Article pubs.acs.org/JPCC
Enhancement of Vertically Aligned Carbon Nanotube Growth Kinetics and Doubling of the Height by Graphene Interface Rahul Rao, Neal Pierce, and Avetik R. Harutyunyan* Honda Research Institute USA, Columbus, Ohio 43212, United States S Supporting Information *
ABSTRACT: Proper size, density, and activity of catalyst nanoparticles are the key prerequisites for the growth of vertically aligned carbon nanotubes (or forests), which are expected to have unprecedented applications. The main challenge during growth is to create a high density of catalyst particles that remain catalytically active, and with a size distribution feasible for nucleation in the course of the nanotube growth. Here, we show that Fe clusters supported on graphene-covered silicon substrates enable forest growth with heights up to twice as compared to bare silicon along with an increase in tube density up to 30%. Detailed comparative analysis of nanotube growth kinetics, catalyst particle size distribution, and Fe nanoparticle phases on graphene-covered and bare substrates strongly suggests that graphene enhances the catalytic activity of Fe nanoparticles.
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properties.11−14 Recently, we have demonstrated that covering conductive substrates with a graphene layer enables the growth of carbon nanotube forests contrary to plain conductive surfaces, which still remain a challenging target.15 CNT forest growth on surfaces like Cu and Pt is achieved by modifying the iron catalyst−metal substrate interaction by the graphene layer, which reduces the catalyst mobility and maintains its size distribution. In this Article, we show that in fact the graphene/ catalyst interface may have a much larger impact on the growth of MWNT forests from Fe catalyst particles, specifically, doubling the heights of the forests and increasing their densities roughly up to 30% as compared to bare silicon substrates. On the basis of a detailed analysis of the growth kinetics of the nanotubes and catalyst particle size, distribution, and phases, we attribute our findings to graphene-driven enhancement of the catalytic activity and increased efficiency of nanotube generation by the Fe nanoparticles.
INTRODUCTION Control over the macroscopic properties of vertically aligned carbon nanotubes (CNTs) such as height and density is of great importance for multifunctional applications. For example, vertically aligned CNTs (or CNT forests) with high densities and lengths are desirable for their use as high current density replacements of copper interconnects.1,2 In addition, appropriate control over the heights and densities of CNT forests is necessary for the production of spinnable yarns.3,4 The typical method to produce single-walled CNT (SWNT) and multiwalled CNT (MWNT) forests involves chemical vapor deposition (CVD) growth of vertically aligned nanotubes from a dense film of catalyst nanoparticles supported on a substrate. In this scheme, a single nanoparticle produces a nanotube, and the growth and termination of the resulting forests are largely determined by the activities of the supported particles. Forest heights can be increased by prolonging catalyst lifetimes, notably by the introduction of water vapor and other oxygen-containing additives during CVD growth.5,6 On the other hand, forest densities can be increased by applying methods to increase nanoparticle densities, such as repeated sputtering/annealing of catalyst films or solution blade casting.7−9 These methods to increase particle densities mainly try to overcome the problem of sintering or coarsening of particles at the high growth temperatures employed (600−900 °C). However, increasing nanotube density during growth causes reduced diffusion of active carbon species to the bottom of the forests, leading to early termination of growth, and, as a result, shorter forests.8,10 Hence, for either of the growth parameters (density and height), control over the catalyst features is the key for achieving simultaneous improvement. It is now well established that catalyst particle thermodynamics and catalytic activities strongly depend on its features as well as on the interaction energy with the substrate and its © XXXX American Chemical Society
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EXPERIMENTAL METHODS The graphene is grown by low-pressure CVD on Cu foil (25 μm thick, 99.8%, Alfa Aesar).16 The foil is first loaded into a 2” diameter tubular quartz furnace and purged with an Ar/H2 gas mixture (4:1) at a flow rate of 50 sccm under 90 mTorr pressure for 20 min, followed by ramping up the furnace temperature to 1000 °C. Once the temperature is reached, the Cu foils are annealed for 30 min, followed by the introduction of CH4 (8 sccm) for 10 min along with the Ar/H2 gases. Following growth, the samples are cooled to room temperature at a rate of 30 °C/min under the Ar/H2 mixture. The graphene is transferred from the Cu foils to Si/SiO2 (280 nm oxide thickness) substrates (hereafter called SiO2-G) Received: May 4, 2014 Revised: August 18, 2014
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using a wet-chemistry polymer-based process.17 PMMA (495, Micro Chem) is first spin-coated (4000 rpm for 60 s) onto the graphene-covered Cu foils. Iron chloride (2 M solution, SigmaAldrich) is then used to etch the Cu such that the PMMA/ graphene film floats to the surface of the solution in a Petri dish. The PMMA/graphene film is then scooped up onto a silicon chip and floated into a second Petri dish containing deionized water. After being rinsed in DI water for several minutes to remove the excess iron chloride, the PMMA/graphene film is placed onto the SiO2 substrate and left to dry for a few hours. After being dried, the PMMA is stripped off in an acetone bath, and the samples are dried in air (see Supporting Information Figure S1 for a representative SEM image and Raman spectrum from SiO2-G). Vertically aligned MWNTs are grown at ambient pressure via an aerosol injection CVD method using ferrocene and xylene as the catalyst and carbon source, respectively.15 Ferrocene (10 wt %) is first dissolved in xylene through mild sonication. The mixture is then loaded into a syringe and delivered into a quartz tube furnace through a capillary connected to a syringe pump. The capillary is placed such that its exit point is just outside the hot zone of the tube furnace. This exit point is at ∼400 °C when the center of the furnace is heated to 750 °C. The SiO2 and SiO2-G substrates are loaded into the center of the quartz tube furnace on a wafer carrier, which is heated to the growth temperature of 700−800 °C under a constant flow of argon (500 sccm) and hydrogen (60 sccm). After the furnace reaches the growth temperature, the ferrocene/xylene mixture is injected continuously into the tube furnace at a rate of 1.2 mL/h for the duration of the CNT growth (few seconds to 4.5 h). At the end of the growth period, the furnace is turned off and allowed to cool to room temperature under the argon/ hydrogen flow. Following growth, the samples are characterized by Raman spectroscopy (Renishaw Raman microscope, laser excitation 633 nm), scanning electron microscopy (SEM, Zeiss Ultra), Xray diffraction (XRD, Bruker D8, Cu Kα, λ = 0.154 nm radiation), and atomic force microscopy (AFM, DI nanoscope, tapping mode).
Figure 1. Scanning electron microscopy images of MWNT forests grown on (a) SiO2-G, and (b) SiO2 for the same growth time (90 min). The dashed horizontal lines indicate the bases of the forests.
Figure 2 shows the effect of the graphene interface on the densities of the forests on SiO2 and SiO2-G. The two forests in
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RESULTS AND DISCUSSION Our CVD process using ferrocene/xylene typically produces vertically aligned MWNTs with an average diameter of ∼15 nm that grow via root growth on substrates placed inside the tube furnace (see Supporting Information Figure S2 for representative Raman spectrum from the MWNTs and TEM image from the MWNTs). SEM images in Figure 1 show a comparison of the MWNT forests grown on SiO2-G and SiO2 for the same growth time (90 min.). The height of the forest grown on SiO2G (∼200 μm) is clearly greater than that of the forest on SiO2 (∼120 μm). In general, we find that forest heights on SiO2-G can be up to twice as much as those on SiO2 for the same growth times. Note that we have employed monolayergraphene-coated SiO2 in this study (cf., Supporting Information Figure S1, and Supporting Information in ref 15), with an ID/IG ratio (ratio of intensities of the defect related Raman D band to the G band) of ∼0.3. The effect of the number of graphene layers and degree of crystallinity on MWNT forest growth have been discussed in detail in ref 15. Briefly, we find that a single graphene layer with an ID/IG ratio greater than 0.12 (i.e., graphene with appropriate defect density) is the most effective at providing the right density of catalyst nanoparticles to support MWNT forest growth on metallic surfaces.
Figure 2. SEM images of MWNT forests on (a) SiO2-G, and (c) SiO2 after ethanol-induced densification. Parts (b) and (d) show false-color overlays of the SEM images in (a) and (c) to show the differences in densities between SiO2 and SiO2-G more clearly.
Figure 2 have similar heights (∼60 μm) and have been densified by infiltration with ethanol, which causes the forests to collapse and the nanotubes to come together through capillary forces.18,19 The solvent-induced densification method has been used previously to estimate the areal density of MWNT forests.7 Note that, although the exact areal density is difficult to determine reliably via the densification method, it is very useful for distinguishing between the two samples B
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qualitatively. Figure 2b and d shows false color versions of the SEM images in Figure 2a and c, respectively. The compacted area of the forest after densification (red colored areas in Figure 2b and d) in SiO2-G is ∼30% higher than the area in SiO2. We consistently observe increases in density in the range of 20− 40% in the forests on SiO2-G (see Supporting Information Figure S3 for additional SEM images showing differences in densities between SiO2 and SiO2-G after densification). Although the method described above is more accurate for larger differences in densities, it is still reliable for a rough qualitative estimation. To gain insight into the mechanism behind the graphene promoted growth, we first analyze the growth kinetics of the MWNT forests on SiO2-G and SiO2. Figure 3 plots the final
Figure 3. Plot of MWNT forest heights versus growth times. The inset shows the evolution of forest heights at short growth times (1010 particles/cm2).22−24 Atomic force microscopy analysis of both the SiO2 and the SiO2-G substrates reveals similar iron nanoparticle densities (Supporting Information Figure S4 and Figure 3 in ref 15), implying that both of the substrates have the necessary particle densities to support CNT forest growth. Thus, the initiation of forest growth earlier on SiO2-G than on SiO2 suggests that, despite similar particle densities, more particles produce MWNTs on SiO2-G than on SiO2. Indeed, we find that SiO2-G supports MWNT forest growth even when the amount of ferrocene is reduced by one-half to 5 wt % (lowering the number of catalyst C
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particles), whereas only disordered MWNT mats are obtained on SiO2 at the lower particle densities (Supporting Information Figure S5). We consider the possibility that an increase in forest height can also be caused by a reduction in the diameters and/or number of walls of the MWNTs, thereby conserving the overall amount of carbon. However, TEM analysis of MWNTs grown on SiO2 and SiO2-G reveals similar diameters (average ∼15 nm) and wall number distribution (Supporting Information Figure S6). The enhancement of forest heights is also not due to different mechanisms governing the kinetics of MWNT growth. Arrhenius plots of forest growth rates versus inverse growth temperatures (see Supporting Information Figure S7 and accompanying discussion on the growth kinetics measurements) reveal activation energies for growth on both SiO2-G and SiO2 to be ∼1.2 eV, indicating that the kinetics are governed by bulk diffusion of carbon through the Fe catalyst.25,26 Our activation energy values are in accord with previously measured values for forest growth using injectionbased floating catalyst CVD.27,28 The mechanism of bulk diffusion hints at one possible reason for the growth enhancement, increased diffusion of carbon through the iron particles supported on graphene as compared to plain SiO2 surfaces. It is known that the iron catalyst can exist either in the metallic state or as a carbide during CNT growth.29−33 Evidence for the presence of BCC (α-Fe) and FCC (γ-Fe) phases during CNT growth has been shown through in situ X-ray diffraction (XRD),29 and for the carbide (Fe3C and Fe2C5) phases by in situ TEM studies.34 In addition, postgrowth XRD measurements have shown the presence of both pure (BCC and FCC) Fe and iron carbide encapsulated within the nanotubes.30,32 At our CVD growth temperature, diffusion of carbon through α-Fe (∼4 × 10−7 cm2/s)35 should be 4 orders of magnitude higher than that through Fe3C (∼10−11 cm2/s).36 Moreover, carbon diffusion through α-Fe is higher than that through γ-Fe (∼10−9 cm2/s).37 Postgrowth XRD analysis (Figure 5a and b) from several samples shows the presence of α-Fe, γ-Fe, and iron carbide (Fe3C as well as mixed FexCy phases). Representative XRD patterns are shown in Figure 5a, with the characteristic peaks corresponding to α-Fe (ICDD no. 06-0696) and γ-Fe (ICDD no. 65-4150), as well as mixed Fe−C phases (ICDD nos. 51-0997 and 44-1290). We also observe peaks in the XRD pattern corresponding to Fe3C (ICDD no. 003-0411). Such a mixture of phases has also been reported previously for MWNT growth under similar conditions (i.e., using ferrocene as a precursor).38,39 To compare the relative amounts of the various phases of Fe in SiO2 and SiO2-G, we plot a histogram showing the amounts of α-Fe, γ-Fe, and carbide (mixed Fe−C phases and Fe3C) in Figure 5b. The data in Figure 5b are compiled from the intensities of the respective phase in the XRD pattern (normalized to the silicon peak) collected from several samples. The histogram in Figure 5b reveals a higher amount of metallic Fe on SiO2-G as compared to that on SiO2. Moreover, the amount of α-Fe is more than γ-Fe on both substrates. According to the diffusion coefficients mentioned above, CNT growth from a metallic Fe particle on SiO2-G as compared to growth from a carbide particle on SiO2 would definitely explain the 2-fold increase in forest heights on SiO2G. However, we note that the XRD measurements are performed after growth when the samples have cooled to room temperature. Moreover, the XRD measurement collects signals from the entire sample, which includes both active and
Figure 5. (a) Representative XRD patterns collected after growth from SiO2 and SiO2-G substrates. (b) Histogram showing the relative amounts of α-Fe, γ-Fe, and mixed carbide phases (FexCy). The amounts in (b) are estimated from the relative intensities of the individual peaks normalized to the silicon peak.
inactive catalyst particles. It is therefore premature to conclude that the phase of the Fe catalyst is solely responsible for the observed enhancement in forest heights on SiO2-G. Next, we discuss another possible scenario based on our observations, enhanced catalytic efficiencies of Fe particles on graphene. It has been shown previously that during high temperature nanotube growth the hydrocarbon does not directly decompose into atomic carbon, but instead goes through decomposition into intermediate hydrocarbons as well as radicals.40−42 For instance, methane decomposition is accompanied by the formation of ethylene (C2H4), acetylene (C2H2), and CO/CO2 molecules.41 It is not necessary that the catalyst particle will also be highly efficient for the decomposition of these byproducts, which is a process that occurs alongside nanotube growth. These competing processes might therefore lead to changes in composition of the gaseous environment surrounding the particle, which in turn can lead to contamination of the catalyst surface and a dramatic reduction of the tube densities. Our growth method employs ferrocene (C10H10Fe) and xylene (C8H10), which are injected just outside the hot zone of the tube furnace, where the temperature is around 400 °C. At this temperature, both xylene and ferrocene vaporize and are carried downstream into the hot zone of the furnace. The byproducts of xylene decomposition at high temperatures (600−1000 °C) have been studied by mass spectrometry, and include toluene (C7H8), benzene (C6H6), methane (CH4), and acetylene (C2H2).43,44 The byproducts of ferrocene also consist D
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of similar hydrocarbon species along with iron atoms that form nanoparticles as they flow downstream through the reactor.44 Indeed, CNT growth has been demonstrated with ferrocene as the sole source of both carbon and Fe, albeit with poorer yield that can be increased with the additives such as sulfur.45,46 Hence, after condensation on the substrates, the Fe nanoparticles interact not only with atomic carbon, but also with various hydrocarbon species produced by the decomposition of ferrocene and xylene. The Fe nanoparticles have to catalytically decompose these species and dissolve carbon to produce the MWNTs. It has recently been shown that metal nanoparticles supported on graphene exhibit enhanced catalytic efficiencies for a variety of chemical processes including water splitting, hydrogen evolution, and hydrocarbon decomposition.47−49 It is therefore possible that the Fe nanoparticles on SiO2-G are more efficient at decomposition of various hydrocarbons than those on SiO2. From the above discussion, we can form a picture of the early stage of the growth process where the ferrocene and xylene precursors decompose in the gas phase, causing Fe atoms and clusters to condense on the substrates and nucleate particles. The particles on graphene-supported SiO2 are catalytically more active than their counterparts on bare SiO2. These particles on SiO2-G initiate collective MWNT growth earlier than on SiO2, as evidenced by the data in Figures 3 and 4. Furthermore, due to their higher activities, more Fe nanoparticles take part in nanotube growth on graphene-supported SiO2, leading to the higher densities of the MWNT forest on SiO2-G. Our results highlight the unique advantages of the graphene interface, which not only enables CNT forest growth on a variety of surfaces,15 but also enhances the growth kinetics of the forests.
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ASSOCIATED CONTENT
S Supporting Information *
Raman spectra, TEM images of the MWNTs, AFM images of the particles, histograms showing distribution of nanotube diameters and number of walls, and Arrhenius plot showing activation energies for MWNT growth on bare and graphenecovered SiO2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank T. Paronyan for assistance with part of the graphene growth and Q. Xu and S. Saber for help with the AFM and TEM imaging, respectively.
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