Two-Dimensional Holey Nanoarchitectures Created by Confined Self

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Two-Dimensional Holey Nanoarchitectures Created by Confined Self-Assembly of Nanoparticles via Block Copolymers: from Synthesis to Energy Storage Property Lele Peng, Zhiwei Fang, Jing Li, Lei Wang, Andrea M. Bruck, Yue Zhu, Yiman Zhang, Kenneth J. Takeuchi, Amy C. Marschilok, Eric A. Stach, Esther S. Takeuchi, and Guihua Yu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08186 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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Two-Dimensional Holey Nanoarchitectures Created by Confined Self-Assembly of Nanoparticles via Block Copolymers: from Synthesis to Energy Storage Property Lele Peng,†,┴ Zhiwei Fang,†,┴ Jing Li,§,ǁ,┴ Lei Wang,‡ Andrea M. Bruck,‡ Yue Zhu,† Yiman Zhang,‡ Kenneth J. Takeuchi,‡,ǁ Amy C. Marschilok,‡,ǁ Eric A. Stach,*,§ Esther S. Takeuchi,*,‡, ǁ,¶ and Guihua Yu*,† †

Materials Science and Engineering Program and Department of Mechanical Engineering, The

University of Texas at Austin, Austin, Texas 78712, USA ‡

Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA

§

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973,

USA ǁ

Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY

11794, USA ¶

Energy Sciences Directorate, Brookhaven National Laboratory, Upton, NY 11973, USA

ACS Paragon Plus Environment

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ABSTRACT: Advances in liquid-phase exfoliation and surfactant-directed anisotropic growth of two-dimensional (2D) nanosheets have enabled their rapid development. However, it remains challenging to develop assembly strategies that lead to the construction of 2D nanomaterials with well-defined geometry and functional nanoarchitectures that are tailored to specific applications. Here we report a facile self-assembly method leading to the controlled synthesis of 2D transition metal oxide (TMO) nanosheets containing a high density of holes. We utilize graphene oxide sheets as a sacrificial template and Pluronic copolymers as surfactant. By using ZnFe2O4 (ZFO) nanoparticles as a model material, we demonstrate that by tuning the molecular weight of the Pluronic copolymers that we can incorporate the ZFO particles and tune the size of the holes in the sheets. The resulting 2D ZFO nanosheets offer synergistic characteristics including increased electrochemically active surface areas, shortened ion diffusion paths, and strong inherent mechanical properties, leading to enhanced lithium-ion storage properties. Post-cycling characterization confirms that the samples maintain structural integrity after electrochemical cycling. Our findings demonstrate that this template-assisted self-assembly method is a useful bottom-up route for controlled synthesis of 2D nanoarchitectures, and these holey 2D nanoarchitectures are promising for improving the electrochemical performance of nextgeneration lithium-ion batteries.

KEYWORDS: 2D holey nanosheet architectures; confined self-assembly; Pluronic copolymers; lithium-ion batteries; energy storage.


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Research progress in liquid-phase exfoliation has enabled facile and large-scale synthesis of two-dimensional (2D) nanomaterials from intrinsically layered compounds. These include graphene, transition metal oxides and metal chalcogenides, as well as MXenes.1-4 Additionally, several wet-chemical routes leading to the synthesis of metal sulfide and metal oxide nanosheets from the intrinsically non-layered materials have also been reported.5,6 2D nanomaterials are novel structures that allow fundamental studies to be conducted and they also lead to the creation of numerous practical devices, due to their unique physical and chemical properties. These include applications in nanoelectronics/optoelectronics, energy storage and conversion devices, heterogeneous catalysis, and sensors, among others.7-13 However, it remains challenging to construct 2D nanomaterials with well-defined geometry and functional nanoarchitecture that are tailored for specific applications. Synthetic protocols are needed to create various nanostructures, and which can be generalized to a wide range of material types. Additionally, it is necessary to demonstrate the ability to modulate material properties. Porosity engineering in a variety of 2D nanomaterials – including carbonaceous materials, transition metal oxides and chalcogenides, as well as metal phosphates and MXene – has been widely applied, and has been shown to enhance their chemical/physical properties in various applications.14-16 Porous 2D nanomaterials in particular provide promising opportunities for enhancing the performance of electrochemical energy storage technology, because pores enable a greater amount of electrochemically active surface and faster ion transport.17-20 Rational synthesis of porous 2D nanomaterials is typically achieved by the chemical-etching, hydrothermal/solvothermal methods or via template-directed synthesis. For example, a chemicaletching method has been used to prepare porous carbonaceous materials in basic media from various parent materials, including activated carbon, CNTs, and biomass compounds.21,22


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Hydrothermal/solvothermal methods were also explored to prepare porous graphene nanosheets, N-doped graphene nanosheets and C3N4 nanosheets, among others.9 Selecting proper solvents, reactants and sometimes surfactants has been shown to be critical to controlling the synthesis of porous 2D nanosheets. Template-directed strategies have recently become another important approach to synthesize 2D nanosheets with well-defined 2D features and porous architectures.2326

For example, a general two-step strategy using graphene oxides as a sacrificial template was

reported recently for controlled synthesis of porous 2D transition metal oxide (TMO) nanosheets with tunable pore sizes.27,28 Apart from the aforementioned synthetic strategies, the ordered selfassembly of anisotropic nanocrystals into macroscopic architectures is one of the most promising routes to construct functional electronic and phonic structures.29-33 In the typical assembly process, block copolymers such as Pluronic molecules have been widely adopted to fine-tune the porosity and/or to obtain the ordered porous architectures.33-36 This approach has proven to be versatile, leading to the synthesis of various nanomaterials with a high degree of control over material composition and structures. However, this method has not yet been applied to the synthesis of porous 2D nanomaterials. Here we report a self-assembly method for the controlled synthesis of 2D holey TMO nanosheets by using graphene oxide (GO) sheets as the sacrificial templates and Pluronic copolymers as surfactants. By using ZnFe2O4 (ZFO) nanoparticles as a model material, we demonstrate the feasibility of this approach to control the hole size in the nanosheets by tuning the molecular weight of the Pluronic copolymers. The hole size of the as-obtained structures can be tuned from 6 nm, 10 nm and 13 nm by controlling the molecular weights of Pluronic from 1100, 4400 and 12600, respectively. The resulting material is composed of interconnected ZFO nanoparticles embedded in a holey nanosheet architecture, and we will show that it offers unique


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characteristics for high-performance electrodes for energy storage. For instance, the ZFO nanosheets obtained by this method exhibit increased surface area and more numerous interfaces, which facilitate interfacial transport and lead to shortened diffusion paths. These structures also show inherently strong mechanical properties, as will be shown via in situ transmission electron microscopy (TEM) characterization of the changes in morphology during the lithiation process, as well as by ex situ, post-cycling STEM and EDX analysis. Furthermore, ex situ TEM imaging of microtomed samples showed the electrodes maintain their structural integrity and that they are relatively well-distributed when fabricated into practical lithium-ion battery electrodes. Our findings demonstrate that the self-assembly method can be a promising bottom-up route for the synthesis of holey nanosheets from the intrinsically non-layered structures, and that these structures may lead to improved electrochemical performance in next-generation lithium-ion batteries. RESULTS AND DISCUSSION The self-assembly synthesis of TMO nanosheets is achieved by using GO as a sacrificial template and Pluronic copolymers as surfactants. Figure 1a presents a schematic illustration of the Pluronic copolymer induced self-assembly of TMO nanoparticles into holey nanosheets. To demonstrate the feasibility of the self-assembly method, ZFO nanoparticles are used here as a model material. Briefly, single-crystalline ZnFe2O4 nanoparticles with ~6 nm in size (Figure S1) were mixed with Pluronic copolymers in ethylene glycol (EG) by magnetic stirring as the first step in the process. Mechanical stirring can promote binding of the Pluronic copolymers onto the ZFO nanoparticles and the subsequent linkage between adjacent ZFO nanoparticles. This mixture was then dispersed in the GO/EG suspension via magnetic stirring. This results in the deposition of ZFO nanoparticles onto the GO templates, mediated by the Pluronic copolymers.


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Notably, the deposition of ZFO nanoparticles onto GO template is critically important for the uniformity of the resulting materials. The resultant precipitates were annealed at 400 ˚C for 2 hours with a ramp rate of 0.5 ˚C/min, leading to the final structure. A low magnification bright field scanning transmission electron microscope (STEM) image (Figure 1b) shows the typical morphology of the ZFO/GO precursors assembled through the use of Pluronic copolymers with molecular weight of 1100 (Pluronic-1100): this image indicates that ZFO nanoparticles were uniformly deposited onto the GO template. A higher magnification STEM image (Figure S2a) indicated that the ZFO nanoparticles were uniformly distributed onto the GO templates. When deionized (DI) H2O was used as solvent for the self-assembly, ZFO nanoparticles were poorly deposited on GO template (Figure S2b). This is probably because the surfactant effects of EG assist in the creation of a uniform dispersion of the ZFO nanoparticles. The above thermal treatment of the as-prepared ZFO/GO precursors induced the transformation of ZFO/GO precursors formed by Pluronic linking into a ZFO nanosheet containing a high density of pores, without altering the 2D morphology. This thermal treatment also led to the simultaneous decomposition of the GO templates. The resulting 2D holey architectures were preserved in the samples, even after thermal treatment at 400 ˚C for 2 hours (Figure 1c), and the SEM image in Figure S3 also indicates that the 2D geometry is preserved. Post-thermal treatment also promotes the interconnection of adjacent ZFO nanoparticles into an interconnected nanosheet network.


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Figure 1. (a) Schematic illustration of Pluronic copolymer induced self-assembly of TMO nanoparticles into holey 2D TMO nanosheets. (b-c) STEM images of the ZFO@GO precursors assembled by Pluronic-1100 in EG, the as-obtained holey ZFO nanosheets. Scale bars in b and c are 1 µm and 200 nm, respectively.

This confined self-assembly synthetic method offers control over both the pore size and the particle size of the resulting ZFO nanosheets through tuning the molecular weights of the Pluronic copolymers. Using Pluronic copolymers with molecular weight of 1100, 4400 and 12600 (Pluronic-1100, Pluronic-4400, Pluronic-12600) as surfactants, the as-obtained ZFO/GO precursors had a controlled occupation ratio of GO template (Figure S4). ZFO/GO precursors obtained by using Pluronic-1100 showed the largest area ratio and nanoparticle densities:


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resulting from the fact that these Pluronic copolymers have the shortest the molecular length. The as-obtained ZFO nanosheets exhibited adjustable hole size after calcination at 400 ˚C for 2 hours. Figure 2a,b,c presents STEM images of the ZFO nanosheets obtained by using Pluronic1100, Pluronic-4400 and Pluronic-12600 copolymers respectively. The as-obtained holey ZFO nanosheets were phase-pure, with the crystalline ZFO being in its most common spinel phase, with Space Group Fd-3m, as identified by X-ray diffraction (XRD) (Figure S5). Figure 2d presents an EDX map of the holey ZFO nanosheets created with Pluronic-4400. The elemental distribution of Zn, Fe and O elements is uniform throughout the sample. Additionally, carbon was only located on the framework of the lacey carbon TEM grid, indicating that the GO templates have been completely removed after annealing at 400 ˚C. Figure 2e presents a highresolution phase contrast transmission electron microscope (HRTEM) image of the holey ZFO nanosheets following thermal treatment at 400 ˚C. The ZFO is crystalline and highly faceted. Lattice fringes of ~0.255 nm and ~0.488 nm are observed, which correspond to the (311) and (111) planes of the spinel ZFO structure, respectively. The concentric rings shown in selected area electron diffraction (SAED) pattern (inset of Figure 2e) indicate that the overall sample is polycrystalline. The diffraction rings can be indexed to spinel ZFO in agreement with the XRD analysis. Moreover, areas indicated by the red, blue and yellow squares in Figure 2e show that there is intimate interconnection between adjacent ZFO nanoparticles. It is also obvious that the pore sizes of the holey ZFO nanosheets increase as the molecular weights of the Pluronic copolymers increase, as shown in Figure 2f. The average pore sizes of the structures synthesized by using Pluronic-1100, Pluronic-4400 and Pluronic-12600 copolymers were ~6.3 nm, ~10.2 nm and ~12.6 nm, respectively, as directly measured from the STEM images in Figure 2a,b,c (Table 1). Direct measurements from the STEM images also showed that these structures had a narrow


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particle size distribution of with average values of ~6.9 nm, ~10.1 nm and ~ 11.5 nm, respectively (Figure S6). The details of pore/particle size measurement in the holey nanosheets are shown in Figure S7. These results were used to determine the ratio of pore to particle size (RPP) presented in Table 1. It was also shown that tuning thermal treatment allows control over the resulting pore and particle sizes. By controlling the calcination temperatures from 400 ˚C, to 500 ˚C and 600 ˚C, the pore size of the nanosheets can be tuned from 6 nm to 14 nm and 22 nm, respectively. Figure S8 presents STEM images of the ZFO nanosheets obtained with Pluronic1100, -4400 and -12600 at 500 ˚C and 600 ˚C, respectively. Pluronic-1100

Pluronic-4400

Pluronic-12600

Pore size (nm)

6.3

10.2

12.5

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6.9

10.1

11.4

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0.9

1.0

1.1

Table 1. Summary of morphological characteristics (hole size, particle size, ratio of pore size) obtained by using Pluronic copolymers with different molecular weight.


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Figure 2. Bright field STEM images of the holey ZFO nanosheets obtained by using Pluronic1100 (a); Pluronic-4400 (b) and Pluronic-12600 (c). (d) EDX mapping of the Zn, Fe, O and C elements in the holey ZFO nanosheets by using Pluronic-4400. (e) HRTEM image of the holey ZFO nanosheets. Red, blue and yellow squared areas in (e) indicate the interparticle interconnection. (f) Hole size distribution of the holey ZFO nanosheets obtained by using Pluronic polymers with different molecular weights (1100, 4400 and 12600). Scale bars in abcd and e are 200 nm and 5 nm, respectively. Scale bars in the insets of a,b,c: 500 nm.

To demonstrate these advantages for lithium ion storage, the holey ZFO nanosheets were explored as anode materials. To evaluate the distribution of the nanosheets in the electrodes, TEM characterization of microtomed section of nanosheets synthesized via thermal treatment at 400 ˚C, 500 ˚C and 600 ˚C was conducted. TEM characterization of microtomed sections is a powerful technique which enables preparation of electron transparent electrode samples at the millimeter scale.37,38 Figure S9 and S10 present TEM images and EDX maps of Fe and C from the nanosheet-based electrodes before testing. The results showed that the individual nanosheets were uniformly dispersed throughout the electrodes, with no obvious aggregation in any of the


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electrodes. In addition, Fe was also evenly distributed throughout the nanosheets. TEM characterization of microtomed sections also allows accurate description of the morphological and structural features of the samples calcined at different temperatures. As shown in Table S1, the average lateral size of the nanosheets obtained at 400 ˚C, 500 ˚C and 600 ˚C are ~2.0, ~1.8 and ~1.7 µm. The average pore sizes obtained at 400 ˚C, 500 ˚C and 600 ˚C are ~4.7, ~8.0 and ~15.5 nm, respectively, and their corresponding average particle sizes are ~10.8, ~14.6 and ~18.4 nm, respectively. These values are consistent with the values obtained in the STEM images (Figure S8).

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400 500 600 700 800 900 1000 Cycle number Figure 3. (a) CV curves from the ZFO nanosheets synthesized by using Pluronic-4400 obtained

at the scan rate of 0.1 mV s─1. (b) Rate performance of three ZFO nanosheet samples and a control ZFO nanoparticle sample at various current densities. P-1100, P-4400 and P-12600 represent the ZFO samples synthesized by using Pluronic polymers with different molecular


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weights (1100, 4400 and 12600). (c) Cycling performance of the porous ZFO nanosheets at 3.2 A g─1 for 1000 cycles.

Figure 3a presents cyclic voltammetry (CV) curves from the ZFO nanosheet anodes during the first 3 cycles between the voltage range of 0.1~3 V, at a scanning rate of 0.1 mV s─1. The sharp peak at ~0.95 V corresponds to the reduction of Zn2+ and Fe3+ to metallic Zn and Fe. The peak at 0.6 V is attributed to the formation of the solid electrolyte interphase (SEI).39,40 The sharp discharge peaks in the CV curves were consistent with the flat plateaus in the charge/discharge curves (Figure S11). The holey ZFO nanosheets delivered an initial discharge capacity of ~1250 mAh g─1, corresponding to the initial conversion reaction of ZFO between Li+ (ZnFe2O4 + 8Li+ + 8e− → Zn + 2Fe + 4Li2O).41 In the second charge/discharge cycle, the ZFO electrodes also exhibited a high reversible capacity of ~1000 mAh g─1, corresponding to the reversible redox reaction in the battery (ZnO + 2Li+ + 2e− ↔ Zn + Li2O, Fe2O3 + 6Li+ + 6e− ↔ 2Fe + 3Li2O). The capacity difference between the first discharge capacity and the reversible capacity is likely due to the formation of the SEI on the ZFO anodes. The charge/discharge profiles in Figure S11 also gave a first-cycle coulombic efficiency (C.E.) of ~73 % for the holey ZFO nanosheets. The higher first-cycle C.E. of the nanosheets can be attributed to the more efficient utilization of active surfaces present in the holey nanosheet samples. After several conditioning cycles, the C.E. of both electrodes improved to ~99%, indicating high reversibility during the conversion reaction between ZFO and Li. The charge/discharge curves of the nanosheets obtained by using different Pluronic (Pluronic-1100 and Pluronic-12600) are shown in Figure S12. Figure 3b presents data for the specific capacities of the samples as a function of pore size and ZFO nanoparticle size. After two activation charge/discharge cycles at 0.2 A g─1, the capacities


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stabilized at different current densities. A high capacity of ~850 mAh g─1 can be obtained at 0.4 A g─1 in the samples created through the use of Pluronic-4400 (ZFO-4400), which is similar with the capacities to the samples created through the use of Pluronic-1100 and Pluronic-12600. For the ZFO-4400 sample, the capacity can be maintained at ~500 mAh g─1, corresponding to a ~60 % capacity retention when the current density is increased to 3.2 A g─1 (by 8 times). ZFO-1100 and ZFO-12600 delivered capacities of ~450 and ~380 mAh g─1 at the same current density. For comparison, ZFO nanoparticle control samples delivered a capacity of only ~300 mAh g─1. The ZFO nanosheets even can deliver reasonable capacities when the current density increased to 5.0 A g─1. The C.E. of the ZFO samples in the rate capability tests are showed in the Figure S13. The charge/discharge curves obtained from the samples made with different Pluronic copolymers as a function of current densities are presented in Figure S14. Figure 3c presents the cycling performance of the holey ZFO nanosheets and the ZFO nanoparticle control sample. After electrochemically activating the electrodes at 0.2 A g─1 for one cycle, a stable capacity of ~420 mAh g─1 can be retained after 1000 cycles at a current density of 3.2 A g─1 for the ZFO-4400 electrodes. The electrochemical activation process at a low current density can facilitate the uniform formation of SEI on the electrode, and thereby help to stabilize the reversible delivery of the electrochemical energy during long-term cycling. These advantages were reflected in the ultra-stable coulombic efficiency (~99.8% in average) of the batteries incorporating the holey nanosheets, which were cycled for 1000 cycles at 3.2 A g─1. ZFO-1100 and ZFO-12600 nanosheets exhibited specific capacities of 380 mAh g─1 and 320 mAh g─1 at 3.2 A g─1 after 1000 cycles, indicating capacity retention rate of 82 % and 80 %, respectively. In order to understand which structural features of the holey nanosheets were responsible for the improved cycling stability, the cycling performance of the ZFO nanoparticle


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control samples were also evaluated. The ZFO nanoparticle control samples were only able to deliver a capacity of ~70 mAh/g after 1000 cycles at 3.2 A g─1. The C.E. of the ZFO samples in the cycling stability tests are showed in the Figure S15. The improved rate capability and cycling stability is believe to be a result of the holey morphology on the nanosheets, which introduces extra active surfaces and facilitate charge transport in the material. Each of these 2D holey nanosheets exhibits high cycling stability, which indicates that this strategy represents a general route for the synthesis of 2D holey nanomaterials with exceptional lithium storage properties. However, there are clear differences in the electrochemical performance among the nanosheets corresponding to differences in their RPP values. The results showed that the holey ZFO nanosheets with the RPP approaching 1 exhibited the best electrochemical performance. This phenomenon may be due to the different hole volume-occupying ratio of the different ZFO nanosheets after Li uptake.42,43


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Figure 4. (a) Scheme of the nanobattery device for the in situ TEM characterization. TEM images of the holey 2D ZFO nanosheets (b) before and (c) after lithiation process. Electron diffraction patterns of the holey 2D ZFO nanosheets (d) before and (e) after lithiation process. Scale bars in b and c: 100 nm.

The exceptional lithium ion storage properties of the holey ZFO nanosheets can be ascribed to the 2D interconnected holey architecture, which offers advantageous structural features, including extra active surfaces for lithium ion storage, open holey structures for rapid ion transport and mechanical properties that suppress structural degradation during charge/discharge processes. To explore this hypothesis, in situ TEM characterization during the lithiation process was conducted to monitor the structural evolution of the materials.44-46 As illustrated in Figure 4a, Li ions were driven towards the nanosheets by a positive external bias. Figure 4b,c shows the morphology of holey ZFO nanosheet before and after the first discharge process. At the initial stage (0 s in lithiation), pristine nanosheet was phase-pure and was polycrystalline in nature, as indicated by selected area electron diffraction (SAED) pattern (Figure 4d). The lithiation process started from the edge and proceeded to the center of the sample gradually. (See Movie S1 in the supporting information). At the lithiated stage (after 370 s of lithiation period), the sample maintained its 2D geometry and holey architecture, with the ultra-fine particles being observed embedded into the Li2O matrix. Electron diffraction of the lithiated ZFO nanosheets (Figure 4e) indicates that the pristine spinel ZFO was fully reduced to metallic Zn, Fe and Li2O, a result that is consistent with previous reports concerning the conversion reaction.27,47-49 Given the previous study of 2D nanosheets and from the observations above, it is apparent that the interconnected ZFO nanoparticles present in the original structure expanded during lithiation/conversion and


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filled in the original pore space. However, because of the high density of pores in the original structure, the overall sample exhibited only a small overall volume change (assuming no expansion along the normal direction). If we simplify the sample as a rectangle in projection and neglecting any changes in the normal direction, the lateral sizes of the nanosheet changed from 618 nm and 439 nm to 642 nm and 473 nm, respectively, indicating an 11% increase in in area. The small metallic Zn and Fe nanoparticles formed during the conversion also appear to be interconnected, leading to the formation of an electrically conductive pathway. These features, including robust mechanical response and small overall volume change are each a result of the unique holey architectures of the original materials.

Figure 5. Direct visualization of phase evolution of the holey ZFO nanosheets in the lithiation process. Scale bars: 5 nm.

In order to investigate the lithiation behavior of the nanosheet in real space, we performed in situ HRTEM observation of the process. Time-sequenced HRTEM images provide information


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about both the morphology and the structure (from its corresponding fast Fourier transformation (FFT)) simultaneously. We tracked the evolution of the spinel structured ZnFe2O4 (highlighted in white) during the entire lithiation process, as shown in Figure 5. The projected area of the spinel ZnFe2O4 nanoparticles (indicated by the yellow-dashed square) decreased concomitant with the formation of fine-sized Zn0 and Fe0 nanoparticles (Figure S16). We propose that the lithiation reaction proceeds as a “core-shell” process: the fine nanoparticles initially formed at the edge of the sample, and then propagate to the center of the pristine ZnFe2O4. Electron energy-loss spectroscopy (EELS) maps of the holey ZFO nanosheets before and after lithiation (Figure S17) indicated the distribution of Zn0 and Fe0 nanoparticles and are consistent with this interpretation.

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Figure 6. (a) STEM image and EDX maps of Zn, Fe and O elements in the holey ZFO nanosheets after 100 cycles at 5 A g─1. (b) EIS profiles of holey ZFO nanosheets before and after cycling. Inset shows the Randle’s circuit used for fitting. Scale bars: 200 nm.

In situ TEM characterization is a powerful technique to study structural evolution and reaction mechanisms in electrode materials in a microscopic manner.50-53 However, it is also necessary correlate these observations with ex situ, mesoscopic characterization as well. We have also obtained STEM images and EDX characterization of the holey nanosheets resulting


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from synthesis at 400 ˚C after cycling tests were conducted. Figure 6a presents a STEM image and corresponding EDX map of the holey ZFO nanosheets after 100 charge/discharge cycles at 5 A g─1. As is shown in the STEM image, the 2D geometry and the holey architecture were preserved after the charge/discharge cycles. The EDX map in Figure 6a also demonstrated that the distribution of Zn, Fe and O remained uniform at the mesoscale in the post-cycled samples. The electrochemical impedance spectroscopy (EIS) profiles – which reflect the internal resistance and charge transfer resistance of the electrodes – decreased dramatically after the cycling (Figure 6b and its inset and Table S2). The decrease in internal resistance is due to the formation of interconnected metallic Zn and Fe nanoparticles in the electrodes, which facilitate electron transport. The decrease in charge transfer resistance is due to the formation of an SEI layer on the electrode, which facilitates the ion transport. Despite the aforementioned advantages to the exceptional lithium ion storage properties, the holey nanoarchitectures may also lead to some unexpected limitations and challenges. For instance, 2D holey ZFO nanosheets tend to consume more electrolytes to form the SEI and suffer from low first-cycle coulombic efficiency (Figure S11 and S12), as they have larger specific surface area and active sites. Moreover, 2D holey ZFO nanosheets provide large contact areas with the electrolyte, which may lead to lower charge transfer resistance. However, the large contact between the 2D holey ZFO nanosheets and electrolyte may also induce the occurrence of unwanted side reaction. CONCLUSIONS In summary, we have demonstrated the controlled synthesis of 2D holey ZFO nanosheets via a facile, confined self-assembly method by using graphene oxide sheets as a sacrificial


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template and Pluronic copolymers as surfactants. By controlling the molecular weights of the Pluronic copolymers from 1100 to 4400 and 12600, the average pore sizes of the nanosheets can be readily tuned from 6 nm to 10 nm and 13 nm, respectively. The as-synthesized nanosheets exhibit facile interfacial transport, shortened ion diffusion paths and excellent mechanical properties for enhanced lithium ion storage properties. The in situ TEM and post-cycling STEM, in combination with the electrochemical characterization, indicate that this nanoarchitecture is a promising material platform for improving electrochemical performance of next-generation lithium-ion batteries. Our findings also demonstrate that the confined self-assembly method can be a promising bottom-up route for the synthesis of holey nanosheets from materials with intrinsically non-layered structures, and may be extended to the fabrication of other holey nanosheet structures beyond transition metal oxides.

EXPERIMENTAL SECTION Preparation of GO nanosheets and ZnFe2O4 (ZFO) nanoparticles. GO is prepared from purified natural graphite by a modified Hummers method.1 ZnFe2O4 (ZFO) nanoparticles were synthesized by using our previously reported method.2 The synthesis of ZFO nanoparticles was done by a precipitation method followed by hydrothermal reaction using DI water as solvent. Briefly, stoichiometric solutions of Zn and Fe were added concurrently to a DI water solution of excess triethylamine in an ice bath. The precipitate was collected, washed, vacuum-dried, and then treated hydrothermally at 220 °C for 12 h. The final sample was washed and vacuum-dried. Confined self-assembly synthesis of holey ZFO nanosheets. The holey ZFO nanosheets was prepared by using graphene oxide (GO) as templates and Pluronic copolymers as


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surfactants. In a typical synthesis, 20 mg Pluronic copolymers were firstly mixed with 20 mg ZFO nanoparticles in 5 mL ethylene glycol (EG). 3 mg GO were dispersed in 5 mL EG. Then the GO-EG dispersion was drop-wisely added into the ZFO-Pluronic mixture. After magnetic stirring the above mixture for 24 hours, the precipitates were centrifuged and washed with ethanol for 4 times. The precipitates were dried in the vacuum oven at the temperature of 50 ˚C. To complete the synthesis of the holey ZFO nanosheets, the as-obtained precursors were annealed in the Lindberg/Blue box furnace at the temperature of 400 ˚C with the ramp rate of 0.5 ˚C/min. To control the hole size of the holey ZFO nanosheets, various Pluronic copolymers with different molecular weights, such as 1100, 4400 and 12600, were used in the synthesis. Cell assembly. Electrochemical measurements of lithium ion battery were carried out in CR 2032 coin cells assembled in an Argon filled glovebox with lithium metal as the anode. The anode was made by mixing the active materials, super P and PVDF (polyvinylidene difluoride), in a weight ratio of 70: 20: 10. The mixture was prepared as slurry and spread onto copper foil. After drying under vacuum at 120 ˚C, the film was cut into circular electrodes with the average mass loading of ~1.0 mg/cm2. The counter electrode was Li metal. The two electrodes were separated by a polymeric material (Celgard 2320). The electrolyte was 1.0 M LiPF6 in a 1:1 ratio of EC (ethylene carbonate) to DMC (dimethylene carbonate). Assembled cells were allowed to soak overnight, and then electrochemical tests were performed. Characterization. X-ray powder diffraction patterns were performed on a Philips X-ray diffractometer (APD 3520) equipped with Cu Kα radiation. SEM and TEM observations were carried out on Hitachi S-5500 scanning electron microspcope (S-5500) and JEOL transmission electron microscope (2010F), respectively. Electrochemical characterization was performed on LAND battery cycler (CT2001A) and Bio-logic potentiostat (VMP3) equipped with impedance


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modules. In situ TEM electrochemical cell was incorporated into a Nanofactory TEM-STM specimen holder in which holey ZFO nanosheets were dispersed onto a TEM half-grid with amorphous carbon support. Li metal was coated onto a piezo-driven W probe as the counter electrode with a thin layer of Li2O formed on Li metal as the solid electrolyte. The Li and ZFO were loaded onto the holder in an Ar-filled glovebox and then transferred to TEM column using a sealed Ar bag to avoid air exposure. During the in situ electrochemical tests, a constant negative dc potential was applied to ZFO electrode against the Li source during the lithiation process, and the lithiation processes were captured by real-time imaging in either TEM mode. To prepare the samples for the post-cycling STEM characterization, the cycled electrodes based on holey ZFO nanosheets were taken out of the coin cell, followed by washing with dimethoxyethane (DME) for several times. The active materials were then taken off from the copper current collector in ethanol by ultrasonic sonication to form a uniform dispersion. Last the dispersion was drop-casted on the copper grid for the STEM characterization.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. TEM images and XRD patterns of ZFO nanoparticles, XRD of holey ZFO nanosheets, STEM images of holey ZFO nanosheets and the precursors, particle size distribution of the holey ZFO nanosheets, pore/particle size measurement details, microtomed TEM images of the porous ZFO nanosheets, additional electrochemical characterization. AUTHOR INFORMATION Corresponding Author


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*[email protected] (G.Y.); *[email protected] (E.T.); *[email protected] (E.S.) Author Contributions ┴

L.P., Z.F. and J.L. contributed equally to this project.

ACKNOWLEDGMENT The authors acknowledge the Center for Mesoscale Transport Properties, an Energy Frontier Research Center from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under award #DE-SC0012673 for financial support on this project. They also acknowledge the Trans-mission Electron Microscopy Facility in the Central Micros-copy Imaging Center (CMIC) at Stony Brook University, Stony Brook, New York for their contribution toward the TEM preparation and data collection. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. A.M.B. acknowledges support from the National Science Foundation Graduate Research Fellowship Program under grant #1109408.

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Two-Dimensional Holey Nanoarchitectures Created by Confined Self

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