收稿日期：13 November 2007
Deposition rate and morphology of carbon nanotubes at different positions in a CVD reactor
Abstract：
Carbon nanotubes (CNTs) were synthesized through the catalytic decomposition of a ferrocene-xylene mixture in a horizontal chemical va- por deposition reactor. The deposition rate of CNTs along the axial direction was measured. The morphology of CNTs was observed by scan- ning electron microscopy (SEM) and transmission electron microscopy (TEM) . The results showed that the deposition rate of CNTs along the axial direction first increased and later decreased, the position achieving the maximum deposition rate was influenced by the operating conditions. The morphologies of CNTs also changed along the axial direction.
Keyword：
carbon nanotubes; deposition rate distribution; chemical vapor deposition; microstructure;
Received： 13 November 2007
1. Introduction
Since their discovery, Carbon nanotubes (CNTs) have been a subject of intensive research.A lot of studies have been carried out for understanding their structure and properties as well as the conditions for their synthesis.CNTs have been produced by several techniques including laser ablation[1], arc discharge[2-3], flame synthesis[4], and a variety of chemical vapor deposition (CVD) methods[5-18].The injection CVD method is one of the most promising methods for producing commercial quantities of CNTs[9-15].
The injection CVD method involves pumping or spraying a catalyst precursor into a suitable furnace.The typical precursor is a metallocene-hydrocarbon solution[9-14].In contrast to other CVD methods, the injection CVD method does not need the catalyst synthesis step, as the catalytic particles are continuously generated in situ throughout the entire growth cycle.This gives the possibility to scale up the method for continuous or semi-continuous production.
The synthesis of CNTs in a CVD reactor depends on reaction conditions such as temperature, gas concentrations, and catalyst level.There are numerous references reporting effects of reaction conditions on the production of single-walled and multi-walled CNTs[8-15].However, hardly any literature deals with the deposition rate and morphology of CNTs at different positions in a CVD reactor.In fact, the reaction conditions can vary throughout the CVD reactor There is no doubt that it is necessary to understand the possible variation of the product throughout the CVD reactor, as a step toward the commercial production of CNTs.The present study is aimed at obtaining an insight into the variation of deposition rate and morphology of CNTs along the axial direction in a horizontal CVD reactor.
2. Experimental
The synthesis of CNTs was carried out using the injection CVD reactor described in Fig.1.The reactor consists of a quartz tube with 46-mm inner diameter, a furnace, and a quartz plate placed in the quartz tube to provide a substrate for CNTs growth.A block with 12-mm inner diameter was set in the quartz tube, which pided the quartz tube into two sections.During the process of synthesizing CNTs, a solution of ferrocene in xylene was injected continuously into the first section of the quartz tube, where ferrocene and xylene were vaporized.The vaporized ferrocene and xylene were swept into the second section of the quartz tube by a flow of carrier gas, which was a mixture of nitrogen and hydrogen.The block inside of the quartz tube ensured that the vapors of ferrocene and xylene were mixed uniformly with the carrier gas, prior to their entry into the second section of the quartz tube.The ferrocene decomposed to provide the iron catalyst particles required for the nucleation of CNT growth, whilst xylene acted as the carbon feedstock.The CNTs were grown on both the substrate and quartz tube in the second section.
Fig.1.A schematic diagram of the CVD reactor.
In all the experiments, the concentration of ferrocene in xylene held at 0.03 wt.%, and the duration of the injection was 30 min.Other operating parameters are listed in Table 1.The CNTs deposited on the substrate were collected and weighted to determine the quantities of CNTs produced at different sites.SEM and TEM were used to analyze their morphologies.
Table 1.Operating conditions 下载原图
Table 1.Operating conditions
3. Results and discussion
3.1. Deposition rate of CNTs along the axial direction
Variation of the CNT deposition rate along the axial direction is shown in Fig.2.For each of the cases, as the distance along the reactor axis increases, the CNT deposition rate first increases and later decreases.The position with a maximum deposition rate varies with a change in operating conditions.
By comparing the distribution of the deposition rate obtained under each operating condition, it can be found that the maximum deposition rate decreases with an increase in the flow rate of carrier gas, and the position with the maximum deposition rate moves slightly to the side with a larger axial distance.The deposition rate increases with an increase of ferrocene-xylene feed rate, and the maximum deposition rate is achieved at the position with a larger axial distance.The reactor temperature has a significant influence on the deposition rate of CNTs.The deposition rate increases with an increase in temperature, and the position with maximum deposition rate shifts to the region with a smaller axial distance.
Fig.2.Variation of the carbon deposition rates on the upside of the substrate along the z-axis.
3.2. Morphologies of CNTs at different positions
Fig.3 shows the SEM image of the CNTs at the three positions obtained under operating condition S2.Their TEM images are shown in Fig.4.It can be seen that the CNTs near the inlet are not so aligned and have many iron particles Near the outlet, the CNTs are sparse, and their diameter is small.In the middle region, corresponding to the place with the maximum deposition rate, the CNTs are well aligned and with high density and larger diameter.No different characteristic was observed on CNTs over a cross section of the CVD reactor.
The CNTs were also examined using quadrant back scattering detector (QBSD) .QBSD images can more clearly show iron particles on CNTs, as the back scattering electron signal is a strong function of the atomic number.As shown in Fig.5, there are more iron particles attached to the CNTs near the reactor inlet than near the reactor outlet, and there are more iron particles at the top of the CNTs than in the bottom.
The different deposition rate and morphologies of CNTs indicate that there are different reaction conditions along the reactor axial direction.In this study, the most important factors that influence the reaction rate and deposition rate of CNTs are the local temperature and concentrations of carbon and catalyst sources.Near the reactor inlet, the concentrations of carbon and catalyst sources are higher.Therefore, the low deposition rate near the inlet can be attributed to a lower reaction temperature, which results in a lower deposition rate of the CNTs as shown in Fig.2.There may be a lower temperature region near the reactor inlet because of the gas entering with a lower temperature and the endothermal effect of decomposition of carbon and catalyst source.It is believed that the size of the catalyst particle controls the diameter of the CNTs formed[19].The diameter of the CNTs grown at a lower temperature is expected to be smaller, because of the decreasing mobility of the iron particles on the substrate surface, which leads to the formation of smaller catalyst particles[12].As expected, the observation results showed that the diameter of the CNTs near the reactor inlet was small.
Fig.3.SEM micrographs of the CNTs deposited in the regions: (a) and (b) 0.13 m
Fig.4.TEM micrographs of the CNTs deposited in the regions: (a) 0.13 m
Fig.5.CNTs imaged with QBSD: (a) 0.13 m
Sparse and small CNTs near the reactor outlet indicate that the ferrocene concentration is low because of fewer and smaller catalyst particles formed at a lower ferrocene concentration.From the observation of iron particles on CNTs it seems that ferrocene decomposes rapidly.The ferrocene concentration rapidly decreases after entering the reactor and most of the ferrocene is consumed before flowing to the outlet region.The concentration of the carbon source also decreases with an increase of distance in the axial direction because of deposition of CNTs, therefore, this decrease of deposition rate of CNTs is because of the depletion of xylene and ferrocene.
The high density of iron particles at the top of the CNTs may be attributed to the continuous injection of ferrocene that generates an excess increase of iron for CNT growth.It seems that the density of CNTs depends on the initial density of iron particles at the nucleation stage.Once the CNT growth starts, new nucleation and growth on the substrate surface are suppressed.As a result, there are excessive iron particles existing at the top of the CNTs even near the reactor outlet where there are not enough iron particles at the nucleation stage.In the growth process of the CNTs, continuously providing iron may favor the growth of the CNTs according to the growth mechanism of the CNTs[20], but the quantity required in the growth process is lower than that at the nucleation stage.
The above experimental results and discussion suggest that the reaction condition can vary throughout the CVD reactor, which results in variations of the deposition rate and morphology of the CNTs at different positions.The uniformity of the CNT deposition can be improved by varying the operating conditions.However, it seems impossible to produce CNTs with uniform quality over a large area because of an inevitable variation in the concentration of carbon and catalyst sources in this type of horizontal CVD reactor.The CVD processes are chemical reactions in the gas phase and on the surfaces, and are influenced by momentum energy, and mass transport.For producing homogenous CNTs over a large area, it is necessary to optimize reactor design and operating conditions based on chemical reactions and transport processes.
4. Conclusion
The deposition rate of CNTs along the axial direction in a horizontal injection CVD reactor first increased and then decreased.The position with maximum deposition rate was influenced by operating conditions.The morphologies of CNTs also varied along the axial direction.By varying the operating conditions, the uniformity of the deposition of CNTs along the axial direction can be improved.It is probably not possible to produce CNTs with uniform deposition rate and same morphology over a large area in this type of horizontal CVD reactor.
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