The role of catalytic nanoparticle pretreatment on the growth of vertically aligned carbon nanotubes by hot-filament chemical vapor deposition

    The role of catalytic nanoparticle pretreatment on the growth of vertically aligned carbon nanotubes by hot-filament chemical vapor deposition Ki-Hwan Kim, Aur´elien Gohier, Jean-Eric Bour´ee, Marc Chˆatelet, CostelSorin Cojocaru PII: DOI: Reference:

S0040-6090(14)00974-2 doi: 10.1016/j.tsf.2014.10.013 TSF 33777

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Thin Solid Films

Please cite this article as: Ki-Hwan Kim, Aur´elien Gohier, Jean-Eric Bour´ee, Marc Chˆ atelet, Costel-Sorin Cojocaru, The role of catalytic nanoparticle pretreatment on the growth of vertically aligned carbon nanotubes by hot-filament chemical vapor deposition, Thin Solid Films (2014), doi: 10.1016/j.tsf.2014.10.013

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The role of catalytic nanoparticle pretreatment on the growth of vertically aligned

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carbon nanotubes by hot-filament chemical vapor deposition

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Ki-Hwan Kim, Aurélien Gohier, Jean-Eric Bourée, Marc Châtelet and Costel-Sorin Cojocaru*

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Laboratoire de Physique des Interfaces et des Couches Minces (LPICM), CNRS UMR 7647,

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Ecole polytechnique, 91128 Palaiseau, France

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Abstract

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The effect of atomic hydrogen assisted pre-treatment on the growth of vertically aligned

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carbon nanotubes using hot-filament chemical vapor deposition was investigated. Iron nanoparticles catalysts were formed on an aluminum oxide support layer by spraying of iron

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chloride salts solutions as catalyst precursor. It is found that pre-treatment time and process temperature tune the density as well as the shape and the structure of the grown carbon

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nanotubes. An optimum pre-treatment time can be found for the growth of long and well aligned carbon nanotubes, densely packed to each other. To provide insight on this behavior, the iron catalytic nanoparticles formed after the atomic hydrogen assisted pre-treatment were analyzed by atomic force microscopy. The relations between the size and the density of the as-formed catalyst and the as-grown carbon nanotube’s structure and density are discussed.

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Corresponding author: fax: +33 (0)1 69 33 43 33, tel: +33 (0)1 69 33 43 56, e-mail: [email protected] (C. S. Cojocaru)

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1. Introduction

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Since the landmark paper by Iijima in 1991 [1], carbon nanotubes (CNTs) have attracted

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much attention from the scientific community. Due to their singular properties related to their

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quasi-1D structure, CNTs have become one of the most promising materials in a huge range of applications including physics, chemistry and biology [2].

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The continuous progress in CNT synthesis has led to substantial control of their structural properties (diameter, number of walls etc.) as well as their organization. Notably, vertical

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carbon nanotube arrays have been widely studied since they allow high density packing of CNTs with the same orientation. Densely packed CNT arrays are of great interest for various

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applications such as supercapacitors [3], self-cleaning “gecko” adhesives [4], nanofiltration

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membranes [5], polymer-CNTs composites [6] and electrodes for lithium (Li)-ions batteries

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[7].

Chemical vapor deposition (CVD) process is generally used to achieve the growth of vertically aligned CNTs (VACNTs). In such process, the catalytic nanoparticles that support

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CNT growth should be sufficiently dense to ensure CNT crowding and hence their vertical alignment [8, 9]. These particles are often obtained from metal thin films dewetting upon temperature annealing. During such thermal treatment, the catalyst support plays a major role since it must restrict the surface mobility of the catalyst in order to limit Ostwald ripening phenomenon [10 - 12]. In this respect, numerous studies have focused on Fe/Al2O3 catalyst/buffer layer system in which strong catalyst/support interactions have been demonstrated [8, 13 - 17]. With this optimal system, many other additional parameters in catalyst preparation have been shown to impact the structure of the resulting VACNT carpet in terms of nanotubes crystalline quality, diameter and length [18 - 20]. In particular, the role 2

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of catalyst particle pre-treatment before growth has been the subject of many studies. However, in such studies, ex situ analysis of the catalyst particles just before CNT growth is

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often hindered by a slow cooling step which may imply a rearrangement of the clusters.

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Here, we study the effect of atomic hydrogen assisted pre-treatment on the growth of vertically aligned carbon nanotubes. After pre-treatment, catalyst nanoparticles were quenched out to provide ex situ analysis as close as possible from the particle status just

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before the CNT growth. Atomic force microscopy (AFM) was performed to analyze particles size and density. Our work reveals remarkable effect of the pre-treatment time and

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temperature on the size of nanoparticles that further strongly impact the as-grown VACNT

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structure.

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2. Experimental

In this study, FeCl3∙6H2O salts were used as precursor for catalyst particles as described in

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detail elsewhere [21, 22]. Briefly, FeCl3∙6H2O solutions were prepared in ethanol and sprayed over the substrate heated at 120 °C. The 30 nm thick aluminum oxide (Al2O3) layer, used here as buffer layer was e-beam evaporated on thermally grown 300 nm thick silicon oxide substrate.

A hot-filament chemical vapor deposition (HF-CVD) system was used for the growth of VACNTs. As shown in figure 1(a), the set-up exhibits two tungsten filaments (0.4 mm diameter) for the decomposition of hydrogen (H2) and methane (CH4) molecules respectively. By applying a voltage of few volts (typically 10-15 V; up to 10 A), they can be heated up to 2000 °C. By forcing H2 passage over its respective hot-filament heated at approximately 3

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2000°C, atomic hydrogen generation is achieved before CNT growth process for the reduction of oxidized metal nanoparticles and appropriate catalytic metal clusters forming.

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Moreover, during CNT growth, atomic hydrogen can prevent parasitic amorphous carbon (a-

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C) deposition. By adjusting each filament power during CVD process, the amount of

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dynamically activated atomic hydrogen and carbon radicals can be controlled independently.

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(b)

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(a)

Regime

Sample position

Process stage

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Cooling zone

Heating

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Process zone

Pre-treatment (Catalyst reduction & formation)

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Process zone

CNT growth

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Cooling zone

Cooling

Figure 1 4

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The CVD system has two zones: i) a cooling zone at room temperature and ii) a process zone uniformly heated by the oven (heater in the figure 1 (a)). The temperature of the process zone

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is measured with a thermocouple directly inside the quartz tube where the sample is in

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thermal equilibrium with the gas. During the process, the sample can be quickly moved from

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one zone to another one. This configuration allows a direct insertion of the sample in the process atmosphere as well as a rapid quenching of the samples after processing.

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Generally, the growth process includes two sequences as shown in figure 1(b): i) catalyst pre-

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treatment under atomic hydrogen atmosphere (ΦH2= 100 sccm, p= 50 mbar, Pfilament = 180 W) during a time duration varying up to 30 minutes; ii) CNT growth using CH4/H2 mixture

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(ΦH2=50 sccm, Pfilament = 180 W, ΦCH4=50 sccm, Pfilament = 205 W, p=50 mbar) during 30

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minutes. Some samples were prepared without pre-treatment and directly introduced in the

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growth atmosphere (step ii).

In order to study the formation of catalytic Fe clusters, the samples were quenched out after the pre-treatment step and their surface was analyzed by atomic force microscopy (AFM,

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Nanoscope). The as-grown vertically aligned carbon nanotubes (VACNTs) carpets were investigated by field emission scanning electron microscope (FESEM, HITACHI S4800), and the crystalline quality of CNTs was analyzed by high-resolution confocal Raman microscope (Labram HR800; HORIBA Jobin Yvon, λ = 633 nm) in the normal incident backscattering configuration as well as by transmission electron microscopy (TEM, Philips CM 30 working at 300 kV).

3. Results and discussion 5

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3.1. Effect of the temperature on the growth of VACNTs

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The effect of the temperature on the growth of vertically aligned carbon nanotubes is first

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discussed in this section as a preliminary study. For this purpose, samples were prepared

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using a standard two steps process: i) atomic hydrogen pre-treatment on dispersed Fe nanoparticles during 15 minutes at fixed temperature in order to reduce oxidized metal nanoparticles followed by ii) CNT growth using CH4/H2 mixture at the same temperature as

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the pre-treatment. Both steps i) and ii) were carried out at oven temperatures ranging from

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500°C to 700°C by 50°C step.

Figure 2 displays SEM images of the set of carpet-like carbon nanotubes synthesized at

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various temperatures. As shown in figure 2(a) and (d), vertically aligned carbon nanotubes

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with a height of 2 µm can be grown at temperature as low as 500 °C. Note that such low

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synthesis temperature could be attained thanks to highly powered hot filaments (heated over 2000 °C) that decompose methane and hydrogen in active species for CNT growth. As the process temperature increases up to 600 °C, the height of the CNT carpet dramatically

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increases up to 20 µm (figures 2(b) and 2(d)). For higher processing temperature, the VACNT carpet height starts to decrease (10 µm at 650 °C) and no carpet is obtained at 700 °C (figures 2(c) and 2(d)).

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Figure 2

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Typically, the CVD process temperature mostly impacts on the diffusion rate of carbon flux in catalytic metal nanoparticles during growth process, and therefore it also can define either the growth rate of the nanotube or the number of walls of grown nanotubes[23]. From this point of view, the increase of VACNT height from 500 °C to 600 °C can be well understood by a kinetic approach: the temperature enhances the CNT growth rate for a given activation energy related to diffusion of carbon atoms onto or into catalytic nanoparticles. Thus, enhanced growth rate may lead to less “curly” CNTs growth as the process temperature increases [24]. Consequently, the shape of the vertically aligned nanotubes changes also towards relatively 7

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straighter and better quality compared to nanotube grown at lower temperature (figure 2(a), (b)). However, the sudden drop-off in carpet height observed at 650 °C as well as the absence

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of VACNT growth at 700 °C is more surprising. Such trend has already been observed in the

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literature [25]. It has been attributed to catalyst poisoning [25, 26], or Ostwald ripening [27].

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Taking this into account, one can expect that in addition to the role of the temperature during nanotube growth stage, the temperature level during pre-treatment stage could also give

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significant impact. Some hints related to this point are provided by, further experiments

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presented in the next section.

Raman spectroscopy (summarized in figure 3) was carried out on the as-grown VACNT to retrieve global information related to CNT crystallinity and diameter. Let us first focus on

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ID/IG ratio which is related to disorder in sp2 carbon network. As expected (tangential mode region in figure 3a), lower ID/IG ratio are observed at higher temperature. We can assume that

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better graphitized carbon nanotubes can be synthesized as the process temperature increases. One may note that Raman signal recorded on the sample prepared at 700 °C could differ

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depending on the laser spot position. In this case, the poor statistics on CNT Raman features is explained by not uniformly grown CNTs in terms of the types of CNTs such as single-, few-, and multi-walled CNTs as well as by the very low density of CNTs synthesized at this temperature. Raman spectra have been taken for three different sample area [(i), (ii), and (iii)] giving three totally different types of signal [(i), (ii), and (iii)], as presented in figure 3(a). It suggests that different types of CNTs have been synthesized on the same substrate at 700 °C growth temperature (in the inset of figure 3(b), red arrows indicate double-walled, thin multiwalled, and defective thick multi-walled carbon nanotubes).

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Figure 3

Let us now discuss on the Radial Breathing Modes (RBM) region were peaks can only be observed for small diameter nanotubes (ω = 248/DNT where, ω is the Raman shift (cm-1), and DNT is the diameter of carbon nanotubes [28]). As shown in Figure 3(a), RBMs which are 9

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corresponding to inner diameter of CNTs (<4nm) [29] could be detected for temperature higher than 600 °C, meaning that single-walled CNTs or thin multi-walled CNTs can only be

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grown at such elevated temperature. These observations were confirmed by transmission

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electron microscopy investigations which revealed small diameter CNTs (<4nm), and

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especially SWCNTs, only for growth temperature higher than 600°C (Figure 3c). As the process temperature decreases below 600 °C, RBM peaks are disappearing and in this case

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one can also observe that the corresponding D-peak is getting larger compared to the G-peak. This suggests that CNTs are getting thickened, and more defective sites on the wall of CNTs

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are created due to the increasing number of graphitic walls in each nanotube. To this respect, the effect of the temperature on the number of CNT walls has already been discussed in the

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literature [25]. Notably, it has been suggested that a low incorporation rate of carbon atoms at

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the edge of the growing CNT structure due to a low temperature could favor the formation of

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nuclei of other walls, leading to MWCNTs growth. As the process temperature decreases (550, and 500 °C), one can expect an increase of the other wall nuclei formation by low

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incorporation rate at the edge of growing CNTs [25].

3.2. The effect of the atomic hydrogen assisted pre-treatment time on the growth of VACNTs

Hydrogen pre-treatment is generally used in the CNT growth process for two main purposes: i) to reduce the catalyst which is often initially oxidized due to air exposure; ii) to form nanoparticles from catalyst thin films. Recently, in situ study has undoubtedly confirmed that iron oxide could not catalyze CNT growth and hence that catalyst particle should be activated, i.e.

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reduced [30]. A pre-treatment step thus appears as an essential for controlled CNT synthesis.

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To verify this assumption, we performed a set of experiments without pre-treatment stage.

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Dispersed Fe nanoparticles were thus, directly exposed to the growth atmosphere of CH4/H2 mixture at 600 °C and respectively 700 °C, as above described, without any pre-treatment. Surprisingly, vertically aligned CNTs could be obtained for both experiments (see Figure 4). In order to explain this efficient growth, we suppose that upon exposure to the CNT growth atmosphere the iron oxide nanoparticles can be quickly reduced to iron clusters. We can 11

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attribute this result to the presence of hot filament-assisted highly activated atomic hydrogen

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which is strongly reactive and may promote such catalyst reduction.

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The VACNT carpet synthesized at 600 °C displays almost identical Raman features in the

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“tangential mode” region as compared to pre-treated samples but the carpet height is significantly shorter as observed in figure 4(a), and (c). For the sample prepared at 700 °C, 20 µm long VACNT carpet is observed as shown in figure 4(b) which starkly contrasts with

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the randomly grown CNT networks obtained at the same temperature when using hydrogen

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pre-treatment. From the Raman data in figure 4(c), one can notice that the CNTs grown at 700 °C show the structural features of few-walled nanotubes (single, double-walled nanotubes, etc) as compared to pre-treated samples. RBM peaks were detected at several frequencies in

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“breathing mode”, and also the ID/IG ratio has low value (~ 0.34).

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These results obtained on the two samples without pre-treatment appear somewhat contradictory: while hydrogen pre-treatment seems to improve VACNT growth rate at 600 °C, it is shown to impede CNT alignment at 700 °C as already discussed in previous paragraph

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(see also figure 2). Thus arises the question about the actual role of the pre-treatment step on the catalyst restructuring and hence on the resulting VACNT growth.

In order to monitor the catalyst restructuring by the atomic hydrogen assisted pre-treatment, a set of samples were prepared at different pre-treatment time in the range of 5 to 30 minutes. After pre-treatment, the samples were quenched out to the cooling zone of the reactor in order to prevent eventual additional catalyst coarsening and modification. For CNT growth, another set of samples were pre-treated under identical conditions and subsequently each sample was exposed to the CNTs growth atmosphere as described in previous paragraph.

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As summarized in figure 5, one can notice that the atomic hydrogen assisted pre-treatment stage significantly affects the diameter, the density and the uniformity of the as-formed

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catalyst nanoparticles. Previous reported studies [27], pointed out that catalyst coarsening

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takes place as the pre-treatment time increases, the catalyst therefore becomes larger.

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However, one may remark an inflection point of this trend for intermediate pre-treatment time as shown in figure 5(g) (around 15 - 20 minutes pre-treatment time).

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This unexpected non-linear dynamics in the iron nanoparticle cluster formation upon pre-

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treatment might be explained by two competitive phenomena: i) Ostwald ripening which yields nanoparticles coarsening and ii) particle splitting by trap site (defects over the Al2O3

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support layer created by hydrogen radicals) [31].

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At the early stage of pre-treatment, Oswald ripening may dominantly take place as the trap

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sites are not created sufficiently as shown in figure 5(b). Catalyst coarsening therefore progresses and particle sizes are getting larger up to a critical point where cracking on Al2O3 layer is sufficiently carried out by atomic hydrogen. Indeed, atomic hydrogen may create and

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stabilize defects on Al2O3 surface with a high trapping energy as pre-treatment time increases (t > 10 min). It could thus increase the pinning of catalyst clusters onto Al2O3 and thus retard the surface diffusion leading to Ostwald ripening of metallic nanoparticles due to the high trapping energy. Hence, the Ostwald ripening effect can be balanced by the increasing defects site number formed during the pre-treatment. As treatment time increases, cluster splitting may become dominant as defects sites on the surface get uniformly distributed and catalytic nanoparticle can be dispersed on the surface until trap sites on Al2O3 layer are stabilized (figure 5c, d). After stabilization of trap sites on Al2O3 layer (t > 20 min) and as pre-treatment time progresses, Ostwald ripening predominates again as compared to catalyst pinning onto 13

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Al2O3 (over a certain nanoparticles size, increasing de-trapping of the trapped metallic nanoparticles in defect sites takes place [17]). Catalyst coarsening leading to bigger cluster

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occurs due to the trapping energy disparity, as irregular size distribution is observed (figure

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5e), and this trend is getting intensified for longer pre-treatment time (figure 5f).

In Figure 6(a) we plotted the height of the VACNT carpets synthesized from Fe nanoparticles formed at various pre-treatment time. We can notice that up to 20 minutes pre-treatment time,

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CNT growth rate continuously increases. At 20 minutes pre-treatment time, the resulting VACNTs carpet exhibit very dense packing, and highest height (~ 25 μm) as well as very good CNT crystalline quality (figure 6b, 6d). For longer pre-treatment time, the CNT growth rate suddenly drops off and the VACNTs grown after 30 minutes pre-treatment show lowest height and very poor CNTs packing (figure 6c). Accounting for the AFM analysis discussed above, the growth rate seems optimum for catalyst particles distribution that exhibits a good compromise between uniformity and small size.

Raman spectroscopy analysis did not highlight significant difference in terms of ID/IG ratio as a function of pre-treatment time (see Figure 6d). We suppose that this can be related to the 14

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fact that CNT growth conditions (which are expected to influence on the average CNTs crystalline quality) were the same except for the pre-treatment time. However, the G bands

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around 1600cm-1 exhibit significant changes (position and splitting) that correlated with the

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corresponding RBM peaks at low frequency provided interesting information on the CNT

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diameter distribution.

Without a pre-treatment stage, a main peak is observed at ~185 cm-1. As the pre-treatment

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time increases up to 15 minutes, the intensity of RBM peaks slightly diminishes even if some

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component at 185 cm-1 can be still distinguished after 15 minutes pre-treatment. After 20 minutes pre-treatment, CNTs exhibit a strong peak at ~130 cm-1 which corresponds to

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relatively larger inner diameter of CNTs compared to non-treated case (185 cm-1). For longer

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pre-treatment, RBM peaks vanish again.

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Figure 6

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The decreasing of the RBM peak intensity can be partially interpreted by loss of uniformity of the CNT diameters. Instead of a main component with a defined frequency, the spreading of

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diameters may yield numerous components that can be hardly distinguished. Hence, the strong single peak observed for 0 and 20 minutes of pre-treatment time could mean that CNT uniformity is close to an optimum. Moreover, the peak displacement from 185 cm-1, in case of no pre-treatment step, to 130 cm-1 after 20 minutes pre-treatment, suggests some thickening effect in the CNT diameters. Such trends well agree with the AFM study carried out on quenched catalyst.

Let us now discuss on the relationship between the VACNT carpet growth rate and the catalyst particle morphology. As observed in this study, the growth rate is low when asformed catalyst particles displays large diameter and poor size uniformity. Comparing 16

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VACNT carpets grown for 20 minutes and 30 minutes pre-treatment time respectively, may help us to explain this behavior. As noticed from the AFM investigation (figure 5d, 5f), and

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the SEM observation (figure 6b, 6c), CNT growth rate increases, the VACNT carpet gets

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dense as the packing density of catalytic nanoparticles increases and as the nanoparticle

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distribution gets more uniform (figure 5d, 6b). On the contrary, when catalyst distribution is irregular (figure 5f), resulting grown VACNTs carpet height decreases, and CNTs are not well

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aligned (figure 6c). An irregular distribution of catalytic nanoparticles may lead to relatively higher nanoparticles interspacing and thus poor CNT density, explaining the observed CNT

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carpet growth rate trend as a function of the pre-treatment time. Previous reported work of Cui et al. [32], also suggests that inter-particle spacing can affect the CNT array height by

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affecting their lengthening time. As a result, the larger is the catalyst interspacing, the lower

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will be the as-grown CNT array height. In addition, the CNT misalignment resulting from

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their low density (less crowding effect) can also partially explain the low CNT array height

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for large particle interspacing.

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Figure 7

Generally, it is expected that pre-treatment temperature [33] will strongly impact the catalytic nanoparticle formation. . In particular, if deposited metal thin films are heated up to a certain temperature, metal atoms have higher mobility resulting in metal nanoparticles coarsening by Oswald ripening or by nanoparticle migration through surface to minimize surface energy and/or the free energy of the substrate/nanoparticle interface [34]. This suggests that change on catalytic nanoparticle formation can take place relatively fast as treatment temperature

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increases. On the contrary, at lower temperature, Ostwald ripening phenomena is expected to

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be strongly limited, leading to smaller catalyst particles formation for the same treatment time.

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In order to confirm the assumption that both atomic hydrogen assisted pre-treatment time and

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the pre-treatment temperature impact the catalyst nanoparticles, we prepared a sample using a two steps process : i) 15 minutes pretreatment at 400°C; ii) 30 minutes CNT growth at 600°C as shown in figure 7. This grown VACNTs carpet is to be compared to the one prepared in

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same growth conditions, except for the pre-treatment step, in figure 4(a) (pre-treatment at

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400°C vs. No treatment). As expected, catalyst particles pretreated at 400°C exhibits smaller diameter as compared to those pretreated at 600 °C. Also, Raman spectra recorded on the

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resulting CNT carpet exhibit strong RBM peaks as well as a lower ID/IG ratio, suggesting the

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presence of smaller diameter CNTs. As discussed above, catalyst particles could be engineered by adjusting the atomic hydrogen assisted pre-treatment step, and such striking

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result shows that the pre-treatment parameters can be used as an “invaluable tool” to tune the

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structural properties of the subsequently grown VACNTs.

4. Conclusion

CNT arrays were synthesized using hot-filament CVD at relatively low temperature down to 500° C. Spayed iron chloride salts solutions were used as catalyst precursor for the synthesis of carbon nanotubes. Upon exposure to highly activated atomic hydrogen, the oxidized particle could be well reduced in the growth atmosphere leading to effective CNT growth. We evidenced that there is an optimum pre-treatment time in order to grow well vertically aligned carbon nanotubes carpets with optimum density and height. We showed that catalyst size and 19

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uniformity did not linearly depend on the pre-treatment time but resulted from a balance between Ostwald ripening and the formation surface defect by atomic hydrogen. Finally, we relatively reduced by lowering the pre-treatment

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showed that CNT’s diameter was

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temperature prior to CNT growth and that the density of the CNT carpet as well as the CNTs

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diameter and length were adjusted by catalytic nanoparticle engineering during the pre-

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treatment stage.

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Acknowledgements

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We appreciate help for iron nanoparticles preparation by spray-gun technique from Paolo

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A list of captions for Figures and Tables

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Figure 1. (a) Schematic image of modified HF CVD system, and (b) general process to

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synthesize VACNTs. In HF CVD system, there are two gas lines, one is to introduce

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hydrogen which plays a role of a reduction agent of oxidized metal catalyst and an etching agent of parasitic amorphous carbon (a-C) during CVD process, and the other one is carbon feedstock precursor. Filaments are set up to each gas inlet inside quartz tube. In quartz tube,

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there are two zones, i.e. cooling zone, and process zone, and samples are placed in either cooling zone or process zone depending on process regime in (b): 1 – heating stage, 2 – pretreatment stage, 3 – growth stage, and 4 – cooling stage.

Figure 2. SEM images of carbon nanotubes synthesized at various process temperatures (a) 500°C, (b) 600°C, (c) 700°C, (d) Height of the VACNT carpet as a function of the growth temperature.

Figure 3. (a) Raman spectra of grown CNTs at various process temperatures and for 3 different laser spot positions [(i),(ii),(iii)] (b) shows the summary of ID/IG ratio and inset 25

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shows the TEM image of grown CNTs at 700 °C. Red arrows indicate double-walled, thin multi-walled, and defective thick multi-walled CNTs. (c) TEM image of grown CNTs at

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600 °C

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Figure 4. SEM images of grown VACNTs without atomic hydrogen assisted pretreatment at (a) 600°C, and (b) 700°C. Inset shows SEM images in high magnification to observe the structural properties of grown VACNTs (c) Raman spectra of grown VACNTs at both

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temperatures.

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Figure 5. Atomic force microscopy (AFM) images of dispersed catalytic Fe nanoparticles on Al2O3 layer after atomic hydrogen assisted pre-treatment process for (a) 0 min, (b) 5 min, (c)

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15 min, (d) 20 min, (e) 25 min, and (f) 30 min before CNT growth process. (g) Average

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height of formed iron nanoparticles as a function of atomic hydrogen assisted pre-treatment

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time. Inset shows RMS roughness of formed iron nanoparticles as a function of pre-treatment time.

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Figure 6. (a) The average height of grown VACNT carpet after various atomic hydrogen assisted pre-treatment time and SEM images of VACNT carpet after atomic hydrogen assisted pretreatment for (b) 20 minutes, and (c) 30 minutes. Inset shows the structural properties of grown CNTs respectively. (d) Raman spectra of grown VACNT carpet after various pretreatment time.

Figure 7. AFM image of Fe nanoparticles after atomic hydrogen assisted pre-treatment at 400°C, and (b) SEM images and (c) Raman spectra of grown VACNTs at 600°C on engineered catalytic iron nanoparticles at 400°C.

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