Anhydrous Liquid-Phase Exfoliation of Pristine Electrochemically

Article Cite This: Chem. Mater. 2018, 30, 2245−2250

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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*,†,‡,§ †

Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208−3108, United States Applied Physics Graduate Program, Northwestern University, Evanston, Illinois 60208-3112, United States § Department of Chemistry and Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208−3108, United States ‡

S Supporting Information *

ABSTRACT: Germanium sulfide (GeS) is an emerging layered material with high promise in its two-dimensional (2D) exfoliated form for energy storage applications. While liquidphase 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 1000 cycles and high rate capability in excess of 10 A g−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.

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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 A W−1.28,29 Thin layers of GeS would further improve 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. For example, 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 (∼1000). While multiple reports have demonstrated solutionphase synthesis of GeS,27,29 this bottom-up approach requires prolonged, high-temperature processing conditions that limit scalability. Similarly, alternative top-down methods such as micromechanical exfoliation28 are hindered by exceptionally low yields and throughput. 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 centrifugationbased postprocessing for lateral-size separation37 or thickness sorting,3,8,10 resulting in highly monodisperse nanosheet size

INTRODUCTION Prior research on layered materials, such as graphene,1−4 transition metal dichalcogenides (TMDCs),5−11 and black phosphorus (BP),12−14 has established the unique properties of two-dimensional (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 form15,16 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 mAh g−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 mAh g−1.19 Furthermore, theoretical calculations predict that GeS has a room temperature Li+ diffusion coefficient 170 times greater than graphene20 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 mAh g−1 for both bulk and exfoliated anodes, but after 50 cycles, the capacity of bulk MoS2 decreases to 226 mAh g−1 while exfoliated MoS2 retains high capacities over 750 mAh g−1.23 It can be anticipated that GeS and other layered materials may show similar benefits for Li-ion © 2018 American Chemical Society

Received: November 5, 2017 Revised: March 15, 2018 Published: March 20, 2018 2245

DOI: 10.1021/acs.chemmater.7b04652 Chem. Mater. 2018, 30, 2245−2250

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Chemistry of Materials

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 1000 rpm for 10 min. (D) Concentration of exfoliated powder GeS as a function of solvent surface tension after centrifugation at 1000 rpm for 10 min. The concentration of NMPexfoliated crystal GeS is plotted as a reference. (E) Raman spectra of a bulk GeS crystal and a film of solution-processed GeS.

Figure 2. (A) AFM micrograph of nanosheets drop-cast onto a 300 nm SiO2/Si substrate. (B) Low magnification TEM image of nanosheets. (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.

distributions.38 Here, an efficient LPE approach is introduced for 2D GeS nanosheets based on ultrasonication in anhydrous Nmethyl-2-pyrrolidone using a customized sealed-tip sonication system. The resulting GeS nanosheets possess high structural and chemical integrity with superlative electrochemical proper-

ties, particularly enabling exceptional capacity retention and high-rate performance in Li-ion battery anodes.

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RESULTS AND DISCUSSION To avoid degradation of chemically reactive GeS during LPE, a sealed ultrasonication apparatus similar to that introduced by 2246

DOI: 10.1021/acs.chemmater.7b04652 Chem. Mater. 2018, 30, 2245−2250

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Chemistry of Materials Kang et al.13 is employed to limit ambient exposure, as shown in Figure 1A. Ultrasonication shears the GeS crystal along the bplane (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, dimethylformamide (DMF): 37.1, N-methyl-2-pyrrolidone (NMP): 40.79). Specifically, 7.5 mg of GeS powder and 15 mL of solvent are placed in the apparatus and sonicated in an ice bath at ∼50 W for 1 h and are subsequently centrifuged at 500 or 1000 rpm for 10 min (see Supporting Information for experimental details). Out of the solvents studied, NMP results in the darkest dispersion postcentrifugation, 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 and then related to the absorbance measured in UV−vis spectroscopy via the Beer−Lambert Law (A = εLc, where A is the absorbance, ε is the molar extinction coefficient, L is the path length, and c is the concentration). Using an estimated ε600nm = 2640 L g−1 m−1 and absorbances measured from UV−vis spectra, the concentrations of GeS exfoliated in different solvents and subsequently centrifuged at 1000 rpm for 10 min 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 in 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 shear vibration of parallel layers in the armchair direction (a-axis). 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 atomic force microscopy (AFM) and transmission electron microscopy (TEM) for thickness and lateral length histograms, respectively. AFM samples are prepared by dropcasting 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−F) are lognormally distributed, as predicted in literature for LPE.40,41 Each histogram is fit to the probability density function of the form y = y0 +

solution due to centrifugation. 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 min. 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

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.

doublet assigned to binding energies of 29.7 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 and 162.4 eV for the 2p3/2 and 2p1/2 peaks, respectively, and an oxide-containing doublet assigned to 161.8 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). TEM further confirms the crystalline structure of LPEprocessed GeS. Figure 4A,B shows 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. To demonstrate that 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

2 x 2 A e−(ln μ ) /(2σ ) xσ 2π

where y0 is a constant offset, A is a constant prefactor, x is either the thickness or length, μ is the log-normal mean, and σ is the log-normal standard deviation. For the as-prepared (AP) dispersion, μThickness,AP = 6.44 nm and μLength,AP = 116.94 nm. Following centrifugation at 500 rpm for 10 min, the log-normal mean for the thickness and lateral length decrease to μThickness,500rpm = 2.87 nm and μLength,500rpm = 66.91 nm, indicating that the larger nanosheets are sedimented out of 2247

DOI: 10.1021/acs.chemmater.7b04652 Chem. Mater. 2018, 30, 2245−2250

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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.

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.

increased to 10 C, the GeS anode still maintains a high specific capacity of 231 mAh g−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 LPEprocessed GeS nanosheet 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

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 (∼1100 mA g−1). The first discharge and charge capacities are 2295 mAh g−1 and 1166 mAh g−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 mAh g−1 and 933 mAh g−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 mAh g−1 at 1000 cycles. Figure 5C further shows the specific capacity as a function of cycling rate. As the cycling rate is 2248

DOI: 10.1021/acs.chemmater.7b04652 Chem. Mater. 2018, 30, 2245−2250

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Chemistry of Materials due to larger surface area to volume ratios and shorter Li+ diffusion lengths.21 Furthermore, prior work on related 2D nanomaterials such as SnS51 and SnS252 demonstrates 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 1000 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.

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

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04652. Experimental methods, additional UV-Vis, AFM, and XPS analysis, and literature comparison with other liquid-phase exfoliated 2D materials as Li-ion battery anode (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(M.C.H.) E-mail: [email protected]. ORCID

Kan-Sheng Chen: 0000-0002-6094-2396 Xiaolong Liu: 0000-0002-6186-5257 Mark C. Hersam: 0000-0003-4120-1426 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Solution processing and structural/chemical characterization were supported by the National Science Foundation (DMR1505849), 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-AC0206CH11357). This work made use of the NUANCE Center, which has received support from the NSF MRSEC (DMR1720139), 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, 2249

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DOI: 10.1021/acs.chemmater.7b04652 Chem. Mater. 2018, 30, 2245−2250