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Disulfide-bridged (Mo3S11) cluster polymer: Molecular dynamics and application as electrode material for rechargeable magnesium battery Quang Duc Truong, Murukanahally Kempaiah Devaraju, Duc N. Nguyen, Yoshiyuki Gambe, Keiichiro Nayuki, Yoshikazu Sasaki, Phong D. Tran, and Itaru Honma Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02593 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 1, 2016
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Nano Letters
Disulfide-bridged (Mo3S11) cluster polymer: Molecular dynamics and application as electrode material for rechargeable magnesium battery Quang Duc Truong†, Murukanahally Kempaiah Devaraju†, Duc N. Nguyen‡, Yoshiyuki Gambe†, Keiichiro Nayuki§, Yoshikazu Sasaki§, Phong D. Tran‡, Itaru Honma*,† †
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Aobaku, Sendai 980-8577, Japan. Department of Advanced Materials Science and Nanotechnology, University of Science and Technology of Hanoi, 18 Hoang Quoc Viet, Hanoi 100000, Vietnam. § Field Solution Division, JEOL Ltd., 1156 Nakagami, Akishima, Tokyo 196-0022, Japan. ‡
Supporting Information Placeholder ABSTRACT: Exploring novel electrode materials is critical for the development of next generation rechargeable magnesium battery with high volumetric capacity. Here, we showed that a distinct amorphous molybdenum sulfide, being a coordination polymer of disulfide-bridged (Mo3S11) clusters, has great potential as rechargeable magnesium battery cathode. This material provided good reversible capacity attributed to its unique structure with highly flexibility and capable of deformation upon Mg insertion. Free terminal disulfide moiety may act as active site for reversible insertion/extraction of magnesium.
Battery technologies based on multivalent-ion, such as Mg, Ca, Al, offer promising opportunities for novel energy storage systems and future application in as hybrid electric vehicles (HEV) or electric vehicles (EV) due to its low cost, safety, high energy density and high power density.1 Among them, the rechargeable magnesium battery have attracted attention owing to the high natural abundance of Mg, safety, high specific capacity (2205 Ah kg−1), and especially high volumetric energy density (3833 mA h cm−3).2−9 However, the strong electrostatic interaction between Mg2+ and host lattice due to its divalency, induces slow intercalation kinetics of Mg ions within the crystal lattice. There are still challenges of exploring novel cathode materials with sufficient capacity to overtake the current lithium-ion battery technology. At present, the Chevrel phase (Mo6S(Se)8) is only reported intercalation-type cathode which provides reversible insertion/extraction of Mg ions with good cyclability and high rate capacity.10,11 The good performance of Chevrel phase was attributed to their advanced crystal structure with unique Mo6 cluster units and multivacant sites. Multi-vacant sites are suitable for ion-hopping in the host lattice, and cluster units of Mo6 can afford multiple electron transfer induced by introduction of Mg ions through delocalization of electrons over cluster.12−14 Thus, a framework with multi vacant sites and cluster units in the crystal structures are expected as promising candidates for reversible intercalation of Mg ions. In this communication, we investigated a distinct amorphous molybdenum sulfide (denoted as a-MoSx), which was recently identified to be a coordination polymer based on (Mo3S11) building block clusters, thus composing of Mo3 cluster units with multi-vacant and active sites,15 as prototype cathode for rechargeable Mg battery. In first part, we report the structural dynamics of the cluster under electron beam irradiation, revealing the decoupling, fluctuation and rotation of cluster molecule. Next, it is elucidated that the cluster materials show good electrode performance with reversible
electrochemical reaction thanks to their unique structural property. The molybdenum sulfide a-MoSx materials was synthesized via chemical oxidation of [MoS4](NH4)2. Typically, a stoichiometric amount of sodium persulfate was added to a deep-red solution of [MoS4](NH4)2 in water, degassed with Ar. The solution was continuously stirred under Ar for 2 h, and then dark brown powder was collected by centrifugation, intensively washed with water, ethanol, CS2, diethyl ether, finally dried under Ar stream and kept under Ar atmosphere.
Figure 1. (a) Schematic illustration for the discrete [Mo3S13]2– building blocks and their ideal coordination polymer structure. (b, c) Sequence of HAADF-STEM images of a-MoSx on graphite flake. The images are acquired with a interval frame time of 7.5375 s. The bright contrast indicates the locations of Mo atoms. The structure of the materials are identified as molecular−based coordination polymers consisting of discrete [Mo3S13]2– building blocks as reported in detail in our previous study.15 In deed, we showed an series of spectroscopic analysis (Raman, X-ray photoelectron spectroscopy, electron paramagnetic resonance), microscopic analysis and elemental analysis that evidenced [Mo3S13]2− as building block units of a-MoSx material. Furthermore, we were able to extract [Mo3S13]2− cluster from a-MoSx nanoparticles and the elemental analysis exhibit a Mo/S ratio close to 4, in accordance with the (Mo3S11)n structure. Actually, within a-MoSx mate-
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1 2 3 Figure 2. (a–e) Experimental series of Z-contrast STEM images as function of time, showing the decoupling and conformation change of cluster. Each subsequent image is acquired with a frame time of 7.5375 s. The superimposed atoms on the inset indicate the locations of Mo atoms with shifting to the right for clarity (top). Schematic atomic model of the corresponding atomic structure (bottom).
rial, [Mo3S13]2− clusters share disulfide ligands providing linear chains or branched networks. Figure 1a illustrates a discrete [Mo3S13]2– building block and their ideal coordination polymer structure. The structure of a single [Mo3S13]2– cluster consists of there Mo atoms arranging into triangle pattern, three terminal S22–, three bridging S22– and one central apical S ligand. Of the three terminal disulfide (S22–) ligands within these clusters, two are shared to form the polymer chain. The atomic structure of the clusters has been unambiguously determined. However, the molecular dynamics of these molybdenum sulfide clusters remain unknown experimentally, although the dynamics behavior undoubtedly influences heterogeneous catalytic activity and electrode functionality in batteries. Scanning transmission electron microscopy (STEM) with probe-forming aberration correctors, is a powerful tool for visualization on the local atomic structure and the chemical composition at the atomic resolution.16,17 For molybdenum sulfide compounds, annular dark field (ADF) image contrast intensity vary strongly among atoms due to large difference in the atomic number (Z), providing direct and accurate tool to identify chemical composition and position of individual Mo atoms.18−21 a-MoSx supported on electrochemically anodized graphite flake was investigated by STEM to image directly cluster dynamic as a function of time (see Supporting Information). HAADF imaging were performed on an aberration-corrected JEM-ARM200F, equipped with a cold field emission electron gun. Camera length is 6 cm. The convergence semi-angle for the incident probe was set to 29 mrad, and probe current of about 34 pA. Most of the ADF images were collected for a half-angle range of 90~370 mrad. Figure 1b shows HAADF-STEM image of sub-monolayer a-MoSx supported on graphite flake. A number of uniform, nanometer-sized bright protrusions are observed clearly, indicating the presence of molybdenum sulfide clusters with different size. The ADF image contrast is roughly correlates to with atomic number according to a Z1.7 relationship. Thus, the bright contrasts in Figures 1b, c exhibit the positions of Mo atoms of a-MoSx. Sulfur atoms (Z=16) are invisible even at high resolution of HAADF mode due to the mobilization of sulfur. The light elements such as sulfur are barely visible in ADF mode when heavy atoms are present. Single Mo atoms are also observed near the cluster or occasionally on the graphite substrate. These Mo atoms, present in the vicinity of cluster, are possibly formed due cluster decomposition under the electron-beam irradiation. These indi-
vidual atoms move frequently from one frame to the next due to the knocking away by energy activation from beam. The significant variation of the contrast intensity (Figure S1) and position of Mo atoms and clusters are observed due to structural fluctuation and rotation under the electron-beam irradiation during the acquisition of the image series. Figure 1c shows such an example of this behavior. The sequential STEM images of clusters in Figure 1b was acquired with interval frame time of 7.5375 s. Direct visualization of dynamics of atoms and clusters is crucial to the understanding the fundamental nanoscience and mechanism of heterogeneous catalytic and electrochemical reactions where the clusters function as nanocatalysts or electrodes. The investigation by direct atomic resolution imaging on the clusters and their dynamics of Pt,24,25 Au,26−29, Ge,30 Si,31,32 Ir,33,34 reveals the fluctuations, transition, reversible atomic displacement, diffusion, coalescence phenomena. Recent reports showed the MoSe wires with width of 3 Mo atoms are flexible, can rotate, bend their entire crystal structure under electron beam irradiation.35 The molecular dynamic of the clusters in this study were observed by acquiring series frames with an individual frame time of 7.5375 s. Figure 2 shows subseries of five consecutive frames of a cluster undergoing conformational transformation. Inset (bottom) show schematic atomic model of the conformation change. In Figure 2a and b, the cluster is pristine threemeric cluster, containing 9±1 Mo atoms with almost same conformation. The positions of Mo atoms are overlaid on the HAADF image, shifting to the right for clarify. Structural transformation was observed throughout the sequence. The clusters was found to decouple into two monomeric and dimeric subclusters as shown in Figure 2c. The denoted neighbor atoms in Figure 2c was separated by 0.58 ± 0.05 nm which is longer than acceptable Mo-Mo distance in cluster (3.17 Å). Another example of direct visualization of decoupling of larger clusters is present in Figure S2. For high quality image and real time observation of these decoupling, please refer to Supporting Movie 01. The driving force for the decoupling is both dissociative electronic excitation and the internal vibration caused by energy excess gained from electron beam. Such decoupling occurred at shared disulfide sites, was thermodynamically unfavorable due to emergence of unsaturated bonds. The molecular rotation was also observed in Figure 2c as the segment 2 show a crystal rotation of 30° to original position. The bridged disulfide play a role as the anchor site for the flexible rotation of the segments. The configuration in Figure 2d is ob-
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Figure 3. (a–j) Sequence of HAADF-STEM images showing the dynamics of cluster with continuous conformation change and structural transition from compact towards an elongated structure, which finally returns original form. Each subsequent image is acquired with a frame time of 7.5375 s. served with reduced number of certain contrast suggesting the spatial overlapping of segment 2 on segment 1. In Figure 2e, the segment 1 rotate 90° and segment 2 returns back to the same orientation in Figure 2b. Such fluctuation and rotation were observed in many clusters as shown in Figure 3 and Supporting Movie 02. The shortening of the polymeric chains under electrochemical activation could improve hydrogen evolution reaction.15 The elastic flexibility is also revealed to induce their self-adapting deformation for the Mg ions hosting.
Figure 4. Electrochemical performances of a-MoSx in Mg ion batteries tested in the potential range of 0.2–2.2 V versus Mg/Mg2+. (a) typical charge/discharge profiles at current rates of 10 mA g–1; (b) cyclic voltammograms of the cell containing aMoSx (dark curves: first cycle, blue curves: second cycle, red curves: third cycle). The electrochemical performance of a-MoSx was measured by galvanostatic charge-discharge method and the result is shown in Figure 4. For electrochemical measurements, the cell is composed of magnesium metal counter, reference electrodes and a-MoSx positive electrode. The cathode and reference electrodes were separated by a microporous polypropylene film. The a-MoSx exhibited a wide and flat voltage plateau at around 1.05 V versus Mg/Mg2+ with an initial discharge capacity of 115 mA h g-1 at current rates of 10 mA g–1 (Figure 4a). From the second cycle, the discharge curve possesses two voltage plateaus, indicating twostep magnesium insertion. The cyclic voltammograms of the cell containing a-MoSx shows broaden reduction peak centered at 0.96 V versus Mg/Mg2+ and one reduction peak at 1.05 V (Figure 4b). From second cycle, the cyclic voltammograms of the cell show two reduction peak centered at 0.9 and 1.05 V versus Mg/Mg2+. The discharge capacities of a-MoSx are 87 and 75 mAhg-1 at se-
cond and third cycle, respectively with capacity retention are 75.6% and 65.2%. At first cycle, only recharge capacity of 99 mAhg-1 was obtained and the coulomb efficiency is 86.1%. From sequential cycle, the coulomb efficiency gradually increases to 99.5% and become stable. The samples obtained by first discharge (to 0.2 V) and recharge (to 2.2 V) were analyzed by X-ray photoelectron spectroscopy to reveal the redox reaction occur during the cycling of the cell. The cathode sample was disassembled in the glovebox and washed with ethanol. Figure S3 show the digital image of the cathode paste mounted on XPS sample holder. Figure 5 shows S 2p spectra of the electrochemically magnesiated MgyMoSx in comparison with that of the original as-prepared material and demagnesiated sample. The as-prepared a-MoSx shows only two characteristic peaks which was fitted with two distinct doublets (2p3/2, 2p1/2). As shown in Figure 5, deconvolution of the S 2p envelop measured for the a-MoSx only reveals two contributions with S 2p3/2 binding energies of 162.9 eV (assigned for the bridging disulfides and apical sulfide) and 161.8 eV (assigned for terminal disulfides).15 We assign electron binding energies for terminal disulfide and shared disulfide by referring to those assigned for the [Mo3S13]2− cluster (ref. 45). Indeed, shared disulfide ligand coordinates to two MoIV centers while terminal disulfide ligand coordinates with only single MoIV center. Thus, it is not surprising that terminal disulfide displays lower binding energy compared with shared disulfide. After discharging to 0.2 V, the chemical bond of sulfur in the electrode change significantly. Notably, a new response appeared at lower binding energies of 159.8 and 161.0 eV. This doublet is assigned to sulfide within MgS2 and MgS,36,37 thus confirming the insertion of magnesium into cluster framework. The presence of Mg 2s and Mg 2p in the survey spectrum was also observed in the discharge sample (Figure S4). At the same time, the relative intensity of S 2p3/2 peak at 161.8 eV decrease from 42% to 17% (Table 1), suggesting the conversion of terminal disulfides into bridging disulfides. After recharging, intensity of the peak associated with MgS2 and MgS as well as bridging disulfides decreased, which confirms the reversible insertion/extraction of magnesium via terminal disulfides (Figure 6). This conclusion was further supported by Raman spectroscopy results (Figure S5). Raman spectra of a-MoSx shows vibration mode at v(Mo-S) of 382–284 cm–1; v(Mo3– µ3S) vibration at 450 cm–1. The bridging/shared disulfide v(S-S)br/sh and terminal disulfide v(S-S)t were detected at 555 and 525 cm–1.15 However, Raman spectra of the discharged
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sample does not show discrete v(S–S)t vibration at 525 cm–1. Instead, vibration mode of Mg–S was found at 360 cm–1 and 480 cm–1. The shift of signal at 450 cm−1 can indeed be a shift of the Mo3–µ3S vibration, following the transition of terminal disulfide ligands that change the geometry of the Mo3 clusters. This signal at 450 cm–1 can also correspond to the formation of bridging Mo– µS–Mo centers produced during electrochemical discharge. Table 1. Representative XPS Data based on peak area Materials SOx S22- bridging S22- terminal a-MoSx 0 58% 42% Discharge Charge
7% 12%
69% 52%
17% (MgS 7%) 36%
The presence of the large peaks located at binding energies of 169.5 and 170.7 eV in the XPS spectra of the discharge/charge samples indicate the formation of sulfur-oxygen bond, which generated from THF solvent degradation. The similar observation has been observed in the cycling of Mg/S battery.37 It is noticed that the peak intensity after the 1st charge is a larger than that of after the 1st discharge (12% and 7%). Thus, we further examined the materials after ten cycle discharge/charge by XPS. The result in Figure S6 and Table S1 show gradually increase in the peak intensity at binding energies of 169.5 and 170.7 eV after cycling, indicating that the decomposition of electrolyte occurred cycle by cycle.
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The low charge capacity (low coulomb efficiency) observed in the initial cycle in Figure 4a is attributed to electrolyte degradation and formation of MgSOx compound as discuss above. The partly oxidation of sulfur inevitably incorporated magnesium due to strong electrostatic interaction, thus, only partly Mg ions were extracted from the frameworks. It also appears that the oxidation of sulfur results in reorganization of cluster from one-dimensional structure into two-dimensional structure via combination at the terminal sulfide sites. The stable coulomb efficiency achieved after first cycle indicates that the oxidation of sulfide only occurs in initial cycle. After that, the structure becomes gradually stable against the deformation. The charge capacity of the sequential cycle is approximate to the discharge capacity of the previous cycle. This result suggests that the unextracted Mg ions in the previous charging reaction remain inactive in the sequential discharging process. Considering that each cluster unit has one free terminal (S2)2− group, which has potential to host one Mg atoms, the theoretical capacity value is calculated to be 85 mA h g–1 (1 Mg per (Mo3S11)). The experimental discharge curve also shows that the capacity reached around 85 mA h g–1 and then the voltage dropped rapidly (Figure 6), in accordance with above calculation. Recent works indicate that clusters such that (MoS2)3 in layer structures or monolayer enable to coordinate one/two magnesium atoms at the bridging disulfide, corresponding to theoretical capacity of 170 and 255 mA h g–1.38,39 In this study, a capacity of 30 mA h g–1 (over theoretical capacity) was obtained between 0.72 and 0.20 V is due to the insertion of second Mg atom at bridging disulfides (Figure S7). The larger capacity may be electrochemically reached at very low potential vs. Mg/Mg2+. However, the voltage cutoff was controlled at 0.2 V to avoid electrolyte degradation and cathode conversion reaction. The rate performance of a-MoSx has also been measured in a three-electrode setup. Figure S8 presents the rate capacity of the cells containing a-MoSx at various discharge rates ranging from 10 to 50 mA g–1. The cell exhibited first discharge capacity of 117, 101, 77 mA h g-1 at 10, 20, 50 mA g–1, respectively. At high discharge rate of 20, 50 mA g–1, the cell exhibited a reduced discharge plateau at 1.0 and 0.9 V, respectively.
Figure 5. XPS analysis of a-MoSx cathode at various stages during the battery cycling: original as-prepared sample; magnesiated sample at first discharge and demagnesiated sample at complete discharge/charge cycle. Experimental data (°°°), orange doublets arises from terminal S22– ligands, dark doublets reflect the bridging S22– ligands and apical S2– ligands, blue doublets are assigned for S–/S2– in MgS2 ang MgS, red doublets are assigned for S-O bond in SOx.
Figure 6. The proposed electrochemical reaction occurs between a-MoSx and Mg2+ during the discharge of the cell (left). The illustration indicates the voltage drop after discharge capacity reaches around theoretical capacity of 85 mA h g–1 (right). Due to the slow diffusion of divalent Mg ions into the intercalation host, most of cathode materials show low specific capacities or rapid capacity fade. The hybrid Mg-Li-ions battery, that involves the lithiation/delithiation at the cathode and magnesiation/demagnesiation at the anode, have been demonstrated to display improved rate capacity and discharge voltage.40–42 The working principle of hybrid Mg-Li-ions battery is shown in Figure 7. The concept is on the basic of the classical Daniel battery
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which used a septum to separate two kind of cations and to prevent the displacement deposition of metal with higher redox potential on the lower one. In the hybrid battery, both kind of cations are dissolved in the same solvent without a septum. During the discharge, Li+ ion dominates intercalation into the host due to the fast diffusion rate of Li+ (several orders of magnitude) compared to Mg2+ in the same host materials. Because the thermodynamic redox potential of Mg/Mg2+ is 0.67 V higher than that of Li/Li+, Mg deposition occurs at the anode side before Li deposition could take place during the charge. The hybrid electrolyte, as an ion reservoir, has to be a compound or a mixture contains both of Li+ and Mg2+ ions. Discharge (Mo3S11) + 2Li+ + 2e → (Li2Mo3S11) Mg → Mg2+ + 2e Mg2+ e
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Figure 7. The working principle of hybrid Mg-Li-ions battery with a-MoSx cathode, Mg plate anodes and (LiCl + [Mg2Cl3]+[AlPh2Cl2]–/THF) electrolyte.
However, from the second lithiation process, the electrodes show a reduced capacity of 101 mA h g–1 and remaining as high as 75 mA h g–1 after 10 cycles. Figure 8b presents the cyclic voltammogram curves of a-MoSx at a sweeping rate of 0.1 mV s–1 in the voltage window of 0.2–2.2 V versus Mg/Mg2+. In the first cycle, the electrode exhibits cathodic peaks at 1.1 V and anodic peak at 1.73 V. The rate performance in the hybrid electrolyte was also improved due to enhanced diffusion rate of Li+ compared to that of Mg2+ (Figure S9). As shown in this study, the discharge capacity of a-MoSx as a cathode material for the rechargeable Mg batteries is comparable to that of current reported materials such as Mo6S(Se)8.10,11 The key functional group is terminal S2 moieties, which is responsible for the hosting of magnesium ions. In order to increase the capacity of the cluster−based cathode, designing clusters with high number of terminal disulfides are rational. Thus, the recent discovered dimeric [Mo2S12]2 clusters,44 or on thiomolybdate [Mo3S13]2– clusters,45 are expected to deliver large capacity for Mg ion battery. The work is now in progress. In summary, we investigated a coordination polymer based on [Mo3S13]2– clusters as a potential rechargeable magnesium ion battery cathode. The atomic structure of the clusters has been observed at atomic-resolution by state-of-the-art HAADF STEM. XPS and Raman study revealed that the insertion/extraction of magnesium into the framework occurs at the terminal disulfide sites. The structural analysis and the correlated electrode properties in present work provide the critical step for rational design of suitable cathode for magnesium battery technology.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Full experimental detail, more ADF-STEM images, digital image of a-MoSx and electrochemical performance of the sample. (PDF) Supporting Movies comprised of two sequential HAADF-STEM frames showing the mechanical decoupling of the clusters from local movements and the structural transition/transformation of the materials (AVI). The frame rates have been changed for compatibility with media formats. The movies were repeated 10 times for clarity.
AUTHOR INFORMATION Figure 8. Electrochemical performances of a-MoSx in Mg ion batteries tested in the potential range of 0.2–2.2 V versus Mg/Mg2+ with LiCl + [Mg2Cl3]+[AlPh2Cl2]–/THF electrolyte. (a) typical charge/discharge profiles at current rates of 20 mA g–1; (b) cyclic voltammograms of the cell containing a-MoSx (dark curves: first cycle, blue curves: second cycle, red curves: third cycle, green curves: fourth cycle). Several intercalation compounds showed high specific capacity and good rate performance, in which Chevrel phase and layered materials such as TiS2 are promising due to their structural flexibility with large layer spacing.41,43 The a-MoSx was also used as cathodes with Mg plate anodes for the hybrid Mg-Li-ion batteries. It was found that the discharge voltage and rate performance were improved with hybrid Mg-Li-ion electrolytes (LiCl + [Mg2Cl3]+[AlPh2Cl2]–/THF). Figure 8a shows typical charge/discharge voltage profiles of a-MoSx at a current rate of 20 mA g–1 in the voltage window of 0.2–2.2 V versus Mg/Mg2+. During the first lithiation process (discharge), the discharge voltage remains 1.20 V with a discharge capacity up to 117 mA h g–1.
Corresponding Author
[email protected].
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This research work was financially supported by Japan Society for Promotion of Science (JSPS, Grant No. PU15903), Japan. Phong D. Tran acknowledges USTH (PECH2 USTH2015 Grant) and Nafosted (Grant 103.99-2015.46) for funding support.
REFERENCES (1) Muldoon, J.; Bucur, C. B.; Gregory, T. Chem. Rev. 2014, 114, 11683−11720. (2) Yoo, H. D.; Shterenberg, I.; Gofer, Y.; Gershinsky, G.; Pour, N.;
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Figure captions Figure 2. (a–e) Experimental series of Z-contrast STEM images as function of time, showing the decoupling and conformation change of cluster. Each subsequent image is acquired with a frame time of 7.5375 s. The superimposed atoms on the inset indicate the locations of Mo atoms with shifting to the right for clarity (top). Schematic atomic model of the corresponding atomic structure (bottom). Figure 3. (a–j) Sequence of HAADF-STEM images showing the dynamics of cluster with continuous conformation change and structural transition from compact towards an elongated structure, which finally returns original form. Each subsequent image is acquired with a frame time of 7.5375 s.
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