High-Density Monodispersed Cobalt Nanoparticles Filled into

Communication pubs.acs.org/cm

High-Density Monodispersed Cobalt Nanoparticles Filled into Multiwalled Carbon Nanotubes Walid Baaziz,† Sylvie Begin-Colin,*,† Benoit P. Pichon,† Ileana Florea,† Ovidiu Ersen,† Spyridon Zafeiratos,‡ Roland Barbosa,‡ Dominique Begin,‡ and Cuong Pham-Huu*,‡ †

Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504 du CNRS, Strasbourg University, 23 rue du Loess, 67037 Strasbourg Cedex 08, France ‡ Laboratoire des Matériaux, Surfaces et Procédés pour la Catalyse, UMR 7515 du CNRS, Strasbourg University, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France S Supporting Information *

KEYWORDS: cobalt nanoparticles, carbon nanotubes, magnetic properties, multifunctional nano-object

M

Hollow vapor-grown multiwalled CNTs (Pyrograf Products) were used as a starting material for the filling of magnetic NPs in the present work. The CNTs exhibit a straight and openended channel with an average inner diameter of about 80 ± 30 nm according to the TEM analyses (see Figure S1 in the Supporting Information). Before filling, the CNTs were annealed under argon flow at 900 °C for 2 h in order to remove the oxygenated functional groups initially present on their surface (see Figure S2 in the Supporting Information).22 The surface concentration of oxygen species as a function of the annealing temperature determined by in situ XPS analysis, is presented in Figure S3 in the Supporting Information. According to the XPS results, the oxygen amount gradually decreases with annealing temperature. At 650 °C, nearly all oxygen species were removed, which renders the surface of CNTs more hydrophobic. This is imperative in preventing cobalt nanoparticles to get attached through these oxygen functionalities at the outside walls of the nanotubes. The cobalt NPs casted inside the CNTs were obtained by introducing the CNTs in an octadecene suspension containing a cobalt stearate complex and oleic acid as surfactant and by submitting this mixture to a thermal decomposition at 318 °C. The details of the synthesis are presented in section 3 in the Supporting Information. One may notice that when the synthesis is performed without introducing CNTs inside the reactant mixture, the thermal decomposition is incomplete and leads to a highly viscous material difficult to wash (see detailed discussion below). The nature of the Co NPs grown in the presence of CNTs was checked by XRD (Figure 1). A good Rietveld refining of the XRD pattern is obtained by considering the presence of two different phases, namely Co and CoO, which were indexed according to JCPDS cards n° 015−0806 and 43−1004, respectively. However, some surface Co3O4 could not be ruled out. For comparison, the XRD pattern of NPs synthesized without CNTs shows only a single phase, namely CoO. The

etal and/or oxide magnetic nanoparticles (NPs) have emerged as a new class of materials that are interesting in several application areas going from electronic and magnetic devices1−6 to catalysis and medicine (drug delivery and MRI contrast agents).7−9 Importantly, their physical and chemical properties are distinctly different from those of the bulk materials and depend mainly on the NP size and morphology.1 Furthermore, the assembling of these magnetic NPs into tailored structures (1D, 2D, and 3D) is a promising strategy for production and design of hybrid materials with new functions.10−12 It is expected that selective casting of these magnetic NPs inside the channel of multiwalled carbon nanotubes (CNTs) could give rise to a new family of hybrid materials with novel physical properties which can find applications in either magnetic devices or catalysis like for the Fischer−Tropsch synthesis. In the field of catalysis, the metal or metal oxide that was entrapped inside the carbon nanotubes channel was found to exhibit unusual catalytic behavior because of the confinement effect provided by the surrounding CNT walls.13−15 Fabrication of nanostructures where the NPs are integrated into a conducting network with a quasi-1D dimension, has great potential interest for applications in several fields of research. Most of the filling techniques reported nowadays are based on the use of capillary forces to fill-up the CNTs channel.16−18 To increase the selectivity of the filling, i.e. outer surface decoration or inner channel filling, CNTs were previously selectively functionalized.19−21 However, the weight loading of the filled NPs is generally low compared to the overall weight of the CNTs and thus, hindered their subsequent use in the field of magnetic devices or organic pollutants removal where high density filled material is often required. It is of interest to find out a new filling technique that could allow on one side achieving the high filling of the CNTs channel with NPs and on the other, making the scaling-up and reproducibility easier. Herein, we report on the development of a new liquid-phase filling of CNT channel with magnetic cobalt-based NPs of uniform size and shape along with an extremely high loading, i.e. up to 60 wt %, which could find use in the fields of magnetic, catalysis, battery, electrochemical devices, or wastewater treatment. © 2012 American Chemical Society

Received: January 27, 2012 Revised: April 11, 2012 Published: April 18, 2012 1549

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Figure 1. (A) XRD pattern of the CNTs containing cobalt composites after synthesis showing the presence of several cobalt species: Co (fcc structure) and CoxOy. (B) XRD pattern of the same cobalt NPs synthesized without CNTs under the same conditions. Blue lines, CoO; red lines, Co.

absence of metallic cobalt, in that case, suggests that the CoO phase is formed by air oxidation during the recovery and storage procedure, as usually observed during the synthesis of iron oxide NPs from iron stearate or iron oleate complex.23,24 In contrast, the presence of a relatively large amount of metallic cobalt inside the casted sample indicates that the oxidation process was significantly hindered by the surrounding CNTs. Such a result has been reported in a previous work dealing with the CoFe alloy casted inside the CNT channels.25 On the other hand, the same synthesis conditions without CNTs conducts to a low decomposition of reactants and so a low yield in NPs which are furthermore, embedded in a viscous material difficult to wash. Without CNTs, the decomposition temperature of the cobalt stearate (see Figure S4 in the Supporting Information) is too high by comparison with the boiling point of the solvent (318 °C) and thus the decomposition yield of cobalt stearate into cobalt NPs is low. The resulting viscous material is mainly due to the presence of viscous free oleic acid. The presence of a high amount of cobalt-based NPs inside CNTs clearly shows that the formation of cobalt based NPs is favored inside the CNTs channel. One may thus conclude that the CNTs act as nanoreactors to promote the nucleation and growth of cobalt based NPs.26,27 The confinement effect of CNT walls has been previously pointed out to favor the stacking of CoFe2O4 nanowires along the channel axis at low temperature.28 Representative TEM micrographs taken from several areas of the sample show that the cobalt-based NPs were mostly inside the CNT channel either in the form of single NPs, with lateral size similar to that of the CNT diameter channel, stacked along the CNT’s channel or as a double range NPs in the case of CNT with larger inner channel (Figure 2A,B). Statistical TEM analysis indicates that the cobalt-based NPs have an average lateral size of around 50 ± 5 nm. An extremely high density of cobalt-based NPs inside the CNT channel is observed regardless their diameter. Statistical two-dimensional TEM analysis also indicates that the filling of the CNTs by the cobaltbased NPs is relatively high, i.e., about 90%. It is worth noting that at the best of our knowledge, it is the first time that such a high filling density is reported. Such a high filling was attributed to the large inner diameter of the CNTs, which is easier to fill with the solution medium during the synthesis. It has been reported previously that CNTs with inner diameter close to 30 nm can be easily filled with NPs after the solution containing the metal salt precursor was drained in the tubes by capillary forces.29 High-resolution TEM micrographs (Figure 2C, D and Figures S5 and S6 in the Supporting Information) also indicate

Figure 2. (A, B) Representative TEM micrographs of the Co NPs with different filling density casted inside the CNT channel. (C) Highresolution STEM-BF and (D) STEM-HAADF of the cobalt oxide NPs encapsulated inside the CNT channel in B showing the faceted and highly porous microstructure of the NPs. The porous structure within each cobalt-based NP is highlighted by a dark contrast.

that the Co NPs were relatively porous. Such a porosity and shape suggest that the formation of colloidal crystals (superlattice) is formed by a crystallization process inside the CNT channel driven by oleic acid molecules.30 The representative STEM and EELS line scan confirms the relatively high Co:O atomic ratio of the encapsulated Co NPs, which indicates that surface oxidation is relatively low on these Co NPs casted inside the CNT channel compared to those synthesized without CNTs, which were rapidly and completely oxidized to CoO (XRD data in Figure 1). TGA analyses (not shown) indicate that cobalt-based loading (weight of cobalt versus the total weight of the final material) was about 60 ± 5% with respect to the overall weight of the composite material. The magnetization measurements were carried out on the nanocomposite under an external applied magnetic field at 5 and 400 K (Figure 3). The magnetization steadily increases as a

Figure 3. Magnetization curves of the CNTs containing cobalt composite at 5 and 400 K. Pictures in inset: Optical photos of the CNT containing a cobalt phase solution after ultrasonication (0 s) and after contacting with an external magnet for 600 s.

function of the external applied magnetic field, which indicates that NPs present in the CNTs are magnetically active. The saturation magnetization value of 57 emu .g−1, measured at 5 K, is lower than bulk cobalt (168 emu g−1) and may be explained by the presence of antiferromagnetic CoO and/or of a spin canted layer.23 Both curves recorded at 5 and 400 K display a 1550

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hysteresis featured by coercive fields (HC) of 470 and 310 Oe, respectively which agree with a ferromagnetic behavior. The decrease of HC with the increase in the recording temperature is related to the increase of the thermal energy versus the magnetocrystalline anisotropy of NPs. The magnetic behavior is also confirmed by exposing the nanocomposite suspension in chloroform alongside with a magnet (picture in inset of Figure 3). The picture demonstrates that the nanocomposite can be easily manipulated by an external magnetic device as all the nanocomposite was removed from the solution within a few minutes after contacting with an external magnetic device. This might act as a clear advantage compared to the conventional filtration or centrifugation processes. In summary, the present work allows one synthesizing cobalt-based NPs with an extremely homogeneous size (50 nm) which are well selectively localized within the CNT channel. The CNTs play a role of nanoreactor to speed-up the decomposition and then the crystallization processes, whereas in the absence of CNTs, almost no cobalt-based NPs are formed. It is worth noting that such a high filling density of 60 wt %, has never been reported up to now in the literature.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the synthesis method, complementary TEM and HRTEM analysis, XPS micrographs, EELS-STEM analysis (PDF). This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.B.-C.); [email protected] (C.P.-H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. Guillaume Dalmas (LMSPC) and Alain Derory (IPCMS) for performing the magnetic separation and magnetic measurements.

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REFERENCES

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