SYNTHESIS OF MATERIALS BASED ON SOLUBILITY PRINCIPLE

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Chapter - 4

SYNTHESIS OF MATERIALS BASED ON SOLUBILITY

PRINCIPLE

C. M. Janet

1. INTRODUCTION

Fundamental to the success of materials science and technology is the availability of

high-quality materials exhibiting specific tailor-made properties together with an

appropriate shape and microstructure. Solution based methods especially water based

mathods offer numerous advantages. Cheap and easy to handle precursors, low cost,

simple equipments, low energy input and the eco-friendly nature are few of them.

Moreover they allow the easy tailoring of synthesis parameters throughout the whole

process, which may be exploited to achieve a more precise control of composition, shape

and size of the resulting material. Since the synthesis route determines the properties of

the material, the preparation method chosen is very important when designing materials

for specific applications. Wet chemical routes for the synthesis of nanostructures are a

valuable alternative to conventional processing and gas phase synthesis, with known

commercial applications. Solvothermal methods and hydrothermal methods are

extensively used in the synthesis of materials of novel shape and properties. The majority

of the metal organic frameworks reported to date have been synthesized using solution

based methods under bench-top conditions (20-80 °C, 1 atm). Hydrothermal synthesis

especially has the attraction that it favors the condensation of M-OH into M-O-M bonds,

allowing the preparation of materials with multidimensional metal-oxygen frameworks

[1]. Recognizing that most metal salts are preferentially soluble in polar solvents, for

example, water, and that the opposite is true for many organic reactants, utilizing a higher

temperature and biphasic solvothermal method appears advantageous for synthesizing

many classes of hybrid materials. Reaction at the interface of two immiscible solvents is

a common technique for crystallizing compounds at low to moderate temperatures (<100

°C) and is commonly used to prepare hybrid inorganic-organic materials, in addition to

other products. The use of biphasic solvothermal synthesis has a number of attractions

over conventional hydro-/solvothermal methods. The basic concept that underlies in wet

4.2

Synthesis of Materials Based on Solubility Principle

chemical methods or solvothermal methods are the principles of solubility. Hence, it is

important to have a clear understanding of the fundamentals of solubility to that its use in

synthetic procedures.

2. BASICS OF SOLUBILITY

A solution is a homogeneous mixture of two or more substances. One of the substances is

called a solvent (a substance in which other substance or substances are dissolved). The

substances dissolved in a solvent are called solutes. A solution can exist in a solid, liquid

or gas form depending on mixed substances and external conditions such as temperature

and pressure. According to a chemists' perspective solubility can be understood as a

maximum amount of solute that can dissolve in a solvent at so called equilibrium. In

chemistry, equilibrium is a state where reactants and products reach a balance which

means that no more solute can be dissolved in the solvent in the set conditions

(temperature, pressure). Such a solution is called a saturated solution. There are two

groups of substances in case of which solubility measure cannot be applied. These are

miscible and immiscible substances. Some solvents, like water and alcohol, can be mixed

together and create a homogenous phase in any proportion. A solubility measure cannot

be applied to such two substances. Such substances are called miscible. On the other hand

if two substances cannot be mixed together (like water and oil), they are called

immiscible [2].

In the process of dissolving, molecules of the solute are inserted into a solvent and

surrounded by its molecules. For this process to take place, molecular bonds between

molecules of solute as well as that of solvent have to be disrupted. Both of these require

energy. For example when sugar dissolves in water, new bonds between sugar and water

are created. During this process energy is given off. The amount of this energy is

sufficient to break bonds between molecules of sugar and between molecules of water.

This example is relevant to any solute and solvent. If the bonds between the solvent and

solute are too strong and there is not enough energy provided to break them while

dissolving, the solute will not dissolve. The same energy rule can be applied to salts. Salts

composed of positive and negative ions which are bound together by the force of

attraction of their opposite charges. In cases where energy needed to break their ionic

Synthetic Strategies in Chemistry

4.3

bonds is lower, the dissolution of salts can take place only if the sufficient energy is given

off by an interaction of the ions with solvent.

Salts that are considered to be soluble are Group I and ammonium (NH4+)

compounds, nitrates, acetates, chlorides, bromides and iodides (except: silver (Ag+), lead

(II) (Pb2+), mercury (I) (Hg2

), copper (Cu+) halides) and sulphates (except: Silver

(Ag+), lead (Pb2+), barium (II) (Ba2+), strontium (II) (Sr2+) and calcium(II) (Ca2+) ). Those

which are insoluble are carbonates except Group I, ammonium (NH4+) and uranyl

compounds, sulfites except Group I and NH4+ compounds, phosphates except Group I

and NH4+ compounds, hydroxides and oxides except Group I, NH4+, barium (Ba2+),

strontium (Sr2+) and thallium (Tl+) and sulfides except Group I, Group II and NH4+

compounds. The solubility principle that holds well in the case of organic compounds is

"Like dissolves like".

3. SOLUBILITY PRODUCT (K SP)

Solubility product constant is a simplified equilibrium constant (Ksp) defined for

equilibrium between a solid and its respective ions in a solution. Its value indicates the

degree to which a compound dissociates in water. The higher the solubility product

constant, the more soluble the compound is. The expression for Ksp for a salt is the

product of the concentrations of the ions, with each concentration raised to a power equal

to the coefficient of that ion in the balanced equation for the solubility equilibrium.

Solubility product constants are used to describe saturated solutions of ionic compounds

of relatively low solubility. A saturated solution is in a state of dynamic equilibrium

between the dissolved, dissociated, ionic compound and the undissolved solid. For Silver

chloride Kc is given in the following equation.

------------------ (1)

Where [Ag+] and [Cl-] reperesent concentrations of ions of Ag+ and Cl- and [AgCl] is a

value representing the amount of moles in a liter of solid AgCl. [AgCl] is a constant and

therefore, the equation can be written as

Kc [AgCl] = [Ag+] [Cl-]

--------------------- (2)

4.4

Synthesis of Materials Based on Solubility Principle

The product of equilibrium concentartions of Ag+ and Cl- is equal to a constant. This

constant is called as solubility product constant or Ksp [3].

3.1 Significance of Le Chatelier's Principle in Solubility Concept

Once a system has reached equilibrium, the relative concentrations or pressures of the

species in the reaction do not change. However, if you disturb the system in some way,

the equilibrium will adjust until a new equilibrium is established. Le Chatelier's principle

states that "If a system at equilibrium is disturbed by a change in temperature, pressure,

or the concentration of one of the components, the system will shift its equilibrium

position so as to counteract the effect of the disturbance". The ionic product (IP) is

simply a measure of the ions present in the solvent. This may sound trivial but, in fact, it

is not always straightforward and the concept opens up a number of interesting features

of how salts behave in solution. The product of the soluble ions of a salt in solution is

called the ionic product. The solubility product (Ksp) is the ionic product when the

system is in equilibrium. When the ionic product exceeds the solubility product,

precipitation will happen according to the Le Chatelier's principle. From equation (2) this

can be understood. Any kind of precipitation is thus governed by the Lechatlier principle.

One of the day to day significance of ionic and solubility products are that they are the

important

basic

chemical

phenomena

underpinning

the

tooth

mineralisation,

demineralisation and stability. Addition of a common ion affects the ionic product. This

has important implications in the mouth since the concentration of calcium and phosphate

ions in saliva and plaque fluid can be influenced by external factors. Solubility principles

in the case of gases are quite different from that of solids and liquids. As the materials of

interest are mostly in solid state the solubility principles of gases are not dealt here.

3.2 Temperature and Pressure Effects on Solubility

Heat + Solid sugar + Water = Dissolved sugar ------------ (3)

The equation (3) represents two processes: dissolution going left to right, and

crystallization going right to left. When the sugar crystals are dissolving at exactly the

same rate that sugar is crystallizing out of solution, the system is at equilibrium. The

balance between dissolution and crystallization can be changed by changing the

Synthetic Strategies in Chemistry

4.5

temperature of the solution. Adding heat will favor dissolution. Cooling the solution will

favor crystallization.

The temperature dependence of solubility is also usually explained using Le Chatelier's

principle. Le Chatelier's principle predicts that heating the solution mixture will shift the

equilibrium in favor of dissolution, to remove the added heat. This explains why sugar is

more soluble in hot water than in cold. The solubility of a substance is its concentration

in a saturated solution. Substances with solubilities much less than 1 g/100 mL of solvent

are usually considered insoluble. The solubility is sometimes called "equilibrium

solubility" because the rates at which solute dissolves and is deposited out of solution are

equal at this concentration.

The solubility of solutes is dependent on temperature. When a solid dissolves in a

liquid, a change in the physical state of the solid analogous to melting takes place. Heat is

required to break the bonds holding the molecules in the solid together. At the same time,

heat is given off during the formation of new solute-solvent bonds.

(a) Decrease in Solubility with Temperature:

If the heat given off in the dissolving process is greater than the heat required to break

apart the solid, the net dissolving reaction is exothermic (energy given off). The addition

of more heat (increases temperature) inhibits the dissolving reaction since excess heat is

already being produced by the reaction. This situation is not very common where an

increase in temperature produces a decrease in solubility.

(b) Increase in solubility with Temperature

If the heat given off in the dissolving reaction is less than the heat required to break apart

the solid, the net dissolving reaction is endothermic (energy required). The addition of

more heat facilitates the dissolving reaction by providing energy to break bonds in the

solid. This is the most common situation where an increase in temperature produces an

increase in solubility for solids.

The use of first-aid instant cold packs is an application of this solubility principle. A

salt such as ammonium nitrate is dissolved in water after a sharp blow breaks the

containers for each. The dissolving reaction is endothermic - requires heat. Therefore the

heat is drawn from the surroundings, the pack feels cold. The Fig. 4.1 represents the

4.6

Synthesis of Materials Based on Solubility Principle

effect of temperature on the solubility of three different salts. Solubility of nitrate salt of

K is more compared to that of chlorides as the temperature increases.

Fig. 4.1. Temperature dependence of solubility of different salts (ref. 2)

4. HETEROGENEOUS EQUILIBRIA AND PRECIPITATION

Phase transitions such as sublimation, deposition, melting, solidification, vaporization,

and condensation are heterogeneous equilibria, so are the formation of crystals from a

saturated solution, because a solid and its solution are separated phases. The equilibrium

constants for saturated solution and solid formation (precipitate) are already defined as

solubility product, Ksp. For unsaturated and supersaturated solutions, the system is not at

equilibrium, and ion products, Qsp, which have the same expression as Ksp is used.

An oversaturated solution becomes a saturated solution by forming a solid to reduce

the dissolved material. The crystals formed are called a precipitate. Often, however, a

precipitate is formed when two clear solutions are mixed. For example, when a silver

nitrate solution and sodium chloride solution are mixed, silver chloride crystals AgCl(s)

(a precipitate) are formed. Na+ and NO3- are by-stander ions.

Ag+(aq) + Cl-(aq) → AgCl(s) (precipitate)

Silver chloride is one of the few chloride that has a limited solubility. A precipitate is also

formed when sodium carbonate is added to a sample of hard water,

Ca2+(aq) + CO32-(aq) → CaCO3(s) (precipitate).

Synthetic Strategies in Chemistry

4.7

4.1 Solubility Products, Ksp, and Ion Products Qsp

Formations of precipitates are chemical equilibria phenomena, and we usually write these

heterogeneous equilibrium in the following manner, and call the equilibrium constants

solubility products, Ksp. If the solution is not saturated, no precipitate will form. In this

case, the product is called the ion product, Qsp.

4.2 Qsp, Ksp and Saturation

For some substances, formation of a solid or crystallization does not occur automatically

whenever a solution is saturated. These substances have a tendency to form oversaturated

solutions. For example, syrup and honey are oversaturated sugar solutions, containing

other substances such as citric acids. For oversaturated solutions, Qsp is greater than Ksp.

When a seed crystal is provided or formed, a precipitate will form immediately due to

equilibrium of requiring Qsp to approach Ksp.

Sodium acetate trihydrate, NaCH3COO.3H2O, when heated to 370 K will become a

liquid. The sodium acetate is said to be dissolved in its own water of crystallization. The

substance stays as a liquid when cooled to room temperature or even below 273 K. As

soon as a seed crystal is present, crystallization occur rapidly. In such a process, heat is

released, and the liquid feels warm. Thus, the relationship among Qsp, Ksp and saturation

is given below [3]:

Qsp < Ksp Unsaturated solution

Qsp = Ksp Saturated solution

Qsp > Ksp Oversaturated solution

5. NANOPARTICLES THROUGH HOMOGENEOUS NUCLEATION

For the formation of nanoparticles by homogeneous nucleation, a supersaturation of

growth species must be created. A reduction in temperature of an equilibrium mixture,

such as saturated solution would lead to supersaturation. Formation of metal quantum

dots in glass matrix by annealing at moderate temperatures is a good example of this

approach. Another method is to generate a supersaturation through in situ chemical

reactions by converting highly soluble chemicals into less soluble chemicals. For

example semiconductor nanoparticles are commonly produced by pyrolysis of

organometallic precursors. Nanoparticles can be synthesized through homogeneous

4.8

Synthesis of Materials Based on Solubility Principle

nucleation in three mediums: liquid, gas and solid: however, the fundamentals of

nucleation and subsequent growth processes are essentially the same. Before discussing

the detailed approaches for the synthesis of uniformly sized monodispersed nanoparticles,

it is essential to review the fundamentals of homogeneous nucleation and subsequent

growth [4].

5.1 Fundamentals of Homogeneous Nucleation

When concentration of a solute in a solvent exceeds its equilibrium solubility or when

temperature decreases below the phase transformation point, a new phase appears. A

solution with solute exceeding the solubility or supersaturation possesses a high Gibbs

free energy. The overall energy of the system will be reduced by segregating solute from

the solution.

Fig. 4. 2. Nucleation and subsequent growth (ref. 4)

When the concentration of a solute increases as a function of time no nucleation would

occur even above the equilibrium solubility. Nucleation occurs only when the

supersaturation reaches a certain value above the solubility. Homogeneous nucleation is

important for the formation of particles of uniform size and shape.

5.2 Examples of Nucleation

Pure water freezes at -42 ° C rather than at its freezing temperature of 0 ° C if no crystal

nuclei, such as dust particles, are present to form an ice nucleus. Presence of cloud

condensation nuclei is important in meteorology because they are often in short supply in

the upper atmosphere.

Synthetic Strategies in Chemistry

4.9

Fig. 4.3. Nucleation of carbon dioxide bubbles around a finger (ref. 5)

Nucleation in boiling can occur in the bulk liquid if the pressure is reduced so that the

liquid becomes superheated with respect to the pressure-dependent boiling point. More

often nucleation occurs on the heating surface, at nucleation sites. Typically, nucleation

sites are tiny crevices where free gas-liquid surface is maintained or spots on the heating

surface with lower wetting properties. Substantial superheating of a liquid can be

achieved after the liquid is de-gassed and if the heating surfaces are clean, smooth and

made of materials well wetted by the liquid. The creation of a nucleus implies the

formation of an interface at the boundaries of the new phase. Some energy is consumed

to form this interface, based on the surface energy of each phase. If a hypothetical

nucleus is too small, the energy that would be released by forming its volume is not

enough to create its surface, and nucleation does not proceed. The critical nucleus size

can be denoted by its radius, and it is when r=r* (or r critical) that the nucleation proceeds

[5].

6. SOLUBILITY AND CRYSTAL GROWTH

Crystal formation is made up of three phases: nucleation, growth, and cessation of

growth. During nucleation, the slowest and most difficult phase in crystal growth, the

smallest crystal capable of growth forms. The barrier to formation of these smallest

crystals results from differences in the stability between molecules at the core and those

on the surface of the crystal. During nucleation, the average stability of a molecule in a

crystal is very low because the attraction between the molecule and the crystal is often

less than that between the molecule and the solvent. Thus, it is significantly easier to

4.10

Synthesis of Materials Based on Solubility Principle

successfully stimulate the birth of a crystal by introducing a smaller crystal (seeding) than

by merely supersaturating a solution. Once nucleation has been accomplished, the growth

phase of crystal formation begins, a phase characterized by the addition of molecules to

the existing crystal. Two opposing forces impact this process: enthalpy and entropy.

Growth of a crystal reduces the entropy (a measure of randomness) of the system because

a molecule free in solution has greater entropy than one tethered to a crystal's growing

surface. Although entropy favors dissolution of crystals, it is energetically favorable for a

molecule to be added to a crystal. Above a critical saturation point, enthalpy overcomes

entropy, and crystal formation can occur.

6.1 Crystal Kinks and Ledges

In addition to temperature and concentration, the geometry of a growing crystal's surface

influences growth. Imagine a spherical molecule approaching a planar surface of a

crystal. In this case, the addition of the molecule is not energetically favorable because

there is only one point of contact. Next, consider a spherical molecule approaching a

"ledge" surface of a crystal. In this case, the addition of the molecule is more

energetically favorable because the molecule is stabilized by two points of contact at

which intermolecular forces hold the molecule to the crystal. In the third case, the "kink"

structure provides three points of contact. The kink structure is the ideal surface geometry

for crystal growth. Because kinks are sparse on a crystal's surface, the addition of even a

small amount of impurities that effectively clog the kink sites is capable of killing crystal

growth. In order for crystal growth to be maintained, "ledges" or kinks must remain

available for molecules to be added to the crystal. At one point in the field of crystal

research, scientists observed that nucleation occurs too quickly to be explained solely by

the addition of molecules to a planar crystal surface. As a result, the spiral dislocation

(SD) theory arose, stating that molecules are added to the surface of a crystal in a manner

resulting in the creation of new ledges or kinks so that crystal growth can continue. With

the development of atomic force microscopy (AFM) for physically detecting changes in

depth on the atomic scale, it was possible to study changes associated with a ledge on a

crystal. To observe crystal formation, the lab uses an atomic force microscope to gather

data on the movement of ledges as organic crystals grow and dissolve. This method

allows the investigator to observe the rate and geometry of ledge movement at various

Synthetic Strategies in Chemistry

4.11

concentrations of solute and in the presence of impurities. It was noticed that dissolution

is not exactly the opposite of growth. During dissolution, the ledges are smooth and

regular, while they are rough and irregular during growth. Although there are several

theories on how the irregularity may be conducive to crystal growth, the differences have

not been fully explained.

6.2 Practical Significance of Solubility and Crystal Growth

Understanding the growth and dissolution of crystals is valuable in a variety of contexts.

Crystallization is an effective and economical purification process, and, in some cases

where substances cannot be purified by distillation, it is the only practical method. More

importantly, approximately eighty percent of all pharmaceutical agents are pure

crystalline solids. Understanding the dissolution of a particular crystalline drug could

allow chemists to alter the drug's dissolution rate. Moreover, understanding the

conditions of crystallization may allow chemists to predict whether drugs crystallize

within the body. In 2004, the journal Heart reported a case of a large crystal forming in

the heart of woman given a continuous dose of a drug for ventricular tachycardia. A

better understanding of this drug's crystallization could have prevented the incident.

Clearly, much of this research is not yet crystal-clear.

7. SOLUTION BASED SYNTHETIC STRATEGIES

Solution based synthetic strategies involve mainly sol gel process, hydrothermal

synthesis, solvothermal synthesis and reduction in solution. Among which solvothermal

synthesis and hydrothermal synthesis are discussed in detail in the present chapter.

7.1 Solvothermal Synthesis

Solvothermal synthesis utilizes a solvent under pressures and temperatures above its

critical point to increase the solubility of solid and to speed up reaction between solids.

Most materials can be made soluble in proper solvent by heating and pressuring the

system close to its critical point. This method allows the easy control on the solubility of

a solute. And it leads to lower super saturation state which is necessary for the

precipitation to happen. The reaction set-up used for solvothermal synthesis is given in

Fig. 4.4.

4.12

Synthesis of Materials Based on Solubility Principle

Fig. 4.4. Solvthermal synthesis set-up (ref. 6)

7.1.1 Synthesis of Semiconductor Chalcogenides

In general, semiconductor chalcogenides of different sizes and shapes are prepared using

solvothermal synthesis. In our laboratory we have synthesized CdS nanostructures

especially nanorods through solvothermal synthesis using a solid precursor CdC2O4. The

reagent used to precipitate CdS was (NH4)2S. The reaction observed was represented in

the equation 6. Here the solvent used was ethylene glycol and the reaction temperature

was 60 °C and the reaction was carried under a constant flow of an inert gas [7].

CdC2O4 + (NH4)2S

CdS + (NH4)2C2O4

---------- (6)

CdC2O4 is an insoluble solid and (NH4)2S was liquid. But the solvothermal synthesis at

low temperatures allowed in this case a slow replacement reaction of the ligand forming

the nanostructures of CdS. The TEM images of the CdS nanorods are given in the Fig.

4.5. By changing the solvents, temperature and the precursor variety of sizes and shapes

can be produced through this method. When ethylene diamine is used the material formed

was also found to be nanorods with high aspect ratio. But, when pyridine was used CdS

nanoparticles are only formed.

Synthetic Strategies in Chemistry

4.13

Fig. 4.5. TEM image of CdS prepared from CdC2O4 using solvothermal method in

ethylene glycol (ref. 7)

Fig. 4.6. TEM image of CdS prepared from CdC2O4 using solvothermal method in

ethylene diamine and pyridine (ref. 6)

7.1.2 Synthesis of Metal Nanoparticles

Pt and Pd nanoparticles were synthesized by microwave-assisted solvothermal method.

The only difference is the heating source is microwave. PVP with an average molecular

weight of 40'000 was used as a capping agent in the experiments. H2PtCl6, and

Palladium(II)2,4-pentanedionate were used as metal precursors. PVP was dissolved in

methanol or ethanol and then the metal salts were added. The reactants were heated for

60 min at 90 ºC when methanol was used as a reducing agent and at 120 ºC when

ethanol was used as a reducing agent for 60 min under microwave irradiation [8].

4.14

Synthesis of Materials Based on Solubility Principle

Fig. 4.7. TEM images of the obtained Pt and Pd nanoparticles under

microwave-assisted solvothermal conditions (ref. 8).

7.1.3 Synthesis of Metal Nitrides and Oxides

In another approach single source precursors of halides of Al, Ga and In were prepared

with urea complexation and further solvothermal synthesis at higher temperatures

resulted in the formation of the respective nitrides. These nitrides are of special relevance

in photovoltaic applications [9].

(a)

(b)

(c)

Fig. 4.8. TEM images of (a) AlN (b) GaN (c) AlN nanowires formed during solvothermal

synthesis (ref. 9).

A new and facile route was developed by Suib et al., to manipulate the growth of

hierarchically ordered Mn2O3 architectures via a solvothermal approach. Various solvents

are employed to control the product morphologies and structures. Mn2O3 with unique

cuboctahedral, truncated-octahedral, and octahedral shapes are obtained. In a typical

synthesis Mn(NO3)2 was dissolved in an organic solvent followed by a vigorous stirring

at room temperature for half an hour in a Teflon liner. Then the Teflon liner was

transferred and sealed in an autoclave for solvothermal treatment at 120 °C for 20 h. A

Synthetic Strategies in Chemistry

4.15

variety of different solvents was used to investigate the effect of solvents on the

morphology of the resultant Mn2O3. In order to investigate the development of Mn2O3

crystals, the reactions were also conducted at different temperatures using ethanol as the

solvent. FESEM images of the Mn2O3 synthesized in different solvents and for different

duration are given in Fig. 4.9 and Fig. 4.10 respectively. The images indicate the shape

evolution of Mn2O3 polyhedra. Mn2O3 is used in catalytic as well as electrocatalytic

applications [10].

Fig. 4.9. FESEM images of products synthesized in different

solvents: (a) ethanol, (b) 1-butanol, (c) 2-butanol, and (d) acetone (ref. 10).

Fig. 4.10. FESEM images of products synthesized under different

reaction periods (a) 1.5 h, (b) 2 h, and (c) 3 h (ref. 10).

7.2 Hydrothermal Synthesis

Hydrothermal synthesis as the name indicates the solvent is always water. If liquid water

is placed in an open container, its temperature cannot be raised above 100 °C. But, if

water is heated in a sealed container, it can be heated to temperatures above 100 °C

which means that supercritical properties of the water can be utilized under this

4.16

Synthesis of Materials Based on Solubility Principle

condition. The advantages of inducing supercritical behavior in hydrothermal synthesis

are that it will provide a single phase behavior and give enhanced permeability, mass

transport capability and dissolving capacities. Hydrothermal synthesis can be defined as a

method of synthesis of single crystals which depends on the solubility of minerals in hot

water under high pressure. Hydrothermal synthesis is a century old synthetic strategy and

it was used for the synthesis of minerals in general. From 80's hydrothermal method was

utilized extensively for newer material synthesis. Advantages of hydrothermal synthesis

are in this method no post-heat treatment is needed and hence agglomeration will be less.

After preparation no milling is required which will reduce impurities. Any complex

chemical compositions can be synthesized by using this method. Particle size or shapes

can be controlled in this approach. It can induce self assembly leading to newer and

complex architectures of materials and the precursors used are relatively cheap raw

materials. And hydrothermal method crystal growth includes the ability to create

crystalline phases which are not stable at the melting point. Also, materials which have a

high vapour pressure near their melting points can be grown by the hydrothermal method.

The method is also particularly suitable for the growth of large good-quality crystals

while maintaining good control over their composition. Disadvantages of the method

include the need of expensive autoclaves, good quality seeds of a fair size and the

impossibility of observing the crystal as it grows. Fig. 4.11 shows the reaction set-up for

hydrothermal synthesis. The pressure generated inside the reactor can be read from the

pressure gauge.

Fig. 4.11. Hydrothermal reactor set-up (ref. 6)

Synthetic Strategies in Chemistry

4.17

7.2.1 Synthesis of Oxides and Mixed Oxides

Barium titanate (BaTiO3) perovskite is widely used in electronic industry in multilayered

ceramic capacitors due to its high dielectric constant. Hydrothermal synthesis route is

promising due to homogeneity, exact stoichiometry and spherical morphology of BT

powders obtained by this synthesis method at low temperature (<200 °C). Using

commercially available titania as Ti precursor (Degussa P-25) and Barium hydroxide

precursor in the Ba: Ti = 1:1 ratio at a temperature as low as 120 °C for 48 h yielded

BaTiO3 crystals [11]. The SEM image of BaTiO3 prepared is given in Fig. 4.12.

Fig. 4.12. SEM image of BaTiO3 crystals (ref. 11).

Alkali treatment of commercially available TiO2 (Degussa p-25) in hydrothermal

conditions (130 °C) will result in the formation of nanotubes of TiO2. These tubes

obtained on further hydrothermal treatment at a high temperature of around 175 °C will

yield nanorods of TiO2 [12]. Thus hydrothermal synthesis provides a route to play around

different morphologies of the same material by simply changing the temperature.

Hydrothermal treatment of zinc chloride hydrazene hydrate at 140 ºC for 12 h

resulted in the formation of flower like microrod bundles. The solution phase is

accelerating the process of self assembly through a dissolution-recrystallization-

decompositiongrowth process [13]. The chemical reactions are represented in the

following equations. ZnO has variety of applications in photonics and optics.

ZnCl2 + 2N2H4

ZnCl2(N2H4)2 ..................(1)

Zn2+ + 2NH3. H2O

Zn(OH)2+ 2NH4+ ................(2)

Zn(OH)2

ZnO + H2O ......................(3)

4.18

Synthesis of Materials Based on Solubility Principle

Fig. 4.13. FE-SEM images of (a) precursor ZnCl2(N2H4)2 obtained at room temperature

(b) ZnO samples obtained at 140 °C and 12 h (ref. 13).

7.2.2 Synthesis of Noble Metal Architectures through Hydrothermal Synthesis

Polymer protected (PDDA-poly (diallyl dimethylammonium) chloride) noble-metal

(including silver, platinum, palladium, and gold) nanostructures in the absence of any

seeds and surfactants can be synthesized using hydrothermal method in which PDDA, an

ordinary and water-soluble polyelectrolyte, acts as both a reducing and a stabilizing

agent.

Under

optimal

experimental

conditions,

nanocubes,

and

nanopolyhedrons, and Au nanoplates are obtained. In typical synthesis, PDDA along with

the respective precursors such as AgNO3 (170 °C for 16 h), H2PtCl6 (140 °C for 40 h),

H2PdCl4 (190 °C for 40 h) and HAuCl4 (170 °C for 12 h) at specific pH was used [14].

Fig. 4.14. Typical SEM and TEM images of the PDDA-protected (a) Ag nanocubes (b) Pt

nanopolyhedrons (c) Pd nanopolyhedrons and (d) Au nanoplates (ref. 14).

Synthetic Strategies in Chemistry

4.19

Apart from metals and metal oxides hydrothermal synthesis method is used in the

preparation of zeolites and mesoporous materials. This method was also found to be

effective in the synthesis of different carbons such as nanotubes, fullerene and diamond

[15].

7.2.2 Synthesis of Polymeric Materials through Hydrothermal Synthesis

Polyaniline (PANI) mesostructures have been synthesized under hydrothermal

conditions. The mesostructures show different forms - fibers, dendrite fibers, textured

plates, featureless plates, and spheres [16]. In a typical synthesis, a complex of

FeCl3·6H2O and methyl orange in hydrochloric acid aqueous solution (pH = 4.0) was

stirred and transferred to an autoclave with aniline monomer and kept at 120 ◦C for 24 h.

The material collected was found to be PANI nanotubes [17]. The formation mechanism

and the TEM images of the tubes formed are given in Scheme 1 and Fig. 4.15. A fibrillar

complex of FeCl3 and methyl orange (MO) acting as reactive selfdegraded templates in

hydrothermal conditions was the driving force for the growth of nanotubes. MO, which

contains a hydrophilic group (SO-3), possesses an anionic characteristic when dissolved

in water. It could dimerize at a particular concentration to form higher oligomers. When

aniline monomer was added into the solution, polymerization occurred on the surface of

MO where the oxidant FeCl3 was adsorbed and MO itself degraded automatically during

the polymerization process.

4.20

Synthesis of Materials Based on Solubility Principle

Fig. 4.15. TEM of products with pH of electrolyte (4.0) Synthesis conditions:

temperature, 120 ◦C; FeCl3, 1.5 mmol; MO, 0.075 mmol; aniline, 1.5 mmol (ref. 17).

CONCLUSIONS

The solubility of a material is having a major role while designing the synthetic strategy

of any new materials. Solution phase synthesis always assisted the self assembly and

gradual growth of the crystals of a variety of materials ranging from metals, metal oxides,

chalcogenides, polymers, zeolites and carbon materials. Easy tailoring of the morphology

and properties can be achieved if solubility concepts are suitably exploited in new

material synthesis. Solution based chemistry is always important because ultimate utility

of materials is going to be in any form of life chemistry which is fully based on aqueous

systems. Drug delivery materials, materials in food processing and preservation,

medicines and cosmetics are much trivial cases where the solubility concepts are

extremely important. Hence, immense care should be taken while designing materials for

day today applications through solution chemistry.

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Synthetic Strategies in Chemistry

4.21

8. Q. Lu, F. Gao, D. Li and S. Komarneni, A to Z of Journal of Materials online, 1(2005)

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