RESEARCH PAPER

Mn–ferrite nanoparticles via reverse microemulsions:synthesis and characterization

Alessandra Scano • Guido Ennas • Francesca Frongia • Aurelio La Barbera •

M. Arturo Lopez-Quintela • Giaime Marongiu • Giorgio Paschina •

Davide Peddis • Martina Pilloni • Carlos Vazquez-Vazquez

Received: 19 April 2010 / Accepted: 21 December 2010 / Published online: 20 January 2011

� Springer Science+Business Media B.V. 2011

Abstract Mn–ferrite nanoparticles were synthesized

by thermal treatment at 800 �C of manganese and iron

oxo-hydroxides obtained via water-in-oil microemul-

sions consisting of n-hexanol as continuous phase, cetyl

trimethyl ammonium bromide (CTAB) as the cationic

surfactant and aqueous solutions of metal salts and

precipitant agent (tetramethyl ammonium hydroxide) as

reagents. Nanoparticles were synthesized using a multi-

microemulsion approach. Two different co-precipitation

routes are described depending on the Fe(II) or Fe(III)

precursor salts. The influence of salt concentration and

digestion process on the final products was examined.

The nanoparticles were characterized by X-ray diffrac-

tion accompanied by Rietveld analysis, transmission

electron microscopy, thermal analysis, infrared spectros-

copy, and SQUID magnetometry. In all the synthesis

reported in this study MnFe2O4 was observed only after

thermal treatment at 800 �C of the as-prepared precur-

sors. Almost spherical nanocrystalline MnFe2O4 ranging

from 12 to 39 nm was obtained starting from chlorides or

mixed chloride–sulfate salts as precursors. Low values of

reduced remanent magnetization (Mr/Ms) and coercive

field (Hc) induce to believe that a fraction of superpara-

magnetic particle is present at room temperature.

Keywords Superparamagnetic materials �Nanocrystalline materials � Nanoparticles �Microemulsion

Introduction

In the last decades nanoparticles have considerably

attracted the attention of researchers for their unique

physical and chemical properties that differ significantly

from those of the bulk material, due to their extremely

small size and large specific surface (Pikethy 2004;

Li et al. 2003). In the field of nanomaterials, ferrite

nanoparticles have received increased attention due to

good catalytic activity in water splitting for hydrogen

production (Alvani et al. 2005) and for their magnetic

properties, which can lead to a broad range of

Electronic supplementary material The online version ofthis article (doi:10.1007/s11051-010-0205-y) containssupplementary material, which is available to authorized users.

A. Scano (&)

Centro de Ingenieria Avanzada de Superficies,

AIN, C/San Cosme y San Damian, Cordovilla,

31191 Pamplona, Spain

e-mail: ascano@ain.es

A. Scano � G. Ennas � F. Frongia � G. Marongiu �G. Paschina � D. Peddis � M. Pilloni

Dipartimento di Scienze Chimiche, Universita di Cagliari,

09042 Monserrato, Cagliari, Italy

A. La Barbera

ENEA C. R. Casaccia, 00123 Rome, Italy

M. A. Lopez-Quintela � C. Vazquez-Vazquez

Departamento de Quımica Fısica, Facultad de Quımica,

Universidad de Santiago de Compostela, Santiago de

Compostela, 15782 Galicia, Spain

123

J Nanopart Res (2011) 13:3063–3073

DOI 10.1007/s11051-010-0205-y

applications, from medicine and pharmacy to electron-

ics and mechanics. Moreover, the magnetic properties

of ferrites, as well as the others iron oxides, change

considerably when they are prepared as nanoparticles.

Metal oxide nanoparticles have been prepared through

several routes, including mixed oxides (Magnetic

Materials Producers Association, Chicago, IL, 1997),

sol–gel (Seki et al. 1988), hydrothermal processing

(Komarneni et al. 1988), ion-exchange resin manufac-

ture (Lee et al. 2003), decomposition of metal organic

precursors or polyols (Feldmann 2001; Feldmann and

Jungk 2001; Feldmann 2003; Ammar et al. 2001;

Rajamathi et al. 2002), spray pyrolysis, and by calcina-

tion of hydroxides co-precipitated from solutions of

pertinent salts (Tourinho et al. 1990; Kodama et al.1996;

Chen et al. 1996). Co-precipitation of hydroxides from

different inorganic bases (Tourinho et al. 1990; Kodama

et al.1996; Chen et al. 1996; Davies et al. 1993; Morales

et al. 1999; Babes et al. 1999; Shen et al. 1999; Carpenter

et al. 1999; Misra et al. 2004) and subsequent calcination

is the simplest method and when carried out in a

microemulsion system it permits the control of the

particles shape and size distribution. Microemulsions

are colloidal ‘‘nano-dispersions’’ of water in oil (or oil in

water) stabilized by a surfactant film. The surfactant

molecules spontaneously adsorb at interfaces and

separate the organic and the aqueous phases, thus

decreasing the interfacial tension. The system is opti-

cally transparent and thermodynamically stable. It

exhibits a dynamic structure of nanosized-aqueous

droplets which are in constant deformation, breakdown

and coalescence via the percolation process. If during

this process two reverse micelles containing reactants

exchange the contents of their aqueous cores, chemical

reaction takes place inside the micellar cores giving rise

to products of nearly uniform size and shape. The size of

the droplets can be modulated varying the water/

surfactant molar ratio (named W0). The underlying

concept behind this technique is the use of these

nanoreactors, through appropriate control of the syn-

thesis parameters, to produce tailor-made product down

to a nanoscale level (Lopez-Quintela 2003; Pileni 2003).

In this work, the synthesis of MnFe2O4 by thermal

treatment of oxo-hydroxide precursors obtained by

water-in-oil microemulsions is reported. Microemul-

sions consisted of n-hexanol as the continuous phase,

cetyl trimethyl ammonium bromide (CTAB) as the

cationic surfactant, and aqueous solutions of metal

salts and precipitant agent (tetramethyl ammonium

hydroxide) as reagents. Since the synthesis conditions

may affect the chemical, structural, and physical

properties of the mixed oxide nanoparticles, the influ-

ence of the different salts, their concentration, and

digestion process on the final products was examined.

Experimental

Synthesis

Chemicals were used without further purification.

n-Hexadecyl trimethyl ammonium bromide (CTAB,

C19H42NBr, 99 wt%), 1-hexanol (C6H13OH, [98 wt%),

iron (II) chloride tetrahydrate (FeCl2�4H2O, [98 wt%),

manganese (II) chloride tetrahydrate (MnCl2�4H2O,

99 wt%), tetramethyl ammonium hydroxide penthahy-

drate ((CH3)4NOH)�5H2O,[97 wt%), hydrogen peroxide

aqueous solution (H2O2, 30 wt%) were purchased from

Fluka. Manganese sulfate monohydrate (MnSO4�H2O,

98 wt%), hydrochloric acid solution (HCl, 37 wt%),

sulfuric acid (H2SO4, 99.8 wt%), and absolute ethanol

(C2H5OH 99.8 wt%) were purchased from Carlo Erba.

Iron (III) chloride (FeCl3�6H2O,[97 wt%) was purchased

from Alfa Aesar and iron (II) sulfate (FeSO4�7H2O,

99.5–102.0 wt%) from Merck. Fresh bidistillate and

deoxygenated water was used.

For the synthesis of MnFe2O4, the reaction was

carried out in a microemulsion with a CTAB/H2O/n-

hexanol weight ratio of 14:7:79 and a W0 molar ratio

([water]/[surfactant]) of 10. These values were selected

according to the phase diagram and literature (Hicks

2004; Ahmad and Friberg 1972), in order to secure the

formation of reverse micelles in the nanometric range.

Two co-precipitation routes were followed: in the first

route, Mn2? and Fe2? salts were used, while in the

second Fe2? salt was substituted by Fe3?.

In the first route microemulsion (I), containing an

acidic (pH 2) Fe2? and Mn2? aqueous solution, where

the Fe2?/Mn2? molar ratio was fixed to 2, was rapidly

mixed (2 min) under N2 atmosphere with microemul-

sion (II), containing an aqueous solution of the

precipitating agent. Iron and manganese chlorides

and sulfates were used as precursor salts in order to

investigate the anionic influence. The final pH was 11.

This resulted in co-precipitation of Fe(OH)2 and

Mn(OH)2 with the formation of the iron-manganese

oxyhydroxide. Microemulsion (III), containing H2O2,

was then rapidly added in order to oxidize the ferrous

3064 J Nanopart Res (2011) 13:3063–3073

123

ions. The resulting mixture was vigorously stirred and

the colloidal precipitate was centrifuged, washed with

ethanol to remove excess surfactant and unreacted

reagent and, dried in the oven at 40 �C. The as-

prepared samples were calcined up to 800 �C in

argon atmosphere and MnFe2O4 was obtained accord-

ing to the overall equation:

Mn2þ þ 2Fe2þ þ 6OH� þ 1=2O2 �!800 �C

MnFe2O4

þ 3H2O

ð1ÞThe effect of concentration, digestion, and tem-

perature on the final product was also taken into

account. Experimental synthesis conditions are

reported in Table 1. In Tables 1 and 2, and through-

out the body of text, syntheses labeled with the letter

‘‘A’’ refer to the use of Fe(II) salt while letter ‘‘B’’

refers to the use of Fe(III) salt. Letters A or B are

followed by the anionic symbol. ‘‘c’’ refers to high

metal concentration. Quoted numerical values refer to

temperature (�C) when different from room temper-

ature (RT).

In the second route starting from Mn2? and Fe3?

salts, microemulsion (I), containing an acidic (pH 2)

stoichiometric Fe3? and Mn2? aqueous solution

where the Fe3?/Mn2? molar ratio is 2, was mixed

with microemulsion (II), containing an aqueous

solution of the precipitating agent. Three preparations

were carried out with this procedure to obtain

MnFe2O4, according the following reaction:

Mn2þ þ 2Fe3þ þ 8OH� �!800 �CMnFe2O4 þ 4H2O:

ð2Þ

Molar concentrations of the starting solutions used in

the syntheses are shown in Table 2.

Characterization

Structural characterization was carried out by X-ray

powder diffraction (Seifert 93000) with a conven-

tional 0-20 Bragg–Brentano focalising geometry,

CuKa wavelength, graphite monochromator on the

diffracted beam and scintillation counter. Qualitative

analysis of XRD spectra was determined using the

PDF database (PCPDF-WIN, JCPDS-International

Center for Diffraction Data, Swarthome, PA) and

average crystallite sizes of the samples were calcu-

lated using the Scherrer equation. The calibration was

performed by means of a standard silicon sample and

using the Warren correction. For each XRD spectrum

different significant peaks was analyzed. Quantitative

phase analysis of selected sample were also evaluated

by the Rietveld method using the MAUD software

(Lutterotti et al. 1999) and recommended fitting

procedures were adopted (Young 1995). Structural

models of the identified phases were obtained by

inorganic crystal structure database (ICSD, Kar-

lsruhe, Germany). Lattice parameters, average crys-

talline size, and weight content of each phase were

refined. Weighted pattern agreement index Rwp was

less than 0.2 for all fitted samples.

Microscopical analysis was carried out by trans-

mission electron microscopy (TEM) with a JEOL

200CX microscope, working at 200 kV. The samples

Table 1 Compositions of the starting solutions

Sample [FeCl2]/M [MnCl2]/M [FeSO4]/M [MnSO4]/M [(CH3)4NOH]/M Digestion

A–Cl 0.066 0.033 0.45

A–Cl–c 0.133 0.066 1

A–Cl–c–50 0.133 0.066 1

A–SO4 0.066 0.033 0.45

A–Cl–SO4 0.066 0.033 0.45 x

A–Cl–SO4–35 0.066 0.033 0.6 x

A–Cl–SO4–c–50 0.25 0.125 1.69 x

Table 2 Compositions of the starting solutions

Sample [FeCl3]/M [MnCl2]/M [(CH3)4NOH]/M

B–Cl–c 0.266 0.133 1

B–Cl 0.133 0.066 1

B–Cl–50 0.133 0.066 1

J Nanopart Res (2011) 13:3063–3073 3065

123

for TEM analysis were dispersed in ethanol and

ultrasonicated. They were then dropped onto a copper

grid, allowed to dry and later viewed on the TEM.

Average particle size was obtained by dark field

images, measuring the average diameter of about 150

particles in different parts of the grid.

The thermal analysis (TG-SDTA) was performed

in an 851 Mettler-Toledo STAR System. TG-DTA

measurements were performed in argon flow (30 mL/

min) using a platinum crucible from room tempera-

ture up to 800 �C, with a 10 �C/min ramp rate.

DC magnetization measurements were performed

with a Quantum Design SQUID magnetometer

(Hmax = 5 T). The sample, in the form of powders,

was immobilized in an epoxy resin in order to prevent

any movement of the nanoparticles during the

measurements. The magnetization loops are mea-

sured up to 2 T.

Results

Bivalent precursor salts

Use of chlorides or sulfates as precursor salts

The as-prepared powder obtained by the first three

syntheses (A–Cl, A–Cl–c, A–Cl–c–50) is an iron

manganese oxide-hydroxide, as confirmed by the

X-ray diffraction pattern (Fig. 1), which matches well

to d-(Fe0.67Mn0.33)OOH (PDF card No. 14-557)

(PCPDF-WIN, JCPDS-International Center for Dif-

fraction Data, Swarthome, PA). Average crystallite

size, as calculated by Scherrer equation, increased

from 2 nm (sample A–Cl) to 6 nm (sample A–Cl–c),

due to a two-fold increase in salt concentration.

Contrary to this a temperature increase from 25 to

50 �C showed no significant effect. In fact the

crystallite sizes increased from 6 to 7 nm. In the

XRD spectra of Fig. 1, an halo centered around 32�20 due to an amorphous or very disordered (Fe,

Mn)OOH phase is also present. A faint peak due to

surfactant residual is also present at 25� 20.

Substitution of the chloride salts with sulfates,

starting with the replacement of manganese chloride

with manganese sulfate and then the replacement of

both manganese and iron chlorides with their sulfate

counterparts caused a change in the sample crystal-

linity. This effect is particularly evident when the

precursor salts are both sulfates, which lead to a

significant crystallite size decrease (Fig. 1).

Transmission electron microscopy (TEM) investi-

gation displayed the effect of the different anions on

the particles shape as shown in Fig. 2. When sulfates

are present there is the tendency to form needles

while in the sample obtained from chloride solutions

under the same conditions agglomerates of mostly

spherical particle are present. Average particle size

obtained by TEM observation is in agreement with

XRD results.

Both IR spectra (Fig. 3) exhibit absorption in the

range 3600–3100 cm-1 for the antisymmetric and

symmetric OH stretching. The simultaneous presence

of bands in the region 3000–2700 cm-1 and around

1400 cm-1 is indicative of the presence of methyl

groups, due to residual CTAB. In the A–SO4 spectrum

the specific band of S–O stretching (1150–1000 cm-1)

is also present indicating that sulfate ions are still present

in the A–SO4 sample even after washing with ethanol.

20 30 40 50 60 70

Inte

nsity

(a.

u.)

A-Cl

A-Cl-c

A-Cl-c-50

A-Cl-SO4

A-SO4

°° ° °

°

° °°

°

° °°

°

°

°

° °

°°

°

Fig. 1 X-ray diffraction patterns of: A–Cl; A–Cl–c; A–Cl–c–50,

A–Cl–SO4; A–SO4 as-prepared samples; open circles refers to

the iron–manganese oxide hydroxide peaks

3066 J Nanopart Res (2011) 13:3063–3073

123

Figure 4 shows the TGA, DTG, and DTA curves

up to 800 �C in Ar for as-prepared samples obtained

from chloride precursor salts. TGA curve shows

different regions of weight loss, which are further

confirmed by the occurrence of peaks on the DTA

curve. A first endothermic peak centered at 135 �C is

present which is due to the water evaporation and

partial loss of residual solvent. A second exothermic

peak, centered at 246 �C, is due to combustion of

residual alcohols. The presence of water and residual

alcohol was confirmed by IR spectra (not reported)

taken after thermal treatment at selected temperature.

The shoulder with a bump around 340 �C is due to

decomposition of residual surfactant as confirmed by

IR spectra and by the strong smell of CTAB in the

exhaust carried gas. Finally, in the range 450–650 �C

the dehydroxilation of oxide hydroxides with the

formation of MnFe2O4 takes place. Figure 5 shows

the XRD patterns of A–Cl sample at intermediate

thermal treatment from 200 to 800 �C, which

confirms the structural evolution of the as-prepared

sample, FexMn1-xOOH, to MnFe2O4 (Jacobsite, PDF

card No. 10-319, PCPDF-WIN, JCPDS-International

Center for Diffraction Data, Swarthome, PA) in

agreement with thermal analysis results. Thermal

treatment of A–Cl samples led to the formation of

Jacobsite (hDi = 12 nm for A–Cl and 23 nm for

A–Cl–c, respectively). Rietveld refinement of XRD

Fig. 2 TEM images of as

prepared samples obtained

from different precursor

salts: a A–Cl and b A–SO4

5001000150020002500300035004000

% T

rans

mitt

ance

Wavenumber (cm -1)

a

b

Fig. 3 IR spectra of as-prepared samples obtained from

different precursor salts: a A–Cl and b A–SO4

50

60

70

80

90

100

110

100 200 300 400 500 600 700 800w

eigh

t los

s %

Temperature, °C

ΔΤ ( )

endo

Fig. 4 TGA (thick line), DTG (dotted line) and DTA (thinline) curves in Argon flow of samples obtained starting from

chlorides precursor salts

20 30 40 50 60 70 80 90

Inte

nsity

(a.u

.)

MnFe2O4

(Fe 0.67 Mn 0.33)OOH

as prepared

200 °C

400 °C

500 °C

800 °C

Fig. 5 X-ray diffraction patterns of A–Cl sample at interme-

diate thermal treatment in the temperature range 200–800 �C

J Nanopart Res (2011) 13:3063–3073 3067

123

data for A–Cl samples thermally treated to 800 �C

were carried out and the results are summarized in

Table 3. The lattice parameter obtained, 0.8442 nm,

is in agreement with literature data for bulk poly-

crystalline Jacobsite (0.8440–0.8500 nm, PCPDF-

WIN, JCPDS-International Center for Diffraction

Data, Swarthome, PA; Kodama et al. 1996).

When the precursor salts are both sulfates thermal

treatment leads to the formation of a hematite type phase

while in the case of sulfate and chloride precursor

salts both Jacobsite and hematite phases are present

(Fig. 6). Rietveld refinement gave (Table 3) 70 wt%

of Jacobsite (lattice parameter a = 0.8500 nm, hDi =

45 nm) and 30 wt% of hematite type phase, rhombo-

hedral a-(Fe,Mn)2O3 (PCPDF-WIN, JCPDS-Interna-

tional Center for Diffraction Data, Swarthome, PA)

with a hDi = 78 nm.

Influence of digestion using mixed sulfate

and chloride as precursor salts

As-prepared samples A–Cl–SO4, A–Cl–SO4–35 and

A–Cl–SO4–c–50 were subjected to a digestion pro-

cess. The first sample was digested for 17 h at RT. The

other two were digested for the same time at 35 �C,

and at 50 �C, respectively. Figure 7a shows the X-ray

diffraction patterns of digested samples. The digested

samples are apparently similar to the ones obtained in

the A-syntheses without digestion. The final product of

the syntheses is an iron–manganese oxide hydroxide

with a hDi = 7 nm. Differences between the three

digested samples appear after thermal treatment at

800 �C (Fig. 7b). In fact Rietveld refinement of the

sample digested at room temperature, thermally

treated to 800 �C showed a mixture of Jacobsite

(70 wt% and hDi = 45 nm) and hematite type phases

(30 wt% and hDi = 78 nm). Samples digested at

Table 3 Rietveld refinement and magnetic measurement results on selected samples

Sample Jacobsite Wustite type Hematite type hDi(nm)

Hc

(Oe)

Ms

(emu/g)

Mr/Ms

a0 (nm) wt% a0 (nm) wt% a0(nm) c0 (nm) wt%

A–Cl 0.8442 (1) 100 (2) 12 60 41 (1) 0.09

A–Cl–SO4 0.8500 (1) 70 (1) 45

0.5030 (1) 1.3730 (5) 30 (1) 78

A–Cl–SO4–c–35 0.8466 (1) 100 (2) 39 80 65 (2) 0.20

A–Cl–SO4–c–50 0.8499 (1) 95 (2) 26 80 45 (1) 0.09

0.4428 (1) 5 (2) 22

B–Cl 0.4355 (1) 100 (2) 45

XRD Rietveld refinements are given in Supporting Information

20 30 40 50 60 70

Inte

nsity

(a.

u.)

Δ Δ

Δ

ΔΔ

ΔΔ

Δ

ΔΔ

Δ

ΔΔ

ΔΔ Δ

Δ Δ

Δ

Δ Δ Δ Δ Δ

##

# # ##

####

# ## # #

# ### #

A-SO4

A-Cl-SO4

A-Cl

A-Cl-c-50

Fig. 6 X-ray diffraction patterns of: a A–SO4 b A–Cl–SO4;

c A–Cl, and d A–Cl–c–50 samples after thermal treatment at

800 �C; open triangles refer to Jacobsite peaks; hash refers to

the Hematite type phase peaks

3068 J Nanopart Res (2011) 13:3063–3073

123

higher temperatures gave rise to Jacobsite. In the

A–Cl–SO4–c–50 a small amount of Wustite type phase

(5 wt%) is also present (PCPDF-WIN, JCPDS-Inter-

national Center for Diffraction Data, Swarthome, PA;

Lutterotti et al. 1999; Young 1995; ICSD, Karlsruhe,

Germany; Jay and Andrews 1946). Average particle

dimensions of Jacobsite phase decreases from 39 to

26 nm in A–Cl–SO4–35 and A–Cl–SO4–50 samples,

respectively.

Trivalent salts

When the Fe precursor salts are trivalent chlorides,

different phases are present in the as-prepared sam-

ples. In fact the iron manganese oxide hydroxide

peaks in the XRD spectra (not reported) are accom-

panied by the presence of peaks corresponding to

lepidocrocite and akaganeite type phases (PCPDF-

WIN, JCPDS-International Center for Diffraction

Data, Swarthome, PA). After thermal treatment

(Fig. 8) all B-samples gave rise to Wustite type phase

(Fe, MnO) only (PCPDF-WIN, JCPDS-International

Center for Diffraction Data, Swarthome, PA; Jay and

Andrews 1946). Rietveld refinement for B–Cl sample

indicates a cubic lattice parameter of a = 0.4355 nm

and an average size dimension hDi of 45 nm.

Magnetic properties

Magnetic measurements were carried out only on

MnFe2O4 samples obtained after thermal treatment at

800 �C. In Fig. 9, the field dependence of the magne-

tization at 300 K for the samples A–Cl, A–Cl–SO4–

35, and A–Cl–SO4–c–50 are reported. The magneti-

zation does not reach saturation at 2 T. As a result the

magnetization saturation values reported in Table 3

were obtained by fitting of the high field data with the

function

20 30 40 50 60 70

Inte

nsity

(a.

u.)

°

° °°

°

°°

°

°

°°

°

A-Cl-SO4

A-Cl-SO4-35

A-Cl-SO4-c-50

20 30 40 50 60 70

Inte

nsity

(a.

u.)

#

# #

## #

##

Δ Δ

Δ

ΔΔ

ΔΔ

Δ

Δ

Δ

Δ

ΔΔ

ΔΔ

Δ

Δ Δ

Δ

Δ ΔΔ

Δ

Δ

A-Cl-SO4-c-50

A-Cl-SO4-35

A-Cl-SO4

a

bFig. 7 a X-ray diffraction

patterns of A–Cl–SO4;

A–Cl–SO4–35; A–Cl–SO4–

c–50 as prepared samples.

b X-ray diffraction patterns

of A–Cl–SO4; A–Cl–SO4–

35; A–Cl–SO4–c–50

samples after thermal

treatment at 800 �C; opentriangles refer to Jacobsite

peaks; hash refers to

Hematite type phase peaks;

open circles refer to the

iron–manganese oxide

hydroxide peaks

20 30 40 50 60 70

Inte

nsity

(a.u

.)

*

*

*

*

Fig. 8 X-ray diffraction pattern of B–Cl after thermal

treatment at 800 �C. asterisks refer to Wustite type phase peaks

J Nanopart Res (2011) 13:3063–3073 3069

123

M ¼ Ms � 1� A

H� B

H2

� �ð3Þ

where H is the field strength, a and b parameters

determined by the fitting procedure (Morrish 1965).

The saturation magnetization decrease with a particle

size as expected for nanoparticles (Haneda and

Morrish 1988). All samples display a lower value

of saturation with respect to the bulk system (Ms

bulk = 80 emu/g at room temperature) (Carpenter

et al. 1999; Liu et al. 2000), with the most important

decrease observed in samples with smaller particles.

The lower values of coercivity, Hc, and reduced

remanent magnetization (the ratio of remanent mag-

netization, Mr, to saturation magnetization, Ms)

(Table 3) may indicate that a fraction of particles

are in the superparamagnetic state (Dormann et al.

1997; Fiorani and Dormann 1991).

Discussion

In all the syntheses reported in this study MnFe2O4

was obtained only after thermal treatment of the

oxide hydroxide precursors. The results reported here

are in agreement with results obtained using various

microemulsion systems (Bellusci et al. 2007; Liu

et al. 2000) and with co-precipitation of hydroxides

by different inorganic bases and subsequent calcina-

tion (Rajamathi et al. 2002; Song et al. 2007). Once

again the synthesis products were disordered iron–

manganese hydroxide, and Jacobsite was only

obtained after a thermal treatment. On the other hand

direct formation of MnFe2O4 can be achieved without

high temperature thermal treatments. Previous reports

have done this through adopting digestion at water

boiling temperature (Ammar et al. 2001; Chen et al.

1996; Carpenter et al. 1999; Misra et al. 2004; Zhang

et al. 1998; Gnanaprakash et al. 2007) and using

either high metal ions and OH- concentrations

(Kodama et al. 1996) and/or different surfactant

(Carpenter et al. 1999). This behavior suggests that

some factors about the nucleation process in the

water droplets of a microemulsion system need

consideration. In fact a mixed metal hydroxide phase

evolves into spinel oxide when the dehydration

process on the previously formed solid aggregates

forms a first nucleus, with an r dimension greater than

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-30

-20

-10

0

10

20

30

H (kOe)

M (

em

u/g

)

-20 -15 -10 -5 0 5 10 15 20

-40

-30

-20

-10

0

10

20

30

40

H (kOe)

M (

em

u/g

)

-1.0 -0.5 0.0 0.5 1.0

-40

-20

0

20

40

M (

em

u/g

)

H (Oe)

-20 -15 -10 -5 0 5 10 15 20

-80

-60

-40

-20

0

20

40

60

80

M (

em

u/g

)

H (kOe)

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-20

-10

0

10

20

M (

em

u/g

)

H (kOe)

-20 -15 -10 -5 0 5 10 15 20

-40

-30

-20

-10

0

10

20

30

40

M (

em

u/g

)

H (kOe)

a

b

c

Fig. 9 Hysteresis loop at room temperature for a A–Cl, b A–

Cl–SO4–35, and c A–Cl–SO4–c–50 samples thermally treated

at 800 �C; insets: details around zero field

3070 J Nanopart Res (2011) 13:3063–3073

123

the r* critical one (Cushing et al. 2004; Jolivet 2000;

Cornell and Schwertmann 1996, 2000). Successive

growth occurs by both solid-state transformation and

through the Ostwald ripening process at the cost of

particles with dimensions below r*. According to the

hypothesis that a first oxide germ nucleates from the

hydroxide phase by an oxolation mechanism, it would

grow until complete phase transformation only if it

reaches and overcomes its critical dimension

r�OX(OX = oxide phase). With a constant metal con-

tent, the change in the composition of the precipitated

precursor particle is accompanied by a shrinkage in

volume, so that the new oxide phase is not able to

reach its critical dimension r�OX and the mixed oxide

does not form. Dehydration process is also governed

by oxolation equilibrium, which depends on water,

OH- and metal ion activities, solubility product

constant Kps (Gnanaprakash et al. 2007). In the

presence of a microemulsion the concentration of

surfactant can influence the final product.

It is worth noting the behavior of samples obtained

when the precursor salts are both sulfates. Generally

iron and manganese cations form complexes that

condense via two basic mechanisms (olation or oxola-

tion), depending on the nature of the coordination

sphere of the cations (Jolivet et al. 2004). In the

presence of chloride the hydrolysis of metal ions begins

with the formation of simple hydroxy complexes, as

monomers and dimers and this step is followed by their

polymerization. The next step in the process is the

formation of oxo-bridges (oxolation mechanism) with

the development of the final structures. The final step is

loss of water and internal crystallization. The presence

of sulfate inhibits the oxolation process (Dousma et al.

1979). This could influence the sample crystallinity,

leading to the formation of small particles, which form

acicular aggregates.

Moreover, thermal treatment leads always to

hematite in absence of digestion when sulfate salts

are used as starting materials. The synthesis per-

formed with Fe3? salts gave rise to mixtures of metal

hydroxide, as the results of a two steps precipitation.

In fact when Fe(III) and Mn(II) are both present,

precipitation cannot occur simultaneously because of

the lower Kps of iron (III) oxyhydroxide (between

10-39 and 10-42) compared to those of Fe(II) and

Mn(II) (10-14 and 10-16), Mn(II) oxyhydroxide

begins to precipitate when Fe(III) oxyhydroxide has

been generated (Gnanaprakash et al. 2007). In this

case a core–shell structure is formed and only through

a severe digestion process it is possible to obtain a

mixed oxide. Contrary to this the close proximity of

Kps values of Mn(II) and Fe(II) oxyhydroxide leads to

their co-precipitation, thus favoring intermixing.

Moreover, in the B samples, where Fe(III) is used,

the presence of residual carbon, due to CTAB

decomposition during thermal treatment, could

reduce oxyhydroxide to Wustite type phase. The

calculated lattice parameter by Rietveld refinement,

0.4355 nm, is in agreement with a reported stoichi-

ometry of Fe0.66Mn0.33O (Jay and Andrews 1946).

The MnFe2O4 spinel structure can be depicted as

(Mn1-iFei)[MniFe2-i]O4 where round and square

bracket denote tetrahedral site occupancy (A) and

octahedral site occupancy (B), respectively, and i rep-

resent the inversion parameter (Zhang et al. 1998). In

MnFe2O4 samples prepared at low temperature the

occupancy of A and B sites is random and depends on

the stoichiometric Fe/Mn ratio (2:1). In fact the

divalent and trivalent cations occupy the site for which

they have higher crystal field stabilization energy

(CFSE) (West 1999). Although Fe(III) and Mn(II) ions

have zero CFSE at low temperature and can occupy

either A and B sites, many authors indicate that there

are significant differences in site preference energy at

high temperature and therefore the ions move to a

stable position. In fact the inversion degree for

MnFe2O4 samples prepared in different ways and with

different average size, but thermally treated over

600 �C, remain almost constant and close to 25%.

(Carta et al. 2009; Liu et al. 2000; Song et al. 2007;

Zhang et al. 1998; Jeyadevan et al. 2000; Harrison et al.

1957). This observation can be used to easily compare

MnFe2O4 samples prepared in different ways, in

particular their interesting magnetic properties.

In all the samples a fraction of superparamagnetic

particles may be present at room temperature, as

shown by low value of Mr/Ms and Hc. A reduction of

saturation magnetization is observed with respect to

the bulk material. In principle, this can be due to

vacancies and/or modification of the lattice parame-

ters or to the presence of non-collinear spin structure

(i.e. spin-canting). The Rietveld refinement of the

XRD spectra induce to believe that any substantial

structural modification are present with respect to the

bulk material, then the most likely reason for the

J Nanopart Res (2011) 13:3063–3073 3071

123

decrease of saturation magnetization seems to be

the presence of spin-canting, as already reported in

literature (Coey 1971; Peddis et al. 2008). It should

also be noted that the Ms values obtained show that

they could be suitable for drug delivery applications

(Pankhurst et al. 2003).

Conclusions

Microemulsion is a convenient technique to produce

nanocrystalline ferrite and is an obstacle in aggregation

and coarsening of particles during thermal treatment. A

new method for the formation of nanocrystalline

MnFe2O4 was achieved by co-precipitation of manga-

nese and iron oxo-hydroxides in water/oil microemul-

sions and subsequent thermal treatment 800 �C in

argon. In this work, MnFe2O4 was only obtained after

thermal treatment of the oxide hydroxide precursors.

The best results were obtained in the Water/CTAB/

Hexanol system with initial concentrations of 0.066 M

and 0.033 M solutions of FeCl2 and MnCl2 at room

temperature, and 0.066 M and 0.033 M solutions of

FeSO4 and MnCl2 after a digestion at 35 �C. The

nanoparticles are spherical in shape and have an

average diameter of 12 and 39 nm, respectively.

Acknowledgment This study was supported by MIUR

(Ministero Istruzione Universita e Ricerca) in the frame of

TEPSI project and by Fondazione Banco di Sardegna. D. Peddis

was granted by RAS (Regione Sardegna – Centro regionale di

Programmazione) co-founded by PO Sardegna FSE 2007-2013

sulla L.R.7/2007 ‘‘Promozione della ricerca scientifica e

dell’innovazione tecnologica in Sardegna’’. We want also to

thank Dr Darragh Gaffney, MSSI (Material and Surface Science

Institute), University of Limerick, Ireland, for helpful discussion.

References

Ahmad SI, Friberg S (1972) Catalysis in micellar and liquid-

crystalline phases. I. System water-hexadecyltrimethyl-

ammonium bromide-hexanol. J Am Chem Soc 94:

5196–5199

Alvani C, Ennas G, La Barbera A, Marongiu G, Padella F,

Varsano F (2005) Synthesis and characterization of

nanocrystalline MnFe2O4: advances in thermochemical

water splitting. Int J Hydrog Energy 30:1407–1411

Ammar S, Helfen A, Jouini N, Fievet F, Rosenman I, Villain F,

Molinie P, Danot M (2001) Magnetic properties of

ultrafine cobalt ferrite particles synthesized by hydrolysis

in a polyol medium. J Mater Chem 11:186–192

Babes L, Denizot B, Tanguy G, Le Jeune JJ, Jallet P (1999)

Synthesis of iron oxide nanoparticles used as MRI con-

trast agents: a parametric study. J Colloid Interface Sci

212:474–482

Bellusci M, Canepari S, Ennas G, La Barbera A, Padella F,

Santini A, Scano A, Seralessandri L, Varsano F (2007)

Phase evolution in synthesis of manganese ferrite nano-

particles. J Am Ceram Soc 90:3977–3983

Carpenter EE, O’Connor J, Harris VG (1999) Atomic structure

and magnetic properties of MnFe2O4 nanoparticles pro-

duced by reverse micelle synthesis. J Appl Phys 85:5175–

5177

Carta D, Casula MF, Falqui A, Loche D, Mountjoy G, San-

gregorio C, Corrias A (2009) A structural and magnetic

investigation of the inversion degree in ferrite nanocrys-

tals MFe2O4 (M = Mn, Co, Ni). Phys Chem C 113:8606–

8615

Chen JP, Sorensen CM, Klabunde KJ, Hadjipanayis GC,

Devlin E, Kostikas A (1996) Size-dependent magnetic

properties of MnFe2O4 fine particles synthesized by

coprecipitation. Phys Rev B 54:9288–9296

Coey JMD (1971) Noncollinear spin arrangement in ultrafine

ferrimagnetic crystallites. Phys Rev Lett 27:1140–1142

Cornell RM, Schwertmann U (2000) The iron oxide in the

laboratory. Wiley-VCH, NewYork

Cornell RM, Schwertmann U (1996) The iron oxide. Wiley-

VCH, NewYork

Cushing BL, Kolesnichenko VL, O’Connor C (2004) Recent

advances in the liquid- phase syntheses of inorganic

nanoparticles. J Chem Rev 104:3893–3946

Davies KJ, Wells S, Charles SW (1993) The effect of tem-

perature and oleate adsorption on the growth of maghe-

mite particles. J Magn Magn Mater 122:24–28

Dormann JL, Fiorani D, Tronc E (1997) Magnetic relaxation in

fine- particle systems. Advances in chemical physics. John

Wiley and Sons, New York, p XCVIII

Dousma J, Den Ottelander D, De Bruyn PL (1979) The influ-

ence of sulfate ions on the formation of iron (III) oxide.

J Inorg Nucl Chem 41:1565–1568

Feldmann C (2001) Preparation of nanoscale pigment particles.

Adv Mater 13:1301–1303

Feldmann C (2003) Polyol-mediated synthesis of nanoscale

functional materials. Adv Funct Mater 13:101–107

Feldmann C, Jungk H-O (2001) Polyol-mediated preparation of

nanoscale oxide particles. Angew Chem Int Ed 40:359–

362

Fiorani D, Dormann JL (1991) Conference proceedings of

Rome, Italy

Gnanaprakash G, Philip J, Raj B (2007) Effect of divalent

metal hydroxide solubility product on the size of ferrite

nanoparticles. Mater Lett 61:4545–4548

Haneda K, Morrish AH (1988) Noncollinear magnetic structure

of CoFe2O4 small particles. J Appl Phys 63:4258–4260

Harrison FW, Osmond WP, Teale RW (1957) Cation distrib-

utionand magnetic moment of manganese ferrite. Phys

Rev 105:865–866

Hicks T (2004) Preparation, characterization and activity of

mono-dispersed supported cataylsts. PhD. Thesis, Georgia

Institute of Technology, USA

ICSD Database (Inorganic Crystal Structure Database) (2007)

Karlsruhe, Germany

3072 J Nanopart Res (2011) 13:3063–3073

123

Jay AH, Andrews KW (1946) Composition of some binary

oxide systems. J Iron Steel Inst 152:15

Jeyadevan B, Tohji K, Nakatsuka K, Narayanasamy A (2000)

Irregular distribution of metal ions in ferrites prepared by

co-precipitation technique structure analysis of Mn–Zn

ferrite using extended X-ray absorption fine structure.

J Magn Magn Mater 217:99–105

Jolivet JP (2000) Metal oxide chemistry and synthesis. From

solution to solid state. Wiley, Chichester

Jolivet JP, Chaneac C, Tronc E (2004) Iron oxides chemistry.

From molecular clusters to extended solid networks.

Chem Commun 5:481–487

Kodama T, Ookubo M, Miura S, Kitayama Y (1996) Synthesis

and characterization of ultrafine Mn(II)-bearing ferrite of

type MnxFe3 - xO4 by coprecipitation. Mater Res Bull 31:

1501–1512

Komarneni S, Fregeau E, Breval E, Roy R (1988) Hydrother-

mal preparation of ultrafine ferrites and their sintering.

J Am Ceram Soc 71:C-26–C28

Lee Y, Rho J, Jung B (2003) Preparation of magnetic ion-

exchange resins by the suspension polymerization of

styrene with magnetite. J Appl Polym Sci 89:2058–2067

Li T, Deng Y, Song X, Jin Z, Zhang Y (2003) The formation of

magnetite nanoparticle in ordered system of the soybean

lecithin. Bull Korean Chem Soc 24:957–960

Liu C, Zuo B, Rondinone AJ, Zhang ZJ (2000) Reverse micelle

synthesis and characterization of superparamagnetic

MnFe2O4 spinel ferrite nanocrystallites. J Phys Chem B

104(6):1141–1145

Lopez-Quintela AM (2003) Synthesis of nanomaterials in mi-

croemulsions: formation mechanisms and growth control.

Curr Opin Colloid Interface Sci 8:137–144

Lutterotti L, Matthies S, Wenk HR (1999) MAUD: a friendly

Java program for material analysis using diffraction. IUCr

Newsl CPD 21:14–15

Misra RDK, Gubbala S, Kale A, Egelhoff WF Jr (2004) A

comparison of the magnetic characteristics of nanocrys-

talline Ni, Zn, and Mn ferrites synthesized by reverse

micelle technique. Mater Sci Eng 111:164–174

Morales MP, Veintemillas S, Montero MI, Serna CJ, Roig A,

Casas L, Martinez B, Sandiumenge F (1999) Surface and

internal spin canting in c -Fe2O3 nanoparticles. Chem

Mater 11:3058–3064

Morrish AH (1965) The physical principles of magnetism.

Wiley, New York

Pankhurst QA, Connelly J, Jones SK, Dobsonb J (2003)

Applications of magnetic nanoparticles in biomedicine.

J Phys D 36:R167–R181

PCPDF-WIN, JCPDS- International Center for Diffraction

Data (1998) Swarthome, PA

Peddis D, Mansilla MV, Morup S, Cannas C, Musinu A, Pic-

caluga G, Orazio F, Lucari F, Fiorani D (2008) Spin

canting and magnetic anisotropy in ultrasmall CoFe2O4

nanoparticles. J Phys Chem B 112:8507–8513

Pitkethy MJ (2004) Nanomaterials—the driving force. Mater

Today 7:20–29

Pileni MP (2003) Role of soft colloidal templates in the control

of size and shape of inorganic nanocrystals. Nat Mater. 2:

145–150

Rajamathi M, Ghosh M, Seshadri R (2002) Hydrolysis and

amine-capping in a glycol solvent as a route to soluble

maghemite gamma-Fe2O3 nanoparticles. Chem Commun

10:1152–1153

Seki M, Sato T, Usui S (1988) Observations of ultrafine

ZnFe2O4 particles with transmission electron microscopy.

J Appl Phys 63:1424–1427

Shen L, Laibinis PE, Hatton TA (1999) Bilayer surfactant

stabilized magnetic fluids: synthesis and interactions at

interfaces. Langmuir 15:447–453

(1997) Soft ferrites. A user’s guide, chap. 1, 2, and 6. Mag-

netic Materials Producers Association, Chicago, IL

Song Q, Ding Y, Wang ZL, Zhang ZJ (2007) Tunning the

thermal stability of molecular precursor for the nonhy-

drolytic synthesis of magnetic MnFe2O4 spinel nano-

crystals. Chem Mater 19:4633–4638

Tourinho FA, Franck R, Massart R (1990) Aqueous ferrofluids

based on manganese and cobalt ferrites. J Mater Sci 25:

3249–3254

West AR (1999) Basic solid state chemistry, 2nd edn. Wiley,

New York

Young RA (1995) The Rietveld method, IUCr monographs on

crystallography 5. Oxford University Press, Oxford

Zhang ZJ, Wang ZL, Chakoumakos BC, Yin JS (1998) Tem-

perature dependence of cation distribution and oxidation

state in magnetic Mn–Fe ferrite nanocrystals. J Am Chem

Soc 120:1800–1804

J Nanopart Res (2011) 13:3063–3073 3073

123