Studies On Structural Control Of Three-Dimensional Graphene Materials And Their Properties

Graphene has emed in recent years as a unique and important class of carbon nanomaterialsdue to its extraordinary mechanical, electrical and thermal properties, and is being used in many fields including electronics, conductive nanocomposites, films, electromagnetic interference shieldingand sensors. Graphene sheets could be constructed to lightweight and three-dimensional (3D) porous structures for special applications in the fields like catalyst support, energy storage and environmental cleaning. The construction of 3D graphene aerogels (GAs) not only avoids the restacking of individual sheets and maintains the intrinsic properties of graphene sheets such as high conductivity and large specific surface area, but also makes the 3D monolith structure with ultralow density and high porosity.Many efforts have been devoted to the fabrication of 3D graphene materials with various microstructures and properties, including self-assembly, template-assisted preparation and chemical vapor deposition. Among these approaches, self-assembly of graphene is commonly used with graphene oxide (GO) as its precursor. Firstly,3D graphene hydrogels are fabricated by hydrothermal process or chemical reduction method, during which GO is converted to graphene by thermal treatment or chemical reducing agents, such as NaHSO3, Na2S, hydroiodic acid, hydrazine, hydroquinone, and Vitamin C. Subsequently, GAs can be obtained by freeze drying or supercritical fluid drying of the graphene hydrogels. When prepared by freeze drying approach, the porous microstructures including pore size and orientation of GAs can be controlled by changing the conditions including freezing temperature and freezing direction.Herein, by comparing different methods for graphene aerogels, highly compressible anisotropic graphene aerogels are prepared by directional freezing method.The detailed work are as follows:1. Graphene oxide is prepared by oxidizing graphite flakes based on the modified Hummers method. Chracterization techniques such as XRD, FTIR, XPS demonstrate the existence of abundant oxygen-containing functional groups on the graphene oxide, which acts as a good precursor for fabricating graphene aerogels. Highly compressible anisotropic graphene aerogels are fabricated by reducing GO with ascorbic acid at 70℃ for four hours, followed by directional-freezing and freeze-drying. SEM images show the pores are aligned in the aerogels and XRD, FTIR, XPS elaborate most of the oxygen-containing groups are removed compared with GO.2. The resultant graphene aerogels exhibit higher compressive strength than those aerogels fabricated by conventional freezing. The stress at 50% strain for the AGA with a density of 6.1 mg cm-3 is about 14.1 kPa, which is among the highest in all graphene aerogels reported in the literature with a similar density. What’s more, it has good compressibility in both axial and radial directions because of the anisotropic graphene pores. In addition, the aerogels also show excellent flexibility in organic liquids.3. Higher compressive strain means better contact between the graphene sheets due to the enhanced density, which results in greater electrical conductivity. The strain-sensitive electrical conductivity of AGA contributes to some applications like pressure sensor and stimulus-responsive graphene systems. Because of the highly porous structure and hydrophobicity, the resultant graphene aerogels exhibit a high ability to absorb organic liquids. Due to its high porosity, excellent flexibility in both air and liquids and satisfactory fire-resistance, they are efficient absorbents with good recyclability for absorption of organic liquids under absorbing-burning, absorbing-distilling, and absorbing-squeezing cycles. And they can be applied for treating oil pollution in the sea.