Supercritical Fluids as Reaction Media for Scalable Production of

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Supercritical Fluids as Reaction Media for Scalable Production of Carbon Nanomaterials Haider Almkhelfe, and Placidus B Amama ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02272 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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ACS Applied Nano Materials

Supercritical Fluids as Reaction Media for Scalable Production of Carbon Nanomaterials

Haider Almkhelfe, Placidus B. Amama* Tim Taylor Department of Chemical Engineering, Kansas State University, Manhattan, KS 66506

ABSTRACT We have demonstrated scalable and selective synthesis of carbon nanotubes (CNTs), carbon nanofibers (CNFs), and onion-like carbons (OLCs) in a batch reactor using supercritical fluids (SCFs) as reaction media. The process utilizes toluene and alcohols (ethanol, propanol, and butanol) as carbon precursors in combination with ferrocene. Growth with supercritical toluene at 600ºC in the absence of water yields large-diameter CNTs while introduction of 92.5 mmol/L of water enhances product yield by 50%, promoting formation of smaller-diameter CNTs and decorating the exterior surface of CNTs with Fe nanoparticles. At 400 and 500ºC, in the absence of water, supercritical toluene produces mainly OLCs and CNFs, respectively. For alcohols, a gradual evolution of the morphology of nanocarbons forms from mainly OLCs to tube-like structures as the ratio of C/O atoms increases, possibly due to a decrease in the tendency of graphitic sheets to minimize their energies by curling into onion-like structures as chain length increases. This study provides a framework for utilizing SCF reaction media in a batch reactor to achieve scalable and selective growth of different nanocarbons and nanocarbon-metal nanocomposites. Keywords: Carbon nanotubes, carbon nanofibers, onion-like carbons, supercritical fluids, batch reactor, ferrocene

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To whom correspondence should be addressed. E-mail: [email protected]; Tel.: 785-532-4318

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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. INTRODUCTION Concerted efforts have been put forth to scale up production of high-purity nanocarbons for applications where large amounts are required such as composites, adsorbents, catalyst supports, and electrodes.1-7 Carbon nanotubes (CNTs), graphene, nanofibers, nanohorns, nanorods, onionlike carbons (OLCs), and fullerenes are some of the nanocarbons that have attracted a great deal of interest due to their unique mechanical, chemical, thermal, and electrical properties.8-13 Nanocarbons are typically synthesized via chemical vapor deposition (CVD),14,15 arc-discharge,16 and laser ablation.17 Different modifications of the aforementioned growth methods exist for producing specific nanocarbons or scaling up their production. OLCs are produced by thermal annealing of a nanodiamond precursor in a vacuum (or inert atmosphere),18,19 underwater arc discharge,20 and heating a carbon filament in liquid alcohol.21 Single-walled carbon nanotubes (SWCNTs) with a productivity of up to 10 g/day are produced by a HiPCO process under severe conditions (3.1 Mpa and 1050˚C).22 Although these vapor-phase synthetic methods can yield macroscopic amounts of nanocarbons, their production rates and yield are still relatively low. CVD, the most preferred growth method, is mostly utilized for CNT or graphene growth; other types of nanocarbons present are usually considered as undesired products, requiring postpurification steps for removal. Use of laser ablation and arc-discharge methods remain unappealing due to apparatus complexity and high operating cost. At present, a suitable growth technique for producing nanocarbons that offers a favorable combination of cost effectiveness, controlled selectivity, and scalability is yet to emerge. Efforts to scale-up production of nanocarbons have not only focused on improving selectivity and yield of catalytic CVD processes, but also on developing new scalable growth approaches. A promising approach for scalable growth of nanocarbons is the supercritical fluid

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(SCF) approach, widely used for synthesis of multi-walled CNTs (MWCNTs) and nanowires as well as processing of nanocarbons.23-25 A major barrier to scalable production of nanocarbons via CVD is the overwhelming number of growth parameters and growth steps that have to be optimized. The SCF method overcomes most of these limitations. The supercritical fluid serves as a carbon source while an organometallic fluid serves as a catalyst source in a batch reactor system. Unlike vapor-phase synthetic methods, supercritical fluid-phase methods are characterized by higher throughputs due to high catalyst dispersion, low viscosity, high diffusivity, zero surface tension, and high precursor concentration and pressure.24,26 The SCF method has been widely used for nanocarbon growth. Li et al.27 reported scalable growth of MWCNTs with a 100% selectivity (based on catalyst weight) using a continuous-flow SCF approach with supercritical carbon monoxide as a carbon source at 750°C and 5.17 MPa. Korgel’s group26 demonstrated use of supercritical toluene in a nitrogen environment at 600˚C and 12.4 Mpa for growth of MWCNTs with Fe and Fe/Pt catalyst nanoparticles; the yield of MWCNTs was found to be dependent on temperature and size of the catalyst particles. Using supercritical ethanol as a carbon source and magnesium as a reductant in a stainless steel autoclave at 650˚C, Liu et al.28 demonstrated scalable growth of bamboo-shaped CNTs. Further investigation by the same group,29 using the same experimental setup, revealed growth of three types of nanocarbons namely, bamboo-shaped CNTs, and cubic and spherical carbon nanocages. The coexistence of cubic and spherical-shaped structures was rationalized as a bending attempt of the graphitic sheets to minimize the highly energetic dangling bonds at the edge of the growing structure.29 While these studies show the SCF approach is suitable for scalable growth of MWCNTs and favors the coexistence of different types of nanocarbons, no clear pathways for tuning nanocarbon selectivity and improving growth efficiency exist. Further, the groundbreaking work of Hata et al.30 revealed

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that activity and lifetime of catalysts during growth of single-walled CNTs are dramatically enhanced by water. Use of water as a growth enhancer in catalytic growth of CNTs has been limited to CVD approaches, but so far has not been implemented in the SCF growth approach. With improved understanding of the role of water in the SCF approach, it is possible growth processes can be designed to enhance growth efficiency and product selectivity. In this work, we report highly selective synthesis of CNTs, CNFs, and OLCs in a batch reactor using SCFs as reaction media. The single-step SCF approach has a simple design and utilizes ferrocene as a catalyst source in combination with an organic solvent as a primary carbon source (alcohols and toluene). The study illuminates the effect of water, type of solvent, and batch reactor temperature on nanocarbon selectivity and growth efficiency. The addition of a miniscule amount of water in the SCF media is demonstrated, for the first time, as a viable pathway to enhance and control selectivity of nanocarbons. By comparing growth behavior of different alcohols, the role of water and the C/O ratio in growth enhancement are rationalized, and by combining results obtained for supercritical toluene, a reasonable mechanism to explain the formation of different nanocarbons is proposed. In addition, the SCF approach presented here is not only a promising one-step process for scalable synthesis of OLCs, CNTs, and CNFs, but is also a viable process for producing nanocomposites — depositing particles on CNTs or filling the interior of CNTs with metallic catalysts.

2. EXPERIMENTAL SECTION 2.1 Synthesis. The apparatus for the SCF-phase method consisted of a 50 ml-capacity stainless steel batch reactor with a pressure gauge and release valve connected to the reactor cap. A picture of the experimental setup is shown in Figure 1. Ferrocene served as the source of catalyst and a

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secondary carbon source, while an anhydrous aromatic (toluene) and anhydrous alcohols (ethanol, propanol, and butanol) were used as the primary carbon precursors. For each experiment, the primary carbon precursor served as a solvent for the ferrocene powder. The resulting solution formed by mixing 30 ml of each solvent and ferrocene powder was introduced into the batch reactor. The reactor was sealed properly and initial pressure was maintained at 1 atm. Thereafter, the reactor was heated to 400-600˚C for 1.5 h using a ceramic heater (Omega) with an iron-chromealuminum ribbon element. After completion of growth at atmospheric pressure (or in a vacuum), the reactor was allowed to cool to room temperature. Reactants were loaded in the batch reactor under two different environments. In the first case, reactants were loaded under atmospheric pressure, and in the second case, reactants were loaded under vacuum (i.e. the batch reactor was vacuumed prior to loading the reactants). Optimization of the catalyst amount was carried out in supercritical toluene containing different amounts of ferrocene (1.79, 17.92, 35.84, and 53.75 mmol/L). Growth experiments with different concentrations of distilled water (46.2, 92.5, 138.7, and 185.0 mmol/L) in the fluid phase were investigated. The resulting black solid powder was collected, sonicated, and washed with ethanol and distilled water, and then dried in a vacuum oven at 80˚C for 5h. The efficacy of amorphous carbon removal via sonication in ethanol has been demonstrated by Rinaldi et al.31 2.2 Material characterization. The morphology and microstructure of the reaction products were characterized by field-emission scanning electron microscopy (SEM; FEI Versa 3D Dual Beam), and transmission electron microscopy (TEM; FEI Tecnai F20 XT) operating at 200 kV and equipped with energy-dispersive X-ray spectroscopy (EDS). For TEM imaging, the final dry products were dispersed in ethanol by sonication for 5 min and dropped on a copper microgrid

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coated with lacy carbon film. The Raman spectra of as-grown nanocarbons were acquired using a Renishaw inVia Raman Microscope with a laser excitation wavelength of 633 nm.

3. RESULTS AND DISCUSSION 3.1 Supercritical toluene in the absence of water. Scalable synthesis of nanocarbons with supercritical toluene in the absence of water was investigated at 400, 500, and 600ºC using a solution consisting of toluene (30 ml) and ferrocene (35.84 mmol/L) for 1 h. Figure 2 shows representative SEM images and corresponding Raman spectra of nanocarbons produced at the different temperatures. At the lowest fluid-phase temperature (400ºC), the structure of the synthesized nanocarbons is mainly spherical (Figure 2a). As temperature is increased to 500ºC, carbon filaments with lengths in the range of 2 – 3μm (Figure 2c) are formed. A close examination of Figure 2c reveals the encapsulation of the Fe catalyst in the hollow interior of the filament. Further increase in the fluid-phase temperature to 600ºC yields dense MWCNTs with catalyst particles at the tips of the nanotubes (Figure 2e). The Raman spectra of samples obtained at the different temperatures are characterized by the omnipresent disorder-induced mode (D-band) at ~1345 cm-1 and tangential stretch mode (G-band) at ~1593 cm-1 associated with graphitic materials. As expected, the degree of graphitization increases with temperature, evidenced by Gbands that become increasingly well defined in combination with full-width at half-maximum (FWHM) that consistently decrease. By studying the Raman spectral features of different surrogate nanocarbons, DiLeo et al.32 showed it is possible to use the G' band to distinguish between nanotubes and other nanocarbons; a distinct G' peak is observed in the Raman spectra of MWCNTs and graphite while it is nonexistent in the case of nanocarbons present as byproducts of CVD. The G' band (~2700 cm-1) is attributed to the long-range order in the sample and originates from the

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two-phonon, second-order scattering process that produces inelastic phonon.33 Given the G' peak originates from a two-phonon process, a high degree of ordering is required for the coupling to occur; therefore, G' peak intensity is directly related to the quality of the sample. The presence of a G' peak for a sample produced at 600°C further demonstrates not only the high selectivity of MWCNTs, but also the high quality. We conclude from Figure 2 that nanocarbon morphology has a strong dependence on the temperature of supercritical toluene, with spherical structures (or OLCs) being highly selective at lower temperatures while higher temperatures induce formation of cylindrical structures (CNFs at 500°C and MWCNTs at 600°C). Figure 3 shows a summary of characterization data of nanocarbons grown with supercritical toluene in the absence of water. SEM/EDS data in Figure 3a confirms the composition of the filament and material occupying the interior of carbon filaments produced at 500ºC (shown in Figures 2c and d). The exterior wall of the filament consists of mainly carbon while the hollow interior is filled with Fe. It is apparent that growth with supercritical toluene at 500ºC yields carbon filament-encapsulated Fe composites. The average diameter of the carbon filaments is quite large (~400 nm), which we attribute to the encapsulation of the Fe catalyst and eventual formation of a rod-like Fe structure. A similar phenomenon was observed by Weissker et al.34 whereby the filling of CNTs with ferromagnetic metals resulted in enlargement of the tube diameter. Note that encapsulation of foreign nanomaterials in the hollow interior of CNTs is an attractive approach for tuning their electronic and mechanical properties.35 To rationalize the filling of the catalyst in the interior of the fibers, it is necessary to consider the widely adopted mechanism for describing growth of CNTs and carbon filament from supported catalysts — a mechanism that has its roots from the VLS (vapor-liquid-solid) theory developed by Wagner and Ellis.36 An important aspect of the growth mechanism is the strength of substrate-catalyst interactions, which usually

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determines whether the mode is tip-growth or base-growth. The complete filling of an Fe catalyst in the hollow interior of the filament at 500°C is inconsistent with the VLS mechanism. However, a folded-growth model, proposed by Louchev et al.,37 which assumes the carbon layer formed wraps around nanoparticles, appears to be more representative of the observed wrapping of Fe nanowire by graphitic layers. Increasing growth temperature to 600ºC yielded MWCNTs as indicated by the Raman spectra and corroborated by SEM images (Figures 2e and f); low- and high-magnification TEM images in Figures 3b and c of this sample confirm catalyst nanoparticles are embedded at the tips of nanotubes. The high-resolution TEM image (insert in Figure 3c) shows the tip of the nanotube with several graphitic layers encapsulating a large catalyst particle (10 – 20 nm). A clear structural difference exists between a CNT and a nanofiber. Unlike CNTs, nanofibers are characterized by poor crystallinity and absence of a hollow interior;5 this difference is apparent in structures formed at 500 ºC (CNFs) and 600ºC (MWCNTs). In agreement with prior studies,26,38 we observed nanoparticles larger than 50 nm in size support the formation of nanofibers, while those that are smaller favor the formation of CNTs. Interestingly, further examination of MWCNTs grown by supercritical toluene at 600ºC reveal the nanotubes unavoidably nucleate metal nanoparticles on their external surface. As shown in Figure S1, highly monodispersed Fe nanoparticles are decorated on the exterior surface of the nanotubes. Ye et al.24 have shown that defects on the walls of CNTs can provide suitable sites for nanoparticle nucleation. The high intensity of the D-band with respect to the G-band in Figure 2f may indicate the existence of a significant amount of defects that could serve as sites for particle nucleation. Having determined the impact of temperature on the selectivity of nanocarbons, we switch our focus to the effect of catalyst amount (or ferrocene concentration) in supercritical toluene

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growth behavior. To assess the efficiency of the SCF method, yield of CNTs is used as our basis for comparison with other growth processes. The yield is defined as the ratio of the actual yield of nanocarbons after purification to the theoretical yield. Figure 3d shows the effect of ferrocene amount on the yield of MWCNTs grown at 600ºC and the pressure of the batch reactor after 90 min. For ferrocene concentrations higher than 0.01 mol/L, the final product was a dry black powder (picture shown in Figure 3e) while lower ferrocene concentrations (< 0.005 mol/L) yielded a thick dark solution (Figure S2), most likely due to insufficient ferrocene to dissociate the feedstock (toluene). The yield of MWCNTs and corresponding pressure in the batch reactor increases with ferrocene concentration, reaching a yield of ~18g and an estimated pressure greater than 17 MPa for a ferrocene concentration of 0.055 mol/L. Due to safety concerns, the reaction was discontinued after the pressure exceeded 17 MPa. We expect higher pressure to result in higher CNT yield. The increasing pressure of the batch reactor with the amount of ferrocene (Figure 3d) is attributed to the increased dissociation rate of the precursor, resulting in the generation of more free radicals and gaseous products. The catalyst amount increases by an order of magnitude between 1.79 and 17.9 mmol and a factor of two between 17.9 and 35.8 mmol. The increased catalyst amount is assumed to enhance the decomposition of toluene, thus producing more free radicals and gaseous intermediates (or species). For the SCF growth process, catalyst synthesis and carbon precursor decomposition occur simultaneously,39 and precursors require sufficient amounts of catalyst for high conversion of precursors to nanocarbons. Increasing the amount of ferrocene is expected to increase free radical reactions and size of particles formed as well as the diameter of resulting CNTs, in consistent with the work of Singh et al.40 3.2 Supercritical toluene in the presence of water. The work of Hata et al.30 on SWCNT forest growth via catalytic water-assisted CVD process (or ꞌsupergrowthꞌ) revealed that introduction of

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miniscule amounts of water in the range of tens or few hundreds of parts per million is an effective way of extending catalyst lifetime.41-43 The role of water in growth enhancement is explained based on two mechanisms: (1) etching of carbon impurities on the surface of catalyst particles 30,42,44 and (2) inhibition of Ostwald ripening of the catalyst.45-48 Given the critical role water plays in enhancing catalyst activity and lifetime, we investigated the effect of adding different amounts of water (46.3, 92.5, and 138.7 mmol/L) in the SCF phase on growth behavior of CNTs at 600ºC. A summary of Raman spectroscopic and SEM data of products formed in the SCF phase with different amounts of water is shown in Figure 4. The Raman spectra exhibit the characteristic Gand D-bands of nanocarbons. The ratio of the G- and D-bands (IG/ID) increases with the amount of water up to 92.5 mmol/L and then decreases upon further increase to 138.7 mmol/L. In addition, the highest product yield increased from 57% (without water) to 78% (with 92.5 mmol/L of water). Based on these results, we conclude the optimum concentration of water for producing highquality CNTs is ~ 92.5 mmol/L. The Raman results are supported by SEM images that reveal CNTs grown from supercritical toluene with 92.5 mmol/L of water are clean and long with diameters less than 80 nm (Figure 4b and c); whereas in the absence of water, mixtures of short and defective nanofibers and nanotubes are obtained. TGA data (Figure S3) indicate the absence of significant amounts of amorphous carbon in samples obtained from supercritical toluene in the presence and absence of water, evidenced by the absence of discernable weight loss in the temperature range where amorphous carbon is expected to oxidize (< 450°C in the presence of a catalyst). A significant decrease in the diameter of CNTs is observed in the presence of water, which we attribute to the inhibition of Ostwald ripening — a phenomenon analogous to the role of water in ꞌsupergrowthꞌ.45,47,49 Since the size of catalyst particles determines the diameter of CNTs, smaller particles formed in the presence of water support growth of smaller-diameter CNTs.

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Growth behavior of supercritical toluene is investigated with added water in the range of 0 – 138.7 mmol/L. The next question we tried to answer is what happens when the concentration of water is increased further. Increasing the amount of water to 185.0 mmol/L under the same operating conditions, which is higher than the range studied in Figure 4, dramatically changes the morphology of the products from nanotubes to spherical nanostructures (OLCs). Under these conditions, high yields of OLCs are produced as shown by SEM images in Figure 5 a and b. TEM images in Figure 5c and d reveal that OLCs consist of an assembly of concentric spherical graphitic cages with a distance of 0.34 nm between adjacent graphene layers. MD simulations by Robertson et al.50 revealed that carbon ribbons, with sizes ranging from 32 to 108 atoms, remain close to planar at low temperatures while at high temperatures edge pentagons induce curvature of the planar structure. This means energy supplied by the SCF process under conditions that favor growth of OLCs is enough to reduce the barrier to closure and consequently increase the probability of forming OLCs.50 Note that for the hexagonal sheets of carbon to grow in a helical orientation, sufficient energy is required; otherwise, formation of curved or closed sheets is favored to eliminate the highly energetic dangling bonds at the edge of growing sheets.51 Under supercritical conditions, water acts as a supercritical oxidant able to eliminate excess amounts of organic compounds and maintain activity of the catalyst.52 In the case of catalytic CVD, the addition of optimum amounts of water effectively improves the initial growth rate and extends the catalyst lifetime, and, as a consequence, results in a higher yield of CNTs.53 3.3 Supercritical alcohols. To further elucidate the role of water in the SCF growth approach, we conducted our study using alcohols since their pyrolysis yield water. Use of alcohols as supercritical fluids in the growth process also yields OLCs of different size ranges. Figure 6 shows detailed SEM images of the structures grown using different alcohol precursors. The yield of OLCs

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obtained with toluene in the presence of 185.0 mmol/L of water was higher than OLCs obtained from pure alcohols by a factor of three. The ability of alcohols to selectively grow tubular structures without a reductant (alkaline earth metal) increases in the following order: C2H5OH< C3H7OH < C4H9OH. This suggests that as the ratio of generated water molecules to carbon atoms (H2O:C) in alcohol precursor increases, the possibility of growing tubular structures increases. The effect of using different mixtures of alcohols (methanol, ethanol, isopropanol, and butanol) and acetylene on soot formation during the pyrolysis process has been reported by Esarte et.al.54 The study reveals the presence of alcohols always inhibits formation of soot and gas products; in other words, the longer the aliphatic chain of the alcohol, the higher the soot yield, largely due to the decrease in the amount of oxygen in the fuel mixture. As shown in Figure 6, OLC formation becomes highly selective as the alcohol chain length decreases: butanol < propanol < ethanol. Our results are consistent with this observation in terms of cylindrical tube formation. A schematic illustration of the synthesis pathways for CNTs and OLCs using the different precursors is presented in Figure 7. To explain the synthesis of OLC using the SCF approach with toluene at either low-growth temperature (400ºC) or in the presence of excess amounts of water at 600 ºC, we hypothesize that OLC formation occurs when available energy is insufficient to reduce the barrier to closure of the graphitic sheets. We attribute this phenomenon to be due to (1) growth at lower temperatures (Figure 2a) and (2) presence of excess water that consumes available energy (Figure 5). Therefore, in the presence of insufficient energy, we hypothesize the graphitic sheets tend to minimize their energies by curling into onion-like carbon structures as shown in Figure 6. There is also a gradual evolution of the morphology of products from mainly OLCs in the case of ethanol/ferrocene to mixtures of OLCs and tube-like structures for propanol/ferrocene. The morphological evolution, observed as the ratio of C/O atoms increases in the different alcohols

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used for synthesis under similar reactor conditions, supports our hypothesis. Note that pyrolysis of alcohol is well known to generate excessive amounts of water (Equation 1). To grow CNTs using supercritical alcohol, a reductant such as potassium, magnesium, or an alkali earth metal is typically required as shown in Equation 2.28,29 CH3CH2OH 2C+H2O + 2H2 ------------------- (1) CH3CH2OH + Mg  2C + MgO + 3H2 --------------- (2) Our results suggest using longer-chain alcohols (higher C/O ratio), may achieve increased nanotube selectivity without a reductant. 4. CONCLUSIONS High yields of CNTs, CNFs, and OLCs were achieved in a one-step process via the SCF approach in a batch reactor. Yields of CNTs, CNFs, and OLCs reached 18.7, 12.5, and 10.5 g per run, respectively, corresponding to greater than 40% conversion. Variation of the influential growth parameters for toluene SCFs revealed dramatic changes in growth properties of nanocarbons produced. Lower growth temperature (400ºC) favors formation of OLCs, while at higher growth temperature (600ºC), CNTs were decorated with Fe nanoparticles. At intermediate temperature (500ºC), Fe nanowires were encapsulated in carbon filaments. The SCF approach in a batch reactor is not only a promising one-step process for scalable synthesis of CNTs and carbon filaments, but is also a viable process for producing nanocomposites — depositing particles on CNTs or filling the interior of CNTs with the metallic catalyst. The tendency for alcohols to produce tubular structures increases in the order C2H5OH < C3H7OH < C4H9OH, which suggests a direct correlation between the aliphatic chain length of alcohol and the tendency to form a tubular

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structure. Use of SCFs for growth not only provides a route for selective and scalable growth of a variety of carbon nanomaterials, but also provides a unique one-step approach free of aggressive acid treatment for synthesis of CNT-supported or -encapsulated Fe composites with potential applications in catalysis and energy storage.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: Additional figures and pictures (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We acknowledge funding from the National Science Foundation (Grant Numbers: 1728567 and 1653527) and support from KSU College of Engineering for material characterization.

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REFERENCES (1) Li, G. Y.; Wang, P. M.; Zhao, X. Pressure-Sensitive Properties and Microstructure of Carbon Nanotube Reinforced Cement Composites. Cement and Concrete Composites 2007, 29, 377382. (2) Kumar, M.; Ando, Y. Chemical Vapor Deposition of Carbon Nanotubes: A Review on Growth Mechanism and Mass Production. J. Nanosci. Nanotechno. 2010, 10, 3739-3758. (3) Zhang, Q.; Huang, J.-Q.; Qian, W.-Z.; Zhang, Y.-Y.; Wei, F. The Road for Nanomaterials Industry: A Review of Carbon Nanotube Production, Post-Treatment, and Bulk Applications for Composites and Energy Storage. Small 2013, 9, 1237-1265. (4) De Volder, M. F. L.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J. Carbon Nanotubes: Present and Future Commercial Applications. Science 2013, 339, 535-539. (5) Serp, P.; Corrias, M.; Kalck, P. Carbon Nanotubes and Nanofibers in Catalysis. Appl. Catal. A: Gen. 2003, 253, 337-358. (6) Zhong, Y. L.; Tian, Z.; Simon, G. P.; Li, D. Scalable Production of Graphene Via Wet Chemistry: Progress and Challenges. Mater. Today 2015, 18, 73-78. (7) Charitidis, C. A.; Georgiou, P.; Koklioti, M. A.; Trompeta, A.-F.; Markakis, V. Manufacturing Nanomaterials: From Research to Industry. Manuf. Rev. 2014, 1, 11. (8) Shanmugharaj, A. M.; Bae, J. H.; Lee, K. Y.; Noh, W. H.; Lee, S. H.; Ryu, S. H. Physical and Chemical Characteristics of Multiwalled Carbon Nanotubes Functionalized with Aminosilane and Its Influence on the Properties of Natural Rubber Composites. Compos. Sci. Technol. 2007, 67, 1813-1822. (9) Iijima, S. Carbon Nanotubes: Past, Present, and Future. Physica B: Condensed Matter 2002, 323, 1-5. (10) Kuan, H.-C.; Ma, C.-C. M.; Chang, W.-P.; Yuen, S.-M.; Wu, H.-H.; Lee, T.-M. Synthesis, Thermal, Mechanical and Rheological Properties of Multiwall Carbon Nanotube/Waterborne Polyurethane Nanocomposite. Compos. Sci. Technol. 2005, 65, 1703-1710. (11) Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162-163. (12) Zeiger, M.; Jackel, N.; Mochalin, V. N.; Presser, V. Review: Carbon Onions for Electrochemical Energy Storage. J. Mater. Chem. A 2016, 4, 3172-3196. (13) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Carbon Nanotubes--the Route toward Applications. Science 2002, 297, 787-792. (14) Li, Y.-L.; Kinloch, I. A.; Windle, A. H. Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis. Science 2004, 304, 276-278.

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(15) Yang, Y.; Liu, X.; Guo, X.; Wen, H.; Xu, B. Synthesis of Nano Onion-Like Fullerenes by Chemical Vapor Deposition Using an Iron Catalyst Supported on Sodium Chloride. J. Nanopart. Res. 2011, 13, 1979-1986. (16) Lange, H.; Sioda, M.; Huczko, A.; Zhu, Y. Q.; Kroto, H. W.; Walton, D. R. M. Nanocarbon Production by Arc Discharge in Water. Carbon 2003, 41, 1617-1623. (17) Scott, C. D.; Arepalli, S.; Nikolaev, P.; Smalley, R. E. Growth Mechanisms for Single-Wall Carbon Nanotubes in a Laser-Ablation Process. Appl. Phys. A 2001, 72, 573-580. (18) Jonathan, C.; John, K. M.; Filipe, P.; Rene, M.; Ioannis, N.; Yury, G.; Sebastian, O. Raman Spectroscopy Study of the Nanodiamond-to-Carbon Onion Transformation. Nanotechnology 2013, 24, 205703. (19) Kuznetsov, V. L.; Chuvilin, A. L.; Moroz, E. M.; Kolomiichuk, V. N.; Shaikhutdinov, S. K.; Butenko, Y. V.; Mal'kov, I. Y. Effect of Explosion Conditions on the Structure of Detonation Soots: Ultradisperse Diamond and Onion Carbon. Carbon 1994, 32, 873-882. (20) Alexandrou, I.; Wang, H.; Sano, N.; Amaratunga, G. A. J. Structure of Carbon Onions and Nanotubes Formed by Arc in Liquids. J. Chem. Phys. 2004, 120, 1055-1058. (21) Fan, J.-C.; Sung, H.-H.; Lin, C.-R.; Lai, M.-H. The Production of Onion-Like Carbon Nanoparticles by Heating Carbon in a Liquid Alcohol. J. Mater. Chem. 2012, 22, 97949797. (22) Bronikowski, M. J.; Willis, P. A.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Gas-Phase Production of Carbon Single-Walled Nanotubes from Carbon Monoxide Via the Hipco Process: A Parametric Study. J. Vac. Sci. Technol. A 2001, 19, 1800-1805. (23) Lu, X.; Hanrath, T.; Johnston, K. P.; Korgel, B. A. Growth of Single Crystal Silicon Nanowires in Supercritical Solution from Tethered Gold Particles on a Silicon Substrate. Nano Lett. 2003, 3, 93-99. (24) Ye, X. R.; Lin, Y.; Wang, C.; Wai, C. M. Supercritical Fluid Fabrication of Metal Nanowires and Nanorods Templated by Multiwalled Carbon Nanotubes. Adv. Mater. 2003, 15, 316-319. (25) Reverchon, E.; Adami, R. Nanomaterials and Supercritical Fluids. J. Supercrit. Fluid 2006, 37, 1-22. (26) Lee, D. C.; Mikulec, F. V.; Korgel, B. A. Carbon Nanotube Synthesis in Supercritical Toluene. J. Am. Chem. Soc. 2004, 126, 4951-4957. (27) Li, Z.; Andzane, J.; Erts, D.; Tobin, J. M.; Wang, K.; Morris, M. A.; Attard, G.; Holmes, J. D. A Supercritical-Fluid Method for Growing Carbon Nanotubes. Adv. Mater. 2007, 19, 3043-3046.

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Page 16 of 27

Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Liu, J.; Shao, M.; Chen, X.; Yu, W.; Liu, X.; Qian, Y. Large-Scale Synthesis of Carbon Nanotubes by an Ethanol Thermal Reduction Process. J. Am. Chem. Soc. 2003, 125, 80888089. (29) Liu, J.; Xu, L.; Zhang, W.; Lin, W. J.; Chen, X.; Wang, Z.; Qian, Y. Formation of Carbon Nanotubes and Cubic and Spherical Nanocages. J. Phys. Chem. B 2004, 108, 20090-20094. (30) Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes. Science 2004, 306, 1362-1364. (31) Rinaldi, A.; Frank, B.; Su, D. S.; Hamid, S. B. A.; Schlögl, R. Facile Removal of Amorphous Carbon from Carbon Nanotubes by Sonication. Chem. Mater. 2011, 23, 926928. (32) DiLeo, R. A.; Landi, B. J.; Raffaelle, R. P. Purity Assessment of Multiwalled Carbon Nanotubes by Raman Spectroscopy. J. Appl. Phys. 2007, 101, 064307. (33) Saito, R.; Grüneis, A.; Samsonidze, G., G. ; Brar, V. W.; Dresselhaus, G.; Dresselhaus, M. S.; Jorio, A.; Cançado, L. G.; Fantini, C.; Pimenta, M. A.; Souza Filho, A. G. Double Resonance Raman Spectroscopy of Single-Wall Carbon Nanotubes. New J. Phys. 2003, 5, 157. (34) Weissker, U.; Hampel, S.; Leonhardt, A.; Büchner, B. Carbon Nanotubes Filled with Ferromagnetic Materials. Materials 2010, 3, 4387-4427. (35) Laasonen, K.; Andreoni, W.; Parrinello, M. Structural and Electronic Properties of La@C82. Science 1992, 258, 1916-1918. (36) Wagner, R. S.; Ellis, W. C. The Vapor-Liquid—Solid Mechanism of Crystal Growth and Its Application to Silicon. Trans. Met. Soc. AIME 1965, 233, 12. (37) Louchev, O. A.; Laude, T.; Sato, Y.; Kanda, H. Diffusion-Controlled Kinetics of Carbon Nanotube Forest Growth by Chemical Vapor Deposition. J. Chem. Phys. 2003, 118, 76227634. (38) Xie, W.; Fang, W.; Li, D.; Xing, Y.; Guo, Y.; Lin, R. Coking of Model Hydrocarbon Fuels under Supercritical Condition. Energy & Fuels 2009, 23, 2997-3001. (39) Dervishi, E.; Biris, A. R.; Watanabe, F.; Umwungeri, J. L.; Mustafa, T.; Driver, J. A.; Biris, A. S. Few-Layer Nano-Graphene Structures with Large Surface Areas Synthesized on a Multifunctional Fe:Mo:Mgo Catalyst System. J. Mater. Sci. 2012, 47, 1910-1919. (40) Singh, C.; Shaffer, M. S. P.; Windle, A. H. Production of Controlled Architectures of Aligned Carbon Nanotubes by an Injection Chemical Vapour Deposition Method. Carbon 2003, 41, 359-368.

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(41) Yoshihara, N.; Ago, H.; Tsuji, M. Chemistry of Water-Assisted Carbon Nanotube Growth over Fe−Mo/Mgo Catalyst. J. Phys. Chem. C 2007, 111, 11577-11582. (42) Almkhelfe, H.; Li, X.; Rao, R.; Amama, P. B. Catalytic Cvd Growth of Millimeter-Tall Single-Wall Carbon Nanotube Carpets Using Industrial Gaseous Waste as a Feedstock. Carbon 2017, 116, 181-190. (43) Almkhelfe, H.; Carpena-Nunez, J.; Back, T. C.; Amama, P. B. Gaseous Product Mixture from Fischer-Tropsch Synthesis as an Efficient Carbon Feedstock for Low Temperature Cvd Growth of Carbon Nanotube Carpets. Nanoscale 2016, 8, 13476-13487. (44) Yamada, T.; Maigne, A.; Yudasaka, M.; Mizuno, K.; Futaba, D. N.; Yumura, M.; Iijima, S.; Hata, K. Revealing the Secret of Water-Assisted Carbon Nanotube Synthesis by Microscopic Observation of the Interaction of Water on the Catalysts. Nano Lett. 2008, 8, 4288-4292. (45) Amama, P. B.; Pint, C. L.; McJilton, L.; Kim, S. M.; Stach, E. A.; Murray, P. T.; Hauge, R. H.; Maruyama, B. Role of Water in Super Growth of Single-Walled Carbon Nanotube Carpets. Nano Lett. 2008, 9, 44-49. (46) Kim, S. M.; Pint, C. L.; Amama, P. B.; Hauge, R. H.; Maruyama, B.; Stach, E. A. Catalyst and Catalyst Support Morphology Evolution in Single-Walled Carbon Nanotube Supergrowth: Growth Deceleration and Termination. J. Mater. Res. 2010, 25, 1875-1885. (47) Kim, S. M.; Pint, C. L.; Amama, P. B.; Zakharov, D. N.; Hauge, R. H.; Maruyama, B.; Stach, E. A. Evolution in Catalyst Morphology Leads to Carbon Nanotube Growth Termination. J. Phys. Chem. Lett. 2010, 1, 918-922. (48) Hasegawa, K.; Noda, S. Moderating Carbon Supply and Suppressing Ostwald Ripening of Catalyst Particles to Produce 4.5-Mm-Tall Single-Walled Carbon Nanotube Forests. Carbon 2011, 49, 4497-4504. (49) Amama, P. B.; Pint, C. L.; Kim, S. M.; McJilton, L.; Eyink, K. G.; Stach, E. A.; Hauge, R. H.; Maruyama, B. Influence of Alumina Type on the Evolution and Activity of AluminaSupported Fe Catalysts in Single-Walled Carbon Nanotube Carpet Growth. ACS Nano 2010, 4, 895-904. (50) Robertson, D. H.; Brenner, D. W.; White, C. T. On the Way to Fullerenes: Molecular Dynamics Study of the Curling and Closure of Graphitic Ribbons. J. Phys. Chem. 1992, 96, 6133-6135. (51) Ugarte, D. Curling and Closure of Graphitic Networks under Electron-Beam Irradiation. Nature 1992, 359, 707-709. (52) Ding, Z. Y.; Frisch, M. A.; Li, L.; Gloyna, E. F. Catalytic Oxidation in Supercritical Water. Ind. Eng. Chem. Res. 1996, 35, 3257-3279.

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(53) Xie, K.; Muhler, M.; Xia, W. Influence of Water on the Initial Growth Rate of Carbon Nanotubes from Ethylene over a Cobalt-Based Catalyst. Ind. Eng. Chem. Res. 2013, 52, 14081-14088. (54) Esarte, C.; Abián, M.; Millera, Á.; Bilbao, R.; Alzueta, M. U. Gas and Soot Products Formed in the Pyrolysis of Acetylene Mixed with Methanol, Ethanol, Isopropanol or NButanol. Energy 2012, 43, 37-46.

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Thermocouple

Heater

Temperature controller

Reactor

Figure 1. Picture of experimental setup for SCF approach in a batch reactor.

20


Page 20 of 27

Page 21 of 27

(a)

400°C

(b)

400C

(d)

500C

1 µm (c)

500°C

1 µm (e)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


D

(f)

600°C

600C G

G'

2 µm

500

1000 1500 2000 2500 3000

Raman Shift (cm-1)

Figure 2. SEM images and corresponding Raman spectra of nanocarbons synthesized in supercritical toluene using ferrocene as a catalyst precursor at different temperatures in the absence of water: 400ºC (a,b), 500ºC (c,d), and 600ºC (e,f).

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(a)

C

Fe

O

5μm (b)

(c)

100 nm

500 nm

(d) 80

Yield Pressure 25

60

20

40

15

20

10

0 0.00

0.02 0.04 Ferrocene (mol/L)

5 nm

(f)

30 Pressure (Mpa)

100

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

(e) 5μm

5 0.06

5 cm

Figure 3. (a) SEM image and EDS analysis (insert) of carbon filament with encapsulated Fe synthesized at 500ºC (sample in Figure 1c and d). (b and c) TEM images of CNTs and carbon filaments synthesized at 600ºC; images depict large catalyst particles at the end of CNTs (b) and CNTs with well-defined hollow interior (c). (d) Plots of CNT yield and corresponding pressure of batch reactor as functions of ferrocene concentration at 600ºC. (e) A picture of bulk quantity ~12.5 g filament and CNTs powder obtained at 0.036 mol/L ferrocene in toluene solution; a standard pen is used as a scaling reference. (f) Low-magnification SEM image of CNTs (insert).

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Page 23 of 27

50

IG/ID = 1.1

(b)

IG/ID = 1.3

92.5 mmol/L

d = 35 ± 13 nm

(c)

40

Counts

138.7 mmol/L

30 20

Model

Gauss

Equation

y=y0 + (A/(w*sqrt(PI/2)))*exp(-2*((x-xc)/w)^2 )

Reduced Chi-Sqr

14.06254 0.91609

Adj. R-Square

Value y0

10

xc ?$OP:F=1

2 µm

46.3 mmol/L

IG/ID = 1.0

0

0

50

(d)

IG/ID = 0.7

100

d = 130 ± 68 nm

(e)

40

0 mmol/L

20 40 60 80 CNT Diameter (nm)

30 20 10

2 µm

500

1000

1500

2000 -1

0

2500

Raman Shift (cm )

0

50

100

150

200

250

300

CNT Diameter (nm)

Figure 4. (a) Raman spectra of nanocarbons formed as a function of water concentration during growth with ferrocene (0.036 mol/L) in toluene at 600ºC. SEM images and corresponding histograms of nanotube diameter distributions with Gaussian analysis fittings for growth with 92.5 mmol/L of water (b and c) and in the absence of water (d and f).

23


5.06447 36.3211

Standard Error 6.02855 1.45598

w

28.62049

6.141

A

1282.02538

432.1577

sigma

Counts

(a)

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14.31025

3.0705

FWHM

33.69806

7.23048

Height

35.74041

5.87794


(a)

100 µm

(c)

50 nm

Page 24 of 27

(b)

50 µm

(d)

(e)

~0.34 nm

10 nm

Figure 5. SEM (a,b) and TEM (c,d) images of OLCs synthesized using 0.036 mol/L ferrocene in toluene with 185.0 mmol/L of water at 600ºC. Inset TEM image of an OLC (e) with concentrically stacked graphite sheets.

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Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

20 µm

40 µm

(d)

(c)

50 µm

(e)

10 µm

(f)

20 µm

10 µm

Figure 6. SEM images of OLCs grown at 600ºC and 0.036 mol/L of ferrocene with ethanol (a,b), propanol (c,d), and butanol (e,f).

25



Figure 7. Schematic illustration of synthesis pathways for CNTs and OLCs using aromatic and alcohol precursors in a batch reactor. The pathway that involves supercritical alcohol with a reductant is based on Refs. 28 and 29.

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GRAPHICAL ABSTRACT

27


Supercritical Fluids as Reaction Media for Scalable Production of

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