Electrochemical Exfoliation to Produce High-Quality Black Phosphorus

Summary

We present a step-by-step procedure for electrochemical exfoliation of black phosphorus (BP), one of the most promising emerging 2D materials with applications in (opto)electronics, from its bulk crystals, as well as the morphological characterization by scanning electron microscopy, atomic force microscopy, and transmission electron microscopy.

Abstract

To obtain high-quality two-dimensional (2D) materials from the bulky crystals, delamination under an externally controlled stimulus is crucial. Electrochemical exfoliation of layered materials requires simple instrumentation yet offers high-quality exfoliated 2D materials with high yields and features straightforward upscalability; therefore, it represents a key technology for advancing fundamental studies and industrial applications. Moreover, the solution processability of functionalized 2D materials enables the fabrication of (opto)electronic and energy devices via different printing technologies such as inkjet printing and 3D printing. This paper presents the electrochemical exfoliation protocol for the synthesis of black phosphorus (BP), one of the most promising emerging 2D materials, from its bulk crystals in a step-by-step manner, namely, cathodic electrochemical exfoliation of BP in the presence of N(C₄H₉)₄∙HSO₄ in propylene carbonate, dispersion preparation by sonication and subsequent centrifugation for the separation of flakes, and morphological characterization by scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM).

Introduction

Due to their superior mechanical, electrical, and optical properties in comparison to their layered bulk analogs, 2D materials have attracted considerable attention among the scientific community. Being the predecessor and the most studied of all 2D materials for several decades, graphene is still in the spotlight of cutting-edge discoveries such as membranes¹, sensors², catalysts³, energy technologies⁴, topological spintronic devices⁵, and condensed matter physics⁶. Inspired by that, numerous other 2D materials have been synthesized and investigated, such as metal chalcogenides⁷, layered double hydroxides⁸, and boron nitride⁹. Including the newest additions to the family of 2D materials (i.e., phosphorene¹⁰), MXenes (2D metal carbides or nitrides)¹¹, and 2D polymers (single/few-layer 2D metal/covalent organic frameworks)^12,13, the family of 2D materials has grown to consist of more than 150 members featuring intrinsic insulators, semiconductors, semimetals, and metals¹⁴.

The emerging 2D materials, such as BP^{15,16,17,18,19,20,21,22}, molybdenum disulfide (MoS₂)^23,24,25,26, and indium(III) selenide (In₂Se₃)^27,28,29, have shown considerable potential in scientific discoveries; however, to extend their excellent physiochemical properties to a macroscopic scale, efficient, reproducible, and low-cost methods are urgently needed. Electrochemical exfoliation is a promising approach for the upscale production of such 2D materials^30,31, mainly due to the fact that it can provide gram scales of high-quality and dispersible exfoliated materials in minutes to a few hours due to the efficient intercalation of ions under the electric force.

The accompanying video demonstrates the step-by-step production of dispersions of BP, one of the most promising emerging 2D materials with applications in (opto)electronics, using electrochemical exfoliation, followed by sonication and centrifugation for the separation of flakes from unexfoliated particles, the preparation of dispersions of exfoliated BP flakes in various solvents, and morphological characterization by SEM, AFM, and TEM.

Protocol

NOTE: See the Table of Materials for details related to the materials and equipment used in this protocol.

1. Synthesis of black phosphorus (BP) by electrochemical exfoliation

1. Chop the large pieces of BP crystal into small pieces of ~1-2 mm (≤5 mg) and confine them inside a platinum gauze to serve as the cathode.
2. Cut a piece of platinum foil with dimensions ~2 cm² to serve as the anode and fix it in a way that it faces the cathode and is 2 cm away from it.
3. To prepare the electrolyte, prepare a fresh solution of 0.1 M tetra-n-butyl-ammonium bisulfate (TBA·HSO₄) in propylene carbonate (use anhydrous and degassed solvent). Fill the electrochemical cell until the solvent level reaches at least 5 mm above the top of the electrodes (Figure 1C).
4. Apply a DC potential of −8.0 V to start the delamination. To prevent degradation during the exfoliation, carry out the whole process under an inert atmosphere (e.g., inside a glove box under argon or nitrogen).
   NOTE: There is no need for continuous purging of the electrolyte by inert gases.
5. Upon completion of the delamination (ensure there is no more visible generation of expanded BP), transfer the whole electrolyte, including the exfoliated BP flakes and the larger unexfoliated particles, into a 50 mL centrifuge tube.
6. Centrifuge at 5,292 × g at 15 °C for 10 min. Discard the electrolyte, add 35 mL of fresh propylene carbonate to the precipitate, and shake it gently. Repeat the centrifugation and washing process 2x with propylene carbonate and 2x with the final solvent of choice (e.g., 2-propanol, N,N-dimethylformamide [DMF], or N-methyl-2-pyrrolidone [NMP]).
7. Add 5-50 mL of the desired solvent to the precipitated products (depending on the required concentration range) and sonicate the resulting product mixture in an ice-cooled sonication bath for 10 min.
8. Centrifuge the mixture at 5,292 × g at 15 °C for 10 min to remove the thick-layered BP flakes.
9. Decant the supernatant containing the monolayer and few-layer (<10 layers) BP flakes into a clean container and keep it in the glove box for further characterization or device fabrication.
10. Calculate the delamination yield (η) using the gravimetric yield formula represented by equation (1):
    η = m₁/m₂ × 100%     (1)
    ​where m₂ represents the mass of the starting BP crystal and m₁ the weight of the dispersed BP flakes.
11. To determine m₁, measure the weight of the BP collected from the vacuum filtration of a certain volume of the BP dispersion (e.g., 0.5 mL). Use polytetrafluoroethylene (PTFE) membrane (0.2 µm pore size) for vacuum filtration.
12. For long-term storage, close the cap of the container bottle and keep it inside the glovebox (up to 3 months).

2. Sample preparation for characterization by SEM, SEM-EDS, AFM, and TEM

NOTE: To explore the quality and morphological aspects of the synthesized BP flakes, it is necessary to perform characterizations such as SEM³² (for studying the surface morphology of the BP flakes), SEM-EDS³³ (for elemental analysis of the flakes), AFM^34,35 (for analysis of the thickness and lateral size of the flakes), and TEM^36,37 (for detection of the structural defects, shape, and size of the BP flakes). Sample preparation protocols for the abovementioned characterization techniques are explained below (sections 2.1-2.4). For operational procedures of the abovementioned characterization techniques, refer to the cited references^{32,33,34,35,36,37}.

1. Sample preparation for characterization by SEM
   1. Cut out a small piece (with dimensions around 5−7 mm) of a silicon wafer.
   2. Clean the silicon wafer piece using acetone, methanol, and water successively. Then, dry the substrate by blowing compressed air or nitrogen over it.
   3. Prepare a 0.5 mL diluted dispersion of the BP dispersion prepared in section 1 by the addition of anhydrous iso-propanol to reach a concentration of ~0.01 mg/mL.
   4. Spin coat 1−2 droplets of the prepared diluted dispersion on the clean and dry substrate in the glovebox and let it dry on a hotplate at 100 °C for 6 h (in the glovebox). After cooling to room temperature, use the sample for characterization.
2. Sample preparation for characterization by SEM-EDS
   1. Follow the procedure mentioned in section 2.1; however, prepare a more concentrated dispersion of ~0.1 mg/mL instead of the one mentioned in step 2.1.3.
3. Sample preparation for characterization by AFM
   1. Follow the procedure mentioned in section 2.1; however, use a silicon oxide wafer instead of the silicon wafer mentioned in step 2.1.1.
4. Sample preparation for characterization by TEM
   1. Prepare the diluted dispersion of BP as explained in section 2.1.3.
   2. Drop cast one droplet of the dispersion on the carbon microgrids. Use the sample for characterization by TEM after drying it in the glovebox argon-atmosphere (without additional heating) for 24 h.

Results

Figure 1 demonstrates the electrochemical exfoliation of BP crystals, the mechanism of intercalation of TBA·HSO₄ and subsequent delamination, and the reaction cell setup.

Figure 1: Schematic demonstration of the mechanism of electrochemical exfoliation of black phosphorus crystals...

Discussion

BP has a valence shell configuration of 3s² 3p³, and each phosphorus atom possesses a lone electron pair, which makes the phosphorus atoms vulnerable to fast oxidative degradation in the presence of oxygen, water, and light⁴¹. To prevent degradation, it is recommended to use degassed and anhydrous solvents and reagents and carry out the production process under an inert atmosphere.

During the exfoliation of BP crystals, part of the produced H^+...

Materials

Name	Company	Catalog Number	Comments
2-Propanol	Sigma Aldrich	278475	anhydrous, 99.5%
Atomic force microscopy (AFM)	Bruker Multimode 8 system
Black phosphorus	Smart Elements	4504	Black Phosphorus 5.0 g sealed under Argon in ampoule
Centrifuge	Sigma 4-16KS
Propylene carbonate	Sigma Aldrich	310328	anhydrous, 99.7%
Scanning electron microscope (SEM)	Zeiss Gemini 500
Tetra-n-butylammonium hydrogen sulfate	Sigma Aldrich	791784	anhydrous, free-flowing, Redi-Dri, 97%
Transmission electron microscopy (TEM)	Zeiss Libra 120 kV