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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.

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.

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].

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

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

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.

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)

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, Ag nanocubes, Pt and Pd 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).

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.

Scheme 4.1. Possible polymerization mechanism for the formation of PANI nanotubes (ref. 17)

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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