1 3
Topics in Current Chemistry (2020) 378:3
combination with nitrogen-containing dopants. Several examples of the use of biomass residues for the preparation of CQDs will be described in this section [71–74].
For instance, citrus limetta waste pulp was treated using a one-step pyrolysis
method to prepare CQDs with interesting optoelectronic features. This carbon-based
nanomaterial was demonstrated to be highly versatile, since it was successfully
employed for photo-electrochemical water splitting and photocatalytic MB degradation, along with bioimaging, iron ion sensing, and bactericidal activity (Fig. 5a–c)
[75]. The photo-electrochemical performance of the prepared CQDs displayed
encouraging results, with an efficient current density of ~6 mA/cm
2
for water splitting (Fig. 5b).
Another example of the use of CQDs as photocatalysts was reported by Reisner
et al., who highlighted the influence of different amorphous and graphite-like CQDs
on photoinduced hydrogen production [76]. The various CQDs were used as photosensitizers to activate a nickel catalytic redox mediator via photoinduced electron
transfer. Although improved hydrogen evolution was observed when using graphitic
nitrogen-doped CQDs, a complete and precise rationalization of the effects of the
CQD morphology and dopant on the photocatalytic performance and light-harvesting properties cannot be drawn, because different carbon sources were used (citric
acid for non-doped CQDs and aspartic acid for the N-doped CQDs), and only graphitic CQDs were tested [77].
Prato et al. recently prepared a family of photoredox N-doped CQDs, revealing
their potential applicability as photocatalysts due to their tunable oxidation/reduction
potential. However, further efforts are still needed in order to expand the knowledge
about the proper design of photoredox-active CQDs, considering synthetic methods,
carbon sources, and possible dopants [78]. In this regard, the size, degree of carbonization, and morphology of CQDs could be controlled by the synthetic strategy. For
instance, hydrothermal approaches generally give rise to incomplete carbonization
and therefore to the presence of molecular fluorophores and amorphous CQDs. In
turn, pyrolytic conditions lead to CQDs with a predominantly graphitic core structure. However, the optical properties and photoredox activity are still difficult to predict. In particular, the emission type and intensity of CQDs are attributed mainly to
the cooperative effects between the molecular fluorophores embedded in both the
nanoparticles and the defect states, and the graphitic cores [79].
Of note, Peroza et al. reported the synthesis of CQDs using two different preparation methods, namely hydrothermal and pyrolytic techniques, and using (1) citric acid and (2) citric acid doped with diethylenetriamine [80]. The hydrothermal
approach gave rise to amorphous CQDs that were either (1) non-doped (a-CDs) or
(2) nitrogen-doped (a-N-CDs). The pyrolytic protocol resulted in the formation of
graphitic CQDs, which were (1) non-doped (g-CDs) or (2) nitrogen-doped (g-NCDs) (Fig. 6a, b). These nanomaterials showed an excitation-dependent emission
band, with a maximum between 420 and 500 nm (Fig. 6c–f). The photocatalytic
activity of the CQDs was investigated for the photoreduction reaction of methyl
viologen (MV) in the absence of redox mediators.
A hydrothermal carbonization method has also been reported as a sustainable and
green option for the preparation of carbon particles in an aqueous medium from a
readily available biomass residue, namely orange peel (Fig. 7a–c). Remarkably, the
11
Reprinted from the journal
Topics in Current Chemistry (2020) 378:3
combination with nitrogen-containing dopants. Several examples of the use of biomass residues for the preparation of CQDs will be described in this section [71–74].
For instance, citrus limetta waste pulp was treated using a one-step pyrolysis
method to prepare CQDs with interesting optoelectronic features. This carbon-based
nanomaterial was demonstrated to be highly versatile, since it was successfully
employed for photo-electrochemical water splitting and photocatalytic MB degradation, along with bioimaging, iron ion sensing, and bactericidal activity (Fig. 5a–c)
[75]. The photo-electrochemical performance of the prepared CQDs displayed
encouraging results, with an efficient current density of ~6 mA/cm
2
for water splitting (Fig. 5b).
Another example of the use of CQDs as photocatalysts was reported by Reisner
et al., who highlighted the influence of different amorphous and graphite-like CQDs
on photoinduced hydrogen production [76]. The various CQDs were used as photosensitizers to activate a nickel catalytic redox mediator via photoinduced electron
transfer. Although improved hydrogen evolution was observed when using graphitic
nitrogen-doped CQDs, a complete and precise rationalization of the effects of the
CQD morphology and dopant on the photocatalytic performance and light-harvesting properties cannot be drawn, because different carbon sources were used (citric
acid for non-doped CQDs and aspartic acid for the N-doped CQDs), and only graphitic CQDs were tested [77].
Prato et al. recently prepared a family of photoredox N-doped CQDs, revealing
their potential applicability as photocatalysts due to their tunable oxidation/reduction
potential. However, further efforts are still needed in order to expand the knowledge
about the proper design of photoredox-active CQDs, considering synthetic methods,
carbon sources, and possible dopants [78]. In this regard, the size, degree of carbonization, and morphology of CQDs could be controlled by the synthetic strategy. For
instance, hydrothermal approaches generally give rise to incomplete carbonization
and therefore to the presence of molecular fluorophores and amorphous CQDs. In
turn, pyrolytic conditions lead to CQDs with a predominantly graphitic core structure. However, the optical properties and photoredox activity are still difficult to predict. In particular, the emission type and intensity of CQDs are attributed mainly to
the cooperative effects between the molecular fluorophores embedded in both the
nanoparticles and the defect states, and the graphitic cores [79].
Of note, Peroza et al. reported the synthesis of CQDs using two different preparation methods, namely hydrothermal and pyrolytic techniques, and using (1) citric acid and (2) citric acid doped with diethylenetriamine [80]. The hydrothermal
approach gave rise to amorphous CQDs that were either (1) non-doped (a-CDs) or
(2) nitrogen-doped (a-N-CDs). The pyrolytic protocol resulted in the formation of
graphitic CQDs, which were (1) non-doped (g-CDs) or (2) nitrogen-doped (g-NCDs) (Fig. 6a, b). These nanomaterials showed an excitation-dependent emission
band, with a maximum between 420 and 500 nm (Fig. 6c–f). The photocatalytic
activity of the CQDs was investigated for the photoreduction reaction of methyl
viologen (MV) in the absence of redox mediators.
A hydrothermal carbonization method has also been reported as a sustainable and
green option for the preparation of carbon particles in an aqueous medium from a
readily available biomass residue, namely orange peel (Fig. 7a–c). Remarkably, the
11
Reprinted from the journal