Graphene is defined as a two-dimensional network of carbon atoms with a single atom thickness and a hexagonal crystalline structure with sp2 hybridization compacted by covalent bonds. Due to its structure and geometry, graphene has unique properties, including a large specific surface area, rapid mobility of load carriers, and high thermal and electrical conductivity. However, these characteristics are limited due to the restructuring of graphene sheets. For this reason, there are many studies devoted to the synthesis of three-dimensional structures that prevent the agglomeration of the sheets and the loss of properties of the graphene structure. These three-dimensional structures have low density, high porosity and surface area, stable mechanical properties, and good mass and electron transfer properties. This chapter aims to summarize the synthesis methods of the different three-dimensional structures derived from graphene as well as their wide range of applications in environmental remediation, sensors, biomedical and energy-related applications, among many others.
INTRODUCTION
Since its discovery, graphene has become one of the most interesting materials in the 21st century. Its chemical bond and structure were first described in the 1930s, but at that time, it was viewed as having little importance as it was assumed to be unstable when undergoing thermal changes. However, in 2004, a research group, led by physicist Andre Gueim, managed to isolate graphite sheets by means of mechanical exfoliation of highly oriented pyrolytic graphite (HOPG). HOPG is a form of high order and high purity synthetic graphite. Thanks to their discovery, this research group was awarded the Nobel Prize in Physics in 2010. (W. Gao, 2012)
Graphene is composed of a series of carbon atoms organized in a regular honeycomb pattern, which is generated by covalent bonds from the superposition of sp2 hybrids from bonded carbon. As a result of this structure, graphene acquires unique properties such as it is extremely hard and resistant, it has high thermal and electrical conductivity, being very light, flexible, and elastic, and it has a low Joule effect. Despite all these characteristics, this material has had few applications, mainly as the graphene sheets agglomerate and restructure, by which some of their properties are removed (Y. Wu et al., 2019). In order to maintain these unique properties, one alternative developed was to manipulate these sheets in order to build three-dimensional structures derived from graphene, thus preventing them from restructuring.
Three-dimensional structures derived from graphene not only retain their intrinsic properties but also acquire other important characteristics such as high porosity, low density, large specific surface area, excellent mechanical properties, etc. (Foster et al., 2017; Xia et al., 2017). Figure 1 shows the upward trend in articles concerning the main three-dimensional structures derived from graphene, such as spheres, adsorbents, foams, and graphene aerogels, showing the growing interest of research into these structures. In addition, note that 2014 showed the greatest year-on-year difference in the number of articles published. That year studies began to focus more on this type of structure, its synthesis, and ways of using it. This book chapter summarises synthesis methods and the main applications of 3D graphene structures. Graphene spheres, aerogels, adsorbents, and porous graphene structures are those considered in this study.
Figure 1. Number of articles published per year related to different graphene structures. Source: Scopus. Key Words Searched: Three-Dimensional graphene, graphene spheres, Graphene Adsorbent Porous Structure, Graphene Foams, and Graphene Aerogels.
BACKGROUND
As mentioned above, three-dimensional structures derived from graphene prevent some of the properties characteristic of two-dimensional structures from being suppressed, by preventing the sheets from restructuring and agglomerating. However, three-dimensional structures have different geometry depending on the synthesis method used and its application.
As a summary, Figure 2 presents the different graphene-derived three-dimensional structures discussed in this book chapter (spheres, aerogels, foams, and adsorbents), as well as their synthesis methods and main applications.
Figure 2. Synthesis and applications of three-dimensional graphene structures.
GRAPHENE SPHERES
These materials are among the most promising options for solving the problems associated with loss of properties in two-dimensional structures by preventing sheets from restructuring. These structures have many porous channels, which provide more free space between the spheres, thereby improving conductivity and structural stability (Yuan et al., 2021).