Synthesis of γ-Fe2O3 by thermal decomposition of ferrous gluconate dihydrate

Article abstract of DOI:10.1023/A:1014924601765

Ferrous gluconate dihydrate (FeC₁₂H₂₂O₁₄·2H₂O), was prepared and its thermal decomposition was studied by means of simultaneous thermal analysis, supplemented with a two probe d.c. electrical conductivity measurements under the atmospheres of static air, dynamic air and dynamic nitrogen. Under all the atmospheres final product was found to be α-Fe₂O₃ with FeO, γ-Fe₂O₃, Fe₃O₄ etc. as probable intermediates. γ-Fe₂O₃ was formed under the atmosphere of dynamic air containing water vapour. γ-Fe₂O₃ thus synthesised was characterised for its structure, morphology, thermal and magnetic behaviour.

Full text of DOI:10.1023/A:1014924601765

Journal of Thermal Analysis and Calorimetry, Vol. 68 (2002) 91–101
SYNTHESIS OF γ-Fe O BY THERMAL
2
3
DECOMPOSITION OF FERROUS GLUCONATE
DIHYDRATE
*
1
M. M. Rahman and A. Venkataraman
2**
1
Nawab Shah Alam Khan Institute of Post Graduate Studies & Research Center, Anwar ul - Ullom
College, Mallepally, Hyderabad – 500 001, India
Department of Chemistry, Gulbarga University, Gulbarga – 585 106, India
2
(
Received June 22, 2001)
Abstract
22 2
Ferrous gluconate dihydrate (FeC12H O14·2H O), was prepared and its thermal decomposition was
studied by means of simultaneous thermal analysis, supplemented with a two probe d.c. electrical
conductivity measurements under the atmospheres of static air, dynamic air and dynamic nitrogen.
Under all the atmospheres final product was found to be α-Fe
probable intermediates. γ-Fe was formed under the atmosphere of dynamic air containing water
vapour. γ-Fe thus synthesised was characterised for its structure, morphology, thermal and mag-
netic behaviour.
2
O
3
2 3 3 4
with FeO, γ-Fe O , Fe O etc. as
2
O
3
2
O
3
2 3
Keywords: electrical conductivity, γ-Fe O , magnetic properties, structure, synthesis, thermal
analysis
Introduction
Commercially, γ-Fe O is prepared by dehydration of synthetic geothite (α-FeO(OH)) to
2
3
α-Fe O , its reduction to Fe O and finally reoxidation to γ-Fe O [1–4].
2
3
3
4
2
3
γ-Fe O particles of nanosized dimensions improves many of the required pa-
rameters for its effective use as a recording material [3–10]. The study on synthesis of
2
3
γ-Fe O for recording and memory devices is continuously attaining greater signifi-
2
3
cance [1, 3–5]. New synthetic routes for the synthesis of γ-Fe O are being continu-
ously investigated, some of them include: the ion exchange reaction of NaFeO with
2
3
2
benzoic acid [11], thermal decomposition of lepidocracite [12], employing sol-gel
techniques [9], decompositions of microemulsions [13], and thermal decompositions
of metal carboxylates under controlled conditions [14–15]. The present investigation
is a continuation of our ongoing programme on study of the new routes and also em-
*
*
This paper is dedicated to late Prof. A. J. Mukhedkar.
Author for correspondence: E-mail: venkat1@rasms.czarsonline.net.in
*
1
418–2874/2002/ $ 5.00
2002 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest
©
Kluwer Academic Publishers, Dordrecht

9
2
2 3
RAHMAN, VENKATARAMAN: SYNTHESIS OF γ-Fe O
ploying new precursors in the synthesis of ultrafine γ-Fe O particles and having opti-
mum properties.
2
3
In our earlier work [14–18], we have used d.c. electrical conductivity technique
as a supplementary tool to thermal analysis in the study of the thermal decomposition
reactions for the formation of γ-Fe O . In the present study we investigate the thermal
2
3
decomposition of ferrous gluconate dihydrate (FeC H O ·2H O), by conventional
thermal analysis (TG/DTA/DTG) supplemented with electrical conductivity mea-
surements, under different atmospheres.
1
2
22 14
2
Experimental
Preparation of FeC H O ·2H O
1
2
22 14
2
Ferrous gluconate dihydrate was prepared by dissolving equal volumes of equimolal
solutions of ferrous sulphate and barium gluconate at 60°C, in a flow of dry nitrogen
under vigorous stirring. The yellowish grey precipitate of ferrous gluconate dihydrate
(
FeC H O ·2H O), thus obtained was washed thoroughly with double distilled wa-
12 22 14 2
ter and dried under vacuum. The elemental analysis of the gluconate gave,
C: 28.64(29.88), H: 4.97(4.56), Fe: 12.32(11.65), the values in the parenthesis are the
calculated values and satisfy the formulae FeC H O ·2H O.
1
2
22 14
2
Thermal analysis
The thermal analysis traces were obtained with Netzsch instrument under the atmo-
–
sphere of static air, dynamic dry nitrogen, and dynamic dry air (100 mL min ) from
1
–
room temperature to 1000°C, with a heating rate of 10°C min .
1
Electrical conductivity
The two-probe method employing a d.c. electrical conductivity method as reported
earlier [16–18] was used in the present study.
Infrared studies
The infrared spectra were recorded on a Perkin Elmer model 337 spectrometer using
nujol mull.
X-ray diffraction
The powder X-ray diffraction patterns was obtained from a PW 1730 Philips X- ray
diffractometer, using CuK radiation.
α
Magnetic hysteresis
The magnetic hysteresis of γ-Fe O particles was investigated using a vibrating sample
magnetometer-VSM (EG & G Princeton Applied Research Model 4500) magnetometer.
2
3
J. Therm. Anal. Cal., 68, 2002

RAHMAN, VENKATARAMAN: SYNTHESIS OF γ-Fe
2
O
3
93
Morphology
The morphology of γ-Fe O particles was investigated using Cambridge Stereo Scan
2
3
1
50 instrument.
Results and discussion
Study under static air
Figure 1 shows the thermal analysis trace of ferrous gluconate sample under static air
atmosphere. In this figure the dehydration step of FeC H O ·2H O, is indicated by a
1
2
22 14
2
broad endothermic peak (~108°C) on the DTA trace. The DTG trace also shows a
broad peak (at 108°C). The TG trace shows a mass loss in the temperature range
4
0–140°C corresponding to the loss of two water molecules. A sample isothermally
heated at 160°C shows that the bands due to water molecules were not present in the
infrared spectrum. Elemental analysis (C, H and metal estimation) of this sample also
agreed with anhydrous gluconate (FeC H O ).
1
2
22 14
The oxidative decomposition of the complex FeC H O , was indicated by the
1
2
22 14
presence of four exothermic peaks on the DTA trace at 220, 293, 350 and 421°C.
Broad peaks on the DTG trace at similar temperatures are noticed. However, the TG
trace shows continuous mass loss from 140 to 478°C. The final mass loss corresponds
to the formation of α-Fe O . The samples isothermally heated at 220 and 293°C gave
2
3
a diffuse X-ray patterns. The X-ray pattern of the sample obtained after heating at
3
50°C shows mixed phases of Fe O and γ-Fe O , and the sample isothermally heated
3
4
2
3
at 421°C shows to be purely α-Fe O . Hence it may be understood that the peaks ob-
served on the DTA and DTG traces at 350 and 421°C are due to the transformation to
2
3
more crystalline forms.
Direct current electrical (d.c.) conductivity of this compound (FeC H O ·
1
2
22 14
2H O) under this atmosphere could not be studied since the pellet of this sample
2
melted above 200°C.
22 2
Fig. 1 Thermal trace of FeC12H O14·2H O under static air atmosphere
J. Therm. Anal. Cal., 68, 2002

9
4
2 3
RAHMAN, VENKATARAMAN: SYNTHESIS OF γ-Fe O
When the reaction has been carried out in static air atmosphere, the gaseous
product act as a gas buffer for the solid state reaction and some of the reactions will be
ill-defined [17]. A study of reaction under dynamic atmospheres viz., dynamic nitro-
gen and dynamic air has been carried out to compare the data and clarify the results
obtained therein.
Study under dynamic nitrogen atmosphere
The thermal analysis traces are shown in Fig. 2. The TG trace shows a mass loss in
the temperature range ambient to 160°C, corresponding to loss of two water mole-
cules indicating a dehydration process. The DTA trace shows an endothermic peak at
9
1
5 followed by a shoulder at 155°C. The DTG trace shows two peaks at 80 and
85°C, both these traces show the dehydration process for gluconate sample.
22 2
Fig. 2 Thermal trace of FeC12H O14·2H O under dynamic nitrogen atmosphere
The d.c. electrical conductivity results for the sample FeC H O ·2H O are shown
12
22 14
2
–1
in Fig. 3, by a plot of logσ vs. 1/T, where σ is the electrical conductivity (ohm cm ) and T
is the temperature in absolute scale. The σ value shows a peak in the region from ambient
temperature to 180°C. This peak can be understood as dehydration step [16]. Figure 4(a)
shows the X-ray diffraction pattern for ferrous gluconate dihydrate and Fig. 4(b) shows
the X-ray diffraction pattern for dehydrated ferrous gluconate. It is seen from these two
traces that the crystalline form is lost when dehydration takes place. The infrared spec-
trum of the sample isothermally heated at 160°C, did not show any bands corresponding
to ν . Elemental analysis also agreed with the composition of the anhydrous formulae,
FeC H O . This anhydrous sample was found to be amorphous in nature with X-ray
study. The transition from crystalline to amorphous gluconate would possibly indicate
OH
12
22 14
the formation of better γ-Fe O , having nanosized particles with high density when car-
ried out under controlled conditions. This transition from amorphous to crystalline phase
2
3
J. Therm. Anal. Cal., 68, 2002

RAHMAN, VENKATARAMAN: SYNTHESIS OF γ-Fe
2
O
3
95
results in better properties for ferrites that are prepared by sol-gel techniques [19]. Hence
γ-Fe O formed from the thermal decomposition of ferrous gluconate may have better
2
3
thermal and structural properties when compared to those prepared from thermal decom-
position of carboxylates studied earlier by us [16–18]. This is also evident from the satu-
ration magnetisation and coercive force measurements, which are discussed later.
In Fig. 2, after the dehydration peaks the DTA trace shows exothermic peaks at
2
2
10, 320, 395 (shoulder) and 440°C. The peaks on the DTG trace were observed at
75, 360, and 415°C. The TG trace shows a two-step mass loss. The first step of mass
loss occurred between 160 and 220°C corresponding to the formation of a mixture of
FeO and anhydrous gluconate. The electrical conductivity plot shows the presence of
a peak in the temperature region 180 to 225°C. A sample isothermally heated in this
region (200°C, for 2 h) indicated the presence of FeO in the X-ray diffractogram (Ta-
–
1
ble 1). The infrared spectrum shows a broad band at 550 cm , which is assigned to
Fe–O stretching frequency [16]. In the electrical conductivity trace (Fig. 3) a steady
increase in σ-value is noticed with a shoulder around 280°C, later a steep increase in
σ-value is noticed upto 460°C thereafter the conductivity trace remained almost con-
stant, which is a typical behaviour for the Fe O [16]. On further heating the final con-
3
4
version to α-Fe O takes place. The X-ray diffraction of the sample isothermally
heated at 280°C, shows a mixture of FeO and Fe O . The X-ray pattern of the sample
isothermally heated at 500°C (2 h), shows the sample to be Fe O (Table 2).
2
3
3
4
3
4
22 2
Table 1 Comparison of X-ray diffraction data of FeO obtained from FeC12H O14·2H O by heat-
ing under dynamic nitrogen at 200°C (2 h)
Observed d-spacing values (µm), present study
Reported [23] d-spacing values µm
4
3
2
2
2
2
2
2
1
1
1
1
1
1
676(40)
180(25)
889(22)
493(75)
304(20)
154(100)
089(25)
058(45)
612(10)
605(22)
524(40)
300(22)
134(10)
075(9)
2490(80)
2153(100)
1523(60)
1299(25)
1077(15)
Figures in parenthesis show the relative line intensities normalised to the strongest intensity line
given by 100)
(
J. Therm. Anal. Cal., 68, 2002

9
6
2 3
RAHMAN, VENKATARAMAN: SYNTHESIS OF γ-Fe O
22 2
Fig. 3 d.c. electrical conductivity trace for FeC12H O14·2H O under nitrogen atmosphere
3 4 22 2
Table 2 X-ray data of Fe O , obtained from FeC12H O14·2H O, by heating under nitrogen atmo-
sphere at 500°C (2 h)
Observed d-spacing values (µm), present study
Fe
3
O
4
, d-spacing in µm [24]
4
2
2
2
2
1
1
1
1
1
1
1
1
820(5)
980(25)
535(100)
420(5)
092(20)
716(5)
620(25)
482(35)
312(5)
288(9)
220(9)
098(4)
060(3)
4850(8)
2967(30)
2532(100)
2424(8)
2099(20)
1715(10)
1616(30)
1485(40)
1419(2)
1328(4)
1281(10)
1266(4)
1212(2)
1
1
1
122(4)
093(12)
050(6)
Figures in parenthesis show the relative intensities normalised to that of the strongest line (given by 100)
J. Therm. Anal. Cal., 68, 2002

RAHMAN, VENKATARAMAN: SYNTHESIS OF γ-Fe
2
O
3
97
22 2 22 2
Fig. 4 XRD pattern for a – FeC12H O14·2H O; b – dehydrated FeC12H O14·2H O
Study under dynamic air atmosphere
The thermal analysis trace for FeC H O ·2H O is shown in Fig. 5. The TG trace
1
2
22 14
2
shows a two-step mass loss. The first step of mass loss occurred in the range from am-
bient temperature to 160°C, corresponding to partial dehydration of the gluconate.
The DTA and DTG traces show peaks at 160 and 178°C. The complete dehydration
of the gluconate complex occurs around 230°C, which is noticed on the TG trace, and
by an endothermic peak on DTA trace at 220°C.
22 2
Fig. 5 Thermal trace of FeC12H O14·2H O under dynamic air atmosphere
J. Therm. Anal. Cal., 68, 2002

9
8
2 3
RAHMAN, VENKATARAMAN: SYNTHESIS OF γ-Fe O
The oxidative decomposition of the complex was indicated by the presence of
three exothermic peaks on the DTA trace at 282, 336 and 402°C respectively. The
DTG trace also shows peaks at 178 and 265°C. The TG trace shows a continuous
mass loss upto 460°C, where a final formation to α-Fe O , took place. The isothermal
2
3
heat treatments were given at 290, 340, and 410°C respectively. The X-ray diffrac-
tion shows that the sample obtained on isothermal heating at 290°C was a mixture of
FeO with some gluconate complex (presence of broad peaks, showing conversion
from amorphous to crystalline nature were observed), at 340°C it was a mixture of
Fe O and γ-Fe O , and at 410°C the formation of α-Fe O was complete. Though the
3
4
2
3
2
3
amorphous → crystalline transition took place in this atmosphere, from the anhy-
drous gluconate to formation of different iron oxides, including γ-Fe O , it is ob-
2
3
served that the stabilisation of this γ-Fe O was not completely possible on isothermal
heating. It is reported in our earlier studies [18] that moisture plays an important role
2
3
in formation and stabilisation of γ-Fe O , phase formed so. The mechanism for the
2
3
stabilisation of γ-Fe O phase is understood with the formation of hydrogen ferrite
2
3
phase with the possible reaction of γ-Fe O and water vapour [12, 18]. The role of wa-
ter vapour and its subsequent decomposition with the formation of hydrogen ferrite is
2
3
very important in understanding the doping features of γ-Fe O [12]. Hence, we have
carried out our experiments under the influence of dry air containing water vapour to
2
3
investigate the feasibility of formation of γ-Fe O and its subsequent stabilisation.
2
3
Study under dynamic air containing water vapour
A complete study of direct current electrical conductivity under the atmosphere of
dynamic air containing water vapour could not be carried out as the ferrous gluconate
pellet melted on heating. The formation of pure γ-Fe O and its stabilisation was pos-
2
3
sible by isothermally heating the gluconate sample at 360°C for an h in a rotating
2 3
Fig. 6 XRD pattern of γ-Fe O synthesised by thermal decomposition of
22 2
FeC12H O14·2H O under dynamic air containing water vapour atmosphere
J. Therm. Anal. Cal., 68, 2002

RAHMAN, VENKATARAMAN: SYNTHESIS OF γ-Fe
2
O
3
99
flask, under this atmosphere. The X-ray diffraction pattern fitted well for this sample
with that of γ-Fe O [20] having a cubic structure as shown in Fig. 6. It is observed
that the gluconate complex under this atmosphere of dynamic air containing water
2
3
vapour first melts, forming froth and than decomposes to give pure γ-Fe O at 360°C.
2
3
Hence the earlier discussion on formation and stabilisation of γ-Fe O is in concur-
rence with the present results.
2
3
Particle morphology
Figure 7 shows the scanning electron micrograph (SEM) of γ-Fe O , prepared from
the gluconate precursor, it is observed from this figure that the particles are nanosized
2
3
(<100 nm). The particles are mainly rectangular and mostly uniformed sized. How-
ever, large agglomerates are also seen.
2 3
Fig. 7 Scanning electron micrograph of γ-Fe O synthesised in the present study
Magnetic properties
To test the feasibility of the above synthesised γ-Fe O , as a recording material, the
results of coercive force (H ), saturation magnetisation (M ), and Curie temperature
2
3
c
s
(
Tc) are obtained and shown in Table 3. These values are compared with that of the
commercially available results. It is reported that the aciculate shape of γ-Fe O parti-
2
3
cle plays a major role in achieving, desired magnetic properties for use in magnetic
recording [4]. Slightly lower values of the magnetic parameters in the present study,
may be due to the particle shape criterion (not acicular, in the present study). The
rectangular nature of the particles indicates that γ-Fe O , prepared from ferrous
2
3
gluconate may also find use as a heterogeneous catalyst, gas sensors and for other
technological purposes, apart from its use as a magnetic tape recording material
[
17, 21, 22].
J. Therm. Anal. Cal., 68, 2002

1
00
2 3
RAHMAN, VENKATARAMAN: SYNTHESIS OF γ-Fe O
2 3
Table 3 Magnetic hysteresis results for γ-Fe O
γ-Fe
2
O
in the present study
3
, obtained
2 3
γ-Fe O results
from literature [4]
Magnetic property
Coercive force/Oe
200
59
250
60
–
1
Saturation magnetisation/emu g
Remanance ratio
0.5
0.6
Conclusions
•
•
•
γ-Fe O as nanomaterial with required magnetic properties, from the ferrous
2
3
gluconate precursor, under the atmosphere containing water vapour is obtained.
It was observed that the water vapour atmosphere stabilises the formation of
γ-Fe O , which is in concurrence with earlier studies.
2
3
Under all the atmospheres it was observed that the anhydrous gluconate formed by
dehydration of ferrous gluconate dihydrate is amorphous, i.e., a change from crys-
talline to amorphous phase. This anhydrous ferrous gluconate later converts to
crystalline iron oxide. The γ-Fe O obtained from this process, has a cubic structure
2
3
as revealed from X-ray data.
γ-Fe O obtained in the present study may find application for magnetic recording,
heterogeneous catalyst, sensor and for other technological applications.
•
2
3
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1
2
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