The Transformation of 0-D Carbon Dots into 1-, 2- and 3-D Carbon Allotropes: A Minireview

Abstract

Carbon dots (CDs) represent a relatively new type of carbon allotrope with a 0-D structure and with nanoparticle sizes < 10 nm. A large number of research articles have been published on the synthesis, characteristics, mechanisms and applications of this carbon allotrope. Many of these articles have also shown that CDs can be synthesized from “bottom-up” and “top-down” methods. The “top-down” methods are dominated by the breaking down of large carbon structures such as fullerene, graphene, carbon black and carbon nanotubes into the CDs. What is less known is that CDs also have the potential to be used as carbon substrates for the synthesis of larger carbon structures such as 1-D carbon nanotubes, 2-D or 3-D graphene-based nanosheets and 3-D porous carbon frameworks. Herein, we present a review of the synthesis strategies used to convert the 0-D carbons into these higher-dimensional carbons. The methods involve the use of catalysts or thermal procedures to generate the larger structures. The surface functional groups on the CDs, typically containing nitrogen and oxygen, appear to be important in the process of creating the larger carbon structures that typically are formed via the generation of covalent bonds. The CD building blocks can also ‘aggregate’ to form so called supra-CDs. The mechanism for the formation of the structures made from CDs, the physical properties of the CDs and their applications (for example in energy devices and as reagents for use in medicinal fields) will also be discussed. We hope that this review will serve to provide valuable insights into this area of CD research and a novel viewpoint on the exploration of CDs.

Structure–Property–Activity Relationships in Carbon Dots

“…In fact, CDs are excellent building blocks for the assembly of many large one-, two-, and three-dimensional carbon-based nanomaterials with either physical or chemical strategies. The physical strategy mainly focuses on calcination, while the chemical strategy primarily indicates covalent conjugation. , In addition, self-assembly of CDs to form supra-CDs can be included into either strategy, which depends on whether there are covalent bonds formed …”

“…Also, it often spontaneously occurs in the media where CDs cannot dissolve and tend to deposit on each other, which provides an opportunity for their interconnections . However, the formed supra-CDs are not permanently stable due to the weak interparticular interactions. , …”

“…The physical strategy mainly focuses on calcination, while the chemical strategy primarily indicates covalent conjugation. 62,63 In addition, selfassembly of CDs to form supra-CDs can be included into either strategy, which depends on whether there are covalent bonds formed. 63 As early as 2012, Cheng et al have succeeded in transforming 3−5 nm CDs into carbon nanotubes (CNTs; 200−300 nm in diameter) by electrophoresis deposition of CDs in a nanoporous anodic aluminum oxide template.…”

“…62,63 In addition, selfassembly of CDs to form supra-CDs can be included into either strategy, which depends on whether there are covalent bonds formed. 63 As early as 2012, Cheng et al have succeeded in transforming 3−5 nm CDs into carbon nanotubes (CNTs; 200−300 nm in diameter) by electrophoresis deposition of CDs in a nanoporous anodic aluminum oxide template. 64 They found the as-prepared CNTs displayed features similar to those of many traditional CNTs, such as a hollow and tubular morphology.…”

“…65 However, the formed supra-CDs are not permanently stable due to the weak interparticular interactions. 63,65 ■…”