Thermoanalytical study of iron(III) phosphate obtained by homogeneous precipitation from different media

Article abstract of DOI:10.1016/j.tca.2003.10.024

Amorphous iron(III) phosphate has been synthesised by homogeneous precipitation from equimolecular Fe(NH₄)₂(SO ₄)₂·6H₂O and NH₄H ₂PO₄ aqueous, water-ethanol and water-iso-propanol solutions at pH=2.0 and ambient temperature using hydrogen peroxide as precipitating agent. The precipitates have been characterised by TG/DTG/DTA techniques, chemical analysis, X-ray powder diffraction (XRD) analysis and scanning electron microscopy (SEM). The presence of ethanol and iso-propanol in the precipitation medium suppressed the co-precipitation of ferric sulphate as it does in aqueous medium. Thermal treatment of the as-precipitates at 750°C in air yields a crystalline quartz-like structured FePO₄ with markedly different morphological features.

Full text of DOI:10.1016/j.tca.2003.10.024

Thermochimica Acta 413 (2004) 81–86
Thermoanalytical study of iron(III) phosphate obtained by
homogeneous precipitation from different media
Silvera Scaccia∗, Maria Carewska, Pier Paolo Prosini
IDROCOMB, Hydrogen Project and Fuel Cells, ENEA, C.R. Casaccia, Via Anguillarese 301, I-00060 Rome, Italy
Received 26 September 2003; received in revised form 30 October 2003; accepted 30 October 2003
Abstract
Amorphous iron(III) phosphate has been synthesised by homogeneous precipitation from equimolecular Fe(NH4)2(SO4)2·6H2O and
NH4H2PO4 aqueous, water–ethanol and water–iso-propanol solutions at pH = 2.0 and ambient temperature using hydrogen peroxide as
precipitating agent. The precipitates have been characterised by TG/DTG/DTA techniques, chemical analysis, X-ray powder diffraction
(XRD) analysis and scanning electron microscopy (SEM). The presence of ethanol and iso-propanol in the precipitation medium suppressed
the co-precipitation of ferric sulphate as it does in aqueous medium. Thermal treatment of the as-precipitates at 750 ◦C in air yields a crystalline
quartz-like structured FePO4 with markedly different morphological features.
© 2003 Elsevier B.V. All rights reserved.
Keywords: FePO4; Thermal analysis; X-ray powder diffraction; SEM
1. Introduction
the ions of opposite charge favouring the formation of insol-
uble species [6]. In a previous work of the authors [7], it has
been studied the preparation of iron(III) phosphate by pre-
cipitation from supersaturated aqueous solutions. Different
synthesis conditions have been investigated, which give rise
to final iron phosphates with different thermal behaviour.
In the present paper we report on the synthesis
of iron(III) phosphate by homogeneous precipitation
from Fe(NH4)2(SO4)2·6H2O and (NH4)H2PO4 aqueous,
water–ethanol and water–iso-propanol solutions at pH =
2.0 and ambient temperature using hydrogen peroxide as
precipitating agent. The precipitates have been characterised
by thermoanalytical techniques, chemical analysis, X-ray
powder diffraction (XRD) analysis and SEM technique.
Iron(III) phosphate is recently gaining more and more
interest as material to be used in the field of catalysis [1],
wastewater purification systems [2], ferroelectrics [3], and
lithium batteries [4,5]. Particularly in the field of lithium
batteries it is used as cathode electrode or as precursor for
phosphate-based anode electrodes. However, the successful
of applicability of this material depends on the morphology
and purity of the material. Powders obtained by the generally
“high temperature” methods induces in fact the sintering
and aggregation of particles, which are deleterious for the
electrochemical performance.
The synthesis of inorganic materials by soft solution pro-
cessing, which includes precipitation methods, sol–gel pro-
cess, hydrothermal techniques, allows obtaining fine grained
particles and high purity materials at low temperatures in en-
vironmentally benign conditions. In precipitation reactions
of inorganic materials the addition of organic solvent misci-
ble in water is a routine way to change the ionic strength of
the medium to increase the electrostatic interaction between
2. Experimental
2.1. Instrumentation
A TG–DTA apparatus SDT 2960 (TA Instruments, Dork-
ing, England) was used. Samples between 5 and 10 mg
were heated over the temperature range from ambient to
800 ◦C at a heating rate of 10 ◦C min−1 in air at a flow rate
of 100 ml min−1. ␣-Al2O3 was used as reference material.
Samples were run in open platinum pans.
∗
Corresponding author. Tel.: +39-06-30483815;
fax: +39-06-30486357.
E-mail address: silvera@casaccia.enea.it (S. Scaccia).
0040-6031/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2003.10.024

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S. Scaccia et al. / Thermochimica Acta 413 (2004) 81–86
Fig. 1. XRD patterns of the as-precipitate iron phosphate sample obtained from water–ethanol solution at pH = 2.0 and ambient temperature using
hydrogen peroxide as precipitating agent (a) and the same sample calcined at 750 ◦C/10 h in air (b).
pH was measured with a SA520 pH-meter (Orion, Bev-
erly, MA, USA), using a C2005-8 red rod combined pH elec-
trode (Radiometer, Copenhagen, Denmark). For calibration,
standard buffer solutions from Aldrich were used.
The elemental composition of the precipitates (Fe, P) was
determined by flame and graphite atomic absorption spec-
trometry (Varian 220 FS, Melgrave Victoria, Australia), af-
ter dissolution of the powders in dilute hydrochloric acid.
The precipitates and the calcined products (in air at 750 ◦C
for 10 h) were characterised by X-ray powder diffraction
analysis (Philips PW 3710 diffractometer) using Cu K␣ ra-
diation.
2.2. Reagents and standard solutions
Ultra-high-purity water (18 Mꢀ cm) obtained from a
Milli-Q water production system (Millipore, Bedford, MA)
and filtered through a 0.22 ␮m membrane filter was used to
prepare all solutions and dilutions. Fe (NH4)2(SO4)2·6H2O
and (NH4)H2PO4 (Reagent Grade, Carlo Erba, Italy) crys-
talline solids were dissolved in de-ionised water to prepare
iron and phosphate stock solutions and standardised by
atomic absorption spectrometry. Stock solutions were al-
ways prepared just before each experiment. Stock standard
solutions of iron and phosphorus (Aldrich) at 1000 ppm
Fig. 2. TG/DTG/DTA curves of the as-precipitate iron phosphate powders obtained from: (a) aqueous solution; (b) water–ethanol solution; and (c)
water–iso-propanol solution. Heating rate of 10 ◦C min−1 in air.

S. Scaccia et al. / Thermochimica Acta 413 (2004) 81–86
83
Fig. 2. (Continued ).
were used for the preparation of the working standard so-
lutions for the iron and phosphorus AAS determination.
All acids used were of analytical reagent grade. Hydrogen
peroxide 30% weight (Reagent grade, Ashland Chemical
Italiana), ethanol and iso-propanol (Reagent Grade, Carlo
Erba) were used.
itate started to form. The precipitates were kept in contact
with the mother liquor for 30 min without stirring. The pre-
cipitates collected on membrane filter (0.8 ␮m) were washed
several times with de-ionised water to make the powder free
from undesired water-soluble impurity species. The recov-
ered precipitates were dried at room temperature and atmo-
sphere for 2 days. The yield of precipitates was near the
theoretical value.
2.3. Preparation
A 0.1 M Fe(NH4)2(SO4)2·6H2O solution (having an ini-
tial pH of 4.9) was added at ambient temperature to a con-
stantly stirred solution of 0.1 M (NH4)H2PO4, previously
dissolved in de-ionised water, water–ethanol (1.5:1, v/v), and
water–iso-propanol (1.5:1, v/v) solutions, in a 1:1 volume
proportion. Three millilitres of concentrated hydrogen per-
oxide solution were added. Instantly, a white gel-like precip-
3. Results and discussion
All the as-precipitate iron phosphate samples have been
identified as being amorphous by X-ray analysis. A poorly
crystalline pattern showing only one low angle reflection
around 16.2◦ has been observed for the as-precipitate

Fig. 3. SEM micrographs of the as-precipitate iron phosphate powders obtained from: (a) aqueous solutions at ambient temperature and (b) the same sample calcined at 750 ◦C/10 h in air; (c) water–ethanol
solution; and (d) water–iso-propanol solution and calcined at 750 ◦C/10 h in air.

S. Scaccia et al. / Thermochimica Acta 413 (2004) 81–86
85
samples derived from water-alcohol solutions at ambient
temperature. The XRD patterns of the amorphous precipi-
tates calcined at 750 ◦C/10 h in air showed that the samples
are crystalline. The major reflections are attributed to the
trigonal quartz-like structure FePO4 [8], whereas other ex-
traneous minor reflections around 16.2, 27.7, 29.3, and 30.4◦
are indicative of the formation of Fe4(P2O7)3 secondary
phase [9]. Exception is made for the calcined precipitate ob-
tained by precipitation from water solution for which all re-
flections are attributed to the anhydrous iron phosphate [7].
By way of example only XRD patterns of the as-precipitate
derived from water–ethanol solution and the same sample
calcined at 750 ◦C are displayed in Fig. 1.
The TG/DTG/DTA curves of the as-precipitate iron phos-
phate obtained from aqueous solution is shown in Fig. 2a.
The TG curve shows a well-defined weight loss between
50 and 500 ◦C (DTG peak at 120 ◦C), which is ascribed to
elimination of crystallisation water. The total mass loss is
22.0%, which based on the total moles of iron and phos-
phorus (0.550) in the precipitate, corresponds to 2 moles of
H2O. The corresponding DTA curve shows one endother-
mic peak at 115 ◦C. At higher temperatures a sudden small
weight loss of about 1.0% between 500 and 600 ◦C oc-
curs, which is immediately followed by another small weight
loss of about 2.0% as seen in the TG curve. These weight
losses were previously [6] associated to co-precipitation of
ferric sulphate or ammonium iron alum. The exothermic
peak at 546 ◦C is displayed in the DTA curve. This event
was assigned to a transition phase from amorphous to crys-
talline as evidenced by XRD [10]. Finally, a width exother-
mic peak at 768 ◦C without appreciable weight loss was
observed.
tal mass loss is 28.0%, which is higher than the expected
value for two moles of crystallisation water. Physically ab-
sorbed iso-propanol on the surface of the as-precipitate pow-
der could take account for this. The corresponding DTA
curve shows one endothermic peak at 105 ◦C. At higher
temperatures a sudden small weight loss of 3.0% between
500 and 600 ◦C was observed. A large exothermic peak at
560 ◦C was displayed on the DTA curve. Again this event
was assigned to a transition phase from amorphous to crys-
talline form. Further, a small exothermic effect at 669 ◦C
followed by a weak endothermic effect at 716 ◦C without
appreciable weight loss was observed in the DTA curve.
The latter effect may be ascribed to the ␣ → ␤ transi-
tion as above-mentioned for the as precipitate derived from
water–ethanol solution.
The scanning electron micrographs of the as-precipitate
iron phosphate powders showed similar uniform morpho-
logical features. The powders consisted of round particles
near 80 nm in size along with a narrow size distribution.
By way of example only the SEM image of iron phosphate
derived from aqueous solution is depicted in Fig. 3a. Con-
versely, morphological variations with temperature were
observed for all the samples. The morphological image of
the 750 ◦C-calcined iron phosphate derived from aqueous
medium showed a slightly increase of grain size (crys-
tallites of 200 nm) (Fig. 3b). The SEM micrographs of
the 750 ◦C-calcined FePO4 derived from alcoholic media
showed that crystallites larger than 10 ␮m in size are yield,
indicating that a sintering process occurs at high tempera-
tures (Fig. 3c and d).
The TG/DTG/DTA curves of the as-precipitate iron phos-
phate obtained from water–ethanol solution is shown in
Fig. 2b. The TG curve shows that the elimination of crys-
tallisation water occurs between 50 and 500 ◦C in two over-
lapped steps with DTG peaks at 117 and 365 ◦C. The to-
tal mass loss is 20.0%, which based on the total moles of
iron and phosphorus (0.540) in the precipitate corresponds
to 2 mol of H2O. The corresponding DTA curve shows one
endothermic peak at 105 ◦C. At higher temperatures, a sud-
den small weight loss of 0.5% between 500 and 600 ◦C was
observed. A large exothermic peak at 559 ◦C was displayed
on DTA curve. Again, this event was assigned to a transi-
tion phase from amorphous to crystalline form. Further, a
small exothermic effect at 669 ◦C followed by a weak en-
dothermic effect at 716 ◦C without appreciable weight loss
was seen in the DTA curve. The latter effect may be ascribed
to the ␣ → ␤ transition as reported for quartz-like materi-
als of type MXO4 (M = B, Al, Ga, Fe, Mn and X = P,
AS) [11].
4. Conclusions
Amorphous iron(III) phosphate has been synthe-
sised by homogeneous precipitation from equimolec-
ular Fe(NH4)2(SO4)2·6H2O and NH4H2PO4 aqueous,
water–ethanol and water–iso-propanol solutions at pH = 2.0
and ambient temperature using hydrogen peroxide as pre-
cipitating agent. The presence of ethanol and iso-propanol
in the precipitation medium suppressed the co-precipitation
of ferric sulphate/or ammonium iron alum as it does in
aqueous medium, but trace level of a new secondary phase,
Fe2(P2O7)3 is formed. Thermal treatment at 750 ◦C yields
a crystalline quartz-like structured FePO4 with markedly
different morphological features.
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