Synthesis of narrow diameter distribution carbon nanotubes on ZnO supported catalysts

https://doi.org/10.1016/j.cplett.2009.03.069Get rights and content

Abstract

Multiwall carbon nanotubes, as indicated by electron microscopy and spectroscopy, were grown over a novel Co(5 wt.%)/ZnO catalyst by using a cold wall radio-frequency catalytic chemical vapor deposition method. The nanotubes were found to have a rather narrow diameter distribution, which can be explained by the formation over the ZnO support particles of uniform Co clusters, as shown by high resolution transmission electron microscopy studies. The growth of carbon nanotubes over such a support could represent a fast and reliable method to develop in one step 3D ZnO–CNTs heterogeneous composites of any size or shape, for use in advanced applications.

Graphical abstract

Multiwall carbon nanotubes were grown over a Co/ZnO catalyst from acetylene by using a cold-wall radiofrequency chemical vapor deposition method.

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Introduction

Carbon nanotubes (CNTs) have exceptional electrical, optical and mechanical properties making them attractive for numerous advanced applications [1], [2], [3]. Such nanotubes with narrow diameter distributions are especially excellent candidates for nano-electronic applications, highly functional composites, and energy applications [4], [5]. From all the synthesis methods, chemical vapor deposition is still the most widely used for the growth of high quality carbon nanotubes.

Synthesis conditions such as reaction temperature, flow rates of the carrier and hydrocarbon gas, hydrocarbon type, catalyst composition and many others have enormous effects on nanotube growth, morphology, and properties. Since the ability to reliably produce CNTs with homogenous diameters is very desirable for many applications, it is crucial to fully understand the influence of the reaction conditions on the morphology of the nanotubes. The diameter, crystallinity, chirality and yield of CNTs are very sensitive to the chemical composition of the catalyst system utilized in the growth process. Different transition metals were employed as active catalyst materials to decompose the hydrocarbon sources and instigate nanotube growth.

For controlled growth of carbon nanotubes with a very narrow diameter distribution, it is of great importance to understand how the reaction parameters affect the size of metal nanoparticles present in the catalytic systems. It has been experimentally and theoretically proven that the diameter of the nanotubes is directly related to the size of the metal nano-particles present on the catalyst support [6]. It is also worth mentioning that the support materials utilized in the catalyst system also play an important role in the nanotube growth. Different inorganic supports such as MgO, Al2O3, SiO2, etc. are currently employed to stabilize the fine nano-particles and prevent them from coalescing or agglomerating into large clusters at high temperatures.

Recently, significant research effort has been devoted to developing and studying advanced composite materials based on zinc oxide and carbon nanotubes, given their unique properties. The interest for these composites has been generated by their possible use as field emission sources and materials with enhanced photocatalytic activity [7], [8]. Moreover, low operating fields and stable emission currents have been obtained by using CNT–ZnO heterojunctions that were grown by a vapor-phase transport process [9], [10], [11], [12]. Furthermore, the deposition of ZnO nanostructures on the surface of single wall carbon nanotubes by an atomic layer deposition process, showed an elongation of the field emission lifetime of the nanotubes [13], [14]. The ZnO coated carbon nanotubes were shown to present higher secondary electron emission capabilities compared to ZnO films deposited on Si substrates, due to the field enhancement at the nanotubes tips [15]. In this context, a special attention was given to the development of synthesis methods that would allow the formation of structurally and morphologically uniform such composites. Heterostructures of ZnO nanowires on carbon nanotubes were successfully synthesized by a catalytic carbothermal reduction process with CNTs as substrate and Cu as a catalyst [16], with distinctive interfaces between ZnO nanowires and nanotubes as well as between the ZnO and Cu catalyst. The photoluminescence properties of these heterostructures are different from those of the separate components. Due to the versatile optoelectronic applications such as room-temperature UV lasers [17], light-emitting diodes [18], solar cells [16], [19], and sensors [20], the morphologically selective synthesis of ZnO has been widely investigated. To accomplish this, advanced ZnO–CNT composites were prepared by hydrothermal processes (both regular heating and microwave induced) under pressure wherein they exhibited a higher photocatalytic activity than ZnO [21], [22], [23].

The plasma enhanced chemical vapor deposition process was used to decorate carbon nanotubes with ZnO nanostructures, which mostly selectively deposited over the oxygen containing functional groups over the CNT surface [24]. Low temperature growth of one-dimensional CNT–ZnO complex nanostructural composites have been obtained on CNT printed films using Au nanoparticles as the catalysts [25]. Three-dimensional ZnO–CNTs hybrids were synthesized by using self-assembled and regular 3D CNTs as frameworks. Such composites showed unique wettability properties [26]. Moreover, the electronic properties of single wall carbon nanotubes were shown to change from p-type into metallic behavior after ZnO was deposited by RF sputtering as the substrate was heated, enabling the use of such composites in advanced nanoelectronics applications [27], [28]. Complex nanocomposites with controllable photoluminescence properties have been obtained by having multi wall carbon nanotubes decorated with ZnO nanostructures through successful ionic deposition [29]. A major potential of the ZnO–CNT composites could be in ultrafast nonlinear optical switching. The optical properties (absorption coefficient, photon absorption) of such devices were found to strongly depend upon the thickness of the ZnO films [30]. The mechanical pressure (0.58 GPa) applied non-hydrostatically to hybrid ZnO-single wall carbon nanotubes structures, was found to strongly influence their optical and electronic properties such as photoluminescence and band gap [31].

Given the increasing number of applications for composites of ZnO-graphitic nanomaterials it is important to develop methods that provide the controlled growth of nanotubes onto the surface of ZnO. The direct synthesis of carbon nanotubes onto ZnO supported catalysts by Radio Frequency Catalytic Chemical Vapor Deposition (RF-cCVD), in our knowledge has not been reported earlier, given the challenge of using ZnO (with relatively low thermal degradation properties) during high temperature catalytic deposition methods. In this report, we have proved the possibility of growing multi wall carbon nanotubes with a narrow diameter distribution by RF-cCVD using a Co/ZnO catalyst. Moreover the integration of 3D ZnO structures as support materials into the catalytic systems used to grow carbon nanotubes, opens the possibility for developing highly functional ZnO–CNTs heterogeneous composites to be used in various advanced applications. Such development would be of high importance for the applications that require a combination of the unique ZnO and CNTs properties, that include composites, energy generation, nano-electronics and optoelectronics.

Section snippets

Experimental

ZnO of 11.8 m2/g surface area was prepared as described in literature [32]. The Co(5 wt.%)/ZnO was obtained by impregnation, pouring 0.500 g ZnO into a solution of 0.111 g of Co(CH3COO)2 · 4H2O in methanol under stirring. After 15 min sonication, the methanol was extracted in rotavapor and the dried product was calcined for 1 h in air at 500 °C resulting in a moss green powder [33]. The nanotube growth reactions were performed in a water-cooled radio-frequency (RF) CCVD installation, previously

Catalyst studies

Zinc oxide is an n-type semiconductor with strong catalytic properties that originate from the presence of surface defects and which can be explained by Zn being in lower oxidation state, due to the hydrogen atoms that act as donors. The reduction of ZnO in CH4 environments proceeds spontaneously only at relatively high temperatures [37]. However, the high instability of acetylene towards decomposition into its elements (ΔG° = +207 kJ/mol) should favor the reduction of ZnO to the metal state at

Conclusion

The direct synthesis of multi-walled carbon nanotubes on zinc oxide was successfully achieved by CCVD on 5%Co/ZnO catalyst at 600 °C from acetylene, using a cold wall RF-CCVD method previously developed in our laboratories. This cold-wall technique minimizes not only the self pyrolysis of acetylene, used as the gas source in the reaction, but also the possible large morphological changes in the catalyst composition due to the evaporation of zinc at the reaction temperature. The direct synthesis

Acknowledgements

This work was partially funded by a grant provided by the U.S. Department of Energy (Grant No. DE-FG 36-06 GO 86072). Also the financial support from Arkansas Science and Technology Authority (ASTA) Grant No. 08-CAT-03 is highly appreciated. The Romanian authors thank the National Authority for Scientific Research (ANCS) for granting the project CEEX 95/2006.

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