Applied Mechanics and Materials
ISSN: 1662-7482, Vol. 320, pp 275-280
doi:10.4028/www.scientific.net/AMM.320.275
© 2013 Trans Tech Publications, Switzerland
Online: 2013-05-27
The Preparation of Nanosized Iron Oxide Using Hydrolysis Enhanced by
CuO and Their Characterization
Li-hua Lin, Jian Li*, Long-long Chen
School of Physical Science & Technology, MOE Key Laboratory on Luminescence and Real-Time
Analysis, Southwest University, Chongqing, 400715, People’s Republic of China
E-mail: [email protected]
Key words: Ferric nitrate, Copper oxide, Hydrolysis, Ferrihydrite, Nanocrystallite
Abstract. By adding CuO into heated Fe(NO3)3 aqueous solution, a precipitation reaction takes
place to form nanosized iron oxide. The product obtained were characterized by transmission
electron microscopy(TEM), vibrating sample magnetized(VSM), X-ray diffraction(XRD), energy
disperse X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy(XPS). The experimental
results showed that the product is weakly magnetic nanoclusters based on smaller ferrihydrite
Fe5O7(OH)·4H2O nanocrystallites. The nanoclusters are of about 40 nm size and absorbed by
Fe(NO3)3. The experimental results are attributed to the Fe(NO3)3 hydrolysis reaction being
enhanced by CuO as hydrolyte. A new route is proposed for the preparation of nanosized oxide
using hydrolysis enhanced.
Introduction
Nanomaterials attract a great deal of interest because of their distinct optical, magnetic, electronic,
mechanical and chemical properties compared with those of the bulk material. Research into the
nanocrystallites or nanoparticles, with controlled size and shape, is expected to provide a
fundamental understanding of phenomenon and materials at the nanoscale, and create useful
structures, devices, and system that have new properties and functions owing to their small and/or
intermediate size. As a result of attractive van der Waals force, and the tendency of the system to
minimize the total surface or interfacial energy, nanoscale crystallites often tend to agglomerate into
cluster, which can occur during any of the following stages: synthesis, drying, handling and/or
post-processing[1]. The development of synthetic technology for material fabrication is of
fundamental importance to the advance of science and technology, and studies of nanoscale
materials have been focused on the development of novel synthetic method[2].
Iron oxides have attracted enormous attention owing to their interesting electrical, magnetic, and
catalytic properties and their wide variety of potential applications in various fields. For example,
the magnetic iron oxide nanoclusters, which consist of smaller iron oxide nanocrystallites, have a
tunable optical response[3]. In the liquid-phase synthesis of inorganic nanocrystallites/
nanoparticles, usual methods are coprecipitation method, sol-gel method, microemulsion method,
hydrothermal and solvothermal methods, etc.[4]. The hydrolysis reaction is used seldom for
preparing nanosized material since the reaction is generally very weak. Nevertheless, using a
modified forced hydrolysis method, iron oxide nanocrystallites were synthesized at 180°C for 24
hr[5]. By dissolving iron salts in corresponding dilute acid solutions, the partial hydrolysis can be
carried out by NaOH addition, and FeOOH polymers were formed[6,7]. And, by adding NH3
aqueous to Fe(NO3)3 solution, low crystalline ferrihydrite(2-XRD lines) was synthesized[8]. In the
work presented, a one-step synthesis is proposed for preparation of iron oxide
nanocrystallites/nanoclusters at 100°C for 0.5hr using hydrolysis of ferric nitrate enhanced by
copper oxide, and the obtained product´s morphology, size, magnetism, crystalline feature and
chemical composition were characterized.
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Experimental
Sample preparation. After Fe(NO3)3 aqueous solution(0.25M, 400ml) was heated to boiling point,
CuO powder (0.1mol) was added into the heated Fe(NO3)3 solution, which was then kept boiling for
30 min with stirring. After heating was stopped, the precipitate was segregated gradually from the
solution. Then the precipitate was washed with acetone and allowed to dry naturally to obtained
final product. And, for research of the precipitated product´s construction, the colloid was
synthesized by dispersing it in acid aqueous solution(0.2M HNO3).
Characterization. For the precipitated product and the colloid, the transmission electron
microscopy(TEM, JEM-2100F) was used to observation of the morphology and analysis of size.
For the precipitated product, the magnetism, crystallization features and chemical composition
was characterized using vibrating sample magnetometer(VSM, HH-15), X-ray diffraction(XRD,
XD-2 ), electron diffraction(ED, in TEM), energy disperse X-ray spectroscopy(EDX, INCA
SEM-350) and X-ray photoelectron spectroscopy(XPS, Thermo ESCA250).
Results and discussion
TEM observation indicated that the matter precipitated is quasi-spherical nanoclusters, which
contained smaller nanocrystallites, as Fig. 1 shown.
Fig. 1
TEM photograph of (a) the precipitate and (b) the colloidal particles
Using statistical analysis from TEM photographs[9], the size distribution of nanoclusters can be
illustrated, as Fig. 2(a) shown, which fits a long-normal distribution, with the median diameter dg as
36.70nm and standard deviation lnσg=0.24. The average diameter d can be calculated as 37.78nm
by
d = exp[ln d g + 0.5 ln 2 σ g ]
(1)
Similarly, the size distribution of the nanocrystallites is illustrated, as Fig. 2(b) shown, which fits
obviously a log-normal distribution also. Their median diameter is 5.96nm and standard deviation is
0.24. By Eq.1, the average diameter is calculated as 6.13nm. According to these data, average
number of the nanocrystallites in a nanocluster n can be estimated as n=(37.78/6.13)3=234.
Applied Mechanics and Materials Vol. 320
1.8
2.0
(a)
1.6
277
(b)
1.6
1.4
dφ /dlnx
dφ /dlnx
1.2
1.0
0.8
0.6
0.4
1.2
0.8
0.4
0.2
0.0
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
0.0
4.4
1.0
lnx
1.2
1.4
1.6
1.8
2.0
2.2
2.4
lnx
Fig. 2 Log-size histogram for (a) the nanoclusters and (b) the nanocrystallites. dφ = dN / ∑ dN is
relative cluster(crystallite) number, where dN is number of particles measured in a interval of the
size, ∑ dN is total number of particles measured, and dlnx(= ln 2 ) is logarithmic interval
measuring size.
1.0
σ (emu/g)
0.5
0.0
-0.5
-1.0
-10
Fig. 3
-5
0
H(kOe)
5
10
The magnetization curve of the sample
Fig. 3 shows the magnetization curve of the nanoclusters at room temperature, which behaviored
as weak magnetism.
The XRD measurement displayed non-crystal like pattern, which is similar to the low crystalline
ferrihydrite with 2-XRD lines[8], as Fig. 4 shown. However, the ED measurement exhibited clear
diffraction rings, as the inset shown in Fig. 4. By the analysis of crystal face spacing d of the
CPS(arb.)
(b)
(a)
10
20
30
40
50
60
70
80
2θ
θ(degree)
Fig. 4 The XRD pattern of (a) the sample and (b) the ferrihydrite from ref.[8]. The inset in (a) is
ED pattern.
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China Functional Materials Technology and Industry Forum
Table 1
The analysis data of the d values from ED
d(nm)
sample
0.2505
0.2240
0.1955
0.1717
0.1496
0.1341
Fe5O7(OH)·4H2O
0.2500
0.2210
0.1960
0.1720
0.1510, 0.1480
0.1339*
(h k l)
(110)
(200)
(113)
(114)
(115),
(106)
I(f)
100.0
80.0
80.0
50.0
70.0,
80.0
(116)
* Calculated value.
diffraction rings corresponding, the electron diffraction agrees with the standard values of
hexagonal Fe5O7(OH)·4H2O ferrihydrite (PDF#29-0712). The detailed data are listed in Table 1.
Counts(arb.)
Fe
O
N
Cu
0
4
2
6
8
10
Energy(keV)
The EDX spectrum of the sample
O1s
Counts(arb.)
Fig. 5
Fe2p3/2
N1s
936
1200
934
1000
932
800
600
400
Binding Energy(eV)
Fig. 6 The XPS spectrum of the sample. The inset is the spectrum range of binding energy
corresponding to Cu
Fig. 5 shows the EDX spectrum, which indicates Fe,N,O and no Cu. XPS measurement
confirmed that there were Fe,N and O, but no Cu in the sample, as shown in Fig. 6.The results of
both EDX and XPS show that the sample contained the nitric compound in addition to
Fe5O7(OH)·4H2O, but no clear Cu. The nitric compound could be Fe(NO3)3 absorbed on the
ferrihydrite crystallites, which is a similar product while the Fe3O4 nanoparticles were treated with
Fe(NO3)3 aqueous solution[10,11]. By dividing the Fe spectrum of XPS into two lines(see fig. 7), it
can be judged that the binding energy of Fe2p3/2 in the Fe(NO3)3 is about 710.23eV, which is ready
the one in Fe(CO)5(709.40eV)[12], and in the Fe5O7(OH)·4H2O ferrihydrite is about 708.97eV.
And, the oxide spectrum is difficult on to be divided, which could be because there are many
oxygen with different binding energies in the sample. The detailed data of binding energy were
listed in Table 2.
Applied Mechanics and Materials Vol. 320
p2
p1
Counts(arb.)
706
279
708
710
712
714
Binding Energy(eV)
Fig. 7
XPS spectrum of Fe in the sample
Table 2 Data of binding energy from XPS(eV)
Fe2p3/2
Sample
710.23(p1)
Fe5O7(OH)·4H2O
△
Fe(NO3)3
708.97(p2)
O1s
N1s
531.25
406.34
△
△
△
△
The experimental results and analysis indicate that the Fe5O7(OH)·4H2O ferrihydrite nanocluster
based smaller nanocrystallites can be synthesized by adding CuO to heated Fe(NO3)3 aqueous
solution. Obviously, the synthesis of ferrihydrite is in relation to Fe(NO3)3 hydrolysis, which is
enhanced by CuO. This can be discussed as follows.
Fe(NO3)3 hydrolysis can be written as
5 Fe(NO3)3+12H2O(H++OH-)→Fe5O7(OH)·4H2O+15H++15NO3(2)
In reality, the hydrolysis reaction of Fe(NO3)3 solution is too weak to product nanocrystallites.
From the experimental results, it can be determined that CuO can well dissolved in heated ferric
nitrate aqueous solution although it is undissolved in pure water. As a consequence, the partial H+ in
Eq. 2 would be neutralized due to the dissolve reaction, i. e.
(3)
CuO+2H+→Cu2++H2O
Thus, the Fe(NO3)3 hydrolysis is enhanced and the production of ferrihydrite increase. Finally,
for the synthesis of ferrihydrite, a simplified reaction can be described by
15CuO+10Fe(NO3)3+9H2O→2Fe5O7(OH)·4H2O+15Cu2++30NO3(4)
The nanocrystallites are so small that they aggregate to form nanoclusters during the
precipitation process. In addition, the hydrolysis reaction of Fe(NO3)3 could be incomplete and
some Fe(NO3)3 could be absorbed on the Fe5O7(OH)·4H2O nanocrystallites nanoclusters.
Conclusion
Although copper oxide remains undissolved in pure water, it can dissolve in heated Fe(NO3)3
aqueous solution, and Fe(NO3)3 hydrolysis can be enhanced by the dissolving reaction of CuO.
Thus, in a precipitation, taking place to form ferrihydrite Fe5O7(OH)·4H2O nanocrystallites with
size of 6 nm or so, these nanocrystallites are so small that they aggregate into nanoclusters about
40nm in size. The ferrihydrite cluster is easy dispersed in acid aqueous solution to form colloid, and
could be used as a precursor to prepare α−Fe2O3 nanoparticles[8,13] And, the binding energy of
Fe3/2 in the Fe5O7(OH)·4H2O is about 710.23eV. In the preparation method, the copper oxide plays
a role of hydrolyte in the ferric nitrate aqueous solution. Such hydrolysis reaction enhanced by CuO
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China Functional Materials Technology and Industry Forum
could be more effective than the stimulation with NaOH[6,7] or NH3·aq[8]. Perhaps, hydrolysis
reaction of metal salt enhanced by oxide is a new route for the preparation of oxide nanocrystallites
or nanocluster, which will be investigated further.
Acknowledgements
Financial support for this work was provided by the National Science Foundation of China
(No.11074205)
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China Functional Materials Technology and Industry Forum
10.4028/www.scientific.net/AMM.320
The Preparation of Nanosized Iron Oxide Using Hydrolysis Enhanced by CuO and their
Characterization
10.4028/www.scientific.net/AMM.320.275
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