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Aug 12, 2008 - conductive layers deposited on polycarbonate substrate. M. R. S. Castro Æ N. Al-Dahoudi Æ P. W. Oliveira Æ. H. K. Schmidt. Received: 26 July ...
J Nanopart Res (2009) 11:801–806 DOI 10.1007/s11051-008-9448-2

RESEARCH PAPER

Multi-walled carbon nanotube-based transparent conductive layers deposited on polycarbonate substrate M. R. S. Castro Æ N. Al-Dahoudi Æ P. W. Oliveira Æ H. K. Schmidt

Received: 26 July 2007 / Accepted: 1 June 2008 / Published online: 12 August 2008 Ó Springer Science+Business Media B.V. 2008

Abstract We characterize 10-nm outer diameter multi-walled carbon nanotubes (powder and dispersion), which will be used for the preparation of conductive layers on polycarbonate (PC) substrates. The optical, electrical, and mechanical characterization of the spin-coated deposited layers is shown and compared with results obtained for layers deposited in borosilicate glass substrates. In both glass and plastic, the layers have shown transmittance higher than 78% in the visible range and have passed the tape and pencil standard tests for adherence and hardness, respectively. However, the sheet resistance presented by layers deposited on PC is still much superior to that of layers deposited on glass and sintered at higher temperatures. Nevertheless, the results obtained still allow the use of such layers in antistatic applications. Keywords Carbon nanotubes  Transparency  Conductivity  Coatings  Antimony tin oxide  Thin layer  Nanocomposite M. R. S. Castro (&)  P. W. Oliveira INM—Leibniz Institute for New Materials GmbH, Campus D2 2, 66123 Saarbru¨cken, Germany e-mail: [email protected] N. Al-Dahoudi Physics Department, Al Azhar University, P.O. Box 1277, Gaza, Gaza Strip, Palestine H. K. Schmidt EPG—Engineered nanoProducts Germany AG, Max-Planck-Str. 2, 66482 Zweibru¨cken, Germany

Introduction Transparent conductive coatings (TCCs) are widely used today as electrodes in optoelectronic devices, infrared reflecting layers in low-emissive glazing and oven windows, heatable layers in defrosting windows, among others (Chappel and Zaban 2002; Hatton et al. 2001; Lewis and Paine 2000). Transparent conductive oxides such as tin-doped indium oxide (ITO) have been the preferred choice for decades (Al-Dahoudi and Aegerter 2006; Minami 2005) as material for the preparation of TCCs with resistivity in the order of 10-4 X cm and transparency higher than 80% in the visible range. However, due to ITO limitations, namely, the scarcity of In and the high production cost of the films, alternative materials have been studied for its replacement, including carbon nanotubes (CNTs) networks (Castro and Schmidt 2008; de Andrade et al. 2007; FerrerAnglada et al. 2004; Gruner 2006; Kaempgen et al. 2005; Wu et al. 2004; Yu et al. 2006). In most cases, however, the electrical conductivity obtained by networks and composites is much inferior to that of ITO films. In our previous work, we have shown that CNTs can enhance the electrical and mechanical properties of antimony tin oxide (ATO) films deposited on borosilicate, with preserved transparency in the visible range (Castro et al. 2008a, b; Castro and Schmidt 2007). We have also produced adherent networks of multi-walled carbon nanotubes (MWNTs) in such substrates,

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obtaining transparency and sheet resistances comparable to that presented by expensive single-walled nanotubes (SWNT) networks (Castro et al. 2008a; Castro and Schmidt 2008). Although such films did not meet the requirements necessary for a proper substitution of ITO in opto-electronic devices, their low cost and simplicity of preparation combined with their optical and electrical response allow them to be used in applications where the high conductivity of ITO is not a requirement. Many challenges for integrations of CNTs including obtaining uniform dispersions and a proper removal of the surfactant from the CNT networks (Castro 2008). There is a growing interest in replacing glass substrates by polymeric ones in many applications (Lee et al. 2000). Therefore, the needs to coat such heat sensitive substrates have considerably increased. The challenge is to find suitable conducting materials which can be deposited at low temperature and to find a method allowing to crystalline the material at low temperatures, without affecting the film optical quality. In this work, we focus on the suitability of using low-cost MWNT-based films as conductive transparent layers in flexible polycarbonate (PC) substrates, comparing the electrical, optical, and mechanical properties of these layers with those previously deposited on glass substrates.

Experimental details Ten-nanometer outer diameter MWNTs from Nanocyl S.A. were dispersed in water containing 5 wt.% of hexadecyl trimethyl ammonium chloride (HDTAC, Fluka, 98%) with the help of a MicrofluidizerÒ high shear processor (M110-Y from Microfluidics, pressure of 1,500 Bar, 15 min, and cylinders of 300 and 87 lm). The solid content of the MWNTs in the suspension was about 10 mg/mL. In order to increase the adhesion and the hardness of the coatings on the substrates, a mixture of Levasil 200S/30 (Bayer) and glycidoxypropyltriethoxysilane (GPTES) (1:1) was added to the dispersion (0.5 wt.%) (Castro and Schmidt 2008) and stirred for 15 min. This new dispersion was used for the preparation of MWNT layers on PC substrate. For the preparation of ATO/MWNT composites, the same procedure presented in (Castro et al. 2008b) was followed in this work. The dispersion of MWNTs in HDTAC (free of Levasil/GPTES) was loaded into

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a colloidal suspension of ATO nanoparticles in different concentrations varying from 2 to 15 wt.% and stirred for 15 min. This sol was used for the preparation of ATO/MWNT layers on PC substrate. Spin coating (spin coater model 1001 CPS II, Convac) was chosen as deposition method, since it is a fast technique which requires only a small amount of sol for the formation of the layers. A final speed of 2,500 rpm for 10 s was used for deposition of all layers. They were further cured in air at 130 °C for 3 h. Characterization methods The morphology of the MWNT powder was examined with a scanning electron microscope (SEM, JSM 6400F, JEOL). The second electron image was formed by exciting the sample with a primary electron beam under 10 kV accelerating voltage. The transmission electron microscopy (TEM) micrograph was provided by the MWNT supplier. The thermal analysis of the powder was performed by thermogravimetric analysis (TG) using a thermal analyzer STA 999C Jupiter (Netzsch). For the measurement, 40 mg of the MWNT powder was placed in an Al2O3 crucible and heated from room temperature to 1,100 °C (heating rate of 10 °C/min) under synthetic air atmosphere. The absolute zeta potential (f) of the MWNT dispersion was recorded at Malvern Instruments facilities using a Zetasizer Nano System apparatus. The sheet resistance (Rsq) of the layers was measured by the four-points technique (Napson Corporation, Model RT-70/RG-7S). The final results presented were the average of at least five measurements in different parts of the surface. The transmittance of the films measured using air as reference was obtained with a CARY 5E UV– VIS–NIR spectrophotometer (Varian). The adhesion and hardness of the coatings were determined by standard DIN and ASTM procedures, respectively (ASTM 1993; DIN 1995).

Results and discussions Characterization of the MWNT powder The MWNT powder was investigated by SEM and TEM, and the results are shown in Fig. 1. Figure 1a

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Fig. 1 (a) SEM and (b) TEM images of MWNTs

shows that the powder consists of agglomerations of MWNTs, what is expected due to van der Waals attractions among the tubes. In Fig. 1b, it is possible to identify individual tubes, each with approximately 10 nm outer diameter. Their curly-like structure could be an indication of the presence of defects in their structure. Moreover, one can detect residues of catalytic particles encapsulated in the MWNTs during the CVD synthesis process, as indicated by arrows in this figure. A precise length of the nanotubes is difficult to predict, although it seems to remain in the tens of micrometer range. It is important that the nanotubes are long in length in order to enable their electrical and mechanical properties. The TG analysis of the MWNT powder under synthetic air atmosphere (10 °C/min) is illustrated in Fig. 2. It is clear that the oxidation of MWNTs in this atmosphere starts at 440 °C and they burn completely 100

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Fig. 2 TG curve of MWNT powder under synthetic air atmosphere (10 °C/min)

at temperatures higher than 670 °C (almost 100% weight loss), also in accordance with other studies (Vincent et al. 2002; Zhu et al. 1999). This parameter could be used as an evidence of the ‘‘purity’’ of the sample, i.e., that the powder consists of MWNTs and not of other kinds of nanotubes or amorphous carbon, which would start oxidizing at different temperatures. Knowing the behavior of the nanotubes with the increase of the temperature is of key importance, once the MWNT-based layers will be also cured. In the case of ATO/MWNT layers, heating is necessary in order to sinter the ATO nanoparticles; in the case of MWNT networks, it was used as a tentative to eliminate all surfactant from the layers. Characterization of the dispersion Besides the difficulty in obtaining stable and homogeneous dispersions of CNTs, another complication is finding a valid method to evaluate their state of dispersion. Techniques such as atomic force microscopy seem not to be representative of the whole sample; others like TEM and SEM require pre-treatment with gold or carbon sputtering, which might cause a defect in the original pattern of the composites. Also characterization methods such as ultrafine particle analysis (UPA) and dynamic light scattering (DLS) are not appropriate, since CNTs are not spherical particles. Therefore, zeta potential measurements were performed in this study as an effective way to determine the stability of MWNTs in water containing the surfactant HDTAC. Figure 3 shows the f distribution of MWNTs dispersed in HDTAC. The existence of only one peak suggests that the particles are monodispersed and an absolute f of 57 mV was measured. Traditionally, if

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Fig. 3 Zeta potential distribution of MWNTs dispersed in water containing 5 wt.% hexadecyl trimethyl ammonium chloride

the absolute value of f is smaller than *25 mV, the repulsive force is not strong enough to overcome the van der Waals attraction between the particles, and hence the particles begin to agglomerate (ASTM 1985). Therefore, the results obtained suggest that the nanotubes are well dispersed in water with the help of HDTAC. Such a surfactant with long tail groups and unsaturated C–C bonds can greatly contribute to the stabilization of CNT dispersions, since the increase in the number of carbon–carbon double bonds per surfactant tail decreases the size of the CNT agglomerate. Similar results were reported in the literature for the dispersion of SWNTs (Lee et al. 2005). Characterization of the layers The sheet resistance of ATO/MWNT layers deposited on PC was studied as a function of the concentration

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Fig. 4 Sheet resistance of MWNT/ATO films deposited on polycarbonate substrate (130 °C, 3 h) as a function of the concentration of MWNT dispersion in the sol

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of the MWNT dispersion in the ATO sol. The results are shown in Fig. 4. It is clear that the presence of nanotubes can decrease the sheet resistance of the films at least until a critical concentration of 12 wt.% of MWNT suspension (corresponding to 0.4 wt.% of CNTs in the film) is used. At this concentration, the lowest sheet resistance of 160 MXsq was reached. This resistance is still very high in comparison with that obtained by the same films deposited in borosilicate and sintered at 400 °C, in the order of 10 kXsq (Castro et al. 2008b). This can be mainly expected due to the low temperature of sintering of 130 °C. At this temperature, the sintering of the ATO nanoparticles is not efficient, and probably, the main contribution for conductivity comes from the few nanotubes present in the film. Their high aspect ratio yields a low percolation threshold so that even a minimum of material can allow the formation of a network. In the case of MWNT networks, the lowest sheet resistance measured was 3 MXsq, against 20– 150 kXsq in the case of networks deposited in glass substrate (Castro and Schmidt 2008). The sheet resistances in the range of MXsq obtained on layers deposited on PC substrate allow them to be used in antistatic applications (Swank 1995). We believe that an inefficient dispersion of MWNTs may take place as high concentrations of MWNTs are present in the matrix (ATO) or in water containing surfactant (in the network case). This could be an explanation for the increase of the sheet resistance observed when concentrations higher than 12 wt.% of MWNT dispersions are studied. The transmittance of films deposited in PC, however, is comparable to that of films sintered at much higher temperatures (Castro et al. 2008b; Castro and Schmidt 2008). As one can see in

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conductivity as high as that of ITO films. Nevertheless, the results obtained allow such films to be used in antistatic applications.

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Acknowledgments This work was financed by the Deutscher Akademischer Austausch Dienst (DAAD) and the Leibniz Association (Grant No. A/03/27558).

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Fig. 5 Sheet resistance versus transmittance of spin-coated MWNT and ATO/MWNT layers deposited on polycarbonate and glass substrates (130 °C for 3 h) Table 1 Classification of ATO/MWNT and MWNT layers deposited on polycarbonate substrate after standard mechanical tests Layer

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Fig. 5, MWNT networks deposited on PC exhibit a high transmittance of 80% at 550 nm, while ATO/ MWNT coatings deposited in the same conditions show a transmission in the visible region of 91%. The classification of the layers after standard mechanical tests is presented in Table 1. Both ATO/ MWNT and MWNT layers deposited on PC substrate passed the adhesion tests using tape (i.e., were left on the substrate) and presented hardness scale of 2H, making them suitable for practical use.

Conclusion MWNTs can be used for the preparation of alternative TCCs in plastic substrates. Stable dispersions of nanotubes (absolute zeta potential of 57 mV) can be obtained using a cationic surfactant in combination with a high shear processor. High transparent MWNT and ATO/MWNT layers can be obtained on plastic substrates by spin coating. The layers have passed the tape test procedure for adhesion as well as the pencil test for hardness. The films studied did not reach

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