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Langmuir 2007, 23, 9046-9049
Direct Growth of Aligned Multiwalled Carbon Nanotubes on Treated Stainless Steel Substrates Charan Masarapu† and Bingqing Wei*,‡ Department of Electrical and Computer Engineering and Department of Mechanical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed April 16, 2007. In Final Form: June 9, 2007 Growth of aligned carbon nanotubes (CNTs) on electrically conductive substrates is promising for many applications; however, the lack of complete understanding of the substrate effects on CNT growth poses a lot of technical challenges. Here, we report the direct growth of aligned multiwalled nanotubes (MWNTs) on chemically treated stainless steel (Type 304) using a chemical vapor deposition (CVD) process. A detailed X-ray photoelectron spectroscopy (XPS) analysis has been carried out for the various treated samples in order to better understand the correlation between the surface properties of the substrates and the MWNT growth. The XPS studies revealed that the CNTs prefer to grow on the enriched surface of iron oxides obtained by the chemical treatment rather than on the passive chromium oxide films present on the surface of the as-received stainless steel substrates. The density and alignment of the MWNTs could therefore be controlled by tuning the ratio of the iron oxides to chromium oxides through the chemical treatment on the stainless steel surfaces. On the basis of this method, selective growth of CNT patterns on stainless steel has also been demonstrated.
Introduction properties1
Carbon nanotubes (CNTs) with their exceptional show great potential in building new and improved devices with better performance. Utilizing CNT-based electrodes in electrochemical devices such as supercapacitors has displayed phenomenal storage capacity and power handling capability2 compared to conventional capacitors. Rechargeable Li-ion batteries utilizing CNT electrodes have exhibited reversible storage capacities in the range of several hundred milliampere hours per gram, even at high cycling rates.3,4 Novel devices such as transistors,5 flat panel displays,6,7 and gas discharge tubes8 utilizing CNTs have already been demonstrated. In most of the above-mentioned applications, the device performance would be dramatically improved if the CNTs were directly synthesized on a conducting substrate. For example, in field emission applications, the cathode is prepared by mixing the CNTs with organic binders and then coating on a metal substrate.6 Similarly, in applications involving the electrochemical intercalation of Li ions, the CNTs were mixed with a polymeric binder in order to increase the adhesion of the electrode material to the current collector.2,4 Several intermediate steps were associated4 with preparing these coatings, and care should be taken in order for the CNTs to be uniformly distributed and properly adhered to the substrate surface. These problems can * Corresponding author. E-mail:
[email protected]. † Department of Electrical and Computer Engineering. ‡ Department of Mechanical Engineering. (1) Ajayan, P. M. Chem. ReV. 1999, 99, 1787. (2) Niu, C.; Sichel, E. K.; Hoch, R.; Moy, D.; Tennent, H. Appl. Phys. Lett. 1997, 70, 1480. (3) Gao, B.; Kleinhammes, A.; Tang, X. P.; Bower, C.; Fleming, L.; Wu, Y.; Zhou, O. Chem. Phys. Lett. 1999, 307, 153. (4) Shin, H. C.; Liu, M.; Sadanadan, B.; Rao, A. M. J. Power Sources 2002, 112, 216. (5) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49. (6) Wang, Q. H.; Setlur, A. A.; Lauerhaas, J. M.; Dai, J. Y.; Seelig, E. W.; Chang, R. P. H. Appl. Phys. Lett. 1998, 72, 2912. (7) Choi, W. B.; Chung, D. S.; Kang, J. H.; Kim, H. Y.; Jin, Y. W.; Han, I. T.; Lee, Y. H.; Jung, J. E.; Lee, N. S.; Park, G. S.; Kim, J. M. Appl. Phys. Lett. 1999, 75, 3129. (8) Rosen, R.; Simendinger, W.; Debbault, C.; Shimoda, H.; Fleming, L.; Stoner, B.; Zhou, O. Appl. Phys. Lett. 2000, 76, 1668.
be overcome by directly synthesizing CNTs on conducting substrates, which prevent the usage of any binder that reduces the weight of the cell, which in turn accounts for the improved energy density of the cell. Furthermore, the mechanical robustness will also be greatly enhanced with very low contact resistance between the CNTs and the conducting substrate. In addition, the alignment of the CNTs can be kept intact in the directly synthesized process, which is not possible when a conventional coating process is employed. Earlier studies involving aligned CNTs grown directly on conducting substrates have shown better stability and overall performance in the respective applications.9-11 In recent years, there has been a considerable increase in the development of new strategies for synthesizing CNTs on conducting substrates. For example, synthesis of CNTs on stainless steel substrates using microwave plasma chemical vapor deposition (MPCVD),12,13 radio frequency-powered plasma enhanced chemical vapor deposition (PECVD),14 and flame synthesis15,16 have been reported. In the above-mentioned methods, either the CNTs were not well aligned on the substrate or the synthesis procedure was laborious. For instance, in the PECVD14 method, the pretreatment of stainless steel involves polishing, etching in hydrofluoric acid, and hydrogen plasma treatment to create catalyst particles, which are quite cumbersome. Recently, the CNTs were grown on Inconel 600 using a floating catalyst CVD method, where the catalyst is allowed to flow along with the carbon source.17 However, no report has hitherto been available on the mechanism behind the growth of CNTs on stainless steel substrates, which is very important for better control of the CNT density and alignment. (9) Croci, M.; Arfaoui, I.; Stockli, T.; Chatelain, A.; Bonard, J. M. Microelectron. J. 2004, 35, 329. (10) Rao, A. M.; Jacques, D.; Haddon, R. C.; Zhu, W.; Bower, C.; Jin, S. Appl. Phys. Lett. 2000, 76, 3813. (11) Chen, J. H.; Li, W. Z.; Wang, D. Z.; Yang, S. X.; Wen, J. G.; Ren, Z. F. Carbon 2001, 40, 1193. (12) Wang, N.; Yao, B. D. Appl. Phys. Lett. 2001, 78, 4028. (13) VanderWal, R. L.; Hall, L. J. Carbon 2003, 41, 659. (14) Park, D.; Kim, Y. H.; Lee, J. K. J. Mater. Sci. 2003, 38, 4933. (15) Lee, G. W.; Jurng, J.; Hwang, J. Carbon 2004, 42, 667. (16) Pan, C.; Liu, Y.; Cao, F.; Wang, J.; Ren, Y. Micron 2004, 35, 461. (17) Talapatra, S.; Kar, S.; Pal, S. K.; Vajtai, R.; Ci, L.; Victor, P.; Shaijumon, M. M.; Kaur, S.; Nalamasu, O.; Ajayan, P. M. Nat. Nanotechnol. 2006, 1, 112.
10.1021/la7012232 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007
Direct Growth of MWNTs on Treated Stainless Steel
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Table 1. Chemical Composition of the Most Commonly Used Austenitic Stainless Steels (Type 304) grade (wt.%) 304
C
Mn
Si
P
S
Cr
Ni
Fe
0.08 2.0 1.0 0.045 0.03 18-20 8-10.5 rest
In this paper, we report a simple processing technique involving cost-effective stainless steel (Type 304, composition is listed in Table 1) that can be used as a conducting substrate for the direct growth of high-density aligned multiwalled nanotubes (MWNTs). The surface properties of the substrates were particularly investigated, and the effect of treatment of the substrates on the CNT growth is critically analyzed. Such thorough understanding is a foundation for the site-selective growth of CNTs on conducting substrates for many prospective applications such as in nanoelectronics, field emission devices, and so forth. On the basis of the understanding of the obtained X-ray photoelectron spectroscopy (XPS) results, here we report a conventional photoresist patterning technique for the selective growth of CNT patterns on stainless steel substrates. This kind of photopatterning technique has been popularly used to synthesize CNT patterns on Si-SiO2 substrates.18-22 Experiment The as-received stainless steel foil (Type 304) with a thickness of 25 µm was subjected to etching in 9 M sulfuric acid solution at room temperature. The foil samples pretreated for different lengths of time were loaded in the thermal CVD furnace for the CNT growth, and MWNTs were synthesized on these foils using a vaporized mixture of ferrocene and xylene. Ferrocene acts as the catalyst precursor and xylene as the carbon source. The experiment was carried out at a temperature of 700 °C in an Ar/H2 gas atmosphere for 30 min to 1 h. The XPS studies were performed with Kratos Axis-165 X-ray photoelectron spectroscope in a high-vacuum environment of 10-11 Torr. An Al-KR X-ray source with pass energy of 40 eV was used to obtain the spectra. Assuming a three-layer model23 the composition of the surface films was determined by applying curve fits to the XPS spectra after satellite and background removal.24
Results and Discussion The stainless steel samples with different acid treatment times of 1, 5, and 10 min along with the as-received sample (as the control sample) were considered to elucidate the effect of surface properties on the CNT growth. Figure 1 shows the scanning electron microscope (SEM) micrographs of MWNTs synthesized on the stainless steel foils treated for different times. It is observed that the density and alignment of the CNTs strongly depend on the duration of the acid etching time. The as-received stainless steel (Figure 1a) barely had any CNTs on its surface, whereas the sample etched for 1 min (Figure 1b) had randomly scattered CNTs. The density of the CNTs gradually increased on the substrates as the etching time increased, and highly aligned CNTs were obtained on the substrate etched for 10 min (Figure 1d). (18) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105. (19) Fan, S. S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. J. Science 1999, 283, 512. (20) Kind, H.; Bonard, J. M.; Emmenegger, C.; Nilsson, L. O.; Hernadi, K.; Maillard-Schaller, E.; Schlapbach, L.; Forro, L.; Kern, K. AdV. Mater. 1999, 11, 1285. (21) Wei, B. Q.; Vajtai, R.; Jung, Y.; Ward, J.; Zhang, R.; Ramanath, G.; Ajayan, P. M. Nature 2002, 416, 495. (22) Wei, B. Q.; Vajtai, R.; Jung, Y.; Ward, J.; Zhang, R.; Ramanath, G.; Ajayan, P. M. Chem. Mater. 2003, 15, 1598. (23) Asami, K.; Hashimoto, K. Langmuir 1987, 3, 897. (24) Sherwood, P. M. A. In Practical Surface Analysis; Briggs, D., Seah, M. P. Eds.; Wiley: New York, 1983; Appendix 3, p 445.
To understand the substrate effect on the CNT growth, all the treated substrate samples along with the control substrate were first heated in the CVD furnace up to the CNT growth temperature of 700 °C and kept at that temperature for 30 min in an Ar atmosphere without any chemical flow. These samples were allowed to cool to room temperature in the Ar atmosphere, and surface property analysis was carried out using the XPS technique. The survey spectra of all the samples revealed that the peaks were mainly contributed by the compounds of iron and chromium. Nickel remained undetectable on all the surfaces of both the as-received and the treated samples. Figure 2a,b shows the XPS spectra of Cr 2p3/2 and Fe 2p3/2 for the as-received and 1, 5, and 10 min treated stainless steel samples. An intense chromium signal is observed on the surface of the as-received stainless steel foil, with almost no detectable iron peak. This is due to the presence of the chromium-rich passive oxide film on the surface of the stainless steel.25 Generally, the chromium-rich passive oxide film is believed to protect stainless steel from corrosion. As the sample is treated in the 9 M sulfuric acid, a gradual decrease in the intensity of the chromium signal and an increase in the iron signal are noticed with the increase in the treatment time from 1 min to 10 min. This suggests that chromium is being depleted from the surface of the treated samples. The composition of the surface films was analyzed by performing a detailed curve fitting of the XPS spectra for all the samples. Figure 2c,d shows typical curve fittings of Cr 2p3/2 and Fe 2p3/2 spectra for the 5 min treated sample. Chromium gave three distinguishable peaks at 575.5, 576.6, and 578.1 eV. The peak at 575.5 eV (blue curve in Figure 2c) corresponds to the CrN due to the absorbed nitrogen from the atmosphere. The peak at 578.1 eV (magenta curve in Figure 2c) corresponds to the Cr6+ state of chromium. The dominant peak is contributed by the Cr3+ located at 576.6 eV (Figure 2c). For the Fe 2p3/2 spectra, there is a small peak at 709.3 eV (blue curve in Figure 2d) corresponding to the Fe2+ state of iron, and the dominant peak at 710.7 eV corresponds to the Fe3+ state (red dashed curve in Figure 2d). Another developed predominant peak at 711.0 eV was observed as the sample treatment time was increased that also corresponded to the Fe3+ state (red dotted curve in Figure 2d). The summary of the dominant peaks of Cr 2p3/2, Fe 2p3/2, and O 1s spectra (spectra not shown) for all the samples is tabulated in Table 2. This reveals that, out of all the compounds of chromium, Cr2O3 is the prevailing oxide present on the surfaces. Similarly, Fe2O3 is the dominant compound of iron present on the surfaces of the acid-treated samples. The presence of oxygen peaks at 530.1 ( 0.1 eV gave additional confirmation for the existence of either Cr2O3 and/or Fe2O3.21 All the peak positions closely matched the standard results in the Handbook of X-ray Photoelectron Spectroscopy.26 The ratios of the areas of Fe2O3 to Cr2O3 peaks of all the samples are shown in Table 3. For the as-received sample, the ratio is much less than 1, revealing that the surface is mainly covered by the Cr2O3 film. Figure 2a shows that there is virtually no CNT on the surface of the as-received stainless steel. This may be due to the fact that the catalyst iron particles coming from the gaseous mixture are poisoned by the passive chromium oxide film present on the substrate, possibly by forming a FeCr-O compound. The surface is therefore devoid of active iron particles acting as catalysts for CNT nucleation and growth. This is analogous to the selective growth mechanism of the CNTs on Si/SiO2 substrates.27 It has to be noted that, by passing only (25) Yang, W. P.; Costa, D.; Marcus, P. J. Electrochem. Soc. 1994, 141, 111. (26) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K.; Chastain, J. Handbook of X-ray Photoelectron Spectroscopy, 2nd edition; Perkin-Elmer: Waltham, MA, 1992.
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Masarapu and Wei
Figure 1. SEM micrographs of the MWNTs synthesized on various stainless steel samples: (a) as-received substrate, (b) substrate treated for 1 min (c) substrate treated for 5 min, and (d) substrate treated for 10 min. Table 2. XPS Peak Analysis of Cr 2p3/2, Fe 2p3/2, and O 1sa surface component
binding energy (eV)
valance state
chemical species
chromium
576.6 ( 0.1 578.1 ( 0.1 710.7 ( 0.1 711 ( 0.1 711.5 ( 0.1 530 ( 0.1 530.2 ( 0.1
3+ 6+ 3+ 3+ 3+ 2+ 2+
Cr2O3 CrO3 Fe2O3 Fe2O3 FeOOH oxide oxide
iron oxygen a
Only dominant peaks from the curve fitting data are tabulated.
Table 3. Ratio of Iron Oxide to Chromium Oxide Obtained from the XPS Spectra Fits
Figure 2. (a,b) XPS spectra of Cr 2p and Fe 2p signals for various treated samples. (c,d) Typical curve fitting of the Cr 2p and Fe 2p spectra for the 5 min treated sample. The red curves represent the corresponding dominant oxide component in both of the plots.
ethylene carbon source, no nanotubes were observed on the stainless substrate without a catalyst precursor. The stainless steel substrate acts as a support for the nanotube growth but is devoid of any catalyst sources such as free metallic iron particles. The surface treatment process as elucidated by XPS studies can be described as follows: The surface passive layer of the as-received sample gives initial protection to the stainless steel from corrosion. When the stainless steel foil is subjected to a (27) Jung, Y. J.; Wei, B. Q; Vajtai, R.; Ajayan, P. M. Nano Lett. 2003, 3, 561.
sample etch time
iron oxide/chromium oxide ratio
as-received (no etching) 1 min 5 min 10 min
0.1 0.9 2.3 8.0
treatment in 9 M sulfuric acid, the passive film weakens on the surface with time, and etching of the stainless steel is observed by dissolution of chromium in the acid. The etching process starts slowly at the beginning and gradually increases as the passive film is completely removed from the surface. When the stainless steel foil is removed from the acid, a mesoporous iron oxide film forms on the surface of the stainless steel, which is believed to be favorable to the CNT growth. For the foil treated for 1 min, the ratio of iron oxide to chromium oxide is about 0.9, indicating that only partial passive Cr2O3 film is removed from the surface, and so the CNTs are not dense on the substrate, as observed in Figure 1b. As the etch time is increased, the formation of the iron oxide film on the foil surface is complete and there is an increase in both the alignment and the density of the CNTs (see Figure 1c,d). It has to be noted that the oxide ratio presented
Direct Growth of MWNTs on Treated Stainless Steel
Langmuir, Vol. 23, No. 17, 2007 9049
patterned with photoresist by conventional microfabrication technique. The characters “NMDL” on the substrate are exposed to the acid while the remaining area is protected by the photoresist. After the acid treatment, the substrate was cleaned thoroughly with acetone to remove all the photoresist and was placed in the CVD furnace for CNT growth. An SEM image of the top view of the sample after the CNT growth can be seen in Figure 3b. It is clear from this figure that the CNTs are present only on the characters NMDL and not on the remaining places that were covered with photoresist. The method reported here on the direct growth of aligned CNTs on the stainless steel foil is superior in various respects compared to other techniques reported before.12-16 For example, when compared with the PECVD14 method where multiple pretreatment steps of stainless steel were involved, the present method has only one pretreatment step, and it is very easy to execute. Also, the present method may be economically scaledup to large-scale production, and the alignment and density of CNTs could readily be controlled compared to MPCVD12 and the diffusion flame synthesis method.15,16 There is no need for the catalyst preparation step since the catalyst is directly sent along with the carbon source into the CVD furnace. This method gives a higher yield of CNTs with growth rates exceeding 10 µm/min compared to the MPCVD method.12 The stainless steel foil utilized is only 25 µm thick and is very flexible to be cut into any size and shape with regular scissors. It can be used to make sturdy and light-weight electrodes for field emission and electrochemical applications.
Conclusions
Figure 3. Selective growth of MWNT patterns on the stainless steel foil. (a) Top view of the stainless steel patterned with photoresist. The characters NMDL are not covered by the photoresist. (b) Nanotube growth only in the exposed regions of letter patterns on the stainless steel foil.
in Table 3 is a relative number and gives an idea of the dominant surface component. This ratio can vary slightly for different samples etched for the same period of time, but the variation in the trend is always the same with an increase in the etch time of the samples. Since the CNTs grow only on the treated stainless steel surfaces, by preventing some areas of the stainless steel surfaces from etching we obtained selective growth of CNTs on patterned stainless steel substrates. Figure 3a shows a stainless steel substrate
In conclusion, an efficient and cost-effective method of synthesizing highly aligned MWNTs directly on stainless steel substrates is reported. XPS analysis reveals that the surface of the as-received stainless steel is covered by a passive chromium oxide film, which is not favorable for the growth of CNTs because of catalyst poisoning. Treating the stainless steel in a strong sulfuric acid for several minutes removes the passive layer and, by enriching the surface with iron oxide, very much supports the CNT growth. The density and alignment of the CNTs depend on the ratio of the iron oxides to the chromium oxides and therefore can be controlled by tuning the oxide ratio through chemical treatment on the stainless steel surface. Controlled etching on the substrate surface proves to be an effective method for the site-selective growth of CNTs on electrically conductive substrates. Acknowledgment. The authors are deeply grateful to the National Science Foundation for financial support of the project under the NSF award number DMI-0457555. LA7012232
