Carbon fiber-promoted activation of catalyst for efficient growth of single-walled carbon nanotubes
Graphical abstract
Micro-space generated by placing carbon fibers on top of supported catalyst particles promotes the reduction of catalyst and the carbon cap “lift off”, resulting in the efficient growth of single-walled carbon nanotubes by chemical vapor deposition.
Introduction
Since the landmark work in 1991 by Iijima [1], carbon nanotubes have evoked much attention because of their unique properties [[2], [3], [4], [5], [6]]. According to number of tube wall, carbon nanotubes can be classified into multi-walled, double-walled and single-walled ones [[7], [8], [9], [10], [11], [12]]. Among them, single-walled carbon nanotubes (SWNTs) are of great importance owing to their extraordinary electronic properties and potential applications in nanoelectronics and optoelectronics [[13], [14], [15], [16], [17]]. To realize these cutting-edge applications, synthesis of SWNTs with controlled diameter, density, conductivity and even chirality is highly desirable. For example, high-density SWNTs are required to fabricate flexible thin film transistors [18], which take the advantages of excellent mobility, high degree of transparency and good flexibility. So far, chemical vapor deposition (CVD) is the most promising technique to synthesize SWNTs because of its low cost and good controllability [19]. Consequently, it is now widely used in lab experiments for controlled synthesis and in industry for large quantity production of SWNTs [[20], [21], [22]].
Despite the progress, SWNT growth usually suffers poor yield and low density [[23], [24], [25]], which would inevitably deteriorate the performances of SWNT-based devices. With the aim to enhance SWNT growth efficiency, a number of strategies have been developed in the past two decades. Particularly, multi-time growth has been widely applied for improving SWNT growth efficiency [26]. In the pioneer work, the substrate supported catalyst was annealed in air to reactivate the catalyst after first time growth, which affords more SWNT synthesis during a second time growth. By repeating the process many times, SWNTs with a high density and a high growth efficiency are achieved. Later, such a multi-time growth approach was adopted to increase the density of horizontal SWNT arrays with or without new catalyst addition [27,28]. Remarkably, Hu et al. proposed a “Trojan” catalyst, in which new catalyst are continuously migrated onto the substrate surface, leading to an efficient growth of horizontal SWNT arrays with a density over 130 SWNTs/μm [28]. Compared with horizontally aligned SWNTs, which are useful for high mobility devices and molecular electronics, films with randomly oriented SWNTs are more applicable and reproducible for practical applications. As most produced SWNTs contain both metallic and semiconducting species, the electronic properties of SWNT-based devices are sensitive to the density of SWNTs. For example, a slight density change in SWNT thin film transistor could cause dramatic change of the on/off ratio [29]. Therefore, the SWNT density should be at least higher than the percolation threshold in SWNT thin film transistor. In addition, a better SWNT-SWNT contact is expected for SWNTs synthesized in one CVD batch, especially compared with SWNT film fabricated by solution process [29]. As a result, it is necessary to synthesize films with high-density SWNTs directly by CVD.
Different from the multi-time growth approach, regulating the gas environments for SWNT growth is a simpler method for improving catalyst efficiency. The gas environment during CVD can by tuned by adding weak oxidizing gas molecules, sulfur-containing molecules, or choosing a suitable carbon source [[30], [31], [32], [33]]. For example, by balancing the relative level of acetylene and H2O, the catalytic activity of Fe nanoparticles can be greatly enhanced, resulting in the growth of highly dense SWNT forest [29]. The catalyst activity of H2O-assisted growth was estimated to be 84% [33], the highest ever reported. Later, oxygen-assisted hydrocarbon CVD was developed by Zhang et al. for ultra-high-yield SWNT growth [31]. The roles of H2O and oxygen are to extend the catalyst lifetime and inhibit the Ostwald ripening of reduced catalyst particles. Similarly, introducing a sulfur-containing compound during CVD also facilitates the nucleation and growth of SWNTs [32]. In the abovementioned experiments, the efficient SWNT growth window is very narrow, a lower or higher amount of oxidizer/sulfur-containing gas molecules would cause the deactivation of catalysts or the dearth of SWNT growth.
In the work reported here, we develop a facile carbon fiber-assisted CVD approach for promoting the growth efficiency of SWNTs. Compared with controlled experiment where no carbon fiber is laid on the substrate surface, SWNTs with a much higher density are achieved on the SiO2 substrate, as schematically illustrated in Fig. 1. The roles of carbon fiber covering are elucidated based on extensive characterizations on the catalysts and SWNTs. It is revealed that micro-spaces are generated between the carbon fibers and the substrate, enhancing the reduction of catalyst and carbon dissolution inside catalyst, which are coherently related to the catalyst activity.
Section snippets
Preparation of Co nanoparticles
Catalysts used in this work are Co nanoparticles derived from block copolymer micelle templates. Experimentally, 0.0130 g poly(styrene-b-4-vinyl pyridine) (denoted PS-b-P4VP) with the Mn 50-b-13 was first dissolved into 10 ml toluene. Cobalt chloride hexahydrate with a mass of 0.0016 g was then added to the solution. After dissolution, a drop of the solution was spin coated onto a surface of SiO2/Si (tox = 500 nm). The dried materials were finally annealed in air at 600 °C for 4 h.
CVD synthesis of carbon nanotubes
CVD growth of
Results and discussion
Fig. 2a presents a typical AFM image of as prepared Co nanoparticles after heat treatment in air. From the height analysis, the diameters of Co nanoparticles are in the range of 0.5–2.3 nm with an average diameter of ∼1.3 nm (ESI Fig. S1). The result suggests that the block copolymer approach is efficient in preparing nanoparticles with a narrow diameter distribution. The average density of the Co particles is ∼160 particles/μm2. Such Co nanoparticles are suitable for catalyzing carbon nanotube
Conclusions
In summary, we propose a simple and feasible method for achieving highly efficient synthesis of SWNTs on flat substrate. Carbon fibers are laid on flat substrate to form micro spaces, which can change the gas flow regime and promote the collision frequency between carbon source and catalysts. Such an enhanced colliding frequency facilitates the reduction of metal oxide and the nucleation of SWNTs, accounting for the efficient growth of SWNTs. Carbon fibers used in the approach can be replaced
Declaration of competing interestsCOI
The authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51972184); Natural Science Foundation of Shandong Province of China (No. ZR2016EMM10); Taishan Scholar Advantage and Characteristic Discipline Team of Eco Chemical Process and Technology; the Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (No. 2016RCJJ001).
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