Synergy of Miniemulsion and Solvothermal Conditions for the Low-Temperature Crystallization of Magnetic Nanostructured Transition-Metal Ferrites

Article abstract of DOI:10.1021/acs.chemmater.6b03467

Crystalline first-row transition-metal (Mn, Fe, Co, Ni, Cu, and Zn) ferrites were prepared by an unprecedented synergetic combination of miniemulsion synthesis and solvothermal route, pursuing unconventional conditions in terms of space confinement, temperature, and pressure. This synergy allowed for obtaining six different crystalline ferrites at much lower temperature (i.e., 80 °C) than usually required and without any postsynthesis thermal treatment. X-ray diffraction (XRD) revealed that analogous ferrites synthesized by miniemulsion at ambient pressure or in bulk (i.e., from an aqueous bulk solution and not in the confined space of the miniemulsion droplets) either at ambient pressure or under solvothermal conditions did not result in comparatively highly crystalline products. To follow the structural evolution at local level as a function of reaction time and depending on the synthesis conditions, X-ray absorption spectroscopy (XAS) was used to determine the cation distribution in these structures. Well-defined nanostructures were observed by transmission electron microscopy (TEM). Concerning their functional behavior, the synthesized ferrites presented superparamagnetism and were found to be active oxidation catalysts, as demonstrated for the oxidation of styrene, taken as a model reaction. Because of the magnetic properties, the ferrites can be easily recovered from the reaction medium, after the catalysis, by magnetic separation and reused for several cycles without losing activity.

Full text of DOI:10.1021/acs.chemmater.6b03467

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Article
Synergy of Miniemulsion and Solvothermal
Conditions for the Low Temperature Crystallization
of Magnetic Nanostructured Transition Metal Ferrites
Alice Antonello, Gerhard Jakob, Paolo Dolcet, Rebecca Momper, Maria
Kokkinopoulou, Katharina Landfester, Rafael Muñoz-Espí, and Silvia Gross
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03467 • Publication Date (Web): 30 Dec 2016
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Chemistry of Materials
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Synergy of Miniemulsion and Solvothermal Conditions for the Low
Temperature Crystallization of Magnetic Nanostructured Transition
Metal Ferrites
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Alice Antonello,a Gerhard Jakob,b Paolo Dolcet,c Rebecca Momper,a Maria Kokkinopoulou,a Katharina Land-
fester,a Rafael Muñoz-Espía,d,* and Silvia Grossc,e,*
a Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany
b University of Mainz, Institute of Physics, Staudingerweg 7, 55128 Mainz, Germany
c Università degli Studi di Padova, Dipartimento di Scienze Chimiche, and INSTM, UdR Padova, via Marzolo 1, I-35131, Padova, Italy
d Institute of Materials Science (ICMUV), Universitat de València, c/ Catedràtic José Beltrán 2, 46980 Paterna, València, Spain.
e Istituto di Chimica della Materia Condensata e di Tecnologie per l’Energia, ICMATE-CNR, and INSTM, UdR Padova, via Marzolo
1 Padova, Italy.
ABSTRACT: Crystalline first row transition metal (Mn, Fe, Co, Ni, Cu and Zn) ferrites were prepared by an unprecedented synergetic com-
bination of miniemulsion synthesis and solvothermal route, pursuing unconventional conditions in terms of space confinement, temperature
and pressure. This synergy allowed for obtaining six different crystalline ferrites at much lower temperature (i.e., 80 °C) than usually required
and without any post-synthesis thermal treatment. X-ray diffraction (XRD) revealed that analogous ferrites synthesized by miniemulsion at
ambient pressure or in bulk (i.e., from an aqueous bulk solution and not in the confined space of the miniemulsion droplets) either at ambient
pressure or under solvothermal conditions did not result in comparatively highly crystalline products. To follow the structural evolution at local
level as a function of reaction time and depending on the synthesis conditions, X-ray absorption spectroscopy (XAS) was used to determine
the cation distribution in these structures. Well-defined nanostructures were observed by transmission electron microscopy (TEM). Concern-
ing their functional behavior, the synthesized ferrites presented superparamagnetism and were found to be active oxidation catalysts, as demon-
strated for the oxidation of styrene, taken as a model reaction. Thanks to the magnetic properties, the ferrites can be easily recovered from the
reaction medium, after the catalysis, by magnetic separation and reused for several cycles without losing activity.
synthesis of ferrites are sol-gel,26 non-aqueous sol-gel,27-29 hydrother-
Introduction
mal and solvothermal routes,30, 31 coprecipitation,32, 33 and micro-
Ferrites are inorganic materials experiencing a great development
emulsion.34, 35 An innovative way to obtain crystalline inorganic ma-
and interest not only for their thermal, mechanical and chemical sta-
terials at low or even room temperature while using a wet-chemical
bility1, 2 and for their magnetic properties,3-5 but also for their out-
strategy is the miniemulsion technique.36, 37 Miniemulsions are kinet-
standing electric and structural properties,6 making them appealing
ically stabilized emulsions (i.e., droplets of a liquid dispersed in a sec-
for a wide array of applications. In particular, ferrites are being ex-
ond immiscible liquid) with droplet sizes in the range of 50 to 500
ploited for catalysis7, 8 and photocatalysis9 of several reactions, such
nm. The formation of a miniemulsion requires high shear forces and,
as C–C and C–X coupling reactions,7 alkylation reactions,10 and ox-
after this homogenization process, a “steady state” is reached. A
idation reactions of alcohol to aldehydes,11 of CO,12 of styrene,13, 14
miniemulsion is considered kinetically stable (metastable) because
and of toluene.15, 16 The ferrite-catalyzed production of hydrogen
it is stable against Ostwald ripening (an osmotic pressure agent is
through the photocatalytic water splitting is as well a promising ap-
added to counterbalance it) but only critically stabilized against co-
plication of these materials.17, 18 Furthermore, ferrites are magnetic
alescence (a small amount of surfactant is required). In contrast, mi-
materials, being hence easily recoverable from the reaction medium,
croemulsions are thermodynamically stable and are formed by sim-
which is important for achieving sustainable and green methodolo-
ple mechanical stirring. Both micro- and miniemulsion have been
gies for chemical reactions.19 Until recently, ferrites have been
employed to produce metal oxides, by exploiting the confined space
mainly prepared by solid-state synthesis and high temperature ap-
of droplets for precipitation/crystallization reactions.34, 38, 39 How-
proaches,20-23 but more sustainable and low-temperature methods of
ever miniemulsions have several advantages compared to micro-
synthesis are searched for.24, 25 The wet-chemistry preparation of
emulsions: generally microemulsions require a high amount of sur-
multinary oxides has led to materials that are usually single-phase,
factant, and the droplets of a microemulsion cannot be considered
more homogeneous and characterized by smaller crystallites than
as nanoreactors since they experience a dynamic equilibrium that
those prepared by conventional solid-state reactions.24 Among wet-
chemistry routes,24 the most commonly employed techniques for
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makes the droplets coalesce and exchange their content and, in ad- grade, Fisher Scientific) were used as received, styrene (Rea-
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dition, the yield for syntheses in microemulsion is very low, due to
the very low solid content.
gentPlus, containing 4-tertbutylcatechol as stabilizer, ≥99%, Sigma
Aldrich) was purified with liquid chromatography on an aluminum
oxide column to remove the stabilizer.
A hydrothermal (or solvothermal) synthesis is defined as “a het-
erogeneous chemical reaction in aqueous media (or organic solvent
Synthetic Protocol
in the case of solvothermal) above room temperature (normally
In a typical synthesis, the inorganic precursors were dissolved in
Milli-Q water to prepare a solution with Fe:metal molar ratio of 2:1.
This aqueous solution was mixed with the organic phase (1 wt% so-
lution of PGPR in cyclohexane), in a weight ratio of 1:4. Both the
aqueous and the organic phase were stirred together to promote a
pre-emulsification. The mixture was then ultrasonified by a Branson
digital sonifier W-450D (½” tip, 2 min sonication time, 70% ampli-
tude, pulse 1 s, pause 0.1 s). Immediately after the ultrasonication,
an excess (i.e., 12 times the moles of Fe) of NaOH 16.6 M aqueous
solution was added to the just prepared stable miniemulsion and this
mixture was ultrasonified again with the same parameters described
above to trigger the penetration of NaOH inside the droplets and to
promote the precipitation of the inorganic precursors within the
confined space of the droplets. The miniemulsion was either (i)
placed immediately at 80 °C and ambient pressure without stirring
in a glass vial or (ii) poured in a Teflon-lined autoclave reactor (45
mL, mod. 4744 Parr Instrument), sealed and placed at 80 °C without
stirring for different heating times (3, 6, 12 and 24 h). After the reac-
tion time, the suspension was centrifuged (13 000 rpm, 5 min) to
collect the solid product. This product was washed afterwards by
centrifuging once with a 1:1 mixture of acetone and ethanol and
three times with distilled water. The solid obtained was dried under
vacuum at room temperature for 15 h and subsequently grinded.
40, 41
above 100 °C) and a pressure greater than 1 atm”.
These non-
standard conditions are usually achieved by employing pressurized
reactors or closed vessels where the developed pressure is autoge-
nous. Non-classical crystallization pathways can be investigated with
solvothermal conditions:30, 42 at high pressure and temperature, rele-
vant physical parameters of the solvent (ionic product, density, vis-
cosity, dielectric constant) change; thus the solubilization and the
crystallization of compounds that normally would not occur are al-
lowed.23, 24 Herein, by exploiting the combination of two different
kinds of not conventional conditions, i.e. space confinement pro-
vided by miniemulsion with the unconventional solvent properties
provided by subcritical solvothermal conditions, we synthesized
crystalline ferrites and assessed the effects of the synthesis condi-
tions on the crystallization process and on the targeted functional
properties of the final materials (namely, the magnetic and catalytic
properties). Our working hypothesis was that the synergy of the un-
conventional conditions provided by the simultaneous use of the
two routes could lead to an improved crystallinity of the products
when compared to materials prepared from conventional miniemul-
sions (i.e., at ambient pressure) and/or simple solvothermal synthe-
sis. Further investigations concerning the role of the synthetic ap-
proach and, in the case of the solvothermal synthesis, also of the re-
action time, were carried out for zinc ferrite. Zinc ferrite was chosen
as a model because, among the spinel ferrites discussed in this work,
is the only one displaying a complete direct structure in the bulk
phase.
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Bulk syntheses (i.e., direct precipitation from an aqueous solu-
tion) were performed for the sake of comparison with the materials
prepared with our miniemulsion method. A typical bulk procedure
started from the aqueous solution of the inorganic salts, keeping the
same amount of precursors and NaOH solution of the miniemulsion
protocol, but the NaOH was added directly to the salts solution and
the precipitation occurred immediately. The mixture was placed at
80 °C without stirring either in the Teflon-lined autoclave reactor for
the solvothermal reaction or at ambient pressure in a glass vial, as the
miniemulsion synthesis. It should be pointed out that the same vol-
ume of cyclohexane was added to the bulk synthesis to have the same
pressure in the reaction vessel. The centrifugation and the washing
procedure was the same performed for the samples produced in
miniemulsion.
Experimental Section
Chemicals
Iron(III) chloride hexahydrate (ACS reagent 97%, Sigma Al-
drich), iron(II) chloride tetrahydrate (puriss., ≥99.0% (RT), Sigma
Aldrich), cobalt(II) chloride hexahydrate (ACS reagent 98%, Sigma
Aldrich), copper(II) chloride dihydrate (ACS reagent ≥99.0%,
Sigma Aldrich), manganese(II) chloride tetrahydrate (99+% for
analysis, Acros organic), nickel(II) chloride hexahydrate (Rea-
gentPlus, Sigma Aldrich), zinc(II) chloride (98+% anhydrous, Alfa
Aesar), and sodium hydroxide (pellets, ACS reagent ≥98%) were
used without further purification. Commercial polyglycerine poly-
ricinoleate (Grinsted PGPR 90 Kosher, Danisco) was used as re-
ceived. Cyclohexane (HiPerSolv, Chromanorm, VWR chemicals)
and Milli-Q water (Millipore, resistance 18.2 MΩ cm) were used to
prepare solutions and suspensions.
Sample Characterization
X-ray diffraction of the washed and vacuum dried powders was
performed with a Philips PW 1820 diffractometer with monochro-
matic Cu Kα radiation (λ = 1.5418 Å, 40 kV, 30 mA, Δθ (scan step)
= 0.02°, 5 s per step). Inductively coupled plasma-mass spectrometry
(ICP-MS, Agilent 7900) was conducted to estimate the molar ratio
between the iron and the other metal present in the ferrite. The solid
samples were digested in aqua regia at 80 °C for 5 h before the anal-
ysis. The interfacial tension between the continuous and the disperse
phase of a typical miniemulsion was determined using the pendant
drop method with an optical goniometer (OCA35, Dataphysics,
Germany). The liquid media (liquid of lower density, in our case,
cyclohexane–PGPR 0.77 g/cm2)) was deposited in a polystyrene
cuvette. A drop of the higher density liquid (in our case, water–salts
For the preparation of samples for transmission electron micros-
copy analyses, D-(+)-trehalose dihydrate (Fisher BioReagents), ura-
nyl acetate dihydrate (Riedel de Haën) and 1-ethyl-3-methylimidaz-
olium tetrafluoroborate (ionic liquid EMI-BF4) (Aldrich) were em-
ployed.
For the catalytic tests 1,2-dichlorethane (ACS reagent, ≥99.0%,
Sigma Aldrich), tert-butyl hydroperoxide solution in decane
(TBHP, 5.5 mol L-1, Sigma Aldrich) and dichloromethane (HPLC
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1.1543 g/cm2) was immersed in the cuvette, evaluating the interfa- transmission mode using an ionization chamber placed downstream
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cial tension through a drop shape analysis software. Averaged values
from data were recorded. The interfacial tension value was also cross
checked with the spinning drop tensiometer Dataphysics SVT 20N.
A FEI Tecnai F20 transmission electron microscope (TEM) oper-
ated at an acceleration voltage of 200 kV was used for the measure-
ments. The micrographs were recorded on a 2k CCD camera (Gatan
Ultrascan 1000). The dispersion was dropped on a carbon coated
grid and the liquid phase was evaporated. To avoid artifacts, the sam-
ples were prepared by embedding the material in a matrix of treha-
lose. To increase the contrast, negative staining with uranyl acetate
was applied. Alternatively, by using another preparation method, the
samples were embedded in a matrix of the ionic liquid 1-ethyl-3-me-
thylimidazolium tetrafluoroborate, which remains liquid due to the
very low vapor pressure under the high vacuum of the electron mi-
croscope chamber. Further details of the preparation method are re-
ported elsewhere.43 Specific surface area and porosity were deter-
mined by measuring the nitrogen physisorption at the critical tem-
perature with an Autosorb MP1 instrument (Quantachrome). The
specific surface area was calculated according to the Brunauer-Em-
mett-Teller (BET) theory in the range of Ps/P0 0–1.0. Before the
measurements, the products were degassed at 200 °C for 24 h under
vacuum to ensure the complete removal of any previously adsorbed
material. Magnetic measurements were performed by a supercon-
ductive quantum interference device (SQUID) magnetometer
(Quantum Design MPMS II). Gelatin capsules were filled with the
weighed samples and mounted in a low magnetic moment sample
holder. Fe (7112 eV) and Zn (9659 eV) K-edge spectra were col-
lected at the bending magnet of the XAFS beamline at the Elettra
synchrotron facility (Trieste, Italy), under standard ring conditions
(2.0 GeV, 300 mA). For energy selection, a Si (111) double-crystal
monochromator was used. Spectra were collected in a detuned con-
figuration of the monochromator. Data collection was carried in
with respect to the sample. Internal energy calibration was accom-
plished by simultaneous measurement of the absorption of a Mn foil
placed between two ionization chambers located after the sample.
Each sample was prepared as a pellet, using cellulose as a dispersant
medium. Data reduction and analysis was performed using the De-
meter package;44 in particular, background removal was performed
by the Autobk routine of the Athena software, which was also used
for data alignment and calibration. The extracted EXAFS functions
were fitted exploiting the software Artemis. Passive electron reduc-
tion factors (S02) were obtained from the fit to bulk metallic refer-
ence (0.8 for Fe and 0.75 for Zn) and then kept constant in the fits
for all the solid solutions.
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Catalysis Experiments
The catalytic activity of the ferrites was tested for the oxidation of
styrene. The reaction was performed at 70 °C: the catalyst (20 mg,
0.09 mmol), 1,2-dichloroethane (25.4 mmol) and purified styrene
(4.3 mmol) were placed in a vial and the addition of the TBHP (13.0
mmol) started the reactions. The closed vial was deposited in a ther-
moshaker (HLC, MKR 23) at 600 rpm and 70 °C. Sampling volumes
of 100 μL were collected at different times and diluted with 2.4 mL
of dichloromethane to quench the reaction. To assure the separation
of the catalyst from the reaction solution a neodymium magnet was
used. 80 μL of the dichloromethane solution were withdrawn and
poured in a vial with 920 μL of dichloromethane to achieve the con-
centration range suitable for the following analysis. Decane, present
in the TBHP solution, was employed as an internal standard. The
samples were analyzed by a gas chromatograph coupled with mass
spectrometer (Shimadzu, GCMS-QP2010 Ultra) to follow the oxi-
dation and to identify the products. The magnetic catalyst was re-
covered with a neodymium magnet after the reaction and the pow-
der was washed three times with dichloromethane.
Surfactant
Spinel
ferrite
NaOH
Centrifugation
Washing
Organic
phase
Ultrasound
1) Ultrasound
Δ
Spinel ferrites
nanoparticles
Drying
(room T, under
vacuum)
2) Transfer in the
autoclave
Autogenous
P = 2–3 bar
reactor
Water
Inorganic
precursor
Figure 1. Combination of inverse miniemulsion process and solvothermal conditions for the synthesis of spinel ferrites.
The oxidation of the styrene was carried out for three additional
cycles using the recovered powder to test the reusability of the cata-
lyst. A blank reaction in the absence of the catalyst was carried out to
demonstrate the catalytic activity of ferrite
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importantly, also about their structural evolution towards the crys-
Results and Discussion
Physicochemical and Structural Characterization of the
Samples
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talline phase as a function of sample composition, pressure higher or
equal to ambient one, reaction time, and temperature. Crystalliza-
tion of the zinc ferrite was therefore followed by analyzing ex-situ
samples after different reaction times (Table 2). Information on the
structural evolution in the long range was integrated and comple-
mented by the one on the local range delivered by XAS measure-
ments (see below).
The preparation of a generic spinel ferrite involves typically the
precipitation of the iron and the divalent metal hydroxides immedi-
ately after the addition of a NaOH solution to an aqueous solutions
of salts, as reported in literature.45 In such a case, the mixture of hy-
droxides is heated at a certain temperature (e.g., 390 °C) to produce
the ternary metal oxides. In our work, we adopted the combination
of miniemulsion synthesis and subcritical solvothermal conditions
to synthesize crystalline spinels at much lower temperature (i.e., 80
°C) than generally reported in literature, exploiting the precipitation
of the hydroxides in the confined space of the miniemulsion droplets
under non-standard conditions. Generally, the inverse miniemul-
sion procedure (Figure 1) involves two phases: a dispersed (aque-
ous) phase and a continuous (organic) phase in which the lipophilic
surfactant is dissolved. In our synthesis, the surfactant employed was
commercial polyglycerol polyricinoleate (PGPR), dissolved in cy-
clohexane. PGPR was selected as a surfactant because it is a lipo-
philic surfactant with a low hydrophilic-lipophilic balance (HLB)
(around 1.5),46 suitable to stabilize inverse miniemulsions. On the
other hand, in our work the disperse phase is an aqueous solution of
the inorganic precursors. After the formation of the miniemulsion by
high sheer forces produced by the ultrasounds and the addition of
the NaOH solution to start the precipitation inside the water drop-
lets, the system was placed in a Teflon-lined autoclave reactor under
subcritical solvothermal conditions, generated upon heating of the
closed vessel. The autogenous pressure is 2∙105 Pa, 2 bar, at 80 °C
and 3∙105 Pa, 3 bar, at 100°C as calculated with Antoine’s equation.47
In the case of the zinc ferrite, a comparison among different ap-
proaches was conducted (the labeling of the samples used in the fol-
lowing is indicated in brackets): i.) miniemulsion under solvother-
mal conditions (ME–HP), ii.) miniemulsion at ambient pressure
(ME) and iii.) bulk procedure, which involves the mixing of the in-
organic precursors and the precipitating agent and directly the pre-
cipitation of the products, either at ambient pressure (bulk) or com-
bined with solvothermal conditions (bulk–HP).
The diffractograms of all ferrites produced from miniemulsion
under solvothermal conditions are reported in Figure 2. The diffrac-
tograms of iron, cobalt and copper ferrites match with the reference
patterns of the corresponding MFe2O4 present in the ICDD PDF-2
database. Although the diffractograms of manganese, nickel and zinc
ferrites are matching with the corresponding references patterns of
MnFe2O4 (ICDD card No. 01-074-2403), NiFe2O4 (ICDD card No.
00-054-0964) and ZnFe2O4 (ICDD card No. 00-022-1012) (see
Supporting Information for the complete comparison) the stoichi-
ometry obtained from Inductively Coupled Plasma Mass Spectrom-
etry (ICP-MS) is not compatible with the ratio M:Fe=1:2 (Table 1).
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Table 1. Results of the ICP-MS analysis measured on ferrites syn-
thesized in miniemulsion–solvothermal conditions.
Metal (M) present
in the ferrite
Reaction
[M] / [Fe]
Molar
temperature / °C
Mn
Co
Ni
80
80
0.99 ± 0.03
0.510 ± 0.005
1.00 ± 0.03
100
80
Cu
Zn
0.50 ± 0.01
80
0.305 ± 0.003
The autogenous pressure present in the autoclave during the experi-
ments was 200 kPa for the reactions at 80 °C and 300 kPa for the reac-
tion at 100 °C
Another suitable wet-chemistry route for the synthesis of ferrites
is the coprecipitation of oxalates: the metal precursors, in the pres-
ence of oxalic acid, form the oxalates after the addition of the precip-
itating agent (generally a concentrated base). This synthesis can be
followed by either thermal treatment at high temperature or by low-
The synthesis of manganese and nickel ferrites required, to obtain
a crystalline material, a molar ratio between the precursors of
metal:Fe 1:1, differently from the other ferrites synthesized with mo-
lar ratio metal:Fe 1:2; hence the molar ratio 1:1 adopted for the syn-
thesis is probably the reason of the values measured by the ICP-MS.
In addition to the pattern of MnFe2O4, the diffraction pattern of the
manganese compound is matching as well with the reference pattern
of Fe3Mn3O8 (ICDD card No. 01-075-0034), as reported in Figure
2. Fe3Mn3O8 is another kind of spinel ferrite, less common, faced-
temperature subcritical hydrothermal processing to afford the final
48, 49
crystalline oxides.
In an initial stage of the work, we employed
the combination of the aforementioned coprecipitation of the oxa-
lates with the miniemulsion technique to achieve the formation of
spinel ferrites. The inorganic precursors and the oxalic acid were dis-
solved in the aqueous disperse phase and the precipitating agent
(NaOH) was added to the miniemulsion. However, this procedure
did not lead to single-phase crystalline materials, and the oxalic acid
was still present despite several washings. This finding can be traced
back to the environment of the confined space of a miniemulsion
droplet, which is completely different from the environment of a
standard bulk synthesis.50 The precipitation of the hydroxides was
found to be a more successful and reliable route.
centered cubic and with the same point group (
Fd 3m ) as the Ja-
cobsite MnFe2O4, but with a slightly contracted lattice. The explana-
tion of the molar ratio Zn/Fe 0.3 obtained by ICP-MS cannot be as-
cribed to a different ratio between the precursors adopted, since the
amounts of zinc and iron chlorides used were in the common ratio
1:2, but probably to the formation of a more favored crystalline
phase, similar to the iron-rich ferrite (Zn0.664Fe0.336) (Fe1.934Zn0.066)O4
corresponding to the ICDD card No. 01-086-0508 (Franklinite fer-
roan, see Figure 2). The crystal structure of this phase is also in this
X-ray diffraction (XRD) provides information not only about
phase purity and crystallite size of the crystalline materials but, more
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case spinel and face-centered cubic (space group Fd 3m ) like the
Franklinite ZnFe2O4.
(a) Fe3Mn3O8
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In the case of the nickel ferrite, the molar ratio Ni:Fe 1:1 corre-
sponds actually to the molar ratio adopted for the synthesis between
Fe and Ni chlorides, as aforementioned. However, the diffractogram
of this sample presents the typical reflections of NiFe2O4 and faint
reflections ascribed to Ni(OH)2, as reported in Figure 2; therefore,
the sample obtained is a mixture of them. The syntheses were per-
formed in an alkaline aqueous solution of the metal salts at a meas-
ured value of pH of 13.8. According to the pH−concentrations dia-
gram of aqueous solution of nickel ions, a description of the behavior
(i.e., concentrations and chemical forms) of the ions as a function of
the pH, nickel hydroxide, and further oxo-hydroxo species, charac-
terized by very low Ksp, already precipitate at pH 7. In our reaction
conditions at pH 13.8, Ni should be present as insoluble hydroxide.
Since the formation of the ferrites, under the chosen experimental
conditions, occurs upon coprecipitation and dehydration of the cor-
responding hydroxides, it can be inferred that part of the hydroxide
has not been decomposed to give the targeted material.
(b) Fe3O4
(c) CoFe2O4
(d) NiFe2O4
9
(e) CuFe2O4
(f) Iron-rich zinc ferrite
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 / °
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2
Figure 2 X-ray diffractograms of the ferrites produced using the sol-
vothermal–miniemulsion route at 80–100 °C: (a) Fe3Mn3O8 with its
reference pattern ICDD card No. 01-075-0034 reported as vertical lines
(b) Fe3O4 with ICDD card No. 00-019-0629 (c) CoFe2O4 with ICDD
card No. 00-022-1086 (d) NiFe2O4 with its reference pattern (continu-
ous line) ICDD card No. 00-054-0964 and the reference pattern of
Ni(OH)2 (dashed line) ICDD card No. 01-073-1520 (e) CuFe2O4 with
its reference pattern (continuous line) ICDD card No. 01-077-0010 and
the reference pattern of CuO (dashed line) ICDD card No. 00-048-
1548 (f) iron-rich zinc ferrite with ICDD card No. 01-086-0508, related
to (Zn0.664Fe0.336)(Fe1.934Zn0.066)O4.
The diffractogram of CuFe2O4 displays, as well, the presence of
the reflections ascribable to CuO, but the stoichiometry assessed by
ICP-MS is as expected 1:2, consistent with nominal molar ratio be-
tween Cu and Fe chlorides of 1:2 used in the synthesis.
To define better the six ferrites produced in miniemulsion–sol-
vothermal conditions in terms of sites occupation and quantify the
relative amount of the spurious phases, Rietveld refinement was per-
formed on the collected diffractograms. These procedures evi-
denced, for most ferrites, the presence of two different populations
with different sizes. The crystallite sizes and relative weights are re-
ported in Table S1.
As anticipated before, to investigate the effect of the different
routes (miniemulsion and bulk, either at ambient pressure or under
solvothermal conditions) on the crystallinity of the samples, a com-
parison of the XRD patterns of the iron-rich zinc ferrites is reported
in Figure 3. Only the sample prepared by the synergetic approach
(miniemulsion under solvothermal conditions) is highly crystalline,
whereas the application of miniemulsion at ambient pressure and
bulk in both of the conditions yield a poorly crystalline material.
Rietveld refinement conducted on the pattern of the sample pre-
pared by the synergetic approach (Table S1) evidences also in this
case the presence of two size populations (4 nm: 57%; 13 nm: 43%).
However, because of the low signal-to-noise ratio, a precise determi-
nation of the cation-site occupancy was not possible.
In conclusion, though slight variation in the metal/Fe ratio, all the
obtained ferrites are classified as spinels, with a face-centered cubic
crystal system and Fd 3m space group: the achievement of such
crystalline spinel ternary oxides from the combination of miniemul-
sion and solvothermal conditions is, to the best of our knowledge, an
unprecedented result. Our synergetic route allowed to obtain (ex-
cept in the case of Ni and Cu, where, though much less intense re-
flections ascribed to Ni(OH)2, and CuO, respectively, were also de-
tected) phase pure crystalline materials and at lower temperature, 80
°C with respect to temperatures above 150 °C employed in other
routes.51 The low temperature synthesis and the absence of post-syn-
thesis thermal treatment are appealing from the perspective of sus-
tainability and to avoid coalescence and coarsening, resulting in un-
controlled growth, of the nanoparticles. The achievement of crystal-
linity and of spinel formation for all samples is the result of a long
process of optimization that involved the accurate selection of rea-
gent molar ratios and reaction temperature: Mn and Ni ferrites were,
as already mentioned, not obtained as expected from the molar ratio
M:Fe 1:2, but rather with a 1:1 M:Fe molar ratio. Furthermore, a
clearly crystalline NiFe2O4 was obtained only working at 100 °C (at
80 °C the product was not crystalline). Interestingly, as visually as-
sessed, the miniemulsions were stable for few hours and no phase
separation occurred. Despite of the boiling point of the cyclohexane
(82 °C), they remained stable even after heating at 80 or at 100 °C
for 24 h.
(a) ME
(b) ME-HP
(c) Bulk
(d) Bulk-HP
10
20
30
40
2  / °
50
60
Figure 3 Iron-rich zinc ferrite produced with different routes compared
with the reference pattern ICDD card No. 01-086-0508: (a) miniemul-
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sion at ambient pressure (ME) (b) miniemulsion under pressure (sol-
vothermal route) (ME–HP) (c) bulk at ambient pressure (bulk) (d)
bulk under pressure (solvothermal route) (bulk–HP).
particular to determine the cation distribution and thus the inver-
sion degree of these structures. Analyses were performed at both
54
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8
Zn and Fe K-edges to enhance the local chemical sensitivity through
a combined picture of both species and corresponding chemical en-
vironments. All the Zn K-edge X-ray absorption near-edge structure
(XANES) spectra (Figure 4) present a white line composed of three
peaks, at 9665 (A), 9668 (B) and 9672 eV (C), with a shoulder (D)
at about 9679 eV. Features B and D were shown to be indicative of
the degree of inversion, with peak B increasing for higher inversion,
while shoulder D decreases.54 In addition, the position of these fea-
tures slightly decreases in ferrites with higher inversion degrees. The
experimental outputs evidence how the cationic distribution is rela-
tively highly inverted for samples prepared by the synergetic ap-
proach or the miniemulsion at ambient pressure, while it decreases
for the bulk preparation. The sample prepared by simple solvother-
mal method is likely a fully direct ferrite.
The enhanced crystallinity of the sample synthesized by the syn-
ergy of miniemulsion and solvothermal route may be explained by
the Laplace pressure acting on droplets.36, 38 The Laplace pressure is
the pressure difference between the inside and the outside of a
curved surface and it is caused by the surface tension at the liq-
uid/liquid interface; this pressure is higher for small droplets and
causes the disappearance of smaller ones in favor of the bigger ones
(Ostwald Ripening). The Ostwald ripening is counteracted in
miniemulsion, to guarantee the stability of the system, by the pres-
ence of an osmotic pressure agent (a substance soluble in the dis-
perse phase—in our case the inorganic salts—but not in the contin-
uous). The Laplace pressure is high in the small droplets of the
miniemulsion and may promote the crystallization of inorganic sys-
tems at lower temperature than required (or even at room tempera-
ture), without any thermal treatment afterwards.52, 53 The additional
external pressure due to the solvothermal route is likely to enhance
this effect. The interfacial tension between the two phases (water
with the salts and cyclohexane with PGPR) was experimentally de-
termined with the pendant drop method and a value of  = (2.9 ±
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The position of the edge in the Fe K-edge XANES spectra (Figure
5) is typical for an oxidation state +3. The pre-edge peak provides
valuable information. First of all, the position of the pre-edge peak is
often a preferred way to determine the oxidation state with respect
to the edge position, since it is less affected by multiple-scattering
phenomena.55-57 In addition, this peak is symptomatic of the sym-
metry around the absorber. It arises from a 1s → 3d transition, which
is forbidden by dipole selection rules and hence weak, but the inten-
sity of which increases for noncentrosymmetric environments, i.e.
tetrahedral ones.58
1
0.7) mN m– was obtained (see Figure S4; value measured at room
temperature, the possible effect of temperature is neglected). This
surprisingly low valued was confirmed by measurements of the in-
terfacial tension with the spinning drop method. The Laplace pres-
sure exerted by droplets is given by the Young-Laplace equation:
2훾water/cylclohexane
∆푃Laplace = 푃inside − 푃outside
=
푟droplet
For ideal spherical droplets of 100 nm of diameter, the resulting
value of ∆푃Laplace is about 1.2∙105 Pa (1.2 bar). Accordingly, the
pressure inside the droplets would be of about 3.4 bar (4.2 bar in the
case of nickel, prepared at 100 °C), more than three times higher
than atmospheric pressure, which evidences the advantages derived
by the synergy miniemulsion–solvothermal conditions. The positive
effect of pressure had been already pointed out in the case of simple
hydrothermal synthesis of ferrites,49 by comparing results obtained
by using hydrothermal synthesis and by using reflux synthesis con-
ditions at ambient pressure. In that case, the beneficial effect of ap-
plied pressure in terms of enhanced purity and yield was pointed out.
A typical cell volume for a ferrite would be of the order of 590 Å3,
which multiplied by Avogadro’s number would lead to a molar vol-
ume of 3.55 × 10-4 m3 mol-1. Multiplying the pressure inside the
droplets (3.4 × 105 Pa) by the molar volume, a value of 120.7 J mol-1
(ca. 29 cal mol-1) is obtained. We are not aware of reliable data on
nucleation and growth activation energies for the studied ferrites,
but we would expect values of the order of tenths up to hundreds of
kilocalories per mol for the nucleation, and of a few kilocalories per
mol up to tenths of kilocalories per mol. The estimated energy of ca.
29 cal/mol exerted by the curved interface of the bubbled is less rel-
evant for the nucleation process (which occurs anyway also in the
bulk solution), but would not be negligible growth processes.
Figure 4 Zn K-edge XANES spectra of samples prepared with different
synthetic approaches.
As anticipated, the effect of the different routes was also deter-
mined on a short-range scale by performing X-ray absorption spec-
troscopy (XAS) experiments at Elettra synchrotron facility. XAS has
been shown a very powerful tool to investigate nanosized ferrites, in
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Chemistry of Materials
a)
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Figure 5 Fe K-edge XANES spectra of iron-rich zinc ferrites prepared
with different synthetic approaches.
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b)
The pre-edge position and relative intensities were analyzed after
background removal with an arctangent function55, 56 and decon-
volution with pseudo-Voigt components. Divalent iron ions usually
show a pre-edge peak centered around 7112.5–7113 eV, while for
trivalent iron ions this is located around 7114–7114.5 eV.55 The cen-
troid positions determined for the experimental samples confirm an
oxidation of +3 for all samples, being centered at 7114.2–7114.3 eV
(Table S3). A small contribution of Fe2+, limited to less than 4%, is
also present, compatible with the ferrite being rich in iron, and thus
requiring the presence of divalent iron to keep electroneutrality.
In addition, the pre-edge peak area decreases in going from tetra-
hedral to octahedral site symmetry54-56. The experimental data at Fe
K-edge disagree with what had been evidenced by measurements at
Zn K-edge, showing an opposite trend. The samples prepared by
miniemulsion at ambient pressure or coupled with the solvothermal
step show similar pre-edge areas (Table S3). Interestingly, the bulk
sample show the greatest area, contrary on the indications given by
the Zn K-edge. This likely indicates the presence of Fe3O4 moieties,
not discernible in the XRD diffractograms due to similar patterns.
Figure 6 FTs of spectra recorded at a) Zn and b) Fe K-edges.
As seen in Figure 6a, the second shells in the Zn K-edge FTs are
clearly formed by two components. Following the previous reason-
ing, the presence of a noticeable component at about 3 Å indicates a
partially inverted spinel. In agreement with the XANES findings, this
component is much less intense in the sample prepared by the sim-
ple solvothermal method, indicating that for this sample the inver-
sion degree is lower. This, as outlined, is a very relevant result.
The Fourier transforms (FTs) of the EXAFS functions recorded
at the two different edges are reported in Figure 6. The second shell
that is the region between 2 and 4 Å is particularly meaningful, since
it is mainly composed by the cation-cation contributions and can
thus provide a qualitative indication of the inversion degree. In par-
ticular, the distance between two cations in octahedral sites is about
3.0 Å, whereas for two cations in tetrahedral sites this distance is
about 3.65 Å; two cations occupying neighboring octahedral and tet-
In the Fe K-edge FTs, the inversion degree is mainly evident in
the contribution at 3.5 Å, corresponding to the interactions involv-
ing tetrahedral sites. For this edge, samples prepared by simple
miniemulsion or by the combined approach present a higher occu-
pation of Fe cations in Td sites, symptomatic of higher inversion. The
in-depth analysis of the EXAFS curves, detailing the features hinted
by XANES and the Fourier Transforms, is reported in the Support-
ing information. Briefly, the Zn-O and Fe-O distances expected from
the direct zinc ferrite structure (Zn in Td A sites, Fe in Oh B ones) are
1.98 and 2.03 Å respectively. For Zn in A sites and Fe in B sites, the
metal-to-oxygen bond distances determined by EXAFS are, for all
samples, in agreement with these values (average values: 1.96 and
2.04 Å respectively). For Zn occupying B sites, an average bond dis-
tance of 2.04 Å was determined, slightly shorter than the expected
rahedral sites are about 3.5 Å apart. (from crystal structure: MO
MO =3.00 Å, MO − MT =3.51 Å, MT − MTd=3.66 Å).
−
h
h
h
d
d
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value for divalent zinc in octahedral symmetry (2.10 Å). Finally, the out that, even with very short processing time, except nickel ferrite,
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insertion of trivalent Fe ions in the A sites leads to a shortening of M-
O distances to about 1.88 Å, in accordance with Fe3+ in Td sites (1.86
Å, Fe2+ in Td: 2.00 Å). The distances determined for metal-metal in-
crystalline and stable materials are produced.
Table 2 Crystallite size (L), calculated with the Scherrer formula, for
all the six ferrites produced in miniemulsion–solvothermal condi-
tions.
teractions were MO − MO =3.00 Å, MO − MT =3.50 Å, MT −
h
h
h
d
d
MTd=3.63 Å
L Mn
ferrite
/ nm
L Fe
ferrite
/ nm
L Co
ferrite
/ nm
L Ni
ferrite
/ nm
L Cu
ferrite
/ nm
L Zn
ferrite
/ nm
Analogously, to understand the effect of the heating time on the
structural evolution, the synthesis of the samples in miniemulsion
under solvothermal conditions at different treatment times (time 0,
3 h, 6 h, 12 h and 24 h) was carried out. In Figure 7 the diffracto-
grams collected at different reaction times of the iron-rich zinc fer-
riteare reported whereas, for sake of comparison, the diffractograms
of the other ferrites are shown in the Supporting Information. As ex-
pected, the crystallinity of the samples increases with time, the re-
flections becoming sharper and more intense. The sample collected
at time 0 was centrifuged and washed immediately after the second
miniemulsification without placing it at 80 °C and, as demonstrated
by its diffractogram, it is poorly crystalline. As reported in Figure 7,
Zn ferrite synthesized after only 3 h of reaction shows instead a crys-
talline pattern: this result is interesting in the pursuing of greener
synthetic routes, because a crystalline product was achieved after a
reaction of just 3 h at 80 °C, without further thermal treatment, only
exploiting the non-standard conditions made available by the com-
bination of miniemulsion and the solvothermal routes. Increasing
the time of the reaction, other conditions being equal, the reflections
become sharper, hence crystallinity increases.
9
3 h
6 h
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–
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10
12 h
24 h
24
29
The time zero is not reported because all these samples were poorly
crystalline and the Scherrer formula could not be applied.
The time-resolved evolution of zinc ferrite nanostructures was
also followed by means of Zn and Fe K-edges XAS. The Zn K-edge
XANES evidences once more the relevance of the combined minie-
mulsion and solvothermal approach. The sample at t = 0 h, thus not
subjected to any solvothermal treatment, indeed shows less defined
features with respect to the other samples, indicating that the local
structure around the Zn absorber sites is different, in agreement with
a poorly crystalline sample. This might also be related to a size effect.
With increasing treatment time, on the other hand, no significant
difference could be detected, in agreement with the similar crystal-
lite size.
(a) Time zero
(b) 3 h
(c) 6 h
(d) 12 h
(e) 24 h
10
20
30
40
50
60
2  / °
Figure 7 Crystallization of iron-rich zinc ferrite synthesized in miniemul-
sion–solvothermal condition at different reaction times: (a) time zero
(b) after 3 h (c) after 6 h (d) after 12 h and (e) after 24 h. The vertical
lines are the reference pattern of (Zn0.664Fe0.336)(Fe1.934Zn0.066)O4 ICDD
card No. 01-086-0508.
Figure 8 Zn K-edge XANES spectra of samples prepared with different
reaction times.
Similar experiments with different reaction times were performed
for all the six ferrites and the results are summarized in Table 2 (the
diffractograms are reported in the Supporting Information). The
values of crystallite size determined by the Scherrer formula59 are,
within the experimental error, constant, for each series of material,
through all the experiments and the size of the crystallites depends
strongly on the composition of the ferrite. Mn spinel has the biggest
crystallites, followed by CoFe2O4; Zn ferrite and Fe3O4 show a simi-
lar size and CuFe2O4 has the smallest crystallites. NiFe2O4 starts to
show distinguishable reflections after 12 hours, so that the Scherrer
formula cannot be applied before. The constancy of the values of the
crystallite size through all the experiment at different times points
In a similar way, the Fe K-edge spectra registered after different
reaction times are very similar, except for the starting sample (Figure
9). In this case, the pre-edge peak is slightly more intense than the
following samples, indicating a less centrosymmetric local structure.
This is confirmed by the fitting of the EXAFS function, since the De-
bye-Waller factors determined at both edges for the starting samples
are higher than samples synthesized with longer times, due to a more
disordered structure, in agreement with a smaller crystallite size (Ta-
ble S6).
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Chemistry of Materials
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Figure 9 Fe K-edge XANES spectra of iron-rich zinc ferrite samples pre-
pared with different solvothermal treatment time.
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X-ray diffraction (XRD) results on crystallinity were comple-
mented with morphological investigations by transmission electron
microscopy (TEM) (Figure 10). The images reported were ob-
tained by embedding the nanoparticles in an ionic-liquid matrix, as
reported in the Experimental Section (alternative measurements
prepared by embedding the nanoparticles in a trehalose matrix with
negative staining of uranyl acetate are reported in the Supporting In-
formation, Figure S10). Despite the natural tendency to aggregate
upon solvent evaporation, single primary particles are distinguisha-
ble. Noticeably, the miniemulsion route has the advantage of yield-
ing samples that are less aggregated than the ones produced in bulk,
since the particles are synthesized independently inside the droplets,
acting as nanoreactors, and not in a single-event precipitation. Parti-
cle sizes were estimated by statistical treatment of TEM images. Two
size populations were observed and a mean value was calculated
from them (Table 3). The average sizes of the two populations are
matching the sizes calculated by the Scherrer formula and by
Rietveld refinement.
Figure 10 TEM images of the ferrites samples: the images are reported
in the order of the periodic table.
The theoretical values, calculated for the ideal case of spherical
and non-porous particles from the average size, are—except for the
case of the manganese ferrite—generally smaller than the experi-
mentally measured ones, which is a hint of the porosity of the mate-
rials that leads to a higher specific surface area. We also compared
the experimental surface areas for the iron-rich zinc ferrites pro-
duced with the different routes. In this case, the sample synthesized
by the miniemulsion–solvothermal route has the highest specific
surface area, followed by the one produced in miniemulsion at am-
bient pressure and the two samples prepared in bulk solution (Fig-
ure S13). The hysteresis loops of the bulk samples seem different
from the ones of the miniemulsion samples and they resemble more
the H2 hysteresis loop officially defined by IUPAC,60 typical of “ink-
bottle” pores (Figure S13). Both miniemulsion and bulk materials
can be considered as mesoporous.
Since these materials are potentially applicable as heterogeneous
catalysts, the specific surface area and the porosity, factors affecting
the interaction with the substrates, are important features to be in-
vestigated. The physisorption of nitrogen at the critical temperature
and the following calculation with the BET method allow assessing
that the produced ferrites are mesoporous materials: their pores
have sizes between 2 and 50 nm (Table 3). The hysteresis loop (Fig-
ure S11) is typical of slit-shaped pores, resembling the hysteresis
loop H4 reported in the official definition of IUPAC.60 The theoret-
ical surface area calculated from the sizes obtained by statistical
treatment of TEM micrographs was compared with the specific sur-
face experimentally obtained from nitrogen physisorption experi-
ments (BET isotherms, Table S7).
Table 3 Crystallite size calculated with the Scherrer formula (L), average particle size measured by TEM of the two size populations, specific
surface area and average pore diameter of the ferrites synthesized in miniemulsion–solvothermal conditions for 24 h.
Metal pre-
sent in the
ferrite
Reaction
temperature /
°C
Specific Sur-
face Area / m2
g-1
Average pore
diameter /
nm
Pressure /
kPa
Average
size / nm
L / nm
36
Average sizes / nm
51 112
Mn
80
200
80
9
82
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Only Fe
Co
80
80
200
200
300
200
200
13
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29
7
93
12
3
61
7.10.9
61
219
112
217
142
123
146
92
1
2
3
4
5
6
149
101
173
203
Ni
100
80
5
135
102
93
Cu
2
61
Zn
80
10
3
61
7
8
9
As outlined in the introduction, these nanomaterials feature also
appealing magnetic properties. The room temperature (RT) mag-
netic behavior of the nanoparticles was characterized by measure-
ments of the magnetic hysteresis in a SQUID magnetometer and is
shown in Figure 11. Saturation magnetization values for each sample
are reported in the corresponding plots.
functions calculated for superparamagnetic particles. The superpar-
amagnetic clusters sizes (between 4–7 nm) calculated from the mag-
netic data are smaller than the crystallite sizes from X-rays outputs.
However, in an X-ray experiment larger particles of a distribution
will dominate as the scattered intensity scales with the scattering am-
plitude of a particle squared, while the magnetization increases line-
arly with the particle volume. In addition, a perfect magnetic spin
alignment is not guaranteed in the nanoparticles such that the mag-
netic data rather give lower bounds on the particle sizes. For the
other MFe2O4 compounds, an essential magnetic defect is the de-
gree of inversion. E.g. for CuFe2O4 with Cu(3d9) and Fe(3d5) in a
high spin state, one calculates 9 B/f.u. (5+5–1) for the normal spi-
nel structure, while the inverse spinel structure has only 1 B/f.u.
(5+1–5).62 This sets the bandwidth from 24 emu g-1 up to 217 emu
g-1. We observe 30 emu g-1 at B = 4 T which points more to an inverse
spinel structure and is in agreement with the measurements on ball
milled nanoparticles of this compound.63 For the Zn based sample
the observed saturation value of 41 emu g-1 at 4 T is in agreement
with reported results on ZnFe2O4 nanoparticles of similar size by
Guo et al.64 A visual inspection of Figure 11 shows a most square like
hysteresis curve for the Mn (Ni) compounds. The fitted values indi-
cate superparamagnetic moments considerably larger than for the
magnetite particles as determined from the magnetization curve. In-
deed, the evaluation of the X-ray diffraction data (see Table 2)
yielded the largest particle size for the Mn compound, followed
closely by NiFe2O4, with both values being a factor 2–3 larger than
that of the Fe3O4 nanoparticles. A pronounced hysteresis with a co-
ercive field of 80 mT is only observed for CoFe2O4. For bulk samples
a saturation magnetization of 74 emu g-1 is observed65 while high
temperature treated nanoparticles of 25 nm size resulted in a satura-
tion magnetization of 61.7 emu g-1 and a coercive field of about 50
mT.66 Our low temperature processed nanoparticles of smaller size
show a maximum magnetization at 4 T external field of 49 emu g-1.
In summary, the magnetic properties of the nanoparticles prepared
by the low temperature solvothermal method are comparable to that
of conventionally processed nanoparticles, which renders them suit-
able for a multitude of applications.
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0
(a) Fe3Mn3O8
(b) Fe3O4
40
20
0
Ms = 45 emu g-1
Ms = 64 emu g-1
-20
-40
-60
-20
-40
-40000
-20000
0
20000
40000
-40000
-20000
0
20000
40000
H
H
H
/ Oe
H
/ Oe
60
40
20
0
30
20
10
0
(c) CoFe2O4
(d) NiFe2O4
Ms = 49 emu g-1
Ms = 24 emu g-1
-20
-40
-60
-10
-20
-30
-40000
-20000
0
20000
40000
-40000
-20000
0
20000
40000
/ Oe
H
/ Oe
30
20
10
0
(e) CuFe2O4
(f) Iron-rich zinc ferrite
40
20
0
Ms = 30 emu g-1
Ms = 41 emu g-1
-10
-20
-30
-20
-40
-40000
-20000
0
20000
40000
-40000
-20000
0
20000
40000
/ Oe
H
/ Oe
Figure 11 Magnetization of the ferrites synthesized in miniemulsion–
solvothermal conditions: (a) Fe3Mn3O8 (b) Fe3O4 (c) CoFe2O4 (d)
NiFe2O4 (e) CuFe2O4 (f) iron-rich Zn ferrite. The saturation magneti-
zation values are reported as inset in the corresponding plots.
All samples show a superparamagnetic behavior except the
CoFe2O4, which exhibits a ferromagnetic hysteresis loop at RT. The
data displayed have been generated by subtraction of the small dia-
magnetic contribution of an empty capsule that was measured sepa-
rately. For the Fe3O4 nanoparticles, a maximum value of 64 emu g-1
is observed at 4 T (i.e. 40000 Oe) that can be compared to a theo-
retical saturation magnetization for magnetite of 96.5 emu g-1 (as-
suming 4 Bohr magnetons per formula unit (B/f.u.) in the inverse
spinel structure). As nanoparticles possess a high surface to volume
ratio and the number of interacting neighbors at a surface is reduced,
a reduction of the saturation moment must be expected and is fre-
quently observed.61 The measured behavior can be reproduced
within the measurement error using a superposition of Brillouin
Catalytic Tests
The ferrite nanoparticles produced in miniemulsion are solvent-
born and, even after several washings, a part of the surfactant used
(PGPR) adsorbed on their surface is still present; the particles are
thus dispersible in organic solvents. For this important property, a
reaction in organic medium, the oxidation of styrene, was selected to
demonstrate the catalytic performances of the synthesized ferrites.
This reaction has a strategic application as well: olefin epoxidation is
one of the most important processes in the chemical industry and
styrene oxide, one of the products (Figure 12), is an intermediate for
fine chemicals and pharmaceuticals.67 New efficient and recyclable
heterogeneous catalysts for this reaction are therefore sought for.
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Chemistry of Materials
test the reproducibility of the results and those evaluations con-
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firmed, in the range of the experimental error, the results reported in
Table 4.
The cobalt, copper and manganese ferrites that showed relevant
catalytic activities after a first cycle of reaction were separated from
the reaction medium and collected with the help of a neodymium
magnet. The recovered catalysts were then washed several times,
dried and employed for other cycles of reaction. Interestingly, the
catalyst showed in all the cases (except a little loss of activity and,
especially in the case of copper ferrite, a slower kinetics), that the
overall conversion is excellent reaching, in the case of copper and
manganese ferrites, 100% of conversion also at 4th cycle of repetition.
Figure 12 Oxidation of styrene with t-butyl hydroperoxide.
Additionally, the ferrites are magnetic particles, therefore are eas-
ily separable from the reaction medium and recoverable to catalyze
further other reactions. The oxidation reactions were carried out us-
ing 1,2-dichloroethane as a solvent because it increases the styrene
conversion when compared to the other halogenated solvents.68 The
oxidant employed was t-butyl hydroperoxide in decane, chosen for
its dispersibility in the organic solvent. In Table 4 the results of the
oxidation with different catalysts are reported. With 100% conver-
sion, manganese, cobalt, and copper ferrites display the highest sty-
rene conversion after 24 h. The catalysts are selective mostly for sty-
rene oxide and benzaldehyde (with comparable percentages) and
only a small amount of phenylacetaldehyde is produced. The other
three ferrites do not show any particular catalytic activity since the
styrene conversion is only slightly higher than the one of the blank
reaction, performed without any catalyst. Possible reasons proposed
to explain the different catalytic activity among the tested ferrites are
the different amount of oxygen vacancies in the spinel materials that
facilitate the adsorption of the peroxide to form the molecular oxy-
gen for the oxidation13 and that the Fe3+ cation from the octahedral
sites of the ferrite reacts with the peroxide molecule to form metal-
oxyradical (Fe4+–O) and hydroxyl radicals as an initiation step.
These species, which are formed on the surface of the catalyst, will
then oxidize the alkene substrates.69, 70 In certain cases also the other
metal of the ferrites can form metal-oxyradical species with a transi-
tional oxidation of the metal.71
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60
100
(a) Fe3Mn3O8
80
60
1 min
40
1st cycle
20
2nd cycle
3rd cycle
4th cycle
0
0
6
12
18
24
t / h
100
80
60
40
20
0
(b) CoFe2O4
Table 4 Styrene oxidation catalyzed by ferrites produced by the
combination of miniemulsion and solvothermal route: conversion
and selectivity after 24 h of reaction.
Metal
present
in the
Phenyl-
acet-alde-
hyde se-
Benz-alde-
hyde se-
lectivity %
Styrene
oxide se-
lectivity %
Styrene con-
version %
1 min
ferrite
lectivity %
1st cycle
Mn
Fe
100
16
48
27
39
28
56
0
16
17
15
18
16
40
36
56
46
54
28
60
2nd cycle
3rd cycle
4th cycle
Co
Ni
100
15
0
6
12
18
24
t / h
Cu
100
8
Zn
Blank
6
23
9
68
no cat-
alyst
The same oxidation of styrene was performed several times with
the same amounts of chemicals and with the same conditions using
the same catalysts (Co, Cu and Mn ferrites) from the same batch to
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Page 12 of 16
them suitable for catalytic applications. The ferrites synthesized
100
80
60
40
20
0
(c) CuFe2O4
1
2
3
4
5
6
7
8
showed promising functional properties such as superparamag-
netism, pointed out with the SQUID (superconductive quantum in-
terference device), and catalytic properties for oxidation of styrene
in organic medium, determined following the reactions with the
GC-MS (gas chromatography coupled with mass spectrometry).
Three of the six ferrites synthesized (Mn, Co and Cu ferrites), well
dispersible in organic media, were proven to be catalytically active
and reached 100% of conversion of styrene after 24 h. The ferrites
can be furthermore, due to the mentioned magnetic properties, eas-
ily recovered from the reaction medium with a magnet and reused
for several cycles without losing activity, pointing out a promising
recyclability of the material.
1 min
9
1st cycle
2nd cycle
3rd cycle
4th cycle
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ASSOCIATED CONTENT
The supporting information is available free of charge via the Internet at
http://pubs.acs.org.
0
6
12
18
24
t / h
Figure 13. Recyclability of the catalysts, ferrites produced by miniemul-
sion–solvothermal route. Conversion after different reactions with the
same catalyst are reported (the solid line is guiding the eye): (a)
Fe3Mn3O8 (b) CoFe2O4 (c) CuFe2O4. In the insets, the pictures of the
catalysts recovered with a magnet are reported.
Comparisons among the diffractograms of manganese, nickel and zinc
ferrites synthesized in miniemulsion–solvothermal route and the refer-
ence patterns; Rietveld refinement of the diffraction data of the ferrites
produced in miniemulsion–solvothermal conditions; interfacial tension
measurement; ICP-MS measurements on the iron-rich zinc ferrites;
structural parameters tables determined by fitting of the EXAFS curves
registered at Zn and Fe K-edges for the Zn ferrites samples; diffracto-
grams of the ferrites produced with different reaction times; TEM im-
ages of the ferrites produced in miniemulsion–solvothermal conditions
with sample preparation by embedding the material in a matrix of treha-
lose and negatively staining with uranyl acetate; adsorption–desorption
isotherms and pore size distribution of the ferrites produced by minie-
mulsion–solvothermal combination; adsorption–desorption isotherms
and pore size distribution of the iron-rich zinc ferrites produced with all
the four routes; comparison between the surface areas measured and the
ones calculated from TEM average particle size; fitting of the magneti-
zation curves and related results.
Conclusions
In this work, we successfully explored, as a “proof of concept”, the
synergy between miniemulsion and solvothermal routes, pursuing
unconventional synthesis conditions for the preparation of crystal-
line first row transition metal ferrites. In particular, the confinement
of “nanoreactors” provided by miniemulsion droplets and the non-
standard temperature and pressure conditions provided by the sol-
vothermal route were proven to be very effective to achieve crystal-
line metal ferrites at lower temperature (i.e. 80 °C) than usually re-
quired and without any post-synthesis thermal treatment. X-ray dif-
fraction (XRD) demonstrated the crystallinity of the six spinel fer-
rites synthesized and that the zinc ones obtained with miniemulsion
at ambient pressure and under bulk solution conditions either at am-
bient pressure or under solvothermal conditions did not result in
comparatively highly crystalline ferrites, thus outlining the relevance
of the combined synthetic approach. X-ray absorption spectroscopy
(XAS) evidenced as Zn ferrites produced in miniemulsion are in-
verse spinels, whereas the samples produced in bulk, especially in the
autoclave, are direct spinels. This is a relevant result since, being the
composition equal, a switch from direct to partially inverse spinel
could be accomplished. The reason for this occurrence could be clar-
ified in future with in situ and time-resolved studies. The evolution
of the crystallization of the ferrites was followed performing ex situ
XRD on samples produced with different reaction times. A very no-
ticeable outcome was the crystallization of the targeted compound
already after 3 h of reaction when the combination of miniemulsion
and solvothermal route, was applied, showing the high potential of
this approach in pursuing more sustainable synthetic routes. Ele-
mental measurements by ICP-MS assessed that the stoichiometries
were not the expected one, MFe2O4, in the case of the Mn, Ni and
Zn ferrite. In particular, Fe3Mn3O8, (Zn0.664Fe0.336) (Fe1.934Zn0.066)O4
and a mixture between NiFe2O4 and Ni(OH)2 were selected as com-
patible compounds, which are in any case all spinel ferrites. Meas-
urements of nitrogen physisorption at critical temperature and the
corresponding calculations with the BET method displayed that the
materials are mesoporous with a quite high surface area, making
AUTHOR INFORMATION
Corresponding Author
Dr. Silvia Gross
* E-mail: silvia.gross@unipd.it
Dr. Rafael Muñoz-Espí
* E-mail: rafael.munoz@uv.es
Author Contributions
The manuscript was written through contributions of all authors. All
authors have given approval to the final version of the manuscript.
ACKNOWLEDGMENTS
The authors thank Michael Steiert, Gunnar Glaßer and Dr. Noemí En-
cinas Garcia for their assistance with XRD, SEM and interfacial tension
measurement, respectively. Beatriz Ma, Saman Ghasimi and Wei Huang
are thanked for their assistance with BET and GC-MS measurements.
Cesare Benedetti is acknowledged for helpful discussions. Elettra Sin-
crotrone Trieste is gratefully acknowledged for providing access to the
synchrotron facility and to the beamline XAFS. Dr. Luca Olivi, Dr. Giu-
liana Aquilanti and Dr. Clara Guglieri, of the XAFS beamline at Elettra
synchrotron facility (Trieste, Italy), are kindly acknowledged for their
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Chemistry of Materials
20.
Crystals. Angew. Chem., Int. Ed. 2004, 43, 4002-4011.
21. Chinnasamy, C. N.; Narayanasamy, A.; Ponpandian, N.;
Chattopadhyay, K.; Shinoda, K.; Jeyadevan, B.; Tohji, K.; Nakatsuka, K.;
Furubayashi, T.; Nakatani, I., Mixed spinel structure in nanocrystalline
NiFe2O4. Phys. Rev. B: Condens. Matter 2001, 63, 184108.
Braga, D.; Grepioni, F., Reactions Between or Within Molecular
technical support and helpful discussions. RME thanks the financial sup-
port from the Spanish Ministry of Economy and Competitiveness
through a Ramón y Cajal grant (grant No. RYC-2013-13451).
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REFERENCES
22.
Harris, V. G.; Fatemi, D. J.; Cross, J. O.; Carpenter, E. E.;
1.
ed.; Butterworth-Heinemann: 1998.
Greenwood, N. N.; Earnshaw, A., Chemistry of the Elements. 2nd
Browning, V. M.; Kirkland, J. P.; Mohan, A.; Long, G. J., One-step processing
of spinel ferrites via the high-energy ball milling of binary oxides. J. Appl.
Phys. 2003, 94, 496-501.
2.
Kung, H. H., Transition Metal Oxides: Surface Chemistry and
Schubert, U.; Hüsing, N., Synthesis of inorganic materials, 2nd
Catalysis. Elsevier: Amsterdam, 1989.
23.
Ed. Wiley-VCH: Weinheim, 2005.
9
3.
4.
Dionne, G. F., Magnetic oxides, 1st Ed. Springer: London, 2009.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Coey, J. M. D., Magnetism and Magnetic Materials, 1st Ed.
24.
Diodati, S.; Dolcet, P.; Casarin, M.; Gross, S., Pursuing the
Cambridge University Press: New York, 2010.
5. Muscas, G.; Yaacoub, N.; Concas, G.; Sayed, F.; Hassan, R. S.;
Greneche, J. M.; Cannas, C.; Musinu, A.; Foglietti, V.; Casciardi, S.;
Sangregorio, C.; Peddis, D., Evolution of the magnetic structure with
chemical composition in spinel iron oxide nanoparticles. Nanoscale 2015, 7,
13576-13585.
Crystallization of Mono- and Polymetallic Nanosized Crystalline Inorganic
Compounds by Low-Temperature Wet-Chemistry and Colloidal Routes.
Chem. Rev. (Washington, DC, U. S.) 2015, 115, 11449-11502.
25.
Mao, Y.; Park, T.-J.; Zhang, F.; Zhou, H.; Wong, S. S.,
Environmentally Friendly Methodologies of Nanostructure Synthesis. Small
2007, 3, 1122-1139.
6.
Fernández-García, M.; Martínez-Arias, A.; Hanson, J. C.;
26.
Javadi, S.; Masoudpanah, S. M.; Zakeri, A., Conventional versus
Rodriguez, J. A., Nanostructured Oxides in Chemistry:ꢀ Characterization
and Properties. Chem. Rev. (Washington, DC, U. S.) 2004, 104, 4063-4104.
microwave combustion synthesis of CoFe2O4 nanoparticles. J. Sol-Gel Sci.
Technol. 2016, 79, 176-183.
7.
Karimi, B.; Mansouri, F.; Mirzaei, H. M., Recent Applications of
27.
Niederberger, M.; Antonietti, M., Nonaqueous Sol–Gel Routes
Magnetically Recoverable Nanocatalysts in C-C and C-X Coupling
Reactions. ChemCatChem 2015, 7, 1736-1789.
to Nanocrystalline Metal Oxides. In Nanomaterials Chemistry, Rao, C. N.
R.; Müller, A.; Cheetham, A. K., Eds. Wiley-VCH Verlag GmbH & Co.
KGaA: 2007; pp 119-137.
8.
Hudson, R.; Feng, Y.; Varma, R. S.; Moores, A., Bare magnetic
nanoparticles: sustainable synthesis and applications in catalytic organic
transformations. Green Chem. 2014, 16, 4493-4505.
28.
Bilecka, I.; Kubli, M.; Amstad, E.; Niederberger, M.,
Simultaneous formation of ferrite nanocrystals and deposition of thin films
via a microwave-assisted nonaqueous sol-gel process. J. Sol-Gel Sci.
Technol. 2011, 57, 313-322.
9.
Casbeer, E.; Sharma, V. K.; Li, X.-Z., Synthesis and
photocatalytic activity of ferrites under visible light: A review. Sep. Purif.
Technol. 2012, 87, 1-14.
29.
Pinna, N.; Niederberger, M., Surfactant-free nonaqueous
10.
Sreekumar, K.; Sugunan, S., Ferrospinels based on Co and Ni
synthesis of metal oxide nanostructures. Angew. Chem., Int. Ed. 2008, 47,
5292-5304.
prepared via a low temperature route as efficient catalysts for the selective
synthesis of o-cresol and 2,6-xylenol from phenol and methanol. J. Mol.
Catal. A: Chem. 2002, 185, 259-268.
30.
Technology. Noyes Publications, Park Ridge, New Jersey, U.S.A: 2001.
31. Modeshia, D. R.; Walton, R. I., Solvothermal synthesis of
Byrappa, K.; Yoshimura, M., Handbook of Hydrothermal
11.
Shi, F.; Tse, M. K.; Pohl, M.-M.; Radnik, J.; Brückner, A.; Zhang,
S.; Beller, M., Nano-iron oxide-catalyzed selective oxidations of alcohols and
olefins with hydrogen peroxide. J. Mol. Catal. A: Chem. 2008, 292, 28-35.
perovskites and pyrochlores: crystallisation of functional oxides under mild
conditions. Chem. Soc. Rev. 2010, 39, 4303-4325.
12.
PalDey, S.; Gedevanishvili, S.; Zhang, W.; Rasouli, F., Evaluation
32.
Cushing, B. L.; Kolesnichenko, V. L.; O'Connor, C. J., Recent
of a spinel based pigment system as a CO oxidation catalyst. Appl. Catal., B
2005, 56, 241-250.
Advances in the Liquid-Phase Syntheses of Inorganic Nanoparticles. Chem.
Rev. (Washington, DC, U. S.) 2004, 104, 3893-3946.
13.
Guin, D.; Baruwati, B.; Manorama, S. V., A simple chemical
33.
Lu, A.-H.; Salabas, E. L.; Schüth, F., Magnetic Nanoparticles:
synthesis of nanocrystalline AFe2O4 (A = Fe, Ni, Zn): An efficient catalyst
for selective oxidation of styrene. J. Mol. Catal. A: Chem. 2005, 242, 26-31.
Synthesis, Protection, Functionalization, and Application. Angew. Chem.,
Int. Ed. 2007, 46, 1222-1244.
14.
Zhang, D.-H.; Li, H.-B.; Li, G.-D.; Chen, J.-S., Magnetically
34.
Sanchez-Dominguez, M.; Pemartin, K.; Boutonnet, M.,
recyclable Ag-ferrite catalysts: general synthesis and support effects in the
epoxidation of styrene. Dalton Trans. 2009, 10527-10533.
Preparation of inorganic nanoparticles in oil-in-water microemulsions: A
soft and versatile approach. Curr. Opin. Colloid Interface Sci. 2012, 17, 297-
305.
15.
Florea, M.; Alifanti, M.; Parvulescu, V. I.; Mihaila-Tarabasanu,
D.; Diamandescu, L.; Feder, M.; Negrila, C.; Frunza, L., Total oxidation of
toluene on ferrite-type catalysts. Catal. Today 2009, 141, 361-366.
35.
Zarur, A. J.; Ying, J. Y., Reverse microemulsion synthesis of
nanostructured complex oxides for catalytic combustion. Nature 2000, 403,
65-67.
16.
Tsoncheva, T.; Manova, E.; Velinov, N.; Paneva, D.; Popova, M.;
Kunev, B.; Tenchev, K.; Mitov, I., Thermally synthesized nanosized copper
ferrites as catalysts for environment protection. Catal. Commun. 2010, 12,
105-109.
36.
Miniemulsions. Adv. Mater. (Weinheim, Ger.) 2001, 13, 765-768.
37. Landfester, K., Miniemulsions for Nanoparticle Synthesis. Top.
Curr. Chem. 2003, 227, 75-123.
Landfester, K., The Generation of Nanoparticles in
17.
Dillert, R.; Taffa, D. H.; Wark, M.; Bredow, T.; Bahnemann, D.
W., Research Update: Photoelectrochemical water splitting and
photocatalytic hydrogen production using ferrites (MFe2O4) under visible
light irradiation. APL Mater. 2015, 3, 104001.
38.
Muñoz-Espí, R.; Weiss, C. K.; Landfester, K., Inorganic
nanoparticles prepared in miniemulsion. Curr. Opin. Colloid Interface Sci.
2012, 17, 212-224.
18.
Kim, J. H.; Kim, J. H.; Jang, J.-W.; Kim, J. Y.; Choi, S. H.; Magesh,
39.
Muñoz-Espí, R.; Mastai, Y.; Gross, S.; Landfester, K., Colloidal
G.; Lee, J.; Lee, J. S., Awakening Solar Water-Splitting Activity of ZnFe2O4
Nanorods by Hybrid Microwave Annealing. Adv. Energy Mater. 2015, 5,
1401933.
systems for crystallization processes from liquid phase. CrystEngComm
2013, 15, 2175-2192.
40.
chemical solutions. Chem. Soc. Rev. 2014, 43, 2187-2199.
41. Rabenau, A., The Role of Hydrothermal Synthesis in Preparative
Chemistry. Angew. Chem., Int. Ed. Engl. 1985, 24, 1026-1040.
Einarsrud, M.-A.; Grande, T., 1D oxide nanostructures from
19.
Zhu, Y.; Stubbs, L. P.; Ho, F.; Liu, R.; Ship, C. P.; Maguire, J. A.;
Hosmane, N. S., Magnetic Nanocomposites: A New Perspective in
Catalysis. ChemCatChem 2010, 2, 365-374.
13
ACS Paragon Plus Environment

Chemistry of Materials
Page 14 of 16
42.
Conditions, 1st Ed. Consultants Bureau: New York, 1973.
43. Renz, P.; Kokkinopoulou, M.; Landfester, K.; Lieberwirth, I.,
Imaging of Polymeric Nanoparticles: Hard Challenge for Soft Objects.
Macromol. Chem. Phys. 2016, 217, 1879-1885.
Lobachev, A. N., Crystallization Processes under Hydrothermal
nanoclusters through emulsion electrospinning. Chem. Commun.
(Cambridge, U. K.) 2015, 51, 3758-3761.
1
2
3
4
5
6
7
62.
Zuo, X.; Yang, A.; Vittoria, C.; Harris, V. G., Computational
study of copper ferrite (CuFe2O4). J. Appl. Phys. 2006, 99, 08M909/1-3.
63.
of nanostructured CuFe2O4. J. Phys.: Condens. Matter 1999, 11, 4063.
64. Guo, P.; Cui, L.; Wang, Y.; Lv, M.; Wang, B.; Zhao, X. S., Facile
Jiang, J. Z.; Goya, G. F.; Rechenberg, H. R., Magnetic properties
44.
Ravel, B.; Newville, M., ATHENA, ARTEMIS, HEPHAESTUS:
data analysis for X-ray absorption spectroscopy using IFEFFIT. J.
Synchrotron Rad. 2005, 12, 537-541.
Synthesis of ZnFe2O4 Nanoparticles with Tunable Magnetic and Sensing
Properties. Langmuir 2013, 29, 8997-9003.
45.
Zhao, D.; Wu, X.; Guan, H.; Han, E., Study on supercritical
hydrothermal synthesis of CoFe2O4 nanoparticles. J. Supercrit. Fluids 2007,
42, 226-233.
65.
Gonzalez-Sandoval, M. P.; Beesley, A. M.; Miki-Yoshida, M.;
8
9
Fuentes-Cobas, L.; Matutes-Aquino, J. A., Comparative study of the
microstructural and magnetic properties of spinel ferrites obtained by co-
precipitation. J. Alloys Compd. 2004, 369, 190-194.
46.
Techniques, Third Edition. CRC Press: 2015.
47. Antoine, C., Tensions des vapeurs; vouvelle relation entre les
tensions et les températures. C. R. Acad. Sci. 1888, 107, 681-684.
48. Diodati, S.; Nodari, L.; Natile, M. M.; Caneschi, A.; de Julián
McClements, D. J., Food Emulsions: Principles, Practices, and
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
66.
Zi, Z.; Sun, Y.; Zhu, X.; Yang, Z.; Dai, J.; Song, W., Synthesis and
magnetic properties of CoFe2O4 ferrite nanoparticles. J. Magn. Magn. Mater.
2009, 321, 1251-1255.
67.
Styrene Epoxidation with Films of MnO2 Nanoparticles and H2O2. J. Am.
Chem. Soc. 2004, 126, 7676-7682.
68.
an efficient catalyst for oxidation of alkenes. Sci. Iran. 2012, 19, 1991-1995.
69. Pardeshi, S. K.; Pawar, R. Y., SrFe2O4 complex oxide an effective
and environmentally benign catalyst for selective oxidation of styrene. J.
Mol. Catal. A: Chem. 2011, 334, 35-43.
70.
in selective oxidation of styrene over fine crystallite spinel-type CaFe2O4
complex oxide catalyst. Mater. Res. Bull. 2010, 45, 609-615.
71.
Fernández, C.; Hoffmann, C.; Kaskel, S.; Lieb, A.; Di Noto, V.; Mascotto, S.;
Saini, R.; Gross, S., Coprecipitation of Oxalates: An Easy and Reproducible
Wet-Chemistry Synthesis Route for Transition-Metal Ferrites. Eur. J. Inorg.
Chem. 2014, 2014, 875-887.
Espinal, L.; Suib, S. L.; Rusling, J. F., Electrochemical Catalysis of
Kooti, M.; Afshari, M., Magnetic cobalt ferrite nanoparticles as
49.
Diodati, S.; Pandolfo, L.; Caneschi, A.; Gialanella, S.; Gross, S.,
Green and low temperature synthesis of nanocrystalline transition metal
ferrites by simple wet chemistry routes. Nano Res. 2014, 7, 1027-1042.
50.
Rossmanith, R.; Weiss, C. K.; Geserick, J.; Hüsing, N.; Hörmann,
U.; Kaiser, U.; Landfester, K., Porous Anatase Nanoparticles with High
Specific Surface Area Prepared by Miniemulsion Technique. Chem. Mater.
2008, 20, 5768-5780.
Pardeshi, S. K.; Pawar, R. Y., Optimization of reaction conditions
51.
Niederberger, M.; Pinna, N., Metal Oxide Nanoparticles in
Rayati, S.; Zakavi, S.; Koliaei, M.; Wojtczak, A.; Kozakiewicz, A.,
Organic Solvents - Synthesis, Formation, Assembly and Applications.
Springer: New York, 2009.
Electron-rich salen-type Schiff base complexes of Cu(II) as catalysts for
oxidation of cyclooctene and styrene with tert-butylhydroperoxide: A
comparison with electron-deficient ones. Inorg. Chem. Commun. 2010, 13,
203-207.
52.
Singh, I.; Landfester, K.; Chandra, A.; Munoz-Espi, R., A new
approach for crystallization of copper(ii) oxide hollow nanostructures with
superior catalytic and magnetic response. Nanoscale 2015, 7, 19250-19258.
53.
Varol, H. S.; Álvarez-Bermúdez, O.; Dolcet, P.; Kuerbanjiang, B.;
Gross, S.; Landfester, K.; Muñoz-Espí, R., Crystallization at Nanodroplet
Interfaces in Emulsion Systems: A Soft-Template Strategy for Preparing
Porous and Hollow Nanoparticles. Langmuir 2016.
54.
Carta, D.; Marras, C.; Loche, D.; Mountjoy, G.; Ahmed, S. I.;
Corrias, A., An X-ray absorption spectroscopy study of the inversion degree
in zinc ferrite nanocrystals dispersed on a highly porous silica aerogel matrix.
J. Chem. Phys. 2013, 138, 054702.
55.
Boubnov, A.; Lichtenberg, H.; Mangoldc, S.; Grunwaldt, J.-D.,
Identification of the iron oxidation state and coordination geometry in iron
oxide- and zeolite-based catalysts using pre-edge XAS analysis. J.
Synchrotron Rad. 2015, 22, 410-426.
56.
Wilke, M.; Farges, F.; Petit, P.-E.; Brown, G. E.; Martin, F.,
Oxidation state and coordination of Fe in minerals: An Fe K-XANES
spectroscopic study. Am. Mineral. 2001, 86, 714-730.
57.
Waychunas, G. A.; Brown, G. E.; Apted, M. J., X-ray K-edge
absorption spectra of Fe minerals and model compounds: II. EXAFS. Phys.
Chem. Miner. 1986, 13, 31-47.
58.
Nilsen, M. H.; Nordhei, C.; Ramstad, A. L.; Nicholson, D. G.;
Poliakoff, M.; Cabañas, A., XAS (XANES and EXAFS) Investigations of
Nanoparticulate Ferrites Synthesized Continuously in Near Critical and
Supercritical Water. J. Phys. Chem. C 2007, 111, 6252-6262.
59.
Scherrer, P., Bestimmung der Groesse und der inneren Struktur
von Kolloidteilchen mittels Roentgenstrahlen. Göttinger Nachrichten
Math. Phys. 1918, 2, 98-100.
60.
Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, l.; Pierotti,
R. A.; Rouquerol, J.; Siemieniewska, T., REPORTING PHYSISORPTION
DATA FOR GAS/SOLID SYSTEMS with Special Reference to the
Determination of Surface Area and Porosity. Pure Appl. Chem. 1985, 57,
603-619.
61.
Bannwarth, M. B.; Camerlo, A.; Ulrich, S.; Jakob, G.; Fortunato,
G.; Rossi, R. M.; Boesel, L. F., Ellipsoid-shaped superparamagnetic
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