Applied Surface Science 278 (2013) 284–288
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Novel Fe@C–TiO2 and Fe@C–SiO2 water-dispersible magnetic nanocomposites Claudiu Teodor Fleaca a,∗ , Florian Dumitrache a , Ion Morjan a , Rodica Alexandrescu a , Catalin Luculescu a , Ana Niculescu a , Eugeniu Vasile b , Victor Kuncser c a b c
National Institute for Plasma, Laser and Radiation Physics (NILPRP), Atomistilor 409, P.O. Box MG 36, R-077125, Magurele, Bucharest, Romania METAV R&D, Rosetti 31, Bucharest, Romania National Institute for Materials Physics (NIMP), Atomistilor 105bis, P.O. Box MG7, R- 077125, Magurele, Bucharest, Romania
a r t i c l e
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Article history: Received 15 June 2012 Received in revised form 22 January 2013 Accepted 24 January 2013 Available online 1 February 2013 PACS: 81.16.Mk 81.20.Fw 75.75.Fk 85.70.−w Keywords: Laser pyrolysis Iron-based magnetic nanocomposites Silica or titania shells
a b s t r a c t We report the synthesis of novel nanocomposites based on Fe@C nanoparticles obtained from Fe(CO)5 and C2 H4 /H2 by laser pyrolysis technique using a three nozzles injector. The ␣Fe–Fex Cy @C particles (below 24 nm diameter) were first functionalized with hydrophilic groups using Na carboxymethylcellulose. Oxidic precursors (Si(OC2 H5 )4 or Ti(OC2 H5 )4 ) dissolved in ethanol were mixed with ethanolic suspensions of hydrophilized Fe@C nanoparticles using strong ultrasonication, then with water (at different pH values) and finally the Fe-containing composites were recovered by magnetic separation. The SiO2 and TiO2 -coated powders were characterized by XRD, FT-IR and TEM techniques and their magnetic hysteresis curves were recorded at different temperatures. Both composites contain submicron aggregates of Fe@C nanoparticles embedded in/surrounded by a disordered porous oxidic matrix/shell. Near superparamagnetic behavior and room temperature and 26 A m2 /kg (for Fe@C/SiO2 ) or 57 A m2 /kg (for Fe@C/TiO2 ) saturation magnetization values were recorded and a blocking temperature around 500 K was extrapolated. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In the last years, an increasing interest was focused in the preparation of magnetic core–shell nanoparticles and composites containing both filler and matrix at nano/submicron scale (usually named nanocomposites) [1]. These systems combine two components: the magnetic core/filler and the non-magnetic (such as a polymer or an oxide) matrix/shell. The second component can have various roles: barrier against oxidation (in case of the metallic core(s)) [2], spacer and hydrophilic support for further anchoring of other functional small molecules and polymers [3] or nanoparticles (such as CdTe quantum dots [4]) or even those of selective activator/support for catalyst [5]. One of the most used material for the coating of magnetic nanoparticles for nanocomposites is the silica, which can be deposed in desired thickness starting from alkoxysilanes by hydrolysis followed by the condensation of Si(OH)4 and other intermediate species using precipitation from solutions (the Stober method [6]) or from microemulsions [5] in acidic or basic media. Moreover, the SiO2 shell can be easily grafted with NH2 terminated APTES (3-aminopropyl-triethoxysilane), allowing further functionalization with useful organic molecules like
∗ Corresponding author. Tel.: +40 21 4574489; fax: +40 21 4574243. E-mail address: claudiufl
[email protected] (C.T. Fleaca). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.172
ETH (3,5-di-tertbutyl-2-hydroxy-benzaldehyde) ligand for Zn2+ fluorescence chemosensing [7], glutaraldehyde for enzyme immobilization [8] or FITC (fluorescein isothiocyanate) for fluorescence dependent pH-sensing [9]. The SiO2 -coated magnetic nanoparticles has been tested as fillers in polymers for microwave devices due to their good magneto-dielectric properties [10] and for electrochemiluminescence sensing using encapsulated Ru(bpy)3 2+ [11]. These nanocomposites were also tested for other biological applications such as MRI contrast agents [12] or for human stem cell labeling [13]. Other valuable material for the embedding of the magnetic nanocores is the titania (anatase) which has been intensively studied due to its remarkable photocatalytic properties [14]. By analogy with the SiO2 case, the deposition of TiO2 shell was achieved by Ti alkoxides (such as Ti(OC4 H9 )4 ) hydrolysis on Co ferrite nanoparticles [15] or on Fe3 O4 /SiO2 [16] nanocomposites. This kind of composite particles can respond to a magnetic field and thus can be magnetically separated from an aqueous suspension when is required, for example after the valuable biomolecules binding or can be magnetically manipulated and retained in specific areas as drug delivery systems [17]. For a facile magnetic manipulation, the composite particles require a high magnetization saturation (Ms ), which can be achieved by lowering the proportion of the non-magnetic component and/or by using metals as magnetic nanocomponents (such as FeCo surrounded by graphitic shells, with Ms = 215 A m2 /kg [18]). We must to underline that the majority
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of the silica and titania covered magnetic nanocomposites reported in the literature have low Ms values (under 10 A m2 /kg). Also, in order to redisperse them without reaggregation in the absence of the magnetic field, a superparamagnetic behavior (reported in [7] or [18]) is desired for these nanocomposites. In this work we coated magnetic Fe@C nanoparticles (Ms = 110 A m2 /kg) with a hydrophilic polymer and subsequent with silica or titania shells in order to obtain water-dispersible nanocomposite particles with Ms room temperature values over 25 A m2 /kg. 2. Experimental In order to obtain the Fe@C nanopowder, we used C2 H4 (3.5), H2 (3.0) and Ar (5.0) from Linde and liquid Fe(CO)5 99.999% from Aldrich. For the nanocomposite synthesis, CMCNa (carboxymethylcellulose sodium salt) medium viscosity from Fluka, Si(OC2 H5 )4 (tetraethylortosilicate, TEOS) 98% from Schuchardt and Ti(OC2 H5 )4 (tetra-ethylortotitanate, TEOT) 98% from Merk were employed. The geometry for the laser pyrolysis experimental set-up required for the one-step gas-phase synthesis of Fe@C nanoparticles was detailed in [19] and those of the three-nozzle concentric gas/vapors injector in [20]. Briefly, the C2 H4 (33 sccm) flow carrying Fe(CO)5 vapors (from a bubbler) pass through the center, the C2 H4 (43 sccm) and H2 (43 sccm) mixture flows through the intermediate nozzle, surrounded by the external Ar (2500 sccm) flow (for confinement and shielding). The working pressure was maintained at 50 kPa. The monomodal continuous wave CO2 laser beam ( = 10.6 m, power density: 3900 W/cm2 ) focused through a ZnSe lens to a 1.5 mm diameter focal spot at 4 mm above the top of the injector leads to the decomposition of the reactive mixture with the formation of Fe-based nanoparticles. The resulted Fe@C hydrophobic powder was first dispersed in 200 ml absolute ethanol (1 g/l) in the presence of hydrophilic polyanion CMCNa (2 g/l) with the aid of a Hielscher Ultrasonic UIP 1000 Sonotrode (1 kW, 20 kHz), under an ice bath during 30 min. Then, 100 ml ethanolic TEOS (as precursor for the silica shell) solution (32 g/l) were added, followed by 400 ml 12% aqueous NH3 in portions, maintaining the ultrasonication and the temperature around at 70 ◦ C during 2 h. Finally, the suspension was diluted with 300 ml distilled water and the magnetic nanocomposite was separated using a NdFeB magnet followed by redispersion in 100 ml distilled water. After repeating this procedure 5 times, the resulted slurry was naturally dried and crushed into a fine powder. For the titania coating, a similar Fe@C–CMCNa ethanolic suspension (30 ml) was mixed with 20 ml of TEOT in ethanol (23 g/l) and diluted by adding 70 ml distilled water in small portions under the action of the sonotrode, followed after 30 min by a separation/washing similar procedure. The samples were investigated with a Tecnai F30 G2 (300 kV) Transmission Electron Microscope (TEM), a PANalytical X’Pert MPD theta–theta X-ray diffraction (XRD) system using a Cu K␣ source (0.15418 nm), a Shimadzu 8400S Fourier Transform Infrared (FTIR) Spectroscope and a Cryogenic Limited Vibrating Sample Magnetometer (VSM). 3. Results and discussions The raw Fe metal/carbide@C nanoparticles result after the carbon atoms (from C2 H4 decomposition and CO disproportionation) absorption, dissolution and precipitation onto freshly formed hot Fe clusters which arise from Fe(CO)5 decomposition due the collision with the laser-excited C2 H4 molecules. The role of the H2 in the coflow is to minimize the unwanted formation of the Polyaromatic Hydrocarbons (PAHs). As can be seen in the TEM image from Fig. 1,
Fig. 1. TEM and inserted HR-TEM images of the initial Fe@C nanoparticles.
these nanoparticles are composed from dense round cores with diameters between 8 and 16 nm, surrounded by a less dense carbon shells (2–4 nm thick). The insert from the same figure presents as example an individual Fe/Fe3 C7 /Fe3 C crystalline nanoparticle ˚ covered with stacked, short and disor(interplanar distance ∼ 2 A) dered graphenes. The XRD diffractogram of this raw powder (from Fig. 3) confirms the presence of both metallic (␣Fe) and carbidic (Fe7 C3 and Fe3 C) phases and expounds a singular weak maghemite peak (at 2 ∼ 36◦ ), proving thus the protective role of the carbon shell against the core oxidation after air exposure. During the TEOS basic hydrolysis, the resulted silanols condense to form tridimensional chains containing Si O Si bonds, part of them attaching to the carboxyl or hydroxyl containing groups CMCNa polymer which surrounds the Fe@C nanoparticles and forming a gel-like shell which finally becomes the dry SiO2 coat/matrix. The upper SEM images from Fig. 2 show a continuous silica shell wrapping the Fe@C aggregates, forming thus submicron Fe@C@SiO2 nanocomposites containing a high density of magnetic nanoparticles. The rough surface and the non-homogeneous contrast of the silica shell suggest a porous structure. When poly(vinyl alcohol) PVP with Mw ∼70,000 was added alongside the CMCNa in a similar SiO2 coating experiment, the resulted shell was thicker, very smooth and homogeneous. The porogenic role of CMCNa was also confirmed for the ZSM-5 zeolite containing irregular pores [21]. The XRD diffractogram (Fig. 3) shows the same peaks as those of Fe@C starting powder with the supplementary presence of the broad peak (∼23◦ ) attributed to amorphous silica, observed also in [7]. The inserted FT-IR spectra from the same Fig. 3 is also similar with those of Fe3 O4 @SiO2 reported in [7,17]. Relevant peaks are found in the 3300–3400 cm−1 zone, attributed to OH stretching in silanols and in the absorbed water, at 1630 cm−1 from the bending mode of OH vibrations and at 1100 and 800 cm−1 from the Si O Si siloxane group vibration [7]. In the experiments involving TEOT hydrolysis, because the reaction is very fast, the great majority of the titania precipitated without attaching to the hydrophilized Fe@C nanocomposite (and was separated being non-magnetic). However, a porous and disordered titania shell (yet with different morphology toward those of the silica shell) can be clearly seen in the SEM images from the lower part of Fig. 2, covering each Fe@C aggregate. It is worth noting that the role of the sonotrode is very important in the small nanoparticle aggregates agglomeration avoiding and in the breaking of the bigger ones, due to the cavitation phenomenon, allowing in the same time their uniform coverage with the oxidic precursor gels by a very
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Fig. 2. TEM images at different magnifications of the SiO2 -coated (up) and TiO2 -coated (down) magnetic nanocomposites.
Fig. 3. Superposed X-ray diffractograms of the Fe@C and Fe@C@SiO2 samples; right insert: superposed FT-IR spectra of magnetic nanocomposites and non-magnetic TiO2 byproduct; left insert: photographs of water suspensions of the oxidic nanocomposites before and after magnetic separation.
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Fig. 4. Magnetic hysteresis cycles recorded at different temperatures for the Fe@C (left), Fe@C–SiO2 (center) and Fe@C–TiO2 (right) and the corresponding inserted details for the zones near origin (for external magnetic field between −200 and 200 kA/m); for the samples presenting higher Ms , the virgin magnetization curves are also presented in the right inserts.
efficient components mixing in the liquid media. The XRD diffractogram of the Fe@C@TiO2 sample is near identical with those of starting Fe@C nanoparticles due to the high degree of titania shell amorphization. In another experience, where a ten times higher quantity of TEOT was used, the X-ray diffractogram of the resulted magnetic nanocomposite with thicker shells (not presented here, as well as the following two XRDs) shows the anatase broad and rounded peaks superposed over the Fe metallic/carbidic peaks (in the 40◦ –46◦ zone). The same anatase peaks (yet narrower) were recorded form the diffractogram of the non-magnetic white titania syntesized in the same experiment. After heating those two supplementary samples three hours in Ar at 300 ◦ C, the anatase peaks became thinner in both, in the case of the magnetic nanocomposite, the Fe/Fex Cy peak at 2 = 46◦ being still visible, with the emergence of a very large maghemite peak around 2 = 30◦ . These supplementary experiments prove that the magnetic nanoparticles from cores influence the anatase crystallinity degree and show the limited protective role of the porous titania shell against oxidation at higher temperatures, even in an inert atmosphere. The FT-IR spectrum (inserted in Fig. 3) of the thin TiO2 coated sample do not show any relevant peak, whereas those from the non-magnetic titania shows the same peaks (only weaker) of the CMCNa polymer. This could be an evidence of the interaction between the Ti(OH)4 and other polyhidroxytitanates from TEOT hydrolysis and the CMCNa hydrophilic polymer. In a previous article, using the same experimental setup, slightly higher laser power density (4100 W/cm2 ), lower Fe(CO)5 vapors carrier C2 H4 central flow (10 sccm) and C2 H4 + Ar (65 + 65 sccm) coflow mixture, we obtained Fe@C nanocomposite with Ms = 77 A m2 /kg [22]. The room temperature Ms of the nanopowder used in this report as starting material for oxide coating, 110 A m2 /kg (extracted from left hysteresis curve of Fig. 4) is considerably higher. Also, from the inserted magnified hysteresis cycle, the corresponding coercivity, 13 kA/m, is lower than those of the previous Fe@C nanoparticles (25 kA/m), indicating a stronger approach to a superparamagnetic behavior (which requires zero coercivity value). Other authors (using a reductive flame system) obtained Fe3 C@C nanopowders with higher Ms (150 A m2 /kg), yet with larger mean nanoparticle diameter (∼30 nm) and higher coercivity (20 kA/m) [23] than our starting Fe-based nanoparticles. Fe–C@SiO2 nanocomposites which were also synthesized by one-step laser pyrolysis from Fe(C5 H5 )2 , C6 H5 CH3 and TEOS [24] show much lower Ms values (1–3.6 A m2 /kg) than the 26 A m2 /kg (acquired from the room temperature magnetization cycle of this sample presented in center of Fig. 4) of our Fe@C@SiO2 particles. This great difference can be attributed to the very poor crystallinity of smaller ␣Fe nanoparticles corroborated with higher carbon (up to 70%) and silica content in their samples. Submicron spherical silica particles containing superparamagnetic small (4–7 nm) Fe nanocrystals obtained by spray pyrolysis followed by Fe oxide
reduction in H2 [25] containing 15%wt. Fe show 24–27 A m2 /kg Ms values (similar with those our SiO2 coated magnetic powder), whereas those with higher Fe content (25%wt) have also a higher Ms (51 A m2 /kg). This last value is comparable with those of our Fe@C@TiO2 porous nanocomposite (57 A m2 /kg), which can be extracted from the corresponding room temperature hysteresis cycle (right part or Fig. 4). The considerable difference between saturation magnetization values of our oxidic nanocomposites can be attributed to differences in the relative amount of non-magnetic part of these powders correlated with the much faster hydrolysis of TEOT compared with those of TEOS. One important parameter of the small magnetic particles is the blocking temperature. Above it, the particle has no coercive force at zero magnetic field, having thus a superparamagnetic behavior. Starting from the coercivity – temperature dependence formula HC (T) = HC0 [1 − (T/TB )0.5 ] in magnetic nanoparticles, where HC0 is coercivity at T = 0 K and TB is the blocking temperature of the largest particle in the system [26], the graphics HC = f(T1/2 ) can be obtained. Using the HC values at different temperatures extracted from Fig. 4 hysteresis cycles and extrapolating the HC linear dependence from T1/2 , we have obtained the following blocking temperatures for our nanocomposite particles: 575 K (for Fe@C), 505 K for Fe@C@SiO2 and 500 K for Fe@C@TiO2 . The supplementary sample with much thicker TiO2 shell and lower Ms (3 A m2 /kg) and Fe content has a TB = 345 K. By comparing these TB values, a clear tendency toward room temperature superpara-magnetism with decreasing both the Fe content and Ms emerges.
4. Conclusions Metallic/carbidic Fe@C nanoparticles submicronic aggregates embedded in/surrounded by defective and porous SiO2 or TiO2 matrices/shells were synthesized by laser pyrolysis followed by sol–gel methods and characterized. They can be easily dispersed in water and present ferromagnetic (near superparamagnetic) behavior at room temperature allowing a facile magnetic separation (as can be seen in the inserted photographs from Fig. 3). Their Ms values, 26 A m2 /kg (for Fe@C/SiO2 ) and 57 A m2 /kg (for Fe@C/TiO2 ) are greater than the majority of those reported for the similar nanocomposites. The resulted composites have potential for application in photocatalysis (those titania-coated) or in the magnetic separation of high-value biomolecules (the silica-coated ones after functionalization with specific ligands).
Acknowledgement Financial support from EU FP7 Project Magpro2 Life is gratefully acknowledged.
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References [1] T. Wen, C.M. Krishnan, Journal of Physics D: Applied Physics 44 (2011) 393001. [2] P. Taraj, C.J. Serna, Journal of the American Chemical Society 125 (2003) 15754. [3] T.-J. Yoon, K.N. Yu, E. Kim, J.S. Kim, B.G. l Kim, S.-H. Yun, B.-H. Sohn, M.-H. Cho, J.-K. Lee, S.B. Park, Small 2 (2006) 202. [4] J. Guo, W. Yang, C. Wang, J. He, J. Chen, Chemistry of Materials 18 (2006) 5554. [5] H. Hayashi, L.Z. Chen, T. Tago, M. Kishida, K. Wakabayashi, Applied Catalysis A 231 (2002) 81. [6] W. Stober, A. Fink, Journal of Colloid and Interface Science 26 (1968) 62. [7] Y. Wang, X. Peng, J. Shi, X. Tang, J. Jiang, W. Liu, Nanoscale Research Letters 7 (86) (2012) 1. [8] T. Georgelin, V. Maurice, B. Malezieux, J.-M. Siaugue, V. Cabuil, Journal Nanoparticle Research 12 (2010) 675. [9] Y. Zhang, W.Y. Gong, L. Jin, S.M. Li, Z.P. Chen, M. Na, N. Gu, Chinese Chemical Letters 20 (2009) 969. [10] T.I. Yang, R.N. Brown, L.C. Kempel, P. Kofinas, Nanotechnology 22 (2011) 10561. [11] L. Zhang, B. Liu, S. Dong, Journal of Physical Chemistry B 111 (2007) 10448. [12] J.L. Campbel, J. Arora, S.F. Cowell, A. Garg, P. Eu, S.K. Bhargava, V. Bansal, PLoS ONE 6 (2011) e21857. [13] C.-W. Lu, Y. Hung, J.-K. Hsiao, M. Yao, T.-H. Chung, Y.-S. Lin, S.-H. Wu, S.-C. Hsu, H.-M. Liu, C.-Y. Mou, C.-S. Yang, D.-M. Huang, Y.-C. Chen, Nano Letters 7 (2007) 149. [14] A. Fujishima, T.N. Rao, D.A. Tryk, Journal of Photochemistry and Photobiology C 1 (2000) 1.
[15] H. Li, Y. Shang, S. Wang, Q. Wu, C. Liu, Journal of Hazardous materials 169 (2009) 1045. [16] R. Wang, X. Wang, X. Xi, R. Hu, G. Jiang, Advanced Materials Science and Engineering (2012) ID409379. [17] C. Vogt, M.S. Toprak, M. Muhammed, S. Laurent, J.-L. Bridot, R.N. Muller, Journal Nanoparticle Research 12 (2010) 1137. [18] W.S. Seo, J.H. Lee, X. Sun, Y. Suzuki, D. Mann, Z. Liu, M. Terashima, P.C. Yang, M.V. McConnell, D.G. Nishimura, H. Dai, Nature Materials 5 (2006) 971. [19] F. Dumitrache, I. Morjan, R. Alexandrescu, R.E. Morjan, I. Voicu, I. Sandu, I. Soare, M. Ploscaru, C. Fleaca, V. Ciupina, G. Prodan, B. Rand, R. Brydson, A. Woodword, Diamond and Related Materials 13 (2004) 362–370. [20] C.T. Fleaca, F. Dumitrache, I. Morjan, R. Alexandrescu, I. Sandu, C. Luculescu, S. Birjega, G. Prodan, I. Stamatin, Applied Surface Science 258 (2012) 9394. [21] H. Tao, C. Li, J. Ren, Y. Wang, G. Lu, Journal of Solid State Chemistry 181 (2011) 1820. [22] F. Dumitrache, I. Morjan, C. Fleaca, R. Birjega, E. Vasile, V. Kuncser, R. Alexandrescu, Applied Surface Science 257 (2011) 5265. [23] I.K. Herrmann, R.N. Grass, D. Mazunin, W.J. Stark, Chemistry of Materials 21 (2009) 3275. [24] O. Bomati-Miguel, Y. Leconte, M.P. Morales, N. Herlin-Boime, S. VeintemillasVerdraguer, Journal of Magnetism Magnetic Materials 290/291 (2005) 272. [25] P. Tartaj, T. Gonzalez-Carreno, O. Bomati-Miguel, C.J. Serna, Physical Review B 69 (2004) 094401. [26] H. Lipert, J. Kazmierczak, I. Pelech, U. Narrkiewicz, A. Slawsjka-Waniewscka, H.A. Lachowicz, Materials Science Poland 25 (2007) 399.