Nanolayers on Carbon Nanofib - American Chemical Society

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In Situ Deposition and Characterization of MoS Nanolayers on Carbon Nanofibers and Nanotubes I. Zafiropoulou, Marios S Katsiotis, Nikolaos Boukos, Michael A Karakassides, Samuel Stephen, Vasilis Tzitzios, Michael Fardis, Radu Valentin Vladea, Saeed M. Alhassan, and George Papavassiliou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp400498x • Publication Date (Web): 22 Apr 2013 Downloaded from http://pubs.acs.org on April 23, 2013

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The Journal of Physical Chemistry

In Situ Deposition and Characterization of MoS2 Nanolayers on Carbon Nanofibers and Nanotubes I. Zafiropoulou1, M.S. Katsiotis2,*, N. Boukos1, M.A. Karakassides3, S. Stephen2, V. Tzitzios1, M. Fardis1, R.V. Vladea2, S.M. Alhassan2, and G. Papavassiliou1

1

Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems, National Centre for Scientific Research “Demokritos”, 153 10 Aghia Paraskevi, Attiki, Greece. 2

Department of Chemical Engineering, The Petroleum Institute, PO Box 2533, Abu Dhabi, United Arab Emirates. 3

Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece

*

Corresponding author: [email protected], Tel: +971-2-6075225, Fax: +971-2-6075200

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ABSTRACT We report on the deposition of thin nanolayers of MoS2 material on carbon nanofibers (CNFs) and multi-walled carbon nanotubes (CNTs) by in-situ thermal decomposition of (NH4)2MoS4 in oleylamine. The synthesized MoS2@CNF and MoS2@CNT nanocomposites were characterized by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Electron Energy Loss Spectroscopy (EELS) and Raman spectroscopy. TEM results indicate that CNFs, as well as CNTs, provide an excellent template for the deposition and growth of MoS2 nano layers. 2-5 layers thick MoS2 nanocrystals and 5-10 nm wide form on the CNFs, while monolayers of the same width form on the CNTs. The advantage of the applied synthetic method is that it provides control over the material morphology, leading to uniform and full cover of the CNFs and CNTs with no individual formations of material away from the carbon. To the best of our knowledge, the deposition of MoS2 on CNFs has not been reported elsewhere.

KEYWORDS Oleylamine, supported hydrotreating catalyst, turbostratic carbon, TEM, EELS, Raman Spectroscopy.


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INTRODUCTION Coating of carbon nanotubes (CNTs) and their related nanomaterials, i.e. nanofibers and onion-like particles, with inorganic materials is a promising area for various applications, ranging from heterogeneous catalysis and hydrogen storage, to biochemical sensing applications and lithium batteries. Prerequisite for this is the matching between the lattice of the inorganic material and the lattice of the carbon nanotemplate. In case of CNTs the structure consists of cylindrical graphene layers, whilst CNFs structure consists of stacking of different graphite sheets, which are oriented at an angle with respect to the fiber axis, displaying prismatic planes with high reactivity for the adsorption of the deposited active phase. On the other hand, Transition Metal Chalcogenides (TMC), such as MoS2 and WS2, produce layered, hollowed closed-cage or tubular nanostructures, which are similar to graphene,1 carbon fullerenes and carbon nanotubes respectively, hence there is a structural affinity between CNT-like materials and TMC nanomaterials. Experiments have shown that when MoS2 is grown on a CNT template tubular layers are formed,2 possessing the typical two-dimensional layered structure of MoS2, where hexagonally packed layers of metal atoms are sandwiched between sulfur layers.3,4 Currently, two methods are most usually applied to produce MoS2/CNTs composites. The first is by wet impregnation of Mo(IV) compounds and CNTs in a reduction atmosphere.5,6 This is a two-step process, the second step being a thermal treatment at ~500 oC, which apart from being energy demanding and time consuming, destroys the morphology and increases the size of particles, especially in the case of nanoscale materials. The second method for the preparation of MoS2/Carbon composites is the hydrothermal reaction of a MoS2 source in the presence of CNTs.7,8 Although this is usually a slow process, which can take up to 72 h duration, it leads to the formation of a homogeneous “sleeve-like” MoS2 film on the CNTs surface.9 In this way, novel composite materials have been synthesized with promising enhanced properties, especially in the field of heterogeneous catalysis.10 Many authors have suggested the application of carbon nanofibers as catalyst support due to: i) their good metal/support interaction caused by the presence of the prismatic planes on the nanofibers surface, ii) their high specific surface area that offers a better contact reactants/active sites and iii) absence of the ink-bottled pores that reduces the diffusion phenomena, mainly in liquid phase reactions. The carbon nanofibers composite can be efficiently used as catalytic support in reactions where diffusional phenomena of the reactants are essential and in reactions with high mass and heat transfers. Moreover,



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the high thermal conductivity of the carbon based support allows a fast homogenization of the heat generated during the reaction throughout the catalyst body, preventing the formation of hot spots which are detrimental to the active phase dispersion and catalyst body conservation. In the present study, both CNFs and CNTs have been considered as supports for the deposition of MoS2. The main scope is to reveal how MoS2 is formed on two different carbon nanostructures using oleylamine, with focus to produce catalytically favorable materials (see below). The CNTs serve more as a comparative material, since the deposition of MoS2 on CNTs is well reported in the literature. CNFs on the other hand may be preferred, mainly due to the lower cost – important for industrial application – high tensile strength and their turbostratic structure.11-15 The latter gives a “defective surface” to the fiber, serving both the deposition of the material, and its catalytic activity.16 More specifically, carbon nanofibers commonly exhibit highly disordered outer regions, where the graphitic layers are either very loosely attached, discontinuous or have collapsed, thus exhibiting decreased sp2 hybridization at the surface. These loosely attached graphite layers provide numerous defects, sites where metals can be deposited and be attached. Molybdenum disulfide, which exhibits a layer-like structure similar to graphite, can “fill” these defects and attach itself by forming Van der Waals bonds with carbon molecules. Should this attachment take place in areas where the graphite walls are greatly defected, then this would lead to the formation of MoS2 particles with small number of layers, which are considered to be highly catalytically active. This is due to the catalytic reaction mostly taking place on the edges of the MoS2 formations17. In addition, it has been shown that for desulphurization reactions, the ideal number of MoS2 layers is between 3 and 5,18 therefore a fractured material consisting of a large number of thin – small surfaced sheets of catalyst would be ideal.19 In this work we present the synthesis and characterization of MoS2@CNF and MoS2@CNT nanocomposites, by using a simple thermolytic synthesis method, based on the use of oleylamine as a multiple role-playing reagent, with high adaptability.20-23 It is a procedure characterized by simplicity and low cost, while the reaction conditions involved are mild, with temperatures up to 350 oC, duration up to 2 h, ambient pressure (no autoclave devise is required) and N2 atmosphere. Also, this synthesis method allows synthesizing the material in a one-step process, avoiding the thermal treatment step required in the case of the impregnation method. Via this technique the MoS2 is in situ synthesized in the presence of the support, all in liquid phase, providing most importantly control over the material morphology. Uniform and full cover of the CNFs/CNTs can be achieved, with no individual formations of MoS2 away from the support. HRTEM measurements indicate that the outer surface of


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CNFs is almost fully coated by the layered MoS2, which are roughly parallel with the basal planes of the CNFs. EXPERIMENTAL METHOD Materials Multiwall carbon nanotubes (CNT) were supplied by Nanocyl (Nanocyl NC7000, 90% purity), while carbon nanofibers were supplied by Pyrograf Products Inc. (Pyrograf III, 95% purity). Preliminary characterization showed that the nanotubes are 89.5% pure (TGA), have BET specific surface of 322±12 m2/g and pore volume of 0.775 cm3/g. Nanofibers exhibited a purity of 98.2%, BET specific surface of 81±6 m2/g and pore volume of 0.116 cm3/g. Carbon nanotubes and nanofibers were used as purchased, without any pretreatment. Synthesis The synthesis is based on the thermolytic decomposition of ammonium tetrathio-molybdate - (NH4)2MoS4 in oleylamine. (NH4)2MoS4 is commonly used as precursor for MoS2 synthesis, dispersed usually in aqueous solutions as it has very low solubility in organic media. However, in the current synthetic procedure, oleylamine would initially form a homogeneous reddish complex with (NH4)2MoS4, which exhibited high solubility in organic solvents. In addition, carbon based materials disperse very well in oleylamine, due to the surface functionalization through the amine group. Although this surface modification is quite weak, it allows for the spontaneous deposition of the produced MoS2 on the surface of carbon material. In a typical synthesis procedure carbon nanofibers or multi-walled carbon nanotubes were suspended in oleylamine via sonication. (NH4)2MoS4 was added, with the quantity adjusted to the desired catalyst loading. The reaction mixture was heated for 2 h at reflux conditions (~350 oC) under N2 flow. After air cooled, the material was received by centrifugation and washed several times with a mixture of hexane and ethanol. Characterization Structural characterization was performed by X-ray diffraction using a Siemens D500 diffractometer, with Cu Kα radiation (λ=1.5418 Å) and by Transmission Electron Microscopy (TEM) with an FEI CM20 microscope equipped with a Gatan GIF200 energy filter and an FEI Tecnai G20 equipped with a Gatan GIF 963 energy filtered camera (0.9 eV energy resolution FWHM



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at zero loss peak). Energy Filtered TEM (EFTEM) mapping was applied to reveal the location of molybdenum and sulfur on the surface of the tubes and fibers. Elemental maps were collected at the relevant core losses of Mo (227eV) and S (165eV) using the well-known three-window method provided with Gatan software.24 In addition, carbon hybridization of nanotubes and nanofibers (at both the surface and core) was identified by collecting EELS spectra at the K-edge carbon core loss. For the preparation of the TEM samples the material was dissolved in hexane and allowed to dry on holey carbon coated Cu grids. For a typical experiment, the sample would be inserted immediately following preparation, in order to avoid contamination. EFTEM and EELS measurements were performed on several areas of low thickness (

Nanolayers on Carbon Nanofib - American Chemical Society

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