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Interconnected Vertically Stacked 2D-MoS2 for Ultrastable Cycling of Rechargeable Li-Ion Battery Congli Sun,†,‡,⊥,∇ Kangning Zhao,§,∇ Yang He,∥,∇ Jianming Zheng,*,∥ Wangwang Xu,# Chenyu Zhang,§ Xiang Wang,∥ Mohan Guo,‡ Liqiang Mai,† Chongmin Wang,*,∥ and Meng Gu*,‡

Downloaded by BUFFALO STATE at 06:47:06:390 on June 03, 2019 from https://pubs.acs.org/doi/10.1021/acsami.9b02359.

†

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, International School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China ‡ Department of Materials Science and Engineering, Shenzhen Engineering Research Center for Novel Electronic Information Materials and Devices, Guangdong Provincial Key Laboratory of Energy Materials for Electric Power, No. 1088 Xueyuan Blvd, Shenzhen, Guangdong 518055, China § Materials Science and Engineering, University of Wisconsin Madison, 1509 University Avenue, Madison, Wisconsin 53706, United States ∥ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ⊥ NRC (Nanostructure Research Centre), Wuhan University of Technology, Wuhan 430070, China # Department of Mechanical & Industrial Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States S Supporting Information *

ABSTRACT: A two-dimensional (2D) layer-structured material is often a high-capacity ionic storage material with fast ionic transport within the layers. This appears to be the case for nonconversion layer structure, such as graphite. However, this is not the case for conversion-type layered structure such as transition-metal sulfide, in which localized congestion of ionic species adjacent to the surface will induce localized conversion, leading to the blocking of the fast diffusion channels and fast capacity fading, which therefore constitutes one of the critical barriers for the application of transition-metal sulfide layered structure. In this work, we report the tackling of this critical barrier through nanoscale engineering. We discover that interconnected vertically stacked two-dimensional-molybdenum disulfide can dramatically enhance the cycling stability. Atomic-level in situ transmission electron microscopy observation reveals that the molybdenum disulfide (MoS2) nanocakes assembled with tangling {100}-terminated nanosheets offer abundant open channels for Li+ insertion through the {100} surface, featuring an exceptional cyclability performance for over 200 cycles with a capacity retention of 90%. In contrast, (002)-terminated MoS2 nanoflowers only retain 10% of original capacity after 50 cycles. The present work demonstrates a general principle and opens a new route of crystallographic design to enhance electrochemical performance for assembling 2D materials for energy storage. KEYWORDS: 2D-MoS2, anode, lithium-ion battery, in situ TEM, volume expansion control

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INTRODUCTION

scale LIBs. In addition, MoS2 electrodes possess much less volume expansion upon lithiation than other conversion materials like silicon. The 35−103% volume expansion was reported for MoS2 with full lithiation.2,3,12,13 This also makes MoS2 a good anode with high-capacity retention and excellent rate capability.5,6,10 The electrochemical process generally involves fast Li intercalation into the unique van der Waals S− S-layered open channels. The volume expansion was minimum during the intercalation process.2,12−16 However, the conversion reaction results in significant volumetric expansion,

Lithium-ion battery has dominated the portable electronic device. However, the energy density of lithium-ion battery fails to keep up with the development of the electronics. In this regard, searching for an electrode with high-energy density is crucial. Two-dimensional (2D) transition-metal sulfides are widely investigated as high-capacity anode materials for lithium-ion batteries (LIBs). Molybdenum disulfide (MoS2) is among the most promising 2D van der Waals transitionmetal sulfides that attract great research focus in recent years.1−10 MoS2 can be used as anode material for LIBs with a high theoretical capacity of 1290 mAh g−1,1,11 which outperforms the commercially used graphite with a capacity of about 370 mAh g−1, leading to potential application in large© XXXX American Chemical Society

Received: February 5, 2019 Accepted: May 22, 2019

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DOI: 10.1021/acsami.9b02359 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) TEM image of P-MoS2. The inset shows the edge of one hierarchical thin nanosheet. (b) STEM image of the surface of P-MoS2. Most nanosheets of P-MoS2 are (002)-surfaced. (c) Schematics of P-MoS2 and the Li insertion, which show that the (002) surface is exposed. (d) STEM high-angle annular dark-field (HAADF) image of V-MoS2; (e) high-resolution transmission electron microscopy (HRTEM) image of the surface of V-MoS2. The surface is mostly along [100] direction, with the 2D open channel exposed to the surface. (f) STEM image showing the interconnected edges of the V-MoS2; (g) schematics of P-MoS2 and the Li insertion pathway. The V-MoS2 is vertically stacked with (002) planes that expose {100}-type surface for 2D open channels; (h) HAADF STEM image of V-MoS2; (i) S EDS map; and (j) Mo EDS map of the region shown in panel (h).

capacity of 533 mAh g−1 retained and excellent rate performance with a capacity of 569 mAh g−1 retained at 5C.

which is mainly caused by rapid volume expansion and the unstable solid electrolyte interphase (SEI), which would consume the active lithium and decreased the electronic conductivity.13,17−19 Improving the performance of MoS2 electrode through many approaches, such as optimized physical structure, enhanced conductivity, and localized conversion has been reported by earlier researchers.20−23 Carbon coating has been demonstrated to be quite successful in protecting the electrode from the electrolyte corrosion.18 However, additional carbon would consume more volume in the electrode and result in a severe decrease of volumetric capacity, which is crucial in portable device application. Thus, a better understanding of cyclability and volume expansion with advanced material engineering remains elusive. To push the cyclability toward commercial applications, volume expansion must be further controlled to maintain structure integrity. Rapid volume expansion would also cause severe safety problem and needs to be carefully modulated. Here, a self-limiting volume expansion mechanism was demonstrated by in situ transmission electron microscopy (TEM), in which MoS2(002) planes are vertically stacked with {100} surface modifying the conversion reaction without blocking the open channels for Li intercalation. The vertically stacked {100}-surfaced MoS2 nanocake (V-MoS2) shows improved cyclability performance for over 200 cycles with a

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METHODS

MoS2 Sample Preparation. The MoS2 nanoflowers comprised (002)-surfaced planar nanosheets (P-MoS2) were prepared by facile hydrothermal reaction. Typically, hexaammonium heptamolybdate tetrahydrate (1 mmol, (NH4)6Mo7O24·4H2O, i.e., 7 mmol Mo) and thiourea (30 mmol) were dissolved in deionized water (35 mL) under vigorous stirring to form a homogeneous solution. Then, the solution was transferred into a 45 mL Teflon-lined stainless steel autoclave and maintained at 220 °C. After 18 h, the reaction system was allowed to cool down to room temperature. The final product was washed with water and absolute ethanol several times to remove any possible ions and dried at 60 °C under vacuum. All of the materials were purchased from SinoPharm and used without further purification. The MoS2 nanocakes bonded by vertically stacked (002) planes with {100} open surface (V-MoS2) were prepared by precipitation method. (NH4)6Mo7O24·4H2O (6 g) is dissolved in 16 mL of purified water. The obtained solution then reacts with 43 mL of 22% (NH4)2S in water at 65 °C with continuous stirring for 2 h. The products were then dried at 75 °C and calcined at 500 °C for 1 h in Ar gas to form MoS2 nanocakes. Vertically stacked (002) stacked with {100} open-surfaced MoS2 lamella was made by in situ lift-out using a Zeiss Auriga focus ion beam (FIB). The final milling voltage is dropped to 2 kV to minimize damage from implanted Ga. The MoS2 lamella is further polished by a B

DOI: 10.1021/acsami.9b02359 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Cycling performance of V-MoS2 at 0.1C and (b) the corresponding charge−discharge curves of the initial three cycles. (c) Comparison of the rate performance of V-MoS2 and P-MoS2 and (d) the corresponding charge−discharge curves at different rates. (e) The highrate cycling performance of V-MoS2 and P-MoS2 at 2C. Fischione 1040 Nano mill with an accelerating voltage of 0.2 kV and an incident angle of 5°. The sample is pasted onto the Cu grid and loaded on the sample side of the nanofactory-scanning tunneling microscope holder. Pure Li metal was loaded onto a tungsten tip that is fixed to piezosystem in the nanofactory holder, which was carried out in an Ar-filled glovebox, and the whole setup was transferred to TEM via a home-made vacuum container. An FEI Titan 80-300 S/ TEM with a probe Cs corrector and environmental transmission electron microscope with an imaging Cs corrector were used for this research. Electrochemical Characterizations. The MoS2 electrodes were prepared by casting a slurry containing the MoS2 active material, acetylene black, and poly(vinylidene fluoride) (Kureha L#9305) binder with a weight ratio of 80:10:10 onto a Cu current collector foil. The MoS2 laminate was then punched into disks with a diameter of 1.27 cm and dried under vacuum overnight before use. The active material loading amount was about 1−2 mg cm−2 on the as-prepared electrodes. The CR2032 coin-type cells consisting of the as-prepared MoS2 electrodes, one piece of polyethylene separator, and the electrolyte were assembled in an argon-filled glovebox with both water and oxygen content lower than 0.1 ppm. The optimized electrolyte is 4.0 M LiFSI in dimethoxyethane (DME). Electrochemical performances were conducted galvanostatically at various current rates in the voltage range of 0.005−3.0 V on a Land-CT2001A battery cycler in a temperature chamber controlled at 30 °C. A current density rate of 1C (i.e., charge or discharge in 1 h) was set to 1000 mA g−1. The rate performance was evaluated with ascending C rates ranging from C/20 to 5C.

composed of nanosheet that is edge-bent to form the typical follower-shape morphology.5,8,24,25 This can be directly viewed by TEM image of Figure S1, with the mass thickness contrast showing a thick edge-bending feature. Figure 1a shows the diffraction contrast TEM image that emphasizes the (002) topdown surface planes of MoS2 from an edge view. Flat nanosheet region of P-MoS2 is shown in scanning transmission electron microscopy (STEM) image of Figure 1b, in which (002) surface is confirmed. The (002) planes are the dominating surface of the nanosheet with an average thickness of ∼7 nm (inset in Figure 1a). Figure 1c shows the schematic illustration of P-MoS2 and the Li intercalation pathways into the 2D open channels. Similar MoS2 nanofollowers comprised (002)-surfaced hierarchical ultrathin MoS2 nanosheets were widely reported with superior capacity and cyclability.5,8,24,25 Typical cake-like morphology of V-MoS2 is shown in Figure 1d−f, which consists of tangling MoS2(002) planes interconnected with each other (Figure 1e). In contrast, with the (002) planes vertically stacked, the surface termination of VMoS2 is dominated by {100}-type planes (see the yellow lines in Figures 1e and S2). As pointed out by the white arrows in Figure 1f, the edges of V-MoS2 are interconnected, which greatly strengthened the MoS2 structure. In addition, the blue arrows in Figure 1f indicate the micropores inside the interconnected V-MoS2, which can greatly relieve the volume expansion during lithiation, as discussed later. The schematics of V-MoS2 morphology is shown in Figure 1g. The chemical identities of MoS2 are further verified by large-scale energydispersive X-ray spectroscopy (EDS) mapping, as shown in

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RESULTS AND DISCUSSION The morphology and microstructure of the V-MoS2 and PMoS2 were analyzed by aberration-corrected (S)TEM. Figures 1a,b and S1 show the morphology of P-MoS2, which is C

DOI: 10.1021/acsami.9b02359 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Figure 1h−j. Similar vertical aligned MoS2 was reported with superior catalyst performance.26−29 The electrochemical performance is evaluated in counter with Li metal in the concentrated electrolyte (4.0 M LiFSI in dimethoxyethane (DME)). The initial cycling performance of V-MoS2 at 0.1C is shown in Figure 2a, and the corresponding charge−discharge curves are shown in Figure 2b. Although the Coulombic efficiency of the first cycle is still unsatisfied (about 77%), it is still higher than P-MoS2. The V-MoS2 shows no obvious capacity fading in 40 cycles. The rate performance is further evaluated in Figure 2c, and the charge−discharge curves at different rates are shown in Figure 2d. The capacities of V-MoS2 are higher than those of P-MoS2 at all different current densities. Noticeably, V-MoS2 shows an excellent highrate capability of 569 mAh g−1 retained at 5C. Thus, to further investigate the cyclability at a high rate, the cycling performance at 1C is further tested, and the V-MoS2 shows a relatively high reversible capacity of 533 mAh g−1 for over 200 cycles, while P-MoS2 failed within only 50 cycles. All of the above tests demonstrate a superior cycling performance of VMoS2 compared with that of P-MoS2. The open-cell setup for in situ TEM observation of lithiation is shown in Figure 3a. The MoS2 serves as a working electrode and Li metal as a counter electrode, with native Li2O as a solid electrolyte. Figure 3b and Movie S1 show sequential HRTEM snapshots of P-MoS2 during lithium insertion. The white arrow in Figure 3b indicates the Li intercalation front. A two-step lithiation process can be clearly viewed. The first step involves fast Li intercalation into (002) van der Waals layers, in which Li only breaks the interlayer S−S bonds but the in-plane structure remains intact. The full lithiation of MoS2 follows the equation similar to conversion reaction4 MoS2 + 4Li+ + 4e− → Mo + 2Li 2S

(1)

+

where four Li ions are allowed for each MoS2 formula unit. However, under thermodynamic equilibrium, the lithiation undergoes the intercalation-and-conversion two-step process3,30 x Li+ + MoS2 + x e− → LixMoS2

Figure 3. (a) Schematic illustration of in situ battery setup for TEM observation. (b) TEM images of P-MoS2 as a function of lithiation time. (c) TEM images of V-MoS2 as a function of lithiation time. (d) In-plane volume expansion of P-MoS2 and V-MoS2 during lithiation (the scale bar for all images is 20 nm).

(2)

(4 − x)Li+ + LixMoS2 + (4 − x)e− → Mo + 2Li 2S

situ SAED patterns in Figure 4a, in which no significant change in the SAED pattern can be found at the beginning. When Li+ intercalation is over a certain threshold, MoS2-related pattern starts to disappear with the simultaneous formation of Li2S and Mo patterns. However, different from the lithiation of the P-MoS2, VMoS2 shows no rapid volume expansion but gradual and mild volume expansion (see Movie S3). As the volume expansion is dominated by the conversion reactions that result in polycrystalline Mo and Li2S with no preferable orientation (Movie S2), the volume expansion is treated as isotropic and extrapolated based on the area changes from 2D in situ TEM images. The volume expansion as a function of lithiation time is plotted in Figure 3d. Compared to that of P-MoS2, the volume expansion of V-MoS2 is considerably reduced, due to the special geometry of V-MoS2, wherein the volume expansion is largely confined in the spaces between the tangled MoS2 nanocrystals. As a result, a global compressive strain gradually builds up ensuing the conversion reaction, which, in turn, modulates the volume expansion in the [001] direction. Energetically, the compressive strain poses an additional barrier for the conversion reaction. Thereby, gentle

(3)

The thermodynamically dominated two-step process was also observed for other 2D transition-metal sulfides like CuS,31 which generally originates from fast Li intercalation along the unique 2D channels. Here, the first intercalation step keeps for about 30 s. Negligible volume expansion for Li intercalation was observed, which agrees with previous reports.2,12−16 With the accumulation of inserted Li+, dramatic conversion reaction starts at around 30 s in Figure 3b, accompanied by significant volume expansion. This step gives rise to a more than 60% expansion within 5 s. The volume expansion as a function of lithiation time is plotted in Figure 3d. The lithiation process of V-MoS2 is shown in Figure 3c. Similarly, Li intercalation reaction happens first, with the dominated {100} planes on the surface providing adequate open channels for fast Li intercalation. The in situ selective area electron diffraction (SAED) patterns of the lithiation process can be found in supporting Movie S2. With the fast accumulation of intercalated Li for about 15 s, V-MoS2 then undergoes almost homogeneous global conversion reaction, which breaks the (002) lattice. This is also supported by the in D

DOI: 10.1021/acsami.9b02359 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) Time sequential SAED of the lithiation process. (b) Radial intensity profile of SAED as a function of reaction time.

Figure 5. (a) STEM image of (002)-surfaced MoS2 lamella prepared by FIB. (b) TEM image of MoS2 lamella after 10 min of lithiation. Yellow circle shows the individual regions that are trapped in the intercalation process. (c) TEM image of one typical incomplete conversion region after 40 min of lithiation.

synchronized intercalation and conversion process in a relatively large region enabled partially by the fast transport of Li+ in between the 2D channels. To further elucidate the self-confinement effect, MoS2 single crystal lamella was prepared by (002) stacking and surfaced with {100} planes by FIB (see Figure 5a). The thickness of the {100}-surfaced MoS2 is ∼9 nm, measured using a log-ratio method from low-loss electron energy loss spectroscopy. The open channel for Li intercalation is exposed, while volume expansion along the [001] direction is self-limited in the bulk. Figure 5b shows lithiated MoS2 for the first 10 min. A lot of area with clear MoS2(002) lattice (yellow circles) can be observed surrounded by a fully reacted matrix. The lithiation is further kept for more than 40 min under a potential of −3.0 V. Many residual MoS2 were found with a largely expanded layer structure, as shown in Figure 5c, which indicates that the initial volume expansion is mainly along [001] direction and the conversion reaction can be incomplete at certain local regions. The surrounding reacted areas result in volume expansion and compressing strain field that hinders further conversion process. The strain field was initiated at the beginning of the conversion reaction and keeps increasing in strength with the proceeding conversion, resulting in the retardation of Li+ diffusion kinetics.32 As a result, the rapid global volume expansion is slowed down. The strain effect is so significant that full conversion reaction is never reached at local regions.32

and mild conversion happens with smaller volume expansion, and the structural integrity of V-MoS2 is largely preserved. Additionally, the special geometry exposes the {100} surface with enormous open channels (stacked (002) planes) for Li injection along the 2D van der Waals layers. Similarly, [001] directional confinement of MoS2 nanosheets by using carbon coating was reported to achieve outstanding long-life cycling capability at high rates, which was mainly attributed to the enhanced structural integrity by the space confinement.7 Figure 4a shows the selective area electron diffraction (SAED) patterns of V-MoS2 before lithiation (0 s), at the right end of the intercalation (15 s), and after the conversion (45 s). The MoS2(002) diffraction spot remains intact before the conversion reaction and disappears with the formation of polycrystalline Li2S and Mo right after the conversion reaction. Figure 4b shows the radial intensity profile of SAED as a function of reaction time, which verifies the overall phase transformation from MoS2 to Li2S and Mo during conversion. This also agrees with the observation in Figure 3 that the electrochemical lithiation of MoS2 is a separate two-step process. Both MoS2(002) peak at 1.51 nm−1 and (100) peak at 0.365 nm−1 show no negative shift during the intercalation process, indicating small lattice expansion corresponding to Li accommodation in the uniquely layered 2D channel. Conversion later happens with significant accumulation of inserted Li+ in the 2D channels. As the SAED aperture covers a circular area of at least 300 nm in radius, it indicates a E

DOI: 10.1021/acsami.9b02359 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. STEM dark-field image (a) and HR-STEM dark-field image (b) of the V-MoS2 after 1 cycle; (c) STEM dark-field image of the V-MoS2 after 10 cycles; TEM overall view (d, e) and HRTEM (f) of the V-MoS2 after 200 cycles.

images in Figure 6d,e, although the surface now contained a thick layer of solid electrolyte interphase (SEI). The HRTEM image in Figure 6f even showed that small portions of the vertically stacked (002) planes are still visible after 200 cycles and large areas were now amorphous. According to the in situ TEM observations, the strain and fatigue caused by repeated insertion and extraction of lithium ions certainly led to structural damages and increasing amount of amorphization of the original crystalline V-MoS2. However, the mesoporous morphology and ultrathin thickness of the nanocakes were kept intact after cycling through the bonded interconnected edges, as shown in Figure 6. Therefore, we believed that the optimized cycling performance of V-MoS2 is closely related to the structural integrity and stability after many cycles.

Lithiation of the bulk (002) MoS2 sheet was widely studied; no residual MoS2 is reported after full lithiation.2−4,12 We have also done the full lithiation of a number of (002)-surfaced MoS2 nanosheet; no residual MoS2 lattice can be observed by HRTEM. This indicates that the conversion reaction first destroys the (002) plane of MoS2, leading to the disappearance of the MoS2 basal plane, which also leads to a [001] orientation-dependent volume expansion at the very beginning of the conversion reaction. More than 60% volume expansion is observed for P-MoS2 nanosheet in Figure 3, while less than 40% size increase can be observed for V-MoS2 after full lithiation.4 The pores inside the V-MoS2 as shown in Figure 1f largely reduced the overall volume expansion. In addition, the interconnected structure and self-confinement limit volume expansion within the in-plane direction for V-MoS2, compared to the rapid volume expansion toward out of plane for P-MoS2. As a result, mild volume expansion happens with structural integrity being well preserved, leading to enhanced cyclability. Stable structures result in better cycling performance for electrodes in a Li-ion battery. As discussed above in Figure 2, the interconnected V-MoS2 exhibited significantly superior battery cycling performance due to better structural integrity and mechanical stability than the loosely arranged P-MoS2. Ex situ S/TEM analysis of the V-MoS2 after different cycles was performed to reveal the structural stability after long charge− discharge cycles in Figure 6. The STEM dark-field image in Figure 6a revealed that the V-MoS2 maintained a similar structure as the fresh V-MoS2 nanocakes after one charge− discharge cycle. High-resolution STEM image in Figure 6b clearly proves that (002) plane lattice fringes were still present after one charge−discharge cycle. The STEM image in Figure 6c revealed the intact structure of V-MoS2 after 10 charge− discharge cycles, indicating the robust structure of the interconnected V-MoS2. In addition, the structural integrity of V-MoS2 was reasonably well maintained even after 200 charge−discharge cycles, as shown by the bright-field TEM

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CONCLUSIONS In conclusion, we have directly observed the intercalation and conversion processes in the lithiation of 2D-MoS2 using in situ TEM. The results demonstrate an effective crystallographic structural design for improving overall battery performance. The interconnected vertically stacked {100}-terminated MoS2 nanocakes show much higher capacity and cyclability than loosely arranged (002)-terminated P-MoS2 nanoflowers. The micropores and a self-confinement effect effectively reduce the volume expansion along the [001] direction and maintain the structural integrity of the materials after many cycles. This study provides much-needed new insights for advanced design and application of 2D materials in lithium-ion batteries.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b02359. Sequential HRTEM snapshots of P-MoS2 during lithium insertion (AVI) F

DOI: 10.1021/acsami.9b02359 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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In situ SAD patterns of the lithiation process of V-MoS2 (AVI) Sequential HRTEM snapshots of V-MoS2 during lithium insertion (AVI) Scanning transmission electron microscopy (STEM and TEM) images of P-MoS2 and V-MoS2; cycling performance of V-MoS2; HRTEM snapshots of P-MoS2 and VMoS2 during lithium insertion; and SAED patterns of the lithiation process of V-MoS2 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Z.). *E-mail: [email protected] (C.W.). *E-mail: [email protected] (M.G.). ORCID

Liqiang Mai: 0000-0003-4259-7725 Chongmin Wang: 0000-0003-3327-0958 Meng Gu: 0000-0002-5126-9611 Author Contributions ∇

C.S., K.Z., and Y.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Shenzhen DRC project [2018] 1433, Guangdong Provincial Key Laboratory of Energy Materials for Electric Power with contract number of 2018B030322001. Part of the work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830. The S/TEM work was also supported at the Nanostructure Research Center (NRC), which is supported by the Fundamental Research Funds for the Central Universities (WUT: 2019III012GX), the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, and the State Key Laboratory of Silicate Materials for Architectures (all of the laboratories are at Wuhan University of Technology).

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REFERENCES

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DOI: 10.1021/acsami.9b02359 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX