Effect of nitrogen and hydrogen on the growth of multiwall carbon nanotubes
on
flexible carbon cloth using thermal chemical vapor deposition
Tsung-Chieh Cheng
Department of Mechanical Engineering, National Kaohsiung University of Applied Science, Kaohsiung, Taiwan
h i g h l i g h t s
< We grew the MWCNTs on flexible carbon cloth.
< We developed the measurement technique to check the density of MWCNTs. < The field emission properties of CNTs depend on the density and geometry of CNTs. < The contact conductance method can predict the field emission properties of CNTs.
a r t i c l e i n f o
Article history:
Received 29 November 2011 Received in revised form 30 May 2012
Accepted 19 June 2012 Keywords:
Thermal chemical vapor deposition Carbon nanotubes
Field emission Carbon cloth
a b s t r a c t
In this study, the effect of various mixturefluxes of nitrogen (N2) and hydrogen (H2) on carbon nanotube (CNT) synthesis grown onflexible carbon cloth using thermal chemical vapor deposition (thermal CVD) with ethylene (C2H4) as the carbon source and nickel (Ni) as the catalyst was investigated. Field emission scanning electron microscopy (FE-SEM) was utilized to study the morphology of CNTs onflexible carbon cloths with various N2and H2inletflow rates. The results indicate that average diameter of MWCNTs decreases with increasing H2and N2flow rates; however, the density of CNTs increases first and then decreases with increasing H2and N2flow rates. On the other hand, in our field emission experiments, the result indicates that thefield emission is strongly dependent on the density and geometry of MWCNTs. In addition, we also found that the contact electrical conductance measurement is an easy method to predict thefield emission characteristics of MWCNTs.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Since the report of carbon nanotube (CNT) synthesis by Dr. Iijima in 1991[1], many unique physical characteristics for the CNTs were studied in several later literatures, including lower electric field, higher aspect ratio, higher thermal conduction coefficient, and perfect mechanical strength[1,2]. Due to their unique perfor-mances, CNTs have been extensively studied and applied in many fields, such as probes for atomic force microscope[3],field emitters
[4], hydrogen storage[5], and biological or chemical sensors[6]. During the development of these applications, many innovative CNT synthesis techniques were evolved such as arc-discharge[7], laser ablation[8], catalysts pyrolysis[9], thermal chemical vapor deposition (thermal CVD) [10], and plasma-enhanced chemical vapor deposition (PECVD) [11]. Among these methods, thermal CVD provides a potential solution for low cost at atmospheric pressure and large scale synthesis for the growth of high quality
CNT on irregular-shaped substrates. However, to obtain this good quality practice, the growth procedure needs to be studied in a systematic manner in order to control the structure and chemical composition for CNTs.
The characteristics and compositions of CNTs can be varied with many preparations and process conditions such as catalyst types, deposition methods, pretreated tie, growth time, processing temperature, and reaction gas types (including carrier gas and carbon source gas). Recently, a few research groups have investi-gated the CNTs growth mechanism to understand the growth parameters of the CNTs on Si or glass by CVD with various gas environments of N2, H2, NH3, or their mixtures[12]. Moshkalyov
[13]and Yang et al.[14]have studied the effect of different gases used for the catalyst surface pretreatment. Their results indicate that the addition of N2gas improves the morphology and the length of multiwall carbon nanotubes (MWCNTs); however, the excessive N2 suppresses the growth rate of MWCNTs. Choi et al. [4] also control the catalysts sizes using NH3treatments and confirmed that the diameter of CNTs depends on the grain size of the metal catalysts.
E-mail address:tcchengme@cc.kuas.edu.tw.
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Materials Chemistry and Physics
j o u rn a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t c h e m p h y s
0254-0584/$e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.matchemphys.2012.06.043
The role of hydrogen on the growth of CNTs for different deposition methods has been widely studied in recent years. Zhang et al. [15] suggested that the ratio between C and H radicals determines the carbon structure (sp2or sp3) in the CVD process. They also reported that a C-rich and H-deficient condition promote the formation of sp2-like graphitic SWCNT. Chung [16]indicated that not all of the carbons participate in growing the CNTs due to the existence of inactive catalyst caused by insufficient H2 gas. Finally, these excess carbons precipitate and form amorphous carbon. Therefore, it is very important to decide the optimal amount of H2 addition to obtain higher nucleation density and better CNT quality. Adding small amount of H2into CH4keeps the size and activity of catalyst particles in the incubation period and enhances the CNT growth. Moreover, Jung et al.[17]observed an easier catalyst passivation with excessive supply of carbons from a gas mixture of H2and C2H2and found randomly tangled CNTs in pure H2environment. According to these studies, H2and N2gasflux plays an important role in CNT formation[12,18].
In recent year,flexible electronic structures capable of making bendable and expandable electronic devices have been developed by many researchers. Carbon cloth is intrinsically a plastic and electrically conducting substrate allowing for the implementation of aflexible electron source for versatile vacuum microelectronics applications. Growing CNTfield emitters on carbon cloths is one of the essential ways forflexible field emission applications. However, few researchers have studied the mixed gas effect of CNT growth on flexible carbon cloths. Therefore, in this paper, the carbon nano-tubes grown onflexible carbon cloths by thermal CVD method were investigated to understand the effect of various mixturefluxes of hydrogen and nitrogen gases using scanning electron microscopy (SEM), Raman spectroscopy, and field emission measurement system.
2. Experimental
The carbon nanotubes were grown on theflexible carbon cloths using thermal decomposition of ethylene (C2H4) at atmospheric pressure. First, Ni thinfilm (5 nm in thickness), the metal catalyst material, was deposited on the carbon cloths using thermal evap-oration. Then, the carbon nanotubes were grown on Ni thinfilm in electric three zones furnace. A uniform temperature reaction region was achieved in the middle area of the furnace. When flexible carbon cloths with Ni catalyst were loaded into the reaction zone of the furnace, in order to reduce the catalysts size effect of CNTs growth, the pre-treatment temperature was ramped up to about 700C for 20 min with nitrogen and hydrogenflow rates fixed at 500 sccm and 100 sccm respectively. This process results in the formation of distributed nanosized Ni particles or islands on the carbon cloth surface and the average sizes of these Ni particles are about 230 nm. After the pre-treatment process, the growth temperature and growth time were set to 700C and 30 min and the nitrogen was introduced atflow rates from 0 to 4000 sccm with a constantflow rate (100 sccm) of H2flow and the hydrogen was also adjusted from 0 to 500 sccm with a constant flow rate (500 sccm) of N2flow, respectively. In this experiment, C2H4was introduced as a carbon source gas for CNTs growth. Finally, the quartz temperature was ramped down to room temperature in nitrogen atmosphere. The specimen morphology of the CNTs was inspected byfield emission scanning electron microscope (FE-SEM Hitachi S-4000) and the structural characteristics of the CNTs were investigated by Raman spectrometer (Horoba, Lab RAM HR). Besides, the field emission properties of the MWCNTs were measured in the vacuum chamber at a pressure of 5 107Torr at room temperature. All samples with the same emission area of 0.5 0.5 cm2 were mounted on a stainless-steel plate as
a grounded cathode. A stainless-steel anode was used to collect all emitting electrons from the CNTs on carbon cloths. The distance between the anode and the cathode was fixed at 550
m
m. The applied voltage was varied from 0 to 1 kV to supply an electricfield to extract electrons from the MWCNTs to enable thefield emission for growing carbon nanotubes on carbon cloths.3. Results and discussions
Fig. 1compares the average diameters of CNTs under various N2 inletflow rates with a constant H2inletflow rate of 100 sccm and the corresponding FE-SEM images of the CNTs for each N2inletflow rate. Apparently, the diameter of the CNTs decreases with increasing N2 flow rates. As nitrogen was introduced into the chamber with higher temperature, the atomic nitrogen reacted easily with hydrogen and formed ammonia. The chemical reaction of NH3can be expressed as[19]:
3H2þ 2N2/2NH3 (1)
Therefore, the amount of free hydrogen molecules or atoms in the process gas required for CNT growth decreases when NH3 reaction occurs. Since the purpose of free hydrogen molecules or atoms are used to avoid CNT suppression which advantages the CNT
Fig. 1. (a) FE-SEM images exhibit the CNTs/Ni/CC (carbon cloth) with various N2gas
inletflow rates (H2inletflow rate was constant at 100 sccm). (b) The scheme of IeV
characteristics with different N2flow rates.
field amplification effect at the emission cite due to the geometric enhancement which has also been discussed in many articles
[28e30]. InFig. 1(a), the SEM image shows the highest density is at 1000 sccm of N2 flow rate. Increasing N2 flow rates beyond 1000 sccm, in fact, decreases the density. This is why the MWCNTs have the lowest thresholdfield and the largest enhancement factor at 1000 sccm of N2flow rate. In addition, it is interesting to note that although the density of MWCNTs at 0 sccm of N2flow rate is larger than 2000 sccm of N2 flow rate, the threshold field of MWCNTs at 2000 sccm is lower than 0 sccm. This lower threshold field is mainly due to the geometry enhancement (since the diameter of MWCNTs at 2000 sccm is smaller than the diameter at 0 sccm)[31]. Another evidence can be seen in the IeV character-istics of Fig. 1(b), the CNTs emitters have the lowest electrical conductance indicating that the density (MWCNT emitters) of MWCNTs is at maximum at 1000 sccm of N2flow rates and the electrical conductance at N2flow rate of 2000 sccm is better than 0 sccm of N2flow rate. Therefore, the field emission strength for MWCNTs on carbon cloth is a balance between density effect and geometry enhancement effect of MWCNTs. Similarfield emission properties using various H2inletflow rates for MWCNTs growth on carbon cloth are also presented inFig. 6(b). The result shows that with 0 sccm, 100 sccm, 300 sccm and 500 sccm of H2flow rates, the thresholdfield of MWCNTs is 12.1 V
m
m1, 10.5 Vm
m1, 15.2 Vm
m1 and 15.7 Vm
m1, respectively, and the enhancement factor of MWCNTs is 741.98, 857.2, 425.1, and 365.9, respectively. This diagram also illustrates that the lowest threshold and the largest enhancement factor were obtained from H2flow rate of 100 sccm due to the strong dependent of thefield emission on the density and geometry of MWCNTs. Moreover, in our experiment, interest-ingly, the contact conductance measurement can be a method to predict thefield emission properties for the growth of MWCNTs on carbon cloths.4. Conclusions
In this study, we successfully grew MWCNTs onflexible carbon cloths with various mixtureflow rates of N2and H2gas by thermal CVD to product flexible electron sources for versatile vacuum microelectronics applications. The quality and field emission characteristics for MWCNTs were measured by Raman spectra and field emission system, respectively. Clearly, the result shows that the average diameter for CNTs decreases with increasing N2and H2 inletflow rates and the quality of MWCNT crystalline increases with increasing N2 and H2 inletflow rates. Although the diameter of MWCNTs decreases with N2and H2inletflow rates, bigger catalyst particles were found on the carbon cloth substrate with various N2 inletflow rates due to the excessive nitrogen simultaneously sup-pressing the carbon atoms into the catalyst surface to generate the passive layer of catalyst. However, with increasing H2 inletflow rates, the size of catalysts decreases due to the hydrogen etching effect. On the other hand, in our experiment, we found that the density of MWCNTs increases first and then decreases with increasing N2and H2inletflow rates. The field emission properties for the growth of MWCNTs on carbon cloth substrate strongly
depend on the density and geometry of MWCNTs. Finally, the contact conductance measurement was successfully conducted to predict thefield emission characteristics of MWCNTs.
Acknowledgment
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for partially supporting this research under Contract No. NSC99-2221-E-151-015. Technical supports from National Nano Device Laboratories are also appreciated.
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