1.1.
Hot Colloidal Chemistry:
A wide range of inorganic with different composition and variety can be synthesized by
hot colloidal chemistry. In fact, such synthesizing chemistry allows the alteration in electronic
and optical properties of nanomaterials in well-defined way. Metallic and Plasmonic
nanoparticles, semiconductor quantum dots, self-doped Plasmonic semiconductors, alloy
semiconductors and lanthanide doped multi-colour upconversion nanoparticles can be
manufactured by hot colloidal chemistry[35, 83].
Nano chemistry normally incorporates with three main components along with their
specific ratios, temperature and ligand controlled coordination solvent. It encompasses;
1. Precursors having Metallic salt or organic based metallic compounds.
2. Surfactant also called ligand or capping agent.
3. High boiling solvent
The precursor provides the inorganic elements, whereas selected ligand anchor with
hydrophilic head ended with a functional group and hydrophilic tail. Growing particles absorb
ligand on their surface which restrict the size and regulate dimension in nanoscale. Surfactant
may be oleic acid, oleylamine, trioctyl phosphine (TOP) and trioctyl phosphine oxide (TOPO).
High boiling solvent (150-350°C) offers thermal energy for whole solubility, solubilization,
and decomposition of reactants and crystallization of resultant nanoparticles[84]. The growth
of particles split into four major processes and graphically represented by La Mer curve as
shown in figure 12;
Figure 1: La Mer curve which explains the growth processes of nanostructures[85]
Phase1: Monomers having ions or small identical molecules are produced from raw metallic
salts by thermolysis (breakdown of macromolecules via heat). These are elementary units of
nanocrystals which accumulate in precursor solution their concentration controls the reaction
kinetics of nanoparticles
Phase 2: Nucleation process in which monomer tend to agglomerate to form nucleus.
Phase 3: Growth of particles and crystallization into nano structures
Phase 4: Recrystallization or Ostwald repining process refers to the development of larger
crystals with low surface to volume ratio from those of smaller size which have high surface
energy. In the process, many small crystals formed initially slowly vanish, with the expense of
the small crystals. The smaller crystals act as fuel for the growth of bigger crystals. Based upon
size importance or for uniform size distribution this process might be in limit
Concentration of monomers specified into two types namely the nucleation and saturation
concentration Cn and Cs respectively. Nucleation does not occur until the accumulated
concentration of monomers is larger than Cn. Furthermore, the concentration decreases after
nucleation process just because consumption of monomers into nucleation growth and
concentration of monomers fall down at saturation level Cs, here crystallization and phase of
nano dimensional material are formed. Precursors can be loaded into hot solution of solvent
and surfactant by two methods a hot injection method and other is heating up method.
1. Hot injection method contains high boiling solution of solvent and surfactant while by the
mean of injection precursor solution discharge into it. Immediate nucleation process occurs
which is separated from growth process, yields uniform size nanoparticles. Size can be
manipulated by temperature, nature of solvent and aging time. Ostwald repining process
controlled by termination the process time for size homogeneity. Semiconductors quantum
dots, Plasmonic metals and semiconductors can be successfully synthesized by hot
injection method [86, 87].
2. Heating up approach contains precursor release into solution which follows two routes of
nucleation and growth. One has thermal decomposition of metallic ion from metallic salt
creating burst nucleation. Other is mixing precursor into solution at room temperature and
follow the heating up approach. Magnetic, lanthanide doped[88] and metallic Plasmonic
nanoparticles are synthesized by this method[87].
1.2.
Synthesis Protocols
1.2.1. Sol Gel Method:
Sol-gel technology is a well-established colloidal chemistry technology, which deals
possibility to produce various materials with unique, predefined properties in simple process
and at relatively low process. There are presently an extensive range of synthesis methods that
are able to yield such materials, and sol-gel is one such optimal synthesis route. In simplest
terms, all sol-gel routes involve two different phases one is solution and other is gelation.
Basically the sol is a name of a colloidal solution made of solid particles tenth to few hundred
nm in diameter, suspended in a liquid phase. The gel can be considered as a solid
macromolecule immersed in a solvent [89]. Sol-gel process contains in the chemical conversion
of a liquid (the sol) into a gel state and with subsequent post-treatment and transition into solid
oxide material. The main benefits of sol-gel method are the high purity and uniform
nanostructure attainable at low temperatures. A sol-gel process occurs in several steps:
• Hydrolysis and condensation of molecules generally refer to hydrolysis & condensation of
alkoxide and metallic alkoxide RO- which yields dispersion of oxide particles in sol which
turned into gel by heating. Hydrolysis of precursor solution is done by catalyst acid or base.
Base induced colloidal formation of gel or more branched chain network, whereas acidic
catalyst formed polymeric form of gel or straight chain network. Rate of hydrolysis and
condensation widely affect the physical properties of nano structures, and particle size can be
control by concentration of precursors, nature of solvent, and type of catalyst, temperature and
pH.
• Gelation: A network formation in which the sol is aggregated & viscosity will rise up to a gel
formation. The sol-gel transition (gel-point) is reached when a continuous network is formed.
• Ageing: It contains rapidly increase in viscosity of the sol, solvent get trapped into the gel.
Structural changes arise depending upon time, pH and temperature of the solvent.
• Drying: When the liquid is removed from the gel quite a lot of effects may happen. When the
liquid in the gel is substituted by air, main change belong to the network structure may happen.
If the structure is preserved, an aerogel is formed. Whereas the structural collapses yield
xerogel. Usual drying of the gel leads to structural collapse due capillary forces maintaining
the walls of the pores together, and dropping the pore size[90].
Mainly, three methods are used to synthesize sol-gel ceramic materials, which are
distinguished by the nature of the initial precursor[91, 92].
1. Aqueous solution of inorganic salt.
2. Aggregation of colloidal particles in solution.
3. Network formation in Aqueous of Organic solution.
Last route is more acceptable due to availability of surface modification and network
formation allows tailor made processing of the gel. Usually precursor solution is prepared in
alcoholic solution[93].
Figure 2: Steps involved in sol-gel method.
Metallic oxide magnetic nanoparticles synthesize by sol gel method is a simplistic and
convenient way. Basic catalyst formed branched chain network linkage or colloidal formation
of gel, it prevents the undesired oxides or variate the oxidation states of metallic cations. Size
and shape of metallic magnetic nanoparticles depend upon precursor salts, ionic strength of
solvent, pH and ratio of Fe cations[89, 94].
1.2.2. Solvothermal / Hydrothermal Method:
Chemical reaction or transformation between precursors in a solvent under closed system
at temperature higher than the boiling temperature of used solvent under ambient conditions of
pressure. This process involves chemical reactions in solvents controlled in sealed containers
in which the temperature of solvents can be carried out at their critical points by heating
alongside with autogenous increasing pressures. The route is referred as “hydrothermal” when
solvent is taken as water. Other solvents could be organic like ethanol, methanol, toluene etc.
Solvothermal process has three main ingredients that are used during synthesis are
following;
1. Precursors or initial reactants in forms of solutions.
2.
Mineralizers or reagents which are inorganic (KOH, NaOH etc.) Or organic (Citric, Acetic
acid etc.) With high concentrations (e.g., 5-10 M) to control the pH of solutions.
3. Additives or surfactants are organics or inorganics in relatively low amount of
concentrations, added to uphold particle dispersion or to enhance the crystal morphology
in controlled manner.
Most hydrothermal or Solvothermal reactions proceed in a sealed vessel, called as a
pressure vessel, an autoclave, or a high-pressure bomb. Often, hydrothermal/ solvothermal
containers are metallic autoclaves with Teflon or alloy linings. Beaker or tube made up of
Teflon metallic materials like platinum, gold, or silver to keep the body of autoclave safe from
the extremely corrosive solvent. In certain circumstances, a Bourdon gauge is attached along
with the autoclaves to directly monitor the variation in pressure, moreover the autoclaves are
equipped with stirring accessories to lowering the concentration gradient inside them. Further
a perfect autoclave should be stress-free to assembly/disassembly as well as should be leakageproof [95-97].
Synthesis of material by hydrothermal/solvothermal route is a procedure of crystallization
straight from solutions that generally has two steps: nucleation of crystal and their successive
growth. Through governing the variables, such as temperature, pH, precursor’s molar
concentrations, and molar ratio of additives with reactant, the final outcome might be fabricated
with preferred sizes and morphologies of nano particles. The phenomena that proceed the
tunable sizes and different morphologies control via changing the synthesis variables are the
overall nucleation and growth rates of crystal structures of nanoparticles, which depend on
super saturation, which means ratio of concentration of precursors in normal conditions to the
concentration under specific conditions in which more species dissolve in a solution[97].
Nucleation arises when solution become supersaturated or amount of solute exceeds in
solution. Solubility of precursors species widely depend upon dielectric constant of solvent and
its density. Once super saturation state established, solute species renewed into crystal cluster
and grow up to microscopic or nano size. On other hand growth of crystal depends upon
sequent series of processes mainly incorporation of growth units that yields same or different
crystal structures. These subsequent processes contain four steps;
1. Transference of unit through solution
2. Unit’s attachment to surface
3. Motion of unit on surface
4. Association of unit to growth sites [98, 99]
Figure 3: Crystal growing mechanism under the hydrothermal/solvothermal route of
synthesis[95]
1.3.
Prime requisite of Synthesis
Size, shape and colloidal efficiency are major attribute of nano materials on which
physical and chemical characteristics of nano materials are highly depend. These attributes can
only be control during synthesis by varying the few parameters. Prime requisite to control the
functionality of nanomaterials contain potential Hydrogen, Temperature, molarity and
synthesis route. All these parameters are material dependent and have no general trend. In fact,
growth rate and nucleation of monomers can be control which is reliant on reaction kinetics
and Gibbs free energy of a system.
Potential hydrogen ranges from 0-14 rely on nature of reagents use during synthesis.
Widely used reagents are hydroxide of barium, potassium, magnesium and barium while citric
acid, acetic acid and boric acid are few examples of acidic reagents. Actually reagent act as a
reductant or oxidant. Basic solution increase the nucleation rate means more monomers tend
to accumulate and produce the particles with bigger size and provides colloidal solubility with
addition of capping agent or surfactant. Acidic medium increases the crystallinity growth of
particle, mostly with low size. Our synthesis requires colloidal synthesis that’s why basic
precursor solution is mandatory while powder synthesis demands acidic channel. Furthermore
shape like spherical, rod, sheets etc., can also control by acidic and basic mediums of solution
[90, 100, 101].
Chemical composition, molar ratio and molarity also influence the shape, size and colloidal
stability of nano material. Low molar concentration usually increases the nucleation rate which
yields larger sized particles, moreover precursors like nitrates, oxides, acetates etc. likewise
effect the shape of nano materials. Reaction time, synthesis temperature and nature of solvent
are further important factors of synthesis.
1.3.1. Surfactant
Hot colloidal or sol-gel methods contain accumulation of minor particles, because of van
der Waals forces and a tendency to drop the total surface energy of particles. Van der Waals
forces are feeble, and its influence extend only for a few nanometers. In order to overcome the
van der Waals interactions, establishment of repulsive forces is necessary.it may be able by
Electrostatic repulsion. By adsorption of species having charge onto the particles’ surface the
cause repulsion between the particles and agglomeration will be prevented. On other hand the
most significant for colloidal systems is steric hindrance.it produce by adsorption of a thick
layer of organic molecules, also called surfactant or capping agent the particles are prevented
from interacting each other vanishing the role of the van der Waals forces with addition of
surfactant in the solution.
Surfactants are amphiphilic molecules which have a water loving part and other is oil
loving. Dual character of surfactant provides variety of properties mainly two behaviors;
adsorption at interface and self-assembly in solution. Multifunctional applications include
detergency, wetting, stabilization and spreading of nanoparticles are provided by the surfactant.
Water loving is usually known as head group and a polar segment, while oil loving part is an
apolar fragment is called tail group. Head group contain oxygen or similar atoms, whereas tail
group is usually long chain of hydrocarbons. The most common surfactants are anionic
surfactant anchoring with anion such as sulfates, phosphate and carboxylate. Cationic
surfactants have cationic head such as ammonium etc. most important class of surfactant is
nonionic poly ethylene glycol PEG and oleic acid. In PEG water soluble part contain polar
character due to oxygen whereas in Oleic acid has characteristics to be colloidal in organic
solvents like chloroforms. Main functions of surfactant are limiting the growth of inorganic
nanoparticles and provide effective colloidal dispersion[84, 102]