoS2→LixMoS2→Li2S (+Mo) [25]. These
⁎ Corresponding author at: Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. E-mail address: [email protected] (Z. Wang).
http://dx.doi.org/10.1016/j.nanoen.2017.09.039 Received 5 August 2017; Received in revised form 3 September 2017; Accepted 19 September 2017 Available online 21 September 2017 2211-2855/ © 2017 Elsevier Ltd. All rights reserved.
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indicate the presence of nano-scaled 2H-MoS2 (JCPDS Card No. 371492) and the amorphous feature of the carbon (broad diffraction peak between 2θ = 22 and 27°). With increasing content of MoS2 in the composite, the diffraction intensity of the MoS2 (002) plane at 14.3° increases, demonstrating that more layers of S-Mo-S slabs are stacked in each MoS2 slice. Raman spectroscopy (Fig. 1b) is sensitive to the amorphous species and was applied to detect the purity of the MoS2/C composite. Each composite presents two Raman peaks at 382 cm−1 (E21g for the opposite vibrations of the S atoms against the Mo atom in 2H-MoS2) and 405 cm−1 ( A1g for the out-of-plane vibration of the S atoms), respectively [30]. No signals for MoO3 or MoO2 (characteristic bands between 250 and 380 cm−1) [31] were detected, demonstrating the absence or negligible content of the MoOx in the composite. The broad peaks at ∼ 1360 cm−1 (the D band) and ∼ 1600 cm−1 (the G band) are characteristic of the amorphous feature of the carbon in the composite. The SEM imaging shows that the pure carbon contains flower-like 3D networks with well-developed sub-micrometer pores (Fig. S1a). The 3D networks are composed of ultrathin corrugated nano-sheets (Fig. S1b). Loading of different contents of MoS2 does not influence the morphology of the carbon nano-sheets (Fig. S1a, b, c, g and h) except that the thickness of the nano-sheet increases slightly with increasing MoS2 content (Fig. S1d, e, f, j and k). Energy dispersive X-ray (EDX) mappings (Fig. S2) show that the MoS2 grains are distributed uniformly on the carbon sheets. The architecture of the MoS2/C composites was characterized by TEM imaging (Fig. 2c and d and Fig. S3). Some dark dots are distributed on the grey background (Fig. 2c and S3a). They become larger with increasing MoS2 content in the composite. With the help of the above SEM imaging, these dots are attributed to MoS2 nano-grains while the grey background is for the carbon sheets. The lattice fringes of the MoS2 can be recognized in the HRTEM images (Fig. 2d and Fig. S3b). The fringes with spacings of 0.65 nm and 0.27 nm on the surface and at the curled edge of grains belong to the MoS2 (002) and (100) planes, respectively. That is, the a-b plane of the MoS2 slices is parallel to the carbon sheets and the S-Mo-S slabs of about 3–6 layers stack along the c-direction (Fig. 2d). In addition, the size of the MoS2 slices is a few tens of nanometers or even smaller. The lattice fringe of the MoS2 (002) planes is bent to some extent when corrugations appear. Moreover, the MoS2 nano-slices were not separated from the carbon sheet host upon ultrasonic treatment for preparing the TEM samples, indicating the strong contact of the MoS2 slices with the carbon nano-sheets, beneficial for improving the electric conductivity and the dynamic properties of the composite. Fig. 3a exhibits the galvanostatic charge-discharge profiles of the MoS2/C composite between 0.01 and 3.00 V at 10 mA g−1 in the first
studies well explain the high discharge capacity of MoS2 as an anode material for the secondary batteries. In comparison with the electrochemical reduction of MoS2, little is known about the oxidation process of these reduction products, especially when a conversion reaction takes place and the MoS2 is transformed from crystalline to amorphous. It was reported that the Li-insertion product of MoS2 (Mo+Li2S) cannot be oxidized to MoS2, probably due to the covalent properties of the Li2S. Rather the Li2S is delithiated at a higher potential (2.3 V vs. Li+/Li) and that the original Li/MoS2 cell works like a Li/S or Li/Li2S cell [26]. Similarly, Xie et al. [16] reported that Na2S is oxidized to S rather than restored to MoS2, on the basis of the cyclic voltammetry (CV) analysis. However, Zhang et al. [27] and Wang et al. [28] claimed to have detected MoS2 in the charged sample after fully discharging MoS2 to Na2S and Mo. Nevertheless, the low signal-to-noise ratio of their XRD patterns made their recognition less persuasive and the reversibility of the conversion reaction remains unclear. There are even authors who supposed the reversibility of the conversion without any structural characterization [19,23,29]. Considering the wide applications of MoS2 and the impacts of reaction pathways on the design of the secondary batteries, a comprehensive understanding and persuasive clarification of the reaction pathway of MoS2 is essential. In this article, a series of MoS2 nano-slices embedded in amorphous carbon nano-sheets were prepared in order to find out the reversibility of MoS2 and its dependence on the dynamic properties of the nanocomposite. On the basis of high-resolution transmission electron microscopy (HRTEM) and other characterizations, we show solid evidence that the discharge product of MoS2 (Mo+Na2S) can be oxidized to MoS2, clarifying the structural reversibility of MoS2 after a complete reduction (an electrochemical conversion reaction). In addition, NaMoS2 was recognized at the atomic scale at the early stage of (Mo +Na2S) oxidation. Finally, the completeness of the oxidation and the reaction pathway are found highly dependent on the carbon content in the composite. The feasibility of this oxidation was supported with the first-principles calculated formation energy. These findings enrich our understanding to the reversible conversion reaction of MoS2. It will help to control the conversion process by material design and enlighten more research on other materials of conversion reactions and promote their applications in secondary batteries and other areas. 2. Results and discussion The content of MoS2 in the MoS2/C composite was determined to be 46.7, 55.9, 65.9 and 84.5 wt%, respectively. Therefore, these composites were named as MoS2/C-47, MoS2/C-56, MoS2/C-66 and MoS2/C85, respectively. The XRD patterns of the Mo2S/C composite (Fig. 1a)
Fig. 1. The XRD patterns (a) and Raman spectra (b) of the MoS2/C composites.
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Fig. 2. Typical morphology (SEM images; a, b) and architecture (TEM images; c, d) of the MoS2/C (MoS2/C-66) composites at various magnifications.
cycle. Since the MoS2 slices in the composite are nano-sized, the discharge plateaus that are usually observed in well-crystallized MoS2 are replaced with a series of slopes in the nano-composite. This evolution process excludes the concern that pseudo-capacitance that usually occurs on or near the surface of an electrode material contribute the major part of the observed capacity. In addition, for most part of the discharge profile, the discharge potential of MoS2/C-47 is higher than that of the
other samples due to its higher electric conductivity and the resultant lower polarization. The reversible capacity of the MoS2/C-47, MoS2/C56, MoS2/C-66 and MoS2/C-85 is 499, 538, 556 and 563 mAh g−1 (all calculated by taking both the MoS2 and the carbon host as the active material), respectively. It increases with the MoS2 content in the composite because the theoretical specific capacity of MoS2 (669 mAh g−1) is higher than that of the carbon host (inset of Fig. 3a). Meanwhile, the
Fig. 3. The galvanostatic discharge/recharge profiles of the MoS2/C composite between 0.01 and 3.00 V at 10 mA g−1 in the first cycle (a; the inset is for that of the pure carbon) and their cyclic voltammograms at a scanning rate of 0.1 mV s−1 (b; the inset is for an overview of the composite and pure carbon).
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with lattice spacing of 0.74 nm and of about 2–7 stacking layers thick come into view (Fig. 4c and S6c). Meanwhile, the number of domains for metallic Mo (Fig. 4c) and for Na2S (Fig. S6c) decreases. As the cell is recharged to 1.50 V, the domains with a lattice spacing of 0.74 nm grow larger; each domain contains more than 10 atomic layers (Fig. 4d and S6d). The increased grain size and the enhanced regularity of the crystal lattice make it easier to observe more of their structural details. It is seen that each light-grey lattice fringe is sandwiched between two dark-grey fringes. The corresponding line scan profile of the image contrast along the blue crossing line (inset of Fig. 4d) makes these features more significant. Considering the lattice spacing value and the alternately changing image contrast, these grains are indexed to “NaxMoS2”, a phase usually observed in Na-intercalated well-defined MoS2 [24] but never reported in the recharge product of fully discharged MoS2. The dark fringes are attributed to the S-Mo-S slabs while the light ones are for the Na layers. These observations demonstrate that NaxMoS2 is formed below 0.50 V and more Na2S is consumed to form more NaxMoS2 above 0.5 V. This process continues in a wide potential range. As the composite is recharged to 3.0 V, the lattice spacing shrinks to 0.61–0.68 nm, with no light lattice fringes observable between the dark ones (Fig. 4e and S6e). This means that the Na-ions are extracted from NaxMoS2 and MoS2 becomes the dominant sulfide; the shrunk fringes are for the MoS2 (002) plane. Meanwhile, domains for metallic Mo are still available but Na2S is absent. These observations and recognitions are supported with the X-ray photoelectron spectroscopic investigation of these materials as well (Fig. S8). Similar evolution process of the crystal lattice fringe is observed in the composite with lower MoS2 contents (Fig. S7 for MoS2/C-47). The slight difference between these composites lies in the fact that MoS2 is observed at a recharge potential as low as 0.5 V. It becomes the dominating phase in the 1.5 V recharged sample while very few NaxMoS2 domains can be observed at and above that potential. This means that de-intercalation of the Na-ions from NaxMoS2 (or transformation from NaxMoS2 to MoS2) can be finished at a lower potential in a composite with better electric conduction. Another point that deserves notice is that metallic Mo can be observed in all the recharged samples though their crystallinity is much lower than that of MoS2 and NaxMoS2. However, Na2S or elemental S was not observed in any of these recharged samples, probably due to their even lower crystallinity than in the initially discharged samples. We tried to characterize the recharge products of these composite with XRD technique but did not receive any viable information, probably due to the poor crystallinity of these samples. In order to have an in-depth understanding of the reversible conversion reaction, first-principles calculations were applied to compare the formation energy of different oxidation products (Table 1). As stated in Section 1 of this article, the Li-inserted MoS2 (final reduction products, Li2S+Mo) was believed unable to be reversibly converted to MoS2. Rather, Li2S was delithiated as the cell is recharged to about 2.3 V (vs. Li+/Li) while the metallic Mo remains untouched in this process [34]. Therefore, de-sodiation from Na2S (Path 3 in Table 1) is considered to be a possible oxidation path as well as the other two paths. It shows that the formation energy of NaxMoS2 (Path 1) is generally lower than (but close to) that of MoS2 (Path 2) as x < 3.0. Actually, as the valence of Mo is ≤ +1 and the valence of S in a sulfide is −2, the x value in NaxMoS2 cannot be ≥ 2. Therefore, the first-principles calculations indicate that both NaxMoS2 (x < 2) and MoS2 can be the oxidation product of (Na2S+Mo) thermodynamically, but the formation of NaxMoS2 is energetically more beneficial than that of MoS2. In addition, the minimal formation energy (3.85 eV) appears at x = 1.0. Therefore, NaMoS2 is the most possible oxidation product. This agrees well with the experiment and explains the presence of NaMoS2 and MoS2 at the very early stage of recharge (as low as 0.44 V). In contrast, much higher energy is required to remove the Na-ions from Na2S (the formation energy of S is ca. 6.896 eV and the oxidation peak observed
coulombic efficiencies of these composites decrease with the reducing content of MoS2 in it. The high carbon content in the composite are supposed to be responsible for the low coulombic efficiency. On the one hand, the specific surface area of the carbon nano-sheets is very high. The defect sites on the basal plane and edge of the carbon will adsorb Na-ions irreversibly. On the other hand, the high specific surface area and the high contents defects on the nano-carbon surface deteriorate the electrolyte decomposition, especially in the first discharge. Another interesting observation is that the recharge profile above 1.60 V is bent towards the high capacity side with the increasing MoS2 content. This bending is related to the enhanced oxidation peak at ca. 1.82 V in the CV profile (Fig. 3b) and will be discussed in detail in this article. The inset of Fig. 3b shows an overview of the CV profiles of the pure carbon and the composite in the initial cycle. The strong and obvious cathodic and anodic peaks exclude the suspicion that the nano-MoS2 contribute much pseudo-capacitance to the composite capacity as these redox peaks become more obvious with decreasing content of MoS2 in the composite. Each composite shows three reduction peaks. The two peaks above 0.50 V are for the Na-ion intercalation in MoS2 (formation of Na0.5MoS2 and NaMoS2, respectively) [24]. These intercalation reactions are believed reversible in the subsequent recharge processes [24]. Further discharge leads to decomposition of the intercalation product (Na1+xMoS2) and a conversion reaction takes place below 0.15 V, Na1+xMoS2+(3-x)Na++(3-x)e- → 2Na2S+Mo [24]. This reduction and the electrolyte decomposition correspond to a strong and broad (the 3rd) reduction peak in the CV profile. Obvious oxidation peaks appear at around 0.44, 1.00 and 1.82 V as the cell is recharged (Fig. 3b). Some authors observed peaks at ca. 1.00 and 1.82 V [21,32] and some others only reported the peak at ca. 1.82 V [15,19,28,33]. To our knowledge, no authors have ever reported the oxidation peak at ca. 0.44 V. Moreover, it is interesting that the relative intensity of the 1.82 V peak increases significantly but the relative intensity of the other two peaks roughly keeps constant, with increasing MoS2 content in the composite. As the pure carbon does not have oxidation peaks in this region (inset of Fig. 3b and Fig. S4a) and that the carbon host enhances the electric conductivity of the MoS2/C composites, we suppose that the distinct features of these CV profiles actually mean that the oxidation path is related to dynamic properties of the composite. That is, which of the oxidation reactions contribute more recharge capacity (integrated area of the oxidation peaks in the CV profile) to the cell is highly dependent on the electric conductivity of the composite. Fig. S4 shows the CV profiles of the MoS2/C composite in the first three cycles. The re-appearance of the oxidation peaks illustrates the good reversibility of these electrochemical reactions though the intensity of the peaks decreases and the polarization becomes severe. HRTEM imaging was conducted in order to recognize the oxidation products and to find out the recharge process of the discharge (0.01 V) products. Fig. 4 and Fig. S6 show the HRTEM images of MoS2/C-66 at various states. As it is discharged to 0.01 V, small (1–3 nm) and large (5–10 nm) grains are observed (Fig. 4a and Fig. S6a). Their lattice spacings are indexed to the Mo(200) (spacing 0.20 nm) and Na2S(220) (spacing 0.23 nm) planes, respectively, agreeing with the previous reports [24,32] and our XRD patterns (Fig. S5). These observations and indexing also indicate that a complete conversion reaction (reduction) occurs as the composite is discharged to 0.01 V. They also demonstrate that we were able to avoid the possible air contamination during sample preparation (solvent rinsing and drying of the electrode sheets, and ultrasonic dispersing) and sample transference (from the glove-box into the vacuum chamber of the instruments). The fact that the MoS2 nano-slices are embedded in a conducting carbon matrix facilitates the oxidation of the ultrafine discharge products of MoS2 (nano-sized Na2S and metallic Mo). When MoS2/C-66 is recharged to 0.50 V, grains with a lattice spacing of 0.74 nm appear but the above discharge products (Mo+Na2S) are still dominant in the view field (Fig. 4b and S6b). As the cell is charged to 0.75 V, more grains 220
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Fig. 4. The HRTEM images of MoS2/C-66 discharged to 0.01 V (a), recharged to 0.50 V (b), 0.75 V (c), 1.50 V (d), and 3.00 V (e) after initially discharged to 0.01 V. The insets in d and e are for the corresponding line profile of contrast along the blue line. (f) Schematic crystal structure of NaxMoS2 and MoS2 from [110] direction (the yellow, blue and grey balls are for the S, Mo and Na atoms, respectively).
Table 1 Formation energies of various oxidation products. ΔE = Eproduct-Ereactant (eV)
Reaction path
Path 1: Mo + 2Na2S → NaxMoS2 + (4-x)Na + (4-x)e Path 2: Mo + 2Na2S → MoS2 + 4Na+ + 4ePath 3: Mo + 2Na2S → Mo + S + 4Na+ + 4e+
-
x = 0.5
x = 1.0
x = 1.5
x = 2.0
x = 3.0
x = 4.0
4.048 4.165 6.896
3.852
4.018
4.009
4.683
5.436
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Fig. 5. The annular bright-field (ABF) images of MoS2/C-66 along the [001] zone axis and partial enlarged views (insets): (a) as-prepared, (b) recharged to 1.5 V after initially discharge to 0.01 V. The blue, yellow, blue/grey and yellow/grey balls are overlaid in the image for Mo, S, Mo/Na and S/Na atoms, respectively.
at 1.82 V). As NaxMoS2 is a phase that has not been reported before as a recharge product, we hereby provide more information on its structure at the atomic scale by STEM imaging. No atom is observed in the center of the hexagonal lattice of the as-prepared MoS2/C-66 (Fig. 5a), typical of the 2H-MoS2. As it is recharged to 1.5 V after the initial complete discharge, the center of the hexagonal lattice of NaMoS2 is occupied (Fig. 5b), similar to that of 1T-MoS2 [24,35,36]. As each spot in an STEM image is for a column of atoms, we suppose that the Na-ions in NaMoS2 are located right under the S or Mo atoms in the interlayer of 1T-MoS2 when viewed in the [001] direction. It was reported that MoS2 transits from 2H to metastable 1T phase as the alkali metals (e.g. Li+10,42, Na+26) or other small ions (e.g. NH4+13) are intercalated in its interlayer, enlarging its interlayer distance and releasing the resultant stress. Here, we recognize that the re-arrangement of NaMoS2 during charging is also in a structure of 1T phase. This structure is favorable in energy and internal stress release. As a result, after Na-ion extraction from NaMoS2, 1T-MoS2 is a preferential phase in fully recharged state. Therefore, the above characterization, recognition and calculations demonstrate the reversibility of MoS2 after the initial conversion reaction. With these, the conversion process of MoS2 during Na-ion insertion and extraction can be summarized as follows (Fig. 6). Insertion of the Na-ions in layered 2H-MoS2 leads to translation of one of its sulfur planes along an intra-layer atomic plane and results in the formation of 1T-NaxMoS2 (x = 0–1.5) [24]. As more Na-ions are inserted, it is reduced to Na2S and metallic Mo [32]. In the early stage of recharge, 1TNaxMoS2 (ca. x = 1) is firstly re-constructed by way of direct reaction of Mo+Na2S at ca 0.44 V due to its lower formation energy than MoS2. Right after that, hexagonal 1T-MoS2 is formed (Eq. (2a)) at ≤ 1.00 V because its formation energy is very close to that of NaMoS2. MoS2 can also be obtained by extracting Na-ions from NaMoS2 at around 1.0 V and above (Eq. (2b)). The higher is the electric conductivity of the composite, the better will these processes be separated. For example, many NaMoS2 domains can be observed in MoS2/C-66 charged to 1.5 V but very few NaMoS2 domains can be detected in MoS2/C-47 recharged to the same potential. However, in a real MoS2 or MoS2-based composite, these three processes may be overlapped in some potential ranges due to dynamic reasons. Therefore, more Na2S is consumed to form 1T-NaxMoS2 and MoS2 as the recharge process continues. The following reactions are typical of MoS2/C-47 or MoS2 composites with better dynamic properties.
Fig. 6. Schematic illustration of the reversible intercalation and conversion in MoS2 composite.
2Na2S + Mo → 1T-NaxMoS2 + (4-x)Na+ + (4-x)e- (x ~ 1) (≥ 0.44 V) (1) Mo + 2Na2S → MoS2 + 4Na+ + 4e- (≤ 1.00 V)
(2a)
1T-NaxMoS2 → 1T-MoS2 + xNa
(2b)
+
-
+ xe (≥ 1.00 V)
Whether or not the 1T-NaxMoS2 and MoS2 can be re-formed depends to a great extent on the dynamic properties, including particle size and electric conductivity of the composite. For MoS2 with poor dynamic properties (MoS2 overloaded and micro-MoS2), oxidation of the metallic Mo becomes difficult. In this case, the Na-ions are directly removed from Na2S and amorphous S is formed at ca. 1.82 V or above. The metallic Mo in the composite remains untouched. This is what happens in most, especially the micro-sized, MoS2 electrode materials,
222
Mo + Na2S → Mo + S + 2Na+ + 2e- (ca. 1.82 V)
(3)
This reaction corresponds to the very strong anodic peak at 1.82 V in the CV profile of MoS2/C-85 (Fig. 3b). In comparison, anodic peaks for the formation of NaxMoS2 at ca. 0.44 V and for formation of MoS2 at ca. 1.00 V become negligible. Clearly the metallic Mo can be directly oxidized to NaxMoS2
ca. the the (≥
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line. The X-ray photoelectron spectra (XPS) were recorded on an Escalab 205Xi spectrometer. The binding energy was calibrated with C1 s spectrum of the contaminated carbon (at 284.8 eV) in the vacuum chamber.
0.44 V) or MoS2 (ca. 1.00 V) for a fully discharged MoS2 composite with good dynamic properties. For the former case, a typical reversible conversion occurs and 1T-MoS2 is formed. In any of these two cases, MoS2 is expected to be one of the final recharge products (cutoff at 3.0 V). As for the case of MoS2 with poor dynamic properties, only elemental S and Mo are expected to be present in the final recharge product. As the dynamic properties of the MoS2 materials are usually somewhere between the “good” and the “poor”, MoS2 and elemental Mo and S can be detected in the final recharge products. That is just what we have observed in this work. Although theoretically different reaction paths of MoS2 lead to the same total specific capacity (a total of 4 electrons are transferred per formula of MoS2, ~ 669 mAh g−1), different specific energies will be delivered as the Na-ions are extracted at different potentials in different reaction paths. Therefore, controlling of the reaction paths by way of designing the material is critical in tuning the battery performances.
4.2. Electrochemical evaluation Button-type (CR2032) test cells were assembled in an Ar-filled glove-box (O2 ≤ 0.5 ppm, H2O ≤ 0.5 ppm), with glass fiber as the separator and 1 mol L−1 NaClO4 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 v/v) as the electrolyte. Galvanostatic discharge/recharge cycling was performed on a Land CT2001A battery tester between 0.01 and 3.00 V (vs Na+/Na) at room temperature. Cyclic voltammetry (CV) was carried out on a CHI600D electrochemistry workstation at a scan rate of 0.1 mV s−1. 4.3. Computational method
3. Conclusions The first-principles calculations were performed in the Vienna ab initio Simulation Package (VASP) code [37]. The generalized gradient approximation (GGA) was employed to describe the exchange-correlation interactions. The Hubbard U correction was taken to calibrate the strong correlation of the d-electrons of the Mo atom [38]. The Hubbard U for the Mo-ion was set to 3.0 eV, following the previous report [39]. The cutoff energy for the wave function was 500 eV. The spin polarization was considered throughout the calculations. Both the cell parameters and the ionic coordinates were fully relaxed, using 11 × 11 × 3 Γ- centered κ- meshes. In comparison with the energy of ferromagnetic configuration of NaMoS2, the anti-ferromagnetic state is believed more stable for its lower total energy.
In summary, a series of composites of MoS2 nano-slices embedded in carbon nano-sheets (MoS2/C) were prepared. Embedding the MoS2 nano-slices in conducting carbon sheets improves the dynamic properties of MoS2. That permits us to draw a complete picture of the reactions, reversible and irreversible, intercalation and conversion, during recharge of the fully discharged MoS2/C composite, including the evolution of the oxidation products and the dependence of the reversibility on the dynamic properties of the composite. On the basis of comprehensive structural characterization and first-principles calculations, the reversibility of the conversion reaction in fully discharged MoS2 is clarified. Either as an intermediate product or as general recharge product, NaMoS2 is detected and recognized as a new phase in the recharged composite. As the oxidation of metallic Mo may occur at different potentials and improving the dynamic properties of the composite is beneficial for promoting the direct or indirect formation of MoS2, designing the structure of the MoS2 composite is critical in enhancing the energy density of the cell though the reaction path does not change the total specific capacity of the material. These findings enrich the understanding of reversible conversion reaction of MoS2 in Na-ion batteries. It could enlighten more research on other material of conversion reaction and promote their application in sodium ion batteries.
Notes The authors declare no completing financial interest. Acknowledgments This work was financially supported by the National 973 Program of China (2015CB251100). Appendix A. Supporting information
4. Experimental section Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.nanoen.2017.09.039.
4.1. Synthesis of nano MoS2/C composites
References
(NH4)6Mo7O24·4H2O, CH4N2S and citric acid were used as the sources for molybdenum (Mo), sulfur (S) and carbon (C), respectively. These reagents were dissolved in 500 mL deionized water by magnetic stirring. The obtained solution was frozen-dried into powder. The powder was then heated and kept at 750 °C for 1 h under argon (Ar) atmosphere in a tube furnace for the MoS2/carbon composite. By tuning the molar ratio of these chemicals, the content of MoS2 in the composite was controlled, resulting in a series of Mo2S/C composites. Pure carbon and pure MoS2 were prepared for comparison. The structure of the Mo2S/C composite was characterized on a X-ray diffractometer (Bruker D8 Advance) equipped with Cu Kα radiation (λ = 1.54 Å). The morphology and microstructure of the sample were examined on a Hitachi S-4800 scanning electron microscope (SEM) and a Tecnai G2 F20 U-TWIN high-resolution transmission electron microscope (HRTEM), respectively. Aberration-corrected scanning transmission electron microscopy (STEM) was performed on a JEOL ARM 200 F transmission electron microscope equipped with double Cs correctors for the condenser lens and objective lens. Annular bright-field (ABF) and high-angle annular dark-field (HAADF) images were acquired at acceptance angles of 11.5–23.0 and 90–370 mrad, respectively. Raman spectroscopy was conducted on inVia-Reflex using the 532 nm laser
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