Size and Shape Selective Metal Oxide Nanomaterials …
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suitable for preparing one and two-dimensional nanostructured powder and films
(on a substrate). The synthetic strategy of hierarchical nanoarrays can be achieved
by single-step self-template growth and multistep-graded growth methods. A multistep-graded growth method is done by making secondary structure on an already
obtained primary nanostructure and it is repeated many times. Applying changes in
the reaction condition during secondary process, the size and shape can be controlled
effectively [25]. On account of the variations in equilibrium with temperature, formed
particles (at lower temperatures) re-dissolve and re-crystallize (at higher temperatures). During heating up or changing the aging period at a constant temperature the
shape control is achieved. The dielectric constant of water is considered important
for controlling factors of reaction equilibrium, rate of reaction and solubility. The
chemical potential of ions is lowered by the usage of high dielectric constant medium,
and dissociation of electrolytes produces reasonable stability [26].
The effects of temperature, concentration of reducing agents such as NaOH, reaction duration and addition of seed particles are known to control the final product
formation such as flower or other particle shapes [27, 28]. The coordination numbers
play a vital role in obtaining specific structures in the same compound. For example,
in preparation of HfO 2 , structures that exhibit coordination number 8 have shown
excellent geometric symmetry and it forms easily. This property is also important in
deciding room temperature stability. In normal temperature, the monoclinic-HfO 2
is very stable as compared to tetragonal crystals [29]. The thermodynamics and
kinetics govern the reaction mechanism. If the kinetic stability is good, then it is
expected to achieve better symmetry at higher alkaline condition. The concentration
of alkalinity has been shown to decide the crystal structure (such as monoclinic or
tetragonal) due to increase in the oxygen ions required for perfect coordination and
symmetry. In terms of nucleation, overall Gibbs free energy change (G) due to
the changes from original particles and to the cluster formation (by new surface) is
maximum at the critical size, r*. This is actually the maximum free energy change
required for nucleation. An excess or changes in this energy is further required for
growth and achieving stability [29].
4.4 Plasma Assisted Methods of Synthesis
Plasma is a collection of freely moving charged particles consisting of electrons,
ions, radicals and neutral species. The low temperature, non-equilibrium plasmas
provides many advantages for synthesizing size and shape specific MONMs at fast
bulk production mode. In this process, the reaction is carried out at room temperature,
and no polymer surfactants or solution agitation are needed. There are different
plasma methods such as dielectric barrier discharge (DBD), soft jet (APPJ), micro
plasma and corona discharge can be used. The NMs precursor solution is mixed
and exposed to the mentioned plasma source for an hour or more. The incoming
plasma species create turbulence and the MONMs nucleate and grow in the presence
of electric field established by plasma thus specific shapes can be achieved. For
89
suitable for preparing one and two-dimensional nanostructured powder and films
(on a substrate). The synthetic strategy of hierarchical nanoarrays can be achieved
by single-step self-template growth and multistep-graded growth methods. A multistep-graded growth method is done by making secondary structure on an already
obtained primary nanostructure and it is repeated many times. Applying changes in
the reaction condition during secondary process, the size and shape can be controlled
effectively [25]. On account of the variations in equilibrium with temperature, formed
particles (at lower temperatures) re-dissolve and re-crystallize (at higher temperatures). During heating up or changing the aging period at a constant temperature the
shape control is achieved. The dielectric constant of water is considered important
for controlling factors of reaction equilibrium, rate of reaction and solubility. The
chemical potential of ions is lowered by the usage of high dielectric constant medium,
and dissociation of electrolytes produces reasonable stability [26].
The effects of temperature, concentration of reducing agents such as NaOH, reaction duration and addition of seed particles are known to control the final product
formation such as flower or other particle shapes [27, 28]. The coordination numbers
play a vital role in obtaining specific structures in the same compound. For example,
in preparation of HfO 2 , structures that exhibit coordination number 8 have shown
excellent geometric symmetry and it forms easily. This property is also important in
deciding room temperature stability. In normal temperature, the monoclinic-HfO 2
is very stable as compared to tetragonal crystals [29]. The thermodynamics and
kinetics govern the reaction mechanism. If the kinetic stability is good, then it is
expected to achieve better symmetry at higher alkaline condition. The concentration
of alkalinity has been shown to decide the crystal structure (such as monoclinic or
tetragonal) due to increase in the oxygen ions required for perfect coordination and
symmetry. In terms of nucleation, overall Gibbs free energy change (G) due to
the changes from original particles and to the cluster formation (by new surface) is
maximum at the critical size, r*. This is actually the maximum free energy change
required for nucleation. An excess or changes in this energy is further required for
growth and achieving stability [29].
4.4 Plasma Assisted Methods of Synthesis
Plasma is a collection of freely moving charged particles consisting of electrons,
ions, radicals and neutral species. The low temperature, non-equilibrium plasmas
provides many advantages for synthesizing size and shape specific MONMs at fast
bulk production mode. In this process, the reaction is carried out at room temperature,
and no polymer surfactants or solution agitation are needed. There are different
plasma methods such as dielectric barrier discharge (DBD), soft jet (APPJ), micro
plasma and corona discharge can be used. The NMs precursor solution is mixed
and exposed to the mentioned plasma source for an hour or more. The incoming
plasma species create turbulence and the MONMs nucleate and grow in the presence
of electric field established by plasma thus specific shapes can be achieved. For