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10.2 Graphitic Carbon Nitride 239
important factor is size, which ranges from 1 to 10 µm. This larger size limits its lumines-
cent properties and water solubility [7].
On the other hand, zero-dimensional particles such as carbon quantum dots (CQDs), gra-
phene quantum dots (GQDs), fullerenes, magnetic nanoparticles (MgNPs), inorganic quantum
dots, noble metal nanoparticles, up-conversion nanoparticles (UCNPs), and polymer dots
(Pdots) possess properties such as biocompatibility, ion detection, biomolecular recognition,
pathogen detection, and other notable chemical and physical properties. One important phe-
nomenon to be noted in zero-dimensional particles is the quantum confinement effect, which
arises due to their ultra-small size, which in turn is responsible for the properties mentioned [8].
Although the invention of these zero-dimensional particles has played a major role in the bet-
terment of society, their possible toxicity is still a threat. CQDs have been shown to be the least
toxic of the zero-dimensional particles and have a wide range of applications due to their
remarkable properties. CQDs are simple to functionalise, chemically inert, low-cost while
being highly biocompatible, and can accept and donate electrons. Their properties make them
stand out from other toxic zero-dimensional particles [9].
As discussed earlier, there is a wide range of applications for CQDs, but they cannot be
generalised as the properties and applications vary for carbon dots synthesised via different
methods and from different precursor molecules [10]. Hence, it is necessary to select the appro-
priate precursor and method of synthesis to produce CQDs, as the quantum yield, characteris-
tics, properties, and applications depend on this. CQDs can be synthesised from various sources,
such as organic compounds, inorganic compounds, and polymers. There are hundreds of
articles reporting the synthesis of biocompatible CQDs obtained from natural products, mostly
through hydrothermal synthesis [11]. However, there is a certain drawback that turns the table
upside-down. Although the duration of synthesis depends on the method of synthesis, generally
the duration may be comparatively longer in the case of biosynthesis. Additional separation
steps might be required for complete separation of CQDs from the precursors. A specific size
and shape are difficult to obtain, and a deep understanding of metal salt reduction and the
chemical configuration of the biological capping has not yet been obtained, as numerous
phytochemicals are present in these natural products, which makes it difficult to identify the
specific phytochemical responsible for the process. These are a few demerits that should be
considered in the case of biosynthesis of CQDs [12].
As noted previously, g-C
3
N
4
contains a significant amount of carbon that has extensive
applications, making it a viable candidate for use as a precursor in the synthesis of CQDs.
Difficulties such as large size, luminescent properties, and water solubility, on the other
hand, can be addressed by doping g-C
3
N
4
with carbon-based nanomaterials such as
graphene, carbon nanotubes, and CQDs, which improve the g-C
3
N
4
’s photo- electrochemical
activity [13]. g-C
3
N
4
/CQD production is widely discussed in this chapter, as are the various
methods of synthesising it and its applications.
10.2 Graphitic Carbon Nitride
g-C
3
N
4
was originally discovered in 1834 [14], although due to a lack of knowledge in
this particular area, nothing much was achieved. But during the 1980s, it came back
into fashion as researchers found it a promising two-dimensional (2D), non-metallic,
ᴫ-conjugated polymeric material that had various applications [15]. In 2012, the properties
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10 Synthesis of Graphitic Carbon Nitride Quantum Dots from Bulk Graphitic Carbon Nitride240
of g-C
3
N
4
were reported such as high intrinsic photo-absorption and photo- responsiveness,
semiconductive properties, high stability under physiological conditions, and good
biocompatibility. Since then it has caught the attention of scientists and has been one of
the top areas of research [16].
Polymerisation of precursors with a C–N core structure or thermal polycondensation can
both be used to create carbon nitride materials. Urea, thiourea, melamine, dicyandiamide,
cyanamide, guanidine hydrochloride, guanidine thiocyanate, and thiourea oxide are a few of
the other compounds that can be produced at between 400 and 600 °C [15, 17]. High surface
regions, outstanding stability in chemistry and physics, excellent electronic band structures,
and outstanding electrical, thermal, optical, and mechanical capabilities are just a few of the
many benefits of this material’s constitution. It was shown to be
responsible for g-C
3
N
4
’s unique
sensing capacity [17]. The degree of polymerisation and the
condensation process determine
the characteristics and reactivity of g-C
3
N
4
. This material has a bandgap of between 2.4 and
2.7
eV, which makes it visibly active [15]. Moreover, the efficiency of g-C
3
N
4
in the visible
spectrum is lower when compared to other semiconductor materials with lower bandgaps. As
a result of g-C
3
N
4
’s bandgap, low specific surface, and crystallisation phase, it has a high number
of lattice defects, which leads to frequent
photogenerated species recombination and hence
inhibits g-C
3
N
4
’s photochemical ability [18]. The mechanism based on g-C
3
N
4
for electron
transfer and redox reactions has not been fully elucidated [19]. This is where g-C
3
N
4
’s unusual
2D layered structure, which encourages hybridisation with other components, comes into play.
As a result, researchers began doping g-C
3
N
4
with a variety of different functional groups,
including metal oxides, noble gases, metal-free composites, and zero-dimensional
particles.
CQDs are being
studied in an attempt to improve their photocatalytic capabilities. g-C
3
N
4
is
visible light active and extensively used as a photocatalyst [16, 20].
g-C
3
N
4
is the most durable of the numerous allotropic forms of carbon nitride. The carbon
and nitrogen atoms in this polyconjugated semiconductor give it a
graphitic structure. g-C
3
N
4
has been extensively explored as a catalyst, but its structure has not been
determined. As
shown in Figure 10.2, a triazine-based 2D structure and a
tri-s-triazine-based 2D structure
have been proposed so far [21]. The most recent work on the
three-dimensional (3D) structure
of g-C
3
N
4
employed X-ray diffraction modelling to answer a question about the structure of
g-C
3
N
4
. However, the lack of long-term resources has resulted in an inappropriate refine-
ment of the structure, and so the structure is still not formally defined [22].
Figure 10.2 Triazine-based and tri-s-triazine-based graphitic carbon nitride structures.
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10.3 Quantum Dots 241
10.3 Quantum Dots
Fluorescent nanoparticles were accidentally obtained as a by-product, while the purification of
single-walled carbon nanotubes was investigated in 2004. These luminescent
carbon nanoma-
terials were found to be zero-dimensional and were named carbon dots, carbon quantum dots,
or carbon nanodots [23]. Carbon dots have emerged as one of the most promising materials for
use as an effective catalyst over the last decade. Because they have distinct photophysical and
chemical features, including light-harvesting, it is feasible to derive a variety of photovoltaic
chemical reactions. Photosensitisers, bivalent redox
character, chemical inertness, low toxicity,
remarkable biocompatibility, and good water solubility are some of the properties of optional
photoluminescence. Carbon dots are more environmentally friendly than other quantum dots
because of their chemical inertness and low toxicity. They are nanoparticles with a quasi-
spherical shape and a diameter of less than 5 nm. Depending on the origin of the CQDs, their
size and shape may change,
allowing them to be used in a wide range of configurations [24].
There are two main classifications of the synthesis of CQDs, the top-down approach and
the bottom-up approach, as shown in Figure 10.3. Chemical oxidation, discharge,
electrochemical oxidation, and ultrasonic techniques are all examples of the top-down
approach to carbon decomposition. This method is more expensive and time-consuming
than the bottom-up technique, in which tiny carbon structures are converted into larger
carbon dots. Thermal degradation, pyrolysis, carbonisation, microwave synthesis, and
Bulk Material
Powder
Nanoparticles
Clusters
Atoms
Nanoparticles
TOP-DOWN APPROACH
BOTTOM-UP APPROACH
Figure 10.3 The manufacture of carbon quantum dots combines top-down and bottom-up
strategies.
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10 Synthesis of Graphitic Carbon Nitride Quantum Dots from Bulk Graphitic Carbon Nitride242
solvothermal procedures are all examples of bottom-up treatment approaches [25]. The
synthesis procedure is solely dependent on the precursor material. Furthermore, it has
been found that the addition of heteroatoms to CQDs improves their quality. It is prefera-
ble to dope N rather than other heteroatoms due to their similar size and shape. N’s strong
electronegativity and five-valence electrons, including two lone pairs, facilitate bonding
with electro-positive ions, particularly trivalent and divalent types [24].
Precursor selection is critical since it determines the size, shape, and characteristics of
the CQDs produced. As in the case of atoms, strong emission peaks are feasible. The varia-
tion in size is due to the changes in the surface-to-volume ratio with size and quantum
confinement effects, which depend on the precursor. As the size changes, the photolumi-
nescence emission colour of the CQD also changes. CQDs are commonly found between
atoms and molecules. Quantum confinement effects can occur due to the size of CQDs and
can occur if the energy level spacing of a nanocrystal surpasses kT when the nanocrystal
becomes a nanoparticle [26]. Hundreds of suitable precursors have been identified over the
course of decades of research, each with a unique set of uses. However, quantum dots
derived from g-C
3
N
4
capture attention due to their important features and applications.
There are several methods for obtaining g-C
3
N
4
quantum dots.
10.4 Methods of Synthesis of Graphitic Carbon Nitride Quantum
Dots and Their Applications
g-C
3
N
4
quantum dots can be synthesised using either top-down or bottom-up approaches
(Figure 10.4). Electrochemical oxidation, hydrothermal treatment, and chemical oxidation
are examples of top-down approaches. Bottom-up approaches include microwave, solid-
phase, hydrothermal, and microwave-assisted thermal methods [27].
Figure 10.4 Various methods of synthesis of graphitic carbon nitride quantum dots.
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10.4 Methods of Synthesis of Graphitic Carbon Nitride Quantum Dots and Their Applications 243
This chapter exclusively covers the synthesis of g-C
3
N
4
quantum dots and their uses
when g-C
3
N
4
is used as an initial precursor. As a general rule, g-C
3
N
4
-based quantum dots
are synthesised using the top-down technique (Table 10.1), but we will also talk about
other ways to make g-C
3
N
4
quantum dots.
10.4.1 Top-Down Approaches
10.4.1.1 Chemical Oxidation
Employing chemical oxidation to produce stable carbon-based nanomaterials such as
CQDs is a dependable method since it is quantitative, suggesting that the amount of oxi-
dation depends on the amount of acid used [28]. Adding a strong acid to bulk g-C
3
N
4
has
been shown to cause simultaneous protonation and exfoliation of the g-C
3
N
4
in mass,
resulting in the production of hydrophilic g-C
3
N
4
quantum dots. Many researchers,
including Song and his colleagues as well as Li and his co-workers, have successfully
produced stable g-C
3
N
4
quantum dots using chemical oxidation. Following HNO
3
oxida-
tion, hydrothermal treatment, and ultrasonication, Song et al. exfoliated bulk g-C
3
N
4
via
ultrasonic treatment. The generated g-C
3
N
4
quantum dots had diameters ranging from 1
to 5 nm and surfaces with hydroxyl, carboxylic acid, and amine groups, making them
very dispersible in water [29]. Li et al. synthesised g-C
3
N
4
quantum dots from melamine-
derived bulky g-C
3
N
4
using an acidic chemical oxidation technique and a liquid exfolia-
tion procedure. It was found that the resulting g-C
3
N
4
quantum dots were highly
crystalline and negatively charged, and they were also water dispersible. When bulk
g-C
3
N
4
oxidises, the instability of hydrogen bonds in the facilities of tri-s-triazine
Table 10.1 Bulk graphitic carbon nitride (g-C
3
N
4
) quantum dot production methods utilising
top-down methods.
Method
Starting
material
Type of
photolumine
scence
(365 nm)
Light
absorp
tion, nm
Light
excitation,
nm
Light
emission,
nm
Average
electron
distribu
tion, nm
Quantum
efficiency
(%) References
Chemical
oxidation
Bulk
g-C
3
N
4
Blue 245 220–280 367 1–5 – [29–31]
228 190–270 368 1–5 8.96
Ultrasoni-
cation
350 365 406 2–6 – [32–36]
Chemical
tailoring
243 230–310 367 2–4 46 [39]
Hydrothermal
treatment
400 300–400 437 5–20 16.9 [40–42]
Electrochemi-
cal oxidation
268 260–420 450 5–8 – [45]
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10 Synthesis of Graphitic Carbon Nitride Quantum Dots from Bulk Graphitic Carbon Nitride244
increases, resulting in attachment and exfoliation [30]. g-C
3
N
4
quantum dots were syn-
thesised in o-dichlorobenzene by chemically oxidising bulk g-C
3
N
4
and then hydrother-
mally treating the solution. At a 2 vol% concentration of quantum dots, polymer solar cell
systems demonstrated a 40% boost in energy conversion efficiency. This highlights the
photovoltaic enhancer properties of the g-C
3
N
4
quantum dots generated [31]. As a result,
this approach is suitable for large-scale manufacturing if certain considerations are met,
such as the demand for further exfoliation treatments and the removal of excess oxidants
from the reaction environment.
10.4.1.2 Ultrasonication
There is simultaneous formation and dissolution of bubbles during ultrasonication. It
is through this process that the material’s bonds are broken because the bubble bursts
and releases its stored energy. There are two types of bonds in C
3
N
4
. When the 0.1316
nm link in the rings of the melon unit and the 0.1442 nm bond that connects them are
disturbed, the rings are subjected to weak van der Waals forces, and robust C–N
interactions are both present in the C
3
N
4
layers under 180 W [32]. The bonds are broken
because of the cavitation process; that is, the energy liberated from the broken bubbles
is utilised to break the bonds. Because the C–N linkage establishing a connection
between the melon components provides poor binding strength, strong ultrasonic
energy might disrupt its C–N interconnection, resulting in reactive radicals like C and
N. However, their activity and the ability to rapidly rearrange make those radicals
invaluable. Wang et al. devised a solution using ethylene glycol as the solvent. Ethylene
glycol is a radical scavenger with good reducibility, which aids in the rapid acceptance
of active radicals. Because of this radical limitation [33], the reformation of bulk C
3
N
4
is limited, resulting in C
3
N
4
quantum dots (Figure 10.5). Furthermore, because of the
proper viscosity of ethylene glycol, the use of ethylene glycol as the solvent has other
advantages, such as the wide dispersion of quantum dots and the limitation of quan-
tum dot collisions. Water can be used as a solvent, but the quantum dots obtained are
larger, and more complex steps might be required in the case of separation [34]. There
have been reports with regard to the creation of g-C
3
N
4
quantum dots utilising water
solvent via ultrasonication of g-C
3
N
4
[35, 36]. Employing CCRF-CEM cells, autosens-
ing investigations were performed on the synthesised g-C
3
N
4
quantum dots, and the
Bulk g-C
3
N
4
Acid
Treatment
NH
3
Treatment
Exfoliated porous g-C
3
N
4
Porous g-C
3
N
4
Ultrasonication
Single Layere
d
g-C
3
N
4
QDs
Figure 10.5 Strategy for the preparation of graphitic carbon nitride quantum dots through
ultrasonication.
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10.4 Methods of Synthesis of Graphitic Carbon Nitride Quantum Dots and Their Applications 245
results demonstrated that these dots have outstanding sensing capabilities [36]. As
already discussed, exfoliated porous nanosheets were obtained through the chemical
oxidation method after treatment with acid. That is where ultrasonication comes into
play. Only through ultrasound were single-layered g-C
3
N
4
quantum dots obtained
[29,
37].
10.4.1.3 Chemical Tailoring
In chemical tailoring, which is also known as chemical cleavage, specific impacts on par-
ticular chemical bonds are used to perform precise molecular cutting [38]. Large molecules
are broken down into smaller, pre-determined pieces. QDs are created from g-C
3
N
4
and
nanowires. Zhang et al. employed larger stacked amounts of polymeric carbon nitride as a
starting material. g-C
3
N
4
quantum dots were created by partially hydrolysing bulk C
3
N
4
in
H
2
SO
4
, with water acting as a protic solvent to disrupt the hydrogen bonds. Hydrogen
atoms were dissolved in H
2
O solvent bonds while partially hydrolysing bulk C
3
N
4
in H
2
SO
4
[39]. The g-C
3
N
4
quantum dots had a unique electronic structure and modified surface
properties that allowed them to detect Fe
3+
ions while being used as light-emitting diodes
(LEDs).
10.4.1.4 Hydrothermal Treatment
Hydrothermal treatment is the most cost-efficient and greener method of all the top-
down methods of synthesis of g-C
3
N
4
quantum dots. When Zhang et al. reported on
hydrothermal
treatment, the mass g-C
3
N
4
was employed in a novel way, especially as
an inducer.
Water-soluble, blue-coloured typical g-C
3
N
4
quantum dots have an aver-
age diameter of about 10 nm and were obtained after 10 hours of heating at 180 °C. A
16.9% greater quantum yield was achieved from quantum dots, and they are excellent
fluorescent probes for metal ion detection [40]. Another interesting method of g-C
3
N
4
synthesis was carried out by Yu and colleagues. Although g-C
3
N
4
nanosheets were
obtained from bulk g-C
3
N
4
by heat etching, the nanosheets were subjected to acidic
cutting, resulting in nanoribbons. Finally, after the nanoribbons underwent hydro-
thermal cutting at 200° C for 10 hours, g-C
3
N
4
quantum dots were obtained. These
g-C
3
N
4
quantum dots were approximately 7 nm in size, had blue emissions with tun-
able photoluminescence (PL), and had the possibility to employ the Vis-NIR (near-
infrared) spectrum of sunlight via photocatalysis [41]. Due to the high temperature and
pressure during hydrothermal treatment, a shear force is applied to the system under
investigation, and 3D g-C
3
N
4
is exfoliated into zero-dimensional (0D) quantum dots.
According to Zhan and colleagues, quantum dots can be made in a single step by heat-
ing the autoclave holding bulk g-C
3
N
4
in a mixture of concentrated potassium hydroxyl
(KOH) and ethanol solution at 180 °C for 16 hours. Adding KOH to g-C
3
N
4
sheets not
only helps g-C
3
N
4
be broken down in the process of exfoliation sheets caused by the
intercalation of potassium and hydroxyl into layers, it also assists in oxidising the mar-
gins of the g-C
3
N
4
sheets, as shown in Figure 10.6 [42].
10.4.1.5 Electrochemical Oxidation
The phenomenon by which the oxidation state of an ion or molecule is altered by the
transfer of electrons from or to the molecule of an ion when an external current of
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10 Synthesis of Graphitic Carbon Nitride Quantum Dots from Bulk Graphitic Carbon Nitride246
chemical energy is applied is called electrochemical oxidation [43]. Quantum dots
obtained through
electrochemical oxidation show high stability. The synthesis of
quantum dots by electrochemical oxidation has been reported in two ways. In one
instance, the carbon–carbon bonds of the precursor are actively broken by the oxida-
tion process. Quantum dots can be oxidatively cleaved by the formation of a hydroxyl
free radical (⋅OH) or an oxygen free radical (⋅O) [44]. The electrolyte was a combina-
tion of 100 mg large bulk g-C
3
N
4
, 50 mg NaOH, as well as 10 mL water mixed together.
The positive and negative electrodes were platinum sheets. The solution was electro-
lysed and diluted (Mw 1000) with H
2
O for 48 hours after being exposed to 10 V (DC)
for three hours and centrifuged at 10,000 rpm for 15 minutes to separate the g-C
3
N
4
quantum dots. The synthesised quantum dots were extremely water stable, had high
fluorescence (FL), and could be reduced at around 60 °C by Ag+ transforming them
into Ag nanoparticles [45]. Although the process produced stable quantum dots, the
pre-treatment of raw materials and purification of quantum dot products took longer
than expected [46].
10.4.2 Other Methods of Synthesis
There are numerous works reporting the synthesis of g-C
3
N
4
quantum dots through differ-
ent methods of synthesis other than those methods discussed here. A few of those methods
and the characteristics of the synthesised quantum dots are given in Table 10.2.
10.5 Conclusion
The methods for g-C
3
N
4
quantum dot synthesis from bulk g-C
3
N
4
were the focus of
this chapter. Using g-C
3
N
4
as a starting material for the synthesis of carbon dots is a
straightforward and efficient method. Despite the fact that there are numerous meth-
ods available, the chapter focuses on the synthesis of quantum dots with bulk g-C
3
N
4
.
Serving as a major contributor, and numerous top-down methods are being employed.
The photon emission efficiency of the type of methods that have been performed can
be seen clearly in their quantum yields. Even though it has proven to be extremely use-
ful, there are still a number of application fields that need to be explored (Figure 10.7)
where there is scope for improvement.
Bulk g-C
3
N
4
Heat
Etching
g-C
3
N
4
Nano sheets
Acid
Cutting
g-C
3
N
4
Nano ribbons
Hydrothermal
Cutting
g-C
3
N
4
QDs
Figure 10.6 Synthesis of graphitic carbon nitride quantum dots through hydrothermal treatment.
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Table 10.2 Bottom-up techniques for making graphitic carbon nitride (g-C
3
N
4
) quantum dots from other factors.
Methods Starting materials
Types of
photolumine-
scence (365nm)
Light absorption,
nm
Light
excitation, nm
Light emission,
nm
Average electron
distribution,
nm
Quantum
efficiency
(%) References
Hydrothermal
process
C
3
H
6
N
6
and
C
5
H
8
O
2
C₆H₈O₇ and
CH
4
N
2
S
blue 282 360 440 3–8 31 [46]
338 369 444 2.78 14.5 [45, 47]
[Bmim]BF
4
and water
240 355–385 458 2.2–6.1 8.34 [48]
Solvothermal
method
CCl
4
and EN 250 360 440 1–5 11 [49]
Citric acid
monohydrate, urea, and
oleic acid
268, 335,
and 400
360–420 450–540 1–5 27.1 [7]
Microwave
method
Guanidine
hydrochloride
and EDTA
260 360 453 3.2–6.5 35 [50]
Citric acid, urea, and oleic
acid
325 425 531 <10 - [51]
DMF and chlorosulfonic acid 266 360 443 1–6 9 [52]
Dimethylamine and H
2
SO
4
262 380 454 2–8 8.9 [53]
Formamide 260–340 340 405 2–15 29 [54]
(Continued)
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Methods Starting materials
Types of
photolumine-
scence (365nm)
Light absorption,
nm
Light
excitation, nm
Light emission,
nm
Average electron
distribution,
nm
Quantum
efficiency
(%) References
Microwave-
assisted
thermal
Thymine,
DAMN, and
NaOH
271 350 445 3–8 46 [55]
Solid-phase
synthesis
Urea and
sodium citrate
344 360 467 2.6–5.5 42 [34]
Melamine and
EDTA
200–500 360 440 3–8 34 [56]
EDTA.2Na
and
thiourea
263–358 360 433 3–13 13.4 [57]
[Bmim]BF
4
, 1-butyl-3-methylimidazolium tetrafluoroborate; CCl
4
, carbon tetrachloride; C
3
H
6
N
6
, melamine; C
5
H
8
O
2
, acetylacetone; C₆H₈O₇, citric acid; CH
4
N
2
S, thiourea;
DAMN, diaminomaleonitrile; DMF, dimethylformamide; EDTA, ethylenediamine tetraacetic acid; EN, ethylenediamine.
Table 10.2 (Continued)
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