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Growth of Dendritic Nano-sized Carbon Spines in a Vortex Field over the Pt-aided Sublimation Frontier

Liang Hong
Published: 13 August 2020
Progress in Petrochemical Science , Volume 3, pp 1-6; doi:10.31031/pps.2020.03.000566

Abstract: Yi’en Zhou1 and Liang Hong2* 1,2Department of Chemical and Biomolecular Engineering, Singapore *Corresponding author: Liang Hong, Department of Chemical and Biomolecular Engineering, Singapore Submission: July 14, 2020;Published: August 13, 2020 DOI: 10.31031/PPS.2020.03.000566 ISSN 2637-8035Volume3 Issue4 When a cool N2-CO2 co-gas stream flows over superheated activated carbon (AC) flakes, a vortex field near the interface is induced as a consequence of the confrontation of the cool co-gas stream and the hot vaporized carbon stream, where the pre-deposited Pt atom clusters on AC catalyze gasification of AC by CO2 to produce CO, and the CO undergoes immediate disproportionation to release carbon atoms in vapor form. The vortex field functions as a dynamic template for deposition of carbon vapor, leading to the proliferation of nano-sized carbon needles with characteristic spikes (ca. 1µm) and short in length through an anisotropic assembling of carbon atoms. A trace amount of Pt pre-coated on the AC flakes is sufficient to catalyze gasifying AC by CO2. This phenomenon is the first observation over the surface of amorphous carbon via catalytic pyrolysis without electric potential assistance. Keywords: Nano carbon spines; Polypyrrole; HEC polymer; Raman spectra; Dendritic nano carbons; AC catalyze gasification; catalytic pyrolysis; Pt atom; CO gas treatment; Reverse boudouard reaction; AC flakes Dendritic growth is characterized by the presence of side branches that evolve under two different ways when the latent heat of fusion is removed from the interface [1]. Growth resulting from an undercooled melt (usually in alloys) results in equiaxed dendritic crystal formations when latent heat is dissipated through the cooler fluid at the interface whilst directional solidification or constrained growth results when the latent heat is dissipated swiftly. This study unveils that the Pt atom clusters assist the generation of carbon atoms forming a vapor stream via a two-step reaction mechanism, which is responsible for the growth of dendritic dense carbon nanofibers from a porous carbon flakes. Such dendritic growth has been observed previously in cells [2-5], crystals [6,7] and metal alloys [8-13] with a characteristic tree-like structure, which is considered as the result of mass transfer under meta-stable thermodynamic state. Namely, the growth happens through a series of thermodynamic instabilities when the growth rate is limited by the rate of diffusion of solute atoms to the interface and the material is supercooled at the same time [14]. Dendrites have shapes that are most suitable for heat and mass transfers at small scales and hence, are highly attractive for applications seeking these properties. Numerous studies undertaken over the years offered the insights in dendritic growth of crystals [7], as well as mathematical models and simulations [6, 12-13,15-18] about the growth. These growths are a result of faster material packing along energetically favorable crystallographic directions and may be due to anisotropy in the surface energy. In trying to minimize the area of these surfaces with the highest surface energy, the dendrite would exhibit a sharper and sharper tip as it grows [19]. When the crystallization front becomes morphologically unstable, small perturbations at the interface will lead to the formation of various polycrystalline structures, especially so for dendritic growth. The dendritic growth theory using Ivantsov transport theory relating to the dendrite tip radius and velocity of growth to the tip has been found to predict the growth rates and limitation of the existence of dendrites in 2D fairly accurately [1,14]. To the best of our knowledge, no reports or discussions on the dendritic carbon spinal growth in N2-CO2 co-gas atmosphere have been published nor observed before. Contrary to the growth of porous carbon fibers described explicitly in our previous work [20] caused by the random stacking of polyaromatic hydrocarbons (PAH) in the axial direction leading to the formation of fibers, this work proposes a different gowth path catalyzed by platinum atom clusters that assist with generation of CO via the reverse Boudouard reaction [21], which subsequently releases carbon atoms in vapor form that condense to form dense dendritic structures in the vortex field as illustrated in Figure 1. Although the incubation environment of this study is similar to that reported in [20], the resulting growth looks drastically different due to mediation by Pt atom clusters or colloids spread on the surface of the sample prior to the co-gas treatment. This paper aims to report and explain the growth mechanism we observed in detail. Figure 1: Preparation of AC Flakes An initial sample of 2-Hydroxyethyel cellulose (HEC) is carbonized by the method discussed elsewhere [22]. The resulting carbonaceous material from HEC was then activated at 700 oC under CO2 for 1 hour and cooled in an Ar purging stream. The carbon powder obtained was washed in water until the filtrate became colorless. This protocol resulted in an AC powder consisting of dense carbon flakes, which was used as the starting material for the preparation of the carbon needles. Carbon Spinal Growth and Characterizations Two separate, independent methods were employed to incorporate platinum onto the samples. The final carbon samples obtained from both Pt-deposition methods were characterized by electron microscopy (JSM-6700F Field Emission Scanning Electron Microscope (FESEM), JEOL), Raman spectroscopy (Renishaw in Via Raman Microscope), and X-ray Diffraction (XRD, Bruker D8 Advance, Cu Ka radiation, l=1.54Å) using Cu target Ka-ray (40kV and 30mA) as X-ray source, respectively. Formation of Dendritic Structures Over the Surface of AC Under Purge of CO-Gas Under close examination using the transmission electron microscope (TEM), tree-like dendritic structures are observed to have formed from the sputtering...
Keywords: Raman spectroscopy / Treatment / Structures / Vortex / Growth and Characterizations / Carbon Needles / Pt Atom Clusters

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