Anhydrous Liquid-Phase Exfoliation of Pristine Electrochemically

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Anhydrous Liquid-Phase Exfoliation of Pristine Electrochemically-Active GeS Nanosheets David Lam, Kan-Sheng Chen, Joohoon Kang, Xiaolong Liu, and Mark C Hersam Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04652 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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Chemistry of Materials

Anhydrous Liquid-Phase Exfoliation of Pristine Electrochemically-Active GeS Nanosheets David Lam†, Kan-Sheng Chen†, Joohoon Kang†, Xiaolong Liu‡, and Mark C. Hersam*,†,‡,! † Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208– 3108, USA ‡

Applied Physics Graduate Program, Northwestern University, Evanston, Illinois 60208-3112, USA

!

Department of Chemistry, Department of Electrical Engineering and Computer Science, Evanston, Illinois 60208–3108, USA

ABSTRACT: Germanium sulfide (GeS) is an emerging layered material with high promise in its two-dimensional (2D) exfoliated form for energy storage applications. While liquid-phase exfoliation (LPE) has been utilized for the low-cost, scalable production of related 2D materials, it has not yet been demonstrated for GeS nanosheets due to its chemical instability in ambient conditions. Here, GeS LPE is achieved in anhydrous N-methyl-2-pyrrolidone using a customized sealed-tip sonication system, yielding sub-10 nm thick GeS nanosheets that are structurally pristine with minimal chemical degradation as revealed by atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. Lithium-ion battery anodes based on these high-quality GeS nanosheets possess superlative electrochemical performance including high cycling stability over 1,000 cycles and high rate capability in excess of 10 Ag-1. Overall, this work establishes a scalable LPE pathway for the production of pristine electrochemically-active GeS nanosheets that are well-suited for high-power lithium-ion battery applications.

INTRODUCTION Prior research on layered materials, such as graphene,1–4 transition metal dichalcogenides (TMDCs),5–11 and black phosphorous (BP),12–14 have established the unique properties of twodimensional (2D) materials and their vast potential for electronic, optical, and energy applications. As a layered IV-VI semiconducting compound that is isostructural to BP, germanium sulfide (GeS) has been studied in its bulk layered form,15,16 but is a recent addition to the family of 2D materials.17 In addition, germanium-based materials such as GeS are ideal for lithium-ion (Li-ion) batteries due to germanium’s high theoretical capacity of 1620 mAhg-1.18 This value is especially impressive compared to graphite, a common commercial Li-ion battery anode material, which has a theoretical capacity of 372 mAhg-1.19 Furthermore, theoretical calculations predict that GeS has a room temperature Li+ diffusion coefficient 170 times greater than graphene,20 and extremely low diffusion barriers for alkali metal ions (0.236 eV for Li+, 0.09 eV for Na+ 0.05 eV for K+, compared to 0.33 eV for Li+ in graphene), indicating possible applications for Na- and K-ion batteries.21 Experiments have also demonstrated the effectiveness of GeS nanoparticles18 and nanoarchitectured GeSe22 as the active materials in Li-ion battery anodes. Other layered materials have shown improved electrochemical capacity retention in the 2D limit; for example, MoS2 exhibits capacities of over 800 mAhg-1 for both bulk and

exfoliated anodes, but after 50 cycles, the capacity of bulk MoS2 decreases to 226 mAhg-1 while exfoliated MoS2 retains high capacities over 750 mAhg-1.23 It can be anticipated that GeS and other layered materials may show similar benefits for Li-ion battery anodes in the exfoliated limit.24–26 In addition to possible electrochemical applications, GeS is a promising material for photovoltaics and photodetectors due to a semiconducting band gap of ~1.6 eV27 and high photoresponsivity of ~200 AW-1.28,29 Thin layers of GeS would further optoelectronic applications by opening up opportunities for gate-tunability.30–32 Despite the significant technological potential for GeS nanosheets, scalable production of 2D GeS through liquidphase exfoliation (LPE) has not yet been achieved due to a variety of challenges. Chemical degradation of GeS in ambient conditions makes exfoliation of pristine GeS sheets difficult. Furthermore, the ratio of intralayer to interlayer force constants in GeS is ~20,33 which indicates a much stronger interlayer coupling than that of MoS2 (~70) and graphene (~1,000). While multiple reports have demonstrated solution-phase synthesis of GeS,27,29 this bottom-up approach requires prolonged, hightemperature processing conditions that limit scalability. Similarly, alternative top-down methods such as micromechanical exfoliation28 are hindered by exceptionally low yields and throughput.

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Figure 1. (A) Schematic of the sealed-tip ultrasonication apparatus. Teflon tape is wrapped around the lip of the centrifuge tube and Parafilm is wrapped around the cap and tip to minimize exposure to ambient conditions. (B) Structural model of GeS, viewed along the c axis. GeS is orthorhombic, with lattice parameters of a = 4.30 Å, b = 10.47 Å, and c = 3.64 Å.27 (C) GeS exfoliated in various solvents and then centrifuged at 1,000 rpm for 10 minutes. (D) Concentration of exfoliated powder GeS as a function of solvent surface tension after centrifugation at 1,000 rpm for 10 minutes. The concentration of NMP-exfoliated crystal GeS is plotted as a reference. (E) Raman spectra of a bulk GeS crystal and a film of solution-processed GeS.

In contrast, LPE has been utilized to provide high-yield, scalable production of electronic-grade 2D materials.34–36 Furthermore, LPE dispersions are amenable to centrifugation-based post-processing for lateral-size separation37 or thickness sorting,3,8,10 resulting in highly monodisperse nanosheet size distributions.38 Here, an efficient LPE approach is introduced for 2D GeS nanosheets based on ultrasonication in anhydrous N-methyl-2-pyrrolidone using a customized sealed-tip sonication system. The resulting GeS nanosheets possess high structural and chemical integrity with superlative electrochemical properties, particularly enabling exceptional capacity retention and high-rate performance in Li-ion battery anodes.

RESULTS AND DISCUSSION To avoid degradation of chemically reactive GeS during LPE, a sealed ultrasonication apparatus similar to that introduced by Kang et al.13 is employed to limit ambient exposure, as shown in Figure 1A. Ultrasonication shears the GeS crystal along the b-plane (Figure 1B), resulting in thin nanosheets. To optimize the choice of solvent used in exfoliation, GeS powder is exfoliated in solvents of different surface tensions (Ethanol: 22.1 mN/m, Acetone: 25.2, Isopropanol (IPA): 23, Hexane: 18.43, Chloroform: 27.5, DMF: 37.1, anhydrous N-Methyl-2pyrrolidone (NMP): 40.79). Specifically, 7.5 mg of GeS powder and 15 mL of solvent of are placed in the apparatus and sonicated in an ice bath at ~50 W for 1 hr and are subsequently centrifuged at 500 or 1,000 rpm for 10 minutes (see Supporting Information for experimental details).

Out of the solvents studied, NMP results in the darkest dispersion post-centrifugation, suggesting its suitability in efficiently stabilizing exfoliated GeS (Figure 1C). This observation is quantified via UV-vis spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS) measurements. ICP-MS is used to measure the concentration of GeS in a given solvent, then related to the absorbance measured in UV-vis spectroscopy via the Beer-Lambert Law ( , where A is the absorbance, ε is the molar extinction coefficient, L is the path length, and c is the concentration). Using an estimated ε600 nm = 2,640 L g-1 m1 and absorbances measured from UV-vis spectra, the concentrations of GeS exfoliated in different solvents and subsequently centrifuged at 1,000 rpm for 10 minutes are calculated and plotted as a function of solvent surface tension in Figure 1D, demonstrating that powder GeS exfoliated in NMP yields the highest concentration out of all of the other solvents.

After determining that NMP is the optimal solvent to exfoliate GeS, 7.5 mg of GeS crystal and 15 mL of anhydrous NMP are exfoliated with the same sonication parameters as in the solvent stability study. The structural integrity of crystals exfoliated this way are characterized using Raman spectroscopy (Figure 1E). GeS nanosheets show peaks that correspond to the A3g (112 cm-1), B3g (213 cm-1), A1g (240 cm-1), and A2g (270 cm-1) modes of the parent crystal.39 The B3g mode corresponds to the in-plane shear vibration of parallel layers in the zigzag direction (the c-axis in Figure 1B), while the Ag modes correspond to


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Chemistry of Materials shear vibration of parallel layers in the armchair direction (aaxis). The positions of the Raman peaks in the exfoliated GeS agree well with those of the parent crystal with similar peak widths, which suggests that exfoliation does not introduce significant structural degradation. Following ultrasonication, the resulting dispersion is characterized via AFM and TEM for thickness and lateral length histograms, respectively. AFM samples are prepared by drop-casting the resulting solution on a SiO2/Si substrate (Figure 2A) while TEM samples are prepared by depositing a drop on a copper grid with a Formvar/carbon film (Figure 2B). The resulting histograms (Figure 2C-2F) are lognormally distributed, as predicted in literature for LPE.40,41 Each histogram is fit to the probability density function of the form / √

where y0 is a constant offset, A is a constant

prefactor, x is either the thickness or length, μ is the lognormal mean, and σ is the lognormal standard deviation. For the asprepared (AP) dispersion, μThickness, AP = 6.44 nm and μLength, AP = 116.94 nm. Following centrifugation at 500 rpm for 10 minutes, the lognormal mean for the thickness and lateral length decrease to μThickness, 500rpm = 2.87 nm and μLength, 500 rpm = 66.91 nm, indicating that the larger nanosheets are sedimented out of solution

due to centrifugation. Inductively coupled plasma mass spectrometry (ICP-MS) measurements measure a GeS concentration of ~0.28 mg mL-1 immediately following sonication and ~0.11 mg mL-1 after centrifugation at 500 rpm for 10 minutes. To verify that the resulting GeS nanosheets do not chemically degrade during exfoliation, X-ray photoelectron spectroscopy (XPS) was conducted on both the as-sonicated and centrifuged dispersions. Figure 3A shows the Ge 3d XPS spectra, with a doublet assigned to binding energies of 29.7 eV and 30.3 eV for the 3d5/2 and 3d3/2 GeS peak, respectively, and a singlet assigned to 31.2 eV for the GeO peak. The GeS doublet indicates that the solution-processed GeS is of high chemical quality (see Figure S3 for comparison with XPS on the bulk crystal), with the smaller GeO peak attributed to surface oxide that is removed after sputtering (Figure S5). Similarly, the S 2p peaks (Figure 3B) confirm the presence of GeS, with a GeS doublet assigned to 161.2 eV and 162.4 eV for the 2p3/2 and 2p1/2 peaks, respectively, and an oxide-containing doublet assigned to 161.8 eV and 162.9 eV. Centrifugation does not noticeably alter the shape of the spectra, indicating that the refined dispersion remains primarily pristine GeS as verified by XPS quantitative analysis (Table S2).

Figure 2. (A) AFM micrograph of nanosheets drop-cast onto a 300 nm SiO2/Si substrate. (C) Thickness and (D) lateral size histograms of the as-exfoliated GeS nanosheets. (E) Thickness and (F) lateral size histograms of the centrifuged GeS nanosheets. Thickness histograms are obtained via AFM while lateral length histograms are generated via statistical TEM analysis. Insets in (C) and (E) are photographs of the dispersions for as-prepared and 500 rpm solutions respectively.



To demonstrate these the LPE-processed GeS nanosheets are chemically pristine, they were incorporated into Li-ion battery anodes by combining them with polyvinylidene fluoride (PVDF) and carbon black (CB) in an 8:1:1 weight ratio. Figure 5A shows the electrochemical voltage profile of the resulting GeS/PVDF/CB anode cycled at 0.2 C, where 1 C is defined as the current density needed to charge up to the first charge capacity in an hour (~1,100 mAg-1). The first discharge and charge capacities are 2295 mAhg-1 and 1166 mAhg-1 respectively, corresponding to an initial Coulombic efficiency of 51% from the irreversible conversion of GeS into Ge and Li2S.18 After 10 cycles, the charge retention stabilizes with discharge and charge capacities of 981 mAhg-1 and 933 mAhg-1, respectively, and a Coulombic efficiency of 95%, which corresponds to reversible Li insertion into Ge. Figure 5B shows the long-term cycling stability where the charge capacity of the GeS/PVDF/CB anode is allowed to stabilize at 0.5 C, after which it is run at 1 C and finally at 1.5 C. The specific capacity at 1.5 C does not decrease with increasing cycle number and stabilizes at a value of 769 mAhg-1 at 1,000 cycles. Figure 5C further shows the specific capacity as a function of cycling rate. As the cycling rate is increased to 10 C, the GeS anode still maintains a high specific capacity of 231 mAhg-1, illustrating the viability of GeS nanosheet anodes for high current density operation. This superlative performance is likely due to lower energy barriers to diffusion21 as well as the shorter lateral distances needed for lithium ions to diffuse into GeS nanosheets.42 In addition, after operating the anode at high currents, the battery recovers to its stabilized value at 0.2 C. As shown in Figure 5D, these metrics compare favorably with other LPE or chemically exfoliated 2D materials in Li-ion battery anodes (with more in-depth information in Table S1),23,24,43–50 particularly showing much higher rate performance. The excellent charge retention and cycling stability of the LPE-processed GeS nanosheets Li-ion battery anodes stem from the nanostructured and electrochemically pristine nature of the material. Lower-dimensional materials such as GeS nanosheets demonstrate better performance than their bulk counterparts due to larger surface-volume ratios and shorter Li+ diffusion lengths.21

Figure 3. XPS of LPE-processed GeS nanosheets for the core levels of (A) Ge 3d and (B) S 2p. The as-exfoliated material shows a small oxide peak, which does not increase substantially following centrifugation.

TEM further confirms the crystalline structure of LPEprocessed GeS. Figures 4A,B show a high-resolution TEM (HRTEM) micrograph of a GeS nanosheet and the corresponding GeS crystal lattice viewed along the out-of-plane direction. A selected-area electron diffraction (SAED) pattern was also collected (Figure 4C), showing sharp diffraction spots that are consistent with crystalline GeS.

Figure 4. (A) HRTEM micrograph, (B) atomic structure, and (C) SAED pattern of a GeS nanosheet along the out-of-plane direction. The highly ordered crystal lattice and sharp diffraction pattern indicates structural integrity of the nanosheets. Secondary spots in the SAED pattern come from restacked flakes.

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Chemistry of Materials

Figure 5. (A) Voltage profile of the LPE-processed GeS anode at 0.2 C. (B) Specific capacity as a function of cycle index. The capacity stabilizes within 10 cycles. (C) Specific capacity as a function of cycle rate after stabilization. The electrode maintains a high specific capacity (231 mAh g-1) at 10 C, showing excellent rate capability. (D) Literature comparison with other LPE or chemically exfoliated 2D materials serving as the active material in Li-ion battery anodes. Li-ion battery anodes based on LPE-processed GeS show exceptional rate performance with high capacity retention. Furthermore, prior work on related 2D nanomaterials such as SnS51 and SnS252 demonstrate that the facile formation of a Li2S interfacial layer leads to high ionic conductivity for following cycles. It has been shown that the efficient formation of this layer in GeS nanoparticles is highly dependent on the crystallinity of the material.18 Thus, the structurally and electrochemically pristine, LPE-processed GeS nanosheets result in Li-ion battery anodes that exhibit excellent performance even at high cycling rates and cycle number. In conclusion, we have demonstrated an effective method for LPE of pristine GeS nanosheets. Using a sealed-tip ultrasonication system, exfoliation of GeS is achieved in anhydrous

NMP with minimal exposure to ambient atmosphere, thus preventing chemical or structural degradation. The chemical and structural integrity of the resulting nanosheets is confirmed by AFM, Raman, XPS, and TEM, revealing sub-10 nm thick GeS nanosheets with excellent crystallinity. Superlative electrochemical performance is verified by incorporating the GeS nanosheets into Li-ion battery anodes, resulting in high cycling stability up to 1,000 cycles and excellent rate capability up to 10 C. Overall, this work establishes a scalable pathway for producing high-performance GeS nanosheets with exceptional potential for high-power lithium-ion battery applications.

ASSOCIATED CONTENT

ACKNOWLEDGMENT

Supporting Information. Experimental methods, GeS crystal XPS, and literature comparison with other liquid-phase exfoliated 2D materials as Li-ion battery anodes can be found in the Supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Solution processing and structural/chemical characterization were supported by the National Science Foundation (DMR-1505849), and battery fabrication and testing were supported by the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences (DE-AC02-06CH11357). This work made use of the NUANCE Center, which has received support from the NSF MRSEC (DMR-1720139), the State of Illinois, and Northwestern University. Metal analysis was performed at the Northwestern University Quantitative Bio-Element Imaging Center, which is supported by NASA Ames Research Center NNA06CB93G. D.L. is also supported by a National Science Foundation Graduate Research Fellowship. The authors further



acknowledge Dr. Junmo Kang, Dr. Josh D. Wood, Dr. Ethan B. Secor and Norman S. Luu for helpful discussions.

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