Gyeong S. Hwang

Aggregation of ion pairs formed during CO2 capture may govern absorption thermodynamics and kinetics, but it is often overestimated by fixed-charge classical force fields. Here, we demonstrate the promise of graph neural network interatomic potentials, which offer quantum-level accuracy, for directly evaluating the aggregation tendencies of ionic products in CO2-loaded aqueous amine solutions.

Silicon carbon nitride (SiCN) is a ceramic material known for its exceptional thermal stability, chemical resistance, and mechanical strength. Compared to silicon nitride (Si3N4), SiCN offers enhanced material properties due to its tunable elemental composition and microstructure. However, the precise relationship between the microstructure of SiCN and its material properties remains inadequately understood. This study aims to elucidate the structure–property relationship by correlating the bonding environment of SiCN with its material properties. Specifically, this work investigates the effects of incorporating [Si–C] and [N=C] bonds into amorphous SiN films. Amorphous SiCN models with diverse Si, N, and C compositions were constructed and analyzed in terms of film stability, dielectric constant, refractive index, bandgap, bulk modulus, and bond densities. We find that [N=C] bonds significantly reduce the dielectric constant and mechanical strength, while [Si–C] bonds preserve mechanical properties but increase the dielectric constant. This study can provide valuable insights for tailoring these properties to meet specific application requirements.

Our study explores a plasma-free approach to remove H+/Cl− pairs, a common reaction by-product, after chlorosilane-type Si precursors (such as dichlorosilane and SiH2Cl2) dissociate on a hydrogen-terminated SiN surface. Although previous studies have suggested that H radicals in NH3 and CH3NH2 plasma may remove some Cl impurities, these methods have limitations that in high-aspect-ratio structures, the recombination of radicals may be too fast rendering them unable to reach the trench bottom intact[H. C. M. Knoops et al., J. Electrochem. Soc. 157, G241 (2010)]. This may lead to Cl accumulation at the trench and passive layer formation, which inhibits further film growth and reduces conformality. Introducing polar molecules, such as NH3, N2H4, CH3NH2, and (CH3)2NH, is an alternative solution. Density functional theory calculations show that when these polar molecules interact with the surface, they first abstract H+ from the H+/Cl− pair and capture Cl−, which may then be solvated by surrounding polar molecules to form a “microsolvation cluster.” This cluster is energetically stable and envelops Cl− while weakening the interactions between Cl− and surface amine groups, creating a pathway for HCl desorption with a lower energy cost. Our metadynamics simulations show that when the surface is saturated with CH3NH2 molecules, the energy barrier for HCl desorption can be reduced significantly from 0.5 to 0.3 eV at 300 K, resulting in a ∼5000 times increase in the desorption rate, assuming an Arrhenius rate expression. Our findings suggest that by-products generated by halogen-containing precursors can be removed at room temperature via “microsolvation cluster” formation, without requiring plasma assistance.

Nickel-based compound catalysts have been shown to have significantly higher alkaline hydrogen evolution activity than pure Ni. We investigate the composite Ni/Ni¬3S2. First, simulated annealing of a previously generated Ni/Ni3S2 heterostructure uncovered S-diffusion to the Ni surface. Grand canonical calculations indicate pristine Ni may be lightly covered by OH at alkaline conditions, in contrast to Pt/Pd, and reveal that S prevents OH adsorption. Static calculations showed no significant effect on kinetics. Using metadynamics to dynamically sample the Volmer step, we show an alignment in entropic effects on Ni and Pt. Low-temperature sampling on Ni resulted in a significantly lower barrier indicating a dependence on solvation consistency. Our characterization of the Ni and Pt interfaces from unbiased ab initio molecular dynamics show similar solvent structures, further supporting this line of thinking. Finally, calculations on OH- and S-modified Ni surfaces found that the former had a greater Volmer barrier by 0.16 eV. This OH-modified surface had a significantly less consistent solvation environment, with an average hydrogen bond lifetime 14 fs less than that of the S-modified surface. These results suggest that the enhanced activity of composite Ni/Ni3S2 originates from adsorbed S on Ni preventing OH-binding and the associated decline in interfacial solvation.

Metal single-atoms embedded in nitrogen-doped graphene (so-called MNCs) are a promising group of electrocatalysts for the oxygen reduction reaction. Previous calculations have simplified the influence of applied potential on electrocatalytic activity by extrapolation from neutral calculations with a linear, Nernstian term. Here we utilize grand canonical density functional theory to account for non-Nernstian charging behavior and evaluate reaction energies at applied potentials for a set of single atom catalysts (Mn to Cu). We briefly illustrate the correspondence between the semi-discrete density of states in these systems and features in their grand canonical free energy curves. We also explore the effect of adsorbed *OO on predicted activity, as treatment of that state has been inconsistent in literature. On surfaces where *OO is most stable, Fe and Co, significant overpotential differences are visible. Importantly, we find significant deviations between Nernstian and non-Nernstian treatments, with effects on overpotential following no clear trend across metals. This lack of a trend is partially caused by a variance in the potential-determining step across metals, which results in the potential responses of different adsorbates contributing to the activities of each metal. This work highlights the importance of explicit consideration of potential when modeling single-atom electrocatalysts.

Theory reveals the reactive oxygen species (ROS) responsible for oxidative degradation (OD) of amine solvents during CO₂ capture. Further computations identify the metal complexes responsible for ROS generation and the initiation step of OD.