38
K. Illath et al.
broad spectrum of variations, are possible. Compared to the other methods discussed
above, crystal structure and surface morphology can be controlled in CVD. Further,
the thin-film coating exhibits high durability, and it is easy to scale up [34, 35].
PVD and CVD methods are usually employed for forming thin films on a substrate.
Later, corresponding NPs or composites can be synthesized, which are generally not
preferred for large scale preparation of pure MNPs. It has been found that nanocomposites containing Ag NPs synthesized using solid-state methods are suitable in
biomedical applications due to their antimicrobial activity and toxic-free nature [17].
3.2 Gas-Phase Methods
Gas-phase methods include spray pyrolysis, laser pyrolysis, and flame pyrolysis.
Spray pyrolysis is done with the help of an apparatus that has three parts; one nebulizer to spray precursor in the vapour form into the hot reactor, a tubular reactor,
which is kept at 200 °C to 120 °C (thermostatically controlled) and a precipitator
for collecting the NPs. The apparatus can be modified by replacing the nebulizer
with ultrasound and adopting atomized techniques like two fluids nozzle or airassisted pumps or sprayers, spinning disk, and vibrating orifice. Out of these modified
methods, ultrasound-based pyrolysis is commonly employed, where ultrasound of
specific frequency is used to produce atomized droplets from the precursor. Atomized droplets are transported to reaction furnace by the carrier gas, and later NPs
are collected in the precipitator. The advantages of spray pyrolysis are low-cost,
simplicity, reproducibility, and easiness in controlling particle size. This is applicable
for metal oxide and mixed metal oxide nanoparticle synthesis. In laser pyrolysis, laser
energy is used to activate a homogeneous nucleation reaction by exposing laser on
the precursor. Commonly employed laser energy heating is infrared carbon dioxide
(CO 2 ) laser energy, which is easily absorbed by inert photosensitizers like sulfur
hexafluoride. Once the sufficient amount of supersaturation of condensable product
is reached in the vapour phase, CO 2 pyrolysis starts immediately. NPs of uniform
size distribution is possible with this approach. Further, the size of the particles can
be controlled by adjusting the flow rate of reagents through the pyrolysis reaction
zone. In flame pyrolysis, the liquid precursor is directly sprayed into the flame, leads
to the delivery of precursors in the form of vapour, which do not possess sufficient
vapour pressure. This method is useful for less volatile raw materials and considered
as a promising approach for producing metal oxide NPs [34].
3.3 Liquid State Synthesis Methods
Liquid state synthesis consists of sol-gel, chemical reduction, hydrothermal and
solvothermal method. The sol-gel process involves the formation of networks of
colloidal suspension (sol) and gelatine in the liquid phase. It can be performed in
K. Illath et al.
broad spectrum of variations, are possible. Compared to the other methods discussed
above, crystal structure and surface morphology can be controlled in CVD. Further,
the thin-film coating exhibits high durability, and it is easy to scale up [34, 35].
PVD and CVD methods are usually employed for forming thin films on a substrate.
Later, corresponding NPs or composites can be synthesized, which are generally not
preferred for large scale preparation of pure MNPs. It has been found that nanocomposites containing Ag NPs synthesized using solid-state methods are suitable in
biomedical applications due to their antimicrobial activity and toxic-free nature [17].
3.2 Gas-Phase Methods
Gas-phase methods include spray pyrolysis, laser pyrolysis, and flame pyrolysis.
Spray pyrolysis is done with the help of an apparatus that has three parts; one nebulizer to spray precursor in the vapour form into the hot reactor, a tubular reactor,
which is kept at 200 °C to 120 °C (thermostatically controlled) and a precipitator
for collecting the NPs. The apparatus can be modified by replacing the nebulizer
with ultrasound and adopting atomized techniques like two fluids nozzle or airassisted pumps or sprayers, spinning disk, and vibrating orifice. Out of these modified
methods, ultrasound-based pyrolysis is commonly employed, where ultrasound of
specific frequency is used to produce atomized droplets from the precursor. Atomized droplets are transported to reaction furnace by the carrier gas, and later NPs
are collected in the precipitator. The advantages of spray pyrolysis are low-cost,
simplicity, reproducibility, and easiness in controlling particle size. This is applicable
for metal oxide and mixed metal oxide nanoparticle synthesis. In laser pyrolysis, laser
energy is used to activate a homogeneous nucleation reaction by exposing laser on
the precursor. Commonly employed laser energy heating is infrared carbon dioxide
(CO 2 ) laser energy, which is easily absorbed by inert photosensitizers like sulfur
hexafluoride. Once the sufficient amount of supersaturation of condensable product
is reached in the vapour phase, CO 2 pyrolysis starts immediately. NPs of uniform
size distribution is possible with this approach. Further, the size of the particles can
be controlled by adjusting the flow rate of reagents through the pyrolysis reaction
zone. In flame pyrolysis, the liquid precursor is directly sprayed into the flame, leads
to the delivery of precursors in the form of vapour, which do not possess sufficient
vapour pressure. This method is useful for less volatile raw materials and considered
as a promising approach for producing metal oxide NPs [34].
3.3 Liquid State Synthesis Methods
Liquid state synthesis consists of sol-gel, chemical reduction, hydrothermal and
solvothermal method. The sol-gel process involves the formation of networks of
colloidal suspension (sol) and gelatine in the liquid phase. It can be performed in