English abstract
Carbon nanotubes (CNTs) are widely used in various fields. Recent reports have shown that by combining the characteristics of CNTs and magnetic nanoparticles (MNPs) in a CNT-MNP nanocomposite matrix, their properties can be improved in comparison to their individual components. There have been several advances made with the nanocomposites in the fields of biomedicine, solid phase extraction, nanoelectronics and nanooptics. However, the influence of catalyst residuals left behind after the synthesis and purification of CNTs has not yet been fully investigated. The purpose of this work is to measure surface, composition, structure, magnetic and electronic properties of nanoferrites and nanoclusters multiwall carbon nanotubes (mCNTs) composites. The pristine mCNTs used in this work exhibit a metallic character along with ferromagnetic behavior which is attributed to the Ni residuals incorporated during the preparation of the tubes. Upon partial encapsulation of zinc ferrite nanoparticles, charge transfer was evident between the mCNTs and the nanoparticles. Ni impurities inside the tubes were attracted to the encapsulated zinc ferrite nanoparticles. The partial encapsulation of ZnFe2O4 nanoparticles in multiwall carbon nanotubes resulted in two interacting sub-systems featured by distinct blocking temperatures and enhanced magnetic properties. However when the ZnFe2O4 was replaced by MnFe204, the nanoparticles which formed inside narrow mCNTs exhibited narrow size distribution, large aspect ratio and small crystallite sizes. These characteristics were in contrast to that of MnFe204 nanoparticles formed inside mCNTs having a larger diameter. Enhanced metallic character of MnFe2O4 was detected upon partial encapsulation. Tube induced stress, size and shape allowed the alteration of MnFe2O4 properties such as its magnetization, coercivity, magnetic anisotropy and hyperfine field.
Further the synthesis of ternary Ni-Co-Cr nanoclusters using a single step method involving the simultaneous sputtering of metallic targets through gas condensation deposition process is also demonstrated. The difference in surface energy between the component atoms was observed to create a preferential surface phase, leading to the formation of multi core/shell structures. Ex-situ structure and surface composition analysis reveal metallic and oxide phases characterized by extraordinarily strong magnetic stripe domains with weak magnetic force microscopy signal attenuation. The ternary nanoclusters also exhibited strong ferromagnetic behavior below the blocking temperature. The enhanced magnetic properties are attributed to Volmer Weber growth mechanism and pave a way for the facile preparation of ternary core/shell magnetic structures, required for applications which mandate strong magnetic anisotropy along their growth direction. Furthermore, when multiwall carbon nanotubes were decorated with NiCoCr core/shell tri nanoclusters, the nanoclusters were able to interact electronically with the mCNTs, affecting a charge transfer from the mCNTs to the nanoclusters which led to enhancement of the metallic properties of the tri nanoclusters. The blocking temperature of NiCoCr nanoclusters attached to mCNTs is 33 K and that of pristine mCNTs is 400 K. Therefore in the temperature range between the two blocking temperatures, the composite combines the ferromagnetic properties of the mCNTs and the superparamagnetic properties of the NiCoCr nanoclusters.
