Preparation of homogeneous titania coatings on the surface of MWNTs

Phys. Status Solidi B 247, Nos. 11–12, 2683–2686 (2010) / DOI 10.1002/pssb.201000190 p s sb

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Preparation of homogeneous titaniacoatings on the surface of MWNTs

Zoltan Nemeth1, Balazs Reti1, Christel Dieker2, Akos Kukovecz1, Duncan T. L. Alexander2, Jin Won Seo3,Laszlo Forro2, and Klara Hernadi*,1

1 Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela ter 1, Szeged 6720, Hungary2 Laboratoire de Physique de la Matiere Complexe, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland3 Department of Metallurgy and Materials Engineering, Katholieke Universitet Leuven, Kasteelpark Arenberg 44-bus, 2450 Heverlee,

Belgium

Received 30 April 2010, revised 30 July 2010, accepted 31 August 2010

Published online 4 October 2010

Keywords composite materials, electron microscopy, homogeneous coverage, X-ray diffraction

* Corresponding author: e-mail [email protected], Phone: þ36 62 544 619; Fax: þ36 62 544 619

The aim of this work was to develop a controllable route to

produce a stable and inorganic layer on the surface of multi-wall

carbon nanotubes. Precursor compounds such as TiBr4, TiCl4,

Ti(OiPr)4, and Ti(OEt)4 were used to cover the surface of

carbon nanotubes (CNTs) under either solvent free or solution

conditions. Various titania precursors were compared in the

formation of homogeneous layers on the surface of CNTs. As-

prepared titania coverages were characterized by transmission

electron microscopy (TEM), high resolution TEM, scanning

electron microscopy, electron energy loss spectroscopy, and X-

ray diffraction techniques. Results revealed that homogeneous

coverage can be achieved in a controllable way.

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Since their discovery [1], carbonnanotubes (CNTs) have induced great interest amongresearchers due to their unique physical and electronicproperties, such as: small dimensions, high strength, andhuge surface areas [2, 3]. Prior to application of CNTs asreinforcing additives in composites, there are two majorobstacles to overcome: the dispersibility of CNTs in thematrix; and undesirable interface reactions between theCNTs and the matrix at high temperatures [4, 5]. To optimizethe use of CNTs in such applications, attaching functionalgroups, or other nanostructures such as inorganic coatings,onto their surfaces, can ameliorate these two problems.There have been several recent investigations concerning theattachment of various inorganic components onto single-wall and multi-wall carbon nanotubes (MWNTs). Oxidenanoparticles, such as Al(OH)3 [6], SiO2 [7], Al2O3 [8], andTiO2 [9], have been coated onto CNTs by an impregnationmethod [10]. The present study reports successful attempts toachieve TiO2 coverages on the surface of MWNTs by a directimpregnation technique. Different precursors, of varyingreactivities, are used as starting materials: TiCl4, TiBr4,Ti(OiPr)4, and Ti(OEt)4.

2 Experimental2.1 Materials MWNTs were prepared by the

decomposition of acetylene (CVD method) in a rotary ovenat 720 8C, using an Fe, Co/CaCO3 catalyst [11]. This growthprocedure using a CaCO3 catalyst support enables highlyefficient, selective formation of MWNTs without anyamorphous carbon, or carbonaceous particles [12] (Fig. 1).The following precursor molecules were used for thepreparation of coatings: TiBr4 (Aldrich), TiCl4 (Aldrich),Ti(OiPr)4 (Aldrich), and Ti(OEt)4 (Fluka). Solvents such asN,N-dimethyl-formamid, acetone, ethanol, and toluene (allHPLC grade from Reanal) were applied.

2.2 Preparation of composite materials For thedeposition of homogeneous titania layers onto the surface ofMWNTs, two different method were used. Before theimpregnation experiments, purified MWNTs were dried inan oven at 100 8C for 1 h. In the solvent-free method, acalculated amount of precursor and 50 mg of MWNTs wereplaced and then sonicated together for 4 min� 15 min in abeaker without any solvent. Subsequently, the titanium


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Figure 1 SEM micrographs of purified MWNTs grown overFe(III) and Co(II)/CaCO3 [11].

Figure 2 TEM (a) and SEM (b) images of titania/MWNT compo-sites prepared under solvent-free conditions, using TiBr4 as theprecursor.

compound was hydrolyzed by adding a few drops of distilledwater to the suspension.

In the solvent method, 50 mg of purified MWNTs werewell dispersed in 10 cm3 of solvent via sonication. After that,calculated amounts of precursors were added under nitrogen(hygroscopic!), and sonicated for an additional 2 min -� 15 min. All precursors are soluble in either of the solventsused in these experiments. After impregnation, 2 cm3 ofdistilled water was added in order to hydrolyze the precursormolecules. At the end of this procedure, samples were driedat 110 8C.

2.3 Sample characterization In order to verify theformation of inorganic coverage on the surface of MWNTs,the products were investigated with transmission electronmicroscopy (TEM) (Philips CM 10). For these investigationsthe sample preparation involved grinding the samplesmechanically and gluing the powder onto a Cu TEM-grid.High resolution TEM (HRTEM) characterization wascarried out using a Philips CM300 FEG microscope operatedat 300 kV. Scanning electron microscopy (SEM) was doneon a Hitachi S-4700 Type II FE-SEM equipped with a coldfield emission gun operating in the range of 5–15 kV. Thesamples were mounted on a conductive carbon tape. Thepresence of titania layers was also confirmed by electronenergy loss spectroscopy (EELS) in scanning TEM (STEM)mode, using a Hitachi HF-2000 cold FEG instrument. Theircrystalline structures were also studied by X-ray diffraction(XRD) (Rigaku Miniflex II Diffractometer).

3 Results and discussion3.1 Composite materials prepared under

solvent-free conditions After the hydrolysis of theprecursor molecules, the quality of composite materialswas investigated by electron microscopy (EM) techniques.SEM and TEM investigations revealed the presence ofinorganic particles covering the CNTs. Even at lowermagnifications, uncovered ends of CNTs (marked by arrow


in Fig. 2a and b) are frequently found, showing that thecoverage can be clearly differentiated from the CNTs. All ofthe precursors led to coatings, although not of the samequality; overall, TiBr4 produced the most uniform coverages.With all precursors, the formation of significant amounts ofseparate inorganic particles was observed. This can becorrelated with the difficulty of creating controllablesynthesis using impregnation under solvent-free conditions.

3.2 Composite materials prepared undersolution conditions

3.2.1 Samples synthesized in DMF Using N,N-dimethyl-formamid solvent, the preparation of compositematerials from TiBr4 was the most effective. However, TEMobservations revealed that the homogeneity of this layer wasstill very poor. Using the TiCl4 precursor, the formation of atitania/MWNT composite was not successful. In the case ofthe Ti(OEt)4 precursor, the product consisted of uncoveredMWNTs together with a significant amount of separatedtitania particles. When the precursor was Ti(OiPr)4, somevery few CNTs were thickly covered, but no homogeneouscoatings were formed.

3.2.2 Samples synthesized in toluene Usingtoluene as a solvent, and a TiBr4 precursor, the titaniacoverage on the surface of MWNTs was inhomogeneous andfragmented. No inorganic layers could be observed on thesurface of the CNTs with the application of the otherprecursors. Instead, only a significant amount of separateinorganic particles were found.

3.2.3 Samples synthesized in acetone Using allprecursors, the majority of CNTs were coated, and only smallamounts of separate particles were formed, from the acetonesuspensions. Although all of the precursors led to coatings,the resulting coating homogeneities differed. In the case ofTi(OEt)4 and Ti(OiPr)4 precursors, the layers were partiallybroken into sections of, e.g., 100 s nm in length (Fig. 3a). Thebest wrapping was obtained with the TiBr4 precursor(Table 1). This coverage was homogeneous, without thepresence of separate inorganic particles (Fig. 3b).

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Phys. Status Solidi B 247, Nos. 11–12 (2010) 2685

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Figure 3 SEM images of titania/MWNT composites prepared inacetone using (a) Ti(OiPr)4 and (b) TiBr4 precursors.

Table 1 Summary of experimental results for achieving titania-coated MWNT nanocomposite materials.

precursor DMF toluene acetone ethanol solvent free

TiBr4 þþ þ þþþ þþþ þþTiCl4

�– þþ þþþ þþ

Ti(OEt)4 � – þþ þþ þþTi(OiPr)4 þ – þþ þ þþ�

no information available; – no coverage, only separate titania particles;

þ rudimentary coverage; þþ rather homogeneous coverage with some

separate particles; þþþ homogeneous coating on CNTs.

Figure 5 (online color at: www.pss-b.com) STEM-EELS Ti line-scan and Z-contrast image (inset) for TiBr4 precursor in ethanol.Linescan location is marked in red; the bright particles in the imagecorrelate with strong Ti intensity, thus these are TiO2 particlesadhered onto the MWNT.

Figure 6 (a) TEM image of titania/MWNT composite prepared inethanol using the Ti(OiPr)4 precursor and (b) HRTEM image oftitania/MWNT composite prepared in ethanol using the TiBr4

precursor. The arrows show the titania layer on the surface.

3.2.4 Samples synthesized in ethanol In ethano-lic solution, the fabrication of titania/MWNT nanocompo-sites was also successful using any of the precursors. EMimages clearly revealed that titania layers formed on thesurface of the CNTs (Fig. 4).

In some cases, Z-contrast imaging using STEM showeda granular structure for the attached titania, as seen in Fig. 5.Because of its higher average atomic number, compared tothe CNT, TiO2 appears bright in the image. EELS analysisconfirmed the presence of titania. For instance, the plot inFig. 5 shows the results of a STEM-EELS linescan on the TiL2,3 ionization edge. EELS intensity is integrated over awindow from 456 to 496 eV, with the background subtractedusing a power-law fit. The linescan shows strong peaks inintensity from the Ti correlating with the adhered particles inthe image.

Figure 4 SEM and TEM images of titania/MWNT compositesprepared in ethanol using (a) TiBr4 and (b) Ti(OEt)4 precursors.

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For the Ti(OEt)4 or Ti(OiPr)4 precursors, samplesprepared from ethanolic solution also contained CNTs withtitania coverage, with, only a small amount of separateparticles being present. At higher magnifications, the titanialayers were found to be inhomogeneous and ragged. Forinstance, the TEM image in Fig. 6a illustrates titania layersfrom Ti(OiPr)4 with a thickness of 8–12 nm. SEM andHRTEM observations showed that the coatings originatingfrom the TiBr4 precursor could instead be more homo-geneous (Fig. 6b).

3.3 Heat treatment In order to transform the amor-phous titania coatings into a crystalline phase, and thus allowtheir identification as TiO2 by XRD, the composite sampleswere heat treated at 400 8C for 3 h. This temperature was lowenough to prevent the CNTs burning (burn threshold 500 8Cin the presence of O2). TEM images of these heat-treatedsamples showed unambiguously that the surface nature ofthe coatings had changed significantly. Formerly smooth


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Figure 7 TEM images of titania/MWNT composites obtainedfrom the TiBr4 precursor after heat treatment.

Figure 8 XRD spectrum confirming the formation of the anataseTiO2 phase by the heat treatment of a titania/MWNT composite.

surface structures transformed to rough, granular materials,probably due to the crystallization process (Fig. 7).

As an example, XRD measurements of samples preparedfrom either Ti(OEt)4 in acetone, or TiBr4 in ethanol, showedthat the heat-treated TiO2 particles had the anatase crystalstructure (Fig. 8). More detailed discussions of the effects ofprecursors on the crystallinity of heat-treated titania layersare reserved for other work.


4 Conclusion Table 1 summarizes the results forproducing titania-coated MWNT composites using differentprecursors and methods. In general, it can be concluded thatthe TiBr4 precursor produced the most homogeneouscoverages. Due to the electronic structure of TiBr4, itssolubility is much better in any solvent than for the otherprecursors. For this reason, it is presumed that the TiBr4

precursor has better sorption properties on MWNTs, andconsequently forms more uniform coverages on theirsurfaces. Concerning the role of different solvents, there isno clear trend, except that those with rather average dipolemoments appear preferable. Such solvents may provide anoptimum for various, sometimes opposing factors. Firstly, ifchemical reaction between surface OH groups and titaniumhalogenides is presumed to result in the rudimentaryformation of coatings, their further growth will be disturbedby strong solvent–surface interactions. Secondly, however,some polar oxygenated groups, such as C–O, C––O andO–C––O, might stimulate formation of the composites, andso enhance the interfacial combination of TiO2 with theCNTs. In contrast, thirdly, the clean surface of MWNTs isalso an important parameter for the strong binding of theCNT surfaces with the coatings, and the presence ofamorphous carbon from strong solvent–surface interactionscould hinder the initial reactions between CNT surfaces andprecursor molecules. Summarizing our results, for theformation of homogeneous coverages on the surface ofCNTs, ethanolic solutions provided the best conditions.

Acknowledgements The work in Lausanne was supportedby the Swiss National Science Foundation IZ73Z0_128037/1 andpartially by the Hungarian Science Foundation (OTKA KM1 15 T K76125). A. K. acknowledges support from the OTKA-NNF grant#78920.

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Preparation of homogeneous titania coatings on the surface of MWNTs

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