Synthesis of carbon-encapsulated iron nanoparticles by pyrolysis of iron citrate and poly(vinyl alcohol): a critical evaluation of yield and selectivity.

Carbon-encapsulated iron nanoparticles were synthesized by pyrolysis at 1000 °C of two solid precursors: poly(vinyl alcohol) and iron citrate. The weight ratio between the precursors controlled the reaction yield, crystallinity, morphological features and magnetic properties of the products. The encapsulation yield of iron nanoparticles in carbon shells was strongly influenced by the iron-to-carbon ratio and depended on the iron citrate content in the initial reactant mixtures. Despite the inherent simplicity of the process and the use of low cost starting materials the demonstrated route possesses limited selectivity, especially at high iron-to-carbon ratios. At these experimental conditions the as-obtained products contained non-encapsulated Fe particles and graphite in addition to magnetic carbon encapsulates. These by-products were effectively removed by a one-pot purification procedure that included acid treatment.

Dispersion and diameter separation of multi-wall carbon nanotubes in aqueous solutions.

Comparative studies on dispersing of multi-wall carbon nanotubes (CNTs) using two anionic surfactants (sodium dodecyl sulphate, SDS, and sodium dodecyl benzenosulfonate, SDBS) are presented. The studies were conducted on the surfactant concentrations that were close to the critical micelle concentration (CMC). The stability of CNTs suspensions obtained for surfactant solutions at concentrations lower than the CMC was investigated. It was also found that the surfactant structure has an influence on the diameter distribution of dispersed CNTs.

Single-walled carbon nanotubes synthesis: a direct comparison of laser ablation and carbon arc routes.

Carbon arc and chemical vapor deposition are at present the most efficient methods for mass production of single-walled carbon nanotubes. However, laser ablation is renowned for high quality nanotubes with narrow diameter distributions and hence is also of great interest. The aim of this work was to compare both the carbon arc and laser ablation techniques with respect to the quality--and relative yield of the produced SWCNTs. For this comparative study we used Fe as the catalyst, which is known not to be very active in laser ablation. However, we show this is not the case when H2 is included in the reaction. The reactions for both synthesis routes were carried out in a N2-H2 (95-5% vol.) atmosphere. The same homogenous carbon rods with different iron contents, between 1 and 5 at.% were used as the carbon feedstock and catalyst supply in both synthesis routes. Additionally, two types of carbon rods containing 1 at.% Fe with different graphitization degrees were also investigated. In the arc-discharge case, the low-graphitized electrode produced a web-like product rich in SWCNTs, while the high-graphitized carbon rods yielded soot containing carbon-encapsulated iron nanocrystallites, amorphous carbon nanoparticles, and surprisingly a small fraction of SWCNTs. With laser ablation synthesis, the Fe content and the reactor temperature significantly influenced the SWCNTs yield. Carbon arc plasma diagnostics were also performed. By using optical emission and Absorption spectroscopy the plasma temperature, C2 and CN radical content in the arc zone were determined.

Formation of SWCnts in arc plasma: effect of graphitization of Fe-doped anode and optical emission studies.

Fe-doped (ca. 1 at%) homogeneous graphite electrodes (with different graphite microcrystallites, degree of graphitization and, thereby, electrical conductivities) were used to produce single-walled carbon nanotubes (SWCNTs) in Ar/Kr/Xe-H2 arc plasma under pressure equal to 26 kPa. The use of electrode with the smaller primary particle size (about 5 nm) comparing to the well-graphitized electrode (25 nm) drastically increased the yield of SWCNTs in Ar-H2 arc plasma, while plasma parameters (temperature, C2 content, and namely carbon vapor pressure) remained on similar levels. However, the use of electrodes with larger grain size (25 nm) can lead to SWCNTs growth when they are arc ablated under the presence of Kr (or Xe)-H2 gas mixture. Thus, the mechanism of CNT formation seems to be more complex that the one involving only simple carbon species (e.g., C2).