High-Yield Growth of Carbon Nanotubes on Composite Fe/Si/O Nanoparticle Catalysts: A Car-Parrinello Molecular Dynamics and Experimental Study

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1 10430 J. Phys. Chem. C 2010, 114, High-Yield Growth of Carbon Nanotubes on Composite Fe/Si/O Nanoparticle Catalysts: A Car-Parrinello Molecular Dynamics and Experimental Study Chad J. Unrau, Richard L. Axelbaum, and Cynthia S. Lo* Department of Energy, EnVironmental, and Chemical Engineering, Washington UniVersity in St. Louis, St. Louis, Missouri ReceiVed: September 25, 2009; ReVised Manuscript ReceiVed: May 4, 2010 Single-walled carbon nanotubes (SWCNTs) have recently been synthesized at high catalyst yield ( 90%) using a composite iron/silicon-oxide nanoparticle catalyst in a gas-phase diffusion flame environment. Since catalyst yields without silicon are less than 10%, the role of silicon in improving catalyst yield must be studied to understand the molecular-scale factors that govern carbon nanotube nucleation and growth. In this work, Car-Parrinello molecular dynamics simulations are employed to investigate the structure of Fe/Si and Fe/Si/O nanoparticle catalysts at synthesis temperatures (1300 K). The simulations show that silicon is uniformly dispersed on the iron surface when oxygen is not present, but covers only one hemisphere of the particle surface when oxygen is present to form a silica cap. These results are consistent with the results of substrate synthesis and the phase diagram of this Fe/Si/O system. The structure of the catalyst particle when oxygen and silicon are present thus facilitates the preferential decomposition of a carbon precursor on the Fe-rich side of the particle. On the basis of this finding, SWCNTs will nucleate preferentially on Fe/Si/O with segregated phases compared to catalyst particles with a uniform surface composition that typically become encapsulated in carbon before nucleation can occur. High catalyst yields are also demonstrated on Fe/Al/O catalysts, which indicate that high yields are not specific to the presence of silicon in the particle. The results of this study support the hypothesis that the addition of silicon or aluminum, in the presence of oxygen, to iron oxidebased catalysts results in a nonuniform surface composition that facilitates SWCNT nucleation. 1. Introduction The unique properties and applications 1 of single-walled carbon nanotubes (SWCNTs) have generated significant academic and industrial interest in designing transition metal-based catalysts 2 (e.g., iron, nickel, cobalt), and more recently transition metal-oxide catalysts, 3 for nanotube nucleation and growth. 4 These catalysts decompose a gas-phase carbon source, such as acetylene or carbon monoxide, 5 which provides carbon atoms that may assemble in ringlike configurations on the catalyst surface to nucleate a carbon nanotube. 6 Numerous methods have been developed for SWCNT synthesis that can generally be classified according to the catalyst environment: affixed to a substrate or freely floating in the gas phase. Substrate methods (e.g., those achieved via chemical vapor deposition (CVD)) 7 have received the most attention, since they provide good control over nanotube length, diameter, and purity. On the other hand, gas-phase methods have the advantage of being volumetric, continuous processes, which are desirable for applications such as composite materials that require large quantities of nanotubes and industrial-scale synthesis methods to make them economically feasible. 8 Although gas-phase synthesis methods have several distinct advantages, they suffer from several disadvantages, such as (1) low catalyst yields, where only a small percentage of catalysts form nanotubes, (2) short catalyst lifetimes, and (3) low catalyst number density, which is necessary to keep the particle size small. Recently, a high catalyst yield (>90%) was achieved in the gas phase by using composite iron/silicon/oxygen catalysts * To whom correspondence should be addressed. Phone: (314) Fax: (314) clo@wustl.edu. synthesized in an enriched-oxygen inverse diffusion flame. 9 Without silicon in the system, however, the catalyst yield was less than 10%. Thus, further investigation is required to understand the growth mechanism of SWCNTs on Fe/Si/O catalysts. Single-walled carbon nanotube formation is thought to begin with the formation of an initial hemispherical carbon cap on the surface of the catalyst particle This has been observed both experimentally and computationally by using molecular dynamics (MD) simulations. MD simulations have proven to be particularly valuable for modeling the growth mechanism of SWCNTs, since the small size of catalysts (on the order of 1 nm) makes experimental observation of SWCNT nucleation difficult. Classical, 13,14 density functional theory-based tight binding, 15 and ab initio 16,17 molecular dynamics have been utilized to study SWCNT formation. The latter approach employs a quantum mechanical treatment of the nuclear and electronic motion that provides a balance between accuracy in simulation of bond breakage and formation and computational time. Thus, ab initio methods are preferred for modeling the initial steps of SWCNT formation, while classical methods are preferred for the simulation of continued nanotube growth over longer simulation times. In this work, we employ Car-Parrinello molecular dynamics (CPMD) 18 to investigate the structure of Fe/Si/O catalysts and the function of silicon in improving catalyst yield. In the case of an Fe/Si catalyst without oxygen, the most stable configuration consists of silicon atoms distributed uniformly on the surface of a spherical iron cluster. On the other hand, when oxygen is present, silicon remains on one hemisphere of the cluster surface, with the majority coordinated with oxygen to /jp909255r 2010 American Chemical Society Published on Web 05/18/2010

2 High-Yield Growth of Carbon Nanotubes J. Phys. Chem. C, Vol. 114, No. 23, form a silica-like phase. We propose that this behavior results in a faster deposition rate of carbon on the iron-rich side of the cluster than on the silicon-rich side. Preferential carbon deposition may increase catalyst yield by allowing sufficient time for SWCNT cap formation before the particle becomes encapsulated in carbon. This hypothesis is supported by experiments that show high catalyst yield may also be achieved by adding elements such as aluminum to the iron oxide catalyst. 2. Computational Model The CPMD code v was utilized to calculate the electronic structure, properties, and reactivity of the catalyst system, using density functional theory (DFT) calculations and Car-Parrinello molecular dynamics (CPMD) simulations. The electron exchange and correlation energies were calculated by using the generalized gradient approximation of Perdew, Burke, and Ernzerhof, 20 and core electrons were treated with use of Vanderbilt ultrasoft pseudopotentials. 21 The local spin density approximation was employed to include spin polarization in the calculations, and an initial multiplicity was estimated based on the number of iron atoms in the cluster. Geometry optimizations were performed with an orbital convergence of 10-5 Ha, an energy convergence of Ha, and a plane-wave cutoff of 25 Ryd. CPMD simulations were performed at 1300 K, which is similar to the temperature at which SWCNTs form in the diffusion flames described in the next section. This temperature was maintained by velocity rescaling of the ions, using the Berendsen thermostat. 22 This thermostat was also used on the electrons to minimize the transfer of energy between ions and electrons. All calculations were performed with use of periodic boundary conditions by placing the catalyst system in a 16 Å cubic box. This box was sufficiently large that the cluster was calculated as if it were an isolated system, rather than part of a periodic structure. All clusters were constructed based on a 0.8 nm iron sphere with atoms in FCC positions. This crystal structure is characteristic of bulk Fe above 1189 K and has been observed previously in iron nanoparticles. 23 The cluster size was chosen so that it would be large enough to represent catalyst particles that form nanotubes under typical experimental conditions while optimizing for computational accuracy and cost. The number of atoms in the cluster was kept constant for all simulations, while varying the ratio of iron, silicon, and oxygen atoms. The Fe:Si and Fe:O atom ratios were set to be 2:1 and 1:1, respectively, which were obtained from experimental characterization of the atomic compositions under optimal conditions for SWCNT formation in diffusion flames. 3. Experimental Methods A detailed description of the experimental setup has been previously published. 9 Briefly, a triaxial inverse diffusion flame was established on a burner with a 12.8-mm-diameter central jet and a 6.35-cm-diameter coannular tube. A second coannular tube was used to introduce nitrogen, which acted as a sheath flow. Pure oxygen was introduced through the jet at 3.7 mg/s while ethylene and nitrogen were introduced through the first coannular tube at 15.6 and mg/s, respectively. Ferrocene and aluminum acetylacetonate were introduced with the fuel stream as precursors for iron and aluminum. The flow rate of ferrocene and aluminum acetylacetonate was 0.5 mg/min and 0.4 mg/s, respectively, to yield a catalyst Fe:Al molar ratio of approximately 2:1. The catalyst yield was estimated from the particle size distribution, which was measured with a fast-quench dilution probe (0.5 mm inlet) coupled to a scanning mobility particle sizer (SMPS). The SMPS consisted of a Kr 85 bipolar ion source, a TSI 3081 differential mobility analyzer (DMA), a TSI 3776 condensation particle counter (CPC), and a PC to run the software. The samples for analysis by scanning electron microscopy (SEM) were collected on a filter downstream of the flame, and subsequently mounted on copper mesh grids with a lacey carbon coating. 4. Results and Discussion 4.1. Influence of Silicon on the Structure of Iron Catalysts. Several studies have shown that the catalyst yield for gas-phase synthesis of single-walled carbon nanotubes on iron or iron oxide catalysts is extremely low ( 10%), even under optimal conditions such as uniform catalyst size and composition Instead, the majority of the catalyst particles become encapsulated in a disordered carbon matrix. On the other hand, a high catalyst yield has been observed in gas-phase diffusion flame synthesis when silicon is added to iron oxide catalysts. 9 Similarly, when iron or iron oxide catalysts are placed on a silicon or silica substrate, the yield for carbon nanotube formation is much greater ( 90%). 27,28 Thus, the presence of silicon likely alters the mechanism controlling catalyst particle encapsulation versus nanotube formation in gas-phase synthesis and possibly even in substrate synthesis. The role of silicon in the nanotube nucleation process may be elucidated by first considering the likely mechanisms for the assembly of carbon atoms on a pure iron or iron oxide particle versus a particle containing some fraction of silicon atoms on the surface. A pure iron or iron oxide catalyst particle on the order of 1 nm in size that is suspended in the gas phase will have a uniform surface composition 29 and is of an appropriate size to produce a SWCNT. As described in the Introduction, the nanotube formation process begins with the decomposition of carbon-containing molecules on the catalyst particle surface. Since the pure iron or iron oxide catalyst has a uniform surface composition, decomposition will occur evenly over the surface of the particle, which results in uniform coverage of carbon atoms. These carbon atoms may either bond together to encapsulate the catalyst or diffuse to one side of the catalyst to form a hemispherical cap. The former scenario is more likely since the characteristic time for the encapsulation process is less than the time needed for diffusion of carbon atoms. It thus appears likely that the addition of silicon to an iron or iron oxide catalyst may result in a different catalyst surface composition, leading to nonuniform behavior of carbon atoms on the catalyst surface. To investigate this possibility, Fe/Si clusters without oxygen were constructed and modeled with use of DFT calculations and CPMD simulations. Although oxygen is known to be present in the catalysts used in oxygen-enriched diffusion flames, it was not considered in this part of the study in order to isolate the effect of silicon on the catalyst surface structure and potential for SWCNT nucleation. In Fe/Si catalysts, several configurations for the silicon phase were considered: (1) Si concentrated in the core of the particle, (2) Si distributed uniformly on the surface, and (3) Si concentrated to one hemisphere of the catalyst surface. The total energies of the different configurations, as computed with DFT, were compared to gain predictive insight on the preferred location of silicon in these catalysts. For these calculations, three catalysts were constructed from a 0.8 nm (43 atom) cluster of iron with atoms in fcc positions. In each case, 13 iron atoms

3 10432 J. Phys. Chem. C, Vol. 114, No. 23, 2010 Unrau et al. Figure 1. Illustrations of 0.8 nm clusters with (a) silicon (yellow) concentrated in the core, (b) silicon concentrated on the catalyst surface uniformly, and (c) on one-half of the surface. The lowest energy configuration is shown in panel b, with the configuration in panel c being 1 ev higher in total energy, and the configuration in panel a being 6 ev higher in total energy. Figure 2. An illustration of the structure of the cluster shown in Figure 1c after a 5 ps CPMD simulation at 1300 K. were replaced with silicon to yield an Fe:Si ratio of approximately 2:1, which was experimentally determined to be the optimum ratio for catalyst yield in oxygen-enriched diffusion flames. 27 Three clusters are shown in Figure 1, each with a different silicon configuration. Iron and silicon are represented by the blue and yellow spheres, respectively. Geometry optimizations were performed on each of these clusters at 0 K, using DFT to determine which configuration is the most energetically favorable. The cluster with silicon concentrated in the core (Figure 1a) was much less stable, being 6 ev higher in total energy than the clusters where silicon was located on the surface (Figure 1b,c). These calculations thus indicate that the lowest energy configuration for Si is similar to that shown in Figure 1b, where Si is distributed uniformly over the catalyst surface. Although the cluster in Figure 1b represents the preferred configuration for Fe/Si clusters at 0 K, the most energetically favorable configuration may be different at the growth temperatures ( 1300 K) present in diffusion flames. To investigate this possibility, a CPMD simulation was conducted at 1300 K for up to 5 ps on the cluster with an initial Si cap, as shown in Figure 1c. This cluster was chosen since the silicon atoms in Figure 1c should distribute uniformly over the surface given sufficiently long simulation times if the preferred configuration of silicon is actually similar to that shown in Figure 1b. The structure of the cluster after 5 ps is shown in Figure 2. Figure 2 shows that over the course of the simulation, silicon redistributes on the surface of the cluster from a cap configuration (Figure 1c) to a more disperse configuration similar to that in Figure 1b. Thus, the most favorable structure of these Fe/Si clusters appears to be one where silicon is distributed uniformly over the surface of the cluster. This result has interesting implications for both substrate and gas-phase synthesis. Several studies have shown that when iron catalysts are placed on silicon substrates, the catalyst yield is high, but catalyst lifetimes are relatively short; this leads to a maximum nanotube length of roughly 1-10 µm. 23,30 Short catalyst lifetimes have been attributed to the formation of an iron silicide, which is thought to be inactive for nanotube growth. The results shown in Figure 2 support this hypothesis, since the preferred structure for Fe/Si catalysts at high temperature appears to be a uniform distribution of Si atoms over the Fe surface. This is in contrast to the configuration showing segregation of iron and silicon (Figure 1c), which corresponds to minimal interaction in a supported catalyst configuration between the iron particles and the silicon substrates. With respect to gas-phase synthesis, Fe/Si catalysts would likely form in the configuration shown in Figure 2. The structure of this Fe/Si catalyst is similar in structure to iron and iron oxide catalysts in that all possess a uniform surface composition. 29 Thus, any dissociation of adsorbed carbon-containing molecules (to provide carbon atoms for nanotube formation) would be expected to occur uniformly over the catalyst surface. This uniform distribution of carbon may not allow for preferential diffusion of carbon to form an SWCNT cap on one side of the catalyst: The result would be that the carbon would encapsulate the gas-phase catalyst particle. This conclusion is supported both by experiment 24,25,31 and ab initio molecular dynamics simulations Influence of Silicon on the Structure of Iron Oxide Catalysts. In contrast, experiments have shown that when oxygen is present during substrate synthesis, catalyst lifetimes are dramatically improved, and nanotubes of over 100 µm in length can be obtained. 23,32 The investigation of the catalyst structure in these studies revealed that the iron and silicon remained segregated over the course of nanotube growth. In the present study, the addition of oxygen to the Fe/Si cluster shown in Figure 1c would likely result in silicon remaining on one side of the catalyst particle instead of dispersing over the surface. To investigate this possibility, a surface oxide layer was added on the silicon side of the cluster in Figure 1c. Oxygen atoms were added to the system to give an Si:O ratio of 1:2, which is consistent with experiment (Fe:O ratio of 1:1). 33 The oxygen was added to the silicon side as opposed to the iron side since the Ellingham diagram indicates that iron will be reduced in the presence of silicon. The resulting Fe/Si/O cluster is shown in Figure 3a. Oxygen atoms are represented by the smaller red spheres. A 5 ps CPMD simulation was performed on the cluster shown in Figure 3a at 1300 K, and the resulting cluster is shown in Figure 3b. Figure 3c shows the total energy of the cluster as a function of time, which indicates that the system has equilibrated after 1 ps. Silicon tends to remain segregated from the iron when oxygen is present (Figure 3b), which is analogous to the experimental observations in the Fe/Si/O catalyst/substrate system. Entropically, one might expect the silicon and oxygen to diffuse around the particle surface but this does not occur due to the large negative enthalpy of formation of SiO 2. Since iron is significantly more active toward acetylene dissociation, 34 carbon precursor dissociation (acetylene in the case of our diffusion flames) will occur more rapidly on the iron-rich side of the catalyst than on the silicon-rich side. Consequently, an SWCNT cap may form and lift off of the iron-rich surface of

4 High-Yield Growth of Carbon Nanotubes J. Phys. Chem. C, Vol. 114, No. 23, Figure 3. An illustration of (a) an Fe/Si cluster with an oxide layer over the silicon, (b) the structure of the catalyst in panel a after a5ps simulation at 1300 K, and (c) a plot of the cluster energy versus time showing that the cluster has equilibrated after just 1 ps. Color key: oxygen, red; iron, blue; silicon, yellow. the catalyst with subsequent carbon atoms adding to the cap edge to form the nanotube wall. This concept is supported by the results of Raty et al., 16 who performed ab initio molecular dynamics simulations on 1 nm iron particles. Their results showed that a pure iron particle becomes encapsulated in carbon. On the other hand, when hydrogen is affixed to one side of the particle to represent a substrate, carbon atoms assembled on the other side of the particle to form a carbon cap that is the precursor for subsequent SWCNT growth Feasibility of Carbon Nanotube Nucleation on Composite Fe/Si/O Catalysts. To confirm that the Fe/Si/O cluster shown in Figure 3b could also result in SWCNT formation, carbon atoms were added to this cluster in two different configurations as shown in Figure 4. Carbon atoms are represented by the small gray spheres. Figure 4a shows six carbon atoms separated from each other on the iron surface of the cluster while Figure 4b shows these carbon atoms arranged in a hexagonal ring. Geometry optimizations were performed on both of these clusters at 0 K to determine which configuration was energetically favorable. The calculations indicated that the ring configuration was favored by 2 ev compared to the isolated carbon configuration. These results indicate that as carbon atoms are supplied to the surface of the catalyst, they will preferentially bond with each other rather than diffusing on the surface of the catalyst particle, which would eventually cause encapsulation. Moreover, since carbon will be supplied more rapidly to the iron-rich side versus the silicon-rich side of the particle, a SWCNT may nucleate preferentially on the iron-rich side as discussed above Experimental Verification of the Role of Silicon and Aluminum in Carbon Nanotube Nucleation. The role of silicon in gas-phase synthesis of SWCNTs thus appears to be similar to that of a substrate in chemical vapor deposition. In CVD, one role of the substrate is to prevent carbon from diffusing on the catalyst particle and encapsulating it before an SWCNT can nucleate. The presence of silicon in gas-phase synthesis of SWCNTs on Fe/Si/O catalysts gives the catalyst a similar structure to that of the substrate system (Figure 3b). Consequently, carbon source dissociation will occur more rapidly on the iron side of the catalyst, allowing an SWCNT to nucleate before the catalyst becomes encapsulated. On the basis of the results presented above, silicon essentially plays a steric or geometric role in improving catalyst yield for gas-phase nanotube synthesis. If this is the case, however, then a high catalyst yield should also be possible if elements other than silicon are added to the catalyst as long as the catalyst structure is similar to that depicted in Figure 3b. Iron catalysts on an alumina substrate have been shown to behave in a similar manner to that of the Fe/Si/O system, in that the iron and aluminum remain segregated in the particle. 35 To determine if

5 10434 J. Phys. Chem. C, Vol. 114, No. 23, 2010 Unrau et al. Figure 4. An illustration of six carbon atoms (gray spheres) added to the cluster of Figure 3b in (a) a separated arrangement and (b) a hexagonal ring arrangement. Figure 5. The particle size distribution of the flame when Fe/Al/O catalysts are present is shown in panel a, while panel b shows many SWCNTs produced from Fe/Al/O catalysts. The large size of the right-most mode in the size distribution relative to the other mode indicates that the catalyst yield is high. an Fe/Al/O catalyst could also result in the high catalyst yield achieved with the Fe/Si/O catalysts, aluminum acetylacetonate and ferrocene were added to the diffusion flame described in the Experimental Methods section. These precursors provide aluminum and iron atoms for particle formation through their decomposition near the flame surface. The flow rates of these precursors were such that the Fe:Al ratio of the catalysts was approximately 2:1, which matches the optimum composition of the Fe/Si/O catalysts. Figure 5a shows the size distribution of particles emerging from the flame when Fe/Al/O catalysts are present. A scanning electron micrograph of SWCNTs produced from these catalysts is shown in Figure 5b. In a previous study, we determined that if bare catalysts and SWCNTs are present in the flame, the size distribution will be bimodal, so the number of particles associated with each mode can be used to estimate the catalyst yield. 36 The size distribution in Figure 5a, with the mode toward the right side of the distribution corresponding to SWCNTs, indicates that the catalyst yield is high ( 90%). Figure 5b shows an SEM of SWCNTs produced from Fe/Al/O catalysts. The measured nanotube diameters were 1-2 nm and the lengths were up to several micrometers. The results shown in Figure 5 are quite similar to those obtained with Fe/Si/O catalysts in terms of catalyst yield and SWCNT size. 9 On the basis of these results and the structure of iron catalysts during SWCNT synthesis on silica or alumina substrates, we expect that Fe/Si/O and Fe/ Al/O catalysts have a similar structure with silicon or aluminum separated from iron. The ternary phase diagrams for the Fe-Si-O and Fe-Al-O systems support this catalyst structure, as both phase diagrams show a region of two immiscible liquids (iron and silica or alumina melts) at high temperature for the molar concentrations of elements employed in this study. 37

6 High-Yield Growth of Carbon Nanotubes J. Phys. Chem. C, Vol. 114, No. 23, preferentially on the iron-rich hemisphere of the particle, which may allow for a SWCNT cap to form. The probability of cap formation appears to depend on the percentage of the catalyst surface covered with silica. Fe/Al/O catalysts were also tested and demonstrated to give high catalyst yields, as seen with similar results achieved with Fe/Si/O catalysts. The addition of other elements to the reaction may thus also result in high catalyst yields, so long as the carbon precursor decomposition occurs preferentially on one side of the catalyst. Figure 6. The catalyst yield as a function of the Fe:Si ratio. Conditions higher than the melting point of iron are considered when evaluating the implications of the phase diagrams since catalysts of approximately 1 nm in size are expected to be liquid or at least exhibit a liquid-like behavior for the temperatures present in our diffusion flames. The phase diagrams also show that when oxygen is not present, a single melt of either Fe/Si or Fe/Al exists. 37 This is consistent with the results presented in Figure 2 and the results of substrate synthesis in the absence of oxygen. 23 The combined results of (1) CPMD simulations on Fe/Si/O clusters, (2) experimental characterization of both Fe/Si/O and Fe/Al/O clusters, and (3) the phase diagrams for both systems are consistent with the hypothesis that a catalyst is formed with a nonuniform surface composition. Part of the surface is ironrich while the other part is iron-deficient. This would be expected to lead to a higher rate of dissociation of the carbon precursor on the iron-rich part of the catalyst, since iron is more catalytically active, and thus a higher concentration of carbon on that part of the particle. Consequently, an SWCNT cap may form before the particle becomes encapsulated in carbon. The probability of cap formation is likely determined by the Fe:Si (or Fe:Al) ratio as shown in ref 3 and reproduced here in Figure 6. For high Fe:Si ratios, only a small percentage of the catalyst surface will be covered with silicon. Thus, the catalyst surface would be similar to that of a pure iron or iron oxide particle, which is known to result in low catalyst yields. For low Fe:Si ratios, most of the catalyst would be covered with silicon (Figure 1b), which would likely render it inactive toward decomposition of the carbon source and prevent the subsequent formation of a nanotube. The optimum Fe:Si ratio of 2:1 found from experimental studies most likely represents the optimum coverage of one hemisphere of the catalyst surface for achieving both carbon source decomposition and SWCNT cap formation. 5. Conclusions The results of this study suggest that silicon may improve iron oxide catalyst yield by reducing carbon precursor decomposition on silicon-rich areas of the catalyst surface. If oxygen is present, the silica phase is segregated from the iron oxide phase, as indicated by Car-Parrinello molecular dynamics simulations on model systems and the phase diagram for this system. Carbon precursor decomposition is expected to occur Acknowledgment. This research was funded by the NASA Missouri Space Grant. References and Notes (1) de Heer, W. A. MRS Bull. 2004, 281. (2) Banhart, F. Nanoscale 2009, 1, 201. (3) Steiner, S. A.; Baumann, T. F.; Bayer, B. C.; Blume, R.; Worsley, M. A.; MoberlyChan, W. J.; Shaw, E. L.; Schlögl, R.; Hart, A. J.; Hofmann, S.; Wardle, B. L. J. Am. Chem. Soc. 2009, 131, (4) Zhao, J.; Martinez-Limia, A.; Balbuena, P. B. Nanotechnology 2005, 16, S575. (5) Resasco, D. E.; Alvarez, W. E.; Pompeo, F.; Balzano, L.; Herrera, J. E.; Kitiyanan, B.; Borgna, A. J. 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