Article pubs.acs.org/cm
Bifunctional FePt@MWCNTs/Ru Nanoarchitectures: Synthesis and Characterization B. Astinchap,∥ R. Moradian,∥ A. Ardu,‡ C. Cannas,‡ G. Varvaro,⊥ and A. Capobianchi*,⊥ ⊥
Istituto di Struttura della Materia, Consiglio Nazionale delle Ricerche, Via Salaria, 0016 Monterotondo, Rome, Italy Dipartimento di Scienze Chimiche, Università di Cagliari, S.S. 554 bivio per Sestu, 09042 Monserrato (CA), Italy ∥ Nano technology research Laboratory, Physics Department, Faculty of Science, Razi University, Kermanshah, Iran ‡
S Supporting Information *
ABSTRACT: The synthesis of novel nanoarchitectures is an important way to combine several properties into the same nanometric object. Magnetic, catalytic, optical, and electrical properties can be embedded and used for heating, moving, or monitoring the nanocomposite. Following this approach, smart materials exhibiting remarkable properties could be obtained. Several nanocomposites are based on carbon nanotubes (CNTs). Because of the presence of empty cavities and very large surface external area, this allotropic form of carbon is especially suitable for this purpose and particularly for catalytic applications. In this work, a new general strategy to synthesize by a wet method three-block, smart nanocomposites based on MWCNTs is described. The new bifunctional material is shortly referred to as FePt@MWCNTs/ Ru(NPs) to point out that nanoparticles (NPs) of a magnetically soft alloy (FePt fcc) fill the MWCNTs cavity, whereas catalytic Ru NPs decorate the external wall. In this way well separated catalytic and magnetic NPs are obtained. All the synthetic steps are described in detail. TEM, HRTEM, XRD, and magnetic measurements by VSM are used to monitor all the steps and to prove the effectiveness of the method. KEYWORDS: nanocomposites, CNTs, catalysis, FePt, filling/decorating
1. INTRODUCTION Heterogeneous catalysis is generally carried out in the presence of heavy metals, which are often expensive and pollutants. The problem becomes worse if the catalysts are used at a nanometric scale because in this case the catalytic materials are easily lost during the working cycle. If we consider the environmental pollution costs due to not only the nature of the metals but also the nanoparticles themselves (NPs),1 the problem, in fact, doubles (see asbestos problems). Usually, nanometric metal catalysts are supported on inert materials like alumina or silica. In the past decade, however, carbon nanotubes (CNTs)2 have come into use as supports in the search of new catalysts. CNTs are single or multiple rolled sheets of graphene exhibiting an empty internal cavity and an external wall. Often the metals NPs are supported on the external surface of the CNTs, whereas the empty nanocavities can be used to insert different NPs with the aim of obtaining smart composites3 with peculiar combinations of properties. As an example the CNTs could be decorated with metals NPs outside and magnetic NPs inside. The magnetic nanoparticles can be used to remove the catalyst by applying an external magnetic field and/or to increase the temperature only at the catalyst site, where the catalytic reaction takes place, by apllying a AC magnetic field, saving the energy needed to heat the entire reactor and perhaps increasing the selectivity. In any case the © 2012 American Chemical Society
individual properties of nanoparticles and CNTs can be combined in many different ways, catalytic-magnetic, catalyticoptics, optics-magnetic, or other, and could be useful in many applications including medicine, electronics, or energy. The use of carbon nanotubes and nanoparticles nanocomposites is being actively explored in many research field including energy conversion (fuel cells4 and solar cells5), chemical sensors, and electronics.7 In particular, carbon nanotubes are very important materials as support of metal nanoparticles for catalytic applications8 because of their high specific surface area, electrical conductivity, chemical stability, high accessibility of the active phase, and absence of any microporosity, thus eliminating diffusion and intraparticle mass transfer limitations. A huge number of papers have been published on this subject in the past decade. With respect to active phases deposited on alumina, silica or activated carbon supports, the nanocomposite CNTs-based catalysts offer many advantages and show higher activity and/or selectivity. For example heterogeneous catalysts made of Ru nanoparticles are much more efficient when supported on CNTs than their analogues using different support materials.9,10 Many metals NPs such as Pt,11 Pd,12 Received: May 18, 2012 Revised: August 1, 2012 Published: August 4, 2012 3393
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grid covered with a carbon thin film for the observation. Highresolution images were obtained by means of a JEM 2010 UHR equipped with a Gatan Imaging Filter (GIF) with a 15 eV window and a 794 slow scan CCD camera. Room temperature magnetization curves were measured by means of a vibrating sample magnetometer (ADE Technologies Model10).
Ag,13 Rh,14 Ru,15 Co,16 and Ni,17 but also magnetic oxides18 and other inorganic compound NPs,19−21 have been linked to the external surface of CNTs. These nanocomposites are generally called metal NPs-decorated-CNTs and shortly referred to as CNTs/Me. In chemical terms one of the most attractive properties of CNTs is their ability to encapsulate metal nanoparticles and to confine them inside the cavities. In spite of the synthetic difficulties met when trying to access the CNTs cavity, many studies have been published on the synthesis of nanocomposites made of CNTs filled with metal nanoparticles, or other compounds.22,23 The catalytic applications of metal-filled CNTs, however, despite being very promising, have been much less explored than metal decorated CNTs. It is known, in fact, that because of the particular reductive environment and to the nanometric cavity dimensions,24 the CNTs filled with catalyst NPs show much higher selectivity with respect to the related metal decorated materials.25,26 The CNTs have been filled with many metals NPs as Ni,27 Ru,28 Fe,29 as well as with inorganic compounds like CdS30 and these nanocomposites are generically indicated as Me@CNTs. Even alloy NPs have been placed into the CNTs cavities. Previously we have developed a new filling method that was shown to fill MWCNTs with FePt in magnetically hard phase L10.31−34 The chemically ordered L10 FePt alloy,35 with alternate atomic planes of Fe and Pt, has proved to be interesting for ultrahigh density magnetic recording36 because of its large uniaxial magnetocrystalline anisotropy (Ku ≈ 7 × 106 J/m3).37 This alloy and in general, all the alloys in chemically ordered L10 phases, exhibit also notable catalytic properties.38 They are oxidation resistant and can be used in catalytic applications without loosing their magnetic properties. The FePt in the chemically disordered form presents a face centered cubic (fcc) crystalline structure and exhibits soft magnetic behavior. This characteristic can also be used for heating in the NPs by application of an external AC magnetic field; this phenomena is usually called hyperthermia and is used in medical applications. In a theoretical article Maenosono and Saita show that the FePt in cubic phase could be the best material for hyperthermia application.39 In this work the authors describe a new synthetic method to produce a three-blocks nanocomposite formed by FePt NPs (fcc phase) inside and Ru NPs outside MWCNTs (hereafter referred to as FePt@MWCNTs/Ru(NPs)) virtually free from impurities deriving from inside/outside particle contamination. In this nanocomposite the catalytic and magnetic properties are both available. To the best of our knowledge, this is the first example of a three-block nanocomposite based on CNTs and NPs.
3. RESULTS AND DISCUSSION 3.1. Synthesis of Nanocomposite. To obtain the nanocomposite made of magnetic FePt in chemically disordered fcc phase NPs inside MWCNTs decorated with catalytic metallic Ru NPs (see Figure 1), we split the synthetic procedure into two separate steps each including more than one stage. (1) Filling MWCNTs with FePt NPs; (2) decoration with Ru NPs.
Figure 1. Schematic representation of final nanocomposite of the synthesis study.
3.1.1. Filling with FePt NPs and Washing. To fill the MWCNTs and wash the external wall, we use a wet-method already developed.30 In our previous works31−34 we showed that the salt called hexaaquairon(II) hexachloroplatinate, ([Fe(H2O)6][PtCl6]), presents a layered structure that after mild reduction becomes an ordered FePt in L10 phase when heated at 400 °C. In this article, we show that lower temperature heating for a longer time almost exclusively produces an fcc phase. Therefore, this complex was used as starting salt to fill the MWCNTs. 3.1.2. Synthesis of the Starting Salt. Crystalline [Fe(H2O)6][PtCl6] is obtained by mixing equal atomic volumes (10 mL) of 0.2 M methanol solutions of H2PtCl6·6H2O and FeCl2, taking care to pour the salt into the acid. In this way the acidic environment (pH 1) prevents oxidation of Fe2+ to Fe3+. The reaction speed is increased by heating the solution at 40 °C, without stirring. The reaction begins immediately with slow development of HCl gas. After slow solvent evaporation at room temperature, yellow hexagonal crystals precipitate. The hexagonal crystals are then purified by recrystallization in ethyl alcohol. The synthesis can also be carried out in water, but the
2. EXPERIMENTAL SECTION All the product and solvent were used as purchased. Solid salt: chloroplatinic acid hexahidrate (H2PtCl6·6H2O), Iron chloride FeCl2 in analytic grade were purchased from Aldrich. Ruthenium(III) chloride hydrate (RuCl3 hydrate) was purchased from Fluka. The solvents: ethylene glycol, benzene and ethanol anhydrous were purchased from Carlo Erba. Mesitylen was purchased from Fluka. Nitric acid was purchased from Carlo Erba. The MWCNTs were purchased purified by Nanocyl S.A. (Namur, Belgium), and they were grown by the carbon catalytic vapor deposition technique. X-ray diffraction patterns were recorded on a Seifert X3000 diffractometer with a θ−θ Bragg−Brentano geometry with Cu Kα wavelength. Electron micrographs were obtained by a transmission electron microscope (JEOL 200CX) operating at an accelerating voltage of 200 kV. Finely ground samples were dispersed in n-octane in an ultrasonic bath. The suspension was then dropped on a copper 3394
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Figure 2. (a) TEM, (b) HRTEM of MWCNTs. (c, d) TEM and (e, f) HRTEM of FePt-filled MWCNTs.
Figure 3. Schematic representation of the FePt@MWCNTs nanocomposite synthesis (a) nanocomposite with the starting salt inside and outside MWCNTs. (b) Washing procedure with benzene protection. (c) Nanocomposite with starting salt only inside MWCNTs. (d) Reduction step. (e) FePt inside MWCNTs indicated as FePt@MWCNTs.
reaction and the subsequent solvent evaporation are slower; 1.1 g of purified [Fe(H2O)6][PtCl6] were obtained. 3.1.3. Filling Procedure. To fill the cavities the tips of the MWCNTs were first opened with nitric acid at 140 °C. The mixture was washed several times with water until pH 5. At this point separating the liquid phase from MWCNTs became difficult and the mixture was frozen in a cold bath at a temperature just below the solution freezing point. Under these conditions, the solvent was sublimated at a pressure close to 1 × 10−2 mmHg for 1 day. The procedure, called “lyophilization” or “freeze and dry”, is used repeatedly in our procedure. In this case, it allows complete separation and dry of the MWCNTs, which are now ready for the subsequent steps. XRD data (shown for convenience in Figure 4) for the empty opened nanotubes show the typical pattern reported in literature for MWCNTs, with a (002) reflection at 2θ =2 5.99, corresponding to an interlayer distance of 0.38 nm. TEM analysis reported in Figure 2a allows us to confirm the efficiency of the CNTs treatment with nitric acid being the
majority (about 80% calculated on 200 nanotubes) of the MWCNTs opened. High-resolution TEM (figure 2b) gives an interlayer distance of 0.38 nm in agreement with XRD data.To fill the MWCNTs cavity a starting solution of [Fe(H2O)6][PtCl6] salt (0.1M) was used following a two-step procedure. First, the opened MWCNTs was added to the aqueous solution of [Fe(H2O)6][PtCl6] salt at room temperature under vacuum (1 × 10−2 mmHg). The filling of the nanotubes occurs just by capillarity near the tips, but when the atmospheric pressure is quickly restored, by flowing nitrogen gas or air, the resulting high difference in pressure between the internal and external nanotube walls induces solution penetration into the MWCNTs cavity. After a few minutes of ultrasonic treatment and successive stirring, the mixture was lyophilized. This procedure allows saving approximately 90% of the expensive salt, which can be reused. Furthermore the amount of material inside the cavities can be controlled.30 At the end of the process, the sample is made of [Fe(H2O)6][PtCl6] salt clusters 3395
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encapsulated inside the MWCNTs, but also deposited on the external MWCNTs’ surface (see Figure 3a). 3.1.4. Cleaning the External Wall. Tessonnier et al.40 describe as a “novelty” the use of two solvents, one organic and the other one inorganic, to direct the reaction and to deposit metal nanoparticles only outside of carbon nanofibers. In a similar way, Bao et al.28 call a “trick” the use of the same twosolvent technique to protect the cavity during the washing step. Actually, the technique of the use of two solvents for the selective filling of cavities or protection from washing has been described30 for the first time, and patented,41 in our previous work, together with other techniques for selective and discrete filling of CNTs. Following this approach, the external wall of the MWCNTs was cleaned, while protecting the particles inside, by treating the solid mixture of filled MWCNTs and salt clusters with an organic solvent, such as benzene (Figure 3b), which does not dissolve the [Fe(H2O)6][PtCl6] salt. The solvent penetrates through the opened tips of the MWCNTs by capillarity and in this way protects the filling material from the washing solvent that is added right after. The washing solvent must be, at the same time, immiscible with benzene and able to dissolve the [Fe(H2O)6][PtCl6] salt present on the external wall. The use of water proved to be suitable because the solvent cannot enter into the tubes, full of benzene, owing to the relatively high surface tension at the water/benzene interface and the small area of the MWCNT ends. As a consequence, benzene protects the salts clusters located inside the MWCNTs from water, whereas the external clusters are in contact with both benzene and water (Figure 3b). Thanks to the well-known phase distribution process, the water-soluble salts linked to the external surface can be extracted by water and washed away. The washing procedure was repeated several times until completely colorless water was obtained. The lyophilization procedure was carried out again to obtain well separated MWCNTs. At this point, the MWCNTs are filled with the starting salt present only in the internal cavities (Figure 3c). 3.1.5. Reduction of [Fe(H2O)6][PtCl6]. At this stage of the process, the [Fe(H2O)6][PtCl6] clusters are present only inside the MWCNTs. To transform the [Fe(H2O)6][PtCl6] salt clusters into the FePt alloy in fcc phase, we heated up the ([Fe(H2O)6][PtCl6])@MWCNTs to 300 °C at a rate of 5 °Cmin−1, for 5 h in a reducing atmosphere of 5% H2 in argon. During the temperature increase several intermediate reactions take place31 and the complete chemical process can be summarized as follows
Figure 4. XRD patterns of opened (down), filled with FePt (middle), filled with FePt, and Ru decorated MWCNTs (top).
and the reduced size of the particles, lead to believe that the cubic disordered phase is mainly present. TEM analyses of the Intermediate nanocomposite FePt@ MWCNTs are showed in figure 2b-e. TEM analysis confirms the efficiency of the two-solvent cleaning procedure; spherical nanoparticles in the 2−5 nm range size being located only inside the CNTs cavities. In no case it has been observed nanoparticles on the outer wall surface or free Figure 2c, d). High-resolution TEM analysis on three single spherical nanoparticles in contact (Figure 2e) and on a single nanoparticles (Figure 2f) indicates a lattice plane distance of 2.2 Å, which corresponds to the (111) reflex of the cubic or tetragonal FePt phase. 3.2. Decoration with Ru NPs. In this section, the Ru NP deposition on the FePt@MWCNTs external wall and the techniques adopted to protect the inner cavities and to avoid the Ru NPs also grow insiding the CNTs walls are described. 3.2.1. FePt@MWCNTs Acid Refunctionalization. Several synthetic methods have been proposed for decorating the CNTs with Ru NPs. Liu and co-workers11 use a supercritical CO2−methanol solution of RuCl3 hydrate as a metal precursor; the deposition of the NPs that are produced is directed by the CNTs acting as a template agent. Lu42 compares different decoration methods as a function of the CNTs surface. In any case all authors agree on the important role played by the −COOH functionalization on the surface of external wall to be decorated. Owing to the treatment at 300 °C under reducing conditions the density of −COOH functions on the external surface of our sample has been greatly reduced and must be restored in order to obtain a satisfactory level of decoration with Ru NPs. This has been accomplished by treating approximately 30 mg of FePt@MWCNTs with 10 mL of 1 M solution of HNO3 for 10 min under ultrasound. Because of the low reactivity of FePt alloy and the reducing environment inside CNTs, the magnetic FePt NPs inside are not affected by mild acid attack, as proved by the fact that the magnetic
[Fe(H 2O)6 ][PtCl 6] + 3H 2 → FePt + 6HCl + 6H 2O
Leading to the MWCNTs filled with FePt NPs in fcc phase (Figure 3d,e). In our previous work32 the L10 phase was obtained working at a higher (400 °C) for 2 h and using a so short reaction time a mix and not isolable products were obtained but with a long reaction time the complete reduction happened and an fcc phase was obtained. The comparison of the XRD patterns of the opened MWCNTs, before and after their filling with FePt nanoparticles is reported in Figure 4. In the XRD pattern of the FePt filled MWCNTs reflections a broad extra peak centered about 41° appear; this peak can be assigned both to the cubic (PDF- Card N. 29−718) or to the tetragonal (PDF- Card N. 43−1349) FePt phase, being the main reflection of both phases. Anyway, the absence of the other important reflections of the tetragonal, 3396
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susceptibility values did not change after the acid treatment. The solution was repeatedly washed with water and separated by centrifugation until pH 5. Finally, 5 mL of water were added and the sample was sonicated and again frozen and dried (lyophilization). At this point the sample resembles soft and light cotton wad, black colored of course. The FePt@ MWCNTs tubes are almost untangled providing more surface available for the subsequent decoration steps. 3.2.2. Cavity Protection. In our case, there are two reasons to protect the cavities. The first one is avoiding the growth of Ru NPs inside the cavities; the second is that, as described by Chen et al.,43 the Ru ions may give rise to a redox reaction in which the Ru atoms replaces Fe in the FePt NPs, and this causes a loss of the magnetic properties. This happens especially if the reaction time is very long as in this case. In paragraph 3.1.3, the external wall of the MWCNTs filled with the starting salt ([Fe(H2O)6][PtCl6]) was cleaned by washing with water, while the inner cavities were protected with benzene. Now the external walls will be decorated by Ru NPs (see paragraph 3.2.3) using ethylene glycol (EG) as the solvent and a temperature reaction of 150 °C. Therefore, to protect the cavities and avoid the problems already mentioned, it is necessary to use a solvent immiscible in EG and boiling at a higher temperature. Mesitylen, which is immiscible with EG even at 150 °C and has a boiling point of 164 °C, was tested as protecting solvent. The FePt@MWCNTs were filled with mesitylen using the same technique applied in the paragraph 3.1.2. The pressure difference generated between the inside and the outside of the cavities pushes the solvent inside and prevents the EG and the reactants from entering inside the nanocomposite by capillarity. Different organic solvents could be probably used for protecting the cavities, but since mesitylen proved to be a suitable medium no additional tests were carried out. After filtering to eliminate the excess mesitylen, 5 mL of ethyl alcohol (EA) were added to the nanocomposite FePt@ MWCNTs (approximately 30 mg), the mixture was sonicated for a few seconds and centrifuged. The EA washings are needed to allow the EG to wet the walls of MWCNTs during the subsequent decoration step. EA, in fact, is miscible with both EG and mesitylen and decreases the surface tension between the two. At the same time a fast washing is necessary to prevent mesitylen from escaping the MWCNTs cavities. On the other hand even if the EA reaches the open tips of the MWCNTs increasing the reaction temperature up to 150 °C causes thermal expansion of the mesitylen and prevents any possible infiltration of the low boiling solvent before it is rapidly boiled off. 3.2.3. Decoration with Ru NPs. Lu et al.42 experimented several synthetic approaches with the aim of understanding the role of surface functionalization. In the following the simplest procedure described by Lu’s will be adopted, slightly modified to meet our needs (see Figure 5). Thirty milligram s of FePt@MWCNTs filled with mesitylen was dissolved in 10 mL of cold EG, with 5 mg of ruthenium chloride hydrate added. In the Lu’s synthesis, the pH of the suspension was adjusted to 9 with 1 M NaOH, but in this case, this high pH value could not be used because the NaOH can dissolve the FePt NPs. therefore the suspension was heated at 150 °C by means of an oil bath and kept at this temperature for 4 h without mechanical stirring. Even in the absence of stirring efficient homogenization is warranted by the high reaction temperature and the mixture, initially dark-red in color slowly becomes almost clear. After the 4 h reaction time, the mixture
Figure 5. Synthesis of final nanocomposite starting from FePt(fcc)@ MWCNTs protect by mesitylen.
was centrifuged, washed with deionized water and frozen and dried under vacuum (lyophilization). From the mesitylen protected reaction, it was possible to obtain the 3-fold nanocomposite: FePt@MWCNTs/Ru(NPs) virtually free from inside/outside particle contamination. The further decoration with ruthenium is confirmed by XRD data (figure 4 top). The XRD pattern of the final nanocomposites FePt@ MWCNTs/Ru(NPs) shows besides the typical reflections of the CNTs the presence of broad peaks at 2θ = 38.33, 42.21, 43.79, 69.53 all assignable to a hexagonal ruthenium nanophase (PDF−Card N. 6−663). The low amount of FePt (FePt@ MWCNTs) and the partial overlapping of the Ru reflections with the most intense peak of the FePt does not allows to clearly evidence the FePt phase. Figure 6a−d show TEM/HRTEM micrographs of ruthenium decorated FePt@MWCNTs nanocomposites. The Ru NPs with irregular shape and a particle size distribution quite narrow with an average size of 5 nm cover homogeneously the outer CNTs surface. Selected area electron diffraction (SAED) on the figure b allows to confirm the presence of ruthenium in agreement with XRD data. High-resolution TEM on several nanoparticles evidence (Figure 6d) the lattice planes of 2.3 and 2.1 Å ascribable respectively to the (100) and (101)−(002) ruthenium reflections. The Ru NP homogeneous coating (Figure 6a,cm and d) make it impossible to observe FePt NPs inside CNTs. To verify their presence inside the tubes, we have carried out the synthesis of a lower amount of RuCl3 (about half of typical synthesis). Images e and f in Figure 6 show respectively TEM and HRTEM of FePt NPs inside the Ru-decorated CNTs. If the same synthesis is carried out without protecting the cavities or using a low boiling solvent such as benzene, the results are dramatically different. TEM images a and b in Figure 7 clearly show that the Ru NPs are also deposited inside the cavities, and they seem to cover the FePt already present inside CNTs. Images c, d, and f show that often core/shell FePt/Ru nanostructures are formed. In particular, the high resolution image (d) and the corresponding fast Fourrier transform (FFT) image (f) indicate both ruthenium (2.1 Å, (101)−(002)) and FePt (1.9 Å, (200)) reflections confirming the core−shell structure. In some other cases, as shown in image e, a ruthenium nanoparticle growth nearby a FePt spherical nanoparticles. As suggested by Chen et al.,43 it cannot be excluded that the Ru not only cover the NPs, but the Ru cations can be exchanged with Fe atoms and give a three-metal alloy as FeRuPt. In that work, the reaction time was 1 h and the reaction temperature 85 °C in our synthesis we use 4 h at 150 °C. In these more dramatic conditions, the iron could probably be replaced more and the material lost the 3397
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Figure 6. (a−d) TEM, HRTEM, and SAED of FePt@ MWCNTs/Ru. (e, f) Images obtained from lower concentration of RuCl3 precursor in the synthesis.
Figure 7. Results of the synthesis without protection or with benzene protection; (a) Ru decoration, (b) Ru NPs inside MWCNTs, (c−e) FePt/Ru core−shell NPs inside MWCNTs.
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Figure 8. Room-temperature magnetization curves of open and empty MWCNTs (− ● −), FePt@MWCNTs (− ○ −), and FePt@MWCNTs:Ru (− Δ −) samples. The curves are normalized to (a) the total weight in and (b, c) the saturation magnetization.
MWCNTs/Ru(NPs), thus specifying that the FePt NPs are inside and Ru NPs are outside. This 3-fold nanocomposite is assumed to be relevant mostly for catalytic applications. The internal magnetic functionalization might easily allow magnetic separation and recycling of the catalyst (a short video demonstrating this property is in the Supporting Information) This multifunctional nanocomposite could also be important for environmental safety, because use of a magnetic trap could avoid loss of the expensive and pollutant catalyst material. Moreover, FePt in fcc phase NPs would exhibit soft properties and would allow to heat the catalyst by means of an alternate external magnetic field. In this way, only the catalytic nanoparticles would be heated, enhancing selectively their reactivity and saving the energy needed to heat the entire reactor. Finally, these kinds of nanocomposites might be relevant in other application fields such as medicine (drug delivery) or energy (photovoltaic, hydrogen storage). The method is not specific, so that different metals or, more generally, different inorganic NPs can be introduced in order to obtain different functionalizations suitable for other applications. The method is also scalable because lyophilization is an old industrial process generally used in the food industry. Catalytic tests are now under way using the nanocomposites described in this paper. Testing the effect of local heating by applying an external alternate magnetic field is in the next step of the reseach. The catalytically relevant synergic effects of the heating in term of selectivity, yield, and energy saving will be investigated.
magnetic properties. On the other hand, after decoration without protection, we do not find free FePt NPs and FePt core bigger than 3 nm, which mean that the smallest FePt NPs have probably been destroyed by ruthenium. So this proves that the protection of the cavities with a high boiling solvent is a crucial step of the synthesis in order to avoid inside/outside contamination. Room temperature magnetization curves of FePt-filled MWCNTs with and without Ru decoration are reported in Figure 8 together with the curve of open and empty MWCNTs. The curves are normalized to the total weight of the measured powder in Figure 8a and to the saturation moment in Figure 8b,c. The empty MWCNTs themselves have a hysteretic behavior, which is due to the presence of magnetic catalyst nanoparticles, which cannot be properly removed by chemical treatments. When MWCNTs are filled with FePt nanoparticles the normalized magnetic signal increases, because of the formation of FePt nanoparticles. As the filled MWCNTs are decorated by Ru nanoparticles (paramagnetic) the percentage of ferromagnetic material of the total weight reduces causing a decrease of the magnetic signal. The shape of the magnetization curves of our systems mainly depends on the combination of the magnetization curves of all present ferromagnetic components, i.e., the FePt nanoparticles and the MWCNTs magnetic catalysts. According to Figure 8b, filling MWCNTs with FePt nanoparticles leads to a reduction of the coercive field and remanence, which indicates that the FePt nanoparticles are magnetically soft (fcc), in agreement with the TEM and XRD results. After decorating the filled MWCNTs with Ru nanoparticles, the shape of the magnetization curve (Figure 8c) does not change significantly respect to FePt@ MWCNTs, indicating that the preparation procedure, especially the protection step, allows decorating the MWCNTs with Ru nanoparticles without destroying the FePt nanoparticles differently from that reported in ref 43, the magnetization curve of decorated but not protect FePt@MWCNTs (not shown) indeed have the same shape of open and empty MWCNTs.
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ASSOCIATED CONTENT
S Supporting Information *
Short video demonstrating how internal magnetic functionalization might easily allow magnetic separation and recycling of the catalyst (MPEG). This material is available free of charge via the Internet at http://pubs.acs.org.
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CONCLUSIONS This work describes a new method for selective filling and decoration of MWCNTs with different nanoparticles inside and outside the nanotubes, without intermixing the two kinds of particles. The new nanocomposite is indicated by FePt@
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 3399
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(37) Inomata, K.; Sawa, T.; Hashimoto, S. J. Appl. Phys. 1988, 64, 2537. (38) Wen, M.; Qi, H.; Zhao, W.; Chen, J.; Li, L.; Wu, Q. Colloids Surf., A 2008, 312, 73. (39) (a) Maenosono, S.; Saita, S. IEEE Trans. Magn. 2006, 42, 1638. (b) Hergt, R.; Dutz, S.; Muller, R.; Zeisberger, M. J. Phys.: Condens. Matter 2006, 18, S2919. (40) Tessonnier, J. P.; Ersen, O.; Weinberg, G.; Pham-Huu, C.; Dang Sheng, S.; Schlogl, R. ACS Nano 2009, 3, 2081. (41) Capobianchi, A.; Foglia S.; Imperatori P. International Patent Application No PCT/IT2006/000119, 2006. (42) Lu., J. Carbon 2007, 45, 1599. (43) Wang, D.-Y.; Chen, C.-H.; Yen, H.-C.; Lin, Y.-L.; Huang, P.-Y.; Hwang, B.-J.; Chen, C-C J. Am. Chem. Soc. 2007, 129, 1538.
ACKNOWLEDGMENTS The authors thank Dr. Donato Attanasio for useful discussions and Regione Autonoma della Sardegna, Programma Master and Back.
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REFERENCES
(1) Auffan, M.; Bottero, J. Y.; Chaneac, C.; Rose, J. Nanomed. 2010, 5, 999. (2) Iijima, S. Nature 1991, 354, 56. (3) Chu, H.; Wei, L.; Cui, R.; Wang, J.; Li, Y. Coord. Chem. Rev. 2010, 254, 1117. (4) Xing, Y. C. J. Phys. Chem. B 2004, 108, 19255. (5) Trancik, J. E.; Barton, S. C.; Hone, J. Nano Lett. 2008, 8, 982. (6) Fam, D. W. H.; Palaniappan, A.; Tok, A. I. Y.; Liedberg, B.; Moochala, S. M. Sens. Actuators, B 2011, 157, 1. (7) Chun, K.-Y.; Oh, Y.; Rho, J.; Ahn, J.-H.; Kim, Y.-J.; Choi, H. R.; Baik, S. Nat. Nanotechnol. 2010, 5, 853. (8) Karousis, N.; Tagmatarchis., N. Chem. Rev. 2010, 110, 5366. (9) Coq, B.; Kumbhar, P. S.; Moreau, C.; Moreau, P.; Warawdekar, M. G. J. Mol. Catal. 1993, 85, 215. (10) Galvagno, S.; Capanelli, G.; Neri, G.; Donato, A.; Pietropaolo, R. J. Mol. Catal. 1991, 64, 237. (11) Yuan, Y.; Smith, J. A.; Goenaga, G.; Liu, D.-J.; Luo, Z.; Liu, J. J. Power Sources 2011, 196, 6160. (12) Sun, Z.; Liu, Z.; Han, B.; Miao, S.; Miao, Z.; An., G. Science 2006, 304, 323. (13) Xin, F.; Li, L. Composites: Part A 2011, 42961. (14) Yoon, B.; Wai, C. M. J. Am. Chem. Soc. 2005, 127, 17174. (15) Lu., J. Carbon 2007, 45, 1599. (16) Ma, X.-M.; Lin, G.-D.; Zhang, H.-B. Catal. Lett. 2006, 111, 141. (17) Ang, L. M.; Hor, T. S. A.; Xu, G. Q.; Tung, C. H.; Zhao, S. P.; Wang., J. L. S. Carbon 2000, 38, 363. (18) Wu, H.; Liu, G.; Wang, X.; Zhang, J.; Chen, Y.; Shi, J.; Yang, H.; Hu., H.; Yang., S. Acta Biomater. 2011, 7, 3496. (19) Zhanga, Y.; Qina, W.; Tanga, H.; Yanb, F.; Tana, L.; Xiea, Q.; Maa, M.; Zhanga, Y.; Yaoa, S. Colloids Surf., B 2011, 87, 346. (20) Moradian, R.; Astinchap, B. NANO 2010, 5, 139. (21) Vatanpour, V.; Madaeni, S. S.; Moradian, R.; Zinadini, S.; Astinchap, B. Sep. Purif. Technol. 2012, 90, 69. (22) Monthioux, M.; Flahaut, E.; Cleuziou, J.-P. J. Mater. Res. 2006, 21 (11), 2774. (23) Sepahvand, R.; Adeli, M.; Astinchap, B.; Kabiri, R. J. Nanopart. Res. 2008, 10 (8), 1309. (24) Serp, P.; Castillejos., E. Chem. Catal. Chem. 2010, 2, 41. (25) Chen, W.; Pan, X.; Bao, X. J. Am. Chem. Soc. 2007, 129, 7421. (26) Pan, X.; Fan, Z.; Chen, W.; Ding, Y.; Luo, H.; Bao, X. Nat. Mater. 2007, 6, 507. (27) Yanga, H.; Songa, S.; R. Raoa, W., X.; Y, Q.; Zhang, A. J. Mol. Catal. A: Chem. 2010, 323, 33. (28) Wang, C.; Guo, S.; Pan, X.; Chen, W.; Bao, X. J. Mater. Chem. 2008, 18, 5782. (29) Camilli, L.; Scarselli, M.; Del Gobbo, S.; Castrucci, P.; Lamastra, F. R.; Nanni, F.; Gautron, E.; Lefrant, S.; D’Orazio, F.; Lucari, F.; De Crescenzi, M. Carbon 2012, 50, 718. (30) Capobianchi, A.; Foglia, S.; Imperatori, P.; Notargiacomo, A.; Giammatteo, M.; Del Buono, T.; Palange, E. Carbon 2007, 45, 2205. (31) Capobianchi, A.; Campi, G.; Camalli, M.; Veroli., C. Z. Kristallogr. 2009, 224, 384. (32) Capobianchi, A.; Colapietro, M.; Fiorani, D.; Foglia, S.; Imperatori, P.; Laureti, S.; Palange., E. Chem. Mater. 2009, 21, 2007. (33) (a) Capobianchi, A.; Laureti, S.; Fiorani, D.; Foglia, S.; Palange, E. J. Phys. D: Appl. Phys. 2010, 43, 474013. (34) Faustini, M.; Capobianchi, A.; Varvaro, G.; Grosso, D. Chem. Mater. 2012, 24, 1072. (35) Sun., S. AdV. Mater. 2006, 18, 393. (36) Xu, Y. F.; Yan, M. L.; Sellmyer, D. J. J. Nanosci. Nanotechnol. 2007, 7, 206. 3400
dx.doi.org/10.1021/cm3015447 | Chem. Mater. 2012, 24, 3393−3400
