Effect of Surface Morphology of Catalysts Nickel ... - Science Direct

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ScienceDirect Procedia Materials Science 8 (2015) 770 – 777

International Congress of Science and Technology of Metallurgy and Materials, SAM CONAMET 2013

Effect of Surface Morphology of Catalysts Nickel Coatings Obtained by Cathodic Arc in the Synthesis of Carbon Nanostructures A. Arias Durán (a,b)*, P. Bozzano(c),D. Grondona(b), S. Goyanes(a). (a)

LP&MC, Dep. de Física, FCEyN_UBA–IFIBA CONICET. Cdad. Univ. Pab I (1428). Bs. As, Argentina. (b) INFIP, CONICET, Dep. de Física, FCEyN_UBA, Cdad. Univ. Pab I (1428). Bs. As, Argentina. (c) Gerencia Materiales, CNEA, Avda Gral Paz 1499, B1650KNA San Martín,Buenos Aires, Argentina

Abstract In this work carbon nanostructures were synthesized by CVD using nickel films deposited onto silicon substrates by cathodic arc technique. In order to modify the surface morphology of the catalyst, the silicon substrate and the nickel coating were exposed to an Ar ion bombardment generated in a dc glow discharge at room temperature. The surface morphology was examined by atomic force microscopy (AFM). The morphology of the nanotubes and other carbon structures was observed by scanning electron microscopy (FE-SEM) and Transmission Electron Microscopy (TEM). It was found that nanofibers with helical geometry grew on the nickel catalyst deposited onto the silicon substrate without the glow treatment, and carbon nanofiber with a linear geometry grew on the nickel catalyst deposited onto the silicon substrate with the glow treatment. Carbon nanotubes (CNTs) were obtained when the Si substrate and the Ni film were treated with the glow discharge.The CNTs obtained are mostly very thin with an average diameter between 8 and 15 nm, with only a few CNTs with external diameters of up to 60 nm. © Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license TheAuthors. Authors.Published by Elsevier ©2015 2014The (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the scientific committee of SAM - CONAMET 2013. Selection and peer-review under responsibility of the scientific committee of SAM - CONAMET 2013

* Corresponding author. Tel Tel.: 54-11-4576-3371; fax: 54-11-4787-2712. E-mail address: [email protected]

2211-8128 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the scientific committee of SAM - CONAMET 2013 doi:10.1016/j.mspro.2015.04.134

A. Arias Durán et al. / Procedia Materials Science 8 (2015) 770 – 777

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Keywords:Carbon nanotubes, CVD, Vacuum arc

1. Introduction Carbon nanotubes (CNTs) have electrical, thermal and mechanical properties which make them attractive for applications such as nano-electronic devices, filters and gas sensors and their use in flexible polymer composites for various applications Bouchet-Fabre B. et. al. (2009), Cantoro M. et. al., (2009), Escobar M. et.al., (2010). Different techniques have been developed for the synthesis of CNTs (C.Journet et. al., 2012), and chemical vapor deposition (CVD) process is a widely used technique for direct CNTs growth on substrates owing to their versatility in terms of the type of the catalyst, carbon sources, and processes variables that can be employed. Several studies have focused on understanding the stages of formation of carbonaceous nanostructures by using CVD techniques. Growth model generally accepted so far assumes three stages. In first place, precursor decomposition of hydrocarbon on metal catalyst particles, then diffusion of carbon atoms within these particles, and last, phase separation in which the carbon atoms precipitate onto the surface of the catalyst particle. That is, the formation of carbon nanostructures is strongly influenced by the structure and morphology of the catalyst. Different physical processes assisted by temperature have attempted to modify the surface of the catalyst. M. Cantoro et. al. (2009), C.Zhang et. al. (2010), Z. Jin et. al. (2011) reported the effect of morphological restructuring of the catalyst using plasma assisted by temperature in the formation of CNTs. Other authors as Terrado et. al. (2009) and Alvarez et. al. (2009) studied the role of the oxidation of the catalyst surface in the formation of carbonaceous nanostructures and proposed the use of metal coatings as a barrier between the catalyst and the substrate to prevent diffusion processes between catalyst and substrate due to the high temperatures applied during the CVD process. Although, there is some background on this subject is still critical to elucidate the influence of the characteristics of the catalyst substrate on the morphology of nanostructures generated. The aim of this paper is to study the influence of the structure and morphology of a Ni film catalyst deposited on Si substrate with a vacuum arc discharge in the growth of different types of carbonaceous nanostructure using CVD technique. The surface morphology of the catalyst was modified exposing the substrate and the catalyst to an Ar ion bombardment generated in a dc glow discharge. Nomenclature CVD CNS

Chemical Vapor Deposition Carbon Nanostructures

2. Experimental Procedure 2.1. Preparation of catalyst. Nickel films The nickel films were deposited onto silicon substrates with a vacuum arc discharge. The arc was produced by discharging an electrolytic capacitor bank with C = 0.075 F, connected to a series inductor-resistor (L = 2 mH, R = 0.33 Ω), which critically damped the discharge. The arc duration is about 35 ms, with a peak current of (450 ± 20) A and an interelectrode voltage of (45 ± 5) V. The charging voltage was 280 V and the arc was ignited with a mechanically controlled tungsten rod. The chamber pressure was maintained at a base pressure < 10-2 Pa during the whole arc discharge with an oil diffusion pump. A grounded Ni cathode (5 cm in length and 1 cm in diameter) was located 1 cm in front of an annular anode with an aperture of 5 cm and a thickness of 2 cm. A rectilinear filter type magnetic island, Kleiman et. al. (2008) was placed at 4 cm from the cathode frontal surface. The magnetic field is generated by an external coil wrapped around a stainless steel tube (22 cm long, 5 cm innerdiameter). The coil (3 layers of 30 turns each) was fed with dc current from an independent power source. The magnetic field strength was measured with a calibrated Hall probe with a precision of 10%, and was measured at the duct center Bext = 43 mT.

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The magnetic island filter consisted of cylindrical permanent magnets enclosed in an aluminum jacket (1 cm diameter and 2 cm long) that generate a magnetic field Bisl = 60 mT. Magnetic island was characterized previously, in order to find the optimal values of Bext and Bisl for which the nickel films were obtained with a considerable reduction of nickel macro particles generated during the arc discharge. The Si substrate was placed 2 cm behind the magnetic island (figure 1).

Figure 1.Schematic diagram of pulsed Vacuum Arc system.

In order to study the effect of surface morphology of Ni film catalyst in the formation of carbon nanostructures synthesized by CVD, both the substrate and the catalyse surfaces, were exposed to a dc glow discharge at room temperature in an Argon atmosphere at a pressure of 200 Pa for 10 minutes and with a discharge power of 10 W. The obtained samples were: M1 (Si / Ni), Ni film and Si substrate surfaces without glow treatment, M2 (Si / Ni / glow) with glow treatment after the Ni deposition, M3 (Si / glow / Ni) with a glow treatment of the Si substrate before the Ni deposition and M4 (Si / glow / Ni / glow) in which both, silicon and Ni deposition were exposed to bombarding of Ar. In table 1 a summary of the sample preparation conditions are presented. The morphology of the different substrate and catalysts was studied using atomic force microscopy (AFM) in contact mode with Nanoscope III of Digital-VEECO.

Glow treatment of the Si substrate

Nickel deposition

Glow treatment of the Ni deposition

M1 (Si/Ni)

No

Yes

No

M2 (Si/Ni/glow)

No

Yes

yes

M3 (Si/glow/Ni)

yes

Yes

No

M4 (Si/glow/Ni/glow)

yes

Yes

Sample

Table 1.Samples preparation conditions

yes


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2.2. Synthesis of carbon nanostructures Carbon nanostructures were grown by CVD on silicon substrates coated with Ni deposited as described above. The substrates were placed in a quartz boat and introduced into a chamber of a tube furnace, figure 2. The chamber was purged by pumping with a mechanical vacuum pump, followed by the injection of a 9% hydrogen/nitrogen mixture, with a flow of 100 sccm at 180 torr. The temperature was raised from room temperature to 700 ◦C, with a ramp of 15 ◦C/s. This treatment allowed the reduction of the nickel oxide that could be generated as consequence of the exposure of the film to air. After the temperature reach 500 ◦C, acetylene (10 sccm) was introduced as carbon source keeping the above condition. The growth time was 60 min. After the growth of CNs, the furnace was cooled to the room temperature in a nitrogen flow. Transmission electron microscopy (TEM-EM Philips 301) and Field Emission Scanning Electron Microscopy (FEG-SEM Zeiss LEO 982 GEMINI) were employed to study the different morphologies. For the TEM studies, the CNs were detached from the substrate by ultrasonic dispersal in ethanol.

Figure 2. CVD reactor.

3. Results 3.1. Influence of Ar ion bombardment on the surface morphology of the catalyst Figure 3 shows AFM images in contact mode of Si surfaces before and after being exposed to glow discharge. As can be observed from figure 3a and 3b, the morphology of the surface of the Si substrate was modified after the glow treatment. The Ni film deposited on Si substrate without the glow treatment presented a poor adhesion, so when it was exposed to the glow treatment the film was removed from the substrate (sample M2). Figure 4 shows the morphology of the Ni film catalyst deposited onto Si surface without and with argon glow treatment, figures 4 a) and b) respectively. Note that the sample without the treatment (sample M1) shows a surface with a grain size between 1 and 2 nm in height with some isolated columns from approximately 15 nm in height, whereas the Ni film catalyst deposited onto Si surface with the glow treatment (sample M3) exhibits a more uniform surface than M1 with a grain sizes around 10 nm in height. The Ar bombardment of the Si substrate before the Ni deposition generated a Ni film more uniform and rough.

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(a)

(b)

Figure 3. AFM images (above) and corresponding profile (below): (a) silicon substrate and (b) silicon substrate exposed to glow discharge in Ar

(a)

(b)

Figure 4. AFM images (top) for: (a) sample M1 (Si / Ni), (b) shows M3 (Si / glow / Ni). And their corresponding profiles (below)


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Figure 5 shows the morphology of the surface of the sample M4 with the glow treatment before and after the Ni deposition. The surface is homogeneous with grain heights range from 1 to 3 nm.

(a)

(b)

Figure 5. (a) shows AFM Image of M4, (Si / glow / Ni / glow), (b) image profile

3.2. Influence of Ar ion bombardment on the morphology of carbon nanostructures formed. After the CVD synthesis, it was observed that carbon nanostructure did not grow on the sample M2; on the whole catalyst surface only uniform amorphous carbon deposition was obtained. This fact could be explained considering that the glow treatment to the Ni deposition cause a considerable reduction of the catalyst material due to a poor Ni adhesion onto Si substrate. Figure 6 a) and b) shows the M2 sample surface before and after CVD synthesis respectively.

(a)

(b)

Figure 6. M2 shows SEM image, (Si / Ni / glow), (a) before the CVD synthesis, (b) after the CVD synthesis.

Figure 7 shows SEM images of the nanostructures grown on samples M1, M3 and M4. On sample M1 (fig. 7a and 7b), the formation of thick carbon nanofibers with helical geometry and diameter estimated up to 60 nm can be observed. A similar behavior was already reported by Escobar et al. (2010). On sample M3, carbon nanofibers with linear geometry and diameters between 20 and 40 nm (Fig. 7c and 7d) were obtained. On the sample M4, thin carbon nanotubes were grown (Fig. 7e and 7f).

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(a)

(c)

(e)

(b)

(d)

(f)

Figure 7. SEM images obtained nanostructures on substrates (a, b) M1 (Si / Ni), (c, d) M3 (Si / glow / Ni) and (e, f) M4 (Si / glow / Ni / glow), (above) corresponds to 100 KX magnification and magnification below corresponds to 400 KX

Figure 8 shows TEM photographs of carbon nanotubes grown on M4. It can be seen that very thin carbon nanotubes with diameters between 8 and 15 nm, and a small amount of nanotubes (Figure 8b) with outer diameters of up to 60 nm were formed.

(a)

(b)

Figure 8. TEM images of carbon nanostructures obtained on M4 substrate (Si / glow / Ni / glow)

The glow discharge improves the anchorage of the Ni deposition and modifies the catalyst roughness. Comparing the morphology of the surface of the catalyst film and the characteristic of the nanostructures grown in each samples (Fig. 7), it is possible to infer the amount of nucleation active centers for the growth of the nanostructures was modified by the ion bombardment. 4. Conclusions The catalyst surface morphology was modified employing argon dc glow plasma at room temperature. It was found that the catalyst surface morphology determined the type of carbon nanostructure obtained.


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Nickel deposited on silicon substrates without glow treatment, sample M1, led to the formation of helical geometries nanofibers, and with the glow treatment, sample M3, led to the formation of carbon nanofibers with a linear geometry. In sample M4, when the glow treatment was applied before and after the Ni deposition was the only sample in which thin carbon nanotubes with diameters between 8 and 15 nm were grown. Acknowledgements We thank the following institutions for their financial support: Universidad de Buenos Aires; Consejo Nacional de Investigaciones Científicas y Técnicas – Argentina and Agencia Nacional de Promoción Científica y Tecnológica - Argentina. References Bouchet-Fabre B., FadjieDjomkam A., Delmas M., Jin C., Antonin O., Hugon M.C., Mayne-L’HermiteM.F., Alvarez F., Mineá T., 2009. Tantalum based coated substrates for controlling the diameter of carbon nanotubes.Carbon. 47, 3424-3426 CantoroM., Hofmann S., Mattevi C., Pisana S., Parvez A., Fasoli A., Ducati C., Scardaci V., Ferrari A. C., and Robertson J., 2009. Plasma restructuring of catalysts for chemical vapor deposition of carbon nanotubes.Journal of Applied Physics. 105,064304. Escobar M., Giuliani L., Candal R.J., Lamas D.G., Caso A., Rubiolo G., Grondona D., Goyanes S., Márquez A., 2010. Carbon nanotubes and nanofibers synthesized by CVD on nickel coatings deposited with a vacuum arc.Journal of Alloys and Compounds. 495, 446–449. Escobar M., Moreno M., Candal R., March M., Caso A., Polosecki P., Rubiolo G., Goyanes S., 2007. Synthesis of carbon nanotubes by CVD: Effect of acetylene pressure on nanotubes characteristics.Applied Surface Science.254, 251–256. Felisberto M., Arias-Durán A., Ramos J.A., Mondragon I., Candal R., Goyanes S., Rubiolo G.H., 2012. Influence of filler alignment in the mechanical and electrical properties of carbon nanotubes/epoxy nanocomposites. Physica B. 407, 3181–3183. GuoP.,Chen, T.,Chen Y., Zhang Z., Feng L.T.,LWang,Z. Lin,Z.H. Sun, 2008. Fabrication of field emission display prototype utilizing printed carbon nanotubes/nanofibers emitters. Solid-StateElectron52, 877–881. Jin C., Delmas M., Aubert P., Alvarez F., Minéa T., Hugon M.C., Bouchet-Fabre B., 2011. Nanostructured tantalum nitride films as buffer-layer for carbon nanotube growth.Thin Solid Films. 519, 4097 – 4100. JournetCatherine, PicherMatthieu, JourdainVincent., 2012. Carbon nanotube synthesis: from large-scale production to atom-by-atom growth.Nanotechnology. 23,142001. Kleiman A, Márquez A and Boxman R, 2008 Plasma Sources Science and Technology17015008 Terrado E.,Tacchini I., Benito A.M., Maser W.K., Martínez M.T., 2009. Optimizing catalyst nanoparticle distribution to produce densely-packed carbon nanotube growth.Carbon.47, 1989 – 2001. Zhang C., Yan F., Allen C. S., Bayer B. C., Hofmann S., Hickey B. J.,Cott D.,Zhong G., and Robertson J., 2010. Growth of vertically-aligned carbon nanotube forests on conductive cobalt disilicide support. Journal of Applied Physics.108,024311.