Black Phosphorus and Black Phosphorus-Based Materials: Synthesis, Characterization and Applications

黑磷及黑磷基材料的製備、表徵與應用

Student thesis: Doctoral Thesis

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Award date15 Nov 2018

Abstract

Black phosphorus (BP) has been gleaning much research interest as a new 2D material due to its intriguing physical and chemical properties. The BP crystals present a puckered, honeycomb-like structure, in which the weak interlayer van der Waals (vdW) interactions and strong in-plane bonds enable the possibility of being exfoliated down to few-layer or single-layer phosphorene. BP has a direct and tunable band gap that varies from 0.3 eV to 2.0 eV, from bulk BP to phosphorene, and a high charge carrier mobility of up to 1000cm2·V-1·s-1. These properties suggest considerable potential for BP not only in electric and optoelectronic applications, but also solar cells, supercapacitors, lithium ion batteries, sensors, thermoelectric devices, and catalysts.

This thesis presents our studies of the synthesis, characterization, and applications of BP and BP-based materials. Our research is introduced and described in five parts, starting with a brief overview of BP in the first chapter that covers properties, preparation methods, and potential applications of BP.

The second chapter reports the thickness-controlled preparation of few-layer BP via hydrogen plasma treatment. Since the thickness (layer number) of BP is a significant parameter which affects its performance in a variety of fields, here, we report a controllable thinning method using hydrogen plasma etching to thin down mechanical exfoliated BP flakes. Atomic force microscope (AFM), optical microscopy (OM), X-ray photoelectron spectroscopy (XPS), and Raman techniques were used to identify the process conditions. The plasma etching treatment could not only achieve the thickness control of the BP flakes, but also remove the defects of the exposed BP surface. It is expected to improve the electrical performance of BP-based field-effect transistor (FET). This method provides a new way to fabricate BP-based electronic and optoelectronic devices in the future.

The third chapter reports a molybdenum diselenide-black phosphorus (MoSe2-BP) heterostructure and its application in electrocatalytic hydrogen evolution. The hydrogen evolution reaction (HER) is promising in the production of clean and renewable energy and MoSe2 is attravtive for this process due to the excellent catalytic ability. However, its low intrinsic conductivity and vulnerability to aggregation have restricted wider application. Herein, a facile strategy to construct a heterostructure composed of MoSe2 nanosheets and BP nanosheets by vdW interactions is designed and described. Compared with the bare MoSe2 nanosheets, the MoSe2-BP heterostructure has improved conductivity and more abundant active sites attributable to BP consequently facilitating electron transfer and preventing MoSe2 from aggregation. In the HER assessment,the MoSe2-BP heterostructure delivers excellent electrocatalytic performance such as a low onset potential (200 mV), small Tafel slope (97mV·dec-1), and excellent stability. The 2D heterostructured semiconductor has great potential in electrocatalytic applications.

The fourth chapter details the fabrication of few-layer BP with liquid phase exfoliation (LPE) as well as applications for gas sensing, focusing on its size effect on gas sensing performance. In this chapter, we chose isopropanol (IPA) for the solvent due to its non-toxicity and low boiling point, to render our BP suspensions more suitable for drop-casting and testing. Three sizes of BP nanosheets were obtained using a step-wise centrifugation process. This fine-grained size control of BP nanosheets yields a rare opportunity to study size-dependent, gas-sensing performance. Our gas-sensing results suggest that smaller and thinner BP nanosheets perform better than larger and thicker ones, likely due to more adsorption sites and better charge transfer in the thinner nanosheets.

The fifth chapter provides conclusions regarding our studies and results; potential future work will be described as well.