Exfoliation of conductive layered materials in solutions

Abstract

This PhD thesis focuses on the scalable exfoliation of van der Waals crystals (e.g. graphite, black phosphorus) into high-quality two-dimensional flakes by means of electrochemical engineering in solutions. In contrast to conventional physical or chemical exfoliation strategies by which the production of these materials has to compromise between scalability and quality, in this thesis, the electrolytic parameters including ionic species, working electrodes, working bias and cell configuration were systematically studied and carefully considered to enable low structural damage as well as high throughput capability of the exfoliated materials. In chapter 2, we demonstrate an efficient approach using reducing agents or radical scavengers during anodic exfoliation process, to avoid the attack of reactive oxygen-containing radicals (e.g. ·OH and ·O·) from water oxidation, which has been considered as the major source for the oxide groups on exfoliated graphene. In chapter 3, we present a novel cell structure with two graphite working electrodes (i.e. anode and cathode) that can be exfoliated simultaneously, by taking advantage of alternating currents. The polarity switch of working bias promotes not only fast exfoliation rate but also high graphene quality. In addition, the potential applications such as printable inks, flexible supercapacitors, lithium ion batteries were explored to evaluate the features of high-quality graphene. Inspired by graphite exfoliation, in chapter 4, we manage to produce large-sized defect-free, few-layer black phosphorus (BP) flakes through the cathodic exfoliation of bulk crystal in organic electrolytes. The exfoliated flakes exhibit excellent hole mobility and remarkable on/off ratio, well comparable with those of mechanically exfoliated BP flakes. This simple and scalable method shows great potential in the future development of BP-based technologies. At last, the challenges and opportunities in the exfoliation of layered conductive or semiconducting materials have been summarized in chapter 5, as the guidelines for the future studies.