How to switch from a tip to base growth mechanism in carbon nanotube growth by catalytic chemical vapour deposition

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How to switch from a tip to base growth mechanism in carbon nanotube growth by catalytic chemical vapour deposition J. Dijon a,*, P.D. Szkutnik a, A. Fournier a, T. Goislard de Monsabert a, H. Okuno a, E. Quesnel b, V. Muffato b, E. De Vito c, N. Bendiab d, A. Bogner e, N. Bernier e a

CEA LITEN/DTNM/LCRE, Grenoble 17 Rue des Martyrs, 38054 Grenoble, France CEA LITEN/DTNM/LTS, Grenoble 17 Rue des Martyrs, 38054 Grenoble, France c CEA LITEN/DTH, Grenoble 17 Rue des Martyrs, 38054 Grenoble, France d Institut Ne´el, CNRS/UJF, 25 Rue des Martyrs, 38042 Grenoble, France e CEA LETI/DPTS/SCPIO, Grenoble 17 Rue des Martyrs, 38054 Grenoble, France b

A R T I C L E I N F O

A B S T R A C T

Article history:

The catalytic growth of carbon nanotubes is performed at 580 C with iron catalyst using a

Received 8 December 2009

hot-wall CVD technique. It is shown that the growth mode can be switched from ‘tip’ to

Accepted 29 June 2010

‘base’ growth mode by an insertion of plasma pre-treatment of catalyst at room tempera-

Available online 3 July 2010

ture before growth. To understand this phenomenon, the oxidation state of catalyst is compared using XPS before and after the pre-treatment. Growth mechanisms induced by different oxidation states of catalyst are proposed to explain the switching of the growth mode. A thermodynamic calculation shows the relation between induced growth mode and resulting CNT diameter.  2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Owing to their excellent electric and thermal properties [1–3], carbon nanotubes (CNTs) have been expected to be used for many applications [4,5]. Since the landmark paper on carbon nanotubes by Iijima [6], great progress has been made on the controlled growth of multiwall (MW) CNTs. Dense forest of vertically aligned (VA) CNTs is a promising material for applications due to their highly anisotropic properties and high tuneable porosity in macroscopic length, where the direct applications are available [7]. In addition, the fine structure of CNTs, such as chirality, diameter, length and the number of graphene layers significantly affects their physical properties. Although there has been vast research performed on the synthesis of CNTs, as well as VACNTs, there still remain fundamental questions regarding processes occurring during growth. The type of grown CNTs sensitively depends on many

parameters such as metal catalysts, substrate materials, feed gases and growth temperatures. Many interesting experimental results on the growth of VACNTs have been presented showing the possibilities of SW or DW CNT forests with small diameters [8], represented by ‘‘Super Growth’’ CNTs [7]. These kinds of materials are known to be easily grown on oxide substrates like alumina [9,10], quartz [11] or magnesium oxide [12]. The use of oxide substrate is also needed for VA-SWCNTs [7,13,14]. However, the use of oxidized substrate limits applications of CNTs in devices where electrical contact between tubes and the structure is necessary such as for interconnects [15]. Conductive substrates should therefore be preferred from an application point of view but their use requires a better understanding of CNT growth mechanisms. The catalytic growth can exhibit two different mechanisms. One is so-called tip growth where the particles are lifted off the substrate and they are found at

* Corresponding author: Fax: +33 438785117. E-mail address: [email protected] (J. Dijon). 0008-6223/$ - see front matter  2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.06.064

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the tip of the CNTs. Another is the base growth with the catalyst particle staying on the substrate surface during the growth. The VA-CNTs attracting much interest for their small diameter and possible high length and density are often grown in the base growth mode [16]. Control of the growth mode should be a first step to determine the type of the grown VACNTs. The choice of the growth mode has been generally explained in terms of adhesion between the catalyst and the substrate [17–20]. The observations of the same substrate catalyst system leading to different growth modes weaken the above argument [21]. Since a strong correlation between the growth mode and the CNT diameter is observed, the control of the mode by the catalyst particle diameter has been recently proposed [21]. However, the choice of the growth mode seems to be a more complicated story. In particular no mechanism explains why catalyst diameter change induces growth mode switching. In this study, we demonstrate a single experimental parameter to determine the growth mode. We focus on the oxidation state of the metal catalyst particle at the moment of feed gas introduction. In addition, the correlation between the CNT size and the growth mode is explained using thermodynamic nucleation models.

2.

Experimental

The catalysts consisting of a continuous iron film are deposited by ion beam sputtering (IBS). The IBS technique allows us to precisely control the catalyst layer thickness because of its low deposition rate (0.05 nm s1). The thickness of Fe film is fixed to be 1 nm for all samples. To obtain a good electrical contact, silicon wafer substrates are deoxidized by in situ argon ion bombardment in the IBS reactor just before the catalyst deposition. Fig. 1 shows cross-sectional TEM images of the catalyst layer deposited before (Fig. 1(a)) and after (Fig. 1(b)) the deoxidation step. It exhibits that the iron is directly deposited on the silicon without any oxide intermediate layer after deoxidation. In the following silicon means deoxidized silicon. CNT arrays are grown in a vertical hot-wall Plassys CVD reactor [22]. Differing from conventional classical CVD

techniques in tubular furnace working at the atmospheric pressure, our reactor allows to perform the growth process at relatively low pressure. The operating pressure is varied typically from 0.2 to 0.4 mbar. First, a step of plasma treatment on the catalyst surface is introduced before the CNT growth. RF-oxidizing plasma with the power of 180 W is performed under air at 0.4 mbar during 20 min at room temperature. Then the reactor is heated up to 580 C in 15 min under H2 atmosphere at 0.2 mbar. After 20 min of thermal stabilization at lower pressure, the feedstock gas is introduced for the CNT growth. The feed gas is composed with acetylene (10 sccm) diluted in a mixture of H2 and He (50 sccm for each gas). The total gas pressure during the growth is fixed at 0.4 mbar. The samples are cooled down under He flow after various growth times. A field-emission scanning electron microscope (FE-SEM) LEO 1530 is used to observe the morphology of the grown CNT arrays. Transmission electron microscopy is performed using a field-emission Jeol JEM 2010FEF operating at an accelerating voltage of 200 kV and equipped with an in column Omega filter to characterize the catalyst–substrate interface and the atomic structure of CNTs. Cross-sectional samples are prepared using a conventional precision ion polishing system (PIPSTM). The CNTs removed from substrate are dispersed in ethanol and dropped on a holey carbon coated copper grid. The XPS device used in this study is a SSI-SProbe XPS spectrometer, equipped with a monochromatic Al Ka radiation (hm = 1486.6 eV). The default take-off angle used for recording the spectra is 35, which enables increased sensitivity at the extreme surface of the samples, and in the particular case of CNT forests, to be more sensitive to the tip of the nanotubes. The data have been computed with the help of the CasaXPS software.

3.

Results

3.1.

Morphology of grown CNT materials

Samples are prepared with and without oxidizing plasma pretreatment on the catalyst layer. All other growth process parameters are kept strictly identical. In all cases, formation of the vertical aligned CNT forests on the substrate is

Fig. 1 – Cross section of our catalyst system (1 nm of Fe) deposited on deoxidized silicon wafer right image as compared with 1 nm of iron deposited on Si with native oxide silica left image. The scale bar is 5 nm.

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Fig. 2 – Role of the catalyst plasma pre-treatment on the CNT height. Typical SEM images of the forest achieved: (A) on sample without plasma and (B) on sample with plasma.

confirmed by SEM observation. Fig. 2 shows SEM images of typical morphologies of CNT forests obtained (a) with and (b) without plasma pre-treatment. The height of CNT forests for 15 min growth time is measured by SEM and the diameter of composing CNTs is determined by TEM observation (Table 1). For the samples grown without plasma pre-treatment, the average height and the minimum diameter are 6 lm and 10 nm, respectively. In the case with plasma pre-treatment, the forest height is found to be much higher (25 lm) but with smaller diameter (>4 nm). Fig. 3 shows TEM images of CNTs grown (a and b) without and (c and d) with plasma pre-treatment. In the sample without plasma treatment, metallic

particles are sometimes observed inside the tip of CNTs as shown in Fig. 3. The average number of walls composing the CNTs is three for the sample with plasma treatment and seven for that without treatment.

3.2.

Identification of the growth mode

Fig. 4 demonstrates SEM images associated with back-scattering mode of CNT tips (a) with and (b) without plasma treatment. In Fig. 4(a), the Fe catalyst particles are recognized at the tip of the CNTs as bright spots (shown by white arrows). On the contrary, in the case with plasma pre-treatment, no

Fig. 3 – TEM images of the CNTs obtained with 1 nm of iron on Si. Top images (A) and (B) sample without plasma pretreatment of the catalyst. Bottom images (C) and (D) sample with plasma treatment of the catalyst before growth.

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Fig. 4 – SEM images of the tip of the tubes grown on samples without plasma pre-treatment (on top) and with plasma pretreatment (on bottom). The same area of both samples is imaged with two different imaging modes the right images correspond to back-scattering mode, the left images correspond to in lens mode. Catalyst particles (arrows) are clearly seen on the tips of all the tube of the untreated sample while they are completely lacking on the treated sample.

any obvious spots are detected at the tip of the CNTs Fig. 4(b). This clearly demonstrates that the plasma pre-treatment induces the tip growth mode. However, this observation is not enough to identify the base growth mode. To identify the growth mode under each condition, two step growth experiments are carried out. When the growth of CNT forest is ruptured and restarted, a conjunction of two separated grown parts is recognized [23]. It is thus possible to determine the growth direction using this conjunction line for levelling the growth front as a function of time. A first growth is performed during 5 min and then the chamber is vacuumed. The sample is kept at growth temperature during 5 min under vacuum, followed by a second growth with duration of 55 min. Fig. 5 shows SEM images after the two step

growth. Although there is no visible conjunction line for the sample without plasma pre-treatment (Fig. 5(a)), a clear conjunction line is observed in the case with plasma (Fig. 5(b)). This line separates the grown CNT forest to upper part of 6 lm and lower part of 36 lm. This indicates that the growth front is at the base of the forest, being evidence of the base growth mode to be induced by the plasma pre-treatment. In contrast, in the case without plasma the forest height corresponds to that for the growth of 5 min. This indicates two important facts in the growth mechanism. First, it is confirmed that the plasma pre-treatment modifies the growth mechanism. Second, as the growth mode without plasma treatment is shown to be the tip growth using SEM back-scattering (Fig. 4(a)), the second growth step can not start after a

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Fig. 6 – Kinetics of CNT growth for samples with 1 nm of iron catalyst film according to the catalyst pre-treatment. With plasma pre-treatment nothing is observed on the surface by SEM after 35 s. First tubes are observed after 1 min of growth. The growth rate is parabolic. The observed incubation time (between 35 and 60 s) is consistent with a reduction step of the iron oxide occurring just before growth start. Without plasma pre-treatment (tip growth mode, dotted line) the growth rate is linear with a rate of 0.4 lm min1.

Fig. 5 – Sequential growth experiment consisting in 5 min growth followed by 5 min stop under vacuum and 55 min growth performed on sample without plasma pre-treatment top image A and with plasma pre-treatment bottom image B. The inset presents a zoom of the upper part of the stack layers and indicates the different layer height.

rupture in the tip growth mode. From these results, it is demonstrated that the growth mode can be identified whether tip growth or base growth using the two step growth operation. Fig. 6 shows the height of the CNT forests as a function of growth time for the samples with and without plasma pretreatment. With the plasma pre-treated catalyst, the growth rate is parabolic with an incubation time close to 1 min before CNT growth start. Without plasma, no similar incubation time has been detected, where the growth rate is observed to be linear up to 15 min. Using the two step growth experiments, the growth mode on the alumina (Al2O3) substrate, commonly used for the dense CNT forest growth, is identified with and without plasma pre-treatment. The characterization results for each condition are summarized in Table 2. Different from the Si substrate, the base growth mode is always induced on the Al2O3 substrates independently of the presence of the plasma pre-treatment step.

3.3.

Oxidation states of the catalyst

To understand the role of the oxidizing plasma step, XPS study on the oxidation state of the catalysts is performed be-

fore and after the plasma pre-treatment. The quantitative results are shown in Table 3. On the Si substrates, both the metallic iron (represented by Feo) and a mixed oxidized iron (Fe 3+,2+) are found before and after the plasma treatment. However, it is clearly shown that the ratio of the oxidized Fe to the metallic one increased after the treatment. The Si2p XPS spectra is found to be composed of three spectral components Sio at 99.2 eV, an oxide contribution at 102.5 eV and a smaller one at 101 eV. Though the highest binding energy peak is lower than standard SiO2 [24,25], we still think that this XPS peak is related to oxidized Si. The smaller contribution energy may be related to the Fe–Si interface. As far as the silicon substrate is concerned, the quantitative XPS results given in Table 3 show that the plasma oxidized Si as well as Fe. This is clearly seen for both materials, for Fe from the ratio Fe2+3+/Feo, and for Si from the ratio Siox/Sio. In the same time, the intermediary peak (Si–Fe interface) does not vary with the plasma application. This indicates a strong influence on the oxidation state of the metal catalyst by the plasma treatment. In the case of Al2O3 substrate, no matter whether the plasma treatment being applied or not, the Fe catalyst is fully oxidized without any metallic Fe (Table 3). These results strongly suggest that oxidation of the catalyst is necessary to induce the base growth mode. To investigate a reduction of Fe catalyst during the growth, XPS measurements are performed on the Fe catalyst deposited on Al2O3 before (Fig. 7(a)) and after (Fig. 7(b)) the growth. As demonstrated above, only oxidized iron is detected without metallic or carbide iron before the growth. On the contrary, the XPS spectrum performed on the iron catalysts after the CNT growth (Fig. 7(b)) shows an important increase of metallic iron, where no more oxide is detected. For the latter experiment, the measurements are done on the catalyst

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Table 1 – The influence of the oxidizing plasma pre-treatment of the catalyst on the morphology of the grown CNT material. The used catalyst is a film of 1 nm of iron catalyst deposited on deoxidized silicon. The other process conditions are identical (see text). Plasma application time (min)

Growth time (min)

0 30

CNT length (SEM) (lm)

15 15

Minimum CNT diameter (SEM), (TEM) (nm)

6 25

10, 8 4

Number of wall (TEM) 5–8 2–5

Table 2 – CNT morphology and associated growth mechanism for the same catalyst system deposited on various substrates. s_MWCNT in the last column is a short for small diameter multiwall CNT. For all these experiments the conditions are identical, growth time is 30 min. Substrate Si Si Al2O3 Al2O3

Plasma treatment No Yes No Yes

Catalyst (nm) Fe Fe Fe Fe

1 1 1 1

Two steps L1, L2 (lm) 2, 6, 5, 9,

Growth mechanism

0 36 90 75

Tip Base Base Base

Total height (lm) 2 42 95 84

Tube diameter nm 12 >4 >4

CNT type MWCNT s_MWCNT s_MWCNT s_MWCNT

Fig. 7 – XPS spectra of 1 nm of iron catalyst deposited on alumina. (A) Before growth on the left (no Fe metal detected); (B) after growth on the right (inside the nanotubes). Only Fe metal is detected (no carbide and no oxide) clear indication of a reduction of the iron oxide during the growth process. particles inside the CNTs in order to avoid the detection of Fe oxidized after the experiments. This indicates a reduction of the iron oxide during the growth process. Since the XPS measurements on the iron catalyst before CNT growth are performed at room temperature, we have no direct measurement of the state of the catalyst after heating just before the introduction of the feedstock gas. Additional experiment to investigate effects of reduction during the heating step is carried out. The experiment is performed with a heating process under O2 instead of H2 after the plasma treatment. In this case we are sure that the iron oxide is not reduced before the introduction of the feedstock gas. For the plasma pre-treated catalyst, the grown CNTs are absolutely identical regardless of the gas used during heating. In both cases the tubes height is 56 lm with a minimum diameter close to 4.5 nm, the mechanism is also base growth.

These results clearly point towards oxide iron as the state of the catalyst necessary to obtain a base growth mechanism. In the case of the oxide substrates, the catalyst is fully oxidized; in the other case (metal phase containing catalysts) the plasma treatment strongly enhances the oxide proportion.

3.4.

Catalyst particle size before growth

The particle size at the end of the thermal stabilization step has been measured using SEM images with both processes. To avoid possible sintering or particle evolution during thermal cooling which cannot be extremely fast with our chamber the cooling step has been done under air atmosphere to oxidize the reduced particles and avoid possible Oswald ripening. Statistical analysis performed on the different images

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indicates that both particle size distributions are well represented by log normal distributions. With plasma pre-treatment step the mean particle diameter is 11.5 nm while without plasma pre-treatment the mean particle diameter is 10.3 nm. Thus the plasma pre-treatment does not modify strongly the particle distribution before feedstock gas introduction.

4. Nucleation models toward the relation between NT size and growth mode We experimentally observe a difference of CNT size following the growth mode. To understand this correlation between CNT size and the mode of their growth, we calculate the Gibbs free energy variation induced by two different carbon nuclei models as a function of CNT diameter. While the proposed thermodynamic model is phenomenological we think it can give a general understanding of the reason for the observed switching growth mode. Furthermore we are dealing with particle size where rigorous microscopic treatment of carbon nucleation is difficult and where thermodynamic approach has proven to be valuable [26]. The difference of the CNT size can be explained by nucleation conditions and difficulty of nucleation for small diameter tubes. This difficulty comes from the very high energy of the edge of the graphite plane that must be overcome. Indeed this energy is close to 4.5 J/m2 while the energy of the graphite basal plane is just 0.077 J/m2 [27]. To have a small nucleation diameter the system must minimise the edge energy and thus it length. One obvious difference between the two growth modes is the initial geometry of the carbon nucleus which allows further growth. In our model, we consider cylindrical catalyst particles with a spherical cap depicted Fig. 8. The height of the cylindrical part h is defined equal to the radius of the particle RP. Indeed this particular geometry minimizes the surface of the particle (thus its energy) for a given volume of the catalyst. It is not too unrealistic to be close to the real shape of catalyst particles and this shape can manage both kinds of nucleus without reshaping the particles. Spherical and cylindrical nuclei are considered for base and tip growth mechanism, respectively. The change in Gibbs free energy between

the initial state (only the catalyst particle without carbon) and the final state (associated with the initial carbon nucleus) is classically expressed as the sum of two terms: surface term DGS and volume term DGV. Following the model by Kuznetsov et al. [28], we also consider two other extra terms. One is edge term, taking into account the interaction of the edges of graphite with the metal. Another is energy strain DGB to roll the graphene sheet into a tube or to bend it into a spherical shape. So here we can describe: DG ¼ DGV þ DGS þ DGedge þ DGB

ð1Þ

These terms depends on the geometry of the carbon deposit and on energetic parameters that will be detailed below. The volume term is express as: a RT ln ¼ VjDgV j ð2Þ DGV ¼ V Vm ao where V is the volume of the deposited graphite with a given geometry, Vm is the volume molar of the graphite (5.3 · 106 m3), ao is the activity of the saturated carbon solution in the metal and a is the activity of the actual carbon solution in the metal. The surface term is expressed for both geometries (because of h = RP) as: DGS ¼ 2pR2P Dr

ð3Þ

where Dr ¼ ðrFe;G þ rG  rFe Þ is the energy variation linked with the creation of the nucleus on the catalyst, rFe;G rG and rFe are iron–graphite interface energy, graphite and iron surface energy, respectively. This energy variation can be expressed conveniently with just one parameter which is the adhesion energy of graphite on the metal, Wad. Thus we have Dr ¼ ðrFe;G þ rG  rFe Þ ¼ ð2rG þ rFe;G  rG  rFe Þ ¼ 2rG  Wad

ð4Þ

Assuming that the carbon atoms at the edge of the nucleus are bounded to the catalyst metal with a zigzag arrangement on the edge, the edge free energy approximated by Kuznetsov is We ¼

DHFeC  DHCC 2NA rCC d

ð5Þ

where DHFeC is the enthalpy of formation of carbon bound with Fe (245.2 kJ mol1), DHCC is the enthalpy of formation

carbon nucleus

Catalyst particle

h Rp Substrate

Fig. 8 – Geometry of the carbon nucleus considered. On the left: for base growth process spherical carbon cap with one graphite edge in interaction with the catalyst metal. On the right: for tip growth process hollow cylindrical carbon nucleus with two edges one in interaction with the substrate the other with the catalyst.

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of carbon–carbon bound (473 kJ mol1), rcc is the distance between two neighbouring carbon atoms in the zigzag edge of a graphite sheet 0.25 nm and d is the graphite interlayer distance 0.34 nm. This bonding decreases significantly the edge energy as compared with the free dangling bond. Using Eq. (5), the edge energy is thus estimated to be 2.23 J/m2 for Fe catalyst. The bending term for the both nucleus geometries, extracted from [29], are DGB ðhalf sphereÞ ¼

2pDð1 þ aÞ t d

DGB ðcylinderÞ ¼

pD t d

ð6Þ

where D is the flexural rigidity with a value of 1.41 eV for graphene with elastic constant parameter a = 0.165. So finally collecting all the above equations, we have: DGBase Growth ¼ 2pR2P tjDgv j  2pR2P ðWad  2rG Þ þ 2pRP tWe þ

2pDð1 þ aÞ t d

DGTip Growth ¼ 2pR2P tjDgv j  2pR2P ðWad  2rG Þ pD t þ 2pRP tðWe þ WS Þ þ d

ð7aÞ

ð7bÞ

These two equations are plotted as a function of the catalyst particle diameter in Fig. 9. We take a thickness of the carbon nucleus as two layers for base growth and 2, 5 and 8 layers for tip growth to stick to our results (Fig. 4). To simplify the discussion, we assume the surface energy of the carbon edge on the substrate WS equal to the surface energy on the catalyst We. Following [28], the over saturation parameter a/ao and Wad are fixed at 2 and 0.25 J/m2, respectively, for carbon adhesion on solid (not melted) catalyst particle. This arbitrary choice is without any influence on the relative position of curves on Fig. 9. Nucleation may occur only with DG < 0. For base growth, this condition is fulfilled only for particle diameters, thus tube diameter in our simplified model, being larger than 4.5 nm (critical diameter). For tip growth, the critical particle diameter is close to 9 nm. This factor of 2 between the minimum tube diameters in both modes is mostly related to the edge

energy which is double in the case of the hollow nucleus as compared with the cap. The nucleus thickness t does not modify significantly the critical values but changes the slope of the energy variation with the catalyst diameter. Energy adhesion between the catalyst particle and the substrate cancels in the energy balance for both kind of carbon nucleus.

5.

Discussion

5.1.

Oxidation state of the catalyst at growth temperature

From our experimental results, the oxidation of iron catalyst has been revealed to be an important parameter to induce the base growth mode. Using the oxidizing plasma pre-treatment, the growth mode is switched from tip growth to base growth on Si substrate. However, VA-CNTs are grown in the base growth mode on Al2O3 substrate without oxidation pretreatment by plasma. In all cases which induce base growth, the catalyst iron is strongly oxidized before the thermal ramping and feed gas introduction (Table 3). The interaction of the catalyst with the substrate may be advocated to understand the difference in iron oxidation state on silicon according to the plasma pre-treatment. Indeed according to its location in Ellingham diagram silicon is able to efficiently reduce iron oxide to pure metal. Without plasma the silicon is slightly oxidized and helps to reduce iron to Feo with plasma treatment silicon and iron are more oxidized (Table 3) and the interfacial exchange of the oxygen is no more possible. Iron stays largely oxidized. The lack of oxygen exchange between iron catalyst and Al2O3 substrates also might be the reason of the complete oxidation. This explains base growth being easily induced on the oxide substrates. On the contrary, it is commonly suggested that the metallic iron contributes to the initial growth of CNTs [20]. The reduction of iron oxide is thus necessary to start the CNT growth. It has been demonstrated in our results that the iron catalysts being oxidized after the plasma pre-treatment fully transform to metallic ones after the CNT growth.

Fig. 9 – Evolution of the free energy of carbon nucleus versus the particle radius for the two geometries considered. The energy variation is negative (gain) and nucleation possible for particle larger than 4.5 nm with the base growth mode associated with a carbon nucleus cap (dotted line). For tip growth mode and a cylinder carbon nucleus the critical catalyst size is close to 9 nm (solid lines). For particles larger than about 11 nm tip growth is the most favourable mode thanks to the higher energy gain provided by this mode.

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Table 3 – XPS quantitative results obtained on 1 nm of iron deposited on silicon and alumina with and without plasma preparation before annealing step. Sample

Plasma

Feo

Fe2+,3+

Al

Sio

Siox

Si–Fe

Fe2+3+/Feo

Siox/Sio

Growth mode

No Yes No Yes

1.8 1 0 0

4.1 3.7 10.2 10.6

0 0 12 14.4

9.1 7 0 0

9.2 15.5 0 0

2.7 2.5 0 0

2.3 3.7 All oxide All oxide

1 2.2 0 0

Tip Base Base Base

Si Si Al2O3 Al2O3

Table 4 – State of oxidation of iron catalyst on silicon after heating. Treatment of the catalyst No plasma Plasma

Xps analysis after plasma Low oxidation Stronger oxidation

Heating atmosphere

Reduction step

Growth mechanism

Reduced Oxidized

During heating During growth

Tip growth Base growth

H2 O2

Let us consider the oxidation state at the moment of the feed gas introduction. Table 4 shows the oxidation states after heating and the growth modes induced in each case. To be sure that the oxidation states of catalyst measured by XPS before heating are conserved at the higher temperature, we use reduced and oxidized atmospheres for the reduced and the oxidized samples, respectively. These two different oxidation states also resulted in different growth modes. This confirms the importance of oxidized state at the moment of the feed gas introduction. Since the plasma pre-treated sample heated under H2 has induced the same growth mode as the sample heated under O2, the iron should be still oxidized at 580 C before the introduction of the feed gas even after ramping under H2. Indeed the difficulty to reduce iron oxide using only hydrogen is thermodynamically well known. The Ellingham diagram plotting the free energy of reactions as a function of temperature tells us that Fe2O3 obtained at room temperature can be easily reduced in Fe3O4 with H2. However as further reduction to Fe with H2 is endothermic [30], it is very difficult to achieve. The kinetic aspect of the reaction is important and strongly dependant on the sample history [30]. The reduction is described as a two steps process, first a reduction to Fe3O4 and then a reduction to Fe. The first reaction must be completed to start further reduction to Fe [31]. The isothermal reduction of Fe3O4 follows sigmoid curves characteristic of an induction period attributed to the metal nucleation in the oxide phase [32]. Under our experimental conditions with low hydrogen pressure, the fast increase in temperature (40 C min1) and the short stabilization time (20 min) before introduction of the feedstock gas, the time for the nucleation of metallic iron is too long which causes a large amount of unreduced oxide irons. The initial amount of metallic iron in the particle as well as the initial substrate state is thus critical for the reduction step, which explains why reduction is nevertheless possible with plasma untreated samples. In the base growth mode, the results point towards reduction of the iron oxide by acetylene with a set of reactions that can be globally written as: xC2 H2 þ FeOx ! 2xC þ Fe þ xH2 O

Iron state at feed gas introduction

ð8Þ

It is thermodynamically possible that acetylene reduces all the iron oxides up to metal phase. The kinetic of CNT growth (Fig. 7) which demonstrates the presence of an incubation period after the feed gas introduction is in good agreement with this scheme. This incubation is interpreted as the reduction time necessary before growth starting. Our results are thus consistent with the catalyst at least partly in a metal phase at the start of the CNT growth.

5.2.

Why CNTs are smaller with base growth mode

In Section 4, we have demonstrated the evolution of the free energy of carbon nucleus as a function of the catalyst particle radius for both growth modes (Fig. 9). This calculation makes it clear why the base grown CNTs appear with smaller diameter. The cap geometry is less energetically costly than the cylinder geometry thus smaller tubes can nucleate with the base growth mode. This is evidenced on CNTs with the same number of walls by comparing the free energies for formation of double wall CNTs in the two different modes. With the base growth, DWCNTs are possible with a particle size larger than 4.5 nm while a tip growth DWCNT becomes possible only above 8.5 nm. More generally, tip growth is exhibited to not occur with a catalyst particle of less than 9 nm in diameter with the tubes structures (5–7 walls) observed in our experiments where the free energy becomes positive for a diameter smaller than 9 nm. The most favourable number of walls nwall in the tip growth mode rapidly increases by increasing the particle radius. Following the results of Tibbetts [33], this wall r thus close to the five walls obnumber is about nwall ffi 2:5d served for 9 nm CNTs (Table 1). The model, with the realistically chosen thermodynamic values, mimics quite well our results and points out the major role of the nucleus geometry and the existence of critical CNT sizes for both modes quantitatively in agreement with the observed tube diameter and structures. Our calculation explains also many experimental results previously presented by other authors [21,34,35]. It those cases, the CNTs grown in tip growth are shown always with large diameters (>9 nm) and the ones in base growth mode with small diameter.

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Reduced Particle

Introduction of C2H2

Metal

Oxidized particle

Incubation: Catalyst reduction

Nucleation

Growth

Fig. 10 – The various steps of the nucleation and growth proposed according to the catalyst oxidation state and the nucleus geometry induced by the two different mechanisms. On top: metal catalyst, on bottom: oxidized catalyst.

The puzzling results described by De Los Arcos between metal iron particles on Al2O3 and TiO2 [34] is explained the same way. In that case, small (6 nm) Fe particles on Al2O3 do not produce growth because they are smaller than the critical diameter for tip growth while large particles (28 nm) on TiO2 allow growth of CNTs. The classical explanation that tube growth is controlled by particle diameter is not sufficient and does not explain at all the growth mode switching. Particles must have the right size but the CNTs must also nucleate and nucleation with a given structure is not possible on arbitrary small particles.

5.3. Role of oxidized catalyst on the growth mode switching If the requested catalyst state is metallic for growth with both modes why is the oxidation state so important for the base growth? The two different nucleation mechanisms are summarized as schematics in Fig. 10. When the metal catalyst particles are already formed at the moment of the feed gas introduction, the carbon species provided by the decomposition of the feed gas immediately starts to diffuse in the whole particle just after the feed gas introduction, where the carbon deposit occurs preferentially on the bottom part of the catalyst. The carbon edge on the bottom links to the substrate and carbon network formed on the catalyst continues to accept carbon atoms provided from the top edge linking to the catalyst particles. This is a mechanism of the growth induced in tip growth mode without oxidation pre-treatment. On the contrary, the base growth induced with the oxidized catalyst particles first starts by the reduction of the particles after the feed gas introduction. This step inserts the incubation period as shown in Fig. 6, and then carbon species start to diffuse in the reduced metal rich part of the particles. Once the metallic part reaches the required size, the growth starts at the top of the particle, where the growth mode is limited to the base growth because of the oxide part found in lower part of the

particles. The carbon edge is linked with the catalyst metal so the carbon structure is able to accept more atoms on bottom. It is commonly accepted that the original catalyst particle size influences the diameter of the final forming CNTs. Considering from our model; the growth mode is first defined by the chemical state of catalyst at the moment of the feed gas introduction. However, it depends on the catalyst particle size if the growth starts or not since both growth modes are valid only with limited catalyst size; larger than 4.5 nm for the base growth and larger than 9 nm for the tip growth as shown in Section 4. We introduce the important ideas that arbitrary CNT diameter cannot be controlled with arbitrary growth mode and that limits in the tubes diameters are governed by nucleation lines. The catalyst chemistry and particularly the oxide states of the iron catalyst play a key role in favouring the initial carbon nucleus geometry by a control of the carbon diffusion in the particle.

6.

Conclusions

We have demonstrated that the insertion of an oxidizing plasma pre-treatment step performed at room temperature changes the growth mode from tip growth to base growth. The growth mode was shown to be governed by the oxidation state of the catalyst at the introduction of the feedstock gas. The oxidation of the catalyst determines the geometry of the carbon nucleus, which in turn determines the growth mode. A simple thermodynamic model based on this hypothesis has lead to a minimum tube diameter for both base and tip growth modes in agreement with our experimental results. The particle diameter is not the key parameter which determines the growth mode but the critical parameter to start the growth in the growth mode triggered by the chemical state of the catalysts. It is thus very important to control both the oxidation state of catalyst and the catalyst particle size to obtain the CNTs with expected diameter and density.

CARBON

4 8 ( 20 1 0 ) 3 9 5 3–39 6 3

Acknowledgements These results have been obtained in the framework of the FP6 project CARBonCHIP funded by the European Commission Contract NMP4-CT-2006-016475. Thanks are due to Patrice Gadelle and Mark Scannell which kindly read the original manuscript and gives us valuable comments and feedbacks. Thanks are also due to Pierre Desre´ for training us in the thermodynamic nucleation theory and for very helpful discussions.

R E F E R E N C E S

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