Laser assisted self-assembly synthesis of porous hollow MoO3-x-doped MoS2 nanospheres sandwiched by graphene for flexible high-areal supercapacitors

Electrochimica Acta 332 (2020) 135499

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Laser assisted self-assembly synthesis of porous hollow MoO3-x-doped MoS2 nanospheres sandwiched by graphene for flexible high-areal supercapacitors Wei Li a, Ting Luo b, Chao Yang c, Xiaopeng Yang a, Shuhua Yang a, Bingqiang Cao a, c, * a

Materials Research Center for Energy and Photoelectrochemical Conversion, School of Material Science and Engineering, University of Jinan, Jinan, 250022, Shandong, China School of Mechanical and Automotive Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250353, Shandong, China c School of Physics and Physical Engineering, Shandong Provincial Key Laboratory of Laser Polarization and Information Technology, Qufu Normal University, Qufu, 273165, Shandong, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 September 2019 Received in revised form 8 December 2019 Accepted 11 December 2019 Available online 14 December 2019

Molybdenum disulphide (MoS2) with two-dimensional (2D) “graphene-like” stacked structure has attracted widespread attention as a type of novel energy storage materials because of its unique electronic properties. However, it suffers from fast capacity fading, low cycling stability and low specific capacitance for flexible devices. This study describes a facile and novel laser irradiation approach to fabricate self-assembled porous hollow MoO3-x-doped MoS2 nanospheres sandwiched by graphene for binder-free flexible supercapacitors (SCs). Due to the one-step laser-induced fragmentation and selfassembly of MoS2 nanoflakes with the photothermal reduction of graphene oxide (GO), the MoO3-xdoped MoS2/graphene nanocomposites show enhanced conductivity, high surface area and increased electrochemical active sites for redox reactions. The symmetric flexible SCs deliver remarkable areal specific capacitance of 121.88 mF cm
2 under a current density of 0.6 mA cm
2, excellent cycle performance of almost no loss after 4000 cycles and high energy density of 73.4 mWh cm
2 under a power density of 240 mW cm
2. This simple and ultra-fast laser-induced self-assembly technique could open up a new avenue towards high-performance flexible energy storage. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Laser irradiation Self-assembly MoO3-x-doped MoS2 nanospheres Supercapacitors

1. Introduction Electrochemical devices such as lithium ion batteries and supercapacitors, continue to attract great attention from both the scientific and industrial applications because of the huge demand on renewable energy to meet the burgeoning energy crisis and address the devastating environmental pollution [1,2]. SCs have rapid charge/discharge capacity, excellent cycling stability, high power density, lightweight and environmentally friendliness [3], which can effectively bridge the gap between conventional capacitors and rechargeable batteries. However, the low energy density of SCs, particularly for areal-capacitance performances, is still a great challenge for researchers [4]. According to the

* Corresponding author. Materials Research Center for Energy and Photoelectrochemical Conversion, School of Material Science and Engineering, University of Jinan, Jinan, 250022, Shandong, China. E-mail address: [email protected] (B. Cao). https://doi.org/10.1016/j.electacta.2019.135499 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

underlying energy storage mechanism, SCs can be generally classified into electrical double-layer capacitors (EDLCs) and pseudocapacitors [5]. EDLCs distribute charges physically by reversible ion adsorption on the electrode/electrolyte interfaces, so they can be charged/discharged within seconds and over 100 000 cycles [6]. Unfortunately, the specific capacitances of such SCs are relatively low [7]. In contrast, pseudocapacitors store energies chemically by redox reactions between electrode and electrolyte, resulting in the higher specific capacitance than that of EDLCs, but sacrificing their rate capability and cycling life to some extent [6]. Naturally, the hybrid capacitors combining EDLC materials with pseudocapacitor materials, such as porous C/MnO2 [8], graphene/polymelamine [9], VN/C [10,11], and MoS2/graphene [12] etc., show enhanced supercapacitive performances. MoS2, as one typical trans11ition metal dichalcogenide material, has attracted widespread attention in energy storage, particularly in flexible SCs, due to its 2D stacked structure similar to graphene and unique electronic properties that can vary from

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semiconducting to metallic depending on its crystalline phase [13e17]. Especially, MoS2 is one truly promising pseudocapacitive electrode material because of its multiple oxidation states from þ2 to þ6 [18,19]. Nevertheless, such 2D nanosheets with interlayer van der Waals forces and high surface energy, have intrinsic low electronic conductivity and serious aggregation, resulting in fast capacity fading and low cycling stability [20,21]. Many researchers designed and synthesized MoS2-based heterostructures or threedimensional (3D) porous structures, such as Ni3S2@MoS2 heterostructure [22], MoS2/graphene heterostructure [23], Ni3S4/MoS2 heterojunction [24], 3D MoS2@CNT/rGO network [3], 3D flowerlike MoS2eCoSe2 structure [17], and 3D MoS2/PANI/rGO [25], which can overcome the aforementioned barriers of MoS2 to a certain extent and enhance the electrochemical performance. In particular, due to the flexible layer structure and good conductivity, graphene is usually combined with MoS2 nanosheets to form SC devices, which acts as a current collector and nano-sized support or flexible medium to improve the electrical conductivity and alleviate the volume changes of MoS2 during charge/discharge process [26,27]. All the research efforts are, up to this date, toward a simple and facile synthetic method for obtaining MoS2-based electrodes on flexible substrates with satisfactory capacitive performance. However, there is still no one work on simple laser-assisted growth method of porous hollow MoS2/graphene composite device that could maximize the available pseudocapacitance of MoS2 and produce high areal specific capacitances. Herein, we develop a facile laser-assisted methodology to construct porous hollow MoO3-x-doped MoS2 nanospheres sandwiched by graphene nanosheets as a binder-free self-assembled flexible electrode for enhanced faradaic reactions to store charge. Laser irradiation method is regarded as a novel and green technique for fabricating various nanomaterials with special microstructures, morphologies and phases under ambient conditions [28e31], such as laser-induced 3D graphene structure used as SCs exhibit higher capacitance compared with other carbon-based SCs prepared by conventional methods [31]. In this work, we introduced the onestep laser irradiation method for self-assembled MoO3-x-doped MoS2/graphene flexible SCs for the first time. The laser-induced MoS2/graphene (L-MoS2/G) nanocomposites are fabricated by laser irradiation of a dispersion composed of MoS2 nanoflakes and GO prepared by modified Hummers method [32]. The MoS2 nanoflakes are fragmented into ultrafine nanosheets and then selfassembled into porous hollow nanospheres on the adjacent graphene nanosheets produced by laser-induced photothermal reduction of GO at ambient conditions. Based on the self-assembled hollow MoS2/graphene electrodes, the symmetric supercapacitor devices show an outstanding energy storage performance. The improved performance of the L-MoS2/G SCs is attributed to their distinctive laser-induced structural configuration: porous, hollow, MoO3-x doping, anti-agglomeration, and dilative interlayer spacing, leading to an enhanced electrical conductivity, high specific surface area and efficient utilization of pseudocapacitance. 2. Experimental section Preparation of the MoS2/graphene nanocomposites: MoS2 nanoflakes were synthesized via a hydrothermal method. Typically, sodium molybdate dihydrate (0.5 g, 99.0% purity, Aladdin), thioacetamide (0.8 g, 99.0% purity, Aladdin) and tungstosilicic acid hydrate (0.2 g, AR, Aladdin) were added into deionized water (80 ml). After sonication for 30 min to dissolve completely, the solution was transferred into a 100 ml Teflon-lined autoclave and heated at 200  C for 24 h. The autoclave was cooled to room temperature naturally. Then MoS2 solution was obtained after being washed at least five times by deionized water. GO solution was

prepared by a modified Hummers method through the exfoliation of graphite (99.8% purity, 325 mesh, Alfa Aesar) [32]. The laser-induced MoS2/graphene (referred as L-MoS2/G) nanocomposite was fabricated by pulse laser irradiation of the mixed solution of the above-prepared MoS2 and GO. The mass ratio of MoS2 and graphene is 10:1. MoS2 solution (25 mL, 3 g L
1) and GO solution (5 mL, 1.5 g L
1) were mixed by magnetic stirring to form a homogeneous colloidal solution. A krypton difluoride (KrF) excimer laser (248 nm, 25 ns, 10 Hz, Coherent, CompexPro 205) was used as light source. The laser beam was focused on the mixed solution through a convex lens with a focal length of 150 mm. The laser irradiation on GO/MoS2 was performed at an energy fluence of 460 mJ pulse
1 cm
1 for 3, 5, 8, 10 min, respectively. During the laser irradiation process, the GO/MoS2 solution was placed in an ice bath and continuously stirred, as shown in Fig. 1a. After laser irradiation, the flexible MoS2/graphene films were prepared by vacuum filtration and then could be directly cut into the desired shapes for the next SC application. As a comparison, same proportional MoS2/graphene nanocomposites were also prepared by hydrothermal method (referred as HeMoS2/G). After being washed, the HeMoS2/G was also filtered by vacuum to form a thin film for the SC application. Material Characterizations. The morphology and microstructure of prepared samples were observed with a scanning electron microscope (SEM, FEI Quanta 250 FEG) and a transmission electron microscope (TEM, JEM-2100F) under 200 kV acceleration voltages. The XRD pattern was collected by an X-ray diffraction apparatus (D8-Advance, Bruker) operated at 40 kV and 40 mA with the Cu-Ka line (l ¼ 0.154184 nm). Raman spectrometer (LabRAM HR Evolution, HORIBA) equipped with a 532 nm laser was used for recording the Raman scattering spectra. The binding energies of Mo and S in different samples were carried out using an X-ray photoelectron spectroscopy (XPS, Thermo Fisher, with an Al Ka X-ray source). Electrochemcal Measurement. The electrodes were prepared use the above-obtained cut film without binder and current collection. The gel electrolyte was prepared as follows: 1 g H3PO4 and 1 g PVA were mixed with 10 mL deionized water and heated at ~80  C under vigorous stirring until the solution became clear. Then, the flexible SC device was assembled by sandwiching two pieces of LMoS2/G films with a PP/PE microporous membrane (Mitsubishi, Japan) as the separator. The SCs were sealed with tape after heated for 5 min at 70  C. As a comparison, the HeMoS2/G devices were assembled according to the same procedure. All electrochemical measurements were taken on a Zahner/ Zennium electrochemical workstation at room temperature. The electrochemical properties of the symmetric SC devices were characterized with galvanostatic charge/discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) techniques. The EIS tests were conducted with 5 mV (rms) perturbation and 0 V dc bias under a frequency range of 0.1 Hze100 kHz. The specific capacitance of the measured SC cell was calculated from the charge and discharge curves, according to C ¼ I Dt/S DV, where C is the specific capacitance (mF cm
2), Dt is the discharge time (s), I is the discharge current (A), DV is the operating potential window (V) during the discharge, and S is the total area of the active material on the two electrodes (cm2). The energy density (E) and power density (P) of the SC cell were estimated according to E ¼ C DV2/2 and P ¼ E/Dt, respectively, where E is the specific energy (mWh cm
2), and P is the specific powder (mW cm
2). 3. Results and discussion As schematically illustrated in Fig. 1, the whole self-assembled growth process of L-MoS2/G can simply conducted with the insitu pulsed laser irradiation in one step. The extreme non-

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Fig. 1. (a) Schematic diagram of the formation process for the L-MoS2/G nanocomposites. Under laser irradiation, GO nanosheets are reduced to rGO nanosheets by laser-induced photothermal effect, meanwhile pristine MoS2 nanoflakes break up into ultrafine fragments and then anchor on the rGO nanosheet to self-assemble into hollow and porous nanospheres. The scale bars are 50 nm. (b) Schematic microstructure of the as-prepared L-MoS2/G nanocomposites. (c) Digital photograph of the bending flexible electrode.

equilibrium conditions around the laser-target interface produced by the transient laser excitation, including plasma formation, photothermal heating, photochemical reaction, and fragmentation, can satisfy the growth conditions of different materials [28e31]. We chose the high energy-density KrF excimer laser (248 nm) as light source, the mixed solution of as-prepared MoS2 and GO as precursor, and an irradiation condition with an ice bath to ensure an ultra-fast cooling system. When the pulse laser beam irradiates the mixed solution, the fragmentation (size reduction) of MoS2 nanoflakes occurs, which may be driven by the implanted high pressure caused by the rapid photothermal effect and/or the laser spot-MoS2 interaction-induced plasma [29,33]. Such ultrafine fragments have high specific surface area and high instability especially at the edges of nanosheets. Thus, these ultrafine nanosheets are easy to self-assemble into spherical shape to reduce surface energy and improve the stability accordingly. The ultra-fast cooling from the ice bath and nanosecond pulses can effectively prevent the agglomeration and merger of nanosheets, and allow these MoS2 nanospheres to remain hollow and porous microstructures (Fig. 1a). Meanwhile, GO is reduced to reduced graphene oxide (rGO) by the laser-induced photothermal effects, as schematically shown in Fig. 1a. This transformation process is also reflected by the color evolution of GO solution from the original yellow golden to black color (see the digital photographs in Fig. 1a). In addition, the rGO nanosheets reduced by laser irradiation contain numerous defect sites, which can provide efficient anchoring sites for ultrafine MoS2 nanosheets to self-assemble into nanospheres. Then, the laminated nanocomposite of porous and hollow MoS2 nanospheres sandwiched by graphene nanosheets (Fig. 1b) can be obtained by the simple one-step laser irradiation. After the vacuum filtration, the binder-free flexible electrodes show excellent flexibility and bending performance, as shown in Fig. 1c. This work presents a facile and scalable laser irradiation approach for fabricating flexible electrodes for energy storage devices. The morphology and structure of the as-prepared L-MoS2/G were characterized by SEM and TEM. The MoS2 nanosphere is homogeneously dispersed on the graphene nanosheets, as shown in Fig. 2a. The high-magnification SEM (Fig. 2b) and the energydispersive X-ray spectroscopy elemental mapping (Fig. S1) images exhibit these nanospheres are tightly sandwiched by the adjacent

graphene sheets and their diameters are approximately 70e160 nm. Due to this favorable composite structure that can effectively prevent the aggregation of 2D nanosheets, the L-MoS2/G nanocomposites possess a much looser structure compared with the HeMoS2/G nanohybrids (Fig. S2). The specific surface area of the L-MoS2/G nanocomposites assessed by the Brunauer-EmmettTeller (BET) method is 167.57 m2 g
1, higher than that of the HeMoS2/G nanohybrids (62.53 m2 g
1), as shown in Fig. S3. The TEM image in Fig. 2c further depicts that the MoS2 nanospheres assembled from ultrafine nanosheets are porous and hollow. HRTEM image (Fig. 2d) display the length of nanosheets around 8e13 nm and the size of the pores around 2e5 nm. The pore size distributions were also calculated by the Barrett-Joyner-Halenda (BJH) method from adsorption branch of the isotherm, as shown in Fig. S3. The interplanar distance of (002) planes is about 0.632e0.674 nm, indicating a 2.8e9.6% dilation of interlayer spacing along the c axis. Such porous hollow structure that can increase the specific surface area and dilative interlayer spacing can effectively enhance the ions transportation through two dimensional planes and lead to an enhanced electrochemical pseudocapacitive performance. In addition, the tight sandwiched graphene layers on MoS2 surface can enhance the conductivity of the L-MoS2/ G nanocomposites. It is worth noting that some MoO3-x grains doped in the MoS2 nanosphere are observed, which may be caused by the partial oxidation of MoS2 during laser irradiation. Further explanation is provided by the below XPS analysis. The crystal structure of the L-MoS2/G nanocomposites was characterized by XRD, as shown in Fig. 3a. The XRD pattern shows all peaks can be indexed to the hexagonal MoS2 in good agreement with JCPDS No. 37e1492. Theses broadened diffraction peaks indicate MoS2 with ultrafine particle size. It is worth noting that the diffraction peak of (002) plan offset to small angular direction due to the dilative interlayer spacing along the c axis, which can also be confirmed by the HRTEM image (Fig. 2d). The lower signal-to-noise may be caused by the ultrafine and MoO3-x grains doped MoS2 nanosheets with lower crystallization induced by laser irradiation [34,35]. Due to few layers and high disorder of graphene nanosheets, the diffraction peaks of carbon species are not detectable, which from the side reveal that the anchored MoS2 nanospheres on graphene nanosheets could effectively avoid the agglomeration and

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Fig. 2. (a) Low- and (b) high-magnification SEM images, (c) TEM image and (d) HRTEM image of L-MoS2/G nanocomposites.

Fig. 3. (a) XRD pattern, (b) Raman spectra, (c) Mo 3d and (d) S 2p core-level spectra of the L-MoS2/G nanocomposites.

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restacking of graphene. Fig. 3b and Fig. S4 displays the representative Raman spectra (excitation 532 nm) of the L-MoS2/G composites, showing both the characteristic MoS2 peaks and the graphene peaks as labeled. Due to the increase of defects in graphene induced by laser induction in liquid, the higher defect ratios (ID/IG) and weaker 2D peaks can be observed [36,37]. To further characterize the valence states of hollow MoS2 microspheres induced by laser irradiation, X-ray photoelectron spectroscopy (XPS) measurement was obtained. The most significant regions of the photoelectron energy spectrum were fitted by Gaussian-Lorentzian curves were performed, as shown in Fig. 3c, d and Fig. S5. The Mo 3d doublet at (229.0, 232.2) eV, the S 2p doublet at (162.2, 163.3) eV, and the S 2s peak at (226.2) eV are all typical features of MoS2. Furthermore, there are two other different chemical states in the Mo 3d separate peaks. Besides the Mo5þ state, Mo 3d XPS spectrum can also be fitted to the doublet Mo5þ 3d5/2 and Mo5þ 3d3/2 peaks at 229.9 and 233.3 eV, and the doublet peaks at 232.0 and 235.2 eV belong to Mo6þ 3d5/2 and Mo6þ 3d3/2 peaks. The relative contents are 88.9% Mo4þ, 6.9% Mo5þ, and 4.2% Mo6þ in the L-MoS2/G composites (Table S1). Therefore, the amount of substance ratio of Mo4þ, Mo5þ, and Mo6þ is 8.9:0.7:0.4. The amount of MoS2, Mo2O5 and MoO3 is 0.067g, 0.009 and 0.003g, respectively. The higher valence state cations Mo5þ and Mo6þ existing in L-MoS2/G suggests that ultrafine MoS2 nanosheets is partially oxidized to Mo2O5 and MoO3 by the oxygen atom from water or air during laser irradiation, which may cause more defects or some degree of distortion in the composites. The study of density functional theory has indicated MoS2 is highly susceptible to oxidation by oxygen atoms, and the lower coordinated Mo sites are more susceptible to oxidation than the more highly coordinated Mo sites [38]. These results also agree with the investigation from HRTEM. The existing defects or some degree of distortion in the structure cause the MoS2 to be in an unstable state, which can improve its electrochemical reactivity to some extent. In addition, due to the presence of the molybdenum oxidation states Mo5þ and Mo6þ, the MoS2 film could exhibit a p-type conductive behavior and higher conductivity leading to an enhanced electrochemical performances for energy storage [35]. To evaluate the electrochemical performances of the selfassembled flexible L-MoS2/G electrodes, two pieces of the electrodes were sandwiched in symmetric planar configuration. The laser irradiation time shows important influence on the capacitive performance of the devices, as illustrated in Fig. S6. The optimized laser irradiation time is 8 min. This could be attributed to the perfect porous spherical MoS2 structure induced by laser irradiation for 8 min, as shown in Fig. S7. When the irradiation time is prolonged to 10 min, the porosity of MoS2 spheres decreases and tending to solid structure, which lead to the decline of capacitive performance. Fig. 4a shows the CV curves performed at different scan rates ranging from 5 to 200 mV s
1 within a potential window from 0 to 1.6 V. The shapes are basically unchanged at different scan rates indicating the electrode materials with an efficient ionic and electronic transport [39]. And more notably, these CV curves show obvious reversible faradaic peaks, which relate to the cation intercalation and reversible redox reactions between Moþ4 and Moþ3 representing the pseudocapacitive process [14]. Usually, only slight deviations from the rectangular shape can be observed because it is quite difficult for MoS2 to capture holes to generate the current and corresponding peaks during anodic polarization process [3,15,19,27,39e41]. During the laser irradiation process, the high laser powder may activate Mo atoms and S atoms to keep them in an unstable state. In addition, due to the MoO3-x presence in the L-MoS2/G nanocomposites, MoS2 can exhibit some p-type behaviors with relatively high conductivity and structure defects, leading to a larger interlayer spacing (Fig. 2d) which promotes

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faster charge storage kinetics [42]. Thus, the intense reversible peaks in L-MoS2/G are observed. The CV data reveal that the supercapacitor capacitance is contributed by the combination of the electrical double-layer capacitance and larger pseudocapacitance. Fig. 4b depicts the GCD curves at different current densities to further understand the capacitance features of the L-MoS2/G electrodes. The shapes of GCD curves exhibit obvious plateau, indicating that the faradaic reaction makes a significant contribution to the total capacitance. Upon increasing the current density from 0.6 to 3 mA cm
2, the areal specific capacitance and volumetric capacitance of the flexible SC decreases from 121.88 to 58.13 mF cm
2 and from 291.58 to 139.07 mF cm
3. The gravimetric capacitance at 1.2, 1.8, 2.4, 3 A g
1 is 243.76, 186.76, 165, and 150 F g
1, respectively. Although the rate performance is not high, the specific capacitance at each current density reaches the same level or the best (Table S2), which is caused by the large significant contribution of redox reactions involved in the charge-storage processes of pseudocapacitance that can greatly increase the specific capacitance and sacrifice rate performance to some extent [43,44]. In addition, the devices exhibit good symmetry and reversibility under higher scan rates and current densities (Fig. S8). To evaluate the electrical performance of L-MoS2/G used as flexible supercapacitors, we bend the device under mechanical deformation to about 90 and 180 . Fig. 4d shows the CV curves with different bending angles at a scan rate of 50 mV s
1. Almost overlapped CV curves are obtained for different bended flexible devices. These results clearly reveal that the self-assembled L-MoS2/G SC device with high flexibility exhibits high specific capacitance benefitting from the enhanced pseudocapacitive charge storage properties. To further understand the enhanced supercapacitor performance of the L-MoS2/G SCs, the electrochemical performances of MoS2/graphene prepared by common hydrothermal method (referred as HeMoS2/G) were also tested. HeMoS2/G nanocomposites were assembled into symmetric supercapacitors following the same process as L-MoS2/G SC. Fig. 5a exhibits the comparison of CV curves at a scan rate of 20 mV s
1, in which it can be seen that L-MoS2/G exhibits much larger capacitance (~10-fold higher) and remarkable shoulders compared to that of HeMoS2/ G. GCD cycles are presented in Fig. S9, which are consistent with capacitive behavior. The smaller capacitance of HeMoS2/G may be due to the small specific surface area caused by the serious aggregation of 2D nanosheets. These results give evidence to the great contribution from the pseudocapacitance associated with the laser irradiation-induced process. Accordingly, the energy density and power density, as two critical parameters in evaluation of actual energy storage device of the symmetric supercapacitors are calculated and the result is shown in Fig. 5b. The flexible L-MoS2/G SC devices show a higher energy density than the HeMoS2/G devices when under the same power output and a higher power density at the same energy density. The symmetric L-MoS2/G supercapacitor can deliver the largest energy density of 73.4 mWh cm
2 under a power density of 240 mW cm
2, which is comparable with or higher than other work reported recently [3,43,45e47], as the Ragone plot presented in Fig. 5b. Cycling stability is also a crucial parameter for actual electrochemical supercapacitors. The cycling performance were further conducted using the galvanostatic charge/discharge technique at 1.5 mA cm
2 for 4000 cycles, as shown in Fig. 6. Instead of a gradual degradation of the specific capacitance with the ongoing cycling, there is an initial large increase (~121%) in the performance over the first ~500 cycles due to insufficient utilization of the excess MoS2 nanospheres [14,43]. After 4000 cycles, about 115% of the

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Fig. 4. (a) CV curves in symmetric device configuration at different scan rates. (b) GCD curves at various current densities. (c) Specific capacitance calculated from GCD curves as a function of current densities. (d) CV curves under different bending angles at a scan rate of 50 mV s
1.

Fig. 5. (a) Comparison of CV performance of L-MoS2/G and HeMoS2/G at a scan rate of 20 mV s
1. (b) Ragone plot (power density vs. energy density).

initial specific capacitance is retained, exhibiting remarkable electrochemical stability, which indicates that the L-MoS2/G nanocomposites could be mechanically stable during the cycling tests. Additionally, the inserted digital photographs in Fig. 6 show three serially connected L-MoS2/G cell SCs could power one LED indicator in both free and bending states. The superior electrochemical performance of the L-MoS2/G SCs is particularly attributed to the distinctive laser-induced structural configuration of MoO3-x-doped porous hollow MoS2 nanospheres tightly sandwiched by graphene nanosheets without using any surface modifying agent or organic additives. MoS2 could store charge in two ways, i.e. inter/intra-sheet EDLC behavior, and redox reactions on the edge of MoS2 nanosheets [45]. The EDLC performance closely related to the electro-active surface area is mainly attributed to two aspects: the loose laminated structure of MoS2

nanospheres embedded in graphene nanosheets that can efficiently avoid the restacking of nanosheets and then accommodate more charge stored on the surface, and the porous hollow MoS2 nanospheres possessing larger surface area leading to an improved higher specific double-layer capacitance. Besides, the hollow MoS2 nanospheres are constructed by ultrafine nanosheets fragmented by laser irradiation, which can provide more active edges for redox reactions. The dilative interlayer spacing along the c axis and MoO3x doped in the L-MoS2/G nanocomposites can also provide effective diffusion channels for the electrolyte ions leading to a significantly improved pseudocapacitive performance. Moreover, the reduction of graphene oxide and the fragmentation and assembling of porous and hollow MoS2 nanospheres were accomplished by one-step laser irradiation. MoS2 spheres are tightly sandwiched by the graphene layers through the bond layer instead of simply attaching on

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Acknowledgments This work was supported by National Natural Science Foundation of China (51872161) and Shandong Provincial Natural Science Foundation (ZR2017ZB0316). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.135499. References

Fig. 6. Cycling performance of the L-MoS2/G device at a current density of 1.5 mA cm
2 and the insets are the digital photographs of three cell SCs (free and bending states) in series which can light up the LED indicators.

the graphene layers, as proved by our previous work [48], which can give one high conductivity in the L-MoS2/G electrode. The Nyquist plot of electrochemical impedance spectroscopy (EIS) for the L-MoS2/G electrode is depicted in Fig. S10. The obtained Nyquist plot was fitted on the basis of an equivalent Randles circuit. RS, RCT, CDL, CF, and Wo in the circuit represents solution resistance, chargetransfer resistance, double-layer capacitance, pseudocapacitance, and the finite-length Warburg diffusion element, respectively. The EIS measurement confirms that L-MoS2/G nanocomposites display a good electrical conductivity and a lower RCT (0.44 U) that can afford facile ion and charge transfer during charge/discharge process and hence a better electrochemical performance. 4. Conclusion In summary, self-assembled porous hollow MoO3-x-doped MoS2/graphene nanocomposites were fabricated via a one-step laser irradiation method at ambient conditions and directly used as a binder-free electrode for SC. The L-MoS2/G nanocomposites possess a distinctive laser-induced structure, which can provide large surface area for EDLC, more active edges for pseudocapacitance, effective diffusion channels for the electrolyte ions, and high conductivity. Therefore, the obtained flexible SCs exhibit superior areal specific capacitance, cycle stability and energy density. These excellent electrochemical performances indicate the considerable potential applications of the self-assembled L-MoS2/G electrodes in energy storage for large-scale, lightweight and flexible electronic devices. Author contributions Wei Li: Conceptualization, Methodology, Formal analysis. Ting Luo: Writing-Original draft preparation, Visualization. Chao Yang: Writing-Reviewing and Editing. Xiaopeng Yang: Validation, Investigation. Shuhua Yang: Investigation. Bingqiang Cao: Conceptualization, Resources, Supervision, Project administration, Funding acquisition. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[1] Z. Yang, J. Zhang, M.C. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, et al., Electrochemical energy storage for green grid, Chem. Rev. 111 (2011) 3577e3613. [2] W.M. Seong, K.-Y. Park, M.H. Lee, S. Moon, K. Oh, H. Park, et al., Abnormal selfdischarge in lithium-ion batteries, Energy Environ. Sci. 11 (2018) 970e978. [3] S. Wang, J. Zhu, Y. Shao, W. Li, Y. Wu, L. Zhang, et al., Three-dimensional MoS2@CNT/RGO network composites for high-performance flexible supercapacitors, Chemistry 23 (2017) 3438e3446. [4] L. Jiang, L. Sheng, C. Long, Z. Fan, Densely packed graphene nanomesh-carbon nanotube hybrid film for ultra-high volumetric performance supercapacitors, Nano Energy 11 (2015) 471e480. [5] Y. Hou, Y. Cheng, T. Hobson, J. Liu, Design and synthesis of hierarchical MnO2 nanospheres/carbon nanotubes/conducting polymer ternary composite for high performance electrochemical electrodes, Nano Lett. 10 (2010) 2727e2733. [6] J. Chen, C. Li, G. Shi, Graphene materials for electrochemical capacitors, J. Phys. Chem. Lett. 4 (2013) 1244e1253. [7] V. Augustyn, P. Simon, B. Dunn, Pseudocapacitive oxide materials for high-rate electrochemical energy storage, Energy Environ. Sci. 7 (2014) 1597. [8] T. Liu, C. Jiang, W. You, J. Yu, Hierarchical porous C/MnO2 composite hollow microspheres with enhanced supercapacitor performance, J. Mater. Chem. A 5 (2017) 8635e8643. [9] A.A. Ensafi, H.A. Alinajafi, B. Rezaei, Thermal reduced graphene oxide/ploymelamine formaldehyde nanocomposite as a high specific capacitance electrochemical supercapacitor electrode, J. Mater. Chem. A 6 (2018) 6045e6053. [10] J. Guo, Q. Zhang, J. Sun, C. Li, J. Zhao, Z. Zhou, et al., Direct growth of vanadium nitride nanosheets on carbon nanotube fibers as novel negative electrodes for high-energy-density wearable fiber-shaped asymmetric supercapacitors, J. Power Sources 382 (2018) 122e127. [11] F.Z. Amir, V.H. Pham, D.W. Mullinax, J.H. Dickerson, Enhanced performance of HRGO-RuO2 solid state flexible supercapacitors fabricated by electrophoretic deposition, Carbon 107 (2016) 338e343. [12] F.N.I. Sari, J.-M. Ting, High performance asymmetric supercapacitor having novel 3D networked polypyrrole nanotube/N-doped graphene negative electrode and core-shelled MoO3/PPy supported MoS2 positive electrode, Electrochim. Acta 320 (2019), 134533. [13] Y. Guo, D. Sun, B. Ouyang, A. Raja, J. Song, T.F. Heinz, et al., Probing the dynamics of the metallic-to-semiconducting structural phase transformation in MoS2 crystals, Nano Lett. 15 (2015) 5081e5088. [14] Y. Yang, H. Fei, G. Ruan, C. Xiang, J.M. Tour, Edge-oriented MoS2 nanoporous films as flexible electrodes for hydrogen evolution reactions and supercapacitor devices, Adv. Mater. 26 (2014) 8163e8168. [15] R. Zhou, C.-j. Han, X.-m. Wang, Hierarchical MoS2-coated three-dimensional graphene network for enhanced supercapacitor performances, J. Power Sources 352 (2017) 99e110. [16] S. Zheng, L. Zheng, Z. Zhu, J. Chen, J. Kang, Z. Huang, et al., MoS2 nanosheet arrays rooted on hollow rGO spheres as bifunctional hydrogen evolution catalyst and supercapacitor electrode, Nano-Micro Lett. 10 (2018) 62. [17] L. Fang, Y. Qiu, W. Li, F. Wang, M. Lan, K. Huang, et al., Three-dimensional flower-like MoS2-CoSe2 heterostructure for high performance superccapacitors, J. Colloid Interface Sci. 512 (2018) 282e290. [18] X.-Y. Yu, L. Yu, X.W.D. Lou, Metal sulfide hollow nanostructures for electrochemical energy storage, Adv. Energy Mater. 6 (2016), 1501333. [19] X. Geng, Y. Zhang, Y. Han, J. Li, L. Yang, M. Benamara, et al., Two-dimensional water-coupled metallic MoS2 with nanochannels for ultrafast supercapacitors, Nano Lett. 17 (2017) 1825e1832. [20] T.-T. Shan, S. Xin, Y. You, H.-P. Cong, S.-H. Yu, A. Manthiram, Combining nitrogen-doped graphene sheets and MoS2: a unique film-foam-film structure for enhanced lithium storage, Angew. Chem. Int. Ed. 55 (2016) 12783e12788. [21] H. Zhang, G. Qin, Y. Lin, D. Zhang, H. Liao, Z. Li, et al., A novel flexible electrode with coaxial sandwich structure based polyaniline-coated MoS2 nanoflakes on activated carbon cloth, Electrochim. Acta 264 (2018) 91e100. [22] J. Wang, J. Liu, H. Yang, D. Chao, J. Yan, S.V. Savilov, et al., MoS2 nanosheets decorated Ni3S2@MoS2 coaxial nanofibers: constructing an ideal heterostructure for enhanced Na-ion storage, Nano Energy 20 (2016) 1e10. [23] C. Zhao, X. Wang, J. Kong, J.M. Ang, P.S. Lee, Z. Liu, et al., Self-assembly-induced alternately stacked single-layer MoS2 and N-doped graphene: a novel van der Waals heterostructure for lithium-ion batteries, ACS Appl. Mater. Interfaces 8

8

W. Li et al. / Electrochimica Acta 332 (2020) 135499

(2016) 2372e2379. [24] W. Luo, G. Zhang, Y. Cui, Y. Sun, Q. Qin, J. Zhang, et al., One-step extended strategy for the ionic liquid-assisted synthesis of Ni3S4-MoS2 heterojunction electrodes for supercapacitors, J. Mater. Chem. A 5 (2017) 11278e11285. [25] J. Chao, L. Yang, J. Liu, R. Hu, M. Zhu, Sandwiched MoS2/polyaniline nanosheets array vertically aligned on reduced graphene oxide for high performance supercapacitors, Electrochim. Acta 270 (2018) 387e394. [26] Y. Zhang, P. Ju, C. Zhao, X. Qian, In-situ grown of MoS2/RGO/MoS2@Mo nanocomposite and its supercapacitor performance, Electrochim. Acta 219 (2016) 693e700. [27] E.G. da Silveira Firmiano, A.C. Rabelo, C.J. Dalmaschio, A.N. Pinheiro, E.C. Pereira, W.H. Schreiner, et al., Supercapacitor electrodes obtained by directly bonding 2D MoS2 on reduced graphene oxide, Adv. Energy Mater. 4 (2014), 1301380. €kce, S. Barcikowski, Laser synthesis and processing of colloids: [28] D. Zhang, B. Go fundamentals and applications, Chem. Rev. 117 (2017) 3990e4103. [29] H. Zeng, X.-W. Du, S.C. Singh, S.A. Kulinich, S. Yang, J. He, et al., Nanomaterials via laser ablation/irradiation in liquid: a review, Adv. Funct. Mater. 22 (2012) 1333e1353. [30] G. Yang, Laser ablation in liquids: applications in the synthesis of nanocrystals, Prog. Mater. Sci. 52 (2007) 648e698. [31] R. Ye, D.K. James, J.M. Tour, Graphene: powder, flakes, ribbons, and sheets, Acc. Chem. Res. 51 (2018) 1609e1620. [32] W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958), 1339-1339. [33] T. Luo, P. Wang, Z. Qiu, S. Yang, H. Zeng, B. Cao, Smooth and solid WS2 submicrospheres grown by a new laser fragmentation and reshaping process with enhanced tribological properties, Chem. Commun. 52 (2016) 10147e10150. [34] X. Zheng, J. Xu, K. Yan, H. Wang, Z. Wang, S. Yang, Space-confined growth of MoS2 nanosheets within graphite: the layered hybrid of MoS2 and graphene as an active catalyst for hydrogen evolution reaction, Chem. Mater. 26 (2014) 2344e2353. [35] P. Qin, G. Fang, W. Ke, F. Cheng, Q. Zheng, J. Wan, et al., In situ growth of double-layer MoO3/MoS2 film from MoS2 for hole-transport layers in organic solar cell, J. Mater. Chem. A 2 (2014) 2742. [36] K.H. Ibrahim, M. Irannejad, M. Hajialamdari, A. Ramadhan, K.P. Musselman, J. Sanderson, et al., A novel femtosecond laser-assisted method for the synthesis of reduced graphene oxide gels and thin films with tunable properties,

Adv. Mater. Interfaces 3 (2016), 1500864. [37] N. Adnan, N. Bidin, N. Taib, H. Haris, M. Fakaruddin, A. Hashim, et al., Passively Q-switched flashlamp pumped Nd: YAG laser using liquid graphene oxide as saturable absorber, Opt. Laser. Technol. 80 (2016) 28e32. [38] T. Liang, W.G. Sawyer, S.S. Perry, S.B. Sinnott, S.R. Phillpot, Energetics of oxidation in MoS2 nanoparticles by density functional theory, J. Phys. Chem. C 115 (2011) 10606e10616. [39] X. Li, X. Li, J. Cheng, D. Yuan, W. Ni, Q. Guan, et al., Fiber-shaped solid-state supercapacitors based on molybdenum disulfide nanosheets for a selfpowered photodetecting system, Nano Energy 21 (2016) 228e237. [40] L. Zheng, T. Xing, Y. Ouyang, Y. Wang, X. Wang, Core-shell structured MoS2@ Mesoporous hollow carbon spheres nanocomposite for supercapacitors applications with enhanced capacitance and energy density, Electrochim. Acta 298 (2019) 630e639. [41] A. Gao, D. Zeng, Q. Liu, F. Yi, D. Shu, H. Cheng, et al., Molecular self-assembly assisted synthesis of carbon nanoparticle-anchored MoS2 nanosheets for high-performance supercapacitors, Electrochim. Acta 295 (2019) 187e194. [42] H.S. Kim, J.B. Cook, H. Lin, J.S. Ko, S.H. Tolbert, V. Ozolins, et al., Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO3-x, Nat. Mater. 16 (2017) 454e460. [43] M.A. Bissett, I.A. Kinloch, R.A. Dryfe, Characterization of MoS2-graphene composites for high-performance coin cell supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 17388e17398. [44] T. Brezesinski, J. Wang, S.H. Tolbert, B. Dunn, Ordered mesoporous alphaMoO3 with iso-oriented nanocrystalline walls for thin-film pseudocapacitors, Nat. Mater. 9 (2010) 146e151. [45] N. Li, H. Zhao, Y. Zhang, Z. Liu, X. Gong, Y. Du, Core-shell structured CeO2@ MoS2 nanocomposites for high performance symmetric supercapacitors, CrystEngComm 18 (2016) 4158e4164. [46] F. Clerici, M. Fontana, S. Bianco, M. Serrapede, F. Perrucci, S. Ferrero, In situ MoS2-decoration of laser induced graphene as flexible supercapacitor electrodes, ACS Appl. Mater. Interfaces 8 (2016) 10459e10465. [47] G.K. Veerasubramani, K. Krishnamoorthy, P. Pazhamalai, S.J. Kim, Enhanced electrochemical performances of graphene based solid-state flexible cable type supercapacitor using redox mediated polymer gel electrolyte, Carbon 105 (2016) 638e648. [48] T. Luo, X. Chen, P. Li, P. Wang, C. Li, B. Cao, et al., Laser irradiation-induced laminated graphene/MoS2 composites with synergistically improved tribological properties, Nanotechnology 29 (2018), 265704.