Nucleation and growth of carbon nanotubes and nanofibers: Mechanism and catalytic geometry control

Abstract

Carbon nanotubes and nanofibers with certain shape, size and structure are desired. The study of the kinetics of nucleation and growth of carbon nanotubes and nanofibers is an important key to understand and control the growth process. That knowledge will greatly improve our capacity to optimize structural shapes and increase growth rates. This Perspectives article draws from the literature on carbon nanotube growth and analyzes them to reveal some aspects underlying reaction mechanisms. In each catalyst nanoparticle a constant carbon bulk diffusion flux operates between two distinct catalytic areas with different roles: 1) Surface catalysis producing carbon atoms in some areas; 2) Graphene nucleation and growth, in other areas. Preliminar solid-state changes – obeying 2nd Fick’s law – may take place. Subsequent kinetic linearity is the sign that a steady-state 1st Fick’s law controlled growth process has been established. Data from the literature on diverse crystal orientations activity are discussed. Catalyst duality may be based on different crystal faces or on solid-state phases prevailing during steady-state growth. Growth of carbon nanotubes from Ni nanoparticles usually described as “octopus” carbon offers evidence of the role of geometry, pentagon formation “catalysis” and catalyst duality operating at low temperatures.

Introduction

There is pressure nowadays to produce nanotubes efficiently and with a given geometry due to its expanding multiple uses [1], [2], [3], [4], [5], [6], [7], [8], [9]. But the mechanism is not well understood: “The central problem of nanotube science is still the mechanism” [3]. The early proposals for the mechanism of catalytic carbon formation involving bulk diffusion of carbon atoms through the catalyst were based on detailed thermogravimetric kinetic studies of carbon formation from hydrocarbons on metals: on Ni in the range 200–350 °C [10], at 1,000 °C [11] and on Ni, Co and Fe and other transition metals in the range 350 °C and 700 °C [12]. Based on the kinetics observed all those authors concluded that C diffusion through Ni was a step in the carbon growth process and calculated the respective activation energy, based in operating conditions where that step was assumed to be rate controlling (Table 1).

Most of the studies on catalytic carbon formation in the period 1970–1990 aimed at minimizing the problem of catalyst poisoning in several processes, particularly steam reforming. Since the 1990’s the scientific interest in understanding carbon formation aims at optimizing the shape, the rate and the density of carbon nanotubes and nanofibers, initially grown mostly at high temperatures - up to 2,000 °C. Several alternative routes of carbon formation are known. The mechanisms of 3 alternative routes in carbon formation have been discussed in a recent paper [14]. Table 2 lists and briefly describes the 3 routes.

The routes of carbon nanotubes (CNTs) formation may be initiated pyrolytically or catalytically. We call hybrid route the formation of carbon particles pyrolytically in the gas phase, impinging on the surface of a catalyst, dissolving carbon atoms and nucleating and growing graphite elsewhere on the surface of the catalyst.

In the nucleation and growth of CNTs by a catalytic route, three factors must be accounted for [14]:

• The dual catalyst: different and separate areas, some active in gas decomposition (or carbon black solubilization, in pyrolysis) and others active in carbon nucleation and growth;

• The meaning of kinetic linearity under steady-state, both in catalytic carbon formation and catalytic carbon gasification;

• The need to form 6 pentagons to get perpendicular CNTs growth, after initial graphene nucleation.

Carbon formation on foils or catalyst layers is also being used to produce graphene layers. The geometry is a key factor in this type of reactions, both in nucleation and in the growth process. This will be discussed below. The duality required to get a sustained growth may be based on different crystal orientations or on distinct solid-state phases (one in equilibrium with graphite and a different one in equilibrium with the reacting gas).

The case of octopus carbon nucleation and growth help illuminate the geometry requirements for CNTs growth [15], [16], [17], [18], [19]. The recent paper by Saavedra et al. is of particular interest in view of the detailed observations and statistics presented [18]. Spheroid metal particles are nowadays deposited on substrates to produce CNTs by CVD (chemical vapor deposition): understanding the initial changes in shape, the crystal nano-faces prevailing and the geometry of carbon nucleation and growth is essential to optimize the process.

The metals active in the dual catalyst route are Ni, Co and Fe. For the hybrid route there is no need for surface catalysis. Pt, Pd, Cu and many other transition metals are active.

It is important to understand the differences in the growth mechanism of carbon tubes (perfectly cylindrical, with one or several graphene walls) and carbon fibers (graphene structure arranged apparently as stacked cones). This will be discussed below, at point 4.