In Situ Study of K+ Electrochemical Intercalating into MoS2 Flakes

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In Situ Study of K Electrochemical Intercalating Into MoS Flakes Faxin Li, Jianli Zou, Lujie Cao, Zhiqiang Li, Shuai Gu, Ying Liu, Jianqiao Zhang, Hongtao Liu, and Zhouguang Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09898 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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The Journal of Physical Chemistry

In situ Study of K+ Electrochemical Intercalating into MoS2 Flakes Faxin Li,#,& Jianli Zou,†,& Lujie Cao,† Zhiqiang Li,† Shuai Gu,† Ying Liu,† Jianqiao Zhang,† Hongtao Liu,#,* Zhouguang Lu†,* # Hunan Provincial Key Laboratory of Chemical Power Sources; Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources; College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P. R. China † Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, P. R. China

ABSTRACT By applying a single flake microelectrode technique, potassium ion (K+) intercalating into a MoS2 flake under potential control was observed via optical microscopy and in situ Raman spectroscopy. The K+ intercalation process showed high reversiblity while cycling between open circuit potential (OCP) and 0.8 V, confirmed by the recovery of the Raman peaks. Further discharging to low potential (~0.5 V) would cause the irreversible loss of the Raman peaks due to decomposition of the K+ intercalated compound (KxMoS2) which was confirmed by XPS analysis. Based on the diffusion behavior of K+ within the MoS2 layer observed visually by optical microscopy, we believed that K+ was inserted into MoS2 via a layer-by-layer fashion on a micrometer scale. K+ intercalation behavior in MoS2 flakes was further studied by using 1 ACS Paragon Plus Environment

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a galvanostatic intermittent titration technique (GITT), in which the abrupt decrease of diffusion coefficient (DK+) suggested the unfavorable energy change within KxMoS2 structure from 0.9 to 0.8 V. The in situ Raman spectra of MoS2 single flakes with a thickness of 2 nm (3 layers) and 47 nm (~72 layers) during potassiation were compared with commercial microcrystalline MoS2 flakes which have a typical thickness of 50-80 nm and a size of 2 µm. Our results reveal important kinetic information of electrochemical K+ insertion into MoS2 and provide useful insights for the investigation of high-rate electrode materials for metal ion batteries.

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1. INTRODUCTION MoS2 is a two-dimensional (2D) transition metal dichalcogenide (TMD) compound in which each layer of MoS2 is comprised of two slabs of S and a slab of Mo in the middle, and the MoS2 layers are bound together at a distance of ~0.65 nm through van der Waals force.

1, 2

The

interlayer gap can be easily intercalated by alkali metal atoms which were intensively investigated in the field of energy storage.

3-7

Intercalation of MoS2 also is known to induce

superconducting transition and improved catalysis.8-10 The ability to electrochemically tune electronic, magnetic, thermal and optical properties of intercalation compounds makes layered materials attractive for applications such as electrochromic displays, optical switches and thermoelectric devices.10-15 However, potassium electrochemically intercalating into MoS2 has only been scarcely studied,16-18 despite that its counterparts, lithium and sodium, were subjected to investigation by a variety of techniques, including transmission electron microscopy, x-ray diffraction and differential optical microscopy.11,

19-24

Raman spectroscopy is a powerful

analytical tool for TMD, including identifying the thickness of 2D materials and studying lattice vibration of TMDs under strain and electron doping.25-29 Analysis of the two main signals in the Raman spectra, in plane vibration of E2g1 band and the out-of-plane vibration of A1g band, offers detailed information of the physical and electronic properties for TMD materials.26, 28, 29 Electron doping in TMD results in softening speciﬁcally of its Raman-active A1g phonon, however, other Ramanmode with E2g1 symmetry is quite insensitive to electron doping. This is due to a stronger electron-phonon coupling of the A1g mode compared with the E2g1 mode. The detailed Raman investigation regarding the electrochemical potassiation process is however still lacking. In situ Raman spectroscopy of the alkali ion intercalating MoS2 3 ACS Paragon Plus Environment


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was hindered probably by the sudden vanish of the Raman peaks at the early stage of electrochemical intercalation. Recently we have observed the unusual Raman peaks shift during Li+ and Na+ intercalation using a microelectrode technique which consists of one single MoS2 flake and a slow discharge rate (0.005 mV/s),31 Herein, the process of K+ intercalating into MoS2 flakes under potential control was stuided by using optical microscopy and in situ Raman spectroscopy. The reversibility of the potassiation process, the diffusion behavior of K+ in the single MoS2 flake and the K+ diffusion coefficient in the microcrystalline MoS2 flake electrode were investigated in detail. 2. EXPERIMENTAL SECTION 2.1. The preparation of microcrystalline MoS2 flake electrode. Synthetic microcrystalline MoS2 flake (2 µm average particle size, Sigma), Poly-vinylidene fluoride (Kynar-flex, Arkema) and dibutyl phthalate (Aldrich) were dispersed in acetone, then cast onto glass at a thickness of 100 µm. Once dry, the free-standing film was removed from the glass plate, and the dibutyl phthalate plasticiser was extracted using diethyl ether, leaving a porous film ca. 80 µm thick, which was cut into 16 mm diameter films. These films were dried under vacuum at 90 oC, weighed and then transferred to an argon filled glovebox (O2, H2O < 1ppm) for cell assembly. The loadings of the electrodes were 2.73 mg·cm-2, with a typical electrode mass being ca. 5.49 mg. 2.2. The preparation of single MoS2 flake electrode. MoS2 flakes were mechanically exfoliated onto a borosilicate glass cover slide. A single flake was selected, isolated using a diamond-tipped glass cutter, and connected to a copper current collector using silver epoxy.


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The position of the flake was aligned to coincide with the small aperture (ca. 1 mm diameter) made in the center of the copper current collector for direct optical observation. 2.3. The assembly of electrochemical cell for in situ Raman spectroscopy observation. The copper current collector with the connected MoS2 flake as the working electrode, potassium metal as the counter electrode and 1 M KPF6 in 1:1 w/w ethylene carbonate /dimethyl carbonate was used as electrolyte. The configuration of the electrochemical cell is illustrated in Figure 1b. 2.4 The setup for electrochemical intercalation and confocal Raman spectroscopy and Imaging. Cyclic voltammetry was performed using a potentiostat to induce electrochemical intercalation of the MoS2 samples. Initially, the cell was discharged at 0.02 mV/s from open circuit voltage (OCP) to 1.25 V (vs. K+/K), then a slow rate of 0.005 mV/s was employed between 1.25 V to 0.005 V (vs. K+/K), whilst Raman spectra were collected at room temperature (ca. 23 oC, Renishaw inVia, laser wavelength 532 nm, < 38 mW/cm2). The successful electronic connection to the MoS2 flakes can be confirmed by the visual change of the flake during K+ intercalation. For Raman imaging, spectra were taken at an area of 50 μm × 50 μm and then plot out using the intensity of A1g after subtracting the baseline. Each image contains 50 pixel × 50 pixel (2500 pixels) in the area of 50 μm × 50 μm with each pixel having a Raman spectrum of a particular spatial position. 3. RESULTS AND DISCUSSIONS



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Figure 1. Schematic illustration of the preparation of MoS2 microelectrode and the assembly of electrochemical cell for in situ Raman study. a) MoS2 flakes were mechanically exfoliated onto a borosilicate glass cover slide. A single flake was connected to a copper current collector, ensuring that the area of interest was aligned with the aperture in the centre for visual observation. b) The copper current collector with the connected MoS2 flake as the working electrode was assembled in an electrochemical test cell. c) Crosssection view of the working electrode shows the connection between the MoS2 flake and the copper current collector. d) The representative microscope image of the connected MoS2 flake. The regional variation in color indicates the different thickness of the MoS2 flake. e) The Raman spectrum of the MoS2 flake with thickness around 47 nm.

Highly crystalline natural MoS2 flakes were mechanically exfoliated onto a borosilicate glass cover slide using the “Scotch tape method”. Figure 1 a and b illustrate the preparation of the MoS2 flake microelectrode and the assembly of the electrochemical cell for in situ Raman study. We selected the flakes with a thin flat region of several square micrometers to allow facile Raman analysis, while the whole flake should be at least a few hundred micrometers at one edge to facilitate electronic connection using silver epoxy. The MoS2 flake acts as a working electrode and a thin metallic potassium foil as the counter electrode in the test cell. Figure 1c shows the cross-section configuration of the MoS2 working electrode. Through the 6 ACS Paragon Plus Environment

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observation window microscope images (Figure 1d) and Raman spectra (Figure 1e) of MoS2 flakes can be acquired. The Raman spectra of MoS2 flakes with thickness of 47 nm exhibit two intense peaks: E2g1 band at ~383 cm-1 and A1g band at ~408 cm-1 (Figure 1e). The position of Raman peaks was in well agreement with the reported data. 30

Figure 2. The reversibility of the potassiation process at different voltages. The microscopic images of the MoS2 flake at a) 1.0 V, d) 0.8 V and g) 0.5 V after holding at that voltage for at least 2 hours, the yellow square (50 µm × 50 µm) indicates where Raman spectra were taken from. Raman mapping images shown in b), e) and h) were plotted out using the intensity of the A1g band. The Raman band vanished in the case of all three discharge voltages. Finally, the voltage was brought back to 2.0 V and held for 2 hours before Raman spectra were taken again in the same area. The full recovery of the Raman band after discharge to c) 1.0 V and f) 0.8 V suggested the potassiation process was reversible, and the partial recovery of the Raman band after discharge to i) 0.5 V indicated the decomposition of the K+ intercalated MoS2 compound (scale bar is 50 µm) .



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We firstly brought the voltage down to1.0 V and found that the intensity of the A1g peak vanished as shown in the Raman mapping image plotted against the intensity of A1g (Figure 1a and b). The vanishing of the Raman band has always been related to the decomposition of alkali (Li or Na) intercalated MoS2 compounds (eqn. 1), where MxMoS2 (M = Li or Na) will undergo a conversion reaction to yield M2S and Mo. However, the intercalation of K+ into MoS2 is still at a very early stage at 1.0 V with x = 0.53 (x in KxMoS2), according to the charge/discharge profile of the microcrystalline MoS2 electrode (Figure S1). For Li+ or Na+ intercalated compounds MxMoS2 (M = Li or Na) the decomposition happens when the value of x is close to 1,31, 32 it is interesting to verify whether the relative early vanishing of the Raman band is indeed related to the decomposition of KxMoS2 compound. Surprisingly when the voltage was brought back to 2.0 V from 1.0 V, the Raman bands were fully recovered (Figure 2c). The Raman bands of MoS2 recovered again from 0.8 V (Figure 2d-f), indicating that the intercalation of K+ was a reversible process when cycled between OCP and 0.8 V, although the discharging process was accompanied by the complete vanishing of the Raman peaks. Clearly the vanishing of Raman peaks is not necessarily related to the decomposition of K+ intercalated MoS2 compounds. We further discharged the Raman cell to 0.5 V and then brought the voltage back to 2.0 V, and this time most of the area on the flake didn’t show any characteristic peak of MoS2 (Figure 2g-i), suggesting the decomposition of KxMoS2, in which the formation of Mo (0) was confirmed by XPS measurement (Figure S2).33 Note that we hold the voltage constant for at least 2 hours before taking Raman spectrum for imaging in the single flake experiments, therefore, it’s expected that potentials at which the E2g1 and A1g peaks change in single flake samples and microcrystalline MoS2 electrode samples will have a very minor discrepancy. 8 ACS Paragon Plus Environment

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Nevertheless, the decomposition of KxMoS2 happened at an early intercalating stage, relative to LixMoS2 and NaxMoS2, where x was less than 0.73 in KxMoS2 (Figure S1).

Figure 3. Using in situ Raman to determine the movement of K+ in MoS2 during electrochemical intercalation. a) The thickness of the MoS2 flake increases along the direction indicated by the red arrow. b) During electrochemical intercalation, Raman spectra were taken from the MoS2 flake shown in a) and the Raman imaging was plotted out using the intensity of A1g band. The vanishing of the Raman A1g peak started from the thick part of the flake and moved toward to the thin part (indicated by the arrows in b) and eventually A1g band vanished from the whole flake. (scale bar is 50 µm.)

It is commonly understood that the intercalation of alkali into layered materials will start from the flake edge with steps of different thickness. Indeed, we often observe that the intercalation happens at the edge part of the flake and moves inwardly. However, it was also observed that the intercalation of K+ started from the thick part of the MoS2 flake (Figure 3). Figure 3a shows the optical microscopy image of the MoS2 flake with different thickness for the K+ intercalation. The boundary between the intercalated region and non-intercalated region, identified by the intensity change of the A1g peak, moved outwardly to the thin part of MoS2 flake as indicated by the yellow arrows in Figure 3b. Thus, it is possible for K+ to insert directly at the studied thin flake edge/electrolyte interface or alternatively diffuse from the thicker part of the MoS2 crystal where being initially intercalated to the thinner part. 9 ACS Paragon Plus Environment


Figure 4. Intercalation dynamics during the potassiation process in the MoS2 flake. Optical microscopy images show the visual change of the MoS2 flake during potassiation at different discharge stages and the red arrows indicate the newly formed boundaries between the intercalated region and non-intercalated region. Accordingly, we propose potassiation progressing in the MoS2 flake via a layer-by-layer fashion, (Scale bar is 100 µm).

Figure 4 shows visually observed intercalation dynamics of K+ during potassiation. For the large MoS2 flakes, it takes time for alkali ions to diffuse from edge of the flake into the center of the flake. This “time-lag” gives us an opportunity to visually observe the change of the MoS2 flake during potassiation at different discharge stages. The red arrows pointed out the newly formed boundaries between the intercalated and non-intercalated regions, recognized by the color difference around the boundaries. In the case of Li+ intercalating MoS2, Li+ was pushed away by the newly intercalated Li+ accompanied by the decomposition of LixMoS2, thus the intercalation frontier moved inwardly via an atom-by-atom fashion.31 During Na+ 10 ACS Paragon Plus Environment

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intercalation of MoS2, the layer-by-layer diffusion behavior was observed by using aberrationcorrected scanning transmission electron microscope, showing that Na+ being intercalated in every other interlayer of MoS2; after these layers were fully occupied, the Na+ would then begin to fill the other interlayers.34 Although it is hard to tell where K+ will prefer to stay using the current set-up, the optical microscopy images clearly show that K+ progresses in the MoS2 flake via a layer-by-layer fashion, in which K+ would slide into different MoS2 interlayers and then distribute through a relatively large area instead of resting at the adjacent locations from the initial intercalation, similar to Na+ intercalating into MoS2.

Figure 5. Kinetics of K+ intercalating in MoS2. a) The chemical diffusion coefficient of K+ during intercalation of MoS2 calculated from GITT data was plotted over the voltage. b) The discharge process of a microcrystalline flake MoS2 electrode plotted out vs. composition of KxMoS2 (OCP was ca. 2.8 V, all potentials quoted measured vs. K/K+).

The chemical diffusion coefficients of K+ (DK+) intercalating into MoS2 was determined from GITT (details see supporting information and Figure S3) and provided more information regarding the kinetics of intercalation. Figure 5a shows DK+ plotted over the voltage (vs. K/K+). When the voltage decreased from OCP to ~1.1 V, corresponding to the first plateau in the 11 ACS Paragon Plus Environment


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discharge profile (Figure 5b), the value of DK+ shows a “V” shape which is the characteristic feature of a phase transformation electrode. 35, 36 These decreased diffusion coefficients at the range of 0.05 < x < 0.44 are the apparent coefficients, which are usually 2-3 orders of magnitude lower than the real coefficient.35 The decrease of diffusion coefficient is likely caused by the structure change associated strain and unfavorable energy between stable intercalated compounds.34 An abrupt drop of the DK+ from 1.8 × 10-10 to 9.4 × 10-13 cm2s-1 between 0.90 V to 0.80 V was also observed. This voltage range happens to be the second plateau in the discharge profile, corresponding to x = 0.56~0.64 in KxMoS2 (Figure 5b). The K+ insertion energy is -1.50 eV at x = 0.5 (x in KxMoS2) and increases to -0.44 eV at x = 0.625, according to the periodic density functional theory (DFT) calculation,37 therefore, we attribute the drop of DK+ to the structure change associated unfavorable K+ insertion energy change. With more K+ inserting into MoS2 layers, K+ insertion energy reaches the highest value of 0.52 eV at x = 0.875, accounting for the increased difficulty of the insertion of K+ into MoS2. During lithiation and sodiation, such abrupt drops of diffusion coefficient have not been observed.31 Overall, the diffusion behavior of K+ into MoS2 interlayers varies greatly from Li+and Na+, which brings complexity to the direct comparison of diffusion coefficients of all three alkali ions.


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Figure 6. Analysis of A1g and E2g1 bands during potassiation. The in situ Raman spectra of a) microcrystalline MoS2 flake, b) 47 nm (~72 layer) MoS2 flake, and c) 3 - layer MoS2 flake electrode during potassiation. d-f) The peak position of A1g and E2g1 corresponding to a-c) MoS2 electrodes plotted out against the potential. All potentials quoted measured vs. K/K+.

Finally we monitored and compared the change of Raman bands of three MoS2 samples, a microcrystalline flake electrode, a 3-layer MoS2 flake electrode and a 47 nm MoS2 (ca. 72 MoS2 layers) flake electrode, during electrochemical intercalation. The thickness of microcrystalline flakes was estimated to be 50-80 nm from SEM images (Figure S4). Single MoS2 flake thicknesses were determined by atomic force microscopy (AFM) and Raman spectroscopy (Figure S5). The 3-layer MoS2 flakes showed Raman peaks at ~383 cm-1 (E2g1) and ~406 cm-1 (A1g). For the thick MoS2 sample, these two Raman peaks were located at ~383 cm-1 (E2g1) and ~408 cm-1 (A1g). Raman spectra were taken a few micrometers away from the edge of the flake, and due to the slow scan rate employed, changes of the spectra of the MoS2 samples were occurring on the timescale of the spectral acquisitions, allowing spectra to be collected at a quasi-equilibrium state.



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The E2g1 band of the microcrystalline MoS2 flake kept almost constant while A1g peaks down-shifted around 2 cm-1 during potassiation (Figure 6a and d). In the 47 nm MoS2 flake, the E2g1 peak didn’t show an obvious shift and A1g peak down-shifted 1 cm-1 (Figure 6b and e). For the flake with 3 MoS2 layers, both A1g and E2g1 peaks didn’t move (Figure 6c and f) before they vanished. Raman spectra of MoS2 flakes have been reported to be sensitive to both strain and electron doping. It has been reported a biaxial compressive strain caused upshift of both A1g and E2g1 bands.38 Electron doping of MoS2 usually soften and broaden the A1g band with the other E2g1 mode remain inert. 39 K+ intercalation will cause both strain and doping effect to MoS2 flake, and the shifts of the A1g peak of microcrystalline MoS2 electrode and 47 nm MoS2 flake electrode is likely suggested that the electron doping was the dominant factor during potassiation.40 However, the A1g peak of 3-layer MoS2 electrode didn’t show similar shift as we expected. We attribute this to the early vanishing of the Raman bands during intercalation. It is worth to mention that we didn’t observe distinctive color change from the microcrystalline MoS2 electrode during intercalation (Figure S6), although the change of Raman peaks was captured, which is likely because the small size of the microcrystalline MoS2 flakes and their random orientation makes the optical observation difficult. 4. CONCLUSION In summary, the electrochemical intercalation of K+ into MoS2 flakes was investigated by using a microelectrode technique combined with in situ Raman spectroscopy. The microelectrode technique allowed the clear observation of the K+ diffusion within a single MoS2 flake. It was observed that the diffusion started from the thick part of the flake and moved towards the thin part. It’s likely that potassiation progresses in MoS2 flakes via a layer-by-layer fashion, in 14 ACS Paragon Plus Environment

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which K+ ions slide into different MoS2 interlayers and distribute through a relative large area instead of resting at the adjacent location from the initial intercalation. The intercalation of K+ was a reversible process when cycled between OCP and 0.8 V, although the discharging process was accompanied by the complete vanishing of Raman peaks. The K+ diffusion coefficient from GITT measurements further revealed important kinetic information of electrochemical K+ insertion into MoS2 and provided useful insights for the investigation of high-rate electrode materials for metal ion batteries. ASSOCIATED CONTENT Supporting Information. The discharge process of a microcrystalline flake MoS2 electrode, XPS test of MoS2 electrodes, GITT of K+ (DK+) during intercalation of MoS2, SEM images of microcrystalline MoS2 flakes and freestanding microcrystalline MoS2 flake electrodes, characterization of MoS2 flakes by optical microscope, AFM and Raman spectroscopy, optical microscopy of microcrystalline MoS2 electrode at different potential during discharge. AUTHOR INFORMATION Corresponding Author *E-mail addressed: [email protected] (H.L.). *E-mail addressed: [email protected] (Z.L.). Author Contributions F.L. and J.Z contributed equally to this work. 15 ACS Paragon Plus Environment


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by the Basic Research Project of the Science and Technology Innovation Commission of Shenzhen (No. JCYJ20170412153139454), the National Natural Science Foundation of China (No. 21875097, and No. 21671096), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06G587), and the Hunan Provincial S&T Plan of China (No. 2017TP1001, 2016TP1007).

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In Situ Study of K+ Electrochemical Intercalating into MoS2 Flakes

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