New Simultaneous Exfoliation and Doping Process for Generating

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A New Simultaneous Exfoliation and Doping Process for Generating MX2 Nanosheets for Electrocatalytic Hydrogen Evolution Reaction Van-Truong Nguyen, Tzu-Yi Yang, Phuoc Anh Le, Po-Jen Yen, Yu-Lun Chueh, and Kung-Hwa Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01374 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 23, 2019

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ACS Applied Materials & Interfaces

A New Simultaneous Exfoliation and Doping Process for Generating MX2 Nanosheets for Electrocatalytic Hydrogen Evolution Reaction Van-Truong Nguyen,† Tzu-Yi Yang,‡ Phuoc Anh Le,† Po-Jen Yen,† Yu-Lun Chueh,*,‡, ∥,⊥ KungHwa Wei*,†,§ †Department

of Material Science and Engineering, National Chiao Tung University,

Hsinchu 30010, Taiwan, ROC. ‡Department

of Materials Science and Engineering, National Tsing Hua University,

Hsinchu, 30013, Taiwan, ROC. §Center

for Emergent Functional Matter Science, National Chiao Tung University,

Hsinchu 30010, Taiwan, ROC. ∥Frontier

Research Center on Fundamental and Applied Sciences of Matters, National

Tsing Hua University, Hsinchu 30013, Taiwan, ROC.

⊥Department

of Physics, National Sun Yat-Sen University, Kaohsiung, 80424, Taiwan, ROC.

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KEYWORDS: Transition metal dichalcogenides MoS2, WS2, MoSe2, WSe2, simultaneous exfoliation and nitrogen doping, Nitrogen-doped MoS2, hydrogen evolution reaction, electrocatalyst.

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ABSTRACT

Doping nonmetal atoms into layered transition metal dichalcogenide MX2 structures has emerged as a promising strategy for enhancing their catalytic activities for hydrogen evolution reaction. In this study, we developed a new and efficient one-step approach that involves simultaneous plasma-induced doping and exfoliating of MX2 bulk into nanosheets —such as MoSe2, WSe2, MoS2 and WS2 nanosheets—within a short time and at a low temperature (ca. 80 °C). Specifically, utilizing active plasma that is generated with asymmetric electrical field during electrochemical reaction at the surface of the submerged cathode tip, we are able to achieve doping of nitrogen atoms, from the electrolytes, into the semiconducting 2H-MX2 structures during their exfoliation process from the bulk states, forming N-doped MX2. We selected N-doped MoS2 nanosheets for demonstrating their catalytic hydrogen evolution potential. We modulated the electronic and transport properties of the MoS2 structure with the synergy of the nitrogen doping and exfoliating for enhancing their catalytic activity. We found that the nitrogen concentration of 5.2 at % at N-doped MoS2 nanosheets have excellent catalytic hydrogen evolution reaction where a low over-potential of 164 mV at a current density of 10 mA cm–2 and a small Tafel slope of 71 dec mV–1—much lower than those of exfoliated MoS2 nanosheets (207 mV, 82 dec mV–1) and bulk MoS2 (602 mV, 198 dec mV–

3



1)—as

well as an extraordinary long-term stability of >25 h in 0.5 M H2SO4 can be

achieved.

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INTRODUCTION Two-dimensional (2D) materials have been studied extensively because of their unusual properties that are suitable for advanced technological applications. For example, transition metal dichalcogenides (TMDs) such as MoS2, WS2, TiS2, TaS2, MoSe2 and WSe2 that can be prepared in the form of a single or a few layers are emerging 2D materials, exhibiting novel electronic and optical properties.1-6 They can have great potentials for applications in electronics, optical devices, actuators, sensors, solar cell and storage devices.1-6 2D TMDs displaying high catalytic activity and stability are particularly suitable for the production of hydrogen fuel, a clean and high-density energy carrier, through the electrolysis of water, and they are promising substitutes for traditional Pt-group metal alloys, which are expensive and scarce. For example, MoS2 nanosheets, a well-established 2D TMD materials, displaying a unusually high electrocatalyst activity; they appear to be an ideal candidate for replacing Pt-based metals in the hydrogen evolution reaction (HER).1,2 Up to date, “bottom-up” and “top-down” are two typical ways for the production of layered MX2 materials where M is Mo or W and X is S or Se. For the “bottom-up” process, physical/chemical vapor deposition (PVD/CVD) 3-4 and hydrothermal synthesis have been used where the concept of self-assembly is the main reason for the formation of layered 2D materials.5 For “top-down” methods, mechanical/thermal cleavage 6-7 and liquid exfoliation 8,9,10 have been demonstrated where the mechanical exfoliation of bulk 5



2D materials into layered 2D materials by physical or chemical intercalation utilizing gas or metal molecules is the main concept. While the “bottom-up” methods utilize specific substrates in a high temperature and vacuum with a limited mass of materials11 , the “top-down” methods often use simple approaches to produce TMD materials in quantity.12,13 Among the top-down approaches, liquid exfoliation is one of the simplest and most convenient methods for producing large amounts of high-quality MX2 nanosheets.14 This method is, however, sensitive to environmental conditions, consumes large amounts of chemical reagents that can cause environmental issues and requires long processing time.15 Several methods—such as engineering the crystal structure and controlling the degree of heteroatom doping16-1718—have been developed to improve the HER activity of layered TMDs.19 Structural engineering usually involves tuning complicate growth parameters and additional template.20 Controlling the degree of heteroatom doping in layered TMDs can in turn tune the active surface area, edge density and defect levels in its basal planes 21-22 —the active sites are determined by these parameters—and thus the electro-catalytic activity since.23,24 For the doping of non-metal atoms, high temperature annealing treatments, however, are necessary steps. For example, Se-doped MoS2 has shown improved electrical conductivity and a greater number of active edge sites, resulting in the enhanced HER activity.25 Si Qui et al. described the tunable N atom doped MoS2 nanosheets through sol–gel process and subsequent annealing treatment26; 6


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the N-doped MoS2 exhibited good performance for the storage of lithium ions

27.

Although N-doped MoS2 nanosheets appear to be one of the most highly active and durable catalysts for HER, particularly in acidic media

28-29,

they are typically

synthesized through multi-step processes involving high temperature annealing

26,30

or

through the use of special equipment (e.g., N2 plasma systems).31 The simultaneous one-step plasma-induced exfoliation and doping of MoS2 for application in the HER process has not been described previously. Herein, we demonstrate such a facile and one-step plasma-induced exfoliation approach— by cleaving of their weak out-of-plane van der Waals interaction—for the production of layered MX2 nanosheets from bulk MX2 powder. A few layers of exfoliated MX2 nanosheets can be successfully prepared by this approach, as confirmed by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), Raman spectra and atomic force microscopy (AFM). Different kinds of MX2 nanosheets such as MoSe2, WSe2, MoS2 and WS2 from bulk counterparts were demonstrated. Furthermore, an active surface plasma can be generated at the submerged cathode tip to induce doping of nitrogen atoms into the MX2 structure, namely N-doped MX2, from electrolytes containing nitrogen atoms. Here, N-doped MoS2 nanosheets were selected for the concept demonstration because of the highly catalytic hydrogen production ability. In addition, selection of electrolytes and plasma-induced exfoliation times were investigated to achieve different 7



concentrations of N atoms in MoS2 nanosheets. The maximum doping concentration can be up to 7.6 at % using the ammonia solution in the electrolyte, as confirmed by X-ray photoelectron spectroscopy and electron energy loss spectroscopy. The optimal Ndoped MoS2 nanosheets with the N concentration of 5.2 at % with the excellent catalytic hydrogen evolution reaction can be found where a low over-potential of 164 mV at a current density of 10 mA cm–2 and a small Tafel slope of 71 dec mV–1—much lower than those of exfoliated MoS2 nanosheets (207 mV, 82 dec mV–1) and bulk MoS2 (602 mV, 198 dec mV–1)—as well as an extraordinary long-term stability of >25 h in 0.5 M H2SO4 can be achieved. The as-made N-doped MoS2 nanosheets displayed highly efficient HER performance, suggesting that the plasma-induced method has an excellent potential for producing N-doped TMDs used in promising HER applications. RESULTS AND DISCUSSION Figure 1(a) provides a schematic of the experimental setup for the plasma-induced exfoliation process. Because a W (or Mo) cathode has a high Mohs hardness of 7.5 (5.5) and a high melting temperature of approximately 3400 °C, it is expected that the surface plasma (ca. 2600 °C) would not damage the cathode. Figure 1(a) inset indicated that the tip of the cathode is always engulfed in many discrete plasma discharge bubbles, instead of a single plasma bubble (see the movie in the supporting information). Figure 1(b) show the energy of the strong and instant plasma leading to in the form of thermal

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and rapid gas exploding energy can simultaneously exfoliate bulk MX2 into MX2 nanosheets and dope nitrogen atoms into MX2 structure. Inside the bubbles, the gas expanded and reached a pressure of several hundred megapascals.32 When the gas pressure surpassed the threshold pressure, the discrete plasma bubbles exploded. The plasma energy can be released into the electrolyte via high-energy jets that attacked the edges of the bulk MX2 and disrupted the weak out-of-plane van der Waals attraction forces between layers to produce the exfoliated nanosheets.33 The local temperature within a vapor plasma envelope can reach above 2000 K.32 The plasma envelope, however, was surrounded by the relatively cool electrolyte that is in a low temperature 120 mV dec−1, with which the rate-determining step follows Volmer reaction.49 However, if the desorption step restricts the reaction, the Tafel slope between 40 and 120 mV dec−1 can be achieved where Heyrovsky reaction is the rate-determinative step. 50

By contrast, the reaction occurs through recombination of two adsorbed hydrogens.

The rate-determining step will be dominated by Tafel reaction, with which the Tafel slope 2 μm) TMD powders (500 mg) into 1.5 м aqueous NaOH (200 mL). Each dispersion was stirred and warmed on a hot plate to 80 °C and then maintained at 80 °C for 3 h. For the setup of the electrolytic cell, a W (or Mo) rod was used as the cathode and a Pt foil as the anode. These electrodes were immersed at depths of 0.2 and 1 cm the electrolyte, respectively. A bias was applied to the system through a DC power supply (100 V/10 A, LinVAC TE CH). The bias was increased gradually to 65 V, at which the point plasma appeared at the immersed part of the cathode. This plasma became stronger as the bias

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was increased further. The cathode and anode were then immersed in the electrolyte at various depths, from 0.2 to 7 cm and from 1 to 5 cm, respectively, while maintaining the bias at 95 V. To exfoliate MX2, the system was stirred at 350 rpm for 30 min. During the plasma-induced treatment process, deionized water was added to keep the electrolyte at 200 mL in the electrolyte. After the plasma treatment, the dispersion was subjected to ultrasonication in a bath for 60 min at 20 kHz under a power of 130 W. After the sonication, the exfoliated MoS2 powder was collected through vacuum filtration onto an AAO membrane filter (Millipore; pore size: 200 nm) and washed repeatedly (at least three times) with distilled water and EtOH and then rinsed with dilute HCl. The powder was dried in a vacuum oven at 70 °C overnight, then dispersed in Ethanol (concentration: 1 mg mL–1) and sonicated for 15 min. The dispersed MoS2 solution was centrifuged (1000 rpm, 30 min) to remove any aggregated or un-exfoliated flakes. The top one-third of the solution was carefully removed and placed dropwise onto a cleaned Si wafer for further characterization. In-situ N doping during Plasma-Induced Exfoliation-N-doped MoS2 nanosheets were prepared by adding 50 mL NH4(OH) 25%, 50mL NH4NO3 1 м or 50 mL (NH4)2SO4 1 м, respectively, in the solution which bulk commercial MoS2 powder (500 mg) were dispersed into an aqueous solution of 2 M NaOH (150 mL). The mixture solution was stirred on a hot plate while warming to 80 °C, maintaining the temperature at 80 °C for 3 h into a three necks flask as the electrolyte. Similar as preparation of TMD nanosheets, 23



the electrolytic cell was setup with a cathode (W rod) and an anode (Pt foil); these electrodes were immersed at depths of 0.2 and 1 cm, respectively, in the electrolyte. A DC potential was applied and regularly increased to plasma-induce point appeared at the immersed part of the cathode; The voltage was increased to 95V, meanwhile the cathode and anode were immersed in the electrolyte with the depths 7 cm and 5 cm, respectively. The plasma-induce was maintained at 95V under 350 rpm stirring for 30 min. To offset the water was steamed, the deionized was added to keep electrolyte around 200 mL. Similarly, 3 mL NH4OH, NH4NO3 or (NH4)2SO4 was slowly dropped into electrolyte twice after 10 and 20 minutes plasma, respectively. After plasmainduced treatment at a DC voltage of 95 V, the N-doped MoS2 nanosheets were collected using the same procedure as described above for the pristine MoS2 nanosheets. Characterizations-Field emission scanning electron microscopy (FE-SEM) images were recorded using a field emission scanning electron microscope (Hitachi SU8000) operated at an accelerating voltage of 15 kV. High-resolution transmission electron microscopy (HRTEM), dark-field scanning transmission electron microscopy (STEM), energy-dispersive spectroscopy (EDS), and electron energy loss spectroscopy (EELS) elemental mapping were performed using a JEM-F200 apparatus operated at an accelerating voltage of 200 kV. Raman spectra were recorded using a Raman spectrometer (HORIBA, LabRAM HR) equipped with an Ar laser source (laser 24


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excitation wavelength: 514.5 nm). UV–Vis spectra were recorded using a Hitachi U-4100 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was performed at beamline (BL) 09A2 U5 Spectroscopy at the National Synchrotron Radiation Research Center (NSRRC), Taiwan. Atomic force microscopy (AFM) was performed using a diInnova scanning probe microscope (Bruker) operated in the tapping mode. Electrocatalytic hydrogen evolution-For HER measurements, linear sweep voltammetry (LSV) was performed at room temperature in 0.5 M H2SO4 at a scan rate of 5 mV s–1 using a CHI 611B electrochemical workstation, with a Pt wire as a counter electrode and a 3 M NaCl Ag/AgCl (CHI equipment) as a reference electrode. A drop (5 μL) of a solution of MoS2 ink at a concentration of 3 mg mL–1 (0.214 mg cm–2) in DMF was placed onto glassy carbon (diameter: 0.3 cm) as the working electrode. The working electrode was protected by doped 5 μL Nafion solution and then natural drying at room temperature. The averages of at least five LSV curves were measured to calculate the Tafel slope. The performance of the hydrogen evolution catalyst was recorded while performing LSV from –0.5 to +0.2 V versus a reversible hydrogen electrode (RHE). The I–t stability was measured at overpotentials of 165 mV for 25 h on N-doped MoS2. Electrochemical impedance spectroscopy (EIS) was performed from 100 kHz to 0.1 Hz (amplitude: 10 mV) under an overpotential of 200 mV (vs. RHE), using a Zahner Zennium workstation. The electrolyte was purged with N2 or Ar gas for 30 min prior to each measurement. All potentials were calibrated with respect to the RHE. 25



ASSOCIATED CONTENT

Supporting Information. SEM images of bulk commercial samples, UV-Vis spectra, XPS spectra, Raman spectra, elements concentrations table, DOCX. Experiment video, AVI. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Tel.: +886-3-5731871; ORCID: 0000-0002-0248-4091 [email protected]; Tel.: (886)3-5715131; ORCID: 0000-0002-0155-9987 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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ACKNOWLEDGMENT This study was supported by the Ministry of Science and Technology (grant numbers 107-2218-E-009-014, 107-2923-E-007-002 -MY3, 107-2112-M-007-030-MY3, 106-2923-E007-006-MY2, 107-2119-M-009-019, 107-3017-F-007-002) and the Center for Emergent Functional Matter Science of National Chiao Tung University (through the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project of the Ministry of Education in Taiwan). Y.-L. C thanks the CNMM for use of their facilities. REFERENCES (1)

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Page 38 of 56

Table 1. Materials, methods, N concentrations and phase of MoS2 at various N-doped MoS2.

Condition Material

Method

Temperature (oC)

Time (hours )

Nitrogen Phase percentag e of MoS2 (at%)

Refere -nces

MoO2@NMoS2

Hydrotherm al

200

24

N-doped MoS2

Sol-gel

350 – 1150

3

N-doped WS2

Sol-gel

550

2

N-doped MoS2

Sol-gel

600

2

6.4

27

47

5.8 - 7.6

26

2H

30

(2 days)

F, N Codoped MoS2

Hydrotherm al

250 – 450

6

2.9

28

N-doped MoS2

N2 plasma surface treatment

300

1

9

31

N-doped MoS2

Hydrotherm al

200

24

5.7

29

N-doped

Plasma -

80

0.5

0.55 - 7.6

38


2H

This

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MoS2

Induced

work

(top-down)

39



Page 40 of 56

Table 2. HER measurements of N-MoS2 nanosheets compared other results from literature with H2SO4 0.5 M electrolyte. Performance Typica l

Catalyst

Overpotential (mV)

Method

@ 10mA cm–2 Pt/C

Undoped

Tafel slope Ref. (mV dec–1)

22

34

52

MoS2 nanosheets

Chemically exfoliated

195

54

53

Metallic MoS2

Hydrothermal

175

41

54

MoS2 nanosheets CVD + plasma 183 plasma treatment treatment

76

22

Strained MoS2

60

55

207

82

This wor k

ca. 200

47.5

47

55

25

ca. 140

69.7

30

vacancy

CVD

170

MoS2 nanosheets

Plasma-Induced

MoO2@N-doped MoS2

Hydrothermal annealing

Se-doped nanosheets

Two-step hydrothermal annealing

MoS2

Doped N-doped nanosheets

WS2 Sol–gel annealing

+

+ ca. 300 +

N-/F-codoped MoS2

Hydrothermal

ca. 220

57

28

N-doped MoS2

Plasma-Induced

164

71

This wor

40


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nanosheets

k

Figure Captions: Figure 1. (a) Experimental setup for plasma-induced exfoliation and (b) proposed mechanism of exfoliation and nitrogen-doping process. Figure 2. SEM images of exfoliated (a) MoS2, (b) MoSe2, (c) WS2 and (d) WSe2 nanosheets. AFM images of exfoliated (e) MoS2, (f) MoSe2 and (h) WSe2 nanosheets. Raman spectra of exfoliated (i) MoS2, (j) MoSe2, (k) WS2 and (l) WSe2 nanosheets.

Figure 3. Low-magnification TEM images of (a) MoS2, (b) MoSe2, (c) WSe2 and (d) WS2 nanosheets. Insets show the corresponding SAED patterns. HRTEM images recorded along the [001] zone axis. Insets: their filtered of (e) MoS2 (f) MoSe2, (g) WSe2, and (h) WS2. STEM bright-field images of (i) MoS2, (j) MoSe2, (k)WS2 and (l) WSe2 nanosheets, and their element mapping images, respectively. Figure 4. (a) Mechanism of the N-doped MoS2 nanosheets. (b-f) Dark-field STEM images of undoped MoS2 and N-doped MoS2 nanosheets and the corresponding EELS elemental mapping images of Mo, S and N with different

41



electrolytes and/or plasma-induced time, respectively. (g) N doping concentrations obtained under various conditions. (h) XPS spectra (N 1s and Mo 3p3/2 regions) of undoped MoS2 and N-doped MoS2. Figure 5. SEM images of N-doped MoS2 after the plasma-induced exfoliation at (a) 200 oC,

(b) 300

oC

(c) 500

oC

and their BF-STEM images(d-f), respectively,

correspond with EDS mapping of Mo, S and N elements. Figure 6. (a) LSV curves (recorded on a glassy-carbon electrode) of bulk MoS2, undoped MoS2 and N-doped MoS2 (b) Corresponding Tafel plots derived from (a). (c) Nyquist plots acquired at –200 mV vs. RHE of the bulk MoS2, undoped MoS2 and N-doped MoS2. (d) Durability test of the N-doped MoS2 catalyst, performed at an overpotential of 165 mV vs. RHE.

42


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Figure 1

43



Figure 2

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Figure 3

45



Figure 4

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Figure 5

47



Figure 6

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TOC

49



Figure 1. (a) Experimental setup for plasma-induced exfoliation and (b) proposed mechanism of exfoliation and nitrogen-doping process. 279x194mm (300 x 300 DPI)


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Figure 2. SEM images of exfoliated (a) MoS2, (b) MoSe2, (c) WS2 and (d) WSe2 nanosheets. AFM images of exfoliated (e) MoS2, (f) MoSe2 and (h) WSe2 nanosheets. Raman spectra of exfoliated (i) MoS2, (j) MoSe2, (k) WS2 and (l) WSe2 nanosheets. 292x157mm (300 x 300 DPI)



Figure 3. Low-magnification TEM images of (a) MoS2, (b) MoSe2, (c) WSe2 and (d) WS2 nanosheets. Insets show the corresponding SAED patterns. HRTEM images recorded along the [001] zone axis. Insets: their filtered of (e) MoS2 (f) MoSe2, (g) WSe2, and (h) WS2. STEM bright-field images of (i) MoS2, (j) MoSe2, (k)WS2 and (l) WSe2 nanosheets, and their element mapping images, respectively. 292x157mm (300 x 300 DPI)


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Figure 4. (a) Mechanism of the N-doped MoS2 nanosheets. (b-f) Dark-field STEM images of undoped MoS2 and N-doped MoS2 nanosheets and the corresponding EELS elemental mapping images of Mo, S and N with different electrolytes and/or plasma-induced time, respectively. (g) N doping concentrations obtained under various conditions. (h) XPS spectra (N 1s and Mo 3p3/2 regions) of undoped MoS2 and N-doped MoS2. 284x157mm (300 x 300 DPI)



Figure 5. SEM images of N-doped MoS2 after the plasma-induced exfoliation at (a) 200 oC, (b) 300oC (c) 500 oC and their BF-STEM images(d-f), respectively, correspond with EDS mapping of Mo, S and N elements. 292x157mm (300 x 300 DPI)


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Figure 6. (a) LSV curves (recorded on a glassy-carbon electrode) of bulk MoS2, undoped MoS2 and N-doped MoS2 (b) Corresponding Tafel plots derived from (a). (c) Nyquist plots acquired at –200 mV vs. RHE of the bulk MoS2, undoped MoS2 and N-doped MoS2. (d) Durability test of the N-doped MoS2 catalyst, performed at an overpotential of 165 mV vs. RHE. 215x157mm (300 x 300 DPI)



ToC 82x44mm (300 x 300 DPI)


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New Simultaneous Exfoliation and Doping Process for Generating

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