Figure 1 . (A) Classification of CDs as GQDs, CQDs, or CPDs. (B)
Schematic diagram of GQDs synthesis. (C) Schematic diagram of CQDs
synthesis. (D) Schematic diagram of CPDs synthesis.
GQDs were first prepared by Ponomarenko et al.[35] in 2008 based on the previous report on CDs
by Xu et al. [13] in 2004. GQDs can be regarded as
a subset of graphene and/or graphene oxide with sp2structure, exhibiting similar chemical and physical properties to it.
However, unlike two-dimensional graphene sheets with zero band gap, the
GQDs is typically anisotropic characterized by single or less layers
atomic graphite planes with lateral size typically <10 nm. The
bandgap of GQDs is opened due to the quantum confinement effects caused
by the small size, and is closely related to the size (mainly refers to
the conjugated sp2 domain), which is the major feature
of GQDs.[36] Density functional theory (DFT)
calculations have shown that the bandgaps of GQDs decrease from around 7
eV for benzene to 2 eV for GQDs consisting of 20 fused aromatic
rings.[37] Moreover, the photoluminescence (PL) of
GQDs can be precisely regulated by control of the attached chemical
functionalities, heteroatom doping, defects, edge configuration, and
shape.[38, 39] Many regard GQDs as giant
polyaromatic molecules.[40] Compared to simple
polyaromatic molecules, GQDs are more complex in that they are larger,
carry abundant functional groups, and have narrower and more complex
band gap structures. The intense PL emissions of GQDs can be roughly
divided into two primary types: PL originating from created or induced
energy bandgaps in single sheet GQDs and PL that is associated with
defects in single- and/or multiple-layer graphene. There are many
different opinions about the number of layers in GQDs, with 1-3
layers,[41] <5
layers,[42] or <10 layers being
advocated in different literature. In the discussion below, we mainly
focus on GQDs with less than 5 layers (GQDs with more layers are
classified as CQDs in the current work).
There are two types of synthetic routes towards GQDs: top-down and
bottom-up methods (Figure 1B). The top-down method involves controlled
fragmentation of bulk carbonaceous materials (e.g., graphite, graphene
sheets, carbon nanofibers, and carbon nanotubes) into small pieces. The
most commonly used methods include oxidative/reductive
cutting,[39, 43] physical
grinding,[44] or combination of cutting and
grinding.[45] Top-down methods have the advantages
of abundant raw materials, simple operation, and large scale production,
but do not allow accurate control over the size and
morphology.[46] The formation of GQDs by top-down
methods does not involve the aggregation of precursors. Accordingly, the
C:H ratio in GQDs prepared by top-down methods is typically near-unity,
with C mainly existing in sp2 form. However, top-down
preparation methods result in partial oxidation of some carbons, whilst
also introducing certain functional groups (e.g. hydroxyl, carbonyl,
carboxylic acid, and epoxy/ether groups) and/or doping of heteroatoms
into the GQDs. These surface modifications give GQDs good
dispersibility, with the introduced defects often acting as fluorescent
centers into the GQDs.[47] π−π stacking
interactions between adjacent layers in GQDs can cause interlayer
quenching, similar to that seen for large conjugated aromatic
hydrocarbons or conjugated fluorescent polymers. The effect of such
“aggregation” on the optical properties of GQDs is explored in section
4.
Compared with the top-down methods, synthesis of GQDs by bottom-up
methods is more chemically challenging, but allows the preparation of
GQDs with more uniform morphologies and controlled size. Progressive
reaction in solution is the dominant approach for the bottom-up
synthesis of GQDs. In such bottom-up methods, aggregation is critical to
the synthesis of GQDs, with the aggregation of precursors being a
critical initial step. Such methods have more stringent precursor
requirements, typically utilizing molecules with conjugated structures
or molecules that combine to form six-membered rings, such as
hexabromobenzene [48],
hexa-peri-hexabenzocoronene,[49]1,3,6-trinitropyrene,[50] citric acid
(CA),[51] glucose,[52] and
so on. The reaction precursors react and aggregate into larger
graphene-like sheets, with aggregation controlling the lateral size and
thickness (number of layers). Most GQDs prepared by such routes are
anisotropic. Liu et al. [49] used
hexa-peri-hexabenzocoronene as precursor to prepare GQDs through a
stepwise process of aggregation, carbonization, oxidization, surface
functionalization, and reduction (Figure 2A). Wide-angle X-ray
scattering show that the unit cell parameter of the GQDs was consistent
with the length of the hexa-peri-hexabenzocoronene molecule, with a
strong diffraction peak observed attributed to the π−π stacking distance
between two overlapped molecules of hexa-peri-hexabenzocoronene.
Moreover, by performing control experiments using
hexa-peri-hexabenzocoronene derivatives with different functional
groups, it was demonstrated that the condensed stacking of
hexa-peri-hexabenzocoronene in the GQDs could be attributed to the lack
of steric hindrance from the substituents, leading to the generation of
graphitic framework with few defects during the carbonization step. Lee
et al. [52] prepared GQDs through catalytic
solution chemistry (Figure 2B). The D-glucose carbon source aggregated
horizontally through spontaneous dehydration, resulting in the formation
of graphene. Dong et al. [51] similarly developed
the synthesis of monolayer GQDs using a CA precursor (Figure 2C). Yan et
al. [53, 54] reported the preparation of GQDs with
a tunable size through solution chemistry (Figure 2D) involving surface
modification with 2, 4, 6-triakyl phenyl. This synthesis route allowed
accurate control of the number of conjugated carbon atoms. Crowdedness
at the edges of the graphene cores caused twisting of the phenyl groups
of 2, 4, 6-triakyl phenyl away from the plane of the core, leading to
alkyl chains projecting in all dimensions. This reduces the face-to-face
interaction between the developing graphene sheets, allowing large GQDs
to be synthesized with good solubility.
For all of the aforementioned GQDs, the band gap is closely related to
the size (especially the size of the sp2 domain) and
number of layers resulting from aggregation. Yeh et al.[55] showed that the band gap in GQDs decreased
with increasing GQDs lateral size (Figure 2E). The PL color varied from
red-orange to blue as the GQDs size was reduced (from 8 to 1 nm).
Results are consistent with quantum confinement with decreasing size in
the sp2 conjugate domains. Dong et al.[56] prepared GQDs by chemically oxidizing CX-72
carbon black (Figure 2F). This synthesis route produced monolayer and
multi-layer GQDs simultaneously, with sizes of 15 nm and 18 nm,
respectively. Monolayer and double-layer GQDs have strong green and
yellow PL emissions, respectively. Yan et al. [57]proposed two different strategies to narrow down the electronic band gap
of GQDs (Figure 2G). Since the band gap of pristine GQDs is negatively
correlated with the size of the sp2 domain, bandgap
narrowing was realized via conjugating GQDs with polyaromatic molecules
to enlarge the sp2 carbon network. The sensitivity of
GQDs to this type of modification is due to the anisotropy and small
size of GQDs. This strategy may not work well for other types of CDs
whose bandgaps are less sensitive to quantum confinement
effects.[40]
2.2. CQDs
Fluorescent carbon nanoparticles were first named CQDs by Sun et al. in
2006 [58]. Like GQDs, CQDs also have quantum
confinement and crystalline core structures. The main different between
GQDs and CQDs is their shape and core composition. GQDs are anisotropic
with crystalline sp2 cores, whereas CQDs are
quasi-spherical carbon nanoparticles with less crystallinity and
containing more defects. CQDs typically possess as crystalline core
containing a mixture of sp2 and sp3carbons. Usually, CQDs are terminated by oxygenic/nitrogenous functional
groups, with heteroatom contents ranging from 5-50 wt.%. The
differences between GQDs and CQDs mainly originate from the different
precursor aggregation pathways in their respective syntheses.