Dopamine-Assisted Synthesis of MoS2 Nanosheets on Carbon

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Dopamine-Assisted Synthesis of MoS2 Nanosheets on Carbon Nanotube for Improved Lithium and Sodium Storage Properties Han Zhou, Ruifang Zhang, Shili Song, Chunhui Xiao, Guoxin Gao, and Shujiang Ding ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01208 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Applied Energy Materials

Dopamine-Assisted Synthesis of MoS2 Nanosheets on Carbon Nanotube for Improved Lithium and Sodium Storage Properties Han Zhou, Ruifang Zhang, Shili Song, Chunhui Xiao, Guoxin Gao, Shujiang Ding*

Department of Applied Chemistry, School of Science, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behaviour of Materials, State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an-710049, China

* Corresponding author: [email protected] Keywords: (CNTs, LIBs, SIBs, MoS2, Polydopamine assisted)

Abstract Hybrid materials with delicate composition and morphology designing usually promote advanced functions. We report a simple synthesis approach to coaxially grow MoS2 nanosheets on carbon nanotubes (CNTs), which was pre-coated with polydopamine (PDA), with readily control phase and morphology. It is found that the thin PDA layeron CNTs surface with abundant catechol base and chinone base can effectively attract both Mo4+ and S2- for subsequently in situ forming MoS2 shell and sheet-like morphology, therefore is essential for the successful deposition of MoS2. The as-prepared MoS2 nanosheets/CNTs (MoS2@CNT) composite with a unique hierarchical hybrid nanostructure with large active surface and enhanced structural robustness has greatly promote the charge transfer process and enhance the

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electrochemical properties when used as the electrode materials for lithium-ion batteries and sodium-ion batteries. As the anode materials for LIBs, the MoS2@CNTs electrode exhibited extremely stable cycling performance with an excellent capacity of 747.3 mA h g-1 over 200 cycles at 200 mA g-1. As for SIBs, MoS2@CNT showed a high capacity of 512.4 mA h g-1 at 200 mA g-1 after 100 cycles. These results demonstrated the importance of the synthesis and the application potentials of MoS2@CNTs electrodes with both long cycling performance and good rate capability in future energy storage devices.

Introduction In order to meet the requirement of the high energy density storage and rapidly growth of electric vehicles, the further improvement is urgently needed of the anode materials for lithium ion batteries (LIBs).

1-5

Recently, sodium ion batteries (SIBs)

have recently received tremendous attentions in energy storage and conversion system because of sodium have much more reserves than lithium on the earth, which makes SIBs have less cost and environmental issues.6-9 Nevertheless, because of the intrinsic disadvantage of larger atomic radius of Na+ (0.102 nm) than Li+ (0.076 nm), SIBs also face the severe challenge of volumetric change along with huge force during the sodium ion charge-discharge process, which can destroy and atomize the anode materials and thus cause the capacity of SIBs falling rapidly.8, 10-11 As a typical graphene-like two-dimensional materials, molybdenum disulfide (MoS2) has caught great attention owning to its inherent crystal structure.12-14In MoS2,


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each Mo atom sites in the center is surrounded by six S atoms each of which combines with two Mo atoms to form a typical triangular prism, the layered structure is formed by the Mo atoms are sandwiched between two sheets of S atoms. Therefore, due to the weak interactions between the sheets of van der Waals, MoS2 enables easy diffusion of Li+ and Na+, therefore supposed as one of the most promising LIBs and SIBs electrode materials.15-18 However, there are still some problems needed solved for MoS2 as the electrode material of LIBs and SIBs. For example, during the charge-discharge process will appear large volume changes which is the major reason for the awful cycling stability due to the destruction of electrode structure and disconnection of the active materials with current collector. In addition, the inherent poor electric conductivity of MoS2 will lead to a depressed reaction kineticwhich is considered as the major reason for the poor capacity at high current rate.19-21 To oversome these disadvanteges, one strategy is to decrease the size and design a loose structure (e.g., hollow, nanosheets, box) of MoS222-25 to reduce the transport length of ions and buffer the volume change. The other strategy is to prepare MoS2/carbon materials composite to promote the electric conductivity.16, 20, 25 Hence, we have proposed a unique structure of MoS2 nanosheets anchored on CNT, which possesses both of the advanced structure strategy and composite strategy. However, for the reasons of minor diameter of CNT and the strong interfacial adhesion between the CNTs, the controllable synthesis of with other desirable composition and nanosheet structure still remains a great challenge. Qian and co-workers synthesized a MoS2@CNTs compoisite via a kirkendall effect, in which a MoO3 nanowire was



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prepared using L-cysteine and glucose as sulphur and carbon sources, respectively, and the prepared MoS2@CNTs shows good lithium and sodium storage properties.20 However, a flaw in an otherwise perfect character is that the electric conductive matrix was prepared by carbonated, which cannot possess as good electric conductive as typical CNTs. Wang reported a facile solvothermal synthetic method of MoS2@CNTs composite with enhanced catalytic activyt for hydrogen evolution reaction.26 Whereas, the component of MoS2 in the composite is low thus may not have an satisfactory energy stroage properties. Dopamine (2-(3, 4-dihydroxyphenyl) ethylamine) is a neurotransmitter that produces in various animals, it can be oxidized in alkaline solutions to forming a strong adhesive polydopamine (PDA) on the surface of the substrate material.

27

The

superior adhesion of PDA is due to the fact that catechol groups combination with the surface of the base material by covalent bonds or non-covalent bonds. Inspired by this spontaneous oxidative polymerization, we generated a layer of PDA on the CNT surface in a controlled way. Herein, we propose a facile synthesis method of MoS2@CNT by virtue of PDA for the first time. The synthetic process is exhibited in Figure 1. For adsorption or of forming complexes with a large variety of functional components, we firstly grafted a PDA layer with abundant catechol groups onto CNTs. Second, the MoS2 nanosheets on CNT was successfully riveted through a typical hydrothermal method. After annealing, the MoS2@CNTs composite is obtained. In addition, the PDA layer will turns to the N-dope amorphous carbon after being calcined, this transition is


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beneficial to enhance both of the cycling stability and rate capacity via enhance the electric conductivity and fix MoS2 on CNT during the

electrode reaction process.

Results and Discussions The morphology information of the as-prepared CNT-PDA material was characterizaed by transmission electron microscopy (TEM). As shown in Figure 2a ,the low-magnification TEM image makes clear that the length of CNTs is about 2~5 um. The high magnification TEM image of CNT-PDA in Figure 2b shows that the CNTs are uniform with 50~90 nm in diameter and most of them are wrapped by ~8 nm-thickness PDA layer. In addition, as shown in Figure S1a, pure MoS2 self aggregates and has a nanosheet sphere structure, which probably consists of densely stacked sheet-like subunits. The low-magnification SEM image of MoS2@CNT (Figure 2c) reveals that these composite materials have a uniform and hierarchical architecture. The higher magnification TEM image Figure 2d and 2e observation further reveals MoS2 nanosheets are uniformly anchored and directly grow on CNT surface. The most of MoS2 nanosheets are about 50 nm in length, thus can provide high specific surface area and lots of active sites, which can greatly promote the storage capacities and rate properties of the materials. By comparison, if untreated CNT were added, MoS2 and CNT mixed was obtained (Figure S1b). Based on these observations, it is clear that the PDA layer plays a critical role in the formation of MoS2@CNT. In the HRTEM (Figure S2a), it shows the thickness of composed MoS2 nanosheets is range 3 to 7 nm, indicating the MoS2 nanosheets consist few MoS2 layers. In figure S2 b), the obvious crystalline lattice is circa 0.84 nm, can be assigned



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to the (002) plane of MoS2. It is noteworthy that the interlayer spacing is a little higher than typical MoS2 (0.65 nm), which is good for metal ions insertion-desertion process. Hence, the as-prepared MoS2@CNT possesses good storage properties. The element distribution of MoS2@CNT is analyzed by Energy dispersive X-ray spectrometry (EDS) mapping (Figure 2f),

it is revealed that carbon element is inside

where sulphur element and molybdenum element covered outside, confirmed the core (i.e., CNTs) and shell (i.e., MoS2 nanosheets) hierarchical structure. To further comfirm the chemical compostition of carbonate polydopamine, we had a XPS test for CNT-PDA after annealing treatment (Figure S3). The C 1s peak is at 287.3 eV and 284.5 eV (after standardized), while the N 1s peak is at 398.4 eV.

All of these

exemplifies the amorphous carbon layer was N-dopped, which possess good electrical conductivity can greatly promote the stroage properties of the composite materials. Figure 3a display the XRD pattern of MoS2@CNTs, the diffraction peak at 26o can be assigned to the (002) plane of CNTs, while the peaks at 33.5o, 39.5o and 58.3o can be assigned to the (101), (103) and (110) planes of MoS2 (JCPDS card no. 37-1492).17, 28

For the (105) plane of MoS2, because the small-size effect on XRD width deduced

by the Scherrer equation, the peak may overlap with the hexagonal phase of CNT (26o). Thus, it’s hard to be distinguished. Thermogravimetric analysis (TGA) was carried out to obtain the amount of MoS2 in initial materials (Figure 3b). The curve clearly demonstrates two weight losses. The first weight loss is attributed to the oxidation of MoS2, which occurs at about 300 oC.28 With the further heating process, the second mass loss platform appears at 400 oC, indicating the combustion of CNTs.


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However, the exact content of amorphous carbon is difficult to confirm. When the heating process is complete, the remaining product is pure MoO3 which the weight percentage calculated to 53.58%. It can be estimate that the content of MoS2 in the initial CNT@MoS2 composite is 60.5% approximately by TGA; the rest of the quality contribution comes from the amorphous carbon and CNT backbone. Next, we investigated the electrochemical properties of the as-prepared MoS2@CNTs composite act as anode material for LIBs. First, a cyclic voltammogram (CV) of MoS2@CNT in the potential range of 0.05~3V was recorded and the obtained CV curves is displayed in Figure 4a. This CV behavior is generally consistent with previous reports.29-30 The are two reduction peaks can be observed loated at 1.01 and 0.54 V respectively

in the first lithiathion progress, in which the peak at 1.01 V is

associated with the intercalation of lithium ions into the MoS2 lattice (Eq.(1)) and the peak at 0.54 V is assigned to the conversion reaction of (Eq.(2)): + + ↔

(1)

4 + + 4 ↔ + 2

(2)

This process contains a continuous decomposition of MoS2 into Mo nanoparticles and the formed Mo nanoparticles continuously embeds into Li2S matrix along with the formation of solid-electrolyte interphase (SEI) layer caused by the degradation of electrolyte. In the fowling delithiation progress, the oxidation peaks at 1.64 V and 2.30V are ascribed to the reversible reaction of Mo to MoS2 and the formation of S from Li2S, respectively. Thus, in the following lithiation progress, the conversion reaction peaks at 0.54 V disappears, whereas a new distinct peak at 1.86 V appears, indicating the formation of Li2S from S. Actually, after the first cycle, the MoS2



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already turns to the Mo nanoparticles and S nanoparticles composite. The second curve is well coincided with the third curve, proving a good cycling stability property of as-prepared MoS2@CNTs composite. The first, second and third cycles charge-discharge voltage profiles of MoS2@CNTs at 200 mA g-1 are shown in Figure 4b. There are two prominent potential plateaus in accord with the CV results above can be observed. The initial charge and discharge capacities are respectively 863.4 and 1089 mA h g-1, which means that the first cycle has a coulombic efficiency of 79%. Besides, the charge and discharge curves are well overlapped from the second cycle,, demonstratinf the excellent reversibility and

super capacity retention of

MoS2@CNTs electrode. Figure 4c exhibits the cycling performance of MoS2@CNTs at 200 mA g-1 current density. The MoS2@CNT electrode shows a stable cycle performance of 907 mA h g-1 after five cycles as well as a high coulombic efficiency nearly 99%. And the capacity of 747.3 mA h g-1 can be remained after 200 cycles with a capacity retention of about 82.3%. Generally, the pure MoS2 flaks can only maintian capacity of 149.4 mA h g-1 and the capacity retention of 30% after 50 cycles, while the capacity and capacity retention of the pure CNTs are 246.3 mA h g-1 and 88.7%, respectively. And to further prove the contribution of polydopamine to the electrochemical properties of MoS2@CNTs, the cycling performance of MoS2@CNT without PDA is tested. From the results shown in Figure S4, the capacity and the capacity retention of 414.5 mA h g-1 and 61% are obtained after 50 cycles. Apparently, the MoS2@CNT shows the best


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cycling performance among these four materials considering both the capacity and stability. The rate capability was further studied by galvanostatic charge/discharge at a variety ofcurrent rates from 0.1 A g-1 to 1.6 A g-1 for MoS2@CNTs electrode. As shown in Figure 4d, the reversible capacities of 1030, 972, 961, 914, 785 mA h g-1 are obtained at the current rates of 0.1, 0.2, 0.4, 0.8 and 1.6 A g-1, respectively. Importantly, when it returns back to a low current density of 0.1 A g-1 after cycling at different current densities, the MoS2@CNTs electrode still reach a high reversible capacity of 1013 mA h g-1. This can further confirm that MoS2@CNTs not only have good rate capacity property, but also owns excellent cycling stability. TEM images of MoS2@CNT after 30 cycles is showd in Figure S5. It’s clearly that after 30 cycles, though part of MoS2 nanosheets fell off from the CNTs, they still kept the sheets form, constituted a conductive network with CNTs. It’s a powerful evidence that as-prepared MoS2@CNT possess good structure stability during charge-discharge process. In order to further understand the superior properties of MoS2@CNT materials, the EIS data of MoS2@CNT, pure MoS2 nanosheets and CNTs are shown in Figure S6. Figure S6a shows the initial state impedance measurement after the batteries just were assembled. All the impedance curves both contain a semicircle and an inclined line. The semicircle is used to indicate the charge transfer resistance of electrolyte and electrode interface, while the inclined line is related with the Li ion diffusion. It’s clearly that the semicircle of CNT is smallest while MoS2@CNT is smaller than pure



MoS2. This result is a convincing demonstration that combination MoS2 with CNT can enhance the electro conductivity. After five cycles (Figure S6b), the semicircle of MoS2@CNT is still smaller than pure MoS2. In particularly, it’s noticed that in low frequency region, the slop of MoS2@CNT is lower than MoS2, which is indicating the combination MoS2 with CNT can enhance the diffusion ability of Li ion. For investigating the storage properties of MoS2@CNT at high current density, we have tested the storage performance of MoS2@CNT at current densities of 5 A g-1 and 10 A g-1. As the Figure S7 shows, at the current density of 5 A g-1, the MoS2@CNT have a capacity of 356.5 mAh g-1 after 200 cycles. Even when the current density rises to 10 A g-1, the capacitance still remains 202 mAh g-1, which indicated the as-prepared MoS2@CNT possesses good cycling and rate performance. Similarly, the electrochemical performance of the obtained MoS2@CNT as the anode electrode of NIBs was studied. Figure 5a are the CV curves of MoS2@CNTs at a scan rate of 0.5 mV s-1 from 0.05 to 3 V. Obviously, the CV curves of MoS2@CNT as NIBs electrode are very different from that as the LIBs electrode, which can be due to the different thermodynamic and kinetic between lithium and sodium insertion mechanism. In the initial sodiathion progress, the peak appear at 0.776 V resulting from

the formation of NaxMoS2 (Eq.(3)) and the generation of peak at 0.456 V can

be assigned to a conversion reaction of (Eq.(4)):31 + + →

(3)

4 + + 4 → + 2

(4)

In addition, there is a broad peak come from the formation of SEI layer in the low voltage range. In the following cycles, the SEI layer is partly reversible, so the


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strength of broad peak remarkablely decreases. In the disodiathion progress, the oxidation peak at 1.83 V indicates the reversible reaction of Mo to MoS2 (Eq.(5)): + 2 → 4 + + 4

(5)

In the following cycles, second cycle and the third cycle overlap well, suggesting that MoS2@CNTs owns excellent cycle stability.

The charge-discharge voltage profiles

at 200mA g-1of MoS2@CNT are shown in Figure 5b,it is obvious that there are two voltage plateaus which is consistent with the CV analysis. The first discharge and charge capacity is 700.74 mA h g-1 and 501.63 mA h g-1, respectively, revealing a first coulombic efficiency is 71.6%. Besides,

the second and third charge-discharge

curves overlap well like the CV curves, further improving the as-prepared MoS2@CNTs owns an excellent cycling stability property. Figure 5c exhibits the cycling performances of MoS2@CNT (Curve I), MoS2 (Curve) II, CNT (Curve III) and coulombic efficiency of MoS2@CNT (Curve IV) at the current density of 200 mA g-1, respectively. The discharge capacity of MoS2@CNT decreases from 700 mA h g-1 to 500 mA h g-1 after 100 cycles with the coulombic efficiency of 99%, which is much better than CNT and pure MoS2. Especially, we further tested the cycling performance with MoS2@CNT without polydopamine as shown in Figure S4, the capacity and the capacity retention is 200.5 mA h g-1 and 43% after 50 cycles. It is obvious that MoS2@CNTs possess the best sodium ion storage properties among these four materials both in capacity and stability. Figure 5d reveals MoS2@CNT shows different specific capacities in different current rates range from 100 to 800 mA g-1. The MoS2@CNT electrode



delivers reversible capacities of 540 480, 425 and 370 mA h g-1 at 100, 200, 400 and 800 mA g-1, respectively. Further, the reversible capacities still can reach 510 mA h g-1 when the current density return back to 100 mA g-1 after cycles, , which reveals MoS2@CNT owns not only good cycling stability but also has excellent rate properties. For comparion, we listed some previous work about MoS2 for sodium stroages(Table S1). Compared with previous works, this work also shows a higher capacity and good cycle stability. The outstanding cycling stability and rate properties of MoS2@CNT in both LIBs and NIBs can be attributed to both the component and structural the superiorities. 1) The MoS2 nanosheets provide high capacity while CNTs provide superior electrical conductivity, which are the most important factors for cycle stability and rate properties; 2) The uniform hierarchical nanosheets on nanotube structure can offer lots of extra volume, which can buffer the large volume change during the charge-discharge progress and is beneficial for cycle stability; 3) The ultrathin nanosheets structure can shorten ions transfer pathway as well as offers lots of reactive sites, which both could raise the rate property substantially. In summary, a facile method of synthesis MoS2@CNT composite materials with the assistance of pre-coated polydopamine (PDA) layers on CNTs have developed. The PDA layer could effectively improve the dispersibility of CNTs as well as induce MoS2 nanosheets in suit growth on CNTs. Owing to the unique structure and the strong coupling effect between MoS2 and CNTs, superior cycle stability as well as excellent rate capacity both in LIBs and NIBs are achieved, More importantly, this


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Hierarchical Quasi-Hollow MoS2 Microspheres Encapsulated Porous Carbon as Stable Anode for Li-Ion Batteries. Small 2014, 10, 4975-4981. (25) Chang, K.; Chen, W. In Situ Synthesis of MoS2/graphene Nanosheet Composites with Extraordinarily High Electrochemical Performance for Lithium Ion Batteries. Chem. Commun. 2011, 47, 4252-4254. (26) Yan, Y.; Ge, X. M.; Liu, Z. L.; Wang, J. Y.; Lee, J. M.; Wang, X. Facile Synthesis of Low Crystalline MoS2 Nanosheet-coated CNTs for Enhanced Hydrogen Evolution Reaction. Nanoscale 2013, 5, 7768-7771. (27) Linert, W.; Herlinger, E.; Jameson, R. F.; Kienzl, E.; Jellinger, K.; Youdim, M. B. H. Dopamine, 6-hydroxydopamine, Iron, and Dioxygen - Their Mutual Interactions and Possible Implication in the Development of Parkinson's Disease. BBA-MOL Basis Dis. 1996, 1316, 160-168. (28) Ding, S.; Chen, J. S.; Lou, X. W. Glucose-Assisted Growth of MoS2 Nanosheets on CNT Backbone for Improved Lithium Storage Properties. Chem. Eur. J. 2011, 17, 13142-13145. (29) Fang, X.; Yu, X.; Liao, S.; Shi, Y.; Hu, Y.-S.; Wang, Z.; Stucky, G. D.; Chen, L. Lithium Storage Performance in Ordered Mesoporous MoS2 Electrode Material. Micropor. Mesopor. Mat. 2012, 151, 418-423. (30) Jiang, H.; Ren, D.; Wang, H.; Hu, Y.; Guo, S.; Yuan, H.; Hu, P.; Zhang, L.; Li, C. 2D Monolayer MoS2–Carbon Interoverlapped Superstructure: Engineering Ideal Atomic Interface for Lithium Ion Storage. Adv. Mater. 2015, 27, 3687-3695. (31) Su, D.; Dou, S.; Wang, G. Ultrathin MoS2 Nanosheets as Anode Materials for Sodium-Ion Batteries with Superior Performance. Adv. Energy Mater. 2015, 5.

Figure 1. Illustration of the synthetic procedure of MoS2@CNT.



Figure 2. a) and b) TEM images of CNT-PDA; c) SEM, d) and e) TEM images of MoS2 @CNT; f) EDS elemental mapping of C, S and Mo of MoS2@CNT.


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Figure 3 a) XRD pattern and b) TGA curves of MoS2@CNT.



Figure 4 a) CV curves of the MoS2@CNT electrode at a scan rates of 0.5 mV s-1 between 0.01 V and 3.0 V. b) Charge-discharge voltage profiles of the MoS2@CNT electrode at a current density of 200 mA g-1. c) Comparison of cycling performance of MoS2@CNT (curve I), pure MoS2 (curve II) and CNT (curve III) at a current density of 200 mA g-1. Curve IV shows the corresponding coulombic efficiency of the MoS2@CNT electrode. d) Rate capability of MoS2@CNT electrode at different current densities.


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Figure 5 a) CV curves of the MoS2@CNT electrode at a scan rates of 0.5 mV s-1 between 0.01 V and 3.0 V. b) Charge-discharge voltage profiles of the MoS2@CNT electrode at a current density of 200 mA g-1. c) Comparison of cycling performance of MoS2@CNT (curve I), pure MoS2 (curve II) and CNT (curve III) at a current density of 200 mA g-1. Curve IV shows the corresponding coulombic efficiency of the MoS2@CNT electrode. d) Rate capability of MoS2@CNT electrode at different current densities.



ASSOCIATED CONTENT Supporting Information Available: Experiment section, SEM images of control group materials, HRTEM images of MoS2@CNT, XPS data, cycling performance of control group materials, TEM iamge of the materials after cycling, Nyquist plots, cycling performance

of the materials at high current density and table of comparison of sodium ion storage performance of this present work with previous works.


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Table of Contents


Dopamine-Assisted Synthesis of MoS2 Nanosheets on Carbon

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