Graphitization by Metal Particles

Abstract

Graphitization of carbon offers a promising route to upcycle waste biomass and plastics into functional carbon nanomaterials for a range of applications including energy storage devices. One challenge to the more widespread utilization of this technology is controlling the carbon nanostructures formed. In this work, we undertake a meta-analysis of graphitization catalyzed by transition metals, examining the available electron microscopy data of carbon nanostructures and finding a correlation between different nanostructures and metal particle size. By considering a thermodynamic description of the graphitization process on transition-metal nanoparticles, we show an energy barrier exists that distinguishes between different growth mechanisms. Particles smaller than ∼25 nm in radius remain trapped within closed carbon structures, while nanoparticles larger than this become mobile and produce nanotubes and ribbons. These predictions agree closely with experimentally observed trends and should provide a framework to better understand and tailor graphitization of waste materials into functional carbon nanostructures.

Investigation of Ni/Ni3C Nanoparticle Synthesis for Application as a Catalyst in Carbon Nanostructure Growth

“…This behavior suggests a higher propensity for the catalytic graphitization of these nanoparticles, leading to the formation of CNOs. 17,21 This process occurs due to the decomposition of carbon-supersaturated Ni 3 C regions, where carbon segregates and crystallizes as graphite carbon on the nanoparticle surface due to the greater thermodynamic stability of graphite carbon compared to Ni 3 C. 37 The presence of Ni 3 C in the surface of the nanoparticles can be observed in Figure 2 through fast Fourier transforms (FFTs) shown in Figure 2e,j, corresponding to the highresolution micrographs of Figure 2d,i for S 1 and S 2 , respectively. The obtained and indexed patterns show only crystalline planes corresponding to Ni 3 C. This observation, in addition to the XRD results, suggests the formation of nanoparticles with a Ni-fcc core and a Ni 3 C shell covered with the organic material adsorbed on their surface.…”

“…15,22,23 As observed, the formation of these structures is dependent on the carbon concentration in the nanoparticles; the higher carbon content in sample S2 resulted in thicker CNOs (see Figure S8 in the Supporting Information). 19,21,24,26 The stability and catalytic performance of these nanoparticles are greatly influenced by the thickness of the graphene layers. As an illustration, the electrical conductivity of these structures enables the active centers to capture photogenerated electrons in applications such as cocatalysts for the photocatalytic HER.…”

“…The increase in the quadrupole frequency (∼230 MHz) for site 2, establishing the formation of graphite around the nanoparticles, confirms that the generation of CNOs occurs during the annealing process and not exclusively by precipitation after cooling. 21 After thermal treatment at 700 K for the nanoparticles (S 1PAC and S 2PAC ), new PAC measurements were taken at room temperature (300 K). Figure 9 shows the R(t) functions and their corresponding FFTs.…”

“…However, the mechanisms governing the growth of graphitic nanostructures from nanoparticles are still under investigation. 21 Previous reports demonstrate that the growth of these nanostructures depends on parameters such as the temperature and synthesis atmosphere, as well as the size, morphology, and carbon content of the precursor nanoparticles. 15,22,23 Thus, transition-metal nanoparticles encapsulated by transition-metal carbides such as Fe/FeC, Co/Co 2 C, and Ni/Ni 3 C, with a well-established structure and morphology, can facilitate the systematic growth of graphitic nanostructures.…”

“…As a result, a number of synthesis protocols have been developed with the goal of controlling the shape, size, and functionality of the surfaces of these materials. 11,21 Among these synthesis methods, "bottom-up" processes such as polyol, 38 sonochemistry, 39 and thermal decomposition 40−42 should be highlighted. These methods are based on the controlled decomposition of metallic salts in the presence of ligands, whose function is to promote precursor reduction and stabilize nanoparticle formation during the growth stage.…”