Supercritical CO₂-assisted fabrication of advanced two-dimensional materials and their heterostructure

Advanced two-dimensional (2D) materials with unique structural features and fascinating physicochemical properties are reviewed for diverse applications. Based on the unique properties of supercritical carbon dioxide (SC CO₂), we focus on the recent progress on fabrication of advanced 2D materials and their heterostructure with the assistance of SC CO₂ and further exploration of fabrication for functional nanomaterials such amorphous materials and quantum dots. Furthermore, promising features, challenges and opportunities in SC CO₂-assisted fabrication techniques are discussed.

Introduction

Since graphene has been successfully exfoliated in 2004 [1], two-dimensional (2D) material with the thickness of single atom or a few atoms have attracted a great deal of research interest due to their distinctive properties and great promise for a broad array of applications [2]. Over the past decade, the preeminent properties of 2D materials have driven a tremendous amount of research, such as hexagonal boron nitride [3], transition metal dichalcogenides, graphitic carbon nitride (g-C₃N₄) and MXenes [4]. The atomic thickness endows 2D nanomaterials with large specific surface area, which can effectively expose internal area with maximum possible active sites. In addition, those confined electrons in two dimensions and hence resulted in dramatic changes in their intrinsic optical and electrical properties [5, 6, 7, 8]. Benefiting from these inimitable features, the ultrathin 2D materials exhibit unique electronic and structural properties and their applications of green chemistry fields for catalysis, electronics/optoelectronics, energy storage, and conversion, and so on. Consequently, in-depth research and development of 2D materials is extremely critical to meet the growing energy demand of today's social for green sustainable chemistry. More importantly, owing to the unique 2D structure, the materials can serve as promising building blocks to expand their family to form heterostructure with tailored properties, toward creating advanced materials [9, 10, 11]. Indeed, the heterostructures have already led to the observation of numerous exciting properties [12, 13, 14, 15].

The prerequisite having unusual properties and potential applications is effective in the synthesis of 2D materials and their heterostructure. Hitherto, the approaches for fabricating 2D materials are mainly divided into two categories of top-down and bottom-up method, which are developed by mechanical exfoliation and liquid exfoliation [16,17], wet-chemical synthesis [18], chemical vapor deposition [19], and molecular beam epitaxy [20]. However, in those methods, irreversible alteration of structure and properties may occur owing to excessive use of solvents. The required harsh preparation conditions limit their widespread use. Therefore, for the exploration, supercritical fluid (SCF) is concerned with a green and sustainable technology owning great potential in nonflammable, nontoxic and environmentally friendly solvent with zero surface tension, low viscosity, and high gas-like diffusivity. There were great efforts devoted to exploration for the fabrication of material with the assistance of SCF [21∗, 22, 23, 24, 25, 26, 27], which illustrated the feasibility and importance of SCF for the future of advanced materials processing. Among them, SCF of carbon dioxide (SC CO₂) reveals the advantages of nonuse of toxic reagents and solvents, nonproduction of toxic by-products, and processing simplicity, meeting the requirements of green processing. Hence, this review focuses on the recent progress in the fabrication of 2D materials and their heterostructure with the assistance of SC CO₂.