Elizabeth A. Whitsitt

LPD silica coating of individual single walled carbon nanotubes

Single walled carbon nanotubes (SWNTs) have been coated with fluorine-doped silica by liquid phase deposition (LPD) using a silica–H2SiF6 solution and a surfactant stabilized solution of SWNTs. The coating of individual SWNTs versus small ropes is controlled by the choice of surfactant. Since the LPD reaction is performed close to the isoelectric point of the silica, some of the SiO2–SWNTs are fused together but the SWNTs remain individual in these composite structures. The SiO2–SWNTs have been characterized by SEM, TEM, Raman and IR spectroscopy, and XPS. Raman fluoresence is maintained even with coatings >50 nm. Using the relative intensity of the Raman G peak and the 8,3 SWNT fluorescence as a convenient measure of bundling, it may be shown that any decrease of fluoresence during growth is not due to changes in ionic strength or pH, as a consequence of addition of the LPD solution or the presence of HF as a side product in the deposition. The Raman D and G modes show no change in intensity, while the Fano line increases, both suggesting that no sidewall functionalization or proton adsorption of the SWNTs occurs during coating. The UV-visible-near infrared spectra shows a red shift in the first Van Hove transitions of the coated SWNTs inferring that the SWNTs in SiO2–SWNTs are in a more polarizable and inhomogeneous environment than that of surfactant solutions. Mats of SiO2–SWNT may be deposited onto silicon and gold substrates and through lithography may be patterned by etching off selected areas of the silica coating.

A study of the formation, purification and application as a SWNT growth catalyst of the nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98]

The synthetic conditions for the isolation of the iron-molybdenum nanocluster FeMoC [HxPMo12O40 [subset]H4Mo72Fe30(O2CMe)15O254(H2O)98], along with its application as a catalyst precursor for VLS growth of SWNTs have been studied. As-prepared FeMoC is contaminated with the Keplerate cage [H4Mo72Fe30(O2CMe)15O254(H2O)98] without the Keggin [HxPMo12O40]n- template, however, isolation of pure FeMoC may be accomplished by Soxhlet extraction with EtOH. The resulting EtOH solvate is consistent with the replacement of the water ligands coordinated to Fe being substituted by EtOH. FeMoC-EtOH has been characterized by IR, UV-vis spectroscopy, MS, XPS and 31P NMR. The solid-state 31P NMR spectrum for FeMoC-EtOH (delta-5.3 ppm) suggests little effect of the paramagnetic Fe3+ centers in the Keplerate cage on the Keggin ions phosphorous. The high chemical shift anisotropy, and calculated T1 (35 ms) and T2 (8 ms) values are consistent with a weak magnetic interaction between the Keggin ions phosphorus symmetrically located within the Keplerate cage. Increasing the FeCl2 concentration and decreasing the pH of the reaction mixture optimizes the yield of FeMoC. The solubility and stability of FeMoC in H2O and MeOH-H2O is investigated. The TGA of FeMoC-EtOH under air, Ar and H2 (in combination with XPS) shows that upon thermolysis the resulting Fe : Mo ratio is highly dependent on the reaction atmosphere: thermolysis in air results in significant loss of volatile molybdenum components. Pure FeMoC-EtOH is found to be essentially inactive as a pre-catalyst for the VLS growth of single-walled carbon nanotubes (SWNTs) irrespective of the substrate or reaction conditions. However, reaction of FeMoC with pyrazine (pyz) results in the formation of aggregates that are found to be active catalysts for the growth of SWNTs. Activation of FeMoC may also be accomplished by the addition of excess iron. The observation of prior works reported growth of SWNTs from FeMoC is discussed with respect to these results.

Silica coated fullerenols: seeded growth of silica spheres under acidic conditions

Liquid phase deposition of silica in the presence of fullerenol C60(OH)n, results in the formation of uniform silica spheres, whereas the use of C60 gives large non-uniform agglomerates as a result of homogeneous nucleation. Raman and UV spectroscopy indicate the C60 is retained as the core of the silica spheres.