Elsevier

Applied Surface Science

Volume 456, 31 October 2018, Pages 657-665
Applied Surface Science

Full Length Article
Co metal nanoparticles deposition inside or outside multi-walled carbon nanotubes via facile support pretreatment

https://doi.org/10.1016/j.apsusc.2018.06.124Get rights and content

Highlights

  • Co/MWCNT hybrids with controllable localization of Co nanoparticles are obtained.

  • Localization of Co nanoparticles depends on the nature and structure of MWCNTs.

  • Hydrophobic/hydrophilic properties of MWCNT surface strongly affect Co deposition.

  • Oxidized MWCNTs with thin walls stabilize Co nanoparticles mainly inside their channels.

  • Oxidation of MWCNTs leads to the decrease of Co nanoparticles average size.

Abstract

Decoration of one-dimensional multi-walled carbon nanotubes (MWCNTs) with zero-dimensional Co nanoparticles leads to hybrid structures with chemical and electromagnetic features that are not available to the individual components. This work addresses the influence of the nature and structure of MWCNTs on the localization of Co nanoparticles. Depending on synthesis conditions, Co can be deposited on the external or in inner surfaces of the nanotubes. Co/MWCNTs hybrids have been characterized by in situ X-ray powder diffraction, high-resolution transmission electron microscopy and 59Co internal field nuclear magnetic resonance. It has been shown that the average diameter (7.2, 9.4 and 18.6 nm), number of walls (5–7, 12–15, 15–20), and functional composition of the MWCNTs have a remarkable effect on the size of Co nanoparticles and their distribution in the structure of MWCNTs. The observed phenomenon has been rationalized in terms of nanotubes surface properties. Parent MWCNTs being hydrophobic and having limited porosity do not stabilize Co nanoparticles and, therefore, they are localized on the outside surface with relatively large average size and broad size distribution. On the other hand, the oxidation of the MWCNTs resulted in the penetration of Co nanoparticles inside of the nanotubes, presumably because of pore opening as well as increased hydrophilicity of the nanotubes.

Introduction

The decoration of multi-walled carbon nanotubes (MWCNTs) by cobalt nanoparticles expands the range of their functional properties by providing them with new magnetic, catalytic, electronic and electro-magnetic characteristics. MWCNTs filled with magnetic Co-containing nanoparticles have been proved to be useful for multiple innovations in nanotechnology [1], [2], [3], Li/air batteries [4], [5], [6], [7], [8], magnetic-storage devices [9], magnetic composites for drug delivery [10], [11], catalysts for different processes [12], [13], [14], [15], [16], [17], [18], as well as absorption and microwave irradiation shielding material [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. It is generally accepted that the magnetic properties of nanoparticles are determined by many factors, such as chemical composition [20], [31], [32], [33], [34], [35], crystallinity [36], [37], size [37], [38], [39], [40], [41] and shape [35], [42], [43], [44], [45]. The possible interaction of the nanoparticles with the surrounding matrix and neighboring nanoparticles [46], [47], [48], [49], [50] should be also mentioned in the list. Hence, changing the size, shape, composition and structure of nanoparticles, it is possible, within certain limits, to control the magnetic characteristics of the composite materials. Moreover, intimate interphase contacts between the Co nanoparticles and MWCNTs result in additional synergistic effects. It is thus possible to further tune the electrophysical properties by adjusting, on the one hand, the dielectric component via the MWCNTs structure and content and, on the other hand, the magnetic properties by means of Co nanoparticles loading. This multiplicity of possible leverage makes MWCNTs promising building blocks for the formation of new hybrid materials with new electromagnetic properties. However, this requires a good control of the location and distribution of Co nanoparticles in the structure of MWCNTs.

Depending on the MWCNTs structure parameters (the average size of outer and inner diameters, number of layers, surface area, aspect ratio, defectiveness and the functional composition of their surface) as well as the preparation method of Co/MWCNT hybrids, the size, shape and location of the Co nanoparticles can be varied. Presently, it is still a great challenge to uniformly deposit 3d group magnetic nanoparticles onto both the inner and outer surfaces of MWCNTs without altering its hollow tubular structure. A common synthetic route to produce Co/MWCNT hybrids is incipient wetness impregnation with cobalt salt solutions. However, the specific features of the cobalt nanoparticles formed by this method in, or on, MWCNTs remain unclear. It is very important to consider various factors that may influence the formation and distribution of cobalt particles in the structure of MWCNTs. Such factors include (i) processes occurring in the impregnating solution; (ii) surface charge of MWCNTs; (iii) structure and functional composition of MWCNTs. According to the literature data [51], [52], structure, surface area, surface functional composition, defects of outer layers, size distribution, agglomeration state, and purity of the samples have considerable influence on the surface charge and the reactivity of carbon nanotubes. MWCNTs are amphoteric by nature, which means that acid and basic functionalities coexist on their surface and that in an aqueous medium. Consequently, depending on the pH and the nature of the MWCNTs surface functionalities, positive or negative charges may be present. In particular, oxidation of pristine MWCNTs leads to the formation of different oxygen-containing groups in the boundary layers, namely carbonyl, carboxyl, lactone, quinone, ether, and hydroxyl groups, the content and distribution of which depend on the oxidation conditions [53], [54]. The appearance of oxygen containing groups can lead under appropriate conditions to the formation of negative charges on the MWCNTs surface, and consequently to a better adsorption of metal cations and, eventually, to a satisfactory dispersion of nanoparticles in the MWCNT structure [54], [55], [56].

Previously [22], we investigated the effect of varying Co content nanoparticles, distributed in the structure of MWCNT, on their magnetic and electromagnetic properties. The present work addresses the issue of Co dispersion when synthesizing Co/MWCNT hybrid materials. A special attention has been paid to the impact of the structure and surface functionalities of MWCNTs on the mechanism of metallic cobalt nanoparticles fixation as well as on their ultimate location and size distribution. The structure and morphology of pure and Co-containing MWCNTs has been monitored by high-resolution transmission electron microscopy (HRTEM) while the structure of the cobalt metal in the hybrids after the reduction has been essentially investigated by 59Co internal field nuclear magnetic resonance (IF-NMR). This technique provides information about the structure (fcc and hcp stackings) and the average size of Co metal nanoparticles through their magnetically single- and multidomain character. It has been complemented by in situ synchrotron X-ray diffraction (in situ XRD) to provide the evolution of all crystalline phases versus temperatures and monitor the evolution of their average sizes.

Section snippets

Synthesis of MWCNTs and functionalization

MWCNTs were synthesized by ethylene decomposition over bimetallic Fe-Co catalysts at 680 °C. The narrowest MWCNTs (labelled MWCNT-7) with an average outer diameter of 7.2 nm and the smaller number of walls (5–7) considered in this study were formed over the 30 wt% Fe2Co/Al2O3 catalyst; MWCNT with medium outer diameter 9.4 nm and wall number (12–15) over the 40 wt% Fe2Co/Al2O3 catalysts (labelled MWCNT-9); and the larger diameter MWCNT (labelled MWCNT-18) of 18.6 nm with 15–20 walls over the

Effect of the MWCNT structure on the Co particles formation

Multi-walled carbon nanotubes are amphoteric by nature, which means that acidic and basic functionalities coexist on their surface. In an aqueous medium, these functionalities, depending on the pH, provoke a distribution of charges. At pH below the point of zero charge, the balance of charges is positive while for pH above, it is negative. Consequently, electrostatic interaction favors the adsorption of anions in aqueous solutions of pH below the PZC and the one of cations at pH above the PZC.

Discussion

Many processes, such as hydrolysis, dissociation, and complexation, must be taken into consideration to determine the different ionic forms existing in aqueous solutions. Taking into account the relevant hydrolysis and dissociation constants, it can be predicted that 99.997% of the Co in the impregnation solution of Co nitrate (II) at pH 3.95 was in the form of Co2+ (see confirmatory calculations in the supplementary materials). The value of the PZC of the oxidized MWCNTs was always around of

Conclusion

The effect of the MWCNTs nature (diameter and number of walls) as well as their oxidative pretreatment on the Co nanoparticle formation by incipient wetness impregnation followed by reduction has been investigated by in situ XRD, HRTEM and 59Co IF-NMR.

Pristine MWCNTs (7, 9 and 18), hydrophobic and with limited porosity, did not stabilize Co nanoparticle which only formed outside of the MWCNTs with an average particles size of 20 nm and a very wide size distribution as well as poor resistance to

Acknowledgements

The reported study was funded by the Russian Foundation for Basic Research via grant 16-32-60046 mol_a_dk (Mariya A. Kazakova). Olga B. Lapina and Andrey S. Andreev are grateful to the Russian Foundation for Basic Research, which provided the 59Co NMR studies via grant 17-53-150018. A.S. Andreev was also supported in part by a PhD grant from the French Embassy in Moscow and by the Société des Amis de l’ESPCI.

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