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Carbon fiber-promoted activation of catalyst for efficient growth of single-walled carbon nanotubes
Carbon 156 (2020) 410e415
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Carbon fiber-promoted activation of catalyst for efficient growth of single-walled carbon nanotubes Benwu Xin a, Wenke Gao a, b, Zhonghai Ji c, Shuchen Zhang d, Liantao Xin b, Lifen Zhao a, Han Xue a, Qianru Wu a, Lili Zhang c, Chang Liu c, Jin Zhang d, Maoshuai He a, b, * a
School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao, 266590, China Key Laboratory of Eco-Chemical Engineering, Ministry of Education, Taishan Scholar Advantage and Characteristic Discipline Team of Eco Chemical Process and Technology, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China c Shenyang National Laboratory for Materials Science, Advanced Carbon Division, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, China d Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, Key Laboratory for the Physics and Chemistry of Nanodevices, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2019 Received in revised form 26 September 2019 Accepted 29 September 2019 Available online 30 September 2019
Carbon fibers are placed onto substrate surface with dispersed Co particles to promote the growth efficiency of single-walled carbon nanotubes (SWNTs). During chemical vapor deposition (CVD) process, micro-spaces are fabricated between the carbon fibers and the substrate surface, which modify the Knudsen number of CO and increase its efficient contact with cobalt oxide nanoparticles, enhancing the reduction of catalysts and the nucleation of SWNTs. Compared with surface grown SWNTs without covering carbon fibers, SWNTs grown under carbon fibers demonstrate a much higher catalyst efficiency, i.e. a SWNT density as high as ~140 SWNTs/mm2. X-ray photoelectron spectroscopy characterizations confirm that the presence of carbon fibers promotes the reduction of cobalt oxide catalyst particles and carbon dissolution inside catalyst particles, which are crucial for catalyst activation and subsequent SWNT nucleation. This work provides a new strategy for the efficient growth of SWNTs, which benefits high-density growth of SWNTs required for future nanoelectronics applications. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Since the landmark work in 1991 by Iijima [1], carbon nanotubes have evoked much attention because of their unique properties [2e6]. According to number of tube wall, carbon nanotubes can be classified into multi-walled, double-walled and single-walled ones [7e12]. Among them, single-walled carbon nanotubes (SWNTs) are of great importance owing to their extraordinary electronic properties and potential applications in nanoelectronics and optoelectronics [13e17]. 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
* Corresponding author. Qingdao University of Science and Technology, Qingdao, 266042, China. E-mail address: [email protected] (M. He). https://doi.org/10.1016/j.carbon.2019.09.089 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
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 [20e22]. Despite the progress, SWNT growth usually suffers poor yield and low density [23e25], 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
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continuously migrated onto the substrate surface, leading to an efficient growth of horizontal SWNT arrays with a density over 130 SWNTs/mm [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, sulfurcontaining molecules, or choosing a suitable carbon source [30e33]. 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 fiberassisted 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. 2. Experimental section 2.1. 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 ¼ 500 nm). The dried materials were finally annealed in air at 600 C for 4 h. 2.2. CVD synthesis of carbon nanotubes CVD growth of carbon nanotubes was carried out under atmospheric pressure and carbon monoxide (CO) was applied as the carbon precursor. In a typical experiment, a layer of carbon fibers
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Fig. 1. Schematic representation for gas flow condition, catalyst reduction and SWNT growth without and with placing carbon fibers on the flat surface. (Fig. 1b and f are cross section pictures. Carbon fibers are omitted in Fig. 1g and h to show catalyst nanoparticles and SWNTs clearly). (A colour version of this figure can be viewed online.)
(T300, TORAY) was placed on top of the SiO2/Si surface with prepared Co nanoparticles. The carbon fibers have an average diameter of ~6 mm and were cut into segments with lengths comparable to that of the substrate. To avoid possible contaminations, the carbon fibers were annealed in Ar at 1000 C prior to placing onto the SiO2/ Si substrate with dispersed Co nanoparticles. After loading the catalyst covered with carbon fibers into a horizontal CVD reactor with a quartz tube (inner diameter: 44 mm), the system was heated to 800 C in a flow of 300 standard cubic centimeter per minute (sccm) Ar. Once being stabilized at 800 C, Ar was switched off and CO with a flow rate of 300 sccm was fed for 30 min. Finally, CO was switched off and the furnace was cooled to room temperature in the protection of Ar. For subsequent characterizations on the catalyst and SWNTs, the carbon fibers were removed by tweezer after CVD growth. In the controlled CVD experiment, carbon nanotubes were synthesized from Co particles dispersed on the SiO2/Si substrate without covering carbon fibers. All the experiments were performed at ambient pressure.
2.3. Characterizations of Co catalysts and carbon nanotubes The surface composition of carbon fibers was analyzed by electron probe microanalysis (EPMA, JEOL JXA-8230). The chemical states of Co nanoparticles before and after CVD growth were examined by X-ray photoelectron spectroscopy (XPS). To minimize the surface oxidation of the catalyst after exposing to air, the treated catalyst or catalyst after CVD process was cooled down to room temperature under the protection of Ar and subjected to XPS characterizations in hours. Photoelectrons were excited by an Al Ka (1486.6 eV) X-ray source and the spectra were collected using an EscaLab 250Xi spectrometer. The morphologies of catalysts and carbon nanotubes were characterized by atomic force microscopy
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(AFM, Bruker Multimode 8) and scanning electron microscopy (SEM, FEI Nova Nano SEM450). A Renishaw Invia Raman system with 532 nm, 633 nm, 785 nm laser wavelengths was performed to characterize the carbon nanotubes. During the Raman characterizations, the objective applied is 50 with a spot diameter of about 2 mm, which is small enough for judging spatial uniformity of the produced SWNT thin film. Transmission electron microscopy (TEM, FEI Tecnai F20, 200 kV) was carried out to characterize the structure of carbon nanotubes transferred onto TEM grid by a PMMAmediated technique.
3. 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 0.5e2.3 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/mm2. Such Co nanoparticles are suitable for catalyzing carbon nanotube growth. Fig. 2b shows an AFM image of the product after normal CVD process. Clearly, carbon nanotubes with small diameters are synthesized (ESI Fig. S2). Based on the height analysis, all the carbon nanotubes have diameters close to 1 nm, indicating that they are all SWNTs. It is noted that during CVD process, carbon coating on catalyst surface might occur, causing the observation of nanoparticles with relatively large diameters. In the 2.5 mm 2.5 mm area of Fig. 2b, the SWNTs generated by normal CO CVD are sparse. The average density of SWNTs grown by conventional CVD is only about 4 SWNTs/mm2, suggesting that the
Co catalyst particles in the reaction environment demonstrate relatively low catalytic activities. Interestingly, the catalyst activities can be greatly enhanced by placing carbon fibers on top of the catalysts. Fig. 2c shows an AFM image of SWNTs grown by “carbon fiber”-assisted CVD, which were characterized after removing the carbon fibers. By counting the number of SWNTs in a unit area of the AFM image, the SWNT density is determined to be ~140 SWNTs/ mm2. Consequently, the catalyst efficiency is estimated to be 87%, which is comparable to the catalyst efficiency record reported by Futaba et al. [33] Such a high SWNT density is also verified by SEM characterizations (Fig. 2d). The structures and morphologies of SWNTs were further verified by TEM characterizations (ESI Fig. S3). All the results suggest that the presence of carbon fibers on the top of catalyst particles facilitates the efficient growth of SWNTs. The uniformity of the SWNT film grown by the carbon fiber-assisted CVD was evaluated by Raman spectroscopy excited with three different laser wavelengths: 532 nm, 633 nm and 785 nm. Fig. 3 presents the Raman spectra acquired from more than 70 spots on the SWNT film for each wavelength. In agreement with the TEM characterization results, the presence of radial breathing modes (RBMs) indicate the production of SWNTs. Raman spectra taken from different spots resemble each other, suggesting the produced SWNT film are quite uniform and the diameter/chirality distribution of SWNTs from different areas are quite similar. Such a uniform SWNT film has the potential to be applied as transparent conductive film and thin film transistors [29]. In order to confirm the effects of carbon fibers, carbon fibers were laid only on the edge area of the substrate surface during CVD growth. In the edge area where there had been carbon fibers on the top of the catalyst particles, high density SWNT growth was observed (ESI Fig. S4). In contrast, the SWNT density is relatively
Fig. 2. (a) AFM image of Co nanoparticles on SiO2/Si surface after air calcination. (b) AFM image of carbon nanotubes grown at 800 C using normal CO CVD. (c) AFM and (d) SEM images of carbon nanotubes synthesized by carbon fiber-assisted CVD. (A colour version of this figure can be viewed online.)
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Fig. 3. Raman spectra of SWNTs acquired with an excitation wavelength of (a) 532 nm, (b) 633 nm and (c) 785 nm. The laser irradiation spot diameter is about 2 mm. (A colour version of this figure can be viewed online.)
low in the middle area where there had been no carbon fibers laid on the Co catalyst particles during CVD. The results further prove that the presence of carbon fibers improves the catalyst efficiency and promote the growth of SWNTs, which could be potentially applied for site-specific growth of high-density SWNTs. Previously, graphene/graphite sheet was reported to assist the growth of highareal-density horizontally aligned carbon nanotubes [34]. The presence of graphene/graphite suppresses the aggregation of metal particles and thus enhances the carbon nanotube density. However, the graphene layers cannot be removed after CVD growth, which might inhibit the further applications of the produced nanotubes. In our work, the carbon fibers are readily removed after CVD growth, and would not affect the properties of the generated SWNT films. To exclude possible metal contamination on the carbon fibers, which can catalyze SWNT growth during CVD, elemental analysis by EPMA was performed on the annealed carbon fibers (ESI Fig. S5). Beside a trace amount of oxygen, carbon is the main element of the fibers, ruling out the metal contamination as a reason for the high density SWNT growth. It is noted that inhibition of catalyst coalescence is also a possible reason for the efficient SWNT growth. As demonstrated by Kobayashi et al., extremely dense nanoparticles may coalesce at high reaction temperatures to form lager particles, unfavorable for SWNT growth [35]. Consequently, in controlled experiments, both carbon fiber covered Co nanoparticles and uncovered Co nanoparticles were subjected to H2 reduction at reaction temperature (800 C) for 45 min. ESI Fig. S6 depict AFM images of the two samples after the reduction process. Obviously, there is no significant difference between the densities of Co nanoparticles, suggesting that the role of carbon fiber is not to inhibit the metal particle coalescence. The reduction of metallic catalysts is an important step in the growth of carbon nanotubes. As previously reported [36,37], the reduction of metal oxide is necessary to activate the catalyst for growing SWNTs efficiently. In bulk catalysts like Ni/SiO2 [32] and Co/SiO2 [37], the threshold temperatures above which SWNT growth occurs are coherently related with the reduction profiles of the catalysts. For example, the H2-temperature programmed reduction profile of Ni/SiO2 catalyst suggests the reduction starts as low as ~400 C, SWNTs are detected to grow at a very low reaction temperature of 450 C [36]. Similarly, in bimetallic catalysts, like FeCu [20], FeRu [38] and FeMn [39], the respective addition of Cu, Ru and Mn, is revealed to facilitate the reduction of iron oxide, thus leading to the growth of SWNT at a temperature of 600 C, which is about 100 C lower than that of SWNT grown on monometallic Fe catalysts [39]. Nessim et al. [40] systematically investigated the areal density of carbon nanotubes through catalyst pretreatment
and suggested that the chemical reduction of the catalyst layer is required for carbon nanotube growth. Similarly, by combining environmental transmission microscopy and in situ, time-resolved X-ray photoelectron spectroscopy (XPS), Hofmann et al. [41] revealed the state of transition metals during CVD growth of carbon nanotubes. The active state of the catalyst for Fe and Ni is determined to be a crystalline metallic nanoparticle. In short, the reduction of such particles could be the prerequisite for the catalytic growth of carbon nanotubes. To explore whether the use of carbon fibers promotes the reduction of the catalyst nanoparticles, XPS characterizations were carried out on supported Co nanoparticles before and immediately after the CVD growth process. Fig. 4a presents the XPS spectrum of Co particles after air calcination. A Co2p3/2 peak is centered at 781.1 eV, corresponding to the Co(II or III) and indicating the existence of cobalt oxide. A peak at 778.2 eV, assigned as Co2p3/2 of metallic Co, appears in both spectra acquired from carbon fiber covered or uncovered Co nanoparticles after CVD growth (Fig. 4b). The results indicate the reduction of cobalt oxide and the formation of metallic Co during CVD. For the Co particles without carbon fiber covering, the metallic Co2p3/2 peak is relatively weak, especially when compared with the Co(O)2p3/2 peak, suggesting that only a small portion of Co are reduced during a normal CO CVD process. In contrast, in the Co2p region of Co particles covered with carbon fibers, mainly a metallic Co2p3/2 peak was observed after CVD. The XPS results prove that when carbon fibers are laid onto the cobalt oxide nanoparticles, the catalyst particles are more easily reduced, accounting for the improved growth efficiency of SWNTs. Such a conclusion is in agreement with the previously reported works [41], which highlight the importance of catalyst reduction in nucleating SWNTs. The enhanced reduction of cobalt oxide could be related to the modulation of the gas flow state. In a previous report, Wang et al. [42] proposed a molecular flow mode to grow large graphene domains on stacked Cu foil with a gap of 10e30 mm. Under such conditions, the gas flow is under molecular flow regime during CVD. In the regime, the CH4 and H2 molecules in the narrow gaps can move forward with a high colliding frequency toward the inner surface of opposite Cu foils. Consequently, the local carbon flux concentration is enhanced, which accelerates the interaction of carbon flux with Cu surfaces, accounting for a high graphene growth rate. Such a mechanism is supposed to be also applicable to our SWNT growth during the carbon fiber-assisted CVD process although our CVD growth is performed at ambient pressure. Owing to the presence of carbon fibers on the SiO2/Si surface, plenty of micro-channels are generated above the cobalt oxide nanoparticles. During CVD process, the introduced CO are trapped inside the
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Fig. 4. XPS spectrum of Co catalysts after (a) calcination in air and (b) CVD growth with or without carbon fiber assistance. (A colour version of this figure can be viewed online.)
microchannels and move forward under transitional flow region. The molecular mean free path of CO is defined as: l ¼ pffiffikffi B T 2 , where 2pd p
kB is the Boltzmann constant 1.38 1023 J K1; T is the reaction temperature, 1073 K; d represents the diameter of CO molecule, 0.376 nm; p is the reaction pressure, 1.013 105 pa. Consequently, the mean free path of CO (l) is calculated to be 0.233 mm. The gap between carbon fiber and the substrate or the fiber is determined by EPMA, which is less than 2 mm (ESI Fig. S7). The calculated Knudsen number (Kn ¼ l/L) is 0.116, which is in a transitional flow region between viscous and molecular flow [43]. Compared with CO gas flowing without the presence of carbon fibers, CO molecules in the micro-spaces have a much higher colliding frequency with the cobalt oxide nanoparticles on the substrate surface, which would substantially enhance the reduction of cobalt oxide. It is noted that the roles of carbon fiber cover are different from those of stacked Si wafer previously reported by Huang et al. [44]. In their work, stacked Si wafers are placed face to face to enhance the carbon nanotube growth rate, which is attributed to the limited H2 diffusion and mild catalyst reduction, favoring the formation of small diameter particles for fast growth of vertically aligned carbon nanotubes. In contrast, our work here demonstrates that carbon fiber covering promotes the reduction of Co catalyst by CO. Such a high catalyst reduction degree can also be readily obtained by H2 prereduction prior to CO CVD growth. The reduction with 200 sccm H2 was carried out at 800 C for 30 min under ambient pressure. As verified by XPS, prereduction in H2 also leads to the almost complete reduction of Co nanoparticles (Fig. S8a). Indeed, SWNTs grown by CO CVD with H2 prereduction process do exhibit an increased growth efficiency. As shown in Fig. S8b, and a SWNT density of ~12 SWNTs/mm2 is obtained, which is 3 times that of SWNTs grown without H2 prereduction step. However, such a density is still much lower than that of SWNTs grown by carbon fiber-assisted CO CVD (~140 tubes/mm2), which indicates that the cover of carbon fibers on the catalyst also promotes the carburization of catalyst, an elementary step for SWNT nucleation. During CVD process, CO not only facilitates the reduction of the catalyst, but also serves as the carbon source for SWNT growth [39,45,46]. After the reduction of catalyst, carbon dissolution inside the metal particles have been proven to be crucial for SWNT nucleation [47,48]. SWNT cap formation starts from the weaving of sp2 carbon on the catalyst surface, which helps to saturate the dangling carbon bonds. Such a carbon cap tends to “lift off” from the metallic particles with incorporated subsurface carbon, which dewets the cap and facilitates the SWNT embryo formation. In contrast, the carbon
cap has strong adhesion on the metal nanoparticle without carbon dissolution, unfavorable for SWNT nucleation and subsequent growth. In the carbon fiber-assisted CVD process, the cover of carbon fiber could promote the carbon dissolution inside the reduced Co nanoparticles by increasing the colliding frequency of CO with the catalyst, facilitating the carbon cap formation and accounting for the high-density growth of SWNTs (Fig. 1). 4. 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 by other materials, and the method is also applicable for other transition metal catalysts like Fe and Ni. This work not only provides a facile approach for increasing the SWNT growth density, but also helps gain more insights into the active phases of catalysts for nucleating SWNTs. 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). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.09.089.
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Carbon fiber-promoted activation of catalyst for efficient growth of single-walled carbon nanotubes Benwu Xin a, Wenke Gao a, b, Zhonghai Ji c, Shuchen Zhang d, Liantao Xin b, Lifen Zhao a, Han Xue a, Qianru Wu a, Lili Zhang c, Chang Liu c, Jin Zhang d, Maoshuai He a, b, * a
School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao, 266590, China Key Laboratory of Eco-Chemical Engineering, Ministry of Education, Taishan Scholar Advantage and Characteristic Discipline Team of Eco Chemical Process and Technology, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China c Shenyang National Laboratory for Materials Science, Advanced Carbon Division, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, China d Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, Key Laboratory for the Physics and Chemistry of Nanodevices, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2019 Received in revised form 26 September 2019 Accepted 29 September 2019 Available online 30 September 2019
Carbon fibers are placed onto substrate surface with dispersed Co particles to promote the growth efficiency of single-walled carbon nanotubes (SWNTs). During chemical vapor deposition (CVD) process, micro-spaces are fabricated between the carbon fibers and the substrate surface, which modify the Knudsen number of CO and increase its efficient contact with cobalt oxide nanoparticles, enhancing the reduction of catalysts and the nucleation of SWNTs. Compared with surface grown SWNTs without covering carbon fibers, SWNTs grown under carbon fibers demonstrate a much higher catalyst efficiency, i.e. a SWNT density as high as ~140 SWNTs/mm2. X-ray photoelectron spectroscopy characterizations confirm that the presence of carbon fibers promotes the reduction of cobalt oxide catalyst particles and carbon dissolution inside catalyst particles, which are crucial for catalyst activation and subsequent SWNT nucleation. This work provides a new strategy for the efficient growth of SWNTs, which benefits high-density growth of SWNTs required for future nanoelectronics applications. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Since the landmark work in 1991 by Iijima [1], carbon nanotubes have evoked much attention because of their unique properties [2e6]. According to number of tube wall, carbon nanotubes can be classified into multi-walled, double-walled and single-walled ones [7e12]. Among them, single-walled carbon nanotubes (SWNTs) are of great importance owing to their extraordinary electronic properties and potential applications in nanoelectronics and optoelectronics [13e17]. 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
* Corresponding author. Qingdao University of Science and Technology, Qingdao, 266042, China. E-mail address: [email protected] (M. He). https://doi.org/10.1016/j.carbon.2019.09.089 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
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 [20e22]. Despite the progress, SWNT growth usually suffers poor yield and low density [23e25], 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
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continuously migrated onto the substrate surface, leading to an efficient growth of horizontal SWNT arrays with a density over 130 SWNTs/mm [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, sulfurcontaining molecules, or choosing a suitable carbon source [30e33]. 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 fiberassisted 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. 2. Experimental section 2.1. 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 ¼ 500 nm). The dried materials were finally annealed in air at 600 C for 4 h. 2.2. CVD synthesis of carbon nanotubes CVD growth of carbon nanotubes was carried out under atmospheric pressure and carbon monoxide (CO) was applied as the carbon precursor. In a typical experiment, a layer of carbon fibers
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Fig. 1. Schematic representation for gas flow condition, catalyst reduction and SWNT growth without and with placing carbon fibers on the flat surface. (Fig. 1b and f are cross section pictures. Carbon fibers are omitted in Fig. 1g and h to show catalyst nanoparticles and SWNTs clearly). (A colour version of this figure can be viewed online.)
(T300, TORAY) was placed on top of the SiO2/Si surface with prepared Co nanoparticles. The carbon fibers have an average diameter of ~6 mm and were cut into segments with lengths comparable to that of the substrate. To avoid possible contaminations, the carbon fibers were annealed in Ar at 1000 C prior to placing onto the SiO2/ Si substrate with dispersed Co nanoparticles. After loading the catalyst covered with carbon fibers into a horizontal CVD reactor with a quartz tube (inner diameter: 44 mm), the system was heated to 800 C in a flow of 300 standard cubic centimeter per minute (sccm) Ar. Once being stabilized at 800 C, Ar was switched off and CO with a flow rate of 300 sccm was fed for 30 min. Finally, CO was switched off and the furnace was cooled to room temperature in the protection of Ar. For subsequent characterizations on the catalyst and SWNTs, the carbon fibers were removed by tweezer after CVD growth. In the controlled CVD experiment, carbon nanotubes were synthesized from Co particles dispersed on the SiO2/Si substrate without covering carbon fibers. All the experiments were performed at ambient pressure.
2.3. Characterizations of Co catalysts and carbon nanotubes The surface composition of carbon fibers was analyzed by electron probe microanalysis (EPMA, JEOL JXA-8230). The chemical states of Co nanoparticles before and after CVD growth were examined by X-ray photoelectron spectroscopy (XPS). To minimize the surface oxidation of the catalyst after exposing to air, the treated catalyst or catalyst after CVD process was cooled down to room temperature under the protection of Ar and subjected to XPS characterizations in hours. Photoelectrons were excited by an Al Ka (1486.6 eV) X-ray source and the spectra were collected using an EscaLab 250Xi spectrometer. The morphologies of catalysts and carbon nanotubes were characterized by atomic force microscopy
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(AFM, Bruker Multimode 8) and scanning electron microscopy (SEM, FEI Nova Nano SEM450). A Renishaw Invia Raman system with 532 nm, 633 nm, 785 nm laser wavelengths was performed to characterize the carbon nanotubes. During the Raman characterizations, the objective applied is 50 with a spot diameter of about 2 mm, which is small enough for judging spatial uniformity of the produced SWNT thin film. Transmission electron microscopy (TEM, FEI Tecnai F20, 200 kV) was carried out to characterize the structure of carbon nanotubes transferred onto TEM grid by a PMMAmediated technique.
3. 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 0.5e2.3 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/mm2. Such Co nanoparticles are suitable for catalyzing carbon nanotube growth. Fig. 2b shows an AFM image of the product after normal CVD process. Clearly, carbon nanotubes with small diameters are synthesized (ESI Fig. S2). Based on the height analysis, all the carbon nanotubes have diameters close to 1 nm, indicating that they are all SWNTs. It is noted that during CVD process, carbon coating on catalyst surface might occur, causing the observation of nanoparticles with relatively large diameters. In the 2.5 mm 2.5 mm area of Fig. 2b, the SWNTs generated by normal CO CVD are sparse. The average density of SWNTs grown by conventional CVD is only about 4 SWNTs/mm2, suggesting that the
Co catalyst particles in the reaction environment demonstrate relatively low catalytic activities. Interestingly, the catalyst activities can be greatly enhanced by placing carbon fibers on top of the catalysts. Fig. 2c shows an AFM image of SWNTs grown by “carbon fiber”-assisted CVD, which were characterized after removing the carbon fibers. By counting the number of SWNTs in a unit area of the AFM image, the SWNT density is determined to be ~140 SWNTs/ mm2. Consequently, the catalyst efficiency is estimated to be 87%, which is comparable to the catalyst efficiency record reported by Futaba et al. [33] Such a high SWNT density is also verified by SEM characterizations (Fig. 2d). The structures and morphologies of SWNTs were further verified by TEM characterizations (ESI Fig. S3). All the results suggest that the presence of carbon fibers on the top of catalyst particles facilitates the efficient growth of SWNTs. The uniformity of the SWNT film grown by the carbon fiber-assisted CVD was evaluated by Raman spectroscopy excited with three different laser wavelengths: 532 nm, 633 nm and 785 nm. Fig. 3 presents the Raman spectra acquired from more than 70 spots on the SWNT film for each wavelength. In agreement with the TEM characterization results, the presence of radial breathing modes (RBMs) indicate the production of SWNTs. Raman spectra taken from different spots resemble each other, suggesting the produced SWNT film are quite uniform and the diameter/chirality distribution of SWNTs from different areas are quite similar. Such a uniform SWNT film has the potential to be applied as transparent conductive film and thin film transistors [29]. In order to confirm the effects of carbon fibers, carbon fibers were laid only on the edge area of the substrate surface during CVD growth. In the edge area where there had been carbon fibers on the top of the catalyst particles, high density SWNT growth was observed (ESI Fig. S4). In contrast, the SWNT density is relatively
Fig. 2. (a) AFM image of Co nanoparticles on SiO2/Si surface after air calcination. (b) AFM image of carbon nanotubes grown at 800 C using normal CO CVD. (c) AFM and (d) SEM images of carbon nanotubes synthesized by carbon fiber-assisted CVD. (A colour version of this figure can be viewed online.)
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Fig. 3. Raman spectra of SWNTs acquired with an excitation wavelength of (a) 532 nm, (b) 633 nm and (c) 785 nm. The laser irradiation spot diameter is about 2 mm. (A colour version of this figure can be viewed online.)
low in the middle area where there had been no carbon fibers laid on the Co catalyst particles during CVD. The results further prove that the presence of carbon fibers improves the catalyst efficiency and promote the growth of SWNTs, which could be potentially applied for site-specific growth of high-density SWNTs. Previously, graphene/graphite sheet was reported to assist the growth of highareal-density horizontally aligned carbon nanotubes [34]. The presence of graphene/graphite suppresses the aggregation of metal particles and thus enhances the carbon nanotube density. However, the graphene layers cannot be removed after CVD growth, which might inhibit the further applications of the produced nanotubes. In our work, the carbon fibers are readily removed after CVD growth, and would not affect the properties of the generated SWNT films. To exclude possible metal contamination on the carbon fibers, which can catalyze SWNT growth during CVD, elemental analysis by EPMA was performed on the annealed carbon fibers (ESI Fig. S5). Beside a trace amount of oxygen, carbon is the main element of the fibers, ruling out the metal contamination as a reason for the high density SWNT growth. It is noted that inhibition of catalyst coalescence is also a possible reason for the efficient SWNT growth. As demonstrated by Kobayashi et al., extremely dense nanoparticles may coalesce at high reaction temperatures to form lager particles, unfavorable for SWNT growth [35]. Consequently, in controlled experiments, both carbon fiber covered Co nanoparticles and uncovered Co nanoparticles were subjected to H2 reduction at reaction temperature (800 C) for 45 min. ESI Fig. S6 depict AFM images of the two samples after the reduction process. Obviously, there is no significant difference between the densities of Co nanoparticles, suggesting that the role of carbon fiber is not to inhibit the metal particle coalescence. The reduction of metallic catalysts is an important step in the growth of carbon nanotubes. As previously reported [36,37], the reduction of metal oxide is necessary to activate the catalyst for growing SWNTs efficiently. In bulk catalysts like Ni/SiO2 [32] and Co/SiO2 [37], the threshold temperatures above which SWNT growth occurs are coherently related with the reduction profiles of the catalysts. For example, the H2-temperature programmed reduction profile of Ni/SiO2 catalyst suggests the reduction starts as low as ~400 C, SWNTs are detected to grow at a very low reaction temperature of 450 C [36]. Similarly, in bimetallic catalysts, like FeCu [20], FeRu [38] and FeMn [39], the respective addition of Cu, Ru and Mn, is revealed to facilitate the reduction of iron oxide, thus leading to the growth of SWNT at a temperature of 600 C, which is about 100 C lower than that of SWNT grown on monometallic Fe catalysts [39]. Nessim et al. [40] systematically investigated the areal density of carbon nanotubes through catalyst pretreatment
and suggested that the chemical reduction of the catalyst layer is required for carbon nanotube growth. Similarly, by combining environmental transmission microscopy and in situ, time-resolved X-ray photoelectron spectroscopy (XPS), Hofmann et al. [41] revealed the state of transition metals during CVD growth of carbon nanotubes. The active state of the catalyst for Fe and Ni is determined to be a crystalline metallic nanoparticle. In short, the reduction of such particles could be the prerequisite for the catalytic growth of carbon nanotubes. To explore whether the use of carbon fibers promotes the reduction of the catalyst nanoparticles, XPS characterizations were carried out on supported Co nanoparticles before and immediately after the CVD growth process. Fig. 4a presents the XPS spectrum of Co particles after air calcination. A Co2p3/2 peak is centered at 781.1 eV, corresponding to the Co(II or III) and indicating the existence of cobalt oxide. A peak at 778.2 eV, assigned as Co2p3/2 of metallic Co, appears in both spectra acquired from carbon fiber covered or uncovered Co nanoparticles after CVD growth (Fig. 4b). The results indicate the reduction of cobalt oxide and the formation of metallic Co during CVD. For the Co particles without carbon fiber covering, the metallic Co2p3/2 peak is relatively weak, especially when compared with the Co(O)2p3/2 peak, suggesting that only a small portion of Co are reduced during a normal CO CVD process. In contrast, in the Co2p region of Co particles covered with carbon fibers, mainly a metallic Co2p3/2 peak was observed after CVD. The XPS results prove that when carbon fibers are laid onto the cobalt oxide nanoparticles, the catalyst particles are more easily reduced, accounting for the improved growth efficiency of SWNTs. Such a conclusion is in agreement with the previously reported works [41], which highlight the importance of catalyst reduction in nucleating SWNTs. The enhanced reduction of cobalt oxide could be related to the modulation of the gas flow state. In a previous report, Wang et al. [42] proposed a molecular flow mode to grow large graphene domains on stacked Cu foil with a gap of 10e30 mm. Under such conditions, the gas flow is under molecular flow regime during CVD. In the regime, the CH4 and H2 molecules in the narrow gaps can move forward with a high colliding frequency toward the inner surface of opposite Cu foils. Consequently, the local carbon flux concentration is enhanced, which accelerates the interaction of carbon flux with Cu surfaces, accounting for a high graphene growth rate. Such a mechanism is supposed to be also applicable to our SWNT growth during the carbon fiber-assisted CVD process although our CVD growth is performed at ambient pressure. Owing to the presence of carbon fibers on the SiO2/Si surface, plenty of micro-channels are generated above the cobalt oxide nanoparticles. During CVD process, the introduced CO are trapped inside the
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Fig. 4. XPS spectrum of Co catalysts after (a) calcination in air and (b) CVD growth with or without carbon fiber assistance. (A colour version of this figure can be viewed online.)
microchannels and move forward under transitional flow region. The molecular mean free path of CO is defined as: l ¼ pffiffikffi B T 2 , where 2pd p
kB is the Boltzmann constant 1.38 1023 J K1; T is the reaction temperature, 1073 K; d represents the diameter of CO molecule, 0.376 nm; p is the reaction pressure, 1.013 105 pa. Consequently, the mean free path of CO (l) is calculated to be 0.233 mm. The gap between carbon fiber and the substrate or the fiber is determined by EPMA, which is less than 2 mm (ESI Fig. S7). The calculated Knudsen number (Kn ¼ l/L) is 0.116, which is in a transitional flow region between viscous and molecular flow [43]. Compared with CO gas flowing without the presence of carbon fibers, CO molecules in the micro-spaces have a much higher colliding frequency with the cobalt oxide nanoparticles on the substrate surface, which would substantially enhance the reduction of cobalt oxide. It is noted that the roles of carbon fiber cover are different from those of stacked Si wafer previously reported by Huang et al. [44]. In their work, stacked Si wafers are placed face to face to enhance the carbon nanotube growth rate, which is attributed to the limited H2 diffusion and mild catalyst reduction, favoring the formation of small diameter particles for fast growth of vertically aligned carbon nanotubes. In contrast, our work here demonstrates that carbon fiber covering promotes the reduction of Co catalyst by CO. Such a high catalyst reduction degree can also be readily obtained by H2 prereduction prior to CO CVD growth. The reduction with 200 sccm H2 was carried out at 800 C for 30 min under ambient pressure. As verified by XPS, prereduction in H2 also leads to the almost complete reduction of Co nanoparticles (Fig. S8a). Indeed, SWNTs grown by CO CVD with H2 prereduction process do exhibit an increased growth efficiency. As shown in Fig. S8b, and a SWNT density of ~12 SWNTs/mm2 is obtained, which is 3 times that of SWNTs grown without H2 prereduction step. However, such a density is still much lower than that of SWNTs grown by carbon fiber-assisted CO CVD (~140 tubes/mm2), which indicates that the cover of carbon fibers on the catalyst also promotes the carburization of catalyst, an elementary step for SWNT nucleation. During CVD process, CO not only facilitates the reduction of the catalyst, but also serves as the carbon source for SWNT growth [39,45,46]. After the reduction of catalyst, carbon dissolution inside the metal particles have been proven to be crucial for SWNT nucleation [47,48]. SWNT cap formation starts from the weaving of sp2 carbon on the catalyst surface, which helps to saturate the dangling carbon bonds. Such a carbon cap tends to “lift off” from the metallic particles with incorporated subsurface carbon, which dewets the cap and facilitates the SWNT embryo formation. In contrast, the carbon
cap has strong adhesion on the metal nanoparticle without carbon dissolution, unfavorable for SWNT nucleation and subsequent growth. In the carbon fiber-assisted CVD process, the cover of carbon fiber could promote the carbon dissolution inside the reduced Co nanoparticles by increasing the colliding frequency of CO with the catalyst, facilitating the carbon cap formation and accounting for the high-density growth of SWNTs (Fig. 1). 4. 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 by other materials, and the method is also applicable for other transition metal catalysts like Fe and Ni. This work not only provides a facile approach for increasing the SWNT growth density, but also helps gain more insights into the active phases of catalysts for nucleating SWNTs. 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). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.09.089.
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