Characterisation of carbon nanotube films deposited by electrophoretic deposition

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Characterisation of carbon nanotube films deposited by electrophoretic deposition b _ Johann Choa, Katarzyna Konopkab, Krzysztof Rozniatowski , Eva Garcı´a-Lecinac, Milo S.P. Shafferd,*, Aldo R. Boccaccinia,* a

Department of Materials, Imperial College London, South Kensington Campus, London SW7 2BP, United Kingdom Warsaw University of Technology, 00-661 Warsaw, Poland c Surface Finishing Department, CIDETEC – Electrochemical Technology Centre, P Miramo´n 196, 20009 Donostia-San Sebastia´n, Spain d Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom b

A R T I C L E I N F O

A B S T R A C T

Article history:

Electrophoretic deposition (EPD) was shown to be a convenient method to fabricate uni-

Received 13 July 2008

form coatings of carbon nanotubes (CNTs) with desired thickness and excellent macro-

Accepted 28 August 2008

scopic homogeneity. The CNT deposition kinetics are controlled by the applied electric

Available online 13 September 2008

field and deposition time which, in turn, prove to be linearly correlated with the deposition yield and thickness. The CNT films were characterised by using a range of techniques including high resolution scanning electron microscopy, nanoindentation and atomic force microscopy. Nanoindentation results revealed differences in the local microstructure of CNT deposits leading to variations of Young’s modulus and hardness, which were ascribed to differences in the packing density of CNTs, as observed also by AFM. A mathematical model for the kinetic of EPD of CNTs based on Hamaker’s law was proposed and the predictions of the model were shown to be in good agreement with experimental results.  2008 Elsevier Ltd. All rights reserved.

1.

Introduction

Owing to their unique atomic structure, outstanding mechanical strength and thermal conductivity, and versatile electronic properties, carbon nanotubes (CNTs) are key components of a wide range of scientific and technological studies [1–5]. In recent years, considerable effort has been directed towards manipulating CNTs, individually or collectively, in order to produce (ideally ordered) CNT arrays, optimised for particular applications [6–10]. It is often a requirement that such processing methods are able to disperse CNTs homogeneously either in space, or in an appropriate composite matrix. One very promising manipulation technique is electrophoretic deposition (EPD), which has recently been reviewed in the context of CNTs [11]. EPD is fun-

damentally a combination of two processes, electrophoresis and deposition. In the first step, material (particles) suspended in a liquid are forced to move towards an electrode by applying an electric field. In the second step, the particles collect at the electrode and form a coherent deposit [11]. EPD has a number of advantages when using particulate suspensions to generate films and coatings, as well as laminar ceramic composites and functionally graded materials [12–14]. The attraction of this method lies in its simplicity; EPD is a cost-effective method usually requiring simple equipment and yet it offers the possibility of forming monolithic or composite coatings with complex shapes and/or surface patterns. In addition, EPD requires only short processing times [15], as much as two orders of magnitude shorter than other suspension-based processes, such as slip casting. Conversely,

* Corresponding authors. Fax: +44 0 20 7594 6757. E-mail addresses: [email protected] (M.S.P. Shaffer), [email protected] (A.R. Boccaccini). 0008-6223/$ - see front matter  2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.08.028

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accurate knowledge of the deposition rate is particularly important if the yield and thickness of an EPD film is to be controlled [16]. The deposition rate is also relevant for controlling the compositional profile of functionally graded materials made by EPD [17]. Previous work on EPD of CNTs [11,18– 24] has mainly focused on the structure and subsequent electronic or electrochemical properties of the deposited CNT films; the kinetics of electrophoresis and deposition has not been studied in detail. In previous studies [18,19], it has been confirmed that relatively high electric fields are necessary to deposit uniform and homogeneous CNT films on metallic substrates. However, no detailed investigation was carried out linking EPD processing parameters to the kinetics of CNT deposition. These issues are addressed quantitatively in the current report. In addition, the EPD CNT films are characterised structurally using SEM and AFM, and phenomenologically by contact angle nanoindentation measurements.

2.

Experimental work

2.1.

Materials

Multi-walled carbon nanotubes were synthesised using the catalytic chemical vapour deposition (CCVD) method. The experimental set-up used to grow aligned arrays of CNTs has been reported elsewhere [25] and it is briefly described here. A solution of 5 wt.% ferrocene in toluene was injected at 5 mL/h into carrier gas stream (2 L/min), a mixture of argon and 10% hydrogen, which was preheated to 200 C to ensure vaporisation of the solution. The vapour was carried into the main furnace at 760 C where the aligned nanotubes grew perpendicularly from the surrounding reaction tube. The ferrocene provided the catalytic particles required to nucleate the nanotubes while the toluene provided the carbon feedstock. The reaction was carried out for 2 h and yielded several grams from each run. The CNT suspension for EPD was prepared by chemical oxidation of as grown CNTs in a concentrated HNO3 and H2SO4 solution (1:3 volume ratio) (Sigma Aldrich, UK), as described elsewhere [26]. Typically, 1 g of raw CNTs was refluxed in 40 mL of the acid mixture at 120 C for 30 min. On cooling, the oxidised CNTs were washed to neutral with distilled

water, with a yield of about 50wt.%. Additional ultrasonication and centrifugation (3000 rpm for 15 min) produced a well dispersed stable CNT suspension. Three aqueous suspensions of different concentrations, specifically 0.25, 0.5 and 0.75 mg/ mL, were prepared for EPD experiments. Thermogravimetric analysis (TGA) (Perkin Elmer Pyris 1) was performed at least 3 times to determine, accurately, the CNT concentration in the starting suspensions.

2.2.

DC power supply Anode

Electrophoretic deposition

EPD was carried out under a range of conditions, including variations of field strength (voltages of up to 35 V), deposition time and CNT concentration, in order to establish the kinetics of the deposition process and the optimal conditions. All EPD experiments were carried out at room temperature. Both electrodes in the EPD cell (Fig. 1) were made of 316 L stainless– steel plates (RS Components, Northants, UK) with dimensions of 15 · 15 · 0.2 mm3, and were used as-received except for a degreasing wash with acetone, followed by distilled water. The CNT suspension was sonicated in an ultrasonic bath (Sonomatic, Langfor Ultrasonics) for 15 min prior to each deposition. For EPD, the two electrodes were fixed parallel at the required distance in the suspension and a constant DC voltage was applied using a laboratory power supply (TTi EL561, Thurlby Thandar Instrument, Huntingdon, UK). The current was recorded as a function of time using a TTi 1906 computing multimeter (Thurlby Thandar Instrument, Huntingdon, UK). Subsequently, the coated samples were carefully removed from the EPD cell to minimise any drag between the wet coating and the remaining suspension. The samples were dried horizontally in air at room temperature for 24 h, and stored in a desiccator at room temperature, until further examination.

2.3.

Characterisation methods

The surface morphology and thickness of the CNT films were characterised by field emission gun-scanning electron microscopy (LEO Gemini, Carl Zeiss, Hertfordshire, UK) (SEM). The thickness of the deposited layer was determined by averaging several measurements on polished cross-sections of CNT films mounted in epoxy resin. In order to deter-

Schematic diagram of EPD of CNTs Cathode

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EPD cell DC power supply

Spacer Stainless steel electrode

Glass vessel Stable CNT suspension Multimeter

CNT suspension

Charged carbon nanotubes

Fig. 1 – Schematic diagram of anodic-electrophoretic deposition of CNTs, showing the cell (left) and the overall set-up (right).

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mine the possible local inhomogeneous CNT packing density, atomic force microscopy (AFM) was carried out using a PicoSPE LE (Molecular Imaging) instrument operated in tappingmode in air using silicon cantilevers with resonance frequency of 204–497 kHz. The wettability of the CNT surfaces was analyzed by static contact angle measurements. Water drops positioned on the CNT coatings in air at room temperature, using a model CAM 200 (KSV Instrument’s) goniometer. Ten lL droplets were dispensed from a Hamilton pipette onto the surface and the angle measurement was recorded immediately. Reported contact angle values are the average of 5 different measurements taken on 3 samples prepared separately. Measurements were compared with an uncoated stainless steel substrate as a reference. Nanoindentation tests on CNT deposits were carried out using a Triboindenter (Hysitron, USA) instrument equipped with a Berkovich diamond indenter, using two indentation loads, 1 mN and 10 mN. Stiffness and hardness were obtained from nanoindentation data using expressions available in the literature [27,28].

3.

Kinetic model for EPD of CNTs

The kinetics of electrophoretic deposition was first derived by Hamaker by simply applying the principle of conservation of mass [29]. The representation of Hamaker’s law is widely accepted for estimating the rate of EPD of inorganic particles, and it is given by Eq. (1) Z t2 f lEAcs dt ð1Þ wðtÞ ¼ t1

4.2.

which states that the deposit yield w (kg) corresponds to the product of the electrophoretic mobility l (m2/(V s), field strength E (V m), electrode surface area A (m2), and particle mass concentration in suspension cs (kg/m3) integrated over time. The efficiency factor f is included to account for the possibility that not all particles that migrate to the electrode will contribute to the formation of the deposit. The electrophoretic mobility, which depends on both particle properties and suspension properties, is given by the Smoluchowski equation [13] (Eq. (2)) l¼

ee0 1 g

ð2Þ

where e is the dielectric constant of the liquid, e0 (C2/Jm) is the permittivity of free space (vacuum permittivity), 1(V) is the zeta potential of the particles, and g(Pa s) is the viscosity of the liquid. The equation is strictly valid for spherical particles, but is related to more complex expressions for the mobility of rod-like particles (partially) oriented in an electric field [30]. Although CNTs have high aspect ratio, Eq. (2) provides a simple means to explore the validity Hamaker’s law when applied to the EPD of CNTs.

4.

Results and discussion

4.1.

EPD of CNTs

groups which are predominately phenolic, carboxylic and lactonic groups. In particular, the presence of carboxylic groups on the surface of acid oxidised CNTs provides a negative surface charge (negative zeta potential), as discussed in the literature [18,31,32]; consequently, the CNTs deposit on the anode during EPD. The acid treatment has the secondary effect of removing impurities such as amorphous carbon and catalyst particles, and cutting the CNTs at defect sites. After treatment, the CNTs are shorter and less entangled, leading to suspensions of well-dispersed individual, charged nanotubesegments [26]. The use of an aqueous, rather than organic, system for EPD has the advantages of lower cost, lower electric potential requirements, and greater environmentally compatibility [33]. However, water introduces the potential problem of gas evolution via electrolysis (1.23 V). Hydrogen gas evolution occurs at the cathode while oxygen gas is generated at the anode. These effects were apparent in the present EPD investigation at high electric field (>35 V/cm) and at long deposition times. Excessive hydrogen gas evolution at the cathode was observed on the upper surface of the CNT suspension, as a layer of bubbles. At the anode, the gas bubbles are trapped within the network of depositing nanotubes and influence the morphology of the CNT film, as observed previously [34]. Generally, the effect is limited by the low ionic conductivity of the suspensions [18]. However, in principle, a membrane could be introduced to minimise the incorporation of gas bubbles into the depositing coating, as discussed in the literature for other systems [12].

As demonstrated previously [26], oxidative treatment of catalytically grown CNTs introduces oxygen containing surface

Characterisation of CNT deposits

Following successful EPD, the relative densities of the CNT films were estimated from their weight, surface area and thickness. Assuming a CNT theoretical density of 1.65 g/cm3 [26], the relative packing density was found to be between 30% and 50%, depending on electric field and deposition time used during EPD, as discussed next. For example, the CNT deposits obtained at the highest electric fields (>20 V/cm) appeared qualitatively more robust than deposits obtained at lower voltages. This improvement may be due to the larger number of contacts between more densely packed CNTs as the individual CNTs are drawn closer to each other during EPD [18], and may also correlate with the formation of more cohesive films. SEM examination both of the upper surfaces and of crosssections of the deposits (see, for example, Fig. 2) shows randomly dispersed but homogeneous 3D networks of CNTs, electrophoretically-deposited on stainless steel substrates. The deposited CNT films have uniform thickness that depends on both the applied electric field and deposition time. The largest voids observed were of the order of 100 nm, comprising only a few CNT diameters. The wettability of the surfaces was analyzed by static contact angle measurements, derived from images such as that shown in Fig. 3. The contact angles of water droplets on the stainless steel and CNT-coated surfaces were 88 ± 2 and 33 ± 2, respectively. The decrease of the contact angle relates to an increase in the hydrophilic character due to the presence of the oxidised CNTs. This result is relevant to current

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Fig. 2 – SEM images of CNT films obtained by EPD on stainless steel substrates: (a) surface view and (b) cross sectional view. The CNT film was produced at 20 V/cm for 5 min.

Fig. 3 – Optical images of water droplets on the surface of: (a) stainless steel substrate and (b) CNT-coated substrate. Water drops positioned on the CNT coatings in air at room temperature.

interest in the application of CNT coatings as bioactive surfaces, for example in tissue engineering scaffolds [19]. Mechanical characterisation of the CNT films was provided by nanoindentation tests. Similar load–depth data were obtained for loads of 10 mN (Fig. 4) and 1 mN. Characteristic features, including the value of Pmax (maximum value of load), A (the area of the impression left by the indenter), hmax (maximum value of indenter penetration), E (reduced Young’s modulus), and H (nanohardness) are presented in Tables 1 and 2 for of 1 mN and 10 mN, respectively. The load–depth curves shift along the depth axis but retain the same characteristic shape, indicating the material response under nanoindentation. The response of the material,

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at a given load, is monotonically related to both the indentation depth and area (see Tables 1 and 2). As the indentation depth increases, the apparent stiffness and hardness decrease. Although the deposits are relatively homogenous at a larger scale, at a microscopic level the porosity changes with location. Fig. 5 shows schematically the relative position and size of the indentation for the measurements given in Table 1. Although the contact area of the indenter is larger than the diameters of individual carbon nanotubes, it is of the same order as the local microstructure. The nature of the local contact will therefore strongly influence the results of nanoindentation experiments on these CNT deposits. A direct contact on a larger, rigidly-pinned nanotube, or group of nanotubes, will yield a small indentation depth and area, and conversely large stiffness and hardness. Alternatively, a contact between nanotubes, will allow them to be pushed sideways, resulting in a large indentation depth and area, and a small stiffness and hardness. Similar effects have been observed previously in nanotube loaded polymer composite films [35]. The values of Young’s modulus vary significantly from 7.7 to 77.7 GPa, for the 1 mN load, and from 70.0 to 157.8 GPa, for the 10 mN load. Hardness varies from 0.15 to 1.19 GPa and from 0.5 to 2.12 GPa, for the same two loads, respectively. The broad ranges are to be expected for a network of rods that are very rigid in tension but flexible in bending [35], and which are probed at the same length scale as the network features. It is also worth noting that the mechanical properties of individual nanoscale object are difficult to measure directly; indeed, nanotubes are particularly, heterogenous, both in dimensions and internal perfection, giving rise to significant variation from one nanotube to another. The smallest contact area in our experiment was around 0.8 lm2, implying an edge length of the triangular cross-section of the Berkovich indenter equal to 0.5 lm. Since the average CNT diameter is 70 nm, nanoindentation does not test the mechanical properties of individual carbon nanotubes; nevertheless, the response is controlled by a small number of nanotubes, and is susceptible to the local variations in microstructure. In order to explore the topography of the CNT coating surface further, AFM studies were performed (Fig. 6 shows a typical coating). The AFM images reveal a surface characterised by a network of cylindrical-like features with lengths of several hundreds of nanometers and much smaller width. These lines represent the segments of the CNTs that approach the surface, and confirm the random planar arrangement observed in the SEM. The height and phase contrast images show the presence of voids and zones of different packing density more clearly than the SEM images, due to the shallow depth of field. The topographic line scans (Fig. 6c) indicate a roughness with a maximum peak-to-valley distance of up to 400 nm. This roughness is significant compared to the typical indentation depths achieved in the nanomechanical study, and confirms that the nature of the contact between the indenter and the nanotube network will vary in different locations. The local inhomogeneities in CNT packing density explain the variability of E and H values determined by nanoindentation, as discussed above. Thus, depending on the region of the sample probed, the indenter will penetrate to a different

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12000

load (uN)

10000 8000 6000 4000 2000 0 0

100

200

300

400

500

600

700

800

900

1000

depth (nm)

Fig. 4 – Typical load–depth curves obtained by nanoindentation tests using a 10 mN force on different regions of CNT coatings obtained at 20 V/cm.

extent, depending on the local topology, density, and nanotube character. During indentation, the individual CNTs first deform and then become displaced, as discussed elsewhere [36]. The weak bonding between the nanotubes, and the relatively low packing density, enhances the plastic deformation of the coating; only a small elastic response is seen in the unloading curve (Fig. 4). In nanoindentation studies, the potential influence of the substrate should be considered. However, in the present experiments, the thickness of the CNT deposits was significantly larger than the depth of the indentation in all cases, and the influence substrate is likely to be negligible. This conclusion is supported by the reducing hardness and stiffness values with increasing penetration depth, which must therefore relate to true differences in the local microstructure tested in each case. The indentation results observed in the current study are similar to the effects described by Biener et al. [37] for nanoporous Au, including shifted load–depth curves and local variations in yield strength. It is interesting to note also that the lowest obtained values of E and H for carbon nanotube layers obtained here are equivalent to values reported for fullerene films, where the Young’s modulus and hardness values were calculated to be 10.5 and 0.24 GPa, respectively, [38].

Table 1 – Values of parameters measured during nanoindentation tests using a load of 1 mN, Pmax (maximum value of force), A (the area of impression left by the indenter), hmax (maximum value of indenter penetration), E (Young’s modulus), and H (nanohardness), arranged in order of increasing indenter penetration hmax (nm) 181 196 195 207 219 261 256 305 438 468 477

A (lm2) 1.2 1.3 1.3 1.4 1.5 2 2.1 2.7 5.1 5.8 6.2

E (GPa)

H (GPa)

Pmax (lN)

65.1 51.7 54.3 37.6 38.9 21.3 41.9 18 7.7 7.9 9.7

0.83 0.74 0.81 0.7 0.62 0.5 0.47 0.36 0.19 0.17 0.15

972 986 1062 994 968 1022 997 989 952 980 957

Table 2 – Values of parameters measured during nanoindentation tests using a load 10 mN, Pmax (maximum value of force), A (the area of impression left by the indenter), hmax (maximum value of indenter penetration), E (Young’s modulus), and H (nanohardness), arranged in order of increasing indenter penetration hmax (nm)

A (lm2)

E (GPa)

H (GPa)

Pmax (lN)

411 553 651 691 754 881

4.7 8.3 11.4 12.7 15 20.3

157.8 118.7 116.6 91.8 79.1 70

2.12 1.19 0.87 0.79 0.66 0.5

10007 9954 9972 10014 9897 10026

4.3.

Analysis of the EPD process

The primary function of the applied electric field in EPD is to induce the movement of charged CNTs towards the oppositely charged electrode promoting their efficient deposition. The CNTs that accumulate close to the electrode eventually deposit due to the pressure exerted on them by CNTs arriving at the outer surface. High electric field strength is necessary to apply sufficient electrophoretic force to CNTs in suspension. This force should overcome the viscous drag as well as the forces exerted by the counter ions surrounding the CNTs in order to move the CNTs along the electric field gradient. For a given CNT suspension (assuming constant resistivity), a higher potential difference between the electrodes is expected to increase the velocity of charged CNTs travelling towards the oppositely charged electrode (the anode in this study). This effect can be quantified simply using J¼rE

ð3Þ

J ¼ nqv

ð4Þ 2

where J is the current density (A/m ), r is electrical conductivity (S/m), E is electric field strength (V/m), n is the particle density in counts per volume (m3), q is the individual particles’ charge (C) and v is the particle drift velocity (m/s). The number and charge of CNTs remain constant for a given CNT suspension of constant electrical conductivity. Thus, increased velocity induced by a higher electric field will increase the deposition rate.

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Fig. 5 – SEM image of a CNT deposit obtained by EPD showing schematically areas of the indenter contact with the sample. The evolution of the current profile with time is shown in Fig. 7, for a range of applied voltages. As expected, the current is proportional to the applied electric field strength, which

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leads to the increase in yield and thickness of the deposited CNT films discussed below. For voltages lower than 15 V, the current intensity decreased slightly immediately after the electric field was applied, before reaching a steady level as deposition progresses. This drop can be attributed to the sudden increase of the concentration of charged CNTs near the anode. Under these conditions, a concentration potential [39,40] is generated that opposes the applied electric field, slows nanotube movement, and reduces the current. After the initial surge of CNTs deposited on the anode, a steady CNT concentration profile is established in the suspension near the anode, and the current remains constant for the remainder of the deposition. This behaviour was also observed by Du et al. [39] during EPD of CNTs under similar conditions. For electric field strength greater than 15 V/cm, the current intensity increased initially before stabilising. This initial increase may be attributed to alignment of CNTs parallel to the electric field. Applied fields are known to induce polarise individual CNTs, generating a dipole moment (as illustrated in Fig. 8) that acts to enhance the movement of CNTs towards the anode [40]. The polarisation has two components, one in radial direction and one parallel to the CNTs

Fig. 6 – Typical atomic force microscope (AFM) images of the surface of the CNT coatings: (a) height contrast, (b) phase contrast, and (c) corresponding topographic line scan.

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Fig. 7 – Current as function of deposition time during EPD for a range of voltages (suspension concentration 0.5 mg/ml and electrode separation 1 cm).

Fig. 8 – Schematic illustration of a polarised single nanotube in an electric field.

axis. The magnitude of both components depends on the polarisability tensor of the nanotubes [41]. The polarisation induced by the applied DC electric field results in a torque, NE, acting on the nanotube, that tends to align parallel to the applied electric field. The final degree of alignment is determined by the balance of the electrical field induced torque and the disordering effects of rotational Brownian motion [30]. For EPD of inorganic (insulating) particles, constant-current conditions are generally preferred because there is no effect of the electrical resistance of the deposit on the kinetics of deposition [13,14]. In contrast, during constant-voltage EPD, the resistance of the deposit increases in proportion to its thickness; thus, at a given constant voltage, the electric current gradually decreases as the film thickens, until the deposition saturates [13]. Under constant-current conditions,

the electric field strength in the suspension is maintained by increasing the total potential difference applied, thus avoiding the limited deposition yield and deposition-rate problems of constant-voltage EPD. Unlike most inorganic colloids, carbon nanotubes are intrinsically electrically conductive, and the growing deposit contributes negligible resistance. For this reason, the constant voltage method employed leads to the constant deposition currents shown in Fig. 7. Similar results were observed for 0.25 and 0.75 mg/mL CNT suspensions (not shown here) but with absolute currents proportional to concentration. Fig. 9 plots the average current during 5 min EPD experiments, against the applied voltage, for three different CNT concentrations, and indicates a broadly linear relationship. However, by plotting the same data as resistance against concentration (Fig. 10), a low voltage transition becomes apparent. All three CNT suspensions show very similar resistance curves that decrease for voltages below around 10 V before attaining a constant value at higher voltages. These data identify a minimum threshold electric field (voltage) for optimal EPD processing of CNT coatings. The high resistance at lower electric fields can be attributed to the presence of disordered (unaligned) CNTs. As the CNTs become orientated at higher field, their mobility will increase, raising the current and the deposition rate. The average yield and thickness of the deposited CNTs are directly proportional to the applied electric field (voltage), as shown in Fig. 11, and as predicted by Eq. (1). It is also clear that higher CNT content in the suspension yields a higher deposition rate.

4.4.

Kinetics of CNT EPD

The yield of deposited nanotubes is plotted in Fig. 12, as function of time. The lines represent the theoretical predictions calculated from Hamaker’s equation (Eq. (1)), using the efficiency factor f to maximise the numerical agreement [13]; the electrophoretic mobilities were determined using Eq. (2), based on zeta potential measurements on the CNT suspensions employed. The linear theoretical trend matches the observed experimental data, with implied efficiency factors of f = 0.26 and f = 0.23 for the CNT suspensions with

Fig. 9 – I–V curve for 5 min EPD of CNTs from three suspensions with different CNT concentrations (electrode separation = 1 cm).

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0.5

16

Yield (mg/cm2)

0.25mg/ml 0.5mg/ml 0.75mg/ml

14

Resistance (kilo-Ω)

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12 10 8 6

R 22 = 0.84

0.5mg/ml 0.5mg/ml 0.75mg/ml 0.75mg/ml

0.4 0.3

R 2 = 0.70

0.2 0.1

4 0

2

0

50

100

0 0

5

150

200

250

300

350

400

450

Deposition time (sec) 10

15

20

25

30

35

40

Electric field (V/cm)

Fig. 10 – Resistance curve during EPD of CNTs for deposition time of 5 min for a range of voltages and three different suspension concentrations (electrode separation: 1 cm).

concentration 0.75 and 0.5 mg/ml, respectively. The large scatter in the experimental data is due to the small masses (in the range 0.0–0.86 mg) of the CNT deposits relative to the substrate. For this reason, it is more common in the literature, to measure the thickness of inorganic nanoparticle deposits, in order to establish the kinetics of the deposition process [42,43]. However, the high aspect ratio of nanotubes, allows greater potential variability in the density of the deposit, and it may be more reliable to consider the mass of the deposit directly. The values of the efficiency factor, f may be influenced by several factors. The most obvious interpretation is that not all of the CNTs that arrive at the electrode are necessarily incorporated into the growing film. More importantly, in

a

Fig. 12 – Relationship between CNT deposition yield and time for EPD from suspensions with two different concentrations of CNTs (0.5 and 0.75 mg/ml). The dotted lines represent fits to Eq. (1).

the present case, is that the estimate of electrophoretic mobility assumes a spherical morphology, likely leading to a systematic error in the predicted deposition rate. With additional information about the double layer thickness, distribution of nanotube dimensions, and their orientation, more detailed calculations could be attempted [30]. Further studies exploring the effect of temperature on deposition rate would be interesting. The theoretical predictions presented in Fig. 12 were derived using values for dielectric constant (80.4) and viscosity (1.002 m Pa s) at 20 C [44]. Increasing the temperature by only 5 C will lead to the decrease of both parameters, suggesting a net increase in electrophoretic mobility, and hence deposition rate, of around 10%.

Yield (mg/cm2)

1 0.5mg/ml 0.75mg/ml

8 0.8

R2 = 0.90

6 0.6 R2 = 0.92

0.4 2 0.2 0 0

5

10

15

20

25

30

35

40

Electric field (V/cm) 20

Thickness (µm)

b

0.5mg/ml 0.75mg/ml

15

R2 = 0.96

R2 = 0.99 10

5

0 0

5

10

15

20

25

30

35

40

Electric field (V/cm)

Fig. 11 – (a) Yield and (b) thickness of CNT films deposited for a range of voltages, deposition time 5 min, and two different concentrations of CNTs in suspension. The experimental data in (a) and (b) follow a linear dependence, as shown in the plots.

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Conclusions

This investigation confirms that EPD represents a very convenient processing technique for the deposition of CNT networks that may be relevant to a variety of applications [11]. The CNT films exhibited excellent macroscopic homogeneity and uniformity of thickness throughout. However, nanoindentation tests highlighted the influence of the local microstructure on properties, specifically Young’s modulus and hardness. Nanoindentation may be a useful method to evaluate the quality (at the micron-scale) of CNT layers, both as deposited by EPD and other methods. The kinetics of the EPD process were analyzed for constant-voltage conditions using a model based on Hamakers’ equation. A useful agreement was found between the mathematical model and experimental results.

Acknowledgements Helpful discussions with Boris Thomas (Imperial College London) are appreciated. The authors acknowledge financial support from the European Commission via EU Network of Excellence ‘‘Knowledge-based Multicomponent Materials for Durable and Safe Performance’’ (KMM-NoE, NMP3-CT-2004502243).

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