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Nanofurry magnetic carbon microspheres for separationprocesses and catalysis: synthesis, phase composition,and properties
Tibor Pasinszki1 • Melinda Krebsz2• Laszlo Kotai3 • Istvan E. Sajo4
•
Zoltan Homonnay1• Ern}o Kuzmann1
• Laszlo F. Kiss5• Tamas Vaczi6 •
Imre Kovacs7
Received: 12 May 2015 / Accepted: 22 July 2015 / Published online: 29 July 2015
� Springer Science+Business Media New York 2015
Abstract A new method is developed to synthesize
magnetic carbon microspheres decorated with carbon
nanofibers and iron nanoparticles (nanofurry microspheres)
for separation techniques in chemistry and biology.
Microspheres are synthesized by carbonizing polystyrene–
divinylbenzene-based, iron-loaded ion exchange resins.
The phase composition, magnetic properties, and surface
area and morphology of these materials are characterized
by various techniques. It is detected that superparamagnetic
(SPM) magnetite is present in microspheres exclusively
upon carbonization at 400–500 �C, elemental iron, both a-and c-Fe, is the major component at 600 �C, and cementite
dominates between 700 and 1000 �C. Nanofiber formation
is observed to be pronounced at high temperatures. The
synthesized carbon microspheres have high surface area
(100–300 m2 g-1) and can be separated easily by a magnet
or by filtration. Saturation magnetization of selected sam-
ples is obtained between 5 and 28 emu g-1, depending on
the phase composition. The novel microcomposites are
expected to be effective adsorbents or support materials in
various chemical processes, for example in water and air
cleaning, catalysis, and biotechnological separations. Pre-
liminary experimental studies for Cr(VI) removal from
water and for platinum deposition are provided.
Electronic supplementary material The online version of thisarticle (doi:10.1007/s10853-015-9292-6) contains supplementarymaterial, which is available to authorized users.
& Tibor Pasinszki
Melinda Krebsz
Laszlo Kotai
Istvan E. Sajo
Zoltan Homonnay
Ern}o Kuzmann
Laszlo F. Kiss
Tamas Vaczi
Imre Kovacs
1 Institute of Chemistry, Eotvos Lorand University,
P.O. Box 32, Budapest 112 1518, Hungary
2 Institute for Geological and Geochemical Research, Research
Centre for Astronomy and Earth Sciences, Hungarian
Academy of Sciences, 45 Budaorsi street, Budapest 1112,
Hungary
3 Institute of Materials and Environmental Chemistry,
Research Centre of Natural Sciences, Hungarian Academy of
Sciences, P.O. Box 286, Budapest 1519, Hungary
4 Janos Szentagothai Research Centre, University of Pecs,
Ifjusag u. 20., Pecs 7624, Hungary
5 Research Institute for Solid State Physics and Optics, Wigner
Research Centre for Physics, Hungarian Academy of
Sciences, P.O. Box 49, Budapest 1525, Hungary
6 Department of Mineralogy, Eotvos Lorand University,
P.O. Box 32, Budapest 112 1518, Hungary
7 DS Development, MOL Plc,
P.O. Box 1., Szazhalombatta 2443, Hungary
123
J Mater Sci (2015) 50:7353–7363
DOI 10.1007/s10853-015-9292-6
Introduction
Carbon and carbonaceous materials are widely used in
industry, in household, and in medicine due to their unique
and versatile properties, for example as adsorbents [1–5],
catalysts [6], support materials and templates [6, 7], fillings
[8], electronic conductors and electrochemical capacitors
[1, 9], and chemical reagents. Nanotubes and nanofibers
(multi-wall nanotubes in the diameter range larger than
about 10 nm) [1, 2, 10–12] expand enormously the appli-
cation possibilities of carbonaceous materials. They have a
high potential to find applications, for example, in catalysis
[13, 14], microelectronics [10, 15], biosensing [16], and
tissue engineering [17]. Anchoring nanotubes to traditional
carbonaceous materials would further increase application
possibilities, especially since nanotubes are difficult to
separate from solutions. Utilizing, for example, that nan-
otubes interact strongly with polyaromatic molecules due
to van der Waals and p-stacking interactions, carbon nan-
otube-decorated microspheres could act as affinity matrices
for biomolecules [18]. Strong interaction between DNA
and carbon nanotubes has been demonstrated [19].
Physicochemical properties of carbonaceous materials can
be strongly influenced by surface modification and chemical
functionalization. Applications of these carbon derivatives as
adsorbents, chemosorbents, or support materials require
active and specific surface area and methods to separate them
easily and effectively. Water and air purification is a major
issue around the globe, and purification techniques require
new and modified adsorbents on a large scale. The present
researchwas partially initiated by this latter need, and our aim
was to develop an adsorbent material which combines
favorable properties of carbon nanostructures and traditional
active carbon, has low hydrodynamic resistivity, and can be
separated from a reactionmedium easily by both filtration and
magnetic separation. We believed that an activated carbon
sphere in the micro or millimeter range containing magnetic
phases inside the sphere and carbon nanotubes on the surface
could fulfill our requirements. Low hydrodynamic resistivity,
important for gas andwater cleaning in flow-through systems,
is expected from the size and spherical shape. Nanotubes may
increase the active surface area, ease filtration due to tangling,
and serve as supports for deposition of biomolecules and
metals. Embedded magnetic phases in the nanometer size
could provide the desired magnetic properties.
In order to produce nanotube-decorated magnetic carbon,
we expected that carbonization of an organic substance
containing finely dispersed iron could lead to the target
material. In addition, if the growth of iron clusters can be
controlled, small iron clusters may catalyze the formation of
carbon nanotubes. For carbon source and for binding iron
ions, we selected a polystyrene–divinylbenzene-based ion
exchange resin containing iminodiacetate functional groups.
Carbonization of iron containing cation exchange resins
are hardly studied to date, and we are not aware of any
detailed experimental study of the dependence of car-
bonization conditions on phase composition, magnetism,
surface area, and morphology of such carbonized resins.
We note, however, that carbonization of Fe(II)-containing
acrylic acid/divinylbenzene copolymer microspheres [20],
CR11 polystyrene-based resin exchanged with Fe3? ions
[21], and chitosan microspheres adsorbing negatively
charged [Fe(C2O4)3]3- ions [22] was investigated recently
at a few selected temperatures (see details below). Nan-
otube formation on the surface of carbon microspheres was
not observed during these previous experiments.
In this paper, we present a novel method for producing
nanotube-decorated magnetic carbon microspheres, and a
study for their phase composition, magnetic properties, and
surface morphology. Preliminary application studies,
namely Cr(VI) removal from water and platinum
nanoparticle deposition for catalysis, are briefly discussed.
Experimental
Materials and fabrication of magnetic carbon
microspheres
VARION BIM-7 commercial cation exchange resin was
used as starting material. This resin is styrene based, with
7 % divinylbenzene crosslinker and 2 % acrylonitrile
modifier, and contains iminodiacetate (–N(CH2COOH)2)
functional groups with binding capacity of 1 mol divalent
metal cation per 1 dm3 resin. The resin was saturated with
Fe3? ions by performing the following consecutive steps:
conditioning with 1 M aqueous NaOH solution (6 bed
volume), washing with distilled water (1BV), saturation
with threefold excess of 1 M aqueous Fe(NO3)3 solution,
and final washing with water. The exchanged resin was first
dried in air and then in a drying box at 120 �C for 1 day.
Our ICP-MS analysis identified an iron content of 6 (m/
m) % for the dried resin.
Carbonization experiments were done as follows: about
2 g of iron-loaded resin was placed into a porcelain com-
bustion boat and the combustion boat was placed into a
horizontally aligned quartz tube, heated along 30 cm with a
tube furnace. The quartz tube was connected to a nitrogen
line and first flushed with oxygen and water-free nitrogen gas
then the flow rate of nitrogen was reduced to a minimal value
and kept there during carbonization. Oxygen and water-free
nitrogen gas was prepared from commercial nitrogen (purity
99.996 %) by passing the nitrogen stream through two con-
secutive columns packed with R3-11G BASF catalyst and
3A molecular sieves, respectively. For carbonization, the
furnace was heated up to the desired temperature in 30 min,
7354 J Mater Sci (2015) 50:7353–7363
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the temperature was kept constant for 2, 4, or 8 h, and then
the furnace was left to cool down naturally.
Characterization
X-ray powder diffraction measurements were done on a
Model PW 3710/PW 1050 Bragg–Brentano diffractometer
using Cu Ka radiation (k = 1.541862 A), secondary beam
graphite monochromator, and proportional counter. Syn-
thetic fluorophlogopite mica (NIST SRM 675) and silicon
powder (NIST SRM 640) were used as internal two theta
standards. Lattice parameters were determined with Le
Bail whole pattern decomposition method using the Full-
prof Rietveld software suite.
The Mossbauer spectra were measured using a KFKI
Mossbauer spectrometer in constant acceleration mode
with a 57Co(Rh) source of 1.5 GBq activity at room tem-
perature. Isomer shifts are given relative to a-Fe reference.Low-temperature measurements were carried out in a bath-
type liquid nitrogen cryostat. The Mossbauer spectra were
analyzed assuming Lorentzian line shapes with the help of
the Mosswinn 3.0i XP software.
TG measurements were performed on a modified Per-
kin-Elmer TGS-2 thermo balance. Typically, 2.5 mg
sample was placed into the platinum sample pan and heated
at 20 �C min-1 up to 900 �C in argon atmosphere.
Magnetic measurements were performed by a Quantum
Design MPMS 5S SQUID (Superconducting Quantum
Interference Device) magnetometer in the temperature and
magnetic field ranges of 5–300 K and 0–5 T, respectively.
Samples were fixed inside a Teflon sample holder by
Apiezon M vacuum grease in order to prevent rotation of
sample particles under the applied magnetic field. The low-
field measurements were made as follows: first, the sample
was cooled down from 300 to 5 K in zero field and then
measured in a field of 10 Oe with increasing temperature
between 5 and 300 K [zero-field-cooled (ZFC) curves].
Second, the sample was cooled down from 300 to 5 K in 10
Oe and then measured in the same field with increasing
temperature between 5 and 300 K [field-cooled (FC)
curves]. Moreover, the magnetization was also measured at
5 K as a function of the magnetic field up to 5 T.
Scanning electron microscopy (SEM) was performed
using a FEI Quanta 3D high-resolution microscope. Res-
olution of the instrument is B1.2 and B2.5 nm using the
secondary electron detector and backscattered electron
detector (BSED), respectively, at 30 keV accelerating
voltage and in high vacuum. Energy resolution of the X-ray
detector is 130 eV at Mn Ka.HORIBA JobinYvon LabRAM HR instrument was used
for confocal Raman microscopic investigations. Raman
spectra were recorded using He–Ne excitation (632 nm)
and a laser power of 0.1 mW.
Spectrophotometric measurements for Cr(VI) concen-
tration study were performed using a Perkin-Elmer UV/Vis
Lambda 25 spectrometer (split width 1 nm, aqueous solu-
tion, see supplementary material).
BET specific surface area was determined using the
volumetric method and nitrogen gas at liquid nitrogen
temperature, and an ASDI RXM-100 Catalyst Characteri-
zation instrument. Samples were pre-treated in vacuum at
300 �C for 2 h.
Results and discussion
Resin loading and carbonization experiments
For producing the desired material, it is essential to select
the appropriate resin and the inorganic iron compound, and
to optimize carbonization conditions. Our preliminary
investigations indicated that using VARION BIM-7 resin
(see ‘‘Experimental’’ section) the spherical shape of the
resin is retained during carbonization, which is a require-
ment for low hydrodynamic resistivity, but powder or
foamy materials were obtained upon carbonizing iron-
loaded ethyl acrylate-based VARION KCM-8 and KCO-8
resins. The effect of inorganic iron salts on the morphology
of carbonized products were also tested; aqueous solutions
of both ferrous and ferric chlorides, sulfates, and nitrate
were used for cation exchange, however, substantial nan-
otube formation during carbonization was observed only
for the Fe(NO3)3 exchanged resin. Therefore, this latter
iron salt and BIM-7 resin are used in the present study. We
note that due to charge balance one nitrate ion stays with
each ferric cation in the resin following cation exchange,
and this may effect carbonization. Details of the ion
exchange procedure are provided in the ‘‘Experimental’’
section.
The thermal stability of iron-loaded resin was first
studied by thermogravimetric (TG) analysis. The weight
loss of the resin starts steadily about 150 �C and the main
decomposition occurs between 400 and 450 �C (see
Fig. 1). The weight loss up to 900 �C is 68.5 %. Based on
this, the carbonization of the resin was investigated in the
400–1000 �C temperature range in 100 �C increments, and
the effect of heating time was studied by performing
experiments for 2, 4, and 8 h. All 21 samples thus prepared
are black in appearance and can be collected by a magnet.
It is apparent, however, that magnetization is higher for
samples prepared at higher temperatures.
Phase analysis
The phase compositions of carbonized resins were deter-
mined using Mossbauer spectroscopy (MOE) and powder
J Mater Sci (2015) 50:7353–7363 7355
123
X-ray diffraction (XRD). Representative Mossbauer spec-
tra are shown in Fig. 2, and all recorded spectra and
diffraction patterns are presented in the supplementary
material (Figs. S1–S15). Results are summarized in
Table S1 and for selected samples in Fig. 3. There is a
good agreement between MOE and XRD results, except
those cases where the crystallite size is very small. Iron
occurs in carbonized samples exclusively in the form of
SPM magnetite (Fe3O4) after carbonization at 400 and
500 �C for 2–8 h, and even at 600 �C if the carbonization
temperature is short (2 h). The crystallite size obtained
from XRD analysis using the Scherrer formula is between
5 and 12 nm (Table S2). Only a doublet, instead of two
sextets of magnetite, was observed in the Mossbauer
spectrum at room temperature (see Fig. 2, top); the col-
lapse of the two ferromagnetic sextets into SPM doublet is
a consequence of small crystallite size. In order to prove
this, low temperature MOE measurements were performed
(Fig. S8). The characteristic sextets of magnetite are well
observable at 20 K, and these sextets disappear gradually
by increasing the temperature. Magnetite is in the SPM
state above 130 K. The SPM transition occurs in a wide
temperature range, which indicates a wide crystallite size
distribution. The average crystallite size is estimated to be
around 10 nm. Upon increasing heating time at 600 �C or
increasing carbonization temperature above 600 �C, the
magnetite content decreases. It is a mere of 2–3 % at
900–1000 �C (the presence of other iron(III) compounds
instead of magnetite cannot be excluded here). Elemental
iron, in both a- and c-form, appears first after carbonization
at 600 �C for 4 h. XRD indicates the presence of a-Fe,however, the diffraction peak has substantial broadening
(Fig. S11). The internal hyperfine magnetic field measured
by MOE is also anomalously low in this particular exper-
iment (27.8 T instead of the regular *33 T). Both findings
are attributed to the low particle size (below 10 nm as
estimated from XRD) where regular bulk properties cannot
be fully developed. Cementite, Fe3C, is the major iron-
containing phase above 700 �C and its amount gradually
increases up to 83 % by increasing the temperature (see
Fig. 1 TG curves of iron(III) nitrate-loaded resin (heating rate:
20 �C min-1, Ar atmosphere)
Fig. 2 Mossbauer spectra of selected carbonized samples. Identified
phases: top SPM Fe3O4, middle: a-Fe, c-Fe, SPM Fe3O4, bottom a-Fe, c-Fe, Fe3C (see detailed decomposition of spectra in the
supplementary material)
Fig. 3 Dependence of iron containing phase composition on the
carbonization temperature (MOE results, heating time 8 h)
7356 J Mater Sci (2015) 50:7353–7363
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Fig. 3 and Table S1). The crystallite size, determined by
XRD, is substantially larger than that obtained at lower
temperatures (30–110 nm, see Table S2). Graphitization of
the product was clearly observed by XRD above 600 �C,and the graphite content was determined to be about 40 %
of all crystalline phases in the temperature range of
700–1000 �C.It is worth to compare our results with those of previous
investigations on similar systems, what clearly indicates
the importance of the selection of resin and carbonization
conditions. The carbonization of Fe(II)-containing acrylic
acid/divinylbenzene copolymer microspheres were inves-
tigated recently [20] at two temperatures. It was concluded
on the basis of XRD analysis that carbonization at 500 and
800 �C produced porous carbon microspheres with a
mixture of embedded c-Fe2O3 and Fe3O4 and nearly pure
Fe3O4, respectively. Sakata et al. [21] prepared porous
carbon composite material by carbonizing CR11 poly-
styrene-based resin exchanged with Fe3? ions between 400
and 800 �C in nitrogen atmosphere. XRD analysis revealed
the formation of FeO, Fe3C, and Fe4C phases. When heat
treatment was performed in CO2 atmosphere, Fe3O4 phases
were obtained. Zhu et al. [22] synthesized magnetic carbon
microspheres by carbonizing, between 700 and 1000 �C,chitosan microspheres adsorbing negatively charged
[Fe(C2O4)3]3- ions. Phase analysis revealed that magnetic
properties appeared due to the presence of c-Fe2O3, a-Fe,and Fe3C phases.
Surface morphology
The shape and surface morphology of synthesized materi-
als were studied by scanning electron microscopy. The
products inherited the spherical morphology of ion
exchange resins. Uncarbonized BIM-7 resin spheres have a
diameter between 600 and 900 lm, while the diameter of
synthesized magnetic carbon microspheres is between 300
and 500 lm (Figs. S16–S36), due to shrinkage during the
heat treatment. Microscopy reveals that iron or iron com-
pound particles are distributed on the surface, too. Tiny
magnetite particles are shown on Figs. S16–S22, in
agreement with MOE and XRD results, and iron and
cementite particles are visualized using the BSED detector,
which is more sensitive to heavy atoms (see for example
Figs. S34–S37). Macropores are well observable on the
surface of samples heat treated at 400 and 500 �C. It is oneof the most interesting findings of this work that nanofibers
grow on the surface of microspheres when carbonization is
performed between 600 and 1000 �C, and the nanofiber
formation corroborates with the formation of elemental
iron and/or cementite (Figs. S23–S36). The nanofiber for-
mation is more pronounced with increasing temperature
and heating time. The diameter of nanofibers is about
20 nm at 600 �C, the thickness gradually increases by
increasing the carbonization temperature, and it is about
200 nm at 900 and 1000 �C. Figure 4 shows, for example,
a nanofiber-decorated carbon microsphere obtained by
carbonization at 900 �C for 8 h.
The shapes of nanofibers widely vary: there are straight
and long tubes, spirals, and curly and densely packed
nanofibers. These latter result in seemingly bald surfaces at
lower magnification (compare Fig. 4 top and bottom). SEM
images reveal that synthesized nanofibers are truly multi-
wall nanotubes (see Figs. 5 and S37). Nanofibers, in gen-
eral, do not contain iron in their inner cavity (compare ETD
and BSED images in Figs. S34–S36), however, iron clus-
ters were detected in some cases at the tip of the fiber
which suggests catalytic effect of iron clusters on nanofiber
formation (Fig. 5). The growing mechanism of nanofibers,
we assume, is that described for chemical vapor deposition
techniques [10], which involves the formation of iron
nanoparticles, dissolution and saturation of carbon atoms in
the iron nanoparticles, and the precipitation of carbon from
the saturated metal particle. Microscopic investigations
suggest that the base-growth mode is dominant compared
to the tip-growth mode.
Raman microscopy
Raman spectra were recorded on several selected spots of
synthesized microspheres to analyze the structural organi-
zation of carbon (Figs. S38–S42), and selected character-
istic spectra are shown in Fig. 6. Graphitic bands appear on
top of a broad luminescence background at low tempera-
ture carbonization, which may indicate imperfect car-
bonization, but this background gradually disappears at
increasing carbonization temperatures (see Fig. S38).
Graphitic bands are narrower at higher carbonization
temperatures (see Figs. S39–S41). The Raman intensity is
decreasing, in general, with increasing carbonization tem-
perature, which is in line with graphitization, namely
replacing sp3 carbon atoms by sp2 carbon atoms. We note
that the Raman intensity strongly drops when the surface of
microspheres becomes crowded with nanofibers due to the
smaller excitation volume (see Fig. 6 bottom and
Figs. S41–S42). In order to prove this, selected samples
were crushed in a mortar and Raman spectra of internal
surfaces were recorded (see Figs. S43–44). Raman spectra
of microspheres support that multi-walled carbon nan-
otubes are formed on the surface because the radial
breathing mode of single-walled carbon nanotubes at
around 180 cm-1 is absent.
The first-order Raman spectrum (1200–1700 cm-1)
exhibits at least five overlapping bands. The two strong
bands at around 1335 and 1595 cm-1 are assigned to the
graphitic D and G bands, respectively. The G band is
J Mater Sci (2015) 50:7353–7363 7357
123
associated to the vibrational mode of sp2-bonded carbon
atoms (Graphene sheets) and the D band is related to
imperfections in the graphitic sp2 carbon structures (Gra-
phene layer edges). The G band is narrower at higher
temperature carbonization and it has an apparent shoulder
Fig. 4 Top SEM image of a microsphere synthesized by carbonizing
at 900 �C for 8 h, middle magnification of a furry area of the
microsphere on top, bottom magnification of the seemingly bald area
of the microsphere on top
Fig. 5 Top SEM image of a nanofiber with iron cluster at the tip,
bottom SEM image of nanofibers, multitubular structure is shown on
the left side of the picture
Fig. 6 Selected Raman spectra of carbon microspheres (carboniza-
tion temperature and heating time: top 600 �C, 4 h, middle: 900 �C,4 h, bottom 1000 �C, 8 h)
7358 J Mater Sci (2015) 50:7353–7363
123
at around 1615 cm-1 (D’ band), also disorder induced. In
addition, the D band exhibits a shoulder at around
1185 cm-1, which may be attributed to sp2–sp3 bonds or
polyene-like structures [23]. Raman spectra also exhibit
second-order bands at about 2660 and 2920 cm-1. The
band at 2660 cm-1 is assigned to the first overtone of D
band, and it is relatively strong and narrow if the car-
bonization temperature is high. The overtone at 2920 cm-1
suggests the presence of an overlapped and hidden first-
order band at around 1470 cm-1. This band can be
assigned to amorphous carbon. The analogous Raman band
is usually observed in amorphous carbon fraction of shoot
[23]. The peak intensity ratios of prominent D and G bands
are indicators of the degree of graphitic content, thus the
ID/IG ratios are calculated after fitting five Lorentzian
curves to the five expected bands of the first-order Raman
spectra (see Fig. S45). The ID/IG ratios gradually decrease
from 6.6 to 1.7 upon increasing the carbonization temper-
ature from 400 to 1000 �C. Both the ID/IG ratios and band
shapes reflect higher degree of graphitization at higher
temperatures.
BET specific surface area
The surface areas of microspheres were determined using
nitrogen adsorption measurements and results are listed in
Table S3 and summarized in Fig. 7. The specific surface
areas of samples obtained by carbonizing at 500 �C are
around 300 m2 g-1 and the specific surface area gradually
decreases with increasing temperature. Applying heating
times longer than 2 h is favorable, but prolonged heating
slightly decreases the surface area (see Fig. 7), possibly
due to the ‘‘blocking’’ effect of nanoparticles within the
porous carbon. The surface area could be certainly
increased by standard activation methods, and we proved
this by treating two carbonized samples with slow water-
saturated nitrogen gas for 4 h at the same temperature than
that was applied for carbonization. The surface area of
microsphere obtained upon carbonization at 800 �C for 4 h
increased by 11 % and that obtained at 1000 �C after 4 h
carbonization increased by 218 %, on the expense of
additional 5 % weight loss. During activation, the amount
of Fe3C is decreased by 17–27 % and the iron content
increased by 19–22 % (see Table S4).
Recent surface area measurements on similar systems
provided comparable results. BET surface area of micro-
spheres obtained by carbonization of Fe(II)-containing
acrylic acid/divinylbenzene copolymer microspheres at
500 and 800 �C were determined to be nearly 200 m2 g-1
[20]. The specific surface area of carbon microspheres
obtained by carbonizing chitosan microspheres adsorbing
negatively charged [Fe(C2O4)3]3- ions between 700 and
1000 �C were determined to be between 226 and
286 m2 g-1 [22].
Magnetic properties
Five selected samples, where MOE and XRD indicated
substantially different phase composition or crystallite size,
were characterized using a SQUID magnetometer between
5 and 300 K (see Figs. 8 and S46–S51). The ZFC and FC
magnetization curves are presented for the sample heat
treated at 400 �C for 4 h in Fig. 8. Blocking of nanopar-
ticles occurs below 150 K at 10 Oe external magnetic field.
This is the highest blocking temperature (TB), where the
ZFC and FC magnetization curves bifurcate from each
other. There is a wide distribution of blocking tempera-
tures, reflecting the size distribution of the particles. The
higher the particle size, the higher the blocking tempera-
ture. Above the highest TB, all particles show SPM
behavior. This is in good agreement with the temperature-
dependent Mossbauer spectroscopic results (see above).
Magnetization curves for the sample obtained by car-
bonizing at 600 �C for 2 h are similar to the previous one
(Fig. S46), not surprisingly since both contain nano mag-
netite, although the highest TB for this sample is higher
(about 280 K). The largest deviation between ZFC and FC
curves is measured for the sample carbonized at 600 �C for
4 h (Fig. 8). The huge deviation between the ZFC and FC
curves measured at low field (10 Oe) hints at a SPM
behavior of magnetic clusters with a wide moment (i.e.,
size) distribution, where the highest TB is well above room
temperature. It means that the average cluster size is
increased compared to that of the sample carbonized at
600 �C for 2 h. This behavior is in line with the X-ray
Fig. 7 Specific surface area of carbon microspheres. Asterisks mark
the surface area of activated (4 h heat treatment in slow water-
saturated nitrogen gas stream at the carbonization temperature)
samples
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123
diffraction measurements which suggest an average grain
size of 10 nm for the major a-Fe phase. The difference
between ZFC and FC curves decreases by increasing the
carbonization temperature (see Figs. 8 top and S47–48),
which indicates larger cluster size. The characteristic
behavior of the ZFC and FC curves for samples obtained
by carbonization above 600 �C clearly indicates the fer-
romagnetic nature of the particles at room temperature.
Magnetization as a function of the external magnetic field
is shown in Figs. 9 and S49–51. Magnetization is higher
for samples obtained at higher temperature than for those
containing SPM magnetite. The saturation magnetization
of sample obtained at 900 �C is around 28 emu g-1 at
50,000 Oe (see Fig. 9).
There is limited information on the magnetic properties
of recently synthesized similar systems. The saturation
magnetization of microspheres synthesized by carbonizing
Fe(II)-containing acrylic acid/divinylbenzene copolymer
microspheres at 800 �C was determined to be
31.5 emu g-1 [20]. The saturation magnetization of
microspheres synthesized by carbonizing chitosan micro-
spheres adsorbing negatively charged [Fe(C2O4)3]3- ions
at 1000 �C was 13.9 emu g-1 [22].
Preliminary application studies
Although the aim of the present work was to find a novel
route to magnetic carbon microspheres and their charac-
terization, we briefly present two of their possible appli-
cations, namely as adsorbents in water purification and
support materials in catalysis. Detailed application studies
are to be published separately.
Carbonaceous materials are known to adsorb heavy
metals from polluted water, and the Cr(VI) removal from
water is a current topic [24, 25]. As an example for
potential application, we also tested the adsorption prop-
erties of our novel magnetic carbon microspheres in the
removal of Cr(VI) from water in neutral solutions (pH
dependence is to be discussed separately). For determining
the adsorption capacity, 50 ml of aqueous Cr(VI) solution
with concentration of 4.2 mg dm-3 was treated with 0.05 g
adsorbent for 24 h. The Cr(VI) concentration before and
after the treatment was determined by UV–Vis spec-
troscopy (see supplementary material). Adsorption capac-
ities of microspheres are summarized in Fig. 10. The
Cr(VI) absorption capacity is small for microspheres pre-
pared by carbonization at 400 �C, the adsorption capacity
increases with increasing carbonization temperatures, up to
700–800 �C, and decreases by further increasing car-
bonization temperatures. This is seemingly in line with
accessible iron and cementite nanoparticles on the surface
of microspheres. At high temperature, nanoparticles are
expected to be encapsulated within nanofibers or carbon
microspheres due to graphitization. The specific surface
area is clearly not the dominant factor for determining the
Cr(VI) removal from water (see BET results above). Both
the surface properties of carbon microspheres and embed-
ded iron nanoparticles play important role in Cr(VI)
removal. Iron nanoparticles reduce chromate ions and thus
take part in chromium removal (the mechanism is dis-
cussed in Ref. [24]).
Fig. 8 ZFC and FC magnetization curves of representative samples
at 10 Oe external field
Fig. 9 Magnetization curves of selected samples at 5 K
7360 J Mater Sci (2015) 50:7353–7363
123
Microspheres obtained by carbonizing at 800 �C for 4 h
possess the highest adsorption capacity of 2.54 mg g-1.
This value is comparable to the recently synthesized
magnetic carbon nanocomposite fabrics (3.74 mg g-1)
[24], carbon-coated magnetic nanoparticles (1.52 mg g-1)
[26], and graphene nanocomposites (1.03 mg g-1) [27],
higher than that of cotton fabrics (0.32 mg g-1) [24], car-
bon fabrics (0.46 mg g-1) [24], and agricultural waste
biomass (0.28–0.82 mg g-1) [28], but lower than that of
nanocomposites derived from cellulose (22.8 mg g-1)
[25], pomegranate husk carbon (35.2 mg g-1) [29], and
activated carbon (112.36 mg g-1) [30]. Note that activated
carbon exhibits a very high specific adsorption capacity
due to its extremely low density.
The efficiency of Cr(VI) removal was tested for mag-
netic carbon microspheres obtained by carbonizing at
800 �C for 4 h. The effect of initial Cr(VI) concentration
on the removal efficiency is shown in Fig. 11. A better than
99 % removal efficiency was achieved for solutions of
initial concentration lower than 1.5 mg dm-3 using an
adsorbent concentration of 1.0 g dm-3. The kinetics of the
adsorption is shown in Fig. 11 for a solution of an initial
concentration of 1.5 mg dm-3. 90 and 99 % of the Cr(VI)
content is removed in 3 and 24 h, respectively, using an
adsorbent concentration of 1.0 g dm-3. Experimental data
is fitted using a pseudo-second-order kinetic model [24,
31]. The adsorption rate constant obtained from the fitting
is 0.037 g mg-1 min-1 (initial adsorption rate is
0.081 g mg-1 min-1), which is comparable to those of
pomegranate husk carbon (\0.032 g mg-1 min-1) [29]
and activated carbon (\0.093 g mg-1 min-1) [30].
The applicability of anchored nanofibers to support
nanoparticles is also tested by depositing platinum metal
nanoparticles onto the surface of nanofurry magnetic car-
bon microspheres. For this, the sample prepared by car-
bonizing the starting material at 900 �C for 8 h was
selected, and iron nanoparticles were removed from the
surface of microspheres by treating them with aqueous HCl
solution. Microspheres obtained are separable magnetically
from the aqueous solution (see Fig. 12, top). Platinum
nanoparticles have been deposited onto the surface of
nanofibers using aq. PtCl4 solution and aq. FeSO4 solution
as reducing agent (Fig. S53, supplementary material).
Deposited Pt nanoparticles with size of about 10–20 nm
are obtained and shown in Fig. 12.
Conclusions
Carbon nanotubes, nanofibers, and activated carbon have a
high potential to find applications in various separation
processes and could serve as efficient support materials in
catalysis and biotechnology (see ‘‘Introduction’’ section).
However, their application is feasible only if their separa-
tion from various reaction media is effective. Magnetic
separation is a possible and desirable method due to its
simplicity. In the present work, a new method was devel-
oped to synthesize carbon nanofibers anchored to magnetic
carbon microspheres. The method, which is based on car-
bonizing iron-loaded ion exchange resins, is simple, cost
Fig. 10 Cr(VI) adsorption capacity of magnetic carbon microspheres
Fig. 11 Cr(VI) removal efficiency from aqueous solutions of differ-
ent initial concentration (top adsorbent concentration: 1.0 g dm-3,
treatment time: 24 h) and time dependence of Cr(VI) removal
efficiency from 1.5 mg dm-3 Cr(VI) aqueous solution (bottom) for
microspheres obtained by carbonization at 800 �C for 4 h
J Mater Sci (2015) 50:7353–7363 7361
123
effective, and provides the possibility for scaling-up for
large-scale production. According to our knowledge this is
the first time when a cheap organic polymer is used to
produce anchored nanofibers. The synthesized nanofurry
magnetic carbon microspheres could potentially be applied
in separation processes due to their high surface area, easy
magnetic separation, low hydrodynamic resistivity result-
ing from their spherical shape, and nanofibers grown on
their surface. This latter can be an advantage in biotech-
nological and biomedical applications. The synthesized
microspheres contain magnetite, iron, and/or cementite on
their surface, depending on carbonization conditions,
which hints toward application in magnetite or iron cat-
alyzed reactions. Due to the presence of iron and cementite
particles on the surface of microspheres, there is a possi-
bility for the deposition of noble metal nanoparticles onto
the surface of anchored nanofibers under very mild con-
ditions, which would provide a novel route to magnetically
separable nanofiber-supported noble metal nanocatalysts.
We have demonstrated in this work that anchored nanofi-
bers can be ideal support for Pt nanoparticles and that
magnetic carbon microspheres are advanced adsorbents in
the removal of Cr(VI) from contaminated water.
Acknowledgements Authors thank Zsuzsanna Czegeny, Gabor
Varga, and Odon Wagner for their assistance in TG, SEM, and UV
spectroscopic investigations.
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