Effects of the restructuring of Fe catalyst films on chemical vapor deposition of carbon nanotubes

Abstract

We investigated the restructuring of Fe catalyst films during high temperature processing and its effects on the morphologies of carbon nanotubes (CNTs) arrays synthesized using the thermal chemical vapor deposition (CVD) techniques. Results show that annealing time and gas environment deeply influence the particle size distribution, which determines the morphologies of the corresponding CNT arrays. Fe particles with narrow size distribution were obtained after proper high temperature processing in NH₃, from which higher purity CNT arrays with better-alignment and uniform diameter distribution have been grown.

Introduction

Since their discovery [1], carbon nanotubes (CNTs) attract extensive interests from the scientists in all kinds of fields because of their electrical, chemical and mechanical properties which give them the corresponding potential applications in electronic devices, metallic catalyst supports and enhanced fibers in composites [2], [3], [4], [5], [6]. Controlled growth of CNTs is critical for their application, due to the close relationship between their morphologies/nanostructures and their measured properties [4], [5]. However, controlled growth of CNTs is still a challenge. To date, the most versatile techniques for synthesizing CNTs have been those based on catalyst-assisted CVD methods. In these CVD methods, catalyst particle size and shape determine the morphologies and nanostructures of CNTs [2]. For this, different methods have been developed to prepare isolated and uniformly distributed metallic catalyst nanoparticles [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. In many cases, metallic catalyst films are used and followed by high temperature processing in H₂, N₂ and NH₃ etc. to form nanoscaled particles [7], [8], [9]. Because thin films have a high surface-to-volume ratio, on heating, metallic films may develop holes and may eventually agglomerate into particles with a lower surface-to-volume ratio [19]. With continuously heating, these particles might coalesce due to Ostwald ripening or surface migration [20], [21], [22], thus modifying the final distribution of catalyst particles, which determine the morphologies of CNTs obtained [18], [23]. This process is strongly dependent on catalyst-substrates interaction [24], the surface morphologies of the pristine metallic films [25], film thickness [26], [27], [28], heating time and gas environment [29] etc., which have never been investigated in the view point of the relation between the chemical or physical state of metallic catalysts and the formation state of CNT to control CNT array structures.

In our previous paper [25], we have reported varying deposition angles, which are the angles subtended by the incoming flux and the substrate surface, can well control the surface morphologies of Fe catalyst films. It was found that the Fe thin films with uniform surface topography could be obtained at 30° of deposition angles. Here, in order to control the formation and distribution of Fe particles, we systematically studied the restructuring of 5 nm of Fe films deposited at 30° of deposition angle during high temperature processing in NH₃ with heating time, and simply discussed the effect of gas environment on the distribution of Fe particles. Further more, we successfully synthesized high-purity and well-aligned CNT arrays with uniform diameters by controlling high temperature processing parameters.