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Synthesis, characterization and growth mechanism of carbon nanopears
Chemical Physics 535 (2020) 110780
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Synthesis, characterization and growth mechanism of carbon nanopears a
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b
a
a
c
a
Chen Wang , Pei-Pei Hou , Peng Liao , Ping Wu , Ming Pan , Hua-Fei Li , Xiao-Di Wang , ⁎ Ning Xiec, Zhi Liua, Zheling Zenga, Gui-Ping Daia, ,1 a b c
T
Department of Chemical Engineering, School of Environmental and Chemical Engineering, Nanchang University, Nanchang 330031, China Department of Cell Research and Development, Farasis Energy Inc., Hayward, CA 94545, USA School of Materials Science and Engineering, Nanchang University, Nanchang 330031, China
ARTICLE INFO
ABSTRACT
Keywords: Carbon nanopears Chemical vapor deposition Sharp tips Formation mechanism
We report a novel carbon morphology that we name “carbon nanopears” using chemical vapor deposition in the presence of silicon wafer as a substrate coating Ni catalyst, thiophene as the growth promoter and benzene as carbon source. The carbon nanopears observed under optimized experimental conditions have nanometer-sized sharp tips, nanometer-sized roots and solid interiors. The carbon nanopears are composed of graphitized carbon and amorphous carbon, with continuous shortening of the length of graphitic sheets from the exterior to interior making them pear-shaped. Furthermore, a probable CNP formation mechanism is proposed in this paper.
1. Introduction Owing to their potential advantages exhibited in novel devices and technologies, a lot of attention has been paid to carbon nanomaterials [1] by using ingenious methods. Morphology control of carbon nanomaterials represents one of the most explored directions. Furthermore, carbon nanostructures with morphologies of cone [2], pipette [3], pipe [4], helix [5], star [6], branch [7], bud [8], bead [9], pearl [10], nanocrosses [11], nanosprout [12], nanoprotrusion [13], which show various one-, and two-, and three-dimensional morphologies, have been widely investigated in recent decades. For instance, Zhang et al. [2] used a microwave plasma assisted chemical vapor deposition (MPCVD) system to obtain tubular graphite cones with the potential for use as tips for scanning probe microscopy. Singhal et al. [3] demonstrated controlled carbon deposition inside/outside glass pipettes in the absence of catalysts by the decomposition of hydrocarbon feedstock gas, which are useful for growing carbon layers of different thicknesses at selective locations on a glass pipette to yield a large number of cellular probes in bulk quantities. Kim et al. [4] described a glass capillary template-based method for forming a single carbon pipe that is integrated with a glass capillary, thus facilitating parallel production of multiple carbon-pipe devices. Hou et al. [5] obtained helically shaped multiwalled carbon nanotubes in large quantities by catalytic pyrolysis.Peng et al. [10] demonstrated the first synthesis of well-aligned, through a CVD process, pearl-like carbon nanotube arrays that will be more effective in reinforcing matrix to produce strong and tough composites. Dai et al. [11]
reported a novel carbon-based nanostructure in the form of crosses with hollow interiors, and that structure would be in favor of the future research for carbon-based nanoelectronic architecture. Xin et al. [12] synthesized carbon nanosprouts on calcined stainless steel by chemical vapor deposition, and they show potential in application in reinforced composites with enhanced mechanical properties. Chamberlain et al. [13] attained nanotube protrusions through the catalytic opening of nanotube sidewalls, which opens up directions for catalytic application to produce novel carbon structures with dramatic functional properties. Here, we report, for the first time, the large-area synthesis of another novel nanostructure of carbon named “carbon nanopear (CNP)”, and they are characterized by the nanometer-sized sharp tip. CNPs are made by chemical vapor deposition (CVD) method, with benzene as source for carbon and hydrogen gas as the reaction carrier gas. Such process is simple and feasible way to prepare CNPs in large scale, thus making them beneficial to prepare easily in the future. 2. Experiment The experimental setup included a horizontal tube furnace, a gas flow/control system, and a quartz tube with an inner diameter of 4.4 cm, outer diameter of 5 cm and length of 100 cm. The experiment was carried out, under optimized experimental conditions, at the atmosphere pressure (1 atm) using chemical vapor deposition (CVD) method, with hydrogen gas (99.999%) only as the reaction carrier gas, benzene as the carbon precursor, and thiophene (0.05 mg/L) as the
Corresponding author. E-mail address: [email protected] (G.-P. Dai). 1 ORCID: 0000-0001-9208-9994. ⁎
https://doi.org/10.1016/j.chemphys.2020.110780 Received 5 February 2020; Received in revised form 28 March 2020; Accepted 28 March 2020 Available online 30 March 2020 0301-0104/ © 2020 Elsevier B.V. All rights reserved.
Chemical Physics 535 (2020) 110780
C. Wang, et al.
Fig. 1. (a) Schematic figure of CVD process used for large-area synthesis of CNPs on the silicon wafer coated with Ni catalyst film. (b) The experiment process of CNPs by CVD, including heating, growth and cooling process.
growth promoter and silicon wafer coated Ni catalyst film (thickness 2.5 nm) made by magnetron sputtering, which is similar as the method used in reference [14], as the substrate. The silicon substrate coated with Ni catalyst film was placed at middle of the quartz boat. Likewise, the quartz boat was positioned at the middle of quartz tube. As the schematic diagram of this system is shown in Fig. 1(a), all reactions took place in the CVD furnace. First of all, high-purity hydrogen gas was introduced into the quartz tube to remove the air in the quartz tube, thiophene was dissolved in the benzene and carried into the quartz tube by hydrogen gas to enhance the growth of these novel carbon nanostructures. Hydrogen gas was introduced into the quartz tube with outer diameter of 50 mm along with the flow rate of 80 standard cubic centimeters per minute (sccm), we simultaneously heated up the quartz tube from atmospheric temperature (around 25 ℃) to 1080 ℃ within 38 min. Then we heated the quartz tube quickly to 1100 ℃ for 2 min. Later, the temperature of quartz tube was remained at 1100 ℃ for 50 min under the flow rate of 50 sccm and 20 sccm for pure H2 and the mixture gas composed of C6H6 and H2, respectively. Lastly, under the flow rate of 20 sccm for H2, the temperature of quartz tube was cooled to 200 ℃ for 2 h. At the end of experiment, as shown in Fig. 2(a), it turned out that CNPs were synthesized in large area and collected on
the silicon wafer. We successfully obtained large-scale CNPs under conditions shown in Fig. 1(a) and (b). 3. Results and discussion Great numbers of benzene gas molecules were pyrolyzed into carbon atoms during the growth stage illustrated in Fig. 1(b), carbon atoms tended to be located at Ni catalyst, then the carbon content in the catalyst particles would increase with time because of its excellent carbon solubility [15]. Eventually, we got the large-scale CNPs on the silicon wafer located at the middle place of quartz tube. Moreover, CNPs are characterized to examine the surface morphology by scanning electron microscopy (SEM). Independent of their sizes, as demonstrated by Fig. 2(a), as-synthesized CNPs are always in similar in shape and grow directly from silicon substrate with Ni catalyst film coating. We could also see that the as-synthesized CNPs are in a large area and aggregate in part of area of Fig. 2(a), which has not been previously reported, confirming the large-scale synthesis of CNPs by CVD. Fig. 2(b) is a higher magnification SEM image of Fig. 2(a) and it clearly shows the random three-dimensional orientation of CNPs. Evidently, among the CNPs in Fig. 2(b), some are vertical, and others 2
Chemical Physics 535 (2020) 110780
C. Wang, et al.
Fig. 2. Scanning electron microscopy (SEM) and Raman spectrum images. (a)-(c) SEM images of large-area synthesis of CNPs on silicon substrate with different magnifications. (d) High-resolution SEM image of an individual CNP deposited on a silicon substrate. (e) Typical SEM image of CNPs prepared under the same condition of Fig. (a)-(d) without using thiophene as growth promoter. (f) Raman spectrum of the CNPs measured with a 532 nm laser excitation.
are flat. Fig. 2(c) is a high-resolution SEM image of Fig. 2(b), and it shows that the CNPs have obvious nanometer-sized sharp tips similar to the tips of pipettes [3], nanometer-sized roots. Fig. 2(d) is the typical SEM image of a single CNP, thus further suggesting that the length of CNPs is ~800 nm and the thickness of CNPs is up to ~300 nm. On the other hand, compared with CNPs in Fig. 2(b), a typical SEM image (Fig. 2e) of CNPs prepared under the same experimental condition of Fig. (a)-(d) without using thiophene as growth promoter, shows less assynthesized CNPs, which suggests that thiophene is effective for facilitating CNP synthesis. CNPs are further characterized by Raman spectroscopy (Fig. 2f). Raman spectroscopy was carried out to identify the quality and homogeneity of the as-grown CNPs samples. As can be seen, Fig. 2(f) exhibits a typical Raman spectrum of the as-prepared CNP, and it manifests two main peaks for the CNP sample including disorder band (D-band) at 1358.4 cm−1 and a graphite band (G-band) at 1603.09 cm−1. The D band is arisen from the breathing mode of sp3 carbon rings and associated with the number of defects in the materials [16]. The G band is originated from the bond stretching of sp2 carbon pairs. The intensity ratio (ID/IG) of D band and G band has been used to determine the relative amounts of the amorphous and the graphitic component of the carbon materials [17,18]. The D-band reveals the finite crystalline size or defects in crystallites and ID/IG = 0.871, which
indicates the disorder feature of the amorphous carbon [19]. To further investigate the internal pear-like structure of CNPs, the morphology of CNPs is studied by transmission electron microscopy (TEM) illustrated in Fig. 3. Relevant TEM images and selected area electron diffraction (SAED) image examine the detailed crystallinity and morphology of the CNP. Fig. 3(a) shows the shape of CNP sample with broken root by ultrasound. As for above shape of CNP in Fig. 3(a), we assume that the CNP in Fig. 3(a) tends to break up during the sonication step owing to the weak van der Waals forces between the molecules in the CNP. Fig. 3(b)-(d) demonstrate the outermost carbon layers of the CNP, which indicates that the CNP contains graphitized carbon. Fig. 3(b)-(e) further reveal high-resolution images of the corresponding positions marked in Fig. 3(a), so we could clearly see continuous shortening of the length of graphitic sheets from the exterior to interior making them pear-shaped. Evidently, high-resolution TEM images in the Fig. 3(b)-(e) show that the interior part of the CNP are composed of thick amorphous carbon layers, and the exterior part of the CNP are made of thin graphitic carbon layers. Additionally, selected area electron diffraction from different part of an individual CNP gives the same pattern. Fig. 3(f) is a typical diffraction pattern taken on the longitudinal upper edge of the CNP, and it reveals that the CNP consists of graphitized carbon and amorphous carbon. However, deriving a detailed mechanism for the formation of such a 3
Chemical Physics 535 (2020) 110780
C. Wang, et al.
Fig. 3. Typical transmission electron microscopy (TEM) images. (a) Low-magnification TEM image of CNPs. (b)-(e) High-resolution TEM images of the corresponding position marked in (a). (f) Corresponding selected area electron diffraction pattern taken on the longitudinal upper edge of the CNP.
pear-like structure is still a challenge. On the basis of the microscopic evidences, we construct a probable CNPs formation mechanism illustrated in Fig. 4(c). Initially, as shown in Fig. 4(a), the diameter of Ni particles is in a range from tens to hundreds of nanometers, and Ni catalyst particles are nearly elliptical. Based on catalyst chemical vapor deposition in the vapor-liquid-solid (VLS), metal particles are to act as a solvent for the carbon atoms before and during carbon nanostructures growth [20–22]. When these particles are supersaturated in carbon, solid-phase carbon nanostructures are grown. It is known from the theory of gas-solid reactions that a certain number of collisions with the surface may be required to activate molecules and initiate the reaction [23,24]. Therefore, we hypothesized one possible explanation that benzene gas molecules have a large number of collisions with the Ni catalyst particles. For a hydrocarbon gas, this brings the molecules to a higher energy (excited) state [3,25], helping to overcome the activation energy barrier required for decomposition. Consequently, benzene gas
molecules would decompose easily to form highly condensed carbon atoms dissolved into the Ni catalyst particles at high temperature, which precipitate and diffuse at the Ni catalyst interface according to the dissolution- precipitation mechanism [26,27]. And benzene gas molecules flow rate would be very slow when benzene gas molecules get close to the Ni catalyst interface because of low Reynold’s number [3]. Owing to the low velocity, the benzene gas molecules could have a longer residence time, thus having a larger number of collisions with the Ni catalyst and facilitating the decomposition of benzene gas molecules. Besides, during the growth of CNPs, Ni particle may undergo elongated deformation which exerts a favorable effect on the formation of CNPs. This result is in line with the growth of nanobells, where the deformation of catalyst particles stems from the wetting or interplay between graphitic sheets and the catalyst particle [20]. The schematic figure of growth process for a single CNP is illustrated in Fig. 4b. Therefore, as typical TEM images (Fig. 3) show, thick carbon layers 4
Chemical Physics 535 (2020) 110780
C. Wang, et al.
Fig. 4. (a) SEM image of catalyst particles made by magnetron sputtering on a Si substrate. (b) Schematic figure of the growth process for a single CNP: (I) active carbon atoms dissolve into the Ni particle through the surface; (II) beginning to precipitate graphitic sheets; (III) the catalyst particle probably undergoing elongated deformation and a CNP grows. (c) Schematic illustration showing large-area synthesis of CNPs through CVD process.
grow near the Ni catalyst, and relatively thin carbon layers develop away from the Ni catalyst. That result was also observed in the case of ethylene [4], which forms carbon nanopipettes with non-uniform layers by catalytic chemical vapor deposition of carbon on the glass surface. Gradually, the repetition of the above process produces sharp tips. Moreover, the amount of decomposition of benzene gas molecules would increase along with time. And sulfur contained in thiophene, as a growth promoter, enhances the growth rate of carbon nanostructure and raises the activity of dissolved carbon in the catalyst particle [28–30]. Furthermore, sulfur could play a complementary role by inducing curvature and therefore influencing the final nanostructure morphology, and S/C ratios of different types of carbon materials correlate with their morphologies and degrees of curvature and complexity [31]. Following above conditions, carbon atoms located at the surface of Ni catalyst on the silicon wafer cluster together to form CNPs, based on “root growth” mechanism [32], then CNPs are produced in a large scale when the temperature is maintained at 1100 ℃. Also, molecular dynamics simulation shows van der Waals forces are of importance in formation of the structures of many systems [33] including graphitic structures [34]. It is possible that van der Waals forces, morphology of
Ni catalysts, and local gas flows can contribute to the formation of pear shape in our case. However, the initiation nucleation process and exact growth process for carbon nanopears are still unclear, and requires consideration of multiple factors, e.g.: hydrogen flow rate and its role, flow dynamics, the role of the underlying Ni in the beginning of carbon nanopears’ nucleation, and the benzene flow rate. This will need further investigation, but it is clear that carbon nanopears are observed under our growth conditions. 4. Conclusions In summary, we have reported, for the first time, that large-scale synthesis of special CNPs on silicon wafer coated with Ni catalyst film by CVD method at atmosphere pressure, which is facile and one-step way to produce CNPs following a root-growth mechanism. In addition, CNPs with nanometer-sized sharps may open a new avenue, because of their unique morphology, for future research in carbon-based nanoelectronics materials, such as conductive materials, electroactive materials and catalyst carriers. 5
Chemical Physics 535 (2020) 110780
C. Wang, et al.
Declaration of Competing Interest
sputter deposited nickel-silica thin films, Mater. Sci. Eng. B 177 (2012) 1073–1079. [15] Ho-je Kwon, J.M. Ha, S.H. Yoo, G. Ali, S.O. Cho, Synthesis of flake-like graphene from nickle-coated polyacrylonitrile polymer, Nanoscale Res. Lett. 9 (1) (2014) 618. [16] H. Huang, S. Yang, Q. Li, Y. Yang, G. Wang, X. You, B. Mao, H. Wang, Y. Ma, P. He, Z. Liu, G. Ding, X. Xie, Electrochemical cutting in weak aqueous electrolytes: the strategy for efficient and controllable preparation of graphene quantum dots, Langmuir 34 (1) (2017) 250–258. [17] G. Dai, M.H. Wu, D.K. Taylor, K. Vinodgopal, Square-shaped, single-crystal, monolayer graphene domains by low-pressure chemical vapor deposition, Mater. Res. Lett. 1 (2013) 67–76. [18] D. Qu, M. Zheng, P. Du, Y. Zhou, L. Zhang, D. Li, H. Tan, Z. Zhao, G. Xie, Z. Sun, Highly luminescent S, N co-doped graphene quantum dots with broad visible absorption bands for visible light photocatalysts, Nanoscale 5 (24) (2013) 12272–12277. [19] M. Liu, L. Gan, W. Xiong, F. Zhao, X. Fan, D. Zhu, Z. Xu, Z. Hao, L. Chen, NickelDoped activated mesoporous carbon microspheres with partially graphitic structure for supercapacitors, Energy Fuel 27 (2013) 1168–1173. [20] G.Y. Zhang, X.C. Ma, D.Y. Zhong, E.G. Wang, Polymerized carbon nitride nanobells, J. Appl. Phys. 91 (11) (2002) 9324. [21] J. Gavillet, A. Loiseau, C. Journet, F. Willaime, F. Ducastelle, J.C. Charlier, Rootgrowth mechanism for single-wall carbon nanotubes, Phys. Rev. Lett. 87 (27) (2001) 275504. [22] R. Seidel, G.S. Duesberg, E. Unger, A.P. Graham, M. Liebau, F. Kreupl, Chemical vapor deposition growth of single-walled carbon nanotubes at 600 °C and a simple growth model, J. Phys. Chem. B 108 (6) (2004) 1888–1893. [23] A. Bukoski, D. Blumling, I. Harrison, Microcanonical unimolecular rate theory at surfaces. I. dissociative chemisorption of methane on Pt (111), J. Chem. Phys. 118 (2) (2003) 843–871. [24] V.A. Ukraintsev, I. Harrison, A statistical model for activated dissociative adsorption: application to methane dissociation on Pt(111), J. Chem. Phys. 101 (1994) 1564. [25] S. Berber, Y.-K. Kwon, David Tománek, Microscopic formation mechanism of nanotube peapods, Phys. Rev. Lett. 88 (18) (2002) 185502. [26] A. Õya, S. Õtani, Influences of particle size of metal on catalytic graphitization of non-graphitizing carbons, Carbon 19 (1981) 391–401. [27] A. Õya, S. Õtani, Catalytic graphitization of carbons by various metals, Carbon 17 (1979) 131–137. [28] M.S. Motta, A. Moisala, I.A. Kinloch, A.H. Windle, The role of sulphur in the synthesis of carbon nanotubes by chemical vapour deposition at high temperatures, J. Nanosci. Nanotechnol. 8 (2008) 2442–2449. [29] G.G. Tibbetts, C.A. Bernardo, D.W. Gorkiewicz, R.L. Alig, Role of sulfur in the production of carbon fibers in the vapor phase, Carbon 32 (1994) 569–576. [30] J. Wei, H. Zhu, Y. Jia, Q. Shu, C. Li, K. Wang, B. Wei, Y. Zhu, Z. Wang, J. Luo, W. Liu, D. Wu, The effect of sulfur on the number of layers in a carbon nanotube, Carbon 45 (2007) 2152–2158. [31] J.M. Romo-Herrera, D.A. Cullen, E. Cruz-Silva, D. Ramırez, B.G. Sumpter, V. Meunier, H. Terrones, D.J. Smith, M. Terrones, The role of sulfur in the synthesis of novel carbon morphologies: from covalent Y-junctions to sea-urchin-like structures, Adv. Funct. Mater. 19 (8) (2009) 1193–1199. [32] J. Robertson, Heterogeneous catalysis model of growth mechanisms of carbon nanotubes, graphene and silicon nanowires, J. Mater. Chem. 22 (2012) 19858. [33] A.C.T. van Duin, S. Dasgupta, F. Lorant, W.A. Goddard III, ReaxFF: a reactive force field for hydrocarbons, J. Phys. Chem. A 105 (2001) 9396–9409. [34] L.A. Girifalco, M. Hodak, Van der Waals binding energies in graphitic structures, Phys. Rev. B 65 (12) (2002) 125404.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements G.-P.D acknowledges the National Natural Science Foundation of China (Grants 51762032 and 51462022) for financial support of this research. And this work was supported by the innovation fund designated for Graduate Students of Jiangxi Province (Grant YC2018-S015). The assistance of Dr. Zhi-Qun Tian (HRTEM measurements) at Guangxi University, is also greatly appreciated. References [1] H.W. Kroto, J.R. Health, S.C. O’Brien, R.F. Curl, R.E. Smalley, C60: Buckminsterfullerence, Nature 318 (1985) 162. [2] G. Zhang, X. Jiang, E. Wang, Tubular graphite cones, Science 300 (2003) 472–474. [3] R. Singhal, S. Bhattacharyya, Z. Orynbayeva, E. Vitol, G. Friedman, Y. Gogotsi, Small diameter carbon nanopipettes, Nanotechnology 21 (2010) 015304. [4] B.M. Kim, T. Murray, H.H. Bau, The fabrication of integrated carbon pipes with submicron diameters, Nanotechnology 16 (8) (2005) 1317–1320. [5] H. Hou, Z. Jun, F. Weller, A. Greiner, Large-scale synthesis and characterization of helically coiled carbon nanotubes by use of Fe(CO)5 as floating catalyst precursor, Chem. Mater. 15 (2003) 3170–3175. [6] M. Sano, A. Kamino, S. Shinkai, Construction of carbon nanotube “stars” with dendrimers, Angew. Chem. Int. Ed. 40 (2001) 4661–4663. [7] G. Meng, Y.J. Jung, A. Cao, R. Vajtai, P.M. Ajayan, Controlled fabrication of hierarchically branched nanopores, nanotubes, and nanowires, Proc. Natl Acad. Sci. U.S.A. 102 (2005) 7074–7078. [8] A.G. Nasibulin, P.V. Pikhitsa, H. Jiang, D.P. Brown, A.V. Krasheninnikov, A.S. Anisimov, P. Queipo, A. Moisala, D. Gonzalez, G. Lientschnig, A. Hassanien, S.D. Shandakov, G. Lolli, D.E. Resasco, M. Choi, D. Tománek, E.I. Kauppinen, A novel hybrid carbon material, Nature Nanotechnol. 2 (2007) 156. [9] W.A. de Heer, P. Poncharal, C. Berger, J. Gezo, Z. Song, J. Bettini, D. Ugarte, Liquid carbon, carbon-glass beads, and the crystallization of carbon nanotubes, Science 307 (2005) 907. [10] H. Peng, M. Jain, Q. Li, D.E. Peterson, Y. Zhu, Q. Jia, Vertically aligned pearl-like carbon nanotube arrays for fiber spinning, J. Am. Chem. Soc. 130 (2008) 1130. [11] G. Dai, S. Deng, Carbon nanocrosses: synthesis, characterization and growth mechanism of a novel carbon nano-structure, Chem. Phys. Lett. 536 (2012) 113–117. [12] B. Xin, G. Sun, C. Lao, D. Shang, X. Zhang, Z. Wen, M. He, Chemical vapor deposition synthesis of carbon nanosprouts on calcined stainless steel, Mater. Lett. 238 (2019) 290–293. [13] T.W. Chamberlain, J.C. Meyer, J. Biskupek, J. Leschner, A. Santana, N.A. Besley, E. Bichoutskaia, U. Kaiser, A.N. Khlobystov, Reactions of the inner surface of carbon nanotubes and nanoprotrusion processes imaged at the atomic scale, Nature Chem. 3 (9) (2011) 732–737. [14] Rajan Waliaa, J.C. Pivinb, A.K. Chawlaa, R. Jayaganthanc, Y.K. Gautamc, Ramesh Chandraa, A comprehensive study of structural and magnetic properties of
6
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Synthesis, characterization and growth mechanism of carbon nanopears a
a
b
a
a
c
a
Chen Wang , Pei-Pei Hou , Peng Liao , Ping Wu , Ming Pan , Hua-Fei Li , Xiao-Di Wang , ⁎ Ning Xiec, Zhi Liua, Zheling Zenga, Gui-Ping Daia, ,1 a b c
T
Department of Chemical Engineering, School of Environmental and Chemical Engineering, Nanchang University, Nanchang 330031, China Department of Cell Research and Development, Farasis Energy Inc., Hayward, CA 94545, USA School of Materials Science and Engineering, Nanchang University, Nanchang 330031, China
ARTICLE INFO
ABSTRACT
Keywords: Carbon nanopears Chemical vapor deposition Sharp tips Formation mechanism
We report a novel carbon morphology that we name “carbon nanopears” using chemical vapor deposition in the presence of silicon wafer as a substrate coating Ni catalyst, thiophene as the growth promoter and benzene as carbon source. The carbon nanopears observed under optimized experimental conditions have nanometer-sized sharp tips, nanometer-sized roots and solid interiors. The carbon nanopears are composed of graphitized carbon and amorphous carbon, with continuous shortening of the length of graphitic sheets from the exterior to interior making them pear-shaped. Furthermore, a probable CNP formation mechanism is proposed in this paper.
1. Introduction Owing to their potential advantages exhibited in novel devices and technologies, a lot of attention has been paid to carbon nanomaterials [1] by using ingenious methods. Morphology control of carbon nanomaterials represents one of the most explored directions. Furthermore, carbon nanostructures with morphologies of cone [2], pipette [3], pipe [4], helix [5], star [6], branch [7], bud [8], bead [9], pearl [10], nanocrosses [11], nanosprout [12], nanoprotrusion [13], which show various one-, and two-, and three-dimensional morphologies, have been widely investigated in recent decades. For instance, Zhang et al. [2] used a microwave plasma assisted chemical vapor deposition (MPCVD) system to obtain tubular graphite cones with the potential for use as tips for scanning probe microscopy. Singhal et al. [3] demonstrated controlled carbon deposition inside/outside glass pipettes in the absence of catalysts by the decomposition of hydrocarbon feedstock gas, which are useful for growing carbon layers of different thicknesses at selective locations on a glass pipette to yield a large number of cellular probes in bulk quantities. Kim et al. [4] described a glass capillary template-based method for forming a single carbon pipe that is integrated with a glass capillary, thus facilitating parallel production of multiple carbon-pipe devices. Hou et al. [5] obtained helically shaped multiwalled carbon nanotubes in large quantities by catalytic pyrolysis.Peng et al. [10] demonstrated the first synthesis of well-aligned, through a CVD process, pearl-like carbon nanotube arrays that will be more effective in reinforcing matrix to produce strong and tough composites. Dai et al. [11]
reported a novel carbon-based nanostructure in the form of crosses with hollow interiors, and that structure would be in favor of the future research for carbon-based nanoelectronic architecture. Xin et al. [12] synthesized carbon nanosprouts on calcined stainless steel by chemical vapor deposition, and they show potential in application in reinforced composites with enhanced mechanical properties. Chamberlain et al. [13] attained nanotube protrusions through the catalytic opening of nanotube sidewalls, which opens up directions for catalytic application to produce novel carbon structures with dramatic functional properties. Here, we report, for the first time, the large-area synthesis of another novel nanostructure of carbon named “carbon nanopear (CNP)”, and they are characterized by the nanometer-sized sharp tip. CNPs are made by chemical vapor deposition (CVD) method, with benzene as source for carbon and hydrogen gas as the reaction carrier gas. Such process is simple and feasible way to prepare CNPs in large scale, thus making them beneficial to prepare easily in the future. 2. Experiment The experimental setup included a horizontal tube furnace, a gas flow/control system, and a quartz tube with an inner diameter of 4.4 cm, outer diameter of 5 cm and length of 100 cm. The experiment was carried out, under optimized experimental conditions, at the atmosphere pressure (1 atm) using chemical vapor deposition (CVD) method, with hydrogen gas (99.999%) only as the reaction carrier gas, benzene as the carbon precursor, and thiophene (0.05 mg/L) as the
Corresponding author. E-mail address: [email protected] (G.-P. Dai). 1 ORCID: 0000-0001-9208-9994. ⁎
https://doi.org/10.1016/j.chemphys.2020.110780 Received 5 February 2020; Received in revised form 28 March 2020; Accepted 28 March 2020 Available online 30 March 2020 0301-0104/ © 2020 Elsevier B.V. All rights reserved.
Chemical Physics 535 (2020) 110780
C. Wang, et al.
Fig. 1. (a) Schematic figure of CVD process used for large-area synthesis of CNPs on the silicon wafer coated with Ni catalyst film. (b) The experiment process of CNPs by CVD, including heating, growth and cooling process.
growth promoter and silicon wafer coated Ni catalyst film (thickness 2.5 nm) made by magnetron sputtering, which is similar as the method used in reference [14], as the substrate. The silicon substrate coated with Ni catalyst film was placed at middle of the quartz boat. Likewise, the quartz boat was positioned at the middle of quartz tube. As the schematic diagram of this system is shown in Fig. 1(a), all reactions took place in the CVD furnace. First of all, high-purity hydrogen gas was introduced into the quartz tube to remove the air in the quartz tube, thiophene was dissolved in the benzene and carried into the quartz tube by hydrogen gas to enhance the growth of these novel carbon nanostructures. Hydrogen gas was introduced into the quartz tube with outer diameter of 50 mm along with the flow rate of 80 standard cubic centimeters per minute (sccm), we simultaneously heated up the quartz tube from atmospheric temperature (around 25 ℃) to 1080 ℃ within 38 min. Then we heated the quartz tube quickly to 1100 ℃ for 2 min. Later, the temperature of quartz tube was remained at 1100 ℃ for 50 min under the flow rate of 50 sccm and 20 sccm for pure H2 and the mixture gas composed of C6H6 and H2, respectively. Lastly, under the flow rate of 20 sccm for H2, the temperature of quartz tube was cooled to 200 ℃ for 2 h. At the end of experiment, as shown in Fig. 2(a), it turned out that CNPs were synthesized in large area and collected on
the silicon wafer. We successfully obtained large-scale CNPs under conditions shown in Fig. 1(a) and (b). 3. Results and discussion Great numbers of benzene gas molecules were pyrolyzed into carbon atoms during the growth stage illustrated in Fig. 1(b), carbon atoms tended to be located at Ni catalyst, then the carbon content in the catalyst particles would increase with time because of its excellent carbon solubility [15]. Eventually, we got the large-scale CNPs on the silicon wafer located at the middle place of quartz tube. Moreover, CNPs are characterized to examine the surface morphology by scanning electron microscopy (SEM). Independent of their sizes, as demonstrated by Fig. 2(a), as-synthesized CNPs are always in similar in shape and grow directly from silicon substrate with Ni catalyst film coating. We could also see that the as-synthesized CNPs are in a large area and aggregate in part of area of Fig. 2(a), which has not been previously reported, confirming the large-scale synthesis of CNPs by CVD. Fig. 2(b) is a higher magnification SEM image of Fig. 2(a) and it clearly shows the random three-dimensional orientation of CNPs. Evidently, among the CNPs in Fig. 2(b), some are vertical, and others 2
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Fig. 2. Scanning electron microscopy (SEM) and Raman spectrum images. (a)-(c) SEM images of large-area synthesis of CNPs on silicon substrate with different magnifications. (d) High-resolution SEM image of an individual CNP deposited on a silicon substrate. (e) Typical SEM image of CNPs prepared under the same condition of Fig. (a)-(d) without using thiophene as growth promoter. (f) Raman spectrum of the CNPs measured with a 532 nm laser excitation.
are flat. Fig. 2(c) is a high-resolution SEM image of Fig. 2(b), and it shows that the CNPs have obvious nanometer-sized sharp tips similar to the tips of pipettes [3], nanometer-sized roots. Fig. 2(d) is the typical SEM image of a single CNP, thus further suggesting that the length of CNPs is ~800 nm and the thickness of CNPs is up to ~300 nm. On the other hand, compared with CNPs in Fig. 2(b), a typical SEM image (Fig. 2e) of CNPs prepared under the same experimental condition of Fig. (a)-(d) without using thiophene as growth promoter, shows less assynthesized CNPs, which suggests that thiophene is effective for facilitating CNP synthesis. CNPs are further characterized by Raman spectroscopy (Fig. 2f). Raman spectroscopy was carried out to identify the quality and homogeneity of the as-grown CNPs samples. As can be seen, Fig. 2(f) exhibits a typical Raman spectrum of the as-prepared CNP, and it manifests two main peaks for the CNP sample including disorder band (D-band) at 1358.4 cm−1 and a graphite band (G-band) at 1603.09 cm−1. The D band is arisen from the breathing mode of sp3 carbon rings and associated with the number of defects in the materials [16]. The G band is originated from the bond stretching of sp2 carbon pairs. The intensity ratio (ID/IG) of D band and G band has been used to determine the relative amounts of the amorphous and the graphitic component of the carbon materials [17,18]. The D-band reveals the finite crystalline size or defects in crystallites and ID/IG = 0.871, which
indicates the disorder feature of the amorphous carbon [19]. To further investigate the internal pear-like structure of CNPs, the morphology of CNPs is studied by transmission electron microscopy (TEM) illustrated in Fig. 3. Relevant TEM images and selected area electron diffraction (SAED) image examine the detailed crystallinity and morphology of the CNP. Fig. 3(a) shows the shape of CNP sample with broken root by ultrasound. As for above shape of CNP in Fig. 3(a), we assume that the CNP in Fig. 3(a) tends to break up during the sonication step owing to the weak van der Waals forces between the molecules in the CNP. Fig. 3(b)-(d) demonstrate the outermost carbon layers of the CNP, which indicates that the CNP contains graphitized carbon. Fig. 3(b)-(e) further reveal high-resolution images of the corresponding positions marked in Fig. 3(a), so we could clearly see continuous shortening of the length of graphitic sheets from the exterior to interior making them pear-shaped. Evidently, high-resolution TEM images in the Fig. 3(b)-(e) show that the interior part of the CNP are composed of thick amorphous carbon layers, and the exterior part of the CNP are made of thin graphitic carbon layers. Additionally, selected area electron diffraction from different part of an individual CNP gives the same pattern. Fig. 3(f) is a typical diffraction pattern taken on the longitudinal upper edge of the CNP, and it reveals that the CNP consists of graphitized carbon and amorphous carbon. However, deriving a detailed mechanism for the formation of such a 3
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Fig. 3. Typical transmission electron microscopy (TEM) images. (a) Low-magnification TEM image of CNPs. (b)-(e) High-resolution TEM images of the corresponding position marked in (a). (f) Corresponding selected area electron diffraction pattern taken on the longitudinal upper edge of the CNP.
pear-like structure is still a challenge. On the basis of the microscopic evidences, we construct a probable CNPs formation mechanism illustrated in Fig. 4(c). Initially, as shown in Fig. 4(a), the diameter of Ni particles is in a range from tens to hundreds of nanometers, and Ni catalyst particles are nearly elliptical. Based on catalyst chemical vapor deposition in the vapor-liquid-solid (VLS), metal particles are to act as a solvent for the carbon atoms before and during carbon nanostructures growth [20–22]. When these particles are supersaturated in carbon, solid-phase carbon nanostructures are grown. It is known from the theory of gas-solid reactions that a certain number of collisions with the surface may be required to activate molecules and initiate the reaction [23,24]. Therefore, we hypothesized one possible explanation that benzene gas molecules have a large number of collisions with the Ni catalyst particles. For a hydrocarbon gas, this brings the molecules to a higher energy (excited) state [3,25], helping to overcome the activation energy barrier required for decomposition. Consequently, benzene gas
molecules would decompose easily to form highly condensed carbon atoms dissolved into the Ni catalyst particles at high temperature, which precipitate and diffuse at the Ni catalyst interface according to the dissolution- precipitation mechanism [26,27]. And benzene gas molecules flow rate would be very slow when benzene gas molecules get close to the Ni catalyst interface because of low Reynold’s number [3]. Owing to the low velocity, the benzene gas molecules could have a longer residence time, thus having a larger number of collisions with the Ni catalyst and facilitating the decomposition of benzene gas molecules. Besides, during the growth of CNPs, Ni particle may undergo elongated deformation which exerts a favorable effect on the formation of CNPs. This result is in line with the growth of nanobells, where the deformation of catalyst particles stems from the wetting or interplay between graphitic sheets and the catalyst particle [20]. The schematic figure of growth process for a single CNP is illustrated in Fig. 4b. Therefore, as typical TEM images (Fig. 3) show, thick carbon layers 4
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Fig. 4. (a) SEM image of catalyst particles made by magnetron sputtering on a Si substrate. (b) Schematic figure of the growth process for a single CNP: (I) active carbon atoms dissolve into the Ni particle through the surface; (II) beginning to precipitate graphitic sheets; (III) the catalyst particle probably undergoing elongated deformation and a CNP grows. (c) Schematic illustration showing large-area synthesis of CNPs through CVD process.
grow near the Ni catalyst, and relatively thin carbon layers develop away from the Ni catalyst. That result was also observed in the case of ethylene [4], which forms carbon nanopipettes with non-uniform layers by catalytic chemical vapor deposition of carbon on the glass surface. Gradually, the repetition of the above process produces sharp tips. Moreover, the amount of decomposition of benzene gas molecules would increase along with time. And sulfur contained in thiophene, as a growth promoter, enhances the growth rate of carbon nanostructure and raises the activity of dissolved carbon in the catalyst particle [28–30]. Furthermore, sulfur could play a complementary role by inducing curvature and therefore influencing the final nanostructure morphology, and S/C ratios of different types of carbon materials correlate with their morphologies and degrees of curvature and complexity [31]. Following above conditions, carbon atoms located at the surface of Ni catalyst on the silicon wafer cluster together to form CNPs, based on “root growth” mechanism [32], then CNPs are produced in a large scale when the temperature is maintained at 1100 ℃. Also, molecular dynamics simulation shows van der Waals forces are of importance in formation of the structures of many systems [33] including graphitic structures [34]. It is possible that van der Waals forces, morphology of
Ni catalysts, and local gas flows can contribute to the formation of pear shape in our case. However, the initiation nucleation process and exact growth process for carbon nanopears are still unclear, and requires consideration of multiple factors, e.g.: hydrogen flow rate and its role, flow dynamics, the role of the underlying Ni in the beginning of carbon nanopears’ nucleation, and the benzene flow rate. This will need further investigation, but it is clear that carbon nanopears are observed under our growth conditions. 4. Conclusions In summary, we have reported, for the first time, that large-scale synthesis of special CNPs on silicon wafer coated with Ni catalyst film by CVD method at atmosphere pressure, which is facile and one-step way to produce CNPs following a root-growth mechanism. In addition, CNPs with nanometer-sized sharps may open a new avenue, because of their unique morphology, for future research in carbon-based nanoelectronics materials, such as conductive materials, electroactive materials and catalyst carriers. 5
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Declaration of Competing Interest
sputter deposited nickel-silica thin films, Mater. Sci. Eng. B 177 (2012) 1073–1079. [15] Ho-je Kwon, J.M. Ha, S.H. Yoo, G. Ali, S.O. Cho, Synthesis of flake-like graphene from nickle-coated polyacrylonitrile polymer, Nanoscale Res. Lett. 9 (1) (2014) 618. [16] H. Huang, S. Yang, Q. Li, Y. Yang, G. Wang, X. You, B. Mao, H. Wang, Y. Ma, P. He, Z. Liu, G. Ding, X. Xie, Electrochemical cutting in weak aqueous electrolytes: the strategy for efficient and controllable preparation of graphene quantum dots, Langmuir 34 (1) (2017) 250–258. [17] G. Dai, M.H. Wu, D.K. Taylor, K. Vinodgopal, Square-shaped, single-crystal, monolayer graphene domains by low-pressure chemical vapor deposition, Mater. Res. Lett. 1 (2013) 67–76. [18] D. Qu, M. Zheng, P. Du, Y. Zhou, L. Zhang, D. Li, H. Tan, Z. Zhao, G. Xie, Z. Sun, Highly luminescent S, N co-doped graphene quantum dots with broad visible absorption bands for visible light photocatalysts, Nanoscale 5 (24) (2013) 12272–12277. [19] M. Liu, L. Gan, W. Xiong, F. Zhao, X. Fan, D. Zhu, Z. Xu, Z. Hao, L. Chen, NickelDoped activated mesoporous carbon microspheres with partially graphitic structure for supercapacitors, Energy Fuel 27 (2013) 1168–1173. [20] G.Y. Zhang, X.C. Ma, D.Y. Zhong, E.G. Wang, Polymerized carbon nitride nanobells, J. Appl. Phys. 91 (11) (2002) 9324. [21] J. Gavillet, A. Loiseau, C. Journet, F. Willaime, F. Ducastelle, J.C. Charlier, Rootgrowth mechanism for single-wall carbon nanotubes, Phys. Rev. Lett. 87 (27) (2001) 275504. [22] R. Seidel, G.S. Duesberg, E. Unger, A.P. Graham, M. Liebau, F. Kreupl, Chemical vapor deposition growth of single-walled carbon nanotubes at 600 °C and a simple growth model, J. Phys. Chem. B 108 (6) (2004) 1888–1893. [23] A. Bukoski, D. Blumling, I. Harrison, Microcanonical unimolecular rate theory at surfaces. I. dissociative chemisorption of methane on Pt (111), J. Chem. Phys. 118 (2) (2003) 843–871. [24] V.A. Ukraintsev, I. Harrison, A statistical model for activated dissociative adsorption: application to methane dissociation on Pt(111), J. Chem. Phys. 101 (1994) 1564. [25] S. Berber, Y.-K. Kwon, David Tománek, Microscopic formation mechanism of nanotube peapods, Phys. Rev. Lett. 88 (18) (2002) 185502. [26] A. Õya, S. Õtani, Influences of particle size of metal on catalytic graphitization of non-graphitizing carbons, Carbon 19 (1981) 391–401. [27] A. Õya, S. Õtani, Catalytic graphitization of carbons by various metals, Carbon 17 (1979) 131–137. [28] M.S. Motta, A. Moisala, I.A. Kinloch, A.H. Windle, The role of sulphur in the synthesis of carbon nanotubes by chemical vapour deposition at high temperatures, J. Nanosci. Nanotechnol. 8 (2008) 2442–2449. [29] G.G. Tibbetts, C.A. Bernardo, D.W. Gorkiewicz, R.L. Alig, Role of sulfur in the production of carbon fibers in the vapor phase, Carbon 32 (1994) 569–576. [30] J. Wei, H. Zhu, Y. Jia, Q. Shu, C. Li, K. Wang, B. Wei, Y. Zhu, Z. Wang, J. Luo, W. Liu, D. Wu, The effect of sulfur on the number of layers in a carbon nanotube, Carbon 45 (2007) 2152–2158. [31] J.M. Romo-Herrera, D.A. Cullen, E. Cruz-Silva, D. Ramırez, B.G. Sumpter, V. Meunier, H. Terrones, D.J. Smith, M. Terrones, The role of sulfur in the synthesis of novel carbon morphologies: from covalent Y-junctions to sea-urchin-like structures, Adv. Funct. Mater. 19 (8) (2009) 1193–1199. [32] J. Robertson, Heterogeneous catalysis model of growth mechanisms of carbon nanotubes, graphene and silicon nanowires, J. Mater. Chem. 22 (2012) 19858. [33] A.C.T. van Duin, S. Dasgupta, F. Lorant, W.A. Goddard III, ReaxFF: a reactive force field for hydrocarbons, J. Phys. Chem. A 105 (2001) 9396–9409. [34] L.A. Girifalco, M. Hodak, Van der Waals binding energies in graphitic structures, Phys. Rev. B 65 (12) (2002) 125404.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements G.-P.D acknowledges the National Natural Science Foundation of China (Grants 51762032 and 51462022) for financial support of this research. And this work was supported by the innovation fund designated for Graduate Students of Jiangxi Province (Grant YC2018-S015). The assistance of Dr. Zhi-Qun Tian (HRTEM measurements) at Guangxi University, is also greatly appreciated. References [1] H.W. Kroto, J.R. Health, S.C. O’Brien, R.F. Curl, R.E. Smalley, C60: Buckminsterfullerence, Nature 318 (1985) 162. [2] G. Zhang, X. Jiang, E. Wang, Tubular graphite cones, Science 300 (2003) 472–474. [3] R. Singhal, S. Bhattacharyya, Z. Orynbayeva, E. Vitol, G. Friedman, Y. Gogotsi, Small diameter carbon nanopipettes, Nanotechnology 21 (2010) 015304. [4] B.M. Kim, T. Murray, H.H. Bau, The fabrication of integrated carbon pipes with submicron diameters, Nanotechnology 16 (8) (2005) 1317–1320. [5] H. Hou, Z. Jun, F. Weller, A. Greiner, Large-scale synthesis and characterization of helically coiled carbon nanotubes by use of Fe(CO)5 as floating catalyst precursor, Chem. Mater. 15 (2003) 3170–3175. [6] M. Sano, A. Kamino, S. Shinkai, Construction of carbon nanotube “stars” with dendrimers, Angew. Chem. Int. Ed. 40 (2001) 4661–4663. [7] G. Meng, Y.J. Jung, A. Cao, R. Vajtai, P.M. Ajayan, Controlled fabrication of hierarchically branched nanopores, nanotubes, and nanowires, Proc. Natl Acad. Sci. U.S.A. 102 (2005) 7074–7078. [8] A.G. Nasibulin, P.V. Pikhitsa, H. Jiang, D.P. Brown, A.V. Krasheninnikov, A.S. Anisimov, P. Queipo, A. Moisala, D. Gonzalez, G. Lientschnig, A. Hassanien, S.D. Shandakov, G. Lolli, D.E. Resasco, M. Choi, D. Tománek, E.I. Kauppinen, A novel hybrid carbon material, Nature Nanotechnol. 2 (2007) 156. [9] W.A. de Heer, P. Poncharal, C. Berger, J. Gezo, Z. Song, J. Bettini, D. Ugarte, Liquid carbon, carbon-glass beads, and the crystallization of carbon nanotubes, Science 307 (2005) 907. [10] H. Peng, M. Jain, Q. Li, D.E. Peterson, Y. Zhu, Q. Jia, Vertically aligned pearl-like carbon nanotube arrays for fiber spinning, J. Am. Chem. Soc. 130 (2008) 1130. [11] G. Dai, S. Deng, Carbon nanocrosses: synthesis, characterization and growth mechanism of a novel carbon nano-structure, Chem. Phys. Lett. 536 (2012) 113–117. [12] B. Xin, G. Sun, C. Lao, D. Shang, X. Zhang, Z. Wen, M. He, Chemical vapor deposition synthesis of carbon nanosprouts on calcined stainless steel, Mater. Lett. 238 (2019) 290–293. [13] T.W. Chamberlain, J.C. Meyer, J. Biskupek, J. Leschner, A. Santana, N.A. Besley, E. Bichoutskaia, U. Kaiser, A.N. Khlobystov, Reactions of the inner surface of carbon nanotubes and nanoprotrusion processes imaged at the atomic scale, Nature Chem. 3 (9) (2011) 732–737. [14] Rajan Waliaa, J.C. Pivinb, A.K. Chawlaa, R. Jayaganthanc, Y.K. Gautamc, Ramesh Chandraa, A comprehensive study of structural and magnetic properties of
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