Chin. Phys. B Vol. 22, No. 10 (2013) 107101 [610268]

Chin. Phys. B Vol. 22, No. 10 (2013) 107101
Growth of monodisperse nanospheres of MnFe 2O4with enhanced
magnetic and optical properties
M. Yasir Rafiquea)b), Pan Li-Qing( 潘礼庆)a)†, Qurat-ul-ain Javedb), M. Zubair Iqbalb),
Qiu Hong-Mei( 邱红梅)b), M. Hassan Farooqc), Guo Zhen-Gang( 郭振刚)b), and M. Tanveerd)
a)College of Science and Research Institute for New Energy, China Three Gorges University, Yichang 443002 , China
b)Department of Physics, University of Science and Technology of Beijing, Beijing 100083 , China
c)School of Material Science and Engineering, University of Science and Technology of Beijing, Beijing 100083 , China
d)Research Center of Materials Science, Beijing Institute of Technology, Beijing 100081 , China
(Received 6 February 2013; revised manuscript received 3 April 2013)
Highly dispersive nanospheres of MnFe 2O4are prepared by template free hydrothermal method. The nanospheres
have 47.3-nm average diameter, narrow size distribution, and good crystallinity with average crystallite size about 22 nm.
The reaction temperature strongly affects the morphology, and high temperature is found to be responsible for growth of
uniform nanospheres. Raman spectroscopy reveals high purity of prepared nanospheres. High saturation magnetization
(78.3 emu/g), low coercivity (45 Oe, 1 Oe = 79.5775 A
cm1), low remanence (5.32 emu/g), and high anisotropy constant
2.84104J/m3(10 times larger than bulk) are observed at room temperatures. The nearly superparamagnetic behavior is
due to comparable size of nanospheres with superparamagnetic critical diameter Dspm
cr. The high value of Keffmay be due to
coupling between the pinned moment in the amorphous shell and the magnetic moment in the core of the nanospheres. The
nanospheres show prominent optical absorption in the visible region, and the indirect band gap is estimated to be 0.98 eV
from the transmission spectrum. The prepared Mn ferrite has potential applications in biomedicine and photocatalysis.
Keywords: Mn ferrite, magnetic materials, hydrothermal method, superparamagnetic, Raman spectroscopy
PACS: 71.10.Ay, 75.50.–y, 75.75.–c, 78.67.Bf DOI: 10.1088/1674-1056/22/10/107101
1. Introduction
Nanostructures of magnetic transition oxides with spinel
structure MFe2O4(M= Mn, Fe, Co, Ni, etc.) possess
unique magnetic properties like moderate magnetizations,
high coercivities, single domain effects, spin filterings, and
so on,[1–3]which leads to impressive industrial and biolog-
ical applications.[4,5]As an important member of the ferrite
family, MnFe 2O4(MFO) has attracted noteworthy research
interest due to its mesmerizing magnetic and electromag-
netic properties. MnFe 2O4is partially inverse spinel so that
the majority of Mn2+ions are located at tetrahedral (A) site
while only 20% of them are located at octahedral (B) site.[6]
The Mn ferrite (MnFe 2O4) has a much lower resistivity than
CoFe 2O4and NiFe 2O4[7]and its magnetic moment is consis-
tent with the Neel coupling scheme.[8]The bulk MnFe 2O4has
a saturation magnetization of 80 emu/g, an anisotropy con-
stant Kof 2.5103J/m3, and exchange stiffness constant A
of 3.221012J/m at room temperature (RT).[1,9,10]How-
ever, nanoparticles, nanostructures, and thin films of MnFe 2O4
show diverse properties, such as, high anisotropy constant K=
5.6104J/m3,[10]size-dependent saturation magnetization
(20 emu/g–70 emu/g),[11]and high Curie temperature.[12,13]
Moreover, nanosized Mn ferrite (MFO) possesses super spinglass state,[14]superparamagnetism,[15,16]and RT spin fil-
tering properties in epitaxial thin films.[17]These proper-
ties of MFO result in many attractive applications such as
magnetic recording,[18]microwave, MRI contrast agent for
liver imaging,[19]ferrofluid, water treatment,[20]site specific
drug delivery,[21]magnetic tunnel junction-based sensor,[22]
photocatalytic,[23]and water-splitting.[24]
To fabricate the nanoparticle of ferrites, a lot of re-
search methods have been reported in the past a few
years.[25]In the previous investigations, nanoparticles/nano-
structures of MnFe 2O4have been synthesized by us-
ing various methods,[26]such as co-precipitation,[10]ther-
mal decomposition,[27]microemulsions[28]and mechano-
chemical[16]microwave-assisted[29]reverse micelles,[10,21]
detonation of emulsion explosive,[30]polymerized com-
plex method,[31]and solvothermal method.[32]Recently,
nanorods,[33,34]nanospheres,[20,35]and a nano-octahedron[20]
of MnFe 2O4have been synthesized by hydrothermal method.
Guo et al.[23]reported on the synthesis of hollow spheres of
250-nm diameter by solvothermal method. Generally, fer-
rite nanoparticle/nanospheres grown by conventional chemical
methods have low saturation magnetizations[11,16,21,27,32]due
to dead layer effect and the presence of non-magnetic impuri-
Project supported by the National Natural Science Foundation of China (Grant Nos. 50472092, 50672008, and 50971023), the Beijing Natural Science
Foundation (Preparation and Magnetic Properties of Ferromagnetic Nanoring Lattice), and Research Foundation for Talented Scholars of China Three Gorges
University. M. Y . Rafique was also supported by the Chancellor Scholarship of the University of Science and Technology of Beijing.
†Corresponding author. E-mail: lpan@ctgu.edu.cn
© 2013 Chinese Physical Society and IOP Publishing Ltd http://iopscience.iop.org/cpb 　 　 　http://cpb.iphy.ac.cn
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Chin. Phys. B Vol. 22, No. 10 (2013) 107101
ties such as ferrihydrite. However, there is still the demand to
synthesize uniform spherical nanoparticles of MnFe 2O4with
high saturation magnetizations, low coercivities, and low re-
manent magnetizations for biomedical applications. Herein,
we demonstrate a one-step, economical, surfactant-free, and
efficient hydrothermal method of large-scale fabricating Mn
ferrite nanospheres. The effect of reaction temperature on
morphology is described. The nanospheres exhibit enhanced
and unique magnetic properties. Furthermore, optical proper-
ties are also investigated for photocatalytic application.
2. Experimental details
2.1. Sample preparation
All chemicals were of analytical grade and directly used
without purification. The 35-ml aqueous solution of 75-mM
MnCl 2
4H2O, 150-mM FeCl 2, and 0.75-M NaOH was pre-
pared in deionized water. In this protocol, the required weight
of NaOH was dissolved separately in 10-ml deionized water
and after the 10-min magnetic stirring of metal salt solution;
it was added dropwise into metal salt. This suspension was
further stirred to acquire the homogeneity for 10 min and 2-ml
N2H4H2O was added as redox agent. Subsequently, the so-
lution was transferred in a Teflon cup and sealed in a stainless
steel autoclave. The temperature of the autoclave was main-
tained at 240C for 8 h, then it was cooled down to RT nat-
urally. A dark brown product was collected at the bottom of
the Teflon cup which was rinsed by water more than 5 times to
remove the alkali salt and other impurities. The final product
was obtained by drying in an oven at 45C for 10 h. The sam-
ples were also prepared at reaction temperatures of 140C,
180C, 200C, and 220C, keeping all the other conditions
the same as those described above.
2.2. Characterization
The morphology and elemental composition of the pre-
pared product was observed with field emission scanning elec-
tron microscope (FESEM) Zessis Ultra-5500. Rigaku smart-
lab X-ray diffractometer (XRD) was used for crystal structure
and average crystallite size analysis. Tecnai F30 transmission
electron microscope was used to characterize the microstruc-
tures. The scanning range was from 20to 90using Cu K a
radiation ( lvalue of 1.5406 ˚A). Raman spectrum was mea-
sured using Horiba Jobin Yvon HR 800 Raman spectrome-
ter with excitation wavelength of 532 nm. Magnetic measure-
ments were carried out using vibrating sample magnetometer
(VSM) option of Quantum Design Versa Lab. Optical prop-
erties were investigated by measuring the transmission spec-
trum with Hitachi U-4100 spectrophotometer using solid sam-
ple measurement system.3. Results and discussion
3.1. Structure, formation, and morphology
Figure 1(a) shows the X-ray diffractometer pattern of
prepared Mn ferrite nanospheres. All diffraction peaks
matched with the standard pattern of bulk Jacobsite MnFe2O4
(JCPDS 10-0319) and it confirms the cubic spinel struc-
tures. The average crystallite size calculated using Scher-
rer formula is 22 nm. No impurity and other phase related
peaks are present in XRD pattern, which indicates the pu-
rity of the prepared sample. The lattice parameter calcu-
lated using refined d value and standard equation for cu-
bic lattice is 8.48993 ˚A which is comparable to bulk value
(8.499 ˚A) (JCPDS 10-0319). EDX spectrum (Fig. 2(b)) shows
20 30 40 50 60 70 80 90
(620)
(444)
(731)(533)(440)(511)(422)(400)(222)(220)
(311)Intensity/arb. units
2θ/(Ο)(a)
(b)
keV Full scale 6177 cts cursor: 12.942 (1cts)
Fig. 1. (color online) (a) XRD pattern and (b) EDX spectrum of Mn
ferrite nanospheres prepared at 240C.
only the peaks of Mn, Fe, and oxygen which further testify the
purity of the sample. From the EDX analysis the elemental
compositions for Mn, Fe, and oxygen are determined to be 17,
29.56, and 53.43 % respectively. The formation of MnFe 2O4
nanospheres can be attributed to the chemical reaction as de-
scribed by Eqs. (1)–(6). By addition of NaOH in the metal
salts, the metal hydroxides were formed according to the fol-
lowing equation:
MnCl 2
4H2O+FeCl 2+4NaOH
!Mn(OH)2+Fe(OH)2+4NaCl . (1)
It is well known that hydrazine hydrate N 2H4can act as both
oxidizing agent and reducing agent.[20]In our protocol, a com-
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Chin. Phys. B Vol. 22, No. 10 (2013) 107101
paratively small amount of hydrazine acts as oxidizing agent
and oxidizes the Fe(OH) 2into Fe(OH) 3according to the fol-
lowing two equations:[20]
N2H4+2H2O+2e!2NH 3+2OH, (2)
2Fe(OH)2+2OH!2Fe(OH)3+2e. (3)
The metal hydroxide is not completely soluble in water and
undergoes the reversible reaction as follows:
Fe(OH)3$Fe3++3OH, (4)
Mn(OH)2$Mn2++2OH, (5)
Mn2++Fe3++8OH!MnFe 2O4+4H2O. (6)
Figure 2(a) shows an FESEM image of MnFe 2O4
nanospheres at low magnification which shows high yield of
nanospheres. Figure 2(b) shows an FESEM image at medium
magnification of the prepared product which clearly indicates
uniform nanospheres with diameters ranging from 45 nm to
60 nm. The shapes and uniformities of nanospheres are appar-
ent from Fig. 2(c), indicating that the prepared nanospheresare nearly monodisperse. Moreover, neither agglomeration
nor other structure of MnFe 2O4is observed from FESEM im-
age. The size distribution of the nanospheres is calculated
from Fig. 2(b) and it is shown in Fig. 2(d) which illustrates the
narrow size distribution of the nanospheres, and the diameters
of most of the nanospheres in a range of 45 nm–50 nm. The av-
erage diameter of the nanospheres is 47.3 nm with a standard
deviation of 1.3 nm. The size distribution is fitted by Gaussian
distribution with R2of value 0.975 and peak at 47.5 nm. The
TEM image of the nanospheres is shown in Fig. 3(a), which
shows the uniform nanospheres with an average size of about
50 nm, which is consistent with SEM images. The high reso-
lution TEM (HRTEM) image of the nanospheres (Fig. 3(b))
shows that the boundary area of the nanosphere (shell) has
an amorphous structure while the core is crystalline, indicat-
ing the core–shell structures of the nanospheres. The lattice
fringes of the core indicate the single crystalline nature and
from HRTEM image the lattice spacing is measured to be the
0.256 nm which corresponds to (311) lattice plane.
(a) (b)
(c)(d)40
30
20
10
040 50 60 70
Diameter/nm Distribution/% 1 mm 100 nm
20 nmDav =47.3 nm
σ=1.3 nm
Fig. 2. (color online) FE-SEM images at (a) low, (b) medium, (c) high magnification, and (d) corresponding size distribution of
MnFe 2O4nanospheres prepared at 240C.
2 nmamorphous shell
Fig. 3. (a) TEM image of nanosphere and (b) HRTEM image of
nanosphere, the double headed arrows show the amorphous area.3.2. Growth mechanism
In order to investigate the formation mechanism of
MnFe 2O4nanospheres, a series of temperature-dependent ex-
periments has been performed. Figure 4 shows FE-SEM
images of samples prepared at different temperatures. The
nanoparticles of sizes less than 30 nm were formed at a re-
action temperature of 140C (Fig. 4(a)). By increasing the
reaction temperature to 180C and 200C, larger particles of
different shapes and sizes are grown as shown in Figs. 4(b) and
4(c) respectively. At a reaction temperature of 220C, most of
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Chin. Phys. B Vol. 22, No. 10 (2013) 107101
the particles present spherical shapes but having a wide distri-
bution of sizes (Fig. 4(d). The nanospheres with a narrow size
distribution are obtained by increasing the reaction tempera-
ture to 240C as shown in Fig. 2. From these results, it can be
noted that the growth rate increases with reaction temperature
increasing, as the size of particle increases with it. The high
reaction temperature favors the growth of the spherical parti-
cle, because at a fast growth rate, the growth rates of various
facets are the same, which results in isotropic morphology.[36]
Furthermore, reaction temperature also facilitates the produc-
tion of nanospheres with narrow size distribution.
Fig. 4. FE-SEM images of samples prepared at reaction temperatures:
(a) 140C, (b) 180C, (c) 200C, and (d) 220C.
nucleation isotropic growth
Ostwald ripening
Fig. 5. (color online) Growth mechanism of nearly monodisperse
nanospheres.
The possible growth mechanism of nanospheres is shown
in Fig. 5. As the reaction takes place at a high temperature, the
nucleation process occurs first by generating the small crystal-
lites. These small crystallites start to grow along all directions
under favorable thermodynamic conditions (isotropic growth).
We suggest that Ostwald ripening starts immediately after nu-
cleation which is responsible for uniform growth in all direc-
tions. Relatively fast nucleation process due to high concentra-
tion of metal ions (Mn2+and Fe3+)and simultaneous nucle-
ation of all species due to high reaction temperature result innarrow size distribution, whereas the fast growth process is re-
sponsible for spherical shape. Therefore nearly monodisperse
nanospheres are generated.
3.3. Raman spectroscopy
Raman spectroscopy provides an important tool to probe
the structural properties of synthesized materials.[37]It is sen-
sitive to purity and many structural disorders such as lat-
tice distortion, local cation distribution, structure transition,
charge–lattice, and spin–lattice coupling, and magnetic order-
ing. For spinel ferrites (MFe 2O4), it is a versatile tool for in-
vestigating the purity of the synthesized sample. The chemical
synthesis of ferrite can introduce some nonmagnetic impuri-
ties such as 2-line ferrihydrite and 6-line ferrihydrite which
cannot be detected by XRD pattern due to the overlapping of
several peaks of ferrites with ferrihydrite phases.[38]However,
the distinct peaks of ferrite and ferrihydrite in Raman spec-
tra facilitate distinguishing them and determining their puri-
ties. MnFe 2O4has a cubic spinel structure that belongs to the
space group O7
h(Fd¯3m). The supercell contains 56 atoms, but
the primitive cell only consists of 14 atoms ( Z=2). Therefore,
42 vibrational modes are possible. The factor group analysis
predicts the five Raman active modes, i.e., A 1g+ E g+ 3T 2g
in MFe 2O4.[39]The Raman spectrum of prepared MnFe 2O4
nanospheres in a frequency range of 200 cm1–1400 cm1
is shown in Fig. 6. The Raman spectrum consists of broad
bands peaking respectively at 340, 456, and 563 cm1and
a strong band peaking at 646 cm1, which can be assigned
to E g, T2g(2), T 2g(3), and A 1grespectively. The mode A 1g
(646 cm1)is due to symmetric stretching of oxygen atoms
at tetrahedral site. The other low frequency modes represent
the characteristics of the octahedral sites (BO 6).[40]In the Ra-
man spectrum, no impurity related modes are observed. The
ferrihydrite phase has Raman-active modes at 710 cm1and
1320 cm1,[38]which are clearly absent in the Raman spec-
trum of prepared nanospheres of MnFe 2O4(Fig. 6). There-
fore, the Raman spectrum revealed the high purity of the pre-
pared nanospheres.
2 6 10 14Intensity/arb. units
Raman shift/102 cm-1
T2gEgT2gA1g
Fig. 6. (color online) Raman spectrum of Mn ferrite nanospheres mea-
sured in the ambient condition.
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Chin. Phys. B Vol. 22, No. 10 (2013) 107101
3.4. Magnetic properties
Magnetic hysteresis measured at RT is shown in Fig. 7(a).
The saturation magnetization ( Ms)of 78.3 emu/g is observed
which is slightly less than the bulk value (80 emu/g).[1]Nev-
ertheless, it is much greater than other reported values for
nanoparticle, 200-nm and 1- m nanospheres.[11,16,21,27,32]The
high value of saturation magnetization (comparatively) is due
to the absence of possible nonmagnetic impurities like ferri-
hydrite as evidenced from Raman spectrum. The coercivity of
value 45 Oe is estimated at RT by linear fitting of data. The
remanence is measured to be only 5.32 emu/g (squareness ra-
tioMr/Ms=0.067) which is rather less than that of the assem-
bly of three-dimensional (3D) noninteracting random particles
with uniaxial anisotropy by Stoner–Wohlfarth (S–W) model
(Mr/Ms=0.5).[41]Low coercivity and low remanence point
out that prepared nanospheres have nearly superparamagnetic
behavior. The enlarged view of MH in the inset of Fig. 3(a)
shows this nearly superparamagnetic behavior. The critical di-
ameter Dspm
crfor a spherical particle, below which the particle
possesses the superparamagnetism, is given by the following
formula:[3,42]
Dspm
cr=6
p25kBT
K1/3
. (7)
Using the value of Kfor bulk material and T=300 K,
Dspm
cris calculated to be 42.9 nm, whereas the prepared
nanospheres have an average diameter of 47.3 nm. Since the
diameter of prepared nanospheres size is close to critical di-
ameter Dspm
cr, nearly superparamagnetic behavior is observed.
M–Hloop at T=60 K is shown in Fig. 7(b) which indicates
the saturation magnetization, coercivity, and remanence to be
91.3 emu/g, 280 Oe, and 22.86 emu/g, respectively (enlarged
view in the inset of Fig. 7(b)). However, MsatT=60 K is
less than the bulk value (110 emu/g). The temperature depen-
dence of magnetization of nanospheres is measured from 55 K
to 400 K, shown in Fig. 7(c). M–Tcurve indicates that block-
ing temperature is slightly above the 400 K (enlarged view in
inset of Fig. 7).
The effective anisotropy coefficient can be calculated us-
ing a technical magnetization curve according to the law of
approach to saturation given by Eq. (3),[43]
M(H) =Ms
10.0762 K2
eff
M2sH2



+cpH, (8)
where M(H),Keff, and cpare magnetization at an applied
field H, effective magnetic anisotropy constant Dspm
cr, and high
field paramagnetic susceptibility respectively. By using the
high field values from RT M–Hcurve in Eq. (3), the effective
anisotropy constant Keffis determined to be 2.84 J/m3which is
10 times higher than the bulk value,[9]which, however, is com-
parable to the value of the nanoparticle of Mn ferrite.[10]Theprepared nanospheres have core–shell structure as evidenced
from the HRTEM image (Fig. 3(a)). The core–shell struc-
ture of magnetic material has a well-ordered magnetic core
surrounded by a shell having an amorphous magnetic struc-
ture. In the shell, poorly ordered magnetic moments could be
isotropically pinned which can be coupled with magnetic mo-
ments of core and produce an effective magnetic drag on the
core. Therefore, magnetic interaction between core and shell
enhances the anisotropy of the nanospheres.[44,45]
-15 -10 -5 0 5 10 15-80-4004080Moment/(emu/g)
H/kOeH/Oe300 K
-10 -5 0 5 10-80-4004080Moment/emu/g)
H/kOe 60 K
100 200 300 4000123456 100 OeMoment/(emu/g)
Moment/(emu/g)
T/K T/K
Fig. 7. (color online) Magnetic hysteresis loops of prepared
nanospheres: (a) at T=300 K and (b) at T=60 K, and (c)
temperature dependence of magnetization at H=100 Oe (Insets
show the enlarged views about origin).
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Chin. Phys. B Vol. 22, No. 10 (2013) 107101
3.5. Optical properties
To explore the potential application of MnFe 2O4as photo
catalyst, its optical properties should be examined. In order to
investigate the optical characteristic of MnFe 2O4nanospheres,
the transmission/absorption spectra are recorded by UV-Vis
spectrophotometer. Figure 8(a) shows the typical transmis-
sion spectrum of prepared Mn ferrite nanospheres with sizes
in a range of 400 nm to 700 nm. It clearly shows the promi-
nent absorption in the visible range. The optical band gap is
an important parameter for optical characteristics and opti-
cal application. The optical band gap is related to absorption
400 500 600 70005101520Transmission/arb. units T %
1.5 1.8 2.1 2.4 2.7 3.0 3.30100200300400500
0.9 1.5 2.1 2.7 3.3012345(αhν)1/2 /eV 1/2 Scm -1/2 (αhν)1/2 /eV 1/2 Scm -1/2
E/eV E/eV
0.98 eV(a)
(b)
(c)Wavelength λ/nm
Fig. 8. (color online) Optical properties of MnFe 2O4nanospheres ; (a)
transmission spectrum, (b) plot of ( ahn)2versus photon energy, and (c)
plot of ( ahn)1/2versus photon energy (the dashed line represents the
linear fitting).coefficient a, which can be obtained from the transmission
spectrum according the following equation:[46]
a=1
dln[1
T
, (9)
where dis the path length and in our case it is the width of
cuvette, and Tis the transmittance. The optical band gap can
be calculated by the following relation:
(ahn)2=A(hneg), (10)
where Ais a constant, Egis the optical band gap, and nis the
exponent that depends on the nature of transition: n=2 for
direct band gap and n=1/2 for indirect band gap. Therefore,
the plots from Eq. (10) for the cases n=2 and 1 /2 are shown
in Figs. 8(b) and 8(c), respectively. It is clear from Figs. 8(b)
and 8(c) that a straight line is obtained only for n=1/2, in-
dicating the indirect band gap nature. The value of the band
gap is obtained to be 0.98 eV by extrapolating the linear fitted
curve of ( ahn)1/2to energy axis at a=0. The narrow band
gap value of 0.98 eV for MnFe 2O4is slightly larger than the
calculated value for Mn ferrite.[47]Therefore, nanospheres of
MnFe 2O4can have a photocatalytic application due to their
strong absorptions in visible and narrow band gaps.
4. Conclusions
Uniform nanospheres of MnFe 2O4are prepared by hy-
drothermal method without assistance of any surfactant. Di-
ameters of nanospheres are in a range from 45 nm to 60 nm.
The nanospheres have a narrow size distribution with an av-
erage size of 47.3 nm and a standard deviation of 1.3 nm.
The reaction temperature affects the morphology remarkably,
and the uniform nanospheres are grown at a high tempera-
ture of 240C. The characteristic Raman mode in Raman
spectrum confirms Mn ferrite phase and high purity of sam-
ple. Nanospheres of MnFe 2O4have high saturation magne-
tization (78.3 emu/g), low coercivity (45 Oe), and low re-
manence (5.32) emu/g at 300 K. The nearly superparamag-
netic behavior is due to the closer diameter of the prepared
nanospheres to the critical diameter for superparamagnetism.
Moreover, high effective anisotropy constant is due to core–
shell structure. Nanospheres with such a narrow size dis-
tribution and magnetic properties have potential applications
in biomedicine. Furthermore, nanospheres of Mn ferrite ex-
hibit strong absorption in the visible region. The indirect band
gap of value 0.98 eV is acquired from transmission spectrum.
These optical characteristics of Mn ferrite nanospheres can
have a photocatalytic application.
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