Effects of ferrite catalyst concentration and water vapor on growth of vertically aligned carbon nanotube

In this study Fe3O4 nanoparticles were used as catalysts for the growth of vertically aligned carbon nanotubes (VA-CNTs) by chemical vapor deposition (CVD). The effect of catalyst concentration and water vapor during the CVD process on the properties of the VA-CNTs was investigated. Monodisperse Fe3O4 nanoparticles (4.5–9.0 nm diameter) prepared by thermal decomposition of iron acetylacetonate compounds were spin-coated on clean silicon substrates which served as a platform for VA-CNTs growth. The results indicated that the length, density and growth rate of CNTs were strongly affected by the catalyst concentration. CNTs grown at 0.026 g ml−1 Fe3O4 catalyst had greater length, density and growth rates than those obtained at 0.01 and 0.033 g ml−1 Fe3O4 catalyst. Addition of water during the CVD process had drastically improved CNTs growth. The length and growth rate of obtained CNTs were 40 μm and 1.33 μm min−1, respectively. The results provided insights into the role of Fe3O4 catalyst and water vapor during VA-CNTs growth process by CVD method and the obtained information might serve as a starting point for further optimization of VA-CNTs synthesis.


Introduction
The discovery of carbon nanotubes (CNTs) in 1991 by Iijima [1] has prompted extensive investigations of the unique physical, chemical, mechanical and electronic properties of this new material [2,3]. Vertically aligned CNTs (VA-CNTs) are carbon cylinders which orient perpendicularly to a substrate [4]. VA-CNTs have advantages of large aspect ratio, good orientation and high purity. CNTs arrays have attractive potential applications in field emission devices, anisotropic conductive materials, filaments, membrane, super springs, and electrochemical and bioelectrochemical sensors [5][6][7].
The growth of VA-CNTs has been studied on various materials such as mesoporous silica, planar silicon substrates, quartz plate using Fe, Co, or Ni catalyst [5]. Fe 3 O 4 nanoparticles have been applied in diverse fields such as biomedicine [8], ferrofluid technology [9], information storage [10] and environmental engineering [11]. Recently, monodispersed Fe 3 O 4 nanoparticles have been investigated as a catalyst for CVD synthesis of boron nanowires [12] and carbon nanotubes [13,14]. Nevertheless, few researches utilize Fe 3 O 4 nanoparticles as a catalyst for synthesis of VA-CNTs via the CVD process [15,16]. The influence of Fe 3 O 4 concentration on CNTs morphology and other properties has not been reported yet. The control of diameter, number of walls, structural defect, length and density of CNTs is very critical [17]. Addition of water is found to be conducive to CNTs growth enhancement, evidenced by increased CNTs length and density [18]. Previous studies demonstrate that water vapor is related to the surface hydroxylation and etching of amorphous carbon during CNTs growth [18,19]. The water vapor helps protect the catalyst from ripening and/or carbide formation which would usually result in early termination of CNT growth. However, the effects of water on CNTs quality apparently differ with different catalysts and substrates [20,21]. So far, scarce information has been available with respect to the effects of water on CNTs growth using Fe 3 O 4 nanoparticles as catalyst.
In this study VA-CNTs growth by the CVD process using Fe 3 O 4 nanoparticle catalyst and water vapor was investigated. The effects of catalyst concentrations and water vapor on CNTs properties were thoroughly investigated.

Preparation of the sample
In this work Fe 3 O 4 nanoparticle catalyst was prepared by thermal decomposition of iron acetylacetonates (Fe(acac) 3 ). Briefly, Fe(acac) 3 (0.69 g, 0.63 mM), oleic acid (OA) (3.6 mL, 372 mM), oleylamine (OLA) (3.6 ml, 372 mM), 1,2hexadecanediol (0.58 g, 75 mM) and 30 mL of 1-octadecene were added to 100 mL three-neck round-bottom flask. The mixture was magnetically stirred under nitrogen flow at room temperature for 30 min. Temperature was then adjusted to 100°C and held for 30 min to remove the remaining water. In the next stage of the reaction, the mixture was maintained at 200°C for 30 min and heated to 295°C at a rate of 5-7°C min −1 . After 60 min, the solution was cooled to room temperature. The product was washed with ethanol, centrifuged, and finally suspended in n-hexane at 0.01, 0.026, 0.033 g ml −1 before use.
A piece of Si/SiO 2 (5 mm × 5 mm) was cut from a 4 inch wafer and sonically cleaned in acetone (20 min), ethanol (20 min) and water (20 min). The substrate was subsequently immersed in a mixture of H 2 SO 4 and H 2 O 2 (molar ratio: 3/1) for 15 min, rinsed by DI water and then dried under nitrogen flow. A volume of 0.002 mL (two drops) catalyst suspension was dropped on Si/SiO 2 surface. Preliminary experiments were conducted to compare drop coating and spin coating methods. It was found that catalyst particles were most uniformly distributed with spin coating at 2000-3000 rpm for 60 s.

Synthesis of vertically-aligned carbon nanotubes
Synthesis of the VA-CNTs was carried out at atmospheric pressure via catalytic decomposition acetylene (C 2 H 2 ). The samples were placed in the furnace and calcined at 600°C in the air for 30 min to remove the residual polymers. The furnace was heated to 750°C at ramping rate of 20°C min −1 under an Ar flow of 800 sccm. The samples were incubated at 750°C under flow of Ar/H 2 (300/100 sccm) for 30 min to deoxidize the catalysts. Then C 2 H 2 (30 sccm) was added for 30 min. After that, the furnace was cooled down to room temperature in Ar gas to prevent oxidation of the CNTs. In this work the effects of Fe 3 O 4 catalyst concentrations (0.01, 0.026 and 0.033 g ml −1 ) on the properties of VA-CNT were studied. The optimal concentration of catalyst was then used to evaluate the effect of water vapor on the growth of VA-CNTs. Water vapor was introduced during the CVD process by bubbling partly argon gas supply (60 sccm) through water prior to entering the furnace.

Characterization of Fe 3 O 4 catalyst and VA-CNTs
The morphology of Fe 3 O 4 nanoparticle catalysts was observed by a transmission electron microscope (TEM) (JEM 1010 JEOL, Japan) at 80 kV and an atomic force microscope (AFM) (N9451A, USA). Sizes of Fe 3 O 4 nanoparticle catalysts were analyzed by ImageJ sofware. The morphology of VA-CNTs was characterized by field emission-scanning electron microscopy (FE-SEM) (Hitachi S-4800) operating at an acceleration voltage of 10 kV. Diameter and length of the CNTs were estimated by analyzing SEM images using Ima-geJ software. Growth rate of the CNTs was calculated by the following equation: where ν is growth rate, l is the length of the tubes and t is CVD time (30 min). Raman spectra were recorded in the range of 500-2000 cm −1 on a JobinYvon Lab RAM-1B (France), using He-Ne laser source of 632.8 nm as the incident light.    When Fe 3 O 4 concentration is 0.026 g ml −1 , obtained VA-CNTs have the most uniform length and highest density ( figure 2(b)). In addition, the length of CNTs produced at this catalyst concentration is highest among three concentrations studied. At 0.033 g ml −1 Fe 3 O 4 , CNTs are shorter than those synthesized with 0.026 g ml −1 Fe 3 O 4 and amorphous carbon can be observed on the surface of the CNTs (figures 2(c) and (f)). This is possibly due to the formation of multilayers of catalyst particles on substrate surface at high Fe 3 O 4 concentrations. The stacking of particles might lead to incomplete removal of residual polymer shell during initial heating stage. Eventually, amorphous carbon formed from the polymer shell would hinder the growth of VA-CNTs during CVD process. At 0.026 g ml −1 Fe3O4, growth rate of CNTs is also highest among three concentrations used (table 1).

Effect of water vapor during CVD on VA-CNTs
Significant changes in properties of VA-CNTs are observed with the addition of water vapor in the CVD process ( figure 3). Water-assisted CVD produces CNTs with greater length (40 μm) (figure 3(b)) than the ones grown without water (6 μm) (figure 3(a), table 1). Some studies previously evidenced the effects of water on CNTs growth using various catalysts such as Fe 2 O 3, cobalt [17,18,21]. The role of water in purifying amorphous carbon is well documented [17,19,20]. Water is considered as a weak oxidant which removes the deposited amorphous carbon on the active sites of the catalyst [21]. Another function of water is to inhibit Oswalt ripening by surface hydroxylation [17]. As a result, the introduction of water during CVD helps extend catalyst lifetime and promote growth rate, which are conducive to notably increased length of CNTs. However, exceeding a certain level, H 2 O showed an overall negative effect on the growth of CNTs. In this case, it was difficult to keep the balance between the carbon supply and the solid carbon precipitation [22]. It was more and more difficult for the acetylene molecules to reach the catalysts. So, an amount of amorphous carbon was increased and the length of CNTs was reduced.
Structures and properties of VA-CNTs grown with and without water were determined by TEM images and Raman  spectroscopy. The TEM images can clearly distinguish amorphous carbon and structural defects of carbon nanotubes. As seen in figure 4(a), the rough surfaces of the CNTs are probably attributed to the formation of amorphous carbon and structural defects. During CNTs growth, under certain conditions, side walls can form cap-like structures and stack in the same direction to produce bamboo-like wall [23] morphology which are considered undesirable structural defects. The graphite layers consisting of one side of tubes are broken and are no longer continuous graphite sheets ( figure 4(a). The addition of water not only reduces the formation of amorphous carbons but also promotes the growth of VA-CNTs into well-defined vertically standing and organized structures. Unlike the CNTs grown without H 2 O, the CNTs have a hollow structure without a bamboo-like wall ( figure 4(b)). It is known that the diameters of CNTs depend on the size of catalyst particle, amount of amorphous carbon formed on the CNTs walls and structural defects. Collected data point out that diameters and diameter distribution of CNTs grown with H 2 O are smaller and more homogeneous than those synthesized without H 2 O (figures 3(d), (e) and figures 4(a), (b)). Results in previous works show that the presence of reactive gases like H 2 or NH 3 can improve the homogeneity of nanoparticles which help narrow the size distribution of CNTs [21]. Hence, enhancement in uniformity of CNTs diameter may suggest a similar effect with respect to the presence of H 2 O in the feed gas.
In this research Raman spectroscopy was used to detect the presence of amorphous and crystalline phases in CNTs samples. The Raman spectra (figure 5) of the CNTs samples grown with and without water vapor show the peaks at 1354 cm −1 (D-line) and at 1594 cm −1 (G-line), which are ascribed to amorphous carbon and to graphitized CNTs, respectively.
The integrated intensity ratio of the G band over the D band can express the graphitization of the CNTs samples. Results from figure 5 show that values of I G /I D are 1.6 and 0.88 for the CNTs samples with and without water vapor. The analysis of Raman spectra evidences increased graphitization of CNTs samples with addition of water vapor during CVD process. These results are consistent with the SEM and TEM images which demonstrate the improvement in VA-CNTs structures in the presence of water vapor.

Conclusion
The vertically-aligned CNTs were successfully synthesized using Fe 3 O 4 nanoparticle catalyst. The length, density, diameter and quality of the VA-CNTs were strongly affected by catalyst concentration as well as by the presence of water vapor during the CVD process. Within the ranges of studied catalyst concentrations, CNTs obtained at 0.026 g ml −1 Fe 3 O 4 had the highest uniformity, density and length. Addition of water vapour drastically reduced formation of amorphous carbon, structural defects which led to significantly enhanced VA-CNTs quality.