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Highly Uniform Atomic Layer Deposited MoS2@3D-Ni-foam: A Novel Approach to Prepare Electrode for Supercapacitor Dip K. Nandi, Sumanta Sahoo, Soumyadeep Sinha, Seungmin Yeo, Hyungjun Kim, Ravindra N Bulakhe, Jaeyeong Heo, Jae-Jin Shim, and soo-hyun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12248 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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ACS Applied Materials & Interfaces

Highly Uniform Atomic Layer Deposited MoS2@3D-Ni-foam: A Novel Approach to Prepare Electrode for Supercapacitor Dip K. Nandi1#, Sumanta Sahoo2#, Soumyadeep Sinha3, Seungmin Yeo4, Hyungjun Kim4,Ravindra N. Bulakhe2, Jaeyeong Heo3, Jae-Jin Shim2, Soo-Hyun Kim1* 1

School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Gyeongbuk

38541, Republic of Korea 2

School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541,

Republic of Korea 3

Department of Materials Science and Engineering, and Optoelectronics Convergence Research

Center, Chonnam National University, Gwangju 61186, Republic of Korea 4

School of Electrical and Electronic Engineering, Yonsei University, Seodaemun-gu, Seoul 120-

749 KEYWORDS: Plasma-enhanced atomic layer deposition, Molybdenum hexacarbonyl, Molybdenum sulfide, Supercapacitor, Areal capacitance

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ABSTRACT: This article takes an effort to establish the potential of atomic layer deposition (ALD) technique towards the field of supercapacitor by preparing molybdenum disulfide (MoS2) as its electrode. While molybdenum hexacarbonyl [Mo(CO)6] serves as a novel precursor towards the low temperature synthesis of ALD-MoS2, H2S plasma helps to deposit its polycrystalline phase at 200 °C. Several ex-situ characterizations like X-ray diffractometry (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) etc. are performed in detail to study the as-grown MoS2 film on Si/SiO2 substrate. While stoichiometric MoS2 with very negligible amount of C and O impurities was evident from XPS, the XRD and high-resolution transmission electron microscopy (HRTEM) analysis confirmed the (002) oriented polycrystalline h-MoS2 phase of the as-grown film. A comparative study of ALD-grown MoS2 as supercapacitor electrode on 2-dimensional (2D) stainless steel (SS) and on 3-dimensional (3D) Ni-foam substrate clearly reflects the advantage and the potential of ALD for growing the uniform and conformal electrode material on 3D-scaffold layer. Cyclic voltammetry measurements showed both double layer capacitance and capacitance contributed by the Faradic reaction at the MoS2 electrode surface. An optimum number of ALD cycles were also found out for achieving maximum capacitance for such MoS2@3D-Ni-foam electrode. Record high areal capacitance of 3400 mF/cm2 was achieved for MoS2@3D-Ni-foam grown by 400 ALD cycles at the current density of 3 mA/cm2. Moreover, the ALD-grown MoS2@3D-Ni-foam composite also retains high areal capacitance even up to a high current density of 50 mA/cm2. Finally, this directly grown MoS2 electrode on 3D-Ni-foam by ALD shows high cyclic stability (>80%) over 4500 charge-discharge cycles which must invoke the research community to explore further the potential of ALD for such applications.


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1. INTRODUCTION Ever growing demand of energy in this 21st century facilitates the urge for energy storage devices in proportion which mainly includes rechargeable batteries and electrochemical capacitors (ECs) commonly known as supercapacitors. The revolutionary discovery of the portable electronic gadgets and the overwhelming popularity of these make them the all-time need for almost everyone in every sphere of life. This fact alone brings a paradigm shift in the industrial market of the rechargeable energy storage. Due to its high power/energy density, very fast charge discharge capabilities and extremely long cyclic stability, supercapacitors are being considered as a better alternative to Li-ion batteries in the field of high power density electronic devices and plug in electric vehicles.1 Supercapacitors can store and release the energy in two different ways either in the form of a double layer capacitor where the energy is stored in the form of a charge separation at the electrode and electrolyte interface, or in the form of Faradic redox reaction with the electrode materials.1-2 The transition metal binaries are one of the most favored classes of electrode materials for this application due to their contribution of Faradic capacitance in addition to their fundamental role as an electrical double layer capacitor (EDLC).3 Among several transition metal binary compounds, transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2), tungsten disulfide (WS2) etc. are considered as a unique class of materials due to their graphitelike layered structures that find a lot of applications in recent energy generation and storage applications.4-5 Particularly, MoS2 could be considered as one of the most researched TMDCs and widely used in several renewable-energy related applications that include solar cell, solar water splitting and Li-ion storage.6-14 In recent time, MoS2 has also been successfully


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investigated by handful number of research groups as an electrode in supercapacitor.15-23 The layered structure of MoS2 can contribute to the double layer capacitance from their inter-layer charge storage in addition to the intra-layer charge storage. Similar to RuO2, MoS2 can also provide additional capacitance from the Faradic reaction at the Mo center with its multiple oxidation states ranging from +2 to +6.16-17 Moreover, the 2-dimensional (2D) layer of MoS2 provides a better electron transport as well as higher ionic conductivity compared to the oxides.20 All of these above mentioned points truly attract several researchers to explore this material as a potential candidate for supercapacitor. Unfortunately, most of the earlier works have adopted some wet-chemical synthesis like hydrothermal process and further combined with some 3dimensional (3D) scaffold layer like graphene oxide (GO),

multi-walled carbon nanotube

(MWCNT) or polyaniline (PANI) to fabricate a hybrid composite to obtain the enhanced capacitance from it.16, 20, 22 In general, a wide range of synthesis processes have been tried out for preparing the electrode material for supercapacitors. Among all kinds of such processes, hydrothermal synthesis has always been the major stake holder, as seen for the case of MoS2 as well. The other relatively less implemented wet chemical processes to prepare MoS2 electrode include the chemical bath deposition (CBD)24 and successive ionic layer adsorption and reaction (SILAR)23. On the other hand, there is also gas-phase deposition technique like chemical vapor deposition (CVD) which also can be found to grow a thin film of the MoS2 for this application.17 Such gas-phase deposition techniques usually have more control over the deposited material which generally includes the stoichiometry, crystallinity etc. compared to the wet chemical synthesis process. Moreover, these deposition techniques involve a single step to deposit the desired material while hydrothermal process requires longer synthesis time (ca. 24‒ 48 h or more) followed by rigorous filtrations steps.19-20


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In recent time, atomic layer deposition (ALD) is considered as one of the most efficient techniques to deposit thin films of materials for energy storage applications which include both secondary batteries and supercapacitors. ALD is a gas-phase, two stage, thin film deposition technique where the precursors are dosed into the ALD reactor sequentially separated by a purging of inert gas like Ar or N2. This sequential dosing of the precursors leads to a self-limiting growth of the film which makes ALD unique from rest of the deposition techniques. Though it finds usage in several types of rechargeable batteries including Li and Na-ion battery, Li-O2 and Li-S battery, unfortunately, this technique has been vastly applied only to Li-ion battery.25-40 In this context it is interesting to notice that, not only in the case of Li-ion battery but also for Li-S and Na-ion battery, the most of attempts are limited to surface engineering of the active electrode by depositing a very thin protecting layer which helps to increase the cyclic stability of the Li-ion battery.25-29 ALD is also successfully demonstrated to grow the active anode materials for such applications.35-36,

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The most interesting thing to notice for the ALD-grown active electrode

material is that when it is combined with a porous and conducting 3D-scaffold layer as a base material.40-41 Such unique combination of conformal and uniform coated ALD material on 3Dstructure truly can establish this technique as a most preferred deposition tool for energy storage applications. There are very few attempts made to apply this process for growing the supercapacitor electrode material. ALD-grown TiO2, VOx, NiO and Co8S9 are among few of them which were successfully demonstrated as an active electrode material for supercapacitor.4247

The earliest studies on ALD-grown metal oxides (VOx and TiO2) as electrodes were carried

out by Boukhalfa et al.47 and by Sun et al.46 Both of these studies used graphene (G) and MWCNT as substrate to deposit the active electrode material to achieve high specific capacitance.46-47 One of the most recent study on ALD-Co8S9 showed very promising results


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towards this application which reflected the potential of this technique for depositing the sulfide materials for supercapacitor electrode fabrication.42 A critical issue of such thin films to be used as an electrode in electrochemical storage devices is associated with its extremely low mass loading of the active material. In most of these cases the active material lies in the range of few µg which would eventually yield into an overestimated specific capacitance (per kg or even per g). Therefore, it is preferable to calculate the areal capacitance (F/cm2) especially for an extremely thin film (in order of nm) electrode grown by a technique like ALD.48-50 Nevertheless, ALD has already been extensively studied and established tool to grow an active electrode material conformally on several 3D scaffold layers. In this regard, Ni-foam is commonly considered as an efficient 3D current collector for supercapacitor application. Moreover, Nifoam further provides an advantage of direct deposition of the electrode material by ALD, avoiding any additional step which is essential in case of other 3D scaffolds like rGO, MWCNT or PANI. In this article, we take an effort to further establish ALD as a very efficient tool to fabricate the electrode for supercapacitor. For this purpose, MoS2 was directly grown on 2D-stainless steel (SS) foil and on 3D-Ni-foam by plasma-enhanced ALD (PEALD) for the first time and was used as an electrode without any further modifications. Molybdenum hexacarbonyl [Mo(CO)6], a novel emerging precursor for low temperature ALD synthesis, was used to deposit MoS2. Previously, a halide precursor (MoCl5) and H2S molecules as reactant were used to deposit MoS2 film at a temperature of 300 °C.51 However, the relatively high temperature of deposition and HCl as a by-product during deposition might hinder this process to deposit it on fragile Ni-foam. Therefore, here we prefer to adopt the hexacarbonyl precursor and H2S plasma as the reactant. After detailed investigations of the material’s properties and the unique growth of it on 3D-Ni-


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foam, the electrochemical studies were carried out to realize the potential of PEALD-grown MoS2 as an electrode in asymmetric supercapacitor. Thickness dependent electrochemical properties were studied and an optimized number of ALD cycles for such application were found out. 2. EXPERIMENTAL SECTION Material Synthesis MoS2 thin films were deposited using Mo(CO)6 [UP chemical Inc., Korea] precursor and H2S plasma reactant by PEALD in a shower-head type ALD reactor (Lucidia-M100, NCD Technology) at a deposition temperature of 200 °C. The Mo(CO)6 is in solid state at room temperature (white powder) and the precursor was kept at a temperature of 40 °C to achieve a minimum amount of vapor pressure (ca. 0.8 Torr) and to prevent the precursor from condensation, the line was heated up to 60 °C. In addition to the heating of the Mo(CO)6 precursor, high purity Ar was used as carrier gas with the flow rate of 50 sccm (standard cubic centimeter per minute) enabling proper chemical transfer as well purging gas with the flow rate of 200 sccm in-between the sequential exposure of the precursor and the reactant. A radio frequency (RF) power of 100 watts (W) was applied to the showerhead to ignite the corresponding plasma during the H2S pulsing. Thermally grown SiO2 (~100 nm) on Si (henceforth written as Si/SiO2) substrates were used to deposit the MoS2 films for material’s characterizations. For electrochemical studies of PEALD-grown MoS2 as an electrode in supercapacitor, the material was directly grown on 2D-stainless steel (SS) foil and on 3D-Nifoam under self-limiting deposition condition.

Material Characterizations


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Several ex-situ characterizations techniques were adopted to study the PEALD-grown MoS2 in details. Grazing incident angle X-ray diffraction (GIAXRD, PANalytical X’-pert PRO MRD, incident angel: ~3º) with Cu-Kα source was used within 2θ of 10‒70 degree to study the crystalline nature of the deposited films. Raman spectroscopy was carried out to find out the Raman active modes in the PEALD-grown MoS2 films. 500 nm laser source was used as an incident excitation energy for this purpose. X-ray photoelectron spectroscopy (XPS, ESCALAP 250 XPS Spectrometer) with Al-Kα source was adopted to investigate the elemental presence and the oxidation states of them in the grown material. Plan-view and cross-sectional view transmission electron microscopy (TEM, Tecnai F20 equipped with 200 kV accelerating voltage and field emission gun) was performed to show the crystal lattice, its corresponding planes and the uniform and conformal deposition of the film grown by PEALD. Energy dispersive spectroscopy (EDS) analysis was also performed to detect the elemental distribution of MoS2 on on 3D-Ni-foam during scanning TEM (STEM) analysis. Plan-view scanning electron microscope (SEM, Hitachi, S-4800) was used to show the uniform deposition and the surface morphology of the film on the 3D-Ni-foam.

Electrochemical measurements The electrochemical studies were carried out in a conventional three-electrode configuration where the PEALD-grown MoS2 on 2D-SS-foil or on 3D-Ni-foam was directly used as a working electrode without any additive or binder. The active electrode area adopted for the working electrode was 1 cm × 1 cm for both SS and Ni-foam substrates. Pt and Ag/AgCl were served as the counter and reference electrodes, respectively. 1M KOH aqueous solution was used as an electrolyte. The cyclic voltammetry (CV) and the galvanostatic charge-discharge cycling


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measurements were performed in a potential stat. (PGSTAT 302N with Nova 1.10 software, Autolab, Netherlands). The electrochemical impedance spectroscopy (EIS) was further carried out within the frequency range of 10-2 to 106 Hz using the same Autolab instrument to realize the electrical behaviors of the electrode electrolyte interface.

3. RESULTS AND DISCUSSION ALD differs from rest of the material synthesis techniques that include wet chemical synthesis as well as gas-phase thin film deposition techniques by its unique feature of self-limiting growth of the material. The similar self-limiting growth condition with 10 s pulsing of both Mo(CO)6 as well as H2S plasma with Ar purging of 10 s in between the alternate exposure of these two was used to deposit the MoS2 thin films as obtained in our previous study.52 A detailed study on the self-limiting ALD reaction and linear growth with ALD cycles could also be found in that study.52 The two-half ALD surface reactions during the PEALD of MoS2 can be written as follows: Mo(CO)x* + H2S Mo‒SH* + H2(g) + CO(g)

(1)

MoSH* + Mo(CO)6 Mo‒S‒Mo(CO)x*+ H2(g) + CO(g)

(2)

where * denotes the surface species. GIAXRD analysis of the as-grown MoS2 film deposited by 200 ALD cycles on Si/SiO2 substrate is shown in Figure 1. The three distinct XRD peaks (at 2θ = 14.37, 32.67 and 39.54 degree) confirm the poly-crystalline nature of the PEALD-grown MoS2 films. These peaks can be assigned to the (002), (100) and (103) planes of hexagonal 2H-MoS2, respectively (space group P63/mmc; JCPDS Ref. 00-037-1492).15-16, 18, 52 Predominantly (002) oriented growth on Si/SiO2 substrate could be expected by the presence of a much higher intense XRD peak at 14.37


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degree compared to the other two planes as reveled by this XRD measurements.16, 52 However, as mentioned earlier, the films were deposited on SS-foil and Ni-foam for supercapacitor applications. Therefore, we have also performed the characterizations of the film deposited both on SS and Ni-foam. The XRD pattern of the as-grown MoS2 film on SS-foil [Figure S1(a)] shows three less intense peaks along (002), (101) and (110) planes at 14.3, 33.5 and 58.9 degree respectively corresponding to the same 2H-MoS2. Unfortunately, we were not able to detect any XRD peaks of the MoS2 on Ni-foam might be due to the complex structure of the substrate, presence of high-intense peaks of bare Ni foam, and low mass content of the target material [Fig. S1(c)]. Yet it does not completely discard the crystalline growth of the film on Ni-foam as shown and discussed later in this article. Therefore, it might be said that the growth and the crystalline nature of the film might depend to some extent on the substrate but gives similar polycrystalline MoS2 on different substrate as well. Here, it is interesting to notice that an earlier study on thermal ALD of MoS2 produced the amorphous film using the same precursor [Mo(CO)6] as a source for Mo.36 Therefore, it could be inferred that the H2S plasma as a reactant facilitates the formation of crystalline MoS2 at a relatively low deposition temperature of 200 °C. This deposition temperature is also considerably lower compared to the 300 °C for ALD of MoS2 using MoCl5 precursor.51 Unlike to MoCl5, the usage of Mo(CO)6 would further help to avoid the formation of corrosive HCl by-product which must affects the MoS2 film itself as well as the substrate during the growth. Therefore, it could be concluded that the Mo(CO)6 served as a better precursor from both deposition temperature and reaction by-products point of view. The crystalline nature of such PEALD-grown MoS2 will certainly help to ease the electron as well as ion transport (for Faradic intercalation-deintercalation reaction) when it is used as an electrode in the supercapacitor.


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Figure 1. GIAXRD pattern of as-grown MoS2 thin film on Si/SiO2 substrate Once the XRD proved the crystalline growth of the PEALD-MoS2 with its most favorable and preferential direction of (002), we further carried out Raman spectroscopy to investigate the material in more details. Raman spectrum was obtained for the as-grown MoS2 film deposited on Si/SiO2 substrate. Two distinct peaks were observed at the frequency of ~383 and ~408 cm‒1 corresponding to the two typical first-order Raman active vibrational modes, i.e., E12g and A1g respectively,53-55 within the S‒Mo‒S layer as shown in Figure 2(a). These are similar to the frequency values as found in bulk MoS2.56-57 The peak at ~520 cm‒1 was obtained from the Si substrate.54-55 The vibrational peak for E12g associates with the planer vibration while the A1g peak indicates the vibration along the out-of-plane direction of the sulfides53 as represented in the schematic Figure 2(b). Similarly, two distinct peaks corresponding to those in-plane and out-ofplane vibrational modes, as shown in Figure S1(b), were also obtained for the MoS2 film deposited on 2D-SS substrate.


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Figure 2. (a-b) Raman spectra and schematic of the major two Raman active modes in MoS2 and (c-d) XPS of Mo3d, S2s and S2p electronic states in ALD-grown MoS2. XPS was performed further to confirm the constituent element, chemical composition and the oxidation state of the Mo in the MoS2 film by determining the corresponding binding energies of Mo and S electrons as shown in Figure 2(c) and (d). The XPS data was taken after etching ca. 10 nm of the film from the surface to avoid the presence of O and C in the spectrum that may arise from the surface. The doublet of Mo3d orbital electrons at the binding energies at ca. 229 and 232.3 eV [Figure 2(c)] can be assigned respectively to the 3d5/2 and 3d3/2 electronic states of Mo in Mo-S bond. These binding energies of Mo3d orbital electrons in the as grown film also indicate the Mo+4 oxidation state of Mo confirming the chemical composition as MoS2.36, 52, 58


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On the other hand, the XPS spectrum corresponding to the 2p binding energies of S is shown in Figure 2(d). The peaks at binding energy of 162 and 163.2 eV, as obtained after the XPS peak fitting, are attributed to the S2p1/2 and S2p3/2 electronic states36, 52, 58 respectively for the divalent sulfide ions (S2‒) present in the film. In addition to these S2p peaks, an additional S2s peak was also detected [Figure 2(c)] at 226.8 eV in the energy range of Mo peaks that also confirms the binding state of S in MoS2.36, 52, 58 The full survey of XPS spectrum of MoS2 film without surface etching on Ni-foam substrate in the binding energy range of 0‒1350 eV with the constituent elemental peaks is given in the SI of this article [Figure S2(a)]. The presence of Mo3d, S2p and Ni2p peaks in the spectrum confirm the successful deposition of MoS2 on the 3D-Ni foam. Figure S2(b), 2(c) and 2(d) shows the further deconvolution of the individual XPS spectra of Ni2p (2p3/2, 2p1/2 and the satellite peaks), Mo3d (3d5/2 and 3d3/2) and S2p (2p3/2 and 2p1/2) respectively at their corresponding binding energies that proves our claim beyond any doubt. The presence of O1s peak at ~532 eV indicates the hydroxide formation due to the absorbed oxygen on film surface after air exposure which can also be observed in other sulfide materials.59-62 The crystallinity of the as-deposited MoS2 thin films was further verified by using TEM analysis. Figure S3(a) shows the plan-view TEM image of the film on Si/SiO2 substrate. Figure S3(b) represents the magnified view of a small portion from Figure S3(a). From the lattice fringes of Figure S3(a) and (b), the interplanar spacing was mostly found to be about 0.60‒0.62 nm, which corresponds to the (002) plane of the hexagonal structure of MoS2 (JCPDS ref. 00037-1492).16, 18, 22 The lattice image clearly exhibits the highly oriented MoS2 thin film with a preferential orientation along [002] direction, which was also obtained from the XRD pattern, as shown in Figure 1. The cross-sectional view TEM images of the as-grown MoS2 films using 200 ALD cycles on Si/SiO2 substrate and trench structure (top opening width of ~ 100 nm) are shown


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in Figure S3(c) and (d) respectively. The cross-sectional view TEM images thus prove the uniform and conformal deposition ability of PEALD which will encourage us to deposit the material on porous 3D-Ni substrate for electrode fabrication. Further cross-sectional TEM analysis of the MoS2@Ni-foam composite was also performed and discussed in detail in the later section of this article. Figure 3(a-d) shows the plan-view SEM images of the MoS2 film deposited on 3D-Ni-foam with different magnifications where the inset of fig. 3(c) shows the SEM image of bare Ni-foam with same magnification as 3(c). The SEM images exhibit uniform film deposition on the entire 3D-substrate surface, which again confirms the conformal film deposition by PEALD process. Figure S4(a) further shows the SEM image of the MoS2 coated 3-D Ni-foam at very low magnification where it is clear that the film growth was extremely uniform and conformal over a large area of this 3D substrate. The MoS2 film grown with 400 ALD cycles also retains uniform micro-porosity throughout the surface of the film. A nano-cauliflower like morphology [Figure S4(b)] can be observed from the SEM image of the MoS2 film on 3D-Ni-foam which is quite different from the films morphology as obtained from the SEM images (Figure S5) on the 2D-SS substrate with a uniform film deposition. Therefore, there are some differences observed in the surface morphologies of the MoS2 grown by same number of ALD cycles on SS and on 3D Nifoam. It is also true that the morphology considerably affects the performance of electrochemical storage devices. However, in this case the performance was predominantly determined by the dimensionality of the substrate. The porous 3D Ni-foam provides much higher active surface area compared to 2D SS substrate, which enables to deposit more materials with same number of ALD cycles. On the other hand, the morphology changes drastically when the film was grown on Ni-foam using 500 ALD cycles. The plan-view SEM images (Fig. S6) clearly show that the


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porosity of the MoS2 films was lost to a great extent as we increased the number of ALD cycles beyond an optimum number. This affects further the performance when it was used as an electrode in supercapacitor. The elemental distribution mapping of energy dispersive spectroscopy (EDS) analysis for the MoS2 film on 3-D Ni-foam is illustrated in Figure 3(e-g). The presence of two constituent elements (Mo and S) is indicated by two different color representations which show the uniform distribution of the elements throughout the Ni-foam captured in the frame due to the uniform and conformal deposition by ALD. Figure 4 (a) and (b) shows the schematic of two-step ALD of MoS2 using Mo(CO)6 and H2S plasma followed by its growth on 2D-SS as well as on 3D-Ni-foam substrate respectively. From this schematic [Figure 4(b)], it is clear that the conformal and uniform coating by ALD on 3D substrate (Ni-foam for this case) will provide higher mass loading by orders of magnitude for same number of ALD cycles.


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Figure 3. (a-d) Plan-view SEM images of MoS2@3D-Ni-foam grown by 400 ALD cycles and in inset of (c) the bare Ni-foam with same magnification; (e-g) EDS mapping of the ALD-grown MoS2 showing the uniform distribution of Mo and S on Ni-foam.


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Figure 4. Schematic of (a) the 2-step ALD-growth of MoS2 using Mo(CO)6 and H2S plasma and (b) the film’s growth on 2D-SS-foil and 3D-Ni-foam substrates.


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Figure 5 shows the cross-sectional view TEM images and the elemental mapping of the MoS2@Ni-foam composite grown by 500 ALD cycles. We prefer this condition to perform the TEM as it makes this analysis easier for the composite with almost without any porosity as observed by the plan-view SEM images of the same. Figure 5(a) and (b) clearly shows the uniform and conformal coating of the material on a complex 3D Ni-foam structure. Unlike to the samples grown by 400 ALD cycles, here the surface of the Ni-foam substrate got covered completely by a 2D thick film of MoS2 [Fig. 5(b)] without any surface-porosity as observed in case of the former. Two different layers could further be observed for this sample mainly because of the surface oxidation of the sample under considerable amount of exposure to the ambient. A catalytic ALD growth adjacent to the Ni-surface might also cause to a more dense film deposition at the beginning. High-resolution TEM (HRTEM) images [Fig. 5(c-d)] clearly show the poly-crystalline lattice grains in the films. The d-spacing measurements further reveal the (100) and (103) planes of h-MoS2 with interplanar spacing of 2.74 and 2.27 Å, respectively (JCPDS Ref. 00-037-1492). Scanning TEM (STEM)-EDS mapping of this MoS2@Ni-foam composite [Fig. 5(e-h)] clearly reflects the uniform distribution of the elemental Mo [Fig. 5(g)] and S [Fig. 5(h)] on Ni [Fig. 5(f)]. Therefore, this cross-sectional view TEM along with STEM analysis univocally proves the formation of poly-crystalline MoS2 on 3D Ni-foam. Conformal and uniform deposition by ALD on 3D Ni-foam structure is also evident from this analysis which is extremely rare in the existing literatures to the best of our knowledge.


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Figure 5. (a-d) Cross-sectional view TEM analysis of the MoS2@Ni-foam grown by 500 ALD cycles and the corresponding HRTEM images showing the poly-crystalline grains in the MoS2 film and (e-h) the STEM elemental mapping of confirming the uniform distribution of Mo and S on Ni.

Electrochemical analysis of the PEALD-grown MoS2 as a supercapacitor electrode was performed in 1M KOH aqueous solution. CV measurements were carried out within a potential window of 0‒0.6 V to study the capacitive response of these MoS2 electrodes. For this purpose, the CV of the 400 cycles PEALD-grown MoS2 on 2D-SS-foil was first tested [Figure S5] for five different voltage scan rates from 5 to 50 mV/s. The current response of MoS2@2D-SS-foil was increased with increasing scan rate, which can be confirmed by the enhancement in the area under the CV curve (or the current). The Faradic capacitance was realized from the distinct peaks in the CV curve of MoS2@SS-foil at 50 mV/s voltage scan rate. The peaks at ca. 0.34 and 0.46 V during the cathodic and anodic sweep respectively confirm such redox reactions in MoS2 electrode in supercapacitor. However, the area under the CV and the proportional maximum current yielded even for the highest voltage scan rate (50 mV/s) was truly negligible. The reason


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for such low capacitive response in this case is due to the less mass loading by ALD on a flat 2D substrate (SS-foil). On the contrary, ALD is best known for its uniform and conformal coating on any randomly shaped 3D substrate. Therefore, it gives further avenue to deposit this material on 3D Ni-foam which can provide much more surface to deposit MoS2 leading to a reasonable mass loading. Figure 6(a) shows the CVs of bare Ni-foam and MoS2@3D-Ni-foam deposited with 200, 400 and 500 ALD cycles at 5 mV/s scan rate. The manifold increase in the area under CV curve compared to the bare Ni-foam clearly confirmed the high capacitive nature of this composite. The capacity increased considerably when the number of ALD cycles was increased from 200 to 400 cycles. The highest capacitive response was observed for the electrode grown with 400 ALD cycles. Further increase in the ALD cycles (MoS2@3D-Ni-foam electrode grown by 500 ALD cycles) increases the resistivity of the composite as the MoS2 films became thicker. The porosity of the electrode was also lost to a great extent which could be observed from plan-view SEM images of this sample (Fig. S6). As a result, the current response in the CVs and hence the capacitance for this electrode was decreased drastically. Thus, an optimum number of 400 ALD cycles was identified to achieve the maximum capacity for MoS2@3D-Ni-foam electrode. However, the nature of the CV for both 200 and 400 ALD cycles grown remained almost unaltered. The Faradic reactions in the MoS2 electrode are clear from the prominent redox reactions during both anodic and cathodic sweep in the CV. The reduction peak during the discharge cycle (anodic sweep) for both MoS2@3D-Ni-foam grown by 200 and 400 ALD cycles in presence of KOH electrolyte occurred at ca. 0.2 V. Therefore the two kinds of non-Faradic and the Faradic reactions in MoS2 could be predicted respectively as follows: (MoS2)surface + K+ + e‒ (MoS2‒ ‒ K+)surface

(3)

MoS2 + K+ + e‒ MoS‒SK

(4)


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A negligible shift in potential by 0.02 V (from 0.42 to 0.44 V) was observed in the reduction peak during the anodic sweep for 200 and 400 cycles MoS2 electrode. The shift in the higher potential for 400 cycles MoS2 electrode, though negligible, could be due to the densification of the thicker film causing slower kinetics of the reduction reaction during the anodic sweep (discharge cycle). The CVs for 400 ALD cycles MoS2 electrode with increasing voltage scan rate (from 5‒100 mV/s) is shown Figure 6(b). The conventional EDLC nature was evident for MoS2@3D-Ni-foam from the increasing area under the CV curves when the voltage scan rate was increased. As mentioned earlier, the capacitance was also obtained from the Faradic reactions. As the voltage scan rate increased, the reduction peak during the anodic (discharge) sweep was continuously shifted to a lower potential which means the chemical reaction between MoS2 and the electrolyte takes place at higher energy for faster voltage scan rate. The opposite phenomena during the cathoodic sweep (during charging higher potential corresponds to higher energy supply) could also be seen from these CVs [Figure 6(b)]. During cathodic sweeps, the Faradic reactions even almost disappeared at 50 and 100 mV/s scan rate that did not allow enough time for the Faradic reversible reaction to take place. Though such Faradic capacitance for MoS2 electrode was not observed in most of the earlier reports where hydrothermal synthesis route was adopted but few others have reported similar CVs like one observed in this study.23 It is interesting to notice that when the MoS2 was prepared in the form of a thin film by CVD or by SILAR (a wet-chemical synthesis route to deposit thin films layer by layer and therefore could be considered as an analogous method to ALD) method, prominent Faradic reactions were observed along with the EDLC behavior.17, 23 It is usually expected for transition metal binary compounds to undergo a reversible Faradic reaction (intercalation and deintercalation) similar to rechargeable secondary batteries (e.g. Li or Na-ion battery) along with the double layer


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capacitance. The capacitance added from such Faradic reactions therefore should be considered as an advantage for this PEALD-grown MoS2 electrode for this case. Therefore in this context, we find ALD-grown films could be a true alternative to grow the active electrode material. Once the electrochemical response (both pseudo-capacitance and electrical double layer capacitance) were investigated, the galvanostatic charge-discharge properties of the PEALD MoS2@3D-Ni-foam at different current densities were carried out in detail within a potential window of 0‒0.45 V. Figure 6(c) shows such charge discharge profiles of MoS2@3D-Ni-foam grown by 200, 400 and 500 ALD cycles at a current density of 3 mA/cm2. Similar to the CVs of these three different electrodes, the charge-discharge time for 400 ALD cycles’ electrode increased considerably compared to the electrode grown by 200 cycles. Moreover, the chargedischarge time decreased drastically when the ALD cycles were increased further to 500 cycles for depositing the MoS2 on Ni-foam which is in complete line with the response obtained during the CV measurements. Such deterioration in the electrochemical property might be caused by two simultaneous factors: (a) more number of ALD cycles beyond a point makes the MoS2@3-D Ni-foam more resistive and (b) the loss of the porosity of the film. However, all of these three electrodes showed similar trend during both charge and discharge profiles. Unlike to the linear charge-discharge profiles for pure EDLC, the prominent plateau in between 0.2‒0.3 V during the discharge process confirmed the Faradic reaction for this ALD-grown MoS2@3-D Ni-foam electrode. The plateau in the charge-discharge profiles are thus in well agreement with the CV measurements. The charge-discharge profiles were performed for a wide range of current densities up to 50 mA/cm2. Figure 6(d) shows the charge-discharge profiles for 400 ALD cycles carried out with current densities 3 to 18 mA/cm2. Similar stable profiles with much less chargedischarge time were also observed for higher current densities (20, 30, 40 and 50 mA/cm2)


22

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[Figure S6(a)]. Thus, this study shows that this ALD-grown electrode can well withstand a high current density and can be applied to any high energy density device in future. The corresponding areal capacitances with increasing current density for 200, 400 and 500 ALD cycles MoS2@3D-Ni-foam electrode were plotted and shown in the Figure 6(e). The areal capacitance of these electrodes could be obtained from the discharge curve of these chargedischarge profiles from the following equation; CA = Itd/VA

(5)

where, I is the current density, td is the discharge time, V is the working potential difference (0.45 V) and A is the area of the electrode (1 cm2). The areal capacitance is increased considerably from 1.4 F/cm2 up to 3.4 F/cm2 corresponding to the 200 and 400 cycles MoS2 respectively at a current density of 3 mA/cm2. On the other hand, the areal capacitance drastically falls (0.22 F/cm2) when the ALD cycles were further increased to 500 cycles. This observation is in well agreement with the earlier CV and charge-discharge results as obtained and discussed earlier. To best our knowledge, the maximum areal capacitance (3400 mF/cm2) found for the MoS2@3D-Ni-foam grown by 400 ALD cycles is to be the highest areal capacitance among several others areal capacitances reported earlier for different electrode materials that includes MoS2 as well.17,

50-69

Highest areal capacitance was obtained for this

composite which provides the maximum active mass (MoS2) loading with optimum porosity and resistivity of the composite assembly. As the MoS2@Ni-foam composite acted as a hybrid supercapacitor (both as EDLC and Faradic), we further employed the Trasatti’s analysis to the optimized electrode (400 ALD cycles) to find the capacitance contributed at the outer surface and the inner surface of the electrode. Areal capacitance at different potential scan rates were calculated and plotted against square root of the scan rates as well as inverse square root of the


23


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same to determine these two contributions separately.63 Figure S9 shows these two plots and the detail of this analysis could also be found in the supporting information of this article. While only ~ 2.7% of the total capacitance (Ctotal = Cin + Cout) was found to be contributed from the EDLC (Cin), ~97.3% was achieved from the Faradic contribution (Cout) at a scan rate of 5 mV/s for this electrode. Similar predominant contribution to the capacitance from Faradic reactions is also evident in earlier reports on transition metal oxide based electrodes.64-65 Table 1 provides a comparative study of areal capacitances reported so far for several other materials and MoS2 as well. The areal capacitances at eight different current densities (3‒18 mA/cm2) were calculated for these three different MoS2@3D-Ni-foam electrodes from their corresponding discharge profiles and shown in Figure 6(e). The areal capacitance of the optimized MoS2@3D-Ni-foam electrode grown by 400 ALD cycles was found as ca. 2.76 F/cm2 at a current density of 18 mA/cm2. Therefore, a high rate capability of ca. 81% was obtained while increasing the current density from 3 to 18 mA/cm2. Thus, this optimized ALD grown electrode is capable to produce much higher areal capacitance (2760 mF/cm2) than the earlier reported materials (see Table 1) even at as high current density as 18 mA/cm2. A general decreasing trend in the areal capacitance with increasing current densities was observed for all of these electrodes with much less value for the 500 ALD cycles electrode for all the current densities. As 400 cycles electrode is optimized to be the best one, the areal capacitances at further higher current densities (20‒50 mA/cm2) were also calculated from those corresponding discharge profiles and plotted in the Figure S6(b). Though the capacitance values fall rapidly as the current density was increased beyond 20 mA/cm2, over 1000 mF/cm2 areal capacitance was obtained even at 50 mA/cm2 current rate. Therefore, the considerably high areal capacitance at such high current density which is more than several others even at much low current density truly reflects the very


24

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efficient way of preparing the electrode by ALD. The Coulombic efficiencies (CE) at different current rates (3‒18 mA/cm2) are calculated by taking the ratio of discharge time to its corresponding charge time [Figure 6(f)]. Excellent CE (~ 100%) was found out for the optimized electrode assembly (400 cycles MoS2) from 8 mA/cm2 current density and beyond. The CE for this electrode at current densities 3 and 5 mA/cm2 were also as high as over 95%. Such relatively lower CE at low current density is a common observation due to the slower discharge kinetics compared to the charge one. Less values of CE were observed for 200 and 500 cycles electrodes which were in good agreement with other observations. The comparatively higher fluctuations in CE obtained for different current rate in case of the 500 cycles electrode further supports the very thick and absence of porosity in the film which resulted in abrupt/random charge-discharge properties under continuous operation.


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Figure 6. (a) C-V plots of bare Ni-foam, MoS2@3D-Ni-foam grown by 200, 400 and 500 ALD cycles at a scan rate of 5 mV/s, (b) CVs at different scan rates for MoS2@3D-Ni-foam electrode grown by 400 ALD cycles, (c) charge-discharge profiles at a current density of 3 mA/cm2 for the above three MoS2@3D-Ni-foam electrodes, (d) charge-discharge profiles at different current densities for MoS2@3D-Ni-foam grown by 400 ALD cycles, (e) areal capacitance at different current densities for these three different electrodes and (f) their corresponding Coulombic efficiencies.


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Table 1: Table showing the areal capacitance of different electrodes reported earlier along with this work.

Sr. No.

1

2

3

4

5

6

Supercapacitor Electrode materials

MoS2@CNT/RGO

NiS@Ni foam Polyaniline/Carbon Cloth FeCo2O4@Ni foam MoS2/CoS2 Nanotube Arrays on Ti plate

Material growth process

Hydrothermal

Performance Areal

Cycling

capacitance

stability

129 mF/cm2 at 0.1 mA/cm

2

94.7% after 10000

2640 mF/cm2

95.7% after

Deposition

at 2.35 A/g

2000 cycles

Chemical Deposition

Hydrothermal

66

cycles

Electrochemical

Electro polymerization

Ref.

3300 mF/cm2 at 1.1 A/g 1880 mF/cm2 at 2 mA/cm

2

91% after

68

69

5000 cycles

142.5 mF/cm2

92.7% after

2

1000 cycles

at 1 mA/ cm

67

Monolayer 1H-

Hot-Injection-

50.65 mF/cm2

240% after

MoS2@Oleylamine

Thermolysis

at 0.37 A/g

5000 cycles

70

71

NixCo3-xS4 7

nanosheet@ NiCo2O4

Hydrothermal

nanowire arrays

followed by

grown on carbon

Electrodeposition

1860 mF/cm2 at 1 mA/cm

2

87.6% after 10000

72

cycles

fiber paper 8

9

10

NiCo2O4@Ni foam

NiO/MnO2@carbon cloth MoS2-graphene

Solvothermal followed by calcination Hydrothermal followed by chemical deposition Ultrasonication

925 mF/cm2 at 5 mA/cm

2

73

6000 cycles

286 mF/cm2 at 0.5 mA/cm

75.8% after

2

11 mF/cm2 at 5

89% after

74

2200 cycles ~250% after

50


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composite

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mV/s scan rate

10000 cycles

11

MoS2 thin films

Chemical Vapor

71 mF/cm at 1

Deposition

mV/s scan rate

Hydrothermal 12

rGO/Ni0.3Co2.7O4

followed by calcination

13

MoS2@3D-Ni-foam

2

2370 mF/cm2 at 2 A/g

-

17

>100% throughout 7000

75

cycles

Atomic layer

3400 mF/cm2

82% after

This

deposition

at 3 mA/cm2

4500 cycles

Work

All of the previous electrochemical studies (CVs and rate capability) univocally proved that the 400 ALD cycles was the optimum number to grow the MoS2 electrode on 3D-Ni-foam that yielded the highest capacitance. Therefore, the cyclic stability of the MoS2@3D-Ni-foam 400 ALD cycles electrode was further investigated by carrying out 4500 charge-discharge cycles. The capacity retention over the cycles and the corresponding CE for 4500 cycles were plotted as shown in the Figure 7. Reasonably good capacity retention of ca. 82% was achieved for this composite at the end of the 4500th charge-discharge cycle with an excellent CE over 90% throughout the cycling which reached beyond the value of 95% after first 2000 cycles of chargedischarge. The cycling performance also revealed that there was no fading in the capacitance retention (or in the areal capacitance) for last ca. 2500 cycles (after 2000) which eventually indicated the stability of this ALD MoS2@3D-Ni-foam electrode even beyond 4500 cycles. The capacity fading in the initial 2000 cycles could be attributed to the slow collapsing of the layered structure of the ALD grown MoS2 which might cause to lose predominantly the Faradic capacitance upon cycling. In addition, the loss of the electrode materials to some extent from the external surface of the electrode also leads to such capacity fading during the initial period of the cycling. However, highest stable areal capacitance over 2700 mF/cm2 could be achieved as per


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best of our knowledge at the end of 4500 cycles. The stability could be further realized from the unaltered nature of the last few charge-discharge cycles which were also shown in the inset of the Figure 7. Such excellent stability could be attributed to the uniform and conformal coating of a polycrystalline MoS2 with an optimum micro-porosity. This study therefore establishes the feasibility and potential of ALD towards the application of supercapacitor where the electrochemistry is surface dominant and therefore ALD can lead to optimize the fabrication of an electrode in a most efficient way.

Figure 7. Cyclic stability as revealed by the capacitance retention and the corresponding Coulombic efficiency for MoS2@3D-Ni-foam grown by 400 ALD cycles (in inset the last few charge-discharge cycles). The electrochemical impedance spectroscopy (EIS) was further carried out for the three different electrodes to understand the kinetics of the electrochemical reactions and the corresponding Nyquist plots were shown and explained with the help of the equivalent circuits. Figure S10 shows the Nyquist, Bode plots, and the schematic of corresponding equivalent circuit


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of these three electrodes. Typical Nyquist plots observed for supercapacitor were also found for this ALD grown electrode material. However, MoS2@3D-Ni-foam grown by 500 ALD cycles exhibited much higher Warburg impedance than the other two electrodes. Complementary information could also be drawn from the corresponding Bode analysis. Thus, these analyses clearly reflect the drastic fall in the capacitance for the electrode grown by 500 ALD cycles. A more detail discussion on the EIS measurements could be found in the supporting information of this article. The robustness and the strongly bonded MoS2@3D-Ni-foam composite was further revealed by the plan-view SEM analysis of the electrode after 4500 charge-discharge cycles as shown in Figure 8. The SEM images proved that the ALD-grown morphology undergoes considerable agglomeration/aggregation [Figure 8(b)] during the large cycling process. Nevertheless, the similar nano-spherical morphologies could be still realized for the MoS2@3D-Ni-foam grown by 400 ALD cycles [Figure 8(a)] which was observed before the cycling. Thus, this post-cycling analysis established the potential of ALD towards a very efficient and direct growth of an electrode material on a 3D-scaffold layer. Therefore, the enhanced electrochemical performance of this MoS2@3D-Ni-foam grown by ALD can be attributed to these three factors: (1) the porous nanostructure of the active material allowed superior ion and electron transport during electrochemical test in aqueous electrolyte, (2) the strong attachment of the active materials on 3D Ni foam substrate allowed the electrode to achieve good mechanical strength and prevented major structural deformation during cycling and (3) the synergistic contribution from both EDLC and Faradaic reactions of MoS2 towards the capacitive response. We believe further research can truly bring ALD into the field of supercapacitor in a similar way it has already been showcased for Li-ion battery.


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Figure 8. Plan-view SEM images of the MoS2@3D-Ni-foam electrode grown by 400 ALD cycles after 4500 charge-discharge cycles at (a) low and (b) high magnification of a selected portion showing the agglomeration and cracks in the electrode material. 4. CONCLUSIONS Molybdenum disulfide (MoS2) was successfully deposited by plasma-enhanced atomic layer deposition (PEALD) using molybdenum hexacarbonyl [Mo(CO)6] precursor and H2S plasma at a relatively low temperature of 200 °C. The thin films of MoS2 deposited on Si/SiO2, SS-foil and Ni-foam substrates were thoroughly characterized by XRD, Raman spectroscopy, XPS and TEM analysis. Stoichiometric and polycrystalline MoS2 with preferential (002) directional growth was evident for the as-grown films. MoS2 was further coated uniformly on 2D-SS-foil and conformally on 3D-Ni-foam by PEALD for the first time and tested as an electrode in supercapacitor. The potential of ALD technique to fabricate a unique and conformal MoS2@3DNi-foam composite electrode was revealed clearly. The 3D-composite electrode showed manifold increase in capacity compared to its 2D-architecture (MoS2@SS-foil) and bare Ni-foam. The optimized number of 400 ALD cycles was estimated for the present MoS2@3D-Ni-foam electrode to achieve the maximum areal capacitance of 3.4 F/cm2 which found to be the highest among several other earlier reported areal capacitances. While the capacitance for PEALD


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MoS2@3D-Ni-foam was contributed from both Faradic reactions and from double layer capacitance, the composite was found to be electrochemically stable up to wide range current densities under operation (3‒50 mA/cm2). Reasonably good cycling stability up to 4500 cycles (>80% capacity retention) with excellent CE (~95%) made this PEALD-grown MoS2 as a promising candidate for such application. We believe that this work will facilitate further to explore ALD technique for the supercapacitor application.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was financially supported by Mid-career Researcher Program through the National Research Foundation of Korea (NRF) (2015R1A2A2A04004945), the Priority Research Centers Program through the NRF funded by the Ministry of Education (2014R1A6A1031189), and Korea Basic Science Institute under the R&D program (Project No. D37700) supervised by the Ministry of Science, ICT and Future Planning, Republic of Korea.

Supporting Information


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Detailed characterizations (XRD, Raman, XPS) of MoS2 grown on SS and Ni-foam substrate. TEM analysis of as-grown MoS2 on Si/SiO2 substrate. SEM images of MoS2 grown on SS and on Ni-foam using 500 ALD cycles. CVs of MoS2@SS electrode. Charge-discharge profiles at high current densities. Detailed Trasatti and impedance analysis of the optimized MoS2@Nifoam composite electrode.

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(6) Tsai, M.-L.; Su, S.-H.; Chang, J.-K.; Tsai, D.-S.; Chen, C.-H.; Wu, C.-I.; Li, L.-J.; Chen, L.-J.; He, J.-H. Monolayer MoS2 Heterojunction Solar Cells. ACS Nano 2014, 8, 8317-8322. (7) Gu, X.; Cui, W.; Li, H.; Wu, Z.; Zeng, Z.; Lee, S.-T.; Zhang, H.; Sun, B. A SolutionProcessed Hole Extraction Layer Made from Ultrathin MoS2 Nanosheets for Efficient Organic Solar Cells. Adv. Energy Mater. 2013, 3, 1262-1268. (8) Singh, E.; Kim, K. S.; Yeom, G. Y.; Nalwa, H. S. Atomically Thin-Layered Molybdenum Disulfide (MoS2) for Bulk-Heterojunction Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 3223-3245. (9) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296-7299. (10) Zhao, Y.-F.; Yang, Z.-Y.; Zhang, Y.-X.; Jing, L.; Guo, X.; Ke, Z.; Hu, P.; Wang, G.; Yan, Y.-M.; Sun, K.-N. Cu2O Decorated with Cocatalyst MoS2 for Solar Hydrogen Production with Enhanced Efficiency under Visible Light. J. Phys. Chem. C 2014, 118, 14238-14245. (11) Chang, K.; Chen, W. l-Cysteine-Assisted Synthesis of Layered MoS2/Graphene Composites with Excellent Electrochemical Performances for Lithium Ion Batteries. ACS Nano 2011, 5, 4720-4728. (12) Xiao, J.; Choi, D.; Cosimbescu, L.; Koech, P.; Liu, J.; Lemmon, J. P. Exfoliated MoS2 Nanocomposite as an Anode Material for Lithium Ion Batteries. Chem. Mater. 2010, 22, 45224524.


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