(604g) Carbon Chemical Speciation and Polymerization in Liquid Metals

The synthesis of high-performance materials, particularly through the integration of metal and carbon, represents a pivotal area in materials science and chemical engineering. The exploration of metal-carbon covetics, characterized by their unique single-phase carbon-metal compositions that retain stability beyond melting temperatures, stands at the forefront of this research. This study investigates the complex chemistry of organic reactions within liquid metals, focusing on covetic materials such as aluminum, copper, and silver covetics in which carbon is polymerized into large metalographenes. Given the current state of knowledge, where the reproducibility and fundamental understanding of covetic synthesis pose significant challenges, this research addresses these gaps but also contributes novel insights into the synthesis mechanisms and properties of covetic materials.

Methods

The scope of this research encompasses the detailed analysis of the chemical speciation, reaction thermodynamics, and transport processes during the synthesis of molecular species within liquid metals. By employing advanced computational methods such as ab initio molecular dynamics (AIMD) and the conductor-like screening model (COSMO) for solvation, this study provides a comprehensive view of the chemical structure, redox chemistry, and polymerization of low molecular weight metalocarbon species. These metalocarbons, identified through AIMD within explicit metal atom solvents, exhibit covalent bonding between carbon atoms and metal atoms, suggesting the formation of novel structures such as metalographenes. Our methodology extends to the validation of these structures and their thermodynamic properties through comparative analysis with COSMO-DFT implicit solvation models. The results confirm the reproducibility of metalocarbon species structures and corroborate the Gibbs energies calculated from COSMO with those obtained from AIMD.

Results

The research explores the influence of charge and electric field on solution reaction mechanisms, providing insights into the transport and speciation of metalocarbons. AIMD simulations on metalocarbon systems subject to electric potential gradients reveal charge speciation by migration direction. Metalocarbons in AIMD are anionic species with cationic metal counter ions in electric fields between 0 and 100 V/m. The metalocarbon solute and metal solvent electric mobilities and anisotropic diffusivities are essential transport properties. Metalocarbon thermodynamic properties calculated by an efficient implicit solvation model accurately approximate average values calculated by AIMD with explicit metal solvent. The congruence between the values of AIMD and COSMO-DFT thermodynamic properties validates our using COSMO-DFT for calculations of metalocarbon thermochemistry and polymerization. Anionic metalocarbons are most probable in equilibrium aluminum, silver, and copper liquids. For all metalocarbons studied cationic metalocarbons exergonicly reduce to neutral metalocarbons, and neutral metalocarbons exergonicly reduce to anionic metalocarbon. These calculations are consistent with the AIMD metalocarbon charges computed by Mulliken population. The thermodynamics of polymerizing metalocarbons via concatenating carbon-carbon bonds are qualitatively similar for aluminum, silver, and copper species. Polymerization of neutral metalocarbons is exergonic without concomitant electron transfer in all three metals. Redox reactions greatly affect polymerization equilibria. Reduction polymerizations combining neutral and anion species producing an anion product have equilibrium constants range 1.4-10 in aluminum, 50-7,000 in copper, and 166-184,000 in silver. Oxidation polymerizations combining two anion species producing an anion product have equilibrium constants 0.9-1.8 in aluminum, 1.1-3.4 in copper, and 1.0-3.8 in silver. All these equilibrium constants increase with increasing reactant molecular weight. Depolymerization reactions can occur by reducing neutral species. Equilibrium constants are much less than unity for oxidation reactions combining a neutral and anion species and releasing an electron from a larger neutral species. Conversely it is highly exergonic for neutral species absorbing an electron to split into smaller neutral and anion species. Therefore, polymerization of large, aromatic metalocarbons is most favorable for anion and neutral species under reduction and for cation and neutral species without oxidation.

The application of inelastic neutron scattering (INS) spectroscopy, complemented by quantum chemical calculations, allows for the in-situ analysis of metalocarbon structures within bulk metal systems, circumventing the limitations of surface analyses of sectioned samples in previous studies. Our aluminum covetic synthesis uses the electro-charging assisted process. The VISION spectrometer at Oak Ridge National Laboratory was used to collect INS spectra. While the covetic spectrum contains several peaks in common with graphene, there are also high intensity, midrange wavenumber peaks in the graphene spectrum that are not present in the covetic spectrum. Ring breathing modes in INS spectra are sensitive to the molecular size and shape of a fused aromatic ring system. The INS spectrum of our experimental covetic does not support the existence of linear aluminoalkenes. The covetic peaks can be matched by linear combinations of Al₂₀C₅₄, Al₂₄C₇₀, Al₃₂C₉₆, Al₃₀C₁₂₆. This supports the hypothesis that large, fused-ring aluminocarbons, at least six rings wide, are present within the experimental aluminum covetic sample. For the first time it is shown feasible to analyze metalocarbon chemical structure in situ within the bulk metal system without being restricted to surface analysis of physically sectioned covetic materials. Inelastic neutron scattering vibrational spectroscopy reveals chemical signatures that can be predicted from ab initio quantum chemical structure and dynamics. This novel finding provides a unique avenue for identifying different metalocarbon compositions within covetics and may become an important tool to develop high performance materials in many different metals. Current work includes utilizing the methods outlined here for quantifying in situ polymerization kinetics within metals.

Implications

The conclusions drawn from this study provide compelling evidence of the formation and polymerization of metalocarbon species within liquid metal solvents. The identification of these species and the understanding of their synthesis mechanisms significantly advance the state of knowledge in covetic materials research. The implications of these findings are profound, offering potential pathways for the development of materials with enhanced mechanical, electrical, and thermal properties.

The research presented herein not only fills existing gaps in the literature concerning the reproducibility and understanding of covetic materials synthesis but also introduces novel computational and experimental methodologies for the study of metal-carbon interactions. The significant enhancements observed in the properties of covetic materials underscore the potential of this research in contributing to the development of next-generation high-performance materials.

Looking forward, this research opens up several avenues for further investigation. The exploration of larger metalocarbon species, their synthesis mechanisms, and the potential for orientational anisotropy within these materials present exciting challenges for future work. Additionally, the application of the methodologies developed in this study to other metal systems could unveil a broader range of high-performance covetic materials. The potential for scaling up the synthesis processes and the exploration of practical applications for these materials in industry are also critical areas for future research.