Ramon Colorado

Growth, new growth, and amplification of carbon nanotubes as a function of catalyst composition.

Carbon nanotubes (CNTs) have been grown using Fe, Co, Ni, and Co/Fe spin-on-catalyst (SOC) systems, involving the metal salt dispersed with a spin-on-glass precursor. During initial growth runs (CH4/H2/900 degrees C), the CNT yield followed the order Co-SOC > Fe-SOC >> Ni-SOC. The Fe catalysts produced the longest nanotubes at the expense of a larger average CNT diameter and broader diameter distribution than the Co-SOC system. A series of Co/Fe-SOCs were prepared where as the atomic percentage of Co is increased nucleation of CNT increases but the CNT length decreases. The linear relationship between the diameter and length of CNTs grown from the Co/Fe-SOC suggests that slow growth is beneficial with respect to control over CNT diameter. After initial CNT growth, the original samples were subjected to additional growth runs. Four individual reactions were observed in the Fe-SOC and binary Co/Fe-SOC: regrowth (amplification), double growth (a second CNT growing from a previously active catalyst), CNT etching, and nucleation from initially inactive catalysts (new growth). CNT etching was observed for the mixed catalyst systems (Co/Fe-SOC) but not for either Fe-SOC or Co-SOC. During the regrowth experiments, CNTs were observed that were not present after the initial growth run (and were not as a result of amplification or double growth). Thus, catalysts, which were initially inactive toward nucleation of CNTs in the original growth run, are capable of becoming activated when placed back into the furnace and submitted to regrowth under identical conditions.

Characterization of the surface structure of CH3 and CF3 terminated n-alkanethiol monolayers self assembled on Au{111}

Surface elemental and structural characterization of hexadecanethiol and heptadecanethiol (C16 and C17 for short) and 16,16,16-trifluorohexadecanethiol (FC16) self-assembled monolayers (SAMs) on a Au{111} surface have been obtained from time-of-flight scattering and recoiling spectrometry. The clean Au surface was also characterized in order to identify the azimuthal orientation of the SAMs with respect to the substrate. Classical ion trajectory simulations were used to relate the experimental scattering and recoiling data to the surface structure. The scattered and recoiled atoms originate from the outermost five–six atomic layers, and azimuthal anisotropy was observed in the measurements. The results provide a model for the SAMs in which the alkyl chains chemisorb with the S atoms situated above the face-centered-cubic (fcc) threefold sites of the Au{111} surface to form a continuous film with a (√3×√3)R30° structure that fully covers the Au surface. The orientation of the molecular axis azimuth of the ...

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.

Improved protein crystallization by vapor diffusion from drops in contact with transparent, self-assembled monolayers on gold-coated glass coverslips

The surfaces of glass coverslips of the type typically used for protein crystallization were modified with four types of transparent, chemically distinct self-assembled monolayers (SAMs). The SAM-functionalized surfaces exhibit a much higher degree of order and chemical uniformity than silanized glass, as judged by contact angle measurements. These characteristics lead to a marked increase in the range of solution conditions under which large crystals of lysozyme, α-lactalbumin, ribonuclease, hemoglobin, thaumatin, and catalase are observed to form. The results are rationalized in terms of a marked reduction in the rate of non-productive nucleation relative to the rate of crystal growth.

Single-walled carbon nanotube growth using [Fe3(µ3-O)(µ-O2CR)6(L)3]n+ complexes as catalyst precursors

We present herein the VLS growth of SWNTs from oxo-hexacarboxylate–triron precursors, [Fe3O(O2CCH3)6(EtOH)3] (1) and [Fe3O(O2CCH2OMe)6(H2O)3][FeCl4] (2), on spin-on-glass surfaces, using C2H4/H2 (750 °C) and CH4/H2 (800 and 900 °C) growth conditions. The SWNTs have been characterized by AFM, SEM and Raman spectroscopy. The characteristics of the SWNTs are found to be independent of the identity of the precursor complex or the solvent from which it is spin-coated. The as grown SWNTs show a low level of side-wall defects and have an average diameter of 1.2–1.4 nm with a narrow distribution of diameters. At 750 and 800 °C the SWNTs are grown with a range of lengths (300 nm–9 µm), but at 900 °C only the longer SWNTs are observed (6–8 µm). The yield of SWNTs per unit area of catalyst nanoparticle decreases with the growth temperature. We have demonstrated that spin coating of molecular precursors allows for the formation of catalyst nanoparticles suitable for growth of SWNTs with a high degree of uniformity in the diameter, without the formation of preformed clusters of a set diameter.

Neutralization of methyl cation via chemical reactions in low-energy ion-surface collisions with fluorocarbon and hydrocarbon self-assembled monolayer films

Low-energy ion-surface collisions of methyl cation at hydrocarbon and fluorocarbon self-assembled monolayer (SAM) surfaces produce extensive neutralization of CH3+. These experimental observations are reported together with the results obtained for ion-surface collisions with the molecular ions of benzene, styrene, 3-fluorobenzonitrile, 1,3,5-triazine, and ammonia on the same surfaces. For comparison, low-energy gas-phase collisions of CD3+ and 3-fluorobenzonitrile molecular ions with neutral n-butane reagent gas were conducted in a triple quadrupole (QQQ) instrument. Relevant MP2 6-31G*//MP2 6-31G* ab initio and thermochemical calculations provide further insight in the neutralization mechanisms of methyl cation. The data suggest that neutralization of methyl cation with hydrocarbon and fluorocarbon SAMs occurs by concerted chemical reactions, i.e., that neutralization of the projectile occurs not only by a direct electron transfer from the surface but also by formation of a neutral molecule. The calculations indicate that the following products can be formed by exothermic processes and without appreciable activation energy: CH4 (formal hydride ion addition) and C2H6 (formal methyl anion addition) from a hydrocarbon surface and CH3F (formal fluoride addition) from a fluorocarbon surface. The results also demonstrate that, in some cases, simple thermochemical calculations cannot be used to predict the energy profiles because relatively large activation energies can be associated with exothermic reactions, as was found for the formation of CH3CF3 (formal addition of trifluoromethyl anion).

Synthesis of silica–ammonium chloride macrofibers generated by anionic surfactant templated nanotubes

Silica-ammonium chloride nanotubes, generated by the condensation of sodium silicate onto ammonium cation-modified anionic sodium dodecyl sulfate (SDS) micellular templates, readily create hierarchical structures through the self-assembly into thicker nanotubes, belts, and ultimately into centimetre long macrofibers. The resulting fibers possess a remarkable range of tubular dimensions (i.e., nanometre to micrometre diameters, and micrometre to centimetre lengths) within a single assembly. The silica-ammonium chloride nanotubes have a high organic content as a result of the surfactant template. The nanotubes lose structural integrity in aqueous solution, but demonstrate retention of structure in organic solvents or when dried. The dependence on each of the component reagents for the generation of these nanotube structures has been investigated. The silica-ammonium chloride nanotubes have been characterized by SEM, TEM, XRD, XPS, and TGA. The creation of three-dimensional mineral structures using anionic surfactants more closely mimics biomineralization processes than prior approaches with cationic or non-ionic organic templates.

Self-assembly of organometallic clusters onto the surface of gold

We report the preparation and characterization of thin films generated via the solution-phase self-assembly of organometallic chalcogen-containing cluster compounds onto the surface of gold. The following anionic complexes were used as thin film precursors: wFe (CO) Ex and wHFe (CO) Ex , where EsS, Se, and Te. The films were prepared by adsorption from organic 2 yy 39 3 9 solvents (i.e. methanol, acetonitrile, acetone, and dicholoromethane ) onto evaporated gold and were characterized by ellipsometry, polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), X-ray diffraction (XRD), atomic force micros- copy (AFM), quartz crystal microbalance (QCM) and X-ray photoelectron spectroscopy (XPS). The data demonstrate that the clusters strongly attach to gold and that the films most likely exist as bilayers rather than monolayers. 2001 Elsevier Science B.V. All rights reserved.

Self-Assembled Monolayer and Multilayer Films of the Nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)68] on Gold

Films of the molybdenum-iron nanocluster [H x PMo 12O 40 subsetH 4Mo 72Fe 30(O 2CMe) 15O 254(H2O) 68] (FeMoC) were generated on gold via the self-assembly technique using two divergent routes. The first route entails the self-assembly of unfunctionalized FeMoC onto a preprepared carboxyl-terminated SAM on gold. The second route involves the preparation of thiol-terminated functionalized FeMoC clusters, which are then allowed to self-assemble onto bare gold surfaces. Monolayer films of FeMoC clusters are attained via both routes, with the second route requiring shorter immersion times (2 days) than the first route (6 days). Multilayer films of FeMoC are formed via the second route for immersion times longer than 2 days. Characterization of these films using optical ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy confirm the self-assembly of the clusters on the surfaces.

Self-Assembled Monolayers Derived from Bidentate Organosulfur Adsorbates

The adsorption of a series of 1,2-bis(mercaptomethyl)-4,5-dialkylbenzenes ( 1 ), spiroalkanedithiols ( 2 ), and aliphatic dithiocarboxylic acids (3) on gold yielded new types of self-assembled monolayers (SAMs). The new SAMs were characterized by optical ellipsometry, contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). These results were compared to those obtained from SAMs derived from normal alkanethiols ( 4 ) of analogous chain length. Comparisons of ellipsometric thickness and contact angle wettability showed that the new SAMs were well packed and highly oriented. Comparisons of the PM-IRRAS data showed that the SAMs generated from 1 and 2 exhibited slightly less crystallinity than their alkanethiolate-based analogs, while the SAMs generated from 3 exhibited comparable crystallinity to their alkanethiolate-based analogs. Moreover, the SAMs derived from 1 and 2 showed no “odd-even” wettability or PM-IRRAS effects, while those derived from 3 showed remarkably large “odd-even” effects compared to those derived from normal alkanethiols.