23. - 25. 10. 2012, Brno, Czech Republic, EU

IN SITU FORMATION OF NANOCOMPOSITES BASED ON CARBON NANOTUBES

AND PHYLLOSILICATES (KAOLINITE, NONTRONITE AND SEPIOLITE)

Magdaléna KADLEČÍKOVÁ a, Katarína BÉDIOVÁ b, Juraj BREZA a, Karol JESENÁK b,

Michal KOLMAČKA a, Jozef KADLEČÍK c, Ľubomír VANČO a, Mária ČAPLOVIČOVÁ b, Filip

LAZIŠŤAN a

a Slovak University of Technology, Bratislava, Slovak Republic, magdalena.kadlecikova@stuba.sk

b Comenius University, Bratislava, Slovak Republic, EU

c Slovak Institute of Metrology, Bratislava, Slovak Republic, EU

Abstract

The article verifies the possibility of synthesizing nanocomposites based on carbon nanotubes and three

minerals – phyllosilicates kaolinite, nontronite and sepiolite. Variability of phyllosilicates allows preparing

catalyst carriers with various contents of active metals. Under the same technology conditions, the type of

catalyst carrier affects the morphology and structure of the final nanotube product markedly. Synthesis of

CNTs was performed by hot filament chemical vapor deposition. The produced nanocomposites were

examined by transmission and scanning electron microscopies and energy dispersive X-ray spectroscopy.

Each of the prepared nanocomposites may be advantageous for a certain field of applications.

Keywords: CVD, CNTs, nanocomposites, kaolinite, nontronite, sepiolite

1. INTRODUCTION

We continue on our previous experiments that were studying the growth of CNTs on montmorillonite and

zeolite [1, 2]. Now we examine the suitability of other clay minerals for the synthesis of CNTs and CNT/clay

nanocomposites using the same technology, hot filament chemical vapor deposition (HF CVD). We have

chosen kaolinite, nontronite and sepiolite as carrier minerals, and – similarly like in our previous works –

incorporated particles of iron as the active phase. The details of experiments growth of multi-walled carbon

nanotubes on kaolinite, nontronite and sepiolite were published in [3]. In this paper, we show the differences

in building the nanocomposites. Numerous substances have been tested as carriers of catalytically active

metals for grown CNTs, for instance SiO2 [4], Al2O3 [5], CaCO3 [6] and MgO [7]. Great attention has been

paid to the synthesis of CNTs on clay carriers. These results are summarized mainly in [8, 9]. In general one

can say that the advantage of clay minerals in the synthesis of CNTs is particularly the small dimension of

their particles, ion exchange properties and – from the application point of view – also their low price.

2. EXPERIMENTAL MATERIALS AND CONDITIONS

The following clay minerals have been used to prepare the catalysts: kaolinite, clay from Warren County,

Georgia, USA, nontronite, natural dark-brown clay from Uley Mine, South Australia, and sepiolite from

Valdemore, Spain. Ferrous forms of kaolinite and sepiolite were prepared from the obtained fractions by ion

exchange reactions. The method of metal infiltrated samples consisted in 12 hour stirring of 100 mg of the

clay mineral with 25 ml of the water solution Fe(NO3)3·9H2O (c=0.04 mol dm–3

). Nontronite was not modified.

The suspensions were stirred for 12 h and afterwards deposited on a polished Si wafer and allowed to dry

quickly under an infrared lamp. In situ synthesis of nanocomposites was carried out in a HF CVD reactor,

where the precursors are activated by five carbonized tungsten filaments heated up to 2200°C. The working

atmosphere was a mixture of methane and hydrogen. During deposition, a DC bias of 100 V was applied to

23. - 25. 10. 2012, Brno, Czech Republic, EU

the substrate holder. The pressure and temperature during deposition were 3000 Pa and approx. 600°C,

respectively, and the synthesis time was 30 min.

Examination of the nanocomposites and CNTs was performed using a transmission electron microscope

JEOL JEM 2000 FX, high resolution transmission electron microscopy characterization was carried out using

a Philips CM300 LaB6 microscope, particles of the catalyst were identified by energy dispersive X-ray

spectroscopy (EDS), and a scanning electron microscope LEO 1550 was used to study the morphology of

the carbon deposits.

3. RESULTS

Kaolinite usually has the form aggregates of planar particles, their size not exceeding several micrometers

(see Fig. 1). These particles create triclinic pseudohexagonal crystallites arranged in parallel into blocks. As

far as the use of for CNTs synthesis kaolinite is concerned, a significant role belongs to its high thermal

stability. The first changes are accompanied by dehydroxylation of the mineral, which leads to a loss of its

mass by approx 15 wt%, at temperatures between 450 and 550°C. X-ray diffraction studies have proved that

dehydroxylation is associated with a marked worsening of the structural order. Nevertheless SEM does not

reveal any changes in macroscopic ordering of the blocks of crystallites. We assume that the slight increase

of porosity and of the amount of defects in the silicate matrix, which are usual concomitants of

dehydroxylation, positively affect creation of the grid of CNTs in kaolinite. CNTs synthesized on kaolinite are

typical by their 3D cross-linking, various shapes and lengths. This fact is demonstrated in Fig. 2. All

investigated CNTs have a multi-walled (MW) structure. The outer diameter of MWCNTs ranges between 10

and 25 nm. EDS of the terminal end of CNTs identifies the presence of iron.

Nontronite is a dioctahedral mineral. Its structure consists of two tetrahedral layers and one octahedral layer

(2:1 sheet structure). The central sites in the octahedrons of nontronite are occupied only by cations Fe3+

. In

the tetrahedral layer the atoms of silicon are partially substituted by atoms of aluminum and in a lower extent

also by iron (cations Fe3+

). Nontronite belongs to clay minerals with a high content of iron. In the frontal

micrograph (Fig. 3) can see a crystal of nontronite. In contrast to kaolinite, CNTs are not incorporated into

the silicate carrier. In nontronite, CNTs do not create grids, they are grown on the surface of nontronite (see

Fig. 4). The amount of CNTs on the surface of nontronite is high, the surface is completely covered. The

nature of the CNTs phase on nontronite is not different from that on/in kaolinite. The fibers have different

shapes and lengths, without alignment. The diameter of MWCNTs grown on nontronite ranges from 10 to

50 nm and the nanotubes are of different types. For example, chain-like nanotubes are made of hollow

carbon cages that are interconnected so that the open end of one cage is coupled with the dome of another

one, the-so-called bamboo type of nanotubes. EDS identifies the presence of iron but also of Al and Ca.

Sepiolite is formed by thin filaments, needles (see Fig. 5). Its structure consists of two tetrahedral layers and

one octahedral layer (2:1 structure). In contrast to other 2:1 structures, the orientation of tetrahedrons in the

tetrahedral layer changes periodically. In the structure of the mineral there are channels filled with reversibly

bond water and exchangeable cations, which allows using this mineral as catalysts, carriers of catalysts or

sorbents. The lumen of the channels in sepiolite is between 0.4 and 0.9 nm. CNTs synthesized on sepiolite

are shown in Fig. 6. SEM examination revealed two remarkable differences in comparison with the previous

cases. First, CNTs were formed both in the layer of the mineral and below the layer of sepiolite, they grew

from the silicate mineral towards the silicon substrate. Second, CNTs created a layer of aligned tubes with a

length of approximately 30 μm and diameter 10 to 20 nm. In spite of these differences the mechanism of

CNTs formation seems to be similar to that in the previous cases because EDS analysis of the terminal ends

of the tubes proves the presence of the catalytically active metal.

23. - 25. 10. 2012, Brno, Czech Republic, EU

Fig 1 SEM image of the crystals of kaolinite on a single

crystal of Si

Fig 2 SEM image of CNT grid and bridges grown

on/in kaolinite pretreated with Fe(NO3)3 ∙ 9H2O

Fig 3 SEM image of the crystals of nontronite on

a single crystal of Si

Fig 4 SEM micrograph of nanocomposite: CNTs

on nontronite (undoped with Fe)

23. - 25. 10. 2012, Brno, Czech Republic, EU

Fig 5 SEM image of the crystals of sepiolite on a single

crystal of Si

Fig 6 SEM image of the CNT grown on crystals of

sepiolite pretreated with Fe(NO3)3 ∙ 9H2O

4. CONCLUSIONS

Nanocomposites based on carbon nanotubes were obtained on three different types of microcrystalline

phyllosilicates. In the case of kaolinite the nanotubes are located between single crystallites, they grow

through the whole volume and create bridges and 3D grids. In the cases of nontronite and sepiolite the CNTs

grow through the volume of the mineral but also create a clearly identifiable separate phase. Whereas on

nontronite the CNTs are non-aligned, on sepiolite they are aligned. The produced CNTs are markedly

different. All nanocomposites are formed from MWCNTs. Also the mechanism of CNTs growth is most likely

the same in the three cases. Under the same technology conditions, the type of the carrier affected the

structure and morphology of the nanotube product significantly. Each of the obtained nanocomposites may

be advantageous for a given field of applications. In terms of application confirms our idea of using these

nanocomposites in high temperature applications. Nowadays, sepiolite is examined in combination with

MWCNTs as a fire retardant nanocomposite, and with a polymer as a nanocomposite with flame retardant

properties [10].

ACKNOWLEDGEMENTS

This work has been partially supported by grand MORTEV of Slovak University of Technology

and by grants VEGA 1/1102/11 and 1/1103/11 of Ministry of Education of the Slovak Republic.

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