Si Nanorods as Efficient Anodes in Micro

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ZIF-8 Cooperating in TiN/Ti/Si Nanorods as Efficient Anodes in Micro-Lithium-Ion-Batteries Yingjian Yu, Chuang Yue, Xionggui Lin, Shibo Sun, Jinping Gu, Xu He, Chuanhui Zhang, Wei Lin, Donghai Lin, Xin Li Liao, Bin-Bin Xu, Suntao Wu, Mingsen Zheng, Jing Li, Junyong Kang, and Liwei Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11287 • Publication Date (Web): 25 Jan 2016 Downloaded from http://pubs.acs.org on January 28, 2016

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ACS Applied Materials & Interfaces

ZIF-8 Cooperating in TiN/Ti/Si Nanorods as Efficient Anodes in Micro-Lithium-Ion-Batteries Yingjian Yu1, Chuang Yue1,4, Xionggui Lin2, Shibo Sun1, Jinping Gu3, Xu He1, Chuanhui Zhang2, Wei Lin1, Donghai Lin3, Xinli Liao3, Binbin Xu2, Suntao Wu1, Mingsen Zheng*2, Jing Li*1,4,5, Junyong Kang1, and Liwei Lin5 1

Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices,

Department of Physics/Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, Fujian, 361005, China. 2

State Key Lab of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian, 361005, China.

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High-field NMR Research Center, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China.

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State Key Laboratory for Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi'an, Shanxi, 710049, China. 5

Department of Mechanical Engineering, University of California, Berkeley, California, 94720, USA

ACS Paragon Plus Environment

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ABSTRACT Zeolite imidazolate framework-8 (ZIF-8) nanoparticles embedded in TiN/Ti/Si nanorod (NR) arrays without pyrolysis have shown increased energy storage capacity as anodes for lithium ion batteries (LIBs). A high capacity of 1650 µAh cm-2 has been achieved in this ZIF8 composited multilayered electrode, which is ~ 100 times higher than the plain electrodes made of only silicon NR. According to the electrochemical impedance spectroscopy (EIS) and 1H nuclear magnetic resonance (NMR) characterizations, the improved diffusion of lithium ions in ZIF-8 and boosted electron/Li+ transfer by the ZIF-8/TiN/Ti multilayer coating are proposed to be responsible for the enhanced energy storage ability. The first principles calculations further indicate the favorable accessibility of lithium with appropriate size to diffuse in the open pores of ZIF-8. This work broadens the application of ZIF-8 to silicon-based LIBs electrodes without the pyrolysis and provides design guidelines for other metal-organic frameworks/Si composite electrodes.

KEYWORDS Lithium ion battery, ZIF-8, silicon nanorod arrays, TiN/Ti layer, metal-organic frameworks

Introduction Given the large surface area and unique channel properties, metal-organic frameworks (MOFs),1 consisting of metal ions/clusters coordinated with organic ligands, have been widely applied in gas storage,2 molecule sensing,3 catalysis,4 and so forth.5 Recently, MOFs also find applications in the fields of clean energy,6 e. g. as adsorbents for hydrogen,7 electrolytes for fuel


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cells,8 electrode materials for supercapacitors9 and platforms for solar cells.10 However, only a few examples of MOFs, such as Zn4O(1,3,4-benzenetribenzoate)2 and copper hexacyanoferrate, have been studied as electrodes for lithium ion batteries (LIBs).11-16 On the other hand, silicon has been intensively investigated both experimentally and theoretically as the electrodes in LIBs or all-solid-state micro-LIBs, due to its highest theoretical capacity of 4200 mAhg-1 with low working potential at 0.5 V, possible integrations with other micro- or nano-electronic devices,1719

and different morphologies, such as nanowires/nanotubes,20-22 and core-shell23 structures to

address the volume expansion24 (>300%) issues during the discharge/charge processes. These aforementioned works usually focused on creating space for Si’s swelling and/or coating a layer, e.g. C or Ge, to realize smaller volume expansion during cycling.25, 26 However, fully addressing the deterioration of the electrode structures caused by the volume expansion, which would generally results in performance fading and safety problem especially when relatively highdensity current imposed unexpectedly in micro/nano-electro-mechanical systems (M/NEMS), is still a challenge issue. Zeolite imidazolate frameworks (ZIFs) , as a new type of MOFs with large surface areas and good stability in thermal and chemical properties, exhibit promising applications in catalyst,27 gas sensor,28 and so forth. Among them, ZIF-8, which has apertures in size of 3.4 Å, just larger than the diameter of Li atom (~ 3.0 Å) or Li ion (~ 1.5 Å), is structurally suitable for the lithium ion diffusion process. Moreover, by compositing with other materials, the specific channels and apertures in ZIF-8 would be of significant help to avoid exceeding and/or nonuniform lithium insertion into the electrode materials, so that the fast fading of the capacity and/or even the damage of electrodes or the whole M/NEMS devices may be effectively prevented. According to


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the up-to-date publications, the composite of ZIF-8 with Si nanostructure applied in micro-LIBs has not been reported yet. Here, this work investigates the ZIF-8 and silicon composite as anodes in micro-LIBs. In order to increase the energy capacity, TiN/Ti/Si nanorod (NR) arrays are chosen as the base electrodes produced by

the nanosphere lithography (NSL) and subsequent dry etching process,29,

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followed by a solution growth using modified method for ZIF-8 coating without pyrolysis.31, 32, 33 As seen in Scheme 1, the ZIF-8/TiN/Ti/Si-NR anode has ~ 100, 4, and 8 times higher capacity and Coulombic Efficiency (CE) than other Si-based anodes in different configurations including only Si-, ZIF-8/Si-, TiN/Ti/Si-. Furthermore, the EIS results reveal the lowest charge transfer resistance in the ZIF-8/TiN/Ti/Si-NR electrodes. In order to investigate the fundamental mechanisms of the high energy storage capacity of ZIF-8 based composite electrodes, first principles calculations and various experimental characterizations have been performed with the focus on the lithium ion diffusion processes.

EXPERIMENTAL SECTION Methods. Fabrication of ZIF-8/Si and ZIF-8/TiN/Ti/Si NRs. As shown in Scheme 2, the Si NR arrays were prepared by the polystyrene (PS) template method and subsequent inductive coupled plasma (ICP) etching as referred to our previous work.29, 34 The Si wafers as substrates were purchased from Shanghai Guang Wei Electronic Materials Co., Ltd. China having the thickness of ~ 300 µm and the conductivity of ~ 2000 S/m. After the fabrication, the Si NR arrays were soaked in a 50 mL 2-methylimidazole solution (0.045 M, 2.25 × 10-3 mol) followed by the addition of a 50 mL Zn(Ac)2•2H2O solution (0.13 M, 6.5 × 10-3 mol) and then kept at room temperature for 24 h to prepare the ZIF-8/Si NRs. To produce the ZIF-8/TiN/Ti/Si NRs, the layer


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of TiN was firstly sputtered on the Si NRs’ surface using radio frequency (RF) magnetron sputtering (JS3X-100B) at the power of 200W for 14 min, and the thickness is ~ 60 nm; subsequently, the Ti layer was sputtered on the TiN/Si NRs’ surface at the power of 250W for 3 min, and its thickness is ~ 20 nm. Then the ZIF-8/TiN/Ti/Si NRs were fabricated by immersing the TiN/Ti/Si NRs into the 2-methylimidazole and Zn(Ac)2•2H2O solutions as mentioned above. After the growth of ZIF-8, the composite NRs were washed by methanol followed by stoving. Characterization. The morphologies of the ZIF-8/Si and ZIF-8/TiN/Ti/Si NRs were researched utilizing the SU70 thermal field emission scanning electron microscope (SEM). The operating voltage for SEM is 5-15 kV and the working distance (WD) is 5 mm. The elemental analyses were conducted by an energy dispersive X-ray (EDX) spectrum analyzer via Tecnai F30 fieldemission transmission electron microscope (TEM). The high-resolution (HR) TEM images of the composite NRs were obtained by JEM-2100 HRTEM. The accelerating voltage for TEM is 200 kV. For the electrochemical testing, the anodes materials were prepared by cutting the fabricated Si substrates into 10 × 10 mm2 pieces, and the current collector was prepared with refer to our previous procedures,29, 34 except for the sputtering time for Au deposition was changed to 7 min. The assembling of ZIF-8/Si or ZIF-8/TiN/Ti/Si composite NR pieces in coin cells (2025) is the same with previous measurements.29,

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Meanwhile, the Si and TiN/Ti/Si NRs pieces were

assembled as comparisons. After aging for 12 h, the cycling and rate properties of the corresponding electrodes were investigated by a Land battery test system at room temperature. The Alternative Current (AC) impedance of the electrodes was measured by Solartron 1260 frequency response analyzer within the frequency range of 1 MHz to 100 Hz. It is well known that in micro-LIBs energy or power densities in a unit area or volume are generally considered.35-


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So, in this work, the areal capacity (µAh cm-2), defined as the discharge/charge capacities

(µAh) divided by the electrode area (1 cm × 1 cm = 1 cm2), was applied to demonstrate the lithium storage ability of the composite electrodes. To prepare the samples for the NMR analysis, the ZIF-8 composited Si NRs before/after discharging were scraped off from the Si substrates, and then dissolved in 500 µL CDCl3 solvent. Theoretical calculations. The first principles methods on the basis of the density functional theory (DFT) were conducted to compute energy variations when Li atom diffuses into the ZIF-8 unit cell. The local density approximation (LDA) and projector-augmented waves (PAW) in Vienna ab-initio simulation package (VASP) code were employed. The 4 × 4 × 4 k-point grids with the plane-wave basis set at 350 eV as the cutoff energy were utilized. RESULTS AND DISCUSSION

1. The Structures of ZIF-8/TiN/Ti/Si NRs Compared with Si NRs’ horizontal size (the distance between two parallel edges of the hexagon) of ~ 120 nm as seen in Figure S1a and b, the average size of TiN/Ti/Si NRs increases to ~ 150 nm after the deposition of the TiN/Ti film, as displayed in Figure 1a and b. After the solution growth of ZIF-8 in the same experimental conditions with those of ZIF-8/Si NRs (Figure S1c and d), the multilayered ZIF-8/TiN/Ti/Si NRs, as shown in Figure 1c, d and Figure S2, were fabricated with the nanoparticles either coherent to the TiN/Ti/Si NRs’ sidewalls or between the NRs. Meanwhile, the TEM characterizations of the multilayered ZIF-8/TiN/Ti/Si NR is presented in Figure 1e, in which the (100) monocrystalline Si inner core is evidently coated by the polycrystalline TiN/Ti layer with TiN (200) and (220) planes resolved according to the crystalline lattices in the corresponding HRTEM image and SAED pattern in the inset of Figure 1f. An individual intact ZIF-8/TiN/Ti/Si NR of ~ 1.4 µm in length and its corresponding


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elemental mappings were also characterized as seen in Figure 1g, identifying the Si, Ti, N, and Zn elements and further evidencing the successful fabrication of the multilayered ZIF8/TiN/Ti/Si NRs. The relative quantities of those elements can be well resolved in the line scanning analysis as shown in Figure 1h. The content of Si increases from the neck to the body, indicating the nature of the bottle-like Si NR; the high contents of Ti and N in the top part and notably more content of Ti than N show the main distribution of TiN with the feature of the RF sputtering method and the existence of Ti layer, respectively; the quantity of Zn is relatively small but uniform, indicating the formation of thin ZIF-8 layer via solution growth process. Further characterizations of the ZIF-8/Si composite NRs and commercial ZIF-8 powder as a reference are detailed in Figure S3-S5 and S6a and b. The elemental mapping, X-ray diffraction (XRD) pattern38 and Raman spectrum39 all evidence the successful growth of ZIF-8 around Si NRs.

2. The electrochemical performances of Si NR, TiN/Ti/Si, ZIF-8/Si and ZIF8/TiN/Ti/Si NR electrodes In order to investigate the interaction between ZIF-8 and lithium ion and electrochemical effect of the ZIF-8 on the composite electrodes, cyclic voltammetry (CV) measurements were conducted, including on Si NR, TiN/Ti/Si, ZIF-8/Si and ZIF-8/TiN/Ti/Si NR electrodes, at a scan rate of 0.5 mV s-1. As shown in the CV profile of Si NRs electrode in Figure 2a, the redox couple at 0.2/0.6 V is related to the Li insertion/extraction into/from the Si active material.40 Meanwhile, during the first 10 cycles the magnitudes of the current peaks increase one by one slightly, indicating an insufficient activation process. Refereeing to Figure 2b, the TiN/Ti/Si NR composite exhibits larger current and higher activation degree than Si NRs as electrodes,


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indicating the positive effect of TiN/Ti layer during lithiation/delithiation processes.41 While, with the cooperation of ZIF-8, the ZIF-8/Si nanocomposite electrode also exhibits the enhanced current and obvious activation effect in corresponding cycles, as shown in Figure 2c. The double peak response at about 0.4 V and 0.6 V in the oxidation branch is probably affiliated to two phases during the delithiation process – Li’s partial and thorough extraction from LixSi alloy. Figure 2d shows the electrochemical response of the multilayered ZIF-8/TiN/Ti/Si NR composite, in which a reduction peak can be evidently observed at ~ 0.1 V along with the largest current compared with other electrodes, revealing a high level of lithiation into the ZIF-8 and Si composite. To discern the peaks assigned to the interaction between ZIF-8 and lithium ion, the first cycle and parts of the first five cycles of the ZIF-8/Si NR electrodes’ CV curves are illustrated individually in Figure 2e and f. Apparently, the two redox couples located at 0.8/1.0V and 1.8/2.0V are attributed to the lithiation/delithiation and/or irreversible processes of ZIF-8 since which are absent in only Si NR electrode during the corresponding cycles. The effects of ZIF-8 are also embodied in the capacities during cycling in the composite electrodes tested by galvanostatic discharge and charge measurements within the voltage window from 0.13 V to 2.0 V vs. Li/Li+, as shown in Figure 3a. Compared with Si and TiN/Ti/Si NRs anodes, ZIF-8/Si composite electrode presents a much higher capacity of ~ 350 µAh cm-2. Reasonably, the improved performance in the composite electrode is believed to be induced by the ZIF-8 cooperation, which would help to hinder the close contact between the Si surfaces and electrolyte. Of course, the efficient interacting of ZIF-8 with Li ions may contribute to the capacity enhancement as well. Meanwhile, the ZIF-8 nanoparticles between Si NRs will definitely enlarge the active surface area to create more adsorption sites. Moreover, the uniform channels in ZIF-8 structure itself may also assist to realize a more homogeneous lithiation


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process and restrict the volume expansion of Si NR due to its appropriate aperture size of 3.4 Å (larger than the diameter of lithium ion or atom) and framework flexibility.42 Thus, the high capacities and stable configuration after cycling can be expected, and as a result electrode safety at higher current density or for longer working lifetime can be foreseen. Evidently, as the post morphologies of ZIF-8/Si NRs anodes after 50 cycles shown in Figure S6a, the configurations are well maintained with the ZIF-8 still coating outside the Si NR as seen in TEM images of Figure S7b and c. The XRD pattern of the cycled ZIF-8/Si NRs anodes shown in Figure S8 indicates that the lithiation/delithiation processes may result in the amorphization of ZIF-8 to some extent.43, 44 Also, the stable cycling performance of the commercial ZIF-8 powder anode, as shown in Figure S9, in certain degree can prove the electrochemical stability of ZIF-8 active material in the composite electrode. The highest capacity of ~ 1650 µAh cm-2 was presented in the multilayered ZIF-8/TiN/Ti/Si composite NR electrode among all the samples even after 40 cycles with a fast activation process, which is suggested to be due to the amorphization in crystalline Si electrodes.29 Therefore, besides the contribution of ZIF-8 itself, the introduction of a conducting layer TiN/Ti also plays an important role to further improve the capacity of the composited electrodes by reducing the charge transfer resistance in the electrode.45,

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This is well revealed in the electrochemical

impedance spectroscopy (EIS) results shown in Figure 4 and its inset, in which the ZIF8/TiN/Ti/Si composite NR electrode exhibits the lowest charge transfer resistance, indicating the enhanced electron/Li+ transfer by the TiN/Ti layer/ZIF-8 coating.46-48 By the way, there is a discrepancy of the capacities in our work compared with the recent result (1752 µAh cm-2) by Han et al49, which may result from the differences in the effective mass of the active materials, impedance of the substrates and voltage windows applied in the cycling measurements.


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In addition, the variation of the areal capacity of the ZIF-8/Si NRs electrode along with the changing of current densities can be found in Figure 3b. The rate dependent Coulombic Efficiency (CE) decreases from ~ 75% to ~60 % in the first 20 cycles, which is probably due to some lithium ions’ exhaustion by some irreversible active sites as inferred from the CV profile of Figure 2f, and then climbs up to ~ 98% at the 45th cycle.50 Apparently, the CE of the ZIF-8/Si NRs electrode is not satisfactory; however, this issue can be addressed by the introduction of TiN/Ti layer to the composite NR electrodes, as shown in the Figure 3c. Excitingly, with the current density increasing from 20 µA cm-2 up to even 1000 µA cm-2, the ZIF-8/TiN/Ti/Si composite NR electrode still maintain its high capacities, which are higher than both of the ZIF8/Si NRs and TiN/Ti/Si NRs anodes. Comparing with the CE of ~ 85 to 99% and ~ 70 to 98% in the TiN/Ti/Si and ZIF-8/Si NR anodes, an stable as high as ~ 99% CE was maintained in the ZIF-8/TiN/Ti/Si composite NR electrode, which is quite promising compared to other recent works on Si-based anodes.34, 51 Understandably, the employment of TiN/Ti layer, which could efficiently provide electrons to the active materials,46 would help to increase the electrode conductivity thereby assisting Li+ ion intercalation, and limit the growth of the SEI on the surface of Si NRs,45 thus improving the overall CE. It is worthy to notice that a significant activation phenomenon still happens after rate cycling for 60 cycles, which is also disparate to our previous work on the Ge/Si system.34 After the rate test till 80th cycle, a subsequent cycling measurement was further conducted in the ZIF-8/TiN/Ti/Si NRs electrodes, as displayed in Figure S10, in which a satisfied capacity retention rate of ~ 90% still can be maintained. Moreover, as identified by the SEM and TEM characterization results shown in Figure S11 and S12, it can be found that the configuration stability of the ZIF-8/TiN/Ti/Si composite NR electrode was still well maintained. Also, the SEI film which formed after the cycling of ZIF-


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8/TiN/Ti/Si NRs electrodes can be visualized in Figure S13a with its main component of Li2CO3, identified by the corresponding XRD pattern in Figure S13b. As a result, the unique structure of ZIF-8 helps to fully activate the ZIF-8/Si NRs electrode to achieve a high capacity, and the introduction of the TiN/Ti layer further realizes its substantial improvement of the cycling and rate properties.

3. The lithiation process in ZIF-8 To further investigate the interactivity between ZIF-8 and lithium ions, one-dimensional 1H nuclear magnetic resonance (NMR) spectra were measured by a Bruker Advance III, 850 MHz spectrometer, as shown in Figure 5 and S14-15. The variations of the peaks and two-dimensional heteronuclear single quantum coherence (HSQC) spectra reveal the differences of the bonds in the composite NRs before and after the charge/discharge processes. Those up-to-date first evidences consistently characterized by both electrochemical and NMR methods demonstrate that there may be the interaction between ZIF-8 and Li+,52 which would definitely contribute to the higher capacities of ZIF-8/TiN/Ti/Si and ZIF-8/Si composite NR electrode than that in only Si NR anodes. Further work is still needed to investigate the specific interaction in more details. Furthermore, with the aim to understand the lithium diffusion process in ZIF-8 in an atomic perspective, the ab initio first principles method, frequently utilized to investigate Li diffusion in Si or other materials and structural/electronic properties of ZIF-8 in recent years,22, 26, 34, 53, 54 was also utilized in this work. As illustrated in Figure 6, when diffusing in the channels of ZIF-8 from the site a to f (corresponding unit cells were illustrated in Figure S16), a Li atom would encounter barriers of ~ 0.29 eV with the relatively stable site c located near the center of the pore. This barrier energy is much lower than that in Si (~ 0.60 eV),55,


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indicating a faster

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diffusion velocity and better electrochemical performances. This theoretical calculation will help to understand the mechanism of much higher capacities presented in ZIF-8 composited TiN/Ti/Si and Si NRs anodes rather than in only Si NRs electrode itself.

CONCLUSIONS In conclusion, ZIF-8/Si composite anodes were initially fabricated without the pyrolysis to demonstrate the efficient lithium ion interaction/diffusion effects of ZIF-8 on their electrochemical performances. Due to effectively hindering in the formation of SEI film on the Si NR, creation of more lithium adsorption sites, promotion of a homogeneous lithiation process and restriction of the volume expansion of Si induced by the ZIF-8 cooperation, an up to 500 µAh cm-2 capacity were achieved in the composited anode. In addition, with the employment of a buffer layer of TiN/Ti to modify the Li+/electronic conductivity of the interfaces between Si inner core and outer ZIF-8, a further enhanced capacity of ~ 1650 µAh cm-2 was realized accompanied with the up-to-date highest Coulombic Efficiency (CE) of ~ 99%. For the first time, the interaction between ZIF-8 without the pyrolysis and lithium was experimentally investigated by both the electrochemical characterizations and 1H NMR measurements. The first principle calculations further suggest that the channels of ZIF-8 are favorable for the lithium diffusion. Besides the ZIF-8/TiN/Ti/Si series NRs, this work also provides experiences for applications of other MOFs in electrode materials for LIBs or all-solid-state micro-LIBs, to achieve higher capacities and durable working lifetime.

ASSOCIATED CONTENT


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Supporting Information.

Additional supplementary figures of the morphology and

electrochemical characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (J. L.) [email protected] (M. Z) ACKNOWLEDGMENT The authors would like to present gratitude to Prof. Yi Cui (Stanford University) and Prof. Nanfeng Zheng (PCSS, Xiamen University) for the in-depth discussion with them and their constructive advices. This work was financially supported by National Basic Research Program of China (2015CB932301), Science and Technology Project of Fujian Province of China (2013H0046), Project (sklms2015005) of State Key Laboratory for Manufacturing Systems Engineering (Xi’an Jiaotong University), and the CSC (China Scholarship Council) scholarship under the State Scholarship Fund. REFERENCES (1) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Design and Synthesis of an Exceptionally Stable and Highly Porous Metal-Organic Framework. Nature 1999, 402, 276-279. (2) Eddaoudi, M.; Kim, J.; Rose, N.; Vodak, D.; Wachter, J.; O’Keefee, M.; Yaghi, O. M. Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage. Science 2002, 295, 469-472. (3) Kreno, L. E.; Hupp, J. T.; Van Duyne, R. P. Metal-Organic Framework Thin Film for Enhanced Localized Surface Plasmon Resonance Gas Sensing. Anal. Chem 2010, 82, 8042-8046.


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Scheme 1. The schematic capacities of Si, TiN/Ti/Si, ZIF-8 and ZIF-8/TiN/Ti/Si NRs as anodes for LIBs.


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Scheme 2. Illustration of the fabrication processes for the ZIF-8/Si NR arrays: (i) modified ICP etching employing SF6/C4F8 gases to fabricate the bottle-like Si NR arrays, (ii) template removal by the ultrasonic cleaning, and (iii) growth of ZIF-8 around the Si NR arrays by a solution method.


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Figure 1. Top-view and section-view SEM images of (a and b) TiN/Ti/Si NRs, and (c and d) ZIF-8/TiN/Ti/Si NRs; (e) TEM image of an individual ZIF-8/TiN/Ti/Si NR; (f) HRTEM images of the interface between Si and TiN/Ti with the inset of SAED patterns showing the (200) and (220) planes of TiN polycrystalline; (g) dark-field TEM image of the ZIF-8/TiN/Ti/Si NR with corresponding EDX elemental mappings of Si K line (i), Ti K line (ii), N K line (iii) and Zn K line (iv); (h) line scanning analysis of the Si, Ti, N and Zn elements along the three parts (top, neck and body) of the ZIF-8/TiN/Ti/Si NR.


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Figure 2. Cyclic voltammograms curves of (a) Si NRs, (b) TiN/Ti/Si, (c) ZIF-8/Si and (d) ZIF8/TiN/Ti/Si NRs electrodes in first ten cycles within the potential window between 2.8 V to 0.01V versus Li/Li+ at a 0.5 mV s-1 scan rate; (e) The first cycle and (f) enlargements of the first five cycles of the ZIF-8/Si NR electrodes’ CV curves.


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Figure 3. (a) Discharge capacities of Si NRs, TiN/Ti/Si NRs, ZIF-8/Si and ZIF-8/TiN/Ti/Si NRs electrodes under the current density of 10 µA cm-2 within the voltage window from 0.13 V to 2.0 V vs. Li/Li+ till 30th cycles; (b) Rate capabilities of the ZIF-8/Si NRs electrode at various current densities with its corresponding Coulombic Efficiency. (c) Rate capabilities of the TiN/Ti/Si NRs and ZIF-8/TiN/Ti/Si NRs electrodes at various current densities (initially at 20 µA cm-2 from 1st to 20th cycle, and back to 20 µA cm-2 after 60th cycle).


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Figure 4. AC impedance spectra of cycled Si, TiN/Ti/Si, ZIF-8/Si and ZIF-8/TiN/Ti/Si NRs at 40th cycle, with its inset showing that of the ZIF-8/TiN/Ti/Si NRs alone.


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Figure 5. (a) The C=C bonds and (b) methyl groups 1H NMR spectra in 2-methylimidazole rings of ZIF-8/Si NRs before discharging processes and after discharging for 50 cycles. (CDCl3 was used as the solvent.)


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Figure 6. Density functional theory (DFT) calculations of the relative system energies upon Li diffusion in the channel of the ZIF-8. (H atoms are not shown here.)


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Table of Contents Graphic


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Si Nanorods as Efficient Anodes in Micro

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