- Email: [email protected]
Carbon nanomaterials synthesized using a spray pyrolysis method
Vacuum xxx (2015) 1e6
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Carbon nanomaterials synthesized using a spray pyrolysis method Wen-Ko Huang a, Kun-Ju Chung a, Yih-Ming Liu b, *, Ming-Der Ger b, Nen-Wen Pu c, Meng-Jey Youh d a
School of Defense Science, Chung Cheng Institute of Technology, National Defense University, 335 Taiwan, ROC Department of Chemical & Materials Science, Chung Cheng Institute of Technology, National Defense University, 335 Taiwan, ROC c Department of Photonic Engineering, Yuan Ze University, 335 Taiwan, ROC d Department of Information Technology, Hsing Wu University, 335 Taiwan, ROC b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2014 Received in revised form 1 February 2015 Accepted 4 February 2015 Available online xxx
Carbon nanomaterials were synthesized using a spray pyrolysis chemical vapor deposition system (SPCVD). By varying the reaction temperature or flow rate of the carbon source, the structure or morphology of the synthesized carbon nanocoils (CNCs) can be controlled. A vertical chemical vapor deposition (CVD) furnace with a three-stage heating zone was employed to synthesize carbonaceous nanomaterials using nano-Pd catalysts at growth temperatures of 600, 700, and 800 C. The morphology of the synthesized carbon products and the relationship between the carbon source concentration and its yield were evaluated. The results showed that CNCs formed at a lower temperature (600 C), whereas straight carbon nanotubes were obtained at a higher temperature (700 or 800 C). The structure and morphology of the carbonaceous samples were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Their graphite crystallinity was analyzed using Raman spectroscopy. When the three heating zones of the vertical CVD chamber were set to different temperatures, a unique nano-carbonaceous material with a special morphology similar to an octopus tentacle was formed. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon nanomaterials Spray pyrolysis Carbon nanocoil (CNC) Octopus-tentacle structure
1. Introduction Carbon nanocoils (CNCs) and straight carbon nanotubes (CNTs) have attracted considerable attention in recent years because of their unique mechanical and electrical properties [1e9]. Due to their special helical structure, CNCs have the potential for applications in different fields such as hydrogen storage media [1], microwave absorbers [2], field emitters [3], microsensors [4], elastic materials [5], composite materials [6], electrode materials [7], and catalyst supports [8,9]. CNCs were first synthesized by Motojima and coworkers [10]. Using a nickel substrate or nickel powder as a catalyst, they synthesized coiled carbon microfibers [11] and suggested a growth mechanism based on the anisotropic extrusion of carbon over a catalyst particle [10,11]. Tang et al. synthesized helical CNTs and helical carbon nanofibers (HCNFs) on Fe nanoparticles and performed systematic experiments to investigate the specific effect of
* Corresponding author. E-mail address: [email protected] (Y.-M. Liu).
catalyst particle size on the selective growth of CNCs [12]. In 2001, Wen et al. successfully used acetylene as the carbon source and NiePeCl composite catalysts to grow helical CNCs; they fabricated several CNC structures including solid and hollow coils [13]. Qin et al. used copper nanocrystals as a catalyst and acetylene as the source gas at a low temperature of 195 C [5]. According to the literature, CNCs were generally synthesized using thermal chemical vapor deposition (TCVD) with metal particles as catalysts, methane or acetylene as the carbon source, and argon as the carrier gas. The common metal catalysts used for growing CNCs include iron (Fe) [12], nickel (Ni) [13], cobalt (Co) [14], and copper (Cu) [5]. In recent years, precious-metal catalysts, such as silver (Ag) [15], gold (Au) [16], titanium (Ti) and palladium (Pd) [17], have also been used for growing CNCs. Chiu and coworkers used liquid-phase metallic K with Ag or Au as the catalyst to grow amorphous CNCs from acetylene on Si substrates; they proposed a vapor-liquidsolid-growth mechanism for explaining the CNC growth [15,16]. Nitze et al. used C60-supported Pd-catalyst particles at 550 C to grow HCNFs [17]. Using Pd nanoparticles, they could grow homogeneous CNCs with a highly periodic pitch and diameter.
http://dx.doi.org/10.1016/j.vacuum.2015.02.003 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003
2
W.-K. Huang et al. / Vacuum xxx (2015) 1e6
Although the traditional TCVD method is a simple and inexpensive process for producing high purity CNCs effectively, the CNC yield of this method is poor and unfavorable for industrialization. Baddour et al. have grown CNTs on stainless steel particles with CVD in a fluidized bed system (FBCVD) to increase the yield [18]. However, compared with the traditional TCVD methods and FBCVD, the spray pyrolysis method (SPCVD) has many advantages such as the possibility of a substrate-free, continuous reaction, and shorter heating or cooling time for the furnace [19e22]. This method can be easily applied for the mass-production of nanocarbonaceous materials and is therefore more favorable for industrialization. Aguilar-Elguezabal et al. successfully used spray pyrolysis of a ferrocene/benzene mixture to produce aligned, multiwalled carbon nanotubes (MWCNTs) [22]. Su et al. demonstrated a simple technique that involved dissolving ferrocene in alcohol and sprayed the solution into the reaction zone for the continuous production of high-purity, single-walled carbon nanotubes (SWCNTs) [20]. Zhang et al. used spray pyrolysis of ethanol to synthesize carbon nanofibers from carbon particles [23]. However, using the SPCVD method to produce pure CNCs has yet to be reported. In this study, we used poly(styrene-co-NIPAAm)/ Pd nanoparticles [24] as a catalyst to synthesize carbonaceous samples with the spray pyrolysis method in a vertical CVD reactor. The morphologies of the synthesized carbon products were observed and their yield was evaluated. 2. Experiments 2.1. Spray-liquid preparation We added a typical poly(styrene-co-NIPAAm)/Pd catalyst solution into ethanol and sonicated the mixture for 10 min to disperse the Pd catalysts homogeneously. The Pd nanoparticles then exhibited satisfactory dispersion in the ethanol without any surfactant.
Fig. 1. Schematic of our vertical spray pyrolysis CVD reactor.
20e30 nm, as shown in Fig. 2c, d. When the growth temperature was increased to 800 C, thinner CNTs with a diameter of approximately 15e20 nm were obtained, as shown in Fig. 2e, f. According to Baker's theory [25], carbon atoms decomposed from a carbon source gas diffuse into the catalyst particles and precipitate on the surface to form carbon fiber, due to the
2.2. Synthesis of carbonaceous samples by using spray pyrolysis chemical vapor deposition Fig. 1 shows the schematic of our SPCVD reactor, which comprises a spray nozzle, a vertical heating furnace, a quartz tube, and a powder collector. The Pd/C2H5OH-mixed liquid was pumped into the vertical CVD chamber with compressed argon gas, hydrogen, and acetylene, to grow carbonaceous products at a temperature of 600e800 C. The synthesis period was 1 h. The synthesized nanocarbonaceous products on the Pd catalysts were then observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Their graphite crystallinity was analyzed using Raman spectroscopy. 3. Results and discussion 3.1. Effect of growth temperature on the morphology of synthesized CNCs Fig. 2 shows the SEM and TEM images of the carbonaceous products synthesized on Pd nanoparticles at various temperatures (i.e., 600, 700, and 800 C). The gas flow rates of Ar, H2, and C2H2 are 5000, 100, and 25 sccm, respectively. The samples synthesized at 600 C is mainly CNCs (Fig. 2a), with very few straight CNFs, as indicated by the arrows. The CNCs grown at 600 C have a solid helical structure and their diameter is approximately 100e150 nm, as shown in Fig. 2a, b. By contrast, the curly CNTs grown at 700 C have a hollow structure and a diameter of approximately
Fig. 2. SEM and TEM images of the nano-carbon samples synthesized at different growth temperatures: (a) (b) 600 C, (c) (d) 700 C, and (e) (f) 800 C with vertical SPCVD reactor. The arrows in (a) indicate the few CNFs as the minority type of material.
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003
W.-K. Huang et al. / Vacuum xxx (2015) 1e6
temperature gradient in the catalyst particles. Because the decomposition of the carbon source and the diffusion of carbon atoms in the catalyst particles are thermally activated reactions, they obey the Arrhenius equation and are sensitive to the reaction temperature. Our results indicated that the acetylene decomposed more slowly at a lower temperature and the carbon diffused and deposited more slowly at 600 C to form helical CNCs mostly and a few straight CNFs; at a higher temperature (700 and 800 C), the acetylene decomposed quickly to provide a greater abundance of carbon atoms and a quicker diffusion and deposit rate to form curly and thin CNTs. The HR-TEM images in Fig. 3a and b show the microstructures of the CNTs and CNCs, respectively. Unlike the crystalline structure of CNTs, our CNCs showed a composite structure with some nano graphite crystallites (indicated with arrows and circles) embedded in the amorphous matrix. In recent years, several research teams, including us, have used CNCs as the cathode material to fabricate field emission lamp (FEL) devices [26,27]. The results of these previous studies have indicated
Fig. 3. The HR-TEM micrographs of the CNT (a) and CNC (b) structures in this work. The arrows and circles in (b) indicate the nano-graphite crystallites embedded in the amorphous matrix.
3
that CNC cathode materials can effectively increase the lighting uniformity and light-spot density of FELs. In this work, the CNTs and CNCs synthesized at 800 C and 600 C had high aspect ratios and low turn-on electric fields (as low as approximately 1.1 V/mm and 2.1 V/mm, respectively). The Raman spectroscopy analysis of the aforementioned carbonaceous samples is shown in Fig. 4. There are two main peaks in the figure: the peak at 1324 cm-1 is the D-band due to structural defects or disorders in the carbonaceous nanomaterials; the peak at 1580 cm-1 is the G-band, which is related to the graphite lattice. When the temperature increased from 600 C to 800 C, the ratio of ID/IG was increased from 0.63 to 0.67, implying that the graphite crystallinity of the CNTs is superior to that of the CNCs. 3.2. Relationship between CNC yield and acetylene flow rate Fig. 5a and b shows the morphologies of CNCs grown at the C2H2 flow rates of 50 and 100 sccm, respectively; the gas flow rates of Ar and H2 were kept at 5000 and 100 sccm, respectively. Compared with the CNCs shown in Fig. 2a, which were grown using 25 sccm C2H2, no apparent morphological difference was observed among these three samples, except that some straight CNFs were present in the 25 sccm C2H2 sample, as indicated by the arrows in Fig. 2a. The results indicated that a change in the concentration of the carbon source gas did not alter the fundamental morphology of the CNCs. However, the growth rate (mg/hr) of nanocarbonaceous materials increased when the C2H2 flow rate increased, as shown in Fig. 6. Also note that the fractions of CNCs in the synthesized carbonaceous materials with the C2H2 flow rates of 25, 50, and 100 sccm were 85, 94, and 96%, respectively. This fraction was calculated from the ratio of the total area of CNCs to that of all nanocarbonaceous materials on the SEM images. As mentioned in
Fig. 4. Raman spectra of the carbonaceous samples synthesized at different temperatures.
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003
4
W.-K. Huang et al. / Vacuum xxx (2015) 1e6
flow rates may be attributed to the lower quantity of decomposed carbon atoms at the 25 sccm flow rate, which was not sufficient for all of the Pd catalysts passing through the heating zones to react. When the flow rate of C2H2 increased to 50 and 100 sccm, more Pd catalysts could participate in the reaction with the abundant carbon atoms, resulting in an increased growth rate. Because the limit of our C2H2 controller in the TCVD system was 100 sccm, we could not identify the optimal value of the C2H2 flow rate. However, we expected the growth rate to increase with the C2H2 flow rate, according to the exponential function. The spray pyrolysis method has many advantages such as the possibility of a catalyst support-free, continuous reaction, and shorter heating or cooling time for the furnace [19e22]. We have compared the yield of our method to those using similar techniques, as shown Table 1. The “yield”, in compliance with the commonly adopted definition in other works, should be calculated from the following equation:
. mcatalyst 100%; yield ¼ mtotal mcatalyst
Fig. 5. The SEM images of the nano carbon samples synthesized at different C2H2 gas flow: (a) 50 sccm and (b) 100 sccm.
Sec.3.1, CNCs (coiled shape; diameter ~ 100e200 nm) can be easily distinguished from CNTs (straight; diameter ~ 20e30 nm) and CNFs (straight; diameter >100 nm) by the shape and diameter. Fig. 6 shows that the growth rate of CNCs increased from approximately 1 mg/h (25 sccm C2H2) to approximately 3.5 mg/h (50 sccm C2H2), and then to approximately 4.5 mg/h (100 sccm C2H2). The growth rate exhibited a larger increment when the C2H2 flow rate increased from 25 to 50 sccm. The reason for the larger increment of the growth rate between the 25 and the 50 sccm C2H2
Fig. 6. Growth rate vs. the flow rate of acetylene: (a) 25, (b) 50, and (c) 100 sccm.
where mtotal and mcatalyst are the total material mass (including catalysts) after CVD and the catalyst mass, respectively. In Table 1, Nitze et al. used C60-supported Pd-catalyst particles to grow HCNFs at 550 C [17]. Using Pd nanoparticles, they could grow homogeneous CNCs with a highly periodic pitch. However, their yield was limited to that allowed by a typical CVD method. Corrias et al. [28] produced CNTs by a fluidized bed method in a vertical CVD furnace. They used Fe/(Al2O3) as the catalyst particles while mesoporous alumina (Al2O3) particles were introduced as the initially fluidized powder. The carbon yield often exceeded 95% with close to 100% selective growth of nanotubes. Su et al. [20] used a simple technique that involved dissolving ferrocene in alcohol and sprayed the solution into the horizontal CVD reaction zone for the continuous production of high-purity SWCNTs. Their method offered a yield of nanocarbonaceous materials up to 50%. In contrast, the yield of our vertical CVD geometry with Pd catalysts and the spray pyrolysis method reached about 128%.
3.3. The special carbonaceous products synthesized in the vertical three-temperature-zone CVD chamber To enhance the CNC yield further, we set the temperatures of the three heating zones of the vertical CVD chamber to 700 C, 650 C and 600 C, from the top to the bottom, to synthesize carbonaceous samples. Compared with the isothermal process, this method can facilitate the pyrolysis of ethanol and acetylene to provide more decomposed carbon atoms in the top heating zones. Before Pd catalysts exit the heating zone (middle and bottom stage), the carbon atoms have more time to diffuse and precipitate to form CNCs in the Pd catalysts. In our research, using the new heating method achieved a CNC yield that was eightfold than that from using the traditional heating method. In addition, an interesting nanoscale carbon structure with a special morphology similar an octopus tentacle was found, as shown in Fig. 7a, b. The TEM observation shown in Fig. 7c, d reveals that this structure contains two parts; the inner part is a hollow structure that looks like the CNTs grown at 700 C, as shown in Fig. 3d; the outside part looks like amorphous carbon deposit, which surrounds the inner tube. This structure was rarely reported in the literature. Mishra et al. demonstrated that gas-flow fields, associated heat, and mass transfer were not homogeneously distributed during CNF-growth in a vertical CVD reactor [30]. They proposed a model for solving
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003
W.-K. Huang et al. / Vacuum xxx (2015) 1e6
5
Table 1 Comparison of catalyst, growth temperature (Tgrowth) and maximum yield (Yieldmax) of synthesis methods in this study and other works. Type of synthesis methods
Products
Catalyst
Catalyst support
Tgrowth
Yieldmax
Ref.
Typical horizontal CVD Spray pyrolysis (Horizontal CVD) Spray pyrolysis (Horizontal CVD) Spray pyrolysis (Horizontal CVD) Spray pyrolysis (Horizontal CVD) Fluidized bed reactor (Vertical CVD) Fluidized bed reactor (Vertical CVD) Spray pyrolysis (Vertical CVD)
CNCs MWCNTs SWCNTs vertically aligned MWCNTs MWCNTs MWCNTs SWCNTs CNCs
Pd CoeFe Ferrocene Ferrocene Ferrocene Fe CoeMo Pd
C60 silica gel none none none A12O3 MgO none
550 C 700 C 1000 C 750 C 900 C 650 C 900 C 600 C
e 33% about 50% e e exceeded 95% 92% 128%
[17] [19] [20] [21] [22] [28] [29] This work
this problem. It can be inferred that the gas-flow fields, associated heat, and mass transfer in the vertical CVD reactor were more complex than in the horizontal CVD reactor. We speculated that the appearance of this octopus-tentacle-like structure was due to the following mechanism: some straight CNTs were preformed at the 700 C zone in the chamber; when they fell into the lower zones with lower temperatures, the decomposed carbon atoms at 650 C or 600 C deposited on the surface of the preformed CNTs, thus forming this special octopus-tentacle-like structure. We have yet to determine the optimal application for this carbon structure. However, the hollow center and the rough and wavy wall surfaces of the octopus-tentacle-like structure can provide a specific surface area substantially larger than those of CNTs and CNCs, which suggests various potential applications such as hydrogen storage media, microwave absorbers, microsensors, and catalyst supports. 4. Conclusion In this work, we used a poly(styrene-co- NIPAAm)/pd catalysts and the SPCVD method with a vertical furnace to synthesize nanocarbonaceous materials. By adjusting the growth temperature, we could control the morphology and structure of the nanocarbon products. Our results indicated that a higher temperature (700 or 800 C) was more conducive to the formation of CNTs, while a lower temperature (600 C) was more favorable for the synthesis of CNCs. The growth rate of nanocarbonaceous materials increased with the increase of the flow rate of the carbon source gas. Moreover, separately setting the temperatures of both the vertical CVD
Fig. 7. The special structure of the octopus-tentacle-like carbonaceous material: (a), (b) the SEM images; (c), (d) the TEM images.
chamber and heating zones enabled synthesizing a new and distinctive octopus-tentacle-like carbonaceous material.
Acknowledgments This work was sponsored by the National Science Council Taiwan under Grant No. NSC 102-2221-E-606 -005 and by the Ministry of Economic Affairs, R.O.C., under Academic TDP No. 100EC-17-A-07-S1-167.
References [1] Furuya Y, Hashishin T, Iwanaga H, Motojima S, Hishikawa Y. Interaction of hydrogen with carbon coils at low temperature. Carbon 2004;42:331e5. [2] Zhao DL, Shen ZM. Preparation and microwave absorption properties of carbon nanocoils. Mater Lett 2008;62:3704e6. [3] Hokushin S, Pan L, Konishi Y, Tanaka H, Nakayama Y. Field emission properties and structural changes of a stand-alone carbon nanocoil. Jpn J Appl Phys 2007;46:20e4. [4] Su CC, Li CH, Chang NK, Gao F, Chang SH. Fabrication of high sensitivity carbon microcoil pressure sensors. Sensors 2012;12:10034e41. [5] Qin Y, Yu L, Wang Y, Li G, Cui Z. Amorphous helical carbon nanofibers synthesized at low temperature and their elasticity and processablity. Solid State Commun 2006;138:5e8. [6] Sanada K, Sanga H, Shindo Y. Effect of ultrasonication on the cryogenic tensile properties of carbon nanocoil/poly-dicyclopentadiene composites. Compos Part A-Appl S 2012;43:1895e900. [7] Rakhi RB, Chen W, Alshareef HN. Conducting polymer/carbon nanocoil composite electrodes for efficient supercapacitors. J Mater Chem 2012;22: 5177e83. [8] Rakhi RB, Cha D, Chen W, Alshareef HN. Electrochemical energy storage devices using electrodes incorporating carbon nanocoils and metal oxides nanoparticles. J Phys Chem 2011;115:14392e9. [9] Hu G, Sharifi T, Nitze F, Barzegar HR, Tai CW, Wagberg T. Phase-transfer synthesis of amorphous palladium nanoparticle-functionalized 3D helical carbon nanofibers and its highly catalytic performance towards hydrazine oxidation. Chem Phys Lett 2012;543:96e100. [10] Motojima S, Kawaguchi M, Nozaki K, Iwanaga H. Growth of regularly coiled carbon filaments by Ni catalyzed pyrolysis of acetylene, and their morphology and extension characteristics. Appl Phys Lett 1990;56:321. [11] Motojima S, Chen X, Yang S, Hasegawa M. Properties and potential applications of carbon microcoils/nanocoils. Diam Relat Mater 2004;13:1989e92. [12] Tang N, Wen J, Zhang Y, Liu F, Lin K, Du Y. Helical carbon nanotubes: catalytic particle size-dependent growth and magnetic properties. ACS Nano 2010;4: 241e50. [13] Wen Y, Shen Z. Synthesis of regular coiled carbon nanotubes by Ni-catalyzed pyrolysis of acetylene and a growth mechanism analysis. Carbon 2001;39: 2369e74. [14] Park KW, Sung YE, Han S, Yun Y, Hyeon T. Origin of the enhanced catalytic activity of carbon nanocoil-supported PtRu alloy electrocatalysts. J Phys Chem B 2004;108:939e44. [15] Tsou TY, Lee CY, Chiu HT. K and Au bicatalyst assisted growth of carbon nanocoils from acetylene: effect of deposition parameters on field emission properties. ACS Appl Mater Interfaces 2012;4:6505e11. [16] Liu WC, Lin HK, Chen YL, Lee CY, Chiu HT. Growth of carbon nanocoils from K and Ag cooperative bicatalyst assisted thermal decomposition of acetylene. ACS Nano 2010;4:4149e57. [17] Nitze F, Abou-Hamad E, Wagberg T. Carbon nanotubes and helical carbon nanofibers grown by chemical vapour deposition on C60 fullerene supported Pd nanoparticles. Carbon 2011;49:1101e7. [18] Baddour CE, Upham DC, Meunier JL. Direct and repetitive growth cycles of carbon nanotubes on stainless steel particles by chemical vapor deposition in a fluidized bed. Carbon 2010:2644e73.
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003
6
W.-K. Huang et al. / Vacuum xxx (2015) 1e6
[19] Afre RA, Soga T, Jimbo T, Kumar M, Ando Y, Sharon M, et al. Carbon nanotubes by spray pyrolysis of turpentine oil at different temperatures and their studies. Micropor Mesopor Mater 2006;96:184e90. [20] Su LF, Wang JN, Yu F, Sheng ZM, Chang H, Pak C. Continuous production of single-wall carbon nanotubes by spray pyrolysis of alcohol with dissolved ferrocene. Chem Phys Lett 2006;420:421e5. [21] Ionescu MI, Zhahg Y, Li R, Sun X, Abou-Rachid H. Hydrogen-free spray pyrolysis chemical vapor deposition method for the carbon nanotube growth: parametric studies. Appl Surf Sci 2011;257:6843e9. [22] Aguilar-Elguezabal A, Antunez W, Alonso G, Delgado FP, Espinosa F, MikiYoshida M. Study of carbon nanotubes synthesis by spray pyrolysis and model of growth. Diam Relat Mater 2006;15:1329e35. [23] Zhang J, Khatri I, Kishi N, Soga T, Jimbo T. Synthesis of carbon nanofibers from carbon particles by ultrasonic spray pyrolysis of ethanol. IEICE T Electron 2009;E92-C(12):1432e7. [24] Chen WD, Sung Y, Chang CP, Chen YC, Ger MD. The preparation of thermoresponsive palladium catalyst with high activity for electroless nickel deposition. Surf Coat Tech 2010;204:2130e5.
[25] Baker RTK, Barber MA, Harris PS, Feates FS, Waite RJ. Nucleation and growth of carbon deposits from the nickel catalyzed decomposition of acetylene. J Catal 1972;26:51e62. [26] Chou YP, Liu YM, Ger MD, Chung KJ, Hou KS, Pu NW. Bulb-shaped field emission lamps using carbon nano-coil cathodes. J Nanosci Nanotechnol 2012;12:8316e22. [27] Zhang Z, He P, Sun Z, Feng T, Chen Y, Li H, et al. Growth and field emission property of coiled carbon nanostructure using copper as catalyst. Appl Surf Sci 2010;256:4417e22. [28] Corrias M, Caussat B, Ayral A, Durand J, Kihn Y, Kalck P, et al. Carbon nanotubes produced by fluidized bed catalytic CVD: first approach of the process. Chem Eng Sci 2003;58:4475e82. [29] Rashidi A, Lotfi R, Fakhrmosavi E, Zare M. Production of single-walled carbon nanotubes from methane over Co-Mo/MgO nanocatalyst: a comparative study of fixed and fluidized bed reactors. J Nat Gas Chem 2011;20:372e6. [30] Mishra P, Verma N. A CFD study on a vertical chemical vapor deposition reactor for growing carbon nanofibers. Chem Eng Res Des 2012;90:2293e301.
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Carbon nanomaterials synthesized using a spray pyrolysis method Wen-Ko Huang a, Kun-Ju Chung a, Yih-Ming Liu b, *, Ming-Der Ger b, Nen-Wen Pu c, Meng-Jey Youh d a
School of Defense Science, Chung Cheng Institute of Technology, National Defense University, 335 Taiwan, ROC Department of Chemical & Materials Science, Chung Cheng Institute of Technology, National Defense University, 335 Taiwan, ROC c Department of Photonic Engineering, Yuan Ze University, 335 Taiwan, ROC d Department of Information Technology, Hsing Wu University, 335 Taiwan, ROC b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2014 Received in revised form 1 February 2015 Accepted 4 February 2015 Available online xxx
Carbon nanomaterials were synthesized using a spray pyrolysis chemical vapor deposition system (SPCVD). By varying the reaction temperature or flow rate of the carbon source, the structure or morphology of the synthesized carbon nanocoils (CNCs) can be controlled. A vertical chemical vapor deposition (CVD) furnace with a three-stage heating zone was employed to synthesize carbonaceous nanomaterials using nano-Pd catalysts at growth temperatures of 600, 700, and 800 C. The morphology of the synthesized carbon products and the relationship between the carbon source concentration and its yield were evaluated. The results showed that CNCs formed at a lower temperature (600 C), whereas straight carbon nanotubes were obtained at a higher temperature (700 or 800 C). The structure and morphology of the carbonaceous samples were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Their graphite crystallinity was analyzed using Raman spectroscopy. When the three heating zones of the vertical CVD chamber were set to different temperatures, a unique nano-carbonaceous material with a special morphology similar to an octopus tentacle was formed. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon nanomaterials Spray pyrolysis Carbon nanocoil (CNC) Octopus-tentacle structure
1. Introduction Carbon nanocoils (CNCs) and straight carbon nanotubes (CNTs) have attracted considerable attention in recent years because of their unique mechanical and electrical properties [1e9]. Due to their special helical structure, CNCs have the potential for applications in different fields such as hydrogen storage media [1], microwave absorbers [2], field emitters [3], microsensors [4], elastic materials [5], composite materials [6], electrode materials [7], and catalyst supports [8,9]. CNCs were first synthesized by Motojima and coworkers [10]. Using a nickel substrate or nickel powder as a catalyst, they synthesized coiled carbon microfibers [11] and suggested a growth mechanism based on the anisotropic extrusion of carbon over a catalyst particle [10,11]. Tang et al. synthesized helical CNTs and helical carbon nanofibers (HCNFs) on Fe nanoparticles and performed systematic experiments to investigate the specific effect of
* Corresponding author. E-mail address: [email protected] (Y.-M. Liu).
catalyst particle size on the selective growth of CNCs [12]. In 2001, Wen et al. successfully used acetylene as the carbon source and NiePeCl composite catalysts to grow helical CNCs; they fabricated several CNC structures including solid and hollow coils [13]. Qin et al. used copper nanocrystals as a catalyst and acetylene as the source gas at a low temperature of 195 C [5]. According to the literature, CNCs were generally synthesized using thermal chemical vapor deposition (TCVD) with metal particles as catalysts, methane or acetylene as the carbon source, and argon as the carrier gas. The common metal catalysts used for growing CNCs include iron (Fe) [12], nickel (Ni) [13], cobalt (Co) [14], and copper (Cu) [5]. In recent years, precious-metal catalysts, such as silver (Ag) [15], gold (Au) [16], titanium (Ti) and palladium (Pd) [17], have also been used for growing CNCs. Chiu and coworkers used liquid-phase metallic K with Ag or Au as the catalyst to grow amorphous CNCs from acetylene on Si substrates; they proposed a vapor-liquidsolid-growth mechanism for explaining the CNC growth [15,16]. Nitze et al. used C60-supported Pd-catalyst particles at 550 C to grow HCNFs [17]. Using Pd nanoparticles, they could grow homogeneous CNCs with a highly periodic pitch and diameter.
http://dx.doi.org/10.1016/j.vacuum.2015.02.003 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003
2
W.-K. Huang et al. / Vacuum xxx (2015) 1e6
Although the traditional TCVD method is a simple and inexpensive process for producing high purity CNCs effectively, the CNC yield of this method is poor and unfavorable for industrialization. Baddour et al. have grown CNTs on stainless steel particles with CVD in a fluidized bed system (FBCVD) to increase the yield [18]. However, compared with the traditional TCVD methods and FBCVD, the spray pyrolysis method (SPCVD) has many advantages such as the possibility of a substrate-free, continuous reaction, and shorter heating or cooling time for the furnace [19e22]. This method can be easily applied for the mass-production of nanocarbonaceous materials and is therefore more favorable for industrialization. Aguilar-Elguezabal et al. successfully used spray pyrolysis of a ferrocene/benzene mixture to produce aligned, multiwalled carbon nanotubes (MWCNTs) [22]. Su et al. demonstrated a simple technique that involved dissolving ferrocene in alcohol and sprayed the solution into the reaction zone for the continuous production of high-purity, single-walled carbon nanotubes (SWCNTs) [20]. Zhang et al. used spray pyrolysis of ethanol to synthesize carbon nanofibers from carbon particles [23]. However, using the SPCVD method to produce pure CNCs has yet to be reported. In this study, we used poly(styrene-co-NIPAAm)/ Pd nanoparticles [24] as a catalyst to synthesize carbonaceous samples with the spray pyrolysis method in a vertical CVD reactor. The morphologies of the synthesized carbon products were observed and their yield was evaluated. 2. Experiments 2.1. Spray-liquid preparation We added a typical poly(styrene-co-NIPAAm)/Pd catalyst solution into ethanol and sonicated the mixture for 10 min to disperse the Pd catalysts homogeneously. The Pd nanoparticles then exhibited satisfactory dispersion in the ethanol without any surfactant.
Fig. 1. Schematic of our vertical spray pyrolysis CVD reactor.
20e30 nm, as shown in Fig. 2c, d. When the growth temperature was increased to 800 C, thinner CNTs with a diameter of approximately 15e20 nm were obtained, as shown in Fig. 2e, f. According to Baker's theory [25], carbon atoms decomposed from a carbon source gas diffuse into the catalyst particles and precipitate on the surface to form carbon fiber, due to the
2.2. Synthesis of carbonaceous samples by using spray pyrolysis chemical vapor deposition Fig. 1 shows the schematic of our SPCVD reactor, which comprises a spray nozzle, a vertical heating furnace, a quartz tube, and a powder collector. The Pd/C2H5OH-mixed liquid was pumped into the vertical CVD chamber with compressed argon gas, hydrogen, and acetylene, to grow carbonaceous products at a temperature of 600e800 C. The synthesis period was 1 h. The synthesized nanocarbonaceous products on the Pd catalysts were then observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Their graphite crystallinity was analyzed using Raman spectroscopy. 3. Results and discussion 3.1. Effect of growth temperature on the morphology of synthesized CNCs Fig. 2 shows the SEM and TEM images of the carbonaceous products synthesized on Pd nanoparticles at various temperatures (i.e., 600, 700, and 800 C). The gas flow rates of Ar, H2, and C2H2 are 5000, 100, and 25 sccm, respectively. The samples synthesized at 600 C is mainly CNCs (Fig. 2a), with very few straight CNFs, as indicated by the arrows. The CNCs grown at 600 C have a solid helical structure and their diameter is approximately 100e150 nm, as shown in Fig. 2a, b. By contrast, the curly CNTs grown at 700 C have a hollow structure and a diameter of approximately
Fig. 2. SEM and TEM images of the nano-carbon samples synthesized at different growth temperatures: (a) (b) 600 C, (c) (d) 700 C, and (e) (f) 800 C with vertical SPCVD reactor. The arrows in (a) indicate the few CNFs as the minority type of material.
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003
W.-K. Huang et al. / Vacuum xxx (2015) 1e6
temperature gradient in the catalyst particles. Because the decomposition of the carbon source and the diffusion of carbon atoms in the catalyst particles are thermally activated reactions, they obey the Arrhenius equation and are sensitive to the reaction temperature. Our results indicated that the acetylene decomposed more slowly at a lower temperature and the carbon diffused and deposited more slowly at 600 C to form helical CNCs mostly and a few straight CNFs; at a higher temperature (700 and 800 C), the acetylene decomposed quickly to provide a greater abundance of carbon atoms and a quicker diffusion and deposit rate to form curly and thin CNTs. The HR-TEM images in Fig. 3a and b show the microstructures of the CNTs and CNCs, respectively. Unlike the crystalline structure of CNTs, our CNCs showed a composite structure with some nano graphite crystallites (indicated with arrows and circles) embedded in the amorphous matrix. In recent years, several research teams, including us, have used CNCs as the cathode material to fabricate field emission lamp (FEL) devices [26,27]. The results of these previous studies have indicated
Fig. 3. The HR-TEM micrographs of the CNT (a) and CNC (b) structures in this work. The arrows and circles in (b) indicate the nano-graphite crystallites embedded in the amorphous matrix.
3
that CNC cathode materials can effectively increase the lighting uniformity and light-spot density of FELs. In this work, the CNTs and CNCs synthesized at 800 C and 600 C had high aspect ratios and low turn-on electric fields (as low as approximately 1.1 V/mm and 2.1 V/mm, respectively). The Raman spectroscopy analysis of the aforementioned carbonaceous samples is shown in Fig. 4. There are two main peaks in the figure: the peak at 1324 cm-1 is the D-band due to structural defects or disorders in the carbonaceous nanomaterials; the peak at 1580 cm-1 is the G-band, which is related to the graphite lattice. When the temperature increased from 600 C to 800 C, the ratio of ID/IG was increased from 0.63 to 0.67, implying that the graphite crystallinity of the CNTs is superior to that of the CNCs. 3.2. Relationship between CNC yield and acetylene flow rate Fig. 5a and b shows the morphologies of CNCs grown at the C2H2 flow rates of 50 and 100 sccm, respectively; the gas flow rates of Ar and H2 were kept at 5000 and 100 sccm, respectively. Compared with the CNCs shown in Fig. 2a, which were grown using 25 sccm C2H2, no apparent morphological difference was observed among these three samples, except that some straight CNFs were present in the 25 sccm C2H2 sample, as indicated by the arrows in Fig. 2a. The results indicated that a change in the concentration of the carbon source gas did not alter the fundamental morphology of the CNCs. However, the growth rate (mg/hr) of nanocarbonaceous materials increased when the C2H2 flow rate increased, as shown in Fig. 6. Also note that the fractions of CNCs in the synthesized carbonaceous materials with the C2H2 flow rates of 25, 50, and 100 sccm were 85, 94, and 96%, respectively. This fraction was calculated from the ratio of the total area of CNCs to that of all nanocarbonaceous materials on the SEM images. As mentioned in
Fig. 4. Raman spectra of the carbonaceous samples synthesized at different temperatures.
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003
4
W.-K. Huang et al. / Vacuum xxx (2015) 1e6
flow rates may be attributed to the lower quantity of decomposed carbon atoms at the 25 sccm flow rate, which was not sufficient for all of the Pd catalysts passing through the heating zones to react. When the flow rate of C2H2 increased to 50 and 100 sccm, more Pd catalysts could participate in the reaction with the abundant carbon atoms, resulting in an increased growth rate. Because the limit of our C2H2 controller in the TCVD system was 100 sccm, we could not identify the optimal value of the C2H2 flow rate. However, we expected the growth rate to increase with the C2H2 flow rate, according to the exponential function. The spray pyrolysis method has many advantages such as the possibility of a catalyst support-free, continuous reaction, and shorter heating or cooling time for the furnace [19e22]. We have compared the yield of our method to those using similar techniques, as shown Table 1. The “yield”, in compliance with the commonly adopted definition in other works, should be calculated from the following equation:
. mcatalyst 100%; yield ¼ mtotal mcatalyst
Fig. 5. The SEM images of the nano carbon samples synthesized at different C2H2 gas flow: (a) 50 sccm and (b) 100 sccm.
Sec.3.1, CNCs (coiled shape; diameter ~ 100e200 nm) can be easily distinguished from CNTs (straight; diameter ~ 20e30 nm) and CNFs (straight; diameter >100 nm) by the shape and diameter. Fig. 6 shows that the growth rate of CNCs increased from approximately 1 mg/h (25 sccm C2H2) to approximately 3.5 mg/h (50 sccm C2H2), and then to approximately 4.5 mg/h (100 sccm C2H2). The growth rate exhibited a larger increment when the C2H2 flow rate increased from 25 to 50 sccm. The reason for the larger increment of the growth rate between the 25 and the 50 sccm C2H2
Fig. 6. Growth rate vs. the flow rate of acetylene: (a) 25, (b) 50, and (c) 100 sccm.
where mtotal and mcatalyst are the total material mass (including catalysts) after CVD and the catalyst mass, respectively. In Table 1, Nitze et al. used C60-supported Pd-catalyst particles to grow HCNFs at 550 C [17]. Using Pd nanoparticles, they could grow homogeneous CNCs with a highly periodic pitch. However, their yield was limited to that allowed by a typical CVD method. Corrias et al. [28] produced CNTs by a fluidized bed method in a vertical CVD furnace. They used Fe/(Al2O3) as the catalyst particles while mesoporous alumina (Al2O3) particles were introduced as the initially fluidized powder. The carbon yield often exceeded 95% with close to 100% selective growth of nanotubes. Su et al. [20] used a simple technique that involved dissolving ferrocene in alcohol and sprayed the solution into the horizontal CVD reaction zone for the continuous production of high-purity SWCNTs. Their method offered a yield of nanocarbonaceous materials up to 50%. In contrast, the yield of our vertical CVD geometry with Pd catalysts and the spray pyrolysis method reached about 128%.
3.3. The special carbonaceous products synthesized in the vertical three-temperature-zone CVD chamber To enhance the CNC yield further, we set the temperatures of the three heating zones of the vertical CVD chamber to 700 C, 650 C and 600 C, from the top to the bottom, to synthesize carbonaceous samples. Compared with the isothermal process, this method can facilitate the pyrolysis of ethanol and acetylene to provide more decomposed carbon atoms in the top heating zones. Before Pd catalysts exit the heating zone (middle and bottom stage), the carbon atoms have more time to diffuse and precipitate to form CNCs in the Pd catalysts. In our research, using the new heating method achieved a CNC yield that was eightfold than that from using the traditional heating method. In addition, an interesting nanoscale carbon structure with a special morphology similar an octopus tentacle was found, as shown in Fig. 7a, b. The TEM observation shown in Fig. 7c, d reveals that this structure contains two parts; the inner part is a hollow structure that looks like the CNTs grown at 700 C, as shown in Fig. 3d; the outside part looks like amorphous carbon deposit, which surrounds the inner tube. This structure was rarely reported in the literature. Mishra et al. demonstrated that gas-flow fields, associated heat, and mass transfer were not homogeneously distributed during CNF-growth in a vertical CVD reactor [30]. They proposed a model for solving
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003
W.-K. Huang et al. / Vacuum xxx (2015) 1e6
5
Table 1 Comparison of catalyst, growth temperature (Tgrowth) and maximum yield (Yieldmax) of synthesis methods in this study and other works. Type of synthesis methods
Products
Catalyst
Catalyst support
Tgrowth
Yieldmax
Ref.
Typical horizontal CVD Spray pyrolysis (Horizontal CVD) Spray pyrolysis (Horizontal CVD) Spray pyrolysis (Horizontal CVD) Spray pyrolysis (Horizontal CVD) Fluidized bed reactor (Vertical CVD) Fluidized bed reactor (Vertical CVD) Spray pyrolysis (Vertical CVD)
CNCs MWCNTs SWCNTs vertically aligned MWCNTs MWCNTs MWCNTs SWCNTs CNCs
Pd CoeFe Ferrocene Ferrocene Ferrocene Fe CoeMo Pd
C60 silica gel none none none A12O3 MgO none
550 C 700 C 1000 C 750 C 900 C 650 C 900 C 600 C
e 33% about 50% e e exceeded 95% 92% 128%
[17] [19] [20] [21] [22] [28] [29] This work
this problem. It can be inferred that the gas-flow fields, associated heat, and mass transfer in the vertical CVD reactor were more complex than in the horizontal CVD reactor. We speculated that the appearance of this octopus-tentacle-like structure was due to the following mechanism: some straight CNTs were preformed at the 700 C zone in the chamber; when they fell into the lower zones with lower temperatures, the decomposed carbon atoms at 650 C or 600 C deposited on the surface of the preformed CNTs, thus forming this special octopus-tentacle-like structure. We have yet to determine the optimal application for this carbon structure. However, the hollow center and the rough and wavy wall surfaces of the octopus-tentacle-like structure can provide a specific surface area substantially larger than those of CNTs and CNCs, which suggests various potential applications such as hydrogen storage media, microwave absorbers, microsensors, and catalyst supports. 4. Conclusion In this work, we used a poly(styrene-co- NIPAAm)/pd catalysts and the SPCVD method with a vertical furnace to synthesize nanocarbonaceous materials. By adjusting the growth temperature, we could control the morphology and structure of the nanocarbon products. Our results indicated that a higher temperature (700 or 800 C) was more conducive to the formation of CNTs, while a lower temperature (600 C) was more favorable for the synthesis of CNCs. The growth rate of nanocarbonaceous materials increased with the increase of the flow rate of the carbon source gas. Moreover, separately setting the temperatures of both the vertical CVD
Fig. 7. The special structure of the octopus-tentacle-like carbonaceous material: (a), (b) the SEM images; (c), (d) the TEM images.
chamber and heating zones enabled synthesizing a new and distinctive octopus-tentacle-like carbonaceous material.
Acknowledgments This work was sponsored by the National Science Council Taiwan under Grant No. NSC 102-2221-E-606 -005 and by the Ministry of Economic Affairs, R.O.C., under Academic TDP No. 100EC-17-A-07-S1-167.
References [1] Furuya Y, Hashishin T, Iwanaga H, Motojima S, Hishikawa Y. Interaction of hydrogen with carbon coils at low temperature. Carbon 2004;42:331e5. [2] Zhao DL, Shen ZM. Preparation and microwave absorption properties of carbon nanocoils. Mater Lett 2008;62:3704e6. [3] Hokushin S, Pan L, Konishi Y, Tanaka H, Nakayama Y. Field emission properties and structural changes of a stand-alone carbon nanocoil. Jpn J Appl Phys 2007;46:20e4. [4] Su CC, Li CH, Chang NK, Gao F, Chang SH. Fabrication of high sensitivity carbon microcoil pressure sensors. Sensors 2012;12:10034e41. [5] Qin Y, Yu L, Wang Y, Li G, Cui Z. Amorphous helical carbon nanofibers synthesized at low temperature and their elasticity and processablity. Solid State Commun 2006;138:5e8. [6] Sanada K, Sanga H, Shindo Y. Effect of ultrasonication on the cryogenic tensile properties of carbon nanocoil/poly-dicyclopentadiene composites. Compos Part A-Appl S 2012;43:1895e900. [7] Rakhi RB, Chen W, Alshareef HN. Conducting polymer/carbon nanocoil composite electrodes for efficient supercapacitors. J Mater Chem 2012;22: 5177e83. [8] Rakhi RB, Cha D, Chen W, Alshareef HN. Electrochemical energy storage devices using electrodes incorporating carbon nanocoils and metal oxides nanoparticles. J Phys Chem 2011;115:14392e9. [9] Hu G, Sharifi T, Nitze F, Barzegar HR, Tai CW, Wagberg T. Phase-transfer synthesis of amorphous palladium nanoparticle-functionalized 3D helical carbon nanofibers and its highly catalytic performance towards hydrazine oxidation. Chem Phys Lett 2012;543:96e100. [10] Motojima S, Kawaguchi M, Nozaki K, Iwanaga H. Growth of regularly coiled carbon filaments by Ni catalyzed pyrolysis of acetylene, and their morphology and extension characteristics. Appl Phys Lett 1990;56:321. [11] Motojima S, Chen X, Yang S, Hasegawa M. Properties and potential applications of carbon microcoils/nanocoils. Diam Relat Mater 2004;13:1989e92. [12] Tang N, Wen J, Zhang Y, Liu F, Lin K, Du Y. Helical carbon nanotubes: catalytic particle size-dependent growth and magnetic properties. ACS Nano 2010;4: 241e50. [13] Wen Y, Shen Z. Synthesis of regular coiled carbon nanotubes by Ni-catalyzed pyrolysis of acetylene and a growth mechanism analysis. Carbon 2001;39: 2369e74. [14] Park KW, Sung YE, Han S, Yun Y, Hyeon T. Origin of the enhanced catalytic activity of carbon nanocoil-supported PtRu alloy electrocatalysts. J Phys Chem B 2004;108:939e44. [15] Tsou TY, Lee CY, Chiu HT. K and Au bicatalyst assisted growth of carbon nanocoils from acetylene: effect of deposition parameters on field emission properties. ACS Appl Mater Interfaces 2012;4:6505e11. [16] Liu WC, Lin HK, Chen YL, Lee CY, Chiu HT. Growth of carbon nanocoils from K and Ag cooperative bicatalyst assisted thermal decomposition of acetylene. ACS Nano 2010;4:4149e57. [17] Nitze F, Abou-Hamad E, Wagberg T. Carbon nanotubes and helical carbon nanofibers grown by chemical vapour deposition on C60 fullerene supported Pd nanoparticles. Carbon 2011;49:1101e7. [18] Baddour CE, Upham DC, Meunier JL. Direct and repetitive growth cycles of carbon nanotubes on stainless steel particles by chemical vapor deposition in a fluidized bed. Carbon 2010:2644e73.
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003
6
W.-K. Huang et al. / Vacuum xxx (2015) 1e6
[19] Afre RA, Soga T, Jimbo T, Kumar M, Ando Y, Sharon M, et al. Carbon nanotubes by spray pyrolysis of turpentine oil at different temperatures and their studies. Micropor Mesopor Mater 2006;96:184e90. [20] Su LF, Wang JN, Yu F, Sheng ZM, Chang H, Pak C. Continuous production of single-wall carbon nanotubes by spray pyrolysis of alcohol with dissolved ferrocene. Chem Phys Lett 2006;420:421e5. [21] Ionescu MI, Zhahg Y, Li R, Sun X, Abou-Rachid H. Hydrogen-free spray pyrolysis chemical vapor deposition method for the carbon nanotube growth: parametric studies. Appl Surf Sci 2011;257:6843e9. [22] Aguilar-Elguezabal A, Antunez W, Alonso G, Delgado FP, Espinosa F, MikiYoshida M. Study of carbon nanotubes synthesis by spray pyrolysis and model of growth. Diam Relat Mater 2006;15:1329e35. [23] Zhang J, Khatri I, Kishi N, Soga T, Jimbo T. Synthesis of carbon nanofibers from carbon particles by ultrasonic spray pyrolysis of ethanol. IEICE T Electron 2009;E92-C(12):1432e7. [24] Chen WD, Sung Y, Chang CP, Chen YC, Ger MD. The preparation of thermoresponsive palladium catalyst with high activity for electroless nickel deposition. Surf Coat Tech 2010;204:2130e5.
[25] Baker RTK, Barber MA, Harris PS, Feates FS, Waite RJ. Nucleation and growth of carbon deposits from the nickel catalyzed decomposition of acetylene. J Catal 1972;26:51e62. [26] Chou YP, Liu YM, Ger MD, Chung KJ, Hou KS, Pu NW. Bulb-shaped field emission lamps using carbon nano-coil cathodes. J Nanosci Nanotechnol 2012;12:8316e22. [27] Zhang Z, He P, Sun Z, Feng T, Chen Y, Li H, et al. Growth and field emission property of coiled carbon nanostructure using copper as catalyst. Appl Surf Sci 2010;256:4417e22. [28] Corrias M, Caussat B, Ayral A, Durand J, Kihn Y, Kalck P, et al. Carbon nanotubes produced by fluidized bed catalytic CVD: first approach of the process. Chem Eng Sci 2003;58:4475e82. [29] Rashidi A, Lotfi R, Fakhrmosavi E, Zare M. Production of single-walled carbon nanotubes from methane over Co-Mo/MgO nanocatalyst: a comparative study of fixed and fluidized bed reactors. J Nat Gas Chem 2011;20:372e6. [30] Mishra P, Verma N. A CFD study on a vertical chemical vapor deposition reactor for growing carbon nanofibers. Chem Eng Res Des 2012;90:2293e301.
Please cite this article in press as: Huang W-K, et al., Carbon nanomaterials synthesized using a spray pyrolysis method, Vacuum (2015), http:// dx.doi.org/10.1016/j.vacuum.2015.02.003