ZeePedia Add to Favourites   |   Contact us


SYNTHETIC STRATEGIES IN CHEMISTRY

<<< Previous SYNTHESIS BY SOLID STATE DECOMPOSITION:DECOMPOSITION METHODS Next >>>
 
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
img
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
a
b
95
90
85
80
75
70
40
60
80 100 120 140 160 180 200
ο
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
img
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 m. After calcinations at 400 °C, MgO with tube-like
morphology is obtained as shown in Fig. 8.2.
ο
Mg5 (CO3 )4 (OH)2 400→ 5 MgO + 4CO2 + H2O
C

(a)
(b)
Fig. 8.2 SEM image of (a) magnesium hydroxycarbonate and (a) MgO [3].
img
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 șC. 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].
img
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]
img
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)).
a
b
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
img
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]
img
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.
a
b
Fig. 8.7. TEM images of (a) AgBr and (b) Ag nanowire [10]
img
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
1.
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,
2.
(2008) (in press).
3.
C. M. Janet, B. Viswanathan, R. P. Viswanath and T. K. Varadarajan, Journal of
Physical Chemistry C, 111 (2007) 10267.
4.
R. N. Nickolov,
B. V. Donkova, K. I. Milenova1 and D. R. Mehandjiev,
Adsorption Science & Technology, 24 (2006) 497.
5.
P. S. Nair, T. Radhakrishnan, N. Revaprasadu, G. A. Kolawole and P.
O'Brien,Chemiacl Communication (2002) 564.
6.
S. Navaladian, C. M. Janet, B. Viswanathan, R. P. Viswanath and T. K.
Varadarajan, Nanotechnology, 19 (2008) 045603.
7.
X. Wang, J. Song, L. Gao, J. Jin, H. Zheng and Z. Zhang, Nanotechnology 16
(2005) 37.
8.
Z. Zhou, Q. Sun, Z. Hu, and Y. Deng, Journal of Physical Chemistry C, 110
(2006) 13387.
Synthetic Strategies in Chemistry
8.11
9.
J. H. Ryu, J.-W. Yoon, C. S. Lim, W.-C. Oh and K. B. Shim, Journal of Alloys
and Compounds, 390 (2005) 245.
10.
C. Liewhiran, S. Seraphin, S. Phanichphant, Current Applied Physics, 6 (2006)
499.
11.
S. Liu, R. J. Wehmschulte, G. Lian and C. M. Burba, Journal of Solid State
Chemistry, 179 (2006) 696.