10.1016/j.mssp.2014.02.048
Synthesis, characterization, magnetic measurements and liquefied petroleum gas sensing properties of nanostructured cobalt ferrite and ferric oxide
All reagents such as ferric chloride, cobaltous acetate and ammonium hydroxide were of analytical grade and used without further purification. The stoichiometric amount of starting materials, such as cobaltous acetate and ferric chloride were taken in 1:1, 1:2, 1:3 and 1:4 M ratios, respectively, and dissolved into required amount of distilled water to form 1 M solution. First of all, we have taken cobaltous acetate and ferric chloride in 1:1 M ratios and dissolved into respective required amount of distilled water to form precursors. The above precursors were refluxed at 60 degC for 4 h to get homogeneous solution. After that both were mixed with each other. The obtained mixed solution was heated at 70-80 degC and magnetically stirred for 4 h. 10 ml of poly-ethylene glycol (PEG) was added to the mixed solution which acts as an encapsulating agent. The resulting solution was precipitated by ammonium hydroxide solution, which was added drop by drop to the above mixed solution and a black colored precipitate was obtained. The pH of the solution was constantly monitored as the NH4OH solution was added until it reached to 12. Then the mixture was mechanically stirred at room temperature for 6 h and the precipitate was washed several times with distilled water until the pH of the filtrate became 7. Here, ammonium hydroxide was used for smooth liberation of the hydroxide ions in place of strong alkali (potassium hydroxide and sodium hydroxide). Strong alkaline solutions are prone to result in the conversion of Fe3+ and Co2+ into CoFe2O4 immediately, usually leading to the formation of severely agglomerated nanoparticles with irregular shapes. Similar procedure was used for the synthesis of nanostructured ferric oxide. 
Initially in aqueous solution, metal ions exist as Co ( H 2 O ) 6 2 + and Fe ( H 2 O ) 6 3 + . As pH was increased, the predominant species existing in solution became Co ( OH ) 6 2 - x and Fe ( OH ) y 3 - y , respectively. At the solubility minima, the predominant species in solution were Co(OH)2 and Fe(OH)3. Further, when the concentration of ammonia solution was increased, cobalt and iron became more soluble as Co ( OH ) 3 - and Fe ( OH ) 4 - species. The synthesis temperature was maintained at 70-80 degC for 4-5 h under vigorous magnetic stirring. The precipitate of CoFe2O4 was obtained through centrifugal settling for 15 min at 3000 rpm. The precipitate was dried overnight at 100 degC. The dried material was grinded into fine powder. The fine powder was then annealed at 450 degC inside a tubular furnace for 2 h with heating and cooling rate of 2 degC per minute. Sensing pellets of the synthesized powder were made by hydraulic press (MB instruments, Delhi) under a uniaxial pressure of 616 MPa. Hereafter, the cobalt ferrite pellets in 1:4, 1:3, 1:2 and 1:1 M ratios were named as P-1, P-2, P-3 and P-4, respectively.
starting materials
M ratios
1:1
starting materials
M ratios
1:2
starting materials
M ratios
1:3
starting materials
M ratios
1:4
solution
form
1 M
M
cobaltous acetate and ferric chloride
M ratios
1:1
precursors
refluxed
60 degC
degC
precursors
refluxed
4 h
h
mixed solution
heated
70-80 degC
degC
obtained mixed solution
magnetically stirred
4 h
h
poly-ethylene glycol (PEG)
added
10 ml
ml
solution
pH
12
filtrate
pH
7
mixture
mechanically stirred
6 h
h
synthesis temperature
maintained
70-80 degC
degC
synthesis temperature
maintained
4-5 h
h
precipitate of CoFe2O4
centrifugal settling
15 min
min
precipitate of CoFe2O4
centrifugal settling
3000 rpm
rpm
precipitate
dried
100 degC
degC
fine powder
annealed
450 degC
degC
fine powder
annealed
2 h
h
fine powder
heating and cooling rate
2 degC per minute
degC per minute
Sensing pellets
uniaxial pressure
616 MPa
MPa
cobalt ferrite pellets
M ratios
1:4, 1:3, 1:2 and 1:1