Parametric Study of the Synthesis of Carbon Nanotubes by Spray

Research Article pubs.acs.org/journal/ascecg

Parametric Study of the Synthesis of Carbon Nanotubes by Spray Pyrolysis of a Biorenewable Feedstock: α‑Pinene J. Lara-Romero,*,† T. Ocampo-Macias,† R. Martínez-Suarez,† R. Rangel-Segura,† J. López-Tinoco,‡ F. Paraguay-Delgado,§ G. Alonso-Nuñez,∥ S. Jiménez-Sandoval,⊥ and F. Chiñas-Castillo# †

Facultad de Ingeniería Química, Universidad Michoacana de San Nicolás de Hidalgo, Avenida Francisco J. Múgica S/N, Edificio V1, Ciudad Universitaria, C.P. 58060, Morelia, Michoacán, México ‡ Departamento de Química y Bioquímica, Instituto Tecnológico de Lázaro Cárdenas, Avenida Melchor Ocampo No. 2555, C.P. 60950, Lázaro Cárdenas, Michoacán, México § Centro de Investigación en Materiales Avanzados, Unidad Chihuahua, Miguel de Cervantes Saavedra No. 120, C.P. 31136, Chihuahua, México ∥ Centro de Nanociencia y Nanotecnología, Universidad Nacional Autónoma de México, Km 107 Carretera Tijuana-Ensenada, C.P. 22800, Ensenada, Baja California Norte, México ⊥ Centro de Investigación y de Estudios Avanzados del IPN, Unidad Querétaro, Libramiento Norponiente No. 2000, C.P. 79230, Querétaro, México # Facultad de Ingeniería Mecánica, Instituto Tecnológico de Oaxaca, Avenida Ing. Víctor Bravo Ahuja No. 125, C.P. 68030, Oaxaca, México S Supporting Information *

ABSTRACT: We present a parametric study on the growth of carbon nanotubes (CNTs) by spray pyrolysis of a biorenewable feedstock obtained from living pine trees, namely α-pinene. The analyzed variables were the type of catalyst, catalyst concentration, flow of reactants, and reaction time; all at a fixed temperature of 800 °C. The evaluation and optimization of these parameters was performed based on the yield and crystallinity of produced CNTs which were monitored by Raman spectroscopy, thermogravimetric analysis, X-ray diffraction, and high resolution transmission electron microcopy. Ferrocene as catalyst produced highly crystalline multiwalled CNTs while Co- and Fe-phtalocyanine produced nitrogendoped CNTs. A ferrocene concentration of 37 mg/mL and 5000 sccm flow were the optimal conditions to obtain the highest yield of crystalline CNTs. The variation of time produced crystalline CNTs with different lengths without modifying their crystallinity. The growth kinetics of MWCNTs follows a Deal− Grove model which indicates that the growth is diffusion-limited and suggests that the root growth mechanism controls the growth process. KEYWORDS: Carbon nanotubes, Spray pyrolysis, α-Pinene, Raman spectroscopy, Growth kinetics

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reaction parameters such as temperature, flow, and reaction time are critical for the production of well-crystallized CNTs. In this direction, many reports considering the effect of different parameters on the synthesis of carbon nanotubes using different reacting systems can be found in the literature.13−18 Several carbon species such as methane, acetylene, benzene, xylene, and toluene have been used as a feedstock to synthesize CNTs. These precursors are obtained from fossil fuel and are projected to diminish in the future making it necessary to look

INTRODUCTION

Carbon nanotubes (CNTs) play an important role in the development of nanomaterials due to their unique electrical, mechanical, and optical properties.1 They are currently being used in various and diverse applications such as catalysts,2−4 sensors,5,6 fuel cells,7−9 lubricant additives,10,11 etc. Because of their enormous application potential, the production of large quantities of quality CNTs at low costs is highly desirable which must be achieved by controlling different parameters for a given synthesis method. Low cost methods such as chemical vapor deposition and spray pyrolysis offer excellent production yield with controlled features of the CNTs produced.12 The combination of carbon source, the type of catalyst and the © 2017 American Chemical Society

Received: December 14, 2016 Revised: March 22, 2017 Published: March 30, 2017 3890

DOI: 10.1021/acssuschemeng.6b03054 ACS Sustainable Chem. Eng. 2017, 5, 3890−3896

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ACS Sustainable Chemistry & Engineering for alternative precursors.19 Botanical feedstocks such as eucalyptus oil,20 camphor oil,21 palm oil,22,23 neem oil,19 castor oil,24 coconut oil, sesame oil, olive oil, and corn oil25 have been proposed in the literature to be good candidates to grow CNTs. The purity of the CNTs obtained from these precursors is approximately 80%. Turpentine oil is a natural mixture of terpenes obtained from the distillation of gum rosin extracted from living pine trees which has also been used to synthesize carbon nanostructures.26−29 We have previously shown that αpinene is the active component of turpentine responsible for producing high yields of highly crystalline multiwalled carbon nanotubes (MWCNTs) at 800 °C.30−32 In this work we present a parametric study on the effect of the type of catalyst, catalyst concentration, flow, and time in the production of MWCNTs using α-pinene as carbon source in a spray pyrolysis system.

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EXPERIMENTAL SECTION

Materials. α-Pinene (Aldrich, 98%) was used as carbon source. Ferrocene (Aldrich, 98%), iron(II) phtalocyanine (Aldrich, 90%), and cobalt(II) phtalocyanine (Aldrich, 97%) were used as catalysts. Synthesis. The spray pyrolysis process used in this work has been described previously.33 Briefly, a vycor tube was attached to a pneumatic device used as a solution atomizer. The overall tube dimensions were 0.9 cm of internal diameter and 23 cm of length. A cylindrical furnace (Thermolyne 1200) equipped with a high precision temperature controller (±1 K) was used to achieve the desired reaction temperature. The MWCNTs films produced during each reaction at the inner surface of the vycor tubing were removed and properly stored for characterization. Characterization Methods. Each MWCNTs sample was analyzed by high resolution transmission electron microscopy (HR-TEM), Raman spectroscopy, thermogravimetric analysis (TGA), and X-ray diffraction (XRD). HR-TEM micrographs were obtained by a Philips CM-200 analytical (TEM) operating at 200 kV. The MWCNTs specimens were prepared by dispersing them in acetone and sonication for 2 min. A drop of the suspension was placed on a perforated carbon coated Cu grid and allowed to dry. Raman spectroscopy was performed using a Labram system model Dilor micro-Raman equipped with a 20 mW He−Ne laser emitting at 632.8 nm and a holographic notch filter made by Kaiser Optical Systems, Inc. (model supertNotchPlus) with a 256 × 1024-pixel charge-coupled device (CCD) used as the detector. All measurements were carried out at room temperature under no special sample preparation. Thermogravimetric analyses were performed on a PerkinElmer Pyris 1 TGA equipment. Samples were analyzed at a heating rate of 1 °C/min to 800 °C in an oxygen atmosphere flowing at 75 mL/min. The sample weight ranged from 5 to 10 mg, and no preparation was required. XRD analyses were carried out using a Philips XPert MPD diffractometer equipped with a curved graphite diffracted beam monochromator using Cu Kα radiation (λ = 1.54184 Å) at 43 kV and 30 mA.

Figure 1. HR-TEM images and their corresponding fast Fourier transforms (FFTs) of MWCNTs produced with different catalysts: (a and b) cobalt phtalocyanine, (c and d) iron phtalocyanine, and (e and f) ferrocene.

Table 1. Yield and Structural Features Determined by HRTEM and Raman Spectroscopy of MWCNTs Grown with Different Catalysts

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catalyst

HR-TEM d002 lattice spacing (nm)

HR-TEM diameter range (nm)

Raman ID/IG

Raman IG′/IG

yield mg CNTs/mL α-pinene

FecH FePh CoPh

0.344 0.354 0.345

70−80 60−70 40−50

0.46 0.95 1.27

1.20 0.15 0.24

32 8 4

obtained by XRD, and structural characteristics obtained from HR-TEM images. Type of Catalyst. MWCNTs were synthesized using three different catalysts: ferrocene (FecH), iron(II) phthalocyanine (FePc), and cobalt(II) phthalocyanine (CoPc). The temperature, injection flow, time, and catalyst concentration where kept constant at 800 °C, 5000 sccm, 30 min, and 4.27 wt % of metal for the three catalysts, respectively. Figure 1a−f shows the HR-TEM images of MWCNTs obtained for each catalyst and their corresponding fast Fourier transform (FFT). Figure 1a and c shows HR-TEM images of MWNTs produced using CoPc and FePc, respectively. The main characteristic of these CNTs is their high disorder, due to the lack of alignment of the carbon layers throughout the entire

RESULTS AND DISCUSION In order to establish the best reaction conditions to obtain the highest yield of carbon and with the purpose of obtaining the best features in terms of crystallinity possible in a spray pyrolysis system, using α-pinene as the carbon source, a parametric study was carried out involving different catalysts, catalyst concentration, flow of reactants, and growing time while the temperature was kept at 800 °C. The criteria to establish the effect of these parameters were the yield (considered as grams of CNTs produced per milliliter of αpinene) ID/IG and IG′/IG ratio intensities obtained from Raman spectroscopy, thermal stability obtained from TGA, the position and width of the (002) crystalline planes of MWCNTs 3891


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Figure 2. (a) Raman spectra of MWCNTs obtained with CoPc, FePc, and FecH catalysts. (b) Plots of ID/IG and IG′/IG for each catalyst tested. Figure 4. HRTEM images and their corresponding fast Fourier transform (FFT) of MWCNTs produced with different FecH concentrations: (a and b) 4 and (c and d) 25 mg/mL.

Figure 3. (a) Raman spectra of MWCNTs obtained using different FecH concentrations, the corresponding concentration in milligrams per milliliter is given adjacent to each spectrum. (b) Plots of ID/IG and IG′/IG for each concentration tested. The solid lines are to guide the eye.

Figure 5. (a) Raman spectra of MWCNTs synthesized with different flows of reactants, the corresponding flow is indicated adjacent to each spectrum. (b) Plots of ID/IG and IG′/IG for each flow tested. The solid lines are to guide the eye.

Table 2. Yield and Structural Features Determined by HRTEM and Raman Spectroscopy of MWCNTs grown with Different FecH Concentrations catalyst concentration (mg/mL)



Raman ID/IG

Raman IG′/IG

yield mg CNTs/ mL αpinene

4 12 25 37 50 62

0.331 0.331 0.336 0.344 0.341 0.347

30−40 50−60 80−90 70−80 70−80 80−90

1.32 0.58 0.44 0.46 0.48 0.53

0.36 0.87 1.50 1.20 1.18 1.04

8 12 16 32 28 24

reveal poor crystallinity of the formed MWCNTs with both catalysts, as indicated by the presence of wide peaks and low intensity of the central spots. It is important to notice that these catalysts formed bamboo-type structures which are characteristic of N-doped CNTs. Highly crystalline MWCNTs were obtained when using FecH as catalyst. The corresponding HRTEM image and FFT analysis are shown in Figure 1e and f. Clearly, the nanotubes exhibit noticeable crystallinity based on the fact that carbon layers are highly aligned and the presence of sharp spots in the FFT analysis and narrow spots in the line scan (inset).34 The average outer diameters as well as the interlayer distance were obtained directly from the HR-TEM images and reported in Table 1.

nanotube structure. The corresponding FFT analysis (Figure 1b and d) displays diffuse spots, and the line scans (inset) 3892


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value was the highest for MWCNTs produced with FecH. This indicates that the highest crystallinity as well as the highest carbon layer smoothness is achieved when using this catalyst. This is in excellent agreement with the HR-TEM results. TGA and XRD analyses for the MWCNTs produced with FecH, FePc, and CoPc catalysts are provided in the Supporting Information. The yield, reported as milligrams of MWCNTs produced per millileter of α-pinene for each catalyst, is given in Table 1. The highest yield was obtained for FecH, a much larger yield than those obtained for FePc and for CoPc. This study demonstrates that for the three catalysts tested, FecH is more active than FePc and CoPc toward the production of crystalline MWCNTs when using α-pinene as carbon source. It has been reported that, when using metal phtalocyanines (Fe, Co, and Ni), N atoms are generated, dissolved into the catalytic nanoparticles and precipitate with carbon producing N-doped carbon nanotubes.38 The incorporation of N atoms in the graphitic layers of the MWCNTs may be the source of the lower crystallinity when using FePc and CoPc as catalysts. FecH does not provide N atoms, hence producing more crystalline MWCNTs as revealed by all the HR-TEM, Raman, TGA, and XRD analyses. Catalyst Concentration. The FecH concentration for the synthesis of MWCNTs was evaluated between 4 and 62 mg of catalyst/mL of α-pinene. The Raman analysis is shown in Figure 3a and provides a clear view of the structural changes of the MWCNTs produced as a function of catalyst concentration. The three characteristic peaks of MWCNTs located at 1328, 1578, and 2657 cm−1 corresponding to the D, G, and G′ bands respectively are clearly identified. The corresponding values of these band intensities are given in Table 2. The lowest catalyst concentration produced the most defective MWCNTs as indicated by the highest ID/IG and the lowest IG′/IG. As the catalyst concentrations increases, more crystalline MWCNTs are formed. The ID/IG ratio reaches the lowest value of at 25 mg/mL concentration and remains practically constant at higher concentrations. The IG′/IG reaches the highest value at 25 mg/mL concentrations and gradually decreases as the concentration is increased. The highest MWCNTs yield was obtained using 37 mg/mL of FecH. Figure 3b shows the tendency of the ID/IG and IG′/IG ratio intensities for all FecH concentrations used. HR-TEM images and FFT analysis of MWCNTs formed with 4 and 25 mg/mL are shown in Figure 4. MWCNTs produced with the lowest FecH concentration display a high degree of disorder (Figure 4a). The corresponding FFT analysis (Figure 4b) display diffuse spots and the line scans (inset) reveal the poor crystallinity of the formed MWCNTs with this catalyst concentration. Highly crystalline MWCNTs were obtained when using 37 mg/mL. The corresponding HRTEM image and FFT analysis are shown in Figure 4c and d. Clearly, the tubes exhibit an extremely high degree of crystallinity based on the fact that the carbon layers are highly aligned and the presence of sharp spots in the FFT analysis and narrow spots in the line scan (inset). These results are in excellent agreement with the Raman analysis. TGA and XRD analyses for the MWCNTs produced using different concentrations of FecH catalyst are provided in the Supporting Information. The catalyst concentration plays an important role in forming the zero oxidation state metal clusters which will promote the adsorption, dissociation, diffusion, and condensa-

Figure 6. HR-TEM images and their corresponding fast Fourier transform (FFT) of MWCNTs produced with different flows of reactants: (a and b) 2500, (c and d) 5000, and (e and f) 7500 sccm.

Table 3. Yield and Structural Features Determined by HRTEM and Raman Spectroscopy of MWCNTs Grown with Different FecH Concentrations flow (sccm)



Raman ID/IG

Raman IG′/IG

yield mg CNTs/mL α-pinene

2500 5000 7500

0.331 0.344 0.343

60−70 70−80 50−60

0.88 0.46 0.58

0.55 1.20 1.00

38 32 15

Raman spectroscopy identifies the presence of MWCNTs relative to other carbon allotropes. Figure 2a shows the Raman spectra of the MWCNTs on a quartz substrate prepared with FecH, FePc, and CoPc as catalysts. All spectra exhibit the three characteristic peaks of MWCNTs: the D band at ∼1340 cm−1, which corresponds to the disorder-induced phonon mode (A1g), the G band at ∼1580 cm−1 assigned to the Ramanallowed phonon mode (E2g), and the G′ band at ∼2660 cm−1 assigned to the first overtone of the D mode.35 The D/G band intensity ratio (ID/IG) is reported to increase with increasing structural disorder. The G′/G band intensity ratio (IG′/IG) increases as the smoothness degree of C deposits is increased.36,37 These values are given in Table 1. The tendency of these band ratios is shown in Figure 2b where it can be determined that the ID/IG value was the lowest and the IG′/IG 3893


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Figure 7. (a) SEM images of MWCNTs grown vertically aligned varying the growth time. (b) Plot of MWCNTs height as a function of growth time.

Figure 8. (a) Raman spectra of MWCNTs synthesized at different reaction times. (b) Plots of ID/IG and IG′/IG for each time tested. The solid lines are to guide the eye.

tion of carbonaceous fragments created by the thermal decomposition of α-pinene. The sizes of the metal clusters dictate whether single-walled, double-wall of multiwalled carbon nanotubes can be produced. In our case, cluster sizes between ∼8 and ∼10.5 nm are formed as indicated by the internal diameter of the carbon nanotubes produced in catalyst concentration range tested. Based on the Raman and XRD results, the most defective MWCNTs are produced at the lowest concentration tested (4 mg/mL). We can assume that at this concentration there is carbon excess with respect to catalytic iron and carbon cannot be efficiently incorporated in the growing nanotubes. At concentrations between 12 and 37 mg/mL, the iron-to-carbon ratio is increased favoring the fundamental steps (adsorption, dissociation, diffusion and condensation) toward the formation of crystalline carbon nanotubes. We observe a decline of yield and crystallinity of MWCNTs at 50 and 62 mg/mL catalyst concentration. We assume that at these higher concentrations, iron particles agglomerate to produce large metallic nanoparticles and there is insufficient amount of carbon with respect to the iron available for the formation of CNTs.

Figure 9. HR-TEM images and their corresponding fast Fourier transform (FFT) of MWCNTs produced at different reaction times: (a and b) 1, (c and d) 3, and (e and f) 45 min.

Flow of Reactants. Three different flows of reactants using argon as carrier gas were tested: 2500, 5000, and 7500 sccm. The corresponding Raman analysis is presented in Figure 5a. All Raman spectra show the D, G, and G′ bands. The lowest ID/IG as well as the highest IG′/IG were found for the 5000 sccm flow (Figure 5b). MWCNTs formed using 2500 sccm flows possess the lowest crystallinity. 3894



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CONCLUSIONS MWCNTs were successfully synthesized using α-pinene, a botanical hydrocarbon, as a carbon source in a spray pyrolysis system. Ferrocene as catalyst produced highly crystalline multiwalled CNTs while Co- and Fe-phtalocyanine produced nitrogen-doped CNTs. By evaluating the reaction conditions such as catalyst, catalyst concentration, flow of reactants and time, mixtures of ferrocene/α-pinene at a concentration of 37 mg/mL and 5000 sccm flow were the optimal conditions to obtain the highest yield of crystalline CNTs at a fixed temperature of 800 °C. The variation of time produced crystalline CNTs with different lengths without modifying their crystallinity. The growth kinetics of MWCNTs follows a Deal− Grove model which indicates that the growth is diffusionlimited and suggests that the root growth mechanism controls the growth process.

The corresponding HR-TEM images and FFT analysis of MWCNTs produced when varying flow are shown in Figure 6. More diffuse spots are observed in the FFT analysis of MWCNTs grown at 2500 sccm (Figure 6b) indicating the presence of more structural defects which agrees well with Raman results. The FFT analysis of nanotubes grown at 5000 and 7500 sccm (Figure 6d and f) shows sharp spots indicative of highly crystalline MWCNTs. The structural features determine directly from these images are reported in Table 3. It has been shown that the diameter of the MWCNTs decreases as the flow rate of the reactive mixture is increased.39 In our case, we did not observe this decrease; on the contrary, we obtained a maximum diameter at the intermediate flow tested (5000 sccm) and lower diameters for the lowest and highest flows evaluated. More important, the highest crystallinity was also obtained for MWCNTs obtained at 5000 sccm. These results may suggest that at this flow the formation of MWCNTs occurs in a more efficient way. Reaction Time. MWCNTs were synthesized at 800 °C, using 37 mg/mL FecH concentration 5000 sccm flow between 1 and 45 min. The length of the MWCNTs was monitored by SEM (Figure 7a). By plotting the length as a function of reaction time (Figure 7b), some information about the growth controlling steps can be obtained. We observe a continuous increase of the tubes’ length with time. As the time is increased, the growth slows down but does not stop. The best fit was achieved using a linear-parabolic equation, h2 + Ah + Bt = 0, where h is the nanotube length, t is time, parameter B is the parabolic rate constant which depends only on temperature and accounts for the diffusion-limited growth, and parameter A is the linear rate constant which depends on reactant pressure and temperature and is related to the surface reaction at the catalytic particles.40 This expression is regarded as the Deal− Grove diffusion law and has been used to fit experimental growth data of vertically aligned single walled nanotubes Fe/ Al2O3 coated Si wafers.41 By fitting the experimental results with this model, we can establish that the growth is diffusionlimited and the root growth mechanism is dominant. The growth gradually slows down as it becomes more difficult for precursors to reach the catalyst at the bottom of the film. Figure 8a shows the Raman spectra of the CNT’s grown at different times. The D, G, and G′ bands are clearly observed. The quality parameters ID/IG and IG′/IG are the highest and lowest respectively for MWCNTs synthesized during 1 min. These parameters reach their lowest (ID/IG) and highest (IG′/ IG) values after 3 min of reaction time and remain essentially unchanged after that, indicating that the crystallinity of the MWCNTs at short reaction time is the same as those formed at longer times (Figure 8b). Poor graphite alignment in the HR-TEM (Figure 9a) as well as diffuse spots and low intensity peaks in the line scan (inset) of the FFT analysis (Figure 9b) reveal the low crystallinity of MWCNTs produced at 1 min of reaction time. For MWCNTs produced at 3 and 45 min the HR-TEM images show higher degree of graphite alignment (Figure 9c and e) and sharp spots and high intensity of peaks in the line scan of the FFT analyses (Figure 9d and e) indicating that more crystalline MWCNTs are formed at these reaction times. In addition, the analysis of the HR-TEM images of MWCNTs reveals that the diameter distribution is 20−30 nm for 1 and 3 min, 70−80 nm for 30 min and 90−100 nm for 45 min. These results are in excellent agreement with the Raman analysis shown previously.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03054. Thermogravimetric and X-ray diffraction data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +52 443-3273584. Fax: +52 443-327-3584. ORCID

J. Lara-Romero: 0000-0003-0216-364X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for valuable technical assistance ́ provided by Francisco Rodriguez-Melgarejo (CINVESTAV). Financial support from the Scientific Research Council of the Universidad Michoacana de San Nicolás de Hidalgo is greatly appreciated.

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REFERENCES

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Parametric Study of the Synthesis of Carbon Nanotubes by Spray

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