Laser Ablation Method
In laser ablation method, carbon precursors firstly absorb the high-energy laser
pulse, and then electrons are removed through photoelectric and thermionic emission
from the atom followed by generation of high electric field which produces strong
repulsive forces and thus breaks down the carbon quantum dots (Xiao et al. 2017).
Laser ablation method is a chemically simple and facile method and generates very
little by-products. Also, laser ablation method does not require extreme temperature
and pressure, and also there is no need of catalysts for the process to be carried out.
Further, preparation of carbon quantum dots was done using carbon nanoparticles in
organic solvent ethanol and acetone (Li et al. 2010a). Here, carbon nanomaterial was
dissolved in organic solvent, with subsequent ultrasonication. The suspension was
then conserved in a glass cell irradiated with laser followed by continuous magnetic
stirring so as to avoid settling of nanoparticles. Other than carbon quantum dots,
fluorescent carbon nanoparticles were also prepared by laser irradiation of carbon
suspension in organic solvent (Hu et al. 2009).
Sun and co-worker (2006) also synthesize carbon dots by using laser ablation
method in argon using water vapor. Carbon precursor was prepared by stepwise
baking, curing, and annealing of the mixture of graphite powder and cement in an
argon flow. The obtained sample was treated with aq. HNO 3 solution to oxidize
surface carbon with refluxing for 12 h followed by surface passivation of carbon
particle through organic species which exhibit luminescence property. Thus, the
surface passivation of carbon precursor by organic molecule polyethylene glycol to
prepare carbon quantum dots can activate the surface and become highly photoactive
in visible and IR region (Wang et al. 2009). Not only carbon quantum dots but
one-dimensional iron-based bimetallic magnetic nanoparticles such as FePt, FeCo,
and FeNi were also synthesized by laser ablation method (Liang et al. 2014). Laser
ablation method is mainly associated with low productivity problem which strongly
depends upon laser power density and liquid medium. Because nanoparticles have
strong absorption and scattering effects in liquid medium, determining their productivity is an important parameter.
Electrochemical Approach
Electrochemical method involves a nonselective top-down approach using various
carbon precursors for the synthesis of carbon quantum dots. Electrochemical method
is a unique method since the size of quantum dots can be adjusted by regulating
potential. Ahirwar et al. (2017) synthesize graphene and graphene oxide quantum
dots via electrochemical exfoliation method with 2–3 nm in size. The two electrodes
used were made up of graphite which is dipped in the solution of electrolyte made up
of combination of weak and strong electrolytes. Further, Devi et al. (2018) synthesize carbon quantum dots via electrochemical method using graphite electrode acting
as the anode and cathode, respectively, and mixture of NaOH/EtOH as an electrolyte
3 Metal and Carbon Quantum Dot Photocatalysts for Water Purification
89
In laser ablation method, carbon precursors firstly absorb the high-energy laser
pulse, and then electrons are removed through photoelectric and thermionic emission
from the atom followed by generation of high electric field which produces strong
repulsive forces and thus breaks down the carbon quantum dots (Xiao et al. 2017).
Laser ablation method is a chemically simple and facile method and generates very
little by-products. Also, laser ablation method does not require extreme temperature
and pressure, and also there is no need of catalysts for the process to be carried out.
Further, preparation of carbon quantum dots was done using carbon nanoparticles in
organic solvent ethanol and acetone (Li et al. 2010a). Here, carbon nanomaterial was
dissolved in organic solvent, with subsequent ultrasonication. The suspension was
then conserved in a glass cell irradiated with laser followed by continuous magnetic
stirring so as to avoid settling of nanoparticles. Other than carbon quantum dots,
fluorescent carbon nanoparticles were also prepared by laser irradiation of carbon
suspension in organic solvent (Hu et al. 2009).
Sun and co-worker (2006) also synthesize carbon dots by using laser ablation
method in argon using water vapor. Carbon precursor was prepared by stepwise
baking, curing, and annealing of the mixture of graphite powder and cement in an
argon flow. The obtained sample was treated with aq. HNO 3 solution to oxidize
surface carbon with refluxing for 12 h followed by surface passivation of carbon
particle through organic species which exhibit luminescence property. Thus, the
surface passivation of carbon precursor by organic molecule polyethylene glycol to
prepare carbon quantum dots can activate the surface and become highly photoactive
in visible and IR region (Wang et al. 2009). Not only carbon quantum dots but
one-dimensional iron-based bimetallic magnetic nanoparticles such as FePt, FeCo,
and FeNi were also synthesized by laser ablation method (Liang et al. 2014). Laser
ablation method is mainly associated with low productivity problem which strongly
depends upon laser power density and liquid medium. Because nanoparticles have
strong absorption and scattering effects in liquid medium, determining their productivity is an important parameter.
Electrochemical Approach
Electrochemical method involves a nonselective top-down approach using various
carbon precursors for the synthesis of carbon quantum dots. Electrochemical method
is a unique method since the size of quantum dots can be adjusted by regulating
potential. Ahirwar et al. (2017) synthesize graphene and graphene oxide quantum
dots via electrochemical exfoliation method with 2–3 nm in size. The two electrodes
used were made up of graphite which is dipped in the solution of electrolyte made up
of combination of weak and strong electrolytes. Further, Devi et al. (2018) synthesize carbon quantum dots via electrochemical method using graphite electrode acting
as the anode and cathode, respectively, and mixture of NaOH/EtOH as an electrolyte
3 Metal and Carbon Quantum Dot Photocatalysts for Water Purification
89