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J. Phys. Chem. C 2007, 111, 16392-16396
The Influence of Catalyst’s Oxidation Degree on Carbon Nanotube Growth as a Substrate-Independent Parameter Teresa de los Arcos,*,† Peter Oelhafen,† Verena Thommen,† and Daniel Mathys‡ Department of Physics, UniVersity of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland, and Center of Microscopy, UniVersity of Basel, Klingelbergstrasse 50-70, 4056 Basel, Switzerland ReceiVed: June 25, 2007; In Final Form: August 9, 2007
The final oxidation state of iron particles, used as catalysts for the growth by chemical vapor deposition of carbon nanotubes, was investigated by means of in situ photoelectron spectroscopy. Although the chemical evolution of the catalyst during the process is known to depend strongly on the nature of the substrate, we could address nanotube growth from metallic and fully oxidized particles grown onto the same type of substrate (Al2O3). While oxide particles promoted carbon nanotube growth efficiently, metallic particles of roughly the same size, known to be active onto other kinds of substrates, showed a very poor performance.
Introduction The understanding of the mechanisms involved in the growth of carbon nanotubes (CNTs) by chemical vapor deposition (CVD) is a critical point. It needs to be elucidated to achieve the degree of growth control necessary for many envisaged applications of CNTs. There are a plethora of experimental parameters that need to be taken into account, and basic knowledge about the way they influence each other is fundamental. One crucial parameter is the shape and chemical state of the catalytic particle employed. In the case of growth directly on substrates, these two factors are strongly dependent on the nature of the selected substrate.1 On the basis of numerous trial and error studies published in the literature, there is nowadays a clear consensus as to the importance of selecting the appropriate catalyst-substrate combination. However, there is still a high degree of confusion as to the exact role played by the chemical composition and structure of the catalytic particles, as the precise CNT growth mechanism is mostly unknown. An additional problem is that because of the high temperatures and aggressive gas environments associated with the CVD process, the initial characteristics of shape and chemical composition of the catalyst change in a complex way.2 In our previous work, we have shown that the catalyst chemistry plays a fundamental role, by using in situ monitoring of the chemical state by photoelectron spectroscopy.1,3-5 Particularly, we saw clear differences in growth mode and individual characteristics of CNTs that were associated with the oxidation state of the iron catalyst employed. In those experiments, the particle formation was achieved by annealing thin layers of catalyst deposited onto different substrates. We determined that the different final oxidation states observed were a consequence of interaction with different buffer layers. In particular, FeO particles formed onto Al2O3 were particularly efficient and promoted denser and faster growth of CNTs than Fe particles formed onto other substrates such as TiN or TiO2.1,5 However, by following this approach it was not possible to clarify separately the role played by the oxidation state of the * To whom correspondence should be addressed. E-mail: t.arcos@ unibas.ch. † Department of Physics. ‡ Center of Microscopy.
catalyst from the influence of the substrate. Additionally, the particle size distributions onto the three substrates were different. The influence of the oxidation state of the catalyst for CNT growth has been investigated recently in other works as well.6,7 However, a controversial point in these cases is that the chemical analysis is performed ex situ with the consequent modification of the original composition due to exposure to air, which makes interpretation of the data difficult. In this work, we have investigated ways to manipulate the final chemical state of the catalyst independently of the employed substrate by tuning the oxidation state of the catalyst with oxygen plasmas. We have used an experimental setup consisting of a photoelectron spectrometer connected to a vacuum reactor adapted for CVD growth that allows for chemical characterization without air exposure of the samples. Experimental Methods The key point of our experimental set up consists on the possibility to perform all steps necessary for the CVD growth of CNTs sequentially in a high vacuum chamber without exposure to the air at any moment. Additionally, the growth chamber is connected directly to an ultrahigh vacuum chamber housing a photoelectron spectrometer in such a way that the growth process can be interrupted at any stage and the sample transferred in vacuo to the spectrometer chamber for characterization by X-ray photoelectron spectroscopy (XPS). With this approach, we can ensure that the chemical state of the sample surface is not altered by exposure to the atmosphere. The background pressure in the growth chamber was ∼1 × 10-4 Pa and could be lowered further to 4 × 10-5 Pa by cooling the chamber walls with a liquid nitrogen trap. We used p-doped Si (100) substrates (resistivity 7-21 Ωcm) that were rinsed consecutively in acetone and ethanol in an ultrasonic bath prior to introduction in the vacuum chamber. No further in situ cleaning procedure was performed. Therefore, all Si substrates used were covered with a native SiO2 layer with a thickness of 1.5-2 nm, as determined by ellipsometry. Al2O3 (or TiO2) layers of 20 nm thickness were deposited by magnetron sputtering of an Al (or Ti) target in a mixture of Ar and O2. Fe was deposited by sputtering of an Fe target by Ar ions using a Kaufman-type ion source. A quartz microbalance was used to monitor
10.1021/jp074928q CCC: $37.00 © 2007 American Chemical Society Published on Web 10/10/2007
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Figure 1. SEM and AFM characterization of the substrates after particle formation, without (a) and with (b) oxidative plasma treatment. The values in parentheses correspond to the mean particle diameter, as evaluated from the SEM images. Ra: mean values of the roughness relative to the center line, determined from the AFM measurements. Scale bars are 100 nm.
deposition rates. Oxygen plasma for the oxidizing of the sample was produced by applying RF (30 W) to a carbon-coated electrode under an O2 pressure of 2 Pa. The spectrometer was a Leybold EA10N equipped with a Mg KR (1253.6 eV) X-ray source and with an experimental resolution of 0.8 eV. The energy position of each spectrum was calibrated with reference to the 4f7/2 level of a clean gold sample (84.0 eV). We have performed CVD growth of carbon nanotubes by annealing the samples in vacuum to a temperature of 800 °C within 10 min. Once the temperature was reached, we let pure acetylene (C2H2) gas in the chamber at a total pressure of 8 × 10-2 Pa during 6 min. The use of low pressures has the advantage of slowing down the nanotube growth process, making it easy to determine relative differences at early growth stages. The annealing of the samples was done by resistive DC heating of a ceramic element onto which the back side of a Mo holder was pressed. The Si samples were fixed to the front side of the Mo holder. The main problem with this approach is the difficulty of determining the real temperature on the top face of the Si surface. We have monitored the temperature of the Mo holder surface by pyrometry, and these are the temperatures given along the text. Mathematical analysis of the XPS data was done using the software UNIFIT. For the mathematical treatment of the spectra, we have employed a Doniaj-Sˇ unjic´ function, which basically consists on the convolution of Gaussian and Lorentzian line shapes with a certain asymmetry factor R (related to the electron density of states at the Fermi level).8 The background of inelastically scattered electrons was simulated by a Shirley function. Ex situ characterization of the samples by scanning electron microscopy (SEM) was done using a Hitachi S-4800. Atomic force microscopy (AFM) was also used to determine the
morphology of the catalyst layer before CVD with a microscope from Digital Instruments, Dimension 3100, in tapping mode. Results and Discussion In our previous experiments,1 metallic iron deposited onto the thin Al2O3 layers used as buffer between the catalyst and the silicon substrate showed a marked tendency toward oxidation during the annealing of the sample to the high temperatures necessary for the CVD. Even when the annealing was performed under vacuum conditions of the order of 10-4 Pa, partial oxidation of the thin Fe layer could not be avoided, possibly due to interaction with the underlying oxide layer. Therefore, our first objective was to find a way to keep the Fe in a metallic state during the annealing onto Al2O3 layers, previously deposited by reactive magnetron sputtering. We found out that preannealing the Al2O3 layer in vacuum to 800 °C before iron deposition resulted in a lower degree of Fe oxidation later on. Additionally, we cooled the walls of the chamber with liquid nitrogen to reduce the background pressure to 4 × 10-5 Pa. Under these conditions, we performed two sets of experiments. In the first, we deposited a 1 nm layer of iron onto the preannealed substrates. The samples were then annealed in vacuum to the CVD temperature, which resulted in the formation of nanosized particles. Finally, they were exposed to acetylene during 6 min. For the second experiment, we produced nanoparticles in the same way with the difference that after particle formation the system was cooled to room temperature, and exposed to O2 plasma. Then the oxidized particles were annealed a second time to the CVD temperature and exposed to acetylene in the same way as above. In Figure 1, we can see SEM and AFM images of two such sets of particles after annealing in the state they present immediately before the beginning of the CVD. From the SEM images, we could determine mean particle diameter values of 5.4 ( 1.5 nm and 6.3 ( 3 nm for nontreated and plasma-treated particles,
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Figure 3. SEM images after CVD onto substrates with Al2O3 buffer layers. (a) CVD performed without previous O2 plasma treatment; (b) CVD performed after O2 plasma treatment. Scale bars are 100 nm.
Figure 2. XPS measurements of the Fe2p and C1s lines after annealing and CVD. (a) Without O2 plasma treatment; (b) with O2 plasma treatment. The spectra have been normalized for comparison, and shiftcorrected horizontally in the figure by the value given by the Ar2p shift (see text). The peaks labeled with an asterisk (/) correspond to shakeup satellites of iron oxides. Dotted lines, measured data; full lines, mathematical fit; dashed lines, background of inelastically scattered electrons.
respectively. The increase of size in what basically is the same particle distribution is due to the incorporation of oxygen. This roughening of the sample due to the plasma treatment can also be seen in the AFM images. In Figure 2, we show XPS measurements of these samples. In Figure 2a, we can see the signals of iron (Fe2p) after the annealing and after CVD and of carbon (C1s) after CVD in the case without plasma treatment. The binding energy position of the Fe2p3/2 component at around 706.8 eV and the typical asymmetry of the line indicate that the iron remains in metallic state both after annealing and CVD. (Note: the actual peak position is calculated to be between 707.4 and 706.2 eV. See Comment on the XPS Data Analysis at the end of the text for an explanation of the binding energy shift correction applied here.) The weak second doublet around 710 eV corresponds to iron oxide. As for the C1s line, it can be decomposed into two components, at approximate positions of 285.1-283.9 eV and 283.8-282.6 eV. We attribute the component at lower binding energy to carbide (possibly Al4C3),9,10 while the component at higher binding energy positions might be associated to a mixture of sp2 (at 284.3 eV) and sp3 (at 285.2 eV) bonds.11
In the case shown in Figure 2b, corresponding to particles oxidized in the O2 plasma, we can see that after annealing (in vacuum) the iron is still in a fully oxidized state. The profile of the Fe2p line is typical of a mixture of Fe2+ (709.7 eV) and Fe3+ (711.1 eV) oxidation states. The mixture of states is confirmed by the presence of their respective shakeup satellites at 714.5 eV for Fe2+ and 719.0 eV for Fe3+.12 After CVD, we can appreciate a partial reduction of the catalyst in the appearance of a component at 707.1-706.9 eV that is attributed to metallic iron. The other two features in the Fe2p line at 710.3-710.1 and 714.8-714.6 eV correspond to Fe2+ and its shakeup satellite.12 In the case of carbon, we have a single C1s line at 284.4 eV corresponding to CNTs. The weak feature at 291 eV is due to the π* plasmon characteristic of sp2 carbon. SEM images of the samples after CVD are shown in Figure 3, where we can appreciate a very clear improvement in CNT yield on the sample pretreated in O2 plasma (Figure 3b). This was to be expected because of the good catalytic activity of iron oxide particles, which was already established in our previous work.1 However, unlike those previous growth experiments (performed onto Al2O3 layers at higher acetylene pressure) we have in this occasion not pure oxide, but a mixture of oxidized and metallic iron. It is not clear at this point if each particle is a mixture of the two phases, or if two different families of particles (metallic and oxidized) are present. If the last situation would be true, the observation of the few thin nanotubes seen in Figure 3a (metallic particles) and the combination of thick and thin nanotubes of Figure 3b (metallic and oxidized particles) could be interpreted as a proof that the two kinds of particles are able to nucleate nanotubes with different characteristics onto the same substrate. However, at
Catalyst’s Oxidation Degree on Carbon Nanotube Growth
J. Phys. Chem. C, Vol. 111, No. 44, 2007 16395 performed low-pressure CVD growth of CNTs from metallic and oxidized Fe particles of similar diameter distributions, grown onto Al2O3 buffer layers. As expected, oxidized particles promoted CNT growth. However, metallic particles, which are efficient catalysts for CNT growth onto other kind of substrates, such as TiN or TiO2, showed a very poor performance. These results point out to the fact that catalyst chemical composition alone does not determine the characteristics of CNT growth, and that more information is needed about the possible role played by the substrate. Comment on the XPS Data Analysis
Figure 4. Top image: SEM image of nanotubes grown onto TiO2 buffer layer. Bottom image: backscattering image and magnification of one of the nanotubes seen on the upper image. The brighter spots on the image correspond to Fe particles. Note the iron-depleted region surrounding the base of the nanotube. The scale bar in the upper image is 1 µm; in the bottom image 100 nm.
the moment this interpretation is purely speculative. Further experiments are envisaged to address this point. The fact that metallic particles provide such a low yield of CNTs when deposited onto Al2O3 is an intriguing result, because metallic particles have already proven to be efficient catalysts for CNT growth when they are deposited onto other substrates, such as TiN or TiO2.1 To check the ability of iron particles to nucleate nanotube growth at low acetylene pressure, we present in Figure 4 an example of CNTs grown at 0.28 Pa onto a 20 nm thick TiO2 buffer layer. Onto this layer, the initial Fe layer (1 nm thick) turns as well into metallic Fe particles, as determined by XPS (data not shown here). However, after performing CVD (800 °C during 8 min) we can see a relatively efficient growth of thick and short carbon filaments on the surface. Nevertheless, in this case the comparison to the growth from metallic Fe onto Al2O3 layers is not straightforward. On the one hand, this growth has been performed without using a liquid nitrogen trap. On the other hand, the particles formed onto TiO2 are clearly bigger with a mean diameter of 28 ( 9 nm, as determined from SEM images. The bigger diameter achieved onto the TiO2 is associated to the higher mobility of Fe onto this substrate. This mobility can be further observed in the iron-depleted areas observed around the grown tubes, which seem to have gathered the iron within their hollow cavity by some capillarity-related process. Conclusions In an attempt to disentangle the actual catalyst chemical composition from the influence of the substrate, we have
The analysis of the chemical composition of the samples from the XPS spectra proved to be somewhat problematic because of the difficulties of correcting the binding energy shifts due to sample charging. Charging in electron spectroscopy results when a nonconductive sample cannot resupply fast enough the emitted photoelectrons. As a consequence, a positive potential builds at the sample surface. This has the effect of slowing down the photoelectrons, which thus arrive to the detector with reduced kinetic energy (i.e., higher apparent binding energy). Because of the deposition of an insulating layer of Al2O3, during the XPS measurement a slight charge of the surface is induced in our experiment. Additionally, because of the geometry of the samples, composed by layers of materials with different conductivity values (carbon/iron-iron oxide/aluminum oxide), the in-depth distribution of the retarding potential is not necessarily uniform, which results into different shift corrections to be applied to elements in different layers.13,14 We have performed an approximate correction using the apparent shift of Ar2p peaks (data not shown) corresponding to argon gas trapped within the Al2O3 film during deposition. Because of the unreactive nature of argon, we assume that any apparent energy shift of this peak with respect to the literature value of 241.9 eV15 can be attributed to the effect of surface charging without consideration to possible chemical shifts. This correction method has proven to give very consistent corrections for the positioning of the Al2p peak in our Al2O3 layers. The value of this shift, which corresponds to the charging of the aluminum oxide layer, gives us an upper limit for the expected shift of the other components. Additionally, in the case of iron, a careful analysis of the relative positions and asymmetry values of the different oxidation states permits the identification of the components with reasonable accuracy, despite the uncertainty in binding energy values introduced by the charging. The carbon signal corresponding to CNTs is not expected to present such shift. Acknowledgment. We thank the National Center of Competence in research for Nanoscience (NCCR) and the Swiss National foundation for financial support. References and Notes (1) de los Arcos, T.; Garnier, M. G.; Seo, J. W.; Oelhafen, P.; Thommen, V.; Mathys, D. J. Phys. Chem. B 2004, 108, 7728. (2) Helveg, S.; Lo´pez-Cartes, C.; Sehested, J.; Hansen, P. L.; Clausen, B. S.; Rostrup-Nielsen, J. R.; Abild-Pedersen, F.; Nørskov, J. K. Nature 2004, 427, 426. (3) de los Arcos, T.; Vonau, F.; Garnier, M. G.; Thommen, V.; Oelhafen, P.; Du¨ggelin, M.; Mathis, D.; Guggenheim, R. Appl. Phys. Lett. 2002, 80, 2383. (4) de los Arcos, T.; Wu, Z. M.; Oelhafen, P. Chem. Phys. Lett. 2003, 380, 419. (5) de los Arcos, T.; Garnier, M. G.; Oelhafen, P.; Mathys, D.; Seo, J. W.; Domingo, C.; Garcı´a-Ramos, J. V.; Sa´nchez-Corte´s, S. Carbon 2004, 42, 187.
16396 J. Phys. Chem. C, Vol. 111, No. 44, 2007 (6) Sato, H.; Hori, Y.; Hata, K.; Seko, K.; Nakahara, H.; Saito, Y. J. Appl. Phys. 2006, 100, 104321. (7) Okita, A; Ozeki, A.; Suda, Y.; Nakamura, J.; Oda, A.; Bhattacharyya, K.; Sugawara, H.; Sakai, Y. Jpn. J. Appl. Phys. 2006, 45, 8323. (8) Doniaj, S.; Sˇ unjic´, M. J. Phys. C 1970, 3, 285. (9) Hinnen, C.; Imbert, D.; Siffre, J. M.; Marcus, P. Appl. Surf. Sci. 1994, 78, 219. (10) Hua, L.; Dian-Hong, S.; Xin-Fa, D.; Qi-Kun, X.; Froumin, N.; Polak, M. Chin. Phys. 2001, 10, 832.
de los Arcos et al. (11) Dı´az, J.; Paolicelli, G.; Ferrer, S.; Comin, F. Phys. ReV. B 1996, 54 (11), 8064. (12) Lin, T. C.; Seshadri, G.; Kelber, J. A. Appl. Surf. Sci. 1997, 119, 83. (13) Barr, T. L. J. Vac. Sci. Technol. A 1988, 7 (3), 1677. (14) Doron-Mor, I.; Hatzor, A.; Vaskevich, A.; van der Boom-Moav, T.; Shanzer, A.; Rubinstein, I.; Cohen, H. Nature 2000, 406, 382. (15) Moulder, J. F.; Sticle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: MN, 1992.
