Article pubs.acs.org/cm
Mechanism and Enhanced Yield of Carbon Nanotube Growth on Stainless Steel by Oxygen-Induced Surface Reconstruction Sebastian W. Pattinson,† Balakrishnan Viswanath,†,∥ Dmitri N. Zakharov,‡ Jinjing Li,§ Eric A. Stach,‡ and A. John Hart*,† †
Department of Mechanical Engineering and Laboratory for Manufacturing and Productivity, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ‡ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973-50000, United States § Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States S Supporting Information *
ABSTRACT: It is well-known that carbon nanotubes (CNTs) can be grown directly on the surface of stainless steel (SS) alloys, because the native composition of SS contains elements that seed CNT growth upon hydrocarbon exposure at elevated temperature. Often such methods use acid immersion or oxidation in air to treat the surface prior to hydrocarbon exposure for CNT growth. However, there lacks a general understanding of how the surface chemistry and morphology influences the nucleation and growth of CNTs. Using environmental transmission electron microscopy, we observe that CNT growth is enabled by surface reconstruction of SS upon oxygen exposure at elevated temperature, followed by further breakup of the surface upon reduction, and subsequent CNT nucleation and growth upon hydrocarbon exposure. Using electron energy loss spectroscopy, we find that catalyst particles consist of both pure iron as well as iron alloys such as Fe−Cr and Fe−Ni. We use these insights to study the synthesis of CNTs on bulk net-shaped porous SS materials and show that annealing of the SS at 1000 °C in air prior to CVD using an ethylene feedstock mixture produces a 70-fold increase in CNT yield. Our findings demonstrate how process conditions can be designed for efficient manufacturing of CNT-enhanced stainless steel materials, and guide improved understanding of CNT growth on other industrially relevant metal substrates.
1. INTRODUCTION Scalable synthesis of carbon nanotubes (CNTs) directly on metallic substrates, such as stainless steel or copper, is an attractive route to manufacturing of CNT-enhanced materials for applications including heat exchangers, filtration membranes, and electrochemical capacitors.1−4 However, the most thorough understanding of substrate-bound CNT growth concerns oxide-supported catalyst systems, e.g., Fe/Al2O3.5−7 In contrast, the direct growth of CNTs on metallic substrates, ideally from catalytic seeds originating in the metal alloy, would enable engineering of bulk materials with functional electrical, thermal, and mechanical contact to the CNTs.4 Moreover, the properties of such hierarchical CNT−metal materials, including electrical and thermal conductivity, wettability, and surface adsorption capacity, depend intimately on the diameter, density, and alignment of the CNTs. To develop manufacturing methods for CNT-enhanced metal surfaces, it is necessary to have a fundamental understanding of how the surface characteristics govern the nucleation and growth of CNTs. CNTs have been grown on metallic substrates such as Inconel and stainless steel by depositing separate catalyst metals © XXXX American Chemical Society
to enable CNT growth, for example, through vaporization of liquid feedstock.4,8 Direct growth, on substrates such as copper, has been achieved but often suffers from poor yield and alignment.9−19 Stainless steel is a very promising candidate for the above-mentioned applications because its native composition can support CNT growth, and because it is a corrosionresistant material widely used in industrial applications that demand mechanical and thermal performance.20,21 Typically, the direct growth of CNTs from stainless steel requires either annealing in air or acid treatment of the stainless steel surface prior to CNT growth.10−14 These steps have also been combined by treating in acid and then growing CNTs in a mixture of oxygen and the carbon source.22 CNTs have also been grown by a flame synthesis method, in effect combining the air pretreatment and synthesis steps.23 Further, pretreatment of SS by heating in air has been shown to enable some degree of chiral selectivity of the CNTs via the formation of Received: November 15, 2014 Revised: December 23, 2014
A
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Chemistry of Materials iron and chromium oxides that stabilize small-diameter catalyst particles for CNT growth.11 Less commonly, growth from stainless steel has been achieved without these pretreatments.15,16,24,25 It has been suggested that nanoscale roughness provides discrete CNT nucleation sites, while the pretreatments remove the native chromium oxide on the stainless steel to expose catalytically active iron; however, the mechanisms by which these occur and their effect on CNT growth, if any, remain unclear.10−12,15−19 Using environmental transmission electron microscopy (ETEM), we provide the first direct observations of how oxygen pretreatment restructures the SS surface to enable direct CNT growth by thermal chemical vapor deposition. We find that surface oxidation and subsequent reduction causes the formation of nanoparticles on the stainless steel surface that are able to nucleate and grow CNTs. Crucial to the ability of these particles to grow CNTs is their ability to lift off the bulk stainless steel surface; this can mediated by the air annealing temperature, which causes roughening and breakup of the stainless steel surface. Using this understanding, we demonstrate significantly increased growth yield through air annealing at 1000 °C and furthermore suggest rational and scalable strategies to extend control CNT growth on this stainless steel and other metallic substrates.
Figure 1. (a) SEM (inset: optical with 10 mm scale bar) image of untreated SS mesh composed of ∼2 μm diameter fibers. (b) SEM image of 2 μm diameter SS fiber mesh following CNT growth.
2. MATERIALS AND METHODS CNT growth experiments were performed on stainless steel 316 filtration media (Pall Corporation). In the tube furnace, CNTs were grown at atmospheric pressure by first annealing the sample in air (AI D300, Airgas) at temperatures ranging from 250 to 1000 °C for 50 min, then flushing the reactor with 1000 sccm of He (HeUHP300, Airgas) for 20 min at 775 °C, and then switching the gas flow to a mixture containing 100 sccm each of C2H4 (EY UHP200, Airgas), H2 (HY UHP300, Airgas), and He for CNT growth for 15 min, also at 775 °C. For the ETEM experiments, 2 μm diameter fibers (SS 316, Pall Corporation) were used, which presented edges that were thin enough to allow transmission of the electron beam. The instrument was an FEI Titan 80/300 E-TEM, equipped with a CEOS postspecimen aberration corrector and operated at 300 kV. Experiments were performed by heating the sample in 40 mTorr oxygen at 500 °C for 30 min, then evacuating the sample area for between 2 and 30 min, and then introducing 50 mTorr C2H2 and 40 mTorr H2 at 775 °C for the desired duration (typically up to 5 min). Growth occurred within 10− 30 s of the carbon source being introduced, and acetylene was passed into the reactor until growth ended, which usually was within 5 min of the carbon source introduction. EELS spectra (Gatan GIF Tridiem, model 863) were acquired in situ analyzed using a power law background subtraction fitted to the edge onset of peaks. To compare the L3/L2 white line ratio, a linear background was subtracted below each L line.26,27 For the L3 line, this was from the edge onset to the first minimum. For the L2 line, this was from the first minimum to the second minimum. XPS was performed using a Physical Electronics Versaprobe II X-ray photoelectron spectrometer (LS). The instrument used a scanning Xray microprobe with a raster scanned 10 μm diameter X-ray beam (pass energy 187.85 eV, pressure 10−9 Torr).
the fibers uniformly, though the CNTs are short enough not to block the pores between the fibers. Using the TEM, we first found that the as-produced stainless steel surface is covered by a thin amorphous carbon coating, approximately 4 nm thick (Figure 2a). Under the amorphous layer, we find a bulk surface with a spacing of 0.2 nm that is compatible with the (022̅ ) reflection of Cr2O3 as well as the (1̅10) reflection of BCC-Fe.28,29 The EELS spectrum taken at room temperature has an iron L3/L2 white line ratio of ∼3, which is consistent with metallic iron,30 while the chromium L3/L2 ratio is ∼2, suggesting a valence between II and III and thus a mixture of Cr2O3 and CrO, or a nonstoichiometric combination of the two.26 Next, we heated the stainless steel sample to 500 °C in 50 mTorr oxygen. After holding the temperature constant for 30 min, the surface of the stainless steel becomes faceted (Figure 2b) and polycrystalline. The 0.34 nm spacing seen at the surface corresponds to the (111) reflection of CrO3, suggesting further oxidation of the surface. The EELS spectrum now also displays a strong oxygen peak, while the L3/L2 ratio of the iron peaks has increased to ∼5, suggesting an increase in the valence to 3+ and, therefore, that the iron has been oxidized to form Fe2O3.15 The chromium peak is too small to identify a change in the L3/L2 ratio. This may be due to a chromium-depleted region forming just beneath the surface layer following preferential migration of the chromium to the surface during oxidation, or may be a result of the beamspot falling on a chromium-poor area.10,31 Therefore, we suggest that the lattice mismatch between the newly formed oxide and the bulk iron induces strain that causes the surface to fracture. This is wellknown in metallurgy where oxide scale forms on stainless steel surfaces that are annealed in air.19,28,31 After oxidation, the sample area of the ETEM must be evacuated before the H2 and C2H2 can be introduced. During
3. RESULTS AND DISCUSSION Initial experiments were performed on an SS mesh substrate (see the Materials and Methods) comprising 2 μm diameter fibers (Figure 1a). Following heating in air at 500 °C and then exposure to CNT synthesis in the tube furnace reactor, the SS fibers are covered by a tangled film of CNTs and carbon nanofibers (CNFs) (Figure 1b). The tangled CNT mesh covers B
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Figure 2. Sequence of high-resolution TEM images and EELS spectra (with linear intensity scale) of a 316L stainless steel surface at (a) room temperature in 4 × 10−7 Torr vacuum with 0.20 nm spacing indicated, (b) in 40 mTorr oxygen at 500 °C with 0.34 nm spacing indicated, and (c) in 4 × 10−7 Torr vacuum after venting the oxygen, but prior to injection of C2H2 and H2, with 0.27 nm spacing indicated. Image (b) was captured by averaging across 13 frames acquired using the Gatan K2-IS Direct Electron Detection camera.
Figure 3. In-situ observation of CNT nucleation and growth on the SS surface: (a) Growth sequence of an individual CNT, where the catalyst particle is elongated and separates from the bulk surface, producing the CNT in its wake (video S4, Supporting Information). (b) EELS spectra of individual Fe−Ni and Fe−Cr catalyst particles after growth completed, but in growth atmosphere and at growth temperature.
by H2, which decreased its surface energy and improved its wettability to the fiber surface. Once C2H2 is introduced, we observe that nanoparticles lift off from the SS fiber surface and grow CNTs and carbon nanofibers (Figure 3a and video S4, Supporting Information). Every CNT we observe features a large catalyst particle at its end and thus occurring by the “tip growth” mechanism, as has been observed for carbon nanofibers,33 though others have observed mixtures of tip and base growth from stainless steel.12 The dissolution of carbon thus promotes separation of the catalyst from the surface, and the fracturing of the surface during air annealing enables the lift-off of catalyst. EELS of individual catalyst particles after growth (Figure 3b) shows both iron−chromium, and iron−nickel mixtures. Interestingly, we did not find any pure chromium or nickel particles that grew CNTs. This suggests that, while iron is necessary to initiate CNT growth, the presence of chromium or nickel will not deactivate the catalyst, as has been previously suggested in the literature on CNT growth from SS.10,15,34 We also note that nickel and chromium have been used previously in catalytic CNT growth by chemical vapor deposition.35 To enable translation of our insights to a scalable production process, we also performed CNT synthesis experiments in a standard tube furnace, operating at atmospheric pressure. Stainless steel mesh (2 μm fiber diameter) identical to that used
evacuation for 30 min, the faceted surface reduces and nanoscale protrusions appear (Figure 2c). Visible in Figure 2c are a discrete nanoparticle, consistent with the [111] zone axis of Fe3O4, as well as solid regions with a spacing of 0.27 nm, consistent with the (01̅4) reflection of Cr2O3. Under these conditions, the EELS spectrum shows a slightly less prominent oxygen peak, though this may also be due to the measurement being taken in vacuum, not in an oxygen atmosphere. The iron L3/L2 ratio has decreased to ∼4, suggesting a decrease in oxidation number during the removal of the oxygen, with likely a majority of the iron present as FeO.32 The chromium L3/L2 ratio has increased to 3, suggesting a decrease in the oxidation number to 2 as compared to before its oxygen treatment.26 The results imply that the oxide crystals produced during treatment in oxygen are partially reduced when the oxygen is removed and thereby contract and become less faceted, increasing the surface roughness. Following the evacuation step, the CNT growth step was performed. However, due to the constraints of the gas inlet system, H2 was introduced approximately 10 s before C2H2. While H2 was present but before C2H2 was added, we occasionally observed the flattening of nanoparticles on the SS surface (Figure S1 and video S3, Supporting Information); this occurred immediately following the introduction of H2 into the microscope. We expect this indicated reduction of the particle C
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Figure 4. Translation of ETEM study insights to bulk synthesis of CNTs on SS fiber mesh. Optical and SEM images of SS mesh (initial size 6 × 6 × 1 mm) (a, c, e) after annealing in air at 500 °C for 50 min at atmospheric pressure, followed by CNT synthesis (see the Materials and Methods), and (b, d, e) after annealing in air at 1000 °C for 50 min at atmospheric pressure, followed by CNT synthesis (see the Materials and Methods). Insets to (a, b) show the pieces of stainless steel mesh prior to air treatment and CNT synthesis.
in the ETEM was annealed in air at 500 and 1000 °C. Our hypothesis, based on the ETEM findings, was that higher annealing temperatures would lead to increased fracturing of the stainless steel surface and, therefore, to a greater density of loosely bound active catalyst particles that would be prone to lift-off for carbon filament growth. The optical images in Figure 4 show that annealing in air at 1000 °C prior to growth results in a very noticeable increase in the volume of the piece after the CVD process, as compared to annealing in air at 500 °C. The mass increase resulting from carbon deposition after air annealing at 500 °C was ∼50%, whereas the mass increase after annealing at 1000 °C was ∼3500%. However, the produced structures appear to be primarily carbon nanofibers rather than nanotubes under these conditions. The nanofibers (Figure S2, Supporting Information) have graphitic walls with stacked lamellae and bamboo-like septa along their core. To investigate whether morphological, and not compositional, changes in the stainless steel led to this increase in CNT growth density, XPS was performed on stainless steel mesh samples annealed in air at 500 and 1000 °C. This showed that the surface of the mesh was composed primarily of oxygen and carbon, with smaller amounts of chromium and iron also detected (Figure 5d). Apart from a decrease in carbon signal intensity, the composition does not significantly change after annealing in air. This supports the conclusion that air annealing enables CNT nucleation and growth primarily through mechanical modification of the stainless steel surface. To further elucidate the morphological changes wrought in the stainless steel by air annealing, SEM images were taken of mesh as delivered, annealed at 500 °C, and at 1000 °C (Figure
5a−c). The as-delivered mesh is relatively smooth, whereas, after annealing at 500 °C, the wires feature noticeable pits. Annealing at 1000 °C causes the segregation of the wire into chains of particles with a diameter ∼50−400 nm on the wire surface. This can be explained by stainless steel 316 recrystallizing relatively rapidly at temperatures greater than 750 °C.36,37 Growing grains eventually consume the entire surface of the wire, leaving large grain boundaries that lead to weak adhesion. Nevertheless, surface-bound CNT nucleation sites form upon the reduction of these particles. As CNTs grow on these particles, the particles fracture and separate, increasing the surface area available for CNT growth and contributing to the 70-fold greater carbon yield upon CVD after annealing at 1000 °C. Our new understanding of how surface restructuring enables CNT growth on bulk SS surfaces suggests a number of strategies to improve CNT growth on these substrates and other metals. The most effective means of increasing CNT yield, i.e., the number of CNTs and/or mass per unit area of the substrate, is likely to be increasing the density of well-defined nanoparticles on the surface. This must occur while imbuing the particles with a chemical composition suitable for CNT growth upon hydrocarbon exposure. We studied air annealing as a means to do so and find that the surface reconstruction is governed by the temperature and duration of the treatment. However, higher air annealing temperatures reduce the mechanical stability of the SS mesh, and the surface restructuring must, therefore, be balanced against the mechanical stability required for handling and to ensure resistance to loads in the desired application context. D
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could be increased by using a different SS alloy with less chromium, or by depositing iron on the surface prior to annealing. Further, the faceting already caused by the air annealing could be complemented by the addition of ammonia, either before or during growth, to increase catalyst activity both through surface morphology changes as well as phase change of the iron to iron carbides.38−40 The CNT yield and morphology can also be influenced by the carbon source and complete reaction mixture through the differences in their breakdown pathways.41−43 Additionally, while CNT growth on SS has been reported without pretreatment, CNT synthesis involves hydrocarbon exposure at elevated temperatures, and therefore, some SS surface reduction and fracturing is likely even without an oxidation step.15,16,24,25 It is also conceivable that the SS production method (e.g., wire drawing) is influential in the CNT growth process, via the surface roughness and microstructure. We also note that some previous studies of CNT growth on SS have found base growth rather than tip growth to be prevalent; base growth is known to result from strong catalyst−substrate interaction and could likely be tuned through the pretreatment conditions.25
4. CONCLUSION We have shown how surface restructuring upon oxygen exposure at elevated temperatures enables CNT growth from bulk SS. The surface forms a primarily chromium oxide (Cr2O3 and CrO3) outer layer, and the change in the lattice constant between the growing oxide crystals and the bulk metal fractures the stainless steel surface and forms the faceted, dense, polycrystalline surface observed. After air is removed from the annealing atmosphere, the oxide crystals are partially reduced, causing a decrease in the volume of the crystals at the surface and a loss of their faceting. The decrease in volume of the previously dense surface leads to the formation of protruding nanoparticles that are not strongly bound to the stainless steel surface. Once hydrogen and the carbon source are passed into the reactor, CNT nucleation occurs as active particles, composed of iron or iron alloys, lift off from the surrounding surface. The density of particles that produce CNTs, and therefore the bulk carbon yield, is strongly affected by the morphology of the stainless steel substrate. Our findings unify previous studies that applied various recipes to activate stainless steel for CNT growth and present guidelines for surface preparation of bulk metals for high yield synthesis of hybrid material architectures.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 5. Influence of air annealing temperature on surface morphology of SS fibers within the mesh. SEM images of fibers (a) as delivered, (b) after annealing in air at 500 °C for 50 min, and (c) annealing in air at 1000 °C for 50 min. (d) XPS of stainless steel mesh following annealing in air at different temperatures, but before CNT growth.
Videos of an iron particle changing shape upon hydrogen exposure and of CNT growth from stainless steel. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
In principle, increased yield could be achieved by surface treatment using many methods instead of, or in addition to, annealing in air, such as higher temperatures than those used in our study, combinations of acid and air treatments, ultrasonication before or after air annealing, ion beam etching, or by tuning the oxide reduction kinetics through the introduction of hydrogen. Also, the concentration of iron at the SS surface
*E-mail:
[email protected] (A.J.H.). Present Address ∥
School of Engineering, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh, India Notes
The authors declare no competing financial interest. E
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ACKNOWLEDGMENTS Primary financial support was provided by Pall Corporation, to S.W.P., J.L., and A.J.H. Additional support to S.W.P was provided by a National Science Foundation Science, Engineering, and Education for Sustainability (NSF SEES) postdoctoral fellowship (Award Number 1415129). Financial support to B.V. and travel for in situ TEM experiments were provided by the Department of Energy, Office of Basic Energy Sciences (DE-SC0004927). We thank Scott Hopkins and Hongbin Zhao of Pall Corporation for technical discussions about the SS filter media and for providing samples. In situ TEM experiments were performed at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences (DE-AC02-98CH10886). XPS analysis made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation (DMR-08-19762). Electron microscopy was performed at the Center for Nanoscale Systems (CNS) at Harvard University, a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation (ECS0335765).
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