Chemical Properties of Metallocene-Filled Carbon Nanotubes to Tailor Toxicity in Plants

Metallocenes are toxic chemicals that are used for the growth of carbon nanotubes (CNTs). The study of the toxicity of metallocenes on plants is very important. It governs the issues of genetics. Toxicity studies should consider (I) the growth kinetics of carbon nanotubes, (II) the chemical reaction of metallocenes inside carbon nanotubes, and (III) investigations into the electronic properties of filled carbon nanotubes. Toxicity studies are influenced by the investigation of the growth processes of metallocene-filled carbon nanotubes, the evaporation of metals, and the formation of multiple-walled carbon nanotubes. Investigations into the modification of the Fermi level of filled carbon nanotubes also play a role in toxicity studies. Metallocenes are filled into carbon nanotubes through the various methods discussed here, including solution methods and gas-phase methods, each differing in methodology and requiring optimization. Many authors have published different methods of filling carbon nanotubes with metallocenes. They lead to high-filling ratios and allow future modifications of the electronic properties of carbon nanotubes. The kinetics of carbon nanotube growth are investigated with different methods. Environmental transmission electron microscopy is applied to studies with a time resolution of several nanoseconds. Here, we use Raman spectroscopy to study the growth process, which is revealed to have activation energies, growth rates, and temperature dependence. The dependence of growth temperature on the tube diameter and metallocene-type is revealed. It is shown that the growth temperature increases with the larger diameter of inner CNTs. The growth temperature increases for ferrocene compared to nickelocene and cobaltocene. With X-ray photoelectron spectroscopy, we show that the heating of metallocene-filled SWCNTs at 360–600 °C causes the n-doping of SWCNTs. Heating at higher temperatures results in p-doping. The obtained data are needed to tailor the toxicity issues of metallocene-filled carbon nanotubes on plants.