Browsing by Author "Esquenazi, Gibran L."

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    Catalytic Growth of Carbon Nanotubes by Direct Liquid Injection CVD Using the Nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y]
    (MDPI, 2018) Esquenazi, Gibran L.; Brinson, Bruce; Barron, Andrew R.
    The growth of carbon nanotubes (CNTs) by direct liquid injection chemical vapor deposition (DLICVD) has been studied using the polyoxometalate cluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y] (FeMoC) as the catalyst with either ethanol or toluene as the carbon source. In order to screen different growth conditions a single large batch of FeMoC is required in order to eliminate variation in the catalyst precursor. The preparation of 6 g of FeMoC is possible by scaling (10×) literature reagent ratios. DLICVD studies of the FeMoC derived carbon product were evaluated by Raman spectroscopy and scanning electron microscopy (SEM) to determine the quality (G:D ratio) and purity of CNT content. With the use of ethanol as the carbon source, increasing the temperature in the injection zone (aspiration temperature) above 250 °C increases the yield, and results in a slight increase in the G:D ratio. The maximum yield is obtained with a growth temperature of 900 °C, while the G:D ratio is the highest at higher temperatures. Faster solution injection rates increase yield, but with a significant decrease in G:D, in fact no CNTs are observed in the product for the highest injection rate (10 mL/h). An optimum catalyst concentration of 1.25 wt.% is found, which influences both the catalyst:C and catalyst:H ratios within the system. Growth at 800 °C is far more efficient for toluene as a carbon source than ethanol. The resulting “process map” allows for large quantities of CNTs to be prepared by DLICVD.
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    Electrodeposition of Cu-SWCNT Composites
    (MDPI, 2019) Raja, Pavan M. V.; Esquenazi, Gibran L.; Gowenlock, Cathren E.; Jones, Daniel R.; Li, Jianhua; Brinson, Bruce; Barron, Andrew R.
    Single walled carbon nanotubes (SWCNTs) are used as a component of a plating solution of CuSO4 for direct current electrodeposition of Cu–SWCNT composites with varying nanotube proportions without the use of either a surfactant, a dispersing agent, or functionalization of the SWCNTs. The Cu–SWCNT composites are characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy. The composites are comprised of metallic Cu and SWCNTs with minor oxide impurities, as well as the residual (Fe) catalyst from the unpurified SWCNTs, in addition to displaying nanotube-mediated morphological differences. EDX analysis of carbon (wt%) is close to quantitative with respect to the wt% of SWCNTs added to the electrolysis solution. The presence of SWCNTs decreases the oxidation of the copper, as well as changing the identity of the oxide from CuO, for electrolysis of Cu, to Cu2O. Hard adherent Cu–SWCNT coatings are prepared by the addition of Cu powder to the electrolysis solution. The approach described in this paper will enable controlled synthesis of metal-nanomaterial composites that can potentially be processed further into high ampacity electrical conductors.
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    Investigation of the Reduction of a Molybdenum/Iron Molecular Nanocluster Single Source Precursor
    (MDPI, 2018) Esquenazi, Gibran L.; Barron, Andrew R.
    The thermolysis of the polyoxometalate cluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)90−y(EtOH)y] (1) under air, argon, and reducing conditions (5%, 10%, 50% H2 with Ar balance) has been investigated. The resulting products have been characterized by XRD, SEM, and EDX analysis. Thermolysis in air at 1100 °C yields predominantly Fe2O3, due to sublimation of the molybdenum component; however, under Ar atmosphere, the mixed metal oxide (Fe2Mo3O8) is formed along with Mo and MoO2. Introduction of 5% H2(1100 °C) results in the alloy Fe2Mo3 in addition to Fe2Mo3O8 and Mo; in contrast, reduction at a lower temperature (900 °C) yields the carbide (Fe3Mo3C) and the analogous oxide (Fe3Mo3O), suggesting that these are direct precursors of Fe2Mo3. Increasing the H2 concentration (10%) promotes carbide rather than oxide formation (Fe3Mo3C and Mo2C), until alloy formation (Fe7.92Mo5.08) predominates under 50% H2 at 1200 °C. The effect of temperature and H2 concentration on the composition, grain size, and morphology has been investigated by EDX, SEM, and XRD. The relationship of the composition of 1 (i.e., Fe:Mo = 30:84) with the product distribution is discussed.
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    Reduction Kinetics of the Nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y]
    (Springer, 2018) Esquenazi, Gibran L.; Barron, Andrew R.
    The reduction of the molecular iron-molybdenum-nanocluster, [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y](FeMoC), was studied using model free isoconversional methods. The reduction kinetics were evaluated using the non-isothermal thermogravimetric measurements at four different heating rates from 5 to 20 °C/min in a 5% hydrogen atmosphere (argon balance). The apparent activation energy dependence on conversion derived from the isoconversional Kissinger–Akahira–Sunose (KAS) and Vyazovkin methods reveals a complex multi-step process with values ranging from 60.8 ± 13.3 to 183 ± 6.3 kJ/mol. The kinetic results were validated by isothermal predictions. The results herein are useful for optimization and development of FeMoC derived Fe–Mo nanoalloy systems.
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    Understanding the “Activation” of the Nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y] for Low Temperature Growth of Carbon Nanotubes
    (Springer, 2018) Esquenazi, Gibran L.; Barron, Andrew R.
    The molecular nanocluster HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y(FeMoC), was the first molecular catalyst precursor (pro-catalyst) that promised controlled growth of carbon nanotubes (CNTs); however, temperatures in excess of ~ 900 °C or the addition of excess iron were required as catalyst promoters for CNT growth. To understand these disappointing results the “activation” of FeMoC for CNT growth was studied by systematic investigation of H2 gas concentration and growth temperature. The pathway for “activation” of FeMoC occurs through the sufficient reduction of both metal oxide components in the pro-catalyst. By ensuring pro-catalyst reduction prior to introduction of growth gases, we demonstrate for the first time, growth of CNTs at temperatures as low as 600 °C without the use of catalyst promoters using the single molecular precursor, FeMoC. To understand the role of catalyst promoters used in prior work, thermogravimetric analysis experiments were performed. The addition of an iron catalyst promoter is observed to play two key roles in the “activation” of FeMoC: (1) to replenish sublimated metal atoms, and (2) to reduce the reduction temperature required for reduction of FeMoC into an “active” catalyst. These results caution the conditions employed in many earlier studies for CNT growth, and create new possibilities for molecular pro-catalysts.