SYNTHESIS BY SOLID STATE DECOMPOSITION:DECOMPOSITION METHODS

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Chapter - 8

SYNTHESIS BY SOLID STATE DECOMPOSITION

S. Navaladian

INTRODUCTION

Synthesis of materials is an important part in materials science and chemistry because the

properties of materials vary drastically based on their synthetic method. Even the stability

of the material varies based on method by which it is synthesized. By varying synthetic

methods, surface area, pore size, crysatallite size, allotropes, morphology, presence of

impurities, defects and other oxidation state of the metal can also be varied. Hence, their

potential to certain applications vary drastically. For example, in catalysis, catalyst with

high surface area is preferred to achieve high conversion. So, synthetic method that yield

high surface of the catalyst is adopted. A lot of methods are known in the synthesis of

materials of various applications in different fields, still efficient and cost-effective

methods are being developed.

DECOMPOSITION METHODS

Decomposition is one of the methods known in the synthesis of various materials. In this

method, a solid or liquid is heated to its decomposition temperatures to obtain the solid of

interest. For example, sugar can be decomposed in an inert atmosphere to get the carbon

material (solid-to-solid). Decomposition of titanium isopropoxide (a liquid) in air

atmosphere is used to get the titanium dioxide (TiO2) (liquid-to-solid). Ni(CO)4, a volatile

complex can be decomposed to get Ni metal (vapour-to-solid). In this particular chapter,

solid state decomposition is considered.

Solid-State Decomposition

Decomposition of any material occurs due to its instability when the particular conditions

are applied. Also, it depends upon the activating source. These activating sources can be

thermal, photochemical, microwave and γ-radiation. For example chemical compound

which need high temperatures to decompose can be photochemically decomposed with

ease at room temperature. Particularly, the decomposition of AgBr to Ag can be achieved

by thermal modes only at 1330°C but it can be easily decomposed at room temperature

under visible light. As we know, this principle is used in photography. Thus, based on the

activating sources, solid-state decomposition can be classified into the following

8.2

Synthesis by Solid State Decomposition

decomposition methods (1) thermal decomposition, (2) Microwave-assisted and (3)

Photochemical decomposition.

Thermal Decomposition Methods

In general, thermal decomposition methods are chosen for synthesis of metal oxides.

Decomposable precursors which posses low decomposition temperature are preferred for

the synthesis because at high temperatures, sintering is also high. So materials formed

will have poor surface area and large crystallite size. Decomposition temperature of any

compounds is mainly decided by the redox properties of the compound. In general, salts

of the silver with easily utilizable organic moiety can be decomposed at lower

temperatures. Various materials such as metal, metal oxides, mixed metal oxides and

metals chalogenides have been synthesized using decomposition method.

Synthesis of Metals

100

80 100 120 140 160 180 200

Temperature ( C )

Fig. 8.1. TGA profile of silver oxalate and (b) silver nanoparticles synthesized by

decomposition of silver oxalate [1].

Synthesis of metallic silver can be achieved using silver formate, silver oxalate, oleate,

maleate and fumarate and their decomposition temperature is 93, 140, 287, 170 and 280

°C respectively (1 and 2). Recently silver dodecanate, myristate and palmitate also have

been utilized for the synthesis of silver nanopartices. Decomposition of these silver

compounds yield various gaseous products like CO, CO2 and organic residues. In the

case of silver oxalate decomposition to Ag metal and CO2 are produced. Hence, this is

one of the methods known for the production pure CO2. In the case of silver oxalate

Synthetic Strategies in Chemistry

8.3

decomposition, reaction was carried out in a water medium under refluxing conditions at

under N2 atmosphere with a capping agent (poly (vinyl alcohol)) (Fig. 8.1). TGA of the

silver oxalate, given in Fig. 8.1 (a) shows the sharp decomposition at around 140 °C. The

corresponding weight loss is due to the loss of two moles of CO2. TEM image of Ag

nanoparticles synthesized by decomposition of silver oxalate is given in Fig. 8.1(b). The

average particle is around 7 nm.

Metal Oxides

Metal oxides can also be synthesized by decomposition of certain precursors like

hydroxides, oxalates, hydroxyl carbonates, hydroxyl sulphates and carbonates. In general,

the direct calcination of metal salt such as nitrates and sulphates is not carried out to

synthesis the metal oxides in order to avoid the impurities and heterogeity of the

materials.

Synthesis of Magnesia

The decomposition of magnesium hydroxycarbonate (Mg5(CO3)4(OH)2)

yields

magnesium oxide (MgO). In this particular case, the morphology of the precursor is

flower like particle of 3 µm. After calcinations at 400 °C, MgO with tube-like

morphology is obtained as shown in Fig. 8.2.

Mg5 (CO3 )4 (OH)2 400→ 5 MgO + 4CO2 + H2O



(a)

(b)

Fig. 8.2 SEM image of (a) magnesium hydroxycarbonate and (a) MgO [3].

8.4

Synthesis by Solid State Decomposition

Similarly MgSO4.5Mg(OH)2.2H2O also yields the MgO by decomposition, but the

complete decomposition needs temperature above 800 °C. In this case, morphology of

the particles is strip-like. In general, MgO is synthesized decomposing the Mg(OH)2 in

the air. But MgO synthesized in the presence of air shows less surface area than that of

MgO synthesized in nitrogen atmosphere. This is due to the high amount of sintering in

presence of oxygen.

Synthesis of Matel Oxides from Oxalate Precursors

Due to the low temperature decomposition of metal oxalates, oxalate precursors are most

widely used for syntheses of metal oxides. Nickel, copper, iron and zinc oxides can be

synthesized directly decomposing their oxalate precursor. Nickel oxalate precursor has

yielded nickel oxide with sizes around 9 nm at 450 ºC. Fe2O3 has been synthesized by

thermal decomposition of ferrous oxalate at 415 °C. When copper oxalate has been

decomposed in air at 300 °C for 1 h mesoporous CuO microspheres have been obtained

(Fig. 8.3). Zinc oxalate has yielded ZnO particles of size around 55 nm. In all these cases,

the formation of CO and CO2 are the by products. Zinc acetate also decomposes and

yields the zinc oxide at 600 °C. Stoichiometric barium titanate (BaTiO3) is synthesized

by thermal decomposition of bariumtitanyl oxalate at 600 °C.

Fig. 8.3. Copper oxide micropshere obtained by the decomposition of copper oxalate [4].

Synthetic Strategies in Chemistry

8.5

Metal Chalcogenides

Cd(II)complex of thiosemicarbazide and selenosemicarbazide are thermally decomposed

to obtain CdS and CdSe. In typical procedure 500 mg of cadmium tetramethylthiourea

complex was mixed with 182 mg of thiosemicarbazide ligand, dispersed well in 3 ml tri-

n-octylphosphine oxide (TOPO). This precursor mixture was injected into TOPO (5 g) at

300 °C. The resulting orange yellow solution was maintained at 300 °C for about 1 h and

then cooled to 70 °C. TEM image of CdS nanorods formed are shown in Fig. 8.4. In this

case the thicknesses of the rods formed are 15 nm and aspect ratio is 3-4.

Cadmium thiosemicarbazide

Fig. 8.4. CdS nanorods synthesized by decomposition of cadmium thiosemicarbazide [5]

8.6

Synthesis by Solid State Decomposition

Microwave-assisted Decomposition

In general, microwave is used to quicken the reactions. The main advantages of

microwave assisted processes are: (i) the rate of the reaction is increased by orders of

magnitude, (ii) the initial heating process is rapid, and (iii) microwaves induce the

generation of localized high temperatures at reaction sites, which enhances the reaction

rates. Moreover, microwave-based syntheses are energy efficient. Hence, microwave is

used in the synthesis of materials as well as organic compounds.

Synthesis of Metals

Decomposition of silver oxalate has been carried out by using microwave radiation in

ethylene glycol and diethylene glycol. Formation of silver nanoparticles has been

observed in 60 s in the case of ethylene glycol. In the case of diethyelene glycol, only

after 75 s of irradiation, the formation of silver nanoparticles has been observed. Ag

nanoparticle formed in ethylene glycol medium is given in Fig. 8.5 (a). Formation of

anisotropic nanoparticle is observed in the case of 75 s of microwave irradiation (see for

example Fig. 8.5 (a)).

Fig. 8.5. TEM images of Ag nanoparticles formed in (a) 60 s and (b) 75 s of microwave

irradiation [6].

Synthesis of Metal Oxides

CaMoO4 has been synthesized at low temperatures by a modified citrate complex method

using microwave irradiation. Synthesizing mixed metal oxides at low temperatures is a

Synthetic Strategies in Chemistry

8.7

tedious process an it needs high temperatures. As a result, the resulting material has poor

surface area and larger crystallite size. For this kind of purpose, microwave can be used

to effectively bring down the temperature required for the formation of mixed metal

oxides. A flow chart for the synthesis of CaMoO4 is given Scheme 8.1. In this case, citric

acid makes a complex with the metal ions and due to the presence of the citrate, the

formation of the nanoparticles is observed. This method is also known as citric acid

combustion method. The corresponding nanoparticles of CaMoO4 formed are shown in

Fig. 8.6.

Scheme 8.1. Flow chart of the synthesis of CaMoO4 by microwave assisted route [9]

8.8

Synthesis by Solid State Decomposition

Fig. 8.6. TEM image of CaMoO4 nanoparticle synthesized at 600 °C for 3 h [9]

Photochemical Decomposition Method

Synthesis of Metals

Silver nanowires have been synthesized using photographic principles. In this case, AgBr

is reduced by in presence of a fluorescent light and followed by the development of the

film. TEM images AgBr and Ag nanowires formed from this process are shown in Fig

8.7.

Fig. 8.7. TEM images of (a) AgBr and (b) Ag nanowire [10]

Synthetic Strategies in Chemistry

8.9

Synthesis of Metal Oxides

Iron oxide Nanoparticles

Nanoparticles of iron oxide have been synthesized by decomposing Fe(CO)5 under

ultraviolet light. During the photolysis, decomposition of Fe(CO)5 takes place to form

nanoparticles of iron metal and further it turns into iron oxide due to the instability of

iron nanoparticles.

Synthesis of EuO nanocrystals

In a quartz vessel, Eu(NO3)3 (37.5 mM) and urea (112.5 mM) were dissolved in methanol

(400 ml) under an N2 atmosphere, and the solution was irradiated with a 500-W high-

pressure mercury arc lamp at 25°C. A yellowish powder precipitated after 30 min. After

24 h of irradiation, the powder was separated by centrifugation and washed with

methanol several times.

Synthesis of CdS nanorod

Fig. 8.8. TEM image of CdS nanowires synthesized by photochemical method [11].

CdS nanorods have been synthesized using cadmium salt, DNA base pairs and

thioacetamide (TAA). Photo-irradiation was performed by directly placing the cuvette

containing the mixture of DNA, Cd salt, and TAA solutions under 260 nm UV light

8.10

Synthesis by Solid State Decomposition

irradiation for six hours. The initially occurring DNA-Cd2+ complex formation was

confirmed by a shift in the UV-vis spectrum of the mixed solution compared to the pure

DNA. After photo-irradiation the UV-vis spectrum was again recorded showing the

formation of CdS nanoparticles. Thioacetamide acts as sulphur source in this reaction.

The corresponding CdS nanowires formed are shown in TEM image in Fig. 8.8.

SUMMARY

Solid state decomposition methods are important in the synthesis of the material both in

nano size ranges as well as in bulk. In the case of thermal decomposition route, the

compound having lower decomposition temperature is preferred. The decomposition

temperature of the compound is mainly based on the redox potential of cation and anion

of the compound. Microwave-assisted syntheses are more advantageous than

conventional heating due to the increased rate of the reaction. Photochemical synthetic

methods are also advantageous due to fact that the material is formed at ambient

conditions so that the sintering of the materials is reduced.

REFERENCES

S. Navaladian, B. Viswanathan, R. P. Viswanath and T. K. Varadarajan,

Nanoscale Research Letter, 2 (2007) 44.

I. K. Shim, Y. L. Lee, K. J. Lee, J. Joung, Materials Chemistry and Physics,

(2008) (in press).

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Physical Chemistry C, 111 (2007) 10267.

R. N. Nickolov,

B. V. Donkova, K. I. Milenova1 and D. R. Mehandjiev,

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P. S. Nair, T. Radhakrishnan, N. Revaprasadu, G. A. Kolawole and P.

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Synthetic Strategies in Chemistry

8.11

J. H. Ryu, J.-W. Yoon, C. S. Lim, W.-C. Oh and K. B. Shim, Journal of Alloys

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