T*P CHi KHOA HQC & C 6 N G NGHg CAC T R I / O N G DAI HQC KV THU^T * S6 91 - 2012
GROWTH OF CARBON NANOTUBES BY MPCVD USING IRON OXIDE NANOPARTICLES AS CATALYST
CHE TAG ONG NANO CAC BON BANG PHUONG PHAP MPCVD SU' DUNG CAC HAT XUC T A C NANO OXIT SAT TU"
Luong Xuan Dien, Huynh Dang Chinh Gemma Rius, Masamichi Yoshimura Hanoi University of Science and Technology Toyota Technological Inslitule. Japan
Din Tda soan 05-3-2012, chdp nhdn ddng 20-7-2012 ABSTRACT
The outstanding electnc, mechanical and thermal propedles of carbon nanotubes (CNTs) have stimulated widespread interest for fundamental research and applications. This work demonstrates the growth of vertically arranged CNTs on Si substrate by microwave plasma enhanced chemical vapor deposition (MPECVD) with optimized operation parameters. Iron oxide (Fe^O,,) nanoparlicles (NPs) were synthesized by co-precipitation method. The catalytic FeiO^ NPs were distributed on Si by spin coating. CNTs were charactenzed by X-ray diffraction (XRD). scanning electron microscope (SEM), transmission electron microscopy (TEt^) and Raman spectroscopy. Through our study, we have successfully grown CNTs based on iron oxide nanoparticles by tVIPECVD at the grown temperature of 65d'C.
Keywords: Chemical vapor deposition (CVD), Carbon nanotubes, Iron oxide T 6 M TAT
Cdc tinh chit nhi&t, co vd tinh chit didn uu vidt cda ing nano cdc bon duxyc quan tdm nghien cOu rdng rdi cho nghidn ciru ca bdn vd cdc ung dung Nghidn c&u ndy chung td cdc ing nano cdc bon mgc sdp xip thdng dung trdn di silic bdng phwang phdp ngung tu hoi hda hgc cd hd trg b&l plasma vd sdng didn tir (MPECVD) v&i cdc thdng si hoat ddng dwac tii wu. Cdc hat nano oxit sit (FejO^ dirac tdng hgp bdng phwang phdp dong kit tua. Cdc hat nano FejO^ diroc phdn Idn Idn di silic bdng phuong phdp quay phu. Cdc ong nano cdc bon dwgc xdc dinh bdng phwang phdp nhiiu xa tia X (XRD), hiin vl dffin tw qu^t (SEtVI), hiin vl didn t& truyin qua (TEM) vd phi tdn xa Raman. Thdng qua nghidn ciru ndy, chOng tdi dd ting hgp thdnh cdng cdc 6ng nano cdc bon dira trdn cdc hat nano oxit sdt FesO^ bang phwong phdp MPECVD v&i nhidt dd phdt triin Id 650°C.
1. INTRODUCTION homogeneous and predetennined structure and The outstanding electrical, mechanical ^ ^ '
and thermal properties of carbon nanotubes There are several methods for CNT (CNTs) have induced widespread interest in fabrication [1],[2],[3]. Chemical vapour research and application [1]-[12]. They are deposition (CVD) is superior to arc-discharge currently being evaluated for a wide range of and laser-ablation because of its lower applications, including field ultra-capacitors [4], operating temperature and higher CNT yield. In paper batteries [5], AFM and STMtips[6HllL this paper, CVD process was used to fabricate biosensors [12]. As a new class of engineering CNTs using iron oxide as catalyst instead of material, CNTs offer the promise of tunable metalic iron. The iron oxide nanoparticles are properties that can be varied without changing reduced to iron during the CVD operation, so the chemical composition, but at the same time that the iron oxide functions the same as iron they face considerable manufacturing catalyst. However, the iron oxide nanoparticles challenges. In order to investigate their intrinsic do nol agglomerate as in the case of iron properties and to realize their promises for nanoparticles, facilitating Ihe growth of CNT.
practical applications, it is critical to develop We also demonstrate that iron oxide is an processes that can yield CNTs with excellent catalyst for the synthesis of high-
quality CNTs on the surfaces of silicon wafers
TAP CHi KHOA HQC & CdNG NGHf: cAC TRlTftNG D^I HQC K* THU^T* S691-2012 at the low growth temperature. By using
methane as carbon source, vertical CNT forest can be generated. Uniform CNTs forest was characterized using scanning electron microscopy (SEM), Raman spectroscopy and transmission electron microscopy (TEM).
Advantages of the present method include nanoscale resolution, easy operation, large production,
2. EXPERIMENTAL SECTION 2.1. Materials
The different organic reactants (anhydrous ethanol 99.5%, acetone 99.5%) were purchased from Sigma Aldrich.
Ammonium iron (11) sulfate hexahydrate ((NH4)2Fe(S04)2.6H20) 99%, iron (111) cloride hexahydrate (FeCh.eHjG) 97%, sodium hydroxide (NaOH) 9&% and ammonium hydroxide solution 28% were also purchased from Sigma Aldrich, Ultrapure water was provided by a MilHpore station, which ensures a resistivity of 18.2 MQ cm at 25 "C. The silicon substrates were cut from [11 l]-oriented double-side polished Float Zone silicon wafer.
2.2. Preparation of silicon surface The silicon substrates were initially cleaned in acetone with supporting of ultrasonicator for 15 minutes. Then, the silicon surfaces were cleaned by anhydrous ethanol and copiously rinsed with ultrapure water. Finally, the surfaces were cleaned by anhydrous ethanol and dried naturally at the room temperature.
2.3. Catalysts preparation and synthesis of carbon nanotubes
The iron oxide (Fe^O^) nano-particles were synthesized by co-precipitation method.
Ammonium iron (11) sulfate hexahydrate ( 0,396 g; 99%) and iron (111) cloride hexahydrate (0,557 g; 97%) were dissolved into distilled water (60 ml) to form a solution that then was stirred constantly in 15 minutes. Then, sodium hydroxide (0.324 g, 98%) was added into the solution. Immediately, the addition of aqueous ammonia hydroxide 28% (NH40H) until the pH of solution is equal to 11 with vigorous stirring at 70''C in 30 minutes, forms magnetite nanoparticles via a co-precipitation in the base.
Next, the obtained suspension was centrifugated to get Fe304 nanoparticles.
Fe304 nanoparticles were distributed in ethanol with adapt content. It was coated silicon substrate using a spin coaler at a speed of 1000 rpm. Then, 80 seem of H; was exposed to the substrate at 650°C for 3 minutes. For the CNT growth the flow rate of H2 and CH4 were 80 and 20 seem, respectively. The total pressure was kept at 1.7 torr and the growth temperature was 650°C. The total growing of CNT was kept for 10 minutes.
2.4. Characterations
Phase identification were performed on powder X-ray diffraction pattern (Rjgaku), using with Cu-Ka irradiation at X = 0.15406 nm, by scanning (0.02°) in the 20 range of 20"- 80°.
The surface morphologies were observed by field emission scanning electron microscopy (FESEM, Hitachi, S-4700), transmission electron microscopy (TEM, JEOL) and the crystallinity of CNTs was examined using 532 nm Raman spectroscopy.
3. RESULTS AND DISCUSSION 3.L Characterzations of catalysts
X-ray powder diffraction proved to be a versatile technique for characterization of the FcjOj nanoparticles. Information on the crystal structure, phase purity, particle size and residual organic material is contained in the data. The average particle diameters of nanocrystalline materials can be estimated from the peak broadening according to the Debye Scherrer equation: B = —^—
Where B is the broadening of the peak (measured as the full-width at half maximum intensity, FWHM) in radians, X is the X-ray wavelength (1.5406 A for Cu-K„), d is the average particle diameters and 0 is the angular location of the peak. Th^^gular position, and corresponding Miller indices (h k 1), for an XRD pattern from Fe304 in the angular range of 2 9 - 2 0 ° - 8 0 ° are: 30.14° (2 2 0), 35.37"(3 1 1), 42.92" ( 4 0 0), 53.40" (4 2 2), 57.10" (5 1 1), 62,64" (4 0 0), which shown in Fig. 1.
Therefore, it was confirmed that the crystalline
T^P CHi K H O A H O C & C A N G N G H $ C A C TRU'6"NC D;^I H O C K * T H U A T • S6 91 - 2012
stmcture of obtained magnetite nanoparticles, agreed with the structure of an inverse spinel type oxide. Accordingly, the average crystallite sizeofFesOj is about 16 nm, which determined using the Debye Scherrer equation.
Fig. 2 shows the typical fransmission electron microscope (TEM) image of Fe304 nanoparticles, from which we can see that the sizes of Fe304 nanoparticles are almost uniform and most of Fe304 nanoparticles are approximately spherical with the mean diameters of 14 ± 4 nm.
Fe304
2 2 0 , 511 I 4 0 0
, I . 4 2 2
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2Tlieta(deg.) Fig 1 XRD image of FejO, nuiioparliclci
Fig. 2: TEM image ofFe3()4 nanoparlicles 3.2, Characterzations of carbon nanotube
Fig. 3a and b show SEM images of our synthesized nanostructures showing that vertical CNTs are uniformly grown on silicon substrate. After only 10 minutes of methane exposure we estimated that the length of the carbon nanotubes reaches 2 /im.
Raman spectrum of the as-grown CNTs is reported in Fig. 4. This image shows characteristic of the formation of multi-walled structure because it only have D, G and G' band without RBM-band. This strongly suggests Ihat our carbon nanotubes are characterized by very few structural imperfections.
Fig 3 SE.M image of CNTs grown on silicon siihslrule: (a) - Cross-section image
(b) - Top-view image
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Fiy. 4. Ram m sp'ctrum of CNTs ^nwn on Si'liams I I I
Fig. 5 TEM iimii^L's of CNTs grown on Silico suh.'<lralc
TEM images proved the multi-walled nature of carbon nanotubes, as reported in Fig.
5. Also, we can see that CNTs are bamboo- shaped multi-waited CNTs. The outer diameter of CNTs was 18 ± 4 nm. Therefore, we suggested the mechanism of the CNT grovrth as in Fig. 6.
CMi
T4P ClH KHOA HQC & CONG NGHj: CAC TRt/flfNG P^l HQC K? THUAT * S6 9 r ^ Q 1 2 ^ _ _ ^ 4. CONCLUSIONS
In summary, a selective growth of CNTs at 650°C for 10 min on Fe304 nanoparticles by a MPECVD process has been demonstrated successfully. The synthesis is made on substrates with spin-coated magnetic nanoparticles and ethanol mixture. The chemical route of the whole process is mainly attributed to the reduction of iron oxide to iron with the aid of hydrogen gas that is product of the methane decomposition. Carbon nanotubes grown with iron oxide revealed bamboo-shaped CNTs. High yield growth of CNTs for all nanoparticles is achieved by MPECVD. CNTs are about 20 nm in diameter and about 2 pm in length. The present approach has the advantages of nanoscale resolution and easy operation, and can be valuable in the construction of CNTs-based nano-electronic devices.
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Fig. 6. Suggested mechanism of CM's grown on Silicon substrate
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Author's Address: Luong Xuan Dien - Tel.: (+84)919.598.986, Email: [email protected] Hanoi University of Science and Technology
No. 1, Dai Co Viet Str., Hanoi, Vietnam
