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Maghemite (γ-Fe 2 O 3)/goethite (α-FeOOH) composite nanometric particles were prepared through an one-pot synthesis protocol, involving a chemical precipitation process performed under ultrasonication of iron sulfate in an aqueous basic solution. As proved by infrared spectroscopy, X-ray diffraction and electronic microscopy, the resulted ferromagnetic composite is composed of goethite acicular nanoparticles of average diameters less than 10 nm and lengths around or higher than 100 nm, surrounded by maghemite spherical nanoparticles with a diameter less than 100 nm. The composite nanoparticles functionalized with OH groups belonging to goethite showed also good magnetic properties specific to maghemite (the saturation magnetization is varying from 59.6 emu/g at room temperature to 75.5 emu/g at low temperature).
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ACADEMIA ROMÂNĂ
Revue Roumaine de Chimie
http://web.icf.ro/rrch/
Rev. Roum. Chim.,
2017, 62(2), 131-138
FERROMAGNETIC MATERIALS OBTAINED THROUGH
ULTRASONICATION.
1. MAGHEMITE/GOETHITE NANOCOMPOSITES
Razvan ROTARU,a Petrisor SAMOILA,a Nicoleta LUPU,b Marian GRIGORASb and
Valeria HARABAGIUa*
a “Petru Poni” Institute of Macromolecular Chemistry, 41A, Aleea Grigore Ghica Vodă, 700487 Iaşi, Roumania
b National Institute of Research and Development for Technical Physics, 47, Mangeron Boulevard, 700050, Iaşi, Roumania
Received July 26, 2016
Maghemite (γ-Fe2O3)/goethite (α-FeOOH) composite nanometric
particles were prepared through an one-pot synthesis protocol,
involving a chemical precipitation process performed under
ultrasonication of iron sulfate in an aqueous basic solution. As
proved by infrared spectroscopy, X-ray diffraction and electronic
microscopy, the resulted ferromagnetic composite is composed of
goethite acicular nanoparticles of average diameters less than 10
nm and lengths around or higher than 100 nm, surrounded by
maghemite spherical nanoparticles with a diameter less than 100
nm. The composite nanoparticles functionalized with OH groups
belonging to goethite showed also good magnetic properties
specific to maghemite (the saturation magnetization is varying
from 59.6 emu/g at room temperature to 75.5 emu/g at low
temperature).
INTRODUCTION*
Ferromagnetic monodispersed or polydispersed
nanoparticles have attracted increasing attention
due to their good biocompatibility, low toxicity,
saturation magnetization higher than 60 emu/g,
superparamagnetic properties and easy preparation
process.1,2 They have considerable potential for use
in the biomedical industry, such as targeted drug
delivery, hyperthermia treatment, cell separation,
magnetic resonance imaging, immunoassays and
the separation of biochemical products.3-5 They are
also useful for environmental processes, such as
the treatment of wastewater.6

* Corresponding author: hvaleria@icmpp.ro
As a member of iron oxide family, maghemite
(γ-Fe2O3) - having a spinel structure with two
magnetically nonequivalent interpenetrating
sublattices - exhibits a strong magnetic behavior.6
Several methods can be used to synthesize
maghemite nanoparticles: sol-gel combustion
process, thermal-decomposition of magnetite
(Fe3O4) nanoparticles, solid-state synthesis, arc-
discharge, mechanical grinding, laser ablation,
microemulsions, co-precipitation and high
temperature decomposition of organic precursors.7-
11 On the other hand, lath- and rod-shaped γ-
lepidocrocite, α-goethite and β-akaganéite
polymorphs of iron oxyhydroxide (FeOOH) show
132 Razvan Rotaru et al.
weaker magnetic properties as compared to
maghemite, but remains of considerable
importance in redox-sensitive environments, being
used as magnetic recording, ferrofluids, magnetic
resonance imaging (MRI) fluids, abrasives or
catalysts.6 Moreover, because of the presence of
the hydroxyl groups, iron oxyhydroxide shows an
increased reactivity as compared to other oxide
materials. Goethite was obtained by oxidation at
temperatures between 20 and 70 oC,12-13 while all
processes described for the preparation of
magnetite request at least one step to be performed
at temperatures higher than 300 oC, Thus, owing
the differences in their preparation conditions, to
obtain both oxides into a single composite, new
routes should be proposed.
Recently, ultrasonic-assisted processes have been
a topic of intense investigation as a low-cost, simple
and effective preparation method for micro or nano-
structures. The well known acoustic cavitation,
thermal, microjets and shockwaves phenomena
occurring during ultrasonication14,15 contribute to the
formation of metal oxide particles of controllable
morphology, rapid crystallization and high purity.
Ultrasonication at high-intensity has been already
used for the synthesis of ferromagnetic materials.16-18
We already proved that barium titanate submicron
particles and viscose/barium titanate composites can
be prepared at lower duration and temperature of the
thermal treatment by a process involving
ultrasonnication.19
Herein, we present an ultrasound-based method
which yields maghemite/goethite ferromagnetic
composite nanoparticles in a single process. Iron
sulfate was used as a source of iron ions in the
presence of sodium hydroxide.
RESULTS AND DISCUSSION
1. Influence of the preparation conditions
on the structure and morphology
of maghemite goethite composites
Ferromagnetic maghemite-like particles were
prepared by a more energy-effective method where
the previously reported high temperature treatments9-
11 were replaced by ultrasonication. Ferrous sulfate
known to undergo oxidation in aqueous alkaline
media was used as iron source. The ultrasonication
has been found much more effective for activation of
the precursors, gaining thus a time-span reduction
and also no need for thermal treatment. As proved
below, the proposed reaction conditions allow the
simultaneous preparation of maghemite/goethite
composite nanoparticles.
To optimize the procedure, several attempts
have been made by varying the sonication duration
between 15 and 60 min. Three samples, M/Ga,
M/Gb and M/G, exemplifying different preparation
conditions are shown in Table 1. The resulted
powder samples of colors depending on the
sonication duration were characterized by FTIR,
SEM, TEM and X-ray diffraction to follow the
influence of the preparation conditions on the
conversion of the reagents into maghemite/goethite
composite. Finally, the ultrasound time-span was
established at 60 minutes that represent the lower
limit for the full conversion of the reagents to the
desired ferromagnetic nanoparticles.
FT-IR analysis
The structures of maghemite/goethite
composite powder (M/G) and of the partially
converted samples (M/Ga and M/Gb) were first
assessed through infrared spectroscopy. The
assignment of the characteristic bands (Table 2)
was made based on literature information6,20-24 and
in correlation with XRD analysis.
As one may see from data in Table 2, for M/Ga
sample, most of the vibrations are specific to
sulfate groups. Goethite was also detected by the
very sharp band located at 879 cm-1, while a very
large and intense absorption band attributed to the
stretching of different OH groups and to water
appeared between 2890 and 3887 cm-1. The
characteristic absorptions of M/Ga sample clearly
indicate that the ultrasonication for 15 minutes
(about 30 kJ dissipated) does not provide the
energy needed for the development of maghemite
crystallites and for full transformation of the iron
sulfate precursor. M/Gb sample obtained at 30
minutes of sonication is characterized by only a
medium intensity band at 870 cm-1 attributed to
goethite, while sulfate bands are still present and
the large OH band was replaced by shorter and
more resolved bands located at 2903 and at wave
numbers higher than 3000 cm-1. Moreover a strong
modification of the spectral absorption in the
region 700-500 cm-1 was noticed that could be
attributed to the superposed maghemite and sulfate
bands. By the contrary, the spectrum of M/G
sample showed only characteristic bands for
maghemite and goethite. The band at 579 cm-1 of a
higher intensity, attributed to ν(Fe-O) (intrinsic
stretching) indicates the formation of γ-Fe2O3 as
Ferromagnetic materials 133
one of the phase of the composite. The bands at
795 and 885 cm-1 (deformational modes of
hydroxyl groups, δ-OH out of plane and in plane),
typical for goethite are well-defined and sharp
indicating also the presence of goethite. However,
adsorbed water (1628 cm-1) and 2–
4
SO (light
traces) are also detectable in the spectrum of M/G
sample. One should mention that sulfate traces
around 1250-1450 cm-1 are present in many
synthesis where FeSO4 is the iron source.
Table 1
Composite samples prepared under different sonication durations
Sample Code Ultrasonic time-span
[minutes]
Energy dissipated
[kJ]
Conversion of the
reagents
Powder product color
M/Ga 15 30,5 Incomplete
transformation
reddish brown
M/Gb 30 59 Incomplete
transformation
reddish-yellow brown
M/G 60 109 Full transformation dark brown
Table 2
Interpretation of spectral bands in FTIR spectra
Sample Band Position [cm-1] Vibration Mode, Interpretation
M/Ga 420, 555, 636, 701 νab SO4: asymmetric bending of SO4 group20
783 δ-OH out of plane: goethite6 p.144, 13
809 νas SO4 asymmetric stretching of SO4 group20
879 δ(OH) in plane: goethite6 p.144, 13
970 νss (SO4): symmetric stretching of sulfates21
1156 ν3(SO42-): sulfate absorptions22,23
1435 ν(SO4)6,22
1642 δ(OH): absorbed water6,22
2790 - 3887, 2861, 2950 OH stretching: surface hydroxyl groups and H2O6 p.143-144
M-Gb 519, 687 νab SO4: asymmetric bending of SO4 group20
870 δ(OH) in plane: goethite6 p.144,13
972 νas SO4 asymmetric stretching of SO4 group20
1157 SO4: adsorbed sulfate groups22,23
1412, 1445 ν(SO4)6,22
1665 δ(OH): absorbed water6,22
2903, 3057 H-OH: intramolecular frequencies24
3323, 3441, 3626, 3543, 3859 OH stretching: surface hydroxyl groups and H2O6 p.143-144
M-G 399, 453 τOH : goethite6 p.143
579 ν(Fe-O): intrinsic stretching: maghemite6 p.146
795, 885 δ-OH out of plane and in plabe, repectively: goethite6 p.144, 13
1047, 1117 Fe-O asymmetric stretching: goethite24
1259, 1369, 1429 ν(SO4): light traces, from synthesis6,22
1628 δ(OH): absorbed water6,22
2183, 2361 possible CO2 from measuring conditions, light traces
2849 ν(OH): goethite or lepidocrocite6 p.143-144
2920 H-OH: intramolecular frequencies24
3152 ν(OH): bulk hydroxyl stretching in goethite structures6 p.143
3435, 3667, 3738, 3842, 3858,
3871, 3890
OH stretching: surface hydroxyl groups and H2O 6 p.143-144
Electron microscopy
The structures and dimensions of the prepared
samples were further analyzed by SEM and TEM
analysis. Fig. 1 shows the SEM micrographs of
M/Ga, M/Gb and M/G samples.
The SEM micrograph for M/Ga shows a porous
surface built by large agglomeration of not
individualized particles indicating that important
inter-particle interactions are characteristic for this
system. The particles become more individualized
and the dimensions decreased as the span-time of
sonication increases. Thus, polydisperse micrometric
particles can be observed for M/Gb, while the size
of the M/G particles became very small. As the
134 Razvan Rotaru et al.
SEM image of M/G composite is not concluding,
the sample was analyzed by TEM (Fig. 3).
Maghemite particles of approximately spherical
shape and different sizes, generally less than 100 nm,
some of them being less than 50 nm are
surrounding the goethite acicular formations with
lengths around or higher than 100 nm and
diameters of around 10 nm. The TEM images
clearly indicate the formation of maghemite/
goethite composite for the sample prepared under
sonication for 60 minutes.
Fig. 1 – SEM micrographs of M/Ga, M/Gb and M/G samples.
Fig. 2 – TEM images of M/G composite.
X-Ray diffraction
The recorded XRD pattern of M/G sample is
shown in Fig. 3. All the peaks observed in the
XRD profile correspond to maghemite (according
to JCPDS card number 39-1346) and goethite
(according to JCPDS card number 03-0251). The
diffraction angles and planes as well as the
interplanar distances (calculated with Bragg
equation (2dsinθ = nλ)) are given in Table 3. Thus,
the XRD analysis is in strong correlation with the
IR spectroscopy and TEM observations and
confirms the formation of maghemite/goethite
composite. These results demonstrate that
ultrasonication of the iron sulfate in basic medium
of sodium hydroxide allowed simultaneous
formation of maghemite and goethite structures at
room temperature.
Ferromagnetic materials 135
Fig. 3 – Diffraction pattern of M/G composite.
Table 3
Diffraction angles, diffraction planes (Miller planes) and interplanar distances
2θ [°] Assignment Miller index
Interplanar Distances [nm]
21.26 Goethite 101 0,41
30.22 Maghemite 213 0,34
35.62 Maghemite 220 0,29
43.24 Maghemite 313 0,25
53.90 Maghemite 400 0,20
57.20 Maghemite 426 0,17
62.94 Maghemite 513 0,16
2. Mechanism of simultaneous synthesis
of maghemite/goethite composite
The following mechanism of the one-pot
preparation of maghemite/goethite composites under
ultrasonication of iron sulphate-sodium hydroxide
mixed solutions in air could be envisaged by
correlating the information obtained from IR, SEM,
TEM and XRD analysis of samples obtained in
different conditions (Scheme 1). Ferrous hydroxide is
first obtained through the reaction between iron
sulfate and sodium hydroxide. In the presence of
oxygen and under ultrasonication, this one is
oxidized to iron oxide hydroxide that further
undergoes the dehydration reaction resulting in the
formation of maghemite. Under ultrasonication for
60 minutes only partial transformation of goethite
into maghemite is achieved. Thus, the resulted
powder contains both iron compounds.
Transformation of iron sulfate in iron oxyhydroxide
FeSO4 + 2 NaOH Fe(OH)2 + Na2SO4 + H2O
Fe(OH)2 + 1/2O2 FeOOH (lepidocrocite or goethite) + H2O
Partial dehydration of iron oxyhydroxide to maghemite
2FeOOH γ-Fe2O3 (maghemite) + H2O
Scheme 1 – Chemistry of simultaneous preparation of maghemite/
goethite composites under ultrasonication at room temperature.
3. Magnetic properties
The magnetic properties of the as-prepared
maghemite/goethite nanocomposite (M/G) were
monitored by measuring the hysteresis loops
(magnetization curves) at different temperatures
(10, 20, 30, 40, 50, 100, 200 and 300 K) (Fig. 4).
The sample shows narrow hysteresis loops. The
saturation values of the magnetic moment are of
0.45 emu at 300 K and 0.57 emu at 10 K (for a
mass of 7.545 mg analyzed).
The dependence of the magnetization on
temperature was measured using zero field cooling
(ZFC) and field cooling (FC) procedures (Fig. 5),
between 10 and 250 K, at decrement and increment
rates of 2 K/min. On this temperature interval, ZFC
and FC curves show a characteristic feature of
ferromagnetic materials, i.e., a slow growth of the
magnetic moment at low temperatures for ZFC and
a much smaller variation for FC on the entire
temperature range. No blocking temperature,
specific to paramagnetic materials, but only a
transition temperature at 144 K (-129 °C) was
observed. Above this temperature an acceleration
of the magnetization was noticed.
136 Razvan Rotaru et al.
Fig. 4 – Evolution of the magnetic hysteresis curves for M/G composites as a function of temperature.
Fig. 5 – Thermomagnetic curves (temperature dependence of magnetization measured using standard zero field cooling (ZFC)
and field cooling (FC) procedure) for M/G.
The variation of the coercive field and of Mr/MS
ratio (Mr: remanent magnetization, Ms: saturation
magnetization) with temperature is shown in
Fig. 6. Ms, Mr and Hc are determined from the
hysteresis curves presented in Fig. 4, where Ms is
the saturation value of the magnetic moment, Mr is
the value of the magnetic moment at H = 0 and Hc
is the coercive field value, i.e. the field at which
the magnetization is passing through 0. MS was
found to vary from 59.6 emu/g at room
temperature (~300 K) to 75.5 emu/g at low
temperature (10 K), lower as compared to 84
emu/g, a maximum value found for pure
maghemite at room temperature,25 but higher than
other early reported values. For example,
Aliahmad prepared γ-Fe2O3 nanoparticles by
Ferromagnetic materials 137
thermal-decomposition of Fe3O4 that showed Ms
of 31 emu/g (room temperatures),9 while Layek
et al..10 and Kluchova et al.11 presented Ms values
of 13.3 emu/g and 53.18 emu/g, respectively, for
maghemite nanoparticles synthesized by chemical
co-precipitation technique. The saturation
magnetization value obtained for maghemite/
goethite composite could be ascribed to surface
effects arising from broken symmetry and also to a
high degree of maghemite/goethite interparticle
interactions.
Both the coercive field and Mr/MS ratio are
decreasing with the increase of temperature (the
coercive field is 7000 Oe at 10 K and 1000 Oe at
300 K, while Mr/MS is 0.87 at 10 K and 0.41 at
300 K, respectively), a normal behavior for a
ferromagnetic material (the magnetic properties are
enhanced at low temperatures).
Fig. 6 – Dependence of remanent magnetization/saturation magnetization ratio (Mr/Ms)
and of coercive field on temperature for M/G composite sample.
EXPERIMENTAL
Materials
Iron sulfate heptahydrate (FeSO47H2O: Sigma-Aldrich,
99% purity), sodium hydroxyde pellets (Merck, 99% purity)
and Milli-Q ultrapure distilled water (our laboratory) were
used without further purification.
2. Preparation of maghemite/goethite nanoparticles (M/G)
M/G nanoparticles were prepared through a multistep
procedure as follows. First, FeSO4/H2O 1/2 w/w and
NaOH/H2O 1/4 w/w solutions were prepared in ultrapure
water and mixed (1/1 w/w ratio) in a ultrasonic bath of a
Sonics Vibracell ultrasound generator (750W nominal electric
power, 20 kHz ultrasound frequencies; amplitude, 50% of the
maximum intensity) equipped with a display for the delivered
energy and a sensor for temperature. A cylindrical ultrasonic
bath was chosen (100 ml Schott-Duran Berzelius beaker) as
previously described by Capelo et al.26. The amounts of iron
sulfate and of sodium hydroxide solutions were chosen based
on the technical requirements of ultrasonic probe generator
(the water level does not exceed 40% of the probe length). The
beaker was placed on an ice bath and the temperature was
rapidly increasing during sonication up to 25 oC. The energy
dissipated in the ultrasonic bath was 30, 59 and 109 kJ after
sonication for 15, 30 and 60 minutes, respectively. The
samples were than centrifuged, washed with Milli-Q water,
centrifuged again and dried in a Trade Raypa vacuum oven for
24 h at 40 °C to obtain powder samples of colors depending
on the ultrasonication duration.
3. Characterization
The structure of the maghemite/goethite ferromagnetic
composite samples was investigated by FTIR spectroscopy on
potassium bromide pellets on a Bruker Vertex 70 Spectrometer.
An ESEM Quanta 200 electronic deflection microscope and a
Hitachi HT 7700 electron transmission microscope were used to
visualize the surface morphologies. The crystallization patterns
were obtained on a Bruker Advance D8 X-ray diffractometer (λ:
0.15405 nm - the wavelength of Cu Kα1 radiation, 2θ from 5 to
100 degrees-angular region). The magnetic measurements
(magnetization curves, magnetization dependence on temperature,
Mr/MS, and coercive field dependence on temperature) were
performed on a Quantum Design-PPMSQD-9 vibrating sample
magnetometer (-20÷20 kOe range for applied magnetic field); the
magnetization dependence on temperature was followed by using
standard zero field cooling (ZFC) and field cooling (FC)
procedures between 10 and 250 K for the applied field of 200 Oe).
138 Razvan Rotaru et al.
CONCLUSION
A ferromagnetic and hydroxyl functionalized
maghemite/goethite composite has been prepared at
room temperature through a chemical precipitation
process and ultrasonication in a basic medium. XRD
and FTIR results indicated the formation of
maghemite and goethite. Particle agglomerates with
average diameter less than 100 nm (maghemite) and
acicular particles (goethite) with lengths higher than
100 nm and diameters of about 10 nm were observed
by TEM analysis. VSM studies have revealed
ferromagnetic character of the composite with a
saturation magnetization (MS) varying from 59.6
emu/g (room temperature) to 75.5 emu/g (low
temperature). The established process parameters
allowed well defined γ-Fe2O3/α-FeOOH nanometric
particles with appropriate properties for applications
as ferromagnetic materials. Moreover, the
maghemite/goethite ferromagnetic composite which
combines the properties of both precursors could be
of interest in the development of new polymeric
composites (higher magnetization and hydroxyl
functional groups able to mutually interact with those
of the polymer matrix).
Acknowledgement: The authors acknowledge the support
of the Project nr. 276/2014, PARTNERSHIPS Program of
MEN-UEFISCDI.
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... The viscose-based composite characterized by magnetic and hydrophobic properties was prepared by a multistep procedure. First MG composite particles composed of goethite (α-FeOOH) acicular nanoparticles surrounded by spherical maghemite nanoparticles (M) were prepared ( Rotaru et al., 2017). A mixture of Vs and MG powder was then submitted to ultrasonication for 15 min, when the temperature increases up to 86 °C to give Vs-MG composite. ...
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The virtuous of two-dimensional (2D) nanomaterials in high aspect ratio and tunable surface functionality have geared towards their implementation in mixed matrix membranes (MMMs) for advanced gas separation. In this study, the effects of functionalized 2D nanosheets in the MMMs for gas separation were investigated. The graphitic carbon nitride (g-C3N4) nanosheets were modified with various functional groups namely sulfuric acid group, aliphatic amino group, aromatic amino group, and sulfonic group via four different synthesis approaches. Polymers of intrinsic microporosity (PIM-1) was selected as the continuous polymer phase as its intrinsic high permeability and comparable selectivity towards different gas pair. The sulfonic acid functionalized g-C3N4 MMMs (e.g., PIM-1/g-C3N4-D) imparted high separation properties ascribed to the great CO2 affinity induced by the sulfonic acid groups. In particular, PIM-1/g-C3N4-D (99:1) MMM demonstrated promising CO2 separation performance, with CO2 permeability of 3740 Barrer and CO2/N2 selectivity of 19.8. Moreover, at 5 wt% loading of g-C3N4-D, the H2/N2 and O2/N2 separation of the MMMs had exceeded the 2008 Robeson upper bound, thanks to the periodic triangular ultramicropores in g-C3N4 that favor precise sieving of small gases. These results pave the way in using the developed membranes in practical H2 purification, air separation and CO2 capture.
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Real-time imaging of gene expression in vivo at high spatial resolutions has been a long-cherished goal in molecular research. If such techniques were available, both endogenous and exogenous (for example, gene therapy) expression could be studied in live animals and potentially in a clinical setting. So far, most of our knowledge about gene expression has come from in vitro studies and is usually limited to observations among different animals. These studies are labor-intensive and time-consuming.
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Nearly monodispersed superparamagnetic maghemite nanoparticles (15-20 nm) were prepared by a one-step thermal decomposition of iron(II) acetate in air at 400 C. The presented synthetic route is simple, cost effective and allows to prepare the high-quality superparamagnetic particles in a large scale. The as-prepared particles were exploited for the development of magnetic nanocomposites with the possible applicability in medicine and biochemistry. For the purposes of the MRI diagnostics, the maghemite particles were simply dispersed in the bentonite matrix. The resulting nanocomposite represents very effective and cheap oral negative contrast agent for MRI of the gastrointestinal tract and reveals excellent contrast properties, fully comparable with those obtained for commercial contrast material. The results of the clinical research of this maghemite-bentonite contrast agent for imaging of the small bowel are discussed. For biochemical applications, the primary functionalization of the prepared maghemite nanoparticles with chitosan was performed. In this way, a highly efficient magnetic carrier for protein immobilization was obtained as demonstrated by conjugating thermostable raffinose-modified trypsin (RMT) using glutaraldehyde. The covalent conjugation resulted in a further increase in trypsin thermostability (T 50 ¼ 61 C) and elimination of its autolysis. Consequently, the immobilization of RMT allowed fast in-solution digestion of proteins and their identification by MALDI-TOF mass spectrometry.
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The Fe3O4-Prussian blue (PB) nanoparticles with core-shell structure have been in situ prepared directly on a nano-Fe3O4-modified glassy carbon electrode by cyclic voltammetry (CV). First, the magnetic nano-Fe3O4 particles were synthesized and characterized by X-ray diffraction. Then, the properties of the Fe3O4-PB nanoparticles were characterized by CV, electrochemical impedance spectroscopy, and superconducting quantum interference device. The resulting core-shell Fe3O4-PB-modified electrode displays a dramatic electrocatalytic ability toward H2O2 reduction, and the catalytic current was a linear function with the concentration of H2O2 in the range of 1 × 10−7~5 × 10−4 mol/l. A detection limit of 2 × 10−8 (s/n = 3) was determined. Moreover, it showed good reproducibility, enhanced long-term stability, and potential applications in fields of magnetite biosensors.
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The present study describes a simple and novel synthesis route for submicrometric acicular goethite (α-FeOOH) using high OH/Fe molar ratio (=5) and moderate temperature (30 and 70 °C). Two different alkaline sources (NaOH and Ca(OH)2) and two iron(III) sources (FeCl3·6H2O and Fe(NO3)3.9H2O) were investigated. FESEM, XRD, FTIR, N2 sorption isotherms, color evolution, and pH monitoring have been used to determine the formation mechanism, the particle size, specific surface area, and morphology of goethite particles. Three pH regions were determined during goethite formation, and each of them was qualitatively associated to (I) the formation of a ferric hydroxide gel, leading to acid conditions (pH < 2.5); (II) the spontaneous nucleation of goethite, leading to alkaline conditions (pH > 11) and fine sedimentable particles; and (III) the growth of goethite in alkaline conditions (11 < pH < 13.5). Both the temperature and the Fe(III) source have a significant effect on the particle size, specific surface area, and morphology of goethite. High acicular goethite particles (<1 μm in length, moderate specific surface area, SBET = 31.2 m2/g) were produced after 7 h of reaction at 70 °C, while about 24 h of reaction are required to produce low acicular goethite particles (<0.5 μm in length, high specific surface area, SBET = 133.8 m2/g) at 30 °C, using in both cases iron chloride. When Ca(OH)2 particles are used as alkaline source, a complex mineral composite with high specific surface area (87.3 m2/g) was synthesized; it was mainly composed of unreacted Ca(OH)2 coated with nanosized particles (possibly amorphous iron hydroxide), calcium iron oxide chloride hydrate, and calcite. Novel conditions to prepare uniform goethite particles, possibly with high potential as adsorbents or pigments, have been established.
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Ferric sulfates were observed on Mars during orbital remote sensing and surface explorations. These observations have stimulated our systematic experimental investigation on the formative conditions, stability fields, phase boundaries, and phase transition pathways of these important minerals. We report here the results from the first step of this project: eight synthesized anhydrous and hydrous crystalline ferric sulfates and their structural characters reflected through spectroscopic studies. A few phenomena observed during the 150 sets of on-going experiments for stability field study are also reported, which reveal the structural distortions that can happen under environmental conditions relevant to Mars.
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