Selective synthesis of metallic and semi-conducting single-walled carbon nanotube by floating catalyst chemical vapour deposition

https://doi.org/10.1016/j.diamond.2019.05.017Get rights and content

Highlights

  • Selective synthesis of metallic and semiconducting SWCNTs using floating catalyst chemical vapour deposition.

  • Chiral selectivity of SWCNTs depend upon the concentration of hydrogen during synthesis.

  • Substrate/support free scalable synthesis of both metallic and semiconducting SWCNTs is reported.

Abstract

Selective synthesis of both metallic and semi-conducting single-walled carbon nanotubes (SWCNTs) has been performed using floating catalyst chemical vapour deposition (FC-CVD). It was observed that partial pressure of hydrogen determined the selectivity of SWCNTs being predominantly metallic at lower hydrogen concentration (20 vol%) and predominantly semiconducting at higher hydrogen concentration (80 vol%). The qualitative and quantitative analysis on type of SWCNTs was performed based on UV–vis-NIR, Raman spectra at different wavelengths and SWCNT-based field effect transistor device performance. Transmission electron microscopy was used to get diameter distribution of SWCNTs. Detailed analysis showed that the type of SWCNTs was decided at the nucleation stage that was governed by the collision frequency between hydrogen molecules and iron nanoparticles.

Introduction

Carbon nanotube (CNT) is one of the widely studied nanomaterials from the carbon family [[1], [2], [3], [4], [5], [6]]. CNTs possess extraordinary mechanical, thermal, chemical, optical and electronic properties, which can be exploited for potential use in various fields [7]. After extensive efforts by various scientists and engineers across the globe, CNTs and CNT based products have been commercialized, majorly consisting of multi-walled CNTs (MWCNTs) [[8], [9], [10], [11], [12]]. Whereas, commercial products based on single-walled carbon nanotubes (SWCNTs) are still in the development phase. Depending upon the chirality, single-walled carbon nanotubes (SWCNTs) can be either metallic or semiconducting [13]. Metallic SWCNTs (m-SWCNTs) are widely used in the conductive coatings, specifically for solar cells, organic light emitting diodes, electrochromic devices, etc. [14] while, semiconducting SWCNTs (s-SWCNTs) are widely used as a channel material for field effect transistors, low-cost printable devices, etc. [15]. Various methods, such as, arc discharge, laser ablation and catalytic chemical vapour deposition (C-CVD) have been studied for the synthesis of SWCNTs. Out of these three methods, C-CVD is widely studied due to inherent potential for large scale production [16]. Most of the synthesis routes end up with producing mixture of m-SWCNTs and s-SWCNTs which impede the process of commercialization of SWCNT based products. In the past two decades enormous efforts have been devoted in order to synthesize SWCNTs with single chirality or either metallic or semi-conducting. The approaches utilized by various researchers can be classified into two, i.e. post synthesis separation and direct controlled growth [17]. In post synthesis separation, the mixture of SWCNTs is separated by exploiting the differences in physical and chemical properties. Density gradient ultracentrifugation [18], gel chromatography [19], dielectrophoresis [20], polymer wrapping [21], aromatic extraction [22], selective oxidation [23], DNA recognition [24], etc. are few examples of post synthesis separation techniques. These techniques are quite effective in term of separation of metallic and semiconducting SWCNTs, but not cost effective. In addition, usage of complex physical and chemical treatments results in contamination of SWCNTs and incorporation of defects in the structure. In order to circumvent the separation problem, it is always desirable to have SWCNTs with uniform type having majorly metallic or semiconducting i.e. by direct controlled growth. Direct synthesis of SWCNTs with specific type (m or s) has been studied by various groups in last two decades [[25], [26], [27], [28], [29]]. Detailed analysis of the reported literature suggests that the type of SWCNTs i.e. either m-type or s-type gets fixed during nucleation stage itself, since along the length of SWCNTs no change in type is observed [30]. Exception to this Zhao et al. [31] recently reported variable chirality along elongation of SWCNTs using tandem plate CVD where the synthesis temperature varied periodically. Hence, in order to have SWCNTs with specific type (m or s), control over the nucleation step during synthesis of nanotubes is of utmost importance while maintaining the synthesis temperature constant throughout the experiment. In C-CVD, various parameters such as type of carbon source [32,33], synthesis temperature [32,[34], [35], [36]], catalyst type [[37], [38], [39]], system pressure [37], growth time [40], carrier gas composition [29,32,41], etc. affect the nucleation and subsequently the type of the grown CNTs. It is quite interesting to observe that more than two decades after discovery of CNTs [42] in 1991 and paramount synthesis of SWCNTs [43,44] in 1993, recent review by Maruyama [45] underlines the lack of insights into the growth mechanism of SWCNTs. Various strategies utilized by different groups for selective growth of SWCNTs (either m-type of s-type) are listed in Table 1. Bachilo et al. [37] reported selective synthesis of s-SWCNTs (~57%) mainly consisting of SWCNTs with chiral index (6,5) and (7,5). CoMoCAT method was utilized for the synthesis with CO as carbon source at 5 atm. In order to have fine control over the active catalyst cluster, lower ratio of Co:Mo (1:3) was used which aids in the formation of smaller diameter s-SWCNTs. Zhang et al. [46] reported highly selective synthesis of s-SWCNTs (~99%) by utilizing selective etching of m-SWCNTs. Ethanol was used as carbon source with cobalt deposited on SiO2/Si as catalyst at 800 °C and 1 atm. Post synthesis of SWCNTs, exposure to methane plasma followed by vacuum annealing leads to selective etching of m-SWCNTs. Harutyunyan et al. [29] reported selective synthesis of m-SWCNTs (~91%). They found that the catalyst morphology and the chirality of SWCNTs are interdependent. In situ TEM studies were also reported in support of the hypothesis. Catalyst morphology was modulated by varying the ambient conditions during thermal annealing of the catalyst. In addition, oxidative species such as water were utilized in order to increase the fraction of m-SWCNTs. Loebick et al. [47] have reported the usage of bimetallic catalyst i.e. CoMn on MCM-41 silica templates with CO as carbon source and hydrogen as carrier gas at 1 atm. Shape and size of catalyst i.e. Co in presence of Mn played a major role in selective synthesis of s-SWCNTs (~93%). In addition, they also reported that the size and shape of catalyst is dependent on the synthesis temperature, while lower synthesis temperature favoured synthesis of s-SWCNTs. Che et al. [33] reported selective synthesis of s-SWCNTs (~97.6%) using isopropanol as carbon source and iron deposited on quartz as catalyst. It was reported that the decomposition of isopropanol leads to the formation of water vapour which is responsible for the selective synthesis of s-SWCNTs by etching out the m-SWCNTs. Hou et al. [48] have reported highly selective synthesis of m-SWCNTs (~91%) using methane, ferrocene and mixture of helium and hydrogen gases as carbon source, catalyst precursor and carrier/etchant gas, respectively. m-SWCNTs were deposited on aluminum foil. It was found that hydrogen gas acted as etchant, which resulted in etching of small diameter s-SWCNTs which are more reactive as compared to large diameters m-SWCNTs due to larger curvature. Yang et al. [38] reported selective synthesis of (12,6) SWCNTs using tungsten based bimetallic alloy nanocrystals as catalyst and ethanol as carbon source. Authors anticipate that high melting point alloy nanocrystals may help in synthesis of chiral selective SWCNTs. Ghorannevis et al. [26] reported selective synthesis of (6,5) SWCNTs using nonmagnetic catalyst. Methane was used as carbon source, while SWCNTs were grown over gold film deposited on SiO2/Si. Recently, Zhang et al. [49] reported selective synthesis of both metallic and semiconducting SWCNTs in the same reactor by tuning chemical composition and size of the catalyst particles. They have reported the usage of SiOx film over SiO2/Si as non-metallic catalyst for the growth of SWCNTs using ethanol as carbon source. Both s-SWCNTs (~91%) and m-SWCNTs (~80%) were synthesized with high selectively using this approach. Biris et al. [50] reported synthesis of SWCNTs by thermal pyrolysis of methane using a bimetallic catalyst i.e. Fe-Mo over MgO as catalyst. They found that amount of hydrogen utilized during synthesis has tremendous effect on the morphological and structural properties of the SWCNTs. An interesting result was reported that at higher concentration of hydrogen, s- SWCNTs were synthesized more favourably in comparison to m- SWCNTs (without any quantitative estimation). Similar results were also reported by Lobiak et al. [51]. Recently Cheng et al. [52] reported selective growth of s-SWCNTs using SiC as a catalyst. They found that, thermal treatment of SiC nanoparticles resulted in extinction of Si atoms resulting in exposure of carbon atoms which leads to the formation of carbon caps. Usage of hydrogen atmosphere preferentially etched the metallic carbon caps and ~95% pure s-SWCNTs were obtained.

Though various strategies have been utilized for the selective synthesis of both semiconducting and metallic SWCNTs, most of the approaches involve usage of substrate/support along with tailor made catalyst. Metal/Support interactions might lead to variations in structure of SWCNTs. In addition, usage of substrate leads to semi-continuous or batch mode of operation and hence batch-to-batch variations is observed in the quality of products. While separation of residual catalyst and support from the desired product increases the post processing steps. As per our current understanding recipe for substrate free selective synthesis of metallic and semiconducting SWCNTs. Here, in the present study, highly selective synthesis of metallic as well as semiconducting SWCNTs has been reported using floating catalyst chemical vapour deposition (FC-CVD) technique without using any substrate or support. In FC-CVD technique, carbon source and catalyst precursor have been introduced simultaneously in the reactor. Here, the catalyst particles are suspended in gas phase which are formed by cracking of catalyst precursors such as ferrocene, cobaltocene, nickelocene, etc. FC-CVD technique reduces the load of post processing treatment of SWCNTs purification, since unsupported metal is used as catalyst. In addition defects generated due to purification steps to SWCNTs is also reduced. In present work the type of the SWCNTs (m-type or s-type) was controlled by the amount of reductive species (H2 gas) present in the reactor. The synthesized SWCNTs were characterized by TEM, Raman spectroscopy, absorption spectroscopy and X-ray diffraction in detail.

Section snippets

Materials

Ferrocene (98%) and sulfur (99.998%) were purchased from Loba Chemie Pvt. Ltd. and used as received without further purification. Hydrochloric acid (37%) was purchased from Sigma Aldrich. Hydrogen, argon and methane gas were supplied by Six Sigma Gases Pvt. Ltd. with 99.995% purity. The mass flow rates of gases were regulated using three mass flow controllers (Alicat Sci MC-Series). De-ionized water was used for all the experiments.

Synthesis of SWCNTs

SWCNTs were synthesized by the floating catalyst chemical

Results

As mentioned earlier, SWCNTs were synthesized using FC-CVD. We studied effects of various parameters such as, synthesis temperature, partial pressure of carbon source (methane), composition of carrier gas, sublimation rate of catalyst precursor (ferrocene), ratio of catalyst to promoter (ferrocene/sulfur). It was found that the composition of carrier gas governs the structure of SWCNTs synthesized. Carrier gas utilized for the synthesis of SWCNTs consists of argon and hydrogen. Process

Discussion

As mentioned earlier, the chirality of nanotubes is fixed during nucleation stage itself. Little et al. [58] reported that magnetic exchange field in the iron lattice controls the magnetic torque experienced during electron spins. In addition the orbital dynamics of carbon during decomposition (hydrocarbon), deposition, accumulation followed by clustering of carbon into CNTs are also depended on magnetic exchange field. Ab initio calculations performed by Reich et al. [59] provides an insight

Conclusions

Protocols for direct and continuous synthesis of highly selective s- and m-SWCNTs have been demonstrated. Qualitative and quantitative analysis on type of SWCNTs (s or m) have been performed using Raman spectroscopy and UV–vis-NIR. Selective synthesis of 88% s-SWCNTs and ~82% m-SWCNTs were possible simply by tuning hydrogen concentration. Role of hydrogen in determining the type of SWCNTs has been explained. More efforts are required in understanding the role of catalyst on the chiral

Acknowledgements

The authors gratefully acknowledge the financial support from United Phosphorous Limited (UPL) Ltd., to carry out this work.

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