Nanofurry magnetic carbon microspheres for separation processes and catalysis: synthesis, phase composition, and properties
Tibor Pasinszki1•Melinda Krebsz2•La´szlo´ Ko´tai3•Istva´n E. Sajo´4• Zolta´n Homonnay1•Ern}o Kuzmann1•La´szlo´ F. Kiss5• Tama´s Va´czi6• Imre Kova´cs7
Received: 12 May 2015 / Accepted: 22 July 2015 / Published online: 29 July 2015 ÓSpringer 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, botha- andc-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 m2g-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 this article (doi:10.1007/s10853-015-9292-6) contains supplementary material, which is available to authorized users.
& Tibor Pasinszki
pasinszki@chem.elte.hu Melinda Krebsz
melinda.krebsz@gmail.com La´szlo´ Ko´tai
kotai.laszlo@ttk.mta.hu Istva´n E. Sajo´
istvan.sajo@gmail.com Zolta´n Homonnay homonnay@caesar.elte.hu Ern}o Kuzmann
kuzmann@caesar.elte.hu La´szlo´ F. Kiss
kissl@szfki.hu Tama´s Va´czi
vaczitamas@caesar.elte.hu Imre Kova´cs
IKovacs@MOL.hu
1 Institute of Chemistry, Eo¨tvo¨s Lora´nd 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 Budao¨rsi 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 Ja´nos Szenta´gothai Research Centre, University of Pe´cs, Ifju´sa´g 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, Eo¨tvo¨s Lora´nd University, P.O. Box 32, Budapest 112 1518, Hungary
7 DS Development, MOL Plc,
P.O. Box 1., Sza´zhalombatta 2443, Hungary 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 research was 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 reaction medium 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 and water 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 dm3resin. 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)3solution, 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,
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 Karadiation (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 Mo¨ssbauer spectra were measured using a KFKI Mo¨ssbauer spectrometer in constant acceleration mode with a57Co(Rh) source of 1.5 GBq activity at room tem- perature. Isomer shifts are given relative toa-Fe reference.
Low-temperature measurements were carried out in a bath- type liquid nitrogen cryostat. The Mo¨ssbauer 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-1up 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 isB1.2 andB2.5 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 Mo¨ssbauer spectroscopy (MOE) and powder
X-ray diffraction (XRD). Representative Mo¨ssbauer 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 Mo¨ssbauer 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 botha- andc-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 Mo¨ssbauer spectra of selected carbonized samples. Identified phases:topSPM Fe3O4, middle:a-Fe,c-Fe, SPM Fe3O4,bottoma- 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)
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 embeddedc-Fe2O3and Fe3O4and 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 CO2atmosphere, Fe3O4phases 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 ofc-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 900lm, while the diameter of synthesized magnetic carbon microspheres is between 300 and 500lm (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 one of 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. Figure4shows, 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.4top and bottom). SEM images reveal that synthesized nanofibers are truly multi- wall nanotubes (see Figs. 5and 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 replacingsp3carbon atoms bysp2carbon 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-1is 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
associated to the vibrational mode of sp2-bonded carbon atoms (Graphene sheets) and the D band is related to imperfections in the graphitic sp2carbon structures (Gra- phene layer edges). The G band is narrower at higher temperature carbonization and it has an apparent shoulder Fig. 4 TopSEM image of a microsphere synthesized by carbonizing at 900°C for 8 h, middle magnification of a furry area of the microsphere ontop,bottommagnification of the seemingly bald area of the microsphere ontop
Fig. 5 TopSEM image of a nanofiber with iron cluster at the tip, bottomSEM image of nanofibers, multitubular structure is shown on theleftside of the picture
Fig. 6 Selected Raman spectra of carbon microspheres (carboniza- tion temperature and heating time:top600°C, 4 h,middle: 900°C, 4 h,bottom1000°C, 8 h)
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). TheID/IGratios gradually decrease from 6.6 to 1.7 upon increasing the carbonization temper- ature from 400 to 1000°C. Both theID/IGratios 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 m2g-1and 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 m2g-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 m2g-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. 8and 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 Mo¨ssbauer 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 highestTBis 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.Asterisksmark the surface area of activated (4 h heat treatment in slow water- saturated nitrogen gas stream at the carbonization temperature) samples
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-3was 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
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-1min-1 (initial adsorption rate is 0.081 g mg-1min-1), which is comparable to those of pomegranate husk carbon (\0.032 g mg-1min-1) [29]
and activated carbon (\0.093 g mg-1min-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. PtCl4solution and aq. FeSO4solution 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-3Cr(VI) aqueous solution (bottom) for microspheres obtained by carbonization at 800°C for 4 h
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 Cze´ge´ny, Ga´bor Varga, and O¨ do¨n Wagner for their assistance in TG, SEM, and UV spectroscopic investigations.
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