Purification and characterization of zeolite-supported single-walled carbon nanotubes catalytically synthesized from ethanol

Noncovalent Grafting of a DyIII2 Single-Molecule Magnet onto Chemically Modified Multiwalled Carbon Nanotubes

While synthetic methods for the grafting of nanoparticles or photoactive molecules onto carbon nanotubes (CNTs) have been developed in the last years, a very limited number of reports have appeared on the grafting of single-molecule magnets (SMMs) onto CNTs. There are many potential causes, mainly focused on the fact that the attachment of molecules on surfaces remains not trivial and their magnetic properties are significantly affected upon attachment. Nevertheless, implementation of this particular type of hybrid material in demanding fields such as spintronic devices makes of utmost importance the investigation of new synthetic protocols for effective grafting. In this paper, we demonstrate a new experimental protocol for the noncovalent grafting of Dy^III₂ SMM, [Dy₂(NO₃)₂(saph)₂(DMF)₄], where H₂saph = N-salicylidene- o-aminophenol and DMF = N, N-dimethylformamide, onto the surface of functionalized multiwalled CNTs (MWCNTs). We present a simple wet chemical method, followed by an extensive washing protocol, where the cross-referencing of data from high-resolution transmission electron microscopy combined with electron energy loss spectroscopy, conventional magnetic measurements (direct and alternating current), X-ray photoelectron spectroscopy, and Raman spectroscopy was used to investigate the physical properties, chemical nature, and overall magnetic behavior of the resulting hybrids. A key point to the whole synthesis involves the functionalization of MWCNTs with carboxylic groups, which proved to be a powerful strategy for enhancing the ability to process MWCNTs and facilitating the preparation of hybrid composites. While in the majority of analogous hybrid materials the raw carbon material (multiwalled or single-walled nanotubes) is heavily treated to minimize the contribution of contaminant traces of magnetic nanoparticles with important effects on their electronic properties, this method can lead easily to elimination of the largest part of the impurities and provide an effective way to investigate/discriminate the magnetic contribution of the SMM molecules.

Carbon atoms in ethanol do not contribute equally to formation of single-walled carbon nanotubes

We propose a unique experimental technique in which isotopically labeled ethanol, e.g., 12CH3-13CH2-OH, is used to trace the carbon atoms during the formation of single-walled carbon nanotubes (SWNTs) by chemical vapor deposition (CVD). The proportion of 13C is determined from Raman spectra of the obtained SWNTs, yielding the respective contribution of ethanol's two different carbon atoms to SWNT formation. Surprisingly, the carbon away from the hydroxyl group is preferably incorporated into the SWNT structure, and this preference is significantly affected by growth temperature, presence of secondary catalyst metal species such as Mo, and even by the substrate material. These experiments provide solid evidence confirming that the active carbon source is not limited to products of gas-phase decomposition such as ethylene and acetylene, but ethanol itself is arriving at and reacting with the metal catalyst particles. Furthermore, even the substrate or other catalytically inactive species directly influences the formation of SWNTs, possibly by changing the local environment around the catalyst or even the reaction pathway of SWNT formation. These unexpected effects, which are inaccessible by conventional techniques, paint a clearer picture regarding the decomposition and bond breaking process of the ethanol precursor during the entire CVD process and how this might influence the quality of the obtained SWNTs.

Low-defect, purified, narrowly (n,m)-dispersed single-walled carbon nanotubes grown from cobalt-incorporated MCM-41

A mild, four-step purification procedure using NaOH reflux, HCl wash, and oxidation by 4 mol % molecular oxygen at 500 degrees C was developed to purify single-walled carbon nanotubes (SWCNTs) with narrow semiconducting (n,m) distribution produced from cobalt-incorporated MCM-41 (Co-MCM-41) in order to obtain bulk low-defect-density nanotubes. Three key features of Co-MCM-41 allow this mild purification technique: (1) ultrathin silica walls versus dense silica or other crystalline oxide supports are soluble in dilute NaOH aqueous solution, which avoids the damage to SWCNTs usually caused by using HF treatment to remove catalytic supports; (2) the small metallic particles are easily dissolved in HCl, a significantly milder chemical treatment compared to HF or HNO(3); (3) the high selectivity to SWCNTs with negligible multiwalled carbon nanotubes or graphite, which facilitates the removal of undesired carbon species by selective oxidation. The effectiveness of this purification procedure was evaluated by high-resolution transmission electron microscopy, scanning electron microscopy, Raman, UV-vis-NIR, and fluorescence spectroscopy, solution redox chemistry on fractionated (6,5) tubes, and SWCNT-based field effect transistor device performance. The results demonstrate that Co-MCM-41 catalyst not only provides tubes with narrow semiconducting (n,m) distribution but also allows a mild purification procedure and, therefore, produces SWCNTs with fewer defects.