We aim to understand the critical reactions of geological and environmental processes in a molecular level. Recent research has focused on sorption and redox reactions occurring in clay minerals, metal oxides and metal sulfides which impact the formation of ore deposits, mobility of contaminants, and climate change. Our research interest also extends to development of advanced materials for energy generation/ storage and for safe storage of nuclear waste. Main research approaches include quantum mechanical computations and molecular dynamics simulations.
PI: Kideok D. Kwon
Many transition metals bind to Mn(IV) vacancy sites in hexagonal birnessite to form two types of interlayer surface complexes: triple-corner-sharing inner-sphere surface complexes (TCS species) and incorporation inside the vacancy site (INC species). The TCS structures are similar to the molecular structure of metal cations found in ernienickelite, jianshuiite, and chalcophanite (Table 1.1). The ratio of INC/TCS differs depending on metal types and sorption pH conditions. Using density functional theory (DFT) computations, we were able to suggest the principles behind the partitioning trends. Important findings in this study are (1) protonation states of the vacancy sites affect the relative ratio of metal species and (2) stereoactive 3d orbitals stabilize the incorporation of metal cations larger than Mn(III) or Mn(IV) inside the vacancy, but Zn does not take advantage of the stabilization because such orbitals are not present in Zn ion (Kwon et al. 2013).
<Table 1.1> Structural parameters (Å) of chalcophanite group minerals predicted by DFT (Kwon and Spositio, 2015)
Lateral edge surfaces of birnessite are also important metal-biding sites particularly in nanoparticles. However, the edge surface itself is little known experimentally. Our DFT geometry-optimized Mn oxide nanodisk (diameter of approximately 2 nm) provided insights on the edge structures by comparison to the planar periodic Mn octahedral sheet (Figure 1.1). The nanodisk exhibited a slightly concave or convex curvature instead of being planar (similar to a potato chip), although the curvature was not as pronounced as the recently-proposed spherically or cylindrically bent model for a δ-MnO2 nanoparticle (Manceau et al., 2013). The nanodisk showed a broad distribution of the interatomic distances mainly arising from distorted Mn octahedra at the edge surfaces.
<Figure 1.1> Distribution of O-O distances in two Mn oxide models calculated by DFT. A nanodisk model exhibited significant structural distortions at the lateral edges compared to a periodic planar sheet model (Kwon and Spisito, 2015).
DFT was also able to resolve highly disordered metal species adsorbed at the edge surfaces (Kwon and Sposito, 2015). New types of surface complexes were found and the effects of Mn(III) presence on the structure of the edge species were explored to complement spectral analysis of edge species (Figure 1.2).
<Figure 1.2> DFT-calculated surface complexes of Ni at the Mn oxide edge sites (Simanova et al., 2015).
Mackinawite (tetragonal FeS) comprises edge-sharing sheets of FeS4 tetrahedra stacked along the c direction with van der Waals (vdW) interactions. The sulfide mineral is very reactive in redox chemistry such reduction of uranium, chromium, and mercury. The high reactivity in the electron transfer is attributed to the fact that Fe in FeS is metallic (Figure 2.1). Fe L-edge X-ray absorption spectra (XAS) and DFT-calculated density of states (DOS) of the FeS electronic states all indicate that Fe of FeS is metallic.
<Figure 2.1> Mackinawite (FeS) is a layer-type iron sulfide mineral, in each sheet of which a metallic Fe plane is sandwiched by two S atomic planes (Kwon et al., 2011).
In sulfide ore deposits, mackinawite possesses transition metals such as Co, Ni and Cu. The transition-metal-incorporated mackinawite may transform into various metal sulfide assemblages. Unfortunately, the structure and stability about the metal-rich mackinawite are little examined in experiments. We first examined systematically the relationships among the chemical composition, structure, and thermodynamic stability of transition-metal-incorporated mackinawite by application of DFT with dispersion correction and DFT atomistic thermodynamics methods. Our results showed that transition metals tend to incorporate into mackinawite by substitution of the Fe sites within the FeS4 tetrahedral sheets (Figure 2.2). Metal substitution tends to enhance the stability of mackinawite. Our findings are useful to understand the formation paths of metal sulfides in ore deposits and develop enhanced metal-sulfide catalysts.
<Figure 2.2> Substitution and intercalation site of trace elements in the crystal structure of FeS(mackinawite) and DFT-calculated formation energy for each case as a function of the chemical potential of sulfur at 300 K, 600 K, and 900 K (Kwon et al., 2015).
Compared to the basal plane, edge surfaces of 2:1 clay minerals are not clearly characterized. We have examined the edge surface structure and stability of pyrophyllite, which has no or few structural charges, by application of DFT atomistic thermodynamics. We found that the stability of the AC and B edge surfaces greatly changes at different temperatures and humid conditions. We also examined defects occurring at the edge surface (Figure 3.1). The defect structure and stability investigation suggests that the edge defects may impact the metal binding activity and the equilibrium morphology of pyrophyllite crystals.
<Figure 3.1> Charge distribution in the AC edge surface of pyrophyllite without a defect and with defects.
Two dimensional MoS2-graphene hybrids are promising energy materials for ultrafast pseudocapacitance. Our collaborator, Prof. Ho Seok Park (Sungkyunkwan U.), observed a phase transition from 2H MoS2 to 1T MoS2 at the interface with reduced graphene oxide (Figure 4.1). We performed DFT calculations for 1H-MoS2 nanosheet/graphene and 1T-MoS2 nanosheet/graphene systems in order to obtain insights into the phase transform. Isolated 1T-MoS2 showed a higher total energy than isolated 1H-MoS2, implying 1T type is more stable than 1H type. However, the binding energy of 1T-MoS2/graphene was greater than that of 1H-MoS2/graphene. This greater binding energy may explain the phase shift at the interface. Because triangular clustering among Mo atoms was found in the 1T-MoS2/graphene hybrid with slight rumpling of the S atomic plane, the structural distortion may play a role in the transformation from 1T to 1H phase.
<Figure 4.1> Electron microscopic image of MoS2-graphene interface and DFT models of two possible interfaces (Mahmood et al., in press).
Phosphorus (P) incorporated graphene exhibits reversible and fast pseudocapacitance (Yu et al., 2015). We performed DFT calculations to obtain insights in the structure of P-doped graphene (P-GRP). Geometry optimization found that P-GRP is more stable when P is outside the graphene sheet than when it is within the sheet plane. The former induced spin polarization on P and neighboring C, but the latter did not. In situ FT-IR spectroscopy identified PO sites from partially oxidized P-GRP (PO-GRP). Our PO-GRP model showed a greater rumpling than P-GRP (Figure 4.2), but it did not induce any spin polarization. Adsorption of proton was much greater at the PO site than at C site in PO-GRP.
<Figure 4.2> Possible proton sites on a partially oxidized P-doped graphene (Yu et al., 2015).