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 synt....

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 ....

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.

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

List of materials used in this article

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