Effect of hydrogen plasma treatment on the growth and microstructures of multiwalled carbon nanotubes

Abstract

The effect of hydrogen plasma treatment of iron oxide films on the growth and microstructure of carbon nanotubes (CNTs) by microwave plasma enhanced chemical vapor deposition process has been investigated. Microwave plasma was characterized in-situ using optical emission spectrometer. Morphology of the films was examined by scanning electron microscopy. Structural analysis was carried out by high resolution transmission electron microscopy (HRTEM) equipped with energy dispersive X-ray spectroscopy (EDS) and micro-diffraction attachments. It is found that oxide films without H 2 plasma pretreatment or treated for lesser time resulted in CNT films with high percentage of carbonaceous particles and with embedded particles/nanorods distributed discontinuously in the cavity of the nanotubes. The embedded particles were found to be of iron carbide (Fe-C) as confirmed by HRTEM, EDS and micro-diffraction analysis. Experimental observations suggested that the iron oxide particles had poor catalytic action for CNT growth and in-situ reduction of oxide clusters to Fe by hydrogen plasma plays a key role in discontinuous filling of the nanotubes by the catalytic particles. [12][13][14] have been used to synthesize CNTs. CVD methods (both thermal and plasma enhanced) are reliable in producing multiwalled CNTs (MWNTs) with proper control over the process parameters. Plasma enhanced CVD, however, can produce CNTs with a higher growth rate, at low temperature, and with better reproducibility. The plasma not only ionizes the gas but also causes a local surface heating [15]. Consequently, growth temperature could be significantly decreased compared to non-plasma CVD processes. In addition, the plasma atmosphere may also influence the detailed catalyst surface kinetics in many ways [16] and hence the growth of carbon nanostructures.Growth of CNTs and their microstructures depend on several parameters such as growth technique, feed gases, growth temperature, catalyst, and catalyst preparation method as well as their state (solid, liquid or vapor). In plasma assisted CVD processes, dilution gases play a critical role in the growth of CNTs and the structure of CNTs can be tailored by judicious control over gas composition [17]. In our earlier report, we investigated the effect of different dilution gases such as H 2 , NH 3 and N 2 and their different composition on the growth and microstructure of CNTs by microwave plasma enhanced CVD (MPECVD) process on Fe coated Si substrates using C 2 H 2 as feed gas [17]. Plasma pretreatment of catalyst film can also affect the growth and structure of CNTs significantly especially in a high frequency plasma process such as MPECVD. To the best of our knowledge, effect of H 2 plasma pretreatment of iron oxide films on the growth and structure of CNTs by MPECVD process have not been reported. Therefore, the motivation for the present

Effect of hydrogen plasma treatment on the growth and microstructures of multiwalled carbon nanotubes

“…In the present study, Ar/H 2 plasma pretreatment of the iron oxide film resulted into high density of nanoparticles (not shown here) which catalyzed the nucleation and growth of CNTs on the introduction of C 2 H 2 in the plasma [17]. The observation of a-C in samples A, and B and relative increase in the percentage of a-C with decreasing H 2 plasma pretreatment time of oxide films suggest the poor catalytic activity of oxide for CNTs growth as also reported by other methods [20,21]. In case of sample C, most of the iron oxide clusters are supposed to be reduced to Fe after 15 minutes of H 2 plasma treatment since plasma contains a lot of active H-species as observed by in-situ OES of the H 2 and H 2 +C 2 H 2 plasma in 200a900 nm wavelength range as shown in Fig.…”

Hydrogen Plasma Treatment of Aligned Multi-Walled Carbon Nanotube Arrays for Improvement of Field Emission Properties

“…Hydrogen plasma treatment is also used for CNT synthesis, e.g., for the regeneration of the catalyst deposited on the substrate [ 32 ]. It was shown that full-cycle CNT growth in hydrogen plasma is possible in the presence of carbon [ 33 , 34 ].…”

Theoretical investigations on the effect of different plasmas on growth and field emission properties of a spherical carbon nanotube (CNT) tip placed over cylindrical surfaces

“…They found that for too high oxygen plasma power, chemical etching occurs at the surface of the CNT, thus destroying its structure. Srivastava et al (2010) studied the effect of hydrogen plasma treatment on oxide films on the growth and microstructures of MWCNTs and reported that the oxide films without the H 2 plasma pretreatment or treated for lesser time resulted in CNT films with high percentage of carbonaceous particles and with embedded particles/nanorods distributed discontinuously in the cavity of the nanotubes. They reported that the size and morphology of Co-catalytic seeds varied with hydrogen plasma treatment time.…”

“…The effect of plasma treatment, plasma parameters and plasma powers on the growth, structure (Lee et al 2002;Felten et al 2005;Zhu et al 2005;Wen et al 2006;Abdi et al 2008;Srivastava et al 2010;Lee et al 2012;Yau and Tsai 2012) and field emission properties (Haaland et al 1996;Bonard et al 1998;Zhi et al 2002;Ahn et al 2003;Srivastava et al 2005Srivastava et al , 2010Xu et al 2006;Feng et al 2007;Lee et al 2009;Sodha et al 2010 Sharma andTewari 2011a) of a carbon nanotube (CNT) has been an active field of research for many years. Yau and Tsai (2012) studied the effect of the localized lateral growth of multiwalled carbon nanotubes (MWCNTs) with ammonia plasma post-treatment and their experimental results showed that upon exposure to ammonia plasma, electrical properties of CNTs are found to increase.…”

“…In this case, it was seen that the deposition of amorphous layer comprising carbon and fluorine during extended CF 4 plasma treatment may hamper the field emission from CNT films. Srivastava et al (2010) studied the effect of hydrogen plasma treatment on oxide films on the growth and microstructures of MWCNTs and reported that the oxide films without the H 2 plasma pretreatment or treated for lesser time resulted in CNT films with high percentage of carbonaceous particles and with embedded particles/nanorods distributed discontinuously in the cavity of the nanotubes. Srivastava et al (2005) synthesized carbon films by microwave plasma enhanced chemical vapour deposition (MPECVD) using the mixture of methane and argon gases on Ni coated and Si substrates, and found that an increase in microwave power causes more ionization of the gas, which increases the density of plasma species of relatively higher energy.…”